PANI-Based Wearable Electrochemical Sensor for pH Sweat Monitoring

Novel interfaces for internet of wearable electrochemical sensors

The integration of wearable devices, the Internet of Things (IoT), and advanced sensing platforms implies a significant paradigm shift in technological innovations and human interactions. The IoT technology allows continuous monitoring in real time. Thus, Internet of Wearables has made remarkable strides, especially in the field of medical monitoring. IoT-enabled wearable systems assist in early disease detection that facilitates personalized interventions and proactive healthcare management, thereby empowering individuals to take charge of their wellbeing. Until now, physical sensors have been successfully integrated into wearable devices for physical activity monitoring. However, obtaining biochemical information poses challenges in the contexts of fabrication compatibility and shorter operation lifetimes. IoT-based electrochemical wearable sensors allow real-time acquisition of data and interpretation of biomolecular information corresponding to biomarkers, viruses, bacteria and metabolites, extending the diagnostic capabilities beyond physical activity tracking. Thus, critical heath parameters such as glucose levels, blood pressure and cardiac rhythm may be monitored by these devices regardless of location and time. This work presents versatile electrochemical sensing devices across different disciplines, including but not limited to sports, safety and wellbeing by using IoT. It also discusses the detection principles for biomarkers and biofluid monitoring, and their integration into devices and advancements in sensing interfaces.

Electrochemical study of an enhanced platform by electrochemical synthesis of three-dimensional polyaniline nanofibers/reduced graphene oxide thin films for diverse applications

This work reports the electrochemical fabrication of thin films comprising polyaniline nanofibers (PANI) in conjunction with graphene oxide (GO) and reduced graphene oxide (rGO) on ITO substrate, along with examining the electrochemical properties, with a focus on the influence of the substrate and electrolyte in the electrodeposition methods. The study explores the electrochemical characteristics of these thin films and establishes a flexible framework for their application in diverse sectors such as sensors, supercapacitors, and electronic devices. It analyzes the impact of the substrate and electrolyte in electrodeposition techniques. The effects were studied using techniques such as cyclic voltammetry and chronoamperometry. The fabrication process of PANI/GO and PANI/rGO thin films involved the integration of rGO within PANI via electropolymerization, conducted under sulfuric acid. GO was synthesized by modifying the well-known Hummers' method and characterized by X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). SEM showed the diameters of the formed PANI were between 40 and 150 nm, which helped to intertwine the rGO nanosheets with PANI nanofibers to form thin films. The electrochemical behavior of the PANI/rGO thin films was examined using cyclic voltammetry (CV) and chronoamperometry in different electrolytes, including sulfuric acid (H₂SO₄) and potassium nitrate (KNO₃). The CV profiles exhibited distinct oxidation and reduction peaks, with variations in the voltammogram morphology attributed to the nature of the electrolyte and the substrate employed during the electrodeposition process. These results highlight the critical role of both the substrate and electrolyte in governing the electrochemical performance of PANI/rGO thin films. The findings from this study demonstrate a versatile approach for the fabrication of PANI/graphene-based thin films with tunable electrochemical properties, and such a strategy has great application to fabricating other thin film composites for supercapacitors or other control source frameworks requiring enhanced charge storage and electrochemical responsiveness.

Measurement of Human and Bovine Exhaled Breath Condensate pH Using Polyaniline-Modified Flexible Inkjet-Printed Nanocarbon Electrodes

The collection, processing, and electrochemical analysis of exhaled breath condensate (EBC) from healthy human and animal subjects is reported on. EBC is a biospecimen potentially rich in biomarkers of respiratory disease. The EBC pH was analyzed potentiometrically using a disposable polyaniline (PANI)-modified inkjet-printed (IJP) carbon electrode. Comparison measurements were performed using a commercial screen-printed carbon (SPC) electrode. The PANI-modified electrodes exhibited reproducible and near-Nernstian responses for pH values between 2 and 9 with slopes from -50 to -60 mV/dec. The PANI-modified IJP carbon electrode exhibited a faster response time and superior reproducibility to the modified SPC electrode. In proof-of-concept studies, the healthy human EBC pH was found to be 6.57 ± 0.09 and the healthy bovine EBC pH was 5.9 ± 0.2. All pH determined using the PANI-modified electrodes were in good agreement with the pH determined using a micro glass pH electrode. An RTube device was used to collect EBC from humans while a modified device was used to collect EBC from calves in the field. EBC volumes of 0.5-2 mL for 5-6 min of tidal breathing were collected from healthy animals. The pH of EBC from healthy calves (17 animals) depends on their age from 1 to 9 weeks with values ranging from 5.3 to 7.2. A distinct alkaline shift was observed for many animals around 20 days of age. The bovine EBC pH also depends on the ambient temperature and humidity at the time of collection. The results indicate that the PANI-modified IJP carbon electrodes outperform commercial SPC and provide reproducible and accurate measurement of pH across various biospecimen types.

