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Paper-based sensors offer an affordable yet powerful platform for field and point-of-care (POC) testing due to their self-pumping ability and utility for many different analytical measurements. When combined with electrochemical detection using small and portable electronics, sensitivity and selectivity of the paper devices can be improved over naked eye detection without sacrificing portability. Herein, we review how the field of electrochemical paper-based analytical devices (ePADs) has grown since it was introduced a decade ago. We start by reviewing fabrication methods relevant to ePADs with more focus given to the electrode fabrication, which is fundamental for electrochemical sensing. Multiple sensing approaches applicable to ePADs are then discussed and evaluated to present applicability, advantages and challenges associated with each approach. Recent applications of ePADs in the fields of clinical diagnostics, environmental testing, and food analysis are also presented. Finally, we discuss how the current ePAD technologies have progressed to meet the analytical and practical specifications required for field and/or POC applications, as well as challenges and outlook. [ABSTRACT FROM AUTHOR]

In this work, a 3,4-ethylenedioxythiophene and tetrahydrophenazine based donor–acceptor–donor-type conjugated molecule was used for the first time as a turn-off fluorescence sensor for the detection of mercury (Hg2+) ions. Eleven metal ions (As3+, Cd2+, Cu2+, Fe2+, Fe3+, Hg+, Hg2+, K+, Ni2+, Se4+ and Zn2+) were tested and the proposed sensor exhibited almost 100% quenching in the fluorescence emission only when interacting with Hg2+ ions. Moreover, the sensor changed its color from yellow to blue upon Hg2+ addition, which indicates that the sensor is not only a fluorescence sensor but also a chromogenic sensor for Hg2+ detection. The solid state behavior was tested by preparing a simple paper-based sensor, and complete fluorescence quenching in the presence of Hg2+ was successfully obtained. The stoichiometry between the sensor and Hg2+ was found to be 1 : 1. The limit of detection (LOD) and the practical quantitation limit (PQL) were calculated to be 8.0 ppm and 26.3 ppm, respectively. The sensor was found to be more sensitive under acidic conditions, and the LOD decreased up to 3.0 ppm in the pH range of 1.6–2.3. [ABSTRACT FROM AUTHOR]

Lateral flow immunochromatographic assays (LFIAs) are analytical devices used to detect the presence of one or more target analytes in a liquid sample. While LFIAs are one of the simplest and inexpensive types of immunoassays, they consist of multiple components (sample pad, conjugate pad, membrane, absorbent pad, backing card) and materials, requiring time-consuming device assembly. Here, we report a unique lateral flow immunochromatographic assay constructed from a single piece of cellulose paper, which is fabricated via laser cutting. Compared with conventional lateral flow immunochromatographic devices, this single-layer immunoassay enables simpler and faster fabrication, while minimizing material consumption and overall device costs. For proof-of-concept, this device was used to detect Plasmodium falciparum histidine-rich protein 2 (PfHRP2), a biomarker for malaria infection, which could be detected at concentrations as low as 4 ng mL−1 by the naked eye with no cross reactivity with other common Plasmodium protein biomarkers. While offering similar speed and ease-of-use as conventional LFIAs with a higher detection sensitivity than existing LFIAs for PfHRP2 detection, this single-layer lateral flow immunoassay has the potential to improve malaria testing, as well as the detection of other important protein biomarkers for point-of-care testing. [ABSTRACT FROM AUTHOR]

The highly sensitive and selective determination of Escherichia coli (E. coli) in urine was achieved using a SYBR™ safe loop-mediated isothermal amplification (LAMP) method with a distance-based paper device. New primers set specific to multi-copy the 16s rRNA gene of E. coli were designed and used in this study. The detection sensitivity of these primers was higher than in related work and they could be incorporated with a low-cost paper-based device to quantify E. coli in urine at a concentration lower than 101 CFU mL−1. Regarding standard artificial urine, a linear range of a 10-fold dilution of E. coli concentration (105–100 CFU mL−1) with an R-square value (R2) = 0.9823 was observed directly using a fluorescent migratory distance of the 4 μL reaction mixture in the detection zone under blue light without the need for postreaction staining process. Based on the device, E. coli infection could be significantly categorized into 3 groups; none, light, and heavy levels, which is beneficial for UTI diagnosis. Hence, this paper-based device is suitable for use with the SYBR™ Safe-LAMP assay to semi-quantify E. coli, especially in resource-limited settings due to advantages of low cost, simple fabrication and operation, and no requirement for sophisticated instruments, as well as its disposability and portability. [ABSTRACT FROM AUTHOR]

This work reports the development of a rapid, simple and inexpensive colorimetric paper-based assay for the detection of the severe acute respiratory symptom coronavirus 2 (SARS-CoV-2) humanized antibody. The paper device was prepared with lamination for easy sample handling and coated with the recombinant SARS-CoV-2 nucleocapsid antigen. This assay employed a colorimetric reaction, which is followed by horseradish peroxidase (HRP) conjugated detecting antibody in the presence of the 3,3′,5,5′-tetramethylbenzidine (TMB) substrate. The colorimetric readout was evaluated and quantified for specificity and sensitivity. The characterization of this assay includes determining the linear regression curve, the limit of detection (LOD), the repeatability, and testing complex biological samples. We found that the LOD of the assay was 9.00 ng μL−1 (0.112 IU mL−1). The relative standard deviation was approximately 10% for a sample number of n = 3. We believe that our proof-of-concept assay has the potential to be developed for clinical screening of the SARS-CoV-2 humanized antibody as a tool to confirm infected active cases or to confirm SARS-CoV-2 immune cases during the process of vaccine development. [ABSTRACT FROM AUTHOR]

Coliforms are one of the most common families of bacteria responsible for water contamination. Certain coliform strains can be extremely toxic, and even fatal if consumed. Current technologies for coliform detection are expensive, require multiple complicated steps, and can take up to 24 hours to produce accurate results. Recently, open-channel, paper-based microfluidic devices have become popular for rapid, inexpensive, and accurate bioassays. In this work, we have created an integrated microfluidic coliform lysis and detection device by fabricating customizable omniphilic regions via direct printing of omniphilic channels on an omniphobic, fluorinated paper. This paper-based device is the first of its kind to demonstrate successful cell lysing on-chip, as it can allow for the flow and control of both high and low surface tension liquids, including different cell lysing agents. The fabricated microfluidic device was able to successfully detect E. coli, via the presence of the coliform-specific enzyme, β-galactosidase, at a concentration as low as ∼104 CFU mL−1. Further, E. coli at an initial concentration of 1 CFU mL−1 could be detected after only 6 hours of incubation. We believe that these devices can be readily utilized for real world E. coli contamination detection in multiple applications, including food and water safety. [ABSTRACT FROM AUTHOR]

