Appl Biochem Biotechnol DOI 10.1007/s12010-015-1635-x
The Photovoltaic Effect of CdS Quantum Dots Synthesized in Inverse Micelles and R-Phycoerythrin Tunnel Cavities Olga D. Bekasova 1 & Alexandra A. Revina 2 & Ekaterina S. Kornienko 1 & Boris I. Kurganov 1
Received: 21 August 2014 / Accepted: 21 April 2015 # Springer Science+Business Media New York 2015
Abstract CdS quantum dots (CdS QDs) 4.3 nm in diameter synthesized in an AOT/isooctane/ water microemulsion and in R-phycoerythrin tunnel cavities (3.5×6.0 nm) were analyzed for photoelectrochemical properties. The CdS QDs preparations were applied onto a platinum electrode to obtain solid films. Experiments were performed in a two-section vessel, with one section filled with ethanol and the other, with 3 M KCl. The sections were connected through an agar stopper. It was found that illumination of the films resulted in a change of the electrode potential. The magnitude of this change and the kinetics of the appearance and disappearance of the photopotential, i.e., the difference between the electrode potential on the light and in dark, depended on the nature of the QD shell. The photovoltaic effect of CdS QDs in Rphycoerythrin, compared to that of CdS QDs in AOT/isooctane micelles, is three to four times greater due to the photosensitizing action of R-phycoerythrin. The photosensitized effect was markedly higher than the photoelectric sensitivity of R-phycoerythrin and had the opposite polarity. Changes in the potential upon turning the light on and off could be observed repeatedly. Keywords R-phycoerythrin . CdS quantum dots . Inverse micelles . Photovoltaic effect
Introduction Technologies based on photochemically active semiconductor nanoparticles are widely used in systems for solar energy conversion, photocatalytic air and water purification, and organic
* Olga D. Bekasova [email protected] 1
Laboratory of Structural Biochemistry of Proteins, Bach Institute of Biochemistry, Russian Academy of Sciences, Leninsky prospect 33, 119071 Moscow, Russia
2
Laboratory of Electronic and Photonic Processes in Polymeric Nanomaterials, Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky prospect 31, 119071 Moscow, Russia
Appl Biochem Biotechnol
waste degradation, as well as in medicine (e.g., as photosensitizers for photodynamic therapy) and other fields [1–3]. The development of new technological approaches to regulating the catalytic properties of nanoparticles is of major interest. In this context, it is relevant that micellar semiconductor systems are capable of performing redox processes much more effectively and selectively, compared to homogeneous solutions. Moreover, the synthesis of nanoparticles in micellar solutions is nearly optimal for producing the particles of desirable size and shape [4]. Such properties make them good candidates for designing new systems of solar energy conversion and storage and novel microheterogeneous assemblies for efficient artificial photosynthesis [5]. Another important direction is the design of next-generation dye-sensitized solar cells and photocatalytic systems based on wide-bandgap semiconductors [6]. A variety of inorganic and organic dyes and plant pigments have been used for this purpose. Of special interest among them is R-phycoerythrin, a water-soluble pigment protein responsible for light harvesting in red algae. As other phycobilin pigments, R-phycoerythrin is also capable of reversible chemical reactions and photosensitization of redox processes [7–10]. A characteristic feature of phycobilin pigments is that they generate a negative electrochemical potential under either oxidizing or reducing conditions [11–13]. In artificial bilayer membrane systems, they enhance photopotential generation when added on the reducing side of the membrane [14]. Such unusual properties of phycobilin pigments are due to their ability to produce free radicals and reactive oxygen species under illumination [15]. Moreover, R-phycoerythrin is nontoxic, highly soluble in water, and its molecules have an identical tunnel cavity 3.5×6.0 nm in size, which makes it a convenient matrix for the synthesis of quantum dots (QDs). In our previous studies, the location of CdS QDs in R-phycoerythrin tunnel cavities has been confirmed by various methods, including capillary electrophoresis, analytical centrifugation, inset electron microscopy, and atomic force microscopy [16–18]. The purpose of this study was to research the influence of the shell on photoelectrochemical properties of QDs, in particular, on the photovoltaic effect of CdS QDs synthesized in the water core of inverse micelles or in the tunnel cavity of R-phycoerythrin, а high-molecular protein capable of photoinduced reversible redox reactions. The effect of R-phycoerythrin on the photoelectrochemical properties of CdS QDs has not been studied previously.
Materials and Methods CdS QDs Synthesis in Inverse Micelles CdS QDs were synthesized in a AOT–water–isooctane solution by a radiochemical method. Micelle-forming anionic surfactant AOT, i.e., sodium bis(2-ethylhexyl) sulfosuccinate (Na2C20H37O7S; 96 %) was from Aldrich (USA); isooctane was from Sigma (USA). Stock solutions of 0.5М Cd(NO3)2 and 0.3М Na2S in deionized water were diluted tenfold with 0.15М AOT solution in isooctane to obtain micellar 0.05М Cd(NO3)2 and 0.03М Na2S solutions with the same hydration level (w=[H2O]/[AOT]=5). The resulting solutions were mixed and γ-irradiated to induce Cd2+ reduction and QD formation in the water core of micelles [19, 20]. Irradiation was performed in a RKhM-γ-20 unit (Special Design Bureau, Russian Academy of Sciences) with a 60Со source at a dose of 7.9 kGy (dose rate 1.25 Gy/s), at room
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temperature. Before irradiation, the solutions in glass ampoules were bubbled with argon to remove dissolved oxygen, and the ampoules were sealed.
