Search

Point-of-care (POC) use is one of the essential goals of biosensing platforms. Because the increasing demand for testing cannot be met by a centralized laboratory-based strategy, rapid and frequent testing at the right time and place will be key to increasing health and safety. To date, however, there are still difficulties in developing a simple and affordable, as well as sensitive and effective, platform that enables POC use. In terms of materials, hydrogels, a unique family of water-absorbing biocompatible polymers, have emerged as promising components for the development of biosensors. Combinations of hydrogels have various additional applications, such as in hydrophilic coatings, nanoscale filtration, stimuli-responsive materials, signal enhancement, and biodegradation. In this review, we highlight the recent efforts to develop hydrogel-assisted biosensing platforms for POC use, especially focusing on polysaccharide hydrogels like agarose, alginate, chitosan, and so on. We first discuss the pros and cons of polysaccharide hydrogels in practical applications and then introduce case studies that test different formats, such as paper-based analytical devices (PADs), microfluidic devices, and independent platforms. We believe the analysis in the present review provides essential information for the development of biosensing platforms for POC use in resource-limited settings. [ABSTRACT FROM AUTHOR]

On-site quick detection of acetylcholinesterase (AChE) activity is of great significance for diagnosing nervous diseases, screening drugs, and monitoring poisoning. Herein, we developed a system for sensitively regulating gold nanobipyramid (AuNBP) morphology on a paper-based analytical device (PAD) via an Au+-disproportionation system, and further developed a high-resolution paper-based multicolor sensor (PMS) for both on-site visual detection and laboratory precision quantification of AChE activity using the system as a signal reporter and a dual-enzyme strategy as a receptor. The paper-based Au+ disproportionation-mediated AuNBP morphology system not only has excellent stability, reproducibility, and robust resistibility against reducing substances existing in biological samples but also can effectively suppress the color interference from reagents and greatly improve the sensitivity of regulating AuNBP morphology transformation, making it capable of generating long-term stable and more colorful/clearer color changes corresponding to Au+ concentrations. The developed PMS exhibited eight color changes corresponding to AChE activity within 0.0–1.0 U/L and the color changes could be stably maintained for 30 days, which makes both on-site visual detection and laboratory sensitive quantification possible. The PMS was successfully used to detect AChE activity in serum and erythrocyte samples with a visual detection limit (LOD) of 50 mU/L, a spectrometry LOD of 26 mU/L, and a recovery of 81–102%. The results obtained with the PMS were consistent with those obtained with Ellman's method, verifying its reliability. All of the above features make the PMS a promising assay for the rapid detection of AChE activity in clinical diagnosis. Importantly, the proposed paper-based Au+ disproportionation-mediated AuNBP morphology system can serve as a promising general signal reporter for developing various sensitive multicolor colorimetric sensors for target detection in various fields. [ABSTRACT FROM AUTHOR]

Point-of-care (POC) use is one of the essential goals of biosensing platforms. Because the increasing demand for testing cannot be met by a centralized laboratory-based strategy, rapid and frequent testing at the right time and place will be key to increasing health and safety. To date, however, there are still difficulties in developing a simple and affordable, as well as sensitive and effective, platform that enables POC use. In terms of materials, hydrogels, a unique family of water-absorbing biocompatible polymers, have emerged as promising components for the development of biosensors. Combinations of hydrogels have various additional applications, such as in hydrophilic coatings, nanoscale filtration, stimuli-responsive materials, signal enhancement, and biodegradation. In this review, we highlight the recent efforts to develop hydrogel-assisted biosensing platforms for POC use, especially focusing on polysaccharide hydrogels like agarose, alginate, chitosan, and so on. We first discuss the pros and cons of polysaccharide hydrogels in practical applications and then introduce case studies that test different formats, such as paper-based analytical devices (PADs), microfluidic devices, and independent platforms. We believe the analysis in the present review provides essential information for the development of biosensing platforms for POC use in resource-limited settings.

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]

The development of simple and accurate detection of uracil-DNA glycosylase (UDG) is of great significance for early clinical diagnosis and biomedical research. Here, we on the first effort introduced the uracil bases into the rolling circle amplification (RCA) reaction to produce the functionalized pure DNA hydrogel (PDH) for UDG detection. During RCA process, methylene blue (MB) molecules as the indicators were encapsulated into PDH. The addition of UDG can remove the uracil bases of PDH to generate abasic sites, which are further cleaved with the assistance of apurinic/apyrimidinic endonuclease (APE), thus resulting in the dissociation of PDH to release blue MB. By combining with the paper analytical devices as the signal readout platform, a colorimetric and electrochemical dual-signal biosensor was constructed for convenient and accurate detection of UDG. The proposed MB@PDH-based dual-signal sensing system exhibited good selectivity and high sensitivity with a detection limit of 6.4 × 10−4 U/mL (electrochemical method). It was also demonstrated that this sensing system showed excellent performance in UDG inhibitor screening, thus providing great potential in UDG-related disease diagnosis and drug discovery.

