Colorimetric sensing of pyrophosphate in aqueous media using bis-functionalised silica surfaces

Bis-functionalised silica surfaces have been designed in order to develop selective and sensitive probes for the chromo-fluorogenic detection of certain guests. The designed system consists of a siliceous support bis-functinalised with thiol and polyamine groups. Thiol groups are suitable reactive centres (R) that are know to react with squaraine dyes (D) inducing bleaching, whereas polyamines act as host sites (H) able to coordinate certain target guests (G). In the absence of G, the reactive groups (R) react with the dye resulting in a bleaching of the solution. On the contrary, the presence of certain guest (G) results in a control of the reaction kinetic between R and D and eventually in a complete reaction inhibition. Different functionalised solids were prepared by reaction of the siliceous surface with different concentrations of mercaptopropyltrimethoxysilane (MPTS) and 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (N3TS). The final materials (solids to ) were characterized employing standard procedures. In a first step the reactivity of the signaling dye D (squaraine ) with the reactive centre R (thiol groups) was studied as a function of the pH using solid that showed a most suitable response. At pH 7 and pH 5 there was a quick reaction between the squaraine and the thiol groups on the surface. On the contrary this reaction is significantly slower at pH 3 due to the different degree of protonation of the amines as a function of the pH. Additionally the reaction of with the squaraine has been studied in the presence of a range of inorganic anions with different structural dimensions and charges, including chloride, perchlorate, nitrate, sulfate, phosphate and pyrophosphate. At pH 3 the reaction of the dye with the thiol groups is still effective in the presence of chloride and perchlorate, but the reaction is highly inhibited in the presence of the anions nitrate, sulfate, phosphate and pyrophosphate. At pH 7 the squaraine reacts very fast with the thiol groups in the presence of all the anions studied. In contrast, at pH 5 only pyrophosphate is able to inhibit, to a certain extent, the reaction of the squaraine with the thiols, resulting in a selective chromo-fluorogenic detection of this anion in pure water. Finally, the observed behaviour was discussed in terms of both, the kinetic rates of the reaction between the thiol and the squaraine groups and the thermodynamic interaction reaction between the anions and the polyamine moieties.