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Synthesis

Discuss about the Study of Gold Nanocluster?

Study of Gold Nanocluster

Introduction

Nanomaterials are the materials which are made up of structural elements which have the characteristic dimensions ranging from 1 nm to 100 nm, all properties of these materials are determined on the basis of size and the arrangement of structural elements. There are four main types of nano-structured materials on the basis of dimensions, which are zero, one, two and three dimensional nanomaterials. Zero-dimensional nanomaterials includes nano-clusters and nano dispersions, so we can say that  the materials where nanoparticles are away from each other or they are not attached with each other. In the 4th century the synthesis of colloidal gold was important due to its property of change of colour depending on the position of light source. In the middle age the gold solutions were highly used for curing the various diseases. In the year 1856, Michael Faraday (Faraday 1857) investigated on the optical properties of gold when he accidentally created a ruby red solution while making gold sheets on microscope slide. In the 20th century, the research on gold nanoparticles accelerated as it was highly stable and can be easily synthesised.

Synthesis

The synthesis of gold nanoclusters (AuNCs) has several stages. Earlier, the focus was on gas state metal clusters. But as these metal clusters in gas phase were short-lived, the solution-phase synthesis was introduced in 1980. These synthetic methods helped in developing gold nanoclusters with high stability and better physicochemical properties. The synthetic methods were of two types: 1. Atoms to Clusters and 2.Nanoparticles to Clusters. In the first one the gold ions are reduced into zerovalent atoms, and further the AuNCs are formed with nucleation of Au atoms. Gold nanoparticles are easily reduced in comparison to the nanoclusters due to the aggregation of AuNCs. For this we use ligands so that the surface can be modified and helps in stabilising the nanoparticle which also helps in enhancing the fluorescent effect. Therefore, the selection of ligand such as phosphine and thiolate is essential to get the stabilised nanoclusters. The other method for synthesizing the AuNCs is Nanoparticles to Cluster route. In this method the etching of surface atoms of gold nanoparticles is done by ligands such as thiols (Mohammed et al. 2008) and bio macromolecules (e.g., BSA). In this procedure firstly the mixing of surface-stabled gold nanoparticles and etchant which are small molecules or polymers is done and then etching of AuNCs surface with ligand exchange is done which generates AuNCs. This method can improve the fluorescence of AuNCs.

In this we have synthesized AuNCs  from a solution of HAuCl4.3H2O (Gold Chloride Tri-hydrate ), NaOH(sodium hydroxide), C6H5Na3O7 (sodium citrate) along with distilled water. In a beaker 0.05 gm of gold chloride trihydrate is mixed with 60 ml distilled water. Another solution, by mixing 0.02 gm sodium hydroxide  with 110 ml of distilled water. NaOH is added to the solution unless it achieves the pH of 6.6. Now, solution is heated  and maintained at 85 °C with continuous stirring for about 30 minutes. Sodium Citrate (C6H5Na3O7) solution is prepared by adding 0.07 gm of sodium citrate in 50 ml of distilled water. Heat 20 ml of this solution  upto 85 °C and then add it to the solution of gold  under the condition of continuous stirring. This temperature of this solution is maintained at 85 for about 15 minutes. Here the gold is reduced  (Au+3→Au°) leading to production of nanoparticles and we observe that the there is change in the solution’s colour which is pinkish red. Gold in macromolecules the face centred cubic structure (fcc) is observed but as we reduce the particle size the fcc structure also changes in centred - icosahedral structure.

Characterisation

Characterisation

 Various characterisation techniques are carried out to study the properties of gold at nanoscale like High Resolution Transmission Electron Microscopy (HRTEM), X-Ray Diffraction, Infrared Absorption and mechanical indentation.

HRTEM

The size and symmetry of AuNC were investigated under TEM with a source of electron emission gun. Images were taken at 300 kV keeping the value of current at 10 μA. HRTEM observation shows that the surface Au-NP is highly sensitive to the source i.e. electron gun as it shows the creation of some coalescence effect between them.(Casillas et al. 2012)

XRD and Infrared Absorption

Powdered XRD was carried out, Cu-Kα rays were passed from the specimen then we found the main reflections at (111), (300), (330) and (421). Broader peak was observed at an angle of 2θ = 25°. Infrared absorption was done . In the infrared absorption spectrum the peaks are found at 3450 cm -1 which shows the presence of OH and 1650 cm−1 which indicates the presence of Chloride .

