Silver Nanoparticles

 

Preparation of Silver Nanoparticles

Silver nanoparticles are currently being studied extensively because of several properties that can be used in science and technology.  Silver nanoparticle preparation is also being experimented upon and studied to find the most cost-efficient and effective way to synthesize them.  The right preparation is important for the size, shape, and chemical surrounding are the determinants on the catalytic, electronic, magnetic, and optical properties of silver nanoparticles. 

There are several ways to synthesize silver nanoparticles, all of them starting with a preparation of silver nitrate (AgNO3). 

Uniform and stable silver nanoparticles can by synthesized by reducing silver ions with the use of ethanol. The following compounds are mixed in a capped tube:   

Ø  20 ml of aqueous solution containing 0.5g of silver nitrate (AgNO3)

Ø  1.5 g sodium linoleate (C18H32ONa)

Ø  8 ml ethanol

Ø  2 ml linoleic acid (C18H32O2)

The solution is agitated and exposed to temperatures 80°C to 100°C under atmospheric conditions for 6 hours.   Ethanol in both liquid and solution phases causes silver ions to shrink into silver nanoparticles.  The silver nanoparticles are nearly circular in shape due to the absoption of linoleic acid on the surface including the alkyl chains.  The solution is then cooled to room temperature and dispersed in chloroform.  The dispersion caused a formation of a homogenous colloidal solution of silver nanoparticles that is reddish brown in color.  The color is achieved by adding linoleic acid, and indicates an almost 100% conversion of silver ions to nanoparticle silver.  The resultant silver nanoparticles were recorded to be of optimum size and stable for over 4 months. 

Silver nanoparticles that were produced in this way were observed, under a transmission electron microscope (TEM).  The morphology of the nanoparticles formed was recorded as smooth and spherical in shape.  The diameter was measured to be 16 nm.  It was also found that the silver nanoparticles produced were generally uniform in size and shape. 

The color of the solution is the result of the optical properties of the produced silver nanoparticles.  Color absorption of the colloidal solution is highly dependent on the size of the particle, the dielectric medium, as well as the chemical surroundings.  A silver nanoparticle that is spherical in size and less than 20 nm shows a single surface Plasmon band.  The UV/Vis absorption peak (SPR) was in the visible range of 410 nm. 

Silver nanoparticle preparation can also be done in a different manner.  An aqueous solution of silver nitrate (AgNO3) along with ethylenediaminetetraacetic acid disodium (EDTA) and poly-vinylpyrrolidone (PVP) was exposed to sonoelectrochemical deposition.  Silver nanoparticles and silver nanowires were produced in the solution.  Electro-chemical reduction with a controlled-current was utilized for this preparation.  Electrolysis was done by using a cathode of stainless steel and a counter-electrode of Rh-Ti alloy web.  A 60mA current was maintained for an electrolysis that lasted 25 minutes.  A 50 ml electrolyte was prepared by adding AgNO3 and EDTA in 50 mL of distilled water.  An ultrasonic bath was used with an electrolytic cell in place along with an ultrasonic field of 20kHz, 100w.  30°C was maintained as a base temperature.  The solution was subject to centrifuge after the reaction and purified several times with the use of distilled water and ethanol. 

Drops of the solution derived under this silver nanoparticle preparation was placed on a carbon film that was supported by a copper mesh grid. The silver nanoparticles formed were spherical in shape and sized 30nm.  The nanowires were measured at 30nm and 200-900 nm long.  The concentration of the silver nitrate as well as the presence of EDTA and PVP was studied comparatively in the synthesis. 

A concentration of 0.0118 mol·L-1 produced silver nanoparticles while higher concentrations caused aggregation and dendrite formation.  It has been concluded that to create silver nanoparticles, low concentrations are favorable.  Higher numbers of silver atoms may create large numbers of silver nanoparticles but because of lack of space, they cluster together, making them form a structure that is a deviation of the intended nanoparticle.

EDTA was found to be a crucial compound in this silver nanoparticle preparation.  If EDTA was not added in the electrolyte, no silver nanoparticles were formed.  A ratio of 1:1 silver nitrate and EDTA formed spherical nanoparticles of 30 nm.  Higher concentrations of EDTA produced spherical particles of 100nm.  The release of EDTA molecules became stabilizers that promote silver nanoparticle formation, but this stabilization decreases as the concentration increases.  Spherical silver nanoparticles of 30nm were formed along with nanowires of 30nm in diameter and 200-900 nm in length were produced when PVP was added. In this experiment, PVP acts as dispersant and a preservative as it complex with silver ions.  Silver nanowires formed were seen to have the protection of PVP in them. 

Biosynthesis of silver nanoparticles was also postulated by some scientists as an alternative form of silver nanoparticle preparation.  A microorganism’s metabolic activity was reported to precipitate nanoparticles outside of the cell.  Fungi were found to be highly efficient for this procedure.  Colletotrichum sp. or Aspergillus fumigates were reported to have extracellular synthesis of nanosilver.  Mukherjee et. al. proposed a biosynthesis of silver nanoparticles with Vercillum under a two-step mechanism.  First, the silver ions were trapped on the fungal surface.  Afterwards, the released enzymes of the fungi reduced the silver ions to nano size.  Duran et. al. described how the Fusarium oxysporum strain of fungus formed silver nanopartilces in the extracellular level because of the presence of hydrogenase.  The enzyme was observed to have exceptional redox properties, acting as an electron shuttle for reduction of metal ions.  The conclusion was that reducing agents (hydroquinones) along with electron shuttles that were given off by microorganism were effective for reduction of ions to nanoparticles.

It is important to prevent aggregation during silver nanoparticle preparation.  Some believe that maintaining the silver nanoparticles in a powdered form will be enough to maintain the size of silver nanoparticles.  However, researchers have discovered that aggregation can occur when silver nanoparticles are in their solid state.  Either way, it is important to check for the stability of produced silver nanoparticles in storage depending on how they were synthesized.

Sources


Preparation of Silver Nanoparticles and Their Characterization:http://www.azonano.com/details.asp?ArticleId=2318

Synthesis of silver nanoparticles using microorganisms: http://www.materialsscience.pwr.wroc.pl/bi/vol26no2/articles/ms_2007_52BIO03.pdf

Synthesis of silver nanoparticles:  http://www.mrsec.wisc.edu/Edetc/nanolab/silver/index.html

Synthesis and characterization of silver nanoparticles by sonoelectrodeposition: http://www.docstoc.com/docs/22838297/Synthesis-and-characterization-of-silver-nanoparticles-by

 


 



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