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Microfluidic paper-based analytical devices are emerging as promising options for on-the-spot detection of chemical contaminants in water. The coupling of these devices with digital imaging technology has attracted immense interest in developing portable sensor applications. However, the potential of digital techniques still remains to be fully explored. In this work, we integrate digital imaging with microfluidic paper-based analytical devices to develop a portable colorimetric assay for iron. The experimental conditions are optimised using image analysis and the effects of imaging equipment and colorimetric analysis methods on the readout of devices are studied using numerical, graphical and statistical techniques. The experimental results are obtained for the estimation of iron contamination in water samples using a colorimetric assay based on iron (III)-thiocyanate reaction. The digital image acquisition approach using smartphone as imaging equipment is adopted for on-the-spot colorimetric data collection. Image processing techniques are employed to estimate the iron concentration in water from digital images. Greyscale colour intensity obtained from a digital image is identified as a readout of the analytical device and employed for quantitative assessment. The limit of detection for iron is estimated to be 0.26 ppm. The microfluidic approach offers good recovery and repeatability with relative standard deviation of 1.4%. The maximum percentage difference for intensities between the image analysis methods is 4.94% and that between the imaging equipment is 9.96%. The gradients of regression lines for pairwise equipment and method outputs with zero intercept range from 0.9931 to 1.0162 with high values of coefficients of determination. The correlation coefficient values for various paired comparisons (>0.99) suggest strong agreement. Statistical test results suggest no significant differences between methods and between imaging equipment (significance level 0.05). The findings suggest the efficacy of digital imaging and colorimetric analysis for paper-based analytical devices. [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]

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. [Display omitted] • Daisy-shaped paper-based analytical devices (PADs) were fabricated by pen drawing. • The PADs were used for a sextuple antioxidant/phenolics/anthocyanins/flavonoids assay. • Reagents were added to the 6 peripheral test zones and sample was added to the central sample zone. • The results of the assay for 18 wines correlated well with those of solution-based assays (R2 > 0.97). • The paper-based analytical methodology is simple, instrument-free, portable, cost-effective, rapid and environment friendly. [ABSTRACT FROM AUTHOR]

[Display omitted] • This review advances in the development of colorimetric and fluorometric paper-based devices (PADs) • PADs with metal nanoparticles and quantum dots employed as sensing probes in the point-of-care diagnosis. • PADs are integrated with desktop scanners and smartphones measure the grey scale/RGB values for quantitative measurement. • This innovative paper-based detection platforms for diverse diagnostic scenarios and their utility in resource-limited settings. This review paper comprehensively explores recent advancements in the development of colorimetric and fluorometric paper-based analytical devices (PADs) for rapid point-of-care diagnosis of specific disease biomarkers. The paper elucidates the principles of colorimetry and fluorometry, focusing on their application in the detection of various biomolecules. Notably, metal nanoparticles (NPs) and quantum dots (QDs) emerge as versatile sensing probes in these platforms. The fabrication of nanomaterials (NMs) on paper substrates is extensively discussed, covering techniques such as dip-coating, drop-casting, wax printing, inkjet printing and screen printing. The integration of PADs with portable devices is explored, including gray scale and RGB measurements using desktop scanners, smartphones and color detector apps for enhanced colorimetric and fluorometric analysis. The application of PADs with metal NPs for the detection of a wide array of biomolecules is highlighted. The versatility of these devices is demonstrated in detecting nucleic acids, proteins, amino acids, bio thiols, urea, uric acid, antigens, antibodies, glucose, creatinine, bilirubin, vitamins and bacteria. The potential for point-of-care diagnosis in diverse healthcare settings is underscored, emphasizing the simplicity, cost-effectiveness and rapidity of these paper-based diagnostic platforms. This review provides an overview of the NMs-enabled colorimetric and fluorometric PADs, offering insights into the fabrication techniques, integration with portable devices and diverse applications in the field of biomarker detection. The amalgamation of metal nanoparticles and quantum dots with paper-based devices holds promising prospects for advancing point-of-care diagnostics of specific disease biomarkers. [ABSTRACT FROM AUTHOR]

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. [Display omitted] [ABSTRACT FROM AUTHOR]

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. [Display omitted] [ABSTRACT FROM AUTHOR]

The metal-organic frameworks (MOFs) nanozyme-mediated paper-based analytical devices (PADs) have shown great potential in portable visual determination of phenolic compounds in the environment. However, most MOF nanozymes suffer from poor dispersibility and block-like structure, which often prompts deposition and results in diminished enzymatic activity, severely hindering their environmental applications. Here, we proposed colorimetric PADs for the visual detection of dichlorophen (Dcp) based on its significant inhibitory effect on the two-dimensional (2D) MOF nanozyme activity. Specifically, we synthesized a 2D Cu TCPP (Fe) (defined as 2D-CTF) MOF nanozyme exhibiting excellent dispersibility and remarkable peroxidase-like (POD-like) activity, which could catalyze the oxidation and subsequent color change of 3,3′,5,5′-tetramethylbenzidine even under neutral conditions. Notably, the POD-like activity of 2D-CTF demonstrated a unique response to Dcp because of the occupation of Fe–N 4 active sites on the 2D-CTF. This property enables the use of 2D-CTF as a highly efficient catalyst to develop colorimetric PADs for naked-eye and portable detection of Dcp. We believe that the proposed colorimetric PADs offer an efficient method for Dcp assay and open fresh avenues for the advancement of colorimetric sensors for analyzing of phenolic toxic substances in real samples. [ABSTRACT FROM AUTHOR]

A dual colorimetric/electrochemical sensor integrated with a microfluidic paper-based analytical device (μPAD) is created for rapid and sensitive detection of cabaryl in fruit. For colorimetric sensor, quercetin modified silver nanoparticle (QCT-AgNPs) is synthesized via self-reduction, resulting in the color change from green to dark green in the presence of cabaryl, which can be readily observed via naked eyes. This colorimetric sensor provides a linear response of 5 – 50 μM quantified by Image J software. For electrochemistry, AgNPs/MXene nanocomposite electrode offers high electrocatalytic activity towards the detection of carbarly with a linear range of 0 – 60 µM and a detection limit of 0.01 µM. The limit of detection values obtained from this dual sensor are lower than a cut-off of carbaryl residue in food, which is 7 mg.kg−1 (35 µM) verifying the desired sensitivity of this platform. Ultimately, this dual sensor is successfully applied for on-site detection of carbaryl in the orange samples read out on the smartphone with satisfactory results. [Display omitted] • Dual colorimetric/electrochemicl sensor is developed on μPAD for cabaryl sensor • QCT-AgNPs is synthesized by self-reduction and applied for colorimetric detection. • AgNPs/MXene modified electrode provided high electrochemical sensitivity. • This platform is applied for on-site carbaryl detection in fruits on smartphone. [ABSTRACT FROM AUTHOR]

