Nano-supramolecular complex synthesis: Switch on/off enhanced fluorescence control and molecular release using a simple chemistry reaction

A nanosensor based on β-cyclodextrin (βCD) macrocycles linked to gold nanoparticles for rhodamine B (RhB) sensing was developed applying the metal-enhanced fluorescence effect (MEF). Hence, we have developed many ways to control the distance of supramolecular systems to nanoparticle surface with different bioconjugation strategies in order to optimize signal detection. Different PEG spacer arm lengths were used to cover the nanoparticle surface with molecular spacers. This type of molecular shell is biocompatible, enabling to switch on/off the MEF effect using a dithiane linker by a simple reduction reaction. In the presence of the nanosensor obtained, an increase was observed in RhB fluorescence emission depending on molecular length, that is a characteristic effect of MEF. The major increase measured was 60% compared with RhB emission in buffer at 1 nM level, for a spacer length of 3.58 nm and an 80% increase as compared with that in the presence of βCD. These differences are ascribed to the fact that, in the presence of macrocycle, we can observe a well-known quenching effect that is overcome by the presence of the metallic core. Even at shorter spacer distances, with PEG lengths of 1.2 and 2.17 nm, increases of 25 and 47% respectively allow the analyte detection by the RhB complexation with βCD. The optimal MEF enhancement was measured with the maximal emission signal stabilized after 50 min due to plasmonic effects based on inter nanoparticle interactions. Moreover, the emission increase, in the presence of the metallic core, was accompanied with a diminution in the fluorescence lifetime's decay value averages, characteristic of MEF. This fact shows that the excited state is protected from the non-radiative emission decays enhancing the analytical signal.

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