The　dispersed　gold　nanoparticles　(AuNPs)　have　weak　photothermal　effect　in　near-infrared　(NIR)　region.　After　the　addition　of　cysteine,　the　AuNPs　are　aggregated　due　to　the　electrostatic　interaction　and　then　exhibited　strong　photothermal　effect.　At　present　of　copper　ion　(Cu2+),　the　cysteine　was　catalytically　oxidized　into　cystine,　leading　to　the　inhibition　of　the　aggregation　of　AuNPs　and　the　photothermal　effect　decreased.　Based　on　this,　a　simple　photothermal　assay　can　be　developed　for　Cu2+　detection　using　a　common　thermometer　as　readout.　The　change　of　the　temperature　(ΔT)　of　the　system　has　a　linear　relationship　with　Cu2+　in　the　range　of　10–300　nM　with　a　detection　limit　of　7.4　nM　(S/N　=　3).　Furthermore,　through　labeling　the　detection　antibody　in　immunoassay　with　CuO　nanoparticles　as　the　source　of　Cu2+,　a　convenient　photothermal　immunoassay　can　be　developed.　Carcinoembryonic　antigen　(CEA),　an　important　biomarker　for　cancer　screening,　was　chosen　as　the　model　target　because　the　rise　of　CEA　level　is　widely　present　in　cancer　blood　serum.　Under　the　optimized　conditions,　ΔT　has　a　linear　relationship　with　CEA　concentration　in　the　range　of　3.0–48.0　ng/mL.　The　detection　limit　is　1.3　ng/mL.　The　proposed　method　had　been　applied　to　detect　CEA　in　serum　samples　with　good　agreement　with　the　reference　method　used　in　hospital.　©　2021　Elsevier　B.V.