Optical Monitoring of the Capillary Filling Dynamics Variation in

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Optical monitoring of capillary filling dynamics variation in nanoporous anodic alumina towards sensing applications Chris Eckstein, Elisabet Xifré-Pérez, Maria Porta-i-Batalla, Josep Ferre-Borrull, and Lluis F. Marsal Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02459 • Publication Date (Web): 25 Sep 2016 Downloaded from http://pubs.acs.org on October 4, 2016

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Optical monitoring of capillary filling dynamics variation in nanoporous anodic alumina towards sensing applications Chris Eckstein, Elisabet Xifré-Pérez, Maria Porta-i-Batalla, Josep Ferré-Borrull, Lluis F. Marsal* Departament d’Enginyeria Electrònica, Elèctrica i Automàtica, ETSE, Universitat Rovira i Virgili. Avda. Països Catalans 26, 43007 Tarragona, Spain. KEYWORDS Nanoporous anodic alumina membrane, biosensor, capillary filling, laser interferometry, pore widening, polyelectrolytes, ABSTRACT Fluid imbibition-coupled laser interferometry (FICLI) is a technique in which the kinetics of a fluid infiltrating into a nanoporous anodic alumina (NAA) membrane is monitored by the interferences of a laser beam at the membrane top and bottom surfaces. A further processing of the measured data results in an estimate of the pore radius. In this work we study the accuracy of FICLI in the detection of small changes in pore radius and we evaluate the possibility of using such detection as a sensing paradigm. The accuracy is estimated by measuring samples with increasing pore radius, obtained by successive wet etching steps, while repeatability is evaluated by using different liquids. For decreasing pore radius, samples obtained by the successive deposition of polyelectrolyte double layers are used. With the aim of evaluating the possibilities of the FICLI method to sense biological binding events, BSA attachment detection is demonstrated by applying FICLI to samples before and after immobilization of the protein. Results show that the technique permits to accurately estimate

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the pore radius, the pore etching rate (with a radius variation of retch,

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DI

= 1.05 nm/min. ± 0.11

nm/min.), and the polyelectrolyte double layer thickness (with a radius variation of rPAH/PSS = 3.2 nm ± 0.2 nm per polyelectrolyte double layer). Furthermore, the pore radius reduction measured after BSA immobilization (dBSA = 4.9 nm ± 1.1 nm) is in good agreement with the protein size, as reported in the literature. With these results we provide a sound basis for the applicability of FICLI as a sensitive technique for the characterization of NAA pore radius modifications.


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INTRODUCTION Technologies employing a fusion of micro- or nanofluidics and optics are an advancing area of research 1 and a large number of chemical and biological sensing methods have been developed 2. Optofluidic systems are dependent on the physico-chemical characteristics of a liquid within a solid or liquid carrier matrix and on the interface between them. To reduce the roughness of the underlying matrix for a more homogenous liquid flow, a fluid-fluid interface systems was described to manipulate light in waveguides, of a core liquid flowing between slabs of a cladding liquid 3. A colloidal system using a microsphere in a microfluidic channel was demonstrated to act as an optically relocatable lens when interacting with the optical path of waveguides4. Moreover, as a fluid-solid interface system, the detection of analytes via refractive index changes in nanoporous platforms has become one of the most popular optofluidic sensing devices5–8. One of the most striking advantages of the integration of optics and nanofluidics lies in the application of very small sample volumes that do not require additional labelling. Nanoporous anodic alumina (NAA) is a material of high interest in nanobiotechnology due to its stability, tissue compatibility and high surface-to-volume ratio for increased analyte immobilization. The fabrication of NAA is simple and reproducible and its regular structure with arrays of a wellordered, hexagonal honeycomb-like arrangement of nanochannels with a narrow pore size distribution provides a well-defined, controllable and homogenous interaction surface

9–12

. The

application of NAA as a powerful optofluidic biosensing platform has been successfully demonstrated via the detection of proteins

