Au NP Honeycomb-Patterned Films with Controllable Pore Size and

Mar 8, 2013 - order and regularity of the honeycomb films. 2. EXPERIMENTAL ... acceleration voltage of 100 kV. Scanning .... 0. B. (1). In eq 1, P is ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Au NP Honeycomb-Patterned Films with Controllable Pore Size and Their Surface-Enhanced Raman Scattering Li Kong, Renhao Dong, Hongmin Ma, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China S Supporting Information *

ABSTRACT: Honeycomb-patterned films (HPFs) of Au nanoparticles (Au NPs) with pore size controlled by varying the quantity of Au NPs or using modified agents of different mercaptans (C14H29SH, C16H33SH, and C18H37SH) were prepared. The strength of the HPFs containing Au NPs can be enhanced because of the addition of polymers including polystyrene, poly(Llactic acid), and poly(methyl methacrylate-co-ethyl acrylate). With an increase in the amount of polymer and the number of Au NPs or the chain length of the modified agents, the pore size of HPFs decreases, indicating that the pore size can be well controlled by adjusting the above factors. Interestingly, HPFs with elliptical pores that were created by the direction of the air flow were observed. The pore diameter on the outer rim is smaller than that in the center, which should be because of the subordinate evaporation of the solvent in the center. Sponge structures were observed in the cross sections of the walls of HPFs, which should be produced by microphase separation. The HPFs consisting of Au NPs with controllable pore size exhibited stronger surface-enhanced Raman scattering. We believe that the HPFs composed of metal NPs such as Au, Ag, and Cu are exploited in multispectral scanners, nanophotons, and sensors.

1. INTRODUCTION Highly ordered structural materials have attracted considerable attention because of their potential applications as templating masks,1 superhydrophobic coatings,2 tissue engineering scaffolds,3 and photocatalytic materials. The highly ordered structural materials can be prepared from both conventional photolithography4−6 and a breath figure method reported by François’s group in 1994.7 Both methods have their own advantages. In our experiments, we used the breath figure method because of its facile property. In this method, water droplets on substrate surfaces act as templates for selfmicrofabrication materials. For solutions consisting of nanoparticles, the formation mechanism can be depicted as follows: a solution of nanoparticles in a volatile solvent such as chloroform, carbon bisulfide, or toluene is dropped onto a solid surface under moist gas flow. The solvent evaporates rapidly, leading to a sharp decrease in the surface temperature. Water vapor condenses on the surface to form microdroplets because of the Pickering emulsion of nanoparticles.8 The water droplets do not coalesce under the effect of the capillary force and Marangoni convection9−11 but pack in hexagonal arrays. When the volatile solvents completely evaporate, the water droplets evaporate and honeycomb films with ordered pores can form © XXXX American Chemical Society

on surfaces. In fact, such porous honeycomb materials upon comparison with lithographic structures are old-fashioned, and there are many kinds of porous materials. However, these material-based porous structures are still fascinating because of functional nanoporous materials such as template-synthesisbased nanoporous materials12,13 and coordination-chemistrybased nanoporous materials.14,15 These porous materials can be easily compared to honeycomb structures in which one can easily appeal to advantages in the latter structures. The influence of factors such as the composition concentration, solvent, relative humidity, and molecular weight of polymers on the formation of honeycomb films has been reported in many studies.16−19 Our group has also reported honeycomb films of C12H25SH−Au and has investigated their optical and magnetic properties.20−22 However, to the best of our knowledge, the influence of different polymers and mercaptans in controlling honeycomb-patterned-film features such as the pore size has not been investigated in detail. Received: December 28, 2012 Revised: March 4, 2013

A

dx.doi.org/10.1021/la305143v | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 1. SEM images of the Au NPs honeycomb films formed by 3.0 g·L−1 C14H29SH−Au (a), 3.0 g·L−1 C16H33SH−Au (b), and 1.0 g·L−1 C18H37SH−Au (c).

