Construction of a Porphyrin-Based Nanohybrid as an Analogue of

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Construction of a Porphyrin-Based Nanohybrid as an Analogue of Chlorophyll Protein Complexes and Its Light-Harvesting Behavior Research Xingming Ning,† Liang Ma,† Shouting Zhang, Dongdong Qin, Duoliang Shan, Yaqi Hu, and Xiaoquan Lu* Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China S Supporting Information *

ABSTRACT: Inspired by the spatial elaborate architecture and the kinetics characteristic of the nature photosynthesis, a nanohybrid of Au@THPP/CNTs with columnar-like shape has been prepared by a combination of the gold-4hydroxyphenyl porphyrin nanohybrid of core−shell structure (Au@THPP) and vertically aligned carbon nanotubes (CNTs) grown on a transparent conducting substrate. A simplified model to uncover the partial electron transfer (ET) of photosystem II (PSII) using the nanohybrid by the chemical approach has been set up. The UV−vis/scanning electrochemical microscopy (UV−vis/SECM) platform, a simple, effective, and novel electrochemistry method, was used to investigate the behavior of photoinduced electron transfer of the model. At 560 nm wavelength, the heterogeneous electron transfer rate constant (keff) of this model is the highest, which is consistent with the maximum absorption wavelength of a different chromophore achieved in photosynthesis. The model is not only of great value to study the complex photosynthesis but also of potential application in a biomimetic system.

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INTRODUCTION Natural photosynthesis shows high efficiency via subtle construction and fast ET and plays a deciding role in the biological process of converting solar energy into storable chemical energy, in which a variety of reactions are derived by light harvesters (e.g., chlorophyll, xanthophylls, and carotenoids, located in the thylakoid membrane). Natural photosynthesis may be regarded as three steps as follows: (1) light harvesting and charge separation in photosystem II (PSII) and photosystem I (PSI); (2) charge separation via photoinduced electron transfer; (3) energy and oxygen production.1 Generally, the second step plays a significant role in the photosynthesis. Recently, an increasing number of people have devoted themselves into artificial photosynthesis by mimicking natural photosynthesis,2 and some simplified models such as protein complexes,3 supramolecular chromophore aggregates,4 quantum dots,5,6 and carbon nanomaterials7,8 have been proposed however, revealing the efficient and selective lightharvesting energy transfer processes in purple bacteria9 and little involvement in photoinduced electron transfer. In this account, a stable artificial photosynthetic model to uncover the partial electron transfer (ET) of photosystem II (PSII) using the nanohybrid by the chemical approach has been set up. Herein, porphyrin was chosen as photosensitized material due to it being an important component of chlorophyll, which has many electrical and optical characterizations.9−11 Moreover, © XXXX American Chemical Society

Au@THPP was chosen as light harvesters due to the performance of signal amplification, making it have a high specific surface area, unusual electronic properties, enhanced bioaffinity, a covalent bond, and the powerful synergy of nanoparticles.12 The oriented carbon nanotubes offer further photoelectric performance compared to randomly dispersed CNTs, which is attributed to the high ET rate along the tube in promoting fast ET.13,14 Currently, the approach of electrostatic interaction or ligand exchange has been applied to form a nanohybrid, which can provide 3D architectures and improve light-harvesting efficiency.4,15−17 In this paper, we fabricated the novel Au@THPP nanohybrid on the surface of vertically aligned carbon nanotubes, which is similar to the light harvesters. The light-harvesting complex like pigment molecule is also embedded in grana within the chloroplast of the cell, which can increase the active surface area and absorb light energy to a certain degree, as illustrated in Scheme 1. The simplified model has been constructed through the composite of Au@THPP/CNTs by the chemical approach, and we apply Au@THPP/CNTs to regenerate benzoquinone (BQ) which are characteristic of quinones homologue, aiming at studying the photoinduced electron transfer of the model Received: November 17, 2015 Revised: December 23, 2015

