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A paper-supported aptasensor was constructed for total IgE using a luminescence resonance energy transfer (LRET) protocol with upconversion nanoparticles (UCNPs) as energy donors and carbon nanoparticles (CNPs) as energy acceptors. This is the first time that zero-dimensional carbon nanoparticles were used as energy acceptors for paper-based LRET assays. The π-π stacking interaction between the aptamer and CNPs brought the energy donor (UCNPs) and energy acceptor (CNPs) in close proximity, induced the LRET process on the surface of paper and thus led to the luminescence quenching of UCNPs. The introduction of IgE inhibited the energy transfer and hence recovered the luminescence of UCNPs in a concentration-dependent manner, as a result of the recognition between IgE and aptamer. This aptasensor can be used to detect IgE concentration in the range of 0.5–80 ng/mL in both buffer solution and human serum samples. The IgE concentrations measured by our method were well correlated to those obtained from chemiluminescence-based clinical assay. Owing to its simplicity and accuracy, the proposed sensor thus showed the potential of clinical applications. [ABSTRACT FROM AUTHOR]

Here, a disposable ultra-low cost and simple paper-based sensor (PBS) for on-site quantification of nitrite in environmental matrices is introduced. The PBS consists of a filter paper disc with 1.2 cm in diameter that is impregnated with the Griess reagents including 3-nitroaniline, 1-naphthylamine and hydrochloric acid with the optimal concentrations obtained by response surface methodology. After introducing 20.0 µL of a sample/standard solution to the sensor, nitrite reacts with the Griess reagents on the sensor and produces the red-pink colored azo dye after 30 min. By use of a digital camera, the image of the PBS is captured and then analyzed by partial least squares for nitrite determination. The use of a blank and sample image reduces bias from variations in ambient light and makes it possible to acquire and process images on-site. In order to demonstrate the potential impact of this technology in the environment monitoring, the device was successfully applied to the analysis of a series of water samples, including tap, rain and wastewater. The results of the method were validated by standard method. [ABSTRACT FROM AUTHOR]

This work reports a new type of disposable electrochemical sensor for the determination of bisphenol A (BPA). The working electrodes were fabricated by sputtering gold nanoparticles on commercial art paper and then modifying the gold layer with multi-walled carbon nanotubes (MWCNTs). The electrode in their intermediated and final stage was characterized by atomic force microscope, scanning electron microscope and electrochemical techniques. To perform electrochemical analysis, the resulting electrode was integrated with a homemade paper-based analytical device, which could also ensure the immobilization of MWCNTs on the electrode surface without any functionalization. The determination of BPA was investigated by linear sweep voltammetry (LSV). A wide linearity in the range from 0.2 to 20 mg/L with a detection limit of 0.03 mg/L (S/N = 3) was obtained. The between-sensor reproducibility was 5.7% (n = 8) for 0.5 mg/L BPA. The proposed sensor showed good resistance against interferences and was applied to detect BPA leached from real plastic samples with satisfying results. This disposable sensor is readily mass-produced and has been verified to serve as an attractive alternative to screen-printed electrodes for practical applications. [ABSTRACT FROM AUTHOR]

Two derivatives of fluorene containing salicylic acid groups are successfully synthesized by palladium-catalyzed coupling reactions and subsequent hydrolysis of salicylate esters. The compounds are characterized by various spectroscopic methods. In phosphate buffer (pH 8.0) solutions, these compounds are well soluble. They show maximum absorption wavelengths in the range of 304-330 nm and exhibit maximum emission wavelength around 420 and 430 nm with the quantum yields of 2.7 and 4.4 %, respectively. The compound with alkynyl salicylate groups ( 2) exhibits a selective fluorescence quenching towards Cu(II) and Fe(II) with a relatively similar sensitivity. The selectivity favoring Cu(II) over Fe(II) and other metal ions can be achieved upon the addition of 30 μM Triton X-100. The Cu(II) detection limit in solution phase is 1.47 ppb. The fluorescence signal recovery upon the addition of EDTA indicate a reversible complexation between 2 and Cu(II) ion. Fabrication of 2 on filter paper using a 50 μM solution in THF affords a naked-eye detection for Cu(II) and Fe(II) in aqueous media at picomole level. [ABSTRACT FROM AUTHOR]

Lead ion (Pb2+) is one of the most toxic and widely polluted heavy metal ions. Given the potential health risks and economic losses associated with Pb2+, the rapid detection of Pb2+ using fluorescent aptasensors is of significant importance in evaluating food safety. A rapid, facile and economic fluorescent aptasensor using convenient paper as the sensing substrate was designed to high-throughput detect Pb2+ in complex samples within about 45 min. The Pb2+ changed the conformation of FAM-modified Apt from a random coil to a stable G-quadruplex structure. And then Dabcyl-labeled cDNA was added to form double-stranded DNA with the Apt that did not form a G-quadruplex structure, resulting in a weak fluorescence due to the fluorescence resonance energy transfer (FRET). The fluorescent aptasensor showed a positive correlation with Pb2+ concentration, and a linear relationship was obtained in the range of 0.01–10 μM with LOD of 6.1 nM. In addition, this method has been successfully used for the determination of Pb2+ in water, soil and various foods containing complex substrates. Meanwhile, the high-throughput detection of Pb2+ has also reached an acceptable level. Therefore, this convenient strategy has potential application value for on-site rapid detection of Pb2+. A high-throughput paper-based fluorescent aptasensor was successfully developed based on the selectivity of aptamer and paper-supported analytical devices. It can detect Pb2+ in multiple real samples (water-soil-food). The strategy has great application prospect for on-site rapid detection. [Display omitted] • High-throughput paper-based FRET aptasensor was successfully developed and used for lightly Pb2+ detection. • Detection of Pb2+ was achieved in a range of 0.01–10 μM with the LOD of 6.1 nM. • The designed strategy was successfully used in the inspection of multiple real samples (water-soil-food). [ABSTRACT FROM AUTHOR]