Figure 2. SEM images of the 2.5 g·L−1 PS/Au NP hybrid honeycomb films with different mercaptans: 3.0 g·L−1 C14H29SH−Au (a), 3.0 g·L−1 C16H33SH−Au (b), and 1.0 g· L−1 C18H37SH−Au (c). ethanol to remove excess thiol. Four hours later, the mixture solution was separated by centrifugation to collect the dark-brown precipitates that were washed with ethanol three times and dried in a vacuum desiccator. Other mercaptan-stabilized Au NPs were prepared by the same method. The as-prepared Au NPs were easily redispersed in chloroform for characterization or honeycomb-patterned film formation. Using this method, three different Au NPs modified by C14H29SH, C16H33SH, and C18H37SH were synthesized, respectively. The Au NPs were characterized by TEM observations (Supporting Information, SI, Figure S1). The mean diameter was calculated to be 4.90 nm for C14H29SH−Au NPs, 4.99 nm for C16H33SH−Au NPs, and 5.11 nm for C18H37SH−Au NPs. With an increase in the chain length of the mercaptan, the diameter of the Au NPs becomes larger. During the experiment, the mercaptan was finally added because of the strong Au−S covalent bonds. Au NPs modified with different mercaptans were characterized by UV−vis measurements (Figure S2). From the UV−vis curves, we can see a red shift with an increase in the diameter of the Au NPs. This might be ascribed to the effect of localized surface plasmon resonance (LSPR). The maximum absorption wavelength was 515 nm for C14H29SH−Au, 517 nm for C16H33SH−Au, and 520 nm for C18H37SH−Au. 2.3. Synthesis of Honeycomb Films. Solutions containing different concentrations of Au NPs in chloroform were prepared. A drop of solution (∼5 μL) was placed on cleaned ITO glass. Moist nitrogen gas was allowed to flow across the solution surface. When the solvent completely evaporates at room temperature, opaque honeycomb films are left on the substrate. To synthesize polymer nanocomposite films, the solution is a mixture of nanoparticles and polymers. The N2 velocity was 1.0 L·min−1, and the relatively humidity was 70−85%. 2.4. Measurements of Surface-Enhanced Raman Scattering (SERS). We prepared samples of HPFs for SERS measurements by dropping 7 μL of a 2 mmol·L−1 R6G/methanol solution onto asprepared HPFs. 2.5. Characterization. Transmission electron microscope (TEM) images were obtained by using a JEOL JEM-100CX II TEM at an acceleration voltage of 100 kV. Scanning electron microscope (SEM) images were obtained by using a JEOL JSM-6700F field emission SEM and a Hitachi S-4800 field-emission SEM at 3.0 kV. AFM images were acquired with a Digital Instruments NanoScope III operating in tapping mode. Absorption spectra were acquired by using a Hitachi U4100 spectrophotometer. SERS spectra were obtained by using LabRAM HR800 (Horiba Jobin Yvon, France). Raman measurements were carried out by laser confocal Raman microspectroscopy

In this Article, we synthesized Au NPs stabilized by three different mercaptans, C14H29SH, C16H33SH, and C18H37SH, and prepared Au NP honeycomb-patterned films (HPFs) on cleaned ITO glass surfaces. Different polymers including polystyrene, poly(L-lactic acid), and poly(methyl methacrylate-co-ethyl acrylate) were added to enhance the strength of the HPFs. The pore size distribution of Au NP honeycombpatterned materials was well controlled by the concentration of the polymers, number of Au NPs, or chain length of the modified agents. Except for the HPFs having spherical pores, HPFs with elliptical pores were also observed. Our results should provide a rich understanding of how to fabricate pores of a controllable size via the modified agents and composition concentration. Surface-enhanced Raman scattering (SERS) of the Au NPs honeycomb-patterned films was primarily tested and revealed that these materials are relevant to the degree of order and regularity of the honeycomb films.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetraoctylammonium bromide (TOAB) was purchased from Fluka. Polystyrene (PS) with an average molecular weight of 250 kD was purchased from Acros Organics. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O), dodecanethiol, and other chemicals were obtained from Guoyao (Shanghai, China). Poly(L-lactic acid) (PLLA, Mw = 500 kD) was purchased from Shandong Institute of Medical Instruments. Poly(methyl methacrylateco-ethyl acrylate) (PMMA, Mw = 101 kD) was purchased from SigmaAldrich. All chemicals were used as received without further purification. 2.2. Synthesis of Au NPs. The synthesis of Au NPs is carried out according to the literature.23 An aqueous solution of HAuCl4·3H2O (10 mL, 24.28 mmol·L−1) was mixed with 20 mL of toluene dissolved with 0.3983 g of TOAB. The two-phase mixture solution was vigorously stirred for 30 min, and the organic phase was collected. A freshly prepared aqueous solution of sodium NaBH4 (0.1010 g of NaBH4 in 10 mL of H2O) was slowly added with vigorous stirring. After being stirred for 0.5 h, the organic phase was separated, a solution of 0.056 g of C14H29SH dissolved in a small amount of toluene was added, and finally the mixture solution was vigorously stirred for 3 h. The mixture solution was evaporated to 5 mL in a rotary evaporator. The residual solution was added to 140 mL of B

