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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Phosphine-Free, Highly Emissive, Water-Soluble Mn:ZnSe/ZnS Core− Shell Nanorods: Synthesis, Characterization, and in Vitro Bioimaging of HEK293 and HeLa Cells Joicy Selvaraj,† Arun Mahesh,‡ Vijayshankar Asokan,§ Vaseeharan Baskaralingam,∥ Arunkumar Dhayalan,‡ and Thangadurai Paramasivam*,† †

Centre for Nanoscience and Technology and ‡Department of Biotechnology, Pondicherry University, Puducherry 605 014, India State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310 027, China ∥ Department of Animal Health and Management, Alagappa University, Karaikudi 630 003, Tamil Nadu, India §

S Supporting Information *

ABSTRACT: Phosphine-free, highly luminescent one-dimensional Mn2+ ion-doped ZnSe(core)/ZnS(shell) nanorods (NRs) were synthesized by heating-up method (core) followed by hot injection route (shell). Effect of Mn2+ doping and shell thickness on structural and optical properties is reported. The NRs were formed with wurtzite-structured Mn:ZnSe core (diameter 2.52 nm) encapsulated epitaxially by a wurtzite-structured ZnS shell (thickness of 3.51 nm) with 3.1% lattice mismatch that alters the band alignment of the overall core−shell structure. A redshift was observed in optical absorption and photoluminescence (PL) emission due to an overall size increase with increasing shell thickness. Because of the reduction of defects/traps by surface passivation, the maximum photoluminescence quantum yield (QY) was obtained to be 49.35%. The exciton radiative lifetime for the core−shell NRs (1.678 ms) was more prolonged than that of the core (0.573 ms). A clear dependence of QY and lifetime was established on the Mn2+ content and ZnS shell thickness. The core−shell structure with thickest shell showed good photostability. Water solubility was achieved by ligand exchange with bifunctional 11-mercaptoundecanoic acid without modifying the optical and microstructural properties. These core−shell NRs were successfully tested for bioimaging of human HEK293 and HeLa cells with good permeability to cells. Toxicity was observed to be 3% for the 100 μg/mL dose. KEYWORDS: core−shell nanorods, Mn:ZnSe/ZnS, epitaxial growth, fluorescent agent, bioimaging

1. INTRODUCTION

based chalcogenides have been regarded as the better choice, particularly in the form of core−shell architectures (e.g., Mn:ZnSe/ZnO, Mn:ZnSe/ZnS, Mn,Cu:ZnSe/ZnS, etc.) for biological applications.10−14 They have additional advantages like large Stokes shift to avoid self-absorption, suppressed host emission, longer excited-state lifetime, improved photostability, and low toxicity. As well, the size and morphology of the semiconductor nanomaterials play an important role in the bioimaging applications. Over the zero-dimensional (0D) spherical quantum dots, one-dimensional (1D) architecturebased semiconductor nanostructures (e.g., nanorods and nanowires) have unique properties including strong linearly polarized emission, faster radiative decay rate, larger absorption cross-section, and larger surface area per particle.15−17 These

Design of two different semiconductors in core−shell geometry have shown their enhanced electrical and optical properties, which are not obtainable in single semiconductor quantum dots, and they possess a lot of applications right from optoelectronics to biotechnology.1−3 In core−shell architecture, shell with wider bandgap material can protect the surface of the core from oxidation and defect generation and thus lead to enhanced photoluminescence as well as photostability. To date, cadmium-based II−VI semiconductor core−shell nanostructures (e.g., CdSe/CdS, CdSe/ZnS, CdS/ZnS, CdSe/ZnSe, etc.) such as dot in dot, dot in rod, dot in tetrapod, rod in rod, dot in hexagonal platelet, etc. have been the most investigated structures owing to their excellent optical properties with tunable narrow emission in visible region as well as high fluorescence quantum yield (QY).4−9 However, the toxicity of Cd limits their usage in biological applications. Alternative to these Cd-based chalcogenides, transition-metal ion-doped Zn© XXXX American Chemical Society

Received: November 17, 2017 Accepted: November 30, 2017 Published: November 30, 2017 A

DOI: 10.1021/acsanm.7b00218 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

2. RESULTS AND DISCUSSION In our synthesis, phosphine-free and highly reactive Se precursor was used. At room temperature, the Se metal powder was dissolved in the mixture of 1-dodecanethiol (DDT) and oleylamine (OAm) to generate a highly reactive alkylammonium selenide in which Se metal powder was reduced by DDT and subsequently dissolved in OAm.33,34 The color of the solution was immediately turned to bright wine red when the Se metal powder was completely dissolved in OAm. The ZnS layer was grown with three different injections of the precursors to attain three different thicknesses, and hereafter they are referred to as ZnS-1, ZnS-2, and ZnS-3, respectively. After the preparation of highly fluorescent oleophilic core−shell NRs, it is a crucial step to make them as hydrophilic NRs usable for biological applications. Thus, the ligand-exchange approach was employed to replace the hydrophobic ligand (OAm) with the hydrophilic ligand. In this study, aliphatic monothiol ligand MUA (HS(CH2)10CO2H) was used as a hydrophilic ligand to obtain the water-soluble core−shell NRs for biological application studies. At pH ≈ 12, the thiol group (−SH) of MUA (the dissociation constant pKa value of thiol group is ∼11) was deprotonated, and this thiolate (−S−) anion more strongly binds to the surface of core−shell NRs than the thiol (binding energy of Zn−Sthiolate and Zn−Sthiol is 194.7 and 31.8 kJ/mol, respectively).26 The other end of the carboxylic group (−COOH) in MUA promotes the solubility in water through its carboxylate (COO−) form. Long alkyl chain length (11 carbon atoms) of MUA improves the separation between NRs and the environment, unlike shortest alkyl chain, which shows the poorest shielding against the environment. The schematic representation of synthesized oleophilic core−shell NRs by heating-up and hot injection methods followed by ligand exchange with MUA is shown in Figure S1 of the Supporting Information. 2.1. Structure and Microstructure Analysis. The X-ray diffraction (XRD) patterns of pure and Mn2+ ion-doped ZnSe are shown in Figure 1a. On the one hand, the pure and Mn2+ ion-doped ZnSe core exhibit a wurtzite structure of ZnSe (ICDD No. 98-002-7685, shown as stick pattern in Figure 1a), and Mn2+ ion doping does not lead to any lattice distortion of the crystal structure. On the other hand, the XRD patterns of ZnS shell (ZnS-3) grown on different Mn:ZnSe core show the wurtzite phase of ZnS (JCPDS No. 36-1450) as shown in Figure 1b. Figure 1c compares the XRD patterns of the core and its corresponding core−shell NRs along with the standard JCPDS data. By comparing the diffraction pattern of Mn:ZnSe core, the peak positions for Mn:ZnSe/ZnS core−shell NRs are systematically shifted to a higher Bragg’s reflection angle and toward standard wurtzite ZnS phase. Thus, the XRD results clearly proved the epitaxial growth of ZnS shell on Mn:ZnSe core. In addition, the peaks lie in the 2θ range from 20 to 36° for the core, and the core−shell NRs are deconvoluted into three peaks by using the pseudo-Voigt function (Figure 1d); the full width at half-maximum (fwhm) values of deconvoluted peaks are given in Table S1 of the Supporting Information. Those deconvoluted three peaks are assigned to the (010), (002), and (011) planes of the ZnSe phase (in the case of core), and their respective fwhm values are 3.54, 0.73 and 3.32°. In the case of core−shell NRs, the three peaks are assigned to (100), (002), and (101) planes of the ZnS wurtzite phase with the respective fwhm of 1.45, 1.12 and 1.24°. In both cases of core and core−

