Photochemically-, Chemically-, and pH-Controlled Electrochemistry

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Langmuir 1997, 13, 1783-1790

1783

Photochemically-, Chemically-, and pH-Controlled Electrochemistry at Functionalized Spiropyran Monolayer Electrodes Amihood Doron, Eugenii Katz, Guoliang Tao, and Itamar Willner* Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received July 23, 1996. In Final Form: December 12, 1996X A photoisomerizable nitrospiropyran monolayer assembled on a Au electrode provides a functionalized interface for the photochemical, pH, and thermal control of electrochemical processes of charged electroactive redox probes. (Mercaptobutyl)nitrospiropyran 1 was assembled as a monolayer on a Au electrode. The monolayer exhibits reversible photoisomerizable features, and illumination of the nitrospiropyran monolayer, SP state, 320 nm < λ < 350 nm, yields at pH ) 7.0 the protonated nitromerocyanine monolayer state, MRH+ state. Further irradiation of the MRH+ monolayer, λ > 495 nm, regenerates the SP state of the monolayer. The light-induced transformation of the monolayer between a neutral and a positively-charged interface allows the control of the electron transfer processes at the electrode interface. Electrooxidation of the negatively-charged (3,4-dihydroxyphenyl)acetic acid, DHPAA, is enhanced at the MRH+ monolayer electrode as compared to the SP-functionalized monolayer electrode. Electrooxidation of the positivelycharged 3-hydroxytyramine (dopamine), DOPA, is inhibited at the MRH+ monolayer electrode as compared to its oxidation by the SP monolayer electrode. The control of the electrochemical oxidation of DHPAA and DOPA at the photoisomerizable monolayer electrode is attributed to the electrostatic interactions of the MRH+ monolayer electrode with the redox-active substrates. Electrostatic attraction of DHPAA and repulsion of DOPA by the MRH+ monolayer results in enhancement or inhibition of the electrochemical processes, respectively. By reversible isomerization of the monolayer between the SP and MRH+ states, cyclic amperometric transduction of the optical signals recorded by the monolayer is accomplished. In the presence of a mixture of oppositely-charged redox substrates, i.e. DHPAA and 2,5-bis[[2-(dimethylbutylammonio)ethyl]amino]-1,4-benzoquinone (3) or pyrroloquinoline quinone, PQQ, (4) and 3, photostimulated selective electrochemistry is accomplished in the presence of the photoisomerizable monolayer electrode. The transformation of the protonated nitromerocyanine monolayer, MRH+ state, generated at pH ) 7.0, to the zwitterionic nitromerocyanine configuration, MR( state at higher pH, allows the pH-controlled electrooxidation of DHPAA and DOPA at the monolayer electrode. Similarly, thermal isomerization of the SP monolayer electrode, pH ) 7.0, 60 °C, yields the MRH+ monolayer electrode. These thermochromic features of the monolayer are employed to respectively activate or deactivate the electrooxidation of DHPAA or DOPA at the functionalized electrode. By cyclic thermal isomerization of the SP monolayer to the MRH+ monolayer followed by photochemical isomerization of the MRH+ monolayer followed by photochemical isomerization of the MRH+ monolayer to the SP state, λ > 495 nm, the thermochromic and photochromic features of the monolayer are amperometrically transduced via the oxidation of DHPAA and DOPA, respectively. Electrochemical oxidation of DHPAA and DOPA is further accomplished by the application of a dinitrospiropyran monolayer (2) electrode in the presence of the dinitrophenyl antibody, DNP-Ab. (Mercaptobutyl)dinitrospiropyran 2 was assembled as a monolayer on a Au electrode. The dinitrospiropyran monolayer, SP state, exhibits antigen features for the DNP-Ab, where the protonated dinitromerocyanine monolayer, MRH+ state, lacks antigen features for the DNP-Ab. Association of the DNP-Ab to the SP monolayer electrode blocks the electrooxidation of DHPAA or DOPA. Photochemical isomerization of the SP monolayer to the MRH+ state, 320 nm < λ < 350 nm, results in the release of DNP-Ab and the activation of the electrooxidation of DHPAA and DOPA. By the reversible photoisomerization of the monolayer between the SP and MRH+ states in the presence of DNP-Ab, cyclic amperometric transduction of the optical signals recorded by the monolayer is accomplished.

Introduction The development of molecular electronic and molecular optoelectronic systems is a rapidly progressing research field.1,2 A basic element of such devices involves the electronic or photonic triggering of a molecular function and the secondary physical transduction of the recorded * To whom correspondence should be addressed. Fax: 972-26527715. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) (a) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 90-112. (b) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304-1319. (c) Fabbrizzi, L.; Poggi, A. Chem. Soc. Rev. 1995, 24, 197-202. (2) (a) Iyoda, T.; Saika, T.; Honda, K.; Shimidzu, T. Tetrahedron Lett. 1989, 30, 5429-5432. (b) Daub, J.; Salbeck, J.; Kno¨chel, T.; Fischer, C.; Kunkely, H.; Rapp, K. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 1494-1496. (c) Daub, J.; Fischer, C.; Salbeck, J.; Ulrich, K. Adv. Mater. 1990, 2, 366-369. (d) Feringa, J.; Jager, W. F.; de Lange, B. J. Am. Chem. Soc. 1991, 113, 5468-5470. (e) Achatz, J.; Fischer, C.; Salbeck, J.; Daub, J. J. Chem. Soc., Chem. Commun. 1991, 504-507.

