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Grating coupled optical waveguide interferometry combined with in situ spectroscopic ellipsometry to monitor surface processes in aqueous solutions
Abstract
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.
... A wide range of analytical techniques can be used to quantify the amount of adsorbed protein on a surface, including quartz-crystal microbalance, infrared spectroscopy, surface plasmon resonance, ellipsometry, optical waveguide lightmode spectroscopy, and atomic force microscopy [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. However, these methods do not provide information about protein adsorption from the reactive metallic surface point of view, which is essential in electrochemical biosensor development. ...
... Fig. 2 shows the optimization of the UPD process, ie the potential sweeps obtained after holding the potential at 400 mV for different periods of time. In accordance with earlier results [19] the oxidation charge of the strongly bonded adsorbed copper reached a constant value after a relatively short time (about t = 1 min), and this charge was about one quarter of the charge of the oxide-reduction peak for the same electrode. Taking these results into consideration, t = 2 min was set as the standard deposition time for copper UPD. ...
We demonstrate the use of copper underpotential deposition (UPD) for measurement of the electrochemically active surface area of a protein-modified gold electrode. The kinetics of adsorption and the subsequent structural rearrangement of β-casein, as a model protein, on the surface of a gold electrode have been followed over time. The protein-free surface area was determined by measuring the charge derived from UPD copper deposition on the non-blocked surface area at different stages of adsorption. The behaviour of β-casein adsorption as a function of surface roughness and the microstructure of the gold electrode, as well as the concentration of the protein solution, are studied using this method.
... GCI is also used in biosensing due to its advantages such as the simple design with no moving components, cost-effectiveness, high-resolution, and excellent sensitivity [22,23]. GCI works on the principle of sensitive optical investigation in which the main component is an optical chip that consists of three layers of substrate, waveguide film, and cladding. ...
... These phase shifts can be calculated and determine the sensing parameters. Agocs et al. [23] used the GCI technique along with spectroscopic ellipsometry (SE) technique to develop the spectroscopic high-sensitive sensor to observe the surface process in aqueous solution. Similarly, Patko et al. [22] developed GCI based labelfree optical sensor to monitor the surface adhesion of extracellular vesicles secreted by liver cells. ...
Various optical sensing techniques to develop different biosensors for detection of biomolecules are summarized here. The identification of biomolecules has led to the major aspect of disease diagnosis. Some critical diseases like cancer can be identified with an increased level of protein markers like vascular endothelial growth factor (VEGF) is a well-known cancer marker. Moreover, viruses and bacterial cells are known to be the major causes of disease which can also be detected with the help of proteins present on their surface. Additionally, a rise in cholesterol levels can cause several diseases like arteriosclerosis. hypertension etc. Thus, in this paper, recent developed optical techniques-based sensors for the detection of different biomolecules (such as proteins, nucleic acid, glucose, DNA, cholesterol, uric acid, amino acids, and dopamine) and microorganisms have been discussed. Various possibilities to develop diverse high sensitive and selective optical sensors to detect the different biomolecules and microorganisms have been also studied.
... 79 New experimental developments also focus on the integration of a grating coupled interferom- eter and spectroscopic ellipsometer into one single tool. 80 ...
... Currently, there is a trend to develop the techniques and to combine several techniques in a single instrument in order to gain further insight into the nature and characteristics of the adsorbed molecular species. For example, OWLS was com- bined with ellipsometry 80 and SPR with mass spectroscopy. 68 In all the presented techniques, efforts are also made to gain in sensitivity, throughput, and other important parame- ters such as minimization of sample volume or even sample recycling. ...
The control over the adsorption or grafting of biomolecules from a liquid to a solid interface is of fundamental importance in different fields, such as drug delivery, pharmaceutics, diagnostics, and tissue engineering. It is thus important to understand and characterize how biomolecules interact with surfaces and to quantitatively measure parameters such as adsorbed amount, kinetics of adsorption and desorption, conformation of the adsorbed biomolecules, orientation, and aggregation state. A better understanding of these interfacial phenomena will help optimize the engineering of biofunctional surfaces, preserving the activity of biomolecules and avoiding unwanted side effects. The characterization of molecular adsorption on a solid surface requires the use of analytical techniques, which are able to detect very low quantities of material in a liquid environment without modifying the adsorption process during acquisition. In general, the combination of different techniques will give a more complete characterization of the layers adsorbed onto a substrate. In this review, the authors will introduce the context, then the different factors influencing the adsorption of biomolecules, as well as relevant parameters that characterize their adsorption. They review surface-sensitive techniques which are able to describe different properties of proteins and polymeric films on solid two-dimensional materials and compare these techniques in terms of sensitivity, penetration depth, ease of use, and ability to perform "parallel measurements."
