Polymeric Colloidal Nanostructures Fabricated via Highly Controlled

Interfaces , 2016, 8 (11), pp 7381–7389. DOI: 10.1021/acsami.6b00375. Publication Date (Web): March 3, 2016. Copyright © 2016 American .... Advance...
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Polymeric Colloidal Nanostructures Fabricated via Highly Controlled Convective Assembly and Their Use for Molecular Imprinting Jin Chul Yang and Jin Young Park* Department of Polymer Science and Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea S Supporting Information *

ABSTRACT: In this work, the formation of various polystyrene (PS) colloidal structures on striped PS patterns is demonstrated based on a simple and novel convective assembly method that controls the electrostatic interactions between the PS colloidal particles and sodium dodecyl sulfate (SDS). Under the optimal conditions (different withdrawal speeds, channel dimensions, suspension concentrations, etc.), highly ordered structures such as highly close-packed, zigzag, and linear colloidal aggregates are observed. In addition, these colloidal arrangements are used for development of molecularly imprinted polymer (MIP) sensors with highly improved sensing properties. Using PDMS replicas, three hemispherical poly(methacrylic acid-ethylene glycol dimethacrylate) (poly(MAAEGDMA)) MIP films, including planar MIP and non-imprinted polymer (NIP) films, are photopolymerized for detection of trace atrazine in an aqueous solution. From gravimetric quartz crystal microbalance (QCM) measurements, a non-close-packed MIP film exhibits highest sensing response (Δf = 932 Hz) to atrazine detection among hemispherical MIP films and shows 6.5fold higher sensing response than the planar MIP film. In addition, the sensitivity of the MIP sensor is equivalent to −119 Hz/ (mol L−1). From the ratio of slopes of the calibration curves for the hemispherical MIP and NIP films, the imprinting factor (If) is as high as 11.0. The hemispherical MIP film also shows excellent selectivity in comparison with the sensing responses of other analogous herbicides. As a result, this molecular surface imprinting using PS colloidal arrays is highly efficient for herbicide detection. KEYWORDS: colloidal lithography, atrazine, poly(MAA-EGDMA), molecularly imprinted polymer, photopolymerization

1. INTRODUCTION Two-dimensional (2D) nano- and microstructures of highly ordered colloidal particles are highly attractive and have the potential to be used in many research fields, including biological and chemical sensing, solar cells, photocatalysis, and field emission because facile, inexpensive, and efficient approaches adopted for their nanofabrication also give high controllability and reproducibility.1−3 Related to higher-order colloidal assemblies, a variety of technical approaches (e.g., spin-coating, drop-casting, dip-coating, electrophoretic deposition, etc.) have been successfully employed to fabricate 2D colloidal monolayers on substrates. In particular, vertical and horizontal deposition techniques, associated with convective assembly, have been widely used to make homogeneous colloidal monolayers.4 Ever since a highly dense polymeric colloidal monolayer was first generated with vertical deposition by Nagayama and Dimitrov in 1996,5 more advanced and efficient methods have been studied toward the fabrication of colloidal layers with highly homogeneous order.6−19 The development of colloidal structures has been extremely helpful to many research fields such as polymer brushes,20 friction,21 plasmonic resonances,22 and molecular imprinting.23 In particular, nano/microstructuring (via the use of colloid) of © XXXX American Chemical Society

recognition units in molecularly imprinted polymer (MIP) sensing systems provides a high surface area-to-volume (S/V) ratio for development of advanced sensing systems.24,25 These approaches can enhance limited sensitivity associated with slow diffusion of an analyte in a thin planar MIP film. Recently, our research group has applied a colloidal lithographic method to develop a less time-consuming and inexpensive fabrication method for MIP sensing systems.26,27 In general, imprinting systems for specific recognition have been achieved through electrochemical polymerization28−33 and photopolymerization34−39 to facilitate noncovalent bonds (i.e., hydrogen bonding, dipole−dipole, and ionic interactions) between templates and MIP matrices. These approaches allow fast and reversible binding of the template to imprinted cavities on the matrices. The molecular imprinting of atrazine (2-chloro-4-ethylamino-6-isopropyl amino-1,3,5-triazine), a common triazineclass herbicide, has been extensively studied to measure trace herbicide levels through electrochemical40−42 and optical43 Received: January 11, 2016 Accepted: March 3, 2016

