AVS2004 Session AS+BI-WeA: Biological Applications of Surface Analysis

Wednesday, November 17, 2004 2:00 PM in Room 210A

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		AS+BI-WeA-1 G-SIMS-MS: Towards Molecular Structure at Surfaces I. Gilmore, F. Green, M. Seah (National Physical Laboratory, UK) SSIMS is a powerful technique for the analysis of complex surfaces. However, many view SSIMS as an excellent research tool but unreliable as an analytical method. This is changing. Modern instruments have superb repeatability and reliability. In the VAMAS 2002 SSIMS inter-laboratory study, the average repeatability of 27 instruments was already 2%. Accessibility to SSIMS measurements is increasing rapidly. However, the complexity of mass spectra makes identification and quantification far from straightforward, even for the experts! This is a major barrier to the wider take-up of SSIMS, especially in new fields. One way around this problem is G-SIMS. G-SIMS or gentle SIMS is a library independent method providing a straightforward way to simplify SSIMS spectra¹²³. SSIMS spectra are composed of parent fragment ions amongst a large number of high intensity degradation products. In G-SIMS, this fragmentation may be quantified in terms of the partition functions of the fragments emitted from a surface plasma with effective temperature, T_p. By extrapolation of the data to low T_p, the intensity of the degradation products rapidly reduces, revealing the parent fragments. The latter peaks are directly characteristic of the material without rearrangement and can enable direct interpretation and identification. This is fine for smaller molecules but, within the plethora of possible larger molecules for which a total mass is insufficient to provide adequate characterisation, an extension of G-SIMS has exciting prospects to elucidate the required structure. Here we use, G-SIMS-MS, to explore the re-building of parent molecules using the fragmentation pathways that are mapped out as T_p is varied. ¹ I S Gilmore and M P Seah, Appl. Surf. Sci., 161 (2000) 465. ² I S Gilmore and M P Seah, Appl. Surf. Sci., 187 (2002) 89. ³ I S Gilmore and M P Seah, Appl. Surf. Sci., 203-204 (2003) 551.
2:20 PM	Invited	AS+BI-WeA-2 Functional Molecular Surfaces for Healthcare - Characterisation, Analysis and Understanding M.C. Davies, S.J.B. Tender, P.M. Williams, C.J. Roberts, S. Allen (University of Nottingham, UK) The characterisation of the surface structure of both conventional and advanced biomedical systems can be an important step in understanding the performance and optimising the function of such healthcare devices. A number of advanced biophysical analytical techniques have emerged for the study of pharmaceutical and biomedical systems. In this talk, we shall explore the role of these analytical tools as complimentary techniques, in the study of surface structure and function of advanced polymeric materials. The value and limitations of visualisation of surface topography and morphology of polymeric devices will be discussed and will include the condensation of polymeric constructs for gene therapy to the single molecule imaging of micro-patterned proteins on nanoengineered tissue-engineering substracts. The role of the force microscope in determining interparticulate and inter-molecular forces in order to explore its potential for the study of biomolecular interactions at polymer interfaces through to the macromolecular stimuli response hydrogels will be reviewed. The potential of the biophysical methodology of high-resolution imaging and force spectroscopy to aid research in biorecognition, development of gene delivery systems and understanding interparticulate and molecular forces, will be highlighted. The ability to identify the chemical structure of the molecules laterally across of a surface for both model and complex multiplayer patterned macromolecular assemblies shows promise but faces major challenges to low surface density ligands and high throughput array systems. The need for new approaches for handling large data sets of surface information and their relationship to surface functionality remains a significant challenge. The talk will aim to provide an objective assessment of current status, future challenges and opportunities.
3:00 PM		AS+BI-WeA-4 Model Calculations for the Quantification of XPS-Results; Application to Self Assembled Monolayers on Gold C. Van der Marel, J.H.M. Snijders, H.R. Stapert (Philips Research, The Netherlands) XPS-analysis is widely applied for the characterization of surfaces and multilayers of thin films. In order to obtain quantitative results, the XPS peak areas generally are divided by appropriate sensitivity factors and normalized to 100 at% to obtain the apparent concentrations. Within the model developed by us, the sample is assumed to consist of a substrate on top of which a number of homogeneous layers are present. Starting from the apparent concentrations, the model calculation provides the thickness, the number of sulphur atoms per surface area and the composition of all layers. The proposed method requires only one measurement at one measuring angle to obtain these results. The method has been verified for a large series of self assembled monolayers made of mercaptoalkyl-polyethylene glycol compounds with various chain lengths on gold-plated Si. The influence of deposition time upon the obtained SAM-layers was examined; also SAM-layers deposited from mixtures of molecules with different chain length were investigated. RBS analysis was applied to determine in an independent way the amount of sulphur at the gold surface (expressed in number of Sulphur atoms per unit area); the RBS results correlated well with the XPS data. Also XPS-results obtained from mixtures of mercaptodeca-triethylene glycol and biotinylated thiols were analyzed in this way. The method resembles the calculation method proposed recently by Petrovykh et al¹. Yet, in the latter method the analysis is restricted to only one layer on top of a substrate. Moreover, ion-etching is required to obtain a reference signal of the gold substrate; the consequence is, that elastic scattering of Au4f-electrons in the top layer is partly neglected in Ref.¹. ¹ D. Y. Petrovykh, H. Kimura-Suda, M.J. Tarlov and L.J. Whitman, Langmuir 20 (2004) 429.
