AVS1997 Session BI+AS-TuP: Macromolecules at Surfaces
Tuesday, October 21, 1997 5:30 PM in Room Exhibit Hall 1
Tuesday Afternoon
Time Period TuP Sessions | Topic BI Sessions | Time Periods | Topics | AVS1997 Schedule
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BI+AS-TuP-1 Effects of Mg++ on the Aggregation of d(CGG)4: Assay by Atomic Force Microscope (AFM)
P. Groening, R. Mu, Y.S. Tung, A. Ueda, D.O. Henderson (Fisk University); F.M. Chen (Tennessee State University) The d(CGG)4 oligonucleotides were suspended in 10 mM sodium citrate buffer (pH 5.4) containing 0.1 M NaCl, 1 mM EDTA and 2 M KCl. Six different samples of d(CGG)4 at 40 µM concentrations were prepared by treating with 0 mM, 2 mM, 4 mM, 8 mM, 16 mM and 32 mM of MgCl2. DNA solutions were prepared for AFM imaging by placing two drops of each sample on a freshly cleaved mica surface. The mica substrates were placed in a water filled desiccator overnight. Twenty milliliters of distilled water was used to gently rinse off the mica substrates. The samples were then allowed to air dry for four hours. A nanoscope-III scanning probe microscope was used for obtaining images. Images were collected in tapping mode using Si tips and an E-scanner (17 µm x 17 µm). Images were captured under the following conditions; scan rate of 2 Hz, a drive frequency of 300 KHz, a set point of 3 volts, an integral to proportional gain of 1 to 10 and 256 scan lines per image. The d(CGG)4 molecules appear as linear structures that exhibit some branching. They have a height of 2 ± 1.4 nm and lengths varying from 57 to 444 nm. The branching and chain length increase with increasing MgCl2 concentration. Circular dichroic (CD), Gel and absorption measurements of d(CGG)4 suggest the formation of four-stranded DNA structures and aggregates in the presence of K+ and Mg++. It is suggested that the aggregation occurs due to vertical stacking and cytosine base pairing, resulting in growth in the vertical and horizontal directions. This is confirmed by the increase in length and the branching pattern observed on AFM images of d(CGG)4. |
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BI+AS-TuP-2 Mechanism of Thermally-Induced Optical Transition in Polypentapeptide (PPP) Elastin
T.J. Morgan (University of California, Santa Barbara); A. Majumdar (University of California, Berkeley) A bioelastic membrane made of polypentapeptide (PPP) elastin undergoes a reversible thermally-induced optical transition such that it is transparent below a transition temperature and opaque above it. To understand the mechanism of this optical transition, transmission and reflection measurements were made in the wavelength range of 500-1050 nm. A two-flux radiative-transfer model was used to analyze the measurements and obtain the scattering albedo as a function of wavelength. The wavelength-dependence of the scattering albedo above the transition temperature was found to be that of Rayleigh scattering. This suggests that the optical transition is not due to absorption but via scattering of light from particles that form when the temperature is raised above the transition temperature. Earlier NMR studies have shown that the transition temperature corresponds to formation of intramolecular bonds between the hydrophobic groups of the PPP molecule when water pentamers surrounding the molecule are broken. It is proposed here that the intramolecular bonding creates folded structures that are large enough to scatter sufficient light for transparent-opaque optical transition, but small enough to be in the Rayleigh scattering regime in the visible and near-infrared range. Current work is focused on direct imaging of folded structures to provide further evidence for the proposed mechanism of optical transition. |
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BI+AS-TuP-3 UV Laser Interactions with a Model Biological Phosphate: Single Crystal Brushite
M.L. Dawes, S.C. Langford, J.T. Dickinson (Washington State University) Laser interactions with biological materials is of high interest due to numerous applications in surgery, surface modification, chemical analysis, and thin film growth by PLD. We explore the interaction of excimer laser light (KrF-248 nm) with single crystal brushite (CaHPO4.2H2O), a model biological phosphate. We first show that defects dominate the interactions as revealed by high sensitivity detection of Ca+ at low fluences and predicts ablation thresholds. The most probable ion energy, which occurs at ~11 eV, is much higher than the incident photon energy of 5 eV. The ion intensities also display a highly nonlinear fluence dependence, typically