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DOI: 10.1021/acsami.6b00375 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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30 min. Air bubbles were removed by vacuum degassing, and the mixture was carefully poured into a line-patterned master mold placed in a plastic Petri dish. After thermal curing at 70 °C for 2−3 h, the striped PDMS molds (PDMS-S1: P = 1400 nm, W = 400 nm, H = 200 nm; PDMS-S2: P = 700 nm, W = 270 nm, H = 135 nm) were successfully replicated and cut to a specific cut size. The two PDMS molds were then stored in a clean Petri dish before use. 2.3. Striped Films. Glass substrates (10 × 25 mm2) were first sonicated with acetone, ethanol, and water for 5 min and then immersed in a piranha solution (H2SO4:H2O2 = 3:1, v/v) for 30 min (CAUTION: piranha solution is a strongly oxidizing solution). Finally, the substrates were rinsed with water. After drying with nitrogen gas, they were immersed in an anhydrous toluene solution containing 1 vol % OTS for 5 min, rinsed with toluene 2−3 times, dried, and then stored in a glass jar prior to use. Various striped films (PS, SU-8, and APTES-modified PDMS) were prepared for PS colloidal arrays as follows. First, soft lithography was used to make two size-defined PS stripes (denoted PS-S1 and PS-S2). A 4 wt% solution of PS in toluene was first released as a droplet from a micropipette on the OTS-modified glass substrate, and each PDMS mold was pressed on the droplet with sufficient pressure for 5 min. After demolding, the PS striped films were dried at 60 °C for 2−3 h to remove residual solvent. Before dip-coating for PS-Cs arrangement, the PS films were exposed to O2 plasma for 3 min using a vacuum plasma system (CUTE, Femto Science Inc.). The substrates were not rinsed with any solvent afterward. The dimension of the two PS stripes on the films is demonstrated in Figure S2. SU-8 stripes were prepared in the same manner. SU-8 solution (30 mg) was released as a droplet on the OTS-modified glass substrate. The PDMS mold (PDMS-S1) was then pressed on the droplet under sufficient pressure. It was continuously photocured for 10 min under a UV lamp (λ = 370 nm, 36 W). After demolding, the substrate was exposed to UV irradiation for 20 min to complete the curing process. Two hydrophobic PDMS molds (10 × 25 mm2) were treated with O2 plasma for 3 min, and a few droplets of 2 wt% APTES in ethanol were placed on the PDMS stripes and left for 10 min to make a hydrophilic surface. Subsequently, the pretreated stripes were rinsed with water and dried with nitrogen gas. 2.4. Stripe-Assisted Assembly. Various assembly parameters were tested to control the PS-C arrays: withdrawal speed (1−5 μm/s), concentration of the colloidal suspension (PS-C/SDS, 0.5−2 wt%), stripe direction (parallel and perpendicular to withdrawal), diameter of PS-Cs, stripe material (PS, SU-8, and APTES-modified PDMS), and stripe dimensions. For the assembly of PS-Cs, dip-coating was performed in a mixed solution of PS-C and SDS as follows. First, SDS was added to a 10 mL vial containing 1.5 mL of a 2.5 wt% PS dispersion solution, and ultrapure water was used to control the concentration of colloidal particles (0.5−2 wt%) and SDS (0.5−2 wt%). Prior to dip-coating, the prepared mixture was ultrasonicated for 10 min. Next, the striped PS substrates were quickly immersed in the mixture at a rate of 4 mm/s using a dip-coater (EF-5100, E-flex) and held until the surface of the solution stabilized. The substrates were withdrawn at various speeds of 1−5 μm/s until they had been raised by a vertical distance of 10 mm. Finally, the substrates were dried under ambient conditions. 2.5. Hemispherical Porous PDMS Replica Molds. The ordered PS-C arrays obtained from colloidal structuring as mentioned above were used as master molds for designing convex hemispherical MIP structures. First, a prepared mixture of silicone elastomer and curing agent was carefully poured onto the PS-C arrays placed in a plastic Petri dish. After thermal curing at 70 °C for 2−3 h, the patterned PDMS replica molds had been successfully made and were stored in a clean Petri dish before use. 2.6. Structured MIP Films. The fabrication of all poly(MAAEGDMA) MIP sensors was performed according to previously reported literature.44 For the MIP matrix, all starting materials (MAA (4 mM), atrazine (1 mM), and EGDMA (20 mM)) were dissolved in 100 μL DMF and sonicated until they had completely dissolved. AIBN (1 mM) was then added to the mixture solution, followed by purging with N2 for 10 min. The imprinting solution was

