Circular Dichroism Spectroscopy as a Powerful Tool for Unraveling

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A: New Tools and Methods in Experiment and Theory

Circular Dichroism Spectroscopy as a Powerful Tool for Unraveling Assembly of Chiral Non-luminescent Aggregates of Photosensitizer Molecules on Nanoparticle Surfaces Anastasia K. Visheratina, Finn Purcell-Milton, Yurii K. Gun'ko, and Anna O. Orlova J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b05500 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Circular Dichroism Spectroscopy as a Powerful Tool for Unraveling Assembly of Chiral Non-luminescent Aggregates of Photosensitizer Molecules on Nanoparticle Surfaces Anastasia K. Visheratinaa*, Finn Purcell-Miltonb, Yurii K. Gun’koa,b, and Anna Orlovaa*

a

ITMO University, 49 Kronverksky Pr., St. Petersburg 197101, Russia

b

School of Chemistry and CRANN, University of Dublin, Trinity College, Dublin 2, Ireland

ABSTRACT Recent developments in nanoscience and nanotechnology significantly help improve the properties of traditional materials. A striking example of this is the formation of hybrid nanostructures based on nanoparticles and photosensitizer molecules, the potential range of applications of which extends from photovoltaics to biomedicine. However, the creation of new and effective hybrid nanomaterials of this form inevitably entails new challenges, one of which is a common and critical problem of aggregation of both nanoparticles and photosensitizer molecules. Therefore, a fundamental challenge is to determine the presence of these aggregates, which will produce a significant step toward creating a new generation of materials and devices of broad-spectrum applicability. Here we report on the key role of circular dichroism spectroscopy as a tool to detect the formation of non-luminescent aggregates of chlorin e6, a second-generation photosensitizer, in a hybrid nanostructure with ZnS:Mn quantum dots. These aggregates are active acceptors of photoexcitation energy from quantum dots and limit the

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photophysical properties of the whole nanostructure. It has been established that circular dichroism spectroscopy reveals the presence of non-luminescent molecule aggregates at chlorin e6 concentrations of ~ 10-6 mol/l, which compares very favorably to absorption spectroscopy which does not show any direct indications of aggregation up to ~ 10-5 mol/l. This result demonstrates the promise and importance of using circular dichroism spectroscopy in the study of organic/inorganic hybrid nanostructures.

INTRODUCTION Photosensitizers (PSs) are species that induce chemical changes in adjacent molecules during a photochemical process1. PSs are commonly utilized in such reactions as photopolymerization2, photocrosslinking3, photodegradation4, and are also used to efficiently generate triplet excited states in organic molecules with uses in photocatalysis4, photon upconversion5, fine chemicals synthesis6, wastewater treatment6, and photodynamic therapy6,7 (PDT). Despite their unique properties, PSs have several disadvantages such as fast photobleaching, low extinction coefficients, low photoluminescence quantum yields, and tendency to aggregate in solutions8. These prevent PSs from achieving their full potential. Therefore, today there is an active search for new materials which could offer improved and expanded properties. One such approach is the combination of nanoparticles and PSs to form a hybrid nanostructure, one which allows the creation of multifunctional systems with advanced properties for application in various fields including photovoltaics9, sensorics, ecology10, and biomedicine11–14. These hybrid structures can be used to expand the PS’s photoexcitation spectral range, prevent aggregation and increase singlet oxygen or other reactive oxygen species generation 14–19. Recent progress in the area of hybrid nanomaterials indicates their excellent prospects20–25. However, during the development of hybrid nanomaterials, a number of difficulties and challenges arise. For instance, it has been reported that the formation of hybrid nanostructures based on colloidal semiconductor quantum dots (QDs) and PSs may be accompanied by the


