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Determining the Sub-Nanometer Thickness of Water-Depletion Layer at the Interface between Water and Hydrophobic Substrate Yongjie Wang, Yingyan Jiang, and Wei Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b02240 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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Analytical Chemistry

Determining the Sub-Nanometer Thickness of Water-Depletion Layer at the Interface between Water and Hydrophobic Substrate Yongjie Wang, Yingyan Jiang, and Wei Wang* State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China *Corresponding Author: Wei Wang Email: [email protected] ABSTRACT: Surface plasmon resonance (SPR) is one of the most popular and powerful techniques for label-free detecting and quantitatively analyzing the interfacial refractive index (RI). So far, most of the SPR measurements are mainly applied to detect the relative change of RI upon biological and chemical events occurring at the interface, while the determinations on the absolute value of RI remains challenging. However, the absolute value of RI has become increasingly urgent in some cases, such as the existence and physical properties of water depletion layer (WDL). WDL refers to a sub-nanometer-thick layer with reduced density between water and hydrophobic substrate. The detailed explanations of how water meets hydrophobic surface have been studied by several kinds of techniques for decades but it remains under debate. In this work, we successfully established a method to measure the absolute RI at a gold-liquid interface by surface plasmon resonance microscopy (SPRM) and 2D Fourier Transformation image processing, and further applied this method to study the existence and physical nature of WDL. It was found that a 0.6-nm thick WDL existed at the interface of water and hydrophobic substrate, leading to a reduced refractive index of 1.3295 ± 0.0006 compared with the standard value of 1.3325. Our results further indicated that the WDL was consisted of a uniform layer rather than numerous isolated surface nanobubbles that distributed at the interface with high density.

Hydrophobic effects are of particular interest owing to their decisive roles in many interfacial phenomena such as protein folding, amphiphilic self-assembly, membrane fusion, and “super-hydrophobicity”.1-3 During the past decades, the physical properties of the interface between water and material have been gradually concerned. For example, it has been hypothesized that a water-depletion layer (WDL) exists at the interface between water and hydrophobic substrate. WDL is the region at the interface where the density of water gradually decreases from a bulk value to virtually no water molecules at the surface, leading to a gas-like layer at the interface.4,5 Various kinds of techniques such as molecular dynamic simulation, atomic force microscopy, neutron reflectivity, and X-ray reflectivity have been utilized to study the water-solid interface on hydrophobic surface.4-15 However, the microscopic details of how water molecules interact with a hydrophobic interface and its physical properties (such as thickness) are still controversial. Refractive index (RI) is sensitive to the composition and density of water at the interface. It’s believed that the existence of WDL will reduce the local RI of water. Therefore, determinations on the absolute RI at the interface provide a non-invasive and in situ capability for clarifying the existence of WDL, and for measuring its thickness. Surface plasmon resonance (SPR) is one of the most popular and powerful techniques to access the RI at a solid-liquid interface.16-18 Recent advancements in prism and objective-based SPR microscopy (SPRM) have rendered sufficient spatial resolutions for imaging single cells and even nanoparticles with implications for various fields such as bio-imaging, catalysis and electrochemistry.19-23 Although numerous studies

have demonstrated that SPR is sensitive to the relative changes in RI,21,23-25 determinations on the absolute value of RI have proven more challenging. In order to overcome the drawbacks such as poor sensitivity and low spatial resolution in resonant angle-based approaches, intensity-based SPR techniques, which monitor the reflectivity changes at a fixed incident angle, have become the major choice. While it exhibits superior sensitivity to the subtle changes in RI, the value of reflectivity is affected by several experimental factors such as the incident angle, the thickness, and surface modification of gold film. It is therefore technically challenging, if not impossible, to calculate the absolute value of local RI solely from the reflectivity. Because parallel measurements and comparisons on the RI of two SPR sensor chips that are modified with hydrophobic and hydrophilic molecules respectively are critical for evaluating the WDL, a capability for determining the absolute value of RI, rather than the relative changes, is certainly desirable. We have recently developed a ring-fitting method to measure the absolute RI at gold-liquid interface by an intensity-based SPRM apparatus.24 This method was built on the quantitative interpretations on the wave vector of surface plasmon polaritons (SPPs). Briefly, several previous works have adopted 2D Fourier Transformation to convert a typical SPR wave-like pattern in the space domain to two adjacent rings in frequency domain (k-space).19,21,25-28 A ring-fitting method was proposed to extract the geometrical features of these rings, which were quantitatively correlated with several experimental parameters such as the wave vectors of SPPs, the propagation direction of SPPs and the incident angle, etc.24 The calculation was straightforward and did not rely on any

