Background-free, highly sensitive surface-enhanced IR absorption of rhodamine 6G molecules deposited onto an array of microholes in thin silver film

Abstract

Selective IR absorption at 1261 cm −1 enhanced by 455 times, was demonstrated for rhodamine 6G molecules, covering a 2D-photonic crystal, represented by a regular array of 4-micron wide holes in a 30 nm thick silver film on a CaF 2 substrate. The reference absorption lines were taken near 2900 cm −1 , where the IR radiation is freely channeling through the microholes, indicating the reference substrate coverage by the dye molecules for its relative internal calibration. The limit of background-free detection for the analyte was determined at the level ~10 −2 monolayer.

Surface-Enhanced IR-Absorption Microscopy of Staphylococcus aureus Bacteria on Bactericidal Nanostructured Si Surfaces

“…Highly sensitive visible and near-IR SERS probes surface-adsorbed molecules only within the nanoscale interfacial region of the optical-range electrical near fields, making available for detection only the exterior elements of (sub) micro-scale viruses and bacteria [14][15][16]. In contrast, less-sensitive mid-and far-IR (typical wavelengths~3-25 microns) SEIRA(R) probing involves the micro-scale interfacial region of the IR electrical near fields, being highly beneficial for characterization of specific internal elements in viruses and bacteria [17,18]. As a result, it is SEIRA(R) that potentially holds promise as a very important spectroscopic modality in characterizing key-enabling interactions of pathogenic bacteria and bactericidal nanostructured surfaces.…”

Diffraction microgratings as a novel optical biosensing platform

“…Then, IR transmission and reflection spectra of the fs-fabricated sensor with the deposited SA-layer exhibit a number of major and minor features (figure 3). First, there are low-frequency sharp strong maximum in transmittance T and sharp strong minimum in reflectance R, which are predicted by the diffraction theory for the specific microhole diameter D ≈ 4 μm [31], and the succeeding plateau-like spectral regions. The diffraction origin of these distinct spectral extrema is clearly demonstrated by the slowly and monotonously diminishing R + T sum spectrum (figure 3), where the minor minima come from the SA-absorption.…”

“…Fabrication and testing of a diffraction micrograting as a SEIRA sensor was presented elsewhere [31]. Briefly, in the fabrication we used a 30 nm thick silver (Ag) film with an optical-quality surface, deposited onto a 1 mm thick CaF 2 substrate by magnetron sputtering of a commercial Ag plate (99.99%) in argon.…”

“…According to its testing regarding rhodamine 6G (R6G) molecules [31], this diffraction grating-like sensor demonstrates the first allowed band until ≈1300 cm −1 , the second allowed band starting from ≈1500 cm −1 and the bandgap in between. Just below the bandgap at 1261 cm −1 , 455-fold selective enhancement of IR-absorption was observed ( figure 1), enabling the background-free detection for the analyte at the level ∼10 −2 monolayer.…”

“…For example, Au-nanoparticle enhanced SERS of Staphylococcus aureus (SA) samples demonstrated broadband plasmonic enhancement of IR absoprtion for molecular vibrations of extracorporal functional groups, related to amide, proteins and DNA, while characteristic intracorporal vibrations of carotenoid fragments were not significanly affected by the nanoscale plasmonic EM near-fields due to their micron-scale spacings [29]. In contrast, micron-scale evanescent EM fields from micro-structured biophotonic sensors can provide efficient SERS or SEIRA(R) excitation owing, apparently, to lightning-guide [30] or diffraction-resonance [19,31] effects, respectively, in corresponding sharpened micro-columnar or diffraction grating-like structures.…”