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A planar solid-state ammonium-selective electrode, employing a composite mediator layer of graphite particles embedded in a polyvinyl butyral matrix on top of an inkjet-printed silver electrode, is presented in this paper. The effect of graphite powder mass fraction on the magnitude of the potentiometric response of the sensor was systematically verified using a batch-mode and a flow injection measurement setup. Under steady-state conditions, the paper electrode provided a Nernstian response of 57.30 mV/pNH4 over the concentration range of 10−5 M to 10−1 M with a detection limit of 4.8 × 10−6 M, while the analytical performance of the array in flow mode showed a narrower linear range (10−4 M to 10−1 M; 60.91 mV/pNH4 slope) with a LOD value of 5.6 × 10−5 M. The experimental results indicate that the prepared electrode exhibited high stability and fast response to different molar concentrations of ammonium chloride solutions. The pH-response of the paper NH4-ISE was also investigated, and the sensor remained stable in the pH range of 2.5–8.5. The potentiometric sensor presented here is simple, lightweight and inexpensive, with a potential application for in-situ analysis of environmental water samples.

We report on a novel colorimetric sensor system for highly sensitive detection of formaldehyde (FA) in the gas phase. The sensor is constructed with paper towel as a substrate coated with the sulfuric acid salt of hydroxylamine ((NH2OH)2·H2SO4) together with two pH indicators, bromophenol blue and thymol blue. Upon exposure to FA, the hydroxylamine will react with the absorbed FA to form a Schiff base (H2C=N-OH), thus releasing a stoichiometric amount of sulfuric acid, which in turn induces a color change of the pH indicator. Such a color change was significantly enriched by incorporating two pH indicators in the system. With the optimized molar ratio of the two pH indicators, the color change (from brown to yellow, and to red) could become so dramatic as to be visible to the eye depending on the concentration of FA. In particular, under 80 ppb of FA (the air quality threshold set by WHO) the color of the sensor substrate changes from brown to yellow, which can even be envisioned clearly by the naked eyes. By using a color reader, the observed color change can be measured quantitatively as a function of the vapor concentration of FA, which produces a linear relationship as fitted with the data points. This helps estimate the limit of detection (LOD), to be 10 ppb under an exposure time of 10 min, which is much lower than the air quality threshold set by WHO. The reported sensor also demonstrates high selectivity towards FA with no color change observed when exposed to other common chemicals, including solvents and volatile organic compounds. With its high sensitivity and selectivity, the proposed paper-based colorimetric sensor thus developed can potentially be employed as a low-cost and disposable detection kit that may find broad application in detecting FA in indoor air and many other environments.

With characters of low cost, portability, easy disposal, and high accuracy, as well as bulky reduced laboratory equipment, paper-based sensors are getting increasing attention for reliable indoor/outdoor onsite detection with nonexpert operation. They have become powerful analysis tools in trace detection with ultra-low detection limits and extremely high accuracy, resulting in their great popularity in medical detection, environmental inspection, and other applications. Herein, we summarize and generalize the recently reported paper-based sensors based on their application for mechanics, biomolecules, food safety, and environmental inspection. Based on the biological, physical, and chemical analytes-sensitive electrical or optical signals, extensive detections of a large number of factors such as humidity, pressure, nucleic acid, protein, sugar, biomarkers, metal ions, and organic/inorganic chemical substances have been reported via paper-based sensors. Challenges faced by the current paper-based sensors from the fundamental problems and practical applications are subsequently analyzed; thus, the future directions of paper-based sensors are specified for their rapid handheld testing.

Quantitatively paper-based senor is performed with simple distance-readout on mixed cellulose ester (MCE) filter paper based on acetylcholinesterase (AChE)-mediated alginate hydrogel. The method is accomplished with the aid of the inhibition effect of target samples on the AChE enzyme-catalyzed hydrolysis of acetylcholine, which changes the pH value of the solution to release Ca2+ and trigger alginate hydrogelation. The viscosity of the solution is thus regulated with the presence of target samples in the reaction mixture, leading to a significant change in the diffusion diameter of the solution spotted on the filter paper. The concentration in the sample is quantitatively determined by ruler-measureable diffusion diameter of the spot on the paper. With successfully application for quantitatively sensing of organophosphorus pesticides (OPs), we show that the method exhibits excellent reproducibility with RSD (n = 5) as low as 0.09% and good selectivity for detection of OPs. The dynamic range of the method is up to 66.7 ng/mL with the limit-of-detection (LOD) of 3.3 ng/mL. The present study provides a new approach for developing paper-based sensors with quantitative distance-readout, by utilizing enzymatic inhibition to modulate liquid viscosity, which would be of value for target detection in complex samples. Image 1 • Quantitative readout via converting viscosity to ruler-measurable distance of paper-based sensor. • Modulation of solution viscosity based on inhibition of enzyme-mediate alginate hydrogelation. • Excellent reproducibility and selectivity for detection of organophosphorus pesticides. [ABSTRACT FROM AUTHOR]

The current work was aimed to develop the paper microzone plates (PµZP) as a scanometric system for determining the total antioxidant capacity of plant extracts. The PµZP was constructed by immobilizing CUPRAC reagent onto 70-well of patterned paper as a sensing zone for the scanometric detection system. The light blue-sensing zone showed a sensitive response to standard antioxidant (rutin) by forming light yellow color adducts which can be scanned and quantitatively measured by image processing program as a scanometric method. The sensor gave a notable signal at 8 min after rutin addition and presented a linear response in the concentration span of 1–10 mM. The developed method was shown to be reproducible (RSD < 3%) and accurate (98–101% recovery) for determining the TAC of plant extracts (as mM rutin equivalents) and showed strong correlation (r = 0.9887) with the standard CUPRAC method, suggesting that it can be applied for the simple and rapid method for TAC determination. [ABSTRACT FROM AUTHOR]

• We devise a SERS substrate prepared by direct nucleation of AuNPs on paper without using reducing agents. • Size of AuNPs was controlled through the changes in pH of Au precursor solution. • The paper-based SERS substrate was used for label-free pesticide detection. • Field applicable test was performed using portable Raman spectrometer on pesticide contaminated apple peels. We introduce a facile and low-cost method for fabricating gold nanostructures on cellulose filter paper (CFP) to prepare a paper-based surface-enhanced Raman scattering (SERS) sensor for label-free molecular detection. Polymerized dopamine (PD) was used as an adhesive layer on the CFP and simultaneously functioned as a reducing agent for gold nanoparticle (AuNP) nucleation. The size of the AuNPs was dependent on the pH of the gold precursor solution, and nanoparticles with an average size of 102 nm were formed on the PD-coated CFP at a pH 3, exhibiting high SERS activity. Finite-difference time-domain (FDTD) simulations of the electromagnetic field enhancement of AuNPs with different sizes and interparticle distances were performed to identify the origin of the SERS effect. The developed paper-based SERS substrate showed uniform and excellent molecular sensitivity with a limit of detection (LOD) of 10−7 M for methylene blue, as measured by a portable Raman spectrometer. Furthermore, as a field application test, surfaces of apples were pretreated with diquat (DQ) and paraquat (PQ) pesticides, which were then detected down to a concentration of 1 ppm after simple attachment of the sensor on the apple peels and performing a SERS measurement. The developed paper-based SERS sensor is expected to be applicable as a label-free sensor for a variety of chemical and biological molecules. [ABSTRACT FROM AUTHOR]

