Adsorption enrichment integrated with paper-based devices for detection of trace levels of hexavalent chromium in water samples

In the present study, a sensitive microfluidic paper-based analytical device (μ-PADs) integrated with adsorption enrichment procedure was developed to analyze Cr(VI) in water samples. The affecting factors, including pH and amounts of reagents were optimized. The limit of detection of 0.0015 mg L-1 and linear range of 0.005-2 mg L-1 were achieved with good intra- and inter-day precision of 5.1 and 7.6% RSD, respectively. The results obtained by the proposed method were validated by inductively coupled plasma-optical emission spectrometry (ICP-OES). The recoveries of the present method and ICP-OES were ranged from 96.3 to 109.0‬% and 106.0 to 109.7%, respectively. The two sets of (μ-PADs and ICP-OES) results were in a good agreement as paired t-test indicated no significant differences. The proposed method could be utilized for analyzing trace levels of Cr(VI) in water samples in the absence of conventional analytical instruments.

Measurement of sucrose concentration using Imbibition length on paper: A device for equipment-free and environmentally-friendly detection

The Lucas-Washburn equation is commonly used to predict the distance (L) that a liquid travels through paper. This equation establishes that L2 is linear with time and inversely proportional to the viscosity of the liquid. However, there is currently no theoretical equation connecting the viscosity of a solution to its concentration. In this study, the imbibition flow of a sucrose solution was measured along the length of a horizontal strip of filter paper, featuring a printed, thermometer-shaped hydrophobic boundary. A sample (38 μL) was dispensed onto the bulb area, and the solution's flow was visually tracked using a red dye added to the sample. The imbibition length (L) was measured by a vernier caliper at 10.0 min after the sample addition. An empirical equation, based on literature values of the viscosity (η) and concentration (C) of sucrose solutions, was proposed. By integrating this empirical equation with the Lucas-Washburn equation, the following equation was derived: L = a⋅exp{-(bC + cC2)}, where 'a', 'b' and 'c' are parameters. This equation was fitted to the dataset of L and C, covering C values from 0 to 60 % w/w standard sucrose solutions, resulting in a coefficient of determination of 0.9987. The plot of L against C was observed to closely follow a linear line, with a fitting providing a coefficient of determination of 0.9986. The sucrose contents in samples, such as soft drinks, syrups, and sugarcanes, determined using the imbibition length method and conventional refractometry, were in statistical agreement via the paired t-test at the 95 % confidence level. This method is simple, instrument-free, requiring only a small amount of safe red food dye, and can be conducted on-site.

Novel molecularly imprinted nanogel modified microfluidic paper-based SERS substrate for simultaneous detection of bisphenol A and bisphenol S traces in plastics

Paper-based surface-enhanced Raman scattering (SERS) optical nanoprobes provide ultrasensitive analyte detection; however, they lack selectivity, making them difficult to use in real-world sample analysis without a pretreatment process. This work describes the design of a microfluidic paper-based SERS substrate based on molecularly imprinted nanogels decorated with silver nanoparticles to simultaneously detect bisphenol A (BPA) and bisphenol S (BPS) traces in plastic toys and receipts. The synthesized nanogels have two characteristics that boost SERS performance: molecularly imprinted cavities that allow for selective adsorption and a wrinkled surface that creates uniformly distributed hot spots. Simple paper-based sensor devices were built as 'drop and read' SERS substrates with a separate reservoir to detect a single target, while advanced SERS platforms were designed as a microfluidic chip with two reservoirs connected by a channel for simultaneous detection of BPA and BPS. The SERS platform with a single reservoir showed outstanding analytical performance for the detection of BPA and BPS, with low detection limits of 0.38 pM and 0.37 pM, respectively. The microfluidic paper-based sensor allowed simultaneous and selective detection of BPA and BPS with detection limits estimated at 0.68 nM and 0.47 nM, respectively. The developed sensors are successfully applied to detect BPA and BPS in plastic products and receipts. Finally, the results obtained with our method showed greater sensitivity than those of commercially available ELISA kits, and the acquired values within the ELISA detection range were in excellent agreement.

Development of a colorimetric sensor based on the coupling of a microfluidic paper-based analytical device and headspace microextraction for determination of formaldehyde in textile, milk, and wastewater samples

A user-friendly, cost-effectively, portable, and environmentally friendly colorimetric sensor for the quantitative determination of formaldehyde was developed based on the combining of microfluidic paper-based analytical device (μPAD), headspace microextraction (HSME), and digital image colorimetry. Coupling HSME and μPAD led to enhancements in selectivity and sensitivity of the sensor through sample cleanup and analyte enrichment. To construct the μPAD-HSME device, two pieces of paper as the sample and detection zone were placed facing each other so that a small common and sealed space was created between them. The color change occurred when the analyte in the gaseous form crossed this gap and reached the detection zone. Colorimetric sensing in the detection zone was performed based on the Hantzsch reaction. The color change in the detection zone was recorded by a smartphone and digital images were processed using image analysis software based on the RGB model. The influence of some key variables on the sensitivity of the method including derivatization reagent composition, sample volume, extraction temperature, and extraction time was studied and optimized. The linear dynamic range of the method was obtained in two ranges of 0.10-0.75 and 0.75-5.0 mg L-1 with a limit of detection of 0.03 mg L-1. The recoveries were in the range 80-126% for the quantification of formaldehyde in textile, milk, and wastewater samples.

Mickey mouse-shaped laminated paper-based analytical device in simultaneous total cholesterol and glucose determination in whole blood

The microfluidic paper-based analytical device (μPAD) platform is gaining attention as a low-cost, portable, and disposable detection tool. However, the limitations of traditional fabrication methods include poor reproducibility and the use of hydrophobic reagents. In this study, an in-house computer-controlled X-Y knife plotter and pen plotter were used to fabricate μPADs, resulting in a simple, more rapid, reproducible process that consumes less volume of reagents. The μPADs were laminated to increase mechanical strength and reduce sample evaporation during analysis. The resulting laminated paper-based analytical device (LPAD) was used to simultaneously determine glucose and total cholesterol in whole blood using the LF1 membrane as a sample zone. The LF1 membrane selectively separates plasma from whole blood by size exclusion and yields plasma for further enzymatic reaction steps while retaining blood cells and larger proteins. The i1 Pro 3 mini spectrophotometer directly detected color on the LPAD. The results were clinically relevant and in agreement with hospital methods, with a detection limit of 0.16 mmol L⁻¹ for glucose and 0.57 mmol L⁻¹ for TC. The LPAD retained color intensity after 60 days of storage. The LPAD offers a low-cost, high-performance option for chemical sensing devices and expands the applicability of markers for diagnosing whole blood samples.

