A first-principles investigation of bio-ion-induced magnetic modulation in defect-engineered Janus WSSe monolayers:...

The emulation of synaptic functionality using two-dimensional (2D) quantum materials offers a promising route toward non-invasive, brain-inspired diagnostic technologies. In this work, we employ first-principles density functional theory (DFT) calculations to investigate ion-induced magnetic modulation in pristine and defect-engineered Janus WSSe monolayers. We show that biologically relevant ions such as Ca²⁺, Na⁺, and Cl^- induce distinct and ion-specific changes in the magnetic moment. This behaviour arises from the intrinsic out-of-plane asymmetry of Janus WSSe combined with defect-induced localized spin states. As a result, external ionic adsorption enables controllable modulation of magnetism, providing a magnetic analogue of synaptic excitatory and inhibitory responses. We systematically explore multiple defect configurations and adsorption geometries to understand defect-ion-spin coupling mechanisms. Our results reveal a synaptic-like hierarchy of responses: Cl^- fully quenches magnetism, corresponding to a synaptic OFF state; Na⁺ preserves a high magnetic moment, representing a synaptic ON state; and Ca²⁺ induces an intermediate partially suppressed state, enabling graded magnetic modulation. The correlation between adsorption height, charge transfer (from Bader analysis), spin density redistribution, and density of states confirms the role of defect-assisted chemisorption in stabilizing non-volatile magnetic states. In contrast, weaker physisorption leads to more volatile magnetic responses. Importantly, the magnetic readout enables a contact-free sensing paradigm, where bio-ionic signatures are transduced into spin-based logic states without direct electrical current flow. Based on these findings, we propose defect-engineered Janus WSSe monolayers as a platform for quantum neuromorphic health sensing. This system can translate ionic imbalances associated with neurological disorders such as Parkinson's disease, epilepsy, and Alzheimer's disease into distinguishable magnetic signals. Overall, this study establishes a theoretical framework for magnetically programmable, bio-ion-responsive neuromorphic sensors based on atomically thin Janus materials.