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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 ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00218 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Applied Nano Materials

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,1 Arun Mahesh,2 Vijayshankar Asokan,3 Vaseeharan Baskaralingam,4 Arunkumar Dhayalan2 and Thangadurai Paramasivam1,* 1. Centre for Nanoscience and Technology, Pondicherry University, Puducherry - 605 014, India. 2. Department of Biotechnology, Pondicherry University, Puducherry - 605 014, India. 3. State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou – 310 027, China. 4. Department of Animal Health and Management, Alagappa University, Karaikudi – 630 003, Tamil Nadu, India. * [email protected] / [email protected]

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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 the shell thickness. Due to the reduction of defects/traps by surface passivation, the maximum PL quantum yield (QY) was obtained to be 49.35%. The exciton radiative lifetime was prolonged for the core-shell NRs (1.678 ms) 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 11mercaptoundecanoic acid without modifying the optical and microstructural properties. These core-shell NRs have been successfully tested for bioimaging of human HEK293 and HeLa cells with good permeability to cells. Toxicity was observed to be 3% for the 100 g/mL dose.

KEYWORDS: Core-shell nanorods, Mn:ZnSe/ZnS, epitaxial growth, fluorescent agent, bioimaging.


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1. INTRODUCTION Design of two different semiconductors in core-shell geometry have shown their enhanced electrical and optical properties which are not obtainable in a single semiconductor quantum dots and they possess a lot of applications right from optoelectronics to biotechnology.[1‒3] In coreshell architecture, shell with wider bandgap material can protect the surface of the core from oxidation and defects generation, and thus leads 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 property with tuneable narrow emission in visible region as well as high fluorescence 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-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 (0-D) spherical quantum dots, one-dimensional (1-D) architecture based 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 features of 1-D nanostructures facilitate conjugation with multiple biomolecules of interest which make them potentially better bio-probes 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 1-



D nanostructures fulfill this necessity at least in a 1-D 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 high-temperature 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 Seorganophosphine 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 expected a smaller (4%) lattice mismatch between wurtzite (WZ) ZnSe and ZnS structures, which in turn Page 4 of 43 ACS Paragon Plus Environment

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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 have been tuned by varying Mn ion content in the core and changing the shell thickness. A thorough structural, microstructural and optical properties are 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). In order to use these materials for bioimaging applications, the PL quantum yield (QY) should be high enough. Based on these, main scope of this work is to prepare the Mn:ZnSe/ZnS with the phosphine free synthesis and to achieve high QY. To the best of our knowledge, this is the firsttime study on this system with core-shell architecture in nanorod morphology. 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 DDT and OAm in order to generate a highly reactive alkylammonium selenide in which Se metal powder was reduced by DDT and subsequently dissolved in OAm.[33,34] The colour 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 injection of the precursors to attain three different thicknesses and hereafter they are referred 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 corePage 5 of 43 ACS Paragon Plus Environment


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-) anions more strongly binds to the surface of core-shell NRs than the thiol (binding energy of ZnSthiolate 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 atom) 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 heatingup and hot injection method followed by ligand exchange with MUA is shown in Figure S1 of supplementary information. 2.1. Structure and Microstructure Analysis The XRD patterns of pure and Mn2+ ion doped ZnSe are shown in Figure 1(a). The pure and Mn2+ ion doped ZnSe core exhibit a wurtzite structure of ZnSe (ICDD # 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 1(b). Figure 1(c) 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 towards standard wurtzite ZnS phase. Thus, the XRD results clearly proved the epitaxial growth of ZnS shell on Mn:ZnSe core.


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Figure 1. XRD patterns of (a) pure ZnSe and Mn:ZnSe NRs with various doping concentrations (mmol) of Mn2+ ions, (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°. In addition, the peaks lie in the 2θ range from 20 to 36° for the core and the core-shell NRs are de-convoluted into three peaks by using the Pseudo-Voigt function (Figure 1d), and the FWHM values of deconvoluted peaks are given in Table S1 of the supplementary 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 are 3.54, 0.73 and 3.32. In the case of Page 7 of 43 ACS Paragon Plus Environment


