Graphene Quantum Dots in the Game of Directing Polymer Self

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Graphene Quantum Dots in the Game of Directing Polymer SelfAssembly to Exotic Kagomé Lattice and Janus Nanostructures Adeeba Shakeel, Rohan Bhattacharya, Sampathkumar Jeevanandham, Dakshi Kochhar, Aarti Singh, Lalita Mehra, Maryam Ghufran, Piyush Garg, Sujata Sangam, Subhrajit Biswas, Amit Tyagi, Dinesh Kalyanasundaram, Sandip Chakrabarti, and Monalisa Mukherjee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04188 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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Graphene Quantum Dots in the Game of Directing Polymer Self-Assembly to Exotic Kagomé Lattice and Janus Nanostructures Adeeba Shakeel†, Rohan Bhattacharya†‡∆, Sampathkumar Jeevanandham‡┴∆, Dakshi Kochhar†∆, Aarti Singh‡∂, Lalita Mehra¶∂, Maryam Ghufran¥, Piyush Garg†, Sujata Sangam†, Subhrajit Biswas¥, Amit Tyagi¶, Dinesh Kalyanasundaram§, Sandip Chakrabarti┴, Monalisa Mukherjee†‡*

†Amity

Institute of Biotechnology, Amity University, Noida, 201303, India

‡Amity

Institute of Click Chemistry Research and Studies, Amity University, Noida, 201303,

India ┴Amity

Institute of Nanotechnology, Amity University, Noida, 201303, India

¶Institute

of Nuclear Medicine and Allied Sciences, Defence Research & Development Organisation, Timarpur, Delhi, 110054, India

¥Amity

Institute of Molecular Medicine and Stem Cell Research, Amity University, Noida, 201303, India

§Centre

for Biomedical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India

*Corresponding Author

: [email protected] 

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ABSTRACT

Graphene Quantum Dots (GQDs) are the harbingers of paradigm shift that revitalize selfassembly of the colloidal puzzle by adding shape and size to the materials-design palette. Although self-assembly is ubiquitous in nature, the extent to which these molecular legos can be engineered reminds us that we are still apprenticing polymer carpenters. In this quest to unlock exotic nanostructures ascending from eventual anisotropy, we have utilized different concentrations of GQDs as a filler in free radical-mediated aqueous copolymerization. Extensive polymer grafting over the geometrically confined landscape of GQDs (0.05%) bolsters crystallization instilling a loom which steers interaction of polymeric cilia into interlaced equilateral triangles with high sophistication. Such two dimensional (2D) assemblies epitomizing the planar tiling of ‘Star of David’ forming molecular kagomé lattice (KL) without metal templation evoke petrichor. Interestingly, a higher percentage (0.3%) of GQDs allow selective tuning of the interfacial property of copolymers breaking symmetry due to surface energy incongruity, producing exotic Janus Nanomicelles (JNMs). Herein, with the help of a suite of characterizations, we delineate the mechanism behind the formation of KL and JNMs which forms a depot of heightened drug accretion with targeted delivery of 5-Fluorouracil in the colon as validated by gamma-scintigraphy study.

KEYWORDS graphene quantum dots, exotic nanostructures, kagomé lattice, janus nanomicelles, gamma scintigraphy


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The rising stellar — ‘Graphene’1 in the frontiers of materials science research has already surpassed its zenith, however, the innate simplicity of each sub lattice represented by a single carbon atom in the unit cell minimises structural modification possibilities.2–4 Increase in the technological demands for sophisticated materials with state-of-the-art properties, their zerodimensional (0D) equivalents Graphene Quantum Dots (GQDs) have emerged as imperative functional material in a myriad of applications owing to their nontoxicity,5 good solubility,6 and better surface grafting.7 Facile control over the spatial arrangement of nanoscopic building blocks has been an obstacle in the bottom-up synthesis of functional materials. Acquiring control using additives to direct the distribution and order within local environment of nanostructures is superlative for designing responsive functional nanocomposites.8 The versatility of block copolymers (BCPs) to form exotic nanostructures9–11 with precisely controlled size, orientation and morphology was dominated by esoteric debate on materials community canvases12 which sowed the seeds for understanding structure-function relationships in synthetic polymers. In order to make ‘design’ more of science and less of art, ensembles of polymeric materials have been fabricated directing self-assembly and engineering defects, albeit exists high level of complexity with illegitimate structural precision where independent control of polymer remains an unmet challenge.13,14 However, regardless of the physical interaction utilized for realizing complex multilevel self-assemblies, there remains a growing bottleneck of precision assembly while designing well-ordered nanostructures across the length scales.15 There has been a concerted effort towards achieving well-ordered nanostructures within sub-10 nm range,16 yet there still exists a relative lack of directionality stemming from structure multiplicity, lack of control and complex low symmetry topologies17,18 which demand the knowledge of target blueprint. Judicious fabrication of exotic nanostructures from BCPs is the game of ‘mix and


