The particles suspended inside evaporating sessile droplets can be assembled into microscopic objects with long-ranged ordered structure. The air-water droplet interface guides the assembly and determines the shape of the resulting micropatches. We report the results of a systematic study of the mechanism of interface-templated assembly on substrates of controlled contact angle. The kinetics of drying were examined by measurements of droplet profiles, and it was found that the rate matched diffusion-limited evaporation well. The shape of the droplets and of the resulting assemblies was correlated to the dynamics of the receding contact line. The effects of major parameters controlling the process, including contact angle, particle concentration, and electrolyte, were investigated in detail. A variety of micropatch shapes were observed and categorized within the parameter space. The in-depth characterization of the process allowed the optimization of the assembly and the formulation of protocols for the deposition of nanostructured patches of different diameter, thickness, and shape.

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Convective assembly at high volume fractions was used to assemble gold nanoparticles into structured porous films templated by colloidal crystals. These gold nanofilms have hierarchical porosity and were proven to be stable and efficient substrates for surface-enhanced Raman spectroscopy (SERS). The control over the film structure allowed optimization of their performance for potential sensor applications.Colloidal crystals are materials with periodic structure on the submicrometre length scale made by self-assembly of colloidal particles. Colloidal crystals can be used as a basis for fabrication of photonic materials, 1-7 optical coatings and filters, [8][9][10] lithographic etching masks, 11-13 and sensors. 14-17 The colloidal crystals can also serve as templates for a variety of other self-assembled materials with controlled and reproducible structure. In this review we highlight results based on our process for rapid and reproducible deposition of colloidal crystal films by convective assembly at high volume fractions. 18These crystal films serve as templates of controlled structure for surface-enhanced Raman spectroscopy substrates. 16,19The hallmark of SERS is selectivity, potential for remote sampling through fiber optics, and capability for detection of analytes in aqueous solvents. Despite these advantages the widespread use of SERS-based analytical technology has been slow. Before SERS-based sensors can find broad application in routine chemical analysis new SERS materials must be developed that yield consistently high signals and provide detection generality towards a wide range of chemical and environmental analytes. The major requirements for the SERS substrate materials include controlled nanoscale structure, periodicity and chemical stability. One of the well-studied methods for making SERS substrates uses mono-and bi-layers of close-packed microsphere

Nanostructured and mesoscopically ordered architectures based on biomaterial templates may be used in making integrated mechanical, optical, and electronic devices. [1] A scheme for fabricating such devices will likely employ patterned functional biomaterials, which are used as scaffolds for the directed growth of inorganic materials. Tobacco mosaic virus (TMV) and M13 bacteriophage are particularly interesting as bioscaffolds owing to the combined chemical functionality of the virus protein coat, low size polydispersity, liquid-crystalline organization, and length scales that bridge the gap between traditional bottom-up and top-down fabrication schemes. [2] Recent advances in the biomimetic synthesis of inorganic materials by using genetically engineered viruses and proteins have opened the door to new organic-inorganic composites containing metals, silica, and semiconductors. [3][4][5][6][7][8][9][10] The major challenge in using such bioscaffolds in technology lies in developing new surface-patterning techniques that are controllable, reproducible, and efficient.We report here a versatile technique for rapidly assembling large-scale nanocoatings and ordered fibers from tobacco mosaic virus and converting them into electrically functional structures. The nanofilms were deposited by pulling, with a constant rate, a meniscus containing the virus suspension. Current molecular and colloidal self-assembly techniques for patterning surfaces with functional biomaterials include Langmuir-Blodgett lithography, [11][12][13] templating in sessile droplets with imposed shear, [14][15][16][17][18] self-assembly driven by dewetting or chemoselective interactions on micropatterned surfaces, [19][20][21][22] and electrostatic layer-by-layer assembly. [23] The technique that we report here uses an alternative flow assembly technique for organizing and aligning TMV into fibers and wires on surfaces. These films were of controlled thickness, structure, and long-range virus orientation, properties achieved by a combination of shear and dewetting. The density and branching of the virus wires were controlled by varying the substrate wettability and the meniscus withdrawal speed. The virus fibers were converted into anisotropically conductive arrays of wires of lengths of multiple centimeters by conjugation of gold nanoparticles followed by silver-metal deposition.TMV is a rod-shaped virus (300 nm in length and 18 nm in diameter), which forms lyotropic liquid crystals. Above a certain critical concentration, suspensions of TMV undergo an isotropic-nematic (I-N) transition. [24] Although the suspensions used here are well below the concentration required for the I-N transition, the formation of phase-separated aggregates can be observed in drying droplets. When droplets of this TMV suspension on a surface were allowed to evaporate, they exhibited alignment of linear virus aggregates normal to the three-phase contact line (Figure 1 b). As the meniscus receded, these aggregates were deposited as fibers and strands onto the substrate. Al...

Highly active and stable substrates for surface-enhanced Raman scattering (SERS) can be fabricated by using colloidal crystals to template gold nanoparticles into structured porous films. The structure-dependent performance of these SERS substrates was systematically characterized with cyanide in continuous flow microfluidic chambers. A matrix of experiments was designed to isolate the SERS contributions arising from nano- and microscale porosity, long-range ordering of the micropores, and the thickness of the nanoparticle layer. The SERS results were compared to the substrate structure observed by scanning electron microscopy (SEM) and optical microscopy to correlate substrate structure to SERS performance. The Raman peak intensity was consistently highest for nanoporous substrates with three-dimensionally ordered micropores, and decreases if the micropores are not ordered or not templated. Removing the nanoscale porosity by fusion of the nanoparticles (without removing the large micropores) leads to a drastic plunge in substrate performance. The peak intensity does not strongly correlate to the thickness of the nanoparticle films. The results make possible the efficient controlled fabrication of stable, reproducible, and highly active substrates for SERS based chemical sensors with continuous sampling.