Paper-based microfluidic analytical devices for colorimetric detection of toxic ions: A review

Microfluidic sensors for the detection of emerging contaminants in water: A review

In recent years, numerous emerging contaminants have been identified in surface water, groundwater, and drinking water. Developing novel sensing methods for detecting diverse emerging pollutants in water is urgently needed, as even at low concentrations, these pollutants can pose a serious threat to human health and environmental safety. Traditional testing methods are based on laboratory equipment, which is highly sensitive but complex to operate, costly, and not suitable for on-site monitoring. Microfluidic sensors offer several benefits, including rapid evaluation, minimal sample usage, accurate liquid manipulation, compact size, automation, and in-situ detection capabilities. They provide promising and efficient analytical tools for high-performance sensing platforms in monitoring emerging contaminants in water. In this paper, recent research advances in microfluidic sensors for the detection of emerging contaminants in water are reviewed. Initially, a concise overview is provided about the various substrate materials, corresponding microfabrication techniques, different driving forces, and commonly used detection techniques for microfluidic devices. Subsequently, a comprehensive analysis is conducted on microfluidic detection methods for endocrine-disrupting chemicals, pharmaceuticals and personal care products, microplastics, and perfluorinated compounds. Finally, the prospects and future challenges of microfluidic sensors in this field are discussed.

Nanomaterial-based optical colorimetric sensors for rapid monitoring of inorganic arsenic species: a review

Health concerns about the toxicity of arsenic compounds have therefore encouraged the development of new analytical tools for quick monitoring of arsenic in real samples with improved sensitivity, selectivity, and reliability. An overview of advanced optical colorimetric sensor techniques for real-time monitoring of inorganic arsenic species in the environment is given in this review paper. Herein, several advanced optical colorimetric sensor techniques for arsenite (As+3) and arsenate (As+5) based on doping chromogenic dyes/reagents, biomolecule-modified nanomaterials, and arsenic-binding ligand tethered nanomaterials are introduced and discussed. This review also highlights the benefits and limitations of the colorimetric sensor for arsenic species. Finally, prospects and future developments of an optical colorimetric sensor for arsenic species are also proposed. For future study in this sector, particularly for field application, authors recommend this review paper will be helpful for readers to understand the design principles and their corresponding sensing mechanisms of various arsenic optical colorimetric sensors.

Microfluidic Platforms for Real-Time In Situ Monitoring of Biomarkers for Cellular Processes

Cellular processes are mechanisms carried out at the cellular level that are aimed at guaranteeing the stability of the organism they comprise. The investigation of cellular processes is key to understanding cell fate, understanding pathogenic mechanisms, and developing new therapeutic technologies. Microfluidic platforms are thought to be the most powerful tools among all methodologies for investigating cellular processes because they can integrate almost all types of the existing intracellular and extracellular biomarker-sensing methods and observation approaches for cell behavior, combined with precisely controlled cell culture, manipulation, stimulation, and analysis. Most importantly, microfluidic platforms can realize real-time in situ detection of secreted proteins, exosomes, and other biomarkers produced during cell physiological processes, thereby providing the possibility to draw the whole picture for a cellular process. Owing to their advantages of high throughput, low sample consumption, and precise cell control, microfluidic platforms with real-time in situ monitoring characteristics are widely being used in cell analysis, disease diagnosis, pharmaceutical research, and biological production. This review focuses on the basic concepts, recent progress, and application prospects of microfluidic platforms for real-time in situ monitoring of biomarkers in cellular processes.

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.

Paper-based analytical device for sensitive colorimetric determination of sulfonamides in pharmaceutical samples

Sulfa drugs are frequently used to treat infections, particularly in antibiotic resistant people. There are several techniques available to determine sulfa drugs, however, they are laborious operation, reagent consumption, expensive, and need specialized types of equipment. Here, a new, very simple and inexpensive paper-based analytical device described for the determination of five sulfa drugs: sulfacetamide, sulfadiazine, sulfamerazine, sulfamethoxazole, and sulfathiazole in pharmaceutical preparations. The method is a one-step reaction, based on the colorimetric reaction between acid-hydrolyzed sulfa drugs and 4-dimethylaminobenzaldehyde. Using a smartphone, the RGB value of color intensity was used as an analytical signal. The paper-based device displayed linear ranges of 0.10-5.00 µg mL-1, linear correlations ranging from 0.9903 to 0.9972, limits of detection 0.0030 to 0.0082 µg mL-1, and RSD of ≤0.258 under optimal conditions. The suggested approach was applied for determining five sulfa drugs in pharmaceutical formulations. This approach is appropriate for pharmaceutical applications since it is inexpensive, simple to utilize, sensitive, and selective.

