Search

Residual chlorine from widespread disinfection processes forms byproducts in water that are harmful to humans and ecosystems. Portable sensors are essential tools for the on-site monitoring of residual chlorine in environmental samples. Here, an inexpensive colorimetric sensor was developed by grafting via amidation the chromogen orthotolidine (OTO) to the surface of a TEMPO-oxidized cellulose filter paper (O-TOFP). A thorough characterization of the sensor strip demonstrated that it was highly stable and that it could be stored for a long period before usage. O-TOFP had a fast response time of 30 s, was highly selective for residual chlorine ions (ClO - ) with an accuracy of at least 95 %, and exhibited an excellent limit of detection of only 0.045 mg/L when combined with smartphone image acquisition. With its many positive features, the easy-to-use and robust O-TOFP sensor described here could become a useful tool for the determination of residual chlorine in different water samples., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

A rapid user-friendly paper-based test strip using zinc microparticles in conjugation with Griess reagent was developed for nitrite and nitrate detection. Test strips were fabricated using a simple and fast method of step-by-step immersion into reagents so that each strip contained a single detection pad for nitrite detection and another separate pad for nitrate detection. To reduce nitrate to nitrite, zinc microparticles suspended in ethanolic solution of polyvinylpyrrolidone (PVP) were uniformly immobilized on the paper strips that were previously impregnated in the Griess reagent and dried. The Griess reagent components were optimized to reach the highest color intensity. The optimized test strip was able to determine both nitrite and nitrate with respective detection limits of 0.43 and 9.43 mg/kg and a detection time of 60 s. The performance of the new test strips was evaluated for the simultaneous colorimetric detection of nitrite and nitrate in water and different food samples., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Monitoring various biomarkers in saliva samples emerges as a dynamic and non-invasive method. However, the high viscosity of saliva presents a distinct challenge when integrating paper-based platforms for on-site analysis. In addressing this challenge, we introduced the capillary-driven microfluidic paper-based analytical devices (μCD-PAD) designed for user-friendly and simultaneous detection of ethanol and tetrahydrocannabinol (THC) in saliva without a sample preparation step. Employing a colorimetric approach, we quantified both analytes. Synthetic salivas of varying viscosity flowed seamlessly to the detection zone without needing a sample preparation step, and no impact on colorimetric detection due to viscosity was observed (RSD <5 %). Within 10 min after the solution reached the detection zone, the device produced a homogeneous color signal, easily analyzed by a smartphone camera. To extend the application for determination to cover a legal limit concentration of ethanol and concentration of salivary THC even 24 h after marijuana consumption, the detection time of 30 min was optimized. Moreover, a saliva sample containing both analytes was used to demonstrate the capability of the developed device to detect ethanol and THC simultaneously. No cross-talk between ethanol and THC occurred and showed recovery in the 98-102 % for ethanol and 95-105 % for THC with acceptable accuracy. This developed device exhibits excellent potential for forensic applications, providing a user-friendly, cost-effective, and real-time screening tool for detecting ethanol and THC in saliva., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

A novel type of colorimetric/fluorescent nanopaper indicator has been developed from the melt-extruded poly (vinyl alcohol-co-ethylene) nanofibers with surface anchored metal-organic frameworks (MOFs) by an interfacial coordination strategy. Specifically, the fluorescein isothiocyanate molecules could be anchored to the nanofiber surface by nickel ions and co-assembled into a hydrophilic nanocoating via a dynamic water/alcohol solvent evaporation method. Interestingly, this hydrophilic surface enables fast adsorption of moistures and interaction with biological amine vapors, resulting a saffron cake-layer of MOF nanocrystals with ultra-sensitive colorimetric/fluorescent responses based on an alkaline pH/ammonia induced competitive coordination mechanism. Finally, these porous nanofibrous matrix and active nanocoating make the nano-paper an ultra-sensitive optical platform for in-situ monitoring of the shrimp freshness from mins to weeks. Therefore, this composite film shows great potential into advanced paper-based indicators for food quality control and safety in processing industry., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Background: 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., Results: 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., Significance: 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Mineral water is a natural water that originated from an underground water table, a well, or a natural spring which is considered microbiologically intact. The revenue from the bottled mineral water industry will be USD 342.40 billion in 2023, and it is expected to grow at a compound annual growth rate (CAGR) of 5.24 %. Consequently, the discrimination of original bottled mineral water from tap water is an important issue that requires designing sensors for simple and portable identification of these two types of water. In this work, we have developed a Dip-Type colorimetric paper-based sensor array with three organic dyes (Bromothymol Blue, Bromophenol Blue, and Methyl Red) followed by chemometrics' pattern recognition methods (PCA and LDA) for discrimination of original bottled mineral waters from tap waters based on differences in ion variety and ion quantity. Forty brands of mineral water and twenty-six Tap water samples from different regions of Shiraz and other Iranian cities were analyzed by this sensor array. Moreover, these experiments were performed in two consecutive years to check the versatility of the sensor with seasonal changes in waters. This sensor array was able to discriminate these two water types from each other with an accuracy of > 95 % based on the analysis of 85 water samples., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Creatinine, an important biomarker for renal functions, is often conventionally estimated using the gold standard Jaffe reaction from blood, which involves the usage of a spectrophotometric measurement, thus restricting its utilization scope in point-of-care settings. Here, we report the development of a method for the single-step quantification of creatinine from whole blood using a paper-based microfluidic device. Our platform uses Whatman filter paper integrated with an LF1 membrane. The on-chip separation of blood plasma is achieved through the LF1 membrane, while the Whatman component of the device contains the embedded reagents for the Jaffe reaction. The combination of two different grades of paper enables a single-step quantification of creatinine as the separated blood plasma traverses to the reaction zone through capillary imbibition. Colorimetric readouts were quantified using an RGB sensor instead of a smartphone, which is highly platform dependent and incurs a relatively higher cost compared to the other components in typical point-of care (POC) devices. Our sensor was integrated within a 3D box, thereby making the detection virtually instrument free and perfectly suited for POC settings. The limit of detection (LOD) of our device was 0.219 mg dL -1 , which falls within the lower range of physiological values. The coefficient of determination ( R 2 ) for the linearity and median accuracy were 0.978 and 94.047%, respectively. The relative standard deviation (RSD) for precision measurements remained below 5% for the developed protocol. Furthermore, we validated the performance of our device with 35 clinical samples in laboratory settings against the gold standard measurements. Our Bland-Altman plot as well as t -test and chi-square test results clearly confirmed the validity of our device within a 95% confidence interval.

