ナノマテリアルエレクトロニクス領域

Noncovalent functionalization of single-walled carbon nanotubes (SWNTs) with metal complexes is an effective strategy to enhance the performance and selectivity of SWNT-based chemiresistive gas sensors toward specific analytes. In this approach, systematic and efficient screening of optimal metal species for a given target gas is crucial for practical sensor design. To this end, we took “post-metal-coordination strategy”. That is, SWNTs functionalized with phenanthroline-based nanobrackets (PNBs) were first deposited onto a substrate and subsequently coordinated various metal ions (M = Co(II), Ni(II), Cu(I), Cu(II), Zn(II), Ru(III), and Pd(II)) with the PNBs to fabricate M-PNB–SWNT sensors. The NH3 sensing performances of the sensors with and without metal ions were then evaluated under almost the same conditions. Among the tested metal ions, Cu(II) provided the largest enhancement in response, followed by Cu(I) and Pd(II). The Cu(II)-PNB–SWNT sensor exhibited the highest response (2.50%/ppm), low noise (2.63 × 10–2%), and the lowest limit of detection (31.5 ppb). In contrast, the Cu(I)-PNB–SWNT sensor, while less sensitive, exhibited a faster response and recovery. The Cu(II)-PNB–SWNT sensor also showed a non-negligible response to ethanol vapor. Density functional theory calculation revealed that the degree of the charge transfer from ethanol to Cu(II)-PNB was approximately half of that from NH3. This result supports the experimentally observed dual responsivity to NH3 and ethanol. Overall, this “post-metal-coordination strategy” offers an efficient platform for screening metal species to identify optimal metal candidates for various gaseous analytes and enables the fabrication of sensors with diverse response patterns by varying the metal species, facilitating their integration into future electronic nose (e-nose) systems.

Although carbon nanotubes (CNTs) are promising candidates for gas-sensing materials owing to their high gas sensitivity and room-temperature operation, they suffer from poor selectivity to specific gas species. Noncovalent functionalization of CNTs with metal complex molecules is an effective and facile way to overcome this problem to enhance their sensitivity. This article reports on sensitive and selective gas sensors to ammonia (NH3) based on single-walled carbon nanotubes (SWNTs) interlocked with metal-tethered tetragonal nanobrackets (M-NBs, M = Cu(II) and Co(II)). The SWNT sensors with Cu- and Co-NBs showed 2.31 ± 0.11 and 2.24 ± 0.04%/ppm in sensitivities to NH3, which are more than 10 times higher than those of the pristine SWNT sensors prepared by removing M-NBs from the interlocked SWNTs. In addition, the detection limits of the SWNT sensors with Cu- and Co-NBs were estimated to be 16 ± 7 and 44 ± 5 ppb, respectively. Such high sensitivities are observed only for NH3, indicating that these sensors are highly selective for NH3 relative to other analyte gases/vapors (H2, CH4, ethanol, and acetone). The transfer characteristics of the field-effect transistor-type sensors and the binding energy to NH3 by density functional theory revealed that the metal ions of the M-NBs provided adsorption sites, resulting in greater charge transfer from NH3. This study suggests that both Cu- and Co-NBs stably and rigidly immobilized on the SWNT surface effectively enhance the sensitivity and selectivity of the gas sensors to NH3.