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A paper-based analytical device (PAD) with an integrated composite electrode has been designed and fabricated for non-enzymatic creatinine sensing. Reduced graphene oxide (rGO) was employed to modify the PAD so that it could function as a solid-contact transducer. A new macrocyclic pyrido-hexapeptide derivative was made and used as a special ionophore in the creatinine membrane sensor. The synthesized PAD showed a detection limit of 1.0 µM (S/N = 3) and a potentiometric response towards creatinine throughout a log-linear range of 2.0 µM–10 mM (R2 = 0.9998). The sensor shows significant selectivity for a few related substances, including ephedrine, codeine, ketamine, caffeine, urea, urate, carbinoxamine, and dextromethorphan. It has been established that the testing method is appropriate for the direct potentiometric detection of creatinine in a variety of human urine sample types. When an indicating electrode and a reference electrode are put on the same flexible disposable, this lets applications with a small sample volume be done. For point-of-care creatinine measurement, the developed paper-based analytical equipment is a good choice because it is affordable, easily accessible, and self-pumping (especially when combined with potentiometric detection). [ABSTRACT FROM AUTHOR]

This study investigated the colorimetric response of standard glucose, serum glucose, and nucleic acid assays on various paper surfaces with different wettability, including hydrophilic, hydrophobic, and nearly superhydrophobic surfaces. Water contact angles (WCA) formed by water droplets on each surface were measured using ImageJ software. The hydrophilic surface showed no contact angle, while the hydrophobic and nearly superhydrophobic surfaces exhibited contact angles of 115.667° and 133.933°, respectively. The colorimetric sensitivity of the standard glucose assay was analyzed on these surfaces, revealing enhanced sensitivity on the nearly superhydrophobic surface due to the high molecular crowding effect owing to its non-wetting behavior and eventually confined reaction product at the sample loading zone. The hydrophobic nature of the surface restricts the spreading and diffusion of the reaction product, leading to a controlled and localized concentration of the assay product leading to moderate colorimetric intensity. On the other hand, the hydrophilic surface showed the least enhancement in colorimetric sensitivity; this is attributed to the high wettability of the hydrophilic surface causing the reaction product to spread extensively, resulting in a larger area of dispersion and consequently a lower colorimetric intensity. The measured limit of detection (LOD) for nucleic acid on nearly superhydrophobic surfaces was found to be 16.15 ng/µL, which was almost four-fold lower than on hydrophilic surfaces (60.08 ng/µL). Additionally, the LODs of standard glucose and clinical serum samples were two-fold lower on nearly superhydrophobic surfaces compared to hydrophilic surfaces. Our findings clearly highlight the promising potential of utilizing superhydrophobic surfaces to significantly enhance colorimetric sensitivity in paper-based diagnostic applications. This innovative approach holds promise for advancing point-of-care diagnostics and improving disease detection in resource-limited settings. [ABSTRACT FROM AUTHOR]

Rapid and sensitive detection of tetracycline (TC) in the aquatic environment is important for the environmental ecosystem and human health. Herein, we presented on the first attempt to construct a fast‐responding gating system by using horseradish peroxidase (HRP)‐loaded functional RNA superstructures (HRP@3D RNA). Proteins were directly loaded into 3D RNA by the rolling circle transcription (RCT) reaction. We found that the specificity of aptamer‐ligand interaction could serve as a triggered manner to release the loaded HRP from 3D RNA. The above general scheme showed a few minutes of rapid release kinetics. According to the controlled‐release mechanism, we designed a disposable origami paper‐based analytical device (doPAD) coupled with HRP@3D RNA for colorimetric detection of TC. This doPAD can realize target recognition and signal readout by taking advantage of the unique spatial control advantages of 3D origami, thus generating a signal of color change that is positively related to the concentration of TC. This doPAD achieved the quantitative analysis of TC with a limit of detection of 10 nM within 41 min. We envision that the platform will be widely used in the field of environmental monitoring and biosensing. [ABSTRACT FROM AUTHOR]

A paper-based analytical device (PAD) is presented with colorimetric/electrochemical dual readouts for the simultaneous sensing of total chromium (Cr) and hexavalent chromium (Cr(VI)). This device consists of a homemade three-electrode system and a patterned paper chip, integrating multiple functions including electrochemical detection, fluid driving, online oxidation, and colorimetric detection. The fiberglass filter paper with a hydrophilic microchannel was used to achieve self-driving fluidics without external equipment. One end of the microchannel was integrated with a homemade three-electrode system to achieve sample loading and electrochemical detection. The middle region on the microchannel was modified with oxidizing reagents to perform online pretreatment, and the yield of Cr(III) oxidation can reach 97.9%, ensuring reliable colorimetric detection of total Cr at another end of the microchannel modified with chromogenic agents. With this device, the signals of Cr(VI) (the signal peak at 0.29 V vs. Ag/AgCl) and total Cr can be obtained in one single injection. After optimization, the limit of detection (LOD) of Cr(VI) and total Cr were 0.01 mg L−1 and 0.06 mg L−1 and the linear ranges were 0.05–3.0 mg L−1 and 0.2–3.0 mg L−1, respectively. The relative standard deviations (RSD) of the electrochemical testing of Cr(VI) results were in a range 1.3%–8.7% (n = 3), and the RSD values of the colorimetric testing of total Cr were between 0.7–9.2% (n = 3). The device's reliability was demonstrated by performing the practical speciation of Cr in tap water, river water, and electroplating wastewater while the recoveries obtained using the present method were in the range 93.5–106%. Overall, the proposed device provides high application prospect in the on-site rapid Cr speciation. [ABSTRACT FROM AUTHOR]

A newly developed research topic, fabricated paper-based microfluidic sensors, was discussed in the field of low-cost environmental detection. Distinguished with the traditional dipstick or lateral-flow setups, these paper-based microfluidic sensors can serve as a tool for onsite quantitative and semi-quantitative measurements, without risks to cause environmental pollution. They have attracted increasing interest since the first easy-fabricated paper-based setup reported by Whitesides group in 2007. Most of the publications utilized paper-based sensors in clinical detection. In recent years, some groups started to use these sensors in environmental measurement, leading to precise, easy operation, low-cost, and eco-friendly methods for onsite detection. In this review, paper-based microfluidic sensors were briefly introduced, followed by literatures review and discussion for future perspectives. [ABSTRACT FROM AUTHOR]

A novel and facile method by sewing low-cost fiber electrodes on paper is proposed to fabricate electrochemical paper-based analytical devices (ePADs). Prussian Blue (PB) and Ag/AgCl were deposited on carbon fibers to prepare the working and reference electrodes. The hydrophilic carbon fiber was employed as the counter electrode. All electrodes were sewed on the paper substrate. The linear voltammetry scanning (LSV) and physical characterization results showed that the physico-chemical characteristics of the prepared electrode were maintained after sewing. As a proof of concept, the ePAD fabricating by sewing was used to detect hydrogen peroxide (H 2 O 2). The limit of detection (LOD) achieved 0.9 µM and the ePAD possessed high anti-interference ability to a variety of common ions and biological molecules. Additionally, the robustness of the ePADs against bending (error 6.2 %) manifested that the ePAD had excellent practical applicability. The good reproducibility (3.65 %) displayed by the current responses confirmed that this strategy can be profitably adopted to easily assemble ePADs in a highly flexible manner. In addition, a user-friendly reagent-free ePAD was proposed to determine the content of H 2 O 2 in real water sample without treatment. The good recovery percentage (95.7–99.7 %) demonstrated the high feasibility of the reagent-free ePAD with facile carbon fiber-sewed electrodes in the detection of H 2 O 2 from complex water samples. These results confirm that the sewing method is feasible and promising. This work provides a new insight into fabrication of highly sensitive and highly selective ePADs with fiber electrodes. [Display omitted] [ABSTRACT FROM AUTHOR]

