Wattle-Bark-Tannin-Derived Carbon Quantum Dots as Multi-Functional Nanomaterials for Intelligent Detection of Cr6+ Ions, Bio-Imaging, and Fluorescent Ink Applications

Abstract

Using natural wattle bark tannin, a kind of stable, nontoxic, fluorescent carbon quantum dots (CQDs) was hydrothermally fabricated and applied as a multifunctional fluorescent nanomaterial for the determination of heavy metal ions (Cr6+ and Co2+), bio-imaging, and fluorescent ink applications. The CQDs could be assembled with polyvinyl alcohol (PVA) through hydrogen bonding to yield a composite fluorescent hydrogel that can be integrated with a smartphone to constitute a novel and convenient intelligent detection system for the rapid (10 s) real-time monitoring of Cr6+. Both the CQDs (with 1.37 μM of LOD (limit of detection)) and CQDs-PVA hydrogel (with 3.36 mg/L of LOD) sensing systems produced fast, sensitive, and selective response to Cr6+ based on the internal filtering effect and electron-transfer effect. The practicability of the CQDs-PVA fluorescence sensor was demonstrated by determining tap water and tanning wastewater samples. Based on their satisfactory fluorescence stability and solubility, the CQDs were further used for HeLa-cell imaging and as fluorescent ink for information encryption. When HeLa cells were incubated for 24 h in CQDs at a high concentration of 200 mg/L, the cell survival remained as high as 90%, and a clear fluorescence image was observed under a laser confocal fluorescence microscope. The CQD solution could be written directly as fluorescent ink on TLC (thin layer chromatography) paper, and the handwritings were invisible after drying, confirming their application potential in the field of information encryption. In summary, the present CQDs derived from wattle bark tannin exhibited excellent stability, sensitivity, and specificity in heavy metal ion sensing and also had great potential in bio-imaging and information encryption applications.