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Ellipsometric and X‐Ray Spectrometric Investigation of Fibrinogen Protein Layers
Abstract
Bovine fibrinogen monolayers on thin gold films and glassy carbon substrate were investigated using grazing incidence X-ray fluorescence (GIXRF) and spectroscopic ellipsometry (SE). The aim was to determine the amount of protein and to develop models and references for the SE measurement. Both methods were capable of measuring protein amount in the range of μg cm−2 with a sensitivity below 10%, which suggests the use of both techniques as complementary, combined methods. To do it with a high confidence, the lateral uniformity and the stability of the layers during transportation has to be investigated in more detail in the future.
... If few nanometers have to be measured in air, the contamination from the ambient is also a significant problem, which consists of hydrocarbons accumulated in a few weeks [61]. Extensive comparative investigations showed that the systematic deviation of some measured thin film parameters is primarily also coming from the contamination and the above mentioned aberrations [62][63][64], which results in a larger thickness and amount of material measured by ellipsometry in air than measured by other techniques such as vacuum ultraviolet reflectometry [63] or X-ray fluorescence [64]. When comparing many different techniques [62], sub-nanometer accuracy is a challenge not only because of a possible contamination, but also because of the different spot-sizes, the different interpretation of the interfaces -even a very small lateral inhomogeneity can cause differences if the spot size or the measurement location cannot be exactly the same in the different techniques. ...
... If few nanometers have to be measured in air, the contamination from the ambient is also a significant problem, which consists of hydrocarbons accumulated in a few weeks [61]. Extensive comparative investigations showed that the systematic deviation of some measured thin film parameters is primarily also coming from the contamination and the above mentioned aberrations [62][63][64], which results in a larger thickness and amount of material measured by ellipsometry in air than measured by other techniques such as vacuum ultraviolet reflectometry [63] or X-ray fluorescence [64]. When comparing many different techniques [62], sub-nanometer accuracy is a challenge not only because of a possible contamination, but also because of the different spot-sizes, the different interpretation of the interfaces -even a very small lateral inhomogeneity can cause differences if the spot size or the measurement location cannot be exactly the same in the different techniques. ...
... When comparing many different techniques [62], sub-nanometer accuracy is a challenge not only because of a possible contamination, but also because of the different spot-sizes, the different interpretation of the interfaces -even a very small lateral inhomogeneity can cause differences if the spot size or the measurement location cannot be exactly the same in the different techniques. Fig. 3 shows an example of a good correlation of the amount of adsorbed fibrinogen protein with systematic differences when comparing the values measured by ellipsometry in air and by X-ray fluorescence investigated in a vacuum chamber [64]. ...
Understanding interface processes has been gaining crucial importance in many applications of biology, chemistry, and physics. The boundaries of those disciplines had been quickly vanishing in the last decade, as metrologies and the knowledge gained based on their use improved and increased rapidly. Optical techniques such as microscopy, waveguide sensing, or ellipsometry are significant and widely used means of studying solid‐liquid interfaces because the applicability of ions, electrons, or X‐ray radiation is strongly limited for this purpose due to the high absorption in aqueous ambient. Light does not only provide access to the interface making the measurement possible, but utilizing the phase information and the large amount of spectroscopic data, the ellipsometric characterization is also highly sensitive and robust. This article focuses on ellipsometry of biomaterials in the visible wavelength range. The authors discuss the main challenges of measuring thickness and optical properties of ultra‐thin films such as biomolecules. The authors give an overview on different kinds of flow cells from conventional through internal reflection to combined methods. They emphasize that surface nanostructures and evaluation strategies are also crucial parts of in situ bioellipsometry and summarize some of the recent trends showing examples mainly from their research.
... A frequently used method is ellipsometry [15], which characterizes a material by the change in polarization of reflected light. Ellipsometry is a well-developed method that has been widely explored in the literature [16][17][18][19][20]. However, devices that can perform ellipsometric measurements are high-cost, and their availability is often limited due to the complexity of the method. ...
