Au Colloid Monolayers Research Articles

Amplified protein sensing using deep purple fluorophores on homogeneous Au substrates

We report here the first observation of appreciable enhancement of fluorescence induced by large Au colloids. Au monolayers with Au colloids of 40 nm, 59 nm, and 81 nm in radii, were formed on silane modified glass surfaces, respectively. The nanoparticle densities were varied by varying the deposition times and documented by scanning electron microscopy. Two types of samples were prepared, with large inter-particle distance and hence little or no inter-particle coupling and small inter-particle distance with inter-particle coupling. The fluorescence enhancement was examined by using a self assembled monolayer of the fluorophore-protein conjugate with Deep Purple as a fluorophore and Bovine Serum Albumin as protein (DP-BSA). The data show the over 15 fold enhancement under optimized conditions and reveal strong variations with both inter-particle distance and particle size. Nanostructures of appropriate size with optimized inter-particle distance thus prepared can produce promising substrates for decent fluorescence enhancement. We demonstrated that the Au colloid monolayers on glass surfaces are promising substrates for fluorescence enhancement with outstanding macroscopic homogeneity. This important feature will pave the way for the application of our substrates in biotechnology and life sciences such as imaging and sensing of biomolecules in proteomics.

A fractal analysis of colloidal Au nanoparticle electrodes

An electrochemical fractal analysis of three types of colloidal Au nanoparticle electrodes is described. One type of electrode comprises multilayers of 12-nm diameter colloidal Au particles on glass substrates; another type consists of analogous films grown on conductive Au substrates. Plots of log( I) vs. log( t) data from electrochemical potential step experiments were used to determine D f, the fractal dimension. On the Au substrate, D f ranged from 2.16 to 2.47 for films consisting of 7–13 colloidal Au layers, respectively, while for the glass substrate D f varied from 1.54 to 2.33 over the same range of layers. A third type of Au nanoparticle-based electrode was prepared by enlargement of nanoparticles in Au colloid monolayers by exposure to NH 2OH/Au 3+. For these samples, D f varied from ca. 1.2 to 2.0. D f determination by atomic force microscopy gave values in the range of 2.3–2.4 for both types of nanoparticle electrodes on glass substrates, regardless of Au film thickness.

Surface-Enhanced Raman Scattering Study of 4-Biphenylcarboxylic Acid and a Liquid Crystalline Oligomer on Different Metal Surfaces

Surface-enhanced Raman spectra of 4-biphenylcarboxylic acid (BPCA) and ethyl 4‘-[4‘-oxy-4-biphenylcarbonyloxy]-4-biphenylcarboxylate with a poly(propylene oxide) coil of 12 (hereafter named P-12-4) adsorbed on two different metal substrates, Ag and Au colloid monolayers, were examined, to determine the relationship between adsorption strength and geometry. A significant downshift of νCOO- by 60 cm-1 and lack of a C−H aromatic stretching vibration band indicated that BPCA lies flat on the Au surface, while a smaller shift of 40 cm-1 to a lower wavenumber and weak appearance of the C−H aromatic stretching mode implied adsorption through mainly oxygen lone pair electrons of carboxylate with the benzene ring tilted on the Ag surface. These orientations were confirmed by the desorption behavior of a liquid crystalline (LC) oligomer, P-12-4, which contains the BPCA moiety, on the Au and Ag colloid monolayers with increasing temperature.

Surface-Enhanced Raman Scattering Study of 4-Biphenylcarboxylic Acid

The interfacial structures of biphenyl (BP), benzoic acid (BA), and 4-biphenylcarboxylic acid (BPCA) on a Au colloid monolayer surface were investigated using surface-enhanced Raman scattering (SERS). The absence of the C−H stretching vibrational mode in the SERS spectrum of BP indicated that the two benzene rings of BP were coplanar on the Au colloid surface, while their twisted structure was suggested in the bulk state. The downward frequency shift of the carboxylate stretching vibrational mode of BA implied the direct chemical adsorption of the carboxylate group onto the Au surface with a flat orientation. In BPCA, an absence of the C−H stretching vibrational mode and the red shift of the carboxylate stretching vibrational mode were observed. This fact strongly indicated that both the two benzene rings and the carboxylate group were adsorbed on the Au surface with flat orientation via their π-systems.

