Fully Inkjet-Printed Paper-Based Potentiometric Ion-Sensing Devices

Miniaturized inkjet-printed flexible ion-selective sensing electrodes with the addition of graphene in PVC layer for fast response real-time monitoring applications

In this letter, we propose a miniaturization scheme of inkjet printed ionic sensing electrodes by adding graphene into the ion-selective PVC film not only to reduce the impedance of the ionic liquid layer of the electrode but also to increase the electrode capacitance for the reduction of the response time. Based on the scheme, we present a fully inkjet-printed electrochemical ion-selective sensor comprising a working electrode and reference electrode, which are inkjet-printed Ag NPs/PEDOT:PSS-graphene/PVC-graphene and Ag/AgCl(s)/ionic liquid PVC-graphene layer structures, respectively. The printed ion-selective working electrode has been miniaturized to a size of 22,400 μm2 equivalent to a square shape of ∼150 × 150 μm2 comparable to the size of a human cell. By adding graphene to the ion selective PVC film, more than 90 % charge transfer resistance reduction can be achieved and the shunt capacitance is increased by 3.4-fold in shunt capacitance compared to the film without graphene, thereby more than 33 % reduction of the response time required to reach equilibrium. Meanwhile, these miniaturized potassium sensors using the working electrodes with/without adding graphene have been integrated with in-lab signal-processing and wireless-transmission module to yield similar results to the one measured by commercial electrochemical workstation showing a great potential for real-time monitoring in portable clinical trials. Specifically, the proposed sensor utilizing graphene-enhanced electrodes demonstrates a linearity uncertainty of 2.9 mV, which is approximately half of the uncertainty observed in the sensors lacking graphene integration.

Chronopotentiometric sensors for antimicrobial peptide-based biosensing of Staphylococcus aureus

Charged antimicrobial peptides can be used for direct potentiometric biosensing, but have never been explored. We report here a galvanostatically-controlled potentiometric sensor for antimicrobial peptide-based biosensing. Solid-state pulsed galvanostatic sensors that showed excellent stability under continuous galvanostatic polarization were prepared by utilizing reduced graphene oxide/poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (rGO/PEDOT: PSS) as a solid contact. More importantly, the chronopotentiometric sensor can be made sensitive to antimicrobial peptides with intrinsic charge on demand via a current pulse. In this study, a positively charged antimicrobial peptide that can bind to Staphylococcus aureus with high affinity and good selectivity was designed as a model. Two arginine residues with positive charges were linked to the C-terminal of the peptide sequence to increase its potentiometric responses on the electrode. The bacteria binding-induced charge or charge density change of the antimicrobial peptide enables the direct chronopotentiometric detection of the target. Under the optimized conditions, the concentration of Staphylococcus aureus can be determined in the linear range 10-1.0 × 105 CFU mL-1 with a detection limit of 10 CFU mL-1. It is anticipated that such a chronopotentiometric sensing platform is readily adaptable to detect other bacteria by choosing the peptides.

Noninvasive and Multiplex Self-Test of Kidney Disease Biomarkers with Graphene-Based Lab-on-a-Chip (G-LOC): Toward Digital Diagnostics in the Hands of Patients

Chronic kidney disease is one of the major health issues worldwide. However, diagnosis is now highly centralized in large laboratories, resulting in low access to patient monitoring and poor personalized treatments. This work reports the development of a graphene-based lab-on-a-chip (G-LOC) for the digital testing of renal function biomarkers in serum and saliva samples. G-LOC integrates multiple bioelectronic sensors with a microfluidic system that enables multiplex self-testing of urea, potassium, sodium, and chloride. The linearity, limit of detection (LOD), accuracy, and coefficient of variability (CV) were studied. Accuracy values higher than 95.5% and CV lower than 9% were obtained for all of the biomarkers. The analytical performance was compared against three reference lab benchtop analyzers by measuring healthy- and renal-failure-level samples of serum. From receiver operating characteristic (ROC) plots, sensitivities (%) of 99.7, 97.6, 99.1, and 89.0 were obtained for urea, potassium, sodium, and chloride, respectively. Then, the test was evaluated in noninvasive saliva samples and compared against reference methods. Correlation and Bland-Altman plots showed good correlation and agreement of the G-LOC with the reference methods. It is noteworthy that the precision of G-LOC was similar to better than benchtop lab analyzers, with the advantage of being highly portable. Finally, a user testing study was conducted. The analytical performance obtained with untrained volunteers was similar to that obtained with trained chemists. Additionally, based on a user experience survey, G-LOC was found to have very simple usability and would be suitable for at-home diagnostics.

