Optical Aggregation of Gold Nanoparticles for SERS Detection of Proteins and Toxins in Liquid Environment: Towards Ultrasensitive and Selective Detection

Abstract

Optical forces are used to aggregate plasmonic nanoparticles and create SERS–active hot spots in liquid. When biomolecules are added to the nanoparticles, high sensitivity SERS detection can be accomplished. Here, we pursue studies on Bovine Serum Albumin (BSA) detection, investigating the BSA–nanorod aggregations in a range from 100 µM to 50 nM by combining light scattering, plasmon resonance and SERS, and correlating the SERS signal with the concentration. Experimental data are fitted with a simple model describing the optical aggregation process. We show that BSA–nanorod complexes can be optically printed on non-functionalized glass surfaces, designing custom patterns stable with time. Furthermore, we demonstrate that this methodology can be used to detect catalase and hemoglobin, two Raman resonant biomolecules, at concentrations of 10 nM and 1 pM, respectively, i.e., well beyond the limit of detection of BSA. Finally, we show that nanorods functionalized with specific aptamers can be used to capture and detect Ochratoxin A, a fungal toxin found in food commodities and wine. This experiment represents the first step towards the addition of molecular specificity to this novel biosensor strategy.

Facile In Situ Photochemical Synthesis of Silver Nanoaggregates for Surface-Enhanced Raman Scattering Applications

“…As a result, more silver nanoaggregates and nanogaps are simultaneously formed, providing strong LSPR and plasmonic coupling. As is known, the SERS activity is highly dependent on the wavelength match between the LSPR resonance and excitation laser [35,36]. We therefore measured the absorbance spectra of the fabricated silver nanoaggregates-based SERS substrates, as shown in Figure 4.…”

A Review on Surface-Enhanced Raman Scattering

“…This technique exploits the optical forces to which metal nanoparticles are subjected when illuminated by strongly focused laser beams. In qualitative terms, a nanoparticle is subjected to two forces: (1) A gradient force that is attractive towards the high intensity region of the laser beam if the excitation wavelength is longer than the surface plasmon resonance of the nanoparticle, and repulsive in the opposite case; (2) a radiation pressure force that propels the nanoparticle along the propagation direction of the beam [329,372]. A proper choice of the experimental parameters allows one to control the aggregation process.…”

“…A proper choice of the experimental parameters allows one to control the aggregation process. Laser tweezers have been used by Foti et al [329] to push and aggregate gold nanorod–biomolecule complexes in 5–10 µm spots at the bottom of a glass microcell.…”

Analysis of Biomolecules Based on the Surface Enhanced Raman Spectroscopy

“…Albumins generally act as transport proteins for numerous compounds in blood plasma [ 26 ]. Bovine serum albumin (BSA) was chosen as a model molecule to research the enhancement of SERS-active substrates in label-free detection.…”

“…To keep proteins in their natural structure and conformation, in-liquid SERS detection of protein was based on nanoparticle aggregating as SERS-active hot spots for label-free detection [ 27 ]. Foti et al [ 26 ] applied optically induced gold nanorods aggregation to the detection of BSA in liquid by combining light scattering, plasmon resonance and SERS. Other Raman resonant biomolecules such as catalase and hemoglobin were used to investigate the application of this methodology, and the limits of detection of catalase and hemoglobin were much lower than that of BSA.…”