Paper Microzone Plates as Analytical Tools for Studying Enzyme Stability: A Case Study on the Stabilization of Horseradish Peroxidase Using Trehalose and SU-8 Epoxy Novolac Resin

The use of biological fluids in microfluidic paper-based analytical devices (μPADs): Recent advances, challenges and future perspectives

The use of microfluidic paper-based analytical devices (μPADs) for aiding medical diagnosis is a growing trend in the literature mainly due to their low cost, easy use, simple manufacturing, and great potential for application in low-resource settings. Many important biomarkers (proteins, ions, lipids, hormones, DNA, RNA, drugs, whole cells, and more) and biofluids are available for precise detection and diagnosis. We have reviewed the advances μPADs in medical diagnostics have achieved in the last few years, focusing on the most common human biofluids (whole blood/plasma, sweat, urine, tears, and saliva). The challenges of detecting specific biomarkers in each sample are discussed, along with innovative techniques that overcome such limitations. Finally, the difficulties of commercializing μPADs are considered, and future trends are presented, including wearable devices and integrating multiple steps in a single platform.

Synthesizing Submicron Polyelectrolyte Capsules to Boost Enzyme Immobilization and Enhance Enzyme-Based Immunoassays

Polyelectrolyte capsules (PCs) exhibit attractive superiorities in enzyme immobilization, including providing a capacious microenvironment for enzyme conformational freedom, highly effective mass transfer, and protecting enzymes from the external environment. Herein, we provide the first systemic evaluation of submicron PCs (SPCs, 500 nm) for enzyme immobilization. The catalytic kinetics results show that SPC encapsulation affected the affinities of enzymes and substrates but significantly enhanced their catalytic activity. The stability test indicates that SPC-encapsulated horseradish peroxidase (HRP) exhibits ultrahigh resistance to external harsh conditions and has a longer storage life than that of soluble HRP. The proposed encapsulation strategy enables 7.73-, 2.22-, and 11.66-fold relative activities when working at a pH as low as 3, at a NaCl concentration as high as 500 mM, and at a trypsin concentration as high as 10 mg/mL. We find that SPC encapsulation accelerates the cascade reaction efficiency of HRP and glucose oxidase. Owing to SPCs enhancing the catalytic activity of the loaded enzymes, we established an amplified enzyme-linked immunosorbent assay (ELISA) for the detection of Escherichia coli O157:H7 using HRP-loaded SPCs. The detection sensitivity of SPC-improved ELISA was found to be 280 times greater than that of conventional HRP-based ELISA. Altogether, we provide an elaborate evaluation of 500 nm SPCs on enzyme immobilization and its application in the ultrasensitive detection of foodborne pathogens. This evaluation provides evidence to reveal the potential advantage of SPCs on enzyme immobilization for enzyme-based immunoassays. It has excellent biological activity and strong stability and broadens the application prospect in urine, soy sauce, sewage, and other special samples.

Beyond Wax Printing: Fabrication of Paper-Based Microfluidic Devices Using a Thermal Transfer Printer

Paper-based microfluidic devices, also known as microPADs, are an emerging analytical platform with the potential to improve point-of-care diagnostics. MicroPADs are fabricated by patterning hydrophobic inks onto sheets of paper to create hydrophilic channels and test zones. One of the main advantages of microPADs is that they are inexpensive and simple to fabricate, making them accessible even to researchers with limited budgets or no prior fabrication expertise. Wax printing, where a solid ink printer is used to pattern wax on paper, has been the most convenient and popular method for fabricating paper-based microfluidic devices. Unfortunately, solid ink printers were discontinued in 2016 and are no longer available commercially. Here we introduce a method for fabricating microPADs using a portable thermal transfer printer that retains the convenience of wax printing. Devices fabricated by thermal transfer printing were comparable to devices fabricated via wax printing and laser printing. The low cost, convenience, and portability of the thermal transfer printer make this approach an exciting prospect for replacing wax printing and facilitating the continued development of paper-based microfluidics.

