The self-organization of the proper subunits of a fluorescence chemosensor on the surface of silica nanoparticles allows the easy design and realization of new effective sensing systems. Commercially available silica particles (20 nm diameter) were functionalized with triethoxysilane derivatives of selective Cu(II) ligands and fluorescent dyes. Grafting of the sensor components to the particle surface ensures the spatial proximity between the sensor components and, as a consequence, binding of Cu(II) ions by the ligand subunits leads to quenching of the fluorescent units emission. In 9 : 1 DMSO-water solution, the coated silica nanoparticles (CSNs) selectively detect copper ions down to nanomolar concentrations. The operative range of the sensors can be tuned either by switching the ligand units or by modification of the components ratio. Sensors with the desired photophysical properties can be easily prepared by using different fluorescent dyes. Moreover, the organization of the network of sensor components gives rise to cooperative and collective effects: on one hand, the ligand subunits bound to the particle surfaces cooperate to form multivalent binding sites with an increased affinity for the Cu( II) ions; on the other hand, binding of a single metal ion leads to the quenching of several fluorescent groups producing a remarkable signal amplification

We present here the study of the photophysical properties of new dye-doped silica nanoparticles (DDNs) bearing dansyl fluorescent derivatives covalently linked to the silica matrix. The described experimental evidences show how the different location of the chromophores induces great changes in their photophysical behavior, suggesting that fluorophores located near the surface of the nanoparticles have a very different behavior with respect to the internal molecules. These latter ones, in fact, are shielded from the solvent and have a strong blue emission, while those at the periphery interact with the solvent and show a weaker red-shifted emission. As a consequence, the fluorescence properties of these nanoparticles are an average between the characteristics of the two different families of dyes. The relative amount of fluorophores located in the two compartments can be controlled simply by changing the size since, from our results, the thickness of the solvent permeable layer is not relevantly affected by the diameter of the nanoparticles. It is noteworthy that the fluorophores located in the outer shell exhibit very peculiar features: they are sensitive and interact with small molecules such as solvent molecules but, at the same time, they are not accessible to big receptor species such as beta-cyclodextrins. Such results indicate that most of the solvent-sensitive dansyl moieties are located within pores large enough to only accommodate solvent but not big molecules as cyclodextrins, giving precious insight on the morphology of the nanoparticles.

There is great interest in the self-organization of the proper subunits as a new strategy for the realization of fluorescent chemosensors. In this article, it is shown that commercially available fluorescent dyes, functionalized with triethoxysilane moieties, can be converted into fluorescent chemosensors by simple inclusion into silica nanostructures. Dye-doped silica nanoparticles and thin films detect Cu(II) ions in the micromolar range by the quenching of fluorescence emission. The different response toward Zn(II), Ni(II), and Co(II) metal ions was also investigated and is reported. The self-organization of the silica structures leads, at the same time, to the formation of metal ion binding sites as well as to the linking of a fluorescent reporter in their proximity. Structural features of the materials, particularly particle size and network porosity, strongly affect their ability to act as fluorescent sensors.