SCIENCE & TECHNOLOGY I L L U M I N A T I N G Belfast graduate student Gareth McClean (left) and postdoctoral researcher Manel Querol Sans examine the optical properties of molecular logic gates.
demonstrates the multiple logic behavior exhibited by the systems. 'A neutral solution of one of our chromophore-receptor compounds passes UV light but blocks blue light virtually completely" he says. "However, the addition of calcium salt to the solution causes the UV light to be blocked and the blue light to be passed. So the change of calcium input from 0 to 1 causes the passage of blue light to go from 0 to 1, which is YES logic. On the other hand, the same change causes the passage of UV light to go from 1 to 0, which is N O T logic."
INDICATORS REVEAL INHERENT LOGIC Ion-indicator dyes can be persuaded to carry out different logic functions at the same time MICHAEL FREEMANTLE, C&EN LONDON
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wavelengths of light to look at the ion-indicator action of certain dyes, chemists in Northern Ireland have unearthed various types of inherent molecular logic in the dyes. Chemistry professor A. Prasannade Silva and postdoctoral associate Nathan D. McClenaghan at Queen's University, Belfast show that compounds consisting of a light-absorbing chromophore integrated with an ion receptor exhibit logic gate configurations that depend on the wavelength of the transmitted light [Chem. £ ^ 8 , 4 9 3 5 (2002)}. "This is the first example of a common ion indicator showing logic gate activity and, more importantly, showing multilogic activity," de Silva tells C&EN. "Ifyou simultaneously shine two beams of light with different wavelengths on the indicator solution, you obtain two different types of logic gates —for example, YES and NOT— simultaneously" YES logic is obtained when a high input
(1) gives a high output (1), and a low input (0) gives a low output (0), whereas N O T logic occurs when inputs of 1 or 0 result in outputs of 0 or 1, respectively. YES and N O T are two of the four types of single-input logic. The other two are PASS 0, which gives an output of 0 when the input is 0 or 1, and PASS 1, which gives an output of 1 when the input is 0 or 1. The new molecular logic systems developed by de Silva and McClenaghan use ions such as Ca2+ as inputs. The outputs are the light transmittance values of the indicators at specif- De Silva ic wavelengths. "The calcium-dependent spectra of our systems are reminiscent of the pHdependent spectra of common pH indicators such as methyl orange," de Silva observes. He describes a simple experiment that
THE SIMPLEST WAY to carry out such an experiment, he explains, is to illuminate the sample with white light and examine it through colored glass filters from different angles while the ionic inputs are being applied. "Looking at the sample from various angles provides an easy way to separate the different signal channels in space at the same time," de Silva explains. A more sophisticated method, he adds, involves sending a beam of white light through the sample into an optical multichannel analyzer, which records the transmittance at several chosen wavelengths. "Up to four light beams give up to four types oflogic simultaneously with some of our compounds," de Silva says. "Polychromatic light allows observation in many distinct channels. Whereas there are only four single-input logic types, there are 16 and 256 two-input and threeinput logic types, respectively The limitation for us will be whether the inputinduced spectral changes are large enough at a number ofwavelengths. Since absorption bands are quite broad, we cannot expect too much. The spectral changes will depend on the spécifie structure of the indicator molecules." He compares the multiple logic behav-
De Silva's molecular logic gates can be considered as the first elements for the development of a chemical computer.' HTTP://WWW.CEN-ONLINE.ORG
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SCIENCE & TECHNOLOGY ior of indicators w i t h t h a t of q u a n t u m computers. "Simultaneous occurrence of strings of input bits is important in quantum computation," he explains. " Q u a n t u m computers are thought to operate simultaneously on all possible combinations of a quantum bit (qubit) string. O u r simple indicators n o w show superposition of logic gates, w h e r e a s quantum computation involves superposition of input bit strings." D e Silva's work demonstrates how far chemists have progressed in the control of matter, notes Luigi Fabbrizzi, chemistry professor at the University of Pavia, in Italy "In de Suva's earlier reports on molecular logic, he used fluorescence as the output," he comments. "Now, color has been added as a further output. D e Silva's molecular logic gates can be considered as the first elements for the development of a chemical computer." Vincenzo Balzani, chemistry professor at the University of Bologna, in Italy points out that there are two approaches to the design and construction of molecular-level logic systems.