Animal derived milk which is an important part of human diet due to its high nutritional value not only supports humans but also presents a growth environment for pathogenic bacteria. Milk may become contaminated with bacteria through udder infections or through contact within the dairy farm environment. Infections are treated with antibiotics, with β-lactams most commonly used in veterinary medicine. However, their frequent use leads to the emergence of β-lactam resistant bacterial strains, which causes difficulties in the treatment of infections in both humans and animals. Detection of pathogens as well as their antibiotic sensitivity is a pre-requisite for successful treatment and this is generally achieved with laboratory-based techniques such as growth inhibition assays, enzyme-linked immunosorbent assays (ELISA) or polymerase chain reactions (PCRs), which are unavailable in resource-limited settings. Here, we investigated paper-based analytical devices (μPADs) for the presumptive detection of Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) and their antibiotic resistant bacterial strains in milk samples. The μPADs were fabricated on filter paper using wax printing, and then impregnated with chromogenic substrates, which reacted with bacterial enzymes to form coloured products. Limits of detection of S. aureus and E. coli and their antibiotic resistant strains in milk samples were found to be 106 cfu mL−1. Enrichment of milk samples in a selective medium for 12 h enabled detection as low as 10 cfu mL−1. The paper devices tested on a set of 640 milk samples collected from dairy animals in Pakistan demonstrated more than 90% sensitivity and 100% selectivity compared to PCR, showing promise to provide inexpensive and portable diagnostic solutions for the detection of pathogenic bacteria in resource-limited settings. [ABSTRACT FROM AUTHOR]

Presently, the use of several pesticides has been continuously rising owing to the increase in the production of food materials to meet the requirements of the growing population of the world. The safety of food materials with regards to pesticides is an important health concern for people. With this aim, we have developed a smartphone-assisted paper-based sensor impregnated with citrate capped Cu@Ag core–shell nanoparticles (NPs) for selective determination of phenthoate pesticides in water and food samples. The mechanism for selective detection is based on the high affinity of phenthoate to interact with silver NPs present on the surface of CuNPs, which results in aggregation and a change in the color of the paper device. Furthermore, the proposed mechanism and interaction of phenthoate with Cu@Ag NPs was theoretically investigated by density functional theory (DFT) using Gaussian 16.0 software. The linear range for the determination of phenthoate was found in the range of 50–1500 μg L−1, with a limit of detection of 15 μg L−1, and a 92.6 to 97.4% recovery, and the interference studies demonstrated the selectivity for the determination of the target analyte from complex sample matrices. Finally, paper impregnated with Cu@Ag was exploited for the monitoring of the phenthoate pesticide in different water and food samples. The advantages of this paper-based sensor, coupled with a smartphone readout system, are that is it is user-friendly, easy-to-use, cost-effective, and can be applied at the sample source compared to sophisticated analytical instruments. [ABSTRACT FROM AUTHOR]

In the current study, noble metal nanoparticle functionalized MoS2 coated biodegradable low-cost paper sensors were fabricated for the selective detection of low concentrations of volatile organic compounds (VOCs). A MoS2 layer was grown on flexible cellulose paper by a two-step hydrothermal route and functionalized further by using noble metal nanoparticles like Au, Pd and Pt by a spray coating technique. MoS2 nanoflakes aggregating into a micro-flower morphology was confirmed by field emission scanning electron microscopy. X-ray photoelectron spectroscopy authenticated the existence of noble metal nanoparticles on the MoS2 surface. Sensors were then tested with seven different VOCs (≤100 ppm) of ketones, alcohols and aromatic hydrocarbon groups. Au–MoS2 and Pd–MoS2 showed inherent acetone and benzene selectivity with 50.5% and 45.7% responses, respectively, after exposure to 100 ppm concentrations at 50 °C. Paper sensors exhibited highly stable baseline resistances, less sensitivity towards humidity (∼75% RH) and an acceptable response/recovery time within 3–6 min at low temperature (50 °C). An additional effort was devoted to discriminating and recognizing different VOCs by using suitable classification algorithms utilizing four types of paper sensors, i.e. pure, Au, Pd and Pt MoS2. Principal component analysis and logistic regression schemes successfully classified five VOCs among the seven. Moreover, the sensing mechanism of the noble metal modified MoS2 paper sensor is discussed in light of the Schottky barrier model. [ABSTRACT FROM AUTHOR]

The enzyme-linked immunosorbent assay (ELISA) is one of the most commonly implemented clinical diagnostic tools for the detection and quantification of protein biomarkers. However, conventional ELISA tests require sophisticated infrastructure, expensive reagents, long assay time, and expertise for operation, which are not often easily accessible in resource-limited settings. Microfluidic ELISA-chip based point-of-care (POC) testing allows miniaturization and integration of complex functions that facilitate their usage in limited-resource settings. The current work demonstrates a simple, portable, low cost and equipment-free paper/poly(methyl methacrylate) (PMMA) integrated microfluidic ELISA-chip as a POC device with a visual distance-based readout for quantitative detection of clinical biomarkers. The integrated paper/PMMA ELISA-chip utilizes the movement of immunoassay complexes with magnetic beads by a permanent magnet in a PMMA part of the compartment. The target concentration is translated into a visual distance signal readout for quantitative detection of biomarkers in a μPAD. Because it does not require sophisticated instruments and has the added advantages of low cost, easy operation, and disposability with quantitative visual readout, the paper/PMMA ELISA-chip holds great promise for portable detection of target bioanalytes as a POC diagnostic tool in resource-limited setups. [ABSTRACT FROM AUTHOR]

Integrated valve microfluidics has an unparalleled capability to automate rapid delivery of fluids at the nanoliter scale for high-throughput biological experimentation. However, multilayer soft lithography, which is used to fabricate valve-microfluidics, produces devices with a minimum thickness of around five millimeters. This form-factor limitation prevents the use of such devices in experiments with limited sample thickness tolerance such as 4-pi microscopy, stimulated Raman scattering microscopy, and many forms of optical or magnetic tweezer applications. We present a new generation of integrated valve microfluidic devices that are less than 300 μm thick, including the cover-glass substrate, that resolves the thickness limitation. This "thin-chip" was fabricated through a novel soft-lithography technique that produces on-chip micro-valves with the same functionality and reliability of traditional thick valve-microfluidic devices despite the orders of magnitude reduction in thickness. We demonstrated the advantage of using our thin-chip over traditional thick devices to automate fluid control while imaging on a high-resolution inverted microscope. First, we demonstrate that the thin-chip provides an improved signal to noise when imaging single cells with two-color stimulated Raman scattering (SRS). We then demonstrated how the thin-chip can be used to simultaneously perform on-chip magnetic manipulation of beads and fluorescent imaging. This study reveals the potential of our thin-chip in high-resolution imaging, sorting, and bead capture-based single-cell multi-omics applications. [ABSTRACT FROM AUTHOR]

This work presents the development of a green paper-based analytical device (Green-PAD) array for pH detection. The array was obtained with natural dyes extracted from red cabbage (Brassica oleracea) and butterfly pea flower (Clitoria ternatea); a filter paper was used as a substrate. The RGB indexes of the PADs' colors were extracted from the pictures taken using a smartphone or using a specifically developed RGB detector (Arduino-based) to obtain RGB indexes not affected by the light and the photocamera sensitivity. Multi-technique chemometric models were developed for calculating the pH value, starting from the RGB triplet of each sensing PAD. A preliminary and explorative chemometric analysis with PCA (partial component analysis) and TWPCA (3-way PCA) was carried out. Partial least square regression, PLS, was then applied to correlate the color of the PAD's picture with the solutions' pH. Different solutions at various pHs, ranging from 1 to 13, were obtained by titrating orthophosphoric acid with standardized NaOH, and they were used to create PLS models. Some real samples were examined as a test set, and the results were validated with pH-meter measurements. The ability of the PLS to model the experimental data was satisfactory since a good agreement between the experimental and fitted pH values was obtained. The proposed PADs were prepared with natural dyes and filter papers, so they are completely biodegradable and eco-friendly. Their fabrication does not require toxic or expensive reagents and sophisticated equipment. Also, the developed RGB detector, built-up with low-cost components and recycled batteries, adds value making the measurement cheap, easy and feasible also by non-expert people. [ABSTRACT FROM AUTHOR]

