(756e) Enhanced Electrochemical Detection Performance of Multiwall Carbon Nanotubes Functionalized by Aspartame

Carbon nanotubes (CNTs) are outstanding materials due to their exceptional properties including excellent mechanical strength, high electrical and thermal conductivities, high surface area, and high aspect ratios [Adsul et al., 2011]. However, their water insolubility is a major bottleneck in the exploitation of these excellent properties. Generally, noncovalent functionalizations of the CNTs sidewalls can preserve their desired properties, while improving their solubilities quite remarkably [Zhao and Stoddart, 2009]. Therefore, inexpensive, nontoxic, and biocompatible materials that can disperse CNTs in aqueous solutions through a noncovalent approach are highly desirable.

In this paper, we report the use of an amphiphilic dipeptide derivative, aspartame, as an effective dispersing agent in preparing highly stable multiwall carbon nanotubes (MWCNTs) suspensions under ultrasonication. In addition, we investigated the stability of the resulting MWCNT/aspartame composites at a wide range of pH values by zeta potential measurement. Furthermore, an investigation into whether the MWCNT/aspartame composites could improve the electrochemical performance of glassy carbon electrode (GCE) was conducted. The main ideas and conclusions are summarized as follows.

(1) The MWCNTs can be well dispersed in water in the presence of aspartame via brief ultrasonication. The resulting MWCNT/aspartame composites demonstrated a high degree of dispersion, which derived from the amphiphilic properties of aspartame. Specifically, L-phenylalanine of aspartame molecule gives aspartame the strong affinity property [Piao et al., 2008], which allows it to be adsorbed onto the MWCNT surface through the p-p stacking of aromatic groups. In addition, The MWCNT/aspartame composites exhibited zeta potentials as low as -36.5 mV at pH values between 6 and 12. The displayed negative zeta potential indicates that the MWCNT surface was negatively charged over a broad range of pH values, which gives the composites great promise in biological applications.

(2) The dispersion qualities and structural details of the MWCNT/aspartame composites was characterized by SEM, HRTEM and FTIR spectroscopy. SEM images showed the majority of the nanotubes exist individually, with some small aggregates of two or three tubes. HRTEM images also demonstrated smooth sidewalls was “polluted” by extra particles of a different species, which are believed to be aspartame debris, after irradiations. Therefore, the morphological observations further suggest that the amphiphilic aspartame was indeed adsorbed onto the nanotube sidewalls. Moreover, FTIR spectra also confirm that aspartame was successfully adsorbed onto the surface of the nanotubes through the p-p stacking interaction between the MWCNTs and the aromatic ring of aspartame.

(3) The MWCNT/aspartame composites could improve the electrochemical performance of GCEs. The enhanced electrochemical reversibility of the MWCNT/aspartame modified electrodes with the strongest peak currents was observed compared with that of the bare electrode. The calculated effective surface area of the MWCNT/aspartame  -coated GCE was 11.68 mm², which is larger compared with 9.14 mm² for the MWCNT-coated GCE and 7.21 mm² for the bare GCE. Furthermore, the MWCNT/aspartame-coated electrode possesses better electrochemical detection performance for hydrogen peroxide, with its greatly enhanced electrocatalytic response and reduced potential. These findings indicate that the utilization of aspartame as a dispersing agent for CNTs provides a favorable avenue for the fabrication of CNT-modified electrodes and offers great promise for amperometric biosensors.

This work was supported by the Natural Science Foundation of China (No. 20806057 and 31071509), the Ministry of Science and Technology of China (Nos. 2012BAD29B05 and 2012AA06A303), and the Ministry of Education (No. NCET-11-0372).

References

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2. Zhao Y. L. and Stoddart J. F. Noncovalent Functionalization of Single-Walled Carbon Nanotubes, Accounts of Chemical Research, 42, 1161-1171, 2009
3. Piao L. Y., Liu Q. R., Li Y.D. and Wang C. Adsorption of L-Phenylalanine on Single-Walled Carbon Nanotubes, The Journal of Physical Chemistry C,112, 2857-2863, 2008