Determination of Fe³⁺ upon Special "Upconversion Luminescence" of Dopamine

A promising technique based on the luminescence with long wavelength excitation and short wavelength emission (LExL, λ_ex-L > λ_em) is developed. This LExL is different from traditional upconversion luminescence (UCL). The LExL, namely, special "UCL", is realized by a xenon light source of a common spectrofluorometer. In this work, we found that dopamine (DA) has this LExL phenomenon. The LExL of DA is mainly caused by the excitations of second-order diffraction light (λ_ex-L/2). The two-photon absorption properties of DA have been calculated employing the density functional response theory. The LExL and Stokes luminescence (SL, λ_ex-S < λ_em) of DA both showed static quenching upon the addition of Fe³⁺. Dual-mode luminescence methods upon LExL (λ_ex-L/λ_em at 565/317 nm) and SL (λ_ex-S/λ_em at 282/317 nm) of DA were applied for the selective determination of Fe³⁺. The detection limits are 0.30 and 0.52 μmol L^-1 for LExL and SL, respectively. In addition, their linear ranges for Fe³⁺ determination are both from 0.70 to 30 μmol L^-1. The LExL method of DA not only meets the basic determination criteria for Fe³⁺ but also offers additional advantages in resisting more interferences and shows satisfactory feasibility performances.