analyses. However, most of the research on MIP sensors for atrazine detection has been focused only on MIP synthetic methods and analyses of sensing properties using related instruments, not surface imprinting via nano/microstructuring associated with enhanced sensor performance. Recently, we described striped MIP films for detection of atrazine at trace levels.44 For atrazine imprinting, methacrylic acid (MAA), ethylene glycol dimethacrylate (EGDMA), and 2,2′-azobis(2isobutyronitrile) (AIBN) were used to synthesize UV-initiated poly(MAA-EGDMA) MIPs. In the work, it was found that the striped films had greater sensing response than planar imprinted films. However, the increased surface active area (patterned/planar surface (A/A0) = 1.057) was not as large as expected. Accordingly, in the striped MIP film the frequency change showed the relative increase of only 100 Hz as compared to that in the conventional planar MIP film (Δf = 158 Hz). Thus, for development of significantly improved MIP sensing systems related to well-designed structures, it is essential to obtain an increased MIP surface area with many functional cavities via lithographic structuring. With this in mind, we have developed a simple new assembly method using physical confinement on striped patterns and surfactant-assisted convective forces to assemble spherical polystyrene (PS) colloidal particles into various aggregates such as zigzags and close-packed (or non-close-packed) straight lines. For practical application to surface imprinting for development of high-performance sensors, such an unconventionally designed colloidal lithographic method is considered to overcome limitations, involving low binding capacity and template leakage. With this 2D highly ordered colloidal structure, structured hemispherical MIP films can increase the number of functional cavities available for the effective detection of template molecules. For this purpose, the potential application of PDMS molds replicated from the PS colloidal structures to poly(MAA-EGDMA) MIP sensing systems is explored, and their capability and selectivity as atrazine MIP sensors are also discussed.

2. MATERIALS AND METHODS 2.1. Materials. PS (Mw = 280 000), sodium dodecyl sulfate (SDS), and trichloro(octadecyl)silane (OTS) were purchased from SigmaAldrich Co. SU-8 50 negative photoresist (PR) (MicroChem Corp.) and 3-aminopropyltriethoxysilane (APTES) were obtained from Tokyo Chemical Industry Co. PS latex beads (referred to as PS-C; dPS‑C = 200, 500, 750 nm, 2.5 wt% dispersion solution (w/v)) were acquired from Alfa Aesar Co. for preparation of 2D colloidal arrays, and ultrapure water (18 MΩ·cm−1) was used to control the total concentration of dispersion solutions containing PS-Cs and SDS. For development of a photopolymerized MIP matrix, MAA (Daejung Chemicals & Metals Co.), EGDMA (Sigma-Aldrich Co.), and atrazine (Sigma-Aldrich Co.) were used as the monomer, crosslinker, and target molecule, respectively. AIBN (Daejung Chemicals & Metals Co.) was used as a photoinitiator. The solvent, dimethylformamide (DMF), was obtained from Tokyo Chemical Industry Co. Three other herbicides (ametryn, prometryn (Sigma-Aldrich Co.), and 2,4dichlorophenoxyacetic acid (2,4-D) (Tokyo Chemical Industry Co.)) were used to investigate the selectivity of the sensor for a specific target molecule (atrazine). All other solvents were analytical reagent grade and were used without any further purification. 2.2. Striped PDMS Molds. To make size-defined PDMS molds, two different types of clean striped master molds (referred to as MM1: periodicity P = 1400 nm, width W = 1000 nm, height H = 200 nm; MM-2: P = 700 nm, W = 430 nm, H = 135 nm) were used (Figure S1). First, the silicone elastomer and curing agent (10:1 weight ratio, Sylgard 184, Dow Corning Co.) were vigorously mixed in a beaker for B