Recent developments in microfluidic paper-based analytical devices for pharmaceutical analysis

In the last decade, due to the global increase in diseases, drugs for biomedical applications have increased dramatically. Therefore, there is an urgent need for analytical tools to monitor, treat, investigate, and control drug compounds in diverse matrices. The new and challenging task has been looking for simple, low-cost, rapid, and portable analytical platforms. The development of microfluidic paper-based analytical devices (µPADs) has garnered immense attention in many analytical applications due to the benefit of cellulose structure. It can be functionalized and serves as an ideal channel and scaffold for the transportation and immobilization of various substances. Microfluidic technology has been considered an effective tool in pharmaceutical analysis that facilitates the quantitative measurement of several parameters on cells or other biological systems. The µPADs represent unique advantages over conventional microfluidics, such as the self-pumping capability. They have low material costs, are easy to fabricate, and do not require external power sources. This review gives an overview of the current designs in this decade for µPADs and their respective application in pharmaceutical analysis. These include device designs, choice of paper material, and fabrication techniques with their advantages and drawbacks. In addition, the strategies for improving analytical performance in terms of simplicity, high sensitivity, and selectivity are highlighted, followed by the application of µPADs design for the detection of drug compounds for various purposes. Moreover, we present recent advances involving µPAD technologies in the field of pharmaceutical applications. Finally, we discussed the challenges and potential of µPADs for the transition from laboratory to commercialization.

Microfluidic paper-based analytical device for determination of sucrose in sugarcane juice using Benedict's reagent

This work presents a microfluidic paper-based analytical device (μPAD) for the determination of sucrose using the Benedict's test. An asymmetric dumbbell-shaped hydrophobic barrier was produced by rubber stamping the barrier pattern onto a laboratory filter paper. Hydrochloric acid and solution containing sucrose were successively deposited onto the sample reservoir of the μPAD attached to a glass slide. The device was placed in a plastic bag and dipped into boiling water for accelerating the hydrolysis of sucrose into the reducing sugars. Then the Benedict's reagent was added at the narrow straight channel connecting the two circular zones of the μPAD, which was replaced in the plastic bag and heated again for reduction of Cu(II) by the reducing sugars. Precipitate of brick-red copper(I) oxide was formed. The image of the μPAD was recorded by a smartphone. The ratio of the red to blue intensities gave linear correlation with the concentration of sucrose in the range of 0.5-10% w/v. The relative standard deviation of the measurement was less than 5% for 2 and 4% w/v sucrose (n = 10), with limit of determination, calculated using standard deviation of regression divided by slope of calibration, of 0.26% w/v sucrose. The method was successfully validated using the dinitrosalicylic acid method for sucrose measurement. Percent recoveries of sucrose were evaluated using ten sugarcane samples. The recoveries were in the range of 89 to 101%, demonstrating that there were no significant sample matrix effects on the quantification.

Dispersive liquid-liquid microextraction coupled with microfluidic paper-based analytical device for the determination of organophosphate and carbamate pesticides in the water sample

A microfluidic paper-based analytical device (µ-PAD) is a promising new technology platform for the development of extremely low-cost sensing devices. However, it has low sensitivity that might not enable to measure maximum allowable concentration of various pollutants in the environment. In this study, a dispersive liquid-liquid microextraction (DLLME) was developed as a preconcentration method to enhance the sensitivity of the µ-PAD for trace analysis of selected pesticides. Four critical parameters (volume of n-hexane and acetone, extraction time, NaCl amount) that affect the efficiency of DLLME have been optimized using response surface methodology. An acceptable mean recovery of 79-97% and 83-93% was observed at 1 µg L^-1 and 5 µg L^-1 fortification level, respectively, with very good repeatability (2.2-6.01% RSD) and reproducibility (5.60-10.41% RSD). Very high enrichment factors ranging from 317 to 1471 were obtained. The limits of detection for the studied analytes were in the range of 0.18-0.41 µg L^-1 which is much lower than the WHO limits of 5-50 µg L^-1 for similar category of analytes. Therefore, by coupling DLLME with µ-PAD, a sensitivity that allows to detect environmental threat and also that surpassed most of the previous reports have been achieved in this study. This implies that the preconcentration step has a paramount contribution to address the sensitivity problem associated with µ-PAD.

Determination of phosphorus in water and chemical fertilizer samples using a simple drawing microfluidic paper-based analytical device

A simple one-step drawing for the cost-effective fabrication of microfluidic paper-based analytical devices (µPADs) for the determination of phosphate content in water and fertilizer samples is presented in this paper. The hydrophobic barrier of µPAD was patterned using a 2-mm tip marker pen using a transparent acrylic sheet template. The molybdenum blue reaction using ascorbic acid as a reducing agent was used. A pre-concentration step of samples is proposed to improve the sensitivity of the measurement. The blue complex produced on the µPADs was recorded using a smartphone camera. The color intensities (red, green, blue and gray) were analyzed using ImageJ program. The proposed µPAD method provides a linear calibration range from 0 to 100 mg L^-1 P. The limit of detection (LOD) was found to be 0.7 mg L^-1 P with a precision of 3.1%RSD for 50 mg L^-1 P (n = 10). The proposed method was successfully applied to the determination of phosphorus contents in water and liquid chemical fertilizer samples. The results obtained from µPAD agreed with a spectrophotometric method using paired t test at a 95% confidence level.

