Abstract
Aqueous solutions are the basis for most biomedical assays, but they quench the upconversion luminescence significantly. Surface modifications of upconverting nanoparticles are vital for shielding the obtained luminescence. Modifications also provide new possibilities for further use by introducing attaching sites for biomolecule conjugation. We demonstrate the use of a layer-by-layer surface modification method combining varying lengths of negatively charged polyelectrolytes with positive neodymium ions in coating the upconverting NaYF4:Yb3+,Er3+ nanoparticles. We confirmed the formation of the bilayers and investigated the surface properties with Fourier transform infrared and reflectance spectroscopy, thermal analysis, and ζ-potential measurements. The effect of the coating on the upconversion luminescence properties was characterized, and the bilayers with the highest improvement in emission intensity were identified. In addition, studies for the nanoparticle and surface stability were carried out in aqueous environments. It was observed that the bilayers were able to shield the materials’ luminescence from quenching also in the presence of phosphate buffer that is currently considered the most disruptive environment for the nanoparticles.
Highlights
The strongest vibrations from poly(acrylic acid) (PAA) and PP could be distinguished at different sections in the recorded spectra, the main vibrations of PAA being around 1000−1750 cm−1 and of PP being around 900−1400 cm−1.41 No quantitative conclusions could be made from Fourier transform infrared (FT-IR) spectra, but with the materials prepared with PAA Mw 100 000 and both lengths of PP having three bilayers, the polyelectrolyte-related vibrations seemed to be the strongest (Figure S2)
We have demonstrated that using various lengths of polyelectrolytes, the layer-by-layer method can be utilized for surface passivation and can offer further functionalization of upconverting nanoparticles
We observed that selected bilayer formations were able to shield the upconversion luminescence and prevent the disintegration of the nanoparticles even in the phosphate buffer that has been proven to enhance the disintegration.[20,21,48]
Summary
Materials exhibiting upconversion luminescence[1] have been studied intensively for the last decade because of their potential use in photovoltaics,[2,3] biomedical imaging and theranostics,[4−6] and in biomedical assays.[7−9] Especially, the research in the biomedical field has benefitted from the extensive development of upconversion material syntheses, resulting in nanoscale materials down to 5 nm.[10−13] Currently, hexagonal β-NaYF4:Yb3+,Er3+ is considered to be the most efficient material in upconversion.[14,15] Its superiority is thought to arise from the low phonon energy in fluoride lattice[16] as well as shorter R−R distance compared with the cubic structure of NaYF4:Yb3+,Er3+.17 the unique properties of upconversion nanoparticles have advantage in the biomedical applications, such as less scattering of light, photodamage of tissue, and minimal autofluorescence, they still have drawbacks that need to be solved.[18]One of the most critical problems of upconverting nanoparticles use in biomedical applications is the significant quenching of the upconversion luminescence in aqueous environment due to OH− vibrations in water.[19]. One method of improving the emission intensity in aqueous environments is to produce more efficient materials. This has been made possible, for example, with core−shell structures[22,23] or using plasmonic enhancement.[24,25] especially in the cases where the shell is composed from similar fluoride structure as the core, it could be debated that it is vulnerable to the disintegration mentioned previously. Preventing the disintegration of the materials is vital, because of the optical changes harming the reproducibility
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