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Covalent-Cross-Linked Plasmene Nanosheets Yiyi Liu,†,§,⊥ Bo Fan,‡,⊥ Qianqian Shi,†,§ Dashen Dong,†,§ Shu Gong,†,§ Bowen Zhu,†,§ Runfang Fu,†,§ San H. Thang,‡ and Wenlong Cheng*,†,§ †
Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia School of Chemistry, Monash University, Clayton, Victoria 3800, Australia § The Melbourne Centre for Nanofabrication, Clayton, Victoria 3800, Australia ‡
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S Supporting Information *
ABSTRACT: Thiol-polystyrene (SH-PS)-capped plasmonic nanoparticles can be fabricated into free-standing, onenanoparticle-thick superlattice sheets (termed plasmene) based on physical entanglement between ligands, which, however, suffer from irreversible dissociation in organic solvents. To address this issue, we introduce coumarin-based photo-cross-linkable moieties to the SH-PS ligands to stabilize gold nanoparticles. Once cross-linked, the obtained plasmene nanosheets consisting of chemically locked nanoparticles can well maintain structural integrity in organic solvents. Particularly, arising from ligand-swelling-induced enlargement of the interparticle spacing, these plasmene nanosheets show significant optical responses to various solvents in a specific as well as reversible manner, which may offer an excellent material for solvent sensing and dynamic plasmonic display. KEYWORDS: covalent-cross-linked, gold nanoparticles, plasmene nanosheets, solvent responsive, plasmonic switching
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maintain their structural integrity in various solvents. This is due to the source of stability of current plasmene, which lies in the physical entanglement among PS ligands. While PS ligands offered sufficient physical stabilities under ambient and vacuum conditions, the plasmene nanosheets would fully decompose into individual nanoparticles when introduced to an organic solvent (such as chloroform (CHCl3)). To address this issue, we herein report on a facile yet robust strategy to fabricate covalent-cross-linked plasmene nanosheets, by introducing coumarin-based photo-cross-linkable moieties into the SH-PS ligands. Coumarin is known as a photo-cross-linkable moiety, and it could undergo a [2π + 2π]cycloaddition under the irradiation of 365 nm UV light.25 Taking gold nanotrisoctahedrons (AuTOHs) consisting of both convex and concave surfaces as the model building blocks, we demonstrate high-quality plasmene nanosheets with
ree-standing, two-dimensional nanoparticle superlattices are advanced materials that have demonstrated a plethora of technical applications in elastic membranes,1,2 pressure3 and vapor sensors,4 stimuli-responsive5−7 and shape-changing thin films,8 microresonators,9 and actuators.10 Recently, free-standing, one-nanoparticle-thick plasmonic nanosheets have been introduced and termed plasmene nanosheets.11 Using thiol-polystyrene (SH-PS) as the generic model ligands, this system can in principle be designed by rich “plasmonic atoms” in the so-called “plasmonic periodic table”,12 including nanospheres,13 nanorods,14 nanocubes,11 nanobipyramids,15 nanotrisoctahedrons,16 etc. The sizes,11,13,15−17 shapes,11,13−17 and even orientations of constituent building blocks14,15 in the plasmene nanosheets can be modulated and give rise to various applications in soft surface-enhanced Raman scattering (SERS),17,18 anticounterfeiting,19 ionic gating,20 flat lenses,21 folding plasmonics,11 doping plasmonics,22 asymmetric nanocrystal growth,23 and memory devices.24 However, despite the promising applicable prospects of plasmene nanosheets, it remains challenging to © 2019 American Chemical Society
Received: February 17, 2019 Accepted: May 30, 2019 Published: May 30, 2019 6760
DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
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Cite This: ACS Nano 2019, 13, 6760−6769
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ACS Nano Scheme 1. Synthesis of SH-PS-b-PAEMC.
the dithioester-terminated PS-b-PAEMC solution was treated with excess amounts of n-butylamine37 to reduce dithioesterterminated PS-b-PAEMC to SH-PS-b-PAEMC (see Figure S6). For dithioester-terminated PS-b-PAEMC, Mn is measured to be 39 000 g mol−1 and dispersity (Đ) to be 1.29 by using gel permeation chromatography (GPC) in PS standard, with MNMR of the PS and PAEMC blocks calculated to be 20 200 and 8800 g mol−1, respectively (see Figures S4 and S5). Based on these data, there are ∼194 styrene units and ∼32 AEMC units in the SH-PS-b-PAEMC ligands. We deliberately design the SH-PS-b-PAEMC ligands with a dominant PS block, which plays a key role in obtaining plasmene nanosheets with good quality during the drying-mediated self-assembly process. SHPS-b-PAEMC ligands with different PS and PAEMC ratios have also been synthesized (MNMR of the PS and PAEMC blocks calculated to be 38 000 and 7000 g mol−1, see Figures S7 and S8), and the effects of the PAEMC block on the selfassembly will be discussed later. For the ease of understanding, we will label our SH-PS-b-PAEMC ligands as SH-PS(x)-bPAEMC(y) in the rest of this paper when needed, where x and y refer to MNMR of the PS and PAEMC blocks, respectively. Next, AuTOHs were synthesized following a previously reported protocol16,38 with a peak location at 553 nm (see Figure S9). The SH-PS(20k)-b-PAEMC(9k) ligands were ligated to the as-obtained AuTOHs through a ligand exchange approach (see Figure S10). Monodispersed aggregation-free discrete AuTOH@SH-PS(20k)-b-PAEMC(9k) nanoparticles were evident from transmission electron microscopy (TEM) characterization, with an average diameter of AuTOHs of 50.4 ± 2.1 nm and an apparent edge length of 24.1 ± 2.4 nm (see Figure S11). Cross-linked plasmene nanosheets were fabricated through a two-step procedure, including initial plasmene nanosheet
superior chemical stability in various organic solvents for both macroscopic nanosheets and shaped microscale nanosheets. We further found that the cross-linked free-standing nanosheets underwent a rapid blue−purple color change from dry to chloroform-wet states due to polymer swelling. Such color switching is reversible, and a similar switching phenomenon can also be observed when exposed to different solvents.
