Langmuir 2003, 19, 3779-3784
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Metal-Ion Chelation and Sensing Using a Self-Assembled Molecular Photoswitch Zhenxin Wang, Michael J. Cook,* Anne-Mette Nygård, and David A. Russell* School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, Norfolk NR4 7TJ, United Kingdom Received November 4, 2002. In Final Form: February 13, 2003 The novel (pyridylazo)phenol derivative 8-[4-(2-pyridylazo)phenoxy]octyl disulfide (1) has been designed and synthesized for the formation of a monolayer film on gold-coated optical waveguides via the technique of self-assembly. The photoinduced trans to cis isomerization of the self-assembled monolayer of 1 (1 SAM) and its interaction with transition-metal ions have been studied by spectroscopic (UV-visible and reflection absorption infrared) and electrochemical methods. The 1 SAM in the trans isomeric form provides a bidentate ligand that can chelate metal ions, such as Ni2+ and Co2+, and results in a strong absorption in the UV-visible absorption spectrum at 433 nm, a red shift of 77 nm from the maximum absorption wavelength for the unchelated 1 SAM. Following irradiation with waveguided UV light (λ ) 365 nm) via the evanescent field, the 1 SAM photoisomerizes to the cis form. No metal chelation was observed with the cis isomer of the 1 SAM. The photoinduced processes are reversible; the cis form SAM is converted to the trans form SAM after visible light irradiation (λ ) 439 nm), again via the evanescent wave. The trans 1 SAM was used to detect Ni2+ in the range 9.6 × 10-5-2.44 × 10-3 M.
Introduction Azobenzenes and related derivatives have been the subject of extensive studies because these compounds are both photon- and redox-active.1-27 The optical and elec* Authors to whom correspondence should be addressed. E-mail:
[email protected]. (1) Yu, H.-Z.; Ye, S.; Zhang, H.-L.; Uosaki, K.; Liu, Z.-F. Langmuir 2000, 16, 6984-6954. (2) Yu, H.-Z.; Zhang, H.-L.; Liu, Z.-F.; Ye, S.; Uosaki, K. Langmuir 1998, 14, 619-624. (3) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 10211-10219. (4) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071-6082. (5) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034-8042. (6) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.-I.; Ichimura, K. Langmuir 1993, 9, 211-218. (7) Moss, R. A.; Jiang, W. Langmuir 1995, 11, 4217-4221. (8) Sekkat, Z.; Wood, J.; Geerts, Y.; Knoll, W. Langmuir 1995, 11, 2856-2859. (9) Wu, A.; Talham, D. R. Langmuir 2000, 16, 7449-7456. (10) Zhang, J.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2001, 13, 2323-2331. (11) Dante, S.; Advincula, R.; Frank, C. W.; Stroeve, P. Langmuir 1999, 15, 193-201. (12) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955-962. (13) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915-1925. (14) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658660. (15) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873-1875. (16) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636-6640. (17) Ichimura, K.; Hayashi, Y.; Akiyama, H.; Ishizuki, N. Langmuir 1993, 9, 3298-3304. (18) Wang, R.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1997, 13, 4644-4651. (19) Cook, M. J.; Nygård, A.-M.; Wang, Z.; Russell, D. A. Chem. Commun. 2002, 1056-1057. (20) Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464-1470. (21) Wang, R.; Iyoda, T.; Jiang, L.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 438, 213-219. (22) Liu, Z. F.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1992, 324, 259-267. (23) Kondo, T.; Kanai, T.; Uosaki, K. Langmuir 2001, 17, 63176324. (24) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436-6440.
