The pH sensing properties of RF sputtered RuO2 thin-film prepared using different Ar/O2 flow ratio

The influence of the Ar/O2 gas ratio during radio frequency (RF) sputtering of the RuO2 sensing electrode on the pH sensing performance is investigated. The developed pH sensor consists in an RF sputtered ruthenium oxide thin-film sensing electrode, in conjunction with an electroplated Ag/AgCl reference electrode. The performance and characterization of the developed pH sensors in terms of sensitivity, response time, stability, reversibility, and hysteresis are investigated. Experimental results show that the pH sensor exhibits super-Nernstian slopes in the range of 64.33-73.83 mV/pH for Ar/O2 gas ratio between 10/0-7/3. In particular, the best pH sensing performance, in terms of sensitivity, response time, reversibility and hysteresis, is achieved when the Ar/O2 gas ratio is 8/2, at which a high sensitivity, a low hysteresis and a short response time are attained simultaneously

Physical-vapor-deposited metal oxide thin films for pH sensing applications: Last decade of research progress

In the last several decades, metal oxide thin films have attracted significant attention for the development of various existing and emerging technological applications, including pH sensors. The mandate for consistent and precise pH sensing techniques has been increasing across various fields, including environmental monitoring, biotechnology, food and agricultural industries, and medical diagnostics. Metal oxide thin films grown using physical vapor deposition (PVD) with precise control over film thickness, composition, and morphology are beneficial for pH sensing applications such as enhancing pH sensitivity and stability, quicker response, repeatability, and compatibility with miniaturization. Various PVD techniques, including sputtering, evaporation, and ion beam deposition, used to fabricate thin films for tailoring materials’ properties for the advanced design and development of high-performing pH sensors, have been explored worldwide by many research groups. In addition, various thin film materials have also been investigated, including metal oxides, nitrides, and nanostructured films, to make very robust pH sensing electrodes with higher pH sensing performance. The development of novel materials and structures has enabled higher sensitivity, improved selectivity, and enhanced durability in harsh pH environments. The last decade has witnessed significant advancements in PVD thin films for pH sensing applications. The combination of precise film deposition techniques, novel materials, and surface functionalization strategies has led to improved pH sensing performance, making PVD thin films a promising choice for future pH sensing technologies

Development, manufacture and application of a solid-state pH sensor using ruthenium oxide

The measurement of pH is undertaken frequently in numerous settings for many applications. The common glass pH probe is almost ideal for measuring pH, and as such, it is used almost ubiquitously. However, glass is not ideal for all applications due to its relatively large size, fragility, need for recalibration and wet-storage. Therefore, much research has been undertaken on the use of metal oxides as an alternative for the measurement of pH. Here, a solid-state potentiometric pH sensor is developed using ruthenium metal oxide (RuO2). Initially, pH sensitive RuO2 electrodes were prepared by deposition with radio frequency magnetron sputtering (RFMS) in a reactive oxygen plasma, onto screen-printed carbon based electrical contacts (substrates). These electrodes performed well, between pH 4 and 10, exhibiting Nernstian pH sensitivity, low hysteresis and low drift rate. However, these electrode were found to exhibit less than ideal properties outside this range (pH 2-12), though this could be overcome using a pH 12 conditioning protocol. Later, improved RuO2 pH sensitive electrodes were developed and characterised. Elimination of the carbon substrate material resulted in electrodes that displayed excellent performance from pH 2 to 12, even without pH 12 conditioning. Whilst this RuO2 electrode displayed excellent pH sensing performance, RuO2 along with all other metal oxide based pH sensors suffer from interference caused by strong oxidising and reducing agents. To reduce this interference, Ta2O5 and Nafion protective layers were studied. Using a combination of sputter deposited Ta2O5 (80 nm) and thermally cured drop-cast Nafion, an electrode was manufactured, which was immune to interference from dissolved oxygen, and resistant to stronger redox species. This electrode was found to outperform an unprotected RuO2 electrode and was suitable for application in several common beverage samples. In order to construct a potentiometric pH sensor a reference electrode is also required. Here, a pH insensitive reference electrode was developed by modification of the pH sensitive RuO2 electrode with a porous polymer junction containing SiO2. The reference electrode showed very low sensitivity to pH and KCl. The reference electrode provided a suitably stable potential over short periods of time, allowing accurate pH measurements to be made. The potential of the reference electrode was found to drift over longer time periods, however, this could be accounted for by recalibration. The developed working and reference electrodes were then used to construct a pH sensor. The sensor displayed excellent performance between pH 2 and 6; close to Nernstian sensitivity (-55.3 mV/pH), linear response (R2=1.0000) and excellent reproducibility (hysteresismV). The sensor was applied to several beverage samples, where it was shown to perform accurately, results within ±0.08 pH of a commercial glass pH sensor. The sensor develop here would be suitable for development into handheld and in-situ type pH sensor devices

