Microfluidic Distance Readout Sweet Hydrogel Integrated Paper-Based Analytical Device (μDiSH-PAD) for Visual Quantitative Point-of-Care Testing

Eco-sustainable point-of-care devices: Progress in paper and fabric based electrochemical and colorimetric biosensors

Monitoring real-time health conditions is a rinsing demand in a pandemic prone era. Wearable Point-of-Care (POC) devices with paper and fabric-based sensors are emerging as simple, low-cost, portable, and disposable analytical tools for development of green POC devices (GPOCDs). Capabilities of passive fluid transportation, compatibility with biochemical analytes, disposability and high degree of tunability using vivid device fabrication strategies enables development of highly sensitive and economically feasible POC sensors in particularly post COVID-19 pandemic outbreak. Herein we focus mainly on development of biosensors for testing body fluids in the last 5 years using microfluidic technique through electrochemical and colorimetric principle which forms the two most competing sensing techniques providing quantitative and qualitative assessment modalities respectively and forms almost 80 % of the diagnostic platform worldwide. Present review highlights use of these popular substrates as well as various fabrication strategies for realization of GPOCDs ranging from costly and highly sophisticated photolithography to low cost, non conventional techniques like use of correction ink or marker based devices to even novel pop-up/origami induced patterning techniques. Insights into the advancements in colorimetric technique like distance, count or even text based semi-quantitative read-out modality as a on-hand diagnostic information has also been provided. Finally, future outlooks with other interdisciplinary modalities like use of novel materials, incorporation of digital tools like artificial intelligence (AI), machine learning (ML) and strategies for sensitivity and reliability improvement of future GPOCDs have also been discussed.

The application of machine learning in 3D/4D printed stimuli-responsive hydrogels

The integration of machine learning (ML) in materials fabrication has seen significant advancements in recent scientific innovations, particularly in the realm of 3D/4D printing. ML algorithms are crucial in optimizing the selection, design, functionalization, and high-throughput manufacturing of materials. Meanwhile, 3D/4D printing with responsive material components has increased the vast design flexibility for printed hydrogel composite materials with stimuli responsiveness. This review focuses on the significant developments in using ML in 3D/4D printing to create hydrogel composites that respond to stimuli. It discusses the molecular designs, theoretical calculations, and simulations underpinning these materials and explores the prospects of such technologies and materials. This innovative technological advancement will offer new design and fabrication opportunities in biosensors, mechatronics, flexible electronics, wearable devices, and intelligent biomedical devices. It also provides advantages such as rapid prototyping, cost-effectiveness, and minimal material wastage.

Development of a Semiquantitative Barcode Readout Approach for Paper-Based Analytical Devices (PADs) for Enzymatic H2O2 and Glucose Detection

The integration of barcode technology with smartphones on paper-based analytical devices (PADs) presents a promising approach to bridging manual detection with digital interpretation and data storage. However, previous studies of 1D barcode approaches have been limited to providing only a "yes/no" response for analyte detection. Herein, a method of using barcode readout for semiquantitative signal detection on PADs has been achieved through the integration of barcode technology with a distance-based measurement concept on PADs. To demonstrate the feasibility of this concept, a PAD fabrication strategy incorporating barcodes was explored, using the enzymatic reaction between horseradish peroxidase (HRP), 3,3'-diaminobenzidine (DAB), and H2O2 as a model system. The enzyme-catalyzed polymerization of DAB to polyDAB in the presence of hydrogen peroxide results in the appearance of color observable by the naked eye inside a paperfluidic channel, with the color-changed length depending on the H2O2 concentration. At the same time, the barcode pattern displayed as a result of this distance-based color evolution overlaid with a paper-based barcode layer can be read using a smartphone application. Parameters affecting the signal readout performance were studied. The developed device can be used to detect H2O2 concentrations in the range of 0.25 to 10 mM within 90 min with 79.6% of barcode signals correctly readable. Additionally, results from different smartphone models showed a consistent reading performance (78.4-79.6%). Finally, the quantification of glucose levels in artificial urine samples was demonstrated. This developed PAD signaling strategy offers end-users more simplicity and can be used as a standalone device or in conjunction with other digital devices.

Polysaccharide Hydrogel-Assisted Biosensing Platforms for Point-of-Care Use

Point-of-care (POC) use is one of the essential goals of biosensing platforms. Because the increasing demand for testing cannot be met by a centralized laboratory-based strategy, rapid and frequent testing at the right time and place will be key to increasing health and safety. To date, however, there are still difficulties in developing a simple and affordable, as well as sensitive and effective, platform that enables POC use. In terms of materials, hydrogels, a unique family of water-absorbing biocompatible polymers, have emerged as promising components for the development of biosensors. Combinations of hydrogels have various additional applications, such as in hydrophilic coatings, nanoscale filtration, stimuli-responsive materials, signal enhancement, and biodegradation. In this review, we highlight the recent efforts to develop hydrogel-assisted biosensing platforms for POC use, especially focusing on polysaccharide hydrogels like agarose, alginate, chitosan, and so on. We first discuss the pros and cons of polysaccharide hydrogels in practical applications and then introduce case studies that test different formats, such as paper-based analytical devices (PADs), microfluidic devices, and independent platforms. We believe the analysis in the present review provides essential information for the development of biosensing platforms for POC use in resource-limited settings.

Distance-based paper device coupled with uracil-rich DNA hydrogel for visual quantification of Uracil-DNA glycosylase

Uracil-DNA glycosylase (UDG), an enzyme for repairing uracil-containing DNA damage, is crucial for maintaining genomic stability. Simple and fast quantification of UDG activity is essential for biological assay and clinical diagnosis, since its aberrant level is associated with DNA damage and various diseases. Herein, we developed a fully integrated "sample in-signal out" distance-based paper analytical device (dPAD) for visual quantification of UDG using a flow-controlled uracil-rich DNA hydrogel (URDH). The uracil base sites contained in the DNA hydrogel are mis-incorporated with dUTP by rolling circle amplification (RCA), which simplifies the preparation process of the functionalized hydrogel. In the presence of UDG, the uracil in URDH can be recognized and removed to induce the permeability change of URDH, resulting in the visible distance signal along the paper channel. Using dPAD, as low as 6.4 × 10-4 U/mL of UDG (within 80 min) is visually identified without any instruments and complicated operations. This integrated dPAD is advantageous for its simplicity, cost effectiveness, and ease of use. We envision that it has the great potential for point-of-care testing (POCT) in DNA damage testing, personalized healthcare assessment, and biomedical applications.

Target-Triggered Ultrafast Chondroitin Gelation Enabled Power-Free and Point-of-Care Bioassays

Point-of-care (POC) tests increasingly highlight the importance of portable, cost-effective, and visually quantitative detection of biomarkers. Herein, we developed a power-free and visual signal-readout POC sensor based on the target-triggered ultrafast gelation process. In the gelation process, the target triggered the cascade reaction catalyzed by oxidase and ferrous glycinate to produce carbon radicals that immediately initiated the rapid polymerization and cross-linking of acryloylated chondroitin sulfate and dimethylacrylamide. This highly efficient enzymatic polymerization process contributed to the ultrafast generation of chondroitin hydrogel within 1 min at 25 °C. The increase in viscosity of aqueous solution resulted from hydrogel formation was then visually measured according to the distance of solution migration on a tick-labeled pH test strip, which thus realized the quantification of a target. By utilizing glucose oxidase as an oxidase model during the gelation process, this POC sensor was successfully employed in the rapid quantitative detection of glucose without the need for any auxiliary instruments. Benefiting from the specificity and stability of the enzymatic polymerization reaction, the sensor exhibited excellent performance in the detection of glucose in clinical blood samples. Moreover, the sensor was further extended to uric acid detection and enabled accurate assay in clinical urine samples, which indicated the versatility and practicability of this sensor in the POC test.

