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This work demonstrates sensitive and low-cost piezoelectric sensors on skin-friendly, ultrathin, and conformable substrates combined with organic electrochemical transistors (OECTs) for the detection and amplification of alternating low-voltage input signals. The fully screen-printed (SP) piezoelectric sensors were manufactured on commercially available tattoo paper substrates, while the all-printed OECTs, relying on an extended gate electrode architecture, were manufactured either by solely using SP or by combining SP and aerosol jet printing (AJP) on PET substrates. Applying a low-voltage signal (±25 mV) to the gate electrode of the SP+AJP OECT results in approximately five times higher current modulation as compared to the fully SP reference OECT. The tattoo paper-based substrate enables transfer of the SP piezoelectric sensor to the skin, which in turn allows for radial pulse monitoring when combined with the SP+AJP OECT; this is possible due to the ability of the conformable sensor to convert mechanical vibrations into voltage signals along with the highly sensitive current modulation ability of the transistor device to further amplify the output signal. The results reported herein pave the way toward all-printed fully conformable wearable devices with high sensitivity to be further utilized for the real-time monitoring of electrophysiological signals.

Paper-based cultural relics often undergo acidification and deterioration during long-term preservation. Accurate detection of paper acidity is of great significance to assess aging status and extend the preservation lifetime of paper-based cultural relics. Rapid identification of the acidification degree and acid distribution across multiple regions of paper is essential. Inspired by fluorescent sensing technology, pH-sensitive cadmium telluride (CdTe) quantum dots (QDs) and rhodamine B (RB) fluorescent probes are synthesized and incorporated onto the nanofibers of a bacterial cellulose (BC) membrane to enable visual acidity detection of paper. Due to the complementary pH detection range of CdTe QDs and RB probes, the composite BC membrane exhibits a clear pH response across an acidic to neutral range (pH 3.0-7.5). Notably, the contrasting fluorescent colors of the two probes within the BC membrane allow for easy visualization of paper pH and acidity distribution with the naked eyes. A distinct color transition from red to green was observed on the fluorescent BC membrane when it is applied to a model paper with a gradient pH distribution. The feasibility of this method was verified by using the flat-headed pH electrode method. Additionally, common metal ions in most paper fillers, inks, pigments, as well as some sugars and amino acids showed minimal interference with the pH response of the composite BC membrane, highlighting its potential and broad applicability for visual acidity detection in paper-based cultural relics.

Inspired by the electric eel, biomimetic, biocompatible energy storage, and power generation technologies show promise for applications in portable and wearable electronic devices by mimicking the electric cell tandem structure of the electric eel and utilizing ionic gradients between hydrogel compartments to generate electricity. Previously, inspired by the unique morphology of the torpedo fish, an artificial flexible power source that can output a large current was introduced. This power source uses a hydrogel-infused paper hybrid to create, accordionize, and reconfigure arbitrary-sized gel films in series and parallel, and the power output of the flexible battery was significantly enhanced. However, maintaining the ionic gradient of hydrogel batteries during storage remains a challenge. Here, by borrowing the isolation properties of the accordion structure, we propose a unique paper accordion structure design to fabricate an Accordion-Structured Hydrogel Battery (ASHB). Pretreatment of hydrogel-injected paper strips improved storage stability and maintained the ionic gradient of hydrogel cells in the nonworking state, so that the cell's gradient retention time after the assembly is completed is increased by at least 30 h compared to stacking, and its per-cell operating voltage is still able to reach. The design also makes the assembly and use of flexible batteries more modular and holistic. In the future, it may be possible to power the cells with ions generated by the human body or the metabolites of living organisms, leading to the development of more efficient, sustainable, and eco-friendly power solutions.

In this work, we report the design and fabrication of a light-addressable, paper-based nanocomposite scaffold for optical pacing and read-out of in vitro grown cardiac tissue. The scaffold consists of paper cellulose microfibers functionalized with gold nanorods (GNRs) and semiconductor quantum dots (QDs), embedded in a cell-permissive collagen matrix. The GNRs enable cardiomyocyte activity modulation through local temperature gradients induced by modulated near-infrared (NIR) laser illumination, with the local temperature changes reported by temperature-dependent QD photoluminescence (PL). The micrometer-sized paper fibers promote the tubular organization of HL-1 cardiac muscle cells, while the NIR plasmonic stimulation modulates reversibly their activity. Given the nanoscale spatial resolution and facile fabrication, paper-based nanocomposite scaffolds with NIR modulation offer excellent alternatives to electrode-based or optogenetic methods for cell activity modulation, at the single cell level, and are compatible with 3D tissue constructs. Such paper-based optical platforms can provide new possibilities for the development of in vitro drug screening assays and heart disease modeling.

The urgent need for real-time and noninvasive monitoring of health-associated biochemical parameters has motivated the development of wearable sweat sensors. Existing electrochemical sensors show promise in real-time analysis of various chemical biomarkers. These sensors often rely on labels and redox probes to generate and amplify the signals for the detection and quantification of analytes with limited sensitivity. In this study, we introduce a molecularly imprinted polymer (MIP)-based biochemical sensor to quantify a molecular biomarker in sweat using electrochemical impedance spectroscopy, which eliminates the need for labels or redox probes. The molecularly imprinted biosensor can achieve sensitive and specific detection of cortisol at concentrations as low as 1 pM, 1000-fold lower than previously reported MIP cortisol sensors. We integrated multimodal electrochemical sensors with an iontophoresis sweat extraction module and paper microfluidics for real-time sweat analysis. Several parameters can be simultaneously quantified, including sweat volume, secretion rate, sodium ion, and cortisol concentration. Paper microfluidic modules not only quantify sweat volume and secretion rate but also facilitate continuous sweat analysis without user intervention. While we focus on cortisol sensing as a proof-of-concept, the molecularly imprinted wearable sensors can be extended to real-time detection of other biochemicals, such as protein biomarkers and therapeutic drugs.

Total antioxidants play a crucial role in human health, and detection of the total antioxidant capacity (TAC) has broad application prospects in fields such as food safety, environmental assessment, and disease diagnosis. However, a long detection time, cumbersome steps, high cost, reliance on professional equipment, and nonportability still remain significant challenges. In this work, an efficient strategy of point-of-care testing (POCT) of the TAC in body fluids by nanozyme-catalyzed colorimetric paper-based microfluidic sensors is proposed. The paper-based microfluidic sensors coupled with a smartphone can reduce testing costs and provide portability. The nanozyme prepared by the solvothermal method presents Michaelis constants of 0.11 and 0.129 mM for H 2 O 2 and TMB, respectively. A method for immobilizing nanozymes and chromogenic agents on a paper-based microfluidic chip is established. Based on smartphone photography and image grayscale extraction, the TAC can be qualitatively detected with a detection limit and linear range of 33.4 and 50-700 μM, respectively. Furthermore, the proposed sensor can realize the one-step quantitative analysis of the TAC in body fluids (blood, saliva, and sweat) within 15 min. The proposed nanozyme-catalyzed colorimetric paper-based microfluidic sensors presented in this study exhibit promising application prospects in the fields of biochemical analysis and POCT.

