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Microfluidic Paper‐based Analytical Devices (µPADs) have emerged as a new class of microfluidic systems, offering numerous advantages over traditional microfluidic chips. These advantages include simplicity, cost‐effectiveness, stability, storability, disposability, and portability. As a result, various designs for different types of assays are developed and investigated. In recent years, µPADs are combined with conventional detection methods to enable rapid on‐site detection, providing results comparable to expensive and sophisticated large‐scale testing methods that require more time and skilled personnel. The application of µPAD techniques is extensive in environmental quality control/analysis, clinical diagnosis, and food safety testing, paving the way for on‐site real‐time diagnosis as a promising future development. This review focuses on the recent research advancements in the design, fabrication, material selection, and detection methods of µPADs. It provides a comprehensive understanding of their principles of operation, applications, and future development prospects. [ABSTRACT FROM AUTHOR]

Iodine is an essential micronutrient for humans due to its fundamental role in the biosynthesis of thyroid hormones. As a key parameter to assess health conditions, iodine intake needs to be monitored to ascertain and prevent iodine deficiency. Iodine is available from various food sources (such as seaweed, fish, and seafood, among others) and dietary supplements (multivitamins or mineral supplements). In this work, a microfluidic paper-based analytical device (μPAD) to quantify iodide in seaweed and dietary supplements is described. The developed μPAD is a small microfluidic device that emerges as quite relevant in terms of its analytical capacity. The quantification of iodide is based on the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by hydrogen peroxide in the presence of iodine, which acts as the catalyst to produce the blue form of TMB. Additionally, powder silica was used to intensify and uniformize the colour of the obtained product. Following optimization, the developed μPAD enabled iodide quantification within the range of 10–100 µM, with a detection limit of 3 µM, and was successfully applied to seaweeds and dietary supplements. The device represents a valuable tool for point-of-care analysis, can be used by untrained personnel at home, and is easily disposable, low-cost, and user-friendly. [ABSTRACT FROM AUTHOR]

Multiplex detection can enhance diagnostic precision and improve diagnostic efficiency, providing important assistance for epidemiological investigation and epidemic prevention. There is a great need for multi-detection sensing platforms to accurately diagnose diseases. Herein, we reported a μPAD-based chemiluminescence (CL) assay for ultrasensitive multiplex detection of AIV biomarkers, based on three DNAzyme/Lum/PEI/CaCO 3. Three time-resolved CL signals were sequentially generated with detection limits of 0.32, 0.34, and 0.29 pM for H1N1, H7N9, and H5N1, respectively, and with excellent selectivity against interfering DNA. The recovery test in human serum displayed satisfactory analysis capabilities for complex biological samples. The μPAD-based CL assay achieved multiplex detection within 70 s, with a high time resolution of 20 s. The proposed strategy has the advantages of low cost, high sensitivity, good selectivity, and wide time resolution, the μPAD-based CL assay has shown great potential in the early and accurate diagnosis of diseases. We reported a μPAD-based CL assay for ultrasensitive multiplex detection of AIV biomarkers, based on three DNAzyme/Lum/PEI/CaCO 3. The platform achieved multiplex detection within 70 s, with a wide time resolution of 20 s, demonstrating great potential in the early and accurate diagnosis of diseases. [Display omitted] • A low-cost, portable μPAD with time-resolved CL signals was fabricated. • The CL assay successfully detected H1N1, H7N9, and H5N1 within 70 s, with a wide time resolution of 20 s. • Dual amplification of the sensor was achieved based on the CHA reaction and DNAzyme catalysis. • The CL assay had a wide linear range (1.0 pM−10 nM) and high detection sensitivity at pM level. • The CL assay was able to visualize trace H1N1, H7N9, and H5N1 down to 10 pM. [ABSTRACT FROM AUTHOR]

The rapid emergence of microfluidic paper-based devices as point-of-care testing (POCT) tools for early disease diagnosis and health monitoring, particularly in resource-limited areas, holds immense potential for enhancing healthcare accessibility. Leveraging the numerous advantages of paper, such as capillary-driven flow, porous structure, hydrophilic functional groups, biodegradability, cost-effectiveness, and flexibility, it has become a pivotal choice for microfluidic substrates. The repertoire of microfluidic paper-based devices includes one-dimensional lateral flow assays (1D LFAs), two-dimensional microfluidic paper-based analytical devices (2D μPADs), and three-dimensional (3D) μPADs. In this comprehensive review, we provide and examine crucial information related to paper substrates, design strategies, and detection methods in multi-dimensional microfluidic paper-based devices. We also investigate potential applications of microfluidic paper-based devices for detecting viruses, metabolites and hormones in non-invasive samples such as human saliva, sweat and urine. Additionally, we delve into capillary-driven flow alternative theoretical models of fluids within the paper to provide guidance. Finally, we critically examine the potential for future developments and address challenges for multi-dimensional microfluidic paper-based devices in advancing noninvasive early diagnosis and health monitoring. This article showcases their transformative impact on healthcare, paving the way for enhanced medical services worldwide. [Display omitted] • Structure design strategies for microfluidic paper-based analytical devices. • Sensing mechanism and application in noninvasive testing. • Theoretical models of capillary-driven fluids within paper. [ABSTRACT FROM AUTHOR]

A dual colorimetric/electrochemical sensor integrated with a microfluidic paper-based analytical device (μPAD) is created for rapid and sensitive detection of cabaryl in fruit. For colorimetric sensor, quercetin modified silver nanoparticle (QCT-AgNPs) is synthesized via self-reduction, resulting in the color change from green to dark green in the presence of cabaryl, which can be readily observed via naked eyes. This colorimetric sensor provides a linear response of 5 – 50 μM quantified by Image J software. For electrochemistry, AgNPs/MXene nanocomposite electrode offers high electrocatalytic activity towards the detection of carbarly with a linear range of 0 – 60 µM and a detection limit of 0.01 µM. The limit of detection values obtained from this dual sensor are lower than a cut-off of carbaryl residue in food, which is 7 mg.kg−1 (35 µM) verifying the desired sensitivity of this platform. Ultimately, this dual sensor is successfully applied for on-site detection of carbaryl in the orange samples read out on the smartphone with satisfactory results. [Display omitted] • Dual colorimetric/electrochemicl sensor is developed on μPAD for cabaryl sensor • QCT-AgNPs is synthesized by self-reduction and applied for colorimetric detection. • AgNPs/MXene modified electrode provided high electrochemical sensitivity. • This platform is applied for on-site carbaryl detection in fruits on smartphone. [ABSTRACT FROM AUTHOR]