A gold nanomaterial-integrated distance-based analytical device for uric acid quantification in human urine samples

In this article, we present the first demonstration of a distance-based paper analytical device (dPAD) for uric acid quantification in human urine samples with instrument-free readout and user-friendliness for the rapid diagnosis and prognosis of various related diseases. By employing gold nanoparticles (AuNPs) as a peroxidase-like nanozyme, our proposed technique eliminates the utilization of horseradish peroxidase (HRP), making the device cost-effective and stable. In our dPAD, uric acid in the sample is oxidized by the uricase enzyme and subsequently catalysed with AuNPs in the sample zone, generating hydroxyl radicals (˙OH). Then, the produced ˙OH reacts with 3,3'-diaminobenzidine (DAB) to form poly DAB (oxDAB), resulting in a coloured distance signal in the detection zone of the dPAD. The variation of the distance of the observed red-brown colour is directly proportional to the uric acid concentration. Our sensor exhibited a linear range from 0.50 to 6.0 mmol L-1 (R2 = 0.9922) with a detection limit (LOD) of 0.25 mmol L-1, covering the clinical range of uric acid in urine. Hence, there is no need for additional sample preparation or dilution. Additionally, this assay is highly selective, with no interferences. We also found that this approach could accurately and precisely determine uric acid in human control samples with the recovery ranging from 99.37 to 100.35 with the highest RSD of 4.05%. Our method is comparable with the use of a commercially available uric acid sensor at a 95% confidence interval. Consequently, the developed dPAD offers numerous advantages such as cost-effectiveness, simplicity, and ease of operation with unskilled individuals. Furthermore, this concept can be applied for extensive biosensing applications in monitoring other biomarkers as an alternative analytical point-of-care (POC) device.

Unveiling the state of the art: a systematic review and meta-analysis of paper-based microfluidic devices

Introduction

This systematic review and meta-analysis present a comprehensive evaluation of paper-based microfluidic devices, focusing on their applications in immunoassays. These devices are emerging as innovative solutions to democratize access to diagnostic technologies, especially in resource-limited settings. Our review consolidates findings from diverse studies to outline advancements in paper-based microfluidic technology, including design intricacies and operational efficacy. Key advantages such as low cost, portability, and ease of use are highlighted.

Materials and Methods

The review categorizes literature based on the design and operational nuances of these diagnostic tools, exploring various methodologies, fabrication techniques, detection methods, and applications, particularly in protein science. The meta-analysis extends to the diverse applications of these technologies, providing a framework for classifying and stratifying their uses in diagnostics.

Results and discussion

Notable findings include a critical analysis of performance metrics, such as sensitivity and specificity. The review addresses challenges, including the need for further validation and optimization for broader clinical applications. A critical discussion on the validation processes, including cross-validation and rigorous control testing, is provided to ensure the robustness of microfluidic devices. This study offers novel insights into the computational strategies underpinning these technologies and serves as a comprehensive roadmap for future research, potentially broadening the impact across the protein science universe.

Semiquantitative Paper-Based Microfluidic Surrogate Virus Neutralization Test for SARS-CoV-2 Neutralizing Antibodies

Neutralizing antibodies (nAbs) produced from infection or vaccination play an important role in acquired immunity. Determining virus-specific nAb titers is a useful tool for measuring aquired immunity in an individual. The standard methods to do so rely on titrating serum samples against live virus and monitoring viral infection in cultured cells which requires high biosafety level containment. The surrogate virus neutralization test (sVNT) reduces the biohazards and it is suitable for designing rapid test device in a lateral flow assay (LFA) format. Here, we introduce the fabrication and development of a unique paper-based LFA device for determining the level of SARS-CoV-2 nAb in a sample with a semiquantitative direct colorimetric readout. A LFA-based gradient assay design was used to facilitate the sVNT, where the spike glycoprotein receptor binding domain (RBD) and angiotensin-converting enzyme 2 (ACE2) stand in as proxies for viruses and cells, respectively. The gradient assay employed multiple test dots of ACE2 spotted in increasing concentration along the sample flow path and gold nanoparticle-conjugated RBD for readout. In this way, the number of developed spots is inversely proportional to the concentration of nAbs present in the sample. The assay was tested with both standard solutions of nAb as well as human serum samples. We have demonstrated that the device can effectively provide semiquantitative test results of nAbs by direct instrument-free colorimetric detection.

A separation-free paper-based hydrogel device for one-step reactive oxygen species determination by a smartphone

Paper-based analytical devices (PADs) are very convenient for determining biomarkers in point-of-care (POC) diagnosis while requiring sample pre-treatment or impurity separation. This study reports a novel hydrogel-coupled, paper-based analytical device (PAD) for separation-free H2O2 colorimetric detection in both aqueous solution and cell lysis with sample-to-answer analysis by directly loading into the sample test zone. By encapsulating an inorganic mimic enzyme and chromogenic substrate into the sodium alginate (SA) hydrogel, amplification of the color signal after catalyzing the substrate could be achieved. Taking advantage of the nanoscale porous structure of the hydrogel and the lateral flow channel of the PAD, large interference fragments or bio-macromolecules are prevented from diffusing into the chromogenic reaction, whereas the small target molecules enter the sensing region to trigger the catalytic reaction. This method demonstrated a rapid and accurate analysis with a limit of detection as low as 0.06 mM and detection selectivity. Our proposed device requires no enzyme and is separation-free, portable, easy-to-fabricate, and low-cost, and may offer a platform for quantitative or qualitative analysis of other analytes in body fluids for POC applications.

Quantitative and equipment-free paper-based agglutination assay of bacterial cells

Background: point-of-care (POC) tests are useful for bedside/home applications, emergencies, frequent follow-ups, and resource-limited areas. Limited quantitative and equipment-free POC assays have been reported. This study aims to develop, validate, and apply a simple, quantitative, paper-based POC assay. Methods: wax-channeled paper treated with specific anti-Brucella and anti-Salmonella antibodies was used for distance-based chromatographic elution of stained bacterial cell agglutinations. Results: a qualitative paper-based agglutination POC test was developed using color intensity, tail appearance, and "+/-" signs that clearly distinguish the positive and negative results. The optimization of the test for paper type, microfluidic channel design, antibody and bacterial cell concentrations, and elution methods was carried out. Quantitative assay transformation was successfully developed using the color intensity of the original reaction zone, intensity of elution tail, and distance-based migration that correspond to bacterial agglutination size. The migration distance of eluted bacterial agglutination bands corresponds to the target concentration with good linearity and minimal variability. Reporting of colored band migration with numbers using microfluidic patterns was used to enhance non-technical end-user applications. A distance-based POC assay prototype was then successfully used for the accurate detection of known and unknown samples in comparison with standard assays. Conclusions: the migration distance of an eluted stained bacterial agglutination correlated with anti-bacterial antibody concentrations. A simple, cheap, quantitative, and equipment-free paper-based POC assay of bacterial cell agglutination was developed. This test can be used for simple "+/-" results, thermometer-like quantification, or text reporting with numbers corresponding to target concentrations. The assay has extended applications to different human disease biomarkers.

