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 3nM, 3nM, 3nM, and 6nM, 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.