• A paper-based piezoresistive sensor with advantages of low-cost, portability, flexibility and disposability was developed. • The resulting device exhibited rapid response and recovery speed (0.6 s and 0.2 s, respectively). • The device was applied to monitoring human physiological signals. A paper-based pressure sensor constructed by sandwiching a piezoresistive film between two pieces of paper substrates with silver electrodes is reported in this work. The resulting devices exhibited rapid response and recovery speed, and the response and recovery times were 0.6 s and 0.2 s, respectively, under a pressure of approximately 1.5 kPa. In addition, the device can be especially sensitive to pressures less than 3 kPa. Finally, the devices were used to monitor physiological signals, including the motions of fingers, vocal cord vibration and wrist pulse. The advantages of paper-based devices, such as low cost, portability and flexibility, will have great potential for expanding the application of resultant sensors in wearable medical devices. [ABSTRACT FROM AUTHOR]

Abstract A S -nitrosothiol (RSNO) is used for highly selective and sensitive copper ion sensing for the first time. Cu(II) in the sample is reduced to Cu(I) by a low amount of thiols generated from hydrolysis of RSNO molecules or added thiols such as l -glutathione or l -cysteine. Cu(I) is able to trigger cleavage of the S NO bond, which converts colored RSNOs to colorless products. The dark green S -nitroso-N-acetylpenicillamine is used as an exemplary RSNO in this report. In the spectrophotometric test, the detection limit toward Cu(II) is 0.23 μM without added thiol reductants, and 0.08 and 0.06 μM in the presence of l -glutathione and l -cysteine, respectively. Furthermore, we prepared fully inkjet printed paper-based sensors by deposition of all reaction reagents and buffers on the same piece of cellulose paper. A smartphone equipped with a color analysis app enables quantification of the color change of the paper-based Cu(II) sensors. In this method, a detection limit of 1.2 μM and a linear range of 0–10 μM were obtained. Finally, we successfully applied this instrumentation-free and reagent-free senor for Cu(II) analysis in real drinking water and river water samples. Graphical abstract Image 1 Highlights • Reduced Cu(II) triggers cleavage of the S NO bond in green S-nitroso-N-acetylpenicillamine, enabling Cu(II) quantification. • The presence of additional thiols enhances the sensitivity of this sensing scheme. • Paper-based sensors were prepared by using an office inkjet printer. • A smartphone equipped with a color analysis app was used as the detector of the paper-based sensors. [ABSTRACT FROM AUTHOR]

A new, simple and low‐cost method for patterning hydrophobic barriers in porous support such as paper by Parafilm® has been introduced. This method is then used for electrochemical paper‐based ammonia sensor construction. Ammonia sensor is based on electrochemical concentration cell which ammonia reaction with electrolyte in halves cell caused in concentration gradient and therefore potential difference dependent on ammonia concentration. The effect of concentrations of the involved chemicals, time periods of the required processes, the presence of Faraday cage as well as the effects of different salts used in the salt bridge on the response of the sensor, were investigated in order to find the optimized conditions. [ABSTRACT FROM AUTHOR]

Development of the portable device is significant for sensitive and rapid detection of an anthrax biomarker dipicolinic acid (DPA), existing in the B. anthracis. In this work, a novel HAp:Tb-EDTA paper-based ratiometric fluorescent sensor was obtained by a simple one-pot method for rapid and sensitive DPA detection. With the increased DPA concentration, the luminescence intensity of HAp (hydroxyapatite) remained constant, and thus applied as the stable reference signal, while the luminescence signal of Tb3+-EDTA was significantly enhanced due to the antenna effect. Therefore, the HAp:Tb-EDTA paper-based sensor was endowed with self-calibrated and ratiometric fluorescent detection performance for DPA. The proposed sensor showed excellent detection performance with a detection limit as low as 10.8 nM in the linear range of 0.5–30 μM. After combination with a smartphone, rapid visual and fluorescent detection of DPA was achieved. The proposed sensor was successfully applied to detect DPA from B. subtilis spore real samples, showing the application prospects of the paper-based sensors and opening a new horizon to develop novel paper-based point-of-care testing (POCT) devices. A sensing platform was constructed by a HAp:Tb-EDTA paper-based ratiometric fluorescent probe prepared by a one-pot method and combined with a smartphone. The sensing platform has good stability and can be used for sensitive detection of DPA released from Bacillus subtilis spores. [Display omitted] • A novel HAp:Tb-EDTA paper-based ratiometric fluorescent probe was facilely constructed. • The Tb3+-EDTA recognition unit uniformly distributed on HAp with a paper-like structure, making it superior to that based on commercial test papers. • A portable platform was constructed through combining with smartphones for real samples detection, showing widespread application prospects. • Opening a new horizon to develop novel paper-based point-of-care testing (POCT) devices. [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]

Ratiometric fluorescent sensors can suppress the interference of factors unrelated to analysis due to their built-in self-calibration characteristics, which exhibit higher sensitivity and more obvious visual detection in the process of qualitative and quantitative analysis. Herein, we constructed a ratiometric fluorescence probe based on fluorescent/colorimetric dual-mode method for the determination of arginine by encapsulating rhodamine B in-situ into UiO-66-NH 2 MOFs (UiO-66-NH 2 @RhB). The as-prepared probe showed dual-emission characteristics under a single excitation wavelength. The fluorescence intensity of UiO-66-NH 2 was increased significantly by arginine, while the emission peak intensity of rhodamine B remained stable, resulting in a single-signal response with fixed reference. Furthermore, the practicality of the presented sensor was successfully validated by quantitative detection of arginine in human serum. More significantly, paper-based sensors for arginine detection were devised by using carboxymethyl cellulose modified filter papers. Under the irradiation of ultraviolet light, the paper-based sensors would produce obvious color variation from lightpink to bluish violet. This work provided a convenient and efficient method for on-site detection of arginine. Schematic illustration of in-situ encapsulation strategy of ratiometric fluorescence probe UiO-66-NH 2 @RhB and paper-based sensor for visual detection of arginine. [Display omitted] [ABSTRACT FROM AUTHOR]

