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RESEARCH PROFILES
Vibrations in situ The body has a nasty habit of treating foreign objects—even biosensors that are meant to be helpful—as intruders. To protect sensors such as ion-selective electrodes that measure electrolytes in the blood, polymers are frequently used to coat the surface and plasticizers are often added to modify the polymers. Plasticizers act on a polymer to reduce the amount of interchain bonding, making the plastic softer and, in the case of ion-selective electrodes, allowing small molecules to travel more easily between the polymer chains and reach the electrode. But plasticizers have proved to be a double-edged sword; although they increase the sensitivity of the biosensor, they also provoke a greater biological response from the body’s immune system than the polymer alone. In the July 15 issue of Analytical Chemistry (pp 3725– 3280), Matthew Clarke, Jie Wang, and Zhan Chen at the University of Michigan apply sum frequency generation vibrational spectroscopy (SFG) to investigate the role of plasticizers at the boundary between the polymer and air, water, and a bovine serum albumin solution. Studying the interaction between the polymer, plasticizer, and the air or liquid in contact with the polymer has proven difficult. “High vacuum is used for most other surface analyses,” says Chen. Because high vacuum is not a very natural environment, he says, “you can’t really tell [about biological compatibility] until you do experiments in water.” The researchers use SFG to look at the junction between a polymer/plasticizer mixture and air or liquid in order to understand the system’s behavior in situ. Because SFG is an optical technique, according to Chen, it can be used under ambient conditions, is nondestructive, and provides good spatial and temporal resolution. “SFG selectively detects vibrations from molecules at the interface between two materials 336 A
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(a) Diagram of how SFG operates. SFG spectra of polyurethane, DOS/polyurethane blends, and DOS-coated polyurethane in (b) air and (c) water. Surface structures change upon contact with water for all surfaces.
or phases and generates vibrational spectra from these molecules or functional groups.” He adds, “I think SFG can be as powerful at looking at molecular interfaces as NMR has been for bulk studies.” The researchers studied polyurethane polymers with either dioctyl sebacate (DOS) or o-nitrophenyl octyl ether as plasticizers and compared the SFG spectra from samples with just polyurethane, polyurethane with DOS, and polyurethane coated with DOS. Previous experiments showed that DOS principally resides on the surface of the polymer. However, SFG showed that the spectra from samples in air were significantly different than those in water. Chen says, “In the water and protein
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solution, the plasticized polyurethane surface changes the structure, which would not have been expected from the experiments in air.” However, the structure reverts back when the water samples are dried. The change in surface structure may contribute to the reduced biocompatibility that occurs when polymers are modified with plasticizers. In particular, the researchers found that albumin, which is thought to protect the biosensor from the immune system, adhered less to the plasticized surface. “What is really important,” says Chen, “is that we have an in situ technique, and detection of molecular structure in situ is very important in analytical chemistry.” a —Michael J. Felton
