The invention provides lipidic microparticles stably associated with at least two different targeting moieties, which targeting moieties are attached to linker molecules comprising a hydrophilic domain and a hydrophobic domain. The new sensors comprise hydrogen-resistant optical fibers which are able to withstand a hot, hydrogen-containing environment as is often found in the downhole well environment. The first optical detector and the side switch are arranged at an interval larger than the diameter of the small diameter disc, while the second optical detector and the side switch are arranged at an interval smaller than the diameter of the small diameter disc.