Screen printed quinone-derivatives-based carbon electrode : a novel universal voltammetric pH sensor in buffered and unbuffered solution

Abstract

Quinone-based working electrodes have been extensively investigated as voltammetric pH sensor. Nonetheless, most of its derivatives could not monitor pH beyond 10 in buffered solution and sense unbuffered solution pH with error less than 5%. In this project, a novel universal voltammetric pH sensor in buffered and unbuffered media using TMNQ has been developed. 1,2-dihydroxyanthraquinone(Alizarin), 2,3,6,7-tetramethylnapthoquinone (TMNQ), and 2,3-dimethyl-1,4-dihydroxyanthraquinone /2,3-dimethylquinizarin (DMQ)-modified electrodes are investigated. Screen printing is used to fabricate two types of electrode (1) homogenized compound and carbon black mix, and (2) bare carbon black followed by subsequent drop on of compound solution. Additionally, varying compoud mix ratio, layer thickness, size and SWV electroanalytical parameter are also explored to optimize their performance. Alizarin, TMNW and DMQ are suggested to have 2e-,2H+ redox mechanism with probable electron hopping and in-film diffusion suggested for 3 layer thickness kinetics. Alizarin exhibits a shift of 56.38mV/pH over pH 1.75-9 in BR Buffer and 0.5% error in unbuffered pH monitoring but it suffers from nucleophilic attack via 1,4-Michael addition to the double bond at high pH, leading to dissolution and production of new compound. Substitution of double bond by methyl group in TMNQ and DMQ do not show similar phenomenon. Drop on DMQ displays a shift of 55.8mV/pH over pH 1-10 and 0.85% error in unbuffered pH sensing. Drop On TMNQ exhibits a potential shift of 62.87mV/pH over a range of pH 0 to14 in BR Buffer, close to theoretical value of 59.2mV/pH and monitored full working range of pH 0-14. It also displays an acceptable error of 3.26% in 0.5M KCl. Since no dissolution and emergence of new compound are not observed, the superior performance of TMNQ and limitation of DMQ in BR Buffer could be attributed to their high and low pKa respectively. In unbuffered media, increasing hydrophobicity shows increasing error in unbuffered media, which reiterates previous literature. Furthermore, ratio of compound:carbon black 1:10, 3 layers, and 2.5mm are found to be the optimal parameter. Further study of compound similar to 2,3-dimethylquinizarin and 2,3,5,6,7,8-hexamethylquinizarin could be done to examine their pKa of napthoquinone and quinizarin group.