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Inorganic salt modified paper substrates were developed and utilized in microfluidic paper based sampling coupled with potentiometric ion sensors for the determination of heavy metals, such as Cd2+ and Pb2+. The application of paper based sampling without any paper substrate pre-treatment were characterized with super Nernstian response, while the paper substrates with inorganic salt pre-treatment were characterized with standard Nernstian response. The application of inorganic salt modified paper substrates was found advantageous when the cation of the salt in the conditioning solution was the same as the primary cations of the ion-selective electrodes (ISEs). A relatively high concentration of inorganic salts during pre-treatment of the paper substrates, namely Cd(NO3)2 or Pb(NO3)2 with subsequent measurement of cadmium(II) and lead(II), by Cd2+- and Pb2+-ISEs, respectively was needed to sustain Nernstian response of sensors. It was also found that paper substrates facilitate sorption of metal ions onto the paper substrates, with stronger binding strength given to Pb2+ over Cd2+. Moreover, the metal-paper interactions suggest concentration dependent sorption-desorption of metal ion, which can have direct limitations of the use of paper based analytics for the determination of low analyte concentration of heavy metals. Nanyang Technological University The authors would like to thank NTU-India Connect Research Internship Program for financial support of Ms Tharini Saravana Sundaram from Coimbatore Institute of Technology, India.

In this study, fabrication of microfluidic paper based analytical devices (μPADs) by microembossing was proposed. Different from the embossing methods described in the literature, the filter paper was embossed such that a protruded structure was formed, which subsequently became the channel after heating the wax at the backside of the filter paper. It was found that, when using the Whatman No. 3 filter paper, the embossing pressure needed to be larger than 50 kg/cm2 in conjunction with using a minimal 2-mm wide channel, and the wax heating time ranged from 15 to 45 s in order to obtain a reliable μPAD with repetitive and consistent results. In addition, there was no need of a hydrophobic material as the backside support. The glucose detection was demonstrated using the μPADs as fabricated and a linear relationship was obtained between 5 and 50 mM glucose concentrations. With the proposed method, the processing time of fabricating μPADs can be reduced within approximately 1 min.

Paper-based devices are always more gaining a relevant position in the field of sensors. The continuous demand for affordable, simple, sustainable, and portable devices, is making paper as the ideal basis towards the realization of analytical tools for the easy self-testing. In this work, we demonstrate, for the first time, the development of a disposable paper-based printed electroanalytical strip for reliable, rapid, and high-throughput detection of glutathione in blood. The detection is based on the thiol-disulfide exchange reaction, which produces a detectable compound easily oxidizable at a Prussian Blue/carbon black nanocomposite involving a favorable low-interference overpotential. This nanocomposite is mixed within a carbon-based conductive ink and successively screen-printed onto a wax-patterned filter paper. The employment of paper provides a reagent-free device, as a consequence of the reagents pre-loading within the testing area. After the experimental conditions have been optimized, glutathione has been detected up to 10 mM, with a detection limit of 60 μM, and a sensitivity of (0.102 ± 0.005) μA/mM. This sensor showed satisfactory repeatability (relative standard deviation equal to 10%, for detection of glutathione 1 mM), especially by considering the hand-made manufacturing process. The “real-world” applicability of this strip has been evaluated by quantifying blood glutathione at physiological levels and by recovery studies achieving satisfactory values.

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.

In the last years, the imprinting technology has raised attention and different polymers with artificial recognition sites towards a target molecule have been developed. In this article, a polymerization-free method ―which only requires the dissolution of nylon-6 polymer and its subsequent incubation with the template molecule― has been employed for the preparation of a molecularly imprinted polymer directly immobilized on filter paper, thus increasing the versatility and manageability of the new material. The molecularly imprinted paper-based analytical device (MIP PAD) has been coupled to a fluorimeter via a custom-built platform, which enables direct measurement of the fluorescence at the surface of the MIP PAD. Extraction efficiency of the MIP PAD was evaluated using quinine as model analyte, measuring the fluorescence directly at the MIP PAD surface after incubation on aqueous standards of quinine and soda drink, showing that the performance of the imprinted material was superior to that of the non-imprinted polymer. In addition, the selectivity of the MIP PAD versus analogous organic fluorescent molecules has been further evaluated using norfloxacin, observing that the molecular imprinting results in selective extraction of quinine. The performance of the method has been evaluated for quantitative analysis, achieving limits of detection (LOD) and quantification (LOQ) for aqueous standards of 0.37 and 1.24 mg L−1, respectively, while in the case of spiked soda drink the LOD and LOQ were 0.63 and 2.11 mg L−1, respectively. The reproducibility of the measurements was also evaluated, observing values within range 2.30 and 9.07% measured as relative standard deviation.

Early and precise diagnosis of cancer is critical for a better prognosis. Here, we describe a simple and cost-effective plasmonic paper as a surface enhanced Raman scattering (SERS) substrate in combination with magnetic separation for cancer screening. The plasmonic paper was fabricated by immersing plain filter paper into gold nanorod solution and the SERS property of the paper was evaluated using 4-mercaptobenzoic acid (4-MBA) and rhodamine 6 G (R6 G), which showed an enhancement factor (EF) in the range of 106–108. HT-29, a colorectal cancer cell line that highly expresses epithelial cell adhesion molecule (EpCAM), served as the target cells; non-EpCAM-expressing cells, namely fibroblasts and red blood cells (RBCs), were used as negative controls. Intrinsic SERS spectra of the target and control cells showed distinctive patterns on the plasmonic paper due to differences in their structure and components. A combination of principal component analysis (PCA) and k-nearest-neighbor algorithm (k-NN) was employed to analyze and distinguish the acquired HT-29 and fibroblast SERS spectra, demonstrating a diagnostic sensitivity and specificity of 84.8% and 82.6%, respectively, whereas the differentiation between HT-29 and RBCs SERS spectra showed a sensitivity and specificity of 96.4% and 100%, respectively. The magnetic separation was applied to capture the target cells from cell mixtures followed by PCA and k-NN analysis. The identification of the captured cells as cancerous cells from the HT-29 and fibroblast mixture indicated an accuracy of 83.7%, while that from a mixture of HT-29 and RBCs was 98.2%. Thus, the simple paper-based SERS substrate with the assistance of magnetic enrichment and multivariate analysis offers a potent new platform for cancer cell detection and screening.

