s21_DLS

Characterization of Nanoparticle Radii Using Dynamic Light Scattering

Andrew Niecikowski, Ibrahim Mohamed

Methods of Experimental Physics II, University of Minnesota, Spring 2021

Abstract

The size of spherical nanoparticles suspended in a viscous liquid were characterized by analyzing the fluctuations of scattered light produced when hit with a beam of monochromatic light. Diffusion coefficients were extracted from the exponential decays of intensity autocorrelation functions and were used to determine the size of the particles, which were then compared to the known values. The results of this were not entirely exact, although the relationships between the different sizes was exactly as predicted. With additional modifications, dynamic light scattering is a valid way to measure the size of particles.

Introduction

Nanoparticles have many applications at the macroscopic scale and are used in a variety of industries including pharmaceuticals, cosmetics, paint manufacture and coatings for solar cells (1). A non-invasive form of size characterization for nanoparticles is dynamic light scattering (DLS)- also known as photon correlation spectroscopy. Particles undergoing Brownian motion will scatter incident monochromatic light with wavelengths exceeding the size of the nanoparticle. The intensity of this scattered light is dependent on the diffusion coefficient of the nanoparticle and so a fluctuating noise-like signal will initially be observed on the intensity-measuring device. These measurements of fluctuating intensities can be observed and correlated over a time delay to obtain a value for the diffusion coefficient from which the unknown radii values will be derived [3]. Thus, there is a great potential for easily checking the sizes of particles without being invasive, which is useful for safety and quality checks of products, and for ensuring precision in both commercial and scientific applications beyond what the product label claims.

Theory

The size of a nanoparticle exhibiting Brownian motion in a viscous solution is related to its diffusivity by the Stokes-Einstein equation

The Stokes-Einstein equation can be related to the autocorrelation function of the intensity through the Siegert Relation.

Where the power factor can be represented as a function of the diffusion coefficient.

The q component can be solved as a function of the wavelength and scattering angle of the beam.

Apparatus

Light from a 633 nm laser was focused into the sample using a 52.25 mm converging lens. The signal from the sample was detected on a silicon photodiode placed in a shielding box and amplified by a SR550 current amplifier and SR560 voltage amplifier. The amplified signal was observed on LabVIEW via a DAQ device. Distilled water was chosen as the fluid, and was mixed in with five different particle sizes of 90nm, 173nm, 304nm, 490nm, and 1030nm and kept at room temperature. Each mixture was placed in a standard cuvette with a total particle concentration of 2 percent. The coherences areas of the detector were set at 8,900 which was adjusted for on the iris. For each particle size, three separate trials of finding the intensity autocorrelation graph were done and then averaged together.

Results

Experimental autocorrelations were compared to the theoretical models and found to follow similar trends.

The decay constants were then obtained from the exponential fit of the autocorrelation function and were used to calculate the hydrodynamic radii of our nanoparticles.

Discussion

The calculated hydrodynamic radii are clearly smaller than our expected hydrodynamic radii. This is however indicative of high diffusivity and not necessarily smaller radii. Higher decay rates could arise due to increased particle motion from the shaking of the sample in an attempt to obtain a uniform distribution of nanoparticles within the medium. The deviations of the experimental results from the predictions can be caused by circuit noise and the non-uniform distribution of nanoparticles within the container. This nonuniformity can cause issues when scattered light is intercepted by other nanoparticles and alter the light traveling to the detector. The medium used for the experiment was not controlled for purity so impurities such as dust could interfere with beam scattering. Considerations could be made to account for the random multiple scattering of the nanoparticles despite low concentrations.

Conclusion

The usage of dynamic light scattering as a method for nanoparticle characterization was experimentally tested and confirmed to be viable. The final results shown below were closely aligned with the expectations, and the general trend of the data was strong enough to conclude that future analysis should be carried out. Additional testing with different fluids should be done as well. The deviations of our data from the expectations can be attributed to electrical noise and sample impurity.

References

[1] SCIENTIFIC COMMITTEE ON EMERGING AND NEWLY IDENTIFIED HEALTH RISKS (SCENIHR);The appropriateness of existing methodologies to assess the potential risks associated with engineered and adventitious products of nanotechnologies;https://ec.europa.eu/health/ph_risk/committees/04_scenihr/docs/scenihr_o_003b.pdf

[2] Stetefeld, J., McKenna, S.A. & Patel, T.R. Dynamic light scattering: a practical guide and applications in biomedical sciences. Biophys Rev 8, 409–427 (2016). https://doi.org/10.1007/s12551-016-0218-6

[3] Robert Finsy, Particle sizing by quasi-elastic light scattering, Advances in Colloid and Interface Science,Volume 52, 1994, ISSN 0001-8686, https://doi.org/10.1016/0001-8686(94)80041-3. (https://www.sciencedirect.com/science/article/pii/0001868694800413)

[4] Dilleys Ferreira CAPES Foundation, Ministry of Education of Brazil, Caixa Postal 250, Brasilia, DF 70040-020, Brazil and Université Côte d'Azur, CNRS, INPHYNI, 06560 Valbonne, France Romain Bachelard Departamento de Física, Universidade Federal de São Carlos, Rodovia Washington Luís, km 235-SP-310, 13565-905 São Carlos, SP, Brazil and Université Côte d'Azur, CNRS, INPHYNI, 06560 Valbonne, France William Guerin, Robin Kaiser, and Mathilde Fouchéa) Université Côte d'Azur, CNRS, INPHYNI, 06560 Valbonne, France, Connecting field and intensity correlations: the Siegert relation and how to test it, American Journal of Physics 88:10, 831-837