David Avnir discusses his recent hot paper with Journal of Materials Chemistry.

Could you explain the significance of your article to the non-specialist? And what has motivated you to conduct this work?

It is well known that molecules can be chiral, that is, they can appear in left-handed or right handed versions. The essence of the chemistry of life, is much due to that structural property. Less known is that materials - that is, large assemblies of small molecules or of polymers - can appear as left-handed or right-handed objects as well. A celebrated example is the crystal of quartz, which appears in nature in both forms. A major challenge in materials chemistry has been to develop methods with which chirality can be induced in materials, and then use these resulting chiral materials for useful processes. These useful processes are chiral catalysis, with which important chiral drugs are synthesized (for instance, the antihypertensive drug Propranolol); chiral chromatography, with which the two forms - the enantiomers - can be separated; and chiral sensing, with which the purity of the desired handedness is evaluated.

How then can chirality be induced within any material, organic or inorganic? A powerful approach known as "Molecular Imprinting" has been to synthesize the material in the presence of a small chiral molecule (the 'template'), and then remove it from the material when ready. If successful, then the removal process leaves behind tiny holes - pores - which have the negative shape of the templating molecule; and the template is chiral, so must be the hole it leaves behind. The material for which this approach has been applied in this study is silica, derived by a polymerization process known as the 'sol-gel process'. 

This approach was shown to work for the sol-gel derived silicas, and the question has been - why? To answer that, the research focused on understanding the interactions between the template, and the forming material - namely the polymerizing silica. This focusing has required sophisticated analytical tools that enable one to probe, on a molecular level, those special chiral interactions within the silica. "Sophisticated" because the analytical tools need to be able to distinguish between left handed and right handed interactions within the material. Two molecular properties were used for that purpose, for the first time in the context of chiral silicas: The magnetic response of the atoms (nuclear magnetic resonance analysis); and the polarized light absorbance of the template (induced circular dichroism).

What did we see? We were able to see the left-handed/right-handed interactions on a molecular level, and evaluate their intensity; and, quite unexpected, we provided evidence that the template produces not only chiral holes, but that the backbone of the material, silica, becomes chiral!

Where do you see this work developing in the future? 

The main future work needed in this field is the development of large scale, simple and cheap processes of the production of chiral silicas by the templating method. It is a potentially economic method, because one can use again and again the same small amount of templating molecules, to produce large quantities of a chiral material. The current approach is to anchor to the surface of silicas chiral molecules, and this requires a large amount of chiral molecules, which are never re-generated. Furthermore, one expects that chiral porosity will be more stable than chirality which is due to anchored molecules.

David Avnir, shown here with a friend, is a chemistry professor at the Institute of Chemistry, The Hebrew University of Jerusalem, where he received all his academic education. His current scientific activities include experimental studies in sol-gel materials, experimental studies in organically doped metals and theoretical/computational studies in symmetry and chirality. Earlier major interests included fractal theory and far-from-equilibrium phenomena. He has co-authored over 290 scientific papers on these topics, five of which are on the list of the 200 most cited chemistry papers for 1982-2002. In addition, he holds a number of key patents on doped sol-gel materials. He serves as the Chairman of the Board of the International Sol-Gel Society. 

	Sharon Fireman-Shoresh recently received her Ph.D. from the Hebrew University of Jerusalem. Her thesis, "Chirality in sol-gel materials", is a comprehensive work which describes three main approaches to induce chirality in sol-gel materials. She received her B.Sc. (1998) and M.Sc. (2000) degrees also from The Hebrew university of Jerusalem.
Sharon Marx is a research scientist in Israel Institute for Biological Research, in the department of Physical Chemistry. She received her Ph.D. from the Hebrew University in Jerusalem at 1996. Her research interests are sensors for small molecules based on molecularly imprinted sol-gels, biosensors and one-dimensional polymer structures.