Maintaining Electrochemical Performance of Flexible ITO-PET Electrodes under High Strain

Flexible electrode materials, particularly indium tin oxide (ITO)-coated polyethylene terephthalate (PET), have attracted the attention of researchers for a wide variety of applications. However, there has been limited attention to the effects of electrode flexibility during electrochemical processes. In this research article, we studied how bending commercially available ITO-PET electrodes impacts the electrodeposition process of polyaniline (PANI). Thicker ITO layers start cracking at a normalized strain of 0.10 (bending radius of 10 mm), and cracking becomes detrimental to full deposition at a normalized strain of 0.16 or higher (bending radius of 6 mm or lower). Thinner ITO layers were evaluated as electrodes in electrochemical applications; however, the higher resistance of these electrodes prevented uniform electrodeposition of PANI. In order to overcome the issues of cracking, conductive thin films and copper tape were explored as low-cost methods for electrically bridging cracks in the electrode. While conductive thin films reduced the resistance effect, copper tape was found to fully restore the original electrochemical activity as measured by chronoamperometry and enable uniform electrodeposition at a bending radius as low as 3 mm. This strategy was then demonstrated by performing electrochromic bleaching of PANI under high-strain conditions. These studies illustrate some of the limitations of ITO-PET electrodes and strategies for overcoming these limitations for future applications that require a high degree of flexibility in a transparent electrode substrate.

Advances in pH Sensing: From Traditional Approaches to Next-Generation Sensors in Biological Contexts

pH has been considered one of the paramount factors in bodily functions because most cellular tasks exclusively rely on precise pH values. In this context, the current techniques for pH sensing provide us with the futuristic insight to further design therapeutic and diagnostic tools. Thus, pH-sensing (electrochemically and optically) is rapidly evolving toward exciting new applications and expanding researchers' interests in many chemical contexts, especially in biomedical applications. The adaptation of cutting-edge technology is subsequently producing the modest form of these biosensors as wearable devices, which are providing us the opportunity to target the real-time collection of vital parameters, including pH for improved healthcare systems. The motif of this review is to provide insight into trending tech-based systems employed in real-time or in-vivo pH-responsive monitoring. Herein, we briefly go through the pH regulation in the human body to help the beginners and scientific community with quick background knowledge, recent advances in the field, and pH detection in real-time biological applications. In the end, we summarize our review by providing an outlook; challenges that need to be addressed, and prospective integration of various pH in vivo platforms with modern electronics that can open new avenues of cutting-edge techniques for disease diagnostics and prevention.

Tailored Polypyrrole Nanofibers as Ion-to-Electron Transduction Membranes for Wearable K+ Sensors

Conductive polymers are recognized as ideal candidates for the development of noninvasive and wearable sensors for real-time monitoring of potassium ions (K+) in sweat to ensure the health of life. However, the low ion-to-electron transduction efficiency and limited active surface area hamper the development of high-performance sensors for low-concentration K+ detection in the sweat. Herein, a wearable K+ sensor is developed by tailoring the nanostructure of polypyrrole (PPy), serving as an ion-to-electron transduction layer, for accurately and stably tracing the K+ fluctuation in human sweat. The PPy nanostructures can be tailored from nanospheres to nanofibers by controlling the supramolecular assembly process during PPy polymerization. Resultantly, the ion-to-electron transduction efficiency (17-fold increase in conductivity) and active surface area (1.3-fold enhancement) are significantly enhanced, accompanied by minimized water layer formation. The optimal PPy nanofibers-based K+ sensor achieved a high sensitivity of 62 mV decade-1, good selectivity, and solid stability. After being integrated with a temperature sensor, the manufactured wearable sensor realized accurate monitoring of K+ fluctuation in the human sweat.

