Synthesis of Spherical Ag Nanoparticles
Author: Rohan Sawhney
University of California San Diego - Jacob’s School of Engineering - Nanoengineering Dept.
Abstract:
Silver nanoparticles (NPs) are capable of exhibiting localised surface plasmon resonance (LSPR), i.e. the ability to absorb and amplify wavelengths of the electromagnetic (EM) spectrum that correspond to the size of the NPs. This occurs due to an NP’s ability to confine a surface plasmon comparable or smaller in wavelength than the NP itself. At the point of resonance - when the wavelength of the incident EM radiation is equal to the natural plasmon frequency of the particle, the NP exhibits maximum absorbance of the incident radiation.
The synthesis of these NPs is relatively easy, taking advantage of Silver’s low reactivity to substitute it out of solution into a relatively pure metallic state. In this synthesis this process is done via rapid injection to ensure even distribution of the silver in solution, leading to evenly shaped and space out NPs, the sizes of which can be regulated by simply altering the growth time.
Introduction:
Ag and Au are preferred metals for plasmon analysis due to their low refractive index and negative permittivity[4] (the electric field vector and the electric displacement vector are opposing in direction). Plasmon resonance also increases the electric field density around the NPs, making organic marker tagged Au and Ag nanowires/NPs useful in sensing technologies such as antibody tests for viruses or other pathogens[1].
The natural resonant frequency of the NP is dependent on multiple factors: the size and shape of the NP, the electron density, the effective mass of the electrons and the charge of the particle, as well as the charge distribution[2]. As this synthesis uses a bottom-up approach, the NPs produced vary in size based on the formation time. The longer the reaction is allowed to run, the larger the resulting NPs are.
The NPs are formed in solution and then analysed to determine their morphology and size using optical microscopy, SEM imaging, and UV-vis spectroscopy. These can then help interpret the resonant frequency of the NPs. Along with NPs, this paper will also go on to consider how, if possible, could this procedure be altered to produce nanowires (NWs) as the following simplest morphology after freeform spherical NP formation.
Method:
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PVP is used as the capping agent to restrict growth, and stabilize the formed nanoparticles.
Add 1.287g of PVP to a 100ml bound bottomed flask.
Add ~50ml of Ethylene glycol to the same round bottom flask.
Immerse the flask in an oil bath at ~150C, and add a stir bar.
Stir the Ethylene glycol until all of the PVP dissolves.
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Add 0.2g of AgNO3 to a glass vial with ~1ml of distilled water. The vial was vortexed and sonicated until all solids were dissolved.
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1. The stirring speed of the PVP solution is increased to the maximum (while avoiding splashing).
2. The temperature of the PVP solution was determined to be ~150C.
3. Using an Eppendorf Pipette (1ml), the AgNO3(aq) solution was quickly added to the round bottom flask.
4. The reaction was allowed to continue (the bath was held at 150oC) for about 20mins.
5. (Note: The reaction rate depends on the concentration of AgNO3 solution; therefore, to finely adjust the exact size of the NPs produced, this procedure can be altered by changing this reaction time.)
6. After the 20min, the round bottom flask was removed from the oil bath and allowed to cool.
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1. 10ml of the cooled NP solution was transferred to two (15ml) centrifuge tubes each.
2. ~5ml of ethanol was added to each tube.
3. The remaining NP solution was transferred to a 50ml Falcon tube for storage.
4. The centrifuge tubes were vortexed to mix the ethanol with the reaction solution thoroughly.
5. Both samples were centrifuged for 20mins at 3500-3600rpm (Repeat this step till a substantial pellet is formed in the tube or the supernatant is clear)
6. Remove the supernatant with a pipette leaving the pellet undisturbed
7. Add ~15ml of ethanol to the centrifuge tubes and vortex/sonicate to disperse the pellet into the solution entirely.
8. Repeat these washing steps two more times
9. In the final run, preserve the ethanolic NP solution in both tubes for analytical use.
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1. Add 5ml of the sample NP solution to a new 15ml centrifuge tube.
2. Centrifuge for ~20mins at 3500-3600rpm, or until the supernatant is clear.
3. Discard the supernatant and add 200μl of ethanol to the pellet.
4. Vortex and sonicate to redistribute the pellet in the ethanol.
5. Transfer the reconstituted solution to a vial and add 2ml of CHCL3 (chloroform).
6. Sonicate the vial till the solution appears homogeneous.
7. Add a thin layer of water to the LB trough and allow many currents to settle.
8. Slowly add the Chloroform based solution to the surface, ensuring that there is a thin layer of Ag nanoparticles suspended on the water's surface.
9. Allow the film to settle across the surface, then use the Teflon coated bumpers to slowly push the film together towards the center of the trough.
