Gold nanorods surface modification and bioconjugation for label-free biosensor

Abstract

Gold nanorods (GNRs) are used in biosensing and imaging applications as they exhibit local surface plasmon resonance (LSPR). The binding of analytes to GNRs alters the local refractive index and produces detectable spectral shifts and this forms the working principle for label-free biosensors. GNRs synthesized via a seed-mediated growth method are capped in a CTAB bilayer. CTAB prevents GNRs from aggregating but free CTAB is cytotoxic and the CTAB bilayer also prevents the bioconjugation of ligands to the GNRs. Several methods have been reported to overcome this problem by modifying the surface of the GNRs. These include the use of small organothiols, mPEG-SH, round-trip phase transfer method and multilayer polyelectrolytes.

We reported the use of 2 methods for GNRs surface modification and demonstrated their bioconjugation and detection as label-free biosensors. UV-VIS was employed for the label-free monitoring of these processes.

Firstly, we showed that CTAB-GNRs could be modified using polyelectrolytes, PSS and PEI, without aggregation while maintaining the positive surface charge of the GNRs. The PSS/PEI-coated GNRs could be further bioconjugated with BSA without aggregation. The bioconjugation process could be detected by the shift in the longitudinal peak of the PSS/PEI-coated GNRs. Hence, PSS/PEI-coated GNRs were feasible as label-free biosensors.

Secondly, we demonstrated a novel method for the surface modification of CTAB-GNRs using PEG-SH first, followed by MPA. PEG-SH was used to first displace the CTAB bilayer and provide stability to the GNRs. Next, MPA containing the carboxylic group was introduced mainly onto the ends of the GNRs, giving PEG/MPA-GNRs. Following EDC/NHS activation, anti-rabbit IgG was bioconjugated onto the MPA molecules present on the PEG/MPA-GNRs. The rabbit IgG was detected by the shift in the longitudinal peak of the antibody conjugated GNRs, which was produced by the formation of its antibody-antigen complex with the anti-rabbit IgG on the PEG/MPA-GNRs. This strategy could be applied for general label-free detection of antigens by bioconjugating the corresponding antibodies onto the PEG/MPA-GNRs.