Raja Gopal Rayavarapu
Gold Nanoparticles as potential contrast agents for light-based Imaging Techniques
To synthesize gold nanorods with certain aspect ratios (length/width) which possess extinction peaks within the "optical imaging and therapeutic window" (spectral window occupying 650-1100 nm where scattering and absorption of biological tissues is low)
To functionalize these gold nanoparticles with affinity biomolecules such as antibodies specific to tumor cells.
To study the affinity of these functionalized gold nanoparticles to target molecules in culture (in vitro) and within the body (in vivo) using various techniques such as optical spectroscopy, confocal microscopy, electron microscopy etc.
From ancient times, it was known that gold powder has properties to produce brilliant ruby red colors for stained glass. These intense optical properties of gold nanoparticles make them ideal as contrast agents for light-based imaging techniques. One dimensional nanostructures such as gold nanorods, have two plasmon bands; the transverse and the longitudinal plasmon bands. The transverse plasmon band is due to the oscillation of free electrons along the short axis and the longitudinal plasmon band is due to the oscillation of free electrons along the long axis of the nanorod. By changing the length and width of the nanorod, the longitudinal plasmon band can be red-shifted towards the near-infrared region where light penetration in tissue is high which is useful for in vivo imaging applications. To make gold nanorods target tumor cells, they are bioconjugated with affinity molecules such as antibodies that bind to tumor cells. These functionalized gold nanorods, when administered into the subject, permeate into leaky vasculature of tumors and bind to tumor cells. We use photoacoustic imaging as a non-invasive imaging technique, which produce strong ultrasound signals when gold nanorods are irradiated and identifies the location of tumor. In addition to imaging, therapy of cancers is also possible. The optical energy absorbed by the nanorods is converted into heat. This causes a temperature rise, which could induce apoptosis in the cancer cells.
The left image shows the extinction spectrum of gold nanorods. The peak at 516 nm is the transverse plasmon band and the peak at 788 nm is the longitudinal plasmon band. The right image, is the corresponding High Resolution Scanning electron micrograph of gold nanorods having an aspect ratio of 3.0 ±0.6 (length 42 nm, width 14 nm). Synthesis of gold nanorods was done using seed-mediated protocols.