Tip enrichment surface-enhanced Raman scattering based on the partial Leidenfrost phenomenon for the ultrasensitive nanosensors

Surface-enhanced Raman scattering (SERS) possesses the advantage that directly detects target analytes in solution with high specificity and sensitivity without complicated pretreatment procedures. It remains a challenge in most practical applications to achieve molecule sensitivity in any highly diluted solutions. Here, we develop a robust and practical molecular enrichment strategy that can effectively confine analyte molecules and Au nanoparticles together in a short time into a small-sized sensitive region on a needle tip, based on the Leidenfrost phenomenon and capillary force. In this strategy, the Leidenfrost evaporation phenomenon maintains the analytes droplet in a Cassie state based on a levitating force, and at the same time, a hung needle tip anchors the droplet based on the capillary force. After 1–2 min quick evaporation, more than 98% Au nanoparticles and analytes can be condensed into an around 0.5 mm small size area on the needle tip. Due to the significant enrichment capability and reproducibility, the SERS measurement enables to achieve the limit of detection (LOD) down to 0.08 nM (at an S/N ratio of 3) for crystal violet (CV) molecules and to nM level for several types of pesticide molecules (glyphosate, carbendazim, thiram and choline) in ethanol solution. The strategy was also applied to the detection of CV molecules in mixture pigments solution, thiram in spiked environment water samples, with good selectivity and sensitivity. In addition, using the high thermal conductivity substrate without additional surface modification, this enrichment SERS detection may open the possibility of universal applications due to its facile and cost-effective. The accurate site of the needle tip offers great practical potentials for on-site identification by using a handheld Raman spectrometer.