Optical and Chiroptical\nStimuli-Responsive Chiral\nAgNPs@H-Leu-Poly(phenylacetylene) Nanocomposites in Water

Dynamic macroscopically chiral nanocomposites are prepared\nby combining\nsilver nanoparticles (AgNPs) and dynamic helical poly(phenylacetylene)s\n(PPAs) bearing pendants functionalized with amino groups. These amino\ngroups provide the nanocomposite with the ability to disperse in water\nalong with high stability due to the interaction between the ammonium\ngroup and the AgNP. Moreover, the equilibrium between NH<sub>3</sub><sup>+</sup>/NH<sub>2</sub> produces a “blinking” contact\nbetween the PPA and the AgNPs, which allows total control of the dynamic\nhelical behavior of the polymer. The use of acidic or neutral pH allows\ncontrolling the morphology of the nanocomposite, which consists of\na nanosphere that has trapped inside it a single AgNP (pH = 2) or\nseveral AgNPs (pH = 7) with ca. 30 nm of diameter. These nanocomposites\ncombine the optical and chiroptical stimuli-responsive properties\nof both components, AgNPs and PPAs. Thus, the controlled aggregation\nof the nanocomposite produced variations in the LSPR band of the AgNPs\nin a reversible manner. In turn, given that the chiral coating is\nselective to Ba<sup>2+</sup>, the presence of this metal ion caused\na helical inversion of the chiral coating of the nanocomposite detected\nby electronic circular dichroism. Moreover, it is possible to distinguish\nbetween three metal ions in different oxidation states, such as Ce<sup>4+</sup>, Fe<sup>3+</sup>, and Hg<sup>2+</sup>, which produce different\nresponses of the nanocomposite when oxidizing the AgNP to Ag<sup>+</sup>.