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What are Quantum Dots?
Quantum dots (QDs) are semiconductor nanoparticles that offer a wide range of optical and electronic properties (optoelectronic). The optoelectronic properties of QDs are often determined by the size of these nanoparticles, which is typically within the range of 1.5 and 10 nanometers (nm), as well as their overall composition.
QDs have already been successfully incorporated into several different electronic and biomedical devices as a result of their unique properties, including, but not limited to, photostability, high extinction coefficient, and brightness, as well as their large Stokes shift.
For sensing devices, some of the most common applications of QDs include those based on carbon and graphene (GQDs) because of their unique luminescent and electronic properties.
Current Gas Sensors
Since the discovery of gas sensors occurred during the mid-1920s, significant advancements have been made in developing innovative sensors for gas detection.
Some of the more notable types of gas sensors include electrochemical, optical, and chemoresistive sensors.
Due to their short lifespan, electrochemical sensor applications are limited, reducing their general popularity, particularly for gas sensing purposes. Comparatively, optical sensors, which are associated with high sensitivity and selectivity capabilities, along with a rapid response time and adequate lifetime, are also associated with limitations because of their large size and high cost.
While chemoresistive sensors are not as selective as optical sensors, they are much less expensive to produce and can be manufactured through simple methods.
Advancing QD Chemical Gas Sensors
To overcome some of the limitations associated with conventional chemical gas sensors, several different attempts have been made to investigate how certain QDs might create more sensitive and stable chemical gas sensors.
Like traditional QDs, colloidal QDs (CQDs) are also semiconductor nanoparticles. However, CQDs are unique due to their suspension in the solution phase. This suspension of CQDs provides strong quantum confinement effects that subsequently enhance the electronic and optical properties of these QDs, as well as increase their absorption, emission, and quantum-size tunability.
To date, CQDs have been incorporated into photodetectors, environmental sensors, and various light-emitting devices, some of which include LEDs or photoluminescent elements.
As a result of the advantageous properties associated with CQDs as a sensing material, one recent study investigated the use of these QDs for the detection of ambient nitrogen oxide (NO2) levels.
In their work, lead sulfide (PbS) QDs were used because of their previous use in the detection of hydrogen sulfide (H2S), methane (CH4), and ammonia (NH3) gases. It should be noted that in addition to PBS, several other types of CQDs have been investigated for their usefulness as gas sensors. Some of these materials include zinc oxide (ZnO) QDs, for example, which have been found to be sensitive to both NO2 and H2S.
Tin (II) oxide (SnO) QD-based sensors have also been found to be sensitive to NO2 and H2S, as well as both ethanol and liquified petroleum gas (LPG).
The 2020 study on PbS QDs found that sensors developed with this material were capable of detecting NO2 levels at room temperature with a detection limit of approximately 0.15 parts per billion (ppb).
As compared to some of the more advanced NO2 sensors that are currently on the market, the researchers in this study proposed that their PbS QD-based sensor might offer better sensitivity capabilities.
Metal oxide QDs
Some of the most widely studied materials for conductometric sensors are metal oxides because of their ideal chemical stability properties for this application.
To this end, the development of novel metal oxide structures at the nanoscale, particularly metal oxide QDs, has been gaining an increasing amount of attention. Tin (IV) oxide (SnO2) QDs within the size range of 3-4 nm, for example, have been found to exhibit a sensing response that is up to three times better compared to conventional SnO2 sensors.
This enhanced sensing response is believed to be the result of both the reduced structural dimensions of SnO2 QDs, as well as a superior surface reactivity that is the result of an increase in the surface-to-bulk ratio. when applied to the detection of carbon monoxide (CO), SnO2 QDs were also found to have enhanced adsorption of this chemical as a result of their greater surface area, allowing for more surface defects and oxygen vacancies.
Metal chalcogenides QDs
Metal chalcogenides, which are metallic materials that contain one or more chalcogen elements such as sulfur (S), tellurium (Te) or selenium (Se), have often been designed as QDs for their ability to enhance the electrical and sensing properties of composite structures.
One example of a metal chalcogenide QD can be found in the recent discovery that by forming a depletion layer between p-type PbS QDs and n-type TiO2 nanotubes, a concentrate increase in active sites within the sensing structure provided better adsorption of gaseous substances on its surface. This composite material was found to selectively sense NH3 at a concentration of up to 100 parts per million (ppm) at room temperature.
The chalcogenide material, comprised of both cadmium sulfide (CdS) QDs and cobalt oxide (Co3O4) microspheres, was also found to provide an enhanced sensing response and recovery time in the detection of H2S.
CdS QDs provide unique chemical stability that has led to their incorporation into other metal oxides, including ZnO, SnO2, and indium oxide (In2O3).
When green light irradiation was applied to each of these composite structures, their sensing capabilities were found to improve significantly.
It should be noted that chalcogenides are relatively new materials in terms of their incorporation into gas sensing devices. Therefore, further studies assessing their performance under various environmental conditions should be conducted.
References and Further Reading
Maxwell, T., Campos, M. G. N., Smith, S., et al. (2020). Chapter 15 – Quantum Dots. Nanoparticles for Biomedical Applications 243-265. doi:10.1016/B978-0-12-816662-8.00015-1.
Li, M., Chen, T., Gooding, J. J., & Liu, J. (2019). Review of Carbon and Graphene Quantum Dots for Sensing. ACS Sensors 4(7); 1732-1748. doi:10.1021/acssensors.9b00514.
Mitri, F., De lacovo, A., De Luca, M., et al. (2020). Lead sulphide colloidal quantum dots for room temperature NO2 gas sensors. Nature Scientific Reports 10; 12556. doi:10.1038/s41598-020-69478-x.
Galstyan, V. (2021). “Quantum dots: Perspectives in next-generation chemical gas sensors” – A review. Analytica Chimica Act. doi:10.1016/j.aca.2020.12.067.