Au nanoparticle-enhanced surface plasmon resonance sensing of biocatalytic transformations

N-(3-Aminopropyl)-N'-methyl-4,4'-bipyridinium is coupled to tiopronin-capped Au nanoparticles (diameter ca. 2 nm) to yield methyl(aminopropyl)viologen-functionalized Au nanoparticles (MPAV(2+)-Au nanoparticles). In situ electrochemical surface plasmon resonance (SPR) measurements are used to follow the electrochemical deposition of the bipyridinium radical cation modified Au nanoparticles on an Au-coated glass surface and the reoxidation and dissolution of the bipyridinium radical cation film. The MPAV(2+)-functionalized Au nanoparticles are also employed for the amplified SPR detection of NAD(+) and NADH cofactors. By SPR monitoring the partial biocatalyzed dissolution of the bipyridinium radical cation film in the presence of diaphorase (DP) NAD(+) is detected in the concentration range of 1x10(-4) M to 2x10(-3) M. Similarly, the diaphorase-mediated formation of the bipyridinium radical cation film on the Au-coated glass surface by the reduction of the MPAV(2+)-functionalized Au nanoparticles by NADH is used for the amplified SPR detection of NADH in the concentration range of 1x10(-4) M to 1x10(-3) M.     

Electrochemical determination of nitrite using a gold nanoparticles-modified glassy carbon electrode prepared by the seed-mediated growth technique.

Seed-mediated growth of gold nanoparticles on glassy carbon (GC) surfaces was developed. The field emission scanning electron microscopy (FE-SEM) and electrochemical characterization confirmed the effective attachment of gold nanoparticles on GC surface with such a wet-chemical method. The as-prepared gold nanoparticles attached glassy carbon electrode (Au/GCE) presented excellent catalytic ability toward the oxidation of nitrite. Compared with bare GCE and planar gold electrode, the Au/GCE obviously decreased the overpotential of nitrite oxidation and improved the peak current. The catalytic current was found to be linearly proportional to the nitrite concentration in the range of 1 x 10(-5) - 5 x 10(-3) M, with a detection limit of 2.4 x 10(-6) M. The Au/GCE was successfully applied to the electrochemical determination of nitrite in a real wastewater sample, showing excellent stability and anti-interference ability.     

Gold nanoparticle formation from photochemical reduction of Au3+ by continuous excitation in colloidal solutions. A proposed molecular mechanism

A photochemical reduction of Au3+ with continuous 250-400 nm excitation is studied in ethylene glycol, and poly(vinylpyrrolidone) (PVP) is used as a capping material. After the absorption of Au3+ disappears, excitation is stopped. The surface plasmon absorption of gold as well as the thermal reappearance of the Au3+ absorption are found to increase as a function of time. The rates of these changes are studied as a function of the mole fraction of ethylene glycol in water. Experimental results show that a small amount of ethylene glycol increases the formation of gold nanoparticles and decreases the reformation of the Au3+ absorption after irradiation. Increasing the glycol concentration first increases the rate of formation of gold nanoparticles to a maximum at a mole fraction 0.40. As the glycol concentration is further increased, the rate of formation of the gold nanoparticles and the rate of re-formation of Au3+ decrease. A mechanism is proposed that involves the reduction of the excited Au3+ to Au2+ by ethylene glycol. This is followed by the disproportionation of Au2+ to Au3+ and Au1+. Both the reduction of Au1+ by ethylene glycol and its disproportionation lead to the formation of Au0, which upon nucleation and growth form Au nanoparticles.     

Selective dissolution of the silver component in colloidal Au and Ag multilayers: a facile way to prepare nanoporous gold film materials

Colloidal Au/Ag multilayer films were prepared by alternate assembly of Au nanoparticles with a size of 5 +/- 1.2 nm and Ag nanoparticles with a size of 10 +/- 2.4 nm by using 1,5-pentanedithiol as cross-linker. Nanoporous gold films with a ligament size of 26.7 +/- 4.6 nm were then prepared by selective dissolution of sacrificial templates of silver particles in colloidal Au/Ag multilayers. The complete dissolution of Ag particles in colloidal Au/Ag multilayers in a mixture solution of 3.0 mM HAuCl(4) and 3 M NaCl took place at room temperature without damage of the colloidal Au film. This method to prepare nanoporous gold films was further extended to the preparation of nanoporous gold nanotubes by depositing colloidal Au/Ag film on the inner wall of anodic aluminum oxides (AAO) followed by dissolution of colloidal Ag and removal of AAO templates.     

