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Electron transfer kinetics across derivatized self-assembled monolayers on platinum: a cyclic voltammetry and electrochemical impedance spectroscopy study
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 574 (2004) 15–22 www.elsevier.com/locate/jelechem
Electron transfer kinetics across derivatized self-assembled monolayers on platinum: a cyclic voltammetry and electrochemical impedance spectroscopy study Rosa Brito a, Rolando Tremont b, Carlos R. Cabrera a
a,*
Department of Chemistry, University of Puerto Rico, Rıo Piedras Campus, P.O. Box 23346, San Juan 00931-3346, Puerto Rico Department of Chemistry, CUH Station, 100 Carr 908, University of Puerto Rico at Humacao, Humacao 00791, Puerto Rico
b
Received 15 July 2003; received in revised form 10 May 2004; accepted 26 May 2004 Available online
Abstract Platinum surfaces derivatized with mercapto and amine compounds have been used to perform this study. The compounds used to derivatize the platinum surfaces were 3-mercaptopropionic acid (HSC2 H4 COOH, 3-MPA), 16-mercaptohexadecanoic acid (HSC15 H30 COOH, 16-MHDA), and 3-aminopropyltrimethoxysilane (H2 NC3 H6 Si(OCH3 )3 , 3-APS). Previously, the derivatization process has been characterized by XPS and FT-IR [Electroanal. Chem. 540 (2003) 53]. In current work, the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, formed by the derivatization of the 3-MPA/Pt and 16-MHDA/Pt surfaces with 3-APS, were characterized by cyclic voltammetry using 2.5 mM K3 Fe(CN)6 in 0.1 M KCl or 2.5 mM Ru(NH3 )6 Cl3 in 0.1 M KCl solution. These redox systems were chosen to study the defect site induced by electrochemical desorption and its effect on the charge transfer processes. Electrochemical impedance spectroscopy was used as an additional electrochemical tool, to study these derivatized platinum surfaces. This technique was useful in the determination of the pinholes formed on the derivatized platinum surfaces, or different desorption potentials. Ó 2004 Published by Elsevier B.V. Keywords: Self-assembled monolayer; 3-Mercaptopropionic acid; 16-Mercaptohexadecanoic acid; 3-Aminopropyltrimethoxysilane; Electrochemical impedance spectroscopy
1. Introduction Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of active surfactants on a solid surface. This is a technique that provides an elegant route to the preparation of well-defined organic assemblies on solid surfaces, especially electrode materials [2]. The formation of monolayers by self-assembly of surfactant molecules at surfaces is an example of the general phenomena that occurs by selfassembly. The advantages of SAMs include their ease preparation, stability, and the possibility of introducing different chemical functionalities. The incorporation of *
Corresponding author. Tel.: +787-764-0000-1-4807; fax: +787-7568242. E-mail address: [email protected] (C.R. Cabrera). 0022-0728/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.jelechem.2004.05.031
the appropriate chemical functionality with some molecular level control into the highly ordered monolayers allows the preparation of surfaces with tailor-made properties [3]. Many of these monolayer systems have characteristic structures and properties and find application in the areas of adhesion, corrosion, lubrication, biocompatibility, and catalysis [3,4]. In the field of electrochemistry, the strong interest in SAMs based on thiols and related molecules is due to the following aspects: (1) They can be employed as insulating barriers between an electrode and a redox couple to study long range electron transfer [5–11]; (2) they can be used to prepare microarray electrodes which have potential applications for creating selective voltammetric detectors or for measuring very fast electron transfer kinetic [12–16]. Cyclic voltammetry (CV), chronoamperometry, and linear scan voltammetry have been the most popular
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R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
electrochemical techniques used to study redox processes at monolayer covered gold [5,8,11,17,18]. However, these methods incur problems due to the current, which is very small, thus making it difficult to measure it at low overpotential, and the double layer charging current is difficult to eliminate [17]. Electrochemical impedance spectroscopy (EIS) can be used to overcome these difficulties and provides a sensitive way to study electron transfer across a thin monolayer. The technique in which the cell or electrode impedance is plotted vs. frequency is called EIS [19]. The electrochemical impedance spectroscopic approach is largely based on similar methods used to analyze circuits in electrical engineering practice [20]. The measured total impedance of the cell, Z, in a bridge is expressed as the series combination of a resistance value RB and the capacitance value CB . These two elements provide the real and imaginary components of Z, that is [16], ZRe ¼ RB and ZIm ¼ 1=xCB . The real part, which must equal the measured ZRe , is
parameter Ra , the method presented by Finklea et al. [15,16,22], was used. Derivation of the faradaic impedance for a microarray electrode with 1 h < 0:05 follows from the formal equivalence of the microarray diffusion problem to an electron transfer step with preceding and succeeding chemical steps [22,23]. Substituting the equivalent expressions for a microarray electrode into the general expression for the faradaic impedance for coupled reaction schemes [16], noting that h is approximately one, and rearranging terms leads to the following expression for the in-phase (Z 0 ) components of the faradaic impedance:
ZRe ¼ RB ¼ RX þ ½Rs =ðA2 þ B2 Þ;
q ¼ D=0:36R2a ;
ð1Þ
where RX is the solution resistance, the series resistance is Rs , A ¼ ðCdl =Cs Þ þ 1
and
B ¼ xRs Cdl ;
where Cdl is the double layer capacitance, Cs is the pseudocapacity, and x is the radial frequency. Similarly, ZIm ¼ 1=xCB ¼ f½ðB2 =xCdl Þ þ ðA=xCs Þ=ðA2 þ B2 Þg; ð2Þ
Z 0 ¼ ½Rct =ð1 hÞ þ ½r=ðxÞ1=2 þ ½r=ð1 hÞ f½ðx2 þ q2 Þ1=2 þ q=ðx2 þ q2 Þg1=2 ;
ð7Þ
where r ¼ ð2Þ1=2 ðRT =F 2 AcðDÞ1=2 Þ
ð8Þ
and ð9Þ
where c is the concentration of either the oxidized or reduced forms, D is the diffusion coefficient, Ra is the pinhole radius, and q is the transition radial frequency. Limiting cases are obtained by setting much larger or much smaller than the transition radial frequency q. The high-frequency case x q, which corresponds to nearly isolated diffusion profiles for each microelectrode, yields the following expression for the faradaic impedance components: Z 0 ¼ ½Rct =ð1 hÞ þ ½r=ðxÞ1=2 þ ½r=ð1 hÞðxÞ1=2 :
where the series resistance, Rs , and the Cs , are Rs ¼ Rct þ r=x1=2 ;
ð3Þ
Cs ¼ 1=rx1=2 ;
ð4Þ
where r is the Warburg impedance slope and Rct is the charge transfer resistance. Substitution for Rs and Cs by (3) and (4) provides
ð10Þ By using these equations, the value Ra was calculated. The contribution of this work is focused on the understanding of the interfacial properties of derivatized platinum surfaces with organic compounds and their possible applications in the field of molecular sensors.