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Scheme 1. (A) Image of the Light-Dependent Components of Photosynthesis Embedded within the Thylakoid Membrane in Natural Photosynthesis; (Right) Schematic Illustration of the Process of Light Harvesting and the ET; (B) Model of Artificial Photosynthesis and the Possible Pathway of ET

nitrogen. Unless otherwise stated, reagents were of analytical grade and used as received. Aqueous solutions were prepared with ultra-high-purity water by a Milli-Q system (Milli-Q, Millipore Corp.). All experiments were performed at normal temperature. Instrumentation. UV−vis and fluorescence measurements were done using a UV-1102 spectrometer (Shanghai, China) and a RF-540 fluorescence spectrometer (Shimadzu, Japan), respectively. The transmission electron microscope (TEM) measurement was carried out using JEM-3010 (JEOL Co., Ltd., Japan). Sample annealing was conducted in the tube furnace (Tianjin Central Experimental Furnace Co., Ltd., model SKG05123K). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI-5702 X-ray photoelectron spectrometer. A 150 W Xe lamp light resource was used as the irradiation source. Photocurrent responses were measured on a CHI900 electrochemical workstation (CH Instruments, Austin, TX). Tin-doped indium oxide (ITO) glasses (with a specific surface conductivity of ca. 10 Ω cm−2) were purchased from Aldrich Chemical Co. The ITO were cut into 1 cm × 1 cm size slices, ultrasonically cleaned in deionized water, acetone, and absolute ethanol for 10 min in turn, and then dried with nitrogen gas. Synthesis of Au@THPP and Vesicle Structure. The Au@THPP microspheres were synthesized via the following procedure. Approximately, 10 mL of 0.2 mM THPP in ethanol was added into 10 mL of 0.2 mM HAuCl4 in water, and then the mixed solution was vigorously stirred in dark conditions for 10 min. Subsequently, it was heated at a temperature of 100 °C under stirring for another 30 min; a clear pink color solution turned green and then crimson. Afterward, the solution was cooled to room temperature; the lower suspension that contained the core−shell of Au@THPP was then filtered

under visible light. Although a simplified model has been set up, it still remains as a challenge to find a way to investigate the process of photoinduced electron transfer. Investigations of the ET in PSII were reported in some literature,18−20 but little involvement in photoinduced electron transfer in PSII by SECM is achieved. Additionally, in contrast to other methods, SECM based on micro- and nanointerface is particularly suitable to reflect the kinetics of ET in a wide range of redoxactive species with high spatial and temporal resolution. A way to combine SECM with UV−vis was reported by our group to study the photoinduced electron transfer of the model.21 In this work, we constructed a simple model by a chemical pathway, and the resulted heterogeneous electron transfer rate constant (keff) of this composite is highest at 560 nm wavelength. Also, the possible mechanism of ET for this model has been described via the UV−vis/SECM platform under visible light illumination. The model also supplies us with a platform to research the complex process of natural photosynthesis and to bridge between natural and artificial photosynthesis.

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EXPERIMENTAL SECTION Chemicals. Sodium nitrite, potassium ferricyanide, hydrochloric acid, 4-phenylenediamine, aniline, and N,N′-dicyclohexylcarbodiimide (DCC) were purchased from Sigma-Aldrich. HAuCl 4 ·3H 2O and BQ were purchased from Aladdin Chemistry Co. and Fuchen Chemical Reagent Factory (TianJin, China), respectively. 5,10,15,20-Tetrakis(4hydroxyphenyl)porphyrin (THPP) has been synthesized in our lab following the same methodology reported previously,22 and carbon nanotubes were purchased from Nano port Co. Ltd. (Shenzhen, China). In this work, pH 7.0 phosphate buffer solution of 0.2 M (PBS) was always employed as the supporting electrolyte after being deoxidized with high-purity B

