Large-scale layer-by-layer inkjet printing of flexible iridium-oxide based pH sensors

Flexible ammonium ion-selective electrode based on inkjet-printed graphene solid contact

Miniaturization and mass-production of potentiometric sensor systems is paving the way towards distributed environmental sensing, on-body measurements and industrial process monitoring. Inkjet printing is gaining popularity as a highly adaptable and scalable production technique. Presented here is a scalable and low-cost route for flexible solid-contact ammonium ion-selective electrode fabrication by inkjet printing. Utilization of inkjet-printed melamine-intercalated graphene nanosheets as the solid-contact material significantly improved charge transport, while evading the detrimental water-layer formation. External polarization was investigated as a means of improving the inter-electrode reproducibility: the standard deviations of E0 values were reduced after electrode polarization, the linear region of the response was extended to the range 10-1-10-6 M of NH4Cl and LODs reduced to 0.88 ± 0.17 μM. Finally, we have shown that the electrodes are adequate for measurements in a complex real sample: ammonium concentration was determined in landfill leachate water, with less than 4 % deviation from the reference method.

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.

A Review on Progress, Challenges, and Prospects of Material Jetting of Copper and Tungsten

Copper (Cu) and tungsten (W) possess exceptional electrical and thermal conductivity properties, making them suitable candidates for applications such as interconnects and thermal conductivity enhancements. Solution-based additive manufacturing (SBAM) offers unique advantages, including patterning capabilities, cost-effectiveness, and scalability among the various methods for manufacturing Cu and W-based films and structures. In particular, SBAM material jetting techniques, such as inkjet printing (IJP), direct ink writing (DIW), and aerosol jet printing (AJP), present a promising approach for design freedom, low material wastes, and versatility as either stand-alone printers or integrated with powder bed-based metal additive manufacturing (MAM). Thus, this review summarizes recent advancements in solution-processed Cu and W, focusing on IJP, DIW, and AJP techniques. The discussion encompasses general aspects, current status, challenges, and recent research highlights. Furthermore, this paper addresses integrating material jetting techniques with powder bed-based MAM to fabricate functional alloys and multi-material structures. Finally, the factors influencing large-scale fabrication and potential prospects in this area are explored.

From Classical to Advanced Use of Polymers in Food and Beverage Applications

Polymers are extensively used in food and beverage packaging to shield against contaminants and external damage due to their barrier properties, protecting the goods inside and reducing waste. However, current trends in polymers for food, water, and beverage applications are moving forward into the design and preparation of advanced polymers, which can act as active packaging, bearing active ingredients in their formulation, or controlling the head-space composition to extend the shelf-life of the goods inside. In addition, polymers can serve as sensory polymers to detect and indicate the presence of target species, including contaminants of food quality indicators, or even to remove or separate target species for later quantification. Polymers are nowadays essential materials for both food safety and the extension of food shelf-life, which are key goals of the food industry, and the irruption of smart materials is opening new opportunities for going even further in these goals. This review describes the state of the art following the last 10 years of research within the field of food and beverage polymer's applications, covering present applications, perspectives, and concerns related to waste generation and the circular economy.

Metal-Oxide Nanomaterials Synthesis and Applications in Flexible and Wearable Sensors

Metal-oxide nanomaterials (MONs) have gained considerable interest in the construction of flexible/wearable sensors due to their tunable band gap, low cost, large specific area, and ease of manufacturing. Furthermore, MONs are in high demand for applications, such as gas leakage alarms, environmental protection, health tracking, and smart devices integrated with another system. In this Review, we introduce a comprehensive investigation of factors to boost the sensitivity of MON-based sensors in environmental indicators and health monitoring. Finally, the challenges and perspectives of MON-based flexible/wearable sensors are considered.

Nanomaterials in Smart Packaging Applications: A Review

Food wastage is a critical and world-wide issue resulting from an excess of food supply, poor food storage, poor marketing, and unstable markets. Since food quality depends on consumer standards, it becomes necessary to monitor the quality to ensure it meets those standards. Embedding sensors with active nanomaterials in food packaging enables customers to monitor the quality of their food in real-time. Though there are many different sensors that can monitor food quality and safety, pH sensors and time-temperature indicators (TTIs) are the most critical metrics in indicating quality. This review showcases some of the recent progress, their importance, preconditions, and the various future needs of pH sensors and TTIs in food packaging for smart sensors in food packaging applications. In discussing these topics, this review includes the materials used to make these sensors, which vary from polymers, metals, metal-oxides, carbon-based materials; and their modes of fabrication, ranging from thin or thick film deposition methods, solution-based chemistry, and electrodeposition. By discussing the use of these materials, novel fabrication process, and problems for the two sensors, this review offers solutions to a brighter future for the use of nanomaterials for pH indicator and TTIs in food packaging applications.

