Picomolar-sensitive impedimetric sensor for salivary calcium analysis at POC based on SAM of Schiff base-modified gold electrode

Analysis of saliva is a potential diagnostic tool in the management of human diseases. Analysis of saliva in healthy individuals is vital to comparison in a diseased state. Salivary glandular secretion constantly bathes the teeth and oral mucosa. The presence of saliva is vital for healthy oral tissue. Positive correlation has been shown in salivary calcium and phosphate and oral health. We have developed a highly sensitive and selective impedimetric calcium sensor for its non-invasive determination in saliva. The sensor is based on 2-hydroxy-4-(2-oxo-1,2-diphenylethylidene)amino) benzoic acid ionophore; self-assembled monolayer (SAM) on gold electrode has been developed. The calcium sensor was constructed by SAM-Au formation of the compound developed by covalently attaching 4-aminothiophenol (ATP) to the ionophore molecule through amide bond formation between its amino group and the carboxylic group of the ionophore. Characterization of the SAM formation on the gold electrode was performed using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The modification of the gold electrode was confirmed by measuring the adhesion and surface morphology using contact angle (CA) measurements and SEM. EIS was used as the measuring technique; the sensor showed a linear analytical range (LR) of 5 x 10(-12)-1 x 10(-6) mol L-1 and a limit of detection (LOD) of 3.6 x 10(-12) mol L-1 calculated on the basis of 3 sigma/s and limit of quantification (LOQ) of 1.2 x 10(-11) mol L-1. Moreover, the sensor was found to exhibit high selectivity for Ca(2+)selectivity over a variety of common interfering ions. The impedance behavior of the proposed calcium sensor has been modeled by an equivalent electrical circuit using a modified Randles model. The covalent immobilization of the ionophore into the modified gold electrode was manifested in its prolonged stability. The sensor was utilized for the determination of calcium concentration in real samples of human saliva; therefore, we believe it is suitable for point of care (POC). Graphical abstract