Langmuir 2001, 17, 4851-4857
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Critical Influence of the Fluorinated Chain Length in the Self-Assembly of Terminally Perfluorinated Alkanethiol Monolayers on Gold Surfaces. An Electrochemical Study C. Naud, P. Calas,* and A. Commeyras Laboratoire Organisation mole´ culaire, Evolution et mate´ riaux fluore´ s, UMR CNRS 5073, CC017, Universite´ de Montpellier II, F. 34095 Montpellier Cedex 05, France Received October 16, 2000. In Final Form: February 27, 2001 Self-assembled monolayers of semifluorinated n-alkanethiols having the general formula F(CF2)n(CH2)mSH (n ) 4, 6, 8, 10 and m ) 2, 11) have been formed from ethanolic solutions, with an incubation time of 96 h, on unannealed polycrystalline Au films vapor deposited on glass supports. Determination of the adlayer capacitances through cyclic voltammetry has been validated from its independence of the potential scan rate and its constancy when using two electrolytes, NaF and KCl, whose anions have different hydrated radii. The thickness of the monolayers has been estimated on the basis of capacitance values, and an indicative average tilt angle deduced. The monolayer’s organization appears to largely depend on the n and m values. Short fluorinated chains (n ) 4) are unable to self-organize in a position near the surface normal. This fact is independent of the length of the hydrogenated spacer (m ) 2, 11), so it appears as an inherent characteristic for these short fluorinated chains. Fluorinated chains containing eight or ten carbon atoms are able to self-organize in a packing near the surface normal (average tilt angle 26°), independently of the length of the hydrogenated spacer length (m ) 2, 11). Compounds with n ) 6 and m ) 2, 11 are in an intermediate situation.
Introduction Self-assembled monolayers (SAMs) of organic compounds on solid substrates, especially monolayers of thiols on gold, have been the subject of considerable interest.1-8 Particularly when preparing mixed SAMs that present ligands for biospecific adsorption of proteins, it appeared necessary to obtain experimental systems having requisite properties. More precisely the surface of the monolayer between the thiols bearing specific ligands has to realize an “inert surface” that resists nonspecific adsorption. Such performances have been attained through the use of polyoxyethylene (POE)-terminated thiols.9 In this field, fluorine-terminated thiols are of great interest since fluorine atoms have inherent properties such as chemical inertness and excellent repellency (practically nothing can stick to fluorine).5,10,11 These properties are essential for research and development of various surface modifiers. Some authors studied semifluorinated n-alkanethiol monolayers, indicating the tilt angle of the molecule from * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 04 67 63 10 46. Phone: 04 67 14 48 37. (1) Ulman, A. Introduction to ultrathin Film, From LangmuirBlodgett Films to Self- Assembly; Acedemic Press: Boston, 1991. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (3) Scho¨nherr, H. Langmuir 1997, 13, 3769. (4) Scho¨nherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (5) Tsao, M.-W.; Hoffmann, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1997, 13, 4317. (6) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705. (7) Gorman, C. B.; Miller, R. L.; Chen, K.-Y.; Bishop, A. R.; Haasch, R. T.; Nuzzo, R. G. Langmuir 1998, 14, 3312. (8) Miura, Y. F; Takenaga, M.; Koini, T.; Graupe, M.; Garg, N.; Graham, R. L., Jr.; Lee, R. T. Langmuir 1998, 14, 5821. (9) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009. (10) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (11) Motomatsu, M.; Mizutani, W.; Nie, H.-Y.; Tokumoto, H. Thin Solid Films 1996, 281-282, 548.
the surface normal. Liu et al.12 estimated a molecular tilt angle of 12° for the SAMs made from F(CF2)12(CH2)2SH, Alves and Porter13 reported 20° for F(CF2)8(CH2)2SH, and Motomatsu et al.11 reported 56-76° for F(CF2)2(CH2)6SH. Tsao et al.5 indicated for F(CF2)8(CH2)11SH that the fluorocarbon helix appears to be slightly tilted (relative to the surface normal) in marked contrast to perfluoroalkylamide thiols having short hydrocarbon sequences which are very near the surface normal. On the contrary for F(CF2)8(CH2)11SH the hydrogenated segment exhibits a smaller tilt than the one found for self-assembled monolayers of octadecanethiol. Recent studies relative to F(CF2)10(CH2)mSH (m ) 2, 6, 11, 17, 33) SAMs on gold14 concluded that, as the length of the methylene spacer was increased, the tilt angle of the perfluorocarbon moiety increased with respect to the surface normal. Thus, on the basis of literature data, it appears that if n-alkanethiols stack on gold surfaces with a tilt angle of 30°,2,5,10,15 independently of their chain length, the organization of semifluorinated n-alkanethiol monolayers F(CF2)n(CH2)mSH appears to largely depend on the n and m values. As we are engaged in the synthesis of functional thiols having the general formula Y(CH2)p(CF2)n(CH2)mSH, Y being a specific ligand, to prepare mixed SAMs with semifluorinated n-alkanethiols F(CF2)n(CH2)mSH, it is of interest to obtain information permitting selection of the semifluorinated n-alkanethiols most adapted for this purpose. In this way we have prepared a series of semifluorinated n-alkanethiols having the general formula F(CF2)n(CH2)mSH (n ) 4, 6, 8, 10 and m ) 2, 11) as reported (12) Liu, G.-Y.; Genter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (13) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (14) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417. (15) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 682.
