Surface Modification of GC and HOPG with Diazonium, Amine, Azide

pubs.acs.org/Langmuir © 2010 American Chemical Society

Surface Modification of GC and HOPG with Diazonium, Amine, Azide, and Olefin Derivatives Mutsuo Tanaka,* Takahiro Sawaguchi,* Yukari Sato, Kyoko Yoshioka, and Osamu Niwa Biomedical Research Institute, Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received September 7, 2010. Revised Manuscript Received October 15, 2010 Surface modification of glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) was carried out with diazonium, amine, azide, and olefin derivatives bearing ferrocene as an electroactive moiety. Features of the modified surfaces were evaluated by surface concentrations of immobilized molecule, blocking effect of the modified surface against redox reaction, and surface observation using cyclic voltammetry and electrochemical scanning tunneling microscope (EC-STM). The measurement of surface concentrations of immobilized molecule revealed the following three aspects: (i) Diazonium and olefin derivatives could modify substrates with the dense-monolayer concentration. (ii) The surface concentration of immobilized amine derivative did not reach to the dense-monolayer concentration reflecting their low reactivity. (iii) The surface modification with the dense-monolayer concentration was also possible with azide derivative, but the modified surface contained some oligomers produced by the photoreaction of azides. Besides, the blocking effect against redox reaction was observed for GC modified with diazonium derivative and for HOPG modified with diazonium and azide derivatives, suggesting fabrication of a densely modified surface. Finally, the surface observation for HOPG modified with diazonium derivative by EC-STM showed a typical monolayer structure, in which the ferrocene moieties were packed densely at random. On the basis of those results, it was demonstrated that surface modification of carbon substrates with diazonium could afford a dense monolayer similar to the self-assembled monolayer (SAM) formation.

Introduction Carbon substrates have been studied and used extensively, especially in analytical electrochemistry fields. For applications in sensing, various surface modification of carbon substrate to introduce functional organic molecules is of particular interest to detect organic and biological molecules.1 It has been reported that electroreduction of phenyl diazonium,2-14 electrooxidation *Corresponding authors. E-mail:[email protected], sawaguchi.t@ aist.go.jp. (1) McCreery, R. L. Chem. Rev. 2008, 108, 2646–2687. (2) Delamar, M.; Hitmi, R.; Pinson, J.; Sav
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170 DOI: 10.1021/la1035757

of alkyl amine,4,15-20 and photoreaction of phenyl azide21-31 and olefin derivatives5,32,33 can modify the surface of carbon substrates. One of attractive features in those modifications is that organic molecules are covalently immobilized on the surface of carbon substrate. Although various modified surfaces have been reported with those methods for application use, a systematic study to compare the features of modified surfaces depending on those methods is relatively rare. Attempts to characterize modified surfaces often come across difficulties in analysis because the surface concentration of immobilized molecule is considerably low for quantitative detection, and analytical methods for thin organic layer on solid substrate are limited compared with those for common organic molecules. Electrochemical analysis is known to be one of useful methods in modified surface characterization. (20) Chr
etien, J.; Ghanem, M. A.; Bartlett, P. N.; Kilburn, J. D. Chem.—Eur. J. 2009, 15, 11928–11936. (21) Dontha, N.; Nowall, W. B.; Kuhr, W. G. Anal. Chem. 1997, 69, 2619–2625. (22) Brooks, S. A.; Ambrose, W. P.; Kuhr, W. G. Anal. Chem. 1999, 71, 2558– 2563. (23) Brooks, S. A.; Dontha, N.; Davis, C. B.; Stuart, J. K.; O’Neil, G.; Kuhr, W. G. Anal. Chem. 2000, 72, 3253–3259. (24) Gao, C.; He, H.; Zhou, L.; Zheng, X.; Zhang, Y. Chem. Mater. 2009, 21, 360–370. (25) Holzinger, M.; Abraha, J.; Whelan, P.; Graupner, R.; Ley, L.; Hennrich, F.; Kappes, M.; Hirsch, A. J. Am. Chem. Soc. 2003, 125, 8566–8580. (26) Holzinger, M.; Steinmetz, J.; Samaille, D.; Glerup, M.; Paillet, M.; Bernier, P.; Ley, L.; Graupner, R. Carbon 2004, 42, 941–947. (27) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002–4005. (28) Liu, L. H.; Yan, M. D. Nano Lett. 2009, 9, 3375–3378. (29) Choi, J.; Kim, K.; Kim, B.; Lee, H.; Kim, S. J. Phys. Chem. C 2009, 113, 9433–9435. (30) Pastine, S. J.; Okawa, D.; Kessler, B.; Rolandi, M.; Llorente, M.; Zetti, A.; Fr
echet, J. M. J. J. Am. Chem. Soc. 2008, 130, 4238–4239. (31) Gross, A. J.; Yu, S. S. C.; Downard, A. J. Langmuir 2010, 26, 7285–7292. (32) Ssenyange, S.; Anariba, F.; Bocian, D. F.; McCreery, R. L. Langmuir 2005, 21, 11105–11112. (33) Sun, B.; Colavita, P. E.; Kim, H.; Lockett, M.; Marcus, M. S.; Smith, L. M.; Hamers, R. J. Langmuir 2006, 22, 9598–9605.

