Blue-red self-calibrating signals for aqueous oxygen from a composite structure of carbon dots and Ru probe

Abstract

The detection of dissolved oxygen in aqueous solutions is an important issue in various fields. In this work, we prepared efficient blue-emitting carbon dots (CDs, emission wavelength=453 nm, quantum yield=25.8%)) using the fibers of Euonymus bungeanus as a starting material through a classic hydrothermal method, serving as the reference, and red-emitting Ru(II) compounds (emission wavelength=705-710 nm, quantum yield=9.2-10.4%), which harvested the blue emission and were thus excited by the CDs, serving as the probe. Ru(II) complexes with terpyridine-derived ligands having different bonding groups (-NH₂ and -COOH) were compared to find the optimal sensing performance. The covalent bonding between the carbon dots and the Ru(II) probe was expected to exhibit ratiometric sensing signals for dissolved oxygen. The Ru(II) probes and the CDs were discussed carefully, including single molecular structure, electronic transition, absorption, emission, excitation, quantum yields, and lifetime, which confirmed an energy transfer from the CDs to the Ru(II) probe. The resulting composite sample (denoted as Ru1@CDs) was analyzed by micromorphology analysis, XRD, IR, and elemental analysis. A probe loading level was determined as 18.45%, with an energy transfer efficiency (CDs → probe) as high as 26.3%. Ru1@CDs showed good sensing selectivity and a linear correlation with the concentration of dissolved oxygen, with a fitting equation of $I_0/I=1.462+3.716·\left[O_2\right]$, a LOD of 0.12 mg/L, and a short response time of ∼140 s. The sensing mechanism was proven as a dynamic collision between the Ru(II) probe and O₂. The novelty of this work was the combination of two luminescent components and the corresponding emission color change from deep-red at low [O₂] levels (∼0.6 mg/L) to blue at high [O₂] levels (∼10 mg/L).