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

Abstract

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.

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films

“…Standardization of enhancements attained for different TERS geometries is necessary for interlaboratory comparison and comparing results from one experimental geometry to another. To this end, Rossi and co-workers determined enhancement factors for reporter molecules in monolayer-coated tips used in three configurations: isolated as single tips, a tip–tip dimer geometry, and a more conventional TERS gap mode geometry in which the tip is brought close to a Ag or Au metallic substrate . The use of a Raman reporter-modified tip eliminates most of the uncertainties associated with scanning probe microscopy control of tip–surface separation and, hence, TERS intensities.…”

“…To this end, Rossi and co-workers determined enhancement factors for reporter molecules in monolayer-coated tips used in three configurations: isolated as single tips, a tip−tip dimer geometry, and a more conventional TERS gap mode geometry in which the tip is brought close to a Ag or Au metallic substrate. 22 The use of a Raman reporter-modified tip eliminates most of the uncertainties associated with scanning probe microscopy control of tip−surface separation and, hence, TERS intensities. Enhancement factors were compared among these geometries utilizing a single excitation source and collection system to allow for appropriate comparison using monolayers of thiophenol, 7-mercapto-4-methylcoumarin, and thiram as Raman reporters.…”

Graphene edge method for three‐dimensional probing of Raman microscopes focal volumes

“…[ 35 ] The production started from commercially available CVD monolayer graphene grown on copper. [ 46 ] A thin layer of poly‐methyl methacrylate (PMMA) was first deposited by spin‐coating on the graphene as a support; then, the copper surface was etched completely; the PMMA was then laid out on the SiO 2 /Si wafer, and after thermal annealing, the PMMA was dissolved in acetone, resulting in wide areas (more than 10 μm for each side) of graphene. A suitable graphene edge was then located, and AFM was performed on it to verify the straightness of the edge and to quantify the difference from an ideal straight edge.…”

“…In this work, the graphene edge method was applied to several experimental configurations on two different dispersive confocal Raman microscopes: an unmodified commercial DXR™xi Raman Imaging system, from Thermo Fisher Scientific™, and a custom setup assembled with a DXR™ Raman spectroscope (by Thermo Fisher Scientific™) with an optical path modified to pass through a Ntegra™ scanning probe microscope (SPM), by NT-MDT ® ; the latter is in fact a tip-enhanced Raman spectroscopy (TERS) apparatus, [45,46] although for these measurements the Raman-enhancing probe was not mounted in order to employ it as a simple confocal Raman microscope. The DXRxi features a magnetic sample stage with a step size of (0.1  0.1  0.2) μm along the three Cartesian axes (x  y  z), where z is the direction of the optical axis.…”

Insight on Mercapto-Coumarins: Synthesis and Reactivity

“…It is also indicated as a suitable matrix for the analysis of small molecular compounds [ 40 ]. The other described incorporation of this highly valuable scaffold is as a reporter molecule [ 41 ], reporters for thiol interactions at the nanoparticle surface [ 42 ], fluorescent probe [ 43 , 44 , 45 ], sugar acceptor [ 46 ], Raman reporter [ 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 ], an excellent substrate for fluorescence spectroscopy [ 57 ], photodimerizable and healable reactant [ 58 ], fluorescent dye [ 59 , 60 ], and probe molecule on gold-coated silicon nanowires [ 61 ].…”