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40. Effects of Multiple-Bond Ruptures on Kinetic Parameters Extracted from Force Spectroscopy Measurements: Revisiting Biotin-Streptavidin Interactions. Senli Guo, Chad Ray, Andrea Kirkpatrick, Nimit Lad, and Boris B. Akhremitchev Biophys. J. 2008, BioFAST article, 10.1529/biophysj.108.133900.
Abstract
Force spectroscopy measurements of the rupture of the molecular bond between biotin and streptavidin often results in a wide distribution of rupture forces. We attribute the long tail of high rupture forces to the nearly simultaneous rupture of more than one molecular bond. To decrease the number of possible bonds we employed hydrophilic polymeric tethers to attach biotin molecules to the atomic force microscope (AFM) probe. It is shown that the measured distributions of rupture forces still contain high forces that cannot be described by the forced dissociation from a deep potential well. We employed a recently developed analytical model of simultaneous rupture of two bonds connected by polymer tethers with uneven length (Gu et al. 2008, J. Phys. Chem. C 112: 5085 -5092) to fit the measured distributions. The resulting kinetic parameters agree with the energy landscape predicted by molecular dynamics simulations. It is demonstrated that when more that one molecular bond might rupture during the pulling measurements there is a noise-limited range of probe velocities where the kinetic parameters measured by force spectroscopy correspond to the true energy landscape. Outside of this range of velocities the kinetic parameters extracted by using the standard most probable force approach might be interpreted as artificial energy barriers that are not present in the actual energy landscape. Factors that affect the range of useful velocities are discussed.
39. Pairwise interactions between linear alkanes in water measured by AFM force spectroscopy. Ray, C.; Brown, J. R.; Kirkpatrick, A.;Akhremitchev, B. B. J. Am. Chem. Soc. 2008, ASAP article, 10.1021/ja801568y.
Abstract
Pairwise interactions between n-alkanes
from decane to octadecane in water have been studied by single molecule force
spectroscopy. The interacting molecules are covalently tethered to the glass
substrate and to the probe of an atomic force microscope (AFM) by water-soluble
linkers to facilitate single-molecule detection. However the measured distribution
of rupture forces deviates significantly from the distribution predicted by
theoretical models for rupture of individual bonds. To describe the statistics
of rupture forces an analytical model that considers near-simultaneous rupture
of two bonds loaded by tethers with different lengths is introduced. The common
most probable force analysis approach is used for comparison. In both data
analyses the possible systematic errors due to non-linear elasticity of polymeric
tethers and variations in the shape of the potential of mean force were considered.
Experimental distributions of rupture forces are well-fit by the two-bond
rupture model using a single set of kinetic parameters for different experiments
while the most probable force approach yields parameters that vary significantly
for different samples. The measured activation energies for dissociation of
alkanes are close to the free energies predicted by cavity models of hydrophobic
interactions. The surface free energy density is estimated ~21 kJ/(mol sq.nm)
and is close to the upper limit of free energies used in the computer simulations
of hydrophobic interactions in proteins. In contrast to the predictions of
the cavity models, the measured activation energy does not increase monotonically
with increase in alkane chain size. To explain this discrepancy and the measured
distance to the transition state barrier (~0.6 nm) it is suggested that alkanes
undergo conformational transition to the collapsed state upon dimerization.
Change in the alkane conformation from extended to helical has been observed
previously for binding of alkanes in water to hydrophobic synthetic receptors.
Here, however conformational change is suggested without geometrical constraints
imposed by small cavitands. The proposed collapsed state of the alkane dimers
has implications for the kinetics of self-assembly of surfactant micelles.
38. Complexity of "A-a" knob-hole fibrin interaction revealed by AFM force spectroscopy. Averett, L. E.; Geer, C. B.; Fuierer, R. R.; Akhremitchev, B. B.; Gorkun, O. V.; Schoenfisch, M. H. Langmuir 2008, 24(9), 4979 -4988. link
Abstract
During blood vessel injury, fibrinogen
is converted to fibrin, a polymer that serves as the structural scaffold of
a blood clot. The primary function of fibrin is to withstand the large shear
forces in blood and provide mechanical stability to the clot, protecting the
wound. Understanding the biophysical forces involved in maintaining fibrin
structure is of great interest to the biomedical community. Previous reports
have identified the "A-a" knob-hole interaction as the dominant
force responsible for fibrin's structural integrity. Herein, biochemical force
spectroscopy is used to study knob-hole interactions between fibrin fragments
and variant fibrinogen molecules to identify the forces occurring between
individual fibrin molecules. The rupture of the "A-a" knob-hole
interaction results in a characteristic profile previously unreported in fibrin
force spectroscopy with two distinct populations of specific forces: 110 ±
34 and 224 ± 31 pN. In the absence of a functional "A" knob
or hole "a", these forces cease to exist. We propose that the characteristic
pattern represents the deformation of the D region of fibrinogen prior to
the rupture of the "A-a" knob-hole bond.
37. Effects of Multiple Bonds Rupture in Force Spectroscopy Measurements of Interactions between Fullerene C60 Molecules in Water. Gu, C.; Kirkpatrick, A.; Ray, C.; Guo, S.; Akhremitchev, B. B. J. Phys. Chem.-C 2008, 112(13), 5085 -5092, link.