Perchlorate, a hazardous pollutant, is mainly found in untreated wastewater from urban and industrial sites and unregulated surface and groundwater sources. Effective monitoring of perchlorate in water is essential to mitigate its potential harmful effects. Microfluidic systems are evolving as promising technologies for detecting chemical contaminants in water due to their ability to enable rapid analysis with minimal consumption of reagents and samples. The integration of paper-based microfluidic devices with digital imaging has garnered enormous attention from the perspective of developing portable analytical techniques. Nevertheless, there is a need for further exploration to fully realize the potential of these systems. This study aimed to develop and evaluate the performance of a microfluidic paper-based device for measuring perchlorate levels in water samples. Smartphone-based digital imaging was integrated with microfluidic paper-based analytical device to establish a reliable colorimetric method for detecting perchlorate contamination. The results demonstrated successful quantitative estimation of perchlorate levels in water samples using a colorimetric assay based on the methylene blue-perchlorate reaction. Real-time, on-site colorimetric data were collected using a digital smartphone, and image processing methods were used to detect the occurrence of perchlorate in water samples from digital images. The developed approach yielded a broad linear response ranging from 4 to 12 µg/L (R2 = 0.97) for perchlorate detection, with a limit of detection of 3.41 µg/L and a limit of quantification of 10.34 µg/L. The findings underscore the effectiveness of colorimetric analysis and digital imaging for paper-based analytical devices. The limitations of this method include the capability to detect only a single analyte and the requirement for additional steps in image processing to obtain analytical results. Future developments should focus on designing devices for simultaneous detection of multiple contaminants and exploring automated methods of image analysis. [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]

Recently, with the development of microfluidic chips, attempts to integrate cell culture and biomedical material testing functions into a single chip have increased to supplement experimental animal models. Among the evaluations of biomaterials, immunogenicity testing, a current priority, is attracting attention. In this study, we developed a simple and easy-to-handle immunogenicity-testing cell chip to evaluate the immunogenicity of biomaterials. On this chip, macrophages were introduced as immunogenicity indicators, and a micro-paper-based analytical device (µPAD) was used for optical analysis. Macrophages are present in all parts of the body and mediate immune reactions against body implants or bio-derived substances, resulting in the production of hydrogen peroxide. In the cell chamber of the developed cell chip, macrophages grow and react to immunogenic materials. Activated macrophages secrete hydrogen peroxide, which is then transferred to the PAD with single-finger actuation. The hydrogen peroxide molecules reaching the PAD detection zone react with the colorimetric detection substrate, resulting in a color that corresponds to the hydrogen peroxide concentration. With the developed testing chip, the immunogenicity of biomaterials can be determined before administration. [ABSTRACT FROM AUTHOR]

The separation of enantiomers plays a critical role in pharmaceutical development, ensuring therapeutic efficacy, safety, and patent protection. It enables the production of enantiopure drugs and enhances our understanding of the properties of chiral compounds. In this study, a straightforward and effective chiral detection strategy was developed for distinguishing between tryptophan (TRP) enantiomers. The approach involved the preparation of a magnetic molecularly imprinted chitosan (MMIC) for preparation of the sample, which was combined with a nitrocellulose membrane (a paper-based analytical device, PAD) integrated with D-TRP covalently grafted with polymethacrylic acid (PAD-PMA_D-TRP). Discriminating between the TRP enantiomers was achieved using AuNPs as a colorimetric probe. Indeed, the presence of D-TRP rapidly induced the aggregation of AuNPs due to its strong affinity to PAD-PMA_D-TRP, resulting in a noticeable change in the color of the AuNPs from red to purple. On the other hand, L-TRP did not induce any color changes. The chiral analysis could be easily performed with the naked eye and/or a smartphone. The developed method exhibited a detection limit of 3.3 µM, and it was successfully applied to detect TRP in serum samples, demonstrating good recovery rates. The proposed procedure is characterized by its simplicity, cost-effectiveness, rapidity, and ease of operation. [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]

Purpose – The purpose of this study was to develop an accurate, selective, low-cost and user-friendly colorimetric pad to detect formaldehyde at low concentration. Design/methodology/approach – 1-phenyl-1,3-butanedione, a reactive chemical, was selected to develop the colorimetric pad for indoor air formaldehyde measurement. Silica nanoparticle impregnated with the reactive chemical was coated on the cellulose filter surface to increase the reactive site. A certified formaldehyde permeation tube was used to generate six varied concentrations between 0.01 and 0.10 ppm in a test chamber. The color intensity on the pads was measured using an image processing program to produce a formaldehyde concentration reading chart. The colorimetric pad was tested for optimum reaction time, accuracy, precision, stability, selectivity and shelf life. Findings – The color of the pads changed from white to yellow and the color intensity varied with the concentrations and appeared to be stable after exposure to formaldehyde for 8 hours. At room temperature, the stability of the pad was 7 days, and shelf life was 120 days. The accuracy, precision and bias of the pad were 12.38%, 0.032 and 6.0%, respectively. Carbonyl compounds, benzene and toluene did not interfere with the reading of this developed colorimetric pad. Originality/value – The developed colorimetric pad meets NIOSH's criteria for an overall accuracy of ±25%, bias = 10%. They were accurate at low concentrations, user-friendly and had low cost compared to an electronic direct reading instrument (cost of chemicals and materials was 21.50 Bath or 0.69 USD per piece) so that favorable for the use of general people for health protection.