Paper-based sensors: affordable, versatile, and emerging analyte detection platforms

Paper-based sensors, often referred to as paper-based analytical devices (PADs), stand as a transformative technology in the field of analytical chemistry. They offer an affordable, versatile, and accessible solution for diverse analyte detection. These sensors harness the unique properties of paper substrates to provide a cost-effective and adaptable platform for rapid analyte detection, spanning chemical species, biomolecules, and pathogens. This review highlights the key attributes that make paper-based sensors an attractive choice for analyte detection. PADs demonstrate their versatility by accommodating a wide range of analytes, from ions and gases to proteins, nucleic acids, and more, with customizable designs for specific applications. Their user-friendly operation and minimal infrastructure requirements suit point-of-care diagnostics, environmental monitoring, food safety, and more. This review also explores various fabrication methods such as inkjet printing, wax printing, screen printing, dip coating, and photolithography. Incorporating nanomaterials and biorecognition elements promises even more sophisticated and sensitive applications.

Designing and prototyping a novel biosensor based on a volumetric bar-chart chip for urea detection

A volumetric bar-chart chip (V-chip) is a microfluidic device based on distance-based quantitative measurement that visualizes analyte concentration without the need for apparatus or data processing. This typically utilizes special receptors and catalysis parts that generate oxygen, so ink can be moved inside the channels, and enables instant visual quantitation of the analyte. However, the low stability of some macromolecules, the use of expensive catalysts, and difficulty in controlling the process cause inaccurate readings, and therefore, limit further development and the use of these systems. In this article, we introduced a novel approach that eliminates the use of catalysts in V-chips and provides an efficient and simple path in the design of biosensors. The product of the enzymatic reaction of urease with urea is bicarbonate, which turns into CO2 gas in an acidic environment. Therefore, the amount of gas produced is proportional to the amount of urea in the sample, and it can be quantitatively measured by visual detection from the amount of ink movement caused by CO2 gas pressure. This biosensor has a linear response range of 0 to 1000 μg ml-1 and a detection limit of 3.6 μg ml-1 in raw milk. The recovery of urea in raw milk at 100 and 400 μg ml-1 concentrations was 96.5% and 98.9%, respectively. This volumetric chip shows potential for determining urea levels in real samples without requiring additional equipment. The combination of the sensitivity and specificity of enzymatic reactions, inherent gas-generating reactions, and the processability of microchips discussed in this paper can be the basis for the comprehensive development of volumetric chips, which can create a new path for the development of efficient and cheap biosensors.

Distance-based paper analytical device for multiplexed quantification of cytokine biomarkers using carbon dots integrated with molecularly imprinted polymer

This article introduces distance-based paper analytical devices (dPADs) integrated with molecularly imprinted polymers (MIPs) and carbon dots (CDs) for simultaneous quantification of cytokine biomarkers, namely C-reactive protein (CRP), tumor necrosis factor-alpha (TNF-α), and interleukin-6 (IL-6) in human biological samples for diagnosis of cytokine syndrome. Using fluorescent CDs and MIP technology, the dPAD exhibits high selectivity and sensitivity. Detection is based on fluorescence quenching of CDs achieved through the interaction of the target analytes with the MIP layer on the paper substrate. Quantitative analysis is easily accomplished by measuring the distance length of quenched fluorescence with a traditional ruler and naked eye readout enabling rapid diagnosis of cytokine syndrome and the underlying infection. Our sensor demonstrated linear ranges of 2.50-24.0 pg mL-1 (R2 = 0.9974), 0.25-3.20 pg mL-1 (R2 = 0.9985), and 1.50-16.0 pg mL-1 (R2 = 0.9966) with detection limits (LODs) of 2.50, 0.25, and 1.50 pg mL-1 for CRP, TNF-α, and IL-6, respectively. This sensor also demonstrated remarkable selectivity compared to a sensor employing a non-imprinted polymer (NIP), and precision with the highest relative standard deviation (RSD) of 5.14%. The sensor is more accessible compared to prior methods relying on expensive reagents and instruments and complex fabrication methods. Furthermore, the assay provided notable accuracy for monitoring these biomarkers in various human samples with recovery percentages ranging between 99.22% and 103.58%. By integrating microfluidic systems, nanosensing, and MIPs technology, our developed dPADs hold significant potential as a cost-effective and user-friendly analytical method for point-of-care diagnostics (POC) of cytokine-related disorders. This concept can be further extended to developing diagnostic devices for other biomarkers.

Portable Sensor for Aflatoxin B1 Based on the Regulation of Resistance of a Microchannel Using a Multimeter as Readout

Changes in the charge density on the inner surface of the microchannel can modulate the ion concentration at the tip, thus causing changes in the resistance of the system. In this study, this property is adopted to construct a portable sensor using a multimeter and aflatoxin B1 (AFB1) is used as the model target. Initially, the cDNA/aptamer complex is modified in the microchannel. The inner microchannel surface's charge density is then altered by the recognition of the target, leading to a change in the system's resistance, which can be conveniently monitored using a multimeter. Critical parameters influencing the performance of the system are optimized. Under optimum conditions, the resistance is linearly related to the logarithm of AFB1 concentration in the range of 100 fM-10 nM and the detection limit is 46 fM (S/N = 3). The resistive measurement is separated from the recognition reaction of the target, reducing the matrix interference during the detection process. This sensor boasts high sensitivity and specificity coupled with commendable reproducibility and stability. It is applied to assay the AFB1 content successfully in an actual sample of corn. Moreover, this approach is cost-effective, user-friendly, and highly accurate.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Paper-based chiral biosensors using enzyme encapsulation in hydrogel network for point-of-care detection of lactate enantiomers

Chiral analysis is of pivotal importance in a variety of fields due to the different biological activities and functions of enantiomers. Here, we develop a simple paper-based chiral biosensor that can perform sample-to-answer simultaneous analysis of lactate enantiomers in human serum samples. By modification of alginate hydrogel with "egg-box" three-dimensional network structure on a glass microfiber paper, reagents of enantiomer-selective enzymatic reactions are efficiently encapsulated forming the sensing regions for chiral analysis. Dual enzyme catalytic system (lactate dehydrogenase and glutamic pyruvic transaminase) is utilized to enhance the response of the biosensor. A smartphone with color analysis software is used to collect and analyze the fluorescence signal from the product nicotinamide adenine dinucleotide. The results show that the sensor has excellent selectivity toward lactate enantiomers with low limit-of-detection of (30.0 ± 0.7) μM for L-lactate and (3.0 ± 0.2) μM for D-lactate, and wide linear detection range of 0.1-3.0mM and 0.01-0.5 mM for L-lactate and D-lactate respectively. The proposed method is successfully applied to the simultaneous detection of L-/D-lactate concentrations in human serum with satisfactory accuracy. Our study provides a robust approach for developing chiral biosensors, which would have promising application prospect in point-of-care testing (POCT) analysis of various biological and food samples.

