Chemical Reaction of Reagents Covalently Confined to a Nanotube

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2008, 112, 8122–8126 Published on Web 05/06/2008

Chemical Reaction of Reagents Covalently Confined to a Nanotube Surface: Nanotube-Mediated Redox Chemistry Elicia L. S. Wong and Richard G. Compton* Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford, OX1 3QZ, United Kingdom ReceiVed: March 20, 2008; ReVised Manuscript ReceiVed: April 20, 2008

We present the use of carbon nanotubes (CNTs) as nanoreaction vessels to confine chemical reactions within the carbon nano-“test tube”. This is achieved by functionalizing two redox reagents, anthraquinonnyl and 4-nitrophenyl groups, on the same CNT. It was found that the CNT can mediate the chemical reaction between the two redox reagents and generate 4-arylamine groups at the CNT surface. The reaction can proceed only when both the reagents are confined on the same CNT; if the reagents are located on different tubes, reaction ceases. Introduction Carbon nanotubes (CNTs) have been promoted as the “world’s smallest test tube”, and the confinement of solids such as NaI within these nano-“test tubes” has been convincingly demonstrated.1–3 In this paper, we report the first chemical reaction between two reagents confined to such a “test tube”. The era of nanotechnology received a huge boost with the discovery of CNTs in 1991.4 The principal interest in CNTs comes from the fact that they contain carbon only and all carbons are locally sp2 bound, as in graphite, which provides a special combination of mechanical and electronic properties. In particular, because these tubes are so narrow, electrons are quantum mechanically restricted to move only parallel to the tube axis, thus opening up a real-life laboratory for onedimensional physics. Over the years, the physical/mechanical properties of CNTs have been well-documented, primarily because the methods for such studies are well-established. Mechanistic testing is accomplished using atomic force microscopy (AFM) to produce a small-angle reversible bending and elongation in CNTs5 and also for direct tensile testing;6 stiffness of CNTs was also investigated by their freestanding vibration at room temperature using a transmission electron microscope (TEM).7 The influences of chemical/electronic properties of CNTs, however, are less well-understood, but the need to understand these properties is just as important. To demonstrate these influences, studies are now exploring ways into chemically functionalizing CNTs in order to shed light on the mechanism of action of CNT-based molecular electronic devices, focusing mainly on their ability to exchange electrons between a conductive substrate and a redox couple either in the solution or chemically attached to the CNTs. The ability for CNTs to function as nanowires is elegantly demonstrated by both Gooding’s8 and Willner’s9 groups, where electron communication was observed between the underlying electrode and * Corresponding author. E-mail: [email protected]. Fax: (+44)1865-275410.

10.1021/jp802467a CCC: $40.75

SCHEME 1: Schematic representation of the proposed chemical reaction at the surface of CNT; A/B and X/Y are the redox couples. The nanotube mediates reduction of Y to X with concomitant oxidation of A to B

electroactive proteins chemically bound on CNTs. Both research groups showed that electrons are transported through CNTs along distances greater than 150 nm from the enzymatic active center to the electrode and that the rate of electron transfer is controlled by the length of the CNTs.9 Although the electron-transfer ability between the underlying electrode and the redox centers covalently immobilized to the end of the CNTs is well-investigated, the electron transportation within the CNT itself is not well-understood. From the perspective of electron-transfer properties, the open ends of CNTs have been attributed to the edge plane defects of highly orientated pyrolytic graphite (HOPG), while the walls are suggested as having properties similar to those of basal planes of HOPG. The ability of CNTs to promote electron-transfer reactions is attributed to the presence of edge plane defects at their end caps. There, of course, exists the intrinsic scientific curiosity about what one may accomplish with the unusual transportation ability of CNTs. Thus far, functionalized CNTs have been used widely as nanowires to bridge the molecules (organic or biomolecules) and solid substrates together. So interest arises to whether one can extend the application of CNTs to more than a nanowire. For instance, can it function as a nanoreaction vessel to confine chemical reaction to the CNT “test tube” as seen in Scheme 1? All chemical reactions typically require the participation of at least two compounds, giving rise to a new chemical structure. As a result, one would envision the prerequisite of having at least two chemical molecules being confined within the vicinity  2008 American Chemical Society

Letters of CNTs in order to dissect its ability as a nanovessel. However, no work has been reported on functionalizing CNTs with more than one chemical molecule on the same nanotube as to date. Herein, we demonstrate the use of CNTs as nanovessels, achieved by chemically modifying the CNTs with two redoxactive functional groups at the same CNT surface. To the best of our knowledge, this is the first reported case of dual-molecule functionalization of CNTs and illustration of the occurrence of a chemical reaction at the CNT surface.

