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Investigation of thin polymer layers for biosensor applications
Abstract
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.
... 56 The authors also underlined the variation in sizes of adsorbed molecules due to polydispersity of bPEI in the solution. The high polydispersity of bPEI and the flattening of the molecules during adsorption on polyimide-coated glass were also reported by Saftics et al. 57 Jackson et al. 54 investigated the radius of gyration of PAMAMDs (R g ) by means of small-angle X-ray scattering by calculating a sphere radius (R) using the formula R = R g /√0.6. The PAMAMD sphere diameter (d = 2R) obtained with this methodology was further compared with the PAMAMD diameter (d TEM ) determined by means of TEM. ...
... On the other hand, the bPEI mass increase observed at prolonged adsorption times can be explained by the aggregation of the large bPEI molecules caused by the high polydispersity index of the macroions. 57 It is worth noticing that for short adsorption times, the PAMAMD dendrimer mass (1 mg m −2 ) agrees with the results obtained by in situ reflectometry in studies by Cahill et al. 65 and Porus et al. 66 with reported PAMAMD adsorbed mass values of 1.2 and 1.1 mg m −2 , respectively. These masses were acquired for the same adsorption conditions, that is, 5 min of adsorption, an initial bulk concentration of 5 mg L −1 , pH 6.0, and the ionic strength of 0.01 M. ...
... These discrepancies can be explained by different solid substrates used for the bPEI adsorption: Si wafers with a 100 nm layer of SiO 2 on the top (reflectometry) versus glass covered by a 170 nm layer of Si 0.78 Ti 0.22 O 2 (OWLS). On the other hand, the mass determined for longer adsorption times (0.7 mg m −2 ) agrees with the mass of the bPEI saturated layer, that is, 0.1−0.7 mg m −2 , determined from OWLS by Saftics et al. 57 In the second step of the OWLS experiments, the adsorption/ desorption kinetics of λ-car was studied on the preadsorbed stable PAMAMD and bPEI layers at the ionic strength of 0.01 M and pH 5.8. The results are the continuation of the kinetic traces shown in Figure 2a,b. ...
The kinetics of lambda carrageenan (λ-car) adsorption/desorption on/from anchoring layers under diffusion- and convection-controlled transport conditions were investigated. The eighth generation of poly(amidoamine) dendrimers and branched polyethyleneimine possessing different shapes and polydispersity indexes were used for anchoring layer formation. Dynamic light scattering, electrophoresis, streaming potential measurements, optical waveguide lightmode spectroscopy, and quartz crystal microbalance were applied to characterize the formation of mono- and bilayers. The unique combination of the employed techniques enabled detailed insights into the mechanism of the λ-car adsorption mainly controlled by electrostatic interactions. The results show that the macroion adsorption efficiency is strictly correlated with the value of the final zeta potentials of the anchoring layers, the transport type, and the initial bulk concentration of the macroions. The type of the macroion forming the anchoring layer had a minor impact on the kinetics of λ-car adsorption. Besides significance to basic science, the results presented in this paper can be used for the development of biocompatible and stable macroion multilayers of well-defined electrokinetic properties and structure.
... 56 The authors also underlined the variation in sizes of adsorbed molecules due to polydispersity of bPEI in the solution. The high polydispersity of bPEI and the flattening of the molecules during adsorption on polyimide-coated glass were also reported by Saftics et al. 57 Jackson et al. 54 investigated the radius of gyration of PAMAMDs (R g ) by means of small-angle X-ray scattering by calculating a sphere radius (R) using the formula R = R g /√0.6. The PAMAMD sphere diameter (d = 2R) obtained with this methodology was further compared with the PAMAMD diameter (d TEM ) determined by means of TEM. ...