sixth order. We show that these observations are entirely consistent with ion emission models we have recently presented [e.g., J. Appl. Phys. 80, 6452 (1996)]. Laser coupling can be enhanced several orders of magnitude by generation of defects, i.e., by mechanical abrasion, heating, or exposure to electron beams. Scanning electron microscope (SEM) images of electron-irradiated and/or heated material show localized delamination along the surface and recrystallization of the underlying material. Thus, the presence of crystalline H2O and HPO42- decomposition play major, related roles in defect production. We show that all stimuli that result in surface fracture and recrystallization promote strong laser-surface and near surface coupling. Gentle dehydration, which occurs slowly in vacuum, shows no delamination or recrystallization structures and ion emission intensities decrease with increasing time under vacuum, whereas rapid processes accompanying abrasion, electron irradiation, and rapid heating produce high densities of the defects responsible for emission. We suggest that similar effects may control the etching and ablation of teeth, bone and related biological materials at sub-bandgap laser irradiation. This work was supported by the Department of Energy under Contract DE-FG06-92ER14252. |
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BI+AS-TuP-4 Observation of Infected Limphocyte Cells by Atomic Force Microscopy
A. Cricenti, R. Generosi, M. Girasole (Consiglio Nazionale delle Ricerche, Italy); C. Colizzi, S. Bach (Universita' di Roma Tor Vergata, Italy); P. Perfetti (Consiglio Nazionale delle Ricerche, Italy) The interaction between limphocyte cell and HIV virus have been studied at membrane level by Atomic Force Microscopy in the repulsive regime of contact mode. Morphological characteristics of non infected lymphoid cells and HIV infected cells were easily imaged from fixed and dried cell preparations. After HIV exposure we observed a decrease in surface protrusions (lost of microvilli) and the creation of many dips. Some particles, presumably of viral origin (120 - 130 nm size), were also observed in proximity of the cell surface. Similar changes have been observed by AFM on cells exposed to intense electromagnetic field thus indicating that such cells undergo modifications of their morphology upon suffering from an external agent. |
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BI+AS-TuP-5 Effect of Processing Variables on Plasma Polymerised Acrylic Acid
S. Candan, A.J. Beck, R.D. Short (University of Sheffield, United Kingdom) Plasma polymers (PPs) of acrylic acid were prepared using inductively coupled, radio frequency-induced plasmas. X-ray photoelectron spectroscopy (XPS) has been used to probe the chemistry of the resultant PPs. External plasma parameters especially the ratio of power to flow rate of monomer (W/F), have a significant effect on the chemical and physical nature of the PP. An inverse relationship between the electrical power supplied to the plasma and the degree of retention of carboxylate functionality in the solid product was observed. At high W/F there is loss of functional groups from the monomer and appearance of new functional groups as a result of fragmentation in the plasma. The PP is also insoluble due to a high degree of cross-linking. At low W/F, high retention of functional groups was achieved but the deposit is soluble as there is a little cross-linking. For use as a coating, for the modification of surfaces, the PP should ideally be insoluble. The addition of ca10 % of 1,7-octadiene to the acrylic acid feed was found to render the PP insoluble. The position of the substrate in the plasma reactor was also found to affect the chemistry of the PP. XPS analysis revealed that the O/C ratio decreased as the distance from the monomer inlet increased. The O/C ratio of PPs was found to be influenced by the substrate material; a higher O/C ratio was observed for deposition onto a silicon wafer than aluminium plate. This was thought to be as a result of the differing topography of the substrates. The stability of the PPs on ageing in the laboratory at room temperature, and under X-radiation have been investigated. The ageing of PPs both in the laboratory and under X-radiation resulted in a reduction in the O/C ratio. |
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BI+AS-TuP-6 Interaction and Stability of Cu Overlayers on Acid-terminated Self-Assembled Monolayers