The effective integration of nanomaterials with underlying current collectors is a key factor affecting the performance of nonenzymatic glucose sensors, where an inappropriate integration structure often leads to poor electron transport and instability. In this work, a seamless integrated electrode was constructed by the in situ immobilizing of a nickel-based metal-organic framework (Ni-MOF) on a three-dimensional (3D) conductive nickel foam (NF) for highly sensitive and durable glucose sensing. Facilitated by a rapid microwave-assisted reaction, a robust interfacial interaction between the Ni-MOF and the substrate was established through in situ conversion from nickel oxide (NiO). The fabricated Ni-MOF/NF electrode exhibits an excellent limit of detection (LOD) of 2.65 μM and an impressive sensitivity (14.31 mA cm-2 mM-1) within the linear range (4-576 μM), which is significantly boosted compared with that of an electrode prepared by a typical drop-casting method (3.56 mA cm-2 mM-1 in 4-1836 μM). Characterization and electrochemical tests reveal that this integrated structure on the one hand contributes to fast electron transport and thus has enhanced sensitivity and on the other hand leads to exceptional durability with its structural integrity maintained under bending, shaking, and ultrasonication. Moreover, this seamless integration method was also employed to immobilize the Ni-MOF converted from the pre-chemically deposited NiO layer on another type of substrate, 3D carbon paper (CP), demonstrating the versatility of this facile strategy in creating diverse electrochemical electrodes for applications beyond glucose sensing. [ABSTRACT FROM AUTHOR]

Plasmonic paper has received attention as a surface-enhanced Raman scattering (SERS) substrate due to several advantages. These include low cost, high rates of sample collection, high physical flexibility and relative ease of fabrication. Here, we report the development of a plasmonic paper based on a combination of graphene oxide (GO) and gold nanorods (AuNRs) on cellulose substrates using a simple layer-by-layer immersion method. Rhodamine 6G (R6G) was selected as the probe molecule to determine the SERS efficiency. The GO–AuNR plasmonic paper composite demonstrated superior signal intensity compared to that of the paper substrate coated only with AuNRs. The signal enhancement was due to synergism between the electromagnetic enhancement (EM) originating from the AuNRs and the chemical enhancement effect (CE) originating from GO. To optimize the fabrication process, the first step was accomplished by varying the GO concentration during the paper pretreatment step. This was followed by determining the number of immersion cycles required for optimal absorption of AuNRs. The highest SERS signals derived from R6G molecules indicated that the 2 mg mL−1 GO pretreatment condition followed by two immersion cycles into the AuNR solution proved to be the best condition for plasmonic paper fabrication. The enhancement factor (EF) of the plasmonic paper was calculated to be 4.56 × 107 and the limit of detection (LOD) for R6G analysis was as low as 0.1 nM. Moreover, the plasmonic paper was tested in the detection of Mitoxantrone (MTX), an anti-cancer drug. We found that the LOD for detection of MTX was on the order of 5 μM. This simple and cost effective plasmonic paper offers an alternative SERS substrate which has potential in trace analysis and drug monitoring. [ABSTRACT FROM AUTHOR]

Herein, a simple approach is demonstrated for the fabrication of a paper-based flexible symmetrical supercapacitor as an energy saving device with composite functional materials of nickel nanoparticles (Ni NPs) and polypyrrole (PPy). Specifically, an Ni@PPy nanocomposite was synthesized through a two-step process involving the growth of Ni NPs, followed by pyrrole polymerization on a paper substrate. The paper fabricated with Ni@PPy showed an electrical conductivity of 105 S cm−1. The paper-based flexible supercapacitor device configured with Ni@PPy/electrolyte/Ni@PPy was evaluated for electrochemical performance, and it showed a good specific capacitance of 544 F g−1 at 1 A g−1. A better specific energy of 48 W h kg−1, specific power of 400 W kg−1, and good cycling stability (68.3% capacitance retention after 3000 cycles at 5 A g−1) were obtained for the paper-based flexible supercapacitor compared with other reported polymer-based nanocomposite materials. The obtained results suggest that the fabricated paper-based supercapacitor is inexpensive, sustainable, highly efficient, portable and flexible for a wide range of electronic applications. [ABSTRACT FROM AUTHOR]

This study describes a microfluidic paper-based analytical device (μPAD) for separating plasma from whole blood and measuring glucose concentration. A two-dimensional μPAD was fabricated by wax printing, using chromatographic paper and was functionalized by the incorporation of chitosan into the plasma separation zone. Red blood cells were isolated from whole blood in the plasma separation zone consisting of a chitosan polymer structure and a wax barrier. The separated plasma, with buffer, was injected into the test zone to cause a color change. The colorimetric results were digitized with a scanner, and the concentration was calculated using a calibration curve. This μPAD, which separated the plasma from whole blood without a separation membrane, will be useful for constructing point-of-care testing devices that detect analytes in small sample volumes. [ABSTRACT FROM AUTHOR]

Fibrinogen (FIB) plays a key role in blood coagulation and thrombosis and its concentration in blood can directly reflect health conditions, thus efficient detection of FIB would benefit the treatments of certain diseases such as liver and heart diseases. However, there is a lack of sensitive, simple, rapid and cheap FIB detection device currently, in lieu of expensive and sophisticated approaches in laboratories. Here, we propose a novel plasma separation and FIB detection platform based on a microfluidic paper-based analytical device (μPAD). It is the first time that dielectrophoretic (DEP) force is combined with capillary force on paper for plasma separation, and the separation efficiency of plasma reaches about 95%, ensuring reliable downstream FIB detection, for which we also propose a new method called the resistance-fibrinogen detection (RFD) method. It not only avoids the use of large-scale instruments for detection, but also possesses high precision and simplicity. The method is found to be reliable in FIB detection for various concentrations ranging from 127.0 to 508.0 mg dL−1. Moreover, the results obtained from the proposed μPAD show an excellent agreement (R2 = 0.9985) with those obtained from an automatic coagulation analyzer with natural human blood samples. Overall, the proposed platform provides a low-cost and reliable approach for FIB detection, especially for clinical use in resource-limited areas. [ABSTRACT FROM AUTHOR]

