T. P. Vinod

Poly(methyl methacrylate)-Supported Polydiacetylene Films: Unique Chromatic Transitions and Molecular Sensing

Polydiacetylenes (PDAs) constitute a family of conjugated polymers exhibiting unique colorimetric and fluorescence transitions, and have attracted significant interest as chemo- and biosensing materials. We spin-coated PDA films upon poly(methyl methacrylate) (PMMA), and investigated the photophysical properties and sensing applications of the new PDA configuration. Specifically, the as-polymerized blue PDA layer underwent distinct transformations to purple, red, and yellow phases, which could be quantified through conventional color scanning combined with application of image analysis algorithms. Furthermore, we recorded a reversible red-purple PDA transition that was induced by ultraviolet irradiation, a phenomenon that had not been reported previously in PDA film systems. We show that distinct color and fluorescence transitions were induced in the PMMA-supported PDA films by amphiphilic substances-surfactants and ionic liquids-and that the chromatic transformations were correlated to the analyte structures and properties. Overall, this study presents a new chromatic PDA film system in which noncovalent interactions between the PMMA substrate and spin-coated PDA give rise to distinct chromatic properties and molecular sensing capabilities.

Transparent, conductive, and SERS-active Au nanofiber films assembled on an amphiphilic peptide template

The use of biological materials as templates for functional molecular assemblies is an active research field at the interface between chemistry, biology, and materials science. We demonstrate the formation of gold nanofiber films on β-sheet peptide domains assembled at the air/water interface. The gold deposition scheme employed a recently discovered chemical process involving spontaneous crystallization and reduction of water-soluble Au(SCN)4(1-) upon anchoring to surface-displayed amine moieties. Here we show that an interlinked network of crystalline Au nanofibers is readily formed upon incubation of the Au(iii) thiocyanate complex with the peptide monolayers. Intriguingly, the resultant films were optically transparent, enabled electrical conductivity, and displayed pronounced surface enhanced Raman spectroscopy (SERS) activity, making the approach a promising avenue for construction of nano-structured films exhibiting practical applications.

Self-guided one-sided metal reduction in te nanowires leading to Au-te matchsticks

Highly anisotropic nanoscale structures are fundamentally important for understanding the properties of hybrid nanoscale systems and self-organization phenomena, but they are typically difficult to prepare experimentally. Hybrid Au and Te matchstick-like nanostructures, which display high structural anisotropy, were synthesized using small Au clusters as seeds. The matchsticks were found to be an exclusive product of this reaction. We hypothesize that the mechanism of the synthesis is based on self-directed site-specific reduction of Au ions due to the gradually increasing dipole moment facilitating this process on one side of Te nanorods, which is supported by quantum mechanical calculations.

A flexible high-sensitivity piezoresistive sensor comprising a Au nanoribbon-coated polymer sponge

Development of pressure sensors which display high sensitivity, and are flexible, inexpensive, and easy to manufacture has drawn significant interest due to their diverse applications such as tactile skin sensors (e.g. “electronic skin”), pulse detectors, speech recognition elements, and others. While varied technologies and molecular constructs have been demonstrated for pressure sensing, considerable conceptual and technical challenges still hamper broad implementation of many such systems. A novel flexible piezoresistive sensor comprising a conductive Au-coated elastomeric polymer sponge is presented. The piezoresistive sponge is prepared through a simple chemical route in which Au nanoribbons are spontaneously grown upon an amine-functionalized polyurethane framework. The Au nanoribbon layer coats the internal surfaces within the polymer pore network, resulting in electrical current modulation upon pressure application/release through changes in the overall contact areas between the conductive surfaces. The Au-polyurethane piezoresistive sensor exhibits excellent functionalities, including enhanced sensitivity, low detection threshold, high fidelity, and physical stability. Application of the sensor is demonstrated for high resolution monitoring of wrist arterial pulses.