dx.doi.org/10.1021/la305143v | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 3. AFM images of the 2.5 g·L−1 PS−Au NPs hybrid honeycomb films with different mercaptans: 0.3 g·L−1 C14H29SH−Au (a), 0.3 g·L−1 C16H33SH−Au (b), and 0.7 g·L−1 C18H37SH−Au (c). (LabRAM HR800, Horiba Jobin Yvon, France) with excitation at 532 nm and 50 mW. All spectra were acquired over 20 s.

3. RESULTS AND DISCUSSION 3.1. Effect of Polystyrene (PS) on Ordered Structures. Au NPs can also form porous structures; however, the degree of order is very poor.21 Pores formed by C14H29SH−Au, as shown in Figure 1a, are not ordered. For C16H33SH−Au, the diameter of the pores, as shown in Figure 1b, becomes larger. The honeycomb films are regularly ordered and look like networks. When the modified agent is C18H37SH, the diameters of the pores grow much larger and are more regularly ordered and the network structures become more obvious, as shown in Figure 1c. No ordered porous structures can be formed with a 2.5 g·L−1 PS solution on the substrate. The degree of ordering of the pores and the strength of the films can be dramatically improved when Au NPs are stabilized with polystyrene. From the SEM images in Figure 2, by comparison with Figure 1, one can see that the networks disappeared completely. The regular ordering of pores for C14H29SH−Au and C18H37SH−Au systems can still be identified; however, the regular ordering of C16H33SH−Au system is perfect and the pore size is uniform at around 830 nm. The regular degree of these honeycomb films was further confirmed by AFM observations. Considering that the sample for AFM should be thin enough and the honeycomb structure should be maintained, we prepared some low-concentration solutions. As shown in Figure 3, the porous films of Au NPs were stable because of the addition of PS, and the honeycomb structures can form more uniformly. The perfectly uniform pores, as shown in Figure 3b, can easily be observed for films of C16H33SH−Au NPs. 3.2. Effect of Concentration of Different Polymers on Pore Size. To investigate the effect of polymer concentration on the pore size of HPFs, we chose C14H29SH−Au as the studied model system and the concentration of Au NPs was fixed at 1.0 g·L−1. Poly(L-lactic acid) (PLLA) was chosen as a polymer by varying the concentration from 0.5 to 2.0 g·L−1. When the PLLA concentration is lower than 0.5 g·L−1, as shown in Figure 4a, the pore size is much larger, around 1.7 μm, exhibiting packed hexagonal arrays of pores. With an increase in the PLLA concentration, the hexagonal array does not damage but deforms and the degree of order decreases, as shown in Figure 4b−d. The relationship between the pore diameter and the PLLA concentration is shown in Figure S3. When the polymer was chosen to be PMMA or PS, we found the same phenomenon (i.e., with an increase in the PMMA or PS concentration, the pore size decreases). Some different phenomena on the structure were obtained. For C14SH−Au/ PMMA honeycomb films, the pore edge of the films is rough,

Figure 4. SEM images of the 1.0 g·L−1 C14SH−Au/PLLA hybrid honeycomb films. The concentration of PLLA changed: 0.5 (a), 1.0 (b), 1.5 (c), and 2.0 g·L−1 (d).

