The demands for multiplexed biological detection have driven the development of fluorescence encoding nanoprobes. We have synthesized water-soluble dual mode emission core–shell rare earth nanoprobes (∼30 nm) for fluorescence encoding. The nanoprobes were composed of ytterbium (Yb), erbium (Er) and/or thulium (Tm) ions co-doped heterogeneous NaYF4/NaLuF4 nanocrystals as cores and amorphous SiO2 embedded with europium (Eu) or terbium (Tb) complexes as shells. Excited by both infrared light and ultraviolet light, the nanoprobes exhibited dual characteristic emissions, which enable a novel spectral encoding strategy. The core nanocrystals exhibited tunable up-conversion emissions through various lanthanide ions doping. Combining these multiple upconversion emissions of core nanocrystals with downconversion emissions of shell containing rare earth complexes, a large amount of distinct fluorescence codes can be generated.

Ultra-small luminescent nanoparticles (NPs) are quite desirable for optoelectronic and biomedical applications. However, it is still a challenge to synthesize ultra-small NPs with high brightness owing to non-radiative energy losses caused by the surface defects as well as from vibrational deactivation ascribed to solvent molecules and ligands absorbed on the NPs. In this paper, we reported a strategy to improve up- and down-conversion luminescence of ultra-small BaLuF5:Yb3+,Er3+ NPs by using multi-layer active-shells (containing Yb3+). Sub-10 nm BaLuF5:Yb3+,Er3+@(X-shell, X = 1–5)BaLuF5:Yb3+ NPs were synthesized via a high boiling solvent process through a layer-by-layer strategy. Up- and down-conversion fluorescence spectra of the NPs were recorded and analyzed by using a 980 nm laser diode as the excitation source. In comparison with optical properties of BaLuF5:Yb3+,Er3+ NPs, the intensities of up- (∼545 nm) and down-conversion (∼1530 nm) fluorescence were enhanced by 52 and 9.8 times after coating 5-layer active-shells (BaLuF5:Yb3+) on the BaLuF5:Yb3+,Er3+ NPs, respectively. In addition, the intensities of up- and down-conversion fluorescence of the BaLuF5:Yb3+,Er3+ NPs with multi-layer active-shells were 1.3 and 1.1 times larger than those of the BaLuF5:Yb3+,Er3+ NPs with a one thick-layer active shell, respectively. These results showed that multi-layer active-shells could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shells.

We demonstrated optical amplification at 650 nm in KMnF3:Yb3+,Er3+@KMnF3:Yb3+ active-core–active-shell nanoparticle (NP) doped polymer waveguides pumped by a 976 nm laser diode for the first time. KMnF3:Yb3+,Er3+ NPs were synthesized via a solvothermal method. With the excitation of a 976 nm laser diode, bright red upconversion (UC) fluorescence was observed from KMnF3:Yb3+,Er3+ NPs owing to the existence of efficient energy transfer between Er3+ and Mn2+:2H11/2,4S3/2 + 6A1 → 4I15/2 + 4T1,2H9/2 + 6A1 → 4I13/2 + 4T1 and 4I15/2 + 4T1 → 4F9/2 + 6A1. The red UC emissions originated from the 4F9/2 → 4I15/2 transition of Er3+. Furthermore, the red UC emissions of KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs were enhanced by 7.5 times compared to that of KMnF3:18 mol% Yb3+,1 mol% Er3+ core-only NPs after coating an active shell containing Yb3+ ions on the core-only NPs. The above results showed that the active-shell could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shell. By using KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs as the gain medium and doping NPs into a polymer waveguide, we constructed polymer-based waveguide amplifiers. For an input signal power of 7.4 mW and a pump power of 45.2 mW, a relative optical gain of ∼3.5 dB was obtained at 650 nm in a 17 mm-long waveguide.