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To apply electrochemical analysis to modified surface characterization, electroactive molecules such as ferrocene, quinone, and nitorobenzene should be incorporated in the molecule immobilized on substrate as a probe. Then, the surface concentration of electroactive molecules immobilized on substrate is detectable electrochemically even in self-assembled monolayer (SAM) concentration order, 10-10 mol/cm2.34 In this report, we synthesized diazonium, amine, azide, and olefin derivatives bearing the ferrocene moiety as an electroactive probe. With those probe molecules, surface modification of GC and HOPG was carried out to shed light on the features of fabricated surfaces depending on the modification methods. There have been several reports to immobilize electroactive molecules on substrate, in which the electroactive molecules were introduced by two-step reaction, namely, reaction of molecules bearing a functional group with substrate and then coupling reaction of electroactive molecules with the immobilized functional group.35-39 With this method, however, the possibility that the obtained surface concentration of electroactive molecules is less than the surface concentration of immobilized molecules cannot be ruled out because complete introduction of the electroactive molecule to the functional group of the immobilized molecule on substrate is impossible. Therefore, to obtain information for the reaction of carbon substrates with diazonium, amine, azide, and olefin moieties exactly, we adopted the procedure that the ferrocene moiety was already incorporated into the molecules, which react with carbon substrates. The features of modified surfaces were evaluated on the basis of the surface concentration of immobilized molecules, blocking the effect of the modified surface against redox reaction, and the surface observation using cyclic voltammetry and electrochemical scanning tunneling microscope (EC-STM).

Experimental Section Materials and Synthesis. All chemicals were used as received without additional purification. Details of synthesis procedures are described in the Supporting Information. Millipore water was used for electrochemical measurements. GC electrode (carbon diameter: 3 mm) and HOPG (mosaic spread 3.5 ( 1.5°) were purchased from BAS and Digital Instruments, respectively. Surface Modification of GC with Diazonium. Before surface modification, GC surface was polished with 0.3 μm alumina slurry and then with 0.05 μm alumina slurry. The GC was rinsed with water under ultrasonication for 8 min for each step. In situ formation of diazonium salts from corresponding amines40-42 by reaction with NaNO2 (15 mM) in the presence of HCl (50 mM) and n-Bu4NPF6 (100 mM) in DMF-H2O (4:1 vol.) mixture at 0 °C was employed. The GC was dipped in the prepared solution of diazonium in an electrochemical cell at 0 °C. The electroreduction to modify GC surface was conducted with potential range of 1.0 V (-480 to 520 mV vs Ag/AgCl) and sweep rate 20 mV/s, where the scan was carried out twice. The (34) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687–2693. :: (35) Noel, J.; Sj€oberg, B.; Marsac, R.; Zigah, D.; Bergamini, J.; Wang, A.; Rigaut, S.; Hapiot, P.; Lagrost, C. Langmuir 2009, 25, 12742–12749. (36) Liu, G.; Liu, J.; B€ocking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136–146. (37) Gautier, C.; Ghodbane, O.; Wayner, D. D. M.; B
elanger, D. Electrochim. Acta 2009, 54, 6327–6334. (38) Vijaikanth, V.; Capon, J.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. Electrochem. Commun. 2005, 7, 427–430. (39) Harper, J. C.; Polsky, R.; Wheeler, D. R.; Brozik, S. M. Langmuir 2008, 24, 2206–2211. (40) Corgier, B. P.; Marquette, C. A.; Blum, L. J. J. Am. Chem. Soc. 2005, 127, 18328–18332. (41) Cougnon, C.; Gohier, F.; B
elanger, D.; Mauzeroll, J. Angew. Chem., Int. Ed. 2009, 48, 4006–4008. (42) Baranton, S.; B
elanger, D. J. Phys. Chem. B 2005, 109, 24401–24410.

