Polymer nanoparticles have been extensively studied as possible drug delivery systems because of their ability to control the discharge of drug contained within them and also because of their biocompatibility [1,2]. With their drug release profiles dependent upon polymer structure, they are comparatively easy to produce. Some of the vital characteristics of polymer nanoparticles for drug delivery applications include their surface chemistry and particle size. They typically range in size from 10 to 1000 nm in diameter allowing them to transverse cell membranes.
Dynamic Light ScatteringA number of different ways are available in which delivery of encapsulated drug from polymer nanoparticles can be controlled [1]. These comprise of chemical (e.g. pH, salt concentration), physical (e.g. sonorphoresis), environmental (e.g. temperature) and biochemical (e.g. enzyme) mechanisms. Dynamic light scattering (DLS) can be used for examining the influence of these mechanisms on the size of the polymer nanoparticles.
Dynamic light scattering is considered to be a non-invasive technique capable of measuring the size of colloidal dispersions and molecular solutions. A sample is contained in a suitable cuvette and illuminated with a laser beam in this technique. The random, Brownian motion of the particles allows the resulting scattered light to fluctuate in intensity. An analysis of these intensity fluctuations via autocorrelation helps determining the diffusion coefficients, which in turn produce the particle size via the Stokes-Einstein equation [3-5].
Experimental
A sample of polymer nanoparticles dispersed in water was measured on a Zetasizer Nano ZS using a temperature range of 50 to 90oC at 1oC intervals. A delay time of 5 minutes was used at each temperature to guarantee that the sample viscosity was equilibrated before taking the measurements. The Zetasizer Nano ZS uses a 4 mW He-Ne laser functioning at a wavelength of 633 nm with a detection angle of 173°. The detection angle of 173° allows for size measurements of concentrated, turbid samples.
Results
Figure 1 is a plot displaying the effect of temperature on the mean count rate (in kilo counts per second (kcps) and z-average diameter (in nanometres) of the polymer nanoparticle dispersion. The z-average diameter refers to the intensity-weighted mean diameter attained from the cumulants analysis as defined in ISO13321 [4] and ISO22412 [5] and is sensitive to the existence of large particles, aggregates or any changes which take place in the size of sample being measured.
The data in Figure 1 reveals that the z-average diameter increases as the temperature increases. An increase in the z-average diameter is normally an indication of particle aggregation. This would also lead to an increase in the mean count rate. However, the results obtained in this study show that the mean count rates decrease upon heating. Thus, the increase in the mean diameter is an indication of the polymer particles swelling with increasing temperature. As the conformation of these swollen particles become more open with increasing temperature, the refractive index of particles decreases (i.e. the relative refractive index decreases) with a resultant decrease in the mean count rate.
Conclusions
The results contained in this article demonstrate the possibility of using dynamic light scattering in order to examine the influence of temperature on the behavior of polymer nanoparticles. In this example, an increase in temperature has resulted in the swelling of polymer nanoparticles bringing about a decrease in the scattering intensity and an increase in their size.
References
[1] D. Bennet and K. Sanghyo (2014) Polymer Nanoparticles for Smart Drug Delivery, Application of Nanotechnology in Drug Delivery 8, 257-310.
[2] Y. Kawashima (2001) Nanoparticulate Systems for Improved Drug Delivery, Advanced Drug Delivery Reviews 47, 1-2.
[3] R. Pecora (1985) Dynamic Light Scattering: Applications of Photon Correlation Spectroscopy. Plenum Press, New York.
[4] International Standard ISO13321 (1996) Methods for Determination of Particle Size Distribution Part 8: Photon Correlation Spectroscopy. International Organization for Standardization (ISO).
[5] International Standard ISO22412 (2008) Particle Size Analysis: Dynamic Light Scattering (DLS). International Organization for Standardization (ISO).
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