m a te ri a l wo rl d
Magnetic dust mobilizes droplets I
JASON DORVEE
which a larger number of the dustt may not be as magical as Tinker coated droplets could be manipuBell’s fairy dust, but the “smart lated with magnetic fields and still dust” developed by Michael Sailor identified optically.” and colleagues at the University of Both Fair and Stroock say that California, San Diego, does some the ability to move microliter-volunique things. The dust is made up ume droplets is not novel and that of tiny, magnetic, amphiphilic partithe current frontier is in the transcles that coat droplets of organic port of nano- to picoliter volumes. and aqueous solvents. In the presDorvee says the droplet volumes ence of an external magnetic field, aren’t limited to the microliter scale. these particles move the droplets He points out that the particles around without the aid of pumps don’t even need to coat a droplet. or valves (Nat. Mater. 2004, 3, “You can condense aqueous liquids 896 – 899). onto one side of the particle and Smart dust was originally develMagnetic smart dust coats a 15-L, 3-mm-diam droplet then transport the particle. You can oped without any magnetic propof water in hexane. get down to picoliter [and] nanoerties in Sailor’s lab (Adv. Mater. netic Fe3O4 nanoparticles, the result was liter [volumes] because [the particles] 2002, 14, 1270–1272). The particles become their own carrier,” says Dorvee. were made out of porous silicon and magnetic smart dust. Fair and Stroock also argue that the modified so that the silicon had both The investigators demonstrated two transport of droplets in microfluidic dehydrophobic and hydrophilic faces. applications for the tiny magnetic partivices has already been accomplished by Once the modified silicon was fragcles. They first showed that a 30-L techniques such as electrowetting and mented into micrometer-sized particles droplet of water in 1:3 dichloromethane– dielectrophoresis. They contend that the by ultrasonication, the resulting partihexane coated with the particles could be use of an external magnetic field is an cles had the ability to interface with formed at a source, moved along a suradditional requirement for a system both organic and aqueous droplets. face by a magnet, and drained at a capilEach face of the particle had an optical lary. Second, they fused two droplets, one based on magnetic smart dust. “Using a “barcode” that would indicate whether containing KI and the other AgNO3, and magnetic field is not necessarily [better] nor does it mean an added requirement. the droplet was organic or aqueous. monitored the changes in the optical It’s just another way of moving things,” Jason Dorvee, a student in Sailor’s spectra of the magnetic smart dust as the counters Dorvee. Sailor also points out lab, tinkered with the fabrication process. reaction occurred. that dielectrophoresis requires a soluHe and a fellow student were trying to However, experts are uncertain about tion that contains ions to carry out its make the particles magnetic because the the utility of magnetic smart dust in mifunction, whereas the magnetic smart dust didn’t have any movement capabilicrofluidic devices. “From a scientific dust doesn’t. ties. The students weren’t having much point of view, [the work is] great,” says Sailor, Dorvee, and their colleagues success, so when Dorvee saw ferrofluids Richard Fair at Duke University. “But featured on a magazine cover (Chem. from a practical application point of view, can think of several ways of using their magnetic smart dust. “We think there Eng. News 2004, 82 [15]), he decided to I [don’t] see a whole lot of use for it.” may be applications in controlled drug take a break from smart dust and play Abraham Stroock of Cornell Univerrelease, diagnosis and treatment of canwith ferrofluids. sity feels that the magnetic smart dust cer, and in performing assays on single Dorvee managed to produce 100 mL may be more useful in generating aerobiological cells contained within these of a ferrofluid; only then did he realize sols. “The smart dust is a potentially drops,” says Sailor. “The main thrust of that he could use the nanoparticles inpowerful tool for analytical chemistry, side the ferrofluid to make smart dust but I am not convinced that the lab-on- the work is now to elaborate on the labmagnetic. When he soaked the silicon a-chip format is the most appropriate. A on-a-drop concept.” a — Rajendrani Mukhopadhyay particles in a solution of superparamagbetter context might be electrospray, in © 2005 AMERICAN CHEMICAL SOCIETY
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