Spectroscopic study of laser-induced phase transition of gold nanoparticles on nanosecond time scales and longer

The pulsed laser induced phase transition of gold nanoparticles in aqueous solution was observed via a transient absorption on nanosecond time scales and longer. Gold nanoparticles were excited with an intense picosecond laser pulse (355 nm, 30 ps), and the subsequent changes were monitored using two continuous wave laser wavelengths (488 and 635 nm). On the nanosecond time scale, below 6.3 mJ cm(-2), no change was observed; however, in the low fluence region between 6.3 and 17 mJ cm(-2), gold nanoparticles produced a bleach signal (488 nm) attributed to the melting of the gold nanoparticles, which decreased linearly with increasing laser fluence. Laser fluences above 17 mJ cm(-2) resulted in a strong absorption at both wavelengths, which is ascribed to vaporization of gold nanoparticles rather than solvated electrons (ejected from gold nanoparticles) or light scattering. The decay of both signals was faster than the 5 ns time resolution used in our experimental system. On the microsecond time scale, increase in absorbance at 635 nm was observed with a time constant of 1.0 micros, while no change was observed at 488 nm. It is considered that this increase is attributed to the formation of smaller gold nanoparticles resulting from pulsed laser induced size reduction of initial gold nanoparticles.     

Rapid synthesis of highly monodisperse Au(x)Ag(1-x) alloy nanoparticles via a half-seeding approach

Gold-silver alloy Au(x)Ag(1-x) is an important class of functional materials promising new applications across a wide array of technological fields. In this paper, we report a fast and facile synthetic protocol for preparation of highly monodisperse Au(x)Ag(1-x) alloy nanoparticles in the size range of 3-6 nm. The precursors employed in this work are M(I)-alkanethiolates (M = Au and Ag), which can be easily prepared by mixing common chemicals such as HAuCl(4) or AgNO(3) with alkanethiols at room temperature. In this half-seeding approach, one of the M(I)-alkanethiolates is first heated and reduced in oleylamine solvent, and freshly formed metal clusters will then act as premature seeds on which both the first and second metals (from M(I)-alkanethiolates, M = Au and Ag) can grow accordingly without additional nucleation and thus achieve high monodispersity for product alloy nanoparticles. Unlike in other prevailing methods, both Au and Ag elements present in these solid precursors are in the same monovalent state and have identical supramolecular structures, which may lead to a more homogeneous reduction and complete interdiffusion at elevated reaction temperatures. When the M(I)-alkanethiolates are reduced to metallic forms, the detached alkanethiolate ligands will serve as capping agent to control the growth. More importantly, composition, particle size, and optical properties of Au(x)Ag(1-x) alloy nanoparticles can be conveniently tuned with this approach. The optical limiting properties of the prepared particles have also been investigated at 532 and 1064 nm using 7 ns laser pulses, which reveals that the as-prepared alloy nanoparticles exhibit outstanding broadband optical limiting properties with low thresholds.     

二苯胺磺酸钠还原法制备粒径可控的金纳米粒子

采用二苯胺磺酸钠还原四氯合金酸的方法,在室温条件下,用SDS(十二烷基硫酸钠)、SDBS(十二烷基苯磺酸钠)作表面活性剂,成功地合成了金纳米粒子.分别讨论了还原剂二苯胺磺酸钠、表面活性剂(SDS、SDBS)及四氯台金酸的浓度等对金纳米粒子的粒径和形貌的影响.通过控制反应条件,可以合成出平均粒径大约为10、14、30、36nm的金纳米粒子.利用透射电镜(TEN)、紫外-可见(UV-Vis)吸收光谱对金纳米粒子进行了表征.研究结果表明不同的SDS或SDBS/HAuCl4的摩尔比,对金纳米粒子的尺寸大小有影响.     