DOI: 10.1021/acs.jpcc.5b11246 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 2. Chemical Steps Involved in the Construction of the Au@THPP/CNTs Hybrid

through a 0.22 μm hydrophilic PVDF filter (Millipore) under a pump. The core−shell of Au@THPP then dispersed in ethanol under ultrasonic wave to form a stable suspension. Vesicle nanostructure was synthesized by heating the 0.2 mM THPP solution at a temperature of 65 °C under stirring for 15 min. Fabrication of Materials. First, ITO was washed by detergent, ultrasonicated in deionized water, absolute acetone, and ethanol, sequentially, and then dried by passing nitrogen gas.23 The aminophenyl (AP) films was prepared by following a similarly reported method.24,25 Typically, 20 μL of 0.1 M NaNO2 was added into 2 mL of 10 mM p-phenylenediamine in hydrochloric acid with stirring for 5 min in the dark at room temperature. This reaction mixture was taken in a electrochemical cell; then the ITO electrode was electrochemically modified by reducing the diazonium cation species present in solution by scanning the potential from 0.4 V to −0.6 V at 100 mV s−1, as shown in Figure S1 (Supporting Information). Modified ITO electrode was sonicated in acetonitrile to eliminate the adsorbed molecules. The acid-functionalized CNTs (1 mg) were prepared by sonicating the CNTs in a 3:1 mixture of concentrated sulfuric acid and nitric acid for 10 h at 20 °C. The acid-functionalized CNTs suspension was filtered and washed thoroughly several times with distilled water until the pH of the filtrate reached 7.0 and then dried in a vacuum oven. The AP-modified ITO electrode was immersed overnight in a suspension of 0.5 mg mL−1 dicyclohexyl carbodiimide (DCC), functionalized CNTs (0.2 mg mL−1) in DMSO, and then sonicated for 30 min, followed by heating at 65 °C for 24 h. The modified electrodes were rinsed with a copious amount of water and then dried with argon gas. Finally, 15 μL of Au@ THPP suspensions was added onto the CNTs-modified electrode in the dark. Au@THPP/CNTs were produced due to electrostatic attraction and π−π stacking interactions between the functionalized CNTs and Au@THPP. Then modified electrodes were washed with distilled water several times and dried with argon gas between each washing step, as shown in Scheme 2. The fabrication of all materials was performed in a dark box. Photoelectrochemical Measurement. Photoelectrochemical measurements were performed at a UV−vis/SECM platform which was designed by our group. SECM experimentation consists of recording approach curves where

the normalized current IT = iT/iT,∞ is plotted versus the normalized distance L = d/a.26 In the SECM operation, the feedback mode has been widely employed to study the kinetics. A 25 μm diameter (ultramicroelectrodes) UMEs (RG = 6, RG is the ratio of the overall electrode radius over the platinum disk radius) was sealed in a borosilicate glass capillary under vacuum and further processed according to a previous report to obtain the kinetics of heterogeneous ET with good resolution.21,27 The UMEs were characterized by cyclic voltammetry (Figure S2, Supporting Information). A typical three-electrode system was used. We used the SECM tip electrode as the working electrode, a platinum wire as an auxiliary electrode, and Ag/ AgCl (1 mol L−1 KCl) as the reference electrode. One millimolar BQ in 0.2 M PBS was used as an electrochemical probe molecule to study the electrochemical properties. A polytetrafluoroethylene electrochemical cell was used to complete all electrochemical experiments. For a defined sample, all approach curves are adjusted using the same zero origin. These curves were obtained via simply changing the wavelength of the illuminated light and the solution containing the mediator between the experiments.21 Besides, CVs are performed on CHI900B by using a standard three-electrode system with modified ITO as working electrode, a platinum wire as an auxiliary electrode, and Ag/AgCl (1 mol L−1 KCl) as the reference electrode. K3Fe(CN)6 in PBS (pH = 7) was used as redox mediator in this section of the experiment.