Rapid and accurate nanoelectrokinetic diagnosis of drug-resistant bacteria

Increased antimicrobial resistance presents a major threat to public health, and it is a global health problem due to the rapid globalization and transmission of infectious diseases. However, fast and precise diagnosis tool is lacking, and inappropriate antibiotic prescription leads to the unforeseen production of drug-resistant bacteria. Here, we report a Rapid and Accurate Nanoelectrokinetic Diagnostic System (RANDx) for detecting drug-resistant bacteria, which cause a common infectious disease called Urinary Tract Infection (UTI), within 7 min. We develop nanoelectrokinetic paper-based analytic device (NEK-PAD) as a sample prep module of RANDx and obtain >100-fold post-wetting preconcentration by balancing between ion concentration polarization (ICP) and radial imbibition for a constant flow rate. Simultaneously with preconcentration, our cathodic nanochannel design enables NEK-PAD to extract drug-resistant enzymes without denaturation and accelerate enzyme-linked reactions under electrical spontaneous heating at approximately 37 °C. Finally, using a cell phone camera, we detect label-free drug-resistant bacteria as low as 10⁴ cfu/mL, which is higher than clinically required threshold (>10⁵ cfu/mL) by enhancing 1000 times of the limit of detection (LOD) of colorimetric nitrocefin assay. We believe that the RANDx will be an innovative precision medicine tool for UTI and other infectious diseases in limited remote settings.

SERS-based CRISPR/Cas assay on microfluidic paper analytical devices for supersensitive detection of pathogenic bacteria in foods

Rapid and point-of-need (PON) detection of bacteria is crucial to directly provide rapid and reliable diagnostics information during on-site tests, allowing more room for taking proactive measures. By taking the multifaceted advantages of CRISPR/Cas12a and surface-enhanced Raman scattering (SERS), for the first time, we designed a recombinase polymerase amplification (RPA)-integrated microfluidic paper-based analytical device (μPAD), coined RPA-Cas12a-μPAD for supersensitive SERS detection. Single-stranded DNAs were designed to "pull down" SERS nanoprobes. The amplicons of the invA gene triggered the trans-cleavage of Cas12a, resulting in the indiscriminate shredding of linker ssDNA. Thus, the degree of aggregation of SERS nanoprobes was dependent on the concentration of Salmonella typhimurium (S. typhi), which was determined on a μPAD and monitored by a Raman spectrometer. The limit of detection for S. typhi was approximately 3-4 CFU/mL for spiked milk and meat samples with a dynamic detection range from 1 to 10⁸ CFU/mL. The RPA-Cas12a-μPAD secured accurate tests for food samples in 45 min. This work expands the reach of CRISPR-based diagnostics (CRISPR-Dx) and provides a novel and robust bacterial PON detection platform.

Portable smartphone-based colorimetric system for simultaneous on-site microfluidic paper-based determination and mapping of phosphate, nitrite and silicate in coastal waters

Early and on-site detection of environmental contaminations and physicochemical parameters of seawater is increasingly preferred to guarantee hazard minimization in many settings. In this paper, we describe a combination of microfluidic paper-based sensors (µPADs) and an Android-based smartphone application (App) for simultaneous on-site quantification of phosphate (PO₄-P), silicate (SiO₃-Si) and nitrite (NO₂-N) in coastal seawater samples. The developed App can on-site capture, process, and quantify the µPAD colorimetric outputs. This App uses an image processing algorithm for quantifying color intensity and relating the RGB components to the analyte concentrations. The GPS-tagged data can be stored on the smartphone or sent via social networks. The significant factors affecting the detection of the analytes were optimized using Box-Behnken design. Under optimized parameters, the proposed method presented the linear ranges between 5 and 100 µg L^-1 for phosphate (R² = 0.9909), 5 to 100 µg L^-1 (R² = 0.9819) for nitrite and 10 to 600 µg L^-1 (R² = 0.9933) for silicate. The LODs of the method for detection of phosphate, nitrite and silicate were 1.52 µg L^-1, 0.61 µg L^-1 and 3.74 µg L^-1, respectively. The device was successfully used to simultaneous analyze and map the PO₄-P, SiO₃-Si and NO₂-N of Bushehr coastal seawater samples (Iran). The results were confirmed by the lab-based conventional colorimetric methods using spectrophotometer.

Reversal of a Fluorescent Fluoride Chemosensor from Turn-Off to Turn-On Based on Aggregation Induced Emission Properties

Here we present a new approach for the development of fluoride chemosensors taking advantage of aggregation induced emission (AIE) properties. Although AIE-based chemosensors have been described, they rely primarily on the analyte causing aggregation and hence fluorescence. We propose a new concept in the use of AIE for the development of fluorescent sensors. Our hypothesis is based on the fact that a turn-off chemosensor in solution can be transformed into turn-on in the solid state if the properties of ACQ and AIE are properly combined between the fluorescent molecules involved. To demonstrate this hypothesis, we have selected a fluorescent chemosensor for the fluoride anion with a conjugated structure of bis(styryl)pyrimidine that, while showing turn-off behavior in solution, becomes turn-on when it is brought to the solid state. We have also combined it with the advantages of a detection system based on the microfluidic paper-based analytical devices (μPAD). The system is fully characterized spectroscopically both in solution and in the solid state, and quantum mechanical calculations were performed to explain how the sensor works. The prepared device presents a high sensitivity, with no interference and with an LoD and LoQ that allow determination of fluoride concentrations in water 2 orders of magnitude below the maximum allowed by WHO.

An automated fast-flow/delayed paper-based platform for the simultaneous electrochemical detection of hepatitis B virus and hepatitis C virus core antigen

Electrochemical paper-based analytical devices (ePADs) are useful analytical devices that serve as point-of-care testing (POCT) devices for various clinical biomarkers in view of their simplicity, portability, and low-cost format. However, multistep reagent manipulation usually restricts the performance of the device for end users. Herein, we developed a sequential ePAD for sequential immunosensing fluid delivery by integrating dual flow behaviors (fast-flow/delayed) within a single paper platform for the simultaneous detection of hepatitis B surface antigen (HBsAg) and hepatitis C core antigen (HCVcAg). In the present work, a fast-flow channel was used for the automated washing of unbound antigens, while a delayed channel was created to store a redox reagent for further electrochemical analysis with a single buffer loading (the analysis time can be completed within 500 s). Hence, the undesirable complex procedure of multi-step reagent manipulation is scarcely needed by the user. The detection limit of the proposed ePAD was as low as 18.2 pg mL^-1 for HBsAg and 1.19 pg mL^-1 for HCVcAg. In addition, this proposed ePAD was also proven to be effective in real clinical sera from patients to verify its biological applicability. The ePAD sensor shows high promise as an easy-to-use, portable, and extendable sensor for other multiplex biological assays.

