Tree-shaped paper strip for semiquantitative colorimetric detection of protein with self-calibration

Dual-spot ratiometric microfluidic paper-based analytical device for accuracy and precision improvement

Instrumental and environmental fluctuations are common sources of error in smartphone-based optical detection, significantly affecting the accuracy of analytical measurements. In this regard, spotting the sample and reference simultaneously and in close proximity compensates for the fluctuations. This "dual-spot" design is similar to the double-beam technique used in spectrophotometry, which reduces fluctuations in the results. The underlying hypothesis is that any instrumental and/or environmental factors influencing the color intensity in the detection zones will similarly impact the color intensity in the control zone under the same conditions. To test our design, a ratiometric microfluidic paper-based analytical device (μPAD), functionalized with a mixture of green-emissive carbon dots (CDs) and red-emissive ethidium bromide, was developed for the selective detection of ascorbic acid (AA). The green emission of the CDs is quenched by both AA and Fe3+; NaF was thus loaded onto the 3D connector as a masking agent to remove the interference effect of the Fe3+ ions. The color variations were monitored under a UV lamp, using a smartphone to capture the images, and the RGB intensities were processed using the Color Grab application. The proposed double-spot method greatly enhanced the analytical precision and accuracy of the device. A linear working range from 0 to 125 μM was obtained, and the limit of detection was 2.71 μM. The μPAD was successfully used for the detection of AA in human serum, with recoveries from 87.27 to 98.52 %.

Paper-based multiplex biosensors for inexpensive healthcare diagnostics: a comprehensive review

Multiplex detection is a smart and an emerging approach in point-of-care testing as it reduces analysis time and testing cost by detecting multiple analytes or biomarkers simultaneously which are crucial for disease detection at an early stage. Application of inexpensive substrate such as paper has immense potential and matter of research interest in the area of point of care testing for multiplexed analysis as it possesses several unique advantages. This study presents the use of paper, strategies adopted to refine the design created on paper and lateral flow strips to enhance the signal, increase the sensitivity and specificity of multiplexed biosensors. An overview of different multiplexed detection studies performed using biological samples has also been reviewed along with the challenges and advantages offered by multiplexed analysis.

The air-gap PAD: a roll-to-roll-compatible fabrication method for paper microfluidics

Paper-based analytical devices (PADs) offer a low-cost, user-friendly platform for rapid point-of-use testing. Without scalable fabrication methods, however, few PADs make it out of the academic laboratory and into the hands of end users. Previously, wax printing was considered an ideal PAD fabrication method, but given that wax printers are no longer commercially available, alternatives are needed. Here, we present one such alternative: the air-gap PAD. Air-gap PADs consist of hydrophilic paper test zones, separated by "air gaps" and affixed to a hydrophobic backing with double-sided adhesive. The primary appeal of this design is its compatibility with roll-to-roll equipment for large-scale manufacturing. In this study, we examine design considerations for air-gap PADs, compare the performance of wax-printed and air-gap PADs, and report on a pilot-scale roll-to-roll production run of air-gap PADs in partnership with a commercial test-strip manufacturer. Air-gap devices performed comparably to their wax-printed counterparts in Washburn flow experiments, a paper-based titration, and a 12-lane pharmaceutical screening device. Using roll-to-roll manufacturing, we produced 2700 feet of air-gap PADs for as little as $0.03 per PAD.

Rapid and inexpensive process to fabricate paper based microfluidic devices using a cut and heat plastic lamination process

Microfluidic paper-based analytical devices (microPADs) are emerging as simple-to-use, low-cost point-of-care testing platforms. Such devices are mostly fabricated at present by creating hydrophobic barriers using wax or photoresist patterning on porous paper sheets. Even though devices fabricated using these methods are used and tested with a wide variety of analytes, still they pose many serious practical limitations for low-cost automated mass fabrication for their widespread applicability. We present an affordable and simple two-step process - cut and heat (CH-microPADs) - for the selective fabrication of hydrophilic channels and reservoirs on a wide variety of porous media such as tissue/printing/filter paper and cloth types, such as cotton and polyester, by a lamination process. The technique presents many advantages as compared to existing commonly used methods. The devices possess excellent mechanical strength against bending, folding and twisting, making them virtually unbreakable. They are structurally flexible and show good chemical resistance to various solvents, acids and bases, presenting widespread applicability in areas such as clinical diagnostics, biological sensing applications, food processing, and the chemical industry. Fabricated paper media 96 well-plate CH-microPAD configurations were tested for cell culture applications using mice embryonic fibroblasts and detection of proteins and enzymes using ELISA. With a simple two-step process and minimal human intervention, the technique presents a promising step towards mass fabrication of inexpensive disposable diagnostic devices for both resource-limited and developed regions.