Conventional airborne virus measurement usually requires appreciable sampling and detection times. Viral aerosols should also be collected or prepared in a liquid medium whose volume typically ranges from milliliters to tens of milliliters; hence, many sampling and detection steps need to be taken with the unit horizontal or immobile. Moreover, viral aerosols need to be sufficiently enriched, which makes real-time monitoring difficult. Herein, we present a near real-time enrichment and quantification system of airborne viruses that consists of a wet-paper-based electrochemical immunosensor with a gel electrolyte and a modified electrostatic particle concentrator. A small amount of phosphate-buffered saline flowed on the electrode, which resulted in sensor electrodes that are barely wet (covered in a thin buffer film measuring several micrometers) to ensure antigen–antibody interaction and the removal of non-target particles on the electrode surface. This system ensures that airborne viruses are highly enriched on the working electrode of the immunosensor, and it is possible to measure the MS2 virus particle concentrations every 10 min for 60 min stably and selectively against non-target airborne viruses and bacteria at horizontal and tilted measurement configurations. This system thus has the potential to be used in the real-time mobile monitoring of airborne microorganisms. [Display omitted] • Wet-paper-based electrochemical immunosensor under an electrostatic field is used to sample and quantify airborne viruses. • The sensors are barely wet to ensure antigen-antibody interaction and the removal of non-targets. • The enrichment ratio for airborne viruses is very high (> 108). • Airborne viruses are sampled and stably measured every 10 min for 60 min. • Airborne viruses are measured selectively against non-targets even when the unit is tilted. [ABSTRACT FROM AUTHOR]

In order to solve the problem of limited sensitivity of paper-based colorimetric sensors in the face of trace analyte, we verified the feasibility of improving the sensitivity by establishing a signal amplification method with high-abundance binding agent and confined nanopores/nanochannels through silica sol-gel treated paper fiber. The elevated abundance of agent serves to increase the absorption efficiency, and the silica nanopores/nanochannels generated on the fiber surface provide confined transport for molecules. By continuously flowing a large volume of sample vertically through the paper-based sensing membrane, the target could be enriched, increasing the signal accumulation, and realizing the sensitive paper-based sensing detection of target analyte. At the same time, we first proposed a novel pesticide detecting protocol based on nanozyme-catalyzed degradation. The chemical sensing of stable pesticides could be realized under the catalysis of nanozymes, and pesticides were degraded into easily detectable active small molecules. The limit of detection of imidacloprid was determined to be as low as 50 ppb with good selectivity. This novel paper-based pesticide sensor with high sensitivity holds great promise for future practical applications. [Display omitted] • A silica film-based paper signal amplification method was studied. • The nanochannels generated on the paper fiber could increase the abundance of captures and provide confined transport. • A brand new idea for pesticide detection based on nanozymes catalysis degradation was developed. [ABSTRACT FROM AUTHOR]

A deep-red emissive fluorescence probe PT-HBI with phenothiazine and 4-(4-hydroxybenzylidene)-1,2-dimethyl-1 H -imidazole-5(4 H)-one (HBI) motif was designed and synthesized for rapid detection of HClO in a fine ratiometric manner. PT-HBI exhibited remarkable ratiometric fluorescence response to HClO within 50 s, achieving a large Stokes shift (120 nm), high sensitivity (LOD = 0.31 μM) and excellent selectivity. Among the tested analytes, only addition HClO to PT-HBI enables result from a significant fluorescence change from red emission (λ max = 660 nm) to yellow emission (λ max = 540 nm) with the ratio of F 540 /F 660 displaying a 175-fold enhancement. In particular, PT-HBI was capable of monitoring exogenous HClO in Hela cells by ratiometric fluorescence imaging. Moreover, the probe PT-HBI has provided an extensive application for detection of HClO in real water samples and on paper-based sensor to detect HClO in different concentration. • Probe PT-HBI displayed a large emission shift (120 nm) and a 175-fold enhancement in the ratio of F 540 /F 660 for the detection of HClO. • Probe PT-HBI exhibited rapid response time (<50 s), high sensitivity and excellent selectivity in response to HClO. • Probe PT-HBI was successfully applied to exogenous cellular HClO imaging and lysosome co-location. • Probe PT-HBI has been successfully used to constructed paper-based sensing material to detect HClO in different concentration. [ABSTRACT FROM AUTHOR]

A novel paper-based colorimetric glucose sensor was proposed employing Prussian blue nanoparticles (PB NPs) as mimic peroxidase. The sensor was manufactured by spraying solution containing PB NPs, glucose oxidase and chromogenic agents into a paper, then coating the filter layer and spreading layer on the top. The layer-by-layer structure enabled the sensor detect glucose in whole blood, as well as elimination of the coffee-ring effect which ensure the performance. As a powerful alternative to natural peroxidase, PB NPs showed the mimic enzymatic activity well preserved in dry environment. The manufacture process of the sensor is easy to be industrialized. Under optimal conditions, the sensor exhibited a linear range from 2.5 mM to 25 mM for glucose in blood with satisfactory reproducibility (the coefficient of variant <4%), great storage stability (1 month at 45 °C) and excellent linearity compared with those commercial kits (R > 0.99). Coupled with a handhold device, the PB NPs-based test strip realized the goal of personal operation, user-friendly control, automatic readouts, and data storage, and able to link the Cloud, showing unique potential in clinical application, especially in community-level medical scenarios. [ABSTRACT FROM AUTHOR]

Since a vast majority of mercury ion (Hg2+)-responsive fluorescence probes suffer from significant false positive interference in practical applications originating from their fluorescence quenching mechanism, it is still a challenge to develop a fluorescence enhancement sensor for its assay. In this work, single hexagonal boron nitride quantum dots (BNQDs)-based probe was presented for Hg2+ assay in a fluorescence enhancement-based ratiometric manner. When Hg2+ was present, the BNQDs exhibited a new fluorescence emission peak located at 560 nm under 280 nm excitation, while the original blue fluorescence of the probe at 461 nm was quenched, realizing a ratiometric fluorescence response to Hg2+. Furthermore, it is found that fluorescence enhancement at 560 nm was dramatically suppressed under 365 nm excitation, the mechanism behind this phenomenon has been explored by experiments and relevant theories. In addition to high selectivity and detection sensitivity with LOD of 0.7 nM, the prepared probe successfully demonstrated its accuracy in Hg2+ detection in environmental water samples. Moreover, the probe could be adopted for paper sensor design, and an accurate and reliable cell phone-based portable platform was demonstrated for Hg2+ assay with LOD of 1.9 nM, suggesting its potential in point-of-care detection application. [Display omitted] • Excitation wavelength-dependent fluorescence probe was synthesized. • BNQDs displayed ratiometric fluorescence response to Hg2+ under 280 nm excitation. • As low as 0.7 nM detection limit could be achieved with high selectivity. • BNQDs-based paper sensor was demonstrated for point-of-care detection of Hg2+. [ABSTRACT FROM AUTHOR]

