Tailoring the longitudinal surface plasmon resonance wavelength of gold nanorods by selective etching

Abstract

Oxidative etching is a common method for regulating the localized surface plasmon resonance wavelength (LSPRW) of gold nanorods (AuNRs), which boasts the advantages of simple operation and short reaction time. However, existing oxidation strategies have a key limitation: they etch only the ends of AuNRs, thus severely restricting the tunable range of LSPRW. To address this issue, we introduced sodium hypochlorite (NaOCl) as the oxidant in the cetyltrimethylammonium chloride (CTAC) system during the preparation of AuNRs. By precisely controlling the concentration of NaOCl, we innovatively achieved the selective etching of both the sides and ends of AuNRs, enabling bidirectional (blue- and red-) shifts of the LSPRW with a tuning range exceeding 400 nm. This versatility arises from concentration-dependent selective etching: low NaOCl concentrations etch the AuNR ends, whereas high concentrations preferentially attack the side facets. Concurrently, fine-tuning the NaOCl concentration allows for independent control of the optical cross-section without perturbing the LSPRW, as corroborated by in situ single-particle oxidation. Notably, our research further reveals that solution pH is a critical determinant of this selective etching process. The methodology demonstrates robust universality across AuNRs of varying aspect ratios and those synthesized in diverse surfactant environments. Moreover, AuNRs derived from oxidative etching retain plasmonic characteristics comparable to those produced by conventional synthesis, exhibiting only a marginal increase in plasmon damping. These insights not only illuminate the fundamental aspects of AuNR oxidation dynamics but also furnish a straightforward, economical, and expeditious strategy for obtaining AuNRs with tailored resonant wavelengths, holding substantial promise for applications in biomedicine and related fields.