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uncontrollable variables, offering an opportunity to estimate the absolute value of local RI. In this work, we established both experimental efforts and imaging processing methods to validate the capability for measuring the absolute RI at a gold-liquid interface with a precision (not sensitivity) of 7×10-4 RI unit (RIU). When comparing the values on different gold films that were respectively modified with hydrophobic and hydrophilic selfassembled monolayers, we applied this method to measure the absolute RI of water layer on hydrophobic and hydrophilic substrates. It was found that the RI value on hydrophobic substrate were 0.003 lower than that on hydrophilic substrates, suggesting the existence of WDL with an effective thickness of 0.6 nm. The influences of solution properties such as surface tension and gas saturation on the thickness of WDL were subsequently examined. Finally, with the help of the superior spatial resolution of SPRM to visualize individual nano-objects, we further discussed the physical nature of WDL and concluded that it is consisted of a uniform layer instead of isolated surface nanobubbles. EXPERIMENTAL SECTION Materials. 1-Octadecanethiol (ODT), 11-mercapto-1undecanol and perfluorohexane were bought from Aladdin. Hexadecyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich (Shanghai). The 150-nm polystyrene nanospheres used in the experiment were purchased from Janus New-Materials as aqueous solution. Other reagents were all of analytical reagent grade and used as received without further purification. The deionized water (DI water, 18.2 MΩ·cm resistivity) produced by Barnstead Smart2Pure3 UF (Thermo Fisher) was used throughout the study. Experimental Setup. The SPRM experiment was performed on an inverted microscope (Nikon TI-E) with a 60× numerical aperture (NA = 1.45) oil immersion objective. A red super luminescent light-emitting diode (SLED) with wavelength of 680 nm (Q-photonics, operating power 0.2 mW) was used as the light source, and a polarizer was inserted in the optical path to generate p-polarized light so as to excite the surface plasmon wave on a gold thin film (47 nm) on a glass coverslip, and the SPR images were recorded by a CCD camera (AVT Pike F-032B) at a frame rate of 13 frames per second. The movement of the stage was achieve by a PI stage (P-561.3CD) fixed on the sample stage of the microscopy and the movement of PI stage was controlled by a Matlab program. The standard RI of de-ionized water (DIW) and ethanol were measured by a commercial Abbe refractometer. Sample Preparation. The SPR chips were #1 type BK7 glass coverslips coated with 3 nm of chromium and then 47 nm of gold. The hydrophobic surface (contact angle ≈ 105 ± 10) is functionalized with self-assembled monolayer of 1octadecanethiol (ODT) and the hydrophilic surface is modified with 11-mercapto-1-undecanol. In a typical experiment procedure, the Au chips were rinsed by DIW and ethanol. After blown dry by N2, the chips were immersed into 5mM ODT or 11-Mercapto-1-undecanol ethanol solution for three days under room temperature. The chips were rinsed with DIW and ethanol after modification and further blown dry by N2 before experiment. Multilayer Simulation. The thickness of WDL in this work was calculated by Winspall simulation. Winspall is an opensource software, developed by Worm Juergen at Max-Planck

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Institute for Polymer Research, to calculate the angledependent reflectivity of a multiple-layer system. The multilayer model consisted of five consecutive layers: glass layer, 3-nm chromium layer, 47-nm gold layer, water depletion layer and water layer. The refractive indexes of each layer are nglass = 1.515, nchromium = 3.06769 + 3.36054i, ngold = 0.16146 + 3.64199i, nWDL = 1.0003, nwater = 1.3325. The thickness of glass and water solution is considered as the bulk media so the values of their thickness were set to 0. The RI of WDL was set to be the same as pure gas. The purpose of the simulation is to find a suitable thickness of WDL, so that the equivalent refractive index equals to the experimental values on hydrophobic substrates. RESULTS AND DISCUSSION Absolute RI detection on gold chip surface. We first demonstrated the capability for calculating the absolute RI by analyzing the wave-like SPRM patterns of polystyrene nanospheres, which were placed on the gold film to enhance the SPRM images by virtue of its interactions with SPPs. The detailed theory on the origin of such wave-like patterns and the conversion from these patterns into two adjacent rings in k-space images can be found in previous reports.27,28 In order to improve the quality of Fourier image and fitting precision, dozens of 150-nm polystyrene nanospheres were bound to the surface before experiment, which brought great improvements in the detection precision. To minimize the influence of those nanospheres on the hydrophobicity of gold substrate, the density of polystyrene nanospheres was extremely low (less than one nanoparticle per 50 μm2, with a surface coverage less than 3.510-4).29 According to our recent work, a ring-fitting method was developed to fit the dual rings in the frequency domain by using the following equation where and are the coordinates of the center of the ring, r is the radius, and σ indicates the broadening effect (thickness) of the ring.24 The theory of spatial Fourier transformation suggests that the radius r in the frequency domain is the wave vector of the SPPs in the space domain. The equation of the wave vector of the SPPs (kspp) is as follow 1