Colorimetric Cotton Swab for Viral Protease Detection

Reporting the activity of a specific viral protease remains an acute need for rapid point-of-care detection strategies that can distinguish active infection from a resolved infection. In this work, we present a simple colorimetric approach for reporting the activity of a specific viral protease through direct color conversion on a cotton swab, which has the potential to be extended to detect the corresponding virus. We use SARS-CoV-2 viral protease as a proof-of-concept model system. We use 4-aminomalachite green (4-AMG) as the base chromophore structure to design a CoV2-AMG reporter, which is selective toward the SARS-CoV-2 M^pro but does not produce any observable color change in the presence of other viral proteases. The color change is observable by the naked eye, as well as smartphone imaging, which affords a lower limit of detection. The simplicity and generalizability of the method could be instrumental in combating future viral outbreaks.

Handmade Paper as a Paper Analytical Device for Determining the Quality of an Antidiabetic Drug

Paper analytical devices (PADs) are a class of low-cost, portable, and easy-to-use platform for several analytical tests in clinical diagnostics, environmental pollution monitoring, and food and drug safety screening. These devices are primarily made from cellulosic paper. Considering the importance of eco-friendly and local or distributed manufacturing of devices realized during the COVID-19 pandemic, we systematically studied the potential of handmade Nepali paper to be used in fabricating PADs in this work. We characterized five different handmade papers made from locally available plant fibers using an eco-friendly method and used them to fabricate PADs for determining the drug quality. The thickness, grammage, and apparent density of the paper samples ranged from 198.6 to 314.8 μm, 49.1 to 117.8 g/m2, and 0.23 to 0.43 g/cm3, respectively. The moisture content, water filtration, and wicking speed ranged from 5.8 to 7.1%, 35.7 to 156.7, and 0.062 to 0.124 mms-1, respectively. Furthermore, the water contact angle and porosity ranged from 76.6 to 112.1° and 79 to 83%, respectively. The best paper sample (P5) was chosen to fabricate PADs for the determination of metformin, an antidiabetic drug. The metformin assay on PADs followed a linear range from 0.0625 to 0.5 mg/mL. The assay had a limit of detection and limit of quantitation of 0.05 and 0.18 mg/mL, respectively. The average amount of metformin concentration in samples collected from local pharmacies (n = 20) was 465.6 ± 15.1 mg/tablet. When compared with the spectrophotometric method, PAD assay correctly predicted the concentration of 90% samples. The PAD assay on handmade paper may provide a low-cost and easy-to-use system for screening the quality of drugs and other point-of-need applications.

Paper based microfluidics: A forecast toward the most affordable and rapid point-of-care devices

The microfluidic industry has evolved through years with acquired scientific knowledge from different, and already developed industries. Consequently, a wide range of materials like silicon from the electronic industry to all the way, silicone, from the chemical engineering industry, has been spotted to solve similar challenges. Although a typical microfluidic chip, fabricated from glass or polymer substrates offers definite benefits, however, paper-based microfluidic analytical devices (μPADs) possess numerous special benefits for practical implementation at a lower price. Owing to these features, in recent years, paper microfluidics has drawn immense interest from researchers in industry and academia alike. These devices have wider applications with advantages like lower cost, speedy detection, user-easiness, biocompatibility, sensitivity, and specificity etc. when compared to other microfluidic devices. Therefore, these sensitive but affordable devices fit themselves into point-of-care (POC) testing with features in demand like natural disposability, situational flexibility, and the capability to store and analyze the target at the point of requirement. Gradually, advancements in fabrication technologies, assay development techniques, and improved packaging capabilities, have contributed significantly to the real-time identification and health investigation through paper microfluidics; however, the growth has not been limited to the biomedical field; industries like electronics, energy storage and many more have expanded substantially. Here, we represent an overall state of the paper-based microfluidic technology by covering the fundamentals, working principles, different fabrication procedures, applications for various needs and then to make things more practical, the real-life scenario and practical challenges involved in launching a device into the market have been revealed. To conclude, recent contribution of μPADs in the 2020 pandemic and potential future possibilities have been reviewed.

Lab-on-a-chip technologies for food safety, processing, and packaging applications: a review

The advent of microfluidic systems has led to significant developments in lab-on-a-chip devices integrating several functions onto a single platform. Over the years, these miniature devices have become a promising tool for faster analytical testing, displaying high precision and efficiency. Nonetheless, most microfluidic systems are not commercially available. Research is actually undergoing on the application of these devices in environmental, food, biomedical, and healthcare industries. The lab-on-a-chip industry is predicted to grow annually by 20%. Here, we review the use of lab-on-a-chip devices in the food sector. We present fabrication technologies and materials to developing lab-on-a-chip devices. We compare electrochemical, optical, colorimetric, chemiluminescence and biological methods for the detection of pathogens and microorganisms. We emphasize emulsion processing, food formulation, nutraceutical development due to their promising characteristics. Last, smart packaging technologies like radio frequency identification and indicators are highlighted because they allow better product identification and traceability.

