A ratiometric SERS gas sensor with high reusability based on spatially separated photocatalysis

Surface-enhanced Raman scattering (SERS) sensors show great promise for trace detection of inorganic gases. However, the development of SERS sensors with high repeatability and accurate quantitative analysis is still a challenge. To address this challenge, a ratiometric SERS sensor with a spatially separated photocatalytic system is designed. This sensor is composed of Ag(3-MET)@TiO₂-CoO_x(4-APD)@ZIF-8 with a hollow multi-shelled structure (HoMS), in which the co-reduction catalyst (Ag) and the co-oxidation catalyst (CoO_x) are spatially separated. The internal standard molecule 3-MET and the probe molecule 4-APD are adsorbed on Ag at the inner surface of the TiO₂ shell and on CoO_x at the outer surface of the TiO₂ shell, respectively. Importantly, the spatially separated photocatalytic system ensures that 4-APD is completely degraded and 3-MET is preserved during the photocatalytic process, providing the sensor with excellent reusability and accurate quantitative analysis. In addition, the outermost ZIF-8 enriches gas molecules to improve the detection sensitivity. The SERS sensor was applied for SO₂ detection, exhibiting excellent linearity (R² = 0.998) and a detection limit as low as 3.2 ppm. During repeated use, the Raman signal from the SERS sensor remained above 95%, with a relative standard deviation (RSD) of only 1.84%.