This study describes the design of a novel fluoroimmunodiagnostic nanoplatform by using tannylated ferritin nanocages (TA-Fn). The tannylation process ensured the tannic acid was modified onto the ferritin nanocages without EDC, NHS or any reagents. The bare ferritin nanocages (Fn) had about 5.6 +/- 0.14 nm average diameter, while tannylated ferritin nanocages had an estimated layer thickness of about 24 +/- 2.2 nm. The FITC labelled thyroglobulin antibodies (FITC-TgAb) were conjugated to TA-Fn via multiple hydrogen bonds and hydrophobic interactions. The FITC labelled TgAb modified tannylated ferritin nanocages (FITC-TgAb-TA-Fn) had an estimated layer thickness of about 32 +/- 3.1 nm. The FITC-TgAb-TA-Fn had good selectivity and specificity to rapidly capture and detect specific antigens via fluorescence quenching with relatively low-cost engineering. The Stern-Volmer plot was used to obtain limit of detection (LOD) and limit of quantification (LOQ). The LOD was 4.3 pg mL(-1) and LOQ was 14.2 pg mL(-1) in artificial human serum medium. These values were lower than the thyroglobulin detection limit with modern assays.