X-ray Reflectivity Studies on the Mixed Langmuir–Blodgett Monolayers

Sep 22, 2017 - Langmuir–Blodgett monolayers of thiolated gold nanoparticles mixed with dipalmitoylphosphatidylcholine/sodium dodecyl sulfate (DPPC/S...
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X‑ray Reflectivity Studies on the Mixed Langmuir−Blodgett Monolayers of Thiol-Capped Gold Nanoparticles, Dipalmitoylphosphatidylcholine, and Sodium Dodecyl Sulfate Yi-Tang Chen,† Han-Shiou Su,† Chin-Hua Hung,† Po-Wei Yang,† Yuan Hu,† Tsang-Lang Lin,*,† Ming-Tao Lee,‡ and U-Ser Jeng‡ †

Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, ROC

‡

S Supporting Information *

ABSTRACT: Langmuir−Blodgett monolayers of thiolated gold nanoparticles mixed with dipalmitoylphosphatidylcholine/sodium dodecyl sulfate (DPPC/SDS) were investigated by combining the X-ray reflectivity, grazing-incident scattering, and TEM analyses to reveal the in-depth and in-plane organization and the 2D morphology of such mixed monolayers. It was found that the addition of a charged single-tail surfactant to the thiolated Au nanoparticle monolayer helps to stabilize the Au nanoparticle monolayer and to strengthen the mechanical property of the mixed monolayer film. For mixing with lipids, it was found that the thiolated gold nanoparticles could be pushed on top of the lipid monolayer when the mixed monolayer is compressed. At a typical comparable total surface area ratio of gold nanoparticle to lipid, the thiolated gold nanoparticles could form a uniform domain on top of the DPPC monolayer. When there are more thiolated gold nanoparticles than that could be supported by the lipid monolayer, domain overlapping could occur to form bilayer gold nanoparticle domains at some regions. At low total surface area ratio of thiolated gold nanoparticle to lipid, the thiolated gold nanoparticles tend to form a connected threadlike aggregation structure. Evidently, the morphology of the thiolated gold nanoparticle monolayer is highly depending on the total surface area ratio of the thiolated gold nanoparticle to lipid. SDS is found to have a dispersion power capable of dispersing the originally uniform Au-8C nanoparticle domain of the mixed Au-8C/DPPC monolayer into a foamlike structure for the mixed Au-8C/SDS/DPPC monolayer. It is evident that not only the concentration ratio but also the size and shape of the template formed by the amphiphilic molecules and their interaction with the thiolated gold nanoparticles can all have great effects on the organizational structure as well as morphology of the thiolated gold nanoparticle monolayer.

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INTRODUCTION The self-assembly of nanoparticles in the form of Langmuir− Blodgett (LB) monolayers has found many applications.1−4 Applying nanoparticles as the building blocks to form twodimensional nanostructural films has become a significant approach in creating functional devices.5−8 There exist several techniques for fabricating a monolayer of self-assembled nanoparticles.3,9,10 Among these techniques, the LB technique and the drop evaporation method were most widely used.3 Even though drop evaporation is a much easier technique to prepare the monolayer film, its final products usually become a film consisting of multilayers.10 In contrast, either a uniform monolayer over a large area or a multilayer from collapsed monolayers under overcompression could be well reproduced and controlled by the LB technique.3,11 The LB technique has been widely used with manufacturing devices by transferring a monolayer from the air−liquid interface onto solid substrates.12 It is important to control the ordering structure of the nanoparticle arrays, since both the optical and electronic © XXXX American Chemical Society

properties of nanoparticle monolayers are greatly affected by the ordering structure.13,14 The self-assembly of nanoparticles at interfaces can be greatly affected by several factors, such as solvent,10 the chain length of ligand,15−17 ligand density,17,18 the size and size distribution of nanoparticles,19 thermal treatment,20 charge interaction,21 and additives.22−27 It is of particular interest to add amphiphilic molecules to control or shape the nanostructure of the nanoparticle array when using the LB technique. Schultz et al. found that the interparticle spacing of dodecanethiol-ligated gold nanoparticles with 6 nm diameter can be slightly tuned from 74 to 85 Å by adding dodecanethiol.22 Several studies also showed that the addition of lipids, fatty acids, and polymers could compel alkanethiolligated gold nanoparticle Langmuir monolayers into distinct structures.23−26,28,29 Hassenkam et al. showed that 0.44 mol % Received: May 13, 2017 Revised: July 20, 2017 Published: September 22, 2017 A

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thickness of the capping thiols. The concentration of Au nanoparticles in the solution was 10 mg/mL of Au-8C. SDS was added to the Au-8C solution to make a mixed Au-8C/SDS solution at a Au-8C to SDS molar ratio of 0.57%. Subsequently, desired amounts of DPPC lipid were mixed with the Au-8C or Au-8C/SDS solutions. The concentration ratios of Au-8C to DPPC were varied from 3.1 to 6.0 to 11.3 mol %. Nima Langmuir trough model 501 was used to produce the LB films of Au-8C monolayers. Sample solutions were spread on deionized water (18.2 MΩ cm resistivity) and the solvent evaporated for 30 min before the monolayer was compressed. Compression was carried out at a constant barrier speed of 10 cm2/min. The monolayers at the air−water interface were transferred onto silicon wafers or copper grids at desired surface pressures for X-ray reflectivity, grazingincident scattering, and TEM studies, respectively. The temperature of the trough were kept at 20 °C by an external circulation bath. X-ray Reflectivity, Grazing-Incident Scattering, and TEM Analysis. The synchrotron X-ray reflectivity and grazing-incident scattering measurements were conducted at beamline 13A of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The photon energy of 12 keV was selected for all X-ray scattering measurements. X-ray reflectivity (XR) data were collected by scintillation detector in a θ−2θ scan mode and analyzed by the Motofit package of IgorPro software.37 The fitting of the XR data is based on the slab model, which is suitable for analyzing the welldefined layered thin films. Due to the strong scattering of the gold monolayer, the errors (uncertainties) of the XR data are quite small. In the low-Q region, the errors (uncertainties) are smaller than 0.1%, and at the high-Q region the errors (uncertainties) are still just around 3− 5%, which are too small to be displayed on the XR curves. The inplane scattering data were collected in grazing-incidence small-angle Xray scattering (GISAXS) mode by using a charge coupled detector (CCD) which was placed 100 cm behind the sample position, and an incident angle of 0.13° was used.38 The TEM images were recorded by a JEOL JEM-1230 transmission electron microscope (JEOL Ltd., Tokyo, Japan) with an operating voltage of 100 kV.

