Encapsulation and Protection of Ultrathin Two-Dimensional Porous

Article Cite This: Chem. Mater. 2019, 31, 4897−4912

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Encapsulation and Protection of Ultrathin Two-Dimensional Porous Organic Nanosheets within Biocompatible Metal−Organic Frameworks for Live-Cell Imaging Jinqiao Dong,†,§ Zhiwei Qiao,‡,§ Yutong Pan,†,§ Shing Bo Peh,† Yi Di Yuan,† Yuxiang Wang,† Linzhi Zhai,† Hongye Yuan,† Youdong Cheng,† Hong Liang,‡ Bin Liu,*,† and Dan Zhao*,†

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†

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore ‡ Guangzhou Key Laboratory for New Energy and Green Catalysis, School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, People’s Republic of China S Supporting Information *

ABSTRACT: Despite the rapid development of ultrathin two-dimensional (2D) organic nanosheets, it still remains a challenge to stabilize them and prevent restacking so that they could be used in aqueous environments for biological applications such as live-cell bioimaging. Herein, we report an effective approach to stabilize and protect ultrathin 2D porous organic nanosheets (PONs) by encapsulating them with biocompatible zeolitic imidazolate framework-8 (ZIF-8) for in vitro live-cell imaging. We rationally design and synthesize few-layered 2D PONs named NUS 27−29 containing flexible tetraphenylethylene units as aggregation-induced emission (AIE) molecular rotors. The micrometersized freestanding 2D nanosheets of NUS 27−29 with thicknesses of 2−5 nm can be easily obtained by exfoliation from their bulk powders. We demonstrate that these 2D nanosheets can be armored by ZIF-8 crystals grown in situ for inhibition of restacking. Importantly, we find that the dynamics of the AIE molecular rotors of NUS 27−29 can be restricted by noncovalent interactions between the 2D nanosheets and ZIF-8 armor, as proved through experimental studies and theoretical simulations. As a result, the integration of these 2D nanosheets in ZIF-8 leads to highly stable, porous, and fluorescent composites. We further demonstrate that these composites can be employed as biological fluorescent probes for in vitro live-cell imaging. Our strategy shows the first example of transporting hydrophobic 2D organic nanosheets into live cancer cells by encapsulating within biocompatible MOFs, which should facilitate the further development of ultrathin 2D nanomaterials for various biological applications.

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from bulk covalent organic framework (COF) powders.11 Zhang and co-workers have also realized highly sensitive DNA recognition using ultrathin 2D COF nanosheets.12 As newly emerged 2D fluorescent nanomaterials, those synthetic ultrathin 2D organic NSs have also exhibited controllable synthesis, flexible structures and excellent optical and molecular sensing properties.13 In this regard, our group has reported few-layered ultrathin 2D porous organic nanosheets (PONs) containing confined molecular rotors for optical sensing applications.14−16 Despite the above-mentioned progress, there is very limited report of highly fluorescent 2D organic NSs for biological applications such as live-cell bioimaging, which has become an indispensable tool for medical diagnostics as well as a powerful technique to monitor physiological and pathological processes of biomolecules in their native environment.17 Moreover, should 2D materials easily penetrate into the live cancer cells, they would become very promising for in vitro biological

INTRODUCTION The rise of graphene1 has inspired the flourish of twodimensional (2D) nanomaterials with complementary properties and functionalities enabling rapid advances in science and engineering,2,3 as well as an accelerated global development of synthetic 2D organic materials for various applications.4,5 Particularly, dimensionality plays a crucial role in determining their fundamental properties.6 For example, ultrathin fewlayered 2D organic nanosheets (NSs) with high aspect ratios are expected to exhibit dimensionally related properties,7 such as large external surface and more accessible active site, compared to that of their stacked 3D bulk powders.8,9 Furthermore, the confinement of excited-state electrons in few-layered 2D NSs can facilitate their fundamental study in optical sensing devices.3 Therefore, those 2D organic NSs have gained increasing attention recently for 2D surface-related chemical sensing or biological sensing due to the strong inplane covalent bond and ultrathin thickness.10 For example, Banerjee and co-workers have reported the fluorescent sensing of volatile organic compounds (VOCs) using ultrathin 2D covalent organic nanosheets (CONs) achieved by exfoliation © 2019 American Chemical Society

Received: April 24, 2019 Revised: June 16, 2019 Published: June 17, 2019 4897

DOI: 10.1021/acs.chemmater.9b01642 Chem. Mater. 2019, 31, 4897−4912

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Chemistry of Materials

Figure 1. (a) Scheme of the challenge in transporting synthetic ultrathin 2D organic NSs into cells for bioimaging. (b) Scheme of the encapsulation and protection of ultrathin 2D organic NSs by biocompatible ZIF-8 for bioimaging.

that the system of enzyme encapsulated in ZIF-8 nanoparticles has a tremendous enhancement in the activity of the catalytic cascades (enzymes@ZIF-8).29 Other ZIF-8 composites have also demonstrated advantages such as high electrocatalytic activity [GOx@ZIF-8(NiPd)],30 efficient and selective catalysis (Pd Nanocubes@ZIF-8),31 as well as selective formation of gallium nitride quantum dots (GaN@ZIF-8).32 Impressively, Zhang and co-workers realized the surface coating of various inorganic 2D nanomaterials including MoS2 NSs, graphene oxide (GO), and reduced graphene oxide (rGO) by ZIF-8, and demonstrated the memory device based on a MoS2@ZIF-8 hybrid with a high ON/OFF ratio and long operating lifetime (MoS2@ZIF-8).33 Very recently, Cosa and co-workers demonstrated the solid-state entrapment of boron dipyrromethene (BODIPY) fluorophores in ZIF-8 (PM605@ ZIF-8), which can effectively enhance the photostability of the dye molecules.34 These successful examples inspire us to overcome the above-mentioned problems of ultrathin 2D organic NSs by coating with ZIF-8, which is stable under physiological conditions and has a pore aperture of around 3.4 Å. We envision that coating ZIF-8 onto the surface of ultrathin 2D NSs containing dynamic molecular rotors with aggregationinduced emission (AIE) property21,35,36 can not only stabilize the 2D nanosheets but also protect the high fluorescence emissions in the composites for possible in vitro bioimaging applications (Figure 1b). In this contribution, we report the design and synthesis of 2D layered π-conjugated aromatic polymers, namely NUS 27− 29, containing flexible tetraphenylethylene (TPE) as the fluorescent molecular rotors. Such material design overcomes the issue with quantum yield while retaining AIE behavior for the synthetic 2D fluorescent organic NSs. The bulk polymer powders can be easily exfoliated into micrometer-sized lamellar 2D NSs with thicknesses of 2−5 nm. For a proof-of-concept

