Mukti S. Rao

Efficient sequestration and reduction of hexavalent chromium with organosilica sol–gels

Sol–gel materials prepared from organically-modified silanes are used for sequestration and reduction of Cr(VI). These sol–gels act as matrices for sequestration and reduction of Cr(VI) as well as binding of Cr(III) ions in the porous structure. Acid treated sol–gels obtained from bis[3-(trimethoxysilyl)propyl]ethylenediamine (EnTMOS) and 3-mercaptopropyltrimethoxysilane (SHTMOS) sequester and reduce Cr(VI) from an external solution. The removal of Cr(VI) from solution was monitored spectrophotometrically and the sol–gels were characterized with FTIR and TEM. The amino and thiol groups in the hybrid sol–gels play a critical role and the gels containing both these functional groups are considerably more effective at removal of Cr(VI) than those containing just one of these functional groups. Finally, the pH of the medium is also found to be an important parameter in kinetics and efficiency of removal of Cr(VI).

Magnetic Behavior of Iron-Oxoclusters Prepared in an Organosilica Sol–Gel Matrix

The crystal structure and magnetization of nanoscale enTMOS–Fe2O3 sol–gel composites with weight iron concentration x, varying from 0.003 to 0.065, have been studied by the transmission electron microscopy technique and a superconducting quantum interference device magnetometer. The clusters are crystallized in a hexagonal crystal structure. All the samples demonstrate a superparamagnetic behavior with antiferomagnetic cluster–cluster coupling at low temperature. The effective paramagnetic moment, μeff, has been found to vary in the range from 5.9 (S=5/2) to 2.5 μB per iron ion. The concentration dependence of the μeff shows a minimum for x∼0.01. At a low iron concentration x<0.01, μeff is practically independent of x and equals about 6 μB per Fe ion. The concentration interval 0.01<x<0.07 is characterized by a monotonical increase of μeff from 2.5 to about 3 μB per Fe ion. Thus, an abrupt variation of μeff (about two times) is observed at x≈0.01. It has been shown that such behavior can be caused by comp...

A Generic Synthesis Route for Matrix Assisted Self-Assembly of Metal-Oxo Clusters in Organosilica Sol-Gels

The use of an organically modified silica sol-gel matrix containing ligating groups that assist in the formation of metal-oxo clusters is described. The metal-ligand interactions can be tuned by change in external environment such that: a) under overload metal ion concentrations, the sol-gel matrix sequesters the metal ions to promote the growth of 4-7 nm size metal-oxo clusters within the porous structure of the matrix, b) a change in pH releases the metal ions from the bound metal-oxo clusters, thereby making the sol-gel matrix behave as ferritin analogues. These characteristics of the matrix assisted self-assembly of metal-oxo clusters are shown using UVVis, TEM, AES, and EDX data.

Molecular Design of Organically-Modified ‘Smart’ Sol-Gel Materials

The design, synthesis and stimuli-responsive behavior of organically-modified silicates (ORMOSILS) is reported. The ORMOSILS behave as ‘smart’ materials by undergoing structural changes at the molecular level to respond to changes in environmental variables. Typical responses in the form of swelling/shrinkage with respect to changes in temperature, pH, and salt concentrations are presented. Finally, the nature of predominant mechanism responsible for the sol-gels behavior towards combined stimuli of high temperature and low pH is discussed.