Advances in Artificial and Biological Membranes: Mechanisms of Ionic Sensitivity, Ion-Sensor Designs and Applications for Ions Measurement

Ion-sensitive membrane-based sensors and ionic processes in bio-membranes are the focus of this book. The chapters are carefully chosen to characterize essential research trends, applications, and perspectives. They include solid contact ion-selective and reference electrodes and their electroanalytical behavior in zero and nonzero-current modes, planar and miniaturized multielectrode platforms, ion monitoring in extreme sports, and transmembrane transport through living endothelial cells to find the volume. This book is crowned by the consideration of a yet unexplored ion status in a mitochondrial matri

Metal oxides based electrochemical pH sensors: Current progress and future perspectives

Electrochemical pH sensors are on high demand in numerous applications such as food processing, health monitoring, agriculture and nuclear sectors, and water quality monitoring etc., owing to their fast response (<10 s), wide pH sensing range (2–12), superior sensitivity (close to Nernstian response of 59.12 mV/pH), easy integration on wearable/flexible substrates, excellent biocompatibility and low cost of fabrication. This article presents an in-depth review of the wide range of MOx materials that have been utilized to develop pH sensors, based on various mechanisms (e.g. potentiometric, conductimetric, chemi-resistors, ion sensitive field effect transistor (ISFET) and extended-gate field effect transistor etc.). The tools and techniques such as potentiometric and electrochemical impedance spectroscopic that are commonly adopted to characterize these metal oxide-based pH sensors are also discussed in detail. Concerning materials and design of sensors for various practical application, the major challenges are toxicity of materials, interfernce of other ions or analytes, cost, and flexibility of materials. In this regard, this review also discusses the metal oxide-based composite sensing (active) material, designs of pH sensors and their applications in flexible/wearable biosensors for medical application are examined to present their suitability for these futuristic applications

Investigation and development of titanium nitride solid-state potentiometric pH sensor

The measurement of pH value is crucial parameter in various fields like, drinking water monitoring, food preparation, biomedical and environmental applications. The most common device for pH sensing is the conventional pH glass electrode. While glass electrodes have several advantages, such as Nernstian sensitivity, superior ion selectivity, excellent stability, and extensive operating range, they have several key disadvantages. pH glass electrodes need to be stored in buffer solutions, they are fragile and have limited size and shape, making them impractical for some applications, such as being potentially used as miniature pH sensors for capsule endoscopy and ambulatory esophageal pH monitoring. To address these issues of limitations of glass electrodes, various metal oxides have been investigated and proposed as potential electrode materials for the development of pH sensors. Solid metal sensors offer unique features such as insolubility, stability, mechanical strength, and possibility of miniaturization. However, the main drawback of the metal oxide pH sensors is the interference caused by oxidizing and reducing agents present in some sample solutions. To reduce the redox interference, metal nitride solid sensors were investigated in this project with the potential for the development of high-sensitivity pH sensing electrodes. Metal nitrides are refractory, have high melting points and interstitial defects, and, at room temperature, they are chemically stable and resist hydrolysis caused by weak acids. There are many reports on different metal nitrides electrodes in literature, of which several have been previously investigated for use as pH sensors. Here, specifically, thin films of titanium nitride (TiN) were manufactured using radio frequency magnetron sputtering. The effect of sputtering parameters (e.g., thickness, sputter power, gas composition) were investigated to optimize the materials for use as pH sensor. Additionally, the underlining mechanism governing the pH sensitivity of these metal nitrides was investigated by examining the pH sensing properties (i.e., sensitivity, hysteresis, and drift) and the effect of redox agents. The successfully optimized material was then used to construct and demonstrate the concept of a solid-state pH sensor using an appropriate reference electrode. The solid-state TiN sensor paves the way for future development of a miniaturised pH sensor capsule for biomedical applications or lab-on-a-chip pH sensor for environmental and industrial applications. Expending the realms of pH monitoring, currently limited by the glass pH electrode

