W. Rudolf Seitz Research
Current Research Interests
Prof. Seitz's research group is developing a new technology for chemical sensing based on swellable polymers. The concept is illustrated in Figure 1. The sensing element consists of lightly crosslinked, derivatized polymer microspheres ca. 1 micrometer in diameter dispersed in a hydrogel membrane.
The microspheres are designed to swell and shrink with changing chemical concentrations in aqueous solutions. For example, we have used diethanolamine derivatized polystyrene to sense pH. Protonation of the amine group introduces a positive charge onto the polymer backbone causing it to swell. This swelling can be viewed as due to repulsion between adjacent charges on the polymer backbone, or it can be considered an osmotic pressure effect. It leads to a continuous change in the diameter of the polymer microspheres over the pH range from approximately 6 to 8 as the nitrogen is protonated. As the polymer microspheres swell, we observe a change in the optical properties of the hydrogel membrane. Two effects contribute to this. One is the increase in the size of the microparticles. This causes them to scatter light more effectively. The other effect is due to a change in refractive index. Because swelling leads to a higher percentage of water in the polymer, the refractive index of the microparticles decreases. This brings it closer to the refractive index of the hydrogel membrane. The amount of light reflected at the interface between two dissimilar phases depends on the difference in the refractive indices of the two phases. For the case of normal incidence, the reflectance is equal to (n1-n2)2/(n1+n2)2. This effect causes a decrease in the amount of light reflected/scattered by the particles. For most of our systems, the change in refractive index is the dominant effect. For example, using typical conditions for diethanolamine derivatized polystyrene, we calculate that the unprotonated polymer has a refractive index ca. 1.48 and the protonated polymer has a refractive index ca. 1.40. We calculate that this should cause the reflected intensity to change by more than a factor of four when the microspheres are suspended in a polyvinyl alcohol membrane with a refractive index of 1.34. In many of our systems we observe changes of this magnitude.
The membranes that we have developed can be used for either transmission or reflection measurements. They can be coupled to optical fibers for remote measurements. This is the subject of ongoing research in the P.I.'s research. Multiple membranes can be prepared on a single substrate to make an array of sensors each responding to a different analyte.
Our sensor technology has the following characteristics:
Stability: Our microspheres are very stable. Unlike most other types of optically sensitive materials, photodegradation is not an issue because the readout does not require absorption of light. Indicator leaching from a substrate is also not a problem because response is a bulk polymer phenomenon. Because microsphere dimensions are on the order of one micrometer, the mechanical stresses associated with swelling and shrinking are minimized, and cracking or other forms of mechanical deterioration are not observed. Our membranes continue to respond after 40 days in an oven at 80 0C (176 oF). The only failure mode we have observed is cracking of the hydrogel when it is allowed to dry out, a problem that we are addressing by using new hydrogels with superior mechanical properties.
Stability is central to our vision of this technology. We expect to prepare sensors that will function for weeks or even months without requiring calibration. We expect them to withstand steam sterilization so that they can be used in bioreactors and other samples that are subject to biological contamination.
Any Wavelength: Membrane reflectance can be measured at any wavelength in the ultraviolet, visible or near infrared. By working at wavelengths used for fiber optic communications, we can use these membranes for remote sensing through fiber lengths up to several kilometers. For the sensor arrays that are the object of this proposal, near infrared wavelengths can be chosen to prevent interferences from absorbing substances in the sample that might diffuse into the membrane. We can also use the same wavelength or range of wavelengths to sense the entire sensor array. We don't have to use different wavelengths for different sensing elements.
Selectivity: We have already demonstrated that we can prepare microspheres that undergo ion-selective swelling. Because our approach involves incorporation of neutral ionophores into the polymer, it allows us to take advantage of the large number of ionophores that have designed for ion-selective sensing.
We have also prepared a molecularly imprinted polymer that swells with increasing template concentration. This approach allows us to selectively sense neutral molecules. Furthermore, unlike biological receptors, the materials prepared by molecular imprinting are stable and suitable for long-term sensing.
Because we can make selective sensors we can design our array to target specific applications.
Response Time: We typically get response times that range from several seconds to a couple minutes, depending on the thickness of the hydrogel, the loading of the hydrogel with microspheres and the extent to which the analyte has to preconcentrated within each individual microspere to get swelling. By working with thinner membranes and using low microsphere loadings (on the order of a few percent by weight of the membrane), we expect to be able to keep response times in this range even for dilute samples. Our response times are acceptable for most industrial and environmental monitoring applications.