which is different from that of C14SH−Au/PS even though the diameter of the honeycomb pores in both cases decreases but without deformation (Figures S4 and S5). The pore size can be explained with Henry law. One can see that the solvent vapor pressure is lower in a more concentrated solution, as shown in eq 1 P = P0(1 − XB)

(1)

In eq 1, P is the vapor pressure of solvent in solution, P0 is the vapor pressure of pure solvent, and XB is the mole fraction of the solute. Low vapor pressure should decrease the evaporation rate of the solvent; therefore, the temperature of the solution should be relatively higher. The temperature gradient (ΔT) between the atmosphere and the solution surface will diminish, as shown in eq 2 ΔT = Tr − Ts

(2)

where Tr is the temperature of the atmosphere and Ts is the temperature of the solution surface. At this nuclear stage, the increase in the radius of droplets R per unit time t is proportional to the temperature gradient (eq 3)16

dR ≈ ΔT 0.8 (3) dt Thus, more concentrated solutions should have lower ΔT values, and the diameter of the droplets grows slowly, resulting in a smaller pore size of the films. Although the water droplets in the more concentrated solution have more time to grow, the viscosity should be correspondingly higher, which inhibits the C

dx.doi.org/10.1021/la305143v | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 5. SEM images of hybrid honeycomb films for PS/Au NPs modified by different mercaptans. (a) 1.0, (b) 3.0, (c) 5.0 g·L−1 C14H29SH−Au; (d) 1.0, (e) 3.0, (f) 5.0 g·L−1 C16H33SH−Au; (g) 1.0, (h) 3.0, (i) 5.0 g·L−1 C18H37SH−Au. cPS = 2.5 g·L−1.

growth of the water droplets. However, the nuclear stage plays a critical role in the size of droplets. 3.3. Effect of Different Modified Agents on Pore Diameter. To detect the effect of the modified agents on the formation of the honeycomb structures, we prepared a series of honeycomb films that were modified by different mercaptans. We chose PS/Au NPs as the studied model system. The PS concentration was fixed at 2.5 g·L −1 by varying the concentration of Au NPs. From the SEM images, as shown in Figure 5, one can see that the honeycomb films are much more highly ordered; however, the degree of ordering is very poor at high C18H37SH−Au concentration. The degree of ordering of the honeycomb films produced from the C16H33SH−Au/PS system has the best regularity. The pore diameter of the honeycomb films decreases from both the horizontal and the longitudinal aspects. We calculated the pore diameter by Nano Measure Software, and the data are listed in Table 1. For the longitudinal aspect, the pore sizes are dependent on the concentration of Au NPs, which we have discussed above. For the horizontal aspect, one can see that the pore diameter decreases with the increase in the length of the modified agents. The relationship between the pore size and the modified agent is shown in Figure S6.

A mechanism for the observations was proposed, as shown in Figure 6. It can be illustrated for three aspects. First, when the

Figure 6. Schematic illustration of a condensed water droplet stabilized by the adsorption of Au NPs at the water/oil interface. θ is the contact angle of a particle at the water/oil interface.

chain length of the modified mercaptans becomes longer, the velocity of the solution becomes correspondingly greater. The higher viscosity can prevent the water droplets from coalescing and also slow the immersion of the water droplets, leading to a slower growth rate. Therefore, the pore size becomes smaller.24,25 Second, the packing and aggregation states of alkyl chains of the mercaptans in covering layers on Au NPs would be a key factor in the aggregation behaviors of the nanoparticles in forming honeycomb films. An asymmetric C− H peak in the IR spectra can provide information about the states of the alkyl chains. Usually, this peak appears below 2920 cm−1 in the crystalline state and above 2920 cm−1 in the disordered state.26 Thus, it can be proposed that the state of C−H alkyl chains adsorbed on the surfaces of Au nanoparticles could be changed during the evaporation process, which should play an important role in the formation of honeycomb films. In the early evaporation stage, the state of C−H alkyl chains adsorbed on the surfaces of Au NPs should be the same as that of alkyl chains of Au NPs modified by mercaptans in chloroform (i.e., the disordered state). During evaporation, the state of the C−H alkyl chains adsorbed on the surfaces of