A paper-based chemiresistive sensor consisting of a single-walled carbon nanotubes network with electrodeposited gold nanoparticles (AuNPs-SWCNTs) was developed for the sensitive detection of As(III). A new sensing strategy was also proposed combined with a paper-based device for As(III) chemiresistive detection based on the electrochemical doping of As(0) on the AuNPs-SWCNTs channel. The proposed paper-based chemiresistive sensor was manufactured by wax printing and vacuum deposition using a water-based SWCNTs ink. The sensor sensitivity was enhanced by optimizing the volume of SWCNTs ink used in the formation of the sensing network in the paper and the sweep segment of cyclic voltammetry used for the electrochemical deposition of AuNPs. Additionally, the mechanism of the sensing strategy based on the electrochemical n-doping of As(0) to the AuNPs-SWCNTs sensing channel was investigated using a liquid-ion gated FET system. The As(III) concentration detected by the as-fabricated chemiresistive sensor is as low as 1.35 × 10−5 μM which is even lower than the maximum permissible concentration in drinking water defined by the WHO. In this paper, we provide a simple strategy for developing paper-based chemiresistive sensors with unique properties for practical As(III) determination. [Display omitted] • A paper-based chemiresistive sensor was developed for the detection of As(III). • The properties of the proposed semiconducting channel were characterized. • A sensing strategy was proposed without using any bio-recognition elements. • The key parameters for the sensor fabrication were optimized. • The concentration of As(III) can be detected as low as 0.001 ppb. [ABSTRACT FROM AUTHOR]

A novel, highly selective and sensitive paper-based colorimetric sensor for trace determination of copper (Cu 2+ ) ions was developed. The measurement is based on the catalytic etching of silver nanoplates (AgNPls) by thiosulfate (S 2 O 3 2− ). Upon the addition of Cu 2+ to the ammonium buffer at pH 11, the absorption peak intensity of AuNPls/S 2 O 3 2− at 522 nm decreased and the pinkish violet AuNPls became clear in color as visible to the naked eye. This assay provides highly sensitive and selective detection of Cu 2+ over other metal ions (K + , Cr 3+ , Cd 2+ , Zn 2+ , As 3+ , Mn 2+ , Co 2+ , Pb 2+ , Al 3+ , Ni 2+ , Fe 3+ , Mg 2+ , Hg 2+ and Bi 3+ ). A paper-based colorimetric sensor was then developed for the simple and rapid determination of Cu 2+ using the catalytic etching of AgNPls. Under optimized conditions, the modified AgNPls coated at the test zone of the devices immediately changes in color in the presence of Cu 2+ . The limit of detection (LOD) was found to be 1.0 ng mL −1 by visual detection. For semi-quantitative measurement with image processing, the method detected Cu 2+ in the range of 0.5–200 ng mL −1 ( R 2 = 0.9974) with an LOD of 0.3 ng mL −1 . The proposed method was successfully applied to detect Cu 2+ in the wide range of real samples including water, food, and blood. The results were in good agreement according to a paired t -test with results from inductively coupled plasma-optical emission spectrometry (ICP-OES). [ABSTRACT FROM AUTHOR]

• A micro-solvothermal method with high safety performance was proposed. • Boron doping caused changes in the microstructure and optical properties of CQDs. • A dual-emission B,N-CQDs was successfully prepared. • A paper-based sensor for the detection of Al3+ was prepared. In this work, dual-emission boron-nitrogen co-doped carbon quantum dots (B,N-CQDs) were synthesized by a micro-solvothermal method, and the prepared B,N-CQDs were used to detect Al3+ and Fe2+. The doping of B elements changed the microstructure of CQDs and endowed them with a dual emission property (F 430 and F 500). The detection basis of Al3+ and Fe2+ with the B,N-CQDs were attributed to the reverse changes of double emission peaks(F 430 and F 500) and fluorescence intensity changes of B,N-CQDs, respectively. Besides, the limit of detection (LOD) of Al3+ and Fe2+as low as 0.187 μM and 0.276 μM, respectively. In addition, B,N-CQDs were loaded onto filter paper to prepare a paper-based sensor for the detection of Al3+ with the feature of being more portable and practical. Meanwhile, B,N-CQDs presented outstanding performance in the detection of Al3+ in tap water and Fe content in ferrous sulfate tablets. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • A new Schiff base based compound 2 with AIEE and reversible piezofluorochromic properties was synthesized. • Compound 2 was used for the sensitive colorimetric and ratiometric fluorescent (LOD; 2.13 nM) detection of Cu2+ based on 2 –Cu2+ complex that was accomplished through intermolecular charge transfer (ICT). • Probe 2 –Cu2+ was further applied for the nanoscale (LOD; 2.02 nM) detection of glutathione (GSH) due to the preferential affinity of Cu2+ for GSH. • Successful use of paper sensor for the detection of Cu2+ and GSH in food validates the authenticity of the developed strategy. A new Schiff base 2 was rationally synthesized that exhibited aggregation induced emission enhancement (AIEE) and unique reversible piezofluorochromic properties. Probe 2 displayed a distinguished ratiometric fluorescent and colorimetric recognition of Cu2+ and further acted as a probe for the selective detection of glutathione (GSH) based on its 2 –Cu2+ complex. Firstly, probe 2 showed a highly selective fluorescence quenching response towards Cu2+ over its competing ions that was attributed to the intermolecular charge transfer (ICT) to paramagnetic Cu2+. Based on ratiometric fluorescence titration, the limit of detection (LOD) of probe 2 for Cu2+ was calculated as 2.13 nM. The complexation mode of probe 2 for Cu2+ was confirmed through 1H NMR titration, Job plot, computational, and DLS studies. Subsequently, the addition of GSH to probe 2 –Cu2+ complex immediately enhanced its fluorescence, and the emission of probe 2 was recovered which was ascribed to the displacement of Cu2+ by GSH and release of probe 2. The developed strategy for the identification of GSH was confirmed through 1H NMR titration experiments. Advantageously, probe 2 and probe 2 –Cu2+ complex coated paper sensors were successfully developed for the nanoscale sensing of Cu2+ and GSH in food samples. The detection of Cu2+ and GSH in food and actual water samples was adequately accomplished through a very effective dip-coating method. Finally, a sequential detection strategy was used to design logic circuits. [ABSTRACT FROM AUTHOR]