The purpose of this study was to evaluate a simple and low-cost colorimetric method based on Fe(III) chloride and potassium hexacyanoferrate(III) for determination of acetaminophen, ascorbic acid and phenylephrine, which can be used in small laboratories using restricted materials and resources. A paper based sensor using the reagent was introduced. Significance of different variables in determination of drugs were studied by Placket-Burman design and optimal values for concentration of hydrochloric acid, Fe(III) chloride and potassium hexacyano ferrate(III), 0.01 M, 0.004 M and 0.004 M, respectively were obtained. Without using any sophisticated instrument and only by image analysis the concentration of acetaminophen, ascorbic acid and phenylephrine was determined in pharmaceutical samples. In this way we were able to determine by analyzing the final color of the reactions. The linear range in ppm for all the analytes was obtained as 0.00–50.00 and limit of detection of 2.52, 2.48 and 2.48 were obtained for acetaminophen, ascorbic acid and phenylephrine, respectively. Percent relative standard deviation (RSD %) and percent relative error (RE %) of the proposed method for analysis of real samples of acetaminophen tablet were 4.77 and 7.64, ascorbic acid tablet, 4.73 and 1.41, and phenylephrine syrup, 2.44 and − 1.61, respectively. [ABSTRACT FROM AUTHOR]

Microfluidic-based biosensors have been developed for their precise automatic reaction control. However, these biosensors require external devices that are difficult to transport and use. To overcome this disadvantage, our group made an easy-to-use, cheap, and light pumpless three-dimensional photo paper–based microfluidic analytical device (3D-μPAD; weight: 1.5 g). Unlike conventional paper-based microfluidic analytical devices, the 3D-μPAD can be used to control fluid flow in a 3D manner, thus allowing sophisticated multi-step reaction control. This device can control fluid flow speed and direction accurately using only the capillary-driven flow without an external device like a pump. The flow speed is controlled by the width of the microfluidic channel and its surface property. In addition, fluid speed control and 3D-bridge structure enable the control of fluid flow direction. Using these methods, multi-step enzyme-linked immunosorbent assay (ELISA) can be done automatically in sequence by injecting solutions (sample, washing, and enzyme's substrate) at the same time in the 3D-μPAD. All the steps can be performed in 14 min, and data can be analyzed immediately. To test this device, thioredoxin-1 (Trx-1), a biomarker of breast cancer, is used as the target. In the 3D-μPAD, it can detect 0–200 ng/mL of Trx-1, and the prepared 3D-μPAD Trx-1 sensor displays excellent selectivity. Moreover, by analyzing the concentration of Trx-1 in real patients and healthy individuals' blood serum samples using the 3D-μPAD, and comparing results to ELISA, it can be confirmed that the 3D-μPAD is a good tool for cancer diagnosis. [ABSTRACT FROM AUTHOR]

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. [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]

Paper-based ion-selective optodes (ISOs) allow for low-cost ion measurements using widely accessible optical detectors such as smartphones, representing an attractive analytical platform for medical testing and environmental monitoring. Previously reported paper-based ISOs rely on plasticized PVC membranes or PVC/plasticizer-free adsorption layers containing hydrophobic sensing chemicals. In this paper, we studied a new format of paper-based ISOs, in which a plasticizer phase containing sensing chemicals is deposited onto cellulose paper without using a hydrophobic polymer like PVC. Compared to ISOs without plasticizers and polymers, the plasticizer-based ISOs show much higher pH sensitivity when using chromoionophore I and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the sensing chemicals. When the sensing layer further contains a solid ionophore with high molecular rigidity, the plasticizer-based ISOs for cations also exhibit enhanced sensitivity than the plasticizer-free ISOs. However, when the ionophore is a liquid at room temperature or has a long alkyl chain, the plasticizer-based and plasticizer-free ISOs have comparable responses because the ionophore behaves as a pseudo-plasticizer. Compared to ISOs with both plasticizers and polymers, the elimination of hydrophobic polymers like PVC makes inkjet printing more efficient and preserves the capillary action of the porous paper. By using the newly launched Samba cartridge with smaller nozzles, the plasticizer-based and polymer-free optodes for sodium, potassium, and calcium ions are prepared by inkjet printing for the first time. In an example application, the inkjet-printed plasticizer-based and polymer-free ISOs are used for colorimetric measurements of urine calcium. • Plasticizer-based and polymer-free ion-selective optodes are fabricated on cellulose paper. • The use of plasticizers enhances the optode performance compared to plasticizer-free optodes. • Inkjet printing of the plasticizer-based optode ink is realized using the newly launched Samba cartridge. • The inkjet-printed optodes are used for colorimetric calcium measurements in urine. [ABSTRACT FROM AUTHOR]

[Display omitted] • A dual-optical method for 2,4-D detection is developed. • oxTMB-mediated colorimetric and ratiometric fluorescent assay was demonstrated. • A high-throughput PAD was fabricated for on-site rapid visual detection. • Dual signals can prove each other with high accuracy. • The proposed platform had facile, portable, and visual advantages in POCT. High-throughput and accurate monitoring of pesticide residues is extremely crucial, but remains challenging. Herein, oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB) mediated dual-optical sensing-based paper analytical device (PAD) was unveiled for on-site visual detection of 2,4-dichlorophenoxyacetic acid (2,4-D) in real samples. Colorimetric blue oxTMB (652 nm) can be reduced to colorless TMB by ascorbic acid (AA). Meanwhile, oxTMB was demonstrated for the first time to switch oxidation and condensation reactions between o -phenylenediamine and AA to produce yellow-emission (564 nm) and blue-emission (429 nm) probes. Thus, colorimetric and ratiometric fluorescent dual-mode detection system was developed for 2,4-D detection by combining with alkaline phosphatase (ALP) initiating AA generation (ascorbic acid-2-phosphate as the substrate). Notably, the developed method can correct itself to improve the accuracy due to the dual-response signals. Using the principle, ALP-loaded PAD (array 3 × 10) was designed for efficient and high-performance detection of 2,4-D in turnip and cauliflower with good sensitivity (0.09 μg/mL for colorimetry, 0.06 μg/mL for fluorometry), satisfactory recovery (98.0 %–107.2 %), and high consistency (relative error ≤ 3.1 %). This work demonstrated a promising strategy for high-throughput visual detection of 2,4-D, which may provide a new pathway for the further development of dual-mode sensing platforms to efficiently detect other targets for food safety. [ABSTRACT FROM AUTHOR]

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

A new strategy for the fluorescent and colorimetric sensing of hydrogen peroxide (H 2 O 2) and glucose based on the metal oxide – carbon-dot hybrid structure was investigated. The sensing system is related to the catalytic oxidation reaction of glucose-by-glucose oxidase (GOx) to H 2 O 2. In this study, a metal oxide hybrid with nitrogen-doped carbon dots (MFNCDs) that showed intrinsic peroxidase-like activity was synthesized and used as a catalyst instead of GOx to oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to blue-emitting oxidized TMB (oxTMB) in the presence of hydrogen peroxide (H 2 O 2). The fluorescence of MFNCDs/TMB at 405 nm was quenched in the presence of H 2 O 2 through the inner filter effect (IFE) and electron transfer within MFNCDs, oxTMB, and glucose system. Therefore, the fluorescence and absorbance intensity can be applied to the quantitative determination of the concentration of H 2 O 2 and glucose with a wide linear range. The detection limit for H 2 O 2 and glucose based on the colorimetric method were as low as 84 nM and 0.41 μM, respectively. In contrast, the detection limit for H 2 O 2 and glucose based on the fluorescent method were as low as 97 nM and 0.85 μM, respectively. Furthermore, the colorimetric readout on the paper device based on the changing color of the solution could also be integrated with a smartphone platform to conduct the on-site analysis of glucose without the use of the spectrometer. In addition, this dual sensor can be applied to detect glucose in real serum with highly accurate results, making it a good candidate for biosensor applications. Image 1 • Metal oxide hybrid with N-doped carbon dots (MFNCDs) as a peroxidase-like catalyst was fabricated by a hydrothermal method. • A colorimetric and fluorescent sensor for H 2 O 2 and glucose was developed. • The MFNCDs exhibited excellent selectivity towards glucose. • MFNCDs showed low cytotoxicity and excellent biocompatibility. [ABSTRACT FROM AUTHOR]