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RESULTS AND DISCUSSION The synthesis strategy of the Au@THPP/CNTs hybrid is shown in Scheme 2. Herein, we first investigated the absorption behavior of Au@THPP and vesicle structure by UV−vis absorption spectroscopy (Figure S3A, Supporting Information) in the 300−800 nm spectral range. As seen clearly, THPP (blue line) shows a strong Soret band at 416 nm and less intense Q bands in the 500−700 nm range due to the presence of highly conjugated π electrons. For AuNPs (red line), there is an evident peak at about 520 nm because of the surface plasmon absorption. The Au@THPP (black line) has a characteristic absorption peak located at about 560 nm due to the surface plasmon resonance (SPR) effect of abundant AuNPs and the strong Soret band arising from porphyrin molecules. Besides, Au@THPP shows a variable band gap compared to Au or C

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The Journal of Physical Chemistry C porphyrin owing to the AuNPs core−shell,28,29 confirming that the THPP has been immobilized via a covalent bond on the gold surface. Compared with the THPP, the Soret band of the vesicle (without AuNPs) is divided into two obvious absorption bands, as shown in Figure S3B (red line, Supporting Information). We can conclude that the aggregate porphyrin increased the energy of the π orbitals which are located in the porphyrin in the vesicle nanostructure by exciton coupling within meso−meso-linked porphyrin subunits.30 At the same time, the fluorescence properties of these nanostructures are discussed (Figure S4, Supporting Information). Compared with THPP, the vesicle structure and Au@THPP show lower intensity, and Au@THPP reveals lower intensity than the vesicle structure. The phenomenon is attributed to the presence of a strong interaction between them, resulting in quenching of the fluorescence of THPP. The morphologies of Au@THPP and vesicle structure can be observed in Figure 1A and 1B,

reasons accounting for the formation of vesicle structure, resulting from high temperature and ethanol reduction in this reaction.31,32 In order to further investigate formation of the core−shell structure, both Au@THPP and THPP were also analyzed by FT-IR (Figure S5, Supporting Information). In our model, the functionalized CNTs which are analogous to plastoquinone (PQ) play an essential role in the ET process. CNTs were successfully anchored to the ITO surface by amide bonds between the amino groups on the surface of the modified ITO and the carboxylic acid groups on the CNTs. Vertically aligned CNTs on the surface of modified ITO were confirmed by atomic force microscopy (AFM), as shown in Figure 2. Figure 2A and 2B illustrates the aminophenyl (AP) films on the surface of the ITO (AP/ITO) and CNTs-modified AP/ITO (CNTs/AP/ITO), respectively. Compared to Figure 2A, Figure 2B shows more obvious roughness, which further illustrates the vertically aligned CNTs were successfully anchored to the ITO surface. Importantly, AFM images also show the length of the vertically aligned CNTs is about 100 nm. The composition of the final Au@THPP/CNTs/AP/ITO products can be further provided by X-ray photoelectron spectroscopy (XPS), as shown in Figure 3. First, a typical survey XPS spectrum in Figure 3A clearly shows the peaks of Au 4f, C 1s, N 1s, and O 1s. The interaction of the nanohybrid can be revealed by examining Au 4f photoelectron spectra (Figure 3B). The binding energies of the Au 4f peak core level are all at 83.9 and 88.0 eV, which is the gold 4f 7/2 (83.9 eV) and 4f 5/2 (88.0 eV),32−34 respectively. The line in 90.8 eV is probably the signal of gold oxide.35 The N 1s spectrum of the functionalized ITO surface reveals a strong N 1s peak at 400.2 eV,29 as shown in Figure 3A. The N 1s spectrum from the APmodified ITO and porphyrin molecules may be present as free molecules and as part of the shell structure and can be separated into two peaks with binding energies at 399.9 and 401.2 eV (Figure 3D), possibly attributed to the amino groups (−NH2) and azo groups (NN).29 The C 1s core level peak (Figure 3C) appearing at 285.2 eV is probably generated from the CNTs carbon (CC) at 284.6 eV, low levels of oxidized