... On the other hand, the bPEI mass increase observed at prolonged adsorption times can be explained by the aggregation of the large bPEI molecules caused by the high polydispersity index of the macroions. 57 It is worth noticing that for short adsorption times, the PAMAMD dendrimer mass (1 mg m −2 ) agrees with the results obtained by in situ reflectometry in studies by Cahill et al. 65 and Porus et al. 66 with reported PAMAMD adsorbed mass values of 1.2 and 1.1 mg m −2 , respectively. These masses were acquired for the same adsorption conditions, that is, 5 min of adsorption, an initial bulk concentration of 5 mg L −1 , pH 6.0, and the ionic strength of 0.01 M. ...
... These discrepancies can be explained by different solid substrates used for the bPEI adsorption: Si wafers with a 100 nm layer of SiO 2 on the top (reflectometry) versus glass covered by a 170 nm layer of Si 0.78 Ti 0.22 O 2 (OWLS). On the other hand, the mass determined for longer adsorption times (0.7 mg m −2 ) agrees with the mass of the bPEI saturated layer, that is, 0.1−0.7 mg m −2 , determined from OWLS by Saftics et al. 57 In the second step of the OWLS experiments, the adsorption/ desorption kinetics of λ-car was studied on the preadsorbed stable PAMAMD and bPEI layers at the ionic strength of 0.01 M and pH 5.8. The results are the continuation of the kinetic traces shown in Figure 2a,b. ...
Physicochemical properties of κ–carrageenan (KC) molecules and their aqueous solutions were determined using molecular dynamics modeling and experimental techniques. Chain conformations, the end to end length, the gyration radius and the density of the molecule were theoretically calculated. Experimentally, the diffusion coefficient and electrophoretic mobility of the molecules were measured for ionic strength range 10⁻⁴ to 0.15 M and different pHs. Using these data, the hydrodynamic diameter, the electrokinetic charge and the effective ionization degree of KC molecules were determined. The dynamic viscosity measurements for dilute KC solutions enabled to determine of its molecule intrinsic viscosity for various ionic strengths. The viscosity attained 50,000 for dilute electrolyte solutions compared to the Einstein value for spheres equal to 2.5. This confirmed that KC molecules assumed extended conformations in accordance with theoretical modeling. It is concluded that the application of complementary theoretical and experimental methods furnishes reliable information about KC molecule conformations in electrolyte solutions.
... Typical values for n A range from 1.36 to 1.5. n A is close to 1.36 when the layers are ultrathin, hydrated, and swollen as is the case for dextran 74 or hyaluronan layers. 75 OWLS has been widely used to study in situ protein adsorption onto surfaces, covalent grafting of biomolecules, such as ethylene glycol dendrons, 76 and growth of layer-by-layer films. ...
... The nature of the adsorption (revers- ible/irreversible) can also be assessed by comparing the signal after washing to the initial baseline value. 74 An analysis was developed in order to take into account a possible anisotropy of the adsorbed layer leading to birefrin- gence. 78 In this case, n A consists of ordinary (n O ) and extraordinary refractive indices (n E ). ...
... It was applied to study the grafting of carboxymethylated dextran on aminosylated or epoxysylated surfaces. 74 Recently, a planar optical waveguide made of a biocom- patible material, Nb 2 O 5 , was designed at the bottom of com- mercially available microplates (Epic) to do high throughput characterization of the adsorption of green tea catechols and subsequent cell adhesion. 79 New experimental developments also focus on the integration of a grating coupled interferom- eter and spectroscopic ellipsometer into one single tool. ...
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."
... BPEI has a spherical conformation in the solution; however, it is slightly flattened due to adsorption on a solid surface [101]. BPEI is polydisperse, which was confirmed in Ref. [102]. The presence of tertiary amine groups allows bPEI to act as the "proton sponge" [44], thus, it can be applied in biotechnology and medicine. ...
