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) × 1010 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