dodecanethiol-capped gold nanoparticles of 1.5−3.0 nm diameter in a matrix of dipalmitoylphosphatidylcholine (DPPC) form connected wires at the air−water interface.25 This indicated that even at a low nanoparticle concentration the nanoparticles still tend to form aggregates and could not be completely dispersed by the DPPC. Although the monolayer morphology of gold nanoparticles mixed with amphiphilic molecules has been observed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), or atomic force microscopy (AFM), the local structures of the mixed monolayers that consist of gold nanoparticles and amphiphilic molecules are still not fully investigated, especially with respect to their ordering structure in the direction perpendicular to the LB film. It is not clear whether the thiolcapped gold nanoparticles are intercalated between the lipid tails or surfactant tails or if they are lifted on top of the monolayer of amphiphilic molecules and how their relative position vary during the compression of the monolayer. The use of X-ray or neutron reflectivity would be needed to determine the in-depth structure (perpendicular to the thin film) of the mixed monolayer to elucidate how the nanoparticles and the amphiphilic molecules are organized in the mixed monolayer.26,30−33 In present work, synchrotron X-ray reflectivity, grazingincident scattering, and TEM were employed to investigate the structure of the mixed thiol-capped gold nanoparticle, DPPC, and sodium dodecyl sulfate (SDS) Langmuir−Blodgett monolayers. As mentioned, the X-ray reflectivity could reveal the in-depth structure of the mixed monolayers. For in-plane ordering of the thiol-capped gold nanoparticle array, it would be necessary to employ grazing-incident scattering. DPPC is a zwitterionic lipid having two hydrocarbon chains and it could form a robust monolayer at the air−water interface. SDS is a single-chain anionic soluble surfactant and it is often used to study the interaction with other amphiphilic molecules at the air−water interface.34 SDS has been found to be able to insert into the DPPC monolayer and could also induce DPPC to become more ordered.35 The molar ratio of the thiol-capped gold nanoparticle to DPPC was varied in this study from 3.1 to 6.0 and 11.3 mol % to reveal the effect of concentration ratio on the organizational structure of the mixed monolayer.

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RESULTS AND DISCUSSIONS Surface Pressure−Area Isotherm. Figure 1 shows the measured surface pressure−area isotherm of Au-8C nanoparticles and the mixtures of Au-8C nanoparticles with amphiphilic molecules. The transverse axis of Figure 1 indicates the surface area per Au-8C nanoparticle. When compressing the

EXPERIMENTAL SECTION

Materials and Sample Preparations. SDS was purchased from Sigma and used without further purification. DPPC was purchased from Avanti and used without further purification. The synthesis of thiol-capped gold nanoparticles followed the procedures described by Brust et al.:36 30 mM Au ion [hydrogen tetrachloroaurate trihydrate (HAuCl4.3H2O), Acros, 99.9%] in 30 mL of aqueous solution was mixed with 45 mL of toluene solution containing 50 mM of tetradodecylammonium bromide (TDAB) (Fluka, 99%) under vigorous stirring. Then, 447.8 μL of 1-octanethiol (Alfa Aesar, 98%) was added into the mixture to have a thiol/Au ion molar ratio of 2.8. A 30 mL portion of 0.4 M sodium borohydride (NaBH4) (Aldrich, 99%) was slowly added in small drops to reduce the Au ion, and the organic phase of the solution turned dark black, indicating the formation of gold nanoparticles. The purification was done by washing the dark crude with large amounts of ethanol and centrifuging at 6000 rpm for 30 min. The dried product was stored at room temperature for further use. The synthesized 1-octanethiol-capped Au nanoparticles (Au-8C) suspended in toluene were examined by small-angle X-ray scattering (SAXS) to determine the size of the synthesized gold nanoparticles. The highly monodisperse gold nanoparticles were found to have a mean diameter of 1.8 ± 0.2 nm, which does not include the shell

Figure 1. Surface pressure−area isotherms of Au-8C and mixtures of Au-8C with different ratios of SDS/DPPC. The transverse axis indicates the area per Au-8C nanoparticle. The surface area per Au-8C nanoparticle at around 19−20 mN/m is indicated in the figure for some of the mixed monolayers in units of Å2. The LB films were pulled from the water surface at constant pressure mode, which resulted in the flattened surface pressure regions shown in the isotherms. B

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thiols and are prone to form wrinkles at higher compression pressure.16,41 When the surface pressure is above the collapse pressure, the surface pressure increases at a much slower rate with compression, since the compression pressure is partially relieved due to wrinkling and domain overlapping. As shown in Figure 1, the isotherm of the mixture of the Au8C nanoparticle/SDS at a Au-8C to SDS ratio of 0.57 mol % shows a plateaulike region around 7 mN/m surface pressure before forming the condensed state at higher compression pressure. The surface area per Au-8C nanoparticle decreases to 545 Å2 when reaching 10 mN/m surface pressure, which is slightly larger than the case without SDS. These findings suggest that SDS molecules must have some association/ interdigitation with the thiol molecules capped on the Au nanoparticle and could affect the formation of the monolayer at the air−water interface. However, the stability of the mixed Au8C/SDS monolayer is not significantly improved by the presence of SDS, at least in terms of the collapsing pressure. The mixed Au-8C/SDS monolayer still collapses at a similar pressure of 12.2 mN/m, as compared with the collapse pressure of pure Au-8C monolayer at 12.5 mM/m. When compressed above the collapse pressure, the increase in surface pressure becomes much slower as compared with the case without SDS. The TEM image (Figure 2c) for the mixed Au-8C/SDS at the collapsed state shows no wrinkles but only overlapped domains as compared with Figure 2b for the case without SDS. It seems that the addition of SDS could ease the formation of wrinkles. Although the domain slipping or folding could not be completely prevented, the addition of an amphiphilic charged single-tail surfactant does improve the mechanical property of the Au-8C nanoparticle monolayer by stabilizing the Au-8C nanoparticles at the air−water interface. The tails of SDS could interdigitate with the thiols at the bottom side of the Au-8C nanoparticles to form a stable amphiphilic complex at the air− water interface. The detailed structure of this mixed monolayer will be discussed in later section on X-ray reflectivity studies. As shown in Figure 1, the mixed monolayers of Au-8C/ DPPC at the Au-8C to DPPC ratio of 3.1, 6.0, and 11.3 mol % could withstand the compression at least to 19−20 mN/m and there is no obvious horizontal break in the isotherm. Since the surface area per Au-8C is at least 515 Å2 and the surface area per DPPC is around 46 Å2 at the liquid condensed state,42 the total surface area ratios of Au-8C to DPPC at the compressed state are estimated to be around 0.35, 0.67, and 1.27 for the particle ratios of 3.1, 6.0, and 11.3 mol %, respectively. This means there are comparable amounts of Au-8C to DPPC at the air−water interface. Since DPPC itself could form a robust monolayer at the air−water interface while the pure Au-8C monolayer could not stand high compression and tend to form overlapped domains, it is expected that the compression pressure would eventually fall on the DPPC but not the Au-8C in the mixed monolayer. Since the DPPC could stand up to the compression, the mixed Au-8C/DPPC monolayer would not collapse as easily as the pure Au-8C monolayer. The surface area per Au-8C nanoparticle reaches about 550, 810, and 1280 Å2 when compressed to about 19−20 mN/m surface pressure, respectively, for the Au-8C to DPPC ratios of 11.3, 6.0, and 3.1 mol %. The addition of DPPC indeed increases the surface area per Au-8C nanoparticle, but it is much less than simply adding up the typical surface area of pure Au-8C and pure DPPC monolayers, which would be around 922, 1282, and 1999 Å2, respectively, for the Au-8C to DPPC ratios of 11.3, 6.0, and 3.1 mol %. These values are calculated from A = AAu−8C + ADPPC/