Iridium Oxide Enabled Sensors Applications

There have been numerous studies applying iridium oxides in different applications to explore their proton-change-based reactions since the 1980s. Iridium oxide can be fabricated directly by applying electrodeposition, sputter-coating method, or oxidation of iridium wire. Generally, there have been currently two approaches in applying iridium oxide to enable its sensing applications. One was to improve or create different electrolytes with (non-)electrodeposition method for better performance of Nernst Constant with the temperature-related system. The mechanism behind the scenes were summarized herein. The other was to change the structure of iridium oxide through different kinds of templates such as photolithography patterns, or template-assisted direct growth methods, etc. to improve the sensing performance. The detection targets varied widely from intracellular cell pH, glucose in an artificial sample or actual urine sample, and the hydrogen peroxide, glutamate or organophosphate pesticides, metal-ions, etc. This review paper has focused on the mechanism of electrodeposition of iridium oxide in aqueous conditions and the sensing applications towards different biomolecules compounds. Finally, we summarize future trends on Iridium oxide based sensing and predict future work that could be further explored.

Advanced Wound Dressing for Real-Time pH Monitoring

The rapid evolution of wearable technologies is giving rise to a strong push for textile chemical sensors design targeting the real-time collection of vital parameters for improved healthcare. Among the most promising applications, monitoring of nonhealing wounds is a scarcely explored medical field that still lacks quantitative tools for the management of the healing process. In this work, a smart bandage is developed for the real-time monitoring of wound pH, which has been reported to correlate with the healing stages, thus potentially giving direct access to the wound status without disturbing the wound bed. The fully textile device is realized by integrating a sensing layer, including the two-terminal pH sensor made of a semiconducting polymer and iridium oxide particles, and an absorbent layer ensuring the delivery of a continuous wound exudate flow across the sensor area. The two-terminal sensor exhibits a reversible response with a sensitivity of (59 ± 4) μA pH-1 in the medically relevant pH range for wound monitoring (pH 6-9), and its performance is not substantially affected either by the presence of the most common chemical interferents or by temperature gradients from 22 to 40 °C. Thanks to the robust sensing mechanism based on potentiometric transduction and the simple device geometry, the fully assembled smart bandage was successfully validated in flow analysis using synthetic wound exudate.

Inkjet Printing Transparent and Conductive MXene (Ti3C2Tx) Films: A Strategy for Flexible Energy Storage Devices

MXene is a generic name for a large family of two-dimensional transition metal carbides or nitrides, which show great promise in the field of transparent supercapacitors. However, the manufacturing of supercapacitor electrodes with a high charge storage capacity and desirable transmittance is a challenging task. Herein, a low-cost, large-scale, and rapid preparation of flexible and transparent MXene films via inkjet printing is reported. The MXene films realized the sheet resistance (R_s) of 1.66 ± 0.16 MΩ sq^-1 to 1.47 ± 0.1 kΩ sq^-1 at the transmissivity of 87-24% (λ = 550 nm), respectively, corresponding to the figure of merit (the ratio of electronic to optical conductivity, σ_DC/σ_OP) of ∼0.0012 to 0.13. Furthermore, the potential of inkjet-printed transparent MXene films in transparent supercapacitors was assessed by electrochemical characterization. The MXene film, with a transmittance of 24%, exhibited a superior areal capacitance of 887.5 μF cm^-2 and retained 85% of the initial capacitance after 10,000 charge/discharge cycles at the scan rate of 10 mV s^-1. Interestingly, the areal capacitance (192 μF cm^-2) of an assembled symmetric MXene transparent supercapacitor, with a high transmittance of 73%, still surpasses the performance of previously reported graphene and single-walled carbon nanotube (SWCNT)-based transparent electrodes. The convenient manufacturing and superior electrochemical performance of inkjet-printed flexible and transparent MXene films widen the application horizon of this strategy for flexible energy storage devices.