CdS QDs Synthesis in R-Phycoerythrin The synthesis was performed as described [16, 17] using 0.4 μM R-phycoerythrin, 0.5 M Cd(NO3)2, and 0.3 M Na2S solutions in deionized water. Initially, R-phycoerythrin solution was supplemented with Cd(NO3)2 to a final concentration of 6 mM Cd2+, which resulted in the formation of a Cd/R-phycoerythrin complex. This process was monitored using an ion-selective electrode for Cd2+ to measure the decrease in the concentration of these ions in the solution. Then, 0.3 M Na2S was added to the Cd/R-phycoerythrin complex to a final concentration of 9.1 mM S2−, with [S2−]/ [Cd2+]=1.2. The mixture was incubated for 30 min and centrifuged at 6000g at 8 °С for 10 min. The CdS particles formed in the solution and on the surface of Rphycoerythrin, which could grow to a large size, were pelleted, while R-phycoerythrin molecules with CdS QDs in the tunnel cavities remained in the supernatant and were then purified by gel filtration on a Sephadex G-200 column. The size of these QDs remained unchanged for several months.
Determining the Diameter of CdS QDs This parameter (D, nm) was determined from the CdS QDs absorption spectra by the following formula [21]: D ¼ 6:6521 108 λ3 þ 1:9557 104 λ2 − 9:2352 102 λ þ ð13:29Þ ð1Þ where λ (nm) is the wavelength of the first extinction absorption peak of CdS QDs. This wavelength corresponds to the position of the minimum in the second derivative absorption spectrum of the sample.
Potentiometric Measurements To prepare photosensitive layers, the preparation of CdS QDs in the AOT/isooctane micelle system or R-phycoerythrin solution was applied onto the platinum electrode (0.3×0.3 cm) and dried as a hanging drop in the dark, at room temperature and normal atmospheric pressure. The electrode was preliminary tested for cleanness by placing it in ethanol and measuring its potential upon illumination in the air and in a vacuum. The electrode was considered clean if changes in the potential did not exceed ±2 mV. Experiments were performed in a two-section vessel with one section filled with ethanol and the other, with 3 M KCl. The sections were connected through a ground glass stopcock. To provide electrical conduction between the sections, the stopper was lubricated with 7 % agar in 2 M KCl [22]. The platinum electrode and the reference calomel electrode were placed in the respective sections, and the redox potential of the system was measured in the dark and upon illumination with UV light directed at a right angle to the film. An OI-18 illuminator with a mercury lamp, work current 1.1–1.2 A (OMO, Russia), was used as a light source. Measurements were made with an EV-74 potentiometer (Russia) with an accuracy of 1–2 mV.
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The schematic diagram of the experimental setup for photopotential measurements is shown in Fig. 1. In control experiments, the air oxygen was removed from ethanol for 3 min using a vacuum pump. The vessel was shaking during degassing.
Absorption and Fluorescence Spectra Absorption spectra were measured with a DU 650 spectrophotometer (Beckman Coulter, USA). Measurements were made at the minimum scanning speed and wavelength interval (0.1 nm) to ensure the maximum possible accuracy in calculating the second derivative of absorption. Fluorescence spectra were recorded in a RF-5301 PC spectrofluorometer (Shimadzu, Japan) in a cuvette with an optical path length of 3 mm, at room temperature.
Results and Discussion Absorption and Emission Properties CdS QDs highly absorb in the UV-blue region of the spectrum (Fig. 2a, b). As follows from the second derivative absorption spectrum, CdS QDs in the AOT/isooctane system have a maximum absorbance peak at 435 nm (Fig. 2a). Calculations by Eq. (1) show that such a peak is characteristic of CdS QDs with a diameter of 4.3±0.2 nm. The second derivative absorption spectrum of CdS QDs synthesized in R-phycoerythrin shows that they have a maximum absorbance at 406 nm (Fig. 2b, curve 2). As determined from the maximum absorption wavelength [21], the size of QDs is 3.4±0.2 nm and coincides with the diameter of the central tunnel cavity in the R-phycoerythrin molecule. This coincidence confirms that CdS QDs are stabilized due to their location in the tunnel cavities, with its size limiting the growth of the QD. On the basis of optical absorption data, the band gap energy values for CdS QDs in the AOT/isooctane system and in R-phycoerythrin have been estimated at 2.7 and 2.8 eV, respectively. This means that the parameters of CdS QDs energy Fig. 1 Schematic image of the experimental setup for photopotential measurements. 1 is Pt-electrode, 2 is calomel electrode, 3 is potentiometer, and 4 is illuminator
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Fig. 2 Absorption spectra (1) and their second derivatives (2) of CdS QDs synthesized in a the AOT/water/ isooctane system and b water solution of R-phycoerythrin; c absorption spectrum of R-phycoerythrin in deionized water
structure in both experimental variants correspond to those at which solid crystalline materials with semiconductor energy structure exhibit their photocatalytic properties [23]. The absorption spectrum of R-phycoerythrin modified by the presence of CdS QDs is characterized by bleaching of the band with peaks at 496, 540, and 564 nm and increasing absorption in the UV spectral region (cf. Fig. 2b (curve 1), c). These substantial changes in the absorption spectra indicate that the chromophore groups of R-phycoerythrin have been modified from their native extended conformation to a cyclohelical conformation. The spectra of CdS QDs fluorescence are shown in Fig. 3. In the inverse micelles, CdS QDs have the maximum fluorescence peak at 625 nm and additional peaks at 440–460 nm (Fig. 3, curve 1). The water solution of R-phycoerythrin with CdS QDs is characterized by the
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fluorescence spectrum with two main peaks, at 470 nm (CdS QDs) and at 575 nm (Rphycoerythrin) (Fig. 3, curve 2). Excitation by photons with different energies has no effect on the position and ratio CdS QDs fluorescence bands either in AOT/isooctane or in Rphycoerythrin. The fluorescence intensity of CdS QDs in the AOT/isooctane system is several times higher than that of CdS QDs in R-phycoerythrin. It is due to the high fluorescence intensity in the red spectral region that aqueous dispersions of CdS QDs are of special interest as donors of energy for photosensitizers, such as phthalocyanine and chlorophyll used in photodynamic therapy [2]. The fluorescence lifetime of single CdS QDs in inverse micelles is approximately 20 times greater than that of CdS QDs in R-phycoerythrin [18, 24].