pure Au-8C monolayer, the surface pressure increases sharply to 515 Å2 when reaching around 10 mN/m surface pressure. This surface area per nanoparticle is equivalent to the surface area of a circle with a diameter of 25.6 Å. The thiol molecules must be interdigitated at this highly compressed state, since the gap between the neighboring nanoparticles will be just about 7.6 Å.39,40 The monolayer of Au-8C nanoparticles collapses at approximately 12.5 mN/m surface pressure. Figure 2a,b shows

Figure 2. TEM image of Au-8C monolayer LB film at 10 mN/m surface pressure (a), of Au-8C monolayer at 19 mN/m surface pressure (b), which is in a collapsed state with both wrinkles in stripe form and domain overlapping, and of the Au-8C/SDS mixed monolayer LB film at 14 mN/m surface pressure (c), which is in a collapsed state.

the TEM images of the Au-8C monolayer at the condensed state and at the collapsed state. At the condensed state, the Au8C nanoparticles are uniformly distributed, while at the collapsed state, the Au-8C nanoparticle domains contain many wrinkles in stripe form, and the Au-8C domains are also partly overlapping each other. Small-sized Au nanoparticles capped with short akyl chain thiols tend to have a lower collapse pressure than those capped with longer akyl chain C

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Langmuir ([Au-8C]/[DPPC]) with AAu−8C = 515 Å2 and ADPPC = 46 Å2. On the other hand, the isotherms can also be plotted in terms of surface area per DPPC molecule, as shown in Figure 3, to

Figure 4. X-ray reflectivity curves and the fitting results (solid lines) of the LB films of Au-8C and its mixtures with DPPC and SDS. The reflectivity curves are shifted vertically for clarity. Here, Q is the scattering vector, defined as Q = (4π/λ) sin(θ/2), where λ is the wavelength of the incident X-rays and θ is the scattering angle between the scattered X-rays and the incident X-rays.

Figure 3. Surface pressure−area isotherm of the mixtures of Au-8C and DPPC at different molar ratios. The transverse axis indicates the surface area divided by the number of lipids in the mixture. The surface areas per lipid at around 19−20 mN/m are also indicated in the figure in units of Å2. The isotherms are indentical to those shown in Figure 1 but plotted as a function of the surface area per DPPC molecule to reveal the effect of adding Au-8C on the packing of the mixed monolayer. The LB films were pulled from the water surface at a constant pressure mode, which resulted in the flattened surface pressure regions shown in the isotherms.

fitting results of the LB films of Au-8C and its mixtures with DPPC and SDS. All these X-ray reflectivity curves have visible interference fringes and distinct features that could reveal the in-depth structure of the LB monolayers. All these X-ray reflectivity curves were satisfactorily fitted by the slab model as shown by the solid lines in Figure 4.31,32 In order to understand how the Au-8c monolayer is formed, the LB films of Au-8C at 0.1 and 10 mN/m were investigated by X-ray reflectivity. As shown in Figure 5, the X-ray reflectivity

reveal the effect of the presence of Au-8C on the ordering of the mixed monolayer. The corresponding surface area per DPPC molecule becomes 40, 49, and 62 Å2 at around 19−20 mN/m, respectively, for the Au-8C to DPPC ratios of 3.1, 6.0, and 11.3 mol %. For the cases of the Au-8C to DPPC ratios of 3.1 and 6.0 mol %, the surface area per DPPC remains around 40−50 Å2, which is within the range of the liquid condensed state of pure DPPC. This means that at these two loading amounts the Au-8C nanoparticles do not affect too much the packing of the DPPC in the mixed monolayer. At the Au-8C to DPPC ratio of 11.3 mol %, the total surface area of the loaded Au-8C is at least 1.27 times the surface area of DPPC, which means that the compression will be not only on the DPPC domains but also on the Au-8C domains. It is obvious that the DPPC could not be compressed to the liquid condensed state in the mixed monolayer at this ratio up to 20 mN/m. It is also worthwhile to note that the surface area per Au-8C nanoparticle decreases from 810 to only 650 Å2 when SDS is added into the mixed Au-8C/DPPC system at a Au-8C to DPPC ratio of 6.0 mol %. The addition of SDS to the Au-8C/DPPC monolayer could make the packing of the mixed monolayer more compact, as found in the study of mixed SDS/DPPC monolayer at the air−water interface.22 Other than making the DPPC become more ordered, SDS could also easily interdigitate with the thiols capped on Au nanoparticles and reduce the interaction of Au-8C with the supporting DPPC monolayer. X-ray Reflectivity/Grazing-Incident Scattering/TEM Analysis. Subsequently, the Au-8C and the mixed monolayers at the air−water interface were transferred onto silicon wafers for the X-ray reflectivity measurements. For comparison, Figure 4 shows several measured X-ray reflectivity curves and the

Figure 5. X-ray reflectivity curves and the fitting results (solid lines) of the LB films of Au-8C at 0.1 and 10 mN/m surface pressures. The reflectivity curve of the 0.1 mN/m case is shifted vertically for clarity.

