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From Nanodroplets by Ouzo Effect to Interfacial Nanolenses Shuhua Peng, Chenglong Xu, Timothy C Hughes, and Xuehua Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/la502821m • Publication Date (Web): 27 Sep 2014 Downloaded from http://pubs.acs.org on October 9, 2014
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From Nanodroplets by Ouzo Effect to Interfacial Nanolenses Shuhua Peng,† Chenglong Xu,‡ Timothy C. Hughes,§ and Xuehua Zhang†,* †
School of Civil, Environmental and Chemical Engineering, RMIT University,
Melbourne, VIC 3001, Australia. ‡
Department of Chemical and Biomolecular Engineering, University of Melbourne,
Parkville, VIC 3010, Australia. §
CSIRO Materials Science and Engineering, Bayview Avenue, Clayton South,
Victoria 3169, Australia.
ABSTRACT: Polymerizing nanodroplets at solid-liquid interfaces is a facile solutionbased approach to the functionalization of large surface areas with polymeric lensshaped nanostructures. In this work, we have applied a one-pot approach to obtain polymeric nanolenses with controlled size and densities. We take advantage of the formation mechanism by the direct adsorption of nanodroplets from a surfactant-free microemulsion onto an immersed hydrophobic substrate. The interfacial nanodroplets were photopolymerized to produce polymeric nanolenses on the substrate surface. The surfactant-free microemulsion of the monomer nanodroplets was obtained through the spontaneous emulsification (i.e., Ozuo effect) in the tertiary system of ethanol, water and precusor monomer. The size of nanolenses on the surface was adjusted by the nanodroplet size, following a linear relationship with the ratio of the components in the microemulsion. This simple approach is applicable to produce nanolenses over the entire surface area or on any specific area at will by depositing a drop of the microemulsion. Possessing high optical transparency, the resulting substrates may have potential application as functional biomedical supporting materials or effective light-harvesting coatings.
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Introduction Nanoscale structures and features implanted on a surface have a profound influence on a broad range of material properties.1,2 Nanostructures with lens shapes such as micro and nano-lenses provide a unique way to tailor surface optical properties, enabling suitability of the substrate to potential applications in the areas of bioimaging, solar cells, optical nano-sensing, surface antireflective coatings, and optical lithography.3-6 In particular, nanolenses are able to overcome the diffraction limit in optical imaging and achieve high resolutions at low light intensities using the nearfield imaging technique.5,7,8
Individual submicrometer lenses with different sizes and morphologies have been prepared by several methods including chemical growth,5 chemical reflow,9 and polymer bread.10 Micro and nano-lens arrays with well-defined patterns and arrangements have also been produced by self-assembly templating,11 dewetting of polymer films,8 lithography,12 inkjet printing,13 surface wrinkling,14 and focused ion beam15 techniques. A unique solution-based approach is to convert liquid nanodroplets at solid-liquid interfaces into polymeric nanolenses.16,17 Several attractive aspects of the solution-based approach have been demonstrated in our previous work.16,17 The shape of the lenses is conveniently controlled by the wettability of the precursor nanodroplets on the substrate and the chemical nature of lenses is determined by the compositions of interfacial droplets. Furthermore, the nanolenses can be situated on top of a nonplanar substrate, for example, to make lenson-lens structure.16
The major prerequisite to achieve the controlled size distribution of nanolenses over an entire surface is the ability to control the size distribution of the precursor nanodroplets before being polymerized. In our previous work, the precursor nanodroplets were produced by a solvent exchange procedure and then cured by insitu photopolymerization. However, it was difficult to realize a narrow size distribution of nanodroplets over the whole surface area by the solvent exchange where the nanodroplets formed through a heterogeneous nucleation at the interface. Similar to the formation of nanobubbles by solvent exchange, this process is very 2 ACS Paragon Plus Environment
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sensitive to various parameters, such as solvent concentration gradients, flow mixing, temperature gradient or physical and chemical heterogeneities on the surface.18-20 Here we explore a different formation mechanism, surfactant-free emulsion templating, to achieve the size control of the interfacial nanodroplets without exchanging the solvents.
It is known that small oil droplets and/or homogenous nanoparticles can form in a tertiary liquid system through a phenomenon, called the ‘Ozuo’ effect, without using any surfactant or dispersing agents.21-28 An example is the formation of decane nanodroplets in ethanol aqueous solution.29 As water and decane are both miscible with ethanol, while water is immiscible with decane. The addition of water into the solution of decane and ethanol leads to the spontaneous formation of small decane droplets. Nucleated by the Ozuo effect, the droplets are uniform in size. Their size is determined by the ratio of the components in the system.21 The previous work has shown that nanodroplets formed by the Ouzo effect can adsorb onto a hydrophobic substrate.29 The size of those interfacial nanodroplets was closely related to the droplet size in the microemulsion, as confirmed by the atomic force microscopy imaging and by the light scattering measurement. The direct adsorption of nanodroplets from the microemulsion formed by the Ouzo effect provides a potential means to control the size of the resulting nanolenses.
In this work, we demonstrate that the adsorption of nanodroplets onto a surface from microemulsions by the Ouzo effect can be developed to an effective one-pot approach to control the size distribution of interfacial nanolenses on an immersed hydrophobic surface. The size of the nanolenses can be readily adjusted by the size of the precursor nanodroplets in the system, following a linear relationship with the composition of the microemulsions.
Experimental Section Materials. Octadecyltrichlorosilane (OTS, >90%, Sigma-Aldrich), n-decane (95% GC, Fluka), crosslinker 1, 6-hexandiol diacrylate (HDODA, 80%, Sigma), monomer 3 ACS Paragon Plus Environment
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methyl
methacrylate
(MMA,
99%,
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Sigma),
initiator
2-hydroxy-2-
methylpropiophenone (97%, Sigma), ethanol (100%, AnalaR, Merck), and chloroform (>90%, AnalaR, Merck) were used without further purification. Milli-Q Water (18.2 MΩ) was used in all experiments.
Preparation of hydrophobic OTS surfaces. Silicon wafers or glass microscope slides were used as model hydrophobic surfaces. Polished silicon wafers (Mitsubishi Silicon, America-Mod 2) or glass slides were cleaned in piranha solution [H2SO4 (70%): H2O2 (30%)] at 75 ˚C for 20 min. (Warning: piranha solution reacts violently with organic materials.) It is important to minimize exposure of OTS to water as this causes polymerization and consequently a heterogeneous interface. All glassware used for the solution was dried for 2 h under 120 ˚C in the oven. The silicon was dried at 120 ˚C for 1.5 h and soaked in 0.5 vol% OTS in toluene for 12 h in a sealed dry container at room temperature. After 12 h, the OTS-silicon was rinsed with chloroform, sonicated in toluene and ethanol, dried with nitrogen, and then stored in a clean container. Before use, the OTS-silicon was cleaned in ultrasound in ethanol and water respectively. Water droplets in air on OTS-silicon forms an advancing contact angle of 112 ± 3˚ and a receding angle of 101 ± 3˚. The rms roughness as measured by AFM of the OTS-Si substrate was 0.4 nm over an area of 5×5 µm2.
Preparation of Microemulsions. The monomer mixture of MMA with initiator and crosslinker (10: 1: 1, v/v) was used as the oil phase in this microemulsion system. Microemulsions were prepared according to the following procedure: mixtures of water and ethanol with different volume ratios were firstly prepared and then a predetermined amount of MMA with initiator and crosslinker was added into the mixtures, and the solution was thoroughly mixed using a Vortex mixer. The ternary phase diagram of the MMA/ethanol/water was determined by direct observation of the transition from transparency to turbidity with water titration and is shown in Fig. 1.
