Fluorescence assay of catecholamines based on the inhibition of peroxidase-like activity of magnetite nanoparticles

Abstract

We report a fluorescence approach for the highly selective and sensitive detection of catecholamines using magnetite nanoparticles (Fe(3)O(4) NPs) in the presence of Amplex UltraRed (AUR) and H(2)O(2). Fe(3)O(4) NPs catalyze H(2)O(2)-mediated oxidation of AUR. The resulting product fluoresces (excitation/emission maxima, ca. 568/587nm) more strongly, relative to AUR. When catecholamines bind to Fe(3)O(4), the complexes that are formed induce decreased activity of Fe(3)O(4) NPs, mediated through the coordination between Fe(3+) on the NP surface and the catechol moiety of catecholamines. As a result, Fe(3)O(4) NPs-catalyzed H(2)O(2)-mediated oxidation of AUR is inhibited by catecholamines. The limits of detection for dopamine (DA), L-DOPA, norepinephrine, and epinephrine were 3 nM, 3 nM, 3 nM, and 6 nM, respectively. The Fe(3)O(4) NPs-H(2)O(2)-AUR probe exhibited high selectivity (>1000-fold) toward catecholamines over other tested biomolecules that commonly exist in urine. Four catecholamines had similar sensitivity because the inhibition of the Fe(3)O(4) NPs activity relies on the presence of the catechol moiety. This approach also allowed the determination of tyrosinase activity because tyrosinase catalyzes the conversion of l-tyrosine to L-DOPA. We validated the practicality of the use of the Fe(3)O(4) NPs-H(2)O(2)-AUR probe for the determination of the concentrations of DA in urine samples.