In this communication, a novel surface modification method of the paper surface using 3-aminopropyltriethoxysilane (APTES) and gold nanoparticles is reported. This functionalized paper (flower-shaped paper-based microfluidic devices (F-PMD) for simultaneous assay of multiple analytes) is used for determination of total lipid profiling (TLP) in a single device. Here, we have determined total cholesterol along with low-density lipoprotein (LDL), high-density lipoprotein (HDL) and triglycerides (TGL). The LDL and HDL were estimated by erasing the non-HDL and non-LDL components with a help of precipitating reagents. Standard graphs prepared by the present paper-based method nicely correlates with the conventional method (calculated by Roche- COBAS C111 autoanalyzer) with Pearson Correlation Coefficient (R) of 0.903, 0.9727, 0.9939 and 0.9866 respectively for total cholesterol, LDL, HDL and triglycerides. The present assay device is intended to assist in the screening of individual’s lipid level for diagnosis of disorders involving high cholesterol level in the blood or for metabolic disorders associated with lipid and lipoprotein. As this is affordable, miniaturized, easily portable, self-testing and rapid, the present assay device can be used easily by pre-clinical, non-professionals, children in schools, labs, etc as a point-of-care diagnostics.

In this study, we designed a rapid colorimetric sensing system using competitive ELISA test on a microfluidic paper-based analytical device for the detection of ketamine, a frequently abused drug. Oral fluid was selected for the test to facilitate the collection of samples based on its advantages of low infection risk, noninvasiveness of sample collection, and decreased chance of sample adulteration. After optimization of the operation parameters, including reaction temperature, vibration washing time, reaction time, and antibody concentration, the resulting assay can be completed in as little as 6 min with a detection limit of 0.03 ng/mL. Moreover, we developed an Android smartphone app to analyze and measure the results of the test, further enhancing the test’s portability, and image recording and data transmission capabilities. In order to test the feasibility and performance of the optimized colorimetric assay, 90 oral fluid samples from drug abuse patients were tested. The paper-based platform features 90% sensitivity (confidence interval (CI): 76.34–97.21%) and 92% specificity (CI: 80.77–97.78%) for ketamine analysis. This competitive paper-based ELISA sensing system provides a rapid, convenient, sensitive, and high-throughput approach for drug monitoring.

A novel visual and fluorometric-sensor paper is prepared for the monitoring of atmospheric humidity and ethanol. The paper contains a combination of copper nanoclusters (CuNCs) and carbon dots (CDs), immobilized on a filter paper. This makes it a dual-emission system. The visual sensor shows bright fluorescence and has a diameter of ∼8 cm with uniform and homogeneous surface. The detection mechanism of the sensing paper is based on the fluorescence intensity variation of CuNCs in different environments. Desirable linear relationship is observed between fluorescence intensity ratio (red/blue) and humidity. Visible color transformation of this sensor paper is observed in the humidity range ∼40-80%, implying utility for easy and facile atmospheric humidity determination. Furthermore, due to the water and ethanol have different impacts on the spectrum, this paper-based fluorescent probe shows excellent response to ethanol. Therefore dual-emitting sensing paper, when suitably calibrated, shows promise for visual and fluorometric sensing platforms aimed at humidity or ethanol detection.

Pressure sensor, as one of the most common components in the sensing systems, has attracted plenty of attention in recent years with urgent needs. Herein, a highly sensitive flexible pressure sensor based on conducting polymer (CP)-coated paper is developed via an ultra-simple approach of synthesis and fabrication. This paper-based sensor displays an ultra-low detection limit as low as 0.3 Pa level, and outstanding sensitivity (∼2 kPa−1, P ≤ 75 Pa), which can rank the top among that of the reported pressure sensors. The advantages based on paper framework such as low cost, portability, and easy disposability further enhance its feasibility. Meanwhile, combining its rapid and precise response at room temperature and the recognized stability of PPy in the ambient environment, the sensor would ensure direct and continuous environment monitoring with low energy consumption.

Graphical abstract, Highlights • Detection of cytokine storm biomarkers with mobile biosensors. • Immunosensors are made solely of cellulose modified with antibody-decorated nanoparticles. • Turnaround time under 10 min. • Detection of biomarkers in respiratory samples allows monitoring local inflammation. • Compatible with decentralized health care schemes., Decentralizing COVID-19 care reduces contagions and affords a better use of hospital resources. We introduce biosensors aimed at detecting severe cases of COVID-19 in decentralized healthcare settings. They consist of a paper immunosensor interfaced with a smartphone. The immunosensors have been designed to generate intense colorimetric signals when the sample contains ultralow concentrations of IL-6, which has been proposed as a prognosis biomarker of COVID-19. This is achieved by combining a paper-based signal amplification mechanism with polymer-filled reservoirs for dispensing antibody-decorated nanoparticles and a bespoken app for color quantification. With this design we achieved a low limit of detection (LOD) of 10−3 pg mL-1 and semi-quantitative measurements in a wide dynamic range between 10−3 and 102 pg mL-1 in PBS. The assay time is under 10 min. The low LOD allowed us to dilute blood samples and detect IL-6 with an LOD of 1.3 pg mL-1 and a dynamic range up to 102 pg mL-1. Following this protocol, we were able to stratify COVID-19 patients according to different blood levels of IL-6. We also report on the detection of IL-6 in respiratory samples (bronchial aspirate, BAS) from COVID-19 patients. The test could be easily adapted to detect other cytokines such as TNF-α and IL-8 by changing the antibodies decorating the nanoparticles accordingly. The ability of detecting cytokines in blood and respiratory samples paves the way for monitoring local inflammation in the lungs as well as systemic inflammation levels in the body.

A simple, low cost, and lightweight three-dimensional (3D) primary alkaline Zn/Fe3O4 origami-battery fabricated from Whatman 1 paper was developed. The battery was created using printed wax to define the hydrophobic and hydrophilic barriers and active electrocatalytic materials for fabricating the electrodes. The boat-shaped 3D-battery consisted of two screen-printed Zn/Fe3O4 batteries connected in series that were activated with a small volume of aqueous KOH solution. The metallic electrodes were first characterized outside the folded battery using electrochemical techniques. Subsequently, the electrodes and separator were assembled together in a triangle-shaped battery resembling the main structure of the boat-shaped 3D-device and this triangle-shaped battery was further optimized. The addition of water triggered the activation of the 3D battery delivering a maximum current and power of 7 mA and 3 mW, respectively. The 3D-Zn/Fe3O4-battery displayed an OCP of 2.2 V and a maximum current and power of 22 mA and 7.5 mW, respectively, in the presence of 6 M KOH electrolyte. These results demonstrate the potential of 3D origami-based structures and Zn/Fe3O4 batteries as an alternative low cost energy source to power portable devices.