Table 1. Relationship between Au Nanoparticle Concentration and Pore Diameter system cAu (g·L−1)

C14H29SH−Au/PS (μm)

C16H33SH−Au/PS (μm)

C18H37SH−Au/PS (μm)

1.0 3.0 5.0

2.65 0.97 0.62

2.41 0.83 0.48

2.28 0.60 0.37 D

dx.doi.org/10.1021/la305143v | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

1.20:1. Correspondingly, for the PS/C18H37SH−Au system, they are 0.60 and 0.49 μm, and the aspect ratio is 1.22:1, respectively (Figure 8b). The mechanism can be illustrated by the direction of the airflow.29 In our experiments, the airflow is blown through an inverse funnel. At some location, the direction of the flow is not perpendicular to the surface (Figure 9). When the air flows past

Au NPs could be in the crystalline state. Third, the ability of adsorbed Au NPs to stabilize the water−solvent interface without a change in the interfacial tension is known to take place in a Pickering emulsion (Figure 6).27 The stabilization energy E for an adsorbed nanocrystal with radius RNC relates to the oil−water interfacial tension γOW and the equilibrium contact angle θOW of the nanocrystal at the liquid−liquid interface:28 E = πRNC 2γOW(1 − |cos θOW |)2

(4)

As is well known, Au NPs stabilized by mercaptans are hydrophobic. With an increase in the chain length of the modified mercaptans, θOW becomes larger. However, in comparison to the difference caused by θOW, the difference in RNC toward these three CnH2n+1SH−Au/PS (n = 14, 16, and 18) systems can be ignored. Therefore, with an increase in the chain length of CnH2n+1SH, the stabilization energy E decreases. Consequently, with the increase of the chain length, the size of water droplets stabilized by the mercaptan-decorated Au NPs becomes smaller. After the evaporation of the water droplets, the pores left on the films should become smaller with a corresponding increase in the chain length. To investigate the cross-section, we cut the substrate containing honeycomb films into two pieces. As shown in Figure 7, the sponge structures can be seen in the PS/

Figure 9. Model of the water droplets as the templates for forming (a) circular and (b) elliptical pores.

the solution surface along the direction making a small angle (θ) with respect to the normal of the solution surface, the water droplets will receive an additional shear (F) and the vector is along the same direction as the air flow. The shear causes the water droplets to change from spherical to ellipsoidal. Finally, the pores templated by water droplets become elliptical, as shown in Figure 8. 3.5. Different Size Distribution of Film Pores. As shown in Figure 10, at the edge of one film, the pore size is smaller

Figure 7. SEM images of a cross-section of honeycomb films produced by 2.5 g·L−1 PS and C16H33SH−Au NPs, where the concentration of Au NPs is (a) 0.5 and (b) 1.0 g·L−1. The inset in panel b is the bottom side of the films.

C16H33SH−Au films, but the spherical structures are also clearly seen from the inset image in Figure 7b. We deduce that the aggregates of the cross-section might be caused by microphase separation. The accurate mechanism is still under further study. 3.4. Formation of Elliptical Pores. From the observations, we found the formation of elliptical pores of the honeycomb films (Figure 8). The long axis of the pores formed by PS/ C14H29SH−Au, as shown in Figure 8a, is 0.79 μm, and the minor axis is 0.66 μm. The aspect ratio of the elliptical pores is

Figure 10. SEM images of the 2.5 g·L−1 PS/1.0 g·L−1 C16H33SH−Au NPs hybrid honeycomb films: (a) the edge, 0.54 μm and (b) the center, 0.70 μm. SEM images of the 2.5 g·L−1 PS/0.5 g·L−1 C18H37SH−Au NPs hybrid honeycomb films: (c) the edge, 1.15 μm and (d) the center, 2.14 μm.

than that of the central part. It is well known that the solvent evaporates first at the edge, compared to the central part, and the droplets at the edge act as the template to have less time to form pores. When the droplets evaporate completely, the pores at the edge should be correspondingly smaller. 3.6. Surface-Enhanced Raman Scattering. As is well known, the interaction of metal nanoparticles with optical fields can result in collective oscillations of conduction electrons within the nanoparticles, which is the “particle plasmon” resonance. The coupling of the plasmon resonance of metal

