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The use of gated mesoporous silica solids as suitable systems for controlled-release protocols is reviewed. These materials are based on mesoporous silica supports that can be prepared with tailor-made pores of around 2 - 10 nm and that show a very large specific surface area (up to 1200 m(2)/g), thus having a large load capacity. The solids can be additionally functionalised in the external surface with gate-like systems that can be opened on command to allow cargo release. Light, redox reactions, pH, temperature, polarity and enzyme-driven protocols are shown. The possible application in drug delivery protocols is discussed.

We report the development of an MCM-41 mesoporous support that is functionalised with saccharides at the pore outlets and contains the dye [Ru(bipy)(3)](2+) in the pores (solid S1; bipy = 2,2'-bipyridyl). For this hybrid system, the inhibition of mass transport of the dye from the pore voids to the bulk solution in the presence of borate is demonstrated in water at neutral pH. The formation of the corresponding boroester derivative is related to the selective reaction of borate with the appended saccharides. This control is selective and only anion borate, among several anions and cations, can act as a molecular tap and inhibit the delivery of the entrapped guest. Additionally, the S1-borate system behaves as pH-controlled gatelike scaffolding. This pH-responsive release can be achieved in an acidic pH (due to hydrolysis of the boroester), whereas the system remains closed at neutral pH. Molecular dynamic simulations using force-field methods have been made to theoretically study the open/close borate-driven mechanism. A mesoporous silica structure was constructed for this purpose, taking the plane (1-11) of the beta-cristobalite structure as a base on which hexagonal nanopores and anchored saccharide derivatives were included. The final model shows a highly flexible nanopore diameter of approximately 12.5 A of similar size to the [Ru(bipy)(3)](2+) complex (ca. 12 A). However, the anchoring of borate to the appended saccharides results in a remarkable reduction of the pore size (down to ca. 6.4 A) and a significant constraint in the flexibility and mobility of the saccharides. The theoretical calculations are in agreement with the experimental results and enable visualisation of the functional borate-driven dye-delivery-inhibition outcome.

This paper proposes a new nanoscopic molecular movable gate-like functional hybrid system consisting of nanoscopic MCM-41-based material functionalized onto pore outlets with a saccharide derivative capable of interacting with boronic acid functionalized gold nanoparticles (AuNPs) acting as nanoscopic caps. The gating mechanism involves the reversible reaction between polyalcohols and boronic acids to form boronate esters. Functionalized AuNPs thus act as a suitable nanoscopic cap via the reversible formation of the corresponding boroester bonds with the saccharide derivative anchored on the external surface of the mesoporous silica-based solid. The developed platform operates in aqueous solution and can be triggered by two simple external stimuli such as pH changes or light. The hydrolysis of the boroester bond takes place at pH 3, which results in rapid delivery of the safranine cargo from the pore voids into the aqueous solution. However, at pH 5 the pores are capped with nanoparticles and the delivery is strongly inhibited. The kinetics of the delivery was studied at pH = 3, assuming a simple diffusion process and that the kinetics of guest release from the pore voids of the hybrid material can be explained by the Higuchi model. It is possible to deliver the cargo in small portions by carrying out on-off aperture cycles via changing the pH from 3 to 5. AuNPs also open the possibility of employing light as a suitable stimulus for release procedures using the AuNPs' capacity for raising their temperature locally by absorption of laser light. The plasmonic heating using a Nd:YAG laser at 1064 nm results in the cleavage of the boronic ester linkage that anchors the nanoparticles to the surface of the mesoporous silica-based material, allowing the release of the entrapped guests. Further studies also demonstrated that it is possible to fine-tune the amount of cargo delivered by simply controlling the laser irradiation opening the possibility to designing laser-induced pulsatile release supports.

A study on the controlled release of vitamin B2 in pure water from mesoporous silica-based materials containing a pH- and anion-controlled nano-supramolecular gate-like ensembles built up by anchoring suitable polyamines on the external surface is reported (solid S1). This solid contains the vitamin (the delivered molecule) onto the pores, whereas the amine-based gate-like ensemble is anchored on the pore outlets. To obtain solid S1 the mesoporous MCM-41 support was first synthesized using tetraethyl orthosilicate (TEOS) as hydrolytic inorganic precursor and the surfactant hexadecyltrimethylammonium bromide (CTAB) as porogen species. Calcination of the mesostructured phase resulted in the starting solid. Then, first the vitamin and the latter an excess of 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane were added to the suspension containing the MCM-41 scaffolding and stirred. Solid S1 was characterized using standard solid state procedures. It was found that the functionalization process and the inclusion of the vitamin on the pores do not modify the mesoporous structure of the starting material. Delivery studies in water were carried out at pH 2 and 7. At pH 2 all the anions studied (sulfate, phosphate, GMP and ATP) strongly hinder vitamin release (Canion = 1 × 10− 2 mol dm− 3), whereas at pH 7 the delivery was observed for sulfate and GMP whereas the gate remained closed in the presence of ATP and phosphate. Selective delivery at neutral pH and no-liberation in acidic conditions can also be controlled with ATP and GMP using a suitable concentration of anion. The remarkable anion-controllable response of the gate-like ensemble at a certain pH can be explained in terms of anion complex formation with the tethered polyamines. Finally, selectivity patterns have been discussed in terms of kinetic rates of vitamin B2 release. The pH-controlled gate-like scaffoldings on S1 might be a suitable prototype for the development of orally applicable delivery systems designed to have the particular ability to protect the cargo from the acidic conditions of the stomach (acid pH, gate closed) but will release the load at the intestine (basic pH, gate open).

The reaction of copper(II) sulfate, copper(II) chloride, 3,5-diacetylamino-1,2,4-triazole, and 3-acetylamino-5-amino-1,2,4-triazole in water yields green, plate-shaped crystals of [[{Cu3(mu3-OH(1/2))L(H2O)2Cl}2{mu-Cu(H2O)2Cl2}].12H2O]n (1), where L is a new triazole-derived macrocyclic ligand. The structure of 1 consists of heptanuclear (H)OCuII(3)-CuII-CuII(3)O(H) entities linked in pairs through symmetric mu3-O...H...O-mu3 hydrogen bonds to form a double-stranded one-dimensional network. A significant overall antiferromagnetic behavior has been observed for 1.

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

Silica nanoparticles functionalized with chromogenic spirobenzopyran and thiourea subunits show selective colour changes in the presence of certain long-chain carboxylates.