Interaction of [HgX4]2- with berberine by absorption and resonance Rayleigh-scattering spectra and their analytical applications.

In a diluted H2SO4 solution, Hg(II) reacts with halide anions X- (including Cl-, Br- and I-) to form anionic complexes [HgX4]2- that can further react with berberine to form ion-association complexes of [Ber]2[HgX4]. As a result, the absorption spectra change, their maximum absorption wavelengths are at 230 nm for [Ber]2[HgCl4], 278 nm for [Ber]2[HgBr4] and 300 nm for [Ber]2[HgI4]. However, among the three complexes, only [Ber]2[HgI4] can lead to distinctly enhanced resonance Rayleigh scattering (RRS), and a new RRS spectrum appears. The maximum RRS wavelength is located at 397 nm, and the RRS intensity is proportional to the concentration of berberine in the range of 0-2.5 microg mL-1. The optimum conditions, the influence factors for the reaction and the effects of coexisting substances have been investigated. A new, simple and fast RRS method for the determination of berberine based on the ion-association reaction of [HgI4]2- with Ber+ was developed. The method has high sensitivity and good selectivity; the detection limit for berberine (3 sigma/K) is 7.22 ng mL-1. The method can be applied to the determination of berberine in some Chinese patent drug and the extracts of Coptis Chinensis. Furthermore, the mechanism of the reaction and the reasons for RRS enhancement have been discussed.