ACS Nano

Publications in this journal

• Article: Diffusive Dynamics of Nanoparticles in Arrays of Nanoposts.

  Kai He, Firoozeh Babaye Khorasani, Scott T Retterer, Darrell K Thomas, Jacinta C Conrad, Ramanan Krishnamoorti
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  ABSTRACT: The diffusive dynamics of dilute dispersions of nanoparticles of diameter 200-400 nm were studied in microfabricated arrays of nanoposts using differential dynamic microscopy and single particle tracking. Posts of diameter 500 nm and height 10 μm were spaced by 1.2-10 μm on a square lattice. As the spacing between posts was decreased, the dynamics of the nanoparticles slowed. Moreover, the dynamics at all length scales were best represented by a stretched exponential rather than a simple exponential. Both the relative diffusivity and the stretching exponent decreased linearly with increased confinement and, equivalently, with decreased void volume. The slowing of the overall diffusive dynamics and the broadening distribution of nanoparticle displacements with increased confinement are consistent with the onset of dynamic heterogeneity and the approach to vitrification.
  ACS Nano 05/2013;
• Article: A Fibrous Hybrid of Graphene and Sulfur Nanocrystals for High Performance Lithium-Sulfur Batteries.

  Guangmin Zhou, Li-Chang Yin, Da-Wei Wang, Lu Li, Songfeng Pei, Ian Ross Gentle, Feng Li, Hui-Ming Cheng
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  ABSTRACT: Graphene-sulfur (G-S) hybrid materials with sulfur nanocrystals anchored on interconnected fibrous graphene are obtained by a facile one pot strategy using a sulfur/carbon disulfide/alcohol mixed solution. The reduction of graphene oxide and the formation/binding of sulfur nanocrystals were integrated. The G-S hybrids exhibit a highly porous network structure constructed by fibrous graphene, many electrically conducting pathways and easily tunable sulfur content, which can be cut and pressed to pellets to be directly used as lithium-sulfur battery cathodes without using metal current-collector, binder and conductive additive. The porous network and sulfur nanocrystals enable rapid ion transport and short Li+ diffuse distance, the interconnected fibrous graphene provides highly conductive electron transport pathways, and the oxygen-containing (mainly hydroxyl/epoxide) groups show strong binding with polysulfides preventing their dissolution into electrolyte based on first-principles calculations. As a result, the G-S hybrids show a high capacity, an excellent high-rate performance and a long life over 100 cycles. These results demonstrate the great potential of this unique hybrid structure as cathodes for high performance lithium-sulfur batteries.
  ACS Nano 05/2013;
• Article: Splitting of a Vertical Multi-Walled Carbon Nanotube Carpet to a Graphene Nanoribbon Carpet and its use in Supercapacitors.

  Chenguang Zhang, Zhiwei Peng, Jian Lin, Yu Zhu, Gedeng Ruan, Chih-Chau Hwang, Wei Lu, Robert H Hauge, James M Tour
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  ABSTRACT: Potassium vapor was used to longitudinally split vertically aligned multi-walled carbon nanotubes carpets (VA-CNTs). The resulting structures have a carpet of partially split MWCNTs and graphene nanoribbons (GNRs). The split structures were characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. When compared to the original VA-CNTs carpet, the split VA-CNTs carpet has enhanced electrochemical performance with better specific capacitance in a supercapacitor. Furthermore, the split VA-CNTs carpet has excellent cyclability as a supercapacitor electrode material. There is a measured maximum power density of 103 kW/kg at an energy density of 5.2 Wh/kg and a maximum energy density of 9.4 Wh/kg. The superior electrochemical performances of the split VA-CNTs can be attributed to the increased surface area for ion accessibility after splitting, and the lasting conductivity of the structure with their vertical conductive paths based on the preserved GNR alignment.
  ACS Nano 05/2013;
• Article: Nano-Layered siRNA Dressing for Sustained Localized Knockdown.