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-shell NRs, the FWHM of the (002) phase 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, this XRD results are in concurrence with HRTEM analysis as will be discussed below. Also, the X-ray diffraction peaks of all samples show the characteristic peak broadening (FWHM) except the peak corresponds to the growth direction for NRs (i.e., (002) plane). Typical low and high magnification HRTEM and HAADF micrographs of Mn:ZnSe core are shown in Figure 2. They (Figure 2a to 2c) clearly show the nanorod shape of the core Mn:ZnSe. It is to be noted that, all the NRs are single and there is no other types such as twins or tripods, and what is observed of these types in Figure 2a and 2b 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 STEM-XEDS elemental maps and spectrum for Mn:ZnSe core NRs are presented in the supplementary information (Figure S2 and S3). From the 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 XEDS spectrum (Figure S3) shows presence of Zn, Se and Mn elements only in the Mn:ZnSe core NRs. It is worth to note that there is a little chance of getting S in the core nanorods; because during the synthesis, DDT was used for the reduction of Se powder. So, there is a possibility that DDT release sulfur Page 8 of 43 ACS Paragon Plus Environment

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at higher temperature and to form ZnSxSe1-x alloy. Therefore, in order to check this, XEDS measurements have been extensively carried out the 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 2. (a) Low magnification HRTEM, (b) HAADF, and (c) high magnification HRTEM micrographs of Mn ion doped ZnSe NRs, the core material. Insert of (a) represents 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. Insert of (c) is the zoomed in of the nanorod marked with dotted circle. Figure 3 shows the HRTEM, HAADF-STEM images and XEDS spectrum of the Mn:ZnSe/ZnS core-shell NRs. As shown in Figure 3(a – 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 insert of Figure 3(b) 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 (Fig. 3) show some broken pieces of NRs which are just the broken pieces of the rods while handling them during the TEM sample preparation process and it is not a separate nucleation of any particles of the core shell components. To further Page 9 of 43 ACS Paragon Plus Environment


confirm the core-shell architecture, the STEM-XEDS elemental mapping was conducted. Figures 4(a – d) show the HAADF image and elemental maps of elements Zn (blue), Se (purple), and S (red) in the core-shell NRs. Figure 4(a) 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 being on the 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 about the width of 2.5 nm, which is of the same 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 4(e). For good visibility, the profiles are overlaid on the schematic of the cross-sectional view of the core-shell NRs as in Figure 4(e). 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 NRs. Because the projected area along the electron beam direction sees more S on the wall side and more Se in the core side, and 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.


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Figure 3. (a) Low magnification and (b) high magnification high-resolution TEM, (c) HAADFSTEM micrographs, and (d) the corresponding XEDS spectrum of the Mn:ZnSe/ZnS core-shell NRs. Insert of (a) is 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. Insert of (b) shows the SAED pattern of the core-shell NRs.

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. Page 11 of 43 ACS Paragon Plus Environment


Size (diameter of the NRs) distributions obtained from the HAADF image analysis for the core and core-shell architecture are presented in insert of Figure 2(a) and 3(a) respectively. The average diameter of the core Mn:ZnSe NR 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 to note 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 wellresolved 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 (Figure S4 and S5). The size distribution is also obtained from the HRTEM images and they are presented in the supporting information (Figure S6). Based on the earlier reports, the possible reasons for the anisotropic growth of core NRs are given as follows: firstly, WZ structure is believed to be a predominant reason for the anisotropic growth, because WZ structure in solution phase is preferably grown anisotropically in a 1-D direction (rod/wire).[35] Secondly, 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 1-D NRs along c-axis of the wurtzite structure.[36,37] 2.2. XPS analysis The XPS survey spectrum (Figure S7 of the supporting information) shows the presence of Zn, Se, Mn and S elements in the sample. The high-resolution Zn-2p, Se-3d, Mn-2p and S-2p core-level XPS spectra of Mn:ZnSe/ZnS NRs are given (Figure 5). The presence of C-1s is resulted from the oleylamine wrapping on the surface of NRs. Also, the observed O-1s peak is due to the adsorbed atmosphere oxygen. The Zn core level spectrum (Figure 5a) has split into ZnPage 12 of 43 ACS Paragon Plus Environment

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2p3/2 (1021.6 eV) and Zn-2p1/2 (1044.7 eV) with a spin-orbit splitting energy of about 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 intense. 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 5(c). 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 ZnS 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 coreshell NRs.

