Tuning the Functionality of Self-Assembled 2D Platelets in the Third Dimension

The decoration of 2D nanostructures using heteroepitaxial growth is of great importance to achieve functional assemblies employed in biomedical, electrical, and mechanical applications. Although the functionalization of polymers before self-assembly has been investigated, the exploration of direct surface modification in the third dimension from 2D nanostructures has, to date, been unexplored. Here, we used living crystallization-driven self-assembly to fabricate poly(ε-caprolactone)-based 2D platelets with controlled size. Importantly, surface modification of the platelets in the third dimension was achieved by using functional monomers and light-induced polymerization. This method allows us to selectively regulate the height and fluorescence properties of the nanostructures. Using this approach, we gained unprecedented spatial control over the surface functionality in the specific region of complex 2D platelets.


■ INTRODUCTION
−5 These advantages have been employed in many applications, for example, as disease-targeted carriers, 6 electronic circuit templates, 7 and local-environment monitors/reactors. 8−18 For elevating the uniformity of assemblies, Manners and Winnik first reported the concept of "living" CDSA for 1D and 2D structures, where they separated the crystallite nucleation and growth processes, reducing the polydispersity of nanostructures formed. 19,20The well-defined assemblies achieved through CDSA are versatile for a broad range of applications, 21−25 from drug delivery, 26−29 electronic devices, 30−33 and reactor substrates 34−37 to emulsion stabilizers. 38urrently, 2D platelets with various functionalities are built using prefunctionalized polymers.However, there are two main drawbacks using this method; first, the functionality within the 2D structure is limited to the chemistry of the prefunctionalized polymers, and second, the incorporation of functional polymers can affect the self-assembly and in turn the nanostructures that they adopt.−42 However, modification in the third dimension from a flat 2D surface has not been investigated before, which allows the formation of more complex structures that could be used for multiple applications.Specifically, the spatial growth from a 2D surface may enable patterning of the 2D structure in the third dimension (extending to the Z axis, not only in the XY plane), introducing variations in 3D topology over the 2D structure.Moreover, the selective addition of functional monomers, such as dyes, at different positions of a 2D platelet would enable the separation of different functionalities within a nanostructure.To facilitate the formation of a range of different structures, this modification should be undertaken postassembly using a methodology that does not disrupt the preformed nanomaterial.−48 However, these conventional approaches have inevitable limitations.For example, heat treatment is unsuitable for heat-sensitive materials, and spin-coating and deposition approaches limit surface interaction and adhesion durability, which hinder the selective and spatial control over the resulting structures. 49,50o overcome these limitations, surface-initiated modification has been explored as an alternative strategy that provides precise control over the density and chemical composition of the modified surface.For this, initiating groups are immobilized on the surface, enabling the growth of the polymer chain from the nanostructure.−54 Here, we report an efficient method to modify 2D CDSA platelets in the third dimension, altering their topology and functionality (Scheme 1).We demonstrate the fabrication of multilayered 2D platelets with different functionality (corona or end group) that can be further surface modified via lightinduced polymerization.Fluorescent monomers can also be attached to platelet surfaces using the same strategy, allowing visible tracking of the 2D nanostructured assemblies.Importantly, we can modify specific regions of the 2D platelet in the third dimension by altering the height or fluorescence properties of the surface.
