In situ Raman spectroelectrochemical study of potential-induced molecular encapsulation of β-carotene inside single-walled carbon nanotubes

The effect of the electrochemical potential of single-walled carbon nanotubes (SWCNTs) on the encapsulation behavior of β-carotene inside the SWCNT cavities was investigated by in situ Raman spectroelectrochemistry. Prior to Raman measurement, the electrochemical behavior of β-carotene at SWCNT electrodes was investigated. Weak adsorption of β-carotene was observed, whereas at glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) electrodes, β-carotene showed diffusion-controlled reactions and was not adsorbed. Furthermore, a negative potential shift of the oxidation reaction of β-carotene was observed at the SWCNT surface compared with that at other carbon materials such as GC and HOPG. To remove the ends of SWCNTs to allow encapsulation of β-carotene and minimize damage of the side wall of SWCNTs, we used finely controlled potential oxidation method. Results of the in situ Raman measurements indicated that encapsulation of β-carotene into the SWCNT cavity occurred most smoothly when a potential of 0.3 V (vs. Ag/Ag+) was applied to the SWCNTs. When the applied potential was > 0.4 V, the encapsulated β-carotene could be oxidized and decomposed through formation of cation radical and dication forms of β-carotene, even though the β-carotene was encapsulated in the SWCNTs. Applied potentials of - 0.2 ~ 0.2 V did not result in smooth encapsulation compared with that at 0.3 V. Overall, we found that the encapsulation behavior of β-carotene in SWCNTs depended on the potential of the SWCNTs.

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