NanoPhotoBioSciences
Volume … (2013), Article ID …, 5 pages
doi:…
Article
Zinc phthalocyanines and quantum dots conjugates: physical
properties and photodynamic activity
E.G. Maksimov1∗, F.-J. Schmitt2, M.G. Strakhovskaya1, D.A. Gvozdev1, T. Friedrich2, V.Z.
Paschenko1 and A.B. Rubin1
Department of Biophysics, Faculty of Biology, M.V. Lomonosov Moscow State University,
119992, Moscow, Russia
1
Institute of Chemistry, Biophysical Chemistry, Berlin Institute of Technology, 10623 Berlin,
Germany
2
Received 08 May 2013; Accepted 19 May 2013
Academic Editor:
Copyright © 2013 Evgeny Maksimov. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.
Abstract
It was shown that semiconductor nanocrystals (quantum dots, QD) can be used to increase
the effective absorption cross section of Zn-phthalocyanines (ZnPcs). ZnPcs and QDs form
stable hybrid complexes due to electrostatic interactions in aqueous solution. The
fluorescence of the QDs in such hybrid complexes is strongly quenched due to the transfer
of the absorbed light energy to the ZnPcs. We discuss the mechsnism of Förster resonance
energy transfer as a possible explanation for the energy migration in donor–acceptor pairs.
Calculations based on the experimental data show a transient enhancement of the ZnPcs
fluorescence by up to 140 % due to efficient excitation energy transfer (EET) from QDs. This
enhanced fluorescence decays with biphasic exponential dynamics indicating ongoing
reactions in the QD-ZnPc hybrid structures. The possible mechanism of increasing the yield
of the reactive oxygen species production due to improved spectral characteristics of hybrid
systems is discussed.
Keywords
Zinc phthalocyanines, Quantum dots, Fluorescence lifetime, Reactive oxygen species
List of Abbreviations
τ – fluorescence lifetime
φfl – fluorescence quantum yield
FWHM – full width at half maximum
TCSPC – time-correlated single photon counting
QD600p – positively charged CdSeCdTe/ZnS quantum dot
QD600n – negatively charged CdTe quantum dot
ZnPc8+ – positively charged octakis-(pyridinemethyl)-phthalocyanine
ZnPc8- – negatively charged octacarboxy phthalocyanine
TEMPOL - 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl
DPIBF - 1,3-diphenylisobenzofuran
1. Introduction
Due to their unique physical and chemical properties, phthalocyanines (Pcs) have found
practical application not only as dyes [1], but as new functional materials in nonlinear optics
[2], as gas sensors [3], catalysts [4,5], in artificial photosynthesis [6, 7, 8] and various
other applications. One of the most rapidly developing application fields of phthalocyanines
and their metal complexes (MPc) is the photodynamic therapy (PDT) of cancer and other
diseases [9-11]. In addition to anticancer PDT, the photosensitizing properties of MPcs are
of increasing interest for photodynamic inactivation of drug-resistant strains of pathogenic
microorganisms for therapeutic purposes [12, 13] as well for water desinfection [14].
Photodynamic inactivation leads to damage of undesirable cells by reactive oxygen species

generated by sensitizers in photoexcited states. Visible light activates triplet oxygen ( 3  g
O2) present in the medium to highly reactive singlet oxygen (3  g O2), which in turn oxidizes
biomolecules leading to the loss of the vital functions of cells and or apopthosis. MPcs are
so-called type II sensitizers forming 1gO2 according to the following scheme [11]:
h

MPc 
1
isc
3

MPc* 
MPc*,
3
MPc* + 3  g O2  MPc + 1gO2,
1
gO2 + biomolecule  oxidation products
where 1MPc* and 3MPc* denote Pc in the singlet or triplet excited state, respectively, isc
describes the inter-system crossing transition from the singlet to the triplet state.
MPcs have a high quantum yield of triplet state formation and singlet oxygen generation,
which determines their high phototoxic effect [12-14]. However, in aqueous solutions,
phthalocyanines tend to form photochemically inactive aggregates. In the aggregates,
excited states are quickly decaying via nonradiative channels, resulting in a decrease of the
quantum yield of singlet oxygen generation. Since water is a universal biological solvent,
the search of sensitizers for photodynamic therapy based on phthalocyanines, being in the
monomeric state in aqueous media, is medically relevant [15].