applications such as live-cell imaging and cancer treatment.18−20 However, to the best of our knowledge, there are still several issues to be overcome before these materials can be considered for biological applications. (1) Those ultrathin 2D organic NSs are too hydrophobic for cellular uptake, and they can easily aggregate into bulk particles in aqueous media, which may increase their size and generate aggregation-caused quenching (ACQ) phenomenon21 that is unfavorable for in vitro biological applications (Figure 1a). (2) Even if those 2D organic NSs can penetrate into living cells, their fluorescence may be easily quenched by various biomolecules and components, such as amino acids, peptides, proteins, and serums in the complicated cellular environment. Therefore, developing new 2D organic NSs to overcome these problems is of paramount importance to realize their potential biological applications. Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal−organic frameworks (MOFs), with high porosity and hydrothermal stability.22−24 Particularly, ZIF-8 is a nontoxic and biocompatible ZIF built from zinc ions and 2methylimidazolate, and is constantly chosen as the support for molecule immobilization to fabricate composites for various applications.25 For example, Zou and co-workers have reported a one-pot synthesis of ZIF-8 with encapsulated targetmolecules (DOX@ZIF-8) for controlled drug delivery.26 Gassensmith and co-workers have realized the enhanced stability of tobacco mosaic virus (TMV) by coating ZIF-8 (TMV@ZIF-8), allowing the virus to be chemically modified using a standard bioconjugation reaction underneath the MOF shell.27 Chu and co-workers have reported protein-encapsulated biomineralized ZIF-8 nanoparticles (protein@ZIF-8), which have the advantages of preserving protein activity for months and protecting proteins from enzyme-mediated degradation.28 Willner and co-workers have demonstrated 4898


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Figure 2. Synthetic route of bulk powders of NUS-27 (a), NUS-28 (b), and NUS-29 (c) via Suzuki coupling reactions. Electrostatic potential surface of NUS-27 (d), NUS-28 (e), and NUS-29 (f) fragments obtained by DFT calculation. Fluorescent photographs of NUS-27 (g), NUS-28 (h), and NUS-29 (i) bulk powders (λex = 365 nm; inset: the corresponding optical photographs of NUS 27−29 bulk powders).

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RESULTS AND DISCUSSION Synthesis and Characterization of NUS 27−29 NSs. In this study, 1,2-diphenyl-1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)phenyl)ethane (DTDE) was employed to synthesize layered bulk powders of NUS 27−29. The DTDE monomer was chosen because of its linear directional effect that, once being connected with three-armed monomers, can afford the expected 2D-layered graphene-like structures with controlled hexagonal pore sizes. In addition, the two free phenyl rings may act as dynamic molecular rotors after the DTDE monomer is incorporated into the porous frameworks.14,37 Furthermore, the highly twisted molecular conformations in the structure can effectively disrupt the

study, we demonstrate a facile approach for the stabilization and protection of these ultrathin 2D NSs by coating them with biocompatible ZIF-8 for in vitro cancer cell imaging. We find that the presence of dangling AIE molecular rotors exposed on the surface of these 2D NSs can be substantially restricted in the composites, leading to highly porous, stable, and fluorescent MOF composites confirmed by extensive experimental studies and theoretical calculations. Finally, we prove that these composites can be employed as fluorescent probes for in vitro live-cell imaging. 4899


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Figure 3. (a) Schematic diagram for the liquid-based exfoliation of bulk layered powders to ultrathin 2D NSs. FE-SEM images of 2D NSs of NUS27 (b), NUS-28 (c), and NUS-29 (d) deposited on AAO substrates. HR-TEM images of 2D NSs of NUS-27 (e), NUS-28 (f), and NUS-29 (g). Inset: the corresponding Tyndall effect of exfoliated NUS 27−29 NSs. AFM images of 2D NSs of NUS-27 (h), NUS-28 (i), and NUS-29 (j) exfoliated in methanol solutions. The bottom of each figure is the height of AFM image for selective area. Inset: the theoretical thickness of seven layers (3.6 nm) of NUS-27, four layers (2.1 nm) of NUS-28, and ten layers (5.0 nm) of NUS-29 based on the optimized AA stacking modeling structures.

intermolecular π−π stacking interactions among the adjacent layers, facilitating efficient exfoliation into ultrathin 2D

NSs.38,39 Simultaneous achievement of AIE performance and quantum yield is beneficial for bioimaging.18,20 However, based 4900


