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Comparative Electrochemical Behaviors of a Series of SH-Terminated-Functionalized Porphyrins Assembled on a Gold Electrode by Scanning Electrochemical Microscopy (SECM) Wenting Wang,†,‡ Xiujuan Li,† Xiaoyan Wang,† Hui Shang,† Xiuhui Liu,† and Xiaoquan Lu*,† College of Chemistry and Chemical Engineering, Northwest Normal UniVersity, Lanzhou 730070, P. R. China, and College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou 730000, P. R. China ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: July 11, 2010
The synthetic sulfhydryl porphyrin compounds (abbreviated as H2TPPO(CH)nSH, n ) 3, 4, 6, 9, 10, 12), with a systematic series of spacers containing large end groups-porphyrin rings and different length alkyl chains, were prepared to investigate the electron transport (ET) kinetics between a gold electrode modified with H2TPPO(CH)nSH and a redox mediator (ferricyanide). The three ET kinetics were clearly distinguishable by scanning electrochemical microscopy (SECM). As shown by the rate constants (kox) of bimolecular ET between H2TPPO(CH)nSH and the redox mediator, the rate constants (kb) of tunneling ET between the underlying gold electrode and the redox mediator, and the rate constants (k′) of pinhole or defects, ET depended upon alkyl chain length, surface coverage, and the concentration of the redox mediator. Among all of them, the bimolecular reaction which contained electron donor (mediators) and electron acceptor (SAMs) was a dynamic cyclic process. That strongly suggested that the bimolecular reaction should mimic ET of photosystem II in chloroplast. Introduction All of the porphyrin and metalloporphyrin compounds are the core of life-activity substance in animals and plants. Because of their larger surface area and special rigid structure, the monolayer with the porphyrins and metalloporphyrin compounds is closest to the perfect model of a genuine biomembrane, which has been used for theoretical research on the basis process of life activities, such as photosynthesis, respiration, energy conversion, and so on, especially photoelectrochemical conversions and electron transfer (ET) reactions in photosynthesis.1,2 It is reported that most porphyrins or analogues have been utilized to mimic a variety of applications on biological ET, oxygen transport, and metalloenzymes for their electrochemical and photophysical properties so far.3,4 The studies on ET greatly depend on the various interfaces, such as solid/liquid, air/water, and liquid/liquid; among them, the properties of porphyrin compounds have become a hot issue in ET processes.5-15 Therefore, the research on ET of porphyrin derivatives is of much importance in modern chemistry and biology circles. The three different ET pathways can be measured by scanning electrochemical microscopy (SECM), which has several advantages over cyclic voltammetry (CV), such as allowing more information to be extracted from a single measurement.16 Bimolecular reaction which contained a sensitive function of parameters relating to the electron donor (such as ferrocene derivatives) and acceptor (such as porphyrin rings) is just one of these three ET pathways. In addition, SECM can research the efficient biological ET system on various interfaces with high spatial resolution, which is a simpler but more elegant research way to mimic the electron donor-acceptor ensembles.17-21 The group of Murray measured the rate of ET between the * To whom correspondence should be addressed. Phone: +86-9317971276. Fax: +86-931-7971323. E-mail:
[email protected]. † Northwest Normal University. ‡ Lanzhou University.
