Nanopaper based test strips: Design a bacterial nanocellulose based ratiometric fluorescent chemoprobe for rapid screening of mercury ions.

Heavy metal ions pollution is a serious issue in environmental protection and human health. Mercury (II) (Hg2+) ions are known as the most hazardous pollutants. Therefore, due to the high toxicity of Hg2+ even at a trace concentration, the development of convenient, rapid, sensitive and portable probe for detection of mercury ions is of particular interest in environmental monitoring [1]. In this study, a simple and effective ratiometric fluorescence nanoprobe has been used to fabricate nanopaper based test strips for visual selective detection of Hg2+. Bacterial cellulose nanopaper (BC) was chosen as a flexible and transparent substrate [2] for developing convenient, rapid, sensitive and portable probe. In designing sensors based on cellulosic substrate, there are always necessary to propose a low cost and simple technique for providing hydrophobic-hydrophilic channel on the surface of substrate. Herein, several nanopaper configurations, including cuvettes and plates have been produced by printing desired pattern with a common laser printer for the first time. The proposed nanosensor is composed by simply mixing blue emitting carbon dots (Citric acid and Urea combination) and orange fluoresce Rhodamine B. The fluorescence emission of CDs is selectivity quench in the presence of Hg2+ through effective electron/energy transfer process, which is arises from the electrostatic interaction and metal-ligand coordination between functional group at the surface of CDs and mercury ions, whereas the fluorescence of RhB is insensitive to Hg2+. This results in an obviously continuous color change from violet to orange with increasing the concentration of Hg2+, which can monitor and analyze with smartphone. As a result of color variation the value of red (R) in RGB based sensor is almost constant, while blue (B) decrease according to quenching efficiency of CDs. Therefore, the ratio of R/B can be linearly related to the concentration of Hg2+. The ratiometric fluorescent test strips has been successfully applied for on-site identification of mercury ions in real samples like lake water and tap water with the detection limit of 0.82 µM.