Highly stable and bio-compatible luminescent ...

Journal of Luminescence 202 (2018) 111–117

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Highly stable and bio-compatible luminescent molybdenum disulfide quantum dots for imaging of alimentary canal in Drosophila

T

Himanshu Mishraa, Sandeep Kumar Singhb, Vijay K. Singha, Jai Singhc, S. Srikrishnab, ⁎ Anchal Srivastavaa, a

Department of Physics, Institute of Science, Banaras Hindu University, Varanasi 221005, India Cell and Neurobiology Laboratory, Department of Bio-chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005, India c Department of Physics, School of Mathematical and Physical Sciences (MPS), Dr. Harisingh Gour University, Sagar 470003, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Molybdenum disulfide quantum dots Fluorescence in vivo bioimaging

Investigations on inorganic luminescent nanomaterials have always gained enormous attention of the science community for their possible applications in the fields of bioimaging and biomedicine. In this succession, various 0D structures including CdSe, CdTe, ZnS, graphene quantum dots (GQDs), carbon nanodots (CNDs) etc. have been explored for their possible applications in the field of biology. Recently, molybdenum disulfide quantum dots (MoS2-QDs) have been explored as an alternative of graphene quantum dots (GQDs) for their possible applications in relevant fields. Herein we report a facile, eco-friendly and single step hydrothermal synthesis of in-situ functionalized molybdenum disulfide quantum dots (f-MoS2-QDs). During the synthesis, variation in the pH of the starting solution provided the controlling over particle size. These as synthesized f-MoS2-QDs have shown excitation dependent broad emission spectra, which could be fitted for a parabolic function. The broadening in the emission spectra might be attributed to the polydispersity of f-MoS2-QDs in colloidal suspension, which was further confirmed with time resolved photoluminescence (TRPL) measurement. Due to surface capping provided by various functional groups present in the colloidal suspension, f-MoS2-QDs have shown excellent stability in aqueous medium and only a 3% of decrement in PL intensity was recorded even after six months. These synthesized f-MoS2-QDs possessed a quantum yield (QY) of 2.3% in aqueous media. Due to their high photostability and biocompatibility, these f-MoS2-QDs have been revealed as a potential entrant for in vivo bioimaging in Drosophila.

1. Introduction Since last few years, molybdenum disulfide (MoS2), a member of transition metal dichalcogenides (TMDs) family has been intensively investigated as an alternative of graphene and graphene derivatives (graphene oxide, reduced graphene oxide etc.) [1,2]. Besides an exhaustive exploration of 2D MoS2, its 0D structure (MoS2-QDs) has also gained an enormous attention due to its strong luminescence property [3]. The luminescence property in QDs rises mainly due to their quantum confinement and edge effect which is better than the conventional fluorescent dyes in terms of long term photostability, highly resistant to photo bleaching and non-blinking nature [4]. MoS2 is a class of layered materials which comprises a metal (Mo) layer sandwiched between two chalcogen (S) layers in the form of S-Mo-S linked with covalent bonds [5,6]. Interlayers of MoS2 are joined together with weak Van der Waals interaction, which makes them suitable to be cleaved easily along their basal planes. Prominent photoluminescence ⁎

Corresponding author. E-mail address: [email protected] (A. Srivastava).

https://doi.org/10.1016/j.jlumin.2018.05.016 Received 1 November 2017; Received in revised form 5 April 2018; Accepted 7 May 2018 Available online 08 May 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.

(PL) has been also reported earlier for monolayer MoS2 due to hybridization of pz orbital of sulfur and d orbital of molybdenum (resulting indirect to direct band gap transition), which make it suitable for various optical applications [7]. However, the synthesis of monolayer MoS2 is very challenging and on the other hand yield is also low making difficult its availability for large scale production. Additionally the PL quantum yield (QY) of MoS2 is very sensitive to the number of layers and a maximum of 4 × 10−3 has been reported for monolayer [8]. On the other hand, there are several reports available regarding the synthesis of MoS2-QDs with good yield, so making it suitable for large scale production [9,10]. Consequently, MoS2-QDs may find applications in new generation photo detectors, microelectronics, nanomedicine, bio-imaging, pH based sensors etc [11–14]. As it is well known that several important biological applications of QDs require PL stability as well as biocompatibility. In this context PL stability and biocompatibility of a number of QDs have been investigated by various groups. Kim et al. used polydentate phosphine