An instrument-free, user-friendly, and cost-effective PAD colorimetric sensor that can quantitative detection of H 2 O 2 has been realized by using bimetallic nickel–cobalt selenides as highly active peroxidase mimic and smartphone integrated with dark cavity as detector. Integrated with an ANN model and a self-compiled easy-to-use smartphone APP, the intelligent and on-site detection of H 2 O 2 was constructed, which exhibited an ultra-wide dynamic range. [Display omitted] • An effective paper-based colorimetric sensor was prepared based on nanozyme. • The optimized Ni 0.75 Co 0.25 Se exhibited excellent peoxidase-mimetic activities. • The on-site detection of H 2 O 2 was constructed by a self-compiled smartphone APP. • The machine learning-assisted sensor can detect H 2 O 2 over a wide dynamic range. Paper-based analytical devices (PADs) integrated with smartphones have shown great potential in various fields, but they also face challenges such as single signal reading, complex data processing and significant environmental impacting. In this study, a colorimetric PAD platform has been proposed using bimetallic nickel–cobalt selenides as highly active peroxidase mimic, smartphone with 3D-printing dark-cavity as a portable detector and an artificial neural network (ANN) model as multi-signal processing tool. Notably, the optimized nickel–cobalt selenides (Ni 0.75 Co 0.25 Se with Ni to Co ratio of 3/1) exhibit excellent peoxidase-mimetic activities and are capable of catalyzing the oxidation of four chromogenic reagents in the presence of H 2 O 2. Using a smartphone with image capture function as a friendly signal readout tool, the Ni 0.75 Co 0.25 Se based four channel colorimetric sensing paper is used for multi-signal quantitative analysis of H 2 O 2 by determining the Grey, red (R), green (G) and blue (B) channel values of the captured pictures. An intelligent on-site detection method for H 2 O 2 has been constructed by combining an ANN model and a self-programmed easy-to-use smartphone APP with a dynamic range of 5 μM to 2 M. Noteworthy, machine learning-assisted smartphone sensing devices based on nanozyme and 3D printing technology provide new insights and universal strategies for visual ultrasensitive detection in a variety of fields, including environments monitoring, biomedical diagnosis and safety screening. [ABSTRACT FROM AUTHOR]

• Surface molecularly imprinted polymer with selective recognition capability to butachlor was fabricated. • A colorimetric paper chip combined with the smartphone was developed to detect butachlor in mung bean. • The sensor has highly selective and sensitive to butachlor with LOD is 1.43 ng/g. • The sensor is simple, cheap, portable and widely used for fast detection. A microfluidic paper chip colorimetric detection system based on surface molecular imprinting of zinc ferrite nanoparticles was established, and the detection images were obtained by smartphone for gray value analysis and determination of butachlor. The best functional monomers and addition ratio were selected by quantum chemical simulation calculation, the properties of the prepared molecularly imprinted polymers were analyzed, and the detection conditions were optimized. The linear range, sensitivity, and selectivity of the method were evaluated. The results showed that under the optimum conditions, the concentration of 2–80 ng/g had a good linear relationship (R2 is 0.9953), the detection limit was 1.43 ng/g, the specificity was good, and the whole detection process did not exceed 20 min. The microfluidic paper chip was applied to detect butachlor in mung bean samples. The results showed that the recovery was 93.4–106.4 %, and the relative standard deviation was less than 5.6 %. [ABSTRACT FROM AUTHOR]

The paper-based device for allopurinol detection was developed by an enzymatic reaction. The concept is that allopurinol will act by blocking the xanthine oxidase enzyme to produce uric acid and H2O2, resulting in a color change from purple to pale purple until it is colorless. The concept was proofed by spectrophotometry and was applied in four types of filter paper including Whatman Qualitative No. 1, No. 4, and No. 6, and Whatman 1 chromatography. The application of this system was observed using ImageJ analysis. The performance of the paper reveals that each Whatman has different characteristics with LOD and LOQ range were 1.62-2.24 ppm and 4.91-6.80 ppm, respectively. The stability study shows that all of the paper was stable for 3 days both at room temperature (around 24°C-26°C) and 4°C. The presence of another drug like paracetamol did not prevent allopurinol detection. The real traditional medicine sample was also analyzed to confirm the applicability of this paper, and the result showed good agreement with spectrophotometry data. [ABSTRACT FROM AUTHOR]

A microcapillary grooved paper-based analytical device capable of dual-mode sensing (colorimetric and electrochemical detection) was demonstrated for analysis of viscous samples (e.g., human saliva). Herein, a hollow capillary channel was constructed via laser engraved micropatterning functions as a micropump to facilitate viscous fluidic transport, which would otherwise impede analysis on paper devices. Using salivary thiocyanate as a model analyte, the proposed device was found to exhibit a promising sensing ability on paper devices without the need for sample pretreatment or bulky instrumentation, as normally required in conventional methods used for saliva analysis. An extensive linear dynamic range covering detection of salivary thiocyanate for both high and trace level regimes (5 orders of magnitude working range) was collectively achieved using the dual-sensing modes. Under optimal conditions, the limit of detection was 6 μmol L−1 with a RSD of less than 5%. An excellent stability for the μpumpPAD was also observed for over 30 days. Real sample analysis using the proposed device was found to be in line with the standard chromatographic method. Benefitting from simple fabrication and operation, portability, disposability, low sample volume (20 μL), and low cost (< 1 USD), the μpumpPAD is an exceptional alternative tool for the detection of various biomarkers in saliva specimens. [ABSTRACT FROM AUTHOR]

In the present study, an alternative platform for trace copper ions (Cu(II)) determination using a diamond electrode in combination with a disposable paper‐based analytical device (d‐PAD) has been proposed. First, the complexation between Cu(II) and 1,10‐phenanthroline ligand was adsorptively accumulated onto a paper that is directly in contact with the diamond electrode and measured via square wave anodic stripping voltammetry. Under the optimal experimental conditions, the peak current was proportional to the concentration of Cu(II) in the range of 0.4–70 ng mL−1, and the detection limit was found to be 0.1 ng mL−1. Most importantly, this platform can be successfully applied to detect trace Cu(II) in different water samples (drinking water, tap water, groundwater and river water) with satisfactory results. Thus, the proposed d‐PAD could be a highly efficient platform for trace Cu(II) determination in environmental samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • A mix-and-measure fluorimetric method was developed for naftazone. • Naftazone quenches the fluorescence of GQDs via inner filter effect. • The method showed nano molar sensitivity for naftazone down to 40 nM. • Successful determination of naftazone in tablets and on equipment surfaces. • Smartphone detection of naftazone by PAD using UV-torch as an excitation source. A simple, fast, and direct mix-and-read spectrofluorimetric method has been developed for the sensitive determination of naftazone (NFZ) utilizing graphene quantum dots (GQDs) as a greener and highly luminescent nanosensor. NFZ effectively quenches the strong fluorescence of GQDs at λ ex /λ em of 350/440 nm via the inner filter effect mechanism. The nanosensor exhibits excellent linearity for NFZ over the concentration range of 0.46 to 186 μM with a limit of detection of 0.04 μM. The proposed method was validated for the successful determination of NFZ in tablets and on manufacturing equipment surfaces with good % recoveries of 98.4–101.6 and 96.3 – 102.2%, respectively. Furthermore, an integrated smartphone-based reader has been implemented and successfully applied for the determination of NFZ. The smartphone-based reader consists of a 365 nm UV torch as an excitation source, a smartphone for recording images, and smartphone-powered image analysis software for signal interpretation, together with a paper-based analytical device (PAD) utilizing filter paper as a substrate and correction fluid as a barrier for creation of detection zones. This smart platform showed excellent sensitivity with a limit of detection down to 0.12 nmol/zone, and it could be used for in-site determination of NFZ, especially for the limited resources laboratories. [ABSTRACT FROM AUTHOR]

