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Supercritical Fluid Facilitated Disintegration of Hexagonal Boron Nitride Nanosheets to Quantum Dots and Its Application in Cells Imaging Pitchai Thangasamy, Manikandan Santhanam, and Marappan Sathish ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04614 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016
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Supercritical Fluid Facilitated Disintegration of Hexagonal Boron Nitride Nanosheets to Quantum Dots and Its Application in Cells Imaging Pitchai Thangasamya,b, Manikandan Santhanamc and Marappan Sathisha,b ⃰ a
b
Functional Materials Division, Academy of Scientific and Innovative Research (AcSIR), Corrosion and Material Protection Division, CSIR-Central Electrochemical Research Institute, Karaikudi – 630 003, INDIA. E. mail:
[email protected];
[email protected] KEYWORDS: Supercritical fluid, BN QDs, fragmentation, few layer, bacterial cells c
ABSTRACT: Preparation of quantum dots (QDs) and exfoliation of two-dimensional layered materials have gathered significant attention in recent days. Though, there are number of attempts have been reported, facile and efficient methodology is yet to be explored. Here, we demonstrate supercritical fluid processing approach for rapid and facile synthesis of blue luminescent BN QDs from layered bulk material via in situ exfoliation followed by disintegration. The microscopic and AFM analysis confirmed the few layer BN QDs formation. The strong luminescent behavior of BN QDs is utilized to stain gram negative bacterial cells specifically in presence of gram positive bacterial cells. properties such as low interfacial tension, excellent wetting of surfaces and high diffusion coefficients. These Recently, carbon nanodots (CDs) and graphene quantum unique features have been employed for the synthesis of dots (GQDs) have received much interest in materials various metal oxide nanoparticles and the exfoliascience owing to their fascinating physical and chemical tion/functionalization of graphene nanosheets.16-20 Very properties. Because of their unique properties, the quantum dots has been considered as a promising candidate in recently, we reported the preparation of h-BN nanosheets various applications such as photocatalyst, sensor, energy and MoS2 nanoscrolls from their respective layered bulk materials using SCF processing. When the exfoliation conversion, bioimaging, biological labelling and drug delivery.1-6 In recent years, plethora of efforts have been condition of BN nanosheets in SCFs was tuned, the formade to synthesize GQDs with high quality and good mation of BN QDs was observed by the disintegration of yield. To date, GQDs have been fabricated using various exfoliated monolayer/few layer BN nanosheets. In a typical preparation, the bulk h-BN powder dispersed in DMF synthetic strategy based on either bottom-up or top-down was subjected to ultra-sonication and sealed in a stainless approach including breaking of graphene sheets by ultrasonication or hydrothermal method, electrochemical synsteel reactor before placing pre-heated (450° C) vertical tubular furnace. After 40 min the reactor was rapidly thesis, mechanical grinding, chemical exfoliation, elecquenched in an ice cold water. The resultant solution was tron beam lithography, chemical synthesis and ruthenium-catalysed cage-opening of fullerene C60 .7-12 Indeed, centrifuged and filtered using 0.22 µm microporous the synthesis of boron nitride quantum dots (BN QDs) membrane filter. The faint yellow colored filtrate containhas not been explored much in literature so far. Recently, ing BN QDs dispersion was collected and subjected to various characterization. Allwood et al., reported the fabrication of monolayered The BN QDs were characterized by XRD, UV-visible, BN QDs with lateral size of 10 nm, where BN QDs prepared by intercalation/de-intercalation process between FT-IR and Raman spectroscopic analysis. The XRD analyh-BN and potassium and disintegration of h-BN edges sis confirms the presence of monolayer BN QDs. Figure S1 represents the XRD patterns of bulk BN, BN QDs and hbased on the top-down approach.13 Wu et al., demonstrated a facile solvothermal treatment combined with liquid BN sediment. The diffraction lines of bulk BN (Figure S1a) exfoliation for the synthesis of BN QDs.14 Very recently, are well matched with hexagonal BN (h-BN) according to the ICDD pattern No: 01-073-2095. The high intense peak Teo et al., also reported a controllable synthesis of highly luminescent BN QDs by the liquid exfoliation and subseobserved at 2θ = 26.72° represents the reflection of layers quent solvothermal process.15 This clearly indicates that stacked along