Abstract
Interactions between fullerene C60 molecules in water were measured by force spectroscopy. Fullerene molecules were covalently connected to bifunctional water-soluble poly(ethylene glycol) (PEG) linkers and subsequently tethered to the substrate and to the tip of the atomic force microscope to facilitate single molecule detection and avoid spurious surface effects. The distributions of rupture forces for substrates prepared with different incubation times of C60-PEG-NH2 exhibit high rupture forces that cannot be explained by the theoretical distribution of single molecule binding. Moreover, the relative amplitude of the high force peak in the histogram increases with incubation time. These observations are explained by attributing the measured high forces to the rupture of multiple bonds between fullerene molecules. Force spectroscopy data analysis based on the most probable forces gives significantly different dissociation rates for samples that exhibit different amplitudes of the high force peak. An approximate analytical model that considers ruptures of two bonds that are simultaneously loaded by tethers with different lengths is proposed. This model successfully fits the distributions of the rupture forces using the same set of kinetic parameters for samples prepared with different grafting densities. It is proposed that this model can be used as a common tool to analyze the probability distributions of rupture forces that contain peaks or shoulders on the high force side of the distribution.
36. Surface Elastic Properties of Human Retinal Pigment Epithelium Melanosomes. Guo, S.; Hong, L.; Akhremitchev, B. B.; Simon, J. D. Photochemistry and Photobiology 2008, 84 (3) , 671–678, link
Abstract
Atomic force microscope (AFM) imaging and nanoindentation measurements in water were used to probe the mechanical properties of retinal pigment epithelium melanosomes isolated from 14-year-old and 76-year-old donors. Topographic imaging reveals surface roughness similar to previous measurements on dry melanosomes. Force-indentation measurements show different types of responses that were catalogued into four different categories. In these measurements no permanent surface damage of melanosomes was observed as revealed by imaging before and after indentation measurements. The indentation measurements that exhibited nearly elastic responses were used to determine the Young’s modulus of melanosomes. The average Young’s modulus values are similar for 14-year-old and 76-year-old melanosomes with a somewhat narrower distribution for the 14-year-old sample. These elastic modulus values are considerably higher than the modulus of organelles with cytoplasm (<1 MPa) and approaching values of the modulus of protein crystals (~100 MPa) indicating rather high packing density of biologic material in melanosomes. The width of the Young’s modulus distributions is considerable spanning from few megapascals to few tens of megapascals indicating large heterogeneity in the structure. A fraction of the force curves cannot be described by the homogeneous elastic sample model; these force curves are consistent with ~10 nm structural heterogeneity in melanosomes. The approach-withdraw hysteresis indicates a significant viscoelasticity, particularly in the samples from the 14-year-old sample. Adhesion of the AFM probe was detected on ~3% and ~20% of the surface of 14-year-old and 76-year-old samples, respectively. In light of previous studies on these same melanosomes using photoelectron emission microscopy, this adhesion is attributed to the presence of lipofuscin on the surface of the melanosomes. This suggestion indicates that part of the difference in photochemical properties between the old and young melanosomes originates from surface lipofuscin.
35. Investigation of Mechanical Properties of Insulin Crystals by Atomic Force Microscopy. Guo, S.; Akhremitchev, B. B. Langmuir 2008, 24(3), 880-887. link.
Abstract
Mechanical properties of protein crystals
and aggregates depend on the conformational and structural properties of individual
protein molecules as well as on the packing density and structure within solid
materials. An atomic force microscopy (AFM)-based approach is developed to
measure the elastic modulus of small protein crystals by nanoindentation and
is applied to measure the elasticity of insulin crystals. The top face of
the crystals deposited on mica substrates is identified as the (001) face.
Insulin crystals exhibit a nearly elastic response during the compression
cycle. The elastic modulus measured on the top face has asymmetric distribution
with a significant width. This width is related to the uncertainty in the
deflection sensitivity. A model that takes into account the distribution of
the sensitivity values is used to correct the elastic modulus. Measurements
performed in aqueous buffer on several crystals at different locations with
three different AFM probes give a mean elastic modulus of 164 ± 10
MPa. This value is close to the static elastic moduli of other protein crystals
measured by different techniques that are usually measured in the range from
100 MPa to 1 GPa. The measured modulus of insulin crystals falls between the
elastic modulus values of insulin amyloid fibrils measured previously at two
orthogonal directions (a modulus of 14 MPa was measured by compressing the
fibril in the direction perpendicular to the fibril axis, and a modulus of
3.3 GPa was measured in the direction along the fibril axis). This comparison
indicates the heterogeneous structure of fibrils in the direction perpendicular
to the fibril axis, with a packing density of the amyloid fibril core that
is higher than the average packing density in insulin crystals. The mechanical
wear of insulin crystals is detected during AFM measurements. In nanoindentation
experiments on insulin crystal, the compressive load by the AFM tip (~1 nN,
corresponding to a pressure of around 5 MPa) occasionally removes protein
molecules from the top or the second top layer of insulin crystal in a sequential
manner. The molecular model of this surface damage is proposed. In addition,
the removal of the multiple layers of molecules is observed during the AC-mode
imaging in aqueous buffer. The number of removed layers depends on the scan
size.
34. Single-Molecule Force Spectroscopy Measurements of Interactions between C60 Fullerene Molecules. Gu, C.; Ray, C; Guo, S.; Akhremitchev, B. B. J. Phys. Chem. -C 2007, 111(35), 12898-12905, link.