Photochemistry of neutral isonitrile gold(I) complexes: modulation of photoreactivity by aurophilicity and pi-acceptance ability

This work demonstrates for the first time that aurophilicity and ligand pi-acceptance ability sensitize the photoreactivity of Au(I) complexes. Photolysis of LAu(I)Cl (L = RNC or CO) complexes leads to free L, Au(III), and Au(0) photoproducts. Solutions of (p-tosyl)CH(2)NCAuCl in dichloromethane undergo significant oligomerization leading to dimers and trimers with formation constants of 1.61 x 10(3) and 6.61 x 10(3) M(-1), respectively, representing the highest values reported to date for complexes that exhibit aurophilic association in solution. The photoproduct quantum yield (Phi) varies with the LAu(I)Cl concentration in solution. For (p-tosyl)CH(2)NCAuCl, metallic gold forms with Phi = 0.0065 and 0.032 in 4.0 x 10(-5) and 4.0 x 10(-3) M dichloromethane solutions, respectively. Meanwhile, irradiation of t-BuNCAuCl primarily produces t-BuNCAuCl(3) with Phi = 0.0045 and 0.013 for 5.0 x 10(-5) and 5.0 x 10(-3) M dichloromethane solutions, respectively. For Au(CO)Cl, metallic gold forms with Phi = 0.013 and 0.065 upon irradiation of 8.0 x 10(-5) and 8.0 x 10(-3) M dichloromethane solutions, respectively. Hence, *[LAuX](n) oligomeric species are more photoreactive than monomeric species. The results also demonstrate intuitive control of Phi via modulation of the pi-acceptance ability of L, as both follow CO > (p-tosyl)CH(2)NC > (alkyl)NC in LAuCl, a trend that is also commensurate with the relative long-term photosensitivity of the corresponding solids and solutions. A new method for preparing stable small gold nanoparticles is described based on the fundamental findings above. Thus, photolysis of different concentrations of LAuX in solutions containing a primary amine-terminated dendrimer leads to clear solutions exhibiting tunable visible plasmon absorptions of gold nanoparticles; these solutions maintain their colors and stability indefinitely. TEM measurements for representative samples prepared by photolysis of (p-tosyl)CH(2)NCAuCl solutions give rise to spherical nanoparticles as small as 5 nm.     

An in situ quick XAFS spectroscopy study on the formation mechanism of small gold nanoparticles supported by porphyrin-cored tetradentate passivants

We previously reported that a porphyrin-cored tetradentate passivant, which has two disulfide straps over one face of the porphyrin plane, can produce monolayer-protected gold nanoparticles, 2-4 nm in size, by the one-pot reduction of HAuCl(4) in DMF. The resulting nanoparticles are smaller than those prepared using the same S/Au molar ratio of a monodentate passivant. To examine the formation mechanism of small gold nanoparticles, the formation of gold nanoparticles in the presence of porphyrin-cored tetradentate passivants or a structurally related monodentate passivant was studied by time-resolved quick X-ray absorption fine structure spectroscopy. The results demonstrated that all of Au ions in solution are reduced to compose small Au clusters, i.e. nuclei, just after the NaBH(4) reduction of HAuCl(4) in both cases, but their size varied with the initial S/Au molar ratios and structure of the passivants. Thus, the size of Au nuclei was kinetically controlled by the passivants. Interestingly, the porphyrin-cored tetradentate passivant could stabilize smaller gold nanoparticles, 2-4 nm in size, but it was less efficient in trapping the Au nuclei formed at a very early stage, in comparison to the monodentate passivant.     

Pyrene-assisted synthesis of size-controlled gold nanoparticles in sodium dodecyl sulfate micelles

Gold nanoparticles prepared by chemical reduction in sodium dodecyl sulfate (SDS) solution are size-controlled with the addition of pyrene. Micellar electrokinetic capillary chromatography (MEKC) is applied to the system to examine the size and polydispersity of gold nanoparticles and to show that pyrene has the extraordinary effect in decreasing the size and narrowing the dispersity of gold nanoparticles. The MEKC electropherograms further suggest that pyrene could be oxidized by the aqueous Au(III) complexes first. All the reduced Au complexes were then solubilized in the pyrene-SDS micelles. The growth of gold nanoparticles beyond the embryonic stage was subsequently inhibited by the encapsulating SDS and electrophilic pyrene.     

