A method for manufacturing stable concentrated colloids containing metal nanoparticles in which the colloid is stabilized by adding a base. This allows the metal particles to be formed in higher concentration without forming larger agglomerates and/or precipitating. The method of manufacturing the stable colloidal metal nanoparticles of the present invention generally includes (i) providing a solution comprising a plurality of metal atoms, (ii) providing a solution comprising a plurality of organic agent molecules, each organic agent molecule comprising at least one functional group capable of bonding to the metal atoms, (iii) reacting the metal atoms in solution with the organic agent molecules in solution to form a mixture comprising a plurality of complexed metal atoms, (iv) reducing the complexed metal atoms in the mixture using a reducing agent to form a plurality of nanoparticles, and (v) adding an amount of a base to the mixture, thereby improving the stability of the nanoparticles in the mixture. The base may be added before or after forming the nanoparticles.