Chemical and Biological Sensing Strategies Based on Upconversion, Metallic and Magnetic Nanoparticles and Their Assemblies

Upconversion nanoparticles (UCNPs) are extremely versatile rare-earth ion (RE) doped nanocrystals capable of converting near-infrared light into shorter wavelengths (ultraviolet/visible) <i>via</i> multi-photon mechanisms. They have been increasingly used for many applications, but it is probably in the field of sensing where they best evidenced their potential thanks to low cytotoxicity, high photostability, large anti-Stokes shifts, long lifetimes, and reduced background noise. Luminescent sensors are systems capable of converting a molecular recognition event into a measurable luminescent signal change. These platforms are gaining attention due to a number of advantages, such as: i) high sensitivity; ii) wide spectral range of fluorescent probes; iii) Stokes' shifts and spectral bandwidth that meet specific requirements; iv) dynamic wide linear range of detection; v) requirement of small amount of sample and reagents; vi) good detection reproducibility; vii) simple operation process and low-cost detection equipment. The sensors can present two types of brightness-based response: “turn-off” and “turn-on”, which corresponds to luminescence quenching and enhancement, respectively. While the first are usually less reliable and more susceptible to external interferences, the “turn-on” sensors have been gaining territory due to their higher intrinsic sensitivity and selectivity. To produce UCNPs-based “turn-on” sensors there are two main steps to follow: 1) to functionalize the UCNPs with recognition molecules that bind to the target analyte and 2) to combine the UCNPs with energy acceptors materials that are able to quench their emission. In this way, upon the analyte’s presence, the originally quenched UCNPs preferentially bind to the target and their emission reappears/increases. In recent years, research carried out in our lab has included the study of luminescent hexagonal NaYF₄:RE@NaYF₄ UCNPs (where RE = Er³⁺, Yb³⁺, Tm³⁺), coated with a mesoporous silica shell, and their association to gold nanostructures as the quenching elements, towards the development of selective sensors for bacteria and molecules. One step further is to develop such platforms from sensing/targeting agents into treatment possibilities and, in that direction, we have also been exploring the combination of the UCNPs with magnetic NPs through core-shell or supramolecular assembly strategies to attain nanothermometry and nanoheating multifunctionality. Via a combination of optical and magnetic excitation, these multifunctional platforms can not only provide heating at the local environment (harmful to malignant cells) but also allow precise temperature control. In this presentation an overview of the work carried out in our lab will be given, along with innovative and exciting possibilities in the field of sensing and theranostics, using luminescent and magnetic NPs.