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wait’ often blighting bulk properties such as processability, swelling and release kinetics, impeding conformational flexibility. Materials fabricated with precisely controlled anisotropy, orientation, topology, molecular functionality, and hierarchical self-assembly are still at their infancy in polymeric materials where a versatile toolbox is indispensable to sail across through these impediments. Low dimensional filler materials, with their inherently large surface-tovolume ratio, are particularly appealing to overcome these challenges as they are known to facilitate an augmented efficiency of innate properties.19 From a structural standpoint, nanoparticles have been paving the way to engineer polymer matrices.20–22 We demonstrate the potential to produce molecularly distinct structural conformations by using different concentrations of GQDs as a filler in free radical mediated aqueous copolymerization with Acrylic acid (AAc) and 2-(Diethylamino)ethyl methacrylate (DEAEMA) without chemically modifying either of the building blocks or filler. Polymer chain grafting at the molecular level has been a desirable task to form exotic nanostructures, however, till date, it has been only achieved by templation with metals23 and annealing in presence of organic solvents24 or first principle calculations.25 At a lower concentration (0.05%), our GQDs act as molecular legos that possess a construction plan allowing autonomous and spontaneous polymer grafting onto their landscape by instilling a molecular loom which weaves an elusive net with interlaced topology to produce single layer crystalline molecular array of long-range ordered Kagomé lattices (KL). To the best of our knowledge, this is the only report mentioning production of trihexagonal tiling26,27 via facile, metal-free, one-step free radical mediated aqueous copolymerisation where GQDs as a filler judiciously directs structural and functional information through DEAEMA grafting at a specific


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target forming self assembled exotic network where each side of a regular hexagon shares an equilateral triangle epitomising the planar tiling of ‘Star of David’. (Scheme 1) Interestingly, BCPs in the presence of a slightly higher concentration of GQDs (0.3%) behave in a different manner and self-assemble towards programmable micelles of well-defined architecture and undergo large structural modifications to produce exotic coacervate Janus Nanomicelles (JNMs). (Scheme 1) Numerous synthetic strategies, for the fabrication of JNMs, have been blossoming throughout the past decade including electrodynamic co-jetting,28 Pickering emulsion interfacial synthesis,29 biphasic electrified jetting,28 olefin metathesis,30 and emulsion polymerisation31; despite technological advancements, these are either tedious, complicated, limited to large diameter Janus particles, or suitable only for conductive polymers. Efforts to manufacture Janus micelles strictly via self-assembly have resulted in multicompartment micelles32 which necessitates transformation using organic solvents,33 restricting their integration in the biological sector. GQDs help in targeting complex self-assembled architecture by adding shape and interaction anisotropy34 to the BCPs controlling their spatial distribution at the nano-scale28 forming soft, homogenous JNMs. The entropic elasticity of the polymer network bestowed with stimuli responsiveness35 extends substantially beyond the promise of homogenous particles. Endowed with tuneable physical properties, these soft materials32 demonstrated excellent swelling and release kinetics, enhanced drug retention as well as targeted delivery of drug in the intestine of rabbits, avoiding gastric degradation eliminating phobia and pain associated with intravenous chemotherapeutics. RESULTS AND DISCUSSION Self-assembling nature of amphiphilic polymers provide a facile way for obtaining well-defined hierarchical superstructures dictated by the balance of hydrophobic attractions and coloumbic


