Effects and anomalies that can occur in SERS spectra of biological molecules when using a wide range of aggregating agents for hydroxylamine-reduced and citrate-reduced silver colloids

SERS spectra of cysteine and proline were measured for citrate-reduced and hydroxylamine-reduced silver colloids using aggregating agents K2SO4, NaNO3, NaCl, Ca(NO3)2, CaCl2, Na2SO4, KNO3 and KCl, to compare the effects of aggregation on two different colloid environments. High intensity anomalous bands were found for both colloids on the addition of the aggregating agents. On the addition of cysteine these anomalous bands disappeared. However, when proline was analysed, the anomalous bands remained, indicating the importance of characterising the colloidal sol to differentiate between band patterns from the analyte and those from the colloid in the presence of an aggregating agent. K2SO4 gave the greatest enhancement (∼103) of any aggregating agent for cysteine with the hydroxylamine-reduced colloid, in contrast to the citrate-reduced colloids where similar enhancement factors (∼102) were found with several different aggregating agents. Through the analysis of different SERS spectra for amino acids with different aggregating agents and colloids, a better understanding of the SERS technique may be achieved and its application in biology may be expanded.