Hydroxylamine Seeding of Colloidal Au Nanoparticles. 3. Controlled Formation of Conductive Au Films

An approach to enlarge preformed colloidal Au nanoparticles in solution based on the Au colloidal surface-catalyzed reduction of Au3+ by NH2OH (“seeding”) has been adapted to 12-nm-diameter colloidal Au nanoparticles immobilized in monolayers and multilayers. Bulk characterization of the ensuing films was carried out by atomic absorption, UV−vis−near-IR optical spectroscopy, cyclic voltammetry, and dc resistance measurements. Exposure of a 12-nm-diameter Au colloid monolayer on organosilane-modified glass surfaces to NH2OH/Au3+ leads to rapid particle growth and coalescence: after roughly 5−10 min, the optical and electrical properties closely resemble that of conductive Au thin films prepared by evaporation. Evolution of the nanometer-scale architecture was followed using atomic force microscopy (AFM), surface-enhanced Raman scattering (SERS), surface plasmon resonance (SPR), and field emission scanning electron microscopy (FE-SEM), leading to the following key findings: (i) Seeding of surface-confined...

Kinetics and Thermodynamics of Au Colloid Monolayer Self-Assembly: Undergraduate Experiments in Surface and Nanomaterials Chemistry

This paper describes three closely related experiments utilizing colloidal Au nanoparticles to teach basic concepts in physical chemistry. This is done in the context of surface chemistry and nanomaterials synthesis, areas of intense current research interest in both academics and industry. First, students investigate the optical properties and stability of aqueous Au colloidal suspensions, determining the amount of a protein (protein A) necessary to prevent salt-induced flocculation of colloidal Au. Then, the kinetics of Au nanoparticle surface immobilization on silanized glass slides is followed by visible spectrophotometry. Data are fit to a (time)1/2 curve that describes particle diffusion and adsorption from an unstirred solution onto a planar substrate. Finally, the adsorption isotherm data are fit to a Frumkin isotherm, demonstrating the effects of interparticle repulsions upon the thermodynamics of Au particle monolayer formation. Each experiment benefits from the intense visible absorption of col...

AFM characterization of the structure of Au-colloid monolayers and their chemical etching

Modification of a glass support with triethoxy propylaminosilane yields an active interface for the assembly of Au colloids. The colloids are imaged by AFM using a low applied load (0.5–0.7 nN). The lateral Au-colloid dimensions, 33±3 nm, deviate from the particle dimensions determined by TEM (19±2 nm) and absorption spectroscopy (15 nm). This deviation is attributed to the intrinsic curvature of the AFM tip. Application of higher loads on the tip (3 nN) results in the sweeping of Au colloids from the monolayer. The Au colloid monolayer is etched in the presence of CN −. The etching proceeds by the initial coincidental etching of Au particles followed by the kinetically favored etching of particles at the edges of the etched domains. This provides means for the micro machining and the chemical manipulation of Au colloids of controlled spatial arrangement.

Au colloid monolayers as templates for nanostructure assembly

A new approach to nanostructure formation based on replication of an existing pattern of colloidal Au nanoparticles is described. The key steps in this strategy involve the coating of a well-defined Au colloid monolayer with an SiO x sol-gel, HF etching until colloidal particles are exposed, and removal of colloidal Au with aqua regia. This process was carried out on a two-dimensional array of 12nm diameter particles on an In-doped SnO 2 (ITO) substrate, and monitored by UV-vis, electrochemistry, and atomic force microscopy (AFM). AFM data indicate the strategy is feasible, with clearly visible differences in surface morphology observed between bare ITO and colloidal Au-depleted, SiO x -coated ITO.

