Vladimir V. Tsukruk

Nanostructured Surfaces and Assemblies as SERS Media

Metallic nanostructures attract much interest as an efficient media for surface-enhanced Raman scattering (SERS). Significant progress has been made on the synthesis of metal nanoparticles with various shapes, composition, and controlled plasmonic properties, all critical for an efficient SERS response. For practical applications, efficient strategies of assembling metal nanoparticles into organized nanostructures are paramount for the fabrication of reproducible, stable, and highly active SERS substrates. Recent progress in the synthesis of novel plasmonic nanoparticles, fabrication of highly ordered one-, two-, and three-dimensional SERS substrates, and some applications of corresponding SERS effects are discussed.

Graphene Oxide−Polyelectrolyte Nanomembranes

Owing to its remarkable electrical, thermal, and mechanical properties, graphene, an atomic layer of carbon, is considered to be an excellent two-dimensional filler for polymer nanocomposites with outstanding mechanical strength along with the potential for excellent electrical and thermal properties. One of the critical limitations with conventional fillers is that the loading fraction required for achieving significant improvement in mechanical properties is relatively high, frequently reaching 50% for maximum strength. Here, we demonstrate that the mechanical properties of ultrathin laminated nanocomposites can be significantly enhanced by the incorporation of small amounts of a dense monolayer of planar graphene oxide (GO) flakes. Negatively charged functionalized graphene oxide layers were incorporated into polyelectrolyte multilayers (PEMs) fabricated in a layer-by-layer (LbL) assembly via Langmuir-Blodgett (LB) deposition. These LbL-LB graphene oxide nanocomposite films were released as robust freely standing membranes with large lateral dimensions (centimeters) and a thickness of around 50 nm. Micromechanical measurements showed enhancement of the elastic modulus by an order of magnitude, from 1.5 GPa for pure LbL membranes to about 20 GPa for only 8.0 vol % graphene oxide encapsulated LbL membranes. These tough nanocomposite PEMs can be freely suspended over large (few millimeters) apertures and sustain large mechanical deformations.

Responsive brush layers: from tailored gradients to reversibly assembled nanoparticles

We present a condensed overview of the recent developments of novel responsive thin polymer films from end-tethered chains (polymer brushes), which are different from conventional, uniform, and planar brush layers. For this discussion, we selected two types of recently introduced surface layers: binary brush layers with variable chemical composition forming a controllable gradient of composition and properties in a selected direction and brush layers either grafted directly to inorganic nanoparticles to form hybrid core-shell structures or combined with inorganic nanoparticles embedded into this layer. Unlike traditional brush layers, such a design brings a novel set of responsive surface properties allowing for capillary-driven microfluidic motion, combinatorial-like multiplexing response, reversible aggregation and dis-assembly of nanoparticles, fabrication of ultrahydrophobic coatings, and switchable mass transport across interfaces.

Ultra-Robust Graphene Oxide-Silk Fibroin Nanocomposite Membranes

Nanocomposite materials in forms of membranes, fi lms, and coatings are gaining surging interests in structural and functional applications, because they are more effi cient in loading transfer than conventional composites and can substantially eliminate catastrophic failure caused by poor loading transfer between components. To enhance the mechanical properties of polymeric nanocomposites, carbon nanotubes, intercalated clay, graphene, and graphene oxide are added as high-performance reinforcing nanofi llers. For example, ultrahigh toughness was reported for polyvinyl alcohol nanocomposite fi lms fi lled with single-walled carbon nanotubes; [ 1 ] and ultrahigh modulus was reported for crosslinked nanoclay containing nanocomposites. [ 2 ] However, improving toughness is usually achieved by increasing the ultimate strain and compromising the strength, which is not desired for high-performance applications. [ 3 ]

Porous Substrates for Label-Free Molecular Level Detection of Nonresonant Organic Molecules

We report on the design of practical surface enhanced Raman scattering (SERS) substrate based upon 3D alumina membranes with cylindrical nanopores chemically modified with polyelectrolyte coating and loaded with gold nanoparticle clusters. These substrates allow for a molecular-level, label-free detection of common plastic explosive materials (TNT, DNT) down to 5-10 zeptograms or 15-30 molecules and a common liquid explosive (HMTD) down to 1 picogram. Such a sensitive detection of organic molecules by utilizing efficient SERS substrates opens the path for affordable and label-free detection of trace amount of practically important chemical compounds.

Responsive microcapsule reactors based on hydrogen-bonded tannic acid layer-by-layer assemblies

We explore responsive properties of hollow multilayer shells of tannic acid assembled with a range of neutral polymers, poly(N-vinylpyrrolidone) (PVPON), poly(N-vinylcaprolactam) (PVCL) or poly(N-isopropylacrylamide) (PNIPAM). We found that properties of the nanoscale shells fabricated through hydrogen-bonded layer-by-layer (LbL) assembly can be tuned changing the interaction strength of a neutral polymer with tannic acid, and by a change in counterpart hydrophobicity. Unlike most hydrogen-bonded LbL films with two polymer components, the produced tannic acid-based multilayer shells are extremely stable in the wide pH range from 2 to 10. We demonstrate that gold nanoparticles can be grown within tannic acid-containing shell walls under mild environmental conditions paving the way for further modification of the capsule walls through thiol-based surface chemistry. Moreover, these shells show reversible pH-triggered changes in surface charge and permeability towards FITC-labeled polysaccharide molecules. The permeability of these LbL containers can be controlled by changing pH providing an opportunity for loading and release of a functional cargo under mild conditions.

