Sensing mode (ratiometric or single-channel) regulated by excitation wavelength: Phosphorescent Ir(III) emitters bearing different substituents and their oxygen-sensing properties after bonding to blue-emitting carbon dots.

The detection of oxygen gas, which is a life-supporting gas and an important oxidant, is an important task in various fields, and optical sensing is an attractive detection technique owing to its multiple advantages. In this work, we designed five neutral Ir(III) probes with a general structure of [Ir+(C^N)2(N^N)-] for oxygen-sensing (C^N and N^N denoted a phenyl-1-phenyl-1H-benzo[d]imidazole ligand and [2,2'-bipyridine]-4,4'-dicarboxylic acid, respectively). The N^N ligand was responsible for charge balance, and the C^N ligands were modified with different groups (-CH3, -CH2Ph, -Ph, and -OMe). The -Ph group improved the luminescence and oxygen-sensing performance of [Ir+(C^N)2(N^N)-], showing a red emission peaking at 585 nm with the highest emission quantum yield (33.6%) and the longest emission lifetime (3.54 μs) among these five Ir(III) probes. Using blue-emitting carbon dots as the host, the optimal Ir(III) probe was covalently bonded to the carbon dots, forming a composite structure of BCDs@Ir-Ph with a probe doping level of 13.8 wt%, which was confirmed by TEM, Zeta potential analysis, XRD, XPS, IR, and elemental analysis. There was energy transfer from the carbon dots to the Ir(III) probe with an efficiency of 34.4%. Due to the separated excitation windows of the carbon dots and Ir(III) probe, BCDs@Ir-Ph could either emitted only the probe emission at 593 nm when excited by 449 nm or simultaneously emitted probe emission at 593 nm and BCDs emission at 450 nm when excited by 295 nm, which endowed the BCDs@Ir-Ph with both single-channel and ratiometric oxygen-sensing abilities. Linear working plots were observed with quenching constants of 0.397 L/mg (when excited by 295 nm) and 0.384 L/mg (when excited by 449 nm), with the LOD (limit of detection) values of 0.033 mg/L and 0.026 mg/L, respectively. The BCDs@Ir-Ph showed a fast-sensing response of 35 s, good storage stability within 14 days, and high selectivity over competing ionic species, but was sensitive to H2O2. The sensing mechanism was revealed to be dynamic by emission lifetime comparison and EPR (electron paramagnetic resonance) evidence.