ZRe ¼ RX þ fðRct þ rx1=2 Þ =½ðCd rx1=2 Þ2 þ x2 Cd2 ðRct þ rx1=2 Þ2 g;
ð5Þ
2
ZIm ¼ f½xCd ðRct þ rx1=2 Þ þ rx1=2 ðx1=2 Cd r þ 1Þ =ðCd rx
1=2
2
þ 1Þ þ x
2
Cd2 ðRct
þ rx
1=2 2
Þ:
2. Experimental 2.1. Chemicals
ð6Þ
Chemical information can be extracted by plotting ZIm vs. ZRe for different x. For simplicity, the limiting behavior at high and low x are considered. Recently, Xing et al. [21] studied the electron transfer behavior across a dodecanothiol monolayer on gold using EIS techniques carried out at various dc potentials. EIS also was used to obtain the pinhole radius (Ra ), formed by organic molecules on the Pt surfaces at different desorption potentials. In order to determine the
3-MPA (Aldrich, 99%), 16-MHDA (Aldrich, 98%), 3-APS (Aldrich, 97%), sulfuric acid (Aldrich, 99,999%), potassium ferricyanide (K3 Fe(CN)6 ) (Aldrich, 99%), hexaammineruthenium (III) chloride [Ru(NH3 )6 Cl3 ] (Aldrich, 98%), potassium chloride (KCl), 4-methylmorpholine (Aldrich, 99.5%), isobutylchloroformate (Aldrich, 98%), acetonitrile anhydrous (Aldrich, 99.8%), ethyl acetate and N,N 0 -dimethylformamide (DMF) were used as received. Tetra-n-butylammonium perchlorate (TBAP) was recrystallized four times from ethyl acetate
R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
(Fisher Scientific) and dried under vacuum for 48 h. The water used in our experiments was previously distilled and pumped through a nanopure system (Barnstead) to give 18 MX cm water. All electrochemical measurements were done at room temperature. The aqueous solutions were degassed with nitrogen for least 30 min prior to an electrochemical experiment.
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et al. [28]. The 3-MPA/Pt and 16-MHDA/Pt films were exposed to a 2 mM solution of 3-APS, in DMF, for 1 h. Finally, the films were removed, rinsed with ethanol, dried with argon and maintained in desiccators for further characterization. The amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems were used to perform this study.
2.2. Cell and instrumentation 3. Results and discussion A three electrode cell was used in all experiments. Electrodes of HgjHg2 SO4 and AgjAgCl were used as the reference electrodes, in aqueous solvent and in nonaqueous solvent, respectively. A platinum mesh electrode was used as the counter electrode. Platinum disk electrodes (geometric area: 0.02 cm2 ) were used as the working electrodes. A Princeton Applied Research (PAR) 273A potentiostat/galvanostat, controlled with PAR 270 Research Electrochemistry software installed in a personal computer, and a frequency response detector Model 1025 (EG&G), were used in our experiments. In our cyclic voltammetry experiments the redox species used were Fe(CN)3 or Ru(NH3 )3þ in 0.1 M 6 6 KCl electrolyte solution. Impedance data for the bare and derivatized platinum surfaces were measured in a solution, containing 2.5 mM K3 Fe(CN)6 and 2.5 mM of K4 Fe(CN)6 in 0.1 M KCl, at their open circuit potentials (Ec ). Sinusoidal waves of voltages (5 mV/rms) in the frequency range between 100 KHz and 0.01 Hz were superimposed on Ec [22–25]. 2.3. Substrate pre-treatment and derivatization All substrates used were new and, before their modification they were submitted to a cleaning treatment. To be sure that the surface of the electrode was clean, CV was carried out in 0.5 M H2 SO4 solution. Then, the electrodes were washed with ethanol and nanopure water. Finally, they were dried with an argon gas flow. The current–potential behavior, characteristic of a clean platinum surface in a clean test solution [26], has been presented in a previous publication [27]. This was used as a criterion of solution and electrode cleanliness in aqueous 0.5 M H2 SO4 . Whenever we used a platinum electrode, we verified its cleanliness by cyclic voltammetry before its chemical modification. The derivatization process for the platinum surface electrodes has been previously reported [1]. The previously cleaned substrates were submerged in 1 mM solutions of 3-MPA and 16-MHDA, in ethanol, for 24 h. Afterward, the substrates were washed with ethanol and dried under an argon gas flow. The chemical derivatization of 3-MPA/Pt and 16-MHDA/Pt surfaces, with the amine-containing molecule (3-APS), was done by a modified version of a method presented by Bruening
3.1. Cyclic voltammetry The electrochemical characterization of the derivatized platinum surface can reveal the presence of a full coverage of the electrode. Cyclic voltammetric studies were done using redox active species in solution. The Pt disk electrodes, bare and derivatized, were characterized by CV using a 2.5 mM solution of Fe(CN)3 or 6 Ru(NH3 )3þ 6 , as the redox active couple, in 0.1 M KCl. The CVs observed in Fig. 1A(b) and 1B(b) correspond to the electrochemical behavior of Ru(NH3 )3þ and 6 Fe(CN)3 6 , respectively, at the amide(3-MPA)/Pt derivatized electrode. The CV in Fig. 1A(b) is similar to that observed at a bare platinum disk electrode (Fig. 1A(a)). The peak currents are clearly defined and this fact demonstrates that the redox couple undergoes the redox process without inhibition. Nevertheless, the voltammogram of Fe(CN)3 6 at an amide(3-MPA)/Pt electrode (Fig. 1B(b)) presents differences in the peak separation and in the peak currents, which are reduced after the derivatization of the Pt electrode with amide(3-MPA). This demonstrates that the peak current change can be attributed to tunneling at defect sites. For the amide(16-MHDA)/Pt electrode we found different cyclic voltammetric behavior of the redox couples. The voltammogram of Fe(CN)3 at an am6 ide(16-MHDA)/Pt electrode, in Fig. 2B(b), shows that the peak currents were significantly suppressed by the
Fig. 1. Cyclic voltammograms of (A) 2.5 mM Ru(NH3 )6 Cl3 and (B) 2.5 mM Fe(CN)3 6 , in 0.1 M KCl solution, at the (a) bare platinum disk electrode and (b) amide(3-MPA)/Pt electrode system. Scan rate 100 mV/s.
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R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
Fig. 2. Cyclic voltammograms of (A) 2.5 mM Ru(NH3 )6 Cl3 and (B) 2.5 mM Fe(CN)3 6 , in 0.1 M KCl solution, at the (a) bare platinum disk electrode and (b) amide(16-MHDA)/Pt electrode system. Scan rate 100 mV/s.