the c-axis (002) plane. Generally, the monothe preparation of BN QDs with controllable size in a faclayered sheet and QDs derived from monolayers should ile manner is a challenging issue. not show the diffraction peak along the (002) direction.21 Here, we report the fabrication of blue luminescent The BN QDs synthesized by SCF processing did not show BN QDs with average lateral size of 5 nm from the layered any peaks, however when the supernatant solution was bulk h-BN using supercritical fluid processing. The repeatedly coated on substrate results very weak peak unique and homogeneous reaction medium, called supercorresponding to (002) reflection (Figure S1b). This clearcritical fluid (SCF), was created by increasing the temperly indicates that the aggregation of quantum dots along ature and pressure of the solvent above its critical levels, the (002) plane during the drying process. In addition, the where distinct liquid and gas phases do not exist. In addiexistence of few layered BN nanosheets in the residual tion, SCFs have the combination of vapor and liquid was confirmed by collecting the supernatant solution of
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low speed (< 2000 rpm) centrifugation (Figure S1c). Indeed, the absence of other diffraction lines clearly indicated the presence of few layer BN nanosheets in the residual sediment. The UV-visible and FT-IR studies confirmed the oxygen functionalities in the BN QDs (see supporting information). Both the as-prepared and water dispersed BN QDs displays strong luminescent behavior and exhibited an excitation independent luminescent spectra owing to the strong quantum confinement effect. The photoluminescence emission (PL) and excitation spectra of as prepared BN QDs shows the prominent emission peak at 414 nm (Figure 1). When the excitation wavelength was increased from 270 to 380 nm the PL intensity decreased without any shift in the peak position. The observed excitation independent luminescent spectra is mainly due to the formation of uniform lateral size of BN QDs after SCF processing. And also due to the solvent molecules are effectively passivated on the surface of BN QDs. In addition to these, the observed strong luminescence may also be attributed to the B-O bonds formation on the surface of BN QDs. It is in good agreement with the earlier reports that the BO2- and BO- ions are acts as strong luminescence centers in BN system.22 In excitation spectrum, two main peaks were observed at 275 and 354 nm with three minor peaks at 286, 322 and 339 nm. It is believed that the minor peaks arise from different coordination environments (e.g., -OH, -NH2, -N3− ligands) however, it needs to be studied in detail.22 The as-prepared BN QDs emits strong blue color under 365 nm UV light illumination (Figure 1 inset). The similar PL emission spectrum was observed for the BN QDs in water (Figure S6). The calculated PL quantum yield of ~ 2.8 % for the asprepared BN QDs using quinine hemisulphate monohydrate as a standard (supporting information), is comparable with recently reported BN QDs (~ 2.5 %) and GQDs.13, 23 The FE-SEM images of BN QDs shows the uniform dispersion of self-assembled agglomerated BN QDs with an average lateral size of 10 to 20 nm (Figure 2 and Figure S7), that contains individual BN QDs with average size of ~ 5 nm. While, the bulk h-BN shows disk-like morphology with lateral size ranges from 50 to 800 nm and a thickness of > 100 nm (Figure S8).
Figure 1. PL emission (at different excitation wavelengths) and excitation spectra (λem=414 nm) of as-prepared BN QDs. Inset shows
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the blue color emmision of BN QDs dispersion in DMF soution under UV light illumination (365 nm).
The SEM images (Figure S9) of residual h-BN sediment reveals that the disintegration of exfoliated BN nanosheets during SCF processing. In order to understand the origin of disintegration, controlled experiments were carried out. Since, the controlled experiments with DMF solution did not provide any clear results due to the high temperature treatment for the removal of DMF solution, controlled experiments were carried out using isopropanol-water (1:1 vol %) mixture. As the SCFs treatment of bulk h-BN at 400 °C for 15 min results only the exfoliation, 19 the reaction time and temperature was increased to 40 min and 450 °C, respectively. The supernatant solution containing BN QDs along with exfoliated BN nanosheets was collected by slow speed centrifugation and coated over the silica substrate. It can be clearly seen the formation of ultra-small nanoparticles in all the h-BN nanosheets edges due to the disintegration of exfoliated BN nanosheets (Figure 2 c-d & S10).