Simple gunshot residue analyses for estimating firing distance: Investigation with four types of fabrics

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.

Paper-based electrochemiluminescence device for the rapid estimation of trimethylamine in fish via the quenching effect of thioglycolic acid-capped cadmium selenide quantum dots

A paper-based electrochemiluminescence device (µPAD-ECL) for the estimation of trimethylamine (TMA) concentration in fish was developed using tris(2,2'-bipyridyl)ruthenium(II) complex coupled with water soluble thioglycolic acid-capped CdSe quantum dots on the inkjet-printed paper-based device. The quenching effect of tertiary amines on the ECL intensity was found to be sensitive and concentration dependent. This effect allows the measurement of TMA at low concentrations. Under the optimal conditions, the linear concentration range was exhibited from 1 × 10^-12 to 1 × 10^-7 M and a detection limit of 2.09 × 10^-13 M, with relative standard deviation of 1.97 %. The applicability of µPAD-ECL is demonstrated by the rapid estimation of trimethylamine concentration in fish tissue, and could be used as a method for screening the total amount of tertiary amines in fishery products in remote communities. The results obtained using the paper-based devices agreed well with those obtained applying high performance liquid chromatography with benzoyl derivatization, at a confidence level of 95%.

Remote Investigation of Total Chromium Determination in Environmental Samples of the Kombolcha Industrial Zone, Ethiopia, Using Microfluidic Paper-based Analytical Devices

Microfluidic paper-based analytical devices (μ-PADs) fabricated in Japan were employed for the determination of total chromium (Cr) in water, soil, and lettuce irrigated with wastewater in Ethiopia. The μ-PADs, which were printed by wax printing in Japan, were transported to Ethiopia and prepared for the determination of total Cr by adding appropriate reagents to the pretreatment and detection zones. Soil and lettuce samples were determined by the μ-PADs and a UV-Vis spectrophotometer in Ethiopia. A paired t-test showed that the mean total Cr concentrations determined in the soil and lettuce samples were not significantly different between μ-PADs and UV-Vis spectrophotometric analysis at the 5% level of significance. This implies that the μ-PADs have good accuracy and reliability, and could be employed to monitor Cr in environmental samples. We found that the total Cr concentrations in all soil and lettuce samples were above the permissible limit. Moreover, evaluating Cr contamination level using the geo-accumulation index indicated that the soils were contaminated with Cr moderately to heavily. Thus, the present work successfully demonstrated the potential of remote investigations of pollution in a less-equipped laboratory by transporting the μ-PADs fabricated in another laboratory.

Speciation of chromium in water samples using microfluidic paper-based analytical devices with online oxidation of trivalent chromium

Speciation of chromium (Cr) was demonstrated using microfluidic paper-based analytical devices (μ-PADs) that permit the colorimetric determination of hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) via online oxidation. The μ-PADs consist of left and right channels that allow the simultaneous measurements of Cr(VI) and total Cr based on the colorimetric reaction of Cr(VI) with 1,5-diphenylcarbazide (DPC). For the determination of Cr(VI), a sample solution was directly reacted with DPC in the left channels whereas total Cr was determined in the right channels, which permitted online oxidation in the pretreatment zone containing cerium (IV) (Ce(IV)) followed by a colorimetric reaction with DPC. We found that the online oxidation of Cr(III) proceeded 100% whereas Ce(IV) inhibited the reaction of Cr(VI) with DPC. Therefore, speciation can be achieved by measuring the Cr(VI) and total Cr in the left and right channels followed by the subtraction of Cr(VI) from total Cr. The limits of detection and quantification were 0.008 and 0.02 mg L^-1 for Cr(VI) and 0.07 and 0.1 mg L^-1 for Cr(III) or total Cr, respectively. The linear dynamic ranges were 0.02-100 mg L^-1 and 0.1-60 mg L^-1 for Cr(VI) and Cr(III), respectively. The RSDs were less than 7.5%. The results obtained using μ-PADs were in good agreement with those obtained via ICP-OES with recoveries of 92-108% for Cr(III) and 108-110% for Cr (VI) using μ-PADs, and 106-110% for total Cr using ICP-OES. Thus, the μ-PADs could potentially be utilized for the speciation of chromium in developing countries where environmental pollution and the availability of sophisticated instruments are significant problems.

Simultaneous Multiplexed Detection of Protein and Metal Ions by a Colorimetric Microfluidic Paper-based Analytical Device

In order to improve the efficiency of disease diagnosis and environmental monitoring, it is desirable to detect the concentration of proteins and metal ions simultaneously, since the current popular diagnostic platform can only detect proteins or metal ions independently. In this work, we developed a colorimetric microfluidic paper-based analytical device (µPAD) for simultaneous determination of protein (bovine serum albumin, BSA) and metal ions [Fe(III) and Ni(II)]. The µPAD consisted of one central zone, ten reaction zones and ten detection zones in one device, in which reaction solutions were effectively optimized for different types of chromogenic reactions. Fe(III), Ni(II) and BSA can be easily identified by the colored products, and their concentrations are in good accordance with color depth based on the established standard curves. The detection limits are 0.1 mM for Fe(III), 0.5 mM for Ni(II) and 1µM for BSA, respectively. Best of all, we demonstrated the efficiency of the µPAD with accurate detection of Fe(III), Ni (II) and BSA from river water samples within 15 minutes. The µPAD detection is efficient, instrument-free, and easy-to-use, holding great potential for simultaneous detection of cross type analytes in numerous diagnostic fields.