Oxygen-induced dual-signal point-of-care testing aptasensor for aflatoxin B1 detection using platinum nanoparticle catalysis in visual fluorometry and gravimetry

Point-of-care testing (POCT) has experienced rapid development owing to its advantages of rapid testing, low cost and strong operability, making it indispensable for analyte detection in outdoor or rural areas. In this study, we propose a novel method for the detection of aflatoxin B₁ (AFB₁) using a dual-signal readout approach within a unified system. This method employs dual channel modes, namely visual fluorescence and weight measurements, as the signal readouts. Specifically, a pressure-sensitive material is utilized as a visual fluorescent agent, its signal can be quenched in the presence of high oxygen pressure. Additionally, an electronic balance, commonly used for weight measurement, is adopted as another signal device, where the signal is generated through the catalytic decomposition of H₂O₂ by platinum nanoparticles. The experimental results demonstrate that the proposed device enables accurate AFB₁ detection within the concentration range of 1.5-32 μg mL^-1, with a detection limit of 0.47 μg mL^-1. Moreover, this method has been successfully applied for practical AFB₁ detection with satisfactory results. Notably, this study pioneers the use of a pressure-sensitive material as a visual signal in POCT. By addressing the limitations of single-signal readout approaches, our method fulfills requirements of intuitiveness, sensitivity, quantitative analysis and reusability.

A portable system for economical nucleic acid amplification testing

Introduction: Regular and rapid large-scale screening for pathogens is crucial for controlling pandemics like Coronavirus Disease 2019 (COVID-19). In this study, we present the development of a digital point-of-care testing (POCT) system utilizing microfluidic paper-based analytical devices (μPADs) for the detection of SARS-CoV-2 gene fragments. The system incorporates temperature tuning and fluorescent detection components, along with intelligent and autonomous image acquisition and self-recognition programs. Methods: The developed POCT system is based on the nucleic acid amplification test (NAAT), a well-established molecular biology technique for detecting and amplifying nucleic acids. We successfully detected artificially synthesized SARS-CoV-2 gene fragments, namely ORF1ab gene, N gene, and E gene, with minimal reagent consumption of only 2.2 μL per readout, representing a mere 11% of the requirements of conventional in-tube methods. The power dissipation of the system was low, at 6.4 W. Results: Our testing results demonstrated that the proposed approach achieved a limit of detection of 1000 copies/mL, which is equivalent to detecting 1 copy or a single RNA template per reaction. By employing standard curve analysis, the quantity of the target templates can be accurately determined. Conclusion: The developed digital POCT system shows great promise for rapid and reliable detection of SARS-CoV-2 gene fragments, offering a cost-effective and efficient solution for controlling pandemics. Its compatibility with other diagnostic techniques and low reagent consumption make it a viable option to enhance healthcare in resource-limited areas.

Facile and portable multimodal sensing platform driven by photothermal-controlled release system for biomarker detection

Portable, maneuverable and reliable versatile-integrated analytical devices are urgently demanded but still extremely challenging to meet the requirements of point-of-care testing in resource-limited areas. Herein, a multifunctional sensing platform with excellent photothermal performance was implanted into the miniature zone of a paper-based electrochemiluminescent (ECL) biosensor for accurate detection of interleukin-6, which could flexibly interconnect the visualized distance and temperature readout with ultrasensitive ECL response. Concretely, the multipurpose MBene and TaSe₂ composites (MBene@TaSe₂) prepared via self-assembly approach as target-associated photothermal element was introduced in the paper-based analytical device (PAD) and served as multi-signals trigger. Under the laser irradiation, MBene@TaSe₂ probe not only generated heat for rapid temperature output, but also triggered the phase transition behavior of thermoresponsive poly (N-isopropylacrylamide) (pNIPAM) hydrogel to release loaded malachite green (MG) dye for distance-based visual readout. Simultaneously, the released MG was also utilized as effective quencher to decrease the ECL signal of luminol. Benefitting from this dexterous architecture, the speedy preliminary screening and precise quantitative analysis could be subsequently obtained in single-drop sample through one-step sandwich immunoreaction, which avoids additional separation operations and greatly simplifies the analysis procedure. Undeniably, this work provides ingenious insights for advancing the development of convenient and fast multifunction-integrated PAD in family surveillance and intelligent diagnosis.

Stimuli-bioresponsive hydrogels as new generation materials for implantable, wearable, and disposable biosensors for medical diagnostics: Principles, opportunities, and challenges

Hydrogels are excellent water-swollen polymeric materials for use in wearable, implantable, and disposable biosensors. Hydrogels have unique properties such as low cost, ease of preparation, transparency, rapid response to external conditions, biocompatibility and self-adhesion to the skin, flexibility, and strain sensitivity, making them ideal for use in biosensor platforms. This review provides a detailed overview of advanced applications of stimuli-responsive hydrogels in biosensor platforms, from hydrogel synthesis and functionalization for bioreceptor immobilization to several important diagnostic applications. Emphasis is placed on recent advances in the fabrication of ultrasensitive fluorescent and electrically conductive hydrogels and their applications in wearable, implantable, and disposable biosensors for quantitative measurements. Design, modification, and assembly techniques of fluorescent, ionically conductive, and electrically conductive hydrogels to improve performance will be addressed. The advantages and performance improvements of immobilizing bioreceptors (e.g., antibodies, enzymes, and aptamers), and incorporating fluorescent and electrically conductive nanomaterials are described, as are their limitations. Potential applications of hydrogels in implantable, wearable, disposable portable biosensors for quantitative detection of the various bioanalytes (ions, molecules, drugs, proteins, and biomarkers) are discussed. Finally, the global market for hydrogel-based biosensors and future challenges and prospects are discussed in detail.

Naked-Eye Readout Distance Quantitative Lateral Flow Assay Based on the Permeability Changes of Enzyme-Catalyzed Hydrogelation

Traditional lateral flow assay (LFA) is restricted to providing qualitative or semi-quantitative results and often requires special equipment for obtaining quantitative results. Herein, we proposed a naked-eye readout distance quantitative lateral flow assay based on the permeability changes in enzyme-catalyzed hydrogelation, which not only has the advantages of being simple, immediate, of high efficiency and low cost, and accurate in quantification but also avoids the use of special equipment. The developed LFA method includes three principal components of a nitrocellulose (NC) membrane containing a control line (C line) loading goat anti-rabbit (GAR) antibodies and a test line (T line) loading specific antibodies, alginate-tyramine conjugates forming a hydrogel in the presence of hydrogen peroxide (H₂O₂) and horseradish peroxidase (HRP), and the HRP-AuNPs-Ab probe only labeling targets captured on the T line. Hemoglobin A1c (HbA1c) was chosen as a representative example to demonstrate the feasibility of our method. Under the optimal conditions, the developed LFA method shows excellent performance in standard samples and real human blood samples where the results of real human blood samples show a high linear correlation with the clinical data obtained by ion exchange chromatography (R² = 0.9929) and the margin of recovery is only 3.8%. All results demonstrated that our developed LFA method not only has enormous potential in the quantitative detection of HbA1c in clinical complex samples but also can serve as a versatile method for highly efficient detection of other target biomolecules due to the fungibility of antibodies.