Rapid and easily identifiable blood typing on microfluidic cotton thread-based analytical devices

In this study, we present a novel swing-elution-based method to achieve rapid, cost-effective, and easily identifiable blood typing assays. Specifically, the method aims to swing the microfluidic cotton thread-based analytical devices (μCTADs) in PBS solution to effectively elute free red blood cells (RBCs) and allow large agglutinated RBCs to remain to precisely determine the blood type. In order to ensure an easily identifiable blood typing assay, fast swing mode needs to be used, and the elution time is evaluated to be >50 seconds. The created μCTADs have been used to successfully classify ABO and RhD blood types in 56 blood samples. Finally, in order to enhance the convenience and portability of blood typing, a blood-typing chip that utilizes a PBS liquid bridge to effectively elute the free RBCs is designed and fabricated based on the above swing-elution principle. Compared with the traditional wicking-elution methods that rely on the wicking effect to weakly elute the RBCs, our method possesses a stronger elution effect to remove the free RBCs inside the inter-fiber gaps or adhered to the fiber surface, resulting in effectively enhancing the identifiability of the elution results and minimizing user interpretation error. Given the simplicity of the blood typing method, we believe that our blood typing method has great potential to be widely applied in resource-limited and developing regions.

Digital colorimetric sensor for assessment of iodate in food-grade salt based on paper-based analytical device

A digital colorimetric sensor was developed for the on-site measurement of iodate in food-grade salt. It involved a paper-based analytical device (PAD) containing an immobilized iodide/starch reagent and a smartphone as the detection system. Iodate ions produce a blue-black coloration on the PAD, the smartphone captures a digital image of the PAD, and an installed mobile app performs an analysis to generate color values related to the intensity of the color in the PAD. Parameters in the image acquisition and PAD preparation were optimized, and a matrix-matched calibration plot was employed for quantification. The plot exhibited a linear response in the concentration range of 10 to 100 mg iodate kg^-1 of salt, and a detection limit of 2.20 mg kg^-1 was determined. The present method was applied to analyze real samples, and the results agreed well with those obtained using the reference method at the 95% confidence level.

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.

Syringe Paper-Based Analytical Device for Thiamazole Detection by Hedysarum Polysaccharides-Mediated Silver Nanoparticles

In this paper, silver nanoparticles (AgNPs) were successfully green-synthesized for the first time using Hedysarum polysaccharide (HPS) as a reducing agent, stabilizer, and modifier (HPS-AgNP). Thiamazole could induce the aggregation of HPS-AgNPs in the residue on a cellulose membrane. A syringe paper-based analytical device was creatively established to ensure the tightness, stability, and good repeatability of the test. The color information remaining on the cellulose membrane was converted into gray values using ImageJ software. Hence, the linear regression curve for thiamazole was established as y = 1 + 0.179x with a detection limit (LOD) of 24.6 nM in the relatively wide range of 0.1~10 μM. This syringe paper-based analytical device was successfully applied to the biological samples.

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.

Development of a new electrochemical method for the determination of copper(ii) at trace levels in environmental and food samples