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appearance of nonradiative channels that deactivate the excited states of the QDs and/or PSs26–28. In turn, this can lead to a decrease in the photoluminescent (PL) properties of PSs and their ability to generate singlet oxygen. Other common difficulties are associated with the aggregation of components of PS-based hybrid nanostructures. It has been recently revealed via absorption and PL spectroscopic study that Al-sulphophthalocyanine can form non-luminescent aggregates on the surface of CdSe/ZnS QDs as a result of partial or complete loss of charge in phthalocyanine upon its binding to QDs29. The authors demonstrated that the appearance of these aggregates inevitably leads to the significant decrease of (i) Förster resonance energy transfer (FRET) from QDs to Al-sulphophthalocyanine molecules and (ii) their PL QY. This is due to the fact that even one non-luminescent aggregate can completely quench the PL of QD and PS monomers. Thus, the reasons for the decline in the functional properties of hybrid nanostructures can be of a different nature, and there is a need for methods that would enable these causes to be fully resolved. To date, in most cases, hybrid nanostructures have been studied using steady-state absorption and photoluminescence (PL) spectroscopy as well as by time-resolved PL spectroscopy. However, in some cases, there are no obvious signs of aggregation of molecules in corresponding spectra, but a decrease in their luminescent and functional properties is observed29. Therefore, in this paper, we propose a new method for identifying aggregates of molecules in PS-based hybrid nanostructures using circular dichroism (CD) spectroscopy, which is based on the differential absorption of right- and left-handed circularly polarized light by a substance30. CD spectroscopy is one of the main methods for investigation of chiral compounds, but it can also be used to study achiral molecules that are assembled into complex structures of a chiral geometry31–36. As for nanoparticles, in this case, hybrid nanostructures can be based on nanoparticles from various materials, sizes, and shapes. For example, a number of PS-based hybrid nanostructures for different applications were formed using nanocarbons, liposomes, as well as polymeric, silica, magnetic, semiconductor, and noble metal nanoparticles37,38. Thus,



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circular dichroism spectroscopy can act as a universal and very informative method for studying hybrid nanostructures on par with standard means of optical spectroscopy. To sum up, the main contributions of the paper are the following. We formed a hybrid nanostructure based on ZnS:Mn QDs with chlorin e6 (Ce6), a promising second-generation photosensitizer with minimal toxicity and remarkable clinical effects for PDT39, as a result of electrostatic interaction between the components in aqueous solution. These QDs and PS molecules were chosen since they are excellent model objects for demonstrating the advantages of using CD spectroscopy to detect the assembly of PS aggregates on the surface of nanoparticles. The resulting systems were studied using standard optical spectroscopy techniques (UV-Vis and PL) and CD spectroscopy. While standard methods did not recognize any evidence of Ce6 aggregations, CD spectroscopy allowed the identification of the presence of the onset of Ce6 assembles into chiral aggregates on the nanoparticle surface. This was accompanied by decreasing PL QY of Ce6, as well as fall in the FRET efficiency from the QDs to Ce6. Thus, CD spectroscopy makes it possible to reveal molecule aggregation at an early stage and, therefore, can help to optimize properties of whole hybrids in order to take a step forward in the development of new materials with improved parameters.

EXPERIMENTAL SECTION Chemicals. All chemical reagents were of analytical grade and used as purchased without further purification. Chlorin e6 (Ce6) was purchased from Frontier Scientific (USA). Cysteine hydrochloride was purchased in Sigma-Aldrich. Zinc chloride (ZnCl2; anhydrous, free-flowing, Redi-Dri, reagent grade, ≥98% Sigma-Aldrich, cat. no. 793523). Manganese chloride (MnCl2; powder and chunks, ≥99% trace metals basis; Sigma-Aldrich, cat. no. 244589). Dibenzylamine (C6H5CH2)2NH, 97%; Sigma- Aldrich, cat. no. D34108). Oleylamine (C18H37N, technical grade, 70% wt/wt; Sigma-Aldrich, cat. no. O7805).


