Described herein is a method of making a reduced metal nanoparticle, the method including mixing a reactive reducing agent with a metal salt in a solution at a temperature of 4-100° C., and forming the reduced metal nanoparticles in the solution. Also described is a kit including a reactive reducing agent that is sensitive to ßgalactosidase, a metal salt, and optionally a modifying agent/functionalizing agent for reduced metal nanoparticles. A 3,4-cyclohexeneoesculetin-B-D-galacto pyranoside (SGNP) gold nanoparticle and its use for measuring ßgalactosidase enzyme activity, comprising by detecting a structural change in the SGNPs caused by the ßgalactosidase are described. Further described are a point of care device, a chip, a biosensor, a laboratory animal, a gene delivery agent, a drug delivery agent, a diagnostic agent, or a disease targeting agent including SGNPs.