Fabrication and characterization of VOC sensor array based on SnO2 and ZnO nanoparticles functionalized by metalloporphyrins
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A volatile organic compound (VOC) sensor array based on metal oxide nanoparticles (MOX NPs) functionalized by metalloporphyrins (MPPs) was demonstrated. The VOC sensor array was composed of four single sensors based on SnO2 NPs/cobalt-porphyrin, SnO2 NPs/zinc-porphyrin, SnO2 NPs/nickel-porphyrin and ZnO NPs/cobalt-porphyrin. The MOX NP/MPP-based sensors were fabricated by drop-casting the MOX NPs dispersion and MPPs solution onto a MEMS platform. The fabricated sensor successfully detected toluene at a concentration as low as 20 ppb, which is below the limit detection concentration of previously reported porphyrin-based VOC sensor arrays. We also confirmed the selectivity between benzene, toluene, ethylbenzene, and xylene (BTEX) by using principal component analysis in contrast to previous studies on MOX/MPP-based sensor. BTEX was classified from 1 to 9 ppm at a resolution of 2 ppm, and the sensor array showed stable performance even after considerable impact.
KeywordsVolatile organic compound Metal oxide Porphyrin Gas sensor array Principle component analysis
Volatile organic compounds (VOCs), such as benzene, toluene, ethylbenzene and xylene (BTEX) are frequently used indoors, e.g., in adhesives or paints. VOCs are harmful when they are absorbed into the human body as they cause skin and respiratory diseases [1, 2, 3, 4]. To prevent health risk caused by VOCs, it is necessary to measure the concentration of VOCs in the atmosphere.
Metal oxides (MOXs) change their resistance when a VOC is adsorbed, and thus have attracted significant attention as a VOC-sensing material . A MOX-based VOC sensor has the advantages of easy processing and low cost, but it suffers from low selectivity and high operating temperature . Recently, several studies were conducted to improve the sensitivity and selectivity to VOC through functionalization by porphyrin. Porphyrins are well-known as functionalizing substances that enhance the sensitivity of VOC-sensing materials owing to the various adsorption sites that can bind VOCs . Belkova et al. improved the sensitivity by functionalizing zinc oxide (ZnO) and tin oxide (SnO2) thin films with porphyrin . Nardis et al. detected methanol at a low temperature by functionalizing a SnO2 thin film prepared via the sol–gel method with cobalt porphyrin . However, very few studies of MOX/porphyrin-based sensors have been performed to confirm the selectivity between various types of VOCs. VOCs must be selectively detected because the severity and nature of the hazards vary from one species to another . Principal component analysis (PCA) uses orthogonal transformations to convert a set of correlated variables into a set of linearly uncorrelated variables, allowing the data to be mathematically or geometrically separated [10, 11]. The PCA method using a sensor array has been studied for the selective detection of VOC type. Shirsat et al. developed a sensor array consisting of several carbon nanotube/porphyrin-based VOC sensors with various metalloporphyrins (MPPs) and confirmed selectivity for acetone, ethanol, methanol, and methyl ethyl ketone . Chen et al. used a sensor array consisting of various MOX nanomaterials and carbon nanotubes to obtain the selectivity between ethanol and other noxious gases . However, the previous sensor arrays have the disadvantages of high detection limit and low concentration resolution.
In this study, a sensor array composed of four sensors was fabricated by functionalizing MOX nanoparticles (NPs) with various kinds of MPPs. The fabricated device exhibited a low detection limit of 20 ppb. Major VOCs such as BTEX were detected at a resolution of 2 ppm from 1 to 9 ppm, and selectivity was confirmed using PCA. Owing to the MPP functionalization, the sensors could react with a low concentration of VOCs, and the sensor response changed significantly even at small concentration changes. In addition, impact test confirmed that the sensor platform, MOX NPs, and MPPs were well bonded.
Design and materials
We used commercially available ZnO NPs and SnO2 NPs (Sigma Aldrich) as VOC-sensing materials. The functional materials were 5,10,15,20-Tetraphenyl-21H, 23H-porphine zinc (ZnPP), 5,10,15,20-Traphenyl-21H, 23H-porphine cobalt (CoPP), and 5,10,15,20-Tetraphenyl-21H,23H-porphine nickel(II) (NiPP) purchased from Sigma Aldrich. Deionized water and chloroform were used as solvents for the MOX NPs dispersion (0.1 wt%) and porphyrin solution (0.07 wt%), respectively.
The VOC sensing tests were performed by measuring the changes in the electrical resistance of the sensors as the sensors were exposed to air-diluted VOC and dry air alternately at atmospheric pressure and room temperature. While the sensor was operating, the microheater was driven at a fixed bias voltage of 3.5 V. The power consumption of the heater was 28 mW, and the corresponding temperature measured by a resistance temperature detector was approximately 353 °C. The concentration of VOC was adjusted by changing the mixing rate using a mass flow controller, and the total flow rate of VOCs diluted in air was maintained as 500 sccm. The response is defined as Ra/Rg, where Ra and Rg are the resistances of the sensors before and after the exposure to VOCs, respectively. For measuring Ra/Rg, a current was monitored using a sourcemeter (KEITHLEY 2400) under a fixed bias voltage of 1 V.
Results and discussions
A VOC sensor array composed of SnO2 NPs/CoPP, SnO2 NPs/ZnPP, SnO2 NPs/NiPP and ZnO NPs/CoPP was developed and its sensing characteristics were evaluated. A single sensor in the sensor array was fabricated by coating MOX NP solutions and solvent-dispersed MPP onto a platform fabricated through a MEMS process. The fabricated device successfully detected toluene at a concentration as low as 20 ppb. We also confirmed the selectivity between BTEX using the arrays via the three-dimensional PCA. BTEX of 1–9 ppm was classified at a resolution of 2 ppm, and the fabricated device showed stable performance even after considerable impact. The fabricated VOC sensor array can be used in indoor environments, such as houses or hospitals, which require low concentration detection and need to distinguish VOCs.
BC and KL performed the experiments, analyzed the data, and wrote the manuscript. SP supported the data analysis and reviewed the manuscript. JK supervised the research and reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIT) (No. NRF-2018R1A2A1A05023070). This material is based upon work supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program. No. 10054548, ‘Development of Suspended Heterogeneous Nanostructure-based Hazardous Gas Microsensor System’.
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