Ag@Au Nano Island for Sensitive SERS Detection of Pesticide Residue

A cost-effective and novel method for making large-area surface-enhanced Raman spectroscopy (SERS) substrates using thermal annealing of ultrathin gold (Au) and silver-gold (Ag-Au) metal films is reported. The evolution of optical properties of thermally annealed Au and Ag-Au discontinuous films were investigated and correlated with morphological changes. Au films (5 nm thick) and Ag-Au films (5 nm thick each) were deposited via electron beam evaporation and subsequently annealed at different temperatures under ambient conditions. Thermal annealing led to the formation of high-density gold nanoislands (Au NIs) and silver-Au NIs (Ag@Au NIs), with average sizes ranging from 10 to 45 nm and 10–50 nm, respectively. The SERS response of the fabricated samples was carried out by using rhodamine 6G (Rd6G) as a Raman probe molecule. The experimental results showed that Ag@Au NIs exhibit the highest SERS signal compared to Au NIs, attributed to enhanced optical field confinement at SERS hot spots facilitated by the presence of Ag. Our study found that the efficiency of Ag@Au NIs in detecting Rd6G achieved a limit of detection (LOD) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{10}}$ </tex-math></inline-formula> M with an enhancement factor (EF) of about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{{5}}$ </tex-math></inline-formula>. In addition, the practical utility of these substrates is demonstrated by detecting Thiram fungicide with an experimental LOD of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{9}}$ </tex-math></inline-formula> M, while the calculated analytical LOD reaches <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{12}}$ </tex-math></inline-formula> M.