J. Phys. Chem. C 2007, 111, 5351-5362
5351
Selective Permeation of a Liquidlike Self-Assembled Monolayer of 11-Amino-1-undecanethiol on Polycrystalline Gold by Highly Charged Electroactive Probes Jose´ M. Campin˜ a,* Ana Martins, and Fernando Silva Departamento de Quı´mica da Faculdade de Cieˆ ncias, UniVersidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal ReceiVed: September 22, 2006; In Final Form: December 13, 2006
Self-assembled monolayers (SAMs) of 11-amino-1-undecanethiol (AUT) have been prepared on polycrystalline Au by immersion of the corresponding surfaces in an AUT 1 mM solution in pure ethanol. The films were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and quartz crystal microbalance (QCM). Results of CV and EIS experiments in NaClO4 solution agree with the fast formation of a well-packed film with low current density (hundreds of nA/cm2) and capacitance values around 2 µF/cm2. The films were stable between -0.7 and 0.7 V (vs Ag/AgCl/NaCl sat.). Average values of 1.6 nm and 26° were obtained for the film thickness and the tilt chain angle in the potential region under study. The kinetic analysis of the adsorption process, monitored in situ by the QCM technique, showed that it occurs in two stages: a fast Langmuir type adsorption step (k1 ) 0.1047 min-1), followed by a much slower process of molecular rearrangement (k2 ) 0.0020 min-1). AFM operated in tapping mode did not reveal any morphological changes on the surface after immersion in AUT discarding multilayer growth. The electron transfer (ET) of Fe(CN)6-3 and Ru(NH3)6+3 species in solution through the AUT layer was investigated by CV and EIS. A mechanism of selective permeation of the electroactive species across the monolayer, controlled by the nature of the electrostatic interactions established at the SAM-solution interface, explains the experimental data obtained and previously reported in the literature for ET processes through substituted SAMs. Analysis of the film structure according to theoretical models (commonly used for SAM characterization) to our experimental data, led to contradictory results clearly affected by the nature of the unconsidered electrostatic interactions (values of θ ) 0.80 and 0.99 were obtained in the presence of Fe(CN)6-3 and Ru(NH3)6+3, respectively, in KCl electrolyte using the Amatore model).
1. Introduction The building of analytical and bioanalytical sensors based on the deposition of functionalized layers on solid substrates are gaining widespread importance. Alkanethiol self-assembly on gold has been used as an efficient method to prepare such stable and well-defined films that can be modified as for example by immobilization of macromolecules.1 The preparation of DNA sensors based on 11-mercaptoundecanoic acid (MUA)3 sets up a well-known example, but the self-assembly processes of many other thiol derivatives on gold and/or other substrates have been deeply investigated. Despite the significant effort and huge number of reported studies on that topic, much of the aspects related to the structure and the nature of the electron transfer (ET) processes through these monolayers still remain unclear.4,5 Chidsey et al.2 studied the alkanethiol, CH3(CH2)nSH, selfassembled monolayer (SAM) formation on gold and evaluated their effect on the redox response of highly charged electroactive species in solution. Almost perfect transfer blocking properties were obtained for the longer chain thiols (n g 9) while the electrochemical behavior resembled the corresponding to a microarray electrode for the shorter ones (n < 7) as the result of the formation of more disordered templates with a higher level of defects (pinholes). Consequently, the SAMs of long chain alkanethiols were described as certainly ordered, close * Corresponding author. E-mail:
[email protected].
packed, compact, and defect-free films being these properties associated with the compulsory exhibition of a near capacitive, effective blockage to electroactive species in solution. However, this description has been contradicted recently for long chain thiol molecules substituted with terminal-charged electroinactive but polar groups. The electrostatic interactions established between these charged groups and the highly charged electroactive species in solution have shown to govern their own redox response.6,7,8 In this case, the SAM behaves as an ideal capacitor or a microarray electrode as a function of the nature of such interactions. Its influence on the concentration of the electroactive species in the vicinity of the ET sites has been pointed out preferably to the hypothesis of pinhole induction and formation. Considering that most of the popular models used to estimate defects in SAMs (as the Sabatini and Rubenstein model,9,10,24 the Amatore model of defects size,25 etc.) are based on the comparison of the electron transfer on covered and bare surfaces, it seems that their application for charged SAMs should originate under- and overestimated results for repulsive and attractive interactions, respectively. In fact, most of the charged films studied in the literature were characterized only in conditions for which repulsive interactions were expected (i.e., in the presence of equally charged electroactive solution species and terminal groups) leading usually to high coverage values around 99-100%.9,24,43 No additional information about the results obtained in the presence of opposite-charged species is usually reported.