Usability as a guiding principle for the design of paper-based, point-of-care devices - A review

Due to their portability, versatility for supporting multiple assay formats, and potential for resulting in low-cost assays, paper-based analytical devices (PADs) are an increasingly popular format as a platform for the development of point-of-care tests. However, very few PADs have been translated successfully to their intended environments outside of academic settings. Often overlooked as a factor that inhibits translation, usability is a vital characteristic of any successful point-of-care test. Recent advancements in PAD design have demonstrated improved usability by simplifying various aspects of user operation, including sample collection, sample processing, device operation, detection, and readout/interpretation. Field testing at various stages of device design can offer critical feedback about device usability, especially when it involves the proposed end-user or other stakeholders. By highlighting advances in usability, we aim to encourage thoughtful and rigorous design at the academic prototyping stage to address one outstanding hurdle that limits the number of PADs that make it from the benchtop to the point-of-care.

Whole-cell paper strip biosensors to semi-quantify tetracycline antibiotics in environmental matrices

A novel, low-cost, and portable paper strip biosensor was developed for the detection of tetracycline antibiotics. Escherichia coli/pMTLacZ containing the tetracycline-mediated regulatory gene used as recognition elements with β-galactosidase as the reporter protein was designed and applied to cheap and portable Whatman filter paper as the carrier to prepare this paper strip biosensor. The detection process was optimized by using EDTA and polymyxin B as a sensitizer to improve the accuracy of detection for complicated matrices. The paper strip biosensor was suitable for tetracycline concentrations in the range of 75-10000 μg/L in water and 75-7500 μg/L in soil extracts. Detection limits of 5.23-17.1 μg/L for water and 5.21-35.3 μg/kg for the EDTA soil extracts were achieved at a response time of 90 min. The standard deviation (SD) of detected values by the biosensor paper strip compared to those determined by HPLC was between 13.4 and 59.6% for tetracycline and 2.01-33.5% for oxytetracycline in water and was between 6.22 and 72.8% for tetracycline and 5.90-43.4% for oxytetracycline in soil. This suggests that the paper strip biosensor was suitable for detecting both tetracycline and oxytetracycline in water, and could provide a suitable detection for extractable oxytetracycline in soils. Therefore, this biosensor provides a simple, economical, and portable piece of field kit for on-site monitoring of tetracyclines in a variety of environmental samples, such as pond water and agricultural soil that are susceptible to tetracycline pollution from feed additives and fertilization with livestock manure.

Selection of appropriate protein assay method for a paper microfluidics platform

Background

Paper-analytical devices (PADs) have gained popularity as a simple and low-cost alternative for determining a wide range of analytes including proteins. Even though several colorimetric PADs methods for protein estimation are reported in literature, they lack justification for the chosen method and parameters therein.

Aim

Major aim of this work was to thoroughly evaluate the most commonly used colorimetric protein assays and recommend the most appropriate method for PADs platform.

Method

We performed following six colorimetric protein assays on PADs: biuret, lowry, bicinchoninic acid, bradford, bromocresol green, and tetrabromophenol blue. We obtained assay signal by analyzing images of the PADs and then assessed analytical figures of merit.

Result

Precision, accuracy, LOD, and LOQ of PADs protein assay methods ranged from 1.2 to 6.4%, 73.3–102.4%, 0.3–3.8 ​mg/mL, and 1.2–12.8 ​mg/mL, respectively. Out of six methods, we determined bromocresol green and tetrabromophenol blue as the best methods for serum and urine samples, respectively based on their optimized parameters and analytical figures of merit. The total average serum and urine protein in human samples were found to be 94.6 ​± ​16.2 ​mg/mL and 2.1 ​± ​1.5 ​mg/mL, respectively using PADs methods. The PADs result on human samples moderately correlated with the results from spectrophotometric determination (r² ​> ​0.6).

Conclusion

Paper-based protein assays were easy to perform and were completed with thousand-fold less volume of samples/reagents without a spectrophotometer compared to conventional assay methods. After testing human samples, we found one protein assay method may not be appropriate for all types of samples.

•

Thorough evaluation of colorimetric protein assays on paper-analytical device.

•

Bromocresol green and tetrabromophenol blue methods were found to be best suited for serum and urine samples, respectively.

•

The selected methods are comparable with spectrophotometric assay protocols for human serum and urine samples.

An Ultrasensitive Fluorescent Paper-Based CO2 Sensor

We demonstrate a versatile and easily fabricated paper-based CO₂ sensor. The sensor consists of a specially designed fluorescent color-shift chromophore infused into standard filter paper. The emission color of the resulting fluorescent paper changes upon exposure to CO₂ due to the formation of carbonic acid, which underlies the sensing mechanism. By using a ratiometric method, the undesirable effects of photobleaching can be eliminated, leading to a stable and repeatable sensor performance. These multiuse sensors have a response time on the order of 1 min and feature low detection limits for a paper-based CO₂ gas sensor, suggesting possible low-cost applications in smart buildings or other facilities in which CO₂ levels are required to be continuously monitored.