The Photovoltaic Effect of CdS QDs The illumination of the films formed on the platinum electrode by CdS QD preparations in the inverse micelles or R-phycoerythrin results in the generation of a positive photopotential, which is revealed as an increment in the redox potential of the system (relative to the standard calomel electrode) and reflects reversible changes in the system upon illumination and in the dark. Figure 4a shows changes in the potential of the platinum electrode with the film prepared from CdS QDs in the AOT/isooctane system (CdS⋅AOT). After the light is switched on, the potential significantly increases and reaches saturation within 2 min. After the light is switched off, the potential drops to a constant level within 2–3 min, but this level is higher than before illumination. The amplitude of photopotential decreases from 80±10 to 40±5 mV. Incomplete reversibility of changes in the potential is observed only when the film is illuminated for the first time. During subsequent episodes, changes in the potential after switching the light on and off can take place repeatedly, with the kinetics and amplitude of photopotential remaining essentially unchanged. The positive sign of photopotential is indicative of hole (p-type) conduction, with the excess electron vacancies (holes) being generated as a result of photoinduced activation of the CdS QD surface. Illumination of the film of CdS QDs in R-phycoerythrin (CdS⋅PE) on the platinum electrode immersed in ethanol also results in the generation of a positive photopotential, with its magnitude reaching +250 mV in some cases. Figure 4b shows a typical picture of change in the potential of the electrode with the CdS⋅PE film in contact with ethanol containing dissolved Fig. 3 Fluorescence spectra of CdS QDs (1) in AOT/isooctane emulsion in water, w=7.9, and (2) in water solution of R-phycoerythrin, λexc =400 nm
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Fig. 4 Changes in the photopotential of films formed on the platinum electrode by a CdS⋅AOT/isooctane, b CdS⋅PE, c R-phycoerythrin, and d two layers (R-phycoerythrin and CdS⋅PE) upon intermittent illumination and darkening. In panels а and b: upwards arrows light on, downwards arrow light off; in panels c and d: downwards arrow light on, upwards arrows light off
oxygen. This potential is stable in the dark, increases upon illumination, and returns to the initial level or approaches it when the light is switched off. Such changes in the potential can be observed repeatedly, without any alterations in the amplitude and kinetics of photopotential. It should be noted that the films of R-phycoerythrin alone, without CdS QDs, generate photopotential with the opposite sign and of lower magnitude (Fig. 4c). Figure 4d shows the picture observed upon intermittent illumination and darkening of a bilayer R-phycoerythrin-CdS⋅PE film. Illumination results in a drop of photopotential, which does not increase immediately after the light is switched off but shows a slight surge. The equilibrium potential reached in the dark is slightly lower than the initial potential and that after previous illumination; thus, the potential of the system gradually decreases. The presence of CdS QDs in the bilayer film manifests itself in altered kinetics of the electrochemical potential upon darkening, whereas the sign of its photopotential being the same as in experiments with the pure R-phycoerythrin film. The average values of photopotentials of pure phycoerythrin, CdS⋅PE, and CdS⋅AOT films are given in Table 1. Thus, changes in the photopotential of CdS⋅PE films are several times greater than those in the photopotential of CdS⋅AOT films. As noted above, the absorbance of R-phycoerythrin at 490–660 nm decreases significantly in the course of CdS⋅QD synthesis. Nevertheless, we consider that the differences in the amplitude and kinetics of changes in the photopotential of
Appl Biochem Biotechnol Table 1 The magnitude and sign of photopotentials Photosensitive films Pure R-phycoerythrin QDs CdS⋅PE
Photopotential, mv −80±20 +250±40
QDs CdS⋅AOT
+75±15
Pure R-phycoerythrin+QDs CdS⋅PE (two layers)
−60±15
Single-walled carbon nanotube–CdS nanocomposites [29]
+200
CdS⋅AOT and CdS⋅PE are due to R-phycoerythrin-sensitized photoelectric processes in CdS QDs. This conclusion is based not only on the fact that R-phycoerythrin intensely absorbs in the UV range after conformational changes resulting from the filling of tunnel cavities by CdS QDs. The main argument is that pigments and dyes have similar influence on the photoeffect of semiconductors, which has been shown in classic studies by A. N. Terenin [25] and observed in our experiments with CdS⋅PE. The following trends should be noted: (1) According to Terenin, the sensitized photoeffect is significantly higher than the photoelectric sensitivity of pigments, and the same follows from our results (cf. Fig. 4b, c). (2) The sign of photoelectric current carriers in the semiconductor is retained irrespective of the sign of current carriers in the solid sensitizer dye (pigment). Thus, R-phycoerythrin generates photopotential with a minus sign, while CdS⋅PE, with a plus sign. (3) The photosensitizing effect is observed when the concentration of photosensitizer is lower than the concentration of semiconductor. In case of CdS⋅PE, this is confirmed by the results of experiments with bilayer films: in the Rphycoerythrin-CdS⋅PE bilayer film, where R-phycoerythrin obviously prevails, no sensitized enhancement of the photoeffect is observed (cf. Fig. 4b, d). Thus, principles of photoeffect sensitization in bulk semiconductors are manifested for nano-sized particles, namely for QDs in CdS⋅PE.