Figure 8. SEM images of the PS/Au NPs hybrid honeycomb films with different CnH2n+1SH: (a) 4.0 g·L−1 C14H29SH−Au and (b) 3.0 g·L−1 C18H33SH−Au. cPS = 2.5 g·L−1. E

dx.doi.org/10.1021/la305143v | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 11. Raman scattering spectra of (a) R6G on C14H29SH−Au honeycomb films formed by different Au concentrations and (b) honeycomb films formed from three different CnH2n+1SH species (n = 14, 16, and 18).

concentration PS/CnH2n+1SH−Au systems result in low vapor pressures, and as the temperature difference between the atmosphere and solution decreases, the templating droplets should grow slowly and the pore size should be smaller. The pore size is also dependent on the length of CnH2n+1SH, which decreases with the length of n. This phenomenon is induced by (i) the long-chain length of CnH2n+1SH, which leads to high velocity and a slow growth rate of water droplets, (ii) the state of alkyl chains of the mercaptans adsorbed on the surfaces of Au nanoparticles can be changed during the evaporation process, and (iii) the hydrophobicity of the modified Au nanoparticles, which becomes stronger with the increasing chain length of CnH2n+1SH to reduce the stabilization energy E until finally the pore size decreases with the increase in hydrocarbon chain length of CnH2n+1SH. The elliptical pores were observed in honeycomb films, which could be the result of the velocity direction. For the same sample, the pore diameter is also varied; generally, the edge contains the smaller pores and the central part contains the larger pores. This phenomenon could be owed to the priority location of solvent evaporation. The solvent first evaporates at the edge, but the droplet acting as a template in the center should need much more time to evaporate. The cross section was investigated, and sponge structures can be found inside. The microstructures inside could be produced by microphase separation. The SERS of honeycomb films was enhanced with much more regular honeycomb films that were due to the nanoscale surface roughness and the large surface, which could provide potential applications in devices and detectors for organic molecules.

nanoparticles with molecules could result in enhanced linear and nonlinear optical properties such as surface-enhanced Raman scattering (SERS) signals. For metal aggregates or assembled nanoparticles, because of the strong additional field enhancement in the gap regions among the particles (the “hot spot”), local field effects could be enhanced by several orders of magnitude that are responsible for the much stronger SERS response.30−32 Besides, the assembling state of Au NPs often influences the gold plasmon peak,33,34 which would give good insight into the aggregation states of Au NPs. Therefore, Au NP aggregates can act as active substrates for SERS. The morphologies of films formed by different concentrations of C14H29SH−Au are different. When the concentration is 5.0 g·L−1, the structure is more ordered than that of the films for two other concentrations (Figure S7). From the Raman scattering spectra, we can see that it has the highest intensity. For the other two concentrations, the morphologies are similar; however, the Raman scattering spectra show no apparent differences (Figure 11a). Subsequently, we used honeycomb films formed by different CnH2n+1SH (n = 14, 16, and 18)-stabilized nanoparticles at the same concentration, 5.0 g·L−1, as shown in Figure 11b. From the optical microscope images (Figure S8), we can see that their structures and their Raman scattering spectra are similar. On the basis of the Raman scattering spectrum measurements, we can deduce that the Raman scattering spectra are closely related to the structure of the honeycomb films. The more regular the honeycomb films, the stronger the SERS. It is suspected that the reason can be attributed to the nanoscale surface roughness. The more regular honeycomb films have higher porosity factors with larger surface roughness values. It may be expected that nanoscale roughness should produce a stronger plasmon resonance, and the SERS signals can be enhanced. Furthermore, the more regular honeycomb films having large surface areas allow them to adsorb more substances from the surrounding solution.

