Reorientation of Polymers in an Applied Electric Field for Electrochemical Sensors.

This mini review investigates the relationship and interactions of polymers under an applied electric field (AEF) for sensor applications. Understanding how and why polymers are reoriented and manipulated by under an AEF is essential for future growth in polymer-based electrochemical sensors. Examples of polymers that can be manipulated in an AEF for sensor applications are provided. Current methods of monitoring polymer reorientation will be described, but new techniques are needed characterize polymer response to various AEF stimuli. The unique and reproducible stimuli response of polymers elicited by an AEF has significant potential for growth in the sensing community.

Application of Voltage in Dynamic Light Scattering Particle Size Analysis.

Dynamic light scattering (DLS) is a common method for characterizing the size distribution of polymers, proteins, and other nano- and microparticles. Modern instrumentation permits measurement of particle size as a function of time and/or temperature, but currently there is no simple method for performing DLS particle size distribution measurements in the presence of applied voltage. The ability to perform such measurements would be useful in the development of electroactive, stimuli-responsive polymers for applications such as sensing, soft robotics, and energy storage. Here, a technique using applied voltage coupled with DLS and a temperature ramp to observe changes in aggregation and particle size in thermoresponsive polymers with and without electroactive monomers is presented. The changes in aggregation behavior observed in these experiments were only possible through the combined application of voltage and temperature control. To obtain these results, a potentiostat was connected to a modified cuvette in order to apply voltage to a solution. Changes in polymer particle size were monitored using DLS in the presence of constant voltage. Simultaneously, current data were produced, which could be compared with particle size data, to understand the relationship between current and particle behavior. The polymer poly(N-isopropylacrylamide) (pNIPAM) served as a test polymer for this technique, as pNIPAM's response to temperature is well-studied. Changes in the lower-critical solution temperature (LCST) aggregation behavior of pNIPAM and poly(N-isopropylacrylamide)-block-poly(ferrocenylmethyl methacrylate), an electrochemically active block-copolymer, in the presence of applied voltage are observed. Understanding the mechanisms behind such changes will be important when trying to achieve reversible polymer structures in the presence of applied voltage.

Insights on the Lower Critical Solution Temperature Behavior of pNIPAM in an Applied Electric Field.

Poly(N-isopropylacrylamide), or pNIPAM, is a free-radical polymer that is commonly studied for uses in surface coatings, tissue engineering, energy storage, biosensing, and more, due to its temperature responsiveness. pNIPAM is known to solubilize at temperatures below its lower critical solution temperature (LCST) and agglomerate above its LCST. This behavior has been shown to be reproducible and reversible. We confirmed this reversibility and the value of the LCST by performing dynamic light scattering (DLS) with a temperature sweep (increase and decrease). However, performing the same experiment under an applied voltage from copper electrodes, we observed a decrease in the LCST of pNIPAM and irreversible aggregation. Here we present preliminary data comparing the LCST behavior of pNIPAM in the presence of applied voltage using copper, aluminum, and carbon electrodes. We present data in support of the hypothesis that a phenomenon is occurring specifically with the use of copper electrodes that is altering pNIPAM LCST behavior.