Conclusions A comparative study has been performed on the photovoltaic effect of two CdS core-shell nanostructures similar in size but differing in shell composition. In one structure, the shell is formed of AOT (a surfactant); in the other, of R-phycoerythrin, a photochemically active pigment protein capable of sensitizing redox reactions. Due to the photosensitizing action of R-phycoerythrin, the photovoltaic effect of QDs in the CdS⋅PE nanocluster reaches 250 % of that in the CdS⋅AOT/isooctane system. The high sensitization efficiency in CdS⋅PE is conditioned by close contact of R-phycoerythrin chromophore groups with QDs. The sensitizer is usually applied in a certain way onto the surface of a semiconductor, but we have synthesized QDs within the sensitizer, i.e., in the R-phycoerythrin tunnel cavity. It is known that the chromophore groups operating as terminal energy acceptors in α and β subunits of R-phycoerythrin are located on the inner surface of its molecule, namely, on the surface of the tunnel cavity [26, 27]. Organic dyes of different classes have been tested as sensitizers for solar cells [28], but, to our knowledge, R-phycoerythrin has not been used for this purpose. Meanwhile, R-
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phycoerythrin with filled tunnel cavity by CdS QDs may well find applications in prospective technologies of photocatalysis, sensors, and energy conversion based on plant pigments and nanoparticles, and also in model systems for photobiological processes. The magnitude of the photopotential of CdS⋅PE films is similar to that of films consisting of 6- to 9-nm CdS nanoparticles synthesized on the surface of single-walled carbon nanotubes [29]. Acknowledgments This study was supported by the Russian Foundation for Basic Research (Grant no. 14-0401530-a) and the Program BMolecular and Cell Biology^ of the Presidium of the Russian Academy of Sciences.
References 1. Grätzel, M. (2001). Photoelectrochemical cells. Nature, 414, 338–344. 2. Chen, W. (2007). Nanoparticle based photodynamic therapy for cancer treatment. In H. Nalwa & T. Webster (Eds.), Cancer nanotechnology: nanomaterials for cancer diagnosis and therapy (pp. 235–258). California: American Scientific Publishers. 3. Ollis, D. F., & Al-Ekabi, H. (1993). Photocatalytic purification and treatment of water and air. Amsterdam: Elsevier. 4. Grzelczak, M., Vermant, J., Furst, E. M., & Liz-Marzan, L. M. (2010). Directed self-assembly of nanoparticles. ACS Nano, 4, 3591–3605. 5. Kamat, P. V. (1997). Native and surface modified semiconductor nanoclusters. Progress in Inorganic Chemistry, 44, 273–343. 6. Kong, F.-T., Dai, S.-Y., & Wang, K.-J. (2007). Review of recent progress in dye-sensitized solar cells. Advances in OptoElectronics, 2007, 1–13. 7. Evstigneev, V. B., & Gavrilova, V. A. (1964). The ability of phycoerythrin to photosensitize redox reactions. Biofizika, 9, 739–741. 8. Evstigneev, V. B. & Bekasova, O. D. (1970). The photochemical properties of algal biliproteins. In: Physiology and biochemistry of healthy and diseased plants. (Andreenko, S. S., ed.), M.G.U., Moscow, Rus., pp. 170–184. 9. Evstigneev, V. B., & Bekasova, O. D. (1973). The photochemical activity of phycocyanin and phycoerythrin. Izvestiya Academy of Sciences U.S.S.R., Biology Series, 3, 344–356. 10. Kumbar, M., & MacColl, R. (1975). Effect of aromatic molecules on the aggregation of C-phycocyanin: quantum chemical calculations on phycocyanobilin and phycoerythrobilin. Research Communications in Chemical Pathology and Pharmacology, 11, 627–637. 11. Evstigneev, V. B., & Bekasova, O. D. (1970). Photochemical effect of phycoerythrin and phycoerythrobilin films. Biofizika, 15, 807–815. 12. Salamon, Z., & Frackowiak, D. (1975). Photosensitization in phycoprotein complexes at electrodes. Photosynthetica, 9, 337–339. 13. Frackowiak, D., Skowron, A., & Salamon, Z. (1976). Photopotential of biliproteins in electrochemical cell. Photosynthetica, 10, 339–342. 14. Berns, D. S., Chen, C.-H., & Ilani, A. (1976). The modification by biliproteins of intensity and direction of electron flow across chlorophyll-containing membranes. In B. Pullman (Ed.), Environmental effects on molecular structure and properties (pp. 547–560). Holland: Dordrecht D. Reidel. 15. He, J. A., Hu, Y. Z., & Jiang, L. J. (1997). Photodynamic action of phycobiliproteins: in situ generation of reactive oxygen species. Biochimica et Biophysica Acta, 1320, 165–174. 16. Brekhovskikh, A. A., & Bekasova, O. D. (2005). CdS nanoparticles in R-phycoerythrin, a protein matrix. Inorganic Materials, 41, 331–337. 17. Bekasova, O. D., Brekhovskikh, A. A., Brykina, G. D., Dubinchuk, B. T., Mochalova, V. S., & Kotelnikov, A. S. (2005). R-Phycoerythrin: a natural ligand for detoxifying cadmium ions and a tunnel matrix for synthesis of cadmium sulfide nanoparticles. Appllied Biochemistry and Microbiology, 41, 269–274. 18. Bekasova, O. D., Chebotareva, N. A., Safenkova, I. V., Rusanov, A. L., & Kurganov, B. I. (2011). Effect of CdS nanoparticles on the properties of a protein matrix. Inorganic Materials, 47, 922–928. 19. Pileni, M. P. (1997). Nanosized particles made in colloidal assemblies. Langmuir, 13, 3266–3276. 20. Dokuchaev, A. G., Mysoedova, T. G., & Revina, А. А. (1997). Effect of various factors on the formation of silver aggregates in reversed micelles under exposure to γ-radiation. High Energy Chemistry (Moscow), 31, 316–319.