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Article modified GC was rinsed with methanol under ultrasonication for 1 min, and then rinsed with water. The ultrasonication more than for 1 min did not influence the surface concentration of molecules immobilized on the GC in control experiments. Surface Modification of HOPG with Diazonium. A fresh HOPG surface was prepared by peeling off method with Scotch tape. The HOPG was mounted on a flanged electrochemical cell. The diazonium solution was prepared in the cell according to the procedure described above, and the surface modification was performed under the same condition as that of the GC. After modification, the solution in the cell was removed, and the cell mounting the modified HOPG was rinsed with methanol. The cell mounting the modified HOPG was filled with methanol, irradiated with ultrasound for 1 min, and then rinsed with water. Surface Modification of GC and HOPG with Amine. The procedures of GC and HOPG treatments were the same as those for diazonium modification. A DMF solution of an amine with n-Bu4NPF6 (100 mM) at room temperature was used. The electrooxidation was carried out with sweep rate 20 mV/s and two scans. The third and subsequent scans did not influence the surface concentration of immobilized molecule, similar to the diazonium modification in control experiments. Surface Modification of GC with Azide. The procedure of GC treatment was the same as that for diazonium modification. A solution of azide dissolved in methanol or a mixture of methanol with chloroform (4:1 vol.) was deposited on the GC, and the GC was dried at room temperature. To the GC, UV light was irradiated for 5 min. The surface modification with azide was completed within 5 min of UV light irradiation in control experiments. A Hg-Xe lamp (200 W, Hamamatsu Photonics K.K.) was used for UV light irradiation. The modified GC was rinsed with methanol under ultrasonication for 1 min and then rinsed with water. A chloroform solution of azide 9 (3.5 mM) was used to fabricate LB membrane, and the LB membrane was deposited on the GC. The GC was allowed to dry under nitrogen atmosphere before UV light irradiation. The following procedure was the same as that for other azides. Surface Modification of HOPG with Azide. The procedure of HOPG treatment was the same as that for diazonium modification. A solution of azide dissolved in methanol or a mixture of methanol with chloroform (4:1 vol.) was deposited on HOPG, and the HOPG was dried at room temperature. The procedure of UV light irradiation was the same as that for the GC modification with azide. The modified HOPG was mounted on the flanged electrochemical cell, and the cell mounting the modified HOPG was rinsed with methanol. After the cell mounting, the modified HOPG was filled up with methanol, and the procedure was the same as that for HOPG modification with diazonium. Surface Modification of GC and HOPG with Olefin. The procedures were the same as those for modification with azide, except for UV light irradiation. In the case of olefin, UV light was irradiated to GC and HOPG for 2 and 3 h, respectively.

Determination of Surface Concentration of Immobilized Molecule. A solution of 0.1 M NaClO4 was used as electrolyte. The sweep rate was always 20 mV/s. Because the ferrocene moiety is oxidized with single electron, the surface concentration of immobilized molecule Ci was determined according to the equation Ci = Q/FA in which Q is the charge of oxidation peak in cyclic volutammogram (CV), F is the Faraday constant, and A is the geometric area of the GC or HOPG. The geometric diameters of areas in CV measurements were 3 and 6 mm for GC and HOPG, respectively. The surface concentration of immobilized molecule was calculated on the basis of the quantity of electricity in CV without considering the roughness of GC and HOPG surfaces.43,44

(43) McDermott, M. T.; McDermott, C. A.; McCreery, R. L. Anal. Chem. 1993, 65, 937–944. (44) McDermott, C. A.; McCreery, R. L. Langmuir 1994, 10, 4307–4314.

