Gold-coated Atomic Force Microscope Tip Research Articles

Conformational changes of a membrane protein determined by infrared difference spectroscopy beyond the diffraction limit

Optogenetics is a revolutionary method for studying neural activity by exploiting the optical stimulation of neurons made possible by artificial genetic expression of light-sensitive transmembrane ``gate'' proteins. Evidence of functional conformational changes of the gate proteins can be obtained by difference infrared (IR) absorption spectroscopy triggered by light stimuli. Here we investigate the effect on the photocycle of the prototype optogenetic protein channelrhodopsin (ChR2) of gold surfaces placed in close proximity to the protein molecules. In order to do this, we bring difference IR spectroscopy to the nanoscale, using a platform based on the coupling of a tunable mid-IR quantum cascade laser and an atomic force microscope (AFM). Sensitivity to individual subwavelength-sized membrane patches, embedding fewer than a hundred ChR2 molecules, is achieved by taking advantage of a plasmonic field enhancement in the nanogap between the gold-coated AFM tip and an ultraflat gold surface used as a sample support. We obtain relative difference absorption variations smaller than ${10}^{\ensuremath{-}2}$ and benchmark nanospectroscopy difference spectra against those taken with the Fourier transform IR (FTIR) spectroscopic technique on the same sample. We identify distinct simultaneous processes in the ChR2 photocycle by performing singular value decomposition of the FTIR difference spectra, and use this procedure to compare IR nanospectroscopy data on individual membrane patches in contact with gold surfaces with those obtained on thick stacks of membrane patches unexposed to metal surfaces. ChR2 proteins maintain their gate function when placed in a 14-nm-wide gap between two gold surfaces, apart from minor modifications to their kinetics. Our results are relevant to optogenetic applications that require physical contact between nanoscale metallic probes and electrodes and the membrane of single neuronal cells. More generally, our work paves the way towards the spectroscopic study of transmembrane proteins at the nanoscale.

Analysis of acid-base interactions at Al2O3 (11-20) interfaces by means of single molecule force spectroscopy

Single molecule force spectroscopy (SMFS) was employed to investigate the interaction forces between aliphatic amino, hydroxyl and ether groups and aluminum oxide single crystal surfaces in an aqueous electrolyte at pH=6. The force studies were based on the variation of the terminal group of polyethylene glycol which was bound via a SAu bond to the gold coated AFM tip. X-ray Photoelectron Spectroscopy (XPS) was performed to characterize the surface chemistry of the substrate. Force distance curves were measured between the PEG-NH2, OH and OCH3 functionalized atomic force microscope (AFM) tip and the non-polar single crystalline Al2O3(11-20) surface. The experimental results exhibit non-equilibrium desorption events which hint at acid-base interactions of the electron donating hydroxyl and amino groups with Al-ions in the surface of the oxide. The observed desorption forces for the NH2, OH/Al2O3(11-20) were in the range of 100–200pN.

The effect of switchable additives on colloidal interactions found in oil sands as measured by chemical force spectrometry

After oil sands separations, settling of clays from aqueous tailings can be promoted by additives such as Ca2+ salts. However, if the liberated water is then recycled, these same additives in the water interfere with bitumen recovery in the separator. Therefore, we have tested CO2-triggered switchable additives to see whether they can switch back and forth between a form that is suitable for the separation stage and a form that promotes tailings ponds settling. CO2-triggered switchable additives can reversibly change water chemistry merely by introduction and removal of CO2, a benign trigger. Here, the effects of CO2-mediated switchable additives on colloidal interactions found in model oil sands were studied by chemical force spectrometry. Self-assembled monolayers of 12-phenyldodecanethiol and 12-mercaptododecanoic acid were used to chemically modify gold-coated atomic force microscope tips. These were subsequently used to study the adhesion force between the modified tips and the minerals silica and mica. The adhesion between the tips and the mineral substrates was studied in aqueous solutions of varying pH and divalent cation concentration and in the presence of cationic switchable additives of varying surfactant potency, both in the presence and in the absence of CO2. In the presence of CO2, the best additive promotes attractive forces, while in the absence of CO2, the forces are repulsive. These results are discussed in the context of the mechanism of colloidal interactions in an oil sands system.

