Spaghettilike surface makes stronger SERS sensor

Spaghettilike arrays of gold-coated metallic carbon nanotubes can amplify the signals of surface-enhanced Raman spectroscopy (SERS) enough to allow the performance of analyses that are more reliable, sensitive and cost-effective. Potential applications include real-time point-of-care monitoring of physiological levels, fast screening of drugs and toxins in law enforcement, and early detection of biological weapons.

Previously, the detection limit of common SERS systems was in the nanomolar range (one billionth of a mole), providing adequate signal strength in isolated cases only, and yielding results with low reproducibility. But a new sensor developed jointly by ETH Zurich and the Lawrence Livermore National Laboratory (LLNL) uses plasmonics to massively amplify the signals of Raman-scattered light: The researchers detected a certain organic species (1,2-bis(4-pyridyl)ethylene, or BPE) in a concentration of a few hundred femtomoles per liter (a 100-femtomolar solution contains around 60 million molecules per liter).

The high-sensitivity sensor is based on the curved tips of carbon nanotubes. The numerous gaps in the spaghettilike construction let the Raman-scattered light pass through. Courtesy of H.G. Park, ETH Zurich.

Raman spectroscopy takes advantage of the fact that molecules illuminated by fixed-frequency light exhibit “inelastic” scattering closely related to the vibrational and rotational modes excited in the molecules. Such light differs from common Rayleigh scattered light in that it has different frequencies than that of the irradiating light and produces a specific frequency pattern for each substance examined, making it possible to use this spectrum information as a “fingerprint” for detecting and identifying specific substances.

To analyze individual molecules, the frequency signals must be amplified, which requires that the molecule in question either be present in a high concentration, or be situated close to a metallic surface that amplifies the signal (surface-enhanced Raman spectroscopy).

The substrate used by doctoral student Ali Altun and professor Hyung Gyu Park, both of ETH Zurich, and LLNL capability leader Tiziana Bond was vertically arranged, caespitose, densely packed carbon nanotubes (CNTs) that guarantee a high density of hot spots. The group developed techniques to grow dense forests of CNTs in a uniform and controlled manner.

CNT tips are sharply curved and coated with gold and hafnium dioxide, a dielectric insulating material. The point of contact between the sensor’s surface and the sample resembles a plate of spaghetti topped with sauce. But between the strands of spaghetti are numerous randomly arranged holes that let scattered light through, with the many points of contact amplifying the signals.

“One method of making highly sensitive SERS sensors is to take advantage of the contact points of metal nanowires,” Park said. The nanospaghetti structure with metal-coated CNT tips is perfect for maximizing the density of these contact points. The wide distribution of metallic nanocrevices in the nanometer range, recognized to be responsible for extreme electromagnetic enhancement, resulted in intense and reproducible amplification.

The CNTs were coated with the insulator before a layer of gold was applied to prevent plasmonic energy leakage. “This was the breakthrough,” Altun said. The insulation layer increased the sensitivity of the sensor substrate by a factor of 100,000 in the molar concentration unit. “For us as scientists, this was a moment of triumph,” said Park, “and it showed us that we had made the right hypothesis and a rational design.”

The work appears in Advanced Materials (doi: 10.1002/adma.201300571). Park and Bond hope to commercialize the technology with an industry partner, but want to improve the sensor’s sensitivity and find potential areas of application.