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.     

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.     

Supramolecular gold(I) compounds with trithiocyanuric acid: Self‐assembly and luminescent properties

[(N₃S₃)Au(AuPMe₃)₂]₂ ( 1 ) and [(N₃S₃)Au(AuPEt₃)₂]₂ ( 2 ) were prepared by treating AuCl(PMe₃) or AuCl(PEt₃) with H₃N₃S₃ upon deprotonation by trimethylamine to give respective Au₆ supermolecules. Using dppm(AuCl)₂ instead of AuCl(PMe₃) or AuCl(PEt₃) to react with H₃N₃S₃ in a similar reaction condition led to a rare heptanuclear supermolecule of [(N₃S₃)₂Au₇(dppm)₄]Cl ( 3 ). It is noted that besides short intramolecular gold(I)?gold(I) distances, both compounds 1 and 2 also show intermolecular gold(I)?gold(I) distances of 3.067(1) and 3.241(1) Å, resulting in two‐dimensional and one‐dimensional polymeric gold(I) solid, respectively. In fact, compound 1 shows a similar two‐dimensional polymeric gold(I) solid to that of the reported [(N₃S₃)Au(AuPPhMe₂)₂]₂ with an intermolecular gold(I)?gold(I) distance of 3.130(2) Å. Significantly, these intermolecular gold(I)?gold(I) distances are well correlated with their cone angles and emission energies. For example, intermolecular gold(I)?gold(I) distances increase in the order of 3.067(1) Å < 3.130(2) Å < 3.241(1) Å for PMe₃ (118°), PPhMe₂ (122°), and PEt₃ (132°), and their emission energies also increase in the order of 542 nm < 530 nm < 504 nm, respectively. This work highlights a very good correlation between intermolecular aurophilic interactions and emission energies for a series of Au₆ supermolecules, where the cone angle plays a vital role in the self‐assembly process as well. Finally, the emissions for 1 – 3 are tentatively assigned to the S → Au charge‐transfer transition, whereas they are most probably modified by gold(I)?gold(I) interactions.     

Poly(phenylhydrosilane): Base-developable polysilane resist

Poly(phenylhydrosilane) becomes soluble in a 2.38% tetramethylammoniumhydroxide aqueous solution after exposure to UV light. This is the first report that the polysilanes not bearing acidic groups can be developed with dilute basic aqueous solutions. Addition of 3,3′,4,4′-tetra(t-butylperoxycarbonyl)benzophenone increases the resist sensitivity. The reaction mechanism is as follows: PS1 photodecomposes to form silyl compounds having SiOH groups, which become soluble in an aqueous base solution, because these silane compounds bearing SiOHs are acidic. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2355–2364, 1997     

Structural control of the horizontal double fixation of oligothiophenes on gold

Quaterthiophenes bearing one (1) or two (2) alkanethiol chains attached at the internal beta-position of the outermost thiophene ring through a sulfide linkage have been synthesized. Cyclic voltammetric analysis of their electrochemical behavior in solution suggests that electrooxidation of the doubly substituted oligomer 2 leads to electrodeposition of a poly(disulfide) on the anode surface. Monolayers of 1 or 2 on gold surfaces have been investigated and characterized by cyclic voltammetry, ellipsometry, contact angle measurement, and X-ray photoelectron spectroscopy. The results of these investigations indicate that introduction of two thiol groups in the structure leads to double fixation of the oligothiophene chain with the main axis of the conjugated system oriented parallel to the surface. The effects of single versus double fixation of the quaterthiophene chain on the electrochemical properties and stability of the corresponding monolayers are discussed.