Two-Dimensional Plasmonic Nanoparticle as a Nanoscale Sensor to Probe Polymer Brush Formation

Conventional analysis and characterization of polymer brush formation relies on laborious methods that use a quartz crystal microbalance, atomic force microscope, microcantilever, or other tools that measure the concentration change of solutions. Herein we develop a simple and easy method that utilizes intrinsically flat two-dimensional (2D) plasmonic nanoparticles as sensors for unveiling the mechanism of polymer brush formation on surfaces. Via ultraviolet-visible spectroscopy, the plasmonic nanoparticles can be used to determine the amount of polymers near the surface in situ. As the amount of polymers increases near the surface, the nanoparticle characteristic localized surface plasmon resonance wavelength redshifts, and the shift amount corresponds linearly to the polymer density near the surface. By functionalizing the nanoparticles in solutions of thiolated polyethylene glycol (PEG-SH) with or without PEG disulfide (PEG-S-S-PEG), the three-regime kinetics of the polymer brush formation is confirmed. The fast adsorption and slow chain rearrangement in the first regime are found to be the causes of the latent regime. In the latent regime, the adsorbed polymer chains rearrange to anchor their ends onto the surface and contract to liberate space so that other polymer chains can graft onto the surface until saturation. The fundamental understanding gained herein enables the design of surfaces with complex chemistries and properties, which can find broad applications in responsive sensors, films, and coatings. Moreover, the novel analytical method of using 2D plasmonic nanoparticle as a sensor to understand the polymer brush formation is applicable to investigating the grafting of other molecules such as self-assembled monolayers, protein, and DNA.