Developments of microfluidic paper-based analytical devices (μPADs) for water analysis: A review

Applications of chemically modified screen-printed electrodes in food analysis and quality monitoring: a review

Food analysis and food quality monitoring are vital aspects of the food industry, ensuring the safety and authenticity of various food products, from packaged goods to fast food. In this comprehensive review, we explore the applications of chemically modified Screen-Printed Electrodes (SPEs) in these critical domains. SPEs have become extremely useful devices for ensuring food safety and quality assessment because of their adaptability, affordability, and convenience of use. The Introduction opens the evaluation, that covers a wide spectrum of foods, encompassing packaged, junk food, and food quality concerns. This sets the stage for a detailed exploration of chemically modified SPEs, including their nature, types, utilization, and the advantages they offer in the context of food analysis. Subsequently, the review delves into the multitude applications of SPEs in food analysis, ranging from the detection of microorganisms such as bacteria and fungi, which are significant indicators of food spoilage and safety, to the identification of pesticide residues, food colorants, chemicals, toxins, and antibiotics. Furthermore, chemically modified SPEs have proven to be invaluable in the quantification of metal ions and vitamins in various food matrices, shedding light on nutritional content and quality.

Development of a simple and rapid dipstick paper-based test strip for colorimetric determination of nitrate and nitrite in water and foodstuffs

A rapid user-friendly paper-based test strip using zinc microparticles in conjugation with Griess reagent was developed for nitrite and nitrate detection. Test strips were fabricated using a simple and fast method of step-by-step immersion into reagents so that each strip contained a single detection pad for nitrite detection and another separate pad for nitrate detection. To reduce nitrate to nitrite, zinc microparticles suspended in ethanolic solution of polyvinylpyrrolidone (PVP) were uniformly immobilized on the paper strips that were previously impregnated in the Griess reagent and dried. The Griess reagent components were optimized to reach the highest color intensity. The optimized test strip was able to determine both nitrite and nitrate with respective detection limits of 0.43 and 9.43 mg/kg and a detection time of 60 s. The performance of the new test strips was evaluated for the simultaneous colorimetric detection of nitrite and nitrate in water and different food samples.

Low-cost precision agriculture for sustainable farming using paper-based analytical devices

The United Nations estimates that by 2030, agricultural production must increase by 70% to meet food demand. Precision agriculture (PA) optimizes production through efficient resource use, with soil fertility being crucial for nutrient supply. Traditional nutrient quantification methods are costly and time-consuming. This study introduces a rapid (15 min), user-friendly, paper-based platform for determining four essential macronutrients-nitrate, magnesium, calcium, and ammonium-using colorimetric methods and a smartphone for data reading and storage. The sensor effectively detects typical soil nutrient concentrations, showing strong linearity and adequate detection limits. For nitrate, the RGB method resulted in an R 2 of 0.992, a detection range of 0.5 to 10.0 mmol L-1, and an LOD of 0.299 mmol L-1. Calcium quantification using grayscale displayed an R 2 of 0.993, a detection range of 2.0 to 6.0 mmol L-1, and an LOD of 0.595 mmol L-1. Magnesium was best quantified using the hue color space, with an R 2 of 0.999, a detection range of 1.0 to 6.0 mmol L-1, and an LOD of 0.144 mmol L-1. Similarly, ammonium detection using the hue color space had an R 2 of 0.988, a range of 0.5 to 2.5 mmol L-1, and an LOD of 0.170 mmol L-1. This device enhances soil fertility assessment accessibility, supporting PA implementation and higher food production.

Preamplification-free viral RNA diagnostics with single-nucleotide resolution using MARVE, an origami paper-based colorimetric nucleic acid test

The evolution and mutation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are urgent concerns as they pose the risk of vaccine failure and increased viral transmission. However, affordable and scalable tools allowing rapid identification of SARS-CoV-2 variants are not readily available, which impedes diagnosis and epidemiological surveillance. Here we present a colorimetric nucleic acid assay named MARVE (multiplexed, preamplification-free, single-nucleotide-resolved viral evolution) that is convenient to perform and yields single-nucleotide resolution. The assay integrates nucleic acid strand displacement reactions with enzymatic amplification to colorimetrically sense viral RNA using a metal ion-incorporated DNA probe (TEprobe). We provide detailed guidelines to design TEprobes for discriminating single-nucleotide variations in viral RNAs, and to fabricate a test paper for the detection of SARS-CoV-2 variants of concern. Compared with other nucleic acid assays, our assay is preamplification-free, single-nucleotide-resolvable and results are visible via a color change. Besides, it is smartphone readable, multiplexed, quick and cheap ($0.30 per test). The protocol takes ~2 h to complete, from the design and preparation of the DNA probes and test papers (~1 h) to the detection of SARS-CoV-2 or its variants (30-45 min). The design of the TEprobes requires basic knowledge of molecular biology and familiarity with NUPACK or the Python programming language. The fabrication of the origami papers requires access to a wax printer using the CAD and PDF files provided or requires users to be familiar with AutoCAD to design new origami papers. The protocol is also applicable for designing assays to detect other pathogens and their variants.