Sensors for Emerging Water Contaminants: Overcoming Roadblocks to Innovation

Ensuring water quality and safety requires the effective detection of emerging contaminants, which present significant risks to both human health and the environment. Field deployable low-cost sensors provide solutions to detect contaminants at their source and enable large-scale water quality monitoring and management. Unfortunately, the availability and utilization of such sensors remain limited. This Perspective examines current sensing technologies for detecting emerging contaminants and analyzes critical barriers, such as high costs, lack of reliability, difficulties in implementation in real-world settings, and lack of stakeholder involvement in sensor design. These technical and nontechnical barriers severely hinder progression from proof-of-concepts and negatively impact user experience factors such as ease-of-use and actionability using sensing data, ultimately affecting successful translation and widespread adoption of these technologies. We provide examples of specific sensing systems and explore key strategies to address the remaining scientific challenges that must be overcome to translate these technologies into the field such as improving sensitivity, selectivity, robustness, and performance in real-world water environments. Other critical aspects such as tailoring research to meet end-users' requirements, integrating cost considerations and consumer needs into the early prototype design, establishing standardized evaluation and validation protocols, fostering academia-industry collaborations, maximizing data value by establishing data sharing initiatives, and promoting workforce development are also discussed. The Perspective describes a set of guidelines for the development, translation, and implementation of water quality sensors to swiftly and accurately detect, analyze, track, and manage contamination.

Dual Colorimetric/Electrochemical Detection of Salmonella typhimurium Using a Laser-Induced Graphene Integrated Lateral Flow Immunoassay Strip

Foodborne illnesses caused by the ingestion of contaminated foods or beverages are a serious concern due to the millions of reported cases per year. It is essential to develop sensitive and rapid detection methods of foodborne pathogens to ensure food safety for producers and consumers. Unfortunately, current detection techniques still suffer from time-consuming operations and the need for highly skilled personnel. Here, we introduce a highly sensitive dual colorimetric/electrochemical detection approach for Salmonella enterica serovar typhimurium (S. typhimurium) based on a laser-induced graphene-integrated lateral flow immunoassay (LIG-LFIA) strip. The LIG electrode was fabricated by laser engraving on a polyimide tape containing a pseudo silver/silver chloride reference electrode from silver sintering and chlorination. Using double-sided tape inserted into the strip, automatic sequential reagent delivery was enabled for the dual-mode signal readout by single-sample loading. A gold-deposited gold nanoparticle strategy was first employed to simultaneously obtain a colorimetric signal for early screening and a signal turn-on electrochemical response for high-sensitivity and -quantitative analysis. A superior performance of the strip was established, characterized by a short analysis time (12 min assay +15 min sample preparation), a broad working concentration range (1 cfu/10 mL to 108 cfu/mL), and the lowest limit of detection (1 ± 0.5 cfu/10 mL; mean ± standard deviation, n = 3) among reported multimode S. typhimurium detection schemes. The strip was successfully applied in the analysis of various food products without any bacterial enrichment or amplification required, and the results were comparable to those of the standard culture method.

Trends in Paper-Based Sensing Devices for Clinical and Environmental Monitoring

Environmental toxic pollutants and pathogens that enter the ecosystem are major global issues. Detection of these toxic chemicals/pollutants and the diagnosis of a disease is a first step in efficiently controlling their contamination and spread, respectively. Various analytical techniques are available to detect and determine toxic chemicals/pathogens, including liquid chromatography, HPLC, mass spectroscopy, and enzyme-linked immunosorbent assays. However, these sensing strategies have some drawbacks such as tedious sample pretreatment and preparation, the requirement for skilled technicians, and dependence on large laboratory-based instruments. Alternatively, biosensors, especially paper-based sensors, could be used extensively and are a cost-effective alternative to conventional laboratory testing. They can improve accessibility to testing to identify chemicals and pollutants, especially in developing countries. Due to its low cost, abundance, easy disposal (by incineration, for example) and biocompatible nature, paper is considered a versatile material for the development of environmentally friendly electrochemical/optical (bio) sensor devices. This review presents an overview of sensing platforms constructed from paper, pointing out the main merits and demerits of paper-based sensing systems, their fabrication techniques, and the different optical/electrochemical detection techniques that they exploit.