simple arithmetic operations (C&EN, May 1,2000, page 12). "De Silva is a pioneer in t h e field of logic systems based o n switching molecular properties by external inputs," Balzani says. "In his most recent paper, de Silva shows that the scope of molecular logic can further be extended just by considering the ability of molecules to exploit the inherent multichannel nature of polychromatic light. T h e paper offers an outstanding example of the novel conceptual interpretation of well-known chemical processes."
These artificial systems reverse the natural role of the eye, which uses light inputs and chemical outputs.
"ONE APPROACH employs molecules as wires and diodes to obtain miniaturized electric circuits t h a t p e r f o r m as logic gates," he says. "The other approach uses the ability of molecules to convert chemical, photochemical, or electrochemical input stimulations into output signals, such as absorbance and transmittance, fluorescence intensity, and redox potentials. "The two approaches differ not only with respect to the chemical structures of the molecules on which they are based, but also with respect to the chemical environment— solid state and solution, respectively—and the nature of the input and output signals," he continues. "They are also different from a philosophical viewpoint. Molecular-scale circuits are devices for solid-state electronics, whereas those based on switching molecular properties work on chemical principles that are more similar to those ruling information transfer in living organisms." Balzani observes that, in previous work, de Silva and colleagues developed molecules and supramolecular species that perform as single logic gates. D e Silva's group also showed that two distinct chemical systems can be operated in parallel to carry out 32
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part of conventional analytical chemistry" de Silva says. "They will also allow molecules to perform more complex logic operations than hitherto demonstrated." DE SILVA and coworkers note that because their molecular logic systems are solution based, they do n o t employ connecting wires and are therefore unlikely to integrate with traditional solid-state systems [Chem. Comm., 2 0 0 2 , 2 4 6 1 } . "Nevertheless, it maybe possible in the future to design 'wet' computers that work more like the brain, relying on membranebound molecular processors similar in nature to ours," they suggest. T h e y point out that their artificial systems reverse the natural role of the eye, which uses light inputs and chemical outputs. W e t molecular logic systems may be used not only for information processing
T h e compounds used by de Silva and McClenaghan for the superposition of logic gate configurations are internal charge-transfer chromophore-receptor systems. W h e n metal ions are added to a system, its electronic absorption spect r u m shifts t o s h o r t e r wavelengths (known as the blue shift), resulting in transmittance values at specific wavelengths being switched on or off. T h e chromophores are cyaChromophore^ nine-dye moieties, and the receptors are chelating amino acids that bind calcium ions and other alkaline earth ions. T h e range of logic gates can be extended by Absorbance incorporating a second receptor site that can be selectively protonated within the chromophore. The Belfast team showed, for e x a m p l e , t h a t t w o - i n p u t X O R logic operation is possible with a receptor-chromophorereceptor system using Ca 2+ and H + as inputs. An X O R logic gate has an output of 1 only if both inputs are digitally different (1 and 0,or0andl). Wavelength, nm T h e t w o c h e m i s t s also demonstrated that the systems M U L T I C H A N N E L Absorption spectrum can be configured by selecting a shifts to shorter wavelengths when Ca2+ suitable excitation wavelength, ions are added to the indicator, allowing to operate in fluorescence mode different logic operations at four rather than transmittance mode. wavelengths. In one such experiment, they configured a system to display two-input N O R logic using Ca 2+ b u t also for information gathering, the and H + as inputs and fluorescence emisgroup adds. They might be used in medical sion as output. T h e output of an N O R g a t e applications, for example, as rniniaturized is 1 only when both inputs are 0. diagnostic systems that work under phys"Our superposed systems allow moleciological conditions. • ular logic behavior to permeate into a large HTTP://WWW.CEN-ONLINE.ORG
