Density of Grafted Chains in Thioglycerol-Capped CdS Quantum Dots

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Density of Grafted Chains in Thioglycerol-Capped CdS Quantum Dots Determines Their Interaction with Aluminum(III) in Water Nassim Ben Brahim,*,† Mélanie Poggi,‡ Jean-Christophe Lambry,§ Naim Bel Haj Mohamed,† Rafik Ben Chaâbane,† and Michel Negrerie*,§ †

Laboratoire des Interfaces et Matériaux Avancés, Faculté des Sciences de Monastir, Boulevard de l’Environnement, 5019 Monastir, Tunisia ‡ Laboratoire de Physique de la Matière Condensée, CNRS UMR7643, Ecole Polytechnique, 91128 Palaiseau, France § Laboratoire d’Optique et Biosciences, INSERM U1182, CNRS UMR7645, Ecole Polytechnique, 91128 Palaiseau, France S Supporting Information *

ABSTRACT: We aimed to quantify the interaction of watersoluble-functionalized CdS quantum dots (QDs) with metal cations from their composition and physical properties. From the diameter of thioglycerol-capped nanoparticles (TG-CdS QDs) measured by electronic microscopy (D = 12.3 ± 0.3 nm), we calculated the molecular mass of the individual particle MAQD = (3 ± 0.5) × 106 g·mol−1 and its molar absorption coefficient ε450 = 21 × 106 M−1·cm−1. We built a three-dimensional model of the TG-CdS QDs in agreement with the structural data, which allowed us to quantify the number of thioglycerol grafted chains to ∼2000 per QD. This value fully matches the saturation binding curve of Al3+ cations interacting with TG-CdS QDs. The reaction occurred with a slow association rate (kon = 2.1 × 103 M−1·s−1), as expected for heavy QDs. The photophysical properties of the functionalized QDs were studied using an absolute QD concentration of 7 nM, which allowed us to investigate the interaction with 14 metallic cations in water. The fluorescence intensity of TG-CdS QDs could be quenched only in the presence of Al3+ ions in the range 0.2−10 μM but not with other cations and was not observed with other kinds of grafting chains.

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INTRODUCTION Semiconductor quantum dots (QDs) with highly tunable fluorescence (FL) properties have gained a high interest in many fields. Water-soluble QDs offer various applications in biology1,2 and environmental technologies as sensors, thanks to the development of surface modifications during their synthesis and to the highly sensitive response due to the existence of their surface states.3−8 These QDs possess photochemical stability, and their size-dependent emission and absorption bands with large extinction coefficients make them attractive in the development of fluorescent probes for analytical biochemistry.9−11 Upon binding of the target analytes, the FL emission of the QDs can be quenched. The nature of the semiconductor QD core and the functionalizing ligands at their surface strongly modulate the FL response, especially the selectivity and sensitivity toward particular ions. The use of QDs as analytical probes for heavy-metal cations started to develop after the pioneering study,3 showing modulation of the FL intensity of L-cysteine-capped CdS QDs in the presence of Cu2+ and Zn2+. Since then, various semiconductor materials were investigated for elaborating either single-core QDs (mainly PbS, PbSe, CdS, CdSe, CdTe, ZnS, ZnSe, and ZnTe) or core/shell QDs (CdSe/ZnS, CdTe/ CdS, etc.),12,14 and many metallic cations (including Ag2+, Hg2+, Cu2+, and Co2+)14−18 were shown to bind to CdS-based © XXXX American Chemical Society

QDs, with different proposed mechanisms. Besides confering the solubility on the QDs, the role of grafted chains in the specificity toward metal cations has been widely demonstrated in a large variety of QDs.6,12−20 However, the profusion of reported studies on functionalized QDs interacting with cations is accompanied by very diverse procedures, and no studies addressed the quantification of the real concentration of QDs, an important parameter. Furthemore, very few studies estimated the grafted ligand density,21 whereas the absorption coefficient was always estimated from empirical relationships,22 whose validity is restricted to particular QDs, size ranges, and conditions. Should semiconductor QDs ever be used to design heavy-metal sensors in cells or water samples, cations−QDs interaction studies require the rigorous evaluation of all chemical and physical parameters, including one of the most important in chemical interactions, the concentration, especially if they are used in living cells.1,2,23 In a functionalized semiconductor QD system, the binding of the analyte to the grafted chain (or in some cases also to the semiconductor core) modulates the quantum yield of QD to produce a response as a change of the FL intensity. The reported concentration working ranges for the QDs−cations Received: December 27, 2017

