Investigation of Complexes of CdTe Quantum Dots with the AlOH

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Investigation of Complexes of CdTe Quantum Dots with the AlOH-Sulphophthalocyanine Molecules in Aqueous Media Anna Orlova, Irina Martinenko, Vladimir G. Maslov, Anatoly V. Fedorov, Yurii K. Gun'Ko, and Alexander V. Baranov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp408802u • Publication Date (Web): 09 Oct 2013 Downloaded from http://pubs.acs.org on October 11, 2013

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The Journal of Physical Chemistry

Investigation of Complexes of CdTe Quantum Dots with the AlOH-Sulphophthalocyanine Molecules in Aqueous Media Anna O. Orlova*†, Irina V. Martynenko†, Vladimir G. Maslov†, Anatoly V. Fedorov†, Yurii

K. Gun’ko†,‡, and Alexander V. Baranov† Saint Petersburg National Research University of Information Technologies, Mechanics and Optics, 197101 Saint-Petersburg, Russia

ACS Paragon Plus Environment

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KEYWORDS: CdTe quantum dots, AlOH-sulphophthalocyanine, tetrapyrroles, FRET, blood plasma, photoluminescence

ABSTRACT: The interaction between negatively charged CdTe quantum dots and AlOHsulphophthalocyanine molecules in blood plasma and in aqueous solutions with different pH levels has been investigated. Similarly charged quantum dots and molecules are found to form complexes that exhibit an effective energy transfer. The efficiency of the energy transfer in these complexes is close to theoretical FRET efficiency for given donor-acceptor pair. It has been shown that in blood plasma, as well as in aqueous solutions, the formation of complexes of quantum dots with AlOH-sulphophthalocyanine molecules without using their charged groups allows preserving the photophysical properties of phthalocyanine molecules. A theoretical model of the formation of complexes between similarly charged quantum dots and molecules is proposed.


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Chart 1. Introduction Today the development of new types of materials and complexes with high efficiency in singlet oxygen generation is a very important task. These compounds have been used in wastewater treatment systems, for sterilization of blood plasma, a synthesis of chemical clear medicines and in the cancer therapy.1 Tetrapyrroles have deserved a special attention among the materials that can generate singlet oxygen as they can be concentrated in cancerous tissues. The properties of tetrapyrroles make them very attractive for use as photosensitizes in photodynamic therapy (PDT).2 The synthesis of colloidal spherical semiconductor nanocrystals with a core diameter from 2 to 10 nm, quantum dots (QDs), is well developed. Quantum dots possess a range of useful properties. Their absorption and photoluminescence spectra depend on a size of nanocrystals. QDs have a high extinction, a high quantum yield of photoluminescence (PL QY), a good chemical and photo stability.3 The unique optical properties of QDs enabled them to compete with conventional dyes and to develop their application in optoelectronics, photonics, biology and medicine.4 For decade there was an increased number of publications which are focused on the study of photophysical properties of complexes of the semiconductor quantum dots with different organic molecules.5 Due to an extended absorption spectrum and high extinction coefficient, the quantum dots are perfect in the role of energy donor in complexes with molecules. Förster Resonant Energy Transfer (FRET) is the most common mechanism of energy transfer in the complexes


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with QDs. In complexes with QDs an increasing generation of singlet oxygen by tetrapyrroles can be realized via an effective energy transfer from quantum dots to tetrapyrroles molecules. Currently, one of the tetrapyrroles (AlOH-sulphophthalocyanine, Pc) is widely used as a photosensitizer in PDT.6 Recently, water-soluble complexes with CdTe quantum dots and Pc molecules were reported.7-9 In the case of electrostatic interaction between oppositely charged quantum CdTe dots and AlOH-sulphophthalocyanine the dramatic fall down of quantum yield of Pc’s photoluminescence was accompanied by change of their spectral form. It was found that in the complexes with negatively charged quantum dots, the AlOH-sulphophthalocyanine molecules do not change their photophysical properties. Blood plasma is complex media, which contains a different type of proteins, enzymes, cofactors, inorganic ions etc.10 Some of the blood plasma components can interact with quantum dots or tetrapyrrole compounds.11 Therefore, in this medium the properties of complexes of quantum dots with tetrapyrroles may differ from ones in water solution. In this work, we investigate formation and photophysical properties of complexes of negatively charged CdTe quantum dots and Pc in blood plasma and in aqueous solutions with different level of pH. We demonstrate that in blood plasma, as well as in aqueous solutions, Pc molecules can form complexes with negatively charged QDs without changing quantum yield of Pc’s photoluminescence. In these complexes, photoexcitation energy transfer from quantum dots to the phthalocyanine molecules was observed. The efficiency of the energy transfer is close to the theoretical FRET efficiency for given donor-acceptor pair. We also propose a theoretical model, which describes the interaction of similarly charged quantum dots and molecules in solutions.