Inexpensive broadband lamps are favorable sources to activate photosensitizers used for
desinfection or treatment of skin infections. However, the light absorption properties of
MPcs are characterized by intensive Q-bands in the far-red spectral region with a maximum
near 680 nm and rather low absorption of other visible light wavelengths [15]. Thus, the
possibilities to potentiate MPcs’ photoactivation by wider range of wavelengths are likely to
be of great interest for biological and medical MPcs applications. This may be achieved by
increasing the effective absorption cross section of MPcs through energy transfer from
additional light-absorbing structures.
Modern nanotechnology allows to produce semiconductor nanocrystals, or so-called
quantum dots (QD), which absorb light in a broad optical range from ultraviolet to near
infrared. The fluorescence spectrum of QDs is rather narrow, have a Gaussian shape, and
the position of their fluorescence emission maximum can be precisely adjusted by the
diameter of the nanocrystal particles [16-18]. While being slightly worse than the best
fluorescent labels in terms of the magnitude of fluorescence quantum yield (up to ~70% at
room temperature), quantum dots exceed the latter by several orders of magnitude in the
light absorption cross section [19]. In addition, the organic coating by bi- or trifunctional
polymers provides water solubility and electrostatic interaction due to polar groups bound to
the surface [20-22]. Functional groups of organic coating are also available for conjugation,
which makes it possible to create artificial light-harvesting complexes based on quantum
dots, which can serve as highly effective energy donors for photosynthetic pigments and
pigment–protein complexes [23-28].
Recently, the possibility of energy transfer from QDs to MPcs photosensitizers was
demonstrated in [29-38]. However, absorption characteristics of the obtained conjugates
differed with the charge of MPcs used in the study. Compared with almost minor changes in
the absorption of anionic MPc in combination with QDs [30] the significant changes were
observed for combination cationic MPc-QD [35]. Moreover, the number of charged
substituents in MPc molecules may influence their aggregation capacity and thus
photophysical and photochemical characteristics [15].
Previously, we experimentally established that quantum dots, used as additional artificial
light collectors, efficiently absorb light in the ultraviolet and visible regions of the spectrum
and transmit energy to native photosynthetic pigment–protein complexes [24-28].
In the present study, we investigate the possibility of creating hybrid structures based on
zinc Pcs, bearing 8 anionic or cationic substituents, and differently charged QDs. We mainly
investigated the enhancement of the effective absorption cross section of ZnPc by efficient
energy transfer from the QDs. Probably, such structures can be used in photodynamic
therapy because QDs can greatly expand the action spectrum of the ZnPcs and reduce their
concentration and additionally allow the creation of switchable PDT dyes, since the coupling
between QDs and organic structures critically depends on external parameters like
temperature [25-26].
Figure 1: Structures of Zn-phthalocyanines used in this study.
2. Materials
Two types of ZnPcs were used. Negatively charged octacarboxy phthalocyanine (ZnPc 8- as
shown in Figure 1) and positively charged octakis-(pyridinemethyl)-phthalocyanine (ZnPc8+)
synthesized in the Organic Intermediates and Dyes Institute (Moscow).
1,4
1,2
1,0
O.D./Fluorescence
0,8
QD600n abs
QD600n em
8+
ZnPc abs
8+
ZnPc em
A
QD600p abs
QD600p em
8ZnPc abs
8ZnPc em
B
0,6
0,4
0,2
0,0
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
300
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Figure 2: Normalized absorption (abs) and emission (em) spectra of quantum dots and
zinc phthalocyanines at room temperature.
Two types of quantum dots with an emission maximum at 600 nm were used. One with a
core of CdSeCdTe/ZnS (further denoted as QD600p), with positively charged polymer shell
"poly T-APS", and another with a core of CdTe (termed QD600n), with negatively charged
carboxyl groups. Both types of QD were synthesized by “Nanotech-Dubna”, Russia.