... The combination of QCM and OWLS data measured in parallel OWLS-QCM experiments have already shown its potential in hydrated nanolayer characterization [29,67,131,271]. Another interesting venture is the combination of GCI with spectroscopic ellipsometry (GCI-SE) in one instrument, which may offer the simultaneous exploitation of the high sensitivity provided by GCI and the spectroscopic capabilities of SE, allowing complex multilayer structures to be analyzed in detail [278]. SE-SPR is a (i) type combination realizing the generation of surface plasmons and simultaneous SE measurement in the proper configuration [279][280][281][282][283]. Whether the data are obtained from (i) or (ii), the postmeasurement combination of QCM and optical data are present complex data analysis challenges. ...
Bacteria repellent surfaces and antibody-based coatings for bacterial assays have shown a growing demand in the field of biosensors, and have crucial importance in the design of biomedical devices. However, in-depth investigations and comparisons of possible solutions are still missing. The optical waveguide lightmode spectroscopy (OWLS) technique offers label-free, non-invasive, in situ characterization of protein and bacterial adsorption. Moreover, it has excellent flexibility for testing various surface coatings. Here, we describe an OWLS-based method supporting the development of bacteria repellent surfaces and characterize the layer structures and affinities of different antibody-based coatings for bacterial assays. In order to test nonspecific binding blocking agents against bacteria, OWLS chips were coated with bovine serum albumin (BSA), I-block, PAcrAM-g-(PMOXA, NH2, Si), (PAcrAM-P) and PLL-g-PEG (PP) (with different coating temperatures), and subsequent Escherichia coli adhesion was monitored. We found that the best performing blocking agents could inhibit bacterial adhesion from samples with bacteria concentrations of up to 107 cells/mL. Various immobilization methods were applied to graft a wide range of selected antibodies onto the biosensor's surface. Simple physisorption, Mix&Go (AnteoBind) (MG) films, covalently immobilized protein A and avidin-biotin based surface chemistries were all fabricated and tested. The surface adsorbed mass densities of deposited antibodies were determined, and the biosensor;s kinetic data were evaluated to divine the possible orientations of the bacteria-capturing antibodies and determine the rate constants and footprints of the binding events. The development of affinity layers was supported by enzyme-linked immunosorbent assay (ELISA) measurements in order to test the bacteria binding capabilities of the antibodies. The best performance in the biosensor measurements was achieved by employing a polyclonal antibody in combination with protein A-based immobilization and PAcrAM-P blocking of nonspecific binding. Using this setting, a surface sensitivity of 70 cells/mm2 was demonstrated.
... The combination of QCM and OWLS data measured in parallel OWLS-QCM experiments have already shown its potential in hydrated nanolayer characterization [29,67,131,271]. Another interesting venture is the combination of GCI with spectroscopic ellipsometry (GCI-SE) in one instrument, which may offer the simultaneous exploitation of the high sensitivity provided by GCI and the spectroscopic capabilities of SE, allowing complex multilayer structures to be analyzed in detail [278]. SE-SPR is a (i) type combination realizing the generation of surface plasmons and simultaneous SE measurement in the proper configuration [279][280][281][282][283]. Whether the data are obtained from (i) or (ii), the postmeasurement combination of QCM and optical data are present complex data analysis challenges. ...
Interfacial layers are important in a wide range of applications in biomedicine, biosensing, analytical chemistry and the maritime industries. Along with the growing number of applications, the analysis of these layer properties and understanding their behavior is becoming crucial. Label-free surface sensitive methods are excellent tools to monitor the structure and formation kinetics of thin layers in real-time even at the nanoscale. In this study, we review the available label-free techniques and demonstrate how the experimentally obtained data can be utilized to extract kinetic and structural information about the formed layers. We outline the most traditional and some novel techniques using optical and mechanical transduction principles. With a special focus, we review the current possibilities o the combination of label-free methods which is a powerful tool to extend the range of detected parameters. We summarize the most important theoretical considerations for obtaining reliable information from measurements taking place in liquid environments and hence layers in a hydrated state. Detailing with both structural and kinetic analysis methods, a thorough overview of the various quantities obtained from the evaluation of the raw label-free data are also provided. Here, thickness, refractive index, optical anisotropy and the related chain orientation, hydration degree, viscoelasticity, footprint, association and dissociation rate constants are all mentioned in detail. To demonstrate the effect of variations in model conditions on the observed data, simulations of kinetic curves at various model settings are also included. Based on our own experience with optical waveguide lightmode spectroscopy (OWLS) and the quartz crystal microbalance (QCM), we have developed software packages for data analysis. Along with this paper, we make the developed software available for the wider scientific community.
