All-Optical Sensing of a Single-Molecule Electron Spin

Abstract

We demonstrate an all-optical method for magnetic sensing of individual molecules in ambient conditions at room temperature. Our approach is based on shallow nitrogen-vacancy (NV) centers near the surface of a diamond crystal, which we use to detect single paramagnetic molecules covalently attached to the diamond surface. The manipulation and readout of the NV centers is alloptical and provides a sensitive probe of the magnetic field fluctuations stemming from the dynamics of the electronic spins of the attached molecules. As a specific example, we demonstrate detection of a single paramagnetic molecule containing a gadolinium (Gd 3+ ) ion. We confirm single-molecule resolution using optical fluorescence and atomic force microscopy to co-localize one NV center and one Gd 3+ -containing molecule. Possible applications include nanoscale and in vivo magnetic spectroscopy and imaging of individual molecules.Precision magnetic sensing is essential to a wide array of technologies such as magnetic resonance imaging (MRI), with important applications in both the physical and life sciences. In particular, in biology and medicine, functional magnetic resonance imaging (fMRI) has emerged as a primary workhorse for obtaining key physiological and pathological information noninvasively, such as blood and tissue oxygen level and redox status [1][2][3]. Developing nanoscale magnetic sensing applicable to individual molecules could enable revolutionary advances in the physical, biological, and medical sciences. Examples include determining the structure of single proteins and other biomolecules as well as in vivo measurements of small concentrations of reactive oxygen species that could lead to insights into cellular signaling, ageing, mutations, and death [4][5][6][7]. The practical realization of these ideas is extremely challenging, however, as it requires sensitive detection of weak magnetic fields associated with individual electronic or nuclear spins at nanometer scale resolution, often under ambient, room-temperature conditions. Many state-of-theart magnetic sensors, including superconducting quantum interference devices (SQUIDs) [8], semiconductor Hall effect sensors [9], and spin exchange relaxation-free atomic magnetometers [10], offer outstanding sensitivity, but their macroscopic nature precludes individual spin sensing. Sensing ensembles of paramagnetic molecules in biological and medical systems is currently performed using bulk electron spin resonance (ESR), which has a limit of roughly 10 7 electron-spins [11]. Magnetic resonance force microscopy has been used to detect individual electronic spins, but operates at cryogenic, milliKelvin temperature [12,13].In this Letter we demonstrate a method for nanoscale magnetic sensing of individual non-fluorescent molecules that employs optical manipulation of nitrogen vacancy (NV) centers in diamond [14][15][16][17][18][19][20][21]. In our approach the target molecules are covalently attached to the diamond surface, and magnetic sensing of these molecules is performed un...

Relaxometry and Dephasing Imaging of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit

“…In a recent experiment, gadolinium-containing molecules were localized on a diamond substrate and then a single molecule's magnetic noise was shown to inhibit the NV center's relaxation time. 18 The NV center has also been used to investigate superparamagnetic particles via spectroscopy, most prominently in the form of ferritin molecules 19 . Recently, it was successfully employed to study their temperature dependent dynamics 20 .…”

Atomic-Scale Nuclear Spin Imaging Using Quantum-Assisted Sensors in Diamond

“…Site-selective isotopic labeling [51] would allow focusing on a small number of 13 C or nitrogen spins, or even protons in molecules in a deuterated solution. The molecules could be functionalized and attached to the diamond surface, for example, by methods such as the EDC/NHS reaction, which forms a chemical bond that is stable over a few weeks [52]. Deterministic binding near a NV center may be obtained using the colocalization techniques developed in Ref.…”

“…Deterministic binding near a NV center may be obtained using the colocalization techniques developed in Ref. [52].…”

Anticrossing Spin Dynamics of Diamond Nitrogen-Vacancy Centers and All-Optical Low-Frequency Magnetometry

“…The detection of external spins with the NV centre is generally achieved through measuring the longitudinal spin relaxation rate (T 1 processes) [9][10][11][12][13] or transverse spin relaxation rate (dephasing, or T 2 processes) [12,[14][15][16] of the NV's electron spin, as they are sensitive to the magnetic field fluctuations produced by the target spins [17][18][19]. To obtain spectral information on the target spins, most studies so far have focussed on using T 2 -based techniques, which have been applied to the spectroscopy of small ensembles of either electronic [20][21][22][23] or nuclear spins [24][25][26][27][28].…”

“…When the transitional energy between two of the NV eigenstates (generally |0 e and |−1 e , where the number refers to the electron spin projection m e ) is equal to that of a target spin, crossrelaxation occurs, which results in an increase in the relaxation rate, 1/T 1 , of the NV [32,33]. This increase can be measured by purely optical means, even for a single NV centre [10][11][12][13]. By scanning across a range of magnetic field strengths, a resonance spectrum of the target spins can be obtained, which can be deconvolved to produce the target spin spectrum [32].…”