An in vitro study of bare and poly(ethylene glycol)-co-fumarate-coated superparamagnetic iron oxide nanoparticles: a new toxicity identification procedure

As the use of superparamagnetic iron oxide nanoparticles (SPION) in biomedical applications increases (e.g. for targeting drug delivery and imaging), patients are likely to be exposed to products containing SPION. Despite their high biomedical importance, toxicity data for SPION are limited to date. The aim of this study is to investigate the cytotoxicity of SPION and its ability to change cell medium components. Bare and poly(ethylene glycol)-co-fumarate (PEGF)-coated SPION with narrow size distributions were synthesized. The particles were prepared by co-precipitation using ferric and ferrous salts with a molar Fe3+/Fe2+ ratio of 2. Dulbecco's modified Eagle's medium (DMEM) and primary mouse fibroblast (L929) cell lines were exposed to the SPION. Variation of cell medium components and cytotoxicity due to the interactions with nanoparticles were analyzed using ultraviolet and visible spectroscopy (UV/vis) and the 3-[4,5-dimethylthiazol-2yl]-2,5-diphenyltetrazolium bromide (MTT) assay methods, respectively. The toxicity amount has been traditionally identified by changes in pH and composition in cells and DMEM due to the tendency of SPION to adsorb proteins, vitamins, amino acids and ions. For in vitro toxicity assessments, a new surface passivation procedure is proposed which can yield more reliable quantitative results. It is shown that a more reliable way of identifying cytotoxicity for in vitro assessments is to use particles with saturated surfaces via interactions with DMEM before usage.

Cell toxicity of superparamagnetic iron oxide nanoparticles

The performance of nanoparticles for biomedical applications is often assessed by their narrow size distribution, suitable magnetic saturation and low toxicity effects. In this work, superparamagnetic iron oxide nanoparticles (SPIONs) with different size, shape and saturation magnetization levels were synthesized via a co-precipitation technique using ferrous salts with a Fe(3+)/Fe(2+) mole ratio equal to 2. A parametric study is conducted, based on a uniform design-of-experiments methodology and a critical polymer/iron mass ratio (r-ratio) for obtaining SPION with narrow size distribution, suitable magnetic saturation, and optimum biocompatibility is identified. Polyvinyl alcohol (PVA) has been used as the nanoparticle coating material, owing to its low toxicity. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay is used to investigate the cell biocompatibility/toxicity effects of the samples. From the MTT assay results, it is observed that the biocompatibility of the nanoparticles, based on cell viabilities, can be enhanced by increasing the r-ratio, regardless of the stirring rate. This effect is mainly due to the growth of the particle hydrodynamic size, causing lower cell toxicity effects.

Highly efficient biocompatible single silicon nanowire electrodes with functional biological pore channels

Nanoscale electrodes based on one-dimensional inorganic conductors could possess significant advantages for electrochemical measurements over their macroscopic counterparts in a variety of electrochemical applications. We show that the efficiency of the electrodes constructed of individual highly doped silicon nanowires greatly exceeds the efficiency of flat Si electrodes. Modification of the surfaces of the nanowire electrodes with phospholipid bilayers produces an efficient biocompatible barrier to transport of the solution redox species to the nanoelectrode surface. Incorporating functional alpha-hemolysin protein pores in the lipid bilayer results in a partial recovery of the Faradic current due to the specific transport through the protein pore. These assemblies represent a robust and versatile platform for building a new generation of highly specific biosensors and nano/bioelectronic devices.

Formation, stability, and mobility of one-dimensional lipid bilayers on polysilicon nanowires

Curved lipid membranes are ubiquitous in living systems and play an important role in many biological processes. To understand how curvature and lipid composition affect membrane formation and fluidity, we have assembled and studied mixed 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) supported lipid bilayers on amorphous silicon nanowires grown around carbon nanotube cores with controlled wire diameters ranging from 20 to 200 nm. We found that lipid vesicles fused onto nanowire substrates and formed continuous bilayers for all DOPC-DOPE mixtures tested (with the DOPE content of up to 30%). Our measurements demonstrate that nanowire-supported bilayers are mobile, exhibit fast recovery after photobleaching, and have a low concentration of defects. Lipid diffusion coefficients in these high-curvature tubular membranes are comparable to the values reported for flat supported bilayers and increase slightly with decreasing nanowire diameter. A free space diffusion model adequately describes the effect of bilayer curvature on the lipid mobility for nanowire substrates with diameters greater than 50 nm, but shows significant deviations from the experimental values for smaller diameter nanowires.