Preparation of Nanolenses. OTS-Si substrates such as silicon wafer and glass slides were immersed into microemulsion solution and precursor interfacial nanodroplets 4 ACS Paragon Plus Environment
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were formed at solid-liquid interface by direct absorption of the bulk droplets in the microemulsion. In situ photopolymerization was subsequently carried out under a UV lamp (20 W, 365 nm, John Morris Scientific Pty Ltd) and the transparent precursor microemulsion solution became opaque after 10 min polymerization. The substrate with polymeric nanolenses was then removed from the microemulsion solution and rinsed with water and sonicated in water bath for 15 min to remove the weakly attached polymer particles. A schematic diagram for the nanolenses preparation procedure is surmised in Scheme 1.
Characterizations. The precursor microemulsions were measured by static light scattering (SLS) using a Malvern 4700 instrument equipped with a 488 nm laser (Malvern Instruments Ltd., Malvern, Worcestershire, United Kingdom). The scattered light was measured at a constant angle of 90˚ and a temperature of 25 ˚C. Intensity of scattered light was recorded to confirm the presence of nanodroplets in the precursor microemulsions.30 All AFM images were captured using an MFD 3D (Asylum Research) in air and cantilevers (NP probe, Veeco) were treated by UV for 20 min before use. AFM images were recorded in contact mode with constant drive frequency and feedback to maintain constant drive amplitude. Image-Pro software was employed to analyse AFM images to extract the information about the size and distribution of nanolenses. The number density of nanolenses was measured over 20× 20 µm2 area based on AFM measurements. It is difficult to image the MMA droplets in liquid by AFM, because heating from the AFM electronics during imaging leads to the phase separation and polymerization of the MMA microemulsion. The transmittance of glass slides coated with
nanolenses was characterized by using the UV/vis spectroscopy (VARIAN, Cary 50Bio). The transmittance was measured against air as the blank reference.
Results 1. Formation of interfacial nanolenses Scheme 1 shows the process to obtain polymeric nanolenses on a hydrophobic substrate. The substrate was brought into contact with the microemulsion prepared by 5 ACS Paragon Plus Environment
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the Ouzo effect. The nanodroplets in microemulsion collide and adhere to the solid surface. The adsorbed nanodroplets adopt a lens shape due to their partial wetting on the solid-liquid interface. The nanodroplets at the interface along with free nanodroplets in microemulsion were in-situ photopolymerized, and the substrate was subjected to the ultrasound cleaning to remove all the weakly attached particles originated from free nanodroplets in the microemulsion. Finally we obtained the substrate decorated with robust nanolenses.
(a) Substrate UV (b) Substrate Clean (c)
Substrate
Water Precursor droplets Polymer Scheme 1. Schematic diagram of nanolens fabrication procedure by microemulsion templating: (a) Interfacial nanodroplets form from the direct adsorption of the nanodroplets in the bulk microemulsion; (b) Photopolymerization of nanodroplets at the interfacial and in bulk; (c) Sonication to clean weakly attached polymer nanoparticles on the surface.
Figure 1(a) shows the partial phase diagram of the tertiary liquid system constructed by measuring the turbidity of monomer in ethanol upon the addition of water. The region of interest within the oil in water microemulsion phase is within the red circle, where the system was just above the spinodal curve before two phases spontaneously form. The relatively low MMA content favours the formation of discrete precursor nanodroplets. Effect of variation in the microemulsion formulations was demonstrated 6 ACS Paragon Plus Environment
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by using six solvent free microemulsions (SFE-1 to SFE-6) with rationally selected compositions as listed in Table 1. All the microemulsions appeared to be stable, homogeneous and optically transparent immediately after the preparation. The photos of the microemulsions displayed in Figure 1 (b) show that SFE-1 to 5 still remain transparent after 24 hours indicating the stability of the microemulsions.
Ethanol
(a)
0.0
1.0
0.2
0.8
0.4
0.6 Spinodal curve
0.6
0.4
0.8
0.2
1.0
H2O
0.0
0.0 0.2
0.4
0.6
0.8
1.0 MMA
Figure 1. (a) Partial phase diagram of microemulsions comprising monomer, ethanol, and water and the region of interest within the oil in water phase was indicated by the red circle; (b) Photos of six microemulsion formulations listed in Table 1.
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Table 1. Precursor microemulsion compositions for the preparation of nanolenses by
in-situ photopolymerization. Formulation No.
Ethanol (ml)
Water (ml)
Monomer (ml/%)
SFE-1
3.5
6.5
0.29/2.8%
SFE-2
3.5
6.5
0.31/3.0%
SFE-3
3.5
6.5
0.33/3.2%
SFE-4
3.6
6.5
0.33/3.2%
SFE-5
3.7
6.5
0.33/3.2%
SFE-6
3.5
6.5
0.45/4.3%
Attempts to measure the size of the microemulsion droplets by dynamic light scattering (DLS) were not successful due to the relatively high concentration. Moreover, dilution of the formulation with higher solvent levels would move the microemulsion into a different position in the phase diagram and was unlikely to represent the size of the microemulsion droplets used here. Instead we used static light scattering (SLS) intensities to characterise the presence of droplets in the microemulsions.30 SLS measurements show the scattering intensities of all five samples (150-165 kcps) was much higher than the binary continuous phase of ethanol and water (>20 kcps), confirming the presence of precursor nanodroplets in the solution.
2. Effects of the content of droplet phase Two parameters, monomer ratio and ethanol concentration were varied in six formulations to examine their effect on the morphology, size, and distribution of the resulting polymerised nanolenses. In four formulations, named SFE-1, SFE-2, SFE-3 and SFE-6, the ratio of the volume of monomer to the volume of ethanol and water increased from 0.29 in 10 (~2.8%) to 0.45 in 10 (~4.3%), while the amount of ethanol and water in the continuous phase were fixed at 3.5 and 6.5 ml, respectively. Optical 8 ACS Paragon Plus Environment
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microscopy and AFM were used to examine the surface over a large area at ambient conditions. Optical images in Figure 2 show that features were uniformly distributed over the entire surface area for all three samples. Specifically, the size of the features on surfaces increased and the number density decreased from Figure 2(a) to (c) with the increase of monomer ratio.
Morphological characteristics of the substrates were further examined by AFM measurements, as shown in Figure 2(d)-(f). The lens-shape of the nanostructures is clearly shown by the cross-sectional profiles of nanolenses in Figure 3. Three representative polymeric nanolenses in Figure 2(f) were all closely fitted by spherical caps having heights of 80-120 nm and lateral diameters of ~700 nm. Over a large surface area in Figure 2(d), the number density of nanolenses was 1.45 per µm2 when the microemulsion SFE-1 contained 2.8% MMA. As the ratio of monomer slightly increased to 3.0% in Figure 2(e), the number density decreased to 0.84 per µm2 accordingly. The number density of nanolenses further decreased to 0.40 per µm2 in SFE-3.
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Figure 2. Optical microscopy (scale bar: 10 µm) and AFM (20×20 µm) images of polymeric nanolenses by microemulsion polymerization with increasing MMA content. The parent microemulsion compositions were summarised in Table 1: (a) and (d), SFE-1; (b) and (e), SFE-2; (c) and (f), SFE-3. Corresponding size and distribution was illustrated by histogram plots, including average diameters ( ) and standard deviations (σ).
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200
Height (nm)
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150
a b
100
c 50 0 -0.5
0.0
0.5
Lateral diameter (µm) Figure 3. Cross-sectional profiles of three polymeric nanolenses indicated by arrows in Figure 2(f). The solid lines are the corresponding spherical-cap fitting.
The statistical analysis of the size distribution of nanolenses is illustrated by histogram plots with average diameters () and standard deviations (σ). At the lowest monomer ratio in the microemulsion formulation, SFE-1, a narrow size distribution of nanolenses with a mean diameter of 0.36 µm and a standard deviation of 0.09 µm was obtained. Polymerisation of SFE-2 resulted in larger nanolenses with relatively broader distributions having a mean diameter of 0.42 µm with a standard deviation of 0.13 µm. The diameter of nanolenses became even larger with SFE-3 and SFE-6, and reached 0.72 µm and 2.11 µm with standard deviations of 0.21 µm and 0.68 µm, respectively.
