AC Mode Atomic Force Microscopy Research Articles

Adsorption Behavior of Perfluorosulfonic Acid Ionomers on a Pt(111) Surface Observed By Atomic Force Microscopy

Understanding of the adsorbed structure of ionomers on the Pt electrocatalyst and carbon support surfaces are very important to improve the cell performance of polymer electrolyte membrane fuel cells (PEMFCs). The effect of the solvent composition and temperature on the adsorbed structure of ionomers have been studied by using various techniques, such as 19F nuclear magnetic resonance (19F NMR) spectroscopy, cryogenic scanning electron microscopy (cryo-SEM), dynamic light scattering (DLS), small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and electron spin resonance (ESR) techniques. However, it has not been fully clarified yet, due to the complexity of the system composed of a variety of ingredients including the Pt electrocatalyst, carbon support, ionomers, solvent and other contaminants. Thus, we have investigated the adsorbed structure of ionomers on the Pt(111) surface as a model of an electrocatalyst by using atomic force microscopy (AFM) at various solvent compositions. A droplet of aqueous solutions of Nafion was placed on Pt (111) surfaces mounted on the AFM sample holder. AFM measurements were performed with Acoustic AC mode AFM using Agilent 5500 AFM microscope (Agilent Technologies, USA) with a silicon nitride cantilever. The images were recorded over a period of ~30 minutes with a scanning area of 1 µm × 1 µm. In a 2 wt% aqueous solution of Nafion, relatively small aggregates of Nafion ionomers with a height of ~2-3 nm were observed at the Pt(111) surface. Amount of adsorption increased with time and, eventually, the surface was entirely covered by randomly oriented aggregates with a thickness of ~4 nm. In a 0.2 wt% aqueous solution, aggregates larger than those observed in the 2 wt% aqueous solution were imaged, suggesting that larger aggregates were formed in the lower concentration solution. Such a concentration-dependent adsorption behavior is further discussed on the basis of solvent composition and conformation of Nafion ionomers in liquid phase. References Masuda, H. Naohara, S. Takakusagi, P. R. Singh, K. Uosaki: Chemistry Letters, 38, 884 (2009).Masuda, S. Faridah, P. Singh, H. Naohara, K. Uosaki, Journal of Physical Chemistry C, 117, 15704 (2013).Masuda, K. Ikeda, K. Uosaki, Langmuir, 29, 2420 (2013).

Nano and micro scale analysis of dentin with in vitro and high speed atomic force microscopy

Abstract

Physical and Chemical Analysis of Elemental Sulfur Formation during Galena Surface Oxidation

The surface oxidation of sulfide minerals, such as galena (PbS), in aqueous solutions is of critical importance in a number of applications. A comprehensive understanding of the formation of oxidation species at the galena surface is still lacking. Much controversy over the nature of these oxidation products exists. A number of oxidation pathways have been proposed, and experimental evidence for the formation of elemental sulfur, metal polysulfides, and metal-deficient lead sulfides in acidic conditions has been shown and argued. This paper provides further insight into the electrochemical behavior of galena at pH 4.5. Utilizing a novel experimental system that combines in situ electrochemical control and AC mode atomic force microscopy (AFM) surface imaging, the formation and growth of nanoscopic domains on the galena surface are detected and examined at anodic potentials. AFM phase images indicate that these domains have different material properties to the underlying galena. Continued oxidation results in nanoscopic pitting and the formation of microscopic surface domains, which are confirmed to be elemental sulfur by Raman spectroscopy. Further clarification of the presence of elemental sulfur is provided by Cryo-XPS. Polysulfide and metal-deficient sulfide could not be detected within this system.

The Effect of Surface Preparation on Apparent Surface pKa's of ω-Mercaptocarboxylic Acid Self-Assembled Monolayers on Polycrystalline Gold

We have measured the apparent surface pKa values of self-assembled monolayers (SAMs) formed from 3-mercaptopropionic acid (3-MPA) and thioctic acid (TA) on evaporated gold film electrodes with a range of surface roughness (Ra from 1.3 to 6.3 nm as measured by AC mode atomic force microscopy). The surface roughness is induced and controlled with electrochemical etching via multiple potential cycles between 0.2 and 1.5 V (versus Ag/AgCl/1.0 M KCl) in 100 mM H2SO4 with KCl concentrations ranging from 0.10 to 10 mM. The pKa's are measured by determining the capacitance of the surface using electrochemical impedance spectroscopy in a range of controlled ionic strength (μ = 0.25 M) buffer solutions. The results for 3-MPA are striking, exhibiting a strong logarithmic correlation (R2 = 0.988) between the KCl concentration used in the electrochemical etching solutions and the apparent surface pKa. Over the range of KCl concentrations, the surface pKa of 3-MPA shifts from 6.5 to 8.4. However, the relationship betwe...

Cantilever dynamics and quality factor control in AC mode AFM height measurements

We show that inconsistent-imaging dynamics, in which the cantilever oscillates in the attractive regime on substrate background but in the repulsive regime on sample, leads to artifacts in apparent height in AC mode Atomic force microscopy. Active Q control can be used to effectively tune the imaging dynamics. Increased effective Q promotes the attractive regime, improves imaging sensitivity, and results in less invasive imaging of soft biological molecules.