Figure 2. FE-SEM images of (a-b) BN QDs in DMF and (c-d) BN QDs along with exfoliated BN nanosheets in isopropanol-water.
Figure 3. (A) HR-TEM images of as prepared BN QDs at different
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magnifications and (B) AFM image and height profile analysis of BN QDs on silica substrate.
The morphology and lateral size of bulk h-BN and BN QDs was confirmed using HR-TEM analysis, where the stacked BN layers can be seen in bulk h-BN (Figure S11). The observed lateral size is in good correspondence with the FE-SEM analysis and the SAED pattern shows clear hexagonal rings. The HR-TEM images (Figure 3A & Figure S12) of BN QDs revealed the formation of uniform size QDs with an average size of 5 nm by disintegration of exfoliated monolayer/few layer BN nanosheets during SCF processing. Furthermore, the observed quantum dots show lattice fringes of 0.21 nm corresponding to the (100) crystal plane of h-BN. This is in good agreement with the recently reported BN QDs.15 However, there are very few BN QDs with lattice fringes of 0.34 nm corresponding to (002) plane of few layered BN. The SAED pattern of the BN QDs suggests that obtained quantum dots are highly crystalline in nature (Figure S12f). The statistical analysis for the particle size distribution of BN QDs using HRTEM images have been placed in Figure S13. The particle size distribution of bulk h-BN and BN QDs in DMF solution obtained using dynamic light scattering (Figure S14), the size distribution of bulk h-BN is in good agreement with FE-SEM and HR-TEM analysis. Whereas, the BN QDs showed majority of the particles between 2 and 6 nm and a small peak corresponding to large particles of 150 to 200 nm size. The observed large size particles may be attributed to the agglomerated quantum dots or exfoliated BN nanosheets without complete disintegration. The thickness of as-synthesized BN QDs was measured using AFM analysis and the height profile analysis (Figure 3B) shows a thickness of < 1.5 nm. This clearly confirms that BN QDs were derived from the monolayer BN nanosheets. However, there are few BN QDs with the thickness of >1.5 nm but not exceeding 4 nm (Figure S15) due to the disintegration of few layered exfoliated BN nanosheets. It is worthy to note here that, after the SCF processing both the lateral size and thickness of BN was reduced significantly. In addition, the SCF processing has several advantages over the solvothermal method for the synthesis of BN QDs. Recently, Wu et al., 14 and Teo et al., 15 demonstrated the fabrication of BN QDs in DMF solvent medium by the combination of ultrasonication and solvothermal treatment. Where, the first step is the exfoliation of BN nanosheets by tip sonication for 8 h followed by disintegration of exfoliated BN nanosheets in second step by solvothermal treatment for 24 h at 140 °C and 24 h at 200 °C. Notably, here the BN QDs was prepared in a simple, one-pot, rapid (with in 40 min), without using any harsh chemicals and energy efficient process using SCF processing. The chemical environment of bulk h-BN and BN QDs was analyzed using XPS analysis (Figure 4A). The sharp peaks observed at 191.8 and 398.3 eV for B1s and N1s spectra of bulk h-BN are in good agreement with reported literature of h-BN.24-27 The XPS spectrum of BN QDs shows two broad peaks upon the deconvolution, the B1s peak can be parted into three individual peaks centered at
191.5, 192.4 and 193.3 eV. These three peaks can be assigned to the presence of B in the B-N, B-O, and B-C bonding environment, respectively.13, 15, 28, 29 It is worthy to note here that the B-N peak intensity was abridged compared to the intensity of B-O peak in the BN QDs. It suggests that more B-O bonds were formed on the surface of BN QDs after SCF processing. Teo et al., also observed the similar efficient oxygen doping on the surface of BN QDs, where the BN QDs synthesized by solvothermal method.15 The possible reason is that the presence of more surface atoms in quantum dots are more vulnerable for surface oxidation in DMF solvent medium under SCF processing. The existence of oxygen doping/defects on the surface of BN QDs was further confirmed from the deconvoluted N1s spectrum. It show two peaks at 398.5 and 400.1 eV, the former is associated with the boron atoms surrounded by nitrogen atoms, while the later is associated with NO/N-C bonds.15 In addition, it could be clearly seen that the peak intensity of N-O/N-C bonds was increased compared to the peak intensity of N-B bonds, that demonstrates more DMF molecules are attached to the surface of BN QDs.
