Supported lipid bilayer self-spreading on a nanostructured silicon surface

We report on the self-spreading behavior of a supported lipid bilayer (SLB) on a silicon surface with various 100 nm nanostructures. SLBs have been successfully grown from a small spot of a lipid molecule source both on a flat surface and uneven surfaces with 100 nm up-and-down nanostructures. After an hour, the self-spreading SLB forms a large circle or an ellipse depending on the nanostructure pattern. The results are explained by a model that shows that a single-layer SLB grows along the nanostructured surfaces. The model is further supported by a quantitative analyses of our data. We also discuss the stability of the SLB on nanostructured surfaces in terms of the balance between its bending and adhesion energies.     

Microchannel device using self-spreading lipid bilayer as molecule carrier

We propose a microchannel device that employs a surface-supported self-spreading lipid bilayer membrane as a molecule carrying medium. The device has a micropattern structure fabricated on a SiO2 surface by photolithography, into which a self-spreading lipid bilayer membrane is introduced as the carrier medium. This system corresponds to a microchannel with a single lipid bilayer membrane height of approximately 5 nm, compared with conventional micro-fluidic channels that have a section height and width of at least several microm. The device is beneficial for detecting intermolecular interactions when molecules carried by the self-spreading lipid bilayer collide with each other in the microchannel. The validity of the device was confirmed by observing the fluorescence resonance energy transfer (FRET) between two dye molecules, coumarin and fluorescein.     

Control of dynamics and molecular distribution in a self-spreading lipid bilayer using surface-modified metal nanoarchitectures

The molecular distribution and spreading dynamics of self-spreading lipid bilayers can be tuned by surface-modified metallic nanoarchitectures. Interactions between lipids and molecules in the surface modification alter the self-spreading behavior at the gate regions between adjacent nanoarchitectures, leading to molecular filtering/concentrating effects and modification of the dynamics. The hydrophilic surface can tune the spreading velocity without changing the molecular distribution in the spreading bilayer, whereas the hydrophobic surface provides a molecular concentrating function to the nanogates. This indicates that a combination of unmodified/hydrophobic/hydrophilic nanoarchitectures has a wide range of potential applications since it can be used to independently control the self-spreading dynamics and the molecular distribution.     

Tuning the dynamics and molecular distribution of the self-spreading lipid bilayer

The self-spreading dynamics of lipid bilayers were investigated at controlled electrolyte concentrations. The self-spreading velocity increased when the concentration of NaCl was increased from 1 to 100 mM. Comparing the experimentally determined spreading energy with that estimated from theoretical models, we found that the self-spreading dynamics were well explained by considering the van der Waals interaction, double layer interaction and hydration interaction energies between the self-spreading bilayer and the substrate. The characteristic behavior at high concentration is attributable to the increase in the density of the lipid layer, originating from the effective shielding of the molecular charges by the electrolyte ions in solution. The distribution of doped dye-labeled molecule within the spreading bilayer was also controllable by tuning the electrolyte concentration. All of these findings were explained by systematic changes in bilayer-substrate or inter-molecular interactions depending on the electrolyte concentration.     

Controlling molecular diffusion in self-spreading lipid bilayer using periodic array of ultra-small metallic architecture on solid surface

Diffusion of target molecules incorporated in the self-spreading lipid bilayer was controlled by the introduction of periodic array of metallic architecture on solid surface. Retardation of the progress of target molecules became significant when the size of gap between small metal architectures was less than a few hundred nanometers. The self-spreading dynamics of the lipid bilayer depending on the size of the small gap were analyzed semiquantitatively. Estimated change in the driving force of the spreading layer suggests that highly localized compression of the spreading layer causes selective segregation of molecules.     

Control of the structure of self-spreading lipid membrane by changing electrolyte concentration

We have controlled the structure of self-spreading lipid bilayer membranes prepared on surface-oxidized silicon substrates by changing electrolyte concentration. Analysis of the fluorescence intensity, considering the optical interference effect, clarified the stacking structure of the lipid membrane. By varying the electrolyte concentration, we can vary the number of single multilamellar lobes adsorbed on the underlying self-spreading bilayer. This dependence of the stacking ability on the electrolyte concentration was investigated on the basis of changes in the bilayer-lobe interaction energies, including van der Waals, electrostatic double layer, and hydration interaction energies. Theoretical estimation suggests that the observed electrolyte concentration dependence can be explained by the combination of the van der Waals attractive interaction energy and the repulsive double-layer interaction energy.     