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signal. For example, a Ru(II)-tris(bipyridine) complex covalently linked to a quinone represents a supramolecular assembly where the electrical signal that reduces the quinone is transduced by the emission features of the chromophore.3 Similarly, the electrical translocation of the bipyridinium cyclophane assembled onto a “molecular wire” consisting of bis(phenol dianisidine) units demonstrates the physical and mechanical activation of molecular functions.4 Physical transduction of recorded optical signals is the basis of molecular optoelectronic devices. Amperometric transduction of recorded optical signals was demonstrated with photoactive compounds that undergo reversible light-stimulated isomerization to electroactive chemical entities.5 For example, dihydroxythiophene ethene undergoes photochemical 6π electrocyclization to (3) Goulle, V.; Harriman, A.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1993, 1034-1036. (4) Bisell, R. A.; Co´rdova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369, 133-137.

© 1997 American Chemical Society

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a redox-active isomer state that allows the electrochemical transduction and storage of the molecular optical switch.6 Photostimulated “ON-OFF” formation of donor-acceptor complexes between eosin and a photoisomerizable azobenzene-bipyridinium diad was reported as a supramolecular optical switch that enables the optical (color or fluorescent) transduction of the recorded optical signals.7 To tailor molecular electronic or optoelectronic devices, the integration of the molecular switch with a transducing support is essential. Modification of the electrode surfaces with functionalized monolayers provides a general means for the electronic (i.e. amperometric) transduction and amplification of recorded optical signals.8-10 Functionalized monolayers assembled onto electrode surfaces were applied as active interfaces for the electronic transduction of recorded optical signals. An eosin monolayer electrode was employed as a functionalized interface for the electrical or microgravimetric transduction of the lightstimulated formation and dissociation of supramolecular complexes between a photoisomerizable bipyridiniumazobenzene diad.9 Also, functionalization of an electrode with a phenoxynaphthacene quinone monolayer generated a photoactive interface for the amperometric transduction of recorded optical signals.10 Photoisomerizable nitrospiropyran and dinitrospiropyran monolayers assembled on the Au electrode represent light-sensitive interfaces. The electrical properties and structural features of these monolayers are controlled by light and the pH of the aqueous electrolyte solution. UV-irradiation of the nitrospiropyran or dinitrospiropyran yields the respective protonated merocyanine states of the monolayer at pH < 8.6, whereas the monolayer turns zwitterionic at more basic pH values.11 Illumination of the protonated nitromerocyanine (or dinitromerocyanine) or the respective zwitterionic monolayer states with visible light regenerates the nitrospiropyran or dinitrospiropyran monolayers.12 The cyclic, photostimulated transformation of the monolayer between a neutral and positively-charged interface was used to control the electrical contact of redox proteins with the electrode surface.13 For example, the electrical communication of a ferrocene-modified glucose oxidase was photoswitched in the presence of the nitrospiropyran monolayer electrode. The system was applied for the amplified amperometric transduction of optical signals recorded by the monolayer via the photostimulated oxidation of glucose by the biocatalyst.13b The photostimulated structural changes of a dinitrospiropyran (5) (a) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 275-284. (b) Kawai, S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 285-293. (6) Kawai, S. H.; Gilat, S. L.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1994, 1011-1013. (7) Willner, I.; Marx, S.; Eichen, Y. Angew. Chem., Int. Ed. Engl. 1992, 31, 1243-1244. (8) (a) Katz, E.; Lion-Dagan, M.; Willner, I. J. Electroanal. Chem. 1995, 382, 25-31. (b) Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1994, 116, 7913-7914. (c) Willner, I.; Lion-Dagan, M.; Marx-Tibbon, S.; Katz, E. J. Am. Chem. Soc. 1995, 117, 6581-6592. (d) Willner, I.; Blonder, R.; Katz, E.; Stocker, A.; Bu¨ckmann, A. F. J. Am. Chem. Soc. 1996, 118, 5310-5311. (e) Willner, I.; Willner, B. Adv. Mater. 1995, 7, 587-589. (9) Marx-Tibbon, S.; Ben-Dov, I.; Willner, I. J. Am. Chem. Soc. 1996, 118, 4717-4718. (10) (a) Doron, A.; Katz, E.; Portnoy, M.; Willner, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1535-1537. (b) Doron, A.; Portnoy, M.; LionDagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 89378944. (11) Kato, S.; Aizawa, M.; Suzuki, S. J. Membr. Sci. 1976, 1, 289300. (12) Guglielmetti, R. In Photochromism. Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; Chapter 8, pp 314-466. (13) (a) Lion-Dagan, M.; Katz, E.; Willner, I. J. Chem. Soc., Chem. Commun. 1994, 2741-2742. (b) Willner, I.; Doron, A.; Katz, E.; Levi, S.; Frank, A. J. Langmuir 1996, 12, 946-954.