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DOI: 10.1021/acs.inorgchem.7b03254 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (A) TEM image of an individual TG-CdS QD (scale bar = 20 nm). (B) HRTEM image of a particular QD. The interplane distance of 0.41 nm corresponds to an orientation with respect to the a axis of the hexagonal structure, which corresponds to the zone axis [001], as demonstrated by FTIR of the image (reciprocal space) in the inset. (C) Histogram of the particle sizes directly measured from the TEM images (n = 84). The histogram of volumes, calculated from this size distribution, is shown in Figure S1. (D) XRD spectrogram of TG-CdS in powder. The positions of the corresponding hexagonal phase peaks are indicated. (E) Hexagonal crystal unit of wurtzite. The Cd2+ cations are blue, and the S2− anions are yellow. (F) 3D model of the TG-CdS QD (see Figure S2 for a larger view).

that, with the same functionalizing chain TG, the nature of the semiconductor core also participates in the specific FL response in the cations−QDs interaction.

interaction depend on the nature of the semiconductor, its size, and the nature19 and density of the grafted ligands. The choice of the synthetic route may also impact the number of surface default traps. Except in some particular cases, these QDs may not be widely used as direct sensors but may be part of more elaborate systems (mutilayer or core/shell QDs to modulate the emission,24 embedded QDs in a material using FL resonance energy transfer25 for signal amplification, associated QDs for multiplexed detection, etc.) for which the fundamentals of the interaction must be known. In the present work, we aimed at quantifying the parameters that drive the interaction between metallic cations and watersoluble QDs based on a semiconductor CdS core, solubilized and functionalized with a thioglycerol chain, that we have synthesized via a one-pot aqueous route.26−28 We report the physical and chemical characteristics of this thioglycerol-capped nanoparticle (TG-CdS QD). From the particle diameter determined by high-resolution electronic microscopy, we calculated its molecular mass and absorption coefficient at the first excitonic band and quantified the number of functionalizing organic grafted chains on an individual QD, in agreement with a three-dimensional (3D) model that we built. We then tested the influence of 14 different metallic cations on the FL intensity of the TG-CdS QDs. Surprisingly, the FL was not quenched by Cd2+ (as it was for TG-CdSe)20 but was quenched in the presence of Al3+. The interaction mechanism between TG-CdS QDs and Al3+ ions is discussed. Remarkably, when the real QD concentration is rigorously determined, here 7 nM, the number of grafted chains per QD (∼2000) corresponds to the saturation of Al3+ cation binding (14 μM). Finally, it appears

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EXPERIMENTAL SECTION

Synthesis of TG-CdS QDs. We have synthesized CdS nanoparticles by an aqueous chemical route using thiourea (NH2CSNH2) and cadmium acetate dihydrate [Cd(CH3COO)2·2H2O] as the starting substrates and thioglycerol (TG) as a capping stabilizer. The synthesis was performed by modifying a previously described procedure.26−28 Briefly, 5.7 mmol of Cd(CH3COO)2·2H2O and 13.8 mmol of TG were dissolved in 200 mL of deionized water, and the solution was adjusted to pH 11 by the dropwise addition of a 1 M solution of KOH. The solution was then placed in a three-necked flask closed with a septum and valves and deaerated for 30 min by bubbling N2. A second aqueous solution of NH2CSNH2 (2.88 mmol in 50 mL) was prepared and added dropwise to the initial one under vigorous stirring. The reactants were converted to CdS nanoparticles by refluxing the mixture at 100 °C for 2 h under N2. The solution became yellow during conversion. To isolate the CdS nanoparticles, the final solution was concentrated down to 25 mL using an evaporator. The particles were then extracted by precipitation in methanol: the solution was stirred for 1 h, and the precipitate was filtered and dried in a desiccator under vacuum. The TG concentration used here gives a stoichiometric ratio Cd/TG/S = 1/2.4/0.5, which leads to saturation coverage of the grafting bonds at the QD surface, maximizing the inhibition of the nonradiative transitions, which gives an intense luminescence.26−29 Caution must be exercised when manipulating and disposing of samples that contain Cd. Solutions of Metal Ions. The metallic cations whose interaction was studied were from the following inorganic soluble salts: NaNO3, LiCl, Ba(NO3)2, CoCl2·6H2O, Pb(NO3)2, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, Al(NO3)3·9H2O, Fe(NO3)3·9H2O, B


