Using a simple UV-vis extinction spectrometer, the scientists recorded the extinction spectrum of gold nanorods adsorbed on glass substrates following the deposition of a monomolecular polymer layer in air and water (see image).

The refractive index of swollen polymer layers can be calculated by fitting the localized surface plasmon resonance (LSPR) shift measured in air and water with an exponential equation. Based on the refractive index of swollen polymer layers, two different effective medium models, namely Maxwell-Garnett and Bruggeman, are employed to calculate the percent swelling of the polymer film (in this case, 26% and 28%).

Validation using AFM

AFM imaging along the edge of an intentional scratch in a PEM film comprising of nine bilayers in air and water was used to confirm that the swelling of the polyelectrolyte layers was in agreement with the value estimated using the plasmonic nanotransducers (gold nanorods).

The technique can be employed to study a variety of phase transitions in ultrathin polymers (such as glass transition, melting, crystallization), which involve perceivable changes in the refractive index of the polymer layer.

Further information can be found in the journal Nanotechnology.