Nanoparticle-enhanced sensitivity of a nanogap-interdigitated electrode array impedimetric biosensor

Interdigitated electrode (IDE) arrays with nanometer-scale gaps have been utilized to enhance the sensitivity of affinity-based detection. The geometry of nanogap IDEs was first optimized on the basis of simulations of the electric field and current density. It was determined that the gap (G) between the electrodes was the most important geometric parameter in determining the distribution and strength of the electric field and the current density compared to the width (W) and height (H) of the IDEs. Several devices were materialized and analyzed for their sensitivity to the electrochemical environment using faradic electrochemical impedance spectroscopy (EIS) as the detection technique. Nanogap optimized IDEs were then employed as biosensors for the label-free, affinity-based detection of antitissue transglutaminase antibodies (αtTG-Abs), a biomarker for the detection of autoimmune disorder celiac sprue, triggered by ingesting gluten. The label-free biosensor assay was found to be less sensitive compared to on-chip ELISA. Gold nanoparticles (GNPs) were then employed to improve the sensitivity of the nanogap IDE-based biosensor. With GNPs, the transducer sensitivity increased by 350% over that of label-free detection. The suitability of nanogap IDEs as biosensor transducers for EIS in label-free and GNP-labeled formats was established. The immunobiosensor assay detection sensitivity with the GNPs was found comparable to ELISA.     

Efficient immobilization of silver nanoparticles on metal substrates through various thiol molecules to utilize a gap mode in surface enhanced Raman scattering

To utilize a gap mode in surface enhanced Raman scattering, we elucidated the interaction between adsorbed species and Ag nanoparticles (AgNPs). Various thiol molecules such as normal alkanethiols, thiols with a phenyl, cyclohexane or naphthalene ring on Ag films immobilized AgNPs through van der Waals force, and electrostatic interaction. Immobilized AgNPs provided enormous Raman enhancement by a factor of 10⁷–10¹⁰ for thiol molecules at a nanogap, in consistent with that anticipated by finite difference time domain calculations. Only alkanethiols with a tert-methyl group and those with a carboxylic group did not immobilize any AgNPs probably owing to steric hindrance. A gap mode is relevant for a variety of metals even with large damping like Pt and Fe, indicating a crucial role of electric multipoles in AgNPs generated by a localized surface plasmon and induced mirror images in metal substrates for markedly enhanced electric field at a nanogap.     

Supported lipid bilayer composition microarray fabricated by pattern-guided self-spreading

We report on the fabrication of a microarray of supported lipid bilayers (SLBs) with different chemical compositions and demonstrate its biosensing application. The technique utilizes the phenomenon of lipid self-spreading on a patterned surface, which offers complete positional selectivity for a supported lipid bilayer. We describe the fabrication of parallel 10-μm-wide lines, each filled with an SLB with a unique composition, at 5 μm intervals. Structures obtained with our new technique are finer and more highly integrated than previously reported structures that employ the vesicle fusion technique on patterned surfaces. We also detected specific binding between biotin and streptavidin with high contrast, indicating that the microarray is valuable for biosensing applications.     

Raman scattering of 4-aminobenzenethiol sandwiched between Ag nanoparticle and macroscopically smooth Au substrate: effects of size of Ag nanoparticles and the excitation wavelength

A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). Accordingly, although no Raman signal is observable when 4-aminobenzenethiol (4-ABT), for instance, is self-assembled on a flat Au substrate, a distinct spectrum is obtained when Ag or Au nanoparticles are adsorbed on the pendent amine groups of 4-ABT. This is definitely due to the electromagnetic coupling between the localized surface plasmon of Ag or Au nanoparticle with the surface plasmon polariton of the planar Au substrate, allowing an intense electric field to be induced in the gap even by visible light. To appreciate the Raman scattering enhancement and also to seek the optimal condition for SERS at the nanogap, we have thoroughly examined the size effect of Ag nanoparticles, along with the excitation wavelength dependence, by assembling 4-ABT between planar Au and a variable-size Ag nanoparticle (from 20- to 80-nm in diameter). Regarding the size dependence, a higher Raman signal was observed when larger Ag nanoparticles were attached onto 4-ABT, irrespective of the excitation wavelength. Regarding the excitation wavelength, the highest Raman signal was measured at 568 nm excitation, slightly larger than that at 632.8 nm excitation. The Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time domain simulation. It is noteworthy that placing an Au nanoparticle on 4-ABT, instead of an Ag nanoparticle, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation, suggesting the importance of the localized surface plasmon resonance of the Ag nanoparticles for an effective coupling with the surface plasmon polariton of the planar Au substrate to induce a very intense electric field at the nanogap.     