An ultra-sensitive paper-based molecularly imprinted sensor for detection of H5N1 was described. The results can not only be detected quantitatively by fluorescence, but also can be directly read with naked eyes under visible light or ultraviolet light within 10 min. With naked eye detection, its sensitivity is obviously better than that of commercial kits. In particular, the cost of the sensor is cheap (Original size: RMB 1.53 Yuan each; Smaller size: RMB 0.08 Yuan each), which is beneficial to application. [Display omitted] • Paper-based molecularly imprinted sensor; • Ultra-sensitive detection of H5N1 (LOD, 0.58 fM); • Rapid visual detection of influenza H5N1 within 10 min; • More intuitive and sensitive results than commercially available kit; • Low cost (Original size, 1.53 yuan/piece; Smaller size, 0.08 yuan/piece). The rapid detection of pathogens is important to the control of epidemics. Here, a paper-based molecularly imprinted sensor has been developed for the visual detection of influenza H5N1 virus. Firstly, the paper-based molecularly imprinted was prepared by a sol–gel reaction. Then, H5N1 ∼ Apt was covalently bound to the surface of the ZIF-8-NH 2 coated with rhodamine B (RhB) through an amide bond, to produce the fluorescent and colorimetric probe ZIF-8-NH 2 @RhB ∼ Apt. When H5N1 and ZIF-8-NH 2 @RhB ∼ Apt are added to the paper-based MIPs, the H5N1 will be combined with MIPs through the synergistic effects of shape and size compatibility and interaction with the functional groups of the monomers, and ZIF-8-NH 2 @RhB ∼ Apt will bind to H5N1 through its aptamers to form a sandwich structure. The sensor has ultra-high sensitivity and the detection limit is 0.58 fmol/L by using fluorescence spectroscopy. A semi-quantitative signal can also be observed with the naked eye within 10 min. A comparative study showed that direct visual detection was highly reliable, and the sensitivity was better than that achieved with a commercially available kit. Importantly, the paper-based sensor is low in produce cost (Original size: RMB 1.53 Yuan each; Smaller size: RMB 0.08 Yuan each) and is easy to cut, store and carry. This sensor is expected to allow self-service, visual, high-throughput, and real-time virus detection, which is of great significance for epidemic prevention and control. [ABSTRACT FROM AUTHOR]

[Display omitted] • The dual-emission ratiometric fluorescent probe was prepared by a facile hydrothermal synthesis method. • The quantification of Pb2+ was based on the efficient FRET between NCDs and AuNCs. • The sensor has good selectivity and anti-interference ability with satisfactory recovery in real water samples. • A paper-based sensor was fabricated for visual semi-quantitative detection by color change. Ratiometric fluorescent sensors could effectively eliminate the interference of spontaneous fluorescence or instrument fluctuation to improve the analysis sensitivity and accuracy. In this work, a dual-emission nitrogen-doped carbon dots/AuNCs (NCDs-AuNCs) nanohybrid was fabricated under mild conditions by the hydrothermal method, without additional coupling agents, which is facile and environmentally friendly. The prepared nanohybrid was employed for the sensitive detection of Pb2+. The sensor exhibited dual emission peaks at 460 and 625 nm with efficient fluorescence resonance energy transfer under a single excitation wavelength of 360 nm, in which the red fluorescence of AuNCs was enhanced and the blue fluorescence of NCDs was slightly quenched with the presence of Pb2+. The quantitative analysis of Pb2+ was performed upon the change of both fluorescence intensities. The sensor was successfully used for Pb2+ detection in real water samples with satisfactory spiking recoveries in a range of 91.0%–101.3% and low relatively standard deviations (RSDs < 3%), which exhibited good selectivity and anti-interference ability. In addition, a paper-based sensor was fabricated by immobilizing the nanohybrids on filter paper, which enables the visual semi-quantitative determination of Pb2+ by the naked eye through the color transition from blue to pink. The prepared NCDs-AuNCs nanohybrids showed great application potential in environmental water monitoring in an efficient and green way. [ABSTRACT FROM AUTHOR]

The smart, rapid, and customizable detection of chemical warfare agents is a huge issue for taking the proper countermeasures in a timely fashion. The printing techniques have established the main pillar to develop miniaturized electrochemical biosensors for onsite and fast detection of nerve and mustard agents, allowing for a lab on a chip in the chemical warfare agent sector. In the fast growth of novel technologies, the combination of miniaturized electrochemical biosensors with flexible electronics allowed for the delivery of useful wearable sensors capable of fast detection of chemical warfare agents. The wearable microneedle sensor array for minimally invasive continuous electrochemical detection of organophosphorus nerve agents, as well as the wearable paper-based origami functionalized with nanomaterials for mustard agents in the gas phase, represent two examples of the forefront devices developed in the chemical warfare agent detection field. This review will highlight the most promising electrochemical biosensors developed by exploiting nanomaterials and cross-cutting technologies for the fabrication of smart and sensitive electrochemical biosensors for the detection of chemical warfare agents.

Immunoassay has the advantages of high sensitivity, high specificity, and simple operation, and has been widely used in the detection of mycotoxins. For several years, time-resolved fluorescence immunochromatography (TRFIA) paper-based sensors have attracted much attention as a simple and low-cost field detection technology. However, a traditional TRFIA paper-based sensor is based on antibody labeling, which cannot easily meet the current detection requirements. A second antibody labeling method was used to amplify the fluorescence signal and improve the detection sensitivity. Polystyrene fluorescent microspheres were combined with sheep anti-mouse IgG to prepare fluorescent probes (Eu-IgGs). After the probe fully reacted with the antibody (Eu-IgGs-Abs) in the sample cell, it was deployed on the paper-based sensor using chromatography. Eu-IgGs-Abs that were not bound to the target were captured on the T-line, while those that were bound were captured on the C-line. The paper-based sensor reflected the corresponding fluorescence intensity change. Because a single molecule of the deoxynivalenol antibody could bind to multiple Eu-IgGs, this method could amplify the fluorescence signal intensity on the unit antibody and improve the detection sensitivity. The working standard curve of the sensor was established under the optimum working conditions. It showed the lower limit of detection and higher recovery rate when it was applied to actual samples and compared with other methods. This sensor has the advantages of high sensitivity, good accuracy, and good specificity, saving the amount of antibody consumed and being suitable for rapid field detection of deoxynivalenol.

We report a paper-based electrochemical immunosensor made with sustainable materials to detect aflatoxin B1 (AFB1), a highly toxic, carcinogenic mycotoxin found in food. The immunosensor was prepared with a waterproof paper substrate and low-cost graphite-based conductive ink through a simple cut-printing method. The working electrode was functionalized with a drop-cast film of multiwalled carbon nanotubes (MWCNT)/chitosan on which a layer of anti-AFB1 monoclonal antibodies was immobilized covalently. The architecture of the immunosensor was confirmed with polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and electrochemical impedance spectroscopy (EIS), including the effective immobilization of the active layer of anti-AFB1. With EIS as the principle of detection, the immunosensor could detect AFB1 in the range from 1 to 30 ng·mL−1, and detection limit of 0.62 ng·mL−1. This sensitivity is sufficient to detect AFB1 in food according to regulatory agencies. The immunosensor exhibited good repeatability, reproducibility, stability, and selectivity in experiments with a possible interferent. Furthermore, detection of AFB1 in maize flour samples yielded recovery of 97–99%, in a demonstration of the possible use of the paper-based immunosensor to detect AFB1 using extraction solutions from food samples.

