Sensing properties of silica nanoparticles functionalized with anion binding sites and sulforhodamine B as fluorogenic signalling unit

Bi-functionalized hybrid silica nanoparticles have been synthesized, characterized and their optical emission properties in the presence of certain anions studied. The hybrid solid S1, coated with sulforhodamine B as fluorogenic signalling unit and imidazolium groups as anion coordination sites, was prepared by the simultaneous grafting of suitable derivatives of these two subunits on silica nanoparticles. For the preparation of the sensing material S2, first the surface of silica nanoparticles was grafted with (3-aminopropyl)trimethoxysilane yielding solid SN. Then, in a second step, the amino groups on the surface were transformed into the corresponding guanidinium binding sites by reaction with pyrazole carboxamidine yielding solid SG. Finally, the grafting of the sulforhodamine B derivative 1 on SG resulted in the hybrid nanoparticles S2. Acetonitrile suspensions of S1 and S2 showed typical sulforhodamine B absorption bands at ca. 550 nm. Upon excitation, an emission band at 575 nm was observed. Signalling properties of S1 and S2, in acetonitrile and acetonitrile:water 9:1 v/v mixtures, in the presence of organic (acetate, benzoate) and inorganic (F−, Cl−, Br−, CN−, NO3−, HSO4− and H2PO4−) anions were studied. S1 showed a fluorogenic response toward iodide and benzoate, whereas S2 selectively responds to dihydrogen phosphate and hydrogen sulfate.