publications
publications by categories in reversed chronological order. generated by jekyll-scholar.
2025
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Addressing Buffer, Size, and Clogging Challenges in LAMP-Coupled Solid-State Nanopores for Point-of-Care TestingMing Dong, Aneesh Kshirsagar, Anthony J. Politza, Muhammad Asad Ullah Khalid, Md Ahasan Ahamed, and Weihua GuanAnalytical Chemistry, Apr 2025Loop-mediated isothermal amplification (LAMP) is a promising method for point-of-care nucleic acid testing due to its simplicity, rapidity, and high sensitivity. Coupling LAMP with solid-state nanopores enables label-free, single-molecule sensing, enhancing diagnostic accuracy. However, conventional LAMP-coupled nanopore protocols require high-salt buffers (\textgreater1 M) to improve signal strength and translocation frequency, complicating workflows and increasing contamination risks. In native LAMP buffers (50 mM KCl), electroosmotic flow (EOF) hinders amplicon transport in sub-10 nm pores, while large amplicons increase the risk of clogging. These challenges limit event rates, data throughput, and device reliability. To address these limitations, we developed a glass nanopore device optimized for direct sensing of amplicons in native buffers, featuring integrated declogging capabilities. Our results revealed that 200 nm pores provided the best balance between minimizing EOF interference and maintaining strong signal strength, achieving the highest event rates. Smaller pores (\textless100 nm) had low event rates due to EOF effects, while larger pores (\textgreater1 μm) showed weakened signal strength. We discovered that clogging in low-salt conditions differs from high-salt environments, with physical vibration effectively resolving clogging in low-salt settings. This led to the integration of an automated vibration motor, extending nanopore lifespan and ensuring continuous data acquisition. Our clog-free, native-buffer sensing platform demonstrated a sensitivity of 0.12 parasite/μL using Plasmodium vivax (P. vivax) as a model organism, exceeding the threshold for detecting asymptomatic infections. These advancements highlight the potential of our nanopore device for rapid, reliable, and user-friendly diagnostics for point-of-care testing.
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Compact multiplex PCR device for HIV-1 and HIV-2 viral load determination from finger-prick whole blood in resource-limited settingsTianyi Liu, Anthony J. Politza, Md Ahasan Ahamed, Aneesh Kshirsagar, Yusheng Zhu, and Weihua GuanBiosensors and Bioelectronics, Mar 2025The human immunodeficiency virus (HIV) remains a major global health concern for which accurate viral load monitoring is essential for the management of HIV infection. The advent of antiretroviral therapy (ART) has transformed once-fatal HIV disease into a manageable chronic condition that now makes the need for VL testing which aims to satisfy international suppression targets 95-95-95 al l the more essential. Therefore, considering the complexity and diversity of HIV infection, it is essential to develop rapid diagnostic technologies suitable for different clinical situations. Here, we report on a multiplexed PCR device developed for simple and efficient quantification of HIV-1 or HIV-2 viral loads using finger-pricked whole blood from rural decentralized settings. This device is comprised of a previously developed RNA extraction module combined with an optimized real-time PCR amplification system. Together, these combine to simultaneously detect and differentiate HIV-1 & 2; as well are adopting a testing control of RNase P allowing for full diagnostic analysis from one sample. Our device also includes an intuitive user interface and is completely autonomous so it can serve individuals in remote areas who are unfamiliar with the field of medical testing. They get the results in a very short time of around 70 min and hence save on testing times without leaving accuracy behind. The efficiency and effectiveness of the device were validated through the analysis of 30 clinical samples, yielding a sensitivity of 100% for both HIV-1 and HIV-2. The specificity was found to be 100% for HIV-1 and 90.91% for HIV-2, demonstrating high diagnostic accuracy. One of the most attractive things about this device is that it comes in comparison to all other counterparts. Given that you can run the assay for less than $10, it could be an economically viable way to use this as a broadscale test in regions where healthcare budgets don’t allow others. Hence it is quite a useful device to aid HIV management in resource-limited settings, where conventional laboratory facilities are out of reach due its simplicity and affordability with rapid output. The point-of-care test is an effective, low-cost, high quality diagnostic tool-promoting rapid testing for HIV-inexpensively overcoming the barriers to efficient control of and care in resource-limited settings.
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CRISPR-based strategies for sample-to-answer monkeypox detection: current status and emerging opportunitiesMd Ahasan Ahamed, Anthony J Politza, Tianyi Liu, Muhammad Asad Ullah Khalid, Huanshu Zhang, and Weihua GuanNanotechnology, Jan 2025The global health threat posed by the Monkeypox virus (Mpox) requires swift, simple, and accurate detection methods for effective management, emphasizing the growing necessity for decentralized point-of-care (POC) diagnostic solutions. The clustered regularly interspaced short palindromic repeats (CRISPR), initially known for its effective nucleic acid detection abilities, presents itself as an attractive diagnostic strategy. CRISPR offers exceptional sensitivity, single-base specificity, and programmability. Here, we reviewed the latest developments in CRISPR-based POC devices and testing strategies for Mpox detection. We explored the crucial role of genetic sequencing in designing crRNA for CRISPR reaction and understanding Mpox transmission and mutations. Additionally, we showed the integration of CRISPR-Cas12 strategy with pre-amplification and amplification-free methods. Our study also focused on the significant role of Cas12 proteins and the effectiveness of Cas12 coupled with recombinase polymerase amplification (RPA) for Mpox detection. We envision the future prospects and challenges, positioning CRISPR-Cas12-based POC devices as a frontrunner in the next generation of molecular biosensing technologies.
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Hydrogel interfaced glass nanopore for high-resolution sizing of short DNA fragmentsMuhammad Asad Ullah Khalid, Md. Ahasan Ahamed, Ming Dong, Aneesh Kshirsagar, and Weihua GuanBiosensors and Bioelectronics, Jan 2025Solid-state nanopores, known for their label-free detection and operational simplicity, face challenges in accurately sizing the short nucleic acids due to fast translocation and a lack of enzyme-based control mechanisms as compared to their biological counterparts with sizing resolutions still limited to ≥100 bp. Here, we present a facile polyethylene glycol-dimethacrylate (PEG-DMA) hydrogel interfaced glass nanopore (HIGN) system by inserting glass nanopore into the hydrogel to achieve sub-100 base pair (bp) resolution in short DNA sizing analysis. We systematically investigated the effects of hydrogel mesh size, spatial configurations of glass nanopores about the hydrogel, applied bias voltage, and analyte concentration on the transport dynamics of 200 bp double-stranded DNA (dsDNA). A 7.5 w/v% PEG-DMA hydrogel induced ∼11x increase in the mean dwell times compared with bare solution nanopore system. The insertion locations and depths of the glass nanopore into the hydrogel resulted in 7.16% and 5.28% coefficients of variation (CV) for mean normalized event frequencies. This enhancement of dwell times and invariability in translocation characteristics enables precise sizing of dsDNA fragments under 400 bp using HIGN, with an achieved size resolution of 50 bp with observable mean normalized peak amplitude (ΔI/Io) of ∼0.005. Furthermore, we have demonstrated the capability of HIGN to perform multiplex detection of influenza A virus (IAV) and severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) through reverse transcriptase-polymerase chain reaction (RT-PCR). These results demonstrated the potential of HIGN as a versatile tool in nucleic acid analysis and multiplexed label-free molecular diagnostics.
2024
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Development and validation of a portable device for lab-free versatile nucleic acid extractionAnthony J. Politza, Tianyi Liu, Aneesh Kshirsagar, Ming Dong, Md. Ahasan Ahamed, and Weihua GuanBioTechniques, Oct 2024Nucleic acid testing (NAT) has revolutionized diagnostics by providing precise, rapid, and scalable detection methods for diverse biological samples. These recent advancements satisfy the increasing demand for on-site diagnostics, yet sample preparation remains a significant bottleneck for achieving highly sensitive diagnostic assays. There is an unmet need for compatible, efficient, and lab-free sample preparation for point-of-care NAT. To address this, we developed a portable, lab-free, and battery-powered device for extracting nucleic acids. We explored using low centrifugal forces with existing commercial chemistry, demonstrating excellent performance. We designed and tested a battery-powered device to enable lab-free extractions, and verified reagents stored out to 6 months, suggesting exceptional deployment capabilities. We evaluated our device, comparing our results against those from a benchtop centrifuge across three types of samples: HIV RNA in buffer, HIV RNA in plasma, and SARS-CoV-2 RNA in saliva. The portable device demonstrated excellent agreement with the benchtop centrifuge, indicating high reliability. By providing an effective on-site sample preparation solution, the widespread adoption of low centrifugal extractions will improve the sensitivity and reliability of NAT and will positively impact other point-of-care technologies such as next generation sequencing (NGS), biomarker detection, and environmental monitoring. This method utilizes a low-power, portable centrifuge to significantly improve the deployment of nucleic acid extractions. As a result, this method offers comparable extraction performance to benchtop devices while offering superior portability and ease of use. Minimizing centrifugal force allows for reliable nucleic acid extraction from a low-power device. Our approach is simple and uses low-cost electronics, presenting high potential for clinical preparation of RNA in field settings. Sample preparation continues to be a major bottleneck for sensitive diagnostic assays.We developed a portable, lab-free, and battery-powered device for extracting nucleic acids.We demonstrated that a commercial extraction kit could be processed at low centrifugal forces, enabling point-of-care development.We designed a battery-powered, semi-automated centrifuge that rivaled a benchtop centrifuge in performance and efficiency.Reagents remained stable for 6 months and extractions were robust without carrier RNA.Our device offers an effective on-site sample preparation solution that enhances NAT sensitivity and reliability while maintaining compatibility with commercial chemistry.Consequently, our portable centrifuge is well-positioned to impact other point-of-care applications like NGS, biomarker detection, and environmental monitoring. Sample preparation continues to be a major bottleneck for sensitive diagnostic assays. We developed a portable, lab-free, and battery-powered device for extracting nucleic acids. We demonstrated that a commercial extraction kit could be processed at low centrifugal forces, enabling point-of-care development. We designed a battery-powered, semi-automated centrifuge that rivaled a benchtop centrifuge in performance and efficiency. Reagents remained stable for 6 months and extractions were robust without carrier RNA. Our device offers an effective on-site sample preparation solution that enhances NAT sensitivity and reliability while maintaining compatibility with commercial chemistry. Consequently, our portable centrifuge is well-positioned to impact other point-of-care applications like NGS, biomarker detection, and environmental monitoring.