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Figure 1. Fabrication of various PS colloidal arrays on striped films via a convective assembly process. precured by irradiation with UV light (λ = 370 nm, 36 W) for a short period of time (60 s) to allow easier PDMS stamping in the lithography step. Similarly, a mixed solution without template molecules was prepared to make a NIP film. The fabrication process of the three different imprinted films was as follows: 5 μL of an imprinting solution was carefully dropped onto a 9 MHz gold-coated AT-cut quartz crystal (active gold area: 0.196 cm2), then the hemispherical pore-patterned PDMS mold was carefully placed on the droplet of the solution, after which sufficient pressure was induced on the mold to maintain well-distributed physical contact between the PDMS and the gold surface during the lithographic process. At the same time, UV-initiated polymerization was employed for about 10 min. After demolding, all the MIP films were dried at 60 °C for 2−3 h to remove residual solvent and complete polymerization. To extract atrazine molecules from the MIP matrix via their association with acid substituents, the films were immersed in a mixture solution of ultrapure water, methanol, and acetic acid (1/4/1, v/v/v) for 15 min. For quantitative analysis in an atrazine rebinding process, the resonant frequency was monitored in situ under various conditions (including different atrazine concentrations, structured MIP films, and analogous herbicides) using a QCA 922 analytical instrument. Each sensing experiment was repeated three times using the same MIP film. 2.7. Characteristics. Scanning probe microscopy (SPM (noncontact mode); NX20, Park instruments) was used to evaluate the dimensions of the master molds and PS stripes. Field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi) was used to investigate the surface topographies of PS-C arrays, PS or SU-8 stripes, and MIP structures. Measurements of water contact angles (AM07007, Surface Technology Co.) were performed to investigate the surface hydrophilicity of the PS films.

withdrawal direction and withdrawn from a PS colloidal suspension containing an ionic surfactant, SDS. In convective assembly, colloidal particles normally nucleate under lateral capillary force along a water contact line on the patterned surface to form arbitrary structural aggregates during evaporation. Lateral attractive capillary forces allow the particles to move toward the crystals when the thickness of the water film drops below the PS-C diameter. However, various conditions (withdrawal speed, channel dimension, and suspension concentration) need to be considered to avoid the unexpected agglomeration of PS-Cs and to allow simple assembly of 2D colloidal structures along the channels in a convective assembly process. Based on the optimum conditions obtained from preliminary experiments (withdrawal rate of 1.0 μm/s; mixture of PS-C and SDS at a concentration of 1:1 wt%) (Figure S3, S4, and S5), PS-Cs (dPS‑C = 500 or 200 nm) were assembled on O2 plasma-treated striped substrates (PS-S1 or PS-S2) parallel to the withdrawal direction. Through O2 plasma treatment, the surface of the planar or striped PS film could be hydrophilically modified to obtain a significant concave meniscus at the interface between the liquid and the substrate (a change in water contact angle from 78° to 32°, Figure S6). In this case, the colloidal particles were assembled along the channels by particle−particle and particle−substrate interactions during water evaporation in an optimum convective assembly. However, highly ordered PS-C aggregates could also be formed on the channels when non-plasma-treated PS films and APTES-modified PDMS were used (Figure S7 and S8). This meant that it was feasible to arrange the PS-Cs on the channels if the water contact angle was 2dPS‑C, respectively.47

sin θ =

WC − dPS − C dPS − C

(1)

As expected, when Wc = 2dPS‑C or 3dPS‑C, the colloidal aggregates formed hexagonally close-packed structures at an angle of 60°, which is identical with the calculated value from eq 1 (Figures S9 and 3a). Even when Wc = 1.75dPS‑C, the colloidal particles were preferentially assembled in the same structure. However, when Wc < 1.2dPS‑C (i.e., Wc = 0.7dPS‑C or 0.8dPS‑C), zigzag structured aggregates with an angle of 35° transformed to almost linear chains with smaller angles (θ ≤ 13°). When Wc ≈ 0.47dPS, highly packed linear PS chains were observed in the entire area, and typical colloidal structures (i.e., the hexagonal array generally shown in colloidal photonic crystals) were absent because of the relatively larger colloidal particle size (dPS‑C = 750 nm), as shown in Figure 3f. Thus, various colloidal aggregates could be assembled by simultaneously controlling the width of the channels and the diameter of the colloidal particles. By summarizing the results mentioned above, it was found that various structural arrangements such as densely packed, zigzagged, and linear colloidal aggregates could be obtained when Wc ≥ 1.75dPS‑C, 1.2dPS‑C ≤ Wc ≤ 1.33dPS‑C, and Wc ≤ 0.86dPS‑C, respectively. 3.2. Use of Assembled PS Colloids for Further Applications. Figure 4 shows the welded structure of 500 nm PS-Cs in zigzag patterns, obtained by heating the arranged colloidal particles on the PS-S1 film to 130 °C, which is slightly above the glass-transition temperature (Tg), for 5 min. This structure is almost consistent with that obtained by heating binary colloidal crystals, as reported elsewhere.48 However, some wave-like formations were also observed in the entire area, including (1) the connection of the melted PS colloidal particles to two adjacent zigzag chains, and (2) linear rods extending from linear PS chains at specific areas (indicated by two arrows in Figure 4a). Furthermore, the wave-like formations varied with the degree of weld with respect to the melted particles due to the unevenly distributed heat (Figure 4b). For further applications to nano/microstructuring, the zigzag colloidal arrangement (dPS‑C = 500 nm) was used as a master

