A series of U(IV) complexes, ((R)ap)2U(THF)2 [R = tert-butyl (t-Bu) (1), adamantyl (Ad) (2), diisopropylphenyl (dipp) (3)], supported by the redox-active 4,6-di-tert-butyl-2-(R)amidophenolate ligand, have been synthesized by salt metathesis of 2 equiv of the alkali metal salt of the ligand, M2[(R)ap] [M = K (1 and 2), Na (3)], with UCl4. Exposure of these uranium complexes to 1 equiv of PhICl2 results in oxidative addition to uranium, forming the bis-(4,6-di-tert-butyl-2-(R)iminosemiquinone) ([(R)isq](1-)) uranium(IV) dichloride dimer, [((R)isq)2UCl]2(μ(2)-Cl)2 [R = t-Bu (4), Ad (5), dipp (6)]. The addition of iodine to 1 forms ((tBu)isq)2UI2(THF) (7), while the reactivity of I2 with 2 and 3 results in decomposition. Complexes 1-7 have been characterized by (1)H NMR and electronic absorption spectroscopies. X-ray crystallography was employed to elucidate structural parameters of 2, 3, 5, and 7.

Single-particle analysis of biosensors that use charge transfer as the means for analyte-dependent signaling with semiconductor nanoparticles, or quantum dots, was examined. Single-particle analysis of biosensors that use energy transfer show analyte-dependent switching of nanoparticle emission from off to on. The charge-transfer-based biosensors reported here show constant emission, where the analyte (maltose) increases the emission intensity. By monitoring the same nanoparticles under various conditions, a single charge-transfer-based biosensor construct (one maltose binding protein, one protein attachment position for the reductant, one type of nanoparticle) showed a dynamic range for analyte (maltose) detection spanning from 100 pM to 10 μM while the emission intensities increase from 25 to 175% at the single-particle level. Since these biosensors were immobilized, the correlation between the detected maltose concentration and the maltose-dependent emission intensity increase could be examined. Minimal correlation between maltose detection limits and emission increases was observed, suggesting a variety of reductant-nanoparticle surface interactions that control maltose-dependent emission intensity responses. Despite the heterogeneous responses, monitoring biosensor emission intensity over 5 min provided a quantifiable method to monitor maltose concentration. Immobilizing and tracking these biosensors with heterogeneous responses, however, expanded the analyte-dependent emission intensity and the analyte dynamic range obtained from a single construct. Given the wide dynamic range and constant emission of charge-transfer-based biosensors, applying these single molecule techniques could provide ultrasensitive, real-time detection of small molecules in living cells.

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