A Customized Microfluidic Paper-Based Platform for Colorimetric Immunosensing: Demonstrated via hCG Assay for Pregnancy Test

Multi-dimensional microfluidic paper-based analytical devices (μPADs) for noninvasive testing: A review of structural design and applications

The rapid emergence of microfluidic paper-based devices as point-of-care testing (POCT) tools for early disease diagnosis and health monitoring, particularly in resource-limited areas, holds immense potential for enhancing healthcare accessibility. Leveraging the numerous advantages of paper, such as capillary-driven flow, porous structure, hydrophilic functional groups, biodegradability, cost-effectiveness, and flexibility, it has become a pivotal choice for microfluidic substrates. The repertoire of microfluidic paper-based devices includes one-dimensional lateral flow assays (1D LFAs), two-dimensional microfluidic paper-based analytical devices (2D μPADs), and three-dimensional (3D) μPADs. In this comprehensive review, we provide and examine crucial information related to paper substrates, design strategies, and detection methods in multi-dimensional microfluidic paper-based devices. We also investigate potential applications of microfluidic paper-based devices for detecting viruses, metabolites and hormones in non-invasive samples such as human saliva, sweat and urine. Additionally, we delve into capillary-driven flow alternative theoretical models of fluids within the paper to provide guidance. Finally, we critically examine the potential for future developments and address challenges for multi-dimensional microfluidic paper-based devices in advancing noninvasive early diagnosis and health monitoring. This article showcases their transformative impact on healthcare, paving the way for enhanced medical services worldwide.

EGaIn-Modified ePADs for Simultaneous Detection of Homocysteine and C-Reactive Protein in Saliva toward Early Diagnosis of Cardiovascular Disease

Homocysteine (Hcy) and C-reactive protein (CRP) are critical biomarkers for numerous chronic diseases, with cardiovascular disease (CVD) being the most prevalent. The ability to simultaneously detect both biomarkers in point-of-care settings is in high demand for CVD early diagnosis and prevention. Herein, we prepared the eutectic gallium indium (EGaIn) nanoparticles decorated with p-phenylenediamine (PPD) on the surface to facilitate the subsequent attachment of gold nanoparticles (AuNPs) to achieve EGaIn-PPD@Au, which was modified on the screen-printed electrochemical paper-based analytical devices (ePADs). Aptamers that are specific to Hcy and CRP were then immobilized on the EGaIn-PPD@Au surface to achieve the sensing interface on ePADs. The presence of EGaIn-PPD@Au significantly enhanced the electrical conductivity, leading to amplified electrochemical signals. This aptasensor demonstrated high specificity, capable of detecting Hcy in a range of 1-50 μM with a detection limit of 0.22 μM, and the detection range for CRP was 1-100 ng/mL with a detection limit of 0.039 ng/mL. The aptasensor also effectively detected Hcy and CRP in clinical saliva samples, yielding an area under the curve (AUC) of about 0.80 when the individual biomarker was considered and 0.93 when both biomarkers were taken into account. The positive correlation observed between salivary and blood concentrations of Hcy and CRP, coupled with their association with cardiovascular disease (CVD), suggested the potential of this methodology as a noninvasive point-of-care strategy for the early diagnosis of CVD.

Point-of-care diagnostic devices for periodontitis - current trends and urgent need

Point of care (POC) diagnostic devices provide a method for rapid accurate identification of disease through analysis of biologically relevant substances. This review focuses on the utility of POC testing for early detection of periodontitis, a critical factor in treating the disease. Accessing the oral cavity for biological sampling is less invasive when compared to other internal test sites, and oral fluids contain biomarkers indicative of periodontitis. The ease of access makes the mouth an excellent target location for the development of POC devices. In this review, accepted standards in industry by which these devices must adhere, provided by the World Health Organization such as REASSURED and CLIA, are discussed. An overview is provided for many periodontal biomarkers currently being investigated as a means of predicting periodontal disease and its progression. POC devices currently being investigated for the identification and monitoring of periodontal disease such as paper-based and lab-on-a-chip based devices are outlined. Limitations of current POC devices on the market are provided and future directions in leveraging biomarkers as an adjunctive method for oral diagnosis along with AI-based analysis systems are discussed. Here, we present the ESSENCE sensor platform, which combines a porous non-planar electrode with enhanced shear flow to achieve unprecedented sensitivity and selectivity. The combination of the ESENCE chip with an automated platform allows us to meet the WHO's ASSURED criteria. This platform promises to be an exciting POC candidate for early detection of periodontitis.