trochemical properties of such materials provide the basis for a number of potential technological applications, such as in photoswitching,15 optical information storage,14 nonlinear,5 and electrooptic3,4 devices. Consequently, during the past decade there has been a considerable research effort to develop stabilized structures of azobenzene derivatives, predominantly based on the surfacefilm techniques of Langmuir-Blodgett6,9,25 and selfassembly.1-5,8-10,12,14,18-24,26 Theformationandcharacterization of electro- and photoactive self-assembled monolayers (SAMs) constructed using functionalized molecules, which incorporate a disulfide or thiol moiety to tether the molecule to either planar or nanoparticulate gold surfaces, are seen as important routes in the development of the next generation of optical and electronic materials. Previously, a (thiazolylazo)phenol derivative formulated as a Langmuir-Blodgett film has been used to complex metal ions.25 We are interested in developing a photoactive, metal-chelating monolayer surface capable of reporting metal-ion concentrations that is driven by waveguided light. To achieve this aim, the new (pyridylazo)phenol derivative 1 has been synthesized and formulated as a SAM on a planar gold-coated surface, with the glass substrate acting as an optical waveguide. The SAM of the (pyridylazo)phenol derivative (1 SAM) forms a photoswitchable surface that can be manipulated using light leaking from the substrate via the evanescent field. UV light (λ1 ) 365 nm) switches the (pyridylazo)phenol derivative from the trans to the cis isomeric form. The trans isomer can be recovered from the cis form with evanescent visible-light (λ2 ) 439 nm) irradiation. The potential for remotely controlling molecular-binding events at such a photoswitchable surface is assessed by chelating the first-row transition metals, Co2+ and Ni2+, with the trans form of the 1 SAM to form a bidentate-ligated compound. Such binding is shown not to occur with the (25) Liu, M.; Ushida, K.; Kira, A.; Nakahara, H. J. Phys. Chem. B 1997, 101, 1101-1104. (26) Saremi, F.; Tieke, B.; Adv. Mater. 1998, 10, 389-391. (27) Hong, J.-D.; Park, E.-S.; Park, A.-L. Langmuir 1999, 15, 65156521.
10.1021/la026792n CCC: $25.00 © 2003 American Chemical Society Published on Web 03/20/2003
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Reagents and Synthetic Methods. The chemical reagents and solvents (Aldrich) employed in the synthesis were used as supplied. The 1H and 13C NMR spectra were recorded at 270 and 67.5 MHz, respectively, using a JEOL EX 270 spectrometer. The low-resolution mass spectra were obtained using a Kratos MS 25 mass spectrometer. The high-resolution fast atom bombardment (FAB; laser ionization mass spectrometry) mass spectra were measured using a Finnegan MAT 900 mass spectrometer. The photoswitch compound (1) deposited as a SAM was prepared according to Scheme 1. 4-(2-Pyridylazo)phenol28 was prepared from 2-hydrazinopyridine29 and benzoquinone via literature procedures. All solvents used in the SAM experiments were of spectroscopic grade. Milli-Q water was used in the aqueous solutions. 8-Hydroxyoctyl Disulfide. 