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Deep Learning Enabled Universal Multiplexed Fluorescence Detection for Point-of-Care ApplicationsAneesh Kshirsagar, Anthony J. Politza, and Weihua GuanACS Sensors, Aug 2024There is a significant demand for multiplexed fluorescence sensing and detection across a range of applications. Yet, the development of portable and compact multiplexable systems remains a substantial challenge. This difficulty largely stems from the inherent need for spectrum separation, which typically requires sophisticated and expensive optical components. Here, we demonstrate a compact, lens-free, and cost-effective fluorescence sensing setup that incorporates machine learning for scalable multiplexed fluorescence detection. This method utilizes low-cost optical components and a pretrained machine learning (ML) model to enable multiplexed fluorescence sensing without optical adjustments. Its multiplexing capability can be easily scaled up through updates to the machine learning model without altering the hardware. We demonstrate its real-world application in a probe-based multiplexed Loop-Mediated Isothermal Amplification (LAMP) assay designed to simultaneously detect three common respiratory viruses within a single reaction. The effectiveness of this approach highlights the system’s potential for point-of-care applications that require cost-effective and scalable solutions. The machine learning-enabled multiplexed fluorescence sensing demonstrated in this work would pave the way for widespread adoption in diverse settings, from clinical laboratories to field diagnostics.
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High Fidelity Machine-Learning-Assisted False Positive Discrimination in Loop-Mediated Isothermal Amplification Using Nanopore-Based Sizing and CountingMing Dong, Aneesh Kshirsagar, Anthony J. Politza, and Weihua GuanACS Nano, Mar 2024Loop-mediated isothermal amplification (LAMP) is a rapid, sensitive, and cost-effective method for developing point-ofcare nucleic acid testing due to its isothermal nature. Yet, LAMP can suffer from the issue of false positives, which can compromise the specificity of the results. LAMP false positives typically arise due to contamination, nonspecific amplification, and nonspecific signal reporting (intercalating dyes, colorimetric, turbidity, etc.). While dye-labeled primers or probes have been introduced for multiplexed detection and enhanced specificity in LAMP assays, they carry the risk of reaction inhibition. This inhibition can result from the labeled primers with fluorophores or quenchers and probes that do not fully dissociate during reaction. This work demonstrated a nanopore-based system for probe-free LAMP readouts by employing amplicon sizing and counting, analogous to an electronic version of gel electrophoresis. We first developed a model to explore LAMP kinetics and verified distinct patterns between true and false positives via gel electrophoresis. Subsequently, we implemented nanopore sized counting and calibrated the event charge deficit (ECD) values and frequencies to ensure a fair analysis of amplicon profiles. This sized counting method, integrated with machine learning, achieved 91.67% accuracy for false positive discrimination, enhancing LAMP’s reliability for nucleic acid detection.
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Differentiating Single Multiple Nanopore Through Conductance Distribution AnalysisShengfa Liang, Yu Liu, Feibin Xiang, Zhihong Yao, Wenchang Zhang, and Weihua GuanAdvanced Sensor Research, Jul 2024Abstract Solid‐state nanopore sensors, a type of resistive pulse sensing, achieve optimal signal‐to‐noise performance with a single nanopore. However, the processes involved in solid‐state nanopore fabrication and subsequent measurements frequently lead to the formation of multiple nanopores, posing a challenge for precise detection. To address this issue, here, a novel and expedient technique to verify the presence of a single nanopore on a chip is developed. The methodology includes measuring the nanopore’s conductance in solutions of various salt conditions, followed by a comparison of these results against a theoretical conductance model. This comparison is instrumental in distinguishing between single and multiple nanopores. Additionally, the study delves into various factors that influence the conductance curve, such as deviations in pore shape from the standard circle and inconsistencies in pore diameter. This approach significantly enhances the practical application of low‐cost nanopore preparation techniques, particularly in scenarios like controlled breakdown nanopore fabrication, where the formation of multiple nanopores is a common concern.
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Sensitive and specific CRISPR-Cas12a assisted nanopore with RPA for Monkeypox detectionMd. Ahasan Ahamed, Muhammad Asad Ullah Khalid, Ming Dong, Anthony J. Politza, Zhikun Zhang, Aneesh Kshirsagar, Tianyi Liu, and Weihua GuanBiosensors and Bioelectronics, Feb 2024Monkeypox virus (MPXV) poses a global health emergency, necessitating rapid, simple, and accurate detection to manage its spread effectively. The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) tech nique has emerged as a promising next-generation molecular diagnostic approach. Here, we developed a highly sensitive and specific CRISPR-Cas12a assisted nanopore (SCAN) with isothermal recombinase polymerase amplification (RPA) for MPXV detection. The RPA-SCAN method offers a sensitivity unachievable with unam plified SCAN while also addressing the obstacles of PCR-SCAN for point-of-care applications. We demonstrated that size-counting of single molecules enables analysis of reaction-time dependent distribution of the cleaved reporter. Our MPXV-specific RPA assay achieved a limit of detection (LoD) of 19 copies in a 50 μL reaction system. By integrating 2 μL of RPA amplifications into a 20 μL CRISPR reaction, we attained an overall LoD of 16 copies/μL (26.56 aM) of MPXV at a 95% confidence level using the SCAN sensor. We also verified the specificity of RPA-SCAN in distinguishing MPXV from cowpox virus with 100% accuracy. These findings suggest that the isothermal RPA-SCAN device is well-suited for highly sensitive and specific Monkeypox detection. Given its electronic nature and miniaturization potential, the RPA-SCAN system paves the way for diagnosing a wide array of other infectious pathogens at the point of care.
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Sample-to-answer salivary miRNA testing: New frontiers in point-of-care diagnostic technologiesZhikun Zhang, Tianyi Liu, Ming Dong, Md. Ahasan Ahamed, and Weihua GuanWIREs Nanomedicine and Nanobiotechnology, May 2024MicroRNA (miRNA), crucial non-coding RNAs, have emerged as key biomarkers in molecular diagnostics, prognosis, and personalized medicine due to their significant role in gene expression regulation. Salivary miRNA, in particular, stands out for its non-invasive collection method and ease of accessibility, offering promising avenues for the development of point-of-care diagnostics for a spectrum of diseases, including cancer, neurodegenerative disorders, and infectious diseases. Such development promises rapid and precise diagnosis, enabling timely treatment. Despite significant advancements in salivary miRNA-based testing, challenges persist in the quantification, multiplexing, sensitivity, and specificity, particularly for miRNA at low concentrations in complex biological mixtures. This work delves into these challenges, focusing on the development and application of salivary miRNA tests for point-of-care use. We explore the biogenesis of salivary miRNA and analyze their quantitative expression and their disease relevance in cancer, infection, and neurodegenerative disorders. We also examined recent progress in miRNA extraction, amplification, and multiplexed detection methods. This study offers a comprehensive view of the development of salivary miRNA-based point-of-care testing (POCT). Its successful advancement could revolutionize the early detection, monitoring, and management of various conditions, enhancing healthcare outcomes.
2023
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Morphology around Nanopores Fabricated by Laser-Assisted Dielectric Breakdown and Its Impact on Ion and DNA Transport and SensingMing Dong, Reza Nouri, Zifan Tang, and Weihua GuanACS Applied Materials & Interfaces, May 2023Laser-assisted controlled dielectric breakdown (LaCBD) has emerged as an alternative to conventional CBD-based nanopore fabrication due to its localization capability, facilitated by the photothermal-induced thinning down in the hot spot. Here, we reported the potential impact of the laser on forming debris around the nanopore region in LaCBD. The debris was clearly observable by scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. We found that debris formation is a unique phenomenon in LaCBD that is not observable in the conventional CBD approach. We also found that the LaCBD-induced debris is more evident when the laser power and voltage stress are higher. Moreover, the debris is asymmetrically distributed on the top and bottom sides of the membrane. We also found unexpected rectified ionic and molecular transport in those LaCBD nanopores with debris. Based on these observations, we developed and validated a model describing the debris formation kinetics in LaCBD by considering the generation, diffusion, drift, and gravity in viscous mediums. These findings indicate that while laser aids in nanopore localization, precautions should be taken due to the potential formation of debris and rectification of molecular transport. This study provides valuable insights into the kinetics of LaCBD and the characteristics of the LaCBD nanopore.
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STAMP-Based Digital CRISPR-Cas13a for Amplification-Free Quantification of HIV-1 Plasma Viral LoadsReza Nouri, Yuqian Jiang, Anthony J. Politza, Tianyi Liu, Wallace H. Greene, Yusheng Zhu, Jonathan J. Nunez, Xiaojun Lian, and Weihua GuanACS Nano, Jun 2023Quantification of HIV RNA in plasma is critical for identifying the disease progression and monitoring the effectiveness of antiretroviral therapy. While RT-qPCR has been the gold standard for HIV viral load quantification, digital assays could provide an alternative calibration-free absolute quantification method. Here, we reported a Self-digitization Through Automated Membrane-based Partitioning (STAMP) method to digitalize the CRISPR-Cas13 assay (dCRISPR) for amplification-free and absolute quantification of HIV-1 viral RNAs. The HIV-1 Cas13 assay was designed, validated, and optimized. We evaluated the analytical performances with synthetic RNAs. With a membrane that partitions ∼100 nL of reaction mixture (effectively containing 10 nL of input RNA sample), we showed that RNA samples spanning 4 orders of dynamic range between 1 fM (∼6 RNAs) to 10 pM (∼60k RNAs) could be quantified as fast as 30 min. We also examined the end-to-end performance from RNA extraction to STAMP-dCRISPR quantification using 140 μL of both spiked and clinical plasma samples. We demonstrated that the device has a detection limit of approximately 2000 copies/mL and can resolve a viral load change of 3571 copies/mL (equivalent to 3 RNAs in a single membrane) with 90% confidence. Finally, we evaluated the device using 140 μL of 20 patient plasma samples (10 positives and 10 negatives) and benchmarked the performance with RT-PCR. The STAMP-dCRISPR results agree very well with RT-PCR for all negative and high positive samples with Ct \textless 32. However, the STAMP-dCRISPR is limited in detecting low positive samples with Ct \textgreater 32 due to the subsampling errors. Our results demonstrated a digital Cas13 platform that could offer an accessible amplificationfree quantification of viral RNAs. By further addressing the subsampling issue with approaches such as preconcentration, this platform could be further exploited for quantitatively determining viral load for an array of infectious diseases.
