Asthma is a chronic disease characterized by recurrent
attacks
of breathlessness and wheezing, which vary in severity and frequency
from person to person. H2S is considered as the biomarker
of asthma. Here, an ultrasensitive chemiresistive H2S gas
sensor based on a γ-Bi2MoO6–CuO
heterostructure with a detection limit of 5 ppb has been fabricated.
It can distinguish asthmatic patients from healthy people roughly
by analyzing the exhaled breaths of 28 asthmatic patients and 28 healthy
people, suggesting that the sensor can be used to assist physicians
in the diagnosis of asthma. Pathologically, it is discovered by this
sensor that with the relief of asthma, the concentration of H2S in one’s exhaled breath gradually increases. This
subtle concentration variation of H2S can be accurately
detected, indicating that this sensor can be used in the asthma severity
monitoring too. Physical models have been built by first-principles
calculation to reveal the causes of the sensor’s ultrasensitivity.
The stable adsorption of H2S on the surface of CuO results
in massive charge transferring and the appearance of the defect states,
which play the major role in the ultrasensitivity of the sensor. Upon
integrating this sensor with circuits, the cheap, smart, and portable
H2S sensing device can be obtained, which can make asthmatic
patients’ access to this device easy and make the severity
monitoring of asthma convenient, especially for children and the aged.
For
a long time, chemiresistive gas sensors based on metal oxide
semiconductors (MOSs) suffer from higher operating temperatures, resulting
in higher energy consumption and instability of the sensors. Generally,
a MOS-based chemiresistive gas sensor being able to work at room temperature
is considered to be outstanding already. Here, a highly sensitive
NO2 gas sensor based on the carbon dots-WO3 heterostructure,
which can work below room temperature at –6 °C, is fabricated. At 18, –1, and –6 °C, its detection limits are 200 ppb, 5 ppm,
and 20 ppm, respectively, and the corresponding response values (R
a/R
g) are 1.11,
1.04, and 1.13, respectively. The sensor exhibits good selectivity,
stability, and linearity between relative humidity and response values
too. A peculiar response behavior was observed. Toward oxidizing gas
NO2, the resistance of the sensor based mainly on n-type
WO3 shows decrease behavior. Its peculiar response behavior
and strong gas sensing ability at lower temperatures were elucidated
theoretically using the results of first-principles calculations.
The reduction of NO2 into NO by surface oxygen vacancies
of WO3 and the following adsorption of NO on the surface
of WO3 lead to electron transfer from NO to WO3, and the Fermi level shifts toward the conduction band, making the
sensor exhibit the peculiar response behavior. The stronger adsorption
capability of carbon dots toward NO2 and a synergistic
effect of carbon dots and WO3 together make the sensor
capable of working at lower temperatures and own higher sensitivity.
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