Figure 4. A) XPS spectra: B1s and N1s spectra of bulk h-BN (a-b) and BN QDs (c-d), respectively. B) Epi-florescence images of exponential phase of (a) Escherichia coli and (b) Pseudomonas aeruginosa and stationary phase of (c) Escherichia coli and (d) Pseudomonas aeruginosa stained by BN QDs.
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In a recent study, Allwood et al., reported that BN QDs could be utilized for luminescent cell imaging of MDCKII cell, where it stained cytosol of a eukaryotic cell13. It is suggested that BN QDs are diffusible through hydrophobic lipid bilayer of the animal cell line. The alike diffusible property has been reported in the literature for GQDs, CDs and MoS2/WS2 QDs (Table S1). However, there is no attempts to imaging the bacterial cells such as gram negative (Escherichia coli and Pseudomonas aeruginosa) and gram positive (Staphylococcus aureus and Bacillus subtilis) using QDs. Here, we studied the fluorescence properties of the BN QDs for staining the above bacterial cells as representative organisms. The exponential and stationary phase cells were collected and stained with BN QDs (~100 µg/mL) for 15 min. The epi-florescence microscopy images show that the gram negative cells were preferentially found to stain with BN QDs (Figure 4B), while, the gram positive cells were not staining with BN QDs (Figure S16). The gram negative cell surface was coated as patches by BN QDs. The binding of BN QDs on the bacterial surface was evidently confirmed by FE-SEM analysis and confocal microscopy (Figure S17 & S18). The bacterial cells differ from animal cells by having a cell wall, peptidoglycan layer, outside the lipid bilayer. However, the gram negative cells contains a lipid layer outside the peptidoglycan layer which is absent in the gram positive cells. Thus, the preferential attachment of BN QDs on gram negative cells may be due to the presence of outer lipid layer in the gram negative cells which lacks in the gram positive cells. This interesting interaction of BN QDs with the outer lipid layer of gram negative cells can be exploited for gram negative cells identification and differential staining of environmental mixed bacterial samples using appropriate combination of staining components. In addition, the similar staining of gram negative and gram positive bacterial cells were also conducted for GQDs prepared using the reported literature.30 interestingly, both the bacterial cells did not stained by GQDs, it may be ascribed to the different physico-chemical properties of GQDs and BN QDs. It reveal that BN QDs was suitable for selective staining of gram negative cells compared to GQDs. In summary, synthesis BN QDs from layered bulk h-BN was demonstrated using a facile and simple SCF processing in a short reaction time of 40 min. The exfoliation followed by disintegration of BN during the SCF processing was confirmed using FE-SEM analysis. HR-TEM and AFM analysis confirmed the formation of BN QDs with an average lateral size of 5 nm by in situ fragmentation/disintegration of mono/few layered BN nanosheets under SCF condition. The obtained BN QDs exhibit the strong luminescent behavior that could be used for identification of gram negative bacteria cells. Based on the experimental observations it is clear that this method is a simple, facile and one step process for the preparation of BN QDs in a short reaction time. To the best of our knowledge, this is the first attempt for the synthesis of BN QDs from layered bulk h-BN using SCF processing. And this methodology could be extended to other layered 2D
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inorganic materials for a viable and facile exfoliation and preparation of QDs. ASSOCIATED CONTENT Supporting Information. Experimental section, structural characterization, additional FE-SEM and HR-TEM images of BN QDs and bulk h-BN are available in the Electronic Supplementary Information (ESI): This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author E. mail:
[email protected] [email protected];
ACKNOWLEDGMENT We thank CSIR, India for financial support through MULTIFUN project, (CSC 0101). PT thanks UGC-CSIR, India for SRF fellowship.
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