Immobilization and Function of nIR-Fluorescent Carbon Nanotube Sensors on Paper Substrates for Fluidic Manipulation

Nanoparticle-based optical sensors are capable of highly sensitive and selective chemical interactions and can form the basis of molecular recognition for various classes of analytes. However, their incorporation into standardized in vitro assays has been limited by their incompatibility with packaging or form factors necessary for specific applications. Here, we have developed a technique for immobilizing nIR-fluorescent single-walled carbon nanotube (SWCNT) sensors on seven different types of paper substrates including nitrocellulose, nylon, poly(vinylidene fluoride), and cellulose. Sensors remain functional upon immobilization and exhibit nIR fluorescence in nonaqueous solvent systems. We then extend this system to the Corona Phase Molecular Recognition (CoPhMoRe) approach of synthetic molecular recognition by screening ssDNA-wrapped SWCNTs with different sequences against a panel of fat-soluble vitamins in canola oil, identifying a sensor which responds to β-carotene with a dissociation constant of 2.2 μM. Moreover, we pattern hydrophobic regions onto nitrocellulose using the wax printing method and form one-dimensional sensor barcodes for rapid multiplexing. Using a sensor array of select ssDNA wrappings, we are able to distinguish between Cu(II), Cd(II), Hg(II), and Pb(II) at a concentration of 100 μM. Finally, we demonstrate that immobilized sensors remain fluorescent and responsive for nearly 60 days when stability is addressed. This work represents a significant step toward the deployment of fluorescent nanoparticle sensors for point-of-use applications.

Simultaneous quantification of multiple biomarkers on a self-calibrating microfluidic paper-based analytic device

In this study, we developed a point-of-care assay platform with simultaneous detection and self-calibration capabilities for multiple targets based on a microfluidic paper-based analytical device (μPAD). This system is easily manufactured using a wax printing method on chromatographic paper. The design pattern consists of a zone of detection and a calibrant zone for controlled loading using wax barriers with different thicknesses. We showed the utility and applicability of this approach by a proof-of-concept study for two clinically important markers: glucose and lactate. With the naked eye, the results could be fully distinguished and recorded to evaluate the analytical performance with a flatbed scanner. The detection limits of glucose and lactate were 0.3125 mM and 0.2975 mM, respectively, and simultaneous detection was possible from a small sample (0.4 μL) with high sensitivity. Furthermore, this device has a self-calibration function, which minimizes the influence of environmental conditions (i.e., ambient light intensity, temperature, humidity, and pressure). Therefore, the developed multiplex paper-based device is promising for clinical multianalyte point-of-care testing since it is easy to manufacture, cost-effective, user-friendly, and highly sensitive.

Single-point calibration and standard addition assays on calibrant-loaded paper-based analytical devices

This article describes a new, simplified approach for performing quantitative colorimetric assays on paper-based analytical devices that uses calibrant-loaded paper devices to perform external calibration and standard addition calibration using one calibration point. Calibrant-loaded devices consist of sensing areas pre-loaded with a colored product which is produced from the reaction of a standard solution of the analyte with the appropriate colorimetric reagents. When the sample is added into the calibrant-loaded sensing zone the analytical signal (i.e. color intensity) increases proportionally to the concentration of the analyte in the tested sample. The total measured signal corresponds to the sum of the concentration of the analyte in the sample and the standard solution pre-stored in the device and is used to calculate the concentration of the analyte in the sample based on the principles of linear calibration. The applicability of this approach was benchmarked in three colorimetric assays (i.e., for the determination of iron, nickel, and amino acids) that use different reaction chemistries. This work demonstrates a simplified calibration-free approach for assays performed on paper-based analytical devices that requires minimum experimental and computational effort, it can be used for either external or standard addition calibration and can be used to identify and eliminate the presence of interferences in a sample.

Fabrication of laser printed microfluidic paper-based analytical devices (LP-µPADs) for point-of-care applications

Microfluidic paper-based analytical devices (µPADs) have provided a breakthrough in portable and low-cost point-of-care diagnostics. Despite their significant scope, the complexity of fabrication and reliance on expensive and sophisticated tools, have limited their outreach and possibility of commercialization. Herein, we report for the first time, a facile method to fabricate µPADs using a commonly available laser printer which drastically reduces the cost and complexity of fabrication. Toner ink is used to pattern the µPADs by printing, without modifying any factory configuration of the laser printer. Hydrophobic barriers are created by heating the patterned paper which melts the toner ink, facilitating its wicking into the cross-section of the substrate. Further, we demonstrate the utilization of the fabricated device by performing two assays. The proposed technique provides a versatile platform for rapid prototyping of µPADs with significant prospect in both developed and resource constrained region.

