Simultaneous Passivation and Encapsulation of Black Phosphorus

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Simultaneous Passivation and Encapsulation of Black Phosphorus Nanosheets (Phosphorene) by Optically Active Polypeptide Micelles for Biosensors Avneesh Kumar ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00265 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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ACS Applied Nano Materials

Simultaneous Passivation and Encapsulation of Black Phosphorus Nanosheets (Phosphorene) by Optically Active Polypeptide Micelles for Biosensors Avneesh Kumar* *International

Center for Materials Science, JNCASR, Jakkur, Bangalore, 560064, India, Email:

[email protected] KEYWORDS 2D materials, black phosphorous nanosheets, helical polymers, micelles, supramolecular assemblies ABSTRACT In a simplistic way, 2D black phosphorus (BP) nanosheets are exfoliated in a polar solvent at room temperature. Afterwards, as-obtained BP or phosphorene nanosheets are passivated and encapsulated simultaneously by using an artificial polypeptide polymer having an ability to form micelles. In first step, thin layers of BP nanosheets were obtained by sonication. Next, helical copolymer based on polyethylene glycol and poly(phenyl isocyanidepeptide) blocks is then allowed to blend with the suspension of phosphorene nanosheets for their inclusion in micelles. The size of nanosheets is reduced after their encapsulation inside the polymeric micelle. The

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copolymer based on polypeptide is also supposed to improve biocompatibility. These 2D nanosheets are investigated for their microstructures by transmission electron microscopy. The transmission electron microscopy results show that the BP nanosheets are included within the helical cavity of the copolymer indicating the hydrophobic nature of the nanosheets. Atomic force microscopy images indicate the formation of smooth and flat nanosheets. Photoluminescence (PL) experiments suggest that the emission from polymer micelles is quenched after nanosheets were embedded within the polymer helical matrix. For polymer micelles, the disappearance of emission proves that electron transfer (ET) occurs between the BP nanosheets and polymer helix. Even after encapsulation, the BP nanosheets are sensitive to light and emits with a sharp signal that shifts slightly towards the blue region. This single step approach for passivation and encapsulation of BP nanosheets provides new solution for protecting the BP nanosheets from oxidation and fabrication without compromising the electronic properties. After fabrication, these 2D active hybrids can be integrated in a device for sensing applications. These 2D nanomaterials can be also introduced in an infected tissue for imaging or delivering a drug particulate. Introduction After graphene’s discovery, 2D materials have been at the helm of material research and have gained tremendous attention both in academia as well as in industry.1,2 Until recently, graphene has been exploited extensively almost for all applications that include energy and biomedical.3,4 Nowadays similar to graphene, a new 2D material known as phosphorene, single or few layered nanosheets of black phosphorus (BP), is gaining momentum as a competitor of graphene.5,6 The distinct features of phosphorene such as its semiconducting, photoluminescence (PL) and biocompatibility have arisen a great interest among the scientific community not only for


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conducting the fundamental research but also exploring this material for various applications.7,8 Recently, the photocatalytic efficiency of 2D black phosphorus nano sheets (BPNS) or phosphorene to produce hydrogen gas from water has been reported.8,9 Recently, few research groups have explored the possibility of using 2D materials for nanomedicines e.g. cancer theranostics.10,11 In these findings, antimonene (AM) coated with polyethylene glycol has been used as a photothermal agent against cancer. Further attempts to employ black phosphorus (BP) nanosheets as drug delivery platform have been also made.12,13 In a separate study, black phosphorus nanosheets were taken into nanocapsule of polydopamine as codelivery system for the targeted chemotherapy.13 Additionally, BP nanosheets decorated with Pt nanoparticles have been investigated for decomposing the H2O2 accumulated in tumor.14 Nevertheless, the challenge to fabricate phosphorene in combination with the polymer or other organic counterpart without sacrificing its optical and electronic efficiency still remains open for materials scientists working across the diverse fields and cutting edge of technology.15,16,17,18,19,20 Covalent functionalization of phosphorene nanosheets may not only alter the properties but reduces its performance as well.27,28 Therefore, in a counter approach, non covalent passivation or wrapping of BP nanosheets by a molecular system must be taken into consideration. However, in this manner either the BP nanosheets or passivating agent may leach out during fabrication and thus exposing the nanosheets to the atmospheric oxygen. Therefore, a molecular system that forms packets or cavities in which these BP nanosheets can be accommodated and wrapped uniformly is strongly sought. In other words, BP nanosheets as guest and molecular cavities as host with chemical functionality must work in a cooperative fashion and be able to electronically communicate to harness their potential for new technological applications. If phosphorene is passivated or protected from oxidation by using a polymer matrix, its properties can decrease significantly.