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Synthesis of ZnS:Mn QDs. 0.4 g of ZnCl2 and 0.02 g of MnCl2.4H2O were dissolved in 54 mL of dibenzylamine and heated under vacuum at 120°C for 2 hours (pot 1). The solution was then cooled to 50°C and removed from vacuum and 0.6g of sulphur powder was added. The solution was then heated to 260°C under reflux and held at this temperature for 15 minutes. The solution was then cooled to 160°. A separate solution must be made up of 0.8 g of ZnCl2 in dibenzylamine and heated under vacuum at 120°C for 1 hour (pot 2). 5 mL of this solution was added to the pot 1 and heated again to 260°C and held at that temperature for a further 15 minutes. The solution was then cooled to around 160°C and then ethanol was added to crash out the quantum dots. The quantum dots were then washed and centrifuged several times with ethanol in order to remove excess sulphur until the supernatant is no longer yellow. Once cleaned, the quantum dots were stabilized with oleylamine (≈ 2 mL) and then dissolved in 20 mL of chloroform. Ligand exchange of ZnS:Mn QDs. A solution of as-synthesized QDs (~ 5 mg) in chloroform (250 μl) was washed with methanol (500 μl) from an excess of oleylamine. After the centrifugation (1 minute; 16000 rpm), the sample was dissolved in 500 μl of chloroform. To replace the initial hydrophobic ligands with hydrophilic, 50 μl of a concentrated solution of cysteamine hydrochloride in methanol (0.1 mg/μl) was added to a QDs solution in chloroform (500 μl). The resulting mixture was actively stirred on a shaker (20 minutes; 600 rpm) to ensure maximum ligand substitution. After that, QDs were precipitated by centrifugation (1 minute; 16000 rpm) and washed with isopropanol by precipitation (1 minute; 16000 rpm). Finally, QDs were dissolved in distilled water (pH ≈ 6). Formation of hybrid nanostructures based on ZnS:Mn QDs with Ce6. Since the surface of ZnS:Mn QDs was positively charged and Ce6 was negatively charged, QDs/Ce6 hybrid nanostructures were formed by simple mixing of solutions of QDs and Ce6 as a result of electrostatic attraction between components. The concentrations ratio n=C Ce6:CQDs, i.e. the ratio of number of Ce6 monomers per QD, was varied from 1.5 to 12.0.



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Characterization of QDs/Ce6 hybrid nanostructures. UV-Vis and PL spectroscopy were carried out using a UV-3600 spectrophotometer (Shimadzu, Japan) with a slit width of 8 nm and a Cary Eclipse fluorescence spectrophotometer (Varian, Australia), respectively. CD spectra were recorded using a J-1500 spectrometer (JASCO, Japan) with a spectral resolution of 3 nm. Transmission Electron Microscopy (TEM) was performed using an FEI Titan electron microscope operating at a beam voltage of 300 kV. A quartz cuvette with a 1 cm path length was used for all spectroscopic measurements.

RESULTS Second generation photosensitizer chlorin e6. Ce6 is a chiral fluorescent tetrapyrrole molecule with three carboxyl groups (Figure 1). This provides the opportunity to form Ce6 based hybrid nanostructures through coordination of carboxyl groups on the nanoparticle surface40, via covalent binding41–45, as a result of electrostatic attraction46 and some others47–50. Electrostatic interaction is the simplest type of hybrid nanostructure formation, which utilizes the fact that oppositely charged components are attracted to each other via Coulomb forces. Hence, it is necessary to prepare solutions of oppositely charged nanoparticles and a PS of the required concentrations and mix them. The carboxyl groups of Ce6 dissociate in an aqueous solution with pH ≈ 6-7 and, therefore, acquire a negative charge. Using nanoparticles in this case, whose surface zeta potential is positive, allows the formation of the hybrid nanostructure as a result of electrostatic interaction.


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Figure 1. Chlorin e6 structure.

As can be seen in Figure 2 and Table 1, the optical properties of Ce6 monomers and dimers, i.e. absorption, PL and CD spectra are different, which allows them to be distinguished in practice. It has been recently reported51 that CD spectroscopy is a more sensitive and therefore informative method for studying the degree of Ce6 aggregation compared to the steady-state absorption and PL spectroscopy. The authors51 demonstrated that Се6 can be in the monomeric form in aqueous solution with pH ≈ 7-8 or dimethyl sulfoxide, in dimeric form in aqueous solution with pH ≈ 6 and in more complex aggregates in aqueous solution with pH < 6. It should be pointed out that the CD spectra of monomeric and dimeric forms of Ce6 are significantly different (see Figure 2). Additionally, it is noted that the first absorption band of Ce6 is in the 600–700 nm region. Therefore, it is preferable to use nanoparticles that do not absorb in this region for a thorough analysis of the optical properties of Ce6.



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Figure 2. Optical properties and structures of Ce6 monomers and dimers.