Growth factors are a class of proteins that play a role in the proliferation (the increase in the number of cells resulting from cell division) and differentiation (when a cell undergoes changes in gene expression becoming a more specific type of cell) of cells. They can have both positive (accelerating the normal healing process) and negative effects (causing cancer) on disease progression and have potential applications in gene therapy and wound healing. However, their short half-life, low stability, and susceptibility to degradation by enzymes at body temperature make them easily degradable in vivo. To improve their effectiveness and stability, growth factors require carriers for delivery that protect them from heat, pH changes, and proteolysis. These carriers should also be able to deliver the growth factors to their intended destination. This review focuses on the current scientific literature concerning the physicochemical properties (such as biocompatibility, high affinity for binding growth factors, improved bioactivity and stability of the growth factors, protection from heat, pH changes or appropriate electric charge for growth factor attachment via electrostatic interactions) of macroions, growth factors, and macroion-growth factor assemblies, as well as their potential uses in medicine (e.g., diabetic wound healing, tissue regeneration, and cancer therapy). Specific attention is given to three types of growth factors: vascular endothelial growth factors, human fibroblast growth factors, and neurotrophins, as well as selected biocompatible synthetic macroions (obtained through standard polymerization techniques) and polysaccharides (natural macroions composed of repeating monomeric units of monosaccharides). Understanding the mechanisms by which growth factors bind to potential carriers could lead to more effective delivery methods for these proteins, which are of significant interest in the diagnosis and treatment of neurodegenerative and civilization diseases, as well as in the healing of chronic wounds.
... The results obtained showed that the stability of PI is poor in aqueous solutions and PI cannot be applied for biosensing as a result of its swelling. However, TiO 2 films of 50 nm thickness obtained using atomic layer deposition (ALD) technique are very promising for the protection of PI [64]. In situ infrared spectroscopic ellipsometry (IRSE) is a powerful tool for the characterization of structural and chemical properties of polymer brushes during stimuli-induced switching experiments [65]. ...
Polymers represent materials that are applied in almost all areas of modern life, therefore, the characterization of polymer layers using different methods is of great importance. In this review, the main attention is dedicated to the non-invasive and label-free optical and acoustic methods, namely spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation (QCM-D). The specific advantages of these techniques applied for in situ monitoring of polymer layer formation and characterization, biomolecule immobilization, and registration of specific interactions were summarized and discussed. In addition, the exceptional benefits and future perspectives of combined spectroscopic ellipsometry and QCM-D (SE/QCM-D) in one measurement are overviewed. Recent advances in the discussed area allow us to conclude that especially significant breakthroughs are foreseen in the complementary application of both QCM-D and SE techniques for the investigation of polymer structure and assessment of the interaction between biomolecules such as antigens and antibodies, receptors and ligands, and complementary DNA strands.
... For decades, polymer thin films have played an important role in many areas of modern life. In addition to classical applications, such as packaging, recent developments indicate promising new applications in the energy sector, for example, high-end coatings in batteries, membranes in fuel cells and especially organic electronics such as light emitting diodes, solar cells, sensors, bio-sensors and detectors [1][2][3][4][5][6][7][8][9][10][11]. The importance and necessity of energy efficient technologies-in which polymer thin films might be key elements-is notably emphasized by the current developments related to climate change. ...
Recent developments in mid-infrared (MIR) spectroscopic ellipsometry enabled by quantum cascade lasers (QCLs) have resulted in a drastic improvement in signal-to-noise ratio compared to conventional thermal emitter based instrumentation. Thus, it was possible to reduce the acquisition time for high-resolution broadband ellipsometric spectra from multiple hours to less than 1 s. This opens up new possibilities for real-time in-situ ellipsometry in polymer processing. To highlight these evolving capabilities, we demonstrate the benefits of a QCL based MIR ellipsometer by investigating single and multilayered polymer films. The molecular structure and reorientation of a 2.5 µm thin biaxially oriented polyethylene terephthalate film is monitored during a stretching process lasting 24.5 s to illustrate the perspective of ellipsometric measurements in dynamic processes. In addition, a polyethylene/ethylene vinyl alcohol/polyethylene multilayer film is investigated at a continuously varying angle of incidence (0∘– 50∘) in 17.2 s, highlighting an unprecedented sample throughput for the technique of varying angle spectroscopic ellipsometry in the MIR spectral range. The obtained results underline the superior spectral and temporal resolution of QCL ellipsometry and qualify this technique as a suitable method for advanced in-situ monitoring in polymer processing.
