An energy transfer kinetic probe for OH-quenchers in the Nd³⁺:YPO₄nanocrystals suitable for imaging in the biological tissue transparency window

Abstract

Tetragonal xenotime-type yttrium orthophosphate (YPO4) Nd(3+) doped nanoparticles suitable for biomedical applications were prepared by microwave-hydrothermal treatment. We applied the energy transfer probing based on the analysis of kinetics of impurity quenching to determine the presence and spatial position of -OH fluorescence quenching acceptors in the impurity-containing nanoparticles. We show that the impurity quenching kinetics of the 0.1 at% Nd(3+) doped YPO4 nanoparticles is a two stage (ordered and disordered) static kinetics, determined by a direct energy transfer to the -OH acceptors. Analyzing the ordered stage, we assume that the origin of the -OH groups is the protonation of the phosphate groups, while analyzing the disordered stage, we assume the presence of water molecules in the mesopores. We determine the dimension of the space of the -OH acceptors as d = 3 and quantify their absolute concentration using the disordered Förster stage of kinetics. We use the late stage of kinetics of fluorescence hopping (CDD ≫ CDA) quenching (the fluctuation asymptotics) at 1 at% Nd(3+) concentration as an energy transfer probe to quantify the relative concentration of -OH molecular groups compared to an optically active rare-earth dopant in the volume of NPs, when energy migration over Nd(3+) donors to the -OH acceptors accelerates fluorescence quenching. In doing so we use just one parameter α = γ(A)/γ(D) = n(A)√[C(DA)]/n(D)√[C(DD)], defined by the relation of concentration of the -OH acceptors to the concentration of an optically active dopant. The higher is the α, the higher is the relative concentration of -OH acceptors in the volume of nanoparticles. We find α = 2.95 for the 1 at% Nd(3+):YPO4 NPs that, according to the equation for α, and the results obtained for the values of the microparameters CDD(Nd-Nd) = 24.6 nm(6) ms(-1) and CDA(Nd-OH) = 0.6 nm(6) ms(-1), suggests twenty times higher concentration for acceptors other than donors. As the main result we have established that the majority of -OH acceptors is located not on the surface of the Nd(3+):YPO4 nanoparticles, as many researchers assumed, but in their volume, and can be either associated with crystal structure defects or located in the mesopores.

Phase composition and morphology of nanoparticles of yttrium orthophosphates synthesized by microwave-hydrothermal treatment: The influence of synthetic conditions

“…After the treatment all the samples were centrifuged, washed several times with deionized water and air-dried at 50 o C for 10 hours. The synthesis method can be easily modified in order to obtain rare-earth doped nanoparticles [15,17].…”

“…It is well known that the luminescent properties of the lanthanide-based materials may vary significantly depending not only on the compound phase, but also on whether they are in the nano or bulk state [15,16]. In turn, luminescent properties of nanomaterials depend on many factors, including sample crystallinity, nanoparticle surface state and, most importantly, the presence of the functional groups that can serve as luminescence quenchers.…”

“…In turn, luminescent properties of nanomaterials depend on many factors, including sample crystallinity, nanoparticle surface state and, most importantly, the presence of the functional groups that can serve as luminescence quenchers. Elsewhere [15,17,18] we report the detailed studies on the luminescent properties of the Nd 3+ and Ce 3+ doped YPO 4 nanoparticles. In particular, we show that -OH groups of water molecules can quench fluorescence of rare-earth ions very effectively, thus decreasing the fluorescence quantum yield.…”

Neodymium-doped nanoparticles for infrared fluorescence bioimaging: The role of the host

“…Along with concentration quenching, other factors such as surface hydroxyls (-OH) and defects in the volume like mesopores filled by the -OH groups are responsible for nonradiative losses in rare earth-doped materials which lead to reduced fluorescence lifetimes. [49][50][51][52] Since neither Nd 3þ concentrations nor surface treatments are the same in all the Nd:NPs studied in this work, the average lifetime values given in Table III do not constitute a general comparison table but rather a comparison of the fluorescence lifetimes of assynthesized Nd:NPs. Despite it cannot be considered as a general comparison table, some general trend can be extracted from data included in Table III.…”

Approaches to contactless optical thermometer in the NIR spectral range based on Nd 3+ doped crystalline nanoparticles

“…3). The fluorescence kinetics, except for short initial stage associated with fluorescence quenching caused by vibrations of -OH groups positioned inside the nanoparticles [13], demonstrates the exponential decay (Fig. 11).…”