10.1021/la0014508 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/03/2001
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Table 1. Linear Regression Results from the Data in Figures 1 and 2 (FnHm, 0.1 M NaF and KCl Electrolytes)a m)2 intercept (µA) n)4 n)6 n)8 n ) 10 a
m ) 11 slope (µA
V-1 s)
intercept (µA)
slope (µA V-1 s)
NaF
KCl
NaF
KCl
NaF
KCl
NaF
KCl
0.0089 0.0036 0.0016 0.0027
0.0099 0.0049 0.0026 0.0057
2.02 0.94 0.57 0.49
2.02 0.96 0.58 0.52
0.0086 0.0003 0.0015 0.0041
0.0250 0.0011 0.0010 0.0019
0.63 0.38 0.32 0.28
0.75 0.40 0.32 0.26
All the linear regression coefficients are superior to 0.99995, except those for F4H11SAu: 0.9983 in NaF and 0.9972 in KCl.
in a separate paper.16 Corresponding self-assembled monolayers have been formed from ethanolic solutions on Au films vapor deposited on glass supports and studied through electrochemical measurements. Results allow us to outline the critical importance of the fluorinated segment length in the final structure of the SAMs formed. Results and Discussion In the following an abbreviated nomenclature is used: F(CF2)n(CH2)mSH will be denoted as FnHmSH and FnHmSAu for the corresponding SAMs; their hydrocarbon analogues H(CH2)mSH will be denoted as HmSH and HmSAu. Electrochemical Capacitance of the Supported Monolayers from Cyclic Voltammetry. The capacitance of monolayers can be determined by cyclic voltammetry, using a linear variation of the applied potential between two chosen limits, in a potential region where no faradaic current exists. If the monolayer is impermeable to ion migration (acting as a simple dielectric), it behaves like an ideal capacitor,10 thus
Q/S ) CU
(1)
I ) dQ/dt ) CS(dU/dt)
(2)
C-1 ) d/0
(3)
where I is half of the current difference between the forward and the reverse scans in the voltammogram (A), dU/dt the scan rate for the linear variation of the applied potential (V/s), C the capacitance of the monolayer per unit area (F/cm2), and S the surface studied area (cm2). The thickness of the dielectric medium separating the two conducting plates is d (cm), its dielectric constant, and 0 the permittivity of free space (8.85 × 10-14 F/cm).17 Therefore, during the linear potential scans of cyclic voltammetry experiments, the charging current is independent of the potential, proportional to the scan rate, and inversely proportional to the film thickness. If the monolayer is not impermeable to ion migration, the apparent capacitance and charging current will be larger. However, numerous studies have shown that this theory fails to account for the variation of the capacitance as a function of the electrolyte, applied potential, and electrode composition, suggesting that and d are dependent on these variables.2,10,18 Despite this, voltammetric data can be considered as significant when data relative to SAMs elaborated from a series of semifluorinated n-alkanethiols F(CF2)n(CH2)mSH (with n ) 4, 6, 8, 10 and m ) 2, 11) are compared. Moreover, cyclic voltammetry gives ready access to information on ion permeation into the mono(16) Naud, C.; Calas, P.; Blancou, H.; Commeyras, A. J. Fluorine Chem. 2000, 104, 173. (17) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (18) Bard, A. J. Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, Wiley: New York, 1980.
Figure 1. Charging current (µA) measured for SAMs of FnH2SH formed at unannealed gold deposited at chromium-primed glass. Electrode area 0.363 cm2. Program of potential: from open circuit to the starting value of -80 mV (vs KSCE), and after 60 s at this value linear scans between -80, -120, 0, -120, 0 and -80 mV. Sweep rates: 0.02, 0.05, 0.1, and 0.2 V/s (see also Figure 4). Measurements obtained in (a) 0.1 M NaF and (b) 0.1 M KCl. Corresponding to the monolayers (1) F4H2SAu, (2) F6H2SAu, (3) F8H2Sau, and (4) F10H2SAu.