Mechanical Nanoindentation (Volinsky, Moody & Gerberich 2004)

In this method the mechanic-structural properties are studied. All the values are achieved by using Continuous Stiffness Measurement (CSM) (Li & Bhushan 2002).The instrument had a  Berkovich diamond indenter whose tip had a very small radius i.e 20 nm, depth limit of the indentor is 35 nm,harmonic displacement of 1 nm at a frequency of 75 Hz is used. For the measurement of elastic modulus, hardness  and elastic stiffness, this technique is used. Elastic Stiffness and hardness are determined from the curve of  load and displacement. Elastic Modulus is illustrated when the max. Penetration is achieved, so  here the CSM technique enables us to measure the  elastic stiffness during the countinous application of load instead of just at the initial unloads. CSM is used to study the mechanical properties of thin films, therefore, here  the AuNPs are mounted on substrate which are made up of silicon.

Mechanical Properties:

The increase of 2nm to 6nm  in the displacement on penetration of indentor ,the hardness of 1.70 ± 0.03 GPa is obtained, close to  the particles in bulk. The graph of elastic modulus is shown in Figure 2, showing average value 100 ± 2.0 GPa which is higher in when compared with macromaterials.

Elastic Stiffness can be calculated from this formula:

Elastic Stiffness Formula

where w is the frequency of excitation, Z0 is the amplitude of displacement, ∅ is the phase angle,F0 is amplitude of excitation, Kf is stiffness of load, Ks is stiffness of supporting spring ,& m is the mass .

Hardness and function of displacement of AuNCs

Figure1. Hardness and function of displacement of AuNCs.

Elastic Modulus from CSM nanoindentation

Figure 2. Elastic Modulus from CSM nanoindentation.

Advantages and Disadvantages

Gold nanoparticles are widely used for high-technology applications within the fields of life science and materials science when the size of gold nanoparticles is reduced upto a diameter of 5 nm or less then it can be used as a catalyst. Normally, gold is highly inert material to be used as a catalyst. However, at nanosize, it can act as a catalyst that can do things such as oxidisation of  carbon monoxide gas. Au nanoparticles further offer a particular unique set of physical, chemical and photonic properties therefore; these are used in drug delivery to treat various diseases like cancer and tumours (Neutrons reveal 2013). It also has the capability of converting certain wavelength of light into heat. With these advantages it has some limitations also, these nanoparticles are toxic in nature which helps in the treatment but along with that it also destroys some cells of the membrane. The presence of gold nanoparticles reduces the presence of a protein named adiponectin which is involved in controlling glucose levels and helps in breakdown of fatty acid to regulate the metabolism of the body.

Uses: AuNPs are used as conductors in microprocessor chips. These nanoclusters have the potential to treat tumours as it heat up rapidly. They also have application in the fuel cells which are used for automobiles.

Environment Impact: The gold nanoparticles cause bio toxicity as they have the potential to be internalized in the exposed plants (Toxicity and efficacy). There are studies which indicate bioaccumulation of Au through tropic transfers in food chains from exposed plant. These also help in oxidation of the carbon monoxide thus resulting in the purification of the harmful gas. It does not have any harmful effect on the water cycle.

References:

Casillas, G Palomares-Báez, JP Rodríguez-López, JL Luo, J Ponce, A Esparza, R Velázquez-Salazar, JJ Hurtado-Macias, A González-Hernández, J José-Yacaman, M 2012, ‘ In situ TEM study of mechanical behaviour of twinned nanoparticles’   , Philosophical Magazine, vol.  92, pp. 4437–4453.

Faraday,M 1857, ‘ The Bakerian Lecture: Experimental Relations of Gold (and Other Metals) to Light’, Philosophical Transactions of the Royal Society, vol.147, pp. 145-181.

Li, X & Bhushan, B 2002, ‘A review of nanoindentation continuous stiffness measurement technique and its applications’, Journal of Material Characterisation, vol. 48, pp. 11–36.

Muhammed, MAH Ramesh, S Sinha, S Pal, SK &  Pradeep, T 2008, ‘Two distinct fluorescent quantum clusters of gold starting from metallic nanoparticles by pH-dependent ligand etching’, Journal of Nano Research, vol. 1, no. 4, pp. 333–340.

Neutrons reveal potential dangers of gold nanoparticles – pharma’s drug delivery agent of the future, 2013. Available from:< https://www.ill.eu/press-and-news/press-room/press-releases/neutrons-reveal-potential-dangers-of-gold-nanoparticles-pharmas-drug-delivery-agent-of-the-future-7062013/ >. [9 February 2016].

Toxicity and Efficacy of Gold Nanoparticle Photothermal Therapy in Cancer, Available from: < https://nano.cancer.gov/action/programs/platforms/emory.asp >. [9 February 2016].

Volinsky, AA Moody, NR & Gerberich, WW 2004, ‘ Nanoindentation of Au and Pt/Cu thin films at elevated temperatures’,  Journal of Material Research, vol. 19,  pp. 2650–2657.

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