Silica-coated NaYF4:Yb/Er(Tm)/Eu nanocrystals (NCs) with a mean size of 35 nm were prepared and characterized. Each of the core/shell NCs can be dispersed in ethanol and water to form stable colloidal solutions and emit bright visible light of two colors (blue and red, green and red) by up- and down-converting excitation modes. As we know, this is the first time to obtain the distinct dual-color photos of NaYF4:Yb/Er(Tm)/Eu NCs which were dispersed in deionized water. In particular, the ability to optically manipulate luminescence color of NCs doped with RE ions opens the door to multiplexed detection for high precision in more complex biotic environment.

Ultra-small luminescent nanoparticles (NPs) are quite desirable for optoelectronic and biomedical applications. However, it is still a challenge to synthesize ultra-small NPs with high brightness owing to non-radiative energy losses caused by the surface defects as well as from vibrational deactivation ascribed to solvent molecules and ligands absorbed on the NPs. In this paper, we reported a strategy to improve up- and down-conversion luminescence of ultra-small BaLuF5:Yb3+,Er3+ NPs by using multi-layer active-shells (containing Yb3+). Sub-10 nm BaLuF5:Yb3+,Er3+@(X-shell, X = 1–5)BaLuF5:Yb3+ NPs were synthesized via a high boiling solvent process through a layer-by-layer strategy. Up- and down-conversion fluorescence spectra of the NPs were recorded and analyzed by using a 980 nm laser diode as the excitation source. In comparison with optical properties of BaLuF5:Yb3+,Er3+ NPs, the intensities of up- (∼545 nm) and down-conversion (∼1530 nm) fluorescence were enhanced by 52 and 9.8 times after coating 5-layer active-shells (BaLuF5:Yb3+) on the BaLuF5:Yb3+,Er3+ NPs, respectively. In addition, the intensities of up- and down-conversion fluorescence of the BaLuF5:Yb3+,Er3+ NPs with multi-layer active-shells were 1.3 and 1.1 times larger than those of the BaLuF5:Yb3+,Er3+ NPs with a one thick-layer active shell, respectively. These results showed that multi-layer active-shells could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shells.

The demands for multiplexed biological detection have driven the development of fluorescence encoding nanoprobes. We have synthesized water-soluble dual mode emission core–shell rare earth nanoprobes (∼30 nm) for fluorescence encoding. The nanoprobes were composed of ytterbium (Yb), erbium (Er) and/or thulium (Tm) ions co-doped heterogeneous NaYF4/NaLuF4 nanocrystals as cores and amorphous SiO2 embedded with europium (Eu) or terbium (Tb) complexes as shells. Excited by both infrared light and ultraviolet light, the nanoprobes exhibited dual characteristic emissions, which enable a novel spectral encoding strategy. The core nanocrystals exhibited tunable up-conversion emissions through various lanthanide ions doping. Combining these multiple upconversion emissions of core nanocrystals with downconversion emissions of shell containing rare earth complexes, a large amount of distinct fluorescence codes can be generated.

We demonstrated optical amplification at 650 nm in KMnF3:Yb3+,Er3+@KMnF3:Yb3+ active-core–active-shell nanoparticle (NP) doped polymer waveguides pumped by a 976 nm laser diode for the first time. KMnF3:Yb3+,Er3+ NPs were synthesized via a solvothermal method. With the excitation of a 976 nm laser diode, bright red upconversion (UC) fluorescence was observed from KMnF3:Yb3+,Er3+ NPs owing to the existence of efficient energy transfer between Er3+ and Mn2+:2H11/2,4S3/2 + 6A1 → 4I15/2 + 4T1,2H9/2 + 6A1 → 4I13/2 + 4T1 and 4I15/2 + 4T1 → 4F9/2 + 6A1. The red UC emissions originated from the 4F9/2 → 4I15/2 transition of Er3+. Furthermore, the red UC emissions of KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs were enhanced by 7.5 times compared to that of KMnF3:18 mol% Yb3+,1 mol% Er3+ core-only NPs after coating an active shell containing Yb3+ ions on the core-only NPs. The above results showed that the active-shell could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shell. By using KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs as the gain medium and doping NPs into a polymer waveguide, we constructed polymer-based waveguide amplifiers. For an input signal power of 7.4 mW and a pump power of 45.2 mW, a relative optical gain of ∼3.5 dB was obtained at 650 nm in a 17 mm-long waveguide.