Medical miracles of platinum grown from plants
Platinum nanoparticles have incredible medical applications, but their creation comes at a high environmental cost. Researchers seek to turn bacteria and plants into green platinum factories.
Platinum is best known as a catalyst – and as a precious metal it is better than gold. However, it also has near-miraculous medical applications.
Chemotherapy drugs like cisplatin and carboplatin exploit platinum‘s ability to inhibit DNA replication and cause cell death. Thanks to their exceptional array of physiochemical properties, platinum nanoparticles (PtNPs) could also be used in targeted drug delivery, photothermal therapy, radiotherapy, antimicrobial ointments, bioimaging and biosensors.
Their possibilities are tempting, but PtNPs have a double cost: monetary and environmental.
PtNPs are created by physical and chemical processes. The first approach uses high pressures and temperatures to produce pure nanoparticles of uniform size and shape, at enormous cost and energy consumption.
This last family of processes, such as the sol-gel process and pyrolysis, applies chemical agents to reduce the precursor metal ions to their corresponding nanoparticles and then to stabilize them. This tends to be more cost effective, but uses chemicals that are dangerous to human and environmental health. These are a hassle to dispose of responsibly, but also leave a toxic coating on the nanoparticles, severely compromising their biocompatibility.
There is a need for greener pathways to synthesize PtNPs, and many researchers hope the answer may lie in biology.
“The main challenges in the therapeutic application of PtNPs include biocompatibility, bioavailability, degradation in the gastrointestinal tract, stability and immune response”, explained Dr. Cristina Satriano, professor of physical chemistry at the University from Catania, Italy. “The pathway of biogenic synthesis […] can overcome these limitations in chemical and physical methods.
Although the biosynthesis of PtNP is at an early stage compared to the biosynthesis of gold and silver nanoparticles, there is a growing body of research on how to grow and harvest these tiny balls of precious metal from bacteria such as E. coli, as well as viruses, fungi. , algae and plants.
Dr Michael Capeness is Laboratory Director at the Horsfall Group at the University of Edinburgh, a biotechnology group whose activities include using bacteria to synthesize metal nanoparticles. Capeness explains: “We kind of hijack the native ability of bacteria to produce these particles from ions. Wherever bacteria have been isolated, they have developed this ability to deal with this insult to their physique. [from that adverse environment]. They do this by making nanoparticles.
The obvious advantage of biological approaches is that, unlike physical and chemical approaches, they tend to be environmentally safe.
Energy consumption is much lower than that of physical processes, and biological entities perform the reduction and stabilization that would otherwise require toxic chemical agents. Biological approaches also do not require pure raw materials, which are increasingly difficult to acquire given the seething geopolitical tensions. Organisms can be genetically engineered to select specific metal ions to synthesize PtNPs from dilute samples, and researchers hope that specialized bacteria will eventually be able to synthesize high-value nanoparticles from waste materials, including e-waste. , on an industrial scale.
The benefits of biosynthesized PtNPs are perhaps most remarkable when it comes to their medical applications. Research has demonstrated that biosynthesized PtNPs tend to have superior antimicrobial, antioxidant, and anticancer activity compared to physically and chemically synthesized nanoparticles. How can this be?
When we talk about nanoparticles, we can refer to any particle that has at least one dimension between 1 and 100 nm. Like a Bombay mix bag, PtNPs come in all shapes and sizes, and a variety of properties such as charge, coating, and crystallinity. Depending on the conditions under which they are synthesized, there can be a world of difference between two PtNPs. In general, biosynthesis – such as through “slow and steady” enzyme-mediated bacterial processes – allows for smaller nanoparticles than “vigorous” physical and chemical processes. Biosynthesized PtNPs can be as small as 1 nm in diameter, while those created by chemical synthesis are typically tens of nm in size and can even clump together into large-scale disordered drops. Smaller nanoparticles are more effective antimicrobial agents (for example, they can easily pass into the cell of an S. aureas and cause the production of reactive oxygen species) and their larger surface area to volume ratio allows for greater anticancer activity.
Biosynthesized PtNPs are also more biocompatible. Their coatings tend to be made up of proteins, sugars, and other substances that are easier for the body to process than the toxic coatings left on chemically synthesized PtNPs.
Biosynthesized PtNPs are not yet produced on a large scale. First, scientists need to optimize these biological processes, for example by genetically modifying platinum-producing bacteria to work faster, better, and without the irritations that prevent them from becoming industrially relevant.