A visual detection method for malachite green (MG) in food was established based on 'double-response-OFF' ratiometric fluorescent paper-based sensor. Biomass carbon quantum dots (BCQDs) using broad bean shell, and Ti 3 C 2 MXene quantum (MQDs) dots modified by ethylenediamine were synthesized by solvothermal method. The MG and two kinds of quantum dots could undergo static quenching, and the fluorescence color of two kinds of quantum dots gradually changed from red to blue, eventually the fluorescence was quenched, and the pattern had a two-stage linear relationship using fluorescent spectrofluorometer in the range of 0.1-140.0 μ M and the detection limit of 0.07 μ M. On this basis, a BCQDs/MQDs ratiometric fluorescence paper-based sensor was constructed and applied to fish sample. Through mobile phone software-Color recognizer, RGB values of fluorescent paper-based sensor at various concentrations of MG were extracted. The results showed that MG concentration was linearly correlated with R ' value of RGB in the range of 20.0-140.0 μ M with 16.5 μ M detection limit. The method had been applied to the determination of canned fish and fresh basa fish samples, and the recovery rates were 97.33%-108.93% and 96.04%-117.97%, respectively. It proved that the ratiometric fluorescent paper-based sensor could be used for the rapid visual quantitative detecting MG in real samples., (© 2024 IOP Publishing Ltd.)

This work discusses surface modification of cellulose paper specimens for compatibility with nitrogen and sulfur co-doped carbon dots (NSCDs) for lead ion sensing. The interaction of carbon dots (CDs) and cellulose fibers was investigated using silane or chitosan-modified cellulose papers. It was found that modified papers could reduce undesirable redistribution of CDs, during paper drying. Also, only chitosan-modified filter paper was suitable for the successful immobilization of NSCDs. The effect of paper type, chitosan amount, pH, and NSCDs concentration was also studied, and a Whatman No. 42 filter paper modified with chitosan (1% w/v), pH 8.0, and an NSCD concentration of 2.5 g L -1 being selected for further studies. The sensor exhibited high selectivity for lead(II) compared with other metal ions because lead(II) resulted in the most significant changes in the emitted light intensity. Variations in NSCDs fluorescence were measured using a fluorescence imaging system. The NSCDs-paper sensor showed a linear relationship between mean fluorescence intensity and lead(II) in the concentration range of 5.00-1.25 × 10 2  μmol L -1 with a correlation coefficient (R 2 ) of 0.9988 and a detection limit of 4.50 μmol L -1 . The suggested method showed satisfying results for lead(II) determination in different samples as a fast and low-cost approach with on-site application., (© 2024 John Wiley & Sons Ltd.)

Gold nanoparticles (GNPs) were synthesised by the Ultrasonic Spray Pyrolysis (USP) process and collected in deionised water with the addition of a stabiliser, i.e. PVP (0.1 wt.%). With the use of a rotary evaporator, a highly concentrated GNPs' suspension was achieved (600 ppm concentration of GNPs), which was used directly as novel nano gold ink for inkjet printing. The physical and chemical characteristics of such prepared nano gold ink were explained in detail by the use of Zeta (?) Potential, ATR-FTIR spectroscopy, UV/VIS spectroscopy, and nanoparticle size was identified through SEM. With nano gold ink the chosen pattern was printed onto photo paper, which was characterised for confirming the presence of gold with optical and SEM/EDX observations. The observations revealed that the tested printed nano gold ink on the paper provided a new route for the fabrication of paper-based electrochemical immunosensors, colorimetric sensors and nanometallic biomedical sensors. [ABSTRACT FROM AUTHOR]

Highlights • MIPs/CDs grafted paper-based sensor for selective and sensitive determination of 3-MCPD. • CDs were intercepted in the paper and MIPs was also directly formed on the paper-based sensor. • The composites exhibited high adsorption capacity and excellent selectivity. • This method could be applied for the detection of 3-MCPD in soy sauce samples. Abstract 3-Monochloropropane-1,2-diol (3-MCPD) is a common food processing contaminant and a simple, rapid, sensitive and low cost monitoring technology is needed due to its potential carcinogenic nature. Carbon dots directly intercepted on filter paper provide high fluorescence intensity and can be adapted for use as a sensor. We synthesized a carbon dot-filter paper in combination with a molecularly imprinted polymeric film to extract 3-MCPD from samples. This grafted paper-based sensor exhibited a high adsorption capacity (68.97 mg g−1), an excellent selectivity (imprinting factor = 4.5) and a low detection limit (0.6 ng mL−1). Recoveries ranged from 97.2% to 105.3% with relative standard deviations <5.6%. The results obtained using this method were linearly correlated to those of the classic GC–MS method (r = 0.998). Based on the Chinese National Standard, this study provides a novel and powerful platform for the simple, rapid, sensitive and on-site analysis of 3-MCPD in soy sauce. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • L -cysteine modified chiral carbon quantum dots (cCQDs) were synthesized. • cCQDs-embedded nanopaper is developed for enantioselective sensing. • Photoluminescent enantioselective sensing of lysine enantiomers is achieved. • The enantiomeric percentage can be determined by the proposed sensor. Abstract The production and application of chiral nanomaterials are expanding to understand the nature of chiral interactions and to detect chiral molecules due to the chirality is a fundamental characteristics of life processes. With the new advances in optical sensors by combining the advantages of optical signaling unit and sensing platform, new avenues can be opened to produce simple and useful devices for chiral sensor applications. Here, we report construction of nanoscale chirality of carbon quantum dots (CQDs) by functionalization with L -cysteine and application for paper-based sensors. Within this respect, nanopaper made of cellulose nanofibers was prepared and the produced chiral carbon quantum dots (cCQDs) were embedded into nanopaper to gain photoluminescence properties. The chiral sensing behavior of cCQDs was investigated in solution phase and nanopaper. cCQDs exhibited an enantioselective response towards L -lysine among the tested enantiomers of amino acids with a LOD value of 0.30 mM and 0.97 mM for solution phase and nanopaper, respectively. [ABSTRACT FROM AUTHOR]

Abstract Wireless chemical sensors are increasingly finding use as analytical devices in healthcare diagnostics, wearable sensing (sweat analysis) and in the rapidly emerging area of the Sensor Internet of Things (SIoT). In such wireless scenarios, the application of fluorescence-based sensors is lagging behind other types of transduction mechanisms. In this work a new low-cost and highly portable wireless fluorimeter for optical chemical fluorescence intensity measurements is presented (bill-of-materials approximately €20). The fluorimeter is programmed and communicates wirelessly with mobile devices or personal computers by radio-frequency identification (RFID) or near-field communication (NFC). This novel mobile analytical system is highly adaptable for use with different fluorescent sensor chemistries and analytes. The fluorimeter has been evaluated in the laboratory in two sets of proof-of-concept experiments: firstly fluorimetry in solution, where the fluorescence intensities of fluorescein were recorded, and secondly chloride sensing via a paper-immobilised quinine sulphate fluorescent indicator. The latter represents the first demonstration of wireless (radio-frequency) fluorescence-based chloride sensing. Satisfactory analytical performance was achieved in the entire range of sweat chloride concentrations. The wireless fluorimeter system presented here could make a significant contribution to several emerging areas of mobile chemical sensor research, including wearable sensors for healthcare and sport, mobile point-of-sample diagnostics as well as more generally for the Sensor Internet of Things. [ABSTRACT FROM AUTHOR]