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Compact Point-of-Care Device for Self-Administered HIV Viral Load Tests from Whole BloodTianyi Liu, Anthony J. Politza, Aneesh Kshirsagar, Yusheng Zhu, and Weihua GuanACS Sensors, Dec 2023Featured on the coverHuman immunodeficiency virus (HIV) is a significant problem to consider as it can lead to acquired immune deficiency syndrome (AIDS). Fortunately, AIDS is manageable through antiretroviral therapy (ART). However, frequent viral load monitoring is needed to monitor the effectiveness of the therapy. The current reverse transcription-polymerase chain reaction (RT-PCR) viral load monitoring is highly effective, but is challenged by being resource-intensive and inaccessible, and its turnaround time does not meet demand. An unmet need exists for an affordable, rapid, and user-friendly point-of-care device that could revolutionize and ensure therapeutic effectiveness, particularly in resource-limited settings. In this work, we explored a pointof-care HIV viral load device to address this need. This device can perform streamlined plasma separation, viral RNA extraction, and real-time reverse transcription loopmediated isothermal amplification (RT-LAMP) semiquantitative testing in an ultracompact device. We developed an absorption-based membrane plasma separation method suitable for finger-prick blood samples, achieving an efficiency of 80%. We also designed a syringe-based RNA extraction method for on-site plasma processing with a viral recovery efficiency of 86%. We created a portable device with a smartphone interface for real-time semiquantitative RT-LAMP, which is useful for monitoring viral load. The device uses lyophilized reagents, processed with our lyophilization method, which remain stable for 16 weeks. The device can accurately categorize viral load into low, medium, and high categories with 95% accuracy. We believe this point-of-care HIV self-test device, offering convenience and longterm storage, could aid patients in home-based ART treatment monitoring.
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Handheld Purification-Free Nucleic Acid Testing Device for Point-of-Need Detection of Malaria from Whole BloodAneesh Kshirsagar, Gihoon Choi, Vishaka Santosh, Tara Harvey, Robert Cory Bernhards, and Weihua GuanACS Sensors, Feb 2023World Health Organization’s aim to eliminate malaria from developing/resource-limited economies requires easy access to low-cost, highly sensitive, and specific screening. We present a handheld nucleic acid testing device with on-chip automated sample preparation to detect malaria (Plasmodium falciparum) infection from a whole blood sample as a feasibility study. We used a simple two-reagent-based purification-free protocol to prepare the whole blood sample on a piezo pump pressure-driven microfluidic cartridge. The cartridge includes a unique mixing chamber for sample preparation and metering structures to dispense a predetermined volume of the sample lysate mixture into four chambers containing a reaction mix. The parasite genomic DNA concentration can be estimated by monitoring the fluorescence generated from the loopmediated isothermal amplification reaction in real time. We achieved a sensitivity of ∼0.42 parasite/μL of whole blood, sufficient for detecting asymptomatic malaria parasite carriers.
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Ensuring fair assessment of solid-state nanopore sensors with reporting baseline currentMing Dong, Zifan Tang, and Weihua GuanApplied Physics Letters, Oct 2023In developing solid-state nanopore sensors for single molecule detection, comprehensive evaluation of the nanopore quality is important. Existing studies typically rely on comparing the noise root mean square or power spectrum density values. Nanopores exhibiting lower noise values are generally considered superior. This evaluation is valid when the single molecule signal remains consistent. However, the signal can vary, as it is strongly related to the solid-state nanopore size, which is hard to control during fabrication consistently. This work emphasized the need to report the baseline current for evaluating solid-state nanopore sensors. The baseline current offers insight into several experimental conditions, particularly the nanopore size. Our experiments show that a nanopore sensor with more noise is not necessarily worse when considering the signal-to-noise ratio (SNR), particularly when the pore size is smaller. Our findings suggest that relying only on noise comparisons can lead to inaccurate evaluations of solid-state nanopore sensors, considering the inherent variability in fabrication and testing setups among labs and measurements. We propose that future studies should include reporting baseline current and sensing conditions. Additionally, using SNR as a primary evaluation tool for nanopore sensors could provide a more comprehensive understanding of their performance.
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Fully integrated point-of-care blood cell count using multi-frame morphology analysisWenchang Zhang, Ya Li, Bing Chen, Yuan Zhang, Ziqiang Du, Feibin Xiang, Yu Hu, Xiaochen Meng, Chunliang Shang, Shengfa Liang, Xiaonan Yang, and Weihua GuanBiosensors and Bioelectronics, Mar 2023Point-of-care testing (POCT) of blood cell count (BCC) is an emerging approach that allows laypersons to identify and count whole blood cells through simple manipulation. To date, POCTs for BCC were mainly achieved by “stationary” images through blood smears or single-laity arranged cells in the microwell, making it difficult to obtain statistically sufficient numbers of cells. In this work, we present a fully integrated POCT device solely using “in-flow” imaging of 3 μL fingertip whole blood for improved identification and counting accuracy of BCC analysis. A miniaturized magnetic stirring module was integrated to maintain the temporal stability of cell concentration. A relatively high throughput (~8000 cells/min) with a 30-fold dilution ratio of whole blood can be tested for as long as 1 h to examine sufficient numbers of cells, and the subclass cell concentration keeps constant. To improve the identification accuracy, multi-frame “in-flow” imaging was used to track the cell motion trails with multi-angle morphology analysis. This proof-of-concept was then validated with healthy whole blood samples and 75 cases of clinical patients with abnormal concentrations of red blood cells (RBCs), white blood cells (WBCs), and platelets (PLT). The average precision (AP) value of WBCs identification was improved from 0.8622 to 0.9934 using the multi-frame analysis method. And the high fitting degrees (\textgreater0.98) between our POCT device and the commercial clinical equipment indicated good agreement. This POCT device is user-friendly and cost-effective, making it a potential tool for diagnosing abnormal blood cell morphology or concentration in the field setting.
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Programmable magnetic robot (ProMagBot) for automated nucleic acid extraction at the point of needAnthony J. Politza, Tianyi Liu, and Weihua GuanLab on a Chip, Mar 2023ProMagBot introduces scalable electromagnetic control of magnetic beads. The device is a handheld, battery-powered, and field-deployable sample preparation device that can extract viral RNA from plasma samples in under 20 minutes. , Upstream sample preparation remains the bottleneck for point-of-need nucleic acid testing due to its complexity and time-consuming nature. Sample preparation involves extracting, purifying, and concentrating nucleic acids from various matrices. These processes are critical for ensuring the accuracy and sensitivity of downstream nucleic acid amplification and detection. However, current sample preparation methods are often laboratory-based, requiring specialized equipment, trained personnel, and several hours of processing time. As a result, sample preparation often limits the speed, portability, and cost-effectiveness of point-of-need nucleic acid testing. A universal, field-deployable sample preparation device is highly desirable for this critical need and unmet challenge. Here we reported a handheld, battery-powered, reconfigurable, and field-deployable nucleic acid sample preparation device. A programmable electromagnetic actuator was developed to drive a magnetic robot (ProMagBot) in X / Y 2D space, such that various magnetic bead-based sample preparations can be readily translated from the laboratory to point-of-need settings. The control of the electromagnetic actuator requires only a 3-phase unipolar voltage in X and Y directions, and therefore, the motion space is highly scalable. We validated the ProMagBot device with a model application by extracting HIV viral RNAs from plasma samples using two widely used magnetic bead kits: ChargeSwitch and MagMAX beads. In both cases, the ProMagBot could successfully extract viral RNAs from 50 μL plasma samples containing as low as 10 2 copies of viral RNAs in 20 minutes. Our results demonstrated the ability of ProMagBot to prepare samples from complex mediums at the point of need. We believe such a device would enable rapid and robust sample preparation in various settings, including resource-limited or remote environments, and accelerate the development of next-generation point-of-need nucleic acid testing.
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Digital CRISPR systems for the next generation of nucleic acid quantificationAnthony J. Politza, Reza Nouri, and Weihua GuanTrAC Trends in Analytical Chemistry, Feb 2023Digital CRISPR (dCRISPR) assays are an emerging platform of molecular diagnostics. Digital platforms introduce absolute quantification and increased sensitivity to bulk CRISPR assays. With ultra-specific targeting, isothermal operation, and rapid detection, dCRISPR systems are well-prepared to lead the field of molecular diagnostics. Here we summarized the common Cas proteins used in CRISPR detection assays. The methods of digital detection and critical performance factors are examined. We formed three strategies to frame the landscape of dCRISPR systems: (1) amplification free, (2) in-partition amplification, and (3) two-stage amplification. We also compared the performance of all systems through the limit of detection (LOD), testing time, and figure of merit (FOM). This work summarizes the details of digital CRISPR platforms to guide future development. We envision that improvements to LOD and dynamic range will position dCRISPR as the leading platform for the next generation of molecular biosensing.
2022
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SLIDE: Saliva-Based SARS-CoV-2 Self-Testing with RT-LAMP in a Mobile DeviceZifan Tang, Jiarui Cui, Aneesh Kshirsagar, Tianyi Liu, Michele Yon, Suresh V. Kuchipudi, and Weihua GuanACS Sensors, Aug 2022Regular, accurate, rapid, and inexpensive self-testing for severe acute respiratory syndrome coronavirus 2 (SARSCoV-2) is urgently needed to quell pandemic propagation. The existing at-home nucleic acid testing (NAT) test has high sensitivity and specificity, but it requires users to mail the sample to the central lab, which often takes 3−5 days to obtain the results. On the other hand, rapid antigen tests for the SARS-CoV-2 antigen provide a fast sample to answer the test (15 min). However, the sensitivity of antigen tests is 30 to 40% lower than nucleic acid testing, which could miss a significant portion of infected patients. Here, we developed a fully integrated SARS-CoV-2 reverse transcription loop-mediated isothermal amplification (RT-LAMP) device using a self-collected saliva sample. This platform can automatically handle the complexity and can perform the functions, including (1) virus particles’ thermal lysis preparation, (2) sample dispensing, (3) target sequence RT-LAMP amplification, (4) real-time detection, and (5) result report and communication. With a turnaround time of less than 45 min, our device achieved the limit of detection (LoD) of 5 copies/μL of the saliva sample, which is comparable with the LoD (6 copies/μL) using FDA-approved quantitative real-time polymerase chain reaction (qRT-PCR) assays with the same heat-lysis saliva sample preparation method. With clinical samples, our platform showed a good agreement with the results from the gold-standard RT-PCR method. These results show that our platform can perform self-administrated SARS-CoV-2 nucleic acid testing by laypersons with noninvasive saliva samples. We believe that our self-testing platform will have an ongoing benefit for COVID-19 control and fighting future pandemics.
