Surface modified gold nanorods in two photon luminescence imaging

Abstract

Gold nanorods (AuNRs) possess unique optical properties which make them good contrast agents for optical microscopy. Their longitudinal plasmon resonance peak can be easily tuned from red to near infrared wavelength by increasing their aspect ratio to match the wavelengths of different imaging modalities. AuNRs are also stronger scatterers of light as compared with gold nanospheres. Nevertheless what sets them apart from other gold nanoparticles is their strong multiphoton luminescence. AuNRs are therefore being increasingly used as contrast agents for multiphoton microscopy of biological samples. In this study, control of the longitudinal resonance peak of gold nanorods is investigated with comparison of two chemical synthesis approaches. Both based on a seed-mediated method, one approach is to tune the aspect ratio through manipulation of the ratio of gold seeds to gold salt and the other is through variation of the volume of hydrochloric acid. The synthesized gold nanorods were made biocompatible by replacing the cytotoxic cetyltrimethylammonium bromide (CTAB) molecules with either silica (SiO2) or polyethylene glycol (PEG). Multiphoton imaging of gold nanorods taken up by cells was demonstrated and the effect of PEG chain length on passive uptake of gold nanorods by cells is discussed.