Nanostructuring substrates

Further enhancement of the SERS can be achieved through precise control over the parameters at the metal particle size scale [10] Most SERS-active substrates were made from pure metallic nanostructures such as metal nanoparticles [33-35], metal particle arrays [5], roughened metal surfaces [36], or a combination with metal nanostructures and other nanomaterials [17, 18, 29, 37-39]. Recently, many strategies have shown the adsorbation of molecules on the surface of Ag and Au substrates for SERS applications [40]. SERS-active Ag nanostructures substrates are required to satisfy certain conditions with good reproducibility and stability [39]. For this reason, it is indispensable to develop and optimize the methods to prepare the SERS-active Ag substrates [41]. 

Luo et al. [61] demonstrated a simple method for the preparation of SERS active Ag nanostructures substrates by deposition of Ag nanoparticles into the designed Si holes. The morphologies of the Ag nanostructures were observed with SEM. The diameters of the Ag nanoparticles were found to be 40-60 nm. With increasing deposition time, flower-like Ag nanostructure commenced crystallization to form near the edge of the bottom surface of the Si microholes. These Ag nanostructures exhibited strong SERS enhancement, which provided an excellent platform for monitoring the R6G molecules by SERS technology [62]. 

Grigorenko AN, Roberts NW, Dickinson MR, Zhang Y (2008) Nanometric optical tweezers based on nanostructured substrates. Nat Photonics 2(6) 365-370... 

Teixeira Al, Abrams GA, Bertics PJ, Murphy CJ, Nealey PF. Epithelial contact guidance on well-defined micro- and nanostructured substrates. J Cell Sci 2003 116 1881-92. 

T.D. Choudhury, N.V.S. Rao, R. Tenent, J. Blackburn, B. Gregg, I.I. Smalyukh, Homeotropic alignment and director structures in thin films of triphenylamine-based discotic liquid crystals controlled by supporting nanostructured substrates and surface confinement. J. Phys. Chem. B... 

Meanwhile, the groups of Van Duyne and Bartlett have made special efforts to broaden the scope of nanosphere lithography to fabricate several new EC-SERS-active structural motifs [94, 99]. This most promising method can be used to produce nanostructured substrates with a precise control over the shape, size and interparticle spacing. In this case, a monolayer or a multilayer of highly ordered nanosphere films is used as the template for vacuum deposition or electrochemical deposition. As a result, three types of structured SERS substrate can be produced (i) physical vapor deposition on the nanosphere template... 

Significant effort, in many research laboratories, is directed towards improving the selectivity and reproducibility of the required nanostructured substrates, in order that these enhanced spectroscopies can be employed as true molecular chemosensing techniques. 

Precisely this latter situatiun aris< if the cuiifining solid surface is endowed with a chemical pattern that is both nanoscopic in size and finite in extent. Such chemical patterns may be created by lithographic methods (1791. Atomic beams have b< n employed to produce hexagonal nanostructures [180]. Other methods capable of creating cliemically nanostructured substrate surfaces involve microphase separation in diblock copolymer films [181] or the use of force micTOscopy to locally oxidize silicon surfaces [182]. 

An excellent example of this was the discovery of Raman signal enhancement when an analyte is placed on a roughened silver substrate." This discovery led to a new analytical technique termed surface-enhanced Raman spectroscopy (SERS). However, a lot is unknown about the mechanism of this anomalous signal enhancement. In the literature, the enhancement has been attributed to a combination of electromagnetic and chemical mechanisms " however, until a great control over the substrates atomistic composition and surface morphology can be achieved, the full extent of each mechanism wiU not be understood. Of course, the production of controllable nanostructured substrates through SERS is currently a heavily studied research area. " ... 

An alternative route to understanding the complications of disordered interfaces is to achieve functional control of intrinsically disordered materials interfaces as well. Such enhanced control can, for example, be achieved by construction of molecular sensitizers capable of binding in a functionally equivalent manner even to disordered nanostructured substrates. An important step in this direction is to find molecular sensitizers that can form interfaces with nanostructured substrates that display consistent interfacial electronic contacting capabilities, enabling systematic investigations of the distance dependence of surface electron transfer processes. 

Other new types of molecular interface that can be expected to become interesting in the near future include soft materials and bio-interfaces, as well as molecular and polymer interactions with new types of nanostructured substrates, such as III-V nanowires. The novelty of such applications means that this is set to continue to offer new computational challenges for many years to come. 

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