Sharp-cornered liquid drops by wetting of nanoscale features.
Sharp-cornered liquid drops by wetting of nanoscale features.
复制标题
通过润湿纳米级特征形成尖角液滴。
DOI：
10.1002/smll.200800564
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发表时间：
2008
期刊：
影响因子：
--
通讯作者：
Birembaut F
中科院分区：
文献类型：
--
作者：
Birembaut F
Nanostructured materials have become of intense interest in recent decades, driven by the realization that conventional properties on the macroscale can be completely modified. Simultaneously, it is clear that an interdisciplinary approach is inherently crucial to understand the properties of new nanostructures. Interest in exploiting enhanced optical properties for biological applications uncovers a critical question for nanobiosensors: where do molecules in solution end up on a nanostructured surface? For instance, molecular sensors can incorporate emitter–quencher coupling, or provide real-time observation of DNA pairing through luminescence tagging.[1] In the application of most interest here, molecules on a metal nanostructured surface produce intense surface-enhanced Raman scattering (SERS), which allows identification of very small numbers of unknown molecules. Discovered by Fleischman et al.,[2] the technique depends on local plasmonic electromagnetic excitations that develop on metal nanostructures and act as antennas to couple light efficiently into and out of the molecules. However, SERS applications suffer from a lack of reproducibility in the substrate preparation, and hence in their quantitative application in real environmental conditions. Recently, a number of research groups have manufactured more reproducible substrates with nanostructured metals.[3–6] Because the huge Raman enhancements (> 106) depend on the optical field to the fourth power and the field enhancements depend strongly on position within the nanostructures, it is critical to understand and control where molecules end up. Herein, we show that microstructured SERS substrates with nanoscale features have a dramatic effect in producing serrated-edged drops, and that deposition of molecules for SERS occurs in specific locations around the drop perimeter. We explore how droplet wetting operates on such nanostructured metal surfaces, to understand how molecules will be distributed on a nanosurface that is drying. This is typified in our recent investigations of the use of a commercially available SERS substrate called Klarite, currently manufactured by D3 Technologies, for examining the potential for mass screening of patients’ teardrops within the UK National Health Service. We show that the drop edges exhibit sharp, square contact lines with extreme curvatures jRj< 100 nm. Molecules within such droplets are preferentially deposited in particular parts of this droplet edge. A simple model describing solvent ffow in this highly folded drop-edge regime explains how molecules experience extreme spatial concentration.The Klarite SERS substrates consist of square-pyramidal pits of side 1.5 mm, spaced 1.8 mm apart on a square lattice, with a maximum depth of 700 nm and an interior apex angle of 708 (Figure 1a, inset).[7] This geometry is selected for optimal control of manufacture using selective anisotropic etching of Si (for details, see Ref.[7]) to obtain atomically ffat surfaces. The surfaces are Au-coated, and used here without additional cleaning procedures. Raman spectra of these clean samples show minimal contamination. To understand the spatial deposition profile of the molecules, we studied the wetting properties of microliter droplets of water. Note that if wetting inside the pit arrays does not happen, then no SERS signal can be observed. We first measured how the droplets sit on the nanostructured substrate by allowing a 1-mL droplet to contract through evaporation (Figure 1a, b).
