Letter Cite This: ACS Macro Lett. 2018, 7, 1062−1067
pubs.acs.org/macroletters
Crystallization-Driven Two-Dimensional Self-Assembly of Amphiphilic PCL‑b‑PEO Coated Gold Nanoparticles in Aqueous Solution Fugui Xu, Pengfei Zhang, Jiacheng Zhang, Chunyang Yu,* Deyue Yan, and Yiyong Mai* School of Chemistry and Chemical Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
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S Supporting Information *
ABSTRACT: This Letter reports the first formation of freestanding plasmonic monolayer nanosheets by the self-assembly of AuNPs without assistance from a planar interface. The strategy involves the coating of poly(caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) diblock copolymers on AuNPs, followed by two-dimensional (2D) self-assembly of the resultant amphiphilic AuNPs in aqueous phase. The crystallization of the PCL blocks, affected by their grafting density and radius of gyration, drives the formation of the AuNP nanosheets, which undergoes a growth process of individual micelles to small nanosheets and eventually to large sheets. Due to the plasmonic coupling of AuNPs in close proximity, the AuNP nanosheets exhibit near-infrared (NIR) absorption with the maximum at about 700 nm. This study not only brings a new approach toward polymer−AuNP hybrid superstructures in solution, but also provides a new system for fundamental study on 2D self-assembly of AuNPs.
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AuNPs (15 nm diameter) in aqueous phase, by taking advantage of the crystallization of PCL blocks. The nanosheets possess average dimensions of 1.2 μm width, 1.5 μm length and different thicknesses of 25−35 nm depending on the PCL block length. The formation of the AuNP nanosheets undergoes a growth process of individual micelles passing through small nanosheets and eventually to large sheets. The sheet formation is allowed in a wide range of the reduced tethering density (õ = 18.5−56.7) of the PCL blocks on the AuNP surface, which is determined by their grafting number density (σ) and radius of gyration (Rg). A lower reduced tethering density (õ = 11.3) results in the formation of individual AuNP micelles coexisted with a small amount of AuNP nanocylinders, while a larger õ value (88.5) leads to the construction of near-spherical AuNP nanoclusters. Dissipative particle dynamics (DPD) simulation validates the physical possibility of the formation of these AuNP assemblies. Compared with the SPR absorption of isolated AuNPs in aqueous solution, the absorptions of these AuNP aggregates show red shift in an increasing order of clusters-cylinderssheets. In particular, the nanosheets exhibit NIR absorption with the maximum at about 700 nm. To synthesize the copolymer-coated AuNPs, a series of thioctate ester (TE) terminated PCLn-b-PEO45 copolymers
ontrolled self-assembly of gold nanoparticles (AuNPs) into ordered plasmonic nanostructures has attracted considerable attention, not only in view of its academic interest but also due to a wide range of their potential applications in sensing, bioimaging, photothermal therapy, and so on.1−9 Selfassembled nanostructures of AuNPs can be obtained by modification of the AuNPs surfaces with appropriate ligands, such as small molecules,2 polypeptide,3 DNA,4 or block copolymers (BCPs),8 followed by the self-assembly of the resulting “amphiphilic AuNPs” in solution. Among the employed ligands, BCPs show great advantages in flexible morphological control of AuNP assemblies.8,10−16 To date, AuNP assemblies of various morphologies have been achieved by the self-assembly of BCP-coated AuNPs in solution, including spheres (or clusters), 8,11,15 cylinders, 17 and vesicles,8,14,15 and so on. For example, Nie and colleagues prepared AuNPs coated with polystyrene-b-poly(ethylene oxide) (PS-b-PEO), the self-assembly of the amphiphilic AuNPs in THF/water produced a variety of ordered AuNP nanostructures, including single micelles, clusters, and vesicles, depending on the copolymer composition and the size of AuNPs.15 Despite the development in the preparation of ordered AuNP assemblies, free-standing 2D AuNP nanostructures have not yet been achieved by the self-assembly of AuNPs without assistance from a planar interface. Here, we report the formation of free-standing monolayer plasmonic nanosheets, through the self-assembly of amphiphilic PCL-b-PEO coated © XXXX American Chemical Society
Received: May 16, 2018 Accepted: August 13, 2018
