20-07-2020 | | By Liam Critchey
Sensors have been scaling down in size for many years now, with ultrasmall active sensing elements becoming commonplace. Over the years, advanced fabrication methods have contributed to sensors which use smaller active sensing areas. Yet, their small size has enabled them to be much more sensitive than other bulk sensors. This is because these types of sensors have used nanomaterials (sizes range from 1 to 100 nm) and their small size (and active surface) can detect much smaller changes within a local environment, and this is what makes them more sensitive than other bulkier sensors. But, there is a step beyond nanoscale sensors, and this is the realm of single atom sensors, where just a single atom is used to detect minute changes in a local environment.
Single-atom sensors are highly sensitive and useful for the right applications. However, they are a type of sensor that is not suitable for many applications. If you need a robust sensor for measuring fluidic flows in a power plant, this is not your type of sensor, nor is it your type if you need to detect structural changes in a building. This is because these areas focus on the macro scale (i.e. measuring substantial changes/deviations). In contrast, a single atom sensor is used to measure the smallest of small changes in an environment. A key application area is to use such sensors in biological environments where small sensors are required to detect minimal changes―because even though some changes can be minimal on a cellular level, the effects of these changes can be considerable.
Here, we're looking at one of the latest developments where a single atom sensor has been developed to detect nitric oxide (NO) gas in cellular environments, as the overproduction of such gases is a sign that a patient may have a disease.
Now, it's not possible to just have one atom by itself as a whole sensing element. Because we're talking about something so small, the single-atom that does the sensing needs to be attached to something that acts as a molecular anchor and keeps the single atom in place. This is where the field of single atomic crystals comes into play. Even though more than a single atom is required within the sensing area for single-atom sensors to work, it is just the single atom that is involved with the sensing response.
Single-atomic crystals are a specialist material where a single metal atom is dispersed onto a solid support material. The support is needed to hold the single metal atom in place (sticking out) so that it can interact with the local environment. Single atomic crystals are rooted in catalysis, but their use outside of this field is growing, and its use in the sensing field is still in its infancy.
Nitric oxide is an essential gas within our bodies. Our bodies and acts produce it as a signalling molecule between cells. While nitric oxide is used for many essential processes, high concentrations can indicate issues within a patient. While the presence of elevated nitric oxide concentrations doesn't pose a risk in itself (i.e. the high gas concentration doesn't have any adverse effects itself), if a cellular environment is found to have a high localised nitric oxide concentration, it can indicate that the release of nitric oxide in the local environment is out of control.
This can be for many reasons, but some of the main reasons are due to cells experiencing mechanical forces (such as shear stress) from specific biological processes. These scenarios are commonplace for patients who have particular diseases, like neurodegenerative diseases, autoimmune processes and cancer. Measuring the local concentration of nitric oxide in a cellular environment is seen as a way of determining if a person could have one of these diseases―which is often easier said than done due to the small measurement environment.
While there are other applications for single-atom sensors, the ability to detect small nitric oxide changes in cellular environments offers a way of seeing if a person could have a disease in real-time, it's an application area that is gathering lots of interest. Researchers from China have now developed a single-molecule sensor based around nickel single atomic crystals (Ni-SAC) to monitor the levels of nitric oxide gas in live cellular environments.
To make a single atomic crystal-based sensing device, the researchers synthesised the single nickel atom onto N-doped carbon spheres. Even though the single atomic crystal was used in a sensing approach, the device still relied on its catalytic properties, as the primary mechanism of detection was through the single atom electrochemically oxidising the nitric oxide. With the released electron from the oxidation reaction providing a detectable response across the single atomic crystal support molecule before recombining with other molecules to form HNO2 as the finished catalysed product. The catalytic approach even showed an enhanced sensitivity over nickel-based nanomaterials (and other bulk materials). It could monitor the levels of nitric oxide within a cellular environment in real-time.
One of the issues of some inorganic atoms and materials is that they are not the most biocompatible. Still, this sensor was found to be suitable for biological environments and has a sensing range at the nanomolar level―meaning that trace amounts could be detected when released from the cells, enabling the smallest concentration changes to be identified.
Single-atom sensors are a lot trickier to produce that other types of sensors, this doesn't take away from the fact that they are highly efficient in specific scenarios―meaning that they could become a viable sensing option alongside the various nanomaterial-based sensors for replacing bulk sensors. Now, they are not going to be useful for all applications, but for measuring tiny changes, they are an excellent option if you're looking for high sensitivity.