Abstract
The hydrophobic effect is important for many biological and technological processes. Despite progress in theory, experimental data clarifying water structure and the interactions between hydrophobic solutes at the nanometer scale are scarce due to very low solubility of hydrophobic species. This article describes single molecule force spectroscopy measurements of interactions between single fullerene C60 molecules in water. C60 molecules are tethered by flexible poly (ethylene glycol) linkers to AFM probes and substrates removing the uncertainty of the aggregation state of solution-based approaches and spurious surface effects. The analysis of the measured most probable rupture force dependence on the most probable loading rate considers the deviations from the conventional Bell-Evans model caused by the anharmonic tether effects as well as by the finite depth and shape of the potential well. The kinetic parameters of activation barrier width, dissociation rate of C60-C60 dimer and activation energy are reported. The measured values differ significantly from the predictions by molecular dynamics simulations indicating that further advances in computer simulations are necessary for the development of a model showing quantitative agreement with experimental results.
33. Rupture force analysis and the associated systematic errors in force spectroscopy by AFM. Ray, C; Brown, J. R.; Akhremitchev, B. B. Langmuir 2007, 23(11) pp 6076-6083. link
Abstract
Force spectroscopy is a new and valuable
tool in physical chemistry and biophysics. However, data analysis has yet
to be standardized, hindering the advancement of the technique. In this article
treatment of the rupture forces is described in the framework of the Bell-Evans
model and the systematic errors associated with the tether effect for approaches
that utilize the most probable, the median and the mean rupture forces are
compared. It is shown that significant systematic errors in the dissociation
rate can result from non-linear loading with polymeric tethers even if the
apparent loading rate is used in the analysis. Analytical expressions for
the systematic errors are provided for the most probable and median forces.
The use of these expressions to correct the associated systematic errors is
illustrated by the analysis of the measured rupture forces between single
hexadecane molecules in water. It is noted that the measured distributions
of rupture forces often contain high forces that are unaccounted for by theoretical
models. Experimental data indicate that the most significant effect of the
high forces “tail” is on the dissociation rate obtained from the
median force analysis while the barrier width appears unaffected.
32. Correction of Systematic Errors in Single-Molecule Force Spectroscopy with Polymeric Tethers by Atomic Force Microscopy. Ray, C; Brown, J. R.; Akhremitchev, B. B. J. Phys. Chem. - B 2007, 111(8) pp 1963 - 1974. link
Abstract
Single-molecule force spectroscopy has become a valuable tool for the investigation of intermolecular energy landscapes for a wide range of molecular associations. Atomic force microscopy (AFM) is often used as an experimental technique in these measurements, and the Bell-Evans model is commonly used in the statistical analysis of rupture forces. Most applications of the Bell-Evans model consider a constant loading rate of force applied to the intermolecular bond. The data analysis is often inconsistent because either the probe velocity or the apparent loading rate is being used as an independent parameter. These approaches provide different results when used in AFM-based experiments. Significant variations in results arise from the relative stiffness of the AFM force sensor in comparison with the stiffness of polymeric tethers that link the molecules under study to the solid surfaces. An analytical model presented here accounts for the systematic errors in force-spectroscopy parameters arising from the nonlinear loading induced by polymer tethers. The presented analytical model is based on the Bell-Evans model of the kinetics of forced dissociation and on the asymptotic models of tether stretching. The two most common data reduction procedures are analyzed, and analytical expressions for the systematic errors are provided. The model shows that the barrier width is underestimated and that the dissociation rate is significantly overestimated when force-spectroscopy data are analyzed without taking into account the elasticity of the polymeric tether. Systematic error estimates for asymptotic freely jointed chain and wormlike chain polymer models are given for comparison. The analytical model based on the asymptotic freely jointed chain stretching is employed to analyze and correct the results of the double-tether force-spectroscopy experiments of disjoining "hydrophobic bonds" between individual hexadecane molecules that are covalently tethered via poly(ethylene glycol) linkers of different lengths to the substrates and to the AFM probes. Application of the correction algorithm decreases the spread of the data from the mean value, which is particularly important for measurements of the dissociation rate, and increases the barrier width to 0.43 nm, which might be indicative of the theoretically predicted hydrophobic dewetting.
31. Investigation of the early stages of amyloid aggregation by single-molecule force spectroscopy. Akhremitchev, B. Nanomedicine 2006, 2(4) pp 270-271. link
30. Single-molecule Force Spectroscopy Measurements of "Hydrophobic Bond" between Tethered Hexadecane Molecules. Ray, C; Brown, J. R.; Akhremitchev, B. B. J. Phys. Chem. - B 2006, 110(35) pp 17578 - 17583. link
Abstract
A nanoindentation approach based on atomic force microscopy was applied to test the elastic properties of insulin amyloid fibrils. Fibrils exhibited a nearly elastic response to the compressive load. The results, corrected for the finite sample thickness effect, reveal that the fibril Young's modulus is considerably lower than the modulus of protein crystals, suggesting lower packing density in amyloid fibrils. Variation in elasticity among and within fibrils has been studied, showing that the Young's moduli of insulin fibrils have a relatively wide distribution of values, ranging from 5 to 50 MPa. Amyloid fibrils with higher modulus were found to be more wear-resistant during AFM scanning. The measured distribution of elasticity values of different fibrils together with wear-resistance tests indicates structural heterogeneity among fibrils, whereas the structure of individual fibrils appears to be homogeneous. The relative simplicity of the method used in this study can facilitate rapid collection of quantitative information related to the packing density and heterogeneity of fibrils formed by different proteins.