10.1021/jp0662146 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/28/2007
5352 J. Phys. Chem. C, Vol. 111, No. 14, 2007 The 11-amino-undecanethiol (AUT) is an 11-fold molecule with interesting properties for derivatization purposes. The reactivity of its terminal amine groups gives an opportunity for further modification and introduction of controlled functionality. In addition to this, the number of papers about supramolecular devices based on AUT is much lower in comparison with others such as MUA. A more thorough characterization of the AUT films becomes, therefore, a fundamental task before proceeding to the modification and building of AUT based devices and, additionally, an opportunity to improve our knowledge about the ET phenomena through these films. In this paper the preparation, structure and performance of AUT monolayers were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). The kinetics of the adsorption process was also studied by means of the quartz crystal microbalance (QCM) technique. The nature of the ET processes of Fe(CN)6-3 and Ru(NH3)6+3 solution species through the AUT layer was additionally investigated by electrochemical means. 2. Experimental Section 2.1. Chemicals and Solutions. 11-Amino-1-undecanethiol hydrochloride (AUT, Dojindo Laboratories), K3Fe(CN)6 (Merck, p.a.), Ru(NH3)6Cl3 (Aldrich, 98%), NaClO4‚H2O (Merck p.a.), NaF (Merck, suprapur), KCl (Merck, p.a.), ethanol (Aga, 96% vol.), N2 (N45, Air Liquid) and all other chemicals were analytical grade and were used as received. Five different solutions were prepared for the electrochemical characterization of the SAM. Solution E refers to a NaClO4 0.3 M electrolyte solution. For the experiments in the presence of electroactive species, the following solutions were prepared: NaF 0.2 M + K3Fe(CN)6 2 mM (A); NaF 0.2 M + Ru(NH3)6Cl3 2 mM (B); KCl 0.2 M + K3Fe(CN)6 2 mM (C); and KCl 0.2 M + Ru(NH3)6 Cl3 2 mM (D). The pH was not adjusted to avoid the unexpected effect of the buffer reagents. The values obtained for solutions A-D, using a Methrom 654 pH meter, were 7.82, 7.86, 5.46, and 5.04, respectively. The pKa of a self-assembled monolayer of AUT on Au has been estimated as ≈7.5 from the results reported by Degefa et al. (Figure 1.I in ref 7). It is shown there that the peak current for the redox process of Fe(CN6)-3/-4 species through an AUT monolayer suffers a great decrease in the pH region between 7 and 8 and that it is not effectively blocked until the pH is higher than 8. CVs obtained for a SAM of cystamine on Au in the presence of the same electroactive probe showed just a partially blocked profile even at a pH of 8.1.8 Consequently we should have at least partially charged monolayers in all cases. A solution of 8 mL of AUT 2 mM in ethanol was prepared for the QCM experiments. All the aqueous solutions were prepared in ultrapure water from a Milli-RO 3 Plus, coupled with a Milli-Q water purification system with a resistivity value of 18 MΩ cm. 2.2. SAM Preparation. AUT films were prepared on a gold polycrystalline rod electrode to apply electrochemical methods as follows. First, the electrode was decontaminated according to the Clavilier method.15 After treatment in a butane-air flame, the electrode was rinsed with ultrapure water then with ethanol and quickly immersed in an AUT 1 mM solution in ethanol. It was kept there for the investigated time. Finally, it was removed and copiously rinsed in ethanol and ultrapure water. This procedure was conducted prior to every electrochemical experiment. Preparation of AUT monolayers on Au polycrystalline slides was performed following a similar procedure. The Au slides were rinsed three times alternately in pure ethanol, acetone, and ultrapure water, dried in a N2 flow, and immersed