features of 1D nanostructures facilitate conjugation with multiple biomolecules of interest, which makes them potentially better bioprobes than the isotropic quantum dots.18 In a more general sense, for cellular uptake, the size of the particles should be small enough to cross the cell membrane, and 1D nanostructures fulfill this necessity at least in a 1D direction (length may be larger to be completely taken by the cells). Synthesis of the core−shell nanostructures with controlled shape and size is highly challenging. Organometallic hightemperature growth by using hydrophobic ligands is the best choice to synthesize high-quality colloidal core−shell nanostructures with monodispersed particles and excellent photoluminescence.19,20 Unfortunately, they are incompatible in aqueous media to be used in biological and medical applications. Therefore, the core−shell nanostructures should have well-defined functional groups on its surface to disperse well in water. In fact, transferring the hydrophobic core−shell nanostructures from organic phase to biocompatible media such as water or buffer solutions is quite challenging to be used in biomedical applications. In the past decades, several functionalization strategies have been developed such as direct ligand exchange, silanization, cross-linked polymer coating, etc. to make biocompatible hydrophilic nanostructures.21−29 Among them, direct surface cap-exchange with bifunctional ligands has been found to be the most promising method for transforming the hydrophobic nanostructures into the aqueous phase with high transform efficiency and a small particle size.26 In this work, we report the synthesis of Mn:ZnSe/ZnS core− shell NRs by anisotropic epitaxial growth by heating-up method followed by hot injection method. Over the two-step procedure, this approach has great advantages such as the controllable growth of core and shell in the same reaction mixture. Moreover, in many of the reported procedures, the hazardous Se-organophosphine complex was used as Se precursor for the preparation of Se-based nanocrystals.30,31 This inspired us to prepare the Mn:ZnSe/ZnS core−shell NRs by using a less toxic organic solvent, and therefore we adopted a phosphine-free synthesis. A wider bandgap material ZnS was chosen as a protective layer for the core (Mn:ZnSe), because there was expected a smaller (4%) lattice mismatch between wurtzite ZnSe and ZnS structures, which in turn reduces the defect formation at the interface between the core ZnSe and the shell ZnS.32 The core ZnSe was doped with different concentrations of Mn ions and coated with ZnS shell of three different thicknesses. The native hydrophobic ligand of the core−shell NRs was successfully replaced with thiolated aliphatic carboxylic acid (11-mercaptoundecanoic acid, MUA) via ligand-exchange approach. Their optical properties were tuned by varying Mn ion content in the core and changing the shell thickness. A thorough study of structural, microstructural, and optical properties is reported. Further, the water-soluble MUA-capped Mn:ZnSe/ZnS core−shell NRs were successfully prepared and applied for in vitro optical fluorescence imaging of human HEK293 (kidney cells) and HeLa cells (cervical cancer cells). To use these materials for bioimaging applications, the photoluminescence (PL) QY should be high enough. On the basis of these, the main scope of this work is to prepare the Mn:ZnSe/ZnS core−shell NRs with the phosphine-free synthesis and to achieve high QY. To the best of our knowledge, this is the first-time study on this system with core−shell architecture in nanorod morphology. B


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except the peak corresponds to the growth direction for NRs (i.e., (002) plane). Typical low- and high-magnification HRTEM and high-angle annular dark-field (HAADF) micrographs of Mn:ZnSe core are shown in Figure 2. They (Figure 2a−c) clearly show the nanorod shape of the core Mn:ZnSe. Note that all the NRs are single and that there are no other types such as twins or tripods, and what is observed of these types in Figure 2a,b are the overlapping of one rod on the other. The interplanar d-spacing value was obtained as 0.34 nm (from Figure 2c) for the planes along the width direction of the NRs, and the same was 0.32 nm in the length direction. The d values 0.34 and 0.32 nm are assigned to the (010) and (002) planes, respectively, of the wurtzite ZnSe phase. Thus, it is concluded that the anisotropic growth of Mn:ZnSe NRs is along the (002) plane direction, which is also consistent with the XRD results. The STEMXEDS elemental maps and spectrum for Mn:ZnSe core NRs are presented in the Supporting Information (Figures S2 and S3). From Figure S2, it can be seen that the elements (i.e.) zinc, selenium, and manganese are homogeneously distributed over the entire region of the NRs. Also, the energy-dispersive X-ray spectroscopy (XEDS) spectrum (Figure S3) shows presence of Zn, Se, and Mn elements only in the Mn:ZnSe core NRs. It is worth noting that there is a little chance of getting S in the core NRs, because during the synthesis, DDT was used for the reduction of Se powder. So, there is a possibility that DDT release sulfur at higher temperature and to form ZnSxSe1−x alloy. Therefore, to check this, XEDS measurement was extensively performed on the Mn:ZnSe core NRs at different places. The acquired XEDS spectra for Mn:ZnSe NRs did not show the presence of S element. Therefore, we can conclude that there is no S incorporation in the Mn:ZnSe NRs in this work. Figure 3 shows the HRTEM, HAADF-STEM (STEM = scanning transmission electron microscopy) images, and XEDS spectrum of the Mn:ZnSe/ZnS core−shell NRs. As shown in Figure 3a−c, the ZnS shell preserves the rod architecture of Mn:ZnSe core. The selected area electron diffraction (SAED) pattern of core−shell NRs presented in the inset of Figure 3b shows sharp rings in it indicating the polycrystalline nature of wurtzite ZnS. The XEDS spectrum (Figure 3d) confirms the presence of Zn, Se, Mn, and S in the Mn:ZnSe/ZnS core−shell NRs. The presence of Cu characteristic peak in the XEDS spectrum is originated from the Cu grid used for the TEM analysis. It is important to note that the TEM images of the Mn:ZnSe/ZnS core−shell NRs (Figure 3) show some broken pieces of NRs that are just the broken pieces of the rods while handling them during the TEM sample preparation process,

Figure 1. XRD patterns of (a) pure ZnSe and Mn:ZnSe NRs with various doping concentrations (mmol) of Mn2+ ions and (b) Mn:ZnSe/ZnS (thickest) core−shell NRs. (c) Comparative XRD patterns of 0.015 mmol Mn2+-doped core and core−shell NRs, and (d) deconvoluted XRD peaks of core and core−shell NRs appeared in the range between angles (2θ) 20° and 36°.

shell NRs, the fwhm of (002) peak is found to be narrower compared to the other two reflections. This clearly shows that the ZnSe core has anisotropically grown in the (002) direction, and shell ZnS has followed the same growth direction (which is expected to be thermodynamically and structurally stable). Obviously, this is an epitaxial growth because of the same crystal structure of the core and overgrown shell materials. Further, these XRD results are in concurrence with highresolution transmission electron microscopy (HRTEM) analysis as will be discussed below. Also, the XRD peaks of all samples show the characteristic peak broadening (fwhm),

