Tunable pore architectures in programmable DNA hydrogels for ultrasensitive magnetic relaxation sensing of Pb²⁺ over a wide dynamic range.

Magnetic relaxation switch (MRS) sensors are widely used in analytical applications due to their excellent performance. Gel-MRS offers enhanced stability, making it particularly advantageous. However, existing stimuli-responsive hydrogel-based MRS sensors are often limited in detection range and limit of detection (LOD) by the physicochemical constraints of the gel network. Herein, we demonstrate wide-range, ultrasensitive MRS detection of Pb²⁺ using programmable DNA hydrogels with tunable pore architectures. By systematically varying the crosslinking linker design, three hydrogels with pore sizes of 0.72, 1.12, and 4.28 μm were fabricated. As pore size increased, the apparent diffusion coefficient (ADC) decreased from 2.03 × 10⁻³ to 1.65 × 10⁻³ mm²/s, while the transverse relaxation time change (ΔT₂) increased proportionally, establishing a quantitative structure-property relationship among pore size, ADC, and T₂ response. Leveraging this relationship, the optimized MRS-DNA hydrogel biosensor achieves a detection limit of 8.4 pg/mL and a linear range spanning four orders of magnitude (0.01-100 ng/mL) with a correlation coefficient of 0.99. Validation in complex matrices yields spike recoveries between 95.21% and 109.21%, underscoring the method's robustness. This programmable pore-engineering strategy offers a new pathway toward reliable, decentralized screening of hazardous contaminants.