Construction of an electrochemical pH sensor using one-pot synthesis of a molybdenum diselenide/nitrogen doped graphene oxide screen-printed electrode

In this study, a one-pot synthesis of a molybdenum diselenide/nitrogen-doped graphene oxide (MoSe2/NGO) composite was demonstrated and used for the fabrication of an electrochemical pH sensor. The MoSe2/NGO composite was characterized using powder X-ray diffraction, infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and Brunauer-Emmett-Teller analysis. The electrochemical behavior at different pH values was determined by recording the open-circuit potential. When applied for pH detection, the MoSe2/NGO modified screen-printed electrode (SPE) showed good linearity with a sensitivity of 61.3 mV pH-1 over a wide pH range of 2-14. In addition, the pH sensor exhibited a remarkably stable response, high reproducibility, and selectivity. The sensor was used to measure the acidity or alkalinity of real food and beverage samples. The results for these samples showed a relative error of less than 10% compared with the results obtained with the commercial pH meter. The portable sensor produced by screen printing electrodes paves the way for the development of simple, cost-effective, real-time, and robust pH sensors for the pH analysis of various sample matrices for clinical diagnostics, biosensing, and cost-effective applications.

Performance Improvement of a ZnGa2O4 Extended-Gate Field-Effect Transistor pH Sensor

ZnGa2O4 sensing films were prepared using an RF magnetron sputtering system and connected to a commercial metal oxide semiconductor field-effect transistor (MOSFET) as the extended-gate field-effect transistor (EGFET) to detect pH values. Experimental parameters were adjusted by varying the oxygen flow rate in the process chamber to produce ZnGa2O4 sensing films with different oxygen ratios. These films were then treated in a furnace tube at an annealing temperature of 700 °C. The sensitivity and linearity of the constant current mode and the constant voltage mode were measured and analyzed in the pH range of 2-12. Under the deposition conditions with an oxygen ratio of 6%, the sensitivity reached 23.14 mV/pH and 33.49 μA/pH, with corresponding linearity values of 92.1 and 96.15%, respectively. Finally, the sensing performance of the ZnGa2O4 EGFET pH sensor with and without annealing processes was analyzed and compared.

Screen-printed wearable skin surface pH sensor for real-time monitoring of the buffering capacity of human skin

This study demonstrated for the first time that skin surface pH can be monitored in real-time, using a screen-printed wearable pH sensor, to evaluate the buffering capacity of the human skin. The screen-printed pH sensor was composed of a polyaniline-based pH-sensitive electrode and a nitrocellulose membrane-based liquid junction type of Ag/AgCl reference electrode. This sensor showed a reliable and reversible potentiometric response to pH with long-term potential stability. Intermittent monitoring of the buffering capacity of skin surface pH demonstrated the reliability of the proposed wearable pH sensor, which was comparable to that of a commercially available flat-tip pH sensor. We found that contact of the wearable pH sensor with the subject's skin via aqueous electrolyte solutions was necessary for the sensor to continuously monitor the skin surface pH while sustaining the natural buffer capacity of the human skin surface.

An electrografted monolayer of polyaniline as a tuneable platform for a glucose biosensor

Polyaniline (PANI), a nanostructured conducting polymer, has shown significant potential in optical and bioelectrochemical devices. However, its performance and stability on various substrates are hindered by weak adhesion to the surface. In this study, a strongly adherent polyaniline conducting polymer layer with a thickness of five nanometers was electrografted onto an initiating monolayer on gold and tin-doped indium oxide substrates. These electrografted monolayers consist of vertically oriented fully oxidized-protonated (pernigraniline salt) and deprotonated (pernigraniline base) forms of polyaniline. The monolayer exhibits pH-dependent colour changes and it is suitable for enzyme compatibility. In light of these findings, we have developed and characterized an electrochemical glucose biosensor based on the monolayer of polyaniline on a gold electrode. The biosensor utilizes glucose oxidase as the biorecognition element for the selective detection of glucose concentrations in real blood plasma samples.

Dual Microfluidic Sensor System for Enriched Electrochemical Profiling and Identification of Illicit Drugs On-Site