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nonplanar π−π stacking of twisted TPE units from adjacent layers.39 This implies that only weak van der Waals interactions exist between the adjacent layers, which are highly conducive for exfoliation. Third, the as-exfoliated few-layered 2D NSs can avoid ACQ due to the reduction of electronic decoupling between adjacent layers, leading to the improvement of luminescence efficiency.35 Lastly, thermogravimetric analyses (TGA) show that NUS 27−29 bulk powders are thermally stable up to 400 °C in nitrogen atmosphere (Figure S12). Excellent chemical stability of these materials was also proven by soaking tests using water, hydrochloric acid (6 M), sulfuric acid (6 M), sodium hydroxide (8 M), and common organic solvents (Table S3). The impressive stability of NUS 27−29 bulk powders increases the feasibility of liquid exfoliation. Free-standing ultrathin 2D NSs of NUS 27−29 were readily obtained by liquid-based exfoliation of their bulk powders in methanol. As a result, the AIE molecular rotors, which are originally suppressed by the interlayer interactions between the adjacent packing layers in the bulk powders, can now be fully liberated and exposed on the surface of these ultrathin 2D NSs (Figure 3a). The morphology of exfoliated NUS 27−29 NSs was studied by FE-SEM, high-resolution transmission electron microscopy (HR-TEM), and atomic force microscopy (AFM). FE-SEM images of the as-exfoliated NUS 27−29 NSs deposited on porous anodic aluminum oxide (AAO) substrates reveal the flat morphology of the NSs and small thicknesses, consistent with their 2D nature (Figure 3b−d and Figure S13). The HR-TEM images of the ultrathin NUS 27−29 NSs further indicate the preservation of shape and micrometer-scale sizes after exfoliation (Figure 3e−g and Figure S14). The substantial retention of structural integrity arises because of the strong covalent bonds that can link the monomers and protect the frameworks from extensive degradation. Thus, the liquid exfoliation process cannot easily destroy the in-plane structures of NUS 27−29. Quantitative measurements of the NSs thickness were performed using AFM. The NUS 27−29 NSs transferred on silicon wafers have a flat morphology, with average thicknesses of around 3.6, 2.0, and 4.8 nm for NUS-27, NUS-28, and NUS-29 NSs (Figure 3h−j and Figure S15), respectively. The thicknesses correspond to structures with 4− 10 unit cell thicknesses based on optimized AA stacking models with the lowest energy (inset of Figure 3h−j). The possible reasons for the small thickness in NUS-28 nanosheets are (1) methanol solvent may easily enter the interlayer space to expand it because of the presence of N atoms in the polymer, and (2) the twisted configuration of TBA monomer, which could cause looser interlayer packing in NUS-28 bulk powder. Notably, the lateral size of NUS 27−29 NSs can reach up to 10 μm, resulting in high aspect ratios of 2D nature. In addition, methanol solutions containing NUS 27−29 NSs exhibit a typical Tyndall effect, indicating the colloidal feature of the solutions containing freestanding and homogeneous ultrathin 2D NSs (inset of Figure 3e−g). FT-IR and XPS characterizations of the ultrathin 2D NSs show that the exfoliated materials maintain the same chemical compositions as that of the bulk powders (Figures S2 and S3). Notably, a structural relaxation was observed in the fluorescence spectra of the ultrathin 2D NSs. Specifically, NUS-27 NSs show a blue shift of about 12 nm relative to the bulk powder (Figure S16). Similar blue shifts of 12 and 13 nm were found in NUS-28 and NUS-29 NSs, respectively. The shifts can be attributed to the reduced interlayer π−π interactions caused by the exfoliation.14,15 Similar structural

on our previous studies,14,15 achievement of this balance in the system of TPE-based 2D organic NSs is extremely challenging. We found that sufficient space is the prerequisite for AIE activity of these dynamic molecular rotors in the cavity.15 On the basis of this observation, we envision that the twoconnected TPE monomer with the three-connected monomer to create [2+3] type 2D frameworks with larger hexagonal-like pores than [2+6] type NUS-25 (triangular-like pores) and [2+4] type NUS-24 (rhombic-like pores) can overcome the issue with quantum yield while retaining AIE behavior for the synthetic 2D fluorescent organic NSs. As shown in Figure 2a−c, NUS 27−29 bulk powders were synthesized using three C3-symmetrical monomers of different planarity and molecular size (Figure S1) by Suzuki−Miyaura reaction:37,40 1,3,5-tribromobenzene (TBB for NUS-27 with a yield of 91%), tris(4-bromophenyl)amine (TBA for NUS-28 with a yield of 88%), and 1,3,5-tris(4-bromophenyl)benzene (TBPB for NUS-29 with a yield of 84%, see synthetic details in the Supporting Information). The Fourier transform infrared spectra (FT-IR) of the bulk powders indicate that the C−Br vibration bands of the monomers almost completely disappeared (Figure S2). X-ray photoelectron spectroscopy (XPS) performed on NUS 27−29 bulk powders indicates that the Br 1s peaks (around 70.4 eV) belonging to the TBB, TBA, and TBPB monomers are absent (Figure S3). Thus, the spectroscopic characterizations indicate the completion of the cross-coupling reactions. The proposed chemical structures of NUS 27−29 were also characterized by solid-state 13C crosspolarized (CP)/magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra (Figure S4) and elemental analysis (Table S1). Density functional theory (DFT) calculations indicate a lower electrostatic surface potential (red isosurface) in the NUS-28 fragment compared with NUS-27 and NUS-29 fragments due to the central N atom in the TBA monomer (Figure 2d−f), allowing stronger interactions with guest molecules for a potentially enhanced chemical sensing ability. Furthermore, UV−vis diffuse reflectance measurements41 show that the optical band gaps (Eg) of NUS 27−29 bulk powders are 2.73, 2.64, and 2.63 eV (Figure S5), respectively, indicating a semiconductor nature of these π-conjugated aromatic polymers, justifying their strong green fluorescence under ultraviolet light (λex = 365 nm, Figure 2g−i). Several characteristics of NUS 27−29 bulk powders prompted us to attempt the exfoliation to obtain ultrathin 2D NSs. First, NUS 27−29 bulk powders exhibit only a surface adsorption behavior as revealed by N2 sorption tests at 77 K, similar to our previously reported ultrathin 2D porous organic NSs.14,15,42 The Brunauer−Emmett−Teller (BET) surface areas of NUS 27−29 are 203, 143, and 201 m2 g−1 (Figure S6), respectively. The pore size distributions calculated using nonlocal density functional theory (NLDFT) reveal the main distributions at 3.1−3.7 nm in NUS 27−29 (Figure S6), matching well with their simulated AA-stacking molecular models (Figures S7−S9 and Table S2). The surface-dominated adsorption suggests that transport to the sensing-active molecular rotors can be improved by exfoliation to expose more external surface. Second, field-emission scanning electron microscopy (FE-SEM) images of NUS 27−29 bulk powders show the layered sheet-like morphology (Figure S10). In addition, powder X-ray diffraction (PXRD) patterns indicate they are amorphous polymers (Figure S11). The lack of longrange order is due to the irreversible formation of C−C covalent bonds during the synthesis,14,43 as well as the weak 4901


