As a severe environmental pollutant, detection and quantitation of nitrogen dioxide (NO 2) have been studied for centuries. In this review, recent progress of NO 2 analysis in the atmosphere will be summarized. Four major types of detection technologies, including traditional chemical detection, optical detection, solid-state field effect transistor detection, and other detection technology are covered. The standard method employed by the US EPA, which is based on luminol, is the most reliable and robust method that is used for fully validated monitoring. In the past two decades, accompanying the fast development of electrical engineering and integrated circuit, micro to nanoscale gas sensors have been gaining more and more attention. Application of novel materials including nano wires and graphene also leads to a new era of research and development of sensors.
Nitrogen dioxide (NO2) is one of the major environmental pollutants, which belongs to the family of nitrogen oxides (NOx). The major source of NO2 in the air comes from combustion of fossil fuels [
Widely used NO2 analysis technologies can be divided into the following four categories based on the mechanism of detection: traditional chemical analysis, optical detection, field-effect transistor detection and other detection techniques.
Analysis of NO2 and related substances such as NH3 and other nitrogen containing compounds can be dated to more than one hundred years ago [
Luminol is achemical reagent that emits chemiluminescence in the presence of appropriate oxidizing agent and metal catalysts. NO2 is a strong oxidizer, which can readily trigger the chemiluminescence reaction of luminol. With the help of photon multiplier tube (PMT), detection of NO2 with luminal is very sensitive. Further
developments of the chemiluminescence detection include utilizing luminol in a solid substrate support to simplify the design. It has become the standard analytical technique of NO2 in the quality evaluation of air. Cavity attenuated phase shift (CAPS) spectroscopy takes advantage of the phase shift caused by light signal in the presence of NO2 in an optical resonant cavity. Most interference can be filtered out by the highly selective and specific absorption of NO2, without compromising the high sensitivity achieved by PMT.
Absorption of NO2 or other gases may lead to changes of the chemical composition of a thin film of sensitive material. Electro-chemical behaviors including impedance, conductivity and resistivity will be changed accordingly. Therefore, detection and analysis of electrical signals can be used to calculate NO2 concentrations. With the help of signal detection and amplification by the rapidly developing field-effect transistor (FET) and integrated circuit (IC), many gas sensors were created and investigated [
There are some other detection technologies for NO2 detection and quantitation. NO2 may lead to unique changes of physical or chemical properties of these materials that absorb NO2. One example is the change of frequencies of a piezoelectric crystal [
Analysis of nitrogen content in organic samples started in the late 19th century. Kjeldahl [
Andres Ferrari proposed a system for continuous digestion and analysis based on the Kjeldahl method [
This automated method has produced similar results as the classic manual method with significantly reduced workload requirement. The rate of analysis using this automated system is up to 10 samples per hour. However, the classic methods are only capable of determining nitrogen content in single digit percentage. Limit of detection of both manual and automatic titration is about 0.1% of the sample weight.
Chemiluminescence was the most prevalent and dominating method for the analysis of NO2 in the air. The United States Environmental Protection Agency (EPA) has published the official guidance of the standard chemiluminescence method for NO2 analysis [
The standard method regulated by EPA employs the creation of chemiluminescence using ozone. In this reaction, NO2 is first reduced to nitric oxide (NO). Then NO reacts with O3 to form the exited state of nitrogen dioxide,
Intensity of light is proportional to the concentration of NO2. Commercially available instruments deliver a fully validated analysis and conform to EPA specifications. The dynamic range used for daily NO2 monitoring is from 0 to 500 ppb. Traceable NO or NO2 standards are also readily available for the calibration of the instruments. Instrument using the luminol method is currently the first and only method available in pursue of the regulated environmental monitoring of NO2 in air. NO and other nitrogen oxides are common interferences for this method. In addition, free radicals in the air may also reduce the amount of exited NO2, leading to a lower result.