Sustainable agricultural production has the aim to maximise the yield and quality of harvests. For that, farmers need to monitor nutrient levels within soils to apply fertilizers accordingly and to prevent land degradation and soil exhaustion. Current methods involve time-consuming sampling and transport to a laboratory; and most farmers do not have the resources to measure frequently at multiple locations, limiting their sustainable production. In order to address this challenge, a paper-based analytical device (PAD) for the multiplexed detection of soil nutrients, i.e. phosphate, nitrate and pH, extracted with a cafetière-based method, is developed in this work. The compact and easy-to-use platform enables an accurate soil analysis in less than 20 min; 3 min for extraction and 15 min for readout. Two detection channels for nitrate and phosphate analysis, and one circular detection area for pH, including the selective reagents to each analyte, are defined within the PAD. Upon contact with the specific nutrients, the platform produces a colorimetric response that is easily readable by naked eye and smartphone camera. The detection reactions were optimized in the range 1 − 22.5 mg L−1, 10 −100 mg L−1 and 5.0 – 8.5 for phosphate, nitrate and pH, respectively, in agreement with the relevant levels in soils. The volume of water, the number of pushes of the plunger, the extraction time and the mass of soil were also optimized for the nutrient extraction method with the cafetière. The optimal workflow was validated with commercial soils and compared with the conventional UV-Vis method and the labels from the soil package, showing excellent agreement. This paper-based platform provides the possibility of a cheap, sensitive and specific monitoring of soil nutrients by minimally trained operators, such as farmers, enabling in-the-field analyses of multiple key soil nutrients in resource-limited settings and therefore addressing the challenge of routine monitoring for sustainable agriculture. [Display omitted] • Monitoring of phosphate, nitrate and pH from soils in less than 20 min. • Extraction of 100 % of soil nutrients with a cafetière-based protocol in 3 min. • Multiplexed paper-based analytical device (PAD) to detect extracted soil nutrients. • Portable, cost-effective and easy-to-use by minimally trained operators. • Validated with commercial soils. [ABSTRACT FROM AUTHOR]

A novel, portable, disposable, affordable, and environmentally friendly paper-based analytical device (PAD) was designed for on-site determination of carmine and carminic acid. This platform utilized paper test strips with a chitosan coating as an adsorption layer, which was characterized using scanning electron microscope, energy-dispersive X-ray analysis, and water contact angle measurement. Carmine and carminic acid could be efficiently adsorbed on chitosan-coated paper test strips, producing distinct colors that could be captured using a smartphone camera without the need for an elution step. Notably, by utilizing the Hue component of the HSL model, it was possible to differentiate between carmine and carminic acid, confirming their presence in a sample. Furthermore, the color saturation intensity changed in a concentration-dependent manner, allowing for the determination of carmine and carminic acid concentrations in the ranges of 200–800 μg/mL and 20–100 μg/mL, respectively. Additionally, the created test strip could be used to measure the percentage of carminic acid in the presence of carmine. The developed PAD enabled the quantification of carmine in various food samples without the need for reagents or complex equipment. The environmental impact of this method was found to be positive based on assessments using GAPI and AGREE tools. [Display omitted] • Adsorption PAD was prepared for differentiation of carmine and carminic acid. • Carmine and carminic acid produced different colors and were captured using a smartphone. • Colour hue and saturation demonstrated carminic acid content and concentration dependence, respectively. • On-site discrimination and quantitation of carmine and carminic acid in food matrices was enabled. • An ecologically benign setup requiring no special reagents or elution is presented. [ABSTRACT FROM AUTHOR]

On-site quantitative detection of organophosphorus pesticides (OPs) is crucial for safeguarding food and public safety. This study presents a novel acetylcholinesterase (AChE)-mediated paper-based Au3+-etching of gold nanobipyramids (AuNBPs) system. The system employs a long-term storable AuNBPs-deposited nylon membrane embedded within a portable and temperature-controlled paper-based analytical device. This system, coupled with a colorimeter-based quantitative method, enables the development of a practical paper-based multicolor sensor (PMS) for on-site quantitative detection of three common OPs (paraoxon, dichlorvos, and trichlorfon). In the absence of OPs, AChE hydrolyzes acetylthiocholine to thiocholine, which reduces Au3+ to Au+. The presence of OPs inhibits AChE activity, thereby preserving Au3+ to etch AuNBPs on nylon membranes, accompanied by multicolor changes. These color changes can be simply quantified by measuring the a∗ parameter of the CIELab color space using a portable colorimeter. Under optimal conditions, the PMS displayed eight OPs-corresponding color changes with a minimum detectable concentration of 1.0–10 μg/L (visual observation) and limits of detection of 0.8–7.2 μg/L (colorimeter) and 0.2–3.4 μg/L (UV–vis spectrometry). The PMS successfully determined the OPs in vegetable and rice samples with recoveries of 89.0–109 % and RSDs (n = 5) of <6 %. These results were consistent with those obtained using the HPLC-MS method. The PMS demonstrates excellent portability, AuNBPs stability, detection sensitivity, and reproducibility, making it a promising tool for the on-site quantitative detection of OPs residues in food. Furthermore, the paper-based etching system coupled with the colorimeter-based quantitative method provides a valuable reference to develop practical PMSs for various targets in diverse fields. [Display omitted] • A novel paper-based gold nanobipyramids etching system was developed. • A portable and temperature-controlled paper-based analytical device was fabricated. • A paper-based multicolor sensor displayed eight OPs-corresponding color changes. • The color changes of gold nanobipyramids were quantified using a portable colorimeter. • The sensor exhibited excellent portability, stability, sensitivity, and reproducibility. [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]