A microfluidic paper-based analytical device (μPAD) is developed in this work to analyze hydroquinone in facial whitening creams using phloroglucinol. The μPAD features a hydrophobic barrier for detection and was fabricated using a wax printer with Whatman chromatographic paper. Detection was achieved by colorimetry based on the formation of an orange hydroquinone-phloroglucinol complex. The colored reaction product formed on the detection zone of the μPAD was scanned, and the images obtained were processed with Image-J software to determine their color intensity (RGB value). Optimization of the process conditions was conducted to achieve sensitive measurements. The optimum conditions yielding maximum sensitivity included a reagent addition sequence of phloroglucinol → NaOH → sample (hydroquinone), 1 μL of 0.5% phloroglucinol, 1 M NaOH, and 10-minute reaction. Under optimal conditions, the μPAD produced two linear calibration curves for hydroquinone at concentrations of 10-100 mg/L (R² = 0.9979) and 250-1000 mg/L (R² = 0.9991). The method demonstrated very good selectivity for the target analyte in the presence of propylene glycol and resorcinol with satisfactory validity and average recovery close to 100%. The proposed μPAD is a very simple and inexpensive technique for hydroquinone analysis and could be applied to cosmetics samples with satisfactory results. [ABSTRACT FROM AUTHOR]

[Display omitted] • Paper-based analytical aptasensor for food safety and quality. • Determination of β-lactoglobulin traces in food products. • Aptamer bioconjugation to fluorescent carbon quantum dots. • Smartphone-assisted sensing platform for portable β-lactoglobulin determination. Herein, we present a new luminescent-based analytical method for sensitive fluorometric determination of a milk allergen, β-lactoglobulin (β-LG), in food products based on a portable, low-cost, and easy-to-use microfluidic paper-based analytical device (µPAD). For this purpose, the developed µPAD consisted on a β-LG specific aptamer immobilized on a cellulose disk, and fluorescent Carbon Quantum Dots (CQDs) bioconjugated with its complementary oligonucleotide (DNA), which worked as a label for the aptamer hybridization and its complementary DNA (CQDs-DNA). In a sample with β-LG, the aptamer preferentially binds to β-LG protein and this leads to release of the CQDs-DNA. After washing the µPAD, where the aptamer is immobilized, detection is achieved by fluorescence intensity decrease. An estimation of β-LG concentration can be calculated thanks to the fluorescence changes, for that, a photo of the µPAD is taken using either a digital camera or a smartphone. The µPAD showed an excellent detection response for β-LG in the range of 1.8 to 500 ng/mL with a limit of detection of up to 0.6 ng/mL using the digital camera. Furthermore, the developed method was successfully applied to the determination of β-LG in food samples such as skimmed milk, crispy rice, cookies and chocolate. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide has been postulated to be present on the surface of Europa and Enceladus. While it could represent a potential source of energy for possible life-forms, H2O2 may also interfere with a number of current detection technologies, including biosensors. To take advantage of the selectivity and portability of these devices, simple and reliable routes to degrade the potential H2O2 present should be developed and implemented to prepare for this possibility. Unfortunately, most of the current approaches for removing H2O2 are slow, may affect the sample, or could interfere with the performance of biosensors. To address these limitations, catalase was immobilized onto silica particles and used as a means to selectively decompose H2O2 prior to the analysis of common biomarkers with a biosensor. For these experiments, glucose, l-leucine, and lactic acid were used as representative examples of biomolecules such as carbohydrates, amino acids, and organic acids, respectively, which could be used as biomarkers on extraterrestrial bodies. While the decomposition reaction between catalase and H2O2 is well known, to our knowledge this is the first instance where catalase has been used in combination with a microfluidic paper-based analytical device (μPAD) to implement selective sample pretreatment. [ABSTRACT FROM AUTHOR]

Despite recent advancements in the field of microfluidic paper-based analytical devices (μPADs), a key challenge remains in developing a simple and efficient μPAD with customized imaging capabilities for antioxidant assays. In the present study, we report a facile approach for μPAD fabrication through the application of transparent nail paint leading to creation of hydrophobic barriers and well-defined channels. The resultant μPADs were then characterized through scanning electron microscopy and contact angle measurements. The resolution and functional features of the fabricated μPAD were amenable to the intended assay. The μPAD's impregnated poly(methacrylic acid) (PMAA)-coated cerium oxide (CeO2) nanoparticles oxidized the 3,3′,5,5′-tetramethylbenzidine (TMB) leading to the formation of a blue-colored charge-transfer complex. The addition of different antioxidant standard solutions resulted in a reduction in the blue color in a dose-dependent manner which could be observed visually. The color intensity of the PMAA-CeO2 nanoparticle@TMB oxidation product was inversely proportional to the antioxidant concentration and was measured using customized in-house MATLAB-based image processing software. Importantly, PMAA-CeO2 nanoparticle-based μPADs demonstrated good analytical characteristics and were able to be stored for long periods without any loss of activity. Moreover, potential interferents did not pose any threat to the colorimetric signal read-out for determination of antioxidant activity. The developed method was further applied for the assessment of antioxidant activity in a variety of tea samples and performed satisfactorily in comparison with a commonly used antioxidant detection method. Collectively, the developed μPAD-based platform holds great potential as a low-cost, convenient, portable and reliable method for pursuing various on-site antioxidant assays. [ABSTRACT FROM AUTHOR]

In the present article, we developed an electrochemical microfluidic paper-based device (EμPAD) for the non-enzymatic detection of Ascorbic Acid (AA) concentration in plasma using whole human blood. We combined LF1 blood plasma separation membrane and Whatman grade 1 filter paper to separate plasma from whole blood through wax printing. A screen-printed electrode (SPE) was modified with spherical-shaped MgFe 2 O 4 nanomaterial (n-MgF) to improve the catalytic properties of SPE. The n-MgF was prepared via hydrothermal method, and its material phase and morphology were confirmed via XRD, FTIR, TEM, SEM, and AFM analysis. The fabricated n-MgF/SPE/EμPAD exhibited detection of AA ranging from 0 to 80 μM. The obtained value of the detection limit, limit of quantification, sensitivity, and response time are 2.44 μM, 8.135 μM, 5.71 × 10−3 mA μM−1 cm−2, and 10 s, respectively. Our developed n-MgF/SPE/EμPAD shows marginal interference with the common analytes present in plasma, such as uric acid, glutamic acid, glucose, urea, lactic acid, and their mixtures. Overall, our low-cost, portable device with its user-friendly design and efficient plasma separation capability offers a practical and effective solution for estimating AA concentration from whole human blood in a single step. [Display omitted] • A simple electrochemical microfluidic paper-based device • Non-enzymatic detection of Ascorbic Acid (AA) • A microfluidic device for blood-plasma separation with high efficacy • AA detection in miniaturized POC setting via single step [ABSTRACT FROM AUTHOR]