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.

Office paper and laser printing: a versatile and affordable approach for fabricating paper-based analytical devices with multimodal detection capabilities

Multiple protocols have been reported to fabricate paper-based analytical devices (PADs). However, some of these techniques must be revised because of the instrumentation required. This paper describes a versatile and globally affordable method to fabricate PADs using office paper as a substrate and a laser printing technique to define hydrophobic barriers on paper surfaces. To demonstrate the feasibility of the alternatives proposed in this study, the fabrication of devices for three types of detection commonly associated with using PADs was demonstrated: colorimetric detection, electrochemical detection, and mass spectrometry associated with a paper-spray ionization (PSI-MS) technique. Besides that, an evaluation of the type of paper used and chemical modifications required on the substrate surface are also presented in this report. Overall, the developed protocol was suitable for using office paper as a substrate, and the laser printing technique as an efficient fabrication method when using this substrate is accessible at a resource-limited point-of-need. Target analytes were used as a proof of concept for these detection techniques. Colorimetric detection was carried out for acetaminophen, iron, nitrate, and nitrite with limits of detection of 0.04 μg, 4.5 mg mL-1, 2.7 μmol L-1, and 6.8 μmol L-1, respectively. A limit of detection of 0.048 fg mL-1 was obtained for the electrochemical analysis of prostate-specific antigen. Colorimetric and electrochemical devices revealed satisfactory performance when office paper with a grammage of 90 g m-2 was employed. Methyldopa analysis was also carried out using PSI-MS, which showed a good response in the same paper weight and behavior compared to chromatographic paper.

Controlling Fluorescent Readout in Paper-based Analytical Devices

Paper is an ideal candidate for the development of new disposable diagnostic devices because it is a low-cost material, allows transport of the liquid on the device by capillary action, and is environmentally friendly. Today, colorimetric analysis is most often used as a detection method for rapid tests (test strips or lateral flow devices) but usually gives only qualitative results and is limited by a relatively high detection threshold. Here, we describe studies using fluorescence as a readout tool for paper-based diagnostics. We study how the optical readout is affected by light transmission, scattering, and fluorescence as a function of paper characteristics such as thickness (grammage), water content, autofluorescence, and paper type/composition. We show that paper-based fluorescence analysis allows better optical readout compared to that of nitrocellulose, which is currently the material of choice in colorimetric assays. To reduce the loss of analyte molecules (e.g., proteins) due to adsorption to the paper surface, we coat the paper fibers with a protein-repellent hydrogel. For this purpose, we use hydrophilic copolymers consisting of N,N-dimethyl acrylamide and a benzophenone-based cross-linker, which are photochemically transformed into a fiber-attached polymer hydrogel on the paper fiber surfaces in situ. We show that the combination of fluorescence detection and the use of a protein-repellent coating enables sensitive paper-based analysis. Finally, the success of the strategy is demonstrated by using a simple LFD application as an example.

Review of paper-based microfluidic analytical devices for in-field testing of pathogens

Pathogens cause various infectious diseases and high morbidity and mortality which is a global public health threat. The highly sensitive and specific detection is of significant importance for the effective treatment and intervention to minimise the impact. However, conventional detection methods including culture and molecular method gravely depend on expensive equipment and well-trained skilled personnel, limiting in the laboratory. It remains challenging to adapt in resource-limiting areas, e.g., low and middle-income countries (LMICs). To this end, low-cost, rapid, and sensitive detection tools with the capability of field testing e.g., a portable device for identification and quantification of pathogens, has attracted increasing attentions. Recently, paper-based microfluidic analytical devices (μPADs) have shown a promising tool for rapid and on-site diagnosis, providing a cost-effective and sensitive analytical approach for pathogens detection. The fast turn-round data collection may also contribute to better understanding of the risks and insights on mitigation method. In this paper, critical developments of μPADs for in-field detection of pathogens both for clinical diagnostics and environmental surveillance are reviewed. The future development, and challenges of μPADs for rapid and onsite detection of pathogens are discussed, including using the cross-disciplinary development with, emerging techniques such as deep learning and Internet of Things (IoT).