RESULTS AND DISCUSSION Current strategies of fabricating cross-linked nanoparticle membranes mainly include layer-by-layer spin coating of nanoparticles with dithiol as surface ligands,26 covalent bond formation between surface ligands,27−30 laser irradiation welding of nanoparticles,31 and electron beam irradiation induced surface ligand cross-linking.32,33 However, these strategies either are not feasible for the fabrication of onenanoparticle-thick nanosheets or involve complicated and expensive procedures. Therefore, it remains appealing to develop a cross-linking strategy with ease of operation and feasibility for the plasmene nanosheets. We began with the synthesis of polystyrene (PS)-based photo-cross-linkable diblock polymeric ligands. Photo-crosslinkable monomer 7-(2-acryloyloxyethoxy)-4-methylcoumarin (AEMC) was synthesized according to the literature34 with slight changes (see Figures S1, S2, and S3), and SH-PS-bPAEMC was then synthesized according to Scheme 1. First, the PAEMC block was synthesized through reversible addition−fragmentation chain transfer (RAFT) polymerization35,36 of AEMC in the presence of a commercially available benzyl benzodithioate RAFT agent and 2,2′-azobis(isobutyronitrile) (AIBN) (see Figure S4). Then, the dithioester-terminated PAEMC was further used as a macro RAFT agent in the polymerization of styrene step to produce dithioester-terminated PS-b-PAEMC (see Figure S5). Lastly, 6761
DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
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Figure 1. Schematic illustration for the fabrication process of covalent-cross-linked plasmene nanosheets.
Figure 2. EM images of cross-linked plasmene nanosheets and stability of cross-linked and non-cross-linked plasmene nanosheets in solvent conditions. (a) SEM image of AuTOH cross-linked plasmene nanosheets with SH-PS(20k)-b-PAEMC(9k) ligands. Inset: High-magnification SEM image showing the close-packed arrangement. Inset scale bar is 100 nm. (b) SEM image of a free-standing cross-linked plasmene nanosheet covering the copper grid. (c) TEM image of free-standing cross-linked plasmene nanosheets with SH-PS(20k)-b-PAEMC(9k) ligands. Inset: High-magnification TEM image showing the close-packed arrangement. Inset scale bar is 80 nm. (d, e) Optical image of (d) cross-linked and (e) non-cross-linked plasmene nanosheets released into acetone (for more information, also see Supporting Movies 1 and 2). (f) UV−vis spectra of monodispersed AuTOH@SH-PS(20k)-b-PAEMC(9k) in CHCl3 (red curve), cross-linked plasmene nanosheets in CHCl3 (green curve), and cross-linked plasmene nanosheets in CHCl3 after two months (blue curve). Inset shows the optical image of monodispersed AuTOH@SH-PS(20k)-b-PAEMC(9k) in CHCl3 and cross-linked plasmene nanosheets in CHCl3.
fabrication and subsequent UV light irradiation enabled crosslinking, as illustrated in Figure 1. First, AuTOH@SH-PS(20k)b-PAEMC(9k) nanoparticles were used to fabricate plasmene nanosheets through a previously reported drying-mediated method.11,13,16 A droplet of highly concentrated AuTOH@SHPS(20k)-b-PAEMC(9k) nanoparticle chloroform solution was drop-casted onto a sessile water droplet sitting on a Si wafer, and initial rapid evaporation of chloroform and subsequent slow evaporation of the water rendered the formation of plasmene nanosheets. Then, the as-fabricated nanosheets were irradiated with 365 nm UV light to enable the dimerization of side coumarin groups in the polymer chains, which led to the cross-linking of polymer ligands and yielded cross-linked plasmene nanosheets.