Use of a rhodamine-based chelator in a microfluidic paper-based analytical device for the in-situ copper quantification in natural waters

This work describes the development of a microfluidic paper-based analytical device (μPAD) for the determination of copper in fresh and marine waters. A functionalized rhodamine-based chelator was synthesized and used as a chromogenic reagent, forming a highly intense pink complex with the analyte. The aim was to create a paper device that offers optimal performance and provides in-situ, rapid and cost-effective analysis in line with World Health Organization guidelines. The influence on the determination of several physical and chemical parameters was evaluated aiming to achieve the best performance. Under optimised conditions, a linear correlation was established in the range of 0.05-0.50 mg L-1 of copper, with a limit of detection of 10 μg L-1. The accuracy of the proposed method was assessed by comparing the results obtained with the developed μPAD and the results obtained with Inductively Coupled Plasma measurements (RE < 10 %). Recovery studies were also performed using different types of water samples with no need for any prior sample pre-treatment: tap, well, river and seawater. The average recovery percentage of 101 % (RSD = 4.3 %) was obtained, a clear indication of no multiplicative matrix interferences.

A dual-mode assay kit using a portable potentiostat connected to a smartphone via Bluetooth communication and a potential-power angle-based paper device susceptible for low-cost point-of-care testing of iodide and dopamine

BACKGROUND

Considering that the brain controls most of the body's activities, it is very important to measure the factors affecting its function, such as dopamine and iodide. Due to the growing population in the world, it is necessary to provide fast, cheap and accurate methods with the capability of on-site analysis and without the need for invasive sampling and operator skill. As a result, there is a strong desire to replace laboratory instruments with small sensors for point-of-care testing. Paper-based analytical devices (PADs) are one of the popular zero-cost approaches to achieve this goal.

RESULTS

We developed a simple and disposable diagnostic paper system based on electroanalytical and potential-power angle-based methods. First, we prepared an angle-based analytical system capable of performing semi-quantitative iodide analysis simply by reading the colored angle traveled. This system design is based on a channel containing complex reagents and two pencil-drawn electrodes to apply a constant voltage accelerating the anions migration. Meanwhile, a three-electrode system based on conductive pencil graphite is developed to measure dopamine concentration based on linear sweep voltammetry. For the quantitative analysis, the voltammetric data was wirelessly transmitted to a mobile device via Bluetooth communication. In this context, a power supply providing the required voltage for the migration of iodide ions, a portable potentiostat system, and a mobile application for measuring dopamine were developed. The calibration curves for I- and dopamine range from 3.5 × 10-4-47.0 × 10-4 and 10.0 × 10-6-1000.0 × 10-6 mol L-1 with LODs of 2.3 × 10-4 and 5.0 × 10-6 mol L-1, respectively.

SIGNIFICANCE AND NOVELTY

A new portable dual-mode voltage-assisted integrated PAD platform was designed for iodide and dopamine analysis. The characteristics of this device allow non-experts to carry out in-field analysis using sub-100 μL saliva sample with a time-to-result of <10 min along with reducing the overall cost and operational complexity.

Preparation of Reactive Indicator Papers Based on Silver-Containing Nanocomposites for the Analysis of Chloride Ions

In recent decades, metal-containing nanocomposites have attracted considerable attention from researchers. In this work, for the first time, a detailed analysis of the preparation of reactive indicator papers (RIPs) based on silver-containing nanocomposites derived from silver fumarate was carried out. Thermolysis products are silver-containing nanocomposites containing silver nanoparticles uniformly distributed in a stabilizing carbon matrix. The study of the optical properties of silver-containing nanocomposites made it possible to outline the prospects for their application in chemical analysis. RIPs were made by impregnating a cellulose carrier with synthesized silver fumarate-derived nanocomposites, which change their color when interacting with chlorine vapor. This made it possible to propose a method for the determination of chloride ions with preliminary oxidation to molecular chlorine, which is then separated from the solution by gas extraction. The subsequent detection of the active zone of RIPs using colorimetry makes it possible to identify mathematical dependences of color coordinates on the concentration of chloride ions. The red (R) color coordinate in the RGB (red-green-blue) system was chosen as the most sensitive and promising analytical signal. Calibration plots of exponential and linear form and their equations are presented. The limit of detection is 0.036 mg/L, the limits of quantification are 0.15-2.4 mg/L, and the time of a single determination is 25 min. The prospects of the developed technique have been successfully shown in the example of the analysis of the natural waters of the Don River, pharmaceuticals, and food products.

Immobilization of diphenylcarbazide on paper-based analytical devices for the pre-concentration and detection of chromium VI in water samples

A novel approach using a smartphone for the detection of Cr (VI) has been developed. In this context, two different platforms were designed for the detection of Cr (VI). The first one was synthesized via a crosslinking reaction of chitosan with 1,5-Diphenylcarbazide (DPC-CS). The obtained material was integrated into a paper to develop a new paper-based analytical device (DPC-CS-PAD). The DPC-CS-PAD exhibited high specificity toward Cr (VI). The second platform (DPC-Nylon PAD) was prepared by covalent immobilization of DPC onto a Nylon paper and then its analytical performances regarding Cr (VI) extraction and detection were evaluated. DPC-CS-PAD presented a linear range of 0.1-5 ppm with detection and quantification limits of about 0.04 and 0.12 ppm, respectively. The DPC-Nylon-PAD exhibited a linear response of 0.1-2.5 ppm with detection and quantification limits of 0.06 and 0.2 ppm, respectively. Furthermore, the developed platforms were effectively applied for testing the effect of the loading solution volume for trace Cr (IV) detection. For the DPC-CS material, a volume of 20 mL allowed the detection of 4 ppb of Cr (VI). In the case of DPC-Nylon-PAD, the loading volume of 1 mL permitted the detection of the critical concentration of Cr (VI) in water.