curve for the 10 mN/m surface pressure case differs from that of 0.1 mN/m. The X-ray reflectivity curve for the 0.1 mN/m surface pressure case can be fitted with a three-layer model. The fitting parameters are listed in Table 1. The thicknesses of the three layers from the air side to the substrate are respectively 6.3, 14.3, and 10.0 Å, with the corresponding X-ray scatter length densities (SLDs) of 5.7 × 10−6, 41.2 × 10−6, and 6.7 × 10−6 Å−2. The recovered depth profile of the SLD is shown in Figure 6a. It is obvious that the top and bottom layers are D

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Langmuir Table 1. Parameters Determined by Fitting the X-ray Reflectivity Curves of the Au-8C Mixed LB Filmsa sample Au-8C (0.1 mN/m)

Au-8C (10 mN/m)

Au-8C/SDS: 0.57 mol % (10 mN/m)

Au-8C/DPPC: 6.0 mol % (0.1 mN/m)

Au-8C/DPPC: 6.0 mol % (5 mN/m)

Au-8C/DPPC: 6.0 mol % (19 mN/m)

Au-8C/DPPC: 3.1 mol % (20 mN/m)

Au-8C/DPPC 11.3 mol % (20 mN/m)

Au-8C + DPPC + SDS Au-8C/DPPC: 6.0 mol % Au-8C/SDS: 0.57 mol % (20 mN/m)

layer

thickness (Å)

SLD (×10−6 Å2)

roughness (Å)

(1) thiol ligand (2) Au NP (3) thiol ligand Si substrate (1) thiol ligand (2) Au NP (2nd) (3) Au NP (1st) (4) thiol ligand substrate Si (1) thiol ligand (2) Au NP (3) thiol ligand/SDS tail (4) SDS head substrate Si (1) thiol ligand (2) Au NP (3) thiol ligand/lipid substrate Si (1) thiol ligand (2) Au NP (3) thiol/lipid tail (4) lipid head substrate Si (1) thiol ligand (2) Au NP (3) thiol ligand/lipid tail (4) lipid head substrate Si (1) thiol ligand (2) Au NP (3) few Au NPs and lipid tail (4) lipid head substrate Si (1) thiol ligand (2) Au NP (2nd) (3) Au NP (1st) (4) thiol ligand/lipid tail (5) lipid head substrate Si (1) thiol ligand (2) Au NP (3) lipid tail/SDS tail (4) lipid head/SDS head substrate Si

6.3 14.3 10.0

5.7 41.2 6.7 20.1 4.5 13.1 40.0 8.8 20.1 8.9 46.2 7.1 9.7 20.1 6.0 18.0 10.9 20.1 7.1 24.0 7.7 11.8 20.1 6.8 27.3 8.0 12.0 20.1 3.5 19.6 14.1 13.4 20.1 4.0 11.0 46.1 8.4 7.1 20.1 5.6 37.5 8.0 11.9 20.1

2.3 4.0 3.9 2.9 5.0 4.9 5.1 4.9 3.9 1.1 3.3 3.6 1.4 3.9 3.1 2.5 2.1 3.0 1.2 1.8 2.2 3.0 1.4 2.3 3.9 3.5 1.0 2.9 3.7 2.5 2.9 1.7 4.0 2.3 9.0 4.0 4.1 1.1 3.4 1.0 3.4 2.8 1.0 3.0

6.8 10.9 12.2 9.8 6.1 14.9 8.9 5.2 7.7 18.7 10.3 6.8 14.3 12.4 8.9 6.3 16.1 16.8 7.2 6.9 18.6 13.1 9.9 7.9 14.1 15.6 10.9 8.0 6.3 18.5 15.6 8.1

a The reflectivity curves were fitted by using the Motofit package of IgorPro software, which uses the slab model to fit the reflectivity curves. The uncertainty of the thickness and X-ray SLD is around 5%, while the uncertainty of the roughness is around 10%.

of the middle layer, 42.9 × 10−6 Å−2, is quite close to the value determined from X-ray reflectivity analysis, 41.2 × 10−6 Å−2. As for the Au-8C monolayer at 10 mN/m surface pressure, a fourlayer model would be needed to fit the X-ray reflectivity curve. The major difference is that it needs an additional layer on top of the Au nanoparticle middle layer with a layer thickness of 10.9 Å and a SLD of 13.1 × 10−6 Å−2. This additional layer indicates that some of the Au-8C nanoparticles are squeezed up a little bit, as shown in the recovered SLD depth profile of Figure 6b. This also results in an increase of the surface roughness from 2.3 to 5.0 Å, respectively, for the 0.1 and 10 mN/m cases. It seems that the Au-8C nanoparticles tend to form irregularities even before arriving at the collapse pressure.

mainly the capped thiols at the top and bottom of the Au nanoparticles, while the high SLD of the middle layer corresponds to the Au nanoparticles and the thiols between the Au nanoparticles. One can estimate the SLD of the middle layer by SLD = (125 × 10−6) × 0.31 + (6 × 10−6) × 0.69 = 42.9 × 10−6 Å−2. Here, 125 × 10−6 Å−2 is the SLD of bulk gold for 12 keV X-rays, and an estimated SLD of 6 × 10−6 Å−2 is used for the SLD of the thiols filling the gap space between the Au nanoparticles. Within this middle layer region (layer thickness 14.3 Å), the volume fraction occupied by Au is only roughly about 31% and the gap space occupies about 69%, as estimated by using a particle center-to-center distance of 25.6 Å and an Au nanoparticle diameter of 18 Å. The estimated SLD E

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7 with a schematic illustration of the Au-8C layer on top the SDS interface layer. When the Au-8C nanoparticle adsorbs

Figure 7. Scattering length density profiles of the LB film of mixed Au8C/SDS at 10 mN/m surface pressure as determined by fitting the Xray reflectivity curve. The position of the silicon surface is set as zero on the X-axis. The corresponding schematic illustration of the Au-8C/ SDS LB film is also shown in the figure to indicate the relative position of the Au-8C and SDS. The red spheres represent the gold nanoparticles, and the surfactant SDS is represented as a blue round sphere (hydrophilic headgroup) with a hydrophobic tail.

some SDS on its bottom side, the Au-8C/SDS becomes an amphiphilic complex and could form a more stable monolayer at the air−water interface. Figure 8 shows the measured X-ray reflectivity curves for the Au-8C/DPPC monolayer at different compression stages of 0.1,

Figure 6. Scattering length density profiles of the LB films of Au-8C at (a) 0.1 mN/m and (b) 10 mN/m surface pressures, as determined by fitting the X-ray reflectivity curves. The position of the silicon surface is set to zero on the X-axis. The corresponding schematic illustrations of the Au-8C LB films are also shown in the figure to indicate their relative positions. The red spheres represent the gold nanoparticles.

