Layer-by-Layer Assembly of Microgel Colloidal Crystals via Photoinitiated Alkyne–Azide Click Reaction

Abstract

Layer-by-layer (LBL) assembly of colloidal crystals (CCs) allows for the fine control of the thickness and architecture of the resulting crystals. Various methods have been developed for the LBL assembly of CCs of hard spheres. However, these methods are inapplicable for microgel CCs owing to the softness and deformability of microgel spheres. In this study, a method was proposed for the LBL assembly of microgel CCs. To build the first monolayer, azide-modified microgel spheres were assembled into a three-dimensional (3D) CC. The first 111 plane of the 3D CC close to the substrate was then fixed in situ onto the substrate via photoinitiated alkyne–azide click reaction between the azide groups on the microgels and the alkyne groups on the substrate surface. The removal of unbonded particles resulted in a microgel monolayer with a high degree of order. The second monolayer was assembled in a similar manner, i.e., a 3D microgel CC was initially assembled followed by in situ fixation of the first 111 plane of the 3D crystal with the underlying microgel monolayer by photoinitiated alkyne–azide click reaction. For this purpose, instead of azide-modified microgel spheres, alkyne-modified microgel spheres were used for the assembly of the second layer. Confocal studies confirmed that the second monolayer was located on top of the first layer. When the lattice constant of the 3D CC approximated that of the underlying microgel monolayer, the second monolayer exhibited a high degree of order. Repeating this process led to alternating deposition of highly ordered monolayers of azide-modified and alkyne-modified microgels onto the substrate. Similar to the microgel CCs obtained by the self-assembly of microgel spheres in bulky dispersions, face-centered cubic and hexagonal-close-packed structures also coexisted in the LBL-assembled microgel CCs.

Light-Triggered Click Chemistry

“…However, the inhibition of reaction is completely reversible; the reaction restarts by purging the solution with argon along with photoirradiation. Because of these features, the light-induced CuAAC reactions have been applied extensively to material sciences. ,,,− …”

Click Chemistry Mediated by Photochemical Energy

“…These properties make light induced CuAAC reactions as an attractive approach to be utilized in material chemistry. [39][40][41][42][43][44][45][46][47]…”

Surface‐Bound Microgels for Separation, Sensing, and Biomedical Applications

“…[86] Liang et al fabricated 3D crystalline microgel arrays via LbL deposition, where covalent crosslinking of the microgels was conducted after every deposition cycle. [87] Laser diffraction, AFM, and optical microscopy were conducted after each layer deposition to evaluate the yield and homogeneity of the coating, revealing a highly structured hexagonally packed microgel multilayer.…”

“…Liang et al also implemented covalent crosslinking to construct surface-bound, highly ordered, crystalline microgel multilayers. [87] P(NIPAm-co-AA) microgels were initially divided in two groups and functionalized via carbodiimide coupling with either azide-and alkyne-displaying moieties (Figure 12). A combination of UV crosslinking and copper-mediated "click" chemistry after each coating was used to immobilize the microgels resulting in a stable 3D LbL-type crystal network of soft microgels.…”