Dual-mode-based sensors have drawn a lot of interest due to their high accuracy and sensitivity compared to single-response systems. A simple electrochemical and colorimetric dual-mode sensor based on enzyme-linked immunosorbent assay (ELISA), without complex electrode surface modification, was developed for accurate and sensitive detection of carcinoembryonic antigen (CEA). The target CEA is recognized by an antibody coupled to horseradish peroxidase (HRP), which then oxidizes the substrate 3,3',5,5'-tetramethylbenzidine (TMB) to generate both a colorimetric and an electrochemical signal. A paper-based analysis device (μPAD) with dual-mode homogeneous sensing microfluidic was created; three paper-based detection areas for colorimetric testing, and a two-electrode embedded detection area for electrochemical testing. When applying colorimetric analysis technology, the linear range of CEA detection is 0.6-40 ng mL -1 , and the limit of detection (LOD) is 0.2 ng mL -1 . The linear range is 0.1-40 ng mL -1 and the LOD is 0.03 ng mL -1 by applying electrochemical analysis. The visibility and intuitiveness of colorimetry provide a reference for higher sensitivity and quick response of the electrochemical method. A smartphone application (APP) was also developed to realize the dual extraction of colorimetric signals. The colorimetric detection system based on ELISA can provide a new path for the development of electrochemical sensing and makes it have inherent self-verification and self-correction functions and is expected to provide more reliable and accurate detection results.

Single-atom nanozymes (SAzymes) as a class of highly active nanozymes with the advantages of high atom utilization, high catalytic activity and stability have attracted great attention. In this work, Fe-N-C SAzymes with exceptional oxidase (OXD)-like activity were achieved utilizing polyvinylpyrrolidone (PVP) as a template. The Fe-N-C SAzymes with remarkable OXD-like activity could oxidize TMB to blue oxTMB, but 8-hydroxyquinoline (8-HQ) as a metal chelator is capable of discoloring oxTMB. Thus, the addition of 8-HQ decolorized the solution. However, upon the introduction of Cr(VI) ions, 8-HQ preferentially chelated with the Cr(VI) ions, reversing the inhibition of the color reaction and restoring the blue color. Based on this phenomenon, we constructed a novel paper-based analytical device (PAD) that exhibited a linear range of 5-1000 μM and an LOD of 1.2 μM. Importantly, the PAD used in this study shows the merits of simplicity, low preparation costs, and rapid reaction times. When combined with smartphone RGB analysis, it enables the simultaneous analysis of eight different Cr(VI) concentrations without the need for large-scale instrumentation. Moreover, the proposed PAD displays high selectivity, accuracy and utility in testing actual short-necked clam samples. This work not only provides a simple and cost-effective method to detect Cr(VI) but also makes a contribution to rapid food testing.

The evolution and mutation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are urgent concerns as they pose the risk of vaccine failure and increased viral transmission. However, affordable and scalable tools allowing rapid identification of SARS-CoV-2 variants are not readily available, which impedes diagnosis and epidemiological surveillance. Here we present a colorimetric nucleic acid assay named MARVE (multiplexed, preamplification-free, single-nucleotide-resolved viral evolution) that is convenient to perform and yields single-nucleotide resolution. The assay integrates nucleic acid strand displacement reactions with enzymatic amplification to colorimetrically sense viral RNA using a metal ion-incorporated DNA probe (TEprobe). We provide detailed guidelines to design TEprobes for discriminating single-nucleotide variations in viral RNAs, and to fabricate a test paper for the detection of SARS-CoV-2 variants of concern. Compared with other nucleic acid assays, our assay is preamplification-free, single-nucleotide-resolvable and results are visible via a color change. Besides, it is smartphone readable, multiplexed, quick and cheap ($0.30 per test). The protocol takes ~2 h to complete, from the design and preparation of the DNA probes and test papers (~1 h) to the detection of SARS-CoV-2 or its variants (30-45 min). The design of the TEprobes requires basic knowledge of molecular biology and familiarity with NUPACK or the Python programming language. The fabrication of the origami papers requires access to a wax printer using the CAD and PDF files provided or requires users to be familiar with AutoCAD to design new origami papers. The protocol is also applicable for designing assays to detect other pathogens and their variants., (© 2024. Springer Nature Limited.)

Herein, oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB) nanobelts were developed to enhance the colorimetric and paper-based sensing of H 2 O 2 . It was found that the minor component of Fe 2+ in Na 2 SO 4 reagent could catalyze the oxidization of TMB by H 2 O 2 into positively charged oxTMB, which was further assembled into dark blue oxTMB nanobelts via electrostatic interaction with SO 4 2- . The extinction originating from the absorption and scattering of oxTMB nanobelts was utilized to quantitatively detect H 2 with a wide linear detection range (1.0-300 μM) and a low limit of detection (0.48 μM). In addition, no coffee-ring effect was observed in the test zone of the paper-based colorimetric array, which was beneficial to judge the color by naked eye. Finally, the colorimetric method was applied to detect H 2 with a wide linear detection range (1.0-300 μM) and a low limit of detection (0.48 μM). In addition, no coffee-ring effect was observed in the test zone of the paper-based colorimetric array, which was beneficial to judge the color by naked eye. Finally, the colorimetric method was applied to detect H 2 O 2 in contact lens care solution. This work not only proposed a new colorimetric sensing platform for H 2 O 2 , but also highlighted the minor component in the reagent might influence the experimental result., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

A paper-based colorimetric sensor array mediated by a novel nanozyme (CuCo 2 O 4 ) was developed using a screen-printing technology. The aim was to facilitate the identification of different kinds of alkaloids. Typically, three chromogenic substrates (3,3',5,5'-tetramethylbenzidine, 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid), and o-phenylenediamine) were selected as sensing elements, which can be catalyzed by a CuCo 2 O 4 nanozyme with peroxidase-like activity to yield corresponding oxidized products, thereby inducing color changes. Owing to the varying inhibitory ability of different alkaloids on acetylcholinesterase (AChE), a decrease in choline (Ch) concentration occurs and subsequently results in the restoration of color within the units of sensor array. Color data can be transformed into hue information with a smartphone. The above color variations generated a unique "fingerprint" pattern on five alkaloids (berberine, palmatine, jatrorrhizine, eserine, and harmane), which can be successfully discriminated through linear discriminant analysis in the range 0.2 to 20 µM. Furthermore, the sensor arrays allowed successful discrimination of the above five alkaloids in Chinese herbal medicine samples and recognition of 22 blind samples. This work presents a novel nanozyme-based paper sensor array, which is a user-friendly and reliable platform for probing different alkaloids. In addition, the developed sensing strategy enables the identification of AChE-related diseases, positively contributing to the screening available of AD-associated drugs., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.)