Micro-PAD card for measuring total ammonia nitrogen in saliva

This work presents a portable microfluidic paper-based analytical device (micro-PAD) card for the quantification of total ammonia nitrogen in human saliva. The amount of total ammonia nitrogen in saliva can be an indicator of the status of the oral microbiome with potential correlation to kidney health problems. The developed micro-PAD card comprises twenty units consisting of three stacked layers of circular discs: the sample layer, paper discs impregnated with sodium hydroxide solution, the PTFE membrane layer, and the detection layer, paper discs impregnated with bromothymol blue. The twenty units were aligned on transparent laminating pouches laminated to form the micro-PAD card (7.5 cm × 10.5 cm). Saliva samples can be directly dispensed onto the micro-PAD card and the detection was achieved by the BTB indicator color change, from yellow to blue, after conversion of ammonium into ammonia and diffusion of the ammonia gas through a hydrophobic layer. The determination of total ammonia nitrogen in saliva using the developed micro-PAD card intended to be very simple method and operated without the need of laboratory equipment. A quantification limit of 11.3 NH₄⁺mg L^-1 and linear application range from up to 150 NH₄⁺mg L^-1 were obtained making it suitable for the expected concentrations of total ammonia nitrogen in human saliva. It was successfully applied to saliva samples and its validation obtained by comparison against a potentiometric method. The card is stable for at least 1 month making it ideal as a portable device for point-of-care diagnosis. Graphical Abstract.

Development of paper-based microfluidic device for the determination of nitrite in meat

This study employed the use of a microfluidic paper-based analytical device (µPAD) to determine the concentration of nitrite in pork and enhanced the limit of detection by analyzing the coffee-ring effect. The µPAD was fabricated by designing and embedding wax channels onto the cellulose-based filter paper through printing and subjecting the paper to heat treatment to allow wax penetration. Nitrite concentration was determined by monitoring the colorimetric reaction that occurred between nitrite and the added Griess reagent. The limit of detection of this device for nitrite in pork was determined to be 19.2 mg kg^-1 by analyzing the inner-chamber reaction, while it could be as low as 1.1 mg kg^-1 if the coffee-ring region was analyzed. The overall analysis could be completed within 15 min. This µPAD-based method has potential applications to routinely screen the nitrite concentration of meat products and ensure food safety and consumer health.

Multiplexed chemiluminescence determination of three acute myocardial infarction biomarkers based on microfluidic paper-based immunodevice dual amplified by multifunctionalized gold nanoparticles

Acute myocardial infarction (AMI) causes significant mortality and morbidity. The determination of multiple AMI biomarkers is very important for the timely diagnosis of AMI. In this work, simultaneous determination of three AMI biomarkers were achieved by virtue of a three-dimensional (3D) microfluidic paper-analytical device (μPAD) with temporally resolved chemiluminescence (CL) emissions for the first time. A dual-signal amplification strategy was introduced including by employing primary antibody functionalized gold nanoparticles (Ab₁-GNPs) immobilized on the detection zone as amplified capture probes, and Co(II) catalyst, secondary antibody, luminol multifunctionalized gold nanoparticles (Co(II)-Ab₂-luminol-GNPs) with excellent CL activity as amplified signal probes. CL immunoreactions were performed at three detection zone of the fabricated 3D μPAD by assembling Ab₁-GNPs, antigen, and Co(II)-Ab₂-luminol-GNPs to form sandwich-type immunocomplexes. Auto separated CL signals with temporal resolution were obtained by time delayed transport of H₂O₂ to different detection zones for multiplexed analysis. The CL signal obtained by using Co(II)-Ab₂-luminol-GNPs as signal probe (10576 a.u.) were about 20-fold higher than that by using conventional horseradish peroxidase labeled antibody modified luminol-GNPs as signal probe (531 a.u.). Finally, three AMI biomarkers including heart-type fatty acid-binding protein (H-FABP), cardiac troponin I (cTnI) and copeptin were quantitatively analyzed in one CL detection run by reading the CL intensity of the obtained three CL emission peaks. The detection range were ultra-wide ranged from 0.1 pg/mL to 1 μg/mL, 0.5 pg/mL to 1 μg/mL and 1 pg/mL to 1 mg/mL with the detection limits down to 0.06 pg/mL, 0.3 pg/mL and 0.4 pg/mL for H-FABP, cTnI and copeptin detection, respectively. The developed μPAD based immunoassay performing multiplexed analysis ability, high sensitivity, ultra-wide dynamic range, favorable selectivity, accessible accuracy and reproducibility, have great application potential for the early diagnosis of AMI.

Photoelectrochemical biosensor of HIV-1 based on cascaded photoactive materials and triple-helix molecular switch

In this work, an ultrasensitive photoelectrochemical (PEC) biosensor was proposed to detect nucleic acids on the basis of cascaded photoactive materials and triple-helix molecular switch. DNA sequence of human immunodeficiency virus type 1 (HIV-1) was chosen as the target DNA (T-DNA). Cascaded photoactive structure was formed via different sizes of CdTe quantum dots (QDs) sensitized ZnO nanorods (ZnO NRs), which was employed as a cascaded photoactive interface to amplify the photocurrent signal. A hairpin structure DNA (H-DNA) as capture probe was conjugated onto the photoactive interface through amide bond, and then a single-stranded DNA modified with gold nanoparticles labeled alkaline phosphatase (ALP-Au NPs-DNA) at each end was introduced to hybridize with the H-DNA to form a triple-helix conformation. The T-DNA detection was based on the photocurrent response change resulted from conformation change of the triple-helix molecule after hybridization with T-DNA. In the absence of T-DNA, the triple-helix molecule was in a closed state and the ALP of ALP-Au NPs-DNA could specifically catalyze the ascorbic acid 2-phosphate (AAP) to generate ascorbic acid (AA) as electron donors, which resulted in a significant photocurrent response due to the rapid electron transfer process. However, in the presence of T-DNA, the T-DNA hybridized with the ALP-Au NPs-DNA molecule, which caused triple-helix molecule in an opened state and compelled ALP-Au NPs-DNA away from the electrode surface, resulting in the absence of ALP which could catalyze AAP to generate AA. Subsequently, the photocurrent response significantly decreased. The proposed PEC biosensor not only had a wide detection range of 1fM-1nM and low detection limit (0.65 fM), but also showed excellent reproducibility, specificity and stability, which had great application prospect and opened up a new research method in the early clinical diagnosis and cancer research.