3:20 PM		AS+BI-WeA-5 Surface Analysis by Friction Force Microscopy G.J. Leggett (University of Sheffield, UK); N.J. Brewer (Dundee University, UK); K.S.L. Chong (University of Sheffield, UK) The characterisation of surface chemical structure on the nanometre scale still presewnts significant challenges. Friction force microscopy (FFM) is a widely accessible technique typically provided as standard on commercial atomic force microscoopes. It is capable of providing significant insights into variations in surface chemical composition and molecular organisation. The sensitivity of FFM to changes in molecular organisation will be illustrated with data from studies of self-assembled monolayers (SAMs) on Au and Ag. It will be shown that unexpected packing density differences, revealed by FFM, correctly predict the variation in the photo-oxidation kinetics of these materials. FFM suggests that while SAMs of methyl temrinated adsorbates on Ag are more closely packed on than they are on Au, the reverse is the case for monolayers of carboxylic acid terminated thiols. Methyl terminated SAMs on Ag oxidise more slowly than similar monolayers on Au, while the reverse is true for carboxylic acid terminated SAMs, reflecting the strong influence of molecular packing on photo-oxidation kinetics. The kinetics of SAM photo-oxidation have also been studied and quantified by FFM. Samples of carboxylic acid terminated thiols were exposed to UV light for varying periods of time and then immersed in solutions of methyl terminated thiols. Oxidised adsorbates were replaced by solution-phase thiols. For macroscopic samples, the variation in the coefficient of friction determined by FFM as a function of SAM photo-oxidation correlates closely with the variation in the contact angle (ie, as oxidation proceeds the SAMs become increasingly hydrophobic, and exhibit an increasingly small coefficient of friction). Similar types of analysis may be used to quantify rates of reactions in photopatterned materials SAMs. For materials with structures as small as a few tens of nm, fabricated by scanning near-field optical lithography, FFM enables the monitoring of chemical reactivity.
3:40 PM		AS+BI-WeA-6 Synthesis, Characterization and Modeling of Tethered Poly (N-isopropylacrylamide) S. Mendez, G.P. Lopez (The University of New Mexico); H. Yim, M.S. Kent, J.G. Curro (Sandia National Laboratories); J.D. McCoy (New Mexico Tech) Tethered polymers are widely used to control surface properties such as adhesion and wettability. By making thin films out of polymers that are thermo-responsive, we can modulate surface properties with changes in temperature. Specifically, we use poly(N-isopropylacrylamide) (PNIPAM) since this exhibits lower critical solution temperature (LCST) behavior near 32 degrees Celsius in water. At temperatures below the LCST, the polymer is hydrated and swollen, whereas above the LCST, the polymer collapses, and when tethered, the surface becomes more hydrophobic. We report a method of growing PNIPAM from mixed self-assembled monoloyers (SAMs) using atom transfer radical polymerization. The use of two-component SAMs with varying composition permits for the control of polymer surface coverage, and the molecular weight can be controlled by the polymerization time. We have used both surface plasmon resonance and neutron reflectivity techniques to make direct measurements of the polymer brush structure at temperatures above and below the solution LCST. The effects of polymer surface coverage and molecular weight on the polymer structure were investigated. To model the temperature-induced structural changes of these brushes, we employed self-consistent field (SCF) theory using as input the chi parameter extracted from the experimental polymer solution phase diagram. The brush structure as predicted by SCF theory is in qualitative agreement with experimental data.
4:00 PM		AS+BI-WeA-7 Reversible Control of Free Energy and Topography of Nanostructured Surfaces Q. Fu, G.V. Rama Rao, S.B. Basame, D.J. Keller, K. Artyushkova, J.E. Fulghum, G.P. Lopez (The University of New Mexico) We describe a facile method for the formation of dynamic nanostructured surfaces based on the modification of porous anodic aluminum oxide with poly(N-isopropyl acrylamide) (PNIPAAm) via surface-initiated atom transfer radical polymerization. These hybrid membranes were characteriazed by FTIR, TGA, SEM. The dynamic structure of these surfaces was investigated by atomic force microscopy (AFM), which showed dramatic changes in the surface nanostructure above and below the aqueous lower critical solution temperature of PNIPAAm. These changes in surface structure are correlated with changes in the macroscopic wettability of the surfaces, which was probed by water contact angle measurements. Principal component analysis was used to develop a quantitative correlation between AFM image intensity histograms and macroscopic wettability. Such correlations and dynamic nanostructured surfaces may have a variety of uses.
4:20 PM		AS+BI-WeA-8 Characterization of Bound Nucleotides using XPS and ATR-FTIR D.B. Barbash, J.E. Fulghum, G.P. Lopez, Y. Wu (The University of New Mexico) In this work we utilize XPS and ATR-FTIR to probe the attachment of nucleotides to self-assembled monolayers. The attachment of DNA onto functional surfaces is utilized in applications ranging from DNA microarray technology to molecular wires. We are utilizing nucleotides as model systems in the development of surface-specific methods for the analysis of bound DNA. The nucleotides or DNA are covalently bound to self-assembled monolayers on glass or gold substrates using a diazotization-based method we recently developed (Dolan, P.L. et al. Nucleic Acids Research 2001, 29, 21e107). Three of the four nucleotides can be attached using this method, and we report unique spectral signatures for the nucleotide bases adenine (dATP), cytosine (dCTP), and guanine (dGTP) based on ATR-FTIR and XPS analyses. Based on these spectral signatures, results of competitive binding experiments will be discussed. XPS is also utilized to characterize the attachment steps and estimate surface coverage. ARXPS results of bound nucleotides will be discussed.