G. Tardajos

Gemini‐Surfactant‐Directed Self‐Assembly of Monodisperse Gold Nanorods into Standing Superlattices

Fabrication of ordered nanoparticle assemblies over extended areas and volumes is still a major challenge in nanomaterials research. The current limitations in the production of such ordered assemblies dramatically hinder the application of nanoparticles in fields such as negative refractive index metamaterials or information technologies. In the particular case of metal nanocrystal assemblies, nanoscale organization of readily accessible spherical gold nanoparticles 4] has been manipulated to produce a diverse range of topologies with interesting optical and electrical properties. 7] However, the use of isotropic nanoparticles strongly limits potential applications that require the formation of lattices with vectorial properties. A recent report demonstrated the formation of 3D gold nanorod (NR) superstructures from liquid-crystalline phases with a limited degree of control over the dimensionality and directionality of the assembly. A major advance is demonstrated herein through the use of a gemini surfactant. Replacement of cetyltrimethylammonium bromide (CTAB) by this unconventional surfactant during nanorod synthesis leads to production of monodisperse NRs that can undergo directional self-assembly into highly ordered 2D and 3D standing superlattices with anisotropic optical properties. The synthesis of highly monodisperse gold NRs is especially appealing because of their strong, polarizationdependent suface-plasmon-based optical properties, which render their assemblies ideal candidates for the preparation of optically anisotropic lattices that allow manipulation of light in the nanoscale. Nowadays, tuning of the longitudinal and transverse localized plasmon resonances of gold NRs by synthetic manipulation is a mature field of research. In particular, the seeded growth method in aqueous solution, based on the reduction by a weak reducing agent of a gold salt on premade small seeds in the presence of CTAB and silver ions provides sufficient flexibility to synthesize nanorods (CTAB-NRs) with diverse sizes and shapes. Besides being a shape-inducing agent, CTAB efficiently prevents aggregation through dynamic adsorption onto the gold NRs surface in a bilayer fashion. This nanoparticle shielding of CTAB in water, together with the intense capillary forces generated at the solvent–air interfaces in aqueous solution and the typical Brownian motion of nanoparticles, brings colloidal stability face-to-face with controlled self-assembly of NRs in water and demands a rational search for new and simple strategies. To date, the construction of assemblies of standing gold NRs has mostly relied on postsynthesis surface functionalization with thiol and silane capping agents and subsequent transfer into organic solvents. However, the degree of order in the self-assembly has still been limited to 2D sub-micrometer areas. Among the different capping agents that have been proposed for the preparation of metal nanoparticle arrays in organic solvents, thiol-functionalized phospholipid derivatives have proven excellent binders in nanoparticle–lipid– nanoparticle assemblies, 21] as they can be efficiently supported as bilayers on different substrates. In this respect, gemini surfactants, made of two hydrophobic tails and two hydrophilic headgroups linked by a spacer chain, are currently available as excellent scaffolds for structurally mimicking the bilayer formation of lipids in water. Moreover, gemini surfactants display exceptional amphiphilic properties, such as high adsorptivities on solid surfaces, which make them ideal candidates to influence the growth 25] and assembly of gold nanorods. We present herein the first study in which cationic gemini surfactants, (oligooxa)alkanediyla,w-bis(dimethyldodecylammonium bromide) (12-EOx-12, see the Supporting Information), were used for the reproducible and controlled synthesis of monodisperse gold NRs, focusing on the role of the chemical structure of these surfactants on the self-assembly of highly ordered, robust 2D and 3D gold NR superlattices with directional optical properties. We synthesized gold nanorods using the gemini surfactant that contains a spacer with one ethylene oxide group, 12-EO112 (Gem1-NRs), as a shape-directing agent by Ag-assisted seeded growth on preformed CTAB-capped gold seeds at 27 8C (see details in the Experimental Section). Controlling the molar ratio between seed and gold salt, the aspect ratio and longitudinal surface plasmon (LSP) resonance of the NRs could be readily tuned (see the Experimental Section). Significantly narrower LSP bands were measured for Gem1NRs than for CTAB-NRs, with typical full width at half LSP maximum (FWHM) reduction of 20–30 % (see the Support[*] Dr. A. Guerrero-Mart nez, Dr. J. P rez-Juste, E. Carb -Argibay, Prof. L. M. Liz-Marz n Departamento de Quimica Fisica and Unidad Asociada CSIC Universidade de Vigo, 36310 Vigo (Spain) Fax: (+ 34)986-812556 E-mail: aguerrero@uvigo.es lmarzan@uvigo.es