μPADs on Centrifugal Microfluidic Discs for Rapid Sample-to-Answer Salivary Diagnostics

A fully integrated device for salivary detection with a sample-in-answer-out fashion is critical for noninvasive point-of-care testing (POCT), especially for the screening of contagious disease infection. Microfluidic paper-based analytical devices (μPADs) have demonstrated their huge potential in POCT due to their low cost and easy adaptation with other components. This study developed a generic POCT platform by integrating a centrifugal microfluidic disc with μPADs to realize sample-to-answer salivary diagnostics. Specifically, a custom centrifugal microfluidic disc integrated with μPADs is fabricated, which demonstrated a high efficiency in saliva treatment. To demonstrate the capability of the integrated device for salivary analysis, the SARS-CoV-2 Nucleocapsid (N) protein, a reliable biomarker for SARS-CoV-2 acute infection, is used as the model analyte. By the chemical treatment of the μPAD surface, and by optimizing the protein immobilization conditions, the on-disc μPADs were able to detect the SARS-CoV-2 N protein down to 10 pg mL-1 with a dynamic range of 10-1000 pg mL-1 and an assay time of 8 min. The integrated device was successfully used for the quantification of the N protein of pseudovirus in saliva with high specificity and demonstrated a comparable performance to the commercial paper lateral flow assay test strips.

Paper-based colorimetric sensors for point-of-care testing

By eliminating the need for sample transportation and centralized laboratory analysis, point-of-care testing (POCT) enables on-the-spot testing, with results available within minutes, leading to improved patient management and overall healthcare efficiency. Motivated by the rapid development of POCT, paper-based colorimetric sensing, a powerful analytical technique that exploits the changes in color or absorbance of a chemical species to detect and quantify analytes of interest, has garnered increasing attention. In this review, we strive to provide a bird's eye view of the development landscape of paper-based colorimetric sensors that harness the unique properties of paper to create low-cost, easy-to-use, and disposable analytical devices, thematically covering both fundamental aspects and categorized applications. In the end, we authors summarized the review with the remaining challenges and emerging opportunities. Hopefully, this review will ignite new research endeavors in the realm of paper-based colorimetric sensors.

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.

A convenient paper-based fluorescent aptasensor for high-throughput detection of Pb2+ in multiple real samples (water-soil-food)

Lead ion (Pb²⁺) is one of the most toxic and widely polluted heavy metal ions. Given the potential health risks and economic losses associated with Pb²⁺, the rapid detection of Pb²⁺ using fluorescent aptasensors is of significant importance in evaluating food safety. A rapid, facile and economic fluorescent aptasensor using convenient paper as the sensing substrate was designed to high-throughput detect Pb²⁺ in complex samples within about 45 min. The Pb²⁺ changed the conformation of FAM-modified Apt from a random coil to a stable G-quadruplex structure. And then Dabcyl-labeled cDNA was added to form double-stranded DNA with the Apt that did not form a G-quadruplex structure, resulting in a weak fluorescence due to the fluorescence resonance energy transfer (FRET). The fluorescent aptasensor showed a positive correlation with Pb²⁺ concentration, and a linear relationship was obtained in the range of 0.01-10 μM with LOD of 6.1 nM. In addition, this method has been successfully used for the determination of Pb²⁺ in water, soil and various foods containing complex substrates. Meanwhile, the high-throughput detection of Pb²⁺ has also reached an acceptable level. Therefore, this convenient strategy has potential application value for on-site rapid detection of Pb²⁺.

PEC/Colorimetric Dual-Mode Lab-on-Paper Device via BiVO4/FeOOH Nanocomposite In Situ Modification on Paper Fibers for Sensitive CEA Detection