Nowadays, great efforts are made in developing new technology for rapid detection of specific DNA sequences, for environmental monitoring, forensic analysis and rapid biomedical diagnosis applications. Microfluidic paper-based analytical devices are suitable platforms for the development of point of care devices, due to their simple fabrication protocols, ease of use and low cost. Herein, a methodology for in situ detection of single strand DNA by using a colorimetric assay based on the formation of a DNAzyme within a paper substrate was developed. A DNAzyme that could only be formed in the presence of a specific sequence of the Y human amelogenin gene was designed. The performance of the DNAzyme was followed colorimetrically first in solution and then in paper substrates. The reaction was found to be specific to the Y fragment selected as analyte. The DNAzyme reaction on paper enabled the unequivocal colorimetric identification of the Y single strand DNA fragment both qualitatively, with the naked eye (143 ng), and quantitatively by image analysis (45.7 ng). As a proof of concept, a microfluidic paper-based device, pre-loaded with all DNAzyme reagents, was characterized and implemented for the simultaneous detection of X and Y single strand DNA fragments. Image 1 • Optimized methodology for in situ DNA detection in a paper device format. • Detection based on a DNAzyme using Y human amelogenin fragment. • Presents a non-enzymatic, colorimetric and rapid way to detect DNA sequences. • Detection: quantitatively by calculating the grey intensity values. • Detection: qualitatively by the presence of a green color "coffee ring". [ABSTRACT FROM AUTHOR]

In this study, we developed a point-of-care assay platform with simultaneous detection and self-calibration capabilities for multiple targets based on a microfluidic paper-based analytical device (μPAD). This system is easily manufactured using a wax printing method on chromatographic paper. The design pattern consists of a zone of detection and a calibrant zone for controlled loading using wax barriers with different thicknesses. We showed the utility and applicability of this approach by a proof-of-concept study for two clinically important markers: glucose and lactate. With the naked eye, the results could be fully distinguished and recorded to evaluate the analytical performance with a flatbed scanner. The detection limits of glucose and lactate were 0.3125 mM and 0.2975 mM, respectively, and simultaneous detection was possible from a small sample (0.4 μL) with high sensitivity. Furthermore, this device has a self-calibration function, which minimizes the influence of environmental conditions (i.e., ambient light intensity, temperature, humidity, and pressure). Therefore, the developed multiplex paper-based device is promising for clinical multianalyte point-of-care testing since it is easy to manufacture, cost-effective, user-friendly, and highly sensitive. Image 1 • Point-of-care assay platform with simultaneous detection and self-calibration capabilities was developed. • As proof of concept, enzymatic colorimetric glucose and lactate detection were successfully performed. • The platform offers low detection limits, accuracy and high specificity for the detection of glucose and lactate in serum. • This approach can be a promising tool for disease screening in medical diagnostics. [ABSTRACT FROM AUTHOR]

Despite recent advancements in the field of microfluidic paper-based analytical devices (μPADs), a key challenge remains in developing a simple and efficient μPAD with customized imaging capabilities for antioxidant assays. In the present study, we report a facile approach for μPAD fabrication through the application of transparent nail paint leading to creation of hydrophobic barriers and well-defined channels. The resultant μPADs were then characterized through scanning electron microscopy and contact angle measurements. The resolution and functional features of the fabricated μPAD were amenable to the intended assay. The μPAD's impregnated poly(methacrylic acid) (PMAA)-coated cerium oxide (CeO2) nanoparticles oxidized the 3,3′,5,5′-tetramethylbenzidine (TMB) leading to the formation of a blue-colored charge-transfer complex. The addition of different antioxidant standard solutions resulted in a reduction in the blue color in a dose-dependent manner which could be observed visually. The color intensity of the PMAA-CeO2 nanoparticle@TMB oxidation product was inversely proportional to the antioxidant concentration and was measured using customized in-house MATLAB-based image processing software. Importantly, PMAA-CeO2 nanoparticle-based μPADs demonstrated good analytical characteristics and were able to be stored for long periods without any loss of activity. Moreover, potential interferents did not pose any threat to the colorimetric signal read-out for determination of antioxidant activity. The developed method was further applied for the assessment of antioxidant activity in a variety of tea samples and performed satisfactorily in comparison with a commonly used antioxidant detection method. Collectively, the developed μPAD-based platform holds great potential as a low-cost, convenient, portable and reliable method for pursuing various on-site antioxidant assays. [ABSTRACT FROM AUTHOR]

[Display omitted] • A series of two-armed indole appended Schiff base conjoined 1,2,3-Triazole based click derived bis-organosilane and bis-organosilatrane is synthesized. • The designed Schiff bases are developed as colorimetric, fluorescent and paper-based selective sensors for Al(III). • The binding capabilities are in sighted by using absorption and emission titrations that determine the 4 and 5 high sensitivity and ultratrace low detection limits of 1.41 × 10−9 M and 0.17 × 10−9 M. • The insights into the optical signal changes are discussed. Metal ions have significant roles in diagnosis, industry, human health, and the environment. To design and develop new lucid molecular receptors for the selective detection of metal ions is important for environmental and medical applications. In the present work, two-armed indole appended Schiff bases conjoined with 1,2,3-Triazole bis-organosilane and bis-organosilatrane skelton sensors for naked eye colorimetric and fluorescent detection sensors for Al(III) are developed. The introduction of Al(III) in sensor 4 and 5 show red shift in UV–visible spectra, changes in fluorescence spectra and immediate color change from colorless to dark yellow. Furthermore, the pH and time response studies were explored for both sensors 4 & 5. The sensors 4 and 5 exhibited significantly low detection limit (LOD) in nano-molar range 1.41 × 10−9 M and 0.17 × 10−9 M respectively from emission titration. The LOD form absorption titration was found to be 0.6 × 10−7 M for sensor 4 and 0.22 × 10−7 M for sensor 5. In addition, the sensing model is developed as paper based sensor for its practical applicability. The theoretical calculations were performed on Gaussian 03 program by relaxing the structures using Density functional theory. [ABSTRACT FROM AUTHOR]

Advancements in technology have led to the use of smartphones as biosensor detectors. However, it is difficult to obtain stable color information of colorimetric sensors using multiple smartphones owing to different light conditions and image correction operations. In this study, we attempted color detection of the colorimetric paper chip using smartphone-embedded light-emitting diode (LED) and simple transformation calculation. This method enables the acquisition of stable color information using different smartphones by reducing the influence of external light sources. Moreover, we studied characteristics such as convergence and distinction of detection result details by the manufacturers and smartphones. These findings suggested the necessity of classification criteria for the use of smartphones to achieve optimal detection. In addition, white and black references were used to set the standard for color correction of various light sources. Based on our results, we conclude that this approach presents the possibility of smartphone-based colorimetric detection in practical applications. Image 1 • We have attempted to quantify the bio-diagnostic assay by using various smartphones. • The effect of the external light environment is reduced by the smartphone LED and correction method. • The deviation between CMOS image sensors in various smartphones is reduced through contrast stretching method. [ABSTRACT FROM AUTHOR]