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Therefore a polymer with precise functionality and tuned properties for example ionic, electronic and optical must be designed and produced to passivate the phosphorene and exploit the entire system in a holistic approach for targeted application such as drug delivery or cancer theranostics.10,11 With regard to biological applications, the polymer used for passivation must be not only responsive towards certain stimuli but also be biocompatible. Therefore, passivation by an active polypeptide chain can offer more opportunities for fabrication and improving biocompatibility. Poly(phenyl isocyanidepeptide)s, a new class of polymers, are known for undergoing self assembly via noncovalent interactions and dynamic changes along the backbone leading to a helical confirmation.20,21 In these polymers, folding and unfolding of helix under certain circumstances can be tuned by introducing an active chemical functionality which in turn can offer multiple purposes such as for memory device, sensor and drug delivery. Additionally, it has been found nontoxic to the tissue.20,21 For obtaining a copolymer, hydrophilic segment for instance PEG has been also linked covalently to the poly(phenyl isocyanidepeptide) chain in which self assembly and responsive nature of the corresponding copolymer was achieved in aqueous phase.21 These copolymers have been reported to show formation of micelles, vesicles and other nanostructures depending on the fraction of polar and non polar segment.21,24,25,26 Thus far, literature lacks any example of passivation and encapsulation of few layered BP nanosheets with a helical shaped artificial polypeptide that is optically active. Herein for the first time, we demonstrate a simple and yet useful strategy to have combined the illusive properties of 2D nanosheets of phosphorene and an artificial helical copolymer engineered chemically for its architecture and supramolecular assembly as reported previously by our group.21 The helical polymer provides multifold benefits 1) it passivates and encapsulates BP nanosheets, 2) it electronically interacts with the BP nanosheets, and 3) it can allow fabrication of inorganic BP


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nanosheets for their integration in devices without sacrificing the inherent properties of BP nanosheets, 4) it protects BP nanosheets from atmospheric oxygen. The major highlight of this research includes the noncovalent passivation and encapsulation of BP nanosheets within the optically active micelles or nanopockets composed of helical chains and thereby creating a shell or coating around the nanosheets while a signal can still be generated via electron transfer (ET).29,30 Scheme 1 illustrates our approach in which first few layered BP nanosheets were obtained by typical solvent exfoliation. These black phosphorus nanosheets (BPNS) were then combined with a helical copolymer composed of PEG and amino acid (L-valine) based monomer.21 In next step, these nanosheets were passivated and encapsulated simultaneously inside the cavity of polymer micelles as formed by molecular self assembly induced by hydrogen bonds, hydrophilic and hydrophobic interactions. Electron transfer (ET) process between BPNSs and helical chains of amphiphilic copolymer 1 was investigated (Scheme1) to verify if the optical properties are sacrificed or not after noncovalent wrapping and encapsulation of these nanosheets in micelles cavities. Here, the innovation is to use an active polypeptide based polymer to simultaneously passivate and encapsulate the BP nanosheets in a simplistic approach. Further, the polypeptide used here is well defined and has multiple amino acid units those have cumulative effect with regard to the biocompatibility of the system and electronic interactions. The issue of uniform functionalization of the surface of nanosheets by small molecules such as amino acid is also addressed by using a polypeptide polymer that can serve the purpose more efficiently. Another major advantage is that by using an active polymer these materials can be fabricated easily into a device without including any further chemical functionality and compromising the properties, which is not the case for nanosheets functionalized with small


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molecules. Obviously, as far as device is concerned, fabrication by using a polymer, that too active, will be rather accessible to manufacturers and useful for the applications.

Scheme 1. Diagrammatic presentation of the strategy to exfoliate, passivate and encapsulate the black phosphorous nanosheets (BPNS) with micelles consisted of helical polypeptide unit and water soluble PEG.

