Effect of ytterbium amount on LaNbO₄:Tm³⁺,Yb³⁺ nanoparticles for bio-labelling applications

This work investigates how Yb³⁺ concentration affects the luminescent properties of LaNbO₄ nanoparticles for medical imaging applications. Due to the highly transparent optical window for organic tissues in the near infrared region (650-1000 nm), upconversion fluorescence allows near infrared wavelengths to penetrate deeply into tissues, which is useful in biomedical areas such as biodetection, activated phototherapy, and screening.

Upconversion nanoparticles based on LaNbO₄ doped with Tm³⁺ and Yb³⁺ were prepared by the one-step industrial process called Spray Pyrolysis. Samples with different Tm³⁺:Yb³⁺ molar ratios (1:4, 1:8 and 1:16) were obtained.

The X-ray powder diffractograms of all the samples displayed the typical peaks of a crystalline material (tetragonal phase). Emission bands emerged in the blue, red, and near infrared regions, and they corresponded to the Tm³⁺¹G₄ → ³H₆ (475 nm), ¹G₄ → ³F₄ (650 nm), ³F_2,3 → ³H₆ (690 nm), and ³H₄ → ³H₆ (803 nm) transitions, which indicated a two-photon absorption process. As for bio-labelling application, the results indicated that Yb³⁺ concentration was directly related to signal intensity.

The intensity of positive conversion emissions depends directly on Yb³⁺ concentration. The bio-labelling tests pointed to the potential application of these materials. The sample containing the highest amount of Yb³⁺ provided better results and was easier to detect than the standard sample.