Ultrasensitive Trace Detection of Dye Molecules Based on Scanning Microsphere-Coupled Surface-Enhanced Raman Spectroscopy

Ultrasensitive detection of trace substances holds significant importance in chemical analysis and biochemical sensing. As an exceptionally robust spectroscopic technique, surface-enhanced Raman spectroscopy offers significant advantages in the trace detection of analytes on various plasmonic noble-metal substrates. The recent integration of plasmonic nanostructures with nonplasmonic microcavities has emerged as a promising approach to enhance the sensitivity of Raman detection further. In this work, we developed an ultrasensitive scanning microsphere-coupled surface-enhanced Raman spectroscopy system and applied it to detect trace dye molecules. We found that based on conventional surface-enhanced Raman substrates with Au nanostructure films, the enhancement factor of Raman intensity can reach the level of ∼109 through optimizing conditions. By incorporating a scanning silica dielectric microsphere, the Raman signals can be further enhanced by two orders of magnitude, resulting in an enhancement factor as high as ∼1011 and corresponding to a detection limit of 10–13 M for an aqueous crystal violet solution. Note that the detection limit reached the best performance ever recorded. The enhancement mechanism of dielectric microspheres is explained well by theoretical simulations. Additionally, our technique allows the spectroscopic imaging of trace analytes over a region, providing a facile strategy to fabricate hybrid Raman enhancers for ultrasensitive, highly efficient, and cost-effective sensing applications.