Temperature-sensitive polymeric nanogels encapsulating with β-cyclodextrin and ICG complex for high-resolution deep-tissue ultrasound-switchable fluorescence imaging

One of the thorny problems currently impeding the applications of the fluorescence imaging technique is the poor spatial resolution in deep tissue. Ultrasound-switchable fluorescence (USF) imaging is a novel imaging tool that has recently been explored to possibly surmount the above-mentioned bottleneck. Herein, a β-cyclodextrin/indocyanine green (ICG) complex-encapsulated poly(N-isopropylacrylamide) (PNIPAM) nanogel was synthesized and studied for ex vivo / in vivo deep tissue/high-resolution near infrared USF (NIR-USF) imaging. To be specific, our results revealed that the average diameter of the as-prepared nanogels was significantly decreased to ~ 32 nm from ~ 335 nm compared to the reported ICG-PNIPAM nanoparticles. Additionally, the excitation/ emission characteristics of the ICG itself in present nanogels were almost completely retained, and the resultant nanogel exhibited high physiological stability and positive biocompatibility. In particular, the signal-to-noise ratio of the USF image for the PNIPAM/ β-cyclodextrin/ICG nanogel (33.01 ± 2.42 dB) was prominently higher than that of the ICG-PNIPAM nanoparticles (18.73 ± 0.33 dB) in 1.5-cm-thick chicken breast tissues. The NIR-USF imaging in 3.5-cm-thick chicken breast tissues was achieved using this new probe. The ex vivo NIR-USF imaging of the mouse liver was also successfully obtained. Animal experiments showed that the present nanogels were able to be effectively accumulated into U87 tumor-bearing mice via enhanced permeability and retention effects, and the high-resolution NIR-USF imaging of in vivo tumor was efficiently acquired. The metabolism and in vivo biodistribution of the nanogels were evaluated. Overall, the results suggest that the current nanogel is a highly promising NIR-USF probe for deep tissue and high-resolution USF imaging.