Draw your assay: Fabrication of low-cost paper-based diagnostic and multi-well test zones by drawing on a paper

Fabrication of paper-based analytical devices by a laminating method with thermal ink ribbons, sticky notes, and office appliances

A stencil printing method utilizing sticky notes, a thermal transfer ink ribbon, and office appliances for paper-based analytical device (PAD) fabrication was proposed. A sticky note was attached to a filter paper, and a mask pattern was cut using a cutting machine. A commercially available thermal ink ribbon was then placed over the mask and laminated. We have characterized the fabricated devices. This approach could be used for the fast and mass prototyping of PADs using simple office appliances with no need for a wax printer.

Patterned Dried Blood Spot Cards for the Improved Sampling of Whole Blood

Dried blood spot (DBS) cards perform many functions for sampling blood that is intended for subsequent laboratory analysis, which include: (i) obviating the need for a phlebotomist by using fingersticks, (ii) enhancing the stability of analytes at ambient or elevated environmental conditions, and (iii) simplifying the transportation of samples without a cold chain. However, a significant drawback of standard DBS cards is the potential for sampling bias due to unrestricted filling caused by the hematocrit of blood, which often limits quantitative or reproducible measurements. Alternative microsampling technologies have minimized or eliminated this bias by restricting blood distribution, but these approaches deviate from clinical protocols and present a barrier to broad adoption. Herein, we describe a patterned dried blood spot (pDBS) card that uses wax barriers to control the flow and restrict the distribution of blood to provide enhanced sampling. These patterned cards reproducibly fill four replicate extraction zones independent of the hematocrit effect. We demonstrate a 3-fold improvement in accuracy for the quantitation of hemoglobin using pDBS cards compared to unpatterned cards. Patterned cards also facilitate the near quantitative recovery (ca. 95%) of sodium with no evidence of a statistically significant difference between dried and liquid blood samples. Similarly, the recovery of select amino acids was conserved in comparison to a recent report with improved intercard precision. We anticipate that this approach presents a viable method for preparing and storing samples of blood in limited resource settings while maintaining current clinical protocols for processing and analyzing dried blood spots.

Fabrication of Paper-Based Microfluidics by Spray on Printed Paper

Since the monumental work conducted by Whitesides et al. in 2007, research and development of paper-based microfluidics has been widely carried out, with its applications ranging from chemical and biological detection and analysis, to environmental monitoring and food-safety inspection. Paper-based microfluidics possesses several competitive advantages over other substrate materials, such as being simple, inexpensive, power-free for fluid transport, lightweight, biodegradable, biocompatible, good for colorimetric tests, flammable for easy disposal of used paper-based diagnostic devices by incineration, and being chemically modifiable. Myriad methods have been demonstrated to fabricate paper-based microfluidics, such as solid wax printing, cutting, photolithography, microembossing, etc. In this study, fabrication of paper-based microfluidics was demonstrated by spray on the printed paper. Different from the normally used filter papers, printing paper, which is much more accessible and cheaper, was utilized as the substrate material. The toner was intended to serve as the mask and the patterned hydrophobic barrier was formed after spray and heating. The processing parameters such as toner coverage on the printing paper, properties of the hydrophobic spray, surface properties of the paper, and curing temperature and time were systematically investigated. It was found that, after repetitive printing four times, the toner was able to prevent the hydrophobic spray (the mixture of PDMS and ethyl acetate) from wicking through the printing paper. The overall processing time for fabrication of paper-based microfluidic chips was less than 10 min and the technique is potentially scalable. Glucose detection was conducted using the microfluidic paper-based analytical devices (µPADs) as fabricated and a linear relationship was obtained between 1 and 10 mM.

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.

