All-Solid-State Ion-Selective Electrodes with Multi-Walled Carbon Nanotubes Based on Glassy Carbon for Biguanide Derivative (Chlorhexidine Digluconate)

This paper presents research on the properties of solid contact electrodes with an intermediate layer of multi-walled carbon nanotubes. They were used as an intermediate layer (Solid Contact SC-ISE electrodes) and as a dispersed material in a polymer membrane (Single Piece SP-ISE electrodes). The properties of neutral macrocyclic compounds as active ion-sensitive substances in membranes, in which plasticizers with varying lipophilicity were also used, were compared. The selected optimal sensor shows a theoretical Nernstian response for the divalent chlorhexidine ion (z = 2) of approximately 29 mV decade−1 over the linearity range of 10−6–10−3 mol L−1. A short response time of approx. 10 s, potential stability (the potential drift is several dozen μV min−1 in a 10−4 mol L−1 chlorhexidine solution over 4 h), and high potential reversibility (the standard deviation in a 10−5 mol L−1 solution is 0.69 mV, whereas in a 10−4 mol L−1 solution it is 0.71 mV) are further advantages and benefits of the developed modern electrodes. In general, it can be concluded that the SC-ISE showed better potential reversibility than the SP-ISE, particularly at chlorhexidine concentrations of 10−4 and 10−5 mol L−1, while also exhibiting improved resistance to water-layer formation. High selectivity was also achieved in relation to ionic substances (sodium, potassium, calcium, magnesium, acesulfame ions, and neutral substances used as excipients in chlorhexidine preparations, i.e., d-mannitol, d-sorbitol, sucralose, propylene glycol, xylitol, aspartame). The sensor with satisfactory parameters was used to determine the API content in the pharmaceutical preparation, Eludril, with a recovery of 95.9–98.6% and a relative standard deviation (RSD) ranging from 0.7 to 3.7%.