29. Packing Density and Structural Heterogeneity of Insulin Amyloid Fibrils Measured by AFM Nanoindentation. Guo, S.; Akhremitchev, B. B. Biomacromolecules 2006, 7(5) pp 1630 - 1636. link
Abstract
A nanoindentation approach based on
atomic force microscopy was applied to test the elastic properties of insulin
amyloid fibrils. Fibrils exhibited a nearly elastic response to the compressive
load. The results, corrected for the finite sample thickness effect, reveal
that the fibril Young's modulus is considerably lower than the modulus of
protein crystals, suggesting lower packing density in amyloid fibrils. Variation
in elasticity among and within fibrils has been studied, showing that the
Young's moduli of insulin fibrils have a relatively wide distribution of values,
ranging from 5 to 50 MPa. Amyloid fibrils with higher modulus were found to
be more wear-resistant during AFM scanning. The measured distribution of elasticity
values of different fibrils together with wear-resistance tests indicates
structural heterogeneity among fibrils, whereas the structure of individual
fibrils appears to be homogeneous. The relative simplicity of the method used
in this study can facilitate rapid collection of quantitative information
related to the packing density and heterogeneity of fibrils formed by different
proteins.
28. Conformational heterogeneity of surface-grafted amyloidogenic fragments of alpha-synuclein dimers detected by atomic force microscopy. Ray, C.; Akhremitchev, B. B. J. Am. Chem. Soc. 2005, 127(42), 14739 - 14744. link
Abstract
A
force-spectroscopy-based approach is used to characterize separation between
amyloidogenic peptide fragments of -synuclein. Interactions between individual
molecules are studied using a scanning-force-microscopy-based technique. -Synuclein
fragments are attached to the solid surfaces via flexible long poly-(ethylene
glycol) linkers removing aggregation state uncertainty of solution-based approaches
and spurious surface effects. Tethering one fragment to the scanning probe
tip and another fragment to the second surface ensures that interactions between
tethered molecules are studied. Control experiments with only one tethered
peptide indicate peptide-peptide interactions as the source of observed interaction
forces in the double-tether experiment. The temperature dependence of rupture
forces from 17.5 deg C to 40 deg C reveals similar molecular parameters indicating
that no significant conformational changes occur in the associated molecules
over this temperature range. Rate-dependent measurements indicate conformational
heterogeneity of joined peptide molecules.
27. Apertureless scanning near-field IR microscopy for chemical imaging of thin films. Akhremitchev, B. B.; Stebounova, L.; Sun, Y. J.; Walker G. C. ACS Symposium Series: Applications of Scanned Probe Microscopy to Polymers 2005, 897, 51-64.
Abstract
The sample's absorption of infrared radiation is used for chemical mapping of thin films using apertureless near-field infrared microscopy. An oscillating metallic probe modulates the scattering of the infrared radiation at the surface. The nearfield signal is registered by demodulation of the scattered signal at twice the probe's oscillation frequency. An apparent image contrast contains contributions from both the topography and the optical properties of the sample. We demonstrate that for a sufficiently thin sample the topography contribution can be small enough to detect the chemical variations unambiguously with a spatial resolution of ca 200 nm. The signal dependence on probe-sample separation is well described by employing a quasi-electrostatic model. This model includes scattering from two sources of different size. We believe that these scattering sources correspond to the different parts of the probe.
26. Single-Molecule AFM Study of Polystyrene Grafted at Gold Surfaces. Al-Maawali, S.; Bemis, J.; Akhremitchev, B. B.; Liu, H.; Walker, G. C. J. Adhesion (2005), 81, 999–1016.
Abstract
Single-molecule studies under poor solvent conditions show that atomic force microscopy (AFM) measurements of contour lengths of polystyrene 12K and 17K relate well to gel permeation chromatography (GPC) data when grafting concentrations are low (not greater than 0.7 mM). Contact angles and ellipsometry have been used to characterize the surfaces and indicate low grafting densities at this grafting concentration. Persistence lengths (p) of different length polystyrene chains, when fitted to a WLC (worm-like chain) model, show values of p = 0.23nm ± 0.10nm and p = 0.25nm ± 0.13 nm, for 12K and 17K polystyrene, respectively, when the persistence lengths are fitted to log-normal distributions. These values are close to the expected theoretical value of 0.23nm and further confirm that mostly single molecules were studied on these polystyrene surfaces. Higher grafting concentrations (>1mM) resulted in pulling multiple molecules.
25. Using the adhesive interaction between atomic force microscopy tips and polymer surfaces to measure the elastic modulus of compliant samples. Sun, Y. J.; Akhremitchev, B.; Walker, G. C. Langmuir (2004), 20(14), 5837-5845.