As discussed in the isotherm section, the addition of SDS could improve the mechanical property of the mixed Au-8C/ SDS monolayer and stabilize the Au-8C at the air−water interface. The organizational structure of Au-8C/SDS monolayer can be revealed by analyzing its X-ray reflectivity curve. As shown in Figure 4, the X-ray reflectivity curve of Au-8C/SDS is quite different from that of pure Au-8C monolayer. The measured X-ray reflectivity curve of Au-8C/SDS monolayer can be fitted with a four-layer model. As compared with the pure Au-8C monolayer, there is an additional layer between the Au8C and the substrate. This additional layer (fourth layer) has a thickness of 5.2 Å and a SLD of 9.7 × 10−6 Å−2. It corresponds to the headgroup layer of the SDS, which has a higher SLD than the thiol/tail layer (third layer). SDS is amphiphilic and it could form a monolayer at the air−water interface to prevent the hydrophobic Au-8C from contacting with the water phase. Since the chain length of SDS is much larger than the thickness of this interface layer, the hydrocarbon chain of the SDS molecule must be interdigitated with the thiol molecules of Au8C, as discussed in the isotherm section. The recovered SLD depth profile of the Au-8C/SDS monolayer is shown in Figure

Figure 8. X-ray reflectivity curves and the fitting results (solid line) of the LB films of Au-8C/DPPC at different surface pressures. The reflectivity curves are shifted vertically for clarity.

5.0, and 19 mN/m surface pressures for the case of a Au-8C to DPPC ratio of 6.0 mol %. The shrinking in the size of the interference fringes indicates that the mixed monolayer becomes thicker due to compression. The reflectivity curves are all well fitted with the slab model to recover the SLD depth profiles, as shown in Figure 9a, together with the schematic illustrations of the monolayers (Figure 9b) and the TEM image for the 19 mN/m case (Figure 9c). The fitting parameters are listed in Table 1. For the low surface pressure case of 0.1 mN/ m, the reflectivity curve can be fitted with a three-layer model F

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between the Au-8C nanoparticles becomes larger, so the average SLD becomes lower. This also indicates that the addition of DPPC could make the gap between neighboring Au-8C nanoparticles larger. As the surface pressure is increased to 5 and 19 mN/m, the position of the Au layer is subsequently pushed up, as indicated in Figure 9a. This is due to the DPPC molecules starting to form a closely packed monolayer at the air−water interface and the Au-8C being pushed on top of this DPPC monolayer. As indicated by Figure 9a and the structural parameters listed in Table 1, the DPPC headgroup layer becomes distinctive when the An-8C nanoparticles are pushed up due to compression. When compressed to 19 mN/m surface pressure, the thiol/lipid tail layer (designated as the third layer) is increase to 16.8 Å, which is quite close to the value for pure DPPC monolayers.43 This also indicates that DPPC in the mixed monolayer forms a well-packed monolayer at 19 mN/m. The DPPC molecules that lie directly below the Au-8C could have their tails interdigitating with the thiols of the Au-8C, and they are more disordered. This causes the average surface area per DPPC in the mixed monolayer to be larger than that for pure DPPC monolayer, as discussed in the isotherm section. From X-ray reflectivity studies, it is clear to see that the Au-8C nanoparticle array can be supported on top of the DPPC monolayer. For this comparable Au-8C to DPPC surface area ratio, the Au-8C nanoparticles do not sink into the DPPC tail regions. The related GISAXS patterns for LB films of Au NPs and its mixtures are shown in Figure S1 of the Supporting Information (SI) and the corresponding in-plane scattering profiles are given in Figures S2−S4 in the SI. As determined from the grazing-incident scattering, the center-to-center spacing of the Au-8C in the mixed Au-8C/DPPC monolayer is found to increase to 34.5 Å, as compared with the 25.6 Å for the pure Au-8C monolayer (Table 2). This indicates that adding DPPC could increase the gap distance between the Au8C nanoparticles. The SLD of the Au layer (second layer, thickness 16.1 Å) is equal to 27.3 × 10−6 Å−2 for the 19 mN/m case, as determined from fitting the X-ray reflectivity curve. This SLD value is significantly lower than that obtained for the Table 2. In-Plane Structural Information Obtained from GISAXSa sample Au-8C (0 mN/m) Au-8C (0.1 mN/m) Au-8C (10 mN/m) Au-8C (19 mN/m) Au-8C/SDS: 0.57 mol % (1 mN/m) Au-8C/SDS: 0.57 mol % (10 mN/m) Au-8C/SDS: 0.57 mol % (14 mN/m)

Figure 9. Scattering length density profiles of the LB films of mixed Au-8C/DPPC monolayers with a Au-8C to DPPC ratio of 6.0 mol %, as determined by fitting the X-ray reflectivity curves (a). The corresponding schematic illustrations of the Au-8C/DPPC LB film at different surface pressures (b), where the numbering of the layers corresponds to the slab model used in fitting the XR curves. TEM image of the LB film of mixed Au-8C/DPPC at 19 mN/m surface pressure (c), where the black dots are gold nanoparticles.

Au-8C/DPPC: 11.3 mol % (20 mN/m) Au-8c/DPPC: 6.0 mol % (19 mN/m) Au-8C/SDS/DPPC (Au-8C/DPPC: 6.0 mol %, Au-8C/SDS: 0.57 mol %) (20 mN/m) Au-8C/DPPC: 3.1 mol % (20 mN/m)

with a slightly thicker middle layer and slightly higher SLD for the bottom layer as compared with the pure Au-8C monolayer. The slightly higher SLD of the bottom layer is due to the presence of the DPPC phosphate headgroup, which has a higher SLD than the hydrocarbon chains. At this low surface pressure, the DPPC is still in the liquid expanded state. The SLD of the Au layer (middle layer) is also lower than that for the pure Au-8C monolayer case, which means that the gap

Qpeak (Å−1)

particle center-tocenter distance (Å)

0.250 0.247 0.246 0.253 0.216 0.215 0.225 0.238 0.226 0.182 0.195

25.1 25.5 25.6 24.8 29.1 29.2 27.9 26.4 27.8 34.5 32.3

0.175

36.0

a

The particle center-to-center distance d is calculated from the Qpeak value by d = 2π/Qpeak. Here, Q is the scattering vector, defined as Q = (4π/λ) sin(θ/2), where λ is the wavelength of the incident X-rays and θ is the scattering angle between the scattered X-rays and the incident X-rays. G

