Advancements in Flexible pH Sensors for Health Monitoring

Measurement of pH using pH sensors is widely used in clinical diagnostics.   For instance, healthcare professionals use pH sensors to assess pH levels of blood and urine and also to monitor and adjust the pH of intravenous solutions to ensure they are compatible with the body’s pH range. However, traditional pH sensors have been limited by their rigid form and lack of wearability, making them not suitable for integration into modern health monitoring systems. To address these challenges, researchers have turned to conducting polymers, such as polyaniline, as promising materials for flexible and wearable pH sensors. Conducting polymers, characterized by π-conjugated double bonds in their chain structures, possess several desirable properties for pH sensing applications. These properties include high electrical conductivity, mechanical flexibility, simplicity, and cost-effectiveness. Notably, polyaniline (PANI) has emerged as a prominent candidate due to its ease of synthesis through various methods, including electrochemical and chemical approaches. Moreover, PANI exhibits tunable electrical conductivity in response to changes in pH, making it an ideal choice for potentiometric pH sensors. One innovative approach in the quest for flexible pH sensors is the incorporation of PANI into composites with carbon-based materials, particularly graphite (G). Graphite offers a unique combination of metallic and non-metallic properties, such as high thermal resistance and low electrical conductivity, which can enhance the performance of pH sensors. PANI/G composites have been explored for their controllable conductivity as a function of pH, enabling precise pH measurements. The synergy between PANI and graphite results in an effective potentiometric pH sensor, where the redox equilibrium between H3O+ ions and PANI phase transitions plays a central role.

In a new research study conducted by PhD candidate Shirin Mahinnezhad, Professor Ricardo Izquierdo, and Professor Andy Shih from the Department of Electrical Engineering at École de technologie supérieure in Canada and published in the peer-reviewed Journal of The Electrochemical Society, sheds light on the development of flexible pH sensors based on polyaniline/graphite (PANI/G) composites.

The research team conducted a comprehensive characterization of PANI/G composites to understand their structural and functional properties. The authors used scanning electron microscopy to reveal the surface morphology of PANI/G composites at various graphite concentrations. Notably, PANI chains were observed attached to graphite active sites, confirming the successful integration of these materials. Fourier transform infrared spectra analysis provided insights into the bonding characteristics of PANI/G composites. The shifts in peaks indicated interlayer bonding between PANI and graphite, which enhanced electron delocalization and conductivity. X-ray diffraction patterns further elucidated the impact of PANI on the crystal structure of graphite. The presence of PANI was observed in all PANI/G composites, indicating charge transfer between PANI and graphite crystallites, resulting in a higher degree of structural ordering. These detailed characterizations shed light on the intriguing interplay between PANI and graphite, underlining the importance of their compositional balance.

The authors evaluated the performance of pH sensors modified with PANI/G composites. The sensors were tested over a pH range from 3 to 10, with excellent results. The PANI/G 3 composite, with the highest graphite loading, exhibited a near-Nernstian sensitivity of 53 mV pH^-1 and a response time of just 15 seconds. This remarkable performance highlights the potential of PANI/G composites in pH sensing applications.

Furthermore, the study explored the role of a graphite layer on the working electrode’s functionality. Sensors with PANI/G composites printed directly on the Ag/AgCl electrode without the graphite layer exhibited inferior performance, emphasizing the importance of graphite in enhancing sensor sensitivity and stability. PANI/G 3, with its higher conductivity, outperformed the other composites, demonstrating the significance of the composite’s electrical properties.

Selectivity is a crucial aspect of pH sensors, especially when they are used in complex environments with various interfering ions. The Canadian researchers assessed the selectivity coefficients of the pH sensor modified with PANI/G 3 composite against interfering ions commonly found in biological fluids. The results were promising, with all selectivity coefficients below 10^-9, indicating the sensor’s ability to accurately measure H3O+ ions over interfering ions. Additionally, the sensor’s repeatability was tested by immersing it in different acidic and alkaline solutions multiple times without cleaning. The sensor exhibited consistent and stable performance across these cycles, reaffirming its reliability for long-term use.

In conclusion, the research conducted by Professor Andy Shih and colleagues represents an important advancement in the field of flexible pH sensors. Their new study demonstrates the potential of PANI/G composites for the development of wearable pH sensors with excellent sensitivity, stability, selectivity, and repeatability. These sensors have the capacity to revolutionize healthcare by enabling real-time monitoring and integration into smart health monitoring systems, bringing us one step closer to a more connected and proactive approach to healthcare.