Radially oriented cellulose triacetate chains on gold nanoparticles

Cellulose triacetate (CTA) derivatives having a disulfide group at the reducing-end (CTA2S, CTA13S, CTA41S), with number average degrees of polymerization (DP_ns) of 2, 13 and 41, respectively, were prepared. The CTA-self-assembled gold nanoparticles (CTA2Au, CTA13Au, and CTA41Au) were obtained through the reduction of gold salt (HAuCl₄) with CTASs. The diameters (d) and the interparticle distances (L) of the gold cores were analyzed by transmission electron microscopy (TEM) observations. The d values of CTA2Au, CTA13Au, and CTA41Au, were 8.7, 7.9, and 13.4 nm respectively. The L values of CTA2Au, CTA13Au, and CTA41Au, were 2.8, 6.3, and 20.9 nm, respectively, and agreed well with the molecular length (l) of CTAS chains (ls of CTA2S, CTA13S, CTA41S = 2.0, 7.5, 21.5 nm, respectively). The hydrodynamic diameters (D) of CTAAu nanoparticles in chloroform solution, measured by dynamic light scattering (DLS), were larger than the d values and increased with the increase in the molecular length of the CTA chains. The CTAS chain was found to work as an excellent stabilizer of the gold nanoparticles in both solid state and solution. The molecular length of CTA chains controlled the size and the alignment of the gold nanoparticles. As a result, the radially oriented CTA chains on the gold nanoparticles were successfully prepared.     

Water-soluble nitric oxide-releasing gold nanoparticles

The synthesis and characterization of water-soluble nitric oxide (NO)-releasing monolayer-protected gold clusters (MPCs) are reported. Tiopronin-protected MPCs ( approximately 3 nm) were functionalized with amine ligands and subsequently exposed to 5 atm of NO to form diazeniumdiolate NO donors covalently bound to the gold MPC. Diazeniumdiolate formation conditions, NO-release, and nanoparticle stability were examined as a function of the structure of the protecting ligand, pH, and storage time. Despite their aqueous solubility, proton-initiated decomposition of the diazeniumdiolate-modified Tio-MPCs resulted in only modest NO-release (<0.023 micromol/mg) for short durations (<1.5 h). To increase the NO storage capacity of gold nanoparticles, polyamine-stabilized MPCs ( approximately 5 nm) were synthesized with significantly enhanced NO-release properties (0.386 micromol/mg) and durations (up to 16 h). Transmission electron microscopy, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, elemental analysis, and UV-vis spectroscopy were used to characterize both nanoparticle systems before and after NO exposure. The MPCs represent the smallest water-soluble NO-release nanoparticles to date (3-5 nm).     

Water-soluble amphiphilic gold nanoparticles with structured ligand shells

Highly water-soluble mixed monolayer protected "rippled" gold nanoparticles were synthesized through a one step reaction with sodium 11-mercaptoundecanesulfonate and octanethiol ligands at various ratios.     

Water-soluble gold nanoparticles protected by fluorinated amphiphilic thiolates

The preparation and the properties of gold nanoparticles (Au NPs) protected by perfluorinated amphiphiles are described. The thiols were devised to form a perfluorinated region close to the gold surface and to have a hydrophilic portion in contact with the bulk solvent to impart solubility in water. The monolayer protected clusters were prepared, in an homogeneous phase using sodium thiolates because of the low nucleophilicity of the alpha-perfluorinated thiols, and fully characterized with (1)H, (19)F NMR spectrometry, IR and UV-vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Au NPs with core diameters ranging from 1.6 to 2.9 nm, depending on the reaction conditions, were obtained. Water-soluble NPs (MPC-F8-PEGs) were obtained with the thiol HS-F8-PEG ending with a short poly(ethylene glycol) unit (PEG-OMe 550), whereas thiols with shorter PEG chains give rise to NPs insoluble in water. MPC-F8-PEGs undergo an exchange reaction with amphiphilic alkyl thiols. ESR investigations, using a hydrophobic radical probe, indicate that the MPC-F8-PEG monolayer shows a greater hydrophobicity compared to the analogous hydrogenated monolayer.     

Water-soluble conjugated polymer-induced self-assembly of gold nanoparticles and its application to SERS

Gold nanochains were prepared by the assembly of citrate-stabilized gold nanospheres induced by cationic conjugated polymers. This assembly method was rapid, and the assembled product was very stable. A longitudinal plasmon resonance band was formed as a result of the coupling of gold nanoparticles and can be tuned from visible to near-infrared by adjusting the polymer/Au molar ratio. The gold nanochains were used as a SERS substrate and gave an enhancement factor of 8.4 x 10 (9), which is approximately 400 times larger than that on the isolated gold nanosphere substrate. The giant SERS enhancement is ascribed to the large electromagnetic fields of coupled gold nanoparticles.     