Concentrations of quantum dots were calculated as described in [39]. Normalized
absorption and fluorescence spectra of QDs and ZnPcs are presented in Figure 2AB.
3. Methods
Fluorescence measurements were performed by time- and wavelength-correlated single
photon counting with the equipment described in [40-41]. The setup consists of a system
with a Hamamatsu R5900 16-channel multi-anode photomultiplier tube with 16 separate
output (anode) elements and a common cathode and dynode system (PML-16,
Becker&Hickl, Berlin, Germany). The polychromator was equipped with 600 grooves/mm
grating resulting in a spectral bandwidth of the PML-16 of about 200 nm (resolution of 12.5
nm/channel). Excitation was performed with a pulsed 405 nm laser diode (IOS, Saint
Petersburg, Russia) delivering 30 ps FWHM pulses, driven at a repetition rate of 50 MHz. To
study the dynamics of fluorescence quenching we measured the changes of initial
fluorescence of the donor after the addition of the acceptor and vice versa. The signal was
recorded form of 64 cycles in time (f(t,T) mode of B&H SPC [40]) with a duration of 3
seconds signal accumulation for each cycle with count rates up to 400.000 photons per
second. This setup allowed us to record the dynamics of the transition of the samples from
the initial to quenched state, by simultaneous measurement of the fluorescence intensity
and the fluorescence decay time. Thus, each experiment resulted in 1024 fluorescence
decay curves (16 spectral sections for each of 64 time windows).
For time-integrated measurements we used Fluoromax 4 (Horiba Jobin Yvon, France) and a
USB-connected fluorometer system with CCD array USB4000 (Ocean Optics, USA). During
the processes of quenching measuring, the fluorescence signal was accumulated for 1 s, this
procedure was repeated 100 times (i.e., 100 fluorescence spectra were recorded in steps of
1 s). Changes in the fluorescence intensity were analyzed in the bands of the QD (600 nm)
and the ZnPc (700 nm) fluorescence spectrum.
Absorption spectra were recorded using a USB2000 spectrometer with a DT-MINI-2-GS
deuterium tungsten halogen light source (Ocean Optics, USA), in standard 10 mm quartz
cuvette.
In all experiments, for mixing the solutions during the measurements, a magnetic stirrer
was used.
The fluorescence decay kinetics were approximated by the sum of the exponential functions
used to fit the experimental data. To compare different kinetic patterns, we calculated the
average decay time according to the expression: τav = ∑ni τi a i , where τi is the lifetime of the
i-th component and a i is the fraction of the amplitude of the i-th component of the
fluorescence decay normalized to ∑  = 1. To obtain the time-integrated fluorescence
spectra, the number of photons in each spectral channel was summed up.
All calculations were performed using Origin 8.0 (OriginLab Corporation, USA) and
SPCImage (Becker&Hickl, Germany) software packages.
To determine the rate of reactive oxygen species, we analyzed the changes in absorption
spectra of 1,3-diphenylisobenzofuran (DPIBF, Sigma Aldrich) as described in [42] as well as
changes in the electronic paramagnetic resonance spectra of 4-hydroxy-2,2,6,6tetramethylpiperidin-1-oxyl (TEMPOL) as described in [15, 42]. To determine the
bactericidal activity of Zn-phthalocyanines alone and hybrid systems we used a
bioluminescent bacterial test system based on a genetically engineered strain E. coli K-12
TG1, which emits bioluminescence due to complete lux-operon (commercially available
biosensor ECOLUM, Russia). The method is based on the correlation between
photosensitized bioluminescence quenching and inactivation of bacterial colony forming
units and is suitable for studying photodynamic effects [43-44]. Bioluminescence intensity
was recorded with a luminometer Sirius Smart Line TL (Titertek, USA).
Each experiment was repeated at least five times.