and are dielectric constants of solution and metal film, respectively. The radius r equals to the wave vector of SPPs, which is a function of wavelength λ, , and . In our experiment, the wavelength λ and dielectric constant of metal film ( ) are constant values, so the dielectric constant of solution ( ) can be directly determined from the wave vector of SPPs, i.e., the radius of the ring. The detailed procedures of image processing are shown in Figure 1a. Firstly, an original SPR image was captured by a CCD camera. Then, background subtraction methods, which will discuss in detail later, were adopted to obtain clear SPR patterns with sufficient signal-tobackground ratio. Next, after two dimensional Fourier transformation, a k-space image with two adjacent rings in frequency domain can be obtained. Finally, with the help of the ring-fitting method mentioned above, we can get access to the radius of the ring and further calculate the local


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Figure 1. (a) shows the procedures of image processing and (b) shows the Fourier ring in frequency domain in four different solutions. (c) shows the relationship between calculated value and standard RI measured by Abbe refractometer (hydrophilic surface, nAir = 1.0003, nPerflexane = 1.2520, nDIW = 1.3318, nEthanol = 1.3565). According to the curve in (c), the calculated RI (calculated from the radius of Fourier ring) of these four solutions have a significant linear correlation with the standard RI of solution with a detection precision around ± 0.0007. Scatter diagrams with error bar of each point in (c) is showed in Figure S1.

Figure 2. Processing procedures of median (a), binding (b) and differential (c) methods and their corresponding k-space images (lower-right panel). The Fourier ring of differential methods (c) is smoother and clearer than the others. (d) is the scatter diagram of calculated RI of DIW by three kinds of background subtraction methods with error bar. It’s obvious that the differential method has an evident advantage in detection precision.

RI ( ) at the interface. In order to verify this method, absolute RIs of four different media including air, perfluorohexane, DIW and ethanol (Figure 1b), have been tested. The standard RIs of DIW and ethanol were measured by a commercial Abbe refractometer, while the standard value of air and perfluorohexane were received from database and