We report here, for the first time, a smartphone-based lab-on-paper method for determination of uranium in ores. The method is based on the reaction with arsenazo-III on a filter-paper strip. The intensity of the green color was determined by a smartphone. The calibration curve is linear up to at least 60 μg mL−1 of uranium with LOD and LOQ values of 0.66 and 2.21 μg mL−1, respectively. The proposed method was applied for the analysis of spiked water samples and real Egyptian ore samples with recoveries ranging from 97.3 to 104.7%. The results are in agreement with the reported reference titrimetric method. [ABSTRACT FROM AUTHOR]

In analytical science, it is always of great scientific and practical significances to explore convenient, rapid, and reliable methods/devices to trace the H 2 O 2 level in various matrices. Herein, we fabricated a ready-to-use paper-based chemosensor by using mesoporous carbon-dispersed Pd nanoparticles (Pd NPs/meso-C) as a promising peroxidase mimic for the visual determination of H 2 O 2 . Thanks to the large exposed surface of the meso-C support and the highly dispersed active Pd NPs, the synthesized Pd NPs/meso-C exhibited favorable catalytic performance to trigger the chromogenic reaction of colorless TMB to blue TMBox mediated by H 2 O 2 , providing a K cat of 326.67 s −1 for H 2 O 2 and 0.55 s −1 for TMB, larger than the bare Pd NPs and even comparable to the activity of natural horse radish peroxidase. As demonstrated, this principle could be employed to colorimetrically detect H 2 O 2 in the linear scope of 5–300 μM. By simply immobilizing the Pd NPs/meso-C mimic and the TMB substrate onto a common filter paper, ready-to-use paper-based sensors were further prepared. By combining these test strips with a smartphone and an easy-to-access color-scanning APP together, the integrated platform could be used for the quantitative analysis of H 2 O 2 with advantages of simple operation, fast response, low cost, and good selectivity and repeatability. Furthermore, reliable sensing of the target in milk matrices suggested great promise of the integrated platform in practical applications. [ABSTRACT FROM AUTHOR]

Paper-based sensors open new avenue to produce simple, rapid, inexpensive and single-use analytical devices for a wide application fields involving medical diagnosis, food analysis and environmental monitoring. In this study, a new optical sensing platform for enantioselective recognition of chiral analytes was introduced by using in-situ synthesized silver nanoparticles-embedded transparent nanopaper. To this aim, nanopaper was obtained by environment-friendly approach using bacterial cellulose made of nanofibers and silver nanoparticles were embedded within nanopaper by an in-situ generation method. The chiral recognition capability of AgNPs was operated in both solution phase and nanopaper for the tested enantiomers. AgNPs showed a discriminative sensing response toward D -cysteine with a LOD value of 4.88 µM. The principle of optical sensing is the selective interaction of the inherently chiral AgNPs with enantiomers causing to aggregation of AgNPs to display a significant colour change from yellow to purple-brown in both aqueous phase and nanopaper. As for practical use, the obtained plasmonic nanopaper was punched into circular pieces and put on wax-printed PET film to produce disposable two-dimensional cuvette which could be inserted in an ordinary spectrophotometer. The enantiomeric percentage of D -cysteine was successfully determined by the fabricated nanopaper-based cuvettes. [ABSTRACT FROM AUTHOR]

The high incidence of Diabetes Mellitus in low-income regions has promoted the development of low-cost alternatives to replace blood-based procedures. In this work, we present a bienzymatic paper-based sensor suitable for the naked-eye detection of glucose in saliva samples. The sensor was obtained by a stamping procedure and modified with chitosan to improve the colorimetric readout. The bienzymatic reaction of GOx-HRP coupled with 2,4,6-tribromo-3-hydroxy benzoic acid was applied for the detection of glucose within a range from 0 to 180 mgdL–1 in buffer and artificial saliva solutions. The visual readout was perceived by the naked eye and registered with an office scanner to evaluate the analytical performance. The results showed a limit of detection of 0.37 mgdL–1 (S/N = 3) with an R.S.D. of 1.69% and a linear range from 1 to 22.5 mgdL–1 with an R2 of 0.99235. The analysis of human saliva samples was performed without pre-processing, achieving recoveries from 92 to 114%. The naked-eye detection was evaluated under two different light settings, showing average recoveries of 108.58 and 90.65% for standard and low illumination. The proposed device showed potential for easy-to-use, sensitive, low-cost, fast, and device-free detection of salivary glucose suitable for untrained personnel operation and limited facilities. [ABSTRACT FROM AUTHOR]

Continuous monitoring and rapid on-site determination of toxic ammonia in various aqueous systems are important for maintaining water quality and preserving the environment. Though several methods such as flow-based spectrophotometry are currently utilized for this purpose, there is still need for more selective ammonia determination. We present a new detection route for quantifying NH 3 in water samples by means of the evaporation of dissolved ammonia into headspace, followed by selective detection using a colorimetric sensor. Gaseous NH 3 detection is attained by the selective formation of blue indophenol dye through modified Berthelot’s reaction on porous paper. To optimize the sensing layer in terms of uniform and intense coloration, the water-soluble powder reagents were homogeneously dispersed in nonpolar cyclohexane and then deposited by a vacuum-assisted filtration on the paper substrate. The formation of indophenol blue was activated under highly humidified conditions through selective chemical reactions between solid-state Berthelot’s reagents and NH 3 . The colorimetric response of the low-cost paper sensors was saturated within 10 min, and color change was observed by the naked eye for an ammonia solution of 10 mg/L. Furthermore, the response magnitude was not affected by addition of 0.1 wt% polar organic solvents (ethanol, acetone, dimethylformamide or acetic acid) or inorganic salts (NaCl, KCl, MgSO 4 or CaSO 4 ), suggesting a good detection cross-selectivity. [ABSTRACT FROM AUTHOR]

This paper describes a paper-based biochemical sensor that realizes redox cycling with close interelectrode distance. Two electrodes, the generator and collector electrodes, can detect steady-state oxidation and reduction currents when suitable potential is held at each electrode. The sensor has two gold plates on both sides of a piece of chromatography paper and defines the interelectrode distance by the thickness of the paper (180μm) without any micro-fabrication processes. Our proposed sensor geometry has successfully exhibited signatures of redox cycling. As a result, the concentration of ferrocyanide as reversible redox molecules was successfully quantified under the interference by ascorbic acid as a strong irreversible reducing agent. This was possible because the ascorbic acids are completely consumed by the irreversible reaction, while maintaining redox cycling of reversible ferrocyanide. This suggests that a sensor based on the redox cycling method will be suitable for detecting target molecules at low concentration. [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]

