(4z) Understanding of Polymers in Confinement: Fluorescence Based Approach

Nanoscale investigation of functional polymers has significant importance in a wide range of applications, such as fuel cells, antifouling, chemobiosensing, optoelectronic materials and so on.  When the film thickness approaches to several tens of nanometers, the optical, mechanical and transport properties are dominantly controlled by the interfaces. The mobility of the polymer chains is constrained due to severe confinement effect. The strength of interaction of water with polymer and substrate governs the film wetting, density and stiffness. Tuning these interactions with a view to approaching to desired material properties is a unique challenge. The key step to improve materials design is to understand the structure-property relationships. There is a large database available for ionomer membranes. However, very little is known about thin films. The challenges start when the conventional bulk characterization techniques (e.g. NMR, TGA, FTIR, standard tensile techniques) fail to predict the nanoscale changes in materials properties and sample preparation for a specific measurement becomes too tough (e.g. making free standing ultrathin films, stringent and specific substrate requirement). Fluorescence based techniques can overcome many of these issues and can be used as an important tool in both energy related research and chemobiosensing.

The knowledge of hydration behavior is truly beneficial for molecular level understanding and advanced level designing of polymer-catalyst interface in fuel cells.  Fluorescent probes sensitive to local viscosity and proton concentration, when incorporated into ionomer thin films, gives valuable insight into mechanical and proton transport properties of thin films as a function of polymer structure, hydration and film thickness when they emit. The application of fluorescence based techniques in fuel cell research is quite new, has already started offering interesting insights and has a lot of scope of work.

One of the most fascinating applications of fluorescence based techniques is solid state nanobiosensing. The fluorescent conjugated oligomers/polymers and/or dyes with high quantum yield, when incorporated into nanoscale self-assembled thin films, respond to subtle changes in the microenvironment (pH, ionic strength) of the films and senses biomolecules (DNA, protein), metal ions and so on. Fluorescence resonance energy transfer (FRET), photoinduced charge transfer (PCT) etc. which requires fluorescent partners within ~10 nm scale distance shows superior response in ultrathin films with unique sensitivity and selectivity to the solid state sensing platform. The cost of detection can be reduced due to simplicity of techniques (e.g. steady state fluorescence), but not at the cost of signal efficiency.

Tailoring the polymer structure, film density and using suitable fluorescent moiety might solve a lot of mysteries hidden inside confined geometries.

Publications:

1. Liu, B.; Dishari, S. K. “Synthesis, Characterization, and Application of Cationic Water-Soluble Oligofluorenes in DNA-Hybridization Detection” Chem. Eur. J. 2008, 14, 7366-7375.
2. Dishari, S. K.; Kan-yi, P.; Liu, B. “Combinatorial Energy Transfer between an End-Capped Conjugated Polyelectrolyte and Chromophore-Labeled PNA for Strand-Specific DNA Detection” Macromol. Rapid Commun., 2009, 30, 1645-1650 (cover paper).
3. Dishari, S. K.; Hickner, M. A. “Antiplasticization and Water Uptake of Nafion Thin Films” ACS Macro Lett., 2012, 1, 291−295 (cover paper).
4. Dishari, S. K.; Hickner, M. A. “Confinement and Proton Transfer in Nafion Thin Films” Macromolecules, 2013, 46 (2), pp 413–421.
5. Modestino, M. A.; Paul, D. K.; Dishari, S.; Petrina, S.; Allen, F. I.; Hickner, M. A.; Karan, K.; Segalman, R. A.; Weber, A. Z. “Self-Assembly and Transport Limitations in Confined Nafion Films” Macromolecules, 2013, 46 (3), pp 867–873.