Cyanide anion can be found in foodstuffs, tobacco smoke and a variety of types of waters, mainly originating from anthropogenic activities. Due to its highly toxic nature, several agencies have established limits for cyanide levels in water. Additionally, monitoring cyanide levels in biological samples, such as blood and urine, is crucial for obtaining clinical information about the health condition of patients. Therefore, there is a pressing need for the development of simple, cost-effective, and reliable analytical methods capable of quantifying cyanide at low concentrations. This study presents a novel analytical method for the selective and sensitive determination of cyanide based on analyte volatilization, pre-concentration via single-drop microextraction (SDME) using a selective reagent, and colorimetric quantification using a paper-based analytical device. For this, 10 mL of a liquid sample was acidified with phosphoric acid and the generated HCN was collected using a single drop of 3 μL of a palladium dimethylglyoximate solution (Pd (DMG) 2 2−) positioned in the flask headspace using a syringe. The reaction of Pd (DMG) 2 2− leads to the formation of Pd(CN) 4 2− and the demasking of the organic ligand. After 15 min of extraction time, the reagent drop was added to a paper-based analytical device that has been previously impregnated with 3 μL of nickel chloride, resulting in the formation of a red precipitate of nickel (II) dimethylglyoximate. Digital images of the paper-based device were captured and the red channel (R) was used for quantification purposes. Under optimized conditions, the method demonstrates a suitable linear relation (r2 > 0.99) ranging from 26 to 286 μg L−1 and a limit of detection of 5 μg L−1. As a proof of concept, cyanide levels were quantified in water and urine samples using this method. The proposed approach offers high sensitivity and selectivity while requiring only a small volume of reagents. Furthermore, it exhibits a high degree of portability for in-situ applications. A selective and sensitive method for the determination of cyanide using HS-SDEM. [Display omitted] • Cyanide in liquid samples was determined by combining HS-SDME and digital image analysis. • Colorimetric reaction is performed on a microfluidic paper-based analytical device. • The reagent is easy to prepare and highly selective for cyanide. • Under optimized conditions, a limit of detection of 5 μg L-1 was achieved. • The point-of-care device is suitable for cyanide determination in water and urine samples. [ABSTRACT FROM AUTHOR]

Severe antimicrobial resistance calls for developing rapid, sensitive and affordable methodological platform for clinical diagnosis of bacterial infection. Herein, a paper-based analytical device (PAD) for fluorescent (FL) detection and antimicrobial susceptibility test (AST) of Pseudomonas aeruginosa (P. aeruginosa) was fabricated by defining 96 hydrophilic microzones on a filter paper using a handheld stamp dipped with liquid wax. The size of microzones was designed to be consistent with traditional 96-well microplate, thus the FL signals can be collected by a commercialized microplate reader. Streptavidin was immobilized into the microzones by chitosan-glutaraldehyde crosslinking reaction, and then biotinylated bacteriophage tail fiber protein (TFP) was conjugated through biotin-streptavidin affinity system. TFP and fluorescein isothiocyanate (FITC) labeled antimicrobial peptide were used as capture agent and signal probe, respectively, for FL detection of P. aeruginosa on the PAD. The linear range for quantifying P. aeruginosa is 1.0 × 103 CFU/mL to 1.0 × 107 CFU/mL, with a detection limit of 137 CFU/mL. The PAD was also applied to conduct AST of P. aeruginosa for imipenem, meropenem, cefepime, amikacin, and gentamicin, and the results are consistent with the traditional broth dilution method. The PAD provides an affordable diagnosis platform for bacterial infection, especially in resource-limited institutes and countries. [ABSTRACT FROM AUTHOR]

Colorimetric sandwich-type biosensors that can both provide sensitivity competitive with fluorescence-based approaches, and leverage reagents that are cost-effective, widely available and as safe as possible, are highly sought after. Herein, we demonstrate an alternative highly-sensitive colorimetric method for paper-based sandwich-type biosensing that uses starch–iodide complexation to simplify practical biosensing using ubiquitous reagents. Targeting the mycotoxin ochratoxin A (OTA), a covalently-immobilised OTA antibody on a cellulose surface captures OTA and forms a sandwich with OTA aptamer-conjugated glucose oxidase. Adding the chromogenic reagents at an optimized concentration, a distinct blue color develops within 30 min, offering excellent contrast with the clear/white of the negative sample. With a sampling volume down to just 5 μL, the assay exhibits concentration limits of detection and quantitation of 20 and 320 pg mL−1, respectively, and a linear range from 10−1 to 105 ng mL−1 (R2 = 0.997). The method displays excellent selectivity against related mycotoxins, excellent %recovery (95–117%) and robust operation in complex matrices (beer, urine and human serum), with no significant difference versus gold-standard liquid chromatography. Along with its excellent analytical performance, this assay benefits from non-toxic and extremely cheap reagents that can be safely disposed of in the field, and presents an attractive alternative to toxic dyes and nanoparticles. Image 1 • Novel colorimetric antibody-aptamer sandwich assay using starch-iodine complexation. • Central composite design for fast and efficient sensor optimization. • Ultrasensitive detection of OTA using PADs with LoD of 20 pg mL−1 using 5 μL. • Applied for OTA detection in beverages, urine and human serum. [ABSTRACT FROM AUTHOR]

Paper-based analytical devices (PADs) have encountered a wealth of applications in recent years thanks to the numerous advantages of paper as a support. A silver nanoflower (AgNF) modified paper-based dual substrate for both surface-enhanced Raman spectroscopy (SERS) and ambient pressure paper spray mass spectrometry (PS-MS) was developed. AgNFs were immobilized on nylon-coated paper modified with silver and ethylenediamine. The developed substrate was characterized via scanning electron microscopy and infrared spectroscopy. The densely packed nanoscale petals of the AgNFs lead to a large number of so-called hot spots at their overlapping points, which result in an enhancement of the Raman signal. In addition, the presence of the AgNFs produces an increase in the sensitivity of the mass spectrometric analysis as compared with bare paper and nylon/Ag-coated paper. The dual substrate was evaluated for the identification and quantification of ketoprofen in aqueous standards as well as human saliva from healthy volunteers. The method enables the determination of ketoprofen with a limit of detection and limit of quantification via PS-MS of 0.023 and 0.076 mg L−1, respectively, with a relative standard deviation (RSD) of 3.4% at a concentration of 0.1 mg L−1. This dual substrate enables the simple and fast detection of ketoprofen with minimal sample preparation, providing complementary Raman and mass spectrometric information. [ABSTRACT FROM AUTHOR]

A new paper-based analytical device based on bulk ion-selective optodes (ISOs) for dual Ag+ and Hg2+ detection has been developed. A plasticized PVC hydrophobic phase composed of 25,27-di(benzothiazolyl)-26,28-hydroxycalix[4]arene (CU1) as an ion-selective ionophore, potassium tetrakis(4-chlorophenyl)borate as an ion-exchanger and chromoionophore XIV as a lipophilic pH indicator was entrapped in the pores of cellulose paper. This paper strip showed higher selectivity for Ag+ and Hg2+ over common alkali, alkaline earth and some transition metal ions with a color change from blue to yellow. With the proposed sensor, Ag+ and Hg2+ can be measured with the range of 1.92 × 10−6 to 5.00 × 10−3 M for Ag+ and 5.74 × 10−7 to 5.00 × 10−5 M for Hg2+ with a limit of detection of 1.92 × 10−6 M for Ag+ and 5.74 × 10−7 M for Hg2+. The proposed sensor was successfully applied to determine the amount of mercury in various water sources and the amount of silver in cleaning product samples containing silver nanoparticles (AgNPs). The results were in good agreement with inductively couple plasma-optical emission spectrometric measurements (ICP-OES). Image 1 • Paper-based bulk ion-selective optode (ISOs) was fabricated. • Benzothiazole calix[4]arene was employed as an ionophore. • The sensor was used for dual colorimetric Ag+ and Hg2+ detection. • High selectivity for Ag+ and Hg2+ over other metal ions was achieved. [ABSTRACT FROM AUTHOR]