Figure 2. (a) Low-magnification HRTEM, (b) HAADF, and (c) high-magnification HRTEM micrographs of Mn ion-doped ZnSe NRs, the core material. (inset, a) The size (nanorod diameter) distribution histogram of core NRs estimated from the HAADF-STEM image analysis. The size distribution was fitted to the log-normal distribution function. (inset, c) Enlarged nanorod marked with dotted circle. C


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size as the pure core NRs. Additionally, the intensity line profiles of the elements Se and S were acquired from XEDS maps across a single core−shell NR are presented in Figure 4e. For good visibility, the profiles are overlaid on the schematic of the cross-sectional view of the core−shell NR as in Figure 4e. The intensity line profile clearly demonstrates that the signal from the element Se is localized only in the core region, whereas the intensity line profile of sulfur looks like a valley shape with two peaks in the shell region of the core−shell NR. Because the projected area along the electron beam direction sees more S on the wall side and more Se in the core side, it therefore gives the profile like a valley in the case of S and single maximum in the case of Se. This intensity profile gives the clear evidence of ZnS shell around the Mn:ZnSe core. Size (diameter of the NRs) distributions obtained from the HAADF image analysis for the core and core−shell architecture are presented in the Inset of Figures 2a and 3a, respectively. The average diameter of the core Mn:ZnSe NRs is 2.52 nm, and the same for the core−shell NRs is 6.03 nm. The average diameter of core NRs (2.52 nm) is smaller than the bulk Bohr excitonic radius of ZnSe (which is 3.8 nm), which may lead to a strong quantum confinement in the radial direction. It is worth noting that the width of the Se distribution in the Se elemental map is equal to the size of the core NRs. The HRTEM images of the core and core−shell NRs have shown well-resolved lattice fringes, which imply the crystalline nature of these samples. Additional HRTEM and HAADF images of both the core and the core−shell structures are given in the Supporting Information (Figures S4 and S5). The size distribution is also obtained from the HRTEM images, and they are presented in the Supporting Information (Figure S6). On the basis of the earlier reports, the possible reasons for the anisotropic growth of core NRs are given as follows: first, wurtzite structure is believed to be a predominant reason for the anisotropic growth, because wurtzite structure in solution phase is preferably grown anisotropically in a 1D direction (rod/wire).35 Second, the anisotropic growth is also favored by the high monomer activity (effective concentration). So, the available highly active precursors in the reaction system lead to the high monomer concentrations, which favored the growth kinetics resulted in the formation of anisotropic 1D NRs along c-axis of the wurtzite structure.36,37 2.2. XPS Analysis. The X-ray photoelectron spectroscopy (XPS) survey spectrum (Figure S7 of the Supporting Information) shows the presence of Zn, Se, Mn, and S elements in the core−shell NRs. The high-resolution Zn-2p, Se-3d, Mn-2p, and S-2p core-level XPS spectra of Mn:ZnSe/ ZnS core−shell NRs are given in Figure 5. The presence of C1s resulted from the oleylamine wrapping on the surface of

Figure 3. (a) Low-magnification and (b) high-magnification highresolution TEM, (c) HAADF-STEM micrographs, and (d) the corresponding XEDS spectrum of the Mn:ZnSe/ZnS core−shell NRs. (inset, a) The size (nanorod diameter) distribution histogram of core/shell NRs estimated from the HAADF-STEM image analysis. The size distribution was fitted to the log-normal distribution function. (inset, b) The SAED pattern of the core−shell NRs.

and it is not a separate nucleation of any particles of the core− shell components. To further confirm the core−shell architecture, the STEM-XEDS elemental mapping was conducted. Figure 4a−d shows the HAADF image and elemental maps of elements Zn (blue), Se (purple), and S (red) in the core−shell NRs. Figure 4a shows the region from which all the elemental maps were acquired. The color elemental mapping indicates that the broken pieces contain all the three elements suggesting that no separate nucleation of ZnS particles has occurred during the shell growth. The size of the elemental distribution is taken as an indication to recognize the core as well as the shell regions. By looking at the Zn and S distribution, the width of the elemental distribution is the same as that of the NRs in the HAADF image (Figure 4a). This implies that the elements Zn and S are distributed throughout the rod thickness. Since the core and shell contain Zn, it cannot be used to differentiate the core from the shell, because it appears throughout the NRs. Though S is present only in the shell, it is always on top of NRs, and thus the projection of these NRs (along the direction of electron beam) will always show the presence of S. Therefore, the element S also cannot be used for understanding the core−shell architecture. The option left out and good indicator here is the Se, which is present only in the core, and its distribution (Figure 4c) in the core−shell NRs is the width of ∼2.5 nm, which is of the same

Figure 4. (a) HAADF-STEM image and the corresponding XEDS elemental maps of (b) Zn (blue), (c) Se (purple), and (d) S (red) of Mn:ZnSe/ ZnS core−shell NRs. (e) Intensity line profiles of Se (from c) and S (from d) elements across a single Mn:ZnSe/ZnS core−shell NR. The core and shell structure is a schematic of the cross section of the core−shell NR to illustrate the distribution of elements in it. D


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the increased overall size of the core−shell NRs upon ZnS coating. Note that the obtained bandgap (Table 1) of the core−shell NRs lies between the respective bulk bandgap value of the core (2.7 eV for ZnSe) and the shell (3.77 eV for ZnS). Decreasing bandgap with growing ZnS layer hinted at the size dependence. The steady-state PL spectra of Mn:ZnSe/ZnS with different shell thicknesses are shown in Figure 6d−f. Importantly, the PL spectra of core alone as a function of doping content is presented in the inset of Figure 6d, and its worth discussing this first to understand the role of Mn ion content on the PL emission of the core NRs. Pure ZnSe shows only one broad emission at 450 nm originated from the surface defects of ZnSe (inset of 6d). When Mn2+ ions are introduced into ZnSe, a new characteristic orange emission appears at 590 nm along with blue emission from the host ZnSe NRs. The newly appeared orange emission originates from the 4T1 → 6A1 internal d−d transition within the 3d shell of Mn2+ ion, thus indicating the successful doping of Mn2+ in ZnSe host NRs.12,13 When the Mn2+ ion content increases, the PL intensity of 590 nm peak is enhanced up to 0.015 mmol of Mn2+, and further increase in dopant has led to a quenching of orange emission, whereas the emission from the host ZnSe (450 nm) keeps on decreasing with increasing Mn2+ ion concentration. On the one hand, initial increment in 590 nm emission intensity is because of the faster nonradiative trapping of conduction band electron by 4T1 level of Mn2+ than the electron capture by defect states. On the other hand, the PL quenching at higher Mn2+ concentration is due to the transfer of photoexcited electrons from one Mn2+ ion site to its nearest neighbor Mn2+ ion site via nonradiative transitions followed by several transfer steps.41 The 590 nm PL emission (Figure 6d−f) of core−shell NRs has significantly enhanced compared to the core NRs without shell. At the same time, interestingly, the PL intensity is found to increase with increasing ZnS layer thickness. This enhanced PL emission had presumably resulted from the suppression of nonradiative carrier trapping by surface defect states and keeping the Mn2+ ions sufficiently far away from the surface traps.11 In addition, the emission peak of core−shell NRs shows a noticeable shift toward longer wavelength (red shift) of ∼9 to 11 nm as well as peak broadening in each step of newly grown ZnS layer, that is, with increasing shell thickness. The wavelength corresponding to the PL maximum is presented in Table 1. Typically for the 0.005 mmol Mn:ZnSe/ZnS NRs, the fwhm varies from 59.1 to 62.0 nm with increasing the ZnS shell thickness. Similar red shift and peak broadening are observed for all the samples (Table 1). Increasing peak width originates from the distribution of particle size in the range from 4.5 to 9 nm as seen in TEM section. In addition, the shift in PL maxima is also influenced by the Mn2+ ion content in the core, and therefore the overall shift is not the same for all the samples. Similar results were also observed by Zeng et al., for Mn:ZnSe coated with ZnS shell. 11 Another important observation made is the huge Stokes shift Δs, that is, the separation between the absorption and emission maxima. This high Stokes shift is high enough to avoid the reabsorption. In our case, the Stokes shift of ∼217.7 nm (Figure 6g) is observed due to the 4T1 → 6A1 transition of Mn2+ ions. Note that this is a crucial parameter for light-emitting devices. To understand the synergistic effect of Mn2+ ion content in the core and the thickness of the shell in the Mn:ZnSe/ZnS core−shell NRs, the PL emission spectra of them with ZnS-3 layer for varying content of Mn2+ ion in the core are plotted and presented in