The polymeric nanolenses produced from SFE-6 were large enough to exhibit welldefined circular Newton rings under 100× objective lens as shown in Figure 4. The formation of the rings is due to the interference between the reflected light from the surface of underlying substrate and the spherical upper face of nanolenses.16,31 The symmetric interference patterns of these Newton rings also indicates that lenses on the OTS-Si substrate possess spherical lens-shape morphology.
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d = 2.11 µm σ = 0.68
(b)
Percentage (%)
45
30
15
0 0
1
2
Diameter (µm)
3
4
Figure 4. (a) Optical microscopy images of polymeric lenses by SFE-6. The emulsion contains 0.45 ml MMA, 3.5 ml ethanol, and 6.5 ml water. Scale bar: 10 µm; (b Corresponding size and distribution was illustrated by histogram plots.
Clearly our AFM measurements show that the nanolens diameter increases with the monomer ratio, while the standard derivation of the diameter remains rather small for all formulations, which is 25-32% of the mean diameter. The correlation between the mean diameter of polymeric nanolenses and the ratio of monomer to the liquid phase is summarized in Figure 5. Importantly, we observed a very good linear correlation between nanolens size and ratio of monomer. These correlations may be used to predict the size of the resulting polymeric nanolenses from the composition of the tertiary system. 3
Diameter (µm)
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2
y = -2.42 + 9.47x 2 R = 0.85
1
0 0.30
0.35
0.40
0.45
MMA volume (ml)
Figure 5. Correlation between average diameter of resulting polymeric nanolenses and volumes of monomer in the precursor microemulsions.
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3. Effect of continuous phase concentration The ethanol content in the continuous phase of the precursor microemulsion was varied in three formulations from SFE-3 to SFE-5. From the AFM images of the nanolenses obtained in SFE-3 and SEF-5 in Figure 6, the number density of nanolenses increased from 0.84 per µm2 to 1.30 per µm2 as the ethanol volume increased from 3.5 to 3.7 ml whilst the oil phase loading was kept constant at 0.33 ml. The mean diameter of nanolenses decreased from 0.72 µm, 0.45 µm to 0.35 µm. So the diameter of nanolenses became smaller as the ethanol concentration increased in the microemulsions. The size of nanolenses was rather uniform for all formulations as reflected by the standard derivation, which was 0.21 µm in SFE-3, 0.13 µm in SFE-4 to 0.09 µm in SFE-5. All of the standard derivations were only 25-29% of the mean diameter. The correlation between the mean diameter of polymeric nanolenses and the amount of ethanol in the tertiary system was summarized in Figure 7. The mean diameter of nanolenses increases linearly with the amount of ethanol for the fixed amount of monomer and water in the tertiary system.
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Figure 6. Optical microscopy (scale bar: 10 µm) and AFM (20×20 µm) images of polymeric nanolenses by microemulsion polymerization with increasing ethanol content. The parent microemulsion compositions were summarised in Table 1: (a) and (d), SFE-3; (b) and (e), SFE-4; (c) and (f), SFE-5.
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1.0
Diameter (µm)
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y = 6.24 - 1.59x 2 R = 0.82 0.5
0.0 3.5
3.6
3.7
Ethanol volume (ml) Figure 7. Correlation between average diameter of resulting polymeric nanolenses and volumes of ethanol in the precursor microemulsion formulations.
4. Morphology of nanolenses We now examine the morphological features of the individual nanolenses prepared from microemulsions SFE-1 to SFE-5. The morphology of spherical-cap droplet in terms of its contact angle at the interface can be calculated by the following equation:
tan = =
(1)
where R is the radius of curvature, L is the lateral diameter, h is the height, and θ is the contact angle. L and h can be directly obtained by the cross-sectional profiles of nanolenses based on AFM measurements. All the nanolenses prepared by the five formulations exhibited similar contact angle values ranging from 15 to 20˚, as shown in Figure 8. This indicates that the variation in the composition of the microemulsions was not significant enough to alter the wettability of the monomer on the solid-liquid interfaces. The morphology of the nanolenses in this work is comparable to that of lenses prepared by the solvent exchange in the presence of CTAB surfactant solution at 0.5 critical micelle concentration (cmc),17 attributed to the similar wetting properties of the monomer on the substrate in contact with the continuous phase of the ethanol solution and with 0.5 cmc CTAB solution. This observation demonstrates another feature of the one-pot approach, that is, the control of the lens size can be achieved without affecting the aspect ratio of the nanolenses.
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The contact angle of a macroscopic MMA droplet was measured to be 2-10° in aqueous solution, which is about 10 degree smaller than the microscopic contact angle of the nanolenses. There was a small variation of the contact angle with the nanolens size. As
we plotted the cosine of the contact angle versus the inverse of the lens radius, the fitting gave a slope of 7.1 nm (Supporting Information). The effect of line tension cannot account for such large difference between microscopic and macroscopic contact angle.32 In our recent paper, we have discussed in detail the alternative explanation of the weak heterogeneity theory.33 30
30
(a)
(b)
SFE-1 20
Contact angle (°)
Contact angle (°)
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SFE-1 SFE-2 SFE-3 SFE-4 SFE-5
10
0
20
SFE-3
SFE-4 SFE-5
SFE-2
10
0 0.0
0.5
1.0
1.5
Diameter (µm)
0
1
2
3
4
5
Microemulsion formulations
Figure 8. (a) Plots of contact angle as a function of diameter of polymeric nanolenses prepared by different microemulsion formulations; (b) Mean contact angles of polymeric nanolenses with different sizes by different microemulsion formulations.
5. Transparency of nanolens-decorated substrates Now we show that the substrates coated with as-prepared polymeric nanolenses exhibit highly optical transmittance to visible light (Figure 9(a)), which is highly desirable for many applications. The transparency of polymeric nanolens with different sizes on glass slides is shown in Figure 9(b). Transmittance of 80-87% over the range of 400-700 nm was observed for all the polymeric nanolenses. Specifically, transparency at 400 nm decreased slightly from 87% for bare glass slide to 85%, 83%, and 80% for nanolenses from SFE-1, SFE-2, and SFE-3 precursor microemulsions, respectively. The decrease in transparency may be induced by increase of nanolens diameter and height. As expected, an increasing trend of transmittance was observed
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when smaller nanolenses were prepared using the parent microemulsions SFE-4 and SFE-5 with higher ethanol contents.
(b) 90
Transmittance (%)
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Bare OTS glass 1
85
5
2 4 80
3
400
500
600
700
Wavelength (nm) Figure 9. High optical transparency of nanolens coated glass: (a) Photograph of glass slides coated with nanolenses with different sizes; (b) Transmittance spectra of glass slides coated with nanolenses with different sizes.
6. Spatial control of nanolens cluster The results shown above were obtained when substrates were fully immersed into microemulsions to produce nanolenses over the entire surface. Now we demonstrate that it is possible to produce clusters of nanolenses on specifically selected areas. When a small sessile drop of the microemulsion SFE-3 (5 µl) was deposited on the substrate, nanolenses were produced specifically on the area beneath the drop as 17 ACS Paragon Plus Environment
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shown in Figure 10(a) and (b) while the surrounding area remained free of nanolenses. Figure 10(c) shows that five clusters of nanolenses were obtained when five sessile drops with different volumes were deposited on the surface. This demonstrates the high flexibility of the one-pot approach in selective surface functionalization.
Figure 10. Nanolens clusters were obtained when sessile drops of precursor microemulsions were deposited on OTS-Si substrates: (a) single nanolens cluster under 4× magnification; (b) single nanolens cluster at 80× magnification; (c) five nanolens clusters under 4× magnification.
Discussion Our results clearly show that the size of nanolenses can be controlled by the monomer and ethanol content in the microemulsion formed by the Ouzo effect. The linear correlations between the mean diameter of lenses and the contents in the system can guide the design of the microemulsion to achieve desired size distribution of the interfacial nanolenses. Therefore this approach has significantly improved the size control of nanolenses by in situ polymerization of nanodroplets, which has been difficult to achieve using the previous solvent exchange method.
