Aqueous Manganese-Doped Core/Shell CdTe/ZnS …

1 < strong >Aqueousstrong > < strong >Manganese-Dopedstrong > < strong >Core/Shellstrong > < strong >CdTe/ZnSstrong > < strong >quantumstrong > Dotswith strong < strong >fluorescencestrong > and < strong >highstrong > RelaxivityLihong Jing, Ke Ding, Sergii Kalytchuk, , Yu Wang, Ruirui Qiao, Stephen V. Kershaw, Andrey L. Rogach,*, and Mingyuan Gao*, Institute of Chemistry, the Chinese Academy of Sciences, Bei Yi Jie 2, Zhong Guan Cun, Beijing 100190, China Department of Physics and Materials Science & Centre for Functional Photonics, City University of Hong Kong, Tat Chee Avenue,Kowloon, Hong Kong Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment, City University of Hong Kong, HongKong *SSupporting InformationABSTRACT: < strong >Core/Shellstrong > < strong >CdTe/ZnSstrong > colloidal < strong >quantumstrong > < strong >dotsstrong > withvarying dopant levels ( ) of paramagnetic manganese ionsspatially distributed within the thin ZnS shell are synthesized by theaqueous approach.

2 They exhibit both strongfluorescence originatingfrom the CdTe core (up to 45% room temperature emission quantumyield) and < strong >highstrong > ionic < strong >relaxivitystrong > in the range of mM 1s 1,which render them promising dualfluorescent/paramagnetic probes. INTRODUCTIONC olloidal semiconductor < strong >quantumstrong > < strong >dotsstrong > (QDs) doped withtransition metal ions have attracted considerable interest as dualfluorescent/paramagnetic 6 Fluorescent QDs synthe-sized by liquid phase-based chemical approaches exhibit usefuloptical properties such as broad excitation combined withstrong, narrow and symmetric emission rendering them suitableas luminescent biomarkers,7 13while paramagnetic transitionmetal ion doping makes them ideal as enhanced contrast agentsfor magnetic resonance 17 Several strategies forintroducing paramagnetic dopants into QDs have beenreported,18 23including coprecipitation,24 26organometallicprecursor thermolysis,20,27and inorganic cluster ,29 Following the early research on Mn-dopedZnS QDs by Bhargava.

3 30significant progress has been achievedin Mn2+doping of both II VI QDs such as Zn(S,Se)27,31,32andCd(S,Se)3,33 36and III V QDs such as In(P,As).37 39 For II VI QDs, substitution of Mn2+ions into Zn2+sites rather thanCd2+sites has been reported as advantageous, due to theintrinsic mismatch between the dopant and host cationic radiifor the latter ,29,33To benefit from the complementaryadvantages of multifunctionalfluorescent/magnetic nano-particles, it is important to maintain the superior emissioncharacteristics of QDs, something that is not always possiblepostdoping as introduced dopant impurities often give rise toadditional nonradiative decay ,40,41 One possiblesolution to minimize the emission quenching involves thedoping of the nanocrystal cores with manganese.

4 Followed byepitaxial shell overgrowth on these as-prepared doped typically results in dopants predominantly located at ornear the core shell interface passivated by the additional approach has been successfully demonstrated for bothMn-doped isocrystalline < strong >Core/Shellstrong > CdS/CdS QDs,42,43as wellas heterocrystalline < strong >Core/Shellstrong > CdS/ZnS,14,18,42 44 CdSxSe1 x/ZnS,45 ZnSe/ZnS,23,46and ZnSe/CdSe47 QDs. Mn dopingdirectly into the shell has been also reported, such as forpresynthesized CdSe cores overgrown with Mn-doped ZnSshell, with emission < strong >quantumstrong > yield up to 20%.15 Most of these QDs have been synthesized by organic phasethermal decomposition of suitable precursors at < strong >highstrong > temper-ature, which requires additional postsynthetic phase transferprocedures for their anticipated use in < strong >Aqueousstrong > phase biologicalapplications.

5 Direct < strong >Aqueousstrong > phase synthetic approaches forstrongly emitting II VI QDs doped with paramagnetic Mn2+ions are highly desirable, and we have addressed this demand inthe present work by choosing strongly emitting CdTeQDs48 50as a core material on which to deposit a wide-bandgap semiconductor ZnS shell with paramagnetic man-ganese dopant ions. The ZnS shell provides a suitable matrix formanganese doping and at the same time maintains the highReceived:June 27, 2013 Revised:August 4, 2013 Published:August 28, 2013 American Chemical | , 117, 18752 18761fluorescence efficiency and enhances the chemical stability ofthe CdTe ,51 The incorporation of Mn2+ions into thegrowing ZnS shell has been accomplished by their coprecipi-tation with Zn2+ions using glutathione (GSH) tripeptide as asulfur source.

