A surface-enhanced Raman scattering substrate of biological origin

Abstract

The surfaces of cicada wings contain a quasi-periodic array of bristle-like structures with a spacing of approximately 200 nm. On this scale, a noble metal film deposited over the structures should be well-suited to generating a localised surface plasmon resonance. This has been confirmed by surface-enhanced Raman scattering (SERS) measurements that have been performed on silvercoated cicada wings. Large enhancements of the Raman scattering intensity can be obtained when a target analyte is adsorbed on a metal surface that is rough on a scale of approximately 10-100 nm. A large number of different SERS-active substrates have been reported over the years. These are typically based on silver or gold surfaces, where the critical structural dimension may, for example, be obtained through electrochemical roughening, mechanical roughening or metal island films [1]. In general however, the most sensitive and reproducible SERS activity requires stable substrates with uniform roughness. A number of fabrication techniques, including those based on metal colloids [2], nanosphere lithography [3], UV photolithography [4] and e-beam lithography [5], have confirmed these basic requirements for the preparation of multiple, high-quality substrates. However, these methods have tended to remain the domain of specialist laboratories, or are prohibitively expensive. Therefore the identification of alternative manufacturing techniques, possibly including biological processes, remains of interest. Nanometer-scale architectures found in biological systems are known to produce a number of striking optical effects. These are of current interest because of the inspiration that they may provide for novel technological applications [6]. Although biological systems are not usually appropriate for direct technological application, this paper describes a novel, indirect use of a natural photonic structure. In particular, it has been observed that the wings of some cicada species contain arrays of bristle-like structures with a spacing of ca. 200 nm and mainly hexagonal symmetry [7]. The structures have a similar spacing to nipple arrays that have been observed in the hawkmoth, where an anti-reflective function has been proposed [8]. The bristles found on the wings of a brown cicada (Cicadetta celis) are somewhat narrower and longer than the hawkmoth structures, as shown in Fig. 1(a). These bristles are comparable in scale to lithographic features that