Below we discuss the possible mechanism behind the relationship between the size of interfacial nanlenses and the contents in the microemulsion. From previous work we know that the interfacial nanodroplets originated from the nanodroplets in the microemulsion formed by the Ouzo effect and the size of interfacial nanodroplets was closely related to the droplet size in the microemulsion.21 The size distribution of nanolenses is narrow for all formulations, determined by the nucleation mechanism of the Ouzo effect that produces homogeneous droplets. The droplet size in a 18 ACS Paragon Plus Environment
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microemulsion formed by the Ouzo effect has been quantitatively studied by Vitale et al.21 To make comparison with their observations, the lateral diameters of nanolenses were converted to the diameters of precursor nanodroplets in the microemulsion before they adsorbed onto the surface, which is straightforward once given the contact angle and the lateral diameter of nanolenses.
The volume of the nanolens was calculated based on its height and lateral size from AFM measurements and then converted to the diameter of a spherical drop (Ddrop) in the precursor microemulsion. According to the work by Vitale et al,21 Ddrop only depends on the ratio of the excess oil concentration to the ethanol concentration, where the excess oil referrs to the concentration of oil in excess of its saturation concentration in the continuous phase. As the ethanol concentration was varied over a very narrow range in our systems, the change of monomer solubility in the continuous phase was also small. Therefore, the plot of Ddrop as a function of the ratio of monomer to ethanol did not reveal the absolute solubility of monomer in the continuous phase. Despite this, a good linear relationship between the drop size and the monomer-to-ethanol ratio in precursor microemulsion was shown in Figure 11. 500
Mean diameter of droplets (nm)
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400
300
y = 8427.5x-660.5 2 R = 0.97
200
100
0 0.06
0.09
0.12
0.15
Ratio of monomer to ethanol
Figure 11. Correlation between average diameter of droplets in microemulsion and the ratio of monomer to ethanol. The diameter of droplets linearly increases with the ratio.
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The number density of polymeric nanolenses on the OTS-Si substrate may also be governed by the number density of oil droplets in the microemulsion. After certain point, the addition of water would cause dilution of the droplets in the microemulsion instead formation of new droplets, leading to the decrease of the number density of droplets in microemulsion with the decrease of ethanol concentration. This is consistent with our observation that the number density of nanolenses decreased with decreasing ethanol concentration. Moreover, for a given ratio of ethanol, the addition of monomer led to larger droplets instead of more droplets. As the droplet size increased with the monomer ratio, the scattering intensity was expected to dramatically increase for a given number density of droplets because a large droplet scatters more strongly than a small droplet. However, all the emulsions had similar scattering intensities, indicating fewer, but larger droplets in the microemulsion with higher monomer ratio. Consequently the number density was lower for those larger nanolenses formed at higher monomer ratios.
Conclusions In summary, we have demonstrated a one-pot approach to functionalize a hydrophobic surface with polymeric nanolenses of controlled size, distribution and location by using nanodroplets from the Ouzo effect. Monodispersed monomer droplets formed in a tertiary system of precursor monomer, ethanol and water and adsorbed onto an immersed hydrophobic substrate. Unlike the previous approach of the nanodroplet nucleation on the interface by switching the solvents, the size of nanolenses in our one-pot approach were readily controlled by the monomer and continuous liquid phase content, as they followed a linear relationship with the ratio of the composition in the tertiary liquid system. This approach can realize functionalization of the entire surface area by nanolenses with well defined size and narrow size distribution. Furthermore, our results demonstrate the flexibility in spatial control of nanolenses by depositing droplets of the microemulsion to obtain nanolens clusters only within specific areas. Surfaces decorated with nanolenses were optically transparent and show promising potential for application as smart surface coatings.
Acknowledgements 20 ACS Paragon Plus Environment
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XHZ and SHP gratefully acknowledge the support from the ARC Future Fellowship Scheme (FFT120100473) and a Postdoctoral Fellowship from Australian Renewable Energy Agency (ARENA).
Corresponding Author *E-mail: [email protected] (XHZ);
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References (1) Dahotre, N. B.; Nayak, S. Nanocoatings for engine application. Surf. Coat. Technol. 2005, 194, 58-67. (2) Dai, L.; He, P.; Li, S. Functionalized surfaces based on polymers and carbon nanotubes
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Nanotechnology 2003, 14, 1081. (3) Kang, D.; Pang, C.; Kim, S. M.; Cho, H. S.; Um, H. S.; Choi, Y. W.; Suh, K. Y. Shape-Controllable Microlens Arrays via Direct Transfer of Photocurable Polymer Droplets. Adv. Mater. 2012, 24, 1709-1715. (4) Chen, Y. Q.; Elshobaki, M.; Ye, Z.; Park, J. M.; Noack, M. A.; Ho, K. M.; Chaudhary, S. Microlens array induced light absorption enhancement in polymer solar cells. Phys. Chem. Chem. Phys. 2013, 15, 4297-4302. (5) Lee, J. Y.; Hong, B. H.; Kim, W. Y.; Min, S. K.; Kim, Y.; Jouravlev, M. V.; Bose, R.; Kim, K. S.; Hwang, I. C.; Kaufman, L. J.; Wong, C. W.; Kim, P. Near-field focusing and magnification through self-assembled nanoscale spherical lenses. Nature 2009, 460, 498-501. (6) Jung, H.; Jeong, K. H. Monolithic polymer microlens arrays with antireflective nanostructures. Appl. Phys. Lett. 2012, 101. (7) Liu, Z.; Lee, H.; Xiong, Y.; Sun, C.; Zhang, X. Far-Field Optical Hyperlens Magnifying Sub-Diffraction-Limited Objects. Science 2007, 315, 1686. (8) Verma, A.; Sharma, A. Enhanced Self-Organized Dewetting of Ultrathin Polymer Films Under Water-Organic Solutions: Fabrication of Submicrometer Spherical Lens Arrays. Adv. Mater. 2010, 22, 5306-5309. (9) Mason, D. R.; Jouravlev, M. V.; Kim, K. S. Enhanced resolution beyond the Abbe diffraction limit with wavelength-scale solid immersion lenses. Opt. Lett. 2010, 35, 2007-2009. (10) Wang, Z.; Guo, W.; Li, L.; Luk'yanchuk, B.; Khan, A.; Liu, Z.; Chen, Z.; Hong, M. Optical virtual imaging at 50 nm lateral resolution with a whitelight nanoscope. Nat. Commun. 2011, 2, 218. (11) Yabu, H.; Shimomura, M. Simple Fabrication of Micro Lens Arrays. Langmuir 2005, 21, 1709-1711. (12) Kunnavakkam, M. V.; Houlihan, F. M.; Schlax, M.; Liddle, J. A.; Kolodner, P.; Nalamasu, O.; Rogers, J. A. Low-cost, low-loss microlens arrays 22 ACS Paragon Plus Environment
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fabricated by soft-lithography replication process. Appl. Phys. Lett. 2003, 82, 1152-1154. (13) Lin, X.; Hosseini, A.; Dou, X.; Subbaraman, H.; Chen, R. T. Low-cost boardto-board optical interconnects using molded polymer waveguide with 45 degree mirrors and inkjet-printed micro-lenses as proximity vertical coupler. Opt. Express 2013, 21, 60-69. (14) Chan, E. P.; Crosby, A. J. Fabricating Microlens Arrays by Surface Wrinkling. Adv. Mater. 2006, 18, 3238-3242. (15) Verslegers, L.; Catrysse, P. B.; Yu, Z.; White, J. S.; Barnard, E. S.; Brongersma, M. L.; Fan, S. Planar Lenses Based on Nanoscale Slit Arrays in a Metallic Film. Nano Lett. 2008, 9, 235-238. (16) Zhang, X. H.; Ren, J. M.; Yang, H. J.; He, Y. H.; Tan, J. F.; Qiao, G. G. From transient nanodroplets to permanent nanolenses. Soft Matter 2012, 8, 43144317. (17) Yang, H.; Peng, S.; Hao, X.; Smith, T. A.; Qiao, G. G.; Zhang, X. Surfactantmediated formation of polymeric microlenses from interfacial microdroplets. Soft Matter 2014, 10, 957-964. (18) Lou, S. T.; Ouyang, Z. Q.; Zhang, Y.; Li, X. J.; Hu, J.; Li, M. Q.; Yang, F. J. Nanobubbles on solid surface imaged by atomic force microscopy. J. Vac. Sci. Technol. B 2000, 18, 2573-2575. (19) Zhang, X. H. Quartz crystal microbalance study of the interfacial nanobubbles. Phys. Chem. Chem. Phys. 2008, 10, 6842-6848. (20) Zhang, X. H.; Quinn, A.; Ducker, W. A. Nanobubbles at the interface between water and a hydrophobic solid. Langmuir 2008, 24, 4756-4764. (21) Vitale, S. A.; Katz, J. L. Liquid droplet dispersions formed by homogeneous liquid-liquid nucleation: "The ouzo effect". Langmuir 2003, 19, 4105-4110. (22) Sitnikova, N. L.; Sprik, R.; Wegdam, G.; Eiser, E. Spontaneously formed trans-anethol/water/alcohol emulsions: Mechanism of formation and stability. Langmuir 2005, 21, 7083-7089. (23) Grillo, I. Small-angle neutron scattering study of a world-wide known emulsion: Le Pastis Colloid Surf. A-Physicochem. Eng. Asp. 2003, 225, 153160.