The arrangement of AuTOH nanoparticles in cross-linked nanosheets on Si wafers was demonstrated by a top view scanning electron microscope (SEM) image (Figure 2a). It clearly shows that AuTOH nanosheet is monolayer. Atomic force microscope (AFM) line scanning further proved that the cross-linked nanosheets were one particle thick, with an average thickness of 52.5 ± 1.3 nm (see Figure S12). The fabrication process can also be adopted for a free-standing system, by simply replacing a Si wafer substrate with a holey copper grid (2000 mesh with an average hole size of 7 μm × 7 μm). As shown in Figure 2b and c, the copper grid was fully covered with monolayered yet flexible cross-linked nanosheets. We further studied the effects of the PAEMC block on the self-assembly of AuTOH nanoparticles. As shown in Figure 6762
DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
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ACS Nano S13a, plasmene nanosheets fabricated from AuTOH@SHPS(50k) possessed the best degree of order. After the PAEMC block was introduced into the polymeric ligands, the degree of order became poorer with a simultaneous decrease in the interparticle spacing (Figure S13b and c). These results show that PAEMC are poorer polymeric blocks than PS for effective encapsulation. Hence, it is essential to have a dominant PS block in the polymer ligands in order to achieve nanosheets with a degree of periodic order. In order to prove the successful cross-linking of plasmene nanosheets, we fabricated them on Si wafers that were precoated with photoresist as sacrificial layers and then released the nanosheets into acetone. Figure 2d shows that a large-scale nanosheet with a diameter of ∼1.2 cm retained its sheet-like structural integrity after being released from the substrate and survived the harsh acetone condition (also see Supporting Movie 1). SEM characterization of the released cross-linked plasmene nanosheets proved that their structural integrity can also be maintained at the microscale after solvent treatment, as shown in Figure S14. A control experiment was carried out by using plasmene nanosheets fabricated from AuTOH@SH-PS(50k), with other conditions remaining unchanged. In contrast, plasmene nanosheets without crosslinking lost their sheet-like structural integrity and broke apart into random fractions immediately after being released from the Si wafers (Figure 2e and Supporting Movie 2). These results clearly show that cross-linked nanosheets are robust enough to maintain their structural integrity over both the macroscale and microscale in the free-standing state, demonstrating successful cross-linking reactions across the entire plasmene nanosheets, holding overall structural integrity and avoiding chemical dissolution. We also checked the stability of cross-linked nanosheets in chloroform, as it is a good solvent for the SH-PS-b-PAEMC ligands. Discrete AuTOH@SH-PS(20k)-b-PAEMC(9k) nanoparticles in CHCl3 exhibited a strong plasmonic peak at ∼557 nm (Figure 2f). After assemblying into nanosheets, the 557 nm peak should disappear and a new collective plasmonic peak with a larger wavelength position should appear due to the coupling of neighboring AuTOH nanoparticles. However, under conventional solution UV−vis measurement conditions, the free-standing cross-linked nanosheets will contribute no signals because they tend to sink to the bottom of a cuvette due to gravity, and there will be only CHCl3 in the light pathway. As shown in Figure 2f, after the cross-linked plasmene nanosheets were dispersed into CHCl3, the plasmonic peak at 557 nm did not recover, indicating no dissolution of nanosheets back into individual AuTOH@SHPS(20k)-b-PAEMC(9k) nanoparticles. It could also be proved from Figure S15 that cross-linked plasmsne nanosheets stayed intact as a whole after being released into CHCl3. To evaluate the long-term stability, the cross-linked nanosheets were left undisturbed in CHCl3 for two months, after which time a UV− vis measurement was taken and signals around 557 nm were not detected. These evidence clearly indicate the excellent stability of the covalent-cross-linked nanosheets in CHCl3. We further extended the fabrication to micrometer-sized shapeable free-standing plasmonic leaves by combining with template-assisted self-assembly, and the procedure is illustrated in Figure S16. Typically, to fabricate micrometer-scale plasmonic leaves, holey copper grids (300 mesh with hole size 70 μm × 70 μm is chosen here) can be used as templates and fixed to the top of sacrificial layers on Si wafers. UV/O3
treatment is performed as follows to ensure the hydrophilicity of the substrate surface, which causes the water droplet to be trapped in the hole areas during the drying-mediated selfassembly. Consequently, the nanosheets are dragged into hole areas and ruptured into patches confined by the holes after the evaporation of water.39 Following the template removal and photo-cross-linking, plasmonic leaves can be obtained on the Si wafers (Figure 3a) and then released into acetone. An optical
Figure 3. Shapeable free-standing plasmonic leaves. (a) Illustration and the reflection mode optical microscope image of square microleaves on the Si wafer with sacrificial layer. Scale bar: 200 μm. (b) Illustration and the reflection mode optical microscope image of a released free-standing plasmonic leaf in acetone. Scale bar: 20 μm.
micrograph shows that dimensions of released plasmonic microleaves matched well with the copper mesh size of 70 μm × 70 μm (Figure 3b). We could obtain other shapes such as kangaroo using the cut tapes with predesigned hollow patterns. As expected, millimeter-scale kangaroo-shaped plasmonic leaves can be fabricated on Si wafers and still maintain their structural features after being released into acetone (Figure S17). We then probed the collective plasmonic properties of the cross-linked plasmene nanosheets. First, the extinction spectrum of the nanosheets on indium tin oxide (ITO) glasses in the dried state was measured, and there was only one dominant peak located at 743 nm (Figure 4a). Upon being wetted with chloroform, the plasmonic peak blue-shifted 26 nm from 743 nm to 717 nm. In contrast, when the same measurements were carried out on the free-standing crosslinked nanosheets, we were able to observe a rapid blue− purple color change under the microscope (see Figure S18) and a significant plasmonic peak shift of 134 nm, from 739 nm in the dried state to 605 nm in the wetted (Figure 4b). This pronounced blue shift indicates a significant interparticle spacing increase.7,16,40 The difference between substrate-supported and freestanding cross-linked nanosheets can be explained using the mechanism as illustrated in Figure 4c and d. For the substratesupported system, there are strong van der Waals (vdW) attraction forces between the nanoparticles and the substrate. In CHCl3, the solvation force needs to overcome not only interparticle bonding force but also nanoparticle−substrate interaction in order to push nanoparticles away from each other (Figure 4c). Unlike previously reported solvent-sensing gold nanoparticle thin films4,7,41−44 in which the nanoparticles used are 3.4−20 nm in size, the constituent building blocks AuTOHs in our system have a diameter of 50 nm. Hence, it is 6763
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Figure 4. CHCl3 wetting induced plasmonic shift for substrate-supported and free-standing cross-linked plasmene nanosheets. (a, b) Extinction spectra of (a) ITO glass supported and (b) free-standing cross-linked plasmene nanosheets in the dry and CHCl3-wet conditions. (c, d) Proposed mechanism for plasmonic shift of (c) ITO glass supported and (d) free-standing cross-linked plasmene nanosheets.
