Sensitive Water Probing through Nearly Monochromatic Red Emission in Er3+-Enriched Core-Shell Upconversion Nanoparticles
Authors: Zhang Ling , Liu Xiaomin , Lu Geyu 
Published: May 2020

Introduction

Water is not only the most precious resource on earth, but also the most important component of the organism, which is vital to life. However, sometimes even a very small amount of water is dangerous. When water is present in fuels, organic solvents, it can sometimes cause serious consequences. Therefore, moisture monitoring and measurement play a very important role in basic research, industrial synthesis and etc.
At present, the standard method for detecting moisture is the Karl Fischer method[1], which is complicated and time-consuming, therefore is not suitable for rapid determination and repeated measurement of moisture. The optical moisture sensor has the advantages of simple operation, fast response, etc. The rare earth-doped upconversion nanoprobes have the advantages of stable chemical properties, large anti-Stokes shift, and stable optical properties, which has attracted great attention in the sensor field[2-3].
Here, a water detection probe based on LiErF4: 0.5% Tm@LiYF4 under 808nm excitation was constructed to detect the trace water in DMF.

Method

TmCl3·6H2O (0.0025mmol) and ErCl3·6H2O (0.4975 mmol) were added to 3 mL oleic acid and 7 mL 1-octadecene. The solution was heated to 150℃ and stirred for 30 minutes before cooled down to 30℃. NH4F (2mmol ) and LiOH (1.25 mmol) dispersed in 5 mL methanol were added to the mixture, then heated to 80℃ lasted for 30 min to remove the methanol. The solution was then heated to 300℃ and keep for 90 min before cool down to room temperature. Finally, the LiErF4:0.5%Tm nanoparticles were washed by acetone and anhydrous ethanol and dispersed in 4ml cyclohexane.
YCl3·6H2O (0.25 mmol) was added to a mixture of 3 mL OA and 7 mL ODE and stirred continuously for 30 min. The solution was heated to 150℃ for 30min, then cooled down to 30℃, 5mL methanol containing NH4F (1mmol) and LiOH (0.625mmol) was added and then heated to 80℃ for 30min to remove the methanol, 2ml cyclohexane containing the core LiErF4:0.5%Tm nanoparticles was added and then heated to 90℃ lasted for 30min to remove the cyclohexane. Finally, the mixture was heated to 305℃ lasted for 90min. The LiErF4: 0.5% Tm@LiYF4
nanoparticles were obtained and washed with acetone and ethanol respectively, then dispersed in 4ml cyclohexane.
Nitrogen protection and continuous stirring are required throughout the experiment.
The oleic acid coated nanoparticles were dispersed in DMF(10mL) containing 0.2mmol NOBF4 and then ultrasonicated for 10 min to remove the oleic acid. The ligand-free nanoparticles were collected by centrifugation at 13000 rpm for 15 min, washed with DMF two times, and redispersed in DMF.

Results and Conclusions

Because of the high concentration of Er3+ ions in the nanoprobe, the interaction among Er3+ ions is very strong and easy to be affected by the environment. When there is water in the solution, the nanoprobe will transfer energy to the water molecules, leading to the upconversion emission quenching. The response sensing of water molecules was realized by detecting the red emission of nanoprobe with the change of water content in DMF.
The results showed that the red emission intensity of the nanoprobe was weakened obviously with the increase of water content in the solution and there is a good linear relationship between the logarithm of the change in emission intensity (ln(F/F0)) and water content (vol%) . The limit of detection in DMF can be down to 60ppm. The results are of great significance for the development of reusable water test strips and highly sensitive detection instruments.

References

[1] Liang Y. Y. Automation of Karl Fischer water titration by flow injection sampling, Analytical Chemistry, 62( 1990), 2504-2506. doi: 10.1021/ac00221a018.
[2] J. Zhou, Q. Liu, W. Feng, Y. Sun, F. Li, Upconversion Luminescent Materials: Advances and Applications, Chemical Reviews, 115(2015), 395-465. doi: 10.1021/cr400478f.
[3] Z. Li, T. Liang, S. Lv, Q. Zhuang, Z. Liu, A Rationally Designed Upconversion Nanoprobe for in Vivo Detection of Hydroxyl Radical, Journal of the American Chemical Society, 137(2015), 11179-11185. doi: 10.1021/jacs.5b06972.