Chitosan stabilization and control over hot spot formation of gold nanospheres and SERS performance evaluation

Abstract

Au nanoparticles tend to aggregate in solution in the presence of several agents, specifically most adsorbates used in SERS experiments, such as rhodamine-6G (R6G). That tendency may be circumvented by stabilizing agents, usually based on modulating electrostatic interactions. The control over stability and formation of SERS hot spots are important factors for application in SERS-based biosensors. Among the stabilizing agents, chitosan (Chit), which is positively charged biopolymer, can provide stability to the AuNS system and control the formation of hot spots. In the present study, Au nanospheres (AuNS) were coated by Chit, which improved stability against KCl and ethanol. The decrease in R6 G SERS intensity relative to the unprotected AuNS was attributed to prevention of hot spots formation by the biopolymer coating; this last characteristic was partially reverted by aggregating with chloride before adding Chit. The AuNS + KCl + Chit system resulted in half R6 G SERS intensity compared to uncoated AuNS, but in an increase in 5 times compared to coating with Chit without the chloride-generated hot spots. Adding Chit improved the temporal stability of AuNS suspension for several weeks. The present study shows that Chit may both stabilize existing hot spots and stop the increase of aggregates, avoiding additional aggregation and agglomeration of Au nanoparticles even in presence of aggregating agents and still resulting in high SERS performance.