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(24) Carteau, D.; Pianet, I.; Brunerie, P.; Guillemat, B.; Bassani, D. M. Probing the initial events in the spontaneous emulsification of trans-anethole using dynamic NMR spectroscopy. Langmuir 2007, 23, 3561-3565. (25) Aubry, J.; Ganachaud, F.; Addad, J. P. C.; Cabane, B. Nanoprecipitation of Polymethylmethacrylate by Solvent Shifting: 1. Boundaries. Langmuir 2009, 25, 1970-1979. (26) Yan, X.; Delgado, M.; Fu, A.; Alcouffe, P.; Gouin, S. G.; Fleury, E.; Katz, J. L.; Ganachaud, F.; Bernard, J. Simple but Precise Engineering of Functional Nanocapsules through Nanoprecipitation. Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201402825 (27) Lepeltier, E.; Bourgaux, C.; Couvreur, P. Nanoprecipitation and the “Ouzo effect”: Application to drug delivery devices. Adv. Drug Deliver. Rev. 2014, 71, 86-97. (28) Aschenbrenner, E.; Bley, K.; Koynov, K.; Makowski, M.; Kappl, M.; Landfester, K.; Weiss, C. K. Using the Polymeric Ouzo Effect for the Preparation of Polysaccharide-Based Nanoparticles. Langmuir 2013, 29, 8845-8855. (29) Zhang, X. H.; Ducker, W. Interfacial oil droplets. Langmuir 2008, 24, 110115. (30) Hab̈ ich, A.; Ducker, W.; Dunstan, D. E.; Zhang, X. Do Stable Nanobubbles Exist in Mixtures of Organic Solvents and Water? J. Phys. Chem. B 2010, 114, 6962-6967. (31) Illueca, C.; Vazquez, C.; Hernandez, C.; Viqueira, V. The use of Newton's rings for characterising ophthalmic lenses. Ophthalmic Physiol. Opt. 1998, 18, 360-371. (32) Weijs, J. H.; Marchand, A.; Andreotti, B.; Lohse, D.; Snoeijer, J. H. Origin of Line Tension for a Lennard-Jones Nanodroplet. Phys. Fluids 2011, 23, 022001. (33) Xu, C.; Peng, S.; Qiao, G. G.; Gutowski, V.; Lohse, D.; Zhang, X. Nanobubble Formation on a Warmer Substrate. Soft Matter 2014, 10, 78577864.
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From Nanodroplets by Ouzo Effect to Interfacial Nanolenses Shuhua Peng, Chenglong Xu, Timothy C Hughes, and Xuehua Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/la502821m • Publication Date (Web): 27 Sep 2014 Downloaded from http://pubs.acs.org on October 9, 2014
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From Nanodroplets by Ouzo Effect to Interfacial Nanolenses Shuhua Peng,† Chenglong Xu,‡ Timothy C. Hughes,§ and Xuehua Zhang†,* †
School of Civil, Environmental and Chemical Engineering, RMIT University,
Melbourne, VIC 3001, Australia. ‡
Department of Chemical and Biomolecular Engineering, University of Melbourne,
Parkville, VIC 3010, Australia. §
CSIRO Materials Science and Engineering, Bayview Avenue, Clayton South,
Victoria 3169, Australia.
ABSTRACT: Polymerizing nanodroplets at solid-liquid interfaces is a facile solutionbased approach to the functionalization of large surface areas with polymeric lensshaped nanostructures. In this work, we have applied a one-pot approach to obtain polymeric nanolenses with controlled size and densities. We take advantage of the formation mechanism by the direct adsorption of nanodroplets from a surfactant-free microemulsion onto an immersed hydrophobic substrate. The interfacial nanodroplets were photopolymerized to produce polymeric nanolenses on the substrate surface. The surfactant-free microemulsion of the monomer nanodroplets was obtained through the spontaneous emulsification (i.e., Ozuo effect) in the tertiary system of ethanol, water and precusor monomer. The size of nanolenses on the surface was adjusted by the nanodroplet size, following a linear relationship with the ratio of the components in the microemulsion. This simple approach is applicable to produce nanolenses over the entire surface area or on any specific area at will by depositing a drop of the microemulsion. Possessing high optical transparency, the resulting substrates may have potential application as functional biomedical supporting materials or effective light-harvesting coatings.
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Introduction Nanoscale structures and features implanted on a surface have a profound influence on a broad range of material properties.1,2 Nanostructures with lens shapes such as micro and nano-lenses provide a unique way to tailor surface optical properties, enabling suitability of the substrate to potential applications in the areas of bioimaging, solar cells, optical nano-sensing, surface antireflective coatings, and optical lithography.3-6 In particular, nanolenses are able to overcome the diffraction limit in optical imaging and achieve high resolutions at low light intensities using the nearfield imaging technique.5,7,8
Individual submicrometer lenses with different sizes and morphologies have been prepared by several methods including chemical growth,5 chemical reflow,9 and polymer bread.10 Micro and nano-lens arrays with well-defined patterns and arrangements have also been produced by self-assembly templating,11 dewetting of polymer films,8 lithography,12 inkjet printing,13 surface wrinkling,14 and focused ion beam15 techniques. A unique solution-based approach is to convert liquid nanodroplets at solid-liquid interfaces into polymeric nanolenses.16,17 Several attractive aspects of the solution-based approach have been demonstrated in our previous work.16,17 The shape of the lenses is conveniently controlled by the wettability of the precursor nanodroplets on the substrate and the chemical nature of lenses is determined by the compositions of interfacial droplets. Furthermore, the nanolenses can be situated on top of a nonplanar substrate, for example, to make lenson-lens structure.16
The major prerequisite to achieve the controlled size distribution of nanolenses over an entire surface is the ability to control the size distribution of the precursor nanodroplets before being polymerized. In our previous work, the precursor nanodroplets were produced by a solvent exchange procedure and then cured by insitu photopolymerization. However, it was difficult to realize a narrow size distribution of nanodroplets over the whole surface area by the solvent exchange where the nanodroplets formed through a heterogeneous nucleation at the interface. Similar to the formation of nanobubbles by solvent exchange, this process is very 2 ACS Paragon Plus Environment
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sensitive to various parameters, such as solvent concentration gradients, flow mixing, temperature gradient or physical and chemical heterogeneities on the surface.18-20 Here we explore a different formation mechanism, surfactant-free emulsion templating, to achieve the size control of the interfacial nanodroplets without exchanging the solvents.