Milk adulteration is life-threatening, and hence, its detection is an integral part of food safety. Most of the currently established detection techniques are expensive, sophisticated, and time-consuming. Though microfluidic devices offer a promising advantage in this regard, they are mostly colorimetric with sophisticated image processing, however, with compromised accuracy. In sharp contrast to this, we develop a low cost, simple yet accurate paper-based microfluidic device for the detection of starch concentration in milk. A 10 μl sample is injected in the inlet zone of the developed device and the resulting color transition length is captured using a smartphone after 5 min of the sample injection, and the starch concentration is measured by an in-house developed app, "starch-app". An excellent correlation (R2 = 0.9981) has been observed between the values of the starch concentration measured by the device and spectrophotometer in the range of 0–10% w/v. This novel, simple and inexpensive device has potential for its deployment in resource-constrained settings for estimation of starch adulteration in milk, especially in developing and under developing nations. [ABSTRACT FROM AUTHOR]

The need to monitor the presence of cyanide (CN−) in water is necessary to minimize the risks to aquatic ecosystems and human health. In this paper, a paper-based analytical device (PAD) was fabricated by immobilizing silver nanoparticles (AgNPs) on filter paper (FP) for the semi-quantitative colorimetric detection of CN− in water. The average diameter of the synthesized AgNPs was estimated to be around 26.23 ± 8.37 nm, with a characteristic optical absorption peak around 420 nm. Scanning electron microscopy and energy-dispersive X-ray spectroscopy results confirmed the successful immobilization of AgNPs on the filter paper via direct immersion technique. The potential of the fabricated FP-AgNPs PAD as a colorimetric sensor for CN− was evaluated using water samples contaminated with various ions and CN− concentration. Here, a color change from yellow to colorless was instantly observed as the FP-AgNPs PAD was exposed to water samples containing CN−. Interestingly, no color change was observed for samples exposed to other analytes suggesting the good selectivity of the FP-AgNPs PAD. Ultraviolet–Visible spectroscopy results and digital image analysis revealed that the fabricated sensor can detect CN− with concentration down to 1.0 ppm. The colorimetric response was also obtained for real water samples spiked with CN−. The results stipulated in this work offer baseline information that can be used in developing highly selective and sensitive digital sensing devices for affordable, accessible, and fast water contaminant monitoring and other related applications. [ABSTRACT FROM AUTHOR]

Herein, a paper-based glucose/air biofuel cell (BFC) was constructed and implemented for self-powered pesticide detection. Our developed paper-based chip relies on a hollow-channel to transport fluids rather than capillarity, which reduces analysis times as well as physical absorption. The gold nanoparticles (Au NPs) and carbon nanotubes (CNTs) were adapted to modify the paper fibers to fabricate the flexible conductive paper anode/cathode electrode (Au–PAE/CNT–PCE). Molecularly imprinted polymers (MIPs) using 2,4-dichlorophenoxyacetic acid (2,4-D) as a template were synthesized on Au–PAE for signal control. In the cathode, bilirubin oxidase (BOD) was used for the oxygen reduction reaction. Based on a competitive reaction between 2,4-D and glucose-oxidase-labeled 2,4-D (GOx-2,4-D), the amount of GOx immobilized on the bioanode can be simply tailored, thus a signal-off self-powered sensing platform was achieved for 2,4-D determination. Meanwhile, the coupling of the paper supercapacitor (PS) with the paper-based chip provides a simple route for signal amplification. Combined with a portable digital multi-meter detector, the amplified signal can be sensitively readout. Through rational design of the paper analytical device, the combination of BFC and PS provides a new prototype for constructing a low-cost, simple, portable, and sensitive self-powered biosensor lab-on-paper, which could be easily expanded in the field of clinical analysis and drug delivery. [ABSTRACT FROM AUTHOR]