To design semiconductor-based optical devices, the optical properties of the used semiconductor materials must be precisely measured over a large band. Transmission spectroscopy stands out as an inexpensive and widely available method for this measurement but requires model assumptions and reconstruction algorithms to convert the measured transmittance spectra into optical properties of the thin films. Amongst the different reconstruction techniques, inverse synthesis methods generally provide high precision but rely on rigid analytical models of a thin film system. In this paper, we demonstrate a novel flexible inverse synthesis method that uses angular spectrum wave propagation and does not rely on rigid model assumptions. Amongst other evaluated parameters, our algorithm is capable of evaluating the geometrical properties of thin film surfaces, which reduces the variance caused by inverse synthesis optimization routines and significantly improves measurement precision. The proposed method could potentially allow for the characterization of "uncommon" thin film samples that do not fit the current model assumptions, as well as the characterization of samples with higher complexity, e.g., multi-layer systems.
Plasmon-enhanced in situ spectroscopic ellipsometry was realized using the Kretschmann geometry. A 10-μL flow cell was designed for multi-channel measurements using a semi-cylindrical lens. Dual-channel monitoring of the layer formation of different organic structures has been demonstrated on titania nanoparticle thin films supported by gold. Complex modeling capabilities as well as a sensitivity of ~40 pg/mm² with a time resolution of 1 s was achieved. The surface adsorption was enhanced by the titania nanoparticles due to the larger specific surface and nanoroughness, which is consistent with our previous results on titanate nanotubes.
The influence of substrate materials on protein adsorption was studied by spectroscopic ellipsometry (SE) and atomic force microscopy. For model proteins fibrinogen and flagellar filaments were chosen and their kinetics of adsorption, surface coverage and adsorbed amount on virgin and chemically activated SiO(2) and Ta(2)O(5) thin films were investigated. In case of flagellar filaments the SE data were analyzed with an effective medium model that accounted for the vertical density distribution of the adsorbed protein layer. Adsorption was measured in situ using flow cells with various fluid volume. Compared to commercially available cells, a flow cell with significantly smaller volume was constructed for cost-effective measurements. The development of the flow cell was supported by finite element fluid dynamics calculations.
The scaling down of critical dimensions for the manufacturing of nanoelectronics requires the continuous introduction of new materials. The results of the analysis of thin high-k films made from Al2O3 as reference samples were used at multiple laboratories to show the power and strength of complementary metrology, e.g. using various techniques, such as synchrotron radiation X-ray spectrometry, ‘table top’ grazing incidence X-ray spectrometry and X-ray reflectometry, and spectroscopic ellipsometry. The layer thicknesses and material parameters validated by several analytical techniques demonstrate the successes of the use of complementary metrology. The requirement for validation, assurance, and support using differing analytical methods is driving the integration of multiple methods into one tool. This paper proposes an integrated metrology approach for reliable characterization of structure and composition. For the analysis of surfaces and materials, light sources in different spectral ranges, e.g. X-rays or infrared light, are used for diffraction, scattering, or excitation of fluorescence. The use of appropriate detectors in the scattering or fluorescence geometry is indispensable. Highly precise metrology requires accurate positioning of the sample with respect to the sources and the detectors. The handling unit for samples and automation are the main contributors to the cost of the semiconductor metrology equipment. For this reason, the approach of integrating multiple analytical techniques has advantages with respect to cost aspects and handling steps. A design study of the 450 mm analytical platform was performed. This design study integrates seven complementary analytical methods into one metrology chamber. Five methods rely on X-ray characterization methods, such as Total Reflection X-Ray Fluorescence Analysis (TXRF), Grazing Incidence X-Ray Fluorescence Analysis (GIXRF/XRF), X-Ray Reflectometry (XRR), X-Ray Diffractometry (XRD), and Grazing Incidence Small Angle X-Ray Scattering (GISAXS). Furthermore, the two methods of spectroscopic ellipsometry and vacuum UV reflectometry using the spectral range of ultra-violet to infrared were supplemented. A novel 5-axis positioning system was designed and patented, enabling the integration of all analytical methods into one chamber under vacuum or atmospheric conditions.
