Behind the buzz and beyond the hype:
Our Nanowerk-exclusive feature articles
Posted: Mar 19, 2008
Novel nanoparticle synthesis techniques could lead to growing functional devices out of solution
(Nanowerk Spotlight) The past few years have seen tremendous progress in developing and fine-tuning fabrication methods for nanoparticles. An important research direction in nanoparticle synthesis is the expansion from single-component nanoparticles to hybrid nanostructures that possess two or more functional properties thanks to the integration of different materials. Multifunctional nanocarriers are a particularly hot topic in nanomedicine where it is hoped that such particles can significantly enhance the efficacy of many therapeutic and diagnostic protocols (see: Creating the nanotechnology wunderkind in pharmaceutics: multifunctional nanocarriers).
What makes hybrid multicomponent nanostructures so alluring is not only the combination of different functionalities, but also the possibility to independently optimize the dimension and material parameters of the individual components. Apart from their multifunctionality, another advantage of these structures is that they can provide novel functions not available in single-component materials.
A recent feature article provides an overview of the synthetic efforts of multicomponent hybrid nanoparticles via high-temperature solution-phase synthesis. The topics include chemical synthesis of multicomponent nanoparticles; characterization of the structural and physical properties, especially the ones arising from the interactions between different components; and potential applications of these multicomponent hybrid nanoparticles.
"Recent synthetic efforts have led to the formation of a large variety of multicomponent nanoparticles with different levels of complexity" Dr. Hao Zeng tells Nanowerk. "Owing to the strong coupling between different components, these systems show novel physical phenomena and enhanced properties, making them superior to their single-component counterparts for biomedicine, nanoelectronics, optoelectronics and spintronics applications. Despite these exciting new developments, the study of multicomponent nanoparticles is still at its infant stage compared to most single-element and alloy nanoparticle systems. However, with the progress in the fundamental understanding of the physics and chemistry in these multicomponent structures, we foresee that novel concepts and applications will be demonstrated in the not-so-distant future."
Zeng is an assistant professor of Physics at the University at Buffalo, the State University of New York. Together with Dr. Shouheng Sun, a professor of Chemistry, Engineering and Diagnostic Imaging at Brown University, he reviews the two group's recent work especially with regard to the synthesis of interesting new morphologies of hybrid structures such as a ternary structure combining magnetic, semiconducting and plasmonic properties.
HRTEM image of a Au-PbS (gold-lead sulfide) dumbbell-shaped hybrid structure – the dark part in the middle is the gold nanoparticle and the two lobes are lead sulfide. (Image: Dr. Zeng, University at Buffalo)
In their research, Zeng and Sun have been trying to find a cost-effective approach to hierarchically assemble nanoscale building blocks for
functional materials and devices. "In the past, this has been largely done by complicated microfabrication techniques involving multi-step lithography" explains Zeng. "The core of our work is the development of a general route for synthesizing multi-component, hybrid nanostructures where different nanoscale building blocks are directly grown onto one another to realize materials with multifunctionality. We envision that one day scientists will be able to grow completely functional sensors or even computer chips out of the solution phase. Our work is one small step towards realizing this goal."
Using their general synthesis approach, the two groups have produced a rather comprehensive list of hybrid materials that can be grouped into four classes: magnetic-metallic, magnetic-semiconductor, semiconductor-metallic, and magnetic-metallic-semiconductor.
The syntheses are based on heterogeneous nucleation and growth of a second and third component onto seed nanoparticles. Zeng points out that the successful synthesis of multicomponent nanoparticles via such seed-mediated growth relies critically on promoting heterogeneous nucleation while suppressing homogeneous nucleation (the formation of separate nanoparticles of the second component). In previous work, Zeng co-authored a paper that demonstrates a general strategy for engineering binary and ternary hybrid nanoparticles based on spontaneous epitaxial nucleation and growth of a second and third component onto seed nanoparticles in high-temperature organic solutions ("A General Approach to Binary and Ternary Hybrid Nanocrystals").
"To achieve this heterogeneous nucleation, the lattice spacing between two components should be well- matched to ensure epitaxial growth of the second component" he explains. "Further, the seed particles often participate in the reaction as catalysts, where charge transfer between the seeds and newly nucleated components is involved. This lowers the energy for heterogeneous nucleation. As long as the reactant concentration, seed-to-precursor ratio, and heating profile are controlled so that the concentration of the precursor is below the homogeneous nucleation threshold throughout the synthesis process, multicomponent heterostructures are formed."
The range of applications for these multicomponent nanoparticles is wide. For instance, they can be used as multi-modal bio-markers combining the functionalities of imaging, guided drug delivery and hyperthermia. Integrating different material properties at the nanoscale may also provide new opportunities for discovering enhanced or entirely novel material properties. Zeng uses the example of a ferroelectric-ferromagnetic multicomponent structure that could be used for electric field control of magnetism. "Such new functionality may one day allow new device concepts in nanoelectronics" he says.
The two scientists say that we will soon see the emergence of more multi-component hybrid structures combining different properties with more diversified morphologies. With most of this research being proof-of-principle type of work, scientists will have to overcome three sets of challenges before we can see large-scale practical applications:
1) gaining a fundamental understanding of the chemistry and materials science issues involved, so that hybrid structures can be designed with a high degree of control;
2) gaining a fundamental understanding of the interactions at the nanoscale between different components, so that the novel physical properties that may originate from such coupling can be predicted and exploited; and
3) finally, the controlled assembly of such hybrid nanoscale building blocks into bulk materials.