Scientists at Rice University in Houston have created a unique nanophotonics sensor capable of amplifying the optical signature of molecules by a factor of approximately 100 billion. Newly published tests have found the device can accurately identify composition and structure of individual molecules containing fewer than 20 atoms.
The new imaging method, which is described this week in the journal Nature Communications, uses a form of Raman spectroscopy in combination with an intricate but mass reproducible optical amplifier. Researchers at Rice’s Laboratory for Nanophotonics (LANP) said the single-molecule sensor is about 10 times more powerful that previously reported devices.
The Nature Communications article, entitled “Coherent anti-Stokes Raman scattering with single-molecule sensitivity using a plasmonic Fano resonance,” (Nature Communications 5, Article number: 4424 doi:10.1038/ncomms5424) is coauthored by Yu Zhang, Yu-Rong Zhen, Peter Nordlander, Oara Neumann, Jared K. Day, Peter Nordlander, and Naomi J. Halas of the Rice University Department of Physics and Astronomy, Laboratory for Nanophotonics, and Department of Electrical and Computer Engineering
The researchers observe that plasmonic nanostructures are of particular interest as substrates for the spectroscopic detection and identification of individual molecules, and show that coherent anti-Stokes Raman spectroscopy (CARS), a nonlinear spectroscopy of great utility and potential for molecular sensing, can be used to obtain single-molecule detection sensitivity, by exploiting the unique light harvesting properties of plasmonic Fano resonances. They say this approach combines unprecedented single-molecule spectral sensitivity with plasmonic substrates that can be fabricated using top-down lithographic strategies.
“Ours and other research groups have been designing single-molecule sensors for several years, but this new approach offers advantages over any previously reported method,” says LANP Director and lead scientist on the study, Naomi J. Halas, in a Rice U. release. “The ideal single-molecule sensor would be able to identify an unknown molecule even a very small one without any prior information about that molecules structure or composition. That’s not possible with current technology, but this new technique has that potential.”
As noted the Rice Halas group’s optical sensor uses Raman spectroscopy, a technique pioneered in the 1930s that took off with the advent of lasers in the 1960s. The Rice researchers explain that when light strikes a molecule, most of its photons bounce off or pass directly through, but a tiny fraction — fewer than one in a trillion — are absorbed and re-emitted into another energy level that differs from their initial level. By measuring and analyzing these re-emitted photons through Raman spectroscopy, scientists can decipher the types of atoms in a molecule as well as their structural arrangement.
The Rice scientists have created a number of techniques to boost Raman signals. In the new study, LANP graduate student Yu Zhang used one of these, a two-coherent-laser technique called coherent anti-Stokes Raman spectroscopy, or CARS. By using CARS in conjunction with a light amplifier made of four tiny gold nanodiscs, Dr. Halas and Yu Zhang were able to measure single molecules in a powerful new way. LANP has dubbed the new technique surface-enhanced CARS, or SECARS.
“The two-coherent-laser setup in SECARS is important because the second laser provides further amplification,” says Zhang, noting that in a conventional single-laser setup, photons go through two steps of absorption and re-emission, and the optical signatures are usually amplified around 100 million to 10 billion times. By adding a second laser that is coherent with the first one, the SECARS technique employs a more complex multiphoton process. (Image courtesy Rice University.)
Zhang says the additional amplification gives SECARS the potential to address most unknown samples — an added advantage over current techniques for single-molecule sensing, which generally require a prior knowledge about a molecules resonant frequency before it can be accurately measured.
Another key component of the SECARS process is the devices optical amplifier, which contains four tiny gold discs in a precise diamond-shaped arrangement. The gap in the center of the four discs is about 15 nanometers wide. Owing to the above-noted optical effect called a Fano resonance, the optical signatures of molecules caught in that gap are dramatically amplified because of the efficient light harvesting and signal scattering properties of the four-disc structure.
Zhang explains that the quadrumer amplifiers are a key to SECARS, in part because they are created with standard e-beam lithographic techniques, which means they can be easily mass-produced.Fano resonance requires a special geometric arrangement of the discs, and one of LANPs specialties is the design, production and analysis of Fano-resonant plasmonic structures like the four-disc quadrumer. In previous LANP research, other geometric disc structures were used to create powerful optical processors.
“A 15-nanometer gap may sound small, but the gap in most competing devices is on the order of 1 nanometer,” Zhang says, noting that “Our design is much more robust because even the smallest defect in a one-nanometer device can have significant effects. Moreover, the larger gap also results in a larger target area, the area where measurements take place. The target area in our device is hundreds of times larger than the target area in a one-nanometer device, and we can measure molecules anywhere in that target area, not just in the exact center.”
Dr. Halas, the Stanley C. Moore Chair of Electrical and Computer Engineering and a professor of biomedical engineering, chemistry, physics and astronomy at Rice, says potential applications for SECARS include chemical and biological sensing as well as metamaterials research, and scientific labs are likely be the first beneficiaries of the technology.
“Amplification is important for sensing small molecules because the smaller the molecule, the weaker the optical signature, Dr. Halas observes. “This amplification method is the most powerful yet demonstrated, and it could prove useful in experiments where existing techniques cant provide reliable data.”
The Halas research group at Rice is focused on four principal missions:
- To design new optically active nanostructures driven by function
- To develop and implement new nanofabrication strategies to build, orient, and pattern these nanostructures into new materials and devices
- To characterize and understand the physical properties of these optically active nanostructures, devices and materials
- To prototype the use of optically active nanostructures in applications of potential technological and broad societal interest
A major goal of the Halas lab’s research program is to produce PhD research scientists with significantly expanded skill sets and expertise who can develop new solutions to research and engineering problems beyond traditional disciplinary boundaries.
The research was supported by the Department of Defense, the Defense Threat Reduction Agency and the Robert A. Welch Foundation.