Fig. 13. (a) Variation in the emission intensity of the sensor assembly (N,S-CDs and o-phenylenediamine) with different concentrations of Cu²⁺ (inset: the digital photograph of sensor system under UV-light after applying Cu²⁺ in different concentrations) and (b) the corresponding linear plot of 557/470 nm emission intensity (F₅₅₇/F₄₇₀) with respect to Cu²⁺ concentration. Reprinted from ref. 200, Copyright (2023), with permission from Elsevier. (c) The UV-vis absorption spectra (inset: variation in absorption intensity), (d) temperature-dependent Stern–Volmer curves, and (e) PL decay profile (inset: variation in the measured lifetime) of bee pollen-derived WBCDs with different concentrations of Fe³⁺. Reprinted from ref. 172, Copyright (2022), with permission from Elsevier. (f) The non-linear plot of (F₀/F − 1) by applying different concentrations of Co²⁺ on pig skin-derived N-CDs (inset: linearity in the Co²⁺ concentration range of 1–100 μM). Reprinted from ref. 208, Copyright (2016), with permission from Elsevier.