Grantee Research Project Results
Final Report: A Low-Cost Handheld Sulfur Dioxide Tester with a Hybrid Nanomaterials-Based Sensor Chip
EPA Contract Number: 68HERC20C0031Title: A Low-Cost Handheld Sulfur Dioxide Tester with a Hybrid Nanomaterials-Based Sensor Chip
Investigators: Lu, Ganhua
Small Business: NanoAffix Science LLC
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2020 through August 31, 2020
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2020) RFA Text | Recipients Lists
Research Category: SBIR - Air and Climate
Description:
This project focused on combining the excellent electronic sensitivity of graphene with sensing materials, such as tin oxide, to develop sensitive and selective sensors toward detecting sulfur oxide. Novel metal oxide hybrid nanoparticles were synthesized, deposited on graphene sheets, and the sensors were evaluated for their ability to detect sulfur dioxide. The resulting sensors are capable of selectively detecting sulfur dioxide at the parts per billion (ppb) concentration level in real time.
Summary/Accomplishments (Outputs/Outcomes):
In this project, sulfur dioxide was successfully detected at ppm levels using our custom testing system and pure reduced graphene oxide (rGO) on gold indigitated electrodes (IDEs) under ambient conditions. The rGO sheets bridge the gaps in the gold IDEs and the device functions as a chemiresistor. The testing system includes a data acquisition program developed in-house to control a Keithley source meter and mass flow controllers to enable exposing the sensors to gases with precisely controlled compositions. The sensors were optimized through exposure to known concentrations of sulfur dioxide, and will be used to measure unknown concentrations of sulfur dioxide using the portable handheld tester and calibration algorithm that is being developed. Sensitivity was improved fourfold by increasing the exposed surface area of the rGO through the controlled corrosion of the graphene via exposure to hydrogen peroxide.
Subsequently, the bridging graphene sheets were coated with various metal oxide nanoparticles. Tin oxide (SnO2) and nickel oxide (NiO) were synthesized via hydrothermal and wet chemical methods to identify the optimum synthesis method for fabricating the sensors to detect low levels of sulfur dioxide (SO2). The deposition of SnO2 nanoparticles synthesized via a facile wet chemical method onto the surface of graphene oxide performed better than other tested alternatives and successfully improved sensitivity over an order of magnitude. The SnO2-rGO sensors successfully lowered the SO2 limit of detection from the ppm range down to the ppb range and improved selectivity of the sensors toward SO2 detection. Nickel oxide (NiO) nanoparticles were also deposited on the graphene and improved sensitivity. The NiO-rGO sensors had additional intriguing properties. Graphene is a near zero band gap semiconductor and surface modifications of rGO can cause it to exhibit p-type or n-type behavior. The unmodified rGO and the SnO2-rGO were p-type devices, whereas the NiO-rGO was an n-type device with a significant change in signal upon exposure to SO2. The addition of NiO improved sensitivity, but the SnO2 had superior consistency and better reversibility.
Therefore, sensors were prepared by depositing a mixture of both NiO and SnO2 on the graphene surface. The mixed transition metal oxide composite nanoparticles were synthesized and tested for their ability to detect SO2. The most promising results were obtained with hybrid Ni/SnO2 composite nanoparticles synthesized via a hydrothermal method and drop cast on to the graphene bridged gold interdigitated electrodes. These Ni/SnO2-rGO sensors showed excellent selectivity with an over 13-fold greater change in signal when exposed to 0.1 ppm sulfur dioxide than when exposed to 10 ppm ammonia. The Ni/SnO2-rGO sensors successfully detected SO2 down to 2 ppb. The excellent sensitivity, selectivity, and reversibility of these sensors toward SO2 under ambient conditions suggests that they are a promising candidate for commercialization with an inexpensive handheld tester.
Conclusions:
Significant progress was made toward developing a portable tester for detecting sulfur dioxide using sensors coated with novel metal oxide composites. SO2 was successfully detected under ambient conditions down to 2 ppb using hydrothermally prepared nickel/tin oxide composite nanoparticles as the probe on graphene sheets (Ni/SnO2-rGO). There is evidence that with further optimization of the fabrication process the sensors could reliably detect even lower concentrations of SO2. The Ni/SnO2-rGO sensors showed excellent selectively for sulfur dioxide when compared with ammonia. Combining these three materials takes advantage of the excellent electrical properties of graphene and pairs it with the selectively and sensitivity of tin oxide and nickel oxide toward sulfur dioxide. The calibration curve generated with the Ni/SnO2-rGO sensors could be used to detect unknown concentrations of sulfur oxide in air under ambient conditions with a portable handheld tester.
Monitoring for pollutants such as SO2 in air can improve the air quality and subsequently the health of many people by mitigating exposure to toxic gases and preventing respiratory and cardio related illnesses caused by SO2. A portable and inexpensive tester capable of real time detection of sulfur dioxide would greatly improve our ability to monitor exposure to this pollutant. NanoAffix has experience in developing graphene-based sensors for the real time detection of toxic compounds. The EPA regulates exposure limits to SO2 and currently only expensive stationary equipment is capable of SO2 monitoring. This would be the first inexpensive and portable SO2 detector on the market and would enable air quality monitoring in real-time and onsite under ambient conditions. NanoAffix has three patents, and a fourth pending patent, in the area of graphene-based sensors. NanoAffix has effectively demonstrated the technical feasibility of this device through the research and development enabled by this phase I SBIR grant, and intends to continue to progress toward commercializing this technology by reducing sensor variability through optimizing the fabrication process and improving the calibration algorithm used to calculate the SO2 concentration. After additional development to improve the product, NanoAffix intends to manufacture and sell this product to various public and private entities that are engaged in air quality monitoring through direct or indirect sales. The target price of the tester, which can be used for several years to make thousands of measurements, is less than $100. The reusable graphene-based sensors will be coated with a proprietary transition metal oxide composite that is selective toward detecting sulfur dioxide and can achieve a limit of detection down to at least 1 ppb sulfur dioxide. These reusable sensors will be sold for use with the aforementioned tester with a target price of less than $20. The sensors in conjunction with the tester will be able to determine unknown concentrations of sulfur dioxide under ambient conditions and using a proprietary calibration algorithm that takes into account variable factors such as temperature and humidity. Alternatively, NanoAffix could license the intellectual property owned in this area to other companies as a source of revenue.
SBIR Phase II:
A Low-cost Hanheld Sulfur Dioxide Tester with a Hybrid Nanomaterials-based Sensor Chip | Final ReportThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.