[Display omitted] • A water-stable new luminescent Cd(Ⅱ) coordination polymer (LCP 1) was constructed. • LCP 1 can be used as a fluorescent sensor for the detection of vanillin. • The limit of detection of vanillin was as low as 1.41 μM. • Paper-based sensor for visual recognition detection of vanillin was developed. Establishing a simple and rapid method for the detection of vanillin is necessary and significant. Herein, we synthesized a new 3D luminescent coordination polymer (LCP) [Cd(4-bmnpd)(BA)](1) {4-bmnpd = N , N ′-bis(4-methylpyridine-4-yl)-2,6-naphthalene dicarboxylamide, H 2 BA = 2[(4-carboxymethyl phenoxy) benzoic acid]} with a semi-rigid bis-pyridine-bis-amide and dicarboxylate mixed ligands under solvothermal condition. X-ray diffraction analysis revealed that 1 is a 5-connected 3D framework structure. The effect of solvents and pH on the luminescence intensity of 1 was investigated. LCP 1 was stable and the luminescence intensity almost unchanged in different solvents as well as in the pH range of 3–11. Fluorescence sensing studies showed that LCP 1 can detect vanillin by fluorescence quenching and has good anti-interference properties toward the substances commonly found in infant formula. The limit of detection of vanillin was as low as 1.41 μM. The fluorescence quenching mechanism of LCP 1 was analyzed in detail. In addition, we developed cheap paper-based sensor for visual recognition detection of vanillin. [ABSTRACT FROM AUTHOR]

This article reports a facile fabrication of a highly sensitive enzyme-free sweat glucose sensor over Whatman filter paper substrate employing a dual-step pencil and pen approach. Initially, two pencil-drawn electrodes (PDEs) are obtained on the filter paper through the manual abrasion of an 8B graphite pencil. Then, the sensing layer is drawn over one of the PDEs using a custom-made copper (Cu) nanoparticle ink (CuNP-ink) pen to form copper nanoparticle ink decorated PDE (CuNP-ink/PDE) which serves as a working electrode, while the other PDE is drawn with silver conductive ink (Ag-ink) pen to form reference electrode. The developed CuNP-ink/PDE is composed of 43.5% of metallic Cu nanoparticles, inducing a highly crystalline and conductive nature, thus promoting fast electron transfer during glucose electrooxidation. The developed sensor offers a sensitivity of 2691.7 μ AmM−1cm−2, a detection limit of 0.5 μM, a linear range of 1.2–40 μM, and a fast response time of ∼ 1.5 s. Besides, the sensor measurements are stable, reproducible, and selective in glucose sensing. Also, the reliability of the sensor is tested by comparing its performance with a UV–Visible spectrophotometer for proving its efficacy in sweat glucose detection. Experimental results evince the effectiveness of the designed sensor for glucose detection in human sweat. [Display omitted] • Sensor is fabricated by hand drawing a layer of copper nanoparticle ink over pencil-drawn electrode (PDE). • Ink deposition allows permeation of glucose sensitive copper nanoparticles. • CuNP-ink/PDE is composed of 44.5% of metallic Cu nanoparticles. • Sensor offers sensitivity of 2616.6 μA mM−1 cm−2 and detection limit of 0.5 μM. • Results are compared with UV–Vis spectrophotometric assay in real sweat samples. [ABSTRACT FROM AUTHOR]

With the increasing interest in the health benefits arising from the consumption of dietary products rich in antioxidants, there exists a clear demand for easy-to-use and cost-effective tests that can be used for the identification of the antioxidant power of food products. Paper-based analytical devices constitute a remarkable platform for such expedient and low-cost assays with minimal external resources but efforts in this direction are still scarce. In this work we introduce a new paper-based device in the form of a sensor patch that enables the determination of antioxidant activity through analyte-driven on-paper formation of gold nanoparticles. The principle of detection capitalizes, for the first time, on the on-paper nucleation of gold ions to its respective nanoparticles, upon reduction by antioxidant compounds present in an aqueous sample. The ensuing chromatic transitions, induced on the paper surface, are used as an optical “signature” of the antioxidant strength of the solution. The response of the paper-based sensor was evaluated against a large variety of antioxidant species and the respective dose response curves were constructed. On the basis of these data, the contribution of each species according to its chemical structure was elucidated. For the analysis of real samples, a concentration-dependent colorimetric response was established against Gallic acid equivalents over a linear range of 10 μM–1.0 mM, with detection limits at the low and ultra-low μM levels (i.e. <1.0 μM) and satisfactory precision (RSD = 3.6–12.6%). The sensor has been tested for the assessment of antioxidant activity in real samples (teas and wines) and the results correlated well with commonly used antioxidant detection methods. Importantly, the sensor performed favorably for long periods of time when stored at moisture-free and low temperature conditions without losing its activity thus posing as an attractive alternative to the assessment of antioxidant activity without specialized equipment. The use of the sensor by non-experts for a rapid assessment of natural products in field testing is envisioned. Importantly, we demonstrate for the first time that analyte-mediated growth of nanomaterials directly on the paper surface could open new opportunities in paper-based analytical devices. [ABSTRACT FROM AUTHOR]

