Q Copyright 1992 American Chemical Society
NOVEMBER 1992 VOLUME 8, NUMBER 11
Letters Cobalt-Mediated Modification of Oxide Surfaces with Redox-Active Molecules Kaimin Chen, Brian R. Herr, Elizabeth T. Singewald, and Chad A. Mirkin' Department of Chemistry, Northwestern University, Evanston, Illinois 60208 Received August 24, 1992 A new and general cobalt carbonyl-mediated method for the modification of indium-tin-oxide (ITO) electrode surfaces with redox-active molecules is reported. IT0 substrates treated with Cl~SiCo(C0)4 yield surfaces with reactivity toward molecules that possess alcohol, amine, thiol, carboxylic acid, and amide functionalities. Surface coverages of the modifying reagents as determined by cyclic voltammetry are consistentwith monolayers. Cyclic voltammetry of the redox-modifiedsubstrates indicatesthat 1% of the active S~-CO(CO)~ sites remain unreacted,and those that do may be rendered inactiveby irreversible oxidation of the surface-bound cobalt species. ClsSi-H modified IT0 also yields surfaces susceptibleto reaction with alcohol and thiol functionalities,but submonolayer coverages were found after equivalent soaking times. Introduction of Co2(CO)a into the modifying solutions increased the rates for surface modification and led to complete monolayer coverages after equivalent soaking times.
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Scheme I
Introduction We report the cobalt-mediated, self-assembly of redoxactive alcohols,amines,thiols, carboxylicacids, and amides (1-5) on indium-tin-oxide substrates (ITO), Scheme I.' Over the paat two decades there has been great deal of interest in the modification of electrode surfaces for potential use in device technology dealing with nonlinear optics,2 sensors,3 molecular electronics,4 and microphotolith~graphy.~ Furthermore, electrodes modified with redox-activemoieties have given insight into fundamental
2, HX, X
-
-p(NHCeHq)FC
3, HX, X = -S(CHz)q&(O)FC 0, HX, X = -OC(O)FC 3, HX, XI -NHC(O)FC n
Fc-
(1) Scheme I ia an idealized representation of an oxide surface treated with a (CH&Cl&iR reagent where x = 0. It is not drawn with the intent of suggesting that all S i 4 1 bonds of the modifying reagent have reactad with the surface to form S i 4 linkages. (2) Li, D.; Ratner, M. A.; Marks,T. J.; Zhang, L. H.; Yang, Y.; Wong, G. J. Am. Chem. SOC.1990,112,7389. (3) (a)Mirkin,C.A.;Valentine,J.R.;Ofer,D.;Hickman,J.J.;Wrighton, M. S. In Chemically Sensitive Microelectrochemical Devices: New Approaches to Sensors; Edelman, P. G., Wang, J.,Eda.;ACSSymposium Series487; American Chemical Society: Washington, DC, 1992; Chapter 17. (b) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991,252,688. (c) Rubinstein, I. Anal. Chem. 1984,56, 1135. (4) (a) Mirkin, C. A.; Ratner, M. A. Ann. Phys. Rev. 1992,43,719. (b) Lower Dimensional S y s t e m and Molecular Electronics; Metzger, R. M., Day, P., Papavaseiliou, E%.; NATO AS1Series; Plenum Press: New York, 1990, Vol. 248. (5) Kang, D.; Wrighton, M. S. Langmuir 1991, 7 , 1342.
& electron transfer processes.6 Advances in methods for modifying surfaces must complement the stringent requirements of the molecular modification reagents of interest. Factors to consider are functional group compatibility, strength of the reagent-aurface interaction, and the environmental stability of the resulting monolayers. The chlorosilyl (CLSi-) and alkoxysilyl ((RO),Si-) funcS the molecular tionalities have become universal ~ O U D for (6) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvineki, T. M.; Mujsce, A. M. J. Am. Chem. SOC.1990,112,4301. (b) Li,T. T.-T.; Weaver, M. J. J. Am. Chem. SOC.1984,106, 6107.
0 1992 American Chemical Society
Letters
2586 Langmuir, Vol. 8, No. 11, 1992
self-assembly of reagents onto oxide surfaces.7 Modification reagents with these functionalities are proposed to undergo condensation reactions with surface -OH or -0sites to anchor the reagent through strong, covalent S i 4 bond(s). The chlorosilyl and alkoxysilylfunctional groups have some inherent drawbacks with respect to surface modification;besides being difficult to work with, the silyl functionalities are often incompatible with the surface modification reagents of interest. Those compounds that may be synthesized with the silyl functionalities are often times difficult to isolate due to condensation reactions with trace amounts of HZO.~Other workers have avoided synthesizing silyl functionalized modification reagents by prior modification of the surface with a molecule possessing organic functionality that lends itself to further modifi~ a t i o n .This ~ prelayer, which is typically a silyl reagent itself,allows for the modification of surfaces using organic methodologies. The results reported herein outline a new and general transition metal-mediated method for the modificationof oxide surfaceswith redox-activemolecules possessing alcohol,thiol, amine, carboxylicacid, and amide functionalities.