product manual because their RIs are beyond the measurement range of our Abbe refractometer. The measurement precision of Abbe refractometer is ± 0.0002 and the standard values are calibrated through temperature correction. Note that the WDL only exists at the liquid-substrate interface with sub-nanometer thickness. It places neglectable influence on the RI of bulk medium and it therefore cannot be directly determined by Abbe refractometer. For the reason that getting clear SPR pattern of the nanoparticles and getting rid of the background interference are essential for improving the accuracy on the RI measurements, several options for background subtraction have been compared and discussed as shown in Figure 2. There were three ways to achieve this goal, including median, binding and differential methods. Median process was a common method to subtract the background disturbance originated from light path by moving the sample stage in a turning shape. Because the backgrounds from the light path didn’t move during the movement while the patterns of nanoparticles on the surface moved accordingly with the stage, the median value of each pixel in this process represented the background value (Figure 2a). By subtracting the median value, a rather clear SPR pattern of nanoparticles on the surface can be obtained. However, due to the influence of surface heterogeneity and mechanical drift of the stage, this method usually led to higher level of low-frequency noise in the k-space image (white arrow in Figure 2a), which cast great obstacles in ring fitting. Second, by subtracting images of nanoparticles before and after binding to the surface, binding was a simple way to get clear SPR pattern of nanoparticles. However, binding was not a general method because it relied on the stochastic nanoparticle-substrate interactions. It was also not applicable under some circumstances such as in air. Finally, differential technique was the most suitable approach to achieve an even and clear signal of ring in the present work. Unlike median and binding process, the purpose of adopting differential technique was to get a differential SPR pattern rather than a typical SPR pattern of nanoparticles. By horizontally moving the sample stage by a very small distance (300 nm) along x direction and subtracting two images before and after movement, we can get a clear differential SPR pattern (Figure S2), leading to superior image contrast. Compared to the median process, steady and tiny movement by piezo stage made the Fourier ring much clearer and smoother (Figure 2c). The periphery of the ring has a fullwidth at half maximum (FWHM) of ~ 0.04 μm-1 (Figure S3). According to our previous study, the FWHM is a constant value determined by the propagation length of surface plasmon polaritons.24 Under the optimized conditions (differential method), the center position of the periphery (i.e., the radius of the ring) can be fit at a precision of 0.0015 μm-1, ~ one thirtieth of the FWHM. Therefore, the radius of the ring can be determined to be 2.0698 ± 0.0015 μm-1, corresponding to the refractive index of 1.3330 ± 0.0007 (Figure S3). According to the super-localization theory, the determination precision can be further improved when increasing the signalto-background ratio (image contrast) in the k-space image containing rings. For instance, in the super-localization of single molecule fluorescence, the position of an emitter can be reliably determined at a precision of ~1 nm even though the FWHM of fluorescent spot is ~300 nm. 30 Because the differential method provided the best image contrast in kspace, the determination precision on refractive



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Figure 3. (a) The schemes and corresponding Fourier space images of DIW on hydrophobic and hydrophilic surface. The small difference of r is invisible to the unaided eye. With the help of accurate fitting, the RI on hydrophobic is smaller than that on hydrophilic surface, which indicates the existence of WDL. (b) The histogram and corresponding data table of the RI of two solutions (DIW and MeOH) between hydrophobic and hydrophilic surface. All experiment values are calibrated by the value of air. The great difference of RI of DIW between hydrophobic and hydrophilic surface clearly indicates the existence of WDL.

index is ~0.0007 for differential method while it is ~0.0015 for the other two methods (Figure 2d). Existence and physical properties of WDL. The presence of WDL will reduce the interfacial RI, so that the wave vectors kspp on the surface with WDL should be smaller than that on the surface without WDL. As discussed above, the difference of the wave vectors kspp between these two conditions can be estimated by measuring the radius of the ring in k-space (Figure 3a). For the purpose of studying whether WDL exists or not when water molecules meet a hydrophobic surface, we systematically compared the absolute values of RI on hydrophobic with hydrophilic surface in water and methanol, respectively. In our experiment, the value of dry air on hydrophobic and hydrophilic surface served as a reference to eliminate the difference of chip and other experiment conditions. The measured refractive index in dried air was set to 1.0003. The same gold film was then used for further measurements in water and methanol. By doing so, the influences of variable hydrophobic or hydrophilic monolayer (coverage, density …) as well as the gold film (thickness, roughness …) on the wavevector of surface plasmon polaritons were eliminated for each gold film. The value of methanol solution served as a verification of the calibration. Figure 3a shows the schematic diagrams and typical k-space images of these two kinds of solutions on hydrophobic and hydrophilic surface. The difference in radius of the ring between hydrophobic surface and hydrophilic surface is not obvious by naked eyes. This finding is not surprising because previous study on WDL shows that the appropriate thickness of WDL ranges from several angstroms to a few nanometers.1 It means that the difference of radius between the existence and absence of WDL is smaller than 0.009 μm-1. Such a small difference of radius is invisible to the unaided eyes. But with the help of accurate fitting, we found that the RI on hydrophobic was systematically smaller than that on hydrophilic surface, which indicated the existence of WDL.

Figure 4. The effect of surface tension (a) and gas saturation (b) of solution on WDL (hydrophobic surface). Both surface tension and gas saturation of solvent have minor influences on the thickness and existence of WDL.