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Article Scheme 1. Synthesized Surface Modification Materials

Evaluation of Blocking Effect. The blocking effect of modified GC and HOPG against redox reaction was evaluated using 1 mM Ru(NH3)6Cl3 with 0.1 M NaClO4 and Ru(BPy)3Cl2 with 0.1 M Na2SO4 solutions as redox reagents. The sweep rate was always 20 mV/s. EC-STM Observation.45 We performed electrochemical STM (EC-STM) measurements in 0.1 M HClO4 solution by using a Nanoscope E system (Digital Instruments, Santa Barbara) with a tungsten tip sharpened electrochemically in 1 M KOH. The tips were coated with nail polish to minimize residual faradaic current. EC-STM images were acquired in the constant current mode. Potential values of substrate and tip were reported with respect to the Ag/AgCl. Langmuir-Blodgett Membrane Fabrication. Surface pressure and molecular area isotherms were measured with a LangmuirBlodgett (LB) trough (12.1
49.0 cm, KSV LB 5000, Finland) at 20 °C. The LB trough and movable barriers were cleaned with wiping several times with chloroform- or ethanol-soaked lint-free BEMCOT cloth. A platinum Wilhelmy plate used in the surface pressure measurements was cleaned with hot chromic acid solution and rinsed with pure water before use. The appropriate amount of freshly prepared 3.5 mM chloroform solution of azide 9 was dropped on a clean surface of water subphase and remained until evaporation of chloroform. The monolayer was gradually compressed and transferred onto the GC electrode with one downstroke into the water subphase at deposition pressure of 40 mN/m using the LB technique. Then, after the subphase surface was cleaned by aspiration of remaining monolayer molecules, the monolayer-deposited GC electrode was slowly taken out from the water subphase and dried under N2 gas flow for 1 h.

Results and Discussion Synthesis of Surface Modification Materials. Synthesized compounds for surface modification, diazonium 1 and 2, amines 3-5, azides 6-9, and olefin derivatives 10 bearing the ferrocene moiety are shown in Scheme 1. In Scheme 1, amine derivatives, 4 and 5 are the starting materials of diazonium derivatives, 1 and 2, respectively. In this study, incorporation of the ferrocene moiety and a long alkyl chain was essential to determine the surface (45) Itaya, K. Prog. Surf. Sci. 1998, 58, 121–248.

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concentration of immobilized molecules and to evaluate blocking effect, respectively. In addition, compounds bearing piperazino group between the ferrocenecarbonyl group and the moiety to react with carbon substrates were also synthesized in cases of diazonium, amine, and azide derivatives. When a monolayer consisting of a long alkyl chain is formed on electrode, it is considered that additional electrochemical reaction to form a multilayer is suppressed effectively.1 Therefore, pairs of diazonium (1 and 2) and amine derivatives (4 and 5) were synthesized to study the influence of molecular structures on the electrochemical surface modification. We considered that molecular orientational order on substrate could influence surface modification in photoreaction of azide derivative because the reactive species from azide is not always produced at the substrate surface, being different from electrochemical reaction. Because a long alkyl chain is known to influence molecular orientational order in SAM formation, a pair of azide derivatives, 6 and 7, was synthesized. Similarly, hydrogen bond formation is recognized to influence the molecular orientational order. Therefore, azide derivative 8, which does not form hydrogen bond, was synthesized to compare with 7. Furthermore, azide derivative 9 was designed to form LB membrane to evaluate the influence of molecular orientational order on surface modification, precisely. Surface Modification with Diazonium Derivatives, 1 and 2. Surface modification of carbon substrates by electroreduction of diazonium has been of great interest because this modification leads to not only strong attachment of organic molecules to the surface of carbon substrates with covalent bond but also formation of ultrathin organic layers. Electrochemical modification of surfaces seems to be desirable for the formation of ultrathin organic layers because the generation of radical is restricted close to the electrode surface, namely, the modification target.1 In several cases, however, the formation of a multilayer has been reported to suggest that there is a difficulty in controlling the radical to form a monolayer.46,47 To use the diazonium modification for applications requiring the formation of a well-defined monolayer, suppression of the multilayer formation is necessary. The formation of a multilayer is fundamentally caused by attack of radical to surface-bound molecules. We considered that two behaviors of diazonium might mainly contribute to the multilayer formation: the tendency of radical produced from diazonium to react with surface-bound molecules rather than carbon surface and the formation of radical by electron tunneling through the formed monolayer. Whereas the tendency of radical has been examined using phenyl diazoniums with various substituents,42,48 we synthesized diazoniums 1 and 2 to shed light on the possibility for monolayer formation by suppression of electron tunneling through the formed monolayer. We adopted in situ formation of diazonium according to the literature.40-42 First, the condition of surface modification was examined with GC electrode. When the electroreduction was carried out using 1 mM solution of diazonium 1 with potential range of 1.0 V (-480 to 520 mV vs Ag/AgCl) and two scans, the surface concentration of molecule immobilized on the GC was 2.0
10-10 mol/cm2. With five scans under the same condition, the surface concentration of immobilized molecule was 2.3
10-10 mol/cm2, showing that additional modification hardly occurred with the following scans. The CV for the first scan afforded a broad irreversible reduction peak, and the second and subsequent scans gave much smaller peaks on the basis of comparison in the (46) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534–6540. (47) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947–5951. (48) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Sav
eant, J. J. Am. Chem. Soc. 1997, 119, 201–207.

