Liganddependent luminescence of ultrasmall EuSu

Authors: Dominika Wawrzynczyk Artur Bednarkiewicz Marcin Nyk Wieslaw Strek Marek Samoc

Publish Date: 2013/05/21

Volume: 15, Issue: 6, Pages: 1707-

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Abstract

Pure cubic phase ultrasmall αNaYF44  Eu3+ colloidal nanoparticles were synthesized by thermal decomposition reaction using three various capping ligands ie oleic acid trioctylphosphine oxide and hexadecylamine To expose as many Eu3+ ions as possible to interactions with the surfacebounded ligands the nanoparticles were fabricated to have the diameters below 10 nm The geometrical structure and properties of surface ligands needed for qualitative estimation of their influence on spectroscopic features of the investigated Eu3+ doped nanoparticles were obtained from DFT quantumchemical calculations Significant changes of luminescence spectra shapes and luminescence lifetime values were observed upon changes in the local chemical environment We show that the ratio R = 5 D 0 → 7 F 1/5 D 0 → 7 F 2 of the intensities of the forced electric dipole J = 2 and magnetic dipole J = 1 transitions in the synthesized Eu3+ doped nanoparticles is highly sensitive to the type of ligand present on the nanoparticle surface Similarly 5 D 0 luminescence lifetimes are found to be sensitive to the refractive index and also to the dielectric constant of ligands used during the synthesis to coat nanoparticles surface We argue that the photophysical and electrooptical properties of colloidal Eu3+ doped inorganic nanoparticles show hypersensitive response to the chemical surroundings in the close vicinity of the nanoparticle itself The behavior of both steadystate luminescence and its kinetics demonstrates the potential suitability of the studied nanoparticles for constructing selfreferencing optical nanosensorsLanthanidedoped nanoparticles NPs are finding increasing interest as they have become an alternative to traditional organic dyes in many biosensing applications Gnach and Bednarkiewicz 2012 They exhibit not only excellent photostability and narrow spectral emission lines but also in the case of codoped upconverting NPs UCNPs efficient conversion of near infrared photoexcitation into visible emission Achatz et al 2011 These features provide a number of advantages including those for micro or nanosensors design and may lead to further improvement of the sensitivity and simplicity of readout methods and devices For example UCNPs were recently used to build FRETbased biosensors for indirect quantification of pH Sun et al 2009 carbon dioxide Achatz et al 2011 ammonia Mader et al 2010 mercury Liu et al 2011 glucose Peng et al 2011 oxygen and for bioimaging of CN− ions distribution in living cells Liu et al 2011 In all of these sensors the spectral behavior of lanthanidedoped UCNPs was indirectly modified by organic dyes attached to the NPs surface The principle of operation of those nanosensors was mostly related to quenching the lanthanides’ luminescence through longrange dipole–dipole interactions between donor UCNPs and acceptor molecules as a response to the occurrence of biochemical reactions eg DNA hybridization protein conformation changes etc For example for the UCNPs based pH sensor Sun et al 2009 the principle of its operation was spectral filtering of the green UC emission of erbium by phenol red a well known pHsensitive dyeNevertheless none of the reported UCNPs based biosensors were related to a direct variation of spectral properties of the lanthanide ion in response to changes in the local surrounding The absorption coefficients of the trivalent lanthanide ions are rather weak due to the Laporte selection rule Hänninen et al 2011 Simultaneously their emission is relatively insensitive to the ligand environment of the central metal ion Lakowicz 2006 and thus little affected by the neighbor environment The latter is due to shielding of the 4f optically active electrons by the filled 5s and 5p orbitals However this is not necessarily true for certain transitions called hypersensitive transitions HST or pseudoquadrupolar PQP transitions They obey selection rules for quadrupolar transitions ΔJ ≤ 2 ΔL ≤ 2 ΔS = 0 and are more susceptible to changes in the lanthanide ion local symmetry or environment in comparison to other f–f transitions Hänninen et al 2011 One of the earliest noticed HSTs was the 5 D 0 → 7 F 2 luminescence transition in Eu3+ ion Sheik Saleem and Srinivasan 1987 It has been proved that the ratio of the intensities of 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 transitions in Eu3+ ions asymmetry ratio R can be considered indicative of the distortion in the local environment for the Eu3+ ions Boyer et al 2004 By measuring such a ratio of HST to standard transition intensity one has access to concentration independent results that can be reliable in complex contexts such as living cells or sensor microarrays It has been shown that the radiative lifetime of the 5 D 0 metastable excited state of Eu3+ ions is sensitive to the changes of refractive index of the surrounding medium Boyer et al 2004 Meltzer et al 1999 Additionally the Eu3+ ions luminescence was successfully used to probe the surface effects originating from high surface to volume ratio in nanosized materials Banski et al 2013 Chang et al 2007 Jia et al 2010 Yan et al 2003 used combined laser selective excitation spectra and lifetime measurements of Eu3+ doped YVO4 materials to detect the change of site symmetry from the interior to the surface of NPs Peng et al 2005 resolved two luminescent centers in La2O3S Eu3+ NPs through the use of timeresolved and laser selective emission and one