Janus electrochemistry: Simultaneous electrochemical detection at multiple working conditions in a paper-based analytical device

The simultaneous detection of multiple analytes from a single sample is a critical tool for the analysis of real world samples. However, this is challenging to accomplish in the field by current electroanalytical techniques, where tuning assay conditions towards a target analyte often results in poor selectivity and sensitivity for other species in the mixture. In this work, an electrochemical paper-based analytical device (ePAD) capable of performing simultaneous electrochemical experiments in different solution conditions on a single sample was developed for the first time. We refer to the system as a Janus-ePAD after the two-faced Greek god because of the ability of the device to perform electrochemistry on the same sample under differing solution conditions at the same time with a single potentiostat. In a Janus-ePAD, a sample wicks down two channels from a single inlet towards two discreet reagent zones that adjust solution conditions, such as pH, before flow termination in two electrochemical detection zones. These zones feature independent working electrodes and shared reference and counter electrodes, facilitating simultaneous detection of multiple species at each species' optimal solution condition. The device utility and applicability are demonstrated through the simultaneous detection of two biologically relevant species (norepinephrine and serotonin) and a common enzymatic assay product (p-aminophenol) at two different solution pH conditions. Janus-ePADs show great promise as an inexpensive and broadly applicable platform which can reduce the complexity and/or number of steps required in multiplexed analysis, while also operating under the optimized conditions of each species present in a mixture.

Electrochemiluminescence cytosensing platform based on Ru(bpy)32+@silica-Au nanocomposite as luminophore and AuPd nanoparticles as coreaction accelerator for in situ evaluation of intracellular H2O2

An electrochemiluminescence (ECL) cytosensor was fabricated onto a microfluidic paper-based analytical device (μ-PAD) in order to detect hydrogen peroxide (H₂O₂) which was released from tumor cells. The ECL probe Ru(bpy)₃²⁺@silica-Au nanocomposite (Ru@SiO₂-Au) was fabricated and served as ECL reagent. The ECL of Ru@SiO₂-Au nanocomposite was quenched by the ferrocene (Fc). AuPd nanoparticles (AuPd NPs), which were modified on the paper working electrode (PWE), were served as the catalyst of H₂O₂ to produce hydroxyl radicals (•OH) for cleaving Fc-labelled DNA to achieve "signal-on", and AuPd NPs also severed as coreaction accelerator. H₂O₂ was released from cells under the stimulation of phorbol myristate acetate. Fc-labelled DNA strand was cleaved via •OH. Fc molecule departed from the PWE surface, The ECL could be recovered. An ECL cytosensor on a 3D origami device was constructed. The ECL response of the Ru@SiO₂-Au-Fc system was related to the number of cells. The ECL intensity was quantitatively related with the logarithm of MCF-7 cells number and H₂O₂ concentration, the detection limit was 30 cells mL^-1. As a consequence, this work provided a really low-cost and disposable μ-PAD for sensitive detection of intracellular H₂O₂. What's more, this work had potential application value for studying cellular biology and pathophysiology.

Paper-based microfluidic devices for glucose assays employing a metal-organic framework (MOF)

This paper describes the development of two microfluidic paper-based analytical devices (μPADs), one well-based and the other based on a lateral flow assay (LFA) configuration, to detect glucose via a colorimetric assay using the solid metal-organic framework (MOF) Zr-PCN-222(Fe), to encapsulate glucose oxidase (GOx). The well-based platform consisted of laminate sheets and multiple layers of wax-printed chromatography paper. Solutions of KI and glucose placed into the well flowed through the device and reacted with the GOx@MOF species sandwiched between the paper layers realizing a yellow-brown color. The LFA platform consisted of chromatography paper between parafilm and polyvinyl acetate (PVA) layers. GOx@MOFs spotted on the paper subjected to solutions of KI and glucose yielded a brown color. The devices were then dried, scanned, and analyzed yielding a correlation between average inverse yellow intensity and glucose concentrations. The development of these devices employing MOFs as biomimetic catalysts should further expand the applications of microfluidic technologies for sensors a variety of analytes.

High-throughput deposition of chemical reagents via pen-plotting technique for microfluidic paper-based analytical devices

The deposition of chemical reagent inks on paper is a crucial step in the development and fabrication of microfluidic paper-based analytical devices (μPADs). A pen-plotting approach, delivering chemical ink deposition using technical pens filled with reagents and inserted into a desktop electronic plotter, is shown herein to be a versatile, low-cost, simple, rapid, reproducible, and high-throughput solution. The volume of the deposited ink was quantified gravimetrically, confirming that nanoliter volumes of reagents can be deposited reproducibly (e.g. 7.55 ± 0.37 nL/mm for a plotting speed of 10 cm/s) in detection zones of μPADs, typically spatially defined using wax printing. This approach was further investigated with regard to deposition of reagents in different geometrical forms (circular and linear), so demonstrating its applicability for preparation of μPADs with flexible design and application. By adjusting the plotting speed for linear deposition, lines with a relatively large range of widths (≈628-1192 μm) were created. Circular deposition was also demonstrated via delivery of reagents within wax printed circular fluidic barriers of a range of diameters (inner diameter = 1.5-7 mm). These capabilities were practically demonstrated via the fabrication of μPADs, based upon differing detection principles for determination of aluminum in natural waters using Morin as the fluorescent reagent. Traditional μPADs based on digital image colorimetry (DIC) were produced using circular deposition, whilst distance-based μPADs exploited linear deposition. Both types of μPADs developed using this method showed excellent precision for determination of trace concentrations of aluminium (average RSDs = 3.38% and 6.45%, and LODs = 0.5 ng (0.25 ppm) and 2 ng (0.5 ppm), for traditional and distance-based detection, respectively).