Figure 5. Core-level XPS spectra (a) Zn-2p, (b) Se-3d, (c) S-2p, and (d) Mn-2p of OAm-coated Mn:ZnSe/ZnS core−shell NRs.

NRs. Also, the observed O-1s peak is due to the adsorbed atmosphere oxygen. The Zn core-level spectrum (Figure 5a) has split into Zn-2p3/2 (1021.6 eV) and Zn-2p1/2 (1044.7 eV) with a spin−orbit splitting energy of ∼23.1 eV, which is corresponding to that of Zn2+, the oxidation state of Zn.38 As core and shell both contain zinc ions, Zn-2p core level has narrow line width (2.1 eV) and intensity. The Se-3d core level spectrum (Figure 5b) is deconvoluted into two Gaussian components located at 53.8 eV (3d5/2) and 54.7 eV (3d3/2) with an energy splitting of 0.9 eV, implying that the chemical state of Se element is −2.39 The deconvoluted core-level spectrum of S-2p is shown in Figure 5c. The bivalent S2− state is confirmed by peaks at 161.2 eV (2p3/2) and 162.4 eV (2p1/2) with its characteristic peak separation of 1.2 eV. The S-2p peak located at 161.2 eV is attributed to the Zn−S bond.38 The binding energy centered at 640.2 eV corresponds to Mn-2p3/2 core level (Figure 5d), which is consistent with Mn2+ valence state of manganese.40 The observed Mn-2p3/2 core-level spectrum is very weak, which signifies that low content of Mn2+ ions at the surface of core−shell NRs. 2.3. Optical Studies. The UV−vis absorption, photoluminescence, and time-resolved photoluminescence (TRPL) spectroscopies were employed to study the optical properties of the core−shell NRs. The UV−vis absorption spectra of all the samples are presented in Figure 6a−c. The exciton absorption maxima for pure, 0.005, 0.015, and 0.025 mmol Mn:ZnSe core NRs are observed at 366.8, 369.0, 370.1, and 373.5 nm, respectively, and all these are blue-shifted in comparison to bulk ZnSe (λ = 460 nm). This blue shift of the core NRs is obviously indicating the quantum confinement effect due to the size (2.52 nm) smaller than its excitonic Bohr radius (3.8 nm). When the ZnS shell is grown, for a given Mn concentration in core, the absorption of the core−shell NRs shows a redshift with increasing shell thickness (Figure 6a−c). The type of shift can provide information about the shell formation if it is really a shell growth or alloy formation with the core.11 Alloy formation usually results in a blue shift, whereas the shell formation results in redshift of absorption maxima.11 Thus, once again the shell formation is confirmed from the observed redshift in our case. The redshift in the absorption with thickness can be ascribed to E


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Figure 6. (a−c) UV−vis absorption spectra and (d−f) PL spectra of Mn:ZnSe NRs coated with ZnS with different injection times containing different Mn ion concentrations (Mn: 0.005, 0.015, and 0.025 mmol), (g) the absorption (blue) and PL emission spectra (orange) of core−shell NRs showing a large Stokes shift (Δs = 217.7 nm), and (h) digital images of the core and core−shell structure with different shell thicknesses under day light and UV light excitation. (inset, d) The PL emission of core ZnSe NRs with different Mn2+ ion (mmol) concentrations.

Table 1. Optical Bandgap Value, PL Peak Position, and fwhm of Emission Peaka PL sample 0.005 mmol Mn:ZnSe/ZnS

0.015 mmol Mn:ZnSe/ZnS

0.025 mmol Mn:ZnSe/ZnS

a

core ZnS-1 ZnS-2 ZnS-3 core ZnS-1 ZnS-2 ZnS-3 core ZnS-1 ZnS-2 ZnS-3

lifetime parameters

bandgap (eV)

peak position (nm)

fwhm (nm)

QY (%)

τ1 (ms)

A1 (%)

τ2 (ms)

A2 (%)

τav (ms)

3.36 3.23 3.19 3.13 3.35 3.30 3.25 3.21 3.32 3.25 3.21 3.16

590 593 596 600 591 597 601 604 591 595 600 603

59.1 60.6 61.5 62.0 61.0 62.4 72.9 74.1 62.6 63.7 67.8 71.9

5.58 10.51 23.26 37.15 8.78 13.99 26.85 49.35 2.89 7.88 15.31 24.53

0.144 0.253 0.283 0.356 0.204 0.309 0.293 0.464 0.135 0.179 0.159 0.231

0.561 0.623 0.476 0.416 0.813 0.761 0.622 0.572 0.540 0.934 1.043 0.663

0.567 0.959 0.935 1.281 0.857 0.955 1.291 2.049 0.465 0.883 1.000 1.059

0.494 0.406 0.529 0.583 0.251 0.239 0.397 0.425 0.452 0.182 0.199 0.384

0.472 0.755 0.795 1.127 0.573 0.627 1.029 1.678 0.380 0.524 0.617 0.832

PL QY and PL lifetime parameters of Mn:ZnSe core coated with different thickness of ZnS shell achieved by different precursor injection times.

