Selective Coating of Single Wall Carbon Nanotubes with Thin SiO

Single walled carbon nanotubes (SWNTs) have been shown to be highly sensitive gas sensors. However, attaching functional groups with selective sensing functions on nanotubes without destroying the intrinsic electronic property of the nanotubes is still challenging. Here, we report a new method of coating SWNTs with a thin layer of SiO2 using 3-aminopropyltriethoxyysilane as coupling layers. The thickness of the SiO2 could be controlled at about 1 nm. The coating of SiO2 on SWNTs was confirmed by burning the SWNTs in air. The effect of 3-aminopropyltriethoxyysilane was also discussed. Carbon nanotubes have been the most actively studied materials in the past few years due to their unique electronic and mechanical properties.1,2 Carbon nanotubes can be either metallic or semiconductive, depending on their helicity and diameter. More importantly, it was observed that the electronic properties of nanotubes, especially semiconducting carbon nanotubes, are very sensitive to the surrounding environments. The presence of oxygen, NH3 and many other molecules that can either donate or remove electrons from the nanotubes can significantly alter the overall conductivity of the nanotubes.3-5 Such properties make carbon nanotubes ideal candidates for nanoscale sensing materials. Actually, it was reported that nanotubes FETs can function as gas sensors.3,4 However, to introduce selectivity to nanotubebased sensors, certain functional groups that can selectively bind to specific target molecules needs to be anchored on the nanotube surface. Unfortunately, the sidewall of carbon nanotubes is chemically stable, making the functionalization of the nanotubes with sensing molecules a challenge. Moreover, covalent modification of the nanotube sidewalls could totally change the electronic properties of the nanotubes, making them insulators rather than semiconductors. So, a reliable, noncovalent modification of the sidewall of carbon nanotubes is required for the development of carbon nanotube based nanosensors. Functionalization of carbon nanotubes with different functional groups have been reported by several groups in the past: 1-pyrenebutanoic acid, succinimidyl ester has been used as anchored molecular for immobilizations of bimolecular;5,6 and anti-Fullerence IgG monoclonal antibody has been bound to SWNTs.7 Certain proteins and DNA have also been demonstrated to modify mutiwall carbon nanotubes (MWNTs).8-11 In addition, wrapping and coating by polymers and surfactants were reported.13-15 More recently, coating with thick layer of SiO2 on MWNTs for composite applications have been reported.12 However, so far, there is no flexible method that could functionalize SWNTs with a broad variety of functionalities. Here, we report a new method to selectively functionalize SWNTs sidewalls with a thin layer of SiO2, which could offer flexible ways for further modification by utilizing the rich chemistry available for silica surface. Our approach involves the use of a promoter, 3-aminopropyltriethanoxysilane (APTES), which could be absorbed on the sidewalls of SWNT by the interaction between amine groups and SWNTs sidewalls, and could be polymerized by simply heating to make the coating irreversible. Then a thin SiO2 layer is subsequently grown on the polymerized APTES layer through a modified Stöber method.16 The thickness of the layer can be controlled by the adjusting the growth conditions of SiO2. Using this process, SWNTs sidewall could be converted into a silica surface, which could be further functionalized with a large variety of functional groups by utilizing the chemistry of SiO2 surface. Because the coating of SiO2 is not covalent, the electronic structure of SWNTs will not be altered significantly (Scheme 1). In addition, because that SiO2 layer is very thin, it is reasonable to assume that the chemical process caused by the binding of target molecules will still have a strong effect on the nanotubes