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Fig. 1. (a) Hydrothermal synthesis of f-MoS2-QDs and (b) explanation of stability for f-MoS2-QDs in aqueous medium due to surface capping in presence of functional groups.

capped near infrared fluorescent type II QDs to perform sentinel lymph node mapping [15]. Dubertret et al. encapsulated semiconductor QDs within phospholipid micelles for in vivo as well as in vitro imaging [16]. Gao et al. developed a multifunctional nanoprobes based on QDs for the targeting of cancer. Structural design of nanoprobes involved the encapsulation of luminescent QDs with an ABC triblock copolymer and linked with amphiphilic polymer [17]. Susumu et al. designed and synthesized a number of modular ligands based on poly(ethylene glycol) (PEG) joined with functional groups terminal to enhance the water solubility and biocompatibility of QDs [18]. Chan et al. synthesized zinc sulphide (ZnS) capped cadmium selenide (CdSe) core shell QDs with 20 times enhanced brightness and 100 times resistant to photo bleach [19]. However, besides the above studies, there are several other reports available showing that the QDs are soluble and stable in water only when it is capped with functional groups. It is also reported that if the surface of QDs were not capped they show agglomeration resulting red shifting as well as decrement in PL intensity with the passage of time [3,20]. Similar results have been also observed for MoS2-QDs. Gan et al. have reported that with the passage of time, MoS2-QDs get agglomerated showing a red shifting with the decrement in its PL intensity [3]. Hence a facile and reliable method is highly required for the synthesis of highly water soluble and stable MoS2-QDs maintaining its PL intensity with the passage of time along with biocompatibility. Present study provides a facile, eco-friendly, single step and cost effective hydrothermal method for the synthesis of highly luminescent f-MoS2-QDs with an average particle size ~ 3.5 nm. For MoS2-QDs synthesis, we have used the hydrothermal method as reported by Wang et al. [21] but with different parameters. This change in synthesis

parameters leads to a size reduction of f-MoS2-QDs. In situ functionalization of the quantum dots provides surface capping, which result high water solubility and stability along with a constant PL intensity with the passage of time. These as synthesized f-MoS2-QDs with highly stable fluorescence and bio-compatible properties have been used as a potential entrant for imaging of the alimentary canal in Drosophila. 2. Experimental section 2.1. Materials Sodium molybdate (Na2MoO4·2H2O) and L-cysteine have been purchased from Hi-media, India and used as reagents for the synthesis of f-MoS2-QDs. HCl was purchased from Molychem, India. All the chemicals were of analytical grade and used without any further purification. De-ionized (DI) water was used for the solution preparation and processing throughout the present study. 2.2. Apparatus Structural and microstructural properties of the f-MoS2-QDs were characterized using High Resolution Transmission Electron Microscope (HRTEM, FEI Technai G2 F20) operated at an accelerating voltage of 200 kV. Size and height profile of the f-MoS2-QDs were examined using Atomic Force Microscope (AFM, Bruker Veeco Innova, USA) in noncontact/tapping mode. Raman spectra of bulk as well as QDs of MoS2 have been recorded using Raman spectrophotometer (Renishaw in-Via, UK) with 532 nm laser excitation. UV–Vis spectrophotometer (Perkin Elmer, USA), has been used to record the absorption spectra of f-MoS2112


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Fig. 2. (a) TEM image of f-MoS2-QDs and inset shows the distribution of particle size fitted with Gaussian function (average particle size ~ 3.5 nm), (b) HRTEM image of f-MoS2-QDs and inset shows interlayer spacing (~ 0.27 nm corresponding to (100) plane), (c) AFM image shows that the f-MoS2QDs are well dispersed and (d) Raman spectra of bulk as well as QDs of f-MoS2. Raman shift difference (Δ = 24 cm−1) of E12g and A1g bands of the f-MoS2-QDs confirms the presence of multilayer structure.