Rapid and Accurate Measurement of the Na+/K+ Balance in Urine for Remote Patient Monitoring Using a Symmetric Electrode Architecture

The daily monitoring of the electrolyte balance in human bodies is challenging due to the lack of a low-cost, user-friendly device that can be used remotely by patients. In this study, a potentiometric sensor for the measurement of the ratio of Na⁺ and K⁺ ions has been fabricated using a solid-state carbon-based internal layer. PVC-type membranes are deposited using an autodispenser with a fabrication rate of approximately one sensor every 10 s, allowing mass production with limited investments. The symmetric architecture of the sensor unit, built without a reference electrode, permits a very fast stabilization of the signal, under 20 s, even without preconditioning. Measurements using buffer solutions in the range Na⁺/K⁺ = 1 to Na⁺/K⁺ = 10 indicate that less than 2 min, including the single-point calibration, is necessary to provide an estimation of the Na⁺/K⁺ balance with the same accuracy as that of a conventional sensing unit (±10%). Five sensors have been tested repeatedly over 30 days, and they maintained a constant level of performance regarding membrane sensitivity and response time. A remote measurement for the ratio between Na⁺ and K⁺ ions in urine samples showed results in agreement with a commercially available sensor. This sensor design could create new perspectives in remote healthcare for the quick detection of several diseases related to electrolyte balance.

Wearable potentiometric biosensor for analysis of urea in sweat

Herein, we introduce wearable potentiometric biosensors on screen-printed carbon electrodes (SPCEs) for on-body and on-site monitoring of urea in sweat. The biosensor architecture was judiciously designed to detect urea at different pHs and incorporate a pH sensor, thus containing polyaniline ink, urease bioink and a polyvinylchloride membrane. Urea detection could be performed in the wide range from 5 to 200 mM at pH 7.0, encompassing urea levels in human sweat. The biosensor response was fast (incubation time 5 min), with no interference from other substances in sweat. Reliable urea detection could be done in undiluted human sweat with a skin-worn flexible device using the pH correction strategy afforded by the pH sensor. The performance of the epidermal biosensor was not affected by severe bending strains. The feasibility of mass production was demonstrated by fabricating epidermal flexible biosensors using slot-die coating with a roll-to-roll technique.

Microfluidic Paper-Based Device for Medicinal Diagnosis

BACKGROUND

The demand for point-of-care testing (POCT) devices has rapidly grown since they offer immediate test results with ease of use, makingthem suitable for home self-testing patients and caretakers. However, the POCT development has faced the challenges of increased cost and limited resources. Therefore, the paper substrate as a low-cost material has been employed to develop a cost-effective POCT device, known as "Microfluidic paper-based analytical devices (μPADs)". This device is gaining attention as a promising tool for medicinal diagnostic applications owing to its unique features of simple fabrication, low cost, enabling manipulation flow (capillarydriven flow), the ability to store reagents, and accommodating multistep assay requirements.

OBJECTIVE

This review comprehensively examines the fabrication methods and device designs (2D/3D configuration) and their advantages and disadvantages, focusing on updated μPADs applications for motif identification.

METHODS

The evolution of paper-based devices, starting from the traditional devices of dipstick and lateral flow assay (LFA) with μPADs, has been described. Patterned structure fabrication of each technique has been compared among the equipment used, benefits, and drawbacks. Microfluidic device designs, including 2D and 3D configurations, have been introduced as well as their modifications. Various designs of μPADs have been integrated with many powerful detection methods such as colorimetry, electrochemistry, fluorescence, chemiluminescence, electrochemiluminescence, and SER-based sensors for medicinal diagnosis applications.

CONCLUSION

The μPADs potential to deal with commercialization in terms of the state-of-the-art of μPADs in medicinal diagnosis has been discussed. A great prototype, which is currently in a reallife application breakthrough, has been updated.

The Effect of Paper on the Detection Limit of Paper-Based Potentiometric Chloride Sensors

While paper is an excellent material for use in many other portable sensors, potentiometric paper-based sensors have been reported to perform worse than conventional rod-shaped electrodes, in particular in view of limits of detection (LODs). Reported here is an in-depth study of the lower LOD for Cl^- measurements with paper-based devices comprising AgCl/Ag transducers. Contamination by Cl^- from two commonly used device materials─a AgCl/Ag ink and so-called ashless filter paper─was found to increase the concentration of Cl^- in paper-contained samples far above what is expected for the spontaneous dissolution of the transducer's AgCl, thereby worsening lower LODs. In addition, for the case of Ag⁺, the commonly hypothesized adsorption of metal cations onto filter paper was found not to significantly affect the performance of AgCl/Ag transducers. We note that in the context of chemical analysis, metal impurities of paper are often mentioned in the literature, but Cl^- contamination of paper has been overlooked.