Modeling electrochemical deposition inside nanotubes to obtain metal-semiconductor multiscale nanocables or conical nanopores

Nanocables with a radial metal-semiconductor heterostructure have recently been prepared by electrochemical deposition inside metal nanotubes. First, a bare nanoporous polycarbonate track-etched membrane is coated uniformly with a metal film by electroless deposition. The film forms a working electrode for further deposition of a semiconductor layer that grows radially inside the nanopore when the deposition rate is slow. We propose a new physical model for the nanocable synthesis and study the effects of the deposited species concentration, potential-dependent reaction rate, and nanopore dimensions on the electrochemical deposition. The problem involves both axial diffusion through the nanopore and radial transport to the nanopore surface, with a surface reaction rate that depends on the axial position and the time. This is so because the radial potential drop across the deposited semiconductor layer changes with the layer thickness through the nanopore. Since axially uniform nanocables are needed for most applications, we consider the relative role of reaction and axial diffusion rates on the deposition process. However, in those cases where partial, empty-core deposition should be desirable (e.g., for producing conical nanopores to be used in single nanoparticle detection), we give conditions where asymmetric geometries can be experimentally realized.

Mechanism of film growth of tellurium by electrochemical deposition in the presence and absence of cadmium ions

The growth morphology and the kinetics of a thin film of Te on Au during electrochemical deposition at -62 mV (vs Ag/AgCl/3 M NaCl) have been studied. The deposition conditions are similar to those used previously by us to grow nanowires inside Au nanotubes by electrochemical deposition in the presence of Cd ions (Cd(2+)). By using electrochemical deposition on a planar Au electrode, we explored the growth of the Te film for two conditions: in the presence of Cd(2+) (0.1 mM TeO(2) + 1 mM CdSO(4) + 50 mM H(2)SO(4) solution) and in the absence of Cd(2+) (0.1 mM TeO(2) + 50 mM H(2)SO(4) solution). We used several surface investigation techniques to study the growth such as: in situ electrochemical atomic force microscopy (EC-AFM), in situ electrochemical surface plasmon resonance (EC-SPR), electrochemical methods, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). In the presence of Cd(2+), in situ electrochemical atomic microscopy showed that Cd(2+) acted as a mediator at the early deposition stage and caused smoothing of the Te deposit obtained. In the absence of Cd(2+), Te had an island growth. The electrochemical surface plasmon resonance showed that the deposit was characterized by a slower deposition rate in the presence of Cd(2+) than in the absence of Cd(2+). Additionally, the thickness of the film was evaluated using EC-AFM measurements, electrochemical stripping analysis, and EC-SPR. The results obtained from all three measurements agree well with the Te film obtained in the presence of Cd(2+), where a continuous and uniform film was formed. In the presence of Cd(2+), a Te film with a thickness of 1.04 nm and atomically flat surface was deposited on an ultraflat Au surface. The XPS spectrum showed no significant amount of Cd in the deposit, indicating that the Cd ion acted as a mediator and not as a co-deposition element.

Controlled electrostatic gating of carbon nanotube FET devices

Carbon nanotube transistors are a promising platform for the next generation of nonoptical biosensors. However, the exact nature of the biomolecule interactions with nanotubes in these devices remains unknown, creating one of the major obstacles to their practical use. We assembled alternating layers of oppositely charged polyelectrolytes on the carbon nanotube transistors to mimic gating of these devices by charged molecules. The devices showed reproducible oscillations of the transistor threshold voltage depending on the polarity of the outer polymer layer in the multilayer film. This behavior shows excellent agreement with the predictions of a simple electrostatic model. Finally, we demonstrate that complex interactions of adsorbed species with the device substrate and the surrounding electrolyte can produce significant and sometimes unexpected effects on the device characteristics.

Growth of ultrathin films of cadmium telluride and tellurium as studied by electrochemical atomic force microscopy

Time dependent, cathodic electrodeposition of ultrathin CdTe and Te films has been studied in 50 mM H(2)SO(4) + 1 mM CdSO(4) + 0.1 mM TeO(2) solutions at room temperature under potential control using electrochemical atomic force microscopy (EC-AFM). The films were also characterized electrochemically and with X-ray diffraction. The growth mechanism and the composition of the films depends on the applied potentials. Island-like growth mode was observed for CdTe films when the deposition potential was -0.35 V (SHE). At a more positive deposition potential of 0.138 V (SHE), Cd was not co-deposited into the film but affected the dynamic growth mode of the deposit. At this voltage smooth Te films were obtained. Depending on the applied potential, Cd acts either as a co-deposition element for CdTe film growth, or as a mediator for layer-by-layer growth of Te films.

