Industrial and academic applications that use iron oxide nanoparticles are vast. Of the nanomaterials, iron oxide has been recently drawing much attention from researchers due to its stability, low toxicity, and magnetic properties. Given the great potential for applications using iron oxide, it is beneficial to understand the effect of chemical changes to the surface of the nanoparticle. The aim of this study was to determine the onset of aggregation due to a change in pH.
A suspension was prepared by adding iron oxide nanopowder Fe3O4 to 20 mL of filtered MilliQ water. A 0.2 µm filter was used to filter the MilliQ water 3 times. The suspension was then sonicated in a Branson 2800 low energy sonic bath for five minutes. A Brookhaven Instruments BI-ZTU Autotitrator was loaded with four reagents: 0.1 M nitric acid, 1 mM nitric acid, 0.1M potassium hydroxide, and 1 mM potassium hydroxide.
The suspension was tested from a pH of 2 to 12 in 2 pH unit steps. The solution was analyzed at each pH, 5 times, each, for 5 minutes per run using dynamic light scattering (DLS).
Measurements were made using Brookhaven Instruments NanoBrook Omni Particle Size Analyzer and BI-ZTU Autotitrator and were analyzed using Particle Solutions, version 3.5 software. The effective diameter of iron oxide nanoparticles was determined from pH 2 to 12 in 2 pH unit steps in a suspension of MilliQ water. Iron oxide nanoparticle aggregates in suspension were examined by plotting the measurements collected over the ranges of tested pHs.
Dynamic light scattering (DLS) technique can detect the effective diameter or size of a nanoparticle and an autotitrator can help automate the process so that changes in particles’ apparent size can be detected as a function of pH. The average effective diameter of iron oxide nanoparticles was in the size range of 1023 to 1029 nm on an intensity weighted basis, in acidic solution. The effective diameter of iron oxide nanoparticles was in the size range of 4258 to 4520 nm on an intensity weighted basis, in basic solution.
The multimodal size distribution shows multiple particle populations in the sample. The iron oxide nanopowder had 3 particle size populations. The particles shifted in size as they aggregated, and 3 populations remained from pH 2 to pH 12.
The isoelectric point for iron oxide is at pH 7.5. This is the range where particle aggregation occurs due to low charge on the particle surface. Measurement of zeta potential with change of pH further supports particle aggregation at the same pH range and can be done using the same Brookhaven Instruments Omni and BI-ZTU.
The Brookhaven Instruments NanoBrook Omni Particle Size Analyzer and autotitrator are useful tools to detect the onset of aggregation of iron oxide nanoparticles because of their ease of use and quick sample preparation. This general method can be applied to many types of nanomaterials and can be customized to particular applications. Different parameters such as time dependence or additive titrations like salts or dispersing agents can be incorporated. The pharmaceutical industry can benefit from the ease of use and efficiency of the NanoBrook Omni together with the ZTU.