Novel Approaches in Tip-Enhanced Raman Spectroscopy: Accurate Measurement of Enhancement Factors and Pesticide Detection in Tip Dimer Configuration

Tip-enhanced Raman spectroscopy (TERS) allows the precise manipulation of a nanometric probe for surface chemical analysis by plasmon-based amplification of Raman signals; however, acknowledged procedures and materials for assessing the enhancement factor in different configurations are still lacking. In this work, we propose a technique for the standardization of TERS intensity measurements, by chemisorption of different organic Raman-active molecules on plasmonic probes, and compare it to the conventional procedures addressed to the same goals. In addition, by ideally considering TERS as a special case of surface-enhanced Raman (SERS) involving a single nanoparticle, we experimentally realized the three most common configurations in SERS: i. isolated particle, ii. single scattering probe on a surface, and iii. nanoparticle dimers. To achieve the latter in an accessible way with plasmonic probes, we established a fabrication procedure for a substrate presenting multiple nanometric tips, intended to be easily approached and mapped for fast, reproducible assembly of tunable tip–tip dimers that further amplify Raman scattering with respect to conventional gap mode TERS; enhancement factors up to (1.4 ± 0.4) × 10^10 were calculated. The three configurations were successfully tested, and enhancement factors were quantified with their associated uncertainties, employing self-assembled monolayers of several probe molecules, including thiophenol, a de facto enhanced Raman standard, and thiram, a common use, law-regulated pesticide, hence opening up TERS to real world applications in agricultural and food analysis.