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. 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. 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. [Display omitted] • The study introduces laser printing to create paper-based ELISA. • Hydroxyapatite was used to immobilize antibody binding and enhance assay performance. • Colorimetric detection using silver-enhanced AuNPs for rapid and sensitive readout. • The LP-pELISA shows a high Greenness score aligning with ecofriendly practices. [ABSTRACT FROM AUTHOR]

Abstract A new promising rhodamine based colorimetric and fluorometric chemosensor, RDV has been designed and synthesized for specific detection of Hg2+ ion. It acts as highly selective "turn-on" fluorescent chemosensor for Hg2+ ion without interference from other competitive metal ions in aqueous acetonitrile medium. The drastic color change with addition of Hg2+, from colorless to pink, indicates RDV can acts as "naked-eye" indicator for Hg2+. The Hg2+ promoted selective hydrolysis of appended vinyl ether group in RDV followed by Hg2+ chelated complex formation with concomitant opening of spirolactam ring is the plausible sensing mechanism. The detailed absorption, fluorescence, 1H NMR, 13C NMR and mass spectrometry confirms the proposed sensing mechanism. The limit of detection (LOD) of Hg2+ by RDV is 136 nM indicating the high sensitivity towards Hg2+. The RDV shows consistent spectroscopic response in biological pH range 4–10. In addition to explore practical applicability of RDV , its paper strips have been made and used to detect Hg2+ in pure water solution up to 10 ppm level. Furthermore, the potential application of RDV for the sensing of Hg2+ in real water samples (tap water and drinking waters from different sources) were also monitored and demonstrated. Graphical Abstract A rhodamine appended vinyl ether probe (RDV) for selective Hg2+ recognition has been synthesized which gives "turn ON" emission response for Hg2+ ion in aqueous medium over other metal ions in a chemodosimetric approach. The LOD is 136 nM. RDV has been explored for paper strip application as well as Hg2+ analysis from real water samples. Unlabelled Image Highlights • A rhodamine appended vinyl ether probe (RDV) for selective Hg2+ recognition has been synthesized. • RDV gives "turn ON" emission response for Hg2+ ion in aqueous medium over other metal ions in a chemodosimetric approach. • The LOD value is as low as 136 nM. • RDV has been explored for paper strip application as well as Hg2+ analysis from real water samples. [ABSTRACT FROM AUTHOR]

Abstract The research progress in the centrifugal microfluidic platform provides great opportunities for simple and effective analytical measurements in a variety of areas including biomedical engineering. In this study, we propose an optical reader that can measure the transmittance in a very sensitive and rapid manner on a hybrid paper-polymer centrifugal disc platform. This device enables real-time monitoring of multiple samples by measuring the absorbance of the light transmitted through the paper integrated on the disc between the light emitting diode (LED) and the photodiode (PD) regardless of the ambient light condition. To confirm its efficiency, we analyzed one of the blood's important indicators, glucose in a successful manner within 10 s without any additional complex image analysis. In addition, we discussed the results by comparing with the reflectance-based methods and with those of the previously reported studies by introducing a figure of merit to evaluate the performance of the assay. Highlights • Optical reader was developed for hybrid paper-polymer centrifugal disc platform. • This device enables real-time detection by measuring light transmission through the paper integrated on the disc between LED and photodiode. • As proof of concept, enzymatic colorimetric glucose detection was successfully performed. • Micromolar detection was achieved using the present technique within 10 s without any additional complex image analysis. [ABSTRACT FROM AUTHOR]

Signal amplification in colorimetric biosensors usually requires labile reagents and/or additional steps, which are not ideal for in-field measurements. Here we show that the limit of detection of a gold nanoparticle-labeled colorimetric immunosensor can be decreased 10-fold by performing the assay on a folded piece of paper. In this approach, the intrinsic porosity of the substrate enables generating multiple signals with a single drop of reagent. This yields combined signals with higher optical density thanks to the semitransparent nature of the wet material. It will be shown that this method can decrease the limit of detection 10 times in a model immunosensor for the detection of immunoglobulins. The procedure is so straightforward that it could be easily implemented to decrease the limit of detection of microfluidic paper-based analytical devices with colorimetric readouts. [ABSTRACT FROM AUTHOR]

We present a paper-based microfluidic colorimetric immunosensor for the detection of carcinoembryonic antigen (CEA), using Co 2 (OH) 2 CO 3 -CeO 2 nanocomposite with extraordinary intrinsic peroxidase like activity. The morphology and composition of the nanocomposite characterized with Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The proposed immunosensor facilely fabricated by loading mixture of ionic liquid and chitosan functionalized primary antibodies (Ab 1 ) on the surface of paper. Compared to traditional paper based immunodevice, when ionic liquid was used the nonspecific binding protein from the paper surface was more effectively removed. Secondary antibodies (Ab 2 ) were stacked on the surface of the carboxylated Co 2 (OH) 2 CO 3 -CeO 2 nanocomposite. The immunosensor response was obtained by a color change resulting from Co 2 (OH) 2 CO 3 -CeO 2 nanocomposite catalyzing the oxidation of 3,3′,5,5′-tetramethyl benzidine in the presence of H 2 O 2 . The colorimetric sensing was accomplished on the paper, using smartphone for taking a photo and then analyzing the colors with an installed application. Detection of CEA was performed by this method with a linear range from 0.002 to 75.0 ng mL −1 and a detection limit of 0.51 pg mL −1 . In this paper we developed simple, cost-effective and portable design for sensitive immunoassay and point-of-care diagnostics of cancer marker. [ABSTRACT FROM AUTHOR]

Detection of bisulfite (HSO 3 −) in food and beverages has vital importance because the excessive amount leads to ill effects on the human body. Colorimetric and fluorometric chromenylium-cyanine-based chemosensor CyR was synthesized and applied for high selective and sensitive analysis of HSO 3 − in red wine, rose wine and, granulated sugar with high recovery ranges and very fast response time without any interference from other competitive species. The limits of detection (LOD) for the UV–Vis and fluorescence titrations were found as 11.5 μM and 3.77 μM, respectively. The on-site and very rapid methods based on paper strips and smartphone which depend on the color changes from yellow to green have been successfully developed to analyze HSO 3 − concentration (10−5-10−1 M for paper strip and 163–1205 μM for smartphone). CyR and the bisulfite-adduct formed in the nucleophilic addition reaction with HSO 3 − were verified by FT-IR, 1H NMR, and MALDI-TOF results as well as Single-Crystal X-Ray Crystallography for CyR. [Display omitted] • Chromenylium-cyanine based probe (CyR) was constructed for bisulfite determination. • CyR platform was decorated with glyoxal group to provide selectivity for bisulfite. • Probe is the first example of NIR Spectrophotometric probes for bisulfite analysis. • Bisulfite was quantified in red wine, rose wine and granulated sugar. • Paper strips and smartphone-based methods have been developed to analyze bisulfite. [ABSTRACT FROM AUTHOR]

