Modern Alchemy: Using Cacao Extract in the Synthesis of Gold Nanoparticles

Samira Hussein

Sophomore (transferred to University of Missouri)

School of Sciences

Independent University, Bangladesh

March 23rd, 2019

Gold is associated by most people with jewelry, and in regions like South Asia, it remains a regular item of gift exchange. Cacao on the other hand is known to us as the source of chocolate. Who would have thought that cacao powder could help in the synthesis of gold nanoparticles? It sounds a bit strange, but a group of scientists have managed to make it happen.

To begin with, we ought to know about nanoparticles; these are extremely tiny particles, about 1 to 100nm in size. Nanoparticles of gold (AuNPs), which form a colloidal suspension in water and other fluids, have been shown to have unique electronic and optical properties, making them useful in a number of applications. AuNPs enhance contrast and thereby enable better visualization of cells and tissues for spotting signs of inflammation, cancer, or other diseases. The large surface area-to-volume ratio of the nanoparticles allows hundreds of molecules of a drug (alongside antibodies that target the particles to the right tissues) to be coated onto their surfaces for efficient delivery. In photodynamic therapy, gold nanoparticles delivered to a specific site, such as a tumor, can be excited by light of specific wavelengths to produce heat (to kill the tumor cells, in this instance).

Traditional methods of synthesizing AuNPs have been plagued by challenging limitations such as toxic byproducts and complex reaction parameters. A recent study published in the journal Nanoparticles by Roy Chowdhury et al. tried to overcome these problems by using cacao extract, that has both reducing and stabilizing properties, in the synthesis of AuNPs. Previous work had demonstrated a key advantage of using plant extracts and microbes for biosynthesis over older approaches using inorganic chemicals: the biological agents and their byproducts are generally not toxic to human and other mammalian cells, as well as the environment.

Schematic of the process provided in the paper

In their experiments, the scientists incubated hydrogen tetrachloroaurate, (HAuCl4), a gold-containing compound, with different concentrations of cacao extract at 100 ̊C. UV-visible spectroscopy was used to monitor the progress of the synthesized AuNPs, alongside other characterization methods. UV-visible spectroscopy measures the absorbance of molecules when hit with light of the ultraviolet and visible electromagnetic spectra, and different structures produce different absorbance peaks. They determined that increasing the concentration of cacao extract beyond a certain point promoted aggregation in the suspension.

Following the determination of the appropriate concentration of cacao extract, they employed a technique called dynamic light scattering (DLS) to measure the diameters of the particles produced. In this technique, light, usually a laser, is shot through a suspension of particles in a solvent, and the resulting scatter indicates what sizes of particles are present in the suspension. They also measured the zeta potential of the products, which was the potential difference the surface of the particles and the water they were immersed in. The zeta potential is predictive of the colloidal stability; nanoparticles with zeta potential values greater than +25 mV or less than -25 mV typically have high degrees of stability. The zeta potential of non-aggregated AuNPs (which were produced at the lower concentrations of cacao extract) were between -11mV to -17mV, indicating the stability of the suspensions. Indeed, these suspensions were so stable that even after a month, no particle agglomeration was observed.

The scientists then used transmission electron microscopy (TEM) to observe the morphology, and measure the size of the particles. In TEM, electrons are blasted through the specimen. The specimen scatters the electrons, and an image of the specimen is interpreted from the pattern of scattering of the electrons by the specimen. The morphology results indicated that most of the AuNPs were spherical and crystalline, and confirmed that there was no aggregation in the samples. The numerical values of the measured sizes obtained from DLS and TEM showed some differences owing to the different principles employed in the techniques, but this did not change the essential conclusion that this method of synthesis was successfully producing non-aggregating gold nanoparticles. Lastly, to detect the presence of any toxic byproducts, the synthesized gold nanoparticles were administered to cultured human dermal fibroblasts, which are normally present in a layer of skin. The cells were not adversely affected.

The one-pot synthesis of biocompatible, spherical, and crystalline AuNPs is a promising breakthrough in the field. So, why does cacao work for this? Cacao contains a chemical called oxalic acid, which reduces the Au3+ in hydrogen tetrachloroaurate to metallic gold, and subsequently stabilizes the product by preventing agglomeration. This simple yet elegant biosynthesis could be part of a revolution in the production of nanoparticles.

Samira writes:

"Completing two semesters in IUB, I realized my passion for knowing the unknown in the biological sciences. It excites my nerves high way beyond the threshold level. I like to read biomedical papers, and hope to join the medical field or go into research in the biomedical sciences."