Bifunctional\nMoO<sub>3–<i>x</i></sub>/CuS Heterojunction Nanozyme-Driven\n“Turn-On” SERS\nSignal for the Sensitive Detection of Cerebral Infarction Biomarker\nS100B

The use of nanozymes has become a promising auxiliary\napproach\nto “turn on” surface-enhanced Raman scattering (SERS)\nsignals for the label-free detection of disease markers. Nevertheless,\nthere are still major challenges to develop bifunctional nanomaterials\nwith both excellent enzyme-like activity and high SERS performance.\nTo this end, a novel Z-scheme MoO<sub>3–<i>x</i></sub>/CuS heterojunction was first constructed as a powerful “two-in-one”\nsubstrate, which can not only catalyze leucomalachite green (LMG)\nto SERS-active malachite green (MG) but also serve as an efficient\nsubstrate to amplify the SERS signal of catalysate. Due to the strong\ninterfacial coupling effect between the MoO<sub>3–<i>x</i></sub> and CuS nanomaterial, which promoted the separation and transport\nof carriers in the heterojunction, the MoO<sub>3–<i>x</i></sub>/CuS heterojunction showed higher peroxidase-like activity\ncompared to individual components and the previously reported heterojunction\nnanozymes. Inspired by these results, a sandwich-type SERS immunoassay\nfor the detection of the cerebral infarction biomarker S100 calcium-binding\nprotein (S100B) was proposed based on the output signal of MG at 1620\ncm<sup>–1</sup>. Furthermore, introducing the antifouling material\nchitosan on the surface of the MoO<sub>3–<i>x</i></sub>/CuS heterojunction can effectively resist nonspecific protein\nadsorption and significantly improve the detection accuracy of the\nimmunoassay. Therefore, the SERS immunoassay based on the MoO<sub>3–<i>x</i></sub>/CuS heterojunction realized highly\nsensitive and selective detection of S100B in the concentration range\nof 0.001 to 100 ng/mL, with a low limit of detection of 0.47 pg/mL.\nThe developed method has been successfully used for the accurate detection\nof S100B in clinical serum. The results showed that the level of S100B\nin the serum of cerebral infarction patients can be distinguished\nfrom those of healthy individuals and intracranial tumor patient controls.\nIn addition, the acquired values of S100B in the serum of cerebral\ninfarction patients based this strategy were well consistent with\nthe results of electrochemiluminescence (ECL) detection with a relative\nerror of less than ±7.3. It is expected that this work may open\nup a paradigm for improving detection sensitivity and accuracy for\nthe early diagnosis and treatment monitoring of cerebral infarction\nin the clinic.