Dissolution-recrystallization mechanism for the conversion of silver nanospheres to triangular nanoplates

A solution chemistry method for transforming polycrystalline Ag spherical particles into single crystalline triangular Ag nanoplates has been developed. The synthesis consists of three consecutive steps: (1) the synthesis of Ag nanospheres by NaBH(4) reduction of AgNO(3) in the presence of sodium citrate; (2) the conversion of citrate-stabilized Ag nanospheres into SDS (sodium dodecyl sulfate)-stabilized Ag nanospheres, and (3) the aging of the SDS-stabilized Ag nanospheres in 0.01 M NaCl solution. Our study indicates that the shape evolved through a Ag nanoparticle dissolution- and re-deposition process; and demonstrated the critical role of SDS in the process: SDS regulates the dynamics in the dissolved O(2)/Cl(-) etching of the Ag nanospheres and the reduction of the released Ag(+) by citrate ions in the same solution. SDS also functions as a shape-directing agent to assimilate the Ag(0) atoms into single crystalline triangular Ag nanoplates. A model for the shape conversion is also proposed which provides the clue for the synthesis of anisotropic Ag nanoparticles with other shapes (rods, wires, cubes, etc.).     

Shape-controlled synthesis of highly monodisperse and small size gold nanoparticles

We describe here that fine control of nanoparticle shape and size can be achieved by systematic varia-tion of experimental parameters in the seeded growth procedure in aqueous solution. Cubic and spherical gold nanoparticles are obtained respectively. In particularly, the Au cubes are highly mono-disperse in 33±2 nm diameter. The experimental methods involve the preparation of Au seed particles and the subsequent addition of an appropriate quantity of Au seed solution to the aqueous growth solutions containing desired quantities of CTAB and ascorbic acid (AA). Here, AA is a weak reducing agent and CTAB is not only a stable agent for nanoparticles but also an inductive agent for leading increase in the face of nanoparticle. Ultraviolet visible spectroscopy (UV-vis), X-ray diffraction (XRD), transmission electron microscopy (TEM) are used to characterize the nanoparticles. The results show that the different size gold nanoparticles displayed high size homogenous distribution and formed mono-membrane at the air/solid interface.     

Phase transfer of large gold nanoparticles to organic solvents with increased stability

In this paper, we describe a new procedure to phase transfer large gold nanoparticles (diameters > 45 nm) from aqueous solution to organic solvents. This is accomplished using a covalent amide coupling reaction that incorporates dicyclohexylamine (DCHA) headgroups on the surface of mercaptoacetic acid (MAA) functionalized gold nanoparticles. Gold nanoparticles are first synthesized in aqueous solution by the citrate-reduction method, and nanoparticle size is controlled by the molar ratio of the reducing agent (sodium citrate) and the gold precursor (KAuCl4). MAA is then adsorbed to the surface of the gold nanoparticles followed by an amide-coupling reaction to covalently attach DCHA to the surface-immobilized MAA. The bulky dicyclohexyl groups entropically stabilize gold nanoparticles in organic solvents. This procedure was used to reliably transfer gold nanoparticles with diameters between 45 and 100 nm from aqueous solution to organic solvents such as dimethyl sulfoxide and chloroform.     

Adsorption of organic acids on TiO2 nanoparticles: effects of pH, nanoparticle size, and nanoparticle aggregation