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CV data of electric charge similar to the literature.49 Because the size of reduction peaks in CV reflected the formation of radical to modify the GC surface, the behavior in CV was consistent with the tendency observed in the immobilized molecular concentrations. The maximum surface concentration of immobilized molecule for a monolayer on GC has been considered to be 12
10-10 mol/cm2 theoretically in the case of nitrobenzene;50 however, the experimental values were lower than that value.51-54 The value for ferrocene has been reported to be 4.5
10-10 mol/cm2 theoretically,55 and it has been considered that the value could be higher in the presence of alkyl chain because of its elastic nature.34 Therefore, the surface concentration of the ferrocene derivatives for a monolayer was expected to be between 4.5 and 12
10-10 mol/cm2 at the maximum concentration in our experiments. Because our result around 2
10-10 mol/cm2 was far less than those concentrations, there might be some reasons to suppress the additional surface modification of the GC with the second and following scans. Control experiments with three kinds of procedures were carried out using 1 mM solution of diazonium 1 to explore the reason for the suppression of additional surface modification. The first procedure was electroreduction with two scans, rinse with water, and then electroreduction with two scans again. The second procedure was electroreduction with two scans, rinse with methanol under ultrasonication for 1 min, and then electroreduction with two scans again. The third procedure was electroreduction with a wider potential range of 1.5 V (-980520 mV vs Ag/AgCl) and two scans. The surface concentrations of the immobilized molecule were 2.1, 3.0, and 3.2
10-10 mol/cm2 for the first, second, and third procedures, respectively. The higher surface concentration of immobilized molecule with the second procedure than with the first procedure suggested that the adhesion of some materials on the GC surface occurred during electroreaction to suppress the surface modification. The rinse with methanol under ultrasonication was effective to remove the materials adhering on the GC surface, but the rinse with water had no effect. The surface modification promoted with the third procedure suggested that the adhesion of some materials occurred during the first scan, as the potential range of electroreduction influenced the surface modification. Those results suggested that the two scans in the surface modification procedure were enough for the GC modification because of the adhesion of some materials on the GC surface to suppress the modification with the subsequent scans. The surface concentration of immobilized molecule depending on the concentration of diazonium 1 solution was examined with GC. For a monolayer modification, a milder electroreductive condition would be preferable, and the electroreduction was carried out with potential range of 1.0 V (-480 to 520 mV vs Ag/AgCl) and two scans. The results are summarized in Figure 1. As seen in Figure 1, the surface concentration of immobilized molecule increased with the increase in solution concentration, but with concentrated solution, the surface concentration of immobilized molecule tended to reach a plateau around the surface concentration of dense monolayer.34,50-55 This result implied that (49) Stockhausen, V.; Ghilane, J.; Martin, P.; Tripp
e-Allard, G.; Randriamahazaka, H.; Lacroix, J. J. Am. Chem. Soc. 2009, 131, 14920–14927. (50) Liu, Y.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254–11259. (51) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791–8798. (52) Downard, A. J. Langmuir 2000, 16, 9680–9682. (53) Cruickshank, A. C.; Tan, E. S. Q.; Brooksby, P. A.; Downard, A. J. Electrochem. Commun. 2007, 9, 1456–1462. (54) Brooksby, P. A.; Downard, A. J.; Yu, S. S. C. Langmuir 2005, 21, 11304– 11311. (55) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301–4306.

