In the second part of this two-part interview, New Materials News speaks to Professor A.T. Charlie Johnson about how his research on molybdenum disulfide has implications for the mobile health and military industries.
Graphene has long been regarded as a “wonder material”, but its use in a number of interesting practical applications has been held back by its poor performance as a semiconductor. This is because graphene does not have an energy band gap, unlike molybdenum disulfide (MoS2), another monolayer material.
Professor Johnson is a Professor of Physics and Director of the Nano/Bio Interface Center at the University of Pennsylvania. He completed his Ph.D at Harvard University before joining the University of Pennsylvania in 1994.
What’s holding MoS2 back in terms of its practical applications?
It’s at a much earlier phase compared to graphene. People started working with graphene using the ‘Scotch tape method’ in 2004 and now graphene can be grown as large area film, even meters in dimension is at least possible. With MoS2 we’re not as good at growing it in large area forms. Even in our case, we’re growing it using a technique that is scalable, but we’re still not growing it in continuous layers.
Does commercial industry know about MoS2, or is it still only found in laboratories?
I would say it’s on the radar screen but it’s about time scale. Graphene is very heavily on people’s radars and people are trying to figure out what is good for and they’re quite interested in it. An angle that my group pursues very strongly is to look at biosensors and chemical sensors that are based on these monolayer materials.
We can already make bio-sensors and chemical sensors based on graphene and we are actively pursuing doing the same thing with MoS2. In fact, there are some reasons to believe that MoS2 might be preferable. The sensitivity of a biosensor of this type comes from the ability of the monolayer material to turn on and off. It turns out that graphene is still good for this application but MoS2 could be better.
Can you give an example of practical uses for biosensors?
There are many things that people are interested although nothing is commercialised at the moment. For example, there’s actually a startup company out of my lab called Graphene Frontiers and they’re looking to make biosensor systems that could, for example, take a blood sample and analyse it for a large number of different biomarker molecules by using a graphene device that is functionalised with receptor proteins for particular targets.
It’s actually interesting to be able to detect drugs, for example, to monitor patient compliance with a medication regiment. I talk to people in the healthcare system here at Penn and they’re saying “wouldn’t it be nice when treating someone to check if they’re taking their medication?” as people don’t always take the medication that they’re given. So imagine, when a patient comes in, we could just ask them to give a urine sample and we’d see straightaway if the drugs are in the urine.
So biosensors can diagnose diseases but also monitor therapy and ensure people are taking the medicine that they have been prescribed.
What about non-healthcare applications?
There are other applications for biosensors in the military. There’s a desire to monitor the performance and the readiness of soldiers with patches that could simply be applied to their bodies for several days. The patches could monitor chemicals in sweat, or by using micro-needles, and could check the soldiers to make sure they’re performing well, that they’re healthy and not suffering from excessive stress.
So there’s a wide number of applications across many different spaces; healthcare, soldiers, law enforcement or even just people who want to monitor their own health, for example, people seeing if their metabolism is sufficiently high if they’re trying to lose weight. So these are all possibilities. There’s a way to go until it actually works but this is the kind of thing in which people are interested.
How long will it take before these biosensors could be used? 5, 10, 20 years or even longer?
I would say it’s more straightforward to deal with a well-controlled sample under well-controlled conditions. A clinical diagnostic test could be seen on the scale of three to five years. I don’t think it has to take that long and we’re working hard to try and achieve it.
If you want something more uncontrolled then I think that’s more in the five to ten year timescale. For example, if you wanted to provide a deep analysis of what’s happening in your body from the sweat you produce then the sweat would be coming out of your body without the instrumentation to process it in any fancy way. You’d have to build that into the patch and then do it “on the fly”.
Of course, there are lots of technical issues that have to be dealt with. We’re talking about things on wearable and flexible platforms that are quite a bit more complicated that anything that’s been shown before. But there is a huge amount of activity and interest, that’s for sure. I think you’re going to see a lot of progress and I don’t think there are fundamental issues that are going to stop us.