Search

Dual-function hybrid material U1 was designed for simultaneous chromofluorogenic detection and removal of Hg(2+) in an aqueous environment. Mesoporous material UVM-7 (MCM41 type) with homogeneously distributed pores of about 2-3 nm in size, a large specific surface area exceeding 1000 m(2) g(-1), and nanoscale particles was used as an inorganic support. The mesoporous solid is decorated with thiol groups that were treated with squaraine dye III to give a 2,4-bis(4-dialkylaminophenyl)-3-hydroxy-4-alkylsulfanylcyclobut-2-enone (APC) derivative that is covalently anchored to the inorganic silica matrix. The solid was characterised by various techniques including X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and nitrogen adsorption. This hybrid solid is the chemodosimeter for Hg(2+) detection. Hg(2+) reacts with the APC fragment in U1 with release of the squaraine dye into the solution, which turns deep blue and fluoresces strongly. Naked-eye Hg(2+) detection is thus accomplished in an easy-to-use procedure. In contrast, U1 remains silent in the presence of other thiophilic transition metal ions, alkali and alkaline earth metal ions, or anions ubiquitously present in water such as chloride, carbonate, sulfate, and phosphate. Material U1 acts not only as chemodosimeter that signals the presence of Hg(2+) down to parts-per-billion concentrations, but at the same time is also an excellent adsorbent for the removal of mercury cations from aqueous solutions. The amount of adsorbed mercury ranges from 0.7 to 1.7 mmol g(-1), depending on the degree of functionalisation. In addition, hybrid material U1 can be regenerated for both sensing and removal purposes. As far as we know, U1 is the first example of a promising new class of polyfunctional hybrid supports that can be used as both remediation and alarm systems by selective signalling and removal of target species of environmental importance. Model compounds based on silica gel (G1), fumed silica (F1), and micrometre-sized MCM-41 scaffolds (M1) were also prepared and studied for comparative purposes.

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.

The development of gate-like systems able to perform certain programmed functions is an interesting way of taking chemistry to the frontiers of nanoscience. In relation to this field, we report a complete study of the behavior of a pH-driven and anion-controlled nano-supramolecular gate-like ensemble obtained by anchoring suitable polyamines on the pore outlets of mesoporous materials of the type MCM-41 (solid N3-S). The release of an entrapped dye (Ru(bipy)3(2+)) from the pore voids into the bulk solution allows us to study the gating effect. A pH-driven open/close mechanism was observed that arises from the hydrogen-bonding interaction between amines at neutral pH (open gate) and Coulombic repulsions at acidic pH between closely located polyammoniums at the pore openings (closed gate). Molecular dynamics simulations using force field methods have been carried out to explain the pH-driven open/close mechanism. For this purpose, a mesoporous silica structure was constructed, taking as base the (111) plane of the beta-crystoballite structure on which large hexagonal nanopores and anchored polyamines were included. From these calculations, it was observed how completely unprotonated amines display poor coverage of the pore (fully open gate), whereas completely protonated amines (simulating a pH 2 or lower) result in a clear reduction of the pore aperture, in agreement with the experimental results. In additional to the pH-driven protocol, opening/closing of the gate-like ensemble can also be modulated via an anion-controlled mechanism. This study was carried out by monitoring the dye released from the pore voids of the N3-S solid at a certain pH in the presence of a range of anions with different structural dimensions and charges, including chloride, sulfate, phosphate, and ATP (C(anion) = 1 x 10(-2) mol dm(-3)). The choice of a certain anionic guest results in a different gate-like ensemble behavior, ranging from basically no action (chloride) to complete (ATP) or partial pore blockage, depending on the pH (sulfate and phosphate). The remarkable anion-controllable response of the gate-like ensemble can be explained in terms of anion complex formation with the tethered polyamines. These experimental studies are also in agreement with computational simulations with fluoride, chloride, iodide, and dihydrogen phosphate anions. In the model, larger anions push the tethered polyamines toward the pore openings more efficiently, and therefore the pore aperture decreases. The studies also show that, for anions showing a strong tendency to form hydrogen-bonding networks (e.g., phosphate), complete pore blockage was observed at acidic pH. Finally, selectivity patterns have been discussed in terms of kinetic rates of the liberation of the Ru(bipy)3(2+) dye from the amine-functionalized dye-containing material N3-S.

Hybrid mesoporous systems containing a gate-like ensemble functionalised with imidazolium groups and a dye are used for the selective colorimetric sensing of long-chain carboxylates. Amoros del Toro, Pedro Jose, Pedro.Amoros@uv.es

An ionically controlled nanoscopic molecular gate has been developed by using functionalized mesoporous materials. The system shows that control of mass transport at nanometric scale can be achieved by using suitable rigid solids and pH-active molecules. The design principle suggests new perspectives in the search of ionically tuned tailored materials and devices with a fine control of mass transport for new applications in fields such as drug delivery, selective removal of toxic species, sensing, or catalysis.

A chromogenic chemosensor for selective pyrophosphate sensing in aqueous environments has been developed using silica functionalised surfaces.