Chiral analysis is of pivotal importance in a variety of fields due to the different biological activities and functions of enantiomers. Here, we develop a simple paper-based chiral biosensor that can perform sample-to-answer simultaneous analysis of lactate enantiomers in human serum samples. By modification of alginate hydrogel with "egg-box" three-dimensional network structure on a glass microfiber paper, reagents of enantiomer-selective enzymatic reactions are efficiently encapsulated forming the sensing regions for chiral analysis. Dual enzyme catalytic system (lactate dehydrogenase and glutamic pyruvic transaminase) is utilized to enhance the response of the biosensor. A smartphone with color analysis software is used to collect and analyze the fluorescence signal from the product nicotinamide adenine dinucleotide. The results show that the sensor has excellent selectivity toward lactate enantiomers with low limit-of-detection of (30.0 ± 0.7) μM for L-lactate and (3.0 ± 0.2) μM for D-lactate, and wide linear detection range of 0.1–3.0mM and 0.01–0.5 mM for L-lactate and D-lactate respectively. The proposed method is successfully applied to the simultaneous detection of L-/D-lactate concentrations in human serum with satisfactory accuracy. Our study provides a robust approach for developing chiral biosensors, which would have promising application prospect in point-of-care testing (POCT) analysis of various biological and food samples. [Display omitted] • Sample-to-answer simultaneous analysis of lactate enantiomers. • Dual enzyme catalytic system to enhance the response of the paper-based sensor. • Excellent reproducibility and selectivity of lactate enantiomers for POCT. [ABSTRACT FROM AUTHOR]

[Display omitted] • A sensory platform which is based on fluorescence quenching and paper-based probe has been developed. • Carbon dots prepared from waste rice straw (RS@CQDs) are used as a precursor source. • The limit of detection (LOD) is as low as 1.33 nM with a wide linear response from 5 nM to 80 μM. • They are highly sensitive and efficient for the quantitative analysis of monosodium glutamate (MSG) ions. • Real food samples were used for MSG sensing with the RS@CQDs based sensor. • The LOD for MSG detection in aqueous samples is as low as 1 nm. In this work, a simple and convenient sensory platform based on the fluorescence quenching and paper-based probe has been developed using carbon dots (CQDs). The CQDs prepared from waste rice straw (RS@CQDs) as a precursor source. Significantly, low value of detection (1.33 nM) and a wide linear response (5 nM to 80 μM), as well as good stability and repeatability endowed them to be highly sensitive and efficient probe for the quantitative analysis of monosodium glutamate (MSG) ions. Interferences from various biological compounds were also investigated for the developed sensor. Further, the RS@CQDs-based sensor efficiently detected MSG ions in real food samples. The portable and highly sensory podium for the visual detection of MSG in aqueous samples with LOD as low as 1 nm has also been prepared. [ABSTRACT FROM AUTHOR]

Carbon paper-based electrochemical sensors offer the possibility of long-term in vivo monitoring of the biomarkers due to their chemical stability and porous structure. However, the serious self-staking and aggregation of the carbon materials seriously block the diffusion of target biomolecules and limit the sensitivity and selectivity of the device. Here we report a controllable interfacial coupling strategy in which a composite paper electrode made by electrostatic self-assembling reduced graphene oxide (rGO) on carbon fiber (CF) modified with cationic polyacrylamide to avoid the self-stacking in scalable wet papermaking and thus increased accessible electrochemically active surface area and accelerate the diffusion of biomarkers. The resulting composite paper shows improved stability, high selectivity, long-durability, and excellent sensitivity in the electrochemical detection of biomarkers such as ascorbic acid, dopamine, and uric acid. Our approach, which combines mature industrial paper technology with interface engineering and electrochemical reduction, offers a potential avenue for low-cost and scalable implantable electrodes for timely and long-term monitoring in the surgical and clinical applications. [Display omitted] • RGO was uniformly coupled on the CFs assisted by interfacial CPAM molecules. • Large-scale preparation of electrodes through traditional wet papermaking. • Enlarged electrochemically active surface area of composite paper electrodes. • Excellent selectivity and sensitivity in detection of biomarkers such as DA. [ABSTRACT FROM AUTHOR]

Rapid detection of indoor airborne viruses is critical to prevent the spread of respiratory diseases. Herein, we present sensitive, highly rapid electrochemical measurement of airborne coronaviruses through condensation-based direct impaction onto antibody-immobilized, carbon nanotube-coated porous paper working electrodes (PWEs). Carboxylated carbon nanotubes are drop-cast on paper fibers to make three-dimensional (3D) porous PWEs. These PWEs have higher active surface area-to-volume ratios and electron transfer characteristics than conventional screen-printed electrodes. The limit of detection and detection time of the PWEs for liquid-borne coronaviruses OC43 are 65.7 plaque-forming units (PFU)/mL and 2 min, respectively. The PWEs showed sensitive and rapid detection of whole coronaviruses, which can be ascribed to the 3D porous electrode structure of the PWEs. Moreover, water molecules condense on airborne virus particles during air sampling, and these water-encapsulated virus particles (<4 µm) are impacted on the PWE for direct measurement without virus lysis and elution. The whole detection takes ∼10 min, including air sampling, at virus concentrations of 1.8 and 11.5 PFU/L of air, which can be due to the highly enriching and minimally damaging virus capture on a soft and porous PWE, demonstrating the potential for the rapid and low-cost airborne virus monitoring system. [Display omitted] • Paper working electrodes (PWEs) are used to detect whole coronaviruses in liquids and in air. • Carbon nanotubes are deposited on a fibrous paper to make porous PWEs. • The detection time of PWEs for liquid-borne coronaviruses is 2 min. • Airborne coronaviruses are water-encapsulated and impacted on PWEs for direct measurement without virus lysis and elution. • Detection of airborne coronaviruses takes ∼10 min including air sampling at 1–10 PFU/L of air. [ABSTRACT FROM AUTHOR]

Paper-based sensors fabricated using the pencil-on-paper method are expected to find wide usage in many fields owing to their low cost and high reproducibility. Here, hydrogen (H2) detection was realized by applying palladium (Pd) nanoparticles (NPs) to electronic circuits printed on paper using a metal mask and a pencil. We confirmed that multilayered graphene was produced by the pencil, and then characterized Pd NPs were added to the pencil marks. To evaluate the gas-sensing ability of the sensor, its sensitivities and reaction rates in the presence and absence of H2 were measured. In addition, sensing tests performed over a wide range of H2 concentrations confirmed that the sensor had a detection limit as low as 1 ppm. Furthermore, the sensor reacted within approximately 50 s at all H2 concentrations tested. The recovery time of the sensor was 32 s at 1 ppm and 78 s at 1000 ppm. Sensing tests were also performed using Pd NPs of different sizes to elucidate the relationship between the sensing rate and catalyst size. The experimental results confirmed the possibility of fabricating paper-based gas sensors with a superior sensing capability and response rate.