Figure 5. Different solvent-induced plasmonic switching of free-standing cross-linked plasmene nanosheets. (a) Schematic showing of homebuilt microscope spectrophotometry setup for the different solvent-induced switching measurements. (b) Extinction spectra of free-standing cross-linked plasmene nanosheets in CHCl3 and acetone, respectively. (c) Proposed mechanism for plasmonic switching of free-standing cross-linked plasmene nanosheets between CHCl3 and acetone. (d, e) Oscillation of extinction wavelengths for free-standing cross-linked plasmene nanosheets with (d) CHCl3 and acetone and (e) CHCl3 and hexane. (f) Oscillation of extinction wavelengths for free-standing cross-linked plasmene nanosheets with different solvents.
expected that core-to-core and core-to-substrate vdW forces are much stronger than those in previous reports. In addition, our polymers are cross-linked and locked close-packed AuTOH arrays in place. Thus, the solvation force cannot easily overcome such vdW forces to enlarge interparticle spacing. Consequently, rather than significant SPR shifts reported in previous works,7,44 our substrate-supported nanosheets showed only a slight response to solvents. The small blue shift indicates the slight increase in the interparticle
spacing, but it is comparable to that under the dried state. In contrast, the solvation force needs to overcome only the interparticle bonding force for the free-standing system. In the absence of vdW force between AuTOHs and the substrate, the strong solvation forces will lead to transition of SH-PS(20k)-bPAEMC(9k) ligands from collapse to fully swollen states when the system is switched from dry to wet. This process will enlarge significantly interparticle spacing (Figure 4d). It is highly possible that cross-linked nanosheets may curve up 6764
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nanosheets exhibited different peak positions within different solvents and stayed at nearly the same peak position each time when it was switched back to being wetted with CHCl3. These evidence clearly reveal the robustness of the free-standing cross-linked nanosheets in different solvents and their excellent reversibility of the solvent-quality-induced plasmonic properties. Additionally, we tested solvent vapors’ effects on freestanding cross-linked nanosheets. We designed a microscope spectrophotometry based real-time monitoring system consisting of a closed glass chamber, with free-standing cross-linked nanosheets supported on a copper grid (2000 mesh with hole size 7 μm × 7 μm) as shown in Figure S20a. The desired solvent in a vial was introduced into the closed chamber at room temperature to generate a solvent vapor atmosphere. As shown in Figure S20b, the extinction spectra of free-standing cross-linked nanosheets in the dry state, under an acetone vapor atmosphere, THF vapor atmosphere, and CHCl3 vapor atmosphere almost overlapped with each other. Unlike the previously reported plasmonic vapor sensing gold nanoparticles films,41−43 our free-standing cross-linked plasmene nanosheets did not respond to solvent vapors because of the strong covalent cross-linking in the nanosheets.
because of strong swollen and solvation forces. Comparison between Figure 4a and b clearly rules out the possibility of refractive index effects of CHCl3 on plasmonic peak shift. It is expected that solvation forces may differ from solvent to solvent. To test their effects on free-standing cross-linked nanosheets, we designed a real-time monitoring system using microscope spectrophotometry (Figure 5a). The device consists of a glass slide, on which the free-standing crosslinked plasmene nanosheets on a copper grid (2000 mesh with hole size 7 μm × 7 μm) is placed, and a square cover slide (24 mm × 24 mm) on top of the free-standing nanosheets. The desired solvent is dropped on one side of the sandwich structure and drained into the interlayer by capillary force to wet the free-standing cross-linked plasmene nanosheets, and switching of solvent can also be easily achieved. Taking the CHCl3/acetone solvent pair as an example, when the crosslinked nanosheets are first wetted with CHCl3, solvent switching to acetone can be enabled through allowing CHCl3 in the interlayer half to dry under ambient conditions and adding acetone to extrude the leftover CHCl3. The quality of a solvent in the polymer chemistry field is a qualitative characterization of the polymer−solvent interaction, and it is defined by International Union of Pure and Applied Chemistry (IUPAC) as follows: “a solution of a polymer in a “better” solvent is characterized by a higher value of the second virial coefficient than a solution of the same polymer in a “poorer” solvent”.45 