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8123 SCHEME 2: Redox pathways of 2-anthraquinonnyl (AQ) and 4-nitrophenyl (NP) functional groups

Experimental Section All of the reagents were obtained from Aldrich (Gillingham, U.K.) with the highest grade available and were used without further purification. Electrochemical measurements were recorded using an Autolab computer-controlled potentiostat (Ecochemie, Utrecht, Netherlands) with a standard three-electrode configuration, consisting of a saturated calomel reference electrode (SCE, Radiometer, Copenhagen, Denmark), a platinum auxiliary electrode (Goodfellow, Cambridge, U.K.) and a bare/ multiwalled CNT (MWCNTs) modified basal plane pyrolytic graphite (BPPG) working electrode. Fifty milligrams of MWCNTs were stirred into 10 mL of solution consisting of a mixture of anthraquinone diazonium chloride and nitrobenzene diazonium tetrafluoroborate. Hypophosphorous acid (H3PO2, 25 mL, 50% w/w in water) was added slowly into the suspension of MWCNTs and diazonium salts mixture and was left stirring for 30 min. The solution was then filtered by water suction to remove any unreacted species from the MWCNT surface. Further washings with Millipore water and acetonitrile were carried out to remove any excess acid and unreacted diazonium salt from the mixture, respectively. The MWCNTs functionalized with both redox species were then airdried. The AQ-NP-MWCNTs were characterized using CV following an established protocol using a standard threeelectrode system.10 All of the potentials quoted here are with respect to SCE. Results and Discussions Functionalization of CNTs with Diazonium Salts. Recently, the effect of electronic structure on covalent functionalized single-walled CNTs was reported.11 The water-soluble diazonium salts are shown to react with CNTs12 through electrochemical activation of the aryl diazonium group by extracting electrons from CNTs, leading to the decomposition of the diazonium functionality and the covalent attachment of the aryl group to the CNTs. The extent of electron donation correlates with the density of states at the Fermi level, which is larger for metallic nanotubes,11 thereby demonstrating highly chemoselective reactions with metallic CNTs. Where covalent attachment of diazonium molecules to the CNT surface has been achieved, this has been carried out by electrochemical reduction of aryl diazonium salts. Lately, our research group reported a simple strategy in functionalizing the CNT with diazonium derivatives in solution without the need for electrochemical activation.10,13 This is achieved by direct reduction of the aryl diazonium salts with hypophosphorous acid in the presence of CNTs. The direct chemical derivatization of CNTs has the advantage over the electrochemical reduction of aryl diazonium salts because it allows inexpensive mass production of the functionalized CNTs. We have covalently linked electroactive groups such as 4-nitrophenyl (NP) and 2-anthraquinnonyl (AQ) onto MWCNTs.14 Both of the chemically derivatized MWCNTs, AQ-MWCNTs and NP-MWCNTs, were characterized successfully using cyclic voltammetry (CV), see the Supporting Information (Figure 1s).