... For decades, polymer thin films have played an important role in many areas of 19 modern life. In addition to classical applications, such as packaging, recent develop- 20 ments indicate promising new applications in the energy sector, e.g., high-end coatings 21 in batteries, membranes in fuel cells and especially organic electronics such as light 22 emitting diodes, solar cells, sensors, bio-sensors and detectors [1][2][3][4][5][6][7][8][9][10][11]. The importance 23 and necessity of energy efficient technologies -in which polymer thin films might be key 24 elements -is notably emphasized by the current developments related to climate change. ...
Recent developments in mid-infrared (MIR) spectroscopic ellipsometry enabled by quantum cascade lasers (QCLs) resulted in a drastic improvement in signal-to-noise ratio compared to conventional thermal emitter based instrumentation. Thus, it was possible to reduce the acquisition time for high-resolution broadband ellipsometric spectra from multiple hours to less than 1 second. This opens up new possibilities for real-time in-situ ellipsometry in polymer processing. To highlight these evolving capabilities we demonstrate the benefits of a QCL based MIR ellipsometer by investigating single and multilayered polymer films. The molecular structure and reorientation of a 2.5m thin biaxially oriented polyethylene terephtalate film is monitored during a stretching process lasting 24.5 s to illustrate the perspective of ellipsometric measurements in dynamic processes. In addition, a polyethylene/ethylene vinyl alcohol/polyethylene multilayer film is investigated at continuously varying angle of incidence ( 0∘ – 50∘) in 17.2 s, highlighting an unprecedented sample throughput for the technique of varying angle spectroscopic ellipsometry in the MIR spectral range. The obtained results underline the superior spectral and temporal resolution of QCL ellipsometry and qualify this technique as suitable method for advanced in-situ monitoring in polymer processing.
... Considering that the antibodies are asymmetric molecules and their recognition sites must be available to interact with their target, numerous strategies for probe immobilization have been used. These include the use of chemically or genetically engineered antibody fragments, antibody immobilization using oxidized oligosaccharide moieties [11], self-assembled thiol monolayers on gold surface or crosslinking of proteins by glutaraldehyde [12][13][14][15][16], and sensibilization with polyethyleneimine or biotin [17,18]. ...
This paper describes the development of an immunosensor for the c-reactive protein (CRP), based on both direct and indirect electrochemical detections. The anti-CRP probe was grafted to the platform sensitized with poly(3-aminothiophenol) and the interaction of the CRP target with the immunosensor was carried either directly or indirectly, using 4-aminoantipyrine (4-APP) as indicator. The immunosensor presents a linear response in the range from 75 ng·mL− 1 to 150 μg·mL− 1 and detection limit of 7.24 μg·mL− 1 (N = 3). The biosensor specificity, which was determined in the presence of non-specific target is also presented in this paper. Assays realized conducted with real samples indicate that the immunosensor discriminates the serum samples from healthy and sick patients with efficiency. In addition, through atomic force microscopy, it was possible to observe differences between the immunosensor surface in the absence or presence of the CRP target.
... Using these techniques a wide range of applications, e.g. high-sensitivity characterization of protein layer formation [11][12][13], cell adhesion [14,15], monitoring of bacteria [14], interfaces [16] and substrate stability [17], has already been reported. Optical waveguide sensors provide high sensitivity, but only within the exponentially decaying region of the evanescent field, of which penetration depth is typically a few hundred nanometers. ...
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 comprehensive discussion of waveguide sensors is available in the review article of P. Kozma et al [21].The combination of ellipsometry with QCM has also been demonstrated [22]. The typical in situ ellipsometry configuration utilizes a flow cell equipped with glass windows for the input and output light [15,[23][24][25][26][27][28], in which not only the effect of the window and the absorption of the water, but also the fixed angle of incidence puts constraints. The so called Kretschmann configuration couples the light into the substrate using prisms [29][30][31][32][33][34], providing the illumination from the substrate, and measuring at the close proximity of the surface with the evanescent field, similar to OWLS and GCI. ...