Fig. 3. Linear sweep voltammetry for (a) platinum bare, (b) amide(3MPA/Pt), and (c) amide(16-MHDA/Pt) in 0.1 M TBAP + CH3 CN solution. Scan rate: 100 mV/s. Geometric area was 0.020 cm2 .
derivatization, compared with the cyclic voltammogram of Fe(CN)3 at a bare platinum disk electrode 6 (Fig. 2B(a)). The CV changes drastically from an electrochemically reversible shape to a capacitive shape after the Pt electrode has been derivatized. In this case, it is probable that the amide(16-MHDA)Pt system shows very low mass transfer effects, which are characteristic of pinhole defects in the amide(16-MHDA)Pt monolayer. It is reasonable to conclude that the redox couple cannot contact the platinum. We found similar voltammetric behavior previously with platinum electrodes modified with 3-MPS and 3-APS [27]. Fig. 2A(b) shows the electrochemical behavior of Ru(NH3 )3þ 6 at an amide(16MHDA)Pt electrode. This voltammogram presents differences in the peak current which decreases and its peak shape becomes more sigmoidal. This electrochemical behavior indicates that the platinum surface has changed with the derivatization. 3þ Compared with Fe(CN)3 6 , Ru(NH3 )6 shows faster electron transfer kinetics and a larger faradaic current at the same amide(3-MPA)Pt and amide(16-MHDA)Pt electrodes. The difference in the voltammograms may derive from the different heterogeneous electron transfer rates for the two reactions, i.e., Ru(NH3 )3þ 6 is a much faster redox couple. The changes in the shapes of the voltammograms can be analyzed by theories of voltammetry at ultramicroelectrodes. Since the effect of the microelectrode assembly is to lower the apparent heterogeneous electron transfer rate, this effect would be less pronounced with the faster couple. In voltammetry, it appears as an increase in the peak separation. As expected, Fig. 1B(b) and Fig. 3B(b) demonstrate that the Fe(CN)3 6 couple has a larger peak separation than Ru(NH3 )3þ 6 .
linear sweep voltammetry (LSV) in 0.1 mol/l TBAP + CH3CN solution. Fig. 3 shows voltammograms obtained for these systems, including the blank. From these voltammograms, it was determined that the desorption potentials for the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems were )1.6 and )1.8 V, respectively. In addition, the amount of compound desorbed from the platinum surface was determined from these voltammograms, relating the area under the desorption curve to the amount of charge involved. For the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems the amounts of thiol desorbed were: 4.4 1011 and 3.1 1011 mol/cm2 , respectively. These amounts correspond to an order of magnitude lower than that for a monolayer on a gold surface (1010 mol/cm2 ) [19]. The amide(16-MHDA)/Pt system was chosen to induce defect sites by electrochemical desorption studies. Fig. 4A(a) and B(a) were obtained from a platinum disk electrode, and Fig. 4A(b) and B(b) were obtained at an amide(16-MHDA)/Pt electrode, in 2.5 mM Ru(NH3 )6 Cl3 + 0.1 M KCl solution and in 2.5 mM
3.2. Electrochemical desorption studies
Fig. 4. Cyclic voltammograms of (A) 2.5 mM Ru(NH3 )6 Cl3 and (B) 2.5 mM Fe(CN)3 6 , in 0.1 M KCl solution, at the bare platinum disk electrode (a), amide(16-MHDA)/Pt electrode system without electrochemical desorption (b), and with electrochemical desorption pretreatment at (c) )1.4, (d) )1.6, and (e) )1.8 V. Scan rate: 100 mV/s.
Desorption of amide(3-MPA) and amide(16-MHDA) from the modified platinum surfaces were determined by
R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
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Fe(CN)3 6 + 0.1 M KCl solution, respectively. The voltammetric behavior, in Fig. 4A(b), is characteristic of probe molecules that cannot contact the surface or penetrate the monolayer. Only electrons that tunnel through the monolayer may reduce them. Fig. 4A(c–e) and B(c–e) shows the behavior of an amide(16-MHDA)/ Pt electrode, prepared with different desorption potentials, in the probe molecule solution. In general, it can be deduced that the currents at the modified platinum surfaces with defect sites are much higher than that at the completely modified platinum electrode. This indicates that the defect sites allow the probe molecule to penetrate through the monolayer and undergo electron transfer with the platinum surfaces. On the other hand, the sigmoidal CV curves, which are expected for an ultramicroelectrode array, show the presence of very small pinhole defects in the monolayer. Compared with 3þ Fe(CN)3 has faster electron transfer ki6 , Ru(NH3 )6 netics and a larger faradaic current at the same amide(16-MHDA)Pt electrode surfaces with different desorption potentials. 3.3. EIS studies EIS was used as an additional electrochemical tool, to study the derivatized platinum surfaces. This technique was useful in the determination of the pinhole size prepared from the derivatized platinum surfaces at different desorption potentials. In order to study the derivatization of platinum surfaces with amide(3-MPA) and amide(16-MHDA), Nyquist and Bode plots were obtained using EIS. Figs. 5 and 6 shows the impedance spectrum, in a Nyquist and Bode presentation, for amide(3-MPA/ Pt) and amide(16-MHDA/Pt) electrode systems, respectively, at different desorption potentials in the presence of 2.5 mM [Fe(CN)6 ]3=4 with 0.1 M KCl as the supporting electrolyte. The electrochemical desorption potential, vs. AgjAgCl, varies negatively as indicated in the figure. It is observed in Figs. 5A and 6A that the curves deflect downwards with increasingly negative desorption potential, indicating that the charge transfer resistance decreases with the more negative desorption potential. This also demonstrates that the applied potential affects the interfacial properties of the derivatized platinum electrodes. Similar results were reported by Boubour and Lennox [29] in studies realized with n-alkanethiols self-assembled monolayers on gold electrodes. The parameters obtained by EIS studies are presented in Table 1. These values were used to calculate the Rct , Cdl , and Ra values. The Rct on the derivatized platinum surfaces decreases until it reaches a minimum resistance of 4.3 and 71.0 kX cm2 , at )1.6 and )1.8 V vs. AgjAgCl, for the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, respectively. The Rct for the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, without an applied
Fig. 5. The electrochemical impedance spectroscopy of amide(3-MPA) on platinum. (A) Nyquist plots and (B) Bode plots for the bare platinum disk electrode (a), amide(3-MPA)/Pt electrode system without electrochemical desorption (b), and with electrochemical desorption pretreatment at (c) )0.5, (d) )1.0, (e) )1.2, (f) )1.4, and (g) )1.6 V, respectively. Geometric area: 0.02 cm2 .
desorption potential, was 10.7 and 1099 kX cm2 , respectively (see Table 1). A diminution of the charge transference resistance, for these two systems, as the desorption potential is increased, is expected since, as more molecules of the electrode surfaces are desorbed, more Pt surface would be exposed and, therefore, there would not be such as impediment for the probe molecule to experience its redox process. The Rct values, for the amide(16-MHDA/Pt) system are much greater than Rct values for the amide(3-MPA/Pt) system. This is because the amide(16-MHDA/Pt) system has a compound with a longer chain, compared with the amide(3-MPA/Pt) system. From the Bode plots, the curve for log jZj vs. log x can yield values of Rct and RX . In addition, at intermediate frequencies, the break point of this curve should be a straight line with a slope of )1. Extrapolating this line to the log jZj axis at x ¼ 1ðlog x ¼ 0Þ yields the value of Cdl from the Eq. (7). The values of Cdl were obtained from the Bode plots. These values are presented in Table 1.