A Multicalibration Urea Potentiometric Sensing Array Based on Au@urease Nanoparticles and Its Application in Home Detection

Home potentiometric sensing devices can real-time monitor personal health status and are widely used in the prevention and management of related diseases. However, variations in the composition and the pH of the sample matrix tend to change the basic potential and response slope of some potentiometric sensors, thus affecting detection reliability. Therefore, this work uses the detection of urea in urine as a model to improve reliability of the potentiometric sensor in home detection. Au@urease nanoparticles were synthesized as the sensing material to improve the stability of the urease-based potentiometric sensor. Meanwhile, a multicalibrated urea potential (MCUP) sensing array was designed, which consists of a urea electrode group, a pH electrode group, and a reference channel. The urea electrode group and the pH electrode group contain respectively a sensing channel and a calibration channel. The basic potential of sensing channels can be calibrated through the corresponding calibration channels. Moreover, the pH electrode group can not only measure the pH values of the samples but also calibrate the response slope of the urea electrode group through the calibration coefficient, thus improving the reliability of home detection. Consequently, the potentiometric sensing array based on the enzyme reaction can be applied in body fluids with a wide pH range.

Printed Potentiometric Nitrate Sensors for Use in Soil

Plant-available nitrogen, often in the form of nitrate, is an essential nutrient for plant growth. However, excessive nitrate in the environment and watershed has harmful impacts on natural ecosystems and consequently human health. A distributed network of nitrate sensors could help to quantify and monitor nitrogen in agriculture and the environment. Here, we have developed fully printed potentiometric nitrate sensors and characterized their sensitivity and selectivity to nitrate. Each sensor comprises an ion-selective electrode and a reference electrode that are functionalized with polymeric membranes. The sensitivity of the printed ion-selective electrodes was characterized by measuring their potential with respect to a commercial silver/silver chloride reference electrode in varying concentrations of nitrate solutions. The sensitivity of the printed reference electrodes to nitrate was minimized with a membrane containing polyvinyl butyral (PVB), sodium chloride, and sodium nitrate. Selectivity studies with sulphate, chloride, phosphate, nitrite, ammonium, calcium, potassium, and magnesium showed that high concentrations of calcium can influence sensor behavior. The printed ion-selective and reference electrodes were combined to form a fully printed sensor with sensitivity of −48.0 ± 3.3 mV/dec between 0.62 and 6200 ppm nitrate in solution and −47 ± 4.1 mV/dec in peat soil.

Photosensitive-Stamp-Inspired Scalable Fabrication Strategy of Wearable Sensing Arrays for Noninvasive Real-Time Sweat Analysis

Wearable sweat sensing is essential to the development of personalized health monitoring in a noninvasive manner with molecular-level insight. Hence, there is an increasing demand for convenient, facile, and efficient fabrication of wearable sensing arrays. Inspired by a photosensitive stamp (PS), we present herein a simple, low-cost, and eco-friendly vacuum filtration-transfer printing method (termed PS-VFTP) for the scalable preparation of single-walled carbon nanotube (SWCNT) based flexible electrode arrays. This method can economically yield customized flexible SWCNT arrays with praiseworthy performance, such as high reproducibility, precision, uniformity, conductivity, and mechanical stability. In addition, the flexible SWCNT arrays can be easily functionalized into high-performance electrochemical sensors for the simultaneous monitoring of sweat metabolites (glucose, lactate) and electrolytes (Na⁺, K⁺). The integration of wearable sensing arrays with a signal acquisition and processing circuit system in the intelligent wearable sensors empowers them to realize noninvasive, real-time, and in situ sweat analysis during exercise. More meaningfully, such a PS-VFTP strategy can be easily expanded to the economical manufacturing of other flexible electronic devices.

Electrochemical Devices to Monitor Ionic Analytes for Healthcare and Industrial Applications

Recent advances in electrochemical devices have sparked exciting opportunities in the healthcare, environment, and food industries. These devices can be fabricated at low costs and are capable of multiplex monitoring. This overcomes challenges presnted in traditional sensors for biomolecules and provides us a unique gateway toward comprehensive analyses. The advantages of electrochemical sensors are derived from their direct integration with electronics and their high selectivity along with sensitivity to sense a wide range of ionic analytes at an economical cost. This review paper aims to summarize recent innovations of a wide variety of electrochemical sensors for ionic analytes for health care and industrial applications. Many of these ionic analytes are important biomarkers to target for new diagnostic tools for medicine, food quality monitoring, and pollution detection. In this paper, we will examine various fabrication techniques, sensing mechanisms, and will also discuss various future opportunities in this research direction.

Dual-mode ion-selective electrodes and distance-based microfluidic device for detection of multiple urinary electrolytes

Here, we developed a microfluidic paper device by combining ion-selective electrodes (ISE) and a distance-based paper device (dPAD) for simultaneous potentiometric and colorimetric detection of urine electrolytes including K+, Na+ and Cl−.

Ion-selective membrane modified microfluidic paper-based solution sampling substrates for potentiometric heavy metal detection

Ion-selective membrane modified paper substrates were used to control the unfavourable super-Nernstian response of Pb2+-ISEs when coupled with microfluidic paper-based solution sampling.