electrodes and attached monolayer (MPCs), which also provide a controllable and quantifiable way to drive ET.22 Besides this, the group of Holt constructed cytochrome c (cyt c) immobilized on a COOH-terminated alkanethiol monolayer.23-32 They obtained the values of k° for tunneling ET and kBI for bimolecular ET and provided some important information about the binding configurations of cyt c for efficient ET by SECM.33 The effect of defects and structure on ET through n-alkanethiol selfassembled monolayer was studied by the Bard group, as they proposed a model using the potential dependence of the measured rate constants to compensate for the pinhole contribution.34 However, the report on a series of macrocyclic molecules of ET kinetics and the rate constants is rare. On the basis of our previous work, the macrocyclic molecules, such as porphyrin compounds, have been confirmed on their capacity to be oxidized and lose the electron at the available positive potential.35-39 In this paper, we extend a series of SH-terminated-functionalized porphyrin and SECM is used to measure the different ET pathways. The objective is to investigate the ET kinetic property of the alkyl chain length of functionalized porphyrin and the different chain length changes around the sulfhydryl porphyrin compounds. A bimolecular ET model is proposed, which is helpful as a mirror for ET processes of the biologic system via porphyrin derivatives. Experimental Section Chemicals. Sulfhydryl porphyrin compounds (abbreviated as H2TPPO(CH2)nSH, n ) 3, 4, 6, 9, 10, 12; the structure is shown in Scheme 1) were synthesized by a previous reported methodology.37 All other reagents were analytical-reagent grade, unless otherwise specified. Solutions were prepared from water that had been purified through an Ultrapure water system Milli-Q Plus (Millipore).
10.1021/jp1026064 2010 American Chemical Society Published on Web 07/27/2010
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J. Phys. Chem. B, Vol. 114, No. 32, 2010 10437 Fe(CN)64- is generated, move to the substrate with porphyrin (H2MPTPP)
SCHEME 1: Scheme of the Sulfhydryl Porphyrin Compounds (H2TPPO(CH2)nSH and Co-H2TPPO(CH2)nSH, n ) 3, 4, 6, 9, 10, 12) Used in This Experiment
[H2MPTPP]+ + Fe(CN)64- f [H2MPTPP] + Fe(CN)63(2) and the kb is the rate constant for the tunneling ET which occupies the main position when the substrate potential (Es) is not very high and the concentration of redox mediator (Fe(CN)63-) is not very low (0.2 mM).
Fe(CN)64--e f Fe(CN)63-
Preparation of Substrate and SAMs. A gold electrode (diameter ) 3 mm) was used as the substrate for these experiments. The electrode was polished to a mirror finish using 0.05 µm alumina powder before careful rinsing with deionized (DI) water, sonication in absolute ethanol and DI water for 10 min in turn, and then drying with nitrogen gas. SAMs were prepared with a solution of 1 mM sulfhydryl porphyrin compounds in chloroform for 24 h. During the self-assembly process, we took the electrode in solution of chloroform and DI water in turn with an ultrasonic instrument every 3 h. Electrochemical Measurements. Electrochemical experiments were carried out using a CHI 900 scanning electrochemical microscope (CH Instruments Co. Ltd., Austin, TX) with a four-electrode cell. The gold (or modified gold) electrode, a platinum wire, and a KCl saturated Ag/AgCl electrode were used as the working, counter, and reference electrode, respectively. The SECM tip was a 25 µm diameter Pt ultramicroelectrode (UME). Before each experiment, the tip was polished with 0.3 µm alumina and rinsed with DI water, and the solutions were purged with pure nitrogen for 15 min before each run. Theory. A 25 µm diameter Pt ultramicroelectrode (UME) was the SECM tip electrode to obtain the kinetics of heterogeneous ET with a good resolution.19,40 The hydrophilic redox mediators on the UME tip, such as Ru(NH3)63+ and Fe(CN)63-, are reduced or oxidized when the tip is moving to the substrate at the steady state, following the tip current by well-known expression
i ) 4nFDca
(1)
