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Branched-chain amino acids (BCAAs) are essential amino acids that play a crucial role in regulating animal physiology and metabolism. The lack of sensitive, rapid, and portable methods for measuring BCAA concentrations in blood samples hinders precise and dynamic monitoring of nutritional status during swine production. In this work, a paper-based analytical device (PAD) was developed for quantifying BCAAs in blood samples. The device utilized a redox reaction between BCAAs and NAD+ catalyzed by leucine dehydrogenase, producing NADH that could be detected by NADH-responsive polymer dots with high selectivity, sensitivity, and accuracy. A separation membrane was incorporated in the PAD for spontaneous blood-plasma separation. The fluorescent images were captured by using a smartphone. It required only 20 μL of blood volume, and the entire assay could be completed in less than 5 minutes. The quantitation range spanned from 35 μM to 1000 μM, covering the normal BCAA concentration range in porcine bloods. The BCAA concentration measured with the PAD assay was in good agreement with those obtained with an ELISA kit. In sum, this PAD-based BCAA assay offers a point-of-care testing approach for monitoring BCAA levels in pig production, especially with a low-protein diet. • A portable paper-based analytical device was developed for BCAAs quantification. • The NADH-sensitive polymer dots were utilized as the ratiometric fluorescent probe. • A ratiometric fluorescent assay was achieved by using a smartphone. • Total BCAA level in pig blood was accurately determined. [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]

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]

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]

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]

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]

As an easily disposable substrate with a microporous texture, paper is a well-suited, generic substrate to build analytical devices for studying bacteria. Using a multi-pass lasing process, cellulose-based laser-induced graphene (cLIG) with a sheet resistance of 43.7 ± 2.3 Ω sq-1 is developed and utilized in the fabrication of low-cost and environmentally-friendly paper-based sensor arrays. Two case studies with Pseudomonas aeruginosa and Escherichia coli demonstrate the practicality of the cLIG sensors for the electrochemical analysis of bacteria. The first study measures the time-dependent profile of phenazines, such as pyocyanin, released from both planktonic (up to 60 h) and on-chip-grown (up to 22 h) Pseudomonas aeruginosa cultures. While similarities do exist, marked differences in phenazine production are seen with cells grown directly on cLIG compared to the planktonic culture. Moreover, in planktonic cultures, pyocyanin levels increase early on and plateau around 20 h, while optical density measurements increase monotonically over the duration of testing. The second study monitors the viability and metabolic activity of Escherichia coli using a resazurin-based electrochemical assay. These results demonstrate the utility of cLIG-based sensors as an inexpensive and versatile platform for monitoring bacteria and could enable new opportunities in high-throughput antibiotic susceptibility testing, ecological studies, and biofilm studies. • A low-cost, easily disposable electrochemical sensor array on cellulose paper for bacterial monitoring. • Optimized laser-graphitization print conditions yield a sheet resistance of 43.7 ± 2.3 Ω sq-1. • Monitoring of phenazine production in planktonic and biofilm Pseudomonas aeruginosa cultures over course of hours to days. • A resazurin-based electrochemical assay for characterizing viability of Escherichia coli cultures. [ABSTRACT FROM AUTHOR]

In this work, an origami paper-based analytical device for glucose biosensor by employing fully-drawn pencil electrodes has been reported. The three-electrode system was prepared on paper directly by drawing with nothing more than pencils. By simple printing, two separated zones on paper were designed for the immobilization of the mediator and glucose oxidase (GOx), respectively. The used paper provides a favorable and biocompatible support for maintaining the bioactivities of GOx. With a sandwich-type scheme, the origami biosensor exhibited great analytical performance for glucose sensing including acceptable reproducibility and favorable selectivity against common interferents in physiological fluids. The limit of detection and linear range achieved with the approach was 0.05 mM and 1–12 mM, respectively. Its analytical performance was also demonstrated in the analysis of human blood samples. Such fully-drawn paper-based device is cheap, flexible, portable, disposable, and environmentally friendly, affording great convenience for practical use under resource-limited conditions. We therefore envision that this approach can be extended to generate other functional paper-based devices. [ABSTRACT FROM AUTHOR]

