(409h) Antioxidant and Antibacterial Silver Nanoparticles from Agro-Industrial Residues Extracts for Biomedical, Pharmaceutical, Cosmetic and Environmental Applications

The field of nanotechnology has revolutionized In the last years, due to the green synthesis of metal nanoparticles with applications in science and engineering. The most versatile nanoparticles are the silver ones, which have optical, electrical, magnetic, chemical, photochemical, catalytic, mechanical, and antimicrobial properties against bacteria, fungi, and viruses. Due to these promising characteristics, they have sought to venture mainly into biomedicine[1]. There are different methods to producing AgNPs, one of them is the chemical synthesis which requires expensive, toxic compounds with negative environmental impact. In contrast, green synthesis is an easier, cheaper and more environmentally friendly methodology because it eliminates the use of chemical substances as reducing/capping agents, replacing them with biological agents [2]. Agro-industrial residues and their extracts can be used for the synthesis of AgNPs, since they possess secondary metabolites as bioactive compounds such as polyphenols, flavonoids, alkaloids, amino acids, that can provide antioxidant, antimicrobial, anti-inflammatory, anti-allergic properties; and can be used for biomedical, pharmaceutical, cosmetic, and environmental applications [3].

Other industries can expand their market to products that can include NPs, such as cosmeceuticals since they need bioactive compounds in their formulations. The AgNPs have a broad spectrum and are well accepted due to their antioxidant, antibacterial, anti-inflammatory and antifungal properties. Nanomaterials are found in many cosmetic products including moisturizer, hair care products, makeup and sunscreen [4]. Nanotechnology also allows for an effective release of ingredients, improving the penetration of substances through the skin barrier. The AgNPs have been tested for the formulation of skin cleansers, lotions, creams, shampoos, toothpastes, and deodorants [5]. Additionally, the anti-inflammatory and anticancer effect of AgNPs allow the development of medicines with specific effects [6]. The antimicrobial activity of AgNPs have major applications in diagnosis, anticancer therapy, diabetes control, biosensor materials, drug delivery systems, among others [7].

According to Singh Prasad et al. (2008) [8], the mechanisms, through which AgNPs exhibited antibacterial properties, were by anchoring to and penetrating the bacterial cell wall and modulating cellular signaling by dephosphorylating putative key peptide substrates on tyrosine residues. AgNps behave in three different ways against Gram-negative bacteria: a) AgNPs (1-10nm) attach to the surface of cell membrane and modify its permeability and respiration; b) AgNPs penetrate inside the bacteria and cause damage to sulfur – and phosphorus – containing molecules such as DNA; c) AgNPs release Ag⁺ ions which, additionally, contribute to its bactericidal effect.

The AgNPs’ antibacterial properties enables their use as cost-effective disinfectants. AgNPs are used as Ag-based air/water filters [9]. In the environmental sector, one of the sustainable development goals for 2030 is providing clean water. Several costly techniques have been used though years such as filtration, advanced oxidation, reverse osmosis, UV irradiation, etc. to remove different contaminants from water. Water filters should be multifunctional to remove the different water pollutants. Hence, nanomaterials that include AgNPs would have the multipurpose approach required to clean contaminated water and remove chemical compounds and moieties as well as microorganisms. Regarding air disinfection, particles, toxic gases, volatile organic compounds, airborne pathogen bacteria, viruses and fungi, that need to be removed for clean air. Techniques such as germicidal irradiations, photocatalyst oxidation and ozonolysis, which are expensive are applied to clean air. Thus, AgNPs–based nanomaterials from agro-industrial residues can help to control air quality, as effective disinfectant agents.

In this research, the antibacterial activity of agro-industrial residue extracts such as cocoa pod husk (CPH) and avocado seed (AS) and their AgNPs was evaluated. The results revealed that AS-AgNPs exhibited antibacterial properties against pathogenic bacteria such as E. coli, while CPH-AgNPs exhibited antibacterial activity against S. aureus and E. coli [3]. Based on these results, other parameters were evaluated such as total polyphenol content, antioxidant activity, that are reported in this study.

The antioxidant activity was measured using the FRAP method of the extract at a concentration of 5 mg/ml and the AgNPs. Results are expressed in µmol Trolox equivalents/mL. The AS extract contains 661.86 (± 16.78), the CPH extract 418.25 (± 14.46). The AS-AgNP and CPH-AgNP showed an antioxidant activity of 698.147 (± 11.18 714.68 (± 8.85) µmol Trolox equivalents/mL, respectively. Comparing the results obtained from the extracts and their the AgNPs, we can identify that the iron-reducing antioxidant power has an increase in the concentration of bioactive compounds in the AgNPs. A significant 1.7-fold increase in antioxidant activity was observed in CPH-AgNPs than in CPH extract, a similar pattern was also identified in the case of AS-AgNPs but in a smaller proportion 1.05 times more than its extract. The increment in the antioxidant capacity of AgNPs may be due to phytochemicals such as terpenoids, flavonoids and phenolic compounds that the extract contains are adsorbed around the spherical Ag NPs and allow them to interact with the silver ion as inhibitors of singlet oxygen acting as hydrogen donors and reducing agents [10].

Regarding the total polyphenol content of the aqueous extracts, the results show that by the Folin-Ciocalteu method at a concentration of 5mg/ml for AS extract is 0.77 (± 0.088) mg GAE/g extract and CPH extract is 0.474 (± 0.059) mg. GAE/g extract. The same methodology was carried out to determine the polyphenol content for the AgNPs, and the results revealed that the AS-AgNPs contained 0.849 (± 0.017) mg GAE/g extract and CPH-AgNPs showed 0.353 (± 0.010) mg GAE/g extract. These results show that the polyphenols content increase in AS-AgNPs compared to the extracts, these compounds can act as a capping and protective agent around the AgNPs [11]. Thus, CPH extract was identified as having a higher concentration of polyphenols compared to CPH-AgNP. This variation can be attributed to the fact that polyphenols can act as reducing agents from the AgNO₃ solution to a colloidal AgNPs solution [12]. The variation in the polyphenol content results may be due the presence of different unique bioactive compounds in extracts that will behave differently as reducing agents when exposed to silver nitrate and LED light.

Based on these results, we can conclude that the use of AgNPs from agro-industrial residues of AS and CPH can be an alternative as antioxidants and antibacterials, since they present particular bioactive characteristics that could be used with specific applications due to their antibacterial and antioxidant capacity that are increased by being included within these nano structures. AgNPs has potential in biomedicine for the coating of biomaterials, medical devices, tissue engineering, diagnostic platforms and therapy against cancer.

This research describes the antioxidant and antibacterial properties as well as polyphenol content of AgNPs from agro-industrial wastes from Ecuador, such as AS and CPH. Additionally, we have evaluated the potential applications that these AgNPs may have in biomedical engineering, cosmetics, pharmaceuticals and environmental sectors. Therefore, AS-AgNPs and CPH-AgNPs are good candidates for new product development, however it requires further studies on cytotoxicity and biodegradability and their impact in the environment.

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