The so-called nitrogen-vacancy (NV) centre in diamond is an especially useful defect for a variety of quantum mechanical applications because it has a long spin coherence time, even at room temperature. This means that the quantum spins in the defect take time to flip from their original positions, which allows them to be read out reliably and reinitialized when needed. The structures can thus be employed as quantum probes to detect magnetic fields in their surroundings.

“The nanodiamonds themselves can also be placed with nanometre precision wherever we wish in a sample and be moved around at will,” explained team leader David Awschalom, “something that has potential applications in sensing, tracking and tagging in submicron biophysical systems.”

The UCSB team’s technique relies on measuring the electron spin resonance of the NV centres in nanodiamonds that have themselves been trapped using optical tweezers. The trapping part of the experiments involves using a single laser beam that is so tightly focused that dielectric particles, like those of nanodiamond, are pulled to the beam focus rather than being pushed forwards by the beam. The particles are thus held in the focus, optically levitated and trapped. “By moving the laser focus with respect to the fluidic environment, we can choose where to position the particles using an all-optical technique (no wires or physical contacts needed),” said Awschalom.

Optical tweezers were first developed in the 1970s, by Arthur Ashkin of Bell Laboratories in the US. They are now routinely used in nanophotonics and optics labs around the world.

Monitoring magnetic fields

Awschalom and colleagues, who report their work in PNAS, used nanodiamonds that had been commercially irradiated to create more than 500 NV centres in every 100 nm-sized diamond particle. The researchers then employed electron spin resonance to measure the energy-level structure of the NV centres. “We take advantage of the so-called Zeeman effect, which shifts the spin energy levels of the NV centre, to monitor the magnetic fields detected by the NV sensors in the nanodiamonds,” explained Awschalom.

“Being able to measure the local magnetic field at a chosen location in a fluidic environment using a laser to position the nanodiamond sensor may have multiple applications,” he told nanotechweb.org. “For example, it could help improve our understanding of biological cellular processes, electrochemical cells, surface catalysis or lipid membranes. It also offers us a new way to visualize important biological and chemical structures that may be difficult to probe with conventional techniques.”

The team says that it is now interested in using functionalized nanodiamonds in microfluidic channels in combination with optical trapping and electron spin resonance to maximize the potential for nanodiamond sensing and on-chip sorting to identify and quantify specific targets.