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Figure 4. (a) Schematic diagram for the preparation of NUS 27−29 NSs@ZIF-8. FE-SEM images of NUS-27 NSs@ZIF-8 (b), NUS-28 NSs@ZIF8 (c), and NUS-29 NSs@ZIF-8 (d). Inset: the FE-SEM images of circulated rectangular in panels b−d. Fluorescence photographs of NUS-27 NSs@ZIF-8 (e), NUS-28 NSs@ZIF-8 (f), and NUS-29 NSs@ZIF-8 (g) (λex = 365 nm). Inset: the corresponding optical photographs of NUS 27− 29 NSs@ZIF-8 in panels e−g. 3D CLSM images of NUS-27 NSs@ZIF-8 (h), NUS-28 NSs@ZIF-8 (i), and NUS-29 NSs@ZIF-8 (j). Inset: the corresponding 2D CLSM images and molecular modellings of NUS 27−29 NSs@ZIF-8 in (h−j). (k) CIE coordinates of emission colors of ZIF-8 and NUS 27−29 NSs@ZIF-8. (l) PXRD patterns of ZIF-8 and NUS 27−29 NSs@ZIF-8 composites. (m) Nitrogen sorption isotherms of ZIF-8 and NUS 27−29 NSs@ZIF-8 measured at 77 K. Inset: the corresponding pore size distributions.

relaxation can also be observed in other ultrathin 2D optical materials such as 2D hybrid perovskites.44,45 Therefore, the

separated few-layered 2D NSs can reduce the electronic decoupling between adjacent layers, leading to the inhibition of 4902


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Figure 5. (a) Temperature-responsive emissions of NUS-28 NSs (c = 20 μg mL−1) in methanol recorded between −10 and +60 °C (λex = 360 nm). (b) Cyclic switching of fluorescence intensity upon heating and cooling for NUS 27−29 NSs. (c) Temperature-responsive emissions of NUS-28 NSs@ZIF-8 (c = 0.5 mg mL−1) in methanol recorded between −10 and +60 °C (λex = 360 nm). (d) Relative fluorescence intensity [I(−10 °C)/I] of NUS 27−29 NSs and NUS 27−29 NSs@ZIF-8 under various temperatures. (e) Relative fluorescence intensity [I(water=90%)/I(water=0%)] of NUS 27− 29 NSs and NUS 27−29 NSs@ZIF-8 in methanol/water mixtures. (f) Relative fluorescence intensity [I(mesitylene)/I(benzene)] of NUS 27−29 NSs and NUS 27−29 NSs@ZIF-8. (g) Schematic diagram for the coating of ZIF-8 into the 2D NSs. Red phenyl rings represent the restricted molecular rotors in the ZIF-8-containing composites.

Synthesis and Characterization of NUS 27−29 NSs@ ZIF-8. In this work, ZIF-8 was chosen as the MOF to encapsulate and protect the NUS 27−29 NSs due to the

ACQ behavior in traditional ultrathin 2D NSs, which is beneficial for preparing highly emissive 2D NS composites for optical sensing or biological imaging applications. 4903


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the composites. The possible reason is that the zinc ions and 2methylimidazole ligands can diffuse through the nanoporous structures of NUS 27−29 NSs during the one-pot synthesis, thereby leading to the growth of ZIF-8 crystallites that can easily cover and even transverse these NSs (Figure 5g). NUS 27−29 NSs@ZIF-8 composites have main emission peaks at 494, 510, and 496 nm (Figure S33), with quantum yields of 33.2%, 29.0%, and 29.2%, respectively. Moreover, the observed emission colors of these composites and pure ZIF-8 are differentiable on the Commission Internationale de L’Eclairage (CIE) chromaticity diagram (Figure 4k). Dynamic Behavior of 2D NSs and NSs@ZIF-8. Next, we sought to understand the dynamic behavior of the AIE molecular rotors within the free-standing 2D NSs and the ZIF8-containing composites. Strong fluorescence is expected should the dynamic behavior of phenyl ring rotors of NUS 27−29 NSs be fully restricted by ZIF-8 coatings.15 First, we tested the temperature-responsive fluorescence of the freestanding 2D NSs and NSs@ZIF-8 composites, as temperature has a strong effect on the degree of rotation of these dynamic phenyl rings. As shown in Figure 5a, upon increasing temperature from −10 to +60 °C, the emission intensity of NUS-28 NSs at 510 nm significantly decreases. Similar phenomena were observed in NUS-27 NSs and NUS-29 NSs (Figure S34). This temperature effect is commonly attributed to activated nonradiative decay in organic fluorophores. However, rotations of the phenyl rings in this system are expected to speed up upon increasing temperature, a typical molecular rotor effort that can lead to increased molecular motions to quench the fluorescence. Our observation of the temperature effect is consistent with other molecular rotor systems such as small organic molecules46 and supramolecular coordination cages,47 as well as TPE-based polymer.48 In particular, the emission intensity of NUS-28 NSs drops by 64.5% over this temperature range, which is higher than that of NUS-27 NSs (45.8%) and NUS-29 NSs (60.2%). Furthermore, the temperature-dependent emission responses of NUS 27−29 NSs can be fitted with linear functions, with average slopes of 1.2%, 2.6%, and 2.1% per °C for NUS-27, NUS-28, and NUS-29 NSs (Figure 5d), respectively. The relatively higher slope in NUS-28 is probably because of the flexible TBA monomer, as the stators in NUS-28 framework can contribute to the dynamic behavior for the whole system compared with the relative rigid TBB and TBPB monomers in NUS-27 and NUS-29 frameworks, respectively. Interestingly, the temperature cycling experiments indicate highly reversible rotation of phenyl ring rotors under temperature control over the five runs attempted (Figure 5b and Figures S35−S37). However, this temperature-responsive behavior of NUS 27−29 NSs is obviously suppressed in their ZIF-8 composites. As shown in Figure 5c, the change in fluorescence intensity of NUS-28 NSs@ZIF-8 is much smaller than free-standing NUS-28 NSs (23.0% vs 64.5%). Similar trends were observed in NUS-27 NSs@ZIF-8 (24.4% vs 45.8%) and NUS-29 NSs@ZIF-8 (24.9% vs 60.2%, Figure S38). The average slopes of change of NUS 27−29 NSs@ZIF-8 were calculated to be 0.43%, 0.48%, and 0.46% per °C, respectively, which are significantly lower than that of their free-standing 2D NSs counterparts (Figure 5d). These results strongly suggest that the phenyl ring rotors of NUS 27−29 NSs are partially restricted by the ZIF-8 coating in the composites. The dynamic behavior of AIE molecular rotors in the composites was further evaluated by AIE characterization in

