Characterization and stability of gold nanocages for use as contrast agents in optoacoustics

Optoacoustic (OA) imaging is a new technique for cancer detection and non-invasive monitoring of cancer therapeutics. This technique is the combination of both optical and ultrasound modalities, in which tissue is irradiated with nanosecond laser pulses. The delivered optical energy is absorbed and converted into heat, leading to thermaoelastic expansion, which produces acoustic waves. Generated sound waves are then detected by ultrasonic transducers to form an image. In order to enhance optoacoustic signals in tissues, gold nanocages (AuNCs) can be used as contrast agents due to their tuneable optical properties in the near infrared region. Gold nanoparticles are biocompatible and their surface chemistry is well known, therefore they can be easily functionalized to targeting specific cells. This work involved three linked investigations involving the synthesis, stability testing and optoacoustic contrast capabilities of AuNCs. Silver nanocubes were synthesized to be used as templates for AuNCs synthesis. More then 95% purity was required but only 20-30% purity was achieved in our experiments due to the sensitivity of the reaction. Gold nanocages of 40-45nm were acquired from Dr. Younan Xia (Georgia Tech). For all OA investigations the system consisted of a Nd:YAG pumped Ti:S laser, which can operate at either 775 nm or 1064 nm, an 8 element annular array transducer with 5 MHz central frequency and bifurcated optic fibre bundles to introduce light to the target. A 6 ns pulse duration and 10 Hz repetition rate were used. For the stability study, different pulse energies (5mJ, 10mJ) were used and at each energy, three different AuNCs concentrations were exposed for 1 min, 3 min and 5 min. Optical absorption measurements and TEM imaging confirmed conformational changes in AuNCs at both energies. For contrast investigations, three different AuNCs concentrations (8.0 X 1011 particles/ml, 2.7 X 1011 particles/ml and 1.6 X 1011 particles/ml) were tested against fluence, of 0.016 J/cm2 at 775nm using seven 1% Intralipid phantoms with different background absorption, μa (0.03cm-1, 0.1 cm-1, 0.5 cm-1, 1.0 cm-1, 1.5 cm-1, 2.0 cm-1, 4.0 cm-1). For the proof of principle study, pork loin tissues of 5mm 10mm thickness were used as background phantoms. A single AuNCs concentration (2.66 X 1011 particles/ml) and 9.2mJ laser energy (0.014 J/cm2) at 775nm was used. Results demonstrate that AuNCs show relevant contrast, even at the highest absorbing background (4.0 cm-1) and for the maximum pork loin thickness (10mm) tested. Results from 1% Intralipid also demonstrate that there were no conformational changes in AuNCs after the illumination. Therefore, the contrast study indicates that AuNCs could be used as contrast agents by knowing specific parameters, such as background optical properties, AuNCs concentration and required laser fluence.