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.
A series of brush copolymers, poly[(2-methyl-2-oxazoline)-random-4-vinylpyridine] (PMOXA-r-4VP), with a variety of compositions was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of the poly(2-methyl-2-oxazoline) methacrylate macromonomer (PMOXA-MA) and 4-vinylpyridine (4VP), and then characterized by ¹H NMR spectroscopy and gel permeation chromatography (GPC). The PMOXA-based coatings on the surfaces of glass, silicon, gold and polydimethylsiloxane (PDMS) substrates were then produced by short-time ultraviolet (UV) irradiation of PMOXA-r-4VP. Water contact angel (WCA), ellipsometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and ζ-potential techniques were used to characterize the coatings. The results showed that copolymers can be successfully bonded on the surfaces of glass, silicon, gold, and PDMS substrate. Besides, the PMOXA-based coatings displayed a superior resistance to bovine serum albumin, human blood platelets, Human Umbilical Vein Endothelial Cells adsorption and good biocompatibility. Finally, stability test indicated that the stability of coatings can be improved with the content of the 4VP segment. Furthermore, PMOXA-r1/2-4VP immobilized surfaces displayed good antifouling property in long-term applications.
A simple and sensitive electrochemical method for the detection of cetyltrimethylammonium bromide (CTAB) by cation exchange processes was developed. The rationale for the amperometric CTAB assay is essentially based on the different binding affinities of polystyrene sulfonate (PSS) toward electrochemically active acridine orange (AO) and electrochemically inactive CTAB. The stronger binding affinity of the PSS toward CTAB than toward AO essentially validates the amperometric CTAB assay through a cation exchange mechanism with the decrease in the redox peak current of AO adsorbed on the PSS. Because of the different binding affinities between PSS and cationic surfactants with different lengths of the alkyl chain, this method can be used to detect quaternary ammonium surfactants with similar structure. The difference value of the anodic peak current showed a linear relationship with CTAB concentration in a concentration range from 0.5 to 20 μg mL-1 and a detection limit of 0.3 μg mL-1 was obtained. More importantly, this detection method has been successfully applied to the detection of CTAB in real samples.
A simple and effective method for the detection of electrochemically inactive sodium dodecyl sulfate (SDS) has been designed, based on different binding affinity of polyethyleneimine (PEI) toward electrochemically active eosin Y and electrochemically inactive SDS. The stronger binding affinity of the PEI toward SDS than eosin Y results in the decrease of the redox peak current of surface confined eosin Y and provides a quantitative readout for the SDS. The difference in value of the cathodic peak current showed a linear relationship with SDS concentration in a concentration range from 1 to 40 μg mL−1, and a detection limit of 0.9 μg mL−1 for SDS was obtained. Furthermore, the method has been successfully applied to the detection of SDS in real samples. The developed approach provided a simple and reliable detection for SDS and might have potential applications in electrochemical methods for inactive molecules.
Considerable efforts have been devoted to the development of rapid and sensitive methods allowing the detection of viral nucleic acid. We herein describe an assay for identification of a specific influenza sequence. The suggested method was based on isolation using paramagnetic particles coupled with electrochemical detection of isolated product. Peptide nucleic acid (PNA) was used as a probe for hybridization and identification of the influenza-derived specific sequence. The use of PNA can show numerous benefits: PNA probe is not degradable by enzymes and the duplex of PNA with RNA/DNA is more thermostable and more resistant to pH changes than DNA/DNA or RNA/RNA duplexes. This PNA probe assay can be applied as a magnetically guidable tool for detection of DNA/RNA samples under different conditions.
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.