following reasons. First, room temperature formation of ZIF-8 in methanol solution is possible. Such a mild synthetic condition mitigates the possible restacking of NUS 27−29 NSs during the one-pot synthesis. Second, the nonemissive feature of ZIF-8 crystals prevents any interference with the fluorescence emissions of these 2D NSs. Third, high stability and hydrophilicity of ZIF-8 are beneficial for in vitro bioimaging applications. Importantly, ZIF-8 provides a window size of approximately 3.4 Å, effectively preventing the fluorescence quenching of these 2D NSs in the complicated cellular environment. All these advantages suggest that ZIF-8 is better than other MOF candidates such as UiO-66, HKUST-1, and MIL-101 for biocompatible MOF encapsulation. The process for one-pot synthesis of NUS 27−29 NSs@ZIF-8 composites is presented in Figure 4a. In a typical synthesis, a methanol solution of Zn(NO3)2, 2-methylimidazole, and exfoliated NUS 27−29 NSs were mixed and sonicated for 30 min at room temperature, and the resultant NUS 27−29 NSs@ZIF-8 composites were collected by centrifugal separation (see synthetic details in Supporting Information). The loading of these 2D NSs in the composites was corroborated by energy-dispersive X-ray spectroscopy (EDX) analyses (Figures S17−S20), indicating that trace amounts of the NSs have been loaded. Consequently, the chemical nature of the composites is largely derived from the ZIF-8 coating. FT-IR spectra (Figure S21), UV−vis spectra (Figure S22), and XPS spectra (Figure S23) indicate that NUS 27−29 NSs@ ZIF-8 possess nearly the same chemical compositions as that of pure ZIF-8. A strong evidence to confirm the loading of 2D NSs in the composites is the change of optical band gaps. For example, the band gap of pure ZIF-8 was calculated to be 5.35 eV. On the contrary, the band gaps of the composites decrease to 5.30 eV for NUS-27 NSs@ZIF-8, 5.27 eV for NUS-28 NSs@ZIF-8, and 5.25 eV for NUS-29 NSs@ZIF-8 (Figure S24). The PXRD patterns of NUS 27−29 NSs@ZIF-8 show that the crystallinity and structure of ZIF-8 are well maintained upon compositing with the ultrathin 2D NSs (Figure 4l). This successful ZIF-8 compositing is further verified by HR-TEM characterization (Figures S25−S27), where the measured lattice fringe spacing of 2.6−3.0 Å in the NUS 27−29 NSs@ ZIF-8 is originated from the (044) crystal plane of the ZIF-8 structure. Furthermore, 77 K N2 sorption tests indicate that the porosity of ZIF-8 is also well maintained in NUS 27−29 NSs@ ZIF-8 (Figure 4m). FE-SEM images show that the NUS 27− 29 NSs@ZIF-8 composites consist of isolated large bulk particles with diameters of several micrometers (Figure 4b−d). In addition, the particle size of these composites is larger than that of pure ZIF-8, and the size distribution broadens as shown by dynamic light scattering (DLS, Figure S28). In order to obtain direct evidence for the NSs-containing composites, we conducted an optical microscopy investigation, which has been used to confirm the encapsulation in other ZIF-8 composites.29 While the pure ZIF-8 crystals did not exhibit obvious fluorescence emission under ultraviolet light (λex = 365 nm, Figure S29), strong green fluorescence was observed in NUS 27−29 NSs@ZIF-8 composites, confirming the successful encapsulation of fluorescent 2D NSs (Figure 4e−g). 3D confocal laser scanning microscopy (CLSM) was further performed on large composite particles to assess the homogeneity of the encapsulation of NSs (Figures S30−S32). As shown in Figure 4h,i, the 3D CLSM images of NUS 27−29 NSs@ZIF-8 large particles show even green fluorescence, suggesting homogeneous distribution of NUS 27−29 NSs in 4904


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Figure 6. Constructed models of NUS-27 (a), NUS-28 (b), and NUS-29 (c) for the calculation of energy barriers of TPE rotors. The theoretical calculations of energy barriers of phenyl rotors in NUS-27 (d), NUS-28 (e), and NUS-29 (f) models. The constructed models of (100) plane (g) and (111) plane (h) of ZIF-8 with NUS-28 fragment after MD simulation. (i) Theoretical calculation of potential energy of phenyl rotor in NUS28@ZIF-8 of the two different planes. TRPL decay curves of NUS-28 NSs (j) and NUS-28 NSs@ZIF-8 (k) in methanol solutions at room temperature (λex = 360 nm). (l) TGA curves of ZIF-8 and NSs@ZIF-8 composites.

NSs. Furthermore, the dynamic molecular rotor in these 2D NSs can be further verified by size-dependent fluorescence emission assay.14,37 As shown in Figure 5f, we can clearly observe the magnified turn-on behavior with the increase of VOC size (Figure S41). The relative intensities (Imesitylene/ Ibenzene) of NUS 27−29 NSs are 1.55, 2.37, and 1.84, respectively, indicating the size-dependent restriction of the dynamic behavior of TPE rotors. However, the AIE characteristic almost fully disappears in their ZIF-8-containing composites (Figure S40). There are only 1.02-fold, 1.07-fold, and 1.07-fold increases in the fluorescent emission than that at f w of 0% for NUS 27−29 NSs@ZIF-8 (Figure 5e), proving that

the mixed solvent of methanol (good solvent) and water (poor solvent). Initial investigations indicate that the DTDE monomer has a typical AIE characteristic in methanol and water mixtures (Figure S39). The fluorescence emission shows an abrupt enhancement when the water fractions (f w) in the mixed solvent increases to 60%, while the highest fluorescence intensity is obtained at f w of 90%. Interestingly, we found that the emission of NUS 27−29 NSs is still 1.33-fold, 1.52-fold, and 1.34-fold higher than that at f w of 0% (Figure 5e and Figure S40). Although the AIE characteristic of NUS 27−29 NSs is weaker than that of the monomer, some dynamic behavior still exists to allow weak AIE activation in the 2D 4905