8-Bromooctanol (4.89 g, 23 mmol) and thiourea (2 equiv, 3.50 g, 46 mmol) in ethanol (40 mL) were heated to reflux under a nitrogen atmosphere. After 22 h, the reaction mixture was allowed to cool to room temperature. A saturated NaOH solution (3 mL) was added, and the mixture was brought to reflux for 1 h. When cool, the reaction mixture was acidified (concentrated HCl) and extracted with ethyl acetate (3 × 50 mL), and the extract was dried (MgSO4). The removal of the solvents under reduced pressure afforded a yellow oil that solidified upon standing. The solid was dissolved in methanol (25 mL), and a saturated solution of iodine in methanol was added until no further discoloration was apparent. The solvent was removed under reduced pressure to yield a light-brown oil that solidified upon standing. The crude product was recrystallized from water/methanol, dried, and then recrystallized from ether to afford 8-hydroxyoctyl disulfide as a light-yellow powder. Mp: 54-55 °C. Yield: 4.70 g, 62%. Anal. Calcd for C16H34O2S2: C, 59.58; H, 10.63. Found: C, 59.70; H, 10.30. 1H NMR (270 MHz, CDCl3) δ: 3.65 (t, J ) 6.6 Hz, 4 H), 2.68 (t, J ) 7.2 Hz, 4 H), 1.7-1.2 (m, 24 H). 13C NMR (67.5 MHz, CDCl3) δ: 62.90 (CH2), 39.04 (CH2), 32.62 (CH2), 29.16 (CH2), 29.05 (CH2), 28.29 (CH2), 25.56 (CH2). MS m/e: 322 (M+). 8-Methylsulfonyloxyoctyl Disulfide. Triethylamine (0.62 g, 6.2 mmol) and methanesulfonyl chloride (0.71 g, 6.2 mmol) were added to a cold (0 °C) solution of 8-hydroxyoctyl disulfide (1.0 g, 3.1 mmol) in dichloromethane (10 mL). The reaction mixture was left to reach room temperature, was then washed with HCl (10%, 20 mL) and a 10% sodium bicarbonate solution (20 mL), and was extracted with dichloromethane (3 × 20 mL). The combined organic extracts were dried (MgSO4) and evaporated to dryness, and the residue was recrystallized from ether to afford 8-methylsulfonyloxyoctyl disulfide as a pale-yellow solid. Mp: 41-42 °C. Yield: 0.76 g, 54%. Anal. Calcd for C18H38O6S4: C, 45.15; H, 8.00. Found: C, 45.43; H, 7.97. 1H NMR (270 MHz, CDCl3) δ: 4.20 (t, J ) 6.6 Hz, 4 H), 2.98 (s, 6 H), 2.65 (t, J ) 7.4 Hz, 4 H), 1.8-1.6 (m, 8 H), 1.4-1.2 (m, 16 H). 13C NMR (67.5 MHz, CDCl3) δ: 70.00 (CH2), 38.90 (CH2), 37.26 (CH3), 29.05 (CH2), 28.98 (CH2), 28.89 (CH2), 28.76 (CH2), 28.20 (CH2), 25.24 (CH2). MS m/e: 450 (M+ - 2CH3), 382 (M+ - CH3SO3).
8-[4-(2-Pyridylazo)phenoxy]octyl Disulfide (1). 8-Methylsulfonyloxyoctyl disulfide (0.70 g, 1.46 mmol) and 4-(2pyridylazo)phenol (2 equiv, 0.58 g, 2.92 mmol) were dissolved in anhydrous dimethylformamide (20 mL) under a nitrogen atmosphere, and excess K2CO3 (0.5 g, 4 mmol) was added. The mixture was stirred at room temperature under nitrogen for 7 days. The reaction mixture was poured onto water (50 mL) and extracted with ethyl acetate (3 × 30 mL). The combined organic extracts were dried (brine and MgSO4) and concentrated. Repeated column chromatography (eluent 4:1 and 9:1 toluene/methanol) separated an orange solid, which was recrystallized (dichloromethane/light petroleum ether) to afford 1 as an orange powder. Mp: 99-101 °C. Yield: 0.26 g, 41%. Anal. Calcd for C38H48N6O2S2: C, 66.63; H, 7.06; N, 12.27. Found: C, 66.73; H, 7.03; N, 11.65. 1H NMR (270 MHz, CDCl3) δ: 8.69 (m, 2 H), 8.03 (m, 4 H), 7.85 (m, 2 H), 7.77 (m, 2 H), 7.34 (m, 2 H), 6.99 (m, 4 H), 4.02 (t, J ) 6.6 Hz, 4 H), 2.67 (t, J ) 7.4 Hz, 4 H), 1.82 (quintet, J ) 6.6 Hz, 4 H), 1.65 (quintet, J ) 6.8 Hz, 4 H), 1.5-1.3 (m, 16 H). 13C NMR (67.5 MHz, CDCl3) δ: 162.90 (Cq), 162.56 (Cq), 149.28 (CH), 146.58 (Cq), 138.12 (CH), 125.61 (CH), 124.50 (CH), 115.07 (CH), 114.62 (CH), 68.24 (CH2), 38.997 (CH2), 29.08 (CH2), 28.99 (CH2), 28.28 (CH2), 25.81 (CH2). MS (FAB): [M + H] calcd, 685.3358; found, 685.3365. Formation of the SAM Photoswitch. Gold-coated glass substrates (cut to the dimensions of a standard microscope slide) were used for the formation of the SAMs. The types of glass substrate and thicknesses of the metal layer used were dependent upon the spectroscopic and electrochemical techniques used. [For the reflection absorption infrared (RAIR) spectroscopic measurements, 5-nm-Cr/45-nm-Au glass