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Inorganic Chemistry Mn(SO4)·2H2O, Cd(SO4)·8H2O, (NH4)2Fe(SO4)2·6H2O, and Hg(ClO4)2·3H2O, which were purchased either from Fluka or from Aldrich Chemicals. A total of 1 mmol of each inorganic salt was dissolved in distilled water (10 mL) to obtain 10−1 M aqueous stock solutions, which were diluted to the desired concentrations as required. Spectrofluorometric Detection of Al3+ Ions Binding to TGCdS QDs. TG-CdS QDs (2 mg per 100 mL) were dissolved in distilled water. This mass concentration corresponds to 7 × 10−9 mol· L−1, using the molecular mass of TG-CdS QDs determined as described in the results (MAQD = 2.88 × 106 g·mol−1). A total of 2 mL of TG-CdS QDs solution was placed in a quartz cell (10 mm optical path length) for FL spectral measurement (Cary Eclipse FL spectrometer) at room temperature. Aliquots (10 μL) of cation solutions were introduced progressively as needed. The excitation wavelength was 360 nm and the slit width 10 nm for both excitation and emission, with a 395 nm cutoff filter on the emission channel. The same cell and concentration were used for steady-state UV−vis absorption (Shimadzu UV-1700) and for time-resolved FL measurements (Horiba Fluorolog spectrometer) performed with excitation at 372 nm with a NanoLED (Horiba), which has a pulse width of 1.3 ns, resulting in an overall instrument function full width at half-maximum (fwhm) = 2.4 ns. Structural Characterization. X-ray diffraction (XRD) measurements were performed on a X’PERT PRO MPD Panalytical powder Xray diffractometer, using the Cu Kα emission (λ = 1.5405 Å) as the incident radiation. Transmission electron microscopy (TEM) samples were prepared by dropping the samples (10 μL) dispersed in water on carbon-coated copper grids, and the excess water was evaporated. High-resolution TEM (HRTEM) images were acquired using a JEOL 2010 field emission gun (FEG) with an accelerating voltage of 200 kV. The Fourier transform infrared (FTIR) spectra were measured with a PerkinElmer FTIR spectrophotometer using the KBr pellet technique. The Raman spectrum was recorded with a LABRAM HR-Raman spectrometer (Jobin-Yvon), and the excitation was set at 514 nm from an argon-ion laser. Energy-dispersive X-ray (EDX) measurement was recorded with a JEOL 2010 FEG apparatus in scanning TEM mode. Differential scanning calorimetry (DSC) was conducted on a Netzsch STA 449C thermal analyzer from room temperature to 600 °C by heating the sample in an argon atmosphere at a speed of 10 °C·min−1.

Before investigating the metal cations−QDs interaction, we aimed at calculating the crucial parameters, including the average molecular mass of an individual crystallite TG-CdS nanoparticle, which is necessary to obtain the true concentration and extinction coefficient. These calculations are based on the geometry and stoichiometry of one crystal unit. From the average diameter (D = 12.3 ± 0.3 nm) measured by HRTEM and the QD volume (VQD = 1017 ± 200 nm3 calculated from the size distribution, assuming a spherical shape; Figure S1) and that of one CdS hexagonal crystal unit, Vunit = 0.3035 nm3 (a = b = 0.416 nm; c = 0.675 nm), we calculated the absolute number of atoms (∼20105 of each Cd and S) in one TG-CdS QD. We then calculated the number of grafted TG chains at the QD surface. From the area of one crystal unit face, we calculated that a 12.3 nm CdS QD, which comprises ∼3350 units in total, possesses a larger number of crystal units exposed to the surface (Ns ∼ 1800) than within the core. Importantly, the synthesis process allows us to link the TG chain to Cd through a S−Cd bond before growth of the core crystal lattice (see Scheme 1), and the number of Cd atoms at the surface is Scheme 1. Synthesis of TG-CdS QDs

larger than that in the core. We used the most plausible working hypothesis that one QD comprises ∼2000 capping TG on average, which was confirmed by saturation binding measurement (see below). This value can be compared to the number of 115−289 ligands per CdSe-ZnS QDs capped with polyethylene glycol−aldehyde,21 having diameters in the range 3.1−4 nm. Both values are proportional with respect to the QD surfaces. From the absolute number of CdS atoms and TG capping chains in one individual QD, we calculated the molecular mass of TG-CdS QDs: MAQD = (3 ± 0.5) × 106 g·mol−1, used in subsequent calculations. In spectroscopic measurements, the molar concentration of TG QDs was 7 nM and the calculated extinction coefficient at the band edge (450 nm) is ε450 = 21 × 106 cm−1·M−1. We note that the diameter of our TG-Cd QDs is out of the range of an often-used empirical fitting curve22 and our determination of ε450 is based directly on the measurement of the QD diameter measured by HRTEM and the real number of atoms. To ascertain the number of grafted chains at the surface, we built a 3D model of a TG-CdS QD based on the structural data.37 A homewritten software allowed us to generate a hexagonal CdS crystal lattice restrained to a 12.24 nm sphere and to insert TG chains at its surface with the Cd−S group of TG embedded in the lattice. We then performed a “free” dynamics (at 293 K) to minimize the energy of the system (CHARMM software), minimizing sterical conflicts between the TG chains. As a result, a total number of 1951 chains were grafted on the model, showing that our working hypothesis of 2000 TG chains is realistic, in agreement with the