Palladium in the form of G-Pd nanocomposite possesses multiple enzyme mimicking activity. It catalyzes the oxidation of dopamine, glutamate, H2O2 and NADH through dopamine oxidase, glutamate oxidase, peroxidase and NADH oxidoreductase biomimetic activity. Kinetic studies were performed to calculate the catalytic parameters of G-Pd nanocomposite towards the substrates for a potential heterogeneous catalyst. Among them, glutamate oxidase activity was coupled with peroxidase activity of G-Pd for the detection of glutamate, in the form of a paper based sensor. Analytical performance of G-Pd nanocomposite catalyzed glutamate detection was characterized, and found to be linear in the range 0–400 μM and 400–8000 μM with limit of detection and quantification of 49.4 μM and 117.9 μM respectively. Results from the present study provides substantial progress in the practical application of nanomaterials as heterogeneous catalysts with multi enzyme mimicking property. G-Pd nanocomposite overcomes the complexity of enzyme based biosensors due to less number of reagents, better stability due to the absence of enzymes with mild storage conditions. Multi-mimetic catalytic activity of Pd will find applications in the field of analytical chemistry, biosensors, energy, environment and food safety.

A bacterial “systematic evolution of ligands by exponential enrichment” protocol was developed herein to identify nucleic acid aptamers capable of binding molecules from three common nosocomial and antibiotic-resistant bacteria: Acinetobacter baumannii, Escherichia coli, and multidrug-resistant Staphylococcus aureus. This high-affinity and high-specificity process featured three selection stages for screening of bacteria-specific aptamers, and the aptamers identified were integrated into a microfluidic system. The biotin-labeled aptamers were first bound to nitrocellulose membranes housed within the chip and then incubated with bacteria; a tetramethyl benzidine (TMB)-streptavidin (blue) color reaction was next exploited upon binding of secondary aptamers to primary ones, thereby permitting bacterial detection. This new dual-aptamer microfluidic chip possesses many advantages over its traditional-scale counterparts, such as faster detection times (35 min), smaller size (7.0 cm × 5.0 cm × 1.2 cm), higher specificity, and the capability to detect multiple pathogens simultaneously; it may therefore be promising for point-of-care bacterial diagnostics.

In analytical science, it is always of great scientific and practical significances to explore convenient, rapid, and reliable methods/devices to trace the H2O2 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 H2O2. 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 H2O2, providing a Kcat of 326.67 s−1 for H2O2 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 H2O2 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 H2O2 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.

A novel luminol derivative combining with azobenzene chromophore (LABD) and its functionalized filter-paper were developed and evaluated for double-wavelength visible pH detection for the first time. By virtue of the reversible protonation or deprotonation of amide or tertiary amino groups, the present sensors displayed high sensitivity and selectivity for reversible double-wavelength colorimetric response to pH at A433 and A529 respectively, with a wide linear recognition range between pH 4.0 and pH 11.0 and a visible color change from purple to bright yellow. After it was confirmed to be quite low-toxic to mouse fibroblast cells, LABD and its functionalized filter-paper were successfully applied to selectively detect pH in some real fruit juices with satisfied results.

Noncovalent side-wall functionalization of SWCNT with a pyrene bearing calix[4]pyrrole is reported. The modified SWCNT has been characterized by scanning electron microscopy, electron dispersive spectroscopy, and thermogravimetric analysis. This calix[4]pyrrole-decorated SWCNT based sensor was implemented on filter paper. The resistance change of SWCNT network upon VOC exposure was monitored in the chemiresistors approach. The calix[4]pyrrole-decorated SWCNT sensor on filter paper was found to show fast and selective sensing of acetone when compared with pristine SWCNT-on-paper sensor due to the binding of acetone by calix[4]pyrrole molecules. The sensor showed logarithmic response toward acetone in a concentration range between 20–500 ppm.

In this paper, a simple but efficient electrochemical biosensing platform was developed for ATP assay based on the preparation of paper-based working electrodes. In contrast to the electrochemical response of commercial glassy carbon electrode (GCE), improved voltammetric signals were achieved at the paper-based electrode. In particular, the surface contamination arising from oxidation product, a prominent challenge for ATP detection, was effectively eliminated by the paper-based system, endowing reliable analysis with high reproducibility. Moreover, the practical application value of the system was verified by assay of ATP in human serums, cancer cells, and normal cells with satisfactory results. Compared with the detection performances of traditional strategies, the advantages involved in the system were exhibited to be rapid, accurate, convenient, and inexpensive, which confer the proposed sensing platform promising for applications in public health as well as the fundamental research of molecular biology.

Point-of-care testing (POCT) is a simple, rapid and cost-efficient diagnostic method in settings where clinical resources are limited, developing countries for instance, and an essential tool for controlling infectious diseases. Previously, we developed a centrifugal-based separation-free visual detection system (SPIN-DNA) for human papillomavirus (HPV) DNA. However, this gel column-based detection system requires a centrifugation device to separate DNA-bound beads from free beads. It would be more feasible in resource-limited settings to employ a smaller force, such as a capillary force, for lateral flow assay. For this purpose, here we describe a paper-based method for separating amplified DNA that we developed. Visualization of amplified DNA to the naked eye using magnetic beads was achieved, without additional centrifugation, addition of capturing DNA probes, incubation time, or dilution. This simple visualized detection method of target DNA (PASS-DNA) was applied for HPV DNA in this study. The detection sensitivity of PASS-DNA was at least 101 copies/mL of HPV DNA, and the results of PASS-DNA showed perfect agreement with those obtained using the Roche Cobas 4800 HPV test (n = 30). No prolonged incubation times or washing steps are required with PASS-DNA; rather, the method involves one mixing step followed by loading the solution onto the paper. In addition, no absorbent pad is needed to provide a capillary driving force. This paper-based DNA detection system could accelerate the development of molecular POCT with further improvements and integration with sample preparation and DNA isolation and amplification in resource-limited settings.

Microfluidic paper-based analysis devices (μPADs) have undergone tremendous development in recent years and now provide a feasible low-cost alternative to traditional laboratory tests for the diagnosis of many common diseases and disorders. As such, they are of great interest and importance in developing regions of the world with a lack of medical resources and associated infrastructures. This review examines the advances made in microfluidic paper-based diagnostic technology in the past five years and describes the application of microfluidic paper-based assays to the detection of many common human diseases using 3 non-invasive samples sources such as saliva, tears and sweat. The review commences by introducing the basic principles of fluid transport in microfluidic paper-based devices. The structures and actuation systems used in common paper-based devices are then introduced and explained. A systematic review of recent proposals for the application of paper-based devices to the diagnosis of common human diseases is then presented. The review concludes with a brief discussion of the challenges facing the microfluidics paper-based diagnosis field in the coming years and the emerging opportunities for future research.

