Reversible assembly and disassembly of gold nanoparticles directed by a zwitterionic polymer

Herein, we have successfully introduced the stimuli-response concept into the controllable synthesis of gold nanoparticles (AuNPs) with designed properties. We used a pH-responsive zwitterionic polymer that acted as a template and a stabilizer. Gold colloids prepared in situ from the polymer solution have a narrow size distribution of about 5 nm. The assembly and disassembly of AuNPs can be finely tuned by modulating the net charges of the zwitterionic polymer so that they are either positive or negative as a function of the solution pH. Different aggregates and colors appear on altering the solution pH. In acidic solutions, gold colloids form large symmetrical aggregates, while the AuNPs disperse in solutions with a pH approximately 9.6. However, as the solution pH increases (>9.6), needle-like aggregates with a small interparticle distances are formed. On the basis of TEM, SEM, 1H NMR and UV/Vis measurements, we attribute pH-triggered aggregation and color changes of the gold colloids to the ionization process and conformational change of the polymer. The ionization process governs the choice of ligand anchored on the surface of AuNPs, and the conformational change of the polymer modulates the interspaces between AuNPs. The present approach, which is based on a rational design of the physicochemical properties of the template used in the synthesis process, provides a powerful means to control the properties of the nanomaterial. Furthermore, the colorimetric readout can be visualized and applied to future studies on nanoscale switches and sensors.     

Surface modification of gold nanoparticles with polyhedral oligomeric silsesquioxane and incorporation within polymer matrices

A new approach to achieve polymer‐mediated gold ferromagnetic nanocomposites in a polyhedral oligomeric silsesquioxane (POSS)‐containing random copolymer matrix has been developed. Stable and narrow distributed gold nanoparticles modified by 3‐mercaptopropylisobutyl POSS to form Au‐POSS nanoparticles are prepared by two‐phase liquid‐liquid method. These Au‐POSS nanoparticles form partial particle aggregation by blending with poly(n‐butyl methacrylate) (PnBMA) homopolymer because of poor miscibility between Au‐POSS and PnBMA polymer matrix. The incorporation the POSS moiety into the PnBMA main chain as a random copolymer matrix displays well‐dispersed gold nanoparticles because the POSS‐POSS interaction enhances miscibility between gold nanoparticles and the PnBMA‐POSS copolymer matrix. This gold‐containing nanocomposite exhibits ferromagnetic phenomenon at room temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 811–819, 2009     

Sensitivity of transmission surface plasmon resonance (T-SPR) spectroscopy: self-assembled multilayers on evaporated gold island films

The distance dependence of the localized surface plasmon (SP) extinction of discontinuous gold films is a crucial issue in the application of transmission surface plasmon resonance (T-SPR) spectroscopy to chemical and biological sensing. This derives from the usual sensing configuration, whereby an analyte binds to a selective receptor layer on the gold film at a certain distance from the metal surface. In the present work the distance sensitivity of T-SPR spectroscopy of 1.0-5.0 nm (nominal thickness) gold island films evaporated on silanized glass substrates is studied by using coordination-based self-assembled multilayers, offering thickness tuning in the range from approximately 1 to approximately 15 nm. The morphology, composition and optical properties of the Au/multilayer systems were studied at each step of multilayer construction. High-resolution scanning electron microscopy (HRSEM) showed no apparent change in the underlying Au islands, while atomic force microscopy (AFM) indicated flattening of the surface topography during multilayer construction. A regular growth mode of the organic layers was substantiated by X-ray photoelectron spectroscopy (XPS). Transmission UV-visible spectra showed an increase of the extinction and a red shift of the maximum of the SP band upon addition of organic layers, establishing the distance dependence of the Au SP absorbance. The distance sensitivity of T-SPR spectroscopy can be varied by using characteristic substrate parameters, that is, Au nominal thickness and annealing. In particular, effective sensitivity up to a distance of at least 15 nm is demonstrated with 5 nm annealed Au films. It is shown that intensity measurements, particularly in the plasmon intensity change (PIC) presentation, provide an alternative to the usually measured plasmon band position, offering good accuracy and the possibility of measuring at a single wavelength. The present distance sensitivity results provide the basis for further development of T-SPR transducers based on receptor-coated Au island films.     

Self‐assembly of polymer‐coated ferromagnetic nanoparticles into mesoscopic polymer chains

The self‐assembly of dispersed polymer‐coated ferromagnetic nanoparticles into micron‐sized one‐dimensional mesostructures at a liquid–liquid interface was reported. When polystyrene‐coated Co nanoparticles (19 nm) are driven to an oil/water interface under zero‐field conditions, long (≈ 5 μm) chain‐like assemblies spontaneously form because of dipolar associations between the ferromagnetic nanoparticles. Direct imaging of the magnetic assembly process was achieved using a recently developed platform consisting of a biphasic oil/water system in which the oil phase was flash‐cured within 1 s upon ultraviolet light exposure. The nanoparticle assemblies embedded in the crosslinked phase were then imaged using atomic force microscopy. The effects of time, temperature, and colloid concentration on the self‐assembly process of dipolar nanoparticles were then investigated. Variation of either assembly time t or temperature T was found to be an interchangeable effect in the 1D organization process. Because of the dependence of chain length on the assembly conditions, we observed striking similarities between 1D nanoparticle self‐assembly and polymerization of small molecule monomers. This is the first in‐depth study of the parameters affecting the self‐assembly of dispersed, dipolar nanoparticles into extended mesostructures in the absence of a magnetic field. © 2008 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys 46: 2267–2277, 2008     