A 3D sliding-strip microfluidic device for the simultaneous determination of mta

A simple paper-based microfluidic device was fabricated to simultaneously detect multiple targets. Microfluidic paper-based analytical devices (μPAD) comprise a single-layer moving sliding PAD (SPAD) to control the flow channel switch together with a folding origami PAD (OPAD) to test the target analytes. The facile assembly without any splicing materials avoids cross-contamination and non-specific adsorption of joining materials that may be caused by multi-target detection. The concentration of Fe(III), Ni(II), Cr(VI), and nitrite in standard solutions and actual aqueous solutions was successfully determined using the designed μPAD. The μPAD was able to achieve LOD of 3.3 mg/L, 1.3 mg/L, 0.35 mg/L, 0.28 mg/L for Fe (III), Ni (II), Cr (VI), and nitrite, respectively. The designed SOPAD exhibits improved stability, with a deviation of less than 7% compared to conventional analytical methods (ICP-OES and UV). Our work demonstrates that this 3D PAD holds great promise and a wide scope in environmental monitoring, biochemical analysis, food testing and other testing industries.

Accuracy improvement via novel ratiometry design in distance-based microfluidic paper based analytical device: instrument-free point of care testing

Developing accurate, precise, instrument-free, and point-of-need microfluidic paper-based devices is highly significant in clinical diagnosis and biomedical analysis. In the present work, a ratiometric distance-based microfluidic paper-based analytical device (R-DB-μPAD), along with a three-dimensional (3D) multifunctional connector (spacer), was designed to improve the accuracy and detection resolution analyses. Specifically, the novel R-DB-μPAD was used for the accurate and precise detection of ascorbic acid (AA) as a model analyte. In this design, two channels were fabricated as detection zones, with a 3D spacer located between the sampling and detection zones to improve the detection resolution by preventing the reagents mixing from overspreading between these zones. Two probes for AA were used: Fe³⁺ and 1,10-phenanthroline were deposited in the first channel, and oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB) was added to the second channel. Accuracy improvement of this ratiometry-based design was achieved by enhancing the linearity range and reducing the volume dependency of the output signal. Moreover, the 3D connector improved the detection resolution by eliminating the systematic errors. Under the optimal conditions, the ratio of the distances of the color bands in the two channels was used to construct an analytical calibration curve in the range from 0.05 to 1.2 mM, with a limit of detection of 16 μM. The proposed R-DB-μPAD combined with the connector was successfully used for the detection of AA in orange juice and vitamin C tablets with satisfactory accuracy and precision. This work opens the door for multiplex analysis of various analytes in different matrices.

Molecular testing devices for on-site detection of E. coli in water samples

Escherichia coli (E. coli) cells are present in fecal materials that can be the main source for disease-causing agents in water. As a result, E. coli is recommended as a water quality indicator. We have developed an innovative platform to detect E. coli for monitoring water quality on-site by integrating paper-based sample preparation with nucleic acid isothermal amplification. The platform carries out bacterial lysis and DNA enrichment onto a paper pad through ball-based valves for fluid control, with no need of laboratory equipment, followed by loop-mediated isothermal amplification (LAMP) in a battery-operated coffee mug, and colorimetric detection. We have used the platform to detect E. coli in environmental water samples in about 1 h, with a limit of quantitation of 0.2 CFU/mL, and 3 copies per reaction. The platform was confirmed for detecting multiple E. coli strains, and for water samples of different salt concentrations. We validated the functions of the platform by analyzing recreational water samples collected near the Atlantic Ocean that contain different concentrations of salt and bacteria.

Gold Nanoparticles-Based Colorimetric Assays for Environmental Monitoring and Food Safety Evaluation

Recent years have witnessed an exponential increase in the research on gold nanoparticles (AuNPs)-based colorimetric sensors to revolutionize point-of-use sensing devices. Hence, this review is compiled focused on current progress in the design and performance parameters of AuNPs-based sensors. The review begins with the characteristics of AuNPs, followed by a brief explanation of synthesis and functionalization methods. Then, the mechanisms of AuNPs-based sensors are comprehensively explained in two broad categories based on the surface plasmon resonance (SPR) characteristics of AuNPs and their peroxidase-like catalytic properties (nanozyme). SPR-based colorimetric sensors further categorize into aggregation, anti-aggregation, etching, growth-mediated, and accumulation-based methods depending on their sensing mechanisms. On the other hand, peroxidase activity-based colorimetric sensors are divided into two methods based on the expression or inhibition of peroxidase-like activity. Next, the analytes in environmental and food samples are classified as inorganic, organic, and biological pollutants, and recent progress in detection of these analytes are reviewed in detail. Finally, conclusions are provided, and future directions are highlighted. Improving the sensitivity, reproducibility, multiplexing capabilities, and cost-effectiveness for colorimetric detection of various analytes in environment and food matrices will have significant impact on fast testing of hazardous substances, hence reducing the pollution load in environment as well as rendering food contamination to ensure food safety.

Microfluidic assessment of nutritional biomarkers: Concepts, approaches and advances

Among various approaches to understand the health status of an individual, nutritional biomarkers can provide valuable information, particularly in terms of deficiencies, if any, and their severity. Commonly, the approach revolves around molecular sciences, and the information gained can support prognosis, diagnosis, remediation, and impact assessment of therapies. Microfluidic platforms can offer benefits of low sample and reagent requirements, low cost, high precision, and lower detection limits, with simplicity in handling and the provision for complete automation and integration with information and communication technologies (ICTs). While several advances are being made, this work details the underlying concepts, with emphasis on different point-of-care devices for the analysis of macro and micronutrient biomarkers. In addition, the scope of using different wearable microfluidic sensors for real-time and noninvasive determination of biomarkers is detailed. While several challenges remain, a strong focus is given on recent advances, presenting the state-of-the-art of this field. With more such biomarkers being discovered and commercialization-driven research, trends indicate the wide prospects of this advancing field in supporting clinicians, food technologists, nutritionists, and others.