Syntheses, structures, and photophysical properties of mono- and dinuclear sulfur-rich gold(I) complexes

The dinuclear gold complexes [{Au(PPh 3)} 2(mu- dmid)] ( 1) ( dmid = 1,3-dithiole-2-one-4,5-dithiolate) and [{Au(PPh 3)} 2(mu- dddt)] ( 2) ( dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate) were synthesized and characterized by X-ray crystallography. Both complexes exhibit intramolecular aurophilic interactions with Au...Au distances of 3.1984(10) A for 1 and 3.1295(11) A for 2. A self-assembly reaction between 4,5-bis(2-hydroxyethylthio)-1,3-dithiole-2-thione ( (HOCH 2 CH 2 ) 2 dmit) and [AuCl(tht)] affords the complex [AuCl{ (HOCH 2 CH 2 ) 2 dmit}] 2 ( 4), which possesses an antiparallel dimeric arrangement resulting from a short aurophilic contact of 3.078(6) A. This motif is extended into two dimensions due to intra- and intermolecular hydrogen bonds via the hydroxyethyl groups, giving rise to a supramolecular network. Three compounds were investigated for their rich photophysical properties at 298 and 77 K in 2-MeTHF and in the solid state; [Au 2(mu- dmid)(PPh 3) 2] ( 1), [Au 2(mu- dddt)(PPh 3) 2] ( 2), and [AuCl{( HOCH 2 CH 2 ) 2 dmit}] ( 4). 1 exhibits relatively long-lived LMCT (ligand-to-metal charge transfer) emissions at 298 K in solution (370 nm; tau e approximately 17 ns, where M is a single gold not interacting with the other gold atom; i.e., the fluxional C-SAuPPh 3 units are away from each other) and in the solid state (410 nm; tau e approximately 70 mus). At 77 K, a new emission band is observed at 685 nm (tau e = 132 mus) and assigned to a LMCT emission where M is representative for two gold atoms interacting together consistent with the presence of Au...Au contacts as found in the crystal structure. In solution at 77 K, the LMCT emission is also red-shifted to 550 nm (tau e approximately 139 mus). It is believed to be associated to a given rotamer. 2 also exhibits LMCT emissions at 380 nm at 298 K in solution and at 470 nm in the solid state. 4 exhibits X/MLCT emission (halide/metal to ligand charge transfer) where M is a dimer in the solid state with obvious Au...Au interactions, resulting in red-shifted emission band, and is a monomer in solution in the 10 (-5) M concentration (i.e., no Au...Au interactions) resulting in blue-shifted luminescence. Both fluorescence and phosphorescence are observed for 4.     

Interactions between water-soluble CdSe quantum dots and gold nanoparticles studied by UV-visible absorption spectroscopy.

The interaction of water-soluble CdSe quantum dots (QDs) with gold (Au) nanoparticles was investigated by ultraviolet visible absorption spectroscopy. The results showed that the aggregation of Au nanoparticles was induced by CdSe QDs. The influences of factors such as the size of Au nanoparticles, acidity, buffer concentration and the concentration ratio of the CdSe QDs to Au nanoparticles were each investigated. The comparison of two different particle sizes (16 and 25 nm) of Au nanoparticles that interact with CdSe QDs in the solution showed that the aggregation of small Au nanoparticles (16 nm) is easier than that of big Au nanoparticles (25 nm). At pH 7.0 phosphate buffer solution (0.02 M), the optimal molar ratio of CdSe:Au is about 3100:1 according to calculations.     

Morphology of gold nanoparticles synthesized by filamentous cyanobacteria from gold(I)-thiosulfate and gold(III)--chloride complexes

Plectonema boryanum UTEX 485, a filamentous cyanobacterium, has been reacted with aqueous Au(S(2)O(3))(2)(3)(-) and AuCl(4)(-) solutions ( approximately 400-550 mg/L Au) at 25-100 degrees C for up to 1 month and at 200 degrees C for 1 day. The interaction of cyanobacteria with aqueous Au(S(2)O(3))(2)(3)(-) promoted the precipitation of cubic (100) gold nanoparticles (<10-25 nm) at membrane vesicles and admixed with gold sulfide within cells and encrusted on the cyanobacteria, whereas reaction with AuCl(4)(-) resulted in the precipitation of octahedral (111) gold platelets ( approximately 1-10 microm) in solutions and nanoparticles of gold (<10 nm) within bacterial cells. Functional groups imaged by negative ion TOF-SIMS on (111) faces of the octahedral platelets were predominantly Cl and CN, with smaller amounts of C(2)H and CNO.     