3D Printed Ion-Selective Membranes and Their Translation into Point-of-Care Sensors

This technical note describes a method for fabricating ion-selective membranes (ISMs) for use in potentiometric sensing by using 3D printing technology. Here, we demonstrate the versatility of this approach by fabricating ISMs and investigating their performance in both liquid-contact and solid-contact ion-selective electrode (ISE) configurations. Using 3D printed ISMs resulted in highly stable (drift of ∼17 μV/h) and highly reproducible (<1 mV deviation) measurements. Furthermore, we show the seamless translation of these membranes into reliable, carbon fiber- and paper-based potentiometric sensors for applications at the point-of-care. To highlight the modifiability of this approach, we fabricated sensors for bilirubin, an important biomarker of liver health; benzalkonium, a common preservative used in the pharmaceutical industry; and potassium, an important blood electrolyte. The ability to mass produce sensors using 3D printing is an attractive advantage over conventional methods, while also decreasing the time and cost associated with sensor fabrication.

Hand-Held and Integrated Tubular Tip-like Sensing Platform Series: Point-of-care Device for Semi-automated Multiplexed Assay

Currently, most of the electrochemical sensors were prepared based on the planar electrode (PE) and utilized in open circumstance. The accompanying issues include fixed and limited sensing area of PE, insufficient usage of the testing sample, tedious operation, and susceptibility to external environment. Herein, a novel tubular tip-like sensor (TTLS) platform was proposed, where a small tip accommodates all electrodes with a curved surface and also acts as a closed detection chamber. Teaming up with a commercial pipette and potentiostat, the TTLS is able to accomplish the whole assay procedure including sampling, detection, rinsing, and regeneration with a single hand. The electrochemical interface area can be easily tuned to adapting for different scenarios with varied sensitivity request. Moreover, two TTLS-based array systems were derived: one integrates multiple working electrodes in one tip for multicomponent quantification and the other assembles eight independent TTLSs for high-throughput analysis. The admirable sensing performance of the TTLS was fully proved by detecting several liver-related biomarkers in 5 μL of the serum sample. The proposed tubular sensor platform is superior to the traditional electrochemical sensor in the aspects of unique sensing surface, fast and simple operation, good portability, and great compatibility. The TTLS could be used as an ideal analytical tool in point-of-care testing and other fields.

Recent advances of environmental pollutants detection via paper-based sensing strategy

Paper has become one of the most promising substrates for building low-cost and powerful sensing platforms due to its self-pumping ability and compatibility with multiple patterning methods. Paper-based sensors have been greatly developed in the field of environmental monitoring. In this review, we introduced the research and application of paper-based sensors in environmental monitoring, focusing on the deposition and patterning methods of building paper-based sensors, and summarized the applications of detecting environmental pollutants, including metal ions, anions, explosives, neurotoxins, volatile organic compounds, and small molecules. In addition, the development prospects and challenges of promoting paper-based sensors are also discussed. The current review will provide references for the construction of portable paper-based sensors, and has implications for the field of on-site real-time detection of the environment.

Home Detection Technique for Na+ and K+ in Urine Using a Self-Calibrated all-Solid-State Ion-Selective Electrode Array Based on Polystyrene-Au Ion-Sensing Nanocomposites

An all-solid-state ion-selective electrode (ASS-ISE) array that is portable and easily miniaturized can meet the needs of home sensing devices for long-term health monitoring. However, their stability and accuracy are affected by the multistep modification required for ASS-ISE manufacturing and the complex background signal of real samples. In this study, a four-channel ISE array with the integration of a calibration channel has been developed based on polystyrene-Au (PS-Au) ion-sensing nanocomposites (PS-Au ISE array) for the home detection of Na⁺ and K⁺. The nanocomposites combine target recognition function and ion-electron transduction function and could be modified on the channel surface by direct drop-casting, thus simplifying the preparation process and then improving the stability. Meanwhile, the integrated calibration channel could automatically deduct complex background signals in real sample analysis and thus improve the accuracy. As a result, the proposed self-calibrated PS-Au ISE array showed a near Nernstian behavior for Na⁺ and K⁺ in the range of 1 × 10^-2 M-1 × 10^-4 M, and the detection limits were 6.8 × 10^-5 M and 5.5 × 10^-5 M in artificial urine. The linear equations can be obtained according to the slopes and intercepts of Na⁺ and K⁺, and thus, the concentration of the target ions can be directly read out by combining this PS-Au ISE array with the smart electronic device. Furthermore, the detection results of Na⁺ and K⁺ in human urine agreed well with those obtained by ICP-AES, suggesting that this proposed self-calibrated PS-Au ISE array is very suitable for home smart sensing devices, facilitating the health monitoring.

