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 (<b>CP</b>) 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 <b>CP</b> molecules were adsorbed on the surface of <b>R6G-GO</b> via π–π stacking interaction. The reaction of thiols with <b>CP</b> on <b>R6G-GO</b> decreases the absorption of <b>CP</b>, resulting in the increase of the intensity of R6G fluorescence. The results showed that the IFE efficiency, sensitivity, and dynamic response time of <b>R6G-GO/CP</b> for biothiols could be significantly improved compared with <b>R6G/CP</b>, and furthermore, <b>R6G-GO/CP</b> 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

Graphene Oxide Assisted Fluorescent Chemodosimeter for High-Performance Sensing and Bioimaging of Fluoride Ions

Fluorescent chemodosimeters for a fluoride ion (F^–) based on a specifically F^–-triggered chemical reaction are characterized by high selectivity. However, they are also subjected to intrinsic limits, such as long response time, poor stability under aqueous solution, and unpredictable cell-member penetration. To address these issues, we reported here that the self-assembly of fluorescent chemodosimeter molecules on a graphene oxide (GO) surface can solve these problems by taking advantage of the excellent chemical catalysis and nanocarrier functions of GO. As a proof of concept, a new F^–-specific fluorescent chemodosimeter molecule, <b>FC-A</b>, and the GO self-assembly structure of <b>GO/FC-A</b> were synthesized and characterized. Fluorescent sensing and imaging of F^– with <b>FC-A</b> and <b>GO/FC-A</b> were performed. The results showed that the reaction rate constant of <b>GO/FC-A</b> for F^– is about 5-fold larger than that of <b>FC-A</b>, so that the response time was shortened from 4 h to about 30 min, while for F^–, the response sensitivity of <b>GO/FC-A</b> was >2-fold higher than that of <b>FC-A</b>. Furthermore, <b>GO/FC-A</b> showed a better bioimaging performance for F^– than <b>FC-A</b> because of the nanocarrier function of GO for cells. It is demonstrated that this GO-based strategy is feasible and general, which could help in the exploration of the development of more effective fluorescent nanodosimeters for other analytes of interest

Graphene Signal Amplification for Sensitive and Real-Time Fluorescence Anisotropy Detection of Small Molecules

Fluorescence anisotropy (FA) is a reliable, sensitive, and robust assay approach for determination of many biological targets. However, it is generally not applicable for the assay of small molecules because their molecular masses are relatively too small to produce observable FA value changes. To address this issue, we report herein the development of a FA signal amplification strategy by employing graphene oxide (GO) as the signal amplifier. Because of the extraordinarily larger volume of GO, the fluorophore exhibits very high polarization when bound to GO. Conversely, low polarization is observed when the fluorophore is dissociated from the GO. As proof-of-principle, the approach was applied to FA detection of adenosine triphosphate (ATP) with a fluorescent aptamer. The aptamer exhibits very high polarization when bound to GO, while the FA is greatly reduced when the aptamer complexes with ATP, which exhibits a maximum signal change of 0.316 and a low detection limit of 100 nM ATP in buffer solution. Successful application of this strategy has been demonstrated that it can be constructed either in a “signal-off” or in a “signal-on” detection scheme. Moreover, because FA is less affected by environmental interferences, FA measurements could be conveniently used to directly detect as low as 1.0 μM adenosine triphosphate (ATP) in human serum. The universality of the approach could be achieved to detect an array of biological analytes when complemented with the use of functional DNA structures