Despite the pivotal role played by the reduction of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the solution phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by reduction through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the reduction kinetics involved. Our quantitative analyses of the reduction kinetics of PdCl and PdBr by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl was reduced in the solution phase while PdBr was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr by ascorbic acid could be switched from surface to solution by raising the reaction temperature.

Melanogenesis inhibition by raspberry ketone (RK) from Rheum officinale was investigated both in vitro in cultivated murine B16 melanoma cells and in vivo in zebrafish and mice. In B16 cells, RK inhibited melanogenesis through a post-transcriptional regulation of tyrosinase gene expression, which resulted in down regulation of both cellular tyrosinase activity and the amount of tyrosinase protein, while the level of tyrosinase mRNA transcription was not affected. In zebrafish, RK also inhibited melanogenesis by reduction of tyrosinase activity. In mice, application of a 0.2% or 2% gel preparation of RK applied to mouse skin significantly increased the degree of skin whitening within one week of treatment. In contrast to the widely used flavoring properties of RK in perfumery and cosmetics, the skin-whitening potency of RK has been demonstrated in the present study. Based on our findings reported here, RK would appear to have high potential for use in the cosmetics industry.

Our previous study showed that a methanol extract from Trifolium pratense exerted potent inhibitory activity on melanogenesis in mouse B16 melanoma cells. In the present study, the active compound in this Chinese herb extract was isolated and identified as biochanin A by mass spectrum, (1)H-NMR, and (13)C-NMR analysis. The inhibitory effects of biochanin A on melanogenesis were investigated in vitro in cultured melanoma cells and in vivo in zebrafish and mice. Biochanin A dose-dependently inhibited both melanogenesis and cellular tyrosinase activity in B16 cells and in zebrafish embryos. Application of a cream containing 2% biochanin A twice daily to the skin of mice also increased the skin-whitening index value after 1 week of treatment, and the increase continued for another 2 weeks. Biochanin A was confirmed as a good candidate for use as a skin-whitening agent in the treatment of skin hyperpigmentation disorders.