An electrochemically reversible wave was observed in the cyclic voltammogram obtained at the AQ-MWCNT-modified basal plane pyrolytic electrode (BPPG) with Ef0 of -0.415 V versus the saturated calomel electrode (SCE) in 0.1 M sodium acetate/ acetic acid buffer (pH 4.2), see Scheme 2. However, the electrochemical behavior of NP-MWCNT is more complicated that of AQ-MWCNT.14 A typical cyclic voltammogram obtained at the NP-MWCNT-modified BPPG electrode shows one irreversible reduction wave (Ec ) -0.745 V) at the first scan, corresponding to the 4e-, 4H+ reduction of 4-nitrophenyl to the 4-arylhydroxyl amine species,15–17 followed by reversible waves at a more positive potential of Ef0 ) 0.072 V in the subsequent scans corresponding to the 2e-, 2H+ exchange between 4-arylnitroso and the 4-arylhydroxyl amine redox species15–17 in 0.1 M sodium acetate/acetic acid buffer (pH 4.2), see Scheme 2. The 4-arylnitroso and 4-arylhydroxyl amine species are stable only over repeated scans within a limited potential range, that is, 0.5 to -0.4 V, in pH 4.2 solution; the peaks decrease with repeated scans over a wider scan range (see the Supporting Information, Figures 2s and 3s), as a result of further electrochemical reduction of the 4-arylhydroxyl amine species to 4-arylamine groups, which is electroinactive over the potential window of 1 to -1 V in pH 4.2 solution. Evidence of the formation of the 4-arylamine group at the MWCNT surface comes from the successful attachment of 2-anthraquinonecarboxylic acids (AQC) onto the 4-arylamine-MWCNT through carbodiimide coupling (see the Supporting Information, Figure 4s). The 4-arylamine-MWCNT is generated by repeated cycling of an NP-MWCNT-modified electrode between 1 to -1 V until the reversible peaks corresponding to 4-arylhydroxyl amine and 4-arylnitroso groups observed at Ef0 ) 0.072 V have diminished completely. Dual Functionalization of CNTS. Although successful derivatization of CNTs is achieved using the aryl diazonium chemistry, the chemical properties of CNTs have not been fully realized using the diazonium-derivatized CNTs. For instance, does the CNT have a preferential binding site for a particular diazonium species? Perhaps more interestingly, does CNT possess any special intrinsic properties to promote any chemical reaction via the electron transfer reaction along the tube when more than one redox-active species were functionalized on the

8124 J. Phys. Chem. C, Vol. 112, No. 22, 2008

Letters

Figure 2. Multiple scans (50) of the cyclic voltammograms obtained at an AQ-NP MWCNT-modified BPPG electrode in 0.1 M sodium acetate/acetic acid buffer (pH 4.2), showing (a) the first pretreatment scan from +0.5 to -1.0 V and subsequent scans (10th, 20th, 30th, 40th, and 50th) between +0.5 and -0.4 V and (b) enlarged voltammograms of the subsequent scans. The arrows show the increase or decrease of peak size with repeated scans. Scan rate is 0.1 V/s.

Figure 1. First scan of the cyclic voltammogram obtained at (a) 1AQ/ 1NP, (b) 1AQ/5NP, and (c) 5AQ/1NP MWCNT-modified BPPG electrode in 0.1 M sodium acetate/acetic acid buffer (pH 4.2). Scan rate is 0.1 V/s.

same tube? Implicit in this concept is whether CNTs accept electrons with ease, which in turn might be transported under nearly ballistic conditions along the surface of CNTs. In our experimental scheme to answering these questions, we used commercially available bamboo-type MWCNTs (research grade with purity >95%, Nanolab Inc., MA). The lengths of the MWCNTs were 5-20 µm with an outer diameter of 30 µm. We then utilized a similar diazonium-CNT derivatization protocol14 to chemically modify MWCNTs with both AQ and NP functional groups on the same CNTs, forming AQ-NPMWCNTs. In a typical experiment, the modified MWCNTs were abrasively immobilized onto the surface of a BPPG electrode, which was then used for voltammetric experiments conducted in a degassed 0.1 M sodium acetate/acetic acid buffer (pH 4.2). When the CNTs were chemically derivatized, they were exposed to different ratios of diazonium species: 1AQ/1NP, 1AQ/5NP, and 5AQ/1NP. The first issue we address is whether the MWCNT has a binding preference for a particular diazonium species. The AQ species is relatively bulkier than the NP, which only contains one phenyl ring. Therefore, one might envisage that the NP species will react preferentially with MWCNT over the AQ species. The first scan of the cyclic voltammograms obtained at the modified BPPG electrode with 1AQ/1NP, 1AQ/ 5NP, and 5AQ/1NP MWCNT are shown in Figure 1, scanning from 1 to -1 V and then back to 1 V. Three distinctive peaks, one reduction peak and two oxidations peaks, were observed in these cyclic voltammograms, which are referred to as systems i, ii, and iii. The irreversible reduction peak (system iii) at Ec ) -0.620 V corresponds to the reduction of the NP species to the 4-arylhydroxyl amine species. The reduction wave of the AQ species, which is normally observed at Ec ) -0.425 V, is overshadowed by the dominant reduction wave of the NP species. The oxidation peak (system i) at Ea ) -0.385 V corresponds to the oxidation of the AQH2 to the AQ species, whereas the oxidation peak (system ii) at Ea ) 0.125 V is the oxidation of 4-arylhydroxyl amine to 4-arylnitroso group. It was found that when the MWCNT is exposed to an even mixture of AQ and NP diazonium species an equivalent amount

