PVP-controlled gold aggregation for in-solution SERS: substrate stability, reproducibility, and analyte-affinity-dependent optimization.

Aggregation agents have been widely used in the in-solution surface-enhanced Raman spectroscopy (SERS) due to the formation of aggregated metallic clusters, more specifically hot spots. However, these clusters can grow with time and sediment within a short period, making the aggregation process time-dependent and difficult to control. This variation is directly related to the enhancement strength of hot spots and the reproducibility of the SERS analysis. Therefore, seeking a controllable aggregation method is a popular subject. Herein, polyvinylpyrrolidone (PVP)-controlled gold aggregates were prepared as in-solution SERS substrates. PVP was used to control the aggregation of gold nanoparticles introduced by NaCl and to maintain the dispersibility of the aggregated gold in the solution. Several conditions, such as laser wavelength, size of the gold nanoparticles, NaCl concentration (as aggregation agents), and PVP concentration (wt%), were optimized using two analytes: 4-mercaptobenzonitrile (MBN) and pyrene (PYR). The aggregation-control ability of PVP was examined by varying detection times within 14 days. The results indicated that the aggregation control by PVP is successful. MBN and PYR have different affinity to gold. The optimal SERS conditions of MBN and PYR were compared, indicating the reaction differences of different affinity analytes in SERS measurement. This variation indicates the necessity of using similar affinity molecules or target analytes in the condition optimization of SERS analysis.