Color correction is an important methodology where a digital image's colors undergo a transformation to more accurately represent their appearance using a predefined set of illumination conditions. Colorimetric measurements in diagnostics are sensitive to very small changes in colors and therefore require consistent, reproducible illumination conditions to produce accurate results, making color correction a necessity. This paper presents an image color correction pipeline developed by HueDx, Inc., using transfer algorithms that improve upon existing methodologies and demonstrates real-world applications of this pipeline in colorimetric clinical chemistry using a smartphone enabled, paper-based total protein diagnostic assay. Our pipeline is able to compensate for a variety of illumination conditions to provide consistent imaging for quantitative colorimetric measurements using white-balancing, multivariate gaussian distributions and histogram regression via dynamic, non-linear interpolating lookup tables. We empirically demonstrate that each point in the color correction pipeline provides a theoretical basis for achieving consistent and precise color correction. To show this, we measure color difference with deltaE (ΔE00), alongside quantifying performance of the HueDx color correction system, including the phone hardware, color sticker manufacturing quality and software correction capabilities. The results show that the HueDx color correction system is capable of restoring images to near-imperceptible levels of difference independent of their original illumination conditions including brightness and color temperature. Comparisons drawn from the paper-based total protein assay calibrated and quantified with and without using the HueDx color correction pipeline show that the coefficient of variation in precision testing is almost twice as high without color-correcting. Limits of blank, detection and quantitation were also higher without color-correction. Overall, we were able to demonstrate the HueDx platform improves reading and outcome of the total protein diagnostic assay and is useful for the development of smartphone-based quantitative colorimetric diagnostic assays for point-of-care testing., Competing Interests: Financial support for the development of this product was provided by HueDx,Inc., an early-stage venture-capital-backed company. All authors are employees of the company. The HueDx system, consisting of the color correction pipeline, HueCard(with custom sticker) and HueTools are commercially marketed products. Code for the color correction pipeline cannot be made available publicly due to legal restrictions., (Copyright: © 2024 Menon et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Sensitive and accurate analysis of pyrophosphate (PPi) is of great importance for preventing health hazard in environment. Nevertheless, most of sensors focus on sensitivity and selectivity, but practicality is also a significant quota. How to reconciling sensitivity, selectivity and practicability in one single sensor is desirable but remains challenging. Here, we created a novel metal-carbon nanozyme V 2 O 5 @C with two-dimensional (2D) morphology and high yet exclusive peroxidase (POD)-like activity via a glucose and NH 4 NO 4 -co-directed avenue, and further showed its application in constructing a portable and disposable paper-based analytical chip (PA-chip) for rapid, visual and onsite analysis of PPi. PPi etched V 2 O 5 to prevent the decomposition of H 2 O 2 into ·OH, resulting in weakened POD-like activity. In comparison with PPi deficiency, colorless TMB couldn't be oxidized into oxidized TMB with a dropped absorption at 652 nm. Therefore, obviously shallowed blue color on PA-chip surface was recorded, and demonstrated a negative relationship with PPi dosage, enabling rapid and visual detection of PPi with a limit of detection of 2.6 nM. This study demonstrated the burgeoning applications of nanozymes with POD-like activity in construction of PA-chips for PPi and will quicken the advancement of practical sensors, guaranteeing environmental safety., Competing Interests: Declaration of competing interest The authors declare no competing financial interest., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Given the significant and potential fatal implications of glyphosate (GLY) residues on human health and the integrity of ecosystems, their presence has garnered substantial global concern and scrutiny. Herein, we introduced a pioneering colorimetric sensing platform, the first of its kind, based on ternary metal-organic frameworks (ZnCo-ZIFs@MIL-101(Fe)). This innovative platform enabled ultra-sensitive, affordable, portable and rapid on-site detection of GLY. This platform achieved a wider linear range for GLY of 0.02-40 μg/mL with an exhibiting remarkable detection limit of 1 ng/mL, which was attributed to the electronic hybridization of the Fe 3+ , Co 2+ , and Zn 2+ metal centers of ZnCo-ZIFs@MIL-101(Fe), significantly enhancing the composite's catalytic performance. The assay was successfully employed to detect GLY in food and herb samples. Moreover, to meet the demand of in-field detection for GLY, a smartphone detection method based on ZnCo-ZIFs@MIL-101(Fe) with visual, intelligent, and portable features was fabricated. This detection concentration range of GLY was 0-1 μg/mL, and the limit of smartphone detection was 23 ng/mL. Furthermore, this sensor seamlessly integrated with smartphones and paper-based microfluidic chips (μPADs), which constructed a portable test strips-smartphone sensing platform for facilitating real-time and on-site visual quantitative detection of GLY. The detection concentration range was 0-1 μg/mL, and the limit was calculated as low as 75 ng/mL. The assay was highly adaptable in practical applications. In summary, our study paved a novel pathway for the design and utilization of multi-metal MOF nanozymes in on-site pesticide monitoring., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Mercury ions (Hg 2+ ) are highly toxic heavy metals that are commonly found in natural environments. Owning to their non-biodegradability and accumulation in the food chain, the precise detection of trace amounts of Hg 2+ is essential for preventing chronic accumulation and ensuring food safety. In this study, we present a dual-mode paper sensor for simultaneous colorimetric and Surface-Enhanced Raman Spectroscopy (SERS) detection of Hg 2+ in tea, achieving ultrasensitive, rapid, and on-site screening. 4-Mercaptopyridine (4-MPY) was effectively chemisorbed onto the gold nanoparticles (AuNPs), acting as a signal probe for colorimetric methods. Moreover, it can produce plasmonic hot spots for SERS by interacting with the pyridine ring. To enhance the signal intensity of both colorimetry and SERS, a silver shell is in-situ grown on the surface of AuNPs captured on the paper sensor by reduction of Ag + , achieving signal amplification. The visual limit of detection (LOD) for the colorimetric biosensor is 2.5 pM, while the LOD of SERS is 0.48 pM with this dual-mode paper sensor. The sensitivity of both the colorimetric method and SERS was improved by approximately 200 and 500 times, respectively, with the designed signal amplification strategy. The system allows for multiple parallel screening of the same sample, ensuring accurate results without any false-positive or false-negative. This study provides a valuable platform for the accurate detection of various other heavy metal ions and provides effective strategies for improving the performance of colorimetric methods., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The improvement of food safety and the reduction of food loss and waste require the development of new bioanalytical tools that provide chemical information about the composition of food that is of great value for improving traceability and extending the shelf life of food. Herein, a Cu-based metal-organic framework has been synthesized and immobilized onto cellulose paper disks for colorimetric and fluorescent detection and quantification of biogenic amines in food. The color of the nano metal-organic framework changes from green to brown in the presence of low amounts of biogenic amine vapors. Also, the fluorescence emission of the nano metal-organic framework greatly decreases after exposing the cellulose disks to amine vapors. The developed sensing paper disk exhibits a quick response to the presence of volatile biogenic amines, very low detection limits, and great selectivity. Also, the paper sensor was used for real-time monitoring of biogenic amines in bass samples at different temperature conditions, being a highly valuable method for evaluating food freshness and safeguarding food safety., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

A wax-patterned paper analytical device (µPAD) has been developed for point-of-care colourimetric testing of serum glutamic oxaloacetic transaminase (SGOT). The detection method was based on the transamination reaction of aspartate with α-ketoglutarate, leading to the formation of oxaloacetate which reacts with the reagent Fast Blue BB salt and forms a cavern pink colour. The intensity of the cavern pink colour grows as the concentration of SGOT increases. UV-visible spectroscopy was utilized to optimize reaction conditions, and the optimized reagents were dropped onto the wax-patterned paper. The coloured PADs, after the addition of SGOT, have been photographed, and a colour band has been generated to correlate the SGOT concentration visually. The images were used to calculate the intensity values using ImageJ software, which inturn was used to calculate the SGOT concentration. The PADs were also tested with serum samples, and SGOT spiked serum samples. The PAD could detect the SGOT concentration ranging from 5 to 200 U/L. The analysis yielded highly accurate results with less than 6% relative error compared to the clinical sample. This colourimetric test demonstrated exceptional selectivity in the presence of other biomolecules in the blood serum, with a detection limit of 2.77 U/L and a limit of quantification of 9.25 U/L. Additionally, a plasma separation membrane was integrated with the PAD to directly test SGOT from finger-prick blood samples., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.)