In this study, we developed a low-cost, handheld optical biosensor for measuring blood urea nitrogen (BUN) levels in fingertip blood. This biosensor can connect to a cloud-enabled smartphone for remote monitoring of renal diseases. The biosensor features a simple, stackable and multilayer paper-based strip design, which not only achieves a linear optical fast response to BUN concentration but also reduces the complexity of the optical reader. Excellent detection performance was obtained via a unique photoelectric detection algorithm. The proportional function for calculating the analyte concentration was established using blood samples with different BUN concentrations, resulting in a wide detection range of 2.46–38.14 mM and a low detection limit of 0.03 mM. Clinical sample testing demonstrated the biosensor's high reliability, showing strong agreement with the bulky biochemistry analyzer (gold standard), with a correlation coefficient R2 = 0.9635. It is also capable of connecting to a cloud-enabled smartphone via Bluetooth to transmit test data, which has an easy-to-use interface, thus enabling proactive health management or remote diagnosis. We believe that the proposed BUN biosensor can be a big boost to the daily monitoring of renal diseases and has promising prospects in contributing to the development of the IoMT. • We developed a low-cost, handheld BUN optical biosensor for renal surveillance in fingertip blood. • The sensor achieves a linear optical fast response through a multi-layer test strip. • Reflection spectroscopy is utilized for colorimetric quantification with high sensitivity. • The biosensor has high reliability and can greatly promote the daily monitoring of kidney diseases. [ABSTRACT FROM AUTHOR]

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

Here, we report a highly water‐soluble colorimetric and fluorescent pH probe that works in the pH range 0.2–3.0. The probe showed a rapid response, high stability, and excellent reversibility to acidity. Moreover, the fluorescence of probe was not influenced by the existence of high concentration of cations. The pKa of probe was calculated to be 2.25 ± 0.06. Furthermore, the pH test papers coated with the probe exhibited a distinct color change in acidic conditions. [ABSTRACT FROM AUTHOR]

Herein, we have developed a simple, sensitive and paper-based colorimetric sensor for the selective detection of Chromium (Ⅵ) ions (Cr (VI)). Silanization-titanium dioxide modified filter paper (STCP) was used to trap bovine serum albumin capped gold nanoparticles (BSA-Au NPs), leading to the fabrication of BSA-Au NPs decorated membrane (BSA-Au NPs/STCP). The BSA-Au NPs/STCP operated on the principle that BSA-Au NPs anchored on the STCP were gradually etched by Cr (VI) as the leaching process of gold in the presence of hydrobromic acid (HBr) and hence induced a visible color change. Under optimum conditions, the paper-based colorimetric sensor showed clear color change after reaction with Cr (VI) as well as with favorable selectivity to a variety of possible interfering counterparts. The amount-dependent colorimetric response was linearly correlated with the Cr (VI) concentrations ranging from 0.5 µM to 50.0 µM with a detection limit down to 280 nM. Moreover, the developed cost-effective colorimetric sensor has been successfully applied to real environmental samples which demonstrated the potential for field applications. [ABSTRACT FROM AUTHOR]

The monitoring of blood glucose is a key aspect of diabetes care in limiting the negative effects of hyperglycaemia to both the microvasculature and macrovasculature. Self-monitoring of blood glucose (SMBG) gives an indication of blood glucose at a specific point in time and is recommended to be carried out four times daily. However, due to the inconvenience and associated pain of blood withdrawal, SMBG is often carried out less frequently than recommended or not at all. Extraction and subsequent determination of glucose in interstitial fluid (ISF) using microneedles (MNs) is an emerging area of research due to their minimally invasive nature and lack of associated pain. In this manuscript, a novel method for the fabrication of a hollow microneedle device is reported. The microneedle produced had a sharp bevelled edge and was 400 µm in length. Additionally, a paper backplate embedded with a colorimetric system for the rapid visual determination of glucose in simulated ISF was developed and paired with the hollow MN. This device rapidly extracted simulated ISF within five seconds and its ability to produce a glucose concentration dependent colour change within 30 s was demonstrated. Using this approach, it was possible to discriminate between glucose concentrations in normal glycaemia (4–7 mM) and hyperglycaemia (>7 mM) ranges using the naked eye. While further development is required, the results herein highlight the potential of this device to be used as a blood-free minimally invasive approach to glucose monitoring. [ABSTRACT FROM AUTHOR]

A novel naphthalimide-based colorimetric and fluorescent turn-on chemosensor for Al3+ was synthesized and characterized with spectroscopic techniques. In MeOH solution, BPAM showed high selectivity and sensitivity to Al3+ by a 60-fold fluorescence enhancement and blue-shift absorption with visible color changes attributed to the contribution of chelation enhanced fluorescence (CHEF) and inhibition of intramolecular charge transfer (ICT). A 1:1 BPAM-Al3+ complex confirmed by job’s plot and HRMS with a binding constant of 6.37 × 104 M− 1, and the detection limit for Al3+ was as low as 1.59 × 10− 7 M. BPAM was successfully applied in real sample detection and assessing the existence of Al3+ by a colorimetric method on filter paper. Furthermore, the fluorescent signals of BPAM were designed to construct an INHIBIT molecular logic gate. [ABSTRACT FROM AUTHOR]

A challenge in developing microfluidic devices for fully portable and automated field analyses is the ability to integrate and store reagents in a stable form while maintaining a low cost for the system. Paper-based field-deployable devices are currently the most extensively explored platforms for field use; however, centrifugal devices offer a means for heightened device automation and portability. The basis for many field devices is chemical sensing via colorimetric reactions. Here, we describe the development of polyester-paper hybrid microfluidic devices using a fabrication method, compounded from the print, cut and laminate (PCL) technique, that allows for simple yet effective dry reagent storage for enhanced use with centrifugal or polymeric-based devices. We utilized various proof-of-principle colorimetric assays for multiple applications, including the analysis of total protein, serum albumin, cocaine, 2,4,6-trinitrotoluene (TNT) and iron content. Localized reagent integration facilitated by paper punches allowed for both reagent stability prior to reaction for storage, color stability post-reaction for image analysis, and was compatible for multi-step analyses. Qualitative and quantitative results were achieved with the broad range of sample applications trialed here, while still maintaining the ability for downstream processing. [ABSTRACT FROM AUTHOR]

At present, most countries or regions use commercial centrifuges for centrifugation, but this is out of reaching for limited-resource areas. To overcome this problem, a portable electric yo-yo as centrifuge was firstly proposed to obtain serum, and this device can be combined with paper-based analytical devices for enzyme-linked immunosorbent assay (ELISA) analysis from human whole blood. In this study, inflammatory biomarkers C-reactive protein (CRP) and serum amyloid A (SAA) were used as target biomarker to verify the performance of the proposed method. The results shows good performance and their detection limits were determined to be 580 pg/mL for CRP and 800 pg/mL for SAA, respectively. We believe this method provides a new platform of low cost and fast detection for inflammatory biomarkers in the limited-resource settings. Using electric yo-yo device to implement inflammatory biomarkers detection in whole blood paper-based microfluidic immunoassay chip. [Display omitted] • A low-cost, portable and effective electric yo-yo centrifugal device is firstly proposed for blood centrifugation. • Paper-based microfluidic immunoassay chip used for inflammatory biomarkers analysis in whole blood. • The method shows good potential to on-site/limited-resource testing. [ABSTRACT FROM AUTHOR]