Campin˜a et al. in the AUT solution. After removal, the slides were rinsed in ethanol. A final drying step in N2 was applied previous to the AFM characterization. 2.3. Cyclic Voltammetry. Experiments were performed in a three-electrode cell under N2 flow using a Voltalab PGZ301 potentiostat (from Radiometer Analytical) controlled by VoltaMaster 4 software. A Ag/AgCl/NaCl saturated electrode (in a separated compartment containing the electrolyte and connected by a salt bridge to the main cell) and a helix-shaped gold wire were used as reference electrode and counter electrode, respectively. A bulk gold polycrystalline electrode with a surface area of 0.11 ( 0.05 cm2, estimated by the method of the electrochemical reduction of an AuO monolayer,11-14 was used as the working electrode in all cases. The contact between the electrode surface and the solution was made by means of the hanging meniscus method.31 Unless otherwise indicated, CV was performed at 50 mV/s scan rate. 2.4. Electrochemical Impedance Spectroscopy. Impedance spectra were registered at 10 mV amplitude, 10 kHz to 500 mHz frequency range, using the same Voltalab PGZ301 potentiostat. Through the use of the Eco Chemie frequency response analyzer (FRA) 4.9 software, the data obtained were fitted to: (a) a RQ equivalent circuit (i.e., a resistor R (electrolyte resistance) in series with a constant phase element (CPE) Q, for experiments in supporting electrolyte) and (b) a Randles circuit in the presence of electroactive species.42 The CPE is a power-law dependent interfacial capacity accounting for the topological imperfections, originated by the different crystal facets, and the surface roughness. FRA considers the admittance of a CPE as
Y ) (Qoiω)n
(1)
The fitted values of n and Qo are supplied by the software. However, the admittance of a CPE is more commonly described as
YCPE ) Yo(iω)n
(2)
Comparing eqs 1 and 2, we must conclude that the fitted values of Qo must be raised to the n power to obtain Yo in their main units of secn Ω-1. When n ) 1, the CPE is equivalent to an ideal capacitor18 and Yo ) Cdl, where Cdl is the corresponding double-layer capacitance in F. For experiments in pure electrolyte, n g 0.97 was obtained in all cases suggesting that the surface of the AUT-modified electrode is only slightly rough and behaves almost as an ideal capacitor. This assumption is not free of small error, but allows considering the area corrected values of Yo as electrode capacitances. However, G. J. Brug et al.49 indicated that the double-layer capacitance of an ideal polarized electrode is more appropriately represented by
Cdl ) (Yo)1/n (RΩ)1-n/n
(3)
This is equivalent to writing
Cdl ) Qo(RΩ)1-n/n
(4)
In these expressions, RΩ represents the solution resistance. Additionally, Hsu and Mansfeld52 suggested the following correction for the case of a depressed semicircle (as observed in the presence of electroactive species):
Cdl) Yo(ωmax)n-1
(5)
Selective Permeation of AUT Monolayers on Au
J. Phys. Chem. C, Vol. 111, No. 14, 2007 5353
Figure 2. CVs registered in NaClO4 0.3 M for an Au electrode previously immersed in AUT 1 mM solution for 30 min (s), 1 h. (-), 2 h. (- ‚), 6 h. (- ‚ ‚), 12 h. (‚ - ‚ -), and 24 h. (...) in NaClO4 0.3 M. Scan rate: 50 mV s-1. Figure 1. The first (s), second (- -), third (- ‚), and fourth (- ‚ ‚) consecutive CVs, obtained for a 24 h AUT modified Au electrode, are compared with the CV for the bare Au (...) in NaClO4 0.3 M. The scan rate was 50 mV s-1 in all cases.