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Fingerpick Blood-Based Nucleic Acid Testing on A USB Interfaced Device towards HIV self-testingTianyi Liu, Gihoon Choi, Zifan Tang, Aneesh Kshirsagar, Anthony J. Politza, and Weihua GuanBiosensors and Bioelectronics, Aug 2022HIV self-testing is an emerging innovative approach that allows individuals who want to know their HIV status to collect their own specimen, perform a test, and interpret the results privately. Existing HIV self-testing methods rely on rapid diagnostic tests (RDTs) to detect the presence of HIV-1/2 antibodies, which could miss a significant portion of asymptomatic carriers during the window period. In this work, we present a fully integrated nucleic acid testing (NAT) device towards streamlined HIV self-testing using 100 μL finger-prick whole blood. The device consists of a ready-to-use microfluidic reagent cartridge and an ultra-compact NAT-on-USB analyzer. The test requires simple steps from the user to drop the finger-prick blood sample into a collection tube with lysis buffer and load the lysate onto the microfluidic cartridge, and the testing result can be easily read out by a custom-built graphical user interface (GUI). The microfluidic cartridge and the analyzer automatically handle the complexity of sample preparation, purification, and real-time reverse-transcription loop-mediated isothermal amplification (RT-LAMP). With a turnaround time of ~60 min, we achieved a limit of detection (LoD) of 214 viral RNA copies/ mL of whole blood at a 95% confidence level. Due to its ease of use and high sensitivity, we anticipate the HIV NAT-on-USB device would be particularly useful for the high-risk populations seeking private self-testing at the early stages of exposure.
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Rapid detection of novel coronavirus SARS-CoV-2 by RT-LAMP coupled solid-state nanoporesZifan Tang, Reza Nouri, Ming Dong, Jianbo Yang, Wallace Greene, Yusheng Zhu, Michele Yon, Meera Surendran Nair, Suresh V. Kuchipudi, and Weihua GuanBiosensors and Bioelectronics, Feb 2022The current pandemic of COVID-19 caused by SARS-CoV-2 (severe acute respiratory syndrome coronavirus-2) has raised significant public health concerns. Rapid and accurate testing of SARS-CoV-2 is urgently needed for early detection and control of the disease spread. Here, we present an RT-LAMP coupled glass nanopore digital counting method for rapid detection of SARS-CoV-2. We validated and compared two one-pot RT-LAMP assays targeting nucleocapsid (N) and envelop (E) genes. The nucleocapsid assay was adopted due to its quick time to positive and better copy number sensitivity. For qualitative positive/negative classification of a testing sample, we used the glass nanopore to digitally count the RT-LAMP amplicons and benchmarked the event rate with a threshold. Due to its intrinsic single molecule sensitivity, nanopore sensors could capture the amplification dynamics more rapidly (quick time to positive). We validated our RT-LAMP coupled glass nanopore digital counting method for SARS-CoV-2 detection by using both spiked saliva samples and COVID-19 clinical naso pharyngeal swab samples. The results obtained showed excellent agreement with the gold standard RT-PCR assay. With its integration capability, the electronic nanopore digital counting platform has significant poten tial to provide a rapid, sensitive, and specific point-of-care assay for SARS-CoV-2.
- Modeling pyramidal silicon nanopores with effective ion transportFeibin Xiang, Ming Dong, Wenchang Zhang, Shengfa Liang, and Weihua GuanNanotechnology, Nov 2022
While the electrical models of the membrane-based solid-state nanopores have been well established, silicon-based pyramidal nanopores cannot apply these models due to two distinctive features. One is its 35.3° half cone angle, which brings additional resistance to the moving ions inside the nanopore. The other is its rectangular entrance, which makes calculating the access conductance challenging.
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Rolling Circle Amplification-Coupled Glass Nanopore Counting of Mild Traumatic Brain Injury-Related Salivary miRNAsMing Dong, Zifan Tang, Steven Hicks, and Weihua GuanAnalytical Chemistry, Mar 2022Mild traumatic brain injury (mTBI) could be underdiagnosed and underreported due to the delayed onset of symptoms and the conventional subjective assessment. Recent studies have suggested that salivary microRNAs (miRNAs) could be reliable biomarkers for objective mTBI diagnosis. In this work, we demonstrated a rolling circle amplification (RCA)-coupled resistive pulse-counting platform for profiling mTBI-related miRNAs, using easy-to-fabricate large glass nanopores (200 nm diameter). The method relies on the linear and specific elongation of the miRNA to a much larger RCA product, which the large glass nanopore can digitally count with a high signal-to-noise ratio. We developed and validated the RCA assay against let-7a, miR-30e, and miR-21. We demonstrated the quantification capability of this large glass nanopore counting platform for purified miRNAs as well as miRNAs in salivary total RNA background. Finally, we quantitatively evaluated the performance of profiling each individual miRNAs in a mixed analyte. Our results showed that the RCA-coupled large glass nanopore counting provides a promising and accessible alternative toward the clinical diagnosis of mTBI using salivary miRNAs.
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Figure of Merit for CRISPR-Based Nucleic Acid-Sensing Systems: Improvement Strategies and Performance ComparisonReza Nouri, Ming Dong, Anthony J. Politza, and Weihua GuanACS Sensors, Mar 2022Clustered regularly interspaced short palindromic repeats (CRISPR)-based nucleic acid-sensing systems have grown rapidly in the past few years. Nevertheless, an objective approach to benchmark the performances of different CRISPR sensing systems is lacking due to the heterogeneous experimental setup. Here, we developed a quantitative CRISPR sensing figure of merit (FOM) to compare different CRISPR methods and explore performance improvement strategies. The CRISPR sensing FOM is defined as the product of the limit of detection (LOD) and the associated CRISPR reaction time (T). A smaller FOM means that the method can detect smaller target quantities faster. We found that there is a tradeoff between the LOD of the assay and the required reaction time. With the proposed CRISPR sensing FOM, we evaluated five strategies to improve the CRISPR-based sensing: preamplification, enzymes of higher catalytic efficiency, multiple crRNAs, digitalization, and sensitive readout systems. We benchmarked the FOM performances of 57 existing studies and found that the effectiveness of these strategies on improving the FOM is consistent with the model prediction. In particular, we found that digitalization is the most promising amplification-free method for achieving comparable FOM performances (∼1 fM·min) as those using preamplification. The findings here would have broad implications for further optimization of the CRISPR-based sensing.
2021
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On Stochastic Reduction in Laser-Assisted Dielectric Breakdown for Programmable Nanopore FabricationZifan Tang, Ming Dong, Xiaodong He, and Weihua GuanACS Applied Materials & Interfaces, Mar 2021The controlled dielectric breakdown emerged as a promising alternative toward accessible solid-state nanopore fabrication. Several prior studies have shown that laser-assisted dielectric breakdown could help control the nanopore position and reduce the possibility of forming multiple pores. Here, we developed a physical model to estimate the probability of forming a single nanopore under different combinations of the laser power and the electric field. This model relies on the material- and experimentspecific parameters: the Weibull statistical parameters and the laser-induced photothermal etching rate. Both the model and our experimental data suggest that a combination of a high laser power and a low electric field is statistically favorable for forming a single nanopore at a programmed location. While this model relies on experiment-specific parameters, we anticipate it could provide the experimental insights for nanopore fabrication by the laser-assisted dielectric breakdown method, enabling broader access to solid-state nanopores and their sensing applications.
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CRISPR-based detection of SARS-CoV-2: A review from sample to resultReza Nouri, Zifan Tang, Ming Dong, Tianyi Liu, Aneesh Kshirsagar, and Weihua GuanBiosensors and Bioelectronics, Apr 2021The current pandemic of the 2019 novel coronavirus (COVID-19) caused by SARS-CoV-2 (severe acute respi ratory syndrome coronavirus-2) has raised significant public health concern. Rapid, affordable, and accurate diagnostics of SARS-CoV-2 is essential for early treatment and control of the disease spread. In the past few years, CRISPR technology has shown great potential for highly sensitive and specific molecular diagnostics. Amid the ongoing COVID-19 pandemic, there is an increasing interest in implementing CRISPR-based diagnostic principles to develop fast and precise methods for detecting SARS-CoV-2. In this work, we reviewed and summarized these CRISPR-based diagnostic systems as well as their characteristics and challenges. We also provided future per spectives of CRISPR-based sensing towards point-of-care molecular diagnosis applications.
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Nanofluidic charged-coupled devices for controlled DNA transport and separationReza Nouri, and Weihua GuanNanotechnology, Aug 2021Controlled molecular transport and separation is of significant importance in various applications. In this work, we presented a novel concept of nanofluidic molecular charge-coupled device (CCD) for controlled DNA transport and separation. By leveraging the unique field-effect coupling in nanofluidic systems, the nanofluidic molecular CCD aims to store charged biomolecules such as DNAs in discrete regions in nanochannels and transfer and separate these biomolecules as a charge packet in a bucket brigade fashion. We developed a quantitative model to capture the impact of nanochannel surface charge, gating voltage and frequency, molecule diffusivity, and gating electrode geometry on the transport and separation efficiency. We studied the synergistic effects of these factors to guide the device design and optimize the DNA transport and separation in a nanofluidic CCD. The findings in this study provided insight into the rational design and implementation of the nanofluidic molecular CCD.
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Detection of SARS-CoV-2 with Solid-State CRISPR-Cas12a-Assisted NanoporesReza Nouri, Yuqian Jiang, Zifan Tang, Xiaojun Lance Lian, and Weihua GuanNano Letters, Oct 2021The outbreak of the SARS-CoV-2 caused the disease COVID-19 to spread globally. Specific and sensitive detection of SARS-CoV-2 facilitates early intervention and prevents the disease from spreading. Here, we present a solid-state CRISPR-Cas12aassisted nanopore (SCAN) sensing strategy for the specific detection of SARS-CoV-2. We introduced a nanopore-sized counting method to measure the cleavage ratio of reporters, which is used as a criterion for positive/negative classification. A kinetic cleavage model was developed and validated to predict the reporter size distributions. The model revealed the trade-offs between sensitivity, turnaround time, and false-positive rate of the SARS-CoV-2 SCAN. With preamplification and a 30 min CRISPR Cas12a assay, we achieved excellent specificity against other common human coronaviruses and a limit of detection of 13.5 copies/μL (22.5 aM) of viral RNA at a confidence level of 95%. These results suggested that the SCAN could provide a rapid, sensitive, and specific analysis of SARS-CoV-2.
