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.
© 2016 Optical Society of America
The sensing of adsorption at solid liquid interfaces is of primary importance in many applications [1–3 ]. Besides high-sensitivity label-free methods like optical waveguide lightmode spectroscopy (OWLS) [4–7 ], grating coupled interferometry (GCI) [8–10 ], quartz crystal microbalance (QCM) [11,12 ] and other resonant waveguide methods [13,14 ], ellipsometry is becoming more and more interesting due to the spectroscopic, and as a result, complex modeling capabilities . Ellipsometry has been used for the in situ measurement of protein adsorption and the investigation of solid liquid interface processes for already several decades [16–19 ]. Both the instrumentation and the modeling methods are mature. We have shown that complex protein structures can be characterized in situ, using proper optical models . However, in spite of the advanced modeling capabilities, the sensitivity is limited, typically 2-3 orders of magnitude smaller than in case of waveguide sensors. The major difference between waveguide sensors and ellipsometry is that ellipsometry measures the change of polarization during one reflection, whereas in waveguide sensors the phase change is accumulated during multiple reflections within the waveguide as the light travels from the location of in coupling to the location of out coupling. A comprehensive discussion of waveguide sensors is available in the review article of P. Kozma et al .The combination of ellipsometry with QCM has also been demonstrated .
The typical in situ ellipsometry configuration utilizes a flow cell equipped with glass windows for the input and output light [15,23–28 ], 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 [29–34 ], providing the illumination from the substrate, and measuring at the close proximity of the surface with the evanescent field, similar to OWLS and GCI. The Kretschmann method in ellipsometry provides the possibility of using plasmon resonance [35,36 ], because in this configuration plasmons can be created in conductive films which are thin enough to allow the propagation of the evanescent field, but thick enough to minimize radiation loss. Also, measuring through the substrate allow the use of a broader wavelength range which would typically be absorbed when measuring through the liquid. This opens the way to many applications, including in situ infra red ellipsometry for solid liquid interface processes .
Our group designed a modified Kretschmann configuration with a semi-cylinder, in which the light can be coupled from angles ranging from 45° to 90° in a broad wavelength range extending from approximately 350 nm to the end of the range (1690 nm) of our ellipsometer . In this article we describe a two-channel version of the new configuration, in which simultaneous in situ measurements can be made in the semi-cylindrical Kretschmann configuration in two differently prepared surfaces in the same liquid cell in the same run. The advantage of the method is that it combines the 2-3 orders of magnitude higher sensitivity of the Kretschmann configuration (compared to conventional measurements through the liquid) with the multi-channel capabilities of advanced biosensing techniques. Based on the mapping of a focused spot, the method can easily be extended to more than two channel measurements, or even for combinatorial materials science investigations .
In this study we chose reference materials such as simple protein structures [40–42 ], and cells, to show the capabilities of the two-channel semi-cylindrical Kretschmann ellipsometry. Also, we utilized a new TiO2 nanoparticle preparation technique , and a masking method to prepare substrates suitable for the two-channel measurement. We obtained enhanced adsorption on the nanoparticle surface, most probably due to the enhanced specific surface area and nanoroughness, in agreement with previous studies [23,44–46 ].
2. Materials and methods
2.1.1 Protein adsorption and cell adhesion measurements
10 mM (millimol/L) phosphate buffer was prepared from phosphate-buffered saline (PBS) tablets (Sigma–Aldrich) dissolved in MQ and sonicated for 10-15 min. This buffer solution was used for the preparation of the protein solution (sample A) and the cell sample (sample B).
For preparing Sample A, fibrinogen powder (Sigma–Aldrich) was dissolved in 10 mM PBS (pH = 7.4). The fibrinogen concentration of the solution was 1 mM. For preparing Sample B, preosteoblast (MC3T3-E1) cells were cultured in an incubator (37 °C, 5% CO2) in Minimum Essential Medium (MEM) Alpha Medium, supplemented with 10% fetal bovine serum (FBS), 2 mM L-glutamine, 100 U/mL penicillin, 100 µg/mL streptomycin solution and 0.25 μg/mL amphotericin. The cells were trypsinized with 0.05% (w/v) trypsin, 0.02% (w/v) ethylenediaminetetraacetic acid (EDTA) warmed to 37 °C. Trypsin was removed before the cells detached completely. Afterwards the cells were taken up in Hank's Balanced Salt Solution (HBSS) containing 20 mM 4-(2-Hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES) buffer (pH 7.0). The preosteoblasts were seeded on the uncoated and the TNP-coated surfaces, and were left (still surrounded by the buffer) for 1 h at room temperature to adhere before we started the ellipsometric measurement.