A dramatic increase in pesticide usage in agriculture highlights the need for on-site monitoring for public health and safety. Here, a paper-based sensor combined with a wet system was developed for the simple and rapid screening of organophosphate (OP) and carbamate (CM) pesticides based on the inhibition of acetylcholinesterase (AChE). The paper-based sensor was designed as a foldable device consisting of a cover and detection sheets pre-prepared with indoxyl acetate and AChE, respectively. The paper-based sensor requires only the incubation of a sample on the test zone for 10 minutes, followed by closing of the foldable sheet to initiate the enzymatic reaction. Importantly, the buffer loading hole was additionally designed on the cover sheet to facilitate the interaction of the coated substrate and the immobilized enzyme. This subsequently facilitates the mixing of indoxyl acetate with AChE, resulting in the improved analytical performance of the sensor. The absence or decrease in blue color produced by the AChE hydrolysis of indoxyl acetate can be observed in the presence of OPs and CMs. Under optimized conditions and using image analysis, the limit of detection (LOD) of carbofuran, dichlorvos, carbaryl, paraoxon, and pirimicarb are 0.003, 0.3, 0.5, 0.6, and 0.6 ppm, respectively. The assay could be applied to determine OP and CM residues in spiked food samples. Visual interpretation of the color signal was clearly observed at the concentration of 5 mg/kg. Furthermore, a self-contained sample pre-concentration approach greatly enhanced the detection sensitivity. The paper-based device developed here is low-cost, requires minimal reagents and is easy to handle. As such, it would be practically useful for pesticide screening by nonprofessional end-users. [ABSTRACT FROM AUTHOR]

Abstract: In this report, a paper-based micro-calorimetric biochemical detection method is presented. Calorimetric detection of biochemical reactions is demonstrated as an extension of current colorimetric and electrochemical detection mechanisms of paper-based biochemical analytical systems. Reaction and/or binding temperature of glucose/glucose oxidase, DNA/hydrogen peroxide, and biotin/streptavidin, are measured by the paper-based micro-calorimeter. Commercially available glucose calibration samples of 0.05, 0.15 and 0.3% wt/vol concentration are used for comparing the device performance with a commercially available glucose meter (electrochemical detection). The calorimetric glucose detection demonstrates a measurement error less than 2%. The calorimetric detection results of DNA concentrations from 0.9 to 7.3mg/mL and temperature changes in biotin and streptavidin reaction are presented to demonstrate the feasibility of integrating the calorimetric detection method with paper based microfluidic devices. [Copyright &y& Elsevier]

Biogenic amines (BAs) are toxicological risks present in many food products. Putrescine is the most common foodborne BA and is frequently used as a quality control marker. Currently, there is a lack of regulation concerning safe putrescine limits in food as well as outdated food handling practices leading to unnecessary putrescine intake. Conventional methods used to evaluate BAs in food are generally time-consuming and resource-heavy with few options for on-site analysis. In response to this challenge, we have developed a transcription factor-based biosensor for the quantification of putrescine in beef samples. In this work, we use a naturally occurring putrescine responsive repressor-operator pair (PuuR- puuO ) native to Escherichia coli . Moreover, we demonstrate the use of the cell-free putrescine biosensor on a paper-based device that enables rapid low-cost detection of putrescine in beef samples stored at different temperatures. The results presented demonstrate the potential role of using paper-based biosensors for on-site testing, particularly as an index for determining meat product stability and quality.

We developed a paper-based colorimetric sensor for facile and cost-effective detection of Pb 2+ in drinking and environmental water samples. The Pb 2+ ion-selective optodes are fabricated by inkjet printing of ionophore, chromoionophore, and ion exchanger on cellulose paper. Pb 2+ in water samples induces deprotonation of the pH chromoionophore and changes the optode color, which is acquired and analyzed by a smartphone. The paper-based optode without any plasticizer or polymer has a dynamic range and selectivity comparable to those of traditional optodes using PVC polymer and/or plasticizer. Furthermore, the response time of the plasticizer/polymer-free paper-based optode is much shorter than those of plasticized PVC-based optodes on paper and glass (5 min vs. 15 and 50 min). Moreover, the plasticizer/polymer-free optode preserves the water-wicking capability of porous cellulose paper, allowing for the design of pump-free microfluidic devices. Chloramine, a widely used disinfectant in drinking water, was found to be a strong and generic interference species for heavy metal ion detection via ion-selective optodes. A fully inkjet-printed lateral-flow paper-based device consisting of a sodium thiosulfate-based chloramine elimination zone and a plasticizer/polymer-free sensing zone was designed for Pb 2+ detection in tap water disinfected by chloramine. The dynamic range of the Pb 2+ sensor may be shifted from the current 10 -6 to 10 -5  M to lower concentrations by using stronger ionophores, but this work lays a foundation for the design of paper-based heavy metal ion sensors without detrimental interference from disinfectants., (© 2022. Springer-Verlag GmbH Germany, part of Springer Nature.)

Cadmium ion (Cd (II)) is a highly toxic heavy metal usually found in natural water. Exposure to Cd (II) can produce serious effects in human organs such as Itai-Itai disease. Therefore, the maximum allowance levels of Cd (II) in drinking water and herbal medicines imposed by the World Health Organization (WHO) are 3 μg L−1 and 300 μg kg−1, respectively. In this work, nitrogen-doped graphene quantum dots (N-GQDs) as a fluorescent sensor for Cd (II) determination was developed in both solution-based and paper-based systems. N-GQDs were synthesized from citric acid (CA) and ethylenediamine (EDA) via the hydrothermal method. The synthesized N-GQDs emitted intense blue fluorescence with a quantum yield (QY) of up to 80%. The functional groups on the surface of N-GQDs measured by FTIR were carboxyl (COO−), hydroxyl (OH−), and amine (NH 2) groups, suggesting that they could be bound to Cd (II) for complexation. The fluorescence intensity of N-GQDs was gradually enhanced with the increase of Cd (II) concentration. This phenomenon was proved to result from the fluorescence enhancement (turn-on) based on the chelation enhanced fluorescence (CHEF) mechanism. Under the optimum conditions in the solution-based and paper-based systems, the limits of detection (LODs) were found to be 1.09 and 0.59 μg L−1, respectively. Furthermore, the developed sensors showed relatively high selectivity toward Cd (II) over ten other metal cations and six other anions of different charges. The performance of the sensor in real water and herbal medicine samples exhibited no significant difference as compared to the results of the validation method (ICP-OES). Therefore, the developed sensors can be used as fluorescent sensors for Cd (II) determination with high sensitivity, high selectivity, short incubation time (5 min). As such, the paper-based strategy has excellent promising potential for practical analysis of Cd (II) in water and herbal medicine samples with a trace level of Cd (II) concentrations. [Display omitted] • Novel paper-based N-GQDs sensor for trace-level probing of Cd (II) was developed. • User-friendly probing involved chelation enhanced fluorescence (turn-on) mechanism. • Sensor had high selectivity toward Cd (II) over 16 other metal cations and anions. • Sensor offers simple fabrication, high sensitivity, and short incubation time. • Sensor was validated by ICP-OES and applicable for real water and medicine samples. [ABSTRACT FROM AUTHOR]