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Microfluidic high-throughput single-cell mechanotyping: Devices and applicationsGihoon Choi, Zifan Tang, and Weihua GuanNanotechnology and Precision Engineering, Dec 2021The mechanical behavior of individual cells plays an important role in regulating various biological activities at the molecular and cellular levels. It can serve as a promising label-free marker of cells’ physiological states. In the past two decades, several techniques have been developed for understanding correlations between cellular mechanical changes and human diseases. However, numerous technical challenges remain with regard to realizing high-throughput, robust, and easy-to-perform measurements of single-cell mechanical properties. In this paper, we review the emerging tools for single-cell mechanical characterization that are provided by microfluidic technology. Different techniques are benchmarked by considering their advantages and limitations. Finally, the potential applications of microfluidic techniques based on cellular mechanical properties are discussed.
2020
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Direct Observation of Redox-Induced Bubble Generation and Nanopore Formation Dynamics in Controlled Dielectric BreakdownMing Dong, Zifan Tang, Xiaodong He, and Weihua GuanACS Applied Electronic Materials, Sep 2020While controlled dielectric breakdown (CBD) emerged as a promising method for accessible solid-state nanopore fabrication, there are still significant challenges in understanding the fabrication dynamics due to the lack of in situ cross-reference characterization beyond current monitoring. In this work, we developed a multimodal method for characterizing the dielectric breakdown-based nanopore formation dynamics. With this capability, we observed for the first time the redox-induced bubble generation at the electrolyte−membrane interface. The randomly generated gas bubble would significantly alter the electric field distribution on the membrane surfaces and is an overlooked factor that can contribute to the random distribution of the nanopores. Besides, we also studied the impact of electric field strength on the number and location of nanopore(s) initially formed and after enlargement. We believe that the direct evidence of redox-induced bubble formation and the impact of the electric field on nanopore formation dynamics presented in this work would provide significant experimental insight for further improving the breakdown-based solid-state nanopore fabrication.
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Sequence-Specific Recognition of HIV-1 DNA with Solid-State CRISPR-Cas12a-Assisted Nanopores (SCAN)Reza Nouri, Yuqian Jiang, Xiaojun Lance Lian, and Weihua GuanACS Sensors, May 2020Nucleic acid detection methods are crucial for many fields such as pathogen detection and genotyping. Solid-state nanopore sensors represent a promising platform for nucleic acid detection due to its unique single molecule sensitivity and labelfree electronic sensing. Here, we demonstrated the use of the glass nanopore for highly sensitive quantification of single-stranded circular DNAs (reporters), which could be degraded under the trans-cleavage activity of the target-specific CRISPR-Cas12a. We developed and optimized the Cas12a assay for HIV-1 analysis. We validated the concept of the solid-state CRISPR-Cas12a-assisted nanopores (SCAN) to specifically detect the HIV-1 DNAs. We showed that the glass nanopore sensor is effective in monitoring the cleavage activity of the target DNA-activated Cas12a. We developed a model to predict the total experimental time needed for making a statistically confident positive/negative call in a qualitative test. The SCAN concept combines the much-needed specificity and sensitivity into a single platform, and we anticipate that the SCAN would provide a compact, rapid, and low-cost method for nucleic acid detection at the point of care.
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Microfluidic deformability-activated sorting of single particlesGihoon Choi, Reza Nouri, Lauren Zarzar, and Weihua GuanMicrosystems & Nanoengineering, Feb 2020Mechanical properties have emerged as a significant label-free marker for characterizing deformable particles such as cells. Here, we demonstrated the first single-particle-resolved, cytometry-like deformability-activated sorting in the continuous flow on a microfluidic chip. Compared with existing deformability-based sorting techniques, the microfluidic device presented in this work measures the deformability and immediately sorts the particles one-by-one in real time. It integrates the transit-time-based deformability measurement and active hydrodynamic sorting onto a single chip. We identified the critical factors that affect the sorting dynamics by modeling and experimental approaches. We found that the device throughput is determined by the summation of the sensing, buffering, and sorting time. A total time of ~100 ms is used for analyzing and sorting a single particle, leading to a throughput of 600 particles/min. We synthesized poly(ethylene glycol) diacrylate (PEGDA) hydrogel beads as the deformability model for device validation and performance evaluation. A deformability-activated sorting purity of 88% and an average efficiency of 73% were achieved. We anticipate that the ability to actively measure and sort individual particles one-byone in a continuous flow would find applications in cell-mechanotyping studies such as correlational studies of the cell mechanical phenotype and molecular mechanism.
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Confocal scanning photoluminescence for mapping electron and photon beam-induced microscopic changes in SiN \textit_\textrmx during nanopore fabricationXiaodong He, Zifan Tang, Shengfa Liang, Ming Liu, and Weihua GuanNanotechnology, Sep 2020Focused electron and laser beams have shown the ability to form nanoscale pores in SiNx membranes. During the fabrication process, areas beyond the final nanopore location will inevitably be exposed to the electron beams or the laser beams due to the need for localization, alignment and focus. It remains unclear how these unintended exposures affect the integrity of the membrane. In this work, we demonstrate the use of confocal scanning photoluminescence (PL) for mapping the microscopic changes in SiNx nanopores when exposed to electron and laser beams. We developed and validated a model for the quantitative interpretation of the scanned PL result. The model shows that the scanning PL result is insensitive to the nanopore size. Instead, it is dominated by the product of two microscopic material factors: quantum yield profile (i.e. variations in electronic structure) and thickness profile (i.e. thinning of the membrane). We experimentally demonstrated that the electron and laser beams could alter the material electronic structures (i.e. quantum yield) even when no thinning of the membrane occurs. Our results suggest that minimizing the unintended e-beam or laser beam to the SiNx during the fabrication is crucial if one desires the microscopic integrity of the membrane.
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Noise in nanopore sensors: Sources, models, reduction, and benchmarkingShengfa Liang, Feibin Xiang, Zifan Tang, Reza Nouri, Xiaodong He, Ming Dong, and Weihua GuanNanotechnology and Precision Engineering, Mar 2020Label-free nanopore sensors have emerged as a new generation technology of DNA sequencing and have been widely used for single molecule analysis. Since the first α-hemolysin biological nanopore, various types of nanopores made of different materials have been under extensive development. Noise represents a common challenge among all types of nanopore sensors. The nanopore noise can be decomposed into four components in the frequency domain (1/f noise, white noise, dielectric noise, and amplifier noise). In this work, we reviewed and summarized the physical models, origins, and reduction methods for each of these noise components. For the first time, we quantitatively benchmarked the root mean square (RMS) noise levels for different types of nanopores, demonstrating a clear material-dependent RMS noise. We anticipate this review article will enhance the understanding of nanopore sensor noises and provide an informative tutorial for developing future nanopore sensors with a high signal-to-noise ratio.
2019
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Calibration-Free Nanopore Digital Counting of Single MoleculesReza Nouri, Zifan Tang, and Weihua GuanAnalytical Chemistry, Sep 2019Nanopore sensor conceptually represents an ideal single molecule counting device due to its unique partitioning-free, label-free electronic sensing. Existing theories and experiments have shown that sample concentration is proportional to the molecule translocation rate. However, a detailed nanopore geometry and size characterization or a calibration curve of concentration standards are often required for quantifying the unknown sample. In this work, we proposed and validated a calibration-free nanopore single molecule digital counting method for isolated molecule quantification. With the background ions as the in situ references, the molecule translocation rates can be normalized to the ion translocation rates (i.e., baseline current). This in situ reference alleviates the requirement for knowing the nanopore geometry and size or generating a calibration curve. In recognition of this effect, we developed a quantitative model for nanopore quantification without the need for prior knowledge of experimental conditions such as nanopore geometry, size, and applied voltage. This model was experimentally validated for different nanopores and DNA molecules with different sizes. We anticipate this calibration-free digital counting approach would provide a new avenue for nanopore-based molecule sensing.
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Quantitative Analysis of Factors Affecting the Event Rate in Glass Nanopore SensorsReza Nouri, Zifan Tang, and Weihua GuanACS Sensors, Nov 2019While the solid-state nanopore sensors have shown exceptional promise with their single-molecule sensitivity and label-free operations, one of the most significant challenges in the nanopore sensor is the limited analyte translocation event rate that leads to prolonged sensor response time. This issue is more pronounced when the analyte concentration is below the nanomolar (nM) range, owing to the diffusion-limited mass transport. In this work, we systematically studied the experimental factors beyond the intrinsic analyte concentration and electrophoretic mobility that affect the event rate in glass nanopore sensors. We developed a quantitative model to capture the impact of nanopore surface charge density, ionic strength, nanopore geometry, and translocation direction on the event rate. The synergistic effects of these factors on the event rates were investigated with the aim to find the optimized experimental conditions for operating the glass nanopore sensor from the response time standpoint. The findings in the study would provide useful and practical insight to enhance the device response time and achieve a lower detection limit for various glass nanopore-sensing experiments.
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Microfluidic Time-Division Multiplexing Accessing Resistive Pulse Sensor for Particle AnalysisGihoon Choi, Erica Murphy, and Weihua GuanACS Sensors, Jul 2019Due to its simplicity and robustness, pore-based resistive pulse sensors have been widely used to detect, measure, and analyze particles at length scales ranging from nanometers to micrometers. While multiple pore-based resistive pulse sensors are preferred to increase the analysis throughput and to overcome the clogging issues, the scalability is often limited. In response, by combining the time-division multiple access technique in the telecommunication field with the microfluidics, we reported a microfluidic time-division multiplexing accessing (TDMA) singleend resistive pulse sensor, in which particles can be analyzed through a scalable number of microfluidic channels. With an eight-channel microfluidic device and polystyrene particles as proof-of-principle, we successfully demonstrated this multiplexed technology is effective in measuring the particle size and concentration, in analyzing the particle arriving dynamics, and in discriminating mixed populations. Importantly, the availability of multiple sensing pores provides a robust mechanism to overcome the clogging issue, allowing the analysis to continue even when some of the pores are clogged. We anticipate this TDMA approach could find wide applications and facilitate future development of multiplexed resistive pulse sensing from the microscale to nanoscale.
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Loop-Mediated Isothermal Amplification-Coupled Glass Nanopore Counting Toward Sensitive and Specific Nucleic Acid TestingZifan Tang, Gihoon Choi, Reza Nouri, and Weihua GuanNano Letters, Nov 2019Solid-state nanopores have shown great promise and achieved tremendous success in label-free single-molecule analysis. However, there are three common challenges in solid-state nanopore sensors, including the nanopore size variations from batch to batch that makes the interpretation of the sensing results difficult, the incorporation of sensor specificity, and the impractical analysis time at low analyte concentration due to diffusion-limited mass transport. Here, we demonstrate a novel loopmediated isothermal amplification (LAMP)-coupled glass nanopore counting strategy that could effectively address these challenges. By using the glass nanopore in the counting mode (versus the sizing mode), the device fabrication challenge is considerably eased since it allows a certain degree of pore size variations and no surface functionalization is needed. The specific molecule replication effectively breaks the diffusion-limited mass transport thanks to the exponential growth of the target molecules. We show the LAMPcoupled glass nanopore counting has the potential to be used in a qualitative test as well as in a quantitative nucleic acid test. This approach lends itself to most amplification strategies as long as the target template is specifically replicated in numbers. The highly sensitive and specific sensing strategy would open a new avenue for solid-state nanopore sensors toward a new form of compact, rapid, low-cost nucleic acid testing at the point of care.