Damage created by ion implantation into single crystalline silicon was characterized with an optical model based on the coupled half-Gaussian model developed by Fried et al [J. Appl. Phys. 71, 2835 (1992)]. In the improved optical model the damage profile was described by sublayers with thicknesses inversely proportional to the slope of the profile. This method allows a better resolution at the quickly changing parts of the profile, and a better approximation of the Gaussian profile with the same number of sublayers. A fitting procedure, which we call “multipoint random search,” was applied to minimize the probability of getting in a local minimum. The capabilities of our method were demonstrated for amorphizing doses using different ions and energies. The improved fit quality and the correlation with results of backscattering spectrometry basically supported the optical model. © 2003 American Institute of Physics.
The continuous dimensional reduction drives the development of metrology, analysis and characterization for nano and micro electronics. An enormous worldwide R&D effort focuses on the understanding and controlling materials properties and dimensions at atomic level. Crucial for groundbreaking new developments is the availability of appropriate analytical infrastructures providing techniques with information depths well adapted to the nanoscaled objects of interest. This requires widely accessible, independent complementary metrology, analytical techniques, and characterization. For example new materials and the demand of improved detection sensitivities for contaminants provide huge challenges for the capabilities of current analysis equipment and expertise. At the same time, the availability of complementary competences is crucial for advancement of analytical methodologies through cross-comparison, round-robin, and benchmarking of results. This paper describes the formation of an independent analytical infrastructure within Europe having the expertise and competence to solve metrology problems for development of nanotechnologies. Furthermore, a strategy is shown to establish independently operating 'Golden Laboratories'(tm) for complementary and reliable metrology, analysis, and characterization adapted to the requirements of industrial partners.
The photodissociation of the amide bond by UV light and soft x-rays is investigated by x-ray absorption spectroscopy at the C, N, and O 1s edges. Irradiation leaves a clear and universal signature for a wide variety of amides, ranging from oligopeptides to large proteins and synthetic polyamides, such as nylon. As the π∗ peak of the amide bond shrinks, two new π∗ peaks appear at the N 1s edge with a characteristic splitting of 1.1 eV. An additional characteristic is the overall intensity reduction of both the π∗ and σ∗ features at the O 1s edge, which indicates loss of oxygen. The spectroscopic results are consistent with the release of the O atom from the amide bond, followed by the migration of the H atom from the N to one of its two C neighbors. Migration to the carbonyl C leads to an imine, and migration to the C(α) of the amino acid residue leads to a nitrile. Imine and nitrile produce the two characteristic π∗ transitions at the N 1s edge. A variety of other models is considered and tested against the N 1s spectra of reference compounds.
Nanometre thin high-k hafnium oxide (HfO2) or Hf silicate layers combined with a sub-nm SiO2 layers have become Si compatible gate dielectrics. Medium energy ion scattering (MEIS) analysis has been applied to a range of such MOCVD grown HfO2/SiO2 and HfSiOx(60%Hf)/SiO2 gate oxide films of thickness between 1 and 2 nm on top of Si(100), before and after decoupled plasma nitridation (DPN). MEIS in combination with energy spectrum simulation provides quantitative layer information with sub-nm resolution on these layer structures and their atomic composition that is in excellent agreement with a) the as grown layer parameters and b) results obtained from techniques, such as SE, XPS, XRF and XTEM. MEIS analysis of a metal gate, high-k TiN/Al2O3/HfO2/SiO2/Si stack shows the interdiffusion, after thermal treatment, of Hf and Al from the caplayer, which was inserted to modify the metal gate work function.
