Red carbon dots as label-free two-photon fluorescent nanoprobes for imaging of formaldehyde in living cells and zebrafishes

Direct, in situ selective detection of intracellular formaldehyde (FA) is of great significance for understanding its function in FA-related diseases. Herein, red carbon dots (RCD) are reported as label-free two-photon fluorescent nanoprobes for detecting and imaging of FA. Upon addition of FA, the -NH₂ groups of RCD could quickly and specially react with aldehydes to form Schiff base and then the strong fluorescence of RCD with blue-shift emission is recovery due to the destruction of the hydrogen bond interaction between RCD and water. In addition, the nanoprobes exhibit outstanding photostability, rapid response (<1 min), high sensitivity (∼9.9 μmol/L) and excellent selectivity toward FA over other aldehyde group compounds. Notably, owing to the good cell-membrane permeability and biocompatibility, as well as the large two-photon absorption cross-section, the as-prepared RCD can be used as label-free nanoprobes for selectively detecting and imaging FA in living cells and zebrafishes through one-photon and two-photon excitation. Moreover, RCD could stain the tissue of zebrafishes at depths interval of up to 240 μm under two-photon excitation. This research implied that RCD are promising tools for directly and in situ imaging FA in vivo, thus providing critical insights into FA-related pathophysiological processes.