Dr. Sultana won funding to advance a nanomaterial-based detector platform that can sense environmental parameters from minute concentrations of target gases and vapors, to atmospheric pressure and temperature, and then transmit that data via a wireless antenna — all from the same 3 × 5’’ self-contained platform.
Tech Briefs: Did you start with the concept of a multifunctional sensor platform?
Dr. Mahmooda Sultana: Initially I started with single gas sensors. When I first came to Goddard, I wanted to develop new instruments for future flight missions. At that time, I was working on graphene — because of its promise, scientists all around the world wanted to explore what potential applications it had to offer. I was intrigued by the set of extreme properties of graphene. So, while identifying possible space applications, I started working on graphene chemical sensors. At the time I was only looking at single-gas sensors. But as the project evolved, I figured that it would make sense to have an array of sensors, and not just gas sensors, but also other environmental sensors and subsystems — the concept of the multifunctional sensor platform was developed over time.
Tech Briefs: How do you sense different gas species?
Dr. Sultana: These nano-materials are highly sensitive — they respond to many different gases for example; the same sensor might respond to hydrogen, as well as methane or ammonia. What we do to induce selectivity is to functionalize these sensors with groups that only interact with one of the target species. Which functional group interacts with which target gas is somewhat known from the field of catalysis; for example, platinum and palladium are known to interact with hydrogen. So, we take a nanosensor, either made with carbon nanotubes, graphene, molybdenum disulfide, or molybdenum diselenide, and then we put nanoparticles of platinum or palladium on it. As a result, this one sensor responds more strongly to hydrogen.
Currently, we are targeting species of gases that are some of the key elements for the origination of life as we know it, but we can also customize the functionalization to target a wide range of other species. We decided to start with gases that are of great importance to planetary science so that we can propose this device for an upcoming mission opportunity to look at the possibility of life and habitability on other planetary bodies.
Tech Briefs: How would you detect temperature and pressure?
Dr. Sultana: We use different materials and/or different device structures to sense different parameters. For example, we print temperature sensors with metal nanoparticles. For pressure sensors, we use two-dimensional material as a membrane over a cavity. Some of the nanomaterials we use for gas sensing are also sensitive to pressure. We came up with a way to decouple the pressure from the concentration of different species of gases.
Tech Briefs: What does "detect" mean — I guess you have electronics that interface with the sensor. What do the electronics sense?
Dr. Sultana: When we have any surface and some gas contained in a volume, some gas molecules will adsorb to the surface, resulting in a change in the electrical properties. For most surfaces, the change is too minute to detect. However, graphene and some of the other nanomaterials have crystalline structures with superior electrical properties and low thermal noise. As a result, even minute changes in electrical resistance can be picked up. When gas molecules or atoms adsorb onto graphene or our sensor material, even though the resistance change is small, it is measurable. Another thing is that these sensors have very high surface to volume ratio. As a result, many gas molecules can adsorb compared to the volume of the sensor material, changing the resistance by a measurable amount. The resistance could either increase or decrease based on the type of molecule that is adsorbing. Some molecules are charge donors — for them it’s one response, and others are charge receptors, so for them, the resistance changes in the opposite direction. We calibrate our sensors based on the target species.
Tech Briefs: I didn’t quite get the significance of the ratio of surface to volume.
Dr. Sultana: I’ll give you a specific example. Graphene is a one-atomic-layer material, which means its thickness is only about 3.35 angstroms — that’s about a million times thinner than a single hair strand. As a result, the volume is extremely small, yet, the surface area is as large as possible with all the atoms at the surface. All the atoms are available to interact with gas molecules.
Tech Briefs: How do you physically connect the electronics to the sensor?
Dr. Sultana: We put electrodes right on top of the sensors through microfabrication. These electrodes are then connected through wire bonding to the printed circuit board we developed. The PC board contains the readout electronics.
Tech Briefs: What kind of power source do you use?
Dr. Sultana: Right now, we are just using a laboratory power supply. For actual applications, we are planning on using either a battery or a source coupled with a battery. We have not worked out that part yet.
Tech Briefs: The wireless transmitter — have you worked on that yet?
Dr. Sultana: Yes, we are incorporating a wireless communication module along with our sensor and readout chip. We have an antenna printed with a 3D printing technique directly on our board. Then we have the RF transmitter integrated with the PCB.
Tech Briefs: Would you transmit a digital signal?
Dr. Sultana: Yes, that’s what we plan. The idea is that once we send a network of these platforms to a planetary body, each one of them can take data and send it to a mother ship or a rover from which the signal can be sent down to the ground. They can even be useful in remote areas of earth — you can drop one of these down a volcano or a cave with a parachute and find out which gas species are present at what quantity.
Tech Briefs: So, you have multiple sensors on the same board. How does the electronics distinguish among them?
Dr. Sultana: Right now, the readout cycles among the sensors. The electronics are really fast, so we can cycle among them and still get a data rate greater than 10Hz, which is more than enough for our target applications.
Tech Briefs: Do you have a plan for how to go about increasing the sensitivity?
Dr. Sultana: I have several ideas, some of which include changing the device parameters and modifying the current electronics design.
An edited version of this interview appeared in the May 2019 issue of Tech Briefs.