Convection, diffusion and reaction in a surface-based biosensor: Modeling of cooperativity and binding site competition on the surface and in the hydrogel

We study theoretically the transport and kinetic processes underlying the operation of a biosensor (particularly the surface plasmon sensor "Biacore") used to study the surface binding kinetics of biomolecules in solution to immobilized receptors. Unlike previous studies, we concentrate mainly on the modeling of system-specific phenomena rather than on the influence of mass transport limitations on the intrinsic kinetic rate constants determined from binding data. In the first problem, the case of two-site binding where each receptor unit on the surface can accommodate two analyte molecules on two different sites is considered. One analyte molecule always binds first to a specific site. Subsequently, the second analyte molecule can bind to the adjacent unoccupied site. In the second problem, two different analytes compete for one binding site on the same surface receptor. Finally, the third problem considers the case of positive cooperativity among bound molecules in the hydrogel using a simple mean-field approach. The transport in both the flow channel and the hydrogel phases of the biosensor is taken into account in this case (with few exceptions, most previous studies assume a simpler model in which the hydrogel is treated as a planar surface with the receptors). We consider simultaneously diffusion and convection through the flow channel together with diffusion and cooperativity binding on the surface and in the hydrogel. In each case, typical results for the concentration contours of the free and bound molecules in the flow channel and hydrogel regions are presented together with the time-dependent association/dissociation curves and reaction rates. For binding site competition, the analysis predicts overshoot phenomena.

Persistence Length Control of the Polyelectrolyte Layer-by-Layer Self-Assembly on Carbon Nanotubes

We have studied layer-by-layer polyelectrolyte self-assembly on pristine individual single-wall carbon nanotubes as a function of solution ionic strength. We report the existence of an ionic strength threshold for the deposition, below which the majority of nanotubes remain uncoated. Once the ionic strength reaches the threshold value, the majority of the individual nanotubes become coated with polyelectrolytes. Our results indicate that the self-assembly process likely involves wrapping of polymer chains around nanotubes and that the polymer chain's ability to bend in order to accommodate the nanotube curvature is one of the critical parameters controlling layer-by-layer electrostatic self-assembly on these one-dimensional templates.

Preparation of a Vast CoFe2O4 Magnetic Monolayer by Langmuir−Blodgett Technique

The preparation of ultrathin films of CoFe2O4 nanocrystallites is reported. TEM images of them showed 2-dimensional assembly of particles, demonstrating the uniformity of these nanocrystallites. The formation of a Langmuir monolayer of the surface coated CoFe2O4 nanocrystallites with oleate at the air/water interface and its stability were studied with pressure-area isotherm curves and Brewster Angle Microscope (BAM) images. Surface pressure vs surface area isotherms and TEM studies demonstrated that the increasing surface pressure resulted in a transition from a complex with well-separated domains of nanocrystallites to well-compressed, monoparticulate layers, and, ultimately, to multiparticulate layers.

Functional One-Dimensional Lipid Bilayers on Carbon Nanotube Templates

Use of biological machines and environments in novel bioinorganic nanostructures is critical for development of new types of biosensors, bio-NEMS devices, and functional materials. Lipid bilayers that mimic a cell membrane have already played an important role in such applications. We present supported lipid bilayers that spontaneously assemble in a continuous nanoshell around a template of a carbon nanotube wrapped with hydrophilic polymer cushion layers. We demonstrate that such 1-D lipid membranes are fluid and can heal defects, even over repeated damage-recovery cycles. A simple diffusion model can describe mobility of lipid molecules in these 1-D nanoshells. These structures could lead to the development of new classes of biosensors and bioelectronic devices.

Fabrication of nanocables by electrochemical deposition inside metal nanotubes

We report a novel route for fabricating Au-Te nanocables. Using nanoporous polycarbonate tract-etching (PCTE) membrane as the template, Au nanotubes were fabricated by electroless Au deposition inside the nanopores of the PCTE membrane. Using the Au nanotube membrane as a second template, Te was deposited on the surfaces of the Au nanotubes by slow electrochemical deposition, taking advantage of underpotential deposition (UPD). The deposition rate was sufficiently slow to radially grow Te nanotubes coaxially within the Au nanotubes to form nanocables.

Using wavelets to analyze AFM images of thin films: surface micelles and supported lipid bilayers

This paper presents micro- and nanoanalysis of thin films based on images obtained by atomic force microscopy (AFM). The analysis exploits the discrete wavelet transform and the resulting wavelet spectrum to study surface features. It is demonstrated that the wavelet technique can characterize micro- and nanosurface features and distinguish between similar surface structures. The use of a feature extraction method is shown. The method involves the separation of certain frequency content from the original AFM images and analyzing the data independently to gain quantitative information about the images. By using the feature extraction method, soft surfaces in water are analyzed and nanofeatures are measured. The packing of surface micelles of sodium dodecyl sulfate on a self-assembled monolayer is analyzed. The characteristics of pore formation, due to penetration of the antibacterial peptide protegrin, into a solid-supported lipid bilayer are quantified. The sizes of the pores are obtained, and it is observed that the line tension of the pores reduces the fluctuations of the lipid bilayer.

