Ultrasensitive spintronic fingerprinting of toxic radicals using single-atom Fe-porphyrin centers in carbon nanotubes

Abstract

Achieving molecule-specific recognition at the single-defect limit demands electronic probes whose transport response is dictated by rigorous microscopic symmetries. Here, we show that a single Fe-centered porphyrinlike defect embedded in an armchair carbon nanotube acts as a symmetry-enforced, zero-bias spintronic sensor capable of distinguishing highly toxic cyanide (modeled as adsorbed CN*) and sulfanyl (HS center dot) radicals. Specifically, CN* denotes the chemisorbed cyanide-derived moiety bound to the Fe center that encodes the transport fingerprint under charge-neutral, spin-polarized conditions. Density-functional theory combined with nonequilibrium Green's function calculations reveals that the Fe-N4 center functions as an efficient spin filter, nearly suppressing the minority-spin 2t-channel transmission eigenvalue (T,down arrow approximate to 0.03) due to a robust orbital-symmetry mismatch between the Fe 3dx2-y2 state and the host 2t channel. Chemical adsorption perturbs this symmetry condition in sharply species-and geometry-dependent ways: HS center dot reopens metallic transport, C-bound CN* maintains the spin blockade through a localized doping inversion, and N-bound CN* restores near-pristine conductance. The distinct electronic fingerprints enable unambiguous discrimination of analytes and their binding configurations at room temperature. Our findings establish metalloporphyrin-functionalized nanotubes as atomically precise platforms for symmetry-driven molecular detection, offering isomer-level resolution in single-defect spintronic sensing.