6

and DNA

13

,

and the quantification of bio-

immobilization 8. Further modifications of NAA, such as increasing the pore radius via wet-chemical etching, are common practice to fine tune the intrinsic properties and geometries of NAA6,9,14–16 Additionally,


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layer by layer (LbL) deposition of polymers, frequently used in drug delivery systems17, allows to reduce the pore radius in a highly controlled manner. As a novel characterization and sensing technique, fluid imbibition-coupled laser interferometry (FICLI) monitors light intensity interferometric fluctuations of a single wavelength in real-time upon capillary-driven fluid imbibition in nanoporous membranes18–20. FICLI allows for a timeresolved estimation of the position of the meniscus of a liquid within nanochannels21. The filling kinetics can then be used as a method to characterize the internal geometries of nanochannels. The time-resolved oscillation patterns corresponding to the light intensity fluctuations are measured by a photodetector and a high-speed data acquisition system, and are analyzed on the basis of mathematical models of the filling kinetics. This analysis leads to an accurate estimate of the top and bottom pore radii 18and even of the complete pore profile 22. The accuracy of the proposed methods permits to think of possible applications to sensing. The simplest sensing paradigm can be based on pore radius variation upon the attachment of some chemical or biological molecule onto the pore walls. Here, we focus on the evaluation of the accuracy of the FICLI method towards the characterization of nanochannels after pore radius modifications and surface chemistry changes. The accuracy of FICLI is experimentally estimated for pore radius increases by pore widening steps and radius decreases by a subsequent stepwise deposition of polymer double layers within the pores. Finally, as a proof-of-concept, we demonstrate the applicability of this novel technique to the detection of a protein attachment event onto the NAA pore walls.

MATERIALS AND METHODS NAA Fabrication. NAA were fabricated following a two-step anodization protocol 12. Aluminum foils of high purity (99.999%) and a thickness of 0.5mm, obtained from Goodfellow Cambridge


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Ltd., were used. Foils were degreased with acetone ((CH3)2CO) followed by an initial electropolishing step in a mixture of ethanol (EtOH) and perchloric acid (HClO4) 4:1 (v/v) at 20 V for 4 min to provide a planar surface for the following anodization steps. A first anodization step was carried out using a mixture of sulphuric acid (H2SO4) and ethanol 5:1 (v/v) at 25V and -5°C to produce the smallest pores radius possible with the given set-up. The anodization time accounted for 20h to obtain self-ordered arrays of nanochannels 23. As the growing nano-channels propagate from disordered to ordered arrays over time during the anodization, the initially disordered nanochannels had to be dissolved by a mixture of phosphoric acid (H3PO4, 6% wt.) and chromic acid (H2CrO7, 1.8% wt.) at 70°C for 3h. A second anodization step was carried out using the same conditions as used for the first step with a total anodization time of 22 h to yield a total average thickness of 56µm. Remainder aluminum was removed by a mixture of CuCl2-HCl (saturated) and 68% HNO3. Finally the barrier layer was removed by 4 cycles of reactive-ion etching (RIE) on an Oxford Instruments Plasma Technology PlasmaPro NGP80 using a mixture of 5ccms argon (Ar) and 35ccms fluoroform (CHF3) for 5min followed by 50ccms Ar for 1min. Sidewall passivation due to condensed polymer ((CF2)n) was removed by a brief wet chemical etching step in 5% H3PO4 at 35°C for 30 seconds. Hereafter, these samples are referred to as sulphuric acid electrolyte NAA. A second set of NAA was fabricated using 0.3M oxalic acid, following the same 2 step protocol described above, with the anodization parameters set to 40V and 5°C until a final thickness of 60µm was reached. After removing the aluminum bottom the barrier layer was removed by 5% H3PO4 in a half permeation cell under current monitoring as reported Lillo M. and Losic D. 2009 24. Hereafter, these samples are referred to as oxalic acid electrolyte NAA.

