Abstract
Naturally occurring paramagnetic species (PS), such as free radicals and paramagnetic metalloproteins, play an essential role in a multitude of critical physiological processes including metabolism, cell signaling, and immune response. These highly dynamic species can also act as intrinsic biomarkers for a variety of disease states, while synthetic paramagnetic probes targeted to specific sites on biomolecules enable the study of functional information such as tissue oxygenation and redox status in living systems. The work presented herein describes a new sensing method that exploits the spin-dependent emission of photoluminescence (PL) from an ensemble of nitrogen-vacancy centers in diamond for rapid, nondestructive detection of PS in living systems. Uniquely this approach involves simple measurement protocols that assess PL contrast with and without the application of microwaves. The method is demonstrated to detect concentrations of paramagnetic salts in solution and the widely used magnetic resonance imaging contrast agent gadobutrol with a limit of detection of less than 10 attomol over a 100 μm × 100 μm field of view. Real-time monitoring of changes in the concentration of paramagnetic salts is demonstrated with image exposure times of 20 ms. Further, dynamic tracking of chemical reactions is demonstrated via the conversion of low-spin cyanide-coordinated Fe3+ to hexaaqua Fe3+ under acidic conditions. Finally, the capability to map paramagnetic species in model cells with subcellular resolution is demonstrated using lipid membranes containing gadolinium-labeled phospholipids under ambient conditions in the order of minutes. Overall, this work introduces a new sensing approach for the realization of fast, sensitive imaging of PS in a widefield format that is readily deployable in biomedical settings. Ultimately, this new approach to nitrogen vacancy-based quantum sensing paves the way toward minimally invasive real-time mapping and observation of free radicals in in vitro cellular environments.
Highlights
Paramagnetic species (PS), which contain at least one unpaired electron in their valence shell, play critical roles both in normal physiology and in the pathophysiology of many diseases.[1−4] Free radicals, including superoxide, hydroxyl, reactive oxygen species, and reactive nitrogen species, are important endogenous PS involved in numerous physiological processes including the immune response to infection,[5] cell signaling,[3] and redox regulation.[6]
The results are indicative of optically detected magnetic resonance (ODMR) spectra obtained from NV− color centers in that there is a reduction in PL
Differential interference contrast images were acquired for the same fields of view to demonstrate colocalization of contrast for the Gd3+-labeled liposomes with DIC images (Figure 6b) and lack of contrast for the control liposomes (Figure 6d). These findings demonstrate the potential the sensing protocol introduced here has for widefield mapping of endogenous PS produced within the intracellular compartments of live cells
Summary
Paramagnetic species (PS), which contain at least one unpaired electron in their valence shell, play critical roles both in normal physiology and in the pathophysiology of many diseases.[1−4] Free radicals, including superoxide, hydroxyl, reactive oxygen species, and reactive nitrogen species, are important endogenous PS involved in numerous physiological processes including the immune response to infection,[5] cell signaling,[3] and redox regulation.[6]. Among the catalog of NV-based sensing protocols is longitudinal spin (T1)-based relaxometry, whereby rapidly fluctuating magnetic fields are quantified based on the rate of mixing of spin states in the ground state triplet after optical polarization into the ground state |ms = 0⟩ This technique has been successfully implemented for detection of various PS, such as Gd3+,41,42 Mn2+, and ferritin.[43,44] While these methods prove to be highly sensitive and can provide spatial as well as quantitative information, they require the application of carefully constructed laser and microwave pulse sequences and measurement times in the order of seconds to hours to achieve an accurate readout of concentration, together with an optimal signal to noise ratio (SNR) necessary for highresolution quantitative imaging. The work reported presents a highly sensitive, minimally invasive dynamic sensing technique based on the contrast of PL emission from an ensemble of shallow NV−
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