Investigating the Optical Properties of Electrodeposited Tungsten Oxide Thin Films for Selective Sodium-Ion Detection

A novel method of sodium-ion detection from aqueous solutions based on a change of optical transmittance is proposed. Tungsten oxide-coated indium tin oxide (ITO) samples for selective sodium-ion detection are prepared from varying concentrations of peroxo tungstic oxide precursors using a simple cathodic electrodeposition method. The optical property changes were recorded in the UV–visible (VIS)–near infrared (NIR) range for the samples in aqueous solutions of NaCl in TRIS/deionized (DI) water solvent. It was observed that transmittance increases with NaCl concentration in a particular wavelength range. A detailed material characterization was performed to support this observation and study Na+ ion diffusion-induced structural changes. The sensor deposited from peroxo tungstic acid (PTA) precursor of molarity 45 mM, at a cathodic potential of −0.45 V, showed a better response. The sensor exhibits good selectivity toward
$\text{N}^{+}$
compared to
$\text{K}^{+}$
and Ca
$^{{2}+}$
ions and linear response in the range of 10–150 mM Na+. Simple construction, higher response, and selectivity make it a promising method for sodium sensing in food industries and health monitoring.