Flower, stem, and leaf extracts from Hypericum perforatum L. to synthesize gold nanoparticles: Effectiveness and antioxidant activity

The significance of gold nanoparticles in nanobiotechnology and the trend towards the development of green methods of synthesis has boosted research on the use of new natural resources, such as plants, as reducing agents for the formation of nanoparticles. This article focuses on Hypericum perforatum L., a plant known since ancient times for its multiple applications in the treatment of a variety of diseases. In this study, the flowers, leaves, and stems of the plant were separated and their corresponding ethanolic extracts (HPF, HPL, and HPS) were extensively characterized and used to synthesize gold nanoparticles. The formation of gold nanoparticles was confirmed in all the cases by UV–Vis spectroscopy, which showed the characteristic SPR band of gold at around 540 nm. Furthermore, the three types of nanoparticles synthesized with each extract (Au@HPF, Au@HPL, and Au@HPS, respectively) were thoroughly characterized by means of HRTEM, XRD, FTIR, DLS and Z-potential. All the nanoparticles obtained were regular in shape, spherical, and with mean diameters between 9.2 and 11.5 nm. Finally, the evaluation of the antioxidant activity of both the extracts and the corresponding nanoparticles was performed by conducting in vitro antioxidant assays, revealing an enhanced DPPH radical scavenging capacity of the nanoparticles after the synthesis (with IC50 values of 0.11 ± 0.05 for Au@HPF, 0.23 ± 0.002 for Au@HPL, and 0.11 ± 0.002 mg/mL for Au@HPS) in comparison with that of the corresponding plant extracts (with IC50 values of 0.71 ± 0.04, 0.71 ± 0.03, and 0.69 ± 0.004 mg/mL for HPF, HPL, and HPS, respectively). To the best of the authors’ knowledge, this is the first detailed study on the capacity of each of the aerial parts of Hypericum perforatum to produce gold nanoparticles in which both the extracts used for the synthesis and the nanoparticles obtained are fully characterized. Given the multiple therapeutic properties of Hypericum perforatum, it is interesting to study the possible synergistic enhancement of those properties by means of its interaction with gold nanoparticles.