Appl Biochem Biotechnol 21. Yu, W. W., Qu, L., Guo, W., & Peng, X. (2003). Effect of various factors on the formation of silver aggregates in reversed micelles under exposure to γ-radiation. Chemistry of Materials, 15, 2854–2860. 22. Evstigneev, V. B. (2008). Photochemical interactions of chlorophyll and its analogues with electron acceptors. In: V. B. Evstigneev: Selected works. A tribute to the 100th anniversary of his birth. (Shuvalov, V. A., ed.), ZAO A-Print, Russia, pp. 78–99. 23. Evstratov, A. A., Kish, K., Malygin, A. A., Taulemesse, G.-M., Gaudon, P., & Vincent, T. (2007). Distribution of free charge carriers over the surface of photosensitive materials: why and how to control? Russian Chemical Journal (Journal of D Mendeleev Russian Chemical Society), 51(6), 52–60. 24. Bekasova, O. D., Revina, A. A., Rusanov, A. L., Kornienko, E. S., & Kurganov, B. I. (2013). Effect of gamma-ray irradiation on the size and properties of CdS quantum dots in reverse micelles. Radiation Physics and Chemistry, 92, 87–92. 25. Terenin, A. N. (1967). Photonics of dye molecules and related organic compounds. Nauka, Leningrad, Russ. 26. Jiang, T., Zhang, J. P., & Liang, D. C. (1999). Structure and function of chromophores in R-phycoerythrin at 1.9 Å resolution. Proteins: Structure, Function, and Genetics, 34, 224–231. 27. Chang, W. R., Jiang, T., Wan, Z. L., Zhang, J. P., Yang, Z. X., & Liang, D. C. (1996). Crystal structure of Rphycoerythrin from Polysiphonia ureolata at 2.8 Å resolution. Journal of Molecular Biology, 262, 721–731. 28. Hara, K., & Mori, S. (2011). Dye-sensitized solar cells. In A. Luque & S. Hegedus (Eds.), Handbook of photovoltaic science and engineering (pp. 642–674). Chichester: Wiley. 29. Robel, I., Bunker, B. A., & Kamat, P. V. (2005). Single-walled carbon nanotube-CdS nanocomposites as light-harvesting assemblies: photoinduced charge-transfer interactions. Advanced Materials, 17, 2458–2463.
The Photovoltaic Effect of CdS Quantum Dots Synthesized in Inverse Micelles and R-Phycoerythrin Tunnel Cavities Olga D. Bekasova 1 & Alexandra A. Revina 2 & Ekaterina S. Kornienko 1 & Boris I. Kurganov 1
Received: 21 August 2014 / Accepted: 21 April 2015 # Springer Science+Business Media New York 2015
Abstract CdS quantum dots (CdS QDs) 4.3 nm in diameter synthesized in an AOT/isooctane/ water microemulsion and in R-phycoerythrin tunnel cavities (3.5×6.0 nm) were analyzed for photoelectrochemical properties. The CdS QDs preparations were applied onto a platinum electrode to obtain solid films. Experiments were performed in a two-section vessel, with one section filled with ethanol and the other, with 3 M KCl. The sections were connected through an agar stopper. It was found that illumination of the films resulted in a change of the electrode potential. The magnitude of this change and the kinetics of the appearance and disappearance of the photopotential, i.e., the difference between the electrode potential on the light and in dark, depended on the nature of the QD shell. The photovoltaic effect of CdS QDs in Rphycoerythrin, compared to that of CdS QDs in AOT/isooctane micelles, is three to four times greater due to the photosensitizing action of R-phycoerythrin. The photosensitized effect was markedly higher than the photoelectric sensitivity of R-phycoerythrin and had the opposite polarity. Changes in the potential upon turning the light on and off could be observed repeatedly. Keywords R-phycoerythrin . CdS quantum dots . Inverse micelles . Photovoltaic effect
Introduction Technologies based on photochemically active semiconductor nanoparticles are widely used in systems for solar energy conversion, photocatalytic air and water purification, and organic
* Olga D. Bekasova [email protected] 1
Laboratory of Structural Biochemistry of Proteins, Bach Institute of Biochemistry, Russian Academy of Sciences, Leninsky prospect 33, 119071 Moscow, Russia
2
Laboratory of Electronic and Photonic Processes in Polymeric Nanomaterials, Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky prospect 31, 119071 Moscow, Russia
Appl Biochem Biotechnol
waste degradation, as well as in medicine (e.g., as photosensitizers for photodynamic therapy) and other fields [1–3]. The development of new technological approaches to regulating the catalytic properties of nanoparticles is of major interest. In this context, it is relevant that micellar semiconductor systems are capable of performing redox processes much more effectively and selectively, compared to homogeneous solutions. Moreover, the synthesis of nanoparticles in micellar solutions is nearly optimal for producing the particles of desirable size and shape [4]. Such properties make them good candidates for designing new systems of solar energy conversion and storage and novel microheterogeneous assemblies for efficient artificial photosynthesis [5]. Another important direction is the design of next-generation dye-sensitized solar cells and photocatalytic systems based on wide-bandgap semiconductors [6]. A variety of inorganic and organic dyes and plant pigments have been used for this purpose. Of special interest among them is R-phycoerythrin, a water-soluble pigment protein responsible for light harvesting in red algae. As other