Paper-based analytical devices for point-of-need applications

Paper-based analytical devices (PADs) are powerful platforms for point-of-need testing since they are inexpensive devices fabricated in different shapes and miniaturized sizes, ensuring better portability. Additionally, the readout and detection systems can be accomplished with portable devices, allying with the features of both systems. These devices have been introduced as promising analytical platforms to meet critical demands involving rapid, reliable, and simple testing. They have been applied to monitor species related to environmental, health, and food issues. Herein, an outline of chronological events involving PADs is first reported. This work also introduces insights into fundamental parameters to engineer new analytical platforms, including the paper type and device operation. The discussions involve the main analytical techniques used as detection systems, such as colorimetry, fluorescence, and electrochemistry. It also showed recent advances involving PADs, especially combining optical and electrochemical detection into a single device. Dual/combined detection systems can overcome individual barriers of the analytical techniques, making possible simultaneous determinations, or enhancing the devices' sensitivity and/or selectivity. In addition, this review reports on distance-based detection, which is also considered a trend in analytical chemistry. Distance-based detection offers instrument-free analyses and avoids user interpretation errors, which are outstanding features for analyses at the point of need, especially for resource-limited regions. Finally, this review provides a critical overview of the practical specifications of the recent analytical platforms involving PADs, demonstrating their challenges. Therefore, this work can be a highly useful reference for new research and innovation.

Portable smartphone-assisted ratiometric fluorescence sensor for visual detection of glucose

Fluorescence-based visual assays have sparked tremendous attention in on-site detection due to their obvious color gradient changes and high sensitivity. In this study, a novel emission wavelength shift-based visual sensing platform is constructed to detect glucose based on the oxidation of Rhodamine B (RhB). MnO₂ nanosheets (MnO₂ NS) with strong oxidizing properties were introduced to oxidize RhB, which resulted in a blue shift in the emission wavelength, and a visual color changed of the fluorescence from orange-red to green. The oxidation reaction could be inhibited via reducing and destroying MnO₂ NS by H₂O₂, which was produced by the oxidizing procedure of glucose in the presence of glucose oxidase (GOx). A series of wavelength shifts and fluorescence color variations appeared with the addition of various amounts of glucose. A ratiometric fluorescence glucose sensor with a lowest recorded concentration of 0.25 μM was developed. Meanwhile, test paper-based assays integrated with the smartphone platform were established for the sensing of glucose by means of the significant fluorescence color changes, offering a reliable, sensitive, and portable on-site assay of glucose.

Development of Pipetteless Paper-Based Analytical Devices with a Volume Gauge

In this work, we propose a new design for paper-based analytical devices (PADs) that eliminate the need to use a micropipette for sample introduction. With this design, a PAD is equipped with a distance-based detection channel that is connected to a storage channel that indicates the volume of a sample introduced into the PAD. The analyte in the sample solution reacts with a colorimetric reagent deposited into the distance-based detection channel as the sample solution flows into the storage channel where the volume is measured. The ratio of the lengths of the detection channel and that of the storage channel (D/S ratio) are constant for a sample containing a certain concentration, which is independent of the introduced volume. Therefore, the PADs permit volume-independent quantification using a dropper instead of a micropipette because the length of the storage channel plays the role of a volume gauge to estimate the introduced sample volume. In this study, the D/S ratios obtained with a dropper were comparable to those obtained with a micropipette, which confirmed that precise volume control is unnecessary for this PAD system. The proposed PADs were applied to the determinations of iron and bovine serum albumin using bathophenanthroline and tetrabromophenol blue as colorimetric reagents, respectively. The calibration curves showed good linear relationships with coefficients of 0.989 for iron and 0.994 for bovine serum albumin, respectively.

Spherical Hydrogel Sensor Based on PB@Fe-COF@Au Nanoparticles with Triplet Peroxidase-like Activity and Multiple Capture Sites for Effective Detection of Organophosphorus Pesticides

The residues of organophosphorus pesticides (OPs) have drawn worldwide increasing attention because of their potential fatal effects on human health and ecological systems. It is of great significance to develop an efficient and portable method for in-field detection of OPs. Herein, a novel core-shell nanocomposite of prussian blue@Fe-covalent organic framework@Au (PB@Fe-COF@Au) was constructed. Fe²⁺ and Fe³⁺ in PB nanoparticle (PBNP) cores, Fe-porphyrin in COF shells, and AuNPs grown on shells all acted as peroxidase-like catalytic active sites, enabling PB@Fe-COF@Au to possess triplet peroxidase-like activity. A colorimetric, affordable, sensitive, and selective strategy was designed to detect OPs. Compared with previous reports, this sensor realized a wider linear range for chlorpyrifos of 10-800 ng mL^-1 with a relatively lower detection limit of 0.61 ng mL^-1, which was attributed to the overlapping triple catalytic sites of PB@Fe-COF@Au and triple response sites to OPs. The assay was successfully employed to detect chlorpyrifos in food and environmental samples. Moreover, to meet the demand of in-field detection for OPs, a spherical hydrogel method based on PB@Fe-COF@Au with visual, portable, and equipment-free features was fabricated. This work provides a new pathway to design and apply effective nanozymes for on-site monitoring of pesticides.

Paper-Based Distance Sensor for the Detection of Lipase via a Phase Separation-Induced Viscosity Change

Human pancreatic lipase is a symbolic biomarker for the diagnosis of acute pancreatitis, which has profound significance for clinical detection and disease treatment. Herein, we first demonstrate a paper-based lipase sensor via a phase separation-induced viscosity change. Lipase catalyzes triolein to produce oleic acid and glycerol. Adding an excess of Ca²⁺ produces calcium oleate. The remaining Ca²⁺ binds with sodium alginate, triggering hydrogelation with an "egg-box" structure. The viscosity change of the aqueous solution induced by the phase separation process can be quantified by measuring the solution flow distance on a pH test paper. The paper-based lipase sensor has high sensitivity with a detection limit of 0.052 U/mL and also shows excellent specificity. Additionally, it is also utilized for quantitative lipase analysis in human serum samples to exhibit its potency in acute pancreatitis detection. This method overcomes the drawbacks of low sensitivity, slow response, and poor reproducibility caused by the nonuniform distribution of the highly viscous hydrogel on the sensing interface in existing approaches. In conclusion, thanks to the prominent characteristics of high portability, low cost, and easy operation, it is prospective for simple quantitative detection of lipase and has great potential for commercialization.

Microfluidic platforms integrated with nano-sensors for point-of-care bioanalysis

Microfluidic technology provides a portable, cost-effective, and versatile tool for point-of-care (POC) bioanalysis because of its associated advantages such as fast analysis, low volumes of reagent consumption, and high portability. Along with microfluidics, the application of nanomaterials in biosensing has attracted lots of attention due to their unique physical and chemical properties for enhanced signal modulation such as signal amplification and signal transduction for POC bioanalysis. Hence, an enormous number of microfluidic devices integrated with nano-sensors have been developed for POC bioanalysis targeting low-resource settings. Herein, we review recent advances in POC bioanalysis on nano-sensor-based microfluidic platforms. We first briefly summarized the different types of cost-effective microfluidic platforms, followed by a concise introduction to nanomaterial-based biosensors. Then, we highlighted the application of microfluidic platforms integrated with nano-sensors for POC bioanalysis. Finally, we discussed the current limitations and perspective trends of the nano-sensor-based microfluidic platforms for POC bioanalysis.

Target-responsive aptamer-cross-linked hydrogel sensors for the visual quantitative detection of aflatoxin B1 using exonuclease I-Triggered target cyclic amplification

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An AFB₁-responsive aptamer-cross-linked hydrogel sensor was successfully constructed.

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Dual signal amplification strategy with Encapsulation of enzymesand exonuclease I.

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This method has great potential for AFB₁ detection in peanut oil.

