The effect of Na⁺ and K⁺ doping on the properties of sol-gel deposited 2-hydroxy-1,4-naphthoquinone thin films

We describe the use of 2-hydroxy-1,4-naphthoquinone (HNQ) thin films as a potential water vapor and electrolyte sensing material towards the goal of non-invasive relative humidity and sweat detection. We have successfully made HNQ sol-gel thin films and studied the effects of sodium and potassium ions on their optical and electrical characteristics. Ultraviolet-visible absorbance and Fourier transform infrared spectroscopy measurements along with scanning electron microscopy demonstrated that we were able to dope HNQ thin films with Na⁺ and K⁺ ions, which are the main electrolyte contents in sweat. While the conductivity of thin films increased by at least an order of magnitude, energy band gaps decreased by doping HNQ with Na⁺ and K⁺ ions. Relative humidity test results showed that HNQ-based thin-films can be used as a sensing material for water vapor. Room temperature current-voltage measurements were also performed to determine the surface conductivity.