Electrochemical sensors have emerged as a new analytical tool for illicit drug detection to facilitate ultrafast and accurate identification of suspicious compounds on-site. Drugs of abuse can be identified using their unique voltammetric fingerprint at a given pH. Today, the right buffer solution is manually selected based on drug appearance, and in some cases, a consecutive analysis in two different pH solutions is required. In this work, we present a disposable microfluidic multichannel sensor system that automatically records fingerprints in two pH solutions (e.g., pH 5 and pH 12). This system has two advantages. It will overcome the manual selection of a buffer solution at the right pH, decrease analysis time, and minimize the risk of human errors. Second, the combination of two fingerprints, the superfingerprint, contains more detailed information about the samples, which enhances the selectivity of the analytical technique. First, real-time pH measurements proved that the sample can be brought to the desired pH within a minute. Subsequently, an electrochemical study on the microfluidic platform with 1 mM illicit drug standards of MDMA, cocaine, heroin, and methamphetamine showed that the characteristic voltammetric fingerprints and peak potentials are reproducible, also in the presence of common cutting agents. Finally, the microfluidic concept was validated with real confiscated samples, showing promising results for the user-friendly identification of drugs of abuse. In short, this paper presents a successful proof-of-concept study of a multichannel microfluidic sensor system to enrich the fingerprints of illicit drugs at pH 5 and pH 12, thus providing a low-cost, portable, and rapid identification system of illicit drugs with minimal user intervention.

High-Sensitivity and Wide-Range Flexible Ionic Piezocapacitive Pressure Sensors with Porous Hemisphere Array Electrodes

The development of high-performance flexible pressure sensors with porous hierarchical microstructures is limited by the complex and time-consuming preparation processes of porous hierarchical microstructures. In this study, a simple modified heat curing process was first proposed to achieve one-step preparation of porous hemispherical microstructures on a polydimethylsiloxane (PDMS) substrate. In this process, a laser-prepared template was used to form surface microstructures on PDMS film. Meanwhile, the thermal decomposition of glucose monohydrate additive during heat curing of PDMS led to the formation of porous structures within PDMS film. Further, based on the obtained PDMS/CNTs electrodes with porous hemisphere array and ionic polymer dielectric layers, high-performance ionic piezocapacitive sensors were realized. Under the synergistic effect of the low-stiffness porous hemisphere microstructure and the electric double layer of the ionic polymer film, the sensor based on an ionic polymer film with a 1:0.75 ratio of P(VDF-HFP):[EMIM][TFSI] not only achieves a sensitivity of up to 106.27 kPa-1 below 3 kPa, but also has a wide measurement range of over 400 kPa, which has obvious advantages in existing flexible piezocapacitive sensors. The rapid response time of 110 s and the good stability of 2300 cycles of the sensor further elucidate its practicality. The application of the sensor in pulse monitoring, speech recognition, and detection of multiple dynamic loads verifies its excellent sensing performance. In short, the proposed heat curing process can simultaneously form porous structures and surface microstructures on PDMS films, greatly simplifying the preparation process of porous hierarchical microstructures and providing a simple and feasible way to obtain high-performance flexible pressure sensors.

Multi-Modal Multi-Array Electrochemical and Optical Sensor Suite for a Biological CubeSat Payload

CubeSats have emerged as cost-effective platforms for biological research in low Earth orbit (LEO). However, they have traditionally been limited to optical absorbance sensors for studying microbial growth. This work has made improvements to the sensing capabilities of these small satellites by incorporating electrochemical ion-selective pH and pNa sensors with optical absorbance sensors to enrich biological experimentation and greatly expand the capabilities of these payloads. We have designed, built, and tested a multi-modal multi-array electrochemical-optical sensor module and its ancillary systems, including a fluidic card and an on-board payload computer with custom firmware. Laboratory tests showed that the module could endure high flow rates (1 mL/min) without leakage, and the 27-well, 81-electrode sensor card accurately detected pH (71.0 mV/pH), sodium ion concentration (75.2 mV/pNa), and absorbance (0.067 AU), with the sensors demonstrating precise linear responses (R2 ≈ 0.99) in various test solutions. The successful development and integration of this technology conclude that CubeSat bio-payloads are now poised for more complex and detailed investigations of biological phenomena in space, marking a significant enhancement of small-satellite research capabilities.

Surface chemistry applications and development of immunosensors using electrochemical impedance spectroscopy: A comprehensive review

Immunosensors are promising alternatives as detection platforms for the current gold standards methods. Electrochemical immunosensors have already proven their capability for the sensitive, selective, detection of target biomarkers specific to COVID-19, varying cancers or Alzheimer's disease, etc. Among the electrochemical techniques, electrochemical impedance spectroscopy (EIS) is a highly sensitive technique which examines the impedance of an electrochemical cell over a range of frequencies. There are several important critical requirements for the construction of successful impedimetric immunosensor. The applied surface chemistry and immobilisation protocol have impact on the electroanalytical performance of the developed immunosensors. In this Review, we summarise the building blocks of immunosensors based on EIS, including self-assembly monolayers, nanomaterials, polymers, immobilisation protocols and antibody orientation.