Second-harmonic spectroscopy of surface immobilized gold nanospheres above a gold surface supported by self-assembled monolayers

We have investigated linear and nonlinear optical properties of surface immobilized gold nanospheres (SIGNs) above a gold surface with a gap distance of a few nanometers. The nanogap was supported by amine or merocyanine terminated self-assembled monolayers (SAMs) of alkanethiolates. A large second-harmonic generation (SHG) was observed from the SIGN systems at localized surface plasmon resonance condition. The maximum enhancement factor of SHG intensity was found to be 3 x 10(5) for the SIGN system of nanospheres 100 nm in diameter with a gap distance of 0.8 nm. The corresponding susceptibility was estimated to be chi((2))=750 pmV (1.8 x 10(-6) esu). In the SIGN system supported with the merocyanine terminated SAMs, the SHG response was also resonant to the merocyanine in the nanogap. It was found that the SHG response of the SIGN systems is strongly frequency dependent. This leads us to conclude that the large chi((2)) is caused by enhanced electric fields at the localized surface plasmon resonance condition and is not due to an increase of the surface susceptibility following from the presence of the gold nanospheres. The observed SHG was consistent with the theoretical calculations involving Fresnel correction factors, based on the quasistatic approximation.     

Frequency dependence of gold nanoparticle superassembly by dielectrophoresis

Dielectrophoresis is an effective method for capturing nanoparticles and assembling them into nanostructures. The frequency of the dielectrophoretic alternating current (ac) electric field greatly influences the morphology of resultant nanoparticle assemblies. In this study, frequency regimes associated with specific gold nanoparticle assembly morphologies were identified. Gold nanoparticles suspended in water were captured by microelectrodes at different electric field frequencies onto thin silicon nitride membranes. The resultant assemblies were examined by transmission electron microscopy. For this system, the major frequency-dependent influence on morphology appears to arise not from the Clausius-Mossotti factor of the dielectrophoretic force itself, but instead from ac electroosmotic fluid flow and the influence of the electrical double layer at the electrode-solution interface. Frequency regimes of technological interest include those forming one-dimensional nanoparticle chains, microwires, combinations of microwires and nanoparticle chains suitable for nanogap electrode formation, and dense three-dimensional assemblies with very high surface area.     

Interface characteristics of thin liquid films in a charged lubricated contact

The interface characteristics of liquid films confined within a nanogap between a steel ball and a smooth glass disk after exposing to an external electric field (EEF) are reported in the paper. The emergence of micro‐bubbles induced by the EEF has been found to be closely related with the electric current flow at both a positive EEF and a negative EEF. The polarity effect of the variation in the electric current flowing through a polar liquid film over time is more pronounced than that through a nonpolar liquid film. The surface of the contacting pair with the polar liquid film confined after the removal of the EEF is more severely damaged at a negative EEF, while no indication of damage has been observed on the surface contacting the nonpolar liquid film at either polarity. The bubbles generated under a positive EEF can collapse more readily than those generated under a negative EEF. Different mechanisms of these experimental phenomena have been discussed in the paper. Copyright © 2014 John Wiley & Sons, Ltd.     

Functionalized Nanogap for DNA Read-Out: Nucleotide Rotation and Current-Voltage Curves

Functionalized nanogaps embedded in nanopores show a strong potential for enhancing the detection of biomolecules, their length, type, and sequence. This detection is strongly dependent on the features of the target biomolecules, as well as the characteristics of the sensing device. In this work, through quantum-mechanical calculations, we elaborate on representative such aspects for the specific case of DNA detection and read-out. These aspects include the influence of single DNA nucleotide rotation within the nanogap and the current-voltage (I-V) characteristics of the nanogap. The results unveil a distinct variation in the electronic current across the functionalized device for the four natural DNA nucleotides with the applied voltage. These also underline the asymmetric response of the rotating nucleotides on this applied voltage and the respective variation in the rectification ratio of the device. The electronic tunneling current across the nanogap can be further enhanced through the proper choice of an applied bias voltage. We were able to correlate the enhancement of this current to the nucleotide rotation dynamics and a shift of the electronic transmission peaks towards the Fermi level. This nucleotide specific shift further reveals the sensitivity of the device in reading-out the identity of the DNA nucleotides for all different configurations in the nanogap. We underline the important information that can be obtained from both the I-V curves and the rectification characteristics of the nanogap device in view of accurately reading-out the DNA information. We show that tuning the applied bias can enhance this detection and discuss the implications in view of novel functionalized nanopore sequencers.     