The Road to Unconventional Detections: Paper-Based Microfluidic Chips

Conventional detectors are mostly made up of complicated structures that are hard to use. A paper-based microfluidic chip, however, combines the advantages of being small, efficient, easy to process, and environmentally friendly. The paper-based microfluidic chips for biomedical applications focus on efficiency, accuracy, integration, and innovation. Therefore, continuous progress is observed in the transition from single-channel detection to multi-channel detection and in the shift from qualitative detection to quantitative detection. These developments improved the efficiency and accuracy of single-cell substance detection. Paper-based microfluidic chips can provide insight into a variety of fields, including biomedicine and other related fields. This review looks at how paper-based microfluidic chips are prepared, analyzed, and used to help with both biomedical development and functional integration, ideally at the same time.

Paper-based multicolor sensor for on-site quantitative detection of 2,4-dichlorophenoxyacetic acid based on alkaline phosphatase-mediated gold nanobipyramids growth and colorimeter-assisted method for quantifying color

On-site quantitative analysis of pesticides is important for food safety. Colorimetric gold nanobipyramids (AuNBPs) sensors are powerful methods for on-site detection. However, a single quantitative method and the instability of AuNBPs in solution limit the practicability of those sensors. Here, a paper-based multicolor AuNBPs sensor involved a colorimeter-assisted method for quantifying color was developed for quantitative detection of 2,4-dichlorophenoxyacetic acid (2,4-D), a common herbicide. The novelty of this study lies in developing a general paper-based quantitative on-site method (PQOM) for colorimetric AuNBPs sensors. Firstly, a paper-based analytical device (PAD) consisting of a nylon membrane, absorbent cotton layers, and two acrylic plates was fabricated to deposit AuNBPs. We demonstrated the PAD could improve the stability of AuNBPs and the detection sensitivity of AuNBPs sensors. Then, a handheld colorimeter was first used to quantify the color change of AuNBPs on the PAD based on the CIELab color space. Finally, as proof of concept, the PQOM was successfully employed to quantify 2,4-D by combining with an alkaline phosphatase-mediated AuNBPs growth method. In this method, 2,4-D specifically inhibited alkaline phosphatase activity to suppress the generation of l-ascorbic acid, thereby mediating AuNBPs growth. The developed sensor exhibited seven 2,4-D concentration-related colors and detected as low as 50 ng mL-1 2,4-D by naked-eye observation and 18 ng mL-1 2,4-D by a colorimeter. It was applied to detect 2,4-D in the spiked rice and apple samples with good recovery rates (91.8-112.0%) and a relative standard deviation (n = 5) < 5%. The success of this study provides a sensing platform for quantifying 2,4-D on site.

Microfluidic Paper-Based Analytical Devices for the Determination of Food Contaminants: Developments and Applications

Food safety is an issue that cannot be ignored at any time because of the great impact of food contaminants on people's daily life, social production, and the economy. Because of the extensive demand for high-quality food, it is necessary to develop rapid, reliable, and efficient devices for food contaminant detection. Microfluidic paper-based analytical devices (μPADs) have been applied in a variety of detection fields owing to the advantages of low-cost, ease of handling, and portability. This review systematically discusses the latest progress of μPADs, including the fundamentals of fabrication as well as applications in the detection of chemical and biological hazards in foods, hoping to provide suitable screening strategies for contaminants in foods and accelerating the technology transformation of μPADs from the lab into the field.

Microfluidics for detection of exosomes and microRNAs in cancer: State of the art

Exosomes are small extracellular vesicles with sizes ranging from 30-150 nanometers that contain proteins, lipids, mRNAs, microRNAs, and double-stranded DNA derived from the cells of origin. Exosomes can be taken up by target cells, acting as a means of cell-to-cell communication. The discovery of these vesicles in body fluids and their participation in cell communication has led to major breakthroughs in diagnosis, prognosis, and treatment of several conditions (e.g., cancer). However, conventional isolation and evaluation of exosomes and their microRNA content suffers from high cost, lengthy processes, difficult standardization, low purity, and poor yield. The emergence of microfluidics devices with increased efficiency in sieving, trapping, and immunological separation of small volumes could provide improved detection and monitoring of exosomes involved in cancer. Microfluidics techniques hold promise for advances in development of diagnostic and prognostic devices. This review covers ongoing research on microfluidics devices for detection of microRNAs and exosomes as biomarkers and their translation to point-of-care and clinical applications.

Speciation of inorganic arsenic in aqueous samples using a novel hydride generation microfluidic paper-based analytical device (µPAD)

The development of the first microfluidic paper-based analytical device (µPAD) for the speciation of inorganic arsenic in environmental aqueous samples as arsenite (As(III)) and arsenate (As(V)) which implements hydride generation on a paper platform is described. The newly developed µPAD has a 3D configuration and uses Au(III) chloride as the detection reagent. Sodium borohydride is used to generate arsine in the device’s sample zone by reducing As(III) in the presence of hydrochloric acid or both As(III) and As(V) (total inorganic As) in the presence of sulfuric acid. Arsine then diffuses across a hydrophobic porous polytetrafluoroethylene membrane into the device’s detection zone where it reduces Au(III) to Au nanoparticles. This results in a color change which can be related to the concentration of As(III) or total inorganic As (i.e., As(III) and As(V)) concentration. Under optimal conditions, the µPAD is characterized by a limit of detection of 0.43 mg L⁻¹ for total inorganic As (As(III) + As(V)) and 0.41 mg L⁻¹ for As(III) and a linear calibration range in both cases of 1.2–8.0 mg As L⁻¹. The newly developed µPAD-based method was validated by applying it to groundwater and freshwater samples and comparing the results with those obtained by conventional atomic spectrometric techniques.