Because PS takes the dominant portion in the SH-PS(20k)-b-PAEMC(9k) ligands, in principle, any good/bad solvent for PS should also be the good/bad solvent for SH-PS(20k)-b-PAEMC(9k). We first chose CHCl3/ acetone as the good/bad solvent pair for the demonstration of solvation force differences. As shown in Figure 5b, the plasmonic peak location of the free-standing nanosheets was 702 nm in acetone versus 603 nm in CHCl3, where a dramatic 99 nm blue shift happened after the solvent switching, and this led to an obvious color change optically (see Figure S19). This could be explained using the proposed mechanism as illustrated in Figure 5c. It has already been proved that the Kuhn length of PS chains will increase with increasing solvent quality, which leads to a larger extent of swelling of the polymer in good solvents.46 Therefore, SH-PS(20k)-bPAEMC(9k) ligands should hold a larger extent of swelling in CHCl3 compared with that in acetone, leading to a larger interparticle spacing and weaker plasmonic coupling of the adjacent AuTOH nanoparticles, resulting in a much bigger blue shift of the peak position of the cross-linked plasmene nanosheets in CHCl3. Furthermore, we studied the reversibility of the solventquality-induced plasmonic switching. Starting from the dry condition, the free-standing cross-linked plasmene nanosheets were first set to wet with CHCl3 and then switched with acetone. We observed nearly fully reversible oscillation of the extinction peak positions upon seven-cycle CHCl3−acetone switching (Figure 5d). We further extended the cycling test using CHCl3/hexane as the good/bad solvent pair, and a nearly reversible peak positions shifting upon seven-cycle CHCl3−hexane switching was observed as well (Figure 5e). To prove the universality of this cross-linked plasmene nanosheet based solvent-quality-induced plasmonic switching, we also carried out the reversibility test using a multisolvent switching cycle in the order of dry−CHCl3−tetrahydrofuran (THF)− CHCl3−acetone−CHCl3−hexane−CHCl3−ethanol−CHCl3. As shown in Figure 5f, the free-standing cross-linked
CONCLUSION In summary, we have demonstrated that plasmene nanosheets could be effectively covalent-cross-linked by introducing coumarin-based photo-cross-linkable moieties to the SH-PS ligands. The cross-linked plasmene nanosheets exhibited great chemical stability in organic solvents, enabling maintenance of overall structural integrity at the centimeter scale. Combined with template-assisted self-assembly, we could shape plasmene arbitrarily at the micrometer scale. Due to polymer ligand swelling-induced enlargement of interparticle spacing, freestanding cross-linked nanosheets exhibited pronounced responses to dry and wet states and also showed characteristic responses to various solvents in a specific and reversible manner. The active control of collective plasmonic properties and tunable interparticle spacing in different solvents renders free-standing cross-linked nanosheets excellent systems in the field of active plasmonics, a burgeoning area in nanoscience.47 The active plasmonic properties demonstrated here could likely be extended to other types of plasmonic nanoparticles, and these cross-linked plasmene nanosheets may be used to design nanodevices and soft metamaterials with chameleon-like properties for solvent sensing and dynamic plasmonic display applications. METHODS Materials. Gold(III) chloride trihydrate (HAuCl4·3H2O, ≥ 99.9%), sodium borohydride (NaBH4), L-ascorbic acid, hexadecyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride solution (CTAC, 25 wt % in H2O), 7-hydroxy-4methylcoumarin (≥98%), ethylene carbonate (anhydrous, 99%), potassium carbonate, styrene (≥99%), benzyl benzodithioate (96%), n-butylamine (99.5%), triethylamine (≥99%), and CDCl3 were purchased from Sigma-Aldrich. 2,2′-Azobis(isobutyronitrile) was purchased from Polysciences Inc. Acryloyl chloride was purchased from Merck. N,N-Dimethylformamide (DMF) and methanol were purchased from Unichrom. Chloroform, n-hexane, and tetrahydrofuran were obtained from Merck KGaA. Acetone and ethanol (absolute) were purchased from AJAX. Diethyl ether and ethyl acetate were purchased from EMD Millipore Corporation. Thiol-terminated polystyrene (Mn = 50 000 g/mol) was obtained from Polymer Source 6765
DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