We report for the first time a highly sensitive and rapid quantitative method for the detection of Salmonella Typhimurium (S. Typhimurium) using a conductive immunosensor on a paper-based device (PAD). S. Typhimurium monoclonal antibodies (MA) were first immobilized on a paper-based device and then captured by S. Typhimurium. After an immunoreaction on the device, the polyclonal antibody-colloidal gold conjugate (PA-AuNPs) was dropped to bind with S. Typhimurium. After a complete sandwich reaction, a dark red color appeared on the paper-based device, which can be observed by the naked eye for a rapid screening test. The electrical conductivity of PA-AuNPs between the screen-printed electrodes on the paper-based device was also measured for an accurate quantitative analysis. The electrical conductivity correlated well with the concentration of S. Typhimurium, which was controlled by the amount of S. Typhimurium attached to the polyclonal antibody-colloidal gold conjugate. The device showed a linear correlation for the concentration of the S. Typhimurium in the range of 10–108 CFU mL−1 in a logarithmic plot, with an R2 value of 0.9882 and a limit of detection (LOD) as low as 10 CFU mL−1. This simple, highly sensitive, and rapid method for the S. Typhimurium detection was successfully performed within 30 min, and it can be developed into small portable measuring devices in order to facilitate preliminary screening tests. [ABSTRACT FROM AUTHOR]

A microfluidic paper-based analytical method is developed for determination of Hg(II) using gold nanoparticles functionalized with N,N′-bis(2-hydroxyethyl)dithiooxamide. Fabrication of the μPAD in this work is free of any expensive equipment and could be performed by untrained personnel. The color of modified AuNPs in the test zone instantly changes after addition of Hg(II), and this color change can be detected by the naked eye, or a digital camera. Under optimized experimental conditions, the linear range is 0.020–8.0 μM of Hg(II) with 12.5 nM (2.5 ppb) detection limit in aqueous solution. In the case of the μPAD, the linear concentration range is 0.025–2.0 μM Hg(II) with a detection limit of 15 nM (3.0 ppb). This method is used for the determination of mercury in real samples such as salmon fish and dusty days in Sanandaj, Iran. The results determined by this method agreed well with those determined according to the ASTM standard method. [ABSTRACT FROM AUTHOR]

In this article, using the integration of paper microfluidics within laser-inscribed commercial contact lenses, we demonstrate the multiplexed detection of clinically relevant analytes including hydrogen ions, proteins, glucose, nitrites and L -ascorbic acid, all sampled directly from model tears. In vitro measurements involved the optimization of colorimetric assays, with readouts collected, stored and analyzed using a bespoke Tears Diagnostics smartphone application prototype. We demonstrate the potential of the device to perform discrete measurements either for medical diagnosis or disease screening in the clinic or at the point-of-care (PoC), with future applications including monitoring of ocular infections, uveitis, diabetes, keratopathies and assessing oxidative stress. [ABSTRACT FROM AUTHOR]

This work reports on a low cost microfluidic electronic tongue (e-tongue) made with carbon interdigitated electrodes, printed on paper, and coated with boronic acid-containing hydrogels. Using capacitance measurements, the e-tongue was capable of distinguishing between different types of sugars (e.g. glucose, fructose and sucrose), in addition to distinguishing between commercial brands of apple juice using a small volume of sample (6 μL). The channels of the microfluidic e-tongue were made using a wax printer, and were modified with hydrogels containing acrylamide copolymerized with 5 or 20 mol% 3-(acrylamido) phenyl boronic acid (Am-PBA), or a crosslinked homopolymeric hydrogel based on N-(2-boronobenzyl)-2-hydroxy-N,N-dimethylethan-1-aminium-3-sulfopropyl acrylate (DMA-PBA). Such hydrogels, containing a phenyl boronic acid (PBA) moiety, can bind saccharides. Combining various hydrogels of this nature in an e-tongue device enabled discrimination between apple juices, which are known to contain higher amounts of fructose compared to glucose or sucrose. Changes in capacitance were captured with impedance spectroscopy in the frequency range from 0.1 to 10 MHz for solutions with varying concentrations of glucose, fructose and sucrose (from 0 to 0.056 g mL−1). The capacitance data were treated with Principal Component Analysis (PCA) and Interactive Document Map (IDMAP), which then correlated overall sugar content from different brands of apple juice. This low-cost, easy-to-use, disposable e-tongue offers great potential in the routine analysis of food and beverages, while offering comparative performance to alternatives in the literature. [ABSTRACT FROM AUTHOR]

In this study, the rational preparation of Kevlar fiber (KF)-reinforced eco-friendly paper based on ultralong hydroxyapatite nanowires (HAPNWs) is reported. The ultralong HAPNWs were used to constitute the paper skeleton, and KF was used to further enhance the mechanical properties of the paper. The KF and ultralong HAPNWs interweaved with each other to form a three-dimensional porous structure and immobilize TiO2 nanoparticles, resulting in high flexibility and mechanical strength of the paper; in particular, the writable paper exhibited superhydrophobic, anti-fouling, self-cleaning and oil/water separation properties after being modified with fluorine-free stearic acid (STA). Owing to the photo-induced hydrophilicity and photocatalysis of TiO2, the surface wettability could be tuned by UV light. Moreover, the as-prepared paper possessed promising thermal stability, and the weight percentage was maintained above 90% even when it was heated to 800 °C. Unlike traditional paper, the as-prepared paper manifested favorable fire-retardant properties even when it was directly exposed to fire for 5 minutes. Furthermore, the soft paper was utilized to efficiently separate an oil/water mixed solution; moreover, it was used to transport water and various organic liquids when lubricant oil was infused into the porous structure. The present study is propitious for the fabrication of multifunctional inorganic paper, which will find practical applications in the fields of paper making, liquid transportation, oil–water separation, fire-retardants, and waterproofing. [ABSTRACT FROM AUTHOR]

In this study, we describe the design and successful synthesis of a new hapten, which we used to prepare an anti-zilpaterol mouse monoclonal antibody (mAb) with an IC50 of 0.31 ng mL−1 and a limit of detection (LOD) of 0.02 ng mL−1. A colloidal gold immunochromatographic test strip was further developed and optimized using the antibody, and cross-reactivity experiments were carried out to prove its good specificity. Semi-quantitative and quantitative detections of zilpaterol were evaluated in swine urine and pork samples using the test strips. For semi-quantitative detection in swine urine and pork samples, the visual limit of detection values were 0.5 μg kg−1 and 0.5 μg kg−1, and the cut-off values were 5 μg kg−1 and 5 μg kg−1, respectively. For quantitative detection, the calculated limit of detection values were 0.08 μg kg−1 and 0.12 μg kg−1, and the remaining values were 10 μg kg−1 and 10 μg kg−1. The developed gold nanoparticle-based immunochromatographic assay is highly suitable for on-site detection and rapid preliminary screening of zilpaterol in swine urine and pork samples. [ABSTRACT FROM AUTHOR]

Electrochemical organic synthesis (EOS) employing electrons to directly activate the reactants can readily complete the chemical conversion under mild conditions. Here, it presented an efficient electrochemical coupling halobenzene into biphenyl on a Pd nanoparticle-coated cathode. The biphenyl product can be obtained with a yield up to 77% at 35 mA, 6 h (3.9 F mol−1). In addition, after consecutive fifth run of the coupling reaction, the yield still remained at ca. 40%, suggesting its considerable recyclable capacity. In addition, the preliminary kinetics study via the off-line gas chromatography analysis of the reaction mixture shows a two-section reaction process, including the introduction process (IP) and fast conversion process (FCP). Further, the estimated reaction kinetics constant value of 0.196 min−1 for FCP suggests a more effective conversion than that obtained by the previous study. This study adopts a simple way to fabricate a low-cost and reusable Pd electrode, achieving a high-efficiency electrochemical strategy for the Ullmann-type coupling reaction at mild conditions, and holds a great promise to extend this synthesis route to other important organic synthesis. [ABSTRACT FROM AUTHOR]