Progress toward a Simplified UTI Diagnostic: Pump-Free Magnetophoresis for E. coli Detection

Urinary tract infections (UTIs) are one of the most common infections across the world and can lead to serious complications such as sepsis if not treated in a timely manner. Uropathogenic Escherichia coli account for 75% of all UTIs. Early diagnosis is crucial to help control UTIs, but current culturing methods are expensive and time-consuming and lack sensitivity. The existing point-of-care methods fall short because they rely on indirect detection from elevated nitrates in urine rather than detecting the actual bacteria causing the infection. Magnetophoresis is a powerful method used to separate and/or isolate cells of interest from complex matrices for analysis. However, magnetophoresis typically requires complex and expensive instrumentation to control flow in microfluidic devices. Coupling magnetophoresis with microfluidic paper-based analytical devices (μPADs) enables pump-free flow control and simple and low-cost operation. Early magnetophoresis μPADs showed detection limits competitive with traditional methods but higher than targets for clinical use. Here, we demonstrate magnetophoresis using hybrid μPADs that rely on capillary action in hydrophilic polyethylene terephthalate channels combined with paper pumps. We were able to detect E. coli with a calculated limit of detection of 2.40 × 10² colony-forming units per mL.

Paper-based microfluidic colorimetric sensor on a 3D printed support for quantitative detection of nitrite in aquatic environments

To ensure safe drinking water, it is necessary to have a simple method by which the probable pollutants are detected at the point of distribution. Nitrite contamination in water near agricultural locations could be an environmental concern due to its deleterious effects on the human population. The development of a frugal paper-based microfluidic sensor could be desirable to achieve the societal objective of providing safe drinking water. This work describes the development of a facile and cost-effective microfluidic paper-based sensor for quantitative estimation of nitrite in aquatic environments. A simple punching machine was used for fabrication and rapid prototyping of paper-based sensors without the need of any specialized equipment or patterning techniques. A reusable 3D printed platform served as the support for simultaneous testing of multiple samples. The nitrite estimation was carried out with smartphone-assisted digital image acquisition and colorimetric analysis. Under optimized experimental conditions, the variation in average grayscale intensity with concentration of nitrite was linear in the range from 0.1 to 10 ppm. The limits of detection and quantitation were 0.12 ppm and 0.35 ppm respectively. The reproducibility, expressed as relative standard deviation was 1.31%. The selectivity of nitrite detection method was determined by performing interference studies with commonly existing co-ions in water, such as bicarbonates, chloride and sulphate. The paper-based sensor was successfully applied for estimation of nitrite in actual water samples and showed high recoveries in the range of 83.5-109%. The results were in good agreement with those obtained using spectrophotometry. The developed paper-based sensor method, by virtue of its simplicity, ease of fabrication and use, could be readily extended for detection of multiple analytes in resource-limited settings.

Wax-Printed Fluidic Controls for Delaying and Accelerating Fluid Transport on Paper-Based Analytical Devices

In this work, we explore a new method for controlling fluid transport rate on paper-based analytical devices that enables both the delay and the acceleration of fluid flow. The delays were incorporated by wax printing linear patterns of variable width within the flow channel and melted to penetrate the paper. In this manner, the surface tension of the fluid decreases while its contact angle increases, causing a pressure drop along the fluid path that reduces capillary flow. The acceleration of flow was accomplished by overlaying hydrophobic stripes (prepared by wax printing and melting the wax) on the hydrophilic path (top or top–bottom). In this manner, the fluid was repelled from two dimensions (vertical and applicate), increasing the flow rate. The combination of these methods on the same devices could adjust wicking time in intermediate time internals. The method enabled a wide timing of fluid transport, accomplishing a change in wicking times that extended from −41% to +259% compared to open paper channels. As a proof of concept, an enzymatic assay of glucose was used to demonstrate the utility of these fluid control methods in kinetic methods of analysis.

Automated Immunoassay Performed on a 3D Microfluidic Paper-Based Device for Malaria Detection by Ambient Mass Spectrometry

Microfluidic paper-based analytical devices (μPADs) are emerging as a prominent platform for disease detection, specifically in developing countries. This paper device offer simplicity and affordability not typically seen in centralized laboratory settings. However, detection limits in μPADs are inadequate and often require test results to be read within a specific time interval to ensure accuracy. To overcome these challenges, we are developing an on-chip mass spectrometry (MS) detection strategy for immunoassays performed on paper substrates. Herein, we present our initial results from a proof-of-concept study toward the development of μPADs capable of storing immunoassay reagents within the confinements of the 3D device, automatic splitting of biofluid into four individual test zones, immuno-capture of the disease biomarker, and on-chip MS detection of the captured species. The reported study encourages the development of point-of-care and direct-to-customer testing using disposable μPADs to collect samples, followed by sensitive analysis using portable MSs. We demonstrate this capability using malaria Plasmodium falciparum histidine-rich protein 2 (PfHRP2) antigen detection.