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RESULTS AND DISCUSSION Chemical and Structural Characterization of the Synthesized Material. The HRTEM images of synthesized TG-CdS nanoparticles (Figure 1A,B) reveal their spherical shape with an average nanoparticle diameter D = 12.3 ± 0.3 nm determined from the Gaussian size distribution of 84 QDs (Figure 1C). The crystal nature of TG-CdS QDs is evident in the HRTEM image (Figure 1B), which reveals an interplane distance a = 0.41 nm corresponding to an orientation with respect to the c axis of the hexagonal structure. The hexagonal wurtzite structure is further characterized by XRD measurement (Figure 1D,E), which identified the (100), (002), (101), (102), (110), (103), (112), and (203) diffraction planes.30 No reflections related to the impurities were identified in the pattern, indicating the high purity of the final product. Because if the size effect in the nanometer range, the XRD peaks broaden and their widths become larger as the diameters of the nanocrystals become smaller, which can thus be estimated as 15.2 nm from this X-ray line broadening using Debye− Scherer’s equation,31 a value close to that obtained from HRTEM images (12.3 ± 0.3 nm). A different aqueous method32−35 using other reagents and conditions resulted in much smaller CdS QDs (2.8−4.8 nm) with cubic crystal phase (or hexagonal in the case of a 2 nm QD35), whereas an organic method with the same reagents yielded TG QDs with a very different absorption spectrum.36 C


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Inorganic Chemistry

Figure 2. (A) FTIR spectra of pure TG (green) and TG-CdS nanoparticles (red) in the solid state in KBr pellets. (B) EDX spectrum of TG-CdS QDs.

structure. The high O and C contents confirm saturation of the QD surface by TG chains. Thermal Stability of TG-CdS QDs. To evaluate the thermal stability of TG-CdS QDs, the temperature-induced phase changes were measured by DSC. The DSC measurement of TG-CdS QDs in the range 50−350 °C (Figure S3) revealed an endothermic peak with a minimum at 112 °C, which is attributed to the evaporation of adsorbed water held by electrochemical interactions to the nanoparticle surface.39,40 Contrastingly, the broad exothermic band centered in the range 200−225 °C can be assigned to structural relaxation processes (relaxation of defects and strains and/or stabilization via crystallization of an amorphous part of the CdS core41 or to decomposition of the TG capping chains). The minor exothermic band appearing at slightly lower temperatures is correlated with nanoparticle growth.39 The major exothermic peak at 275 °C is due to the total transformation to the hexagonal phase.42 This temperature, lower than the phase transition of bulk CdS (300 °C), agrees with the dependence of the phase transition temperature with the QD size and with the surface or interface energy.21 Electronic and Optical Characterization of Synthesized QDs. We further demonstrated the crystal state by measuring the phonon modes in the Raman spectrum of TGCdS QDs (Figure 3A). The peaks located at 299 and 596 cm−1 correspond to the fundamental band (1LO) and the overtone (2LO) of the longitudinal-optical (LO) phonon modes of CdS. The peak at 900 cm−1 is assigned to a combination of phonons (1LO + 2LO).43 The overtone to the fundamental intensity ratio is small, as expected because the excitation energy is larger than the band gap. The LO mode at 305 cm−1 of TG-CdS QDs is shifted with respect to its position in the Raman spectrum of bulk CdS because of the spatial confinement of the phonons.32,33 The absorption spectrum (Figure 3B) exhibits three resolved electronic transitions with absorption maxima located at 320, 340, and 450 nm. The first excitonic peak position (lower energy) at 450 nm of TG-CdS is considerably blue-shifted compared to that of the bulk phase CdS (515 nm),44 as expected from the quantum size effect. The change of the band gap along with exciton features results in the discrete energy spectrum of the individual nanoparticles. The band-gap energy of the nanocrystal was calculated to be 2.7 eV from a simple energy-wave equation.

stoichiometric ratio used (Cd/TG/S = 1/2.4/0.5), which ensures saturation of the QD surface (Figures 1F and S2). The presence of the TG chains grafted at the surface of CdS was verified by FTIR. In the spectrum of solid-state TG-CdS (Figure 2A), the characteristic bands of TG are observed at 619 cm−1 (C−S stretching), 960 cm−1 (C−C stretching), 1000− 1260 cm−1 (C−O stretching), 1420 cm−1 (C−H stretching), 2932 cm−1 (C−H2 stretching), and 3360 cm−1 (O−H stretching vibration of the hydroxyl group).29,38 Importantly, the S−H vibration located within 2400−2560 cm−1 in the spectrum of pure TG ligands completely disappeared in the FTIR spectrum of TG-CdS QDs (Figure 2A), indicating that cleavage of the S−H bond of TG occurred because of the formation of S−Cd bonds in functionalized CdS nanocrystals. The atomic composition of the synthesized QDs was analyzed by EDX (Figure 2B). The incorporation of grafted chains of TG in CdS QDs is further demonstrated by the presence of atomic peaks assigned to C and O together with Cd and S (with a Si trace from the grid). The X/Cd ratios of elements are reported in Table 1. The measured atomic percent ratio C/O = 1.59, close to the theoretical ratio (1.5) in TG (C3H8O2S), demonstrates the integrity of the grafted TG chain. Table 1. Relative Compositions of TG-CdS QDs from the EDX Spectrum 112

