A method for photothermally modulating a chemical polymerase chain reaction, the method comprising: preparing a reaction solution comprising: target nucleic acid molecules, primer nucleic acid strands, free nucleotides, a nucleic acid polymerase, fluorescent probe molecules that bind to said target nucleic acid molecules, and a metal nanocrystal sample in which at least 85% of the metal nanocrystals are plasmonic penta-twinned bipyramid-shaped gold nanocrystals, the plasmonic penta-twinned bipyramid-shaped gold nanocrystals having a polydispersity of less than 8%, based on transmission electron micrograph images of the penta-twinned bipyramid-shaped gold nanocrystals, and having a pegylated silica coating; and cycling the reaction solution through a plurality of photothermal cycles, each photothermal cycle comprising: irradiating the reaction solution with radiation having wavelengths in the infrared region of the electromagnetic spectrum, at which said plasmonic penta-twinned bipyramid-shaped gold nanocrystals absorb the radiation and heat the reaction solution, via plasmonic photothermal radiation-to-heat conversion, thereby causing said target nucleic acid molecules to denature; and cooling said reaction mixture to a temperature at which said primer nucleic acid strands anneal to the denatured target nucleic acid molecules and new strands of nucleic acids are synthesized starting from an annealed primer nucleic acid strand to form new target nucleic acid molecules; and exciting said fluorescent probe molecules and monitoring a resulting fluorescence. preparing a reaction solution comprising: target nucleic acid molecules, primer nucleic acid strands, free nucleotides, a nucleic acid polymerase, fluorescent probe molecules that bind to said target nucleic acid molecules, and a metal nanocrystal sample in which at least 85% of the metal nanocrystals are plasmonic penta-twinned bipyramid-shaped gold nanocrystals, the plasmonic penta-twinned bipyramid-shaped gold nanocrystals having a polydispersity of less than 8%, based on transmission electron micrograph images of the penta-twinned bipyramid-shaped gold nanocrystals, and having a pegylated silica coating; and cycling the reaction solution through a plurality of photothermal cycles, each photothermal cycle comprising: irradiating the reaction solution with radiation having wavelengths in the infrared region of the electromagnetic spectrum, at which said plasmonic penta-twinned bipyramid-shaped gold nanocrystals absorb the radiation and heat the reaction solution, via plasmonic photothermal radiation-to-heat conversion, thereby causing said target nucleic acid molecules to denature; and cooling said reaction mixture to a temperature at which said primer nucleic acid strands anneal to the denatured target nucleic acid molecules and new strands of nucleic acids are synthesized starting from an annealed primer nucleic acid strand to form new target nucleic acid molecules; and exciting said fluorescent probe molecules and monitoring a resulting fluorescence. irradiating the reaction solution with radiation having wavelengths in the infrared region of the electromagnetic spectrum, at which said plasmonic penta-twinned bipyramid-shaped gold nanocrystals absorb the radiation and heat the reaction solution, via plasmonic photothermal radiation-to-heat conversion, thereby causing said target nucleic acid molecules to denature; and cooling said reaction mixture to a temperature at which said primer nucleic acid strands anneal to the denatured target nucleic acid molecules and new strands of nucleic acids are synthesized starting from an annealed primer nucleic acid strand to form new target nucleic acid molecules; and exciting said fluorescent probe molecules and monitoring a resulting fluorescence.