“Generally we try to design them to be more resistant to the metal we give them,” Capeness explains. “This way we can make nanoparticles faster […] and since we use impure raw materials, we can genetically adjust them to only react to certain metals.
One of the most tedious parts of the process is recovering the platinum from the bacteria after reducing the metal ion to a nanoparticle. Like a pearl in an oyster, the nanoparticle is trapped inside the cell. Retrieving nanoparticles produced intracellularly requires approaches such as grinding, applying sound waves, and chemically treating cells to make them more porous. Then, once the ion has been extracted, it must be purified. This adds yet another recovery cost. Capeness explains, “You have to open the cell, so to speak. This releases all the guts of the bacteria into the soup, from which you then need to remove your nanoparticles. So that’s a problem.
Facilitating the fight against bacterial PtNPs – such as genetically modifying them to produce nanoparticles extracellularly – is a major focus of research in this area.
In the meantime, some researchers are turning to organizations more willing to give up their precious loot. Dr Sougata Ghosh, microbiologist and associate professor at RK University of Gujarat, says: “Although I am a microbiologist, I mostly prefer phytochemistry, plant-mediated synthesis, where the nanoparticle is extracellular and therefore easier to recover .”
Plants, which have not been explored as thoroughly as bacteria have been than PtNP factories, produce these nanoparticles by entirely different mechanisms. For starters, the process is generally not enzyme-mediated. “We like green tea because it has a lot of phenols; it has a lot of antioxidants. These antioxidants are a very, very good reducing agent and that’s why green tea extract is one of the best methods to reduce metal ions into metal nanoparticles,” says Ghosh. “When it comes to [plant-based nanoparticle synthesis]this is usually due to polyphenols, flavonoids, ingredients or chemicals present in the plant, not enzymes.
Of course, plants come with their own complications, such as slow growth and enormous complexity compared to bacteria. The tea plant Ghosh works with in India has a distinct phytochemistry compared to a Western European tea plant thanks to differences in soil quality, age and climate. This makes it considerably more difficult to replicate studies. With their added layer of complication, however, this wild taxonomic realm offers an expanded world of possibilities.
There is a lot of work to be done before platinum grown in bacteria and tea plants is a regular fixture in NHS services. To compete with physical and chemical approaches, biosynthesized PtNPs must be produced faster and with more consistent size and shape.
“Fine control of the size and shape of PtNPs can be easily achieved by chemical or physical synthesis with control over the precursor reduction conditions,” says Satriano from the University of Catania. “The biggest challenge facing biosynthesized PtNPs is controlling their size, morphology, as well as polydisperity, all of which are necessary for the commercial use of biogenic nanoparticles. Another challenge is the difficulty of replicability due to the intrinsic differences – and associated biochemical activities – of the living systems that generate them.
It is a nascent field of research, but with enormous potential. A multitude of medical treatments could be derived from the latter-day goose: bacteria that lay platinum particles.
Precious metals in air pollution
A UK-based project investigated whether high-value metals, including PtNPs, could be captured from plants exposed to air pollution. Platinum particles from aging catalytic converters are emitted in car exhaust, contributing to particulate pollution, with serious human and environmental health implications.
Scientists have planted rock cress by roadsides in northeast England where they could accumulate nanoparticles. The goal was to keep contaminants out of the soil, grow biomass for carbon-neutral energy, and enable the recovery of high-value metal nanoparticles, on which a business model could be based.
Watercress has been genetically engineered with the help of academics from the University of York to stimulate relevant metabolic pathways. “[The researchers] helped us to introduce certain genes responsible for the bioaccumulation of this nanoparticle, so that we could improve the process of synthesis”, explains Dr. Pattanathu Rahman, founder of TeeGene Biotech Ltd and scholar at the Center for Natural Products Discovery at John University Liverpool Moores. “The normal plant can probably accumulate a few milligrams, but by using this minor modification, we could improve the efficiency by 10 to 100 times.”
Launched in 2015, the EU and BBSRC-funded proof-of-concept project was successfully completed a few years ago. Although TeeGene has turned to biosurfactants, Rahman remains interested in this work: “We would like to pursue the bioaccumulation of metals in this area. If industrial or academic partners are interested, we would be happy to support and continue this journey. »
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