Development of a generic multi-analyte optical sensor platform for fluorescence-based sensing

This work describes the development of two advanced sensor platforms based on different spectroscopic techniques. The first, and the primary focus of this work, is an enhanced generic multi-analyte sensor platform for fluorescence-based sensors and the second is an absorbance-based portable sensor for the detection of nitrates in groundwater. A generic multi-analyte sensor platform can be applied to a broad range of areas such as food packaging and blood gas analysis. A multi-analyte optical sensor platform for enhanced capture of fluorescence was modelled, designed and fabricated. The sensor platform was developed using a range of microfabrication techniques. Films sensitive to oxygen, relative humidity and carbon dioxide respectively were developed for the context of indoor air-quality monitoring. Deposition methods for printing the sensor solutions onto the sensor platforms were also investigated. The sensor films and platforms were integrated into a working sensor chip with both a fluorescence intensity and phase fluorometric detection system. An absorbance-based portable sensor for the detection of nitrates in groundwater was also developed. This was based on the direct absorbance of UV-light by the nitrate ion. Other contaminants, which could be found in groundwater and interfere with the nitrate detection, such as humic acid and chlorides, were investigated and compensated for

Titanium nitride thin film based low‐redox‐interference potentiometric pH sensing electrodes

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. In this work, a solid‐state potentiometric pH sensor is designed by incorporating a thin film of Radio Frequency Magnetron Sputtered (RFMS) Titanium Nitride (TiN) working electrode and a commercial Ag|AgCl|KCl double junction reference electrode. The sensor shows a linear pH slope of −59.1 mV/pH, R2 = 0.9997, a hysteresis as low as 1.2 mV, and drift below 3.9 mV/hr. In addition, the redox interference performance of TiN electrodes is compared with that of Iridium Oxide (IrO2) counterparts. Experimental results show −32mV potential shift (E0 value) in 1 mM ascorbic acid (reducing agent) for TiN electrodes, and this is significantly lower than the −114 mV potential shift of IrO2 electrodes with sub‐Nernstian sensitivity. These results are most encouraging and pave the way towards the development of miniaturized, cost‐effective, and robust pH sensors for difficult matrices, such as wine and fresh orange juice

Flexible and Polymer-based CO2 Sensors for Food Packaging and Other Potential Applications

CO2 sensing is important in many applications ranging from air-quality monitoring to food packaging. Despite all the advancements in CO2 sensor technology, they are typically qualitative, bulky, expensive, and cross-sensitive to humidity, require high operating temperatures, external power sources, and complicated manufacturing processes making them incompatible with integration into food packaging. In light of this, the present study aims to develop chemiresistive, flexible, miniaturised, low-cost, lowpower, and simple-to-manufacture sensors capable of CO2 measurement at room temperature and high humidity conditions for food packaging and other potential applications. This thesis aims to develop chemiresistive, flexible, miniaturised, low-cost, low-power CO2 sensors for applications such as food packaging. The sensors are based on CO2-responsive polymers that change their electrical properties upon CO2 absorption. The interaction between CO2 and the polymer relies on acid-base chemistry, resulting in protonation of amine groups and altering the resistance. Initially, poly(N-[3-(dimethylamino)propyl] methacrylamide) (pDMAPMAm) is synthesised, but it exhibits irreversible response due to hindered proton hopping. To address this, poly(N-[3- (dimethylamino)propyl]-methacrylamide-co-2-N-morpholinoethyl methacrylate) (p(D-co-M)) with adjusted composition and basicity is developed, showing a reversible response to CO2. However, it has relatively long response and recovery times and cross-sensitivity to ammonia. To improve these shortcomings, a thin layer of Nafion-Na coating is applied to the p(D-co-M) sensor, denoted P-NafionNa sensors, reducing cross-sensitivity, shortening recovery time, and enabling Bluetooth® communication. The developed materials and sensors show promise for creating the next generation of miniaturised, flexible, wireless, and cost-effective CO2 sensors for various applications, including food quality monitoring