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R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
Fig. 7. Variation of the capacitance (Cdl ) at different desorption potentials in the (A) amide(3-MPA/Pt) and (B) amide(16-MHDA/Pt) systems.
Fig. 6. The electrochemical impedance spectroscopy of amide(16MHDA) on platinum. (A) Nyquist plots and (B) Bode plots for the bare platinum disk electrode (a), amide(16-MHDA)/Pt electrode system without electrochemical desorption (b), and with electrochemical desorption pretreatment at (c) )1.0, (d) )1.2, (e) )1.4, (f) )1.6, and (g) )1.8 V, respectively. Geometric area: 0.02 cm2 .
jZj ¼ 1=Cdl :
ð11Þ
The Cdl values as the desorption potential increases, in the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, are presented in Fig. 7A and B, respectively. The
amide(3-MPA) system produces a larger double layer capacitance (Cdl ) of 3.09 lF/cm2 at desorption potentials of )1.6 V. A similar result is obtained for the amide(16MHDA/PT) system at )1.8 V vs. AgjAgCl, where the capacitance value is 2.54 lF/cm2 (see Table 1). This behavior is explained by the presence of fewer molecules on the surface. In this manner, the surface will be more accessible to form a double layer with the electrolyte in solution. At lower desorption potentials, the double layer capacitance, for both systems, decreases in a similar way. The values obtained for Ra , for the two systems in this study, increase as the desorption potential becomes more negative. This behavior is observed in Fig. 8. In
Table 1 Parameters obtained by EIS for the platinum surfaces derivatized and treated at different desorption potentials Desorption potential (V)
Rs (kX cm2 )
Pt bare –
0.32
Amide(3MPA/Pt) Without )0.5 )1.0 )1.2 )1.4 )1.6
0.35 0.34 0.34 0.35 0.34 0.32
Amide(16-MHDA/Pt) Without )1.0 )1.2 )1.4 )1.6 )1.8
0.33 0.34 0.34 0.34 0.34 0.32
Rp (kX cm2 ) 0.96 11.0 10.1 9.3 8.1 6.0 4.6 1100 909 659 180 169 72
Rct (kX cm2 ) 0.64 10.7 9.8 9.0 7.8 5.7 4.3 1099 909 659 179 168 71
Cdl (lF/cm2 ) 127
r (X s1=2 )
1.21
108 D (cm2 /s)
q (s1 )
106 Ra (cm)
3770
–
–
1.46 1.52 1.57 1.62 2.71 3.09
40.5 33.5 32.6 30.0 29.3 29.0
3.4 5.0 5.2 6.1 6.4 6.6
543 526 421 394 361 318
13.0 16.0 19.0 21.0 22.0 24.0
1.25 1.34 1.55 1.72 2.51 2.54
54.0 48.0 46.0 33.0 25.0 21.0
1.9 2.4 2.6 5.1 8.8 12.5
646 303 221 160 125 104
9.0 14.0 18.0 29.0 44.0 58.0
R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
Fig. 8. Variation of the Ra at different desorption potentials in the (A) amide(3-MPA/Pt) and (B) amide(16-MHDA/Pt) systems.
both case a linear relation is observed. In the case of the amide(3-MPA/Pt) system (see Fig. 8A), the Ra variation was between 13.0 106 and 24.0 106 cm, and for amide(16-MHDA/PT) (see Fig. 8B), the Ra variation was between 9.0 106 and 58.0 106 cm.
4. Conclusions Taking into account the results obtained from the applied electrochemical studies of the platinum surfaces derivatized with 3-MPA, 16-MHDA, and 3-APS, the following conclusions were reached. The analysis of the cyclic voltammetric results indicates that the degree of coverage, the presence of holes and their sizes, and the possible interactions between the different species involved are the principal factors that determine the shape of the voltammograms. The present study has shown that, in the presence of a derivatized platinum surface, the electron transfer between redox species and a platinum electrode is reduced significantly and mass transfer effects minimized, indicating that the amide(16-MHDA)/Pt system, compared with the amide(3-MPA)/Pt system, is free of mass transfer effects. The blocking effect produced by amide(16-MHDA/Pt) was much more noticeable than that produced by am4 ide(3-MPA/Pt) in the case of Fe(CN)3 6 /Fe(CN)6 charge transfer where almost no current flowed. The effect of the amide(3-MPA/Pt) system is much less 3þ evident in the case of Ru(NH3 )2þ 6 /Ru(NH3 )6 ; its voltammetric behavior remains almost intact when amide(3-MPA) modifies the Pt electrode. On the other hand, in the electrochemical desorption potential studies 3þ carried out, compared with Fe(CN)3 6 , Ru(NH3 )6 , as the desorption potential is increased, has faster electron
21
transfer kinetics and a larger faradaic current at the same amide(16-MHDA)/Pt electrode. In the EIS studies it is observed that the charge transfer resistance (Rct ), using the amide(16-MHDA/Pt) system, is much larger than the Rct obtained for the amide(3-MPA/Pt) system and the clean platinum surface. Moreover, it was found that Rct diminished in the two systems studied, as the desorption potential increased. This is due to the exposure of the platinum surface, as molecules are desorbed from the platinum surface, allowing the probe molecule to undergo its redox process. The Cdl values increased, for the two systems in this study, as the desorption potential increased. This behavior is explained by the presence of fewer molecules on the surface. In this manner, the surface is more accessible to form the double layer with the electrolyte in solution. On the other hand, the Ra values also increased, for the two systems in this study, as the desorption potential increased.
Acknowledgements The authors acknowledge the use of facilities of the Materials Characterization Center of the University of Puerto Rico. This work had the financial support of the NSF-EPSCoR program, Grant No. EPF-9874782. R.B. acknowledges financial support from the Fondo Nacional de Ciencia, Innovaci on y Tecnologıa, FONACIT, from Venezuela.
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[16] H.O. Finklea, D.A. Snider, J. Fedyk, E. Sabatini, Y. Gafni, I. Rubinstein, Langmuir 9 (1993) 3660. [17] C.J. Miller, P. Cuendet, M. Gr€atzel, J. Phys. Chem. 95 (1991) 877. [18] C.J. Miller, M. Gr€atzel, J. Phys. Chem. 95 (1991) 5225. [19] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 2001, pp. 376–384. [20] J.R. Macdonald, W.B. Johnson, Impedance Spectroscopy, Wiley, New York, 1987, p. 1. [21] Y.F. Xing, S.J. O’Shea, S.F.Y. Li, J. Electroanal. Chem. 542 (2003) 7. [22] H.O. Finklea, S. Avery, M. Lynch, T. Furtsch, Langmuir 3 (1987) 409.