Figure 1. TEM images of Au@THPP (A) and vesicle structure (B).

which were initially characterized with TEM. Figure 1A and 1B reveals that the structure is a uniform hexagon at the nanoscale and indicates the average sizes of Au@THPP and vesicle structure with a size of ∼60 nm. At the same time, TEM further reveals the presence of a Au core (Figure 1A) and the absence of a Au core (Figure 1B). The size of AuNPs is measured to be 40 nm, and the thickness of Au@THPP and vesicle structure aggregated by THPP are about 20 nm. There are a few possible

Figure 2. AFM images of AP/ITO (A) and CNTs/AP/ITO (B). D

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In natural photosynthesis, PQ works as an electron acceptor to promote the efficiency of photoinduced electron transfer. To research the photoinduced ET process, in our system, we choose BQ instead of PQ and a visible light source instead of sunlight. First, the photoelectric properties of the Au@THPP/ ITO composite was checked by SECM in the presence of BQ under illumination and dark condition, as shown in Figure 5.

Figure 3. Typical XPS spectra of the Au@THPP/CNTs/AP/ITO: survey spectra (A), XPS spectra of Au 4f for Au@THPP (B), C1S region XPS spectrum (C), and N1S region XPS spectrum (D).

Figure 5. SECM probe approach curves of Au@THPP-modified ITO electrode with light (blue curve) and without light (red curve) in 1 mM PBS solution containing 1 mM BQ. Etip = −0.4 V versus Ag/ AgCl.

carbon species at 285.5 eV, the ether carbon species (C−O−C) at 286.2 eV, porphyrin carbon species i.e., amide carbon species at 287.2 eV, and the carboxylic acid carbon species at 288.5 eV.24,29,32 Therefore, XPS data suggests that an Au@THPP/ CNTs/AP composite combined by covalent linkage and Au@ THPP is surely attached on CNTs. Above all, Au@THPP/ CNTs/AP/ITO was accurately prepared, namely, a model has been successfully constructed by a chemical method. Successful preparation of AP/ITO and CNTs/AP/ITO has been checked with cyclic voltammetry (CVs) in phosphate buffer solution (PBS, pH = 7) using ferricyanide as redox species, as illustrated in Figure 4. No electrochemical response

When the tip approached the substrate under visible light illumination, a “positive feedback” approach curve (blue) was seen. On the contrary, a “negative feedback” approach curve (red) was observed in the dark condition. These behaviors reflect the fact that Au@THPP is photoactive and can be used as a light harvester. It is worth noting that the Au@THPP/ITO and vesicle/ITO show different photoelectric properties, as shown in Figure 6A.

Figure 6. (A) Probe approach curves of Au@THPP and vesiclemodified ITO electrode in 1 mM PBS solution containing 1 mM BQ. (B) Probe approach curves of different modified substrate in 1 mM PBS solution containing 1 mM BQ. Etip = −0.4 V versus Ag/AgCl. Figure 4. Typical cyclic voltammograms of naked ITO electrode (red), AP/ITO (pink), and CNTs/AP/ITO (violet) obtained for solutions of 250 μM ferricyanide and 1 mM PBS (PBS, pH = 7.0) at a scan rate of 100 mV s−1.

When the tip approaches both substrates under visible light, positive feedback approach curves are obtained by SECM. This positive feedback behavior indicates the occurrence of a rapid regeneration of BQ at the surface of the substrate and implies direct ET from Au@THPP and vesicle to BQ. Compared to the vesicle structure, the “positive feedback” current of Au@THPP is higher under visible light. This phenomenon is most likely due to the presence of AuNPs, which reduce charge recombination and promote interfacial charge transfer, and increases light absorption of composite due to the strong SPR.32,36−38 When the tip approaches the naked ITO and AP/ ITO under visible light, the negative feedback approach curves are observed. This negative feedback behavior indicates slow

is observed in the AP/ITO (pink curve) because nonconducting AP films on the surface of the ITO prohibit the reaction between ITO electrode and solutions. When the naked ITO was used, a pair of redox peaks was obtained (red curve). Compared with naked ITO (red curve), the corresponding peak current of CNTs/AP/ITO (violet curve) significantly increased because of the presence of CNTs. Generally speaking, AP/ITO and CNTs/AP/ITO have been successfully prepared, supporting the result obtained from AFM. E