Table 1. Characteristics of Ce6 monomers and dimers. Parameter λmax of the 1st absorption band Extinction coefficient at λmax g-factor PL Quantum Yield

Ce6 monomers 655 nm 3.86∙104 M-1cm-1 3∙10-4 13%

Ce6 dimers 640 nm 3.14∙104 M-1cm-1 8∙10-4 8%

ZnS:Mn Quantum dots. The advantages of using ZnS:Mn QDs as a platform for hybrid nanostructures with Ce6 are the following: (1) ZnS:Mn QDs are composed of biocompatible materials. (2) ZnS:Mn QDs absorb in the UV spectral region, which does not overlap with the Ce6 absorption spectrum and therefore does not interfere with the analysis of the Ce6 optical properties. (3) The g-factor of Ce6 in the region of its first absorption band does not change in the case of QD/Ce6 hybrid nanostructures formed in aqueous media51. It can be associated with low degree of hybridization of the Ce6 HOMO with the QD valence band states due to the considerable spatial distance between the QDs and Ce640. Thus, the analysis of the Ce6 optical activity, in this case, will not be impeded under the QDs influence.


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(4) ZnS:Mn QDs act as effective energy donors for Ce629 due to the resonance of the lower Ce6 excited states with the QD’s electronic transition, which is an intercombination transition 4T 1

→ 6A1 between the intrinsic levels of the impurity Mn2+ ions. It was reported that the

rate-determining step for the production of singlet oxygen is the energy transfer from the QDs to the photosensitizer molecules52. Therefore, energy transfer efficiency is a good indicator of the functional potential of ZnS:Mn QD/PS hybrid structures. Oleylamine-stabilized ZnS:Mn QDs were synthesized according to the previously reported high-temperature metal-organic procedure53 and displayed PL QY of approximately 35%. Synthesized QDs were of spherical shape, highly monodispersed with an average diameter of 5.3±0.7 nm. Following this, the initial oleylamine ligands were replaced by cysteamine using a standard phase transfer method (see Experimental section). Cysteamine provides a positive charge on the QD surface in aqueous solution with pH ≈ 6. Figure 3 shows that after the ligand exchange, the positions and shapes of the QD’s absorption and PL bands remained unchanged, but analysis of these spectra revealed a slight decrease in PL QY from 35% to 29% with the edge of the first exciton absorption band and the maximum of the QD’s phosphorescence peak located at 300 nm and 590 nm, respectively. The extinction coefficient of ZnS:Mn QDs at the first exciton absorption peak (300 nm) was determined to be of approximately 2.75·105 M-1cm-1.

(a) Oleylamine Cysteamine

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

350

400

Wavelength, nm

Figure 3. (a) UV-Vis and (b) PL spectra of ZnS:Mn QDs before (Oleylamine) and after (Cysteamine) the phase transfer. Positions of exciton absorption and PL bands are marked by



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vertical bars.

Optical properties of ZnS:Mn QD/Ce6 hybrid nanostructures. Figure 4 shows the absorption spectra of cationic ZnS:Mn QDs (СQDs ~ 2·10-6 mol/l) and their mixed aqueous solutions with Ce6 at different ratios of the components. The inset shows the absorption spectrum of QDbonded Ce6 monomer (at n=1.5) in the mixture with QDs compared with the absorption spectrum of free Ce6 dimer in an aqueous solution at pH = 6, as well as the schematic illustration of Ce6 transition from dimeric to monomeric form as a result of interaction with QDs. The spectrum of the bonded monomers was calculated by subtracting the QD absorption spectrum from the corresponding mixture spectrum. At the smallest components’ ratio, the Ce6 absorption spectrum in the mixture with QDs underwent the following changes: (1) The intensity of the first absorption band of Се6 Q (I) increased and 20 nm bathochromic shift was observed (from 642 nm to 662 nm); (2) 20 nm hypsochromic shift of the Q (IV) band took place (from 525 nm to 505 nm); (3) The half-width of the Soret band, which is located at 400 nm, increased. Similar changes of the absorption spectrum of Ce6 were previously observed in its mixed solutions with QDs because of the formation of QD/Ce6 hybrid nanostructures with the Ce616,54 accompanied by Ce6 monomerization51 in the hybrid nanostructures (a schematic illustration of the process is in the Inset in Fig.4).


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Figure 4. Absorption spectra of solutions (pH = 6) of ZnS:Mn QDs (СQDs ~ 2·10-6 mol/l) and Ce6 with different component ratios (n). Inset: absorption spectra of QDs-bonded Ce6 monomer in the mixture solution with QDs, free Ce6 dimers in an aqueous solution at pH = 6, and the schematic illustration of Ce6 transition from dimeric to monomeric form as a result of interaction with QDs. The QDs-bonded Ce6 monomer spectrum was calculated as a difference between the spectra of QD-Ce6 mixture (at n=1.5) and QDs.