We developed a paper-based analytical device based on the electropolymerization of poly (3,4-ethylenedioxythipohene) (PEDOT) and graphene oxide (GO) composites on the ITO substrate for the detection of uric acid (UA) in authentic human saliva. Scanning electron microscopy, UV–vis spectrum and X–ray diffraction confirmed the formation of porous PEDOT combined with GO film during the electropolymerization process. The nanocomposite based sensor showed an enhanced electrocatalytic activity toward UA with high sensitivity and stability. We demonstrate that UA can be directly detected in undiluted saliva using the paper-based electroanalytical device with no interference from ascorbic acid and dopamine that are normally present in biological fluids. The results indicated that the developed device is promising for non-invasive monitoring of salivary UA in human body. A paper-based electroanalytical device integrated with a disposable ITO electrode based on PEDOT/GO nanocomposite was applied for the evaluation of salivary uric acid in authentic human saliva. Image 1 • A paper-based electroanalytical device was developed for the salivary uric acid analysis. • The PEDOT-GO nanocomposite based disposable sensor showed an enhanced electrocatalytic activity toward uric acid. • Salivary uric acid in human saliva can be directly detected with the sensor. • The sensor is low-cost, portable and promising for non-invasive monitoring of salivary uric acid. [ABSTRACT FROM AUTHOR]

This work presents a novel development that exploits the concept of in-situ gas-separation together with a specific enzymatic colorimetric detection to produce a portable biosensor called "Blood Alcohol Micro-pad" for direct quantitation of ethanol in whole blood. The thin square device (25 mm × 25 mm × 1.8 mm) comprises two layers of patterned filter paper held together with a double-sided mounting tape with an 8-mm circular hole (the headspace). In operation, the reagent is deposited on one layer and covered with sticky tape. Then 8 μL of a blood sample is dispensed onto the opposite layer and covered with sticky tape. Diffusion of ethanol across the 1.6 mm narrow headspace permits selective detection of ethanol by the enzymatic reagents deposited on the opposite layer. This reagent zone contains alcohol oxidase, horseradish peroxidase and 2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt, as the chromogenic reagent. The color intensity, measured from the recorded digital image, resulting from the enzymatic assay of ethanol, correlates with the concentration of blood alcohol. The results obtained with spiked mice and sheep blood samples, using an external calibration in the range of 1–120 mg dL−1ethanol, gave recoveries of 93.2–104.4% (n = 12). The "Blood Alcohol Micro-pad" gave good precision with %RSD <1 (50 mg dL−1 ethanol, n = 10) and limit of quantification (10SD of intercept/slope) of 11.56 mg dL−1. The method was successfully validated against a headspace gas chromatography-mass spectrometric method. It has good potential for development as a simple and convenient blood alcohol sensor for on-site testing. Image 1 • The first paper-based biosensor for quantifying alcohol content directly from whole blood. • In-situ headspace separation with colorimetric enzymatic assay for rapid readout within 5 min. • The device is portable, low cost and disposable. • Results are promising for roadside check and toxicological test. [ABSTRACT FROM AUTHOR]

Herein we report the first two distance-based microfluidic paper-based analytical devices (μPADs) using fluorescence to quantify aluminum. In addition to their read-by-eye quantification, the devices are simple to fabricate, require no sample pretreatment or preconcentration, and have a shelf life of >5 months. The first device is designed in a "chemometer" format where the length of a fluorescent band linearly responds to an Al(III) concentration. The second device uses a radial design where the fluorescent diameter also linearly responds to an Al(III) concentration. The chemometer device has a detection limit of 2.5 ppm (100 μM) and a linear range from 2 to 54 ppm Al(III) (100 μM–1 mM), R2 = 0.989). The radial device has a detection limit of 0.9 ppm (33 μM) and a linear range from 2 to 24 ppm Al(III) (100–900 μM, R2 = 0.968). The utility of the μPADs were successfully demonstrated by measuring Al(III) in two water effluent samples from the Gold King Mine near Silverton, CO. Image 1 • Distance-based detection of Al(III) in water samples. • Radial and linear distance based detection demonstrated. • Method gives accurate results in natural waters. [ABSTRACT FROM AUTHOR]

A paper-based analytical device (PAD) with an integrated composite electrode for non-enzymatic creatinine sensing was developed. The electrode was produced and optimization was efficiently accomplished using a rapid digital dispensing approach. The electrochemical sensor was fabricated using an HP D300 digital dispenser to deliver a copper oxide and ionic liquid composite onto an electrochemically reduced graphene modified screen-printed carbon electrode (CuO/IL/ERGO/SPCE) on a PAD. The modified electrode was characterized using electrochemical and microscopic techniques. Electrochemical detection of creatinine was performed on the SPCE using amperometry at a constant potential. Under optimized conditions, the paper-based sensor exhibited a linear range of 0.01–2.0 mM (R2 = 0.99) and the limit of detection was 0.22 μM (S/N = 3, IUPAC definition) for creatinine. The simple fabrication process, low cost, and clinically appropriate creatinine sensitivity make this device applicable for point-of-care use. Image 1 • A CuO/IL/ERGO modified paper-based screen printed electrode was constructed and used as a non-enzymatic creatinine sensor. • An HP D300 dispenser was used to enable efficient electrode modification, leading to improved sensitivity and reproducibility. • The sensitivity was sufficient for detecting clinically relevant creatinine levels in acute kidney injury diagnosis. [ABSTRACT FROM AUTHOR]

Field-amplified stacking (FAS) is a commonly used method for enhancing the sensitivity of charged species from low conductive media in capillary electrophoresis. FAS also showed significant sensitivity enhancement effect on a uniform paper fluidic channel by proper design of the electrolyte. In this paper, a novel method of introducing electric field gradient is proposed by geometry design of a 2D paper fluidic channel, and field amplification effect was successfully demonstrated with reduced requirement on the sample's conductivity. Sensitive colorimetric detection of microalbuminuria (MAU) from urine samples was demonstrated by mobile phone camera. Experimental results showed that, with active electric field motivation, up to 93.5% of the loaded protein probe could be effectively transferred and stacked into a narrow band on the newly designed paper fluidic channel. A limit of detection (LOD) of 6.5 mg‧L−1 HSA was achieved with a dynamic range of 10–300 mg‧L−1 (linear in the range of 10–100 mg‧L−1, R2 = 0.991). Combined with selective staining of albumin with bromophenol blue (BPB), the established method was applied to the detection of MAU from clinical urine samples, and consistent results with that of the clinical method were obtained. With this paper-based analytical device (PAD), MAU from highly conductive urine samples can be directly loaded and detected without any pretreatment. This method provides a way to develop highly sensitive point-of-care test (POCT) for rapid screening of some diseases. Image 1 • Novel field amplification achieved mainly by the change of paper channel geometry, rather than the conductivity difference. • Protein probe was actively transported with a high transfer rate up to 93.5%, vs. less than 50% only by capillary force. • Sensitivity was enhanced by 2 orders of magnitude as demonstrated by colorimetric detection of human serum albumin (HSA). • Detection of microalbuminuria (MAU) from clinical urine samples was successfully demonstrated. [ABSTRACT FROM AUTHOR]