Paper and thread as media for the frugal detection of urinary tract infections (UTIs)

Urinary tract infections (UTIs) make up a significant proportion of the global burden of disease in vulnerable groups and tend to substantially impair the quality of life of those affected, making timely detection of UTIs a priority for public health. However, economic and societal barriers drastically reduce accessibility of traditional lab-based testing methods for critical patient groups in low-resource areas, negatively affecting their overall healthcare outcomes. As a result, cellulose-based materials such as paper and thread have garnered significant interest among researchers as substrates for so-called frugal analytical devices which leverage the material's portability and adaptability for facile and reproducible diagnoses of UTIs. Although the field may be only in its infancy, strategies aimed at commercial penetration can appreciably increase access to more healthcare options for at-risk people. In this review, we catalogue recent advances in devices that use cellulose-based materials as the primary housing or medium for UTI detection and chart out trends in the field. We also explore different modalities employed for detection, with particular emphasis on their ability to be ported onto discreet casings such as sanitary products.

"Scratch it out": carbon copy based paper devices for microbial assays and liver disease diagnosis

We present a facile paper-based microfluidic device fabrication technique leveraging off-the-shelf carbon paper for the deposition of hydrophobic barriers using a novel "stencil scratching" method. This exceedingly frugal approach (0.05$) requires practically no technical training to employ. Hydrophobic barriers fabricated using this approach offer a width of 3 mm and a hydrophilic channel width of 849 μm, with an ability to confine major aqueous solvents without leakage. The utility of the device is demonstrated by porting a cell viability assay showing a limit-of-detection (LOD) of 0.6 × 10⁸ CFU mL^-1 and bilirubin assay with human serum showing a detection range of 1.76-6.9 mg dL^-1 and a limit-of-detection (LOD) of 1.76 mg dL^-1. The intuitiveness and economic viability of the fabrication method afford it great potential in the field of point-of-care diagnostics geared towards providing testing infrastructure in resource-scarce regions globally.

Nucleic acid analysis on paper substrates (NAAPs): an innovative tool for Point of Care (POC) infectious disease diagnosis

The cost-effective rapid diagnosis of infectious diseases is an essential and important factor for curing such diseases in the global public health care picture. Owing to poor infrastructure and lack of sanitation, these diseases have an extreme impact on remote and rural areas, especially in developing countries, and there are unresolved challenges. Molecular diagnosis, such as nucleic acid analysis, plays a key role in the significant treatment of numerous infectious diseases. Current molecular diagnostic assays require a sophisticated laboratory setup with expensive components. Molecular diagnosis on a microfluidic point-of-care (POC) platform is attractive to researchers for disease detection with proper prevention. Compared to various microfluidic substrate materials, paper-based POC technologies offer significant cost-effective solutions over high-cost clinical instruments to fill the gap between the needs of users and affordability. Low-cost paper-based microfluidic POC technologies provide portable and disposable diagnostic systems for multiple disease detection that may be extremely useful in remote areas. This article presents a critical review of paper-based microfluidic device technology which has become an imminent platform to adjust the current health scenario for the detection of diseases using different stages of nucleic acid analysis, such as extraction, amplification and detection of nucleic acid, with future perspectives for paper substrates.

A Portable Smartphone-based Platform with an Offline Image-processing Tool for the Rapid Paper-based Colorimetric Detection of Glucose in Artificial Saliva

In this study, a microfluidic paper-based analytical device (μPAD) was integrated with a smartphone app capable of offline (without internet access) image processing and analysis for the rapid colorimetric detection of glucose. A self-inking stamp was used to form hydrophobic channels on a piece of paper-towel due to its superior water absorption efficiency. As demonstrated, the developed sensor was employed for the colorimetric detection of glucose in artificial saliva in the linear scope of 0 - 1 mM with a calculated detection limit of 29.65 μM. The experimental results show that the quantitative analysis of glucose with the proposed smartphone platform could be completed in less than one minute. The app developed for the smartphone platform is capable of extracting the color-changing area with an embedded image processing tool which could address the problem of color uniformity in the detection zones of μPAD. The integrated platform has great potential to be used for non-invasive measurements of glucose in body fluids, like tears, sweat and saliva.