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8A. Photoinduced electron transfer was regarded as a quasireversible reaction, and Keff at different concentrations of BQ

regeneration of BQ at the ITO surface. The positive feedback occurs in CNTs-modified AP/ITO, suggesting the good photoelectric property of CNTs,39 as compared with AP/ ITO. As shown in Figure 6B, once Au@THPP is loaded onto the CNTs, the highest probe curve can be obtained by Au@ THPP/CNTs among the sample tested, indicating the feasibility of our strategy to construct a platform by combination of Au, THPP, and vertically aligned CNTs. A multistep ET process is shown in Scheme 1, which is similar to the photoinduced electron transfer process in PSII. The observation of positive feedback suggests a bimolecular reaction occurring at the modified substrates and the ability of Au@ THPP/CNTs for catalytic oxidation and regeneration of BQ under light irradiation. Chlorophyll, a kind of metal porphyrin, is widely known for its ability to absorb a certain wavelength of light and located in the thylakoid membrane. In this work, the approach probe curve of Au@THPP/CNTs under series of wavelengths (469, 489, 515, 531, 546, 560, 570, and 580 nm) was investigated, as shown in the Figure 7A. It is found that positive feedback is

Figure 8. (A) SECM probe approach curves of Au@THPP/CNTs/ ITO-modified electrode under different concentrations of BQ at 560 nm. (B) Dependence of keff on the concentration of BQ with SD definition (n = 5).

were calculated by fitting the experimental curves to theoretical ones. It is found that the rate constant of ET is elevated along with an increase of the concentration of BQ. In addition, reaction rate constants present a linear relation with the concentration of BQ, as shown in Figure 8B. In other words, the rates of the photoinduced electron transfer are closely related to the content of electrically active molecules, which is the same as PQ in plants.40 Therefore, considering all the information obtained above together, possible mechanism of ET for this process could be described as follows Au@THPP + hv → Au − THPP*

Figure 7. (A) SECM probe approach curves of Au@THPP/CNTs/ ITO-modified electrode under different wavelengths in 1 mM PBS solution containing 1 mM BQ. (B) Dependence of keff on the light wavelength with SD definition (n = 5). Etip = −0.4 V versus Ag/AgCl.

Au − THPP* → Au* − THPP

Au* − THPP* → Au(e−) − THPP•+ Au(e−) − THPP•+ + CB(CNTs)

observed at wavelengths of 531, 546, 560, 570, and 580 nm, and meanwhile negative feedback is seen at wavelengths of 469, 489, and 515 nm. The results suggest that light with wavelengths of 469, 489, and 515 nm cannot excite the sensitizer system and further reflect the fact that light with certain wavelengths can be use to ignite the process of photosynthesis. Importantly, the highest keff for Au@THPP/ CNTs is achieved at 560 nm (Table S1, Supporting Information), a light wavelength lower or higher than 560 nm could retard charge transfer process, and consequently the lower keff is given. This wavelength is consistent with the maximum absorption wavelength of Au@THPP/CNTs, and more excited electrons can be gained in this case. Another reason for this highest rate constant is that the Fermi level of the Au@THPP core−shell matches well with BQ and CNTs.27 Therefore, under optical excitation, excited porphyrin can inject electrons efficiently to AuNPs. Even more important, the kinetic parameters such as the rate constant keff of ET could be directly and easily recorded by using SECM, which is a quicker and easier technique so far. Figure 7B reflects the keff at different wavelengths, and Table S1 (Supporting Information) shows the certain value with standard deviation (SD); nevertheless, there was no linearity between the wavelength and keff. The effect of BQ concentration on ET at 560 nm was studied in the concentration range of 0.5−4.0 mM, as shown in Figure