A dual-readout paper-based sensor with smartphone detection is developed for quantitative determination of penicillinase. The sensor is accomplished with the aid of penicillinase-mediated alginate hydrogelation, which changes both the viscosity and pH of the reaction solution. The solution is dropped on a mixed cellulose ester filter paper for sensing, which is coated with a color indicator, bromothymol blue-cetyltrimethylammonium bromide-alginate complex. Dual signals of the targets are easily achieved from the sensing spot on the paper since the modulations of viscosity and pH of the reaction solution would result in changes of diffusion diameter and color of the spot, respectively. A smartphone-based readout device is developed to record and analyze both the diffusion diameter and color signal for quantitative detection of penicillinase. The wide linear range (8.00 × 10−3 - 2.67 mU/μL for distance-readout and 8.00 × 10-2 to 2.67 mU/μL for color-readout) with low limit-of-detection (2.67 × 10-3 mU/μL for distance-readout and 2.67 × 10-2 mU/μL for color-readout) have been achieved using the dual-readout paper-based sensors. The proposed method is successfully applied for the detection of penicillinase in milk samples with excellent sensitivity and reproducibility, showing its potential value for analysis of complex real samples.

Paper-based microfluidics is the branch of microfluidics involving devices made out of paper, or other porous membranes, that wick fluids by capillary action. Paper-based microfluidic devices have several advantages over conventional microfluidic devices including simpler fabrication, lower cost, easier disposal, and the ability to operate without pumps or other supporting equipment. The most common application of paper-based microfluidic devices is in the development of point-of-care (POC) diagnostic devices, which could eliminate the need for costly and time-consuming laboratory-based analytical procedures. This review provides an overview of current methods of fabricating paper-based microfluidic devices, examples of applications of these devices, a discussion of their current limitations, and an outlook on their future.

We report here the fabrication of laminated graphene paper solid-contact ion-selective electrodes (SCISE) with performance characteristics equal to the state of the art SCISEs. Several identical and flexible graphene paper electrodes were first prepared from commercially available graphene paper with a standard pouch laminator in a single lamination step to a price of less than $1 per electrode. After that the solid contact consisting of polyaniline (PANI) doped with dinonylnaphthalene sulfonic acid and the K+-selective plasticized poly(vinyl chloride) membrane were deposited in two steps by drop casting on the surface of the graphene paper. The lamination of 10–15 identical graphene paper electrodes takes only one minute and we obtained fully functional graphene paper electrodes in ca. 40 min, including all manual pre-fabrication steps and 10 min of argon plasma cleaning to remove organic contaminants from the graphene surface. We found that plasma cleaning was necessary to improve the reversibility of the electron transfer of the Fe(CN)63−/4− redox couple. The laminated graphene paper K-SCISEs had a Nernstian slope of 56.9 ± 0.1 mV pK-1, a high E° reproducibility of ±4.4 mV (n = 6) still after more than 24 h, high selectivity against Na+, Li+, H+, Mg2+ and Ca2+, and they showed no water layer formation, light and oxygen gas sensitivity.· However, we observed a minor response to carbon dioxide (pH) because of the pH response of PANI. In this work, we show that laminated graphene paper electrodes are a cost-effective alternative for example to commercially available glassy carbon and platinum rod electrodes.

Electrochemiluminescence (ECL) is a powerful technique with the advantages of high sensitivity and a wide dynamic range. However, most of previous strategies based on ECL could only detect one analyte so that it is inevitable to investigate the selectivity and influences of interferences. Herein ECL integrated with paper chromatography was showed to be a potential platform for separation and detection of two environmental hormones (bisphenol A, BPA, and dibutyl phthalate, DBP). In our approach, carbon conductive tape was attached to an ITO glass and then modified with multi-walled carbon nanotubes (MWCNTs) and CdSe quantum dot (CdSe QDs) to fabricate the working electrodes for ECL detection. The synthesis of CdSe QDs was optimized based on their photoluminescence spectra and the ECL intensities. The modified electrode was then coupled in the paper-based analytical devices consisting of the filter paper modified with chitosan. Based on their quenching effect on the ECL emission of CdSe QDs, excellent linear relationships could be obtained between decreased ECL intensities and both the concentrations of DBP and BPA. Besides, DBP and BPA could be separated based on their retention time by tilting the filter paper for 30° and then detected with ECL. This study suggested that paper-based analytical devices could be potentially applied for separation and detection of multiple targets due to their ease of fabrication and convenient operation.

A flexible free-standing paper sensor was fabricated for the first time via in situ growth of Cu nanoflower on reduced graphene oxide (RGO) sheets towards amperometrically non-enzymatic detection of glucose. The low cost of copper (Cu) and graphene made the prepared sensor inexpensive in comparison with sensors made from noble metal catalysts. This Cu-RGO paper sensor exhibited high sensitivity and good selectivity when utilised in glucose sensing attributed to the synergistic effects of graphene paper and Cu nanoflowers. The combination of excellent catalytic activity, largely reactive surface area of Cu nanoflower and good conductivity of graphene paper enabled the sensor to perform well in glucose detection. The prepared glucose sensor holds a great promise for inexpensive flexible electrochemical non-enzymatic glucose sensors.

A concept of composable paper-based analytical devices (composable μPADs) is proposed. The key idea of composable μPADs is to develop a paper-based analytical platform that can be easily and quickly assembled into a desired architecture from standard pre-made paper-based parts. Here it is illustrated by the determination of several flavonoids, as an important class of natural polyphenol antioxidants. Two methods for the determination of flavonoids, at high (a two-part configuration) and at low (a three-part configuration including an additional preconcentration zone) concentrations, were developed. The principle for flavonoid detection was based on their ability to reduce silver nitrate to silver nanoparticles (AgNPs) producing an intense surface plasmon resonance band. The effects of main factors on the formation of AgNPs and the analytical response in proposed μPADs were studied. The concentration limits of detection were 2.3, 5.2, 14 μg mL−1 and the absolute limits of detection were 0.12, 0.26, 0.70 μg for quercetin, morin and dihydroquercetin, respectively. To determine flavonoids at low concentration level, a three-part configuration of a composable μPAD utilizing preconcentration on magnetic hypercrosslinked polystyrene was used. It was shown that such preconcentration increases 5 times sensitivity of morin determination. The method was applied to the analysis of hawthorn tincture and onion peel.