Flow-Injection Methods in Water Analysis-Recent Developments

Widespread demand for the analysis and control of water quality and supply for human activity and ecosystem sustainability has necessitated the continuous improvement of water analysis methods in terms of their reliability, efficiency, and costs. To satisfy these requirements, flow-injection analysis using different detection methods has successfully been developed in recent decades. This review, based on about 100 original research papers, presents the achievements in this field over the past ten years. Various methodologies for establishing flow-injection measurements are reviewed, together with microfluidics and portable systems. The developed applications mostly concern not only the determination of inorganic analytes but also the speciation analysis of different elements, and the determination of several total indices of water quality. Examples of the determination of organic residues (e.g., pesticides, phenolic compounds, and surfactants) in natural surface waters, seawater, groundwater, and drinking water have also been identified. Usually, changes in the format of manual procedures for flow-injection determination results in the improvement of various operational parameters, such as the limits of detection, the sampling rate, or selectivity in different matrices.

Development of a simplified spectrophotometric method for nitrite determination in water samples

A new eco-friendly, rapid, and sensitive spectrophotometric method was developed to determine small quantities of nitrite, based on a diazotization mechanism. In an acidic solution, sulfathiazole was first diazotized with sodium nitrite, followed by adding phosphate buffer to form a yellow-colored compound, which showed maximum absorption at 450 nm, without the need for the addition of coupling agents such as N-(1-naphthyl) ethylenediamine. The effects of reagents amount and the optimal experimental conditions were examined by Central composite design. The simplified method presented a wide linear range of nitrite between 0.091 μg mL^-1 and 1.47 μg mL^-1, a sensitivity of 0.447 Abs mL µg^-1, a determination coefficient of 0.998, and a low limit of detection of 0.053 μg mL^-1. The simplified method was found to be comparable to the Griess method. It was evaluated for the measurements of nitrite using the accuracy profile approach. The validation procedure results established that 80% of the future results would be within the acceptability limit of 10% over the validation domain ranging from 0.174 μg mL^-1 to 1.37 μg mL^-1. The developed method was furtherly applied in the determination of nitrite using a developed paper-based analytical device that detected a nitrite concentration of 3 μg mL^-1 which is considered by the World Health Organization to be the maximal permissible limit of nitrite in drinking water.

Tuning the Structure, Conductivity, and Wettability of Laser-Induced Graphene for Multiplexed Open Microfluidic Environmental Biosensing and Energy Storage Devices

The integration of microfluidics and electrochemical cells is at the forefront of emerging sensors and energy systems; however, a fabrication scheme that can create both the microfluidics and electrochemical cells in a scalable fashion is still lacking. We present a one-step, mask-free process to create, pattern, and tune laser-induced graphene (LIG) with a ubiquitous CO₂ laser. The laser parameters are adjusted to create LIG with different electrical conductivity, surface morphology, and surface wettability without the need for postchemical modification. Such definitive control over material properties enables the creation of LIG-based integrated open microfluidics and electrochemical sensors that are capable of dividing a single water sample along four multifurcating paths to three ion selective electrodes (ISEs) for potassium (K⁺), nitrate (NO₃^-), and ammonium (NH₄⁺) monitoring and to an enzymatic pesticide sensor for organophosphate pesticide (parathion) monitoring. The ISEs displayed near-Nernstian sensitivities and low limits of detection (LODs) (10^-5.01 M, 10^-5.07 M, and 10^-4.89 M for the K⁺, NO₃^-, and NH₄⁺ ISEs, respectively) while the pesticide sensor exhibited the lowest LOD (15.4 pM) for an electrochemical parathion sensor to date. LIG was also specifically patterned and tuned to create a high-performance electrochemical micro supercapacitor (MSC) capable of improving the power density by 2 orders of magnitude compared to a Li-based thin-film battery and the energy density by 3 orders of magnitude compared to a commercial electrolytic capacitor. Hence, this tunable fabrication approach to LIG is expected to enable a wide range of real-time, point-of-use health and environmental sensors as well as energy storage/harvesting modules.

Dip-and-Fold Device: A Paper-Based Testing Platform for Rapid Assessment of Insecticides in Water Samples

The contamination of water and food in agricultural areas, where an enormous volume of pesticides is widely employed to enhance crop production, is a challenging reality. The rapid assessment of these contaminants is fundamental to assure water and food quality and safety, particularly for local community members. This work presents a nonexpensive and easy-operational paper-based testing device for the fast detection of insecticides (carbamates and organophosphates) in water samples. The structural design "dip-and-fold" allows us to carry out the analysis without introducing reagents or samples. The device is prepared using different high-quality papers to support the active acetylcholinesterase (AChE) and the customized chemical formulation for colorimetric detection. The chemical principle is based on the AChE inhibition reaction and Ellman's method. The experiments using standard solutions of carbofuran, propoxur, and chlorpyriphos indicated satisfactory detection at concentrations between 0.1 and 0.0001 mM, and the color results are revealed within 10 min. Therefore, this technique represents a promising alternative for implementing low-cost and efficient water monitoring and management solutions.

