Phenothiazine-cyanine-functionalized upconversion nanoparticles for LRET and colorimetric sensing of cyanide ions in water samples

• Phenothiazine-cyanine-functionalized upconversion nanoparticles are devised.
• The probe utilizes upconverting luminescence to detect cyanide ions.
• The detection limit is much lower than the record from a similar platform.
• The platform could detect cyanide ions with high sensitivity and selectivity.
• The cyanide probe is able to work well in real water samples.

Upconversion nanoparticles (UCNPs) could serve as an excellent signaling unit for sensing due to their advantages such as near-infrared excitation, no autofluorescence, low damage to samples and no photobleaching. In this work, a detection platform comprising UCNPs modified with phenothiazine-cyanine (PTZCy) is achieved for efficient cyanide detection based on the luminescent resonance energy transfer (LRET) process. The UCNPs is modified with γ-cyclodextrin (γ-CD) to increase the solubility in water and meanwhile to generate a hydrophobic cavity for PTZCy and realize detection of CN− in pure water. PTZCy has a significant absorbance with a maximum at 542 nm, which ideally overlaps the emission band of UCNPs at around 540 nm and results in a LRET process to quench the green emission. After addition of CN−, the absorbance intensity of PTZCy at 542 nm decreases dramatically, and the LRET process is terminated and the green emission from UCNPs will be recovered, concurrently, the solution color of PTZC-UCNPs changes from light purple to colorless. This upconverting luminescence recognition of cyanide ions could be finished as soon as the CN− is added, while all the other anions have no influence on the detection process. The detection limit (0.84 μM) is much lower than the reported value obtained from a UCNP-Ir complex based probe (37.6 μM). In addition, the CDs could protect the organic dyes from photobleaching and photodegradation. The results indicate that the cyanide probe in this work is superior when compared with the other UCNPs based cyanide probes. This newly developed sensor allows efficient sensing of CN− in pure water, moreover, it also works well in real water samples taken from underground water or a lake.