Molecular Methods for Pathogenic Bacteria Detection and Recent Advances in Wastewater Analysis

With increasing concerns about public health and the development of molecular techniques, new detection tools and the combination of existing approaches have increased the abilities of pathogenic bacteria monitoring by exploring new biomarkers, increasing the sensitivity and accuracy of detection, quantification, and analyzing various genes such as functional genes and antimicrobial resistance genes (ARG). Molecular methods are gradually emerging as the most popular detection approach for pathogens, in addition to the conventional culture-based plate enumeration methods. The analysis of pathogens in wastewater and the back-estimation of infections in the community, also known as wastewater-based epidemiology (WBE), is an emerging methodology and has a great potential to supplement current surveillance systems for the monitoring of infectious diseases and the early warning of outbreaks. However, as a complex matrix, wastewater largely challenges the analytical performance of molecular methods. This review synthesized the literature of typical pathogenic bacteria in wastewater, types of biomarkers, molecular methods for bacterial analysis, and their recent advances in wastewater analysis. The advantages and limitation of these molecular methods were evaluated, and their prospects in WBE were discussed to provide insight for future development.

Paper-Based Analytical Device for the On-Site Detection of Nerve Agents

We report a colorimetric paper-based microfluidic device based on an enzyme inhibition assay that allows the on-site detection of nerve agents by sampling and wicking. The sample and reagents are automatically transported through the channel where an enzyme inhibition reaction is conducted, followed by an enzyme-substrate reaction and a color reaction. This device can detect 0.1 μg/mL of the nerve agent VX in a 2.5 μL drop and is nerve agent selective and robust against temperature, pH, and several liquids. We confirmed that sampling procedures (dilution and wiping) are applicable to this device. Furthermore, the fabrication procedure is easy, and the cost is at most a few tens of cents. Thus, the present device provides a practical method for the urgent detection of nerve agents in suspected chemical terrorism incidents.

Evaluation of the photocatalytic performance of molecularly imprinted S-TiO2 by paper microzones

The paper microzones method (PMZs) is a green chemical method that uses the principle of the three primary colors of red, green and blue (RGB) to detect the water quality of the droplets on white paper. However, this method is rarely used in the performance evaluation of photocatalysts. The paper details the first use of paper microzones utilized in the evaluation of photocatalyst performance. A sol-gel method was used to prepare molecularly imprinted modified TiO₂ photocatalysts for the treatment of different wastewaters, and characterized the catalysts using XRD and several other methods. The reliability of PMZs on the evaluation of photocatalytic activity and selectivity was also analyzed. The following results were obtained: EP-TiO₂ catalysts (EP, ethyl paraben, the imprinting molecule) with different S doping levels were synthesized using a one-step sol-gel method, and the best S doping ratio was found to be n(Ti):n(S) 3:1. S-EP-TiO₂ was found to be 100% anatase and showed excellent photocatalytic performance, while the PMZs method accurately determined changes in RGB levels for the photocatalytic degradation process of pollutants using S-EP-TiO₂ as the photocatalyst. A photocatalytic kinetic analysis showed the PMZs method was quite suitable for the evaluation of photocatalyst activity, but the evaluation of selectivity needs improvement. This method is a promising green chemistry way to evaluate photocatalyst performance and the rapid detection of outdoor sewage water quality.

Dry storage of multiple reagent types within a paper microfluidic device for phenylalanine monitoring

The degradation of biochemical reagents on the timescale of weeks can severely limit the utility of microfluidic assays intended for field use, and is a challenging aspect of microfluidic device development in general. Our study focuses on the evaluation of the dry storage stability of three types of reagents: (i) the colorimetric reagents nitroblue tetrazolium and 1-methoxy-5-methylphenazinium methylsulfate, (ii) the enzyme phenylalanine dehydrogenase, and (iii) the coenzyme β-nicotinamide adenine dinucleotide hydrate, within the context of a phenylalanine monitoring device. We have demonstrated stable dry storage of each of the reagents, over the time span of approximately one month. Drying the colorimetric reagents under nitrogen was found to largely suppress reagent degradation and the appearance of nonspecific signal, while the enzyme and coenzyme retained activity when stored dry for a month without additional processing or chemical additives. Finally, phenylalanine monitoring devices with all three reagent types dried down and stored for 15 days showed comparable functionality to devices containing freshly-dried reagents - a key milestone to enable future clinical testing.