In this study, the adsorption of two organic acids, oxalic acid and adipic acid, on TiO2 nanoparticles was investigated at room temperature, 298 K. Solution-phase measurements were used to quantify the extent and reversibility of oxalic acid and adipic acid adsorption on anatase nanoparticles with primary particle sizes of 5 and 32 nm. At all pH values considered, there were minimal differences in measured Langmuir adsorption constants, K ads, or surface-area-normalized maximum adsorbate-surface coverages, Gamma max, between 5 and 32 nm particles. Although macroscopic differences in the reactivity of these organic acids as a function of nanoparticle size were not observed, ATR-FTIR spectroscopy showed some distinct differences in the absorption bands present for oxalic acid adsorbed on 5 nm particles compared to 32 nm particles, suggesting different adsorption sites or a different distribution of adsorption sites for oxalic acid on the 5 nm particles. These results illustrate that molecular-level differences in nanoparticle reactivity can still exist even when macroscopic differences are not observed from solution phase measurements. Our results also allowed the impact of nanoparticle aggregation on acid uptake to be assessed. It is clear that particle aggregation occurs at all pH values and that organic acids can destabilize nanoparticle suspensions. Furthermore, 5 nm particles can form larger aggregates compared to 32 nm particles under the same conditions of pH and solid concentrations. The relative reactivity of 5 and 32 nm particles as determined from Langmuir adsorption parameters did not appear to vary greatly despite differences that occur in nanoparticle aggregation for these two different size nanoparticles. Although this potentially suggests that aggregation does not impact organic acid uptake on anatase particles, these data clearly show that challenges remain in assessing the available surface area for adsorption in nanoparticle aqueous suspensions because of aggregation.     

Amphiphilic polyethylenes leading to surfactant-free thermoresponsive nanoparticles

Linear copolymers of ethylene and acrylic acid (PEAA) were prepared by catalytic polymerization of ethylene and tert-butyl acrylate followed by hydrolysis of the ester groups. The copolymers contained COOH groups inserted into the crystalline unit cell with formation of intramolecular hydrogen-bonds, as established on the basis of differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) studies. A solvent-exchange protocol, with no added surfactant, converted a solution in tetrahydrofuran of a PEAA sample containing 12 mol % of acrylic acid (AA) into a colloidally stable aqueous suspension of nanoparticles. Transmission electron microscopy (TEM), dynamic light scattering (DLS), and high sensitivity differential scanning calorimetry (HS-DSC) were used to characterize the nanoparticles. They are single crystals of elongated shape with a polar radius of 49 nm (σ = 15 nm) and an equatorial radius of 9 nm (σ = 3 nm) stabilized in aqueous media via carboxylate groups located preferentially on the particle/water interface. The PEAA (AA: 12 mol %) nanoparticles dispersed in aqueous media exhibited a remarkable reversible thermoresponsive behavior upon heating/cooling from 25 to 80 °C.     

纳米氧化锌的机械力化学表面改性

采用机械力化学表面改性工艺,以硬脂酸为改性剂,在气流粉碎机中对纳米氧化锌进行解团聚和表面改性,并借助SEM、XRD、FF—IR和XPS对改性前后的氧化锌粉体进行结构表征．结果表明：硬脂酸分子化学键合在氧化锌表面．改性前后氧化锌的晶体结构相同,其颗粒的团聚性降低,二次粒径明显减小．通过测定改性样品的活化指数和亲油化度,选择最佳的改性剂用量为氧化锌质量的10％,氧化锌表面亲油疏水,在有机溶剂中有较好的分散性．     

Optical and electrical behavior of organic/inorganic hybrid with embedded gold nanoparticles

Gold nanoparticles (AuNPs) with different diameters (from 4 up to 10 nm) were immobilized within a amine-alcohol-silicate matrix [AA(600)] by mixing a preformed Au nanoparticle colloidal solution with the precursors of amine-alcohol-silicate, prior to the sol–gel transition. The organic–inorganic hybrid (OIH) nanocomposites were synthetized by sol–gel method by reaction of amino-functionalized polyether and a siloxane functionalized with a terminal epoxy group. The obtained homogeneous, high transparent and stable materials exhibit enhanced optical and electrical properties derived from plasmonic effects associated with the size and form of the nanoparticle dopants which show to be preserved during the synthesis steps. Performed electrochemical impedance spectroscopy revealed that OIH gels doped with AuNPs exhibit low conductivity that shows to be slightly dependent on dispersed Au particle sizes. The characterization of this materials by current–voltage (I–V) measurements shows that these materials exhibit an electrical stability within an range of applied potential of about 5 V and suggests that charge transfer mechanism is strongly dependent on the potential applied across the OIH gel as observed by testing different charge transfer models: space-charge-limited current, Poole–Frenkel, Schottky emission and hopping conduction or the Schottky-Simmons. The results obtained from the characterization the electrochemical properties shows that the produced material to be relevant for the potential application of OIH embedded AuNPs nanocomposites in non-volatile organic memory devices.     

