Doping the polymer brush with gold nanoparticles results in a switchable composite that changes its thickness depending on the pH. Research by physicists at the Darmstadt University of Technology in the journal Soft Matter can be used to design chemical nanosensors for diagnostic or environmental analysis.
Polymer brushes are macromolecular chains that are densely grafted on the surface. Due to the electrostatic force, the chain extends from the surface and forms a fur-like layer having a thickness of several hundred nanometers. Currently, research has focused on the design of polymer systems that respond to various environmental stimuli such as pH, temperature or specific biomarkers. Physicists at TU Darmstadt and TU Berlin demonstrated for the first time how the thickness of polymer brushes can be switched by combining pH-sensitive gold nanoparticles.
"The combination of polymer chains and gold nanoparticles is promising, especially in medical diagnostics or environmental analysis," Dikran Boyaciyan said. The 30-year-old doctoral student works in the "Interfacial Soft Matter" group led by Professor Regin von Kleinzin.
“This technology is still in the early stages of development, and the main goal is how the interaction between the polymer system and the nanoparticles can be adjusted and calibrated in a controlled environment,” Boyaciyan explained. Smart polymer materials can be used to report chemical nanosensors for toxins or cancer cells, monitor organ parameters or target drug release into the human body.
Boyaciyan tested two types of pH-insensitive polymers for its long-term use as a sensor: non-ionic PNIPAM and cationic PMETAC. The former is considered to be unsuitable because the gold particles are washed away from the brush at high pH. However, in cationic PMETAC brushes, gold particles are not affected by changes in pH.
In addition, Boyaciyan was able to demonstrate how to prepare a reversible pH switch complex from PMETAC by incorporating gold nanoparticles and how their complex formation works. In an acidic environment, the particles lose charge and particle-particle interactions and particle-brush interactions occur. This causes the brush to swell because its chain is less restricted.
In contrast, an alkaline environment creates a negative charge on the particles and interaction with a positively charged brush is preferred. The collapse of the chain causes the brush layer to become thinner.
Since the thickness variation also affects the spectral composition of the reflected light, the material can be used as a colorimetric nanosensor. Due to its very small size, it may be coupled to microlasers and spectrometers, which in the future can be used in laboratory chip systems and even in human cells.
