Credit: © 2006 ACS

Surface-enhanced Raman spectroscopy (SERS) — a technique that measures the inelastic scattering of a photon near a metallic surface — is well suited for biosensing applications because of its high sensitivity and selectivity. However, the lack of stable SERS substrates has hindered its widespread use. Now, researchers at Northwestern University and Argonne National Laboratory in the USA have found a way to improve traditional SERS substrates by coating them with an ultra-thin layer of aluminium oxide (alumina). When used to detect anthrax spores, this alumina-modified substrate showed a twofold better detection limit and a sixfold faster data acquisition time.

Richard Van Duyne and colleagues1 made the substrate by first coating a glass surface with polystyrene nanospheres. A silver film that acts as the SERS-active component was deposited over the nanospheres, followed by an ultrathin (sub-1-nm) layer of alumina on top. The highly inert alumina helps to stabilize SERS activity during measurements, while presenting a polar surface chemistry for the binding of hydrophilic analytes. Calcium dipicolinate (a marker for anthrax spores) has a five times greater affinity toward alumina compared with silver, and so improved detection limits were achieved with this sensor.

This modified SERS-active substrate was reported to be stable for at least nine months. The work by Van Duyne and colleagues represents a general strategy for making better chemical and biological detection systems based on SERS.