• Synthesis of novel oxazolidine derivatives with ionochromic properties. • Application of the oxazolidine molecules for photodetection of metal ions by colorimetric and fluorometric methods. • Selective sensing of Fe3+ and Ag+ in aqueous solution by development of metal ion chemosensors. • Preparation of ionochromic papers with the printing ability of security marks by using metal ion solutions as the ink. Photodetection of metal ions in different environments is a noteworthy research. Some sensitive oxazolidine derivatives were synthesized and their responsivities toward different metal ions were investigated. The OX 1 and OX 2 derivatives were synthesized from 2,3,3-trimethylindolenin and 1,1,2-trimethylbenz[e]indole, respectively. In addition, two hydroxyl and naphthalene functionalities were applied to both of the OX 1 and OX 2 derivatives to obtain OX 1 -OH and OX 2 -OH and also OX 1 -Naph and OX 2 -Naph derivatives, respectively. Investigation of ionochromic properties of oxazolidine derivatives showed their potential applications as highly-selective and sensitive optical chemosensors for detection of metal ions. The results showed colorimetric and fluorometric photodetection of Fe3+ ions in aqueous solution, where OX 2 -NaPh displayed selective photodetection of Ag+ in addition to Fe3+. For the development of paper-based colorimetric/fluorometric metal ion chemosensors and also high-security anticounterfeiting technology, ionochromic cellulosic papers based on oxazolidine derivatives were prepared via layer-by-layer coating strategy, which showed fluorescence emission in different colors with impregnation in aqueous solution of Fe3+ or Ag+. In addition, the Fe3+ and Ag+ aqueous solutions were used as anticounterfeiting ink for handwriting and printing on the ionochromic papers with pink, cyan blue, and dark blue fluorescence emissions of the written or printed marks. Therefore, the prepared ionochromic cellulosic papers have potential applications as colorimetric/fluorometric metal ion chemosensors and also in anticounterfeiting technology based on ionochromic security papers. Photodetection of metal ions by development of fluorometric/colorimetric chemosensors, and also anticounterfeiting technology based on oxazolidine derivatives. [Display omitted] [ABSTRACT FROM AUTHOR]

A novel tetrahedral DNA nanostructure (TDN) functionalized paper-based colorimetric sensor was developed for ultrasensitive analysis of mercury ion (Hg2+). On the paper-based platform, TDN was used as scaffold to anchor aptamer to improve the capture efficiency owing to the controllably specific orientation, low steric hindrance effect and well-defined spacing. In the absence of Hg2+, biotinylated aptamer would bind with streptavidin-labeled HRP. Whereas, in the presence of Hg2+, aptamer would preferentially bind to Hg2+ owing to the higher binding constant of T–Hg–T than that of T–A, which consequently leading to the biotinylated aptamer releasing from TDN. After introducing TMB-H 2 O 2 solution, strong variation of color signal could be observed. Therefore, the quantitative analysis of Hg2+ was carried out by using smartphone to monitor the color intensity on paper chip. The proposed strategy exhibits good linearity over a wide range of 10–13–10–7 M with a low detection limit of 30 fM. And this method had been successfully applied in real water samples. Our strategy provided a versatile, low-cost and user-friendly paper-based monitoring platform for rapid and ultrasensitive analysis of heavy metal ions and thus had potential applications in food safety and public health. [Display omitted] • A sensitive paper-based platform based on tetrahedral DNA nanostructure. • DNA nanostructure confers controllable anchoring aptamer in a consistently specific orientation and well-defined spacing. • Employment of high affinity of aptamer toward target mercury ion with 30 fM detection limit. • The developed platform is simplicity, rapidity, user-friendly, low cost and sensitive monitor food quality. [ABSTRACT FROM AUTHOR]

[Display omitted] • The TPA-SB sensor showed rapid absorbance, excellent selectivity, and high sensitivity towards Hg2+ ions. • The TPA-SB sensor could detect Hg2+ ions from colourless to reddish-orange via naked-eye, exhibiting fluorescence OFF response to Hg2+ ions. • The detection limit achieved for Hg2+ ions (0.27 nM). • Low-cost, easy-to-make filter paper strips are developed to detect Hg2+ ions for field application. • Solid-supported TPA-SB@SiO 2 was developed and demonstrated for onsite analysis. A new, sensitive and highly selective chemosensor (N, N-diphenyl-4-(E)-(E)-thiophen-2-ylmethylene)hydrazono)methyl) aniline (TPA-SB) for Hg2+ was designed and synthesised by combining the triphenylamine motif and thiophene group. The TPA-SB exhibited a remarkable colour change from light yellow to reddish-orange with Hg2+, which was visible to the naked eye. The detection limit of Hg2+ was 0.27 nM. We fabricated a TPA-SB-coated paper sensor to detect Hg2+ rapidly, with excellent selectivity and sensitivity. Adding KI to TPA-SB-Hg2+ recovered the colour and retained the probe TPA-SB as a reversible sensor for real-time applications in solution and paper matrix. Further, we developed a solid-supported adsorbent (TPA-SB-SiO 2) and utilised it for the complete removal (100%) of Hg2+ ions from the mercury-contaminated water. Based on the NMR titration, ESI – MS, Job's plot and DFT study, probable binding mode and biding ratio (1:1) of TPA-SB with Hg2+ have been proposed. Importantly, we can apply the present sensing system for recognising Hg2+ ions in Escherichia coli cells. [ABSTRACT FROM AUTHOR]

Fe3+ ion plays a very active role in life, agriculture, and industry. Human health and the environment are seriously affected by the abnormal presence or excess of this cation. Therefore, the development of a fast, reliable, sensitive, and simple fluorescent probe to detect this cation is crucial. In the present paper, polycaprolactone-capped zinc oxide quantum dots were prepared for the determination of Fe3+ ions. The proposed fluorescent chemosensor exhibited a fluorometric and strong quenching effect toward Fe3+ ions at two wavelengths (303 and 602 nm). The limit of detection (LOD) was calculated as 0.410, and 0.333µM at the mentioned wavelengths. Also, the binding stoichiometric ratio was calculated as 1:1 by Job's plot. The findings indicated that the PCL@ZnO colorimetric chemosensor could be successfully applied with reliable, and good accuracy for the detection of Fe3+ ions in real seawater samples. [ABSTRACT FROM AUTHOR]

• A new spiropyran probe was designed and synthesized by modifying a bis(pyridin-2-ylmethyl)amino group as the acceptor at 5-position. • The probe displays a remarkable property of selective naked-eye recognizing copper ions in aqueous media and on test paper strips. • The linking of Cu2+ to the probe caused a dramatic change from colorless to pink. • Job's plot analysis displayed the coordination ratio between the probe and copper ions is 2:1. Using a strategy of modifying a bis(pyridin-2-ylmethyl) amino group in the 5-position, a new spiropyran dye ((1′-ethyl-3′,3′-dimethyl-N, N-bis(pyridin-2-ylmethyl) was synthesized. The dye was able to distinguish copper ions with remarkable selectivity by naked eye on paper strips and in aqueous media. Copper ions are selectively coupled to the receptor, which stimulates the opening of the spirocycle. Consequently, the π -electrons are localized on the spiropyran and a new absorption peak appears between 450 and 600 nm. Due to its functional properties, the receptor facilitates the rapid, selective, and sensitive detection of copper ions with a detection limit of 19 nM. According to Job's plot analysis, the correlation ratio between the probe and copper ions appears to be 2:1. Further, the probe was successfully applied to test papers strips that demonstrated distinct colors changes in response to different concentrations of copper ions. [Display omitted] [ABSTRACT FROM AUTHOR]