It is known that small oil droplets and/or homogenous nanoparticles can form in a tertiary liquid system through a phenomenon, called the ‘Ozuo’ effect, without using any surfactant or dispersing agents.21-28 An example is the formation of decane nanodroplets in ethanol aqueous solution.29 As water and decane are both miscible with ethanol, while water is immiscible with decane. The addition of water into the solution of decane and ethanol leads to the spontaneous formation of small decane droplets. Nucleated by the Ozuo effect, the droplets are uniform in size. Their size is determined by the ratio of the components in the system.21 The previous work has shown that nanodroplets formed by the Ouzo effect can adsorb onto a hydrophobic substrate.29 The size of those interfacial nanodroplets was closely related to the droplet size in the microemulsion, as confirmed by the atomic force microscopy imaging and by the light scattering measurement. The direct adsorption of nanodroplets from the microemulsion formed by the Ouzo effect provides a potential means to control the size of the resulting nanolenses.
In this work, we demonstrate that the adsorption of nanodroplets onto a surface from microemulsions by the Ouzo effect can be developed to an effective one-pot approach to control the size distribution of interfacial nanolenses on an immersed hydrophobic surface. The size of the nanolenses can be readily adjusted by the size of the precursor nanodroplets in the system, following a linear relationship with the composition of the microemulsions.
Experimental Section Materials. Octadecyltrichlorosilane (OTS, >90%, Sigma-Aldrich), n-decane (95% GC, Fluka), crosslinker 1, 6-hexandiol diacrylate (HDODA, 80%, Sigma), monomer 3 ACS Paragon Plus Environment
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methyl
methacrylate
(MMA,
99%,
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Sigma),
initiator
2-hydroxy-2-
methylpropiophenone (97%, Sigma), ethanol (100%, AnalaR, Merck), and chloroform (>90%, AnalaR, Merck) were used without further purification. Milli-Q Water (18.2 MΩ) was used in all experiments.
Preparation of hydrophobic OTS surfaces. Silicon wafers or glass microscope slides were used as model hydrophobic surfaces. Polished silicon wafers (Mitsubishi Silicon, America-Mod 2) or glass slides were cleaned in piranha solution [H2SO4 (70%): H2O2 (30%)] at 75 ˚C for 20 min. (Warning: piranha solution reacts violently with organic materials.) It is important to minimize exposure of OTS to water as this causes polymerization and consequently a heterogeneous interface. All glassware used for the solution was dried for 2 h under 120 ˚C in the oven. The silicon was dried at 120 ˚C for 1.5 h and soaked in 0.5 vol% OTS in toluene for 12 h in a sealed dry container at room temperature. After 12 h, the OTS-silicon was rinsed with chloroform, sonicated in toluene and ethanol, dried with nitrogen, and then stored in a clean container. Before use, the OTS-silicon was cleaned in ultrasound in ethanol and water respectively. Water droplets in air on OTS-silicon forms an advancing contact angle of 112 ± 3˚ and a receding angle of 101 ± 3˚. The rms roughness as measured by AFM of the OTS-Si substrate was 0.4 nm over an area of 5×5 µm2.
Preparation of Microemulsions. The monomer mixture of MMA with initiator and crosslinker (10: 1: 1, v/v) was used as the oil phase in this microemulsion system. Microemulsions were prepared according to the following procedure: mixtures of water and ethanol with different volume ratios were firstly prepared and then a predetermined amount of MMA with initiator and crosslinker was added into the mixtures, and the solution was thoroughly mixed using a Vortex mixer. The ternary phase diagram of the MMA/ethanol/water was determined by direct observation of the transition from transparency to turbidity with water titration and is shown in Fig. 1.
Preparation of Nanolenses. OTS-Si substrates such as silicon wafer and glass slides were immersed into microemulsion solution and precursor interfacial nanodroplets 4 ACS Paragon Plus Environment
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were formed at solid-liquid interface by direct absorption of the bulk droplets in the microemulsion. In situ photopolymerization was subsequently carried out under a UV lamp (20 W, 365 nm, John Morris Scientific Pty Ltd) and the transparent precursor microemulsion solution became opaque after 10 min polymerization. The substrate with polymeric nanolenses was then removed from the microemulsion solution and rinsed with water and sonicated in water bath for 15 min to remove the weakly attached polymer particles. A schematic diagram for the nanolenses preparation procedure is surmised in Scheme 1.
Characterizations. The precursor microemulsions were measured by static light scattering (SLS) using a Malvern 4700 instrument equipped with a 488 nm laser (Malvern Instruments Ltd., Malvern, Worcestershire, United Kingdom). The scattered light was measured at a constant angle of 90˚ and a temperature of 25 ˚C. Intensity of scattered light was recorded to confirm the presence of nanodroplets in the precursor microemulsions.30 All AFM images were captured using an MFD 3D (Asylum Research) in air and cantilevers (NP probe, Veeco) were treated by UV for 20 min before use. AFM images were recorded in contact mode with constant drive frequency and feedback to maintain constant drive amplitude. Image-Pro software was employed to analyse AFM images to extract the information about the size and distribution of nanolenses. The number density of nanolenses was measured over 20× 20 µm2 area based on AFM measurements. It is difficult to image the MMA droplets in liquid by AFM, because heating from the AFM electronics during imaging leads to the phase separation and polymerization of the MMA microemulsion. The transmittance of glass slides coated with
nanolenses was characterized by using the UV/vis spectroscopy (VARIAN, Cary 50Bio). The transmittance was measured against air as the blank reference.
Results 1. Formation of interfacial nanolenses Scheme 1 shows the process to obtain polymeric nanolenses on a hydrophobic substrate. The substrate was brought into contact with the microemulsion prepared by 5 ACS Paragon Plus Environment
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the Ouzo effect. The nanodroplets in microemulsion collide and adhere to the solid surface. The adsorbed nanodroplets adopt a lens shape due to their partial wetting on the solid-liquid interface. The nanodroplets at the interface along with free nanodroplets in microemulsion were in-situ photopolymerized, and the substrate was subjected to the ultrasound cleaning to remove all the weakly attached particles originated from free nanodroplets in the microemulsion. Finally we obtained the substrate decorated with robust nanolenses.
(a) Substrate UV (b) Substrate Clean (c)
Substrate
Water Precursor droplets Polymer Scheme 1. Schematic diagram of nanolens fabrication procedure by microemulsion templating: (a) Interfacial nanodroplets form from the direct adsorption of the nanodroplets in the bulk microemulsion; (b) Photopolymerization of nanodroplets at the interfacial and in bulk; (c) Sonication to clean weakly attached polymer nanoparticles on the surface.
Figure 1(a) shows the partial phase diagram of the tertiary liquid system constructed by measuring the turbidity of monomer in ethanol upon the addition of water. The region of interest within the oil in water microemulsion phase is within the red circle, where the system was just above the spinodal curve before two phases spontaneously form. The relatively low MMA content favours the formation of discrete precursor nanodroplets. Effect of variation in the microemulsion formulations was demonstrated 6 ACS Paragon Plus Environment
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by using six solvent free microemulsions (SFE-1 to SFE-6) with rationally selected compositions as listed in Table 1. All the microemulsions appeared to be stable, homogeneous and optically transparent immediately after the preparation. The photos of the microemulsions displayed in Figure 1 (b) show that SFE-1 to 5 still remain transparent after 24 hours indicating the stability of the microemulsions.
Ethanol
(a)
0.0
1.0
0.2
0.8
0.4
0.6 Spinodal curve
0.6
0.4
0.8
0.2
1.0
H2O
0.0
0.0 0.2
0.4
0.6
0.8
1.0 MMA
Figure 1. (a) Partial phase diagram of microemulsions comprising monomer, ethanol, and water and the region of interest within the oil in water phase was indicated by the red circle; (b) Photos of six microemulsion formulations listed in Table 1.
