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Here we provide a complete review on the use of dynamic light scattering (DLS) to study the size distribution and colloidal stability of magnetic nanoparticles (MNPs). The mathematical analysis involved in obtaining size information from the correlation function and the calculation of Z-average are introduced. Contributions from various variables, such as surface coating, size differences, and concentration of particles, are elaborated within the context of measurement data. Comparison with other sizing techniques, such as transmission electron microscopy and dark-field microscopy, revealed both the advantages and disadvantages of DLS in measuring the size of magnetic nanoparticles. The self-assembly process of MNP with anisotropic structure can also be monitored effectively by DLS.

Convective current driven by momentum transfer between magnetic nanoparticles (MNPs) and their surrounding fluid during magnetophoresis process under a low gradient magnetic field (<100 T m(-1)) is presented. This magnetophoresis induced convective flow, which imposed direct hydrodynamic effects onto the separation kinetics of the MNPs under low gradient magnetic separation (LGMS), is analogous to the natural convection found in heat transportation. Herein, we show the significance of the induced convection in controlling the transport behavior of MNPs, even at a very low particle concentration of 5 mg L(-1), and this feature can be characterized by the newly defined magnetic Grashof number. By incorporating fluid flow equations into the existing magnetophoresis model, we reveal two unique features of this convective flow associated with low gradient magnetophoresis, namely, (1) the continuous homogenization of the MNPs solution and (2) accompanying sweeping flow that accelerates the collection of MNPs. According to both simulation and experimental data, the induced convection boosts the magnetophoretic capture of MNPs by approximately 30 times compared to the situation with no convection.

Iron oxide cores of 35 nm are coated with gold nanoparticles so that individual particle motion can be tracked in real time through the plasmonic response using dark field optical microscopy. Although Brownian and viscous drag forces are pronounced for nanoparticles, we show that magnetic manipulation is possible using large magnetic field gradients. The trajectories are analyzed to separate contributions from the different types of forces. With field gradients up to 3000 T/m, forces as small as 1.5 fN are detected.

Both magnetic and plasmonic particles are of increasing interest for biomedical applications. Magnetic particles are currently used for magnetic separation, to collect tagged cells or DNA sequences, [1][2][3] and for magnetically guided drug delivery. [4][5][6] A key advantage of magnetism lies in the ability to control motion at a distance without perturbing the biological system, as would occur with a large electric field. Noble-metal nanostructures are beneficial not only because of their relative ease of biofunctionalization, but also for plasmonic biosensing. [7][8][9][10] Unlike quantum dots, [11] these particles do not show optical bistability or blinking making them competitive with fluorophores for quantifying the number of cell surface markers. [12,13] The main advantages of plasmonic particles are their extremely large molar extinction coefficients and resonant Rayleigh scattering efficiencies, [8] and the exceptional sensitivity of the surface plasmon resonance peak wavelength to changes in the local dielectric environment. Individual plasmonic nanoparticles and nanorods have been detected by using dark field optical microscopy. [13][14][15][16][17] In fact, by using nonmagnetically responsive gold-coated silica particles ($150 nm), Halas and co-workers have demonstrated the capability of gold-shell nanoparticles in detecting low concentrations of analytes in whole blood within minutes without any sample preparation.[18]Here we describe the preparation of iron oxide/gold core/ shell nanoparticles that can be moved magnetically and imaged optically. The synthesis of large-diameter ($150 nm) magnetic plasmonic three-layer composite particles has previously been reported, [19] but our focus here is on smaller particles. The small size of these nanoparticles approaches that of cellmembrane-bound antigens, and is in a range in which particles can be spontaneously internalized by endocytosis. We expect that the combination of magnetic responsiveness, facile bioconjugation, and a localized surface plasmon resonance in these smaller nanoparticles will open new possibilities for in vitro molecular and cell biological applications, including intracellular mechanical and chemical composition analyses, and magnetophoretic cell sorting according to the expression level of cell surface receptors. The magnetic forcẽ F m ¼ ðm pt Á rÞB * that can be applied to a particle depends on the particle magnetic moment m pt . Since m pt ¼ M s V pt , where M s is the saturation magnetization of the material and V pt is the particle volume, a larger size is generally preferable for a strong magnetic response. Thus there is a compromise between large magnetic moments and the desirability of small sizes for new applications.Gold, silver, and SiO 2 core/Au shell particles have been used for plasmonic sensing, where the surface plasmon peak wavelength shifts in response to small changes in the dielectric environment near the particle surface. A core/shell structure with the noble metal in the shell enables the surface plasmon resona...

Nanoparticles with monodisperse, spherical magnetic iron oxide cores and contiguous gold shells (Fe/Au NPs) have been synthesized in order to combine magnetophoretic responsiveness and localized surface plasmon resonance in a single nanoparticle. Such particles are sufficiently charged to be stable against flocculation in low ionic strength media, but they require surface modification to be stably dispersed in elevated ionic strength media that are appropriate for biotechnological applications. Dynamic light scattering and ultraviolet-visible spectrophotometry are used to monitor the colloidal stability of Fe/Au NPs in pH 7.4 phosphate buffered saline containing 154 mM NaCl (PBS). While uncoated particles flocculate immediately upon introduction to PBS, Fe/Au NPs with adsorbed layers of bovine serum albumin or the amphiphilic triblock copolymers Pluronic F127 and Pluronic F68 resist flocculation after more than 5 days in PBS. Adsorbed dextran allowed flocculation that was limited to the formation of small clusters, while poly(ethylene glycol) homopolymers ranging in molecular weight from 6000 to 100 000 were ineffective steric stabilizers. The effectiveness of adsorbed Pluronic copolymers as steric stabilizers was interpreted in terms of the measured adsorbed layer thickness and extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory predictions of interparticle interactions.

Magnetic collection of the microalgae Chlorella sp. from culture media facilitated by low‐gradient magnetophoretic separation is achieved in real time. A removal efficiency as high as 99% is accomplished by binding of iron oxide nanoparticles (NPs) to microalgal cells in the presence of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) as a binder and subsequently subjecting the mixture to a NdFeB permanent magnet with surface magnetic field ≈6000 G and magnetic field gradient <80 T m−1. Surface functionalization of magnetic NPs with PDDA before exposure to Chlorella sp. is proven to be more effective in promoting higher magnetophoretic removal efficiency than the conventional procedure, in which premixing of microalgal cells with binder is carried out before the addition of NPs. Rodlike NPs are a superior candidate for enhancing the magnetophoretic separation compared to spherical NPs due to their stable magnetic moment that originates from shape anisotropy and the tendency to form large NP aggregates. Cell chaining is observed for nanorod‐tagged Chlorella sp. which eventually fosters the formation of elongated cell clusters.

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