We developed a simple and low-cost cell culture monitoring system utilizing a paper-based analytical device (PAD) and a smartphone. The PAD simultaneously analyses glucose and lactate concentrations in the cell culture medium. Focusing on the fact that animal cells consume glucose and produce lactate under anaerobic conditions, oxidase- and horseradish peroxidase (HRP) enzyme-mediated colorimetric assays were integrated into the PAD. The PAD was designed to have three laminated layers. By using a double-sided adhesive tape as the middle layer and wax coating, a bifurcated fluidic channel was prepared to manipulate sample flow. At the inlet and the outlets of the channel, a sample drop zone and two detection zones for glucose and lactate, respectively, were positioned. When sample solution is loaded onto the drop zone, it flows to the detection zone through the hydrophilic fluidic channel via capillary force. Upon reaching the detection zone, the sample reacts with glucose and lactate oxidases (GOx and LOx) and HRP, immobilized on the detection zone along with colorless chromophores. By the Trinder’s reaction, the colorless chromophore is converted to a blue-colored product, generating concentration-dependent signal. With a gadget designed to aid the image acquisition, the PAD was positioned to the smartphone-embedded camera. Images of the detection zones were acquired using a mobile application and the color intensities were quantified as sensor signals. For the glucose assay using GOx/HRP format, we obtained the limit of detection (LOD ∼0.3 mM) and the limit of quantification (LOQ ∼0.9 mM) values in the dynamic detection range from 0.3 to 8.0 mM of glucose. For lactate assay using LOx/HRP, the LOD (0.02 mM) and the LOQ (0.06 mM) values were registered in the dynamic detection range from 0.02 to 0.50 mM of lactate. With the device, simultaneous analyses of glucose and lactate in cell culture media were conducted, exhibiting highly accurate and reproducible results. Based on the results, we propose that the optical sensing system developed is feasible for practical monitoring of animal cell culture. [ABSTRACT FROM AUTHOR]

We described a new paper-based device for performing flow injection analysis (FIA) in this article. A homemade three-electrode system, an origami paper strip, and a ceramic microheater were attached to a modular plastic supporting structure. Continuous flow in the origami paper strip was generated by the combination of capillary and the evaporation enhanced by a microheater. The proposed device was optimized and characterized by using K 4 Fe(CN) 6 /K 3 Fe(CN) 6 as a probe of chronoamperometry. Under optimal conditions, the device can perform a single analysis within 1 min and enable over 200 injections in one sequential test. To demonstrate its applicability, the device was utilized to detect uric acid in human serum and urine samples. The limit of detection was 1.1 μM and the linear range was from 0.01 mM to 1 mM. Moreover, the testing results of the real samples agreed with the reference values. The proposed method exhibits great promise for developing a convenient and high-performance FIA system on paper substrates. • A paper-based analytical device for flow injection analysis was developed based on locally enhanced evaporation. • The device combines the advantages of short analysis time and high injection capacity. • The device can perform a single analysis within 1 min and enable over 200 injections in one sequential test. • This device was successfully applied to detect uric acid in human serum and urine samples. [ABSTRACT FROM AUTHOR]