Abstract The facile detection of phosphopeptides is important for clinical screening and phosphoproteomic research. This work develops an instrument-free, cost-effective, convenient paper-based method for quantitative analysis of phosphorylated peptides. With a novel portable device, Phos-PAD, this method can achieve selective enrichment and colorimetric detection of phosphopeptides within 15 min TiO 2 nanoparticle-based chemisorption and tetrabromophenol blue-based colorimetric assay were integrated into the single paper-based analytical device. The color change can indicate the presence of phosphopeptides and the mean pixel intensity of the red channel can be used for phosphopeptide quantification. With capability of quantifying phosphopeptides in serum samples, this Phos-PAD assisted phosphopeptide assay may attract significant attention to clinical analysis of endogenous serum phosphopeptides. Graphical abstract Image 1 Highlights • A novel portable device, Phos-PAD, was developed for the analysis of phosphopeptides. • The instrument-free quantification of phosphopeptides can be finished within 15 min. • TiO 2 nanoparticles were in-situ synthesized for the enrichment of phosphopeptides. • The potential of this method in real sample analysis was explored with serum samples. [ABSTRACT FROM AUTHOR]

Rapid, low-cost, and sensitive nucleic acid detection and quantification assays enabled by microfluidic paper-based analytical devices (μPADs) hold great promise for point-of-care disease diagnostics and field-based molecular tests. Through the capillary action in μPAD, flexible manipulation of nucleic acid samples can be achieved without the need for external pumps or power supplies, making it possible to fabricate highly integrated sample-to-answer devices that streamline the nucleic acid extraction, separation, concentration, amplification, and detection. To detect minute amounts of genetic materials from clinical and biological samples, it is also critical to develop sensitive signal readouts that generate physically detectable signals for in-device nucleic acid detection and/or quantification. In this review, we will focus on μPAD approaches for the facile manipulation of nucleic acids and emerging signal transduction strategies allowing sensitive and specific nucleic acid detection in μPAD. [ABSTRACT FROM AUTHOR]

• Paper-based isoelectric focusing was applied to the separation and fractionation of amino acids. • Ampholytes were used to form a pH gradient in the folding paper. • Formation of a pH gradient was confirmed using pH indicators. • HPLC confirmed the separated amino acids following extraction from the paper. We demonstrated the fractionation of two amino acids, glutamic acid and histidine, separated via isoelectric focusing (IEF) on filter paper folded and stacked in an origami fashion. Channels for electrophoresis were fabricated as circular zones acquired via wax printing onto the filter paper. An ampholyte solution with amphiphilic samples was deposited on all the circle zones, which was followed by folding to form the electrophoresis channels. IEF was achieved by applying an electrical potential between the anodic and cathodic chambers filled with phosphoric acid and sodium hydroxide solutions, respectively. A pH gradient was formed using either a wide-range ampholyte with a pH of 3 to 10 or a narrow-range version with a pH of 5 to 8, which was confirmed by adding pH indicators to each layer. The origami IEF was used to separate the amino acids, glutamic acid and histidine, by mixing with the ampholytes, which were deposited on the layers. The components in each layer were extracted with water and measured by high-performance liquid chromatography using pre-column derivatization with dansyl chloride. The results indicated that the focus for glutamic acid and that for histidine were at different layers, according to their isoelectric points. The origami isoelectric focusing achieved the fractionation of amino acids in less than 3 min using voltage as low as 30 V. [ABSTRACT FROM AUTHOR]

A novel smartphone and paper-based analytical device is constructed for glucose monitoring using acetylene black (AB)-hemin nanozyme as catalyst. [Display omitted] • A significant increase in peroxidase-like activity of acetylene black loaded hemin. • Point of care test can be realized by combined with smartphone for blood monitoring. • Test results in undiluted serum are in good consistency with commercial glucometer. Glucose management is an important part of disease control for diabetes patients, thus the development of a rapid and real-time point of care testing (POCT) device for monitoring blood glucose is of great significance. In this work, a paper-based analytical device (PAD) is constructed by combining acetylene black (AB)-hemin complex modified filter paper as sensing platform with a smartphone as signal detector. Large specific surface area of AB decreases the self-associate and aggregate of hemin in aqueous solution, resulting in improved peroxidase-like activity of hemin. Compared with graphene oxide supported hemin, AB-hemin exhibits superior signal response on paper. Glucose oxidase (GOx) catalyzes the conversion of blood glucose to hydrogen peroxide, and then AB-hemin complex catalyzes the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue TMB oxidized products (TMB+) in the presence of hydrogen peroxide, thus achieving the visual detection of blood glucose. In optimal conditions, PAD provides an applicable linear range from 0.2 mM to 30 mM and a low limit of detection (LOD) (0.06 mM). Notably, the detection accuracy of the developed paper-based sensor is in good agreement with that of the commercially available blood glucose meter (p > 0.05). Moreover, the proposed PAD presents high recoveries from 95.4% to 112% (RSD ≤ 3.2%), and therefore holds great potential for glucose monitoring and diabetes diagnosis. [ABSTRACT FROM AUTHOR]

The colorimetric sensor usually depends on enzyme-mediated signal amplification to achieve trace analysis of ochratoxin A (OTA) residues in food samples. However, the enzyme labeling and manual addition of reagents steps increased assay time and operation complexity, restricting their application in point-of-care testing (POCT). Herein, we report a label-free colorimetric device integrating a 3D paper-based analytical device and a smartphone as handheld readout for rapid and sensitive detection of OTA. Using vertical-flow design, the paper-based analytical device enables the specific recognition of target and self-assembly of G-quadruplex (G4)/hemin DNAzyme to be performed, then employs DNAzyme for transducing the OTA binding event signal into a colorimetric signal. The design of independent functional units, including biorecognition unit, self-assembly unit and colorimetric units, which can address crowding and disorder of biosensing interfaces and improve the recognition efficiency of aptamer (apta). In addition, we eliminated signal losses and nonuniform coloring by introducing carboxymethyl chitosan (CMCS) to obtain perfectly focused signals on colorimetric unit. On the basis of parameter optimization, the device exhibited a detection range of 0.1–500 ng/mL and a detection limit of 41.9 pg/mL for OTA. Importantly, good results were obtained in spiked real samples, indicating applicability and reliability of developed device. [Display omitted] • A label-free and enzyme-free colorimetric 3D μPAD was designed with a smartphone as a readout device. • The G4/hemin DNAzyme with high peroxidase-like activity was acted as signal amplification element. • The device displayed low detection limit of 41.9 pg/mL for OTA. • The favorable practical applicability was obtained for OTA detection in real samples. [ABSTRACT FROM AUTHOR]

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

Abstract 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. Graphical abstract Image 1 Highlights • Dual channel, dual working electrode ePAD has been developed and characterized. • Enables detection at two conditions in the same device using the same potentiostat. • Example application showing detection of norepinephrine and serotonin. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Molecularly imprinted paper-based sensor was prepared via polymerization-free method. • The synthesis was carried out by dissolving nylon-6 in formic acid. • The use of filter paper increases the versatility and manageability of the material. • Sensor was evaluated by extracting aqueous solutions of quinine and a soda drink. • Molecularly imprinted paper-based analytical device can be reused at least six times. Abstract In the last years, the imprinting technology has raised attention and different polymers with artificial recognition sites towards a target molecule have been developed. In this article, a polymerization-free method ―which only requires the dissolution of nylon-6 polymer and its subsequent incubation with the template molecule― has been employed for the preparation of a molecularly imprinted polymer directly immobilized on filter paper, thus increasing the versatility and manageability of the new material. The molecularly imprinted paper-based analytical device (MIP PAD) has been coupled to a fluorimeter via a custom-built platform, which enables direct measurement of the fluorescence at the surface of the MIP PAD. Extraction efficiency of the MIP PAD was evaluated using quinine as model analyte, measuring the fluorescence directly at the MIP PAD surface after incubation on aqueous standards of quinine and soda drink, showing that the performance of the imprinted material was superior to that of the non-imprinted polymer. In addition, the selectivity of the MIP PAD versus analogous organic fluorescent molecules has been further evaluated using norfloxacin, observing that the molecular imprinting results in selective extraction of quinine. The performance of the method has been evaluated for quantitative analysis, achieving limits of detection (LOD) and quantification (LOQ) for aqueous standards of 0.37 and 1.24 mg L−1, respectively, while in the case of spiked soda drink the LOD and LOQ were 0.63 and 2.11 mg L−1, respectively. The reproducibility of the measurements was also evaluated, observing values within range 2.30 and 9.07% measured as relative standard deviation. [ABSTRACT FROM AUTHOR]