  Steven Castleberry, Mary Wang, Paula T Hammond
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  ABSTRACT: The success of RNA interference (RNAi) in medicine relies on the development of technology capable of successfully delivering it to tissues of interest. Significant research has focused on the difficult task of systemic delivery of RNAi; however its local delivery could be a more easily realized approach. Localized delivery is of particular interest for many medical applications, including the treatment of localized diseases, the modulation of cellular response to implants or tissue engineering constructs, and the management of wound healing and regenerative medicine. In this work we present an ultra-thin electrostatically assembled coating for localized and sustained delivery of short interfering RNA (siRNA). This film was applied to a commercially available woven nylon dressing commonly used for surgical applications, and was demonstrated to sustain significant knockdown of protein expression in multiple cell types for over one week in vitro. Significantly, this coating can be easily applied to a medically relevant device and requires no externally delivered transfection agents for effective delivery of siRNA. These results present promising opportunities for the localized administration of RNAi.
  ACS Nano 05/2013;
• Article: Effects of Functionalization on Thermal Properties of Single-Wall and Multi-Wall Carbon Nanotube - Polymer Nanocomposites.

  Richard Gulotty, Micaela Castellino, Pravin Jagdale, Alberto Tagliaferro, Alexander A Balandin
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  ABSTRACT: Carboxylic functionalization (-COOH groups) of carbon nanotubes is known to improve their dispersion properties and increase the electrical conductivity of carbon-nanotube - polymer nanocomposites. We have studied experimentally the effects of this type of functionalization on the thermal conductivity of the nanocomposites. It was found that while even small quantities of carbon nanotubes (~1 wt%) can increase the electrical conductivity, larger loading fraction (~3 wt%) is required to enhance the thermal conductivity of nanocomposites. Functionalized multi-wall carbon nanotubes performed the best as the filler material leading to a simultaneous improvement of the electrical and thermal properties of the composites. Functionalization of the single-wall carbon nanotubes reduced the thermal conductivity enhancement. The observed trends were explained by the fact that while surface functionalization increases the coupling between carbon nanotube and polymer matrix it also leads to formation of defects, which impede the acoustic phonon transport in the single wall carbon nanotubes. The obtained results are important for applications of carbon nanotubes and graphene flakes as fillers for improving thermal, electrical and mechanical properties of composites.
  ACS Nano 05/2013;
• Article: Scanning-Raman-Microscopy for the Statistical Analysis of Covalently Functionalized Graphene.

  Jan M Englert, Philipp Vecera, Kathrin C Knirsch, Ricarda A Schäfer, Frank Hauke, Andreas Hirsch
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  ABSTRACT: We report on the introduction of a systematic method for the quantitative and reliable characterization of covalently functionalized graphene based on Scanning-Raman-Microscopy (SRM). This allows for recording and analyzing several thousands of Raman spectra per sample and straightforward display of various Raman properties and their correlations with each other in histograms or coded 2D-plots. In this way information about the functionalization efficiency of a given reaction, the reproducibility of the statistical analysis and the sample homogeneity can be easily deduced. Based on geometric considerations we were also able to provide for the first time a correlation between the mean defect distance of densely packed point defects and the Raman ID/IG ratio directly obtained from the statistical analysis. This proved to be the prerequisite for determining the degree of functionalization, termed θ. As model compounds we have studied a series of arylated graphenes (GPh) for which we have developed new synthetic procedures. Both graphite and graphene grown by chemical vapor deposition (CVD) were used as starting materials. The best route towards GPh consisted of the initial reduction of graphite with a Na/K alloy in 1,2-dimethoxyethane (DME) as it yields the highest overall homogeneity of products reflected in the widths of the Raman ID/IG histograms. The Raman results correlate very well with parallel thermogravimetric analysis (TGA) coupled with mass spectrometry (MS) studies.
  ACS Nano 05/2013;
• Article: Mapping the Pharyngeal and Intestinal pH of Caenorhabditis elegans and Real-Time Luminal pH Oscillations Using Extended Dynamic Range pH-Sensitive Nanosensors.

