Synthesis and Characterization of Colloidal Silver nanoparticles of about 16 nm Diameter by a Chemical Reduction Method

Introduction

Metal nanoparticles are the good number studied nanopartcles among which AgNPs paid an excellent interest due its vast applications. Last 30 years has noticed the tremendous use of AgNPs in different fields like electronics, medicines, healthcare, cosmetics, food packaging, agriculture, environmental cleaningup programme, energy production and tank-storages.^1–8AgNPs have potential antimicrobialactivity and for this reason this nanoparticles are able to prevent infections.⁹ Water filter coated with AgNPs have been marketed by different manufacturers.¹⁰ AgNPs can be synthesized by using variety of methods in solution such as electrochemical, photochemical, chemical reduction, microwave processing, ultra-sound processing. ^11–15 Several strategies are derived to produce metal nanoparticles like AgNPs of meticulous geometry depending on precise necessities for variety of utilization are based on its geometry. ^16,17  In the past nearly 22 years, a fresh route emerged allied with the research of non-spherical nanoobjects both planar like circular plates, triangles, penta- and hexagons, etc. and 3D like cubes, prisms, tetrahedra, rods, wires, etc.

Now scientists paying good attention on making nanoparticles via colloidal system as in solution phase synthesis methods are very much adaptable and suitable method. Here the prime aim to synthesize colloidal silver nanoparticles which are spherical in nature because spherical particles that have the least amount surface for a given volume are thermodynamically more stable.¹⁸ The chemical reduction technique is preferable for this particular nanoparticles synthesis because it uses ordinary and very economical reducing agents like NaBH₄, salt of ascorbic acid, salt of citric acid, and elemental hydrogen.^19–21

Experimental

Chemicals and reagents

AgNO₃ was purchased from Indian Drug and Pharmaceuticals LTD., Hyderabad, India. Sodium borohydride (NaBH₄) and Milli-Q waterwere purchased from Merk, Mumbai, India. All the chemicals used for the synthesis of colloidal nano particles are of highest purity available. All the reactions were performed by using Milli-Q water.

Synthesis of Silver nanoparticles

Yellow coloured colloidal AgNPs were synthesized by reducing AgNO₃ with a mild reducing agent sodium borohydride. Ag⁺ ions were reduced to Ag° by BH₄ˉ ions which is used as a reducer as well as a stabilizer.^22,23 One millimolar of aqueous silver nitrate solution (1 mM) of about 1 ml was taken in a 2 ml micro-centrifuge tube and cooled it by keeping it in ice for 10 minutes. In a small beaker, aqueous NaBH₄ solution (2 mM) of about 3 ml was also cooled in ice. After 10 minutes, drop wise AgNO₃ solution was added to the NaBH₄ solution with constant stirring. The ratio of the reactants AgNO₃ and NaBH₄ in the recation was (1:2).^24,25

AgNO₃ + milli Q-water → AgNO₃ (aq)

NaBH₄ + milli Q-water → 2NaBH₄ (aq)

2AgNO₃(aq) + 2NaBH₄(aq) → 2Ag⁰ (aq) + B₂H₆ + H₂ ↑+ 2NaNO₃ (aq)

UV-Vis Spectroscopy

The formation AgNPs were performed by utilizing UV-Visible spectroscopy. This study was performed by SHIMADZU Spectrophotometer model no. TCC-240A at 25°C. A rectangular cell of path lenghth 1 cm was used for this purpose. AgNPs were prepared by reduction of AgNO₃ with sodium borohydride.

X- ray diffraction study (XRD)

Colloidal AgNPs were lyophilized to get solid AgNPs for the purpose of powdered XRD study. The X- ray diffraction pattern of AgNPs was recorded with the help of the intrument Brucker AXS (Model D8, WI, USA) by using CuKα supply ( λ =1.5409 Å) at the time of diffractogram recording. The 2q range and the scan rate were set up 0° to 80° and 5°/minute respectively. This experiment was done by keeping the current 40 mA at 25°C.