Abstract
An atomic force microscope (AFM) method for measuring surface elasticity based on the adhesive interactions between an AFM tip and sample surfaces is introduced. The method is particularly useful when there is a large adhesion between the tip and soft samples, when the indentation method would be less accurate. For thin and soft samples, this method will have much less interference from the substrate than is found using the indentation method because there is only passive indentation induced by tip-sample adhesion; in contrast, a large indentation with a sharp tip in the sample may break its stress-strain linearity, or even make it fracture. For the case where it is difficult to accurately locate the tip-sample contact point, which is problematic for the indentation method, the method based on adhesive interactions is helpful because it does not require locating the tip-sample contact point when fitting the whole retraction force curve. The model is tested on PDMS polymers with different degrees of cross-linking.
24. Imaging of optical field confinement in ridge waveguides fabricated on very-small-aperture laser. Chen, Fang; Itagi, A.; Bain, J. A.; Stancil, D. D.; Schlesinger, T. E.; Stebounova, L.; Walker, G. C.; Akhremitchev, B. B. Applied Physics Letters (2003), 83(16), 3245-3247.
Abstract
Optical field confinement in a ridge waveguide nanostructure (C aperture) designed for ultrahigh-d. recording was obsd. using an apertureless near-field scanning optical microscope. The aperture was fabricated on a com. edge-emitting semiconductor laser as the light source. High-contrast near-field images at both 1´ and 2´ lock-in detection frequencies were obtained. The emission patterns are in agreement with theor. simulation of such structures. A 90 nm´70 nm full width half max. spot size was measured and is comparable to the ridge width of the aperture.
23. Enhancement of the weak scattered signal in apertureless near-field scanning infrared microscopy. Stebounova, Larissa; Akhremitchev, Boris B.; Walker, Gilbert C. Review of Scientific Instruments (2003), 74(8), 3670-3674.
Abstract
An interferometric method is used to enhance the weak scattered signal in apertureless near-field scanning IR microscopy. The method involves introducing a homodyning ref. field, and amplifies the desired signal field by the magnitude of the ref. field. This method markedly improves the signal-to-noise ratio of the detected signal, over the nonhomodyned expt. A model for the dependence of the near-field signal, as a function of the normal distance of the tip from the surface, is discussed. Application of a model in which the tip is represented by two spherical scatterers, one large and one small, indicates the electromagnetic field enhancement is 90-fold greater at the sharp apex of the metallic probe tip.
22. Application of scanning force and near field microscopies to the characterization of minimally adhesive polymer surfaces. Akhremitchev, Boris B.; Bemis, Jason E.; Al-Maawali, Sabah; Sun, Yujie; Stebounova, Larissa; Walker, Gilbert C. Biofouling (2003), 19(Suppl.), 99-104.
Abstract
A review on the use of scanning probe microscopy in characterizing the function and aging of textured, minimally adhesive polymer surfaces used for antifouling applications in the marine environment.
21. Conductive probe atomic force microscopy of conducting polymers. Tivanski, Alexei V.; Bemis, Jason E.; Akhremitchev, Boris B.; Liu, Haiying; Walker, Gilbert C. Polymeric Materials Science and Engineering (2003), 88 618.
Abstract
This work examines how the adhesion force between a conducting probe and a conductive surface influences the elec. properties of conductive polymers using conducting probe at. force microscopy (CP-AFM). When a voltage is applied between the sample and the tip, an attractive electrostatic capacitance force is added to the adhesion force. The tip-sample capacitance force in the CP-AFM of poly(decylthiophene) monolayers is described through theor. modeling and compared with expt. Expts. were performed in insulating org. solvent that decreased adhesion force approx. ten times relative to the air measurements. The results for the adhesion force measurements as a function of applied bias show good agreement with the theor. prediction. Based on the adhesion force vs applied bias dependence and the current-voltage characteristics of poly(decylthiophene) mols., we conclude that characterization of elec. properties of conducting polymers using CP-AFM at a desired force requires knowledge of the adhesion force.
20. Dynamic force spectroscopy of adhesion at polymeric interfaces. Bemis, Jason E.; Al-Maawali, Sabah; Meadows, Pamela Y.; Akhremitchev, Boris B.; Walker, Gilbert C. Polymeric Materials Science and Engineering (2003), 88 522.
Abstract
We use single mol. force and spectroscopic techniques for studying protein (un)folding at designer surfaces. An AFM probe is used to pull macromols. away from surfaces which causes them to change their conformational states. In the case of homopolymers, elastic extension and unraveling transitions between folded and unfolded states. We have used colloidal particles deposition dynamics to model the adsorption of living organisms such as algal spores and bamacle cyprids onto minimally adhesive materials. Topog. plays strong role in controlling where the particles absorb.
19. Homodyne detection in infrared near field microscopy of surfaces. Stebounova, Larissa; Akhremitchev, Boris B.; Walker, Gilbert C. Polymeric Materials Science and Engineering (2003), 88 451.
Abstract
An aperture-less scanning near-field microscope was developed and constructed in the authors' lab. The app. is based on a com. AFM. The exptl. setup is depicted and described. A DNA sample was deposited on a patterned Au substrate and studied in the aperture-less near-field IR microscope. The near-field signal amplified by homodyne detection was recorded at different frequencies in the range between 900-1100 cm-1.
18. Studying the surface modulus of polymers using atomic force microscopy. Sun, Yujie; Akhremitchev, Boris; Walker, Gilbert. Polymeric Materials Science and Engineering (2003), 88 417.