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The accuracy and consistency repeatability of this method are close to those of UPLC-HRMS.

For the on-site detection of aflatoxin B₁ (AFB₁), a DNA hydrogel was prepared as a biosensor substrate, while an AFB₁ aptamer was used as the recognition element. An AFB₁-responsive aptamer-cross-linked hydrogel sensor was constructed using an enzyme-linked signal amplification strategy; AFB₁ binds competitively to the aptamer, causing the hydrogel to undergo cleavage and release horseradish peroxidase (HRP). The addition of exonuclease I (ExoI) to the hydrogel causes the release of AFB₁ from the aptamer, promoting additional hydrogel cleavage to release more HRP, ultimately catalysing the reaction between 3,3′,5,5′-tetramethylbenzidine and H₂O₂. The hydrogel sensor exhibited an outstanding sensitivity (limit of detection, 4.93 nM; dynamic range, 0–500 nM), and its selectivity towards seven other mycotoxins was confirmed. The feasibility and reliability were verified by measuring the AFB₁ levels in peanut oil (recoveries, 89.59–95.66 %; relative standard deviation, <7%); the obtained results were comparable to those obtained by UPLC-HRMS.

Shape-Encoded Functional Hydrogel Pellets for Multiplexed Detection of Pathogenic Bacteria Using a Gas Pressure Sensor

Gas pressure is a promising signal readout mode in point-of-care testing for its merits such as rapidity, simplicity, affordability, and no need for sophisticated instrumentation. Herein, a gas pressure sensor for multiplexed detection of pathogenic bacteria was developed on a hydrogel platform. Spherical and square hydrogel pellets prepared by cross-linking of sodium alginate were functionalized with nisin and ConA for the capture of Staphylococcus aureus and Escherichia coli O157:H7, respectively. By using the shape-encoded functional hydrogel pellets and aptamer-modified platinum-coated gold nanoparticles (Au@PtNPs), a dual-molecule recognition mode was established for rapid and specific detection of the two pathogenic bacteria. Au@PtNPs were applied as signal probes to efficiently catalyze the decomposition of H₂O₂ for generating abundant O₂, which was converted into an amplified gas pressure signal. In two closed containers, the significant gas pressure signals were monitored with a portable pressure meter to quantitate the two pathogenic bacteria. The sensor was successfully applied to detect the pathogenic bacteria in various environmental, biological, and food samples. Thus, the proof-of-principle work paves a new avenue for multiplexed detection of pathogenic bacteria with shape-encoded hydrogel pellets combined with gas pressure signal readout.

Modification of Microfluidic Paper-Based Devices with an Oxidant Layer for Distance Readout of Reducing Substances

We developed a novel strategy for modification of paper cellulose with water-insoluble oxidants for distance readout of reducing substances on microfluidic paper-based analytical devices (μPADs). Water-insoluble oxidants were formed and modified onto paper cellulose through the redox reaction that occurred between paper cellulose and potassium permanganate deposited on the paper channel, developing a yellowish-brown color on the channel. As aqueous solutions containing reducing substances flowed along the channel, reducing substances were consumed owing to the redox reaction that occurred between oxidants and reducing substances until the reducing substances were depleted, forming a discolored zone on the yellowish-brown channel. The redox reaction between insoluble oxidants and reducing substances on the paper cellulose could be used for distance-based detection of a wide variety of reducing substances, which is similar to the classical potassium permanganate titration that employs the redox reaction that occurred between potassium permanganate and reducing substances. We believe that this method will broaden the analytical applications of distance-based detection on μPADs. This method was applied to ascorbic acid assay and captopril assay in real samples with analytical results comparing well with the labeled values, demonstrating its great potential in real sample analysis.

Distance-based β-amyloid protein detection on PADs for the scanning and subsequent follow-up of Alzheimer's disease in human urine samples

We report on the first development of a simple distance-based β-amyloid (Aβ) protein quantification using a paper-based device (dPAD) to screen for Alzheimer's disease (AD) and to subsequently follow up on its influence, i.e., clinical dementia. This sensor method is based on the transformation of a free acid form and its binding with a basic form of bromocresol purple (BCP) through its electrostatic interaction with an Aβ protein. This sensor can measure the length of color change from yellow to blue-green on a paper strip, with this change proportional to the amount of Aβ protein level. We found that the linearity for Aβ protein monitoring was in the range from 0.50 to 10.0 ng mL^-1, and the subsequent naked-eye detection limit for Aβ was 0.20 ng mL^-1. This system also provided high reproducibility and with no apparent interference effect for Aβ protein analysis in human urine samples. Furthermore, our developed dPAD constituted an accurate and effective device to precisely determine an Aβ protein concentration in real samples, with percentage recoveries in the range of 97-103%, and with the highest relative standard deviation of 5.41%. Subsequently, the validation of our assay was assessed by comparison with a commercial ELISA approach, with favorable results. Finally, the proposed dPAD was successfully applied to the determination of an Aβ protein in human urine samples and showed more benefits for the unskilled user, such as cost-efficiency, simplicity, low reagent usage, and low time consumption. It is also suitable for point-of-care monitoring.

Hydrogel-Involved Colorimetric Platforms Based on Layered Double Oxide Nanozymes for Point-of-Care Detection of Liver-Related Biomarkers

Monitoring the liver status in a convenient and low-cost way is significant for obtaining a warning about drug-indued liver diseases promptly. Herein, we designed a novel colorimetric point-of-care (POC) platform for the determination of three liver-related biomarkers─aspartate transaminase (AST), alanine transaminase (ALT), and alkaline phosphatase (ALP). This platform integrated agarose hydrogels into a portable device, where hydrogels were loaded with nanozymes and different reaction substances for triggering specific reactions and generating colorimetric signals. Typically, Au-decorated CoAl-layered double oxide (Au/LDO) was for the first time developed as the nanozyme with peroxidase (POD) mimic activity, which can accelerate the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxTMB with the coexistence of hydrogen peroxide (H₂O₂). The detection mechanism of AST and ALT is based on the fact that they can cause individual cascade reactions to generate H₂O₂, and H₂O₂ further activates the Au/LDO nanozyme to catalyze the chromogenic reaction of TMB. As for ALP, it can catalytically hydrolyze l-ascorbic acid-2-phosphate to ascorbic acid. The latter then discolored the oxTMB that was produced with the assistance of Au/LDO. Teaming up with a smartphone, the color information of hydrogels can be converted to hue values, which allow quantitative analysis of ALT, AST, and ALP with detection limits of 15, 10, and 5 U/L, respectively. Moreover, the simple and cost-effective platform was successfully applied for the simultaneous determination of the three analytes in human plasma. Additionally, since the hydrogel is disposable and can be replaced by new ones loaded with different reaction regents, the platform is expected to serve the POC testing of various chem/bio targets.