A simple paper-based sensor with electrochemical detection was first developed for rapid, selective, and sensitive determination of acetylcholinesterase (AChE). The screen-printed graphene electrode was used as working electrode providing sensitivity for the sensor. The amperometric detection of AChE is based on the determination of thiocholine (TCh) produced from hydrolysis of acetylthiocholine chloride (ATCh) by AChE. To detect TCh, the ATCh immobilized sheet was stacked onto the detection sheet using double adhesive tape, then samples of AChE were dropped on the back side of an ATCh immobilized sheet with only 1 min of incubation time. To avoid interference, glutathione (GSH), the potential of 0.5 V vs. Ag/AgCl was applied onto a graphene electrode and the current, which depends on AChE concentration, was measured. Under optimized conditions, the limit of detection (LOD) from the experiment of AChE determination was 0.1 U/mL with AChE concentration in range of 0.1–15 U/mL. The data correlated well with the data obtained using spectrophotometric method. The developed sensor had been successfully applied to detect AChE in blood samples. [ABSTRACT FROM AUTHOR]

A paper-based colorimetric sensor was fabricated using polymethylacrylic acid-templated silver nanoclusters (AgNCs) as a sensing probe for the detection of Fe 2+ and employing an ion exchange polymeric membrane as a coating layer. AgNCs were impregnated on a cellulose filter paper. The AgNC-impregnated filter paper was then lacquered with a hydrophobic membrane of polyvinyl chloride plasticized with o -NPOE incorporating a hydrophobic cation exchanger. Due to the ion exchange phenomenon at the membrane surface, Fe 2+ was extracted into the paper layer, and the bound Ag + on the AgNCs was reduced by Fe 2+ . The growth of the cluster could be observed by measuring the increase in the silver nanoparticle (AgNP) plasmon band. The maximum plasmon band on the paper at 447 nm increased as a linear function of the increasing Fe 2+ concentration. The proposed sensor was highly selective to Fe 2+ over Fe 3+ and other cations. The proposed sensor was optimized and could be used to detect Fe 2+ solution concentrations as low as 50 μM, with a wide working linear concentration range of 0.2–1.0 mM. The sensor was applied to determine Fe 2+ in iron supplement tablet samples with satisfactory results. [ABSTRACT FROM AUTHOR]

Three new pyrene-substituted benzimidazole-isoquinolinones ( 2–4 ) which are different in the positions and numbers of the pyrene groups are designed and synthesized via imidation-condensation reaction and Suzuki coupling. The structural elucidations are performed using standard spectroscopic techniques and computational calculations. These compounds exhibit interesting aggregation-induced emission enhancement (AIEE) in highly aqueous THF media and selective fluorescent quenching toward trinitrotoluene (TNT). The compound with two pyrene units ( 4 ) has the best selectivity toward TNT and has a quenching efficiencies ( K sv ) of 6.0 × 10 4 M −1 , which leads to the estimation of the detection limit of 0.25 ppm TNT. The paper-based sensors prepared from 4 can readily detect TNT in aqueous media by naked-eye observation at the concentration as low as 50 μM. It can also detect TNT vapor in closed chamber within 5 min of exposure at room temperature. [ABSTRACT FROM AUTHOR]

In this work, we demonstrated that enhancement of electrochemical current due to redox cycling could be accomplished by paper-based biosensor without any expensive microfabrication process. The paper-based sensor had layered structure to generate higher current than a conventional one. We took advantage of the fact that the paper thickness was micrometer-sized (180um), and it defined the distance between two electrochemical electrodes on both sides of the paper. Experimental results showed signatures of the redox cycling, where the electrochemical current from low concentration molecules could be arbitrarily increased by decreasing the distance between electrodes. Such a structure was advantageous for detecting target molecules at very low concentration, proposing a low-cost highly-sensitive biochemcal sensor. [ABSTRACT FROM AUTHOR]

Nanomaterials with different characteristics are offering many ingenious sensing approaches with interest for simple and disposable paper-based (bio)sensing applications. In this study, the colorimetric discrimination of alanine enantiomers is examined and, more importantly, AuNPs-embedded paper-based lab-in-a-syringe (LIS) device is developed as a sensing strategy. The LIS consists of two cellulose acetate membranes: the conjugate pad capturing the analyte and the detection pad signaling the presence of the captured analyte, both are sandwiched between reusable plastic filter holders connected to a disposable syringe. The principle of LIS assay is based on the enantioselective interaction occurring between the inherently chiral AuNPs and enantiomers in the first filter holder, which results in aggregation of AuNPs to give a distinct colour change from red to purple in solution and finally the aggregated AuNPs is kept on the detection pad through vertical-flow operation. AuNPs show an enantioselective recognition response toward L -Alanine and limit of detection (LOD) value is determined as 0.77 mM. In addition, we demonstrate the efficiency of the LIS device for detecting L -Alanine in human serum. The proposed LIS assay has some advantages such as useful for naked-eye observation, disposable, not time-consuming, inexpensive, no need of advanced instruments, easy to prepare and easy to handle. In the field, the approach which is the first demonstration of applicability of LIS device to show simple colorimetric enantioselective sensing of chiral species with a fast readout in less than 5 min is truly new and may have broad interest in enantiosensing of various chiral molecules. [ABSTRACT FROM AUTHOR]

A novel paper-based potentiometric sensor with an enhanced response for the detection of glucose in biological fluids is presented. The electrode consists on platinum sputtered on a filter paper and a Nafion membrane to immobilize the enzyme glucose oxidase. The response obtained is proportional to the logarithm of the concentration of glucose, with a sensitivity of −119±8 mV·decade −1 , a linear range that spans from 10 −4 M to 10 −2.5 M and a limit of detection of 10 −4.5 M of glucose. It is shown that Nafion increases the sensitivity of the technique while minimizing interferences. Validation with human serum samples shows an excellent agreement when compared to standard methods. This approach can become an interesting alternative for the development of simple and affordable devices for point of care and home-based diagnostics. [ABSTRACT FROM AUTHOR]

A facile and reliable colorimetric sensor in both solution and paper-based membrane was presented for the determination of nickel ions. Commercial organometallic reagent zincon had been incorporated within hollow ZnSiO 3 nanospheres to serve as hybrid ionophore, the blue color of which faded after successive addition of Ni 2+ due to competition and displacement of Zn 2+ in the ionophore. The selectivity was very good and can be further improved using Na 2 -EDTA as a masking agents which eventually becomes a co-ionophore. The detection solutions were further used to coat cellulose membrane modified with TiO 2 and silylating reagent to fabricate paper-based sensors for naked-eye detection of Ni 2+ . With optimum conditions selected, the assays in solution obtained a detection limit of 35.6 nM with linear range from 32 nM to 9.2 mM, while for the paper-based assays, the detection limit goes to 83 nM with linear range between 78 nM and 76 μΜ. Besides, the assay enable naked-eye differentiation of nickel ions down to 0.1 μΜ. Testing the present paper sensor on real river turbulent water shows a maximum 5% relative error, demonstrating the feasibility of present assays for real applications. [ABSTRACT FROM AUTHOR]

The development of paper-based sensors with the distance readout is particularly attractive for the point-of-care testing (POCT) of biomarkers. Herein, we report a paper-based sensor that can convert the viscosity change caused by the hydrogel degradation into the variance of water flow distance on the paper. The detection of alkaline phosphatase (ALP) and alpha-fetoprotein (AFP) is demonstrated as examples. Copper alginate (Cu-Alg) hydrogel can be formed when mixing the aqueous solutions of Cu2+ and alginate. The introduction of pyrophosphate ion (PPi) induces the gel-sol transition due to the high affinity between Cu2+ and PPi. However, in the presence of ALP, the gel-sol transition is prevented due to the hydrolysis of PPi by ALP. In these processes, the viscosity changes of the hydrogels are translated into the differences of the flow distances on the pH test papers. In addition, the tilt of the papers can increase the accuracy of detection. The detection of AFP is also fulfilled via the construction of the paper-based immunosensor using the ALP-linked AFP detection antibody. The developed paper-based sensors show excellent performance for the quantitative detection of biomarkers in real samples with remarkable selectivity. Overall, this convenient method has great potential in POCT applications. • A paper-based sensor is developed to detect ALP and AFP via distance readout. • It converts viscosity changes during the gel-sol transition into distance signals. • Copper alginate hydrogel can be degraded by PPi which is the substrate of ALP. • Paper-based immunosensor is built for the detection of AFP. • It shows excellent performance in the detection of ALP and AFP in human serum. [ABSTRACT FROM AUTHOR]

