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-CoOx(4-APD)@ZIF-8 with a hollow multi-shelled structure (HoMS), in which the co-reduction catalyst (Ag) and the co-oxidation catalyst (CoOx) 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 CoOx 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%.