Diamond Quantum Sensing Microscopy of Spin Crossover and Cytochrome C Molecules

Diamond Quantum sensing (DQS) microscopy based on nitrogen vacancy (NV) centers has become a unique tool to probe weak static and fluctuating magnetic fields with an excellent combination of spatial resolution and magnetic sensitivity, opening new routes to study solid-state [1] and biological materials [2]. In this work, we present two examples of using DQS in wide-field geometry to probe nanoscale magnetic phenomena in molecules.<br/>First, we discuss DQS wide-field microscopy measurements on individual Fe(Htrz)₂(trz)](BF₄)] (Fe triazole) spin-crossover (SCO) nano-rods of size varying from 20 to 1000 nm. Fe triazole SCO molecules show thermal switching between low spin (LS) and high spin (HS) states which are applicable in thermal sensors and molecular switches. While the bulk magnetic properties of these molecules are widely studied by bulk magnetometry techniques, their properties at the individual level are not yet explored. Scanning electron microscopy (SEM) and Raman spectroscopy are performed to determine the size of the nano-rods and to confirm the spin state (LS vs HS) of the Fe triazole molecule respectively [3]. The stray magnetic fields produced by individual nano-rods are imaged by DQS microscopy as a function of temperature (up to 150 ⁰C) and applied magnetic field (up to 350 mT) and correlated with SEM and Raman. We found that in most of the nanorods the LS state is slightly paramagnetic, probably originating from the surface oxidation and/or the greater Fe⁺³ presence along the nanorods’ edges. NV measurements on the Fe-triazole LS state nanoparticle clusters revealed both diamagnetic and paramagnetic behaviors [3].<br/>Then, we show DQS wide-field microscpy measurements on cytochrome C (Cyt-C) nanoclustered molecules. Cyt-C is an iron-containing biomolecule that plays an important role in the electron transport chain of mitochondria [4]. Under ambient conditions, the heme group remains in the Fe⁺³ paramagnetic state [5]. Initially, we performed NV <i>T</i>₁ relaxometry measurements on a carboxylated diamond chip doped with 8-nm thick NV-layer without any Cyt-C, which revealed a relaxation time <i>T</i>₁ of ~ 1.2 ms. Subsequently, we varied the concentration of Cyt-C from 6 mM to 54 mM and show a reduction of the NV relaxation time <i>T</i>₁ from ~ 800 μs to 150 μs respectively, explained by the spin noise generated from the intracellular iron spins in the Cyt-C molecules [6]. Additionally, we performed magnetic relaxometry imaging of Cyt-C proteins on a nanostructured diamond substrate by which we estimate the density of adsorbed iron from 1.44 × 10⁶ to 1.7 × 10⁷ per μm² [6]. [1] A. Laraoui, K. Ambal, Appl. Phys. Lett. 121, 060502 (2022). [2] I. Fescenko, A. Laraoui, et <i>al</i>., Phy. Rev. App. 11(3), 034029 (2019). [3] S. Lamichhane, A. Laraoui, et <i>al</i>., ACS Nano 17 (9), 8694-8704 (2023). [4] I. Bertini, et <i>al.</i>, Chem. Rev., 106 (1), 90–115 (2006). [5] J. Liu, et<i> al.</i>, Chem. Rev. 114 (8), 4366–446 (2014). [6] S. Lamichhane, A. Laraoui, et <i>al.</i>, under review, arXiv:2310.08605 (2023).<br/>Acknowledgment: This material is based upon work supported by the NSF/EPSCoR RII Track-1: Emergent Quantum Materials and Technologies (EQUATE) Award OIA-2044049, and NSF Award 2328822. I.F. acknowledges support from Latvian Council of Science project lzp- 2021/1-0379. K.A. would like to acknowledge the support of the National Science Foundation/EPSCoR RII Track-4 Award OIA-2033210 and NSF Award 2328822. The research was performed in part in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience (and/or NERCF), which are supported by NSF under Award ECCS: 2025298, and the Nebraska Research Initiative.