Characterization of size and dispersion stability of colloidal photonic crystal solutions made easy

Abstract

Highly charged Fe₃O₄ nanocrystals have found applications in telecommunication devices, drug delivery and contrast agents. The Zetasizer Nano can characterize the size and stability of these photonic crystal dispersions easily and quickly.

Introduction

Recently, a magnetically tuneable photonic crystal system has been developed by assembling highly charged Fe₃O₄ colloidal nanocrystal clusters (CNCs) [1,2]. They have attracted much attention because of their optoelectronic applications such as photonic components in tele-communication devices, lasers, sensors, drug delivery system (DDS) and MRI contrast agents [1,2]. Stabilized by the balance of attractive (magnetic) and repulsive(electrostatic) forces, the colloids form ordered structures along the direction of the external magnetic field with a regular interparticle spacing on the order of hundreds of nanometers. As a result, the solutions strongly diffract visible light [1,2]. CNCs can tune the diffraction wavelength throughout the entire visible spectrum by changing the strength of the external field [1,2]. The tuning range of the diffraction wavelength is related to the average size of the colloidal nanocrystal clusters. In general, crystals of large clusters (160-180 nm) preferentially diffract red light in a relatively weak magnetic field. Small clusters (60-100 nm) form ordered structures only when the magnetic field is strong and the crystals preferentially diffract blue light [1,2]. The size and dispersion stability of the photonic crystal system can be characterized by measuring the mean particle size and zeta potential respectively, because the optical response is sensitive to both the size of the colloids and the ionic strength of the solution.

Experimental

Alkanol solutions of Fe₃O₄@SiO₂ colloids show significant long-term stability in photonic activity [2]. These, Fe₃O₄@SiO₂ core/shell particles were dispersed in ethanol and measured, in this work, using the Zetasizer Nano ZS. Size and zeta potential measurements were performed on five different samples. All measurements in this study were taken at a temperature of 25°C. The Nano ZS uses a 4mW He-Ne laser operating at a wavelength of 633nm. Both size and zeta potential measurements were performed using folded capillary cells. All samples were measured 3 times to check for result repeatability. The z-average diameter (the intensity-weighted mean diameter) and the polydispersity index values (an estimate of the distribution width) were calculated from the cumulants analysis as defined in ISO13321 [3] and ISO22412 [4]. The intensity particle size distributions (PSDs) were obtained from analysis of the correlation functions using the General Purpose algorithm in the Zetasizer software. This algorithm is based upon a non-negative least squares fit [5, 6].

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