Novel approach for the preparation of metal colloid monolayers on modified surfaces

The morphology of Au colloid monolayers on modified silicon oxide surfaces (glass, silicon wafer, and ITO slides) has been studied. These monolayers were prepared by a new method described here. We also demonstrate several applications of this colloid monolayer surface. The preparation route creates a rough metal surface, the morphology of which can be easily controlled. The colloid monolayers are prepared in two steps: (1) modification of the substrates with starburst dendrimers; (2) noble metal colloid deposition onto the dendrimer layer. Au particles of approximately 28 nm in diameter were deposited onto the dendrimer-modified surfaces. The spontaneous adsorption of the dendrimers to the silicon oxide surface was proven by x-ray photoelectron spectroscopy, contact angle, and grazing angle Fourier transform infrared spectroscopy. The thickness of the dendrimer layer has been calculated to be in the range of d=14–25 Å. The structure and properties of the resulting particle arrangement have been studied by atomic force microscopy (AFM) and ultraviolet (UV-vis) spectroscopy. AFM data show that the colloids spontaneously form continuous films on the dendrimer-modified surfaces. The noble metal particles are well isolated, confined to a single layer, and aggregation does not occur on the surface. UV-vis spectroscopic data show that the microstructure controls the optical properties of the layer. The article demonstrates the practical applications of this rough metal surface as a platform for surface-enhanced Raman scattering and electrochemistry.

Chemical and Electrochemical Ag Deposition onto Preformed Au Colloid Monolayers: Approaches to Uniformly-Sized Surface Features with Ag-Like Optical Properties

Two approaches to preparation of Ag-clad Au colloid monolayers are described. Each begins with a preformed monolayer of colloidal Au particles on glass, Au-coated quartz, or In-doped SnO2. Chemical reduction of Ag+ from a commercial Ag coating formulation (LI Silver) or electrochemical reduction of Ag+ leads to surfaces for which the amount of Ag deposited can be controlled. On transparent substrates, Ag deposition can be followed in real time by UV−visible spectroscopy. When the substrate is an electrode, electrochemical deposition can be monitored by coulometry; for the chemical process, anodic stripping voltammetry yields accurate values of the amount of Ag present, as confirmed by quartz crystal microgravimetry (QCM). Chemical reduction yields surfaces that are extremely enhancing for surface enhanced Raman scattering (SERS), although the deposition process required must be precisely tuned. In contrast, electrochemical deposition, while affording more accurate control of the reduction rate, produces only mildly enhancing surfaces. Atomic force microscopy (AFM) images of electrochemically-produced surfaces show formation of very large nonuniform Ag particles that are distinct from the Au colloid monolayer, while those prepared by LI Silver show both growth of Ag on Au and formation of smaller colloidal Ag particles attached to Ag-coated Au particles.

Preparation and Characterization of Au Colloid Monolayers

The design and initial characterization of two-dimensional arrays of colloidal Au particles are reported. These surfaces are prepared by self-assembly of 12 nm diameter colloidal Au particles onto immobilized polymers having pendant functional groups with high affinity for Au (i.e., CN, SH, and NH 2 ). The polymers are formed by condensation of functionalized alkoxysilanes onto cleaned quartz, glass, and SiO 2 surfaces. The assembly protocol is carried out completely in solution: cleaned substrates are immersed in methanolic solutions of organosilane, rinsed, and subsequently immersed in aqueous colloidal Au solutions. Two-dimensional arrays spontaneously form on the polymer surface. The resulting substrates have been characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), and surface-enhanced Raman scattering (SERS). TEM data show that the particles are spatially separated but close enough to interact electromagnetically (small spacing compared to λ). The UV-vis data show that collective particle surface plasmon modes are present in the 650-750 nm region, suggesting that these assemblies are SERS-active. This is indeed the case, with enhancement factors of roughly. Au colloid monolayers possess a set of features that make them very attractive for both basic and applied uses, including uniform roughness, high stability, and biocompatibility