QDs. Steady state fluorescence spectra of f-MoS2-QDs have been recorded using Photoluminescence spectrophotometer (Perkin Elmer, UK). Dynamic fluorescence studies have been performed using time resolved photoluminescence spectrophotometer (TRPL, FLS920, Edinburg, UK). 10 mm path length cuvette has been used for all the optical measurements. Fourier Transformation Infrared (FTIR, Varian Excalibur 3000, Palo Alto, CA) spectroscopy has been used for the confirmation of functional groups present along with f-MoS2-QDs in colloidal solution. Nikon Eclipse NIU fluorescence microscope has been used for bio-imaging purposes.

Here H2S produced in the Eq. (1) will serve as sulfur source for MoS2. 2.4. Storage of f-MoS2-QDs for fluorescence stability test To test the stability of PL intensity of f-MoS2-QDs, it was stored in a tube under ambient conditions. The PL spectra of the f-MoS2-QDs had been recorded at a regular interval of time (30 days) up to six months. 2.5. Fly strain and culture

2.3. Synthesis of f-MoS2-QDs

Wild type D. Melanogaster (Oregon R+) strain was used for the imaging study of f-MoS2-QDs. Male and female flies were reared at 24 ± 1 °C on standard Drosophila diet containing agar-agar, maize powder, sugar, yeast, nepagin (methyl-p-hydroxy benzoate) and propionic acid. Additional yeast suspension was provided for healthy growth of the organism.

f-MoS2-QDs have been synthesized using a facile, ecofriendly and cost effective hydrothermal method reported by Wang et al., with slight modification in synthesis parameters viz. pH of the solution and reaction temperature [21]. This modification in synthesis process helped in tuning the particle size and in-situ functionalization of QDs to maintain their solubility as well as stability in aqueous media (Fig. 1) up to several months. 0.25 g of sodium molybdate (Na2MoO4·2H2O) was dissolved into 25 ml of DI water followed by stirring for 10 min at 40 °C. In another beaker, 0.50 g of L-cysteine was dissolved into 50 ml of DI water followed by stirring at 40 °C for 10 min. After that, both the solutions were mixed into a single beaker under constant stirring for 10 min at 40 °C while maintaining its pH at ~ 5 using 0.1 M HCl. Finally the mixed solution was transferred into a 100 ml capacity stainless steel lined Teflon autoclave and kept for hydrothermal reaction at 220 °C for 40 h. After the completion of the reaction, the light yellowish colloidal solution containing f-MoS2-QDs was taken out for further studies. The proposed reaction during hydrothermal reaction isHSCH2CHNH2COOH + H2O → CH3COCOOH + NH3 + H2S

2.6. Treatment schedule Two different concentrations (40 and 80 µl/ml) of f-MoS2-QDs were used to feed adult flies. Further, third instar larvae from the treated as well as normal food were collected for imaging purpose, since from embryonic stage up to adult flies were fed on f-MoS2-QDs supplemented food of above mentioned concentrations in respect to control in the present study. 3. Results and discussion 3.1. Structural/microstructural characterizations of f-MoS2-QDs

(1) Fig. 2(a) shows the transmission electron microscope (TEM) image and particle size distribution of f-MoS2-QDs (inset). The particle size distribution data has been collected statistically for more than hundred

4MoO42- + 9H2S + 6CH3COCOOH → 4MoS2 + SO42- + 6 CH3COCOO+ 12 H2O (2) 113


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particles and the plot was fitted for the Gaussian function. Histogram shows the polydispersity of particles ranging from ~ 1 nm to ~ 8 nm which is the well-known characteristic of the nanostructure colloidal synthesis. Gaussian function fitted histogram shows an average particle size of ~ 3.5 nm. High resolution transmission electron microscopy (HRTEM) image has been shown in Fig. 2(b). Yellow encircled regions show the particle diameter ~ 3 nm. Inset of Fig. 2(b) shows the lattice fringes of the yellow square region with interlayer spacing ~ 0.27 nm corresponding to (100) plane of the MoS2 [22]. Colloidal f-MoS2-QDs have been spin coated and dried in open air for 8 h on a SiO2/Si substrate before performing atomic force microscopic (AFM) characterization in noncontact/tapping mode. AFM image of well dispersed fMoS2-QDs has been shown in the Fig. 2(c). AFM result suggests an average particle diameter of ~ 4 nm which is in good agreement with our TEM result. Fig. 2(d) shows Raman spectra of f-MoS2-QDs as well as its bulk counterpart. Here two characteristic peaks E12g (in plane vibrational mode) and A1g (out of plane vibrational mode) of MoS2 have been identified. The position of E12g peak remains same at wavenumber ~ 380 cm−1 for bulk MoS2 as well as QDs structure. However, A1g peak appears at wavenumbers ~ 406 cm−1 for bulk MoS2 and ~ 404 cm−1 for f-MoS2-QDs, respectively. The wavenumber separations (Δ) between the E12g and A1g peak positions for bulk MoS2 are found to be ~ 26 cm−1 and ~ 24 cm−1 for f-MoS2-QDs. The wavenumber separation (Δ) of ~ 24 cm−1 for f-MoS2-QDs suggests the presence of multilayer structure [23]. In case of f-MoS2-QDs, Raman intensity also get diminished in comparison to its bulk counterparts, which may be attributed to the small crystallinity of f-MoS2-QDs [9,23,24].