F


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ACS Applied Nano Materials the Supporting Information (Figure S8). It is very clear, for a given shell thickness, increasing Mn2+ content in the core has significantly increases the PL emission. This is a clear evidence for the synergistic effect contributed to the enhancement of PL emission by Mn2+ ion content as well as by shell thickness in this core−shell NRs. The red shift from 3 to 13 nm in the PL emission peak (also 5−28 nm in the absorption peak in Figure 6a−c) in the Mn:ZnSe/ZnS core−shell NRs can be explained by band alignment model of the core−shell structure. The bulk bandoffset of ZnSe/ZnS system exhibit type-I heterostructure; that is, the core material possesses a lower conduction band (CB) and a higher valence band (VB) compared to those of the shell material (that is, the exciton wave functions are confined within the core), with a CB offset of 0.35 eV and a VB offset of 0.55 eV.32,42 There are studies in the ZnSe/ZnS core−shell structures reported that the CB offset is depending on the crystal structure of the core and shell materials by using effective mass approximation model. In particular, when both the ZnSe (core) and ZnS (shell) possess cubic sphalerite structure, its CB offset is smaller than the same with WZ phases.32 Strain at the heterostructure interface plays a significant role in deforming the band alignment. Li et al. have theoretically proved the strain-induced changes in the band offset of CdSe/ZnS core− shell QDs.43 It has also been proved by Brusilovski et al. that the strain by even a small lattice mismatch (3%) can influence the band offset in PbSe/PbS core−shell colloidal QDs.44 Additionally, Smith et al. have demonstrated by using the ab initio calculations using deformation potentials that the CB minimum is strongly affected compared to the VB maximum by deformation lattice (strain in the lattice) in semiconductors.45 Even though the growth is epitaxial in our case, the lattice parameter c along the (002) direction (which is the c-axis) was calculated from the XRD patterns for ZnSe and ZnS as 6.472 and 6.277 Å, respectively. This is 3.1% mismatch between the lattices causing strain. In line with the reported theoretical and experimental observations, in the present case of Mn:ZnSe/ ZnS, the anisotropic strain induced by the lattice mismatch (3.1%) at the interface of core−shell structure modifies the band alignment. Therefore, the strain effect contributes to the spilling of electrons’ wave function partially into the shell, while holes’ remain in the core material (quasi type II; see Scheme 1). Thus, the switching from type-I to quasi type-II band alignment induced by strain leads to the red shift in the optical property (PL emission and absorption).42,46 In addition to quasi type-II band alignment, a mechanism for optical processes occurring in Mn:ZnSe/ZnS core−shell NRs under UV light irradiation is also depicted in Scheme 1. The optical process begins with the creation of electron−hole pairs in the ZnS shell predominantly by absorption of incident photons. Then holes in the VB of ZnS shell rapidly moved toward VB of the core (hole localized in the core NRs), whereas the electron wave function is delocalized in the CB of core and shell. Further, the electrons in the CB of core−shell NRs are nonradiatively trapped by the 4T1 level of Mn2+ ion and then recombine with the holes trapped by the ground state of 6A1 level of Mn2+ ion by radiative emission of orange light. Note at this point that the large Stokes shift discussed earlier has played a key role in the reduction of reabsorption and enhanced PL emission. Since QY determines the quality of the Mn:ZnSe/ZnS core− shell NRs for bioimaging, it was experimentally obtained by

Scheme 1. Schematic Band Diagram of the Mn Ion-Doped ZnSe/ZnS Core−Shell NRs Illustrating the Optical Processes Taking Place in Them under UV Light Excitation

using a standard dye (Rhodamine B) as reference. The obtained PL QY values for Mn:ZnSe core as well as for Mn:ZnSe/ZnS core−shell NRs with different Mn2+ concentrations and ZnS shell thicknesses are presented in Table 1. The QY for the Mn:ZnSe NRs is obtained as 5.58, 8.78, and 2.89% for 0.005, 0.015, and 0.025 mmol of Mn2+-doped ZnSe, respectively. There are two variations observed in the QY in the present case. First, when the Mn:ZnSe is coated with ZnS, obviously the QY has increased to a maximum of 37.15, 49.35, and 24.53%, respectively, for the Mn2+ concentrations of 0.005, 0.015, and 0.025 mmol in the core. Second, for a given Mn2+ concentration, the QY is increasing with increasing the ZnS shell thickness. It is obvious from the PL emission that, when the layer thickness increases, the electron wave function is allowed to be throughout the core and shell. In addition, increasing shell thickness also reduces the surface defects (which tend to quench the emission) and hence increases the QY. Another important observation is that, for a given ZnS shell thickness (including Mn:ZnSe), the QY has increased from 0.005 to 0.015 mmol of Mn2+ content and that further increase in Mn2+ concentration has reduced the QY. For example, for the thickest ZnS shell the QY for 0.005 mmol Mn:ZnSe/ZnS is 37.15%, and it is increased to 49.35% for 0.015 mmol, but the same is reduced to 24.53% for the 0.025 mmol of Mn2+. This is obviously due to the transfer of photoexcited electrons from one Mn2+ ion site to its nearest neighbor Mn2+ ion site via nonradiative transitions followed by several transfer steps because of the more available Mn2+ sites. The present QY can be compared with the already reported values for similar systems prepared by various methods. Zhu et al. had achieved 25% QY for the Mn:ZnSe/ZnS core−shell QDs prepared by microwave-assisted aqueous phase method.47 In many cases, the QY achieved in Mn:ZnSe/ZnS core−shell systems prepared in aqueous phase was less than 40% (e.g., 9% by Aboulaich et al., 24% by Dong et al., and 35% by Fang et al.).48−50 Zhang et al. reported 8% QY for the Mn:ZnSe QDs prepared by phosphine-free nucleation doping strategy, whereas they achieved the highest QY of 35% once the Mn:ZnSe QDs were overcoated with ZnS layer.51 In another case, ∼50% of QY was achieved by Yuan et al. at high temperature (500 K) but G


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Figure 7. PL decay curves of (a) 0.005, (b) 0.015, and (c) 0.025 mmol Mn2+ ion-doped ZnSe/ZnS core−shell NRs. ZnS-1, ZnS-2, and ZnS-3 represents the Mn:ZnSe core coated with ZnS shell with three different thicknesses, ZnS-3 being the thickest. Data points are the experimental observation, and the continuous lines are the fit with the eq 1.

not at room temperature as in our case as well as other cases.52 It is very clear in our case that Mn:ZnSe/ZnS had possessed a 49.35% with the thickest ZnS coating and 0.015 mmol of Mn2+ in the core. To summarize, the improvement in the PL QY is achieved by passivating the core with the wide bandgap ZnS, so that the recombination sites present on the surface of core are greatly reduced by the ZnS shell. However, the QY shown by these NRs is not comparable to the core−shell structures made with ternary and quaternary alloys.53,54 Figure 6h shows the digital photograph of samples acquired from the reaction of Figure 6e under day light and UV light (in dark) excitation, and the emission is seen to be orange only under UV light excitation. Also note that the brightness of orange emission increases with increasing the shell thickness. The CIE diagram showing the evidence of orange emission by the NRs are presented in the Supporting Information (Figure S9). To understand the relaxation dynamics of the excited-state electrons, room-temperature TRPL was measured for the core−shell NRs excited at 320 nm, and the respective PL decay curves are presented (Figure 7). The PL decay curves were fitted with the biexponential function, and the average lifetime (τav) was calculated by using the following Equation9 τav =