Numerical simulations and analytical calculations are performed to support the design of grating-coupled planar optical waveguides for biological sensing. Near cut-off and far from cut-off modes are investigated, and their characteristics and suitability for sensing are compared. The numerical simulations reveal the high sensitivity of the guided mode intensity near the cut-off wavelength for any refractive index change along the waveguide. Consequently, it is sufficient to monitor the intensity change of the near cut-off sensing mode, which leads to a simpler sensor design compared to those setups where the resonant wavelength shift of the guided mode is monitored with high precision. The operating wavelength and the sensitivity of the proposed device can be tuned by varying the geometrical parameters of the corrugated waveguide. These results may lead to the development of highly sensitive integrated sensors, which have a simple design and therefore are cost-effective for a wide range of applications. These numerical findings are supported with experimental results, where the cut-off sensing mode was identified.
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.
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.
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.
Grating Coupled Interferometry (GCI) using high quality waveguides with two incoupling and one outcoupling grating areas is introduced to increase and precisely control the sensing length of the device; and to make the sensor design suitable for plate-based multiplexing. In contrast to other interferometric arrangements, the sensor chips are interrogated with a single expanded laser beam illuminating both incoupling gratings simultaneously. In order to obtain the interference signal, only half of the beam is phase modulated using a laterally divided two-cell liquid crystal modulator. The developed highly symmetrical arrangement of the interferometric arms increases the stability and at the same time offers straightforward integration of parallel sensing channels. The device characteristics are demonstrated for both TE and TM polarized modes.
An interferometric sensor based on gratings on a planar waveguide is introduced. The device combines the advantages of known
interference-based waveguide sensors with the simplicity of grating couplers. In the presented configuration, two parallel
and coherent light beams, laterally separated in the direction of mode propagation, are coupled into a planar waveguide through
a grating. One of the coupled beams is phase modulated using a periodically relaxing liquid crystal modulator, resulting in
a time varying intensity signal at the end face of the waveguide. Refractive index changes within the waveguide section between
the two coupling regions are monitored by observing characteristic changes in the intensity signal.
The effect of the attachment and spreading of living cells on the modes of a grating coupled reverse symmetry waveguide sensor is investigated in real time. The reverse symmetry design has an increased probing depth into the sample making it well suited for the monitoring of cell morphology. As a result, significant changes in the incoupling peak height and peak shape were observed during cell attachment and spreading. It is suggested that the area under the incoupling peaks reflects the initial cell attachment process, while the mean peak position is mostly governed by the spreading of the cells.
An optical fiber magnetic field sensor based on the no-core fiber (NCF) and magnetic fluid (MF) is proposed and demonstrated. By using the temperature sensing property of fiber Bragg grating (FBG), the magnetic field sensor with dynamic temperature compensation can be achieved. The transmittance of the NCF that is sealed in a capillary with MF is highly sensitive to the surrounding magnetic field. Experimental results show that the sensor has the magnetic field sensitivity of 9.2 pm/mT ranging from 0 to 9 mT and the temperature sensitivity of 11.9 pm/°C ranging from 15 to 45 °C. The prototype might be attractive due to its low fabrication cost and simple configuration, which are desirable features in magnetic field measurement.
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.
Novel biosensors made of polymers may offer advantages over conventional
technology such as possibility of mass production and tunability of the
material properties. With the ongoing work on the polymer photonic chip
fabrication in our project, simple model samples were tested parallel
for future immobilization and accessing conditions for applications in
typical aqueous buffers. The model samples consist of a thin, high
refractive index polyimide film on top of TEOS on Si wafer. These model
samples were measured by in situ spectroscopic ellipsometry using
different aqueous buffers. The experiments revealed a high drift in
aqueous solutions; the drift in the ellipsometric parameters (delta,
psi) can be evaluated and presented as changes in thickness and
refractive index of the polyimide layer. The first molecular layer of
immobilization is based on polyethyleneimine (PEI). The signal for the
PEI adsorption was detected on a stable baseline, only after a long
conditioning. The stability of polyimide films in aqueous buffer
solutions should be improved toward the real biosensor application.
Preliminary results are shown on the possibilities to protect the
polyimide. Optical Waveguide Lightmode Spectroscopy (OWLS) has been used
to demonstrate the shielding effect of the thin TiO2 adlayer
in biosensor applications.