microscope slides (BDH Ltd., U.K.) were used; for the UV-visible measurements, 1-nm-Cr/ 8-nm-Au quartz slides (Chemglass, U.S.A./Crystran Ltd., U.K.) were used; and for the electrochemical measurements, 1-nmCr/8-nm-Au ITO glass substrates (Crystran Ltd., U.K.) were used.] In each case, the glass substrate was cleaned and the metals chromium (to aid adhesion) and gold were thermally evaporated (Edwards Auto 306 vacuum evaporator) onto the surface, as previously described.30,31 The SAMs were formed by immersing the freshly prepared gold-coated substrate in an ethanolic solution of 1 (3.50 × 10-5 M) for more than 15 h. The coated substrates were then washed in ethanol and dried in a stream of argon. Spectroscopic and Electrochemical Characterization. RAIR measurements of the 1 SAM were performed using a BioRad FTS40 Fourier transform infrared spectrometer coupled with a Spectra-Tech FT85 reflectance unit, using p-polarized light at an incidence angle of 85°. The spectra were obtained from the coaddition of 1500 scans at a resolution of 4 cm-1. The UV-visible spectra of the photoswitch monolayer were obtained using a Hitachi U-3000 spectrophotometer, with the corresponding reference-blank gold-coated substrate placed in the reference beam. Cyclic voltammograms (CVs) were recorded using a µAutolab II electrochemical analyzer (Autolab, The Netherlands) with an in-house-built electrochemical cell. A gold-coated ITO glass substrate (ca. 4 cm2, bare or coated with 1 SAM) was used as the working electrode, a saturated Ag/AgCl electrode as the reference electrode, and a platinum disk as the counter electrode. During the electrochemical experiment, a phosphate buffer solution (pH ) 6.92, 0.05 M) was used. Photoswitching and Chelation Experiments. The freshly prepared 1 SAMs were irradiated using UV light (λ1 ) 365 nm) via the evanescent wave for 9 h to obtain the cis form of the 1 SAM. The reverse photoswitch, that is, the formation of the trans isomeric form, required that the SAM be irradiated using evanescent-wave visible light (λ2 ) 439 nm) again for 9 h. To chelate the Ni2+ or Co2+ metal ions from solution, the freshly prepared 1 SAM (in either the trans or cis form) was immersed into a solution of the metal nitrate (2.38 × 10-3 M) for 24 h to form the respective Ni2+ or Co2+ bidentate-ligand compound. Prior to spectral or electrochemical measurement of the 1 SAMs,
(28) Alhaider, A. A.; Abdelkader, M. A.; Lien, E. J. J. Med. Chem. 1985, 28, 1394-1398. (29) Betteridge, D.; John, D. Analyst (Cambridge, U.K.) 1973, 98, 377-389.
(30) Revell, D. J.; Chambrier, I.; Cook, M. J.; Russell, D. A. J. Mater. Chem. 2000, 10, 31-37. (31) Simpson, T. R. E.; Revell, D. J.; Cook, M. J.; Russell, D. A. Langmuir 1997, 13, 460-464.
cis isomeric form. Subsequently, the use of the photoswitchable SAM as a chemical sensor was established through the detection of a range of Ni2+ concentrations. Experimental Section
Metal-Ion Chelation and Sensing
Langmuir, Vol. 19, No. 9, 2003 3781 Table 1. Absorption Maxima Dataa of 1 with Ni2+ or Co2+ trans isomer λmax (nm) cis isomer λmax (nm)
1
1 + Ni2+
1 + Co2+
356 330
413 427
401 409
a The concentrations of 1, Co2+, and Ni2+ were 1.70 × 10-5, 4.88 × 10-4, and 3.55 × 10-4 M, respectively.
Figure 1. UV-visible absorption spectra of 1 in ethyl acetate before (solid line) and after (dashed line) UV irradiation and in the presence of Ni2+ before (dotted line) and after (dotteddashed line) UV irradiation. the 1 SAMs with a coordinated metal ion and their respective references were washed in a solvent, dried in a stream of argon, and then stored in clean amber sample jars with airtight lids.