Helical copolymer 1 (Scheme 1) was synthesized and has been investigated for its optical properties by us recently.21 The chemical structure of the copolymer is shown in Scheme 1. In this copolymer the ratio of each individual block i.e. (PEG and poly(phenyl isocyanidepeptide) was about 1:10 in order to induce the formation of micelles and a rigid helix. The presence of chiral information in this block copolymer is considered to further increase the potential of BP nanosheets for incorporating into the cell membrane. Also, dangling alkyl chains in repeating


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unit can bind with hydrophobic lipid or drug molecules. By following a typical technique, BP crystals were exfoliated in EtOH under ultrasonication to obtain few–layered BP nanosheets (BPNS).31 Sonication for longer duration was carried out to obtain thin layered BP nanosheets. Few–layered BP nanosheets were separated from bulk by centrifugation. The formation of few– layered BP nanosheets was confirmed by Raman spectrum in which vibration peaks were found to be shifted towards higher wave numbers as compared to the ones reported for bulk BP (Figure 1d). For bulk BP, three vibrational bands at 359, 436 and 464 cm-1 are typical, while for few– layered BP nanosheets these bands appeared at 371, 449 and 476 cm-1 (Figure 1a).

Figure 1. a) Raman spectra of exfoliated few-layered BP nanosheets, b) three types of phonon modes; c) AFM height image (2×2 µm), d) and e) TEM images of exfoliated few-layered BP nanosheets demonstrating crystalline 2D layers, and a top view of the individual nanosheet of layered BP is also shown on the top far right side


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These bands were linked with the three types of phonon modes namely A1g, B2g and A2g as shown in Figure 1a. To describe the chemical structure of monolayer of BP, it consists of phosphorus atoms packed hexagonally like in graphene (top far right, Figure 1). However, unlike in graphene, the hexagonal ring is puckered in BP nanosheet and thereby generating three phonon modes in Raman spectra. The phonon mode A1g resulted from out-of-plane vibration in puckered ring, whereas B2g and A2g were contributed from in-plane vibration along the zigzag and armchair direction respectively (Figure 1b). Surface morphology of exfoliated BP nanosheets was investigated by AFM (atomic force microscopy). The topographic or height image of few layered BP nanosheets is given in Figure 1c. The size distribution of nanosheets was found to be heterogenous ranging from 600 nm to 400 nm. For larger nanosheet with grain size about 530 nm the root mean square roughness (Rq) was 5.49 nm, whereas nanosheet with grain size about 340 nm had Rq of 4.33 nm (Figure 1c). The lower Rq is considered to be useful while fabricating the device in order to minimize the interfacial boundaries between the surface and BP nanosheets. With regard to further topographic features of BP nanosheets, the larger nanosheets (530 nm) showed a Kurtosis value (Rku) 3 (5.13) signifying a surface with more peaks than valleys. Furthermore, crystalline layers and dimensions of these exfoliated BP nanosheets were investigated by transmission electron microscopy (TEM). Figure 1d and 1e show the TEM images of free BP nanosheets under low and high resolution respectively. For the few–layered BP nanosheets, the lateral dimension was found to be ca. 200 nm (Figure 1d). It has been reported in literature that by increasing the time of sonication, nanosheets of smaller dimension can be obtained as shearing force can weaken Van der walls interactions between two layers and thereby facilitating subsequent exfoliation.32 Interestingly, we have also noticed similar case and


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found some nanosheets with size smaller than 200 nm (SI3). Under high resolution or high beam energy in TEM, these nanosheets revealed layer-by-layer crystal structure corresponding to hexagonal othorhombic shape (Figure 1e). The interlayer distance was obtained to be 0.51 (SI1). The interlayer distance was obtained from FFT of selected region in HRTEM image (SI1b). To passivate and encapsulate these nanosheets into polymer micelles, a certain concentration (5 mg) of block copolymer 1 was dissolved in two solvent mixture of THF and EtOH (70/10 v/v) as polymer was less soluble in solvent (EtOH) used for exfoliation. This solution was added to the suspension of already exfoliated BP nanosheets. In order to allow an effective passivation and inclusion of BP nanosheets, the entire solution was again sonicated for 4-6 hrs. The dimensions of BP nanosheets were significantly reduced after their encapsulation inside the polymeric micelles. One possible reason for thinning of BP layers can be explained by the fact that polymer chain may have added further force to interrupt Van der walls force between two layers of nanosheets.


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Figure 2. TEM images of a) copolymer micelles; b, c) encapsulated BP nanosheets within the micelles; d) Raman spectrum of BP nanosheets after encapsulation and e) phonon modes comparison between the free BP nanosheets and encapsulated ones; f) illustration of single micelle of copolymer 1.