Cotton fiber-based assay with time-based microfluidic absorption sampling for point-of-care applications

Aim: Time-based microfluidic absorption sampling was proposed using cotton fiber-based device made in swab stick. The assay was optimized and compared with conventional pipetted drop sampling using the same device. Materials & methods: Reagents were integrated into cotton fiber device for assessing concentration of analytes by the colorimetric detection method through time-based absorption sampling microfluidic system. All assay parameters were first optimized using conventional pipette-based drop sampling. Results: The color intensity is linear in the relevant concentration range of the analytes. The LOD are 0.189 mM for glucose and 6.56 μM for nitrite, respectively. These values are better than conventional drop sampling. The fiber-containing swab itself functions as sampling, assay and calibration device. Conclusion: Microfluidic cotton fiber-based assay device was fabricated and can determine analyte concentration in artificial salivary samples, colorimetrically, by time-based absorption sampling without the need of complex equipments.

A whole blood sample-to-answer polymer lab-on-a-chip with superhydrophilic surface toward point-of-care technology

An innovative sample-to-answer (S-to-A) polymer lab-on-a-chip (LOC) with a blood plasma separator based on asymmetric capillary force has been proposed, developed, and completely characterized for point-of-care technology (POCT) applications. A spray layer-by-layer (LbL) nanoassembly coating has been applied for the superhydrophilic surface onto the cyclic olefin copolymer (COC). Then, the developed superwetting surfaces were designed and optimized for three device applications such as lateral transportation of whole blood in the device by capillary pumping, on-chip whole blood/plasma separation with an asymmetric capillary force, and detection using a capillary-driven lateral flow colorimetric assay. Integrating three primary components of the devices, the S-to-A polymer LOC platform has been effectively confirmed for the lateral flow colorimetric assay of bovine serum albumin (BSA) from unprocessed human whole blood without an external power resource.

A review on microscale polymerase chain reaction based methods in molecular diagnosis, and future prospects for the fabrication of fully integrated portable biomedical devices

Since the advent of microfabrication technology and soft lithography, the lab-on-a-chip concept has emerged as a state-of-the-art miniaturized tool for conducting the multiple functions associated with micro total analyses of nucleic acids, in series, in a seamless manner with a miniscule volume of sample. The enhanced surface-to-volume ratio inside a microchannel enables fast reactions owing to increased heat dissipation, allowing rapid amplification. For this reason, PCR has been one of the first applications to be miniaturized in a portable format. However, the nature of the basic working principle for microscale PCR, such as the complicated temperature controls and use of a thermal cycler, has hindered its total integration with other components into a micro total analyses systems (μTAS). This review (with 179 references) surveys the diverse forms of PCR microdevices constructed on the basis of different working principles and evaluates their performances. The first two main sections cover the state-of-the-art in chamber-type PCR microdevices and in continuous-flow PCR microdevices. Methods are then discussed that lead to microdevices with upstream sample purification and downstream detection schemes, with a particular focus on rapid on-site detection of foodborne pathogens. Next, the potential for miniaturizing and automating heaters and pumps is examined. The review concludes with sections on aspects of complete functional integration in conjunction with nanomaterial based sensing, a discussion on future prospects, and with conclusions. Graphical abstract In recent years, thermocycler-based PCR systems have been miniaturized to palm-sized, disposable polymer platforms. In addition, operational accessories such as heaters and mechanical pumps have been simplified to realize semi-automatted stand-alone portable biomedical diagnostic microdevices that are directly applicable in the field. This review summarizes the progress made and the current state of this field.

Patterned polycaprolactone-filled glass microfiber microfluidic devices for total protein content analysis

Membrane based microfluidic devices have gained much popularity in recent years, as they make possible rapid, inexpensive analytical techniques that can be applied to a wide variety of areas. The ability to modify device hydrophilicity and hydrophobicity is critically important in fabricating membrane based microfluidic devices. Polar hydrophilic membranes, such as glass microfiber (GMF) membranes, hold great potential as they are inexpensive, chemically inert, and stable. Filling of these membranes with non-polar polymers such as polycaprolactone (PCL) converts the hydrophilic GMF into a hydrophobic medium. Controlled alteration of the surface chemistry of PCL/GMF substrates allows for the fabrication of microfluidic patterns on the surface. Using this approach, we have developed a simple and rapid technique for fabrication of highly adaptable complex multidimensional (2D and 3D) microfluidic pathways on a single membrane. PCL-filled GMF media were masked and selectively exposed to oxygen radicals so that the exposed surface became permanently superhydrophilic in its behavior. The desired microfluidic pattern was cut into the mask prior to assembly and exposure, and the mask was removed after exposure to reveal the ready-to-use microfluidic device. To verify and demonstrate the performance of this novel fabrication method, a colorimetric total protein assay was applied to the determination of protein concentrations in real samples.