Reagent-Pencil and Paper Spray Mass Spectrometry: A Convenient Combination for Selective Analyses in Complex Matrixes

The recent developments on fieldable miniature mass spectrometers require efforts to produce easy-to-use and portable alternative tools to assist in point-of-care analysis. In this paper, the reagent-pencil (RP) technology, which has been used for solvent-free deposition of reagents in paper-based microfluidics, was combined with paper spray ionization mass spectrometry (PS-MS). In this approach, named RP-PS-MS, the PS triangular piece of paper was written with the reagent pencil, consisting of mixtures of graphite and bentonite (used as a support) and a reactive compound, and allowed to react with a given analyte from a sample matrix selectively. We conducted typical applications as proof-of-principles to verify the methodology's general usefulness in detecting small organic molecules in distinct samples. Hence, various aldehydes (2-furaldehyde, valeraldehyde, and benzaldehyde) in spiked cachaça samples (an alcoholic drink produced from fermentation/distillation of sugarcane juice) were promptly detected using a reagent pencil doped with 4-aminophenol (the reactive compound). Similarly, we recognized typical ginsenosides and triacylglycerols (TAGs) in ginseng aqueous infusions and soybean oil samples, respectively, using lithium chloride as the reactive compound. The results indicate that the reagent-pencil methodology is compatible with PS-MS and provides an easy and fast way to detect target analytes in complex samples. The advantage over the usual solution-based deposition of reagents lies in the lack of preparation or carrying different specific solutions for special applications, which can simplify operation, especially in point-of-care analysis with fieldable mass spectrometers.

Signal-Enhanced Detection of Multiplexed Cardiac Biomarkers by a Paper-Based Fluorogenic Immunodevice Integrated with Zinc Oxide Nanowires

Using a single test to comprehensively evaluate multiple cardiac biomarkers for early diagnosis and prevention of acute myocardial infarction (AMI) has faced enormous challenges. Here, we have developed paper-based fluorogenic immunodevices for multiplexed detection of three cardiac biomarkers, namely, human heart-type fatty acid binding protein (FABP), cardiac troponin I (cTnI), and myoglobin, simultaneously. The detection is based on a strategy using zinc oxide nanowires (ZnO NWs) to enhance fluorescence signals (∼5-fold compared to that on pure paper). The immunodevices showed high sensitivity and selectivity for FABP, cTnI, and myoglobin with detection limits of 1.36 ng/mL, 1.00 ng/mL, and 2.38 ng/mL, respectively. Additionally, the paper-based immunoassay was rapid (∼5 min to complete the test) and portable (using a homemade chamber with a smartphone and an ultraviolet lamp). The developed devices integrated with ZnO NWs enable quantitative, sensitive, and simultaneous detection of multiple cardiac biomarkers in point-of-care settings, which provides a useful approach for monitoring AMI diseases and may be extended to other medical diagnostics and environmental assessments.

A Simple Imaging Device for Fluorescence-Relevant Applications

This article unveiled the development of an inexpensive, lightweight, easy-to-use, and portable fluorescence imaging device for paper-based analytical applications. We used commercial fluorescent dyes, as proof of concept, to verify the feasibility of our fluorescence imaging device for bioanalysis. This approach may provide an alternative method for nucleotide detection and semen analysis, using a miniaturized fluorescence reader that is more compact and portable than conventional analytical equipment.

Creating compact and microscale features in paper-based devices by laser cutting

In this work we describe a fabrication method to create compact and microscale features in paper-based microfluidic devices using a CO₂ laser cutting/engraving machine. Using this method we are able to produce the smallest features with the narrowest barriers yet reported for paper-based microfluidic devices. The method uses foil backed paper as the base material and yields inexpensive paper-based devices capable of using small fluid sample volumes and thus small reagent volumes, which is also suitable for mass production. The laser parameters (power and laser head speed) were adjusted to minimize the width of hydrophobic barriers and we were able to create barriers with a width of 39 ± 15 μm that were capable of preventing cross-barrier bleeding. We generated channels with a width of 128 ± 30 μm, which we found to be the physical limit for small features in the chromatography paper we used. We demonstrate how miniaturizing of paper-based microfluidic devices enables eight tests on a single bioassay device using only 2 μL of sample fluid volume.

