Separation and application of immature nanoparticles from the synthesis system of polymeric nanoparticles (PNPs) is a cost-effective route to improve the utilization rate of monomers. Here, for the first time, mature and immature polydopamine nanoparticles (mPNPs and imPNPs) with a total yield of up to 75.7% were separated from the ammonia-driven synthesis system via centrifugation and dialysis, and further transformed into the polyelectrolyte-modified PNPs (mPPNPs and imPPNPs) in a simple strategy. These PPNPs exhibited respectable colloidal stability and tribological properties as lubricant additives of PEG200. Tribological evaluations reflected that the optimal doses of mPPNPs and imPPNPs are 1.0   wt% and 0.5   wt%, respectively. At these doses, the friction and wear reductions by imPPNPs (38.2% and 63.7%) were higher than those of mPPNPs (37.6% and 61.6%), which is attributed to the smaller size and higher charge density of imPPNPs than mPPNPs. Moreover, the tribological stability of imPPNPs is better than that of mPPNPs under heavy-load, high-speed, long-duration, and start-stop tests. Potential-controlled friction experiments and wear analyses confirmed that the good adsorption ability of the polyelectrolyte shell, the strong adhesion and chelation effects of polydopamine structures, and the deposition of hybrid tribofilm should be responsible for the robust tribological behaviors of imPPNPs. This study highlights the prospect of imPNPs in tribology, enabling waste-to-resource conversion and providing new insights into green lubrication.