Abstract
Ln(III) complexes of macrocyclic ligands are used in medicinal chemistry, for example as contrast agents in MRI or radiopharmaceutical compounds, and in diagnostics using fluorescence imaging. This paper is devoted to a spectroscopic study of Ln(III) ternary complexes consisting of macrocyclic heptadentate DO3A and bidentate 3-isoquinolinate (IQCA) ligands. IQCA serves as an efficient antenna ligand, leading to a higher quantum yield and Stokes shift (250–350 nm for Eu, Tb, Sm, Dy in VIS region, 550–650 nm for Yb, Nd in NIR region). The shielding-quenching effect of NAD(P)H on the luminescence of the Ln(III) ternary complexes was investigated in detail and this phenomenon was utilized for the analytical determination of this compound. This general approach was verified through an enzymatic reaction during which the course of ethanol transformation catalyzed by alcohol-dehydrogenase (ADH) was followed by luminescence spectroscopy. This method can be utilized for selective and sensitive determination of ethanol concentration and/or ADH enzyme activity. This new analytical method can also be used for other enzyme systems coupled with NAD(P)H/NAD(P)+ redox pairs.
Highlights
The specific spectroscopic, electrochemical and magnetic properties of Ln(III) ions make them perfect candidates for use in many chemical, biological and environmental systems
The macrocyclic ligands H2 DO2A and H3 DO3A, which differ in their denticity, are capable of forming Ln(III) complexes with high thermodynamic stability and kinetic inertness [19,20,21,23,24]
The formation of these Ln(III) complexes with characteristic sharp-peak emission spectra is accompanied by an increase of Ln(III) ion luminescence intensity due to the release of water molecules coordinated to Ln(III) ion [24,28]:
Summary
The specific spectroscopic, electrochemical and magnetic properties of Ln(III) ions make them perfect candidates for use in many chemical, biological and environmental systems. The luminescence spectra of Ln(III) complexes can be recorded in time-resolved mode due to relatively long lifetimes (in the μs-ms scale). This enables the filtering of their signal from the in vivo background originating from organic molecules with luminescent decay on the ns time scale [1,2,3,4,5,6,7,8]. The excited states of Ln(III) ions in their complexes are not quenched by O2 molecules but are influenced by water molecules coordinated to the Ln(III) ion [1,2,3,4,5,6,7,8]. The introduction of a chromophoric group into the ligand significantly increases the luminescence of Ln(III) complexes as consequence of the so-called antenna effect, through which energy strongly absorbed by the chromophoric ligand is efficiently transferred to the Ln(III) ion
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