Abstract In this work, a rapid and sensitive paper-based analytical device (PAD) method was developed for screening detection of total tetracyclines (TCs) in environment water. TCs form firstly a narrow stacking band on a paper channel by field amplification stacking, and then the band was visualized by fluorescent imaging which was induced by an ultraviolet light-emitting diode (UV LED). The image was acquired by a smartphone camera for quantitative analysis. Under optimal conditions, the sensitivity of this portable PAD system was 250 times enhanced within 2 min by online electrokinetic stacking and linear response was demonstrated in the range of 5–80 ng mL−1(R2 = 0.997) with detection limit of 4.5 ng mL−1. The applicability was demonstrated by detection of total TCs in environmental water with recovery ranging from 87 to 112% (RSD = 4.1–16.3%). Graphical abstract Unlabelled Image Highlights • Fluorescent colorimetric detection of total tetracyclines was implemented simply with UV LED and smartphone camera on a portable PAD system. • The sensitivity for tetracyclines was 250 times enhanced by field amplification sample stacking. • Fast instrument-free tetracyclines detection was achieved within 5 min in range of 5–80 ng mL−1 (R2 = 0.997) with detection limit of 4.5 ng mL−1. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A rapid colorimetric sensing system using competitive ELISA test on a paper-based analytical device was investigated for the detection of ketamine. • Oral fluid was selected for test based on its advantages of low infection risk, noninvasiveness, and decreased chance of sample adulteration. • After optimization of the operation parameters, the resulting assay can be completed in as little as 6 min with a detection limit of 0.03 ng/mL. • A smartphone app was developed to analyze the results of the test, further enhancing the test's portability and data transmission capabilities. • The paper-based platform features 90% sensitivity and 92% specificity for ketamine analysis of 90 oral fluid samples from drug abuse patients. Abstract In this study, we designed a rapid colorimetric sensing system using competitive ELISA test on a microfluidic paper-based analytical device for the detection of ketamine, a frequently abused drug. Oral fluid was selected for the test to facilitate the collection of samples based on its advantages of low infection risk, noninvasiveness of sample collection, and decreased chance of sample adulteration. After optimization of the operation parameters, including reaction temperature, vibration washing time, reaction time, and antibody concentration, the resulting assay can be completed in as little as 6 min with a detection limit of 0.03 ng/mL. Moreover, we developed an Android smartphone app to analyze and measure the results of the test, further enhancing the test's portability, and image recording and data transmission capabilities. In order to test the feasibility and performance of the optimized colorimetric assay, 90 oral fluid samples from drug abuse patients were tested. The paper-based platform features 90% sensitivity (confidence interval (CI): 76.34–97.21%) and 92% specificity (CI: 80.77–97.78%) for ketamine analysis. This competitive paper-based ELISA sensing system provides a rapid, convenient, sensitive, and high-throughput approach for drug monitoring. [ABSTRACT FROM AUTHOR]

Microfluidic paper-based analytical devices (µPADs) have attracted much attention over the past decade because they offer clinicians the ability to deliver point-of-care testing and onsite analysis. Many of the advantages of µPADs, however, are limited to work in a laboratory setting due to the difficulties of processing data when using electronic devices in the field. This review focuses on the use of µPADs that have the potential to work without batteries or with only small and portable devices such as smartphones, timers, or miniaturized detectors. The µPADs that can be operated without batteries are, in general, those that allow the visual judgment of analyte concentrations via readouts that are measured in time, distance, count, or text. Conversely, a smartphone works as a camera to permit the capture and processing of an image that digitizes the color intensity produced by the reaction of an analyte with a colorimetric reagent. Miniaturized detectors for electrochemical, fluorometric, chemiluminescence, and electrochemiluminescence methods are also discussed, although some of them require the use of a laptop computer for operation and data processing. [ABSTRACT FROM AUTHOR]

Abstract Cronobacter spp. are recognized as a world-wide emerging opportunistic food-borne pathogens that can persist in various food and processing environment for a long time. Therefore, the prevention and detection of them are particularly important. In this study, a micro-spot paper-based analytical device (μPADs) was created by combining PVC pad with filter paper. Detection is achieved by measuring the color change (from colorless to indigo) when a species-specific enzyme associated with the Cronobacter spp. of interest reacts with a chromogenic substrate. When combined with the optimization of specific enrichment process, the method allows for a testing time of 10 h or less and is capable of detecting live bacteria on the inoculated surface of samples in concentrations as low as 101 CFU cm−2. We are surprised to discover that C. dublinensis species and their subspecies had the highest ability to produce α-glucosidase in all genus of Cronobacter spp.. This work demonstrated that the manufacturing method is novel, simple, well reproducible (RSD less than 5%) and low-cost (less than $ 0.15 per micro-spot). Graphical abstract Image 1 Highlights • The fabrication of μPADs using PVC pad is described for the first time, which is simple and low-cost (less than $ 0.15). • The colorimetric detection of Cronobacter spp. using μPADs for the first time. • We found that C. dublinensis and its subspecies had the highest ability to produce α-glucosidase. • The LOD and RSD (n = 6) are 101 CFU cm−2 and less than 5%. In addition, the detection time is about 10 h. [ABSTRACT FROM AUTHOR]

The rapid on-site detection of organophosphate compounds such as nerve agents and pesticides is essential due to their lethal effect, which irreversibly inhibits the activity of acetylcholinesterase (AChE). In this study, we demonstrate a foldable paper-based analytical device (PAD) that is capable of detecting an AChE inhibitor in a semiquantitative manner. An angle-based readout of the yellow area in the ring-type paper channel was used for the semiquantitative analysis with the naked eye. The color of the paper channel was changed to yellow as a result of oxidized cerium oxide nanoparticles by hydrogen peroxide produced from the enzymatic reactions. All reagents required for enzyme reactions were preadsorbed on the pop-up card shaped PAD to minimize the manual steps for dealing with multiple reagents for sequential enzyme reactions. As a proof of concept, we were able to semiquantitatively analyze a nerve agent simulant called dimethyl methylphosphonate (DMMP), one of the AChE inhibitors, by simply reading the angle of the yellow area in the paper channel. The foldable PAD proposed here would serve as a valuable tool that allows unskilled people to semiquantitatively detect AChE inhibitors in the field, including nerve agents and organophosphate pesticides. [ABSTRACT FROM AUTHOR]

Abstract We developed a novel micro-patterning technique for single-walled carbon nanotubes (SWCNTs) on nitrocellulose (NC) membranes and characterized the mechanical and electrical properties of the patterned SWCNTs. The SWCNT patterns were successfully fabricated on wax-printed NC membrane by a vacuum filtration method within 15 min. The well-deposited SWCNT layer on the NC membrane exhibited strong adhesion and high conductivity due to the interconnection of the SWCNT layer in the porous structure of the NC membrane. The distinguishable patterns were resolved down to approximately 100 μm. The patterned SWCNT had an average resistivity of less than 100 Ω/sq and showed excellent mechanical properties in repeated bending tests. The suitability of the patterned SWCNT electrodes for electronic devices was demonstrated successfully via a simple light emitting diode array. Moreover, for the application of the patterned SWCNT to electrochemical paper-based analytical devices (ePADs), the gold nanoparticles (AuNPs) were directly deposited on the SWCNT electrode for non-enzymatic glucose determination. The oxidation peak had a linear response with glucose concentration in the range between 0.5 and 10 mM. The ePADs exhibited a sensitivity of 240 μA/mM·cm2 and a detection limit of 148 μM, which was derived from the electro-catalytic property of AuNPs toward glucose. Based on these results, we believe that the developed SWCNT patterning technique on paper substrates has great potential in ePADs. Graphical abstract Unlabelled Image Highlights • Micro-patterns of single-walled carbon nanotubes (SWCNTs) were successfully fabricated on wax-printed papers. • AuNPs were deposited on the SWCNT electrodes for the application to electrochemical paper-based analytical devices. • The developed devices were applied successfully to non-enzymatic glucose determination. [ABSTRACT FROM AUTHOR]