  Veeren M Chauhan, Gianni Orsi, Alan Brown, David I Pritchard, Jonathan W Aylott
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  ABSTRACT: Extended dynamic range pH-sensitive ratiometric nanosensors, capable of accurately mapping the full physiological pH range, have been developed and used to characterize the pH of the pharyngeal and intestinal lumen of Caenorhabditis elegans in real-time. Nanosensors, 40 nm in diameter, were prepared by conjugating pH-sensitive fluorophores, carboxyfluorescein (FAM) and Oregon Green® (OG) in a 1:1 ratio, and a reference fluorophore, 5-(and-6)-carboxytetramethylrhodamine (TAMRA) to an inert polyacrylamide matrix. Accurate ratiometric pH measurements were calculated through determination of the fluorescence ratio between the pH-sensitive and reference fluorophores. Nanosensors were calibrated with an automated image analysis system and validated to demonstrate a pH measurement resolution of ± 0.17 pH units. The motility of C. elegans populations, as an indicator for viability, showed nematodes treated with nanosensors, for concentrations ranging from 50.00 to 3.13 mg/mL, were not statistically different to nematodes not challenged with nanosensors up to a period of 4 days (p<0.05). The nanosensors were also found to remain in the C. elegans lumen >24 hours after nanosensor challenge was removed. The pH of viable C. elegans lumen was found to range from 5.96 ± 0.31 in the anterior pharynx to 3.59 ± 0.09 in the posterior intestine. The pharyngeal pumping rate, which dictates the transfer of ingested material from the pharynx to the intestine, was found to be temperature dependent. Imaging C. elegans at 4 °C reduced the pharyngeal pumping rate to 7 contractions/minute and enabled the reconstruction of rhythmic pH oscillations in the intestinal lumen in real-time with fluorescence microscopy.
  ACS Nano 05/2013;
• Article: Studying Quantum Dot Blinking Through the Addition of an Engineered Inorganic Hole Trap.

  Ron Tenne, Ayelet Teitelboim, Pazit Rukenstein, Maria Dyshel, Taleb Mokari, Dan Oron
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  ABSTRACT: An all-inorganic compound colloidal quantum dot incorporating a highly emissive CdSe core, which is linked by a CdS tunneling barrier to an engineered charge carrier trap composed of PbS is designed and its optical properties are studied in detail at the single particle level. Study of this structure enables a deeper understanding of the link between photoinduced charging and surface trapping of charge carriers and the phenomenon of quantum dot blinking. In presence of the hole trap, a 'grey' emissive state appears, associated with charging of the core. Rapid switching is observed between the 'on' and the 'grey' state, although the switching dynamics in and out of the dark 'off' state remain unaffected. This result completes the links in the causality chain connecting charge carrier trapping, charging of QDs and the appearance of a 'grey' emission state.
  ACS Nano 05/2013;
• Article: High-Performance, Highly Bendable MoS2 Transistors with High-K Dielectrics for Flexible Low-Power Systems.

  Hsiao-Yu Chang, Shixuan Yang, Jongho Lee, Li Tao, Wan-Sik Hwang, Debdeep Jena, Nanshu Lu, Deji Akinwande
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  ABSTRACT: While there has been increasing studies of MoS2 and other two-dimensional (2D) semiconducting dichalcogenides on hard conventional substrates, experimental or analytical studies on flexible substrates has been very limited so far, even though these 2D crystals are understood to have greater prospects for flexible smart systems. In this article, we report detailed studies of MoS2 transistors on industrial plastic sheets. Transistor characteristics afford more than 100x improvement in the ON/OFF current ratio and 4x enhancement in mobility compared to previous flexible MoS2 devices. Mechanical studies reveal robust electronic properties down to a bending radius of 1mm which is comparable to previous reports for flexible graphene transistors. Experimental investigation identifies that crack formation in the dielectric is the responsible failure mechanism demonstrating that the mechanical properties of the dielectric layer is critical for realizing flexible electronics that can accommodate high strain. Our uniaxial tensile tests have revealed that atomic-layer-deposited HfO2 and Al2O3 films have very similar crack onset strain. However, crack propagation is slower in HfO2 dielectric compared to Al2O3 dielectric, suggesting a subcritical fracture mechanism in the thin oxide films. Rigorous mechanics modeling provides guidance for achieving flexible MoS2 transistors that are reliable at sub-mm bending radius.
  ACS Nano 05/2013;
• Article: Record Mobility in Transparent p-Type Tin Monoxide Films and Devices by Phase Engineering.