NAA Modification. In order to evaluate the sensitivity of FICLI towards small pore radius increases, the pores of the sulphuric acid electrolyte NAA were widened in three consecutive steps


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of wet etching in 5% H3PO4 at 35°C for a given pore widening time (tPW) and the pore radius was estimated after every widening step. After concluding the final widening step and pore estimations, the pores were further modified with polyelectrolyte double layers as follows: The pores were hydroxylated in > 30% hydrogen peroxide (H2O2) at 70 °C for 30 min and thereafter handled in nitrogen environment. The sulphuric acid electrolyte

NAA

was

placed

in

20

mL

anhydrous

toluene

and

0.2

mL

of

(3-

aminopropyl)triethoxysilane (APTES) was added drop wise and stirred for one hour. 10 min. washing steps in anhydrous toluene in nitrogen environment and in toluene and deionized water at ambient environment followed. Incubation overnight at 120 °C facilitated silane crosslinking. Polymer deposition was performed by dipping the NAA, in poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) solutions (0.5 mg/mL in 0.15 M NaCl) for 30 min, for a number of polyelectrolyte double layers given as NPDL, following a protocol reported elsewhere 25. In order to estimate the pore radius reduction due to electrostatic immobilization of proteins, the oxalic acid electrolyte NAA was first hydroxylated as described above to provide sufficient hydroxyl groups. Subsequently the NAA was incubated with 100 µg/mL bovine serum albumin (BSA) in phosphate buffered saline (PBS) at 4 °C overnight.

NAA Characterization. Scanning electron microscopy (SEM FEI Quanta 600) in conjunction with a standard image processing package (ImageJ, public domain program developed at the RSB of the NIH, USA) served to estimate the sample thickness, pore radius of the as-produced NAA and interpore distance. Confocal microscopy (NIKON TE2000-E) was used to confirm the immobilization of the BSA protein on the inner pore walls. By superimposing 12 images taken at advancing depths of 7.6 µm from top to bottom of the BSA immobilized oxalic acid electrolyte NAA a transverse view of the NAA was produced.


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Interferometry and Data Handling. The light interference of a He-Ne laser beam (emission wavelength 632.8nm) reflected off the two interfaces of a NAA thin film is registered as a function of time. A liquid drop is released onto the NAA and readily drawn into the pores by capillary forces (supporting information Fig. S1). As the liquid interface moves within the nanopores, the optical path between the two reflected waves varies leading to successive intensity maxima and minima, represented as time-resolved interferograms (supporting information Fig. S2, left). Each extremum in the pattern is indexed with integers and the time of occurrence is registered. Then, the time difference between two consecutive extrema is plotted against the extremum index (supporting information Fig. S2, right), giving a linear trend. It can be shown that the pore radius can be estimated from the slope of this linear trend (S) through the expression:

, with

and

,

where θ is the liquid contact angle with the inner pore surface, γ is the liquid surface tension and µ is the liquid viscosity. The refractive indices of the liquid and air are n2 and n3, respectively, the laser wavelength is λ and the number of pores per unit of area is δ. Imbibition experiments were conducted on three different positions on the NAA; triplicates were performed on every position contributing to a total of up to 9 measurement replicates. Measurements were conducted with three liquids: deionized water, ethanol, and 2-propanol. Values for the liquids’ properties (i.e. surface tensions and viscosities) and for the liquid-surface interfaces (i.e. contact angles), necessary for the pore radius estimation, were obtained from the literature and summarized in table S1, in the Supplementary Information. The number of pores per unit area was estimated from the ESEM pictures and the value was taken as δ=268·1010 pores/cm2.