of both groups were covalently attached onto the MWCNT. This is evidenced by the similar charge calculated from the oxidation peaks at systems i and ii obtained in the first scan of the cyclic voltammogram (Figure 1a), which gives an indication of the coverage of the surface-confined species. This observation indicated that all of the reactive sites of MWCNT are equally accessible to both the AQ and NP species regardless of the physical size of the diazonium species. However, this is consistent with the likely free-radical nature of the reactions. The size-independent modification of CNTs using different diazonium salt is further exemplified by the relatively bigger 4-arylhydroxyl amine peak obtained at a 1AQ/5NP MWCNTmodified BPPG electrode (Figure 1b) and vice versa at a 5AQ/ 1NP MWCNT-modified BPPG electrode (Figure 1c). In both cases, the calculated charge (area underneath the oxidation peaks corresponding to systems i and ii) is five times bigger than that of the other species. The observed voltammetric responses not only suggest that the attachment chemistry is independent of the type of diazonium species but also provide further evidence that both species are covalently linked to the surface of MWCNTs, rather than chemisorb to the MWCNTs. If both redox species are not covalently confined to the MWCNT, then it is unclear why the charge calculated for systems i and ii corresponds to the ratio of the diazonium species used in the derivatization of CNTs. Next we were interested to investigate whether the presence of two redox-active species will affect their usual electrochemical behavior at the MWCNT surface. This is investigated by performing CV at a narrower potential window, +0.5 to -0.4 V (after the initial electrochemical treatment of one potential cycle from +1 to -1 V, see Figure 2a), in order to obtain a stable peak potential for the 4-arylhydroxyl amine/4-arylnitroso groups. The cyclic voltammograms of 50 subsequent scans obtained at the 1AQ/1NP-MWCNT-modified BPPG electrode are shown in Figure 2b. The voltammogram depicted three peaks referred to as systems i, ii, and iV. The reduction peak, system iV, at Ec ) 0.025 V corresponds to the reduction of the 4-arylnitroso group to the 4-arylhydroxyl amine group, whereas the oxidation peak at Ea ) -0.385 V (system i as seen in Figure 1) is attributed to the oxidation of AQH2 to AQ, and the oxidation peak at Ea ) 0.125 V (system ii as seen in Figure 1) corresponds to the oxidation of 4-arylhydroxyl amine to the 4-arylnitroso group. The resultant voltammogram showed the gradual decrease in peak currents corresponding to systems ii and iV with each

Letters

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8125 SCHEME 3: Proposed mechanism of the chemical reaction occurs at the surface of the CNT modified with two redox species (AQ and NP). The orange box shows the redox pathway between AQ and AQH2; the blue box shows the redox pathway between 4-arylhydroxyl amine and 4-arylnitroso groups; and the green box shows the further reduction of 4-arylhydroxyl amine to the 4-arylamine group, where CNTs accepts electrons from the AQH2 species and donate the electrons to 4-arylhydroxyl amine. Electron transfer at the CNT surface can occur through either electron tunneling (green arrow) or charge hopping (yellow arrow)