A pictorial illustration of aggregation of copolymer chains and their assembly with BP nanosheets, and subsequently reformation of micelles is shown in Scheme 2 for better understanding. It is well known that peptides can undergo self assembly and form ordered nanostructures. These assemblies can be stabilized by interpeptide and hydrogen bond networks.33,34 Similarly, as the copolymer 1 contains polypeptide unit and thus it is supposed to form self assembly giving rise to well defined nanostructures e.g. micelles. These micelles of copolymer 1 acted as nanocontainers to capture the BP nanosheets under mechanical shearing


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assisting the disintegration of micelles into aggregated copolymer chains with no well defined shape, and thereby allowing BP nanosheets to come in contact with the inner helical compartment. As soon as the mechanical force was removed, the entire system including copolymer chains reorganized and reformed the micelles capturing the BP nanosheets within (Scheme 2). In few micelles, the inclusion of BP nanosheets was not observed as the content of micelles was higher as compared to the nanosheets (Figure 2b).

Scheme 2. Illustration of disintegration of micelle under mechanical force and assembly of nanosheets with copolymer chains followed by reformation of micelle and encapsulation of BP nnaosheet after mechanical force was removed.

During refolding process of polymer chains, some nanosheets may have remained unencapsulated due to the fact that stacking of nanosheets may have prevented polymer chains from folding completely with lesser curvature and hence spherical shape did not re-form. As a result, only aggregation of nanosheets and polymer was observed (SI2). Furthermore, in these micelles, the helical segment remained inside the cavity and can open/close its helix under certain solvent as explained and reported before.21 Herein these nanostructured micelles acted as a coating or shell for BP nanosheets to be passivated and encapsulated in a single step. The


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inclusion of BP nanosheets in polymer micelles was observed by TEM experiments. In Figure 2, TEM images of polymeric micelles, encapsulated BP nanosheets within micelles, and Raman spectrum of encapsulated BP nanosheets are depicted for drawing a comparison with that of free BP nanosheets. For encapsulating purpose, the size of micelles must be larger than the BP nanosheets. From TEM image of micelles formed by copolymer 1 (Figure 2a), it is clear that the size of micelles ranged from 400-800 nm facilitating inclusion of BP nanosheets with ca. 200 nm dimension. However, as mentioned before that already exfoliated BP nanosheets were sonicated again together with the polymer to allow simultaneous passivation and encapsulation that favored a reduction in dimension of nanosheets to ca. 50-100 nm (Figure 2c). It is assumed that the driving force and mechanism behind the inclusion of nanosheets into micelles may have originated from hydrophobic-hydrophobic interactions of BP nanosheets and polymer helix. In addition to this, the lone pair of P and pi electrons of polymer helix have assisted the encapsulation further as described in next section. Crystalline structure investigations of BP nanosheets with and without polymer were done by selected area diffraction (SAD) in TEM. These studies have indicated that the crystalline layers (hexagonal packing) of BP nanosheets were intact (SI4). With polymer, the diffraction patterns of BP nanosheets appeared rather intense (SI4b). The intense diffraction patterns are related with the thinning of nanosheets in the presence of polymer. Also, there were very few crystal defects observed. Further evidences for BP nanosheets residing as a host within the micelle cavity were obtained by Raman studies (Figure 2d-e). For Raman spectra, the range of wave number was 300-2000 cm-1 (Figure 2d). In Raman studies of encapsulated BP nanosheets, a noticeable shift of wave number was observed. Raman bands for copolymer were very strong and distinguishable whereas the typical three phonon modes of BP nanosheets were found to be shifted towards red region (Bathochromic). In