• Ruthenium-based fluorescent probes with BSA as a scaffold have been prepared. • Detection of ethylene without reliance on organic solvents. • BSA hydrophobic pocket provides screening for analytes. • Achieved the paper-based sensor to monitor ethylene. • Achieved time-based fluorescence imaging of plant tissues. The detection of phytohormone ethylene is significant for extending the shelf-life of postharvest and controlling plant maturity. In this work, we report a biocompatible fluorescent probe for trace detection of the phytohormone ethylene based on the mechanism of the olefin metathesis reaction. The probe was prepared by simply incubating the ruthenium-based complex (FPRu) with bovine serum albumin (BSA). Then the structure of the probe was characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and circular dichroism circular dichroism (CD). The probe possesses a satisfactory detection of limit (LOD) for ethylene (1.17 ppm) and a decent linear range (4–250 ppm). Moreover, the probe-based test paper could also detect ethylene vapor with the naked eye, completing the visualization effect. Ethylene concentration monitoring at plant maturity could be done in solution and vapor. In particular, due to the protection of the hydrophobic pocket of BSA, the ruthenium-based fluorescent probe was also effectively applied to the fluorescence imaging in plant tissues. This work provides a new strategy to achieve highly selective detection and fluorescence imaging of ethylene in plant tissues without complicated operations and expensive instruments, which is of great significance for plant biology, agriculture and the food industry. [ABSTRACT FROM AUTHOR]

Surging interests in point-of-care diagnostics have led to the development of many lightweight and cost-effective paper-based sensors. Particularly, sensors using colorimetric readouts are considered highly advantageous because no additional detector or device is required for signal display. Herein, we introduce an electrochemically operated colorimetric sensor that can compensate for the disadvantages of traditional colorimetry, hence enhancing response time, reusability and color uniformity. On a single paper substrate, carbon/graphite paste was screen printed to form the working and counter electrodes, and Ag/AgCl ink was applied for the reference electrode. Prussian blue and Glucose oxidase were employed on the one of the carbon electrodes for the detection of analytes, hydrogen peroxide and glucose. For the colorimetric readout, indium tin oxide nanoparticles and polyaniline were consecutively introduced on the other carbon electrode, which is used as the counter electrode. The color change of electrochromic polyaniline could be clearly observed, and its application as a colorimetric sensor was demonstrated by the quantitative analyses of hydrogen peroxide and glucose. This paper-based electrochromic glucose sensor showed a short response time of 30 s and exhibited a detection limit of 126 μM for glucose. Along with its rapid and easy detection by incorporating the merits of electrochemical sensing and colorimetry, the paper-based electrochromic sensor could potentially contribute to the development of point-of-care devices by combination with portable power sources. • An electrochromic glucose sensor is formed on a paper substrate. • Polyaniline/ITO/carbon electrode is the counter electrode for electrochromic display. • Glucose oxidase/Prussian blue/carbon working electrode selectively detects glucose. • Concentrations of glucose are reflected by the color change of polyaniline. • Glucose concentration of real samples is analyzed and its reliability is confirmed. [ABSTRACT FROM AUTHOR]

[Display omitted] • A reliable diagnostic test presents based on the paper based system. • The three electrode system constructed on paper as platform. • The system shows sensitive behavior for MCF-7 cell in serum samples. Rapid diagnosis of circulating tumor cells (CTC) is of great importance towards patients' treatment. In this work, a simple paper-based sensing platform for label-free detection of circulating tumor cells in the breast cancer cells (MCF-7) is reported. This paper-based aptasensor (P-apt) consists of three-electrode system which constructed based on the manual screen-printed process through a patterned sticker on the paper. To manufacture the MCF-7 sensor, the working electrode of P-apt was functionalized with aptamers that are selective to mucin 1 proteins on the surface of MCF-7 cells. The performance of the prepared P-apt was investigated with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV). The linear range was between 20 and 1×106 cell mL−1, and the detection limit was 7 cell mL−1. Furthermore, the P-apt was able to detect MCF-7 cells in serum samples. The prepared P-apt with simple manufacturing process can potentially be developed to detect various circulating tumor cells. In conclusion, presented P-apt as a low-cost, rapid and robust sensor can be use to detect breast cancer metastases and follow up of metastases treatment. [ABSTRACT FROM AUTHOR]

Bacillus anthracis is a fatal biological warfare agent, with a mortality rate of almost 100% when the bacteria are inhaled. Therefore, developing a biosensor for rapidly detecting B. anthracis spores in the ambient air is critical. Conventional techniques tend to be time-consuming, labor-intensive, and require highly trained personnel, thereby preventing rapid onsite detection. Moreover, low-cost, stable probes are required for continuous monitoring of multiple airborne targets. This study presents a paper-based electrochemical peptide sensor for rapid, label-free, sensitive, and economical quantification of liquid-borne and airborne Bacillus subtilis spores, widely used nonpathogenic B. anthracis simulant spores. These sensors were fabricated using wax printing on Whatman filter paper along with stencil-printed carbon paste electrodes and Ag/AgCl ink, as well as subsequent peptide (NHFLPKV) immobilization. These low-cost quickly fabricated sensors were characterized by differential pulse voltammetry and could detect spore concentrations as low as 6.9 × 102 colony-forming units (CFU) mL-1 in 30 min with high specificity towards the spores and stability. Furthermore, spore concentrations were determined from air samples obtained using the BioSampler, which demonstrated that these measurements agreed with those using the plate-counting assay; therefore, the surface proteins of the spores were intact through sampling with the BioSampler. These paper-based peptide sensors can be used for onsite rapid and continuous airborne spore-monitoring systems. • The first study to use a peptide-immobilized electrochemical paper-based sensor for detection of whole bacteria. • Rapid, label-free, sensitive, and economical quantification of airborne B. subtilis spores is presented. • The sensors detected spore concentrations as low as 6.9 × 102 CFU/mL in 30 min with low-cost and fast fabrication. • Measurements of air-sampled spores agreed with those using the plate-counting assay. • The surface proteins of the spores were intact through sampling with the BioSampler. [ABSTRACT FROM AUTHOR]