where n is the number of electrons transferred per molecule, F is Faraday’s constant, D is the diffusion coefficient of the electroactive molecule, c is the bulk concentration of the electroactive molecule, and a is the radius of the tip.41 The approach curve is recorded by resulting current curves versus distance when the tip is infinitely closing to the substrate and the hydrophilic redox mediators (Ru(NH3)63+ or Fe(CN)63-) at the moving UME tip regenerate.42,43 The heterogeneous rate constant (keff) is obtained by the experimental ones fitting to the theory ones. Scheme 2 shows SECM and the simple model, in which the substrate is the porphyrin SAM on the gold electrode, and at the UME tip the redox mediator (Fe(CN)63-) is reduced to Fe(CN)64- at a more negative potential, and then it is regenerated at the substrate surface. The bimolecular ET and the rate constant, kox (mol-1 cm3 s-1), following the UME tip, where
(3)
Otherwise, the pinhole or defects of ET cannot be neglected. The rate constants for different chain lengths of alkanethiol SAM between the gold electrode and redox species were measured by the new developed methodologies of SECM.16 The reaction scheme (Scheme 2) can be adapted for the above methodologies of SECM. When reaction 2 is not reversible and the reaction 3 is quasi-reversible, the heterogeneous rate constant (keff) via experimental curves fitting to theory ones is
keff )
koxkbΓ* + k' koxc° + kb + kf
(4)
There are two limiting cases: (a) At quite high substrate potential (Es), where Es is much more negative than the formal potential (E°ads), and koxc° , kb + kf, the heterogeneous rate constant (keff) in eq 3 becomes
keff ) koxΓ* + k'
(5)
From the Butler-Volmer equations, kb and kf should be
[
kf ) k° exp -
[
kb ) k° exp -
RF(ES - E°ads) RT
]
(1 - R)F(ES - E°ads) RT
(6a)
]
(6b)
where k° is the standard ET rate constant, R is the transfer coefficient, F is Faraday’s constant, R is the gas constant, and T is the absolute (Kelvin) temperature. Equation 5 could be shown as
1 + exp[-F(Es - E°ads)/RT] Γ* c° + ) keff - k' kb kox
(7)
where Es is the substrate potential, E°ads is the formal potential of the SAM-bound redox species, keff is the heterogeneous rate constant, c° is the concentration of the redox mediator, Γ* is the surface concentration of oxidized and reduced forms of redox centers in the monolayer, kox is the bimolecular oxidation rate constant, kb is the tunneling rate constant, and k′ is the defect and pinhole rate constant. The values of kox and kb could be calculated by eq 7. (b) At little low substrate potential (Es), in this Article, Es is equal to the formal potential (E°ads), and koxc° . kb + kf, the
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SCHEME 2: (A) The Mode on SECM; (B) the Simple Model for the ET of Photosystem II in the Chloroplast; (C) Three Modes for SECM Operation: (a) Electrons of the Redox Mediater Directly Transfer through the Monolayer, Pinholes, and Defects; (b) between the Monolayer-Attached Redox Species and the Underlying Au; (c) Bimolecular Reaction between the Attached SAMs and the Redox Mediator
rate constant (kb) for the tunneling ET occupies main position in the ET processes, and then the heterogeneous rate constant (keff) in eq 3 becomes
keff )
kbΓ* + k' c°
(8)
The values of kb could be calculated by eq 8. Results and Discussion The Surface Coverage Γ to Porphyrin Modified SAMs. For assembling molecular components on the substrate, SAMs have emerged as an alternative and available strategy and the surface coverage, Γ, is the most characteristic investigative
method, which is obtained by Γ ) Q/nFA (Q is the peak area in coulombs, A is the electrode surface area, F is the Faradaic constant, and n is the number of electrons involved in the electrode interaction). Here, the peak area, Q, was obtained by redox stripping at the negative potential (-0.9 V), and then the surface coverage, Γ, was calculated by the above equation where Q is the peak area in coulombs. Table 1 shows Γ values and molecular areas of different alky chain length porphyrins. The Γ values ((0.41-3.01) × 10-10 mol cm-2) are consistent with previous reports ((0.4-2.0) × 10-10 mol cm-2).44,45 It is also well-known by calculation when the values of Γ are smaller than 70 pmol cm-2 and the molecular areas are around 400 Å for the not long enough spacers (n ) 3, 4), the porphyrin rings are densely packed with a parallel orientation to the gold electrode. However, by the alkyl spacers growing, the porphyrin