In this work, indoxyl-glucoside was used as the substrate to develop a cost-effective, paper-based analytical device for the fluorescent and colorimetric dual-mode detection of β-glucosidase activity through a smartphone. The β-glucosidase can hydrolyze the colorless substrate indoxyl-glucoside to release indoxyl, which will be self-oxidized to generate green products in the presence of oxygen. Meanwhile, the green products emit bright blue-green fluorescence under ultraviolet–visible light irradiation at 365 nm. Fluorescent or colorimetric images were obtained by a smartphone, and the red-green-blue channels were analyzed by the Adobe Photoshop to quantify the β-glucosidase activity. Under the optimum conditions, the relative fluorescent and colorimetric signals have a good linear relationship with the activity of β-glucosidase, in the range of 0.01–1.00 U/mL and 0.25–5.00 U/mL, and the limits of detection are 0.005 U/mL and 0.0668 U/mL, respectively. The activities of β-glucosidase in a crude almond sample measured by the fluorescent and colorimetric methods were 23.62 ± 0.53 U/mL and 23.86 ± 0.25 U/mL, respectively. In addition, the spiked recoveries of normal human serum and crude almond samples were between 87.5% and 118.0%. In short, the paper-based device, combined with a smartphone, can provide a simple, environmentally friendly, and low-cost method for the fluorescent and colorimetric dual-mode detection of β-glucosidase activity. [ABSTRACT FROM AUTHOR]

The separation of enantiomers plays a critical role in pharmaceutical development, ensuring therapeutic efficacy, safety, and patent protection. It enables the production of enantiopure drugs and enhances our understanding of the properties of chiral compounds. In this study, a straightforward and effective chiral detection strategy was developed for distinguishing between tryptophan (TRP) enantiomers. The approach involved the preparation of a magnetic molecularly imprinted chitosan (MMIC) for preparation of the sample, which was combined with a nitrocellulose membrane (a paper-based analytical device, PAD) integrated with D-TRP covalently grafted with polymethacrylic acid (PAD-PMA_D-TRP). Discriminating between the TRP enantiomers was achieved using AuNPs as a colorimetric probe. Indeed, the presence of D-TRP rapidly induced the aggregation of AuNPs due to its strong affinity to PAD-PMA_D-TRP, resulting in a noticeable change in the color of the AuNPs from red to purple. On the other hand, L-TRP did not induce any color changes. The chiral analysis could be easily performed with the naked eye and/or a smartphone. The developed method exhibited a detection limit of 3.3 µM, and it was successfully applied to detect TRP in serum samples, demonstrating good recovery rates. The proposed procedure is characterized by its simplicity, cost-effectiveness, rapidity, and ease of operation.

Sildenafil (SIL) is regarded as an illegal adulterant in functional foods. Some functional foods doped with SIL have posed significant concern about their safety risks. However, the facile colorimetric detection of SIL is rarely investigated. Herein, we prepared a monodispersed spherical composite nanozyme (Fe 3 O 4 –NH 2 /GONRs), possessing excellent peroxidase-like (POD-like) and catalase-like (CAT-like) activities and strong superparamagnetic property. The enzyme-like activities of Fe 3 O 4 –NH 2 /GONRs can be selectively inhibited by SIL due to the synergistic effect of hydrogen bonds and π-π stacking between Fe 3 O 4 –NH 2 /GONRs and SIL. Leveraging this mechanism, a highly sensitive and selective colorimetric detection for SIL with a detection limit (LOD) of 0.26 ng/mL was developed. In addition, we prepared a three-dimensional paper-based analytical device (3D-PAD) for SIL colorimetric detection with naked-eyes and the semi-quantitative analysis with a LOD of 88 ng/mL. The proposed colorimetric and PAD detections demonstrated the advantages of low-cost, highly sensitive and selective, thus have promise application potential in the rapid detection of adulterated functional foods. [Display omitted] • The Fe 3 O 4 –NH 2 /GONRs composite nanozyme with peroxidase-like and catalase-like activities. • The enzyme-like activities of Fe 3 O 4 –NH 2 /GONRs can be inhibited by sildenafil. • A highly sensitive and selective colorimetric detection for SIL was developed. • A three-dimensional paper-based analytical device for SIL detection was designed. [ABSTRACT FROM AUTHOR]

The microfluidic paper-based analytical devices (μPADs) have been highly regarded as effective tools that offer a cost-effective and portable solution for point-of-care testing (POCT) and on-site detection. Utilizing paper substrates such as cellulose and nitrocellulose membranes, μPADs have proven beneficial for a range of applications from medical diagnostics to environmental monitoring. Despite their advantages, the fabrication of μPADs often requires sophisticated techniques and equipment, posing challenges for widespread adoption, especially in resource-limited settings. This study addresses the need for a simplified, low-cost method for fabricating μPADs that is accessible without specialized training or equipment. This research introduces a novel, efficient method for producing μPADs using 3D-printed slidable chambers and super glue vapor, bypassing traditional, more complex fabrication processes. The method utilizes super glue (ethyl-cyanoacrylate) vapor to create hydrophobic barriers on paper substrates. By optimizing the exposure sequence to super glue and water vapors and the heating conditions, we achieved rapid hydrophobization within 5 min, creating effective hydrophobic barriers and hydrophilic channels on paper substrates. The technique's simplicity allows for use by individuals without specialized training. The practical application of the fabrication method is demonstrated by the fabrication of μPADs that can detect multiple target analytes. We perform the simultaneous detection of glucose, proteins, and also the simultaneous detection of heavy metal ions nickel (Ni2+) and copper (Cu2+), highlighting its potential for broad applications in point-of-care diagnostics. This study is the first to report a method for selective exposure of ethyl-cyanoacrylate vapor for the fabrication of μPADs. This method significantly reduces the complexity, time, and fabrication cost, making it feasible for use in various settings. It also eliminates the need for specialized equipment and can be executed by individuals without specialized training. We believe that the proposed fabrication method contributes to the wider adoption and deployment of μPADs across various sectors. [Display omitted] • The hydrophobic barrier is patterned on paper using commercialized super glue vapor. • 3D-printed chamber and stencil are used to fabricate μPADs in less than 5 min. • Detection of glucose, proteins and heavy metals are demonstrated. [ABSTRACT FROM AUTHOR]