Homogeneous immunoassay for human IgG using oriented hen egg IgY immobilized on gold sol nanoparticles

Homogeneous immunoassays using (red) gold nanoparticles represent an attractive detection scheme because of the option of photometric readout. We have applied oriented immobilization of hen egg immunoglobulin Y (IgY) on gold nanoparticles when developing a homogeneous immunoassay for human IgG. In oriented immobilization, as opposed to random immobilization, the antigen binding capabilities of the antibodies are retained. It is shown that such immunoassay has significantly better sensitivity in comparison with methods based on conventional immobilization of affinity-purified antibodies. It is also shown that hen egg IgY is better suited than rabbit antibodies, because much more antibody can be immobilized on gold nanoparticles without any destabilization, probably because of the more acidic nature of these antibodies. In addition, hen egg IgY can be supplied in higher quantity and can be prepared more easily than IgG from rabbits. Bleeding and slaughtering of animals is not needed. The assay presented here has a wide detection range (30–500?ng?.mL^?1) and a limit of detection as low as 30?ng.mL^?1 of human IgG.

Grafting of monodisperse low-molecular-weight polystyrene onto cellulose acetate

Well-defined low-molecular-weight polystyrene was grafted onto cellulose acetate in a homogeneous solution. The grafting was performed by esterifying the free hydroxyls in the cellulose acetate (acetyl DS 2.5) with anionically prepared polystyrene having a carboxylic acid group at one end of the chain. The carboxylic acid end group of the polystyrene was activated by either conversion to the corresponding acid chloride, or by reaction with trifluoroacetic anhydride. Pyridine and the more active 4-dimethylaminopyridine were used as catalysts in the esterifications. The polystyrene contents of the copolymers varied between 10 and 80% and the molecular weights of the polystyrene grafts were 2500, 12,100 and 17,100 (M?_w/M?_n = 1.1).     

Development of nanofibrillated cellulose coated with gold nanoparticles for measurement of melamine by SERS

The objective of this study was to develop nanofibrillated cellulose (NFC)-based substrate for rapid detection of melamine in milk by surface-enhanced Raman spectroscopy (SERS). NFC were served as a highly porous platform to load with gold nanoparticles (AuNPs), which can be used as a flexible SERS substrate with nanoscale roughness to generate strong electromagnetic field in SERS measurement. The NFC/AuNP substrate was characterized by UV–Vis spectroscopy and electron microscopy. Milk samples contaminated by different concentrations of melamine were measured by SERS coupled with NFC/AuNP substrate. The spectral data analysis was conducted by multivariate statistical analysis [i.e. partial least squares (PLS)]. Satisfactory PLS result for quantification of melamine in milk was obtained (R = 0.9464). The detection limit for melamine extracted from liquid milk by SERS is 1 ppm, which meets the World Health Organization’s requirement of melamine in liquid milk. These results demonstrate that NFC/AuNP substrate has improved homogeneity and can be used in SERS analysis for food safety applications.     

Photoswitchable rotaxanes on gold nanoparticles

We studied rotaxanes that consisted of a molecular axle, with a photoactive 9-Aryl-9-methoxy-acridane moiety at one end, and a tetracationic ring of cyclobis(paraquat-p-phenylene) (CBQT(4+)). The aim of the study was to deposit the axle ends onto gold nanoparticles (AuNPs). First, we introduced thioctic acid into the axle molecules. Then, rotaxanes were deposited on AuNPs by two methods: 1) Pseudorotaxanes were deposited on the gold surface by forming rotaxanes with the AuNP as a terminator to prevent unthreading of the ring structure; and 2) a chain containing the thioctic ester was introduced into a complete rotaxane, and then it was deposited on the AuNP with the aid of an exchange process. The photoheterolysis of the acridane unit resulted in formation of the corresponding acridinium methoxide; this, in turn, could thermally react to return to the acridane moiety. Due to the creation of a positive charge, the ring moved from the acridane station to a second, evasive station within the axle. This switching cycle could also take place when deposited on the gold surface. However, on the gold surface, the ring movement associated with the switching process was unidirectional.     

Formation of gold nanoparticles in aqueous solutions of cellulose derivatives and a study of the properties of these nanoparticles

It was demonstrated that gold nanoparticles can be obtained by using cellulose ethers, methyl hydroxyethyl cellulose and carboxymethylcellulose as reducing agents and also as nanoparticle stabilizers. IR spectral studies revealed a difference between the mechanisms of reduction and nanoparticle stabilization by these cellulose derivatives. A scanning tunnel microscope was used to examine composite films formed from nanoparticle dispersions on the surface of polycrystalline gold films. It was demonstrated that, in the case of gold nanoparticles, densely packed globular structures are formed in a carboxymethyl cellulose solution. A fibril-like structure of layers is formed in the Au+(methyl hydroxyethyl cellulose) system.     

