Introduction

Synthesis of nanoparticles is a remarkable improvement in the area of nanotechnology. Nanoparticles will have special properties due to their large ratio between surface and volume and larger surface energy. Metal based nanoparticles are known to have many applications. In the process of synthesis of nanoparticles, the toxic reagents and organic solvents that are used here are found to generate environmental pollution. Performing the synthesis of nanoparticles in an eco-friendly manner has led to the way of using biological system which is effective and cost efficient.

The AuNPs are known to have biomedical applications. AuNPs absorb light in the visible section of the spectrum because of surface plasmon resonance and converted it into heat. AuNPs are beneficial for the imaging and photothermal therapy. Gold nanoparticles or AuNPs are useful in various biomedical applications like leukemia therapy, bio-sensor production, bio-molecular immobilization, and labeling for contrast enhancement in cryoelectron microscopy.

Organisms like Pseudomonas aeruginosa, Actinobacter sp., Rhodopseudomonas capsulata, and eukaryotes like Fusarium oxysporum are functional in the green synthesis of gold nanoparticles. Depending on the location of the reducing agents, these organisms reduce the metal ions available intra or extracellularly into corresponding nano metals.

AuNPs can be synthesized also by various parts of the plants like Neem leaf broth and roots of alfalfa and so on. The current study focuses on the synthesis of AuNPs by micro-alga Tetraselmis suecica after the algal cell extracts are incubated with HAuCl₄ solution.

Methodology

The T. suecica axenic culture was grown and stored in the sterile medium of f/2 seawater-agar. The culture is further sub cultured to be used afresh in some more experiments. Sufficient algal mass was prepared by first inoculating the microorganism into flasks.

AuNP preparation

The algal cultures of T.suecica were harvested in the logarithmic phase and were centrifuged to collect the cells of the algae. The harvested algal cells are taken in a mortar and ground with liquid nitrogen using a sterile pestle. The disturbed cells were centrifuged to collect the cell extract from the supernatant. The synthesis of AuNPs was monitored by mixing the T.suecica cell extracts with the aqueous 10^-3 M HAuCl₄ . The mixture is incubated at various temperature ranging between 30 and 90 degree centigrade in the water bath. The conversion of yellow colored HAuCl4₄into red color designates the formation of AuNPs.

Analysis

The spectrum of the reaction mixture was read by Double beam PC scanning spectrophotometer. The monitoring was done by taking the blank as the solution of cell extract and de-ionized water. The UV-visible patterns for aqueous 10^-3 M HAuCl₄ after disclosure to various temperatures were scrutinized. The transmission electron microscopic analysis was performed by dropping the AuNP solution into the carbon coated copper TEM grids. The X-Ray diffraction analysis of algal cell extract which reduced AuNPs was conducted on the solution films that were placed on the glass substrates.

Fourier transform infrared spectroscopy (FTIR) analysis was also done with the centrifuged pellet having dried powder of AuNPs solution. The pellet was washed with other chemicals and oven dried before it is analyzed for FTIR spectroscopy.

Results of the study

To replace the procedure of the chemical reduction of aqueous 10-3 M HAuCl4 with the green synthesis methods, cell extracts of microalga T. suecica were used to synthesize gold nanoparticles. Gold nanoparticles display red color in aqueous medium due to scattering of visible light based on the size of the particle.

Synthesis of AuNP occurred in the mixture consisting of 1 ml of algal cell extract when incubated at the temperature of 90 degree celcius for five minutes. The AuNP formation did not happen in other mixtures at other temperatures.

It is investigated and understood that the formation of AuNPs at the highest temperature was due to the formation of reducing agents by the algal cells after the incubation time. The reduction alters the color of the gold nanoparticles from yellow to red.

AuNP synthesis is detected by UV-visible spectroscopy. Light absorbance occurs in the wavelength of metallic nanoparticles. If the gold particles are of the size < 25 nm diameter, the shift of the surface plasmon peak will be short. For the particles of size > 25 nm in diameter, the surface plasmon peak reveals the red shift. The contact of light with wavelength shorter than the particle size, with AuNPs led to the polarization of free electrons giving rise to absorbance at 510 to 540 nm. The absorbance for aqueous HAuCl₄ was observed at 308 nm. Once the algal cell extract was added and incubated at 90 degree Celsius a vivid absorbance band was created at 530 nm.

The separated spherical AuNPs without any aggregation was observed under transmission electron microscope. The gold nanoparticles were formed with the diameter range of 51 nm to 120nm. The surface area of AuNPs was specifically 57.9 m^2*g^-1.

Synthesis of AuNPs with the use of T.suecica cell extracts and incubating for 5 mints at 90 degree Celsius resulted in the most frequent formation of particles with the diameter of 79 nm. The differences might arise if there is any change in reducing agents present in the various algal species.

The X-Ray diffraction showed the formation of AuNPs at four peaks placed at 38.2 degrees, 44.6 degrees, 63.1 degrees, and 76.6 degrees. This pattern was similar to the Bragg reflection that was performed for AuNPs earlier. FTIR spectroscopic evaluation revealed the peaks for AuNPs at 3431.58 and 1675.11 wavenumbers.cm^-1 .

Reference

Mojtaba Shakibaie, Hamid Forootanfar, Kamyar Mollazadeh-Moghaddam, Zahra Bagherzadeh, Nastaran Nafissi-Varcheh, Ahmad Reza Shahverdi and Mohammad Ali Faramarzi. Green synthesis of gold particles by the marine microalga Tetraselmis suecica. Biotechnology Applied Biochemistry (2010) 57, 71-75. DOI: 10.1042/BA20100196.


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