Simple and green method for direct quantification of hypochlorite in household bleach with membraneless gas-separation microfluidic paper-based analytical device

This work presents development of a microfluidic paper-based analytical device (µPAD) for direct determination of hypochlorite in household bleach. The recent design of a membraneless gas-separation microfluidic paper-based analytical device (MBL-GS µPAD) was employed to fabricate the hypochlorite-µPAD. Chlorine gas is generated in the µPAD via acidification of an aliquot of sample loaded on to the donor reservoir located at the bottom layer of the μPAD. The liberated chlorine gas diffuses through the air space to oxidize iodide ion previously impregnated in the acceptor reservoir at the top layer of the μPAD, leading to formation of the brown color of the tri-iodide ions. Digital image of the brown zone was captured at exactly 5 min after loading the acid. Image J program is used for analysis of the image for quantification of the hypochlorite in unit of g Cl₂ L^-1. It was found that employing a relatively large volume of the air space (ca. 270 µL) direct analysis of the high concentration of hypochlorite in the bleach was achieved without prior dilution. The method thus provides a linear working range of 25-100 g Cl₂ L^-1, which is suitable for most commercial household products. The calibration line has a coefficient of determination of 0.999. The precision of measurements is 0.96% RSD and 0.30% RSD at 30 g Cl₂ L^-1 and 80 g Cl₂ L^-1 (n = 10), respectively. Using the paired t-test (P = 0.05, n = 8), the method agreed well with the iodometric titration method. Our μPAD for hypochlorite is portable and cost-effective. The method is also "green" since there is a significant reduction in use of reagents compared to other conventional methods.

Highly sensitive microfluidic paper-based photoelectrochemical sensing platform based on reversible photo-oxidation products and morphology-preferable multi-plate ZnO nanoflowers

A microfluidic paper-based analytical device (μPAD) was simply constructed for highly sensitive detection of L-glutamic acid and L-cysteine. The μPAD featured with two functional zones on one strip of paper achieved by preferable multi-plate ZnO nanoflowers (ZnO NFs) and molecularly imprinting polymer (MIP) membranes. The as-designed μPAD was established based on the inherent relation between the photo-oxidation products and photoelectrochemical (PEC) performance with the highly sensitive detection of biomolecules. The ZnO NFs were utilized to produce photo-oxidation products by driving the reaction between ferrocenemethanol and photogenerated holes under ultraviolet light. The photo-oxidation products easily flowed to MIP membranes along the hydrophilic channel via capillary action. MIP membranes as the receptors specifically recognized the analytes as well as decreased the electron loss by blocking the reduction reaction between electrons and photo-oxidation products. The PEC response was obtained in the processes of electrons transfer and exhibited the direct relationships corresponding to the concentrations of target analytes. The μPAD showed the detection limits toward L-glutamic acid and L-cysteine as low as 9.6 pM and 24 pM, respectively. Moreover, it is interesting to point out that ZnO NFs nanostructure shows superior PEC signal compared with those of ZnO nanospheres, nanosheets, and nanorod arrays. In current work, photo-oxidation products are utilized to achieve highly sensitive PEC detection for biomolecules under ultraviolet light as well as avoid the effects of multiple modifications in the same region on the reproducibility, which is beneficial for opening up rich possibility for designing more efficient analytical strategy.

Laminated and infused Parafilm® - paper for paper-based analytical devices

Numerous fabrication methods have been reported for microfluidic paper-based analytical devices (μPADs) using barrier materials ranging from photoresist to wax. While these methods have been used with wide success, consistently producing small, high-resolution features using materials and methods that are compatible with solvents and surfactants remains a challenge. Two new methods are presented here for generating μPADs with well-defined, high-resolution structures compatible with solvents and surfactant-containing solutions by partially or fully fusing paper with Parafilm® followed by cutting with a CO₂ laser cutter. Partial fusion leads to laminated paper (l-paper) while the complete fusion results in infused paper (i-paper). Patterned structures in l-paper were fabricated by selective removal of the paper but not the underlying Parafilm® using a benchtop CO₂ laser. Under optimized conditions, a gap as small as 137 ± 22 μm could be generated. Using this approach, a miniaturized paper 384-zone plate, consisting of circular detection elements with a diameter of 1.86 mm, was fabricated in 64 × 43 mm² area. Furthermore, these ablation-patterned substrates were confirmed to be compatible with surfactant solutions and common organic solvents (methanol, acetonitrile and dimethylformamide), which has been achieved by very few μPAD patterning techniques. Patterns in i-paper were created by completely cutting out zones of the i-paper and then fixing pre-cut paper into these openings similar to the strategy of fitting a jigsaw piece into a puzzle. Upon heating, unmodified paper was readily sealed into these openings due to partial reflow of the paraffin into the paper. This unique and simple bonding method was illustrated by two types of 3D μPADs, a push-on valve and a time-gated flow distributor, without adding adhesive layers. The free-standing jigsaw-patterned sheets showed good structural stability and solution compatibility, which provided a facile alternative method for fabricating complicated μPADs.

An Instrument-free Detection of Antioxidant Activity Using Paper-based Analytical Devices Coated with Nanoceria

This work reports a portable distance-based detection paper device that has a thermometer-like shape for rapid, instrument-free determination of antioxidant activity using a nanoceria assay. The assay is based on partial reduction of cerium ion from Ce⁴⁺ to Ce³⁺ on nanoceria deposited along the detection channel by antioxidants present in food, giving highly reactive oxidation products. Either these products or the parent antioxidant compounds could then bind to the OH-rich ceria nanoparticles and generate charge transfer ceria-antioxidant complexes resulting in a yellow to brown color change. The distance of the brown color on the detection channel is directly proportional to antioxidant activity, and can be easily measured using an integrated ruler without the need of any external sophisticated instrument for detection. The paper sensor has been studied for the analysis of common antioxidants and its performance was validated against traditional antioxidant assays for 11 tea sample analyses. Using the Spearman rank correlation coefficient method, the antioxidant activity of tea samples obtained from the paper device correlated with the traditional assay at the 95% confidence level. The developed sensor provided a high recovery and tolerance limit and was stable for 50 days both when stored at ambient and low temperature (6 and -20°C). The results demonstrated that the developed paper device is an alternative to allow for fast, simple, instrument-free, cheap, portable and high-throughput screening of antioxidant activity analysis in real samples.