Abstract
Young's moduli of different crosslinked polymers were studied using at. force microscopy (AFM). Phase shifts in tapping mode AFM data indicate the change of elasticity and adhesion. Polymers with different degrees of crosslinking were studied with force vol. using AFM. Based on Johnson-Kendall-Roberts (JKR) Theory, the distribution of Young's moduli of the polymers were calcd. from their AFM force vol. data. The Young's moduli vs. crosslink d. of polymer data were fitted and the resultant fits were consistent with expectations from the kinetic theory of rubber elasticity.
17. Apertureless scanning near-field infrared microscopy for chemical imaging of thin films. Akhremitchev, Boris B.; Stebounova, Larissa; Walker, Gilbert C. Polymeric Materials Science and Engineering (2003), 88 144.
Abstract
Application of the apertureless near-field IR microscopy to spatially resolve different chem. compn. of thin films is discussed. The sample's absorption of IR radiation is used for chem. mapping. An oscillating metallic probe modulates the scattering of the IR radiation at the surface. The near-field signal is registered by demodulation of the scattered signal at twice the probe's oscillation frequency. An apparent image contrast contains contributions from both the topog. and the optical properties of the sample. We demonstrate that for a sufficiently thin sample the topog. contribution can be small enough to detect the chem. variations unambiguously with a spatial resoln. of ca 200 nm. The topog. contribution to the detected signal is related to the probe's motion in the direction perpendicular to the substrate. The signal dependence on probe-sample sepn. is well described by employing a quasi-electrostatic model. This model includes scattering from two sources of different size. We believe that these scattering sources correspond to the different parts of the probe.
16. Adhesion Forces in Conducting Probe Atomic Force Microscopy. Tivanski, Alexei V.; Bemis, Jason E.; Akhremitchev, Boris B.; Liu, Haiying; Walker, Gilbert C. Langmuir (2003), 19(6), 1929-1934.
Abstract
This paper examines how the adhesion force between a conducting probe and a conductive surface influences the elec. properties of conductive polymers. Conducting probe at. force microscopy (CP-AFM) was employed. When a voltage is applied between the sample and the tip, an attractive electrostatic capacitance force is added to the adhesion force. The tip-sample capacitance force in the CP-AFM of polythiophene monolayers is described through theor. modeling and compared with expt. Expts. were performed in insulating org. solvent that decreased the adhesion force by .apprx.10 times relative to measurements in air. The results for the adhesion force measurements as a function of applied bias show good agreement with the theor. prediction. From the dependence of the adhesion force vs. applied bias and the current-voltage characteristics of polythiophene, characterization of elec. properties of conducting polymers using CP-AFM at a desired force requires knowledge of the adhesion force.
15. Monolayer-sensitive infrared imaging of DNA stripes using apertureless near-field microscopy. Akhremitchev, Boris B.; Sun, Yujie; Stebounova, Larissa; Walker, Gilbert C. Langmuir (2002), 18(14), 5325-5328.
Abstract
Monolayer-sensitive chem. imaging with a lateral spatial resoln. of approx. 200 nm is demonstrated. Apertureless near-field scanning IR microscopy was employed to study samples patterned with regions of DNA and hexadecanethiol. The scattering of the IR radiation was modulated by an oscillating metallic probe, and the scattered signal was demodulated at twice the probe's oscillation frequency. The maps of the near-field signal reveal patterns that are not readily observable in the topog. images, indicating the absence of topog.-related artifacts. The decrease in the scattering from areas coated with 24 base pair long DNA mols. is attributed to a phosphate stretching absorption band. The near-field absorption is obsd. to be larger than the far-field absorption.
14. Developing vibrational infrared near field spectroscopy to characterize polymer structures on surfaces: identification and reduction of topographic coupling artifacts. Akhremitchev, Boris B.; Walker, Gilbert C. Bulletin of the Chemical Society of Japan (2002), 75(5), 1011-1018.
Abstract
We report efforts to develop IR near-field microscopy for the characterization of polymer surfaces. The new near field microscope is adapted from an at. force microscope with custom optics to create a high spatial resoln. imaging platform. In addn., we report developments of anal. and finite difference approaches relating the rough sample/light interaction, the resultant near field optical signal, and the chem. structure of the sample. Simulations of the role of a topog. coupling artifact of the obsd. near field signal, and a method to overcome that artifact are presented.
13. Force modulation elasticity mapping of plastic-embedded, thin-sectioned skeletal muscle. Akhremitchev, Boris B.; Brown, Henry G.; Graner, Scott R.; Walker, Gilbert C. Microscopy and Microanalysis (2001), 7(1), 32-38.
Abstract
We have been researching the capability of at. force microscopy to reveal nontopog. properties of tissue embedded in plastic and sectioned with std. electron microscopic techniques. We present topog. and elasticity maps of plastic-embedded, thin sections of muscle tissue. The images show topog. correlated with the normal repeating structure of the sarcomere. Elasticity mapping using force modulation revealed contrast between the actin- and myosin-rich areas. We attribute the obsd. contrast in elasticity to the difference in local concns. of biol. material in embedding plastic.
12. Apertureless Scanning Near-Field Infrared Microscopy of a Rough Polymeric Surface. Akhremitchev, Boris B.; Pollack, Steven; Walker, Gilbert C. Langmuir (2001), 17(9), 2774-2781.