Electrochemical sensing technology integrated with a paper-based analytical device (PAD) offers an effective method for analyte detection. Nevertheless, it is still challenging to realize reliable and sensitive analysis owing to the basic shortcomings (inferior conductivity and high batch-to-batch variability) of paper-based working electrodes (WE) and lack of efficient signal amplification label. Herein, we proposed a novel electrochemical aptasensor to detect ochratoxin A (OTA), with MXene-Au as sensing substrates and Pt nanoparticles (NPs) anchored hollow structure NiCo-layered double hydroxides (Pt@NiCo-LDH) as signal amplification materials. The complementary DNA (cDNA) was firstly assembled onto the MXene-Au/WE via Au-S bonds. Then aptamer (apta)-binding Pt@NiCo-LDH with excellent peroxidase (POD)-like activity was immobilized through hybridization to trigger the "signal on" state, generating a significant electrochemical signal. The presence of OTA enabled the dissociation of the apta-cDNA hybrid, releasing signal amplification labels to achieve "signal off" state. Based on the "signal on/off" strategy, the aptasensor exhibited a wider linear range from 20 fg/mL to 100 ng/mL and a lower limit of detection of 8.9 fg/mL (S/N = 3). This work provided a new avenue for the versatile design of electrochemical paper-based platform that enabled the on-site detection of various analytes. [Display omitted] • Mxene-Au as sensing substrates exhibited high conductivity, good stability and low batch-to-batch variation. • Pt@NiCo-LDH showed excellent peroxidase-like catalytic activity and high durability to environmental conditions. • The electrochemical OTA aptasensor based on "signal on/off" strategy showed a wider linear and a lower limit of detection. • This work provided a new avenue for broad spectrum detection of analytes by simply altering the specific aptamer. [ABSTRACT FROM AUTHOR]

In this work, we developed a paper-based analytical device (PAD) that utilizes molecularly imprinted polymer (MIP) as the biomimetic receptor and a very simple colorimetric assay combined with a smartphone readout. Sulfamethoxazole was used as a model analyte to evaluate the feasibility of the developed MIP-PAD. After the synthesis of MIP, its adsorption properties (isotherm, kinetic and thermodynamic) were scrutinized. Thereafter, The MIP- PAD was fabricated through the vacuum-assisted deposition of MIP suspension onto porous paper forming a 7 mm diameter of MIP layer. The MIP-PAD was characterized by Fourier-transform infrared spectroscopy and scanning electron microscopy. The sulfamethoxazole transforms into azo dye through a colorimetric reaction. The smartphone was utilized for the read-out of results. The LOD was 0.21 ppm. The proposed MIP-PAD showed high long-term stability. The determination of SMX was carried out in spiked tap and river water samples to evaluate the applicability of the proposed analytical strategy in real-world applications. The developed MIP-PAD would provide a new platform for real-time, selective, rapid, user-friendly, low-cost, portable, and straightforward colorimetric assays in environmental monitoring, public health, and food control. [Display omitted] • MIP integrated into paper-based analytical device (PAD) was successfully constructed. • A colorimetric assay was developed on MIP-PAD for sulfamethoxazole. • The smartphone was utilized as a signal read-out. • The proposed MIP-PAD showed high long-term stability. • The developed sensor is selective, user-friendly, portable, and intuitive to operate. [ABSTRACT FROM AUTHOR]

Pathogens are a persistent threat to human health, causing infectious disease and millions of deaths annually. The accurate and timely detection of pathogens is crucial in disease prevention, treatment, and monitoring. Although reliable detection methods are well established, many of them are still limited in use to clinical laboratories due to the need for costly and specialized instrumentation. In this study, we demonstrate a handheld and low-cost pathogen sensor consisting of a paper-based analytical device (μPAD) that can perform immunoassays and quantify analyte concentration by integrating with an automated color detection system that analyzes the color intensity of the μPAD. The core of the proposed sensor is the portable color detection system that can read the red-green-blue color of the paper emitted light from fluorescent nanomaterials, including graphene quantum dots (GQDs) and gold nanoclusters (AuNCs), which are conjugated with antibodies to indicate the immunoassay results, converting the presence of a pathogen to a colorful fluorescence signal. By adopting GQDs and AuNCs with high quantum yield and relatively high fluorescence intensity as the sensing signal, the paper-based detection system decreases the detection limit to as low as subnanogram/mL. Furthermore, GQDs and AuNCs can emit distinguishable fluorescence under the same light source (UV light) and possess limited background interference from the cellulose, enabling two or more analytes to be simultaneously detected with one UV light. Furthermore, a reaction time of just 10 min is needed, enabling diagnoses to be made in a timely manner and with high sensitivity. As a result, the proposed handheld pathogen sensor can rapidly detect the presence of pathogens with enhanced sensitivity and multiplexity, along with low instrumentation requirements, making it suitable for use in resource-limited settings where medical infrastructure is lacking. [Display omitted] • A handheld sensor consisting of a paper device and a color detection system was developed for on-site pathogen detection. • The system can read the color of the paper emitted light converting the presence of a pathogen into a fluorescence signal. • The high quantum yield GQDs and AuNCs help decrease the detection limit for multiple pathogens detection. [ABSTRACT FROM AUTHOR]