→ Au − THPP·+ + e−(CNTs)

Au − THPP* + CB(CNTs) → Au − THPP•+ + e−(CNTs)

BQ + E Tip → HQ THPP•+ + HQ → THPP + BQ− + 2H+ THPP•+ + BQ− → THPP + BQ

THPP is stimulated immediately under illumination. The excited electrons in THPP transfer directly to the Au core or the conduction band of CNTs and became an oxidation state (THPP·+).21,27 At the time when UME, named as Tip, is inflicted a potential, BQ is reduced to HQ on the surface of Tip, and the sacrificial agent HQ is oxidized to BQ by THPP oxidation state because porphyrin positive ions have higher oxidizability.21 At this time, a whole cycle of ET finished and energy conversion from light to electricity is achieved.

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CONCLUSIONS In summary, we successfully prepared the novel functional Au@THPP/CNTs nanohybrid, and a new model to uncover the partial ET of PSII using the composite by the chemical F

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(5) Jin, S.; Son, H.; Farha, O.; Wiederrecht, G.; Hupp, J. Energy Transfer from Quantum Dots to Metal−Organic Frameworks for Enhanced Light Harvesting. J. Am. Chem. Soc. 2013, 135, 955−958. (6) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (7) Ham, M.-H.; Strano, M. S.; et al. Photoelectrochemical Complexes for Solar Energy Conversion That Chemically and Autonomously Regenerate. Nat. Chem. 2010, 2, 929−936. (8) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. Ordered Assembly of Protonated Porphyrin Driven by Single-Wall Carbon Nanotubes. Jand H-Aggregates to Nanorods. J. Am. Chem. Soc. 2005, 127, 11884− 11885. (9) Ziessel, R.; Harriman, A. Artificial Light-Harvesting Antennae: Electronic Energy Transfer by Way of Molecular Funnels. Chem. Commun. 2011, 47, 611−631. (10) Yu, J.; Mathew, S.; Flavel, B. S.; Johnston, M. R.; Shapter, J. G. Ruthenium Porphyrin Functionalized Single-Walled Carbon Nanotube Arrayss-A Step Toward Light Harvesting Antenna and Multibit Information Storage. J. Am. Chem. Soc. 2008, 130, 8788−8796. (11) de la Torre, G.; Bottari, G.; Sekita, M.; Hausmann, A.; Guldi, D. M.; Torres, T. A Voyage into the Synthesis and Photophysics of Homo- and Heterobinuclear Ensembles of Phthalocyanines and Porphyrins. Chem. Soc. Rev. 2013, 42, 8049−8105. (12) Mallouk, T. E.; Yang, P. Chemistry at the Nano-Bio Interface. J. Am. Chem. Soc. 2009, 131, 7937−7939. (13) de Fuentes, O. A.; Ferri, T.; Frasconi, M.; Paolini, V.; Santucci, R. Highly-Ordered Covalent Anchoring of Carbon Nanotubes on Electrode Surfaces by Diazonium Salt Reactions. Angew. Chem. 2011, 123, 3519−3523. (14) Diao, P.; Liu, Z. Electrochemistry at Chemically Assembled Single-Wall Carbon Nanotube Arrays. J. Phys. Chem. B 2005, 109, 20906−20913. (15) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. Photovoltaic Cells Using Composite Nanoclusters of Porphyrins and Fullerenes with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 1216−1228. (16) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. Organized Assemblies of Single Wall Carbon Nanotubes and Porphyrin for Photochemical Solar Cells: Charge Injection from Excited Porphyrin into Single-Walled Carbon Nanotubes. J. Phys. Chem. B 2006, 110, 25477−25484. (17) Saito, K.; Troiani, V.; Qiu, H.; Solladié, N.; Sakata, T.; Mori, H.; Ohama, M.; Fukuzumi, S. Nondestructive Formation of Supramolecular Nanohybrids of Single-Walled Carbon Nanotubes with Flexible Porphyrinic Polypeptides. J. Phys. Chem. C 2007, 111, 1194− 1199. (18) Karge, O.; Bondar, A. N.; Dau, H. Cationic Screening of Charged Surface Groups (carboxylates) Affects Electron Transfer Steps in Photosystem-II Water Oxidation and Quinone Reduction. Biochim. Biophys. Acta, Bioenerg. 2014, 1837, 1625−1634. (19) Shinopoulos, K. E.; Yu, J.; Nixon, P. J.; Brudvig, G. W. Using Site-Directed Mutagenesis to Probe the Role of the D2 Carotenoid in the Secondary Electron-Transfer Pathway of Photosystem II. Photosynth. Res. 2014, 120, 141−152. (20) Hammarströ m, L.; Styring, S. Proton-Coupled Electron Transfer of Tyrosines in Photosystem II and Model Systems for Artificial Photosynthesis: the Role of a Redox-Active Link Between Catalyst and Photosensitizer. Energy Environ. Sci. 2011, 4, 2379−2388. (21) Wang, W.; Shan, D.; Yang, Y.; Wang, C.; Hu, Y.; Lu, X. A Novel Method for Dynamic Investigation of Photoinduced Electron Transport Using Functionalized-Porphyrin at ITO/Liquid Interface. Chem. Commun. 2011, 47, 6975−6977. (22) Lu, X.; Geng, Z.; Wang, Y.; Lv, B.; Kang, J. Synthesis and Characterization of Three New Unsymmetrical Porphyrins and Their Cobalt Complexes. Synth. React. Inorg. Met.-Org. Chem. 2002, 32, 843− 851. (23) Yao, C.; Xu, X.; Wang, J.; Shi, L.; Li, L. Low-Temperature, Solution-Processed Hole Selective Layers for Polymer Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 1100−1107.