 Scanning electron microscopy (SEM)

Surface morphological study and size of the synthesized colloidal AgNPs were done by SEM (model Zeiss EVO 18 special edition, Japan) with operating voltage 15 Kv. Dilution of the colloidal AgNPs was done by ethanol and then a droplet was casted on glass slit. Then it was reserved in vacuum desiccators for drying. Finally it was gold coated for SEM.

 HRTEM for morphological study

HRTEM is a dominant instrument to study their interface structures, morphology, particle size and bare surfaces of the synthesized metal nanoparticles like AgNPs. The geometric shape and size of the sample was measured with the help of JEOL-HRTEM-2011-Tokyo(Japan) by keeping the accelerating voltage range 80-85Kv. A drop of 50 times diluted AgNPs solution was casted on copper grid (carbon coated) having mesh size 300C (PRO Sci Tech). AgNPs containing Cu grids were incubated for about 6 hour before taking the final images.

Result and discussion

UV-visible Spectroscopy

The synthesized AgNPs were primarily characterized by UV-vis spectroscopic study (Fig.1). The generation of UV-visible yellow colour obviously indicates the creation of AgNPs and it confirmed by the surface Plasmon resonance. The UV-visible spectroscopy clearly shows the Plasmon absorption band of AgNPs with peak maxima at ~404 nm. This value indicates that the size of the synthesized AgNPs is of about 16±2 nm which is already mentioned in various literature.²⁶

Figure 1: UV-vis absorption spectra of AgNPs Click here to View Figure

X- ray diffraction study (XRD)

The pattern of XRD recorded for the sample AgNPs (shown in Fig. 2) is  displayed  that the four peaks were stand for the (111), (200), (220) and (311) planes of fcc silver respectively. The Bragg’s peaks are broad in nature which indicates the development of nanoparticles. The four distinct diffraction peaks were represented in Table 1. This value in accord with the reported literature (JCPDS file no. 84-0713 and 04-0783).^27,28

Table 1: The different values of 2q vs planes hkl

2q (degree)	38.15	46.10	67.35	77.70
hkl	111	200	220	311

 

Figure 2: XRD pattern of AgNPs Click here to View Figure

Scanning electron microscopy (SEM)

Fig. 3a represents SEM image of the AgNPs which reveals the surface morphology of the synthesized nanoparticles. Most of the AgNPs are spherical in nature represented by Fig.3b-c. which are the zoomed in picture of Fig. 3a.

Figure 3: SEM images of AgNPs nanoparticles with different magnification (a-c) Click here to View Figure

HRTEM for morphological study

HRTEM study proofs (Fig. 4d-f) that the morphological structure of synthesized AgNPs is spherical in nature and average diameter is of about 16±2 nm. The SAED picture with many bright points is shown by Fig. g which is the evidence of the formation of crystalline AgNPs.

Figure 4: TEM image of synthesized colloidal AgNPs (d). Zoomed in images are (e and f) of the same. Bright SAED pattern is represented by Fig. 4g. Click here to View Figure

Conclusion

This article represents successful synthesis of colloidal AgNPs by chemical reduction method where aqueous silver nitrate was reduced by aqueous sodium borohydride. The UV-vis study shows absorption maxima of synthesized nanoparticles is ~404 nm which is a preliminary indication of generation of AgNPs having average diameter of about 16 ±2 nm. Further SEM and TEM images confirm this diameter. The nature of the metal nanoparticles fcc and crystalline was confimed by the XRD study and SAED pattern respectively. This article is significant in the fileld of synthesis of AgNPs because the unique properties of colloidal AgNPs makes them precious building blocks for developing sophisticated materials and technologies with a broad range of applications.

Acknowledgement

The author gleefully acknowledges Prof. Umesh Chandra Halder, Jadavpur University, Department of Chemistry, for giving me the necessary research amenities.

Conflict of Interest

The author declares no conflict of interest.

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