Figure 2. SEM images of PS colloidal arrays on O2-plasma pretreated striped PS films: (a, c) PS-S1 film, dPS‑C = 500 nm; (b, d) PS-S2 film, dPS‑C = 200 nm. The striped PS films were placed either (a, b) parallel or (c, d) perpendicular to the withdrawal direction and were raised from the aqueous solution of colloidal particles (1 wt%) and SDS (1 wt%) at a constant withdrawal rate of 1 μm/s. All scale bars are 1 μm.

the pattern developed in the withdrawal direction, the physical barriers severed the continuous colloidal organization even though a lateral capillary force was applied. Instead, the colloidal particles filled the channels that were perpendicular to the withdrawal direction. Thus, this phenomenon suggests that the pattern direction is a factor to be considered when attempting to use surfactant-assisted convective assembly to obtain specific colloidal structures. The pattern dimension is another parameter of the convective assembly. To further study the relationship between dPS and Wc, various structural arrangements of the colloidal particles were investigated. Figure 3 shows assembled colloidal particles of three different sizes (dPS‑C = 200, 500, and 750 nm) on the PS-S1 or PS-S2 patterns parallel to the withdrawal direction under optimized conditions (withdrawal rate =1 μm/s and PS-C:SDS = 1:1 wt%). Based on the relationship between dPS‑C and Wc on the striped patterns in a colloidal assembly (eq 1), 2D colloidal aggregates can be predicted to form straight and linear chains for Wc ≤ dPS‑C, a zigzag structure for dPS‑C ≤

Figure 3. SEM images of PS colloidal chains arranged on O2-plasma pretreated striped PS films: (a−c) PS-S1 films and (d−f) PS-S2 films. The striped PS films were placed parallel to the withdrawal direction and raised from an aqueous solution consisting of colloidal particles (1 wt%) and SDS (1 wt%) at a constant withdrawal rate of 1 μm/s. The inset in (b) represents the angle (θ) between two colloidal particles. All scale bars are 1 μm. D

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Figure 4. (a) SEM images of wave-like PS patterns. Zigzag PS colloidal arrays (dPS‑C = 500 nm) were formed by withdrawing the PS-S1 film at 1 μm/s from a suspension of colloidal particles (1 wt%) and SDS (1 wt%), followed by heat-treatment at 130 °C for 5 min. (b) Magnified image of the highlighted rectangular area in (a). All scale bars are 2 μm.

mold to make replica polymeric structures (i.e., PDMS). A hemispherical porous PDMS replica was created from the zigzag master mold (as shown in Figure 5a) through

Figure 6. (a) Fabrication process of various convex hemispherical MIP films using the three PDMS replicas (500PS-S1, 750PS-S1, and 750PSS2) and SEM images of (b) zigzag MIP (hs-MIP500PS‑S1), (c) linearly isolated discontinuous MIP (hs-MIP750PS‑S1), and (d) non-closepacked MIP (hs-MIP750PS‑S2) made from 500PS-S1, 750PS-S1, and 750PS-S2 molds and (e−g) their corresponding cross-sectional images. All scale bars are 1 μm.

