All-Liquid White-Light-Emitting Diode and Its Solid-State Counterpart from the Photophysical and Anionic Responses of a 2-Hydroxy-5-methylbenzaldehyde Molecule: Fabrication and Its Application in Ion Sensing

In this work, the simple fabrication of all-liquid white-light-emitting diodes (WLEDs) based on 2-hydroxy-5-methylbenzaldehyde (HMBZ) and gold nanoclusters (AuNCs) was reported for the first time, effective over a broad pH range (7.0–11.0) and a wide temperature window (20–60 °C). For the neutral to mildly alkaline pH region (up to ∼8.0), white-light emission (WLE) was primarily governed by the excited-state intramolecular proton-transfer (ESIPT) emission of HMBZ. In contrast, in strongly alkaline conditions (pH ≥ 9.6), the emission was predominantly contributed by the anionic form of the HMBZ molecule. The chromaticity parameters of the as-fabricated HMBZ-AuNCs system were adjusted from ice blue to cream white by varying the AuNCs concentration. For optoelectronic applications, the HMBZ-AuNCs white-light-emitting gel embedded in an agarose matrix was coated onto a commercial UV LED chip, resulting in intense white emission under UV irradiation, with Commission Internationale de l’Eclairage (CIE) coordinates of ∼(0.33, 0.36) and a high color rendering index (CRI) of ∼97, suggesting its potential in solid-state lighting. Remarkably, the WLEDs exhibited strong selectivity toward Zn2+ and Hg2+ ions, with impressive detection limits of 40 nM for Zn2+ and 0.26 μM for Hg2+.