This paper presents the fabrication of a new modified carbon paste electrode (CPE) with N 1-hydroxy-N 1,N 2-diphenylbenzamidine (HDPBA) and functionalized multi-walled carbon nanotubes (MWCNTs) (HDPBA-MWCNTs/CPE) for highly sensitive and selective determination of Cu(ii) using the square wave anodic stripping voltammetry (SWASV) technique. The fabricated electrode was characterized using various spectroscopic techniques to study its morphological, structural, and electrochemical properties. The accumulation of Cu(ii) on the surface of HDPBA-MWCNTs/CPE was done in 0.1 M ammonium chloride (NH4Cl, pH 5) solution at an applied potential of -0.70 V versus Ag/AgCl for 180 s, followed by electrochemical stripping in the positive scan of the voltammetry after a resting time of 10 s. The developed HDPBA-MWCNTs/CPE was found to be highly selective, sensitive and reproducible. At optimal conditions of the experiment, the proposed method exhibited a very low limit of detection (0.0048 nM Cu(ii)), a wide linear dynamic range (0.00007-1.5000 μM Cu(ii)), and good reproducibility with relative standard deviation (RSD) value of 3.7%. The effect of various foreign ions on the voltammetric response of Cu(ii) was investigated and the electrode was found to be highly selective to Cu(ii). The practical applicability of the proposed HDPBA-MWCNTs/CPE was studied by applying the electrode for the quantification of Cu(ii) contents in environmental water (wastewater and tap water), soft drink (Fanta and Sprite), and food supplement (commercially available multi-mineral/vitamin tablets) samples. The present method was validated with atomic absorption spectrometry (AAS). The results found from the two methods are in good agreement with a 95% confidence level.

Porous Cellulose Substrate Study to Improve the Performance of Diffusion-Based Ionic Strength Sensors

Microfluidic paper-based analytical devices (µPADs) are leading the field of low-cost, quantitative in-situ assays. However, understanding the flow behavior in cellulose-based membranes to achieve an accurate and rapid response has remained a challenge. Previous studies focused on commercial filter papers, and one of their problems was the time required to perform the test. This work studies the effect of different cellulose substrates on diffusion-based sensor performance. A diffusion-based sensor was laser cut on different cellulose fibers (Whatman and lab-made Sisal papers) with different structure characteristics, such as basis weight, density, pore size, fiber diameter, and length. Better sensitivity and faster response are found in papers with bigger pore sizes and lower basis weights. The designed sensor has been successfully used to quantify the ionic concentration of commercial wines with a 13.6 mM limit of detection in 30 s. The developed µPAD can be used in quantitative assays for agri-food applications without the need for any external equipment or trained personnel.

Metal-organic frameworks for pharmaceutical and biomedical applications

Metal-organic framework (MOF) materials provide unprecedented opportunities for evaluating valuable compounds for various medical applications. MOFs merged with biomolecules, used as novel biomaterials, have become particularly useful in biological environments. Bio-MOFs can be promising materials in the global to avoid utilization above toxicological substances. Bio-MOFs with crystallin and porosity nature offer flexible structure via bio-linker and metal node variation, which improves their wide applicability in medical science.

A paper-based microfluidic sensor array combining molecular imprinting technology and carbon quantum dots for the discrimination of nitrophenol isomers

Paper-based microfluidic analytical devices (μPADs) have recently attracted attention as a rapid test kit owing to their low cost and nonrequirement for external driving pump. However, low accuracy and poor anti-interference ability of μPADs under complex detection condition limit their practical applications. Here, we present a facile way to prepare a novel fluorescence sensor-array μPAD for multi-analyte discrimination based on molecular imprinting technology, and its sensing behavior was studied by using three nitrophenol (NP) isomers (2-, 3-, and 4-NP) as the testing models. Carbon quantum dots (CQDs) emitting blue light were grafted on glass-fiber paper, followed by in-situ modification of three types of molecularly imprinted polymers (MIPs) with 2-, 3-, and 4-NP as template. Each sensing unit on the array showed differential yet cross-reactive binding affinity to NP isomers, resulting in distinct fluorescence quenching efficiency. Thus, precise distinguishment of the three NPs was realized with the MIPs/CQDs/paper-based sensor array. Furthermore, the discrimination ability of the platform was evaluated in mixtures of the NP isomers. Practicability of this apparatus was validated by identification of blind samples and 100% accuracy was achieved. The μPAD has proven to be highly sensitive and accurate, which will serve as an ideal analytical tool in the fields of environment monitoring, disease prognosis, food safety and so on.