375 nm were observed. Among these three absorption bands, ~ 375 nm band could be assigned to the transitions from the deep level of the valence band to the conduction band and the other two (~ 215 nm and ~265 nm) as blue shifted Z, C and D excitonic bands of f-MoS2-QDs [25]. Steady state PL spectra of f-MoS2-QDs have been recorded under different excitation wavelengths ranging from ~ 265 nm to ~ 425 nm (Fig. 3(b)). These PL spectra of f-MoS2-QDs show excitation dependent behavior (Fig. 4(a)) with broad emission spectra ranging from ~ 420 nm to ~ 475 nm, whose position as well as intensity changes with the change in excitation wavelengths. A maximum PL intensity has been observed at ~ 473 nm for an excitation wavelength of ~ 265 nm. PL spectra have shown blue shifting for the lower excitation wavelengths ranging from ~265 nm to ~ 325 nm, while for higher excitation wavelengths (λex > 325 nm), red shifting was observed (Fig. S2). This excitation dependent PL behavior could be attributed to the polydispersity nature of the QDs which is also supported by our TEM and AFM results [26–29]. However, for the excitation wavelengths ~265 nm and ~ 375 nm (corresponding to the blue shifted C and D excitonic absorption bands, and deep level transitions) maximum PL intensities were observed with narrow emission range. To understand the PL dynamics, TRPL spectra of f-MoS2-QDs has been recorded with an excitation wavelength ~ 370 nm and is shown in the Fig. 3(c). TRPL spectrum was fitted for an exponential function,

t t t y = A exp ⎛− ⎞ + B exp ⎛− ⎞ + C exp ⎛− ⎞ τ τ τ ⎝ 1⎠ ⎝ 2⎠ ⎝ 3⎠ ⎜

⎟

⎜

⎟

⎜

⎟

(3)

Here three decay times in nanosecond scale have been observed from TRPL spectrum. The values of the decay times with standard deviation values and their contribution in the PL spectra of f-MoS2-QDs are listed in the Table 1. The presence of three decay times in the TRPL spectra also confirms the polydispersity nature of particles in colloidal f-MoS2-QDs. Again this result is in good agreement with our TEM data (shown in inset of Fig. 2(a)) where the particle size varies from ~1 nm to ~8 nm.

3.2. Optical & spectroscopic characterizations of f-MoS2-QDs Inset shown in Fig. 3(a) are the optical images of f-MoS2-QDs under normal and UV light (λ = 365 nm). From the UV–Vis spectra of f-MoS2QDs (Fig. 3(a)), three absorption bands at ca. 215 nm, 265 nm and

Fig. 3. (a) UV–Vis absorption spectrum showing three absorption bands (~ 215 nm, 265 nm and 375 nm) corresponding to blue shifted Z, C and D excitonic peaks, and deep level transitions of MoS2, inset shows the optical images of f-MoS2-QDs under normal light and UV light (λ = 365 nm) (b) excitation dependent PL spectra, (c) TRPL spectrum in aqueous medium and (d) FT-IR spectra of f-MoS2-QDs.

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Fig. 4. (a) Excitation dependent PL peak position is fitted for parabolic function, showing a dip at excitation wavelength ~ 340 nm and (b) stability of f-MoS2-QDs in water tested under a regular interval of 30 days for six months.