The present work reports on a sensitive and affordable non-instrumental method for the determination of bromine species in environmental waters by combination of paper-based headspace sampling and smartphone-based fluorescence detection. The proposed approach takes advantage of the luminescence quenching of a probe (5-oxo-3,5-dihydro-2 H-thiazolo[3,2- a ]pyridine-7-carboxylic acid, TPCA) derived from natural compounds, namely, citric acid and cysteamine, when exposed to molecular bromine. The decreased luminescence intensity of the probe was attributed to the formation of a dibrominated derivative of TPCA at the detection area of a paper-based analytical device (PAD). Remarkably, PADs modified with the luminescent probe showed excellent stability. Under optimal conditions, the proposed approach showed limits of detection of 5.4 µg L-1 and 0.9 µg L-1 for Br- and BrO 3 -, respectively, with a repeatability lower than 10% for both bromine species. The non-instrumental method was validated against two certified reference materials, namely BCR-611 and BCR-612 (low and high level Br- in groundwater), and successfully applied to the analysis of environmental water samples with recoveries in the range 93–106%. [Display omitted] • A citric acid-derived luminescent probe (TPCA) enables Br 2 sensing. • Paper-based analytical devices modified with TPCA showed excellent stability. • Formation of a dibrominated derivative of TPCA led to fluorescence quenching. • The proposed non-instrumental method shows excellent sensitivity and selectivity. • The method enables determination of Br- and BrO 3 - in environmental waters. [ABSTRACT FROM AUTHOR]

Storing highly concentrated reagents and releasing them with high efficiency is crucial for developing highly sensitive biosensors. However, the performance of reservoirs in biosensors made of filter paper is limited by the intrinsic physicochemical properties of untreated cellulose. In this article, the impact of 9 different polymer modifications on the storage and release of enzymes and nanoparticles in paper-based biosensors is studied for the first time. Carboxymethylcellulose, polyvinyl alcohol, carrageenan, and chitosan, which contain hydroxyl groups, yielded paper-based reservoirs with the highest concentration of reagents in them. Polymer reservoirs made of carboxymethylcellulose release enzymes rapidly and at high concentration, which results in colorimetric glucose biosensors with an ultralow limit of detection of 0.005 mM. Reservoirs made with a blend of polymers concentrate antibody-decorated nanoparticles and decrease 10 times the limit of detection of a model immunosensor. An adaption of this design was used for detecting the pathogen Klebsiella pneumoniae in urine samples from hospital patients at the infectious dose threshold rapidly and with high specificity. The fabrication of the reservoirs only requires drop-casting a polymer solution on the paper and drying. This procedure is compatible with the fabrication of origami biosensors, which are made from a single piece of filter paper. • Dry polymer drops in paper substrates concentrate reagents and release them with high efficacy. • Carboxymethylcellulose (CMC) enhances the release of highly concentrated enzymes. • A colorimetric glucose biosensor using CMC reservoirs had a low limit of detection of 0.005 mM. • Using CMC and PSS enhanced the release of antibody-decorated nanoparticles and decreased 10 times the LOD the immunosensor. • Immunosensors for detecting urinary tract infections by K.pneumoniae were developed. [ABSTRACT FROM AUTHOR]

