Add a Touch of TOPO to Put Cobalt In Its Place

Add a Touch of TOPO to Put Cobalt In Its Place

by John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Mar-Apr 02	Rank Last Period Jan-Feb 02
1	A.K. Chatterjee, et al., "Synthesis of functionalized olefins by cross and ring-closing metatheses," J. Amer. Chem. Soc., 122(15):3783-4, 19 April 2000. [Caltech, Pasadena] *308TT	13	6
2	C.P. Collier, et al., "A [2]catenane-based solid state electronically reconfigurable switch," Science, 289(5482):1172-5, 18 August 2000. [Univ. Calif., Los Angeles] *346JE	11	2
3	V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, "Colloidal nanocrystal shape and size control: The case of cobalt," Science, 291(5511):2115-7, 16 March 2001. [U. Calif., Berkeley; Lawrence Berkeley Lab., CA] *412PP	11	†
4	A.F. Littke, C. Dai, G.C. Fu, "Versatile catalysts for the Suzuki cross- coupling of arylboronic acids with aryl and vinyl halides and triflates under mild conditions," J. Amer. Chem. Soc., 122(17):4020-8, 3 May 2000. [MIT, Cambridge] *312NZ	10	1
5	T.A. Taton, C.A. Mirkin, R.L. Letsinger, "Scanometric DNA array detection with nanoparticle probes," Science, 289(5485):1757-60, 8 September 2000. [Northwestern U., Evanston, IL] *352BT	10	7
6	S.S. Shiratori, M.F. Rubner, "pH-dependent thickness behavior of sequentially adsorbed layers of weak polyelectrolytes," Macromolecules, 33(11):4213-9, 30 May 2000. [MIT, Cambridge; Keio U., Yokohama, Japan] *320AF	10	8
7	M. Zweckstetter, A. Bax, "Prediction of sterically induced alignment in a dilute liquid crystalline phase: Aid to protein structure determination by NMR," J. Amer. Chem. Soc., 122(15):3791-2, 19 April 2000. [NIDDKD, NIH, Bethesda, MD] *308TT	10	†
8	Y.-J. Han, J.M. Kim, G.D. Stucky, "Preparation of noble metal nanowires using hexagonal mesoporous silica SBA-15," Chem. Materials, 12(8):2068-9, August 2000. [U. Calif., Santa Barbara] *349DN	10	†
9	R.S. Martin, et al., "Dual-electrode electrochemical detection for poly(dimethylsiloxane)-fabricated capillary electrophoresis microchips," Anal. Chem., 72(14):3196-202, 15 July 2000. [U. Kansas, Lawrence; Mississippi St. U., MS] *337FQ	10	†
10	J.P.M. Lommerse, et al., "A test of crystal structure prediction of small organic molecules," Acta Crystall. B, 56:697-714, August 2000. [11 institutions worldwide] *341XA	9	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

he development of new materials, based on the assembly of atoms and molecules into nano-sized components, is an exceptionally active area of chemistry but it has its problems. For example, how do you go about assembling atoms of a magnetic material like cobalt, which by its very nature will exert forces that interfere? The answer is to be found in paper #3, which reports a remarkable breakthrough in doing just that. The answer is to coat the cobalt atoms and nano-sized assemblies with the correct ligand molecules, and ones that will overcome cobalt’s willfulness, and so allow the formation of designer assemblages. Such a ligand is TOPO.

Paper #3 reports the work of Victor Puntes, Kannan Krishnan, and Paul Alivisatos who are based at the University of California at Berkeley. They have adapted methods used to tailor-make cadmium/selenium nanocrystals, to suit cobalt. This metal is rather daunting, not only because of its magnetic properties, but because of its various crystal phases which may be at odds with the desired design. Despite the problems, the Berkeley team have managed to produce a range of skillfully crafted cobalt nanocrystals.

Their method involved injecting a solution of cobalt carbonyl, Co₂(CO)₈, dissolved in 0-dichorobenzene, into a refluxing solution of trioctylphosphine oxide (TOPO) in the same solvent, and to which oleic acid had also been added. Almost immediately, crystals of cobalt form as rods and disks and, if the refluxing system is quenched within 10 seconds, these can be separated. However, if the mixture is allowed to continue refluxing then other crystals form with diameters between 2 and 4 nanometres, and lengths up to 80 nm. TOPO serves to etch the corners of the forming particles, giving them a smoother profile, as well as producing a narrower range of crystal types.

When a colloidal solution of mixed crystals is allowed to evaporate slowly under a controlled atmosphere, and from a high boiling solvent like o-dichlorobenzene (b.p. 182° C), then they will self-assemble into superstructures. Under such conditions the particles diffuse to their lowest energy sites, producing well-defined structures such as ribbon-like materials.

So how did all this come about? "We adapted the knowledge we gained in quantum dot synthesis to the technologically challenging case of cobalt," says Puntes, adding that they deliberately chose this metal because of its magnetic, electronic, and catalytic properties. "The curious behavior of cobalt has spurred intensive studies into nanocrystal synthesis with the object of making magnetic storage devices, but these have always been hampered by the difficulty in making suitable nanocrystals."

Now that the California group has overcome this, they might well have opened the door to new applications. For example, they have studied the synthesis and magnetic behaviour of cobalt monolayers (see V.F. Puntes, et al., Applied Physics Letters, 78[15]:2187-9, 2001), and even investigated possible medical applications—see European Journal of Cells and Materials (2002, in press). They have also developed a method of making large amounts of cobalt nano-disks with magnetic moment in two dimensions, details of which should be published in Journal of the American Chemical Society later this year. Thanks to their researches, there may one day be nano-sized magnetic recorders for data storage, on cobalt devices that are too small for the human eye to see.

All this is exciting, but what Puntes and colleagues may also have achieved is the reawakening of interest in inorganic chemistry, a branch of chemistry that was thought to be in decline. Puntes strongly believes that further developments are in the offing: "Every new nanoparticle with controlled size, shape, and chemical composition represents a new inorganic ‘molecule.’ At some point, we will be able to develop a large library of inorganic molecules and then, as organic chemists and biochemists have done, we will develop customized materials and systems at the molecular level, maybe even making materials that will self-assemble and self-replicate."

It would appear that cobalt, the element that was named after unruly German goblins, might yet have a few tricks still to play.

Dr. John Emsley is Science Writer in Residence
at the Department of Chemistry, University of Cambridge, U.K.

Science Watch^®, September/October 2002, Vol. 13, No. 5
Citing URL: http://www.sciencewatch.com/sept-oct2002/sw_sept-oct2002_page7.htm