Professor A Prasanna de Silva

Image		Professor A Prasanna de Silva BSc (University of Colombo), 1976 PhD (Queen’s University of Belfast), 1980 Chair of Organic Chemistry Tel:     + 44 (0) 28 9097 4422 Fax:    + 44 (0) 28 9097 6524 E-mail: a.desilva@qub.ac.uk

Research Keywords Supramolecular science Molecular-scale photonic devices Fluorescent/luminescent sensors Switchable molecular systems Molecule-based logic Molecular switches Molecular computation

Publications

Research

These are interesting times for chemists. Single molecules are no longer fairy-tale creatures, since they are now available for direct interrogation by the ultrasensitive techniques of scanning probe microscopy and fluorescence spectroscopy. Therefore the onus is on us to provide suitable molecules to take advantage of these breakthroughs, while displaying important functions previously held exclusively in the realm of bulk matter. We are addressing this challenge by designing and constructing supermolecules which can gather or process information on our behalf. Fluorescent molecular sensors can gather information about atomic or molecular behaviour from environments of down to nanometre dimensions. We have helped to establish fluorescent PET (photoinduced electron transfer) sensing, a design principle of wide utility which has already led to successes in targeting several chemical species crucial in biological and medical contexts, e.g. protons, sodium and calcium. The design concept is straightforward. A fluorescent unit and a receptor unit are joined through a spacer module. The supermolecule so assembled loses its fluorescence capability owing to an inter-module photoinduced electron transfer quite similar to that seen in green plant photosynthesis. This PET process is arrested the moment the receptor module captures its target, e.g. a sodium ion, thereby switching the suppressed fluorescence back ‘on’. Thus the fluorescence signal measures the concentration level of the target species (Figure1).

A real-life application of our research has achieved considerable commercial success. The Fluorescent PET sensor design was used as the platform of a portable diagnostic tool – a blood gas analyzer for hospital critical care units and ambulances. The specific sensors for sodium, potassium and calcium were designed, synthesized and tested in collaboration with scientists at AVL Bioscience Corporation, Roswell, GA. The product was rolled out in 1997 and has been sold by AVL, Roche Diagnostics and OSMETECH in turn. The total sales of the sensor cassette is around 40M USD so far. Further information is available on the following website: http://www.osmetech.com (look under OPTI products)

Fluorescent PET sensors are modular, so that the system is flexible and expandable to meet various demands. Delayed luminescence phenomena such as molecular phosphorescence and lanthanide emission can be smoothly brought on-line to permit the successful addressing of intrinsically fluorescent environments. At the smaller end of the scale, fluorescent PET sensors are being outfitted with targeting units to permit their operation as submarine periscopes near water-membrane interfaces. Towards the larger end of the scale, these PET sensors are being anchored on submillimetre inorganic particles and embedded in organic polymer films to produce robust systems compatible with harsh industrial environments.

As the information technology revolution encroaches more and more into our lives, attention is turning to how the revolution started by silicon logic gates can be carried forward. Smaller-scale information processors can be one approach to which chemical solutions can be imagined. The first molecular logic gates were built in Belfast a few years back and a range of gates such as YES, NOT, AND, OR, NOR and INHIBIT are now available (Figure 2). These artificial systems use chemical inputs and light output, reversing the natural roles existing within the eye.

We have built on this breakthrough by generalizing and applying molecular logic. We are delighted that more laboratories around the world are also joining in this endeavour. One of our own contributions has been to persuade molecules to perform arithmetic operations. Small molecules can now add one and one to get two, just like children. It is clear that small molecules can perform small-scale computational operations in small spaces where semiconductors cannot go in spite of all their power.

 

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