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Diabetics must currently take a blood sample a few times a day. This can be unpleasant for the patient, and it does not accurately convey the continually changing blood glucose levels that can lead to hypo- or hyperglycemic episodes. Thus, minimally invasive implantable glucose sensors that can provide continuous monitoring are sought. Optical-based glucose sensing presents a promising strategy for such a device.
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PET sensors. A schematic of how photoinduced electron transfer provides a means of analyte (e.g., glucose) detection, and the structure of the rhenium bipyridine boronic acid derivative used in this study.
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In this direction, scientists from the Lawrence Livermore National Laboratory (Livermore, CA) and Minimed, Inc. (Sylmar, CA), are looking to use transition metal complexes, such as rhenium bipyridine compounds, that can monitor glucose levels based on fluorescence. The sensing mechanism would rely on the process of intramolecular photoinduced electron transfer (PET, see figure), in which part of a molecule is photoexcited and its fluorescence is quenched by an electron donor species also present in the molecule. If another chemical species, such as glucose, specifically binds to the “surplus” electrons of the donor, then the quenching mechanism is limited and a fluorescence increase will be indicative of the glucose concentration.
In a recent study, the researchers synthesized three different rhenium bipyridinecompounds and characterized them by fluorescence and electrochemical analysis (Inorg. Chem. 2002, 41 (6), 1662–1669). The boronic acid derivative, shown in the figure, had the highest PET driving forces in the longer-wavelength portion of the visible spectrum, which shows the greatest transmission through skin. In a pure methanol solution, upon increasing glucose concentration from 0 to 400 mg/dL (the region of physiological interest), a 55% increase in fluorescence intensity was observed for this complex, indicating a significant sensitivity to sugar level. However, in a methanol–saline solution, which is closer to biological conditions, this compound failed to show a large rise in intensity over the same range. The group is now seeking related molecules that are effective in aqueous solvents.
The overall goal is to find a glucose-sensing complex that can be attached to a membrane for implantation under the skin.
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