  Jesus A Caraveo-Frescas, Pradipta K Nayak, Hala A Al-Jawhari, Danilo B Granato, Udo Schwingenschlögl, Husam N Alshareef
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  ABSTRACT: Here, we report the fabrication of nanoscale (15 nm) fully transparent p-type SnO thin film transistors (TFT) at temperatures as low as 180 °C with record device performance. Specifically, by carefully controlling the process conditions, we have developed SnO thin films with a Hall mobility of 18.71 cm(2) V(-1) s(-1) and fabricated TFT devices with a linear field-effect mobility of 6.75 cm(2) V(-1) s(-1) and 5.87 cm(2) V(-1) s(-1) on transparent rigid and translucent flexible substrates, respectively. These values of mobility are the highest reported to date for any p-type oxide processed at this low temperature. We further demonstrate that this high mobility is realized by careful phase engineering. Specifically, we show that phase-pure SnO is not necessarily the highest mobility phase; instead, well-controlled amounts of residual metallic tin are shown to substantially increase the hole mobility. A detailed phase stability map for physical vapor deposition of nanoscale SnO is constructed for the first time for this p-type oxide.
  ACS Nano 05/2013;
• Article: Enhancing the Efficiency of Solution-Processed Polymer:Colloidal Nanocrystal Hybrid Photovoltaic Cells Using Ethanedithiol Treatment.

  Renjia Zhou, Romain Stalder, Dongping Xie, Weiran Cao, Ying Zheng, Yixing Yang, Marc Plaisant, Paul Holloway, Kirk S Schanze, John R Reynolds, Jiangeng Xue
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  ABSTRACT: Advances in colloidal inorganic nanocrystal synthesis and processing have led to the demonstration of organic-inorganic hybrid photovoltaic (PV) cells using low-cost solution processes from blends of conjugated polymer and colloidal nanocrystals. However, performance of such hybrid PV cells has been limited due to the lack of control at the complex interfaces between the organic and inorganic hybrid active materials. Here we show that the efficiency of hybrid PV devices can be significantly enhanced by engineering the polymer-nanocrystal interface with proper chemical treatment. Using two different conjugated polymers, poly(3-hexylthiophene) (P3HT) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), we show that treating the polymer:nanocrystal hybrid film in an ethanedithiol-containing acetonitrile solution can increase the efficiency of the hybrid PV devices by 30-90%, and a maximum power conversion efficiency of (5.2±0.3)% was obtained in the PCPDTBT:CdSe devices at 0.2 suns (AM1.5G), which was slightly reduced to (4.7±0.3)% at 1 sun. The ethanedithiol treatment did not result in significant changes in the morphology and UV-vis optical absorption of the hybrid thin films; however, infrared absorption, NMR, and x-ray photoelectron spectroscopies revealed the effective removal of organic ligands, especially the charged phosphonic acid ligands, from the CdSe nanorod surface after the treatment, accompanied by the possible monolayer passivation of nanorod surfaces with Cd-thiolates. We attribute the hybrid PV cell efficiency increase upon the ethanedithiol treatment to the reduction in charge and exciton recombination sites on the nanocrystal surface and the simultaneous increase in electron transport through the hybrid film.
  ACS Nano 05/2013;
• Article: Cellular Interaction and Toxicity Depends On Physiochemical Properties and Surface Modification of Redox Active Nanomaterials.

  Janet M Dowding, Soumen Das, Amit Kumar, Talib Dosani, Rameech McCormack, Ankur Gupta, Thi X T Sayle, Dean C Sayle, Laurence Vonkalm, Sudipta Seal, William T Self
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  ABSTRACT: The study of the chemical and biological properties of CeO2 NPs (CNPs) has expanded recently due to its therapeutic potential, and the methods used to synthesize these materials are diverse. Moreover, conflicting reports exists regarding the toxicity of CNP. To help resolve these discrepancies, we must first determine whether CeO2 NPs made by different methods are similar or different in their physiochemical and catalytic properties. In this paper, we have synthesized several forms of CNPs using identical precursors through a wet chemical process but using different oxidizer/reducer H2O2 (CNP1), NH4OH (CNP2) or hexamethylenetetramine (HMT-CNP1). Physiochemical properties of these CeO2 NPs were extensively studied and found to be different depending on the preparation methods. Unlike CNP1 and CNP2, HMT-CNP1 were readily taken into endothelial cells and their aggregation can be visualized using light microscopy. Exposure to HMT-CNP1 also reduced cell viability (MTT) at a 10-fold lower concentration than CNP1 or CNP2. Surprisingly, exposure to HMT-CNP1 led to substantial decreases in the ATP levels. Mechanistic studies revealed that HMT-CNP1 exhibited substantial ATPase (phosphatase) activity. Though CNP2 also exhibits ATPase activity, CNP1 lacked ATPase activity. The difference in catalytic (ATPase) activity of different CeO2 NPs preparation may be due to differences in their morphology and oxygen extraction energy. These results suggest the combination of increased uptake and ATPase activity of HMT-CNP1 may underlie the biomechanism of the toxicity of this preparation of CNPs, and may suggest ATPase activity should be considered when synthesizing CNPs for use in biomedical applications.
  ACS Nano 05/2013;
• Article: Nanotopography Modulates Mechanotransduction of Stem Cell and Induces Differentiation Through Focal Adhesion Kinase.