Figure 5. The 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. Page 13 of 43 ACS Paragon Plus Environment


2.3. Optical Studies The UV-Vis absorption, photoluminescence, and time resolved photoluminescence (TRPL) spectroscopy 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 6(a – 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, 6b and 6c). 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 the increased overall size of the core-shell NRs upon ZnS coating. It is to be noted that the obtained bandgap (Table 1) of the core-shell NRs lies in 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 the size dependence. The steady state PL spectra of Mn:ZnSe/ZnS with different shell thicknesses are shown in Figure 6(d – f). Importantly, the PL spectra of core alone as a function of doping content is presented in the insert of Figure 6d and its worth discussing this first in order 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 (insert of 6d). When Mn2+ ions are introduced into ZnSe, a new characteristic orange emission appears at 590 nm along with blue Page 14 of 43 ACS Paragon Plus Environment

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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 the 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. 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 photo-excited electrons from one Mn2+ ion site to its nearest neighbour Mn2+ ion site via non-radiative transitions followed by several transfer steps.[41]

Table 1. Optical bandgap value, PL peak position and FWHM of emission peak, PL QY, and PL life time parameters of Mn:ZnSe core coated with different thickness of ZnS shell achieved by different precursor injection times. Sample

Bandgap (eV)

0.005 mmol Mn:ZnSe/ZnS



PL FWHM (nm) 59.1 60.6 61.5 62.0 61.0

QY (%) 5.58 10.51 23.26 37.15 8.78

τ1 (ms) 0.144 0.253 0.283 0.356 0.204

Life time parameters A1 τ2 A2 (%) (ms) (%) 0.561 0.567 0.494 0.623 0.959 0.406 0.476 0.935 0.529 0.416 1.281 0.583 0.813 0.857 0.251

τav (ms)

Core ZnS-1 ZnS-2 ZnS-3 core

3.36 3.23 3.19 3.13 3.35

Peak position (nm) 590 593 596 600 591

ZnS-1

3.30

597

62.4

13.99

0.309

0.761

0.955

0.239

0.627

ZnS-2

3.25

601

72.9

26.85

0.293

0.622

1.291

0.397

1.029

ZnS-3 Core

3.21 3.32

604 591

74.1 62.6

49.35 2.89

0.464 0.135

0.572 0.540

2.049 0.465

0.425 0.452

1.678 0.380

ZnS-1

3.25

595

63.7

7.88

0.179

0.934

0.883

0.182

0.524

ZnS-2

3.21

600

67.8

15.31

0.159

1.043

1.000

0.199

0.617

ZnS-3

3.16

603

71.9

24.53

0.231

0.663

1.059

0.384

0.832

0.472 0.755 0.795 1.127 0.573

The 590 nm PL emission (Figure 6d-6f) 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 Page 15 of 43 ACS Paragon Plus Environment


resulted from the suppression of non-radiative 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 towards longer wavelength (redshift) of about 9 to 11 nm as well as peak broadening in each step of newly grown ZnS layer, i.e., with increasing shell thickness. The wavelength corresponding to the PL maximum are 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 redshift and peak broadening are observed for all the samples (Table 1). Increasing peak width originates from the distribution of particle size in the range of 4.5 nm 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 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, i.e., 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. It is to be noted that, this is a crucial parameter for light emitting devices. In order 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 the supporting information (Fig. S8). It is very clear, for a given shell thickness, increasing the Mn2+ in core has significantly increases the PL emission. This is a clear evidence for the synergistic effect contributed to the enhancement of PL emission by Mn ion content as well as by shell thickness in this core-shell NRs. The redshift of 3 to 13 nm in the PL emission peak (also 5 – 28 nm in the absorption peak in Figure 6a, 6b and 6c) in the Mn:ZnSe/ZnS core-shell NRs can be explained by band alignment model of the core-shell structure. The bulk band-offset of ZnSe/ZnS system exhibit type – I heterostructure i.e. the core material possesses a lower conduction band (CB) and a higher valence Page 16 of 43 ACS Paragon Plus Environment

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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]

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



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 bandoffset 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 Å 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 redshift 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 towards 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 non-radiatively trapped Page 18 of 43 ACS Paragon Plus Environment

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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. It is to be noted at this point that the large Stokes shift discussed earlier has played a key role in the reduction of reabsorption and enhanced the PL emission.

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.