Biosensors and Microfluidic Biosensors: From Fabrication to Application

Biosensors are ubiquitous in a variety of disciplines, such as biochemical, electrochemical, agricultural, and biomedical areas. They can integrate various point-of-care applications, such as in the food, healthcare, environmental monitoring, water quality, forensics, drug development, and biological domains. Multiple strategies have been employed to develop and fabricate miniaturized biosensors, including design, optimization, characterization, and testing. In view of their interactions with high-affinity biomolecules, they find application in the sensitive detection of analytes, even in small sample volumes. Among the many developed techniques, microfluidics have been widely explored; these use fluid mechanics to operate miniaturized biosensors. The currently used commercial devices are bulky, slow in operation, expensive, and require human intervention; thus, it is difficult to automate, integrate, and miniaturize the existing conventional devices for multi-faceted applications. Microfluidic biosensors have the advantages of mobility, operational transparency, controllability, and stability with a small reaction volume for sensing. This review addresses biosensor technologies, including the design, classification, advances, and challenges in microfluidic-based biosensors. The value chain for developing miniaturized microfluidic-based biosensor devices is critically discussed, including fabrication and other associated protocols for application in various point-of-care testing applications.

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.

A Three-Reagent “Green” Paper-Based Analytical Device for Solid-Phase Spectrometric and Colorimetric Determination of Dihydroquercetin

Microfluidic paper-based analytical devices (µPADs) represent one of the promising green analytical strategies for low-cost and simple determination of various analytes. The actual task is the development of such devices for quantitation of antioxidants, e.g., flavonoids. In this paper, possibilities of a novel three-reagent µPAD including silver nitrate, 4-nitrophenyldiazonium tetrafluoroborate, and iron(III) chloride as reagents are assessed with respect to the determination of dihydroquercetin. It is shown that all the three reagents produce different colorimetric responses that can be detected by a mini-spectrophotometer–monitor calibrator or by a smartphone. The method is applicable to direct measuring high contents of dihydroquercetin (the linearity range is 0.026–1 mg mL⁻¹, and the limit of detection is 7.7 µg mL⁻¹), which is favorable for many dietary supplements. The analysis of a food supplement was possible with the relative standard deviations of 9–26%, which is satisfactory for quantitative and semiquantitative determinations. It was found that plotting a calibration graph in 3D space of the three reagents’ responses allows us to distinguish dihydroquercetin from its close structural analogue, quercetin.

Microfluidics-based strategies for molecular diagnostics of infectious diseases

Traditional diagnostic strategies for infectious disease detection require benchtop instruments that are inappropriate for point-of-care testing (POCT). Emerging microfluidics, a highly miniaturized, automatic, and integrated technology, are a potential substitute for traditional methods in performing rapid, low-cost, accurate, and on-site diagnoses. Molecular diagnostics are widely used in microfluidic devices as the most effective approaches for pathogen detection. This review summarizes the latest advances in microfluidics-based molecular diagnostics for infectious diseases from academic perspectives and industrial outlooks. First, we introduce the typical on-chip nucleic acid processes, including sample preprocessing, amplification, and signal read-out. Then, four categories of microfluidic platforms are compared with respect to features, merits, and demerits. We further discuss application of the digital assay in absolute nucleic acid quantification. Both the classic and recent microfluidics-based commercial molecular diagnostic devices are summarized as proof of the current market status. Finally, we propose future directions for microfluidics-based infectious disease diagnosis.

A One-Dollar, Disposable, Paper-Based Microfluidic Chip for Real-Time Monitoring of Sweat Rate

Collecting sweat and monitoring its rate is important for determining body condition and further sweat analyses, as this provides vital information about physiologic status and fitness level and could become an alternative to invasive blood tests in the future. Presented here is a one-dollar, disposable, paper-based microfluidic chip for real-time monitoring of sweat rate. The chip, pasted on any part of the skin surface, consists of a skin adhesive layer, sweat-proof layer, sweat-sensing layer, and scale layer with a disk-shape from bottom to top. The sweat-sensing layer has an impressed wax micro-channel containing pre-added chromogenic agent to show displacement by sweat, and the sweat volume can be read directly by scale lines without any electronic elements. The diameter and thickness of the complete chip are 25 mm and 0.3 mm, respectively, permitting good flexibility and compactness with the skin surface. Tests of sweat flow rate monitoring on the left forearm, forehead, and nape of the neck of volunteers doing running exercise were conducted. Average sweat rate on left forearm (1156 g·m⁻²·h⁻¹) was much lower than that on the forehead (1710 g·m⁻²·h⁻¹) and greater than that on the nape of the neck (998 g·m⁻²·h⁻¹), in good agreement with rates measured using existing common commercial sweat collectors. The chip, as a very low-cost and convenient wearable device, has wide application prospects in real-time monitoring of sweat loss by body builders, athletes, firefighters, etc., or for further sweat analyses.