Foam films in the presence of functionalized gold nanoparticles

Dry aqueous foams made of anionic surfactant (SDS) and spherical gold nanoparticles are studied by small angle X-ray scattering and by optical techniques. To obtain stable foams, the surfactant concentration is well above the critical micelle concentration. The specular reflectivity signal obtained on a very thin film (thickness 20 nm) shows that functionalized nanoparticles (17 nm typical size) are trapped within the film in the form of a single monolayer. In order to isolate the film behavior, investigations are made on a single film confined in a tube. The film thinning according to the ratio of functionalized nanoparticle and SDS micelles (1:1, 1:10, 1:100) is mainly governed by the structural arrangement of SDS micelles. In thick films, nanoparticles tend to form aggregates that disappear during drainage. In particular self-organization of nanoparticles (with different surface charge) inside the film is not detected.     

Bimodal Size Distribution of Gold Nanoparticles under Picosecond Laser Pulses

The evolution of size distributions of gold nanoparticles under pulsed laser irradiation (Nd:YAG, lambda = 355 nm, pulse width 30 ps) was carefully observed by transmission electron microscopy. Interestingly, the initial monomodal size distribution of gold nanoparticles turned into a bimodal one, with two peaks in the number of particles, one at 6 nm and the other at 16-24 nm. The sizes for small particles depended very little on the irradiated laser energy. This change is attributed to laser-induced size reduction of the initial gold nanoparticles followed by the formation of small particles. In our analysis, we extracted a characteristic value for the size-reduction rate per one pulse and revealed that laser-induced size reduction of gold nanoparticles occurred even below the boiling point. When laser energy is insufficient for the boiling of particles, formation of gold vapor around liquid gold drops is thought to cause the phenomenon. With enough laser energy for the boiling, the formation of gold vapor around and inside liquid gold drops is responsible for the phenomenon. We also observed particles with gold strings after one pulse irradiation with a laser energy of 43 mJ cm(-2) pulse(-1), which is sufficient energy for the boiling. It is considered that such particles with gold strings are formed by the projection of gaseous gold from liquid gold drops with some volume of liquid gold around the bubble. On the basis of comparison with previous work, picosecond laser pulses are thought to be the most efficient way to cause laser-induced size reduction of gold nanoparticles.     

Control of shell thickness in silica-coating of Au nanoparticles and their X-ray imaging properties

This paper describes a performance of precise control of shell thickness in silica-coating of Au nanoparticles based on a sol-gel process, and an investigation into X-ray imaging properties for the silica-coated Au (Au/SiO(2)) particles. The Au nanoparticles with a size of 16.9±1.2 nm prepared through a conventional citrate reduction method were used as core particles. The Au nanoparticles were silica-coated with a sol-gel reaction using tetraethylorthosilicate (TEOS) as a silica source, sodium hydroxide (NaOH) as a catalyst, and (3-aminopropyl) trimethoxysilane (APMS) as a silane coupling agent. An increase in TEOS concentration resulted in an increase in shell thickness. Under certain concentrations of Au, H(2)O, NaOH, and APMS, the Au/SiO(2) particles with silica shell thickness of 6.0-61.0 nm were produced with varying TEOS concentration. Absorption peak wavelength of surface plasmon resonance of the Au/SiO(2) colloid solution depended on silica shell thickness, which agreed approximately with the predictions by Mie theory. The as-prepared colloid solution could be concentrated up to an Au concentration of 0.19 M with salting-out and centrifugation. The concentrated colloid solution showed an X-ray image with high contrast, and a computed tomography value for the colloid solution with an Au concentration of 0.129 M was achieved 1329.7±52.7 HU.     

