Detecting the invisible: Fluorescence-based identification of chemical threats

What are some of your responsibilities as director of the Centre for Organic Photonics and Electronics (COPE) at the University of Queensland?
I hold a number of responsibilities. Within COPE this includes setting research directions in consultation with the management team, raising research funding, and pursuing commercialisation of our technologies. As director I also support the wider research community within Australia through chairing the Australasian Community of Advanced Organic Semiconductors. This is a one-stop portal for connecting the groups working in the field across Australian and New Zealand, and they run a popular annual symposium.

What are some of the current projects at COPE that you’re most excited about?
It has been exciting to see our program on chemical-threat detection move from fundamental science to the point where we have technology in market. We still do fundamental science in the area, as this critical in ensuring that the next development and commercialisation steps are on firm scientific footing. We also have substantial programs on organic and perovskite solar cells. In the former, we are exploring the creation of high dielectric constant organic semiconductor materials for use in homojunction devices. The aim is to replace the need for bulk heterojunction films in organic solar cells, which although currently offer the most efficient devices require complex materials and processing conditions.

What led to your interest in chemistry? And, more specifically, to your interest in chemical-threat detection? 
I really just fell into chemistry at university. However, I particularly enjoy the organic semiconductor field as it brings together chemistry, materials science, and physics.

Investigating chemical-threat detection arose from a serendipitous discovery. My team in Oxford (UK) had been developing light-emitting dendrimers for organic light-emitting diodes, and a mass spectrum of an intermediate came back with an unusual mass pattern. We determined that the mass pattern arose from the fluorescent material binding the matrix that was used for the measurement. The matrix had a high electron affinity and remembering the pioneering work of Professor Timothy Swager on the use of conjugated polymers for nitro group-containing explosive detection, we wondered whether we could also detect such explosives. We could, and the rest is history.

How would you explain fluorescence sensing to an undergraduate chemistry student?
It would depend on what they had already learnt about the spectroscopy of molecules. That being said, I find that a visual demonstration with tennis balls as electrons is helpful. For a chemistry student early in their career, one way of explaining fluorescence sensing is to get them to think about the bonding between atoms, the difference between single and double bonds, and what happens if we provide sufficient energy to an electron to excite it from a bonding to an antibonding orbital – that is, a ground state to the first excited state. We can then discuss the fate of the excited electron: does it fall back down to the ground state? And, if so, how is the energy released? In the case of fluorescence, the energy is as light, with the colour of the light dependent on how far the electron falls. Once this is understood, we can use the process to consider how photoinduced electron transfer can be used to detect explosives that have a high electron affinity through thinking about how, instead of the excited electron simply decaying back to the ground state and giving out light, it can be captured by the high electron affinity explosive and then decay back to the ground state of the sensing material non-radiatively (without giving out light). The fundamental processes that occur in fluorescent materials can then be expanded on and used to explain the different sensing mechanisms used in the field.

What do you see as the most important aspect of your work at this time?
The most important aspect of my work at this time is provide mentoring and a supportive environment for my staff and students. I have a fantastic, dedicated team of young scientists that I need to keep resourced. One of the key drivers of our research is that it has the potential for social impact, e.g., next-generation solar cells to reduce our carbon footprint as well as chemical-detection systems that provide a safer environment for our communities, first responders, and security and defence personnel.

What is most exciting or surprising about your work? What are some of the challenges?
One of the most exciting aspects of our sensing work is the way it has developed over the years. We started by detecting nitro-group-based explosives and taggants, moved onto detecting organo-peroxide explosives and then chemical warfare agents and narcotics. Along the way we have developed new measurement techniques that have given unique insights into sensing mechanisms and shown that some of the concepts and approaches of earlier published work needs to be re-evaluated.

Reporting concepts resulting from application of new experimental methods that go against widely held views in the field has proved to be a major challenge. For example, we have shown that the widely held view of conjugated polymers having a unique amplified fluorescence response is not necessarily true in the real-time detection of explosive analytes.

What do you see as the future of fluorescence-based sensing? What would you like to see?
There are already commercial fluorescent-based sensing devices in the market, and I would like to see our technologies augmenting these existing devices to provide increased safety to our communities and security personnel at home and abroad. Fluorescence-based sensing is a fantastic technology as it is capable of trace detection – ppb or less – of chemical vapours, rapid response – seconds rather than minutes – and can be selective. The effort that needs to be expended on fluorescent organic semiconductor-based sensing is much smaller than other areas such as organic light-emitting diodes, transistors and solar cells. It would be great if more of the organic-semiconductor community brought their collective knowledge into the sensing field. I believe we are just at the beginning of the fluorescence-based sensing journey and that it can be applied in new areas within and beyond chemical threats.

What do you want attendees to learn from your talk at SPIE Optics + Photonics?
I would hope that the attendees would go away from my talk with an appreciation of how fluorescence-based sensing can be used for detecting chemical vapours, including understanding the modes of detection, the pitfalls to avoid, and the steps that are needed to move from a basic concept to a detector that is deployable in the field.