Vertical Paper Analytical Devices Fabricated Using the Principles of Quilling and Kirigami

Here we report the vertical paper analytical devices (vPADs) fabricated using the principles of quilling and kirigami. What differentiates the vPADs from conventional paper microfluidic devices is that the paper substrate used to fabricate the device is placed vertically to the device plane. The fabrication of vPADs with high precision is instrument-free, requiring no photolithography, printing or heating. Two- and three-dimensional vPADs are fabricated for multiplex colorimetric assays of four biochemical indicators and automated enzyme-linked immunosorbent assay of human myoglobin, respectively.

Lipopolysaccharides detection on a grating-coupled surface plasmon resonance smartphone biosensor

We report a smartphone label-free biosensor platform based on grating-coupled surface plasmon resonance (GC-SPR). The sensor system relies on the smartphone's built-in flash light source and camera, a disposable sensor chip with Au diffraction grating and a compact disk (CD) as the spectra dispersive unit. The Au grating sensor chip was modified with a synthetic peptide receptor and employed on the GC-SPR detection of lipopolysaccharides (known as endotoxins) with detection limit of 32.5ng/mL in water. Upon incubation of various small and macro-molecules with the synthetic peptide modified sensor chips, we concluded the good selectivity of the sensor for LPS detection. In addition, the sensor shows feasibility for the detection of LPS in commonly used clinical injectable fluids, such as clinical-grade 0.9% sodium chloride intravenous infusion, compound sodium lactate intravenous infusion and insulin aspart. The developed sensor platform offers the advantage of portability and simplicity, which is attractive for point-of-care and remote detection of biomedical and environmental targets.

Microfluidic paper-based analytical devices for potential use in quantitative and direct detection of disease biomarkers in clinical analysis

Clinicians, working in the health-care diagnostic systems of developing countries, currently face the challenges of rising costs, increased number of patient visits, and limited resources. A significant trend is using low-cost substrates to develop microfluidic devices for diagnostic purposes. Various fabrication techniques, materials, and detection methods have been explored to develop these devices. Microfluidic paper-based analytical devices (μPADs) have gained attention for sensing multiplex analytes, confirming diagnostic test results, rapid sample analysis, and reducing the volume of samples and analytical reagents. μPADs, which can provide accurate and reliable direct measurement without sample pretreatment, can reduce patient medical burden and yield rapid test results, aiding physicians in choosing appropriate treatment. The objectives of this review are to provide an overview of the strategies used for developing paper-based sensors with enhanced analytical performances and to discuss the current challenges, limitations, advantages, disadvantages, and future prospects of paper-based microfluidic platforms in clinical diagnostics. μPADs, with validated and justified analytical performances, can potentially improve the quality of life by providing inexpensive, rapid, portable, biodegradable, and reliable diagnostics.

A Chemically Patterned Microfluidic Paper-based Analytical Device (C-µPAD) for Point-of-Care Diagnostics

A chemically patterned microfluidic paper-based analytical device (C-µPAD) is developed to create fluidic networks by forming hydrophobic barriers using chemical vapor deposition (CVD) of trichlorosilane (TCS) on a chromatography paper. By controlling temperature, pattern size, and CVD duration, optimal conditions were determined by characterizing hydrophobicity, spreading patterns, and flow behavior on various sized fluidic patterns. With these optimal conditions, we demonstrated glucose assay, immunoassay, and heavy metal detection on well-spot C-µPAD and lateral flow C-µPAD. For these assays, standard curves showing correlation between target concentration and gray intensity were obtained to determine a limit of detection (LOD) of each assay. For the glucose assays on both well-spot C-µPAD and lateral flow C-µPAD, we achieved LOD of 13 mg/dL, which is equivalent to that of a commercial glucose sensor. Similar results were obtained from tumor necrosis factor alpha (TNFα) detection with 3 ng/mL of LOD. For Ni detection, a colorimetric agent was immobilized to obtain a stationary and uniform reaction by using thermal condensation coupling method. During the immobilization, we successfully functionalized amine for coupling the colorimetric agent on the C-µPAD and detected as low as 150 μg/L of Ni. These C-µPADs enable simple, rapid, and cost-effective bioassays and environmental monitoring, which provide practically relevant LODs with high expandability and adaptability.

Zinc oxide nanorods functionalized paper for protein preconcentration in biodiagnostics

Distinguishing a specific biomarker from a biofluid sample containing a large variety of proteins often requires the selective preconcentration of that particular biomarker to a detectable level for analysis. Low-cost, paper-based device is an emerging opportunity in diagnostics. In the present study, we report a novel Zinc oxide nanorods functionalized paper platform for the preconcentration of Myoglobin, a cardiac biomarker. Zinc oxide nanorods were grown on a Whatman filter paper no. 1 via the standard hydrothermal route. The growth of Zinc oxide nanorods on paper was confirmed by a combination of techniques consisting of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS,) scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDX) analysis. The Zinc oxide nanorods modified Whatman filter paper (ZnO-NRs/WFP) was further tested for use as a protein preconcentrator. Paper-based ELISA was performed for determination of pre-concentration of cardiac marker protein Myoglobin using the new ZnO-NRs/WFP platform. The ZnO-NRs/WFP could efficiently capture the biomarker even from a very dilute solution (Myoglobin < 50 nM). Our ELISA results show a threefold enhancement in protein capture with ZnO-NRs/WFP compared to unmodified Whatman filter paper, allowing accurate protein analysis and showing the diagnostic concept.

