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Distinctly Different Morphologies of Bimetallic Au-Ag Nanostructures and Their Application in Submicromolar SERS-Detection of Free Base Porphyrin.

ABSTRACT: Core-shell Au-Ag nanostructures (Au-AgNSs) are prepared by a seed-meditated growth, i.e., by a two-step process. The synthetic parameters greatly influence the morphologies of the final bimetallic Au-AgNSs, their stability and application potential as surface-enhanced Raman scattering (SERS) substrates. Direct comparison of several types of Au NPs possessing different surface species and serving as seeds in Au-AgNSs synthesis is the main objective of this paper. Borohydride-reduced (with varying stages of borohydride hydrolysis) and citrate-reduced Au NPs were prepared and used as seeds in Au-AgNSs generation. The order of reactants in seed-mediated growth procedure represents another key factor influencing the final Au-AgNSs characteristics. Electronic absorption spectra, dynamic light scattering, zeta potential measurements, energy dispersive spectroscopy and transmission electron microscopy were employed for Au-AgNSs characterization. Subsequently, possibilities and limitations of SERS-detection of unperturbed cationic porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridyl)21H,23H-porphine (TMPyP), were investigated by using these Au-AgNSs. Only the free base (unperturbed) SERS spectral form of TMPyP is detected in all types of Au-AgNSs. It reports about a well-developed envelope of organic molecules around each Au-AgNSs which prevents metalation from occuring. TMPyP, attached via ionic interaction, was successfully detected in 10 nM concentration due to Au-AgNSs.

SUBMITTER: Vilimova I

PROVIDER: S-EPMC8464754 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:It is well-known that Ag-Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS). In recent years, there have been many studies on the fabrication of bimetallic nanostructures. However, controlling the shape, size, and structure of bimetallic nanostructures still has many challenges. In this work, we present the results of the synthesis of silver nanoplates (Ag NPls), and Ag-Au bimetallic core/shell and alloy nanostructures, using seed-mediated growth under green LED excitation and a gold salt (HAuCl4) as a precursor of gold. The results show that the optical properties and crystal structure strongly depend on the amount of added gold salt. Interestingly, when the amount of gold(x) in the sample was less than 0.6 μmol (x < 0.6 μmol), the structural nature of Ag-Au was core/shell, in contrast x > 0.6 μmol gave the alloy structure. The morphology of the obtained nanostructures was investigated using the field emission scanning electron microscopy (FESEM) technique. The UV-Vis extinction spectra of Ag-Au nanostructures showed localized surface plasmon resonance (LSPR) bands in the spectral range of 402-627 nm which changed from two peaks to one peak as the amount of gold increased. Ag-Au core/shell and alloy nanostructures were utilized as surface enhanced Raman scattering (SERS) substrates to detect methylene blue (MB) (10-7 M concentration). Our experimental observations indicated that the highest enhancement factor (EF) of about 1.2 × 107 was obtained with Ag-Au alloy. Our detailed investigations revealed that the Ag-Au alloy exhibited significant EF compared to pure metal Ag and Ag-Au core/shell nanostructures. Moreover, the analysis of the data revealed a linear dependence between the logarithm of concentration (log C) and the logarithm of SERS signal intensity (log I) in the range of 10-7-10-4 M with a correlation coefficient (R 2) of 0.994. This research helps us understand better the SERS mechanism and the application of Raman spectroscopy on a bimetallic surface.

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Distinctly Different Morphologies of Bimetallic Au-Ag Nanostructures and Their Application in Submicromolar SERS-Detection of Free Base Porphyrin.

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