From Biochemical Sensor to Wearable Device: The Key Role of the Conductive Polymer in the Triboelectric Nanogenerator

Triboelectric nanogenerators (TENGs) have revolutionized energy harvesting and active sensing, holding tremendous potential in personalized healthcare, sustainable diagnoses, and green energy applications. In these scenarios, conductive polymers play a vital role in enhancing the performance of both TENG and TENG-based biosensors, enabling the development of flexible, wearable, and highly sensitive diagnostic devices. This review summarizes the impact of conductive polymers on TENG-based sensors, focusing on their contributions to triboelectric properties, sensitivity, detection limits, and wearability. We discuss various strategies for incorporating conductive polymers into TENG-based biosensors, promoting the creation of innovative and customizable devices tailored for specific healthcare applications. Additionally, we consider the potential of integrating TENG-based sensors with energy storage devices, signal conditioning circuits, and wireless communication modules, ultimately leading to the development of advanced, self-powered diagnostic systems. Finally, we outline the challenges and future directions in developing TENGs that integrate conducting polymers for personalized healthcare, emphasizing the need to improve biocompatibility, stability, and device integration for practical applications.

Modular Design in Triboelectric Sensors: A Review on the Clinical Applications for Real-Time Diagnosis

Triboelectric nanogenerators (TENGs) have garnered considerable interest as a promising technology for energy harvesting and stimulus sensing. While TENGs facilitate the generation of electricity from micro-motions, the modular design of TENG-based modular sensing systems (TMSs) also offers significant potential for powering biosensors and other medical devices, thus reducing dependence on external power sources and enabling biological processes to be monitored in real time. Moreover, TENGs can be customised and personalized to address individual patient needs while ensuring biocompatibility and safety, ultimately enhancing the efficiency and security of diagnosis and treatment. In this review, we concentrate on recent advancements in the modular design of TMSs for clinical applications with an emphasis on their potential for personalised real-time diagnosis. We also examine the design and fabrication of TMSs, their sensitivity and specificity, and their capabilities of detecting biomarkers for disease diagnosis and monitoring. Furthermore, we investigate the application of TENGs to energy harvesting and real-time monitoring in wearable and implantable medical devices, underscore the promising prospects of personalised and modular TMSs in advancing real-time diagnosis for clinical applications, and offer insights into the future direction of this burgeoning field.

Paper-Based Sandwich-Structured Wearable Sensor with Sebum Filtering for Continuous Detection of Sweat pH

Wearable sweat sensors, a product of the development of flexible electronics and microfluidic technologies, can continuously and noninvasively monitor abundant biomarkers in human sweat; however, sweat interferences, such as sebum, can reduce sensor reliability and accuracy. Herein, for the first time, the influence of sebum on the potentiometric response of an all-solid-state pH sensor was studied, and the obtained experimental results show that sebum mixed in sweat can decrease the potential response of the sensor and the slope of its calibration curve. A paper-based sandwich-structured pH sensor that can filter the sebum mixed in sweat was proposed based on commonly used oil-control sheets. Moreover, the hydrophilic properties, microstructure, and microfluidic performance of the sensor were investigated. The detection performance of the paper-based sandwich-structured pH sensor was comprehensively evaluated in terms of calibration in the presence of sebum and potentiometric response upon the addition of sebum. Furthermore, the anti-interference ability of the sensor was evaluated using different analytes under various deformation conditions. On-body trials were conducted to verify the performance, and their results showed that the proposed sensor can filter over 90% of the sebum in sweat, significantly enhancing sensor reliability and accuracy. Additionally, microfluidic channels could be simply fabricated using a scissor and paper, obviating the need for complex micromachining processes, such as photolithography and laser engraving. Overall, this work illustrates the influence of sebum on the detection performance of traditional potentiometric wearable sensors and paves the way for their development for real-world applications.

Green and Integrated Wearable Electrochemical Sensor for Chloride Detection in Sweat