A dual-mode lab-on-paper device based on BiVO4/FeOOH nanocomposites as an efficient generating photoelectrochemical (PEC)/colorimetric signal reporter has been successfully constructed by integration of the lab-on-paper sensing platform and PEC/colorimetric detection technologies for sensitive detection of carcinoembryonic antigen (CEA). Concretely, the BiVO4/FeOOH nanocomposites were in situ synthesized onto the paper-working electrode (PWE) through hydrothermal synthesis of the BiVO4 layer on cellulose fibers (paper-based BiVO4) which were initially modified by Au nanoparticles for improving the conductivity of three dimensional PWE, and then the photo-electrodeposition of FeOOH onto the paper-based BiVO4 to construct the paper-based BiVO4/FeOOH for the portable dual-mode lab-on-paper device. The obtained nanocomposites with an FeOOH needle-like structure deposited on the BiVO4 layer exhibits enhanced PEC response activity due to its effective separation of the electron-hole pair which could further accelerate the PEC conversion efficiency during the sensing process. With the introduction of CEA targets onto the surface of nanocomposite-modified PWE assisted by the interaction with the CEA antibody from a specific recognition property, a signal-off PEC signal state with a remarkable photocurrent response decreasing trend can be achieved, realizing the quantitative detection of CEA with the PEC signal readout mode. By means of a smart origami paper folding, the colorimetric signal readout is achieved by catalyzing 3,3',5,5'-tetramethylbenzidine (TMB) to generate blue oxidized TMB in the presence of H2O2 due to the satisfied enzyme-like catalytic activity of the needle-like structure, FeOOH, thereby achieving the dual-mode signal readout system for the proposed lab-on-paper device. Under the optimal conditions, the PEC and colorimetric signals measurement were effectively carried out, and the corresponding linear ranges were 0.001-200 ng·mL-1 and 0.5-100 ng·mL-1 separately, with the limit of detection of 0.0008 and 0.013 ng·mL-1 for each dual-mode. The prepared lab-on-paper device also presented a successful application in serum samples for the detection of CEA, providing a potential pathway for the sensitive detection of target biomarkers in clinical application.

Microfluidic Paper-Based Device for Medicinal Diagnosis

BACKGROUND

The demand for point-of-care testing (POCT) devices has rapidly grown since they offer immediate test results with ease of use, makingthem suitable for home self-testing patients and caretakers. However, the POCT development has faced the challenges of increased cost and limited resources. Therefore, the paper substrate as a low-cost material has been employed to develop a cost-effective POCT device, known as "Microfluidic paper-based analytical devices (μPADs)". This device is gaining attention as a promising tool for medicinal diagnostic applications owing to its unique features of simple fabrication, low cost, enabling manipulation flow (capillarydriven flow), the ability to store reagents, and accommodating multistep assay requirements.

OBJECTIVE

This review comprehensively examines the fabrication methods and device designs (2D/3D configuration) and their advantages and disadvantages, focusing on updated μPADs applications for motif identification.

METHODS

The evolution of paper-based devices, starting from the traditional devices of dipstick and lateral flow assay (LFA) with μPADs, has been described. Patterned structure fabrication of each technique has been compared among the equipment used, benefits, and drawbacks. Microfluidic device designs, including 2D and 3D configurations, have been introduced as well as their modifications. Various designs of μPADs have been integrated with many powerful detection methods such as colorimetry, electrochemistry, fluorescence, chemiluminescence, electrochemiluminescence, and SER-based sensors for medicinal diagnosis applications.

CONCLUSION

The μPADs potential to deal with commercialization in terms of the state-of-the-art of μPADs in medicinal diagnosis has been discussed. A great prototype, which is currently in a reallife application breakthrough, has been updated.

A Paper-Based Photoelectrochemical Sensing Platform Based on In Situ Grown ZnO/ZnIn2S4 Heterojunctions onto Paper Fibers for Sensitively Detecting AFP

Nowadays, developing a cost-effective, easy-to-operate, and efficient signal amplification platform is of important to microfluidic paper-based analytical devices (μPAD) for end-use markets of point-of-care (POC) assay applications. Herein, an ultrasensitive, paper-based photoelectrochemical (PEC) bioassay platform is constructed by in situ grown ZnO/ZnIn2S4 heterojunctions onto paper fibers, which acted as photoactive signal amplification probes for enhancing the sensitivity of antibodies-based diagnostic assays, for the sensitive detection of alpha-fetoprotein (AFP) targets. The crystalline flake-like ZnIn2S4 composited with hexagonal nanorods (NRs) morphology of ZnO is an in situ grown, at the first time, onto cellulose fibers surface supported with Au nanoparticle (Au NP) modification to improve conductivity of the device working zone. The obtained composites on paper fibers are implemented as a flexible paper-based photoelectrode to realize remarkable performance of the fabricated μPAD, resulting from the enhanced PEC activity of heterojunctions with effective electron-hole pair separation for accelerating photoelectric conversion efficiency of the sensing process under light irradiation. Once the target AFP was introduced into the biosensing interface assistant, with a specific recognition interaction of AFP antibody, a drastically photocurrent response was generated, in view of the apparent steric effects. With the concentration increase of AFP targets, more immune conjugates could be confined onto the biosensing interface, eventually leading to the quantitative decrease of photocurrent intensity. Combined with an ingenious origami design and permitting the hydrophobic/hydrophilic conversion procedure in the bioassay process, the ultrasensitive PEC detection of AFP targets was realized. Under the optimized conditions, the level of AFP could be sensitively tracked by the prepared μPAD with a liner range from 0.1 to 100 ng mL-1 and limit of detection of 0.03 ng mL-1. This work provides a great potential application for highly selective and sensitive POC testing of AFP, and finally, developments for clinical disease diagnosis.