Long-term stabilization of hydrogen peroxide by poly(vinyl alcohol) on paper-based analytical devices

Stabilizing reagents that can be deposited onto paper is an important issue for researchers who depend on paper-based analytical devices (PADs), because long-term stability of the devices is essential in point-of-care testing. Here, we found that poly(vinyl alcohol) (PVA) would stabilize hydrogen peroxide placed on a paper substrate following exposure to air. Horseradish peroxidase was employed as a sample in colorimetric measurements of PADs after hydrogen peroxide and 3,3',5,5'-tetramethylbenzidine were deposited as substrates in an enzymatic reaction. The addition of PVA to hydrogen peroxide significantly suppressed its degradation. Concentrations of PVA that ranged from 0.5 to 2%, increased the duration of the stability of hydrogen peroxide, and the results for a PVA concentration of 1% approximated those of 2% PVA. Storage of the PADs at 4 °C in a refrigerator extended the stability of the hydrogen peroxide containing 2% PVA by as much as 30 days. The stability of hydrogen peroxide without PVA was degraded after one day under room temperature.

Wax-Printed Fluidic Time Delays for Automating Multi-Step Assays in Paper-Based Microfluidic Devices (MicroPADs)

Microfluidic paper-based analytical devices (microPADs) have emerged as a promising platform for point-of-care diagnostic devices. While the inherent wicking properties of microPADs allow for fluid flow without supporting equipment, this also presents a major challenge in achieving robust fluid control, which becomes especially important when performing complex multi-step assays. Herein, we describe an ideal method of fluid control mediated by wax-printed fluidic time delays. This method relies on a simple fabrication technique, does not utilize chemicals/reagents that could affect downstream assays, is readily scalable, and has a wide temporal range of tunable fluid control. The delays are wax printed on both the top and bottom of pre-fabricated microPAD channels, without subsequent heating, to create hemi-/fully-enclosed channels. With these wax printed delays, we were able to tune the time it took aqueous solutions to wick across a 25 mm-long channel between 3.6 min and 13.4 min. We then employed these fluid delays in the sequential delivery of four dyes to a test zone. Additionally, we demonstrated the automation of two simple enzymatic assays with this fluid control modality. This method of fluid control may allow future researchers to automate more complex assays, thereby further advancing microPADs toward real-world applications.

Paper Capillary Enables Effective Sampling for Microfluidic Paper Analytical Devices

The paper capillary is introduced to enable effective sampling on microfluidic paper analytical devices. By coupling the macroscale capillary force of paper capillary and the microscale capillary forces of native paper, fluid transport can be flexibly tailored with proper design. Subsequently, a hybrid-fluid-mode paper capillary device was proposed which enables fast and reliable sampling in an arrayed form with less surface adsorption and bias for different components. The resulting device thus supports high-throughput, quantitative, and repeatable assays by manual operation. With all these merits, multiplex analysis of ions, proteins, and microbes have all been realized on this platform, which has paved the way to higher analysis on μPADs.

Detection of Biomarkers of Periodontal Disease in Human Saliva Using Stabilized, Vertical Flow Immunoassays

We report methods for stabilizing cellulose-based immunoassays and using this platform to analyze human saliva. Stabilization treatments of immunoassays for matrix metalloproteinases (MMP)-8 and -9, biomarkers of periodontal disease, were conducted and compared, revealing that anti-MMP-8 and -9 capture antibodies could be stabilized with the addition of a 5% trehalose solution to the test zones, followed by drying in a vacuum oven. After stabilization, the paper devices retained equivalent binding activity to that of freshly prepared tests for 14 days-a time frame that enables US-based clinical testing of this diagnostic assay. A saliva pretreatment method was developed to remove viscous elements without reducing the concentration or binding activity of dissolved proteins. Immunoassays were stored in ziplock bags containing desiccant, and used to detect nanomolar concentrations of MMP-9 in human saliva across the relevant clinical concentration range. These methods and findings facilitate rapid, affordable validation studies of this and other biomarkers that are found in saliva using vertical flow immunoassays.