Colorimetric quantification of linking in thermoreversible nanocrystal gel assemblies

Abstract

Nanocrystal gels can be responsive, tunable materials, but designing their structure and properties is challenging. By using reversibly bonded molecular linkers, gelation can be realized under conditions predicted by thermodynamics. However, simulations have offered the only microscopic insights, with no experimental means to monitor linking leading to gelation. We introduce a metal coordination linkage with a distinct optical signature allowing us to quantify linking in situ and establish structural and thermodynamic bases for assembly. Because of coupling between linked indium tin oxide nanocrystals, their infrared absorption shifts abruptly at a chemically tunable gelation temperature. We quantify bonding spectroscopically and use molecular simulation to understand temperature-dependent bonding motifs, revealing that gel formation is governed by reaching a critical number of effective links that extend the nanocrystal network. Microscopic insights from our colorimetric linking chemistry enable switchable gels based on thermodynamic principles, opening the door to rational design of programmable nanocrystal networks.

Assembling Inorganic Nanocrystal Gels

“…(d) TEM images of all-inorganic linker gels with insets of the corresponding NC dispersions (left) and gels (right). Adapted with permission from (a) ref , Copyright 2020 American Chemical Society; (b) ref , Copyright 2016 John Wiley and Sons; (c) ref , Copyright 2022 The Authors, under CC BY-NC 4.0 () (replotted from the original); (d) ref , Copyright 2010 American Chemical Society.…”

“…More recently, coordination bonding was employed to induce thermoreversible gelation of ITO NCs whose terpyridine-terminated ligands were interconnected via the addition of Co 2+ . With increasing temperature in the presence of concentrated Cl – , the linking bonds (cobalt­(II)–(bis)­terpyridine complexes) were gradually diminished in favor of the formation of cobalt­(II)–tetrachloride complexes, ultimately inducing phase transition of a gel into a free-flowing dispersion.…”

“…More recently, coordination bonding was employed to induce thermoreversible gelation of ITO NCs whose terpyridine-terminated ligands were interconnected via the addition of Co 2+ . 54 With increasing temperature in the presence of concentrated Cl − , the linking bonds (cobalt(II)− (bis)terpyridine complexes) were gradually diminished in favor of the formation of cobalt(II)−tetrachloride complexes, ultimately inducing phase transition of a gel into a free-flowing dispersion. The gel-to-sol transition at a threshold temperature was confirmed visually as well as via a decrease in scattering at low momentum vector (q) using in situ temperature-dependent small-angle X-ray scattering (SAXS) (Figure 4c).…”

Linker-Templated Structure Tuning of Optical Response in Plasmonic Nanoparticle Gels

“…Assemblies of metal nanoparticles, whose conduction electrons interact collectively with light by localized surface plasmon resonance (LSPR), display rich optical properties. − Structure-dependent LSPR coupling in these materials produces frequency-tunable absorption and strong electromagnetic fields in gaps between closely spaced nanoparticles (“hot spots”), enabling applications from molecular detection and characterization to energy harvesting and data storage. ,− This sensitivity to nanoparticle spatial organization can render plasmonic assemblies responsive, capable of dynamically reconfiguring due to physicochemical, − electromagnetic, ,− or mechanical − stimuli to form structures with distinct optical near- and far-field spectral features.…”

Depletion-driven assembly of polymer-coated nanocrystals

“…Through assembly, NC-based materials acquire emergent properties as evidenced by, e.g., shifts in optical absorption of tin-doped indium oxide (ITO) NCs upon gelation. [3][4][5] The spatial arrangement, i.e., structure, of nanoparticles in hybrid materials can significantly impact ion or proton conductivity [6][7][8][9] and mechanical properties. [10][11][12][13] Develop-ing strategies to systematically tune the interactions that influence nanoparticle structure and structure-dependent properties of functional nanomaterials is essential for their design.…”