In vivo photoacoustic monitoring using 700-nm region Raman source for targeting Prussian blue nanoparticles in mouse tumor model.

Photoacoustic imaging (PAI) is a noninvasive imaging tool to visualize optical absorbing contrast agents. Due to high ultrasonic resolution and superior optical sensitivity, PAI can be used to monitor nanoparticle-mediated cancer therapy. The current study synthesized Food and Drug Administration-approved Prussian blue (PB) in the form of nanoparticles (NPs) with the peak absorption at 712 nm for photoacoustically imaging tumor-bearing mouse models. To monitor PB NPs from the background tissue in vivo, we also developed a new 700-nm-region stimulated Raman scattering (SRS) source (pulse energy up to 200 nJ and repetition rate up to 50 kHz) and implemented optical-resolution photoacoustic microscopy (OR-PAM). The SRS-assisted OR-PAM system was able to monitor PB NPs in the tumor model with micrometer resolution. Due to strong light absorption at 712 nm, the developed SRS light yielded a two-fold higher contrast from PB NPs, in comparison with a 532-nm pumping source. The proposed laser source involved cost-effective and simple system implementation along with high compatibility with the fiber-based OR-PAM system. The study highlights the OR-PAM system in conjunction with the tunable-color SRS light source as a feasible tool to assist NP-mediated cancer therapy.