In cancer metastasis, where mortality rates remain high despite advancements in medical treatments, understanding the molecular pathways and cellular dynamics underlying tumor spread is critical for devising more effective therapeutic strategies. Here, a folding paper system was proposed and developed to mimic native tumor microenvironment. This system, composed of 7 stacked layers of paper enclosed in a holder, allows for the culture of cancer cells under conditions mimicking those found in solid tumors, including limited oxygen and nutrients. Because of the migratory capabilities of cancer cells, the cells in the center layer could migrated to outer layers of the paper stack, enabling the differentiation of cells based on their migratory potential. Subsequent gene expression analysis, conducted through RT-PCR and RNA sequencing, revealed significant correlations between cancer cell migration distance and the expression of genes associated with hypoxia, metabolism, ATP production, and cellular process. Moreover, our study identified cells with aggressive phenotypic traits from the outer layers of the paper stack, highlighting the potential of this system for enabling the study of aggressive cancer cell characteristics. Validation of the folding paper system against clinical carcinoma tissue demonstrated its ability to faithfully mimic the native tumor microenvironment. Overall, our findings underscore the utility of the folding paper system as a valuable tool for investigating and identifying critical molecular pathways involved in cancer metastasis.

Perovskite quantum dots (PQDs) have attracted emerging attention as fluorescent and light-absorbing materials for next-generation optoelectronics due to their outstanding properties and cost-efficiency. However, PQD thin film suffers significant instability due to structure and material failures, which hinders their application in flexible and reliable PQD-based advanced wearable devices. Herein, we use commercial cellulose fiber-based filter paper as a substrate to synthesize PQDs in situ and fabricate PQD-paper free-standing flexible composite film. The abundant hydroxy capping ligands of cellulose fibers and the unique dense network structure of the filter paper can facilitate confined crystallization, forming strong interactions between the PQDs and substrate, the unpackaged PQD composite film showed extraordinary stability (>30 days) in the air with high humidity (90%). Meanwhile, the strong interaction between PQDs and paper enables an ultrasimple drop-cast synthesis process with excellent process tolerance, making it customizable and easy to scale up (10 cm in diameter). Due to the uniformly dispersed PQDs on cellulose fibers of the substrate, the composite demonstrates impressive photo-responsive properties. Photodetector (PD) arrays were designed on free-standing PQD paper and flexible graphitic electrodes, and circuits were fabricated by drawing. The PD arrays can work as optical and electrical dual-mode image sensors with incredible bending robustness, enduring up to 100,000 cycles at 180°.

An alarming increase in the use of pesticides and organoarsenic compounds and their toxic impacts on the environment have inspired us to develop a selective and highly sensitive sensor for the detection of these pollutants. Herein, a bio-friendly, low-cost Al-based luminescent metal-organic framework ( 1' )-based fluorescent material is demonstrated that helps in sustaining water quality by rapid monitoring and quantification of a long-established pesticide (pendimethalin) and a widely employed organoarsenic feed additive (roxarsone). A pyridine-functionalized porous aluminum-based metal-organic framework (Al-MOF) was solvothermally synthesized. After activation, it was used for fast (<10 s) and selective turn-off detection of roxarsone and pendimethalin over other competitive analytes. This is the first MOF-based recyclable sensor for pendimethalin with a remarkably low limit of detection (LOD, 14.4 nM). Real-time effectiveness in detection of pendimethalin in various vegetable and food extracts was successfully verified. Moreover, the aqueous-phase recyclable detection of roxarsone with an ultralow detection limit (13.1 nM) makes it a potential candidate for real-time application. The detection limits for roxarsone and pendimethalin are lower than the existing luminescent material based sensors. Furthermore, the detection of roxarsone in different environmental water and a wide pH range with a good recovery percentage was demonstrated. In addition, a cheap and bio-friendly 1'@chitosan@paper strip composite was prepared and successfully employed for the hands-on detection of pendimethalin and roxarsone. The turn-off behavior of 1' in the presence of pendimethalin and roxarsone was examined systematically, and plausible mechanistic pathways were proposed with the help of multiple experimental evidences.

Point-of-care (POC) devices can provide inexpensive, practical, and expedited solutions for applications ranging from biomedicine to environmental monitoring. This work reports on the development of low-cost microfluidic substrates for POC systems suitable for analytical assays, while also satisfying the need for social and environmentally conscious practices regarding circular economy, waste reduction, and the use of local resources. Thus, an innovative greener process to extract cellulose from plants including abaca, cotton, kozo, linen, and sisal, originating from different places around the world, is developed, and then the corresponding paper substrates are obtained to serve as platforms for POC assays. Hydrophobic wax is used to delineate channels that are able to guide solutions into chambers where the colorimetric assay for total cholesterol quantification is carried out as a proof of concept. Morphological and physicochemical analyses are performed, including the evaluation of fiber diameter, shape and density, and mechanical and thermal properties, together with peel adhesion of the printed wax channels. Contact angle and capillary flow tests ascertain the suitability of the substrates for liquid assays and overall viability as low-cost, sustainable microfluidic substrates for POC applications.

With the continuous improvement of living standards and advancements in science and technology, composite materials with multiple functionalities are gaining high practical value in modern society. In this paper, we present a multifunctional conductive paper-based composite with electromagnetic (EMI) shielding, sensing, Joule heating, and antimicrobial properties. The composite is prepared by growing metallic silver nanoparticles inside the cellulose paper (CP) modified with polydopamine (PDA). The resulting CP@PDA@Ag (CPPA) composite has high conductivity and EMI shielding properties. Furthermore, CPPA composites demonstrate exceptional sensing, Joule heating, and antimicrobial properties. In addition, Vitrimer, a polymer with excellent cross-linked network structure, is introduced into CPPA composites to obtain CPPA-V intelligent electromagnetic shielding materials with shape memory function. These excellent properties show that the prepared multifunctional intelligent composite has exceptional EMI shielding, sensing, Joule heating, and antibacterial and shape memory properties. In short, this multifunctional intelligent composite material has great application prospects in flexible wearable electronics.

The penciling part on a paper is colored by the formation of structural color, and the coloring of a paper without ink has been achieved. In a previous study, our group reported that structural color is formed by plasma irradiation (40-120 s) of the surface of a pencil lead or paper painted with a pencil. The formation of structural color due to the thin-layer interference of components of the pencil lead was observed. The clay exposed by removal of the surface graphite through plasma etching plays the role of a "thin layer". The pencil lead can be colored blue, red, and green by the method. In the case of the paper painted with a pencil (6B), the paper turned blue but could not form the other colors by the method. The reason is that the graphite layer on the paper is not thick enough to form thin-layer interference to show colors other than blue. We now present the method that forms structural colors by plasma irradiation of the paper painted by a graphite-rich pencil lead (9B and 12B). The formation of various structural colors, such as blue, yellow, red-purple, and green, on the paper was achieved. The colored site can be effaced by an eraser. This method is a novel coloring method without using colored inks. This environmentallly friendly coloring method can be applied to various activities, such as studies and art, and can contribute to the achievement of a sustainable society.

Moisture-activated electric generators (MEGs) that harvest clean energy from atmospheric humidity offer exciting opportunities for upgraded energy conversions. However, it is challenging to obtain MEGs that are both easy to fabricate and of high output power, due to the requirement for particular functional materials and the cumbersome manufacturing process. Herein, a simple and general method is adopted to prepare MEGs with chemically gradient structures. As a specific example, a gradient distribution of citric acid was successfully constructed inside an A4 printer paper by asymmetric drying, which can generate a continuous voltage of tens of millivolts by ambient humidity, and even to volts (275 mV and 7.6 μA cm -2 ) under asymmetric humidity stimulation, and the maximum power density output was 2.1 μW cm -2 . The driving force behind this energy conversion is a self-maintained ionic gradient created within the paper by the asymmetric ionization of gradient organic acids when exposed to gradient or nongradient humid air. This work broadens the class of materials and possibilities for the rapid development of MEGs, shedding new light on the revolution of generators that harvest green and sustainable energy for power generation.

