Click Chemistry Functionalization of Harmonic Nanoparticles with Lanthanide Complexes Towards Tunable Platforms for Multimodal Imaging

Nanoplatforms combining multiple imaging contrast modalities are gaining interest across life sciences and beyond. Here, we disclose a proof-of-concept series of harmonic nanoparticles (HNPs) conjugated with a variety of lanthanide (Ln) complexes, enabling tunable imaging properties. Building on our previous approach for the conjugation of Gd(III) complexes at the surface of HNPs through copper-catalyzed click chemistry, we first establish a copper-free alternative by benchmarking the signals of the resulting conjugates in magnetic resonance imaging phantoms. We then extend this system to Eu, Tb and Yb conjugates and investigate their photophysical properties, successfully detecting long-lived Ln emissions spanning the visible and near-infrared spectrum. Interestingly, the Ln ion can be efficiently removed and exchanged, allowing reuse of the same HNP with a new optical signature. Most notably, we demonstrate that the Eu luminescence can be indirectly activated via second-harmonic generation from the HNP core upon femtosecond-pulsed irradiation in parallel to direct two-photon excitation. This nonlinear activation scheme paves the way for the preparation of mixtures with multidimensional optical signatures using a single excitation source. Altogether this work provides a versatile framework to further explore HNP-Ln conjugates as multimodal imaging probes.