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

Abstract

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.

A portable optical human sweat sensor

“…31 It turns from pale orange to dark orange (see Figure 2(a) inset). The main reason behind the color change is that the hydroxyl group of HNQ (see Figure 1 inset) is deprotonated, and the negatively charged oxygen atom is attached to positively charged sodium or potassium ions, forming a doped HNQ structure.…”

“…The main reason behind the color change is that the hydroxyl group of HNQ (see Figure 1 inset) is deprotonated, and the negatively charged oxygen atom is attached to positively charged sodium or potassium ions, forming a doped HNQ structure. 31 This effect changes the electron density of the FIG. 1.…”

“…We have previously demonstrated that a natural dye known as HNQ can be used as a sweat sensing material based on color changes. 31,32 Our previous work focused on the optical properties of HNQ under various hydration conditions where the optical absorbance is linearly related to the amount of ions that are present in a liquid media along with HNQ. 32 In the current work, optical absorbance versus concentration of sweat components is analyzed.…”

Hydration detection through use of artificial sweat in doped- and partially-doped nanostructured CuO films

“…Correct and immediate determination of dehydration level in the body is vital because any change from normal levels may induce health problems. Developing highly sensitive and selective sweat sensors has attracted much attention due to its importance in areas of biotechnology and clinical diagnostics [20][21][22][23].…”

Highly improved hydration level sensing properties of copper oxide films with sodium and potassium doping

“…Indeed, researchers have recently focused on the investigations of un-doped and doped nanostructured metal oxide films. For example, doping with alkali metals such as Sodium and Potassium enhances the conductivity, sensitivity and selectivity of humidity sensors [10][11][12]. It was reported that doping thin films with alkali metals affects luminescence and electrical properties of films [13,14].…”