(136e) Dual-Signal Downconversion Luminescent Nanoparticles Harnessing Changes in the Surface Dipole As a Novel Approach for Small Molecule Detection

Detection of biomolecules is vital for several applications in diagnostics such as drug discovery, food safety standards, defense, security, and environmental monitoring. Traditional analytical tools such as the enzyme-linked immunosorbent assay (ELISA), enable the capture and detection of proteins through antibody-antigen binding interactions, while oligonucleotide microarray and sequencing technologies (e.g., polymerase chain reaction-PCR) allow for the identification and discovery of specific nucleic acid targets. However, these approaches are generally incompatible with the growing need for point-of-care (POC) diagnostics according to the set of criteria proposed by the World Health Organization (WHO). Alternatively, POC diagnostics are moving toward nanomaterial-based label-free biosensors that utilize a dynamic photoluminescence (PL) response to the local environment for high sensitivity detection of (bio)molecules. Rare earth (RE)-doped metal oxides have been employed as sensing agents due to their unique optical properties. The well-defined energy levels of RE elements are relatively insensitive to the crystal host due to the shielding of the partially filled 4f orbitals by the filled 5s and 5p shells, which is a bottleneck for label-free dynamic PL sensing. This can be mediated by incorporating non-RE dopants such as Bi³⁺ to dynamically sensitize the RE PL. Our previous work demonstrated that by spatially controlling the position of Bi³⁺ and Eu³⁺ with respect to the NP surface, using a core-shell (CS) architecture, it is possible to systematically tune the luminescent intensity via the surface chemistry. Specifically, when the Bi³⁺ ions are separately doped in the shell layer, closer to the surface, while the Eu³⁺ ions in the core are shielded from the surface quenchers. The hybridization of the Bi³⁺/VO₄^3− energy levels was observed to be a function of the dipole generated by the surface decorated ligands (NH₂-BZA and NO₂-BZA). The dipole-induced difference in the hybridization of the Bi³⁺/VO₄^3− energy levels resulted in changes in energy level overlap between Bi³⁺/VO₄^3− and Eu³⁺ ions, changing the energy transfer (ET) mechanism and altering the Eu³⁺ luminescence. Moreover, only a small decrease in luminescence lifetimes of the functionalized CSNPs has been observed for both ligands in CSNPs compared to core NPs.

As a proof of concept, the developed CSNPs (YVO₄: Eu³⁺|YVO₄: Bi³⁺) were functionalized with biotin for label-free sensing of avidin based on the changes in the local surface dipoles. The CSNPs exhibited high avidin selectivity and sensitivity with a detection limit of ~7.8 nM, signal-to-noise ratio (SNR) of 25.1, and a wide dynamic range (1 nM-10 µM) in DI water. The application of the assay in a complex biological matrix was then verified with good avidin sensitivity (detection limit of ~34.7 nM, SNR of 11.7). Single molecule detection is valuable; however, there is a growing need to develop multiplexed biosensors capable of sensitive monitoring of multiple analytes. Lack of multiplexing capabilities is one of the major limitations involved with upconversion-based sensors, which can be addressed using downconversion PL CSNPs. To add an extra detecting element to the existing CSNPs, Tb³⁺, Eu³⁺, and Bi³⁺ were systematically doped in a NaYF₄ core-YVO₄ multi-shell architecture. NP size, morphology, and dopant distribution were optimized through multiple annealing steps and further characterized using XRD, TEM, and ICP-OES, respectively, resulting in comparable PL signals from both Eu³⁺ and Tb³⁺. The structuring, spatial composition, and surface functionalization were simultaneously manipulated to produce dual-signal dynamic luminescent core/shell/shell materials with unique ET pathways. Specifically, the PL signals of the two RE dopants were selectively tuned by modulating the direction and magnitude of the surface dipole moment, which confirmed the dual-signal detection capability of the CSNPs. The added Tb³⁺ signals in the core-multi shell design can be used as either a control signal for detecting single target analytes or as an extra detecting channel for multi-target analytes detection. Leveraging the known ET mechanisms between the three dopants and host materials highlights future potential for RE and none-RE dopants that can be added to engineer more PL sensing channels. This inexpensive, label-free, and multiplexed CSNP has the potential to be coupled with different surface ligands (e.g., peptides and aptamers) for the detection of small molecules, enzymes, antibodies, and toxins with high sensitivity and reliability in clinical and POC diagnosis settings.