Biosensing of DNA oxidative damage: a model of using glucose meter for non-glucose biomarker detection

Non-glucose biomarker-DNA oxidative damage biomarker 8-hydroxy-2′-deoxyguanosine (8-OHdG) has been successfully detected using a smartphone-enabled glucose meter. Through a series of immune reactions and enzymatic reactions on a solid lateral flow platform, 8-OHdG concentration has been converted to a relative amount of glucose, and therefore can be detected by conventional glucose meter directly. The device was able to detect 8-OHdG concentrations in phosphate buffer saline as low as 1.73 ng mL⁻¹ with a dynamic range of 1–200 ng mL⁻¹. Considering the inherent advantages of the personal glucose meter, the demonstration of this device, therefore, should provide new opportunities for the monitoring of a wide range of biomarkers and various target analytes in connection with different molecular recognition events.

Development of a point-of-care diagnostic for influenza detection with antiviral treatment effectiveness indication

Currently, diagnosis of influenza is performed either through tedious polymerase chain reaction (PCR) or through rapid antigen detection assays. The rapid antigen detection assays available today are highly specific but not very sensitive, and most importantly, lack the ability to show if the strain of influenza detected is susceptible to antiviral agents, such as Tamiflu and Relenza. The ability to rapidly determine if a patient has an infectious disease and what type of treatment the infection will respond to, would significantly reduce the treatment decision time, shorten the impact of symptoms, and minimize transfer to others. In this study, a novel, point-of-care style μPAD (microfluidic paper-based diagnostic) for influenza has been developed with the ability to determine antiviral susceptibility of the strain for treatment decision. The assay exploits the enzymatic activity of surface proteins present on all influenza strains, and potential false positive responses can be mitigated. A sample can be added to the device, distributed to 4 different reagent zones, and development of the enzymatic substrate under different buffer conditions takes place on bottom of the device. Analysis can be performed by eye or through a colorimetric image analysis smartphone application.

A simple method to produce 2D and 3D microfluidic paper-based analytical devices for clinical analysis

This paper describes the fabrication of 2D and 3D microfluidic paper-based analytical devices (μPADs) for monitoring glucose, total protein, and nitrite in blood serum and artificial urine. A new method of cutting and sealing filter paper to construct μPADs was demonstrated. Using an inexpensive home cutter printer soft cellulose-based filter paper was easily and precisely cut to produce pattern hydrophilic microchannels. 2D and 3D μPADs were designed with three detection zones each for the colorimetric detection of the analytes. A small volume of samples was added to the μPADs, which was photographed after 15 min using a digital camera. Both μPADs presented an excellent analytical performance for all analytes. The 2D device was applied in artificial urine samples and reached limits of detection (LODs) of 0.54 mM, 5.19 μM, and 2.34 μM for glucose, protein, and nitrite, respectively. The corresponding LODs of the 3D device applied for detecting the same analytes in artificial blood serum were 0.44 mM, 1.26 μM, and 4.35 μM.

Ultraminiaturized assay for rapid, low cost detection and quantification of clinical and biochemical samples

Herein, we report a simple, sensitive, rapid and low-cost ultraminiaturized assay technique for quantitative detection of 1 μl of clinical or biochemical sample on a novel ultraminiaturized assay plate (UAP). UAP is prepared by making tiny cavities on a polypropylene sheet. As UAP cannot immobilize a biomolecule through absorption, we have activated the tiny cavities of UAP by 1-fluoro-2-nitro-4-azidobenzene in a photochemical reaction. Activated UAP (AUAP) can covalently immobilize any biomolecule having an active nucleophilic group such as amino group. Efficacy of AUAP is demonstrated by detecting human IgE, antibody of hepatitis C virus core antigen and oligonucleotides. Quantification is performed by capturing the image of the colored assay solution and digitally quantifying the image by color saturation without using costly NanoDrop spectrophotometer. Image - based detection of human IgE and an oligonucleotide shows an excellent correlation with absorbance - based assay (recorded in a NanoDrop spectrophotometer); it is validated by Pearson's product-moment correlation with correlation coefficient of r = 0.9545088 and r = 0.9947444 respectively. AUAP is further checked by detecting hepatitis C virus Ab where strong correlation of color saturation with absorbance with respect to concentration is observed. Ultraminiaturized assay successfully detects target oligonucleotides by perfectly hybridizing with their respective complementary oligonucleotide probes but not with a random oligonucleotide. Ultraminiaturized assay technique has substantially reduced the requirement of reagents by 100 times and assay timing by 50 times making it a potential alternative to conventional method.