Online and offline preconcentration techniques on paper-based analytical devices for ultrasensitive chemical and biochemical analysis: A review

Microfluidic paper-based analytical devices (μPADs) have attracted much attention over the past decade. They embody many advantages, such as abundance, portability, cost-effectiveness, and ease of fabrication, making them superior for clinical diagnostics, environmental monitoring, and food safety assurance. Despite these advantages, μPADs lack the high sensitivity to detect many analytes at trace levels than other commercial analytical instruments such as mass spectrometry. Therefore, a preconcentration step is required to enhance their sensitivity. This review focuses on the techniques used to separate and preconcentrate the analytes onto the μPADs, such as ion concentration polarization, isotachophoresis, and field amplification sample stacking. Other separations and preconcentration techniques, including liquid-solid and liquid-liquid extractions coupled with μPADs, are also reviewed and discussed. In addition, the fabrication methods, advantages, disadvantages, and the performance evaluation of the μPADs concerning their precision and accuracy were highlighted and critically assessed. Finally, the challenges and future perspectives have been discussed.

Citizen-led sampling to monitor phosphate levels in freshwater environments using a simple paper microfluidic device

Contamination of waterways is of increasing concern, with recent studies demonstrating elevated levels of antibiotics, antidepressants, household, agricultural and industrial chemicals in freshwater systems. Thus, there is a growing demand for methods to rapidly and conveniently monitor contaminants in waterways. Here we demonstrate how a combination of paper microfluidic devices and handheld mobile technology can be used by citizen scientists to carry out a sustained water monitoring campaign. We have developed a paper-based analytical device and a 3 minute sampling workflow that requires no more than a container, a test device and a smartphone app. The contaminant measured in these pilots are phosphates, detectable down to 3 mg L^-1. Together these allow volunteers to successfully carry out cost-effective, high frequency, phosphate monitoring over an extended geographies and periods.

Design of an Integrated Microfluidic Paper-Based Chip and Inspection Machine for the Detection of Mercury in Food with Silver Nanoparticles

For most of the fast screening test papers for detecting Hg2+, the obtained results are qualitative. This study developed an operation for the μPAD and combined it with the chemical colorimetric method. Silver nanoparticle (AgNP) colloids were adopted as the reactive color reagent to combine and react with the Hg standards on the paper-based chip. Then, the RGB values for the color change were used to establish the standard curve (R2 > 0.99). Subsequently, this detection system was employed for the detection tests of actual samples, and the detected RGB values of the samples were substituted back to the formula to calculate the Hg2+ contents in the food. In this study, the Hg2+ content and recovery rate in commercially available packaged water and edible salts were measured. The research results indicate that a swift, economical, and simple detection method for Hg2+ content in food has been successfully developed.

Simple, fast, and instrumentless fabrication of paper analytical devices by novel contact stamping method based on acrylic varnish and 3D printing

A new contact stamping method for fabrication of paper-based analytical devices (PADs) is reported. It uses an all-purpose acrylic varnish and 3D-printed stamps to pattern hydrophobic structures on paper substrates. The use of 3D printing allows quickly prototyping the desired stamp shape without resorting to third-party services, which are often expensive and time consuming. To the best of our knowledge, this is the first report regarding the use of this material for creation of hydrophobic barriers in paper substrates, as well as this 3D printing-based stamping method. The acrylic varnish was characterized and the features of the stamping method were studied. The PADs developed here presented better compatibility with organic solvents and surfactants compared with similar protocols. Furthermore, the use of this contact stamping method for fabrication of paper electrochemical devices was also possible, as well as multiplexed microfluidic devices for lateral flow testing. The analytical applicability of the varnish-based PADs was demonstrated through the image-based colorimetric quantification of iron in pharmaceutical samples. A limit of detection of 0.61 mg L^-1 was achieved. The results were compared with spectrophotometry for validation and presented great concordance (relative error was < 5% and recoveries were between 104 and 108%). Thus, taking into account the performance of the devices explored here, we believe this novel contact stamping method is a very interesting alternative for production of PADs, exhibiting great potentiality. In addition, this work brings a new application of 3D printing in analytical sciences.

Efficient and portable cellulose-based colorimetric test paper for metal ion detection

Fabrication of metal ion detection materials generally involved problems such as high cost and complicated processes of pretreatment and operation. Herein, a novel colorimetric test paper for metal ions detection was developed based on functionalized cellulose fibers. Acetoacetyl groups were introduced on cellulose fibers by a surface esterification process. The obtained cellulose acetoacetate (CAA) fibers were made into CAA paper via a paper-making process. The CAA paper possessed robust mechanical property, thermal stability selectivity and rapid response to Fe³⁺ and Cu²⁺ ions, with an obvious naked-eye color change within 5 s. The mechanism of this visual recognition for metal ions due to that the acetoacetyl groups coordination chelated with metal ion to form six-membered ring structure, further leading to the color change of the materials. It provided a facile and universal method to prepare efficient and portable cellulose-based test paper, which has great potential in metal ion detection field.

Reliable environmental trace heavy metal analysis with potentiometric ion sensors - reality or a distant dream

Over two decades have passed since polymeric membrane ion-selective electrodes were found to exhibit sufficiently lower detection limits. This in turn brought a great promise to measure trace level concentrations of heavy metals using potentiometric ion sensors at environmental conditions. Despite great efforts, trace analysis of heavy metals using ion-selective electrodes at environmental conditions is still not commercially available. This work will predominantly concentrate on summarizing and evaluating prospects of using potentiometric ion sensors in view of environmental determination of heavy metals in on-site and on-line analysis modes. Challenges associated with development of reliable potentiometric sensors to be operational in environmental conditions will be discussed and reasoning behind unsuccessful efforts to develop potentiometric on-site and on-line environmental ion sensors will be explored. In short, it is now clear that solely lowering the detection limit of the ion-selective electrodes does not guarantee development of successful sensors that would meet the requirement of environmental matrices over long term usage. More pressing challenges of the properties and the performance of the potentiometric sensors must be addressed first before considering extending their sensitivity to low analyte concentrations. These are, in order of importance, selectivity of the ion-selective membrane to main ion followed by the membrane resistance to parallel processes, such as water ingress to the ISM, light sensitivity, change in temperature, presence of gasses in solution and pH and finally resistance of the ion-selective membrane to fouling. In the future, targeted on-site and on-line environmental sensors should be developed, addressing specific environmental conditions. Thus, ion-selective electrodes should be developed with the intention to be suitable to the operational environmental conditions, rather than looking at universal sensor design validated in the idealized and simple sample matrices.

