The present work introduces ternary Ln(III) (Ln = Eu, Yb, Lu) complexes with thenoyltriflouro1,3-diketonate (TTA−) and phosphine oxide derivative (PhO) as building blocks for core-shell nanoparticles with both Eu(III)- or Yb(III)-centered luminescence and the dual Eu(III)-Yb(III)-centered luminescence. Solvent-mediated self-assembly of the complexes is presented herein as the procedure for formation of Eu–Lu, Eu–Yb and Yb–Lu heterometallic or homometallic cores coated by hydrophilic polystyrenesulfonate-based shells. Steady state and time resolved Eu-centered luminescence in homolanthanide and heterolanthanide Eu–Lu and Eu–Yb cores is affected by Eu → Eu and Eu → Yb energy transfer due to a close proximity of the lanthanide blocks within the core of nanoparticles. The Eu → Yb energy transfer is highlighted to be the reason for the enhancement of the NIR Yb-centered luminescence. Efficient cellular uptake, low cytotoxicity towards normal and cancer cells, and sensing ability of Eu–Yb nanoparticles on lomefloxacin additives via both red and NIR channels make them promising as cellular imaging agents and sensors.