Fig. 1. Schematic representation of the NC@FMS all-fiber temperature sensor. (a)–(c) Schematic design of temperature sensing using the NC@FMS: (a) temperature-sensing strategies of nanomaterials relying on emission intensity, lifetime, peak position, and fluorescence intensity ratio (FIR); (b) schematic of total internal reflection of light in an FMS; (c) schematic of excitation and emission in an NC@FMS. (d) Schematic design of all-fiber temperature sensing based on the NC@FMS. The insets in panel (d) illustrate the purpose of different fiber segments.

Fig. 2. Properties of the designed NC@FMS all-fiber temperature sensor. (a) Transmission electron microscope (TEM) image of the as-synthesized NaYF4:20%Yb3+,2%Er3+@NaYF4 NCs. (b) SEM image of a silica FMS. (c) SEM images of the NaYF4:20%Yb3+,2%Er3+@NaYF4 NC@FMS and its surface structures. (d) Photoluminescence (PL) spectrum of the NaYF4:20%Yb3+,2%Er3+@NaYF4 NC@FMS all-fiber sensor under 980 nm fiber laser excitation. The insets are the corresponding microscopic fluorescent photograph and the enlarged spectrum from 365 to 490 nm. (e) PL spectra of the NaYF4:20%Yb3+,2%Er3+@NaYF4 NC@FMS all-fiber sensor with different fiber A [as labeled in Fig. 1(d)] lengths under 980 nm fiber laser excitation. The inset is the photograph of fiber A. (f)–(h) PL spectra of the NaYF4:20%Yb3+,2%Er3+@NaYF4 NC@FMS all-fiber sensor before, during, and after immersion in different environments for 1 min: (f) in cyclohexane, (g) in water, and (h) in alkali. The insets are the fluorescent photographs of the NC@FMS in different environments.

Fig. 3. Thermometric performance of the NaYF4:20%Yb3+,2%Er3+@NaYF4 NC@FMS all-fiber temperature sensor. (a) Green fluorescence response of the all-fiber sensor to temperature ranging from 225 to 465 K. The inset on the left is the microscopic fluorescent photograph of the NC@FMS at room temperature. Scale bar: 15μm. The inset on the right shows the energy levels from which emissions are observed. (b) Experimental data and fitted curve of FIR (H211/2/S43/2) against the absolute temperature. (c) Absolute and relative sensitivities of the all-fiber temperature sensor. (d) FIR (H211/2/S43/2) fluctuations of the all-fiber temperature sensor over time at the constant temperatures of 225, 300, and 465 K. (e) Temperature cycling test of the all-fiber temperature sensor between 225 and 465 K. (f) SEM images of the NC@FMS and its surface structures after the thermometric testing. (g) Comparison of the temperature response between the commercial thermistor and the all-fiber temperature sensor. (h) Emission intensity fluctuations of the annealed NC@FMS all-fiber temperature sensor over time when in an acidic environment.

Fig. 4. Applications of the NaYF4:20%Yb3+,2%Er3+@NaYF4 NC@FMS all-fiber temperature sensors in enclosed spaces and microscale areas. (a) Photograph of the commercial silica fiber wrapping around a glass rod. Scale bar: 5 mm. (b) Schematic of the NC@FMS all-fiber temperature sensor performing fixed-point temperature detection in an enclosed space. (c) Temperature changes with time in an enclosed space monitored using the NC@FMS all-fiber temperature sensor. The inset shows the corresponding photograph. (d) Temperature changes with time in 20μL of heated 1-octadecene (ODE) monitored using the NC@FMS all-fiber temperature sensor. The inset is the photograph of the NC@FMS when in the heated ODE. (e) Photograph of the NC@FMS all-fiber temperature sensor inserted in a capillary tube. (f) End face photograph of the capillary tube in panel (e). (g) Interior temperature distribution of the capillary tube in the thermal field measured using the NC@FMS all-fiber temperature sensor. (h) Interfacial temperature distribution between water and ODE measured using the NC@FMS all-fiber temperature sensor.

Fig. 5. Applications of the NaYF4:20%Yb3+,2%Er3+@NaYF4 NC@FMS all-fiber temperature sensors in complicated structures. (a) Temperature distributions of different planes in the micro-area 3D temperature field with a dimension of 5mm×5mm×5mm by measuring the temperature at 1 mm intervals using the NC@FMS all-fiber temperature sensor. (b) Photographs of the NC@FMS all-fiber temperature sensor measuring the interior temperature of an S-type capillary tube. (c) and (d) Interior temperature distributions of the S-type capillary tube when in (c) a cooling field or (d) a thermal field measured using the NC@FMS all-fiber temperature sensor. (e) Temperatures at different positions of the artificial blood vessel measured using the NC@FMS all-fiber temperature sensor. The insets show the photograph of the all-fiber sensor inserted in the artificial blood vessel and its inner diameter. The temperature at the location of the heating source in the temperature field is 38°C.