Ternary Z-Scheme Ag-Embedded TiO2–Ag2S Nanojunction as a Novel Photoelectrochemical Converter for CD44 Detection

Nanoarrays (NAs) with stable signal output have become the most promising photoelectrochemical (PEC) biosensing substrate. However, a general issue is that interfacial charge-carrier recombination in a single-component semiconductor cannot be easily prevented, resulting in a low photocurrent density. Herein, a biosensor utilizing a Ag-embedded TiO₂-Ag₂S nanojunction (TiO₂-Ag-Ag₂S) as a signal converter was developed for the detection of CD44 protein─a transmembrane glycoprotein highly expressed in breast cancer cells. The ternary Z-scheme heterojunction was prepared by a distinctive scheme in which the Ag layer is introduced onto the surface of rutile TiO₂ NAs by magnetron sputtering, whereas the Ag₂S is rooted in the local sulfuration of Ag. With a sufficient density of oriented nanorods, TiO₂-Ag-Ag₂S exhibits a smooth photocurrent output and minimal variation among different batches; it is undoubtedly a satisfactory PEC sensing carrier, which enables highly specific identification of target CD44 on the surface of MDA-MB-231 cells due to DNA strand displacement reactions (SDRs) and host-guest recognition between hyaluronic acid (HA) and CD44. The biosensor shows a sensitive PEC response to CD44 over a wide range of 37 to 5.0 × 10⁵ cells/mL. We can conclude that this approach will provide an alternative solution to breast cancer diagnosis.