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Figure 2c an expanded Raman spectra for encapsulated BP nanosheets is shown in which a shift towards lower wave number can be noticed clearly. Three typical bands for BP nanosheets in micelles were at 370, 447 and 474 cm-1.This has further confirmed the successful encapsulation of these nanosheets inside micelle cavity. Direct or non-covalent passivation, and encapsulation of BP nanosheets by using polypeptide copolymer protected these nanosheets from oxidation as no Raman bands for PxOy oxide were observed even after exposing the BP nanosheets to atmospheric oxygen. Clearly, the micelles formed a shield in which the nanosheets were not only encapsulated but also electronically interacted through lone pair of P and pi electrons of helix. Optical properties of few–layered BP nanosheets with and without passivation/encapsulation were also investigated to further verify the electronic interactions and inclusion of BP nanosheets within polymer micelles. Figure 3a and b show the UV–Vis absorbance and PL spectrum respectively for few–layered BP nanosheets, polymer micelles, and encapsulated BP nanosheets. In UV–Vis spectrum of few–layered BP nanosheets, a typical broad absorption was seen with an absorption peak at around 250 nm (Figure 3a). Polymer micelles without any BP nanosheets displayed an absorption peak at 280 nm. As expected, a red shift absorption peak for passivated and encapsulated BP nanosheets appeared at 257 nm as compared to the free BP nanosheets. This has again supported the conclusion that BP nanosheets were included in micelle cavities in which they were wrapped around by helical chains of polymer. The photon energy transfer occurred between the helical polymer chains and BP nanosheets which would have not been possible if the nanosheets were to be out of the micelles or lying outside freely. Usually, the light absorbance properties of BP nanosheets can decrease significantly after passivation by a polymer or other inactive organic molecule.35 In contrast to this, from our UV–Vis results, it is clear that optical properties were not diminished, and the light absorbance with a broader peak was shifted


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towards visible range. This shift in absorbance for encapsulated BP nanosheets in active polymer micelles can be a determining factor with regard to the applications and fabrication of BP nanosheets without compromising their electronic properties.

Figure 3. a) UV–Vis absorbance and b) PL emission bands of free few layered BP nanosheets, micelles and encapsulated BP nanosheets in micelles.

To

determine

the

fluorescence

properties

of

BP

nanosheets

before

and

after

encapsulation/passivation, photoluminescence (PL) experiments were carried out from which further insight about the photon energy transfer between BP nanosheets and helical polymer chains was gained.36 In Figure 3b, the PL spectrum of few–layered BP nanosheets, micelles and encapsulated BP nanosheets in micelles are presented. All samples were excited at similar wavelength of 300 nm. For few–layered BP nanosheets, a broad emission band appeared at 353 nm. For polymer micelles, two emission bands at 347 and 405 nm were observed. These two bands were attributed to pi-pi* electrons of polymer helix. Furthermore, the emission for polymer helix was found to be rather strong as compared to that of free BP nanosheets.


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Interestingly, the emission band for helix at 405 nm disappeared when measured for encapsulated BP nanosheets in micelles. A stiff emission band at 352 nm reemerged for encapsulated BP nanosheets (Figure 2b). In presence of BP nanosheets, the fluorescence of polymer helix was quenched to a large extent which would happen only when nanosheets were in close vicinity of helix so that photon energy transfer can occur. The quenching of PL signal of polymer helix proved the insertion of BP nanosheets inside the micelles. As the emission band for encapsulated BP nanosheets became intense and narrow, any effect of attractive polarization forces between absorber and solvent was excluded. From these findings the possibility for intermolecular electron transfer (ET) between BP nanosheets and polymer helix can be concluded. Furthermore, the structural transformations and dynamics in helix under polar solvent may have also occurred in order to accommodate BP nanosheets and for electron transfer process to take place.21,37 It is assumed that polar solvent may have disrupted hydrogen bonds of amide linkage in helix and assisted BP nanosheets to fit in the free space originated from the dynamic changes in helix. Nevertheless, the stability of polymer helix was not an issue. Additionally, it must be noted that both UV–Vis and PL measurements were done at ambient conditions and samples were exposed to the atmospheric oxygen. Despite oxygen exposure, the fluorescence properties of BP nanosheets were not destroyed owing to the fact that these nanosheets remained protected inside micelle cavities and were able to interact with the external source of photon energy as well as with the helical polymer chains. Conclusion In summary, simultaneous passivation and encapsulation of BP nanosheets were achieved in a single step by using an optically active polypeptide that not only interacts with the nanosheets but also provides further chemical functionality available for external molecular agents.