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Table 1. Precursor microemulsion compositions for the preparation of nanolenses by
in-situ photopolymerization. Formulation No.
Ethanol (ml)
Water (ml)
Monomer (ml/%)
SFE-1
3.5
6.5
0.29/2.8%
SFE-2
3.5
6.5
0.31/3.0%
SFE-3
3.5
6.5
0.33/3.2%
SFE-4
3.6
6.5
0.33/3.2%
SFE-5
3.7
6.5
0.33/3.2%
SFE-6
3.5
6.5
0.45/4.3%
Attempts to measure the size of the microemulsion droplets by dynamic light scattering (DLS) were not successful due to the relatively high concentration. Moreover, dilution of the formulation with higher solvent levels would move the microemulsion into a different position in the phase diagram and was unlikely to represent the size of the microemulsion droplets used here. Instead we used static light scattering (SLS) intensities to characterise the presence of droplets in the microemulsions.30 SLS measurements show the scattering intensities of all five samples (150-165 kcps) was much higher than the binary continuous phase of ethanol and water (>20 kcps), confirming the presence of precursor nanodroplets in the solution.
2. Effects of the content of droplet phase Two parameters, monomer ratio and ethanol concentration were varied in six formulations to examine their effect on the morphology, size, and distribution of the resulting polymerised nanolenses. In four formulations, named SFE-1, SFE-2, SFE-3 and SFE-6, the ratio of the volume of monomer to the volume of ethanol and water increased from 0.29 in 10 (~2.8%) to 0.45 in 10 (~4.3%), while the amount of ethanol and water in the continuous phase were fixed at 3.5 and 6.5 ml, respectively. Optical 8 ACS Paragon Plus Environment
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microscopy and AFM were used to examine the surface over a large area at ambient conditions. Optical images in Figure 2 show that features were uniformly distributed over the entire surface area for all three samples. Specifically, the size of the features on surfaces increased and the number density decreased from Figure 2(a) to (c) with the increase of monomer ratio.
Morphological characteristics of the substrates were further examined by AFM measurements, as shown in Figure 2(d)-(f). The lens-shape of the nanostructures is clearly shown by the cross-sectional profiles of nanolenses in Figure 3. Three representative polymeric nanolenses in Figure 2(f) were all closely fitted by spherical caps having heights of 80-120 nm and lateral diameters of ~700 nm. Over a large surface area in Figure 2(d), the number density of nanolenses was 1.45 per µm2 when the microemulsion SFE-1 contained 2.8% MMA. As the ratio of monomer slightly increased to 3.0% in Figure 2(e), the number density decreased to 0.84 per µm2 accordingly. The number density of nanolenses further decreased to 0.40 per µm2 in SFE-3.
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Figure 2. Optical microscopy (scale bar: 10 µm) and AFM (20×20 µm) images of polymeric nanolenses by microemulsion polymerization with increasing MMA content. The parent microemulsion compositions were summarised in Table 1: (a) and (d), SFE-1; (b) and (e), SFE-2; (c) and (f), SFE-3. Corresponding size and distribution was illustrated by histogram plots, including average diameters ( ) and standard deviations (σ).
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150
a b
100
c 50 0 -0.5
0.0
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Lateral diameter (µm) Figure 3. Cross-sectional profiles of three polymeric nanolenses indicated by arrows in Figure 2(f). The solid lines are the corresponding spherical-cap fitting.
The statistical analysis of the size distribution of nanolenses is illustrated by histogram plots with average diameters () and standard deviations (σ). At the lowest monomer ratio in the microemulsion formulation, SFE-1, a narrow size distribution of nanolenses with a mean diameter of 0.36 µm and a standard deviation of 0.09 µm was obtained. Polymerisation of SFE-2 resulted in larger nanolenses with relatively broader distributions having a mean diameter of 0.42 µm with a standard deviation of 0.13 µm. The diameter of nanolenses became even larger with SFE-3 and SFE-6, and reached 0.72 µm and 2.11 µm with standard deviations of 0.21 µm and 0.68 µm, respectively.
The polymeric nanolenses produced from SFE-6 were large enough to exhibit welldefined circular Newton rings under 100× objective lens as shown in Figure 4. The formation of the rings is due to the interference between the reflected light from the surface of underlying substrate and the spherical upper face of nanolenses.16,31 The symmetric interference patterns of these Newton rings also indicates that lenses on the OTS-Si substrate possess spherical lens-shape morphology.
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d = 2.11 µm σ = 0.68
(b)
Percentage (%)
45
30
15
0 0
1
2
Diameter (µm)
3
4
Figure 4. (a) Optical microscopy images of polymeric lenses by SFE-6. The emulsion contains 0.45 ml MMA, 3.5 ml ethanol, and 6.5 ml water. Scale bar: 10 µm; (b Corresponding size and distribution was illustrated by histogram plots.
Clearly our AFM measurements show that the nanolens diameter increases with the monomer ratio, while the standard derivation of the diameter remains rather small for all formulations, which is 25-32% of the mean diameter. The correlation between the mean diameter of polymeric nanolenses and the ratio of monomer to the liquid phase is summarized in Figure 5. Importantly, we observed a very good linear correlation between nanolens size and ratio of monomer. These correlations may be used to predict the size of the resulting polymeric nanolenses from the composition of the tertiary system. 3
Diameter (µm)
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2
y = -2.42 + 9.47x 2 R = 0.85
1
0 0.30
0.35
0.40
0.45
MMA volume (ml)
Figure 5. Correlation between average diameter of resulting polymeric nanolenses and volumes of monomer in the precursor microemulsions.
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3. Effect of continuous phase concentration The ethanol content in the continuous phase of the precursor microemulsion was varied in three formulations from SFE-3 to SFE-5. From the AFM images of the nanolenses obtained in SFE-3 and SEF-5 in Figure 6, the number density of nanolenses increased from 0.84 per µm2 to 1.30 per µm2 as the ethanol volume increased from 3.5 to 3.7 ml whilst the oil phase loading was kept constant at 0.33 ml. The mean diameter of nanolenses decreased from 0.72 µm, 0.45 µm to 0.35 µm. So the diameter of nanolenses became smaller as the ethanol concentration increased in the microemulsions. The size of nanolenses was rather uniform for all formulations as reflected by the standard derivation, which was 0.21 µm in SFE-3, 0.13 µm in SFE-4 to 0.09 µm in SFE-5. All of the standard derivations were only 25-29% of the mean diameter. The correlation between the mean diameter of polymeric nanolenses and the amount of ethanol in the tertiary system was summarized in Figure 7. The mean diameter of nanolenses increases linearly with the amount of ethanol for the fixed amount of monomer and water in the tertiary system.
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Figure 6. Optical microscopy (scale bar: 10 µm) and AFM (20×20 µm) images of polymeric nanolenses by microemulsion polymerization with increasing ethanol content. The parent microemulsion compositions were summarised in Table 1: (a) and (d), SFE-3; (b) and (e), SFE-4; (c) and (f), SFE-5.
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0.0 3.5
3.6
3.7
Ethanol volume (ml) Figure 7. Correlation between average diameter of resulting polymeric nanolenses and volumes of ethanol in the precursor microemulsion formulations.
4. Morphology of nanolenses We now examine the morphological features of the individual nanolenses prepared from microemulsions SFE-1 to SFE-5. The morphology of spherical-cap droplet in terms of its contact angle at the interface can be calculated by the following equation:
tan = =
(1)
where R is the radius of curvature, L is the lateral diameter, h is the height, and θ is the contact angle. L and h can be directly obtained by the cross-sectional profiles of nanolenses based on AFM measurements. All the nanolenses prepared by the five formulations exhibited similar contact angle values ranging from 15 to 20˚, as shown in Figure 8. This indicates that the variation in the composition of the microemulsions was not significant enough to alter the wettability of the monomer on the solid-liquid interfaces. The morphology of the nanolenses in this work is comparable to that of lenses prepared by the solvent exchange in the presence of CTAB surfactant solution at 0.5 critical micelle concentration (cmc),17 attributed to the similar wetting properties of the monomer on the substrate in contact with the continuous phase of the ethanol solution and with 0.5 cmc CTAB solution. This observation demonstrates another feature of the one-pot approach, that is, the control of the lens size can be achieved without affecting the aspect ratio of the nanolenses.
