Hydrated Ionic Liquids Boost the Trace Detection Capacity of Proteins on TiO₂ Support

Trace detection based on surface-enhanced Raman scattering (SERS) has attracted considerable attention, and exploiting efficient strategies to stretch the limit of detection and understanding the mechanisms on molecular level are of utmost importance. In this work, we use ionic liquids (ILs) as trace additives in a protein-TiO₂ system, allowing us to obtain an exceptionally low limit of detection down to 10^-9 M. The enhancement factors (EFs) were determined to 2.30 × 10⁴, 6.17 × 10⁴, and 1.19 × 10⁵, for the three systems: one without ILs, one with ILs in solutions, and one with ILs immobilized on the TiO₂ substrate, respectively, corresponding to the molecular forces of 1.65, 1.32, and 1.16 nN quantified by the atomic force microscopy. The dissociation and following hydration of ILs, occurring in the SERS system, weakened the molecular forces but instead improved the electron transfer ability of ILs, which is the major contribution for the observed excellent detection. The weaker diffusion of the hydrated IL ions immobilized on the TiO₂ substrate did provide a considerably greater EF value, compared to the ILs in the solution. This work clearly demonstrates the importance of the hydration of ions, causing an improved electron transfer ability of ILs and leading to an exceptional SERS performance in the field of trace detection. Our results should stimulate further development to use ILs in SERS and related applications in bioanalysis, medical diagnosis, and environmental science.