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lifetime of NUS-28 NSs@ZIF-8 is extended approximate 2.5fold over free-standing NUS-28 NSs (834 vs 339 ps, Figure 6j,k). Importantly, we found that the degree of change in decay time appears to be monomer dependent. NUS-28 NSs@ZIF-8 has the highest degree of change relative to the freestanding NSs among the three composites (2.5 vs 1.1 vs 1.1). The increased flexibility of the TBA monomer in NUS-28 compared to TBB and TBPB monomers may cause such an increased dynamic behavior in NUS-28 NSs. Accordingly, when the ZIF-8 coating was applied, the restriction of the flexible monomers led to the most pronounced effect on lifetime decay. Size-Selective Protection of Fluorescence for NUS 27−29 NSs@ZIF-8. While the AIE molecular rotors of NUS 27−29 NSs encountered different degrees of restriction by benzene and mesitylene, the same discrimination effect was not present in the NUS 27−29 NSs@ZIF-8 composites (Figure S43). The I(mesitylene)/I(benzene) relative intensities of NUS 27− 29 NSs@ZIF-8 are only 0.88, 1.0, and 0.95, respectively, which are much smaller than that of their freestanding 2D NSs. This is probably due to the large molecular sizes of benzene and mesitylene, which are larger than the window size of ZIF-8 coating (3.4 Å) so that they cannot get access to the NSs. To prove our hypothesis further, we chose geometric butanol isomers, namely n-butanol (6.5 Å × 2.2 Å), i-butanol (5.7 Å × 3.1 Å), and t-butanol (4.4 Å × 3.6 Å) to investigate the potential size-selective molecular sensing capabilities of NUS 27−29 NSs@ZIF-8 (Figure S44a). It is known that separation and recognition of the isomers, in particular n-butanol, is highly challenging due to their similar chemical and physical properties. To compare the ability of the composites in discriminating the three isomers, we defined two relative intensities using the fluorescence response of the material to nbutanol as a reference. Specifically, IT and II are defined as IT = It‑butanol/In‑butanol and II = Ii‑butanol/In‑butanol, respectively. The molecular sensing experiments carried out using the freestanding NUS 27−29 NSs (Figure S44b,c and Figure S45) showed that the emission intensities in t-butanol are higher than those in n-butanol and i-butanol. The trend of sizedependent turn-on fluorescence for the butanol isomers is the same as that of the above-mentioned VOC experiments. Upon compositing with ZIF-8, IT and II values increase relative to the freestanding 2D NSs (Figure S46). In the best-performing composite (NUS-28 NSs@ZIF-8), the IT and II are 1.54 and 1.20, respectively, compared to 1.27 and 0.88 for the freestanding 2D NSs (Figure S47). Similar results can also be observed in NUS-27 and NUS-29 composites. Thus, a distinct enhancement in size-selective molecular recognition of geometric isomers was achieved by the MOF coating approach. Based on the above results, the limited window size of the ZIF-8 coating may lead to new strategy in the protection of fluorescence of NUS 27−29 NSs in the MOF composites by size-selective effect, as only those molecules with sizes smaller or comparable to the aperture size of ZIF-8 may diffuse through and interact with the fluorescent 2D NSs core. Therefore, the applicability of size-selective effect to protect the fluorescence of the 2D NSs for the potential applications of in vitro cancer cell imaging was evaluated using the composites. We chose NUS-28-NSs@ZIF-8 as the candidate material due to its abundant N species which are beneficial for electron or energy transfer based on hydrogen-bonding interactions. Five amino acids with different molecular sizes, namely L-alanine

the ZIF-8 coating is able to restrict the dynamic molecular rotors exposed on the surface of these 2D NSs. Theoretical Calculations. To have a better understanding on the dynamic behavior of the molecular rotors in freestanding NUS 27−29 NSs and their ZIF-8-containing composites, molecular dynamics (MD) simulations were conducted to estimate the rotational energy barriers and provide microscopic insights into the noncovalent restrictions of the AIE molecular rotors in the composites (see simulation details in Supporting Information). We thus constructed the molecular models of NUS 27−29 to examine the energy barriers for phenyl ring rotors (Figure 6a−c). Figure 6d−f shows the potential energy of one phenyl ring within the hexagonal pore models as a function of rotor angle varying from 0° to 360° with an interval of 15°. Two stable conformations were observed at ca. 45° and 225°. On the contrary, less favorable conformations could be observed at ca. 135° and 315° with relative higher energy barriers. The rotation energy barriers were respectively calculated to be 6.9 kcal mol−1 in NUS-27, 7.3 kcal mol−1 in NUS-28, and 10.0 kcal mol−1 in NUS-29, which are lower than that of the partially restricted molecular rotors in the smaller triangular pores of NUS-25 (12.1 kcal mol−1).15 The MD simulation results indicate the presence of dynamic AIE molecular rotors in NUS 27−29 NSs, which is well consistent with the above experimental results. Notably, the calculated energy barriers of phenyl rings in the NUS 27−29 are similar to other phenyl ring rotor systems which are also confined in porous materials such as MOFs (11.3 kcal mol−1),49 porous organic frameworks (6.7 kcal mol−1),50 and mesoporous organosilica (9.6 kcal mol−1).51 In order to understand the confinement effect of phenyl ring rotors in the composites, we further constructed two composite models for (100) and (111) planes of ZIF-8 with NUS-28 NSs. MD simulation was performed to derive the binding energies of the two models. For the (100) and (111) planes of ZIF-8, binding energies with NUS-28 were calculated to be −18.81 and −11.33 kcal mol−1, respectively. There are short distances of 3.6 and 4.2 Å between the center N atom of NUS-28 and Zn of ZIF-8 in the optimized structures (Figure 6g−h), indicating the possible strong interactions. Meanwhile, hydrophobic supramolecular interactions such as CH−π (2.7 Å − 2.8 Å) between NUS-28 and ZIF-8 can also contribute to the strong binding in the composites (Figure 6g−h). This is verified by TGA results, showing that the composites decompose at higher temperatures compared to pure ZIF-8 (Figure 6l). For example, the NUS-28 NSs@ZIF-8 composite is stable to around 460 °C, which is higher than that of ZIF-8 (400 °C). In light of the increased level of molecular interactions, we sought to determine the rotational energy barriers of the phenyl rings to rationalize the change in AIE effect. For the model of (100) plane of ZIF-8 with NUS-28, the potential energy of the phenyl ring is 4.3 kcal mol−1 when the dihedral angle is 30°, which is similar to that of free NUS28 (4.6 kcal mol−1). However, when the dihedral angle increases to 45°, the potential energy rapidly rises to 46.4 kcal mol−1 (Figure 6i), which is over 8-fold higher than that of free NUS-28 NSs (5.6 kcal mol−1), suggesting greatly hindered rotation caused by compositing with ZIF-8. Hence, the simulation results are consistent with the experimentally observed restriction of phenyl ring rotors of NUS-28 in the composites. Moreover, time-resolved photoluminescence measurements (TRPL) indicate significantly increased lifetimes for the composites (Figure S42). For example, the 4906


