Ag–Au–Cu Trimetallic Alloy Microflower: A Highly Sensitive SERS Substrate for Detection of Low Raman Scattering Cross-Section Thiols

Abstract

Trimetallic Ag−Au−Cu alloy microflowers (MFs) with various surface compositions were synthesized on a glass coverslip and used as efficient surface-enhanced Raman spectroscopy (SERS) substrates for highly sensitive label-free detection of smaller Raman scattering crosssection molecules, namely, L-cysteine and toxic thiophenols. MFs of different compositions were synthesized via appropriate mixing of metal− alkyl ammonium halide precursors followed by a single-step thermolysis at 350 °C. While the Ag percentage was kept constant at 90% for all the substrates, the composition of Au and Cu was varied between 1 and 9% sequentially. The synthesized MFs were thoroughly characterized by using field emission scanning electron microscopy (FE-SEM), wide-angle X-ray scattering, X-ray photoelectron spectroscopy (XPS), and X-ray fluorescence techniques. FE-SEM studies revealed that the MFs were present throughout the substrate, and the average size varied from 20 to 40 μm. XPS studies showed that the top surface of the alloy substrates was rich in either Au or Cu atoms, while Ag remained underneath. The performance of the trimetallic MFs as SERS substrates was evaluated using Rhodamine 6G as a probe molecule, which showed that the MFs with Ag−Au−Cu compositions 90−7−3 and 90−3−7 were found to be the best and of equal SERS efficiency. The SERS enhancement factor (EF) of both these MFs was found to be the same, approximately 9 × 10 7 , when calculated using 1,2,3benzatriazole as the probe molecule. Between the two, the trimetallic substrate with a higher Au percentage (Ag−Au−Cu as 90−7− 3) was used for the sensitive SERS-based detection of thiols to exploit the strong Au−S binding interaction. By virtue of the high EF of the substrate, the inherently low Raman scattering cross-sections of the probe molecules were greatly enhanced in SERS mode. The 'limit of quantification (LOQ)' values were found to be 1 nM for aliphatic L-Cysteine and 1−0.1 pM for aromatic thiols using the trimetallic SERS sensor.

Mono-, di- and trimetallic coinage nanoparticles prepared via the Brust–Schiffrin method

“…1,7–9 Trimetallic materials, such as Ag–Au–Cu microflowers, have been applied to SERS substrates to detect molecules with a smaller Raman scattering cross-section. 10 Cu–Ag–Au trimetallic NPs have shown higher catalytic activity than those of Cu–Ag or Au–Ag bimetallic NPs for reducing 4-nitrophenol to 4-aminophenol. 2 The optical and catalytic activities of the multimetallic nanostructures of coinage metals have been optimized and multiplexed, owing to the synergistic effect between different components.…”