% mass % atoms X/Cd ratio

Cd

51.27 13.79 1

32

S

17.33 16.33 1.184

12

O

14.32 26.98 1.956

16

C

17.08 42.90 3.111

The integrated signals from the S and Cd atoms indicate a nonstoichiometric atomic ratio S/Cd = 1.18, showing that the QDs are richer in S than in Cd, in agreement with the incorporation of TG. Importantly, the amount of S atoms is lower than what would be obtained from the mere sum of the CdS core and TG chain contents. This observation is readily interpreted by the incorporation of most of the S atoms of TG within the crystal units at the surface of the QD structure. Indeed, this mode of chain grafting directly results from the synthesis route (Scheme 1), in which TG reacts first with cadmium acetate to form a Cd−S bond. Then, the resulting TG−Cd2+ derivative is incorporated, together with S released by NH2CSNH2 and the remaining Cd2+, to build the crystal D


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Inorganic Chemistry

Figure 3. (A) Raman spectrum of TG-CdS QDs probed with the 476.5 nm argon laser line. (B) UV−vis absorption and FL spectra of TG-CdS QDs. Optical path length = 1 cm. [QDs] = 7 nM.

Figure 4. FL spectra of functionalized QDs in solution before and after the addition of different metal cations: (A) TG-CdS at 7 nM with cations at 10 μM; (B) MPA-CdS at 10 nM with cations at 2 μM; (C) TG-CdSe at 200 nM with cations at 20 μM. The excitation wavelength is 360 nm.

Figure 5. (A) FL spectra of TG-CdS QDs as a function of the Al3+ concentration up to 11 μM. TG-CdS QD concentration = 7 nM. pH = 8.0. T = 20 °C. The excitation was set at 360 nm. (B) FL intensity as a function of [Al3+]. The dotted line is the asymptote of the fitted curve. Inset: Stern− Volmer plot of the FL intensity as a function of the Al3+ concentration. The Stern−Volmer quenching constant was Ksv = 0.19 μM−1 and the limit of detection estimated as 0.2 μM. (C) FL intensity at 500 nm of TG-CdS QDs (7 nM) in the absence and presence of Al3+ (5 μM) with the presence of other metal cations at 10 μM. Excitation at λex = 360 nm. pH = 8.0.

S4A), a phenomenon attributed to energy dissipation in a nonradiative mode (rather than to electron−hole recombination).47 Consequently, the temperature was controlled and stabilized at 20 °C for all spectroscopic measurements. Interaction of TG-Functionalized QDs with Metal Cations. Because the FL of QDs arises from recombination of the excitons, it is expected that changes of the surface states or ligands of QDs, which affect the electron−hole recombination efficiency, will consequently also affect the luminescence yield.48 Indeed, a seminal work demonstrated the effect of metallic cations on the FL of QDs.3 On the basis of this principle, various functionalized QD−optical systems were developed, via either FL quenching or enhancement, for different kinds of soluble ligands, including metal ions.13 The

The FL spectrum of the same solution under excitation at 360 nm shows an intense band centered at 495 nm, with a fwhm of 140 nm, corresponding to the band-edge emission. The broad emission band at higher wavelength is due to the surface trap emission.45 Indeed, in the case of CdSe, the broader width of the FL spectrum (Figure 3) has been assigned to a larger number of surface defects in QDs and internal defects (vacancies or extra Cd atoms).23 These defects favor nonradiative trap states,46 which increase the propensity for emission quenching (we must note that the use of a broad-band lamp in the fluorimeter does not allow us to resolve the emission originating from Cd vacancies, contrary to the use of laser lines2). The FL intensity of TG-CdS QDs decreased as a function of the temperature in the range 5−70 °C (Figure E


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Inorganic Chemistry

Figure 6. (A) Absorption spectra of TG-CdS QDs (7 nM) as a function of the Al3+ concentration. (B) Difference of absorption spectra minus the spectrum of TG-CdS QDs in the absence of Al3+. The black dotted line is the absolute spectrum of the TG-CdS QDs alone. pH = 8.0.

