Spin Dynamics and Ionic Strength Sensitivity in Nitroxide-Doped Micelles

Molecular quantum information science seeks to develop and understand molecular systems demonstrating quantum phenomena such as superposition, entanglement, and coherence for enhanced applications in computing, communication, and sensing. Paramagnetic spin quantum bits (qubits) offer an attractive option for quantum sensing applications due to their scale and chemical tunability. Furthermore, spin qubits compatible with biological systems can be leveraged to probe complex microenvironments. Thus, integrating spin qubits into biological model systems and investigating relaxation and decoherence dynamics yield mechanistic insight and inform us which quantum observables are correlated with chemically relevant parameters. Here, a nitroxide spin-labeled phospholipid was doped into nonionic Triton X-100 micelles to investigate relaxation and decoherence near biological interfaces. At low temperatures (5-80 K), the spin-lattice relaxation (T₁) mechanism was found to vary relative to nitroxides in nonmicelle controls. Furthermore, the relaxation rate was found to be sensitive to KCl concentration within the physiological range (0-500 mM) with rate magnitude and variance decreasing with increasing salt concentration. Critically, greater sensitivity to KCl concentrations was observed for T₁ relaxometry (30%) compared to other techniques, such as pyrene fluorescence (<1%). Thus, this work provides a proof-of-concept study for investigating interfacial relaxation dynamics in biological systems and observing ionic strength sensitivity for spin relaxation.