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This paper describes the synthesis and optical properties of PbS/CdS quantum dots for in vivo fluorescence imaging.

By extensively examining the experimental conditions for time-resolved spectrophotometry of non-transparent light scattering systems, we demonstrated the feasibility of quantitative analysis of both the fluorescence lifetime and intensity of reduced pyridine nucleotides in living tissues, suspensions of isolated liver mitochondria, and hepatocytes, as well as hemoglobin-free perfused rat liver being used systematically for measurements. The fluorescence decay was analyzed by the maximum likelihood method with a 4-component decay model. The lifetime of NADH observed in mitochondria (mean: 2.8 +/- 0.2 ns) was much longer than that of the free form in an aqueous solution (mean: 0.43 +/- 0.01 ns), and it was characterized as a protein-bound form. The lifetime was not affected by either aerobic or anaerobic conditions nor by the energy state, though the intensity changed markedly. The decay curves of isolated hepatocytes under normal aerobic conditions were the same as those of isolated mitochondria, though cytosolic NADH and NADPH were superimposed. Under the conditions of "unphysiological" acidosis, the mean lifetime became about 1.5 times longer than that under normal conditions. With perfused liver, the relative contributions of cytosolic NADH and NADPH were determined by infusing lactate and tert-butylhydroperoxide. Cytosolic NADH did not contribute to the overall fluorescence of pyridine nucleotides. In contrast, about 70% of the total fluorescence intensity was due to cytosolic NADPH, but its decay parameters were essentially the same as those of mitochondrial NADH. No free form of either NADH or NADPH was detected in the cytosolic and mitochondrial spaces. We concluded that the changes in fluorescence intensity observed under the various conditions can be simply explained by a change in the amount of reduced pyridine nucleotides in tissues, rather than by changes in the microscopic environment. The wide applicability of time-resolved fluorescence photometry to in vivo studies is well documented.

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