Figure 5 presents PL spectra of Ce6 mixed with QDs in solution at different concentration ratios. Obviously, at the smallest Ce6 concentration (at n=1.5), only PL of Ce6 monomeric form centered at 670 nm is observed in the spectrum. At the same time, the increase of the component ratio up to n=9.0 led to the disappearance of the Ce6 monomer PL band and to the appearance of the PL band centered at 655 nm, i.e. the luminescence of free Ce6 dimers51 was only observed at high Ce6 concentration. This means that the PL of Ce6 monomers is completely quenched in the QD/Ce6 hybrid nanostructures at the concentrations ratio equal to 9.0 and higher. ACS Paragon Plus Environment


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Figure 5. PL spectra of mixed solutions of ZnS:Mn QDs (2·10-6 mol/l) with Ce6 at different ratios of the components (n). The inset shows the normalized spectra. PL excitation wavelength is 610 nm.

We have also analyzed PL excitation (PLE) spectra of the samples presented in Figure 6, in order to estimate the efficiency of FRET, which confirms QD/Ce6 nanostructure formation in solution11,12,16,28,30.


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Figure 6. PLE spectra of mixed solutions of ZnS: Mn QDs and Ce6, PLE spectrum of free Ce6 monomers as well as ZnS:Mn QDs absorption spectrum. PL registration wavelength is 680 nm.

The FRET model55 for QDs-based hybrid nanostructures was used to estimate energy transfer efficiency in the samples. The analysis of PL properties of the QDs/Ce6 hybrid nanostructures, i.e. QDs PL quenching, the appearance of sensitized PL of Ce6 and contribution of the QD’s absorption spectrum to the PLE spectrum of Ce6, revealed FRET from the ZnS:Mn QDs to Ce6 with the efficiency of approximately 40% at the smallest Ce6 concentration in the samples. An increase of the component concentration ratio up to n=3 has led to an increase in the intensity of the Ce6 first absorption band corresponding to the QD-bonded Ce6 monomers (see Figure 4). At the same time, Ce6 PL intensity (data not shown) and the contribution of the QD’s absorption spectrum to the Ce6 PLE spectrum decreased (see Fig. 6). In addition, a drop in FRET efficiency from 40±3% to 20±2% was also observed. It should be noted that the shape and position of the Ce6 first absorption band and PL peak corresponded to that of the Ce6 monomers bonded to the ZnS:Mn QDs. A similar picture was observed in hybrid nanostructures with CdSe/ZnS QDs and ACS Paragon Plus Environment


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Al-sulphopthalocianine molecules because of the formation of non-luminescent molecules aggregates55. It should be pointed out that absorption spectra of the samples concerned (see Fig. 4) include only patterns of dimeric and monomeric forms of Ce6, but no new absorption bands that could be attributed to non-luminescent Ce6 aggregates. Chiral properties of ZnS:Mn QD/Ce6 hybrid nanostructures. Because of low sensitivity, absorption spectroscopy does not allow detecting non-luminescent Ce6 aggregates. Therefore, at the next step we have applied CD spectroscopy to the study of Ce6 optical properties in the QDs/Ce6 hybrid nanostructures. Figure 7 presents CD and absorption spectra of the samples in the spectral range of the first Ce6 absorption band, i.e. 580-720 nm.

Figure 7. CD (the top panel) and Absorption (the bottom panel) spectra of mixed aqueous solutions (pH = 6) of ZnS:Mn QDs and Ce6 with different ratios of components (n) in the spectral range of 580 – 720 nm.


