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Exosome analysis in gastric cancer (GC) serum requires sensitive readout with reduced matrix interference. Circulating GC-related CD63-positive exosomes can indicate vesicle burden and stress-induced secretion. Herein, we developed a dual-mode photothermal/colorimetric aptasensor using CD63 aptamer-functionalized core-shell AuPt nanozymes for detecting GC-related CD63-positive exosomes in serum samples. The innovation of this platform lies in integrating aptamer recognition, Pt-shell peroxidase-like catalysis, and Au-core near-infrared (NIR) photothermal conversion within one nanoprobe. The porous Pt shell showed high TMB affinity (K_m = 0.18 mM), and the Au core enabled 808 nm-responsive photothermal enhancement. Arrhenius analysis showed that 808 nm NIR irradiation decreased the apparent activation energy by 45.9%, while NIR-XPS, photocurrent, impedance, and wavelength-dependent analyses supported localized photothermal heating and plasmon-assisted interfacial charge transfer in the catalytic enhancement. The colorimetric mode achieved a limit of detection of 5.0 × 10² particles/mL and a linear range of 1.0 × 10³ to 1.0 × 10⁶ particles/mL. The photothermal mode covered 1.0 × 10⁵ to 1.0 × 10⁸ particles/mL and reduced hemolysis-induced optical interference. In a preliminary double-blind cohort with 60 serum samples, the assay showed a significant difference between GC patients and healthy donors (P < 0.0001). Biologically, the assay tracked increased particle-level exosome release from SGC-7901 cells under hypoxia and 5-FU stress over 48 h, supporting dynamic monitoring of stress-related vesicle abundance. A portable format combining smartphone RGB analysis and pocket-sized thermal imaging also showed good linearity under fixed imaging conditions. This strategy provides a dual-mode tool for quantitative GC-related exosome analysis and preliminary extracellular vesicle monitoring.