While applied frequently in physical biochemistry to the study of protein complexes, the
quantitative use of analytical ultracentrifugation (AUC) for nanocrystal analysis is relatively
rare. Its application in nanoscience is potentially very powerful as it provides a measure of
nanocrystal density, size and structure directly in the solution phase. Towards that end, this
paper examines the best practices for applying data collection and analysis methods for AUC,
geared towards the study of biomolecules, to the unique problems of nanoparticle analysis.
Using uniform nanocrystals of cadmium selenide, we compared several schemes for analyzing
raw sedimentation data. Comparable values of the mean sedimentation coefficients (s-value)
were found using several popular analytical approaches; however, the distribution in sample
s-values is best captured using the van Holde–Weischt algorithm. Measured s-values could be
reproducibly collected if sample temperature and concentration were controlled; under these
circumstances, the variability for average sedimentation values was typically 5%. The full
shape of the distribution in s-values, however, is not easily subjected to quantitative
interpretation. Moreover, the selection of the appropriate sedimentation speed is crucial for
AUC of nanocrystals as the density of inorganic nanocrystals is much larger than that of
solvents. Quantitative analysis of sedimentation properties will allow for better agreement
between experimental and theoretical models of nanocrystal solution behavior, as well as
providing deeper insight into the hydrodynamic size and solution properties of nanomaterials.
Magnetic separations: From steel plants to biotechnology
Magnetic separations have for decades been essential processes in diverse industries ranging from steel production to coal desulfurization. In such settings magnetic fields are used in continuous flow processes as filters to remove magnetic impurities. High gradient magnetic separation (HGMS) has found even broader use in wastewater treatment and food processing. Batch scale magnetic separations are also relevant in industry, particularly biotechnology where fixed magnetic separators are used to purify complex mixtures for protein isolation, cell separation, drug delivery, and biocatalysis. In this review, we introduce the basic concepts behind magnetic separations and summarize a few examples of its large scale application. HGMS systems and batch systems for magnetic separations have been developed largely in parallel by different communities. However, in this work we compare and contrast each approach so that investigators can approach both key areas. Finally, we discuss how new advances in magnetic materials, particularly on the nanoscale, as well as magnetic filter design offer new opportunities for industries that have challenging separation problems.
Size-dependent sedimentation properties of nanocrystals
J. A. Jamison, K. M. Krueger, C. T. Yavuz, J. T. Mayo, D. LeCrone, J. J. Redden, V. L. Colvin
Centrifugation is an increasingly important technique for nanomaterial processing. Here, we examine this process for gold, cadmium selenide, and iron oxide nanocrystals using an analytical ultracentrifuge. Such data provide an accurate measure of the sedimentation coefficients for these materials, and we find that this parameter has a significant dependence on the size and surface coating. Conventional models for particle sedimentation cannot capture the behavior of these nanocrystals unless the density of the nanocrystals is described by a size-dependent term that accounts for both the inorganic core and the organic coating. Using this modification in the particle sedimentation framework, it is possible to estimate sedimentation coefficients from information about the nanocrystal core and surface coating dimensions. Such data are useful in choosing the speeds for a centrifugation process and are particularly important when bimodal nanocrystal distributions are present.
Electrically driven phase transition in magnetite nanostructures
S. Lee, A. Fursina, J. T. Mayo, C. T. Yavuz, V. L. Colvin, R. G. S. Sofin, I. V. Shvets, D. Natelson
Magnetite (Fe3O4), an archetypal transition-metal oxide, has been used for thousands of years, from lodestones in primitive compasses1 to a candidate material for magnetoelectronic devices2. In 1939, Verwey3 found that bulk magnetite undergoes a transition at TV≈120 K from a high-temperature ‘bad metal’ conducting phase to a low-temperature insulating phase. He suggested4 that high-temperature conduction is through the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering on cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial5,6,7,8,9,10,11. Here, we show that magnetite nanocrystals and single-crystal thin films exhibit an electrically driven phase transition below the Verwey temperature. The signature of this transition is the onset of sharp conductance switching in high electric fields, hysteretic in voltage. We demonstrate that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. We anticipate that further studies of this newly observed transition and its low-temperature conducting phase will shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound.
The effect of nanocrystalline magnetite size on arsenic removal
J.T. Mayo, C. T. Yavuz, S. Yean, L. Cong, H. J. Shipley, W. W. Yu, J. C. Falkner, A. T. Kan, M. Tomson, V. L. Colvin
Higher environmental standards have made the removal of arsenic from water an important problem for environmental engineering.
Iron oxide is a particularly interesting sorbent to consider for this application. Its magnetic properties allow relatively routine dispersal
and recovery of the adsorbent into and from groundwater or industrial processing facilities; in addition, iron oxide has strong and
specific interactions with both As(III) and As(V). Finally, this material can be produced with nanoscale dimensions, which enhance both
its capacity and removal. The objective of this study is to evaluate the potential arsenic adsorption by nanoscale iron oxides, specifically
magnetite (Fe3O4) nanoparticles. We focus on the effect of Fe3O4 particle size on the adsorption and desorption behavior of As(III) and
As(V). The results show that the nanoparticle size has a dramatic effect on the adsorption and desorption of arsenic. As particle size is
decreased from 300 to 12 nm the adsorption capacities for both As(III) and As(V) increase nearly 200 times. Interestingly, such an
increase is more than expected from simple considerations of surface area and suggests that nanoscale iron oxide materials sorb arsenic
through different means than bulk systems. The desorption process, however, exhibits some hysteresis with the effect becoming more
pronounced with small nanoparticles. This hysteresis most likely results from a higher arsenic affinity for Fe3O4 nanoparticles. This work
suggests that Fe3O4 nanocrystals and magnetic separations offer a promising method for arsenic removal.
r 2006 NIMS and Elsevier Ltd. All rights reserved.
Low-field magnetic separation of monodisperse Fe3O4 nanocrystals
C. T. Yavuz, J. T. Mayo, W. W. Yu, A. Prakash, J. C. Falkner, S. Yean, L. Cong, H. J. Shipley, A. Kan, M. Tomson, D. Natelson, V. L. Colvin
Magnetic separations at very low magnetic field gradients (<100 tesla per meter) can now be applied to diverse problems, such as point-of-use water purification and the simultaneous separation of complex mixtures. High–surface area and monodisperse magnetite (Fe3O4) nanocrystals (NCs) were shown to respond to low fields in a size-dependent fashion. The particles apparently do not act independently in the separation but rather reversibly aggregate through the resulting high-field gradients present at their surfaces. Using the high specific surface area of Fe3O4 NCs that were 12 nanometers in diameter, we reduced the mass of waste associated with arsenic removal from water by orders of magnitude. Additionally, the size dependence of magnetic separation permitted mixtures of 4- and 12-nanometer–size
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