Fluorescence Modulation by Absorbent on Solid Surface: An Improved Approach for Designing Fluorescent Sensor

Inner filter effect (IFE), a well-known phenomenon of fluorescence quenching resulting from absorption of the excitation or emission light of luminescent species by absorbent, has been used as a smart approach to design fluorescent sensors, which are characterized by the simplicity and flexibility with high sensitivity. However, further application of IFE-based sensors in complex environment is hampered by the insufficient IFE efficiency and low sensitivity resulting from interference of the external environment. In this paper, we report that IFE occurring on a solid substrate surface would solve this problem. As a proof of concept, a fluorescent sensor for intracellular biothiols has been developed on the basis of the absorption of a newly designed thiols-specific chromogenic probe (CP) coupled with the use of a thiols-independent fluorophore, rhodamine 6G (R6G), operative on the IFE on graphene oxide (GO). To construct an efficient IFE system, R6G was covalently attached to GO, and the CP molecules were adsorbed on the surface of R6G-GO via π–π stacking interaction. The reaction of thiols with CP on R6G-GO decreases the absorption of CP, resulting in the increase of the intensity of R6G fluorescence. The results showed that the IFE efficiency, sensitivity, and dynamic response time of R6G-GO/CP for biothiols could be significantly improved compared with R6G/CP, and furthermore, R6G-GO/CP functioned under complex system and could be used for assaying biothiols in living cells and in human serum samples. This new strategy would be general to explore the development of more effective IFE-based sensors for other analytes of interest.