Silicon nanocrystals (Si-nc) present several plus points as advanced fluorescent biomarkers but suffer from difficulties met in controlling their intrinsic photoluminescence (PL). Here, we first consider the reasons for this difficulty, showing results that support an interface defect-related origin of the PL. Attainment of a controlled PL emission would then require tuning of defects in the capping oxide, a hard and yet unaddressed task. Alternatively, we demonstrate the possible use of Si-nc as antennas, or sensitizers, of a luminescent rare-earth ion in an engineered fluorophore. In this approach the relatively high and broadband optical absorption of Si-nc was exploited, keeping the advantages of a near-infrared inorganic light emitter. Another fundamental part of the assessment of Si-nc for bioimaging is their biocompatibility. Here, we report toxicity tests based on the lactate dehydrogenase release and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays on epithelial cells and fibroblasts, confirming that Si-nc in concentration suitable for luminescent labeling do not affect significantly the cells viability.