This study explores a smartphone-based spot detection framework for glucose in a rapid, simple, and affordable paper-based analytical device (PAD), which employs machine-learning algorithms to estimate various glucose concentrations. Herein, two different detection mixtures were chosen with chitosan (C) and without chitosan (WC) for the color change analysis. Being a biopolymer, chitosan improves the analytical performance of PADs when used with a chromogenic agent. Moreover, the influence of the illumination conditions and camera optics on the professed color of glucose strips was observed by choosing various illumination conditions and different smartphones. Hence, this study focuses on developing a framework for smartphone-based simple and user-friendly spot-based glucose detection (with a concentration range of 10-40 mM) at any illumination conditions and in any direction of illumination. Additionally, the combination of color spaces and machine-learning algorithms was applied for the signal enhancement. It was observed that the machine learning classifiers, cubic support vector machine (SVM) and narrow neutral network show higher accuracy for the WC samples, which are 92.7 and 92.3%, respectively. The samples with chitosan show higher accuracy for the linear discriminant and quadratic SVM classifiers, which are 94.1 and 93.9%, respectively. Simultaneously, cubic SVM shows ∼93% accuracy for both cases. In order to assess the performance of the devices, a blind test was also conducted. This study demonstrates the potential of the developed system for initial disease screening at the user end. By incorporating machine learning techniques, the platform can provide reliable and accurate results, thus paving the way for estimating the accuracy of the results for improved initial healthcare screening and diagnosis of any disease.

Paper-based flexible sensors are of great significance for promoting the development of green wearable electronic devices due to their good degradability and low cost. In this work, a paper-based wearable pressure sensor with a sandwich structure is proposed, which is assembled from a sensing layer printed with Ti 3 C 2 T x MXene ink, an interdigitated electrode printed in the same simple and economical way, and two polyethylene terephthalate films. The demonstrated paper-based pressure sensor exhibits excellent sensitivity in a wide pressure sensing range, as well as cyclic stability at a certain pressure. The sensor can be attached to the human body's surface to monitor various pressure-related physical activities. Using a self-designed mobile phone APP, the special pressure signals collected from the sensor can be transmitted and translated, and an intelligent and encrypted information transmission system can be established. Since only ordinary printing paper and Ti 3 C 2 T x MXene ink are used, the pressure sensor is easy to prepare, economical, and environmentally friendly, and it can be degraded by stirring in water without generating electronic waste. It can be foreseen that the proposed sensor shows bright application potential in the sustainable development of healthcare and human-computer interaction fields.

Heavy metal ion pollution problems have had a terrible influence on human health and the environment. Therefore, the monitoring of heavy metal ions is of great practical significance. In this paper, an electrochemical three-electrode system was fabricated and integrated on nitrocellulose membrane (NC) by the use of magnetron sputtering technology, which exhibited a uniform arrangement of porous structure without further film modification. This paper-based sensor chip was used for Cu2+ detection by square-wave stripping voltammetry (SWSV). Within the ranges of 5~200 μg·L−1 and 200~1000 μg·L−1, it showed good linearity of 99.58% and 98.87%, respectively. The limit of detection was 2 μg·L−1. On the basis of satisfying the detection requirements (10 μg·L−1), the integrated sensor was small in size and inexpensive in cost. Zn2+, Cd2+, Pb2+ and Bi3+ were also detected by this paper-based sensor chip with good linearity.

[Display omitted] • A novel PVP-capped NiNCs were simply synthesized. • A ratiometric luminescence sensor for TCs was developed based on NiNCs and Eu3+. • The sensor exhibited high selectivity and low detection limit for TC and OTC. • A portable paper-based device for on-site detection of TCs was developed. In this work, a ratiometric fluorometric sensor based on nickel nanoclusters (NiNCs)-europium complex (NiNCs-Eu3+) was constructed for the highly selectivity detection of tetracyclines (TCs) in water samples. In the presence of TCs, the blue fluorescence of the sensor NiNCs-Eu3+ was quenched at 430 nm and the characteristic red fluorescence of Eu3+-TCs appeared at 620 nm because of the combined help of inner filter effect (IFE) and antenna effect. Under the optimized conditions (100 mM Eu3+ (100 µL); temperature (25℃); reaction time (10 min), HEPES buffer solution (pH = 7.0)), the sensor offered a wide detection range of tetracycline (TC) and oxytetracycline (OTC) from 0.1 to 50 μM with the detection limit (LOD) of 25 nM and 21 nM, respectively. Moreover, the sensor was able to detect of TC and OTC in tap and lake water with high recovery rate (89.10%-97.60%). In addition, the portable paper-based sensor was constructed using filter paper embedded with NiNCs-Eu3+. The distinct fluorescent color of the paper-based sensor varied from bright blue to red against different concentrations of TC and OTC. These above findings demonstrated the potential for wide application of as-prepared ratio metric fluorescence sensor for visual detection of TCs in water samples. [ABSTRACT FROM AUTHOR]

In recent years, paper-based nucleic acid sensors have been demonstrated for the ability to detect DNA and RNA molecules extracted from viruses and bacteria. In clinical samples, these nucleic acids are mostly encapsulated in lipid membranes and need to be released before being analyzed using paper-based sensors. For the nucleic acid amplification tests (NAATs), it is also desirable to remove the interfering molecules that can inhibit the nucleic acid amplification. To achieve a field deployable NAAT, we report a portable sensor system that combines the thermolysis and paper-based NAATs to detect target RNA molecules carried by viral and exosomal nanoparticles. The sensor cartridge includes a lysis chamber with a pressure-controlled diaphragm valve, paper flow channels, and three paper-based NAAT reaction chambers to extract, transport, and detect nucleic acids respectively. A compact instrument was prototyped to automate the assay, collect fluorescence images of the nucleic acid amplification, and generate amplification curves for NAATs. The pump-free and paper-based sensor achieved quantitative analysis of influenza A virus (IAV) RNA and exosome microRNA within 1 h, with the lowest detect concentration of 104 TCID 50 /mL and 106 EV/mL for IAV and exosome, respectively. Owing to the advantages of easy storage, simple operation, and low cost, such as system has great potential to be used as a point-of-care test for in-field diagnosis of viral and bacterial infections. • Integration of thermolysis and nucleic acid amplification tests (NAATs) for quantitative detection of exosomal microRNA and viral RNA. • Pump-free and automated assays with RNA extraction, transportation, amplification, and detection. • Storage of NAAT reagents in paper for in-field applications • A fully automated, low-cost, and compact instrument [ABSTRACT FROM AUTHOR]