Solar desalination is expected to solve the problem of global water shortage. Yet its stability is plagued by salt accumulation. Here, a paper-based thermal radiation-enabled evaporation system (TREES) is demonstrated to achieve sustainable and highly efficient salt-collecting desalination, featuring a dynamic evaporation front based on the accumulated salt layer where water serves as its own absorber via energy down-conversion. When processing 7 wt % brine, it continuously evaporates water at a high rate─2.25 L m -2 h -1 under 1 sun illumination─which is well beyond the input solar energy limit for over 366 h. It is revealed that such enhanced evaporation arises from the unique vertical evaporation wall of the paper-TREES, which captures the thermal energy from the heated bottom efficiently and gains extra energy from the warmer environment. These findings provide novel insights into the design of next-generation salt-harvesting solar evaporators and take a step further to advance their applications in green desalination.

The exact fabrication of precise three-dimensional structures for piezoresistive sensors necessitates superior manufacturing methods or tooling, which are accompanied by time-consuming processes and the potential for environmental harm. Herein, we demonstrated a method for in situ synthesis of zinc oxide nanorod (ZnO NR) arrays on graphene-treated cotton and paper substrates and constructed highly sensitive, flexible, wearable, and chemically stable strain sensors. Based on the structure of pine trees and needles in nature, the hybrid sensing layer consisted of graphene-attached cotton or paper fibers and ZnO NRs, and the results showed a high sensitivity of 0.389, 0.095, and 0.029 kPa -1 and an ultra-wide linear range of 0-100 kPa of this sensor under optimal conditions. Our study found that water absorption and swelling of graphene fibers and the associated reduction of pore size and growth of zinc oxide were detrimental to pressure sensor performance. A random line model was developed to examine the effects of different hydrothermal times on sensor performance. Meanwhile, pulse detection, respiration detection, speech recognition, and motion detection, including finger movements, walking, and throat movements, were used to show their practical application in human health activity monitoring. In addition, monolithically grown ZnO NRs on graphene cotton sheets had been integrated into a flexible sensing platform for outdoor UV photo-indication, which is, to our knowledge, the first successful case of an integrated UV photo-detector and motion sensor. Due to its excellent strain detection and UV detection abilities, these strategies are a step forward in developing wearable sensors that are cost-controllable and high-performance.

Composite adsorbents based on metal-organic frameworks (MOFs) are excellent candidates for solid-phase extraction (SPE) due to their diverse chemical functionality and multilevel porosity. MOF superstructures based on self-assembly at room temperature (RT) could have less energy consumption and easier manipulation due to the larger complex geometry. The π-π stacking of the benzene ring could not only enhance the interaction toward hydrophobic or plane-structured targets but also be expected to promote the formation of the MOF superstructure. In this work, in the established RT self-assembly synthesis system, several factors were investigated to see how to obtain functional MOF superstructures with a regular geometry, among which the number of benzene rings in the ligand was mainly tested for its impact on self-assembly and adsorption capacity. By means of adsorption experiments and computational fluid dynamics (CFD) simulation, the relationship between structure and activity (SARs) was further explored. Interestingly, the MOF unit with the lowest specific surface area performed the best in adsorption. Then, the selected functional MOF superstructure and ionic liquid were used to produce the composite paper membrane facilely applied in the SPE device. After optimization of the preparation conditions and operation parameters, the established SPE-HPLC-UV method could selectively analyze tetracycline-like antibiotics in the range of 16.6-833.3 ng/g (ppb) in a meat sample. This work provided an RT synthesis method to produce a microsize MOF superstructure, with experimental and theoretical insights into the SARs, which could be expanded in the design of other MOF-based SPE composite membranes toward one group of analogues.

Graphene paper (GP), a macroscopic self-supporting material, has exceptional flexibility and preserves the excellent physical and chemical properties of graphene nanomaterials. But its applications in regenerative medicine remain to be further explored. Here, we biologically functionalized GP with hydroxyapatite (HA) nanorods by the use of GP-binding peptides as an affinity linker. This strategy solved two daunting challenges for regenerative medicine applications of GP: the lack of good hydrophilicity for supporting cell growth and the difficulty in forming composites by binding with nanobiomaterials. Briefly, we first screened a high-affinity GP-binding peptide (TWWNPRLVYFDY) by the phage display technique. Then we chemically conjugated the GP-binding peptide to the synthetic HA nanorods. The GP-binding peptide on the resultant HA nanorods enabled them to be bound and assembled onto the GP substrate with high affinity, forming a GP-peptide-HA composite with significantly improved hydrophilicity of GP. The composite promoted the attachment and proliferation of mesenchymal stem cells (MSCs), demonstrating its outstanding biocompatibility. Due to the unique compositions of the composite, it was also found to induce osteogenic differentiation of MSCs in vitro in the absence of other inducers in the medium, by verifying the expression of the osteogenic markers including collagen-1, bone morphogenetic proteins 2, runx-related transcription factor 2, osteocalcin, and alkaline phosphatase. Our work suggests that the GP-binding peptide can be used to link inorganic nanoparticles onto GP to facilitate the biomedical applications of GP.

Though the widely available, low-cost, and disposable papers have been explored in flexible paper-based pressure sensors, it is still difficult for them to simultaneously achieve ultrahigh sensitivity, low limit and broad range of detection, and high-pressure resolution. Herein, we demonstrate a novel flexible paper-based pressure sensing platform that features the MXene-coated tissue paper (MTP) sandwiched between a polyimide encapsulation layer and a printing paper with interdigital electrodes. After replacing the polyimide with weighing paper in the MTP pressure sensor, the silver interdigital electrodes can be recycled through incineration. The resulting pressure sensor with polyimide or paper encapsulation exhibits a high sensitivity of 509.5 or 344.0 kPa -1 , a low limit (∼1 Pa) and a broad range (100 kPa) of detection, and outstanding stability over 10 000 loading/unloading cycles. With ultrahigh sensitivity over a wide pressure range, the flexible pressure sensor can monitor various physiological signals and human movements. Configuring the pressure sensors into an array layout results in a smart artificial electronic skin to recognize the spatial pressure distribution. The flexible pressure sensor can also be integrated with signal processing and wireless communication modules on a face mask as a remote respiration monitoring system to wirelessly detect various respiration conditions and respiratory abnormalities for early self-identification of opioid overdose, pulmonary fibrosis, and other cardiopulmonary diseases.

Microfluidic paper-based sensors as a new type of microsample detection technology are widely used in medical diagnosis, environmental monitoring, and food safety testing. Inkjet printing has the advantages of simplicity, speed, flexibility, high resolution, low cost, and efficient mass production and has become one of the most cutting-edge technologies in the manufacture of paper-based sensors. In this work, a fully inkjet printing preparation method was proposed for paper-based sensors, which can achieve high-precision, multichannel, and visual fluorescence detection. Three kinds of fluorescent carbon dots (CDs; r-CDs, b-CDs, and y-CDs) were fabricated into inkjet ink by adding a suitable ratio of solvent, PEG, and surfactant FS3100 to control its viscosity, surface tension, and other influencing factors, obtaining the best-visualized fluorescence response on paper. To optimize the full inkjet printing process of the paper-based sensor, we studied the influence of factors such as the hydrophobic material AKD formula, postprocessing conditions, and the structure of the hydrophilic and hydrophobic channels on the paper-based detection accuracy, and it was found that proper AKD concentration, curing time, and temperature can make AKD fully react with paper-based surface groups and produce more hydrophobic groups on the surface and inside of the filter paper, which can form paper-based microfluidic sensors with clear boundaries and fast transmission speed at low cost and high efficiency. The fabricated sensor is used for the fluorometric determination of vitamin C (AA), NO 2 - , and sunset yellow (SY) at the same time, and the limits of visual detection by eyes are 6 mmol/L (NO 2 - ), 60 μmol/L (SY), and 40 mmol/L (AA). The mechanism of inkjet printing is investigated in detail, which is simple, reliable, and easy to realize mass production and can realize highly sensitive, on-site, and visual detection for food additives.