  Benjamin Kim Kiat Teo, Sum Thai Wong, Choon Kiat Lim, Terence Y S Kung, Chong Hao Yap, Yamini Ramgopal, Lewis H Romer, Evelyn K F Yim
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  ABSTRACT: Regulated biophysical cues, such as nanotopography, have been shown to be integral for tissue regeneration and embryogenesis in the stem cell niche. Tissue homeostasis involves the interaction of multipotent cells with nano-scaled topographical features in their ECM to regulate aspects of cell behaviour. Synthetic nanostructures can drive specific cell differentiation but the sensing mechanism of nanocues remains poorly understood. Here we report that nanotopography-induced human mesenchymal stem cells (hMSC) differentiation through cell mechanotransduction is modulated by the integrin-activated focal adhesion kinase (FAK). On nano-gratings with 250nm line width on polydimethylsiloxane, hMSCs developed aligned stress fibers and showed an upregulation of neurogenic and myogenic differentiation markers. The observed cellular focal adhesions within these cells were also significantly smaller and more elongated on the nano-gratings compared to micro-gratings or unpatterned control. In addition, our mechanistic study confirmed that the regulation was dependent on actomyosin contractility, suggesting a direct force dependent mechanism. The topography induced differentiation could be observed on different ECM compositions but the response is independent of direct ECM-induced hMSC differentiation pathway. Essentially, FAK phosphorylation was required for topography-induced hMSC differentiation while FAK overexpression overruled the topographical cues in cell lineage bias. The results obtained indicated a direct effect of FAK activity on topography-induced gene expression but less dependent on cell shape. Mechanically, the findings may explain how hMSCs can sense and transduce nanotopographical signals through focal adhesions and actomyosin cytoskeleton contractility to induce differential gene expression.
  ACS Nano 05/2013;
• Article: Si Microwire Solar Cells: Improved Efficiency with a Conformal SiO2 Layer.

  Kwanyong Seo, Young J Yu, Peter Duane, Wenqi Zhu, Hyunsung Park, Munib Wober, Kenneth B Crozier
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  ABSTRACT: Silicon microwire arrays have attracted considerable attention recently due to the opportunity they present as highly efficient and cost-effective solar cells. In this study, we report on efficient Si microwire array solar cells with areas of 1 cm(2) and Air Mass 1.5 Global conversion efficiencies of up to 10.6 %. These solar cells show an open-circuit voltage of 0.56 V, a short-circuit current density of 25.2 mA/cm(2), and a fill factor of 75.2 %, with a silicon absorption region that is only 25 µm thick. In particular, the maximum overall efficiency of the champion device is improved from 8.71 % to 10.6 % by conformally coating the wires with a 200 nm thick SiO2 layer. Optical measurements reveal that the layer reduces reflection significantly over the entire visible range.
  ACS Nano 05/2013;
• Article: Chemical Nano-Gardens: Growth of Salt Nanowires from Supramolecular Self-Assembly Gels.