A robust hollow metal–organic framework with enhanced diffusion for size selective catalysis

Single crystalline (SC) hollow metal–organic frameworks (MOFs) are excellent host materials for molecular and nanoparticle catalysts. However, due to synthetic challenges, chemically robust SC hollow MOFs are rare. This work reports the construction of a defect-free and chemically stable SC hollow MOF, MOF-801(h), through templated growth from a unit cell mismatched core, UiO-66. Under the protection of excess MOF-801 ligand, fumaric acid, the MOF-801 shell was perfectly retained while the isoreticular UiO-66 core was selectively and completely etched away by formic acid. The combination of a large cavity, small aperture and short diffusion length allows the Pt nanoparticle encapsulated composite catalyst, Pt⊂MOF-801(h), to perform size selective hydrogenation of nitro compounds at an accelerated speed. Impressively, the catalyst can undergo concentrated HCl or boiling water treatment while maintaining its crystallinity, morphology, catalytic activity, and size selectivity. In addition, Au nanoparticles encapsulated catalyst, Au⊂MOF-801(h), was used for the size selective nucleophilic addition of HCl to terminal alkynes for the first time, which is a harsh reaction involving high concentrations of a strong acid.

A chemically robust single crystalline hollow MOF-801 containing Pt and Au nanoparticles was constructed by using UiO-66 as a sacrificial template for size selective catalysis with fast reaction kinetics under harsh chemical conditions.     

One-step synthesis and functionalization of hydroxyl-decorated magnetite nanoparticles

Magnetite nanoparticles covered by a layer of omega-hydroxycarboxylic acid were synthesized in one step by high-temperature decomposition of iron(III) omega-hydroxycarboxylates in tri- and tetra-ethylene glycol. The nanoparticles were characterized by TEM, XRD, IR, XPS and NMR techniques in order to show that they comprise a crystalline magnetite core and actually bear on the outer surface terminal hydroxy groups. The latter ones are convenient "handles" for further functionalization as opposed to the chemically-inert aliphatic chains which cover conventionally synthesized nanoparticles. This was shown by several examples in which the hydroxy groups on the nanoparticle surface were easily transformed in other functional groups or reacted with other molecules. For instance, the hydroxyl-decorated nanoparticles were made water soluble by esterification with a PEGylated acetic acid. The reactive behavior of the surfactant monolayer was monitored by degrading the nanoparticles with aqueous acid and isolating the surfactant for NMR characterization. In general, the reactivity of the terminal hydroxyl groups on the nanoparticle surface parallels that observed in the free surfactants. The reported hydroxyl-decorated magnetite nanoparticles can be thus considered as pro-functional nanoparticles, i.e., a convenient starting material to functionalized magnetic nanoparticles.     

A new kind of polyion complex nanoparticles and the covalent drug-loading pattern for doxorubicin and pH-controlled release

The one-pot synthesis method was developed for the preparation of complex nanoparticles with a narrow size distribution and stable morphology. The vinyl monomers of (2-dimethylamino)ethyl methacrylate (DEMA) and diacetone acrylamide (DAA) were copolymerized in the presence of alginic acid in an aqueous solution without any organic solvents or surfactants, yielding stable complex nanoparticles in one-pot synthesis. The nanoparticle was composed of the complex of poly(DEMA-co-DAA) and alginic acid. The complex was formed via electrostatic interaction between polycations of DEMA and polyanions of alginate. The residual alginate segment around the core formed the shell of the nanoparticles. The average diameter of the nanoparticles varied from 120 to 213 nm when the molar percentage of DAA changed from 0.5 to 0 with respect to DEMA. The anti-cancer drug doxorubicin could be loaded onto the nanoparticles with a high-loading efficiency through the formation of polymer–drug conjugate. The drug release could be controlled by adjusting the pH value of the medium.     

