Zinc oxide nanowire as a novel platform for optical imaging

Objectives With properties such as biocompatibility, versatile chemistry, and intrinsic fluorescence, zinc oxide nanowires (ZnO NWs) can serve as a novel nanoplatform for cancer imaging and therapy. Herein we aim to prove the principle that ZnO NWs can be adopted for optical imaging.

Methods Fluorescent ZnO NWs, with high point defect concentration, were synthesized by chemical vapor deposition. The resulting fluorescent ZnO NWs were conjugated to cyclic RGD peptides (integrin αvβ3 antagonist) through PEG linkers, which were fully characterized. Cellular toxicity of NW, PEG-NW, and RGD-PEG-NW were tested via MTT assay. Lastly, U87MG cancer cells (αvβ3-positive) were incubated with various concentrations of NW, PEG-NW, or RGD-PEG-NW and the fluorescence signal of the cells were examined at different time points. MCF-7 human breast cancer cells (αvβ3-negative) served as a negative control.

Results Ultra-small ZnO NWs (~20 nm in diameter and ~100-500 nm in length) were synthesized with intrinsic green fluorescence (Em: 490 nm). Significant changes in zeta-potential confirmed the success of PEG and RGD-PEG conjugation at the NW surface. MTT assay revealed that at a concentration of 30 μg/mL, the viability of U87MG cells treated with NW was significantly lower (70.2 ± 5.4 %) than cells treated with PEG-NW (85.4 ± 5.8 %) or RGD-PEG-NW (83.2 ± 6.2 %), indicating that PEGylation could enhance the biocompatibility of ZnO NWs. Incubation with RGD-PEG-NW resulted in good fluorescence signal of the U87MG cells as early as 15 min post-incubation. The fluorescence intensity plateaued at 1 h and declined after 4 h, possibly due to the dissolution of internalized ZnO NWs. Several control studies were carried out to confirm that the fluorescence signal observed in U87MG cells upon RGD-PEG-NW incubation is αvβ3-specific.

Conclusions We showed that ZnO NWs can be used for optical imaging. Upon further optimization, ZnO NW could serve as a novel biodegradable platform for cancer-targeted imaging and therapy