The similar transmission patterns and early symptoms of respiratory viral infections, particularly severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), influenza (H1N1), and respiratory syncytial virus (RSV), pose substantial challenges in the diagnosis, therapeutic management, and handling of these infectious diseases. Multiplexed point-of-care testing for detection is urgently needed for prompt and efficient disease management. Here, we introduce an electrochemical paper-based analytical device (ePAD) platform for multiplexed and label-free detection of SARS-CoV-2, H1N1, and RSV infection using immobilized pyrrolidinyl peptide nucleic acid probes. Hybridization between the probes and viral nucleic acid targets causes changes in the electrochemical response. The resulting sensor offers high sensitivity and low detection limits of 0.12, 0.35, and 0.36 pM for SARS-CoV-2 (N gene), H1N1, and RSV, respectively, without showing any cross-reactivities. The amplification-free detection of extracted RNA from 42 nasopharyngeal swab samples was successfully demonstrated and validated against reverse-transcription polymerase chain reaction (range of cycle threshold values: 17.43–25.89). The proposed platform showed excellent clinical sensitivity (100 %) and specificity (≥97 %) to achieve excellent agreement (κ ≥ 0.914) with the standard assay, thereby demonstrating its applicability for the screening and diagnosis of these respiratory diseases. [Display omitted] • The PNA-based ePAD was developed for multiplex detection of respiratory viruses. • This platform can be used to diagnose SARS-CoV-2, H1N1, and RSV infection. • The developed device achieves high specificity and sensitivity for the detection. • This system can be applied to clinical samples without requiring RNA amplification. • The analysis results corresponded well with those of the RT-PCR assays. [ABSTRACT FROM AUTHOR]

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]

We report on the design of a simple all-DNA circuit with dual functions of signal amplification and signal reporting and its use for detection of human telomerase activity from cancer cells. The system utilizes a catalytic hairpin assembly (CHA) reaction for amplification, which produces split G-quadruplex outputs that assemble to form complete guanine quadruplex structures as reporting modules. As designed, a linear DNA sequence (the target) functions as a catalyst to drive cyclic programmed assembly of two hairpins, producing a DNA duplex with two guanine-rich sequences that assemble to form a complete Gq structure. The formation of the Gq element allows either fluorescence or colorimetric detection of the target. Examples are provided to demonstrate fluorescence detection of cancer cells' telomerase activities in solution and the first example of a CHAmodulated colorimetric assay for detecting telomerase activities of cancer cells using a simple paper device. [ABSTRACT FROM AUTHOR]

In this study, a paper-based sensor was developed for the detection of hydrogen-peroxide-related biomarkers, with glucose oxidase catalyzing as an example. Potassium iodide can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine in the presence of hydrogen peroxide to colorize the paper-based biosensor detection area, which was imaged by a scanner, and the color intensity was analyzed by the Adobe Photoshop. Under the optimal conditions, the color intensity shows a good linear relationship with hydrogen peroxide and glucose concentrations in the ranges of 0.1–5.0 mM and 0.5–6.0 mM, respectively. The detection limit of hydrogen peroxide is 0.03 mM and the limit of quantification of glucose is 0.5 mM. Besides, the method was employed in measuring glucose concentration in fruit samples, and the spiked recoveries are in the range of 95.4–106.1%. This method is cost-effective, environmentally friendly, and easy to be operated, which is expected to realize the point-of-care testing of more hydrogen-peroxide-related biomarkers. [ABSTRACT FROM AUTHOR]

