Reversible assembly and disassembly of gold nanorods induced by EDTA and its application in SERS tuning

Abstract

A facile and reversible method for assembling and disassembling gold nanorods (GNRs) using a common chelating agent, ethylenediaminetetraacetic acid (EDTA), is reported. Assembly was induced by the electrostatic interaction between the cetyltrimethylammonium bromide (CTAB) bilayer present on GNRs and EDTA. At lower concentrations of EDTA, end-to-end assembled chains were formed. At higher concentrations of EDTA, these chains come together to form sheet-like structures. The complex of CTAB and EDTA, being labile, disassembles in the presence of stronger chelating agents. Upon addition of metal ions having higher formation constants, EDTA detaches from the GNRs and forms stronger complexes with metal ions, resulting in disassembly. Characteristic changes were observed in the UV/vis spectra. Addition of EDTA resulted in a red shift of longitudinal surface plasmon (LSP) resonance at lower concentrations, indicating an end-to-end assembly. At higher concentrations, the characteristic of side-by-side assembly was seen in the UV/vis spectra. TEM analysis proved the existence of end-to-end chains at lower concentrations of EDTA and side-by-side assembled sheet-like structures at higher concentrations. The addition of metal ions induced disassembly. Even 2 ppb of metal ion was detected using the spectral changes. Disassembly was studied in detail, taking Pb(II) as the model system. Upon addition of Pb(II), TSP showed a blue shift and decreased in intensity while the LSP showed a red shift and increased in intensity. A new peak at a higher wavelength region emerged, pointing to the existence of both side-by-side and end-to-end assembly in the system. TEM analysis showed that the disassembly involves the formation of bundled chains which may be the reason for the observed spectral changes. Surface-enhanced Raman scattering (SERS) activity of the system could be tuned by controlling the concentration of EDTA and the metal ion, Pb(II).