A two-photon fluorescent probe for bio-imaging of formaldehyde in living cells and tissues

Formaldehyde (FA) plays an important role in living systems as a reactive carbonyl species (RCS). An abnormal degree of FA is known to induce neurodegeneration, cognitive decrease and memory loss owing to the formation of strong cross-link DNA and protein and other molecules. The development of efficient methods for biological FA detection is of great biomedical importance. Although a few one-photon FA fluorescent probes have been reported for imaging in living cells, probes excited by two photons are more suitable for bio-imaging due to their low background fluorescence, less photobleaching, and deep penetration depth. In this study, a two-photon fluorescent probe FATP1 for FA detection and bio-imaging in living cells and tissues was reported. The detection is based on the 2-aza-Cope sigmatropic rearrangement followed by elimination to release the fluorophore, resulting in both one- and two-photon excited fluorescence increase. The probe FATP1 showed a high sensitivity to FA with a detection limit of 0.2 μM. Moreover, FATP1 enabled the two-photon bio-imaging of FA in live HEK-293 cells and tissues with tissue-imaging depths of 40–170 μm. Furthermore, FATP1 could be applied for the monitoring of endogenous FA in live MCF-7 cells, presaging its practical applications in biological systems.