Optimizing gold nanostars as a colloid-based surface-enhanced Raman scattering (SERS) substrate

Abstract

The one-pot seedless protocol provides a facile approach in the synthesis of gold nanostars (AuNS) that involves only three reagents, gold (III) chloride (HAuCl 4 ), silver nitrate (AgNO 3 ) and ascorbic acid (C 6 H 8 O 6 ). While studies correlating the synthesis parameters of the seedmediated protocol to surface-enhance Raman scattering (SERS) enhancement is well reported, the same understanding of the one-pot seedless protocol is limited. Here, we aim to elucidate how the synthesis parameters of AuNS from the one-pot seedless protocol, the AuNS concentration, surface passivation and aggregation level affect the colloidal SERS enhancement. Using crystal violet (CV) as a Raman probe molecule, we found that the SERS enhancement increases with Au 3+ /C 6 H 8 O 6 molar ratio up to 0.60 and Au 3+ /Ag + molar ratio up to 18. Although the surfactant, cetyltrimethylammonium bromide (CTAB) maintained colloidal stability, it reduced the SERS enhancement. Interestingly, the SERS enhancement did not increase monotonically with AuNS concentration, but decreased when AuNS concentration was beyond 15 pM. The SERS enhancement also increased with the increasing level of salt-induced aggregation of AuNS, but only within a few minutes. While the concept of SERS with colloidal nanostructures is not new, we have shown for the first time, a detailed systematic study of various parameters that affect the SERS enhancement of AuNS synthesized using a one-pot seedless protocol. This study enables us to optimize the SERS enhancement of AuNS at the synthesis level to make them effective colloid-based SERS substrates for potential use in intracellular biosensing.

Experimental Studies and Modeling of “Starlike” Plasmonic Nanostructures for SERS Application

“…In addition to the high efficiency of SERS effect on nanostar‐based substrates, they possess very good long‐term stability of SERS signal over time . Previously we have studied enhancement of Raman signals of R6G molecules using type I substrates; other authors studied SERS substrates with nanostars using other molecules …”

“…As will be shown below, both SERS mechanisms are working for the gold starlike NPs that were synthesized and characterized in the present study. There are many papers that analyze these objects, which is caused by the perspectives of their applications . However, there are still many unsolved problems in this field and the values of enhancement coefficients reported for similar SERS substrates and molecules differ by orders of magnitude .…”

“…There are many papers that analyze these objects, which is caused by the perspectives of their applications . However, there are still many unsolved problems in this field and the values of enhancement coefficients reported for similar SERS substrates and molecules differ by orders of magnitude . Therefore, the goal of the present study is a systematic investigation of the SERS substrates based on starlike gold NPs, particularly the relation between their morphology, optical absorption spectra, and efficiency of Raman scattering by the deposited analyte molecules.…”

Ag Nanostructures with Spikes on Adhesive Tape as a Flexible Sers-Active Substrate for In Situ Trace Detection of Pesticides on Fruit Skin

“…The peak enhancement brought by Ag NSPs_0.05/tape is due to sufficient amounts of agglomeration that consequently results in optimized interspike distances. On the other hand, the decrease in peak intensities with increasing suspension concentration is due to heavy agglomeration that results in more radiation being scattered by the system, thus less laser energy is available to form "hot spots" [15][16][17]. In addition, it cannot be explicitly concluded that the enhancement comes from resonance effects brought by the laser wavelength and the LSPR of the substrate since the LSPR information was not investigated.…”

“…Aside from the intra-spike gap, "hot spots" formed on a single Ag NSP, the clustering of these nanostructures also plays an important role in contributing to the SERS effect since the distance among the nanostructures is also critical in providing the enhancement of SERS signals. The agglomeration of Ag NSPs should be sufficient enough to create an effective density of "hot spots" in obtaining an optimized SERS enhancement [17]. Considering the effects of each signal contributing to single or overlapped multiple Ag NSPs, larger regions of high electrical field are created and, consequently, the overall effect of SERS are generated.…”