Kinetics of Aggregation and Magnetic Separation of Multicore Iron Oxide Nanoparticles: Effect of the Grafted Layer Thickness

Abstract

In this work, we have studied field-induced aggregation and magnetic separation—realized in a microfluidic channel equipped with a single magnetizable micropillar—of multicore iron oxide nanoparticles (IONPs) also called “nanoflowers” of an average size of 27 ± 4 nm and covered by either a citrate or polyethylene (PEG) monolayer having a thickness of 0.2–1 nm and 3.4–7.8 nm, respectively. The thickness of the adsorbed molecular layer is shown to strongly affect the magnetic dipolar coupling parameter because thicker molecular layers result in larger separation distances between nanoparticle metal oxide multicores thus decreasing dipolar magnetic forces between them. This simple geometrical constraint effect leads to the following important features related to the aggregation and magnetic separation processes: (a) Thinner citrate layer on the IONP surface promotes faster and stronger field-induced aggregation resulting in longer and thicker bulk needle-like aggregates as compared to those obtained with a thicker PEG layer; (b) A stronger aggregation of citrated IONPs leads to an enhanced retention capacity of these IONPs by a magnetized micropillar during magnetic separation. However, the capture efficiency Λ at the beginning of the magnetic separation seems to be almost independent of the adsorbed layer thickness. This is explained by the fact that only a small portion of nanoparticles composes bulk aggregates, while the main part of nanoparticles forms chains whose capture efficiency is independent of the adsorbed layer thickness but depends solely on the Mason number Ma. More precisely, the capture efficiency shows a power law trend Λ∝Ma−n, with n ≈ 1.4–1.7 at 300 < Ma < 104, in agreement with a new theoretical model. Besides these fundamental issues, the current work shows that the multicore IONPs with a size of about 30 nm have a good potential for use in biomedical sensor applications where an efficient low-field magnetic separation is required. In these applications, the nanoparticle surface design should be carried out in a close feedback with the magnetic separation study in order to find a compromise between biological functionalities of the adsorbed molecular layer and magnetic separation efficiency.

Kinetics of field-induced phase separation of a magnetic colloid under rotating magnetic fields

“…3b and 3c. The PDMS lid was fabricated by soft photolithography as described in detail in Ezzaier et al (2018). The channel had a length of 1 cm, a width of 2 mm and a thickness h=200±5 µm.…”

“…Similar experiments on superparamagnetic nanoclusters or nano-emulsion droplets of 30-200 nm-size range also revealed the appearance of long needle-like aggregates growing with time due to the absorption of single nanoclusters from the dilute phase surrounding the aggregates followed by the coalescence of aggregates due to dipolar attractions. This coalescence was however hindered at long times by dipolar repulsions [Socoliuc et al (2013); Ezzaier et al (2017); Ezzaier et al (2018); Mohapatra and Philip (2019)]. From a theoretical perspective, the driving force of the field-induced phase separation is the supersaturation ∆= − ′ defined as the difference of the nanoparticle volume fraction of the phase separating colloid and ′the threshold volume fraction below which there is no phase separation.…”

Multi-modal imaging probe for assessing the efficiency of stem cell delivery to orthotopic breast tumours

“…No diameter measurement was performed due to aggregation. Particle aggregation is mainly due to the large size (100 nm) and a multi-domain core of original fluidMAG-CT. 34 , 35 The hydrodynamic diameter (nm) and zeta potential (mV) of SPIONs alone and modified SPIONs were measured in deionised water at 25 °C using dynamic light scattering (DLS). The modified SPIONs had a decreased hydrodynamic diameter (98.6 ± 0.7 nm) compared to unmodified SPIONs (103.1 ± 0.2 nm).…”

From Single-Core Nanoparticles in Ferrofluids to Multi-Core Magnetic Nanocomposites: Assembly Strategies, Structure, and Magnetic Behavior

“…Once in contact due to magnetic attraction, the van der Waals attraction may prevent clusters disintegration after the field removal (Figure 27b). The MMC clusters morphology and formation kinetics were investigated both theoretically and experimentally (optical microscopy) [273,274], and static light scattering [271,273] was investigated as well. After external magnetic field application, about a micron thick and from tens up to a hundred microns-long spindle-like clusters begin to form, grow, and coalesce (Figure 28).…”