Results and Discussion Photoisomerization of 1 in Solution. The spectra of the photoisomers of 1 (8.76 × 10-6 M) in ethyl acetate before and after UV irradiation (λ ) 365 nm) are shown in the UV-visible absorption spectra (Figure 1). As previously reported for other azobenzene derivatives, the trans form of 1 is the stable, dominant isomer prior to irradiation by UV light. The spectrum of the trans isomer of 1 exhibits an intense π-π* transition band at 356 nm ( ) 2.89 × 104 M-1 cm-1) and weak n-π* absorption at 433 nm ( ) 1.59 × 103 M-1 cm-1).9,10 The broad absorption band at about 233 nm ( ) 1.45 × 104 M-1 cm-1) has been previously attributed to a π-π* electronic-transition moment roughly parallel to the short axis of the trans azobenzene chromophore (sNdNs).9,11 Following UV irradiation, the π-π* transition band of 1 blue shifts from 356 to 330 nm ( ) 1.18 × 104 M-1 cm-1), and the n-π* absorption band becomes more intense ( ) 2.40 × 103 M-1 cm-1), suggesting that the trans form of 1 has converted to the cis form in solution.9,12 The cis form of the (pyridylazo)phenol derivative can be converted back to the trans isomer by irradiating the solution with visible light (λ ) 439 nm) for a few minutes or after thermal relaxation (put in the dark at room temperature) for about 24 h. Chelation of Ni2+ and Co2+ by 1 in Solution. The UV-visible spectra of the two isomers of 1 in the presence of Ni2+ are also shown in Figure 1. It is apparent that both isomers can bind to the Ni2+ ions to form coordinated compounds. The absorption λmax data for the chelation of Ni2+ and Co2+ with 1 in solution are shown in Table 1. The large red shift of the absorption maximum upon the
Figure 2. (A) IR spectrum of 1 formulated as a KBr disk, (B) RAIR spectrum of the 1 SAM, and (C) RAIR spectrum of the 1 SAM following UV irradiation.
binding of the metal ion, 57 nm for Ni2+, is similar to that observed for the parent pyridine-azo complex in solution.32 RAIR Spectroscopic Characterization of the 1 SAM Photoswitch. RAIR spectroscopy was used to establish whether the 1 SAM had been deposited on the substrate and to determine whether cis-trans isomerization could be detected from the gold-coated surface. The RAIR spectra of the 1 SAM, before and after irradiation with UV light, together with the IR spectrum of 1 in a KBr pellet are shown in Figure 2. It is apparent that the RAIR spectrum of the as-prepared 1 SAM (Figure 2B) is similar to that of 1 in the KBr pellet, suggesting that the stable isomer of the SAM is the trans form. Upon UV irradiation, some spectral changes are observed in the RAIR spectrum (Figure 2C) of the 1 SAM. The intensities of some of the absorption bands decrease and shift in wavelength, suggesting that, following irradiation, the molecular orientation of the photoswitch on the surface has changed; upon visible-light irradiation, the absorption bands return to their original intensities and positions. Specifically, the absorption bands at 1603 and 1583 cm-1, assigned to the C5H4N ring stretching vibration and C6H5 ring stretching vibration, shift to higher wavenumbers, namely, 1617 and 1595 cm-1, with decreased intensities. The absorption band at 1139 cm-1, assigned to the phenyl-N stretch, splits to two bands at 1155 and 1130 cm-1. These spectral variations are consistent with the molecular orientational changes expected following photoswitching from the trans to the cis isomeric forms of the SAM following UV irradiation. The assignments of the absorption bands seen in Figure 2 are summarized in Table 2. UV-Visible Study of the 1 SAM. The photoisomerization of the 1 SAM was investigated by UV spectroscopy; the spectra of the two isomers are shown in Figure 3 (solid and dashed lines). The as-prepared (trans isomer) SAM exhibits a strong π-π* transition at 356 nm and a medium absorption at 251 nm (not shown). The long-wavelength (32) Klotz, I. M.; Loh Ming, W.-C. J. Am. Chem. Soc. 1953, 75, 41594162.