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

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

Enhancement of the Detection Performance of Paper-Based Analytical Devices by Nanomaterials

Paper-based analytical devices (PADs), including lateral flow assays (LFAs), dipstick assays and microfluidic PADs (μPADs), have a great impact on the healthcare realm and environmental monitoring. This is especially evident in developing countries because PADs-based point-of-care testing (POCT) enables to rapidly determine various (bio)chemical analytes in a miniaturized, cost-effective and user-friendly manner. Low sensitivity and poor specificity are the main bottlenecks associated with PADs, which limit the entry of PADs into the real-life applications. The application of nanomaterials in PADs is showing great improvement in their detection performance in terms of sensitivity, selectivity and accuracy since the nanomaterials have unique physicochemical properties. In this review, the research progress on the nanomaterial-based PADs is summarized by highlighting representative recent publications. We mainly focus on the detection principles, the sensing mechanisms of how they work and applications in disease diagnosis, environmental monitoring and food safety management. In addition, the limitations and challenges associated with the development of nanomaterial-based PADs are discussed, and further directions in this research field are proposed.

Surface engineering of silica based materials with Ni-Fe layered double hydroxide for the efficient removal of methyl orange: Isotherms, kinetics, mechanism and high selectivity studies

Herein, low-cost diatomite (DE) and bentonite (BE) materials were surface modified with Ni-Fe layered double hydroxide (LDHs) (represented as NFD and NFB respectively), using a simple co-precipitation procedure for the removal of methyl orange (MO) dye from water. The adsorbents of both before and after MO adsorption have been studied by XRD, N₂ adsorption-desorption isotherm, FTIR, FESEM-EDX and XPS characterization. The zeta potential analysis was used to observe the surface charge of adsorbents within the pH ranges of 4-10. The MO removal efficiency was significantly improved after LDHs modification, showing a 94.7% and 92.6% efficiency for NFD and NFB at pH 6, respectively. Whereas bare DE and BE have shown removal efficiency of 15.5% and 4.9% respectively. The maximum adsorption capacities of NFD and NFB using the Langmuir isotherm model were found to be 246.9 mgg^-1 and 215.9 mgg^-1 respectively. The designed NFD showed high selectivity towards anionic-based dyes from water and also the effect of salts shows the dye removal percentage was increased and decreased for the addition of Na₂SO₄ and NaCl, respectively. The reusability of NFD and NFB have been studied for a maximum of five cycles and they can remove MO up to four cycles. Therefore, the designed adsorbents can be very effective towards the removal of MO from water and they may be useful for dye-based wastewater treatment.

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.

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.

Application of Capillary Electrophoresis for Determination of Inorganic Analytes in Waters

Aside from HPLC and GC, capillary electrophoresis (CE) is one of the most important techniques for high-performance separations in modern analytical chemistry. Its main advantages are the possibility of using different detection techniques, the possibility of in-capillary sample processing for preconcentration or derivatization, and ease of instrumental miniaturization down to the microfluidic scale. Those features are utilized in the separation of macromolecules in biochemistry and in genetic investigations, but they can be also used in determinations of inorganic ions in water analysis. This review, based on about 100 original research works, presents applications of CE methods in water analysis reported in recent decade, mostly regarding conductivity detection or indirect UV detection. The developed applications include analysis of high salinity sea waters, as well as analysis of other surface waters and drinking waters.

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.

