Green, microwave-assisted synthesis of gold nanoparticles using Caesalpinia sappan heartwood extract and chitosan

Abstract

The aims of this work were to develop microwave (MW) assisted synthesis methods for gold nanoparticles (AuNPs) using green reagents as both reducing agents and stabilizing agents and to apply the fabricated products for analytical application. In the first work in which the Caesalpinia sappan (CS) heartwood extract was employed, the MW-assisted extraction performed at 300 W for 3 min was used to prepare the extract with high and reproducible brazilin content (13.1 ± 0.4% w/w) and it could be readily used for the synthesis of AuNPs. Under the optimal AuNP synthesis conditions i.e. using 5 mL of 1.5 mM HAuCl4 solution, 0.4 mL of the CS extract and a MW irradiation fixed at 60 °C for 1 min, roughly spherical CS-AuNPs with an average diameter size of 49.6 nm and acceptable 28-day stability were obtained. The resulting CS-AuNPs were capable of selective binding to Fe2+, Fe3+ and Al3+, leading to particle aggregation as well as noticeable change of color and shift of UV-vis absorption maxima at 650 nm. Therefore, they had potential for the colorimetric sensing of certain metal ions in pharmaceutical products. In the second work, chitosan-capped gold nanoparticles (CTS-AuNPs) were synthesized. The synthesis was carried out by using 0.75 mL of 10 mM HAuCl4 and 5 mL of 0.1% w/v CTS solution and a microwave irradiation at 125 °C for 40 s. The resulting red colloidal solution, showing the maximum absorption peak at 520 nm, contained monodispersed, spherical, and stable CTS-AuNPs with a mean diameter of 17.8 nm. Due to the positively charged surface, CTS-AuNPs were applied to the assay of the polyanionic compound i.e. disodium edetate based on the particle aggregation which eventually induced red-to-blue color change of the solution. It was found that the absorbance at 670 nm related to the disodium edetate concentration with a 4-parameter logistic model. The assay was accurate and precise; its results were comparable to those obtained from the titrimetric pharmacopeial method. Moreover, gold (Au0) was shown to be easily recovered from CTS-AuNPs through alkaline-induced precipitation followed by acid treatment and heating, providing a facile and efficient means for treatment of laboratory waste.

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