Fluidics of a nanogap

We have determined the filling properties of nanogaps with chemically heterogeneous walls. The quantitative criteria we present allow the prediction of the liquid loading of the nanostructure. They can easily be applied in combination with contact-angle measurements on planar substrates of the nanogap materials. We present an application of the theory to a recently developed nanogap biosensor. Chemical force microscopy (CFM) is employed to characterize the initial silanol properties of the gap. The functionality of the complex surface chemistry of the biosensor is demonstrated by the observation of functionalized nanoparticles in the gap with its resulting characteristic current-voltage relationship.     

Dielectrophoretic placement of quasi-zero-, one-, and two-dimensional nanomaterials into nanogap for electrical characterizations

DEP is one of promising techniques for positioning nanomaterials into the desirable location for nanoelectronic applications. In contrast, the lithography technique is commonly used to make ultra-thin conducting wires and narrow gaps but, due to the limit of patterning resolution, it is not feasible to make electrical contacts on ultra-small nanomaterials for a bottom-up device fabrication. Thus, integrating the lithography and dielectrophoresis, a real bottom-up fabrication can be achieved. In this work, the device with the nanogap in between two nanofinger-electrodes is made using electron-beam lithography from top down and the ultra-small nanomaterials, such as colloidal PbSe quantum dots, polyaniline nanofibers, and reduced-graphene-oxide flakes, are placed in the nanogap by DEP from bottom up. The threshold electric field for the DEP placement of PbSe nanocrystals was roughly estimated to be about 8.3 × 10(4) V/cm under our experimental configuration. After the DEP process, several procedures for reducing contact resistances are attempted and measurements of intrinsic electron transport in versatile nanomaterials are performed. It is experimentally confirmed that electron transport in both PbSe nanocrystal arrays and polyaniline nanofibers agrees well with Prof. Ping Sheng's model of granular metallic conduction. In addition, electron transport in reduced-graphene-oxide flakes follows Mott's 2D variable-range-hopping model. This study illustrates an integration of the electron-beam lithography and the DEP techniques for a precise manipulation of nanomaterials into electronic circuits for characterization of intrinsic properties.     

Experimental study of the creation of a surface-wave-sustained argon plasma column at atmospheric pressure

The creation and stabilization processes of an argon surface-wave-sustained discharge at atmospheric pressure have been studied by means of a power interruption technique. The analysis of the temporal behavior of the radial electric field intensity outside the plasma has permitted us to study the advance of the ionization front along the plasma column. However, the temporal evolution of the intensity of some atomic emission lines has also been analyzed during the creation process of the discharge. Some relevant results of this study are presented and discussed in terms of the ionization front and the stabilization of the electric field as well as the temporal evolution of excited level populations of the species.     

Dielectrophoretic trapping of DNA-coated gold nanoparticles on silicon based vertical nanogap devices

We report on the successful dielectrophoretic trapping and electrical characterization of DNA-coated gold nanoparticles on vertical nanogap devices (VNDs). The nanogap devices with an electrode distance of 13 nm were fabricated from Silicon-on-Insulator (SOI) material using a combination of anisotropic reactive ion etching (RIE), selective wet chemical etching and metal thin-film deposition. Au nanoparticles (diameter 40 nm) coated with a monolayer of dithiolated 8 base pairs double stranded DNA were dielectrophoretically trapped into the nanogap from electrolyte buffer solution at MHz frequencies as verified by scanning and transmission electron microscopy (SEM/TEM) analysis. First electrical transport measurements through the formed DNA-Au-DNA junctions partially revealed an approximately linear current-voltage characteristic with resistance in the range of 2-4 G? when measured in solution. Our findings point to the importance of strong covalent bonding to the electrodes in order to observe DNA conductance, both in solution and in the dry state. We propose our setup for novel applications in biosensing, addressing the direct interaction of biomolecular species with DNA in aqueous electrolyte media.     

Fabrication of a nano-scale gap by selective chemical deposition

An electrode nanogap of 45 nm has been prepared by a new method in which the initial gap of 1-2 microns obtained by conventional lithography was shortened by selective chemical deposition of copper onto the electrodes.