Water-soluble iron oxide nanoparticles with high stability and selective surface functionality

The water dispensability and stability of high quality iron oxide nanoparticles synthesized in organic solvents are major issues for biomedical and biological applications. In this paper, a versatile approach for preparing water-soluble iron oxide nanoparticles with great stability and selective surface functionality (-COOH, -NH(2), or -SH) was demonstrated. The hydrophobic nanoparticles were first synthesized by the thermal decomposition of an iron oleate complex in organic solvent. Subsequently, the hydrophobic coatings of nanoparticles were replaced with poly(acrylic acid) , polyethylenimine, or glutathione, yielding charged nanoparticles in aqueous solution. Two parameters were found to be critical for obtaining highly stable nanoparticle dispersions: the original coating and the surfactant-to-nanoparticle ratio. These charged nanoparticles exhibited different stabilities in biological buffers, which were directly influenced by the surface coatings. This report will provide significant practical value in exploring the biological or biomedical applications of iron oxide nanoparticles.     

Growth and characterization of highly branched nanostructures of magnetic nanoparticles

Magnetite nanoparticles of Fe(3)O(4) have been found to grow into large highly branched nanostructures including nanochains and highly branched nanotrees in the solid state through a postannealing process. By varying the preparation conditions such as annealing time and temperature, the nanostructures could be easily manipulated. Changing the starting concentration of the magnetic nanoparticle solution also caused significant changes of the nanoarchitectures. When the magnetic nanoparticle concentration is low, the nanoparticles formed straight rods mainly with an average diameter of 80 nm and a length of several microns. With increasing concentration of the nanoparticles, treelike structures began to form. With further increase of the concentration, well-ordered nanostructures with the appearance of snowflakes were generated. The driving force for the formation of the highly ordered nanostructures includes interaction between the nanoparticles and interaction through surface-capping molecules. This experiment demonstrates that novel nanostructures can be generated by self-assembly of magnetic nanoparticles under the solid state.     

Functionalization of monodisperse magnetic nanoparticles

We report a new strategy for the preparation of monodisperse, water-soluble magnetic nanoparticles. Oleic acid-stabilized magnetic nanocrystals were prepared by the organic synthesis route proposed by Sun et al. (J. Am. Chem. Soc. 2004, 126, 273.), with size control obtained via seeded-mediated growth. The oleic groups initially present on the nanoparticle surfaces were replaced via ligand exchange reactions with various capping agents bearing reactive hydroxyl moieties. These hydroxyl groups were (i) exploited to initiate ring opening polymerization (ROP) of polylactic acid from the nanoparticle surfaces and (ii) esterified by acylation to permit the addition of alkyl halide moieties to transform the nanoparticle surfaces into macroinitiators for atom transfer radical polymerization (ATRP). By appropriate selection of the ligand properties, the nanoparticle surfaces can be polymerized in various solvents, providing an opportunity for the growth of a wide variety of water-soluble polymers and polylectrolyte brushes (both cationic and anionic) from the nanoparticle surfaces. The nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), electron microscopy, and light scattering. Light scattering measurements indicate that the nanoparticles are mostly present as individual nonclustered units in water. With pH-responsive polymers grown on the nanoparticle surfaces, reversible aggregation of nanoparticles could be induced by suitable swings in the pH between the stable and unstable regions.     