Since QY determines the quality of the Mn:ZnSe/ZnS core-shell NRs for bioimaging, it had experimentally obtained by using a standard dye (Rhodamin-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 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. Firstly, when the Mn:ZnSe is coated with ZnS, it is 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. Secondly, for a given Mn2+ concentration, the QY is increasing with increasing the ZnS shell Page 19 of 43 ACS Paragon Plus Environment


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 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 photo-excited electrons from one Mn2+ ion site to its nearest neighbour Mn2+ ion site via non-radiative 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% (for example, 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 over coated with ZnS layer.[51] In another case, about 50% was achieved by Yuan et al. at high temperature (500 K) but 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 6(h) shows the digital photograph of samples acquired from the reaction of Figure 6(e) under day light and UV light (in dark) excitation and the emission is seen Page 20 of 43 ACS Paragon Plus Environment

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to be orange only under UV light excitation. It is also to be noted 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). In order 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 bi-exponential function and the average lifetime (τav) was calculated by using the following Equation [9]

 AV 

( A1 12  A2 22 ) ( A1 1  A2 2 )

(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 1 varies from 0.135 to 0.464 ms whereas 2 varies from 0.465 to 2.049 ms and the av varies from 0.380 to 1.678 ms for various samples. The 1 is less compared to the 2 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 in 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 non-radiative process which occurs on the surface of the NRs.[55]



Figure 7. PL decay curve 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 Equation 1. There are two different trends observed in the average lifetime. The first one is that, for a given Mn2+ content, the av increases with the ZnS shell thickness (for example, when the Mn2+ content is 0.015 mmol, the av is 0.573 ms for the Mn:ZnSe core NRs and the same is 0.627, 1.029 and 1.678 ms when 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 i.e. 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 av 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 av 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 a 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


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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 av of 0.57 ms and 0.41 ms.[56,51] It is worth to note that all these three cases, the morphology of the core shell structure is the quantum dots (zero-dimensional 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 life time (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: Firstly, the elongated nanocrystals known as NRs have: 1) larger absorption cross section, which makes it possible to photo excite the multiple electrons in NRs by a single excitation source, 2) aspect ratio which significantly minimize the nonradiative carrier losses 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 is compared in Figure S10(a) 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 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 ZnS1 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 photo-corrosion. 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 photo-oxidation.[61] The MUA capped core-shell NRs show initial enhancement in the PL intensity is observed up to a certain time and then decreases under UV exposure and this behavior is due to the photo-induced oxidation process 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 photo-corrosion.[61,62] It is worth to note that there is no precipitation observed when the MUA capped core-shell NRs was stored at 4 °C, and also they preserved their fluorescence yield (monitored for two months). In addition, microstructural analysis is carried out on MUA capped Mn:ZnSe/ZnS core-shell NRs. The HRTEM micrographs (Figure S11 of the supporting information) shows the rod like structure and dispersity of core-shell QDs is not significantly Page 24 of 43 ACS Paragon Plus Environment

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affected after ligand exchange with MUA. So, it is clear that 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 has been determined by the zeta potential measurements. The zeta potential measurements (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 In order to investigate the potential of water soluble MUA capped Mn:ZnSe/ZnS core-shell NRs for biomedical applications, the in-vitro cell imaging has been performed on human HEK293 and HeLa cells. Based on the optical characteristics carried out in the earlier sections, the sample with the highest QY (the 0.015 mmol Mn:ZnSe/ZnS core shell NRs) has been chosen for bioimaging studies. These cells were treated with MUA capped core-shell NRs at the optimal concentration of 100 µg/mL for 6 h and the treated cells were (fluorescence) imaged by confocal laser scanning microscopy. Figure 8 presents the bright field (left column), fluorescence (middle column), overlay (right column) confocal images of HEK293 (top row) and HeLa (bottom row) cells treated with water soluble core-shell NRs. It is to be noted 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. In order 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 Page 25 of 43 ACS Paragon Plus Environment


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.

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.

When we use materials for bioimaging, especially in bio-system, there are certain parameters such as low toxicity and biocompatibility of the fluorescence imaging agent in the cellular environment are the most important requirements. In this regard, in-vitro cytotoxicity of the MUA capped Mn:ZnSe/ZnS core-shell NRs have been evaluated in the HEK293 and HeLa cells at different concentrations (0 to 1000 µg/mL) for 48 h through 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 Page 26 of 43 ACS Paragon Plus Environment

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HeLa cells upon the treatment of core-shell NRs are presented in Figure 9 (a and b). It is found that only 3% reduction in the viability of the cells after being incubated with 100 µg/mL coreshell NRs for 48 h is observed, and also there is no any obvious morphological changes between the control cells and the cells treated with the core-shell NRs (Figure 9a and 9b respectively). Even in the case where the HEK293 and HeLa cells were treated with the very high concentration (of 1000 g/mL) of 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.

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 concentration of MUA capped Mn:ZnSe/ZnS core-shell NRs for 48 h.