We present the most recent results of EU funded project P3SENS
(FP7-ICT-2009.3.8) aimed at the development of a low-cost and medium
sensitivity polymer based photonic biosensor for point of care
applications in proteomics. The fabrication of the polymer photonic chip
(biosensor) using thermal nanoimprint lithography (NIL) is described.
This technique offers the potential for very large production at reduced
cost. However several technical challenges arise due to the properties
of the used materials. We believe that, once the NIL technique has been
optimised to the specific materials, it could be even transferred to a
kind of roll-to-roll production for manufacturing a very large number of
photonic devices at reduced cost.
p>In this work, we report advances in the fabrication and anticipated performance of a polymer biosensor photonic chip developed in the European Union project P3SENS (FP7-ICT4-248304). Due to the low cost requirements of point-of-care applications, the photonic chip is fabricated from nanocomposite polymeric materials, using highly scalable nano-imprint-lithography (NIL). A suitable microfluidic structure transporting the analyte solutions to the sensor area is also fabricated in polymer and adequately bonded to the photonic chip.
We first discuss the design and the simulated performance of a high-Q resonant cavity photonic crystal sensor made of a high refractive index polyimide core waveguide on a low index polymer cladding. We then report the advances in doped and undoped polymer thin film processing and characterization for fabricating the photonic sensor chip. Finally the development of the microfluidic chip is presented in details, including the characterisation of the fluidic behaviour, the technological and material aspects of the 3D polymer structuring and the stable adhesion strategies for bonding the fluidic and the photonic chips, with regards to the constraints imposed by the bioreceptors supposedly already present on the sensors.</p
Creating optical quality thin films with a high refractive index is increasingly important for waveguide sensor applications. In this study, we present optical models to measure the layer thickness, vertical and lateral homogeneity, the refractive index and the extinction coefficients of the polymer films with nanocrystal inclusions using spectroscopic ellipsometry. The optical properties can be determined in a broad wavelength range from 190 to 1700 nm. The sensitivity of spectroscopic ellipsometry allows a detailed characterization of the nanostructure of the layer, i.e. the surface roughness down to the nm scale, the interface properties, the optical density profile within the layer, and any other optical parameters that can be modeled in a proper and consistent way. In case of larger than about 50 nm particles even the particle size can be determined from the onset of depolarization due to light scattering. Besides the refractive index, the extinction coefficient, being a critical parameter for waveguiding layers, was also determined in a broad wavelength range. Using the above information from the ellipsometric models the preparation conditions can be identified. A range of samples were investigated including doctor bladed films using TiO2 nanoparticles.
Reverse symmetry waveguide biosensors employ substrates with a refractive index less than the analyzed aqueous cover sample (1.33). This design offers higher sensitivities for detecting micron scale biological objects such as bacteria and living cells. In the present paper, the fabrication details of reverse symmetry polymer waveguide sensors using nanoporous silica substrates with refractive index 1.2 are presented. Using nanoporous substrates the direct spin-coating deposition of polymer films is not feasible, since the solvent used to dissolve the polymer fills up the nanopores leading to a substrate RI more than 1.33. Instead, a technique—referred to as dip-floating—was applied to create freely floating 150-nm thick polymer films on the surface of water. The films were transferred to the nanoporous substrates simply by pressing the substrates through the air-floating film–water interface. A heat molding technique using a PDMS grating was used to create integrated gratings on the polymer films for light coupling.
A grating-coupled planar optical waveguide sensor is presented for sensing of bacteria by evanescent waves. The waveguide design results in increased depth of penetration into the sample volume, which makes it suitable for detecting micrometer-sized biological objects. We tested the sensor's performance by monitoring the adhesion of Escherichia coli K12 cells to the sensor surface.