■ RESULTS AND DISCUSSION Controlling Platelet Growth.Poly(ε-caprolactone)-bpoly(N,N-dimethylacrylamide) (PCL-b-PDMA) was synthesized using a hydroxy-terminated dual-head reversible addition/fragmentation chain transfer (RAFT) agent, which could undergo ring-opening polymerization (ROP) followed by RAFT polymerization (see Schemes S1 and S2, Figures S1− S5 for details). 55Following a reported procedure, the resulting PCL 50 -b-PDMA 198 block copolymer (BCP) was used to form polydisperse cylinders via CDSA, then sonication to obtain small crystalline seeds for further living CDSA (Figures 1a, S6, and S7), to form 2D platelets (seeds located on the center of platelets and parallel to the long axis, Figure S8). 55The addition of unimers (PCL/PCL-b-PDMA) to these preformed seeds led to controlled growth of 2D platelets.The presence of PCL homopolymer directs the epitaxial growth toward 2D structures as expected, while a preference for 1D cylindrical structures is observed when only PCL-b-PDMA is used. 44The ratio of the homopolymer to block copolymer in the unimer solution governs the eventual size of the platelet.By increasing PCL content (m PCL :m PCL-b-PDMA from 0.25 to 5.00 wt %), the average size of platelets increased in length from ca. 770 to 1462 nm (Figure S9).Notably, the dimensions of the platelets could be controlled by varying the unimer-to-seed ratio (m PCL :m PCL-b-PDMA = 1:1).This was characterized by transmission electron microscopy (TEM, Figure 1b), atomic force microscopy (AFM, Figure S10), and confocal laser scanning microscopy (CLSM, Figure S10).A linear relationship between the measured area and unimer-to-seed mass ratio indicated predictable epitaxial growth of 2D platelets that could be observed up to a ratio of 80 (Figure S10).However, the aspect ratio (ratio of length to width) remained relatively stable at around 1.85 (1.77−1.91)across the range of unimerto-seed ratios.Low dispersity (<1.02) was observed for all 2D platelets (Table S1).
Multilayer 2D Platelet Construction.Having accomplished 2D epitaxial growth using 1D short seeds, we investigated whether a secondary layer could be grown from prepared platelets (Figure 2a).Double-layered platelets containing two regions, an inner layer and an outer layer, can be generated by the sequential addition of blending unimers (m homopolymer :m block copolymer = 1:1, Figure S25).PCL and PCL-b-PDMA blend unimers have a preference for growing from the exposed crystalline plane, as it is more energetically favored.Consequently, when additional unimers are introduced, they tend to deposit on the edges of PCLbased platelets rather than initiating spontaneous nucleation. 20urthermore, well-defined triple-layered and quadruple-layered platelets were prepared following a similar procedure, as observed by AFM and TEM (Figures 2b, S25).To further explore heterocorona chemistry, the multilayered platelets with different coronas were targeted.All corona modifications demonstrated good compatibility with heterocoronas in a sequential seeded growth process (Figures 2b, S26).
To further prove the precision and scope of this method, the fluorescent dye aminochloromaleimide (ACM) was utilized to functionalize polymers (homopolymer PCL-ACM and block copolymer PCL-b-PDMA-ACM, Scheme S3, Figures S11−  S13). 56The modified polymers can generate spatially defined fluorescent layered platelets by living CDSA.Through adjusting the addition sequences of fluorescent and nonfluorescent polymer unimers into a seed solution, various kinds of patterned platelets (dark and green areas within the "patterned platelets" correspond to nonfluorescent and fluorescent regions, respectively) were fabricated and visualized by CLSM and stimulated emission depletion (STED) microscopy (Figures 2c, S27).Using these methods, we could observe the formation of complex 2D structures with a spatially confined surface functionality.Moreover, no detectable diffusion of luminescent polymers across the area of the 2D platelets was observed over time (2, 10, and 35 days, Figure S28).Furthermore, this shows that no chain transformation in the platelet local environment occurred through this process.
In Situ Surface Modification.The surface modification of 2D PCL-based platelets was investigated in order to modify the structure in the third dimension to enhance the tunability of the nanostructures (Figure 3a).The assemblies were observed by AFM, exhibiting a uniform height of 10 nm across the platelets (Figure S29).Photoiniferter polymerization of DMA on the surface of platelets was carried out under UV irradiation and a nitrogen atmosphere.Considering the degradable nature of PCL, prolonged radiation time may induce photodegradation of the platelets; 57,58 therefore, the UV radiation was minimized to 4 h.Following dialysis to remove unreacted monomer, the height change of platelets was observed to 20 nm (100 mass equivalents of DMA to platelets), and platelet morphology was maintained by AFM analysis, revealing a successful surface functionalization via in situ light-induced polymerization.Moreover, a clear shift in the molecular weight of the bimodal distributions was confirmed by SEC, with good overlap of the refractive index and UV traces (λ abs = 309 nm, Figure S29), showing the successful polymerization and retention of the RAFT end group.To further confirm the applicability of this approach, the aminobromomaleimide methacrylate (ABMMA)-based fluorescent monomer was polymerized to the surface of platelets via the same procedure (Scheme S6).As observed by CLSM, the overlap of fluorescent and bright field channels confirmed that the fluorescent dye was distributed uniformly on the surface of the platelets (Figure S31).