Polymers and surfactants are used in many technological and industrial applications such as the manufacture of functional materials and coatings, personal care and pharmaceutical products, food science, paints, anti-icing fluids, tertiary oil recovery and the paper industry. Polymer–surfactant mixtures are particularly important in shampoos and conditioners. However, as in almost every other industry over the past five or more decades, the performance of hair care formulations has been significantly improved by the use of petrochemical-derived ingredients. As a result, cosmetic formulations, and hair care formulations in particular, have been based primarily on polymers and surfactants that are neither renewable, nor derived from environmentally friendly processes, nor have a positive environmental impact. This contrasts with the extensive use of natural and renewable products, mainly plant extracts, in cosmetics in ancient times. Therefore, the substitution of currently used ingredients with others of natural origin has been a top priority for the cosmetic industry over the last two decades, and in order to achieve greater consumer acceptance, it is crucial to maintain and, where possible, improve the technical performance of such products. This paper describes the complexities and challenges of developing greener shampoo and conditioner ingredients and formulations to meet current and future needs, and outlines a methodological approach based on model hair surfaces and a selection of appropriate experimental and numerical techniques to achieve our goals. Some encouraging technical routes using biosurfactants, biopolymers and bio-based polymers are presented, along with the significant opportunity to obtain a wide range of green ingredients through molecular design and well-controlled biotechnological processes. Similar concerns apply to other cosmetic products such as waxes, fragrances, bleaching agents, etc. [ABSTRACT FROM AUTHOR]

A well-designed support material between catalyst and substrate can always significantly enhance the performance of an electrode on water oxidation. In this work, a functional Ni and N-doped carbon layer (NNC) was designed on carbon paper (CP) via pyrolysis by using a controlled electrodeposited polyporphyrin as a precursor. Consequently, the fabricated NiFe-LDH/NNC/CP achieved a catalytic current density of 100 mA cm−2 at a small overpotential of 231 mV with a low Tafel slope of 26.0 mV dec−1, as well as high durability for more than 360 h. The insights are that N-doping reinforces the hydrophilicity and the catalyst binding capacity, while Ni-doping intensifies the conductivity. [ABSTRACT FROM AUTHOR]

We report a cyclic organosulfide synthesized via a condensation reaction. It can be cycled for 1000 times in half cells. Impressively, it can work with lithiated carbon paper as the anode in ether electrolyte in a full cell. This work shows the promising property of the organosulfide cathode in lithium batteries. [ABSTRACT FROM AUTHOR]

Here, we present a new diacetoxy-functionalized UiO-66 metal–organic framework (MOF) for the trace level detection of hydrazine in water. The MOF material (1) was solvothermally prepared by the reaction between ZrOCl2·8H2O and 2,5-diacetoxy-1,4-benzenedicarboxylic acid (H2BDC–(OCOCH3)2). The desolvated material (1′) showed a highly selective fluorescent turn-on signal towards hydrazine in water, which can be visualized by the naked eye under a UV lamp. Within 1 min of hydrazine addition, there was 14-fold fluorescence enhancement. The probe can detect hydrazine up to the nanomolar level (detection limit = 78.8 nM) in water. This detection limit is the lowest among MOF-based fluorescent probes for hydrazine. The material was also utilized for the sensing of hydrazine in paper strips and environmental water samples. Hydrazine-selective deprotection of ester groups anchored with the ligand is the principal reason behind the switch-on nature of sensing. [ABSTRACT FROM AUTHOR]

The wide spread of smartphones and QR codes for various end-user applications has had an impact beyond traditional fields of use, recently also reaching point-of-care testing (POCT). This work presents the integration of QR code recognition into paper-based analytical devices (PADs) with "distance-based" colorimetric signalling, resulting in semiquantitative readout fully relying on straightforward barcode reader solutions. PADs consist of an array of QR codes arranged in series inside a paperfluidic channel. A mask dye concept has been developed, which enables utilisation of colour changing indicators by initially hiding QR codes. The colour change of the indicator induced by the presence of an analyte of interest results in gradual unmasking of QR codes, which become recognisable by the smartphone barcode reader app. To reproducibly fabricate devices, all fabrication steps were performed by commercial desktop solid ink and inkjet printing. The QR code masking function was optimised by controlling the amount of printed mask dye through adjusting the opacity of printing patterns during the inkjet deposition process. For proof-of-concept, a model assay in the form of colorimetric copper ion (Cu2+) detection in the concentration range of 0.4 mM to 3.2 mM was evaluated. Consistent results independent of the smartphone model and environmental light condition were achieved with a free barcode reader app. To the best of our knowledge, this work is the first demonstration of a semiquantitative assay approach fully relying on QR code readout without digital colour analysis, customised app or hardware modification. [ABSTRACT FROM AUTHOR]

Nucleic acid amplification tests (NAATs), which amplify and detect pathogen nucleic acids, are vital methods to diagnose diseases, particularly in cases where patients exhibit low levels of infection. For many blood-borne pathogens such as HIV or Plasmodium falciparum, it is necessary to first extract pathogen RNA or DNA from patient blood prior to NAAT analysis. Traditional nucleic acid extraction methods are expensive, resource-intensive and are often difficult to deploy to resource-limited areas where many blood-borne infections are widespread. Here, we describe a portable, paper-and-plastic device, called SNAPflex, for instrument-free nucleic acid extraction from whole blood, which builds upon our previous work for RNA extraction using a pressure-driven extraction system. SNAPflex shows improved HIV RNA extraction from simulated patient samples compared to traditional extraction methods as well as long-term stability of extracted RNA without the need for cold storage. We further demonstrated successful extraction and recovery of P. falciparum DNA from cultured parasites in whole blood. SNAPflex was designed to be easily manufacturable and deployable to resource-limited settings. [ABSTRACT FROM AUTHOR]

The SARS-CoV-2 virus is known as the causal agent for the current COVID-19 global pandemic. The majority of COVID-19 patients develop acute respiratory distress syndrome (ARDS), while some experience a cytokine storm effect, which is considered as one of the leading causes of patient mortality. Lipids are known to be involved in the various stages of the lifecycle of a virus functioning as receptors or co-receptors that controls viral propagation inside the host cell. Therefore, lipid-related metabolomics aims to provide insight into the immune response of the novel coronavirus. Our study has focused on determination of the potential metabolomic biomarkers utilizing a Teslin® Substrate in paper spray mass spectrometry (PS-MS) for the development of a rapid detection test within 60 seconds of analysis time. In this study, results were correlated with PCR tests to reflect that the systemic responses of the cells were affected by the COVID-19 virus. [ABSTRACT FROM AUTHOR]