Figure 2. Charging current (µA) measured for SAMs of FnH11SH formed at unannealed gold deposited at chromiumprimed glass. Electrode area 0.363 cm2. Program of potential: from open circuit to the starting value of -80 mV (vs KSCE), and after 60 s at this value linear scans between -80, -120, 0, -120, 0 and -80 mV. Sweep rates: 0.02, 0.05, 0.1, and 0.2 V/s. Measurements obtained in (a) 0.1 M NaF and (b) 0.1 M KCl. Corresponding to the monolayers (1) F4H11SAu, (2) F6H11SAu, (3) F8H11Sau, and (4) F10H11SAu.
layers, even if it occurs over tens of seconds. Ionic permeation can also be evidenced through the determination of the capacitance using successively two electrolytes, NaF and KCl, whose anions have different hydrated radii. Indeed chloride ion has a smaller hydrated radius than fluoride ion and permeates semiordered SAMs to a greater degree than fluoride ion, as indicated by a larger apparent capacitance when using KCl compared with NaF as electrolyte. On the contrary, well-ordered SAMs exhibit similar capacitance for the two anions.7 SAMs of Semifluorinated n-Alkanethiols. When SAMs obtained from the thiols FnHmSH with n ) 4, 6, 8, 10 and m ) 2, 11 were studied in linear sweep voltammetry with 0.1 M NaF or KCl as supporting salt, the charging current observed varied linearly with the scan rate, as shown in Figures 1 and 2. Thus, for F4H11SAu, calculated regression lines exhibit an intercept far from the origin (Table 1), more particularly when KCl is the supporting salt. Moreover, for F4H11SAu the linear regression coefficient (0.9983 in NaF and 0.9972 in KCl) deviates from values found for the others (>0.99995). So the monolayer
Self-Assembly of Alkanethiol Monolayers
Langmuir, Vol. 17, No. 16, 2001 4853
Table 2. Capacitance Values (0.1 M NaF and KCl Electrolytes, 50, 100, and 200 mV/s Scan Rates) for F(CF2)n(CH2)mSH Monolayers at a Gold Electrode capacitance (µF/cm2) NaF SAM F4H2SAu F6H2SAu F8H2SAua F10H2SAu F4H11SAu F6H11SAu F8H11SAu F10H11SAu
KCl
50 100 200 50 100 200 mV/s mV/s mV/s mV/s mV/s mV/s uncertainty 6.25 2.81 1.65 1.53 2.37 1.06 0.94 0.88
5.76 2.69 1.65 1.47 2.07 1.06 0.92 0.89
5.70 2.62 1.60 1.38 1.86 1.06 0.90 0.81
6.12 2.87 1.75 1.78 3.61 1.19 0.94 0.81
5.94 2.88 1.68 1.60 2.88 1.13 0.90 0.78
5.70 2.69 1.63 1.50 2.37 1.12 0.90 0.75
(0.30 (0.15 (0.10 (0.08 (0.15 (0.06 (0.05 (0.05
a For F H SAu, Chidsey et al.10 report for the capacitance value 8 2 in 0.1 M NaF at 100 mV/s: 1.65 µF cm-2 with a potential scan range of (200 mV and 1.55 µF/cm-2 for (50 mV.
Figure 3. Capacitance values vs number of carbon atoms (n + m) for the monolayers FnHmSH on the basis of linear sweep voltammetry data. (a) Capacitance values at 200 mV/s in 0.1 M NaF (+) and 0.1 M KCl (O). (b) Capacitance values in 0.1 M NaF at 50 mV/s (O) and 200 mV/s (+). (Data from Table 2.)
