Bimetallic Ag–Cu Alloy Microflowers as SERS Substrates with Single-Molecule Detection Limit

Abstract

Bimetallic Ag–Cu alloy microflowers with tunable surface compositions were fabricated as surface-enhanced Raman spectroscopy (SERS) substrates with a limit of detection in the zeptomolar range for the analyte molecule rhodamine 6G (R6G). The substrates were prepared on a glass coverslip through a bottom-up strategy by simple thermolysis of metal-alkyl ammonium halide precursors. The reaction temperature and composition of the alloy were varied sequentially to find out the maximum SERS efficiency from the substrates. While UV–vis spectroscopy was employed to characterize the optical properties of the substrates, the bulk and surface compositions of the microflowers were determined using energy-dispersive X-ray fluorescence (ED-XRF) and X-ray photoelectron spectroscopy (XPS) techniques, respectively. Also, the structural and morphological characterizations of the substrates were performed by X-ray diffraction and scanning electron microscope (SEM), respectively. For alloys, the ED-XRF studies confirmed that the bulk compositions matched with the feed ratio, while the surface compositions were found to be rich in copper in the form of both elementary copper and copper oxide, as revealed by XPS studies. From the efficiency studies for different compositions prepared, it was found that 10% Ag–Cu alloy microflowers produced the maximum SERS intensity for resonant R6G molecules as probes. In fact, R6G evidences a 50-fold enhancement in SERS spectra with 10% alloy microflowers as against pure Ag microflowers. Using 1, 2, 3-benzotriazole as a nonresonant Raman probe, uniform enhancement factors on the order of ≈108 were achieved from different parts of the 10% Ag–Cu alloy microflower. The same substrate showed excellent Raman response for detecting R6G at very low concentrations such as 10 zM, leading to detection and analysis of SERS spectra from a single R6G molecule.

Facile Method to Fabricate Cactus-like Ag NPs/CuO/Cu2O Nanocomposites for Recyclable SERS Detection of Trace Carbendazim Residues

“…Surface-enhanced Raman scattering (SERS) is a convenient, nondestructive, and ultrasensitive molecular fingerprinting spectral method, and has been widely applied in chemical, biological, and food fields. − In principle, SERS is mainly based on electromagnetic (EM) field enhancements localized in the gaps or junctions (hot spots) of nanostructures due to the interaction of incident laser light with the nanostructures. The enhancement factor depends on the size, shape, distribution, and material composition of the nanostructures. ,− The most widely used nanostructures are made of noble metallic nanomaterials, such as Ag or Au nanoparticles (NPs), Ag NPs@SiO 2 , Ag or Au nanowires, − and bimetallic Ag–Cu alloy materials, , which exhibit high detection sensitivity. Especially, Ag nanostructures have been used to detect trace carbendazim residue recently ,, because the carbendazim tends to adsorb on the Ag surface due to interaction between the benzimidazole group of the carbendazim and Ag. , …”

“…The enhancement factor depends on the size, shape, distribution, and material composition of the nanostructures. 8,11−13 The most widely used nanostructures are made of noble metallic nanomaterials, such as Ag or Au nanoparticles (NPs), Ag NPs@SiO 2 , Ag or Au nanowires, 14−17 and bimetallic Ag−Cu alloy materials, 18,19 which exhibit high detection sensitivity. Especially, Ag nanostructures have been used to detect trace carbendazim residue recently 7,20,21 because the carbendazim tends to adsorb on the Ag surface due to interaction between the benzimidazole group of the carbendazim and Ag.…”

Trends in Application of SERS Substrates beyond Ag and Au, and Their Role in Bioanalysis

“…By measuring the SERS intensity of different microflower (MF) substrates, Sravani et al demonstrated that with an increasing amount of Cu in the Ag-Cu alloy, the intensity of a probe molecule R6G gradually increases compared with pure Ag MFs and can achieve the highest SERS intensity at 10% Cu. They achieved a 50-fold enhancement in SERS spectra with 10% Ag-Cu alloy microflowers, which supersedes the efficiency of pure Au-Ag MFs [ 180 ]. Bimetallic nanostructures with tunable morphology are of current interest in plasmonics, catalysis and SERS [ 175 ].…”

A highly sensitive SERS substrate based on a mesoporous Ag–TiO2 thin film for the detection of dye molecules

“…17,18 Several sources in literature report the superior SERS activity of Ag-Au bimetallic nanoparticles, Ag-Cu nanostructures, and Ag-Au-Pt trimetallic nanostructures compared to the SERS activity of a single metal. [19][20][21] The substrates exhibit tunable plasmonic wavelengths by varying their composition, length and morphology. We have recently reported that the Ag capped Al nanorods (NRs) based an SERS substrate fabricated by the glancing angle deposition (GLAD) technique shows excellent SERS activity in the visible-NIR region.…”