Malondialdehyde (MDA) is a mutagenic secondary product of the peroxidation of polyunsaturated fatty acids, often used as a target for the food oxidation state, whose determination is frequently performed by its slow reaction with thiobarbituric acid (TBA), dissolved in acidified aqueous-ethanol medium, to produce a pink colored MDA-TBA adduct. In this investigation, we have verified that this reaction also occurs in comparable times in a deep eutectic solvent (DES) such as choline chloride-malonic acid (ChCl-MA) displaying intrinsic acid properties. This finding has allowed us to set up a MDA detection procedure suitable for simply assembling colorimetric smart labels of possible use in packaged food. It is based on the use of paper discs soaked with a controlled concentration of TBA dissolved in the mentioned DES, which are exposed to the vapors released from controlled amounts of analyzed samples, whose temperature is of course a constraint for the formation of the pink colored adduct. It profits from the negligible volatility of DESs which makes them ideal candidates to replace volatile organic and aqueous media in colorimetric sensing and, moreover, it allows any preliminary sample extraction to be avoided. In order to avoid only rough MDA determinations by color comparison with standard discs, also a device allowing MDA quantification by smartphone-based reflectance measurements was assembled. The application of this procedure to real samples consisting of fried seed oil gave results in good agreement with values previously reported in literature for similar samples. Finally, it deserves to be underlined that this approach could be easily applied to the colorimetric sensing of other several analytes.

Porcine pseudorabies is one of the most serious infectious diseases influencing the pig-breeding industry. It inflicts enormous losses, underscoring the urgent need for the development of a clinical method for accurate and quick diagnosis of wild-type PRV infection. PRV-gE antibodies are an excellent indicator of PRV source, as their presence in the serum of individuals infected by wild-type PRV but not in those administered PRV gE-deleted vaccines enables distinction of wild-type PRV infection from vaccine immunization. For this study, we developed a miniaturized paper-based smartphone biosensor detection system (paper-biosensor) based on a blocking immunoassay for differential diagnosis of wild-type pseudorabies virus infection versus vaccination immunization. Latex beads (LBs) were used to label PRV, and the test line (T-line) was coated with PRV gE-mAb. The ambient light sensor of the smartphone was used to measure the transmitted light intensity from the T-line. Under optimized conditions, the paper-based smartphone biosensor achieved excellent sensitivity and selectivity. In tests of swine clinical samples (n = 101), the agreement between this method and the most widely used commercial gE-ELISA kit was 98%. Moreover, our method was rapid (15 min), low cost, and easy to operate, endowing it with great promise as an on-site differential diagnosis tool for PRV infection.

Antibiotics such as tetracycline (TET) and chloramphenicol (CAP) are widely used, but their abuse leads to antibiotic residue which is harmful to human health. To obtain sensitive and accurate TET and CAP detections, we design the paper-based antibiotic sensor (PAS) relying on the colorimetric indirect competitive ELISA. PAS can be simply fabricated only via wax printing and surface modification, making the cost of single detection rather cheap as ∼0.15 US dollar; and additionally, it can be fabricated and conserved in good stability. Moreover, the PAS supporting smartphone application with the functions of image recording, correction and analysis is also designed for quantitative antibiotic detections. Using a series of standard solutions, it is proved that the PAS could reach the extremely low limits of detection as 0.5 ng/mL and 0.05 ng/mL as well as the wide effective linear detecting ranges as 1∼103 ng/mL and 0.1∼100 ng/mL in the TET and CAP detections, respectively. Moreover, using milk and fish samples, it was also verified that the PAS could quantify the TET and CAP concentrations accurately even in the practical applications.

Highly selective azophenol-based chromogenic probe was synthesized that gave sharp color change in presence of cyanide. Based on colorimetric response of probe, a simple and economic colorimetric device has also been developed. Recently smartphones were employed for colorimetric analysis however there are number of limitations associated with it. Therefore, colorimeter was built using color sensor (TCS3200) and Arduino microcontroller for quantification of analytes using sensor coated paper chip. Initial colorimetric experiments revealed that sensor coated paper chip gave most linear response for change in the intensity of green component with change in the concentration of the cyanide. Thus, the device was calibrated using sensor coated paper chip and known concentrations of cyanide. It produced a best linear response over the range of 0−20 μM concentration of cyanide with R2 value of 0.9858 and limit of detection was calculated to be 0.86 μM which is lesser than WHO’s permissible limit of 1.9 μM. Finally, the applicability of device was successfully evaluated for quantification of cyanide concentration in spiked river water and food samples. Thus, the device can be successfully calibrated and used for quantitative analysis of other hazardous analytes such as cyanide through colorimetric sensing chips.

Electrochemical non-enzymatic detections of glucose and fructose were based on gold nanoparticles (AuNPs) onto graphene paper. Electrodes based on AuNPs have been obtained inducing dewetting, by thermal (furnace) or by laser, of sputter deposited 8 nm-thick Au layer onto graphene paper. The electrodes were characterized by Scanning Electron Microscopy, Micro Raman Spectroscopy, X-ray Diffraction, Rutherford back-scattering Spectroscopy and Cyclic Voltammetry. The main difference exhibited by thermal and laser dewetting processes lies in the size and shape of the resulting gold nanoparticles. Laser dewetting originates smaller particles than that obtained by thermal dewetting. The particles are almost spherical and mainly localized onto graphene nanoplatelets. The size of AuNPs is in the ranges 10–150 nm. Electrodes obtained by thermal process present gold nanostructures characterized by faceted AuNPs. Typical sizes are in the range of 20–40 and 200–400 nm. The electrocatalytic activity toward glucose and fructose oxidation in alkaline phosphate buffer solution are presented and discussed. Glucose was detected at a potential of 0.17 V (laser dewetting) or 0.19 V (thermal dewetting) vs SCE, which corresponds to the intense peak of two electrons oxidation. Fructose was detected at potential of 0.4 V vs SCE. Sensitivity up to 1240 μA mM−1 cm−2 for glucose detection was obtained. The resulting analytical performances for glucose and fructose detection are very promising since comparable to the actual state of art for nanostructured gold electrodes which are, however, produced by complex multi-steps wet processes and/or enzymes.

Carbon dots (C-dots), a new class of non-toxic carbogenic nanomaterials have attracted lots of attention. By using simple microwave heating of a biocompatible biological media such as potato-dextrose agar (PDA), the newly synthesized C-dots not only showed its potential application on the ultrasensitive and highly selective Lead (Pb 2+ ) detection in solution but also in paper based sensor strips. The detection limit (DL) of Pb 2+ was found to be 106–110 pM both in solution and on a paper based strips sensor. The fast kinetic data suggested on spot real time Pb 2+ detection. Finally, it showed the potential Pb 2+ sensing in human cells.