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Figure 1. Surface concentration of immobilized molecule depending on concentration of diazonium 1 solution. The modification was carried out with potential range of 1.0 V (-480 to 520 mV vs Ag/AgCl) and two scans.

the formation of a multilayer was not predominant in the modification of the electrode surface with diazonium 1. Although diazonium 1 possessed a shorter linker, piperazino group between the ferrocene and diazonium moieties, it was found that the electron tunneling through the modified surface with diazonium 1 was suppressed effectively. Similar modification tendency was observed for diazonium 2, and the surface concentrations of immobilized molecule with 5 and 10 mM solutions were 6.3 and 8.2
10-10 mol/cm2, respectively, under the same condition as that in Figure 1. The results suggested that the incorporation of a linker between the ferrocene and diazonium moieties to suppress the electron tunneling through modified surface could be a promising method to obtain a monolayer surface in diazonium modification. The blocking effect against redox reaction was examined using 1 mM Ru(NH3)6Cl3 solution with 0.1 M NaClO4 and 1 mM Ru(BPy)3Cl2 solution with 0.1 M Na2SO4 as redox reagents to evaluate the modified surface. The surface modification of GC was carried out using 10 mM solutions of diazoniums 1 and 2 with potential range of 1.0 V (-480 to 520 mV vs Ag/AgCl), and the surface concentrations of immobilized molecule on the GC surface were (7.9 and 8.2)
10-10 mol/cm2 for diazoniums 1 and 2, respectively. The CVs of the GC modified with diazoniums 1 and 2 in the presence or absence of redox reagents are summarized in Figure 2, and the CVs of redox reaction with the intact GC are depicted with a dotted line. The blocking effect was evaluated with positively charged complexes, Ru(NH3)63þ and Ru(BPy)32þ. The modification with diazonium 1 showed a slight blocking effect against Ru(NH3)63þ redox reaction, as seen in Figure 2c, and the blocking effect against Ru(BPy)32þ redox reaction in Figure 2d was significant. This tendency seemed to reflect not only the size of redox reagents, in which Ru(BPy)32þ was bulkier than Ru(NH3)63þ, but also the charge repulsion of Ru(BPy)32þ with the electrode surface positively charged by the ferricinium ion. By the surface modification with diazonium 2, the blocking effect against Ru(NH3)63þ redox reaction became clear, as seen in Figure 2e compared with Figure 2c. It was suggested that the dodecyl group was more effective to show blocking effect than the piperazino group. In Figure 2f, an irreversible oxidation peak around 0.7 V versus Ag/AgCl was observed in the first scan, but that disappeared in the second scan. (See Figure S1 in the Supporting Information.) Such an irreversible peak was sometimes observed for the surface modified with electroactive molecules, and some DOI: 10.1021/la1035757

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Figure 2. CVs of GC modified with (a) diazonium 1 in 0.1 M NaClO4, (b) diazonium 2 in 0.1 M NaClO4, (c) diazonium 1 in 1 mM Ru(NH3)6Cl3-0.1 M NaClO4, (d) diazonium 1 in 1 mM Ru(BPy)3Cl2-0.1 M Na2SO4, (e) diazonium 2 in 1 mM Ru(NH3)6Cl3-0.1 M NaClO4, and (f) diazonium 2 in 1 mM Ru(BPy)3Cl2-0.1 M Na2SO4. The modification was carried out using 10 mM solution with potential range of 1.0 V (-480 to 520 mV vs Ag/AgCl) and two scans. The CVs of redox reaction with the intact GC are depicted with a dotted line.

surface rearrangement to form insulated surface might occur during the first scan, resulting in the disappearance of the peak. The modification of HOPG was also examined with 10 mM solution of diazonium 1 and 2 under the same condition as that of GC. The surface concentrations of immobilized molecule were (6.4 and 6.6)
10-10 mol/cm2 for diazoniums 1 and 2, respectively. The surface concentrations of immobilized molecule for HOPG were smaller than those for GC, being (7.9 and 8.2)
10-10 mol/ cm2 for diazoniums 1 and 2, respectively. Taking into account the fact that HOPG surface is smoother than GC surface,43,44 this difference could be likely because the surface concentration of immobilized molecule was calculated without considering the roughness factor of GC and HOPG surfaces. Therefore, there might be no significant difference in the reactivity between GC and HOPG with diazonium. The blocking effect against redox reaction for the modified HOPG was evaluated with the same method as that for the modified GC. The CVs for the HOPG modified with diazonium 1 and 2 are summarized in Figure 3. As shown in Figure 3c-f, the modified HOPG showed more significant blocking effect than the modified GC to afford insulating surface, especially in the case of diazonium 2 modification, although the surface concentration of immobilized molecule on the HOPG surface was smaller than that on the GC surface. This tendency in blocking effect could be derived from the smoothness of the HOPG surface compared with the GC surface.43,44 EC-STM Observation for HOPG Surface Modified with Diazonium Derivative 2. The CV analysis of the surface modified with diazonium derivative suggested the formation of a dense monolayer. To evaluate the modified surface directly, ECSTM observation was carried out for the HOPG surface modified with diazonium 2, in which the surface concentration of immo174 DOI: 10.1021/la1035757

