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Electrostatic self-assembly is a well-known technique to form nearly monodisperse nanoparticles using coulombic interactions between oppositely charged species in the aqueous phase. In this study, the opposite charges originate from cationically charged polyelectrolytes and multivalent organic counterions. Self-assembly of these charged, multi-responsive building blocks leads to forming a wide range of shapes, for example, spheres, rods, ellipsoids, cylinders, etc. Various external triggers such as pH, light irradiation, and charge ratio could be used to tune these electrostatic self-assembled structures.
In the present work, small-angle neutron scattering (SANS) plays a crucial role in determining the formation mechanisms and structure-directing effects. As a model system, a divalent azo dye (AY38, Acid Yellow 38) and cationic polyamidoamine (PAMAM) dendrimers were used as the key components to construct self-assembled structures in aqueous solution. The isomerization capability of AY38 and pH-responsiveness of the dendrimers make them suitable candidates to alter the size and shape of self-assembled particles through light irradiation and degree of protonation. In particular, the preparation method has been modified to gain insight into the formation mechanism (see Scheme). Slow assemblies' growth was observed time-dependently whilst the dye-molecules reconverted to the trans-isomer. A home-built device is designed to analyze the structural changes using in-situ UV irradiation on the SANS instrument. Various complementary methods are used towards understanding the formation and restructuring mechanism, especially dynamic and static light scattering (DLS/SLS), zeta potential, isothermal titration calorimetry (ITC), and UV/Vis spectroscopy.
