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Precise manipulation of optical properties through the structure‐evolution of plasmonic nanoparticles is of great interest in biomedical fields including bioimaging and phototherapy. However, previous success has been limited to fixed assembled structures or visible–NIR‐I absorption. Here, an activatable NIR‐II plasmonic theranostics system based on silica‐encapsulated self‐assembled gold nanochains (AuNCs@SiO2) for accurate tumor diagnosis and effective treatment is reported. This transformable chain structure breaks through the traditional molecular imaging window, whose absorption can be redshifted from the visible to the NIR‐II region owing to the fusion between adjacent gold nanoparticles in the restricted local space of AuNCs@SiO2 triggered by the high H2O2 level in the tumor microenvironment (TME), leading to the generation of a new string‐like structure with strong NIR‐II absorption, which is further confirmed by finite‐difference‐time‐domain (FDTD) simulation. With the TME‐activated characteristics, AuNCs@SiO2 exhibits excellent properties for photoacoustic imaging and a high photothermal conversion efficiency of 82.2% at 1064 nm leading to severe cell death and remarkable tumor growth inhibition in vivo. These prominent intelligent TME‐responsive features of AuNCs@SiO2 may open up a new avenue to explore optical regulated nano‐platform for intelligent, accurate, and noninvasive theranostics in NIR‐II window.

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j-Carrageenan was degraded by an oxidative method involving hydrogen peroxide (H 2 O 2 ). The molecular weight was measured by gel permeation chromatography. The effects of the concentration of H 2 O 2 and initial j-carrageenan, pH value, and degradation time on the molecular weights of the degraded products were studied. The structural change of the degraded j-carrageenans was characterized by Fourier transform infrared spectroscopy and determination of the sulfate content. The antioxidant activity of the degraded j-carrageenans was evaluated as scavengers of superoxide anions and hydroxyl radicals by application of flow injection chemiluminescence technology. The values of the 50% inhibition concentration (IC 50 ) against the superoxide anion of degraded j-carrageenans labeled A, B, C, and D (with weight-average molecular weights of 3250, 5820, 15,080, and 209,000, respectively) were 2.65, 3.22, 6.66, and 8.13 mg/mL, respectively. As for hydroxyl radical scavenging, the IC 50 values of j-carrageenans A, B, C, and D were 0.014, 0.049, 0.062, and 0.110 mg/mL, respectively. The results indicated that the j-carrageenans with lower molecular weights had better antioxidant activity.