Large size citrate‐reduced gold colloids appear as optimal SERS substrates for cationic peptides

Abstract

Surface‐enhanced Raman spectra of two cyclic cationic peptides with therapeutic interest, somatostatin‐14 and its synthetic analog octreotide, were simultaneously analyzed as a function of concentration and colloidal size. It appeared that the large size (~95 nm) citrate‐reduced gold nanoparticles are the most adequate substrates for providing an optimal Raman signal enhancement within the 10−6–10−7 m peptide concentration range. It could also be evidenced that these large size particles can give rise to such a remarkable result within the first month following their elaboration. In fact, the recorded Raman spectra after this period mainly originate from the molecular species in the coverage of gold particles and not from the adsorbed peptides. To bring clarification to the temporal evolution of gold colloids, a systematic analysis was performed during 3 months on different particle sizes in the 15–150 nm range. The data obtained by a combined use of different techniques (Ultraviolet‐visible absorption, z‐potential, transmission electron microscopy, and Raman scattering) are consistent with a permanent evolution of the colloidal coverage, assignable to the gradual transformation of citrate anions and their substitution by oxidized products. It can be concluded that only small and medium size nanoparticles (≤ 40 nm) are able to regenerate their initial features after the first month, while large size colloids are subjected to a continuous degradation of their plasmonic and electrostatic properties. Copyright © 2016 John Wiley & Sons, Ltd.

Unexpected Dual Action of Cetyltrimethylammonium Bromide (CTAB) in the Self‐Assembly of Colloidal Nanoparticles at Liquid–Liquid Interfaces

“…However, in general the SERS signals were dominated by vibrational bands which can be attributed to Au-Cl (250 cm −1 ), citric acid, and various oxidants of citric acid (1537, 1432, 1374, 1300, 1246, 1165, 994, 760, and 492 cm −1 ) initially present on the surface of the Au NPs. [31][32][33] These data suggest that in this concentration range the amount of CTAB adsorbed on the surface of the particles is small and thus it should have a negligible effect on their surface properties. Indeed, the ζ potential of CRGC is barely affected by the addition of CTAB at this feedstock concentration range and the NPs remain highly charged (≤−30 mV), as shown in Figure 2b-i.…”

Stimulated Adsorption of Humic Acids on Capped Plasmonic Ag Nanoparticles Investigated by Surface-Enhanced Optical Techniques

“…The high affinity of HA for AgCT is unusual, as the AgNPs exhibit a negative zeta potential , produced by the large amount of CT and its oxidation products adsorbed on the interface, which may avoid to a large extent the adsorption of the negatively charged humic substances. This is what also occurs on AgHX, where a large amount of adsorbed chloride ions decorates the surface (Figure A).…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XII

“…Lopez‐Tobar et al introduced large size citrate‐reduced Au colloids as optimal SERS substrates for the detection of cationic peptides. SERS of two cyclic cationic peptides of therapeutic interest, somatostatin‐14 and its synthetic analog octreotide, were simultaneously analyzed as a function of concentration and colloidal substrate size . Mehrvar et al studied SERS of dye molecules on Ag‐modified silicon nanowire substrates to determine the influence of photoinduced probe degradation on enhancement factors.…”

“…SERS of two cyclic cationic peptides of therapeutic interest, somatostatin-14 and its synthetic analog octreotide, were simultaneously analyzed as a function of concentration and colloidal substrate size. [19] Mehrvar et al studied SERS of dye molecules on Ag-modified silicon nanowire substrates to determine the influence of photoinduced probe degradation on enhancement factors. The performance of their nanostructure design was assessed for different concentrations, and a limit of detection of 10 pm was obtained.…”