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repulsions at varying length scales via nanostructuring. Polymerization was carried out by free radical mediated aqueous copolymerization (Table S1) (Scheme S1). A polyelectrolytic complex of DEAEMA and AAc was formed via electrolytic interactions and H-bonding which further self-assembled into crosslinked hydrophobic core comprising of p(DEAEMA-co-AAc) surrounded by the hydrophilic corona embracing the remaining PAAc. This orchestrates the formation of most energetically favored centrosymmetric core-shell polymeric micelles. As evident from the TEM images (Figure 1A, B), micelles of different sizes were obtained owing to the formation of aggregates by the amphiphilic polymeric structures. PAAc encompassing short chains form a thin hydrophilic shell as validated by the low intensity (O—C=O) and (C=O) peaks in the deconvoluted C(1s) spectra of pAcD at 289 eV and 287.3 eV respectively (Figure S1). As a result, the hydrophobic and van der Waals interactions between the hydrophobic core dominates over the repulsive forces of the hydrophilic segments, ultimately, leading to aggregation.36,37 High compressive modulus and fracture strength of pAcD indicates a higher crosslinked network density supported by the synergistic effects of covalent bonding as well as non-covalent interactions (Figure S2) (Table S2). Moreover, the low values of tan δ in pAcD clearly affirm minimal imperfections in the cross-linked network (Figure 2B). GQDs were synthesized as earlier reported by our group.6 Excellent solubility of GQDs (0.05%) in the water proved rewarding where the abundant phenolic O—H groups de-protonate increasing hydrophobia enhancing interfacial linkages overcoming energy penalty. Absence of overlapping domains and rotational stacking faults which are intrinsic to turbostratic graphite were observed as distinct hexagonal spots in the SAED pattern pointing towards single crystalline GQDs with well-maintained integrity and uniformity of the lattice structure.6 As GQDs are introduced in the medium, its functional groups such as O—H, —COOH, —SO3H,


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de-protonates and forms electrolytic complex with N+ (Tertiary amine) of DEAEMA which is corroborated from the intense peak centred at 402.8 eV (C—N+) and 404.7 eV (SO3-N+) in the deconvoluted N(1s) spectra of KL (Figure 3A, C) as well as the sharp peak centred at ∼3434 cm-1 in the FTIR spectra of KL. (Figure S3) (SI1) With the aim of harnessing intelligent macromolecular design, we added AAc to lay the foundation of multi-electrolyte complexation where AAc interacts with the N+ of remaining DEAEMA, affecting polarity contrast in the aqueous milieu. Addition of APS and TEMED resulted in the formation of free radicals of AAc and DEAEMA, the latter being more stabilized by the inductive effect of CH3 group facilitates the reaction with GQDs compared to AAc˙ which is in good agreement with the higher binding energy shift of C—C/C—H component in the deconvulated C(1s) spectra of KL (287.5 eV) (Figure 3B) compared to pAcD (285 eV) (Figure S1A, B). The free radical wreaks havoc on the GQDs perturbing the delocalization of electrons. The nanoscale strain induced local corrugation facilitates grafting of PDEAEMA chains on GQDs due to their in-plane polarisability38 via geometric matching to form highly oriented structure owing to van der Waals interaction between alkyl groups and GQDs. This has been validated by the Raman spectra where the small sharp peak appearing at 1353 cm-1 in KL corresponds to D band of disrupted sp2 domains of GQDs arising from the distorted framework and obliterating the G band (Figure 4A). Moreover, the absence of (002) peak in XRD spectra of KL further confirms our hypothesis (Figure S4). These linear PDEAEMA chains serve as macro-initiators to initiate the graft copolymerization of DEAEMA and AAc, leading to branched P(DEAEMA-co-AAc) side chains along the edges of GQDs. Geometric confinement of GQDs enhances nucleation process encouraging intimate polymer-nanomaterial interaction augmenting polymer grafting density and steering a


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conformational change of the tethered polymer. This further aids genesis of nascent crystal embryo on GQDs surface, maneuvering crystallization of melt39 that forms the independent isolated ciliary structure on disrupted GQDs. These crystal embryos further promote polymer attachment to GQDs landscape, polynomially boosting crystallization kinetics heterogeneously by reducing nucleation activation energy due to their high aspect ratio, leading to the formation of well ordered crystalline cilia (SI 1). This has been validated by the drastic changes in the chemical shift observed in the deconvoluted C(1s) spectra including (C—C/C—H), (C—N/C— O), and (C=O) from 285 eV, 286.07 eV and 287.3 eV in pAcD (Figure S1B) to 287.5 eV, 286.2 eV and 288.8 eV respectively in KL (Figure 3B). The peak at 402.8 eV in the deconvoluted N(1s) spectra corresponds to C—N+ of KL indicating effective grafting of PDEAEMA chains on the surface of GQDs (Figure 3C). Similar results were observed in Raman spectroscopy where the appearance of a sharp peak at ~1296 cm-1 in KL testifies the existence of crystalline alkyl chains grafted on GQDs (Figure 4). The sharp and intense Raman bands at ~2946 cm-1 and ~2933 cm-1 in KL underpins considerable conformational reorganization manifesting high crystallinity and maximum polarisation9 (Figure 4). We envisage, the ciliated structure continuously grows in size reducing the distance between the adjacent tethering points, where overlaying of adjoining polymer chains becomes conspicuous. The stretching of polymer chains orthogonal to GQDs lay the cornerstone of interdigitated packing among its nearest neighbor chains forming an interlinked reticular network. Possibly, the entropic drive for polymer attachment and growth is counterbalanced by the energetic drive for interdigitated packing to crystallize, reducing the induction period and conformational entropy of the polymer chains. Electrolytic and hydrophobic interactions between the grafted chains impart bending rigidity which leads to the formation of triangular