New technological and scientific advances in the development of sensors and actuators demand the development of new devices to deal with recent problems and challenges in these new and emerging processes. Moreover, paper-based devices have tremendous potential for developing actuators as paper exhibits capillary transport and hygroexpansion due to swelling of the fibers when absorbing water. Therefore, this paper proposes a mini actuator that is based on a hygro-thermal-paper-based cantilever beam that is activated by means of a droplet of an aqueous solution in combination with a circulating electrical current to analyze its response. The contribution of this proposal includes the analysis of the flexural response of the mini actuator when it is tested by using two different solutions: distilled water and a water/alcohol solution. Additionally, four cases related to the droplet volume are studied and a statistical analysis of the bending responses is presented. The results achieved show that that water-alcohol solutions have a lower deviation in comparison with water only. Moreover, it is demonstrated that a specific change in the maximum displacement is obtained according to the volume and the type of solution. Thus, it is suggested that the response of the mini actuator can be tuned using different aqueous solutions. [ABSTRACT FROM AUTHOR]

Hypochlorous acid (HClO), the core bactericidal substance of the human immune system, plays a vital role in many physiological and pathological processes in the human body. In this work, we designed and synthesized a novel deep-red fluorescent probe TCF-ClO for the determination of hypochlorous acid through theoretical analysis. The results showed that probe TCF-ClO exhibited excellent characteristics of long-wavelength emission (635 nm), fast response (< 1 min), and low detection limit (24 nM). In addition, it had been successfully used for imaging of HClO in living HeLa cells. More importantly, the TCF-ClO composited paper-based sensing material was successfully constructed. The RGB/gray value was obtained from a mobile phone and computer, which could achieve the quantitative detection of HClO, with a linear detection range of 0-50 μM and a detection limit of 1.09 μM (RGB mode)/3.38 μM (gray mode). The function of the paper-based sensor extended from qualitative to quantitative detection of HClO, and it is expected to become a portable device widely used in the environmental area., (© 2022. Springer-Verlag GmbH Germany, part of Springer Nature.)

[Display omitted] • GQDs-AuNCs ratiometric fluorescent sensor was prepared by electrostatic interaction. • The sensor was used to detect Cu2+ and Cd2+ in scallops. • The paper-based sensor can be used for semi-quantitative detection of Cu2+ and Cd2+. • The detection limits are 4.12 nM for Cu2+ and and 943 nM for Cd2+, respectively. Based on the electrostatic interaction, we constructed a ratiometric fluorescence nanomixture of graphene quantum dots-gold nanoclusters (GQDs-AuNCs) for the quantitative detection of Cu2+ and Cd2+. When Cu2+ or Cd2+ was added into the reaction system, the fluorescence of GSH-AuNCs at 565 nm can be quenched by Cu2+ and enhanced by Cd2+ while the intensity of N -GQDs at 403 nm stayed constant. Under the optimized conditions, the fluorescence intensity ratio (I 565 /I 403) of the GQDs-AuNCs system was proportional to the concentration of Cu2+ and Cd2+ in the range of 8×10−8 mol/L-6×10−6 mol/L and 1×10−6 mol/L-4×10−5 mol/L, respectively, with detection limits of 4.12×10−9 mol/L and 9.43×10−7 mol/L, respectively. In the presence of Cu2+ and Cd2+, the paper-based vision sensor would produce visible fluorescent color changes, which can be used for rapid detection on site. The method has been successfully applied to the determination of Cu2+ and Cd2+ in scallops with satisfactory results. [ABSTRACT FROM AUTHOR]

Volatile sulfur compounds (VSCs) are considered as indicators of halitosis and precursors of periodontal diseases. Trace VSCs determination in-breath can provide noninvasive data for clinical diagnostics. In this work, a dual-mode colorimetric/ chemiluminescent (CL) paper-based device with a copper metal-organic framework (Cu-MOF) was developed for trace VSCs detection in exhaled breath. The Cu-MOF with 2,4,6-tris(4-carboxyphenyl)− 1,3,5-triazine (TATB) as a ligand was in situ synthesized on a paper surface by the solvothermal method. The porous Cu-TATB served as a gaseous confinement cavity to selectively capture and adsorb VSCs. When Cu-TATB reacted with VSCs, significant and sensitive color changes can be observed from aqua blue to greenish-brown. Also, the Cu-TATB had an excellent catalytic effect on the luminol-H 2 O 2 CL system. The CL intensity was inhibited by VSCs because of sulfur in VSCs reacting with copper active sites in Cu-TATB to form CuS. In this way, dual-mode detection methods for VSCs in breath were established with limits of detection (LOD) of 0.2 µM for colorimetry and 8 nM for CL. This portable, low-cost and stable paper-based sensor realizes accurate and reproducible signals read-out and shows potential feasibility as a prospect sensor platform for exhaled breath analysis. • A dual-mode colorimetric/ CL paper-based sensor was developed for VSCs detection in breath. • The Cu-TATB was in situ synthesized on paper substrates to form Cu-TATB@paper. • The Cu-TATB@paper can be stored in the air for 18 months with good precision. • The Cu-TATB selectively adsorbs VSCs to form CuS, realizing colorimetry with LOD of 0.2 µM. • The formation of CuS inhibited the catalytic effect of Cu-TATB-luminol-H 2 O 2 with LOD of 8 nM. [ABSTRACT FROM AUTHOR]