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Langmuir pure Au-8C monolayer case. It is due to the particle center-tocenter distance being larger for the mixed Au-8C/DPPC monolayer than for pure Au-8C monolayer. The SLD value of this Au layer can be roughly estimated from a similar calculation of SLD = (125 × 10−6) × 0.16 + (6 × 10−6) × 0.84 = 25.0 × 10−6 Å−2, which is close to the measured value of 27.3 × 10−6 Å−2. Again, the volume fraction of Au within this Au layer region (layer thickness 16.2 Å) is estimated to be about 16% and the gap space occupies about 84%, as estimated by using a particle center-to-center distance of 34.5 Å and an Au nanoparticle diameter of 18 Å. Schematic illustrations of the mixed LB film at different surface pressures are shown in Figure 9b. The TEM image of the mixed Au-8C/DPPC monolayer at 19 mN/m is shown in Figure 9c. As shown in the TEM picture, for the case of a Au-8C to DPPC ratio of 6 mol %, the Au-8C nanoparticles could form uniform domain without wrinkles, but there are some holes within the Au-8C domain. It would be interesting to see how the Au-8C nanoparticles are organized when the total surface area of added Au-8C is larger than the total surface area of DPPC in the mixed Au-8C/ DPPC monolayer. The Au-8C to DPPC ratio of 11.3 mol % is such a case, in that it has a nominal Au-8C to DPPC total surface area ratio of about 1.27 in the compressed state. As shown in Figure 4, the X-ray reflectivity curve from this mixed LB monolayer film prepared at 20 mN/m surface pressure shows more complex features than that in the case of a Au-8C to DPPC ratio of 6.0 mol %, which means the LB film has a different structure. It needs to add one extra layer on top of the high SLD Au layer in order to fit this reflectivity curve as compared with the model used to fit the reflectivity curve from the Au-8C to DPPC 6.0 mol % film. The fitting parameters are listed in Table 1 and the corresponding SLD depth profile, together with the schematic illustration of the Au-8C/DPPC monolayer, is shown in Figure 10, parts a and b, respectively. It is clear that some of Au-8C nanoparticles are squeezed to form a bilayer structure. Since the total surface area of Au-8C is larger than the total surface area of DPPC, the DPPC could not have enough surface area to support all the Au-8C on top of the DPPC in a single-layer form. This means that the Au-8C domain will eventually be compressed even before the DPPC is tightly compressed and Au-8C is prone to overlapping at higher compression pressure. The DPPC layer thickness (head plus tail) for the Au-8C/DPPC 11.3 mol % case is 18.9 Å, which is smaller than that for the Au-8C/DPPC 6.0 mol % at 19 mN/m case (24 Å). This also indicates that the DPPC layer of the Au8C/DPPC 11.3 mol % case is less ordered than that of the Au8C/DPPC 6.0 mol % at 19 mN/m case. The position (height) of the main gold layer of the Au-8C/DPPC 11.3 mol % case is thus lower than that for the Au-8C/DPPC 6 mol % at 19 mN/ m case due to the thinner (less ordered) DPPC layer underneath. As determined from grazing-incident scattering, the Au particle center-to-center distance for this case is found to be 27.8 Å (Table 2), which is much smaller than the value of 34.5 Å for the Au-8C to DPPC ratio of 6.0 mol % case. This value is, however, close to the value for the pure Au-8C case of 24.8 Å at 19 mN/m surface pressure. This also indicates that the Au-8C nanoparticles are in a highly compressed state for the 11.3 mol % case, as compared with the Au-8C nanoparticles for the 6.0 mol % case. Domain overlapping is observed in the TEM image of Figure 10c. The overlapped region would have a bilayer structure of Au-8C nanoparticles. The SLD of the main Au layer is found to be 46.1 × 10−6 Å−2, which is more close to that found for the pure Au-8C case and it differs a lot from that

Figure 10. Scattering length density profile of the LB film of mixed Au-8C/DPPC for the Au-8C to DPPC ratio of 11.3 mol % at 20 mN/ m surface pressure as determined by fitting the X-ray reflectivity curve (a), where the corresponding schematic illustration of the mixed Au8C/DPPC LB film is also shown in the panel to indicate their relative positions. The schematic of the LB film of mixed Au-8C/DPPC for the Au-8C to DPPC ratio of 11.3 mol % at 20 mN/m surface pressure (b), where the numbering of the layers corresponds to the slab model used in fitting the XR curve. TEM image of the LB film of the mixed Au8C/DPPC for the Au-8C to DPPC ratio of 11.3 mol % at 20 mN/m surface pressure (c), where the black dots are gold nanoparticles. Some overlapping of the Au-8C monolayers can be observed in the TEM image. In the illustrations, the red spheres represent the gold nanoparticles. The DPPC is represented as a blue round sphere (hydrophilic headgroup) with two hydrophobic tails.

found for the Au-8C to DPPC ratio of 6 mol % case. This finding is also consistent with the finding of a tighter packing of the Au-8C for the 11.3 mol % case than the 6.0 mol % case. For the case of a lower Au-8C to DPPC ratio of 3.1 mol %, the X-ray reflectivity curve resembles that of the 6 mol % one, but it is not exactly the same. The four-layer slab model is used to fit the reflectivity curve, and the obtained parameters are listed in Table 1. The corresponding SLD depth profile, H

