Diamonds Detect Fleeting Chemical Signals in Real Time

Emma Herbst and colleagues have detected the nitroxide radical 2,2,6,6-Tetramethylpiperidinyloxyl (TEMPO) using nitrogen-vacancy (NV) centres in nanodiamonds. They observed a sharply concentration-dependent shortening of the longitudinal spin relaxation time (T₁) from 197s ±21s to 66s ±30s at a 1 M TEMPO concentration, achieving sensitivity in the nanomolar range and a signal-to-noise ratio of 1.6 to 3. The detection enables real-time monitoring of radical behaviour within liquid-phase chemical systems.

Nitroxide radical detection via nanodiamonds enables real-time monitoring of transient chemical

A dramatic shortening of the longitudinal spin relaxation time (T₁), from 197μs ±21μs to 66μs ±30μs, occurred with the in-situ detection of the nitroxide radical TEMPO using nanodiamonds containing nitrogen-vacancy (NV) centres. This represents the first direct monitoring of such short-lived radical intermediates in liquid solutions, overcoming a long-standing limitation in chemical kinetics. Previously, indirect methods such as electron spin resonance (ESR) spectroscopy, which often require freezing the sample and thus lose information about dynamic processes, or insurmountable signal limitations hindered the observation of these fleeting species. The ability to achieve sensitivity now extending into the nanomolar range offers a significant advancement in the field. The nitrogen-vacancy centre, a point defect in the diamond lattice where a carbon atom is replaced by a nitrogen atom and an adjacent vacancy, possesses a unique spin-dependent fluorescence that is exquisitely sensitive to its surrounding magnetic environment.

A concentration-dependent effect was confirmed, with substantial reduction in the T₁ relaxation time as TEMPO concentration increased. At 1 M TEMPO, the signal reached 66μs ±30μs, demonstrating the nanodiamonds’ sensitivity to the radical species. This shortening of T₁ is attributed to the magnetic dipole-dipole interaction between the unpaired electron spin of the TEMPO radical and the NV centre’s electronic spin. The closer the radical, the stronger the interaction and the faster the relaxation of the NV centre’s spin state. Confocal microscopy revealed that identifying nanodiamonds became more difficult at higher TEMPO concentrations, with only two bright nanodiamonds visible in a 20 × 20μm area compared to eight at zero concentration. This decrease in visible nanodiamonds is likely due to increased quenching of the NV centre’s fluorescence by the surrounding TEMPO radicals, a phenomenon where the excited state of the NV centre is non-radiatively decayed by energy transfer to the radical. Data from solutions six days old mirrored the results from freshly prepared samples, indicating stability in the measurement over time; T₁ values for 10^-9 M TEMPO ranged from 115μs to 143μs. This long-term stability is crucial for applications requiring prolonged monitoring of chemical reactions.

Nanoscale diamond sensors reveal previously undetectable chemical reactivity in liquids

Detecting fleeting radical intermediates has long been a bottleneck in understanding reaction mechanisms, and this new technique offers a pathway to observe these species directly within liquid environments. Radical species are crucial in a wide range of chemical and biological processes, including combustion, polymerisation, atmospheric chemistry, and enzyme catalysis. Understanding their behaviour is essential for optimising these processes and developing new technologies. This sensitivity opens avenues for studying complex biological systems and industrial chemical processes, although complex systems present challenges. For instance, biological samples often contain multiple paramagnetic species, requiring careful control experiments and data analysis to isolate the signal from the target radical. Acknowledging potential interference from other paramagnetic substances remains important for future development, potentially through the use of spectral filtering or advanced data processing techniques.

The observed changes in spin relaxation time now confirm the feasibility of in-situ detection using nanodiamonds containing nitrogen-vacancy (NV) centres, nanoscale defects within the diamond structure sensitive to magnetic fields. NV centres possess a ground state spin triplet, allowing for optically-detected magnetic resonance (ODMR) measurements. The NV centre is initialised in a specific spin state using laser excitation, and the subsequent decay of the spin state is monitored through changes in fluorescence intensity. The rate of this decay, quantified by T₁, is directly influenced by the local magnetic field. Monitoring these changes has bypassed the need for indirect measurements or sample extraction, which previously hindered the study of short-lived intermediates. Larger particles exhibited longer T₁ times, suggesting further optimisation is needed before widespread application, and the influence of nanodiamond size on signal strength warrants further investigation. Smaller nanodiamonds generally offer a higher surface area to volume ratio, increasing the probability of interaction with the target radical. This raises questions regarding the application of this approach to more complex, multi-radical systems and the potential for real-time analysis of changing chemical processes in diverse environments. Future research could focus on developing nanodiamonds with tailored surface functionalities to enhance radical capture and improve signal amplification, as well as exploring the use of multiple NV centres to create spatially resolved maps of radical distribution within the liquid sample.

The research successfully detected the nitroxide radical TEMPO in-situ using nanodiamonds containing nitrogen-vacancy centres. This demonstrates a new method for sensing short-lived radical species directly within a chemical reaction, avoiding the need to remove or indirectly measure them. The study observed a decrease in the longitudinal spin relaxation time from 197 s to 66 s as TEMPO concentration increased to 1 M, with sensitivity reaching the nanomolar range. Researchers suggest future work may focus on optimising nanodiamond properties and addressing potential interference from other paramagnetic substances.