[23] J. Xu, H.L. Li, Y.J. Zhang, J. Phys. Chem. 97 (1993) 11497. [24] E. Sabatini, I. Rubintein, J. Phys. Chem. 91 (1987) 6663. [25] H. Seigenthaler, K. Juttner, J. Electroanal. Chem. 163 (1984) 327. [26] D.L. Allara, R.G. Nuzzo, Langmuir 1 (1985) 45; L. Yan, C. Marzolin, A. Terfort, G.M. Whitesides, Langmuir 13 (1997) 6704. [27] R. Brito, J. Figueroa, C.R. Cabrera, J. Electroanal. Chem. 520 (2002) 47. [28] L.M. Bruening, Y. Zhou, G. Aguilar, R. Agee, D.E. Bergbreiter, R.M. Crooks, Langmuir 13 (1997) 770. [29] E. Boubour, R.B. Lennox, J. Phys. Chem. B 104 (2000) 9004.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 574 (2004) 15–22 www.elsevier.com/locate/jelechem
Electron transfer kinetics across derivatized self-assembled monolayers on platinum: a cyclic voltammetry and electrochemical impedance spectroscopy study Rosa Brito a, Rolando Tremont b, Carlos R. Cabrera a
a,*
Department of Chemistry, University of Puerto Rico, Rıo Piedras Campus, P.O. Box 23346, San Juan 00931-3346, Puerto Rico Department of Chemistry, CUH Station, 100 Carr 908, University of Puerto Rico at Humacao, Humacao 00791, Puerto Rico
b
Received 15 July 2003; received in revised form 10 May 2004; accepted 26 May 2004 Available online
Abstract Platinum surfaces derivatized with mercapto and amine compounds have been used to perform this study. The compounds used to derivatize the platinum surfaces were 3-mercaptopropionic acid (HSC2 H4 COOH, 3-MPA), 16-mercaptohexadecanoic acid (HSC15 H30 COOH, 16-MHDA), and 3-aminopropyltrimethoxysilane (H2 NC3 H6 Si(OCH3 )3 , 3-APS). Previously, the derivatization process has been characterized by XPS and FT-IR [Electroanal. Chem. 540 (2003) 53]. In current work, the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, formed by the derivatization of the 3-MPA/Pt and 16-MHDA/Pt surfaces with 3-APS, were characterized by cyclic voltammetry using 2.5 mM K3 Fe(CN)6 in 0.1 M KCl or 2.5 mM Ru(NH3 )6 Cl3 in 0.1 M KCl solution. These redox systems were chosen to study the defect site induced by electrochemical desorption and its effect on the charge transfer processes. Electrochemical impedance spectroscopy was used as an additional electrochemical tool, to study these derivatized platinum surfaces. This technique was useful in the determination of the pinholes formed on the derivatized platinum surfaces, or different desorption potentials. Ó 2004 Published by Elsevier B.V. Keywords: Self-assembled monolayer; 3-Mercaptopropionic acid; 16-Mercaptohexadecanoic acid; 3-Aminopropyltrimethoxysilane; Electrochemical impedance spectroscopy
1. Introduction Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of active surfactants on a solid surface. This is a technique that provides an elegant route to the preparation of well-defined organic assemblies on solid surfaces, especially electrode materials [2]. The formation of monolayers by self-assembly of surfactant molecules at surfaces is an example of the general phenomena that occurs by selfassembly. The advantages of SAMs include their ease preparation, stability, and the possibility of introducing different chemical functionalities. The incorporation of *
Corresponding author. Tel.: +787-764-0000-1-4807; fax: +787-7568242. E-mail address: [email protected] (C.R. Cabrera). 0022-0728/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.jelechem.2004.05.031
the appropriate chemical functionality with some molecular level control into the highly ordered monolayers allows the preparation of surfaces with tailor-made properties [3]. Many of these monolayer systems have characteristic structures and properties and find application in the areas of adhesion, corrosion, lubrication, biocompatibility, and catalysis [3,4]. In the field of electrochemistry, the strong interest in SAMs based on thiols and related molecules is due to the following aspects: (1) They can be employed as insulating barriers between an electrode and a redox couple to study long range electron transfer [5–11]; (2) they can be used to prepare microarray electrodes which have potential applications for creating selective voltammetric detectors or for measuring very fast electron transfer kinetic [12–16]. Cyclic voltammetry (CV), chronoamperometry, and linear scan voltammetry have been the most popular
16
R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
electrochemical techniques used to study redox processes at monolayer covered gold [5,8,11,17,18]. However, these methods incur problems due to the current, which is very small, thus making it difficult to measure it at low overpotential, and the double layer charging current is difficult to eliminate [17]. Electrochemical impedance spectroscopy (EIS) can be used to overcome these difficulties and provides a sensitive way to study electron transfer across a thin monolayer. The technique in which the cell or electrode impedance is plotted vs. frequency is called EIS [19]. The electrochemical impedance spectroscopic approach is largely based on similar methods used to analyze circuits in electrical engineering practice [20]. The measured total impedance of the cell, Z, in a bridge is expressed as the series combination of a resistance value RB and the capacitance value CB . These two elements provide the real and imaginary components of Z, that is [16], ZRe ¼ RB and ZIm ¼ 1=xCB . The real part, which must equal the measured ZRe , is
parameter Ra , the method presented by Finklea et al. [15,16,22], was used. Derivation of the faradaic impedance for a microarray electrode with 1 h < 0:05 follows from the formal equivalence of the microarray diffusion problem to an electron transfer step with preceding and succeeding chemical steps [22,23]. Substituting the equivalent expressions for a microarray electrode into the general expression for the faradaic impedance for coupled reaction schemes [16], noting that h is approximately one, and rearranging terms leads to the following expression for the in-phase (Z 0 ) components of the faradaic impedance:
ZRe ¼ RB ¼ RX þ ½Rs =ðA2 þ B2 Þ;
q ¼ D=0:36R2a ;
ð1Þ
where RX is the solution resistance, the series resistance is Rs , A ¼ ðCdl =Cs Þ þ 1
and
B ¼ xRs Cdl ;
where Cdl is the double layer capacitance, Cs is the pseudocapacity, and x is the radial frequency. Similarly, ZIm ¼ 1=xCB ¼ f½ðB2 =xCdl Þ þ ðA=xCs Þ=ðA2 þ B2 Þg; ð2Þ
Z 0 ¼ ½Rct =ð1 hÞ þ ½r=ðxÞ1=2 þ ½r=ð1 hÞ f½ðx2 þ q2 Þ1=2 þ q=ðx2 þ q2 Þg1=2 ;
ð7Þ
where r ¼ ð2Þ1=2 ðRT =F 2 AcðDÞ1=2 Þ
ð8Þ
and ð9Þ
where c is the concentration of either the oxidized or reduced forms, D is the diffusion coefficient, Ra is the pinhole radius, and q is the transition radial frequency. Limiting cases are obtained by setting much larger or much smaller than the transition radial frequency q. The high-frequency case x q, which corresponds to nearly isolated diffusion profiles for each microelectrode, yields the following expression for the faradaic impedance components: Z 0 ¼ ½Rct =ð1 hÞ þ ½r=ðxÞ1=2 þ ½r=ð1 hÞðxÞ1=2 :
where the series resistance, Rs , and the Cs , are Rs ¼ Rct þ r=x1=2 ;
ð3Þ
Cs ¼ 1=rx1=2 ;
ð4Þ
where r is the Warburg impedance slope and Rct is the charge transfer resistance. Substitution for Rs and Cs by (3) and (4) provides
ð10Þ By using these equations, the value Ra was calculated. The contribution of this work is focused on the understanding of the interfacial properties of derivatized platinum surfaces with organic compounds and their possible applications in the field of molecular sensors.