A simple, cost-effective, instrumental free and user-friendly analytical method based on the combination of paper-based headspace extraction and digital scanning image analysis was developed for the sensitive detection of cyanide in water and wastewater samples. The method relied on chloramine-T/pyridine-barbituric acid reaction, which is a well-known process for measuring cyanide in standard analytical methods. During the reaction, cyanide was converted to cyanogen chloride, which was extracted and collected on a paper impregnated with pyridine-barbituric acid. The color appeared on the paper was used to quantify the analyte using a scanning-assisted image processing software. Experimental parameters affecting the sensitivity of the method such as stirring rate, paper size, chloramine-T and barbituric acid concentration, equilibrium and extraction time were studied. Under optimal conditions, the calibration curve was linear in the range of 3.0–100 μg L −1 with the detection limit of 0.7 μg L −1 . The intra- and inter- day relative standard deviations of the method at 10 and 70 μg L −1 concentration levels were less than 6%. Analyte recovery in real samples was in the range of 69–111%. [ABSTRACT FROM AUTHOR]

Detecting variations in the localized surface plasmon resonance (LSPR) of gold nanoparticles is a widespread approach for developing colorimetric biosensors. This is usually performed with a large spectrophotometer, which is not suitable for in-field measurements, or with the naked eye, which only allows detecting large spectral variations leading to clear changes in color. Here we demonstrate that patterns printed on paper can transduce LSPR variations caused by the aggregation of plasmonic nanosensors using a mobile device as the reader. The sensitivity of the proposed paper transducers is tested with a new enzyme-less signal generation mechanism for biosensors based on triggering the aggregation of gold nanoparticles in the presence of neutravidin. A competitive immunoassay using this mechanism and the proposed transducers can detect the model analyte C-reactive protein with a limit of detection of 3·10 −8  g mL −1 within 1 h, which is comparable to an ELISA using a spectrophotometer to detect the signal. The fabrication of the transducers only requires a common toner printer, and the signal can be detected with the ubiquitous smartphone using a downloadable app, which makes the proposed detection platform ideal for developing mobile biosensors for global health applications. [ABSTRACT FROM AUTHOR]

A sensitive and stable cellulose paper platform modified with carboxymethyl cellulose (CMC) was investigated for use as a rapid and accurate immunoassay for tuberculosis. Absorbing CMC to the paper substrate enables it to act as a linker for immobilizing biomolecules in a hydrophilic environment to promote their functionality and stability. This CMC-modified cellulose material can also be crosslinked with 1-ethyl-3-(-3-dimethylaminopropyl) carbodiimide hydrochloride/ N -Hydroxy-succinimide (EDC/NHS) to generate NHS ester groups on the paper surface that can be covalently conjugated with ligands or target analytes for highly specific binding. As a result, with this system we were able to achieve a detection limit of 0.03 ng/mL for tuberculin-purified protein derivative. In addition, multiple tests can be completed simultaneously in as little as 21 min. These results demonstrate how this surface-modified paper analytical device can provide sensitive and high-throughput on-site disease diagnosis for resource-limited settings. [ABSTRACT FROM AUTHOR]

[Display omitted] • The silver nanocomposite is in situ synthesized on paper substrate. • Detection of Fe3+ is based on the aggregation of silver nanocomposite. • The sensitivity for Fe3+ detection can be tuned by the pre-concentration steps. • On site detection of Fe3+ can be realized with the aid of a smartphone. In this study, a green and novel method for in situ synthesis of silver nanocomposites on paper substrate was developed and used for the pre-concentration and determination of Fe3+. Silver nanocomposites were in situ synthesized using tannic acid as reducing and capping agent under a mildly alkaline condition. The mechanism for the Fe3+ detection is based on its coordination with the tannic acid present on the surface of the silver nanocomposites and the formation of aggregates. The colorimetric response between Fe3+ and tannic acid modified silver nanocomposites deposited on the surface of the paper substrate was collected by a smartphone followed by processing with Adobe Photoshop CS 5.1 software. Under the optimal conditions, the color response showed a good linear relationship with Fe3+ concentration in the range of 25.0–500.0 μM. The limit of detection for Fe3+ was calculated to be 10.0 μM. Finally, the developed smartphone assisted paper-based chemical sensor was used to detect Fe3+ in two lake water samples, and the recoveries are in the range of 83.5 %−106.7 %. The sensitivity for Fe3+ detection can be tuned through the pre-concentration steps based on the porous nature of the filter paper. The platform highlights the advantage of simplicity, ease of synthesis, fast response time, and low reagent/analyte volume requirement, which is suitable for point-of-use application in iron ions contamination, especially in the remote or resource-limited regions. [ABSTRACT FROM AUTHOR]

A rapid, simple and affordable method for arsenic speciation analysis is described in this work. The proposed methodology involves in situ arsine generation, transfer of the volatile to the headspace and its reaction with silver nitrate at the detection zone of a paper-based analytical device (PAD). Thus, silver nitrate acts as a recognition element for arsine in the paper-based sensor. The chemical reaction between the recognition element and the analyte derivative results in the formation of a colored product which can be detected by scanning the detection zone and data treatment with an image processing and analysis program. Detection and injection zones were defined in the paper substrate by formation of hydrophobic barriers, thus enabling the formation of the volatile derivative without affecting the chemical stability of the recognition element present in the PAD. Experimental parameters influencing the analytical performance of the methodology, namely color mode detection, composition of the paper-based sensor and hydride generation and mass transfer conditions, were evaluated. Under optimal conditions, the proposed method showed limits of detection and quantification of 1.1 and 3.6 ng mL −1 , respectively. Remarkably, the limit of detection of the method reported herein was much lower than the maximum contaminant levels set by both the World Health Organization and the US Environmental Protection Agency for arsenic in drinking water, unlike several commercially available arsenic test kits. The repeatability, expressed as relative standard deviation, was found to be 7.1% (n = 8). The method was validated against the European Reference Material ERM ® -CA615 groundwater and successfully applied to the determination of As(III), As(V) and total inorganic As in different water samples. Furthermore, the method can be used for the screening analysis of total arsenic in waters when a cut-off level of 7 ng mL −1 is used. [ABSTRACT FROM AUTHOR]

An easily performed miniaturized, cheap, selective and sensitive procedure for the determination of H 2 S in fuel oil samples based on a headspace liquid-phase microextraction followed by a cyclic voltammetry detection using a paper-based analytical device (PAD) was developed. A modified wax dipping method was applied to fabricate the PAD. The PAD included hydrophobic zones of sample and supporting electrolyte connecting by hydrophilic channel. The zones of sample and supporting electrolyte were connected with nickel working, platinum auxiliary and Ag/AgCl reference electrodes. The analytical procedure included separation of H 2 S from fuel oil sample based on the headspace liquid-phase microextraction in alkaline solution. Then, sulfide ions solution obtained and supporting electrolyte were dropped on the zones followed by analyte detection at + 0.45 V. Under the optimized conditions, H 2 S concentration in the range from 2 to 20 mg kg −1 had a good linear relation with the peak current. The limit of detection (3σ) was 0.6 mg kg −1 . The procedure was successfully applied to the analysis of fuel oil samples. [ABSTRACT FROM AUTHOR]

