The CDC estimates that annually 9 million people get sick and over 55,000 are hospitalized as a result of foodborne illness in the United States alone . The need for innovations designed to minimize exposure to harmful bacteria is ever-present. Photochemical reduction of AgNO3 solution can extend the shelf life of food through photon generation of silver nanoparticle films onto polyethylene and glass packaging materials as a means of inhibiting bacterial growth.
Silver has long been used for its antibacterial properties. Roman soldiers applied silver and nitrate dressings to wounds, and soldiers wounded in the First World War were treated with silver foil applications . Today, many bandages are infused with a thin layer of silver which functions as a protective film over the exposed wound. The antimicrobial properties of silver can also be applied to food packaging systems, known as active and smart packaging, to extend the shelf life of food and decrease spoilage. Using silver nanoparticles has additional advantages over silver nitrate solutions due to their unique physical, chemical, biological, and optical properties derived from their nano-size.
The silver nanoparticles (AgNPs) of active packaging systems are created from a solution of AgNO3 poured directly onto food packaging plastic wraps such as low density polyethylene film or polyvinyl chloride. A UV spot light then irradiates the sample to produce photochemically generated free radicals which reduce the Ag+ ions in the solution to create a layer of AgNPs (3-11nm in diameter). Photochemical generation of AgNPs is both a cost effective and accessible method of creating active packaging systems. Traditional techniques such as thermal deposition of nanoparticles with a laser require expensive equipment and a carefully controlled environment, while photochemical process can take place in ambient conditions. Generating AgNPs from solution is also safer than laser ablating dry powder nanoparticles onto a substrate, as dry powder nanoparticles have the potential to aggregate, which decreases silver’s antimicrobial properties and can cause a cytotoxic build-up in the lungs if inhaled. This process also eliminates the use of toxic stabilizers or additional reducing agents, further improving the safety of the product .
The exact mechanism of silver’s antimicrobial activity is yet undefined, and has been hypothesized to be attributed to several different factors including AgNPs direct interaction with the bacterial cell (disruption of transmembrane electron transfer, penetration of the cell envelope, oxidation of cellular components), the by-products of AgNPs (reactive oxygen species (ROS) damaging DNA and oxidizing amino acids and key enzymes), and as a result of the interactions of Ag+ ions formed from the dissolution of AgNPs in situ . Silver ions are thought to be antimicrobial due to their interactions with thiol groups in enzymes and proteins, as well as their inhibition of bacterial growth and structural damage of the cell when accumulated in the vacuole and cell wall . In the nanoparticle form, higher concentrations of silver ions are released compared to a non-nano particle of equivalent mass due to the particles increased surface size. Additionally, due to the nanoparticles unique interactions with biological receptor molecules, the AgNPs are able to adsorb farther into the cell before dissolution and release cytotoxic silver ions, increasing its effectiveness against bacteria .
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Many current methods of nanoparticle production incorporate electron-beams, X-rays, and other devices. To prepare silver nanoparticles chemically, common practice involves the use of reducing agents and other toxic stabilizers which may be hazardous to both the environment and the efficacy of the nanoparticle. Photochemical reduction of silver nitrate is a new, more sustainable means of adsorbing silver nanoparticles directly onto glass or Low-density polyethylene (LDPE) food packaging materials. The photochemical process is advantageous in that light energy drives the reaction, offering more energy per photon compared to thermal methods. This process also does not use reducing agents such as citrate or any stabilizers capable of contaminating the food .
A concern with silver nanoparticle packaging is the unknown effect that potential silver migration into polymer matrices and thus food products may have on human cellular functions. While packaging may initially demonstrate migration levels within the approved guidelines, increased nanoparticle release rates were found to be caused by exposure to microwave energy from certain ovens . Further research of silver nanoparticle migration through packaging materials into food is needed to fully assess the associated health hazards with the technology.