Accompanied by the rapid development of modern technology, more and more nanomaterial-bearing products are being invented: antibacterial phone shells, antifungal socks, carbon fiber-reinforced rackets, quantum dot displays, etc. These products tremendously improve the quality of our daily lives, but also raise a serious problem: nanomaterial pollution.
Literally, nanomaterial refers to the material with a size of 1-1000 nanometers (10-9 – 10-6 meters) (Fig 1). In comparison, the size of a microbe normally ranges from 10-6 meters to 10-5 meters, and the size of a human cell is around 10-5 meters. This means that the volume of a microbe or a human cell is at least over 1,000,000 times larger than a nanoparticle. Hence, dispersed nanomaterial could easily penetrate microbes and human cells. Based on this mechanism, nanomaterials with luminescent properties are used as biomarkers to track cancer cells in the human body.
Carefully designed nano-particles can be used for medical purposes, while many nano-materials used in consumer products are used to inhibit the growth of a microbe. There are three reaction mechanisms of nanomaterials aiding in microbe control: (1) nanomaterials have large specific surface area and can release toxic ions such as Ag+ to inhibit the activity of microbe; (2) due to the small physical size, nanomaterials can easily enter microbe cell and disturb its normal functions, such as energy transfer and DNA replication; (3), nanomaterials are highly chemically reactive and can catalyze the formation of reactive oxygen species (such as peroxides, superoxides, hydroxyl radicals, and singlet oxygen) under sunlight (ultraviolet). These reactive oxygen species can oxidize the cell wall and cell membrane of microbe and damage its structure. These reaction mechanisms would lead to the death of microbe and thus achieve the advertised functions such as antibacterial and odor-control.
Fig 1. Comparison about Ag2S nanoparticles (left) and microbes cultured in bioreactor serving for wastewater treatment (right). Please note the different plotting scales. Credit: Lu Wang
Although nanomaterials are effective for microbe control, they can be released into the water after usage or washing. These particles are too tiny to be captured by the current water treatment technologies and may enter the human body through drinking water and air. For the crops and livestock exposed to polluted water, the nanomaterial could accumulate in tissues and eventually be consumed by humans through the food chain. Similar to microbe cells, nanomaterials are highly toxic to human cells. Investigating the environmental impacts of nanomaterial and eliminating them from polluted water are critical issues faced by environmental science in recent decades.
Fortunately, for the nano-materials released into water, they undergo many different physical and chemical processes such as coagulation, adsorption, desorption, reduction, and settlement. Natural organic matter is a class of organics that includes proteins, saccharides, humic acids, etc. that come from the decomposition of creatures. These organics ubiquitously exist in natural waters. Scientists found that although lab-prepared nanomaterials are highly toxic, their toxicity decreased after they were released into natural waters and interacted with aquatic creatures. Some theories speculate that the nanomaterial was coagulated with the natural organic matter and became large particles, which are hard to be taken by a microbe and aquatic creatures. However, detailed information about this micro-process is ambiguous.
In their recently published paper, researchers found that for the nanomaterials in natural water, the background organic matter coats the surface of nanomaterials and inhibits their aggregation. The organic coating layer could simultaneously inhibit the release of toxic metal ions from nanomaterials, prevent the formation of reactive oxygen species by screening sunlight, and cushion the physical interaction of nanomaterials with the microbe cell, thus alleviating the toxic effects of nanomaterials.
By interpreting the effects of natural organic matter on antimicrobial activity of nanomaterials with the soft particle theory, researchers found that the thickness and softness of the organic coating layer negatively correlate with the biotoxicity of the nanomaterial. For example, the protein coating layer is thicker and softer than the alginate coating layer, and the toxicity of protein covered nanomaterials is less than the alginate-covered nanomaterials. The study provides a quantitative analysis tool for investigating environmental behavior and understanding of the toxicity effect of nanomaterial.
Credit: Lu Wang
These findings are described in the article entitled Interpreting the effects of natural organic matter on antimicrobial activity of Ag2S nanoparticles with soft particle theory, recently published in the journal Water Research. This work was conducted by Yulei Liu, Tao Yang, Lu Wang, Zhuangsong Huang, Juan Li, Haijun Cheng, Jin Jiang, Jingyao Qi, and Jun Ma from the Harbin Institute of Technology, and Suyan Pang from Jilin Jianzhu University.