Measurement of thermopower and current-voltage characteristics of molecular junctions to identify orbital alignment

We report an experimental technique that concurrently measures the Seebeck coefficient and the current-voltage (I-V) characteristics of a molecular junction to determine the identity and the effective energetic separation of the molecular orbital closest to the electrodes’ Fermi level. Junctions created by contacting a gold-coated atomic force microscope tip with a monolayer of molecules assembled on a gold substrate were found to have a Seebeck coefficient of (+16.9±1.4) μV/K. This positive value unambiguously shows that the highest occupied molecular orbital (HOMO) dominates charge transport. Further, by analyzing the (I-V) characteristics, the HOMO level is estimated to be ∼0.69 eV with respect to the Fermi level.

Force Spectroscopy on Single Polymer Incorporated into Polymer Gels

We conducted the extraction experiments of single polymer incorporated into hydrogels with an atomic force microscope (AFM) as a model for investigating nonspecific intermolecular interactions between macromolecules in a semidilute region at the single molecule level. Small amount of poly(ethylene glycol) (PEG) terminated with a thiol group was inserted in poly(acrylamide) gels, and a part of PEG polymer segments on the gel surface was attempted to pull out of the gels with a gold-coated AFM tip. The observed force-distance curves were classified into two kinds of extraction force profiles: a plateau force, which is almost constant irrespective of the tip-surface distance and a nonlinear force, which nonlinearly increases with the extraction length. Characteristic interaction length, L, and force, F, of these extraction force profiles were measured with changing the crosslinker concentration of gels which strongly affects the network structures. As a result, L of these extraction profiles significantly decreased at crosslinker concentrations higher than a standard one at which most gels have been prepared for investigating their physical properties. On the other hand, F showed no obvious difference on the change in crosslinker concentrations both on the plateau and the nonlinear force profiles. The origin of the observed forces was discussed in terms of gel network structures.

STEP-BY-STEP SURFACE ASSEMBLY

MOLECULAR MANUFACTURING'S ultimate goal of assembling molecular machines atom by atom may be a long way off, but a research team in Europe has come a step closer to assembling structures molecule by molecule. In an elegant meeting of chemistry and mechanics at the single-molecule level, Anne-Sophie Duwez, a materials scientist at Belgium's University of Liege, and colleagues use an atomic force microscope (AFM) to covalently attach individual molecules to a surface, making and breaking each bond individually ( Nat. Nanotechnol ., DOI: 10.1038/nnano.2006.92). The ultimate physical limit in the design and fabrication of organic surfaces can be reached using this approach, according to Duwez. To achieve their mechanosynthesis, Duwez' group grafts poly- N -succinimidyl acrylate onto a gold-coated AFM tip. They then carefully bring this modified AFM tip into contact with a silicon surface functionalized with primary amines. A nucleophilic amine attacks a reactive carbonyl on the polymer, forming an amide lin...

Electrical nanoprobing of semiconducting carbon nanotubes using an atomic force microscope.

We use an atomic force microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p region, with no Schottky barrier. In the n region, large contact resistances were found which dominate the transport properties.

Effect of molecular structure of self-assembled monolayers on their tribological behaviors in nano- and microscales

The tribological behavior of self-assembled monolayers (SAMs) of ω-functional n-alkanethiol compounds were experimentally investigated at the nanoscale using friction force microscopy (FFM) and at the microscale using a microtribometer. Molecularly thin and well-ordered films of the surfactants with different terminal groups and chain length were prepared on gold substrates, gold-coated atomic force microscope (AFM) tips and steel balls by the spontaneous adsorption of the surfactant molecules. Nanoscale adhesion and friction forces between the films and gold-coated AFM tip were measured by FFM whereas microscale friction and wear properties between the films and gold-coated steel balls and uncoated steel balls were measured by the microtribometer. The nano- and microscale tribological behaviors between the films are compared and discussed in terms of the chemical structure such as terminal group and chain length of the film molecules. This investigation revealed that the molecular structure of the films significantly affects their tribological behaviors both in nano- and microscales.

