Whenever I measure the nanoparticles size I get a higher size using Dynamic Light Scattering but a considerably small size with TEM images. What can be the reason behind that?
DLS gives you the hydrodynamic radius (not the exact size of the particle/the hyration layer lets the particles appear larger)
DLS gives you the hydrodynamic radius (not the exact size of the particle/the hyration layer lets the particles appear larger)
@antranik is know this reason but can you suggest at what level they differ between each other??? because I synthesized the particles they give 90 nm with DLS and 15 nm with TEM images? is this difference fair enough?
Intuitively it seems that the difference between the DLS and TEM values is too large to attribute it just to the hydrodynamic boundary layer alone.
DLS measurements can be quite tricky. In order to analyze this phenomenon, you must look at the following questions:
Are your nanoparticles monodisperse in TEM or aggregated ? If they are aggregated you will measure the size of your agglomerates.
If they are monodisperse you may have not adjusted the stability of the suspension in such a way to get a stable colloid. Self assembly of the particles may then lead to "particle sizes" which are too high.
Measure the zetapotential of your dispersion and stabilize the dispersion at a pH value of maximum zetapotential.
I agree with Frank, that aggregation might be part of the as well as the hydrodynamic effect.
You can try to dilute the concentrations and see if that affect the measured size or use a mass sensitive method for characterizing the aggregation state. See for instance the attached paper
@mihail yes i am using oleic aicd as a surfactant.
@frank thanks for giving a detailed answer. In my TEM image i am getting aggregates. Also I did the pH analysis the particles aggregate more if the pH is decreased to 6.5
In nanotechnology,in my opinion,in comparatively & in both the studies,and the case for iron oxide nanoparticles,when our studies go for the formation of hydrodynamic layer or by using acid as for the preparation of double layer as other described for surface study through diffused layer by other charge carriers,there always a lag (1).for Ist,in DLS,between the incoming ray & the ray which has been scattered (2).for IInd,in T.E.S.,a path difference
Your nanoparticles are agglomerated when you measure DLS in my opinion. Because if you know that an oleic acid molecule is 1.97 nm, you know that if your nanoparticles are monodisperse you should get a hydrodynamic diameter of approximately 15 nm (from TEM) + 1.97 nm x 2 = 18.94 nm in DLS.
I agree with Frank Kern that you should ask yourself if the sample you used for DLS and for TEM are showing the same conditions (because you need to dilute your sample for TEM much more, which sometimes can lead to a different result in DLS).
However, you need to be carefull with zeta potential measurements. I guess your particles are stabilized in a non polar solvent, because they are capped with oleic acid. Than you need to use the hückel (instead of the smoluchowski) approximation when determining your zeta potential (probably you did, but I just wanted to let you know).
It might be useful to sonicate and dilute a suspension before DSL measurements. Could help to get more accurate and consistent numbers.
1. TEM gives diameter average on number of particles, and DLS - average on intensity (z-average). Therefore for comparison you should count z-average hydrodynamic diameter in n-average (modern spectrometers of DLS do it automatically).
2 . You have to consider possibility of aggregation of particles in solution (colleagues already reported to you about it).
3 . it is necessary to consider also тольшину the adsorptive layer of the stabilizer on particles
Hi! Although it concerns gold nanoparticles, maybe you find the following article interesting:
Comparative method evaluation for size and size-distribution analysis of gold nanoparticles
J Sep Sci. 2013 Sep;36(17):2952-61. doi: 10.1002/jssc.201300460.
It is closer:
Characterization of magnetic nanoparticle by dynamic light scattering
Jit Kang Lim, Swee Pin Yeap, Hui Xin Che and Siew Chun Low
Nanoscale Research Letters 2013, 8:381
http://www.nanoscalereslett.com/content/pdf/1556-276X-8-381.pdf
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