approach has been constructed. Further, we analyzed the model in detail, and the kinetics of the photoinduced electron transfer process by regenerating BQ in the presence of visible light was shown by UV−vis/SECM. The proposed model exhibited strong absorption and a faster electron transfer rate constant at 560 nm, which is consistent with the maximum absorption wavelength of Au@THPP/CNTs hybrid. In addition, the possible ET pathways and the mechanism involved have been discussed. This work, we believe, could strongly propel research about the photoinduced electron transfer and provide a new pathway for insight into the process of natural photosynthesis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11246. Process of the in-situ-generated aminophenyl (AP) films; cyclic voltammograms of 1.0 × 10−3 mol L−1 K3Fe(CN)6 + 0.1 mol L−1 KCl solution at UMEs; UV−vis spectra of the core−shell nanostructure, gold nanoparticles, and porphyrin; UV−vis spectra of THPP vesicle structures and the porphyrin; fluorescence emission spectra of the Au@THPP, vesicles nanostructure, and porphyrin; FTIR spectrum of the core−shell nanostructure and THPP; heterogeneous electron transfer rate constant of ITO modified with CNTs/Au@THPP for different light source wavelengths (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-931-7971276. Fax: +86-931-7971323. E-mail: [email protected], [email protected]. Author Contributions †

X.N. and L.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21327005, 21175108, 21575115, 21565022, 21445006), the Program for Chang Jiang Scholars and Innovative Research Team, Ministry of Education, China. (Grant No. IRT1283), the Program for Innovative Research Group of Gansu Province, China (Grant No. 1210RJIA001), and the Program of Innovation and Entrepreneurial for Talent, Lan Zhou, Gansu Province, China (Grant No. 2014-RC-39)

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DOI: 10.1021/acs.jpcc.5b11246 J. Phys. Chem. C XXXX, XXX, XXX−XXX