Plasmon-enhanced fluorescence sensor based on Au nanocages for sensitive detection of norepinephrine

Norepinephrine (NE) as a crucial monoamine neurotransmitter in the central and sympathetic nervous system, plays an important role in different physiological and pathophysiological processes. Brain NE can modulate cerebrospinal fluid flux and neurovascular coupling, regulate cortical and hippocampal neuronal circuitry, and participate the immune system. In addition, the reduced concentration of NE in brain was currently deemed to be the internal reason of major depression. However, development of detection method of NE with high spatiotemporal resolution in living systems remains a great challenge.

Herein, a plasmon-enhanced fluorescence (PEF) sensor based on Au nanocages (Aucages) were designed and prepared for ultra-sensitive detection of NE. Aucages with porous walls, hollow interior and systematically tunable localized surface plasmon resonance (LSPR) wavelengths (536 nm, 654 nm, 754 nm) were prepared to obtain a highly fluorescent enhancement of Au nanoclusters (Au NCs). Moreover, polyethylene glycol (PEG) with different molecular weight (1000, 5000, 10000 Da) were applied to control the distance between the Aucages and Au NCs. 3D-FDTD simulation results indicated that the fluorescence enhancement was primarily due to the internal and external enhanced electric field effects of Aucages. This sensor was applied for the turn-on detection of NE in commonly used clinical injectable norepinephrine bitartrate with the recovery rate of 98.06-105.34 %. Meanwhile, real-time fluorescence imaging of NE in living pheochromocytoma (PC-12) cells was explored with a red-emitted fluorescence.

This study first employed Aucages with more "hot spot" for red-emitted Au NCs to realize fluorescence enhancement. It provides a new method for the development of more sensitive, accurate and convenient analysis of NE in clinical drug analysis, cell monitor and metabolism study.