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Hz, 1H), 6.95 (m, 2H), 6.19 (d, J = 0.98 Hz, 1H), 4.93 (s, 1H), 4.11 (t, J = 4.88 Hz, 2H), 3.75 (t, J = 4.88 Hz, 2H), 2.39 (s, 3H). Synthesis of 7-(2-Acryloyloxyethoxy)-4-methylcoumarin (AEMC). AEMC was synthesized according to literature procedures with slight changes.48 First, 7-(2-hydroxyethoxy)-4-methylcoumarin (4.3 g, 19.6 mmol, 1.0 equiv) and triethylamine (5.9 mL, 43.1 mmol, 2.2 equiv) were dissolved in 70 mL of chloroform. Acryloyl chloride (3.8 mL, 47.0 mmol, 2.4 equiv) was then added dropwise to the solution at room temperature. After stirring for 16 h, 200 mL of dichloromethane was added. The combined mixture was washed with saturated sodium chloride solution three times and then dried over anhydrous MgSO4. The solvent was removed under reduced pressure to obtain a white solid, which was dissolved in 40 mL of ethanol under heat (50 °C) and crystallized twice to yield a white product (49%). 1H NMR (400 MHz, DMSO-d6): δ 7.67 (d, J = 8.79 Hz, 1H), 7.02 (m, 2H), 6.34 (dd, J = 1.46, 17.59 Hz, 1H), 6.21 (m, 2H), 5.98 (dd, J = 1.46, 10.25 Hz, 1H), 4.47 (t, J = 4.39 Hz, 2H), 4.36 (t, J = 4.39 Hz, 2H), 2.40 (s, 3H). Polymerization of AEMC. AEMC was polymerized using a RAFT polymerization process.35,36 Typically, AEMC (1.0 g, 3.65 mmol, 50.0 equiv), benzyl benzodithioate (14.8 μL, 73.0 μmol, 1.0 equiv), and AIBN (1.2 mg, 7.3 μmol, 0.1 equiv) were dissolved in 2.0 mL of DMF and transferred to a Young’s vessel. After three cycles of freeze−vacuum−thaw to remove oxygen, the flask was immersed in an oil bath at 70 °C for 16 h. The solution was then precipitated in 100 mL of diethyl ether twice to obtain pink dithioester-terminated PAEMC powder in 61% yield. 1H NMR (400 MHz, CDCl3): δ 7.67− 7.85 (b, 3H), 7.27−7.56 (b, 34H), 6.95−7.22 (b, 7H), 6.46−6.90 (b, 63H), 5.87−6.14 (b, 29H), 4.30−4.55 (b, 64H), 4.04−4.28 (b, 64H), 2.42−2.65 (b, 28H), 2.17−2.45 (b, 97H), 1.93−2.11 (b, 15H), 1.40− 1.90 (b, 67H). Synthesis of Dithioester-Terminated PS(20k)-b-PAEMC(9k). Dithioester-terminated PAEMC (0.2 g, 25.0 μmol, 1.0 equiv), styrene (1.4 mL, 12.5 mmol, 500.0 equiv), and AIBN (0.82 mg, 5.0 μmol, 0.2 equiv) were dissolved in 2.0 mL of DMF and transferred to a Young’s vessel. After three cycles of freeze−vacuum−thaw to remove oxygen, the flask was immersed in an oil bath at 70 °C for 16 h. The solution was then precipitated in 100 mL of methanol to obtain pink dithioester-terminated PS(20k)-b-PAEMC(9k) powder in 40% yield. 1 H NMR (400 MHz, CDCl3): δ 7.31−7.40 (b, 32H), 6.87−7.22 (b, 584H), 6.28−6.85 (b, 454H), 5.87−6.12 (b, 31H), 4.30−4.55 (b, 64H), 4.04−4.28 (b, 67H), 2.42−2.65 (b, 33H), 2.17−2.40 (b, 114H), 1.69−2.13 (b, 254H), 1.20−1.66 (b, 440H). SEC: Mn = 39 000 g/mol, Mw = 50 000 g/mol, Đ = 1.29. Synthesis of SH-PS(20k)-b-PAEMC(9k). Dithioester-terminated PS(20k)-b-PAEMC(9k) (0.5 g, 16.7 μmol, 1.0 equiv) was dissolved in 4.0 mL of THF and transferred to a round bottle flask. n-Butylamine (165.0 μL, 1.67 mmol, 100.0 equiv) was added after the solution was bubbled with nitrogen flow. The mixture was stirred at room temperature for 2 h, and it was then precipitated in 100 mL of methanol (bubbled with nitrogen flow) to obtain white SH-PS(20k)b-PAEMC(9k) powder in 91% yield. 1H NMR (400 MHz, CDCl3): δ 7.31−7.45 (b, 68H), 6.87−7.21 (b, 536H), 6.28−6.85 (b, 433H), 5.87−6.12 (b, 40H), 4.29−4.48 (b, 65H), 4.04−4.28 (b, 71H), 2.39− 2.57 (b, 30H), 2.17−2.40 (b, 87H), 1.64−2.13 (b, 231H), 1.16−1.48 (b, 431H). Synthesis of Dithioester-Terminated PS(38k)-b-PAEMC(7k). Dithioester-terminated PS(38k)-b-PAEMC(7k) was synthesized in the same way used for dithioester-terminated PS(20k)-b-PAEMC(9k), except 800.0 equiv of styrene was used for polymerization. 1H NMR (400 MHz, CDCl3): δ 7.32−7.41 (b, 32H), 6.80−7.17 (b, 1384H), 6.14−6.69 (b, 1039H), 5.82−6.06 (b, 28H), 4.29−4.52 (b, 50H), 3.97−4.18 (b, 53H), 2.37−2.52 (b, 23H), 2.07−2.28 (b, 111H), 1.60−2.03 (b, 621H), 1.14−1.58 (b, 1798H). SEC: Mn = 63 000 g/mol, Mw = 82 000 g/mol, Đ = 1.30. Synthesis of SH-PS(38k)-b-PAEMC(7k). SH-PS(38k)-bPAEMC(7k) was synthesized in the same way used for SHPS(20k)-b-PAEMC(9k). 1H NMR (400 MHz, CDCl3): δ 7.30− 7.44 (b, 38H), 6.85−7.22 (b, 1132H), 6.29−6.86 (b, 815H), 5.91− 6.11 (b, 27H), 4.29−4.52 (b, 50H), 4.05−4.26 (b, 56H), 2.43−2.60
Inc. Positive photoresist AZ 1512 was received from Microchemicals GmbH. Bare silicon wafer ⟨100⟩ was purchased from Electronics and Materials Corporation Limited, Japan. ITO-coated glass slides was purchased from South China Science & Technology Company Limited, China. The styrene was distilled at room temperature under high vacuum right before using. All other chemicals and solvents were reagent grade and used without further purification unless otherwise noted. Deionized water was used in all aqueous solutions, which were further purified with a Milli-Q system (Millipore). All glassware used in the gold nanoparticle synthesis procedures was cleaned in a bath of freshly prepared aqua regia and was rinsed thoroughly in H2O prior to use. Gilder fine bar