As most of the known systems are flashtype, long-lasting chemiluminescence (CL) emissions are extremely needed for the application of cold light sources, accurate CL quantitative analysis, and biological mapping. In this work, the flashtype system of luminol was altered to a long lasting CL system just because of the paper substrate. The Cu(II)-based organic complex was loaded on the paper surface, which can trigger luminol-H 2 O 2 to produce a long lasting CL emission for over 30 min. By using 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) as the ligand, a hexacoordinated Cu(II)-based organic complex was synthesized by the simple freeze-drying method. It is interesting that the complex morphology can be controlled by adding different amounts of water in the synthesizing procedure. The complex with a certain size can be definitely trapped in the pores of the cellulose. Then, slow diffusion, which can be attributed to the long lasting CL emission, was produced. With the high catalytic activity of the complex, reactive oxygen species from H 2 O 2 was generated and was responsible for the high CL intensity. By using the paper substrate, the flash-type luminol system can be easily transferred to the long-duration CL system without any extra reagent. This long-lasting emission system was used for hydrogen sulfide detection by the CL imaging method. This paper-based sensor has great potential for CL imaging in the clinical field in the future.

Timely and rapid detection of biomarkers is extremely important for the diagnosis and treatment of diseases. However, going to the hospital to test biomarkers is the most common way. People need to spend a lot of money and time on various tests for potential disease detection. To make the detection more convenient and affordable, we propose a paper-based aptasensor platform in this work. This device is based on a cellulose paper, on which a three-electrode system and microfluidic channels are fabricated. Meanwhile, novel nanomaterials consisting of amino redox graphene/thionine/streptavidin-modified gold nanoparticles/chitosan are synthesized and modified on the working electrode of the device. Through the biotin-streptavidin system, the aptamer whose 5'end is modified with biotin can be firmly immobilized on the electrode. The detection principle is that the current generated by the nanomaterials decreases proportionally to the concentration of targets owing to the combination of the biomarker and its aptamer. 17β-Estradiol (17β-E2), as one of the widely used diagnostic biomarkers of various clinical conditions, is adopted for verifying the performance of the platform. The experimental results demonstrated that this device enables the determination of 17β-E2 in a wide linear range of concentrations of 10 pg mL -1 to 100 ng mL -1 and the limit of detection is 10 pg mL -1 (S/N = 3). Moreover, it enables the detection of targets in clinical serum samples, demonstrating its potential to be a disposable and convenient integrated platform for detecting various biomarkers.

Versatile sensing matrixes are essential for the development of enzyme-immobilized optical biosensors. A novel three-dimensional titanium dioxide nanotubes/alginate hydrogel scaffold is proposed for the detection of sweat biomarkers, lactate, and glucose in artificial sweat. Hydrothermally synthesized titanium dioxide nanotubes were introduced to the alginate polymeric matrix, followed by cross-linking nanocomposite with dicationic calcium ions to fabricate the scaffold platform. Rapid colorimetric detection (blue color optical signal) was carried out for both lactate and glucose biomarkers in artificial sweat at 4 and 6 min, respectively. The superhydrophilicity and the capillarity of the synthesized titanium dioxide nanotubes, when incorporated into the alginate matrix, facilitate the rapid transfer of the artificial sweat components throughout the sensor scaffold, decreasing the detection times. Moreover, the scaffold was integrated on a cellulose paper to demonstrate the adaptability of the material to other matrixes, obtaining fast and homogeneous colorimetric detection of lactate and glucose in the paper substrate when image analysis was performed. The properties of this new composite provide new avenues in the development of paper-based sensor devices. The biocompatibility, the efficient immobilization of biological enzymes/colorimetric assays, and the quick optical signal readout behavior of the titanium dioxide nanotubes/alginate hydrogel scaffolds provide a prospective opportunity for integration into wearable devices.

Hypoxia is a nonphysiological oxygen tension which is common in most malignant tumors. Hypoxia stimulates complicated cell signaling networks in cancer cells, e.g., the HIF, PI3K, MAPK, and NFκB pathways. Then, cells release a number of cytokines such as VEGFA to promote the growth of peripheral blood vessels and lead to metastasis. In the current work, understanding of the internal hypoxic environment in solid tumor tissue was attempted by developing a folding paper system. A paper-based solid tumor was constructed by folding a filter paper cultured with cancer cells. The cellular response in each layer could be analyzed by disassembling the folded paper after the culture course. The result showed that an internal hypoxic environment was successfully reproduced in the paper-based solid tumor. The cells in the inner layer expressed high levels of HIF1-α and VEGFA. Hence, proliferation and migration of endothelial cells were shown to be induced by the cells located in the internal hypoxic environment. Moreover, the paper-based solid tumor was transplanted into nude mice for the study of hypoxic response and angiogenesis. The crosstalk between internal and external parts of solid tumor tissue could be analyzed by sectioning each layer of the paper-based solid tumor. This approach provides a favorable analytical method for the discovery of the interaction between cancer cells, hypoxia, and peripheral angiogenesis.

In this work, we fabricated a novel origami paper-based analytical device (oPAD) assisted by the nanostructuring effect of in situ Pd decoration of Cu/Co-doped CeO 2 (CuCo-CeO 2 -Pd) nanospheres, functionalized with their strongly enhanced electrocatalytic properties to realize an electrochemical and visual signal readout system in oPAD, for highly sensitive detection of amyloid-β (Aβ). The CuCo-CeO 2 -Pd nanospheres were introduced as an enhanced "signal transducer layer" on account of the electron transfer acceleration caused by catalyzing glucose to produce H 2 O 2 for differential pulse voltammetry signal readout and further 3,3'5,5'-tetramethylbenzidine (TMB) oxidation for colorimetric analysis. Meanwhile, for achieving superior performance of the proposed oPAD, in situ growth of urchin-like gold nanoparticles (Au NPs) onto cellulose fibers was adopted to improve "the recognition layer" in favor of immobilizing antibodies for targeting Aβ through specific antigen-antibody interactions. Combined with the delicate design of oPAD, exhibiting actuation of the conversion procedure between hydrophobicity and hydrophilicity on paper tabs in the assay process, the oPAD successfully enabled sensitive diagnosis of Aβ in a linear range from 1.0 pM to 100 nM with a limit of detection of 0.05 pM (S/N = 3) for electrochemical detection, providing a reliable strategy for quantifying the Aβ protein in clinical applications.

Cellulose nanopaper (CNP) has been considered as a promising material with great application potential in diverse fields. However, the hydrophilic nature of CNP significantly limits its practical application. In order to improve its water resistance, we demonstrate a facile approach to functionalize CNP by impregnating it with chitosan (CS), followed by in situ polymerization of polypyrrole (PPy). The results indicate that the obtained CNP/CS/PPy shows excellent water resistance with the wet tensile strength of up to 80 MPa, which is more than 10 times higher than that of the pure CNP. Intriguingly, new features ( e.g. , electrical conductivity, antibacterial activity, and so forth) are achieved at the same time. The functionalized CNP/CS/PPy shows a high conductivity of 6.5 S cm -1 , which can be used for electromagnetic interference shielding applications with a high shielding performance of around 18 dB. In addition, the CNP/CS/PPy exhibits good antibacterial activity toward Staphylococcus aureus and Escherichia coli , with the bacterial reductions of 99.28 and 95.59%, respectively. Thus, this work provides a simple and versatile approach to functionalize CNP for achieving multifunctional properties.