2.1.2 Polyelectrolyte adsorption measurements
For the polyelectrolyte adsorption measurements 0.5 mM Poly(allylamine hydrochloride) (PAH) solution was prepared from powder (Alfa Aesar), 0.5 mM Poly(styrenesulfonate) (PSS) was prepared from powder (Sigma Aldrich) dissolved in MQ. The pH of both solutions was adjusted to pH 4 and pH 8 with HCl (VWR Chemicals) and NaOH (VWR Chemicals) solutions respectively. The pH of the solutions was measured with Hydrion MicroFine pH test paper set (Sigma Aldrich.)
2.2 Experimental methods
2.2.1 Preparation of substrates
The adsorption measurements were carried out on substrates consisting of four layers: a cover glass, a 2 nm thick Cr2O3 layer, a 20 or 30 nm Au layer and a 10-12 nm thick coating made of TiO2 nanoparticles (TNPs). The cover glass slides coated by e-beam evaporation with Cr2O3 and gold thin film were purchased from the Optilab Ltd., Hungary and were used as substrates for the spin coating of the TNP-s. Prior to the preparation of the TNP coatings the substrates were steeped in ultrapure milli-Q water (MQ), methanol (VWR Chemicals) and acetone (VWR Chemicals); each step lasted for ~10 s. The substrates were dried with nitrogen gas between and after the cleaning steps.
The synthesis of TiO2 nanoparticles and the sol preparation was similar to that described [47–49 ] with the exception that neither doping agents nor solvothermal treatment were applied. 50 mL of 2-propanol and then 100 mL of distilled water was added carefully to 13.3 mL of TiCl4. 250 mL of 1.5 M NaOH solution was added dropwise to the above solution under vigorous stirring. The resulting white precipitate was thoroughly washed with water and ethanol. Finally, the precipitate was dispersed in ethanol ultrasonically, to form stable dispersion of TNPs. To obtain monodisperse TNPs the ethanolic dispersion was centrifuged at 12 000 rpm (13225 × g) for 30 min, and then the resulting supernatant with a solid content of 0.45 w/v% was used for the spin coating process. The size distribution of the TNPs in the ethanolic solution was inspected by Zetasizer Nano Zs (Malvern Instruments). The average size of the nanoparticles is 11.34 ± 1.97 nm.
The fast and simple spin coating procedure was carried out at room temperature on the gold coated glass slides. To obtain the partially coated substrates for the multichannel in situ ellipsometric measurements, we wrapped one half of the sample surface with a specific stick-on foil before spin coating the TNPs on it. The spinning time was 20 s, the spinning speed was 3000 rpm and the volume of the dropped sol was 50 μL. After the spin coating, the foil could be removed without a trace (the clean removal was checked by ellipsometric measurements), thus half of the surface remained clean and uncoated. After some minutes of drying at room temperature, the substrates were ready to use.
2.2.2 Characterization of the TiO2 nanoparticle coating
The morphology of the coatings was characterized by atomic force microscopy (AFM) (Aist-NT, DigiScope1000) in tapping mode. The AFM images (Fig. 1 ) had an area of 1 μm × 1 μm and were recorded at different parts of the surface, thus they clearly showed that TNPs covered the whole surface of the substrates homogeneously. The AFM images were processed using the data leveling, background subtraction and false color mapping operations of Gwyddion 2.37 software.
The thickness of the coating was studied by spectroscopic ellipsometry and the measurements revealed that the thickness of the coatings were 10-12 nm, which is in good agreement with the size of the nanoparticles determined by dynamic light scattering (11.3 nm by Zetasizer). The thickness of the coatings and the volume fraction of the nanoparticles were perfectly fitted by a model assuming a homogeneous effective medium approximation (EMA)  of TiO2 (50-55%) and void (45-50%). Furthermore, the volume fraction of void was also in agreement with a spherical geometry of the TiO2 particles. (If we calculate the volume fraction of a monolayer of contiguous spheres, we got similar results.) The large difference between the refractive indices of TNP and gold layer makes the uncertanity of the evaluation acceptable (0.04% for the TNP thickness, and 0.21% for the EMA% of void).
2.2.3 Multiple angle of incidence two-channel Kretschmann ellipsometry
We developed a new flow-cell using an internal reflection Kretschmann configuration [31,36 ] (Fig. 2 ), in which the substrate is positioned upside-down and a semi-cylindrical glass lens is placed above it. This results in two advantages. While in conventional in situ flow-cell ellipsometry we have to use a fix angle of incidence, which is determined by the layout of the cell, in the semi-cylindrical Kretschmann configuration any angle can be chosen between 45° and 90° due to the semi-cylinder. We used the angle of incidence where the plasmon resonance was the highest on the coated surface. The other benefit of the semi-cylinder is that in contrary to the conventional method, where the light beam propagates through the liquid, the infrared range is not absorbed. With our method, we can evaluate the spectrum from 350 nm to 1690 nm (at wavelengths under 350 nm the glass absorbs the light).