4. Results and Discussion
The absorption and fluorescence spectra of individual solutions of ZnPcs and quantum dots
(Figure 2AB) were used to calculate the corresponding overlap integrals according to the
formula [45]:
∞
 = ∫0  () ()4 ,
where  () is the normalized fluorescence spectrum of the donor, εa (λ) denotes the
absorption spectrum of the acceptor, λ is the light wavelength. The Förster radius was
calculated as:
0 = √8,8 × 10−25 ( 2 −4  ),
6
where  is the quantum yield of the donor in absence of acceptor,  is the refractive index
of the surrounding medium, k 2 denotes the orientation factor between the transition dipole
moments of the donor and acceptor. The value of the Förster radius was calculated to be
about 60 Å for the QD600p and 62 Å for the QD600n (as the fluorescence spectra of the
latter is a bit broader, Figure 2) for all ZnPc acceptors, due to the high similarity of the
absorption spectra of all studied compounds.
9
8-
ZnPc
QD600n
8ZnPc + QD
8
9
A
8+
ZnPc
QD600n
8+
ZnPc + QD
Fluorescence (a.u.)
7
B
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
0
20
40
8-
60
80
0
-8
10
20
8+
ZnPc (10 M)
30
40
-8
ZnPc (10 M)
12
11
8-
ZnPc
QD600p
8ZnPc + QD
10
9
Fluorescence (a.u.)
8
C
8+
ZnPc
QD600p
8+
ZnPc + QD
D
12
11
10
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
0
20
40
8-
60
-8
ZnPc (10 M)
80
0
10
20
8+
30
40
-8
ZnPc (10 M)
Figure 3: Relative quantum yields of fluorescence of quantum dots and zinc
phthalocyanines during the titration. Fluorescence excitation at 405 nm, CW mode. Red
curves – QDs, black – ZnPcs without QDs, blue – ZnPcs in presence of quantum dots. Initial
concentrations of QD600p and QD600n were 7·10-9 M and 2·10-8 M, respectively.
Titrations were performed to estimate the quenching of the QDs by Pcs. Initially, an
aqueous solution of the QD (donor of energy) was prepared and then gradually increasing
concentrations of ZnPc (acceptor of energy) were added. After the addition of each new
concentration of acceptor, the solution was incubated for 5 minutes with intense stirring.
The steady-state fluorescence spectra of QD and ZnPc were recorded to analyze the
changes of the related quantum yield. To determine the enhancement of the acceptor
fluorescence, the same sequence of experiments was carried out with ZnPc in the absence
of quantum dots. The results of the titration are shown in Figure 3. It clearly shows that in
pairs of similarly charged donor and acceptor of energy, the quenching of donor is not as
strong as in the case of opposite charged pairs. After mixing QD600n and ZnPc 8+ as well as
in solution of QD600p and ZnPc8-, significant quenching of both, donor and acceptor, was
observed at high acceptor concentrations. The same procedure of titration was studied with
time-resolved fluorescence spectroscopy (data not shown), the changes of the overall
photon numbers in the spectral channels corresponding to the QD and ZnPc fluorescence
were in a good agreement with the results presented in Figure 3. Those series of
experiments were used to estimate the efficiency of energy transfer () from QDs to ZnPc
as a result of changes of the fluorescence lifetime (  ) of the donor (QD):
 =1−

0
where   is the fluorescence lifetime of the donor in presence of the acceptor and τd0 denotes
the fluorescence lifetime of the donor in absence of the acceptor. Despite of a significant
reduction in the fluorescence intensity of quantum dots in pairs of QD600n-ZnPc8- and
QD600p-ZnPc8+, the efficiency of energy migration does not exceed the values of 0.1 and
0.5, respectively, and might be even lower due to radiationless dynamic quenching of the
donor by the acceptor opening concurring decay channels that lead to reduction of the
lifetime without real “transfer” of energy. In contrast, in pairs of oppositely charged QD and
ZnPc, the efficiency of energy migration due to the reduction of lifetime of the donor
exceeded 0.9, corresponding to ~ 30 Å distance between the donor and the acceptor. Thus,
the quantum dots can interact with oppositely charged ZnPc due to electrostatic interactions
and form hybrid structures with highly efficient energy transfer.It is also important to note
that the absorption spectrum of QD600n-ZnPc8+ hybrid system cannot be obtained by
simple summing the optical densities of the individual solutions (see Figure 4B). Significant
changes in the red region of the absorption spectrum indicate the formation of dimers of
ZnPc8+, as characterized by low fluorescence quantum yield and the low yield of singlet
oxygen [44, 46]. Therefore, it is assumed that two (or more) molecules of ZnPc 8+ can
simultaneously bind to a single QD of this type. We can assume that this explains the
significant decrease of the fluorescence intensity of ZnPc 8+ in the presence of QD600n. On
the contrary, the absorption spectrum of the hybrid system QD600p-ZnPc8- in the red region
corresponds well to the absorption spectrum of an aqueous solution of the ZnPc 8-. Thus,
from the point of view of the possible increase in the photodynamic effect, the most
interesting hybrid structure is QD600p-ZnPc8- hybrid system, since it indicates a highly
efficient transfer of energy, and the ratio of QD to ZnPC in the hybrid system is probably 1
to 1, despite a much larger number of phthalocyanine molecules in solution.
