Grantee Research Project Results
2006 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, 2006 through August 23, 2007
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 by dispersion polymerization at 65°C and deposited onto a gold SPR slide by spin coating a methanol suspension of the particles onto a gold SPR slide. Ten drops of the methanol suspension were placed in the center of the sample slide, which was spun at 500 rpm for 5 seconds and 3000 rpm for 25 seconds. Two or three additional applications of two drops of the polymer suspension were added to build a nearly uniform polymer layer as determined by visual inspection using a Leitz Orthoplane microscope. The slide was then allowed to dry for two days in a desiccator before rehydration. The particles were held on the slide by electrostatic attraction. The polymer particles formed a layer that was 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 (see Figure 1). 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.) The full scale response of the imprinted particles to the template occurred in less than 10 minutes. Swelling was also reversible and replicate precision was less than 10-4 refractive index units. A unique aspect of the prepared particles is the use of light crosslinking rather than heavy crosslinking. This is a significant development, as it indicates that heavy crosslinking is not necessary for selectivity in molecular imprinting with polyacrylamides.
-
Figure 1. Refractive Index Values of the Spin Coated Imprinted Polymer Particles as a Function of the Theophylline Concentration in the Solutions in Contact With the Particles. The refractive index values were determined by fitting the SPR spectra to a five layer model: the first layer is the prism, the second layer is chromium (1 nm layer), the third layer is gold (50 nm layer), the fourth layer is the polyNNPA particles, and the fifth layer is the solution in contact with the polymer.
Swellable polymer particles that respond to pH have also been prepared by dispersion polymerization at ambient temperatures. When these polymer particles are dispersed in a hydrogel, there are large changes in absorbance as the pH or metal ion content of the solution in contact with the membrane is varied. Changes of approximately one-half of an absorbance unit have been observed in the pH range of 3.5 to 5.5 because of the swelling of the polymer particles, which is reversible at ambient (see Figure 2) and physiological temperatures. Swelling is also observed to be independent of ionic strength. The polymer particles show a larger response over a narrower pH range than predicted by the Henderson-Hasselbach equation. The pKa of the particles can be tuned by varying the degree of crosslinking, the pKa of the pH sensitive comonomer used, or the amount of N-tertbutylacrylamide n the formulation. One potential application of the pH sensitive polymer particles developed in our research is monitoring the progress of open-heart surgery, where pH serves as a measure of tissue ischemia. Gastric pH sensing is yet another possible application. Monitoring the pH of rivers and streams is a third application.
Figure 2. pH Titration Curve (0.1 M Ionic Strength) for NK 1-60 Particles at Ambient Temperature
Polymer particles prepared from N-isopropylacrylamide (NIPA) have been investigated as potential adsorbents for removal of metal ions in wastewater. Because little is known about the adsorption of metal ions by these polymers, polymers and copolymers of NIPA were prepared and immobilized on an SPR slide to study noncompetitive metal ion binding. Of particular interest was studying the effects of metal ions on the swelling and shrinking behavior of NIPA particles copolymerized with methacrylic acid. The solutions placed in contact with the polymer particles consisted of a metal salt dissolved in distilled water. For alkali and alkaline earth metal ions, binding occurs via ion pairing with the carboxylic acid group leading to an increase in the water content of the particles (see Figure 3). The lack of selectivity which is exhibited by these particles towards the alkaline metals can be attributed to the low level of crosslinking used (approximately 10%). Hydration occurs because of solvation, which decreases the density of binding sites and their selectivity for the Group I and Group II metal ions.
-
Figure 3. Refractive Index Versus the Log of the Concentration of Na+, K+, Li+, and Ca2+ Obtained From SPR Spectra for polyNIPA-methacrylic acid Particles Spin Coated Onto a Gold Surface.