A phase shifting optical waveguide interferometer is demonstrated for label-free evanescent wave biosensing in aqueous solutions. In the proposed configuration, the reference and measurement arms of the interferometer are combined inside an integrated optical Ta2O5 waveguide using an ion implanted grating. Biomolecules adsorbing on the waveguide surface shift the phase in the measurement arm, which is modulated at the same time by a periodically relaxing liquid crystal phase modulator. It is demonstrated that by analyzing the periodic intensity response at the end facet of the waveguide, the phase shifts in the measurement arm can be monitored in real-time with a precision of 10−4rad. The high phase resolution allows the detection of surface adsorbed molecule densities below 1pg/mm2 without using any labeling or on-chip referencing. The instrument performance is demonstrated by monitoring in situ protein adsorption and affinity binding of low molecular weight pure biotin to an immobilized avidin layer.
Ellipsometry is well-suited for bioadsorption studies and numerous reports, mainly using null ellipsometry, are found on this subject whereas investigations addressing structural properties of thin biolayers are few. Here two examples based on the use of spectroscopic ellipsometry (SE) on the latter are briefly discussed. In the first example, time evolution of thickness, spectral refractive index and surface mass density of a fibrinogen matrix forming on a silicon substrate are investigated with SE and a structural model of the protein matrix is discussed. In the second example a model dielectric function concept for protein monolayers is presented. The model allows parameterization of the optical properties which facilitates monitoring of temperature induced degradation of a protein layer. More recently, photonic structures in beetles have been studied with SE. It is shown here that full Mueller-matrix SE can resolve very complex nanostructures in scarab beetles, more specifically chiral structures causing reflected light to become circularly polarized.
Spectroscopic ellipsometry has become an essential metrology tool for the semiconductor industry. It is widely used where precise film thicknesses and optical constants are required. Faster measurement speeds are opening new doors for ellipsometry in thin-film processing environments, primarily due to its ability to maintain high precision when measuring very thin or multilayered films (such as gate oxides). The technique works well for all varieties of films, including semiconductors, dielectrics, metals, and polymer coatings.
The buildup of polyelectrolyte multilayers (PEMs), fabricated by the layer-by-layer (LBL) assembly, was followed in situ by the combination of a quartz crystal microbalance with dissipation monitoring (QCM-D) and spectroscopic ellipsometry in a single device. PEMs composed of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) polyelectrolyte pairs and of poly(diallyldimethyl ammonium chloride) (PDADMAC) and PSS were built up to 17 layers. The combination of ellipsometry and QCM-D allowed simultaneous determination of the acoustic mass, which comprises both the mass of the polymer and solvent, and the optical mass which corresponds to the polymer mass alone. From these parameters, the hydration of the PEM was calculated layer by layer. The linearly growing PAH/PSS PEMs showed a constant absolute content of water throughout the assembly, while the relative contribution of water to the PEM mass content approached zero, when grown in 0.5 M NaCl. Rinses with water between polyelectrolyte deposition steps resulted in a hydration of approximately 40%. The supralinearly growing PDADMAC/PSS PEMs exhibited a remarkable dependence of the hydration on the polyelectrolyte that was deposited last. Implications for the mechanism of assembly of the PEMs are discussed.
Generalized ellipsometry and quartz crystal nanogravimetry are combined to determine adsorption isotherms and changes in the optical properties of biaxial TiO2 thin films by monitoring changes in the Mueller matrix. Individual Mueller matrix elements, corresponding to a variety of polarization states, exhibit dramatically different sensitivities to the adsorption of toluene. While some elements are sensitive to structural anisotropy and orientation, others report uniquely on the refractive index. The fast (na) optical axis reflects the greatest change in refractive index due to the adsorption, leading to a decrease from Δn800nm = 0.4 to 0.1. This change is discussed in terms of the Bragg−Pippard (B−P) effective medium approximation, which is shown to accurately describe changes in optical behavior in response to adsorption. The integration of generalized ellipsometry with quartz crystal nanogravimetry establishes a highly sensitive technique for acquiring adsorption isotherms and for chemical optical sensing of structurally anisotropic thin films.Keywords (keywords): glancing angle deposition; reactive ballistic deposition; ellipsometric porosimetry; Mueller matrix; titanium dioxide; anisotropy
The quality of flagellin films in terms of thickness and homogeneity was investigated by spectroscopic ellipsometry. Flagellin films were prepared in three steps: silanization, glutaraldehyde activation and finally the coating with proteins. The process of film preparation was optimized by varying the duration of the silanization and by testing sticking on different substrates including Si wafer covered with different thickness of silicon-oxide and also covered with a thin film of tantallum pentoxide. Spectroscopic ellipsometry was applied to gain in-depth information on the film properties for the optimization of the immobilization. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
A variety of simple non-destructive techniques were investigated to suppress backside reflections from transparent glass and plastic substrates. Different “glassy” materials such as translucent adhesive tape, white glue, grease, paint, and modelling clay were investigated and found to work well for substrate materials with refractive indices in the range of 1.35 to 1.60, including materials such as fused silica, float glass, display glass, microscope slides, plastic sheets, etc. These back-surface suppression techniques are also useful in suppressing anisotropic effects from plastic substrates. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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.
