Triple-negative　breast　cancer　(TNBC)　accounts　for　nearly　one-quarter　of　all　breast　cancer　cases,　but　effective　targeted　therapies　for　this　disease　remain　elusive　because　TNBC　cells　lack　the　expression　of　the　most　common　three　receptors　seen　in　other　subtypes　of　breast　cancers.　The　medium-term　diagnosis　of　breast　cancers　is　essential　for　development　and　prognosis.　According　to　reports,　patients　with　TNBC　may　be　converted　to　a　positive　epidermal　growth　factor　receptor　2(HER-2)　after　chemotherapy,　and　trastuzumab　treatment　will　have　a　better　prognosis.　Therefore,　it　is　important　to　accurately　quantify　the　expression　of　HER-2　in　breast　cancer　cells.　Herein,　we　design　a　red　fluorescent　Au-25　probe　synthesized　with　BSA-biotin　as　the　ligand,　which　is　accurately　quantified　by　HER-2　primary　antibody-biotin　using　the　avidin　system.　The　quantitative　detection　of　the　expression　of　HER-2　in　breast　cancers　is　helpful　for　the　companion　diagnostic　of　breast　cancer　treatment　and　provides　follow-up　treatment.