Adsorption of synthetic homo- and hetero-oligodeoxynucleotides onto highly oriented pyrolytic graphite: Atomic force microscopy characterization

DNA adsorption on electrode surfaces is of fundamental interest for the development of DNA-based biosensors. The free adsorption of 10-mer synthetic oligodeoxynucleotides (ODNs) onto highly oriented pyrolytic graphite (HOPG) surfaces was studied using Magnetic AC mode atomic force microscopy (MAC Mode AFM). The mechanism of interaction of nucleic acids with carbon electrode surfaces was elucidated, using 10-mer synthetic homo- and hetero-ODNs sequences of known base sequences, because they allow clear interpretation of the experimental data. AFM images in air revealed different adsorption patterns and degree of HOPG surface coverage for the ODNs, and correlation with the individual structure and base sequence of each ODN molecule will be presented. The results demonstrated that the hydrophobic interactions with the HOPG hydrophobic surface explain the main adsorption mechanism, although other effects such as electrostatic and Van der Waals interactions may contribute to the free adsorption process. The ODNs interacted differently with the HOPG surface, according to the ODN sequence hydrophobic characteristics, being directly depending on the molecular mass, the hydrophobic character of the individual bases and on the secondary structure of the molecule. The importance of the type of base existent at the ODN chain extremities on the adsorption process was investigated and different adsorption patterns were obtained with ODN sequences composed by the same group of bases aligned in a different order.

Cell volume increase in murine MC3T3-E1 pre-osteoblasts attaching onto biocompatible Tantalum observed by magnetic AC mode Atomic Force Microscopy

Magnetic AC mode (MACmode) atomic force microscopy (AFM) was used to study murine (mouse) MC3T3-E1 preosteoblastic cells attached to biocompatible tantalum substrates. Cell volumes of attached cells derived from AFM images were compared to volumes of detached cells in suspension measured by the Coulter sizing technique. An increase of approximately 50% in cell volume was observed when the cells attached to planar tantalum substrates and developed a flattened structure including lamellipodia. We address thoroughly the issues general to the AFM determination of absolute cell volumes, and compare our magnetic AC mode AFM measurements to hitherto reported cell volume determinations by contact mode AFM.

Atomic Force Microscopy of DNA Immobilized onto a Highly Oriented Pyrolytic Graphite Electrode Surface

Magnetic AC mode atomic force microscopy (MAC Mode AFM) was used to characterize the process of adsorption of DNA on a highly oriented pyrolytic graphite (HOPG) electrode surface using different concentrations of DNA and adsorption procedures. AFM of DNA immobilized on the HOPG showed that both single-stranded DNA and double-stranded DNA molecules have the tendency to spontaneously self-assemble from solution onto the solid support and the process was very fast. DNA condensed on the substrate in a tight and well-spread two-dimensional lattice covering the entire surface uniformly. The interaction of DNA with the hydrophobic HOPG surface induced DNA superposition, overlapping, and intra- and intermolecular interactions. The application of a positive potential of 300 mV (vs Ag wire) to the HOPG electrode during adsorption was studied. The applied potential considerably enhanced the robustness and stability to mechanical stress of the DNA films, through multiple electrostatic interactions between the negatively charged hydrophilic sugar-phosphate backbone and the positively charged carbon surface. The characteristics of the DNA films and the apparent height of the network wires were dependent on the DNA concentration and the immobilization procedure. The DNA lattices were held together on the substrate surface only by noncovalent interactions such as hydrogen bonding, base stacking, electrostatic, van der Waals, and hydrophobic interactions.

Lipid Phase Dependence of DNA−Cationic Phospholipid Bilayer Interactions Examined Using Atomic Force Microscopy

Magnetic AC mode atomic force microscopy was used to study DNA adsorption onto supported cationic phospholipid (CL) bilayers in the fluid (L α ) and gel (L β ) phases. A L α phase CL bilayer was used as a model system to study DNA-CL complex formation at the single molecule level. Pure and mixed bilayers of dioleoyltrimethylammonium propane (DOTAP), dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylcholine (DOPC), and dipalmitoyltrimethylammonium propane (DPTAP) were investigated. The adsorption of small oligodeoxynucleotide DNA (14 base pairs) and large (1000-3000 base pairs) was examined and revealed that the behavior of the DNA-CL complex depends on the length and density of DNA. Once adsorbed to the bilayer, large DNA induced changes in the bilayer coverage of the mica support, while short DNA did not destabilize the bilayer. There was a strong dependence of the bilayer destabilization on the type of neutral helper lipid mixed with the cationic lipid, with DOTAP/DOPE being much less stable to DNA adsorption than DOTAP/DOPC. The characteristics of pure and mixed DOTAP, DOPE, and DOPC supported phospholipid bilayers were investigated to understand the stability changes induced by DNA. Images of the DNA-free phospholipid bilayer revealed the dependence of supported planar bilayer structure and stability on the chemical nature of the headgroup and on the phase of the bilayer. Finally, it was illuminating to image DNA on DPTAP bilayers as a function of temperature. When the system was examined above the L α -L β phase transition, images similar to those recorded for DNA-DOTAP (L α ) and DNA-DOTAP/DOPC (L α ) were observed.

Force interaction in low-amplitude ac-mode atomic force microscopy:. cantilever simulations and comparison with data from Si(111)7×7

Force interaction in low-amplitude ac-mode atomic force microscopy:. cantilever simulations and comparison with data from Si(111)7×7

Inequivalent atoms and imaging mechanisms in ac-mode atomic-force microscopy of Si(111)7 x 7.

ac-mode atomic-force microscopy (AFM) has been used to image the Si(111)7\ifmmode\times\else\texttimes\fi{}7 reconstruction. The corner holes and adatoms in the 7\ifmmode\times\else\texttimes\fi{}7 unit cell as well as isolated atomic defects are clearly resolved. In addition, we observe a contrast between inequivalent adatoms, the center adatoms appearing 0.13 \AA{} higher than the corner adatoms. We show that AFM does not image the true atom positions nor the charge density in the dangling bonds. Rather, our data suggest that the contrast is due to a variation in the chemical reactivity of the adatoms or to a tip-induced atomic-relaxation effect reflecting the stiffness of the surface lattice.