Wearable sensors for sweat biomarkers can provide facile analyte capability and monitoring for several diseases. In this work, a green wearable sensor for sweat absorption and chloride sensing is presented. In order to produce a sustainable device, polylactic acid (PLA) was used for both the substrate and the sweat absorption pad fabrication. The sensor material for chloride detection consisted of silver-based reference, working, and counter electrodes obtained from upcycled compact discs. The PLA substrates were prepared by thermal bonding of PLA sheets obtained via a flat die extruder, prototyped in single functional layers via CO₂ laser cutting, and bonded via hot-press. The effect of cold plasma treatment on the transparency and bonding strength of PLA sheets was investigated. The PLA membrane, to act as a sweat absorption pad, was directly deposited onto the membrane holder layer by means of an electrolyte-assisted electrospinning technique. The membrane adhesion capacity was investigated by indentation tests in both dry and wet modes. The integrated device made of PLA and silver-based electrodes was used to quantify chloride ions. The calibration tests revealed that the proposed sensor platform could quantify chloride ions in a sensitive and reproducible way. The chloride ions were also quantified in a real sweat sample collected from a healthy volunteer. Therefore, we demonstrated the feasibility of a green and integrated sweat sensor that can be applied directly on human skin to quantify chloride ions.

Highly Sensitive and Selective Sol-Gel Spin-Coated Composite TiO2-PANI Thin Films for EGFET-pH Sensor

A highly selective and sensitive EGFET-pH sensor based on composite TiO2-PANI had been developed in this work. A sol-gel titanium dioxide (TiO2) and the composite of TiO2 with semiconducting polyaniline (PANI) were deposited using a simple spin-coating method on an indium tin oxide (ITO) substrate. The films have been explored as a sensing electrode (SE) of extended gate field-effect transistor (EGFET) for pH applications in the range of pH 2 to 12. The pH sensitivities between TiO2, TiO2-PANI bilayer composite, and TiO2-PANI composite thin films were discussed. Among these, the TiO2-PANI composite thin film showed a super-Nernstian behavior with high sensitivity of 66.1 mV/pH and linearity of 0.9931; good repeatability with a standard deviation of 0.49%; a low hysteresis value of 3 mV; and drift rates of 4.96, 5.54, and 3.32 mV/h in pH 4, 7, and 10, respectively, for 6 h. Upon applying the TiO2-PANI composite as the SE for nitrate measurement, low sensitivity of 12.9 mV/dec was obtained, indicating that this film is a highly selective sensing electrode as a pH sensor. The surface morphology and crystallinity of the thin films were also discussed.

Electrochemical Quantification of H2O2 Released by Airway Cells Growing in Different Culture Media

Quantification of oxidative stress is a challenging task that can help in monitoring chronic inflammatory respiratory airway diseases. Different studies can be found in the literature regarding the development of electrochemical sensors for H2O2 in cell culture medium to quantify oxidative stress. However, there are very limited data regarding the impact of the cell culture medium on the electrochemical quantification of H2O2. In this work, we studied the effect of different media (RPMI, MEM, DMEM, Ham's F12 and BEGM/DMEM) on the electrochemical quantification of H2O2. The used electrode is based on reduced graphene oxide (rGO) and gold nanoparticles (AuNPs) and was obtained by co-electrodeposition. To reduce the electrode fouling by the medium, the effect of dilution was investigated using diluted (50% v/v in PBS) and undiluted media. With the same aim, two electrochemical techniques were employed, chronoamperometry (CH) and linear scan voltammetry (LSV). The influence of different interfering species and the effect of the operating temperature of 37 °C were also studied in order to simulate the operation of the sensor in the culture plate. The LSV technique made the sensor adaptable to undiluted media because the test time is short, compared with the CH technique, reducing the electrode fouling. The long-term stability of the sensors was also evaluated by testing different storage conditions. By storing the electrode at 4 °C, the sensor performance was not reduced for up to 21 days. The sensors were validated measuring H2O2 released by two different human bronchial epithelial cell lines (A549, 16HBE) and human primary bronchial epithelial cells (PBEC) grown in RPMI, MEM and BEGM/DMEM media. To confirm the results obtained with the sensor, the release of reactive oxygen species was also evaluated with a standard flow cytometry technique. The results obtained with the two techniques were very similar. Thus, the LSV technique permits using the proposed sensor for an effective oxidative stress quantification in different culture media and without dilution.

Electrochemical Soft Actuator: Deciphering the Difference in the Characteristics of Polaronic and Bipolaronic Forms of Polyaniline