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structure over the hexagonal architecture of graphene mimicking exotic nets. Such interactions self-corrects the imperfectly aligned bonds where kinetically formed intermediate defects transform to favored structure, tessellating40 to form a single domain 2D molecular Kagomé lattice (KL) with long-range order (Figure 1C - I).25 Furthermore, a sharp peak at 14.6˚ 2θ corresponding to (010) plane emphasizes a well-ordered structure with extremely narrow FWHM (Figure S4). Our KL demonstrates high stability despite the existence of 60˚ bond angle in the interlacing triangles enclosing quasi-hexagonal nanocavities27 (Figure S5) giving a bifurcated sharp Raman band at ~2982 cm-1 which can be explained by resonance transition dipole interaction (Figure 4A).41 The increase in d-spacing from 0.21 nm to 0.26 nm fortifies honeycomb6 to kagomé transformation aided by the polymer.38 The intense C=O band in the C(1s) spectra centered at 288.8 eV (Figure 3B) and C=O/O—C=O component at 533.33 eV in O(1s) spectrum of KL (Figure 3D) indicates that the polymer chains are well exposed in the basketry framework. We believe, the strain induced crossover geometry of the entwining threads in KL creates an extra pressure on the element thereby increasing the binding energy in XPS compared to GQDs and pAcD, as well as broadening the symmetric and asymmetric stretching vibration mode of -CH2groups in the Raman spectra (Figure 4A).42 In presence of 0.3 (w/v) % GQDs, the —OH, —SO3H and —COOH moieties de-protonate which self assembles to form J-type aggregates minimizing hydrophobia.6 As DEAEMA and AAc monomers are introduced in the media, the change in interfacial tension between water, the electrolytic complex of DEAEMA- J aggregates (EC-1) and DEAEMA-AAc (EC-2), along with rich PAAc domains impact thermodynamics and kinetics of self-assembly favoring majorly micellar assemblage enthalpically.43 The immiscibility of EC-1 and EC-2 eventually induces


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large internal strain driving intramicellar nano-phase separation within the nano-regime. Addition of APS and TEMED in the reaction mixture bolsters in-plane and out-plane polymerization of PDEAEMA on J aggregates with minimal entanglements forming a well exposed hairy architecture sculpting the C—N/C—O component at 287.75 eV and C—N+ at 402.5 eV in the C(1s) and N(1s) spectra of JNMs respectively (Figure 3F, G). J aggregates enable selective tuning of the interfacial properties of block copolymer by localizing themselves at the desired location forming a hemispherical cap (Figure 1J - L). The apical portion of JNMs appears dark due to the high electron density on J aggregates. Under monomer starvation of DEAEMA, AAc unimers repel Poly EC-1 due to the presence of negative charges, stretching itself towards the periphery of the micelles producing exotic coacervate Janus nanomicelles (JNMs). Meanwhile, the EC-2 remains intact at the interface of the two hemispheres maintaining the polar symmetry whose amphiphilicity screens the electrostatic repulsion between like charges. The solubility of PAAc in water permanently fixates the phase separated state which controls the Janus balance.32 The surface energy incongruity between the phase-separated hemispheres arising from differential hydrophilia and hydrophobia leads to homogenous mixing of the two phases at the interface producing noncentrosymmetric JNMs.44 This results in the reduction of C=O component in the C(1s) spectra at 286.5 eV of JNMs (Figure 3F) which is in stark contrast to KL (Figure 3B) and pAcD (Figure S1B). The competence of our GQDs is greatly amplified due to their single crystallinity, nano-sized dimension and high aspect ratio.6 This steers a partial change in crystal orientation of J aggregates from graphene hexagonal (002) phase to cyclohexane monoclinic (011) at 20.6˚ 2θ and (111) at 24.6˚ 2θ owing to grafting assisted reduction of symmetry in J aggregates. The


