Polymer nanoparticles and related methods include polymer dots having a coating including a polyelectrolyte polymer. The polymer dots can have a polyelectrolyte coating that can improve colloidal stability of the particles as compared to polymer dots not having the coating. A method of preparing a population of nanoparticles. The methods can include, e.g., providing the population of nanoparticles having a condensed semiconducting polymer; and combining, in a first aqueous solution comprising polyelectrolytes, the population of nanoparticles having the condensed semiconducting polymer to form a population of nanoparticles having a polyelectrolyte coating surrounding the condensed semiconducting polymer of each of the nanoparticles in the population. The methods can include a step of forming the condensed semiconducting polymer using nanoprecipitation or miniemulsion techniques. The polyelectrolyte coating can completely surround the condensed semiconducting polymer.