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Our data has shown a good correlation with this model. In addition, we discuss a reason of a relatively high QD luminescence in solutions of similarly charged quantum dots and molecules. Chart 2. Experimental details The materials used were of highest purity available and used as received. Aqueous solutions with required pH level have been prepared by adding a necessary amount of NaOH into solutions. Water-soluble semiconductor CdTe quantum dots with average size of 3.5 nm were prepared according to the previously reported procedure.12 Stabilization of quantum dots by thioglycolic acid (TGA) molecules allows producing stable colloidal QD solutions. Due to the dissociation of carboxyl groups of TGA in aqueous solutions at pH=11, the quantum dots passivated by TGA molecules have a negative charge at the surface. Complexes of quantum dots with AlOH-sulphophthalocyanine molecules have been prepared by mixing the solutions of the pure QDs and Pc molecules with different concentrations in the range of 10-6÷5·10-5 mol/L. All experiments were performed in two hours after preparing the mixture solutions. UV-visible absorption spectra were recorded using UV3600 (Shimadzu) spectrophotometer. Steady-state photoluminescent (PL) spectra were measured by using Cary Eclipse (Varian) spectrofluorometer. Investigation of absorption and PL spectra of solutions was performed in 2 mm length quartz cuvette. Time-resolved PL spectroscopy was performed using a timecorrelated single photon counting (TCSPC) spectrometer (PicoQuant, Inc.). A pulse laser (405 nm) with an average power of 1 mW operating at 40 MHz with a pulse duration of 70 ps was used for PL excitation.


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Chart 3. Results and discussion Chart 3.1 Characterization of complex components Absorption and photoluminescence spectra of CdTe quantum dot water solution at neutral pH are presented in Figure 1. It should be noted that the optical properties of the QD solution are the same in blood plasma and in water solutions within used range of pH values. PL decay of QD solutions was well fitted by biexponential function with characteristic times τ1 = 5 ns and τ2 = 20 ns.

Figure 1. Absorption and photoluminescence spectra of CdTe quantum dots in alkaline solution (pH=11). PL excitation was performed at 475 nm. The chemical structure, absorption and photoluminescence spectra of AlOHsulphophthalocyanine in aqueous solution at different level of pH and in blood plasma are presented in Figure 2. In these media Pc demonstrated an exponential decay with characteristic time is equal to 4.5 ns.


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Figure 2. Absorption (a) and photoluminescence (b) spectra of Pc in different media: 1 – aqueous solution with neutral pH; 2 – alkaline solution (pH=11); 3 – blood plasma. PL excitation was performed at 350 nm. Insert in (a) shows structural formula of Pc. Chart 3.2 Complexes of quantum dots with AlOH-sulphophthalocyanine molecules It is well known that strong alkaline solution is unsuitable medium for biological species. Obviously, that for use in biology and in medical applications the complexes is of interest, which is stable and efficiently generate singlet oxygen in a neutral medium. Therefore, despite of the trend for spontaneous aggregation of colloidal quantum dots solubilized by TGA in neutral medium, initially we have investigated the photophysical properties of our complexes in water solutions at neutral pH. Absorption and PL spectra of CdTe QDs with concentration CQD ~ 2·10-6 mol/L and Pc with concentration CPc ~ 3·10-5 mol/L mixture solution are presented in Figure 3.


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Figure 3. QD/Pc complexes in aqueous solution at the neutral pH. Absorption (a) and PL (b) spectra of: 1 – the mixture of CdTe QDs and Pc; 2 – pure QD solution; 3 – pure Pc solution. PL excitation was performed at 475 nm. Inset in (b) shows increased part of the PL spectra. Clearly, that absorption spectrum of mixture solutions (see curve 1 in Figure 3a) is an additive sum of absorption spectra of pure components. At the same time, in the mixture of solutions of CdTe QDs and Pc molecules the 47 % quenching of QD PL accompanied by sevenfold increasing phthalocyanine PL is observed (see Figure 3b). Thus, these results provide an evidence of an effective energy transfer from CdTe QDs to Pc molecules. Analysis of decay of photoluminescence of quantum dots has shown than the QD PL lifetime in mixture solutions does not depend on concentration of Pc and equals to the QD PL lifetime in pure QD solution. This means, that quenching of quantum dot PL is the static, i.e. nonluminescent complexes of QDs with phthalocyanine molecules are formed. So in the mixture solutions only photoluminescence from free QDs has been observed. A complex formation via electrostatic interaction should be excluded in our experiments since both quantum dots and Pc molecules have negative charge at their surface. Complexes of