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. [Display omitted] • A 3D paper-based microfluidic device (μPAD) was designed via origami methods. • Sliding paper tape as a valve for fluid control. • μPAD successfully applied to the detection of Fe (III), Ni (II), Cr (VI) and nitrite. • 3D μPAD is a convenient and efficient rapid detection device. [ABSTRACT FROM AUTHOR]

The medical field and public health rely on diagnostics carried out by sensors and laboratory testing methods. However, conducting laboratory tests is a demanding task, requiring both substantial equipment and the skilled touch of experienced operators. The microfluidic field has tackled this challenge over recent years. Paper microfluidic devices have become popular recently due to their testing speed, low cost, and accessibility. They are more suitable for field operations and developing regions with limited access to centralized healthcare centers and medical laboratories since they do not require external flow control instruments and are much cheaper to make when compared to conventional microfluidic chips. However, the majority of paper-based analytical devices tend to be qualitative or semi-quantitative; consequently, there is a significant need for paper-based quantitative transduction methods, given their expanding range of applications. Here, we introduce a novel microwave paper-based metamaterial-inspired transduction method that relies on dielectric sensing. Using paper as both the microfluidic channel and the resonator substrate hugely reduces the sensing device's costs, complexity, and environmental impact. We designed the metamaterial-based transducer using computational methods and demonstrated its fabrication and operation experimentally. Dielectric sensing of multiple analytes with different permittivities is demonstrated as a proof of concept. Although the device has a non-linear response curve in a wide range of permittivity change, the response can be assumed linear in the shorter spans. The results for permittivities lower than 30 show a 2.14 MHz/RPU sensitivity. This design is a prototype demonstrating the possibility of integrating a porous media microfluidic channel and paper-based microwave resonators. The linear sensing performance illustrated by the platform indicates its potential applications in the biosensing field. [Display omitted] • A novel paper-based metamaterial-inspired transducer is introduced. • Paper serves as microfluidic channel and substrate, reducing costs and eco impact. • This unique transduction method is a new type of quantitative analysis for μ PADs. • The linear sensing concept is demonstrated using various solutions. • This approach has implications for affordable quantitative μ PADs. [ABSTRACT FROM AUTHOR]

• A μPAD with two layers was developed to monitor Cu2+, Fe2+ and free chlorine simultaneously. • The detection limits of μPAD for single analyte were less or equal to each Chinese Drinking Water Standard concentration. • The μPAD exhibited good applicability for real use in different kinds of drinking water. Cu2+, Fe2+ and free chlorine are three important analytes in drinking water. Firstly, the μPAD for single analyte was easily fabricated. Detection limits for Cu2+, Fe2+ and free chlorine were 0.1 mg L−1, 0.1 mg L−1 and 0.2 mg L−1, respectively, less or equal to each Chinese Drinking Water Standard concentration and lower than those of most commercially available testing strips. Another μPAD with two layers was then developed to monitor and report the safety level of these three analytes in drinking water simultaneously. Moreover, the μPADs for Fe2+ and free chlorine provided more distinguishable colors to users with naked eyes. The as-fabricated μPAD not only demonstrated excellent anti-interference capability, production reproducibility, but also exhibited good applicability for real use in different kinds of drinking water. [ABSTRACT FROM AUTHOR]

Abstract The design, optimization and evaluation of a micro paper-based analytical device (μPAD) for the simultaneous determination of fluoride and nitrite in real water samples was developed. In order to create hydrophilic and hydrophobic zones on the laboratory filter paper device, a wax ink printer was used. The μPAD has four reaction zones for each analyte where the respective colorimetric reactions took place using the SPADNS method and Griess method for fluoride and nitrite respectively. A Smartphone was used to capture the corresponding images which were then converted to the RGB colour space by the software ImageJ using a personal computer. Under the optimal conditions, the linear range was 0.23–2.26 mg L−1 for fluoride with limit of detection (LOD) and limit of quantification (LOQ) of 0.13 mg L−1 and 0.25 mg L−1. The working range for nitrite was 0.05–10.0 mg L−1 with LOD of 0.03 mg L−1 and LOQ 0.13 mg L−1. The μPAD was used to determine the analytes in water samples obtaining satisfactory results in both cases. The validation of fluoride determination was performed comparing the μPAD with the reference method and for nitrite determination, a recovery study was carried out. In both cases, the obtained results demonstrated a good agreement. Graphical abstract Unlabelled Image Highlights • The design, optimization and evaluation of a micro paper-based analytical device was developed. • This user-friendly μPAD simultaneously determines fluoride and nitrite in real water samples. • A commercial wax printer was employed to create this portable inexpensive device on filter paper. • It requires minimum volumes of sample and reagents and uses biodegradable raw materials. • Results demonstrate this device is a promising analytical tool for a wide field of applications. [ABSTRACT FROM AUTHOR]

The utility of strikingly simple and cheap optoelectronic detectors operating according to a Paired-Emitter-Detector-Diode (PEDD) principle for microfluidic paper-based analytical devices (μPAD) has been experimentally confirmed. The prototypes of these compact detectors have been made of only two customary light emitting diodes without any additional optical parts like lens, filters, fibers etc. Moreover, for their operation economic and portable equipment is required (low-power circuit and ordinary voltmeter). Photometric and fluorometric PEDDs for paper devices are developed. In both cases the possibility of their applicability in transmittance and reflectance modes of measurements are presented. In these investigations as model analytes some coloured and fluorescence substances have been handled, but the utility of several developed systems for real chemical analysis has also been demonstrated. The results of photometric determination of hemoglobin in human blood, as well as fluorometric determination of quinine in tonics and calcium ions in mineral waters using various PEDD-μPAD systems have been shown. [ABSTRACT FROM AUTHOR]