An Insight into Skeletal Networks Analysis for Smart Hydrogels

Hydrogels are 3D cross‐linked polymer networks. Benefiting from the flexible designs and reasonable constructions of these networks, a large number of smart hydrogels with response characteristics to specific stimuli have received widespread attention and developed rapidly. The skeletal networks composed of the skeletal polymer chains and effectual cross‐links are the soul of such soft materials, and the response behaviors fundamentally depend on the dynamic characteristics of skeletal networks. Herein, the novel concepts of skeletal networks analysis to describe, understand, and guide the advanced designs and applications of smart hydrogels are proposed. Representative glucose‐sensitive hydrogels and DNA‐based smart hydrogels are reviewed to demonstrate the principle of skeletal networks analysis and clarify its practical guidance. Summarizing and classifying the characterizations and conversions of skeletal networks dynamics based on different response mechanisms provides a realistic solution. On this basis, advanced applications of smart hydrogels guided by skeletal networks dynamics including biochemical detection, cell mechanics sensing, drug delivery systems, and dynamic complex soft materials are typically reviewed. The skeletal networks analysis for smart hydrogels is of great significance for understanding the microstructures of hydrogels and guiding the designs of soft materials and their smart applications in the fields of analytical science and advanced materials.

Shaping Macromolecules for Sensing Applications—From Polymer Hydrogels to Foldamers

Sensors are tools for detecting, recognizing, and recording signals from the surrounding environment. They provide measurable information on chemical or physical changes, and thus are widely used in diagnosis, environment monitoring, food quality checks, or process control. Polymers are versatile materials that find a broad range of applications in sensory devices for the biomedical sector and beyond. Sensory materials are expected to exhibit a measurable change of properties in the presence of an analyte or a stimulus, characterized by high sensitivity and selectivity of the signal. Signal parameters can be tuned by material features connected with the restriction of macromolecule shape by crosslinking or folding. Gels are crosslinked, three-dimensional networks that can form cavities of different sizes and forms, which can be adapted to trap particular analytes. A higher level of structural control can be achieved by foldamers, which are macromolecules that can attain well-defined conformation in solution. By increasing control over the three-dimensional structure, we can improve the selectivity of polymer materials, which is one of the crucial requirements for sensors. Here, we discuss various examples of polymer gels and foldamer-based sensor systems. We have classified and described applied polymer materials and used sensing techniques. Finally, we deliberated the necessity and potential of further exploration of the field towards the increased selectivity of sensory devices.

Stimuli-responsive DNA-based hydrogels for biosensing applications

The base sequences of DNA are endowed with the rich structural and functional information and are available for the precise construction of the 2D and 3D macro products. The hydrogels formed by DNA are biocompatible, stable, tunable and biologically versatile, thus, these have a wide range of promising applications in bioanalysis and biomedicine. In particular, the stimuli-responsive DNA hydrogels (smart DNA hydrogels), which exhibit a reversible and switchable hydrogel to sol transition under different triggers, have emerged as smart materials for sensing. Thus far, the combination of the stimuli-responsive DNA hydrogels and multiple sensing platforms is considered as biocompatible and is useful as the flexible recognition components. A review of the stimuli-responsive DNA hydrogels and their biosensing applications has been presented in this study. The synthesis methods to prepare the DNA hydrogels have been introduced. Subsequently, the current status of the stimuli-responsive DNA hydrogels in biosensing has been described. The analytical mechanisms are further elaborated by the combination of the stimuli-responsive DNA hydrogels with the optical, electrochemical, point-of-care testing (POCT) and other detection platforms. In addition, the prospects of the application of the stimuli-responsive DNA hydrogels in biosensing are presented.

Colorimetric and Electrochemical Screening for Early Detection of Diabetes Mellitus and Diabetic Retinopathy-Application of Sensor Arrays and Machine Learning

In this review, a selection of works on the sensing of biomarkers related to diabetes mellitus (DM) and diabetic retinopathy (DR) are presented, with the scope of helping and encouraging researchers to design sensor-array machine-learning (ML)-supported devices for robust, fast, and cost-effective early detection of these devastating diseases. First, we highlight the social relevance of developing systematic screening programs for such diseases and how sensor-arrays and ML approaches could ease their early diagnosis. Then, we present diverse works related to the colorimetric and electrochemical sensing of biomarkers related to DM and DR with non-invasive sampling (e.g., urine, saliva, breath, tears, and sweat samples), with a special mention to some already-existing sensor arrays and ML approaches. We finally highlight the great potential of the latter approaches for the fast and reliable early diagnosis of DM and DR.

Recent Developments in Microfluidic Paper-based Analytical Devices for Pharmaceutical Analysis

In the last decade, due to the global increase in diseases, drugs for biomedical applications have increased dramatically. Therefore, there is an urgent need for analytical tools to monitor, treat, investigate, and control drug compounds in diverse matrices. The new and challenging task has been looking for simple, low-cost, rapid, and portable analytical platforms. The development of microfluidic paper-based analytical devices (μPADs) has garnered immense attention in many analytical applications due to the benefit of cellulose structure. It can be functionalized and serves as an ideal channel and scaffold for the transportation and immobilization of various substances. Microfluidic technology has been considered an effective tool in pharmaceutical analysis that facilitates the quantitative measurement of several parameters on cells or other biological systems. The μPADs represent unique advantages over conventional microfluidics, such as the self-pumping capability. They have low material costs, are easy to fabricate, and do not require external power sources. This review gives an overview of the current designs in this decade for μPADs and their respective application in pharmaceutical analysis. These include device designs, choice of paper material, and fabrication techniques with their advantages and drawbacks. In addition, the strategies for improving analytical performance in terms of simplicity, high sensitivity, and selectivity are highlighted, followed by the application of μPADs design for the detection of drug compounds for various purposes. Moreover, we present recent advances involving μPAD technologies in the field of pharmaceutical applications. Finally, we discussed the challenges and potential of μPADs for the transition from laboratory to commercialization.

Microfluidic Ruler-Readout and CRISPR Cas12a-Responded Hydrogel-Integrated Paper-Based Analytical Devices (μReaCH-PAD) for Visible Quantitative Point-of-Care Testing of Invasive Fungi

Invasive fungi (IF) have become a significant problem affecting human health. However, the culture-based assay of IF, known as the most commonly used clinical diagnostic method, suffers from time consumption, complicated operation, and the requirement of trained operators, which may cause the delay diagnosis of the disease. In this report, a microfluidic ruler-readout and CRISPR Cas12a-responded hydrogel-integrated paper-based analytical device (μReaCH-PAD) was established for visible and quantitative point-of-care testing of IF. Using the genus-conserved fragments of 18s rRNA as the detection target, this platform relied on a CRISPR Cas12a system for target recognition, a DNA hydrogel coupled with a cascade of enzymatic reactions for signal amplification and transduction, and paper-based microfluidic chips for visual quantitative readout by naked eyes. The 18s rRNA fragments of Candida or Aspergillus were employed as a model target and introduced with PAM sites for Cas12a-recognition during reverse transcription recombinase-aided amplification. Using μReaCH-PAD, as low as 10 CFU/mL Candida and Aspergillus were visually identified by unaided eyes. The calculated detection limits were 4.90 and 4.13 CFU/mL (in 1 mL samples), respectively. The quantitative detection results can be obtained in the range from 10 to 10⁴ CFU/mL with reasonable specificity and accuracy compared with qRT-PCR. Furthermore, μReaCH-PAD can analyze complex biological samples by Candida, Aspergillus, and Cryptococcus detection systems and identify specific genera of different IF by naked eyes, indicating a good agreement with the culture-based assay and the advantages over G-testing and GM-testing systems. With the benefits of high sensitivity, selectivity, quantitative readout, low cost, and ease of operation, μReaCH-PAD is expected to provide a portable detection tool of IF in resource-limited settings by untrained personnel and technical support for early diagnosis.