A novel microfluidic paper-based analytical device with dual colorimetric and electrochemical detection (dual μPAD) was developed for the assessment of transferrin saturation (TSAT) in samples from ischemic stroke patients. TSAT was calculated from the ratio between transferrin-bound iron, which was colorimetrically measured, and the total iron-binding capacity, which was electrochemically measured. To this end, a μPAD was smartly designed, which integrated both colorimetric and electrochemical detection reservoirs, communicating via a microchannel acting as a chemical reactor, and with preloading/storing capabilities (reagent-free device). This approach allowed the dual and simultaneous determination of both parameters, providing an improvement in the reliability of the results due to an independent signal principle and processing. The μPADs were validated by analyzing a certified reference material, showing excellent accuracy ( E r ≤ 5%) and precision (RSD ≤ 2%). Then they were applied to the analysis of diagnosed serum samples from ischemic stroke patients. The results were compared to those provided by a free-interference method (urea-PAGE). Impressively, both methods exhibited a good correlation ( r = 0.96, p < 0.05) and no significant differences were found between them (slope 1.0 ± 0.1 and the intercept 1 ± 4, p < 0.05), demonstrating the excellent accuracy of our approach during the analysis of complex samples from ischemic stroke patients, using just 90 μL of clinical samples and taking less than 90 min in comparison with the 18 hours required by the urea-PAGE approach. The developed fully integrated colorimetric-electrochemical μPAD is a promising ready to use reagent-free device for the point-of-care testing of TSAT, which can be used to assist physicians in the fast diagnosis and prognosis of ischemic strokes, where the decision-time is crucial for the patient's survival.

This work reports the development of low-cost and rapid multiplexed colorimetric assay of antioxidants (total phenolics, antioxidant capacity, flavonoids and anthocyanins) in wines at daisy-shaped fluidic paper-based analytical devices (PADs). The desired fluidic patterns were formed on paper by pen drawing and colorimetric reagents were immobilized at the 6 peripheral test zones. The sample was added at the central sample zone, migrated to the test zones and reacted with the immobilized reagents producing characteristic colors that were captured and analyzed. The paper-based approach was applied to the analysis of several wine samples and the results were statistically correlated to standard solution-based colorimetric assays, indicating that it could be reliably used for ranking wines according to their antioxidants content. In addition, the paper-based analytical methodology is simple, instrument-free, portable, cost-effective, rapid and environment friendly., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

A colorimetric sensing method based on a paper-based vapor-test kit was successfully developed for the selective and sensitive real-time monitoring of formalin in food samples. The device was specifically designed to efficiently extract and detect formalin simultaneously. A microcentrifuge tube was used as the sample solution container, with the inner cap serving as the reaction and detection zone. Formalin was converted into gaseous formaldehyde through controlled heating, which was then extracted and collected on a filter paper coated with Nash's reagent. The color change on paper was used for formalin quantification using a smartphone for detection and image analysis. Under optimal conditions, our method provided a linear range of 0.5-75 mg L -1 with a detection limit of 0.11 mg L -1 . This method effectively determined formalin in fresh food and vegetable samples, with recoveries ranging from 92 to 111%, demonstrating comparable accuracy to the standard method for practical food quality control and safety., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Food adulteration presents a significant challenge due to the evasion of legal oversight and the difficulty of identification. Addressing this issue, there is an urgent need for on-site, rapid, visually based small-scale equipment, along with large-scale screening technology, to enable prompt results without providing opportunities for dishonest traders to react. Colorimetric reactions offer advantages in terms of speed, visualization, and miniaturization. However, there is a scarcity of suitable colorimetric reactions for food adulteration detection, and interference from colored food impurities and easily comparable color results affects accuracy. To overcome limitations, this study introduces a novel approach utilizing polydopamine magnetic nanoparticles to enrich DNA in food samples, effectively eliminating interfering components. By employing gold nanoparticles to generate magnetic-gold nanoparticles, a single magnetic bead achieves simultaneous enrichment, impurity removal, and detection. The use of paper-based biosensors and visualization equipment allows for the visualization and digital analysis of results, achieving a low detection limit of 4.59 nmol mL -1 . The method also exhibits high accuracy and repeatability, with a RSD ranging from 1.6 % to 4.0 %. This innovative colorimetric method addresses the need for rapid, miniaturized, and large-scale detection, thus providing a solution for food adulteration challenges., Competing Interests: Declaration of Competing Interest The author declare that there are no conflicts of interest associated with this manuscript. This research was conducted with utmost integrity and impartiality, adhering to the highest ethical standards. We have no financial or personal relationships that could inappropriately influence the content or interpretation of this work., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Cervical cancer is among the most common malignant tumors in women. The development of rapid screening techniques plays an important role in early screening for cancer treatment. We have developed an HPV screening method, which effectively combines the high-efficiency nucleic acid enrichment of chitosan-modified filter paper and the rapid visual detectability of colorimetric LAMP, along with the enhancement of the tolerance ability of the pH-sensitive LAMP reagent to acidic original samples, making the detection of HPV 16/18 easy to carry out and reliable, which is helpful for the epidemiological prevention and control strategies of HPV-induced cancer. This technique can simultaneously exhibit the " in situ amplification" capability of chitosan-modified filter paper and the nontemperature cycle dependence of visual LAMP detection. Therefore, DNA extraction and amplification can be performed efficiently and quickly within a single reaction where all DNA is concentrated in the QF paper disc. By embedding amino-modified filter paper into the plastic chip, a simple and reliable disposable chip was prepared for rapid HPV16 and HPV18 detection from clinical endometrial samples, and the results were 100% consistent with clinical diagnosis. More importantly, even after the sample was diluted 100-fold, HPV16/18-infected cells could be accurately identified, showing the advantages of the system in early cancer screening. Moreover, for endometrial samples containing plenty of cells, the filter paper could be used to enrich cells by filtration, preventing the acidic fluid from impacting pH-induced colorimetric LAMP detection and realizing direct amplification for HPV identification without nucleic acid extraction. This easy-to-operate system that can analyze a wide range of samples will be suitable for routine on-site HPV screening, dramatically extending the applications and utility for rapid, near-patient nucleic acid testing.

