TiO2 nanotube-integrated microwave planar resonator sensor for ultraviolet transmission-based liquid characterization

Abstract

This work presents a planar microwave resonator sensor equipped with vertically self-aligned TiO2 nanotubes to enable real-time and contactless liquid characterization. The microwave sensor operates based on detecting a resonator response as it changes under exposure to UV light. This was used for liquid characterization by quantifying the transmission of ultraviolet (UV) light through a liquid sample and its subsequent absorbance by the TiO2 nanotube membrane. A microwave split ring resonator (SRR) was designed and simulated to operate with a resonant frequency of 8.73 GHz, resonant amplitude of -12 dB, and a quality factor of 103. The SRR was fabricated alongside an anatase-TiO2 nanotube membrane with a nanotube length of 20 μm, a wall thickness of 10 nm, and a tube diameter of 120 nm. The membrane was placed within the ring gap of the resonator to absorb any transmitted UV light which in turn would alter its dielectric properties. In this way, the designed sensor measured the ultraviolet transmission of a solution of S1813 photoresist in ethylene glycol (EG) and related the solvent concentration to a change in the transmission profile, without requiring contact or near-field proximity between the resonator sensor and the solution. Measuring different concentrations of S1813 photoresist in ethylene glycol, ranging from 0 to 100 v/v%, resulted in accurately calculated sensitivities of 0.18 MHz/%, 0.029 dB/%, and 0.45/% in resonant frequency, resonant amplitude, and quality factor, respectively. The achieved integration of TiO2 nanotube membranes for liquid characterization by analyzing microwave property variation may offer unique avenues for real-time liquid and UV detection and enable more comprehensive liquid and chemical analysis in comparison to conventional microwave sensor devices.