Biosensing Based on Upconversion Nanoparticles

The phenomenon of photon upconversion is the sequential absorption of two or more long-wavelength photons leading to the emission of a shorter-wavelength photon, which is defined as an anti-Stokes emission. Upconversion mechanisms include excited state absorption (ESA), energy transfer upconversion (ETU), energy migration-mediated upconversion (EMU), photon avalanche (PA), and cooperative sensitization upconversion (CSU). Upconversion systems show high resistance to photobleaching and photochemical degradation, the use of infrared light has greater sample penetration depths than ultraviolet excitation and reduces the unwanted effects of light scattering, and autofluorescence and is less harmful to biological samples. Upconverting nanoparticles (UCNPs) have been investigated for use in biological systems, as well as improving compatibility with the environment to eliminate cytotoxic effects. Furthermore, as technology advances, new possibilities emerge when combining additional layers with UCNPs, resulting in unique and versatile nanosystems with multiple applications in both the solution phase and solid systems. In this context, the development of efficient host matrices becomes crucial. Core/multi-shell UCNPs can be created with multiple independent luminescence channels and simultaneously provide multiple detection parameters for bioimage acquisition or for sensors and therapies. In this context, we highlight two widely studied matrices for accommodating UCNPs to enhance their optical properties. By changing the optical characteristics, the performance of UCNPs can be customized for a variety of applications, including photodynamic therapy, high-resolution displays, bioimaging, biosensors, real-time temperature detection, and drug administration. Since upconverting nanomaterials can convert NIR light into visible photons and enable activate photosensitizers, these alterations can be improved by designing efficient matrices such as NPs NaYF4:RE3+ and NaGdF4:RE3+ (RE3+ = Er3+/Yb3+; Er3+/Tm3+; Yb3+/Nd3+; Tm3+/Yb3+), as discussed in this chapter.