Journal: Nano Letters
Authors: Constantin Ungureanu†, Rene Kroes†, Wilma Petersen†, Tom A. M. Groothuis‡, Felicia Ungureanu‡, Hans Janssen§, Fijs W. B. van Leeuwen, Rob P. H. Kooyman‡, Srirang Manohar†, and Ton G. van Leeuwen†
Affiliation: †Biomedical Photonic Imaging Group, MIRA Institute for Biomedical Technology and Technical Medicine, Faculty of Science and Technology, University of Twente; ‡Nanobiophysics Group, Faculty of Science and Technology, University of Twente; §Division of Cell Biology, The Netherlands Cancer Institute; Division of Diagnostic Oncology, The Netherlands Cancer Institute; Biomedical Engineering and Physics, Academic Medical Center, University of Amsterdam
Gold nanorods (AuNRs) have a very intense absorption at certain peaks, because of a phenomenon known as surface plasmon resonance. One peak, the so called longitudinal plasmon (LP) absorption peak, is in the far-red and near-infrared region of the electromagnetic spectrum – a region where living tissue is largely transparent. The other peak is called the transverse plasmon (TP) peak and is located around 520 nm. The LP peak is tunable by changing the structure of the AuNRs, while the TP peak remains around 520 nm. Scientists are now trying to exploit this property and use gold nanorods as contrast agents in photoacoustic imaging and photothermal therapeutics.
It is known that short high energy pulses of light can cause AuNRs to melt or fragment. Once melted or fragmented they no longer exhibit these intense absorption peaks, making them far less useful as contrast agents or therapeutics. This study looks at the threshold at which the AuNRs are reshaped by light pulses. They then study the survival of cancer cells incubated with AuNRs and irradiated with light above and below the determined threshold.
The researchers took AuNRs in solution and irradiated them with 1 to 20 mJ/sq cm per pulse at the LP peak. They were able to quantify the number of AuNRs that had reshaped by looking at the decrease in amplitude of the LP peak. They found that 1 min pulses at 10 Hz of 6 mJ/sq cm and larger, more than 95% of the AuNRs are reshaped. They determined the threshold where 50% of the AuNRs are reshaped to be a 3.5 mJ/sq cm.
The authors then took cancer cells incubated with AuNRs and irradiated them with light at the LP peak. They found that reshaping of the AuNRs did take place within the cells, but “no direct lethal damage to cells was inflicted.” However, when the cells were irradiated with light at the TP peak “extensive cell death was achieved.”
The authors also found evidence of plasmonic interactions with the cell. So although the LP wavelength could be finely tuned in the laboratory once the AuNR is introduced into a cell this peak could change significantly, which would greatly affected its ability to perform the intended function. Although great strides have been made in AuNR technology much remains to be investigated including the use of the LP peak of AuNRs to kill cancer cells and the changes in the LP peak that occur in cells.