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monolayer were used to control the association of the dinitrophenyl antibody, DNP-Ab, and to establish a basic principle to tailor reversible immunosensor devices.14 The electrochemistry of charged electroactive substrates is controlled at electrodes modified by charged monolayers.15-18 Electrooxidation of ascorbic acid was found to be retarded at electrodes modified by a negativelycharged monolayer16 while the oxidation of 3-hydroxytyramine (DOPA) was found to be enhanced at negatively-

charged interfaces.17b In these systems, electrostatic attractive or repulsive interactions between the charged electroactive substrates and the monolayer control the electroactivity at the electrode interface (Frumkin effect19). The alteration of the electrical properties of the nitrospiropyran monolayers by light and pH suggests the possibility of controlling by light and pH the electrochemistry of charged electroactive probes. Here we wish to report on the light, pH, and thermal control of the electrooxidation of (3,4-dihydroxyphenyl)acetic acid, DHPAA, and 3-hydroxytyramine (dopamine), DOPA, at a nitrospiropyran monolayer-modified electrode. We demonstrate that the light-stimulated control of the electrooxidation of the two substrates at the functionalized electrode enables the amperometric transduction of optical, thermal, or pH signals recorded by the monolayer. We also address (14) (a) Willner, I.; Blonder, R.; Dagan, A. J. Am. Chem. Soc. 1994, 116, 9365-9366. (b) Willner, I.; Blonder, R. Thin Solid Films 1995, 266, 254-257. (c) Willner, I.; Lion-Dagan, M.; Katz, E. J. Chem. Soc., Chem. Commun. 1996, 623-624. (15) (a) Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1973, 77, 14011410. (b) Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1973, 77, 14111421. (16) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37-41. (17) (a) Takehara, K.; Ide, Y. Bioelectrochem. Bioenerg. 1992, 27, 207-219. (b) Takehara, K.; Takemura, H.; Aihara, M.; Yoshimura, M. J. Electroanal. Chem. 1996, 404, 179-182. (18) Katz, E.; Schlereth, D. D.; Schmidt, H.-L. J. Electroanal. Chem. 1994, 367, 59-70. (19) Delahay, P. In Double Layer and Electrode Kinetics: Advances in Electrochemistry and Engineering; Delahay, P., Tobias, C. W., Eds.; Wiley Interscience: New York, 1965; Chapter 3.

Functionalized Spiropyran Monolayer Electrodes

the application of the photoisomerizable monolayer electrode as a functionalized surface to photostimulate selectivity in electrochemical transformations.

Langmuir, Vol. 13, No. 6, 1997 1785 Scheme 1. Assembly of the Nitrospiropyran 1 Monolayer on a Au Electrode

Experimental Section 1. Materials. 1-(4-Mercaptobutyl)-3,3-dimethyl-6′-nitrospiro[2′H-1-benzopyran-2′,2-indoline] (1) was synthesized according to the published procedure,13b and 1-(4-mercaptobutyl)-3,3dimethyl-6′,8′-dinitrospiro[2′H-1-benzopyran-2′,2-indoline] (2) was synthesized by a similar technique using the corresponding dinitro derivative as the precursor. 2,5-Bis[[2-(dimethylbutylammonio)ethyl]amino]-1,4-benzoquinone dichloride (3) was synthesized by a modification of the reported procedure for the related naphthoquinone derivatives.20 To a suspension of 10.8 g (0.1 mol) of 1,4-benzoquinone in 100 mL of ethanol was added 19.36 g (0.22 mol) of 2-(dimethylamino)ethylamine. As the amine was added, a deep red color developed. The reaction mixture was stirred at room temperature overnight and then refluxed for 1 h. After cooling, the mixture was acidified with HCl and the bright red precipitate was collected by filtration to give 33.18 g (94% yield) of crude product, 2,5bis[[2-(dimethylammonio)ethyl]amino]-1,4-benzoquinone dihydrochloride (5). The free base of 4 was then prepared by treating the crude product with excess aqueous Na2CO3, followed by extraction into CH2Cl2. The solution was dried over MgSO4 and filtered. The CH2Cl2 was removed under vacuum to yield 5. Alkylation of 5 was accomplished by stirring 5, 24 g (0.085 mol), with excess 1-butyl chloride, 200 mL, at 70 °C until the product (3) precipitated (ca. 5 h), 29.14 g (87% yield). All synthesized materials gave satisfactory 1H-NMR and elementary analyses and the quinone (3) showed electrochemical behavior at a nonmodified Au electrode similar to those of other 2,5-diamino-1,4-benzoquinone derivatives.21 All other materials, including 3-hydroxytyramine (dopamine), DOPA, (Aldrich), (3,4-dihydroxyphenyl)acetic acid, DHPAA, (Aldrich), pyrroloquinolino quinone, PQQ (4), (Fluka), and dinitrophenyl antibody (monoclonal mouse IgE anti-DNP), DNPAb (Sigma), were applied as supplied without further purification. Ultrapure water from a Nanopure (Barnstead) source was used throughout this work. 2. Electrodes and Electrochemical Setup. A gold electrode (0.5 mM diameter Au wire of geometrical area ca. 0.2 cm2) was used for all modifications and measurements. A cyclic voltammogram recorded in 0.5 M H2SO4 was used to determine the purity of the electrode surface just before modification. The real electrode surface area and coefficient of roughness (ca. 1.2) were estimated from the same cyclic voltammogram by integrating the cathodic peak for the electrochemical reduction of the oxide layer on the electrode surface.22 To remove the previous organic layer and to regenerate a bare metal surface, the electrode was treated with a boiling 2 M solution of KOH for 1 h, rinsed with water, and stored in concentrated sulfuric acid. Immediately and prior to modification, the electrode was rinsed with water, soaked for 10 min in concentrated nitric acid, and then rinsed with water again. A clean bare gold electrode was soaked in a solution of 1 or 2, 1 mM, in ethanol for 2 h. The electrode was then rinsed thoroughly with ethanol to remove the physically adsorbed modifier. Electrochemical measurements were performed using a potentiostat (EG&G VersaStat) connected to a personal computer (EG&G research electrochemistry software model 270/250). All the measurements were carried out in a three-compartment electrochemical cell comprising the chemically-modified electrode as a working electrode, a glassy carbon auxiliary electrode isolated by a glass frit, and a saturated calomel electrode (SCE) connected to the working volume with a Luggin capillary. All potentials are reported with respect to this reference electrode. Argon bubbling was used to remove oxygen from the solutions in the (20) (a) Zee-Cheng, R. K.-Y.; Podrebarac, E.; Menon, C. S.; Cheng, C. C. J. Med. Chem. 1979, 22, 501-505. (b) Calabrese, G. S.; Buchanan, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105, 5594-5600. (21) (a) Cameron, D. W.; Giles, R. G. F.; Pay, M. H. Tetrahedron Lett. 1970, 2049-2050. (b) Katz, E.; Shkuropatov, A. Y.; Sviridov, B. D.; Shuvalov, V. A.; Vagabova, O. I. Russian J. Phys. Chem. 1986, 60, 786-787. (22) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 9, pp 1-162.

