5 ILs-BASED MULTIFUNCTIONAL COMPOUNDS FOR ELECTROCATALYSIS AND BIOSENSORS
Figure 4.14 (a) Examples of cations and anions commonly used for the formation of ionic liquids and SWNT-IL-X. (b) Cyclic voltammograms (CVs) of SWNT-IL-POM modified GC electrode (d = 3 mm) at different scan rate in 0.5 M H2SO4. Scan rate: 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 V/s from inner to outer. The inset shows the peak current (square) and peak potential (triangle) of the third reduction wave as a function of scan rate. From Zhang, Y. J., Shen, Y. F., Yuan, J. H., Han, D. X., Wang, Z. J., Zhang, Q. X., and Niu, L. (2006). Design and synthesis of multifunctional materials based on an ionic-liquid backbone. Angew. Chem. Int. Edit., 45, pp. 5867-5870. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. One of the most fantastic features of ILs is that their properties can be easily and well tuned by rationally selecting proper combination of organic cations and anions. That also means that we can delicately utilize one ionic component to deliver a unique function and the other ionic component to deliver a different, completely independent function. Moreover, the components and their combination of anions and cations are various, for example, the cation of ILs can have several substituting groups (R1, R2, R3, etc., as shown in Fig. 4.14a) and these substituting groups are tunable. Therefore, it offers us a promising and facile way to combine individual functions into a target compound. In contrast, for a commonly seen compound, to achieve this multifunctional combination would encounter all
sorts of synthetic challenges. We have illustrated this flexibility of ILs by selecting SWNT (R1) as a model substituting group of the imidazolium cation, and Br-, PF6-, BF4-, polyoxometalates (POMs), and even glucose oxidase (GOD) as the counter-anions (Fig. 4.14a, SWNT-IL-X) . The properties of SWNT and the various anions were facilely and successfully delivered into the resulting compounds. For example, the rich redox activity was also successfully transferred into SWNT-IL-POM merely by a simple and facile anions exchange. The surface-confined SWNT-IL-POM shows three couples of welldefined redox waves at scan rates up to 2 V/s (Fig. 4.14b), which was presumably attributed from electron conduction of SWNT, ionic conduction of IL, and redox conduction of POM. It was unusual for an often-seen compound in electrochemical systems. Figure 4.15 (a) SECM arrangement (not to scale) for feedback measurement at pure (left) and SWNTs-Im-sandwiched (right) water/chloroform interface; (b) Experimental approach curves (CHI 900) for a tip in aqueous solution approaching SWNTs-Im-sandwiched (I) and pure (II) water/chloroform interface. Currents are normalized to the steady-state diffusion limiting current, i
T,∞ and distance to tip radius. The aqueous solution contained 0.5 mM Ru(NH3)63+ and 100 mM KCl. The tip (Pt, 10 mm radius, RG = 10) was held at -0.35 V vs. Ag|AgCl (saturated KCl) for Ru(NH3)63+ reduction and approached at 1 mm/s. The counter electrode was Pt wire . Reproduced by permission of The Royal Society of Chemistry. Moreover, due to the controllable wettability by counter-anion exchange [56, 57], SWNT-IL-Br, which was both hydrophobic
and hydrophilic, could assemble at the water/oil interface in a controllable manner, i.e., from monolayer to multilayers . Thus, it enabled a novel SWNT-sandwiched water/oil interface for the
electron transfer, one of the most fundamental chemical processes across the interfaces. It will promote both fundamental electron transfer research such as in biological and artificial membrane and homo/heterogeneous catalysis. For example, it indicated SWNTs accelerated the electron transfer at water/oil interface by using scanning electrochemical microscopy (Fig. 4.15).