Influence of intense pulsed laser irradiation on optical and morphological properties of gold nanoparticle aggregates produced by surface acid-base reactions

Gold nanoparticles were surface modified with an ionizable and pH-sensitive monolayer of thiobarbituric acid (TBA). By variation of the pH value of the solution, nanoparticle aggregates can be produced in a controlled way. The aggregates thus prepared were irradiated with an intense pulsed laser at 532 nm. The products in solution were examined by transmission electron microscopy (TEM) and optical absorption spectroscopy. The TEM images of the products revealed that the nanoparticle aggregates dissociate upon laser irradiation and form much smaller gold nanoparticles. The optical absorption spectra measured simultaneously show the gradual disappearance of the absorbance band of the aggregates at around 680 nm. Additionally, a blue shift (from 534 to 524 nm) of the resonance absorbance corresponding to isolated nanoparticles has been observed. All the observations suggest that the colloidal solution becomes more stable after laser irradiation. Both the reduced nanoparticle size and the stabilizing TBA ligands present on the particle surface contribute to the acquired stability of the colloidal solutions.     

Formation of gold@polymer core-shell particles and gold particle clusters on a template of thermoresponsive and pH-responsive coordination triblock copolymer

Template synthesis of various morphological gold colloidal nanoparticles using a thermoresponsive and pH-responsive coordination triblock copolymer of poly(ethylene glycol)-b-poly(4-vinylpyridine)-b-poly(N-isopropylacrylamide) is studied. The template morphology of the thermoresponsive and pH-responsive coordination triblock copolymer, which can be tuned by simply changing the pH or temperature of the triblock copolymer aqueous solution, ranges from single chains to core-corona micelles and further to micellar clusters. Various morphological gold colloidal nanoparticles such as discrete gold nanoparticles, gold@polymer core-shell nanoparticles, and gold nanoparticle clusters are synthesized on the corresponding template of the triblock copolymer by first coordination with gold ions and then reduction by NaBH4. All three resultant gold colloidal nanoparticles are stable in aqueous solution, and their sizes are 2, 10, and 7 nm, respectively. The gold@polymer core-shell nanoparticles are thermoresponsive. The gold nanoparticle cluster has a novel structure, and each one holds about 40 single gold nanoparticles.     

In situ synthesis of gold and silver nanoparticles by using redox-active amphiphiles and their phase transfer to organic solvents

An in situ reduction approach to synthesizing gold and silver nanoparticles by using a series of newly designed, redox-active amphiphiles at basic pH is described. These amphiphiles are the conjugates of a fatty acid (e.g., oleic acid, stearic acid, and lauric acid) and a redox-active amino acid (e.g., tryptophan or tyrosine). The amphiphile-coated nanoparticles are then efficiently transferred from water to different nonpolar organic media (such as benzene, toluene, xylene, cyclohexane, and hexane) simply by acid treatment. The phase-transfer process was monitored by UV/visible spectroscopy and transmission electron microscopy, and the results showed that the average particle size and size distribution remain almost unchanged after transferring to the organic media. The anchoring of the amphiphile to the nanoparticle surface was confirmed by FTIR spectroscopy and thermogravimetric analysis. A mechanism is proposed to describe the stability of colloidal Au and Ag nanoparticles formed in situ and their phase transfer to organic solvents. The presence of the amphiphile increases the thermal stability of the colloidal gold nanoparticle conjugates in organic solvents.     

FTIR study of the mode of binding of the reactants on the Pd nanoparticle surface during the catalysis of the Suzuki reaction

In the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by palladium nanoparticles, our previous studies suggested that the phenylboronic acid adsorbs on the nanoparticle surface and then interacts with the iodobenzene that is present in solution. In the present study, FTIR is used to examine the change in the vibrational frequencies of phenylboronic acid in films with and without the addition of palladium nanoparticles. The large change in the B-O stretching frequency of phenylboronic acid from 1348 to 1376 cm(-1) in the presence of sodium acetate and palladium nanoparticles strongly suggests that the mode of binding of phenylboronic acid to the Pd nanoparticle surface involves a B-O-Pd type of bonding. Shifts in the B-C stretching mode and the out-of-plane phenyl C-C ring deformation bands associated with phenylboronic acid provide additional confirmations of the binding process. It is also shown that the phenylboronic acid needs to be in the deprotonated form in the presence of sodium acetate (phenylboronate anion) to bind to the palladium nanoparticle surface. No changes in the characteristic bands of iodobenzene were observed in films made in the presence of the palladium nanoparticles. The FTIR studies provide proof of the mode of binding that occurs in the nanoparticle surface for the first time and also confirms the mechanism of the Suzuki reaction that we proposed previously.     