Polyaniline (PANI) has been projected as an efficient electrochemical actuator due to its ease of synthesis, lightweight, biocompatibility, low cost, and possible low operating potential and high stress generation. However, challenges such as low inherent ionic and electronic conductivity of the polymer lead to small accumulation of ions and high ionic diffusion path length inside the polymer remain. In the present study, a highly conjugated, planar, conducting polaronic form of PANI with a nanofiber morphology is synthesized using in situ electrochemical polymerization on a reduced graphene oxide (rGO) electrode. The polymerization is carried out in the Schaefer mode at the air-water interface under controlled surface pressure in a Langmuir trough. Electrochemical, UV-visible, XPS, and Raman spectroscopic studies confirm the formation of the planar polaronic PANI form. Polymerization without surface pressure leads to the bipolaronic form of PANI. The two forms are subsequently used to understand their contributions toward electrochemical actuation in a bilayer configuration. The conducting polaronic PANI/EGO (exfoliated graphene oxide) exhibits a remarkably larger total angular displacement of 220° in aqueous 1 M NaClO₄ during a potential scan in the range ±0.9 V than the bipolaronic counterpart which exhibits a total angular displacement of 125°. Current imaging in the scanning electrochemical microscopy mode confirms a high volumetric expansion in the case of the polaronic form as compared to its bipolaronic counterpart. Raman spectroscopy reveals the oxidation to the emeraldine form in the polaronic PANI and to the pernigraniline form in the bipolaronic form during actuation. Electrochemical impedance spectroscopy study evidences the existence of a small charge transfer resistance with high bulk capacitance for the polaronic structure.

Wearable Sensor for Continuous Sweat Biomarker Monitoring

In recent years, wearable sensors have enabled the unique mode of real-time and noninvasive monitoring to develop rapidly in medical care, sports, and other fields. Sweat contains a wide range of biomarkers such as metabolites, electrolytes, and various hormones. Combined with wearable technology, sweat can reflect human fatigue, disease, mental stress, dehydration, and so on. This paper comprehensively describes the analysis of sweat components such as glucose, lactic acid, electrolytes, pH, cortisol, vitamins, ethanol, and drugs by wearable sensing technology, and the application of sweat wearable devices in glasses, patches, fabrics, tattoos, and paper. The development trend of sweat wearable devices is prospected. It is believed that if the sweat collection, air permeability, biocompatibility, sensing array construction, continuous monitoring, self-healing technology, power consumption, real-time data transmission, specific recognition, and other problems of the wearable sweat sensor are solved, we can provide the wearer with important information about their health level in the true sense.

Excellent physicochemical and sensing characteristics of a Re x O y based pH sensor at low post-deposition annealing temperature

pH monitoring in clinical assessment is pivotal as pH imbalance significantly influences the physiological and extracellular functions of the human body. Metal oxide based pH sensors, a promising alternative to bulky pH electrodes, mostly require complex fabrication, high-temperature post-deposition treatment, and high expenses that inhibit their practical applicability. So, there is still room to develop a straightforward and cost-effective metal oxide based pH sensor comprising high sensitivity and reliability. In this report, a novel solution-processed and low-temperature annealed (220 °C) mixed-valence (vii/vi) oxide of rhenium (Re x O y ) was applied in an electrolyte-insulator-semiconductor (EIS) structure. The annealing effect on morphological, structural, and compositional properties was scrutinized by physical and chemical characterizations. The post-annealed Re x O y exhibited a high pH sensitivity (57.3 mV pH-1, R 2 = 0.99), a lower hysteresis (4.7 mV), and a reduced drift rate (1.7 mV h-1) compared to the as-prepared sample for an analytically acceptable pH range (2-12) along with good stability and reproducibility. The magnified sensing performance originated due to the valence state of Re from Re6+ to Re7+ attributed to each electron transfer for a single H+ ion. The device showed high selectivity for H+ ions, which was confirmed by the interference study with other relevant ions. The feasibility of the sensor was verified by measuring the device in real samples. Hence, the ease-of-fabrication and notable sensing performance of the proposed sensor endorsed its implementation for diagnosing pH-related diseases.

Electrochemical Synthesis of Zinc Oxide Nanostructures on Flexible Substrate and Application as an Electrochemical Immunoglobulin-G Immunosensor

Immunoglobulin G (IgG), a type of antibody, represents approximately 75% of serum antibodies in humans, and is the most common type of antibody found in blood circulation. Consequently, the development of simple, fast and reliable systems for IgG detection, which can be achieved using electrochemical sandwich-type immunosensors, is of considerable interest. In this study we have developed an immunosensor for human (H)-IgG using an inexpensive and very simple fabrication method based on ZnO nanorods (NRs) obtained through the electrodeposition of ZnO. The ZnO NRs were treated by electrodepositing a layer of reduced graphene oxide (rGO) to ensure an easy immobilization of the antibodies. On Indium Tin Oxide supported on Polyethylene Terephthalate/ZnO NRs/rGO substrate, the sandwich configuration of the immunosensor was built through different incubation steps, which were all optimized. The immunosensor is electrochemically active thanks to the presence of gold nanoparticles tagging the secondary antibody. The immunosensor was used to measure the current density of the hydrogen development reaction which is indirectly linked to the concentration of H-IgG. In this way the calibration curve was constructed obtaining a logarithmic linear range of 10-1000 ng/mL with a detection limit of few ng/mL and good sensitivity.