Quercetin-rGO based mercury-free electrode for the determination of toxic Cd (II) and Pb (II) ions using DPASV technique

Metal ion pollution poses serious threat to environment. Analysis of Cd (II) and Pb (II) ions using chemically modified mercury free electrode is a feasible routine analytical tool. Developing an electrode surface modified with conductive 2D carbon and metal complexing ligand created a synergetic effect towards sensitive and selective electrochemical determination of metal ions. The present study focused on green chemistry approach towards synthesis of reduced graphene oxide using a natural flavonoid (Quercetin) that acts as a reducing, functionalizing agent and also as metal complexing agent. This quercetin reduced graphene oxide (Q-rGO) was surface modified over paraffin wax impregnated graphite electrode. The resulting Q-rGO electrode was used as a mercury-free electrode for simultaneous analysis of Pb (II) and Cd (II) ions. Physico-chemical parameters of the synthesized Q-rGO and modified electrodes were characterized using X-ray diffraction, UV-Vis, FT-IR, and Raman spectrometer. The morphology of the material and surface topography of the modified electrode was observed using HR-TEM and FESEM, respectively. Cyclic voltammetry (CV) and AC impedance (EIS) were adopted for electrochemical characterization and Differential pulse anodic stripping voltammetry (DPASV) was chosen for simultaneous sensing of metal ions using Q-rGO electrode. Analytical parameters such as effect of electrolyte, effect of pH, preconcentration time and deposition potential were optimized. The experimental results suggested that the Q-rGO electrode is capable of sensing Pb (II) and Cd (II) ions individually and simultaneously. Inference from the calibration plot showed that the Q-rGO electrode was capable of sensing the concentration range of Cd (II) ion form 0.19 to 2.5 μgL^-1 with LOD-0.05 μgL^-1 and Pb (II) ions from 0.19 to 3.1 μgL^-1 with LOD 0.06 μgL^-1.

Microfluidic devices based on textile threads for analytical applications: state of the art and prospects

Microfluidic devices based on textile threads have interesting advantages when compared to systems made with traditional materials, such as polymers and inorganic substrates (especially silicon and glass). One of these significant advantages is the device fabrication process, made more cheap and simple, with little or no microfabrication apparatus. This review describes the fundamentals, applications, challenges, and prospects of microfluidic devices fabricated with textile threads. A wide range of applications is discussed, integrated with several analysis methods, such as electrochemical, colorimetric, electrophoretic, chromatographic, and fluorescence. Additionally, the integration of these devices with different substrates (e.g., 3D printed components or fabrics), other devices (e.g., smartphones), and microelectronics is described. These combinations have allowed the construction of fully portable devices and consequently the development of point-of-care and wearable analytical systems.

Recent Developments and Applications of Microfluidic Paper-Based Analytical Devices for the Detection of Biological and Chemical Hazards in Foods: A Critical Review

Nowadays, food safety has become a major concern for the sustainability of global public health. Through the production and distribution steps, food can be contaminated by either chemical hazards or pathogens, and the determination of these plays a critical role in the processes of ensuring food safety. Therefore, the development of analytical tools that can provide rapid screening of these hazards is highly necessary. Microfluidic paper-based analytical devices (µPADs) have advanced significantly in recent years as they are rapid and low-cost analytical screening tools for testing contaminated food products. This review focuses on recent developments of µPADs for various applications in the food safety field. A description of the fabrication of selected papers is briefly discussed, and evaluation of the μPADs' performance with regard to their precision and accuracy as well as their limits of detection is critically assessed. The advantages and disadvantages of these devices are highlighted.

Paper-Based Electrochemical Biosensors for Voltammetric Detection of miRNA Biomarkers Using Reduced Graphene Oxide or MoS2 Nanosheets Decorated with Gold Nanoparticle Electrodes

Paper-based biosensors are considered simple and cost-efficient sensing platforms for analytical tests and diagnostics. Here, a paper-based electrochemical biosensor was developed for the rapid and sensitive detection of microRNAs (miRNA-155 and miRNA-21) related to early diagnosis of lung cancer. Hydrophobic barriers to creating electrode areas were manufactured by wax printing, whereas a three-electrode system was fabricated by a simple stencil approach. A carbon-based working electrode was modified using either reduced graphene oxide or molybdenum disulfide nanosheets modified with gold nanoparticle (AuNPs/RGO, AuNPs/MoS2) hybrid structures. The resulting paper-based biosensors offered sensitive detection of miRNA-155 and miRNA-21 by differential pulse voltammetry (DPV) in only 5.0 µL sample. The duration in our assay from the point of electrode modification to the final detection of miRNA was completed within only 35 min. The detection limits for miRNA-21 and miRNA-155 were found to be 12.0 and 25.7 nM for AuNPs/RGO and 51.6 and 59.6 nM for AuNPs/MoS2 sensors in the case of perfectly matched probe-target hybrids. These biosensors were found to be selective enough to distinguish the target miRNA in the presence of single-base mismatch miRNA or noncomplementary miRNA sequences.