Abstract: The present work describes for the first time the coupling of graphite pencil electrodes with paper-based analytical devices (μPADs) for glucose biosensing. Electrochemical measurement for μPADs using a two-electrode system was also developed. This dual-electrode configuration on paper provides electrochemical responses similar to those recorded by conventional electrochemical systems (three electrode systems). A wax printing process was used to define hydrophilic circular microzones by inserting hydrophobic patterns on paper. The microzones were employed one for filtration, one for an enzymatic reaction and one for electrochemical detection. By adding 4-aminophenylboronic acid as redox mediator and glucose oxidase to the reaction microzone, it was possible to reach low limits of detection for glucose with graphite pencil electrodes without modifying the electrode. The limit of detection of the proposed μPAD was found to be 0.38μmolL−1 for glucose. Low sample consumption (40μL) and fast analysis time (less than 5min) combined with low cost electrodes and paper-based analytical platforms are attractive properties of the proposed μPAD with electrochemical detection. Artificial blood serum samples containing glucose were analyzed with the proposed device as proof of concept. [Copyright &y& Elsevier]

• A novel chemiluminescence (CL) system was established for the determination of deltamethrin (DM). • The system is based on the enhancing effect of polyphosphate (PP) on graphene quantum dots (GQDs)-KMnO 4 CL reaction. • Two peaks were observed, at 490 and 695 nm as a characteristic emission for this unique CL system. • A simple paper-based analytical device (PADs) was used as platform for the analysis. A paper-based analytical device (PADs) combined with a powerful chemiluminescence (CL) system was established for the determination of deltamethrin (DM), based on the enhancing effect of polyphosphate (PP) on graphene quantum dots (GQDs)-KMnO 4 CL reaction. A possible mechanism for the obtained emission was proposed using the CL spectra, fluorescence and ultraviolet-visible patterns. The reaction of KMnO 4 and GQDs can lead to generation of GQDs in excited state (GQDs*), which can emit at 490 nm. Interestingly, PP changes the CL mechanism and the main emitter becomes Mn2+ instead of GQDs leading to a strong emission at 695 nm. Furthermore, the obtained multi-emission CL system was examined for analytical applications. The initial experiments showed that the amplified CL emission of the GQDs-KMnO4 system was selectively quenched in the presence of trace levels of DM, probably due to its effective interaction with GQDs or reaction with KMnO 4. This observation led to a facile, reliable and sensitive PADs-CL probe, developed for the determination of DM residue in food samples. Using this CL system and under the optimized experimental conditions, the generated signal is decreased by increasing DM concentration in the range of 0.3−10 μg mL-1 with limit of detection (LOD) of 0.15 μg mL-1. [ABSTRACT FROM AUTHOR]

• "Signal-on" electrochemical biosensor based on a competitive immunoassay was created. • The device pattern was designed to reduce the unnecessary processing and analysis time. • The amounts of OTC were successfully quantified in agricultural products. • This sensor provides high selectivity, sensitivity, portability, and low cost. In this work, we describe a new "signal-on" electrochemical biosensor based on a competitive immunoassay with a label-free format for the facile, sensitive and cost-effective determination of oxytetracycline (OTC). The device pattern was designed and printed on low-cost filter paper to construct a disposable paper-based analytical device (PAD) using a wax printing technique. The sensor consisted of a capture anti-OTC antibody (anti-OTC) and OTC-conjugated bovine serum albumin (OTC-BSA). Anti-OTC was first anchored onto the functionalized PAD, followed by the competitive binding of the OTC target and OTC-BSA. The amount of the captured OTC was examined by observing the signal response of the redox [Fe(CN) 6 ]3−/4−. In the presence of OTC, the current response was significantly increased with increasing OTC concentration, whereas the signal response was negligible in the absence thereof. The detected OTC concentration was linear over a range of 1−200 ng mL-1, and the corresponding values of 0.33 and 1.1 ng mL-1 were obtained for the limit of detection (LOD) and limit of quantitation (LOQ), respectively. This introduced sensor displayed high sensitivity and specificity, and the amounts of OTC were successfully quantified in agricultural products. This developed signal-on sensor may be an alternative tool for determining OTC residues and broader antibiotic targets. [ABSTRACT FROM AUTHOR]