A dye-loaded fluorescent polymeric nanoparticle as nano-antenna, comprising: a) a polymer chosen from: - a polymethacrylate or its derivative, - a polystyrene or its derivative, - an aliphatic polyester or its derivative, or - a copolymer of aforementioned polymers with polyethylene glycol (PEG) or with charged monomers, b) an energy donor, which content is from 5 to 50 % by weight of the polymer, preferably from 10 to 30 % by weight of the polymer, c) an energy acceptor, which content from 0.001 to 0.04 % by weight of the polymer, preferably from 0.002 to 0.02% by weight of the polymer ; said polymer forming a matrix in which from 1000 to 50000 of said energy donor molecules are encapsulated per nanoparticle, wherein the energy donor is a salt of a donor dye with bulky fluorinated anions, said donor dye being chosen from: (i) a rhodamine derivative represented by formula (I) in which: - R 1 , R 2 , R 3 and R 4 are identical or different and each represent a hydrogen or a (C 1 -C 8 ) alkyl group, - R 5 is a (C 1 -C 24 ) alkyl, or (ii) a cyanine derivative represented by formula (II) n is a integer chosen from 1, 2 or 3 ; Ra and Rb are identical or different and each represent a (C 1 -C 24 ) alkyl group; and wherein the energy acceptor is a salt of another cyanine derivative of formula (II) defined above with a counterion, said energy donor and said energy acceptor being different. a) a polymer chosen from: - a polymethacrylate or its derivative, - a polystyrene or its derivative, - an aliphatic polyester or its derivative, or - a copolymer of aforementioned polymers with polyethylene glycol (PEG) or with charged monomers, - a polymethacrylate or its derivative, - a polystyrene or its derivative, - an aliphatic polyester or its derivative, or - a copolymer of aforementioned polymers with polyethylene glycol (PEG) or with charged monomers, b) an energy donor, which content is from 5 to 50 % by weight of the polymer, preferably from 10 to 30 % by weight of the polymer, c) an energy acceptor, which content from 0.001 to 0.04 % by weight of the polymer, preferably from 0.002 to 0.02% by weight of the polymer ; said polymer forming a matrix in which from 1000 to 50000 of said energy donor molecules are encapsulated per nanoparticle, wherein the energy donor is a salt of a donor dye with bulky fluorinated anions, said donor dye being chosen from: (i) a rhodamine derivative represented by formula (I) in which: - R 1 , R 2 , R 3 and R 4 are identical or different and each represent a hydrogen or a (C 1 -C 8 ) alkyl group, - R 5 is a (C 1 -C 24 ) alkyl, or (ii) a cyanine derivative represented by formula (II) n is a integer chosen from 1, 2 or 3 ; Ra and Rb are identical or different and each represent a (C 1 -C 24 ) alkyl group; (i) a rhodamine derivative represented by formula (I) in which: - R 1 , R 2 , R 3 and R 4 are identical or different and each represent a hydrogen or a (C 1 -C 8 ) alkyl group, - R 5 is a (C 1 -C 24 ) alkyl, or - R 1 , R 2 , R 3 and R 4 are identical or different and each represent a hydrogen or a (C 1 -C 8 ) alkyl group, - R 5 is a (C 1 -C 24 ) alkyl, or (ii) a cyanine derivative represented by formula (II) n is a integer chosen from 1, 2 or 3 ; <EOS>