Abstract: To achieve in-situ detection of bioavailable Cr(Ⅵ) in aquatic systems, this study developed a novel composite material comprising polydopamine (PDA)-coated silicon dioxide (SiO₂) (designated as SiO₂@PDA), which was subsequently incorporated as the binding phase in a Diffusive Gradients in Thin-films (DGT) device (referred to as SiO₂@PDA-DGT). The fundamental performance and operational conditions of the device were systematically investigated, and its feasibility for determining bioavailable Cr(Ⅵ) in diverse water matrices was evaluated. The results show that the SiO₂@PDA-DGT device exhibited excellent selectivity and accumulation efficiency for Cr(Ⅵ). In water samples containing a high concentration of Cr(Ⅲ)Cr(Ⅵ)∶Cr(Ⅲ) = 1∶10, the device achieved precise and selective determination of Cr(Ⅵ)(R=1.03, where R=C_DGT/C_soln). The device reliably quantified bioavailable Cr(Ⅵ) concentrations within pH 5.0-7.0 and ionic strength ranges of 0.1-800 mmol·L^-1, with a maximum effective adsorption capacity of 100 mg·L^-1, a blank value of 2.43 μg·L^-1, and a method detection limit of 0.49 μg·L^-1. The diffusion coefficient (D_cell) of Cr(Ⅵ) in the device was determined as 6.70 × 10^-6 cm²·s^-1, confirming its robust practical performance. In conclusion, the SiO₂@PDA-DGT device not only retains the advantages of in-situ detection but also demonstrates high stability across diverse environmental conditions, highlighting its broad application potential. This technology provides a powerful tool for environmental monitoring and remediation, particularly in scenarios requiring accurate assessment of bioavailable Cr(Ⅵ) contamination in complex aquatic systems.