electrochemical cell. The electrochemical cell was thermostated at 25 °C (except when specially indicated in the text) using circulated water in a jacket around the cell. The SCE was maintained at room temperature, ca. 25 °C, during all measurements. Electrochemical measurements were done in 0.02 M phosphate buffer, pH 7.0, as supporting electrolyte. In some experiments, the electrolyte was titrated to other pH values directly in the cell. 3. Photochemical Isomerization of the MonolayerModified Electrodes. Photochemical isomerization of the monolayer-modified electrodes was accomplished outside the electrochemical cell in air. Isomerization of nitrospiropyran, SP, derivatives 1 and 2, to nitromerocyanine states, MRH+, was stimulated by 1 min of irradiation at 320 nm < λ < 350 nm using an 18 W mercury pencil lamp (Oriel-6042). Photoisomerization of nitromerocyanine derivatives, MRH+, to nitrospiropyran or dinitrospiropyran states, SP, was induced by 1 min of illumination with a Xe arc lamp (150 W Oriel), fitted with a 495 nm cutoff filter. The electrodes modified with photoisomerizable monolayers were protected from room light during the electrochemical measurements. 4. Treatment of the 2-Monolayer-Modified Electrode with DNP-Antibody. The 2-monolayer-modified electrode was illuminated for 1 min at λ > 495 nm to produce the SP state. The SP monolayer electrode was then incubated in a 5 g mL-1 DNPAb solution in 0.02 M phosphate buffer, pH ) 7.0, at 35 °C for 5 min and rinsed with the phosphate buffer. The DNP-Ab-treated electrode was used for electrochemical measurements. To remove the DNP-Ab from the surface, the electrode was illuminated for 1 min at 320 nm < λ < 350 nm to produce the MRH+ state and rinsed thoroughly with 0.02 M phosphate buffer, pH ) 7.0.

Results and Discussion A photoisomerizable nitrospiropyran (1) monolayer was assembled on a Au electrode as outlined in Scheme 1. Photoirradiation of the nitrospiropyran monolayer, SP state, 320 nm < λ < 350 nm, yields at pH ) 7.0 the protonated merocyanine monolayer, MRH+ state. Further irradiation of the MRH+ monolayer, λ > 495 nm, regenerates the SP monolayer electrode. The photoisomerization of the monolayer from the neutral SP state to the positively-charged MRH+ interface enables the control of the interactions of charged electroactive components with the electrode interface. Figure 1 shows the cyclic voltammograms of (3,4-dihydroxyphenyl)acetic acid, DHPAA, in the presence of the SP monolayer electrode (curve b) and with the MRH+ monolayer electrode (curve c). A substantially higher anodic current is observed with the MRH+ electrode. This is attributed to the electrostatic attraction of the negatively-charged DHPAA electroactive probe to the positively-charged MRH+ monolayer interface. Cyclic photoisomerization of the monolayer between the SP and the MRH+ states enables the reversible transduction of the characteristic amperometric responses of the two electrode states in the presence of DHPAA (curves d and e and inset, Figure 1). The photoisomerizable 1 monolayer-modified electrode was similarly examined as a functionalized interface to control the electrooxidation of dopamine, DOPA (Figure 2). At pH ) 7.0 DOPA is