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Table 2. Assignment of RAIR and IR Absorption Bands18,30 absorption bands (cm-1)
a
KBr
SAMs
assignment
2923 and 2853 1603 and 1583 1501 1472 and 1464 1257 1139
2918 and 2853 1603 and 1586 (1617 and 1595)a 1501 (1501)a 1472 and 1456 (1468 and 1455)a 1257 (1257)a 1139 (1155 and 1130)a
νas(CH2) and νs(CH2), respectively C5H4N and C6H5 ring stretching vibration C6H5 ring stretching vibration CH2 deformation [δ(CH2)] ν(CN) phenyl-N stretch
Absorption bands following UV irradiation for 3 h.
Figure 3. UV-visible absorption spectra of the 1 SAM: spectrum of the as-prepared SAM (solid line), spectrum of the SAM following UV irradiation (dashed line), and spectrum of the SAM following subsequent visible irradiation (dotted line).
absorption band of the 1 SAM is similar to that of the photoswitch in solution, although there is a red shift of the short-wavelength band. Fox, Whitesell, and co-workers have previously reported that stilbene derivatives selfassembled on planar gold-coated surfaces are unable to undergo trans-cis photoisomerization,10,12,20 although the same authors have shown recently that azobenzene derivatives, similar to 1, can photoisomerize on gold nanoparticle surfaces.10,20 These authors suggested that the closely packed structure of the SAMs on the planar gold surface might prevent the photon-induced isomerization. However, upon UV irradiation of the 1 SAM, the absorption band at 356 nm decreases in intensity, which suggests that the trans form of the SAM has, indeed, been converted to the cis isomer. In this photoisomerization process, the absorption band has not blue shifted upon formation of the cis isomer, as observed with 1 in solution (cf. Figure 1). It is possible that the formation of the cis isomer in the SAM is not as complete as that observed in solution. Following waveguided irradiation (with visible light for 9 h) of the cis isomer of the 1 SAM, the spectrum reverted back to the trans form (Figure 3, dotted line). A similar recovery was observed after thermal relaxation (the SAM stored in the dark at room temperature) for >14 days. It is apparent that a full recovery of the spectrum to that of the original as-deposited SAM did not occur, possibly as a result of the incomplete photoisomerization of the molecules on the gold surface from the cis form or as a result of the polarization of the incident-light beam.
Figure 4. CVs of the 1 SAM before (solid line) and after (dashed line) UV irradiation. Following further visible-light irradiation, the CV represented by the dotted line was obtained. The inset shows the CVs of the two isomers of the 1 SAM following Ni2+ binding: the trans (solid line) and cis isomers (dotted-dashed line) formed following UV irradiation.
The time taken for the photoisomerization using waveguided light, namely, 9 h, was considerably shortened when direct illumination, with the respective light, of the SAM surface was used. Typically, the trans-cis (or cis-trans) photoisomerization with direct illumination would take 3 h. This suggests that further optimization of the coupling efficiency of the waveguided light would shorten the time taken for the photoisomerization of the SAM. Electrochemical Study of the 1 SAM. Electrochemical studies were undertaken to probe further the extent of conversion of the trans to cis isomerization in the SAM. Figure 4 shows the CVs of the 1 SAM modified gold electrode before and after light irradiation measured in 0.05 M phospate-buffered saline (pH ) 6.92) with a potential sweep rate of 20 mV s-1 between +0.25 and -0.5 V. No faradaic current was observed while a blank gold film substrate was used as the working electrode. Before UV light irradiation, the 1 SAM exhibited good electrochemical behavior, with oxidation and reduction peaks due to the redox of the 1 SAM observed at +0.083 and -0.390 V, respectively (Figure 4, solid line). The cationic peak current was larger than the anionic peak current and was linear with the scan rates (1-20 mV s-1; data not shown). Following UV irradiation, the shape of the CV changed, with both the cationic peak current and anodic peak current decreasing (Figure 4, dashed line). This result