Electrochemical detection of chloride ions using Ag-based electrodes obtained from compact disc

In this work electrochemical sensors fabricated from compact disc material (waste or new) are used to quantify chloride ions in different types of samples. All three electrodes, working, counter, and pseudo-reference electrodes, were fabricated from the compact disc and directly used. Different parameters were studied in order to demonstrate the possibility of using this waste material for efficient and low-cost electrochemical sensors. Chloride sensing performance was evaluated using linear scan voltammetry as the detection technique. A sensitivity of 0.174 mA mM^-1 cm^-2 with a limit of detection of 20 μM and excellent selectivity against many interferents was observed. Selectivity and reproducibility tests were also carried out, showing excellent results. Sensors were also validated with real samples (drinking and sea water, milk, sweat and physiological solutions) with results comparable to conventional techniques. Our results show the applicability and suitability of these low-cost sensors, for detection of those analytes for which, silver, has high sensitivity and selectivity.

Printed microfluidic sweat sensing platform for cortisol and glucose detection

Wearable sweat biosensors offer compelling opportunities for improved personal health monitoring and non-invasive measurements of key biomarkers. Inexpensive device fabrication methods are necessary for scalable manufacturing of portable, disposable, and flexible sweat sensors. Furthermore, real-time sweat assessment must be analyzed to validate measurement reliability at various sweating rates. Here, we demonstrate a "smart bandage" microfluidic platform for cortisol detection and continuous glucose monitoring integrated with a synthetic skin. The low-cost, laser-cut microfluidic device is composed of an adhesive-based microchannel and solution-processed electrochemical sensors fabricated from inkjet-printed graphene and silver solutions. An antibody-derived cortisol sensor achieved a limit of detection of 10 pM and included a low-voltage electrowetting valve, validating the microfluidic sensor design under typical physiological conditions. To understand effects of perspiration rate on sensor performance, a synthetic skin was developed using soft lithography to mimic human sweat pores and sweating rates. The enzymatic glucose sensor exhibited a range of 0.2 to 1.0 mM, a limit of detection of 10 μM, and reproducible response curves at flow rates of 2.0 μL min^-1 and higher when integrated with the synthetic skin, validating its relevance for human health monitoring. These results demonstrate the potential of using printed microfluidic sweat sensors as a low-cost, real-time, multi-diagnostic device for human health monitoring.

Electrochemical detection of dopamine with negligible interference from ascorbic and uric acid by means of reduced graphene oxide and metals-NPs based electrodes

Dopamine is an important neurotransmitter involved in many human biological processes as well as in different neurodegenerative diseases. Monitoring the concentration of dopamine in biological fluids, i.e., blood and urine is an effective way of accelerating the early diagnosis of these types of diseases. Electrochemical sensors are an ideal choice for real-time screening of dopamine as they can achieve fast, portable inexpensive and accurate measurements. In this work, we present electrochemical dopamine sensors based on reduced graphene oxide coupled with Au or Pt nanoparticles. Sensors were developed by co-electrodeposition onto a flexible substrate, and a systematic investigation concerning the electrodeposition parameters (concentration of precursors, deposition time and potential) was carried out to maximize the sensitivity of the dopamine detection. Square wave voltammetry was used as an electrochemical technique that ensured a high sensitive detection in the nM range. The sensors were challenged against synthetic urine in order to simulate a real sample detection scenario where dopamine concentrations are usually lower than 600 nM. Our sensors show a negligible interference from uric and ascorbic acids which did not affect sensor performance. A wide linear range (0.1-20 μm for gold nanoparticles, 0.1-10 μm for platinum nanoparticles) with high sensitivity (6.02 and 7.19 μA μM^-1 cm^-2 for gold and platinum, respectively) and a low limit of detection (75 and 62 nM for Au and Pt, respectively) were achieved. Real urine samples were also assayed, where the concentrations of dopamine detected aligned very closely to measurements undertaken using conventional laboratory techniques. Sensor fabrication employed a cost-effective production process with the possibility of also being integrated into flexible substrates, thus allowing for the possible development of wearable sensing devices.