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Figure 1. Cyclic voltammograms at the photoisomerizable 1 monolayer electrode in the presence of (a) electrolyte only at the SP monolayer electrode produced by illumination (λ > 495 nm), (b and d) 5 × 10-4 M DHPAA at the SP monolayer electrode, and (c and e) 5 × 10-4 M DHPAA at the MRH+ monolayer electrode produced by illumination (320 nm < λ < 350 nm). Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0; potential scan rate, 200 mV s-1. (Inset) Reversible transduction of anodic currents by the photoisomerizable monolayer electrode, extracted from cyclic voltammograms of DHPAA at E ) 470 mV: (b) SP monolayer; (9) MRH+ monolayer.

Figure 2. Cyclic voltammograms at the photoisomerizable 1 monolayer electrode in the presence of (a) electrolyte only at the SP monolayer electrode produced by illumination (λ > 495 nm), (b and d) 5 × 10-4 M DOPA at the SP monolayer electrode, and (c and e) 5 × 10-4 M DOPA at the MRH+ monolayer electrode produced by illumination (320 nm < λ < 350 nm). Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0; potential scan rate, 200 mV s-1. (Inset) Reversible transduction of anodic currents extracted from the cyclic voltammograms of DOPA at E ) 470 mV: (b) SP monolayer; (9) MRH+ monolayer.

positively charged, and hence its oxidation by the modified electrode should be controlled by the charged monolayer, but in an opposite way to DHPAA. In the presence of the SP monolayer electrode, a high anodic current is observed (curve b) where, upon photoisomerization of the monolayer to the MRH+ state, the electrooxidation of DOPA is inhibited (curve c). By cyclic photoisomerization of the monolayer between the SP and MRH+ states, reversible high and low anodic currents are observed as a result of DOPA oxidation (curves d and e and inset, Figure 2). The lower anodic current observed upon the oxidation of DOPA in the presence of the MRH+ monolayer electrode is attributed to the electrostatic repulsion of the redox-active component from the monolayer interface. Figure 3 shows the anodic currents developed by the SP and MRH+ monolayer electrodes at different concentrations of DOPA and DHPAA. While the anodic currents of the two substrates are similar (at the selected potential E ) 0.47 V) in the presence of the SP monolayer electrode, they differ substantially with the MRH+ monolayer electrode.

Doron et al.

Figure 3. Concentration dependencies of the anodic currents at E ) 470 mV extracted from cyclic voltammograms (200 mV s-1) of (a) DHPAA at a MRH+ monolayer electrode, (b) DHPAA at a SP monolayer electrode, (c) DOPA at a MRH+ monolayer electrode, and (d) DOPA at a SP monolayer electrode. Scheme 2. Photochemically- or Thermally-Controlled Electrochemical Oxidation of Positively and Negatively-Charged Electroactive Probes (i.e., DOPA and DHPAA) at a 1 Monolayer Electrode and Principles for the Photostimulated Selective Oxidation of Charged Electroactive Probes at the Photoisomerizable 1 Monolayer Electrode

The electrooxidation of DHPAA is enhanced in the presence of the MRH+ monolayer due to the electrostatic attraction of the electroactive substrate to this monolayer state. Electrical repulsion of DOPA from the MRH+ monolayer results in the retardation of the oxidation of the latter electroactive substrate. The results reveal that the light-induced activation or deactivation of the electrooxidation of DHPAA or DOPA is accomplished by the photoisomerizable electrode (Scheme 2). Furthermore, the results imply that the two electroactive compounds, DOPA and DHPAA, can be employed as electroactive probes for the amperometric transduction of optical signals recorded by the monolayer interface. The light-induced control of the electroactivity of charged redox-active substrates by means of the photoisomerizable monolayer allows the application of the functionalized electrode as an active interface to photostimulate selective electrochemical transformations. Figure 4 shows the cyclic voltammograms of a mixture of DHPAA and 2,5-bis[[2-(dimethylbutylammonio)ethyl]amino]-1,4-benzoquinone (3) in the presence of the photoisomerizable monolayer electrode. DHPAA and 3 exhibit opposite charges, and hence their electrochemical response

Functionalized Spiropyran Monolayer Electrodes

Figure 4. Cyclic voltammograms at the photoisomerizable 1 monolayer electrode in the presence of (a) electrolyte only at the SP monolayer electrode produced by illumination (λ > 495 nm), (b) DHPAA, 3 × 10-4 M, and 3, 3 × 10-4 M at the SP monolayer electrode, and (c) DHPAA, 3 × 10-4 M, and 3, 3 × 10-4 M at the MRH+ monolayer electrode produced by illumination (320 nm < λ < 350 nm). Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0; potential scan rate, 200 mV s-1.