The chemical nature of soil organic nitrogen (N) is still poorly understood and one-third to one-half of it is typically classified as ‘unknown N’. Nitrogen K-edge XANES spectroscopy has been used to develop a systematic overview on spectral features of all major N functions in soil and environmental samples. The absolute calibration of the photon energy was completed using the 1s → π* transitions of pure gas-phase N2. On this basis a library of spectral features is provided for mineral N, nitro N, amino acids, peptides, and substituted pyrroles, pyridines, imidazoles, pyrazoles, pyrazines, pyrimidines and purine bases. Although N XANES was previously considered ‘non-destructive’, effects of radiation damage were shown for two compound classes and an approach was proposed to minimize it. This new evidence is integrated into a proposal for the evaluation spectra from environmental samples with unknown composition. Thus a basis is laid to develop N K-edge XANES as a complementary standard research method to study the molecular composition and ecological functions of ‘unknown N’ in soil and the environment.
Two surface-sensitive label-free optical methods, grating coupled interferometry (GCI) and spectroscopic ellipsometry (SE) were integrated into a single instrument. The new tool combines the high sensitivity of GCI with the spectroscopic capabilities of SE. This approach allows quantification with complex optical models supported by SE and accurate measurements with the evanescent field of GCI. A flow cell was developed to perform combined and simultaneous investigations on the same sensor area in liquid (or gas) environments. The capabilities of the instrument were demonstrated in simple refractometry and protein adsorption experiments.
Integrated planar optical waveguide interferometer biosensors are advantageous combinations of evanescent field sensing and optical phase difference measurement methods. By probing the near surface region of a sensor area with the evanescent field, any change of the refractive index of the probed volume induces a phase shift of the guided mode compared to a reference field typically of a mode propagating through the reference arm of the same waveguide structure. The interfering fields of these modes produce an interference signal detected at the sensor׳s output, whose alteration is proportional to the refractive index change. This signal can be recorded, processed and related to e.g. the concentration of an analyte in the solution of interest. Although this sensing principle is relatively simple, studies about integrated planar optical waveguide interferometer biosensors can mostly be found in the literature covering the past twenty years. During these two decades, several members of this sensor family have been introduced, which have remarkably advantageous properties. These entail label-free and non-destructive detection, outstandingly good sensitivity and detection limit, cost-effective and simple production, ability of multiplexing and miniaturization. Furthermore, these properties lead to low reagent consumption, short analysis time and open prospects for point-of-care applications. The present review collects the most relevant developments of the past twenty years categorizing them into two main groups, such as common- and double path waveguide interferometers. In addition, it tries to maintain the historical order as it is possible and it compares the diverse sensor designs in order to reveal not only the development of this field in time, but to contrast the advantages and disadvantages of the different approaches and sensor families, as well.
The authors undertook to compile a database of recent values of the atomic parameters required for fundamental parameters (FP) calculation of X-ray fluorescence (XRF) spectra, calculation of X-ray absorption in crystals and other samples, and correction of X-ray absorption spectra for self-absorption effects. All values were obtained from published sources and include the elements hydrogen (atomic number 1) through californium (atomic number 98). The data were collected into a single unstructured ASCII text file.
Ever progressive miniaturization of integrated circuits and breakthroughs in knowledge of biological macromolecules deriving from DNA and protein surface research are propelling ellipsometry, a measurement technique based on phase and amplitude changes in polarized light, to greater popularity in a widening array of applications. Ellipsometry, without contact and non-damaging to samples, is an ideal measurement technique to determine optical and physical properties of materials at the nano scale.
In this study, we have reconstructed the statistical 3D structure of hundreds of nanometers thick surface immobilized flagellar filament protein layers in their native environment, in buffer solution. The protein deposition onto the surface activated Ta2O5 film was performed in a flow cell, and the immobilization process was followed by in situ spectroscopic ellipsometry. A multilayer optical model was developed, in that the protein layer was described by five effective medium sublayers. Applying this method, an in-depth analysis of the protein layer formation was performed. Based on the kinetics in the distribution of the surface mass density, the statistical properties of the filamentous film could be determined computationally as a function of the measurement time. It was also demonstrated that the 3D structure of the protein layer can be reconstructed based on the calculated in-depth mass density profile. The computational investigation revealed that the filaments can be classified into two individual groups in approximately equal ratio according to their orientation. In the first group the filaments are close to laying position, whereas in the second group they are in a standing position, resulting in a significantly denser sublayer close to the substrate than at a larger distance.Highlights► Flagellar filaments were immobilized onto Ta2O5 substrates in flow-cell. ► The surface was monitored in situ with spectroscopic ellipsometry. ► New optical model was developed for the characterization of the filamentous layers. ► The depth profile of protein mass density was determined. ► The statistical 3D structure of the filamentous layer was reconstructed.
