Grantee Research Project Results
Final Report: A Low-Cost Chemosensor for Measuring Phosphate in Water and Soil
EPA Contract Number: EPD07048Title: A Low-Cost Chemosensor for Measuring Phosphate in Water and Soil
Investigators: Coleman, Thomas E.
Small Business: dTEC Systems LLC
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2007 through August 31, 2007
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2007) RFA Text | Recipients Lists
Research Category: SBIR - Animal Waste and Waste to Energy , Small Business Innovation Research (SBIR)
Description:
The proposed Phase I research was aimed at developing a low- cost portable instrument capable of making real -time measurements for phosphate (PO4-3) in water and soil samples. The sensor system for this instrument is based on the PO4-3- specific selective binding properties of a specially designed polymer coating. The key objective of this Phase I research was to demonstrate a quantitative response to PO4-3 concentration in water using a sensor fabricated by applying the PO4-3 -binding polymer onto a surface as a thin film. The dielectric response of the PO4-3 -binding polymer was measured with fringing electric field (FEF) dielectrometry when exposed to varying concentrations of PO4-3 . The chemoflourescent activity of the thin film coating also was measured in an attempt to verify the degree to which the polymer selectively binds PO4-3 in aqueous sample solutions.
The specific technical objectives for the proposed Phase I Small Business Innovation Research were to :
- Demonstrate orthophosphate ion selective binding properties of a polymer matrix, which was to be developed in this research.
- Demonstrate a measurable shift in complex dielectric permittivity of the PO4-3 -binding polymer in response to binding of orthophosphate molecules.
- Determine the practical detection limits, range, and reproducibility of measurements for orthophosphate in water, based on monitoring dielectric properties of the PO4-3 -binding polymer film measured with FEF dielectrometry over a range of orthophosphate concentrations.
- Determine the potential for interferences from other anions (such as nitrate, sulfate, and arsenate) that probably are present in the environment, and which might limit the practical application of the instrument for measuring orthophosphate in water samples.
Summary/Accomplishments (Outputs/Outcomes):
A key intermediate compound for the synthesis of a polymer with selective orthophosphate binding properties was synthesized. Incorporation of this intermediate compound into a larger polymer structure proved to be more difficult than anticipated. Problems encountered included the low solubility of the compound in solvents, such as toluene, which are typically used in the polymerization reactions, and the presence of impurities, which can greatly reduce the yield of the reactions. A somewhat similar, but less complex, diamine functionalized polyfluorene polymer was synthesized in Professor Jenehke’s laboratory. Because of the availability of the amine groups on this polymer, it also should have a binding affinity for PO4-3 even if it lacks the high degree of specificity for PO4-3 that this research aimed to achieve. This polymer was used in carrying out experiments aimed at determining the dielectric response of a polymer- coated FEF sensor in the presence of PO4-3 and other anions.
The results of the experimental measurements of the coated FEF sensors proved to be much more complex than anticipated. Even the initial calibration/verification measurements indicated that there is possibly an unexpected phenomenon occurring with the sensor when measuring these liquid samples. One of the possible explanations for the phenomena is the formation of the Debye layer (often referred to as electrical double layer in dielectrometry) near the electrode surface.
One of the issues encountered in the experiments done with the coated sensors was the very significant variation in sensor signal, which was consistently observed. Frequently, the measurement in the same sample produced significantly different results. These variations were far more significant than any error that could be expected to originate in the measurement system itself. Therefore, it is assumed that there is an electrochemical process, possibly a Debye layer formation, which was described above, occuring during the measurements that has a significant effect on the observed values of those measurements.
A promising response was obtained with a sensor coated with the intermediate compound described above, where it was observed that the capacitance and resistance signals obtained for the 1,000 mg/L PO4-3 solution are separated from the other measurements of the solutions of the other anions. This suggests that there is some selectivity in the binding of the orthophosphate molecules to the coating of this compound.
A series of dielectrometry experiments involved measurement of the sensor response to PO4-3 solutions over a wide range of concentrations—10, 100, and 1,000 mg/L. For the sensor coated with the compound synthesized in this research, capacitance values are the highest for 1,000 mg/L and lowest for 10 mg/L PO4-3 concentrations. The order of the values agrees with the prediction that the ionic binding will increase sensor capacitance by increasing the total number of electric dipoles in the measurement layer. Resistance showed an opposite trend, with the lowest resistance measured at the highest PO4-3 concentration. This also was predicted because the resistance of the sample is directly proportional to its ion concentration.
For two other polymer coatings synthesized in Professor Sam Jenekhe’s Laboratory at the University of Washington, the capacitance spectra order was reversed. There are a number of possible explanations that could explain why the capacitance actually increased with decreasing PO4-3 concentration. Because the capacitance values increased with time as the experiment progressed, there simply might be a delay in the sensor response, which could be attributed to slow sample diffusion into the bulk polymer. Another possible explanation for the reversing of the capacitance lines order for the Jenekhe Lab polymer is formation of a thin, high conductivity layer at the surface of the polymer. A high conductivity layer could be formed by the ions attracted by the amine groups and which cannot diffuse further into the sample. A conductive layer in the vicinity of the FEF sensor surface would be capable of distorting the electric field pattern such that lowered sensor transcapacitance would result. If the conductivity of that layer drops lower ( because of higher PO4-3 concentration), then the sensor capacitance would decrease further.
Because of the wide variations observed in the dielectrometry measurements, even at relatively high concentrations of 1,000 mg/L, it was not possible to develop a standard curve for PO4-3, which would enable the detection limits for an instrument based on this method to be predicted reliably.
Photoluminescence experiments were conducted as an independent method of determining if PO4–3 binding to the polymer coating was actually occurring. These experiments were inconclusive as well. Either the binding of PO4–3 was not occurring, or if it was, there was no photoluminescence response.
Conclusions:
In summary, it must be reported that, while some encouraging results were obtained using a FEF sensor coated with a PO4-3 selective binding intermediate compound synthesized in this research, overall these results fell considerably short of achieving the technical objectives identified in the Phase I proposal. As a result, it cannot be stated conclusively that the concept for a low cost, handheld instrument for measuring PO4-3 as described in the proposal has been proven. Most significantly, the Phase I experimental data did not provide definitive support of the fundamental hypothesis that the binding of the analyte of interest (PO4-3 in this case) would result in a predictable and reproducible increase in capacitance in the fringing field sensor coating. The results of the photoluminescence experiments aimed at confirming the binding of PO4-3 to an amine functionalized polyfluorene polymer also were inconclusive.
Based on observations and analyses of these Phase I research results, several research and development (R&D) objectives have been identified that need to be addressed to advance this technology to the prototype development stage. These future R&D efforts could include investigating the mechanisms that affect the dielectric response of the sensor coating systems to better understand the possible effects of a Debye layer formation and other interferences. Specific research methodologies should include multiple penetration depth sensors and/or specially designed multi layer coatings; investigation of finer electrode structures that might be possible using nanofabrication techniques; and investigation of other polymers and coating thicknesses.
Supplemental Keywords:
SBIR, small business, chemosensor, phosphate, PO4-3, fringing field, dielectric spectroscopy, eutrophication, confined animal feeding operations,, RFA, Scientific Discipline, Sustainable Industry/Business, Sustainable Environment, Technology for Sustainable Environment, Environmental Monitoring, CAFOs, chemical sensors, euthrophicationThe 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.