Microbial nanotechnologists

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  • Published: Aug 1, 2009
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Microbial nanotechnologists

The bacterium Bacillus licheniformis is an expert nanotechnologist, according to scientists in India. They have used the microbe to help them synthesise gold nanocubes, as verified by UV spectroscopy and other techniques. The approach offers an alternative approach to making these important nanoparticles without using high temperatures or toxic solvents.

Kalimuthu Kalishwaralal, Venkataraman Deepak, Sureshbabu Ram Kumar Pandian, and Sangiliyandi Gurunathan of the Department of Biotechnology, at Kalasalingam University, in Anand Nagar, Tamil Nadu, India, have used the bacterial skills of B. licheniformis to make gold nanoparticles just 10 to 100 nanometres across. UV spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction reveal details of the products. B. licheniformis is well known as a microbe cultured for its protease, which is used in biological washing powder.

"Gold nanocubes syntheses have recently emerged in the field of nanotechnology and scientists are exploring various applications of them," Sangiliyandi told SpectroscopyNOW, "recent major applications of nanocubes are tumour cell detection and targeting cancer cells for various kinds of treatments."

The teams approach to nanocubes builds on earlier efforts by other researchers to use microbes as nanotechnologists, however, it offers two distinct advantages, first it works in an aqueous solution and also avoids toxic solvents and high temperatures. Other advantages of biological methods are increased control over the formation of nanoparticles, a highly reproducible synthesis, as well as the possibility to make biocompatible particles.

All that glitters is not gold, of course, but in the world of nanoscience, gold is one of the elements of choice for the construction of useful particles on the sub-microscopic scale. "Nanotechnology encompasses the synthesis of nanoscale materials, the understanding and the utilization of their physicochemical and optoelectronic properties, and the organization of nanoscale structures into predefined superstructures," says Gurunathan.

Making gold nanoparticles usually requires high temperatures, organic solvents and toxic reagents, such as borohydrate reducing agents and acetylene. Some researchers have turned to nature in the hope of greening the field of nanoparticle synthesis. Commercially viable and environmentally clean approaches for highly stabilized gold particles have been developed, using microbes such as Bacillus subtilis, Rhodopseudomonas capsulata, Fusarium oxysporum, Sargassum wightii, Lactobacillus species, and Helminthosporum solani.

Gurunathan adds that one of the most prominent reasons for turning to microbes as nanotechnologists is that bacteria are relatively easy to handle and can be manipulated genetically so that the end products can be fine tuned for particular applications.

The researchers grew B. licheniformis in a nitrate medium with yeast extract and incubated them at room temperature for 24 hours. The cultures were then centrifuged and wet cells re-suspended in 100 ml of a 1 millimolar aqueous gold(IV) chloride (chloroaurate) solution and incubated for 48 hours. The method essentially involves the reduction of aqueous gold(IV) chloride ion by B. licheniformis at room temperature in a single-step. They were able to remove the gold nanoparticles from the final suspension by slow evaporation for analysis.

XRD confirmed the crystalline nature of the nanoparticles, while the SEM analysis revealed the particles to be cubic. They used UV-Vis spectra and the XRD pattern to calculate particle size.

The team explains that their UV-Vis spectra indicate that the reaction solution has an absorption maximum at approximately 540 nm. This, they say can be attributed to the surface plasmon resonance band (SPR) of the gold nanoparticles. "Observation of this peak, assigned to a surface plasmon, is well documented for various metal nanoparticles with sizes ranging from 2 to 100 nm," the researchers add. Based on their additional calculations they were able to fit the theoretical values for particle size in the range 5-100 nm. At this point, however, the actual physical mechanism by which the gold nanocubes form is unknown.

"This single-step greener approach is general and cost effective. The flexibility of gold nanocubes could find applications in drug delivery and recently, gold nanocubes have extended its application to fields such as cancer diagnosis and treatment," the team concludes.

Sangiliyandi told SpectroscopyNOW that the team now has a paper in press in which they discuss size control of silver nanoparticles produced in a similar way in which temperature, silver nitrate concentration and pH are the controlling factors applicable to nanocube size.

 

B. licheniformis (Credit: Milton Wainwright, Sheffield University)
Bacteria that build nanocubes

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