of the optically active centers was attributed to a lower symmetry site occupied by surface Eu3+ ionsBasing on what has been discussed above we have designed synthesized and studied both the steady state and time dependent luminescence response of Eu3+doped ultrasmall NaYF4 NPs to the ligand type used to stabilize these nanocrystallites in liquid colloids as well as in powders The NaYF4 nanocrystalline host was chosen because of relatively low phonon energy Wang and Liu 2009 and reported low toxicity Kumar et al 2009 Moreover we have implemented a method to synthesize ultrasmall 7–9 nm NPs uniquely developed for NaYF4 NPs Chen et al 2010 These ultrasmall NPs allowed reducing the “core–shell” effect ie studying the impact of ligands and solvents on spectral properties of luminescent nanolabels is hindered within larger nanoparticles since the luminescence of superficial shell lanthanides is dominated by the spectral properties of much more numerous volumetric core lanthanide ions NPs were intentionally fabricated to have the diameters below 10 nm which was supposed to expose as many Eu3+ ions as possible to interactions with the surface bound ligands and the local chemical environment Properties of ligand molecules were assessed based on quantumchemical calculations to allow qualitative discussion of the differences between themAll chemicals for synthesis of NaYF4 NPs were purchased from Sigma Aldrich or POCH SA Poland and used without further purification Xray diffraction XRD patterns were recorded on a STOE diffractometer with Gefiltered CuKα1 radiation Transmission electron microscopy TEM studies were performed using a Philips CM20 SuperTwin microscope at 200 kV with 025 nm resolution Fourier transform infrared FTIR spectra were registered on a Biodra 575C spectrometer with resolution of 2 cm−1 for samples in KBr pellets The emission spectra and luminescence decay curves were obtained under excitation from the second harmonic 394 nm of a TiSapphire laser Belarus pumped by a NdYAG LOTIS TII laser Belarus The luminescence signals were detected by a Synapse CCD camera HORIBA JOBIN–YVON Fluorescence decay curves were recorded with a photomultiplier HAMAMATSU R928 output fed into a LeCroy Wave Surfer 425 digital oscilloscopeIn order to determine geometrical structure of ligands quantumchemical calculations have been performed using the Kohn–Sham formulation of the density functional theory DFT In particular geometry optimization followed by the evaluation of force constants was carried out using the B3LYP exchange–correlation functional Becke 1993 and the 631Gd basis set Dipole moment and electronic polarizability were determined at equilibrium geometries using the Coulomb–attenuated CAM–B3LYP functional Yanai et al 2004 The latter functional has been proved to outperform B3LYP functional as far as polarizabilities of extended conjugated systems are concerned Jacquemin et al 2007 Electricdipole properties were computed using the 631+Gd basis setOleic acid capped NaYF44  Eu3+ NPs OANPs were synthesized using the method previously reported by Chen et al 2010 Trioctylphosphine oxide caped NPs TOPONPs were synthesized using a previously reported thermolysis method Shan et al 2007 with some modifications For the TOPONPs synthesis aliquots of 1 mmol lanthanide oxides Y2O3 Eu2O3 were mixed and dissolved in ~50  concentrated trifluoroacetic acid The transparent solution was evaporated to dryness using a rotary evaporator The mixture of 15 ml of octadecane OD and 20 g TOPO was preheated to form homogeneous solution and added into the reaction flask together with 125 mmol of sodium trifluoroacetate The solution was then heated to 120 °C under vacuum with magnetic stirring for total oxygen and water removal during which the flask was purged with dry argon every 5 min The resulting yellow solution was then heated to 300 °C under inert atmosphere and kept at these conditions for at least 1 h After the synthesis the solution was cooled to room temperature precipitated by acetone and then collected by centrifugation Hexadecylamine capped NPs HDANPs were synthesized based on approaches previously proposed for quantum dots Mntungwa et al 2011 Nyamen et al 2011 In an optimized synthesis protocol fixed amounts of 125 mmol lanthanide oxides Y2O3 Eu2O3 were mixed and dissolved in ~50  concentrated trifluoroacetic acid The optically transparent solution was evaporated to dryness using a rotary evaporator The mixture of 15 ml of octadecane and 12 g HDA was preheated and next added into the reaction flask together with 125 mmol of sodium trifluoroacetate The solution was then heated to 120 °C under vacuum with magnetic stirring for total oxygen and water removal during which the flask was purged with dry argon every 5 min The resulting yellow solution was then heated to 300 °C under inert atmosphere and kept at these conditions for at least 1 h The solution was cooled to room temperature precipitated by acetone and collected by centrifugation In OANPs TOPONPs and HDANPs synthesis protocols the volume ratio of OAOD ODTOPO ODHDA was kept at 11 Additionally selection of 4  Eu3+ doping amount allowed to achieve strong luminescence without evident concentration quenching Kumar et al 2009 After precipitation NPs were equally dispersed in four solvents chloroform cyclohexane toluene and THF For transferring synthesized NPs to water solutions we performed a ligand removal procedure Bogdan et al 2011 To further investigate the influence of the nearest chemical surroundings on spectroscopic properties of obtained NPs we have also prepared NPs powders by a simple water evaporation method

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