Paper-based analytical device (PAD) with image-based colorimetric detection is advantageous in low-cost, fast, simple and suitable for on-site rapid analysis. However, the detection capability is normally limited. In this paper, a new method was proposed that combined a complex reaction and electrokinetic stacking, and the sensitivity of PAD was effectively increased as demonstrated by the colorimetric detection of Cu 2+ with smartphone. A T-type paper fluidic channel was designed to realize the on-line complex reaction between polyethyleneimine (PEI) and Cu 2+ . Under electric field, positively charged PEI and Cu 2+ were driven through the two channels and eventually met on the same paper fluidic channel to form the positively charged complex of dark blue color. Meanwhile, the complex was stacked into a narrow and enhanced color band by field amplification effect. Under optimal experiment conditions, the band intensity was increased by three orders of magnitude compared with that of direct Cu 2+ imaging on the same paper substrate. A linear relationship in the range of 0.1–1.0 mM Cu 2+ was obtained (R 2 = 0.991), and a limit of detection of 30 μM was achieved, which is close to that by a desktop spectrophotometer. The selectivity of this method to other co-existing metal ions was also investigated. This work shows that by integration of a complex reaction and electro kinetic stacking, the sensitivity of image-based colorimetric detection can be greatly enhanced for rapid detection of Cu 2+ by PAD. [ABSTRACT FROM AUTHOR]

Sensitive detection and determination of hardness and fluoride in the ground water is an important issue for the water safety. The present work reports a sensitive, selective, rapid and visual hybrid fluorescent nanosensor by hybridizing carbon dots and hexametaphosphate capped gold nanoparticles through intrinsic interactions of the two components. The hybrid nanosensor can be used for hardness (Ca 2+ , Mg 2+ ) detection with the fluorescence intensity decreasing at 439 nm and increasing at 608 nm via the aggregation induced fluorescence resonance energy transfer on mechanism. In addition, the hybrid nanosensor mixed with calcium ions can detect fluoride in water with a lower detection limit of 0.339 ppm with the fluorescence color recovery based on the precipitation induced fluorescence resonance energy transfer off process. Experimental results indicate that the as-synthesized hybrid nanosensor has shown efficient feasibility and practicality for hardness and fluoride detection in actual groundwater samples. Furthermore, a paper-based analytical device has been fabricated by immobilizing the hybrid nanosensor on the cellulose filter paper for the visual and on-site detection of fluoride in water. The hybrid nanosensor would provide new sensing materials and mechanisms for the determination and detection of hardness and fluoride in the groundwater. [ABSTRACT FROM AUTHOR]

Electrokinetic stacking (ES) is effective for improving sensitivity of paper-based analytical device (PAD) for charged analytes. In this paper, we successfully demonstrated ES of electrically neutral analytes on PAD, and the performance was characterized by smartphone-based colorimetry and fluorescence. Firstly, SDS from cathode reservoir stacked as a micelle band on an open paper fluidic channel by ES, and the target analyte was swept by the micelle. Meanwhile, the probes at the other side were carried by electroosmotic flow (EOF). Eventually, neutral components preloaded on the channel were concentrated as the narrow stacking band. Taking the rhodamine B as a probe, the effects of EOF, background electrolyte concentration and anionic surfactant concentration were investigated. Fluorescence detection of rhodamine B and colorimetric analysis of Sudan III demonstrated the sensitivity enhanced and its potential for the semi-quantitative test. Under the optimized conditions, fluorescence detection limit of 50 nM of rhodamine B was achieved with a linear range of 1.0–10 μM (R 2 = 0.99). The colorimetric detection limit for Sudan III was 5.2 μM and the linear range was 5–40 μM (R 2 = 0.99). Compared with direct analysis without stacking, the signal levels of rhodamine B and Sudan III were increased by 30-fold and 6-fold, respectively. This study showed that with ES, sensitive and rapid PAD detection of electrically neutral analytes could be achieved. [ABSTRACT FROM AUTHOR]

In this work, we studied the feasibility of interaction among gold nanoparticles (AuNPs) and a cationic dye in an aptasensor system for the detection of potassium ions. The presence and absence of potassium in the solution was distinguishable by different colors (between orange and green) appeared after reaction. Cationic dye (Y5GL) acts as a new aggregator for AuNP-based sensors which changes the aggregated AuNP solution color from blue-purple to green. In the presence of K + ions, the aptamer dissociated from the surface of the AuNP so that free AuNPs and cationic dye make the solution green. The aptasensor showed that the analytical linear range was from 10 nM to 50 mM and the detection limit was 4.4 nM. Also, we examined the practicality of this method on a simple paper based platform. The linear range of the colorimetric paper sensor covered of K + concentration from 10 μM to 40 mM and the detection limit of 6.2 μM was obtained. The selectivity of AuNP aggregation-based sensor improved by the use of cationic dye. Rapidity, simplicity, high sensitivity and excellent selectivity made this assay suitable for practical determination of K + in real urine samples. [ABSTRACT FROM AUTHOR]

On-site rapid monitoring of nitrite as an assessment indicator of the environment, food, and physiological systems has drawn extensive attention. Here, electrokinetic stacking (ES) was combined with colorimetric reaction on a paper-based device (PAD) to achieve colorless nitrite detection with smartphone. In this paper, nitrite was stacked on the paper fluidic channel as a narrow band by electrokinetic stacking. Then, Griess reagent was introduced to visualize the stacking band. Under optimal conditions, the sensitivity of nitrite was 160-fold increased within 5 min. A linear response in the range of 0.075 to 1.0 μg mL−1 (R2 = 0.99) and a limit of detection (LOD) of 73 ng mL−1 (0.86 μM) were obtained. The LOD was 10 times lower than the reported PAD, and close to that achieved by a desktop spectrophotometer. The applicability was demonstrated by nitrite detection from saliva and water with good selectivity, adding 100 times more concentrated co-ions. High recovery (91.0~108.7%) and reasonable intra-day and inter-day reproducibility (RSD < 9%) were obtained. This work shows that the sensitivity of colorless analyte detection-based colorimetric reaction can be effectively enhanced by integration of ES on a PAD.Schematic of the experimental setups (left) and the corresponding images (right) of the actual portable device. [ABSTRACT FROM AUTHOR]

In this study, polyethersulfone (PES) paper embedded with a polymethyl methacrylate (PMMA) platform was developed for simultaneous DNA amplification and colorimetric detection. DNA was amplified using loop-mediated isothermal amplification (LAMP) in isothermal conditions in the presence of the common magnesium-dependent colorimetric indicator, eriochrome black T (EBT), to detect DNA. Here, the DNA of Staphylococcus aureus was used as the DNA template. The developed device was successfully used to simultaneously amplify and detect the DNA by the naked eye within 30 min with a detection limit of 1 fg/µL. [ABSTRACT FROM AUTHOR]

Quantification is a fundamental aspect of performance of an analytical system. Paper-based analytical device (PAD) as an on-site detection platform has drawn wide attention mainly due to its portability and cost effectiveness. In this work, a portable and low-cost PAD for online preconcentration and sensitive determination of fluorescent whitening agent (FWA) was demonstrated, which was consisted of ultra violet light-emitting diode (UV LED), macro-focusing lens, smartphone and miniaturized DC voltage source. Taking a widely used FWA component VBL as the analyte, the performance of the PAD enhanced with electrokinetic stacking (ES) and fluorescence imaging detection was systematically investigated. With ES, the sensitivity of the PAD system was 160-fold enhanced, and a limit of detection (LOD) of 0.06 μg mL −1 was achieved. The dynamic range was 0.1–10.0 μg mL −1 (linear in 0.1–0.7 μg mL −1 , R 2 = 0.99). With manual operation, the relative standard deviation (RSD) of intra-day and inter-day were all below 15%. Eventually, VBL from different napkin samples and toilet paper was determined with average recovery rates in the range of 90%–95% (RSD = 8.0%–12.0%). This work shows that with ES, the sensitivity of PAD can be greatly improved, and a LOD close to a desktop fluorescent spectrophotometer can be achieved as demonstrated by the detection of FWA component. [ABSTRACT FROM AUTHOR]