Optomechanical Time-Gated Fluorescence Imaging Using Long-Lived Silicon Quantum Dot Nanoparticles.

We demonstrate a novel optomechanical synchronization method to achieve ultrahigh-contrast time-gated fluorescence imaging using live zebrafish as models. Silicon quantum dot nanoparticles (SiQDNPs) with photoluminescence lifetime of about 16 µs were used as the long-lived probes to enable background autofluorescence removal and multiplexing through time-gating. A continuous-wave 405 nm laser as the excitation source was focused on a rotating optical chopper on which the emission light beam obtained from an inverted fluorescence microscope was also focused but with a phase difference such that in a short delay after the excitation laser is blocked, the emission light beam passes through the optical chopper, initiating the image acquisition by a conventional sensor. Both excitation and detection time windows were synchronized by one optical chopper, eliminating the need for pulsed light source and image intensifier which is often used as ultrafast optical shutter. Through use of the cost-effective time-gating method, nearly all background autofluorescence emitted from the yolk sac of a zebrafish embryo microinjected with the SiQDNPs was removed, leading to a 45-fold increase in signal-to-background ratio. Furthermore, two kinds of fluorescence signals emitted from the microinjected SiQDNPs and the intrinsic green fluorescent protein of transgenic zebrafish larvae can be clearly separated through time-gating.

Phototoxicity Generated by Silicon Quantum Dot Nanoparticles on Zebrafish Embryos.

We demonstrate phototoxicity generated by silicon quantum dot nanoparticles (SiQDNPs) using zebrafish as an animal model. Having long exciton lifetime, the SiQDNPs can function as photosensitizers which absorb incident optical light and transfer the energy to oxygen molecules in close proximity, generating cytotoxic singlet oxygens. First, the zebrafish embryos were soaked in the SiQDNP suspension in E3 medium, while being illuminated under blue light or kept in the dark for 6 h. Through neutral red staining immediately afterward, the illuminated embryos showed more prominent injuries at their head, yolk sac and tail parts than those in the dark. Furthermore, prolonged observation after the treatment revealed that the illuminated embryos had mortality rates significantly higher than those without illumination, clearly showing the phototoxicity effect generated by the SiQDNPs. However, adverse effect due to the immersion of whole embryos in the SiQDNP suspension was also observed. To alleviate this issue, minute amounts of the SiQDNPs were microinjected to the embryos, followed by blue light illumination. By acridine orange staining subsequently, cell apoptosis localized near the microinjection site was revealed, whereas no apoptosis was found for those also microinjected with the SiQDNPs but without illumination. The phototoxicity effect demonstrated on zebrafish embryos in this work manifests the potential of using the SiQDNPs as a photosensitizer for photodynamic therapy.