Figure 3. Multiple scans (50) of the cyclic voltammograms obtained at an AQ-MWCNT and NP-MWCNT-modified BPPG electrode in 0.1 M sodium acetate/acetic acid buffer (pH 4.2), showing (a) the first pretreatment scan from +0.5 to -1.0 V and subsequent scans (10th, 20th, 30th, 40th, and 50th) between +0.5 to -0.4 V and (b) enlarged voltammograms of the subsequent scans. Scan rate is 0.1 V/s.

subsequent scan while the peak current of AQ (system i) increases with each successive scan. This electrochemical observation suggests that there is a continuous loss of 4-arylhydroxyl amine/4-arylnitroso groups with concomitant increase in AQH2 species at the MWCNT surface. The same observations were obtained using the 5AQ/1NP-MWCNT and 1AQ/5NPMWCNT-modified BPPG electrode, see the Supporting Information (Figures 5s and 6s). We next consider the following question: Is this electrochemical behavior normal to both redox-active species when they are the sole redox molecules on the MWCNT, that is, AQMWCNT located separately from NP-MWCNT? This is investigated through abrasive immobilization of equivalent amounts of AQ-MWCNT and NP-MWCNT onto the BPPG electrode such that the modified BPPG electrode still contained two redoxactive species, but on separate MWCNTs. The cyclic voltammograms of 50 successive scans of this modified BPPG electrode were shown in Figure 3. Although the voltammogram still shows three distinct peaks, corresponding to the oxidation species of AQH2 (system i) and 4-arylhydroxyl amine (system ii), and the reduction species of 4-arylnitroso groups (system iV), their peak currents remained stable over repeated cycles. This contrasts with the electrochemical observations as seen in Figure 2b. Nonetheless, the difference in the electrochemical behaviors as seen in Figures 2b and 3b provides strong evidence that we have derivatized two redox-active species onto the same tube successfully. Proposed Mechanism. The question arises as to why the peak currents for systems i, ii, and iV varies only when both the redox-active species were attached onto the same CNTs. The gradual loss of 4-arylhydroxyl amine/4-arylnitroso groups with each scan in CV is similar to the phenomenon observed when the NP-MWCNT-modified BPPG electrode was scanned at a wider potential range (see the Supporting Information, Figure 3s) which is due to the formation of the 4-arylamine group under electrochemical reduction at a higher electrode potential. We propose that the decrease in peak currents observed in systems ii and iV as seen in Figure 2 is also a result of the further reduction of 4-arylhydroxyl amine to 4-arylamine, through an alternate route other than electrochemical reduction at a higher electrode potential. In this scenario, the MWCNT plays a role in accepting electrons from the AQH2 species (oxidizing AQH2 back to AQ, orange box in Scheme 3) and donating the electrons to the 4-arylhydroxyl amine group to further reduce it to 4-arylamine (green box in Scheme 3). When the redox-active species are located at different tubes, electron

hopping between different CNTs is not observed as evidenced by the stable peak currents observed in systems ii and iV with repeated cycling. Therefore, it is believed that when both redox species are located at the same surface the CNT helps to confine the electron at the surface, acting not only as a nanowire to transfer the electron but also as a nanovessel to allow chemical reaction (production of 4-arylamine group) to occur. The proposed mechanism for this chemical reaction is as follows: The 4-arylhydroxyl amine and AQH2 species were generated from the first cycle starting from 1 to -1 V, see eqs 1 and 2.

AQ-MWCNT + 2H++2e- f AQH2-MWCNT

(1)

NO2-C6H4-MWCNT + 4H++4e- f NHOH - C6H4MWCNT + H2O (2) The AQH2 group reacts with 4-arylhydroxyl amine to form both the 4-arylamine and AQ species at the surface (eq 3):

AQH2-MWCNT + NHOH-C6H4-MWCNT f AQ MWCNT+NH2-C6H4-MWCNT + H2O (3) This resulting AQ-MWCNT can in turn produce more AQH2MWCNTs, which explains the increased in peak current corresponding to the AQH2 species (system i in Figure 2). The AQH2-MWCNT can then further reduce the remaining 4-arylhydroxyl amine species into the 4-arylamine group as seen in eq 3. As a result, the cycles between eqs 1 and 3 continue until all of the 4-arylhydroxyl amine species are converted to

