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Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07 Page 21 of 29

Figure 12. Properties of CNTs-filled in a thermoelectric generation (Reproduced with permission [149] . Copyright 2017, Springer Nature):
(A) Schematic diagram of measuring equipment. (B) TEM photos of measuring device details. (C and D) The relationship between κ
and S of CNTs and temperature after filling C , Gd@C , and Er @C .
60 82 2 82
Sensor
The 1D electronic properties and large adsorption area of SWCNTs can also be applied to gas sensors .
[154]
The ideal sensor should be highly selective , highly sensitive , completely recyclable , durable , and
[156]
[155]
[158]
[157]
[159]
low in cost . Although pure SWCNTs show neither specificity for gases nor are they gas-sensitive
materials , the selection of suitable materials for filling can confer selectivity and sensitivity to
[160]
[24]
X@SWCNTs heterostructures. Chimowa et al. used DWCNTs filled with ZnI as a formaldehyde sensor .
2
The study demonstrates that it is possible to enhance the sensitivity and selectivity of the sensor by filling
DWCNT. The performance of this filling is better than the surface function of nanotubes. Quang et al.
reported the effects of ammonia adsorption at various temperatures on the electrical performance of
SWCNT . When the concentration of NH was as low as 5 ppm, they were still able to detect the sensor’s
[161]
3
reaction to NH and showed a good linear relationship. There is no obvious sign of saturation in the linear
3
response area of the ~40 ppm concentration. Under a higher concentration, the reaction is sub-linear, but
the reaction continues to increase as the concentration level increases. Nguyen et al. tested NH sensors
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based on SWCNT devices under constant current conditions. The sensor’s recovery and exposure time were
[162]
determined by controlling the NH concentration at 5 ppm . After 10 min of exposure to NH , the
3
3
electrical resistance increased by 8%. Qi et al. prepare a large-scale array of low-noise electronic CNT
sensors for detecting gas molecules . The functionalization of polymers makes SWCNTs resistant with
[163]
high sensitivity and selectivity. Polyethylene amine coatings enable nanotube devices to detect nitric oxide
at levels as low as 1 ppb (one billionth). Ramachandran et al. filled multi-wall carbon nanotubes with Ni-Co
[164]
alloy nanowires for non-enzyme electrochemical sensor probes that reliably detect glucose . The