Hydrogen Peroxide (H 2 O 2 ) is a highly hazardous, toxic, and carcinogenic chemical compound utilised in various industries-based applications. Despite strict restriction, they are deliberately added to food items such as milk as preservatives to increase its shelf life. Herein, we have formulated a green rapid colorimetric nanosensor for detection of H 2 O 2 in milk using cotton leaves as both reducing and functionalizing agent for synthesis of silver nanoparticles (AgNPs). UV-Vis spectra exhibit a strong plasmonic peak at around 434 nm. X-Ray Diffraction (XRD) analysis was performed to determine the crystallinity of the nanoparticles. Field Emission Scanning Electron Microscope (FESEM) and Transmission Electron Microscope (TEM) characterizations revealed spherical morphology with size approximately ∼16 nm. This functionalized nanoparticle could colorimetrically sense presence of H 2 O 2 in milk samples both in liquid media and on paper substrates with Limit of Detection (LOD) of 8.46 ppm even in presence of other interfering substances in milk. This inexpensive route will pave the way for in depth research., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The monitoring of foodborne bacterial contamination requires simple and convenient biosensors. This work describes a novel paper-based colorimetric biosensor for the rapid and sensitive bacteria detection. The biosensor was constructed via the encapsulation of D-alanyl-D-alanine capped gold nanoparticles (DADA-AuNPs) in a modified paper that was fabricated by the freeze-drying of TEMPO-oxidized cellulose nanofibers/cationic guar gum composite hydrogel-modified filter paper. The results indicated that the size of DADA-AuNPs largely determined the color of their aqueous system and they exhibited light red to dark red as their size increased from around 6 to 36 nm. All these different sized DADA-AuNPs turned into colorless when encountered with either S. aureus or E. coli. In particular, the smaller the DADA-AuNPs size, the faster the discoloration. The encapsulation of DADA-AuNPs into modified paper negligibly changed their responsiveness towards bacteria. In comparison to the original filter paper and oven-dried hydrogel-modified filter paper, the freeze-dried hydrogel-modified paper was demonstrated to be a better substrate for the encapsulation of DADA-AuNPs since they could be loaded with a larger amount of DADA-AuNPs in a faster way and showed a better perceivable color. This work demonstrated a promising paper-based colorimetric biosensor for the facile and rapid detection of bacteria., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Total antioxidants play a crucial role in human health, and detection of the total antioxidant capacity (TAC) has broad application prospects in fields such as food safety, environmental assessment, and disease diagnosis. However, a long detection time, cumbersome steps, high cost, reliance on professional equipment, and nonportability still remain significant challenges. In this work, an efficient strategy of point-of-care testing (POCT) of the TAC in body fluids by nanozyme-catalyzed colorimetric paper-based microfluidic sensors is proposed. The paper-based microfluidic sensors coupled with a smartphone can reduce testing costs and provide portability. The nanozyme prepared by the solvothermal method presents Michaelis constants of 0.11 and 0.129 mM for H 2 O 2 and TMB, respectively. A method for immobilizing nanozymes and chromogenic agents on a paper-based microfluidic chip is established. Based on smartphone photography and image grayscale extraction, the TAC can be qualitatively detected with a detection limit and linear range of 33.4 and 50-700 μM, respectively. Furthermore, the proposed sensor can realize the one-step quantitative analysis of the TAC in body fluids (blood, saliva, and sweat) within 15 min. The proposed nanozyme-catalyzed colorimetric paper-based microfluidic sensors presented in this study exhibit promising application prospects in the fields of biochemical analysis and POCT.

Paper-based analytical devices (PADs) are very convenient for determining biomarkers in point-of-care (POC) diagnosis while requiring sample pre-treatment or impurity separation. This study reports a novel hydrogel-coupled, paper-based analytical device (PAD) for separation-free H 2 O 2 colorimetric detection in both aqueous solution and cell lysis with sample-to-answer analysis by directly loading into the sample test zone. By encapsulating an inorganic mimic enzyme and chromogenic substrate into the sodium alginate (SA) hydrogel, amplification of the color signal after catalyzing the substrate could be achieved. Taking advantage of the nanoscale porous structure of the hydrogel and the lateral flow channel of the PAD, large interference fragments or bio-macromolecules are prevented from diffusing into the chromogenic reaction, whereas the small target molecules enter the sensing region to trigger the catalytic reaction. This method demonstrated a rapid and accurate analysis with a limit of detection as low as 0.06 mM and detection selectivity. Our proposed device requires no enzyme and is separation-free, portable, easy-to-fabricate, and low-cost, and may offer a platform for quantitative or qualitative analysis of other analytes in body fluids for POC applications.

New strategies for the simultaneous and portable detection of multiple enzyme activities are highly desirable for clinical diagnosis and home care. However, the methods developed thus far generally suffer from high costs, cumbersome procedures, and heavy reliance on large-scale instruments. To satisfy the actual requirements of rapid, accurate, and on-site detection of multiple enzyme activities, we report herein a smartphone-assisted programmable microfluidic paper-based analytical device (μPAD) that utilizes colorimetric and photothermal signals for simultaneous, accurate, and visual quantitative detection of alkaline phosphatase (ALP) and butyrylcholinesterase (BChE). Specifically, the operation of this μPAD sensing platform is based on two sequential steps. Cobalt-doped mesoporous cerium oxide (Co-m-CeO 2 ) with remarkable peroxidase-like activities under neutral conditions first catalytically decomposes H 2 O 2 for effectively converting colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB). The subsequent addition of ALP or BChE to their respective substrates produces a reducing substance that can somewhat inhibit the oxTMB transformation for compromised colorimetric and photothermal signals of oxTMB. Notably, these two-step bioenzyme-nanozyme cascade reactions strongly support the straightforward and excellent processability of this platform, which exhibit lower detection limits for ALP and BChE with a detection limit for BChE an order of magnitude lower than those of the other reported paper-based detection methods. The practicability and efficiency of this platform are further demonstrated through the analysis of clinical serum samples. This innovative platform exhibits great potential as a facile yet robust approach for simultaneous, accurate, and on-site visual detection of multiple enzyme activities in authentic samples.

Hyperuricemia (HUA) has gradually become a public health burden as an independent risk factor for a variety of chronic diseases. Herein, a user-friendly point-of-care (POC) detection system (namely "Smart-HUA-Monitor") based on smartphone-assisted paper-based microfluidic is proposed for colorimetric quantification of HUA urinary markers, including uric acid (UA), creatinine (CR) and pH. The detection limits of UA and CR were 0.0178 and 0.5983 mM, respectively, and the sensitivity of pH were 0.1. The method was successfully validated in artificial urine samples and 100 clinical samples. Bland-Altman plots showed a high consistency between μPAD and the testing instruments (HITACHI 7600 Automatic Analyzer, URIT-500B Urine Analyzer and AU5800B automatic biochemical analyzer) in hospital. Smart-HUA-Monitor provides an accurate quantitative, rapid, low-cost and reliable tool for the monitoring and early diagnosis of HUA urine indicators., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

We reported a colorimetric paper-based device by integrating the modified acid RNA-cleaving DNAzymes (MaRCD-EC1) for highly sensitive (detection limit = 10 2 CFU mL -1 ), and rapid (within 30 min) detection of E. coli without amplification. This device exhibited a clinical sensitivity of 100% and a specificity of 100% in identifying E. coli -associated urinary tract infections (UTIs) using the clinical urine samples.