Photocatalytic hybrid Au/ZnO nanoparticles assembled through a one-pot method.

Growth of metal domains on semiconductor nanoparticles is known to enhance their photocatalytic properties. We prepared ZnO nanoparticles decorated with metallic Au domains through a new one-pot microwave-based strategy. The synthetic route utilized microwave-heating of a mixture of only three components: Zn(2+) salt, Au(SCN)4(-) which served as a precursor for metallic gold, and Tris base. The Tris molecules had a dual role in the process, both shaping the morphology of the ZnO particles, as well as constituting docking and nucleation sites for the Au(SCN)4(-) ions. The Au complex subsequently underwent spontaneous crystallization/reduction without co-addition of reducing or stabilizing agents, yielding Au nanoparticles attached to the ZnO surface. We show that the hybrid Au/ZnO nanoparticles exhibited enhanced photocatalytic properties compared to the plain ZnO nanoparticles.

Unilamellar Vesicles from Amphiphilic Graphene Quantum Dots

Graphene quantum dots (GQDs) have attracted considerable interest due to their unique physicochemical properties and various applications. For the first time it is shown that GQDs surface-functionalized with hydrocarbon chains (i.e., amphiphilic GQDs) self-assemble into unilamellar spherical vesicles in aqueous solution. The amphiphilic GQD vesicles exhibit multicolor luminescence that can be readily exploited for membrane studies by fluorescence spectroscopy and microscopy. The GQD vesicles were used for microscopic analysis of membrane interactions and disruption by the peptide beta-amyloid.

Nanostructure Synthesis at the Solid–Water Interface: Spontaneous Assembly and Chemical Transformations of Tellurium Nanorods

Bottom-up synthesis offers novel routes to obtain nanostructures for nanotechnology applications. Most self-assembly processes are carried out in three dimensions (i.e. solutions); however, the large majority of nanostructure-based devices function in two dimensions (i.e. on surfaces). Accordingly, an essential and often cumbersome step in bottom-up applications involves harvesting and transferring the synthesized nanostructures from the solution onto target surfaces. We demonstrate a simple strategy for the synthesis and chemical transformation of tellurium nanorods, which is carried out directly at the solid-solution interface. The technique involves binding the nanorod precursors onto amine-functionalized surfaces, followed by in situ crystallization/oxidation. We show that the surface-anchored tellurium nanorods can be further transformed in situ into Ag2Te, Cu2Te, and SERS-active Au-Te nanorods. This new approach offers a way to construct functional nanostructures directly on surfaces.

Microscale screen printing of large-area arrays of microparticles for the fabrication of photonic structures and for optical sorting

There are a limited number of methods applicable to the large-scale fabrication of arrays of discrete microparticles; however, such methods can be applied to the fabrication of structures applicable to photonics, barcoding, and optoelectronics. This manuscript describes a universal method, “microparticle screen printing” (μSP), for the rational patterning of micron-scale particles onto a variety of 2D substrates with diverse mechanical and chemical properties. Specifically, an array of microparticles of different sizes and compositions were patterned onto an array of materials of varying chemistry and stiffness using μSP yielding a diversity of homo/heterogeneous microparticle-based structures. Further, this manuscript reports how the Youngs moduli of the substrate can be used to calculate contact area and thus interaction energies (quantified using Hamaker constants) between the particle/substrate during μSP. Generally, μSP is most effective for substrates with low Youngs moduli and large Hamaker constants (A132) with the target particles, as confirmed by the performance (quantified using yield and accuracy metrics) of μSP for the different empirically investigated particle/substrate combinations. These understandings allow for the design of optimal surface/particle pairing for μSP and were applied to the fabrication of a diversity of heterogeneous structures, including those with periodic vacancies in HCP (hexagonally closed packed) 2D photonic crystal useful to structural optics, optical particle screening useful to chemical assays, and the fabrication of structural barcodes useful for labeling and anticounterfeiting.