(A1τ1 + A 2 τ2 2) (A1τ1 + A 2 τ2)

coated with ZnS-1, ZnS-2, and ZnS-3, respectively). This increased lifetime has enhanced the PL emission performance of the Mn:ZnSe/ZnS core−shell NRs by suppressing the surface defects/traps; that is, the recombination is slower in this case to give a sustained PL emission. The second variation is that, for a given shell thickness (typically for ZnS-3), the τav increases from 1.127 ms (for 0.005 mmol Mn2+ content) to 1.678 ms (for 0.015 mmol of Mn2+ content). But, with further increase in Mn2+ content to 0.025 mmol, the τav is found to be decreased to 0.832 ms. The initial increase is due to the less available Mn2+ sites for the electron trapping, so that the excited electrons have a longer lifetime. Once the Mn2+ content increases, the increased Mn2+ sites will act as trapping centers for these electrons at a faster rate and hence the shorter lifetime. The similar dependence of the PL lifetime on the dopant and shell thickness is also shown by the PL QY. That is, for the higher doping content (0.025 mmol), the PL QY has decreased to 2.89% (from 8.78% for 0.015 mmol), while the lifetime also decreased to 0.832 ms (from 0.573 ms for 0.015 mmol). Analogously, for a 0.015 mmol Mn2+ content, the PL QY increases from 8.78 to 49.35%, and the lifetime has also increased from 0.573 to 1.678 ms with increase in shell thickness. The present PL lifetime is compared with the other reported values for the similar system. Aboulaich et al. have obtained a PL lifetime of 1.39 ms for Mn:ZnSe/ZnS quantum dots prepared by aqueous-based route.48 In the other cases, the Mn:ZnSe/ZnS quantum dots (prepared by nucleation doping strategy) have been shown to have τav of 0.57 ms and 0.41 ms.56,51 It is worth noting that, in all these three cases, the morphology of the core−shell structure is the quantum dots (0D nanodots). But in our case, the 0.015 mmol Mn:ZnSe/ZnS core−shell NRs showed 1.678 ms, which is much higher than the values reported above. It is also significant to note that the magnitude of the lifetime (with the maximum of 1.678 ms) in the core−shell structure can also be compared with the system with the Mn ion diffused into the ZnS shell. There are reports that with the Mn in ZnS possessed lifetime in the range from 0.11 to 2.0 ms.57−59 In our case also there is a possibility of Mn ion diffusion into the shell, but it is expected to be too minimum because of the very low concentration of Mn2+ ions in the core. The valid reasons for the enhanced PL QY and lifetime of Mn:ZnSe/ZnS core−shell NRs compared to the concentric core−shell structure are as follows: First, the elongated nanocrystals known as NRs have (1) larger absorption cross section, which makes it possible to photoexcite the multiple

(1)

where τ1 and τ2 represent the PL decay times, and A1 and A2 represent the normalized amplitudes of the decay components. The τav along with individual lifetime components are summarized in Table 1. The magnitude of the τ1 varies from 0.135 to 0.464 ms, whereas τ2 varies from 0.465 to 2.049 ms, and the τav varies from 0.380 to 1.678 ms for various samples. The τ1 is less compared to the τ2, and therefore the former is related to the fast process, and the latter is related to the slow process. Since additional energy states are incorporated between the VB and CB, the return transition of the excited electrons takes place via the intermediate states introduced by the Mn2+ ions (see Scheme 1), and therefore the PL lifetime in this case is expected to be more, and the overall luminescence yield is high. So, the slow decay component (τ2) is due to Mn2+ related emission, whereas the fast decay component (τ1) is attributed to the nonradiative process that occurs on the surface of the NRs.55 There are two different trends observed in the average lifetime. The first one is that, for a given Mn2+ content, the τav increases with the ZnS shell thickness (e.g., when the Mn2+ content is 0.015 mmol, the τav is 0.573 ms for the Mn:ZnSe core NRs, and the same is 0.627, 1.029, and 1.678 ms when H


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6 h, and the treated cells were (fluorescence) imaged by confocal laser scanning microscopy. Figure 8 presents the

electrons in NRs by a single excitation source, (2) aspect ratio that significantly minimizes the nonradiative carrier losses that originate from the Auger recombination in material. The suppression of Auger processes in NRs leads to the longer excited-state lifetime.17,60 In addition because of the higher aspect ratio, NRs have been reported to be better emitting species compared to lower aspect-ratio colloidal nanorods/ nanodots due to high polarized emission.15 The PL emission spectra of core−shell NRs before and after ligand exchange with MUA are compared in Figure S10a of the Supporting Information, and they look almost similar. This indicates that the core−shell NRs are able to retain their luminescence even after the ligand exchange. For many practical applications, especially for biological studies, stability against photo-oxidation of the water-soluble sample is crucial, and that criterion has also been considered in this work. Therefore, the photostability test was done under continuous UV (365 nm, 12 W) irradiation at normal temperature and pressure (NTP) followed by PL measurements at regular intervals. The PL intensity (relative intensity) after UV irradiation (Figure S10b of the Supporting Information) is increased initially and then decreased due to surface oxidation. After 24 h of UV irradiation, the PL intensity of ZnS-1 and ZnS-2 is dropped by 72.6% and 9.3%, respectively, from its initial value, while no change is observed for ZnS-3. Thus, the core with thickest ZnS shell showed complete stability against photocorrosion. In the case of MUA-capped core−shell NRs, the PL intensity is dropped to 43.75% from its initial value after 24 h of irradiation, and precipitation of particles was also observed, which could be due to the formation of disulfide bonds (COOH−(CH2)10−SS−(CH2)10−COOH) by photooxidation.61 The MUA-capped core−shell NRs show initial enhancement in the PL intensity which is observed up to a certain time and then decreases under UV exposure, and this behavior is due to the photoinduced oxidation process that led to the formation of a thin oxide layer around the NRs. With irradiation time, the thickness of the oxide layer grows further leading to more photocorrosion.61,62 It is worth noting that there is no precipitation observed when the MUA-capped core−shell NRs were stored at 4 °C, and also they preserved their fluorescence yield (monitored for two months). In addition, microstructural analysis is performed on MUA-capped Mn:ZnSe/ZnS core−shell NRs. The HRTEM micrographs (Figure S11 of the Supporting Information) show the rodlike structure, and dispersity of core−shell QDs is not significantly affected after ligand exchange with MUA. So, clearly the core− shell NRs did not undergo aggregation, and the morphology is maintained even after ligand-exchange process. Moreover, the surface charge of MUA-capped Mn:ZnSe/ZnS core−shell NRs was determined by the zeta potential measurement. The zeta potential measurement (Figure S12 of the Supporting Information) showed the negative potential of −57.7 mV. This negatively charged surface is due to the deprotonation of carboxylic group of MUA. 2.4. Bioimaging and Cell Viability Studies. To investigate the potential of water-soluble MUA-capped Mn:ZnSe/ZnS core−shell NRs for biomedical applications, the in vitro cell imaging was performed on human HEK293 and HeLa cells. On the basis of optical characteristics performed in the earlier sections, the sample with the highest QY (the 0.015 mmol Mn:ZnSe/ZnS core−shell NRs) was chosen for bioimaging studies. These cells were treated with MUA-capped core−shell NRs at the optimal concentration of 100 μg/mL for

Figure 8. Confocal fluorescence images of HEK293 and HeLa cells treated with MUA-capped Mn:ZnSe/ZnS core−shell NRs (100 μg/ mL) for 6 h acquired with the excitation wavelength of 405 nm. The left, middle, and right columns are the bright field, fluorescent, and overlapped images, respectively, of the HEK293 and HeLa cells. Scale bar is 20 μm.