F4H11SAu with the short fluorinated chain F4 and the long H11 hydrogenated spacer does not exhibit the behavior of an ideal capacitor. Table 2 reports capacitance data obtained using two electrolytes, NaF or KCl, 0.1 M, and at three different scan rates. Uncertainty on the capacitance values can be discussed as follows: a systematic error exists on the surface value related to its geometric determination, (2%. See also our comments on the effective surface value in the Experimental Section. Determination of the charging current introduces a random error evaluated to be ((0.5-3%) as the chain length increases, thus decreasing the measured current to lower values by which our electrochemical apparatus exhibits some limitations (see the Experimental Section). The resulting uncertainty on capacitance values is (3-5%. Capacitance values for F4H11SAu in the two electrolytes (Table 2) differ largely, indicating the ionic permeability of the monolayer at least for chlorides (also evidenced in Figure 2), and also for fluorides as capacitance varies with the scan rate. For F4H2SAu, capacitance data are similar with the two salts and differ with the scan rate in a limited range (10%). Thus, this adlayer can be considered to resist ionic permeation. Capacitance values observed for the other monolayers studied are remarkably constant for the two supporting salts used and the three scan rates reported. These monolayers can be considered to resist ionic permeation, toward the strongly hydrated fluorides and also the smaller chlorides. The behavior of the studied monolayers is displayed in a more comprehensive fashion in Figure 3a for the influence of the supporting salt used and Figure 3b for the scan rate. From the data in Table
Figure 4. Cyclic voltammogram of a gold electrode bearing an F8H11SH monolayer. Supporting salt 0.1 M NaF, scan rate 100 mV/s. Program of potential: from open circuit to the starting value of -80 mV (vs KSCE), and after 60 s at this value linear scans between -80, -120, 0, -120, 0, -80 mV.
Figure 5. Reciprocal of the monolayer’s capacitance vs n + m, the number of carbons in the chain, with as electrolyte 0.1 M NaF (from LSV data at a scan rate of 200 mV/s). The monolayers are as follows: (solid line) H(CH2)mSAu from literature data;2 (O) F(CF2)n(CH2)2SAu, n ) 4, 6, 8, 10; (4) F(CF2)n(CH2)11SAu, n ) 4, 6, 8, 10. The plus sign indicates the reciprocal of the capacitance value for F(CF2)8(CH2)2SAu reported by Chidsey et al.10 in 0.1 M NaF at 100 mV/s.
2 one can note, for the series FnH2SAu, the remarkable integrity of the monolayer F8H2SAu. For the series FnH11SAu, the integrity of the monolayers F8H11SAu and F6H11SAu can be particularly outlined. When the reciprocal capacitance of the monolayers F(CF2)n(CH2)mSH (n ) 4, 6, 8, 10 and m ) 2, 11) is plotted versus their total number of carbon atoms (n + m) (Figure 5), one can note that the data follow, as the n value increases from 4 to 10 and for each m value of 2 or 11, a monotonic variation toward the line related to the hydrogenated thiols H(CH2)mSH (from ref 2). The point indicated by a plus sign corresponds very closely to the one reported by Chidsey et al.10 for the F8H2SAu studied in similar conditions. As (see above) most of the compounds studied have been found to resist ionic permeation, C-1 values can be considered as significant with the thickness of the monolayer. Thus, on the basis of capacitance values, it is possible to calculate the thickness of the monolayers (the monolayer F4H11 must be ruled out of such a calculation because we know it is permeable to chlorides and we are not sure that it resists fluoride permeation, so corresponding results are reported in parentheses). We introduce a simplified model, considering that the two F and H chains exhibit the same tilt angle from the surface normal (Chart 1). The tilt angle value is deduced from the comparison of the electrochemical thickness, deduced from
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Chart 1. Simplified Model for Semifluorinated SAMs F(CF2)n(CH2)mSH, Based on a Similar Tilt Angle r from the Surface Normal for the Fluorinated and the Hydrogenated Segmentsa
Chart 2. Simplified Model for Semifluorinated SAMs F(CF2)n(CH2)mSH, Based on a Similar Tilt Angle r from the Surface normal for the Fluorinated and the Hydrogenated Segmentsa
a The model does not imply collinearity for the two segments (see the text and Chart 2).