In this work, a self-circulation oxygen-hydrogen peroxide-oxygen (O 2 -H 2 O 2 -O 2 ) system with photogenerated electrons as fuel and highly active hemin monomers as operators was engineered for ultrasensitive cathode photoelectrochemical bioassay of microRNA-141 (miRNA-141) using a stacked sealed paper device. During the circulation, the photogenerated electrons from BiVO 4 /Cu 2 O photosensitive structures assembled on a reduced graphene oxide paper electrode first reduced the electron acceptors (dissolved O 2 ) to H 2 O 2 , which was then catalytically decomposed by hemin monomers to generate O 2 again. The regenerated O 2 continued to be reduced, which made O 2 and H 2 O 2 stuck in the infinite loop of O 2 -H 2 O 2 -O 2 accompanied by the fast consumption of photogenerated electrons, generating an amplified photocurrent signal. When a target existed, a duplex-specific nuclease-induced target recycling reaction with dual trigger DNA probes as the output was performed to initiate the assembly of bridge-like DNA nanostructures, which endowed the self-circulation system with dual destruction functions as follows. (i) Reduced fuel supply: the assembled DNA bridges acting as a negatively charged barrier prevented the photogenerated electrons from participating in the O 2 reduction to H 2 O 2 . (ii) Incapacitation of operators: DNA bridging induced the dimerization of hemin monomers linked on the DNA hairpins to catalytically inactive hemin dimers, leading to the abortive regeneration of O 2 . These destruction functions resulted in the circulation interruption and a remarkably decreased photocurrent signal. Thus, the developed cathode photoelectrochemical biosensing platform achieved ultrasensitive miRNA-141 detection with a linear range of 0.25 fM to 1 nM and a detection limit of 83 aM, and it also exhibited high accuracy, selectivity, and practicability.

Novel ways to track and verify items of a high value or security is an ever-present need. Taggants made from deoxyribonucleic acid (DNA) have several advantageous properties, such as high information density and robust synthesis; however, existing methods require laboratory techniques to verify, limiting applications. Here, we leverage DNA nanotechnology to create DNA taggants that can be validated in the field in seconds to minutes with a simple equipment. The system is driven by toehold-mediated strand-displacement reactions where matching oligonucleotide sequences drive the generation of a fluorescent signal through the potential energy of base pairing. By pooling different "input" oligonucleotide sequences in a taggant and spatially separating "reporter" oligonucleotide sequences on a paper ticket, unique, sequence-driven patterns emerge for different taggant formulations. Algorithmically generated oligonucleotide sequences show no crosstalk and ink-embedded taggants maintain activity for at least 99 days at 60 °C (equivalent to nearly 2 years at room temperature). The resulting fluorescent signals can be analyzed by the eye or a smartphone when paired with a UV flashlight and filtered glasses.

On-site detection of multiple small-molecule analytes in complex sample matrixes would be highly valuable for diverse biosensing applications. Paper electrochemical devices (PEDs) offer an especially appealing sensing platform for such applications due to their low cost, portability, and ease of use. Using oligonucleotide-based aptamers as biorecognition elements, we here for the first time have developed a simple, inexpensive procedure for the fabrication of aptamer-modified multiplex PEDs (mPEDs), which can robustly and specifically detect multiple small molecules in complex samples. These devices are prepared via an ambient vacuum filtration technique using carbon and metal nanomaterials that yields precisely patterned sensing architecture featuring a silver pseudo-reference electrode, a gold counter electrode, and three gold working electrodes. The devices are user-friendly, and the fabrication procedure is highly reproducible. Each working electrode can be readily modified with different aptamers for sensitive and accurate detection of multiple small-molecule analytes in a single sample within seconds. We further demonstrate that the addition of a PDMS chamber allows us to achieve detection in microliter volumes of biological samples. We believe this approach should be highly generalizable, and given the rapid development of small-molecule aptamers, we envision that facile on-site multi-analyte detection of diverse targets in a drop of sample should be readily achievable in the near future.

Simultaneous detection of multi-biomarkers not only enhances the accuracy of disease diagnosis but also improves detection efficiency and reduces cost. It is vital to achieve portable, simple, low-cost, and simultaneous detection of biomarkers for point-of-care (POC) diagnostics in a low-resource setting. Herein, a multichannel paper chip-based gas pressure bioassay was developed for the simultaneous detection of multiple biomarkers by combining multichannel paper chips with a portable gas pressure meter. Four DNA tetrahedral probes (DTPs) were used as capture probes and were immobilized in different detection zones of the paper chips to improve hybridization efficiency and reduce nonspecific adsorption. The formation of a sandwich structure between target microRNAs (miRNAs), the capture probe, and platinum nanoparticles (PtNPs)-modified complementary DNA (PtNPs-cDNA) transformed biomolecular recognition into quantitative detection of gas pressure. Four lung cancer-related miRNAs were detected simultaneously by a portable gas pressure meter. There is a good linear relationship between gas pressure and the logarithm of miRNA concentration in the range of 10 pM to 100 nM. Compared with single-stranded DNA capture probe, the signal-to-noise (S/N) of DNA tetrahedral probes improved more than 3 times. Using ring-oven washing, the unbound reagents in all channels of the paper chip were simultaneously and continuously washed away, leading to a more cheap, simple, and fast separation than magnetic separation. Therefore, it offers a promising multichannel paper chip-based gas pressure bioassay for portable and simultaneous detection of multiple biomarkers.

The ability to synthesize laser-induced graphene (LIG) on cellulosic materials such as paper opens the door to a wide range of potential applications, from consumer electronics to biomonitoring. In this work, strain and bending sensors fabricated by irradiation of regular filter paper with a CO 2 laser are presented. A systematic study of the influence of the different process parameters on the conversion of cellulose fibers into LIG is undertaken, by analyzing the resulting morphology, structure, conductivity, and surface chemistry. The obtained material is characterized by porous electrically conductive weblike structures with sheet resistances reaching as low as 32 Ω sq -1 . The functionality of both strain (gauge factor of ≈42) and bending sensors is demonstrated for different sensing configurations, emphasizing the versatility and potential of this material for low-cost, sustainable, and environmentally friendly mechanical sensing.

Biomarkers in sweat are a largely untapped source of health information. Most of the currently available sweat harvesting and testing devices are incapable of operating under low-sweat rates such as those experienced by humans at rest. Here we analyze the in vitro and in vivo sampling of sweat through osmosis via the use of a hydrogel interfaced with the skin, without need for active perspiration. The hydrogel also interfaces with paper-based microfluidics to transport the fluid via capillary forces toward a testing zone and then evaporation pad. We show that the hydrogel solute content and area of the evaporation pad regulate the long-term extraction of sweat and its associated biomarkers. The results indicate that the platform can sample biomarkers from a model skin system continuously for approximately 12 h. On-skin testing of the platform on both resting and exercising human subjects confirms that it can sample sweat lactate directly from the surface of skin. The results highlight that lactate in sweat increases with exercise and as a direct result of muscle activity. Implementation of such new principles for sweat fluid harvesting and management via wearable patch devices can contribute toward the advancement of next generation wearables.