Thermosensitive gold nanoparticles

Thermosensitive gold nanoparticles were fabricated by conjugating Au with a thiol-terminated poly(N-isopropylacrylamide) or PPA; this polymer stabilizer exhibits a temperature transition while undergoing a hydrophilic to hydrophobic transformation. The introduction of PPA onto gold nanoparticles has sensitized Au nanoparticles with unique temperature dependence. At low temperature (25 degrees C), the solutions containing PPA-functionalized gold nanoparticles are transparent, whereas higher temperatures (30 degrees C) lead to opaque suspensions. The thermosensitive property of PPA-functionalized Au nanoparticles is reversible, and the clear-opaque suspensions can be repeated many times.     

Alkanetelluroxide-protected gold nanoparticles

The synthesis and characterization of the first air-stable tellurium-containing ligand-protected gold nanoparticles (NPs) are reported. Although the synthesis largely followed the well-known Brust two-phase approach, the starting ligand was dioctyl ditelluride rather than alkanetellurol, which is an analogue of the widely used alkanethiol. Dioctyl ditelluride was used because alkanetellurol is unstable. The 1H and 13C NMR spectra, as well as infrared spectra (IR) of the formed Au NPs, indicated that the Te-Te bond in the starting ligand was broken but the octyl group was intact. This was further corroborated by the solid-state 125Te NMR spectrum that displayed a very broad and significantly downfield-shifted peak, indicating that tellurium was directly bound to the Au core. Furthermore, the O 1s and Te 3d XPS spectra of the Au NPs indicated that the capping ligands were octanetelluroxide. An average particle size of 2.7 nm diameter as measured by transmission electron microscopy (TEM) corresponded to an Au607 core. A two-step weight loss of approximately 22.2% in total was observed in the thermogravimetric analysis, which indicated about 53% ligand monolayer coverage (i.e., Au607(Te(=O)C8H17)133). Additionally, dioctyl ditelluride demonstrated an intriguing reductive power that led to a more sophisticated chemistry of forming the air-stable octanetelluroxide-protected gold NPs. It has been found that (1) when the ratio of Au to Te was about 1.5 a colorless intermediate state similar to Au(I)-SR (the intermediate state widely accepted in the synthesis of thiolate-protected Au NPs) could be obtained and (2) this kind of intermediate state played a key role in the formation of stable Au NPs.     

Ciprofloxacin-protected gold nanoparticles

The antibacterial drug ciprofloxacin (cfH) has been used to protect gold nanoparticles of two different mean diameters, 4 and 20 nm. The protection is complete with about 65 and 585 cfH molecules covering 4 and 15 nm particles, respectively. The nature of binding has been investigated by several analytical techniques. The nitrogen atom of the NH moiety of piperazine group binds on the gold surface, as revealed by voltammetric and spectroscopic studies. The cfH-adsorbed particles are stable in the dry state as well as at room temperature, and as a result, redispersion is possible. The rate of release of the drug molecule from the nanoparticles is more in the basic medium than in pure water, and the kinetics depend on the size of the particle; faster desorption is seen in smaller particles. The bound cfH is fluorescent, and this property could be used in biological investigations. This study shows that metal nanoparticles could be useful carriers for cfH and fluoroquinolone molecules. Most of the bound molecules could be released over an extended period of time.     

Paclitaxel-functionalized gold nanoparticles

Here we describe the first example of 2 nm gold nanoparticles (Au NPs) covalently functionalized with a chemotherapeutic drug, paclitaxel. The synthetic strategy involves the attachment of a flexible hexaethylene glycol linker at the C-7 position of paclitaxel followed by coupling of the resulting linear analogue to phenol-terminated gold nanocrystals. The reaction proceeds under mild esterification conditions and yields the product with a high molecular weight, while exhibiting an extremely low polydispersity index (1.02, relative to linear polystyrene standards). TGA analysis of the hybrid nanoparticles reveals the content of the covalently attached organic shell as nearly 67% by weight, which corresponds to approximately 70 molecules of paclitaxel per 1 nanoparticle. The presence of a paclitaxel shell with a high grafting density renders the product soluble in organic solvents and allows for detailed (1)H NMR analysis and, therefore, definitive confirmation of its chemical structure. High-resolution TEM was employed for direct visualization of the inorganic core of hybrid nanoparticles, which were found to retain their average size, shape, and high crystallinity after multiple synthetic steps and purifications. The interparticle distance substantially increases after the attachment of paclitaxel as revealed by low-magnification TEM, suggesting the presence of a larger organic shell. The method described here demonstrates that organic molecules with exceedingly complex structures can be covalently attached to gold nanocrystals in a controlled manner and fully characterized by traditional analytical techniques. In addition, this approach gives a rare opportunity to prepare hybrid particles with a well-defined amount of drug and offers a new alternative for the design of nanosized drug-delivery systems.     