3. CONCLUSIONS From the results of the present study, the following conclusions can be drawn: (1) highly emissive 1-D 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 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 Mn 2+ content in the core. (5) The PL lifetime in the core-shell NRs was effectively prolonged for about three 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 (water soluble) 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%), 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%), oleylamine Page 28 of 43 ACS Paragon Plus Environment

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(OAm, 70%), 1-dodecanethiol (DDT, ≥98%), and 11-mercaptoundecanoic acid (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 1.2 mmol of sulfur powder was dissolved 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), 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 minutes 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 minutes. After completion of particle (NR) growth, the temperature of the reaction mixture was raised to 240 °C for 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 3 batches at an interval of 15 minutes (each batch contain a 2 mL 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 down to room temperature. In order to extract the unreacted precursors and byproducts, the assynthesized oleophilic OAm capped Mn:ZnSe/ZnS core-shell NRs were purified three times with Page 29 of 43 ACS Paragon Plus Environment


the addition of CHCl3/acetone (v/v, 1:2) mixture solvent followed by centrifugation and decantation. Afterwards, 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 have been prepared with three different concentrations of Mn2+ ion (Mn: 0.005, 0 015 and 0.025 mmol) and used for all the studies. It is to be noted 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 exchange process, the initial oleophilic core-shell NRs were purified with ethanol for 3 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 vigorous stirring for 30 mins at room temperature, the mixture was heated at 70 °C for 20 mins. After cooling down to room temperature, 1 mL of double distilled water (DDW) was added and stirred for another 10 mins. The phase transformation of core-shell NRs from chloroform phase to water phase has 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 X-Ray diffraction pattern were acquired in a Rigaku (Ultima-IV) X-ray diffractometer by using a Cu-Kα radiation with K-line filtered. The UV-Vis absorption spectra were recorded in a Shimadzu (UV-3600 plus) UV-Vis-NIR spectrophotometer. Page 30 of 43 ACS Paragon Plus Environment

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Steady state fluorescence and time resolved photoluminescence (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]

I A  n  QYX  QYS X S exS  X  I S AX exX  nS 

2

(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 around 0.05 for all the samples in order to avoid the reabsorption), λex is the excitation wavelength (both reference and sample were excited at 350 nm), 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, Bandpass). Zeta potential measurement was performed in a HORIBA scientific SZ-100 instrument. The high-resolution transmission electron microscopy (HRTEM) measurement was carried out in an FEI Tecnai G2 F20 TEM with an acceleration voltage of 200 kV. The scanning TEM-high angle annular dark field (STEMHAADF) and X-ray energy dispersive spectroscopy (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. X-ray photoelectron spectroscopy (XPS) measurements were done in a PHI 5000 VersaProbe-II equipped with a monochromatic Al-Kα XPage 31 of 43 ACS Paragon Plus Environment


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 DMEM (Himedia) medium supplemented with 10% FBS (Himedia) and LGlutamine-Penicillin-Streptomycin solution (Himedia) at 37 ºC 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 PBS three times and fixed with 4% formaldehyde for 10 mins at room temperature and embedded with Mowiol (Sigma). Confocal fluorescence images were taken using a Carl Zeiss LSM 710 instrument equipped with 20X 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 coreshell NRs for 48 h. The drug MG-132 (Sigma-Aldrich) 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 96 well plate, and the plates were incubated for another 3 h. The resulting formazan crystals were solubilized by adding 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 Page 32 of 43 ACS Paragon Plus Environment

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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. ASSOCIATED CONTENT Supporting Information: 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 MUA capped Mn:ZnSe/ZnS core-shell NRs for 6 h.



Conflicts of interest: There are no conflicts of interest to declare. ACKNOWLEDGMENTS This work was financially supported by DST-SERB, India (SR/FTP/PS-137/2010). The author JS acknowledges UGC-India for the Maulana Azad National Fellowship (F1-17.1/2016-17/MANF2015-17-TAM-58815). The CIF of Pondicherry University is acknowledged for the characterization facilities. Dr. Amirthapandian and Dr. Jagadeesan of MSD, IGCAR are acknowledged for TEM measurements. REFERENCES (1)

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of

Cadmium-Free

Cu-Doped

Electroluminescence. Chem. Mater. 2014, 26, 1204–1212. Page 41 of 43 ACS Paragon Plus Environment

Zn−In−S

Nanocrystals

and


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GRAPHICAL ABSTRACT


ZnS Core

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