Srikanth Singamaneni

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.

Paper-Based SERS Swab for Rapid Trace Detection on Real-World Surfaces

One of the important but often overlooked considerations in the design of surface-enhanced Raman scattering (SERS) substrates for trace detection is the efficiency of sample collection. Conventional designs based on rigid substrates such as silicon, alumina, and glass resist conformal contact with the surface under investigation, making the sample collection inefficient. We demonstrate a novel SERS substrate based on common filter paper adsorbed with gold nanorods, which allows conformal contact with real-world surfaces, thus dramatically enhancing the sample collection efficiency compared to conventional rigid substrates. We demonstrate the detection of trace amounts of analyte (140 pg spread over 4 cm2) by simply swabbing the surface under investigation with the novel SERS substrate. The hierarchical fibrous structure of paper serves as a 3D vasculature for easy uptake and transport of the analytes to the electromagnetic hot spots in the paper. Simple yet highly efficient and cost-effective SERS substrate demonstrated here brings SERS-based trace detection closer to real-world applications.

Magnetic nanoparticles: recent advances in synthesis, self-assembly and applications

Nanostructured magnetic materials have a variety of promising applications spreading from nano-scale electronic devices, sensors and high-density data storage media to controlled drug delivery and cancer diagnostics/treatment systems. Magnetic nanoparticles offer the most natural and elegant way for fabrication of such (multi-) functional materials. In this review, we briefly summarize the recent progress in the synthesis of magnetic nanoparticles (which now can be done with precise control over the size and surface chemistry), and nanoscale interactions leading to their self-assembly into 1D, 2D or 3D aggregates. Various approaches to self-organization, directed-, or template-assisted assembly of these nanostructures are discussed with the special emphasis on magnetic-field enabled interactions. We also discuss new physical phenomena associated with magnetic coupling between nanoparticles and their interaction with a substrate and the characterization of the physical properties at the nanoscale using various experimental techniques (including scanning quantum interferometry (SQUID) and magnetic force microscopy). Applications of magnetic nanoparticle assemblies in data storage, spintronics, drug delivery, cancer therapy, and prospective applications such as adaptive materials and multifunctional reconfigurable materials are also highlighted.

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.

Highly Sensitive Surface Enhanced Raman Scattering Substrates Based on Filter Paper Loaded with Plasmonic Nanostructures

We report a novel surface enhanced Raman scattering (SERS) substrate platform based on a common filter paper adsorbed with plasmonic nanostructures that overcomes many of the challenges associated with existing SERS substrates. The paper-based design results in a substrate that combines all of the advantages of conventional rigid and planar SERS substrates in a dynamic flexible scaffolding format. In this paper, we discuss the fabrication, physical characterization, and SERS activity of our novel substrates using nonresonant analytes. The SERS substrate was found to be highly sensitive, robust, and amiable to several different environments and target analytes. It is also cost-efficient and demonstrates high sample collection efficiency and does not require complex fabrication methodologies. The paper substrate has high sensitivity (0.5 nM trans-1,2-bis(4-pyridyl)ethene (BPE)) and excellent reproducibility (~15% relative standard deviation (RSD)). The paper substrates demonstrated here establish a novel platform for integrating SERS with already existing analytical techniques such as chromatography and microfluidics, imparting chemical specificity to these techniques.

Plasmonic Planet–Satellite Analogues: Hierarchical Self-Assembly of Gold Nanostructures

In the past few years, a remarkable progress has been made in unveiling novel and unique optical properties of strongly coupled plasmonic nanostructures, known as plasmonic molecules. However, realization of such plasmonic molecules using nonlithographic approaches remains challenging largely due to the lack of facile and robust assembly methods. Previous attempts to achieve plasmonic nanoassemblies using molecular ligands were limited to dipolar assembly of nanostructures, which typically results in polydisperse linear and branched chains. Here, we demonstrate that core-satellite structures comprised of shape-controlled plasmonic nanostructures can be achieved through self-assembly using simple molecular cross-linkers. Prevention of self-conjugation and promotion of cross-conjugation among cores and satellites plays a key role in the formation of core-satellite heteroassemblies. The in-built electromagnetic hot-spots and Raman reporters of core-satellite structures make them excellent candidates for surface-enhanced Raman scattering probes.

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.

Bilayered Biofoam for Highly Efficient Solar Steam Generation.

A novel bilayered hybrid biofoam composed of a bacterial nanocellulose (BNC) layer and a reduced graphene oxide (RGO)-filled BNC layer is introduced for highly efficient solar steam generation. The biofoam exhibits a solar thermal efficiency of ≈83% under simulated solar illumination (10 kW m-2 ). The fabrication method introduced here is highly scalable and cost-efficient.

Multifunctional analytical platform on a paper strip: separation, preconcentration, and subattomolar detection.

We report a plasmonic paper-based analytical platform with functional versatility and subattomolar (<10(-18) M) detection limit using surface-enhanced Raman scattering as a transduction method. The microfluidic paper-based analytical device (μPAD) is made with a lithography-free process by a simple cut and drop method. Complex samples are separated by a surface chemical gradient created by differential polyelectrolyte coating of the paper. The μPAD with a starlike shape is designed to enable liquid handling by lateral flow without microchannel patterning. This design generates a rapid capillary-driven flow capable of dragging liquid samples as well as gold nanorods into a single cellulose microfiber, thereby providing an extremely preconcentrated and optically active detection spot.

Bilayered Raman‐Intense Gold Nanostructures with Hidden Tags (BRIGHTs) for High‐Resolution Bioimaging

Conventional SERS probes suffer from limited brightness and poor reproducibility and stability making them unsuitable for routine in vivo applications. A novel class of layered SERS probes is demonstrated in which individual nanostructures host electromagnetic hotspots, thus increasing brightness by more than two orders magnitude compared to conventional individual nanostructures.