The strongly enhanced electromagnetic “hotspots” from the edges and tips of nanoplates are highly preferred to significantly promote the surface enhanced Raman scattering (SERS) activity. Here, Ag nanotriangles with a sharp edge of 20 nm were prepared by the reduction of AgNO 3 using N,N-Dimethylformamide (DMF) with the assistance of Polyvinyl pyrrolidone (PVP). By loading of Ag nanotriangles onto the filter paper, a flexible SERS substrate with good activity and reproducibility was obtained and the ultratrace detection of p -aminothiophenol (pATP) with a concentration as low as 10 −8 M was achieved. The sharp edges and tips boosted SERS activity was proved by the finite element method modeling of the plasmonic properties of Ag nanotriangles with different edge thicknesses (10 − 80 nm), Ag nanosheets with different tip acutances (squares and hexagons) together with Ag nanorods, nanocubes and nanospheres. The experimental and theoretical results confirmed that the interspacing created by filter paper plays an important role in enhancing the overall SERS activity. Furthermore, it is shown that the present silver nanotriangles-loaded filter paper SERS substrate can be employed to sensitive and selective detecting ultratrace military explosives picric acid (PA) of 10 −6 M.

A simple, cheap and effective functional paper has been fabricated to absorb and detect Cu2+ with good sensitivity and selectivity. Using commercial cellulose filter papers as substrate, Magneli phase structure of Ti6O11 was coated at the surface to serve as intermediate linker to modify N-[3-(Trimethoxysilyl) propyl] ethylenediamine for Cu2+ binding. Under optimized conditions, i.e. at 50 °C with electromagnetic stirring, the functional paper can adsorb Cu2+ with high performance and give clear color response using cupron as chromogenic agent. The concentration-dependent colorimetric response (ΔB value) matches well with the exponential linear curve under dynamic adsorption conditions, where the linear range was from 33.6 nM to 39.8 μM and the detection limit was measured to be about 33.6 nM. Measurement of a group of possible interfering ions and spiked real water samples demonstrated the good selectivity of the proposed paper. Furthermore, the performance can be further improved using microwave irradiation, where a lower detection limit of 16.8 nM was achieved within a wider linear range from 16.2 nM to 0.98 mM. The reaction time can be shortened to 20 min to reach final equilibrium and the saturated adsorption amount of Cu2+ for a single piece of paper (diameter = 1.0 cm) was as high as 4.5169 μg/cm2. The proposed functional papers would offer an effective alternative for potential adsorption and determination of copper ions in real applications.

This paper describes for the first time the use of a 3D pen to fabricate paper-based analytical devices. 3D pens are usually commercialized as “toy for children” and a kit containing four units and a handheld flashlight can be found at cost of US$ 10−15. The protocol involves the 3D pen drawing of the desirable layout on paper using an acrylic resin followed by a curing stage through a handheld flashlight. The barriers created with 3D pen revealed good chemical resistance when exposed to acid and alkaline solutions, surfactants and organic solvents, except ethanol. The proposed approach enabled the creation of zones with 2 mm diameter and channels with 3 mm wide. The analytical feasibility of the devices created with 3D pen was investigated through colorimetric measurements of clinically and environmentally relevant analytes using a free application for image capture and analysis. Considering the instrumental simplicity and the satisfactory analytical performance, we believe the reported approach exhibits potentiality to be implemented in laboratories or research centers with limited resources, opening possibilities to create the desirable devices for the point-of-need applications using only a pen and a flashlight.

Preserving the three dimensions (3D) fibrous structure of paper-based working electrodes during the addition of conductivity to the filter paper sheets could be challenging. Protecting the paper electrode porosity could easily lead a higher surface area and therefore, higher sensitivity for electrochemical analyte sensing. Herein, various shapes of Au dendritic nanostructures have grown on the paper fibers by an electrodeposition process from HAuCl4 precursor and in the short time of 4.0 min. The effect of different experimental parameters including deposition potential, concentration of HAuCl4 solution and paper thickness on the electrodeposition results were investigated and discussed. The electrochemical behavior of the different deposited Au nanostructures was compared. Finally, the paper-based electrode modified by Au dendrites showed an efficient catalytic role for the non-enzymatic oxidation of glucose. The parameters effecting the performance of the sensor were investigated and optimized. Finally, it was used as sensor for measurement of glucose. The obtained linear calibration curve for the designed biosensor showed a wide concentration range of 10.0 μM to 15.0 mM with the limit of detection of 0.6 μM.

Accumulation of excessive total serum bilirubin (TSB) induces hyperbilirubinemia and jaundice, especially in neonates. Diagnostics of TSB from bench to bedside is a clinical need requiring development of low-cost point-of-care analytical devices. This work describes a bilirubin paper-based analytical device (BiliPAD), for detection of TSB following diazotization by a colorimetric method. The chemistry of diazotization of TSB by using different accelerating reagents (dimethyl sulfoxide (DMSO) and caffeine-benzoate) on paper substrate is investigated with focus. It is found that paper assay is sensitive to the DMSO solvent system, which has interaction with the sample matrix; such sensitivity cannot be observed in solution reactions. Further study shows that the use of DMSO results in the precipitation of albumin on BiliPAD, interfering with the colorimetric evaluation of TSB; however, the caffeine-benzoate accelerating system produces a homogeneous colorimetric result. This work has also shown that the interferent tolerance of hemoglobin by the BiliPAD method for TSB assay is up to 100 mg/dL. The accuracy of the BiliPAD is verified by comparison with the accredited hospital laboratory method, obtaining a good linearity with the slope of 0.99 and the correlation coefficient R2 of 0.96. The 3D-BiliPAD device reported in this study also includes the design of 3D-printed wells, which is essential for realizing pre-treatment of serum samples using accelerating reagent and quantitative measurement of TSB without requirement of lab-based micropipettes.

Diagnostic assays made of microfluidic paper-based analytical devices have become the focus of considerable attention in developing countries because of simplicity, low cost, user friendliness, and no requirement for complex equipment, which is suitable to point-of-care testing. Considering the intrinsic shortcomings of nature enzyme, synthesis of artificial peroxidase mimics possessing improved catalytic activities relative to native enzymes is of considerable interest for the development of highly sensitive and stable sensor. In this paper, water-soluble molybdenum oxide quantum dots (MoOx QDs) were developed as highly effective biomimetic catalysts which were prepared by a facile ultrasonic-assisted hydrothermal method. The resulted MoOx QDs displayed peroxidase-like activity with high sensitivity and specificity, and were constructed for efficient colorimetric quantitative detection of H2O2 based on microfluidic paper-based device. As low as 0.175 μmol/L H2O2 could be determined at a linear range from 1 to 20 μmol/L. Moreover, this biosensing device was successfully applied for visual detection of H2O2 released from PC12 cells with the advantages of low cost, rapid response and portability, indicating that the designed sensor of H2O2 has great potential in biotechnology and clinical diagnosis.