■

ASSOCIATED CONTENT

* Supporting Information S

TEM images of gold nanoparticles. UV−vis absorption of different nanoparticles. Relationship between the pore diameter and the PLLA concentration. SEM images of C14SH−Au/ PMMA and C14SH−Au/PS hybrid honeycomb films. Relationship between the alkyl chain of the modified mercaptans and pore diameter. Optical microscope images formed by different nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS In this paper, we propose methods to control the pore size of honeycomb films, and the methods are facile and convenient. We expect this report to open a new field of view for controlling honeycomb films that has potential applications in cell culturing, filtration, and segregation. The addition of polymer such as PS, PLLA, and PMMA can enhance the strength and regularity of the honeycomb films. The pore size decreases with the increase in the concentration of the polymer. The pore size is dependent on the vapor pressure. Higher-

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-531-88363532. Fax: +86531-88564750. F

dx.doi.org/10.1021/la305143v | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Notes

(20) Ma, H.; Hao, J. Evaporation-Induced Ordered Honeycomb Structures of Gold Nanoparticles at the Air/Water Interface. Chem. Eur. J. 2010, 16, 655−660. (21) Ma, H.; Cui, J.; Chen, J.; Hao, J. Self-Organized Polymer Nanocomposite Inverse Opal Films with Combined Optical Properties. Chem.Eur. J. 2011, 17, 655−660. (22) Ma, H.; Cui, J.; Song, A.; Hao, J. Fabrication of Freestanding Honeycomb Films with Through-Pore Structures via Air/Water Interfacial Self-Assembly. Chem. Commun. 2011, 47, 1154−1156. (23) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-Derivatized Gold Nanoparticles in a 2-Phase Liquid-Liquid System. J. Chem. Soc., Chem. Commun. 1994, 801−802. (24) Dong, R.; Ma, H.; Yan, J.; Fang, Y.; Hao, J. Tunable Morphology of 2D Honeycomb-Patterned Films and the Hydrophobicity of a Ferrocenyl-Based Oligomer. Chem.Eur. J. 2011, 17, 7674−7684. (25) Peng, J.; Han, Y.; Yang, Y.; Li, B. The Influencing Factors on the Macroporous Formation in Polymer Films by Water Droplet Templating. Polymer 2004, 45, 447−452. (26) Casal, D. L.; Mantasch, H. H. Polymorphic Phase Behaviour of Phospholipid Membranes Studied by Infrared Spectroscopy. Biochim. Biophys. Acta 1984, 779, 381−401. (27) Binks, B. P. Particles as Surfactants-Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (28) Saunders, A. E.; Shah, P. S.; Sigman, M. B.; Hanrath, T.; Hwang, H. S.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. Inverse Opal Nanocrystal Superlattice Films. Nano Lett. 2004, 4, 1943−1948. (29) Li, J.; Peng, J.; Huang, W.; Wu, Y.; Fu, J.; Cong, Y.; Xue, L.; Han, Y. Ordered Honeycomb-Structured Gold Nanoparticle Films with Changeable Pore Morphology: From Circle to Ellipse. Langmuir 2005, 21, 2017−2021. (30) Gupta, R.; Weimer, W. A. High Enhancement Factor Gold Films for Surface Enhanced Raman Spectroscopy. Chem. Phys. Lett. 2003, 374, 302−306. (31) Kuncicky, D. M.; Prevo, B. G.; Velev, O. D. Controlled Assembly of SERS Substrates Templated by Colloidal Crystal Films. J. Mater. Chem. 2006, 16, 1207−1211. (32) Polavarapu, L.; Xu, Q. Water-Soluble Conjugated PolymerInduced Self-Assembly of Gold Nanoparticles and Its Application to SERS. Langmuir 2008, 24, 10608−10611. (33) Mandal, S.; Shundo, A.; Acharya, S.; Hill, J. P.; Ji, Q.; Ariga, K. Hydrogen-Bond-Assisted “Gold Cold Fusion” for Fabrication of 2D Web Structures. Chem. Asian J. 2009, 4, 1055−1058. (34) Lin, S.; Li, M.; Dujardin, E.; Girard, G.; Mann, S. OneDimensional Plasmon Coupling by Facile Self-assembly of Gold Nanoparticles into Branched Chain Networks. Adv. Mater. 2005, 17, 2553−2559.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the NSFC (21033005 & 21273134), the National Basic Research Program of China (973 Program, 2009CB930103), and GIFSDU (yyx10099).