Figure 3b shows the histogram and its corresponding table of the calculated values of the absolute surface RI. The result indicates that the RI of DIW on hydrophobic surface is smaller than that on hydrophilic surface when the RIs of methanol on these two surfaces are almost the same. This is an obvious evidence of the existence of WDL on hydrophobic surface. The thickness of WDL was calculated to be 0.6 ± 0.2nm (the average value of five independent experiments) by Winspall simulation. Detailed procedures and simulation parameters are shown in experimental section.31,32 Multi-layer Fresnel reflection model has been widely used to estimate the thickness of self-assembled monolayers of small molecules (thiols) and biomacromolecules (DNA and proteins), whose thickness is also ~1 nm.32 This result is also in agreement with previous experiments.1 In addition to the surface hydrophobicity, we further examined the influences of two important physical factors of solution on WDL, including gas saturation and surface tension. As shown in Figure 4a, 0.5 critical micelle concentration CTAB solution and 0.5 critical micelle concentration SDS solution only have a marginal difference with DIW, which


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Figure 5. The schemes, SPR differential images and the corresponding Fourier space images of hydrophobic surface in DIW (a) and MeOH (b) solution and with 15nm Au NPs in DIW (c). The intensity of the ring reflected the roughness of the surface and the result suggested that WDL is a continuous gas layer rather than surface nanobubbles. means that the surface tension plays a minor role on the existence and thickness of WDL. Next, as shown in Figure 4b, untreated DIW, gas saturated (CO2) DIW and degassed DIW shows little difference in calculated RIs. So gas saturation of solution also has little influence on the existence of WDL. In brief, instead of solution properties, the hydrophobicity of surface plays a decisive role in the existence and physical properties of WDL. The physical nature of WDL is also an interesting and inconclusive question that needs to be addressed. Whether WDL is a kind of continuous layer or is consisted of sparsely isolated surface nanobubbles is still controversial today.6-8,13,33 The existence and properties of surface nanobubbles on hydrophobic surface has been investigated by various kinds of techniques for several decades.34-36 Both of these two models can lead to the reduction of averaged RI at the interface, which has been validated by the present work and many other studies. Early studies which used neutron reflectivity (NR) measurements and high-resolution x-ray reflectivity suggested that WDL is a layer rather than many small nanobubbles for some particular reasons.6,7 However, the spatial resolution of these techniques was not sufficient to reach a clear conclusion. Here, we took advantage of the high spatial resolution of SPRM to differentiate the monolayer and nanobubbles models. If the WDL is consisted of a uniform layer, the scattering of SPPs by WDL should be rather small due to the large dimension of WDL. However, if the WDL is consisted of sparsely distributed surface nanobubbles, the combination of scattering effects from all individuals should be accumulated. The scattering effect can be quantitatively evaluated by analyzing the intensity of rings in k-space. According to the experimental results shown in Figure 5, we can conclude that monolayer is more appropriate to describe the physical nature of WDL. First of all, compared with surface without WDL, we did not find any obvious additional SPR pattern of nanoobjects in the SPR image of the surface with WDL while our previous studies have demonstrated the capability of SPRM to

detect single nanobubble as small as 40 nm.37,38 Secondly, in order to further push the detection limit and improve the image contrast of SPRM, the differential SPRM image was obtained by laterally moving the sample stage by 300-nm. Intensity of the adjacent rings in the k-space image of such differential SPRM images was able to reflect the existence of surface nano-objects with much smaller sizes. As shown in Figure 5a and b, the intensity of the Fourier ring has little difference between DIW and methanol solution, which should not have WDL theoretically. Next, to further rule out the possibility of small surface nanobubbles with diameters smaller than 40 nm, we used a surface which was covered with many isolated 15nm Au nanospheres as a reference. According to early study in SPRM, 15nm Au nanospheres is nearly the lowest detection limit of SPRM, so that the SPR pattern of them is hardly to recognize in untreated SPR images.39 If there were small nanobubbles on the hydrophobic surface in DIW, the intensity of the Fourier ring should be stronger than that in MeOH solution and be similar to surface covered with 15nm Au nanospheres. The SPR intensity of a 15-nm Au nanosphere (nAu = 0.16 + 3.64i) is equivalent to that of a 22-nm spherical nanobubble (RI = 1.00), according to a COMSOL simulation method that we previously established.38 Because the surface nanobubble usually has a spherical cap (micro-pancake) shape with typical contact angle of 30°, the volume of a 22-nm spherical nanobubbles equals to that of a surface nanobubble with a spanning diameter of 75 nm. It is a typical size that was often reported in literatures.34 According to Figure 5b and c, it is found that the intensity of the surface with 15nm Au nanospheres is much stronger than the surface with WDL. Therefore, our measurements by using 15-nm gold nanospheres, at least, excluded the existence of isolated surface nanobubbles larger than 75 nm. The WDL is likely consisted of a continuous layer rather than isolated small surface nanobubbles. It should be noted that the present work is not against the previous studies on surface nanobubbles,40 because of the