1,0
1,0
0,8
O.D. (units)
A
QD600p
8ZnPc
Hybrid system
B
QD600n
8+
ZnPc
Hybrid system
0,8
0,6
0,6
0,4
0,4
0,2
0,2
0,0
300
400
500
600
700
Wavelength (nm)
800
300
400
500
600
700
0,0
800
Wavelength (nm)
Figure 4: Absorption spectra of quantum dots and Zn-phthalocyanines and their hybrid
systems at room temperature. The concentration ratio was selected from Figure 3,
corresponding 1/133 and 1/500 for QD600p/ZnPc8- and QD600n/ZnPc8+, respectively,
showing the most efficient quenching of the QDs.
It was found that the fluorescence intensity of the donor and the acceptor species is
strongly time-dependent after the mixing, and relative quantum yields can change
dramatically in a few minutes. This indicates that a slow chemical reaction proceeds (in
contact of QDs and ZnPCs) when QDs and ZnPcs are in contact. To analyze such a slowly
proceeding chemical reaction by continuous imaging of the EET efficiency, time-resolved
spectra were taken with highest possible time resolution for the single decay curves. Since
the relative fluorescence quantum yield may change due to static and dynamic fluorescence
quenching [45] that develops on a macroscopic time scale of up to minutes, the dynamics of
the quenching process was investigated by measuring the time-resolved fluorescence in
steps of 3 seconds. For these experiments, we prepared a solution with a certain
concentration of the acceptor. Simultaneously, the registration of the fluorescence decay
kinetics and the steady-state fluorescence spectra was started with an increment of 3
seconds. Then, the donor was added to the measured volume. The concentration ratio for
the dynamic experiments was selected based on the results of the titration (Figure 3). Of
greatest interest are the points with the largest difference between the relative quantum
yields of the individual acceptor and the acceptor in the presence of the donor (black and
blue curves in Figure 3). However, to obtain a high count rates (up to 400,000 photons per
second) by the single photon-counting method, the concentrations of acceptor and donor
were increased 10-fold and were exactly the same as in Figure 4 (1/133 and 1/500 for
QD600p/ZnPc8- and QD600n/ZnPc8+, respectively). The most interesting results are
presented in Figure 5.
It was found that the fluorescence intensity and lifetime of QD600p are sharply reduced
after the injection into a solution of ZnPc8-, and this decrease continues with a typical
biphasic behavior comprising characteristic time constants of about 8 s (major component)
and 45 s (minor component). The efficiency of energy migration, calculated as a change of
fluorescence lifetime of QD600p, reaches 0.94. Monitoring the ZnPc 8- fluorescence shows
that after injection QD600p, there is a fast increase of fluorescence intensity of ZnPc 8- by
almost 140% compared to the initial level of fluorescence. The enhancement factor
gradually decreases with characteristic time constants of about 8 s (major component) and
100 s (minor component). It should be noted that after addition of the QD600p the
fluorescence spectrum of the ZnPc8- is red-shifted by approximately 5 nm. Simultaneous
measurements of the fluorescence lifetime of the acceptor shows that upon the addition of
QD, the fluorescence lifetimes increases by 7% and essentially does not change with time.