concentration (14 μM) perfectly agrees with the number of grafted TG chains on one QD (∼2000) deduced from its diameter and the number of crystal units at the surface and thus agrees with a direct interaction of Al3+ with grafted TG chains. Because we waited for equilibrium before recording the emission of TG-CdS QDs interacting with Al3+, we expect a static FL quenching, as observed in the Stern−Volmer relationship (Figure 5B, inset). The kinetics of the reaction between Al3+ (5 μM) and TGCdS QDs (7 nM) reached a plateau after ∼5 min (Figure S4B), a time that we have chosen as the standard reaction time for all interaction measurements. With a calculated association rate kon = 2.1 × 103 M−1·s−1, this reaction appears to be slow, a property that was expected because the TG binding sites are immobilized on the much heavier crystal core, so that only the Al3+ cations experience diffusion relative to the QDs, in contrast to a protein−ligand interaction, for example. Here, the concentration of TG sites is estimated to 14 μM. To test whether the binding of Al3+ is reversible, we try removing it by centrifuge dialysis. After measurement of the emission spectrum of QDs in the presence of 11 μM Al3+, the sample was centrifuged in a dialysis tube and then resuspended in the same volume of pure water and the spectrum was measured again. The FL spectrum after dialysis recovered almost the same intensity and shape as those in the absence of Al3+ (Figure S5), demonstrating that the quenching is indeed reversible. This observation is in line with the binding of Al3+ to the hydroxyl group of TG, not directly to the surface of QD, as deduced from the asymptotic saturation of the signal (Figure 5B), which corresponds to the density of grafted chains. The FL intensity of QDs can be strongly influenced by the pH of the solution,52 and we tested the emission quenching of TG-CdS QDs in the presence of Al3+ in the 3−12 pH range (QDs were equilibrated at the final pH before Al3+ cations were added). A maximum of FL emission was observed at pH = 8.0 (Figure S4C). The decrease of the FL intensity in an acidic medium can be assigned to dissociation of the QDs and capping TG due to the attack of the surface binding S rather than to a change of the ionization state of TG because the acidic transition observed is far from the pKa value of the −OH groups. However, the decrease of FL at pH values greater than 9.0 was probably due to the precipitation of Al as aluminum hydroxide because of the reaction with OH− ions.53 Therefore,

mechanism of ligand interaction can be either direct adsorption on the semiconductor core or chelation of ions or small molecules to the chains grafted on the QD surface.49−51 TG contains hydroxyl groups that can efficiently coordinate with many divalent metal ions (such as Cd, Cu, and Fe),3,47 and we therefore investigated the effect of various metal ions on the FL intensity of TG-CdS QDs (Figure 4). Saliently, only the Al3+ cation induced a drastic quenching of the FL intensity, in contrast to all other metal ions tested at the same concentration, demonstrating an unexpected selectivity of the interaction between TG-CdS QDs and Al3+. We note that, in the pioneering work of Chen and Rosenzweig,3 the TG-CdS QDs FL was quenched by Cu2+ ions but at a high concentration of ions (100 μM) and using a high concentration of QDs of 150 μM (here we used 7 nM). Importantly, the sensitivity and selectivity depend also on the concentration of both interacting partners. We compared the selectivity of TG-CdS with two other QD systems that we have also synthesized: substituting the functionalizing TG chain with mercaptopropionic acid (MPA) leads to a remarkable change of the specificity from Al3+ to Co2+ (Figure 4C), whereas using the different semiconductor core CdSe instead of CdS with the same TG chain results in less specificity, with Cd2+ having the larger effect while the quenching effect of Al3+ is greatly reduced (Figure 4B). In the latter case, a different quenching mechanism may take place. These comparisons demonstrate the influence not only of the capping chain but also of the nature of the semiconductor core. The FL emission from TG-CdS QDs was gradually quenched with increasing Al3+ concentration (Figure 5A), together with a progressive small red shift of the emission maximum. It should be noted that Al(OH)3 is not formed when mixing a Tris-HCl (pH = 8.0) buffer solution with Al ions, even at millimolar concentrations. Thus, we ruled out that the interaction with the QDs results from the formation of Al(OH)3 on their surface. The luminescence decrease is therefore attributed to the interaction between the Al3+ cations and the grafted TG at the surface of the QD, in agreement with the observed saturation binding behavior (Figure 5B). The concentration dependence of FL quenching follows the binding of Al ions to the surface of the TG-CdS QDs and no longer evolves after [Al3+] = ∼14 μM, tending toward an asymptotic value when the concentration of the QDs is 7 nM. This F


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Inorganic Chemistry

Figure 7. (A) Time-resolved FL decay curves of TG-CdS QDs measured at the maximum of the emission peak (500 nm) at varying concentrations of Al3+. λex = 372 nm. The intensity is on a logarithmic scale. The curves are not normalized but were each recorded up to a maximum intensity of 104 counts. The instrument function has a fwhm = 2.4 ns. The parameters of the fitted curves are given in Table 2. (B) Normalized FL spectra of TG-CdS in the absence and presence of Al3+.