phycobilin pigments, R-phycoerythrin is also capable of reversible chemical reactions and photosensitization of redox processes [7–10]. A characteristic feature of phycobilin pigments is that they generate a negative electrochemical potential under either oxidizing or reducing conditions [11–13]. In artificial bilayer membrane systems, they enhance photopotential generation when added on the reducing side of the membrane [14]. Such unusual properties of phycobilin pigments are due to their ability to produce free radicals and reactive oxygen species under illumination [15]. Moreover, R-phycoerythrin is nontoxic, highly soluble in water, and its molecules have an identical tunnel cavity 3.5×6.0 nm in size, which makes it a convenient matrix for the synthesis of quantum dots (QDs). In our previous studies, the location of CdS QDs in R-phycoerythrin tunnel cavities has been confirmed by various methods, including capillary electrophoresis, analytical centrifugation, inset electron microscopy, and atomic force microscopy [16–18]. The purpose of this study was to research the influence of the shell on photoelectrochemical properties of QDs, in particular, on the photovoltaic effect of CdS QDs synthesized in the water core of inverse micelles or in the tunnel cavity of R-phycoerythrin, а high-molecular protein capable of photoinduced reversible redox reactions. The effect of R-phycoerythrin on the photoelectrochemical properties of CdS QDs has not been studied previously.
Materials and Methods CdS QDs Synthesis in Inverse Micelles CdS QDs were synthesized in a AOT–water–isooctane solution by a radiochemical method. Micelle-forming anionic surfactant AOT, i.e., sodium bis(2-ethylhexyl) sulfosuccinate (Na2C20H37O7S; 96 %) was from Aldrich (USA); isooctane was from Sigma (USA). Stock solutions of 0.5М Cd(NO3)2 and 0.3М Na2S in deionized water were diluted tenfold with 0.15М AOT solution in isooctane to obtain micellar 0.05М Cd(NO3)2 and 0.03М Na2S solutions with the same hydration level (w=[H2O]/[AOT]=5). The resulting solutions were mixed and γ-irradiated to induce Cd2+ reduction and QD formation in the water core of micelles [19, 20]. Irradiation was performed in a RKhM-γ-20 unit (Special Design Bureau, Russian Academy of Sciences) with a 60Со source at a dose of 7.9 kGy (dose rate 1.25 Gy/s), at room
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temperature. Before irradiation, the solutions in glass ampoules were bubbled with argon to remove dissolved oxygen, and the ampoules were sealed.
CdS QDs Synthesis in R-Phycoerythrin The synthesis was performed as described [16, 17] using 0.4 μM R-phycoerythrin, 0.5 M Cd(NO3)2, and 0.3 M Na2S solutions in deionized water. Initially, R-phycoerythrin solution was supplemented with Cd(NO3)2 to a final concentration of 6 mM Cd2+, which resulted in the formation of a Cd/R-phycoerythrin complex. This process was monitored using an ion-selective electrode for Cd2+ to measure the decrease in the concentration of these ions in the solution. Then, 0.3 M Na2S was added to the Cd/R-phycoerythrin complex to a final concentration of 9.1 mM S2−, with [S2−]/ [Cd2+]=1.2. The mixture was incubated for 30 min and centrifuged at 6000g at 8 °С for 10 min. The CdS particles formed in the solution and on the surface of Rphycoerythrin, which could grow to a large size, were pelleted, while R-phycoerythrin molecules with CdS QDs in the tunnel cavities remained in the supernatant and were then purified by gel filtration on a Sephadex G-200 column. The size of these QDs remained unchanged for several months.
Determining the Diameter of CdS QDs This parameter (D, nm) was determined from the CdS QDs absorption spectra by the following formula [21]: D ¼ 6:6521 108 λ3 þ 1:9557 104 λ2 − 9:2352 102 λ þ ð13:29Þ ð1Þ where λ (nm) is the wavelength of the first extinction absorption peak of CdS QDs. This wavelength corresponds to the position of the minimum in the second derivative absorption spectrum of the sample.
Potentiometric Measurements To prepare photosensitive layers, the preparation of CdS QDs in the AOT/isooctane micelle system or R-phycoerythrin solution was applied onto the platinum electrode (0.3×0.3 cm) and dried as a hanging drop in the dark, at room temperature and normal atmospheric pressure. The electrode was preliminary tested for cleanness by placing it in ethanol and measuring its potential upon illumination in the air and in a vacuum. The electrode was considered clean if changes in the potential did not exceed ±2 mV. Experiments were performed in a two-section vessel with one section filled with ethanol and the other, with 3 M KCl. The sections were connected through a ground glass stopcock. To provide electrical conduction between the sections, the stopper was lubricated with 7 % agar in 2 M KCl [22]. The platinum electrode and the reference calomel electrode were placed in the respective sections, and the redox potential of the system was measured in the dark and upon illumination with UV light directed at a right angle to the film. An OI-18 illuminator with a mercury lamp, work current 1.1–1.2 A (OMO, Russia), was used as a light source. Measurements were made with an EV-74 potentiometer (Russia) with an accuracy of 1–2 mV.
