Credit: © 2007 ACS

The small size of nanostructures, combined with their easily tailored physical and chemical properties, makes them attractive materials for encoding applications. In particular, striped nanorods in which the sequence, size and location of the different chemical blocks can encode information have been used as probes in molecular diagnostic systems. These nanorod codes are read-out by looking at the reflectivity of the different metal segments or the fluorescence of chemical labels, and this imposes a number of practical limitations on this approach.

Now, Chad Mirkin and co-workers1 at Northwestern University in the US have used their on-wire lithography (OWL) methodology to create nanodisk arrays that turn some of the limitations of striped nanorods — such as fluorescence quenching — into advantages by using Raman spectroscopy. OWL involves creating segmented nanorods from two materials, such as gold and nickel, by template-directed electrochemical synthesis. After coating one side of these rods with silica, the nickel segments are removed by selective wet-chemical etching. This results in nanodisk arrays consisting of separated gold disk pairs with well-defined gap separations, and represents a physical encoding reminiscent of barcodes. The gold nanodisks can then be functionalized with different Raman-active dyes, making the code spectroscopically observable.

The use of the nanodisk codes for biomolecule detection was demonstrated by functionalizing the disks with single-stranded DNA molecules. These strands contained a sequence complementary to one-half of a target DNA sequence, the other half of which was complementary to a reporter DNA strand containing a Raman-active probe. This three-strand detection system, which takes advantage of surface-enhanced Raman scattering of over eight orders of magnitude, could be used to detect very low concentrations of the target DNA.