I measured by DLS the size of a TiO2 nanoparticles suspension sample to compare the results against a TEM measurement (5-6nm). As I expected I got different results from the two experiments due to the agglomeration behaviour of the particles in their liquor.
The DLS results are reported in three different ways : volume distribution, intensity distribution and number distribution. The volume distribution shows ONLY a peak centered at 20 nm , whereas the intensity distribution shows TWO peaks: one centered at 20 nm (low intensity) and the other centered at 100 nm (high intensity). Based on the theory the intensity signal is proportional to the 6th power of the radius, so my conclusion is that the main cluster present in suspension is the one of 20nm and the 100 nm cluster is present in lower percentage. Assuming my reasoning is correct, I wonder if I can quantify the ratio of the two cluster populations. Should I use the number distribution?
Giovanni Zummo
Using the DLS method, information is obtained about the inhomogeneous dynamics of TiO2 nanoparticles or about fluctuations in space and time. In this case, the diffusion coefficient and the diameter of the hydrodynamic sphere can be calculated using data on the constant decay of the correlation function Г. The picture of the Brownian motion of crystalline nanoparticles, for example, gold, obtained by the DLS method, is clearer. They are poorly hydrated. The hydrodynamic sphere is a hydrated solid particle. For soft substances (TiO2 + Ti(OH)4 micelles), summary information was obtained on fluctuations in TiO2 concentration, fluctuations in the density, entropy of Ti(OH)4. At the same time, the software of the device, using the mathematical model of Einstein-Smoluchowski, "considers" these oscillations as the Brownian motion of solid particles.
According to the DLS data, it cannot be stated with certainty that the 100 nm peak belongs to TiO2. The size distribution also does not say anything. The correlation function is not related to the law of the 6th power of the radius.
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@ Giovanni, I personally believe that in DLS method intensity distribution results is more accurate as it gives the amount of light scattered by the particles in the different size bins.
Yuri Mirgorod thank you for your answer. I know that DLS measures how fast the scatterred light intensity fluctuations versus time take to show a correlation function going to zero. From the decay constant of this function is possible to obtain the diffusion coefficient of the particles in the sample and then from the stokes-einstein equation, having D (diffusion coeffiecient) , the hydronamic diameter is calculated. Having said that, I wonder where the volume, intensity and number distribution versus size come from. Isn't the intensity plot linked with light scattered (hence related to the law of the 6th power of the radius) ? Sorry if I am asking something again, but I am afraid I am a bit confused about the way the software displays the results.
Giovanni Zummo
I repeat that the correlation function is related to Brownian motion and not to light scattering. Using light scattering, the Brownian motion of a solid nanoparticle in time is studied and the diffusion coefficient is obtained. For a deeper understanding, you need to read the author of the DLS
Pecora, R. Eds. Dynamic light scattering: applications of photon correlation spectroscopy. Springer, Plenum Press: New York, 1985, 420 p. DOI: 10.1007/978-1-4613-2389-1
Giovanni Zummo Take a look at this post by Ulf Nobbmann:
https://www.materials-talks.com/intensity-volume-number-which-size-is-correct/
Intensity size distributions (obtained from the diffusion coefficients) are converted to volume via Mie Theory (and thus a knowledge of the particle's optical properties is required). The scattering intensity in the Rayleigh region (say < 60 nm) is proportional to r6 or (volume)2. The conversion from intensity to volume is only reasonable if the size distribution is relatively narrow as this is an ill-conditioned mathematical problem: small changes in data or noise can bring large changes in derived result. Example: 5 + 4 = ? is easily solved; the reverse (9 = ? + ?) is not so easy. You can get the steak from the cow easily but not the cow from the steak...
Note that the international (ISO and ASTM) standards for DLS deprecate the conversion of intensity to number even though the math would indicate that this is possible.
Be aware that if you want sensitivity to small numbers of large particles in a system (e.g. aggregation in proteins) the intensity distribution tells you something about this. 'Cleaning up' the distribution by converting to volume (which will tell you about the mass content if you know or assume the density) may be deceptive if you use it (say) to a regulatory body to 'confirm' that there is no large matter present... Numbers of particles are never sold to a customer - you don't buy 4 billion particles of Aspirin - rather a dosage such as 250 or 500 mg.
Good luck with your research!
thank you all for your answer.
Alan F Rawle If the most reliable signal is the intensity, in case of two populations of particles with different size, it would not possible to quantify in what ratio they are present in the colloid. What technique can be used to quantify the two kind of particles with different size?
Giovanni Zummo They can be quantified in terms of intensity or, with narrow distributions, in terms of volume (assuming the optical properties are known). DLS is a powerful technique allowing many billions of particles to be ’seen’ at once, but it is low resolution.
Hello Giovanni Zummo
in addition to the comments above, yes you could use mathematics to come up with a quantification. This might work for either volume/mass or number. But there are certain caveats to keep in mind, and you must have really good data. For some guidance in principle on how this works, take a look atArticle Dynamic Light Scattering as a Relative Tool for Assessing th...
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