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Langmuir together with the schematic illustration of the mixed monolayer, is shown in Figure 11a,b. The major difference is a higher SLD third layer as compared with the SLD depth profile of the 6.0 mol % case at 19 mN/m surface pressure (Figure 9a). The higher SLD third layer suggests that some of

the Au-8C nanoparticles sink a little bit more into the lipid tail layer, similar to the phenomenon discussed by Hassenkam et al.25 Since the Au-8C nanoparticles distribute more widely in vertical positions in both the second and third layers (total thickness 31.7 Å), the average SLD of these two layers are lower than the SLD of the Au layer (second layer) for the 6.0 mol % case. As shown in the TEM image of Figure 11c, the Au8C nanoparticles aggregate into connected threadlike structures for this lower Au-8C to DPPC ratio case. A similar aggregation structure was also observed in several other studies with very low Au nanoparticle to lipid molar ratios.23,29,30 It was suggested that Au nanoparticles preferentially aggregate at interfaces between liquid-expanded and liquid-condensed phases, especially for low Au to lipid concentrations.23 This means that lipids right beneath the Au-8C threadlike aggregates must be in a more disordered state, even when it is compressed to the surface pressure of 20 mN/m, and the vertical position of the Au-8C nanoparticles is not as uniform as that for the 6 mol % case. This indicates that the concentration ratio of Au nanoparticle to lipid is a also a crucial factor in determining the aggregation structure of the Au nanoparticles in a mixed monolayer system. For sufficiently high Au nanoparticle to lipid ratios, the Au nanoparticle could form continuous large domains and could be well-supported on top of the compact DPPC monolayer when compressed. The effect of adding SDS to the mixed Au-8C/DPPC monolayer at a Au-8C to DPPC ratio of 6.0 mol % was also investigated by X-ray reflectivity. The X-ray reflectivity curve was successfully modeled by a four-layer slab model and the obtained fitting parameters are listed in Table 1. The corresponding SLD depth profile and the schematic illustration of the mixed monolayer are respectively shown in Figure 12, parts a and b. The recovered depth profile is very similar to that of the mixed Au-8C/DPPC monolayer case, except that the interlayer roughness is smaller. Both the Au-8C layer and the supporting DPPC layer become more distinctive due to the presence of SDS. As mentioned previously, SDS could induce DPPC to become more ordered and SDS is more easily interdigitated with the thiols capped on Au nanoparticle since it is a single-chain surfactant.35 The interdigitation between the DPPC tails and the thiols will be reduced due to the presence of SDS, and better layer ordering is achieved. However, the inplane aggregation structure of Au-8C nanoparticles is greatly affected due to the presence of SDS. As shown in the TEM image of the mixed Au-8C/SDS/DPPC monolayer in Figure 12c, the originally continuous Au-8C domain (Figure 9c) is dispersed into a very fine meshlike foam structure due to the presence of SDS. SDS is known to be able to affect the domain formation of DPPC in a mixed DPPC/SDS monolayer.44,45 According to the study of McConlogue et al., it was found that in the mixed DPPC/SDS monolayer the DPPC domain boundary was uniformly dispersed into smaller domains at higher surface pressure.45 The foamlike aggregation structure of Au-8C must originate from the dispersion of the DPPC domains by SDS. This indicates that the aggregation morphology of Au-8C depends strongly on the formation and shape of the lipid domains, especially in the presence of additional additives.

Figure 11. Scattering length density profile of the LB film of mixed Au-8C/DPPC for the Au-8C to DPPC ratio of 3.1 mol % at 20 mN/m surface pressure as determined by fitting the X-ray reflectivity curve (a), where the corresponding schematic illustration of the mixed Au8C/DPPC LB film is also shown in the panel to indicate their relative positions. The schematic illustration of the LB film of mixed Au-8C and DPPC for the Au-8C to DPPC ratio of 3.1 mol % at 20 mN/m surface pressure (b), where the numbering of the layers corresponds to the slab model used in fitting the XR curves. TEM image of the LB film of mixed Au-8C/DPPC for the Au-8C to DPPC ratio of 3.1 mol % at 20 mN/m surface pressure (c), where the black dots are gold nanoparticles. The Au-8C forms a threadlike structure. The inset is the enlarged picture of the threadlike structure. In the illustrations, the red spheres represent the gold nanoparticles. The DPPC is represented as a blue round sphere (hydrophilic headgroup) with two hydrophobic tails.

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CONCLUSIONS In this study, the LB monolayers of the thiolated gold nanoparticles mixed with DPPC/SDS were investigated by combining the X-ray reflectivity, grazing-incident scattering, I

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Langmuir

film collapses only due to domain overlapping. The addition of charged single-tail surfactant to the thiolated Au nanoparticle monolayer could help to stabilize the Au nanoparticle monolayer, and it could strengthen the mechanical property of the film. The hydrocarbon chain of the single surfactant at the air−water interface could interdigitate with the thiols capped on Au nanoparticles and could stabilize the Au-8C/SDS monolayer at the air−water interface. For mixing with lipids, it is found that the thiolated gold nanoparticles are pushed on top of the lipid monolayer when the mixed monolayer is compressed. Unlike the single-chain surfactant, the double-tail lipid could form a robust monolayer at the air−water interface and it could support the thiolated gold nanoparticle arrays on its top surface to keep the hydrophobic thiolated gold nanoparticles from contacting with the water phase. When the total surface area ratio of thiolated gold nanoparticles to lipid is greater than 1, the compression of the mixed monolayer eventually causes the thiolated gold nanoparticle domains to partially overlap and to form bilayer thiolated gold nanoparticle domains at some regions. On the other hand, when the total surface area of thiolated gold nanoparticles is significantly smaller than that of the lipid, the thiolated gold nanoparticles form a connected threadlike structure. At a comparable ratio of 0.67 for the total surface area ratio of thiolated gold nanoparticle to lipid, it is found that thiolated gold nanoparticles could form a uniform domain on top of the lipid monolayer. It is evident that the morphology of the thiolated gold nanoparticle monolayer is highly dependent on the total surface area ratio of the thiolated gold nanoparticle to lipid. SDS is found to have the power to disperse the originally uniform Au-8C nanoparticle domain of the mixed Au-8C/ DPPC monolayer into a foamlike structure. It is evident that not only the concentration ratio but also the size and shape of the template formed by the amphiphilic molecules and their interaction with the thiolated gold nanoparticles can all have great effects on the organization structure as well as the morphology of the thiolated gold nanoparticle monolayer. The particle center-to-center distance of the thiolated gold nanoparticles becomes slightly larger in the presence of SDS and can be increased about 35% in the mixed Au-8C/DPPC monolayer for the Au-8C to DPP ratio of 6.0 mol %. Tuning the gold nanoparticle gap distance can control the optical properties of the gold nanoparticle monolayer, and it is useful for fabricating various functional devices.

