Surface-Enhanced Raman Scattering Trace-Detection Platform Based on Continuous-Rolling-Assisted Evaporation on Superhydrophobic Surfaces

Surface-enhanced Raman scattering (SERS) has proved to be an efficient approach for ultrasensitive detection of highly diluted analytes, and the metal colloid-based SERS platform is one of the most successful techniques to perform evaporation-assisted collection and enrichment of highly diluted molecules in hot spot regions on various substrates. However, there are still no effective and facile ways to achieve high SERS sensitivity at femtomolar levels on superhydrophobic (SH) surfaces. Here, for the first time, an efficient and versatile strategy, termed continuous-rolling-assisted evaporation on an SH surface (CRESH), is developed for the ultrasensitive colloid-based SERS platform, which overcomes the inherent limitations of the pinning effect during evaporative enrichment on unpatterned SH substrates. This strategy combines an SH surface with a rolling property and droplet continuous-rolling motion in a synergistic way to achieve trace analyte detection at the femtomolar level. A 50 μL droplet based on CRESH can be considerably condensed from approximately 0.6 cm to 0.2 mm in diameter, and final sizes are independent of the initial droplet volumes, showing remarkable condensing efficiency and potential applications for a lower detection limit. Our results of numerical analysis of nanoparticle motions inside the droplet show, in contrast to common belief, that the CRESH strategy endows the SH surface with nearly pinning-free peculiarity by means of significantly reducing the critical contact line values when the Cassie-Wenzel transition occurs. As a proof of concept, CRESH is further evaluated by detecting trace quantities of Raman probes with quantitative SERS detection down to the femtomolar level (1 × 10-15 mol/L). Our platform may open the possibility of universal applications of unpatterned SH surfaces that have long been suspected to play a role in effective, facile, cost-effective, and ultrasensitive SERS detection.