is expected to differ in the presence of the photoisomerizable monolayer electrode. With the SP monolayer electrode (curve b) a low electrochemical response of DHPAA is observed, where 3 exhibits a reversible redox wave at E° ) -0.53 V (pH ) 7.0). In the presence of the protonated nitromerocyanine monolayer electrode, MRH+ state (curve c), the electrochemical response of DHPAA is substantially enhanced, but 3 shows an ill-defined, irreversible, low amperometric response. These results are consistent with the anticipated electrostatic interactions of the photoisomerizable monolayer with the electroactive components. With the MRH+ monolayer, DHPAA is attracted to the electrode while 3 is repelled from the electrode surface. This results in the effective electrooxidation of DHPAA and concomitant perturbation of the redox process of 3 at the charged interface. In the presence of the neutral SP monolayer, the oxidation of DHPAA is inefficient but 3 shows the reversible redox wave characteristic of this redox substrate at a bare Au electrode. Thus, the lack of electrostatic repulsive interactions between 3 and the electrode allows the reversible redox process of 3 at the SP functionalized electrode. The feasibility of photostimulating selective electrical transformations at the functionalized monolayer electrode is further demonstrated using the oppositely-charged quinones, i.e. 3 and pyrroloquinoline quinone, PQQ (4), as redox probes (Figure 5). With the SP monolayer electrode (curve a), no redox response of PQQ is observed at its thermodynamic redox potential, E° ) -0.125 V (pH ) 7.0),8a,18 but 3 exhibits a reversible redox wave at E° ) -0.53 V.23 With the MRH+ monolayer electrode (curve b), PQQ reveals its characteristic redox response but 3 shows an irreversible low amperometric signal. These results are attributed to the electrostatic interactions of 3 and PQQ (4) with the monolayer electrode. The electrochemical redox response of PQQ was found to be controlled by the electrical properties of monolayerfunctionalized electrodes.8a,18 At negatively-charged and neutral interfaces, the redox processes of PQQ are irreversible. Its redox process at positively-charged interfaces is reversible due to the electrostatic attraction (23) The redox wave of 3 is unsymmetrical, and the cathodic peak current, ipc , is higher than the anodic peak current, ipa. This is due to the appearance of the irreversible reduction wave of PQQ within the reduction signal of 3. This was independently confirmed by examination of the redox behavior of PQQ alone at the SP monolayer electrode. (24) Katz, E.; Willner, I. Electroanalysis 1995, 7, 417-419.

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Figure 5. Cyclic voltammograms at the photoisomerizable 1 monolayer electrode in the presence of PQQ, 3 × 10-4 M, and 3, 3 × 10-4 M: (a) at the SP monolayer electrode produced by illumination (λ > 495 nm); (b) at the MRH+ monolayer electrode produced by illumination (320 nm < λ < 350 nm). Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0; potential scan rate, 200 mV s-1. Scheme 3. pH-Controlled Electrochemical Oxidation of DOPA and DHPAA at a 1 Monolayer Electrode in the Merocyanine State

of PQQ. At the same time, repulsion of 3 from the positively-charged electrode surface yields the irreversible, inefficient reduction of 3. With the neutral SP monolayer electrode, 3 exhibits reversible redox features where the electrical signal of PQQ turns irreversible. By cyclic photoisomerization of the monolayer between the SP and MRH+ states, reversible selective redox processes of PQQ or 3 are stimulated (Scheme 2). The electrical properties of the functionalized monolayer can also be controlled by variation of the pH of the electrolyte or by thermal treatment of the electrode. At pH ) 7.0, the illuminated electrode, 320 nm < λ < 350 nm, exists in the protonated merocyanine state, MRH+. At pH ) 9.2, this monolayer is in the zwitterionic merocyanine configuration. As the redox probes DOPA or DHPAA retain their positive and negative charges at pH ) 7.0 and pH ) 9.2, respectively, the electrical responses of the electrode could be controlled by pH signals as well (Scheme 3). Figure 6A shows the cyclic voltammograms of DHPAA in the presence of the illuminated electrode, 320 nm < λ < 350 nm at pH ) 7.0 (curve a) and at pH ) 9.2 (curve b). The anodic current that corresponds to the oxidation of DHPAA is enhanced at pH ) 7.0 as compared to pH ) 9.2. This is attributed to the pH-

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Figure 7. Cyclic voltammograms at the isomerizable (1)monolayer electrode in the presence of DHPAA, 5 × 10-4 M: (a) SP monolayer electrode produced by illumination (λ > 495 nm) at 20 °C; (b) MRH+ monolayer electrode produced by heating at 60 °C; (c) MRH+ monolayer electrode at 20 °C; (d) SP monolayer electrode produced by illumination (λ > 495 nm) at 20 °C. Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0; potential scan rate, 200 mV s-1. (Inset) Reversible variation of anodic currents extracted from cyclic voltammograms of DHPAA at E ) 470 mV: (b) SP monolayer at 20 °C; (9) MRH+ monolayer at 60 °C; (2) MRH+ monolayer at 20 °C.