This study reports an economical and portable point-of-care (POC) monitoring device based on artificial multi-enzyme cascade systems for multiple detection purposes. The device was made up of a disposable three dimensional microfluidic paper-based analytical device (3D μPAD) with multiple detection zones and a smartphone readout. On-paper synthesis of a multifunctional mimetic composite, based on the CeO 2 nanoparticles embedded in the amino-functionalized Fe metal-organic frameworks (CeO 2 @NH 2 -MIL-88B(Fe)), for cascade reactions was the main achievement of this work. The 3D μPAD was applied for simultaneous quantification of glucose, fructose, sucrose and maltose, and the detection process consisted of the enzymatic reaction of each sugar by anchored enzymes on the metal-organic frameworks (MOF) and successive oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB). Utilizing the new artificial mimicking system improved the color development uniformity and resulted in a reliable detection tool, with excellent detection limits in the range of 20–280 μM. It was directly applied to analyze the sugars levels of human total blood, urine, semen, honey and juice samples with the relative errors of less than 7.7% compared with the HPLC method. The cost-effective and easy-to-use μPAD has a great potential to be used in either medical diagnostics or the food industry. Also, it can be considered as a competitive POC method for patients in disadvantaged communities or emergencies. [ABSTRACT FROM AUTHOR]

A simple colorimetric paper-based sensor array (optical tongue), which can quickly and accurately identify and discriminate twenty-five kinds of heavy metal ions, is developed. It functions by immobilizing six metal indicators on the paper substrate using polyvinyl chloride. Exposing the device to the aqueous solution of metal ions produces a unique colorimetric signature for each ion, enabling discrimination of the ions by visual inspection. Further analysis of the colorimetric patterns by chemometrics data analysis methods leads to a clustering map for accurate classifying of the metal ions at different concentrations (10.0, 5.0, 2.5, and 1.0 mg L−1). In addition, the device was applied to analyze binary and ternary mixtures of metal ions as well. An interesting feature of this tasting device is speciation analysis of an element of different oxidation states and their mixtures. Beside to qualitative analysis, it represented quantitative analysis ability, e.g., for Zn2+ and Co2+, in the concentration ranges of 0.10–10.00 mg L−1 (R2 = 0.99) and 0.25–12.50 mg L−1 (R2 = 0.99) and detection limits of 0.03 and 0.08 mg L−1, respectively. This optoelectronic tongue shows promising results to be used potentially of on-site analysis of water for its quality. • An optical tongue, which can quickly identify and discriminate 25 heavy metal ions. • Immobilizing of the indicators on the paper surface hinders licking of the indicators. • The device can analyze sample solution, without need to any pretreatment. • The device produces a unique colorimetric signature for each ion. • Has potentially in on-site analysis of water for its quality. [ABSTRACT FROM AUTHOR]

To ensure safe drinking water, it is necessary to have a simple method by which the probable pollutants are detected at the point of distribution. Nitrite contamination in water near agricultural locations could be an environmental concern due to its deleterious effects on the human population. The development of a frugal paper-based microfluidic sensor could be desirable to achieve the societal objective of providing safe drinking water. This work describes the development of a facile and cost-effective microfluidic paper-based sensor for quantitative estimation of nitrite in aquatic environments. A simple punching machine was used for fabrication and rapid prototyping of paper-based sensors without the need of any specialized equipment or patterning techniques. A reusable 3D printed platform served as the support for simultaneous testing of multiple samples. The nitrite estimation was carried out with smartphone-assisted digital image acquisition and colorimetric analysis. Under optimized experimental conditions, the variation in average grayscale intensity with concentration of nitrite was linear in the range from 0.1 to 10 ppm. The limits of detection and quantitation were 0.12 ppm and 0.35 ppm respectively. The reproducibility, expressed as relative standard deviation was 1.31%. The selectivity of nitrite detection method was determined by performing interference studies with commonly existing co-ions in water, such as bicarbonates, chloride and sulphate. The paper-based sensor was successfully applied for estimation of nitrite in actual water samples and showed high recoveries in the range of 83.5–109%. The results were in good agreement with those obtained using spectrophotometry. The developed paper-based sensor method, by virtue of its simplicity, ease of fabrication and use, could be readily extended for detection of multiple analytes in resource-limited settings. [ABSTRACT FROM AUTHOR]

Aflatoxin B 1 (AFB 1) is a powerful carcinogen, and the pollution it causes has become a global public health safety issue. Therefore, microfluidic paper-based analytical devices (μPADs) with smartphones to read colorimetric signals were constructed for rapid AFB 1 detection. The colorimetric mechanism of μPADs was based on the common starch–iodine reaction, and the addition of gelatin/chitosan enhanced the colorimetric effect. During detection and analysis, the competitive analysis occurred in the process of AFB 1 monoclonal antibody's recognition of free AFB 1 and functionalized antigen. The glucose oxidase-labeled IgG was used as the secondary antibody. Hydrogen peroxide, an oxidation product of the glucose, caused a chroma change in μPADs. Simultaneously, the change in chroma was consistent with the change in AFB 1 concentration. Under the best optimized conditions, the limits of detection and quantitation of AFB 1 in the buffer were 9.45 and 12.00 ng mL−1, respectively. A comparison of the detection performance of commercial kits with that of the method introduced herein confirmed the superiority of our method in actual food samples with known AFB 1 concentrations. This colorimetric μPADs analysis met the detection requirements of some countries, such as China, and had great potential for development in the direction of daily extensive food safety control and environmental monitoring. [Display omitted] • Portable and low-cost smartphone-based microfluidic paper-based analytical devices (μPADs). • The colorimetric μPAD detection system can detect aflatoxin B1 as low as 9.45 ng mL−1 through Lab data analysis. • The colorimetric mechanism is based on the common starch–iodine reaction. • The addition of gelatin and chitosan mixture enhances the colorimetric effect. [ABSTRACT FROM AUTHOR]

[Display omitted] • High-efficiency (2 orders of magnitude), reproducible (RSD 2.5%), fast (within 2 min) parallel electric stacking PAD was created; • The wet real-time photographic colorimetry results of the parallel PAD system are insensitive to the changes of light conditions; • On-line colorimetric detection of pigments in drinks and off-line MS/MS detection of serum amino acids are used to demonstrate their applicability. A microfluidic paper-based analytical device (PAD) with both pre-concentration and parallel operation functions is of great promise for sensitive and high throughput detection. In this paper, a fast and multiplexed PAD was designed based on wax-screen-printing with glass fiber paper substrate and validated by image-based colorimetry. Driven by the same DC power supply, parallel electrokinetic stacking with the PAD of 8 channels can be completed within 2 min, and each channel can be loaded with different samples. Enrichment factor of 2 orders of magnitude was demonstrated with RSD below 2.5% between the chips and the channels of a chip. Reproducible and multiplexed stacking operations of this PAD provide a solid foundation for online calibration and sample detection on the same chip at the same time. Applications of this PAD was demonstrated by online colorimetric detection of food pigments from beverages and offline MS/MS detection of amino acids from a serum sample. Besides high-throughput and sensitive point-of-care test detection, this PAD method could also find applications for efficient pre-separation and pre-concentration of complex samples. [ABSTRACT FROM AUTHOR]