A Paper-Based Potentiometric Platform for Determination of Water Hardness

A novel paper-based potentiometric platform for the simple and fast monitoring of water hardness is presented. First, potentiometric ion-selective electrodes for calcium and magnesium printed on a paper substrate were built and optimized. These sensors, which display near-Nernstian sensitivity, were used for the determination of the concentration of these cations and the calculation of the water hardness. Second, the incorporation of a solid-state reference electrode allowed building an integrated paper-based potentiometric cell for the determination of the hardness of artificial and real samples (mineral waters). The validation of the results shows good ability to predict hardness in the conventional scale. Truly decentralized measurements were demonstrated by integration of a miniaturized instrument and dedicated software in a portable device. The measurements were able to be performed in just under two minutes, including a two-point calibration. Since the method is simple to use and cost-effective, it can be implemented in domestic and industrial settings.

Ultrasensitive Microfluidic Paper-Based Electrochemical/Visual Analytical Device via Signal Amplification of Pd@Hollow Zn/Co Core-Shell ZIF67/ZIF8 Nanoparticles for Prostate-Specific Antigen Detection

An effective signal amplification strategy is essential to enhance the analytical performance of microfluidic paper-based analytical devices (μPADs) for tracing biomarkers. Here, a simple but efficient approach with superior electrocatalytic performance of Pd@hollow Zn/Co core-shell ZIF67/ZIF8 nanoparticles for regulating the efficacious signal amplification process was utilized to realize the detection of prostate-specific antigen (PSA). By rationally designing the core-shell structure of ZIF67/ZIF8 with hollow characteristics on the nanoscale and introducing the noble metal element Pd into the cavity, the diffusion limitation and porous confinement reduction of the obtained nanomaterials with uniform morphology and satisfactory chemical stability could be realized, which endowed it with better catalytic performance than solid metal-organic frameworks (MOFs) and ensured effective signal amplification of H₂O₂ reduction for achieving enhanced electrochemical signals. Moreover, with the assistance of signal probes, the remaining H₂O₂ could flow to the color area to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine to form a colored product by changing the spatial configuration of the μPAD, thus realizing the visual detection of PSA. On the basis of this novel analytical device, dual-mode ultrasensitive detection of PSA could be achieved with a lower limit of detection of 0.78 pg/mL (S/N = 3) and a wider linear range from 5 pg/mL to 50 ng/mL. This work provided the opportunity of introducing the noble metal element Pd into the cavity of the MOF hollow structure to improve its electrocatalytic efficiency and construct a high-performance μPAD for clinical detection of other biomarkers.

Long-Term Quantitatively Imaging Intracellular Chloride Concentration Using a Core-/Shell-Structured Nanosensor and Time-Domain Dual-Lifetime Referencing Method

Luminescence lifetime-based nanosensors for chloride ions were designed by incorporating a luminescent ruthenium dye [Ru(1,10-phenanthroline)₃] inside silica nanoparticles and chemically labelling their outer surface with chloride ion-sensitive fluorescent dyes (N,N'-bis(carboxypropyl)-9,9'-biacridine). The nanosensor surface was further functionalized with positively charged amino groups to facilitate intracellular uptake via endocytosis and target lysosomes. The nanosensors have an average diameter of 52 nm and are monodispersed in aqueous solutions. Because of the long lifetime of the reference ruthenium dye, the sensor response can be analyzed using the time-domain dual-lifetime referencing (td-DLR) approach. The use of pulsed excitation in td-DLR rather than intense continuous illumination in ratiometric measurements greatly prevents the dye from photobleaching which significantly improves its measurement stability and reproducibility for long-term monitoring. At optimum conditions, the sensor can measure chloride concentration in the range of 0-200 mM with a large ratiometric signal change from 140.9 to 40.2. Combined with our custom-built microscopic td-DLR system, variations of intracellular chloride concentration in lysosomes were imaged quantitatively with a high spatial resolution and accuracy.

Engineering strategies for enhancing the performance of electrochemical paper-based analytical devices

Applications of electrochemical detection methods in microfluidic paper-based analytical devices (μPADs) has revolutionized the area of point-of-care (POC) testing towards highly sensitive and selective quantification of various (bio)chemical analytes in a miniaturized, low-coat, rapid, and user-friendly manner. Shortly after the initiation, these relatively new modulations of μPADs, named as electrochemical paper-based analytical devices (ePADs), gained widespread popularity within the POC research community thanks to the inherent advantages of both electrochemical sensing and usage of paper as a suitable substrate for POC testing platforms. Even though general aspects of ePADs such as applications and fabrication techniques, have already been reviewed multiple times in the literature, herein, we intend to provide a critical engineering insight into the area of ePADs by focusing particularly on the practical strategies utilized to enhance their analytical performance (i.e. sensitivity), while maintaining the desired simplicity and efficiency intact. Basically, the discussed strategies are driven by considering the parameters potentially affecting the generated electrochemical signal in the ePADs. Some of these parameters include the type of filter paper, electrode fabrication methods, electrode materials, fluid flow patterns, etc. Besides, the limitations and challenges associated with the development of ePADs are discussed, and further insights and directions for future research in this field are proposed.