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The schematic diagram of the experimental setup for photopotential measurements is shown in Fig. 1. In control experiments, the air oxygen was removed from ethanol for 3 min using a vacuum pump. The vessel was shaking during degassing.
Absorption and Fluorescence Spectra Absorption spectra were measured with a DU 650 spectrophotometer (Beckman Coulter, USA). Measurements were made at the minimum scanning speed and wavelength interval (0.1 nm) to ensure the maximum possible accuracy in calculating the second derivative of absorption. Fluorescence spectra were recorded in a RF-5301 PC spectrofluorometer (Shimadzu, Japan) in a cuvette with an optical path length of 3 mm, at room temperature.
Results and Discussion Absorption and Emission Properties CdS QDs highly absorb in the UV-blue region of the spectrum (Fig. 2a, b). As follows from the second derivative absorption spectrum, CdS QDs in the AOT/isooctane system have a maximum absorbance peak at 435 nm (Fig. 2a). Calculations by Eq. (1) show that such a peak is characteristic of CdS QDs with a diameter of 4.3±0.2 nm. The second derivative absorption spectrum of CdS QDs synthesized in R-phycoerythrin shows that they have a maximum absorbance at 406 nm (Fig. 2b, curve 2). As determined from the maximum absorption wavelength [21], the size of QDs is 3.4±0.2 nm and coincides with the diameter of the central tunnel cavity in the R-phycoerythrin molecule. This coincidence confirms that CdS QDs are stabilized due to their location in the tunnel cavities, with its size limiting the growth of the QD. On the basis of optical absorption data, the band gap energy values for CdS QDs in the AOT/isooctane system and in R-phycoerythrin have been estimated at 2.7 and 2.8 eV, respectively. This means that the parameters of CdS QDs energy Fig. 1 Schematic image of the experimental setup for photopotential measurements. 1 is Pt-electrode, 2 is calomel electrode, 3 is potentiometer, and 4 is illuminator
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Fig. 2 Absorption spectra (1) and their second derivatives (2) of CdS QDs synthesized in a the AOT/water/ isooctane system and b water solution of R-phycoerythrin; c absorption spectrum of R-phycoerythrin in deionized water
structure in both experimental variants correspond to those at which solid crystalline materials with semiconductor energy structure exhibit their photocatalytic properties [23]. The absorption spectrum of R-phycoerythrin modified by the presence of CdS QDs is characterized by bleaching of the band with peaks at 496, 540, and 564 nm and increasing absorption in the UV spectral region (cf. Fig. 2b (curve 1), c). These substantial changes in the absorption spectra indicate that the chromophore groups of R-phycoerythrin have been modified from their native extended conformation to a cyclohelical conformation. The spectra of CdS QDs fluorescence are shown in Fig. 3. In the inverse micelles, CdS QDs have the maximum fluorescence peak at 625 nm and additional peaks at 440–460 nm (Fig. 3, curve 1). The water solution of R-phycoerythrin with CdS QDs is characterized by the
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fluorescence spectrum with two main peaks, at 470 nm (CdS QDs) and at 575 nm (Rphycoerythrin) (Fig. 3, curve 2). Excitation by photons with different energies has no effect on the position and ratio CdS QDs fluorescence bands either in AOT/isooctane or in Rphycoerythrin. The fluorescence intensity of CdS QDs in the AOT/isooctane system is several times higher than that of CdS QDs in R-phycoerythrin. It is due to the high fluorescence intensity in the red spectral region that aqueous dispersions of CdS QDs are of special interest as donors of energy for photosensitizers, such as phthalocyanine and chlorophyll used in photodynamic therapy [2]. The fluorescence lifetime of single CdS QDs in inverse micelles is approximately 20 times greater than that of CdS QDs in R-phycoerythrin [18, 24].