After a systematic analysis of different databases and a comparison with empirical data, we decided to describe photoelectric absorption coefficients, coherent and incoherent scattering coefficients and mass attenuation coefficients in the energy range from 1 to 300 keV for elements from Z = 1 to 94 by a modified version of Scofield's tables in combination with scattering coefficients from Elam et al. Modifications include check and correction of edge positions, description of M4- and M5-ranges of elements from promethium upwards by sawtooth responses and reduction of data by introduction of least-squares fits of fifth order to Scofield's and Elam et al.'s numerical data. Our database was developed especially for application in fundamental parameter programs for quantitative x-ray analysis (XRFA, EPMA, XPS and TEY). Copyright © 2003 John Wiley & Sons, Ltd.
The application of ellipsometry of the study of the adsorption behavior of proteins and synthetic macromolecules at the air-water interface has been investigated. It is shown that for macromolecules the amount adsorbed per unit area, Γ, as determined by ellipsometry, only has a well-defined physical meaning if the refractive-index increment remains constant up to high concentrations present in the adsorbed layer. It has been found experimentally that this conditioned is fulfilled for proteins. The ellipsometric Γ values of some protein agree satisfactorily with those obtained by two independent techniques has been used to investigate the adsorption from solution of κ-casein, bovine serum albumin, and polyvinyl alcohol. For bovine serum albumin, Γ reaches a plateau value of 2.9 mg/m2 for concentrations ≥ 0.05 wt%. The thickness of the adsorbed molecules. For κ-casein, Γ steadily increases with increasing centration and multilayers are formed. The technique provides interesting information on conformational changes in adsorbed macromolecules, on the rate of the process, and on the conditions under which these occur.
Optical constants have been obtained for glassy carbon from 0 to 82 eV by means of reflection measurements. The data have been analyzed by analogy with those for graphite in terms of single electron excitations and of collective oscillations of the π and σ electrons.
Total reflection X-ray fluorescence (TXRF) analysis is a well-established method to monitor lowest level contamination on semiconductor surfaces. Even light elements on a wafer surface can be excited effectively when using high-flux synchrotron radiation in the soft X-ray range. To meet current industrial requirements in nondestructive semiconductor analysis, the Physikalisch-Technische Bundesanstalt (PTB) operates dedicated instrumentation for analyzing light element contamination on wafer pieces as well as on 200- and 300-mm silicon wafer surfaces. This instrumentation is also suited for grazing incidence X-ray fluorescence analysis and conventional energy-dispersive X-ray fluorescence analysis of buried and surface nanolayered structures, respectively. The most prominent features are a high-vacuum load-lock combined with an equipment front end module and a UHV irradiation chamber with an electrostatic chuck mounted on an eight-axis manipulator. Here, the entire surface of a 200- or a 300-mm wafer can be scanned by monochromatized radiation provided by the plane grating monochromator beamline for undulator radiation in the PTB laboratory at the electron storage ring BESSY II. This beamline provides high spectral purity and high photon flux in the range of 0.078-1.86 keV. In addition, absolutely calibrated photodiodes and Si(Li) detectors are used to monitor the exciting radiant power respectively the fluorescence radiation. Furthermore, the footprint of the excitation radiation at the wafer surface is well-known due to beam profile recordings by a CCD during special operation conditions at BESSY II that allow for drastically reduced electron beam currents. Thus, all the requirements of completely reference-free quantitation of TXRF analysis are fulfilled and are to be presented in the present work. The perspectives to arrange for reference-free quantitation using X-ray tube-based, table-top TXRF analysis are also addressed.
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