Analytical chemists often turn to surface-enhanced Raman spectroscopy to detect low concentrations of compounds including toxins in drinking water or drugs in blood. Unfortunately the technique is expensive. Now scientists have demonstrated a cheaper way to perform SERS using gold nanoparticles and barrel-like molecules (Nano Lett., DOI: 10.1021/nl303345z).
SERS works by trapping molecules between metal surfaces. Chemists call these crevices “hot spots” because the electronic properties of the metal surfaces amplify the characteristic Raman signals that the trapped molecules produce when they’re hit with near-infrared light. Typically, researchers pattern nanosized hot spots onto gold surfaces for use in SERS experiments. These nanopatterned surfaces are expensive, so some groups have started using suspensions of gold nanoparticles instead. These less expensive experiments rely on molecules getting trapped in the spaces between particles.
But nanoparticle SERS methods aren’t as consistent as ones that use the nanostructured surfaces. This is because the signal enhancement created by a SERS hot spot changes with the size of the gap. Researchers have struggled to control the distances between nanoparticles in suspension, leading to SERS signals of varying strengths. Such variable data leads to less accurate concentration measurements.
To make nanoparticle methods more reliable, Oren A. Scherman, Sumeet Mahajan, and their colleagues at the University of Cambridge found a way to hold gold nanoparticles at a fixed distance from one another. They use barrel-shaped molecules called cucurbiturils that, along their rims, can bind to and bridge gold nanoparticles. Scherman says the cucurbiturils keep the particles 0.9 nm apart.
Other groups have used molecules such as DNA and cyclodextrins to space nanoparticles, says Mahajan, who is now at the University of Southampton, in the U.K. But, he adds, those molecules either aren’t very rigid, allowing the spacing between particles to change, or they occupy the entire gap between the nanoparticles, leaving no room for analytes in the hot spot. Meanwhile, cucurbiturils are both rigid and hollow.
To make the barrel molecules selectively trap the molecules they’d like to detect, the researchers first found a small molecule that would bind selectively to their target. Once bound, this pair of molecules could fit inside the cucurbituril, confining the target molecule in the SERS hot spot.
The researchers tested their technique on solutions containing low concentrations of anthracene, a common pollutant that comes from oil. They first prepared a solution of cucurbituril and methyl viologen, which readily binds anthracene. Next they added this solution to one of anthracene and 60-nm-wide gold nanoparticles. To measure the anthracene concentration, the scientists then irradiated the resulting mixture with near-IR light and measured the SERS signals. The team reports that it could detect anthracene down to micromolar concentration, which makes their technique as sensitive as any other SERS method for anthracene.
Duncan Graham at the University of Strathclyde, in the U.K., calls the cucurbituril approach elegant. By changing the molecules used with cucurbituril, he says, researchers could easily adapt the method to other analytes.
But José Vicente García Ramos at the Spanish National Research Council’s Institute for the Structure of Matter worries that using two molecules to bind the target molecule will lead to complicated Raman spectra with multiple peaks, obscuring the analyte’s signals.
The paper’s authors do not think this is a concern. They say that researchers can identify peaks corresponding to about 10 compounds in a single SERS spectrum. So, they say, looking for one among three should be straightforward.