A Guanine-Enhanced Graphene-DNA Paper-Based Sensing Platform Enabling Sensitive Hg²⁺ Detection

Mercury ions (Hg²⁺) are highly toxic and pose severe risks to human health and ecosystems, necessitating sensitive detection methods for environmental monitoring. Here, we report a paper-based graphene sensor functionalized with single-stranded DNA (ssDNA) probes for Hg²⁺ detection based on T-Hg²⁺-T coordination chemistry. To elucidate the effect of probe structure on sensing performance, we designed DNA constructs with varying numbers of guanine (G) bases (3-6, designated DNA2-DNA5) in the bridging fragment and systematically evaluated their influence on hairpin stability, Hg²⁺ binding affinity, and sensor response. The DNA3-based sensor (four G bases) exhibited optimal electronic stability and sensitivity, achieving a detection limit of 0.673 pM with effective real-time monitoring capability in aqueous media. These findings highlight the critical role of DNA sequence design in T-Hg²⁺-T-based biosensors and provide a promising strategy for sensitive and selective Hg²⁺ detection in environmental samples.