Background: Alpha-fetoprotein (AFP) is a fetal protein that can indicate congenital anomalies such as Down syndrome and spinal canal blockage when detected at abnormal levels in pregnant women. Current AFP detection methods rely on invasive blood or serum samples, which require sophisticated equipment. From the many solutions proposed, colorimetric paper-based assays excel in point-of-care settings. The concept of paper-based ELISA (p-ELISA) enhances traditional methods, aligning with the ASSURED criteria for diagnostics in resource-limited regions. Despite success in microfluidic paper-based assay devices, laser printing remains underexplored for p-ELISA. Additionally, modifying the paper surface provides an additional layer of sensitivity enhancement., Results: In this study, we developed a novel laser-printed paper-based ELISA (LP-pELISA) for rapid, sensitive, and noninvasive detection of AFP in saliva samples. The LP-pELISA platform was fabricated by printing hydrophobic barriers on filter paper using a laser printer, followed by depositing hydroxyapatite (HAp) as an immobilization material for the antibodies. The colorimetric detection was achieved using AuNPs functionalized with anti-AFP antibodies and silver nitrate enhancement. The LP-pELISA exhibited a linear response for AFP detection in both buffer and saliva samples over a range of 1.0-800 ng mL -1 , with a limit of detection (LOD) reaching 1.0 ng mL -1 . The assay also demonstrated good selectivity, repeatability, reproducibility, and stability. The LP-pELISA was further validated by testing spiked human saliva samples, showing its potential for point-of-care diagnosis of congenital disabilities., Significance: The LP-pELISA is a noninvasive platform showcasing simplicity, cost-effectiveness, and user-friendliness, utilizing laser printing, hydroxyapatite modification, and saliva samples to efficiently detect AFP. Beyond its application for AFP, this method's versatility extends to other biomarkers, positioning it as a catalyst for the evolution of paper-based biosensors. The LP-pELISA holds promise as a transformative tool for point-of-care diagnostics, fostering advancements in healthcare with its innovative technology., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The highly contagious nature and 100% fatality rate contribute to the ongoing and expanding impact of the African swine fever virus (ASFV), causing significant economic losses worldwide. Herein, we developed a cascaded colorimetric detection using the combination of a CRISPR/Cas14a system, G-quadruplex DNAzyme, and microfluidic paper-based analytical device. This CRISPR/Cas14a-G4 biosensor could detect ASFV as low as 5 copies/μL and differentiate the wild-type and mutated ASFV DNA with 2-nt difference. Moreover, this approach was employed to detect ASFV in porcine plasma. A broad linear detection range was observed, and the limit of detection in spiked porcine plasma was calculated to be as low as 42-85 copies/μL. Our results indicate that the developed paper platform exhibits the advantages of high sensitivity, excellent specificity, and low cost, making it promising for clinical applications in the field of DNA disease detection and suitable for popularization in low-resourced areas.

We present a colorimetric probe based on polyvinylpyrrolidone-capped gold nanoparticles (PVP-AuNPs) that is sensitive and selective for cysteine (Cys). A microfluidic paper-based analytical device (μ-PAD) with embedded dried PVP-AuNPs at the polyethersulfone (PES) paper surface is used for Cys detection. When thiol molecules attach to PVP-AuNPs in the presence of Cys, they clump together, and this causes the solution's color to shift from red to blue within 5 minutes. The device is capable of detecting Cys levels between 1.0 μM and 50.0 μM with a limit of detection (LOD) of 0.2 μM under optimized conditions. The stability of the μ-PAD was tested for 100 days, demonstrating re-dispersibility to detect Cys levels in blood. Dried PVP-AuNP-μPADs were integrated with blood plasma separation modules for point-of-care (POC) Cys detection. Consequently, the device shows potential as a self-sustaining, quantification platform with a recovery percentage ranging from 98.44 to 111.9 in clinical samples.

Plastics are ubiquitous in today's lifestyle, and their indiscriminate use has led to the accumulation of plastic waste in landfills and oceans. The waste accumulates and breaks into micro-particles that enter the food chain, causing severe threats to human health, wildlife, and the ecosystem. Environment-friendly and bio-based degradable materials offer a sustainable alternative to the vastly used synthetic materials. Here, a polylactic acid and carbon nanofiber-based membrane and a paper-based colorimetric sensor have been developed. The membrane had a surface area of 3.02 m 2 g -1 and a pore size of 18.77 nm. The pores were evenly distributed with a pore volume of 0.0137 cm 3 g -1 . The membrane was evaluated in accordance with OECD guidelines and was found to be safe for tested aquatic and terrestrial models. The activated PLA-CNF membrane was further used as a bio-based electrode for the electrochemical detection of nitrates (NO 3 - ) in water samples with a detection limit of 0.046 ppm and sensitivity of 1.69 × 10 -4 A ppm -1 , whereas the developed paper-based colorimetric sensor had a detection limit of 156 ppm for NO -2 , whereas the developed paper-based colorimetric sensor had a detection limit of 156 ppm for NO 3 - . This study presents an environment-friendly, low-carbon footprint disposable material for sensing applications as a sustainable alternative to plastics.

A paper-based colourimetric assay for the Point-of-Care Testing (PoCT) of bilirubin has been developed based on the formation of a green-coloured copper-bilirubin complex from a blue-coloured tetraamminecopper(II) sulphate complex. The reaction was studied and optimized by UV-Visible absorption spectroscopy and translated onto a paper strip. Hydrophobic circular well patterns on Whatman chromatography paper were created by wax printing. The tetraamminecopper(II) sulphate complex was drop cast and dried on the reagent zones in the wax-patterned paper. The images of reagent zones captured using a scanner were analyzed using ImageJ software. Bilirubin spiked blood serum was tested in the concentration range of 1.2 to 950 µM. The PAD exhibited sensitivities of 0.4197 a.u/μM and 0.1040 a.u/μM for concentration ranges of bilirubin 1.2 to 96 μM and 105 to 950 μM respectively and a low detection limit of 0.799 µM. The method is highly selective to bilirubin, even in the presence of other biomarkers in serum. A plasma separation membrane incorporated PAD was fabricated for the final testing and quantification of bilirubin from whole blood., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

A novel, rapid, and facile method for the colorimetric determination of calcium using micro-analytical paper-based devices (μ-PADs) was developed. The proposed analytical method utilizes the color differences developing, after the addition of calcium, on the surface of the devices because of the complexation reaction of calcium with Methylthymol Blue (MTB) at room temperature, in alkaline pH. The devices were manufactured with chromatographic paper, using wax barriers, and the analytical protocol was easily implemented without the need of any experimental apparatus except for a simple imaging device. The user must regulate the pH, add the solutions on the paper, and measure the color intensity of the formed Ca(II)-MTB complex with a flatbed scanner. The experimental conditions for optimum color development, the possible interfering substances, and the reliability of the paper devices in different preserving conditions were optimized, with satisfactory results. The method exhibited acceptable detection limits (2.9 mg L -1 ) with sufficiently good precision, which varied from 4.2% (intra-day) to 6.4% (inter-day). Saliva samples from healthy volunteers were successfully analyzed, and the calcium levels were calculated in the range of 30.71 to 84.15 mg L -1 .