Metal-Ion Chelation and Sensing
is in contrast to that obtained for azobenzene SAMs,1,2,14,21,22 where the cis isomer of the SAM exhibits good electrochemical behavior. The difference in the CVs between azobenzene SAMs and the 1 SAM is likely to be a consequence of the structural difference between azobenzene and the current compound, which possesses a pyridyl moiety. This moiety would enable the trans isomer of the 1 SAM to be readily reduced to the hydraazobenzene derivative. Upon irradiation with visible light, the CV reverted back to its original shape (Figure 4, dotted line), which again confirms that the photoisomerization from the cis to the trans form of the 1 SAM was occurring at the waveguide surface. The electrochemical data clearly show that the 1 SAM readily converts from the trans to the cis and back to the trans isomer upon irradiation with waveguided light of the appropriate wavelength. Further electrochemical studies were undertaken to establish the stability of the (pyridylazo)phenol derivative 1 formulated as a SAM. Identical CVs were obtained from the 1 SAM following two complete photoisomerization cycles (viz. trans-cis-trans-cis-trans). Similarly, an identical CV was obtained after the SAM was stored in a sample jar for 1 month. There has been a great deal of interest in the oxidative stability of the SAMs following exposure to UV light.33-35 Typically, the wavelength of light used to initiate photooxidation is 254 nm or lower.35 In this current study, the wavelength used to produce the cis isomer was 365 nm. It is thought that the energy of the light at this wavelength is insufficient to oxidize the thiolate root at the gold surface. Indeed, the aromatic nature of the terminal moieties of the 1 SAM may afford a degree of protection of the thiol moiety from oxidation, as previously observed with macrocycles self-assembled onto planar gold surfaces.30 The surface coverage of the redox-active centers on the electrode (Γ) of the 1 SAM was calculated by integrating the charge (Q) passing on each reduction peak at 10 mV s-1 or the peak current using the equations36
Langmuir, Vol. 19, No. 9, 2003 3783
Figure 5. UV-visible absorption spectra for the two isomers of the 1 SAM following interaction with Ni2+: the trans (solid line) and cis isomers (dotted-dashed line).
Γ ) Q/nFA or ip ) n2F2ΓAv/4RT where A is the electrode surface area, F is Faraday’s constant, and n is the number of electrons involved in the electrode reaction. A value of Γ ) 6.21 × 10-10 mol cm-2 was obtained. It should be noted that the true electrode area, rather than the geometric surface area, should be used to estimate the surface coverage. However, a true electrode area is difficult to determine because of the microscopic roughness of the gold electrode surface. Even though the value of Γ reported herein should only be considered as an approximate value, it should be noted that it is in agreement with those values previously reported37,38 for azobenzene SAMs, which range from 1 × 10-10 to 8 × 10-10 mol cm-2. Interaction of the 1 SAM with Ni2+ and Co2+. The CVs of Ni2+ binding with the 1 SAM in both the trans and the cis isomeric forms are shown in Figure 4 (inset). Both the cationic and the anionic peak currents decrease, as (33) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 33423343. (34) Horn, A. B.; Russell, D. A.; Shorthouse, L. J.; Simpson, T. R. E. J. Chem. Soc., Faraday Trans. 1996, 92, 4759-4762. (35) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089-4090. (36) Wang, J. Analytical Electrochemistry, 2nd ed.; Wiley: New York, 2000. (37) Yu, H.-Z.; Wang, Y.-Q.; Cheng, J.-Z.; Zhao, J. W.; Cai, S.-M.; Inokuchi, H.; Fujishima, A.; Liu, Z.-F. Langmuir 1996, 12, 2843-2848. (38) Zhang, W.-W.; Ren, X.-M.; Li, H.-F.; Lu, C.-S.; Hu, C.-J.; Zhu, H.-Z.; Meng, Q.-J. J. Colloid Interface Sci. 2002, 255, 150-157.
