A multi-functional, stimuli-responsive material includes a substrate functionalized with a pH-sensitive Azo-QPS compound or co-assemblies containing Azo-QPS compounds. The Azo-QPS compound includes a positively-charged phenyl-azo-pyridinium core, an anion affiliated with the core, a head group, a tail group, a surface bonding group coupling the pH-sensitive Azo-QPS compound to the substrate, and a spacer connecting the pH-sensitive Azo-QPS compound to the surface bonding group.