Isothermal compressibilities of n-1-alcohols from methanol to 1-dodecanol at 298.15, 308.15, 318.15, and 333.15 K

Abstract Isothermal compressibility for twelve n -1-alcohols, from methanol to 1-dodecanol, have been determined at 298.15, 308.15, 318.15, and 333.15 K. Except for methanol, and perhaps ethanol, the compressibility decreases regularly with chain length.

The Aggregation of Cyclodextrins as Studied by Photon Correlation Spectroscopy

Photon correlation spectroscopy has been used to study theaggregation processes of natural and some modified cyclodextrins (CDs) in diluted aqueoussolutions. α-, β-, and γ-CD form large, polydisperse aggregatesin water, although the aggregation capability is different depending on the macrocycle considered. γ-CD solutions filtered through 0.2~μm give a single-modaldistribution of aggregates of 224 nm in size. The monomeric γ-CD can be isolated by filtering through 0.1 μm. α-CD displays a bimodaldistribution (monomer + aggregates) with both pore sizes. At the concentrations studied (0.012 M) the contribution in mass of the aggregates with both CDs isnegligible. β-CD is much more persistent in its aggregation, even after sievingits solutions through 0.02 μm filters, and time dependent. The aggregation displayed byCDs with partial substitution of the OH groups (Methyl-β-CD and Hydroxypropyl-β-CD) is much weaker, indicating the implication of the hydrophilic rims ofthe CDs in the process. High temperatures, addition of urea or electrolytes andionisation of the OH groups by raising the pH, prevent the aggregation.

Isothermal compressibilities of n-alkanes and benzene

Abstract The isothermal compressibilities of benzene and of 16 n -alkanes, from n -hexane to n -hexadecane, have been measured at zero pressure and 298.15, 308.15, 318.15, and 333.15 K.

Speed of sound in pure liquids by a pulse-echo-overlap method

An ultrasonic apparatus for the measurement of the speed of sound in pure liquids is described. To test its performance the speed of sound in several liquids at 298.15 K was measured and the results were compared with literature values. Isentropic compressibilities ks were also calculated from the experimental results and for the n-alkanes the variation with the chain number of atoms was studied.

Speeds of sound and isentropic compressibilities of (cyclohexane + benzene and (1-chlorobutane + n-hexane or n-heptane or n-octane or n-decane) at 298.15 K

Abstract An ultrasonic technique, described previously for pure liquids and based on the pulse-echooverlap method, has been used now to obtain the speeds of sound in binary mixtures. Measurements were carried out over the whole composition range at 298.15 K, for (cyclohexane + benzene) and for (1-chlorobutane + n -hexane or n -heptane or n -octane or n -decane). Values for the first mixture are in good agreement with literature values. Isentropic compressibiities κ S and corresponding excess functions κ S E have been obtained for all measured mole fractions, and the behaviour with composition and number of carbon atoms of the n -alkane is discussed.

Relationships observed in the structure and spectra of uracil and its 5-substituted derivatives.

The effects on the geometry structure, atomic charges and vibrational wavenumbers of the main different substituents in the 5th position of the uracil ring were analysed, and relationships were established. The 5-monosubstituted derivatives studied were 5-XU (X=F, Cl, Br, I, CH(3), NH(2), NO(2)). The geometry and vibrational wavenumbers were determined in these molecules. The FT-IR and Raman spectra were studied with the support of B3LYP calculations using several basis sets. Several general conclusions were underlined.