Integrities and Skin Permeation-Enhancing Characteristics of Behenic Acid Nanoparticles

Fatty acid nanoparticles containing hinokitiol (HKL) were prepared by a melt-emulsification method. Behenic acid (BA) was used as a lipid for the matrix material of the nanoparticle. The size distribution was quite mono-dispersive and the mean diameter was around 200 nm. On a differential scanning calorimeter, no endothermic peak of HKL was observed when it was encapsulated in the nanoparticles. This indicates that HKL in the lipid matrix of the nanoparticles is not in a crystalline state, but rather in a dissolved state. The nanoparticles were stable over the range of pH 2–10 in terms of size. The absolute values of the zeta potentials in the pH range were high enough to prevent a particle-to-particle aggregation. Compared with stearic acid (SA) nanoparticles, which were completely disintegrated in sodium lauryl sulfate (SLS) solution of 4%, the BA nanoparticles were robust and remained intact even in SLS solutions of 14%. Therefore, BA nanoparticles would be applicable to cleansing products containing detergents for personal care. When encapsulated either in SA nanoparticles or in BA nanoparticles, the in vitro flux of HKL through hairless mouse skin was 10–15 times higher than dissolved in propylene glycol. BA nanoparticles, however, enhanced the transdermal delivery of HKL less than SA nanoparticles did.     

Simple reductant concentration-dependent shape control of polyhedral gold nanoparticles and their plasmonic properties

We report a facile seed-mediated method for the synthesis of monodisperse polyhedral gold nanoparticles, with systematic shape evolution from octahedral to trisoctahedral structures. The control over the particle growth process was achieved simply by changing the concentration of the reductant in the growth solution, in the presence of small spherical seed nanoparticles. By progressively increasing the concentration of the reductant used in the growth solution (ascorbic acid), while keeping the amount and type of added surfactant constant, the morphology of the gold nanoparticles was varied from octahedral to truncated octahedral, cuboctahedral, truncated cubic, cubic, and finally trisoctahedral structures. These nanoparticles were monodisperse in size, possessed similar volumes, and were naturally oriented so that their larger crystal planes were face down on quartz substrates when deposited from the solution. By adjusting the volume of gold seed nanoparticle solution added to a growth solution, the size of the simplest gold nanoparticles (with a highly symmetric cubic morphology) could be tuned from 50 ± 2.1 to 112 ± 11 nm. When other seed nanoparticles were used, the size of the cubic Au nanoparticles reached 169 ± 7.0 nm. The nanoparticle growth mechanism and the plasmonic properties of the resulting polyhedral nanoparticles are discussed in this paper.     

Monodisperse cobalt ferrite nanomagnets with uniform silica coatings

Ferro- and ferrimagnetic nanoparticles are difficult to manipulate in solution as a consequence of the formation of magnetically induced nanoparticle aggregates, which hamper the utility of these particles for applications ranging from data storage to bionanotechnology. Nonmagnetic shells that encapsulate these magnetic particles can reduce the interparticle magnetic interactions and improve the dispersibility of the nanoparticles in solution. A route to create uniform silica shells around individual cobalt ferrite nanoparticles--which uses poly(acrylic acid) to bind to the nanoparticle surface and inhibit nanoparticle aggregation prior to the addition of a silica precursor--was developed. In the absence of the poly(acrylic acid) the cobalt ferrite nanoparticles irreversibly aggregated during the silica shell formation. The thickness of the silica shell around the core-shell nanoparticles could be controlled in order to tune the interparticle magnetic coupling as well as inhibit magnetically induced nanoparticle aggregation. These ferrimagnetic core-silica shell structures form stable dispersion in polar solvents such as EtOH and water, which is critical for enabling technologies that require the assembly or derivatization of ferrimagnetic particles in solution.