A microfluidic paper-based analytical device (μPADs) immunoassay for detection of the blood native biomarker D-dimer is reported. The μPAD is created by wax printing on a single piece of chromatographic paper and combined with an anti-D-dimer capture antibody and conjugates of anti-D-dimer antibody with 40 nm gold nanoparticles. The presence of D-dimer in buffer/simulated plasma samples is successfully reported for concentrations as low as 15 ng D-dimer/mL. Linearity between signal intensity and D-dimer concentration is observed up to 100 ng/mL. Using an appropriate dilution, the test could be used to yield positive results only for those samples with a D-dimer concentration above the clinically relevant threshold range of 250–500 ng/mL. Finally, the merits and pitfalls of using μPADs as compared to lateral flow devices in immunoassays are discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Iron speciation of Iron(II) and Iron(III) in wines using a handheld-size microfluidic paper-based device. • Single device assembled for quantification of both ionic forms accounting for sample colour interference. • Application to both white and red wines in a portable, in situ, approach. During the winemaking process, wine is frequently exposed to oxygen which can affect the organoleptic characteristics and therefore the quality of the finished wine. Most phenolic molecules in wine serve as oxidation substrates affecting colour and flavour. Transition metal ions namely copper and iron act as electron pumps; in fact, Fe(III) is reduced to Fe(II) as a consequence of the oxidation of phenols to quinones and, in the presence of oxygen, Fe(II) is oxidised back to Fe(III). Consequently, speciation analysis by the oxidation state is critical and even more relevant than the total ion concentration when one wants to evaluate the risk for wine oxidation. Interestingly the different reactivity of Fe(III) and Fe(II) makes spectrophotometric methods, namely colorimetric reactions, ideally suited for speciation analysis. In this work, a microfluidic paper-based analytical device (µPAD) was designed for the iron speciation in wine samples. The colorimetric reaction between bathophenanthroline and iron(II) was chosen and the oxidation of Fe(III) to Fe(II), using hydroxylamine, was carried out within the same device. Aiming for application to different types of wine, the developed device also enabled to account for the potential sample colour (subtraction of interfering signal). The designed µPAD enabled the iron speciation in both white and red wines. [ABSTRACT FROM AUTHOR]

Microfluidic paper-based analytical devices (μPADs) have a significant potential in developing portable and disposable point-of-care testing (POCT). Herein, a facile, rapid, cost-effective and environment friendly strategy for μPADs fabrication is proposed. Specifically, the substrate paper was hydrophobized by coating with trimethoxysilane (TOS), and then the selected area was hydrophilized by treating with surfactant. The whole fabrication process was implemented within 7 min, with no need for complex pre-treatment, high-temperature and special equipment. As a proof-of-concept application, the as-prepared μPAD was applied to determination of the glucose content in human serum samples. The results agreed well with those obtained by a glucometer. We believe that the μPADs fabrication method proposed here could provide a facile, rapid and low-cost reference for other related studies. [ABSTRACT FROM AUTHOR]

A microfluidic biosensor paper-based analytical device, or μPAD, was developed as a point-of-care assay platform for the simultaneous determination of three clinically important markers: uric acid, urea, and triglycerides. To achieve a simultaneous determination, thermosensitive valves based on N-isopropylacrylamide polymers controlled the flow of fluid in the μPAD. The evaluation of the analytical characteristics of the μPAD was done using a photographic camera at room temperature. The detection limits were 4.5·10−5, 2.5·10−3, and 1.5·10−3 mg mL−1, for uric acid, urea, and triglycerides, respectively, and the precision expressed as relative standard deviation (% RSD) 3.2, 1.2, and 1.6%. The obtained μPAD devices were validated with real human plasma samples, demonstrating high accuracy and precision. Multiple analyte point-of-care tests such as those developed here have excellent characteristics, being easy to manufacture, cost-effective, easy to use, and highly sensitive. [Display omitted] • A point-of-care assay platform based μPAD device was developed for the simultaneous determination of three clinically important markers: uric acid, urea, and triglycerides. • Thermosensitive valves based on isopolyacrylamide-acrylamide hydrogels controlled the flow of fluid in the μPAD. • The obtained μPAD devices were validated with real human plasma samples, demonstrating high accuracy and precision. • μPAD device has excellent characteristics: easy to manufacture and to use, cost-effective and high sensitivity. [ABSTRACT FROM AUTHOR]

The synthesis of a novel spirooxazine (1-benzyl-3,3-dimethylspiro[indoline-2,3′-naphtho [2,1-b] [1,4] oxazine, BSP-SP) was developed as a colorimetric probe for the quantitative analysis of Fe2+ and Pb2+ in water samples. The device is based on a microfluidic paper-based analytical device (μPAD). BSP-SP was characterized by various techniques, e.g. single-crystal X-ray diffraction (SC-XRD) and time-dependent density functional theory (TD-DFT). In addition, the density functional theory (DFT) allowed us to understand better the adsorption of Fe2+ and Pb2+ over other cations adsorption on the BSP-MO structure. The structural isomerization of BSP-SP occurs as an open-form merooxazine (BSP-MO), which acts as a ligand to form Fe2+/BSP-MO and Pb2+/BSP-MO complexes. We observed these complexes under UV irradiation as having a light blue coloration. The μPADs were also proposed as detection platforms to maintain photostability and photochromic efficiency and extend the lifetime of the spirooxazine. Under optimal conditions, the developed μPAD enables rapid and sensitive quantitative analysis of Fe2+ at pH 6 and Pb2+ at pH 11. The limits of detection (LOD) were 0.036 mg L−1 for Fe2+ and 0.152 mg L−1 Pb2+. Overall, the developed sensors, particularly the μPADs, promise the practical analysis of Fe2+ and Pb2+ in water samples. [Display omitted] • Our novel spirooxazine (BSP) was successfully synthesized as colorimetric probe. • BSP-coated μPAD was illustrated to enhance photochromic efficiency. • This method features selectivity toward Fe2+ and Pb2+ determination. [ABSTRACT FROM AUTHOR]

Paper based microfluidics (µPADs) with advantages of portability, low cost, and ease of use have attracted extensive attention. Here we describe a novel method that integrates glucoamylase-trapped aptamer-crosslinked hydrogel for molecular recognition with cascaded enzymatic reactions for signal amplification and a µPAD for portable readout. Upon target introduction, the hydrogel decomposes to release glucoamylase, which catalyzes the hydrolysis of amylose to produce a large amount of glucose. With a simple folding of the µPAD, the sample solution containing glucose product wicks and diffuses in parallel to each test-zone to carry out homogeneous assays, where glucose is used to produce I 2 for brown color visualization through multiple enzymatic and chemical cascade reactions. Through color gradient changes based on different concentrations of the target, a semiquantitative assay is achieved by the naked eye, and quantitation can be obtained by handheld devices. Detection of cocaine in buffer and urine was performed to demonstrate the utility of the hydrogel–µPAD system. More importantly, the hydrogel–µPAD system can be extended to the detection of various targets by incorporating the corresponding aptamer into the hydrogel. The hydrogel–µPAD system reported here provides a new platform for portable, disposable and visual detection of a wide range of targets. [ABSTRACT FROM AUTHOR]