Here, a simple and portable paper‐based analytical device (PAD) based on the inherent capability of carbon quantum dots (CQDs) to serve as a great emitter for the bis(2,4,6‐trichlorophenyl)oxalate (TCPO)–hydrogen peroxide (H₂O₂) chemiluminescence (CL) reaction is introduced for the detection of harmful mercury ions (Hg2+). The energy is transferred from the unstable reaction intermediate (1,2‐dioxetanedione) to CQDs, as acceptors, and an intensive orange‐red CL emission is generated at ~600 nm, which is equal to the fluorescence emission wavelength of CQDs. The analytical applicability of this system was examined for the determination of Hg2+. It was observed that Hg2+ could significantly quench the produced emission, which can be attributed to the formation of a stable and nonluminescent Hg2+–CQDs complex. Accordingly, a simple and rapid PAD was established for monitoring Hg2+, with a limit of detection of 0.04 μg ml−1. No interfering effect on the signal was found from other examined cations, indicating the acceptable specificity of the method. The designed assay was appropriately utilized to detect Hg2+ ions in cosmetic samples with high efficiency. It was characterized by its low cost, ease of use, and was facile but accurate and high selective for the detection of Hg2+ ions. In addition, the portability of this probe makes it suitable for on‐site screening purposes. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide is a key reagent in many analytical assays. At the same time, it is rather unstable and prone to evaporation. For these reasons, its application in sensors requiring reagents in solid state, for example in paper-based microfluidics, is hindered. Usually in paper-based analytical devices reagents are stored in a dried form within paper matrix until the device is used. This approach is not feasible in case of hydrogen peroxide. Here, hydrogen peroxide stabilization on paper with the aid of silica xerogel was studied and optimized to create long-term stable systems which rapidly deliver hydrogen peroxide. The variables affecting hydrogen peroxide stability such as gelation time, silica to H 2 O 2 ratio, type of solid support and storage conditions were optimized to find the combination of variables providing stable H 2 O 2 concentration for the longest time possible. Such paper-silica-H 2 O 2 composites allow to maintain steady hydrogen peroxide concentration for at least 27 days in the optimal conditions. Hydrogen peroxide is rapidly released from silica-paper matrix within a few minutes upon contact with water, without any byproducts. The obtained systems were characterized using scanning electron microscopy with energy dispersive spectroscopy and infrared spectroscopy, revealing that silica is present as a thin film covering cellulose fibers. Finally, to test the developed hydrogen peroxide stabilization method in real sensing scenario, a proof-of-concept paper-based sensor was created for phenolic content determination in fruits and wine. The outcome of this research will open new avenues in the development of user-friendly, long-term stable paper-based analytical devices which utilize hydrogen peroxide as one of reagents. Owing to the fact, that silica matrix is insoluble in water, the proposed H 2 O 2 stabilization method is compatible with most detection schemes without the risk of interfering with the assay. [Display omitted] • Hydrogen peroxide stabilization on paper with silica xerogel was studied. • Factors affecting hydrogen peroxide stability were optimized. • H 2 O 2 -silica-paper composites retain a stable H 2 O 2 amount for at least 27 days. • Paper-based analytical device for phenolic content determination was developed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The accurate measurement of creatinine (Ctn) in human biofluid samples is a challenge. • A stable carboxyl (C=O, COOH) groups were introduced on the electrode surface during electrochemical activation. • ePAD* was applied for the selective non-enzymatic detection of Ctn. • ePAD* detects Ctn in blood serum and urine samples with high reliability. • ePAD* is economic and accurate compared to existing methods for Ctn detection. Selective detection of creatinine (Ctn) is crucial for the clinical diagnosis of early-stage kidney dysfunction. A non-enzymatic electrochemical method for a highly selective and sensitive detection of Ctn in blood serum and urine samples was developed using an electrochemically activated carbon-coated paper-based analytical device (ePAD*). During activation, the growth of stable carboxyl (C = O, COOH) groups on the edge planes of the carbon surface was observed. The prepared ePAD* was characterized using various microscopic, spectroscopic, and electrochemical techniques. The ePAD* was utilized as a medium exchanger, and the voltammetric response of Ctn detection in the linear concentration from 0.03 to 45 mM with a detection limit of 5.41 µM was achieved. In addition, the selective detection of Ctn in the presence of potential interferences (i.e., uric acid, ascorbic acid, ammonia, and urea) was examined. The examination revealed a remarkable selectivity toward Ctn detection. Real-time detection of Ctn in blood serum and urine samples was performed. The results were correlated with the classical Jaffé colorimetric method. Our ePAD* exhibited a better electrochemical response. Thus, it is a highly potential diagnostic method for further development as a rapid and precise detection platform for point-of-care (POC) devices for examining patients with kidney disease. [ABSTRACT FROM AUTHOR]

Herein, a paper-based glucose/air biofuel cell (BFC) was constructed and implemented for self-powered pesticide detection. Our developed paper-based chip relies on a hollow-channel to transport fluids rather than capillarity, which reduces analysis times as well as physical absorption. The gold nanoparticles (Au NPs) and carbon nanotubes (CNTs) were adapted to modify the paper fibers to fabricate the flexible conductive paper anode/cathode electrode (Au–PAE/CNT–PCE). Molecularly imprinted polymers (MIPs) using 2,4-dichlorophenoxyacetic acid (2,4-D) as a template were synthesized on Au–PAE for signal control. In the cathode, bilirubin oxidase (BOD) was used for the oxygen reduction reaction. Based on a competitive reaction between 2,4-D and glucose-oxidase-labeled 2,4-D (GOx-2,4-D), the amount of GOx immobilized on the bioanode can be simply tailored, thus a signal-off self-powered sensing platform was achieved for 2,4-D determination. Meanwhile, the coupling of the paper supercapacitor (PS) with the paper-based chip provides a simple route for signal amplification. Combined with a portable digital multi-meter detector, the amplified signal can be sensitively readout. Through rational design of the paper analytical device, the combination of BFC and PS provides a new prototype for constructing a low-cost, simple, portable, and sensitive self-powered biosensor lab-on-paper, which could be easily expanded in the field of clinical analysis and drug delivery.