After focusing, the light beam is projected on the substrate in a small spot (~0.9 mm × 0.3 mm). The reflected beam is collimated, before reaching the detector. The intensity is the same as using the basic focusing tool of the ellipsometer. As a result, the whole spectrum can be measured in less than 1 s at one spot with low noise.
The liquid with the dissolved protein molecules or cells is flowing continuously under the substrate in a flow cell where the adsorption is taking place, therefore we can measure the process real-time. The volume of the flow-cell [Fig. 2(d)] is about 10 µl, what facilitates the rapid blending of the samples and reduces the needed amount of the sample.
During the measurement the substrate (together with the semi-cylinder and the flow-cell) is shifted back and forth relative to the light beam, so that one spot on the uncoated area and another one on the TNP-coated area are measured one-by-one alternately. The advantage of this approach is that the experiments on the two different surfaces are carried out in the same process, under the same conditions (temperature, pH, concentration etc.), thus the comparison of these measurements are more reliable than before, ruling out most of the systematic errors, and minimizing concerns about repeatability.
On the surface of the substrate where the adsorption occurs, a 10-50 nm thick gold film was evaporated on a glass slide. It facilitates the plasmon resonance phenomenon, which shows up as sharp peaks in the spectrum at certain angles of incidence in certain ranges of wavelength. The resonance peaks allow several orders of magnitude higher sensitivity than without plasmon enhancement.
The measurements were carried out using a Woollam M2000DI spectroscopic ellipsometer; the measured data were evaluated with CompleteEASE 4.72 software. The spectral range of the measurements was 191-1689 nm, but because of the absorption of the glass semi-cylinder in the UV range, we used only the data between 350 and 1689 nm in the evaluation process. The spectral step was 1.59 nm between 191 and 999 nm and 3.46 nm between 999 and 1689 nm.
The ellipsometric model can be seen in Fig. 3 with the cross-sectional view of the physical and corresponding model layers. The semi-cylindrical lens and the glass substrate are on the top, described by a BK7 glass (from SCHOTT) ambient in the model. Below, there is an intermediate 2 nm thick chromium-oxide layer to enhance the adhesion of the gold film. The optical constants of both the Au and the chromium oxide layers were determined using B-spline parametrization. The optical constants determined by this method were in good agreement with those found in the database of the ellipsometer used for the measurement. TNPs are deposited on the surface of the gold layer by the spin coating method. This is a ca. 12 nm thick EMA layer in the model, containing TiO2 (50-55%) and water (45-50%). The TiO2 reference was also taken from the database of our Woollam ellipsometer. The good fit quality and the agreement of the thickness with the diameters obtained by dynamic light scattering supported the use of this reference, for the optical constants of the water Sellmeier's dispersion equation fitted to Palik's data  was used.
The next layer is the adsorbed film itself, measured in situ during the process. In Fig. 3, it is a monolayer of protein molecules, which was modeled using the Cauchy dispersion (n = A + B/λ2 + C/λ4) with the parameters of A = 1.45, B = 0.01 and C = 0 [25,26 ] (using these parameters, the refractive index is 1.47 at the wavelength of 632.8 nm). The thickness of this adsorbing layer was monitored during the measurement. The refractive index of the polyelectrolyte layers was also described by the Cauchy formula with the same dispersion parameters as for proteins (B = 0.01), and by adjusting parameter A so that the refractive index at the wavelength of 632.8 nm is the same as that published in Ref . The lowermost medium was the PBS buffer, for which the water with the Sellmeier dispersion was used as a substrate in the model.
Figure 4 shows the map of complex reflection coefficients (ρ) as a function of the angles of incidence and wavelength. The range of angles of incidence used in this work was chosen to be close to the largest plasmon resonance regions (the lowest ρ values on the map), which is approximately around the angles of 64° and 65°.
According to our calculation from the mean standard deviation of the 5 minute long interval of the baselines of the fibrinogen adsorption measurements, the limit of detection in the most sensitive range is 2.3° for Δ, which is equal to a sensitivity of ~40 pg/mm2 in surface adsorbed mass density of the proteins.
3. Results and discussion
3.1 Protein adsorption measurements
The adsorption of a widely used stable protein, fibrinogen (Fgn), was investigated by the recently developed configuration of spectroscopic ellipsometry. After achieving a stable baseline by flowing pure PBS buffer, the Fgn solution (1 mg/mL) was introduced for 30 min, which was followed by a washing step with the buffer.