[Display omitted] • ABO and Rh blood type detection using a three-dimensional origami microfluidic technology. • Blood moves through paper thickness instead of paper length. • The device simply works based on the counting the number of layers through which blood moves. • The proposed method can determine the blood type of 95% of samples compared to the gold method. We report a three-dimensional origami paper-based analytical device (3D-µPAD) for ABO and Rh blood type detection using a three-dimensional origami microfluidic technology. A piece of rectangular shaped paper sizing (28.5 × 2) cm was folded into thirteen square layers of 2 cm edge. The first two layers formed the sample zone and distribution zones. Other layers were composed of 4 hydrophilic circle zones, which provide cylindrical channels when the paper layers are folded. Three circle zones on layers from 3 to 8 were pre-deposited with antibodies A, B and D and the reminder circle zone was considered as control. According to the design principle, when blood moves through channels of different antibodies it forms agglutination with those having its complementary blood group antigen. This lowers down the speed of blood movement through the channels. Therefore, the ability of blood movement through each channel was used as the basis of our detection (i.e., counting the number of layers through which blood moved for each channel, separately). The effects of the experimental parameters on the sensor's performance were investigated by using Placket-Burman and Central Composite Design methods. The device operation was validated by analyzing 67 blood samples. It was found that the if blood had complementary antigens, it moved 3–5 layers in the channels pre-deposited with anti-A or anti-B and 5–7 layer in channels pre-deposited with anti-D. Based on this observation, by single analysis we could determine the blood type of 95 % of samples compared to the gold standard method, which is currently used in blood transfusion labs. Replicate analysis resulted in 100 % accuracy. The device was stable for 14 days before using. However, after using, it was stable for at least 1 year. [ABSTRACT FROM AUTHOR]

This paper reviews the development and recent use of everyday communications and IT equipment (mobile phones, digital cameras, scanners, webcams, etc) as detection devices for colorimetric chemistries. Such devices can readily be applied for visible detection using reaction formats such as microfluidic paper based analytical devices (µPADs), indicator papers, and well plate reaction vessels. Their use is highly advantageous with respect to cost, simplicity and portability, and offers many opportunities in the areas of point of care diagnosis, and at-site monitoring of environmental, agricultural, food and beverage parameters. [ABSTRACT FROM AUTHOR]

A microfluidic paper-based analytical device integrating a chromoreactand – a formylazo dye– has been fabricated and used for a colorimetric assay of sulfites. The chromoreactand was covalently linked to paper by vinyl sulfone chemistry. This work presents two robust capillary microfluidic devices to determine sulfite in wine without any pretreatment. One of them based on thread (µTPAD) useful to determine it in white wine and another based on paper (µPAD) to specifically determine sulfite in red wine as well as in white wine. Both are based on the selective recognition of sulfite by means of a chromoreactand that turns from orange to yellow in the presence of sulfite. The colour information acquired (H coordinate) using a digital camera readout allows for a range of application of the µTPAD from 7.8·10−5 M (8.1 mg L−1) to 2.7·10−3 M (279.3 mg L−1) with a limit of detection (LOD) of 78 µM. The strong interference caused by the dyes present in red wine is eliminated by including a laminated paper channel in the µPAD structure that allows for the separation of colorants from red wine before the recognition of the sulfite. This makes it possible to adjust the µPAD procedure to the usual sulfite concentration in wine, with an LOD of 2.2·10−4 M (22.7 mg L−1) and a CV of 2.6%. [Display omitted] • μPAD based on a novel chromoreactand for fast, easy and selective sulfite determination in wine. • Buffering and decolorization of wine sample is performed in the μPAD. • Red and white wine can be analysed without any kind of sample pre-treatment. [ABSTRACT FROM AUTHOR]

Sample loading, Element enrichment and Visualization processes. [Display omitted] • Novel designed lateral flow strip based on functional Pb (II) MIPs paper chip was successfully fabricated. • A newly designed μ PAD platform and visualization methodology was proposed. • The presented platform demonstrated sensitivity, rapidness and application ruggedness for real sample with 1.375 mg kg−1 (LOD) and 2.5–100 mg kg−1 (linear range) A high-performance lateral flow and paper-based microfluidic device (μ PAD) was constructed to develop a rapid read-out analytical platform for heavy metal elements (Pb (II)) in green tea with complex matrices. The entire analytical device was integrated with several functional parts, including a liquid flow channel and an enrichment visualization zone prepared by the molecularly imprinted polymer (MIP) protocol. Under the optimal conditions, the Pb (II) content in tea products can be sensitively and specifically analyzed using an in situ visualization readout. Herein, a rapid visual read-out analysis of the lead level can be achieved by overcoming the other co-existing interfering elements in the traditional green tea sample. The ionic imprinted paper chip can effectively capture the Pb (II) of the digested sample solution, and a visualization sensing process occurred between the metal elements and a chelating visual reagent with high sensitivity and selectivity in situ. Owing to the stable visualization performance, the constructed chip demonstrated good contrast, with a low background and easily recognizable colored spots. The linear range was 2.5–100 mg kg−1, and the limit of detection for Pb (II) in traditional green tea was 1.375 mg kg−1. In addition, other co-existing metal elements did not interfere with the analysis because of the appropriate pretreatment and selectivity of the paper-based chip. Furthermore, the analytical results for tea samples were verified by atomic absorption spectrophotometer (AAS) tests, which demonstrated the robustness and reliability of the proposed method. Therefore, the high-performance visualization lateral flow- μ PAD is a promising platform for the rapid analysis of metal element levels in food products. [ABSTRACT FROM AUTHOR]

Herein, a simple microfluidic paper-based analytical device (μPAD) by using platinum nanoparticles (Pt NPs) as highly active peroxidase mimic for simultaneous determination of glucose and uric acid was fabricated. The μPAD consisted of one sample transportation layer, four paper-based detection chips, and two layers of hydrophobic polyethylene terephthalate (PET) films. The four detection chips were immobilized with various chromogenic reagents, Pt NPs, and specific oxidase (glucose oxidase or uricase). H 2 O 2 generated by specific enzymatic reactions could oxidize co-immobilized chromogenic reagents to produce colored products by using Pt NPs as efficient catalyst. The multi-layered structure of μPAD could effectively improve the color uniformity and color intensity. Total color intensity from each two detection chips modified with distinct chromogenic reagents were used for quantitative analysis of glucose and uric acid, respectively, resulting in significantly improved sensitivity. The linear range for glucose and uric acid detection was 0.01–5.0 mM and 0.01–2.5 mM, respectively. Satisfied results were obtained for glucose and uric acid detection in real serum samples. An easy-to-use smartphone APP was developed for convenient and intelligent detection. The developed μPAD integrated with smartphone as detector holds great applicability for simple and portable on-site analysis. [Display omitted] • A multi-layered μPAD by using Pt NPs as highly active peroxidase mimic was fabricated. • The multi-layered structure of μPAD could effectively improve color uniformity and intensity. • Simultaneous determination of glucose and uric acid in mixture was achieved by using smartphone as detector. • Improved sensitivity was obtained by using total colorimetric responses from each two detection chips as quantitative signal. • Easy-to-use smartphone APP was developed for convenient and intelligent detection. [ABSTRACT FROM AUTHOR]