Abstract
IR near-field microscopy using an apertureless probe technique has been accomplished to study the surface of a cast copolymer film. Two basic models for the predicted signal and the exptl. data are presented. The first model includes plane wave light scattering by a conductive sphere and an infinitely wide absorptive layer placed on a semi-infinite conductor. This model shows IR signal dependence on the layer absorption and predicts topog. coupling into the IR signal. The exptl. data also indicate that a significant component in the IR contrast arises from the probe following the sample's topog., and a method to eliminate the influence of topog. following is demonstrated. The images cor. by such a procedure show spatial resoln. of approx. l/80. A more complex model based on a three-dimensional finite difference time domain method was used to calc. scattering from a rough surface. Both const. tip-sample gap and const. tip-substrate height analyses were made, and it is found that const. height imaging is a preferred mode of operation. Calcns. for dielec. and Lorentzian materials are reported. These calcns. indicate that the near-field IR signal attenuation for an absorptive object is larger than for a bare layer of the same thickness. This effect may be used to enhance chem. contrast in near-field imaging.
11. Study of the Polydispersity of Grafted Poly(dimethylsiloxane) Surfaces Using Single-Molecule Atomic Force Microscopy. Al-Maawali, Sabah; Bemis, Jason E.; Akhremitchev, Boris B.; Leecharoen, Rojana; Janesko, Benjamin G.; Walker, Gilbert C. Journal of Physical Chemistry B (2001), 105(18), 3965-3971.
Abstract
Single-mol. at. force microscopy (AFM) was used to study the statistical distribution of contour lengths (polydispersity) of polymer chains grafted to a surface. A poly(dimethylsiloxane) (PDMS) monolayer was grafted on a flat silicon substrate by covalently bonding Cl-terminated PDMS (Mw = 15000-20000) to an OH-silicon surface and characterized using contact angle measurements, ellipsometry, and single-mol. AFM. A model for the single-chain dynamics is presented. The statistical distributions of the polymer contour lengths were found to depend on the concn. of the PDMS polymer used in the grafting solns. Shifts of the statistical distributions toward higher contour lengths indicated preferential adsorption of longer chains with increasing PDMS:CH2Cl2 vol. ratios of 0.005-0.16. The gel permeation chromatog. (GPC) profile was found to correlate with the most dil. (0.005 vol. ratio) AFM data. The polydispersity index (PI) calcd. using AFM data was found to be 1.56 compared to 1.62 by GPC. A surface grafted with two PDMS polymer samples of av. mol. wts., 3000 and 15000-20000, was found to have a bimodal distribution of contour lengths, with peaks corresponding to the two grafting samples.
10. Ultrafast infrared and visible spectroscopy of intermolecular electron transfer from dimethylaniline to Coumarin 337. Akhremitchev, Boris; Wang, Chengfei; Walker, Gilbert C. Laser Chemistry (1999), 19(1-4), 403-405.
Abstract
Ultrafast visible and IR polarization spectroscopy were used to reveal the orientation of donor and acceptor mols. in the intermol. electron transfer between Coumarin 337 and PhNMe2.
9. Vibrational mode coupling to reverse electron transfer in (CN)5FeCNRu(NH3)5- in solution. Wang, Chengfei; Akhremitchev, Boris; Walker, Gilbert C. Laser Chemistry (1999), 19(1-4), 385-387.
Abstract
We present charge transfer absorption, resonance Raman and time-resolved IR spectral data for (CN)5FeCNRu(NH3)5- in various solvents. The transient IR spectra and anisotropies reveal both non-equil. vibrational populations of high frequency modes and local solvent heating.
8. Ultrafast Infrared Spectroscopy of Vibrational States Prepared by Photoinduced Electron Transfer in (CN)5FeCNRu(NH3)5-. Wang, Chengfei; Mohney, Brian K.; Akhremitchev, Boris B.; Walker, Gilbert C. Journal of Physical Chemistry A (2000), 104(18), 4314-4320.
Abstract
The picosecond time-resolved IR spectra of (CN)5FeCNRu(NH3)5- were collected following optical excitation and reverse electron transfer. The measured reverse electron transfer rates are >3 ´ 1012 s-1. In both formamide and D2O solns., vibrational excitation in CN stretch modes is found after reverse electron transfer. The transient spectra at both earlier (1-35 ps) and later (10 ns) times give evidence of environment-solute coupling that can be accounted for by solvent heating and ion pair dynamics. A simulation of the spectral dynamics in formamide soln. is presented using a kinetic model for vibrational excitation and relaxation. The simulation includes minor excitation in vibrational modes consistent with resonance Raman derived Franck-Condon factors, but also a non-totally sym. mode is equally important as an acceptor.
7. Finite Sample Thickness Effects on Elasticity Determination Using Atomic Force Microscopy. Akhremitchev, Boris B.; Walker, Gilbert C. Langmuir (1999), 15(17), 5630-5634.
Abstract
Finite sample thickness effects on material elasticity measurements made using an at. force microscope have been calcd. The model includes an elastic layer on an elastic foundation and simulates sample indentation under an applied load. Rigid axisym. tips with conical, paraboloidal, and hyperboloidal profiles are considered. The results show that a common approach to estg. elastic moduli from force-displacement curves can lead to a significant error that depends on the units of measurement. A method to unambiguously est. and correct this error is proposed. In addn., it is shown that elasticity ests. for monolayer thick samples using the force-modulation technique can contain a substantial, sample thickness-dependent error. Local thickness variations can result in misleading contrast in force-modulation images for samples that are several nanometers thick.