Nanoparticle‐Decorated Nanocanals for Surface‐Enhanced Raman Scattering

The surface-enhanced Raman scattering (SERS) effect is considered important for fast detection of characteristic ‘‘fingerprint’’ signatures of analytes. In the SERS effect, a substantial Raman enhancement arises on localized spots (‘‘hot spots’’) in metallic nanostructures owing to strong local electromagnetic fields associated with the surface plasmon resonances of metal nanostructures. SERS on colloidal metal aggregates and nanowires and Raman resonances have been utilized for trace detection of explosives, chemical and biological warfare agents, internal mechanical stress, distribution and stress of carbon nanotubes, and toxic environmental pollutants. However, an outstanding challenge of SERS-based detection is the lack of robust and facile fabrication routines for SERS substrates with considerable enhancements. Traditionally, electrochemically roughened metal surfaces, colloids, island films, nanowires, periodic arrays, and self-assembled nanoparticles are employed as SERS substrates (for a recent review, see Reference [27]). However, the long-term stability of aggregated nanoparticles is the main obstacle for assembled structures, and the fabrication of complex surface structures (e.g. with microfabrication) is laborand cost-demanding and sometimes impossible to extend to large dimensions. Moreover, the sensitivity of simple 2D SERS substrates remains modest owing to a limited number of hot spots (usually below 10). To increase the sensitivity of SERS substrates, 3D porous structures have been suggested as active SERS substrates with the advantage of having a large surface area available for the formation of hot spots and the adsorption of target analytes. Consequently, 3D SERS substrates have been fabricated by depositing Au or Ag films on porous silicon, GaN, and filter paper. Alternatively, deposition of metal nanoparticles on the porous aluminum membranes, colloidal crystal templates, or microwires have been exploited. For instance, nanopores in gold films have been proven to show enhanced Raman scattering owing to an intense electromagnetic field generated by the surface plasmons. However, most of these studies have not fully utilized the advantages of 3D structures for SERS effects, mainly because of limited light propagation through porous materials owing to

Probing Soft Matter with the Atomic Force Microscopies: Imaging and Force Spectroscopy

The development of atomic force microscopy has evolved into a wide variety of microscopy and characterization techniques well beyond conventional imaging. The focus of this review is on characterization methods based on the scanning probe and their application in characterizing physical properties of soft materials. This consideration is broken into three major categories focusing on mechanical, thermal, and electrical/magnetic properties in addition to a brief review of high-resolution imaging. Surface spectroscopy is discussed to great extent and consideration includes procedural information, common pitfalls, capabilities, and their practical application in characterizing soft matter. Key examples of the method are presented to communicate the capabilities and impact that probe-based characterization techniques have had on the mechanical, thermal, and electrical characterization of soft materials.

Molecular Lubricants and Glues for Micro‐ and Nanodevices

Research advances in molecular coatings from functional polymeric and organic molecules designed as molecular lubricants or molecular glues for micro- and nanodevices are presented here, with a focus on organized molecular films from amphiphilic molecules, molecules with reactive ends and functional oligomers. The interfacial properties of molecular coatings critical for their lubrication (see Figure) or adhesive performance at the nanoscale are discussed in conjunction with results on molecular structure and morphology of these coatings. Examples of the latest developments in the field of nanocomposite molecular coatings and applications of molecular lubrication concepts for computer hard drives are presented.

Assembly of supramolecular polymers in ultrathin films

Abstract This review presents a sketch of the current status and trends in the field of organized assemblies from supramolecular polymeric systems. Among various classes of polymeric systems used to fabricate organized films, we select supramolecular polymers able to self-organize or be organized in complicated superlattices in the bulk state and/or at an interface with low-dimensional ordering. We focus on the microstructural behavior in different classes of polymers and their abilities to form various organized superstructures at surfaces and interfaces. Among the supramolecular polymers discussed are: mesomorphic polymers with chromophore and non-linear optic fragments, chiral and amphotropic polymers, systems with hydrogen bonding and discotic compounds, hairy-rod and electroactive macromolecules, block-polymers and fullerene-based systems, dendritic and latex nanocomposites, organic-inorganic organized composites and biomolecular complexes. The variation of molecular shapes, architecture of polymer backbones and specific intermolecular interactions are very effective tools for tailoring supramolecular organization of these materials in ultrathin films. We discuss the merits of various fabrication techniques as applied to ultrathin organized films, such as Langmuir-Blodgett, self-assembly and forced solvent removal methods, as well as some characterization techniques. The physical properties of these films are discussed briefly with emphasis on optical, electric, transport, sensing and nanomechanical behavior. Finally, some thoughts about the future development in this field are presented.