  Ronan Daly, Oxana Kotova, Markus Boese, Thorfinnur Gunnlaugsson, John J Boland
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  ABSTRACT: In this article we examine the phenomenon of single crystal halide salt wire growth at the surface of porous materials. We report the use of a single-step casting technique with a supramolecular self-assembly gel matrix that upon drying leads to the growth of single crystal halide (e.g. NaCl, KCl and KI) nanowires with diameters ca. 130-200 nm. We demonstrate their formation using electron microscopy and electron dispersive X-ray spectroscopy, showing that the supramolecular gel stabilizes the growth of these wires by facilitating a diffusion driven base-growth mechanism. Critically, we show that standard non-supramolecular gels are unable to facilitate nanowire growth. We further show that these nanowires can be grown by seeding; forming nano-crystal gardens. This study helps understand the possible pre-functionalisation of membranes to stimulate ion-specific filters or salt efflorescence suppressors, while also providing a novel route to nanomaterials growth.
  ACS Nano 05/2013;
• Article: Repeated and Controlled Growth of Monolayer, Bilayer and Few-Layer Hexagonal Boron Nitride on Pt Foils.

  Yang Gao, Wencai Ren, Teng Ma, Zhibo Liu, Yu Zhang, Wen-Bin Liu, Lai-Peng Ma, Xiuliang Ma, Hui-Ming Cheng
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  ABSTRACT: Atomically thin hexagonal boron nitride (h-BN), as a graphene analogue, has attracted increasing interest because of many fascinating properties and a wide range of potential applications. However, it still remains a great challenge to synthesize high-quality h-BN with pre-determined number of layers at a low cost. Here we reported the controlled growth of h-BN on polycrystalline Pt foils by low-cost ambient pressure chemical vapor deposition with ammonia borane as the precursor. Monolayer, bilayer and few-layer h-BN domains and large-area films were selectively obtained on Pt by simply changing the concentration of ammonia borane. Moreover, using a bubbling method, we have achieved the nondestructive transfer of h-BN from Pt to arbitrary substrates and the repeated use of the Pt for h-BN growth, which not only reduces environmental pollution but also decreases the production cost of h-BN. The monolayer and bilayer h-BN obtained are very uniform with high quality and smooth surfaces. In addition, we found that the optical band gap of h-BN increases with decreasing the number of layers. The repeated growth of large-area, high-quality monolayer and bilayer h-BN films, together with the successful growth of graphene, opens up the possibility for creating various functional heterostructures for large-scale fabrication and integration of novel electronics.
  ACS Nano 05/2013;
• Article: Three-Dimensional Hierarchical Plasmonic Nano-Architecture Enhanced Surface-Enhanced Raman Scattering Immuno-Sensor for Cancer Biomarker Detection in Blood Plasma.

  Ming Li, Scott K Cushing, Jianming Zhang, Savan Suri, Rebecca Evans, William P Petros, Laura F Gibson, Dongling Ma, Yuxin Liu, Nianqiang Nick Wu
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  ABSTRACT: A three-dimensional (3D) hierarchical plasmonic nano-architecture has been designed for a sensitive surface-enhanced Raman scattering (SERS) immuno-sensor for protein biomarker detection. The capture antibody molecules are immobilized on a plasmonic gold triangle nano-array pattern. On the other hand, the detection antibody molecules are linked to the gold nano-star@Raman-reporter@silica sandwich nanoparticles. When protein biomarkers are present, the sandwich nanoparticles are captured over the gold triangle nano-array, forming a confined 3D plasmonic field, leading to the enhanced electromagnetic field in intensity and in 3D space. As a result, the Raman reporter molecules are exposed to a high density of "hot spots", which amplifies the Raman signal remarkably, improving the sensitivity of the SERS immuno-sensor. This SERS immuno-sensor exhibits a wide linear range (0.1 pg/mL to 10 ng/mL), and a low limit of detection (7 fg/mL) toward human immunoglobulin G (IgG) protein in the buffer solution. This biosensor has been successfully used for detection of the vascular endothelial growth factor (VEGF) in the human blood plasma from clinical breast cancer patient samples.
  ACS Nano 05/2013;
• Article: Controlling Molecular Self-Assembly on an Insulating Surface by Rationally Designing an Efficient Anchor Functionality that Maintains Structural Flexibility.