Figure 5. SEM images of (a) the zigzag colloidal master mold (dPS‑C = 500 nm) and (b) the hemispherical zigzag PS patterns made using the PDMS replica obtained from the master mold. All scale bars are 1 μm.

sufficient pressure and UV light was induced for polymerization. After demolding, the imprinted atrazine molecules were extracted in a mixture solution of ultrapure water, methanol, and acetic acid. All the MIP films successfully formed on the substrates during photopolymerization. From the zigzag colloidal structure, distorted and interconnected hemispherical MIP colloids with diameters (dMIP) of 389 ± 14 nm and heights (hMIP) of 241 ± 12 nm (denoted hs-MIP500PS‑S1) were established along the central line of the zigzag-aligned colloids (Figure 6b). Because photo-cross-linking occurred during MIP film formation, the poly(MAA-EGDMA) imprinted structure was slightly different from the PS hemispheres, as shown in Figure 5b. Moreover, the two other replicated molds (750PSS1 and 750PS-S2) generated isolated MIP structures (denoted hs-MIP750PS‑S1 and hs-MIP750PS‑S2) due to the shapes and locations of the porous hemispheres in swollen state of the molds in contact with the solvent (Figure 6c and 6d). More interestingly, when the 750PS-S2 mold was used, a non-closepacked imprinted array (hs-MIP750PS‑S2) was formed following the regularity of the PS-C arrangement, but random sharp shapes were formed on individual particles. Formation of this imprinted structure strongly depended on the influence of adjacent hemispherical pores in the swollen state of the PDMS mold (Figure S10). The atrazine imprinted in all the films was extracted under identical conditions. The extraction process left specific cavities with functional shapes in the MIP films and enabled selective chemical recognition of atrazine via hydrogen bonding during a rebind process in solution. Thus, the extraction process described in the experimental section probably removed the atrazine templates by breaking the hydrogen bonding between the carboxylic group on the MAA monomers and the amino group on the atrazine templates.50

conventional replica molding. The replica was then placed on a substrate with a few droplets of the PS solution. After demolding, the zigzagging hemispheres were formed on the substrate as shown in Figure 5b. Similarly to the close-packed hemispherical array from 2D ordered microspheres,49 the interconnected PS zigzag hemispheres were formed with reduced diameter (dPS‑hemi ≈ 430 nm) due to the swelling of the porous PDMS by solvent absorption during molding.45,46 The result allows one to use this method for molecular surface imprinting in development of high-performance MIP sensors. 3.3. Formation of Hemispherical MIP Structures. MIP systems that recognize a specific template molecule via noncovalent interactions with functional monomers were used for development of sensors for detection of atrazine. Compared to a planar imprinted film, lithographically structured MIP films that provide increased surface areas allow the generation of a greater relative number of crossreactive cavities for molecular recognition. Three well-ordered PS-C arrays (zigzag colloidal chains from dPS‑C = 500 nm on PS-S1, linearly isolated discontinuous arrays from dPS‑C = 750 nm on PS-S1, and non-close-packed arrays from dPS‑C = 750 nm on PS-S2) were used as the master molds for designing convex hemispherical MIP structures. To evaluate the capabilities of the structured MIP films, hemispherical porous PDMS replicas (referred to as 500PS-S1, 750PS-S1, and 750PS-S2, respectively) were first prepared. As shown in Figure 6a the molecular imprinting process was simply performed as follows: the imprinting solution including MAA, EGDMA, AIBN, atrazine was dropped on the substrate. Then, the PDMS replica was carefully contacted on it under E

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ACS Applied Materials & Interfaces Although all the MIP films demonstrated no significant changes in surface morphology after removal of atrazine (Figure S11), the extraction process has been proven to be highly effective according the literature.44 3.4. Evaluation of MIP Sensing Systems. The efficiency of the imprinting process was quantified by determining the sensitivity and selectivity of the imprinted films. Gravimetric quartz crystal microbalance (QCM) sensors were employed to investigate and evaluate molecular recognition in the imprinted polymers prepared by the noncovalent approach. For these sensing systems, extracted MIP (three structured and one planar) and NIP films were prepared on gold-coated quartz crystals, and their capacities were assessed in equilibrium binding assays. For investigations into the binding characteristics of atrazine to an extracted, imprinted film, the measured change in resonant frequency as a function of atrazine concentration allowed us to evaluate the recognition properties of the three structured MIP films. The resonant frequency shift in a 10−6 mol L−1 atrazine solution (in a 1:1 v:v mixture of C2H5OH:H2O) was monitored over a 30 min rebind process of (Figure 7). In general, the NIP film (the reference polymer)

