The research objective of this award is to develop a new class of surface modification techniques that use programmable ferroelectric substrates to control the local hydrogen ion concentration (pH) at the solid/liquid interface. The key concept exploited in this work is that polarized ferroelectric films produce intense electric fields which can substantially alter the counterion concentration near the surface of the ferroelectric. By controlling the local polarization state of the ferroelectric film, it will be possible to precisely tune the local pH over a very large operating range (i.e. from as low as pH=0 to as high as pH=14). The research approach will employ a combination of characterization tools, including AFM, pH dyes, and self-assembled monolayers to study the local pH above polarized ferroelectric films. Computational techniques will also be developed to compare and interpret the experimental results.
If successful, the results of this research will provide an opportunity to create a compact device for synthesizing polypeptide micro-arrays via step and repeat synthesis using pH sensitive Fmoc protective blocking chemistry. Additionally, this research may be used to modulate surface wetting properties, providing an additional degree of control in microfluidic systems. Due to the highly interdisciplinary nature of this work, graduate and undergraduate students will have broad exposure to the fields of electromagnetism, chemistry, biology and computer science. Minority studies from Historically Black College and Universities (HBCU?s) or Historically Minority Universities (HMU?s), will be included in this work for two summer months. In addition, undergraduate students from Duke University will participate in this research through the Duke University Pratt Fellows program.
Ferroelectric thin films (FETFs) have the unique ability to maintain either a positive or negative surface charge in the absence of external power. The prefix "ferro" is not used to imply that it is a magnetic material (which it is not) but rather that the material can remember its electrical state much like a ferromagnet. Ferroelectric thin films act like a bistable switch for storing electric field. An external electric field can reprogram the electric polarization of the material, allowing for either negative or positive charge to be presented on the surface of the film. Most often, FETFs are used in electronic applications, such as in memory storage, capacitors, and other electronic devices. However, the biological applications of FETFs are few and far between. The lack of development in this area is curious, since many biological materials such as proteins, DNA, and cells, are charged and thus can be deposited on pre-programmed regions of the substrate. Likewise, ions in a fluid, including H+ and OH-, can be attracted to pre-programmed substrate regions, leading to locally high or low pH. We found that the fabrication of FETFs that can withstand exposure to water is extremely challenging, and we were not able to demonstrate the broader goal of implementing a locally programmable pH switch. Despite our struggle, we were able to develop new techniques for fabricating ultra-smooth, crack free FETFs that can withstand exposure to water for prolonged periods without virtually any affect on its electrical properties, crystal structure, surface roughness, or any adverse delamination effects. This is an important first step that is required for future biological applications. Our main findings are listed below: 1) We developed a low cost method for coating surfaces with FETFs through a technique known as "sol gel", which consists of spinning a liquid precursor onto a substrate and evaporating it to form a solid thin film. We discovered that smooth, crack free, thin films are very sensitive to the processing conditions. A specific temperature is needed for the first baking step, which is below the evaporation temperature of the solvent. If too high or too low temperature was used, then the films displayed significant cracking. We also determined that rapid annealing of the film at 600 degrees Celcius was the optimal procedure for producing crack free films. Normal baking procedures at high temperature led to significant cracking. 2) We demonstrated that the FETFs could be electrically programmed, and that its electrical state could be stored for prolonged periods. We also showed that the same spot could be re-programmed multiple times with no changes in its electrical properties. 3) We demonstrated that the FETFs could withstand water exposures of at least 18 hours without any change to its electrical properties, surface roughness, crystal structure, or any adverse peeling effects. We expect that these FETFs can withstand significantly longer time periods in liquid environments, however we have not tested this yet 4) We tested the force of attraction between the polarized regions of a FETF with a charged tip in water. This result demonstrated that the charged region of the FETF could interact with other charges in a fluid at long range. We believe these results pave the wave for new biological devices that employ FETFs. One of the reasons why we chose to investigate sol gel techniques to produce FETFs was because it is significantly lower cost than other approaches that rely on high temperature sputtering and evaporation of thin films. Thus, these results are promising because it will enable devices to be built more cheaply with similar properties.