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graphitic broad peak at 26.6˚ 2θ (002) arising from the J aggregates suggests that the graphene layers are exfoliated (Figure S4).45 Furthermore, GQDs disrupted the formation of stable micelles, thereby decreasing the crosslinking density during polymerization by a factor of 0.5829 (KL) to 0.2633 (JNMs) as derived from storage modulus with increasing concentration of the nanofiller (Figure 2A). This in turn affected the nanostructures converting them into "soft" JNMs, which is further affirmed by thermal analysis (Figure S6). Moreover, the diverging curves of complex modulus of hydrogels, initiating from intermediate (ω = 40 rad/s-1) to higher frequency region can be attributed to highly interconnected PAAc-rich and PDEAEMA-rich phases which is monotonically increasing in pAcD, almost uniform in KL and decreasing in JNMs due to prominent phase separation in the latter (Figure 2C). The decrease in strength was further confirmed by the compressive moduli which decreased with the increasing concentration of GQDs (Figure S2B) (Table S2). Consequently, the dynamic elastic response G', decreased by more than two orders of magnitude in the case of JNMs as compared to pAcD (SI 2) (Figure 2A). Interestingly, the attraction between the hydrophobic hemispherical caps brings two JNMs at closer proximity where the frustrated hairy alkyl side chains develop partial interdigitated packing stabilizing orientation order. This gives rise to distinct diffraction spots in the SAED pattern indicating reduced intermicellar steric interaction between JNMs (Figure 1N). The presence of extra diffraction spots arises from irregularly sheared GQDs due to out-plane polymerization of DEAEMA restricting positional long-range ordering of the structure along (100) normal to the plane. Particular volume fraction of the hydrophilic and hydrophobic domains in amphiphilic BCPs possess concomitant influence over the morphology of the self-assembling nanostructures.43


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Intricate reciprocity of various thermodynamic factors and differences in free energy46, the hydrophobic patch on JNMs furnishes directionality encouraging both intra and interchain interaction weakening polymer-solvent interaction coalescing with one another guiding insurgence of Janus Tadpoles (Figure 1O - Q). We observed multi-tadpole assemblies in Figure 1Q formed via anisotropic amalgamation of numerous hydrophobic patches over J-aggregates which was further confirmed through particle size distribution analyses where the mean diameter of the particles were ~11.44 nm (Figure S7). The elemental mapping analyses confirmed the distribution of C, O, N (Figure S7). At elevated pH, the AAc backbone becomes charged where electrostatic repulsion between individual solvated polymeric chain bends the tadpoles generating curved cylindrical JNMs (Figure 1S). The outer surface of the tadpole is hydrophilic because of the presence of short chains of PAAc which can be observed in the less intense O— C=O band centered at 289 eV in C(1s) and C=O/O—C=O band at 533.2 eV in O(1s) spectra (Figure 3F, H). Eventually, the attractive interaction between cylindrical JNMs overcomes the energy penalty for bending at higher pH, forming a cage bell structure driven by rim energy stabilized by entropic and kinetic factor (Figure 1T).47 Diblock copolymers have crucial implications in obtaining self-assembled core-shell micelle architectures. The outer shell, composed of the hydrophilic blocks enable stable dispersion in aqueous environments while providing protection of encapsulated payload. At physiological pH, the hydrophilic PAAc domain dissociates as a result of electrostatic repulsion between the carboxylate anions formed by the deprotonation of the carboxylic acid moieties.48 The hydrophobic core undergoes transformation to hydrophilic state forming large voids that connect the micellar core with the surrounding medium making channels for the movement of drug and


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solvent. KL and JNMs underwent deprotonation at pH 7.4 exhibiting superior swelling behavior owing to an increase in repulsive forces and a reduction in crosslinking density. Furthermore, the tertiary amine groups of DEAEMA undergo protonation as a consequence of their interaction with GQD functional groups forming a quaternary ammonium salt which induces coulombic repulsions between the cationic moieties enhancing the swelling ability of the matrix. Comparing the swelling profiles (Figure S8A) of the three variants in PBS solution (pH 7.4), it was noted that the ESR values for different hydrogels intensified in the order pAcD

Graphene Quantum Dots in the Game of Directing Polymer Self

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