Protein filaments composed of thousands of subunits are promising candidates as sensing elements in biosensors. In this work in situ spectroscopic ellipsometry is applied to monitor the surface immobilization of flagellar filaments. This study is the first step towards the development of layers of filamentous receptors for sensor applications.Surface activation is performed using silanization and a subsequent glutaraldehyde crosslinking. Structure of the flagellar filament layers immobilized on activated and non-activated Si wafer substrates is determined using a two-layer effective medium model that accounted for the vertical density distribution of flagellar filaments with lengths of 300–1500 nm bound to the surface. The formation of the first interface layer can be explained by the multipoint covalent attachment of the filaments, while the second layer is mainly composed of tail pinned filaments floating upwards with the free parts. As confirmed by atomic force microscopy, covalent immobilization resulted in an increased surface density compared to absorption.
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.
The refractive index increment of a protein solution is a property not only of the protein, but also of the solvent. This is demonstrated theoretically and confirmed experimentally using analytical interferometry. © 1998 John Wiley & Sons, Inc. Biopoly 46: 489–492, 1998
Imaging ellipsometry is presented as a technique for quantification and visualization of the lateral thickness distribution of thin (0–30 nm) transparent layers on solid substrates. The main advantage of imaging ellipsometry is that every point on a surface is measured at the same time with a high lateral resolution. The method is based on the use of combined null and off‐null ellipsometry at an incident angle close to the pseudo‐Brewster angle of a high index substrate such as silicon. In the present experimental setup, a xenon lamp, a collimator, and a wavelength‐selective filter provide an expanded collimated probe beam with a diameter of 25 mm. Other major components in the system are a polarizer, a compensator, and an analyzer. In this way, a 15×30 mm2 image of a sample surface can be focused onto a charge‐coupled‐device video camera and transferred to a computer for further evaluation by image processing. Thickness measurements are performed for calibration purposes with ordinary null ellipsometry. The imaging ellipsometer has an accuracy of better than 0.5 nm at a lateral resolution of 5 μm in the present configuration, but improvements of at least a factor of 5 can be foreseen. Several aspects of the ellipsometric imaging system are illustrated in selected applications including continuous protein thickness distributions, stepped silicon dioxide thickness distributions, and visualization of protein patterning of surfaces. The latter can be used in a biosensor system as illustrated here by antigen–antibody binding studies. © 1996 American Institute of Physics.
We introduce a new measurement system called Nanopolar interferometer devoted to monitor and characterize single nanoparticles which is based on the interferometric phase modulated ellipsometry technique. The system collects the backscattered light by the particles in the solid angle subtended by a microscope objective and then analyses its frequency components. The results for the detection of 2 μm and 50 nm particles are explained in terms of a cross polarization effect of the polarization vectors when the beam converts from divergent to parallel in the microscope objective. This explanation is supported with the results of the optical modelling using the exact Mie theory for the light scattered by the particles.
Biodegradable polyesters such as poly(lactic-co-glycolic acid) copolymers (PLGA) are preferred materials for drug carrier systems although their surface hydrophobicity greatly limits their use in controlled drug delivery. PLGA thin films on a solid support blended with PEG-containing compound (Pluronic) were used as model systems to study the interfacial interactions with aqueous media. Degree of surface hydrophilization was assessed by wettability, and X-ray photoelectron spectroscopy (XPS) measurements. Protein adsorption behavior was investigated by in situ spectroscopic ellipsometry. The degree of protein adsorption showed a good correlation with the hydrophilicity, and surface composition. Unexpectedly, the layer thickness was found to have a great impact on the interfacial characteristics of the polymer films in the investigated regime (20-200 nm). Thick layers presented higher hydrophilicity and great resistance to protein adsorption. That special behavior was explained as the result of the swelling of the polymer film combined with the partial dissolution of Pluronic from the layer. This finding might promote the rational design of surface modified biocompatible nanoparticles.
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