Nitrite (NO2-) and nitrate (NO3-) ions are important components of agricultural fertilizers. However, they are pollutants when they find their way into drinking water, causing birth defects and other health problems if the concentration is high enough.
The Environmental Protection Agency recommends limiting exposure to 1 part per million. Unfortunately, current methods of detecting these ions are expensive, complicated, and nonideal for testing in remote/rural locations, e.g., where testing is most needed.
Chad Mirkin (Northwestern University, Illinois) and coworkers have addressed this limitation by developing an assay for nitrite and nitrate ions using gold nanoparticles. The basis of the assay is a clearly observable loss of color in the solution when the nanoparticles chemically react with the target ion.
The chemical reaction.
The scientists' gold nanoparticles are an even mixture of those appended with an aniline derivative, and those appended with a naphthalene derivative. The nanoparticles are also appended with inert water-soluble molecules to impart solubility of the nanoparticles in water.
When nitrite ions, in hot acidic pH, react with an even mixture of these nanoparticles, there is a chemical reaction that aggregates the nanoparticles. The aggregation causes the nanoparticles to precipitate out of solution.
The nanoparticles impart a red color to the solution when they are freely dispersed. However, after aggregation, the color of the solution is lost.
When nitrite is present, the solution is colorless (the nanoparticles precipitate out of solution). When nitrite is not present, the solution is red (the nanoparticles remain in solution).
This is the basis for the assay. There is a loss of color in the solution when nitrite ions are present.
Concentration range of nitrite detection.
It is important that an assay be useful for relevant concentrations of the analyte of interest. In this case, the assay should be useful near 1 part per million nitrite ions (the limit set by the Environmental Protection Agency).
The scientists' assay is useful for nitrite concentrations between 0.9 and 1.6 parts per million. This is a narrow concentration range, but it is a useful concentration range because it is near the desired 1 part per million limit.
The onset of color change can be tuned by changing the incubation time of the chemical reaction. This enables one to tune the assay for optimal response at different nitrite concentrations.
Nitrate can be detected in this assay as well. Here, nitrate must first be chemically converted into nitrite using the enzyme nitrate reductase.
Environmentally relevant concentrations of nitrite (and, with deliberate processing, nitrate) ions can clearly be detected with this assay.
Selectivity of the assay.
It is important that an assay work in complex solutions. In the real world, extraneous molecules will be present in the sample besides the analyte of interest, and they must not lead to false readings.
The scientists checked for this by performing the assay with large extraneous concentrations of other environmentally relevant ions, such as nitrate, fluoride, sulfate, bromide, perchlorate, acetate, thiosulfate, oxalate, azide, and carbonate. None of these ions interfered in the assay.
It is clear that this assay is selective for nitrite ions, the ion of interest for this assay.
Overall evaluation.
These scientists' assay can be readily performed in remote locations, with cheap, common, portable instrumentation if quantitative data is required. It works with real-world samples, and is useful on a practical level for environmental monitoring of nitrite and nitrate ions in rural water sources.
for more information:
Daniel, W. L., Han, M. S., Lee, J.-S., & Mirkin, C. A. (2009). Colorimetric Nitrite and Nitrate Detection with Gold Nanoparticle Probes and Kinetic End Points Journal of the American Chemical Society, 131 (18), 6362-6363 DOI: 10.1021/ja901609k