B
Experimental Section Instruments and Equipment. IR spectra were recorded using a Nicolet 520 FT-IR spectrometer with MCT detector. X-ray photoelectron spectra were obtained with an ESCALAB Mark I1 X-ray photoelectron spectrometer (XPS) with Mg Ka source. Cyclic voltammograms were recorded with a Pine RDE4 bipotentiostat coupled with a Kipp and Zonen BD90 X-Y recorder. Either acetonitrile or methylene chloride was used as electrochemistry solvent, tetrabutylammonium hexatluorophosphate (TBAPF8) as supporting electrolyte, Pt mesh as counter electrode, and silver wire as a quasi-reference electrode. All electrochemistrywas performed in a dryboxunder N2 atmosphere. Materials. All solvents were dried and purified by literature methods.10 Indium-tin-oxide coated glass and quartz were purchased from DeltaTechnology, Stillwater, MN. Compounds 1,4, HSiCl3, and HSi(CH3)Clz were purchased from Aldrich and used without further purification. Co~(C0)a was purchased from Strem Chemical Co. Compounds 2,3, and 5 were synthesized and characterized according to literature methods.ll C13SiCo(CO)4 and Clz(CH3)SiCo(CO)4 were synthesized according to literature methods12 and characterized via FTIR spectroscopy and mass spectroscopy: Cl3SiCo(CO)4, MS M+ = 304 m/z, IR (toluene) YCO = 2120 (m), 2066 (m), 2034 (vs) cm-'; MeClzSiCo(COl4,MS M+ = 284 m/z, IR (toluene) YCO = 2111 (m), 2054 (m), 2026 (vs), 2016 (vs) cm-l.
Results and Discussion I T 0 surfaces pretreated with (CH3),(Cl)s-,SiCo(CO)r, x = 0, 1,yield surfaces susceptible to metathesis with NH (aminesand amides),SH, and OH (alcoholsand carboxylic acids) functionalities, Scheme I. Other workers have shown that (CH3)3SiCo(C0)4 reacts with amines and (7) For reviews see (a) Murray, R. W. Electroanalytical Chemistry; Bard, A. J., Ed.;Marcel Dekker: New York, 1984, Vol. 13, p 191. (b) Molecular Design of Electrode Surfaces, Murray, R., Ed.;Wiley: New York, 1992. (8)(a) Fischer, A. B.; Kinney, J. B.; Stanley, R. H.; Wrighton, M. S. J. Am. Chem. SOC. 1979,101,7863. (b) Wrighton, M.S.; Palazzotto, M. C.; Bocareley, A. B.; Bolta, J. M.; Fiecher, A. B.; Nadjo, L. J. Am. Chem. SOC.1978,100, 7264. (9) (a) Haller, I. J. Am. Chem. SOC.1978,100, 8050. (b) Fox, M. A.; Nobs, F. J.; Voynick, T. A. J. Am. Chem. SOC.1980,102,4029. (10) Gordon,A.J.; Ford, R. A. The Chemist's Companion;Wiley: New York, 1972. (11)2: Little, W.F.; Clark, A. K. J. Org. Chem. 1960, 25, 1979. 3 Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J . Am. Chem. SOC.1991, 113, 1128. 6 Little, W.F.; Eieenthal, R. J . Am. Chem. SOC.1960,82,1577. (12) (a) Baay, Y. L.; McDarmid, A. G. Inorg. Chem. 1969,8,986. (b) Chalk, A. J.; Harrod, J. F. J. Am. Chem. SOC.1965,87, 1133.
0.5 Wsec 0.1 M TBAPFe
in CH$N +I
* . L I
8 0.4
0
I
I
.
0.4
Potential, V
1
1
I
0.8 VS.
.
I
.
I
1
I
4
2.0
1.2 1.6
Ag wire
Figure 1. (A) Cyclic voltammetry of a C&SiCo(CO)dmodified IT0 electrode (0.25cmz)afte.r soakingin a 0.01 M toluene solution of ferrocenylmethanol for 48 h. The first scan shows two surface species, SiCo(CO)4(irreversible wave at lees positive potential) and ferrocenylmethanol (reversible wave at more positive potential). The second scan shows only one reversible wave for the ferrocenylmethanol. (B)Cyclic voltammetry of aClz(CH3)SiCo(COI4modified IT0 electrode (0.25 cm2). First scan shows one irreversible wave assigned to siCo(C0)~species. The second scan resembles the response observed for an unmodified IT0 electrode.