The lanthanide functionalized MOFs with an exceptional performance for early diagnosis of pheochromocytomas and paragangliomas was constructed as a multifunctional sensing platform based on postsynthetic modification strategy. [Display omitted] • It is the first report in designing a luminescent sensor to determine the biomarker 3-methoxytyramine. • The proposed sensor Eu3+@Al-MOF is applicable in the detection of 3-methoxytyramine in urine system with excellent performance. • The test paper creates a portable analysis tool for visual and selectivity detection of 3-methoxytyramine. As a main extracellular metabolite of dopamine, 3-methoxytyramine (3-MT) is considered a potential biomarker of pheochromocytomas and paragangliomas. Therefore, the determination of 3-MT is of great significance in the early diagnosis of disease. However, it remains challenging for detecting 3-MT in consideration of sensitivity and accuracy. Here, a luminescent Eu3+ functionalized metal-organic frameworks （Eu3+@Al-MOF）with ultra-high chemical stability was constructed based on postsynthetic modification. Such a rational design greatly enhances the fluorescence signal of Eu3+@Al-MOF and it is endowed with excellent properties as a luminescent sensor to detect 3-MT in urine system. Intriguingly, the strong red emitting derived from antenna effect was significantly interdicted upon addition of 3-MT through the interaction between 3-MT and the ligand. The proposed sensing system exhibited many appealing analytical performances, such as excellent selectivity, high sensitivity and quick response. Remarkably, the developed paper-based sensor not only provides a portable and reliable strategy for direct detection of 3-MT but also expands the application of visual analysis tools. This work represents the first effort in designing a luminescent sensor to determine the metabolite biomarker 3-MT level and provides a new method for biomedical analysis. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this work, green-emitting carbon quantum dots were successfully prepared through a facile one-step solid-state reaction method. The obtained green-emitting carbon dots (G-CDs) showed good fluorescence stability in NaCl aqueous solution and different pH values. Moreover, the G-CDs showed high sensitivity and selectivity for detecting hypochlorite by both fluorometry and colorimetry. Under the optimized condition, a highly sensitive detection of hypochlorite was established in the range of 0.2-100 μM and 10-150 μM for fluorescent and colorimetric methods, respectively. The corresponding limits of detection (LOD) were 0.0781 μM and 1.82 μM, respectively. Therefore, the G-CDs were successfully applied to determinate hypochlorite in actual water samples. In addition, a paper-based sensor loading with the G-CDs was also developed for rapid visual detection of hypochlorite. The results suggested that the G-CDs could be a promising candidate to detect hypochlorite., (© 2022. Springer-Verlag GmbH Germany, part of Springer Nature.)

Microcystin-LR (MC-LR) is a cyanobacterial toxin produced as a result of eutrophication in polluted water in warm weather conditions. The MC-LR could cause health problems in mammal organs such as the liver, heart, and muscle. Therefore, the World Health Organization (WHO) has stipulated a limit of <1.0 ng/mL in drinking water. Thus, detection and quantification are vital, but current techniques require complex and expensive offsite analysis. We have developed an inexpensive, sensitive, and field-deployable sensor based on bioactivated multiwalled carbon nanotubes (MWCNTs, diameter 20 nm) and micropore filter paper (0.45-μm pore size) for the detection of MC-LR. A specially designed DNA oligonucleotide (5-NH 2 -C 6 -AN 6) was used as the MC-LR targeting aptamer (MCTA). For bioactivation, MCTA was immobilized on the carboxylated MWCNTs via the formation of amide bonds. The bioactivated MWCNTs were deposited on the micropore filter paper by suction filtering. The detection of MC-LR in freshwater was possible within 1.5 h, achieved by measuring the changes in electrical resistance caused by the selective MC-LR and MCTA interactions. Despite suffering from some matrix effects, the detection limit of the sensor was 0.19 ng/mL for low-concentration MC-LR (≤0.5 ng/mL). This method is much cheaper (biosensor price: < $2.5) than liquid chromatography coupled with tandem mass spectroscopy analysis (ca. $50/sample) which is a standard method for MC-LR detection in a modern laboratory. Thus, this cheap and straightforward MC-LR sensor has applications for detection in remote locations. • Inexpensive, simple, and sensitive paper sensor for Microcystin-LR (MC-LR) detection. • MC-LR targeting aptamer (DNA oligonucleotide) - activated multi-wall carbon nanotubes biosensorl. • Acceptable limit of detection (0.19 ng/mL) for drinking water. • High selectivity to MC-LR among other MC congeners. • A special advantage to use in remoted location. [ABSTRACT FROM AUTHOR]

• CsPbBr 3 perovskite was synthesized as a photoluminescent signal transducer for detecting water content. • A facile water assay device was designed by using the perovskite as the sensing element. • The device is fit for rapid screening of large amounts of samples with high throughput. Excessive water in herbal medicines (HMs) can cause pests infestation, mildew, and decomposition of active ingredients during storage. The classical detection methods for water content are still unsatisfactory due to the requirement of sophisticated manipulation, long time consumption and usage of large amounts of toxic toluene. Herein, hot injection method was adopted to synthesize humidity-sensitive CsPbBr 3 perovskite, which was used to develop a novel photoluminescent (PL) sensor on a paper substrate for rapid detection of water content in HMs with a turn-off mode. Due to the strong ionic nature of CsPbBr 3 and the high aqueous solubility of CsBr, CsPbBr 3 perovskite is very unstable upon the exposure of water. The signal off can be attributed to the phase change from luminescent CsPbBr 3 to non-luminescent CsPb 2 Br 5 according to the results of XRD pattern analysis of CsPbBr 3 perovskite before and after water exposure. The PL intensity of CsPbBr 3 is linearly related to the water content within the range of 1–17 %. For Seutellaria baicalensis and Astragalus flavone , the detection limits are 0.75 % and 0.67 %, respectively. The results of recovery tests for the two HMs are within the range of 96.7–102.5 %. This work realizes the application of CsPbBr 3 perovskite in the field of pharmaceutical analysis, and shows the application potential for the control of water content in other types of samples sensitive to humidity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]