Core or Shell? Er³⁺ FRET Donors in Upconversion Nanoparticles

Abstract

Upconversion nanoparticles (UCNPs) are of high interest for biosensing because of their unique near-infrared-excitation and visible-emission features. An emerging field within UCNP biosensing is the detection of biological interactions through Förster resonance energy transfer (FRET). However, the relatively large size, the distribution of emitting lanthanide ions within the nanoparticle, the unknown photoluminescence (PL) quantum yields (QY) of these emitting ions, and the many available core-shell architectures make the interpretation of UCNPbased FRET data extremely difficult. Here, we present a detailed

Upconversion nanoparticles for sensing pH

“…The estimated value for QD was based on the range provided by Muhr et al 21 and Mattsson et al 31 QD is different from the overall quantum yield of the UCNP, which is much lower due to multiple possible transitions between the various lanthanide energy levels and surface quenching. 29 For UCNP/CaR, the efficiency goes from 99+% to 4% when distance between UCNP and dye is 0.4 nm and 3 nm, respectively. Similarly for UCNP/ARS, the efficiency decreases Please do not adjust margins Please do not adjust margins from 99+% to 2% when distance is 0.4 nm versus 3 nm, respectively.…”

“…Equation 2 describes the dependence of the Förster distance on various factors of the system of interest: k 2 is the dipole orientation factor, QD is the quantum yield of the Er 3+ donor, J is the spectral overlap integral of the UCNP and dye, NA is Avogadro's number, and n is the refractive index of the medium. The Förster distance values for the UCNP/CaR and UCNP/ARS UC-RET pair combinations were estimated to be 1.78 nm and 1.52 nm, respectively, assuming a QD of 0.01, k 2 of 2/3 (due to the long lifetime of UCNPs), 29 and n of 1.48 (for NaYF4). 30 We calculated a J of 2.6 ´ 10 14 nm 4 M -1 cm -1 for the UCNP/CaR pair and 1.0 ´ 10 14 nm 4 M -1 cm -1 for the UCNP/ARS pair.…”

“…Nevertheless, the UC-RET efficiencies allow us to estimate distances between donor and acceptor using Equations 1 and 2 (Table 4), the reverse of the theoretical calculations of RET efficiencies performed earlier. Based on acceptable boundary values for QD (on the order of 0.1% to 5%), 29,31 the UCNPanthraquinone distances were estimated to be 1-3 nm given a UC-RET efficiency of 0.09. With a 23.8 nm UCNP core, shell thickness of 0.4 nm, and PEI thickness of 2.1±0.1 nm (from TEM), this suggests that only the Er 3+ ions close to the surface of the UCNP, within the Förster distance, are involved in RET, while the inner Er 3+ ions contribute to radiative energy transfer to the dyes through reabsorption.…”

Energy Transfer in Dye‐Coupled Lanthanide‐Doped Nanoparticles: From Design to Application

“…In this regard, early efforts have been taken to prepare core–shell NPs in which all of the lanthanide emitters are confined to the shell layer (Figure b). However, this design was less effective because the near‐surface emitters suffered from more serious surface‐quenching effects . To overcome this issue, a three‐layered sandwich structure was developed (Figure c) .…”

“…However,t his designw as less effective because the near-surfacee mitters suffered from more seriouss urface-quenching effects. [12] To overcome this issue, at hree-layered sandwich structure was developed (Figure 3c). [13] As ap rominent example, Liu et al preparedathreelayered NaYF 4 @NaYF 4 :Yb,Er(Tm)@NaYF 4 core-shell-shell NP in which the active dopants (Yb 3 + ,E r 3 + and Tm 3 + )w ere confined to the middle layer and further protected by an outer layer of inert NaYF 4 .…”