as standard conditions, all analyses were performed in a TrisHCl buffer at pH = 8.0. The interference between cations and the selectivity were further evaluated by measuring the QDs FL in the presence of a mixture of Al3+ (5 μM) and other metal ions (10 μM), showing that a strong quenching was induced only by Al3+ (Figure 5C). The presence of these selected metal ions does not interfere significantly with Al3+ ions binding to the QDs, implying that the present system could effectively act as a chemical sensor for Al3+ in an aqueous medium.54 A system based on indole− carboxylic acid grafted on Ag nanoparticles was also reported to detect Al3+ down to 0.6 μM through a decrease of its absorption,55 whereas the FL of organic dyes increases upon binding Al3+ ions in the 2−20 μM range56−59 or even higher.60−62 These molecules have in common one or several −HO groups linked to aromatic cycles and experience an increase of absorbance upon Al3+ binding. In our system, the Al3+ ions are inferred to bind also to −HO groups of TG, inducing a quenching of QD FL. So far, only ZnO semiconductor nanoparticles functionalized with imine groups have been reported63 to interact with Al3+. However, they interact with all other cations so that the FL increase cannot be specific. Functionalized organic64 and Au nanoparticles show a FL increase with Al3+ in the 5−500 μM range, whereas C nanodots linked to rhodamine do not bring more sensitivity (500 μM).65 Considering the specificity and the demonstrated concentration range 0.2−20 μM for QD quenching by Al3+, these 12.3 nm TG-CdS QDs can be used to elaborate an Al sensor in aqueous media. Quenching Mechanism of TG-Capped QDs by Al3+ Ions. Physical and chemical interactions between QDs and certain metal ions can induce changes of the surface properties. To address the FL quenching mechanism of TG-CdS QDs in solution, the influence of Al3+ on the UV−vis absorption and on the FL lifetime was investigated. The UV−vis absorption spectrum of the TG-CdS nanoparticles evolves as the concentration of Al3+ increases up to 11 μM (Figure 6A). The absorbance did not change significantly at the 450 nm peak, whereas the absorbance band located at 340 nm (second excitonic transition) increased with the addition of Al3+ ions together with a small red shift of its maximum to 347 nm. In addition, we observed that the band centered at 316 nm (third excitonic transition) gradually disappeared with increasing Al3+

concentration. A similar observation was reported in TGcapped CdSe QDs interacting with the Cd2+ ion.20 We can readily discard a direct contribution of Al3+ absorption because it has a lower molar absorption coefficient (0.93 × 104 M−1· cm−1 at 320 nm; Figure S6), which leads to OD = 0.1 at 11 μM and whose increase is in contrast to the evolution of the 316 nm band from QDs. Besides the changes of the second and third excitonic transition peaks, an overall increase of the spectrum with small intensity occurred at 8 μM and higher. This can be due to Rayleigh scattering induced by a slight aggregation of QDs upon complexation with Al3+ ions, a phenomenon that occurs with a concentration threshold. Possibly, this mechanism may favor a nonradiative energy transfer at large Al3+ concentration but is not predominant at low concentration. To visualize the real spectral evolution, we plotted the difference of spectra at a given Al3+ concentration minus the spectrum in the absence of Al3+ (Figure 6B), which makes the decrease of the third excitonic peak resulting from strong interactions of Al3+ onto the surface of TG-CdS QDs obvious. The fundamental electronic and optical properties of semiconductor nanoparticles depend on their size, which induces a shift of the absorption peak, as a result of the electron and hole confinement.66 There is a slight shift of the second excitonic peak and no shift of the first one as the Al3+ cations interact with the grafted chain, so that the decrease of the third excitonic peak is not a size effect but may result from an electric-field effect induced by the presence of the Al3+ ions close to the QD surface. A fundamental explanation of this effect awaits a quantum-mechanics description, which is out of the scope of the present study. However, such a change in the absorption spectrum could also be used as a signature of Al3+ binding. The FL of QDs in solution usually decays with two components, corresponding to two distinct processes of excitonic relaxation.67,68 The lifetimes of the two decays depend on both the size of the QDs and the nature of the capping ligands. The faster one (τ1) is associated with exciton recombination in internal states, whereas the slower one (τ2) is associated with recombination by trap states localized at surface atoms.69 Here, the FL decays of TG-CdS QDs at various concentrations of Al3+ (Figure 7A) were also fitted with a biexponential function [At = A1 exp(−t/τ1) + A2 exp(−t/τ2) + G


Article

Inorganic Chemistry

concentration. We showed that only the Al3+ cations (in the concentration range 0.2−11 μM) induce a large FL quenching of these QDs (at 7 nM). The binding reaches saturation after Al3+ concentration (14 μM), which fully agrees with the estimated number of TG grafted chains (∼2000) around the QDs. This quenching appears to be static and can be attributed to the interaction of Al3+ with the TG capping layer around the QDs. By a comparison with other reported TG-CdS QDs, these results demonstrate the importance of the nature of the semiconductor material of the QD core, as well as its size and nature and the density of the grafted chains for the specificity of cation binding. Many capped QDs were reported to date as possible “sensors” for metallic divalent cations, the true effective detection of which most often stays within the range 0.1−20 μM (it may be lower for some CdTe systems74,75). These observations have raised huge interest for possible applications in detecting cations, either for the study of biological systems or for the measurement of pollutants in waters. However, beyond the issue of the detection range, no real sensor can be designed without knowing the true involved concentrations, which drive the physical chemistry, and having a clear understanding of the specificity. The very mechanism at the origin of the specificity of the QDs toward the cations is not yet fully understood, and many parameters interverne in this interaction. Even the choice of the synthesis procedure may impact the specificity by influencing the surface default traps and density of grafted chains. Because many reported QDs respond to several metallic cations by a change of the FL intensity measurable even if particular cations give rise to a large quenching,3,76 we foresee that a better defined QD-based sensing system would be multiplexed. Such a system would incorporate two or three QDs, with their response toward the cations being essentially different in amplitude, wavelength, and specificity. Then, the deconvolved overall spectral change of the FL intensity of the QD mixture will provide an unambiguous measurement of the cations. This methodology requires the rigorous quantification of all QD properties and all chemical parameters defining the metal−QD interaction, as presented here.