bright field (left column), fluorescence (middle column), and overlay (right column) confocal images of HEK293 (top row) and HeLa (bottom row) cells treated with water-soluble core− shell NRs. Note that these fluorescence images were acquired with the excitation light wavelength of 405 nm by using a diode laser, and this excitation wavelength falls within the visible region of the electromagnetic spectrum, and therefore no harm is expected to the normal cells during fluorescence imaging. Bright field images show the shape of the cells, and the fluorescent images show a strong fluorescence signal in both the cell lines. The fluorescence image shows the uniform illumination throughout the cell, which clearly confirms the effective and uniform uptake of the MUA-capped core−shell NRs by HEK293 and HeLa cells. To understand the effective emission from the cells, the fluorescence images are overlapped with the bright field image, and a good correlation is obtained (right column of Figure 8). Another set of confocal images of HEK293 and Hela cells treated with MUA-capped core−shell NRs are given in the Figure S13 of the Supporting Information. When we use materials for bioimaging, especially in biosystem, there are certain parameters such as low toxicity and biocompatibility of the fluorescence imaging agent in the cellular environment that are the most important requirements. In this regard, in vitro cytotoxicity of the MUA-capped Mn:ZnSe/ZnS core−shell NRs were evaluated in the HEK293 and HeLa cells at different concentrations (0 to 1000 μg/mL) for 48 h through 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide (MTT) assay. In addition, the morphological changes of the cells incubated with 100 μg/mL of MUA-capped core−shell NRs for 48 h were also investigated. The results of morphological changes and viability of HEK293 and HeLa cells upon the treatment of core−shell NRs are presented in Figure 9a,b. It is found that only 3% reduction in the viability of the cells after being incubated with 100 μg/mL core−shell NRs for 48 h is observed, and also there are no obvious morphological changes between the control cells and the cells treated with the core−shell NRs (Figure 9a,b, respectively). Even in the case where the HEK293 and HeLa cells were treated with very high concentration (of 1000 μg/ I


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Figure 9. (a) Bright field optical images of HEK293 and HeLa cells treated without (column −) and with (column +) MUA-capped Mn:ZnSe/ZnS core−shell NRs (100 μg/mL) for 48 h to understand the morphological changes in these cells, and (b) cell viability results of HEK293 and HeLa cells upon treatment with varying concentrations of MUA-capped Mn:ZnSe/ZnS core−shell NRs for 48 h. methanol (CH3OH), acetone (C3H6O), and chloroform (CHCl3) were purchased from Merck, India. Manganese(II) acetate tetrahydrate (Mn(OAc)2·4H2O, ≥98%), selenium powder (100 mesh, ≥99.5%), OAm (70%), DDT (≥98%), and MUA (95%) were obtained from Sigma-Aldrich, India. All the chemicals were used directly for the synthesis without any further purifications. 4.2. Synthesis of Oleophilic Mn:ZnSe/ZnS Core−Shell NRs. Preparation of Stock Solutions. The Zn monomer was prepared by dissolving 1.2 mmol of Zn(Ac)2 in OAm and ODE (v/v, 1:9, in 3 mL) at 160 °C under Ar bubbling until getting a clear solution. The sulfur monomer was prepared by dissolving 1.2 mmol of sulfur powder in OAm and ODE (v/v, 1:9, in 3 mL) at 120 °C under Ar bubbling to get a clear yellow solution. These core−shell NRs were synthesized via the already reported procedure with slight modifications.63 Initially, Mn:ZnSe NRs were grown as core, and then ZnS shell was coated on it by introducing a solution of Zn and S precursors in three batches to form different ZnS shell thickness. For the preparation of Mn:ZnSe NRs, Zn(Ac)2 (0.2 mmol, 43.9 mg), Mn(Ac)2 (0.005 mmol, 1.225 mg), and Se powder (0.2 mmol, 15.79 mg) were mixed together with 4 mL of DDT and 6 mL of OAm in a three-necked flask equipped with thermometer, condenser tube, and oil bath placed on hot stir plate. This mixed solution was degassed for ∼15 min by purging with argon gas at 100 °C. Under Ar flow, the reaction mixture was heated to 220 °C and maintained at this temperature for 30 min. After completion of particle (NR) growth, the temperature of the reaction mixture was raised to 240 °C for the ZnS shell growth. At this temperature, an equal volume of Zn and S monomers mixture was swiftly injected into the hot crude core solution in three batches at an interval of 15 min (each batch contained a 2 mL aliquot of Zn and S monomers mixture). Before every injection, 1 mL aliquots were collected from the reaction mixture to monitor the growth of ZnS shell. After completion of the reaction, the resultant colloidal solution was naturally cooled to room temperature. To extract the unreacted precursors and byproducts, the as-synthesized oleophilic OAm-capped Mn:ZnSe/ZnS core−shell NRs were purified three times with the addition of CHCl3/acetone (v/ v, 1:2) mixture solvent followed by centrifugation and decantation. Afterward, they were dissolved in 10 mL of chloroform to form a stable clear colloidal solution or dried at 50 °C under vacuum to get powder for other characterizations. Oleophilic pure ZnSe NRs were prepared with three different concentrations of Mn2+ ion (Mn: 0.005, 0 015, and 0.025 mmol) and used for all the studies. Note that the concentrations of Mn ions in ZnSe NRs used throughout the manuscript is based on the initial concentrations added during the synthesis. Ligand Exchange. The initial OAm-coated oleophilic core−shell NRs were made water-soluble by ligand exchange with hydrophilic long-chain thiol carboxylic acid (MUA).64,26 Before performing ligand-

mL) of the core−shell NRs, the amount of cytotoxicity observed was just 10% and 20% on HEK293 and HeLa cells, respectively, upon the treatment for 48 h. Thus, there is enough time available to remove these particles from the cells after using them for bioimaging. The successful cellular uptake and low cytotoxicity of the MUA-capped Mn:ZnSe/ZnS core−shell NRs suggested that it could be an excellent fluorescent imaging agent to penetrate the cell membrane and image the live cells without affecting them.

3. CONCLUSIONS From the results of the present study, the following conclusions can be drawn: (1) highly emissive 1D core−shell NRs consisting of Mn:ZnSe core and ZnS outer shell were successfully synthesized by using phosphine-free Se precursor. (2) The Mn:ZnSe/ZnS core−shell NRs have been grown epitaxially and anisotropically along the (002) plane direction with the core and shell possessing wurtzite crystal structure. (3) The core−shell architecture has been clearly traced by XEDS elemental mapping with the average core size of 2.52 nm and core−shell size of 6.03 nm. (4) A good tunability was achieved in the band gap and PL emission (from 590 to 604 nm) of the Mn:ZnSe/ZnS core−shell NRs with the highest PL QY up to 49.35% by varying either the ZnS shell thickness or the Mn2+ content in the core. (5) The PL lifetime in the core−shell NRs was effectively prolonged for ∼3 times compared to that of core due to the suppression of surface traps by the ZnS shell thickness. (6) The oil-soluble OAm-coated core−shell NRs were successfully transformed into an aqueous phase (watersoluble) via ligand exchange by using MUA without affecting the luminescence intensity. (7) The water-soluble core−shell NRs were used as a fluorescent agent for imaging the human HEK293 and HeLa cells, and the cell viability test has proved its harmlessness against these cells. In summary, the present study revealed the immense potential of the Mn:ZnSe/ZnS core−shell NRs for applications in the biomedical field as an alternative for Cd-based core−shell nanostructures. 4. EXPERIMENTAL SECTION 4.1. Materials. The precursors zinc acetate dihydrate (Zn(OAc)2· 2H2O, 98%), sulfur powder (99%), tetramethylammonium hydroxide pentahydrate (TMAHP, 97%), and Rhodamine B were purchased from Himedia (India) company. 1-Octadecene (ODE, ≥91%), J