Lab-on-chip technology for chronic disease diagnosis

Various types of chronic diseases (CD) are the leading causes of disability and death worldwide. While those diseases are chronic in nature, accurate and timely clinical decision making is critically required. Current diagnosis procedures are often lengthy and costly, which present a major bottleneck for effective CD healthcare. Rapid, reliable and low-cost diagnostic tools at point-of-care (PoC) are therefore on high demand. Owing to miniaturization, lab-on-chip (LoC) technology has high potential to enable improved biomedical applications in terms of low-cost, high-throughput, ease-of-operation and analysis. In this direction, research toward developing new LoC-based PoC systems for CD diagnosis is fast growing into an emerging area. Some studies in this area began to incorporate digital and mobile technologies. Here we review the recent developments of this area with the focus on chronic respiratory diseases (CRD), diabetes, and chronic kidney diseases (CKD). We conclude by discussing the challenges, opportunities and future perspectives of this field.

Magnetic Two-Way Valves for Paper-Based Capillary-Driven Microfluidic Devices

This article presents a magnetically actuated two-way, three-position (+, 0, -), paper-based microfluidic valve that includes a neutral position (0)-the first of its kind. The system is highly robust, customizable, and fully automated. The advent of a neutral position and the ability to precisely control switching frequencies establish a new platform for highly controlled fluid flows in paper-based wicking microfluidic devices. The potential utility of these valves is demonstrated in automated, programmed, patterning of dyed liquids in a wicking device akin to a colorimetric assay but with a programmed fluid/reagent delivery. These valves are fabricated using facile methods and thus remain cost-effective for adoption into affordable point-of-care/bioanalytical devices.

Stacking chip for quantitative bioanalysis

This paper describes a microwell-based microdevice for performing quantitative bioanalysis. This microdevice combined the passive pumping by degassed polydimethylsiloxane (PDMS) with serial operations including solution dispensing, plates splitting and plates stacking. We name this microdevice "stacking chip". To use the stacking chip in quantitative bioanalysis, nanoliter solutions were first dispensed into the microwells through the degassed PDMS microchannels. Next, we split the microwell and microchannel plates assisted by the application of one drop of silicone oil, which resulted in a microwell array containing the reagent solutions. Microreactor arrays were formed by stacking the two microwell arrays containing the reagent solutions. With this microdevice, the enzymatic kinetics of alkaline phosphatase during the dissociation of the fluorescein diphosphate was measured and analyzed by the Michaelis-Menten model. The stacking chip is simple to fabricate and operate, and amenable to automation for high throughput analysis.

Rethinking the Design of Low-Cost Point-of-Care Diagnostic Devices

Reducing the global diseases burden requires effective diagnosis and treatment. In the developing world, accurate diagnosis can be the most expensive and time-consuming aspect of health care. Healthcare cost can, however, be reduced by use of affordable rapid diagnostic tests (RDTs). In the developed world, low-cost RDTs are being developed in many research laboratories; however, they are not being equally adopted in the developing countries. This disconnect points to a gap in the design philosophy, where parameterization of design variables ignores the most critical component of the system, the point-of-use stakeholders (e.g., doctors, nurses and patients). Herein, we demonstrated that a general focus on reducing cost (i.e., "low-cost"), rather than efficiency and reliability is misguided by the assumption that poverty reduces the value individuals place on their well-being. A case study of clinicians in Kenya showed that "zero-cost" is a low-weight parameter for point-of-use stakeholders, while reliability and standardization are crucial. We therefore argue that a user-driven, value-addition systems-engineering approach is needed for the design of RDTs to enhance adoption and translation into the field.