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similarly charged QDs and molecules could be formed via counterion interaction. However, this type of Pc and QDs interaction results in strong reduction of the Pc PL quantum yield,8 therefore, we believe that this complex formation mechanism is unlikely. The complex formation via any type of binding of the carboxyl groups of TGA with central Al metal atom of tetrapyrrole ring of Pc seems the most probable.9 The enhancement of phthalocyanine PL in solutions with QDs is a strong evidence of energy transfer from QDs to Pc molecules. Since the phthalocyanine absorption in the spectral range from 450 to 500 nm is negligible (see Figure 3a), the Pc PL excited at 475 nm is mainly due to the energy transfer from CdTe QDs to phthalocyanine molecules. FRET is the most probable mechanism of intracomplex energy transfer in the QDs/Pc complexes because of very good overlapping the QD PL band with phthalocyanine absorption peak. Importantly, attachment of the Pc molecules to the negatively charged CdTe QDs does not decrease of the Pc PL quantum yield. This fact allows ones expecting an enhancement of the singlet oxygen generation because of effective FRET from QDs to Pc molecules in these complexes. At neutral pH the colloidal solutions of the CdTe QDs solubilized by TGA molecules have a trend to spontaneous aggregation. This is usually accompanied by a monotonic decrease of QD photoluminescence that complicates the analysis of optical properties of QD/Pc complexes. That is why we performed studies of photophysical properties of QD/Pc complexes in alkaline solutions at pH=11, where QDs have the best stability of their optical properties. In alkaline solution, we also observed the formation of the QD/Pc complexes accompanied by the QD PL quenching and the enhancement of Pc photoluminescence. It should be noticed that in this solution our complexes showed stable optical properties for at least two months.


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Utilization of the complexes of QDs with tetrapyrrole molecules in PDT or for a sterilization of blood plasma is the most important of their applications. For these reasons, conditions of the formation and the existence of these complexes in blood plasma should be investigated. In general, the preparation of complexes between charged quantum dots and molecules in blood plasma is a nontrivial task. This is because both QDs and tetrapyrroles molecules could interact with components of blood plasma and this probably could lead to the instability of the complexes.11 Analysis of absorption and PL spectra of CdTe quantum dots in blood plasma performed by us has shown that QDs solubilized by TGA molecules do not form aggregates. Quantum yield of QD PL in blood plasma increased by 10% compared to their alkaline solutions. The increase in PL QY may be caused by the interaction of QDs with proteins and other species in blood plasma that results in an additional protection of QDs with a shell, which prevents QDs interaction with water molecules and reduces quenching. The absence of spontaneous aggregation of QDs enables us to expect a formation of stable complexes of CdTe QDs with Pc molecules in blood plasma. Figure 4 presents an example of absorption and PL spectra of the mixture of CdTe quantum dots and Pc molecules for concentrations of CQD ~ 1·10-6 mol/L and CPc ~ 3·10-5 mol/L. The spectra of pure components in the blood plasma at the same concentrations are also shown for comparison.


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Figure 4. QD/Pc complexes in human blood plasma. Absorption (a) and PL (b) spectra, excited at 475 nm: 1 – mixture of CdTe QDs and Pc; 2 – pure CdTe QDs; 3 – pure Pc; 4 – the absorption spectrum of human blood plasma. Inset in (b) shows increased part of the PL spectra. Taking into account the absorption of human blood plasma (see Figure 4a, curve 4), the absorption spectrum of the mixture of QDs with Pc molecules in the plasma, similarly to that in aqueous solutions, was found to be an additive sum of individual components. At the same time, strong QD PL quenching and enhancement of Pc PL were observed, e.g. twofold QD PL quenching and sevenfold enhancement of Pc PL were measured in human blood plasma at relative Pc concentration n = CPc: CQD equal to 30:1. These facts manifest formation in the human blood plasma of the QD/Pc complexes with photoexcitation energy transfer from QD to Pc molecule. Chart 3.3 Quenching of quantum dots PL and enhancement of phthalocyanine PL in mixture solutions In order to clarify physical origin of the QD PL quenching and the Pc PL enhancement in mixture solution we studied dependencies of QD and Pc PL intensities on n.


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The QD and Pc PL intensities were measured for mixture solution (CQD ~ 10-6 mol/L) after sequential addition to the QD solution of portions of Pc solution with desired concentration. The quenching curves for QD PL in human blood plasma and in alkaline mixture solution (pH=11) are presented in Figure 5.