ZRe ¼ RX þ fðRct þ rx1=2 Þ =½ðCd rx1=2 Þ2 þ x2 Cd2 ðRct þ rx1=2 Þ2 g;
ð5Þ
2
ZIm ¼ f½xCd ðRct þ rx1=2 Þ þ rx1=2 ðx1=2 Cd r þ 1Þ =ðCd rx
1=2
2
þ 1Þ þ x
2
Cd2 ðRct
þ rx
1=2 2
Þ:
2. Experimental 2.1. Chemicals
ð6Þ
Chemical information can be extracted by plotting ZIm vs. ZRe for different x. For simplicity, the limiting behavior at high and low x are considered. Recently, Xing et al. [21] studied the electron transfer behavior across a dodecanothiol monolayer on gold using EIS techniques carried out at various dc potentials. EIS also was used to obtain the pinhole radius (Ra ), formed by organic molecules on the Pt surfaces at different desorption potentials. In order to determine the
3-MPA (Aldrich, 99%), 16-MHDA (Aldrich, 98%), 3-APS (Aldrich, 97%), sulfuric acid (Aldrich, 99,999%), potassium ferricyanide (K3 Fe(CN)6 ) (Aldrich, 99%), hexaammineruthenium (III) chloride [Ru(NH3 )6 Cl3 ] (Aldrich, 98%), potassium chloride (KCl), 4-methylmorpholine (Aldrich, 99.5%), isobutylchloroformate (Aldrich, 98%), acetonitrile anhydrous (Aldrich, 99.8%), ethyl acetate and N,N 0 -dimethylformamide (DMF) were used as received. Tetra-n-butylammonium perchlorate (TBAP) was recrystallized four times from ethyl acetate
R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
(Fisher Scientific) and dried under vacuum for 48 h. The water used in our experiments was previously distilled and pumped through a nanopure system (Barnstead) to give 18 MX cm water. All electrochemical measurements were done at room temperature. The aqueous solutions were degassed with nitrogen for least 30 min prior to an electrochemical experiment.
17
et al. [28]. The 3-MPA/Pt and 16-MHDA/Pt films were exposed to a 2 mM solution of 3-APS, in DMF, for 1 h. Finally, the films were removed, rinsed with ethanol, dried with argon and maintained in desiccators for further characterization. The amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems were used to perform this study.
2.2. Cell and instrumentation 3. Results and discussion A three electrode cell was used in all experiments. Electrodes of HgjHg2 SO4 and AgjAgCl were used as the reference electrodes, in aqueous solvent and in nonaqueous solvent, respectively. A platinum mesh electrode was used as the counter electrode. Platinum disk electrodes (geometric area: 0.02 cm2 ) were used as the working electrodes. A Princeton Applied Research (PAR) 273A potentiostat/galvanostat, controlled with PAR 270 Research Electrochemistry software installed in a personal computer, and a frequency response detector Model 1025 (EG&G), were used in our experiments. In our cyclic voltammetry experiments the redox species used were Fe(CN)3 or Ru(NH3 )3þ in 0.1 M 6 6 KCl electrolyte solution. Impedance data for the bare and derivatized platinum surfaces were measured in a solution, containing 2.5 mM K3 Fe(CN)6 and 2.5 mM of K4 Fe(CN)6 in 0.1 M KCl, at their open circuit potentials (Ec ). Sinusoidal waves of voltages (5 mV/rms) in the frequency range between 100 KHz and 0.01 Hz were superimposed on Ec [22–25]. 2.3. Substrate pre-treatment and derivatization All substrates used were new and, before their modification they were submitted to a cleaning treatment. To be sure that the surface of the electrode was clean, CV was carried out in 0.5 M H2 SO4 solution. Then, the electrodes were washed with ethanol and nanopure water. Finally, they were dried with an argon gas flow. The current–potential behavior, characteristic of a clean platinum surface in a clean test solution [26], has been presented in a previous publication [27]. This was used as a criterion of solution and electrode cleanliness in aqueous 0.5 M H2 SO4 . Whenever we used a platinum electrode, we verified its cleanliness by cyclic voltammetry before its chemical modification. The derivatization process for the platinum surface electrodes has been previously reported [1]. The previously cleaned substrates were submerged in 1 mM solutions of 3-MPA and 16-MHDA, in ethanol, for 24 h. Afterward, the substrates were washed with ethanol and dried under an argon gas flow. The chemical derivatization of 3-MPA/Pt and 16-MHDA/Pt surfaces, with the amine-containing molecule (3-APS), was done by a modified version of a method presented by Bruening
3.1. Cyclic voltammetry The electrochemical characterization of the derivatized platinum surface can reveal the presence of a full coverage of the electrode. Cyclic voltammetric studies were done using redox active species in solution. The Pt disk electrodes, bare and derivatized, were characterized by CV using a 2.5 mM solution of Fe(CN)3 or 6 Ru(NH3 )3þ 6 , as the redox active couple, in 0.1 M KCl. The CVs observed in Fig. 1A(b) and 1B(b) correspond to the electrochemical behavior of Ru(NH3 )3þ and 6 Fe(CN)3 6 , respectively, at the amide(3-MPA)/Pt derivatized electrode. The CV in Fig. 1A(b) is similar to that observed at a bare platinum disk electrode (Fig. 1A(a)). The peak currents are clearly defined and this fact demonstrates that the redox couple undergoes the redox process without inhibition. Nevertheless, the voltammogram of Fe(CN)3 6 at an amide(3-MPA)/Pt electrode (Fig. 1B(b)) presents differences in the peak separation and in the peak currents, which are reduced after the derivatization of the Pt electrode with amide(3-MPA). This demonstrates that the peak current change can be attributed to tunneling at defect sites. For the amide(16-MHDA)/Pt electrode we found different cyclic voltammetric behavior of the redox couples. The voltammogram of Fe(CN)3 at an am6 ide(16-MHDA)/Pt electrode, in Fig. 2B(b), shows that the peak currents were significantly suppressed by the
Fig. 1. Cyclic voltammograms of (A) 2.5 mM Ru(NH3 )6 Cl3 and (B) 2.5 mM Fe(CN)3 6 , in 0.1 M KCl solution, at the (a) bare platinum disk electrode and (b) amide(3-MPA)/Pt electrode system. Scan rate 100 mV/s.