Polyamidoamine starburst dendrimer-activated chromatography paper-based assay for sensitive detection of telomerase activity

Telomerase is extensively expressed in various cancer cells and recognized as a target for cancer drug discovery. In the present study, a simple and amplification-free fluorescence assay based on polyamidoamine starburst dendrimer (PAMAM dendrimer)-activated paper device is proposed for sensitive detection of telomerase activity through hybridization of Cy5 modified single strand DNA probes with telomerase extension products. The paper substrate is fabricated by hand drawing according to a template, which is low cost, instrument free and easy operation. PAMAM is rich in amino groups on its surface and employed to immobilize the telomerase substrate (TS) primer. Highly sensitive detection of telomerase activity in HeLa cell lysate of 10 cells is achieved since the PAMAM dendrimer-activated paper surface can provide high density of binding sites for immobilization of TS primer. The experimental results also demonstrate that the assay can be employed to evaluate telomerase activity levels of various cell lines and screen telomerase inhibitors.

Materials for Microfluidic Immunoassays: A Review

Conventional immunoassays suffer from at least one of these following limitations: long processing time, high costs, poor user-friendliness, technical complexity, poor sensitivity and specificity. Microfluidics, a technology characterized by the engineered manipulation of fluids in channels with characteristic lengthscale of tens of micrometers, has shown considerable promise for improving immunoassays that could overcome these limitations in medical diagnostics and biology research. The combination of microfluidics and immunoassay can detect biomarkers with faster assay time, reduced volumes of reagents, lower power requirements, and higher levels of integration and automation compared to traditional approaches. This review focuses on the materials-related aspects of the recent advances in microfluidics-based immunoassays for point-of-care (POC) diagnostics of biomarkers. We compare the materials for microfluidic chips fabrication in five aspects: fabrication, integration, function, modification and cost, and describe their advantages and drawbacks. In addition, we review materials for modifying antibodies to improve the performance of the reaction of immunoassay. We also review the state of the art in microfluidic immunoassays POC platforms, from the laboratory to routine clinical practice, and also commercial products in the market. Finally, we discuss the current challenges and future developments in microfluidic immunoassays.

Paper-Based Colorimetric Biosensor for Tear Glucose Measurements

This paper describes a paper-based colorimetric biosensor for measuring glucose concentration levels in human tear samples. Colorimetric biosensors were wax printed on paper platforms and modified with chitosan previously prepared in acetic acid. The proposed device was explored to measure the glucose levels in human tear samples using 3,3′,5,5′-tetramethylbenzydine (TMB) as the chromogenic reagent. The paper-based colorimetric biosensor exhibited a linear behavior for the glucose concentration range between 0.1 and 1.0 mM. The achieved analytical sensitivity and limit of detection (LOD) were 84 AU/mM and 50 µM, respectively. Moreover, the device provided analytical reliability and no statistical difference when compared to the data recorded with a commercial glucometer. The proof-of-concept of our device was successfully demonstrated by measuring the glucose levels in six tear samples from nondiabetic subjects. In general, the results showed that the colorimetric biosensor has noticeable potential to be used as a powerful tool for tear glucose monitoring, since this fluid offers lower potential interferences, non-invasive sample collection and is pain-free. Furthermore, the proposed device could facilitate the treatment of diabetic patients who need constant control of glucose levels and cannot tolerate multiple finger sticks per day.

