Claudio Rottman

Doped sol-gel glasses as chemical sensors☆

Abstract It is reported that by trapping suitable analytical reagents, porous sol-gel glasses can be used for the preparation of a wide variety of chemical-sensing materials. The key to the successful preparation of these materials is based on the observation that a substantial fraction of the trapped molecules is exposed to the liquid or gaseous intra-pore volume. The glass matrix thus serves as a solid support for analytical reagents which interact with analyzates present in the adjacent phase. The phenomenon is quite general, and has been demonstrated for a variety of color tests for metal cations, proton (pH indication), anions, and organic molecules.

Doped sol-gel glasses as pH sensors

Abstract A series of pH indicators were trapped in sol-gel porous glasses by polymerization of tetramethoxysilane in the presence of a surface active agent. The properties of these novel sensing materials including spectral shifts, shifts in the pH-sensing range, cycle repeatability, leachability, rates of response and isosbestic points are described. A prototype of a pH meter based on a pH-sensing glass was constructed.

Advanced Sunscreens: UV Absorbers Encapsulated in Sol-Gel Glass Microcapsules

Undesirable phototoxic and photoallergic reactions accompanying a justified increased use of sunscreen active ingredients within cosmetic products have encouraged the development of new products safer for human use. The sol-gel microencapsulation technology developed utilizes an interfacial polymerization process, allowing for the achievement of transparent silica glass microcapsules with sizes ranging between 0.3–3 microns and a characteristic core-shell structure. Within the sol-gel microcapsule structure a UV absorber core, constituting roughly 80% of the final product weight, is enclosed within a silica shell. These advanced sunscreen actives are then incorporated into a suitable cosmetic vehicle to achieve high Sun Protection Factors (SPF), while affording an improved safety profile, as the penetration of the UV absorbers is markedly reduced.

Sol-Gel Physical and Covalent Entrapment of Three Methyl Red Indicators: A Comparative Study

We report the results of a comparative study of the properties of three azo dyes from the methyl red indicator family, which were coentrapped with the surfactant cetyltrimethylammonium bromide within tetramethoxysilane-derived silica sol-gel matrices. Characteristic parameters studied were spectral shifts, shifts in indicating ranges and in pKi values, isosbestic-point behavior, changes in the titration curves and stability to leaching. Physical entrapment of methyl red and para-methyl red (pMR) is compared to the covalent entrapment of a trialkoxysilane derivative of methyl red. High stability for leaching was found for all three entrapped indicators. The pKi change in entrapped pMR is interpreted in terms of intracage motion induced by pH changes.

Recent Developments in Organically Doped Sol-Gel Sensors: A Microns-Scale Probe; Successful Trapping of Purified Polyclonal Antibodies; Solutions to the Dopant-Leaching Problem

We describe recent advances made in our laboratories in the general field of organically and bio-organically doped sol-gel sensors. The developments described are: (a) The first miniaturization of a sol-gel sensor down to the microns scale, with potential applications to near-field optical microscopy, using a fluorescent pH-indicator. (b) The first successful sol-gel encapsulation of purified polyclonal antibodies, and in particular an anti-nitroaromatics immunoglobulin, with which selective sensing of nitroaromatics, an important class of environmental pollutants, was demonstrated, (c) The leaching problem, occasionally encountered in doping procedures, is solved by two methodologies: First, TMOS polymerization at high acidity and low water content was found to result in non-leachable yet reactive matrices, as demonstrated with O 2 sensing by excited state pyrene and with H + sensing by excited state pyranine; and second, doping with molecules capable of forming a covalent bond within the encapsulating cage results in the permanent anchoring of the dopant. Thus, Methyl-Red, a pH indicator, was derivatized with a silylating residue, and a polymerizing TMOS was doped with it forming a pH-shifted indicator. With both methodologies, leachability was practically zero.

The Evolution of Microstructure in Nonhydrolytic Alumina Xerogels

The effect of drying, aging and thermal treatment of alumina xerogels prepared by the nonhydrolytic route was investigated using SAXS, BET and HR-SEM techniques. The microstructure of the fresh xerogels prepared under different procedures varied drastically, ranging from aerogel-like mass fractals to narrow pore size distribution materials. By variation of the drying conditions the N2-BET surface area was varied from an immeasurable low level up to 600 m2/g. The initial microstructure has a significant influence on the xerogel behaviour during the post-drying heating stage. The ability to produce aerogel-like mass fractal materials from the nonhydrolytic systems is discussed. Finally, a brief theoretical treatment of the drying process of mass fractals is presented as well.

Effects of water/silane r-ratio and humidity on properties of sol-gel-entrapped indicators

When entrapping indicators in sol-gel materials for optical sensing purposes, one should be careful to take into account that changes in the cage properties, caused by variations in the synthetic procedures, might affect the performance of the dopant. We show - for SiO2 sol-gel entrapped pH and polarity indicators - that such variations are brought about by changing the water-silane r-ratio preparation conditions and by changing the environmental humidity to which the material is exposed. The observations are interpreted in terms of changes in the acidity and polarity of the cage surface.

Photometric sensors based on sol-gel porous glass doped with organic reagents

A new class of sensitive disposable sensors for determination of trace concentrations of water pollutants has been developed. It utilizes porous transparent silica glasses doped with organic photometric reagents. The silica sensors are produced at room temperature by the sol-gel procedure, i.e. by hydrolysis followed by polycondensation of tetraalkoxysilanes. Thus, we produced porous glasses doped with organic photometric reagents for the determination of typical cations, anions, pH, oxidation agents (e.g. disinfection agents) as well as typical air pollutants. A mathematical model for a flat plate monolithic iron sensor was developed and the predicted calibration curves were compared with the experimental response.

Dowex® 1-supported PtCl4 ion pair as a recycle hydrogenation catalyst

Abstract Polystyrene-supported ion pairs, generated from PtCl 4 and Dowex® 1 anion exchangers in EtOH, were shown to be leach-proof and recyclable catalysts for hydrogenation of alkenes and for other unsaturated compounds at