significantly different experimental conditions. Numerous literatures have shown that pancake-shaped nanobubbles existed on hydrophobic substrate after particular treatment such as solvent exchange and heating.34 The key is to create local oversaturation of dissolved gas by regulating the gas solubility. In fact, we have employed the same SPR microscopy to validate the existence of surface nanobubbles under such conditions.37 However, quite different conditions were applied in the present work – no solvent exchange or heating is utilized at all. AFM image of hydrophobic surface in DIW solution without solvent exchange process also proved the absence of surface nanobubble in our experiment (Figure S4). Our results indicated that surface nanobubble may not exist without solvent exchange. CONCLUSIONS In this work, we have developed a method to achieve the detection of absolute local RI by SPRM and 2D Fourier Transformation at the solid-liquid interface. By taking advantage of this method, we have successfully studied the existence and thickness of the WDL on hydrophobic surface and the relevant physical properties of this layer. The present work opens up the possibility that the SPRM image have the potential to provide more powerful and richer information by making the best of 2D Fourier Transformation and other image processing methods. By further improving the image quality of SPRM image and precision and accuracy of ring fitting program, we anticipate that this method can be applied to better detect and understand the reactions and phenomena on surface.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1. The scatter diagrams with error bar of each point in Fig.1 (c). Figure S2. Detailed sketch maps of differential method. Figure S3. The ring width fitting curve of Fig.3 (a). Figure S4. AFM image of hydrophobic Au chip in DIW.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]..

ORICD Yongjie Wang: 0000-0002-6300-053X Yingyan Jiang: 0000-0002-5367-1225 Wei Wang: 0000-0002-4628-1755

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge financial supports from the National Natural Science Foundation of China (Grants No. 21874070 and 21527807).

REFERENCES (1) Ball, P. Nature 2003, 423, 25-26. (2) Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15739-15746.

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Figure 1. (a) shows the procedures of image processing and (b) shows the Fourier ring in frequency domain in four different solutions. (c) shows the relationship between calculated value and standard RI measured by Abbe refractometer (hydrophilic surface, nAir = 1.0003, nPerflexane = 1.2520, nDIW = 1.3318, nEthanol = 1.3565). According to the curve in (c), the calculated RI (calculated from the radius of Fourier ring) of these four solutions have a significant linear correlation with the standard RI of solution with a detection precision around ± 0.0007. Scatter diagrams with error bar of each point in (c) is showed in Figure S1. 85x76mm (300 x 300 DPI)


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Figure 2. Processing procedures of median (a), binding (b) and differential (c) methods and their corresponding k-space images (lower-right panel). The Fourier ring of differential methods (c) is smoother and clearer than the others. (d) is the scatter diagram of calculated RI of DIW by three kinds of background subtraction methods with error bar. It’s obvious that the differential method has an evident advantage in detection precision. 85x71mm (300 x 300 DPI)



Figure 3. (a) The schemes and corresponding Fourier space images of DIW on hydrophobic and hydrophilic surface. The small difference of r is invisible to the unaided eye. With the help of accurate fitting, the RI on hydrophobic is smaller than that on hydrophilic surface, which indicates the existence of WDL. (b) The histogram and corresponding data table of the RI of two solutions (DIW and MeOH) between hydrophobic and hydrophilic surface. All experiment values are calibrated by the value of air. The great difference of RI of DIW between hydrophobic and hydrophilic surface clearly indicates the existence of WDL. 160x80mm (300 x 300 DPI)


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Figure 4. The effect of surface tension (a) and gas saturation (b) of solution on WDL (hydrophobic surface). Both surface tension and gas saturation of solvent have minor influences on the thickness and existence of WDL. 80x33mm (300 x 300 DPI)



Figure 5. The schemes, SPR differential images and the corresponding Fourier space images of hydrophobic surface in DIW (a) and MeOH (b) solution and with 15nm Au NPs in DIW (c). The intensity of the ring reflected the roughness of the surface and the result suggested that WDL is a continuous gas layer rather than surface nanobubbles. 160x109mm (300 x 300 DPI)


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