Enhanced Vibrational Spectroscopies as Tools for Small Molecule Biosensing

Abstract

In this short summary we summarize some of the latest developments in vibrational spectroscopic tools applied for the sensing of (small) molecules and biomolecules in a label-free mode of operation. We first introduce various concepts for the enhancement of InfraRed spectroscopic techniques, including the principles of Attenuated Total Reflection InfraRed (ATR-IR), (phase-modulated) InfraRed Reflection Absorption Spectroscopy (IRRAS/PM-IRRAS), and Surface Enhanced Infrared Reflection Absorption Spectroscopy (SEIRAS). Particular attention is put on the use of novel nanostructured substrates that allow for the excitation of propagating and localized surface plasmon modes aimed at operating additional enhancement mechanisms. This is then be complemented by the description of the latest development in Surface- and Tip-Enhanced Raman Spectroscopies, again with an emphasis on the detection of small molecules or bioanalytes.

Keywords: IRRAS; SERS; biosensors; enhanced spectroscopies; infrared; plasmonic.

Figure 1

Surface IR spectroscopy techniques: (A) principle of Attenuated Total Reflection InfraRed (ATR-IR); (B) principle of (phase-modulated) InfraRed Reflection Absorption Spectroscopy (IRRAS/PM-IRRAS); and (C) experimental setup for PM-IRRAS measurements.

Figure 2

Step by step monitoring of β-lactoglobulin immobilization on acid-terminated SAMs (left, mercaptoundecanoic acid grafting, activation, then protein immobilization and on amine-terminated SAMs (right, cysteamine grafting and attachment of activated protein), adapted from reference [24].

Figure 3

Preparation of a PEG based diclofenac biosensor chip or microarray followed at each step of cleaning and functionalization by Surface IR in Attenuated Total Reflection (ATR) mode, adapted from reference [32].

Figure 4

Direct detection of benzo-[a]-pyrene using PM-IRRAS immunosensors adapted from reference [43].

Figure 5

(A) Schematic arrangement of the Au nanodisc array for collective excitation of coupled propagating and localized surface plasmon modes; (B) SEM image of the nanodisc array; (C) reflectivity spectrum of a nanodisc array with a disc diameter of d = 100 nm, and a disc-disc separation distance of s = 550 nm. The (simulated) spatial distribution of the optical intensity associated with the two resonance modes at λ = 568 nm and λ = 625 nm, seen in the reflectivity spectrum are given in (D) (λ = 568 nm) and (E) (λ = 635 nm), respectively.

Figure 6

(A) IR absorption measurements of nanodisc arrays showing the tunability of the plasmon resonances by variation of the disc diameters; (B) Blue circles represent the measured spectral positions of the IR resonances of the nanodisc arrays. The black curve was calculated for a three dimensional model of the nanodisc array using finite difference time domain (FDTD) simulations, in good agreement with the experimental data.

Figure 7

(A), schematic representation of a self-assembled monolayer of dodecanethiol on gold; the hydrocarbon chains consisting of CH₂ groups and a CH₃ terminal group; (B) schematics of the asymmetric (blue) and symmetric (magenta) stretching modes of CH₂ groups: (C) IR spectrum of the asymmetric (blue) and symmetric (magenta) stretching modes of CH₂ vibrations; (B) ATR IR absorption spectrum of dodecanethiol corresponding to the stretching modes depicted in (B); (D) Schematic picture of two-dimensional arrays of Ta₂O₅/Au nanodiscs with different radii, however, identical disc-disc separation distance; (E) reflection/absorption spectrum of a nanodisc array with the spectral position of the plasmon excitation at higher wavenumbers than the molecule vibrations (spectral difference δ < 0) enhances the molecular absorption, while the array with larger particle diameters (F), showing a plasmonic resonance at lower wavenumbers than the molecular vibration, i.e., with spectral difference δ > 0, suppress the molecular vibrations which lowers the total absorbance; (G) infrared absorbance spectra of dodecanethiol monolayers adsorbed onto nanodisc arrays. The intensities of methyl and methylene stretching bands increase for lower δ values, i.e., for structures which exhibit the plasmon resonance closer to the molecular vibrations; (H) baseline-corrected IR spectra of the C–H vibrations for different δ-values. Negative δ-values lead to enhanced absorption, whereas positive δ-values result in negative difference spectra; (I) normalized band areas measured at various spectral differences.

Figure 8

Principle of the Surface Enhanced Raman Scattering.

Figure 9

Principle of SERS sensors (a) in solution or (b) on surface.