grids (300 mesh with 70 × 70 μm2 square holes) and extrafine bar grids (2000 mesh with 7 × 7 μm2 square holes) were purchased from Ted Pella. Characterization. The NMR spectra were measured using a Bruker Advance III 400 (9.4 T magnet) with a 5 mm broadband autotunable probe with Z-gradients and BACS 60 tube autosampler. NMR chemical shifts (δ) are reported in ppm and were calibrated against residual solvent signals of DMSO-d6 (δ 2.50) and CDCl3 (δ 7.26). Gel permeation chromatography was performed on a system comprising a Shimadzu LC-20AT pump, Shimadzu RID-20A refractive index detector, and SPD-20A UV−visible detector. The GPC is equipped with a guide column GPC-800P (1-X4.6 mm) and 4 × Shim-pack GPC columns (GPC-805, GPC-804, GPC-803, GPC02), each 300 mm × 8 mm, providing an effective molecular weight range of 300−2 000 000. The eluent was THF at 40 °C (flow rate: 1 mL min−1). Number-average (Mn) and weight-average (Mw) molecular weights were evaluated using Shimadzu software. The GPC columns were calibrated using poly(styrene) (Agilent) ranging from 580 to 1 044 000 g/mol; molecular weights are reported using the calibration stated for each experiment. A third-order polynomial was used to fit the log(Mp) vs time calibration curve, which appeared approximately linear across the molecular weight range 2 × 102 to 2 × 106 g mol−1. The morphologies of monodispersed AuTOH@SH-PS(20k)-bPAEMC(9k) and free-standing cross-linked plasmene nanosheets were obtained through TEM (FEI Tecnai G2 T20 TWIN LaB6 TEM operating at 200 kV). The morphologies of cross-linked plasmene nanosheets on Si wafers were observed through SEM (FEI Helios Nanolab 600 FIB-SEM operating at 5 kV). The absorption spectra measurements of AuTOH nanoparticles in water, monodispersed AuTOH@SH-PS(20k)-b-PAEMC(9k) in CHCl3, cross-linked plasmene nanosheets in CHCl3, and cross-linked plasmene nanosheets in CHCl3 after two months were performed by using an Agilent Cary 60 UV−vis spectrophotometer. Extinction spectra of ITO glass supported cross-linked plasmene nanosheets, free-standing cross-linked plasmene nanosheets, and solvent-induced plasmonic switching of free-standing cross-linked plasmene nanosheets were obtained through a J&M MSP210 microscope spectrometry system, and the samples were illuminated by a highintensity fiber light source under a 50× objective. The spectra acquisition in a local area was 3 × 3 μm2. The thickness of cross-linked plasmene nanosheets was recorded using a Veeco Dimension Icon AFM in tapping mode using Bruker silicon probes (MPP-1120-10), and the spring constant of the cantilever was 40 N m−1. The AFM data were processed using NanoScope Analysis software. Synthesis and Preparation of Materials. Synthesis of 7-(2Hydroxyethoxy)-4-methylcoumarin. 7-(2-Hydroxyethoxy)-4-methylcoumarin was synthesized according to the literature with slight changes.34 Typically, 7-hydroxy-4-methylcoumarin (8.8 g, 0.05 mol, 1.0 equiv) and ethylene carbonate (4.4 g, 0.05 mol, 1.0 equiv) were dissolved in 40 mL of DMF. Following addition of potassium carbonate (13.8 g, 0.10 mol, 2.0 equiv), the mixture was then stirred for 16 h at 100 °C under a nitrogen atmosphere. The formed solid was precipitated in 500 mL of cold deionized water and collected via vacuum filtration. The product was further purified by recrystallization in ethyl acetate (320 mL, dissolved at 70 °C) to produce white crystal with 84% yield. 1H NMR (400 MHz, DMSO-d6): δ 7.65 (d, J = 9.77 6766
DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
Article
ACS Nano
copper grids (300 mesh with hole size 70 μm × 70 μm) were chosen as templates. First, holey copper grids were fixed to Si wafers that were precoated with photoresist sacrificial layers. UV/O3 treatment (3 min, O2 flow rate 0.5 L/min) was then used to make the substrate surface hydrophilic, which caused the water droplet to be trapped in the hole areas during the drying-mediated self-assembly approach. Then, the cross-linked plasmene nanosheet fabrication process described above was applied to grow plasmene nanosheets on top of the holey grids, and the nanosheets were dragged into the hole areas and ruptured into patches confined by the holes after the evaporation of water. Then the copper grids were peeled off using tweezers, and the square-shaped plasmonic leaves left on the Si wafers were irradiated with UV light for 7200 s to enable the cross-linking. Plasmonic leaves could be released from the Si wafers by immersing the Si wafers with sacrificial layers into acetone. The fabrication process described above could also be adopted to fabricate millimeter-scale plasmonic leaves, by simply replacing the holey copper grids with other templates with desired shapes.