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R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
Fig. 2. Cyclic voltammograms of (A) 2.5 mM Ru(NH3 )6 Cl3 and (B) 2.5 mM Fe(CN)3 6 , in 0.1 M KCl solution, at the (a) bare platinum disk electrode and (b) amide(16-MHDA)/Pt electrode system. Scan rate 100 mV/s.
Fig. 3. Linear sweep voltammetry for (a) platinum bare, (b) amide(3MPA/Pt), and (c) amide(16-MHDA/Pt) in 0.1 M TBAP + CH3 CN solution. Scan rate: 100 mV/s. Geometric area was 0.020 cm2 .
derivatization, compared with the cyclic voltammogram of Fe(CN)3 at a bare platinum disk electrode 6 (Fig. 2B(a)). The CV changes drastically from an electrochemically reversible shape to a capacitive shape after the Pt electrode has been derivatized. In this case, it is probable that the amide(16-MHDA)Pt system shows very low mass transfer effects, which are characteristic of pinhole defects in the amide(16-MHDA)Pt monolayer. It is reasonable to conclude that the redox couple cannot contact the platinum. We found similar voltammetric behavior previously with platinum electrodes modified with 3-MPS and 3-APS [27]. Fig. 2A(b) shows the electrochemical behavior of Ru(NH3 )3þ 6 at an amide(16MHDA)Pt electrode. This voltammogram presents differences in the peak current which decreases and its peak shape becomes more sigmoidal. This electrochemical behavior indicates that the platinum surface has changed with the derivatization. 3þ Compared with Fe(CN)3 6 , Ru(NH3 )6 shows faster electron transfer kinetics and a larger faradaic current at the same amide(3-MPA)Pt and amide(16-MHDA)Pt electrodes. The difference in the voltammograms may derive from the different heterogeneous electron transfer rates for the two reactions, i.e., Ru(NH3 )3þ 6 is a much faster redox couple. The changes in the shapes of the voltammograms can be analyzed by theories of voltammetry at ultramicroelectrodes. Since the effect of the microelectrode assembly is to lower the apparent heterogeneous electron transfer rate, this effect would be less pronounced with the faster couple. In voltammetry, it appears as an increase in the peak separation. As expected, Fig. 1B(b) and Fig. 3B(b) demonstrate that the Fe(CN)3 6 couple has a larger peak separation than Ru(NH3 )3þ 6 .
linear sweep voltammetry (LSV) in 0.1 mol/l TBAP + CH3CN solution. Fig. 3 shows voltammograms obtained for these systems, including the blank. From these voltammograms, it was determined that the desorption potentials for the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems were )1.6 and )1.8 V, respectively. In addition, the amount of compound desorbed from the platinum surface was determined from these voltammograms, relating the area under the desorption curve to the amount of charge involved. For the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems the amounts of thiol desorbed were: 4.4 1011 and 3.1 1011 mol/cm2 , respectively. These amounts correspond to an order of magnitude lower than that for a monolayer on a gold surface (1010 mol/cm2 ) [19]. The amide(16-MHDA)/Pt system was chosen to induce defect sites by electrochemical desorption studies. Fig. 4A(a) and B(a) were obtained from a platinum disk electrode, and Fig. 4A(b) and B(b) were obtained at an amide(16-MHDA)/Pt electrode, in 2.5 mM Ru(NH3 )6 Cl3 + 0.1 M KCl solution and in 2.5 mM
3.2. Electrochemical desorption studies
Fig. 4. Cyclic voltammograms of (A) 2.5 mM Ru(NH3 )6 Cl3 and (B) 2.5 mM Fe(CN)3 6 , in 0.1 M KCl solution, at the bare platinum disk electrode (a), amide(16-MHDA)/Pt electrode system without electrochemical desorption (b), and with electrochemical desorption pretreatment at (c) )1.4, (d) )1.6, and (e) )1.8 V. Scan rate: 100 mV/s.
Desorption of amide(3-MPA) and amide(16-MHDA) from the modified platinum surfaces were determined by
R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
19
Fe(CN)3 6 + 0.1 M KCl solution, respectively. The voltammetric behavior, in Fig. 4A(b), is characteristic of probe molecules that cannot contact the surface or penetrate the monolayer. Only electrons that tunnel through the monolayer may reduce them. Fig. 4A(c–e) and B(c–e) shows the behavior of an amide(16-MHDA)/ Pt electrode, prepared with different desorption potentials, in the probe molecule solution. In general, it can be deduced that the currents at the modified platinum surfaces with defect sites are much higher than that at the completely modified platinum electrode. This indicates that the defect sites allow the probe molecule to penetrate through the monolayer and undergo electron transfer with the platinum surfaces. On the other hand, the sigmoidal CV curves, which are expected for an ultramicroelectrode array, show the presence of very small pinhole defects in the monolayer. Compared with 3þ Fe(CN)3 has faster electron transfer ki6 , Ru(NH3 )6 netics and a larger faradaic current at the same amide(16-MHDA)Pt electrode surfaces with different desorption potentials. 3.3. EIS studies EIS was used as an additional electrochemical tool, to study the derivatized platinum surfaces. This technique was useful in the determination of the pinhole size prepared from the derivatized platinum surfaces at different desorption potentials. In order to study the derivatization of platinum surfaces with amide(3-MPA) and amide(16-MHDA), Nyquist and Bode plots were obtained using EIS. Figs. 5 and 6 shows the impedance spectrum, in a Nyquist and Bode presentation, for amide(3-MPA/ Pt) and amide(16-MHDA/Pt) electrode systems, respectively, at different desorption potentials in the presence of 2.5 mM [Fe(CN)6 ]3=4 with 0.1 M KCl as the supporting electrolyte. The electrochemical desorption potential, vs. AgjAgCl, varies negatively as indicated in the figure. It is observed in Figs. 5A and 6A that the curves deflect downwards with increasingly negative desorption potential, indicating that the charge transfer resistance decreases with the more negative desorption potential. This also demonstrates that the applied potential affects the interfacial properties of the derivatized platinum electrodes. Similar results were reported by Boubour and Lennox [29] in studies realized with n-alkanethiols self-assembled monolayers on gold electrodes. The parameters obtained by EIS studies are presented in Table 1. These values were used to calculate the Rct , Cdl , and Ra values. The Rct on the derivatized platinum surfaces decreases until it reaches a minimum resistance of 4.3 and 71.0 kX cm2 , at )1.6 and )1.8 V vs. AgjAgCl, for the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, respectively. The Rct for the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, without an applied
Fig. 5. The electrochemical impedance spectroscopy of amide(3-MPA) on platinum. (A) Nyquist plots and (B) Bode plots for the bare platinum disk electrode (a), amide(3-MPA)/Pt electrode system without electrochemical desorption (b), and with electrochemical desorption pretreatment at (c) )0.5, (d) )1.0, (e) )1.2, (f) )1.4, and (g) )1.6 V, respectively. Geometric area: 0.02 cm2 .
desorption potential, was 10.7 and 1099 kX cm2 , respectively (see Table 1). A diminution of the charge transference resistance, for these two systems, as the desorption potential is increased, is expected since, as more molecules of the electrode surfaces are desorbed, more Pt surface would be exposed and, therefore, there would not be such as impediment for the probe molecule to experience its redox process. The Rct values, for the amide(16-MHDA/Pt) system are much greater than Rct values for the amide(3-MPA/Pt) system. This is because the amide(16-MHDA/Pt) system has a compound with a longer chain, compared with the amide(3-MPA/Pt) system. From the Bode plots, the curve for log jZj vs. log x can yield values of Rct and RX . In addition, at intermediate frequencies, the break point of this curve should be a straight line with a slope of )1. Extrapolating this line to the log jZj axis at x ¼ 1ðlog x ¼ 0Þ yields the value of Cdl from the Eq. (7). The values of Cdl were obtained from the Bode plots. These values are presented in Table 1.