~425 nm. This parabolic PL spectrum of f-MoS2-QDs may be attributed to the polydispersity of QDs which was earlier confirmed with TEM and TRPL data. As it is well known from the quantum theory that the smaller particles possess larger band gap hence require lower wavelength for their excitation while bigger particles with smaller band gap need higher excitation wavelength. Here, for smaller excitation wavelength smaller f-MoS2-QDs may be excited and for higher excitation wavelength bigger f-MoS2-QDs. Solubility and stability of the f-MoS2-QDs have been tested in terms of its PL intensity under a regular interval of 30 days for a period of six months and is shown in the Fig. 4(b). After a period of six months, no significant change (only ~3% decrement) in PL intensity was observed, which means that particles are not get agglomerated with time and hence highly soluble and stable in water. This high solubility and stability of f-MoS2-QDs is supposed mainly due to the surface passivation as well as improved hydrophilicity in presence of –COOH, -NH2, SO2etc. functional groups, confirmed in the FT-IR spectra [31–33]. Here it is supposed that these functional groups are get adsorbed over the surface of MoS2-QDs. As all these functional groups are electron rich species and hence provide a slight negative charge (δ-) to the surface of f-MoS2-QDs. Due to presence of this negative charge (δ-) an electrostatic repulsion (surface passivation) may be possible among the particles which prevents the agglomerations of f-MoS2-QDs and hence provided a high dispersibility and PL stability in aqueous media. Zeta potential measurement of f-MoS2-QDs yields a value > −25 mV with narrow charge distribution over the QDs (Fig. S3). This suggests the presence of negative charges over the surface of QDs providing a good solubility as well as stability in aqueous medium [34,35].

Table 1 Decay times and their contributions into the fluorescence of f-MoS2-QDs. Decay time (ns) τ1 τ2 τ3

0.22 ± 0.01 0.97 ± 0.05 3.38 ± 0.08

% Contribution

χ2

21 43 37

1.001

To understand the presence of functional groups and their bonding with f-MoS2-QDs, FT-IR characterization has been performed and the obtained result is shown in Fig. 3(d). FT-IR peak at wavenumber ~ 467 cm−1 represents the Mo-S vibration of MoS2 [30]. The intensity of Mo-S vibration in FT-IR spectra is found to be very feeble due to the stiffness of Mo-S bond. The FT-IR peak appearing at wavenumber position ~ 950 cm−1 may be assigned to the O-H bending present in carboxylic (-COOH) group. Presence of primary amine (-NH2) group has been confirmed with the appearance of FT-IR peak ranging from 1108 cm−1 to 1193 cm−1 showing C-N stretching. FT-IR peak at wavenumber ~ 1395 cm−1 corresponds to the S˭O vibration of sulfate (–SO2−) ions. FT-IR peak corresponding to C˭N has been found at wavenumber ~ 1628 cm−1. A broad FT-IR peak ranging from ~ 3083 cm−1 to ~3625 cm−1 corresponds to the dimer –OH stretching confirming the presence of carboxylic and amine groups [11]. An FT-IR peak is also recorded at ~615 cm−1 for C-Br which may be due to the KBr pellets used for FT-IR measurement. So, here, FT-IR spectrum confirmed the presence of different functional groups present over the QDs surface and hence justifies the in-situ functionalization of MoS2-QDs. We hypothesized that these adsorbed functional groups over the surface of MoS2-QDs makes it more hydrophilic, soluble and stable in aqueous medium with the passage of time.

3.4. Quantum yield (QY) calculation of f-MoS2-QDs Quantum yield (QY) of f-MoS2-QDs has been calculated with respect to the reference quinine sulfate of known QY (0.546) using following Eq. (4).

3.3. Excitation dependent PL and stability test of MoS2-QDs PL spectra of f-MoS2-QDs have shown excitation dependent behavior. It shows either blue shifting or red shifting in PL spectrum depending upon the excitation wavelength. For an excitation wavelength of ~325 nm, maximum PL intensity was obtained at ~ 420 nm. A blue shifting in PL spectra was observed for an excitation wavelength starting from ~ 265 nm to ~ 325 nm and a red shifting for an excitation wavelength greater than this. PL spectra of f-MoS2-QDs shows a maximum intensity for excitation wavelengths ~ 265 nm and ~ 375 nm, which is the position of absorption bands in UV–Vis absorption spectrum, while for other it is diminished along with shifting in its position. Fig. 4(a) shows the excitation dependent maximum PL peak position of f-MoS2-QDs. In Fig. 4(a) the observed excitation dependent PL peak position has been fitted for a parabolic function which shows minima for an excitation wavelength ~350 nm with an emission wavelength of