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High fidelity moving Z-score based controlled breakdown fabrication of solid-state nanoporeKamyar Akbari Roshan, Zifan Tang, and Weihua GuanNanotechnology, Mar 2019We investigate the current transport characteristics in the electrolyte-dielectric-electrolyte structure commonly used in the in situ controlled breakdown (CBD) fabrication of solid-state nanopores. It is found that the stochastic breakdown process could lead to fidelity issues of false positives (an incorrect indication of a true nanopore formation) and false negatives (inability to detect initial nanopore formation). Robust and deterministic detection of initial physical breakdown to alleviate false positives and false negatives is critical for precise nanopore size control. To this end, we report a high fidelity moving Z-score method based CBD fabrication of solid-state nanopore. We demonstrate 100% success rate of realizing the initial nanopore conductance of 3±1 nS (corresponds to size of 1.7±0.6 nm) regardless of the dielectric membrane characteristics. Our study also elucidates the Joule heating is the dominant mechanism for electric field-based nanopore enlargement. Single DNA molecule sensing using nanopores fabricated by this method was successfully demonstrated. We anticipate the moving Z-score based CBD method could enable broader access to the solid state nanopore-based single molecule analysis.
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Microfluidic multiple cross-correlated Coulter counter for improved particle size analysisWenchang Zhang, Yuan Hu, Gihoon Choi, Shengfa Liang, Ming Liu, and Weihua GuanSensors and Actuators B: Chemical, Oct 2019Coulter counters (a.k.a. resistive pulse sensors) were widely used to measure the size of biological cells and colloidal particles. One of the important parameters of Coulter counters is its size discriminative capability. This work reports a multiple pore-based microfluidic Coulter counter for improved size differentiation in a mixed sample. When a single particle translocated across an array of sensing pores, multiple time-related resistive pulse signals were generated. Due to the time correlation of these resistive pulse signals, we found a multiple crosscorrelation analysis (MCCA) could enhance the sizing signal-to-noise (SNR) ratio by a factor of n1/2, where n is the pore numbers in series. This proof-of-concept is experimentally validated with polystyrene beads as well as human red blood cells. We anticipate this method would be highly beneficial for applications where improved size differentiation is required.
2018
- Kirigami‐Inspired 3D Organic Light‐Emitting Diode (OLED) Lighting ConceptsTaehwan Kim, Jared S. Price, Alex Grede, Sora Lee, Gihoon Choi, Weihua Guan, Thomas N. Jackson, and Noel C. GiebinkAdvanced Materials Technologies, Jul 2018
As the technical performance and reliability of white organic light‐emitting diodes (OLEDs) begin to mature, broader adoption in the lighting market will increasingly rely on exploiting the unique aesthetic opportunities that OLEDs afford. Here, kirigami‐based concepts that enable ultraflexible thin film OLEDs to be cut, folded, and “popped up” into a variety of 3D shapes ranging from globes to candle flame mimics are used. It is demonstrated that large area (\textgreater15 cm 2) OLEDs fabricated and encapsulated on sub‐50 µm thick polyimide films can be cut and folded into 3D with negligible impact on their efficiency and electrical characteristics. These results point to an alternate paradigm for OLED lighting that moves beyond traditional 2D panels toward 3D designs that deliver unique and creative new opportunities for lighting.
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Sample-to-answer palm-sized nucleic acid testing device towards low-cost malaria mass screeningGihoon Choi, Theodore Prince, Jun Miao, Liwang Cui, and Weihua GuanBiosensors and Bioelectronics, Sep 2018The effectiveness of malaria screening and treatment highly depends on the low-cost access to the highly sensitive and specific malaria test. We report a real-time fluorescence nucleic acid testing device for malaria field detection with automated and scalable sample preparation capability. The device consists a compact analyzer and a disposable microfluidic reagent compact disc. The parasite DNA sample preparation and subsequent realtime LAMP detection were seamlessly integrated on a single microfluidic compact disc, driven by energy efficient non-centrifuge based magnetic field interactions. Each disc contains four parallel testing units which could be configured either as four identical tests or as four species-specific tests. When configured as species-specific tests, it could identify two of the most life-threatening malaria species (P. falciparum and P. vivax). The NAT device is capable of processing four samples simultaneously within 50 min turnaround time. It achieves a detection limit of ~0.5 parasites/µl for whole blood, sufficient for detecting asymptomatic parasite carriers. The combination of the sensitivity, specificity, cost, and scalable sample preparation suggests the real-time fluorescence LAMP device could be particularly useful for malaria screening in the field settings.
2017
- High-throughput and label-free parasitemia quantification and stage differentiation for malaria-infected red blood cellsXiaonan Yang, Zhuofa Chen, Jun Miao, Liwang Cui, and Weihua GuanBiosensors and Bioelectronics, Dec 2017
This work reports a high throughput and label-free microfluidic cell deformability sensor for quantitative parasitemia measurement and stage determination for Plasmodium falciparum-infected red blood cells (PfiRBCs). The sensor relies on differentiating the RBC deformability (a mechanical biomarker) that is highly correlated with the infection status. The cell deformability is measured by evaluating the transit time when each individual RBC squeezes through a microscale constriction (cross-section ~5 µm×5 µm). More than 30,000 RBCs can be analyzed for parasitemia quantification in under 1 min with a throughput ~500 cells/s. Moreover, the device can also differentiate various malaria stages (ring, trophozoite, and schizont stage) due to their varied deformability. Using Pf-iRBCs at 0.1% parasitemia as a testing sample, the microfluidic deformability sensor achieved an excellent sensitivity (94.29%), specificity (86.67%) and accuracy (92.00%) in a blind test, comparable to the gold standard of the blood smear microscopy. As a supplement technology to the microscopy and flow cytometry, the microfluidic deformability sensor would possibly allow for label-free, rapid and costeffective parasitemia quantification and stage determination for malaria in remote regions.
2016
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A field-deployable mobile molecular diagnostic system for malaria at the point of needGihoon Choi, Daniel Song, Sony Shrestha, Jun Miao, Liwang Cui, and Weihua GuanLab on a Chip, Dec 2016In response to the urgent need of a field-deployable and highly sensitive malaria diagnosis, we developed a standalone, “sample-in-answer-out” molecular diagnostic system (AnyMDx) to enable quantitative molecular analysis of blood-borne malaria in low resource areas. The system consists of a durable battery-powered analyzer and a disposable microfluidic compact disc loaded with reagents ready for use. A low power thermal module and a novel fluorescence-sensing module are integrated into the analyzer for real-time monitoring of loop-mediated isothermal nucleic acid amplification (LAMP) of target parasite DNA. With 10 μL of raw blood sample, the AnyMDx system automates the nucleic acid sample preparation and subsequent LAMP and real-time detection. Under laboratory conditions with whole-blood samples spiked with cultured Plasmodium falciparum, we achieved a detection limit of ∼0.6 parasite per μL, much lower than those for the conventional microscopy and rapid diagnostic tests (∼50–100 parasites per μL). The turnaround time from sample to answer is less than 40 minutes. The AnyMDx is user-friendly requiring minimal technological training. The analyzer and the disposable reagent compact discs are cost-effective, making AnyMDx a potential tool for malaria molecular diagnosis under field settings for malaria elimination.
- Arbitrarily Accessible 3D Microfluidic Device for Combinatorial High-Throughput Drug ScreeningZhuofa Chen, Weizhi Li, Gihoon Choi, Xiaonan Yang, Jun Miao, Liwang Cui, and Weihua GuanSensors, Sep 2016
Microfluidics-based drug-screening systems have enabled efficient and high-throughput drug screening, but their routine uses in ordinary labs are limited due to the complexity involved in device fabrication and system setup. In this work, we report an easy-to-use and low-cost arbitrarily accessible 3D microfluidic device that can be easily adopted by various labs to perform combinatorial assays for high-throughput drug screening. The device is capable of precisely performing automatic and simultaneous reagent loading and aliquoting tasks and performing multistep assays with arbitrary sequences. The device is not intended to compete with other microfluidic technologies regarding ultra-low reaction volume. Instead, its freedom from tubing or pumping systems and easy operation makes it an ideal platform for routine high-throughput drug screening outside traditional microfluidic labs. The functionality and quantitative reliability of the 3D microfluidic device were demonstrated with a histone acetyltransferase-based drug-screening assay using the recombinant Plasmodium falciparum GCN5 enzyme, benchmarked with a traditional microtiter plate-based method. This arbitrarily accessible, multistep capable, low-cost, and easy-to-use device can be widely adopted in various combinatorial assays beyond high-throughput drug screening.
2015
- Droplet Digital Enzyme-Linked Oligonucleotide Hybridization Assay for Absolute RNA QuantificationWeihua Guan, Liben Chen, Tushar D. Rane, and Tza-Huei WangScientific Reports, Sep 2015
We present a continuous-flow droplet-based digital Enzyme-Linked Oligonucleotide Hybridization Assay (droplet digital ELOHA) for sensitive detection and absolute quantification of RNA molecules. Droplet digital ELOHA incorporates direct hybridization and single enzyme reaction via the formation of single probe-RNA-probe (enzyme) complex on magnetic beads. It enables RNA detection without reverse transcription and PCR amplification processes. The magnetic beads are subsequently encapsulated into a large number of picoliter-sized droplets with enzyme substrates in a continuous-flow device. This device is capable of generating droplets at high-throughput. It also integrates in-line enzymatic incubation and detection of fluorescent products. Our droplet digital ELOHA is able to accurately quantify (differentiate 40% difference) as few as ~600 RNA molecules in a 1 mL sample (equivalent to 1 aM or lower) without molecular replication. The absolute quantification ability of droplet digital ELOHA is demonstrated with the analysis of clinical Neisseria gonorrhoeae 16S rRNA to show its potential value in real complex samples.