Figure 6. (A, top) Cyclic voltammograms at the 1 monolayer electrode in the MR(/MRH+ state produced by illumination (320 nm < λ < 350 nm) in the presence of (a) DHPAA, 5 × 10-4 M, pH ) 7.0; (b) DHPAA, 5 × 10-4 M, pH ) 9.2; (c) electrolyte only, pH ) 7.0; and (d) electrolyte only, pH ) 9.2. Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0 or pH ) 9.2, respectively; potential scan rate, 200 mV s-1. (Inset) Reversible transduction of anodic currents as a function of pH, elucidated from cyclic voltammograms of DHPAA at E ) 300 mV: (b) pH ) 7.0; (9) pH ) 9.2. (B, bottom) Cyclic voltammograms at the 1 monolayer electrode in the MR(/MRH+ state produced by illumination (320 nm < λ < 350 nm) in the presence of (a) DOPA, 5 × 10-4 M, pH ) 7.0; (b) DOPA, 5 × 10-4 M, pH ) 9.2; (c) electrolyte only, pH ) 7.0; (d) electrolyte only, pH ) 9.2. Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0 or pH ) 9.2, respectively; potential scan rate, 200 mV s-1. (Inset) Reversible transduced anodic currents at different pH values of the electrolyte, elucidated from cyclic voltammograms of DOPA at E ) 300 mV: (b) pH ) 7.0; (9) pH ) 9.2.

controlled electrical features of the monolayer electrode. At pH ) 7.0, the positively-charged monolayer attracts DHPAA and its oxidation is accelerated. The zwitterionic structure of the monolayer at pH ) 9.2 perturbs the localization of the negatively charged substrate at the monolayer, and its oxidation is inhibited. Note, however, that the onset oxidation potential of DHPAA occurs at less positive potentials at pH ) 9.2 as compared to pH ) 7.0. This is consistent with the pH-dependence of the oxidation of the dihydroxyphenyl unit, which is favored at basic pH values. By cyclic variation of the pH of the electrolyte between pH ) 7.0 and pH ) 9.2, high and low amperometric responses of the merocyanine monolayer electrode are transduced (inset, Figure 6A). Similar pHcontrolled electroactivities of DOPA are observed at the functionalized electrode. Figure 6B shows the cyclic voltammograms of DOPA in the presence of the illuminated electrode, 320 nm < λ < 350 nm (nitromerocyanine monolayer electrode) at pH 7.0 (curve a) and at pH ) 9.2 (curve b). The anodic current resulting from the oxidation of DOPA at pH ) 7.0 is low due to the repulsion

of the electroactive substrate from the electrode interface. At pH ) 9.2 the monolayer transforms into the zwitterionic merocyanine configuration and the electrocatalyzed oxidation of DOPA is enhanced. Note that the high amperometric response of DOPA, at pH ) 9.2, occurs at a less positive potential as compared to that for pH ) 7.0. This is consistent with the fact that oxidation of the dihydroxyphenyl unit is favored at basic pH values. By cyclic variation of the pH of the electrolyte between pH ) 7.0 and pH ) 9.2, reversible low- and high-current responses of DOPA are stimulated (inset, Figure 6B). These results demonstrate that the electrooxidation of DHPAA and DOPA is controlled by the functionalized merocyanine monolayer and triggered to “ON” and “OFF” states by the pH of the solution. The nitrospiropyran undergoes thermal ring-opening to the merocyanine state.24 Thus, thermal treatment of the SP monolayer at pH ) 7.0 generates the MRH+ monolayer interface capable of enhancing the electrooxidation of DHPAA (Scheme 2). Figure 7 shows the cyclic voltammograms of DHPAA in the presence of the SP monolayer (curve a) and after thermal treatment of the electrode at 60 °C (curve b). The amperometric signal of the thermally-treated electrode (60 °C) at 20 °C (curve c) is high and identical to that of the MRH+ electrode generated photochemically from the SP monolayer electrode. Thus, in the presence of the SP monolayer electrode, the electrical response of DHPAA is low, since the electroactive probe does not interact with the monolayer. Thermal treatment of the SP monolayer electrode (60 °C) results in isomerization to the MRH+ monolayer state, and the electrooxidation of the substrate is enhanced (curve b). Upon cooling the electrode to 20 °C, the electrode retains the MRH+ configuration and the high amperometric response of the electrode (curve c) is preserved. The slightly lower anodic current observed at 20 °C as compared to 60 °C is attributed to the temperature effect on the diffusion of DHPAA. Irradiation of the thermallytreated electrode, λ > 495 nm, results in the photoisomerization of the MRH+ monolayer to the SP monolayer state, and the low inactive electrode for DHPAA oxidation is regenerated (curve d). By cyclic thermal and photochemical triggering of the SP monolayer electrode, an active

Functionalized Spiropyran Monolayer Electrodes

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Scheme 4. Photostimulated Electrochemical Oxidation of DOPA and DHPAA at a 2 Monolayer Electrode in the Presence of DNP-Ab