Ultrasensitive microfluidic paper-based electrochemical/visual biosensor based on spherical-like cerium dioxide catalyst for miR-21 detection

In this work, an electrochemical biosensor based on Au nanorods (NRs) modified microfluidic paper-based analytical devices (μPADs) were constructed for sensitive detection of microRNA (miRNA) by using cerium dioxide - Au@glucose oxidase (CeO₂-Au@GOx) as an electrochemical probe for signal amplification. Au NRs were synthesized by in-situ growth method in μPADs surface to enhance the conductivity and modified hairpin probe through Au-S bonds. The construction of "the signal transducer layer" was carried out by GOx catalyzing glucose to produce H₂O₂, which was further electrocatalyzed by CeO₂. After the biosensor was constructed, an obvious electrochemical signal was observed from the reduction of H₂O₂. In order to make the detection more convincing, the visual detection was performed based on the oxidation of 3,3',5,5'-tetramethylbenzidine by H₂O₂ with the help of Exonuclease I. The electrochemical biosensor provided a wide linear range of 1.0fM to 1000fM with a relatively low detection limit of 0.434fM by the electrochemical measurement. Linear range of 10fM to 1000fM with a relatively low detection limit of 7.382fM was obtained by visual detection. The results indicated the proposed platform has potential utility for detection of miRNA.

Microfluidic paper-based analytical devices for potential use in quantitative and direct detection of disease biomarkers in clinical analysis

Clinicians, working in the health-care diagnostic systems of developing countries, currently face the challenges of rising costs, increased number of patient visits, and limited resources. A significant trend is using low-cost substrates to develop microfluidic devices for diagnostic purposes. Various fabrication techniques, materials, and detection methods have been explored to develop these devices. Microfluidic paper-based analytical devices (μPADs) have gained attention for sensing multiplex analytes, confirming diagnostic test results, rapid sample analysis, and reducing the volume of samples and analytical reagents. μPADs, which can provide accurate and reliable direct measurement without sample pretreatment, can reduce patient medical burden and yield rapid test results, aiding physicians in choosing appropriate treatment. The objectives of this review are to provide an overview of the strategies used for developing paper-based sensors with enhanced analytical performances and to discuss the current challenges, limitations, advantages, disadvantages, and future prospects of paper-based microfluidic platforms in clinical diagnostics. μPADs, with validated and justified analytical performances, can potentially improve the quality of life by providing inexpensive, rapid, portable, biodegradable, and reliable diagnostics.

A selective distance-based paper analytical device for copper(II) determination using a porphyrin derivative

Meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin sulfonate (TDMPzP), a water-soluble porphyrin derivative, was synthesized and used as a colorimetric reagent for Cu²⁺ detection on a microfluidic paper-based analytical device (µPAD) using distance-based quantification. TDMPzP showed a high selectivity for Cu²⁺ detection in aqueous solutions. When Cu²⁺ was added to the TDMPzP under acidic conditions, a color change from green to a pink was observed by the naked eye. Under optimized conditions, the application of this system to a distance-based μPAD exhibited good analytical response. The presence of common metal ions (Al³⁺, Fe³⁺, Mg²⁺, Co²⁺, Mn²⁺, Zn²⁺, Pb²⁺, Cd²⁺, Sn²⁺, and Ni²⁺) did not interfere with Cu²⁺ detection within reasonable tolerance ratios. The lowest concentration of copper that could be measured was 1mgL^-1 (1ppm) which meets the requirements for drinking water contamination regulations from the US Environmental Protection Agency (EPA) and World Health Organization (WHO) guidelines for drinking water. Real drinking water samples were analyzed to confirm the practical application of this system and the results showed good agreement with ICP-MS data. This distance-based µPAD based on TDMPzP for Cu²⁺ detection is convenient and effective for real-time drinking water analysis.

Microfluidic paper-based analytical device for photoelectrochemical immunoassay with multiplex signal amplification using multibranched hybridization chain reaction and PdAu enzyme mimetics

Combining multibranched hybridization chain reaction (mHCR), the photoelectrochemical (PEC) immunosensor was fabricated with a microfluidic paper-based analytical devices using different sizes of CdTe quantum dots (QDs) sensitized flower-like 3D ZnO superstructures as photoactive materials. Firstly, 4-aminothiophenol (PATP) functioned ZnO was anchored on gold-paper working electrode. With the aid of PATP, large-sized CdTe-COOH QDs (QDs1) were conjugated onto the ZnO surface because of the formation of a strong bond (Zn-S) between the thiol of PATP molecule and the ZnO, and the remaining amino group formed an amide bond with carboxylic acid group capping CdTe. Then the small-sized CdTe-NH2 QDs (QDs2) were modified on the QDs1 by forming amide bond, which leaded to a very strong photocurrent response because of the formation of cosensitized structure. The designed mHCR produced long products with multiple branched arms, which could attached multiple PdAu nanoparticles and catalyze the oxidation of hydroquinone (HQ) using H2O2 as anoxidant. Double strands DNA with multiple branched arms (mdsDNA) was formed by mHCR. In the presence of carcinoembryonic antigen (CEA), PdAu-mdsDNA conjugates-labeled CEA antibody was captured. The concentrations of CEA were measured through the decrease in photocurrent intensity resulting from the increase in steric hindrance of the immunocomplex and the polymeric oxidation product of HQ. In addition, the oxidation product of HQ deposited on the as-obtained electrode, which could efficiently inhibit the photoinduced electron transfer. Under optimal conditions, the PEC immunosensor exhibited excellent analytical performance: the detection range of CEA was from 0.001 to 90 ng mL(-1) with low detection limit of 0.33 pg mL(-1). The as-obtained immunosensor exhibited excellent precision, prominent specificity, acceptable stability and reproducibility, and could be used for the detection of CEA in real samples. The proposed assay opens a promising platform of clinical immunoassay for other biomolecules.