This chapter describes ellipsometric characterization of thin films. The chapter illustrates that the newly activated interest is driven by the demand for rapid, nondestructive analysis of surfaces and thin films—particularly films and surfaces occurring in different device technologies. The fact that ellipsometric measurements can be performed under any ambient conditions is a definite advantage over other surface-science (electron or ion beam) techniques for industrial applications. In ellipsometry, the change in polarization state of a linearly polarized beam of light has to be measured after non-normal reflection from the sample to be studied. The polarization state can be defined by two parameters—for example, the relative phase and relative amplitude of the orthogonal electricfield components of the polarized light wave. The technique of principles of ellipsometry found its first practical use with the development of so-called rotating element ellipsometers and in computers to solve complex equations. These polarimeter-type ellipsometers measure continuously thus, wavelength scanning can be performed.
We report here on the structural characterization of polyelectrolytes multilayer films formed by poly(l-glutamic acid) and poly(l-lysine) (PGA/PLL). The growth of this system is compared to that of poly(styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) multilayers by means of in situ atomic force microscopy (AFM) and by optical waveguide lightmode spectroscopy (OWLS). In contrary to the (PSS/PAH)i films that are growing linearly with the number of deposited layer pairs i, optical data evidenced that the (PGA/PLL)i films are characterized by an exponential growth. The analysis of the structure of the (PSS/PAH)i films reveals a smooth featureless surface covered by small globules. On the other hand, (PGA/PLL)i films form extended structures that appear with a vermiculate pattern. We propose a new growth mechanism based on polyelectrolyte diffusion in and out of the film coupled to the formation of polyanion/polycation complexes at the surface of the film in order to explain the whole results.
Surfaces of a hydrolyzed PMDA-ODA polyimide have been examined by contact angle titrations. From an analysis of the data and the free energy changes occurring during ionization, an estimate can be made of the number of carboxylic acid groups created by hydrolysis. Calculations estimate that (5−7) × 1017 −CO2H/m2 are formed during alkaline hydrolysis (0.25 M NaOH) from times of 5−120 min. These results are consistent with X-ray scattering data and unit cell area values obtained on aromatic polyimides.
The adsorption features of aqueous PEI/SDS solutions on silica surfaces were investigated by ellipsometry. Results were compared with the interfacial behavior of the SDS solutions on preadsorbed PEI layers. It was shown that the properties of the mixed surface layers can be well interpreted via the individual adsorption properties of PEI as well as the characteristics of its interaction with SDS in the bulk solution.
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 adsorption kinetics of three model proteins—human serum albumin, fibrinogen and hemoglobin—has been measured and compared using three different experimental techniques: optical waveguide lightmode spectroscopy (OWLS), ellipsometry (ELM) and quartz crystal microbalance (QCM-D). The studies were complemented by also monitoring the corresponding antibody interactions with the pre-adsorbed protein layer. All measurements were performed with identically prepared titanium oxide coated substrates. All three techniques are suitable to follow in-situ kinetics of protein–surface and protein–antibody interactions, and provide quantitative values of the adsorbed adlayer mass. The results have, however, different physical contents. The optical techniques OWLS and ELM provide in most cases consistent and comparable results, which can be straightforwardly converted to adsorbed protein molar (‘dry’) mass. QCM-D, on the other hand, produces measured values that are generally higher in terms of mass. This, in turn, provides valuable, complementary information in two respects: (i) the mass calculated from the resonance frequency shift includes both protein mass and water that binds or hydrodynamically couples to the protein adlayer; and (ii) analysis of the energy dissipation in the adlayer and its magnitude in relation to the frequency shift (c.f. adsorbed mass) provides insight about the mechanical/structural properties such as viscoelasticity.