This work presents two simple methods for estimating the firing distance from the gunshot residues (GSRs) on fabric targets. Four types of fabric targets, namely twill weave denim cotton-polyester (80/20), jersey knitting 100% cotton, plain weave cotton-polyester (80/20) and plain weave cotton-polyester (60/40), were employed. The firing tests were carried out using these white fabrics as targets at distances of 5-100 cm, respectively. In the first method, digital images of the black GSRs on fabric materials were recorded inside an illuminated box and the inverted gray intensity values were plotted against the firing distances. Since the plots of all fabrics are not significantly different, the estimation of firing distance employs the same exponential curve for all test fabrics. Although simple, the imaging method is not suitable for dark-colored materials. A chemical-based method was therefore developed as an alternative method. In the second method, a small disposable microfluidic paper-based analytical device (μPAD) was employed for detecting Pb(II) extracted from the GSRs. The μPAD method uses the measurement of the length of a narrow band of a pink color resulting from reaction between rhodizonate reagent and the Pb(II) extract. The plots indicated that the data of thick denim material are significantly different to other test fabrics which are much thinner. These three fabrics share the same estimation curve. However, it is recommended that the separate estimation curve for denim materials must be used. Both methods are suitable for short range firing distance, no further than 60 cm, since at greater distances the inverted gray intensity and the 'band-length' methods are unable to detect the GSRs. [ABSTRACT FROM AUTHOR]

Microfluidic paper combines pump-free water transport at low cost with a high degree of sustainability, as well as good availability of the paper-forming cellulosic material, thus making it an attractive candidate for point-of-care (POC) analytics and diagnostics. Although a number of interesting demonstrators for such paper devices have been reported to date, a number of challenges still exist, which limit a successful transfer into marketable applications. A strong limitation in this respect is the (unspecific) adsorption of protein analytes to the paper fibers during the lateral flow assay. This interaction may significantly reduce the amount of analyte that reaches the detection zone of the microfluidic paper-based analytical device (µPAD), thereby reducing its overall sensitivity. Here, we introduce a novel approach on reducing the nonspecific adsorption of proteins to lab-made paper sheets for the use in µPADs. To this, cotton linter fibers in lab-formed additive-free paper sheets are modified with a surrounding thin hydrogel layer generated from photo-crosslinked, benzophenone functionalized copolymers based on poly-(oligo-ethylene glycol methacrylate) (POEGMA) and poly-dimethyl acrylamide (PDMAA). This, as we show in tests similar to lateral flow assays, significantly reduces unspecific binding of model proteins. Furthermore, by evaporating the transport fluid during the microfluidic run at the end of the paper strip through local heating, model proteins can almost quantitatively be accumulated in that zone. The possibility of complete, almost quantitative protein transport in a µPAD opens up new opportunities to significantly improve the signal-to-noise (S/N) ratio of paper-based lateral flow assays. [ABSTRACT FROM AUTHOR]

To increase milk production, antibiotics are administered to animals to provide weight gain and to prevent or treat diseases. The inappropriate use of these substances can lead to antibiotic resistance and allergic reactions and toxic effects to milk consumers. We describe the development of a simple, fast, portable, and low-cost microfluidic paper-based analytical device (μPAD) to quantify sulfonamides in milk using the inhibition of the colorimetric reaction between carbonic anhydrase (CA) and 4-nitrophenyl acetate. The main advantages presented by the μPAD include reproducible batch production, simple application, and precise analysis without previous treatment. The µPAD displayed good linearity (R2 ≥ 0.986) in a wide range of sulfonamides in milk (2.5 to 40.0 µmol L−1), being selective for the drugs even in a highly complex matrix. We expect that this device allows in situ monitoring of milk quality, reducing the prejudicial conditions associated with high concentrations of sulfonamides in milk. [ABSTRACT FROM AUTHOR]

We report a novel three-dimensional microfluidic paper-based analytical device (3D-μPAD) with colorimetric detection, using Mn–ZnS quantum dot embedded molecularly imprinted polymer (Mn–ZnS QD-MIP), for selective glyphosate determination in whole grain samples. Detection is based on the catalytic activity of Mn–ZnS QD-MIP in the H 2 O 2 oxidation of ABTS. Glyphosate imprinted polymer is successfully synthesized on the Mn–ZnS QD surface using a poly (N-isopropylacrylamide) (NIPAM) and N, N′-Methylenebisacrylamide (MBA) as the functional monomers. The catalytic activity depends on binding or non-binding of glyphosate molecules on the synthetic recognition sites of the Mn–ZnS QD-MIP. Glyphosate selectively binds to the cavities embedded on the Mn–ZnS QD surface, and subsequently turns-off or inhibits the ABTS oxidation and color change to light green. The change of reaction color from dark green to light green depends on the concentration of glyphosate. We report, for the first time, using the relatively new penguard enamel colour to create a hydrophobic barrier. The foldable 3D-μPAD comprises three layers (top/center/bottom), named as the detection zone, immobilized Mn–ZnS QD-MIP disc, and sample loading. Assay on the 3D-μPAD can determine glyphosate by ImageJ detection, over an operating range of 0.005–50 μg mL−1 and with a detection limit of 0.002 μg mL−1. Our 3D-μPAD exhibits high accuracy, with a 0.4% (intra-day) and 0.7% (inter-day) relative difference from the certified CRM value. Moreover, the fabricated 3D-μPAD provides good reproducibility (1.7% RSD for ten devices). The developed 3D-μPAD was successfully applied to determine the glyphosate concentration in whole grain samples and shows great promise as an alternative highly selective and sensitive colorimetric method. The 3D-μPAD is well suited to food-quality control and onsite environmental-monitoring applications, without sophisticated instrumentation. Image 1 • Mn–ZnS QD-MIP was synthesized for selective and sensitive glyphosate determination. • Colorimetric detection is based on glyphosate inhibit activity of Mn–ZnS QD-MIP in the H 2 O 2 oxidation of ABTS. • 3D-μPAD fabricated from Penguard enamel colour was firstly present. • The proposed 3D-μPAD exhibits high accuracy, sensitivity, selectivity and suitable for whole-grain samples. [ABSTRACT FROM AUTHOR]