amplification by controlling the binding sites by increasing the surface area. Based on this strategy, the three lithographically designed MIP structures demonstrated much higher sensing responses. The frequency shift was increased in the following order: linear structure (Δf = 490 Hz), zigzag colloidal structure (Δf = 702 Hz), and isolated non-close-packed MIP (Δf = 932 Hz). Although the 30 min rebinding process was undergone, all the structured MIP films showed the tendency of continuously decreasing in frequency because of enough cavities available for detection. More interestingly, the imprinted structure in Figure 6d showed 6.5-fold higher sensing response than the planar MIP film due to its substantially increased surface area due to the lithographic structuring process. In addition, in the same sensing conditions the structured MIP film exhibited 5.9-fold higher sensing response than the striped MIP film in our previous work.44 Thus, hemispherical MIP structures with comparatively higher S/V ratios could provide increased numbers of binding sites and easy access for template molecules to cavity surfaces during the rebinding process. To test sensing response reliability, each MIP sensor was reused several times while the resonant frequency was measured. Despite the extraction process being repeated 10 times, there was no damage or loss on the MIP films, indicating that all the films were sufficiently stable. In addition, the dependence of the sensing response on atrazine concentration was investigated (Figures 8 and S12a).

Figure 7. Resonant frequency change as a function of time for planar NIP, planar MIP, and three different convex hemispherical MIP films (hs-MIP750PS‑S1, hs-MIP500PS‑S1, and hs-MIP750PS‑S2 made from 750PSS1, 500PS-S1, and 750PS-S2 molds, respectively) in a 10−6 mol L−1 solution of atrazine (solvent = H2O:C2H5OH, 1:1, v:v) during the atrazine rebinding process (30 min).

Figure 8. Resonant frequency change for the hemispherical MIP (hsMIP750PS‑S2) and NIP films during the rebinding process as a function of atrazine concentration. The measurements were performed in solution (H2O:C2H5OH = 1:1 v:v) at various concentrations of atrazine (10−10−10−6 mol L−1) for 30 min.

demonstrated negligible changes in frequency in the presence of atrazine (Δf NIP ≈ 47 Hz) due to nonspecific binding effects (chemisorption) of the template molecules onto the film surface. After the elapsed time of 7 min in sensing response (point “1” in Figure 7), the frequency was stabilized. This implies that the atrazine molecules bind nonselectively on the surface and no more analytes are detected on the NIP film. This is in agreement with our previous results published elsewhere.44 However, the planar imprinted poly(MAAEGDMA) film showed a higher recognition for atrazine. The strong interaction of MAA residues with atrazine through hydrogen bonding caused a shift in frequency that was 3 orders of magnitude higher than the NIP control film (Δf plane ≈ 144 Hz) during the rebinding process. During the first 25 min the frequency was decreased as the trace atrazine was diffused into the planar MIP film and bound on molecularly selective cavities and then stabilized (point “2” in Figure 7) due to the limitation of sensing capability. Although the planar MIP film showed enhanced sensing, it was possible to increase the frequency

Using the isolated non-close-packed MIP film (Figure 6c), which had the highest sensing response, a rebinding process was performed for 30 min in various solutions with atrazine levels ranging from 10−6 to 10−10 mol L−1. An increase in atrazine concentration in the solution led to a corresponding increase of the shift in frequency. The detection of the atrazine at concentrations of 0.1 nmol L−1 demonstrated a frequency shift of 474 Hz, corresponding to a mass of 474 ng. In the case of a 9 MHz AT-cut quartz crystal, a frequency change of 1 Hz was observed for a mass change of ∼1 ng or 5.682 ng/cm2.51−53 The calibration graph shows a linear plot with a correlation coefficient (R) of −0.993 in the examined concentration range, giving a measure of the linear correlation between the two variables (resonant frequency change and logarithm of concentration). The sensitivity of the MIP sensor was F

DOI: 10.1021/acsami.6b00375 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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solution. Thus, these results showed that the non-close-packed hemispherical imprinted film built up with colloidal lithography showed excellent selectivity for a specific template, atrazine.