Both bending stability and high sensitivity are two important problems to be solved in developing flexible sensors. The flexible sensor based on flower-like polyaniline coated filter paper (PCFP) was developed by spraying for the first time, achieving the non-contact and fast detection of nitroaromatic explosives. Hierarchical structure, high permeability of filter paper and the interaction between amino groups of PANI and nitro groups of nitroaromatic explosives contributed to the sensitive and fast response to 2, 4, 6-trinitrotoluene (TNT), picric acid (PA) and 2, 4-dinitrotoluene (DNT) at room temperature. The responses of PCFP to TNT, PA and DNT run up to about 237.7, 100.6 and 80.1%, respectively. The average response time and recovery time to TNT, PA and DNT are no more than 8.1 and 1.9 s, respectively. The theoretical limit of detection for TNT and PA is 0.094 and 0.029 ppb, respectively. More importantly, the special interlaced flower-like structures of polyaniline contributed to the good flexural stability. The response fluctuation to TNT, PA and DNT do not exceed 7% in the bending process and the decreases of responses are less than 7.9% after 2000 bends of 40°. In addition, the spraying method is suitable for economical, efficient and large-scale preparation of paper-based flexible sensors.

This study presents a simple, inexpensive and environment-friendly fabrication strategy for microfluidic paper-based analytical devices which can resist the penetration of surfactant solutions and organic solvents, by using water-based polyurethane acrylate via UV light curing. The filter paper’s barrier created using cured PUA could withstand surfactant solutions (10 wt%, CTAB, SDS and Triton X-100) and organic solvents (methanol, isopropanol, DMF, DMSO, etc). This is very useful for analyzing complicated biological samples on the microfluidic paper-based analytical devices. In addition, the expense of water-based polyurethane acrylate is very cheap (about $8/500 g) and PUA developer is water that is environmental-friendly. To further verify its advantage, we successfully demonstrated the proposed microfluidic devices for detection of E. coli targets in tap water and seawater via colorimetric analysis in a fast and convenient manner. Our results revealed that the linear response to E. coli BL21 was in the range of 104∼109 cfu/mL. The proposed method can effectively avoid the damage for the hydrophobic barriers from the solution even some aggressive liquids, and shows great potential in on-site analysis, environmental monitoring, and food safety.

Hydrogen sulfide (H2S) is a lipid-soluble gas signal molecule that plays a vital role in numerous abnormal pathological courses. Therefore, monitoring the changes of intracellular H2S level is undoubtedly of great scientific significance. Here, a red fluorescent probe SXR was reported, which is in view of the ICT mechanism. SXR showed rapid response (∼6 min), excellent selectivity and large Stokes shift (∼90 nm) to H2S. In addition, fluorescent imaging experiments further proved that SXR could detect endogenous and exogenous H2S in mitochondria, which provided a powerful tool for in-depth study of H2S function in various physiological courses. More importantly, a convenient visual paper-based sensor platform was designed, proving the favorable practicability for quantitative scanning and detecting of H2S with smart phone.

Microfluidic paper-based analytical devices (μPADs) have experienced rapid growth over the past decade due to their simple design, low cost, minimal sample requirement, and good sensitivity, selectivity and accuracy. While designed originally for point-of-care medical diagnostics, biological, and food safety applications, μPADs are now used increasingly for environmental monitoring purposes. This review provides a detailed overview of the μPADs developed over the past ten years for the environmental analysis of soil, air, ecology (pesticides) and river water. The review commences by introducing the fabrication techniques and detection methods used in μPAD technology. A detailed description of the main μPAD frameworks proposed in the past decade for environmental monitoring is then provided. The review concludes by examining the challenges facing μPADs for environmental monitoring and identifying probable avenues of future research.

Despite the tremendous achievements of existing sensing techniques, sensitive and portable sensing of organophosphorus pesticides (OPs) in a non-polluting manner remains a challenging issue. This work proposed a facile colorimetric paper sensor using γ-MnOOH nanowires (NWs) as a degradable nanozyme and 3,3′,5,5′-tetramethylbenzidine (TMB) as a chromogenic indicator for rapid and sensitive screening of OPs and acetylcholinesterase (AChE) activity. The sensing mechanism was based on the easy disintegration of the oxidase-like γ-MnOOH NWs into invalid Mn2+ ions by the selective reaction with thiocholine (TCh) produced by AChE and acetylthiocholine iodide (ATCh), triggering a remarkable activity loss of MnOOH nanozymes towards TMB oxidation. The concentration of OPs, as an AChE inhibitor, was measured by the changes in absorbance at 652 nm or blue color of oxTMB products. Such detection platform in solution state achieved relatively low limits of detection (LODs) for AChE activity (0.007 mU mL−1), and for two typical OPs, i.e., omethoate (0.35 ng mL−1) and dichlorvos (0.14 ng mL−1), respectively. Further, the portable assembly of such AChE-nanozyme tandem reaction on test paper performed well, reaching LODs of 0.1 mU mL−1 for AChE, 10 ng mL−1 for omethoate, and 3 ng mL−1 for dichlorvos, respectively. Meanwhile, this sensing system showed great selectivity and anti-interfering capacity in both solution and solid states, and worked well in real serum and vegetable samples. The abovementioned results, along with evidence of its good ecological sustainability, demonstrated the superiority of this well-performing platform for application in the food, environmental, and medical fields.

Developing sensors for food safety, soil analysis, water quality monitoring and healthcare often requires distinguishing between different species of bacteria. The most rapid, sensitive and specific method to identify bacteria is to analyse their DNA sequence, which comprises of disinfection and lysis of bacterial cells, amplification of the isolated DNA and detection of the amplified sequence. Seamless integration of these steps on a paper substrate is a big challenge. This problem is even more complex for mycobacteria as its thick cell wall structure impedes lysis and the high GC-content of the genome requires careful optimization of enzymatic denaturation. Here we successfully combine thermal lysis and loop-mediated isothermal amplification (LAMP) into a single reaction step on paper. We demonstrate our integrated assay by amplifying DNA from 100 CFU/mL of Escherichia coli (MG1655) and Mycobacterium smegmatis (mc2155) cells in 30 min on a paper substrate. We also confirm that E. coli and M. smegmatis can be completely disinfected on paper by heating at 60 °C for 5 min and 15 min respectively, making this assay safe and suitable for incorporation into diverse paperfluidic sensors for field use.