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TABLE 1: Γ Value and Molecular Area of Porphyrins with Different Length Spacers system
Γ/pmol cm-2
molecular area/Å2 molecule-1
n)3 n)4 n)6 n)9 n ) 10 n ) 12
49 41 184 301 295 312
338.0 404.8 90.3 55.4 56.2 49.3
rings self-assembled on the gold electrode may change from the parallel to the perpendicular orientation, which could be full proof of the more and more regular permutation and denser monolayer between sulfhydryl porphyrin molecules by the alkyl spacers increasing.44,45 On the basis of the above discussion, it is easy to understand the difference of structure on the series of alkyl chain length on porphyrin SAMs. When n ) 4 in Table 1, the molecular area (Å2) is largest in the group. For a single porphyrin molecule, the porphyrin ring occupies the electrode area much more when the methylene number (n) is even.45 Conversely, the porphyrin molecule with odd methylene occupies less electrode area, so the surface coverage (Γ) is much more, such as n ) 3. In comparison, the surface coverage of the even methylene porphyrin molecule is less than the adjacent odd ones with shorter chain length in Table 1, that is, similar to the odd-even effect of biphenyl SAMs.46 The Influence of Different Alkyl Chain Lengths. The theory shown in eq 4 with the heterogeneous rate constant (keff) via experimental curves fitting to theory ones contained two limiting cases. Among them, the different limiting cases were according with the different experimental conditions including the substrate potential, E°ads, and the concentration of the redox mediator. Here, the series of SH-terminated-functionalized porphyrins was self-assembled on the substrate (gold electrode) in the model on SECM and then applied the new developed methodologies of SECM on the porphyrin SAMs contained different alkyl chain lengths.16 The approach curves to the electrode modified with the different length spacers containing the macrocyclic molecule as the terminal group were recorded by using 1 mM ferricyanide redox mediator in 0.1 M KCl and a tip potential of -0.1 V versus Ag/AgCl to generate ferrocyanide at a steady state. The peaks for the reversible oxidation in the CVs indicate the formal potential of the porphyrin ring (n ) 12) is about 0.75 V (Figure 1). It is according with our previous works that the porphyrin ring could be oxidized and lose electrons at available potentials.36 By controlling the potential of the substrate above 0.75 V and a steady state at the UME tip to deoxidize the mediator (Fe(CN)63- in all experiments), the bimolecular ET appears as the UME tip is moving to the substrate. Figure 2 shows the theoretical curves are fitting to the experimental data.
Figure 1. Cyclic voltammogram of H2TPPO(CH2)nSH (n ) 12) modified on a gold electrode in 0.1 M KCl at a scan rate of 10 mV/s.
According to the relevant theories, if the three ET pathways are all existent in SAMs on the gold eletrode, the kinetic constants satisfy eq 7 when the chain length of the selfassembled molecule is relatively short.47 The requirement of eq 7 is much more positive substrate potential (Es) than the formal potential (E°ads). Taking an example (n ) 3), the formal potential is about 0.25 V (Figure S3, Supporting Information), as the bimolecular rate constant, kox, was obtained with a slope by eq 7, equal to kox ) 1.23 × 108 mol-1 cm3 s-1 by the Figure 2A inset (Es ) 0.8 V). However, for the long alkyl chain length, the bimolecular rate constant, kox, was obtained with a slope by eq 5 under the condition of an extremely low concentration of the mediator and a little low substrate potential compared