Nitrogen-Vacancy Magnetometry of Individual Fe-Triazole Spin Crossover Nanorods

Abstract

[Fe(Htrz)2(trz)](BF4) (Fe-triazole) spin crossover molecules show thermal, electrical, and optical switching between high spin (HS) and low spin (LS) states, making them promising candidates for molecular spintronics. The LS and HS transitions originate from the electronic configurations of Fe(II) and are considered to be diamagnetic and paramagnetic, respectively. The Fe(II) LS state has six paired electrons in the ground states with no interaction with the magnetic field and a diamagnetic behavior is usually observed. While the bulk magnetic properties of Fe-triazole compounds are widely studied by standard magnetometry techniques, their magnetic properties at the individual level are missing. Here we use nitrogen vacancy (NV) based magnetometry to study the magnetic properties of the Fe-triazole LS state of nanoparticle clusters and individual nanorods of size varying from 20 to 1000 nm. Scanning electron microscopy (SEM) and Raman spectroscopy are performed to determine the size of the nanoparticles/nanorods and to confirm their respective spin states. The magnetic field patterns produced by the nanoparticles/nanorods are imaged by NV magnetic microscopy as a function of 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, possibly originating from the surface oxidation and/or the greater Fe(III) presence along the nanorods’ edges. NV measurements on the Fe-triazole LS state nanoparticle clusters revealed both diamagnetic and paramagnetic behavior. Our results highlight the potential of NV quantum sensors to study the magnetic properties of spin crossover molecules and molecular magnets.

Nitrogen-Vacancy Magnetic Relaxometry of Nanoclustered Cytochrome C Proteins

“…To measure the diameter and height of Cyt-C nanoclusters, we further diluted the Cyt-C solution to a concentration of ∼0.1 mM and drop-casted 5 μL on silicon and diamond substrates, allowing it to evaporate and form a thin film. To alleviate the charging effects in scanning electron microscope (SEM) measurements, we drop-casted Cyt-C nanoclusters on carbon tape and on gold (Au) coated diamond substrates, see the Supporting Information (SI) Section S1. Figure b shows the SEM image of nanoclustered Cyt-C proteins on carbon tape with an aggregated diameter that varies from 30 to 150 nm (SI Section S1).…”

“…Microwave (MW) excitation allows spin transitions from m s = 0 to m s = ± 1 sublevels. The applied magnetic field breaks the degeneracy of the m s = ± 1, leading to a pair of spin transitions ( m s = 0 to m s = +1 and m s = 0 to m s = −1) that can be interrogated via optically detected magnetic resonance (ODMR) spectroscopy. , This method results in a set of resonance peaks that are sensitive to the magnetic field (Figure S5b). In addition, if the laser excitation is abruptly turned off, the amplitude of the ODMR signal decays exponentially as the NV electron spins relax from their polarized states.…”

“…Numerous techniques have been utilized to detect Cyt-C, including electron paramagnetic resonance (EPR) spectrometry − and X-ray photoelectron spectroscopy (XPS). , The variation of the Cyt-C concentration in the cells depending on its current physiology state makes it hard to use bulk techniques such as EPR to detect it at the subcellular level. Among the currently available high-sensitivity and spatial resolution detection techniques, magnetic imaging based on the nitrogen-vacancy (NV) center in diamond − emerges as one of the most promising tools for high-resolution nanoscale imaging. The NV center is a spin-1 defect with optically addressable electron spin properties − and millisecond spin relaxation times ( T 1 , T 2 ) at room temperature .…”

“…These capabilities have opened new opportunities in quantum sensing, nanoscale magnetometry, − and biosensing . For example, it enabled the initial detection of single proteins and the realization of nanoscale nuclear magnetic resonance (NMR) spectroscopy. − NV magnetometry has been used very recently to measure the magnetic properties of individual (size <1 μm) malarial hemozoin biocrystals and [Fe­(Htrz) 2 (trz)]­(BF 4 ) spin crossover molecules . Another approach to NV magnetometry is to detect spin noise from metal ions through the decrease of the spin–lattice T 1 relaxation time.…”

“…In Figure d, we show the schematic of the wide-field ODMR microscope. We used a 532 nm laser (power = 180 mW) to excite the NVs over an area of ∼36 μm × 36 μm, and the NV fluorescence (650–800 nm) is mapped onto a sCMOS camera. , The diamond chip was placed on top of a glass coverslip with patterned gold loops for MW excitation (Figure d). A magnetic field B app (3.2 mT) is applied along the [111] direction of the (100) diamond enabling the separation of the ODMR peaks for m s = 0 to m s = −1 and m s = 0 to m s = +1 spin transitions (Figure S5b).…”