Separation of nanoparticles in a density gradient: FeCo@C and gold nanocrystals.

Size and geometric control of nanomaterials are important to the discovery of intrinsic size/shape dependent properties and bottom up approaches for the fabrication of functional nanodevices. Two general strategies have been employed to create size-uniform nanocrystals. One method is direct particle size control during synthesis by adjusting growth parameters; 7–9] the other is post-synthesis separation. Much capacity exists to improve size separation efficacy in the latter case. Differential centrifugation can remove large and unstable particles from colloidal systems, but lacks precise control over particle size. Addition of adjustable amounts of “anti-solvent” (including CO2) [12] into colloidal systems may make precipitation processes more controllable. Other methods include filtration (including diafiltration), electrophoresis, and chromatographic methods that can produce particle fractions with narrow shape and size distributions. To maintain or improve the quality of nanoparticle (NP) separation, whilst addressing the issues of adhesion and clogging in liquid–solid phase separation processes, a completely liquid phase separation method is highly appealing. Isopycnic centrifugation, which is often used for biomacromolecule separation, relies upon a density gradient and ultracentrifugation to separate components according to subtle density differences, and has been applied for diameter and electronic-dependent separation of single-walled carbon nanotubes (SWNT). However, the isopycnic densitygradient centrifugation method reaches a limitation when it is extended to the separation of metal nanoparticles. Such a method requires that the components for separation have densities within a gradient range. Aqueous density gradient media usually have densities less than 1.4 gcm , which is much less than the density of metal nanoparticles. Size or shape separation of such heavy nanocrystals remains an issue, both in their preparation and utility for various applications. In contrast to isopycnic separation, ultracentrifugal rate separation can utilize density gradients to separate nanocrystals with higher densities than the gradient media itself. We have previously applied such a method to achieve length separation of suspended SWNTs and pegylated graphene oxide. In this report, the method was extended to metallic NP size separation. Nanoparticles of various size, suspension chemistry, and composition, including FeCo@C and gold nanoparticles (Au NPs), were separated using the method. FeCo nanocrystals coated in graphitic shells have superior magnetic properties, and have shown promise for applications in biolabeling and magnetic resonance imaging (MRI). However, the chemical deposition method used for their preparation produces nanocrystals with wide size distribution. They are thus ideal candidates for post-synthesis separation. FeCo@C NPs with average diameters of about 4 nm were separated first by our density gradient rate (DGR) separation method by using a 10+ 20+ 30+ 40% gradient and centrifugation for 3.5 h. TEM results of typical fractions (Figure 1A) indicate that fraction 8 (labeled as “f8” in Figure 1B) contained circa 1.5 nm NPs. The average particle diameter of subsequent fractions (f11, 15, 19, 24, and 27) gradually increased from 2.5 to 5.6 nm. By varying the step gradient densities and centrifuge exposure time, this method could be used for separation of nanoparticles of a larger size range, which was demonstrated by (on average) 7 nm FeCo@C NP separation (see TEM images of initial 7 nm FeCo@C NPs in the Supporting Information). A gradient of higher density steps (20+ 30+ 40+ 60%) was used. The use of higher density gradient media helps to control the sedimentation rate by reducing the density difference between the NPs and the environmental medium, and increasing the medium viscosity. After centrifugation for 2.5 h, several bands formed in the centrifuge vessel (Figure 2A), just as in the 4 nm NP case. Sampling fractions along the centrifuge vessel yielded nanoparticles of increasing size (with increasing density), as revealed by TEM (Figure 2B). From f5 to f16, the average particle size increased from 2 to 6.5 nm. It is noteworthy that the Fe/Co atomic ratios of f8 and f16 were measured by energy dispersive spectra (EDS) and found to be different (Figure 2B, bottom right). For f8, the ratio Fe/Co= 48:52; for f16, Fe/Co= 40:60. Previously such stoichiometry was analyzed by calcination/burning of the graphitic shells at 500 8C, dissolving the metal species in an HCl solution, and measuring the iron and cobalt concentrations on the basis of the ultraviolet[*] Prof. Dr. X. M. Sun, L. Bai State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology Beijing 100029 (China)