Gold nanorods are rod-shaped gold nanoparticles which show distinctive double surface plasmon (SP) bands in visible region (~520 nm) and near infrared (IR) regions (750 ~ 1200 nm). Pulsed-laser irradiation on the gold nanorods induced photothermal conversion of absorbed photons and resulted in reshaping of the gold nanorods. These photoreactions can be applied for a trigger of controlled release of gene or drag. We reported photo-triggered release of DNA from conjugates of gold nanoparticle and DNA. In this study, we evaluated gene expression induced by the pulsed-laser irradiation using in vitro system.

Gold nanorods were provided from Mitsubishi Materials Corporation. Characteristics of the gold nanorods in transverse and longitudinal directions were 65 ± 5 nm and 11 ± 1 nm, respectively. Hexadecyltrimethylammonium bromide (CTAB, 480 mM), which was essential in the preparation of nanorods, was replaced with phosphatidylcholine (PC). The PC-modified gold nanorods (PC-NRs) showed positive zeta potential (ca. +15 mV). The PC-NRs were mixed with DNA in water, and then the mixed solution was added in phosphate buffered saline (pH=7.0). A Nd:YAG laser (1064 nm, 10 Hz, 120 sec, 160 mJ/pulse) was used for the laser irradiation.

Gel electrophoresis of the buffered solution of DNA and PC-NR indicated that the DNA was tightly conjugated with the PC-NRs. After the pulsed-laser irradiation, gel electrophoresis showed a new band that could be assigned to released DNA from the conjugates. Fluorescence intensities of the released DNA, which was labeled with SYBR®Green I, was 1.2% of the total amount of DNA. This value would be a release efficiency of DNA from the conjugates by the pulsed-laser irradiation. We also estimated the bioactivity of released DNA after pulsed-laser irradiation by luciferase assay in vitro (TnT® T7 Quick Coupled Transcription Translation System, production of Promega). The released DNA was picked up from a part of gel, and extracted by Wizard SV Gel And PCR Clean-up System (Promega). The conjugates before the pulsed-laser irradiation did not show luciferase activity. After pulsed-laser irradiation, the activity of the released DNA was evaluated as about 0.5% to the initial DNA.