New Functionalities for Paper-Based Sensors Lead to Simplified User Operation, Lower Limits of Detection, and New Applications

In the last decade, paper analytical devices (PADs) have evolved into sophisticated yet simple sensors with biological and environmental applications in the developed and developing world. The focus of this review is the technological improvements that have over the past five years increased the applicability of PADs to real-world problems. Specifically, this review reports on advances in sample processing, fluid flow control, signal amplification, and component integration. Throughout, we have sought to emphasize advances that retain the main virtues of PADs: low cost, portability, and simplicity.

Distance-based microfluidic quantitative detection methods for point-of-care testing

Equipment-free devices with quantitative readout are of great significance to point-of-care testing (POCT), which provides real-time readout to users and is especially important in low-resource settings. Among various equipment-free approaches, distance-based visual quantitative detection methods rely on reading the visual signal length for corresponding target concentrations, thus eliminating the need for sophisticated instruments. The distance-based methods are low-cost, user-friendly and can be integrated into portable analytical devices. Moreover, such methods enable quantitative detection of various targets by the naked eye. In this review, we first introduce the concept and history of distance-based visual quantitative detection methods. Then, we summarize the main methods for translation of molecular signals to distance-based readout and discuss different microfluidic platforms (glass, PDMS, paper and thread) in terms of applications in biomedical diagnostics, food safety monitoring, and environmental analysis. Finally, the potential and future perspectives are discussed.

Biomarker detection for disease diagnosis using cost-effective microfluidic platforms

Early and timely detection of disease biomarkers can prevent the spread of infectious diseases, and drastically decrease the death rate of people suffering from different diseases such as cancer and infectious diseases. Because conventional diagnostic methods have limited application in low-resource settings due to the use of bulky and expensive instrumentation, simple and low-cost point-of-care diagnostic devices for timely and early biomarker diagnosis is the need of the hour, especially in rural areas and developing nations. The microfluidics technology possesses remarkable features for simple, low-cost, and rapid disease diagnosis. There have been significant advances in the development of microfluidic platforms for biomarker detection of diseases. This article reviews recent advances in biomarker detection using cost-effective microfluidic devices for disease diagnosis, with the emphasis on infectious disease and cancer diagnosis in low-resource settings. This review first introduces different microfluidic platforms (e.g. polymer and paper-based microfluidics) used for disease diagnosis, with a brief description of their common fabrication techniques. Then, it highlights various detection strategies for disease biomarker detection using microfluidic platforms, including colorimetric, fluorescence, chemiluminescence, electrochemiluminescence (ECL), and electrochemical detection. Finally, it discusses the current limitations of microfluidic devices for disease biomarker detection and future prospects.

Conversion of a laboratory-based test for phenylalanine detection to a simple paper-based format and implications for PKU screening in low-resource settings

Laboratory-based testing does not reach many individuals in lower-resource settings who could benefit from access to appropriate tests for diagnosis and therapy. A critical issue is laboratory-based testing often requires an environment with a high level of resources and supporting infrastructure that is not available in many areas of the world. The current report describes the conversion of a laboratory-based test for phenylalanine detection to a simple paper-based test appropriate for use in low-resource settings. The paper-based test is easy to operate, with all reagents stored dry on the card, is compatible with visible detection for clinically relevant concentrations of phenylalanine, and has a time to result of 10 minutes. Next steps for test development are discussed in the context of the potential for the paper-based Phe test to be used as a newborn PKU screening test in settings that are not well served by existing screening approaches.

Enabling robust quantitative readout in an equipment-free model of device development

A critical constraint in the design of appropriate medical devices for the lowest-resource settings is the lack of access to maintenance or repair on instrumentation. There are numerous point-of-care applications for which quantitative readout would have clinical utility. Thus, a challenge to the device developer is to enable quantitative device readout in an equipment-free model that is appropriate for use in even the lowest-resource settings. Paper microfluidics has great potential for enabling equipment-free devices that are low-cost, operable by minimally-trained users, and provide quantitative readout. The focus of this critical review is to describe the work, starting several decades ago and continuing to the present, to enable assays with quantitative readout in a fully-disposable device.

Paper-based chemical and biological sensors: Engineering aspects

Remarkable efforts have been dedicated to paper-based chemosensors and biosensors over the last few years, mainly driven by the promise of reaching the best trade-off between performance, affordability and simplicity. Because of the low-cost and rapid prototyping of these sensors, recent research has been focused on providing affordable diagnostic devices to the developing world. The recent progress in sensitivity, multi-functionality and integration of microfluidic paper-based analytical devices (µPADs), increasingly suggests that this technology is not only attractive in resource-limited environments but it also represents a serious challenger to silicon, glass and polymer-based biosensors. This review discusses the design, chemistry and engineering aspects of these developments, with a focus on the past few years.