To detect a chemical of interest, analytical chemists sometimes toss antibody-coated beads into a solution to scavenge for it. To analyze their captured chemicals, chemists have to herd the beads together using centrifugation or magnetic fields. Now researchers report a faster, simpler method to concentrate their samples that relies on beads that float in a bubble (J. Am. Chem. Soc., DOI: 10.1021/ja208463f).
Keith T. Carron of the University of Wyoming, Aaron D. Strickland of iFyber, a small technology company, and their colleagues use surface-enhanced Raman scattering (SERS) spectroscopy to detect chemicals. In a typical application, they use paramagnetic beads that glom onto gold nanoparticles only in the presence of a compound of interest. When the researchers capture the beads with a magnet and shine a laser on them, the compound adsorbed onto the gold nanoparticles produces a characteristic SERS signal.
The problem, explains Carron, is that a magnetic field’s strength falls dramatically with distance. In a container with a volume greater than 250 μL, he says, “it just takes forever for the magnets to pull down all the particles.”
To speed things up, Carron and his team developed what they call a lab on a bubble. The key components are hollow 50-μm-diameter silica spheres studded with 50-nm-diamter gold nanoparticles, which can capture certain chemicals in solution. Like ping-pong balls, the spheres float in water. When the researchers mix the nanoparticle-covered spheres into a sample, they pop up to the surface, where the scientists can probe them with a Raman laser. With this method, the sample’s volume isn’t an issue, Carron says: “I could throw them in a lake and they would float to the top.”
To prove the principle, the researchers used the lab-on-a-bubble method to measure known concentrations of cyanide ions. The technique could detect the ions down to 173 ppt, a similar sensitivity to using gold nanoparticles alone. But the SERS signal from the bubble method has considerably less noise, Carron says, because the spheres stay in a relatively small volume. Without their flotation devices, he explains, nanoparticles wouldn’t concentrate in the laser’s beam and would drift in and out of the detection area, which leads to signal noise. The bubbles also produce about 28 times larger SERS signals than gold nanoparticles do alone because the spheres prevent the gold from aggregating.
Carron says he has adapted the lab-on-a-bubble method to detect biological molecules using immunoassays. Potential applications, he says, include point-of-care diagnostics.