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Deionized water was chosen as the reference liquid for the experiments. In order to assess the reproducibility of the pore radius estimates, ethanol and 2-propanol were further used to estimate the pore radius throughout the pore widening experiment and the results are presented with respect to the results obtained using deionized water. RESULTS AND DISCUSSION ESEM . Fig. 1 a and b show ESEM images of the top surface of NAA produced in sulphuric acid electrolyte, for tPW = 0 min. and tPW = 6 min., respectively. The pictures show well-ordered pores in a hexagonal honeycomb-like distribution. ESEM image analysis revealed estimated average pore radius rpore,0 min. = 14.5 nm ± 1.1 nm at tPW = 0 min. and rpore,6 min = 17.6 nm ± 1.1 nm at tPW = 6 min (supporting information Fig. S3). Average interpore distances were estimated to dint,sulphuric = 66.4 nm ± 3.9 nm (supporting information Fig. S4). Fig. 1 c shows the membrane cross section (membrane thickness of 56 µm) with a homogenous perpendicular growth of pores from top to bottom (Fig. 1 c inset). Fig. 1 d shows the top surface of a NAA membrane produced with oxalic acid electrolyte. The pores show, as in the previous case, a well-ordered structure. After a wet-chemical pore opening process of 3 h 20 min, the radius was estimated to be rpore,oxalic = 27.5 nm ± 1.7 nm and the interpore distance was estimated to dint,oxalic = 102.1 nm ± 5.8 nm. Narrow pore size and interpore distance distributions were also found (supporting information Fig. S3 and S4).

Fluid Imbibition-Coupled Laser Interferometry (FICLI). Fig. 2 shows the estimated pore radius (r), as a function of pore widening time (tPW), obtained by FICLI, using deionised water (DI water) in a sulphuric acid electrolyte NAA membrane. The values of the parameters in eq. (1) for the liquids and NAA samples employed for the estimation of pore radius are specified in table S1. The as-produced average pore radius is estimated to rpore,0min = 16.0 nm ± 0.5 nm at tPW = 0 min.


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Consecutive pore widening steps (tPW = 2, 4, 6 min.) lead to a linearly proportional increase in estimated average pore radius, as indicated by the trend line in Fig. 2. The slope of this trend line reveals that the pore etching rate is retch, DI = 1.05 nm/min. ± 0.11 nm/min. In order to validate the values oftained with this method, Fig. S3 of the Supporting Information shows the histogram of pore radius values estimated from the ESEM pictures in Fig. 1, together with a graphical indication of the value estimated with FICLI. The result shows a good agreement between the two techniques. This result is also in good agreement with previously reported etching rates investigated by ESEM and AFM image analysis6,26 and underlines the accuracy and sensitivity of FICLI for nanopore characterization. The error bars in Fig. 2, corresponding to the standard deviation for all the measurements of the particular pore widening steps, indicate that the uncertainty in pore radius is at most 1.7 nm, which further illustrates the accuracy of the method. Figure S2c in the Supporting Information shows the time-resolved interferograms and the corresponding analysis of the time differences versus extremum order index, for one of the measurements of each pore widening time. The graphs show that in all cases, the filling dynamics are uniform and the linear fits have good regression coefficients. This result confirms a uniform imbibition of the pores and the absence of pore blockage.

Consistency of Pore Radius Estimates Using Different Liquids. Addressing the consistency of the method, Fig. 3 a and b shows the results of pore radius estimations with FICLI for the consecutive pore widening steps, using different liquids. Fig. 3 a depicts the percent deviation of ethanol and 2-propanol with respect to the values obtained with DI water, shown in Fig. 2. The error bars in Fig. 3 a represent the standard deviation of the measurement replicates for ethanol and 2propanol. The estimated radius using a given liquid corresponding to a set of replicates were first normalized with respect to the correspoending average pore radius obtained for DI water, then, the


