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Core/shell nanoparticles have applications in bioanalysis

Mar 2007
Michael J. Lander

High performance often comes at a price, and nanoparticles with dual magnetic and luminescent properties are no exception. Because researchers can manipulate and detect the particles, they are useful in MRI agents and in targeted drug delivery and cell tagging. The colloidal solution techniques most often used to produce them, however, are expensive and complicated.

Ian M. Kennedy and colleagues at the University of California, Davis, have used spray pyrolysis to synthesize particles with a magnetic core and luminescent shell for a reasonable price and with comparative ease. They went on to test the particles in immunoassays.

Researchers used flame pyrolysis to create particles with magnetic cores and fluorescent shells. This transmission electron microscopy image shows an individual particle — antibodies on its surface appear as hairlike structures. Reprinted from the February 2006 issue of Analytical and Bioanalytical Chemistry.

The flame pyrolysis method the scientists used required two steps. To make the magnetic cores, they sprayed a solution of iron (III) nitrate, cobalt (II) nitrate and neodymium nitrate in ethanol through a nebulizer. At the outlet of the device, a hydrogen diffusion flame at 2000 °C created Nd:Co:Fe2O3 nanoparticles, which were collected on a cold finger via thermophoresis.

The scientists next added these particles to a solution of europium (III) nitrate and gadolinium nitrate, also in ethanol. When they sprayed the mixture into an H2 flame, a Eu:Gd2O3 luminescent shell 10 to 20 nm thick formed over the magnetic cores. Final particle diameters ranged from 200 nm to 1 μm. Luminescent properties of the particles were characterized using excitation from an Optotek tunable Nd:YAG laser.

Dosi Dosev, one of the researchers, noted that their adaptation of spray pyrolysis has several advantages. Other groups have produced particles with these properties by placing separate magnetic and fluorescent particles in a composite polymer bead through an emulsion-based process. Pyrolysis creates them forcefully with an adjustable flame and reagent flow rate, which can allow high throughput. Dosev added that the technology is readily scalable, which could allow industry to synthesize usable quantities of the particles at a low cost.

The core/shell particles were paramagnetic, in contrast to the cores, which displayed ferromagnetic behavior. The particles can be extracted from water with a commercial permanent magnet, which makes them useful in biochemical separation applications.

To demonstrate the nanoparticles’ utility as substrates for the immobilization of biological receptors, the researchers used them in two immunoassays. First, anti-rabbit IgG-coated particles were incubated with a fluorescently labeled antigen, rabbit IgG-Alexa Fluor 350. After magnetic extraction and resuspension, the researchers excited the shell material and the label with 350-nm light and plotted the label’s 445-nm signal both alone and relative to Eu’s 615-nm emission using a Molecular Devices plate reader. The results revealed that use of the nanoparticle’s signal as an internal standard improves quantification and reduces experimental errors caused by particle handling and variability in the excitation source. A competitive immunoassay with labeled and unlabeled rabbit IgG confirmed the scientists’ conclusions.

According to Dosev, synthesis of high-quality particles was challenging at first. Some particles incorporated more than one magnetic core, and some contained none at all. The scientists collected the usable, more strongly magnetic particles with a magnet. In addition, they obtained a relatively wide size distribution. To alleviate this, they eliminated the large agglomerates by settling. Also, recent improvements to their synthesis process have allowed them to obtain a more uniform powder.

The nanoparticles themselves have a comparatively narrow emission peak and high photostability — properties typical for lanthanide luminescence. Dosev also said that doping with terbium, samarium or other lanthanides in gadolinium or yttrium oxides might allow a variety of luminescent spectra. Because of their economy, the particles may prove most useful for high-volume applications such as environmental monitoring of bioterrorism agents.

Nanotechnology, Feb. 7, 2007, 055102.

Basic ScienceBiophotonicsenergyimmunoassaysmagneticMicroscopyMRINews & Features

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