Optical and electrochemical probes-assisted dual-mode nanosensor for ultrasensitive monitoring of serotonin in single cells.

5-Hydroxytryptamine (5-HT), a vital neurotransmitter, plays a critical role in the development of various neurodegenerative diseases. Thus, in situ tracking and monitoring of 5-HT levels in single living cells and their fluctuations under nonylphenol (NP)-induced stimulation are of great significance. Herein, we constructed a novel aptamer and methylene blue (MB)-assisted nanosensor (AM-NS) for in situ monitoring of 5-HT by combining electrochemical with fluorescence sensing. The dual-mode AM-NS was prepared through covalent coupling of dopamine (DA) and aptamers with electrochemical conjugation strategy and insertion reaction, which exhibited excellent specificity and sensitivity in complex cellular microenvironment. When 5-HT existed, the electrochemical response signal of "Signal-OFF" and the fluorescence response signal of "Signal-ON" are simultaneously output, with low detection limits of 0.156 nM and 0.042 nM, respectively. Furthermore, we also delved into the impact of NP stimulation on the release of 5-HT in sirtuin 1 (SIRT1)-mediated signaling pathway and explored the protective effect of nicotinamide mononucleotide (NMN) on the nervous system as a therapeutic intervention. Electrochemical and fluorescence assays revealed that NMN-mediated treatment led to over 95 % signal recovery, signifying a remarkable therapeutic efficacy of NMN against NP-induced nerve cell damage at the cellular level. This research offers a more robust scientific foundation for the early diagnosis and prompt treatment of neural disorders.