Fireworks for the information age.

W hat is the information content of a molecule? What is the data transmission rate associated with a chemical reaction? In contemplating such questions, Thomas et al. in this issue of PNAS (1) arrive at an interesting prototype device that stores and retrieves information by using only chemistry and chemical processes. This combination of information technology and chemistry, in a new field that Thomas et al. call ‘‘infochemistry,’’ provides unusual strategies for communication and data storage with the potential for important practical applications. Information processing is central to nearly all forms of modern technology. The most well-established systems for data storage and retrieval are based on electronic, photonic, and/or magnetic phenomena. Flash memory, for example, uses the control of charge on a f loating gate to modulate the threshold voltage of a metal oxide field effect transistor, as a means for writing and reading information (2). Hard drives use inductive/magnetoresistive effects to record/extract data in the form of spatial patterns of magnetization on a spinning disk (3). Magneto-optical drives (4) manipulate magnetization at somewhat larger scales with heating induced by focused light and uniform magnetic fields. Information retrieval occurs by monitoring changes in ref lectivity, much like a purely optical drive (5). Some of the newest and most powerful technologies use holograms written into polymer films to achieve unprecedented storage densities and retrieval rates (6). Although these and related approaches offer spectacular levels of functionality at very low cost per bit, there may be opportunities for alternatives; biology, where information is encoded in base-pair sequences of DNA and transmitted via RNA, represents a compelling existence proof. This bio-inspired idea of using molecules themselves as a form of information technology could lead to entirely new classes of devices. Early work on DNA computers, in which base-pairing in large numbers of DNA provide parallel routes for examining many outcomes for certain classes of problems, might be viewed as among the first attempts to exploit such concepts in a technology (7). More recent, related embodiments provide paths in which input and output information is in molecular form; devices that monitor and respond to biomolecular species represent impressive examples (8). These approaches, however, focus on information processing, rather than storage and communication; they are also slow and often require nonchemical means to program the systems and read out the results. Although the ideas of infochemistry introduced in Thomas et al.’s article (1) have some features in common with previous work, the emphasis is instead on outcomes that provide direct alternatives to the sorts of devices described in the previous paragraph. Flares are the simplest abiotic embodiments of the concepts of infochemistry, where chemical reactions transmit information on location and serve as distress signals. Fireworks can be considered as a similar class of technology, in which the transmitted messages are abstract, but valuable nevertheless. Thomas et al. (1) show how advanced chemistries and materials processing technologies can yield qualitatively more sophisticated versions, capable of storing and communicating alphanumeric messages, in formats that are compatible with further scaling. In particular, they demonstrate their ideas of infochemistry in a type of device that they call an ‘‘infofuse’’: an ink jet-printed pattern of different alkali metal salts on a thin strip of nitrocellulose (NC). Igniting an end of such a structure leads to controlled burning that thermally excites atomic emission from the metals to produce short pulses of colored light in specific, programmed time sequences. The chemistry of the salts and their spatial