Figure 8. Cyclic voltammograms at the photoisomerizable 2 monolayer electrode in the presence of (a) Electrolyte only at the MRH+ monolayer electrode produced by illumination (320 nm < λ < 350 nm); (b) DHPAA, 5 × 10-4 M, at the MRH+ monolayer electrode; (c) DHPAA, 5 × 10-4 M, at the SP monolayer electrode produced by illumination (λ > 495 nm) and treated with a 5 µg‚mL-1 DNP-Ab solution for 5 min; (d) DOPA, 5 × 10-4 M, at the MRH+ monolayer electrode; and (e) DOPA, 5 × 10-4 M, at the SP monolayer electrode treated with a 5 µg‚mL-1 DNP-Ab solution for 5 min. Background electrolyte, 0.02 M phosphate buffer, pH ) 7.0; potential scan rate, 200 mV s-1. (Inset) Reversible amperometric transduction of optical signals recorded by the dinitrospiropyran 2 photoisomerizable monolayer in the presence of DNP-Ab. values elucidated from the cyclic voltammograms at E ) 470 mV: (O) DOPA at the MRH+ monolayer; (4) DHPAA at the MRH+ monolayer; (b) DOPA at the SP monolayer treated with DNP-Ab; (2) DHPAA at the SP monolayer treated with DNP-Ab.

and inactive electrode interface for the oxidation of the substrate is obtained (inset, Figure 7). The thermal isomerization of the SP monolayer to the MRH+ state represents a thermochromic assembly, while the lightinduced isomerization of the MRH+ monolayer to the SP state is a photochromic transformation. Coupling of the functionalized monolayer electrode to the oxidation of DHPAA enables the amperometric transduction of the thermochromic and photochromic processes at the monolayer interface. A further means to control the electrical properties of a spiropyran monolayer-modified electrode involves the modification of the electrode with a dinitrospiropyran 2-monolayer and coupling of the functionalized electrode to the dinitrophenyl antibody, DNP-Ab. Previous studies14 have revealed that the photoisomerizable dinitrospiropyran shows photoswitchable antigen affinities for DNPAb. Dinitrospiropyran exhibits high affinity for DNP-Ab while the protonated dinitromerocyanine state lacks affinity for the antibody. Furthermore, it was shown that the association of DNP-Ab to a dinitromerocyanine monolayer insulates the electrode toward a redox probe in solution.14a This allows the addition of a further element to control by light the electrical performance of the functionalized electrode for electrooxidation of DOPA and DHPAA, (Scheme 4). The DNP-Ab associates to the dinitrospiropyran monolayer electrode. This insulates the electrode interface, and electrooxidation of DOPA and DHPAA is blocked. Photoisomerization of the monolayer to the protonated dinitromerocyanine states results in a monolayer that lacks affinity for the antibody that is released. The positively charged interface enhances the oxidation of DHPAA while electrooxidation of DOPA is inhibited. Dinitrospiropyran 2 was assembled on the Au electrode similarly to the organization of the 1 monolayer. Figure 8 shows the cyclic voltammograms of DHPAA and DOPA in the absence and presence of DNP-Ab and upon photoactivation of the monolayer to the respective isomer

states. With DHPAA as substrate, the protonated merocyanine monolayer electrode yields a high amperometric signal (curve b) due to the electrostatic attraction of the substrate to the monolayer. Photoisomerization of the monolayer to the dinitrospiropyran state completely blocks the electrical response of the electrode (curve c), since DNP-Ab associates to the electrode. Similarly, when DOPA is applied as a substrate, a low current response is observed with the protonated dinitromerocyanine monolayer electrode, since the electroactive material is repelled from the electrode surface (curve d). Photoisomerization of the monolayer to the dinitrospiropyran configuration blocks the electrical response of DOPA (curve e) due to the binding of the DNP-Ab monolayer. Note that the dinitrospiropyran 2 monolayer electrode behaves differently for oxidation of DOPA in the absence and presence of DNP-Ab. The dinitrospiropyran monolayer electrode induces the effective oxidation of DOPA, since no electrical repulsion between the electrode interface and the substrate is operative. The electrooxidation of the substrate is, however, completely blocked in the presence of DNP-Ab. By cyclic photoisomerization of the monolayer between the protonated dinitromerocyanine monolayer state and the dinitrospiropyran configuration in the presence of DNP-Ab, reversible effective oxidation of DHPAA and inefficient oxidation of DOPA are triggered ON and complete blocking of the electrooxidation of the two substrates is affected, respectively (inset Figure 8). Conclusions This study described the application of photoisomerizable (or thermoisomerizable) monolayers assembled on Au electrodes as functionalized interfaces that control the electrooxidation of DOPA and DHPAA by means of electrostatic interactions (Frumkin effect).19 The positivelycharged protonated merocyanine monolayer, which is the active interface in controlling the electrooxidation of DOPA

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or DHPAA, was generated photochemically or thermally or by pH-variation of the electrolyte medium. The complementary nitrospiropyran monolayer was always generated by photochemical isomerization of the protonated merocyanine monolayer state. Thus, the functionalized electrode provides a means for the amperometric transduction of optical, thermal, or pH signals recorded by the monolayer. Furthermore, the study reveals a novel method to induce by light, thermal, or pH signals selectivity in electrochemical transformations. Finally,

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the study demonstrated that the application of a photoisomerizable antigen monolayer electrode consisting of a dinitrospiropyran monolayer enables the control of the electrooxidation of DHPAA and DOPA in the presence of DNP-Ab. Acknowledgment. This study was supported by a grant from The Israel Science Foundation. LA960729E