The monosaccharide 1,5-anhydroglucitol (1,5-AG) is a known indicator of glucose levels. Conventional 1,5-AG quantification methods with enzyme-based sensors using pyranose oxidase (PROD) require elimination of interference from the sample (a laborious and time-consuming process), as PROD cannot distinguish 1,5-AG from other sugars. We developed a one-step paper-based sensor for detecting 1,5-AG using glucose oxidase, catalase, and mutarotase that eliminates excess glucose, which interferes with 1,5-AG detection. This sensor consists of two compartments for the quantification of glucose and 1,5-AG and reflects the concentration of these targets after reaction with water or spiked human urine. The limit of detection of the sensor was 0.9 mg dL−1 for glucose and 3.2 μg mL−1 for 1,5-AG. [ABSTRACT FROM AUTHOR]

A low-cost, sensitive, and disposable paper-based immunosensor for instrument-free colorimetric detection of pancreatic cancer biomarker PEAK1 was reported for the first time by capitalizing the catalytic properties of gold nanoparticles in colour dye degradation. This simple signal amplification method enhances the detection sensitivity by about 10 fold. [ABSTRACT FROM AUTHOR]

In this paper, a paper-supported analytical device based on a sandwich immunoreaction and luminescence resonance energy transfer (LRET) was reported for the visual and quantitative determination of a cancer biomarker, in which upconversion nanoparticles (UCNPs) were located on the surface of the paper as energy donors and gold nanoparticles (AuNPs) were used as energy acceptors. Upon the recognition of the cancer biomarker by two rationally selected antibodies, the upconversion luminescence was quenched by the AuNPs in a biomarker concentration-dependent manner. As a model target, CEA was detected using this immunosensor, and a linear relationship within 0.5–30 ng mL−1 was obtained in buffer solution, with a detection limit of 0.21 ng mL−1. The immunosensor was also applicable in 20-fold diluted human serum with a linear range of 0.5–30 ng mL−1 and a detection limit of 0.36 ng mL−1. This technique also realized the qualitative judgment of the critical concentration of CEA in serum samples by the naked eye. This approach displays great application potential for point-of-care testing in clinical applications, as well as the potentiality to be extended to other kinds of disease-related biomolecules. [ABSTRACT FROM AUTHOR]

There is growing interest in the development of simple, fast, sustainable and low-cost analytical methodologies on paper-based platforms. However, sensitive detection strategies that fit properly with these devices are still required. In this work, a calibration-free method is proposed for analytical determinations performed on paper-based electrochemical devices, in this case, for ascorbic acid. Carbon ink is deposited on a hydrophilic working area of the paper delimited with a hydrophobic wax. This maskless procedure is fast and cuts down ink waste. The connection of this working electrode to the potentiostat is provided by reusable gold-plated connector headers that provide also the reference and counter electrodes. The thickness of the paper substrate defines the electrochemical cell and confines a sample volume, ideal for thin-layer coulometry. Controlled-potential coulometry is performed applying a potential of +0.6 V for 50 s. The charge is calculated by measuring the area under the fast chronoamperogram and the concentration is determined following Faraday's law (known number of transferred electrons). This methodology was applied to the determination of ascorbic acid, with a limit of detection of 40 μM. Its concentration in commercial fruit juices can be directly determined in diluted samples. The absence of matrix effects is observed by comparing the results obtained before and after enzymatic reaction of the sample with cucumber ascorbate oxidase. Good accuracy and precision makes this method suitable for quality control of ascorbic acid in commercial juices. Underexploited coulometric readout can be applied as a fast (calibration-free) and low-cost (standards not required) transduction principle for the newly developed paper devices. [ABSTRACT FROM AUTHOR]

Regulating the fluid flow in microfluidic devices enables a wide range of assay protocols for analytical applications. A programmable, photo-paper-based microfluidic device fabricated by using a method of cutting and laminating, followed by printing, is reported. The flow distance of fluid in the photo-paper-based channel was linearly proportional to time. By printing silver nanoparticle (AgNP) and poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene] (PTFE) patterns on the surface of a photo-paper-based channel, we were able to either increase or decrease the fluid flow in the fabricated microfluidic devices, while maintaining the linearity in the flow distance–time relation. In comparison to the speed of fluid flow in a pristine channel, by using hydrophilic AgNP patterns, we were able to increase the speed in the channel by up to 15 times while we were able to slow the speed by a factor of 3 when using hydrophobic PTFE dots. We then further demonstrated a single-step protocol for detecting glucose and a multi-step protocol for detecting methyl paraoxon (MPO) with our methods in photo-paper-based microfluidic devices. This approach can lead to improved fluid handling techniques to achieve a wide range of complex, but programmable, assays without the need for any additional auxiliary devices for automated operation. [ABSTRACT FROM AUTHOR]

In the present article we report a new hybrid microfluidic device (hyFlow) comprising a disposable paper electrode and a three-dimensional (3D) printed plastic chip for the electrochemical detection of a magnetic bead–silver nanoparticle (MB–AgNP) bioconjugate. This hybrid device evolved due to the difficulty of incorporating micron-scale MBs into paper-only fluidic devices. Specifically, paper fluidic devices can entrap MB-containing conjugates within their cellulose or nitrocellulose fiber matrix. The hyFlow system was designed to minimize such issues and transport MB conjugates more efficiently to the electrochemical detection zone of the device. The hyFlow system retains the benefit of fluid transport by pressure-driven flow, however, no pump is required for its operation. The hyFlow device is capable of detecting either pre-formed MB–AgNP conjugates or conjugates formed in situ. The detection limit of AgNPs using this device is 12 pM, which represents just 22 AgNPs per MB. [ABSTRACT FROM AUTHOR]

Escherichia coli O157:H7 is a severe foodborne pathogen that causes lots of life-threatening diseases. In the search for a rapid, sensitive, portable and low-cost method to detect this pathogen, we developed a wax-printed paper-based enzyme-linked immunosorbent assay (P-ELISA) based on microfluidic paper-based analytical devices (μPADs), with the whole operation time being less than 3 h and only needing 5 μl samples for detection. The limit of detection (LOD) of E. coli O157:H7 reached 104 CFU ml−1, which is an order of magnitude higher than that of conventional ELISA (C-ELISA). The LOD in artificially contaminated beef samples is 1 CFU per 25 g after enriching the culture for 8 h. This method is superior to the molecular biology method in detection sensitivity and superior to C-ELISA and the national standard method in detection time and cost. Thus, the established P-ELISA method has good sensitivity, specificity and repeatability. It can be suitable for point-of-care testing without expensive and bulky instruments and can also provide a platform for detecting other pathogens, especially in areas that lack advanced clinical equipment. [ABSTRACT FROM AUTHOR]