Deposition of gold nanoparticles on silica spheres by electroless metal plating technique

A previously proposed method for metal deposition with silver [Kobayashi et al., Chem. Mater. 13 (2001) 1630] was extended to uniform deposition of gold nanoparticles on submicrometer-sized silica spheres. The present method consisted of three steps: (1) the adsorption of Sn(2+) ions took place on surface of silica particles, (2) Ag(+) ions added were reduced and simultaneously adsorbed to the surface, while Sn(2+) was oxidized to Sn(4+), and (3) Au(+) ions added were reduced and deposited on the Ag surface. TEM observation, X-ray diffractometry, and UV-vis absorption spectroscopy revealed that gold metal nanoparticles with an average particle size of 13 nm and a crystal size of 5.1 nm were formed on the silica spheres with a size of 273 nm at an Au concentration of 0.77 M.     

Electron self-exchange between Au140(+/0) nanoparticles is faster than that between Au38(+/0) in solid-state, mixed-valent films

The well-defined one-electron steps in the voltammetry of solutions of the nanoparticles Au38(SC2Ph)24 and Au140(SC6)53 (SC2Ph = phenylethanethiolate; SC6 = hexanethiolate) enable preparation of solutions containing, for example, Au38(SC2Ph)24 and Au38(SC2Ph)24+(ClO4)- nanoparticles in known relative proportions. From these solutions can be cast dry, mixed-valent films demonstrably containing the same proportions. Electronic conduction in such mixed-valent films is shown to occur by a bimolecular electron self-exchange reaction at a rate proportional to the concentration product, [Au38][Au38+]. The observed Au38(+/0) rate constant, approximately 2 x 10(6) M(-1) s(-1), is much smaller than that previously observed for Au140(+/0) films (ca. 4 x 10(9) M(-1) s(-1); Wuelfing, W. P.; et al. J. Am. Chem. Soc. 2000, 122, 11465). To our knowledge, this is the first example of a significant size effect in metal nanoparticle electron-transfer dynamics. Thermal activation parameters for the electron-hopping conductivities of the two nanoparticles reveal that the rate difference is mainly caused by energy barriers (EA) for Au38(+/0) electron transfers that are larger by approximately 3-fold than those for Au140(+/0) electron transfers (ca. 20 vs 7 kJ/mol). Differences in pre-exponential terms in the activation equations for the two nanoparticles are a smaller contributor to the rate constant difference and can be partly ascribed to differences in tunneling distances.     

Surface-enhanced Raman scattering of p-aminothiophenol on a Au(core)/Cu(shell) nanoparticle assembly.

Surface-enhanced Raman scattering (SERS) of p-aminothiophenol (PATP) molecules adsorbed onto assemblies of Au(core)/Cu(shell) nanoparticles is reported. We compare it with the SERS spectrum of PATP adsorbed onto gold nanoparticles: both the absolute and relative scattered intensities of various bands in the two spectra are very different. The difference in relative intensity can be ascribed to chemical effects; the chemical enhancement ratio of the two substrates is approximately 3-5. A theoretical analysis based on a charge-transfer model is carried out, which yields a consistent result and shows that the difference in chemical enhancement is mainly due to the state densities and Fermi levels of the substrates. The difference in absolute intensity originates from electromagnetic (EM) enhancement. EM enhancement of Au(core)/Cu(shell) nanoparticles is unlike that of single-component gold or copper SERS-active substrates. The core/shell particle size for optimal enhancement is about 20 nm in the case of a 632.8 nm incident laser (the size ratio of the core and shell layers is about 0.6).     

Nanoparticle‐Imprinted Polymers for Size‐Selective Recognition of Nanoparticles

Citrate‐stabilized gold nanoparticles 15 nm and 33 nm in diameter were transferred concomitantly with a monolayer of positively charged polyaniline by Langmuir–Blodgett transfer at pH 5 onto a conducting indium‐doped tin oxide (ITO) support. Films consisting of one to three layers of polyaniline with thicknesses of 1–3 nm were prepared and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy. After electro‐oxidation of the Au nanoparticles in 0.1 M KCl, cavities were left behind in the film that could be analyzed by SEM. These cavities were able to recapture analyte nanoparticles from a solution of pH 10 and showed size‐exclusion properties. The amount of nanoparticles taken up by the cavities was conveniently analyzed by measuring the charge associated with the electro‐oxidation of these particles in 0.1 M KCl after the film had been rinsed with water. The size‐exclusion properties improved with the number of Langmuir–Blodgett layers transferred.