Wendel described a continuous NO2 monitoring system capable of detecting NO2 in the sub ppb range [
Besides chemiluminescence, Cavity Attenuated Phase Shift Spectroscopy (CAPS) developed by Aerodyne Inc. was also used for NO2 analysis (
light-absorbing species in the cavity [
The main oscillation chamber used in CAPS was constructed using two highly reflective mirrors facing each other, similar to CRDS. A beam of modulated broadband light created by LED is directed into the chamber. Presence of NO2 causes a phase shift in the signal output, which is proportional to the concentration of NO2. 430 nm was chosen as the light wavelength because it matches the maximum absorption of NO2. With the use of an interference filter, the limit of detection is 0.5 ppb. This sensor is capable of measuring 0 - 200 ppb of NO2 in the air. However, the noticeable curvature seen in the calibration towards 200 ppb is caused by loss of light during absorption, which is comparable to the phase shift effect at higher concentrations [
Though chemical and optical analysis techniques are good options for NO2 analysis, researchers are continuously seeking for new sensors that are in solid state, easy to operate, maintenance-free and small in size. Without the requirement of any chemical solutions, gas sensors have been made perfect candidates for varies of applications both in the lab and the field. Its simplicity also enables the mass production with relatively low cost for each unit. Integration with modern electrical engineering allows direct control and digital signal transmission to a data collection device. Interest for the FET gas sensors is increasing, accompanying hundreds of research articles and communication papers in this field. The study of sensor design and material has been divided into two ways. One is the optimization of the sensing performance. The other is the additional feature and utilizations on top of the good performance.
Early work on FET gas sensors can be tracked back to 1980s. In a study by Kolesar et al. [
A versatile sensor platform of NO2 in the air was developed by Oprea et al. in 2007 (
Technology | Limit of detection | Typical range | Reference/Note | |
---|---|---|---|---|
Chemiluminescence | NO2 detector based on reaction between Luminol and NO2 in air | 10 ppt | 65ppt - 18 ppb | Fast response ~1 s [ |
Poly-gel Immobilized Luminol NO2 sensor | 460 ppt | 0.5 - 20 ppb | The gel substrate needs to be replaced every day [ | |
US EPA standard NO2 test method | ~1 ppb | 1 - 500 ppb | LOD and range will vary slightly on different conditions [ | |
Non- Chemiluminescence | Cavity Attenuated Phase Shift (CAPS) NO2 detector | 0.3 ppb | 0.5 - 200 ppb | Very fast response, suitable for online monitoring [ |
CAPS NO2 detector, second generation | 0.06 ppb | 0 - 320 ppb | Improved accuracy and sensitivity [ | |
Miniature fiber-optic spectroscopy | ~10 ppb | 15 - 50 ppb | Semi quantitative with a relatively large error (±20%) [ | |
DPPD-polymer NO2 detector | 0.12 ppm | 0.1 - 25 ppm | Small scale integrated sensor [ |
platform was based on capacitive coupled field effect transistors (CC-FET). Unlike other common gas sensors, the sensing material in this study was suspended on top of the substrate, which is called suspended gates (SG) [
In the past two decades, a great deal of research work has been focused on resistance and conductivity gas sensors [
NO2 is an oxidizing gas so it induces an increase of resistance and impedance of n-type semiconductors. Sensors based on semiconducting metal oxides have been developed and reviewed by a number of researchers [
ZnO has been proved to be a stable, low cost and non-toxic sensing material. However, the sheet resistance of pure ZnO thin film is substantially high. If impedance of the sensor becomes higher, the resulting weak signal of current change will considerably increase the complexity of the associated controlling and measuring circuitry of instruments, though sensing of NO2 is based on the increase of resistance. Data may also become distorted at such week signal levels. Ferro et al. [
In addition, sensors of oxidizing gases usually utilize high temperature to enhance the desorption kinetics. In studies of gas sensors, response time (T90) is defined as the time required for the signal to reach 90% of the final value upon exposure of the target gas, while the recovery time (T10) is the time for the signal to reduce to 10% of the highest level when the target gas is removed. Slow response and recovery within 10 - 60 min is commonly observed across this type of sensors (
