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 ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ NCs. (b) SEM image of a silica FMS. (c) SEM images of the ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ NC@FMS and its surface structures. (d) Photoluminescence (PL) spectrum of the ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ 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 ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ 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 ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ 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 ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ 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: $15\mu \mathrm{m}$. The inset on the right shows the energy levels from which emissions are observed. (b) Experimental data and fitted curve of FIR (${{}^2\mathrm{H}}_{11/2}/{{}^4\mathrm{S}}_{3/2}$) against the absolute temperature. (c) Absolute and relative sensitivities of the all-fiber temperature sensor. (d) FIR (${{}^2\mathrm{H}}_{11/2}/{{}^4\mathrm{S}}_{3/2}$) 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 ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ 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 $20\mu \mathrm{L}$ 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 ${\mathrm{NaYF}}_4:20\%{\mathrm{Yb}}^{3+},2\%{\mathrm{Er}}^{3+}@{\mathrm{NaYF}}_4$ 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 $5\mathrm{mm}\times 5\mathrm{mm}\times 5\mathrm{mm}$ 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.