Rapid Detection of Dimethoate in Soybean Samples by Microfluidic Paper Chips Based on Oil-Soluble CdSe Quantum Dots

Given the imperative of monitoring organophosphorus pesticides (OPs) residues in the ecosystem, here a novel, facile and sensitive fluorescence sensor is presented for the rapid detection of dimethoate. In this work, surface molecularly imprinted polymer (SMIP) and microfluidic technology had been introduced to enhance the selectivity and portability of the described methodology. Oil-soluble CdSe quantum dots (QDs) synthesized in a green way were used as fluorescent material for the selective detection of dimethoate on the basis of static quenching and photoinduced electron transfer mechanism. Among many kinds of paper materials, glass fiber paper was used as the novel substrate of paper chip due to low pristine fluorescence and better performance when combining CdSe QDs. In the process of molecular imprinting, the interaction between several functional monomers and dimethoate molecule was investigated and simulated theoretically by software to improve the selectivity of the sensor. Consequently, the fabricated novel detection platform could effectively respond to dimethoate in 10 min with the concentration range of 0.45–80 μmol/L and detection limit of 0.13 μmol/L. The recovery in the spiked experiment soybean sample was in an acceptable range (97.6–104.1%) and the accuracy was verified by gas chromatography-mass spectrometry, which signified the feasibility and potential in food sampling.

Paper-Based Analytical Devices for Colorimetric and Luminescent Detection of Mercury in Waters: An Overview

Lab-on-paper technologies, also known as paper-based analytical devices (PADs), have received increasing attention in the last years, and nowadays, their use has spread to virtually every application area, i.e., medical diagnostic, food safety, environmental monitoring, etc. Advantages inherent to on-field detection, which include avoiding sampling, sample preparation and conventional instrumentation in central labs, are undoubtedly driving many developments in this area. Heavy metals represent an important group of environmental pollutants that require strict controls due to the threat they pose to ecosystems and human health. In this overview, the development of PADs for Hg monitoring, which is considered the most toxic metal in the environment, is addressed. The main emphasis is placed on recognition elements (i.e., organic chromophores/fluorophores, plasmonic nanoparticles, inorganic quantum dots, carbon quantum dots, metal nanoclusters, etc.) employed to provide suitable selectivity and sensitivity. The performance of both microfluidic paper-based analytical devices and paper-based sensors using signal readout by colorimetry and luminescence will be discussed.

Vertical-Flow Paper Sensor for On-Site and Prompt Evaluation of Chloride Contamination in Concrete Structures

Corrosion occurring in reinforced concrete has turned into a primary concern of the current century, concrete being the most ubiquitous and predominant material used in the construction industry. Among the many interrelated processes that trigger corrosion of metallic reinforcements, the penetration of chloride ions into the concrete matrix is the most insidious threat. Herein, we developed the first electrochemical device entirely made of paper that allows for the direct, prompt, and noninvasive evaluation of free chloride ion contamination in concrete-based constructions. Our device is based on a three-layer wax-modified filter paper, consisting of two Ag/AgCl screen-printed electrodes that are interfaced by a junction pad in a sandwich-like configuration. Filter paper allows for generating a vertical-flow potentiometric device capable of measuring the electrochemical potential between two solutions containing different concentrations of chloride ions, which are separately drop-cast on the top and bottom layers. After demonstrating the analytical performance of the device, the same principle was applied to the evaluation of the chloride contents in different concrete samples, exploiting paper as a suitable interfacing material for potentiometric measurements on the cement solid surface. Laboratory-prepared concrete samples with known chloride contents were first assessed, and then, the paper-based vertical-flow device was applied to real concrete structures at the Giacomo Manzù Museum (Ardea, Italy) for the evaluation of chloride contamination caused by the proximity to the seaside. The capability of our device to provide timely warning of the risk conditions of concrete-based artifacts was demonstrated.

New microfluidic paper-based analytical device for iron determination in urine samples

Iron is an important micronutrient involved in several mechanisms in the human body and can be an important biomarker. In this work, a simple and disposable microfluidic paper-based analytical device (µPAD) was developed for the quantification of iron in urine samples. The detection was based on the colorimetric reaction between iron(II) and bathophenanthroline and the reduction of iron(III) to iron(II) with hydroxylamine. The developed µPAD enabled iron determination in the range 0.07-1.2 mg/L, with a limit of detection of 20 µg/L and a limit of quantification of 65 µg/L, thus suitable for the expected values in human urine. Additionally, targeting urine samples, the potential interference of the samples color was overcome by incorporating a sample blank assessment for absorbance subtraction. Stability studies revealed that the device was stable for 15 days prior to usage and that the formed colored product was stable for scanning up to 3 h. The accuracy of the developed device was established by analyzing urine samples (#26) with the developed µPAD and with the atomic absorption spectrometry method; the relative deviation between the two sets of results was below 9.5%.

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.