After successful surface modification with a homogeneous monomer and fluorescent monomer by light-induced polymerization, we sought to explore whether the platelet height could be precisely tuned by varying the equivalents of the monomer added.Arrangements from 10 to 400 mass equivalents of DMA were used for the photoiniferter polymerization process.AFM analysis confirmed that the height of the platelets changed as expected and increased in a linear relationship between platelet height and monomer concentration, ranging from 11 to 20 nm with 10−100 mass equivalents of DMA (Figures 3b,c, S32).Interestingly, when the concentration of DMA was increased to higher equivalents (more than 200 mass equiv), the height remained at ca. 22 nm (Figure S32).Furthermore, NMR spectroscopic analysis showed an increase in the ratio of the PDMA block to the PCL block, indicating the DP of PDMA increased.It exhibited a linear curve below 200 equiv and then plateaued, which is associated with the height results (relationship of DMA concentration and platelet height, Figures S34 and 35, Table S3).This phenomenon may be ascribed to three reasons: (1) the chain ends aggregating on the surface of the platelets or the active polymer chains becoming bulkier, limiting the active RAFT group's ability to further react with the free monomer; (2) the degradation of the RAFT agent (trithiocarbonate), especially the fragmented thiyl radical; 59 and (3) the uncontrolled polymerization rate when [DMA]/[RAFT agent] was very high. 60Furthermore, the behavior of the chains standing upright and polymer chain conformation on the surface of PCL-based platelets is intricately linked to factors such as grafting density, molecular weight, and mean square radius of gyration (R g ) of corona segments in ethanol, requiring further investigation in the future.Encouraged by the promising results that platelets assembled via CDSA could act as a template for surface decoration, we investigated whether heterogeneous monomers could also be applied in this system (Figures S36 and S37).Photoiniferter polymerization successfully achieved the addition of the heterogeneous DMAEMA monomer.A significant increase in molecular weight was detected by SEC analysis, requiring further investigation of the photo-induced polymerization for this system.
Spatial Selective Modulation in the Third Dimension.To satisfy more specific application scenarios such as pharmaceutical encapsulation and microelectronic devices, selective functionalization on a specific region of the platelet was investigated using in situ surface modification (Figure 4a).Modulating the position of the RAFT active group in the specific layer of the platelets is a facile way for postassembly surface modification, providing the chain-extending ability to postpolymerize only on the layer containing the RAFT group (Scheme S7 and Figures S38−S40).To achieve this, multilayered platelets were generated by living CDSA using alternate layers of unimers with/without RAFT agent.This offers an efficient route to construct complex hierarchical structures by selective spatial modification on the surface of 2D platelets.In this work, light-induced polymerization (photoiniferter) was used to extend the chain of the polymers that had a living chain end (Figure 4a).