  Christopher M Hauke, Ralf Bechstein, Markus Kittelmann, Christof Storz, Andreas F M Kilbinger, Philipp Rahe, Angelika Kuhnle
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  ABSTRACT: Molecular self-assembly on surfaces is dictated by the delicate balance between intermolecular and molecule-surface interactions. For many insulating surfaces, however, the molecule-surface interactions are weak and rather unspecific. Enhancing these interactions, on the other hand, often puts a severe limit on the achievable structural variety. To grasp the full potential of molecular self-assembly on these application-relevant substrates therefore requires strategies for anchoring the molecular building blocks towards the surface in a way that maintains flexibility in terms of intermolecular interaction and relative molecule orientation. Here, we report the design of a site-specific anchor functionality that provides strong anchoring towards the surface, resulting in a well-defined adsorption position. At the same time, the anchor does not significantly interfere with the intermolecular interaction, ensuring structural flexibility. We demonstrate the success of this approach with three molecules from the class of shape-persistent oligo(p-benzamide)s adsorbed onto the calcite(10.4) surface. These molecules have the same aromatic backbone with iodine substituents, providing the same basic adsorption mechanism to the surface calcium cations. The backbone is equipped with different functional groups. These have a negligible influence on the molecular adsorption on the surface, but significantly change the intermolecular interaction. We show that distinctly different molecular structures are obtained that wet the surface due to the strong linker while maintaining variability in the relative molecular orientation. With this study, we thus provide a versatile strategy for increasing the structural richness in molecular self-assembly on insulating substrates.
  ACS Nano 05/2013;
• Article: Thermodynamic and Structural Insights into Nanocomposites Engineering by Comparing Two Materials Assembly Techniques for Graphene.

  Nicholas A Kotov, Jian Zhu, Huanan Zhang
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  ABSTRACT: Materials assembled by layer-by-layer (LBL) assembly and vacuum assisted flocculation (VAF) have similarities but a systematic study of their comparative advantages and disadvantages is missing. Such a study is needed from both practical and fundamental perspectives aiming at better understanding of structure-property relations of nanocomposites and purposeful engineering of materials with unique properties. Layered composites from polyvinyl alcohol (PVA) and reduced graphene (RG) are made by both techniques. We comparatively evaluate their structure, mechanical, and electrical properties. LBL and VAF composites demonstrate clear differences for atomic and nanoscale structural levels, but reveal similarities for micron and submicron organization. Epitaxial crystallization and suppression of phase transition temperatures are more pronounced for PVA in LBL than for VAF composites. Mechanical properties are virtually identical for both assemblies at high RG contents. We conclude that mechanical properties in layered RG assemblies are largely determined by the thermodynamic state of PVA at the polymer/nanosheet interface rather than the nanometer scale differences in RG packing. High and nearly identical values of toughness for LBL and VAF composites reaching 6.1 MJ/m3 observed for thermodynamically optimal composition confirm this conclusion. Their toughness is the highest among all other layered assemblies from RG, cellulose, clay, etc. Electrical conductivity, however, is more than 10x higher for LBL than for VAF composites for the same RG contents. Electrical properties are largely determined by the tunneling barrier between RG sheets, and therefore strongly dependent on atomic/nanoscale organization. These findings open the door for application-oriented methods of materials engineering using both types of layered assemblies.
  ACS Nano 05/2013;
• Article: Tiny Grains Give Huge Gains: Nanocrystal-Based Signal Amplification for Biomolecule Detection.

  Sheng Tong, Binbin Ren, Zhilan Zheng, Han Shen, Gang Bao
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  ABSTRACT: Nanocrystals, despite their tiny sizes, contain thousands to millions of atoms. Here we show that the large number of atoms packed in each metallic nanocrystal can provide a huge gain in signal amplification for biomolecule detection. We have devised a highly sensitive, linear amplification scheme by integrating the dissolution of bound nanocrystals and metal-induced stoichiometric chromogenesis, and demonstrated that signal amplification is fully defined by the size and atom density of nanocrystals, which can be optimized through well-controlled nanocrystal synthesis. Further, the rich library of chromogenic reactions allows implementation of this scheme in various assay formats, as demonstrated by the iron oxide nanoparticle linked immunosorbent assay (ILISA) and blotting assay developed in this study. Our results indicate that, owing to the inherent simplicity, high sensitivity and repeatability, the nanocrystal based amplification scheme can significantly improve biomolecule quantification in both laboratory research and clinical diagnostics. This novel method adds a new dimension to current nanoparticle-based bioassays.
  ACS Nano 05/2013;