Chain Length and Concentration Dependence of β-Cyclodextrin−Ferrocene Host−Guest Complex Rupture Forces Probed by Dynamic Force Spectroscopy

The rupture forces of individual s-cyclodextrin (s-CD)-ferrocene host-guest complexes in an aqueous medium were measured by dynamic single molecule force spectroscopy using an atomic force microscope (AFM). The thiol-derivatized ferrocene guest was immobilized in self-assembled monolayers on gold-coated AFM tips, while the heptasulfide s-CD host was self-assembled onto atomically flat Au(111) substrates. The effects of the alkyl spacer length of the ferrocene adsorbates, the relative concentration of ferrocene in the mixed monolayer on the AFM tip, and the unloading rate on the observed molecular unbinding events were studied. Depending on the concentration of ferrocene moieties on the AFM tip, multiple or predominantly single pull-off events were observed. A statistical analysis showed that the observed rupture forces are integer multiples of one fundamental force quantum of 55 ± 10 pN, which is attributed to the rupture of a single host-guest complex. This force quantum is found to be independent of the number of interacting host-guest pairs, independent of the spacer length, and independent of the unloading rate. These results indicate that the host-guest complex rupture forces were probed under conditions of thermodynamic equilibrium.

Fabrication of gold nanowires on insulating substrates by field-induced mass transport

A method for the fabrication of nanometer size gold wires on insulating surfaces is presented. An oscillating gold-coated atomic force microscope tip is brought into close proximity of a silicon dioxide surface. The application of a negative sample voltage produces the transport of gold atoms from the tip to the surface. The voltage is applied when there is a tip–surface separation of ∼3 nm. The finite tip–surface separation enhances the tip lifetime. It also allows the application of sequences of multiple voltage pulses. Those sequences allow the fabrication of continuous nanowires. The atomic force microscope gold deposition is performed at room temperature and in ambient conditions which makes the method fully compatible with standard lithographic techniques. Electron transport measurements of the wires show a clear metallic behavior. Electrical resistivities of ∼3×10−7 Ω m and current densities of up to 5×1011 A m−2 are reported.

An apertureless near-field scanning optical microscope and its application to surface-enhanced Raman spectroscopy and multiphoton fluorescence imaging

We describe a home-built apertureless near-field scanning optical microscope and present preliminary results of its operation. Raman scattering from samples of polydiacetylene para-toluene sulphonate, and two-photon-induced fluorescence from crystallites of coumarin I dye are strongly enhanced in the presence of a sharp gold-coated atomic force microscope tip. We verify the dependence of the scattered intensity on the polarization of the incident beam relative to the tip axis. Finally, we show near-field fluorescence images taken in the presence of a strong far-field background whose spatial resolution is limited by the size of the tip.

Locally enhanced Raman spectroscopy with an atomic force microscope

An atomic force microscope (AFM) tip has been used to selectively produce surface enhanced Raman scattering (SERS) for localized Raman spectroscopy. Spectra of thin films, undetectable with a Raman microprobe spectrometer alone, were readily acquired in contact with a suitably gold-coated AFM tip. Similarly, an AFM tip was used to remove sample layers at the nanometer scale and subsequently served as a SERS substrate for ultratrace analysis. This work demonstrates the interface of an AFM with a Raman spectrometer that provides increased sensitivity, selectivity, and spatial resolution over a conventional Raman microprobe. An AFM guiding the SERS effect has the potential for targeted single molecule spectroscopy.

A novel cleaning method of gold-coated atomic force microscope tips for their chemical modification

For chemical modification of gold-coated AFM tips with thiol or sulfide compounds, a new two-step precleaning procedure was studied. The two-step cleaning procedure involves (i) oxidation of organic contaminants on the AFM tips with ozone treatment and (ii) reduction of the oxidized gold surface by immersing the oxidized tip into pure hot ethanol at ca. 65°C. The chemically modified tips prepared from gold-coated AFM tips precleaned by the two-step procedure gave almost the same tip characteristics as those chemically modified immediately after gold vapor deposition in a factory. The present two-step cleaning procedure can be used widely for chemical modification of commercially available gold-coated AFM tips with thiol or disulfide compounds for chemical force microscopy.