Near-infrared (NIR) persistent luminescence nanoparticles (PLNPs) with strong tissue penetration can avoid light scattering and fluoresce interference of tissues caused by in situ excitation, and they can be employed in the research on biological imaging and tumor diagnosis and treatment. Currently, the majority of reported PLNPs are based on Cr³⁺ as luminescent centers. The toxicity of heavy metal Cr³⁺-doped materials poses a potential safety hazard to long-term in vivo imaging tracking and therapy. However, Fe³⁺ as a basic element of the human body is a good candidate for NIR luminescence center with broadband emission. The longer and stronger emission wavelength of NIR, coupled with its superior penetration ability, further enhances the tissue penetration depth for biological applications. Thus, it is imperative to develop a friendly NIR-PLNP with enhanced luminescence performance for Fe³⁺-doped materials. We aim to develop Bi₂Ga_3.985O₉∶1.5%Fe³⁺, 1%Eu³⁺ (BGO∶1.5%Fe³⁺, 1%Eu³⁺) NIR-emission PLNP materials with stronger luminescence intensity and longer emission wavelength by co-doping Eu³⁺ ions based on BGO∶1.5%Fe³⁺ PLNP material. The prepared PLNP has excellent NIR luminescence and plays an important role in the in-vivo imaging without background noise and deep tissues.

BGO∶1.5%Fe³⁺,xEu³⁺ (x=0-2%) PLNP materials are prepared by the co-precipitation method. Meanwhile, we investigate the effects of the Eu³⁺ concentration and the calcination temperature on the luminescent properties and crystal structure of BGO∶1.5%Fe³⁺ PLNP material. The surface shape, element distribution mappings, valence distribution, energy transfer between Fe³⁺ and Eu³⁺, and luminescence lifetime of the BGO∶1.5%Fe³⁺, 1%Eu³⁺ PLNP material are observed and analyzed.

Firstly, the PLNP material characterization and X-ray diffraction peaks of the BGO∶1.5%Fe³⁺,xEu³⁺ (x=0-2%) PLNP materials are consistent with the Bi₂Ga₄O₉ planes crystal (PDF#76-2240). The TEM picture shows that the average grain diameter of BGO∶1.5%Fe³⁺, 1%Eu³⁺ PLNP material is about 100 nm (Fig. 1). The EDS spectra and the element distribution mappings of BGO∶1.5%Fe³⁺, 1%Eu³⁺ PLNP material indicate the presence of Bi, Ga, O, Fe, and Eu elements (Fig. 2). The XPS spectra of BGO∶1.5%Fe³⁺, 1%Eu³⁺ PLNP material reveal the presence of Bi, Ga, O, Fe, and Eu elements in a trivalent state (Fig. 3). Additionally, the BGO∶1.5%Fe³⁺, 1%Eu³⁺ PLNP material exhibits strong NIR emission at 798 nm with the highest luminescence intensity. The intensity of BGO∶1.5%Fe³⁺ PLNP material is enhanced by co-doping the Eu³⁺ ions to obtain BGO∶1.5%Fe³⁺, 1%Eu³⁺. The excitation spectra show that the four peaks are at 307 nm, 422 nm, 464 nm, and 636 nm, and then BGO∶1.5%Fe³⁺, 1%Eu³⁺ with the strongest is obtained [Figs. 4(a)-(b)]. BGO∶1.5%Fe³⁺, 1%Eu³⁺ improves the luminescence intensity and duration due to the energy transfer from Eu³⁺ to Fe³⁺ [Fig. 5(a)]. The two-dimensional thermoluminescence curves of BGO∶1.5%Fe³⁺, 1%Eu³⁺ PLNP picture show that the average electron trap energy level depth is 0.676 eV [Fig. 5(b)]. The CIE color coordinates picture shows that the co-doping of Eu³⁺ increases the red luminescence intensity of the material (Fig. 6). The intensity of BGO∶1.5%Fe³⁺ PLNP material is improved, and the average luminescence lifetime (τ_av) increases from 13.77 s to 15.56 s by co-doped Eu³⁺ ions (Table 2). The luminescence time of PLNPs is extended from 3 h to more than 8 h [Fig. 7(b)]. Finally, under the calcination temperature of 900 ℃ and calcination time of 1 h, the BGO∶1.5%Fe³⁺, 1%Eu³⁺ PLNP material has good crystallinity and NIR luminescence intensity (Fig. 8).

BGO∶1.5%Fe³⁺, xEu³⁺ (x=0-2%) PLNPs are prepared by the co-precipitation method. The effects of calcination temperature and co-doping amount of Eu³⁺ ions on the luminescence properties of BGO∶1.5%Fe³⁺ PLNP are investigated. The excitation and emission spectra analysis demonstrates the existence of energy transfer from Eu³⁺ to Fe³⁺, which enhances the luminescence intensity and time of PLNPs in the NIR emission (798 nm). The optimal form is obtained to the BGO∶1.5%Fe³⁺, 1%Eu³⁺ with 798 nm emission, and the average electron trap energy level depth is 0.676 eV. The average luminescence lifetime (τ_av) of BGO∶1.5%Fe³⁺ and BGO∶1.5%Fe³⁺, 1%Eu³⁺ increases from 13.77 s to 15.56 s, and the luminescence time extends from 3 h to more than 8 h. Thus, NIR luminescence has a high penetration depth by doped Fe³⁺, which is conducive to luminescent imaging. The NIR luminescence with 798 nm emission can eliminate the influence of spontaneous and scattered light, and improve the sensitivity and signal-to-noise ratio of detection and imaging. Therefore, the proposed material will have great potential applications in bio-sensing, deep tissue imaging, and image-guided therapy.