Amplified fluorescence response of chemosensors grafted onto silica nanoparticles.

In conventional fluorescent chemosensors, the recognition of the target by the receptor unit affects the fluorescence properties of a single covalently coupled fluorescent moiety. Here we show for the first time that when a suitable TSQ derivative is densely grafted onto the surface of preformed silica nanoparticles electronic interactions between the individual chemosensor units enable the free units to recognize the state of the surrounding complexed ones. As a result, the fluorescence transduction is not limited to the local site where binding occurs, but it involves a wider region of the fluorophore network that is able to transfer its excitation energy to the complexed units. Such behavior leads to an amplification of the fluorescence signal. What we report here is the first example of amplification in the case an off-on chemosensor due to its organization onto the surface of silica nanoparticles. We also describe a simple general model to approach amplification in multifluorophoric systems based on the localization of the excited states, which is valid for assemblies such as the supramolecular ones where molecular interactions are weak and do not significantly perturb the individual electronic states. The introduction of an amplification factor f in particular allows for a simple quantitative estimation of the amplification effects.

Silica nanoparticles for fluorescence sensing of Zn(II): exploring the covalent strategy.

Silica nanoparticles (about 15 nm diameters), which contain a derivative of 6-methoxy-8-(p-toluensulfonamido)-quinoline (TSQ) as a Zn(II) fluorescent probe covalently linked to the silica network, were prepared and studied as Zn(II) fluorescent chemosensors. The systems selectively detect Zn(II) ions in water rich solutions with a submicromolar sensitivity: 0.13 microM concentrations of Zn(II) can be measured with the only interference of Cu(II) and Cd(II) ions. Compared with free TSQ, the nanoparticles based systems have the advantage that they can be employed in aqueous solutions without aggregation problems while at the same time, they maintain a similar Zn(II) affinity and sensing ability. Addition of a second, substrate insensitive, fluorophore to the particles leads to the realization of a ratiometric sensor.