Table 3. Thickness and Tilt Angle of Semifluorinated Thiol Monolayers (FnHmSH) on Gold thickness, Å n
m
C-1 a measdb maxc
Rd (deg)
0.18 0.38 0.63 0.72
64 ( 2 44 ( 3 22 ( 9 ∼20 27 ( 7 12 ( 2 56-76 (56) ( 3f 31 ( 5 26 ( 7 (F8) >20 (H11) 20° and (H11) 99 mol % as judged by
Figure 6. Scheme of the cell used for electrochemical measurements on monolayer-covered electrodes. Electrolyte H2O, 0.1 M NaF or 0.1 M KCl. integration of the corresponding nuclear magnetic resonance spectra (1H and 19F NMR, 250 MHz), and probably to a higher degree of purity, as they are obtained by recrystallization or distillation.16 Substrate Preparation. Thin glass slides (1 in. × 1.5 in., 0.15 mm thick, also usable for future surface tension measurements) used as electrodes were cleaned by sulfochromic acid etching, rinsed with deionized water, ethanol, and acetone, and dried under a warm air flow. Vapor deposition of 100 Å of chromium (99.99% purity) under vacuum was followed by 2000 Å of gold (99.99% purity). The evaporation rate, measured with a quartz crystal thickness monitor, was 10-15 Å/s. The deposition was conducted under a vacuum of 7 × 10-7 Torr. Monolayer Preparation. SAMs were prepared by immersing freshly prepared gold-coated slides in ethanolic solutions deoxygenated by nitrogen, containing the appropriate thiol at 4 × 10-3 M concentration. At this concentration the compounds with n ) 8, 10 and m ) 11 are not completely soluble. The gold slides were allowed to remain in the ethanolic solution at ambient temperature for 96 h, unless otherwise stated. Some experiments have been run with shorter times for comparison. After removal from the solution, the slides were rinsed thoroughly with ethanol; the solvent was then removed by passing the slides under a warm air flow. The electrodes were transferred to the electrochemical setup with minimal delay. Electrochemical Measurements. The electrochemical measurements were performed with an ESII computerized apparatus. In this unit the voltage scans used for cyclic voltammetry are generated by a classical analogic electronic circuit. Consequently smooth ramps are produced and no additional filtering is necessary (contrary to an apparatus generating a numerical voltage ramp). The limit of sensitivity is (2 nA. The threeelectrode cell used is described in Figure 6. Typically the cyclic voltammograms were run in the potential range between 0 and -120 mV versus a potassium-saturated calomel electrode (KSCE) used as reference. The auxiliary electrode was a platinum wire. The gold working electrode exposed an area of 0.363 ( 0.01 cm2. All measurements were carried out at room temperature, under nitrogen and after bubbling for 15 min. Active Surface of the Gold Electrodes Used. The knowledge of the active surface area in the study of the electrodeelectrolyte interface is of fundamental importance. The calculation of the capacitance from electrochemical data, as shown in the above, is dependent on the value of the active area, which differs from the geometric one by the roughness factor. For the determination of the area of an electrode surface, some different approaches have been used, for example, iodine chemisorption29
Self-Assembly of Alkanethiol Monolayers and gold oxide measurements.30 Unannealed gold vapor deposited at chromium-primed glass is known to exhibit pronounced (111) crystallinity. The value of the roughness factor based on electrochemical measurements can be considered as equal to 1.3 ( 0.3 and from STM as 1.11 ( 0.03.31 In addition, it can be noted that roughness factors as low as 1.01 have been attained from metallographic polishing of massive gold electrodes.32 The geometric surface, limited by an O-ring in our arrangement, is itself determined with a given uncertainty (measurement of the inner contact diameter under the charge of the pinch clamp, using a profile projector). An important remark in the work of Walczak et al.31 is that the average structure of the hydrocarbon portion of the SAMs appears as unaffected by the substrate crystallinity, as shown by the similar values observed for octanethiol monolayer capacitance when working at gold electrodes vacuum deposited at mica or glass, and this in spite of the fact that for these two supports the roughness factor differs significantly. However, the reactivity of the thiol group may be more sensitive to substrate crystallinity, for example, when its (30) Finklea, H. O.; Averty, S.; Lynch, M. Langmuir 1987, 3, 409. (31) Walczak, M. M.; Alves, C. A.; Lamp. B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (32) Thoden van Velzen, E. U.; Engbersen, J. F. J.; J. de Lange, P.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 6853.
Langmuir, Vol. 17, No. 16, 2001 4857 electroreductive removal is studied.19,31 When measuring the capacitance of uncoated bare gold (vacuum deposited as above) in 0.1 M KCl, we deduced from CV (cyclic voltammetry) at 100 mV/s a value of 27.6 µF/cm2 on the basis of the geometrical surface. A value of 27 µF/cm2, obtained through EIS (electrochemical impedance spectroscopy) measurements has been previously reported.33 In this situation, for our calculations of the monolayer’s capacitance, we have chosen to use the geometric surface value. Thus, reported values are not corrected for the surface roughness of the gold electrode.
Acknowledgment. We are grateful to the ELF-ATOCHEM Co., through M. Pierre DURUAL, for providing some of the starting compounds and financial support, to Jean Lyonnet (ATEMI, Universite´ de Montpellier II) for the preparation of gold substrates, and to the European Commission, TMR Research Networks, Contract Number ERBFMRXCT 970120, for financial support. LA0014508 (33) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197.
