Development, Optimisation and Applications of Screen-Printed Electrochemical Sensors

The sustainability of healthcare delivery depends on the adoption of new low-cost devices to support the transition of services from centralised generic models to home and community-based care models, through which the patient status can be monitored remotely. Easily accessible body fluids (like saliva, sweat and interstitial fluids) represent alternative sampling media to blood that in principle can be conveniently analysed through wearable sensors. For instance, continuous monitoring of pH in saliva would allow a better clinical management of pathologies that alter acid contents within the mouth. Similarly, the real-time tracking of sodium levels in sweat and other body fluids can assist clinicians in the diagnosis and treatment of Cystic Fibrosis. Furthermore, athletes could reap many benefits from an optimal strategy for personalised rehydration, which might be informed by continuously measuring the amount of minerals lost in sweat. Electrochemical sensors based on the combination of screen-printed working and solid-contact reference electrodes are versatile and low-cost tools that are effective in facing many of the challenges in current sensing technology. They can be readily adapted for the detection of several ionic species, and in this thesis, as an example, two electrochemical platforms to monitor pH in saliva and sodium in sweat are going to be presented. The final devices are minimally-invasive and wearable, with a compact format due to the integration of miniaturised solid state ion-selective and reference electrodes. The technological advancements developed for their realisation are significant contributions for the more flexible design of novel miniaturised sensors for remote monitoring in general. Future developments of this technology could be pivotal for realising devices for applications as diverse as sensors integrated into fabrics for personal health monitoring, or autonomous sensors deployed in rivers and lakes for monitoring water quality

Disposable potentiometric strips: a versatile tool for low-cost sensing

The integration of sensors within wireless networks represents a pivotal step for remote analytical monitoring, not limited to pollutants in the environment but also suitable for diagnostics and personal health monitoring. For instance, higher spatial and temporal resolutions for toxic metals are fundamental for epidemiology studies, where they would help to establish definitive associations between the exposure to specific chemicals and the health of individuals and communities. In addition, accessible body fluids such as saliva and sweat represent an interesting medium for the realization of wearable sensors. These fluids contain important personalized physiological information. For example, a continuous monitoring of pH in saliva and electrolyte concentrations in sweat would be beneficial for controlling healthy mouth conditions or to improve endurance performances and avoid critical situations, e.g., dehydration and hyponatremia in athletes. Low-cost potentiometric strips based on screen printed substrates consisting of a solid-contact ion-selective electrode (SC-ISE) combined to a solid-contact reference electrode (SC-RE) may offer a versatile tool in sensing technology. These strips can be readily adapted for the detection of several ionic species. Here, as an example, we present on the monitoring of pH in saliva, Na+ in sweat and Pb2+ in water. The strip is plugged into an electronic platform of reduced dimensions which converts the chemical information into an electric signal, wireless transmitted to a base station. This technology may have an impact in remote monitoring but also in wearable sensing thanks to the device miniaturization

A potentiometric disposable sensor strip for measuring pH in saliva

In this paper, the preparation of a potentiometric strip for pH monitoring in saliva samples is reported. The potentiometric strip consists of a solid contact pH-selective and of a solidcontact ionogel reference electrode prepared on a dual screen printed substrate. The screen printing protocols, i.e., type of inks and number of deposits, were adjusted to relatively improve the batch reproducibility and the stability of the pH sensor. The pH of real saliva samples was monitored using the optimised potentiometric strip, and results were validated through parallel measurements with a standard laboratory method

Screen printed electrochemical sensors for real-time sodium monitoring in sweat

We report on the preparation of disposable potentiometric sensor strips for monitoring sodium in sweat. We also present their integration in a microfluidic chip used to harvest sweat in-situ during exercise. The sensor-chip is integrated with a miniaturized electronic platform able to transmit data wirelessly in real time during a stationary cycling session in a controlled environment

Non invasive detection of biological fluids: a new perspective in monitoring pH in saliva and sodium in sweat

The chemical composition of body fluids contains crucial information about the state of health of an individual. While many efforts have been already directed toward real time analysis of blood and urine, there is still a pressing need for new solutions to non-invasively monitor other fluids like saliva and sweat1. Towards this aim, the main technological challenge is the development of devices that are at the same time low-cost, minimally invasive and wearable, so that they could be used for in situ and real-time monitoring of physiological conditions2. For example, continuous recording of sodium levels in sweat could be an informative tool to assist clinicians in prescribing a more personalised treatment of diseases such as Cystic Fibrosis3 and in assessing athletes’ performances4. Similarly, the monitoring of pH levels in saliva provides valuable information for the treatment of pathologies where physiological mouth conditions are compromised, like in Gastroesophageal Reflux Disease (GERD)5. Ion Selective Electrodes (ISEs) are potentiometric sensors designed to detect specific ions in blood and saliva. Using dual-screen printed electrodes as substrates, we were able to reduce their production cost, improve reproducibility, and combine pH5 and sodium ISEs with solid contact reference electrodes. In our design, the sensors will be interfaced to two miniaturized potentiometric platforms (WIXEL for pH and Tyndall Mote for sodium detection) that were wirelessly connected to a base station. For pH measurements, the device will be accommodated into a gum shield. For sodium detection instead, we will use a microfluidic channel to convey sweat to the electrodes. The mote communication platform was adapted so that it could be worn on the upper shoulder through a fiber strip