Digital Quantification Method for Sensitive Point-of-Care Detection of Salivary Uric Acid Using Smartphone-Assisted μPADs

Uric acid (UA) is an important biomarker for many diseases. A sensitive point-of-care (POC) testing platform is designed for the digital quantification of salivary UA based on a colorimetric reaction on an easy-to-build smartphone-assisted microfluidic paper-based analytical device (SμPAD). UA levels are quantified according to the color intensity of Prussian blue on the SμPAD with the aid of a MATLAB code or a smartphone APP. A color correction method is specifically applied to exclude the light effect. Together with the engineering design of SμPADs, the background calibration function with the APP increases the UA sensitivity by 100-fold to reach 0.1 ppm with a linear range of 0.1-200 ppm. The assay time is less than 10 min. SμPADs demonstrate a correlation of 0.97 with a commercial UA kit for the detection of salivary UA in clinical samples. SμPADs provide a sensitive, fast, affordable, and reliable tool for the noninvasive POC quantification of salivary UA for early diagnosis of abnormal UA level-associated health conditions.

Microfluidic Sliding Paper-Based Device for Point-of-Care Determination of Albumin-to-Creatine Ratio in Human Urine

A novel assay platform consisting of a microfluidic sliding double-track paper-based chip and a hand-held Raspberry Pi detection system is proposed for determining the albumin-to-creatine ratio (ACR) in human urine. It is a clinically important parameter and can be used for the early detection of related diseases, such as renal insufficiency. In the proposed method, the sliding layer of the microchip is applied and the sample diffuses through two parallel filtration channels to the reaction/detection areas of the microchip to complete the detection reaction, which is a simple method well suited for self-diagnosis of ACR index in human urine. The RGB (red, green, and blue) value intensity signals of the reaction complexes in these two reaction zones are analyzed by a Raspberry Pi computer to derive the ACR value (ALB and CRE concentrations). It is shown that the G + B value intensity signal is linearly related to the ALB and CRE concentrations with the correlation coefficients of R2 = 0.9919 and R2 = 0.9923, respectively. It is additionally shown that the ALB and CRE concentration results determined using the proposed method for 23 urine samples were collected from real suffering chronic kidney disease (CKD) patients are in fine agreement with those acquired operating a traditional high-reliability macroscale method. Overall, for point-of-care (POC) CKD diagnosis and monitoring in clinical applications, the results prove that the proposed method offers a convenient, real time, reliable, and low-spending solution for POC CKD diagnosis.

Diagnoses Based on C-Reactive Protein Point-of-Care Tests

C-reactive protein (CRP) is an important part of the immune system's reaction to various pathological impulses such as bacterial infections, systemic inflammation, and internal organ failures. An increased CRP level serves to diagnose the mentioned pathological states. Both standard laboratory methods and simple point-of-care devices such as lateral flow tests and immunoturbidimetric assays serve for the instrumental diagnoses based on CRP. The current method for CRP has many flaws and limitations in its use. Biosensor and bioassay analytical devices are presently researched by many teams to provide more sensitive and better-suited tools for point-of-care tests of CRP in biological samples when compared to the standard methods. This review article is focused on mapping the diagnostical relevance of CRP, the applicability of the current analytical methods, and the recent innovations in the measurement of CRP level.

Rapid and facile detection of HBV with CRISPR/Cas13a

A rapid point-of-care assay based on CRISPR-Cas13a for HBV was built with a LOD of 1 copy per μL was achieved in 50 min. The lateral flow test strip method can achieve naked-eye results, and the fluorescence readout can achieve real-time detection.