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DOI: 10.1021/acsmacrolett.8b00383 ACS Macro Lett. 2018, 7, 1062−1067
Letter
ACS Macro Letters Table 1. Parameters of TE-PCLn-b-PEO45 Samples and AuNP-n copolymersa
Mn (g mol−1)b
dispersityb
RTEc
AuNP-n
σ (chains/nm2)d
Rg (nm)e
õf
TE-PCL20-b-PEO45 TE-PCL30-b-PEO45 TE-PCL65-b-PEO45 TE-PCL85-b-PEO45 TE-PCL150-b-PEO45
4350 5970 10410 12850 22520
1.20 1.23 1.25 1.28 1.36
0.74 0.70 0.63 0.68 0.66
AuNP-20 AuNP-30 AuNP-65 AuNP-85 AuNP-150
2.2 2.4 2.1 2.6 2.3
1.3 1.6 2.3 2.6 3.5
11.3 18.5 35.0 56.5 88.5
a
The DPs (n) of the PCL blocks were determined by 1H NMR spectra. bMolecular weight and dispersity were measured by GPC in THF against the polystyrene standard, the GPC curves are shown in Figure S2. cRatio of TE end group modification. dNumber density of the PCL-b-PEO chains coated on the AuNP surface, which is determined from TEM images. eRg = bN1/2/6, where N is the DP of PCL block and b = 0.7 for PCL.20 f Reduced tethering density of polymer chains on a surface: õ = σπRg2.21
the effect of solvent evaporation on the aggregate morphology.23 Sheet-like AuNP aggregates were formed in the selfassembly of AuNP-30, AuNP-60, or AuNP-85. As a typical example, the TEM images and AFM topological profile of the nanosheets from AuNP-85 are shown in Figure 1. The
(denoted as TE-PCLn-b-PEO45, the subscript expresses the degree of polymerization) were synthesized by ring-opening polymerization using PEO45−OH as the macroinitiator, followed by the esterification of the resulting PCLn-b-PEO45 copolymers with thioctic acid (Scheme S1a).18 Nuclear magnetic resonance spectroscopy (1H NMR, Figure S1) and gel permeation chromatography (GPC, Figure S2) analyses demonstrated the successful synthesis of the PCL-b-PEO copolymers and gave degrees of polymerization (DPs) of 20− 150 and dispersity of 1.20−1.36 for the PCL-b-PEO samples (Table 1). The production of TE-PCLn-b-PEO 45 was evidenced by 1H NMR spectra, in which signals attributed to the protons in thioctic acid group were clearly seen in addition to those belonging to the protons in PCL-b-PEO (Figure S3). The ratios of the TE group modification, determined from 1H NMR spectra (Figure S3), were in the range of 63∼74% (Table 1). AuNPs were synthesized according to the reported sodium citrate reduction method19 (see Supporting Information, SI); the average diameter of the AuNPs, based on the statistics of 200 particles in TEM images, was 15.3 ± 1.4 nm (Figure S4a), which was supported by dynamic light scattering (DLS) analysis (Figure S4b). Then, ligand exchange method was employed for the preparation of TE-PCLn-b-PEO45 coated AuNPs (Scheme S1b). The copolymer-coated AuNPs were named as AuNP-n, where n is the DP of the PCL block in the attached TE-PCLn-b-PEO45. The successful coating of the copolymers was demonstrated by TEM images, in which a shadow shell derived from the phosphotungstic acid stained PCL-b-PEO around the Au core was clearly seen (Figure S5). Based on the average thickness of the shells, the number density of the coated copolymer chains on the AuNPs were estimated to be 2.1−2.6 chains/nm2 for the different AuNP-n samples (see Table 1 and the calculations in page S7). In contrast, the control experiment using PCL-b-PEO without TE modification could not yield copolymer-coated AuNPs. To perform the self-assembly, the copolymer-coated AuNPs were first dispersed in dioxane and their molar concentrations were calculated to be ∼3 nM based on UV−vis spectra and a theoretical extinction coefficient of 7.8 × 108 M−1 cm−1 at 520 nm.22 The self-assembly of the AuNPs was induced by slow addition (1 mL/min) of 5 mL of water into 1 mL of dioxane solution of the AuNPs under mild stirring (400 rpm). Subsequently, the mixed solutions were dialyzed against pure water to remove dioxane and then the resulting aqueous solutions were incubated without stirring/shaking for 1 week at room temperature (25 °C). The morphologies of the aggregates were examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The TEM and AFM samples were prepared by freeze-drying, in order to avoid
Figure 1. (a) Schematic illustration of the formation of the AuNP nanosheets by the self-assembly of the TE-PCL-b-PEO coated AuNPs in solution. (b, c) TEM images of the AuNP-85 nanosheets. (d) AFM topological profile of the AuNP-85 nanosheets. (e) UV−vis spectra of individual AuNPs and the plasmonic nanosheets formed by AuNP-30, AuNP-65, and AuNP-85, respectively.