Wearable chemical sensing – sensor design and sampling techniques for real-time sweat analysis

Wearable chemical sensors have the potential to provide new methods of non-invasive physiological measurement. The nature of chemical sensors involves an active surface where a chemical reaction must occur to elicit a response. This adds complexity to a wearable system which creates challenges in the design of a reliable long-term working system. This work presents the design of a real-time sweat sensing platform to analyse sweat loss and composition. Sampling methods have an impact on composition therefore skin encapsulation needs to be avoided so as not to disrupt normal sweating patterns. Sensors ideally need to be placed close to the sampling site which may be subject to motion artefacts [1]. The design of this device takes into account sample collection and delivery, sensor placement and associated electronics. The overall design is ergonomic to interface with the contours of the body. Results of lab-based simulations and real-time exercise trials are presented. This device can offer valuable information regarding hydration status and electrolyte balance which may be especially important for optimised rehydration during or after sports activities. [1] Curto, V. F. S. Coyle, R. Byrne, N. Angelov, D. Diamond, F. Benito-Lopez., Sens. Actuators, B, 2012, 175, 263-270

Disposable solid-contact ion-selective electrodes for environmental monitoring of lead with ppb

Solid-contact Pb2+-selective-electrodes and solid contact reference electrodes suitable for use as disposable sensing devices for environmental monitoring of lead have been prepared on screen-printed substrates. Accurate control over the fabrication procedures leads to excellent reproducibility of their calibration characteristics such as slope, offset and limit of detection. In particular, the limit of detection in the nanomolar range opens the possibility of their use for trace analysis of Pb2+ in environmental water samples. Significantly, the potentiometric measurements correlate well with data determined using inductively coupled plasma mass spectrometry (ICP-MS) in a number of real samples taken from local rivers. Ways in which these sensors might be employed in autonomous platforms for monitoring water quality in-situ are discussed. The possibility of including arrays of virtually identical sensors is highlighted as a possible route to achieve long-term deployments

‘SWEATCH’ – A platform for real-time monitoring of sweat electrolyte composition

Since the initial breakthroughs in the 1960’s and 70’s that led to the development of the glucose biosensor, the oxygen electrode, ion-selective electrodes, and electrochemical/optochemical diagnostic devices, the vision of very reliable, affordable chemical sensors and bio-sensors capable of functioning autonomously for long periods of time (years), and providing access to continuous streams of real-time data remains unrealized. This is despite massive investment in research and the publication of many thousands of papers in the literature. It is over 40 years since the first papers proposing the concept of the artificial pancreas, by combining the glucose electrode with an insulin pump. Yet even now, there is no chemical sensor/biosensor that can function reliably inside the body for more than a few days, and such is the gap in what can be delivered (days), and what is required (minimum 10 years) for implantable devices, it is not surprising that in health diagnostics, the overwhelmingly dominant paradigm for reliable measurements is single use disposable sensors. Realising disruptive improvements in chem/bio-sensing platforms capable of long-term (months, years) independent operation requires a step-back and rethinking of strategies, and considering solutions suggested by nature, rather than incremental improvements in available technologies

A liquid-junction-free reference electrode based on a PEDOT solid-contact and ionogel capping membrane

Liquid-junction-free reference electrodes were prepared on screen printed substrates using poly-3,4-ethylenedioxythiophene (PEDOT) as solid-contact and novel ionogels as capping membrane. The chemico-physical properties of the PEDOT layer were tuned by changing the electropolymerization media and electrodeposition technique. Particularly, electrodepositing PEDOT films potentiostatically or potentiodynamically impacted on the traces of the potential of the electrodes during the conditioning step. In addition, the choice of the capping membrane formulation, e.g., acrylate monomers, ionic liquid, cross-linkers and photo-initiators, was adjusted to obtain electrodes with properties almost equivalent of a standard reference electrode. Thus, calibration plots of Na+ ion-selective electrodes against the optimized solid-contact ionogel reference electrodes (SCI-REs) or against a double-liquid junction Ag/AgCl electrode did not present any significant difference. Such SCI-REs may provide an effective route to the generation of future low-cost components for potentiometric sensing strips