In the ellipsometric model, the adsorbing protein layer was approximated by a Cauchy layer with a fixed thickness and fitted refractive index. It can be an appropriate model, because it can be visualized as a fixed volume above the surface that is pure buffer in the beginning, and filled up with protein molecules during the measurement. This process also causes the increase of the refractive index. The curves of the ellipsometric models could be fitted to the measured spectra sufficiently in the wavelength range of 350-1690 nm, and the resonance peaks are distinct at around 1000-1200 nm [Fig. 5(a) ].
From the refractive index data and the other known parameters, the surface adsorbed mass density of the proteins can be calculated using de Feijter's formula . The kinetic curves of the adsorption proved that the TiO2-coated surface enhanced the adsorption of the proteins [Fig. 5(b)], which can be explained by the increased specific surface area due to the TNPs . If we assume (according to the AFM images) that the nanoparticles are densely deposited on the surface, we can estimate the surface area, approximated with the area of contiguous hemispheres. The increase of the area - compared to a plane surface - is about 81%.
3.2 Cell adhesion measurements
The adhesion of living cells, preosteoblasts was also studied on the TiO2 coating, using the same concept. This type of cells were already investigated by us on another nanostructured coating made of titanate nanotubes [46,54 ] and proved the adhesion enhancement effect of the coating.
In the beginning of the experiments the preosteoblasts were seeded on the uncoated and the titania-coated surfaces, and were left to adhere. After 1 h they were measured being at rest for 30 min, then buffer was flowing slowly for 30 min. Afterwards trypsin-EDTA was injected into the cuvette for 3 min for detaching the cells, and then the buffer was flowed rapidly for 1 min and slowly for 2 more hours. In the evaluation no proper ellipsometric model was found, so only the Δ is plotted here as a function of time at the wavelengths where the plasmonic effect was the highest. The changes in the Δ at the different periods of the measurement is clearly seen [Fig. 6(a) ].
After the measurement, inverted phase-contrast microscopic images were taken to investigate the residual cells on the surfaces [Fig. 6(b)]. Significantly more cells could be observed on the nanostructured surface, which indicates that the cells adhered stronger to the TiO2-coated surface.
The cells were also seeded on a control substrate which was a similar glass slide covered with gold thin film, and partly coated with TNPs. The cells on the control substrate were incubated, and phase-contrast images were taken of them each day [Fig. 6(c)].
3.3 Polyelectrolyte deposition
The process of polyelectrolyte layer-by-layer adsorption on the coatings was also investigated by the same in situ Kretschmann ellipsometric method. Poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) was applied as positively and negatively charged polyelectrolyte, respectively. Ten pairs of layers were built and measured in real-time, at pH 8, where both the titania and the gold thin film has negative charges on its surface. Each layer was being deposited for 10 min, followed by a 5 min long washing step with ultrapure water.
In the ellipsometric model the polyelectrolyte layers were modeled as a Cauchy layer with a refractive index of 1.47 . The thicknesses of the layers were demonstrated to be increased by the TNP coating [Fig. 7(a) ].
The measurement was also carried out at pH 4, where the surfaces are positively charged. The thickness of the deposited layers was significantly larger at the basic pH than at the acidic pH [Fig. 7(b)]. The difference between the thicknesses of the bilayers at the acidic and basic pH can be explained by the strong pH-dependence of the polyelectrolyte layer deposition .
A 10-μL flow cell with a semi-cylindrical lens in the Kretschmann geometry has been built for multi-channel, multiple angles of incidence, multi-wavelength, internal reflection, plasmon-enhanced ellipsometry, to study various adsorption processes. The cell can be used with a standard spectroscopic ellipsometer and mapping stage that allows measurements in the wavelength range of 350-1690 nm, for angles of incidence from 45° to 90°. Custom lenses and mountings have been designed to focus the light in a spot smaller than approximately 0.5 mm at the axis of the semi-cylinder. Multi-channel measurements can be made by scanning the stage parallel to the axis of the cylinder and moving the spot in the flow cell. We also have utilized a custom sample preparation method that allows covering only half of the sample surface with 10-nm TiO2 nanoparticles, leaving the other half as a reference that can be measured in the flow cell in the same run, used as an in situ reference channel. By optimizing the thickness of the plasmonic layer, the angle of incidence and the wavelength range, a sensitivity of ~40 pg/mm2 was achieved with a time resolution of 1 s, together with the capability of using complex optical models with numerous parameters fitted to the large amount of measured spectroscopic data. The surface adsorption was enhanced by the TiO2 nanoparticles due to the larger specific surface and nanoroughness, which is consistent with our previous results on titanate nanotubes.
Support from OTKA K115852, Lendület Program, M-ERA-NET 117847, ENIAC E450EDL, SEA4KET and TÉT_12_DE-1-2013-0002 projects are gratefully acknowledged.
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