- Direct Observation of Charge Inversion in Divalent Nanofluidic DevicesSylvia Xin Li, Weihua Guan, Benjamin Weiner, and Mark A. ReedNano Letters, Aug 2015
Solid-state nanofluidic devices have proven to be ideal systems for studying the physics of ionic transport at the nanometer length scale. When the geometrical confining size of fluids approaches the ionic Debye screening length, new transport phenomena occur, such as surface mediated transport and permselectivity. Prior work has explored these effects extensively in monovalent systems (e.g., predominantly KCl and NaCl). In this report, we present a new characterization method for the study of divalent ionic transport and have unambiguously observed divalent charge inversion at solid/fluid interfaces. This observation has important implications in applications ranging from biology to energy conversion.
2014
- Highly specific and sensitive non-enzymatic determination of uric acid in serum and urine by extended gate field effect transistor sensorsWeihua Guan, Xuexin Duan, and Mark A. ReedBiosensors and Bioelectronics, Jan 2014
A potentiometric non-enzymatic sensor using off-chip extended-gate field effect transistor (EGFET) with a ferrocenyl-alkanethiol modified gold electrode is demonstrated for determining the uric acid concentration in human serum and urine. Hexacyanoferrate (II) and (III) ions are used as redox reagent. This potentiometric sensor measures the interface potential on the ferrocene immobilized gold electrode, which is modulated by the redox reaction between uric acid and hexacyanoferrate ions. The device shows a near Nernstian response to uric acid and is highly specific. The interference that comes from glucose, bilirubin, ascorbic acid and hemoglobin is negligible in normal concentration range of these interferents. The sensor also exhibits excellent long term reliability. This extended gate field effect transistor based sensors can be used as a point of care UA testing tool, due to the small size, low cost, and low sample volume consumption.
- Voltage gated ion and molecule transport in engineered nanochannels: theory, fabrication and applicationsWeihua Guan, Sylvia Xin Li, and Mark A ReedNanotechnology, Mar 2014
Nanochannels remain at the focus of growing scientific and technological interest. The nanometer scale of the structure allows the discovery of a new range of phenomena that has not been possible in traditional microchannels, among which a direct field effect control over the charges in nanochannels is very attractive for various applications, since it offers a unique opportunity to integrate wet ionics with dry electronics seamlessly. This review will focus on the voltage gated ionic and molecular transport in engineered gated nanochannels. We will present an overview of the transport theory. Fabrication techniques regarding the gated nanostructures will also be discussed. In addition, various applications using the voltage gated nanochannels are outlined, which involves biological and chemical analysis, and energy conversion.
2013
- Quantitative probing of surface charges at dielectric–electrolyte interfacesWeihua Guan, Nitin K. Rajan, Xuexin Duan, and Mark A. ReedLab on a Chip, Mar 2013
The intrinsic charging status at the dielectric–electrolyte interface (DEI) plays a critical role for electrofluidic gating in microfluidics and nanofluidics, which offers opportunities for integration of wet ionics with dry electronics. A convenient approach to quantitatively probe the surface charges at the DEI for material pre-selection purpose has been lacking so far. We report here a low-cost, off-chip extended gate field effect transistor configuration for direct electrostatic probing the charging status at the DEI. Capacitive coupling between the surface charges and the floating extended gate is utilized for signal transducing. The relationship between the surface charge density and the experimentally accessible quantities is given by device modeling. The multiplexing ability makes measuring a local instead of a globally averaged surface charge possible.
2012
- Electric Field Modulation of the Membrane Potential in Solid-State Ion ChannelsWeihua Guan, and Mark A. ReedNano Letters, Dec 2012
Biological ion channels are molecular devices that allow a rapid flow of ions across the cell membrane. Normal physiological functions, such as generating action potentials for cell-to-cell communication, are highly dependent on ion channels that can open and close in response to external stimuli for regulating ion permeation. Mimicking these biological functions using synthetic structures is a rapidly progressing yet challenging area. Here we report the electric field modulation of the membrane potential phenomena in mechanically and chemically robust solid-state ion channels, an abiotic analogue to the voltage-gated ion channels in living systems. To understand the complex physicochemical processes in the electric field regulated membrane potential behavior, both quasi-static and transient characteristics of converting transmembrane ion gradients into electric potential are investigated. It is found that the transmembrane potential can be adequately tuned by an external electrical stimulation, thanks to the unique properties of the voltage-regulated selective ion transport through a nanoscale channel.
- Tunable Aqueous Virtual MicroporeJae Hyun Park, Weihua Guan, Mark A. Reed, and Predrag S. KrstićSmall, Mar 2012
A charged microparticle can be trapped in an aqueous environment by forming a narrow virtual pore—a cylindrical space region in which the particle motion in the radial direction is limited by forces emerging from dynamical interactions of the particle charge and dipole moment with an external radiofrequency quadrupole electric field. If the particle satisfies the trap stability criteria, its mean motion is reduced exponentially with time due to the viscosity of the aqueous environment; thereafter the long‐time motion of particle is subject only to random, Brownian fluctuations, whose magnitude, influenced by the electrophoretic and dielectrophoretic effects and added to the particle size, determines the radius of the virtual pore, which is demonstrated by comparison of computer simulations and experiment. The measured size of the virtual nanopore could be utilized to estimate the charge of a trapped micro‐object.
2011
- Field-effect reconfigurable nanofluidic ionic diodesWeihua Guan, Rong Fan, and Mark A. ReedNature Communications, Oct 2011
Several types of nanofluidic devices based on nanopores and nanochannels have been reported to yield ionic current rectification, with the aim to control the delivery of chemical species in integrated systems. However, the rectifying properties obtained by existing approaches cannot be altered once the devices are made. It would be desirable to have the ability to modulate the predefined properties in situ without introducing external chemical stimuli. Here we report a field-effect reconfigurable nanofluidic diode, with a single asymmetrically placed gate or dual split-gate on top of the nanochannel. The forward/reverse directions of the diode as well as the degrees of rectification can be regulated by the application of gate voltages. Compared with the stimuli-responsive tuning of the rectification properties, the electrostatic modulation offers a fully independent and digitally programmable approach for controlling the preferential conduction of ions and molecules in fluids. This device would serve as a building block for large-scale integration of reconfigurable ionic circuits.
- Paul trapping of charged particles in aqueous solutionWeihua Guan, Sony Joseph, Jae Hyun Park, Predrag S. Krstić, and Mark A. ReedProceedings of the National Academy of Sciences, Jun 2011
We experimentally demonstrate the feasibility of an aqueous Paul trap using a proof-of-principle planar device. Radio frequency voltages are used to generate an alternating focusing/defocusing potential well in two orthogonal directions. Individual charged particles are dynamically confined into nanometer scale in space. Compared with conventional Paul traps working in frictionless vacuum, the aqueous environment associated with damping forces and thermally induced fluctuations (Brownian noise) exerts a fundamental influence on the underlying physics. We investigate the impact of these two effects on the confining dynamics, with the aim to reduce the rms value of the positional fluctuations. We find that the rms fluctuations can be modulated by adjusting the voltages and frequencies. This technique provides an alternative for the localization and control of charged particles in an aqueous environment.
- Non-vanishing ponderomotive AC electrophoretic effect for particle trappingWeihua Guan, Jae Hyun Park, Predrag S Krstić, and Mark A ReedNanotechnology, Jun 2011
We present here a study on overlooked aspects of alternating current (AC) electrokinetics—AC electrophoretic (ACEP) phenomena. The dynamics of a particle with both polarizability and net charges in a non-uniform AC electric trapping field is investigated. It is found that either electrophoretic (EP) or dielectrophoretic (DEP) effects can dominate the trapping dynamics, depending on experimental conditions. A dimensionless parameter γ is developed to predict the relative strength of EP and DEP effects in a quadrupole AC field. An ACEP trap is feasible for charged particles in ‘salt-free’ or low salt concentration solutions. In contrast to DEP traps, an ACEP trap favors the downscaling of the particle size.
2010
- A long DNA segment in a linear nanoscale Paul trapSony Joseph, Weihua Guan, Mark A Reed, and Predrag S KrsticNanotechnology, Jan 2010
We study the dynamics of a linearly distributed line charge such as single stranded DNA (ssDNA) in a nanoscale, linear 2D Paul trap in vacuum. Using molecular dynamics simulations we show that a line charge can be trapped effectively in the trap for a well defined range of stability parameters. We investigated (i) a flexible bonded string of charged beads and (ii) a ssDNA polymer of variable length, for various trap parameters. A line charge undergoes oscillations or rotations as it moves, depending on its initial angle, the position of the center of mass and the velocity. The stability region for a strongly bonded line of charged beads is similar to that of a single ion with the same charge to mass ratio. Single stranded DNA as long as 40 nm does not fold or curl in the Paul trap, but could undergo rotations about the center of mass. However, we show that a stretching field in the axial direction can effectively prevent the rotations and increase the confinement stability.
2009
- Multilevel resistive switching with ionic and metallic filamentsMing Liu, Z. Abid, Wei Wang, Xiaoli He, Qi Liu, and Weihua GuanApplied Physics Letters, Jun 2009
The resistive random access memory (ReRAM) device with three distinguishable resistance states is fabricated by doping Cu into a portion of the ZrO2 oxide layer of the Ti/ZrO2/n+-Si structure. The temperature-dependent measurement results demonstrate that filaments due to ionic trap-controlled space charge limited current conduction and metallic bridge are formed at different voltages. The formation and rupture of these different conducting filamentary paths in parallel are suggested to be responsible for the multilevel switching with the large resistance ratio, which can be used to establish a reliable multilevel ReRAM solution with variation tolerance.
- Resistive switching characteristics of MnOx-based ReRAMSen Zhang, Shibing Long, Weihua Guan, Qi Liu, Qin Wang, and Ming LiuJournal of Physics D: Applied Physics, Mar 2009
The resistive switching characteristics of MnOx thin film were investigated for resistive random access memory (ReRAM) applications. The devices in the form of metal–insulator–metal structure exhibited reversible resistive switching behaviour under both sweeping voltages and voltage pulses. Formation and rupture of conductive filaments were proposed to explain the resistive switching. When Al was used as the top electrode instead of Pt, the device had a better endurance performance. Additionally, the Pt/MnOx/Al device showed fast switching speed and long retention ability. The experiment result suggested that Pt/MnOx/Al device had a potentiality for practical memory application.