This phenomenon indicating that there is a significant EET from the quantum dots that leads
to appearance of the fluorescence rise kinetic and therefore to a virtual prolongation of the
fluorescence decay time. That means that even after a long time, hybrid structures with
efficient EET can be detected in the measured volume. Thus, QD600p form stable hybrid
complexes with ZnPc8-. Due to a highly efficient energy transfer, the effective absorption
cross section of ZnPc can be strongly increased. In other investigated pairs of QDs and
ZnPcs, the increase of the acceptor fluorescence was insignificant, probably due to weak
electrostatic interactions, radiationless quenching, or the formation of dimers (Figure 4).
3200
2,4
1,0
3150
2,2
0,8
I/I0 QD600p
QD600p
0,4
0,2
3100
1,8
Enhancement coef.
8 ZnPc
1,6
3050
3000
1,4
1,2
Lifetime (ps)
I/I0,  (units)
0,6
Enhancement (units)
2,0
2950
1,0
0,0
A
0
30
60
90
120
Time (s)
150
180
B
0,8
0
30
60
90
120
150
2900
180
Time (s)
Figure 5: (A) Changes in the relative quantum yield and the fluorescence lifetime of the QD600p
(compared to individual QD aqueous solution) when adding it to the solution of ZnPc 8-. At zero time,
fluorescence lifetime and intensity of QD correspond to those in case of donor in the absence of
acceptor. (B) The enhancement of ZnPc8- fluorescence (comparing to initial level) and changes of the
fluorescence lifetime as a result of the addition of quantum dots with positive surface charges. The
ratio of donor and acceptor is exactly the same as in Figure 4A. Fluorescence excitation - 405 nm, 30
ps, 50 MHz. A magnetic stirrer was used with constant stirring rate. Arrows indicate the time of QD
injection.
Since the ZnPc8- in the excited state can generate singlet oxygen, it is reasonable to assume
that the energetic interaction between the positively charged QD600p and the negatively
charged ZnPc8- should lead to increased amount of 1gO2 generation, which also might
cause the degradation of the Zn-phthalocyanine molecules and reducing the measured
fluorescence amplitude. Such photodegradation would lead to the gradual decay of the
increased fluorescence intensity of ZnPc without influence on the fluorescence decay time
(Figure 5). However, the question remained to be answered, if quantum dots, when used as
additional light-harvesting antennas, can increase the rate of 1gO2 generation by ZnPc.
Therefore, standard methods to detect reactive oxygen were used [42]. The presence of
5·10-6 M ZnPcs caused characteristic changes of the DPIBF absorption spectrum and lightinduced oxidation of the TEMPOL spin label was registered. The addition of a solution of
sodium azide, which is known to bind singlet oxygen, caused a sharp decrease in the
amplitude of the EPR signal.
Intensity (a.u.)
8-
ZnPc
Hybrid system
8-
ZnPc
Hybrid system
3000
3000
2000
2000
1000
1000
0
0
-1000
-1000
-2000
-2000
-3000
A
B
Magnetic Field
-3000
Magnetic Field
Figure 6: Left (A) - EPR signal of TEMPOL after 5 minutes of white light illumination of
solution of ZnPc8- (black) and QD600p/ZnPc8- (red). Right (B) - EPR spectrum of ZnPc8before (black) and after addition of QD600p (red).
These results clearly show that ZnPcs exposed to light are able to generate singlet oxygen.
However, no significant increase in the rate of generation of singlet oxygen could be
registered with the described methods after additionally adding quantum dots to ZnPcs.
Conversely, in the presence of hybrid systems, changes of the optical density of DPIBF were
not longer detected, and the amplitude of the EPR signal of TEMPOL (and ZnPcs themselves)
decreased dramatically (see Figure 6), wich points out the reduction of TEMPOL. Moreover,
it was shown in control experiments that the quantum dots themselves are able to interact
with DPIBF and TEMPOL, probably causing their reduction. Thus, we assume that these
methods of singlet oxygen detection do not give adequate results, since QDs can interact
directly with DPIBF and TEMPOL (this mechanism is interesting by itself), and, obviously,
other methods of testing our hypothesis are required. The most interesting test would be to
measure the direct interaction between bacteria and QD-ZnPc hybrid complexes.