C; parameters in Table 2], clearly indicating that two different radiative processes are responsible for the FL emission. The Table 2. Parameters of Fitted FL Decays as a Function of the Al3+ Concentration [Al3+] (μM)

A1 (%)

τ1 (ns)

A2 (%)

τ2 (ns)

0 2 4 8 10 11

99.5 97.5 97.5 97.3 96.1 95.9

3 6 6 6 7 7

0.5 2.5 2.5 2.7 3.9 4.1

68 89 92 101 130 131

time constants of both decays slightly increased upon Al3+ binding, but the variation is limited compared to that obtained when increasing the QD size.67 Saliently, the evolution is opposite to that observed with Co2+ binding to cysteine-capped CdSe QDs70 or with Cu2+ binding to glutathione-capped ZnSe QDs,71 for which the quenching mechanism is dynamic, with both FL decay times decreasing. The FL emission is composed of two bands, the more intense at ∼500 nm is the band-edge emission, and the much less intense at 700 nm is due to surface defects (trap-state) emission.47 Because the intensity of the 700 nm emission band is not affected (Figure 5A), the binding of Al3+ has very little influence on the surface trap states. When the entire emission spectra at different Al3+ concentrations are normalized (Figure 7B), we can see that the main band-edge emission (500 nm) experienced a small shift of 13 nm toward lower energy upon Al3+ binding (Figure 5A), whereas the surface state emission did not change. In a pioneering study,3 the red shift of the TGCdS QDs emission was assigned to an electron transfer from CdS to Cu2+, yielding CdS+/Cu+ through the TG capping ligand but at very high Cu2+ concentration (100 μM), which could easily saturate the 3.5 nm TG-CdS QDs.3 A much larger shift (>100 nm) of the band-edge emission was also observed when the cations directly interacted with the surface of the semiconductor in the case of Cu2+ binding to nonfunctionalized CdS QDs.72 In the present study, in which the concentrations of QDs and cations are much lower compared with those in ref 3, negligeable binding of Al3+ occurs at the surface of the TGCdS QDs and Al3+ interacts preferentially with the TG chains, a conclusion based on several observations: first, the saturation level of the Al3+ binding curve (Figure 5B) corresponds to the calculated number of TG grafted chains; second, the shape was unchanged, and the amplitude of the emission spectrum was recovered after the removal of 11 μM Al3+ (Figure S5), indicating the integrity of the QD surface. From the absence of overlap between the emission spectrum of the QDs and the absorption of Al3+ (Figure S6), we also exclude a quenching by resonance energy transfer, so that the nonradiative channel due to Al3+ binding, which induces FL quenching, remains to be identified. Electron transfer from thioglycolic acid capped CdTe QDs to bound Eu3+ was considered to explain the emission quenching.73 In contrast to Eu3+, Al3+ has a much lower redox potential, which prevents us from retaining this hypothesis.

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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/acs.inorgchem.7b03254. Histogram of the volume of QDs, 3D model of the TGCdS QD, DSC thermogram of the TG-CdS, FL intensity of the TG-CdS QDs as a function of the pH, time, and temperature, recovery of the TG-CdS QDs, and absorption spectrum of Al3+ in solution (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +216 96 400 499. *E-mail: [email protected]. Phone: +331 69 33 50 52.

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CONCLUSION AND PERSPECTIVES In this work, we quantified on a rigorous basis the interaction of Al3+ with TG-CdS QDs, based on the determination of the absolute molecular mass of an individual QD, which allowed us to derive both its molar extinction coefficient and its true

ORCID

Michel Negrerie: 0000-0001-9918-031X Notes

The authors declare no competing financial interest. H


Article

Inorganic Chemistry

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ACKNOWLEDGMENTS N.B.B. acknowledges a travel research fellowship, “Bourse d’Alternance”, from the Tunisian Government. We thank the CIMEX team (Centre Interdisciplinaire de Microscopie Electronique de l’Ecole Polytechnique) for help in acquiring and interpreting the TEM data.

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Density of Grafted Chains in Thioglycerol-Capped CdS Quantum Dots

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