Aggregation-induced emission luminogen@manganese dioxide core-shell nanomaterial-based paper analytical device for equipment-free and visual detection of organophosphorus pesticide

Organophosphorus pesticide (OP) residues have gathered considerable attention because of their significant threat to society development and healthy life. Developing a sensitive and practical OPs sensor is highly urgent, whereas remains a huge challenge. To this end, we fabricated a high-performance fluorescence paper analytical device (PAD) for apparatus-free and visual sensing of OPs based on aggregation-induced emission (AIE) luminogen's bright emission in aggregated state, unique response of MnO₂ to thiol compounds, and difference of MnO₂ and Mn²⁺ in quenching fluorescence. AIE nanoparticles PTDNPs-0.10 and MnO₂ respectively acted as core and shell to prepare PTDNPs@MnO₂, which possessed high stability and were dripped on cellulose paper's surface to fabricate AIE-PAD. The sensing mechanism is that OPs-treated acetylcholinesterase (AChE) prevents the formation of thiocholine, thereby minimizing the reduction of MnO₂ into Mn²⁺ and changing the output signal. As a result, equipment-free and visual sensing of OPs was acquired with limit of detection of 1.60 ng/mL. This work justifies the feasibility of applying core-shell material to develop high-performance sensor and substituting complex/expensive solution-phase sensor with PAD, providing a new avenue to bring OPs analysis out of the lab and into the world.

New optofluidic based lab-on-a-chip device for the real-time fluoride analysis

A PDMS (Polydimethylsiloxane) microfluidic channel coupled with UV-vis fibre-optic spectrometer and new synthesized colorimetric probe was integrated into an optofluidic based Lab-on-a-chip device for highly sensitive and real-time quantitative measurements of fluoride ions (F¯). An 'S' shaped microchannel in a microfluidic device was designed to act as microreactor to facilitate the continuous reaction between synthetized colorimetric probe (sensor) and F¯ ions. Following this reaction, the UV-vis optical probe in the downstream detection zone of the microfluidic device was used to capture their spectrum and present as F¯ concentration in real-time conditions. An initial study of the developed colorimetric probe with multi-colour change with several binding and chromophore groups such as -OH, -NH and -NO₂ groups confirmed its high sensitivity and selectivity for F¯ ions with a detection limit of 0.79 ppm. The performance of the developed optofluidic device was evaluated for the selective, sensitive detection of F¯ ions including real samples out-performing conventional methods. The technology has advantages such as low sample consumption, rapid analysis, high sensitivity and portability. Presented new Lab-on-a-chip device provides many competitive advantages for the real-time analysis of F¯ ions needed across broad sectors.

Flow Control in Porous Media: From Numerical Analysis to Quantitative μPAD for Ionic Strength Measurements

Microfluidic paper-based analytical devices (µPADs) are a promising technology to enable accurate and quantitative in situ assays. Paper’s inherent hydrophilicity drives the fluids without the need for external pressure sources. However, controlling the flow in the porous medium has remained a challenge. This study addresses this problem from the nature of the paper substrate and its design. A computational fluid dynamic model has been developed, which couples the characteristics of the porous media (fiber length, fiber diameter and porosity) to the fluidic performance of the diffusion-based µPAD sensor. The numerical results showed that for a given porous membrane, the diffusion, and therefore the sensor performance is affected not only by the substrate nature but also by the inlets’ orientation. Given a porous substrate, the optimum performance is achieved by the lowest inlets’ angle. A diffusion-based self-referencing colorimetric sensor was built and validated according to the design. The device is able to quantify the hydronium concentration in wines by comparison to 0.1–1.0 M tartaric acid solutions with a 41.3 mM limit of detection. This research showed that by proper adjustments even the simplest µPADs can be used in quantitative assays for agri-food applications.