Flexible paper-based sensors may be applied in numerous fields, but this requires addressing their limitations related to poor thermal and water resistance, which results in low service life. Herein, we report a paper-based composite sensor composed of carboxylic carbon nanotubes (CCNTs) and poly- m -phenyleneisophthalamide (PMIA), fabricated by a facile papermaking process. The CCNT/PMIA composite sensor exhibits an ability to detect pressures generated by various human movements, attributed to the sensor's conductive network and the characteristic "mud-brick" microstructure. The sensor exhibits the capability to monitor human motions, such as bending of finger joints and elbow joints, speaking, blinking, and smiling, as well as temperature variations in the range of 30-90 °C. Such a capability to sensitively detect pressure can be realized at different applied frequencies, gradient sagittas, and multiple twists with a short response time (104 ms) even after being soaked in water, acid, and alkali solutions. Moreover, the sensor demonstrates excellent mechanical properties and hence can be folded up to 6000 times without failure, can bear 5 kg of load without breaking, and can be cycled 2000 times without energy loss, providing a great possibility for a long sensing life. Additionally, the composite sensor shows exceptional Joule heating performance, which can reach 242 °C in less than 15 s even when powered by a low input voltage (25 V). From the perspective of industrialization, low-cost and large-scale roll-to-roll production of the paper-based sensor can be achieved, with a formed length of thousands of meters, showing great potential for future industrial applications as a wearable smart sensor for detecting pressure and temperature, with the capability of electric heating.

The plasmonic properties of gold nanoparticles (AuNPs) are a promising tool to develop sensing alternatives to traditional, enzyme-catalyzed reactions. The need for sensing alternatives, especially in underdeveloped areas of the world, has given rise to the application of nonenzymatic sensing approaches paired with cellulosic substrates to biochemical analysis. Herein, we present three individual, low-step, wet-chemistry, colorimetric assays for three target biomarkers, namely, glucose, uric acid, and free cholesterol, relevant in diabetes control and their translation into paper-based assays and microfluidic platforms for multiplexed analysis. For glucose determination, an in situ AuNPs synthesis approach was applied into the developed μPAD, giving semiquantitative measures in the physiologically relevant range. For uric acid and cholesterol determination, modified AuNPs were used to functionalize paper with a gold-on-paper approach with the optical properties changing based on different aggregation degrees and hydrophobic properties of particles dependent on analyte concentration. These paper-based assays show sensitivity ranges and limits of detection compatible for target analyte level determination and detection limits comparable to those of similar enzymatic, colorimetric systems, relying only on plasmonic transduction without the need for enzymatic activity or other chromogenic substrates. The resulting paper-based assays were integrated into a single 3D, multiplex paper-based device using paper microfluidics, showing the capability for performing different colorimetric assays with distinct requirements in terms of sample flow and sample uptake in test zones using a combination of both horizontal and vertical flows inside the same device. The presented device allows for multiparametric, colorimetric measures of different metabolite levels from a single complex sample matrix drop using digital color analysis, showing the potential for development of low-cost, low-complexity tools for diagnostics toward the point-of-care.

There is currently a growing demand for flexible strain sensors with high performance and water repellency for various applications such as human motion monitoring, sweat or humidity detection, and certain underwater tests. Among these strain sensors, paper-based ones have attracted increasing attention because they coincide with the future development trend of environment-friendly electronic products. However, paper-based electronics are easy to fail when they encounter water and are thus unable to be applied to humid or underwater circumstances. Herein, based on a strategy of coupling bionics inspired by lotus leaf and scorpion, which exhibit superhydrophobic characteristics and ultrasensitive vibration-sensing capacity, respectively, a paper-based strain sensor with high sensitivity and water repellency is successfully fabricated. As a result, the strain sensor exhibits a gauge factor of 263.34, a high strain resolution (0.098%), a fast response time (78 ms), excellent stability over 12,000 cycles, and a water contact angle of 164°. Owing to the bioinspired structures and function mechanisms, the paper-based strain sensor is suitable to not only serve as regular wearable electronics to monitor human motions in real-time but also to detect subtle underwater vibrations, demonstrating its great potential for numerous applications like wearable electronics, water environmental protection, and underwater robots.

Development of selective and sensitive methods for on-site assay of tetracycline (TC) is of great significance for public health and food safety. Herein, a valid ratiometric fluorescence strategy using g-C 3 N 4 nanosheets coupled with Eu 3+ is designed for the assay of TC. In this strategy, both Eu 3+ and g-C 3 N 4 nanosheets serve as the recognition units of TC. The blue fluorescence of g-C 3 N 4 nanosheets can be quenched by TC via the inner filter effect (IFE); meanwhile, the red fluorescence of Eu 3+ can be enhanced by TC through the antenna effect (AE). The synergistic effect of AE and IFE caused by TC makes the developed ratiometric fluorescent sensor display a wide linear range for TC from 0.25 to 80 μM with a detection limit of 6.5 nM and a significant fluorescence color evolution from blue to red. Given its simplicity, free-label, excellent selectivity, high sensitivity, and recognizable color change, point-of-care testing systems, including smartphones and test paper-based assays, are developed for the visual sensing of TC. The integration of smartphones and test paper on a ratiometric fluorescent sensor greatly reduces the detection cost and time, providing a promising method for the qualitative discernment and semi-quantitative assay of TC on-site. Moreover, the potential application of the approach is also verified by detecting TC in milk.

A soft body area sensor network presents a promising direction in wearable devices to integrate on-body sensors for physiological signal monitoring and flexible printed circuit boards (FPCBs) for signal conditioning/readout and wireless transmission. However, its realization currently relies on various sophisticated fabrication approaches such as lithography or direct printing on a carrier substrate before attaching to the body. Here, we report a universal fabrication scheme to enable printing and room-temperature sintering of the metal nanoparticle on paper/fabric for FPCBs and directly on the human skin for on-body sensors with a novel sintering aid layer. Consisting of polyvinyl alcohol (PVA) paste and nanoadditives in the water, the sintering aid layer reduces the sintering temperature. Together with the significantly decreased surface roughness, it allows for the integration of a submicron-thick conductive pattern with enhanced electromechanical performance. Various on-body sensors integrated with an FPCB to detect health conditions illustrate a system-level example.

As new infectious disease outbreaks become more likely, it is important to be able to develop and deploy appropriate testing in time. Paper-based immunoassays are rapid, cheap, and easy to produce at scale and relatively user friendly but often suffer from low selectivity and sensitivity. Understanding the molecular mechanisms of paper immunoassays may help improve and hasten development and therefore production and market availability. Here, we study how the behavior of nanoparticle-antibody immunoprobes in paper dipstick immunoassays is impacted by synthesis strategy and surface chemistry architecture. We conjugate gold nanoparticles to polyclonal anti-immunoglobulin G (IgG) and anti-zika NS1 antibodies by electrostatic adsorption and N -hydroxysuccinimide (NHS) and hydrazide (Hz) chemistries. The immunoprobes were used in paper immunoassays and the effective affinity for the antigen was quantified from the test line intensities, as well as the distribution of the immunoprobes throughout the strips. The results show that nanoparticle colloidal stability, both post synthesis and during antigen binding, is a key factor and affects immunoassay results and performance, often through reduction or loss of signal.

As a global shift continues to occur in high burden diseases toward developing countries, the importance of medical diagnostics based on point-of-care testing (POCT) is rapidly increasing. However, most diagnostic tests that meet clinical standards rely on high-end analyzers in central hospitals. Here, we report the development of a simple, low-cost, mass-producible, highly sensitive/quantitative, automated, and robust paper/soluble polymer hybrid-based lateral flow biosensing platform, paired with a smartphone-based reader, for high-performance POCT. The testing architecture incorporates a polymeric barrier that programs/automates sequential reactions via a polymer dissolving mechanism. The smartphone-based reader with simple opto-mechanical parts offers a stable framework for accurate quantification. Analytical performance of this platform was evaluated by testing human cardiac troponin I (cTnI), a preferred biomarker for the diagnosis of myocardial infarction, in serum/plasma samples. Coupled with catalytic/colorimetric gold-ion amplification, this platform produced results within 20 min with a detection limit of 0.92 pg mL -1 and a coefficient of variation <10%, which is equivalent to the performance of a high-sensitivity standard analyzer, and operated within acceptable levels stipulated by clinical guidelines. Moreover, cTnI clinical sample tests indicate a high correlation ( r = 0.981) with the contemporary analyzers, demonstrating the clinical utility of this platform in high-performance POCT.