L.S. Dake, A.W. Czanderna, D.E. King (National Renewable Energy Laboratory) Because it has been shown that metal/self-assembled monolayer (SAM) interfaces can be prepared and studied in vacuum, there is a need to determine the critical factors governing the behavior and stability of these interfaces. Important parameters include deposition rate, temperature, and stability of the metal films to aging. Copper deposited onto SAMs with different organic-functional end groups exhibits a wide range of behavior, from chemical interactions with the end group, to complete penetration through the SAM. Previous studies at NREL of Cu deposited on mercaptoundecanoic acid (MUA) at room temperature showed that Cu reacts with the acid to form Cu+1; but the penetration behavior was not characterized. Studies of Cu interactions with an acid ester end group (mercaptomethylhexadecanoate, MMHD) showed only a weak interaction of Cu with the acid ester end group, and penetration also occurred, but only after several hours delay. In this study, we have examined the chemical interactions and penetration behavior of Cu with an acid-functionalized SAM (mercaptohexadecanoic acid, MHA) as a function of deposition rate and temperatures from well below room temperature up to 50 C, using XPS, ISS and contact angle measurements. Aging of the films in vacuum was also studied, along with the effects of prolonged X-ray exposure. The results will be compared with those observed for Cu on MUA and MMHD. This work was performed under DOE contract No. DE-AC36-83CH10093. |
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BI+AS-TuP-7 XPS and FTIR Analysis of UV/Ozone Modified PMMA
D.E. King (National Renewable Energy Laboratory) Low cost, highly durable, polymer reflectors are essential elements of modern solar thermal concentrator systems. Silvered polymethylmethacrylate (PMMA) has been selected as a leading solar mirror candidate because of it's high reflectance, environmental stability, and ease of use and manufacture. The low to moderate adhesion of vacuum evaporated silver to PMMA sometimes results in delamination failures especially with exposure to moisture. A critical aspect of optical materials research at NREL has been to improve the adhesion of silver to PMMA. Conventional plasma surface modification methods are not successful with PMMA, typically resulting in inadequate metal/polymer adhesion. A 20 minute exposure with an ozone generating UV lamp lowers the PMMA water contact angle by more than 50 degrees and provides an improvement in silver adhesion. XPS analysis of the treated surface shows a 5 percent increase in the surface oxygen content and changes in the carbon 1s peak consistent with additional surface oxygen functionality. Results from XPS and ATR-FTIR analysis that were used to determine the extent and nature of the surface chemical modification of the PMMA will be discussed. *Performed under DOE contract DE-AC36-83CH10093 |
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BI+AS-TuP-8 Surface Studies of PSII-Treated Polystyrene
Y.K. Lee, S. Han, J. Lee, J. Yoon (Korea Institute of Science and Technology, Korea); H. Lim, K. Kim (Korea University) The Plasma Source Ion Implantation (PSII) technique has been utilized to improve the hardness, friction, wear, and corrosion properties of material surfaces, and to substitute the doping steps in semiconductor manufacturing. Very recently, our group investigated the use of PSII in the modification of polymeric materials. Polystyrene was treated with different kinds of plasma ions to render the surface more hydrophilic or hydrophobic. Hydrophobic recovery of PSII-treated polystyrene was also observed as a function of aging time, aging temperature, and treatment parameters. Treatment parameters involve kinds of gases, pressure, radio frequency, pulse voltage, etc. To study the effect of inert gas on hydrophobic recovery, polystyrene samples were prepared by helium, argon, or gas-mixture treatment. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) has been used to interpret the PSII-treated polystyrene surface and its hydrophobic recovery, with assistance of X-ray Photoelectron Spectroscopy (XPS) and water contact angle measurements. TOF-SIMS spectra of 18O2 PSII-treated samples showed the presence of 18O-containing peaks in clusters from the modified surfaces. Isotopically enriched polystyrene samples gave the information about aging time- and temperature-induced changes in the hydrophobic recovery mechanism. PSII modifications provide more stable surfaces of polystyrene as a function of aging time than plasma treatments. The comparison of aging behavior data allows for examination of the differences in the stability of the functionality introduced by the two different treatment techniques. |