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The analysis of the CD spectra shown in Figure 7 demonstrates that at the smallest Ce6 concentration in the samples (at n=1.5), the CD spectrum is characterized with a band centered at 660 nm, which corresponds to the first absorption band of Ce6 monomers. A gradual increase of the components ratio (n) up to 6.0 in the samples leads to appearance of a new CD band centered at 700 nm in the spectra (see Fig. 7). It should be pointed out that the increase of Ce6 concentration in the mixture with QDs has been accompanied by increasing of intensity of this band and the QD-bonded Ce6 monomer band. It indicates that the increase of Ce6 concentration in the samples leads not only to increase of QD-bonded Ce6 monomers, but also to increase of a new spectral form of Ce6. We can suppose that this form of Ce6 is QD-bonded non-luminescent Ce6 aggregate because all experiments were carried out in aqueous solutions with pH 6 and no pattern of this form was observed in PL spectra of the samples or in free Ce6 absorption spectra. The increase of the component ratio up to 9.0 also led to the appearance of free Ce6 dimers in the samples, as evidenced by the presence of a short-wavelength shoulder in the first absorption band of Ce6 (see Figure 7) and the appearance of Ce6 PL band corresponding to free Ce6 dimers in aqueous solution (see Figure 5). It means that the formation of QDs/Ce6 hybrid nanostructures has reached saturation at these component concentrations in the samples, and new portions of Ce6 are no longer bonded to the ZnS:Mn QDs. It is important to note that there are no significant changes in the first absorption band corresponding to the monomers of Ce6 bonded to the QDs.



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Figure 8. Absorption (a) and CD (b) spectra of non-luminescent Ce6 aggregates bonded with


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QDs. The figure also shows the absorption and CD spectra of a mixture of QD and Ce6 (with n=CCe6:CQDs is 12.0), as well as free Ce6 dimers and QD-bonded Ce6 monomers.

Analysis of the absorption spectra of mixed solutions of ZnS:Mn QDs and Ce6 (at n=12.0), as well as the corresponding spectra of free Ce6 dimers and QDs-bonded Ce6 monomers presented in Figure 8а, indicates that in addition to the previously mentioned Ce6 forms, these solutions contain at least one more Ce6 with the first absorption band centered at 675 nm. The spectrum of this Ce6 form was obtained by successively subtracting the absorption spectra of free Ce6 dimers and QDs-bonded monomers from the recorded absorption spectra of the mixtures of QDs and Ce6 (at n=12.0). Also, we can deduce that this form is non-luminescent since a corresponding PL was not registered. Figure 8b shows the CD band with a maximum of 700 nm, which belongs to this non-luminescent Ce6 form. As evidenced by the transition through the zero in its first absorption band (675 nm), this form is aggregated. The g-factor of this form, estimated as Δε/ε= (εL – εR)/(εL + εR), (where εL and εR are the extinction coefficients of the left and right circularly polarized light, respectively) is about 7∙10-4, the value of which is almost equally comparable to the g-factor of free Ce6 dimers (8∙10-4). In order to establish the exact type of these aggregates (dimers, tetramers, or higher-ordered aggregates), it is necessary to perform density functional theory calculations, which is the subject of our ongoing research.



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Figure 9. The concept of QDs/Ce6 hybrid nanostructures formation in aqueous solution. Thus, CD spectroscopy demonstrates higher sensitivity to Ce6 aggregation since this reveals the presence of Ce6 aggregates at low Ce6 concentration in the samples (at n=3.3), which compares very favorably to UV-Vis spectroscopy that does not allow the detection of any direct patterns of Ce6 aggregation up to n=9.0. We believe that the following picture of the formation of QD/Ce6 hybrid nanostructures in aqueous solutions, presented in Figure 9, is the most likely. At low Ce6 concentration in mixtures with ZnS:Mn QDs, Ce6 molecules are monomerized because of binding to the QDs. The increase of Ce6 concentration in the mixtures leads to the formation of non-luminescent Ce6 aggregates on the surface of the QDs as a result of partial and complete charge loss of Ce6. Moreover, the available monomers of Ce6 may be the centers of formation of non-luminescent Ce6 aggregates, but most of the monomers remain luminescent but are effectively quenched by non-luminescent aggregates as a result of efficient energy transfer through the FRET mechanism. Probing the FRET, in the case of Ce6 aggregation, suggests that: 1) the performance of these hybrid nanostructures in regards to the singlet oxygen generation greatly decreases52 and 2) that non-luminescent Ce6 aggregates are bonded with the ZnS:Mn QDs55. Similarly, as what was reported for hybrid nanostructures based on QDs and


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phthalocyanine molecules, the formation of Ce6 aggregates in this type of hybrid nanostructures can be caused by partial or complete loss of charge in Ce6 upon it’s binding to QDs used.