- Unipolar resistive switching of Au-implanted ZrO2 filmsLiu Qi, Long Shibing, Guan Weihua, Zhang Sen, Liu Ming, and Chen JunningJournal of Semiconductors, Apr 2009
The resistive switching characteristics of Au+-implanted ZrO2 films are investigated. The Au/Cr/Au+implanted-ZrO2/n+-Si sandwiched structure exhibits reproducible unipolar resistive switching behavior. After 200 write-read-erase-read cycles, the resistance ratio between the high and low resistance states is more than 180 at a readout bias of 0.7 V. Additionally, the Au/Cr/Au+-implanted-ZrO2/n+-Si structure shows good retention characteristics and nearly 100% device yield. The unipolar resistive switching behavior is due to changes in the film conductivity related to the formation and rupture of conducting filamentary paths, which consist of implanted Au ions.
- Resistance Switching Characteristics of Zirconium Oxide Containing Gold Nanocrystals for Nonvolatile Memory ApplicationsWeihua Guan, Shibing Long, Yuan Hu, Qi Liu, Qin Wang, and Ming LiuJournal of Nanoscience and Nanotechnology, Feb 2009
Resistance switching characteristics of ZrO2 films with gold nanocrystals (nc-Au) embedded are investigated for nonvolatile memory applications. The sandwiched structure of top Au electrode/ZrO2 (with nc-Au embedded)/n+ Si exhibits two stable resistance states (high-resistance state and lowresistance state). By applying proper voltage bias, resistance switching from one state to the other can be achieved. This resistance switching behavior is reproducible and the ratio between the high and low resistance states can be as high as two orders. A long memory retention time and over 102 times DC sweep cycles are demonstrated. The ZrO2 films employing gold nanocrystals are promising to be used in the nonvolatile memory devices.
2008
- Resistive switching memory effect of ZrO2 films with Zr+ implantedQi Liu, Weihua Guan, Shibing Long, Rui Jia, Ming Liu, and Junning ChenApplied Physics Letters, Jan 2008
The Au∕Cr∕Zr+-implanted-ZrO2∕n+-Si sandwiched structure exhibits reversible bipolar resistive switching behavior under dc sweeping voltage. The resistance ratio (Rratio) of high resistive state and low resistive state is as large as five orders of magnitude with 0.5V readout bias. Zr+-implanted-ZrO2 films exhibit good retention characteristics and high device yield. The impact of implanted Zr+ ions on resistive switching performances is investigated. Resistive switching of the fabricated structures is explained by trap-controlled space charge limited current conduction.
- On the resistive switching mechanisms of Cu/ZrO2:Cu/PtWeihua Guan, Ming Liu, Shibing Long, Qi Liu, and Wei WangApplied Physics Letters, Dec 2008
We use convincing experimental evidences to demonstrate that the nonpolar resistive switching phenomenon observed in Cu/ZrO2:Cu/Pt memory devices conforms to a filament formation and annihilation mechanism. Temperature-dependent switching characteristics show that a metallic filamentary channel is responsible for the low resistance state (ON state). Further analysis reveals that the physical origin of this metallic filament is the nanoscale Cu conductive bridge. On this basis, we propose that the set process (switching from OFF state to ON state) and the reset process (switching from ON to OFF state) stem from the electrochemical reactions in the filament, in which a thermal effect is greatly involved.
- Nonpolar Nonvolatile Resistive Switching in Cu Doped {}hbox{ZrO}_{2}\Weihua Guan, Shibing Long, Qi Liu, Ming Liu, and Wei WangIEEE Electron Device Letters, May 2008
In this letter, the unique reproducible nonpolar resistive switching behavior is reported in the Cu-doped ZrO2 memory devices. The devices are with the sandwiched structure of Cu/ZrO2:Cu/Pt. The switching between high resistance state (OFF-state) and low resistance state (ON-state) does not depend on the polarity of the applied voltage bias and can be achieved under both voltage sweeping and voltage pulse. The ratio between the high and low resistance is on the order of 106. Set and Reset operation in voltage pulse mode can be as fast as 50 and 100 ns, respectively. No data loss is found upon continuous readout for more than 104 s. Multilevel storage is considered feasible due to the dependence of ON-state resistance on Set compliance current. The switching mechanism is believed to be related with the formation and rupture of conducting filamentary paths.
- Resistance switching of Au-implanted-ZrO2 film for nonvolatile memory applicationQi Liu, Weihua Guan, Shibing Long, Ming Liu, Sen Zhang, Qin Wang, and Junning ChenJournal of Applied Physics, Dec 2008
The resistive switching characteristics and switching mechanisms of the Au-implanted-ZrO2 film are extensively investigated for nonvolatile memory applications. Reversible resistance-switching behavior from a high resistance to low resistance state can be traced by dc voltage and pulse voltage. After more than 200 dc switching cycles, the resistance ratio between the high and low resistance states is more than 180 times under 0.7 V readout bias. In the voltage pulse test, the “write” and “erase” speeds can be as fast as 50 and 100 ns, respectively. No data loss is observed for more than 106 s. The formation and rupture of conducting filamentary paths related to the implanted Au ions are suggested to be responsible for the resistive switching phenomenon. The dependence of resistance on temperature indicates that the variable-range hopping conduction mechanism is dominated in the low-resistance state, while the current characteristics are governed by the trap-controlled space limited conduction mechanism in the high-resistance state.
- Comparison of discrete-storage nonvolatile memories: advantage of hybrid method for fabrication of Au nanocrystal nonvolatile memoryQin Wang, Rui Jia, Weihua Guan, Weilong Li, Qi Liu, Yuan Hu, Shibing Long, Baoqin Chen, Ming Liu, Tianchun Ye, Wensheng Lu, and Long JiangJournal of Physics D: Applied Physics, Feb 2008
In this paper, the memory characteristics of two kinds of metal-oxide-semiconductor (MOS) capacitors embedded with Au nanocrytals are investigated: hybrid MOS with nanocrystals (NCs) fabricated by chemical syntheses and rapid thermal annealing (RTA) MOS with NCs fabricated by RTA. For both kinds of devices, the capacitance versus voltage (C–V ) curves clearly indicate the charge storage in the NCs. The hybrid MOS, however, shows a larger memory window, as compared with RTA MOS. The retention characteristics of the two MOS devices are also investigated. The capacitance versus time (C–t) measurement shows that the hybrid MOS capacitor embedded with Au nanocrystals has a longer retention time. The mechanism of longer retention time for hybrid MOS capacitor is qualitatively discussed.
- Organic thin-film transistor memory with gold nanocrystals embedded in polyimide gate dielectricLijuan Zhen, Weihua Guan, Liwei Shang, Ming Liu, and Ge LiuJournal of Physics D: Applied Physics, Jul 2008
An all-organic memory device based on a copper phthalocyanine (CuPc) thin-film transistor (TFT) using gold nanocrystals embedded in a polyimide gate dielectric is demonstrated. Both the gate dielectric and the active semiconductor layer are organic materials. Discrete gold nanocrystals are adopted as the charge storage medium. Under proper gate bias, gold nanocrystals are charged and discharged, resulting in the modulation of the channel conductance. Current–voltage (I –V ) measurements at room temperature show the memory behaviour of the fabricated devices. The detailed programming and erasing operations are discussed. Low fabrication temperature and low cost are two benefits of the fabricated memory devices, which could provide a low-cost solution for the all organic circuits.
- Charge storage characteristics of metal-induced nanocrystalline in erbium-doped amorphous silicon filmsZhigang Li, Weihua Guan, Ming Liu, Shibing Long, Rui Jia, Jin Lv, Yi Shi, and Xinwei ZhaoThin Solid Films, Sep 2008
Amorphous silicon film (α-Si) doped with uniform erbium (Er) impurities is deposited by laser ablation technology. High density silicon nanocrystals (nc-Si) can be formed after the rapid thermal annealing (RTA) process. The crystalline process can be explained as the “metal-induced nanocrystalline mechanism”, i.e., doped erbium atoms introduce additional strain in the amorphous silicon matrix and behave as nucleation centers during the thermal annealing process. Through this method, Si nanocrystals with high density and self-limited size distribution can be obtained. The experimental results demonstrate that the Er-induced nanocrystalline silicon film has good charge storage characteristics. It is shown that the optimal condition for the Er-induced nanocrystalline layer in α-Si is with 1 wt.% Er concentration and 1100 °C RTA process for 30 s.
2007
- Nonvolatile resistive switching memory utilizing gold nanocrystals embedded in zirconium oxideWeihua Guan, Shibing Long, Rui Jia, and Ming LiuApplied Physics Letters, Aug 2007
Resistive switching characteristics of ZrO2 films containing gold nanocrystals (nc-Au) are investigated for nonvolatile memory applications. The sandwiched top electrode/ZrO2 (with nc-Au embedded)/n+ Si structure exhibits two stable resistance states (high-resistance state and low-resistance state). By applying proper voltage bias, resistive switching from one state to the other state can be achieved. This resistive switching behavior is reproducible and the ratio between the high and low resistances can be as high as two orders. The intentionally introduced nc-Au in ZrO2 films can improve the device yield greatly. ZrO2 films with gold nanocrystals embedded are promising to be used in the nonvolatile resistive switching memory devices.
- Fabrication and charging characteristics of MOS capacitor structure with metal nanocrystals embedded in gate oxideWeihua Guan, Shibing Long, Ming Liu, Zhigang Li, Yuan Hu, and Qi LiuJournal of Physics D: Applied Physics, May 2007
Metal–oxide–semiconductor capacitor structure with metal nanocrystals embedded in the gate oxide for the application of nonvolatile memory (NVM) is fabricated. Optimal process parameters are investigated and Au nanocrystals are adopted in this paper. High-frequency capacitance versus voltage (C–V ) and conductance versus voltage (G–V ) measurements demonstrate the memory effect of the structure which is shown to originate from the confined states of metal nanocrystals. Capacitance versus time (C–t) measurement under a constant gate bias is executed to evaluate the retention performance and an exponential decaying trend is observed and discussed. It is found that with respect to semiconductor counterparts, Au nanocrystals can provide enhanced retention performance, which confirms the high capacity of Au nanocrystals for NVM applications.
- Modeling of retention characteristics for metal and semiconductor nanocrystal memoriesWeihua Guan, Shibing Long, Ming Liu, Qi Liu, Yuan Hu, Zhigang Li, and Rui JiaSolid-State Electronics, May 2007
The charge retention characteristics of metal nanocrystal (MNC) and semiconductor nanocrystal (SNC) memory devices are comparatively studied in this paper. A charge retention model is proposed, taking into account the quantum confinement effect, to account for the better retention characteristics of metal nanocrystal memory observed in the experiment. Simulation results are in good agreement with experimental data, which confirms the validity of this model. The impact of the nanocrystal size, tunneling dielectric materials (especially high-j dielectrics), and tunneling dielectric thickness on the retention characteristics are all investigated for both the metal nanocrystals and the semiconductor nanocrystals.