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Figure 7. (a) Chemical structures and molecular sizes of amino acid guest molecules being tested in this study. (b) Fluorescence emission spectra of NUS-28 NSs (c = 20 μg mL−1) in different amino acid solutions (1 × 10−3 M) at room temperature (λex = 365 nm). (c) Optimized host−guest configuration and binding energy of NUS-28 fragment with L-alanine based on MD simulation. (d) Fluorescence emission spectra of NUS-28 NSs@ZIF-8 composites (c = 0.5 mg mL−1) in different amino acid solutions at room temperature (λex = 365 nm). (e) QP of NUS-28 NSs and NUS-28 NSs@ZIF-8 by the different amino acid guest molecules.

(5.5 Å × 3.3 Å), L-threonine (6.8 Å × 4.2 Å), L-cysteine (6.9 Å × 3.3 Å), L-phenylalanine (9.5 Å × 4.4 Å), and L-tryptophan (9.9 Å × 6.0 Å), were chosen and tested because they may exist in living cells and have the similar functional groups (NH or COOH) with the peptides and proteins (Figure 7a). As shown in Figure 7b, the fluorescence of NUS-28 NSs can be easily quenched by these amino acids. For example, the quenching percentages [QP, QP = (I0 − I)/I0)] were calculated to be 24.5%, 29.8%, 20.1%, 43.7%, and 37.6% for L-alanine, L-cysteine, L-threonine, L-phenylalanine, and Ltryptophan, respectively. Furthermore, we found that the emission peak of NUS-28 NSs has a red-shift of around 5 nm after interacting with these amino acids, which is probably caused by electron or energy transfer in the host−guest system.

To gain more insight into host−guest recognition of NUS-28 NSs, we performed the DFT calculation for the electronic and optical properties of the NUS-28 model fragment. As shown in Figure S48, we can see that the lowest unoccupied molecular orbitals are mainly delocalized on the TPE backbone. In addition, the calculated bandgap (2.27 eV) matches with the experimental value of 2.64 eV, indicating that NUS-28 is a semiconducting porous polymer, which can transfer electron or energy to the guest molecules. In order to understand the host−guest interactions between NUS-28 NSs and amino acids further, the binding energies of L-alanine@NUS-28 NSs were calculated by MD simulations. The results show that there is a −15.44 kcal mol−1 binding energy for L-alanine@NUS-28 NSs (Figure 7c), suggesting a strong binding affinity within the 4907


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Figure 8. Contact angle measurements of NUS-27 bulk powder (a), NUS-28 bulk powder (b), NUS-29 bulk powder (c), NUS-27 NSs@ZIF-8 (d), NUS-28 NSs@ZIF-8 (e), and NUS-29 NSs@ZIF-8 (f). Cytotoxicity assay of NUS 27−29 NSs and NUS 27−29 NSs@ZIF-8 for NIH-3T3 cells (g) and HeLa cells (h).

host−guest system for possible electron or energy transfer.52,53 As expected, NUS-28 NSs@ZIF-8 composites display a very limited quenching degree under the same conditions. The QP are 5.9%, 1.9%, 2.4%, 10.9%, and 8.2% for L-alanine, Lcysteine, L-threonine, L-phenylalanine, and L-tryptophan (Figure 7e), respectively, and we cannot observe any redshift in the emission spectra of the composites (Figure 7d). These results indicate that the fluorescence of ultrathin 2D NSs can be effectively protected after coating with ZIF-8, leading to highly emissive and stable composites. Live-Cell Imaging by 2D NSs@ZIF-8. Besides stabilization of 2D NSs and protection of their fluorescence by coating with the biocompatible ZIF-8, other advantages prompted us to attempt the live cancer cell imaging using the 2D NSs@ZIF8 composites. (1) The contact angle measurement showed that NUS 27−29 bulk powders exhibit hydrophobic characteristic (Figure 8a−c), which directly prevents the 2D NSs from penetrating into the live cancer cell for bioimaging due to the serious aggregation in aqueous media. On the contrary, their 2D NSs@ZIF-8 composites exhibit a more hydrophilic characteristic (Figure 8d−f). The transform from hydrophobic to hydrophilic characteristic induced by ZIF-8 coating would be an effective approach to overcome the self-aggregation problem. (2) Cytotoxicity assay showed that these 2D NSs have no toxicity for NIH-3T3 cells and HeLa cells (Figure 8g,h). In addition, no obvious in vitro toxicity was observed for the composites. For example, at a 200 μM loading concentration of 2D NSs@ZIF-8, the cell viability is around