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error bar was computed as the standard average of these normalized values. Small discrepancies between the pore radius estimates for the different liquids were found. However, these discrepancies are constant for all the pore widening times and present variations in pore radii between 3 - 5 % using ethanol and between 5 - 8 % using 2-propanol. The pore etching rates obtained with ethanol of retch,Ethanol = 0.95 nm/min. ± 0.05 nm/min. and 2-propanol of retch, 2-Propanol = 0.94 nm/min. ± 0.05 nm/min., presented in Fig. 3 b, are consistent with the pore etching rates obtained with DI water. The observed discrepancy of the estimated radius for the different liquids is probably caused by the fact that the values of the different liquid parameters (i.e. viscosity, surface tension and contact angle) considered in eq. (1) to estimate the radius do not correspond to the actual values in the experimental conditions. Previously reported changes in liquid properties in nano-channels, as opposed to (micro) capillaries, might in part explain the variation of the estimated pore radii between the liquids. For example, opposed to bulk material, higher viscosities of aqueous liquids have been reported due to the nanometer range of nanochannels27 and their rectangular geometry28. Furthermore, direct effects of pore radius on the liquid contact angle were reported29. Nonetheless, our presented results have proven reproducible and pore etching rates acquired using different liquids are in good agreement with previously reported etching rates obtained by different methods 6,14–16,26

.

Pore Radius Decrease by Layer-by-Layer filling. In order to evaluate the sensitivity of the method towards the reduction of the pore radius, PAH/PSS polyelectrolyte double layers were deposited inside the pores. Fig. 4 shows the pore radius (r) as a function of the number of double layers of PAH/PSS polyelectrolytes (NDPL) deposited inside the pores. The radius reduction rate during the LbL deposition of PAH/PSS was found to be constant at rPAH/PSS = 3.2 nm ± 0.2 nm per


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NPDL, as illustrated by the trend line in Fig. 4. The thickness of every added double layer is slightly larger than the thickness reported in other works20, where elipsometry was used to obtain an estimated PAH/PSS double layer thickness on flat surfaces of about 2 nm. The slightly larger thickness presented in this study might be explained by spatial restrictions during the LbL deposition, where the geometry of the nanochannels is confined as opposed to a flat surface. The error bars in Fig. 4, which correspond to the standard deviation for the replicates on the same sample, are smaller than the radius difference between two consecutive double layers. This demonstrates that FICLI is able to resolve each deposited double layer. The use of different liquids and the change from non-modified to PAH/PSS coated NAA illustrates the robustness of the method towards estimating NAA pore radii across different liquid-surface interfaces. Figure S2 e and f in the Supporting Information shows the time-resolved interferograms and the corresponding data analysis to confirm the absence of artifacts in the experiments and that the pores remained open after the deposition of the 3rd polyelectrolyte double layer (Fig S2 e, inset).

Detection of a Biological Binding Event. Finally, in order to demonstrate the ability of FICLI to sense biological binding events on the inner pore surface of NAA, the pore radius was estimated before and after the immobilization of BSA inside the pores. Results showed a radius reduction from 35.6 ± 0.8 nm to 30.7 nm ± 0.4 nm which lead to an estimate of BSA diameter of dBSA = 4.9 nm ± 1.1 nm. The immobilization of BSA was confirmed by confocal microscopy by conjugated fluorescence marker of the protein. Fig. 5 shows uniform protein coverage of the pores along the sample thickness. The dimensions of BSA were previously reported to 2.7 x 5.0 x 6.3 nm 30 and the estimated pore radius reduction upon BSA immobilization found in this study are in good agreement.


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CONCLUSIONS In this study the ability of fluid imbibition-coupled laser interferometry to determine very small variations in the pore radius of nanoporous anodic alumina was demonstrated and the accuracy and reproducibility of the method have been evaluated. Pore etching rate of the pore widening process has been estimated using deionized water as retch,

