Grantee Research Project Results
2005 Progress Report: Compound Specific Imprinted Nanospheres for Optical Sensing
EPA Grant Number: R830911Title: Compound Specific Imprinted Nanospheres for Optical Sensing
Investigators: Lavine, Barry K.
Current Investigators: Lavine, Barry K. , Seitz, William Rudolf , Fendler, Janos
Institution: Oklahoma State University
Current Institution: Oklahoma State University , Clarkson University , University of New Hampshire
EPA Project Officer: Hahn, Intaek
Project Period: August 24, 2003 through August 23, 2006 (Extended to August 31, 2010)
Project Period Covered by this Report: August 24, 2005 through August 23, 2006
Project Amount: $323,000
RFA: Environmental Futures Research in Nanoscale Science Engineering and Technology (2002) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Objective:
The objective of this research project is to investigate the use of molecularly imprinted polymers as the basis of a sensitive and selective method for the detection of pharmaceuticals and other emerging organic contaminants at parts per billion (ppb) levels in aquatic environments. Moderately cross-linked, molecularly imprinted polymeric nanospheres (ranging from 100 nm to 1,000 nm in diameter), which are designed to swell and shrink as a function of analyte concentration in aqueous media, are being prepared. The nanospheres are dispersed in a hydrogel membrane. Chemical sensing is based on changes in the optical properties of the membrane that accompany swelling of the molecularly imprinted nanospheres. Two effects contribute to this change. One is an increase in the size of the nanospheres, resulting in an increase in the amount of light scattered. The other is a change in their refractive index. Because swelling leads to an increase in the percentage of water in the polymer, the refractive index decreases as the nanospheres swell. This brings them closer to the refractive index of the hydrogel membrane, leading to a decrease in the amount of light scattered/reflected by the nanospheres. For the systems that we have been studying, the change in refractive index is the dominant effect. This change can be measured by absorbance or surface plasmon resonance (SPR) spectroscopy. Using SPR, our prototype sensor will be capable of detecting pollutants and hazardous materials selectively at ppb levels in the environment.
Progress Summary:
Theophylline-imprinted polymer particles (approximately 0.3 microns in diameter) suitable for SPR were prepared and deposited by spin coating a methanol suspension of particles onto a gold SPR slide followed by drying. The particles were held on the slide by electrostatic attraction. The polymer particles formed were both sensitive and specific. The addition of as little as 1.0x10-6 M theophylline was sufficient to cause a change in the refractive index, which we were able to detect by SPR. Higher concentrations of theophylline produced larger changes in the refractive index. In contrast, the particles showed no response to distilled water or 1.0x10-2 M caffeine. (Caffeine and theophylline differ by only a single methyl group.) Furthermore, particle swelling was unaffected by ionic strength of the medium. This result, we believe, is significant for three reasons. First, selectivity has been introduced into SPR analyses though use of these particles. Studies where biological receptors have been used to functionalize Au or Ag surfaces with analyte specific receptors for pollutant monitoring have been unsuccessful because of problems associated with antigen stability and cross reactivity. Second, the likelihood is high that ppb detection limits for theophylline and other so-called emerging organic contaminants can be achieved with this approach to chemical sensing once a membrane is used to incorporate the polymeric particles and the polymeric formulation used to develop the imprinted polymer and hydrogel membrane is optimized. Third, because swelling is not affected by ionic strength, these particles are well-suited for analyses in media other than freshwater.
Swellable molecularly imprinted polymer particles that respond to pH also have been prepared. When these polymer particles are dispersed in a hydrogel, there are large changes in absorbance as the pH of the solution in contact with the membrane is varied. Changes of approximately one absorbance unit have been observed in the pH range of 3.5 to 5.5 because of the swelling of the polymer particles, which was reversible in both low (0.1 M) and high (1.0 M) ionic strength buffered solutions. The membrane obeys the Henderson-Hasselbach equation in buffers of low ionic strength, but the sensitivity of the pH response is greater than that predicted by the Henderson-Hasselbach equation when the membrane is immersed in media of high ionic strength. Increasing the temperature of the membrane produces a response similar to that of increasing the ionic strength of the buffer. The pH response of the membrane also can be increased by increasing the weight percent of polymer in the hydrogel. One possible application of pH sensitive polymer particles includes monitoring the progress of open-heart surgery, where pH serves as a measure of tissue ischemia. Gastric pH sensing is another possible application.
Future Activities:
Currently, we are using a polymer formulation developed from N-isopropylacrylamide (for pH sensitive particles) or N-N-propylacrylamide (for theophylline-sensitive particles), t-butyl acrylamide, methacrylic acid (recognition monomer), moderate concentrations of methylenebisacrylamide (crosslinker), and template to prepare polymer particles that swell in the presence of the targeted analyte. Recent studies performed in our laboratory indicate that pH swelling is dependent upon the concentration of methacrylic acid in the formulation, with 10 percent being the optimum. In all likelihood, tuning the concentration of methacrylic acid in the formulation also will promote polymer swelling for the theophylline-sensitive particles. Previously, the concentration of recognition monomer used in the formulation for molecular imprinting has been less than or equal to 5 percent. To promote polymer swelling, we might need to increase the concentration of methacrylic acid in the formulation. Optimizing the formulation to promote polymer swelling for theophylline will require us to understand the nature of the imprinting process, and this will be a major focus of our research during the next reporting period.
The thickness of the hydrogel membrane is approximately 100 microns, and the size of the microspheres for theophylline sensing is approximately 300 nm. It would be advantageous if the membrane were thinner to minimize diffusion distances, ensuring facile mass transfer. In addition, using smaller nanospheres (approximately 100 nm in diameter) would be advantageous because it would mean that a larger number of polymer particles could be immobilized on the SPR slide, and the entire particle would lie within the region of the evanescent wave.
Currently, dilute methanol solutions of polymer particles are being spin coated onto the SPR slide. The solution is allowed to dry and the particles are held by electrostatic attraction. Preliminary data from our laboratory suggests that particle rearrangement occurs on the surface because of swelling. During repeated swelling and shrinking cycles, they are lost from the surface in the region where the evanescent wave is observed. To mitigate this effect, mercaptoamine SAMs will be deposited onto the SPR slide with the imprinted particles, which contain methacrylic acid deposited onto the SAM modified gold via spin coating. Alternatively, polyethylenimine will be spin coated onto the SPR slide. It then would be a simple matter to spin coat PolyNNPA particles onto the modified gold substrate.
Journal Articles:
No journal articles submitted with this report: View all 19 publications for this projectSupplemental Keywords:
groundwater, chemicals, nanotechnology, hydrogel, N-isopropylacrylamide, N-N-propylacrylamide, molecular imprinting, environmental monitoring, pollutant monitoring, methods, techniques, colloidal polymerization, analytical chemistry, new/innovative technologies, environmental measurement, nanosensors, environmental chemistry,, RFA, Scientific Discipline, Water, Ecosystem Protection/Environmental Exposure & Risk, Sustainable Industry/Business, Environmental Chemistry, Sustainable Environment, Technology for Sustainable Environment, Monitoring/Modeling, Environmental Monitoring, Engineering, Chemistry, & Physics, aqueous impurities, aquatic ecosystem, nanosensors, chemical sensors, membranes, nanotechnology, environmental sustainability, chemical detection techniques, aquatic toxins, analytical chemistry, surface plasma resonance spectroscopy, optical sensing, nanoporous membranes, hydrogel membranes, membrane technologyProgress and Final Reports:
Original AbstractThe 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.