Single-Particle Polydiacetylene Sensors: Harnessing Monomer Reservoirs for Reconfigurable Chromic Architectures

ConspectusPolydiacetylene (PDA) is widely used in chromic sensing because it converts a broad range of stimuli into large, easy-to-read color and fluorescence changes. These responses have been extensively studied across diverse material formats, including nanovesicles, thin films, and fibrous assemblies, where chromism is typically interpreted in terms of stimulus-induced distortion of the conjugated backbone and headgroup packing. In this Account, we focus on microscale PDA particles produced by emulsion-based or microfluidic approaches. These particles are characterized by a polymerized outer layer that encloses an internal domain rich in unpolymerized diacetylene (DA) monomers, arising from the finite penetration depth of UV-induced polymerization. This structural feature introduces an additional chemical dimension to PDA chromism. Under solvent or thermal stimuli, these monomers can dissolve, migrate, and polymerize, creating a monomer-driven reconfiguration pathway involving dynamic structural rearrangement that complements conventional backbone-distortion chromism and enables chromic behavior with new forms of structural and optical response.This Account takes a single-particle view of PDA chromism and shows how that hidden monomer reservoir can be turned into a design feature. Using optical tweezers, we place individual PDA microparticles at controlled fluid-fluid interfaces and follow, in real time, how solvent infiltration generates chromic fronts and internal voids within one particle rather than across an ensemble. These experiments connect capillarity, particle topology, and chromic response, and they reveal how residual monomer participates in the restructuring. Mechanical inputs can likewise be read at the level of a single particle: a fluorogenic PDA sphere trapped in a stenosis-mimicking microchannel integrates shear and impact events into a mechanofluorescent signal, distinguishing viscous loading from discrete collisions of nanoparticles, red blood cells, or yeast.Building on these mechanistic insights, we design PDA particle architectures that deliberately use monomer mobility, confinement, and interfacial control. Core-shell PDA@PDMS particles contain a PDA core that houses a reservoir of unpolymerized PCDA, surrounded by a permeable PDMS shell. Upon solvent exposure, PCDA dissolves and diffuses into the PDMS layer, and subsequent UV-induced polymerization converts the migrated monomer into new blue phase PDA domains in both the core and shell. This solvent-driven monomer redistribution and polymerization constitute a reconfiguration process that yields semireversible, dual-region chromic signatures, enabling discrimination of even closely related organic solvents. Thermally driven reorganization at tuned interfacial tension produces Janus PDA microparticles whose two lobes, shaped by distinct monomer histories, respond differently to heat and solvent, enabling directional and ratiometric sensing. A gas-shearing microfluidic platform encapsulates PDA in alginate, producing monodisperse, biocompatible beads with strong polarity-selective solvatochromism.Together, these systems illustrate how a dynamic monomer reservoir, coupled with confinement and interface engineering, links molecular-scale reconfiguration to device-level performance. The single-particle PDA platform developed here offers practical molecular insight into chromic behavior in particulate systems and provides a foundation for designing semireversible, directional, and tunable sensors. These concepts may also be adaptable to other chromic conjugated polymers.