statistics of 200 sheets in the TEM images gives a roughly estimated average dimension of about 1.2 μm width and 1.5 μm length (Figure 1b, we measured the maximum widths and lengths in the irregular nanosheets). In the flat nanosheets, the AuNPs pack closely with one next to another while no obvious particle overlap is observed (Figure 1c), suggesting a singlelayer structure of the nanosheets. Both microdifferential scanning calorimetry (μDSC) curve of the nanosheets in aqueous phase and DSC curve of the nanosheets in dry state show distinct signals attributed to PCL crystallization (Figure S6),24,25 demonstrating the presence of PCL crystals in the nanosheets. Meanwhile, curved or folded nanosheets were also observed (Figure S7). No vesicles were found, probably 1063
DOI: 10.1021/acsmacrolett.8b00383 ACS Macro Lett. 2018, 7, 1062−1067
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ACS Macro Letters
can be attributed to the depletion of the isolated micelles as well as the sedimentation of the nanosheets. Interestingly, the formation of nanosheets limited in the selfassembly of AuNP-30, AuNP-60, and AuNP-85. As a control sample, AuNP-20 formed single micelles with an average diameter of 16 nm under the similar experiment conditions, which coexisted with a small amount of AuNP nanocylinders with a 16 nm mean diameter and different lengths (Figures 3a,b and S11a,b). The coexistence was confirmed by the UV−
because the PCL crystallization results in a bending energy higher than the edge energy of the nanosheets, which is unfavorable for the close of the sheets.26,27 AFM analysis suggested a mean thickness of 35 ± 5 nm for the nanosheets (Figure 1d). Moreover, super-resolution multiphoton confocal microscopy images confirmed the formation of the freestanding nanosheets in aqueous solution (Figure S8). Similarly, AuNP-30 and AuNP-60 also self-assembled into free-standing single-layer nanosheets with analogous lateral dimensions but smaller average thicknesses of 25 and 31 nm, respectively (Figure S9a−d). The nanosheets of AuNP-30, AuNP-60, and AuNP-85 show similar surface plasmon resonance (SPR) absorptions with the maximum at near 700 nm in the NIR region (Figure 1e). The parallel plasmonic absorption suggests a similar interparticle distance between neighboring AuNPs in the different nanosheets,14,15 in good agreement with the TEM measurements. This result indicates that the length of the PCL block does not affect the average interparticle distance of the AuNPs in the nanosheets. The formation mechanism of the nanosheets was studied by tracking the intermediate aggregates. Figure 2 shows the TEM
Figure 3. (a) Schematic illustration of the formation of single micelles, cylinders, and clusters. (b) Typical TEM image of the individual AuNP-20 micelles and cylinders. (c) UV−vis spectra of the AuNP-n aggregates in aqueous solutions. (d) Typical TEM image of the AuNP-150 clusters.
vis spectrum, which includes two SPR bands attributed to the two types of AuNP aggregates, respectively (Figure 3c). μDSC curve does not show evident crystal signals, indicating no obvious crystallization occurred in the AuNP-20 aggregates (Figure S6). Different from the formation of the nanosheets, the morphology of the AuNP-20 aggregates did not alter apparently with the aging of their aqueous solution. As another control sample, AuNP-150 with longer PCL blocks aggregated into near-spherical nanoclusters with a mean diameter of 93 ± 20 nm (Figures 3d and S11c). As well, the morphology did not vary with the aging of the aggregate solution. Since the radii of the clusters are much smaller than the length of a fully stretched zigzag PCL150 chain, the PCL chains can be drawn from the internalized AuNPs to the interface of the aggregates by the hydrophilic PEO segments, as illustrated in Figure 3a. Interestingly, the UV−vis spectra show that the plasmonic coupling of AuNPs in these AuNP aggregates results in redshift SPR absorptions in an increasing order of clusterscylinders sheets (Figure 3c), due to the increased 1D/2D plasmonic coupling dimension in these AuNP aggregates.29,30 The above-described phenomena and results indicate that the formation of the various AuNP aggregates is affected by the PCL crystallization, which is governed by the number density of the PCL blocks on the AuNP surface and their average length. It is known that the onset of the crystallization of attached crystalline polymer chains on a surface relies on their reduced tethering density, õ = σπRg2,21,31,32 where σ expresses the number density of the attached chains and Rg is their radius of gyration in solution (Rg = bN1/2/6, in which b denotes the Kuhn monomer length of a free jointed polymer chain and N is the DP).20 If õ is over 14.3, the polymer chains can be highly stretched and crystallize.31,32 In the present work, the calculated õ values of the different PCL chains on the AuNP surfaces are listed in Table 1. Obviously, õ is larger than 14.3
Figure 2. (a−e) AuNP aggregates trapped at increasing aging time of the AuNP-85 aqueous solution after dialysis. (f) Corresponding UV− vis spectra of the AuNP-85 solution at different aging time after dialysis.