• Paper-based microfluidic device combined with a graphene heater for silicon nanodots synthesis. • Total carbohydrates determination based on fluorescence intensity of the silicon nanodots. • Heater temperature controlled by portable electronic system with wireless communication. We report a simple, rapid, low-resource and one-step method for formation of fluorescent silicon nanodots (SNDs) in a microfluidic paper-based analytical device (μPAD) incorporated in a reusable, portable and flexible heater for determination of total carbohydrates. The synthesis of SNDs is based on the redox reaction between (3-aminopropyl) triethoxysilane (APTS) reagent and carbohydrates, which act as a reducer. The graphene-based heater was fabricated by laser ablation of Kapton polyimide. Thereby, the developed system heat the μPAD during the synthesis of SNDs at 80 °C for 30 min. The blue emitting SNDs formed have an emission peak wavelength at 475 nm. We used a digital camera and smartphone for the quantitative analysis of total carbohydrates expressed such as index of glucose or fructose with grey scale value as the analytical parameter. Under the optimal conditions, the method has a low detection limit (0.80 μM for glucose and 0.51 μM for fructose, respectively), and a linear response (10–200 μM for glucose and 10–100 μM for fructose). The method has been applied to the determination of glucose in biological fluids (serum and urine samples). In addition, determination of total carbohydrates in commercial juices and teas have been carried out. [ABSTRACT FROM AUTHOR]

• A machine learning based platform is proposed for glucose determination with a μ PAD. • A smartphone app was developed to perform machine learning in colorimetric analysis. • Over 98% accuracy was obtained in glucose determination with TMB. • First study to link machine learning, μ PAD and smartphone for glucose determination. • The platform has great potential for non-invasive measurement of glucose. Potassium iodide (KI) and 3,3 ′ ,5,5 ′ -tetramethylbenzidine (TMB) are frequently used as chromogenic agents in μ PADs for glucose determination. Chitosan (Chi) has peroxidase like activity and improves the analytic performance of μ PADs when used in combination with a chromogenic agent. Here, a portable platform incorporating a μ PAD with a smartphone application based on machine learning was developed to quantify glucose concentration in artificial saliva. The detection zones of the μ PAD were modified with three different detection mixtures containing; (i) KI, (ii) KI+Chi and (iii) TMB. After the color change, the images of the μ PADs were taken with four different smartphones under seven different illumination conditions. The images were first processed for feature extraction and then used to train machine learning classifiers, resulting in a more robust and adaptive platform against illumination variation and camera optics. Different machine learning classifiers were tested and the best machine learning classifier for each detection mixture was obtained. Next, a special application called " GlucoSensing " capable of image capture, cropping and processing was developed to make the system more user-friendly. A cloud system was used in the application to communicate with a remote server running machine learning classifiers. Among the three different detection mixtures, the mixture with TMB demonstrated the highest classification accuracy (98.24%) with inter-phone repeatability under versatile illumination. [ABSTRACT FROM AUTHOR]

The main problem for the expansion of the use of microfluidic paper-based analytical devices and, thus, their mass production is their inherent lack of fluid flow control due to its uncontrolled fabrication protocols. To address this issue, the first step is the generation of uniform and reliable microfluidic channels. The most common paper microfluidic fabrication method is wax printing, which consists of two parts, printing and heating, where heating is a critical step for the fabrication of reproducible device dimensions. In order to bring paper-based devices to success, it is essential to optimize the fabrication process in order to always get a reproducible device. Therefore, the optimization of the heating process and the analysis of the parameters that could affect the final dimensions of the device, such as its shape, the width of the wax barrier and the internal area of the device, were performed. Moreover, we present a method to predict reproducible devices with controlled working areas in a simple manner. [ABSTRACT FROM AUTHOR]

• A practical concept of a solid reference electrode was introduced. • The reference electrode was integrated with paper-based microfluidics and ISEs. • The paper substrates defined the solution wicking dynamics and sample filtration. • The developed device was used for direct analysis of wastewater sludge and sweat. A new concept of a solid reference electrode integrated with microfluidic paper-based sampling was introduced and applied in potentiometric ion sensing. The new reference electrode consisted of an Ag/AgCl reference element (reusable) that is in contact with a disposable paper substrate (DPS) containing solid KCl. Thus, no KCl reference solution was applied during the analysis step, but the applied sample solution dissolved the solid KCl in the paper providing the electrolyte for the Ag/AgCl reference element. Such reference electrode was found to give a relatively constant potential after less than ca 1 min of equilibration. Moreover, the response of the reference electrode was not affected by concentrations of background electrolytes in the sample. Thus the proposed reference electrode was integrated with a paper-based microfluidic device and solid-contact ion-selective electrodes (ISEs) sensitive to K+, Na+ and Cl−. The optimized systems were characterized with near-Nernstian sensitivities (59.1 ± 1.5, 57.5 ± 0.5 and –56.4 ± 0.6 mV dec−1) and detection limits (10−4.1 ± 0.1, 10−3.3 ± 0.1 and 10−4.1 ± 0.1 mol dm−3) for K+, Na+ and Cl− ions, respectively. The paper-based measurement setup was favourably used for potentiometric determination of ions in environmental (wastewater sludge) and clinically relevant (sweat) samples. [ABSTRACT FROM AUTHOR]

To date, 55% of inhabitants of the developing world still live in rural regions and they have a very urgent need to improve the level of disease diagnosis, especially in resource-limited settings. Herein, we present a microfluidic system that centrifuges whole blood and quantifies contained biomarkers in a rapid, feasible and integrated way. Such a device provides a blood-in/answer-out capability and is small enough to be carried by any individual in any environment. The successful integration a hand-powered centrifuge and immunoassay unit within a rotational paper-based device allows for diagnostic application by untrained users and in environments where access to electricity cannot be assumed. In addition, the low cost (~$ 0.5), light weight and small instrumental footprint make the device ideally suited for rapid on-site detection. To validate the applicability of the system in a clinical diagnostic testing, we successfully perform enzyme-linked immunosorbent assay (ELISA) analysis of carcinoembryonic antigen and alpha fetoprotein from human blood samples. We expect that this powerful platform technology will provide the opportunities for point-of-care diagnosis in resource-limited settings. • Integrated hand-power blood centrifugation and ELISA analysis was firstly proposed on paper based device. • The device provides a rapid, low-cost and robust diagnostic test with blood-in/answer-out capability. • Two typical tumor markers CEA and AFP are successfully detected on this platform. [ABSTRACT FROM AUTHOR]