Indicator-Free Argentometric Titration for Distance-Based Detection of Chloride Using Microfluidic Paper-Based Analytical Devices

We described an indicator-free argentometric titration strategy using a microfluidic paper-based analytical device. This strategy was based on the formation of insoluble silver salts by reactions occurring between analytes and titrant (Ag⁺) on a paper channel. After the insoluble silver salts were formed and precipitated on the channel, the paper substrate modified with the surplus titrant on the channel turned reddish-brown by exposure of the devices to a simple and cheap UV light source for 5 min, generating a colored band on the channel. Distance-based detection of chloride was achieved by measuring the length of the colored band with a detection limit of 1.7 mg L^-1 Cl^-. This method was used to detect chlorides in tap water, with an analytical result (10.1 ± 1.2 mg L^-1) agreeing well with that obtained by a classical conventional precipitation titration (9.8 mg L^-1), which was based on the measurement of the consumed volume of titrant. This paper-based precipitation titration method is free of skilled personnel and has advantages of low reagent/sample consumption, disposability, portability, and simple operation over the conventional precipitation titration. More importantly, being free of any indicator, this method may be used to detect more species than the conventional precipitation titrations, which are limited by the indicator, for example, CO₃ ^2- and SO₄ ^2-, which could form insoluble silver salts in aqueous liquids. Additionally, comparing with most of those paper-based titrimetry reported previously, this presented precipitation titration is free of any indicator or ion selective electrode to detect the end point of titration.

Recent Advances in Microfluidic Paper-Based Analytical Devices toward High-Throughput Screening

Microfluidic paper-based analytical devices (µPADs) have become promising tools offering various analytical applications for chemical and biological assays at the point-of-care (POC). Compared to traditional microfluidic devices, µPADs offer notable advantages; they are cost-effective, easily fabricated, disposable, and portable. Because of our better understanding and advanced engineering of µPADs, multistep assays, high detection sensitivity, and rapid result readout have become possible, and recently developed µPADs have gained extensive interest in parallel analyses to detect biomarkers of interest. In this review, we focus on recent developments in order to achieve µPADs with high-throughput capability. We discuss existing fabrication techniques and designs, and we introduce and discuss current detection methods and their applications to multiplexed detection assays in relation to clinical diagnosis, drug analysis and screening, environmental monitoring, and food and beverage quality control. A summary with future perspectives for µPADs is also presented.

Modulating the mixed potential for developing biosensors: Direct potentiometric determination of glucose in whole, undiluted blood

The growing demand for tools to generate chemical information in decentralized settings is creating a vast range of opportunities for potentiometric sensors, since their combination of robustness, simplicity of operation and cost can hardly be rivalled by any other technique. In previous works, we have shown that the mixed potential of a Pt electrode can be controlled with analytical purposes using a coating of Nafion, thus providing a way to develop a potentiometric biosensor for glucose. Unfortunately, the linear range of this device did not match the relevant clinical range for glucose in blood. This work presents a novel strategy to control the mixed potential that allows the development of a potentiometric biosensor for the direct detection of glucose in whole, undiluted blood without any sample pretreatment. By changing the ionomer, the analytical response can be tuned, shifting the linear range while keeping the sensitivity. Aquivion, a polyelectrolyte from the same family as Nafion, is used to stabilize the mixed potential of a platinized paper-based electrode, to entrap the enzyme and to reduce the interference from negatively charged species. Factors affecting the generation of the signal and the principle of detection are discussed. Optimization of the biosensor composition was achieved with particular focus on the characterization of the linear range and sensitivity. The accurate measurement of blood sugar levels in a single drop of whole blood with excellent recovery is presented.