Enhanced feature analysis using wavelets for scanning probe microscopy images of surfaces

In this work we develop wavelet theory for the analysis of surface topography images obtained by scanning probe microscopy (SPM) such as atomic force microscopy (AFM). Wavelet transformation is localized in space and frequency, which can offer an advantage for analyzing information on surface morphology and topography. Wavelet transformation is an ideal tool to detect trends, discontinuities, and short periodicities on a surface. Additionally, wavelets can be used to remove artifacts and noise from scanning microscopy images. In terms of 3-D image analysis, discrete wavelet transform can capture patterns at all relevant frequency scales, thus providing a level of image analysis that is not possible otherwise. It is also possible to use the methodology for analyzing surface structures at the molecular level. The results demonstrate superior capabilities of wavelet approach to scanning probe microscopy image analysis compared to traditional analysis techniques.

Layer-by-Layer electrostatic self-assembly of polyelectrolyte nanoshells on individual carbon nanotube templates

Carbon nanotubes have been featured prominently in the nanotechnology research for some time, yet robust strategies for noncovalent chemical modification of the nanotube surface are still missing. Such strategies are essential for the creation of functional device architectures. Here, we present a new general procedure for carbon nanotube modification based on polyelectrolyte layer-by-layer assembly. We have built multilayer structures around individual carbon nanotube bridges by first modifying the nanotube surface with a pyrene derivative followed by layer-by-layer deposition of polyelectrolyte macroions on the nanotube. Transmission electron microscopy and scanning confocal fluorescence microscopy images confirm the formation of nanometer-thick amorphous polymer nanoshells around the nanotubes. These multilayer polyelectrolyte shells on individual carbon nanotubes introduce nearly unlimited opportunities for the incorporation of various functionalities into nanotube devices, which, in turn, opens up the possibility of building more complex multicomponent structures.

Phosphatidylserine/Cholesterol Bilayers Supported on a Polycation/Alkylthiol Layer Pair

1-Stearoyl-2-oleoyl phosphatidylserine (SOPS)/cholesterol bilayers, supported on a polycation/alkylthiol layer pair on a gold surface, were investigated by surface plasmon resonance (SPR) and fluorescence recovery after photobleaching. The substrate was formed by electrostatic adsorbance of a hydrated poly(diallyldimethylammonium chloride) (PDDA) layer on the negatively charged surface of a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) on gold. Lipid membranes with different SOPS/cholesterol compositions were deposited on the PDDA/MUA layer pair by vesicle fusion. When the cholesterol content was below 20%, single bilayers were deposited. Fluorescence recovery after the bleaching experiments revealed that the SOPS/cholesterol bilayers were mobile at room temperature; lateral diffusion coefficients of a fluorescence probe were approximately 1x10(-9) cm(2)/s. The kinetics of the addition of the ion-channel-forming peptide gramicidin to the supported bilayers was detected by SPR. Copyright 2000 Academic Press.

Dental OCT

We present here the first in vivo optical coherence tomography (OCT) images of human dental tissue. A novel dental optical coherence tomography system has been developed. This system incorporates the interferometer sample arm and transverse scanning optics into a handpiece that can be used intraorally to image human dental tissues. The average imaging depth of this system varied from 3 mm in hard tissues to 1.5 mm in soft tissues. We discuss the application of this imaging system for dentistry and illustrate the potential of our dental OCT system for diagnosis of periodontal disease, detection of caries, and evaluation of dental restorations.

Studies on the Diffusion of Molecules through Pores in Monolayer and Multilayer Films

A one-parameter model for species flux through pores in a crystalline monolayer or multilayer film is developed. The model is based on surface diffusion of the species on the upstream surface of the monolayer and one-directional diffusion of the species into the pores of the monolayer. It is found that the Fickian diffusivity depends on the concentration of adsorbed species on surface sites and on the pore coverage. The model contains an important parameter, the rate constant of activation, which is function of the diffusing species and film surface properties. The model is compared with experimental data on ion and gas flux through single and multilayer films. Good agreement between theory and experiment is found with all sets of the data with the rate constant as a sole adjustable parameter. Copyright 1998 Academic Press.

Imaging of hard- and soft-tissue structure in the oral cavity by optical coherence tomography

We have developed a prototype optical coherence tomography (OCT) system for the imaging of hard and soft tissue in the oral cavity. High-resolution images of in vitro porcine periodontal tissues have been obtained with this system. The images clearly show the enamel-cementum and the gingiva-tooth interfaces, indicating OCT is a potentially useful technique for diagnosis of periodontal diseases. To our knowledge, this is the first application of OCT for imaging biologic hard tissue.