The Influence of Surface Modification on the Shortwave Infrared Emission of Rare-Earth-Doped Nanoparticles

Purpose

Deep tissue imaging can be achieved using shortwave infrared (SWIR) light, ranging from 900 to 2500 nm in wavelength. SWIR light has several advantages, such as low scattering, reduced photobleaching and autofluorescence, and high sensitivity for biological samples. One of the most efficient materials for SWIR emission is rare-earth-doped nanoparticles (RENP), but they are hydrophobic and incompatible with biological systems. Therefore, we use common surface modifiers, such as polyethylene glycol (PEG) and Tween 20 (Tw), to improve the biocompatibility and dispersibility of RENP in aqueous media. This study aims to evaluate the effects of PEG and Tw as surface modifiers on the stability and SWIR emission intensity of RENP.

Methods

Using the thermal decomposition method, we prepared RENP (NaYF₄: Yb, Er) and modified their surface with PEG and Tw. RENP were modified with PEG and Tw using simple phase inversion and sonication-assisted methods, respectively. We characterized the RENP-PEG and RENP-Tw by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, zeta potential, and thermogravimetric analysis (TGA). We also measured the SWIR emission and spectra of the RENP-PEG and RENP-Tw using a Ninox 640 VIS-SWIR InGaAs camera with a 980 nm NIR laser excitation. To assess the biocompatibility of RENP-Tw, we performed an MTT assay with L929 cells.

Results

The XRD and FTIR analyses confirmed the successful surface modification of RENP and the formation of the hexagonal phase β-NaYF₄. The FTIR spectra showed the characteristic peaks of the functional groups associated with PEG and Tw. The surface modification also changed the zeta potential values of RENP, indicating different surface charges. The stability studies revealed that RENP-Tw remained well-dispersed in aqueous media after 24 h, while RENP-PEG aggregated over time. The RENP-Tw showed bright SWIR emission and a prominent peak at 1385 nm. The biocompatibility assay revealed that RENP-Tw did not cause significant cytotoxicity even at high concentrations (400 μg/mL) for 24 h.

Conclusion

Based on our findings, we propose that Tw is a suitable modifier for rare-earth-doped nanoparticles' performance as a SWIR agent, as it improves their stability properties in aqueous media, biocompatibility, and luminescence emissions.