Recent Developments of Flexible and Stretchable Electrochemical Biosensors

The skyrocketing popularity of health monitoring has spurred increasing interest in wearable electrochemical biosensors. Compared with the traditionally rigid and bulky electrochemical biosensors, flexible and stretchable devices render a unique capability to conform to the complex, hierarchically textured surfaces of the human body. With a recognition element (e.g., enzymes, antibodies, nucleic acids, ions) to selectively react with the target analyte, wearable electrochemical biosensors can convert the types and concentrations of chemical changes in the body into electrical signals for easy readout. Initial exploration of wearable electrochemical biosensors integrates electrodes on textile and flexible thin-film substrate materials. A stretchable property is needed for the thin-film device to form an intimate contact with the textured skin surface and to deform with various natural skin motions. Thus, stretchable materials and structures have been exploited to ensure the effective function of a wearable electrochemical biosensor. In this mini-review, we summarize the recent development of flexible and stretchable electrochemical biosensors, including their principles, representative application scenarios (e.g., saliva, tear, sweat, and interstitial fluid), and materials and structures. While great strides have been made in the wearable electrochemical biosensors, challenges still exist, which represents a small fraction of opportunities for the future development of this burgeoning field.

Electrochemical paper-based devices: sensing approaches and progress toward practical applications

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.

A novel trimodal system on a paper-based microfluidic device for on-site detection of the date rape drug "ketamine"

Paper-based microfluidic device was designed with wax-printing to combine potentiometric, fluorimetric and colorimetric detection zones. This newly developed trimodal paper chip has been used for on-site determination of ketamine hydrochloride (KET) as a date rape drug in beverages. The device employed polyaniline nano-dispersion as conducting polymer in ion sensing paper electrodes designed to fit USB plug connector. Carbon dots-gold nanoparticles and cobalt thiocyanate were used in fluorescence and color detection zones, respectively. Cellular phone's camera facilitated the on-site fluorimetric and color detection. The implemented trimodal detection system exhibited specificity for KET detection in the presence of several other beverage interferences i.e., biogenic amines. This innovative sensor brings together analytical figures of merit for effective KET detection in single aliquot of spiked beverages. The proposed paper-based chip also fulfils WHO criteria for point-of-care devices posing the proposed trimodal paper device as an active part for rapid, on-site drug diagnostics and to be applied further for other similar drugs.

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).

Stable Full-Inkjet-Printed Solid-State Ag/AgCl Reference Electrode

With a growing demand for the availability of inexpensive, simple, and rapid prototyped devices, the prospect of miniaturization of the reference electrodes using printing techniques becomes promising. A stable and reusable full-inkjet-printed solid-state reference electrode (IPRE) was developed. The reference electrode was fully produced by consecutive inkjet printing of several layers. Ag ink was printed and chlorinated by NaClO printing, forming a Ag/AgCl pseudoreference electrode. Then a surface protection by printing a Cl^--saturated polyvinyl butyral membrane finally gave a reference electrode that demonstrated an outstanding performance comparable to commercial ones. This full inkjet printing fabrication strategy will improve the viability of producing low-cost miniaturized reference electrodes with interest in many electrochemical sensor-dependent areas.

Controlling the mixed potential of polyelectrolyte-coated platinum electrodes for the potentiometric detection of hydrogen peroxide

The use of a Pt electrode coated with a layer of Nafion has been described in previous works as an attractive way to perform the potentiometric detection of hydrogen peroxide. Despite of the attractive features of this approach, the nature of the non-Nernstian response of this system was not properly addressed. In this work, using a mixed potential model, the open circuit potential of the Pt electrode is shown to be under kinetic control of the oxygen reduction reaction (ORR). It is proposed that hydrogen peroxide acts as an oxygenated species that blocks free sites on the Pt surface, interfering with the ORR. Therefore, the effect of the polyelectrolyte coating can be understood in terms of the modulation of the factors that affects the kinetics of the ORR, such as an increase of the H⁺ concentration, minimization of the effect of the spectator species, etc. Because of the complexity and the lack of models that accurately describe systems with practical applications, this work is not intended to provide a mechanistic but rather a phenomenological view on problem. A general framework to understand the factors that affect the potentiometric response is provided. Experimental evidence showing that the use of polyelectrolyte coatings are a powerful way to control the mixed potential open new ways for the development of robust and simple potentiometric sensors.

Fabrication, Flow Control, and Applications of Microfluidic Paper-Based Analytical Devices

Paper-based microfluidic devices have advanced significantly in recent years as they are affordable, automated with capillary action, portable, and biodegradable diagnostic platforms for a variety of health, environmental, and food quality applications. In terms of commercialization, however, paper-based microfluidics still have to overcome significant challenges to become an authentic point-of-care testing format with the advanced capabilities of analyte purification, multiplex analysis, quantification, and detection with high sensitivity and selectivity. Moreover, fluid flow manipulation for multistep integration, which involves valving and flow velocity control, is also a critical parameter to achieve high-performance devices. Considering these limitations, the aim of this review is to (i) comprehensively analyze the fabrication techniques of microfluidic paper-based analytical devices, (ii) provide a theoretical background and various methods for fluid flow manipulation, and iii) highlight the recent detection techniques developed for various applications, including their advantages and disadvantages.