RAFT made methacrylate copolymers for reversible pH‐responsive nanoparticles

In this study, we designed and investigated pH‐responsive nanoparticles based on different ratios of monomers with primary, secondary or tertiary amino groups. For this purpose, copolymers of methyl methacrylate (MMA) with different compositions of amino methacrylates (2‐(dimethylamino)ethyl methacrylate (DMAEMA), 2‐(tert‐butylamino)ethyl methacrylate (tBAEMA) and 2‐aminoethyl methacrylate hydrochloride (AEMA·HCI)) were synthesized using the reversible addition‐fragmentation chain transfer (RAFT) polymerization process. The controlled nature of the radical polymerization was demonstrated by kinetic studies. All copolymers show low dispersities (?_M < 1.2) with amino contents between 9 and 21 mol %. For the nanoparticle formation, nanoprecipitation with subsequent solvent evaporation was used. All suspensions were characterized by dynamic light scattering (DLS) and scanning electron microscopy (SEM). Different initial conditions of the formulations resulted in differently sized nanoparticles that have monomodal size distributions, relatively narrow polydispersity index (PDI) values and positive zeta potential values. The pH‐stability test results demonstrated that, depending on the structure and amount of the amino content, the obtained nanoparticles reveal a reversible pH‐response, such as dissolution at acidic pH values. The ability of the nanoparticles to encapsulate guest molecules was confirmed by pyrene fluorescence studies. The cytotoxicity assay results showed that the nanoparticles did not have any significant cytotoxic effect. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2711–2721     

One-step synthesis and stabilization of gold nanoparticles and multilayer film assembly

Au nanoparticles (NPs) were synthesized in the one-pot procedure in water at room temperature with the wheel-shaped V^V-V^IV mixed-valence tungstovanadate [P₈W₄₈O₁₈₄{V₄^VV₂^IVO₁₂(H₂O)₂}₂]³²⁻ (V12) acting as both reducing and stabilizing agents. The V12 stabilized Au NPs (Au@V12 NPs) were characterized by SEM, TEM, DLS, UV-vis spectroscopy, XPS, and XRD analyses and the negatively charged surface of the Au@V12 NPs was proved by the zeta potential analysis. Based on the layer-by-layer assembly (LbL), the Au@V12 NPs-containing multilayer films have been fabricated on ITO-coated glass slide and quartz substrates with poly(ethyleneimine) (PEI). The regular growth of the multilayer films was monitored by UV-vis spectroscopy and cyclic voltammetry, the composition was characterized by XPS. The Au@V12 NPs based composite films showed electrocatalytic activities towards the reduction of dioxygen and the oxidation of methanol. This approach is expected to open the way towards procedures aimed at the one-step fabrication of Au NPs and polyoxometalates (POMs) into the multilayer films.     

Interfacing colloidal graphene oxide sheets with gold nanoparticles

Aqueous solutions of graphene oxide (GO) and citrate-stabilised gold nanoparticles (AuNPs) are two classic, negatively charged colloids. Using the surface plasmon resonance spectra of AuNPs as a probe, we illustrate how the two like-charged colloids interact with each other and in so doing, reveal the unique solution behaviour of GO. We demonstrate that the electrical double layer of the GO sheets in water plays a key role in controlling the interaction between GO and AuNPs, which displays a one-way gate effect. It is shown that GO can capture and disperse AuNPs in water in a controllable fashion, without the need for additional chemical linkers. This discovery allows the successful synthesis of uncapped, yet solution-dispersible metal-nanoparticle assemblies. Such metal nanostructures have long been pursued for nano-plasmonics and sensing applications, but have remained difficult to prepare using conventional polymer dispersants. This work also makes clear that the combination of the two-dimensional conformation of GO along with its large molecular size and self-contained functional groups allows it to act as a unique soluble nanocarrier/substrate (the thinnest, functionalised flat substrate possible in nature) for the synthesis of new, soluble functional materials.     

Colloidal assembly: the road from particles to colloidal molecules and crystals

Colloidal particles may be considered as building blocks for materials, just like atoms are the bricks of molecules, macromolecules, and crystals. Periodic arrays of colloids (colloidal crystals) have attracted much interest over the last two decades, largely because of their unique photonic properties. The archetype opal structures are based on close-packed arrays of spheres of submicrometer diameter. Interest in structuring materials at this length scale, but with more complex features and ideally by self-assembly processes, has led to much progress in controlling features of both building blocks and assemblies. The necessary ingredients include colloids, colloidal clusters, and colloidal "molecules" which have special shapes and the ability to bind directionally, the control over short-range and long-range interactions, and the capability to place and orientate these bricks. This Review highlights recent experimental and theoretical progress in the assembly of colloids larger than 50 nm.     

Proton-fueled DNA-duplex-based stimuli-responsive reversible assembly of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have received much attention in nanotechnology because of their potential applications in molecular electronics, field-emission devices, biomedical engineering, and biosensors. Carbon nanotubes as gene and drug delivery vectors or as "building blocks" in nano-/microelectronic devices has been successfully explored. However, since SWNTs lack chemical recognition, SWNT-based electronic devices and sensors are strictly related to the development of a bottom-up self-assembly technique. Here we present an example of using DNA duplex-based protons (H(+)) as a fuel to control reversible assembly of SWNTs without generation of waste duplex products that poison DNA-based systems.     

Studies on surface plasmon resonance and photoluminescence of silver nanoparticles

Silver nanoparticles of different sizes were prepared by citrate reduction and characterized by UV-vis absorbance spectra, TEM images and photoluminescence spectra. The morphology of the colloids obtained consists of a mixture of nanorods and spheres. The surface plasmon resonance (SPR) and photoemission properties of Ag nanoparticles are found to be sensitive to citrate concentration. A blue shift in SPR and an enhancement in photoluminescence intensity are observed with increase in citrate concentration. Effect of addition of KCl and variation of pH in photoluminescence was also studied.