Water-soluble surface-anchored gold and palladium nanoparticles stabilized by exchange of low molecular weight ligands with biamphiphilic triblock copolymers

A study is presented of the stabilization of gold and palladium nanoparticles (NPs) via a place-exchange reaction. Au and Pd NPs of approximately 3.5 nm were prepared by a conventional method using tetraoctylammonium bromide (TOAB) as the stabilizing agent. The resulting nanoparticles, referred to as Au-TOAB or Pd-TOAB, were later used as templates for the replacement of TOAB ligand with poly(ethylene oxide)- b-polystyrene- b-poly(4-vinylpyridine) (PEO- b-PS- b-P4VP) triblock copolymer. This biamphiphilic triblock copolymer was synthesized by atom transfer radical polymerization (ATRP) with control over the molecular weight and polydispersity. The place-exchange reaction was mediated through strong coordination forces between the 4-vinylpyridine copolymer and the metal species located on the surface of the nanoparticles. In addition, the displacement of the outgoing low molecular weight TOAB ligands by high molecular weight polymers is an entropy-assisted process and is believed to contribute to stabilization. The prepared complex, polymer-NP, exhibits greatly improved stability over the metal-NP complex in common organic solvents for the triblock copolymer. Self-assembly in water after ligand exchange resulted in micellar structures of about approximately 20 nm (electron microscopy) with the metal NP found located on the surface of the micelles. The stability of the nanoparticles in water was shown to depend greatly on the grafting density of the copolymer, with high stability (more than 6 months) at high grafting density and low stability, accompanied with irreversible agglomeration, at relatively low grafting densities. The surprising location of the metal NP (for both Au and Pd) on the surface of the micelles in water is explained by the fact that, upon self-assembly in THF/water system, the most hydrophobic chains (i.e., PS) undergo self-assembly first at low water content forming the core, followed by the P4VP (whether or not associated with the metal) forming a shell, and finally the PEO forming the corona. In lower metal content assemblies, the P4VP chains located in the shell undergo swelling in an acidic medium causing a substantial increase in micellar corona size, as confirmed by dynamic light scattering measurements. The present study offers a simple approach for the stabilization of various metal nanoparticles of catalytic interest, using a unique polymeric support that can be dispersed in organic solvents as well as aqueous solutions.     

Facile fabrication of noble metal nanoparticles encapsulated in hollow silica with radially oriented mesopores: multiple roles of the N-lauroylsarcosine sodium surfactant

A facile one-pot method was reported to fabricate noble metal nanoparticles encapsulated in silica hollow nanospheres with radially oriented mesopores, and the anionic amino acid surfactant, N-lauroylsarcosine sodium, played multiple roles: reducing agent, stabilizer, emulsion droplets and mesopore template.     

Adsorption of cellulose derivatives on flat gold surfaces and on spherical gold particles

The adsorption of hydroxyethylcellulose (HEC), ethyl(hydroxyethyl)cellulose (EHEC), and their hydrophobically modified counterparts HM-HEC and HM-EHEC has been studied on planar gold and citrate-covered gold surfaces by means of quartz crystal microbalance with dissipation monitoring (QCM-D), and on citrate-covered gold particles with the aid of dynamic light scattering (DLS). The QCM-D results indicate that larger amounts of polymer are adsorbed from aqueous solutions of HM-HEC and HM-EHEC on both substrates than from solutions of their unmodified analogues. The adsorption affinity for all the polymers, except EHEC, is higher on the citrate-covered surfaces than on the bare gold substrate. This indicates that more adsorption sites are activated in the presence of the citrate layer. The experimental adsorption data for all the polymers can be described fairly well by the Langmuir adsorption isotherm. However, at very low polymer concentrations significant deviations from the model are observed. The value of the hydrodynamic thickness of the adsorbed polymer layer (delta h), determined from DLS, rises with increasing polymer concentration for all the cellulose derivatives; a Langmuir type of isotherm can be used to roughly describe the adsorption behavior. Because of good solvent conditions for HEC the chains extend far out in the bulk at higher concentrations and the value of delta h is much higher than that of HM-HEC. The adsorption of EHEC and HM-EHEC onto gold particles discloses that the values of delta h are considerably higher for the hydrophobically modified cellulose derivative, and this finding is compatible with the trend in layer thickness estimated from the QCM-D measurements.     

Chemical transformations in the chains of cellulose dialdehydes and cellulose ethers

Nucleophilic exchange reactions of cellulose dialdehydes and ethers with a series of nucleophilic reagents have been investigated and the structural features of the reaction products have been established.Institute of the Chemistry and Physics of Polymers, Academy of Sciences of the Republic of Uzbekistan, fax (3712) 44 26 61. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 212–217, March–April, 1998.