(293g) Observation of Nonlinear Optical Effects from Carbon Dots and Applications for Fluorescence Bioimaging from Cells through Tissues to Animals

Carbon dots (CDs), also known as carbon quantum dots or carbon nanodots, generally refer to small carbon nanoparticles with a typical size under 10 nm in all three dimensions, which have found great potential for a wide range of applications, from chemical sensing through bioimaging to optoelectronics. As for fluorescence bioimaging, the penetration of visible light into biological tissues is quite limited because of the large photon absorption and scattering, which results in a distinct photon attenuation and is unfavourable for deep-tissue fluorescence imaging. In this presentation, I will introduce our work on rapid and large-scale production of CDs, followed by the observation of their nonlinear fluorescence behaviors under excitation of femtosecond (fs) laser in both traditional near-infrared spectral region (NIR-I, 700–900 nm) and second near-infrared region (NIR-II, 1000–1700 nm). The two-photon excited fluorescence spectrum of the CDs by a 750 nm fs laser was observed in the wavelength range of 400–600 nm, with peak maximum located at ~460 nm and long fluorescence lifetime of ~7.2 ns, which were similar to the counterpart under one-photon excitation. However, the wavelength-tunable two-photon fluorescence from the CDs could be obtained with spectral peaks from 580 nm to 740 nm, under 1000–1560 nm fs laser excitation. Interestingly, when an fs excitation wavelength as long as 1560 nm was adopted, two typical nonlinear optical signals, namely two-photon fluorescence and third harmonic generation (THG), could be observed. Based on the nonlinear optical effects of CDs under NIR fs laser excitation, CDs-assisted two-photon fluorescence microscopy realized high-contrast fluorescence lifetime imaging of cancer cells, deep-tissue imaging of zebrafish embryos and the brain neuron networks of mice. These studies show the potential of CDs as fluorescent probes for deep-tissue and high-contrast functional bioimaging and related applications in the future.