Polyelectrolytes Assembly: A Powerful Tool for Electrochemical Sensing Application

The development of sensing coatings, as important sensor elements that integrate functionality, simplicity, chemical stability, and physical stability, has been shown to play a major role in electrochemical sensing system development trends. Simple and versatile assembling procedures and scalability make polyelectrolytes highly convenient for use in electrochemical sensing applications. Polyelectrolytes are mainly used in electrochemical sensor architectures for entrapping (incorporation, immobilization, etc.) various materials into sensing layers. These materials can often increase sensitivity, selectivity, and electronic communications with the electrode substrate, and they can mediate electron transfer between an analyte and transducer. Analytical performance can be significantly improved by the synergistic effect of materials (sensing material, transducer, and mediator) present in these composites. As most reported methods for the preparation of polyelectrolyte-based sensing layers are layer-by-layer and casting/coating methods, this review focuses on the use of the latter methods in the development of electrochemical sensors within the last decade. In contrast to many reviews related to electrochemical sensors that feature polyelectrolytes, this review is focused on architectures of sensing layers and the role of polyelectrolytes in the development of sensing systems. Additionally, the role of polyelectrolytes in the preparation and modification of various nanoparticles, nanoprobes, reporter probes, nanobeads, etc. that are used in electrochemical sensing systems is also reviewed.

Nanochannel Templated Iridium Oxide Nanostructures for Wide-Range pH Sensing from Solutions to Human Skin Surface

Herein we report the fabrication of highly sensitive solid-state pH sensors based on iridium oxide nanowires (IONWs) for a wide-range of pH measurements. IONWs were confined electrodeposits on the indium tin oxide (ITO) electrode using a highly ordered silica nanochannel membrane as the template. Subsequently removing the template produced amorphous IONWs consisting of hydrated iridium oxyhydroxides. The IONW/ITO sensor can rapidly respond to the pH of the aqueous solutions in a wide range (from 0 to 13), avoiding the acid and alkaline errors encountered by conventional pH electrodes and exhibiting a super-Nernst analytical sensitivity as high as 235.5 mV/pH in the very acidic range of ∼0-2.5 and 90.1 mV/pH beyond (pH = ∼2.5-13). The sensitivity was associated with the interconversion of oxidation states of iridium oxyhydroxides. While in the very acidic range, intercalation of Cl^- was proved to be responsible for the exceptionally high pH sensitivity. Moreover, the sensor was also demonstrated to work in organic solutions too. Finally, the flexible IONW/ITO electrode was prepared and interfaced to a wireless electrochemical device for real-time epidermal pH analysis with smartphones.

Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes

Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 μm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li₂SO₄), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit.

Potentiometric performance of flexible pH sensor based on polyaniline nanofiber arrays

We report potentiometric performance of a polyaniline nanofiber array-based pH sensor fabricated by combining a dilute chemical polymerization and low-cost and simple screen printing process. The pH sensor had a two-electrode configuration consisting of polyaniline nanofiber array sensing electrode and Ag/AgCl reference electrode. Measurement of electromotive force between sensing and reference electrodes provided various electrochemical properties of pH sensors. The pH sensor show excellent sensor performances of sensitivity of 62.4 mV/pH, repeatability of 97.9% retention, response time of 12.8 s, and durability of 3.0 mV/h. The pH sensor could also measure pH changes as the milk is spoiled, which is similar to those of a commercial pH meter. The pH sensors were highly flexible, and thus can measure the fruit decay on the curved surface of an apple. This flexible and miniature pH sensor opens new opportunities for monitoring of water, product process, human health, and chemical (or bio) reactions even using small volumes of samples.

Greenhouse Gas Sensors Fabricated with New Materials for Climatic Usage: A Review

With the increasing utilization of fossil fuels in today’s technological world, the atmosphere’s concentration of greenhouse gases is increasing and needs to be controlled. In order to achieve this goal, it is imperative to have sensors that can provide data on the greenhouse gases in the environment. The recent literature contains a few publications that detail the use of new methods and materials for sensing these gases. The first part of this review is focused on the possible effects of greenhouse gases in the atmosphere, and the second part surveys the developments of sensors for greenhouse gases with coverage on carbon nano-materials and composites directed towards sensing gases like CO2, CH4, and NOx. With carbon dioxide measurements, due consideration is given to the dissolved carbon dioxide gas in water (moisture). The density functional calculations project that Pd-doped single-walled carbon nanotubes are ideal for the development of NOx sensors. The current trend is to make sensors using 3D printing or inkjet printing in order to allow for the achievement of ppb levels of sensitivity that have not been realized before. This review is to elaborate on the need for the development of greenhouse gas sensors for climatic usage by using selected examples.