fulltext10 - Toward Single-metal-ion Sensing by Forster...

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Toward Single-metal-ion Sensing by F¨orster Resonance Energy Transfer J ENS U. S UTTER , A LEXANDER M. M ACMILLAN , D AVID J. S. B IRCH , AND O LAF J. R OLINSKI Photophysics Group, Department of Physics, University of Strathclyde, Glasgow, Scotland, United Kingdom Here we describe progress toward our objective of detecting single nonfluorescent hydrated metal ions. Single-ion detection represents detection and spectroscopy at the ultimate sensitivity level of approximately 1.6 × 10 24 M. Achieving this goal would provide a breakthrough in analytical science and allow much more detailed insight into sensor–ion interaction than that available with conventional bulk detection methods. We combine recent advances in confocal microscopy with the sensitivity and the noninvasive nature of fluorescence by analyzing F¨orster resonance energy transfer between sensor fluorophores and transition metal ions. Key words: ion sensing; F¨orster resonance energy transfer (FRET); quantum dots Introduction Transition Metal Ions Transition metal ions play an important role in bi- ology as nutritional microelements as well as impor- tant ligands in proteins and small molecules. Uptake of these ions by a living organism is small compared with the flux of bulk nutrients, such as potassium, sodium, or calcium. The concentration of transition metal ions in water, soil, or tissue is small, and selective detection is difficult. With cobalt, chromium, copper, iron, man- ganese, and molybdenum, half of the essential mineral micronutrients are within the group of transition metal ions. The important cellular labeling metals osmium and gold fall within this group as well. Many elements within the group play important roles as contaminants in water or soil. Sensing transition metal ions in biological systems by chemical methods proves to be difficult for con- centrations that are small, and chemical sensing often interferes with the very process that one desires to mon- itor. 1 Light Absorption by Metal Ions When a transition metal ion interacts with one or more ligands, the electrons of the ligand as well as the d-orbital of the ion repulse each other. This repulsion raises the energy level of the electrons in the d-orbital and leads to a split into two distinct energy bands. Address for correspondence: Jens U. Sutter, Photophysics Group, De- partment of Physics, John Anderson Bldg., University of Strathclyde, Glas- gow G4 0NG, Scotland, UK. [email protected] With the d-orbitals incompletely filled, absorption of photons can lift an electron between the d-orbitals. Thus, the ligands of the ion and its close environ- ment influence the split of the d-orbitals and the sub- sequent absorption spectrum. Under fully controlled experimental conditions, we can determine the ab- sorption spectrum by provision of the ligand (F IG . 1).
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