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Figure 3. CVs of HOPG modified with (a) diazonium 1 in 0.1 M NaClO4, (b) diazonium 2 in 0.1 M NaClO4, (c) diazonium 1 in 1 mM Ru(NH3)6Cl3-0.1 M NaClO4, (d) diazonium 1 in 1 mM Ru(BPy)3Cl2-0.1 M Na2SO4, (e) diazonium 2 in 1 mM Ru(NH3)6Cl3-0.1 M NaClO4, and (f) diazonium 2 in 1 mM Ru(BPy)3Cl2-0.1 M Na2SO4. The modification was carried out using 10 mM solution with potential range of 1.0 V (-480 to 520 mV vs Ag/AgCl) and two scans. The CVs of redox reaction with the intact HOPG are depicted with a dotted line.

Figure 4. EC-STM images of HOPG surface modified with diazonium 2 obtained in 0.1 M of aqueous perchloric acid solution at room temperature: (a) 100
100 and (b) 20
20 nm. Electrode potential and tunneling current were 0.31 V versus Ag/AgCl and 0.2 nA, respectively.

bilized 2 was 6.6
10-10 mol/cm2. The obtained EC-STM images are shown in Figure 4. As seen in Figure 4, there was no convex or aggregated structure to suggest the formation of a multilayer on the HOPG surface. This observation result supported the fact that additional electroreduction to produce a multilayer was suppressed by the long alkyl chain effectively. In the expanded image Figure 4b, the surface was covered with spots, where the average diameter was 6.2 ( 0.6 A˚. This diameter was consistent with the size of ferrocene moiety of A˚,56,57 which showed that the ferrocene moieties were densely packed on the HOPG surface as a monolayer. The result of the EC-STM observation revealed that the (56) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687–2693. (57) Seiler, P.; Dunitz, J. D. Acta Crystallogr. 1979, 1335, 1068–1074.

Langmuir 2011, 27(1), 170–178

Tanaka et al.

Article

Table 1. Modification of GC with Aminesa

entry

amine

amine (mM)

potential range (V)

1 2 3 4 5 6 7

base (mM)

3 5 1.5 0 3 5 1.5 50 3 5 2.0 0 3 10 1.5 0 4 5 1.5 0 4 5 1.5 50 5 5 1.5 0 a Modification was carried out with two scans.

