Electrochemically Tunable Surface-Plasmon-Enhanced Diffraction

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Electrochemically Tunable Surface-Plasmon-Enhanced Diffraction Gratings and Their (Bio-)sensing Applications Shengjun Tian,† Neal R. Armstrong,‡ and Wolfgang Knoll*,† Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany, and Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received December 28, 2004. In Final Form: February 28, 2005 Electrochemistry was combined with surface-plasmon-enhanced diffraction (ESPD) to investigate a redox-switchable polymer grating and its (bio-)sensing applications. Patterned arrays of polyaniline (PANI)/ poly(styrenesulfonate) (PSS) were fabricated by the combination of electropolymerization and micromolding in capillaries (MIMIC) and were used as an optical grating for surface-plasmon-enhanced diffraction experiments. The diffraction efficiency (DE) could be tuned by changes in the applied potential, and by changes in the pH of the surrounding solution (dielectric medium). The response of the DE to the pH depends strongly on the redox state of the PANI/PSS grating. If the polymer grating is mainly in its reduced state, the DE shows a linear dependence on the pH. The DE of the PANI/PSS grating can also be modulated by an electrocatalytic event: by keeping PANI/PSS in its oxidized form, the addition of β-nicotinamide adenine dinucleotide (NADH) increases the DE with the increase of NADH concentration, which points to the possibility of the use of ESPD technologies for biosensing.

Introduction During the past decades, optical diffraction techniques and diffraction-based optical assays have attracted great interest.1-25 Diffraction occurs if light interacts with a periodically patterned surface with a periodic contrast of * Corresponding author. Phone: +49-6131-379160. Fax: +496131-379360. E-mail: [email protected]. † Max-Planck-Institute for Polymer Research. ‡ University of Arizona. (1) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60-62. (2) Gupta, V. K.; Abbott, N. L. Science 1997, 276, 1533-1536. (3) Zhang, J.; Carlen, C. R.; Palmer, S.; Sponsler, M. B. J. Am. Chem. Soc. 1994, 116, 7055-7063. (4) Kawatsuki, N.; Hasegawa, T.; Ono, H.; Tamoto, T. Adv. Mater. 2003, 15, 991-994. (5) Bergstedt, T. S.; Hauser, B. T.; Schanze, K. S. J. Am. Chem. Soc. 1994, 116, 8380-8381. (6) Hauser, B. T.; Bergstedt, T. S.; Schanze, K. S. J. Chem. Soc., Chem. Commun. 1995, 1945-1946. (7) Schanze, K. S.; Bergstedt, T. S.; Hauser, B. T.; Cavalaheiro, C. S. P. Langmuir 2000, 16, 795-810. (8) Massari, A. M.; Stevenson, K. J.; Hupp, J. T. J. Electroanal. Chem. 2001, 500, 185-191. (9) Bailey, R. C.; Hupp, J. T. J. Am. Chem. Soc. 2002, 124, 67676774. (10) Mines, G. A.; Tzeng, B.-C.; Stevenson, K. J.; Li, J.; Hupp, J. T. Angew. Chem., Int. Ed. 2002, 41, 154-157. (11) Bailey, R. C.; Hupp, J. T. Anal. Chem. 2003, 75, 2392-2398. (12) Tsay, Y. G.; Lin, C. I.; Lee, J.; Gustafson, E. K.; Appelqvist, R.; Magginetti, P.; Norton, R.; Teng, N.; Charlton, D. Clin. Chem. 1991, 37, 1502-1505. (13) Nakajima, F.; Hirakawa, Y.; Kaneta, T.; Imasaka, T. Anal. Chem. 1999, 71, 2262-2265. (14) John, P. M. St.; Davis, R.; Cady, N.; Czajka, J.; Batt, C. A.; Craighead, H. G. Anal. Chem. 1998, 70, 1108-1111. (15) Morhard, F.; Pipper, J.; Dahint, R.; Grunze, M. Sens. Actuators, B 2000, 70, 232-242. (16) Goh, J. B.; Loo, R. W.; McAloney, R. A.; Goh, M. Cynthia. Anal. Bioanal. Chem. 2002, 374, 54-56. (17) Goh, J. B.; Tam, P. L.; Loo, R. W.; Goh, M. C. Anal. Biochem. 2003, 313, 262-266. (18) Rothenha¨usler, B.; Knoll, W. Opt. Commun. 1987, 63, 301-304. (19) Rothenha¨usler, B.; Knoll, W. Appl. Phys. Lett. 1987, 51, 783785. (20) Fischer, B.; Rothenha¨usler, B.; Knoll, W. Thin Solid Films 1995, 258, 247-251. (21) Yu, F.; Tian, S. J.; Yao, D.; Knoll, W. Anal. Chem. 2004, 76, 3530-3535. (22) Yu, F.; Yao, D.; Knoll, W. Nucleic Acids Res. 2004, 32, e75-e81. (23) Yu, F.; Knoll, W. Anal. Chem. 2004, 76, 1971-1975. (24) Fayer, M. D. Annu. Rev. Phys. Chem. 1982, 33, 63-87.

the refractive index between the patterned lattice and the surrounding medium (normally air or water). Any event that induces a change in the refractive index contrast will result in a modulation of the diffracted light intensity, and thus in a change of the diffraction efficiency (DE).26 Different materials have been used to fabricate such gratings, for example, liquid crystals,2-4 redox polymers,5-8 microporous supramolecular coordination compounds,10 gelatin films containing pH indicator,13 biomolecules,12-17,21-23 and even condensed water drops (condensation figures, CFs).1 Diffraction-based signal transduction has successfully been demonstrated in the sensing of environmental humidity,1 pH,13 volatile organic compounds,9-11 aqueous phase metal ions,10 DNA hybridization,22 proteins,12,16,17,21,23 and even whole cells.14,15 Among the above-reported diffraction-related sensing schemes, most are based on the traditional transmittance or reflectance measurements. Knoll and co-workers demonstrated earlier the development of a surface-plasmonenhanced diffraction (SPD) platform.18-20 In this scheme (cf., Figure 1), the light is coupled into surface plasmon modes through a prism, and a dielectric grating on the planar metal surface (normally Au or Ag) diffracts the nonradiative plasmon surface polariton (PSP) field into light radiation. The grating structure with a periodicity Λ provides an additional multiple of a small momentum g with |g| ) 2π/Λ, which thus delocalizes the surface plasmon field, generating a typical diffraction pattern. The DE of this SPD configuration was shown to be strongly enhanced as compared to that of the conventional diffraction schemes such as the total internal reflection (TIR) (enhancement factors from about 6 to 20 were once demonstrated based on different systems).18,27 Recently, this SPD scheme has been successfully used by our group to construct biosensors for the label-free detection of protein and DNA hybridization.21-23 (25) Nelson, K. A.; Casalegno, R.; Miller, R. J. D.; Fayer, M. D. J. Chem. Phys. 1982, 77, 1144-1152. (26) The diffraction efficiency (DE) here was defined as the ratio between the diffracted light intensity of the first-order Id1 and that of the zeroth-order Id0 (cf., Figure 1). (27) Yu, F. Ph.D. dissertation, Universita¨t Mainz, Germany, 2004.

10.1021/la0467741 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/02/2005

Surface-Plasmon-Enhanced Diffraction Gratings

Figure 1. Scheme of the electrochemically tunable surfaceplasmon-enhanced diffraction (ESPD) setup.

In this paper, we report the combination of electrochemistry with an SPD scheme (ESPD) to investigate a redox-active polymer grating created from polyaniline (PANI) and its polymeric counterion, poly(styrenesulfonate) (PSS). Hupp and co-workers recently reported micrometer-scale PANI diffraction gratings based on a transmittance configuration, and their chemo- and electrochemical responses.8 The loss of redox activity of PANI in neutral solutions28,29 generally precludes the use of such pure PANI gratings for biosensing purposes. It has been demonstrated, however, that the redox activity of PANI and the interconversion between its different oxidation forms can be sustained in neutral pH environments by doping it with different polyanions (such as PSS)30-34 or modified gold nanoparticles.35,36 Here, we use PANI/PSS composite instead of pure PANI for the fabrication of the grating to probe its potential biosensing applications near pH ) 7.0. We demonstrate that the DE of a PANI/PSS grating, interrogated in the SPD mode, can be electrochemically tuned in a neutral pH environment, offering the possibility for bioassays. The DE is also very sensitive to environmental pH changes. The observed DE response to changes in pH strongly depends on the redox state of PANI/PSS and shows almost a linear dependence if the PANI/PSS composite is mainly in its reduced state. Moreover, we demonstrate that PANI/PSS composite gratings can also be used to monitor an electrocatalytic event: by keeping PANI/PSS in its oxidized form, the DE can be modulated by adding β-nicotinamide adenine dinucleotide (NADH). Due to the electrocatalytic oxidation of NADH by PANI, the DE increases with the increase of NADH concentration, which means that the ESPD platform described here will be well-suited for certain biosensor applications. (28) Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980, 111, 111. (29) Ohsaka, T.; Ohnuki, Y.; Oyama, N.; Katagiri, K.; Kamisako, K. J. Electroanal. Chem. 1984, 161, 399. (30) Karyakin, A. A.; Strakhova, A. K.; Yatsimirsky, A. K. J. Electroanal. Chem. 1994, 371, 259-265. (31) Bartlett, P. N.; Birkin, P. R.; Wallace, E. N. K. J. Chem. Soc., Faraday Trans. 1997, 93, 1951-1960. (32) Bartlett, P. N.; Wallace, E. N. K. J. Electroanal. Chem. 2000, 486, 23-31. (33) Raitman, O. A.; Katz, E.; Bu¨ckmann, A. F.; Willner, I. J. Am. Chem. Soc. 2002, 124, 6487-6496. (34) Tian, S. J.; Baba, A.; Liu, J. Y.; Wang, Z. H.; Knoll, W.; Park, M.-K.; Advincula, R. Adv. Funct. Mater. 2003, 13, 473-479. (35) Tian, S. J.; Liu, J. Y.; Zhu, T.; Knoll, W. Chem. Commun. 2003, 21, 2738-2739. (36) Tian, S. J.; Liu, J. Y.; Zhu, T.; Knoll, W. Chem. Mater. 2004, 16, 4103-4108.

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Figure 2. Schematic illustration of the procedure used for fabricating PANI/PSS grating by combining MIMIC with electropolymerization (EMIMIC).

Experimental Section Materials. Aniline (99%), PSS (Mw ca. 70 000), and 3-mercapto-1-propanesulfonic acid (sodium salt, MPS) were obtained from Aldrich. β-Nicotinamide adenine dinucleotide (NADH, reduced form, disodium salt, approximately 98%) was obtained from Sigma. All of these materials were used as received. Other materials used were of analytical grade. Millipore water (18 MΩ cm) was used throughout the experiments. Grating Fabrication. The fabrication of the PANI/PSS composite polymer grating was accomplished by the combination of micromolding in capillaries (MIMIC)37,38 with electropolymerization (EMIMIC, Figure 2). A freshly prepared PDMS stamp (parallel line shape, line width 10 µm, spacing 15 µm) was placed on an Au substrate (50 nm Au evaporated onto a LaSFN9 glass slide with 2 nm Cr adhesion layer between) to form an array of microchannels. To facilitate the subsequent filling of these channels with polymer solution, the Au substrate was first functionalized with a layer of MPS. After filling in the polymer solution (0.02 M aniline in 0.5 M H2SO4 with 0.01 M PSS) by capillary force and carrying out electropolymerization, the PDMS stamp was carefully peeled off, and well-shaped PANI/PSS gratings were obtained (cf., Figure 3). The grating was rinsed carefully first with 0.5 M H2SO4, followed by 0.1 M PBS buffer (pH 7.2), and then used for the diffraction measurements. Instrumentation and Measurement. The scheme of the ESPD is shown in Figure 1. An electrochemical flow cell is combined with a SPR setup based on the Kretschmann configeration, which has been described in detail elsewhere.39 The Au substrate coated with the polymer grating was used both for the coupling of the PSP and as the working electrode for the electrochemistry measurement. A p-polarized HeNe laser (λ ) 632.8 nm) was used for the excitation of the PSP. The reflected or diffracted light was measured by photodiodes. The electrochemical measurements were carried out with a potentiostat (EG&G 273A), with a coiled platinum wire being used as the counter electrode, and an Ag/AgCl (3 M NaCl) electrode as the reference electrode. All potentials reported here are with respect to this reference electrode. Before the diffraction measurements, the quality of the prepared polymer grating was checked first by an optical microscope and then further by AFM (Dimension 3100, Veeco) in the tapping mode. A silicon cantilever was used, which has a spring constant of 42 N/s and a resonant frequency of 300 kHz. (37) Xia, Y.; Kim, E.; Whitesides, G. M. Chem. Mater. 1996, 8, 15581567. (38) Kim, E.; Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1996, 118, 5722-5731. (39) Baba, A.; Park, M.-K.; Advincula, R. C.; Knoll, W. Langmuir 2002, 18, 4648.

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Figure 3. Example of PANI/PSS composite polymer grating prepared via EMIMIC and its corresponding diffraction patterns. (A) Optical image (light area is the polymer grating); (B) AFM image; (C) section analysis as shown in (B); (D) digital diffraction image; and (E) corresponding SPD pattern. Electropolymerization conditions: 0.02 M aniline in 0.5 M H2SO4 with 0.01 M PSS, scan rate 20 mV/s, 6 cycles.

Results and Discussion Grating Characterization. The images of a representative region of a PANI/PSS grating prepared via EMIMIC are shown in Figure 3. The optical image (Figure 3A) reveals a highly periodic polymer structure over a large area (samples up to 1 cm × 1 cm in area were successfully fabricated with only a few observable defects). Further characterization using AFM (Figure 3B) confirmed the well-shaped micropattern, which is exactly the reverse replica of the used PDMS stamp, with an average PANI/PSS thickness of 25 nm (Figure 3C). The typical surface plasmon diffraction patterns from this grating are also shown in Figure 3. Figure 3D is the digital image of the diffraction, while Figure 3E shows the corresponding diffraction pattern, obtained from the SPD platform. It should be noted that, for the calculation of DE,26 we are interested only in the first few diffraction orders, so we deliberately reduced the laser intensity to avoid any saturation of diffraction signals of the first few orders. If we increase the laser intensity, at least 15 diffraction orders can be observed even with the naked eye. Electrochemical Modulation of the Diffraction Efficiency. The cyclic voltammograms (CV) of the

prepared PANI/PSS polymer grating (Figure 3) measured in 0.1 M PBS buffer (pH7.2) are shown in Figure 4A. It is clear that the polymer grating can remain redox-active in a neutral pH environment, with a midpoint potential near +0.04 V vs Ag/AgCl at a scan rate of 20 mV/s. The redox behavior is similar to that reported previously, where this redox peak was ascribed to the overlap of the two redox processes of PANI normally found in acid conditions, that is, transition between the fully reduced leucoemeraldine base (LEB) form and the half-oxidized emeraldine base (EB) form, and between the EB form and the fully oxidized pernigraniline base (PNB) form (vide post, also cf. Figure 5B).30,31,34,40 The DE of the grating changes with the change of the applied potential (Figure 4B), with a relatively good signal reproducibility, after the first cycle. Examining these changes in greater detail, we observe that the DE decreases if the polymer grating changes from its reduced state to its oxidized state. If the potential is scanned back to re-form the reduced state of PANI, the DE increases again, with only a little hysteresis (Figure 4C). This change of the DE of the SPD grating with the (40) Huang, W.-S.; Humphrey, B. D.; MacDiarmid, A. G. J Chem. Soc., Faraday Trans. 1 1986, 82, 2385-2400.

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Figure 5. (A) DE changes of the polymer grating with the potential in different pH buffer solutions. Inset shows the linear relationship between the DE and pH in the range pH 4-8 when the grating is kept at -0.15 V where it is mainly in its reduced state. The arrows show the corresponding potential scan directions. (B) CV curves of the polymer grating measured in buffer solutions with different pH. Buffer solutions: pH 1, HCl solution; pH 3-9, 0.1 M citrate-phosphate buffer solution. CV scan rate was 20 mV/s.

Figure 4. Cyclic voltammograms (A) and diffraction efficiency changes with the potential change (B and C) of a PANI/PSS polymer grating (as shown in Figure 3) measured in 0.1 M PBS buffer, pH 7.2. CV scan rate was 20 mV/s. The electrode was held at -0.2 V for 1 min before CV scanning.

change in redox state of PANI/PSS is in good accordance with the change of the real part of the refractive index n (but contrary to the change of imaginary part of the refractive index κ) of PANI with change in potential.41,42 We believe this DE change of PANI/PSS polymer grating with the potential was predominated by the change of n that was induced by the change in the film absorptivity (i.e., κ), insertion or expulsion of counterions or solvent, and changes in grating thickness and density, that occurred concomitant with the oxidation or reduction of the grating film during the potential scanning. In earlier work, Schanze and co-workers also demonstrated that the DE is considerably more sensitive to changes in the real part of the refractive index than to changes in absorptivity.7 pH Modulation of the Diffraction Efficiency. It has been demonstrated previously that the redox behavior of PANI/polyelectrolyte films depends strongly on its (41) Baba, A.; Tian, S. J.; Stefani, F.; Xia, C. J.; Wang, Z. H.; Advincula, R. C.; Johannsmann, D.; Knoll, W. J. Electroanal. Chem. 2004, 562, 95-103. (42) Kim, D. R.; Cha, W.; Paik, W.-K. Synth. Met. 1997, 84, 759-760.

environmental pH.30-32,34 The DE of PANI/PSS gratings should therefore also be modulated by a change of pH. Shown in Figure 5A are the DE changes of the polymer grating upon changes in potential, measured in solutions of different pH. It is found that the ∆DE versus ∆E response is quite different in different pH solutions. For solutions near pH ) 7, the DE systematically decreases with increasing applied potential (increasing oxidation of the PANI film), with some hysteresis noted in the ∆DE versus ∆E response as the potential was returned negative to reduce the PANI film. For lower pH solutions, the DE decreases as the PANI film is oxidized, and then increases as the potential is driven positive of ca. 0.0 V, with some hysteresis again noted upon return of the potential to that for the reduced state of PANI. As mentioned above, the oxidation of PANI films occurs through two pH-dependent steps: (i) the oxidation of the fully reduced LEB form to the half-oxidized EB form, followed by (ii) the oxidation of the EB form to the fully oxidized PNB form (cf., Figure 5B, black curve). The EB form is of course in equilibrium with its protonated emeraldine salt (ES) form. Any change in the surrounding pH will therefore cause a change in the equilibrium forms of PANI within the grating structure, causing a shift in voltammetric activity (as shown in Figure 5B), and changes to both n and κ that change the DE in an SPD platform. At higher pH, where the transition from LEB to EB and PNB forms of PANI tends to be unresolved voltammetrically, these optical changes are still observable and can be used to track changes in solution pH by

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in its oxidized state. With the first addition of NADH, the DE decreases a little, which was caused by the increase in refractive index of the surrounding buffer, due to the addition of NADH, which results in a smaller refractive index contrast between the polymer grating and its surrounding dielectric. With further addition of NADH, however, the DE increases gradually with the increase of NADH concentration, as a result of the electrocatalytic oxidation of NADH by the polymer grating. At the same time, part of the polymer grating was reduced, which results in an increase in the average refractive index of the polymer grating, and thus the increased DE. Figure 6. Diffraction efficiency changes of a PANI/PSS grating measured in 0.1 M PBS buffer (pH 7.2) with different amounts of NADH when the potential was fixed at +0.4 V.

monitoring changes in DE. At sufficiently negative potentials (e.g., -0.15 V), the DE shows a linear relationship with pH in the range between pH 4 and pH 8 (Figure 5A, inset). This is a potential where the PANI film is predominantly in the LEB form, but still coupled to its other redox forms, which can take up or release protons (and counterions), thereby ensuring adequate changes primarily to n, and less so to κ, which change DE. Electrocatalytic Modulation of the Diffraction Efficiency. Because PANI/PSS gratings remain electroactive in neutral pH (vide supra), and because previous studies of unpatterned PANI/PSS composites showed that it can electrocatalytize the oxidation of NADH,32,34 we undertook an evaluation of the change in DE of the PANI/ PSS grating during the electrocatalytic oxidization of NADH. Figure 6 shows the DE change with the change of NADH concentration, measured in 0.1 M PBS buffer (pH 7.2) at +0.4 V where the polymer grating is primarily

Conclusions PANI/PSS gratings were successfully fabricated by the combination of MIMIC with electropolymerization and were used as optical diffraction devices operated in the ESPD mode. The DE of PANI/PSS grating can be tuned both by potential changes and by changes to the pH of the contacting aqueous medium. Moreover, because the PANI/ PSS grating remains electroactive in neutral pH conditions, its DE can also be tuned by the electrocatalytic oxidation of NADH, which points to the possibility of using the present system for developing ESPD-based biosensors. Acknowledgment. We thank Dr. Emmanuel Delamarche (IBM Zurich Research Laboratory) for providing some of the stamps and for valuable suggestions on the preparation of the gratings. N.R.A. would like to gratefully acknowledge support by the Alexander von Humboldt Stiftung for a Senior Research Prize while at the MPIP-Mainz. LA0467741