6. Structural Characterization and Electron Tunneling at n-Si/SiO2/SAM/Liquid Interface. Gu, Y.; Akhremitchev, B.; Walker, G. C.; Waldeck, D. H. Journal of Physical Chemistry B (1999), 103(25), 5220-5226.
Abstract
Alkylsilane-based self-assembled monolayer films were prepd. on n-Si with a thin oxide (nominally SiO2) layer. The surface morphol. was characterized by at. force microscopy (AFM). Photocurrents between the n-Si and a redox species in liq. soln. were also examd. as a function of the alkane chain length of the self-assembled monolayer (SAM) mols. The films are compact and current flow is blocked by the alkane monolayer, displaying a significant decrease with increasing chain length of the alkane.
5. Single Polymer Chain Elongation by Atomic Force Microscopy. Bemis, Jason E.; Akhremitchev, Boris B.; Walker, Gilbert C. Langmuir (1999), 15(8), 2799-2805.
Abstract
The elastic deformation of single polystyrene-b-poly-2-vinylpyridine chains from spun cast films was investigated by at. force microscopy. A nonlinear elastic response is shown to be present hundreds of nanometers above the bulk surface. The length of the elastic response monotonically increases with mol. wt. of the polymer. These nonlinear elastic responses are fit to wormlike chain and freely joined chain models giving persistence and Kuhn lengths of approx. 3 and 4 .ANG., resp. The entropic models reveal that the polymer chains are stretched to 80-90% of their contour length before the attachment to the tip is ruptured.
4. Femtosecond IR spectroscopy of optically excited molecules. Wang, Chengfei; Mohney, Brian; Akhremitchev, Boris; Walker, Gilbert C. Los Alamos National Laboratory, [Report] LA (United States) (1997), (LA-13290-C, Seventh International Conference on Time-Resolved Vibrational Spectroscopy, 1995), 113-117.
Abstract
The phys. and anal. optics necessary for ultrafast time-resolved IR vibrational spectroscopy are discussed. The authors first explain the exptl. procedure by which they generate 100 fs tunable visible and IR pulses, and then they describe optical pump-if probe measurements on neat liqs., and dye-solvent mixts. A Ti-doped sapphire laser system was constructed for the generation of pulses throughout the visible and IR spectral regions. Two kinds of IR detection are presented: first, a pair of HgCdTe detectors, 1 for the IR ref. channel, and the 2nd for an IR signal channel.
3. Atomic Force Microscopy Studies of Hydration of Fluorinated Amide/Urethane Copolymer Film Surfaces. Akhremitchev, Boris B.; Mohney, Brian K.; Marra, Kacey G.; Chapman, Toby M.; Walker, Gilbert C. Langmuir (1998), 14(15), 3976-3982.
Abstract
A polyamide-polyurethane block copolymer was studied using at. force microscopy and IR spectroscopy to examine the effects on the surface of immersion underwater. A film initially prepd. in air and annealed above the glass transition temp. undergoes significant surface morphol. change upon immersion, with noteworthy formation of surface particles. The particles are compliant and reduce surface friction probed by at. force microscopy. The pH dependence of the force of adhesion of the at. force microscope tip to the polymer is examd. and found to vary with surface morphol. and with tip functionality. Double layer forces as well as tip-induced extensions of the polymeric particle are identified. The IR measurements indicate that both hydrophobic and hydrophilic blocks are partially hydrated within minutes and undergo further hydration on longer time scales, up to days.
2. Femtosecond Infrared and Visible Spectroscopy of Photoinduced Intermolecular Electron Transfer Dynamics and Solvent-Solute Reaction Geometries: Coumarin 337 in Dimethylaniline. Wang, Chengfei; Akhremitchev, Boris; Walker, Gilbert C. Journal of Physical Chemistry A (1997), 101(15), 2735-2738.
Abstract
We present time-resolved IR and visible spectroscopic data that reveal the relative orientations of donor and acceptor mols. in the átETñ = 4 ps intermol. electron transfer between dimethylaniline solvent and Coumarin 337 solute. Although consideration of the dipolar attraction suggests the two species are most likely to be aligned with their mol. long axes parallel, the electron transfer occurs between species with their long axes perpendicular.
1. A femtosecond absorption spectrometer tunable from 50 000 to 800 cm-1: nonlinear optics and pump/probe geometries. Akhremitchev, Boris; Wang, Chengfei; Walker, Gilbert C. Review of Scientific Instruments (1996), 67(11), 3799-3805.
Abstract
Solid state Ti:sapphire lasers were constructed and combined with nonlinear optics to produce a 100 fs resoln. absorption spectrometer with tunability form 50,000 to 800 cm-1 and <1 m o.d./s sensitivity. Tunable light generation and array detection technol. are presented, including a discussion of the suitability of different IR nonlinear optical materials. A systematic approach to stable IR pulse generation is provided. Crit. parameters of the pump/probe beam geometry are enumerated, revealing detrimental nonlinear contributions to the obsd. pump-induced change in probe absorbance. These effects include pump-induced probe phase modulation and pump-induced refractive index lenses. Measurements and simulations of the unwanted features in various system geometries reveal their systematic control and elimination.