This article reports a simple and inexpensive leak-proof paper pad with an initial selection of a paper substrate on the grounds of surface morphology and fluid absorption time. Herein, a drying method is used for glucose detection on a paper pad through colorimetric analysis, and the spot detection of glucose is analyzed by optimizing the HRP concentration and volume to obtain accurate results. The rapid colorimetric method for the detection of glucose on the paper pad was developed with a limit of detection (LOD) of 2.92 mmol L -1 . Furthermore, the effects of the detection conditions were investigated and discussed comprehensively with the help of chemometric methods. Paper pads were developed for glucose detection with a range of 0.5-20 mM (apropos to the normal glucose level in the human body) and 0.1-0.5 M (to test the excessive intake of glucose). The developed concept has huge potential in the healthcare sector, and its extension could be envisioned to develop the reported paper pad as a point-of-care testing device for the initial screening of a variety of diseases.

In this paper, simultaneous enrichment and separation of ions and amphoteric components were successfully demonstrated by using electric field (E) and pH gradient (double gradient) in the ion depletion zone of anion concentration polarization interface established on a paper fluid channel. Experimental results were visualized with colored ions (bright blue and amaranth) and protein probes (phycocyanin and cytochrome C). With optimization, colored phycocyanin and bovine hemoglobin with similar pI as that of albumin and immunoglobulin respectively were well separated in 900 s with 10-fold enrichment effect. Based on the separation and enrichment function of this paper-based analytical device (PAD) and rapid selective staining of human serum albumin (HSA) with bromophenol blue, a rapid colorimetric detection of HSA was implemented with smartphone camera. A limit of detection (LOD) of 5.2 mg·L - 1 was achieved in the clinically significant range of 10-300 mg·L -1  (R 2  = 0.99). This method was applied to real human urine samples with good agreement (ɑ = 0.01) to clinical detection method (immunoturbidimetry). With the separation and enrichment functions of PAD, both the specificity and sensitivity were enhanced, which provides a solid basis for point-of-care test of microalbuminuria. Therefore, this PAD device is potential for sample pretreatment and detection of target components from complex physiological samples., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Microfluidic paper-based analytical devices (µPADs) represent one of the promising green analytical strategies for low-cost and simple determination of various analytes. The actual task is the development of such devices for quantitation of antioxidants, e.g., flavonoids. In this paper, possibilities of a novel three-reagent µPAD including silver nitrate, 4-nitrophenyldiazonium tetrafluoroborate, and iron(III) chloride as reagents are assessed with respect to the determination of dihydroquercetin. It is shown that all the three reagents produce different colorimetric responses that can be detected by a mini-spectrophotometer-monitor calibrator or by a smartphone. The method is applicable to direct measuring high contents of dihydroquercetin (the linearity range is 0.026-1 mg mL -1 , and the limit of detection is 7.7 µg mL -1 ), which is favorable for many dietary supplements. The analysis of a food supplement was possible with the relative standard deviations of 9-26%, which is satisfactory for quantitative and semiquantitative determinations. It was found that plotting a calibration graph in 3D space of the three reagents' responses allows us to distinguish dihydroquercetin from its close structural analogue, quercetin.

Early and on-site detection of environmental contaminations and physicochemical parameters of seawater is increasingly preferred to guarantee hazard minimization in many settings. In this paper, we describe a combination of microfluidic paper-based sensors (µPADs) and an Android-based smartphone application (App) for simultaneous on-site quantification of phosphate (PO 4 -P), silicate (SiO 3 -Si) and nitrite (NO 2 -N) in coastal seawater samples. The developed App can on-site capture, process, and quantify the µPAD colorimetric outputs. This App uses an image processing algorithm for quantifying color intensity and relating the RGB components to the analyte concentrations. The GPS-tagged data can be stored on the smartphone or sent via social networks. The significant factors affecting the detection of the analytes were optimized using Box-Behnken design. Under optimized parameters, the proposed method presented the linear ranges between 5 and 100 µg L -1 for phosphate (R 2  = 0.9909), 5 to 100 µg L -1 (R 2  = 0.9819) for nitrite and 10 to 600 µg L -1 (R 2  = 0.9933) for silicate. The LODs of the method for detection of phosphate, nitrite and silicate were 1.52 µg L -1 , 0.61 µg L -1 and 3.74 µg L -1 , respectively. The device was successfully used to simultaneous analyze and map the PO 4 -P, SiO 3 -Si and NO 2 -N of Bushehr coastal seawater samples (Iran). The results were confirmed by the lab-based conventional colorimetric methods using spectrophotometer., (© 2022. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

The existence of pesticide residues in the hydrosphere, biosphere, and anthroposphere can cause acute or chronic diseases and deteriorate the environment. Therefore, efficient detection of pesticide residues is of great significance to prevent food poisoning, control food pollution, and protect human lives by recognizing their distribution and concentration. Herein, a novel smartphone-coupled three-layered paper-based microfluidic chip is proposed as a facile platform to detect the pesticides. The stereoscopic capillary-driven fluid transport is enabled by the three-layered microfluidic chip configuration. The detection mechanism is based on the enzyme inhibition reaction and the chromatic reaction. The detection results are obtained by a smartphone and figured out by colorimetric quantitative analysis. Taking advantages of the above merits, we demonstrate the utilization of this smartphone-coupled three-layered paper-based microfluidic chip for the effective analysis of typical pesticides (profenofo and methomyl). The linear ranges of profenofo and methomyl are 0.27-2.1 μmol L -1 and 0.14-1.85 μmol L -1 , respectively. The corresponding limits of detection in the chips are 55 nM and 34 nM, respectively. The paper-based chips are also highly cost-effective with a total cost of 0.082 ¥ per piece. It can be anticipated that this technique will open new avenues for the mass fabrication of paper-based microfluidic chips and provide state-of-the-art methods in the field of analytical chemistry., (© 2021. Springer-Verlag GmbH Germany, part of Springer Nature.)

MicroRNAs (miRNAs), found in blood and body fluids, have emerged as potential non-invasive biomarkers for disease and injury. miRNAs are quantitatively evaluated using typical RNA analysis methods such as the quantitative reverse transcription polymerase chain reaction, microarrays, and Northern blot, all of which require complex procedures and expensive reagents. To utilize miRNAs as practical biomarkers, it will be helpful to develop simple and user-friendly sensors. In this study, a paper-based miRNA sensor was developed by combining two methods: (1) target-recycled DNAzyme (Dz) amplification and (2) graphene oxide-assisted Dz blotting on paper. The Dz spots on paper caused a miRNA-dependent color change in presence of colorimetric reagents and facilitated the quantification of absolute amount of the target miRNA, irrespective of the volume, with high reproducibility. This approach is technologically straightforward and enables quantification of as low as 7.75 fmol miRNA using a portable smartphone., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.)