80% (Figure 8g,h). The low toxicity is the necessary prerequisite for in vitro cell imaging. (3) We also tested the photophysical properties of NUS 27−29 NSs@ZIF-8 composites in water conditions including fluorescence emission and UV−vis absorbance (Figure S49). The quantum yields are 27.0%, 23.2%, and 22.1% for NUS 27−29 NSs@ZIF8, respectively, which inspire us to explore the cancer cell imaging using these fluorescent biocompatible MOF composites. Taking all the above-mentioned features, NUS 27−29 NSs@ ZIF-8 composites could potentially be good candidates for livecell imaging. To explore this possibility, 2D NSs and 2D NSs@ ZIF-8 composites were incubated with HeLa, MCF-7, 231, and NIH-3T3 cell lines at 100 μg mL−1 for 24 h, followed by imaging with a confocal microscope. During this process, we used the same amount of 2D NSs under the same condition for comparison. Although poor colocalization of NUS 27−29 NSs@ZIF-8 in HeLa cells was found (Figure S50), living cancer cell imaging using these ZIF composites can be easily achieved compared with their pure 2D NSs. First, we found that pure NUS 27−29 NSs could barely enter either HeLa (Figure 9a−c and Figure S51) or MCF-7 cells (Figure 9g−i and Figure S51). For NUS-28 NSs, we can hardly observe any cell imaging in either HeLa cells (Figure 9b) or MCF-7 cells (Figure 9h). On the contrary, we can clearly observe their cell imaging using NUS-28 NSs@ZIF-8 composites (Figure 9e for HeLa cells and Figure 9k for MCF-7 cells). The acquired images manifest bright green emission in the cytoplasm of the 4908


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Figure 9. HeLa cell confocal microscopy imaging using NUS-27 NSs (a), NUS-28 NSs (b), NUS-29 NSs (c), NUS-27 NSs@ZIF-8 (d), NUS-28 NSs@ZIF-8 (e), and NUS-29 NSs@ZIF-8 (f). MCF-7 cell confocal microscopy imaging using NUS-27 NSs (g), NUS-28 NSs (h), NUS-29 NSs (i), NUS-27 NSs@ZIF-8 (j), NUS-28 NSs@ZIF-8 (k), and NUS-29 NSs@ZIF-8 (l). All scale bars are 25 μm.

cells without changing their morphologies. Furthermore, such similar results are also observed in NUS-27 NSs@ZIF-8 (Figure 9d,j, and Figures S52 and S53) and NUS-29 NSs@ ZIF-8 (Figure 9f,l, Figures S52 and S53), further demonstrating that the MOF encapsulation strategy is able to transport ultrathin 2D NSs into the living cancer cells for bioimaging.

Second, we also compare the cell imaging behavior among different kinds of cells using the same material under the same conditions. Here we also choose NUS-28 NSs@ZIF-8 as an example. The confocal images showed that the cell imaging of cancer cells (including HeLa cells, MCF-7 cells, and 231 cells, Figure 9e,k, Figure S54d) are slightly better than that of the 4909


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Chemistry of Materials Notes

normal NIH-3T3 cells (Figure S55b). Such higher cellular uptakes in cancer cells than normal NIH 3T3 cells are also observed in NUS-27 NSs@ZIF-8 and NUS-29 NSs@ZIF-8 (Figure 9, and Figures S52−S55). Third, we further compare the three ZIF composites under the same cell imaging. We choose HeLa cells as an example. As shown in Figure 9d−f, we can see that they have a similar behavior in cell imaging, as well as for MCF-7 cells (Figure 9j−l). This successful cell imaging could be attributed to the biocompatible feature of the composites caused by ZIF-8 coating. These contrast results evidently demonstrate that the strategy of biocompatible MOF encapsulation can effectively transport hydrophobic 2D organic NSs into live cells for in vitro biological applications.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation Singapore (NRF2018-NRF-ANR007 POCEMON), the Ministry of Education−Singapore (MOE AcRF Tier 1 R-279-000-540-114, R-279-000-482-133), the Agency for Science, Technology and Research (PSF 1521200078, IRG A1783c0015, and IAF-PP A1789a0024), National Natural Science Foundation of China (No. 21676094 and 21576058), Fundamental Research Funds for the Central Universities (2017MS083), and the technical support from National Supercomputer Center in Guangzhou (Tianhe-2). We thank Ms. Fengyin Wu for drawing liposome pictures (hemispherical hollow sphere) in Figure 1,.

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CONCLUSION In summary, we have synthesized three 2D layered πconjugated aromatic polymers (NUS 27−29) containing flexible TPE moieties as AIE molecular motors. NUS 27−29 bulk powders can be easily exfoliated into ultrathin few-layered 2D NSs with thicknesses of 2−5 nm. For the first time, a versatile approach based on in situ biocompatible MOF encapsulation was applied to prepare composites containing ultrathin 2D NSs (2D NSs@ZIF-8). This approach leads to stabilization of the exfoliated 2D NSs while enabling the nearcomplete restriction of the dynamic AIE molecular rotors of these 2D NSs as elucidated by extensive experimental studies and theoretical calculations, resulting in high stability and porosity, as well as highly emissive MOF composites. Moreover, the limited window size of the ZIF-8 coating can effectively protect the fluorescence of NUS 27−29 NSs by a size-selective effect, as well as change the hydrophobic characteristic of NUS 27−29 NSs. Eventually, we demonstrate that these 2D NSs@ZIF-8 composites with highly fluorescent nature can penetrate into the cancer cells for bioimaging application. This study sheds light on the further development of biocompatible MOF composites containing ultrathin 2D organic nanomaterials for various biological applications.

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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/acs.chemmater.9b01642.

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REFERENCES

Experimental details on the synthesis and characterization of NUS 27−29 bulk powders, ultrathin freestanding 2D NSs and NUS 27−29 NSs@ZIF-8 composites, size-selective molecular recognition experiments, live-cell imaging, MD simulations and DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.Z.). *E-mail: [email protected] (B.L.). ORCID

Zhiwei Qiao: 0000-0002-1264-3762 Bin Liu: 0000-0002-0956-2777 Dan Zhao: 0000-0002-4427-2150 Author Contributions §

J. Dong, Z. Qiao, and Y. Pan contributed equally to this work. 4910


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