6 The latter is both a ligand and releases sulfideions during its thermal decomposition. The optical andmagnetic properties of the resultant Mn-doped CdTe/ZnScore shell QDs have been systematically studied as a functionof variable amounts of Mn2+dopant ions. EXPERIMENTAL perchlorate hexahydrate (Aldrich, ), thioglycolic acid (TGA; Fluka, 97%+), 3-mercaptopro-pionic acid (MPA; Aldrich, ),L-glutathione reduced(GSH; Sigma-Aldrich, ), manganese(II) chloride(Aldrich, 98%), and zinc chloride (Fluka, ) were usedas received. Octadecyl-p-vinylbenzyl-dimethylammonium chlor-ide (OVDAC) was synthesized according to Aoyagi et of CdTe Core QDs weresynthesized according to the previously reported ,54 Briefly, g of Cd(ClO4) 6H2O ( mmol) was dissolvedin 160 mL of water, and g of TGA ( mmol) wasintroduced under stirring.

7 The pH value of the reaction mixturewas adjusted to by dropwise addition of 1 M NaOHaqueous solution, followed by deaeration under nitrogenflowfor 1 h. H2Te gas (note: H2Te gas is highlyflammable and toxicby inhalation), generated by dropwise addition of 13 mL of an oxygen-freeflask containing g ( ) of Al2Te3lumps, was introduced into solution driven bya slow stream of N2. The resultant solution was refluxed underopen-air conditions, until CdTe QDs reached the desired of < strong >CdTe/ZnSstrong > and < strong >Manganese-Dopedstrong > < strong >CdTe/ZnSstrong > < strong >Core/Shellstrong > precursor solution for ZnS shellgrowth was prepared by dissolving ZnCl2( mmol/L), GSH( mmol/L), and MPA ( mmol/L) in water, followedby pH adjustment to by the dropwise addition of 1 MNaOH.

8 The precursor solution for deposition of the ZnS shelldoped with manganese ions was prepared in the same way, withthe further addition of appropriate amounts of Mn2+ molar ratios of Mn2+-to-Cd2+ions (defined as the molarratios of Mn2+precursor introduced into the reaction mixtureto the amount of Cd in the CdTe QD core determined bycompositional analysis) were used, namely: , , As-prepared CdTe core QDs were precipitated byisopropanol and introduced into precursor solutions at aconcentration of 10 6mol/L. After deaerating bynitrogen bubbling for 30 min, the mixture was heated to refluxunder open-air conditions, and the progress of the reaction wasmonitored by absorption andfluorescence and Compositional electron microscopy (TEM) and < strong >highstrong > resolution TEM(HRTEM) images were recorded with a Philips CM 200 FEGmicroscope and a FEI Tecnai20 JEM-2100F microscope,respectively.

9 Samples for TEM were prepared by drying adrop of diluted QD solution in toluene on the copper gridscoated with a thin carbonfilm. To achieve better contrast andavoid aggregation on the grids, QDs were transferred fromwater into toluene utilizing OVDAC as a phase transfer agentaccording to Zhang et X-ray diffraction (XRD)patterns of the QD samples on glass substrates were recordedon a Regaku D/Max-2500 diffractometer. The composition ofMn-doped QDs was determined by an inductively coupledplasma optical emission spectrometer (ICP-OES) using aThermo Fisher IRIS Intrepid II XSP. Samples for elementalanalysis were prepared by decomposition of QDs in a HCl/HNO3mixture (aqua regia) with subsequent dilution by Milli-Q water.

10 The manganese doping levels, defined as [Mn]/([Mn] + [Cd]CdTe+ [Zn]), were evaluated after three cycles ofprecipitation of QDs with isopropanol, centrifugation, andredissolution in water, followed by further purification byfilterdialysis on a 10-K MWCO centrifugal device (Millipore YM-10) against an < strong >Aqueousstrong > medium solution containing MPA ( ) for ligand exchange at pH in order to elute theexcess surface ligand bound Mn2+ions. The same purificationprocedure was applied to the samples for electron paramagneticresonance (EPR) studies. X-ray photoelectron spectroscopy(XPS) was performed on an ESCALAB 220i-XL photoelectronspectrometer (VG Scientific). The binding energies fordifferent elements were calibrated with respect to the C1sline at eV from adventitious carbon.