This article reports on the development and validation of a disposable microfluidic paper-based analytical device (μPAD) for on-hand, in-situ, and cheap Fe(III) determination in natural waters complying with World Health Organization guidelines. The developed μPAD used 3-hydroxy-4-pyridinone (3,4-HPO) as a colour reagent due to its considerably lower toxicity than traditionally used iron analytical reagents. It was selected among a group of hydrophilic 3,4-HPO chelators containing ether-derived chains in their structure which were prepared using green methods. The relatively high water solubility of these chelators improved the detection limit and applicability as μPAD reagents. Under optimal conditions, the μPAD is characterised by a quantification range between 0.25 and 2.0 mg/L, a detection limit of 55 μg/L and 15 min of analysis time. The signal stability extends up to 4 h and the device is stable for at least one month. The reagent consumption is below 0.2 mg per analysis and the μPAD method was validated by analysis certified reference materials and by comparison with atomic absorption results (RD < 10%). The newly developed μPAD was successfully applied to the determination of iron in river, well and tap waters with no need of any prior sample pre-treatment. Image 1 • First μPAD for iron analysis using specially designed hydrophilic 3,4-HPO chelator. • Arrangement of three staked hydrophilic layers for improving the limit of detection. • Successful application to natural water samples with no need of sample pretreatment. • Dynamic range complying with WHO guidelines for Fe(III) content in drinking waters. [ABSTRACT FROM AUTHOR]

• A novel analytical method based on origami paper-based microfluidic device as a platform for paper chromatography. • Device fabrication processes is a simple single side printing and curing paper. • Consumption low amount of reagents/solvents and cost-effective analysis. • The method does not require any sample pretreatment step, applying a raid analytical procedure by the green aqueous solvent Herein, we report three-dimensional paper chromatography (3D-PC) as a micro-chromatographic platform. The method was based on applying the origami microfluidic device for separation, coupled by colorimetric methods for simultaneous determination. The microfluidic device fabrication was a facile printing approach. Two azo food dyes, Tartrazine (E102) and Indigo carmine (E132), were selected as a model analyte, while carbonate-bicarbonate buffer was used as the mobile phase. Our micro-chromatographic device is associated with two big advantages including needing very small volume of mobile phase (~12 µL) and ultrafast separation time (~35 s). Under the optimal conditions, the method provided acceptable linear ranges of 0. 0 g L 1–18.0 g L 1 (R2 = 0.997) for E102 and 0.070 g L 1–10.0 g L 1 for E132 and the limits of detection (3σ/slope) were evaluated as 0.620 and 0.060 g L 1, respectively. The proposed method was successfully applied in the separation and quantification of these dyes in commercial food products such as jelly, candy, and four kinds of drink samples without any sample preparation prior to analysis. The mean recovery values for the real sample analysis were in the range of 100.14%–102.38% for E132 and E102 respectively. The inter-device relative standard deviations were in the ranges of 1.5%–11.8%. In total, our chromatographic μPAD is small (1.0 cm × 1.0 cm × 0.5 cm), portable, inexpensive, no need of specialized user, requires low volumes of sample (0.5 μL), and can perform separation using 12 μL of aqueous mobile phase in very short time. [ABSTRACT FROM AUTHOR]

In this note we describe a four-channel paper microfluidic device (μPAD) that utilizes gold nanoparticles (AuNPs) as colorimetric sensors and specifically tailored aptamers to capture a set of targeted drugs, including cocaine, codeine and methamphetamine. The design utilizes salt-induced aggregation of AuNPs to produce a color change that indicates the presence of target analytes. In the absence of the targets, the aptamers bind to AuNPs, preventing aggregation. This μPAD was developed by optimizing the concentrations of aggregating agents, AuNPs and aptamers. The resultant device is sensitive and specific, producing a positive result upon detection of the target analyte. • A four-channel paper microfluidic device with gold nanoparticles and aptamers has been developed for siezed drugs. • The design is optimized for length and width of channels, moving solution, quantity of gold nanoparticles, and aptamers. • The runtime of this μPAD is under 10 min and the LOD is 4 μg for cocaine, codeine and methamphetamine. [ABSTRACT FROM AUTHOR]

In this work, a double-layered three-dimensional (3D) microfluidic paper-based analytical device (μPAD) with high resolution temporally resolved chemiluminescence (CL) emissions were designed for multiplexed CL analysis. The temporally resolved CL emissions were obtained by virtue of the 3D branched microfluidic channel design, which create time delays for luminol transported from one detection zone to another. The peak intensity and peak shape of the temporally resolved CL emissions were quite stable and base-line separated with resolution as high as 21.2–24.4. Then, the fabricated μPAD was applied to multiplexed determination of glucose, lactate, cholesterol, and choline as model analytes. The sample was added to four detection zone modified with CL catalyst cobalt ion and different oxidase by virtue of chitosan. When luminol flowed to μPAD, four temporally resolved CL peaks were successively generated from the cobalt ion catalyzed CL reactions between luminol and generated H 2 O 2 from the specific enzymatic reactions between the oxidase and the analytes. The generated four CL emission peaks in the CL kinetic curve increased in proportion to the concentrations of glucose, lactate, cholesterol, and choline, respectively. Finally, four linear calibration curves were obtained for the detection of glucose (0.01–1.0 mmol/L), lactate (0.02–5.0 mmol/L), cholesterol (0.01–0.4 mmol/L), and choline (0.001–1.0 mmol/L). The detection limits were as low as 8 μmol/L, 15 μmol/L, 6 μmol/L, and 0.07 μmol/L for glucose, lactate, choline, and cholesterol detection, respectively. The present work provides a new strategy for the fabrication of simple and sensitive 3D μPAD with high resolution temporally resolved CL emissions for multiplexed CL analysis, which holds great application potential for point-of-care diagnosis. • A double-layered 3D μPAD with temporally resolved CL emissions was fabricated. • Stable, steep and base-line separated temporally resolved CL emissions with resolution higher than 21.2 were obtained. • Sensitive, selective and simultaneous determination of glucose, lactate, cholesterol, and choline was achieved. • The first example of μPAD based CL biosensor for lactate, cholesterol and choline detection. [ABSTRACT FROM AUTHOR]

Paper-based microfluidic devices have advanced significantly in recent years as they are affordable, automated with capillary action, portable, and biodegradable diagnostic platforms for a variety of health, environmental, and food quality applications. In terms of commercialization, however, paper-based microfluidics still have to overcome significant challenges to become an authentic point-of-care testing format with the advanced capabilities of analyte purification, multiplex analysis, quantification, and detection with high sensitivity and selectivity. Moreover, fluid flow manipulation for multistep integration, which involves valving and flow velocity control, is also a critical parameter to achieve high-performance devices. Considering these limitations, the aim of this review is to (i) comprehensively analyze the fabrication techniques of microfluidic paper-based analytical devices, (ii) provide a theoretical background and various methods for fluid flow manipulation, and (iii) highlight the recent detection techniques developed for various applications, including their advantages and disadvantages. [ABSTRACT FROM AUTHOR]