The generation of DNase type I 3′OH DNA ends is closely related to the harm of endogenous reactive oxygen species (ROS) and environmental genotoxic agents. The evaluation of this type of DNA damage plays an important role in clinical intervention and environmental toxicity assessment. Terminal deoxynucleotidyl transferase (TdT)-assisted isothermal amplification (TAIA) offers a facile and versatile way to detect DNase type I 3′OH DNA ends. Its ability of templated-independent isothermal amplification is one unique feature. Here, we reported a paper-based analytical device (PAD) coupled with a smartphone for the detection of DNase type I 3′OH DNA ends using TAIA and colorimetric signal readout. We achieved the integration of cell lysis, DNA extraction, TAIA, horseradish peroxidase (HRP)-enabled colorimetric reaction, and signal readout. This device could achieve a limit of detection of 264 cells with a total assay time of less than 45 min. By combining PAD with a smartphone, the integrated platform could be used for the visual and quantitative analysis of DNA damages with the advantages of ease-to-use, fast response, inexpensive, and instrument free. Furthermore, successful assessment of the genotoxicity in wastewater effluents suggested the great promise of the integrated platform for on-site testing in practical applications.

In this work, indoxyl-glucoside was used as the substrate to develop a cost-effective, paper-based analytical device for the fluorescent and colorimetric dual-mode detection of β-glucosidase activity through a smartphone. The β-glucosidase can hydrolyze the colorless substrate indoxyl-glucoside to release indoxyl, which will be self-oxidized to generate green products in the presence of oxygen. Meanwhile, the green products emit bright blue-green fluorescence under ultraviolet–visible light irradiation at 365 nm. Fluorescent or colorimetric images were obtained by a smartphone, and the red-green-blue channels were analyzed by the Adobe Photoshop to quantify the β-glucosidase activity. Under the optimum conditions, the relative fluorescent and colorimetric signals have a good linear relationship with the activity of β-glucosidase, in the range of 0.01–1.00 U/mL and 0.25–5.00 U/mL, and the limits of detection are 0.005 U/mL and 0.0668 U/mL, respectively. The activities of β-glucosidase in a crude almond sample measured by the fluorescent and colorimetric methods were 23.62 ± 0.53 U/mL and 23.86 ± 0.25 U/mL, respectively. In addition, the spiked recoveries of normal human serum and crude almond samples were between 87.5% and 118.0%. In short, the paper-based device, combined with a smartphone, can provide a simple, environmentally friendly, and low-cost method for the fluorescent and colorimetric dual-mode detection of β-glucosidase activity.

In this study, a paper-based sensor was developed for the detection of hydrogen-peroxide-related biomarkers, with glucose oxidase catalyzing as an example. Potassium iodide can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine in the presence of hydrogen peroxide to colorize the paper-based biosensor detection area, which was imaged by a scanner, and the color intensity was analyzed by the Adobe Photoshop. Under the optimal conditions, the color intensity shows a good linear relationship with hydrogen peroxide and glucose concentrations in the ranges of 0.1–5.0 mM and 0.5–6.0 mM, respectively. The detection limit of hydrogen peroxide is 0.03 mM and the limit of quantification of glucose is 0.5 mM. Besides, the method was employed in measuring glucose concentration in fruit samples, and the spiked recoveries are in the range of 95.4–106.1%. This method is cost-effective, environmentally friendly, and easy to be operated, which is expected to realize the point-of-care testing of more hydrogen-peroxide-related biomarkers.

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]

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]

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]

We conducted electrochemical immunochromatography using a developed electrochemical immunochromatography platform. We used immunoglobulin G (IgG) as a representative protein. We also conducted enzyme-linked immunosorbent assay (ELISA) as a control test. We used the same reagents except for the substrate and compared their limits of detection (LODs) and detection ranges. We succeeded in measuring IgG concentrations ranging from 30 ng/ml to 10 μg/ml and obtained 23.00 ng/ml as the LOD for electrochemical immunochromatography. The detection signal was not saturated even at 10 μg/ml, so it seems that higher concentrations of IgG can also be measured. The LOD was approximately 80-fold higher than that of ELISA (0.30 ng/ml); however, the detection range was one order of magnitude higher than that of ELISA. While electrochemical immunochromatography is less sensitive than ELISA, we demonstrated sufficient sensitivity for its use in a point-of-care testing (POCT) device. Furthermore, this information will be very helpful when introducing an assay system already established in ELISA to POCT. [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]