The use of synthetic stimulants, including designer cathinones, remains a significant concern worldwide. Thus, the detection and identification of synthetic cathinones in biological matrices is of paramount importance for clinical and forensic laboratories. In this study, distribution of mephedrone and its metabolites was investigated in fingerprints. Following a controlled human mephedrone administration (100 mg nasally insufflated), two mass spectrometry-based methods for fingerprint analysis have been evaluated. The samples deposited on triangular pieces of chromatography paper were directly analysed under ambient conditions by paper spray-mass spectrometry (PS-MS) while those deposited on glass cover slips were extracted and analysed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The LC-MS/MS method was 5–6 times more sensitive than PS-MS but required sample preparation and longer analysis time. Mephedrone was detected in 62% and in 38% of all post-administration samples analysed by LC-MS/MS and PS-MS, respectively. Nor-mephedrone was the only metabolite detected in 3.8% of all samples analysed by LC-MS/MS. A large inter- and intra-subject variation was observed for mephedrone which may be due to several factors, such as the applied finger pressure, angle and duration of contact with the deposition surface and inability to control the 'amount' of collected fingerprint deposits. Until these limitations are addressed, we suggest that the sole use of fingerprints can be a useful diagnostic tool in qualitative rather than quantitative analysis, and requires a confirmatory analysis in a different biological matrix. [ABSTRACT FROM AUTHOR]

A wide range of microfluidic paper-based analytical devices (μPADs) have been developed in the last decade. Despite this, the quality of colorimetric analysis has not substantially improved as the data is vulnerable to heterogeneous color distribution (e.g., coffee ring effects), non-uniform shapes of colored detection area, and noise from the underlying paper structure. These limitations are here addressed by a colorimetric method to quantify freely discharged dye on paper substrate, without the need for a defined channel or hydrophobic barrier. For accurate quantification, colorimetric absorbance values are calculated for each pixel based on the recorded RGB values and noise from the paper structure eliminated, to extract accurate absorbance information at the pixel level. Total analyte quantity is then calculated through the conversion of absorbance values into quantity values for each pixel followed by integration across the entire image. The resulting quantity is shown to be independent of the shape of the applied colored dye spot, with a cross, circle or rod shape all giving the same quantity information. The approach is applied to a capillary-based potassium-selective sensor, where the sample solution is loaded with the dye thioflavin T (ThT) obtained by quantitative exchange with K+ in a sensing capillary, which is discharged onto a bare paper substrate without any channels. The resulting dye quantity is successfully obtained by flatbed scanner and smartphone. The successful automated computation of colorimetric data on μPADs will help realize simpler paper-based assay and reaction systems that should be more applicable to addressing real world analytical problems. [ABSTRACT FROM AUTHOR]

In this study, we report for the first time few-layered MoS2 nanosheets grown onto graphene dipped cellulose filter paper (M-G-CFP) using a simple hydrothermal method towards efficient adsorption of cationic methylene blue (MB) dye. Detailed morphological and chemical characterization studies of M-G-CFP are performed using FESEM, TEM, XRD and Raman spectroscopy that reveal micro flower-like MoS2 grown onto graphene modified paper with few layers of MoS2 nanosheets and its high purity nature. Due to electrostatic interactions between cationic dye and the anionic adsorbent surface, the hybrid M-G-CFP exhibits ultrafast removal of MB within 2 min with a maximum adsorption capacity of 485.4 mg g−1 which is superior in comparison to other reported MoS2 based adsorbents. This high adsorption capacity is attributed to large accessible active sites on the surface of the hybrid M-G-CFP structure. The adsorption mechanism of MB dye on the MoS2 nanosheets are well described using kinetic and isotherm models with the effects of parameters like pH, contact time and temperature. The kinetic and isotherm study reveals that the adsorption of MB dye follows a pseudo-second order model, indicating the chemisorption and Langmuir model of monolayer adsorption, respectively. Furthermore, this hybrid structure can be efficiently reused for subsequent dye adsorption without compromising on its dye removal efficiency suggesting it is a feasible and promising material towards wastewater treatment applications. [ABSTRACT FROM AUTHOR]

Cobalt(II) phthalocyanine (CoPc) was suspended in aqueous medium and the colloidal system was used as catalyst for the electrochemical determination of hydrogen peroxide on paper-based electrodes modified with carbon nanomaterials. H2O2 was oxidised at 0.275 V vs. Ag pseudoreference electrode. This system was adapted to develop a glucose sensor with glucose oxidase immobilized on the cellulose electrode. CoPc suspended nanoparticles acted as nanoenzyme mimicking peroxidase activity and were combined with different carbon nanomaterials to form hybrids with optimised catalytic performance. GO-CoPc paper-based electrodes yielded the best results with a linear range of ∼12 μM to 49 mM for H2O2 and 0.1 mM to 1 mM for glucose. Glucose was determined in physiological serum and juice samples with recoveries of 93.3 and 94.2% respectively. CoPc could replace HRP for the catalytic sensing of H2O2, without the need to be dissolved. This material can be used in situ in a simple protocol with other nanomaterials for electrode modification. The sensor described has the advantage of easy preparation, using the catalyst in colloidal form, long term stability, and versatility to be adapted to other low cost and disposable enzymatic systems. [ABSTRACT FROM AUTHOR]

Rapid analysis of metabolites in biofluids is of great importance for disease diagnosis or new-born disease screening. Herein, we introduce an agarose hydrogel conditioning method to enhance the performance of paper spray ionization mass spectrometry. With facile and fast hydrogel conditioning, the signal intensity of therapeutic drugs spiked in urine was 5 to 15 fold higher than that in direct paper spray ionization mass spectrometry analysis. Consequently, the sensitivity of metabolites in urine was improved via hydrogel conditioning, resulting in 9 to 15 fold decrease in the possibility of detection (POD) levels. These results show that agarose hydrogel conditioning coupled with paper spray ionization mass spectrometry could serve as a facile ionization method for ambient mass spectrometry, which might be useful in fast screening of metabolites and therapeutic drugs in raw biofluids. [ABSTRACT FROM AUTHOR]

Screening for a deficiency of glucose-6-phosphate dehydrogenase (G6PD) in red blood cells is vital for determining the potentially life-threatening presence of congenital, hereditary or induced hemolytic anemias. In this study, a "sample-to-readout" paper-based point-of-care (POC) colorimetric biosensor was developed for direct detection of G6PD in whole blood by simple visual comparison to a color card. The G6PD paper sensor was highly stable with no observable loss in performance after room temperature storage for at least 6 weeks, and worked equally well at room temperature and 37 °C. The simple printed paper format and the stability of the colorimetric reagents facilitates scalable manufacturing. The ability to utilize well established sample collection and preparation protocols along with a colorimetric visual readout should facilitate future transfer of this proof-of-concept POC biosensor to remote or resource-poor locations. [ABSTRACT FROM AUTHOR]

The recent boom of nanomaterials printing in the fields of biomedical engineering, bioanalysis and flexible electronics has greatly stimulated researchers' interest in printing technologies. However, specifically formulated nanomaterial inks have limited the types of printable nanomaterials. Here, a unique non-powered capillary force-driven stamped (CFDS) approach, combining a 3D-printed stamper with a paper substrate, is developed for directly printing patterned nanomaterials aqueous solution. The CFDS approach has two processes, including the loading process in which the capillary force of the stamper channel is stronger than gravity, and the deposition process, in which the synergistic action of the capillary force of the paper fibre tubes and gravity is approximately 20 times the capillary force of the stamper channel. Four additive-free nanomaterial aqueous solutions, including nanowires, nanosheets, nanostars and nanogels, are used to print patterns, and show slight diffusion and desired uniformity with a diffusion rate and roundness of 1.12 and 0.78, respectively, demonstrating the feasibility of this approach. Four kinds of nanogel with different fluorescence labels are simultaneously printed to challenge the approach and demonstrate its flexibility and scalability. The resolution of the approach is 0.3 mm. Without any post-processing, the stamped paper substrates directly serve as paper-based surface enhanced Raman scattering substrates with an enhancement factor of 4 × 106 and as electrodes with a resistance of 0.74 Ω, demonstrating their multi-functionality. Due to its general, flexible and scalable applicability, this simple, low-cost and non-powered approach could be widely applied to the personalized printing of nanomaterials on paper substrates. [ABSTRACT FROM AUTHOR]