(b, 29H), 2.14−2.39 (b, 107H), 1.69−2.15 (b, 493H), 1.23−1.65 (b, 869H). Synthesis of Gold Nanotrisoctahedrons (AuTOHs). AuTOH nanoparticles were synthesized through a seed-mediated growth method as described in previous papers.16,38 First, a gold seed solution was prepared by adding HAuCl4 (0.1 mL, 25 mM) and NaBH4 (0.6 mL, 10 mM) into a CTAB solution (10 mL, 0.1 M) at 30 °C. After stirring at 1000 rpm for 2 min, the brownish seed solution was kept undisturbed in a 30 °C water bath for 2 h. Then a diluted seed solution was prepared by adding 0.1 mL of the gold seed solution into 9.9 mL of Milli-Q water. Next, a growth solution was prepared by adding HAuCl4 (1.667 mL, 20 mM) and ascorbic acid solution (40.8 mL, 38.8 mM) into a CTAC solution (22 mM, 120 mL) at room temperature. After the growth solution was mixed well, 0.667 mL of the diluted seed solution was quickly added, and then the mix solution was stirred at 800 rpm for 5 min until no further color change occurred to yield the AuTOH nanoparticles. The obtained pinkishcolored solution was centrifuged at 6000 rpm for 4 min, and the AuTOH nanoparticles were dispersed into 40 mL of Milli-Q water for further use. Fabrication of Cross-Linked Plasmene Nanosheets. The fabrication of cross-linked plasmene nanosheets consisted of three steps. The first step was the ligand exchange of CTAC-protected AuTOHs with SH-PS-b-PAEMC to yield SH-PS-b-PAEMC-capped AuTOH nanoparticles. The second step was the fabrication of AuTOH plasmene nanosheets using drying-mediated self-assembly,11,13,16 and the third step was the UV-light-induced cross-linking. AuTOH nanoparticles (10 mL) were centrifuged at 6000 rpm for 10 min and redispersed into 5 mL of THF containing 20 mg of SHPS-b-PAEMC (4 mg/mL). The nanoparticles/THF solution was mixed well and kept in the dark and undisturbed overnight. Then, the nanoparticles were washed once with THF and once with CHCl3 (6000 rpm, 10 min), respectively. To fabricate AuTOH plasmene nanosheets, a 1 μL droplet of concentrated AuTOH@SH-PS-b-PAEMC CHCl3 solution was dropped onto the surface of a water droplet on a 0.8 cm × 0.8 cm silicon wafer or an ITO glass (for substrate-supported plasmene nanosheets) or on a copper grid (2000 mesh with 7 × 7 μm2 square holes, for free-standing plasmene nanosheets). A golden-colored reflective film was formed upon the fast evaporation of CHCl3, indicating the successful fabrication of plasmene nanosheets. After the evaporation of water droplets, patches of monolayer plasmene nanosheets were left on the Si wafers, ITO glasses, or copper grids. The cross-linking of SH-PS(20k)-b-PAEMC(9k)-modified AuTOH plasmene nanosheets was carried out by using a Spectrolinker XL-1000 UV cross-linker (wavelength 365 nm). Typically, the as-fabricated AuTOH plasmene nanosheets were put into a closed chamber and 365 nm UV irradiation was given for 7200 s under time mode to produce the cross-linked plasmene nanosheets. To fabricate large-scale cross-linked plasmene nanosheets on Si wafers, a 2.5 μL droplet of concentrated AuTOH@SH-PS(20k)-bPAEMC(9k) CHCl3 solution was dropped onto the surface of a large water droplet on a 1.5 cm × 1.5 cm silicon wafer, with all other conditions remaining the same. To fabricate normal AuTOH plasmene nanosheets, SH-PS (Mn = 50 000 g/mol) was used to replace SH-PS-b-PAEMC in the ligand exchange step with AuTOH nanoparticles. The rest of the fabrication process remained the same as that for cross-linked plasmene nanosheets. To release cross-linked plasmene nanosheets into acetone, the nanosheets need to be fabricated on Si wafers that were previously spin-coated with thin layers of photoresist as sacrificial layers. First, bare silicon wafers were spin coated with positive photoresist AZ 1512 at 500 rpm for 15 s and 4000 rpm for 45 s with a SUSS Delta 90 spinner. Then, the resist was baked at 95 °C for 2 min. The crosslinked plasmene nanosheets were then fabricated on the photoresistcoated Si wafers and could be released by immersing the substrate with cross-linked plasmene nanosheets into acetone. Preparation of Shapeable Free-Standing Plasmonic Leaves. To fabricate micrometer-scale plasmonic leaves, typically, holey
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b01343. Covalent-cross-linked plasmene nanosheets (AVI) Non-cross-linked plasmene nanosheets (AVI) NMR spectra of synthesized polymers, UV−vis spectrum and TEM characterization of gold nanoparticles, AFM and optical characterization of cross-linked plasmene nanosheets (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Qianqian Shi: 0000-0001-8787-8227 Dashen Dong: 0000-0001-9169-4260 San H. Thang: 0000-0003-2629-3895 Wenlong Cheng: 0000-0002-2346-4970 Author Contributions
W.C. conceived the project; W.C., Y.L., and B.F. designed the experiments. Y.L., Q.S., D.D., and R.F. carried out experiments on synthesis and characterization of covalent-cross-linked plasmene nanosheets. Y.L., B.F., and S.H.T. designed the SH-PS-b-PAEMC polymer ligands, and B.F. carried out the experiments on polymer synthesis and characterization. Y.L., Q.S., S.G., and B.Z. carried out experiments on the fabrication of shapeable plasmonic leaves. Y.L. carried out the plasmonic switching experiments. W.C., Y.L., B.F., and S.H.T. analyzed the experimental data and co-wrote the paper. Y.L., B.F., Q.S., and D.D. prepared figures. All authors have given approval to the final version of the manuscript. Author Contributions ⊥
Y.L. and B.F. contributed equally to this work.
Funding
This work was financially supported by ARC discovery project DP170102208. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the 6767
DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
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ACS Nano
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DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
Article
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DOI: 10.1021/acsnano.9b01343 ACS Nano 2019, 13, 6760−6769
