Silver‑Embedded Polymer Microfibers for Enhanced Up‑Conversion Optical Sensing

Abstract

We report a facile fabrication route for polymer microfibers that incorporate up‑conversion nanoparticles (UCNPs) co‑doped with silver (Ag). The resulting LiYF₄:Yb³⁺/Er³⁺/Ag microfibers exhibit a transmission loss of α = 108.94 cm⁻¹ and a temperature sensitivity of 0.0095 K⁻¹ at 303 K, outperforming their Ag‑free counterparts (0.0065 K⁻¹). The enhanced performance stems from efficient photon‑to‑plasmon conversion in the Ag nanoinclusions, which amplifies the up‑conversion luminescence and improves wave‑guiding. This platform offers a low‑cost, scalable route to high‑performance optical temperature sensors and photonic components.

Background

Up‑conversion nanoparticles (UCNPs) doped with lanthanide ions are increasingly exploited in bio‑imaging, lasers, displays, and photovoltaics due to their large anti‑Stokes shift, chemical stability, and resistance to photobleaching. However, the weak absorption cross‑section of lanthanide ions limits the fluorescence yield. Embedding metallic nanostructures, particularly silver, into UCNPs or polymer matrices has been shown to enhance local electromagnetic fields, thereby boosting up‑conversion efficiency. While Ag‑doped UCNPs have been studied in bulk or thin‑film form, their integration into microfibers—key building blocks for on‑chip photonics—remains unexplored.

Experimental and Method Section

Materials

Analytical‑grade silver powder, chloroform, cyclohexane, NaOH, NH₄F, and ethanol were sourced from Shanghai Chemical Company and used without further purification.

Preparation of LiYF₄:Yb³⁺/Er³⁺ Nanoparticles

UCNPs were synthesized via thermal decomposition. A 100 mL three‑neck flask contained LnCl₃ salts (Lu:Yb:Er = 78:22:1) in 15 mL 1‑octadecene and 6 mL oleic acid, heated to 150 °C to clear the solution. After degassing, 4 mmol NH₄F and 2.5 mmol NaOH were added to 10 mL methanol, stirred for 30 min, then the mixture was heated to 300 °C (50 °C min⁻¹) under Ar for 1 h. Nanoparticles were centrifuged (4000 rpm), washed with ethanol, and dried at 60 °C for 12 h.

Fabrication of Ag‑Co‑Doped Fibers

0.003 g Ag, 0.005 g UCNPs, and 0.6 g PMMA were dispersed separately in 15 mL, 12 mL, and 18 mL of cyclohexane or chloroform. The PMMA solution was gradually added to the Ag and UCNP solutions and stirred for 30 min until transparent. A flame‑heated drawing technique was used to pull microfibers from the mixed solution, which were then cut into segments (Fig. 1).

Fabrication process of Ag‑co‑doped microfibers: (a) pulling; (b) cutting.

Spectra Measurement

Figure 2 illustrates the experimental setup. Microfibers were excited with a 980 nm CW laser (0.998 mW) and positioned on a glass substrate. An ×20 objective (NA = 0.4) collected the transmitted light, while a CCD camera recorded emission spectra. Temperature‑dependent measurements were performed with an Ocean Optics spectrometer.

Experimental setup for wave‑guiding.

Results and Discussion

Structure and Transmission Properties

XRD patterns confirmed the pure tetragonal LiYF₄ phase (JCPDS #17‑0874). SEM images (Fig. 3b) show smooth, uniform‑diameter fibers (~6 µm). TEM and EDS (Fig. 3c‑d) verify homogeneous Ag dispersion within the polymer matrix. XPS spectra (Fig. 4) confirm the presence of Li, Y, F, Yb, Er, and Ag, with Ag 3d binding energy at 359.08 eV, indicating successful tri‑doping.

Characterization of LiYF₄:Yb³⁺/Er³⁺ and Ag‑co‑doped microfibers.

FTIR (Fig. 5a) shows characteristic C–H, C=O, and Ag–O vibrations, confirming polymer and Ag incorporation. TGA (Fig. 5b) reveals two degradation steps: moisture loss below 333 K and polymer decomposition between 333–393 K, indicating thermal stability up to ~332 K.

FTIR and TGA of Ag‑co‑doped microfibers.

Photoluminescence imaging (Fig. 6) demonstrates that Ag‑co‑doped fibers transmit green emission efficiently along the entire length, whereas Ag‑free fibers exhibit stronger self‑absorption and Rayleigh scattering. Quantitative analysis of intensity versus guiding distance (Fig. 7) yields loss coefficients of 108.94 cm⁻¹ (Ag) and 231.72–274.84 cm⁻¹ (Ag‑free). The reduced loss in Ag‑doped fibers is attributed to enhanced photon‑to‑plasmon coupling and tighter mode confinement.

Transmission loss vs. guiding distance for Ag‑co‑doped and Ag‑free microfibers.

Energy Levels and Thermal Effects

Up‑conversion spectra show green bands at 522 nm (2H_11/2→4I_15/2) and 541 nm (4S_3/2→4I_15/2) and a red band at ~660 nm (4S_3/2→4F_9/2). These emissions arise from sequential energy transfer between Yb³⁺ pump ions and Er³⁺ activators.

Temperature‑dependent measurements (Fig. 9) show that the fluorescence intensity ratio (FIR) between the two green bands follows an exponential relationship (Eq. 2) with a linear fit of the natural logarithm versus 1/T. The absolute sensitivity (Eq. 3) reaches 0.0095 K⁻¹ at 303 K for Ag‑co‑doped fibers, surpassing the 0.0065 K⁻¹ of Ag‑free fibers. This enhancement confirms that Ag nanoparticles significantly boost the thermometric performance of UCNP microfibers.

3D up‑conversion spectra (Ag) and FIR vs. temperature.

3D up‑conversion spectra (Ag‑free) and FIR vs. temperature.

Conclusions

We have demonstrated a scalable, low‑cost method to produce silver‑embedded polymer microfibers loaded with LiYF₄:Yb³⁺/Er³⁺ UCNPs. Comprehensive structural, optical, and thermal analyses confirm that Ag nanoinclusions enhance wave‑guiding, reduce transmission loss, and increase up‑conversion intensity, leading to a 45 % improvement in temperature sensitivity at room temperature. These findings open a pathway to compact, high‑performance optical sensors and photonic devices that harness plasmonic‑enhanced up‑conversion.

Availability of Data and Materials

All data are fully available without restriction.