Fig. 1. (a) TEM images, (b) size distribution, (c) EDXA, and (d) schematics and upconversion process of the UCNPs. (e) Emission spectra of the UCNPs dispersed in water under different excitation powers. The concentration of the UCNPs is set at 0.1% w/v. The inset graph shows a linear relationship between emission intensity and excitation laser power. (f) Transmission spectra of hydrogel incorporated with various concentrations of UCNPs.

Fig. 2. (a) Fabrication of the UCNPs-HOF by molding and UV-induced crosslinking. (b) Coupling of a 532 nm laser to the UCNPs-HOF. (c) Mechanical flexibility. The UCNPs-HOF can effectively guide light even when tied into a knot. (d) Photograph showing blue UCL emission of the UCNPs-HOF under the illumination of an excitation laser at 980 nm. (e) Optical setup for interrogation of the sensing UCNPs-HOF. (f) Long-term stability.

Fig. 3. (a) Dependence of the UCL spectrum on temperature. The inset image describes the corresponding experimental setup, where the sensor is immersed in a heating water bath (Tris-HCl buffer, pH=8.4, ionic strength=150 mM) and a thermocouple is employed for temperature calibration. (b) UCL intensities at 450 nm under different temperatures (Tris-HCl buffer, pH=8.4, ionic strength=150 mM). (c) UCL intensities at 450 nm under different pH values at room temperature.

Fig. 4. (a) Mechanism of the UCNPs-HOF for DA sensing. (b) Absorption spectra of DA and ox-DA, and emission spectrum of the UCNPs-HOF. (c) Emission spectra of UCNPs-HOF immersed in different samples containing DA, CaCl2, KCl, NaCl, glycine (Gly), L-glutamate (L-Glu), glucose (GLU), uric acid (UA), ascorbic acid (AA), and S-adenosylmetionine (SAM). The concentration of each sample is kept constant at 100 μM. (d) Selectivity of the UCNPs-HOF sensor.

Fig. 5. (a) UCL quenching ratios of the UCNPs-HOF for DA sensing at different pH values. The DA concentration is set at 100 μM. (b) Emission spectra of the UCNPs-HOF versus the concentration of DA (Tris-HCl buffer, pH=8.4, ionic strength=150 mM). (c) Corresponding calibration curve of the UCNPs-HOF for DA detection in the range of 0–200 μM. Inset shows a linear plot in a small range of 0–1 μM. (d) Time response of the UCNPs-HOF sensor.