■

REFERENCES

(1) Galeotti, F.; Mróz, W.; Bolognesi, A. CdTe Nanocrystal Assemblies Guided by Breath Figure Templates. Soft Matter 2011, 7, 3832−3836. (2) Yabu, H.; Shimomura, M. Single-Step Fabrication of Transparent Super- hydrophobic Porous Polymer Films. Chem. Mater. 2005, 17, 5231−5234. (3) Yamamoto, S.; Tanaka, M.; Sunami, H.; Ito, E.; Yamashita, S.; Morita, Y.; Shimomura, M. Effect of Honeycomb-Patterned Surface Topography on the Adhesion and Signal Transduction of Porcine Aortic Endothelial Cells. Langmuir 2007, 23, 8114−8120. (4) Govor, L. V.; Bashmakov, I. A.; Kaputski, F. N.; Pientka, M.; Parisi, J. Self-Organized Formation of Low-Dimensional Network Structures Starting from a Nitrocellulose Solution. Macromol. Chem. Phys. 2000, 201, 2721−2728. (5) Govor, L. V.; Bashmakov, I. A.; Kiebooms, R.; Dyakonov, V.; Parisi, J. Self-Organized Networks Based on Conjugated Polymers. Adv. Mater. 2001, 13, 588−591. (6) Hill, J. P.; Lee, M. V.; Yu, X. Y.; Okamoto, K.; Linford, M. R.; Ariga, K. Macroporous Poly(aromatic amine): Synthesis and Film Fabrication. Colloids Surf., A 2010, 354, 156−161. (7) Widawski, G.; Rawiso, M.; Francois, B. Self-Organized Honeycomb Morphology of Star-Polymer Polystrene Films. Nature 1994, 369, 387−389. (8) Sun, W.; Ji, J.; Shen, J. Rings of Nanoparticle-Decorated Honeycomb-Structured Polymeric Film: The Combination of Pickering Emulsions and Capillary Flow in the Breath Figures Method. Langmuir 2008, 24, 11338−11341. (9) Kralchevskyt, P. A.; Nagayama, K. Capillary Forces Between Colloidal Particles. Langmuir 1994, 10, 23−36. (10) Garcia, P. L.; Castillo, J. L.; Velarde, M. G. Bénard-Marangoni Convection with a Deformable Interface and Poorly Conducting Bounaries. Phys. Fluids 1987, 30, 2655−2661. (11) Golovin, A. A.; Nepomnyashchy, A. A.; Pismen, L. M. Pattern Formation in Large-Scale Marangoni Convection Deformable Interface. Physica D 1995, 81, 117−147. (12) Ariga, K.; Vinu, A.; Yamauchi, Y.; Ji, Q.; Hill, J. P. Nanoarchitectonics for Mesoporous Materials. Bull. Chem. Soc. Jpn. 2012, 85, 1−32. (13) Li, Z.; Barnes, J. C.; Bosoy, A.; Stoddart, J. F.; Zink, J. I. Mesoporous silica nanoparticles in biomedical applications. Chem. Soc. Rev. 2012, 41, 2590−2605. (14) Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Luminescent Functional Metal-Organic Frameworks. Chem. Rev. 2012, 112, 1126−1162. (15) Li, J. R.; Sculley, J.; Zhou, H. C. Metal-Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869−932. (16) Xu, Y.; Zhu, B.; Xu, Y. A Study on Formation of Regular Honeycomb Pattern in Polysulfone Film. Polymer 2005, 46, 713−717. (17) Tang, P.; Hao, J. Formation Mechanism and Morphology Modulation of Honeycomb Hybrid Films Made of Polyoxometalates/ Surfactants at the Air/Water Interface. J. Colloid Interface Sci. 2009, 333, 1−5. (18) Ke, B.; Wan, L.; Chen, P.; Zhang, L.; Xu, Z. Tunable Assembly of Nanoparticles on Patterned Porous Film. Langmuir 2010, 26, 15982−15988. (19) Tang, P.; Hao, J. Macroporous Honeycomb Films of SurfactantEncapsulated Polyoxometalates at Air/Water Interface and Their Electrochemical Properties. Adv. Colloid Interface Sci. 2010, 161, 163− 170. G

dx.doi.org/10.1021/la305143v | Langmuir XXXX, XXX, XXX−XXX