ODr 1 ⎞ ⎛ In2 ⎞ QY = QYr ⎜⎛ X 2⎟ n r ⎠ ⎝ OD ⎠ ⎝ Ir ⎜

⎟

(4)

Here QYr is the quantum yield of the reference; n and nr are the refractive index of solvents used for sample and reference solutions. Quinine sulfate had been dissolved in 0.1 M H2SO4 and the f-MoS2-QDs in DI water. The refractive index (nr) of the 0.1 M H2SO4 is 1.63 [36] and for DI water it is 1.33. Here I and Ir are the integrated intensities (area) of the sample and reference PL spectra respectively. OD and ODr are the optical densities of the sample and reference respectively, measured from the UV–Vis absorption spectra at excitation wavelength 308 nm (Table 2). 115


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readily allows observation of luminescence in alimentary canal or any other part of the third instar larvae. Using the standard corn syrupsucrose-agar food treated with optimized concentrations of f-MoS2-QDs (40 & 80 µl/ml of Drosophila food), few male and female flies of Oregon R+ were fed continuously up to next generation. In parallel, the few pairs of Oregon R+ were reared in normal food. These flies were allowed to grow separately with f-MoS2-QDs treated as well as normal food to complete their life cycle by 12 days. The luminescence of fMoS2-QDs in third instar larvae were observed by optical fluorescence microscopy (Nikon Eclipse NIU), in comparison with the auto-fluorescence from a control Drosophila melanogaster which were fed in normal food. Fig. 5 shows f-MoS2-QDs exposed larvae (and control) under different concentrations and observed under green filter. Fig. 5(a), (b) and (c) present the representative images of the bright larvae of Drosophila melanogaster. Interestingly, we observed green luminescence of f-MoS2-QDs in intestinal part of third instar larvae fed on two different concentrations (40 & 80 µl/ml) of f-MoS2-QDs ((Fig. 5(e)) and (h)) and their respective merged images showing clear green luminescence in intestinal part (Fig. 5(f) & (i)). Since the Drosophila survives at f-MoS2QDs concentration ~ 80 µl/ml, so we can conclude that these f-MoS2QDs also have good bio-compatibility. In addition to this, there are several other reports about the biocompatibility of MoS2 [37,38]. Thus f-MoS2-QDs can be used for imaging purposes in different systems.

Table 2 QY of the f-MoS2-QDs. Sample

Integrated emission intensity (I)

Absorbance at 308 nm

Refractive index of the solvent (n)

Quantum yield (QY)

Quinine Sulfate f-MoS2QDs

38,907.6

0.013413

1.63

0.546 (known)

2722.592

0.01498

1.33

0.023

3.5. Band gap calculation of f-MoS2-QDs Tauc's plot has been used for the band gap calculation of f-MoS2QDs. Two band gaps Eg = 2.5 eV and 3.7 eV were found (Fig. S4), which is larger than bulk MoS2, because reduction in particle size leads to increased band gap. Presence of multiple band gaps may be assigned to the polydispersity nature since smaller particles possess larger band gap (~ 3.7 eV), while bigger have smaller one (~ 2.5 eV). 3.6. Imaging of alimentary canal in Drosophila using f-MoS2-QDs These highly water soluble and stable f-MoS2-QDs with fluorescence property have been used for imaging of alimentary canal in Drosophila (Fig. 5). For imaging purpose of any nanoparticle in Drosophila, oral ingestion of the luminescent probe is the simplest approach, where doses may readily be controlled in dietary uptake. Furthermore, this

4. Conclusions In summary, f-MoS2-QDs have been synthesized using a facile and

Fig. 5. This figure is showing the green fluorescence of f-MoS2-QDs inside the intestine of the drosophila third instar larvae fed on f-MoS2-QDs supplemented food. First panel showing the alimentary canal of larvae fed on normal food i.e. control one ((a), (b) and (c) are showing bright field, green filter and merged images, respectively). Middle and lower panel showing the alimentary canal of third instar larvae feed on 40 and 80 µl/ml of f-MoS2-QDs supplemented food ((d) and (g), (e) and (h), (f) and (i) are bright field, green filter and merged images respectively). 116


