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Invent your own barcode
All the coding schemes described herein are afflicted by one handicap or another. Thus while one can only marvel at the baroque elegance of Still¡¯s original coding scheme, he and his colleagues must have sweated blood to get it to work. While less unwieldy, the metallic and optical barcodes, do not, as Still¡¯s scheme does, inextricably link the code to what is on the particle. The digital DNA codes4-6 (see Box) are prone to contamination from templates not intended to be present in the labelling mixture. And so on. In short, there is plenty of room for other ideas.
One very simple possibility, suggested by isotope ratios and insect pheromone blends, is to vary rations and insect pheromone blends, is to vary the amounts of just two different substances. This will not lead to very many codes as it stands, but this scheme could be used in the same way that isotope ratios are used to identify the provenance of ancient timbers and samples of cocaine.7,8
There is no reason, either, why different pairs of substances should not be used.
Another possibility is to use the alternative configurations, R or S, of an asymmetric centre as the basis for a digital code. It is well known that some complex natural products ca have a significant number of a symmetric centres, and the framework of one of these molecules could be used for coding, with the code residing in a single molecule. Care would be residing in choosing a suitable framework, however, since in some cyclic natural products (eg steroids) ring formation will predispose some asymmetric centres towards one particular configuration. Alternatively, a complex code could be generated by connecting a small number of different monomers in a variety of different ways, in the same manner as branched carbohydrates.
Perhaps the scheme with the greatest coding potential, however, is a tiny-dimensional array of several different coloured quantum dots. Fixing four different coloured dots to a 10¡Á10 array, for example, would generate over 2000m patterns.
These are just three straightforward examples. I would bet money that there are other miniature barcoding schemes out there waiting to make more jaws drop in astonishment.
Acknowledgements: I am grateful to Alan Armstrong for telling me about the Still-Ohlmeyer incident and Michael Natan for telling me about how the metallic barcodes came about. Laurie Peter, Chris Flost and other colleagues in the chemistry department at Batch kindly discussed various aspects of miniature barcodes with me, and Pooja Kumat made helpful comments on the manuscript.
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