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R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
Fig. 7. Variation of the capacitance (Cdl ) at different desorption potentials in the (A) amide(3-MPA/Pt) and (B) amide(16-MHDA/Pt) systems.
Fig. 6. The electrochemical impedance spectroscopy of amide(16MHDA) on platinum. (A) Nyquist plots and (B) Bode plots for the bare platinum disk electrode (a), amide(16-MHDA)/Pt electrode system without electrochemical desorption (b), and with electrochemical desorption pretreatment at (c) )1.0, (d) )1.2, (e) )1.4, (f) )1.6, and (g) )1.8 V, respectively. Geometric area: 0.02 cm2 .
jZj ¼ 1=Cdl :
ð11Þ
The Cdl values as the desorption potential increases, in the amide(3-MPA/Pt) and amide(16-MHDA/Pt) systems, are presented in Fig. 7A and B, respectively. The
amide(3-MPA) system produces a larger double layer capacitance (Cdl ) of 3.09 lF/cm2 at desorption potentials of )1.6 V. A similar result is obtained for the amide(16MHDA/PT) system at )1.8 V vs. AgjAgCl, where the capacitance value is 2.54 lF/cm2 (see Table 1). This behavior is explained by the presence of fewer molecules on the surface. In this manner, the surface will be more accessible to form a double layer with the electrolyte in solution. At lower desorption potentials, the double layer capacitance, for both systems, decreases in a similar way. The values obtained for Ra , for the two systems in this study, increase as the desorption potential becomes more negative. This behavior is observed in Fig. 8. In
Table 1 Parameters obtained by EIS for the platinum surfaces derivatized and treated at different desorption potentials Desorption potential (V)
Rs (kX cm2 )
Pt bare –
0.32
Amide(3MPA/Pt) Without )0.5 )1.0 )1.2 )1.4 )1.6
0.35 0.34 0.34 0.35 0.34 0.32
Amide(16-MHDA/Pt) Without )1.0 )1.2 )1.4 )1.6 )1.8
0.33 0.34 0.34 0.34 0.34 0.32
Rp (kX cm2 ) 0.96 11.0 10.1 9.3 8.1 6.0 4.6 1100 909 659 180 169 72
Rct (kX cm2 ) 0.64 10.7 9.8 9.0 7.8 5.7 4.3 1099 909 659 179 168 71
Cdl (lF/cm2 ) 127
r (X s1=2 )
1.21
108 D (cm2 /s)
q (s1 )
106 Ra (cm)
3770
–
–
1.46 1.52 1.57 1.62 2.71 3.09
40.5 33.5 32.6 30.0 29.3 29.0
3.4 5.0 5.2 6.1 6.4 6.6
543 526 421 394 361 318
13.0 16.0 19.0 21.0 22.0 24.0
1.25 1.34 1.55 1.72 2.51 2.54
54.0 48.0 46.0 33.0 25.0 21.0
1.9 2.4 2.6 5.1 8.8 12.5
646 303 221 160 125 104
9.0 14.0 18.0 29.0 44.0 58.0
R. Brito et al. / Journal of Electroanalytical Chemistry 574 (2004) 15–22
Fig. 8. Variation of the Ra at different desorption potentials in the (A) amide(3-MPA/Pt) and (B) amide(16-MHDA/Pt) systems.
both case a linear relation is observed. In the case of the amide(3-MPA/Pt) system (see Fig. 8A), the Ra variation was between 13.0 106 and 24.0 106 cm, and for amide(16-MHDA/PT) (see Fig. 8B), the Ra variation was between 9.0 106 and 58.0 106 cm.
4. Conclusions Taking into account the results obtained from the applied electrochemical studies of the platinum surfaces derivatized with 3-MPA, 16-MHDA, and 3-APS, the following conclusions were reached. The analysis of the cyclic voltammetric results indicates that the degree of coverage, the presence of holes and their sizes, and the possible interactions between the different species involved are the principal factors that determine the shape of the voltammograms. The present study has shown that, in the presence of a derivatized platinum surface, the electron transfer between redox species and a platinum electrode is reduced significantly and mass transfer effects minimized, indicating that the amide(16-MHDA)/Pt system, compared with the amide(3-MPA)/Pt system, is free of mass transfer effects. The blocking effect produced by amide(16-MHDA/Pt) was much more noticeable than that produced by am4 ide(3-MPA/Pt) in the case of Fe(CN)3 6 /Fe(CN)6 charge transfer where almost no current flowed. The effect of the amide(3-MPA/Pt) system is much less 3þ evident in the case of Ru(NH3 )2þ 6 /Ru(NH3 )6 ; its voltammetric behavior remains almost intact when amide(3-MPA) modifies the Pt electrode. On the other hand, in the electrochemical desorption potential studies 3þ carried out, compared with Fe(CN)3 6 , Ru(NH3 )6 , as the desorption potential is increased, has faster electron
21
transfer kinetics and a larger faradaic current at the same amide(16-MHDA)/Pt electrode. In the EIS studies it is observed that the charge transfer resistance (Rct ), using the amide(16-MHDA/Pt) system, is much larger than the Rct obtained for the amide(3-MPA/Pt) system and the clean platinum surface. Moreover, it was found that Rct diminished in the two systems studied, as the desorption potential increased. This is due to the exposure of the platinum surface, as molecules are desorbed from the platinum surface, allowing the probe molecule to undergo its redox process. The Cdl values increased, for the two systems in this study, as the desorption potential increased. This behavior is explained by the presence of fewer molecules on the surface. In this manner, the surface is more accessible to form the double layer with the electrolyte in solution. On the other hand, the Ra values also increased, for the two systems in this study, as the desorption potential increased.
Acknowledgements The authors acknowledge the use of facilities of the Materials Characterization Center of the University of Puerto Rico. This work had the financial support of the NSF-EPSCoR program, Grant No. EPF-9874782. R.B. acknowledges financial support from the Fondo Nacional de Ciencia, Innovaci on y Tecnologıa, FONACIT, from Venezuela.
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