In surface biology there are numerous studies carried out using single wavelength ellipsometry, especially in the area of macromolecular adsorption on solid surfaces. The results obtained contribute significantly to the understanding of the basic mechanisms of adsorption and surface dynamics of organic molecules, especially of proteins. An example of an area of great importance is biomaterials, where ellipsometry is used as a tool in the process of acquiring knowledge about the biological acceptance of new as well as currently used implant materials. In the area of affinity biosensors, ellipsometry has been suggested as a potential readout principle. Ellipsometry is also a tool in emerging technologies, such as surface molecular engineering with the aim to construct molecular superstructures with predesigned biological functions and to interface biology with electronics. However, in most cases when ellipsometry is applied in biology, it has been used for surface mass determination. The potential in using spectroscopic data for resolving microstructural and dynamic information has not been exploited fully. From the above perspective, this report reviews the use of spectroscopic ellipsometry for studies in surface biology and highlights the advantages it offers. Two main themes are developed. The first is spectroscopy on monolayers of macromolecules with emphasis on determination of their dielectric functions and microstructure. A specific example discussed is ferritin adsorption on gold. The results, including dynamics of both the surface mass and layer microstructure, indicate an adsorption model based on a two-state adsorption mechanism. The second theme is ellipsometrically based biosensor systems. The discussion covers aspects of what imaging ellipsometry can provide in this context and is exemplified by results from affinity biosensor and gas sensor systems.
In sensor applications it is often necessary to cover the sensor surface or the interacting area of the sensor with a shielding layer. We present a well reproducible and easy method to shield a sensor surface with an aromatic polyimide layer. The shielding capacity was tested with surface acoustic wave (SAW) devices which are commercially available and furthermore meet the requirements for (bio)sensor applications in aqueous media. All experiments described here were done with these devices. One major advantage of this technique was the prevention of corrosion processes on the sensor surface, especially the damage of interdigital transducers which often consist of aluminium, because this material is known to have the best acoustoelectric properties for SAW-devices. Besides, we show that a thin polyimide film enhance the typical sensor characteristics in terms ‘of sensitivity and stability’.
We have examined the adsorption of a nonionic surfactant, penta(ethylene glycol) monododecyl ether (C12E5), at the poly(methyl methacrylate) (PMMA)−water interface using spectroscopic ellipsometry. The solid PMMA surface was deposited by spin casting an ultrathin film onto a freshly cleaned silicon wafer. Measurements by both spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) showed that the thin PMMA film was uniform with no prominent structural features on the surface. The adsorption of C12E5 at the solid PMMA−aqueous solution interface was studied using a specially designed cell with a fixed angle of incidence of 75°, and the measurements were made over a wide concentration range around the critical micellar concentration (cmc). It was found that the adsorption is completely reversible and that there is no observable penetration of C12E5 into the PMMA. The adsorption was found to reach equilibrium well within seconds. Although spectroscopic ellipsometry cannot allow a reliable measurement of layer thickness as a result of coupling between refractive indices and layer thickness for ultrathin layers, the surface excess at a given concentration can be determined reliably. The limiting area per molecule at the cmc was calculated to be 50 ± 3 Å2, in good agreement with the value obtained from a previous neutron reflection study.
The role of polymer charge density in the kinetics of the adsorption and desorption, on silica, of the polyelectrolyte poly(ethyleneimine) (PEI) was investigated by stagnation-point flow reflectometry. In the first series of experiments, PEI solutions were introduced at the same ionic strength and pH as the background solvent. It was found that the adsorbed amount of PEI increased by increasing pH. In the second series of investigations, several PEI solutions with ascending pH were introduced consecutively into the cell. In these cases, a stepwise buildup of the adsorbed amount was observed and the "final" adsorbed amounts were observed to be roughly equal with the adsorbed amounts of the first series of measurements at the same pH. Finally, adsorption/desorption experiments were performed where the preadsorption of PEI was followed by the introduction of PEI solutions of descending pH. No desorption was detected when the pH changed from pH = 9.7 to pH = 5.8. However, when there was a 9.7 --> 3.3 or 5.8 --> 3.3 decrease in the pH, the kinetic barriers of desorption seemed to completely disappear and roughly the same adsorbed amount as in the first series of experiments at pH = 3.3 was quickly attained by desorption of the PEI. This study reveals the high impact of pH, affecting parameters such as charge density of the surface and polyelectrolyte as well as the structure of the adsorbed macromolecules, on the desorption properties of weak polyelectrolytes. The observed interfacial behavior of PEI may have some important consequences for the stability of alternating polyelectrolyte multilayers containing weak polyelectrolytes.
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