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ACS Applied Nano Materials exchange process, the initial oleophilic core−shell NRs were purified with ethanol for three times to remove the excess hydrophobic ligands on the surface of core−shell NRs for a successful ligand exchange. In a typical procedure, 100 mg of MUA was dissolved in 1 mL of methanol, and the pH of the solution was adjusted to ∼12 by addition of TMAHP. Then, 1 mg of oleophilic core−shell NRs (0.015 mmol Mn:ZnSe/ZnS-3) dissolved in 1 mL of chloroform was added into the above MUA−methanol solution rapidly. After it was vigorously stirred for 30 min at room temperature, the mixture was heated at 70 °C for another 20 min. After this mixture cooled to room temperature, double-distilled water (DDW; 1 mL) was added and stirred for another 10 min. The phase transformation of core−shell NRs from chloroform phase to water phase occurred (see Figure S1b). Finally, the upper part containing the core−shell NRs was collected, precipitated with addition of acetone, and centrifuged. Finally, the precipitate was dissolved in 1 mL of DDW and stored at 4 °C for further use. Characterization Techniques. Powder XRD patterns were acquired in a Rigaku (Ultima-IV) X-ray diffractometer by using a Cu Kα radiation with Kβ-line filtered. The UV−vis absorption spectra were recorded in a Shimadzu (UV-3600 plus) UV−vis−NIR (NIR = nearinfrared) spectrophotometer. Steady-state fluorescence and TRPL spectra were measured with Fluorolog FL3-11, Horiba Jobin Yvon, fluorescence spectrometer with a time-correlated single-photon counting (TCSPC) accessory. The PL spectra were acquired with the excitation wavelength of 320 nm. The PL QY was determined by the method of comparison with a standard known QY (Rhodamine B dye in methanol, QY = 70%, emission range: 560−590 nm), which can be calculated from the following expression.65 2 I A λ ⎛n ⎞ QYX = QYS X S exS ⎜ X ⎟ IS AX λexX ⎝ nS ⎠

instrument equipped with 20× immersion objective. A near-UV diode laser (λ = 405 nm) was used for excitation. Cell Viability Assay: The cell viability assay was performed on human HEK293 and HeLa cells by using MTT assay as described previously.66 Briefly, 1 × 104 cells were seeded in a 96-well culture plate and incubated with or without the indicated amount of sterile MUA-capped core−shell NRs for 48 h. The drug MG-132 (SigmaAldrich) was included in the assay as a positive control, which is a potent proteosomal inhibitor and known to induce apoptosis.67,68 After the incubation period, the cells were washed one time with DMEM medium. Then, 90 μL of fresh medium and 10 μL of sterile MTT solution (5 mg/mL in PBS) were added to each well of the 96well plate, and the plates were incubated for another 3 h. The resulting formazan crystals were solubilized by adding dimethyl sulfoxide (DMSO), and the resulting solution was read by a plate reader at 570 nm. MTT assay was performed with triplicate samples to ensure the reproducibility. Data are presented as means ± standard deviation. The percentage of cell viability was calculated with respect to control as following Equation.

Cell viability(%) =

IntensitySample IntensityControl

× 100 (3)

where Intensitysample and Intensitycontrol represent the optical density values of cells incubated with NRs and culture medium, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.7b00218. Schematic representation of preparation method of oleophilic core−shell NRs, and ligand exchange process, peak parameters of the XRD peaks located in the 2θ range from 20° to 36° after deconvolution, STEM-XEDS mapping and spectrum of Mn:ZnSe NRs, additional TEM, HRTEM, and HAADF images of Mn:ZnSe core and Mn:ZnSe/ZnS core−shell NRs, size distribution histogram of Mn:ZnSe core and Mn:ZnSe/ZnS core− shell NRs estimated from the HRTEM images, XPS survey spectrum, PL spectra of Mn:ZnSe/ZnS core−shell NRs with various Mn2+ content in the core for a given thickness of the ZnS shell (ZnS-3), CIE diagram of Mn:ZnSe core and Mn:ZnSe/ZnS core−shell NRs, PL spectra of Mn:ZnSe/ZnS core−shell NRs before and after ligand exchange, photostability test of OAm capped core−shell NRs with different shell thicknesses and MUA capped Mn:ZnSe/ZnS core−shell NRs (thickest shell) under UV irradiation for different period of times, HRTEM micrographs of Mn:ZnSe/ZnS core−shell NRs after ligand exchange with MUA, zeta potential measurement for the MUA-coated core−shell NRs, confocal images of HEK293 and HeLa cells incubated with MUAcapped Mn:ZnSe/ZnS core−shell NRs for 6 h (PDF)

(2)

where subscripts S and X refer to the standard and the sample under investigation respectively, I represent the integrated area of the PL spectrum, A is the optical density at the excitation wavelength used for the PL measurement (optical density was kept at ∼0.05 for all the samples to avoid the reabsorption), λex is the excitation wavelength (both reference and sample were excited at 350 nm), and n is the refractive index of the solvents (n = 1.4458 for chloroform, n = 1.329 for methanol). During the QY measurement, both the UV−vis absorption and fluorescence spectra were recorded with a constant slit width (2 nm, Bbandpass). Zeta potential measurement was performed in a HORIBA scientific SZ-100 instrument. The HRTEM measurement was performed in an FEI Tecnai G2 F20 TEM with an acceleration voltage of 200 kV. The STEM-HAADF and XEDS were performed on FEI Titan ChemiSTEM 80−200. For TEM analysis, the specimens were prepared on carbon film coated copper grids by placing a drop of dilute solution of the samples in chloroform solvent and evaporating the solvent in an air atmosphere. Analysis of HRTEM images and SAED patterns of samples was performed by using the ImageJ software. XPS measurements were done in a PHI 5000 VersaProbe-II equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) and a hemispherical analyzer. Electrical compensation was applied during the analysis. All the obtained binding energies were calibrated with C-1s core-level peak at 284.6 eV as a reference. Bioimaging and Cell Viability Studies. Cell Culture: The HEK293 and HeLa cells were purchased from NCCS, Pune, India. The cells were cultured in Dulbecco’s Modified Eagle Medium (DMEM; Himedia) supplemented with 10% fetal bovine serum (FBS; Himedia) and L-glutamine-penicillin-streptomycin solution (Himedia) at 37 °C in 5% CO2 condition. Bioimaging Studies: For bioimaging experiments, the HEK293 or HeLa cells were seeded on coverslips and treated with MUA-capped core−shell NRs at the concentration of 100 μg/mL for 6 h. After the treatment period, the cells were washed with phosphate-buffered saline (PBS) three times and fixed with 4% formaldehyde for 10 min at room temperature and embedded with Mowiol (Sigma). Confocal fluorescence images were taken using a Carl Zeiss LSM 710

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or thangaduraip.nst@ pondiuni.edu.in. ORCID

Vijayshankar Asokan: 0000-0003-0989-8083 Thangadurai Paramasivam: 0000-0002-6757-9016 Notes

The authors declare no competing financial interest. K


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ACKNOWLEDGMENTS This work was financially supported by DST-SERB, India (SR/ FTP/PS-137/2010). J.S. acknowledges UGC-India for the Maulana Azad National Fellowship (F1-17.1/2016-17/MANF2015-17-TAM-58815). The CIF of Pondicherry Univ. is acknowledged for the characterization facilities. Dr. Amirthapandian and Dr. Jagadeesan of MSD, IGCAR are acknowledged for TEM measurements.

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