For that purpose, we investigated the photoinactivation of the special strain E. coli K-12
TG1 after addition of ZnPcs, QDs and hybrid structures. The level of the K-12 TG1
bioluminescence was measured immediately after the addition of ZnPcs and QDs to the cell
suspension, and then at regular intervals after irradiation with white light. The main results
are presented in Figure 7.
It is seen from Figure 6, not only the zinc phthalocyanines [15, 46], but also the aqueous
solution of quantum dots display bactericidal activity [47-50]. Treatment of the cell
suspension by a solution of hybrid structures leads to a more efficient photoinactivation.
The effect of bacteria inactivation by ZnPc-QD hybrids, however, is smaller compared to the
summarized effect in the presence of both, QDs and ZnPC. Probably, positively charged QDs
bind to negatively charged bacterial cell walls and protect cells from singlet oxygen
generated in solution by the anionic photosensitizer. Therefore, it is assumed that our
investigated structure does not release more 1gO2 than ZnPc or QD only.
100
Bioluminescence, %
80
60
40
control
QD600p
8ZnPc
Hybrid
20
0
0
2
4
6
8
10
12
Dose of white light, J/cm
14
16
18
20
2
Figure 7: Level of E. coli K-12 TG1 bioluminescence comparing to initial level (I/I0 ).
Thus, the effect of quantum dots on the ability of zinc phthalocyanine to generate singlet
oxygen is unclear and requires alternative research methods.
5. Conclusion
Creation of hybrid structures in solution due to electrostatic interactions, without the use of
additional reagents for the formation of covalent bonds, opens up a number of promising
new areas of research and corresponding applications. It was shown that in a mixture of
ZnPcs and QDs, stable hybrid complexes can be formed due to electrostatic interactions.
The fluorescence of QDs in such hybrid complexes is strongly quenched due to quenching of
the absorbed energy by ZnPcs. In the framework of FRET, the distance between the donor
and the acceptor of energy in hybrid complexes should not exceed ~ 30 Å. According to
steady-state and time-resolved spectral measurements, quantum dots can transfer the
excitation energy to the zinc phthalocyanine, increasing the effective absorption cross
section of ZnPc and the number of excited states. Especially in ZnPc 8- /QD-600p complexes,
a characteristic fluorescence rise (prolongation of the acceptor fluorescence) was observed
indicating a strong EET. Calculations based on the experimental data show that the
enhancement of ZnPcs fluorescence can reach a factor of 2.4 due to efficient energy
migration from QDs. Interestingly, this EET is transient and seems to occur concomitant to
strong degradation of ZnPc, possibly due to the interaction with 1 gO2. This degradation
might be caused by the generation of reactive oxygen species, which can reduce the
concentration of ZnPcs and QDs itself. However, the rate of reactive oxygen species
generation through improved spectral characteristics of hybrid systems is unclear. It was
shown that quantum dots can interact with DPIBF and TEMPOL making them insensitive to
singlet oxygen, so alternative methods are necessary to study this process.
Additionally the strong photodegradation of ZnPc might occur due to the interaction with
localized 1gO2 therefore preventing the release of 1gO2 into the solution. This finding is
supported by the observation that the biocidal effect on bacteria is not significantly
enhanced in the case of hybrid complexes compared to the effects of ZnPc or QDs only. It is
important to note that a very strong bactericidal action by QDs alone was shown.
Some additional questions should be studied in the future works. For example, what is the
nature of the dynamics of the amplitude decrease and virtual destruction of the acceptor?
What is the exact mechanism of donor quenching? The role of diffusion should be analyzed
in future studies by the means of temperature-dependent investigation. The exact
dependency of the interaction between donors and acceptors in the QD-ZnPc hybrid
structures on the light intensity and the ionic strength of the buffer medium will be targeted
by future studies, as well as investigations of the stability of the complexes.
Acknowledgements
Authors are grateful to Dr. K.N. Timofeev for assistance with the registration of the EPR
spectra. Financial support by BMBF bilateral cooperation funds RUS 10/026 and RUS 11/014
is gratefully acknowledged. V.Z. Paschenko and E.G.Maksimov also thank the Russian
Foundation for Basic Research (project no. 11-04-01617 and no. 12-04-31100).
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