Figure 12. Scattering length density profile of the LB film of mixed Au-8C/SDS/DPPC monolayer for the Au-8C to DPPC ratio of 6.0 mol % and Au-8C to SDS ratio of 0.57 mol % at 20 mN/m surface pressure as determined by fitting the X-ray reflectivity curve (a), where the corresponding scheme is also shown in the panel to indicate their relative positions. Schematic of the LB film of mixed Au-8C/SDS/ DPPC monolayer for the Au-8C to DPPC ratio of 6.0 mol % and Au8C to SDS ratio of 0.57 mol % at 20 mN/m surface pressure (b), where the numbering of the layers corresponds to the slab model used in fitting the XR curve. TEM image of the LB film of mixed Au-8C/ SDS/DPPC monolayer for the Au-8C to DPPC ratio of 6.0 mol % and Au-8C to SDS ratio of 0.57 mol % at 20 mN/m surface pressure (c), where the black dots are gold nanoparticles. The Au-8C forms a foamlike aggregation structure. The inset is the enlarged picture of the aggregation structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01559. Representative GISAXS patterns for LB films of Au NPs and its mixtures and in-plane scattering profiles from the GISAXS patterns for the LB films of Au-8C NPs, Au-8C NPs mixed with SDS, and Au-8C NPs and its mixtures (PDF)

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and TEM analyses to reveal the in-depth and in-plane organization and the 2D morphology of such mixed monolayers. It was found that the addition of charged surfactant SDS could not suppress the collapse of the Au-8C monolayer, but the collapsing behavior is altered. In the presence of SDS, the wrinkle formation is suppressed and the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tsang-Lang Lin: 0000-0001-7638-2066 U-Ser Jeng: 0000-0002-2247-5061 J

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Langmuir Notes

(17) Griesemer, S. D.; You, S. S.; Kanjanaboos, P.; Calabro, M.; Jaeger, H. M.; Rice, S. A.; Lin, B. The role of ligands in the mechanical properties of Langmuir nanoparticle films. Soft Matter 2017, 13, 3125−3133. (18) Takae, S.; Akiyama, Y.; Otsuka, H.; Nakamura, T.; Nagasaki, Y.; Kataoka, K. Ligand density effect on biorecognition by PEGylated gold nanoparticles: regulated interaction of RCA120 lectin with lactose installed to the distal end of tethered PEG strands on gold surface. Biomacromolecules 2005, 6, 818−824. (19) Oxtoby, D. W. New perspectives on freezing and melting. Nature 1990, 347, 725−730. (20) Bera, M. K.; Sanyal, M. K.; Pal, S.; Daillant, J.; Datta, A.; Kulkarni, G. U.; Luzet, D.; Konovalov, O. Reversible buckling in monolayer of gold nanoparticles on water surface. Eur. Phys. J. 2007, 78, 56003−56008. (21) Srivastava, S.; Nykypanchuk, D.; Fukuto, M.; Gang, O. Tunable nanoparticle arrays at charged interfaces. ACS Nano 2014, 8, 9857− 9866. (22) Schultz, D. G.; Lin, X.-M.; Li, D.; Gebhardt, J.; Meron, M.; Viccaro, P. J.; Lin, B. Structure, wrinkling, and reversibility of Langmuir monolayers of gold nanoparticles. J. Phys. Chem. B 2006, 110, 24522−24529. (23) Mogilevsky, A.; Volinsky, R.; Dayagi, Y.; Markovich, N.; Jelinek, R. Gold Nanoparticle Self-Assembly in Saturated Phospholipid Monolayers. Langmuir 2010, 26, 7893−7898. (24) Mogilevsky, A.; Jelinek, R. Gold nanoparticle self-assembly in two-component lipid Langmuir monolayers. Langmuir 2011, 27, 1260−1268. (25) Hassenkam, T.; Nørgaard, K.; Iversen, L.; Kiely, C. J.; Brust, M.; BJørnholm, T. Fabrication of 2D gold nanowires by self-assembly of gold nanoparticles on water surfaces in the presence of surfactants. Adv. Mater. 2002, 14, 1126. (26) Hansen, C. R.; Westerlund, F.; Moth-Poulsen, K.; Ravindranath, R.; Valiyaveettil, S.; Bjørnholm, T. Polymer-templated self-assembly of a 2-Dimensional gold nanoparticle network. Langmuir 2008, 24, 3905. (27) Gagnon, B. P.; Meli, M.-V. Effects on the self-assembly of nalkane/gold nanoparticle mixtures spread at the air−water interface. Langmuir 2014, 30, 179−185. (28) Choudhuri, M.; Datta, A. Time-structuring in the evolution of 2D nanopatterns through interactions with substrate. Soft Matter 2016, 12, 5867−5875. (29) Tatur, S.; Badia, A. Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant. Langmuir 2012, 28, 628−639. (30) Paczesny, J.; Sozanski, K.; Dziecielewski, I.; Zywocinski, A.; Hołyst, R. Formation of net-like patterns of gold nanoparticles in liquid crystal matrix at the air−water interface. J. Nanopart. Res. 2012, 14, 826−836. (31) Yang, P.-W.; Lin, T.-L.; Liu, I.-T.; Hu, Y.; James, M. In-situ neutron reflectivity studies of the adsorption of DNA by charged diblock copolymer monolayers at the air-water interface. Soft Matter 2012, 8, 7161−7168. (32) Lin, T.-L.; Wu, J.-C.; Hu, Y.; Jeng, U.-S.; Lee, H.-Y. Effect of different divalent ions on the DNA adsorption by lipid monolayers. Chin. J. Phys. 2012, 50, 332−343. (33) Yang, P.-W.; Lin, T.-L.; Liu, I.-T.; Hu, Y.; Jeng, U.-S.; Gilbert, E. P. Small-angle neutron scattering studies on the multilamellae formed by mixing lamella-forming cationic diblock copolymers with lipids and their interaction with DNA. Langmuir 2016, 32, 1828−1835. (34) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. Organization of polymer-surfactant mixtures at the air-water interface: poly(dimethyldiallylammonium chloride), sodium dodecyl sulfate, and hexaethylene glycol monododecyl ether. Langmuir 2002, 18, 5139− 5146. (35) Harper, K. L.; Allen, H. C. Competition between DPPC and SDS at the Air-Aqueous Interface. Langmuir 2007, 23, 8925−8931. (36) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of thiol-derivatised gold nanoparticles in a two-phase liquid− liquid system. J. Chem. Soc., Chem. Commun. 1994, 0, 801−802.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the NSRRC for providing the beam time and help with the X-ray reflectivity and GISAXS measurements. We also extend our thanks to the microscopy center of Chang Gung University for assistance in TEM analyses. This research is supported by the projects MOST 105-2113-M-007-015, MOST 103-2113-M-007-014-MY2, NSC 102-2113-M-007-013, and NSC 99-2113-M-007-014-MY3 (T.L.L.). This research is also partly supported by the Frontier Research Center on Fundamental and Applied Sciences of Matters of National Tsing Hua University, Project 102N2012E1.

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