CONCLUSIONS Here we report on formation and study of QD/Ce6 hybrid nanostructures based on secondgeneration photosensitizer Сe6 and ZnS:Mn quantum dots, which are formed as a result of electrostatic interaction between negatively charged Ce6 and the positively charged QDs. Using spectral-luminescent methods, we demonstrate that an increase of the Ce6 concentration in the ZnS:Mn QDs solution lead to the decrease of Ce6 PL QY and FRET efficiency from the QDs to Ce6. Interestingly, the shape and position of the photoluminescence band of Ce6 corresponded to its monomeric form. However, the analysis of CD spectra of the samples suggests the appearance of new QD-bonded non-luminescent Ce6 aggregates, which can efficiently quench QD and QD-bonded Ce6 monomers luminescence due to efficient FRET. Thus, as is well known in this field, aggregation of photosensitizers in hybrid nanostructures is a common phenomenon, which needs to be detected at its earliest onset in order to obtain hybrids with the greatest of performance. Therefore, from the results demonstrated in this study, we believe that CD spectroscopy is a highly sensitive technique that can successfully tackle this task and detect PS aggregates far before that of more standard methods of absorption and luminescence.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected], [email protected]

Author Contributions



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Government of Russian Federation, Grant 08-08 and the Ministry of Education and Science of the Russian Federation, State assignment, Passport 2019-1080. A.K.V. thanks the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students (2018−2020). Y.K.G. thanks Bioeconomy SFI Research Centre (grant number SFI 16/RC/3889) for further financial support.

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TOC Graphic



Optical properties and structures of Ce6 monomers and dimers 338x190mm (96 x 96 DPI)


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Figure 3. (a) UV-Vis and (b) PL spectra of ZnS:Mn QDs before (Oleylamine) and after (Cysteamine) the phase transfer. Positions of exciton absorption and PL bands are marked by vertical bars. 385x138mm (120 x 120 DPI)



Absorption spectra of solutions (pH = 6) of ZnS:Mn QDs (СQDs ~ 2•10-6 mol/l) and Ce6 with different component ratios (n). Inset: absorption spectra of QDs-bonded Ce6 monomer in the mixture solution with QDs, free Ce6 dimers in an aqueous solution at pH = 6, and the schematic illustration of Ce6 transition from dimeric to monomeric form as a result of interaction with QDs. The QDs-bonded Ce6 monomer spectrum was calculated as a difference between the spectra of QD-Ce6 mixture (at n=1.5) and QDs. 288x201mm (300 x 300 DPI)


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PL spectra of mixed solutions of ZnS:Mn QDs (2•10-6 mol/l) with Ce6 at different ratios of the components (n). The inset shows the normalized spectra. PL excitation wavelength is 610 nm. 297x210mm (300 x 300 DPI)



PLE spectra of mixed solutions of ZnS: Mn QDs and Ce6, PLE spectrum of free Ce6 monomers as well as ZnS:Mn QDs absorption spectrum. PL registration wavelength is 680 nm. 297x207mm (150 x 150 DPI)


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CD (the top panel) and Absorption (the bottom panel) spectra of mixed aqueous solutions (pH = 6) of ZnS:Mn QDs and Ce6 with different ratios of components (n) in the spectral range of 580 – 720 nm. 297x207mm (150 x 150 DPI)



Absorption (a) and CD (b) spectra of non-luminescent Ce6 aggregates bonded with QDs. The figure also shows the absorption and CD spectra of a mixture of QD and Ce6 (with n=CCe6:CQDs is 12.0), as well as free Ce6 dimers and QD-bonded Ce6 monomers. 297x207mm (150 x 150 DPI)


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Absorption (a) and CD (b) spectra of non-luminescent Ce6 aggregates bonded with QDs. The figure also shows the absorption and CD spectra of a mixture of QD and Ce6 (with n=CCe6:CQDs is 12.0), as well as free Ce6 dimers and QD-bonded Ce6 monomers. 297x207mm (150 x 150 DPI)



Figure 9. The concept of QD/Ce6 hybrid nanostructures formation in aqueous solution. 308x184mm (120 x 120 DPI)


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Figure 1. Chlorin e6 structure.



236x131mm (72 x 72 DPI)


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Circular Dichroism Spectroscopy as a Powerful Tool for Unraveling

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