A paper-based colourimetric assay for the detection of alanine transaminase has been developed. In the presence of alanine transaminase, 2,4-dinitrophenyl hydrazine changes to pyruvate hydrazone leading to a colour change from pale yellow to dark yellow. Reaction conditions were optimized using absorption spectroscopic studies. Hydrophobic patterns on the Whatman chromatographic paper were created by wax printing, and the reagents were drop cast at the reagent zone. On the paper device, the intensity of the yellow colour increases with ALT concentration in the range of 20-140 U/L in human serum. For the quantification of ALT, coloured images were captured using a digital camera and were processed with Image J software. The machine learning approach was also explored for the ALT analysis by training with colour images of the paper device and testing using a cross-validation procedure. The results obtained with real clinical samples on the paper device showed good accuracy of less than 5% relative error with the clinical lab results. Furthermore, the paper device shows high selectivity to ALT in the presence of various interfering species in blood serum with a sensitivity of 0.261 a.u/(U/L), a detection limit of 4.12 U/L, and precise results with an RSD of less than 7%. For the testing of whole blood, a plasma separation membrane was integrated with the patterned paper., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

In this study, we describe a monolithic and fully integrated paper-based device for the simultaneous detection of three prognostic biomarkers in saliva. The pattern of the proposed multiplexed device is designed with a central sample deposition zone and three identical arms, each containing a pre-treatment and test zone. Its one-step fabrication is realized by CO 2 laser cutting, providing remarkable parallelization and rapidity (ca. 5 s/device). The colorimetric detection is based on the sensitive and selective target-induced reshaping of plasmonic multibranched gold nanoparticles, which exhibit a clear spectral shift (and blue-to-pink color change) in case of non-physiological concentrations of the three salivary biomarkers. A rapid and multiplexed naked-eye or smartphone-based readout of the colorimetric response is achieved within 10 min. A prototype kit for POCT testing is also reported, providing robustness and easy handling of the device.

To increase milk production, antibiotics are administered to animals to provide weight gain and to prevent or treat diseases. The inappropriate use of these substances can lead to antibiotic resistance and allergic reactions and toxic effects to milk consumers. We describe the development of a simple, fast, portable, and low-cost microfluidic paper-based analytical device (μPAD) to quantify sulfonamides in milk using the inhibition of the colorimetric reaction between carbonic anhydrase (CA) and 4-nitrophenyl acetate. The main advantages presented by the μPAD include reproducible batch production, simple application, and precise analysis without previous treatment. The µPAD displayed good linearity (R 2  ≥ 0.986) in a wide range of sulfonamides in milk (2.5 to 40.0 µmol L -1 ), being selective for the drugs even in a highly complex matrix. We expect that this device allows in situ monitoring of milk quality, reducing the prejudicial conditions associated with high concentrations of sulfonamides in milk., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Versatile sensing matrixes are essential for the development of enzyme-immobilized optical biosensors. A novel three-dimensional titanium dioxide nanotubes/alginate hydrogel scaffold is proposed for the detection of sweat biomarkers, lactate, and glucose in artificial sweat. Hydrothermally synthesized titanium dioxide nanotubes were introduced to the alginate polymeric matrix, followed by cross-linking nanocomposite with dicationic calcium ions to fabricate the scaffold platform. Rapid colorimetric detection (blue color optical signal) was carried out for both lactate and glucose biomarkers in artificial sweat at 4 and 6 min, respectively. The superhydrophilicity and the capillarity of the synthesized titanium dioxide nanotubes, when incorporated into the alginate matrix, facilitate the rapid transfer of the artificial sweat components throughout the sensor scaffold, decreasing the detection times. Moreover, the scaffold was integrated on a cellulose paper to demonstrate the adaptability of the material to other matrixes, obtaining fast and homogeneous colorimetric detection of lactate and glucose in the paper substrate when image analysis was performed. The properties of this new composite provide new avenues in the development of paper-based sensor devices. The biocompatibility, the efficient immobilization of biological enzymes/colorimetric assays, and the quick optical signal readout behavior of the titanium dioxide nanotubes/alginate hydrogel scaffolds provide a prospective opportunity for integration into wearable devices.

Enzyme-linked immunosorbent assay (ELISA) is widely used for the detection of disease biomarkers. However, it utilizes time-consuming procedures and expensive instruments, making it infeasible for point-of-care (POC) analysis especially in resource-limited settings. In this work, a multicolorimetric ELISA biosensor integrated on a paper/polymer hybrid microfluidic device was developed for rapid visual detection of disease biomarkers at point of care, without using costly equipment. This multicolormetric ELISA platform was built on multiple distinct color variants resulted from the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and the etching of gold nanorods (AuNRs). The vivid color changes could be easily distinguished by the naked eye, and their red mean values allowed quantitative biomarker detection, without using any sophisticated instruments. When this multicolorimetric ELISA was integrated on a paper/polymer hybrid analytical device, it not only provided integrated processing and high portability but also enabled fast assays in about 50 min due to the unique advantages of paper/polymer hybrid devices. The limit of detection of 9.1 ng/μL of the hepatitis C virus core antigen, a biomarker for hepatitis C, was achieved using this multicolorimetric ELISA platform. This multicolor ELISA analytical device provides a new versatile, user-friendly, affordable, and portable immunosensing platform with high potential for on-site detections of various viruses, proteins, and biomarkers for low-resource settings such as at home, public venues, rural areas, and developing nations.

In recent years, microfluidic paper analytical devices (μPADs) have been extensively utilized to conduct multiplex colorimetric assays. Despite their simple and user-friendly operation, the need for patterning paper with wax or other physical barriers to create flow channels makes large-scale manufacturing cumbersome. Moreover, convection of rehydrated reagents in the test zones leads to nonuniform colorimetric signals, which makes quantification difficult. To overcome these challenges, we present a device called a barrier-free μPAD (BF-μPAD) that consists of a stack of two paper membranes having different wicking rates-the top layer acting as a fluid distributing layer and the bottom layer containing reagents for colorimetric detection. Multiple analytes can be detected using this assembly without the need to pattern either layer with wax or other barriers. In one embodiment, a device is capable of delivering the sample fluid to 20 distinct dried reagent spots stored on an 8 cm × 2 cm membrane in as few as 30 s. The multiplexing feature of the BF-μPAD is demonstrated for colorimetric detection of salivary thiocyanate, protein, glucose, and nitrite. Most importantly, the device improves the limit of detection of colorimetric assays performed on conventional μPADs by more than 3.5×. To understand fluid imbibition in the paper assembly, the device geometry is modeled in COMSOL Multiphysics using the Richards equation; the results obtained provide insights into the nonintuitive flow pattern producing perfectly uniform signals in the barrier-free assembly.

In this study, a microfluidic paper-based analytical device (μPAD) was integrated with a smartphone app capable of offline (without internet access) image processing and analysis for the rapid colorimetric detection of glucose. A self-inking stamp was used to form hydrophobic channels on a piece of paper-towel due to its superior water absorption efficiency. As demonstrated, the developed sensor was employed for the colorimetric detection of glucose in artificial saliva in the linear scope of 0 - 1 mM with a calculated detection limit of 29.65 μM. The experimental results show that the quantitative analysis of glucose with the proposed smartphone platform could be completed in less than one minute. The app developed for the smartphone platform is capable of extracting the color-changing area with an embedded image processing tool which could address the problem of color uniformity in the detection zones of μPAD. The integrated platform has great potential to be used for non-invasive measurements of glucose in body fluids, like tears, sweat and saliva.