Paper-based microfluidic devices by asymmetric calendaring

We report a simple, efficient, one-step, affordable method to produce open-channel paper-based microfluidic channels. One surface of a sheet of paper is selectively calendared, with concomitant hydrophobization, to create the microfluidic channel. Our method involves asymmetric mechanical modification of a paper surface using a rolling ball (ball-point pen) under a controlled amount of applied stress (σ_z) to ascertain that only one side is modified. A lubricating solvent (hexane) aids in the selective deformation. The lubricant also serves as a carrier for a perfluoroalkyl trichlorosilane allowing the channel to be made hydrophobic as it is formed. For brevity and clarity, we abbreviated this method as TACH (Targeted Asymmetric Calendaring and Hydrophobization). We demonstrate that TACH can be used to reliably produce channels of variable widths (size of the ball) and depths (number of passes), without affecting the nonworking surface of the paper. Using tomography, we demonstrate that these channels can vary from 10s to 100s of microns in diameter. The created hydrophobic barrier extends around the channel through wicking to ensure no leakages. We demonstrate, through modeling and fabrication, that flow properties of the resulting channels are analogous to conventional devices and are tunable based on associated dimensionless numbers.

A Review on Microfluidic Paper-Based Analytical Devices for Glucose Detection

Glucose, as an essential substance directly involved in metabolic processes, is closely related to the occurrence of various diseases such as glucose metabolism disorders and islet cell carcinoma. Therefore, it is crucial to develop sensitive, accurate, rapid, and cost effective methods for frequent and convenient detections of glucose. Microfluidic Paper-based Analytical Devices (μPADs) not only satisfying the above requirements but also occupying the advantages of portability and minimal sample consumption, have exhibited great potential in the field of glucose detection. This article reviews and summarizes the most recent improvements in glucose detection in two aspects of colorimetric and electrochemical μPADs. The progressive techniques for fabricating channels on μPADs are also emphasized in this article. With the growth of diabetes and other glucose indication diseases in the underdeveloped and developing countries, low-cost and reliably commercial μPADs for glucose detection will be in unprecedentedly demand.

Effect of Substrate Morphology on the Odd-Even Effect in Hydrophobicity of Self-Assembled Monolayers

Surface roughness, often captured through root-mean-square roughness (R_rms), has been shown to impact the quality of self-assembled monolayers (SAMs) formed on coinage metals. Understanding the effect of roughness on hydrophobicity of SAMs, however, is complicated by the odd-even effect-a zigzag oscillation in contact angles with changes in molecular length. We recently showed that for surfaces with R_rms > 1 nm, the odd-even effect in hydrophobicity cannot be empirically observed. In this report, we compare wetting properties of SAMs on Ag and Au surfaces of different morphologies across the R_rms ∼ 1 nm limit. We prepared surfaces with comparable properties (grain sizes and R_rms) and assessed the wetting properties of resultant SAMs. Substrates with R_rms either below or above the odd-even limit were investigated. With smoother surfaces (lower R_rms), an inverted asymmetric odd-even zigzag oscillation in static contact angles (θ_s) was observed with change from Au to Ag. Asymmetry in odd-even oscillation in Au was attributed to a larger change in θ_s from odd to even number of carbons in the n-alkanethiol and vice versa for Ag. For rougher surfaces, no odd-even effect was observed; however, a gradual increase in the static contact angle was observed. Increase in the average grain sizes (>3 times larger) on rough surfaces did not lead to significant difference in the wetting properties, suggesting that surface roughness significantly dominated the nature of the SAMs. We therefore infer that the predicted roughness-dependent limit to the observation of the odd-even effect in wetting properties of n-alkanethiols cannot be overcome by creating surfaces with large grain sizes for surfaces with R_rms > 1 nm. We also observed that the differences between Au and Ag surfaces are dominated by differences in the even-numbered SAMs, but this difference vanishes with shorter molecular chain length (≤C₃).

