Designing nanostructures with optimized surface-enhanced Raman scattering behavior

Abstract

On-wire lithography is a new nanofabrication technique that can be used to design functionalized nanowires for both encoding and biosensing applications.Investigations in the burgeoning field of nanophotonics have demonstrated that certain nanostructures act in previously unanticipated ways. One such behavior is surface-enhanced Raman scattering (SERS), an area of substantial interest, as demonstrated by the hundreds of scientific papers published on the subject during the past 5 years. SERS was discovered over 3 decades ago. 1-3 It transforms the basic Raman technique (a measure of the structure and properties of molecules based on their interaction with light that normally has very weak intensity relative to other spectroscopies) into one that has highly sensitive detection capabilities. Whereas Raman scattering is useful for analyzing the structure of small molecules and for addressing traditional analytical issues such as a material's composition in bulk, SERS promises advances in ultrasensitive detection of a variety of molecule classes. The most important among these are biomolecules such as DNA, RNA, and proteins. Such detection capabilities (which can approach single-molecule limits) could lead to very sensitive and even mobile medical diagnostic assays. Yet the SERS phenomenon is still perplexing to scientists.The enhanced effect is most commonly attributed to electromagnetic or chemical mechanisms. [4][5][6] The latter involves charge transfer excitation between analyte molecules (i.e., molecules of interest) and the metal particles that they coat in nanostructures such as nanowires. (The closer an analyte is to the SERSenhancing metal nanostructure, the stronger the SERS effect, hence the coating. Coating is used for detection beyond SERS as well.) The electromagnetic mechanism is dominated by plasmon excitation, leading to 'hot spots' of Raman signals around nanosized metal particles. In SERS, these signals are highly ampli- fied, generating a signal orders of magnitude more intense than a nonenhanced Raman signal. Together with the unique molecular fingerprints provided by Raman spectra, SERS signals constitute an extraordinarily powerful detection and spectroscopic modality that few other techniques can match in terms of sensitivity and data density. However, optimizing and harnessing this phenomenon have proven difficult, due in part to nanostructure morphology, which significantly affects optical properties. The fabrication of nanoparticles with controllable structures is therefore extremely important for understanding and exploiting SERS fully. 7 Our group recently developed a process called on-wire lithography (OWL) that allows fabrication of 1D nanowires with nanoscale gaps along the long axis of the wire (see Figure 1). 8 The process starts with the synthesis of a segmented nanorod, consisting, for example, of alternating gold and nickel segments electrochemically grown on a template such as anodized aluminum oxide, which contains cylindrical pores. Some segments (e.g., nickel) are deliberate...