Abstract
Methods and techniques to measure and image beyond the state-of-the-art have always been influential in propelling basic science and technology. Because current technologies are venturing into nanoscopic and molecular-scale fabrication, atomic-scale measurement techniques are inevitable. One such emerging sensing method uses the spins associated with nitrogen-vacancy (NV) defects in diamond. The uniqueness of this NV sensor is its atomic size and ability to perform precision sensing under ambient conditions conveniently using light and microwaves (MW). These advantages have unique applications in nanoscale sensing and imaging of magnetic fields from nuclear spins in single biomolecules. During the last few years, several encouraging results have emerged towards the realization of an NV spin-based molecular structure microscope. Here, we present a projection-reconstruction method that retrieves the three-dimensional structure of a single molecule from the nuclear spin noise signatures. We validate this method using numerical simulations and reconstruct the structure of a molecular phantom β-cyclodextrin, revealing the characteristic toroidal shape.
Highlights
We considered a simple molecule of β -cyclodextrin as a molecular phantom
We compute the fluctuating magnetic field amplitude (r.m.s.) produced by proton spins at the location of the NV spin by using the expression given by Rugar et al.[23]
We subject the molecule to the magnetic field gradients of 3 G/nm, and this produces spread in Larmor frequencies of 30.6 kHz for the hydrogen spins in the examined volume ( 3 times molecule size)
Summary
Several different schemes are currently being considered for nanoscale magnetic resonance imaging (MRI) using NV-sensors: scanning the probe[12,15,41], scanning the sample[23,24] and scanning the gradient[12,35]. Directions of field gradient applied to the molecule are changed by moving the magnetic tip to several locations This gradient gives distinct projection perspectives while the spin noise spectrum contains the corresponding nuclear spin distribution information. These signals are indexed using the coordinates of the magnetic tip with respect to the NV defect as Θ and Φ (measured using high-resolution ODMR) and are stored in a 3D array of S(Θ, Φ, ω) values. The reconstruction procedure is as follows: by knowing the complete magnetic field distribution from the tip[42], we can calculate the gradient orientation (θ, φ) at the sample location for any tip position (Θ, Φ) and rescale the spectral information to spatial information in 1D: ω = γr∇ B In this way, the encoded data set matrix dimensions are transformed into θ, φ and r. The resulting 3D matrix carries nuclear spin density in every element and contains three-dimensional image of the molecule
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