In this study, a highly flexible, patternable multiwalled-carbon nanotube (MWCNT) paper electrode was specially designed and fabricated. The addition of a nitrocellulose (NC) polymer binder at less than the critical amount (≤2 wt %) was found to be effective for maintaining both the flexibility and electrical conductance of the resulting MWCNT paper electrode. The fabricated MWCNT paper electrode was then employed as a heating platform to ignite Al/CuO nanoparticle-based nanoenergetic materials (nEMs). The nEM layer was drop-cast on the surface of the MWCNT paper electrode with specially patterned shapes using a plotter, and its ignition was evaluated by applying various voltages through the MWCNT paper electrode. To increase the adhesion between the nEM layer and MWCNT paper electrode and to decrease the sparking sensitivity of the nEM layer, it was essential to incorporate NC in the nEM matrix. However, the combustion and explosion properties of nEM layers deteriorated with the addition of NC, enabling the estimation of the optimum amount of NC to be incorporated. The fabricated igniter can be employed in various thermal engineering applications, such as in the ignition of explosives and propellants, and in pyrotechnics. To demonstrate this, a compact, flexible, and patternable igniter composed of the NC/nEM layer (NC/nEM = 2:8 wt %) on an MWCNT paper electrode was used to successfully ignite solid propellants for launching a small rocket.

Lots of beetles, moths, and birds in the natural world present stunning unique structural colors as well as excellent hydrophobic performances. Herein, a novel bioinspired variable structural-color film with organic-solvent responsiveness and surface hydrophobicity was fabricated. Cellulose nanocrystals (CNCs) provided structural color with left-handed helicity. PEG-PPG-PEG triblock copolymers (PPPTCs) were blended with CNCs, giving rise to the organic-solvent-responsive structural color and wider red-shift window of the reflectance peak. The color of the film could be regulated repeatedly under the stimulus of cyclohexanone with an obvious red shift up to 107 nm, corresponding to a macroscopic color change from blue to yellow. Low-surface-energy compound hexadecyltrimethoxysilane (HDTMS) was covalently grafted on the surface in a one-step method to introduce hydrophobicity, successfully preventing the effect of water on the ordered nanostructure. Based on the bionics principle, the as-prepared CNC/PPPTC nanocomposite films with variable structural colors and hydrophobicity are beneficial to their prospective applications in display screens, rewritable hydrophobic structural-color-changing paper, biomimetic sensors, and so forth.

Radiation dosimeters are critical for accurately assessing the levels of radiation exposure of tumor sites and surrounding tissues and for optimizing therapeutic interventions as well as for monitoring environmental exposure. To fill the need for a simple, user-friendly, and inexpensive dosimeter, we designed an innovative colorimetric nanosensor-based assay for detecting ionizing radiation. We show that hydroxyl radicals generated by ionizing radiation can be used to etch gold nanorods (AuNRs) and silver nanoprisms (AgNPRs), yielding reproducible color changes for radiation dose detection in the range of 50-2000 rad, broad enough to cover doses used in hyperfractionated, conventional, and hypofractionated radiotherapy. This range of doses detected by this assay correlates with radiation-induced DNA damage response in mammalian cells. Furthermore, this AuNR- and AgNPR-based sensing platform has been established in a paper format that can be readily adopted for a wide range of applications and translation.

A new class of biosensing transducer based on alternating-current electroluminescent (ACEL) display is demonstrated. Unlike conventional ACEL displays where they have been rigidly used in flexible screens and advertising applications, here, the display is integrated with immunoassay and functioned as an optical transducer. Taking advantage of the reversed ACEL architecture, the display can be simply fabricated on an unconventional paper material without requiring the transparent indium tin oxide (ITO) electrode. The sensing mechanism relies on the promoted electronic conduction from the immunocomplex formation between immobilized antibody, antigen, and nanoparticle labeled antibody. As a result, the electroluminescence could be triggered off instantaneously. To demonstrate the device effectiveness, C-reactive protein (CRP), a particular biomarker of an inflammatory process and cardiovascular disease, is chosen as a model analyte in this work. Additionally, the applicability of the proposed platform is proved efficacious in human serums, showing negligible interference from nontargeting proteins. The sensing display is also capable of performing multiple assays (up to 8) within a single device. This bio-optoelectronic device represents a straightforward yet highly sensitive approach. This ACEL transducer is believed to explore new possibilities for biosensing and exploit in point-of-care testing.

We demonstrate a versatile and easily fabricated paper-based CO 2 sensor. The sensor consists of a specially designed fluorescent color-shift chromophore infused into standard filter paper. The emission color of the resulting fluorescent paper changes upon exposure to CO 2 due to the formation of carbonic acid, which underlies the sensing mechanism. By using a ratiometric method, the undesirable effects of photobleaching can be eliminated, leading to a stable and repeatable sensor performance. These multiuse sensors have a response time on the order of 1 min and feature low detection limits for a paper-based CO 2 gas sensor, suggesting possible low-cost applications in smart buildings or other facilities in which CO 2 levels are required to be continuously monitored.

Paper-based packaging is widely employed in industries ranging from food to beverages to pharmaceuticals because of its attractive advantages of biodegradability, recyclability, good strength, low cost, and lightweight. However, paper products usually have poor water barrier resistance properties because of paper and fibers porous microstructure. In this study, an ecofriendly water-resistant (hydrophobic) oil from biological origin, namely, palm kernel oil (PKO) was used to coat paper by using a facile and cost-effective dip-casting approach. PKO formulation was prepared by mixing with a solvent and furfuryl alcohol (FA). The water resistance, structural properties, and thermal and mechanical properties of the coated papers obtained under different processing conditions were reported and compared to understand the performance of coated paper. Contact angle (CA), Fourier transform infrared (FTIR), and thermal gravimetry (TGA) were used for analysis and characterization of coated papers. Data from contact angle measurements showed that the PKO formulation could considerably improve the liquid water barrier property of the paper, with a measured water contact angle (CA) of ∼120° and reduce the water vapor transmission rate (WVTR) by 22%. This novel, green, low-cost, and water-resistant paper coating made with biological and biodegradable oil is a potential candidate for replacing petroleum-based coatings used in a broad range of applications and will also be able to make an additional full use of the palm kernel oil.

In this work, a lab-on-paper cathode photoelectrochemical (PEC) sensing platform was constructed for ultrasensitive microRNA-141 (miRNA-141) assay using cascaded multiple photo-active structures as signal generators and hemin/Pt nanoparticle (Pt NP) trunk-branching-decorated DNA dendrimers as signal reinforcers. Specifically, pyramid-like Cu 2 O was first in situ grown on the Au nanoparticle-functionalized tangled cellulose fibers network, followed by the sensitization of trepang-like BiVO 4 -Bi 2 S 3 heterostructures, forming the cascaded sensitization structures. Then, the DNA dendrimer was introduced into the photocathode sensing interface by coupling the duplex-specific-nuclease (DSN)-induced target recycling reaction with multiple-branched hybridization chain reaction (MHCR). The programmed target recycling procedures propelled using DSN guaranteed the highly amplified transduction of miRNA-141 to the exposed initiator strand, which triggered the cascaded MHCR accompanied by the formation of the DNA dendrimer with unique trunk-branching structures. Finally, the hemin/Pt NP trunk-branching-decorated DNA dendrimer (HPTD) was acquired by the assembly of Pt NPs and hemin on the trunk and branch, respectively. The resulting HPTD with the synergy catalysis of Pt NPs and hemin could efficiently catalyze the decomposition of H 2 O 2 for in situ generation of O 2 as the electron acceptor, leading to an enhanced photocurrent response. Based on the target-dependent photocurrent enhancement, ultrasensitive determination of miRNA-141 was realized with persuasive selectivity, high stability, and excellent reproducibility. Thus, the proposed paper-based cathode PEC sensing platform possessed promising application prospect in clinical miRNA diagnosis.