Figure 5. The dependence of normalized PL intensity of CdTe QDs (СQD ~ 10-6 mol/L) on the relative Pc concentration (n) in mixture solutions: 1 and 2 – the experimental data and their fitting by function y=0.55·exp(-0.08·n)+0.45, respectively, for the human blood plasma; 3 and 4 – the experimental data and their fitting by function y=0.57·exp(-0.2·n)+0.43, respectively, for alkaline solution with pH=11. From Figure 5 it is clear that an increase in the concentration of phthalocyanine in the mixture solution leads to a decrease of the QDs photoluminescence. Transition from alkaline solution to human blood plasma does not qualitatively change the shape of the quenching curve of QDs. This means that the QD/Pc complex formation in human blood plasma and in aqueous solutions is similar.


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In the both media, the relatively high level of residual photoluminescence of quantum dots that does not depend on the concentration of phthalocyanine molecules has been observed. Because the PL of QDs bound to phthalocyanine molecules is fully quenched, about 50% of QDs in the mixture solutions with Pc cannot bind in the complexes with these molecules. We believe that the binding of AlOH-sulphophthalocyanine molecules with quantum dots occurs via TGA molecule that attached to the QD surface. A number of TGA molecules attached to the QD surface depends on the size of QDs and can vary within QD ensembles. According to estimations, up to fifty TGA molecules can be attached to the surface of CdTe quantum dot with an average core diameter equal to 3.4 nm.13 It is obvious that similarly charged QDs and Pc molecules must overcome an energy barrier, which appears due to Coulomb interaction between these components. A magnitude of energy barrier between QDs and Pc molecules depends on the surface charge of QD and the charge of molecule. It should be noticed that in alkaline solutions with pH=11 due to dissociation of carboxyl groups practically all TGA molecules which are attached to the QDs have a negative charge. Therefore on the surface of studied CdTe QDs the negative charge may reach value of “50”.13 Accordingly, only the QD-Pc pairs, which can overcome the energy barrier at room temperature, could form the complexes. Dependences of normalized Pc PL intensity (IPc/IPc0) on Pc relative concentration n in the mixture with QDs in human blood plasma at two different excitation wavelengths, 475 nm and 640 nm are presented in Figure 6. Here, IPc0 is the PL intensity of free Pc molecules with CPc = 10-6 mol/L equal to concentration of QDs in the mixture solution.


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Figure 6. The Pc PL intensity dependencies on n in human blood plasma: 1 and 2 – the experimental data and their fitting by linear function y = n, respectively; PL excitation at 640 nm. 3 – the experimental data for PL excitation at 475 nm; 4 – the guideline for eyes. The CdTe QDs with core diameter 3.4 nm do not absorb light at 640 nm (see Figure 1). Therefore, in the mixture solution the Pc PL excited at 640 nm is due to only Pc absorption. At the same time, at the 475 nm the Pc absorption is negligible and practically all Pc PL is that sensitized by QDs due to effective FRET from QDs to Pc molecules. At 640 nm excitation the dependence of normalized Pc PL on n is well fitted by a linear function y=n (see Figure 6). It means that the complexing with QDs does not change quantum yield of the Pc photoluminescence. This is a benefit of these complexes in comparison with complexes formed via charged groups of tetrapyrroles molecules.7,8,14 The normalized Pc PL intensity excited by 475 nm radiation is significantly higher than that excited at 640 nm (see Figure 6). This is an evidence of effective intracomplex FRET from QDs to phthalocyanine molecules. The dependence of Pc PL intensity on n excited at 475 nm is nonlinear. At small n the sensitized photoluminescence of Pc molecules due to FRET from QDs


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is seven times higher than that caused by intrinsic Pc absorption. When the quenching of QD PL by Pc reaches its maximum value (50%) in the human blood plasma, the ratio IPc/IPc0 is reduced to 4·n (see Figure 6). Such a nonlinear dependence of Pc PL intensity on n could be observed in two cases. First, if intracomplex FRET efficiency from QD to Pc molecule is close to 100%, the increasing number of phthalocyanine molecules attached to one QD does not increase the sensitized PL intensity. Second, when not all added Pc molecules bind to form the complex with QDs. It should be mentioned that the similar dependences of the PL intensity of Pc molecules on their relative concentration (n) were observed in alkaline solution. Chart 3.4 Model of complex formation between similarly charged QDs and organic molecules Using our experimental data, we propose the following model of complex formation between similarly charged semiconductor quantum dots and quencher molecules. 1. At certain concentration of the QDs and quencher molecules in the mixture solution there is a equilibrium:

[QM]