images of the assemblies trapped by freeze-drying at increasing aging time during the formation of the AuNP-85 nanosheets. Apparently, the sheet formation experienced a growth process from isolated AuNP micelles passing through small nanosheets and finally to the large ones (Figure 2a−e). This course is analogous to that occurred in the crystallization-driven nanosheet formation of some PCL-containing BCPs.26−28 However, the sheet formation has been rarely observed in water due to the slow dynamics of PCL crystallization.26 UV− vis spectra of the AuNP aggregates at increasing aging time show an obvious red-shift of the maximum absorption (Figure 2f), confirming the growth of the aggregates. Meanwhile, DLS analysis confirmed the increase in the size of the AuNP aggregates, supporting their growth in solution (Figure S10). Further, the almost unchanged UV−vis and DLS spectra of the AuNP aggregate solution after aging for over 36 h suggest a stable state of the aggregates without detectable morphology and size variation, which is in agreement with TEM observation. The reason of the nanosheets not further growing 1064
DOI: 10.1021/acsmacrolett.8b00383 ACS Macro Lett. 2018, 7, 1062−1067
Letter
ACS Macro Letters for AuNP-30, 60, and 85, which might account for their nanosheet formation. Meanwhile, the possible residual dioxane in the PCL domains and the low Tg (ca. −60 °C) of PCL allow the movement of the PCL blocks in the aggregates, which is necessary for the morphological transition from the AuNP-n micelles to the crystalline nanosheets.26,33 In contrast, the PCL blocks in AuNP-20 cannot crystallize as its õ is smaller than 14.3. For AuNP-20, the hydrophobic interaction drives the aggregation and collapse of the PCL blocks on the surface of AuNPs, forming isolated micelles with the PCL layer covered by the PEO corona, which reduces the system energy and also prevents the micelles from their aggregation. The coexistence of the nanocylinders can be ascribed to the effect of dispersity in PCL block lengths33 as well as a possible nonequilibrium state of the aggregates on the limited experiment time scale.34 On the other side, although the õ value of AuNP-150 is larger than 14.3, AuNP-150 forms near-spherical nanoclusters. This is probably due to the much longer PCL length in AuNP-150, which affords the PCL blocks stronger hydrophobicity and, thus, reduces the critical aggregation concentration and the critical water content. Under the similar self-assembly condition, AuNP-150 particles preferentially aggregate into nanoclusters driven by hydrophobic interaction, rather than slowly associate along a 2D direction into nanosheets. Some PCL blocks may probably crystallize in the nanoclusters after their formation, but their morphology does not change in the time span of our experiment. Herein, it is also noted that with increasing PCL length, the micelle−sheet−cluster morphological transition of AuNP-n is analogous to a previously reported transition of PCL-b-PEO aggregates.35 This similarity suggests that the formation of AuNP-n aggregates also follows the principle of hydrophilic-to-hydrophobic balance (or packing parameter) determining morphology. At this point, it should be mentioned that the initial concentration of the AuNPs can also influence the nanosheet formation. It is found that at higher initial AuNP concentrations (>4 nM), AuNP-65 and AuNP-85 aggregate into nanoclusters rather than nanosheets. Akin to AuNP-150, the nanoclusters form right after water addition without experiencing a slow growth process. This is most likely because the higher concentrations reduce the critical water content and favor the preferential aggregation of the AuNPs that are in a closer proximity into nanoclusters. Thereby, we believe that the effect of the reduced tethering density on the nanosheet formation is selectively applicable to a low AuNP concentration range (