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The contact angle of a macroscopic MMA droplet was measured to be 2-10° in aqueous solution, which is about 10 degree smaller than the microscopic contact angle of the nanolenses. There was a small variation of the contact angle with the nanolens size. As
we plotted the cosine of the contact angle versus the inverse of the lens radius, the fitting gave a slope of 7.1 nm (Supporting Information). The effect of line tension cannot account for such large difference between microscopic and macroscopic contact angle.32 In our recent paper, we have discussed in detail the alternative explanation of the weak heterogeneity theory.33 30
30
(a)
(b)
SFE-1 20
Contact angle (°)
Contact angle (°)
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SFE-1 SFE-2 SFE-3 SFE-4 SFE-5
10
0
20
SFE-3
SFE-4 SFE-5
SFE-2
10
0 0.0
0.5
1.0
1.5
Diameter (µm)
0
1
2
3
4
5
Microemulsion formulations
Figure 8. (a) Plots of contact angle as a function of diameter of polymeric nanolenses prepared by different microemulsion formulations; (b) Mean contact angles of polymeric nanolenses with different sizes by different microemulsion formulations.
5. Transparency of nanolens-decorated substrates Now we show that the substrates coated with as-prepared polymeric nanolenses exhibit highly optical transmittance to visible light (Figure 9(a)), which is highly desirable for many applications. The transparency of polymeric nanolens with different sizes on glass slides is shown in Figure 9(b). Transmittance of 80-87% over the range of 400-700 nm was observed for all the polymeric nanolenses. Specifically, transparency at 400 nm decreased slightly from 87% for bare glass slide to 85%, 83%, and 80% for nanolenses from SFE-1, SFE-2, and SFE-3 precursor microemulsions, respectively. The decrease in transparency may be induced by increase of nanolens diameter and height. As expected, an increasing trend of transmittance was observed
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when smaller nanolenses were prepared using the parent microemulsions SFE-4 and SFE-5 with higher ethanol contents.
(b) 90
Transmittance (%)
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Bare OTS glass 1
85
5
2 4 80
3
400
500
600
700
Wavelength (nm) Figure 9. High optical transparency of nanolens coated glass: (a) Photograph of glass slides coated with nanolenses with different sizes; (b) Transmittance spectra of glass slides coated with nanolenses with different sizes.
6. Spatial control of nanolens cluster The results shown above were obtained when substrates were fully immersed into microemulsions to produce nanolenses over the entire surface. Now we demonstrate that it is possible to produce clusters of nanolenses on specifically selected areas. When a small sessile drop of the microemulsion SFE-3 (5 µl) was deposited on the substrate, nanolenses were produced specifically on the area beneath the drop as 17 ACS Paragon Plus Environment
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shown in Figure 10(a) and (b) while the surrounding area remained free of nanolenses. Figure 10(c) shows that five clusters of nanolenses were obtained when five sessile drops with different volumes were deposited on the surface. This demonstrates the high flexibility of the one-pot approach in selective surface functionalization.
Figure 10. Nanolens clusters were obtained when sessile drops of precursor microemulsions were deposited on OTS-Si substrates: (a) single nanolens cluster under 4× magnification; (b) single nanolens cluster at 80× magnification; (c) five nanolens clusters under 4× magnification.
Discussion Our results clearly show that the size of nanolenses can be controlled by the monomer and ethanol content in the microemulsion formed by the Ouzo effect. The linear correlations between the mean diameter of lenses and the contents in the system can guide the design of the microemulsion to achieve desired size distribution of the interfacial nanolenses. Therefore this approach has significantly improved the size control of nanolenses by in situ polymerization of nanodroplets, which has been difficult to achieve using the previous solvent exchange method.
Below we discuss the possible mechanism behind the relationship between the size of interfacial nanlenses and the contents in the microemulsion. From previous work we know that the interfacial nanodroplets originated from the nanodroplets in the microemulsion formed by the Ouzo effect and the size of interfacial nanodroplets was closely related to the droplet size in the microemulsion.21 The size distribution of nanolenses is narrow for all formulations, determined by the nucleation mechanism of the Ouzo effect that produces homogeneous droplets. The droplet size in a 18 ACS Paragon Plus Environment
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microemulsion formed by the Ouzo effect has been quantitatively studied by Vitale et al.21 To make comparison with their observations, the lateral diameters of nanolenses were converted to the diameters of precursor nanodroplets in the microemulsion before they adsorbed onto the surface, which is straightforward once given the contact angle and the lateral diameter of nanolenses.
The volume of the nanolens was calculated based on its height and lateral size from AFM measurements and then converted to the diameter of a spherical drop (Ddrop) in the precursor microemulsion. According to the work by Vitale et al,21 Ddrop only depends on the ratio of the excess oil concentration to the ethanol concentration, where the excess oil referrs to the concentration of oil in excess of its saturation concentration in the continuous phase. As the ethanol concentration was varied over a very narrow range in our systems, the change of monomer solubility in the continuous phase was also small. Therefore, the plot of Ddrop as a function of the ratio of monomer to ethanol did not reveal the absolute solubility of monomer in the continuous phase. Despite this, a good linear relationship between the drop size and the monomer-to-ethanol ratio in precursor microemulsion was shown in Figure 11. 500
Mean diameter of droplets (nm)
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400
300
y = 8427.5x-660.5 2 R = 0.97
200
100
0 0.06
0.09
0.12
0.15
Ratio of monomer to ethanol
Figure 11. Correlation between average diameter of droplets in microemulsion and the ratio of monomer to ethanol. The diameter of droplets linearly increases with the ratio.
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The number density of polymeric nanolenses on the OTS-Si substrate may also be governed by the number density of oil droplets in the microemulsion. After certain point, the addition of water would cause dilution of the droplets in the microemulsion instead formation of new droplets, leading to the decrease of the number density of droplets in microemulsion with the decrease of ethanol concentration. This is consistent with our observation that the number density of nanolenses decreased with decreasing ethanol concentration. Moreover, for a given ratio of ethanol, the addition of monomer led to larger droplets instead of more droplets. As the droplet size increased with the monomer ratio, the scattering intensity was expected to dramatically increase for a given number density of droplets because a large droplet scatters more strongly than a small droplet. However, all the emulsions had similar scattering intensities, indicating fewer, but larger droplets in the microemulsion with higher monomer ratio. Consequently the number density was lower for those larger nanolenses formed at higher monomer ratios.
Conclusions In summary, we have demonstrated a one-pot approach to functionalize a hydrophobic surface with polymeric nanolenses of controlled size, distribution and location by using nanodroplets from the Ouzo effect. Monodispersed monomer droplets formed in a tertiary system of precursor monomer, ethanol and water and adsorbed onto an immersed hydrophobic substrate. Unlike the previous approach of the nanodroplet nucleation on the interface by switching the solvents, the size of nanolenses in our one-pot approach were readily controlled by the monomer and continuous liquid phase content, as they followed a linear relationship with the ratio of the composition in the tertiary liquid system. This approach can realize functionalization of the entire surface area by nanolenses with well defined size and narrow size distribution. Furthermore, our results demonstrate the flexibility in spatial control of nanolenses by depositing droplets of the microemulsion to obtain nanolens clusters only within specific areas. Surfaces decorated with nanolenses were optically transparent and show promising potential for application as smart surface coatings.
Acknowledgements 20 ACS Paragon Plus Environment
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XHZ and SHP gratefully acknowledge the support from the ARC Future Fellowship Scheme (FFT120100473) and a Postdoctoral Fellowship from Australian Renewable Energy Agency (ARENA).
Corresponding Author *E-mail: [email protected] (XHZ);
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