Kp ← →

[Q] + [ M] ,

(1)

where [QM] is concentration of complexes the QDs with molecules; [Q] is concentration of free QDs; [M] is concentration of free quencher molecules. 2. The equilibrium is characterized by dissociation constant (Kp) of QM complexes: ∆E

Kp =

− [Q][ M ] = K 0e kT , [QM ]

(2)


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where K 0 ~ e

∆S k

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, ∆S is change of entropy, ∆E is the energetic effect of the reaction (change of

enthalpy), k is the Boltzmann constant. We assume ∆E > 0. Exact values of K0 and ∆E are unknown. Therefore, we consider them as parameters of the model. Since the bonding of QDs in complex with the quencher molecules is assumed to occur through the solubilizer molecules covalently linked with the QD surface, the equilibrium of complexing between the quencher and free solubilizer molecules could also be considered:

[SM]

K (pS ) ← →

[S] + [M] ,

(3)

with the equilibrium constant K p( S ) = K 0( S ) e

−

∆E kT

, that is determined by the same energetic effect

value ∆E as in case of Eq. (1) with, in general case, other value of entropy factor K 0( S ) . 3. The presence of charge of similar sign at the surfaces of QDs and on the molecules leads to appearance of additional energy barrier, which must be overcome by these particles to bind them into the complex. Due to this barrier Eq. (2) for equilibrium constant becomes as follows: mze 2 R + r0 kT

∆E +

K p = K 0e

−

,

(4)

where R is the average radius of QDs; r0 is the distance from centre of molecule to QDs surface; m and z are the values of charge of QD and quencher molecule, respectively. An ensemble of QDs is characterized by average value of surface charge m , which is determined by the number of the solubilizer molecules. We will assume that the value of the


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∞

surface charge of QD obeys the distribution p(m) (

∑ p(k ) = 1 ), where p(m) is the probability of

k = −∞

that QD has surface charge equal to m. The presence of QDs with different surface charge in the ensemble with non-zero size distribution leads to appearance of Kp dependence on m. It is obvious that there is a certain value of QD surface charge mth, at which the energy barrier between QD and quencher molecules cannot be overcome. We assume that the certain part of quantum dots from the ensemble have a surface charge m > mth, and therefore they are not involved in the formation of complexes with the quencher molecules. 4. The model proposed suggests the formation of only one type of complexes of QDs with quencher molecules, i.e. one QD could attach only one molecule. It is clear that existence of several solubilizer molecules at the surface of QDs allows a formation of complexes with different number of quencher molecules. Attachment of one quencher molecule to quantum dot leads to an increase of the surface charge of this dot by z (z is value of quencher molecule charge). After attaching one molecule with the charge equal to z to all QDs having the charge ∞

m m + z th 

(5)

While sequential attachment of quencher molecules to QD leads to an increase in the surface charge of QDs, it should be expected that one QD could attach a limited number of quencher molecules. We assume that in the complexes no more than the limited number (e.g. five) of


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molecules of quencher could be attached to one QD. In this case, it can be also assumed that a sequential increasing number of quencher molecules, which are attached to one quantum dot, does not change the energy ∆E that is determined by the Eq. (3). This leads only to increase of the surface charge of this quantum dot that, in turn, leads to an enhancement of the height of the energy barrier between this QD and free quencher molecules according to Eq. (4). In other words, we propose that free quencher molecules do not “see” the identical molecules, which were attached to the surface of QD earlier. Therefore, if we take into account the Eq. (4), our model is suitable for description of formation of complexes with different stoichiometry. Let us consider the character of the dependence of the QD PL intensity from the concentration of quencher molecules in the solution. In this model, we assume that attaching one quencher molecule to quantum dot leads to complete quenching of photoluminescence of this QD. This means that in solution we observe only photoluminescence from free QDs. If complexes of QDs with one quencher molecule are only formed and a constant of dissociation of such complexes is equal to zero, the QD PL intensity will be proportional to concentration of quencher molecules until QDs with surface charge m < mth will present in the solution. In the case of absence of charges on the QD surface and neutral quencher molecules, all the solubilizer molecules would be characterized by an equal equilibrium constant determined by Eq. (3). Then, numbers of quencher molecules attached to one QD should obey the Poisson distribution.15 Therefore the dependence of QD PL intensity on concentration of quencher molecules in the mixture solution should be described by the exponential function y=exp(-x), where x is the number of quencher molecules bound with a single quantum dot.15 A presence of a


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charge of similar sign at the surface of QDs and quencher molecules leads to the transformation of a simple exponential dependence to the form y=exp (-k·x), where k

Investigation of Complexes of CdTe Quantum Dots with the AlOH

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