Grantee Research Project Results
Final Report: Indoor Formaldehyde Detection by a Low-Cost Chemical Sensor Based on Organic Nanofibers
EPA Contract Number: EPD17032Title: Indoor Formaldehyde Detection by a Low-Cost Chemical Sensor Based on Organic Nanofibers
Investigators: Later, Douglas W
Small Business: Vaporsens, Inc.
EPA Contact: Richards, April
Phase: I
Project Period: September 1, 2017 through February 28, 2018
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2017) RFA Text | Recipients Lists
Research Category: SBIR - Air and Climate , Small Business Innovation Research (SBIR)
Description:
Formaldehyde poses a significant health threat. It was classified as a probable carcinogen by the Environmental Protection Agency (EPA) in 1991 and classified as a carcinogen by the International Agency for Research on Cancer in 2004 (Chem. Rev., 110, 2536 (2010)). In fact, one study indicates that formaldehyde ranks in the top three sources of decreased lifespan due to indoor air pollutants, which affects approximately 1% of the American population (Environ. Health Perspectives, LBNL Report 5267E (2011)). The World Health Organization’s guideline for maximal concentration of formaldehyde is 81 parts per billion (ppb). While the Occupational Safety and Health Administration regulates a time-weighted average of 750 ppb, the National Institute for Occupational Safety and Health recommends a lower exposure limit of 16 ppb.
People are exposed to formaldehyde from a variety of sources inside their homes, places of employment, and other indoor environments. These sources include furniture and building materials, particularly those featuring laminated wood products (Environ. Sci. Technol., 33, 81 (1999)). Paint and carpet are other sources (Indoor Air, 12, 10 (2002)). Formaldehyde exposure in modern buildings, which are tightly sealed from the outdoors for energy conservation purposes, is exacerbated by decreased air turnover (Environ. Res., 102, 1 (2006)). With the growing health threat that formaldehyde poses, it is becoming more important to monitor homes and other indoor spaces so that inhabitants can take actions to minimize their exposure. Current products on the market require consumers to sacrifice sensitivity and selectivity for cost; low cost sensors do not meet the program goal of 50 ppb or give false positives to common chemicals such as ethanol.
The main failure of the market to meet the program requirements lies in the cost of the instruments and their performance. While there are sensors that cost less than $100, they are typically only sensitive in the parts per million (ppm) range and have poor selectivity against chemicals commonly found in homes, such as ethanol. More sensitive and selective instruments, such as those based on liquid chromatography or colorimetric sensing, meet the performance requirements, but are very expensive and require extensive training to use. These instruments are also very slow and do not provide the temporal resolution required for a user to take immediate action.
Vaporsens produces sensors based on organic nanofibers. The nanofibers are self-assembled from building block molecules (Acc. Chem. Res., 41, 1596 (2008)). These building block molecules are functionalized to interact specifically with certain chemical species. The use of organic structures provides virtually limitless possibilities in functionality. Once assembled, the nanofibers are coated onto an electrode pair to create chemiresistive sensors (i.e., a sensor that signals detection of an analyte by changing electrical resistance). The change in resistance is due to a change in charge carrier density caused by electron transfer with the detected chemicals (J. Am. Chem. Soc., 129, 6354 (2007)).
The purpose of the research performed during the reporting period was to evaluate Vaporsens’ chemical sensors for monitoring formaldehyde in simulated real-world conditions, including varying temperatures, levels of humidity, and chemical interferents. Three technical questions were stated in the Phase I Statement of Work:
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What are the sensitivity and resolution of the nanofiber-based sensors toward formaldehyde? If they are inadequate, what is the path to reach the specifications?
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How do the sensors behave in different environmental conditions (e.g., temperature, humidity) when sensing formaldehyde? If there is an impact, can it be removed algorithmically?
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How do the sensors behave in the presence of other chemicals when sensing formaldehyde? If there is an impact, can it be removed algorithmically?
Summary/Accomplishments (Outputs/Outcomes):
Vaporsens performed detailed tests on fourteen of its sensors that showed promise for detecting formaldehyde in indoor environments. Three sensor arrays were assembled and showed very similar behavior when exposed to different concentrations of formaldehyde. The sensors were tested against formaldehyde concentrations of 10 ppb, 50 ppb, 100 ppb, 500 ppb, 1 ppm, and 5 ppm. Repeated exposures showed that eight sensors typically gave responses within a 10% range, indicating the ability of the sensors to quantify formaldehyde.
Environmental tests were performed by testing the sensors at different temperatures and levels of humidity. Again, the sensors performed consistently, with the top sensors showing less than a 16% deviation across the ranges tested. Humidity was shown to have the larger impact. The impact of humidity was modeled and can be removed algorithmically if humidity is known (e.g., from the inclusion of a low-cost humidity sensor).
False positives have been a major issue with commercially available sensors. Electrochemical cells, for example, tend to measure alcohol as formaldehyde. Vaporsens tested its sensors’ abilities to detect 50 ppb of formaldehyde in the presence of chemicals at much higher concentrations. These chemicals include: 2 ppm of ethanol, 100 ppm of methanol, 300 ppm of carbon dioxide, 64,000 ppm of hexane, and 0.9 ppm of urea. Ethanol, carbon dioxide, and hexane did not significantly influence sensor response, with sensors responding within the 10% reproducibility described above. Methanol and urea caused measurement errors on certain channels. For example, one sensor decreased in response by 37% in the presence of the high concentration of methanol. Given the unrealistically high concentrations of these chemicals and their small influence on the performance of the sensors, selectivity is not expected to be an issue. Cross-reactivity with ethanol was also investigated because it was a common complaint that Vaporsens heard from its customers about competing sensors. Vaporsens’ worst-performing sensor showed a cross-reactivity of just 0.3%, while a leading sensor showed a cross-reactivity of 189% during the same test.
Conclusions:
Vaporsens’ sensors showed excellent sensitivity, with demonstrated sensitivity down to 10 ppb and a projected limit of detection of 1 ppb. With repeated exposures, several sensors demonstrated relative standard deviations less than 10%. The impacts of temperature and humidity were investigated. The responses were not significantly impacted over ranges that are relevant to indoor environments. Finally, sensor responses to formaldehyde were performed in the presence of different chemicals. These chemicals were selected either because they are challenging for sensors available today or are expected to co-exist with formaldehyde in certain situations. No chemical hindered the ability of the sensors to detect formaldehyde. Ethanol is a frequent problem for formaldehyde sensors. Vaporsens’ most responsive sensor to ethanol demonstrated a cross-reactivity of only 0.3%, whereas a leading commercially-available sensor had a cross-reactivity of 189%. These results clearly demonstrate the sensors ability to address the goals of the EPA’s SBIR program.
These results demonstrate the technical feasibility of using Vaporsens’ nanofiber technology in a handheld instrument. The higher selectivity of the sensors will allow for the size of the sensor array to be reduced, possibly to as few as one or two sensors. This will greatly reduce the cost of the instrument. Sensitivity enables further reduction in cost, which makes it possible to use inexpensive (noisy) electronics while still meeting the program requirements. Finally, the reproducibility of the sensors, as demonstrated by the results from the three sensor array cards, demonstrates the potential for the sensors to be manufactured on a production scale. Vaporsens feels strongly that its sensors can meet or exceed all program requirements.
SBIR Phase II:
Indoor Formaldehyde Detection by a Low-Cost Chemical SensorBasedonOrganic Nanofibers | 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.