with the formal potential when the three ET processes coexist on SAM, so keff became independent of the concentration of the mediator. For instance (n ) 12), the little low substrate potential (Es ) 0.9 V) was near the formal potential (E°ads ) 0.75 V), and then a plot of keff derived from the approach curves versus the coverage by eq 5, which yields a value for the bimolecular ET rate constant, kox, of 3.13 × 1010 mol-1 cm3 s-1, as shown in the Figure 2B inset. In summary, the spacers containing the porphyrin rings as the terminal groups with the long alkyl chain length acted as a more excellent blocking layer than the short ones, according to what was reported previously.36 The same results about the values of kox were obtained, which can be consistent with this manner: n12 ≈ n10 > n9 > n6 > n4 > n3. The reason for the change trend on the values of kox may be both sides: the mediator Fe(CN)63- and the porphyrin rings-terminated SAMs. The different concentrations of the redox mediator of 0.2, 0.4, 0.6, 0.8, and 1.0 mM Fe(CN)63- in 0.1 M KCl were used to carry out the experiment. When the tip potential is so negative (-0.1 V), Fe(CN)63- is deoxidized to the Fe(CN)64- and ensures the ET is a bimolecular reaction, so for the ET rate constant the concentration of the redox mediator is an important influencing factor. For the porphyrin rings-terminated SAMs, the ET rate constant depends on the arrangement around the porphyrin rings on the gold electrode as well as the surface coverage of electroactive SAMs. Table 2 shows all of the rate constants. The bimolecular ET rate constants, kox, increase by the alkyl chain length growing, following the change on the surface coverage of different alkyl chain lengths. That indicates the surface coverage of the modified electrode is the important influencing factor to the rate constant. In addition, the porphyrin rings were immobilized on the gold electrode with the alkyl chain, so the length of the alkyl chain in SAMs may directly influence the porphyrin ring arrangement and indirectly influence the ET rate to a certain extent. The tunneling ET constants of short chain length, kb, are much bigger than those of long chain length. From the condition of the calculating equation, the substrate potential, Esub, is different between the porphyrin with short chain length and the long ones, so the substrate potential plays a key role in SECM and may influence the tunneling ET. At low substrate potential for short ones, the tunneling ET constants, kb, are much greater than that with a high substrate potential. By the rate constant of the pinhole, it is obvious that the alkyl chain length increasing improves SAMs, according with the calculating result of surface coverage. When the alkyl chain length is n ) 12, the surface coverage and the bimolecular ET constant are the greatest and the tunneling ET rate constant and the pinhole rate constant are the least. The Bimolecular Reaction Model for Partial ET of Photosystem II in the Chloroplasts. Consider the bimolecular interaction [A]+ + Fe(CN)64- ) A + Fe(CN)63-. This model
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Figure 2. (A) Probe approach curves of H2TPPO(CH2)nSH from bottom to top; the corresponding numbers of methylenes are 3, 4, and 6. Inset: Dependence of the effective heterogeneous rate constants of the H2TPPO(CH2)nSH (n ) 3) monolayer in 0.1 M KCl solution. Etip ) -0.1 V vs Ag/AgCl, Esub ) 0.80 V vs Ag/AgCl. (B) Probe approach curves of H2TPPO(CH2)nSH from bottom to top; the corresponding numbers of methylenes are 9, 10, and 12. Inset: Dependence of the effective heterogeneous rate contants of the H2TPPO (CH2)nSH (n ) 12) monolayer. Etip ) -0.1 V vs Ag/AgCl, Esub ) 0.90 V vs Ag/AgCl. The diamonds are experimental approach curves, and the lines are theoretical approach curves calculated for different values of heterogeneous rate constant, keff. (L is the distance between the normalized tip electrode and the substrate, d is the distance between the tip electrode and the substrate, and a is the radius of the tip electrode).
