Janus-Faced Fluorescence Imaging Agent for Malondialdehyde and Formaldehyde in Brains

Carbonyl stress caused by reactive carbonyl species (RCS) is closely related to various brain diseases. As the highly reactive, highly toxic, and lipophilic RCS, malondialdehyde (MDA) and formaldehyde (FA) could easily cross the blood-brain barrier (BBB) and induce protein dysfunction or cross-linking in the brain. Do MDA and FA coordinately regulate the physio-pathological processes of the brain? To answer the question, first of all, powerful identification and sensing tools are needed. However, competent probes for simultaneously analyzing MDA and FA in living brains are lacking, which originates from the following three challenges: (1) MDA and FA are difficult to distinguish due to their great similarity in structure and reactivity; (2) to achieve simultaneous and discriminable imaging, same excitation and different emissions are preferable; and (3) the detection of MDA and FA in living brains require the materials to pass through the BBB. Thus, we created a two-photon fluorescent agent, TFCH, for MDA/FA. The hydrazine group in TFCH could successfully differentiate MDA/FA at 440/510 nm under same excitation. Moreover, the lipophilic trifluoromethyl group (-CF₃) in TFCH prompts it to traverse the BBB, thereby realizing the coinstantaneous visualization of MDA and FA in the living brain. Using TFCH, we observed the excessive production of MDA and FA in living PC12 cells under carbonyl stress and oxidative stress. Notably, for the first time, two-photon fluorescence imaging indicated the synchronous increase of MDA and FA in living brains of mice with depression. Altogether, this work provides a promising tool for revealing the carbonyl stress-related molecular mechanism involved in brain diseases.