8126 J. Phys. Chem. C, Vol. 112, No. 22, 2008 4-arylamine groups. Because the electrochemical reaction between 4-arylhydroxyl amine and 4-arylamine is not reversible, a concomitant decrease in 4-arylhydroxyl is observed as exemplified by the decrease in peak current over repeated cycling. The exact mechanism of electron transfer from AQH2 to the 4-arylhydroxyl amine group, whether it occurs by electron tunneling between reagents (green arrows in Scheme 3) or by hopping in the CNT (yellow arrows in Scheme 3) is unknown to us. However, the fact that the reaction proceeds to completion suggests the latter because statistically path that the reagents need to pass must be further apart than the electron distance for tunneling. Conclusions In conclusion, we have demonstrated the ability to chemically derivatize two redox-active molecules onto the same MWCNT. We found that the binding site for the diazonium species is independent of the size of the molecules. Using this strategy, we proceed to investigate the electrochemical behaviors of both redox-active species immobilized on the same CNT. We found that the chemical reaction (reduction of 4-arylhydroxyl amine to 4-arylamine) can proceed at an electrode potential that is inert to 4-arylamine at the modified MWCNT surface with the CNT acting as a nanovessel to confine and supply the electron from the reduced from of AQ to the 4-arylhydroxyl amine species. This phenomenon is not observed, however, if the redox-active species are individually modified on separate CNTs, indicating the inability of CNTs to transfer electrons between tubes. The present findings may not only broaden the chemical horizon of smart CNT utilization but also stimulate research on these unique materials in the nanotechnology field.

Letters Supporting Information Available: Additional cyclic voltammograms of the MWCNT-functionalized BPPG electrode surface. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shao, L. D.; Tobias, G.; Huh, Y.; Green, M. L. H. Carbon 2006, 44, 2855–2858. (2) Ilie, A.; Bendall, J. S.; Roy, D.; Philp, E.; Green, M. L. H. J. Phys. Chem. B 2006, 110, 13848–13857. (3) Philp, E.; Sloan, J.; Kirkland, A. I.; Meyer, R. R.; Friedrichs, S.; Hutchison, J. L.; Green, M. L. H. Nat. Mater. 2003, 2, 788–791. (4) Iijima, S. Nature 1991, 354, 56–58. (5) Bozovic, D.; Bockrath, M.; Hafner, J. H.; Lieber, C. M.; Park, H.; Tinkham, M. Phys. ReV. B 2003, 67, 033407/1-033407/4. (6) Yu, M. F.; Lourie, O.; Dyer, M. J.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Science 2000, 287, 637–640. (7) Krishnan, A.; Dujardin, E.; Ebbesen, T. W.; Yianilos, P. N.; Treacy, M. M. J. Phys. ReV. B 1998, 58, 14013–14019. (8) Gooding, J. J.; Wibowo, R.; Liu, J. Q.; Yang, W. R.; Losic, D.; Orbons, S.; Mearns, F. J.; Shapter, J. G.; Hibbert, D. B. J. Am. Chem. Soc. 2003, 125, 9006–9007. (9) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113–2117. (10) Pandurangappa, M.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Analyst 2003, 128, 473–479. (11) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H. W.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519–1522. (12) Dyke, C. A.; Tour, J. M. J. Am. Chem. Soc. 2003, 125, 1156– 1157. (13) Pandurangappa, M.; Lawrence, N. S.; Compton, R. G. Analyst 2002, 127, 1568–1571. (14) Heald, C. G. R.; Wildgoose, G. G.; Jiang, L.; Jones, T. G. J.; Compton, R. G. ChemPhysChem 2004, 5, 1794–1799. (15) Darchen, A.; Moinet, C. J. Electroanal. Chem. 1975, 61, 373–375. (16) Laviron, E.; Meunierprest, R.; Lacasse, R. J. Electroanal. Chem. 1994, 375, 263–274. (17) Rubinstein, I. J. Electroanal. Chem. 1985, 183, 379–386.

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