Processing: The processing required to make a membrane involves preparation of a microsphere suspension in a solution of monomer or polymer following by either polymerization or crosslinking to form the hydrogel. The processing depends on the hydrogel rather than the microspheres and, therefore, is the same for all sensor elements. Most of our membranes have been prepared by molding. However, we have already demonstrated the feasibility of preparing membranes by spin-coating. We also anticipate that our membranes can be screenprinted onto a substrate. This requires that we start with a microsphere suspension with the appropriate high viscosity for screenprinting. After it has been forced through a mesh onto the substrate, exposure to heat or light can be used to prepare the final hydrogel.
All of these technologies are easily adapted to make sensor arrays. Spin- coating can be following photopolymerization through a mask to selectively form membranes at particularly locations on a substrate. Screenprinting through a succession of screens can be used to prepare a membrane array. Molding can be employed to make an array. This involves dispensing different microsphere suspensions into each element of a multielement array followed by polymerization.
The ability to easily process our membranes to make sensor arrays is also central to our technology vision. The use of multiple sensors is required to correct for interferences and for matrix effects due to variations in parameters such as temperature and ionic strength. The us eof multiple sensors also allows us to build in redundance into our system by using multiple sensing elements of the same type and to monitor whether or not the array is functioning properly.
Low Cost: Both the preparation and instrumentation costs of the proposed sensing technology are low. We expect that screenprinting will be the preparation method of choice because it has minimal capital costs and will allow multiple arrays to be prepared on a single substrate.
Because membrane turbidity changes are large, array response is easily monitored with a low cost videocamera. Any radiation source can be used. A broadband near infrared transmitting filter can be used to exclude light in the visible and ultraviolet as necessary to avoid interferences from absorbers in the sample. Alternatively, we can use LEDs as the source with photodiode detectors to produce low cost, compact sensor system that would easily be battery powered for field use.
We envision that our research will lead to sensor array systems that can be used for process control and other monitoring applications involving aqueous media where the investigator has some idea of the composition of the sample. The microspheres would be chosen to detect the analytes in the particular sample. The array would respond to all analytes of interest that can be detected by swellable microspheres. It would have built-in redundance so that sensing element failures can be detected. Once calibrated, it could be used for monitoring the sample for extended periods of time. Factory precalibration is a possibility since multiple arrays can be prepared on a single substrate at the same time. Bioreactor monitoring is an example of a typical application. The measured parameters might include ion concentration, ionic strength, temperature, pH, food (glucose), carbon dioxide (coupled to pH sensing) and product buildup. By a combination of the approaches that are described in this proposal, we expect to be able to measure all these parameters. Another type of application would be environmental monitoring. For example, we could use a sensor array designed to detect atrazine detection. It would include molecularly imprinted microspheres designed to bind atrazine, reference microspheres prepared without atrazine as a template, temperature and reference sensors as well as possibly pH and ionic strength sensors. This array would use LEDs and photodiodes and would be battery powered for remote use. By using a low duty cycle, it would be suitable for longterm field applications.
Current research in this area includes the following specific projects:
- Synthesis of lightly crosslinked, porous polymer microspheres using emulsions prepared by forcing monomer solution through porous glass.
- Using HYPAN copolymers of polyacrylonitrile and hydrophilic monomers as a more permeable and stronger hydrogel for our membranes.
- Preparation of ion selective membranes based on neutral ionophores immobilized in polynitrophenol microspheres.
- Use of molecular imprinting to prepare microspheres that selectively respond to polar organics.
- Use of lightly crosslinked 2,4,5-trichlorophenylacrylate-vinylbenzyl chloride copolymers as substrates for preparing derivatized microspheres of controlled hydrophilicity and porosity.
- Development of poly(vinyl-4-pyridine) as a pH sensitive material.
- Development of sensor arrays.
- Remote sensing through fiber optics using our membranes.
- Remote sensing through fiber optics using our membranes.
In collaboration with Prof. Craig Grimes at the University of Kentucky, we are developing magnetochemical sensors. These sensors consist of a magnetoelastic strip coated by a swellable polymer membrane. The resonant frequency shifts as the polymer swells. These polymers have the important advantage that they can be interrogated remotely without an electrical connection or a line of sight.
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