Electrochemical Characterisation of a Real-Time pH Sensor

Real-time monitoring of pH levels is important for many reasons.  Abnormal tissue pH is an indicator of altered cellular metabolism in illnesses such as stroke and cancer.  Tumour pH, for example, is more acidic than that of normal tissue in both animals and humans due to elevated levels of anaerobic and aerobic glycolysis in tumours¹.  In the brain, the release of neurotransmitters result in a series of metabolic processes causing an influx of various ions that result in pH changes in the surrounding matrix². Transient local pH changes in the brain are important markers of neuronal activity that can be used to follow metabolic processes which underlie the biological basis of behaviour, learning and memory³.

In this study a carbon paste electrode (CPE) was modified with a quinone containing diazonium salt by electrochemical deposition.  Various deposition conditions were employed to give the optimum and most efficient method.  Shown in Figure 1 is a SEM micrograph of a modified CPE alongside the mechanism for the electrodeposition of the diazonium salt onto the electrode.

Figure 2 shows a bare CPE cycled in a PBS solution compared to a modified CPE cycled in the same solution.  This indicates that the diazonium salt was successfully deposited onto the electrode surface.  Potentiometric pH responses of the modified electrodes showed a response slope of −60.36 ± 0.89 mV/pH, n = 23, as shown in Figure 3.  The electrode stability was tested for a period of up to 28 days.  Modified electrodes were also subjected to treatment with physiologically relevant proteins and lipids to investigate any implications for real-time monitoring of brain pH.

References:

1. J. Wike-Hooley, A. van den Berg, J. van der Zee and H. Reinhold, European Journal of Cancer and Clinical Oncology, 1985, 7, 785-787.
2. M. Makos, Y.Kim, K. Han, M. Heien and A. Ewing , Analytical Chemistry, 2009, 81, 1848–1854.
3. P. Takmakov, M. Zachek, R. Keithley, E. Bucher, G. McCarthy and R. Wightman, Analytical Chemistry, 2010, 82, 9892–9900.