Lithium-sulfur (Li-S) batteries are highly attractive for their theoretical energy density and natural abundance, but the drawbacks of low sulfur utilization and rapid capacity fade in high-sulfur-loading cathodes still retard their practical use. To enhance kinetics in high-sulfur-loading Li-S cells, it is important to first understand and control the deposition of Li 2 S/Li 2 S from highly soluble lithium polysulfide (LiPS) during discharge processes. Here, we presented a series of multiphase-derived self-standing papers with diverse electronic conductivity and LiPS affinity for highly concentrated LiPS discharge processes and explained the Li 2 S/Li 2 S deposition behavior in detail. We demonstrated that high rate capacity and long cycle life of as-assembled paper-LiPS cathodes can be greatly depended on their phase material with high conductivity and LiPS affinity. A high-performance self-standing LiPS host-multiwalled carbon nanotube (MWCNT)/cellulose nanofiber (CNF)/NiCo 2 S 4 (3.5 mg cm -2 ) can catalyze 2.85 mg cm -2 (based on sulfur) loaded LiPS to deliver a high specific capacity of 1154 mAh g -1 at 0.1C and a high rate performance of 963 mAh g -1 at 1C. We suggest that the insulating phase defect of nano-CNF and both highly electronic conductive (above 50 S cm -1 ) and LiPS adsorptive NiCo 2 S 4 can promote the local concentration effect of LiPS, thus contributing to fast and stable heterogeneous particle-shaped deposition of Li 2 S 2 /Li 2 S and leading to high kinetics of the LiPS cathode.

Because of numerous inherent and unrivaled features of nanofibers made of chitin, the second most plentiful natural-based polymer (after cellulose), including affordability, abundant nature, biodegradability, biocompatibility, commercial availability, flexibility, transparency, and extraordinary mechanical and physicochemical properties, chitin nanofibers (ChNFs) are being applied as one of the most appealing bionanomaterials in a myriad of fields. Herein, we exploited the beneficial properties offered by the ChNF paper to fabricate transparent, efficient, biocompatible, flexible, and miniaturized optical sensing bioplatforms via embedding/immobilizing various plasmonic nanoparticles (silver and gold nanoparticles), photoluminescent nanoparticles (CdTe quantum dots, carbon dots, and NaYF 4 :Yb 3+ @Er 3+ &SiO 2 upconversion nanoparticles) along with colorimetric reagents (curcumin, dithizone, etc.) in the 3D nanonetwork scaffold of the ChNF paper. Several configurations, including 2D multi-wall and 2D cuvette patterns with hydrophobic barriers/walls and hydrophilic test zones/channels, were easily printed using laser printing technology or punched as spot patterns on the dried ChNF paper-based nanocomposites to fabricate the (bio)sensing platforms. A variety of (bio)chemicals as model analytes were used to confirm the efficiency and applicability of the fabricated ChNF paper-based sensing bioplatforms. The developed (bio)sensors were also coupled with smartphone technology to take the advantages of smartphone-based monitoring/sensing devices along with the Internet of Nano Things (IoNT)/the Internet of Medical Things (IoMT) concepts for easy-to-use sensing applications. Building upon the unrivaled and inherent features of ChNF as a very promising bionanomaterial, we foresee that the ChNF paper-based sensing bioplatforms will emerge new opportunities for the development of innovative strategies to fabricate cost-effective, simple, smart, transparent, biodegradable, miniaturized, flexible, portable, and easy-to-use (bio)sensing/monitoring devices.

Real-time monitoring of neonicotinoid pesticide residues is of great significance for food security and sustainable development of the ecological environment. Herein, a paper-based surface-enhanced Raman scattering (SERS) amplified approach was proposed by virtue of multilayered plasmonic coupling amplification. The unique plasmonic SERS multilayer was constructed using three-dimensional (3D) silver dendrite (SD)/electropolymerized molecular identifier (EMI)/silver nanoparticle (AgNP) sandwich hybrids with multiple hotspots and a strong electromagnetic field in nanogaps. Dendritelike 3D silver materials with remarkably high accessible surface areas and the lightning rod effect constituted the first-order enhancement of paper-based sensors. Molecular identifiers coated upon an SD layer as the interlayer were used for target capture and enrichment. Subsequently, AgNPs featuring rough surface and local plasma resonance decorated as the top layer formed the secondary enhancement of the amplification strategy. As the most brilliant part, dendritelike 3D silver coupled with AgNPs has established double Ag layers to accomplish a multistage enhancement of SERS signals based on the superposition of their electromagnetic fields. Owning to the distinctive design of the multiple coupling amplification strategy, the fabricated SERS paper chips demonstrated impressive specificity and ultrahigh sensitivity in the detection of imidacloprid (IMI), with a detection limit as low as 0.02811 ng mL -1 . More importantly, the multiple SERS enhancement paper chip holds great potential for automated screening of a variety of contaminants.

Design and fabrication of flexible Li-ion batteries (FLIBs) with excellent electrochemical and structural stability via scalable fabrication techniques are important for their practical applications. A wide range of FLIBs with excellent flexibility have been reported. However, sophisticated designs and complex fabrication techniques are often used in fabricating FLIBS, making them difficult to be realized in industrial production. Here, we fabricate FLIBs with an integrated structure by assembling the LiFePO 4 cathode, Li 4 Ti 5 O 12 anode, graphene current collectors, and poly(vinylidene fluoride) (PVDF) electrolyte all together on commercial printing paper via conventional and scalable Meyer rod coating. In the design, the commercial paper serves as a flexible substrate to enable good flexibility of the device, and the paper is coated twice with PVDF to avoid the short-circuit problem and create a strong binding to integrate the device. The resultant integrated FLIBs exhibit excellent internal structural stability and good electrochemical performance under cycling bending for 100 times.

Paper-based microfluidic analytical devices (μPADs) have recently attracted attention as a point-of-care test kit because of their low cost and nonrequirement for external forces. To directly detect biomarkers in whole blood, however, they need to be assembled with a filter such as a plasma separation membrane (PSM) because the color of the blood cells interferes with the colorimetric assay. However, this assembly process is rather complicated and cumbersome, and the fluid does not uniformly move to the detection zone when the adhesion between the paper and PSM is not perfect. In this study, we report a simple three-dimensional (3D) printing method for fabricating PSM-integrated 3D-μPADs made of plastics without the need for additional assembly. In detail, PSM was coated with parylene C to prevent its dissolution from organic solvent during 3D printing. Then, the coated PSM was superimposed on the paper. Detection zones and a reservoir were printed on the paper and PSM via liquid photopolymerization, using a digital light processing printer. The limit of detection of the PSM-integrated 3D-μPADs for glucose in whole blood was 0.3 mM, and these devices demonstrated clinically relevant performance on diabetes patient blood samples. Our 3D-μPADs can also simultaneously detect multiple metabolic disease markers including glucose, cholesterol, and triglycerides in whole blood. Our results suggest that our printing method is useful for fabricating 3D-μPADs integrated with PSM for the direct detection of biomarkers in whole blood.