alcohols to form (CH3)3SiNHR and (CH3)3SiOR, eqs 1 and 2, respectively.12 We rationalized that the SiCo(C0)r
+ R'NH2
(CH3)3SiCo(C0)4
+ R'OH
(CH3)3SiNHR' (CH&SiOR'
+
+
(C0)dCoH
(C0)4CoH
(1)
(2)
unit, if immobilized onto a surface, would serve as a versatile synthetic intermediate for further surface modification. Although the S i 4 1 and S i 4 0 bonds of &SiCo(C0)r are susceptible to hydrolysis reactions and therefore possible reaction with the IT0 surface, the observed reactivity is with the S i 4 1 bonds leaving the Si-Co(CO)r unit intact. In a typical experiment, an IT0 electrode is soaked in a 0.5 M KOH solution (EtOH/H20) for 1 h and rinsed with hot, distilled water and ethanol, respectively, and dried under vacuum. The IT0 electrode is then soaked in a 0.01 M toluene solution of (CH3),Cl3-,SiCo(CO)r ( x = 0 or 1) for 24 h at 22 "C under a N2 atmosphere. Subsequent treatment of the Si-Co(CO)r modified IT0 with a 0.01 M acetonitrile solution of any one of the redoxactive surface modification reagents 1-5 leads to efficient molecular self-assembly onto the I T 0 electrode surface, Scheme I. Figure 1A shows the cyclic voltammetry of a Cl3SiCo(CO)4 modified IT0 electrode (0.25 cm2) after
Langmuir, Vol. 8, No. 11, 1992 2587
Letters soaking in a 0.01 M solution of ferrocenylmethanol(1) for 48 h at 22 "C. On the first scan two Oxidation waves are observed,one large electrochemicallyreversible wave (Ell2 = 0.64 V vs Ag) due to the adsorption of the ferrocenylmethanol and one considerably smaller irreversible wave (E, = 0.34 V vs Ag) which we assign to unreacted SiCo(C0)r sites. On the second and subsequent scans one persistent, electrochemically reversible wave for the ferrocenylmethanol is observed with an electrochemical response consistent with monolayer coverage (-6 X 1013 molecules/cm2),Figure 1A. The cyclicvoltammetry of a Cl2(CH3)SiCo(CO)&reated IT0 electrode (0.25 cm2)shows one irreversible oxidation wave (Epa= 0.5 V vs Ag), Figure lB, at a potential similar to the small irreversible wave observed in Figure 1A. Similar results are observed for ClsSiCo(C0)r surfaces. Significantly, compounds 1-5 will not react with Si-Co(C0)r modified surface after it has been electrochemically oxidized. X-ray photoelectron spectroscopy of Cia+ (CH3),SiCo(CO)r-treated electrode after electrochemical oxidation shows no Co signal. The electrochemically irreversible processes observed in parts A and B of Figure 1presumably are due to the oxidation of SiCo(CO)4species and are responsible for the removal of potential surface modification sites. In Figure lA, a comparison between the currents associated with Co and ferrocene (Fc) oxidation, IFJIco = 99:l reveals that 1% of the cobalt surface sites remains unreacted after treatment with the ferrocenylmethanol solution. Cyclic voltammograms similar to the one depicted in Figure 1A may be recorded for compounds 2-5 adsorbed onto the cobalt-modifiedIT0 in a similar fashion. Surface coverages for compounds 2,3,4,and 5 were found to be 4 x lO13,5 X lO13,7 X 1013, and 4 X 1013 molecules/cm2, respectively. The electrochemicalresponse for assemblies formed from 2-5 via this method are also persistent and stable to the conditions employed.13 Compounds 1 and 4 are presumably attachedto the surfacevia Si-0 covalent bonds formed from the metathesis of the SiCo(C0)r unit and an 0-H bond. Likewise compounds 2 and 5 must be anchored to the surface via Si-N covalent bonds formed from the metathesis of the SiCo(C0)r unit with the N-H bonds of 2 and 5,respectively. It occurred to us that the surface SiCo(C0)r sites might be reacting with one of two acidic H sites on compound 3, either the S-H bond or one of the C-H bonds CY to the carbonyl group. However, solution and surface-bound Cl@iCo(CO)rdo not react with acetylferrocene, a molecule that possesses acyl functionality but not thiol functionality, indicating that the mode of modification for 3 is most likely through a Si43 bond. The reaction between 3 and the (CO)&oSi-modified surfaceis particularly interestingsince thiols are commonly used to modify noble metal surfaces (Pt, Au, Ni, Ag).14 With the chemistry reported herein, thiols may now be
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(13) Modified electrodeawere subjectedtocontinuow cyclingat variow scan rates (1000,800,500,300,200, and 100 mV/s).
Scheme I1
used as a universal functional group for noble metal and oxide electrode surface modification. It is interesting to note that compounds 1and 3 will also react slowly and inefficiently (several days) at room temperature with an C13SiH treated IT0 electrode (0.3 cm2) to yield submonolayer coverages (