determined from the slope of the calibration curve and was equivalent to −119 Hz/(mol L−1). Because of the diffusion of trace atrazine toward more accessible imprinted molecular cavities in the film, the sensitivity of this hemispherical MIP film is 5.7-fold greater than that of the striped MIP film in our previous study.44 To confirm the imprinting, an NIP film was used for the atrazine determination in a control experiment (Figures 8 and S12b). In view of no imprinted cavities, the atrazine-binding to NIP film is much less efficient than the analyte-to-MIP binding, as indicated by only minor decrease of the resonant frequency (Δf = 40−80 Hz) with the increase of the atrazine concentration in solution. Thus, the NIP films show extremely low sensitivity (−10.78 Hz/(mol L−1)) in the same detection range. In addition, the imprinting effect is evaluated from the If commonly calculated as the ratio of slopes of the calibration curves for the hemispherical MIP and NIP films,54 which is as high as 11.0. The high If means that the MIP film retains the analyte more strongly than the NIP film due to selective interactions between the analyte and MIP. Furthermore, to evaluate the selectivity of the same imprinted film, atrazine analogues such as ametryn, prometryn, and 2,4-D were used under the same sensing conditions (10−6 mol L−1 in C2H5OH:H2O, 1:1 v/v) (Figures 9 and S13). In

4. CONCLUSIONS In conclusion, we developed a novel approach to the fabrication of highly ordered colloidal structures on striped patterns via soft lithography and dip-coating. The 2D colloidal structures were significantly influenced by the concentration of the suspension, withdrawal rate, colloid diameter, and the dimensions of the striped patterns. In addition, controlling the amount of SDS surfactant was significantly important for the formation of highly ordered structures in convective assemblies. From these results, the appropriate use of the relationship between the width of striped channels and the diameter of colloidal particles under optimum conditions (e.g., withdrawal rate and concentration) may enable the design of particular colloidal structures on various patterns, and could allow for potential applications such as sensors, optical devices, PDMS molds, etc. As an example of a potential application, the practical use of these colloidal structures for atrazine imprinting was tested. Three different poly(MAA-EGDMA) MIP films, obtained from replicated PDMS molds through soft lithography and UVinitiated polymerization, were used to detect traces of the herbicide atrazine. The sensing response was significantly improved compared to the planar MIP film due to increased surface area provided by the structuring. In addition, the MIP system exhibited very high sensitivity and selectivity. As a consequence, this lithographical method of template imprinting enabled rapid and ultrasensitive detection as well as superior selectivity. Thus, from the results this imprinting method will be extensively applied to the detection of particular target molecules for applications in a variety of fields including separations, diagnostics (sensors and assays), and catalysts, etc.

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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/acsami.6b00375. AFM images of two master molds; SEM and AFM images of striped PS films prepared using PDMS molds; SEM images of PS-Cs arranged on the channels of the O2-plasma pretreated striped PS films under various conditions; water contact angles on the planar films before and after O2-plasma treatment; SEM images of PS colloidal particles of various sizes arranged on the channels of various patterned substrates; SEM images of three MIP films after atrazine extraction; resonant frequency changes as a function of atrazine concentrations; resonant frequency changes of the hemispherical MIP film as a function of time in various herbicide solutions (PDF)

Figure 9. Mass change upon molecular sensing response by the hemispherical MIP film (hs-MIP750PS‑S2) in a mixture of H2O:C2H5OH (1:1 v:v) with individual herbicides (10−6 mol L−1), three mixed herbicides (ametryn, prometryn, and 2,4-D) (3 × 10−6 mol L−1, molar ratio =1:1:1), or all herbicides (ametryn, prometryn, 2,4-D, and atrazine) (4 × 10−6 mol L−1, molar ratio =1:1:1:1) during the 30 min rebinding process (Inset of the Figure is the chemical structure of atrazine and three analogous herbicides).

each case, the change in resonant frequency was not significantly distinct from the nonspecific chemisorption observable in the NIP film (Δfametryn = 78 ± 5 Hz, Δf prometryn = 57 ± 9 Hz, and Δf 2,4‑D = 40 ± 7 Hz). In addition, the sensing response reached a frequency shift of 123 ± 12 Hz in an aqueous solution containing all three herbicides together at a concentration of 3 × 10−6 mol L−1 (1:1:1 molar ratio). This phenomenon may result from the relative increase in total concentration. On the other hand, when atrazine was added to this solution at a concentration of 10−6 mol L−1, a frequency shift of 858 ± 16 Hz was detected due to hydrogen bonding of atrazine on the cavities. This response corresponded to 92% of the original sensing response seen for a 10−6 mol L−1 atrazine

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected] Author Contributions

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

DOI: 10.1021/acsami.6b00375 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2013R1A1A2061434).

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DOI: 10.1021/acsami.6b00375 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX