Reversible Electroaddressing of Self-assembling Amino-Acid Conjugates

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Yi Liu , Eunkyoung Kim , Rein V. Ulijn , William E. Bentley , and Gregory F. Payne *

Reversible Electroaddressing of Self-assembling Amino-Acid Conjugates

The triggered assembly of organic and biological materials in response to imposed electrical signals (i.e., electroaddressing) provides interesting opportunities for applications in molecular electronics, biosensing and nano-biotechnology. Recent studies have shown that the conjugation of aromatic moieties to short peptides often yields hydrogelator compounds that can be triggered to self-assemble over a hierarchy of length scales in response to a reduction in pH. Here, we examined the capabilities of fl uorenyl-9-methoxycarbonyl-phenylalanine (Fmoc-Phe) to electrodeposit in response to an electrochemically-induced pH gradient generated at the anode surface. We report that the electrodeposition of Fmoc-Phe; is rapid (minutes), can be spatially controlled in normal and lateral directions, and can be reversed by applying a brief cathodic current. Further more, we show that Fmoc-Phe can be simultaneously deposited on one electrode address (anode) while it is being cathodically stripped from a separate electrode address of the same chip. Finally, we demonstrate that these capabilities can be extended for electroaddressing within microfl uidic channels. The reversible assembly/disassembly of molecular gelators (Fmoc-amino acids and Fmoc-peptides) in response to spatiotemporally imposed electrical signals offers unique oppor-tunities for electroaddressing that should be especially valuable for lab-on-a-chip applications.

1. Introduction

In principle, electroaddressing is an attractive method for the spatially-controlled surface-assembly of organic and biological materials because electrical signals can be readily imposed with high fi delity. For instance, microfabrication allows electrodes to be patterned onto surfaces (e.g., silicon) to generate chips and devices capable of imposing electrical signals with exquisite

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2011, 21, 1575–1580

DOI: 10.1002/adfm.201002020

Dr. Y. Liu , Dr. E. Kim , Prof. W. E. Bentley , Prof. G. F. Payne Center for Biosystems ResearchUniversity of Maryland5115 Plant Sciences Building, College Park, MD 20742, USA E-mail: [email protected] Prof. R. V. Ulijn WestCHEM Department of Pure and Applied ChemistryThe University of Strathclyde295 Cathedral Street, Glasgow G1 1XL, Scotland, UK Prof. W. E. Bentley , Prof. G. F. Payne Fischell Department of BioengineeringUniversity of MarylandCollege Park, MD 20742, USA

spatiotemporal control. In addition, these electrical signals can be imposed across arbitrarily large and complex elec-trode patterns to permit effi cient parallel processing.

The attraction of electroaddressing has spurred numerous studies, [ 1 , 2 ] and several mechanisms have been identifi ed to enlist electrical signals to guide assembly. One common approach for electroaddressing is to directly electropolymerize a monomer (e.g., pyrroles [ 3–7 ] ) onto the electrode. Alter-natively, applied electrical potentials can induce oxidation/reduction reactions: to promote direct grafting to the electrode surface (e.g., via the molecule’s amine [ 8–11 ] or aryl-diazonium moieties [ 12–17 ] ), to con-vert surface substituents into reactive moieties [ 18 ] (e.g., by oxidation to generate surface quinones [ 19–26 ] or reduction to gen-erate surface primary amines [ 27 , 28 ] ), or to generate diffusible mediators (e.g., HOCl) that can selectively remove an overlayering protective resist layer. [ 29–32 ] In addition, electrochemical reactions can generate pH-gradients adjacent to electrode sur-faces that can be employed for electroad-

dressing. [ 33 ] In fact, several groups are exploring the ability of pH-responsive, fi lm-forming polysaccharides (e.g., chitosan [ 34–39 ] and alginate [ 40–42 ] ) to electrodeposit at electrode surfaces.

Recently, it was discovered that short peptides that are con-jugated to alkyl or aromatic [ 43 , 44 ] moieties can be triggered to self-assemble into hydrogels. [ 45 , 46 ] Specifi cally, the conjuga-tion of fl uorenyl-9-methoxycarbonyl (Fmoc) to the N-terminus of amino acids and short peptides yields hydrogelator com-pounds [ 47 , 48 ] that can be triggered to undergo self-assembly in response to a reduction in pH. [ 49 ] Studies with these self-assembling peptides indicate that they undergo complex struc-tural transitions and organize over a hierarchy of length scales to form fi brous hydrogels. [ 50–54 ] One recent study reported that a low pH generated at a gold anode could induce the Fmoc-dipeptide (Fmoc-Leu-Gly) to form a fi lm near the gold sur-face. Further, these studies demonstrated that the addition of hydroquinone facilitated gelation by allowing the pH-gradient to be established at lower anodic potentials and thus limited damage to the gold electrode. [ 55 ] We extended this prior work using a simpler molecular gelator, Fmoc-phenylalanine, and demonstrate that: (i) deposition is spatially-selective in lateral directions; (ii) separate electrical signals can be enlisted to

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simultaneously assemble and disassemble Fmoc-Phe fi lms on separate electrode addresses of the same chip; and (iii) revers-ible electroaddressing allows assembly/disassembly within fl u-idic environments.

2. Results and Discussion

2.1. Spatiotemporal Control of Fmoc-Phe Assembly

The initial evidence for the electrodeposition of Fmoc-phenyla-lanine (Fmoc-Phe) was provided by in situ measurements with an electrochemical quartz crystal microbalance (EQCM). In this experiment, the gold-coated QCM crystal was assembled in the EQCM cell, the reaction solution (50 m M hydroquinone, 0.1 M NaCl, pH 7.5) containing Fmoc-Phe (1.0 m M ) was added, and the potential was swept in the oxidizing direction (0.02 V s − 1 ) while the current and frequency change ( Δ F ) were recorded. As shown in the upper plot of Figure 1 a, anodic currents are observed at potentials above 0.09 V (vs Ag/AgCl) consistent

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Figure 1 . Electrodeposition of Fmoc-Phe is controllable. (a) In situ EQCM measurements of Fmoc-Phe (1.0 m M ) using a linear sweep in voltage (0.02 V s − 1 ) while the crystal’s resonance frequency was monitored. (b) Ex situ QCM measurements of dried fi lms electrodeposited from reaction solution (50 m M hydroquinone, 0.1 M NaCl, pH 7.5) containing 10 m M Fmoc-Phe at a constant current density (1 A m − 2 ).

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with the oxidation of hydroquinone. The lower plot in Figure 1 a shows a substantial reduction in Δ F beginning at 0.09 V indi-cating that mass is accumulating on this electrode under these oxidizing conditions. These observations are consistent with a pH-mediated electrodeposition of Fmoc-Phe.

Results from two controls are also shown in Figure 1 a. The fi rst control solution contained Fmoc-Phe and NaCl but not hydroquinone. Figure 1 a shows no oxidation currents and no changes in Δ F are observed with this control, which is con-sistent with the expectation that hydroquinone is required for a pH-gradient to be generated at low oxidation potentials. The second control solution contained hydroquinone and NaCl but not Fmoc-Phe. An oxidation current but no change in Δ F are observed with this control indicating that Fmoc-Phe (and not hydroquinone) electrodeposits.

To demonstrate that electrodeposition can be controlled, we performed ex situ QCM measurements using conditions that allowed the rapid generation of thicker, more stable fi lms (e.g., with higher Fmoc-Phe concentrations). Specifi cally, the gold-coated QCM crystal was immersed in the reaction solution con-taining 10 m M Fmoc-Phe; a constant anodic current (1 A m − 2 ) was applied to the gold electrode for various times; the crystal was removed, washed gently with water and dried; and then the frequency was measured and compared to the initial frequency measured before deposition. Quantitatively, the measured Δ F can be related to the mass of the deposited Fmoc-Phe by the Sauerbrey equation. [ 56 ] Figure 1 b shows a linear increase in the added mass with the deposition time for up to 7 minutes. Longer deposition times were not investigated because the voltage required to maintain a constant current increased with time and we were concerned that increased anodic voltages could damage the QCM electrode. The control in Figure 1 b is a crystal that was incubated in the reaction solution of hydroquinone (50 m M ) plus NaCl (0.1 M ) that lacked Fmoc-Phe and a constant current was applied for 7 min. No added mass was observed for this control which further indicates that hydroquinone does not electrode-posit at the electrode. Thus, the results in Figure 1 b indicate that the growth of the electrodeposited Fmoc-Phe fi lm can be controlled by the deposition conditions (e.g., deposition time). Analogous observations were made for the controlled growth of thin fi lms of Fmoc-Leu-Gly ( < 200 nm) using in situ measure-ments by surface plasmon resonance spectroscopy. [ 55 ]

In addition to controlling fi lm growth in the normal direc-tion, we also examined the spatial selectivity of peptide deposi-tion in the lateral dimensions. The schematic in Figure 2 shows that we fabricated a chip to have six individually-addressable gold electrodes (250 μ m wide lines). This patterned chip was partially-immersed in the reaction solution containing Fmoc-Phe (10 m M ) that also contained fl uorescein-labeled dextran (0.005 m M ) to aid visualization (initial observations indicated that dextran could be co-deposited and partially retained within the Fmoc-Phe fi lm). As illustrated in the schematic in Figure 2 , deposition was induced on three of the electrodes by anodically biasing (1 A m − 2 ) the individual electrodes for different times (2, 4, and 6 min). After deposition, the chip was removed from the solution, disconnected from the power supply, gently rinsed with deionized water, and then dried. The fl uorescence photo-microscopy image in Figure 2 indicates good spatial-selectivity for deposition of the Fmoc-Phe.

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Figure 2 . Spatial-selectivity of Fmoc-Phe electrodeposition. Chip study with patterned gold electrodes (250 μ m wide gold lines patterned onto a silicon wafer) demonstrates spatial selectivity of electrodeposition of 10 m M Fmoc-Phe (0.005 m M fl uorescein-labeled dextran was included for visualization).

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The deposit in Figure 2 was further analyzed to quantify the fl uorescence intensity using ImageJ software (http://rsb.info.nih.gov/ij/) and fi lm thickness (using profi lometry). Both results demonstrate that electrodeposition is spatially selec-

Figure 3 . Simultaneous assembly/disassembly of Fmoc-Phe on separate electrodes of the same chip. (a) Photograph and profi le measurement of the patterned chip. (b) Fluorescence photo-microscopy images and profi le measurements after individual steps. Step 1 – simulta-neous anodic assembly (1 A m − 2 ) on the left and middle electrodes. Step 2 – simultaneous cathodic disassembly (4 A m − 2 ) on the middle electrode and anodic assembly (1 A m − 2 ) on the right electrode. Step 3 – simultaneous cathodic disassembly (4 A m − 2 ) on the left electrode and anodic assembly (1 A m − 2 ) on the middle electrode. Step 4 – treatment of chip with basic phosphate buffer. See text for experimental details.

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tive in the lateral dimensions. Further, both results indicate a systematic growth of the fi lm with deposition time.

In sum, Figures 1 and 2 show that the elec-trodeposition of Fmoc-Phe can be controlled spatially in normal and lateral directions. The results are consistent with the expectation that anodic oxidation of hydroquinone gen-erates a pH gradient that triggers Fmoc-Phe gelation.

2.2. Simultaneous Assembly/Disassembly of Fmoc-phe

Previous studies report that Fmoc-Leu-Gly fi lms can be reversibly formed and dis-solved by altering the potential. [ 55 ] Specifi -cally, anodic reactions that generate a low pH induce deposition (i.e., assembly) while cathodic reactions that generate high pH dis-solve (i.e., disassemble) the fi lm. Here, we examined the possibility of simultaneously

© 2011 WILEY-VCH Verlag G© 2011 WILEY-VCH Verlag GAdv. Funct. Mater. 2011, 21, 1575–1580

electrodepositing Fmoc-Phe from one electrode address while dissolving a previously-deposited Fmoc-Phe fi lm from a sepa-rate address of the same chip. In this experiment, we patterned 3 electrically-independent 1 mm × 5 mm gold-electrodes onto a silicon chip as indicated by the photograph shown in Figure 3 a.

In the fi rst step, the chip was partially immersed in the reac-tion solution containing Fmoc-Phe (10 m M ) and Rhodamine-labeled dextran (0.005 m M ), and the left and middle electrodes were biased anodically (1 A m − 2 ) for 4 min. After gentle rinsing and drying, the chip was examined using both fl uorescence microscopy and profi lometry. As shown in Figure 3 b, both meas-urements indicate that Fmoc-Phe was successfully deposited on these two electrodes. In addition, the measured fi lm thickness of about 0.9 μ m is similar to that observed in Figure 2 for the electrode biased under the same conditions (4-min at 1 A m − 2 ).

In the second step, the same chip was immersed back into the Fmoc-Phe reaction solution and simultaneously: (i) a cathodic signal (4 A m − 2 ) was applied to the middle electrode for 1 min to dissolve the previously deposited fi lm; and (ii) an anodic signal (1 A m − 2 ) was applied to the right electrode for 2 min to deposit a new Fmoc-Phe fi lm. (Note: two power sup-plies were used in this experiment and separate Pt wires were used as the counter electrodes.) As indicated in Figure 3 b, both fl uorescence and thickness measurements (after drying) indicate that the Fmoc-amino acid fi lm on the middle electrode was dis-assembled by cathodic treatment while a new fi lm was depos-ited on the right electrode. A small decrease in fi lm thickness is observed for the left electrode that was unbiased during this second step – possibly due to a partial dissolution of the fi lm.

The third step is similar to the second step except in this case a cathodic signal was applied to the left electrode and at the same time an anodic signal was applied to the middle electrode. As expected, Figure 3 b indicates that the Fmoc-amino acid on the left electrode was removed during cathodic disassembly, while a new Fmoc-Phe fi lm was deposited on the middle elec-trode. Finally, the chip was immersed in a phosphate buffered

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solution (pH 8.5) for 15-min without applying any potential. The results in Figure 3 b indicate all of the Fmoc-Phe fi lms were dissembled by this treatment.

The results in Figure 3 demonstrate that Fmoc-Phe fi lms can be reversibly assembled and dissembled in response to the imposed electric signals. The ease of assembly and disassembly allows Fmoc-Phe fi lms to be simultaneously deposited and dis-solved at separate electrode addresses on the same chip.

Figure 5 . Assembly of Fmoc-Phe fi lms in a covered fl uidic channel. (a) Fluorescence photo-microscopy image of electrodeposited Fmoc-Phe on sidewall electrodes. (b) Calculated volumes of deposited fi lms. [Deposition solution 10 m M Fmoc-Phe, 50 m M hydroquinone, 0.1 M NaCl, 0.005 m M Rhodamine B-dextran, pH 7.5; deposition condition 1 A m − 2 with opposing electrode as counter electrode.]

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2.3. Assembly/Disassembly Within Fluidic Channel

Our fi nal studies examined the electroaddressing of Fmoc-Phe in the microfabricated fl uidic channel illustrated in Figure 4 . [ 57 ] This device was generated from two polydimethylsiloxane (PDMS) layers that formed the base and cover (cover not shown), and two glass slides (75 mm × 25 mm × 1 mm) that were spaced 1-mm apart to form the fl uidic channel. As indi-cated in Figure 4 , the glass slides were patterned with 1 mm gold stripes which formed the sidewall electrode addresses and their corresponding leads. Each of the sidewall electrodes had a working area of 1 mm × 1 mm and was aligned with an opposing electrode across the channel. The fabrication of this fl uidic channel using transparent layers (glass and PDMS) facil-itates visualization of electrodeposited fi lms.

Initially, we examined the growth of the electrodeposited Fmoc-Phe fi lm on a side-wall electrode address. In this experi-ment, the reaction solution containing Fmoc-Phe (10 m M ) and Rhodamine B-dextran (0.005 m M ) was introduced into the channel using a syringe. After adding the fl uid, fl ow was stopped and a constant current (1 A m − 2 ) was applied to an opposing electrode pair as illustrated in the schematic of Figure 4 . Deposition was performed for various times with separate electrode pairs. After deposition, the channel was rinsed briefl y with water which was then displaced with an acidic buffer (20 m M sodium phosphate pH 5) that was held in the channel for 5 minutes to stabilize the electrodeposited fi lms making them resistant to dissolving in water. Rhodamine B-dextran was also included in this acidic buffer because it was found to enhance visualization of the deposited fi lm. Immedi-ately after stabilization, the channel was washed with a pH 5 buffer (lacking Rhodamine B-dextran), the liquid was drained from the channel and fl uidic device was examined using a fl uo-rescence microscope.

The fl uorescence photo-microscopy images in Figure 5 a show that the thickness of the deposited Fmoc-Phe fi lms increases with


deposition time. To quantify the volume of the electrodeposited fi lm, we assumed a channel height of 1 mm and determined the cross-sectional area by tracing the fl uorescence profi les using the available software package. As shown in Figure 5 b, the volumes of the deposited Fmoc-Phe fi lms increase as a function of deposition time consistent with the results from the chip studies (Figure 1 b). These results indicate the Fmoc-Phe can be controllably electrodeposited at an electrode address within a covered fl uidic channel.

In a fi nal experiment we examined the parallel assembly and sequential disassembly of Fmoc-Phe fi lms at three electrodes within the fl uidic channel of Figure 4 . In Step 1 of the experi-ment in Figure 6 , we introduced the reaction solution containing Fmoc-Phe (10 m M ) and Rhodamine B-dextran (0.005 m M ) into the channel and anodically biased three neighboring electrodes at 1 A m − 2 for 2 min (the opposing electrodes served as the counter electrode). After deposition, we rinsed the channel with water, stabilized the fi lms for 5 min with pH 5 buffer (containing Rhodamine B-dextran) and then fi lled the channel with label-free buffer. The fl uorescence photo-microscopy image in Figure 6 (Step 1) shows that Fmoc-Phe fi lms were deposited on these three neighboring gold electrodes.

Next, we examined the disassembly of an Fmoc-Phe fi lm from one electrode address. In Step 2, a Fmoc-Phe-free reac-tion solution containing Rhodamine B-dextran (0.005 m M ) was introduced into the channel; a cathodic signal (1 A m − 2 ) was applied to the middle electrode for 2 min to remove the previously-deposited fi lm while the two neighboring electrodes served as the counter electrodes (i.e., as the anodes). After this disassembly step, we rinsed and stabilized the fi lms as described


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Figure 6 . Parallel assembly and sequential disassembly of Fmoc-Phe fi lms. Step 1: Parallel anodic assembly of Fmoc-Phe fi lms at 3 neighboring elec-trodes (opposing counterelectrodes). Step 2: Cathodic disassembly from middle electrode (neighboring counter electrodes). Step 3: Cathodic disas-sembly from left electrode (right electrode served as anode). Step 4: Disassembly by treatment with basic phosphate buffer (pH 8.5 for 15 min). See text for experimental details.


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above. The fl uorescence photo-microscopy images in Figure 6 (Step 2) shows that the Fmoc-Phe fi lm on the middle electrode was completely removed upon cathodic treatment while fi lms on the two neighboring electrodes were retained.

A second deposited fi lm was removed in Step 3 using the same conditions described for Step 2 by introducing the Fmoc-Phe-free reaction solution containing Rhodamine B-dextran (0.005 m M ), but applying a cathodic signal (1 A m − 2 ) to the left electrode for 2 min with the right electrode serving as the anode. The image in Figure 6 (Step 3) shows that the fi lm on the left electrode was completely removed while the fi lm on the right electrode was retained. The remaining fi lm was then removed by pumping phosphate buffer (pH 8.5) into the channel (50 μ L min − 1 for 15 min) and this disassembly is indicated by the right-most image in Figure 6 . The results in Figure 6 demonstrate that imposed electrical signals can be used to assemble/disassemble Fmoc-Phe fi lms within a fl uidic channel.

3. Conclusions

We demonstrate that: 1) Fmoc-Phe fi lms can be reversibly assembled (i.e., electrodeposited) in response to anodic signals and disassembled (i.e., dissolved) in response to cathodic sig-nals; 2) Fmoc-Phe assembly/disassembly is rapid and spatially-controllable; and 3) Fmoc-Phe fi lms can be simultaneously assembled at one electrode address while being disassembled from a separate electrode address. We anticipate that reversible electroaddressing will be especially useful for enlisting local-ized electrical signals in lab-on-a-chip applications and believe the above results expand and complement the established capabilities of Fmoc-peptides for applications in molecular electronics [ 58 , 59 ] and regenerative medicine. [ 53 , 54 , 60 , 61 ]

4. Experimental Section The following chemicals were purchased from Sigma-Aldrich; Fmoc-D-phenylalanine (Fmoc-Phe, ≥ 98%), DMSO (99.94 + %), hydroquinone ( ≥ 99%), Fluorescein isothiocyanate–Dextran (MW ∼ 40,000), and Rhodamine B isothiocyanate–Dextran (MW ∼ 70,000).

An Fmoc-Phe solution was prepared by; fi rst dissolving Fmoc-Phe in DMSO (100 mg mL − 1 ), then diluting this concentrate in an aqueous reaction solution of hydroquinone (50 m M fi nal concentration) plus NaCl (0.1 M fi nal concentration), and fi nally adjusting the pH to 7.5 with 0.5 M NaOH. After vortex-mixing, the solution was fi ltered using a syringe fi lter

© 2011 WILEY-VCH Verlag G© 2011 WILEY-VCH Verlag GAdv. Funct. Mater. 2011, 21, 1575–1580

(1.5 μ m) to remove undissolved particles. These solutions were prepared immediately prior to use ( < 30 min).

Electrodeposition was performed on either gold-coated QCM crystal (ICM, Oklahoma City, OK) or chips with gold electrodes patterned onto silicon. The patterned gold chips were prepared using standard microfabrication methods as described elsewhere. [ 62 ] Before use, the gold electrodes were cleaned by immersing in piranha solution (7:3 concentrated H 2 SO 4 :30% H 2 O 2 ) for 3 min. [Caution: piranha solution is a highly reactive mixture and results in a severely exothermic reaction. It should be kept out of contact with oxidizable organic materials.] For deposition, the chips were partially-immersed in the reaction solution containing Fmoc-Phe, an alligator clip was used to connect the chip to a DC power supply (2400 Sourcemeter, Keithley), and the gold electrode was biased to serve as the anode (1 A m − 2 ) while a platinum foil served as cathode. After electrodeposition, the chip was immediately removed from the deposition solution, gently rinsed with water, and stabilized in a pH 5 buffer solution (20 m M sodium phosphate). The chip was then rinsed with water and gently blown dry with a stream of air.

Electrochemical quartz crystal microbalance (EQCM) measurements were carried out on a CHI420a Electrochemical Analyzer (CH Instruments, Inc., Austin, TX) and the experimental procedure was described elsewhere. [ 63 ] For in situ EQCM experiments, the gold-coated crystal (working area: 0.205 cm 2 ) with 8 MHz resonating frequency served as the working electrode, Ag/AgCl as a reference electrode and Pt wire as a counter electrode. For ex situ QCM experiments, the frequency of the dried crystal before and after electrodeposition was measured in the air, and the difference of frequency ( Δ F ) was used to estimate the mass change using the Sauerbrey equation. [ 56 ]

Fabrication of the fl uidic device with paired sidewall gold electrodes is reported elsewhere. [ 57 ] The solutions were introduced into the fl uidic channel through a PTFE #24 AWG thin wall tubing (Cole-Parmer, Vernon Hills, IL) connected to a syringe (BD, Frankin Lakes, NJ) by a 22 ga × ½” Luer stub (Instech Solomon, Plymouth Meeting, PA). The procedures to electrodeposit and dissolve the Fmoc-Phe fi lms within the channel are similar to those used for the chip studies except both anode and cathode were sidewall gold electrodes.

The chips and fl uidic channel were examined using an Olympus MVX10 MacroView microscope. Fluorescence microscopy images were obtained using an Olympus DP72 digital camera connected to the fl uorescence microscope and the images were analyzed using either an Olympus cellSens Standard software or ImageJ (http://rsb.info.nih.gov/ij/). The patterned chips were also examined using profi lometry (Veeco Dektak 6M stylus profi lometer).

Acknowledgements The authors gratefully acknowledge fi nancial support from the R.W. Deutsch Foundation, the National Science Foundation (NSF; EFRI-0735987), and the Department of Defense, Defense Threat Reduction Agency (W91B9480520121) and the Offi ce of Naval Research (N000141010446). We acknowledge the Maryland NanoCenter and its

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FabLab for the chip and device fabrication. We thank Dr. Yi Cheng for his help with fl uidic device fabrication and Mr. Peter Dykstra for his help with profi lometric thickness measurements.

Received: September 24, 2010Published online: March 7, 2011

[ 1 ] S. Cosnier , Biosens. Bioelectron. 1999 , 14 , 443 – 56 . [ 2 ] X. W. Shi , X. H. Yang , K. J. Gaskell , Y. Liu , E. Kobatake , W. E. Bentley ,

G. F. Payne , Adv. Mater. 2009 , 21 , 984 – 8 . [ 3 ] C. A. Marquette , E. Imbert-Laurenceau , F. Mallet , C. Chaix ,

B. Mandrand , L. J. Blum , Anal. Biochem. 2005 , 340 , 14 – 23 . [ 4 ] C. Farre , N. Spinelli , A. Bouchet , C. Marquette , B. Mandrand ,

F. Garnier , C. Chaix , Synth. Met. 2007 , 157 , 125 – 33 . [ 5 ] L. M. Torres-Rodriguez , A. Roget , M. Billon , G. Bidan , Chem.

Commun. 1998 , 1993 – 4 . [ 6 ] N. Haddour , S. Cosnier , C. Gondran , J. Am. Chem. Soc. 2005 , 127 ,

5752 – 3 . [ 7 ] H. Xu , K. Malladi , C. Wang , L. Kulinsky , M. Song , M. Madou , Bio-

sens. Bioelectron. 2008 , 23 , 1637 – 44 . [ 8 ] R. S. Deinhammer , M. Ho , J. W. Anderegg , M. D. Porter , Langmuir

1994 , 10 , 1306 – 13 . [ 9 ] M. Delamar , R. Hitmi , J. Pinson , J. M. Saveant , J. Am. Chem. Soc.

1992 , 114 , 5883 – 4 . [ 10 ] M. A. Ghanem , J. M. Chretien , A. Pinczewska , J. D. Kilburn ,

P. N. Bartlett , J. Mater. Chem. 2008 , 18 , 4917 – 27 . [ 11 ] A. Adenier , M. M. Chehimi , I. Gallardo , J. Pinson , N. VilÃ , Langmuir

2004 , 20 , 8243 – 53 . [ 12 ] B. P. Corgier , S. Bellon , M. Anger-Leroy , L. J. Blum , C. A. Marquette ,

Langmuir 2009 , 25 , 9619 – 23 . [ 13 ] B. P. Corgier , A. Laurent , P. Perriat , L. J. Blum , C. A. Marquette ,

Angew. Chem.-Int. Edit. 2007 , 46 , 4108 – 10 . [ 14 ] B. P. Corgier , C. A. Marquette , L. J. Blum , J. Am. Chem. Soc. 2005 ,

127 , 18328 – 32 . [ 15 ] B. P. Corgier , C. A. Marquette , L. J. Blum , Biosens. Bioelectron. 2007 ,

22 , 1522 – 6 . [ 16 ] R. Polsky , J. C. Harper , S. M. Dirk , D. C. Arango , D. R. Wheeler ,

S. M. Brozik , Langmuir 2007 , 23 , 364 – 6 . [ 17 ] R. Polsky , J. C. Harper , D. R. Wheeler , S. M. Dirk , D. C. Arango ,

S. M. Brozik , Biosens. Bioelectron. 2008 , 23 , 757 – 64 . [ 18 ] E. Pavlovic , A. P. Quist , U. Gelius , L. Nyholm , S. Oscarsson , Lang-

muir 2003 , 19 , 4217 – 21 . [ 19 ] Y. L. Bunimovich , G. Ge , K. C. Beverly , R. S. Ries , L. Hood ,

J. R. Heath , Langmuir 2004 , 20 , 10630 – 8 . [ 20 ] K. Kim , J. Hwang , I. Seo , T. H. Youn , J. Kwak , Chem. Commun. 2006 ,

4723 – 5 . [ 21 ] K. Kim , M. Jang , H. S. Yang , E. Kim , Y. T. Kim , J. Kwak , Langmuir

2004 , 20 , 3821 – 3 . [ 22 ] K. Kim , H. Yang , S. Jon , E. Kim , J. Kwak , J. Am. Chem. Soc. 2004 ,

126 , 15368 – 9 . [ 23 ] K. Kim , H. Yang , E. Kim , Y. B. Han , Y. T. Kim , S. H. Kang , J. Kwak ,

Langmuir 2002 , 18 , 1460 – 2 . [ 24 ] M. N. Yousaf , M. Mrksich , J. Am. Chem. Soc. 1999 , 121 , 4286 – 7 . [ 25 ] W. S. Yeo , M. N. Yousaf , M. Mrksich , J. Am. Chem. Soc. 2003 , 125 ,

14994 – 5 . [ 26 ] M. Curreli , C. Li , Y. Sun , B. Lei , M. A. Gundersen , M. E. Thompson ,

C. Zhou , J. Am. Chem. Soc. 2005 , 127 , 6922 – 3 . [ 27 ] S. Dauphas , A. Corlu , C. Guguen-Guillouzo , S. Ababou-Girard ,

O. Lavastre , F. Geneste , New J. Chem. 2008 , 32 , 1228 – 34 . [ 28 ] P. M. Mendes , K. L. Christman , P. Parthasarathy , E. Schopf ,

J. Ouyang , Y. Yang , J. A. Preece , H. D. Maynard , Y. Chen , J. F. Stoddart , Bioconjugate Chem. 2007 , 18 , 1919 – 23 .

[ 29 ] H. Kaji , M. Hashimoto , M. Nishizawa , Anal. Chem. 2006 , 78 , 5469 – 73 .


[ 30 ] H. Kaji , M. Kanada , D. Oyamatsu , T. Matsue , M. Nishizawa , Lang-muir 2004 , 20 , 16 – 9 .

[ 31 ] H. Kaji , K. Tsukidate , M. Hashimoto , T. Matsue , M. Nishizawa , Langmuir 2005 , 21 , 6966 – 9 .

[ 32 ] H. Kaji , K. Tsukidate , T. Matsue , M. Nishizawa , J. Am. Chem. Soc. 2004 , 126 , 15026 – 7 .

[ 33 ] C. Kurzawa , A. Hengstenberg , W. Schuhmann , Anal. Chem. 2002 , 74 , 355 – 61 .

[ 34 ] L. Q. Wu , A. P. Gadre , H. M. Yi , M. J. Kastantin , G. W. Rubloff , W. E. Bentley , G. F. Payne , R. Ghodssi , Langmuir 2002 , 18 , 8620 – 5 .

[ 35 ] H. M. Yi , L. Q. Wu , W. E. Bentley , R. Ghodssi , G. W. Rubloff , J. N. Culver , G. F. Payne , Biomacromolecules 2005 , 6 , 2881 – 94 .

[ 36 ] J. Redepenning , G. Venkataraman , J. Chen , N. Stafford , J. Biomed. Mater. Res. A 2003 , 66 , 411 – 6 .

[ 37 ] X. Pang , I. Zhitomirsky , Mater. Chem. Phys. 2005 , 94 , 245 – 51 . [ 38 ] X. L. Luo , J. J. Xu , Y. Du , H. Y. Chen , Anal. Biochem. 2004 , 334 , 284 – 9 . [ 39 ] Q. M. Zhou , Q. J. Xie , Y. C. Fu , Z. H. Su , X. Jia , S. Z. Yao , J. Phys.

Chem. B 2007 , 111 , 11276 – 84 . [ 40 ] M. Cheong , I. Zhitomirsky , Colloid Surf. A-Physicochem. Eng. Asp.

2008 , 328 , 73 – 8 . [ 41 ] X. W. Shi , C.-Y. Tsao , X. Yang , Y. Liu , P. Dykstra , G. W. Rubloff ,

R. Ghodssi , W. E. Bentley , G. F. Payne , Adv. Funct. Mater. 2009 , 19 , 2074 – 80 .

[ 42 ] X. H. Yang , E. Kim , Y. Liu , X. W. Shi , G. W. Rubloff , R. Ghodssi , W. E. Bentley , Z. Pancer , G. F. Payne , Adv. Funct. Mater. 2010 , 20 , 1645 – 52 .

[ 43 ] Y. Zhang , H. Gu , Z. Yang , B. Xu , J. Am. Chem. Soc. 2003 , 125 , 13680 – 1 . [ 44 ] Z. Yang , H. Gu , D. Fu , P. Gao , J. K. Lam , B. Xu , Adv. Mater. 2004 , 16 ,

1440 – 4 . [ 45 ] R. V. Ulijn , A. M. Smith , Chem. Soc. Rev. 2008 , 37 , 664 – 75 . [ 46 ] L. Chen , K. Morris , A. Laybourn , D. Elias , M. R. Hicks , A. Rodger ,

L. Serpell , D. J. Adams , Langmuir 2010 , 26 , 5232 – 42 . [ 47 ] G. Cheng , V. Castelletto , C. M. Moulton , G. E. Newby , I. W. Hamley ,

Langmuir 2010 , 26 , 4990 – 8 . [ 48 ] D. J. Adams , L. M. Mullen , M. Berta , L. Chen , W. J. Frith , Soft Matter

2010 , 6 , 1971 – 80 . [ 49 ] D. J. Adams , M. F. Butler , W. J. Frith , M. Kirkland , L. Mullen ,

P. Sanderson , Soft Matter 2009 , 5 , 1856 – 62 . [ 50 ] A. M. Smith , R. J. Williams , C. Tang , P. Coppo , R. F. Collins ,

M. L. Turner , A. Saiani , R. V. Ulijn , Adv. Mater. 2008 , 20 , 37 – 41 . [ 51 ] V. Jayawarna , M. Ali , T. A. Jowitt , A. F. Miller , A. Saiani , J. E. Gough ,

R. V. Ulijn , Adv. Mater. 2006 , 18 , 611 – 4 . [ 52 ] C. Tang , A. M. Smith , R. F. Collins , R. V. Ulijn , A. Saiani , Langmuir

2009 , 25 , 9447 – 53 . [ 53 ] A. Mahler , M. Reches , M. Rechter , S. Cohen , E. Gazit , Adv. Mater.

2006 , 18 , 1365 – 70 . [ 54 ] R. Orbach , L. Adler-Abramovich , S. Zigerson , I. Mironi-Harpaz ,

D. Seliktar , E. Gazit , Biomacromolecules 2009 , 10 , 2646 – 51 . [ 55 ] E. K. Johnson , D. J. Adams , P. J. Cameron , J. Am. Chem. Soc. 2010 ,

132 , 5130 – 6 . [ 56 ] D. A. Buttry , M. D. Ward , Chem. Rev. 1992 , 92 , 1355 – 79 . [ 57 ] Y. Cheng , X. L. Luo , J. Betz , S. Buckhout-White , O. Bekdash ,

G. F. Payne , W. E. Bentley , G. W. Rubloff , Soft Matter 2010 , 6 , 3177 – 83 . [ 58 ] L. Chen , S. Revel , K. Morris , D. J. Adams , Chem. Commun. 2010 , 46 , 4267 – 9 . [ 59 ] H. X. Xu , A. K. Das , M. Horie , M. S. Shaik , A. M. Smith , Y. Luo ,

X. F. Lu , R. Collins , S. Y. Liem , A. M. Song , P. L. A. Popelier , M. L. Turner , P. Xiao , I. A. Kinloch , R. V. Ulijn , Nanoscale 2 , 960 – 6 .

[ 60 ] M. Zhou , A. M. Smith , A. K. Das , N. W. Hodson , R. F. Collins , R. V. Ulijn , J. E. Gough , Biomaterials 2009 , 30 , 2523 – 30 .

[ 61 ] V. Jayawarna , S. M. Richardson , A. R. Hirst , N. W. Hodson , A. Saiani , J. E. Gough , R. V. Ulijn , Acta Biomater. 2009 , 5 , 934 – 43 .

[ 62 ] L. Q. Wu , H. M. Yi , S. Li , G. W. Rubloff , W. E. Bentley , R. Ghodssi , G. F. Payne , Langmuir 2003 , 19 , 519 – 24 .

[ 63 ] E. Kim , Y. Liu , X. W. Shi , X. Yang , W. E. Bentley , G. F. Payne , Adv. Funct. Mater. 2010 , 20 , 2683 – 94 .


Reversible Electroaddressing of Self-assembling Amino-Acid Conjugates

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