Aptamer functionalized DNA hydrogels: Design, applications and kinetics

DNA hydrogels have received considerable attention in various promising applications due to their excellent biocompatibility, controlled biodegradability, adjustable mechanical properties, stability against proteases, self-healing ability, and stimuli responsiveness. To obtain the specific molecular recognition capability, aptamers and many other functional motifs are utilized. Aptamers are short single-stranded DNA or RNA selected through SELEX to bind with specific target with high affinity and specificity. With advantages of broad range of targets, good stability, easy modification, and low cost, aptamer functionalized DNA hydrogels become popular in a wide range of promising applications. In this review, the recent progress on aptamer functionalized DNA hydrogels including general design principles, applications and kinetics has been summarized. Finally, the current challenges and prospects are discussed.

Low-Cost and Convenient Microchannel Resistance Biosensing Platform by Directly Translating Biorecognition into a Current Signal

We report a low-cost and convenient microchannel resistance (MCR) biosensing platform that uses current signal to report biorecognition. The biorecognition behavior between targets and biometric molecules (antigens, antibodies, or oligonucleotides) immobilized on magnetic beads and polystyrene (PS) microspheres induces a quantitative change in the unreacted PS microspheres. After magnetic separation, the unreacted PS microsphere solution is passed through the microchannel, leading to an obvious blocking effect, resulting in an increase in resistance, which can in turn be measured by monitoring the electric current. Thus, the biorecognition is directly converted into a detectable current signal without any bulky instruments or additional chemical reactions. The MCR biosensing platform is cost-effective and user-friendly with high accuracy. It can be an appropriate analysis technique for point-of-care testing in resource-poor settings.

Detection of hydrogen peroxide and glucose with a novel fluorescent probe by the enzymatic reaction of amino functionalized MOF nanosheets

Amino-functionalized two-dimensional (2D) MOFs have great potential in biosensors due to their excellent water solubility, high fluorescence, large specific surface area, good adsorption properties and good ability to enrich the target analytes. Fluorescence detection of hydrogen peroxide and glucose mostly relies on monitoring the single fluorescence intensity changes in a single excitation wavelength. Here, a ratiometric fluorescence sensor based on NH₂-MIL-53(Al) nanosheets to sensitively detect H₂O₂ and glucose through enzymatic reactions was developed. o-Phenylenediamine (OPD) was oxidized by H₂O₂ in the presence of horseradish peroxidase (HRP). Then, the oxidation product could be self-assembled on NH₂-MIL-53(Al) nanosheets by hydrogen bonding and π-π stacking. The orbital interaction or the fluorescence resonance energy transfer (FRET) between the nanosheets and the oxidation product could effectively quench the fluorescence of the nanosheets at 433 nm. At the same time, the oxidation product provided a new emission peak at 564 nm. The fluorescence ratio signal changes generated by this oxidation process were used to stably and sensitively detect H₂O₂ and glucose. Structural and mechanistic analysis was carried out by calculation methods such as AICD and ORCA to explore the π electron structure characteristics, the hole/electron orbitals and the quenching phenomenon. The detection limit was 26.9 nM for H₂O₂ and 0.041 μM for glucose. The detection of glucose in human serum has a satisfactory recovery of 97.4-102.8%. It is clear that the sensor has a good application prospect in real sample analysis.

Hydrogel-assisted paper-based lateral flow sensor for the detection of trypsin in human serum

The detection of trypsin and its inhibitor is significantly important for both clinical diagnosis and disease treatment. Herein, we demonstrate a hydrogel-assisted paper-based lateral flow sensor for the detection of trypsin and its inhibitor for the first time. The gelatin hydrogel is hydrolyzed based on the gel-to-sol transition in the presence of trypsin, which results in the release of the trapped water molecules in the gelatin hydrogel. By placing one end of a pH indicator strip onto the hydrolyzed gelatin hydrogel, water is flowing along the pH indicator strip. However, in the absence of trypsin, water cannot flow along the pH indicator strip as the water molecules are trapped in the gelatin hydrogel. The detection limit of the system reaches as low as 1.0 × 10^-6 mg/mL, and it is also applied to the quantitative detection of trypsin in human serum. In addition, the detection of a clinical drug aprotinin that is an inhibitor of trypsin is also successfully achieved. Noteworthy, only the gelatin hydrogel, pH indicator strip, and PS substrate are needed to fulfill the detection of trypsin without the need of other chemicals or reagents. Overall, we develop a particularly simple, elegant, robust, competitive, high-throughput, and low-cost approach for the rapid and label-free detection of trypsin and its inhibitor, which is very promising in the development of commercial products for sensing, diagnostic, and pharmaceutical applications. Besides, the hydrogel-assisted paper-based lateral flow sensor can also be employed to detect other analytes of interest by use of different stimuli-responsive hydrogel systems.

Lab-on-Paper Devices for Diagnosis of Human Diseases Using Urine Samples-A Review

In recent years, microfluidic lab-on-paper devices have emerged as a rapid and low-cost alternative to traditional laboratory tests. Additionally, they were widely considered as a promising solution for point-of-care testing (POCT) at home or regions that lack medical infrastructure and resources. This review describes important advances in microfluidic lab-on-paper diagnostics for human health monitoring and disease diagnosis over the past five years. The review commenced by explaining the choice of paper, fabrication methods, and detection techniques to realize microfluidic lab-on-paper devices. Then, the sample pretreatment procedure used to improve the detection performance of lab-on-paper devices was introduced. Furthermore, an in-depth review of lab-on-paper devices for disease measurement based on an analysis of urine samples was presented. The review concludes with the potential challenges that the future development of commercial microfluidic lab-on-paper platforms for human disease detection would face.

Bi2S3@MoS2 Nanoflowers on Cellulose Fibers Combined with Octahedral CeO2 for Dual-Mode Microfluidic Paper-Based MiRNA-141 Sensors

An effective dual-mode microfluidic paper-based analysis device (μPAD) was proposed via Bi₂S₃@MoS₂ nanoflowers combined with octahedral CeO₂ for ultrasensitive miRNA-141 bioassay. To obtain the amplified electrochemical signal, Bi₂S₃@MoS₂ nanoflowers were first in situ grown onto the surface of cellulose fibers to promote the reduction of H₂O₂. The prism-anchored octahedral CeO₂ nanoparticles with a great catalytic function on the reduction of H₂O₂ were linked up to the functionalized cellulose fibers through the hybridization chain reaction to further enhance the electrochemical signal. By means of the catalysis effect of Bi₂S₃@MoS₂ nanoflowers and octahedral CeO₂ nanoparticles, the obtained signal was amplified, thereby achieving ultrasensitive electrochemical detection of the target. With the help of duplex specific nuclease, the octahedral CeO₂ could be released from the electrochemical detection area and flow to the color channel through capillary action, which could initiate the oxidation reaction of 3,3',5,5'-tetramethylbenzidine in the existence of H₂O₂ to generate a blue visual band, avoiding the error of distinguishing color depth caused by the naked eye and thus improving the accuracy of the visual method. Under the optimal conditions, satisfactory prediction and accurate detection performance were achieved in the range of 10 fM-1 nM and 0.5 fM-1 nM, respectively, by measuring the length of the blue product and the electrochemical signal intensity. The electrochemical/visual detection limits of the proposed μPAD for miRNA-141 were as low as 0.12 and 2.65 fM (S/N = 3). This work provides great potential for the construction of low-cost and high-performance dual-mode biosensors for the detection of biomarkers.