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eco-friendly hydrothermal method and confirmed using Raman and TEM characterization techniques. Particle size distribution data fitted for Gaussian function yields an average particle size ~ 3.5 nm. Presence of various functional groups viz. –COOH, -NH2, SO42- etc. have been confirmed using FT-IR spectrum which are supposed to passivate the surface of these QDs to make them water soluble with stable PL intensity up to several months. The stability of f-MoS2-QDs has been tested for six months and a slight decrement (~3%) in the PL intensity was observed. Further, these highly stable, biocompatible and fluorescent f-MoS2-QDs have been used as an entrant for the imaging of alimentary canal of Drosophila. We believe that the present study may provide an insight to the in-situ functionalization of MoS2-QDs for making them soluble & stable in aqueous medium as well as biocompatible with their possible application in the field of bio-imaging.

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Acknowledgements HM is thankful to the UGC, New Delhi, India for JRF and CSIR, New Delhi, India for SRF (Letter No.: 09/013(0752)/2018-EMR-I) fellowships. VKS acknowledges UGC for providing Junior Research Fellowship (Fellowship Grant No: F.25-1/2014-15/(BSR). AS acknowledges to DST, India (PURSE Scheme 5050 and DST/TSG/PT/ 2012/68), DST-SERB India (Project Code: EMR/2016/007720) and CAS, Department of Physics, BHU for financial support. Authors are thankful to Biophysics Lab, Department of Physics, Institute of Science, BHU, for UV–Vis, PL and TRPL measurements. We are thankful to Dr. R. K. Srivastava for AFM characterizations. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jlumin.2018.05.016. References [1] S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutiérrez, T.F. Heinz, S.S. Hong, J. Huang, A.F. Ismach, Progress, challenges, and opportunities in twodimensional materials beyond graphene, ACS Nano 7 (2013) 2898–2926. [2] R. Mas-Balleste, C. Gomez-Navarro, J. Gomez-Herrero, F. Zamora, 2D materials: to graphene and beyond, Nanoscale 3 (2011) 20–30. [3] Z. Gan, L. Liu, H. Wu, Y. Hao, Y. Shan, X. Wu, P.K. Chu, Quantum confinement effects across two-dimensional planes in MoS2 quantum dots, Appl. Phys. Lett. 106 (2015) 233113. [4] H. Dong, S. Tang, Y. Hao, H. Yu, W. Dai, G. Zhao, Y. Cao, H. Lu, X. Zhang, H. Ju, Fluorescent MoS2 quantum dots: ultrasonic preparation, up-conversion and downconversion bioimaging, and photodynamic therapy, ACS Appl. Mater. Interfaces (2016). [5] A. Molina-Sánchez, D. Sangalli, K. Hummer, A. Marini, L. Wirtz, Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2, Phys. Rev. B 88 (2013) 045412. [6] J. Yang, D. Voiry, S.J. Ahn, D. Kang, A.Y. Kim, M. Chhowalla, H.S. Shin, Two‐dimensional hybrid nanosheets of tungsten disulfide and reduced graphene oxide as catalysts for enhanced hydrogen evolution, Angew. Chem. Int. Ed. 52 (2013) 13751–13754. [7] G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, M. Chhowalla, Photoluminescence from chemically exfoliated MoS2, Nano Lett. 11 (2011) 5111–5116. [8] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Emerging photoluminescence in monolayer MoS2, Nano Lett. 10 (2010) 1271–1275. [9] D. Gopalakrishnan, D. Damien, M.M. Shaijumon, MoS2 quantum dot-interspersed exfoliated MoS2 nanosheets, ACS nano 8 (2014) 5297–5303. [10] R.J. Smith, P.J. King, M. Lotya, C. Wirtz, U. Khan, S. De, A. O'Neill, G.S. Duesberg, J.C. Grunlan, G. Moriarty, Large scale exfoliation of inorganic layered compounds in aqueous surfactant solutions, Adv. Mater. 23 (2011) 3944–3948. [11] R. Das, H. Mishra, A. Srivastava, A.M. Kayastha, Covalent immobilization of βamylase onto functionalized molybdenum sulfide nanosheets, its kinetics and

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