Crystallization-driven two-dimensional assemblies from a phenothiazine-conjugated poly(l-lactide): redox-responsive tunable emission, white-light harvesting and surface-enabled nanoparticle decoration

Abstract

Crystallization-driven self-assembly (CDSA) of amphiphilic crystallizable block copolymers represents a powerful tool for constructing precision nanostructures with uniform shapes and dimensions. To date, major efforts have been made towards achieving structural control by employing living-CDSA. Herein, we unravel a new approach for precision two-dimensional (2D) architectures that can exhibit redox-responsive surface-emission color tunability, including white-light harvesting from a single phenothiazine end-capped poly(L-lactide) homopolymer (PTZ-P1) without compromising the intrinsic 2D morphology. By CDSA in 20% isopropanol/water, PTZ-P1 produced lozenge-shaped nanoplatelets with multilayered thickness via crystallization of the poly(L-lactide) core, while the phenothiazine (PTZ) moieties occupying the 2D surface act as the soluble corona. These surface-bound PTZ units show selective and ratiometric sensing toward hypochlorite ions (ClO⁻), a key component of the reactive oxygen species, with a low detection limit of 1.54 μM. The sensing response is triggered by NaClO-induced oxidation of the surface-anchored PTZ to phenothiazine sulfoxide (OPTZ), as confirmed by X-ray photoelectron spectroscopy (XPS) and mass spectrometry data, which alters the emission color of the nanoplatelets from orange to cyan via an intermediate white-light-emitting state with CIE coordinates (0.31, 0.36), depending on the analyte concentration. The in situ formation of the white-light-emitting platelets was attributed to the partial oxidation of PTZ (orange-emissive) to OPTZ (cyan-emissive) on the 2D surface, as confirmed by confocal microscopy imaging. This yielded a broad-emission spectrum, ranging from 400 to 750 nm, producing white light. Building on this, co-assembly of PTZ-P1 with its oxidized counterpart, OPTZ-P1, separately synthesized from a phenothiazine sulfoxide initiator (OPTZ-OH), was successfully achieved for better control over the surface emission properties. At a composition of 7 : 1 (OPTZ-P1 : PTZ-P1), stable white-light-emitting platelets (CIE: 0.31, 0.32) were produced due to the dual color emission and contribution from Förster resonance energy transfer (FRET), with a 32% efficiency between the donor (OPTZ) and acceptor (PTZ) chromophores. In contrast, mixing preformed PTZ-P1 and OPTZ-P1 platelets at a 2 : 1 ratio also yielded near-white light via only additive color mixing, without FRET, from the spatially separated orange- and cyan-emitting platelets. Interestingly, surface-grafted PTZ moieties enabled photo-induced silver nanoparticle deposition, imparting conductive properties to the PTZ-P1 nanoplatelets.