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Encapsulation was proven by TEM investigations in which hydrophobic nanosheets were included inside the cavity of polymeric micelles. Encapsulation process and electronic interactions between BP nanosheets and helical segment of the micelles were studied by UV–Vis and PL indicating electron transport process between the two, and supporting the successful encapsulation of BP nanosheets. After encapsulation, the optical properties of BP nanosheets were not compromised and thereby highlighting the importance of an active polymer for the fabrication of these materials. Our findings have opened new avenues for passivating and encapsulating BP nanosheets by using an active biocompatible polymer. These assemblies of BP nanosheets with biocompatible artificial polypeptide can be integrated with biomedical devices for suitable applications such as sensor38,39 and drug delivery40,41. Our research has clearly created more possibilities to encapsulate BP nanosheets with other artificial polypeptides designed and synthesized with tuned optical properties in such a way that optical properties of BP nanosheets-polymer micelles can be exploited further for phototherapy applications.42,43 Due to the presence of amino acid building blocks, these new 2D materials will be exploited in biosensors applications. Further potential application will include in cancer imaging after slight modulation in their optical properties towards visible light range. Our future goal will be on testing cell toxicity of encapsulated BP nanosheets and exploring their sensing properties with analytes that can be detected via non-covalent interactions involving amide linkage and aliphatic chains of polymer helix. Such detection can be readable by observing a change in optical properties of the system described here with BP nanosheets and active polymer micelles. Experimental Helical copolymer 1 was synthesized and investigated for its self assembly and optical properties according to the literature as reported previously by us.21 All solvents for preparing samples were


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ordered from Sigma Aldrich. For sonication, optical properties measurements (UV–Vis and PL) and TEM film, the solvents were dried and degassed with nitrogen prior to their usage. For AFM, the samples were prepared by drop casting method and deposited on Si surface. Samples for UV–Vis, photoluminescence (PL), Raman, AFM and TEM were handled at ambient conditions without employing any specific techniques such as glove box. Crystals of Black Phosphorus (99.998 % purity) were ordered from Smart-Elements. BP crystals were stored under nitrogen atmosphere in a glove box. Samples for exfoliation were sonicated in Bransonic Ultrasonic 3510. . UV–Vis absorption and PL spectrum of few layered BP nanosheets and their hybrids with polymer micelles were monitored by using Perkin Elmer UV/VIS/NIR Spectrometer Lambda 750. Circular dichroism (CD) studies were done on a Jasco J-815 CD Spectrometer. A quartz cuvette with 1 mm path length was used for CD experiments. Raman spectrums were recorded on a LabRAM HR high-resolution Raman spectrometer (Horiba- Jobin Yvon) installed with an Ar laser with wavelength of 514.5 nm. Exfoliation of BP Crystals BP crystals (10 mg) were taken in a Schlenk tube (10 mL). The Schlenk tube was sealed by a rubber septum. A 10 mL degassed EtOH was added to this via a syringe under nitrogen atmosphere. The mixture was degassed for 5-6 hrs in order to remove any oxygen.. The suspension of BP crystals in EtOH was put in a sonication bath for 24 hrs to obtain thin layers of BP nanosheets. The temperature of bath was maintained by adding ice in a timely manner. After this, suspension was centrifuged in glass vials at the rate of 3000 rpm for one hr. The supernatant was extracted from Schlenk tube by syringe to afford the few layered BP nanosheets. Passivation and encapsulation of few–layered BP nanosheets


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In a Schlenk tube (5 mL), 5 mg copolymer dissolved in dried THF (2 mL) was introduced. To this, suspension of above BP nanosheets in EtOH (1 mL) was added. Schlenk tube was sealed with a rubber septum and was degassed subsequently. After this, the mixture was sonicated again for 10 hr to allow disintegration of micelles followed by encapsulation of BP nanosheets under mechanical shearing. Sample was centrifuged at 3000 rpm twice each for one hour in order to remove any excess of free polymer and large BP nanosheets. The supernatant was collected in a separate glass vial. Corresponding Author *Avneesh Kumar, International Center for Materials Science, JNCASR, Jakkur, Bangalore, 560064, India Email: [email protected] Funding Sources DST (India) and Sheikh Saqr Scholarship (UAE) ACKNOWLEDGMENT Author acknowledges electron microscopy and Raman spectrometer facilities group (ICMS, JNCASR, Bangalore, India) for TEM and Raman experiments respectively. Special thanks to Professor C. N. R. Rao for his generous support. Financial support from DST (India) and Sheikh Saqr Scholarship (UAE) is greatly appreciated. ABBREVIATIONS BPNSs, black phosphorous nanosheets; ET, electron transfer; TEM, transmission electron microscopy; PL, photoluminescence


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Simultaneous Passivation and Encapsulation of Black Phosphorus

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