Table 2. Modification of HOPG with Aminesa

surface concentration (
10-10 mol/cm2) 3.0 2.8 3.1 3.8 2.9 1.6 0.77

surface modification with diazonium derivative could afford a dense monolayer similar to SAM formation. Surface Modification with Amine Derivatives 3-5. It has been known that alkyl amine is immobilized on the surface of carbon substrates via electrooxidation, and primary amine is more reactive than secondary and tertiary amines.58,59 Recently, it was reported that aniline derivative was also immobilized on GC surface under basic condition.60 To study surface modification with amines, we synthesized alkyl amine 3 and aniline derivatives 4 and 5, and the electrooxidative surface modification was carried out using those amines. The results of surface modification of GC depending on various conditions are summarized in Table 1. In entry 1, the electrooxidation of amine 3 with potential range of 1.5 V (-20 to 1480 mV vs Ag/AgCl) afforded the modified surface with the surface concentration of immobilized molecule of 3.0
10-10 mol/cm2. Next, according to the literature,60 the surface modification with amine 3 was performed in the presence of 50 mM trimethylpyridine. However, the basic condition did not promote the surface modification at all, as seen in entry 2. The electrooxidation of amine 3 was carried out with wider potential range of 2.0 V (-20 to 1980 mV vs Ag/AgCl) in entry 3, but the surface concentration of immobilized molecule was almost the same as that with potential range of 1.5 V in entry 1. To increase the surface concentration of immobilized molecule, we examined the surface modification using 10 mM solution of 3 in entry 4. As seen in entry 4, the concentrated solution was effective to obtain a higher surface concentration of immobilized molecule. However, the surface concentration of immobilized molecule was only 3.8
10-10 mol/cm2, which was considerably less than the surface concentration for the dense monolayer on GC surface.34,50-55 The surface modification with aniline 4 in entry 5 afforded the surface concentration of immobilized molecule, 2.9
10-10 mol/ cm2. The addition of trimethylpyridine to the aniline 4 solution deteriorated the surface concentration of immobilized molecule in entry 6, unexpectedly. Finally, the surface concentration of immobilized molecule by electrooxidation of aniline 5 was no more than 7.7
10-11 mol/cm2. On the basis of those results, we concluded that there was significant difficulty to attain a dense surface modification of GC by electrooxidation of amines because of their low reactivity. Similarly, the surface modification of HOPG with amines was examined, and the results are tabulated in Table 2. Whereas the surface modification of HOPG with amine 3 afforded a considerably low surface concentration of immobilized molecule of 8.6
10-11 mol/cm2 in entry 1, aniline 4 showed a high surface concentration of immobilized molecule of 5.4
10-10 mol/cm2, as seen in (58) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306–1313. (59) Adenier, A.; Chehimi, M. M.; Gallardo, I.; Pinson, J.; Vila, N. Langmuir 2004, 20, 8243–8253. (60) Buriez, O.; Labb
e, E.; Pigeon, P.; Jaouen, G.; Amatore, C. J. Electroanal. Chem. 2008, 619-620, 169–175.

Langmuir 2011, 27(1), 170–178

entry 1 2 3 4 5

amine

amine (mM)

potential range (V)

3 5 1.5 4 5 1.5 4 5 2.0 4 10 1.5 5 5 1.5 a Modification was carried out with two scans.

surface concentration (
10-10 mol/cm2) 0.86 5.4 5.3 28 1.3

entry 2. The adoption of a wider potential range of 2.0 V (-20 to 1980 mV vs Ag/AgCl) for electrooxidation in entry 3 did not influence the surface concentration of immobilized molecule again. In entry 4, the surface modification with 10 mM solution of aniline 4 gave far higher surface concentration of immobilized molecule of 2.8
10-9 mol/cm2 than that of the dense monolayer,34,50-55 indicating multilayer formation.61-63 Aniline is wellknown to afford a conductive polymer by electrooxidation; therefore, it was plausible that electron tunneling through the conductive modified surface generated the reactive species from aniline, resulting in the formation of a multilayer. In entry 5, the surface concentration of immobilized molecule with aniline 5 was much lower than that with aniline 4 in entry 2. This result reflected the fact that incorporation of a long alkyl chain was effective to suppress the multilayer formation. The surface concentration of immobilized molecule of the HOPG modified with amine 3 (Table 2, entry 1) was lower than that of the GC (Table 1, entry 1). On the contrary, it was observed that the modification of HOPG with anilines 4 and 5 (Table 2, entries 2 and 5) gave higher surface concentration of immobilized molecule than that of GC (Table 1, entries 5 and 7). The surface of GC was reported to be rougher than that of HOPG;43,44 then, a higher surface concentration of immobilized amine 3 was expected for the GC modification. Taking into account the formation of a multilayer, as seen in Table 2 entry 4, it was considered that the higher surface concentration of immobilized anilines 4 and 5 with HOPG than with GC was derived from the polymerization to form the multilayer but not the reaction with the HOPG surface. The comparison of HOPG with GC in the reactivity with amine derivative suggested that the HOPG surface was reluctant to react with alkyl amines and enhanced polymerization of aniline derivatives. Therefore, a dense monolayer modification of the HOPG surface with amine derivative was considered to be difficult because of those tendencies. Surface Modification with Azide Derivatives 6-9. It has been known that irradiation of UV light on phenyl azide causes the production of a reactive intermediate.64 The key reactive intermediate is considered to be phenyl nitrene65-67 with some ambiguous aspects left to study in the reaction mechanism. There are abundant applications of this photoreaction, for example, (61) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791–8798. (62) Ceccato, M.; Nielsen, L. T.; Iruthayaraj, J.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2010, 26, 10812–10821. (63) An electrochemical analysis using CV has been evaluated extensively in refs 61 and 62, and it was reported that this method was applicable when the thickness of the film was