with ωmax corresponding to the frequency at which the imaginary component of the impedance reaches a maximum in the semicircle region of the Nyquist plot. The capacitance values obtained using eqs 4, 5 were divided by the working electrode area (A). The final capacitance values presented in this work, consist on the average of five independent measurements (with their corresponding standard deviation) performed at each potential. This point becomes very important because details on the capacitance calculation are not generally discussed in the literature. 2.5. Atomic Force Microscopy. AFM imaging was performed in air using a PicoLe atomic force microscope (Molecular Imaging, USA) operated in tapping mode using n+-silicon cantilever/tips (Nanosensors, cantilever force constant of 2550 N/m). Commercial 1.8 × 1.8 cm gold polycrystalline slides (EMI) were used as substrates. The images were scanned in topography, amplitude, and phase mode with a resolution of 512 × 512 pixels and were representative of at least five 1 × 1 micrometer images taken over different locations on the studied surfaces. 2.6. Quartz Crystal Microbalance. The AUT adsorption process was studied in situ using a Picobalance V3 oscillator, from Technobiochip Scarl, controlled by Picoview 3.0 software. 10 MHz overtone polished AT-cut plano-plano quartz crystals coated with gold electrodes (5 mm diameter) on both sides and connected to the oscillator by chromium spring contacts were purchased from International Crystal Manufacturing (ICM) Co, Inc. The experiments were performed exposing just one side of the quartz plate to the solution phase. Pure ethanol was injected into the cell after probe equilibration in ambient air. It was followed by the addition of the concentrated thiol solution. The resonance frequency shift of the quartz crystal was then recorded in a time-resolved manner. 3. Results and Discussion 3.1. Electrochemical Characterization in Electrolyte Solution. Four consecutive CVs obtained (increasing progressively the potential limits from (0.5 V to (1.250 V) for a 24 h AUT immersed Au electrode in solution E are compared with the CV obtained for the bare Au electrode in Figure 1. A significantly lower current density (>10 times at 0.5 V) was observed for the first recorded cycle when compared to the bare
Au. The blocking properties of the modified electrode showed to be stable within a potential region of ( 0.7 V. As the potential window was extended beyond these values, an oxidation current was observed consistent with the electrochemical desorption of SAMs.16 The process showed to be irreversible reverting to a voltammetric profile that resembled the one for the bare Au just after four cycles. The effect of the immersion time was also investigated from 30 min to 24 h. The modified electrodes were studied by CV within the previously determined region of potential stability and their comparison is shown in Figure 2. No significant differences in shape and current density (which fell in the order of few hundreds of nA/cm2 in all cases) were observed. EIS experiments showed good agreement with the voltammetric data. The Nyquist plots recorded at 0.150 V for the modified electrodes and the bare Au in NaClO4 0.3M are shown in Figure 3A. No significant differences were observed between the different AUT immersed electrodes, which exhibited a linear response close to the typical behavior of a pure capacitor. The Bode and Bode phase plots obtained for two different immersion times (30 min and 24 h) and for the bare Au electrode are shown in Figure 3B. The values of log |Z| obtained for the latter were lower than those obtained for the AUT immersed electrodes in the whole frequency domain, which confirms that these modified electrodes exhibit stronger capacitor behavior. Theoretically, the SAM behavior is well described by the Helmholtz ideal capacitor model when the phase angle at 1 Hz is 90°.18 At higher frequencies, the total impedance is controlled by the solution resistance. Phase angles higher than 88° in the medium-lowfrequency region (1-1000 Hz) are considered in practical terms to correspond to pure capacitor behavior. Bode phase plots for the modified electrodes exhibited a value close to 90° in a wider range of frequencies than for the bare Au confirming their enhanced capacitor character again. No significant difference was observed in the Bode and Bode Phase plots for both immersion times. The impedance data were fitted to an RQ equivalent circuit. The capacitances obtained at 0.150V, as described in the experimental section, for the different immersion times and for the bare Au are presented in Table 1. After immersion in the thiol solution, the capacitance strongly decreased from values of 20 µF/cm2 (obtained for Au) to approximately 2 µF/cm2 found for the modified electrodes. These values are in good agreement with the reported for alkanethiol SAMs with more than 10 methylene groups (1-5 µF/cm2 1,21) and with the ones for Au electrodes (between 20 and 50 µF/cm2 depending on the applied potential and the electrolyte composition1,45). Capacitances obtained at three different potentials for the films prepared with 24 h and 30 min
5354 J. Phys. Chem. C, Vol. 111, No. 14, 2007
Campin˜a et al.
Figure 3. (A) Nyquist plots registered at 0.150 V for the different AUT immersed electrodes: 30 min (0), 1h (O), 2h (4), 6h (3), 12h ("), 24h (