The Photovoltaic Effect of CdS QDs The illumination of the films formed on the platinum electrode by CdS QD preparations in the inverse micelles or R-phycoerythrin results in the generation of a positive photopotential, which is revealed as an increment in the redox potential of the system (relative to the standard calomel electrode) and reflects reversible changes in the system upon illumination and in the dark. Figure 4a shows changes in the potential of the platinum electrode with the film prepared from CdS QDs in the AOT/isooctane system (CdS⋅AOT). After the light is switched on, the potential significantly increases and reaches saturation within 2 min. After the light is switched off, the potential drops to a constant level within 2–3 min, but this level is higher than before illumination. The amplitude of photopotential decreases from 80±10 to 40±5 mV. Incomplete reversibility of changes in the potential is observed only when the film is illuminated for the first time. During subsequent episodes, changes in the potential after switching the light on and off can take place repeatedly, with the kinetics and amplitude of photopotential remaining essentially unchanged. The positive sign of photopotential is indicative of hole (p-type) conduction, with the excess electron vacancies (holes) being generated as a result of photoinduced activation of the CdS QD surface. Illumination of the film of CdS QDs in R-phycoerythrin (CdS⋅PE) on the platinum electrode immersed in ethanol also results in the generation of a positive photopotential, with its magnitude reaching +250 mV in some cases. Figure 4b shows a typical picture of change in the potential of the electrode with the CdS⋅PE film in contact with ethanol containing dissolved Fig. 3 Fluorescence spectra of CdS QDs (1) in AOT/isooctane emulsion in water, w=7.9, and (2) in water solution of R-phycoerythrin, λexc =400 nm
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Fig. 4 Changes in the photopotential of films formed on the platinum electrode by a CdS⋅AOT/isooctane, b CdS⋅PE, c R-phycoerythrin, and d two layers (R-phycoerythrin and CdS⋅PE) upon intermittent illumination and darkening. In panels а and b: upwards arrows light on, downwards arrow light off; in panels c and d: downwards arrow light on, upwards arrows light off
oxygen. This potential is stable in the dark, increases upon illumination, and returns to the initial level or approaches it when the light is switched off. Such changes in the potential can be observed repeatedly, without any alterations in the amplitude and kinetics of photopotential. It should be noted that the films of R-phycoerythrin alone, without CdS QDs, generate photopotential with the opposite sign and of lower magnitude (Fig. 4c). Figure 4d shows the picture observed upon intermittent illumination and darkening of a bilayer R-phycoerythrin-CdS⋅PE film. Illumination results in a drop of photopotential, which does not increase immediately after the light is switched off but shows a slight surge. The equilibrium potential reached in the dark is slightly lower than the initial potential and that after previous illumination; thus, the potential of the system gradually decreases. The presence of CdS QDs in the bilayer film manifests itself in altered kinetics of the electrochemical potential upon darkening, whereas the sign of its photopotential being the same as in experiments with the pure R-phycoerythrin film. The average values of photopotentials of pure phycoerythrin, CdS⋅PE, and CdS⋅AOT films are given in Table 1. Thus, changes in the photopotential of CdS⋅PE films are several times greater than those in the photopotential of CdS⋅AOT films. As noted above, the absorbance of R-phycoerythrin at 490–660 nm decreases significantly in the course of CdS⋅QD synthesis. Nevertheless, we consider that the differences in the amplitude and kinetics of changes in the photopotential of
Appl Biochem Biotechnol Table 1 The magnitude and sign of photopotentials Photosensitive films Pure R-phycoerythrin QDs CdS⋅PE
Photopotential, mv −80±20 +250±40
QDs CdS⋅AOT
+75±15
Pure R-phycoerythrin+QDs CdS⋅PE (two layers)
−60±15
Single-walled carbon nanotube–CdS nanocomposites [29]
+200
CdS⋅AOT and CdS⋅PE are due to R-phycoerythrin-sensitized photoelectric processes in CdS QDs. This conclusion is based not only on the fact that R-phycoerythrin intensely absorbs in the UV range after conformational changes resulting from the filling of tunnel cavities by CdS QDs. The main argument is that pigments and dyes have similar influence on the photoeffect of semiconductors, which has been shown in classic studies by A. N. Terenin [25] and observed in our experiments with CdS⋅PE. The following trends should be noted: (1) According to Terenin, the sensitized photoeffect is significantly higher than the photoelectric sensitivity of pigments, and the same follows from our results (cf. Fig. 4b, c). (2) The sign of photoelectric current carriers in the semiconductor is retained irrespective of the sign of current carriers in the solid sensitizer dye (pigment). Thus, R-phycoerythrin generates photopotential with a minus sign, while CdS⋅PE, with a plus sign. (3) The photosensitizing effect is observed when the concentration of photosensitizer is lower than the concentration of semiconductor. In case of CdS⋅PE, this is confirmed by the results of experiments with bilayer films: in the Rphycoerythrin-CdS⋅PE bilayer film, where R-phycoerythrin obviously prevails, no sensitized enhancement of the photoeffect is observed (cf. Fig. 4b, d). Thus, principles of photoeffect sensitization in bulk semiconductors are manifested for nano-sized particles, namely for QDs in CdS⋅PE.
Conclusions A comparative study has been performed on the photovoltaic effect of two CdS core-shell nanostructures similar in size but differing in shell composition. In one structure, the shell is formed of AOT (a surfactant); in the other, of R-phycoerythrin, a photochemically active pigment protein capable of sensitizing redox reactions. Due to the photosensitizing action of R-phycoerythrin, the photovoltaic effect of QDs in the CdS⋅PE nanocluster reaches 250 % of that in the CdS⋅AOT/isooctane system. The high sensitization efficiency in CdS⋅PE is conditioned by close contact of R-phycoerythrin chromophore groups with QDs. The sensitizer is usually applied in a certain way onto the surface of a semiconductor, but we have synthesized QDs within the sensitizer, i.e., in the R-phycoerythrin tunnel cavity. It is known that the chromophore groups operating as terminal energy acceptors in α and β subunits of R-phycoerythrin are located on the inner surface of its molecule, namely, on the surface of the tunnel cavity [26, 27]. Organic dyes of different classes have been tested as sensitizers for solar cells [28], but, to our knowledge, R-phycoerythrin has not been used for this purpose. Meanwhile, R-
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phycoerythrin with filled tunnel cavity by CdS QDs may well find applications in prospective technologies of photocatalysis, sensors, and energy conversion based on plant pigments and nanoparticles, and also in model systems for photobiological processes. The magnitude of the photopotential of CdS⋅PE films is similar to that of films consisting of 6- to 9-nm CdS nanoparticles synthesized on the surface of single-walled carbon nanotubes [29]. Acknowledgments This study was supported by the Russian Foundation for Basic Research (Grant no. 14-0401530-a) and the Program BMolecular and Cell Biology^ of the Presidium of the Russian Academy of Sciences.
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