A novel platform of microfluidic paper-based analytical devices (μPADs) for dual detection of bisphenol A (BPA), a model analyte, was fabricated using an electronic digital plotter to create the stacked layer of μPADs and generate the lateral-flow channel without using an external pump. Two detection techniques, including electrochemical detection and laser desorption ionization mass spectrometric detection (LDI-MS), were used complementarily to improve the precision in the detection of BPA. The fluid sample was delivered to both detection zones by the capillary action, automatically generated from the fabricated microfluidic device. For an electrochemical sensor, two ballpoint pens filled with silver nanoparticles (AgNPs) and multiwall carbon nanotube (MWCNT) ink were used to print onto the paper with a contact printing method using a digital plotter. To further improve the sensitivity, zinc oxide (ZnO) was used to modify both electrochemical and LDI-MS detection zones. For BPA detection, high electrocatalytic properties and strong UV absorption of ZnO promote the electron transfer in the electrochemical sensor and ionization efficiency in LDI-MS with low interferences compared with a conventional organic matrix. Under optimal conditions, this platform showed a dual detection capability for BPA with a detection limit of 0.35 μM for electrochemical detection and with an ultralow detection limit of 0.01 pM for LDI-MS. This novel platform might be very useful for trace analyses requiring high precision detection of various analytes. [ABSTRACT FROM AUTHOR]

The development of viable point-of-care diagnostic formats is integral to achieving better patient care and improved outcomes. The need for robust and low-cost tests is especially important in under-resourced and rural settings. Perhaps the greatest challenge is ensuring that an untrained individual is capable of operating and interpreting the test, out with a care facility. Here we present a paper-based diagnostic device capable of sensing miR-29a using both colorimetric and surface enhanced Raman scattering (SERS) analysis. Rather, than carry out the two types of analyses in tandem, we envisage that the colorimetric output is easy enough to be interpreted by the untrained-individual administering the test to provide them with qualitative feedback. If deemed positive, the test can be further validated at a centralized care facility using a handheld-Raman spectrometer to provide a semi-quantitative result. Detection of miR-29a, a microRNA associated with myocardial infarction, was achieved at a level of pg μL−1 through the combination of three-dimensional paper-based microfluidics, colorimetric detection, and surface enhanced Raman scattering (SERS) analysis. RGB analysis of the colorimetric output generated from samples containing miR-29a at different concentrations (18–360 pg μL−1) showed differentiation from the control sample, however significant repeat variability indicated that it could not be used for quantifying miR-29a levels. However, the SERS analysis exhibited greater reproducibility at varying concentrations, achieving an LoD of 47 pg μL−1. The union of the paper-based device and the two analysis methods resulted in the production of a sensitive, reproducible and facile, point of care test (POCT), which paves the way for future implementation in the diagnosis of a range of diseases. [ABSTRACT FROM AUTHOR]

Fluorescence detection of pesticide contamination enables timely control of food safety. This study aims to construct novel and facile microfluidic paper-based analytical devices for the ratiometric fluorescence determination of pesticides. Through fluorescence resonance energy transfer (FRET) of nitrobenzoxadiazole (NBD) and CdTe quantum dots (QDs), the new microfluidic paper chips allowed the rapid and selective visual detection of 2,4-dichlorophenoxyacetic acid (2,4-D). The performance changes of the fluorescent material on solid matrix material were studied in detail. The sensor products exhibited visual observability and fluorescence characteristics. Under optimized conditions, the sensors showed satisfactory linearity in the range of 0.56–80 μM, and achieved detection limits as low as 90 nM. The sensors were successfully applied for soybean sprouts and lake water samples. Four levels of spiked-in 2,4-D concentrations were obtained with high recovery rates ranging from 86.2% to 109.5% and the RSD less than 4.19%. Thus, the present work described the integration of surface imprinted grafts on cellulose paper and ratiometric fluorescence techniques for highly sensitive separation and detection of pesticides in real food and environmental samples. Ultimately, this study paved the way for the development of novel ratiometric fluorescence detection to address food safety and environmental issues. [ABSTRACT FROM AUTHOR]

Herein, natural celery leaves and glutathione were used as the precursors to prepare carbon dots (CDs) as novel fluorescent paper sensors, which exhibited high sensitivity and selectivity for the detection of explosive nitrophenols (including 2-nitrophenol (2-NP), 3-nitrophenol (3-NP) and 4-nitrophenol (4-NP)). The prepared CDs featured blue fluorescence emission under irradiation using an UV lamp, and the fluorescence quantum yield was as high as 53%. As fluorescent probes, the CDs showed a particular response to 2-NP, 3-NP and 4-NP, and the detection limits of 2-NP, 3-NP and 4-NP were determined to be 39 nM, 43 nM and 26 nM, respectively. Due to the multiple advantages including good water solubility, low toxicity and excellent stability, CDs are expected to be widely used. Meanwhile, a CD-based test paper was exploited and it displayed a distinctive quenching response when exposed to 3-NP and 4-NP, with the visual detection limits of 0.5 μM and 0.1 μM, respectively. Furthermore, the low toxicity and biocompatibility of CDs were also demonstrated by evaluating cell culture experiments, and CDs showed good potential applications in imaging. In summary, the prepared CDs are promising for fluorescent paper sensors and bioimaging. [ABSTRACT FROM AUTHOR]

More than 60% newborns experience hyperbilirubinemia and jaundice within the initial week after birth due to the accumulation of total bilirubin in blood. Left untreated high levels of bilirubin may result in brain impairment. Simple, fast, accurate, low-cost and timely point-of-care (POC) analysis of total bilirubin is an unmet need especially in resource-limited areas. This work introduces a novel sensing device, named a "tape-paper sensor", capable of separating plasma from whole blood and measuring total bilirubin by a colorimetric diazotization method. The tape-paper sensing method overcomes non-homogeneous color distribution caused by the "coffee stain" effect, which improves the accuracy of colorimetric evaluation on paper-based analytical devices. The level of hemolysis in the plasma extracted by the device is evaluated, confirming no interference in the detection of total bilirubin. The accuracy of the tape-paper sensing approach for neonatal blood sample measurement is verified by comparison with the hospital pathology laboratory method. The small volume of samples and reagents, minimal equipment (an office scanner), fast detection (<10 min) and low fabrication cost (∼A$ 0.6) reveal the suitability of the device for POC use and in resource-limited settings. The tape-paper sensor is a low-cost, fast, and user-friendly device for measurement of blood total bilirubin levels in neonatal jaundice diagnostics. [ABSTRACT FROM AUTHOR]