Upconverting luminescent nanoparticles (UCNPs) represent an interesting class of nanomaterials for bioanalytical applications. Due to their excitation in the near infrared region of the spectra, no fluorescence of biological compounds is trigged. Compared to other nanomaterials like quantum dots they exhibit low cytotoxicity, high photostability, no blinking and chemical inertness. Nevertheless, UCNPs suffer from low quantum efficiency. Here we report on two different core-shell particle systems which have a core consisting of NaYF<sub>4</sub> doped with Yb<sup>3+</sup>/ Tm<sup>3+</sup> and an additional inert shell (NaYF<sub>4</sub>) or an active shell (NaYF<sub>4</sub> doped with Yb<sup>3+</sup>/Nd<sup>3+</sup>). Nanoparticles without Yb<sup>3+</sup> as sensitizer can be excited at 980 nm. However, water has an absorption band in this region. This results in a reduction of the upconversion efficiency in aqueous systems and a heating of the solution. For bioanalytical application, more beneficial is the shifting of the wavelength to 808 nm by additional doping of the shell with Nd<sup>3+</sup>. Both core-shell systems were investigated in respect to the monitor enzymatic reactions of dehydrogenases and oxidases involving the generation of either NADH or FADH<sub>2</sub>.