Exploring Reliable and Efficient Plasmonic Nanopatterning for Surface- and Tip-Enhanced Raman Spectroscopies

Abstract

Surface-enhanced Raman scattering (SERS) is of growing interest for a wide range of applications, especially for biomedical analysis, thanks to its sensitivity, specificity, and multiplexing capabilities. A crucial role for successful applications of SERS is played by the development of reproducible, efficient, and facile procedures for the fabrication of metal nanostructures (SERS substrates). Even more challenging is to extend the fabrication techniques of plasmonic nano-textures to atomic force microscope (AFM) probes to carry out tip-enhanced Raman spectroscopy (TERS) experiments, in which spatial resolution below the diffraction limit is added to the peculiarities of SERS. In this short review, we describe recent studies performed by our group during the last ten years in which novel nanofabrication techniques have been successfully applied to SERS and TERS experiments for studying bio-systems and molecular species of environmental interest.

Keywords: Raman spectroscopy; biophotonics; surface- and tip-enhanced Raman scattering.

Figure 1

Simple description of SERS: (a) a metal nanoparticle illuminated by light causes the oscillation of the free electrons of the surface, so that a molecule placed at nanometric distance from the NP surface is affected by an amplified optical field; (b) amplification is further increased in the gap region in a dimer structure (hot-spot).

Figure 2

Schematic representation of the bottom-up and top-down approaches.

Figure 3

General scheme for the fabrication of the plasmonic template, according to the steps described in the text.

Figure 4

(a) Example of AFM morphological characterization of the micelle film on the glass substrate; (b) TEM micrograph of the plasmonic hexagonal lattice coated on glass and transferred on TEM carbon grid. Adapted from [123].

Figure 5

Comparison between typical signals in the case of RBCs adhered to a glass coverslip (a) and a SERS substrate (b). In both cases, a cartoon of the experimental configuration is provided on the right. Adapted from [123].

Figure 6

(a) Sketch of the sample cell used for SERS analysis, formed by a sandwich structure consisting in a nano-patterned Ag-substrate covered by a glass coverslip on which SKOV3 cells were cultured. (b) Bright field image from SKOV3 cells. (c) Four background subtracted SERS spectra acquired in the single cell shown in (d). Measurements were acquired with a laser power of 10 μw (on the sample) and an integration time of 2 s. (d) Two-dimensional scatter plot (PC1-PC3 plane). The 95% confidence ellipses relative to CAIX+ (red) and CAIX− (green) are also shown. Adapted with permission from [125], under the Creative Commons 3.0 license (https://creativecommons.org/licenses/by/3.0/ accessed on 8 September 2023).

Figure 7

(a) SEM micrograph of an Arrow©-type AFM probe coated with clusters of AgNPs, showing a uniform monolayer on the pyramidal tip with dewetted regions at the base. (b) Magnified version of (a), with inset showing a region close to the apex where contrast is increased to resolve the structure of close-packed nano-islands. (c) Spatial distribution of TERS signal intensity relative to the Raman band at 1558 cm⁻¹, acquired in a large area of bundles of SWCNTS spin-coated on glass. (d) Tip-in and tip-out TERS signals acquired on the bundle of SWCNTS. The band at 1588 cm⁻¹ is highlighted in blue, and the inset shows the topographic cross section along the red line drawn in (c). (Adapted with permission from [126], under the Creative Commons 3.0 license (https://creativecommons.org/licenses/by/3.0/ accessed on 8 September 2023).

Figure 8

(a) Schematics of the procedure designed to fabricate Ag coral-like probes. (b) SEM images of a treated TESPA tip: (i) probe after sputtering of a 30 nm thick Ag layer; (ii) Ag-coated tip after air-based plasma treatment; (iii) TERS probe after Ar plasma reduction. (c) AFM height map of a multi-walled CNT. (d) Comparison between AFM (i) and TERS (ii) profiles crossing the CNT along the blue line drawn in (c). Adapted from [141].

Figure 9

(a) Raman spectra of solid pyrene (iii), SERS spectrum (ii) acquired at a concentration of 1 µM pyrene, and, for comparison, (i) SERS spectrum of distilled water evaporated on a SERS substrate. (b) Linear trend of band intensity at 1029 cm⁻¹ in the lower concentration range with the relative best-fit line. Adapted from [146].