Graphene has recently been reported as a potent material for construction of sensors, small-scale electronics and solar cells. The unique wide-spread 2D structure of graphene reassembles a honeycomb lattice, which forms π bonds across the hexagonal skeleton. π bondis half filled and allows free-flowing electrons, which contributes to the most notable electronic properties of graphene, the quantum Hall effect [
When reduced graphene oxide is sprayed withasoluble semiconducting polymer, a double-layered sensing film is formed. Xie et al. [
Type | Pros | Cons |
---|---|---|
Chemical | Simple processing without the need of expensive equipment/instrument | Poor sensitivity; Requires the use of chemicals; Low throughput and high labor demand |
Optical | Highly reliable and fully validated detection with the use of luminal; Very high sensitivity with CAPS detector | High cost of instrument purchase and maintenance; Unable to use in small scale sensors |
FET | Solid state sensors without chemicals; Continuously monitoring (on-line analysis) | The accuracy and reliability can still be improved by developing new sensing materials |
The response to exposure of NO2 showed a very interesting three-stage curve, the rapid response, the slow response and the recovery, especially at higher concentrations of NO2. The mechanism that caused the 3 stages can be explained that at higher concentrations (over 60 ppm), the rapid rise of signal (stage 1) is due to the adsorption of gas molecules onto low energy bonding sites such as sp2-bonded carbon. With gradual saturation of these sites, the gas molecules will interact with higher energy sites such as vacancies and structural defects, which is much slower (stage 2). Stage 3 showed an exponential decay of the signal when the analyte (NO2) was purged out with air. However, at lower NO2 concentrations, only very few NO2 molecules can reach to the surface of graphene through the P3HT top layer. The fast responding low energy sites will not be saturated. Thus all the NO2 molecules interact with the low energy sites, which lead to a relatively fast over-all response.
One of the major drawbacks of graphene based sensing material is the baseline shift. As showed also in the response curve over time, the recovery of the signal did not go back to the original level, especially after exposure to high levels of NO2. Within 90 min, the signal will return to baseline close to the original value, but long recovery is not desirable in practice and needs further investigation for improvement.
Besides these dominating detection techniques mentioned above, there are other methods to analyze NO2, such as Gas Chromatography/Mass Spectrometry (GC/MS), Residual Gas Analyzer (RGA) and immunochemical methods [
A brief review of NO2 analysis technologies has been presented in this paper. The advantages and disadvantages of the categories are compared in
The research of NO2 detection has gone a long way from the first discovery of chemiluminescence with Luminol. The reviewed technologies were classified into categories by the sensing properties. Selected important studies that represent the key characteristics are included. Factors that affect the sensing characters are analyzed. Trends of improvements are also discussed. The current active studies of NO2 analysis focus on two major paths. One is the environmental analysis regulated by the EPA or other governments, which is also known as the regulatory measurements. Another more cutting edge area of study is the development of gas sensing material, especially semi-conductive thin films. These materials have great potential of applications on versatile, solid state, small scale and maintenance-free devices.
Jun Wang,Wei Zhang,Rui Cao,Xiangyu You,Hong Lai, (2016) Analysis of Nitrogen Dioxide in Environment. Advances in Bioscience and Biotechnology,07,278-288. doi: 10.4236/abb.2016.76026
NO2: nitrogen dioxide
US: United States of America
EPA: Environmental Protection Agency
NH3: ammonia
PMT: photon multiplier tube
CAPS: cavity attenuated phase shift
FET: field-effect transistor
IC: integrated circuit
NO: nitric oxide
O3: ozone
NOx: nitrogen oxides
CRDS: cavity ring-down spectroscopy
IGE-FET: interdigitated gate electrode field-effect transistor
CuPc: copper phthalocyanine
CC-FET: capacitive coupled field effect transistors
SG: suspended gates
SnO2: tin oxide
WO3: tungsten oxide
In2O3: indium oxide
TiO2: titanium oxide
ZnO: zinc oxide
CVD: chemical vapor deposition
XRD: X-ray powder diffraction
SEM: scanning electron microscope
TEM: Transmission electron microscopy
AFM: atomic force microscope
XPS: X-ray photoelectron spectroscopy
P3HT: poly-(3-hexylthiophene
OTFT: organic thin film transistor
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