Femtosecond Laser-Controlled Tip-to-Tip Assembly and Welding of Gold Nanorods.

Directed assembly of gold nanorods through the use of dithiolated molecular linkers is one of the most efficient methodologies for the morphologically controlled tip-to-tip assembly of this type of anisotropic nanocrystals. However, in a direct analogy to molecular polymerization synthesis, this process is characterized by difficulties in chain-growth control over nanoparticle oligomers. In particular, it is nearly impossible to favor the formation of one type of oligomer, making the methodology hard to use for actual applications in nanoplasmonics. We propose here a light-controlled synthetic procedure that allows obtaining selected plasmonic oligomers in high yield and with reaction times in the scale of minutes by irradiation with low fluence near-infrared (NIR) femtosecond laser pulses. Selective inhibition of the formation of gold nanorod n-mers (trimers) with a longitudinal localized surface plasmon in resonance with a 800 nm Ti:sapphire laser, allowed efficient trapping of the (n – 1)-mers (dimers) by hot spot mediated photothermal decomposition of the interparticle molecular linkers. Laser irradiation at higher energies produced near-field enhancement at the interparticle gaps, which is large enough to melt gold nanorod tips, offering a new pathway toward tip-to-tip welding of gold nanorod oligomers with a plasmonic response at the NIR. Thorough optical and electron microscopy characterization indicates that plasmonic oligomers can be selectively trapped and welded, which has been analyzed in terms of a model that predicts with reasonable accuracy the relative concentrations of the main plasmonic species.

Excess enthalpies at 298.15 K of binary mixtures of cyclohexane with n-alkanes

Abstract Molar excess enthalpies for binary mixtures of cyclohexane + n -hexane, + n -octane, + n -decane, + n -undecane, + n -hexadecane, and n -heptadecane, were measured at 298.15 K. All these mixings were endothermic. The Redlich-Kister equation was used to correlate these mixtures H E with composition.

Natural cyclodextrins as efficient boosters of the chemiluminescence of luminol and isoluminol: exploration of potential applications.

The chemiluminescent oxidation of luminol (LUM) and isoluminol (ISOL) is notably enhanced, both in intensity and duration, in the presence of natural cyclodextrins (alpha-, beta-, gamma-CD). The experiments have considered some of the most widespread applications of these compounds: the determination of metal cations and the revealing of bloodstains by oxidation with hydrogen peroxide in alkaline solution in the presence of Co(II), Fe(III), human hemoglobin, and blood, in order to explore potential applications. The largest enhancement in the emitted intensity occurs for the reaction of LUM with Co(II) in the presence of beta-CD. The use of the more soluble gamma-CD permits to expand the range of concentration and obtain more intense emission, although soluble derivatives of the beta-CD (methyl, hydroxypropyl-beta-CD, and a soluble cross-linked epichlorhydrin polymer) do not improve the chemiluminescence (CL) yield. In the case of hemoglobin and diluted human blood, the CDs aid in producing more light but only at high concentration of CDs, with a more lasting luminescence, up to three times longer. The changes in CL when glucose is used instead, much lower than with any of the CDs, imply that the cyclic structure of these oligosaccharides plays a key factor in the boosting of the emission. The results are explained in terms of the binding between the luminescent intermediate of the reaction, 3-aminophthalate (3-AP) and the CD, rather than to the luminescent reactant itself. The association constants obtained by steady-state fluorescence by assuming 1:1 stoichiometries reveal that the most stable association occurs between beta-CD and the intermediate, in accordance with the trend in the chemiluminescence. The topology of the complex deduced via ROESY experiments confirms a shallow inclusion of the double-charged intermediate by the primary rim of the CD, which accounts for the low stability of the complexes.