Paper-based analytical devices (PADs) with four new designs could be fabricated using commercially available home-based scan-and-cut printer. They serve for miniaturised platforms for chemical analysis. Replication analysis of a sample together with the calibration (using the analyte standards at different concentrations) can be completed in a single run, by utilising smartphone as the detector. Some new approaches for choosing detection zones were suggested. The four proposed PAD designs here were used as models in microliter scale operation to demonstrate the well-known chemistries of colorimetric determinations of iron, phosphate, and hardness using 1,10-phenanthroline and simple aqueous guava leaf extract; molybdate, and EBT-EDTA complexometric titration, respectively, through calibrations: where Blue (B) value = 88.2log [Fe3+] - 80.8, R2 = 0.989; B value = 1.75 [Fe3+] + 0.198, R2 = 0.999; Grey scale (I) value = 1.77 [Fe3+] – 1.22, R2 = 0.997; Red (R) value = 16.1log [PO 4 3−] + 8.95, R2 = 0.999; Hue (H) value = 43.3log [Ca2+] + 233, R2 = 0.994, respectively. For the hardness, using one of the PAD designs, true titration was also possible. Applications of the proposed devices and procedures were demonstrated for real world samples with validation. Additionally, kinetic study of the molybdenum blue for phosphate was demonstrated using one of the PADs. [Display omitted] • New designs of paper based analytical devices (PADs) by simple fabrication. • The PADs for replication analysis of a sample within a single run with smartphone. • Via video recorded by smartphone allows new way for optimisation. • Demonstrated for real world uses with iron, phosphate, and hardness as model assays. • Reengineered PADs as powerful analytical tools in low resource settings. [ABSTRACT FROM AUTHOR]

This work presents a novel cellulose-based aptasensor for the colorimetric detection of a cancer biomarker, osteopontin (OPN), in point-of-care (PoC) analysis. For this purpose, the cellulose paper was chemically modified with (mercaptopropyl)methyldimetoxisilane to attach the thiolated aptamer, which acts as a biological detection layer. The surface modification was checked by Fourier transform infrared spectroscopy and thermogravimetric analysis. Colorimetric detection was performed using a conventional staining solution, Bradford reagent. The color analysis was performed by evaluating the RGB coordinates provided by the ImageJ program from the photographs taken with a smartphone. Overall, the biosensor shows good sensitivity with a wide linear range (R > 0.998) of 5–1000 ng/mL and a detection limit lower than 5 ng/mL in buffer and commercial human serum solution, after 30 min of incubation. In addition, this aptasensor shows good selectivity to some interfering species such as bovine serum albumin and recombinant OPN. Analytical data obtained from spiked serum samples confirm the accuracy of the method. Importantly, it is a broad-spectrum method that tends to meet the criteria of REASSURED (real-time connectivity, ease of sampling, affordability, specificity, ease of use, speed and robustness, device freedom, and deliverability) for global testing. [Display omitted] • Development of a cellulose-based colorimetric test strip for the detection of a cancer biomarker. • Aptamer as a biological recognition layer for an OPN detection. • Bradford as a detection reagent. • Visual quantitative detection of a cancer biomarker. [ABSTRACT FROM AUTHOR]

Mercury contamination in the environment has reached alarming levels. Due to its persistence and bioaccumulation, mercury is one of the most widespread toxic heavy metals found in air, water and food. Thus, it is mandatory to monitor mercury and its compounds, and the availability of sensitive and rapid biosensors is highly valuable. We developed a low-cost biosensor for orthogonal detection of mercury(II) integrating three different biorecognition principles on a three-leaf paper: i) a mercury-specific bioluminescent Escherichia coli bioreporter strain expressing NanoLuc luciferase as reporter protein, ii) a purified β-galactosidase (β-gal) enzyme which is irreversibly inhibited by mercury and other metal ions, and iii) an Aliivibrio fischeri bioluminescent strain which is used to quantitatively assess sample toxicity and correct the analytical signal accordingly. Both sensory elements and substrates, Furimazine for the bioluminescent reporter strain and chlorophenol red-β-D-galactopyranoside for colorimetric detection of β-gal, were integrated in the paper sensor to provide a stable all-in-one disposable cartridge which can be easily snapped into a smartphone with a clover-shaped 3D printed housing. This is the first integration of bioluminescence and colorimetric detection on a smartphone-paper sensor, providing a readout within 15 and 60 min for the colorimetric and bioluminescent detection respectively. The biosensor was applied to water samples spiked with different concentrations of mercury, interferents and toxic chemicals providing a limit of detection for Hg(II) at the ppb levels. [Display omitted] • Orthogonal smartphone biosensor for mercury detection. • Colorimetric enzyme sensor and bioluminescent bacterial bioreporters on a paper sensor. • Mercury(II) detection within 60 min at ppb levels. • Applicable to toxic and complex samples. [ABSTRACT FROM AUTHOR]

A microfluidic colorimetric detection (MCD) platform consisting of a sliding hybrid PMMA/paper microchip and a smart analysis system is proposed for the convenient, low-cost and rapid analysis of human urine and whole blood samples. The sliding PMMA/paper microchip comprises a PMMA microfluidic chip for sample injection and transportation, a paper strip for sample filtration (urine) or separation (blood), and a sealed paper-chip detection zone for sample reaction and detection. In the proposed device, the paper-chip is coated with bicinchoninic acid (BCA) and biuret reagent and is then assembled into the PMMA microchip and packaged in aluminum housing. In the detection process, the PMMA/paper microchip is slid partially out of the housing, and 2 μL of sample (urine or whole blood) is dripped onto the sample injection zone. The chip is then slid back into the housing and the sample is filtered/separated by the paper strip and transferred under the effects of capillary action to the sealed paper-chip detection zone. The housing is inserted into the color analysis system and heated at 45 °C for 5 min to produce a purple-colored reaction complex. The complex is imaged using a CCD camera and the RGB color intensity of the image is then analyzed using a smartphone to determine the total protein (TP) concentration of the sample. The effectiveness of the proposed method is demonstrated using TP control samples with known concentrations in the range of 0.03–5.0 g/dL. The detection results obtained for 50 human urine samples obtained from random volunteers are shown to be consistent with those obtained from a conventional hospital analysis system (R2 = 0.992). Moreover, the detection results obtained for the albumin (ALB) and creatine (CRE) concentrations of 50 whole blood samples are also shown to be in good agreement with the results obtained from the hospital analysis system (R2 = 0.982 and 0.988, respectively). [Display omitted] • A MCD platform is presented for detection of urine and whole blood samples. • The platform comprises a sliding hybrid microchip and an intelligent analysis system. • The hybrid microchip has the functions of filtering urine and separating whole blood. • The hybrid microchip drives the fluid by capillary force without external driving force. • The platform can perform the detection of urine TP, whole blood ALB and CRE concentration. [ABSTRACT FROM AUTHOR]