Using this strategy, the ability to conduct surface polymerization is restricted to the layer containing the RAFT group.From AFM analysis, the height of the inner layer containing the RAFT group was shown to increase by about 10.1 ± 1.6 nm, while the outer layer, free from the RAFT group, remained at the original height (Figures 4b, S41, and S42).Furthermore, when the RAFT group was located in the outer layer, the height of the outer layer was shown to increase in height by 6.2 ± 1.1 nm as compared to the inner layer, which again remained the same (Figures 4b, S43, and S44).This work to our knowledge demonstrated for the first time selective surface modification from 2D platelets in the third dimension.To further demonstrate this selective modification method, a fluorescent dye was incorporated using photoiniferter polymerization to visualize the spatially defined regions of the multilayered platelets (Figure 4a).By performing photoiniferter polymerization with dye functionalized monomer, the platelets were successfully modified to emit a green fluorescence in specific regions, which allowed for their tracking using CLSM and STED.However, limited dye incorporation due to inefficient photoiniferter polymerization resulted in poor fluorescent image resolution (Figure S45).To overcome this limitation and enhance the polymerization efficiency, photoinduced electron/energy transfer RAFT (PET-RAFT) polymerization was introduced into this system (Figure S46).PET-RAFT polymerization of prepared platelets was carried out using the fluorescent monomer (10 equiv) and photocatalyst eosin Y (0.01 equiv) under green light radiation for 4 h; see the SI for details.Upon incorporation of the fluorescent monomer to the regions containing an active RAFT end group, the segregation of domains could be observed by fluorescence imaging.The centrosymmetric multilayered platelets formed by sequentially adding RAFT or RAFT end group removed polymer to a solution of seeds, and were then polymerized with fluorescent dye by PET-RAFT.The green ring-like multilayered platelets were visualized by CLSM and STED, proving the formation of a complex hierarchical structures spatially defined by post surface modification (Figure 4c).

■ CONCLUSIONS
In summary, we report a living crystallization-driven selfassembly method for the formation of uniform 2D platelets by adding PCL-based homopolymer and different corona chemistry block copolymer blends with controlled spatial dimensions and uniform dispersity.Multilayered platelets were accessed by selective sequential addition of fluorescent modified PCL blends to the 1D or 2D precursors, exhibiting excellent trace tracking ability and colloidal stability with integral structural morphology.Furthermore, in situ lightinduced (photoiniferter or PET-RAFT) polymerization was used to selectively modify and spatially functionalize the surface of the platelets using DMA or dye as a monomer.Moreover, uniform platelets with different heights were prepared by tuning the monomer concentration during the light-induced polymerization process.This exhibited good control and a linear relationship of surface height growth upto and including the addition of 100 mass equivalents of DMA.The ability of in situ photodecoration of the surface of 2D platelets opens the possibility of controlled selective and spatial surface functionalization of soft materials, possibly exploiting the huge diversity of applications in material science and polymer chemistry.

Figure 3 .
Figure 3.In situ surface modification via light-induced polymerization (photoiniferter polymerization).(a) Strategy of surface modification of CDSA platelets prepared via light-induced polymerization growth.Typical light-induced polymerization procedure: prepared platelets (in ethanol) and monomer mixed solution was degassed under a nitrogen flow for 30 min in an ice bath, then was placed in the UV cross-linker and irradiated with 405 nm light under 25 °C for 4 h.(The role of PCL-b-PDMA is to act as a stabilizer, enhancing colloidal stability in the solvent and preventing precipitation, also no change of Tyndall effect before and after cooling as shown in Figure S30.)(b) Relationship of platelet height against the concentrations of DMA monomer (0, 10, 25, 50, 75, 100, 200, and 400 mass equivalents to platelets mass) by photoiniferter polymerization.(c) AFM height profiles of platelets reacted with different concentrations of DMA.

Figure 4 .
Figure 4. Spatial selective modulation on 2D platelets' surface via light-induced polymerization (PET-RAFT polymerization).(a) Design of spatially selective modification on the surface of platelets.Reaction conditions: platelets, monomer, and photocatalysts (eosin Y, 0.01 equiv) mixed solution was degassed under a nitrogen flow for 30 min in an ice-bath, then was placed in the self-made light setup and irradiated with green light under 25 °C for 4 h.(b) AFM images and height profiles of double-layered platelet before and after light-induced polymerization with DMA monomer.Scale bar = 1 μm.(c) CLSM (hybrid detector) images of a multilayered platelet with 2, 3, 4, and 5 layers, respectively (left to right).Specific layers in the multilayered platelet, containing active RAFT end groups, were chain extended with the ABM dye monomer through PET-RAFT polymerization.Scale bar = 5 μm.