NOVELTY - A quantum dot is formed by functionalizing with ligands bearing a dithiolane group and an acid group salified with counteractions chosen from sodium ion, lithium ion, potassium ion, zinc ion, ferrous ion, copper ion, and 1-8C tetralkylammonium.    USE - Quantum dot is used as luminescent probe/label for in vitro biological applications e.g. medical diagnostics and medical imaging, and used as component of photosensitizers for photodynamic therapy, and for light absorbing material for manufacture of solar cells and light-emitting material for forming lighting apparatus and display (all claimed).    ADVANTAGE - The quantum dot has excellent optical properties, electronic properties and stability in solution, and compatibility with polar solvents.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) manufacture of quantum dot, which involves adding a resin loaded with boron tetrahydride to a solution of ligand bearing a dithiolane group and a salifiable acid group in solvent (S1) which dissolves the ligand and does not dissolves the resin, removing the solvent (S1) in order to obtain the resin with the attached ligand, adding a solution of a base or a salt to the resin, in which the cation of base or salt is chosen from sodium ion, lithium ion, potassium ion, zinc ion, ferrous ion, copper ion, 1-8C tetralkylammonium, and stirring the mixture to extract the reduced ligand from the resin, washing the resin with the solvent (S1) to obtain a solution containing the reduced ligand, adding quantum dots covered with hydrophobic ligands, in which the quantum dots is in solid form or dissolved in a solvent (S2) not miscible with the solvent (S1), to the solution thus obtaining a biphasic mixture, shaking the mixture to allow the transfer of the quantum dots to the solution to allow exchange of native hydrophobic ligands with the reduced, hydrophilic ones, and isolating the quantum dots; and    (2) use of the quantum dot as luminescent probe/label for in vitro biological applications.