10. Using a clean glass slide, use the dip-coater to apply the NP thin film to the glass slide.
11. This NP film will be used for the aggregated NP tests.
Measurements:
1. UV-Vis Spectroscopy (Suspension):
1. Fill one cuvette with ethanol (blank) and one with the sample solution (test sample)
2. Using the UV-Visible spectrophotometer, measure the absorbance of the Ag NP solution compared to the blank solution and record the intensity and frequency of any peaks.
UV-Vis Spectroscopy (Aggregate):
1. Repeat the steps from the previous UV-Vis spec.
2. Use a clean glass slide as the blank.
SEM Imaging:
1. Apply a drop (~0.5ml) of NP suspension to the SEM stage and allow it to dry completely.
2. Conduct a scan of the NPs under an SEM using BSE imaging modes.
Optical Dark Field Microscopy (Suspension):
1. Use a drop (~0.5ml) of the sample solution on a glass slide. Allow the sample to dry.
2. Using a camera-equipped Optical Microscope, capture a dark field image of the sample under a Vis-spectrum backlight.
Optical Dark Field Microscopy (Aggregate):
1. Repeat the steps from the previous microscopy, using the glass slide with the NP film directly as the sample and obtain a dark field image.
Results:
The reactant mixture changes from a colourless solution to first a reddish-brown colour, and then finally to a bluish-green colour (Figure 1.0). A silver mirror can also be seen forming on the inner surface of the round bottom flask during the reaction (Figure 1.1). The cleaning steps provided a metallic silver pellet between dispersions. However, in solution, the NPs fluctuated between bluish and greenish.
As shown in Figure 2.0, in the dark field image of the suspension of nanoparticles, each smaller dot appears to be a singular NP emitting a wavelength in visible light due to resonance. (The NPs themselves are too small to be seen due to the diffraction limit of visible light.) For the aggregated film of NPs, the optical image (Figure 2.1) shows how multiple NPs appear to merge to form a larger unordered structure. The NP’s emission also shifts to be more in the longer wavelengths of the visible spectrum, appearing silver or golden in the dark field image. From the SEM images, individual NPs can be seen having a roughly spherical morphology as well as forming small clusters in suspension. The SEM images also allow us to estimate the average diameter of the NPs (~80-100nm).
The UV-Vis spectra of the suspended NPs shows only one distinct peak at ~460nm (Figure 4.0) while the NP film shows two peaks, one distinct at ~435nm and one very broad peak at ~680nm (Figure 4.1)
Discussion:
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Colour Change of Reaction Mixture:
The reaction occurring is simply the precipitation of Ag+ (aq) as metallic Ag(s) in solution (using ethylene glycol removes the polar dissolution effects of water).
However, as the solution is being stirred vigorously, the metallic Ag(s) cannot nucleate and crystalise into one large crystalline block, but instead forms small clusters of atoms resulting in the wanted nanoparticles. PVP is required as a capping agent to prevent full crystallisation of the Ag atoms, slowing and eventually halting the growth of each NP when the heating is stopped.
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UV-Vis Spectrum:
As the suspension shows a very rough UV-Vis spectrum but agrees roughly with the film sample on the position of the first peak (~435nm as per the film spectra), this will be taken as the λmax for the NPs. (λmax = 435nm, with absorption =0.05360)
The full-width half max:
A = 0.0268
λ = ~350nm
Using Beer-Lambert’s Law: α = 𝐴/𝑑
α - Absorption coefficient
A - Absorption
d - Optical path length (in cm)
However as the absorbance appears to be too low, the absorption coefficient cannot be calculated accurately, and thus the estimated diameter of the NPs would be highly inaccurate.
A more accurate size can be determined from the SEM images, placing the avg. diameter of the NPs at ~80nm.
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Darkfield Optical Microscopy:
In this dark field (DF) image of the Ag NPs in solution, we can see multiple coloured dots comprising all the visible spectrum colours. However, as the NPs are smaller than the visible wavelength (400-700nm), these dots cannot directly reflect the visible backlight. Instead, the multiple colours are due to the slight variations in the size of the NPs, causing them to have slightly different LSPR, some of which fall within the visual spectrum and thus can be seen visually.
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Darkfield optical Image of NP film:
In the film the NPs are in a ‘solid’ closely packed state. As can also be seen, the film is visible and can be seen easily by the naked eye. The aggregation also allows for the electrical fields of neighboring Ag NPs to overlap and extend into each other, thus changing the resonant frequency. This can be seen by the distinct lack of any individual colour being amplified, instead, the entire film appears to be roughly uniform, with a few out-of-plane aggregates appearing metallic (silver).
Acknowledgements:
This experiment was made possible by Prof. Sheng Xu, with the lab assistance of Dr Stephen Horvath. The lab time, reagents and facilities were provided in the Structural and Material Engineering complex, by Jacob’s School of Engineering - Nanoengineering department.