Controlled etching and tapering of Au nanorods using cysteamine

While gold nanorods (AuNRs) have found many applications due to their unique optical properties, a few challenges persist in their synthesis. Namely, it is often difficult to reproducibly synthesize AuNRs with specific and monodisperse sizes, especially at shorter aspect ratios. Here, we report a method of post-synthesis precise tailoring of AuNRs by etching with cysteamine. Cysteamine selectively etches AuNRs from their ends while preserving the initial rod shape and monodispersity, making this a viable means of obtaining highly monodisperse short AuNRs down to aspect ratio 2.3. Further, we explore the effect of this etching method on two types of silica-coated AuNRs: silica side-coated and silica end-coated AuNRs. We find that the etching process is cysteamine concentration-dependent and can lead to different degrees of sharpening of the silica-coated AuNRs, forming elongated tips. We also find that cysteamine behaves only as a ligand at concentrations above 200 mM, as no etching of the AuNRs is observed in this condition. Simulations show that excitation of plasmon resonances in these sharpened AuNRs produces local electric fields twice as strong as those produced by conventional AuNRs. Thus, cysteamine etching of AuNRs is shown to be an effective means of tailoring both the size and shape of AuNRs along with their corresponding optical properties. At the same time, the resulting cysteamine coating on the etched AuNRs displays terminal amino groups that allow for further functionalization of the nanorods.