Inorganic nanomaterials applied as an efficient alternative to natural enzymes, because of their low cost and high stability. Herein, we report the facile synthesis of Co3O4-CeO2 nanosheets (Co/Ce ratio is 3/1) with synergistic properties as novel biosensing platform. Significantly, this new nanocomposite possessed the properties of both Co3O4 and CeO2 and exhibited intrinsic peroxidase-like activity, which could effectively catalyze oxidation of the substrate 333′,5,5′-Tetramethylbenzidine (TMB) by H2O2, which produced during oxidation of glucose by glucose-oxidase enzyme. Based on this finding, a novel paper-based analytical devices (PADs) biosensor is developed for sensitive and visualized detection of glucose. This biosensor platform is easily fabricated through equipment free method by using hydrophobic polystyrene solution. Thus, we utilized the application of Co3O4-CeO2 nanocomposite in the presence of glucose oxidase enzyme for glucose detection using the paper as measuring platform. The color change of the platform is recorded via a mobile camera and analyzed by smartphone application. This biosensor is showed to enable rapid and sensitive quantification of glucose in the concentration range of 0.005–1.5 mM with a limit of detection (LOD) of 0.21μM. The application of the proposed assay for colorimetric measuring of glucose in human serum samples was evaluated with satisfactory results. The good catalytic activity and low-cost make of Co3O4-CeO2 nanosheets as an effective biocatalyst developed for a wide range of potential applications in biotechnology, environmental chemistry and clinical diagnostics.

In this paper, two novel pH sensitive reactive dyes with intramolecular color matching based on the combination of anthraquinone and azo chromophores were design and synthesized. The incorporation of azo chromophore into anthraquinone structures enlarged visible absorption wavelength. Smart color-changing paper packaging sensors with pH sensitive chromophores were developed by printing patterns. Two reactive dyes on paper cellulose fibers could not be dissolved by environmental solvents due to covalent bonds. The printed patterns showed an obvious, sensitive and timely color change from green to purple for the paper printed with D-1, and from red to green for the paper printed with D-2 under different pH conditions. The maximum wavelengths of the color-changing samples shifted from 460 nm to 520 nm for D-1 and from 502 nm to 420 nm for D-2. The freshness experiment of cooked crabs indicated that cooked fresh crabs began to deteriorate after 1.5 h, and completely deteriorated after 4 h. The results showed that the printed patterns as the smart color-changing packaging sensors could be used in freshness judgment of the food. The color-changing mechanism of the printed patterns was revealed by 1H NMR and FT-IR spectra. The smart packaging sensors have potential application in the rapid identification of food freshness during delivery or before eating.

The development of paper-based diagnostics should provide facile fabrication and user-friendliness. A promising solution is to obviate hydrophobic barriers and display intuitive results. Herein, we report a novel “Pen-Writing” technique for providing hydrophobic barrier-free patterned biosensor fabrication and user-friendly bioassay operation. We functionalized the rollerball pen with reagent ink to directly write on-demand patterned paper biosensors of any design. The single-step glucose biosensor and multi-step immunosensor were prepared, and the multi-analyte detection capability was proved. Besides, the reagent ink-based rollerball pen was used to directly write bioassays on paper pre-treated with analytes. To perform continuous, high-throughput fabrication and analysis, we also proposed an automatic platform via combining the reagent ink-based rollerball pen and a desktop pen plotter. Our work provides an easy, affordable and generic solution to prepare customized barrier-free patterned paper biosensors and develop on-site bioassays in low-resource settings.

Double-sided electrohydrodynamic jet printing of circuitry using silver nanowires (AgNWs) and dispensing of high viscosity silver nanoparticles (AgNPs) is demonstrated for the first time in the fabrication of a two-dimensional array of electrodes (3D circuitry) on a single sheet of paper. The penetration of AgNW ink through the paper’s thickness by capillary imbibition allows for multilayer electrical access to connect the individual electrodes. This makes it an automated and efficient choice as the functional area of the device is conserved and the surface of the paper is preserved. This allows for two-dimensional droplet manipulation on a paper-based digital microfluidics (PBDMF) platform. Compared to printed circuit board technology, paper-based electronics offer a range of desirable properties: they are light weight, portable, economical, flexible, biodegradable, and the materials are abundantly available. The fabricated PBDMF chip is demonstrated to be a low-cost, fully reconfigurable, and disposable point-of-care diagnostic platform for the environmental sensing of pesticide using organophosphorus hydrolase enzyme through colorimetric detection via smartphone.

Human interferon-gamma (IFN-γ) plays an important role in diagnosis of tuberculosis susceptibility, which is one of the crucial health problems worldwide. Here, a novel platform of paper-based microfluidic device coupled with label-free electrochemical impedance immunosensor was developed for sensitive detection of human IFN-γ. Polyaniline (PANI) was deposited on a graphene (G) screen-printed paper electrode by using cyclic voltammetry. The modified electrode provides high electrochemical conductivity and surface area for antibody immobilization, which significantly enhances the sensitivity. A linear relationship between impedance and logarithmic concentrations of human IFN-γ was found in a range of 5―1000 pg mL―1 with a detection limit (LOD) of 3.4 pg mL―1. Interestingly, the PANI-G modified electrodes display 31-fold higher sensitivity compared to PANI-modified electrodes. The PANI-G modified electrode was successfully applied to the determination of human IFN-γ in human serum with a good reproducibility. Moreover, the system offers several advantages including low cost, low sample volume requirement, disposability, and rapid analysis relative to traditional methods, allowing the platform to be used as an alternative tool for human IFN-γ screening.

The screening of highly toxic chemical vapor is critical for environmental monitoring and human health maintenance. Herein, we reported a chemosensor (AC-HPD) for rapid and specific detection of highly toxic oxalyl chloride vapor, which adopted anthracene carboxyimide as a fluorophore and 3-aminopropanol as the reaction site. Upon coupling of oxalyl chloride with NH and OH groups, AC-HPD exhibited remarkable colorimetric and ratiometric fluorescence dual-mode responses towards oxalyl chloride via the intramolecular charge transfer effect. AC-HPD could determine oxalyl chloride with features of rapid response, excellent specificity, low detection limit (95 nM), as well as obvious color and fluorescence change. Furthermore, AC-HPD-embedded test strip was successfully fabricated and used as a flexible sensing device for rapid, sensitive and visible monitoring oxalyl chloride vapor.