TABLE 2: ET Rate Constant of Porphyrins with Different Length Spacers -1
3 -1
-1
-1
system
kox (mol cm s )
kb (s )
n)3 n)4 n)6 n)9 n ) 10 n ) 12
1.23 × 10 6.75 × 108 8.44 × 109 1.76 × 1010 6.27 × 1010 3.13 × 1010
8.44 × 10 1.55 × 105 7.42 × 104 5.23 × 104 1.45 × 104 4.75 × 103
8
k′ (cm s ) 5
1.33 × 10-4 1.95 × 10-4 3.32 × 10-4 3.82 × 10-4 5.12 × 10-4 1.89 × 10-5
is just used to mimic partial ET of photosystem II in the chloroplasts via the applications of SAMs with the spacers containing the metal porphyrin ring. Scheme 2 shows the reaction: (A) the substrate under study is the sulfhydryl porphyrin compound SAM on a gold electrode; (B) the simple model of bimolecular reaction for the ET reaction in the photosystem II. The porphyrin in SAMs on the substrate can be an excited electron (e-) by the light, and the redox mediator can be the electron carrier in the ET process in photosystem II, and then the tip in SECM can measure the electron in the electron carrier by the probe approve curves. In the process of generating NADPH and ATP, electron (e-) transport required carbon chain fixation plays a dominant role in chloroplasts. When it comes to chlorophyll electrochemistry, ET could be studied by the bimolecular reaction model and the relative rate constants. The process is described as follows: kox
R + Mox {\} O + Mred kred
This represents the bimolecular reaction at the solid/solution interface; Mox could be used as an electron donor, reducing matter R could be an electron acceptor, and the rate constant (kox, kred) is the bimolecular oxidation-reduction rate constant (mol-1 cm3 s-1). Conclusions In conclusion, the kinetics investigation of SAMs containing macrocyclic porphyrin as the terminal group was presented via three different ET reaction systems. The SAMs with different alkyl chain lengths were studied by the two limiting cases of the new developed methodologies of SECM. What’s more
interesting is the bimolecular ET could evaluate the ET of photosystem II, distinctive from other electrochemical analytical methods. Although we apply electrical energy instead of light energy to excite electrons, the study could contribute to study the partial photosynthetic ET process to a certain extent. The calculation of the kinetics constant with the different equations showed a phenomenon that the alkyl chain length was an important factor in the ET process. It provided a method of the selection of the chain length in SAMs to investigate the kinetics by corresponding methodologies. Acknowledgment. This work was supported by the Natural Science Foundation of China (Nos. 20775060, 20927004, and 20875077), the Key Project of Scientific Research Base of education department (08zx-07), and the Key Laboratory of Ploymer Materials of Gansu Province. Supporting Information Available: Figures showing cyclic voltammograms, probe approach curves, and the dependence of the effective heterogeneous rate constants. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Furukawa, Y.; Ishimori, K.; Morishima, I. Biochemistry 2000, 39, 10996. (2) Sakai, H.; Onuma, H.; Umeyama, M.; Takeoka, S.; Tsuchida, E. Biochemistry 2000, 39, 14595. (3) Biesaga, M.; Pyrzn´ska, K.; Trojanowicz, M. Talanta 2000, 51, 209. (4) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato, N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, Y. J. Phys. Chem. B 2000, 104, 1253. (5) Mclendon, G. A. Chem. Res. 1998, 21, 160. (6) Onuchic, J. N.; Beraten, D. N.; Winkler, J. R.; Gray, H. B. ReV. Biophys. Biomol. Struct. 1992, 21, 349. (7) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. J. Am. Chem. Soc. 2003, 125, 6644. (8) Chen, S. J. Am. Chem. Soc. 2000, 122, 7420. (9) Chen, S. Langmuir 2001, 17, 2878. (10) Chen, S. Langmuir 2001, 17, 6664. (11) Chen, S.; Pei, R. J. Am. Chem. Soc. 2001, 123, 10607–10615. (12) Liljeroth, P.; Quinn, B. M.; Ruiz, V.; Kontturi, K. Chem. Commun 2003, 1570. (13) Liljeroth, P.; Vanmaekelbergh, D.; Ruiz, V.; Kontturi, K.; Jiang, H.; Kauppinen, E.; Quinn, B. M. J. Am. Chem. Soc. 2004, 126, 7126. (14) Yang, Y.; Pradhan, S.; Chen, S. J. Am. Chem. Soc. 2004, 126, 76. (15) Quinn, B. M.; Liljeroth, P.; Kontturi, K. J. Am. Chem. Soc. 2002, 124, 12915. (16) Biao, L.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc. 2004, 126, 1485.
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