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Analytical Chemistry and Chemical Composites
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start this editorial with my best wishes for the world of analytical chemistry and all it encompasses—academic science; industrial, pharmaceutical, biomedical, and environmental applications; and our students, leaders, followers, and high priests. May Good Chemistry be with you in 2007. I want to discuss composites: mixtures that, while retaining the properties of their individual chemical components, offer added desirable ones. In materials chemistry, composites are an old notion; for instance, the blending of carbon black into vulcanized tire rubber is a pre-20th-century process. The addition of carbon microfibers to the polymers used to fabricate fly rods, wind-turbine blades, and the airframes of stealth aircraft is a more modern example. Physical strengthening of the materials in some manner is one of many goals for polymer composites containing particles or fibers. Interactions of the surfaces of particles, nanoparticles, and fibers with their host polymer matrices are crucial determinants of composite properties. These interactions may be electrostatic or dipolar or may involve the formation of chemical bonds. Wrapping a polymer around a particle presents an unfavorable entropy loss for the polymer chain, and this loss must be offset by a strong interaction between the polymer chain and particles. Inadequately strong polymer–particle interactions can lead to particle aggregation and to failure to attain the desired composite properties. The aggregation tendency decreases with decreasing particle size. Many composites being investigated involve nanoscopic components such as metal and semiconductor nanoparticles and carbon nanotubes. This thrust of composite materials into molecular dimensions is lucidly discussed in a recent review (Science 2006, 314, 1107–1110). Designing desirable chemical properties, including those for analytical applications, is also possible using composites. This journal considers numerous manuscripts that include descriptions or uses of composite materials—films on electrodes, chromatographic stationary phases, and interfaces of films in surface plasmon resonance measurements, among others. The host materials are organic polymers or sol–gels, and they may contain carbon nanotubes to increase film conductivity and/or internal surface area, enzymes or other chemically reactive recognition chemicals, fluorophores that respond to some solute’s permeation into a film, or combinations of such © 2007 AMERICAN CHEMICAL SOCIETY
additives. It is evident that judicious choices of compositions are generally made by those who prepare analytically directed composites. In some studies, however, the composite design reminds me of the proverbial “kitchen sink”. At such times, I feel that analytical chemists would be well served to follow the materials-chemistry literature more carefully. An investment in the analysis of structure–composition– property relationships is as important to the analytical chemist as it is to the materials chemist. A paper in Macromolecules (2003, 36, 7199–7204) illustrates a thorough application of measurements to a new chemical material designed for composites. A wider range of analytical chemists should begin to make comparable investments in the study of their new, analytically directed composite materials. The topic of molecularly imprinted polymers (MIPs) was given impetus in 1990 by Klaus Mosbach and is an example of the rational evolution of design rules for composites that have a chemical purpose. MIPs are molecular composites that contain a guest substance; its purpose is to form a templated cavity that, after removal of the initial molecular guest by some means, exhibits a capacity for selective, reversible reincorporation of the guest component. Early MIP composites were used as chromatographic phases. The essential ideas of MIPs have since been extended to a broader range of uses in chemical-recognition phases, solid-state extractions, membrane permeation, chemical sensors, and even drug delivery. The most sophisticated, successful designs go beyond the cavity shape/size “fit” idea and involve coincorporation into the polymer of other chemicals known to undergo specific, reversible interactions with the guest. In these, it is interesting to see how the composite design rules that have emerged for analytical composite materials have converged with those developed by materials chemists for other purposes. Thus, there is a common importance of entropy effects in preventing guest aggregation and of designing chemical interactions into the relationship between the polymer host and the molecular or nanoparticle guests. Nonetheless, each discipline will usefully learn from the other as these important topics evolve.
J A N U A R Y 1 , 2 0 0 7 / A N A LY T I C A L C H E M I S T R Y
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