We were given a few samples of silver nanoparticles (AgNPs) samples (of different colours, green and yellow solutions) for the investigation of the antifungal efficacy of AgNPs. The AgNPs were synthesised using chemical reduction method but we do not know the concentration of the nanoparticles.
We determined the average sizes of the nanoparticles in the sample solutions using Malvern Zetasizer software, therefore we thought that we can easily get the molar extinction coefficient (from literature search).
Applying the Beer-Lambert Law A=ECl, assuming that the extinction coefficients found are correct (using size-dependent table), we can use UV-vis spectroscopy to measure absorbance of our AgNPs solutions samples and then find the concentration of each of the samples.
1.Do we need to use a AgNP solution of known concentration to compare the absorbance (in a calibration curve)? If we need to do a calibration curve, does the solution have to be AgNP solution or does it have to be the solvent (in this case sodium trinitrate)?
Another question is, AgNP aggregates quickly so the shape may change. How do we study the sizes of nanoparticles? Can this be done using microscopy? We are aware that pH is important for the stability and can affect the shape of nanoparticles, instead of microscopy can we use pH measurement to determine the stability of the AgNP samples, but what is the pH that has the most stable AgNPs?
Dear Sir. Concerning your issue about determination of the concentration of silver nanoparticles using UV-visible spectrometry. The colour of the colloidal solutions varied from light yellow to dark brown depending on the concentration of nanoparticles. There was no obvious aggregation of particles as the colour remained stable even after autoclaving at 15 psi for 20 min. Aggregation is generally visualized by a change in colour as the yellow darkens, turns violet then grayish as the particles settle out. Finally a colourless solution with black precipitates of silver is seen. The formation process and the optical properties of the silver nanoparticles can also be identified from both the colour change and the UV-Vis spectra of the solutions (He et al., 2002). For more details, I think the following below link may help you in your analysis:
http://shodhganga.inflibnet.ac.in/bitstream/10603/23638/11/11_chapter%203.pdf
Thanks
Hi,
To answer some points in the latter part of your question, to understand the stability of your samples, there are a number of measurements that you can do using the Zetasizer.
Measurement of the particle size distribution as a function of time will reveal quite easily if your sample is stable, as you will be able to track the size of aggregates and their relative abundance. Temperature, pH and concentration effects can also be understood by comparing how the particle size changes in response to these parameters.
Depending on the size of your particles, it can also be informative to do a Stokes law calcualtion to determine whether sedimentation is likely to be a significant factor in their stability.
With particular focus on pH stability, measurement of the zeta potential as a function of dispersant pH can reveal the isoelectric point, where aggregation is most likely. A pH with a strongly negative or positive zeta potential indicates that your particles are electrostatically stable.
In terms of calculation of concentration, as well as UV-Vis, there are a number of particle counting technolgies available that may be able to help, including the Malvern Panalytical Nanosight with performs particle tracking measurements, or the Archimedes which can measure the mass and size of individual particles.
Imaging with methods like TEM can of course give you important information of size and shape, but this may be of less relevance if the properties of the particles in dispersion are of most interest.
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