Figure 6. Changing UV-visible absorption spectra of the 1 SAM with increasing concentrations of Ni2+ [0 (solid line), 9.6 × 10-5 (dashed line), 2.88 × 10-4 (dotted line), 1.22 × 10-3 (dotted-dashed line), and 2.44 × 10-3 M (dashed-dotteddotted line)]. The inset shows the calibration curves obtained by monitoring the change in absorbance at 356 (circles) and 433 nm (squares) as a function of added Ni2+ concentration.
compared with the trans 1 SAM, suggesting that an electrochemically inactive compound forms upon Ni2+ binding with the 1 SAM. Using the peak-current ratio of the 1 SAM and Ni2+-1 SAM, it was estimated that about 80% of the 1 SAM is binding with Ni2+. No significant change in the CV was observed when the Ni2+ ions were added to the 1 SAM in the cis form. This suggests that the cis isomeric form of the SAM cannot chelate the Ni2+ ions. A similar result was obtained when Co2+ ions replaced the Ni2+ ions.
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Figure 7. Diagrammatic summary of the photoswitching and metal-ion chelation of the 1 SAM.
The UV-visible absorption spectra of the Ni2+ ions binding with the cis and trans 1 SAM are shown in Figure 5. No significant change was observed in the absorption spectrum when the metal ions were added to the cis form of the 1 SAM (cf. Figures 3 and 5). A similar result was obtained with the Co2+ ions. However, with the trans isomer 1 SAM, the spectral band shape becomes broad and the maximum absorption wavelength shifts from 356 to 433 nm when the Ni2+ ions are ligated with the SAM photoswitch. These spectral results indicate that the Ni2+ ions (and those of Co2+) can be chelated by the trans form of the 1 SAM, whereas no chelation can occur when the SAM is in the cis isomeric form. This result is different from that in solution, where the metal ions could be chelated by both the cis and the trans isomers. It is possible that in solution two molecules of the 1 azobenzene derivative are required to chelate the metal ions when the photoswitch is in the cis form. Such a coordination would not be possible with the photoswitch self-assembled on the planar surface. Following the successful chelation of the metal ions to the trans 1 SAM isomer, the photoswitch was assessed for use as a chemical sensor for the detection of Ni2+ ions. The UV-visible absorption spectra as a function of the Ni2+ concentration are shown in Figure 6. Two broad absorption bands, centered at 356 and 433 nm, with an isosbestic point at about 385 nm, are evident in this figure. The two absorption bands are due to the unchelated and Ni2+-chelated 1 SAM, respectively. The calibration curves of absorbance intensity, at the peak maxima at 356 and 433 nm, versus the Ni2+ concentration are plotted in Figure 6 (inset). From the range of the Ni2+ concentrations studied, namely, 9.6 × 10-5-2.44 × 10-3 M, it is apparent that the formation constant of the 1 SAM chelate is low. This would be in agreement with the data for the parent pyridine-azo complex, which is reported32 to have a value of Kf for the Ni2+ chelate of 1.72 × 104. The data shown
in Figure 6 were collected from one sensor surface over a period of 16 h of continual use. Not only do the data show the stability of the surface, but also they show the potential of this photoswitch for use as a chemical sensor. Conclusion The formation of a metal-ion-chelating SAM photoswitch has been described. Once deposited, the SAM forms the stable trans isomer of the (pyridylazo)phenol derivative. The monolayer can be photoisomerized using UV light via the evanescent wave to the inactive cis isomer and, subsequently, returned to the trans isomer using visible light, again via the evanescent wave. Characterization of the SAM photoswitch was achieved using UVvisible and infrared spectroscopies and electrochemical techniques. Chelation of the Ni2+ and Co2+ metal ions occurred only when the SAM photoswitch was in the trans isomeric form. The overall scheme of photoswitching and metal-ion chelation is shown in Figure 7. The active trans isomer of the 1 SAM was used to detect the Ni2+ ions in the concentration range of 9.6 × 10-5-2.44 × 10-3 M. This paper has shown that the ability to switch the 1 SAM from the trans to the cis isomeric forms using waveguided light enables the surface to be modulated from an “active” to an “inactive” receptor, which not only enables the sensing of the metal ions but also will have other molecular applications, such as in photon-induced metalion transport. Acknowledgment. This work was financially supported by the EPSRC (Grant GR/M07434) and the CEC (Contract HPRN-CT2000-00004). The mass spectrometry service of the EPSRC, Swansea, is gratefully acknowledged for the MS data for compound characterization. LA026792N