DI

= 1.05 nm/min. ± 0.11 nm/min., with an

accuracy in the determination of pore radius of 1.7 nm. Using different liquids to determine the pore etching rate lead to similar etching rates and pore radius discrepancies no larger than 8%. These minor discrepancies may arise from the data analysis, where values for the liquid parameters (surface tension, viscosity, contact angle) are used to obtain the radius estimate. Actual vales may deviate slightly from nominal values as they can be influenced by factors not taken into account in the analysis such as the confinement in nanometer-sized pores. It was further demonstrated that the stepwise deposition of polyelectrolyte double layers lead to a pore radius reduction of rPAH/PSS = 3.2 nm ± 0.2 nm per polyelectrolyte double layer. Finally, immobilized BSA was sensed and its size was accurately estimated to dBSA = 4.9 nm ± 1.1 nm due to a pore radius reduction from 35.6 ± 0.8 nm to 30.7 nm ± 0.4 nm. The accuracies in the determination of pore radius change estimated in this work permit to propose FICLI as a useful tool for different applications, such as sensing. The determination of the pore radius reduction due to the binding of a biological species is an example of future possibilities of the technique.

FIGURES


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Figure 1. (a) ESEM images of NAA produced in sulphuric acid at pore widening time tPW = 0min, (b) tPW = 6min and (c) cross-sectional view with (c, inset) perpendicular orientation of the pores without unordered or dead pores. (d) A well-ordered, as produced NAA produced in oxalic acid.

Figure 2. Estimated pore radius obtained with FICLI using deionized water as a function of pore widening time tPW.


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Figure 3. (a) Percent deviation of estimated pore radius obtained with ethanol and 2-propanol relative to the estimated pore radius obtained with DI water as a function of pore widening time tPW. (b) Estimated pore radius obtained with FICLI using deionized water, ethanol and 2-propanol as a function of pore widening time tPW.

Figure 4. Estimated pore radius obtained by FICLI using 0.15M NaCl as a function of the number of PAH/PSS double layers NPDL.


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Figure 5. Confocal microscopy image showing the cross section of NAA produced in oxalic acid after immobilization of labeled BSA. A clear brighter coloration is visible where the BSA is immobilized; this bright area corresponds to the sample thickness of 60µm.

AUTHOR INFORMATION Corresponding Author *Prof. Dr. L. F. Marsal; E-mail: [email protected]; telephone.: (0034) 977559625; Fax: (0034) 977 559605 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This work was supported in part by the Spanish Ministry of Economy and competitiveness TEC2015-71324-R (MINECO/FEDER), the Catalan authority AGAUR 2014SGR1344, and ICREA under the ICREA Academia Award.


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Figure 1. (a) ESEM images of NAA produced in sulphuric acid at pore widening time tPW = 0 min, (b) tPW = 6 min and (c) cross-sectional view with (c, inset) perpendicular orientation of the pores without unordered or dead pores. (d) A well-ordered, as produced NAA produced in oxalic acid. 197x197mm (96 x 96 DPI)

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Figure 2. Estimated pore radius obtained with FICLI using deionized water as a function of pore widening time tPW. 304x212mm (150 x 150 DPI)


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Figure 3. (a) Percent deviation of estimated pore radius obtained with ethanol and 2-propanol relative to the estimated pore radius obtained with DI water as a function of pore widening time tPW. (b) Estimated pore radius obtained with FICLI using deionized water, ethanol and 2-propanol as a function of pore widening time tPW. 304x212mm (150 x 150 DPI)


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Figure 3. (b) Estimated pore radius obtained with FICLI using deionized water, ethanol and 2-propanol as a function of pore widening time tPW. 304x212mm (150 x 150 DPI)


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Figure 4. Estimated pore radius obtained by FICLI using 0.15M NaCl as a function of the number of PAH/PSS double layers NPDL. 304x212mm (150 x 150 DPI)


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Figure 5. Confocal microscopy image showing the cross section of NAA produced in oxalic acid after immobilization of labeled BSA. A clear brighter coloration is visible where the BSA is immobilized; this bright area corresponds to the sample thickness of 60 µm. 120x110mm (96 x 96 DPI)


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Graphical abstract 169x83mm (96 x 96 DPI)


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Optical Monitoring of the Capillary Filling Dynamics Variation in

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