Tuneable fluorescence and structural colour in PNIPAM microgel assemblies

Abstract

The combination of fluorescence and structural colour in colloidal photonic glasses could be the starting point for the design of dynamic multiple responsive colloidal photonic materials for advanced applications in which the fluorescence intensity could be modulated by stimulus-induced colour changes. The merocyanine functionalization of the PNIPAM-based microgel system made it possible to obtain tuneable photonic colloidal glasses displaying vivid structural colours and fluorescence. For this purpose, the MC was introduced in some of the crosslinking points of the inner structure of a PNIPAM-based microgel system. These building blocks were left to organize in water (pH 4.5) to obtain photonic glasses. The structural colour was studied as a function of water concentration, so that increasing the microgel concentration led to a blue colour shift. The PBG was characterized by diffuse reflectance spectroscopy. Reflection imaging microscopy was used to understand the microstructure responsible for the PBG. It could be determined that the microgel particles decreased their diameter from 430 nm up to 220 nm to adjust to the glass organization. It has been found that MC fluorescence intensity in these assemblies depended on the photonic band gap (PBG) position in such manner, that strong quenching of MC fluorescence was observed when the PBG matched the MC excitation wavelength. Besides, the MC fluorescence excitation band was 10 nm shifted to higher wavelengths.