A solvent-free microbial-activated air cathode battery paper platform made with pencil-traced graphite electrodes

We present the fabrication of an ultra-low cost, disposable, solvent-free air cathode all-paper microbial fuel cell (MFC) that does not utilize any chemical treatments. The anode and cathode were fabricated by depositing graphite particles by drawing them on paper with a pencil (four strokes). Hydrophobic parchment paper was used as a proton exchange membrane (PEM) to allow only H⁺ to pass. Air cathode MFC technology, where O₂ was used as an electron acceptor, was implemented on the paper platform. The bioelectric current was generated by an electrochemical process involving the redox couple of microbial-activated extracellular electron transferred electrons, PEM-passed H⁺, and O₂ in the cathode. A fully micro-integrated pencil-traced MFC showed a fast start-time, producing current within 10 s after injection of bacterial cells. A single miniaturized all-paper air cathode MFC generated a maximum potential of 300 mV and a maximum current of 11 μA during 100 min after a single injection of Shewanella oneidensis. The micro-fabricated solvent-free air cathode all-paper MFC generated a power of 2,270 nW (5.68 mW/m²). The proposed solvent-free air cathode paper-based MFC device could be used for environmentally-friendly energy storage as well as in single-use medical power supplies that use organic matter.

A Microfluidic Paper-Based Origami Nanobiosensor for Label-Free, Ultrasensitive Immunoassays

Microfluidic paper-based analytical devices (μPADs) represent a promising platform technology for point-of-care diagnosis. Highly sensitive, rapid, and easy-to-perform immunoassays implemented on μPADs are desirable to fulfill the promise of the μPAD technology. This article reports the first microfluidic paper-based origami nanobiosensor (origami μPAD), which integrates zinc oxide nanowires (ZnO NWs) and electrochemical impedance spectroscopy (EIS) biosensing mechanism, for label-free, ultrasensitive immunoassays. The EIS mechanism features simple and label-free assay operations which take less than 25 min to be finished, while the ZnO NWs allow covalent bonding for immobilizing probe proteins and improve the biosensing performance with such features as high surface-area-to-volume ratios and high sensitivity to surface binding. The calibration of the device reveals an ultralow limit of detection (LOD) of 60 fg mL(-1) (>100 times lower than those of existing μPADs) for rabbit immunoglobulin G in phosphate-buffered saline. The detection of human immunodeficiency virus p24 antigen in human serum with a low LOD of 300 fg mL(-1) (>33 times lower than that of a commercial p24 antigen test kit) is also demonstrated. This novel μPAD design offers ultrahigh sensitivity, short assay time, and ease of operation, and thus possesses significant potential for low-cost, rapid molecular diagnosis of early-stage diseases.

Simple and Sensitive Paper-Based Device Coupling Electrochemical Sample Pretreatment and Colorimetric Detection

We report the development of a simple, portable, low-cost, high-throughput visual colorimetric paper-based analytical device for the detection of procaine in seized cocaine samples. The interference of most common cutting agents found in cocaine samples was verified, and a novel electrochemical approach was used for sample pretreatment in order to increase the selectivity. Under the optimized experimental conditions, a linear analytical curve was obtained for procaine concentrations ranging from 5 to 60 μmol L(-1), with a detection limit of 0.9 μmol L(-1). The accuracy of the proposed method was evaluated using seized cocaine samples and an addition and recovery protocol.

Surface polymerization of perfluorosilane treatments on paper mitigates HF production upon incineration

Burning perfluoro trichlorosilanes (R^F) treated paper leads to depolymerization of the crosslinked polysilane, distilling off liquid R^F and emitting CO₂ and H₂O as the only gaseous products.

Lignocellulose-based analytical devices: bamboo as an analytical platform for chemical detection

This article describes the development of lignocellulose-based analytical devices (LADs) for rapid bioanalysis in low-resource settings. LADs are constructed using either a single lignocellulose or a hybrid design consisting of multiple types of lignocellulose. LADs are simple, low-cost, easy to use, provide rapid response, and do not require external instrumentation during operation. Here, we demonstrate the implementation of LADs for food and water safety (i.e., nitrite assay in hot-pot soup, bacterial detection in water, and resazurin assay in milk) and urinalysis (i.e., nitrite, urobilinogen, and pH assays in human urine). Notably, we created a unique approach using simple chemicals to achieve sensitivity similar to that of commercially available immunochromatographic strips that is low-cost, and provides on-site, rapid detection, for instance, of Eschericia coli (E. coli) in water.