The response profiles obtained for the transition metal ions are different (see Figure 4). At low concentrations of Pb2+ or Cu2+ (e.g., 10-7 M and 10-6 M), the decrease in refractive index is probably due to an increase in the lower critical solution temperature of the polymer, which can be attributed to deprotonation of the carboxylic acid groups by Pb2+ or Cu2+ through an ion exchange mechanism. At higher concentrations of Pb2+ or Cu2+, the refractive index of the polymer particles increases, suggesting that water is being squeezed out of the polymer. We attribute this to Pb2+ or Cu2+ being bound to both the carboxylic acid by ion exchange and to the nitrogen atom of NIPA, through coordination with its lone pair, forming a chelate. Pb2+ or Cu2+ via complexation through coordination and ion exchange is effectively acting as a crosslinker reducing the particle diameter and lowering the affinity of the polymer for the solvent, which is water. The binding of Pb2+ or Cu2+ to the nitrogen atom of NIPA also disrupts the hydrogen bonding between water molecules and the NH2 group on the polymer backbone, reducing the affinity of the polymer for water. In either case, the refractive index of the polymer would increase, which is consistent with our experimental data.
-
Figure 4. Refractive Index Versus the Log of the Concentration of Cu2+ and Pb2+ Obtained from SPR Spectra for polyNIPA-MAA Particles Spin Coated Onto a Gold Surface
Future Activities:
The plot of Brewster angle versus concentration of theophylline for the data shown in Figure 1 is nonlinear. When the template concentration is low, the binding of theophylline to the molecular recognition sites will increase with theophylline concentration and so will the SPR angle shift. As the number of recognition sites on the polymer becomes saturated with the template, the SPR response will become less dependent on the concentration. Whether the SPR angle shift data is following a Langmuir (binding sites of equivalent reactivity) or sigmoid (binding sites of differing reactivity) relationship is an open question, which we are currently investigating.
Lai and coworkers in a previous study have demonstrated that a surface plasmon resonance sensor using a theophylline imprinted polymer as the sorbent could successfully assay theophylline in a complex mixture consisting of eight drug molecules similar in structure to the template. The detection limit reported by Lai in his study was approximately 0.022 M. The lower detection limit achieved in our studies can be attributed to the use of swelling as the transduction mechanism to sense theophylline binding. However, our goal is to detect theophylline at 4 ppb, which corresponds to 2x10-8 M, the detection limit for theophylline by solid-phase extraction-liquid chromatography/mass spectrometry (SPE-LC/MS). Currently, our detection limit is 50 times higher. Since the formulation used to develop the swellable molecularly imprinted polymer particles has not yet been optimized, it is our view that even lower detection limits can be achieved. For example, a different recognition monomer, ethacrylic acid, will be used in future studies in lieu of methacrylic acid since acrylamide monomers, when copolymerized with ethacrylic acid, exhibit greater swelling. Furthermore, the binding constant of theophylline to recognition sites formed by etharcylic acid should be larger since etharcylic acid is more hydrophobic than methacrylic acid. (The detection limit of the proposed SPR method is largely determined by the binding constant of the analyte to the recognition sites in the polymer. In our current study, the binding constant is approximately 10,000, which corresponds to the reciprocal of the concentration of theophylline at the inflection point on the sigmoid shape curve shown in Figure 1.) In aqueous media, hydrophobicity is an important force controlling the interactions between recognition sites in the polymer and the template, which is in the solution.
Particle adhesion to the gold surface has been problematic, contributing to higher detection limits. The two-step spin coating procedure that has been developed in our laboratory has helped to obviate this problem. Another approach is to apply a polyvinyl alcohol hydrogel membrane to the spin coated particles. Our previous attempts to spin coat the membrane formulation on top of the particles prior to polymerization of the polyvinyl alcohol (PVA) were not successful. Recently, we have learned that placing gold nanoparticles on top of the spin coated polymer particles prior to deposition and polymerization of the PVA formulation can facilitate the formation of a membrane that will contain the particles. We plan to further investigate this procedure during the next reporting period.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 19 publications | 5 publications in selected types | All 5 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Lavine BK, Westover DJ, Kaval N, Westover, DJ, Oxenford L. New approaches to chemical sensing-sensors based on polymer swelling. Analytical Letters 2006;39(9):1773-1783. |
R830911 (2006) |
Exit |
Supplemental Keywords:
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.