A new platform for ion sensing in small volumes – talking papers series, #2
This paper delivers on Professor Monro’s vision of new platforms for sensing that push the boundaries of what can be done with extremely versatile micro-structured optical fibres when resarchers take a trans-disciplinary approach to their collaboration. It is well known that these fibres have the potential to deliver results in real-time, in-situ, by directly accessing tiny volumes of fluids smaller than those typically found in living cells. The work done for the paper “Fluorescence-Based Aluminum Ion Sensing using a Surface Functionalized Microstructured Optical Fiber” combines commercially available chemicals with micro-structured fibres in a way that retains the useful characteristics of each, thus paving the way for a new platform in (ion) sensing. I spoke to two of the paper’s authors and IPAS director Professor Tanya Monro to find out more about the achievements and challenges in this important quest.
Stephen Warren-Smith and Sabrina Heng joined forces under Tanya’s guidance and with support from Andrew Abell, worked in the space between their two disciplines – Physics and Chemistry. Sabrina and her colleagues adapted an existing commercially available synthetic compound (a ‘flurophore’ called lumogallion). This new ‘compound 3’ is designed to stick to the glass surfaces of the micro-structured fibre while retaining or enhancing fluorescence in the presence of aluminium ions. Sabrina tells me they made good progress with ‘compound 3’ because it only takes a few steps to make, is made from low cost ingredients and provides a reasonably high yield.
While the compound did bind well to the fibre surface, it did so in such a way that may have impacted on sensitivity and was easily removed. There is room for improvement here. Stephen tells me there are many ways in which the binding may be improved – by tweaking the chemistry of the flurophore and/or that of the polyelectrolyte intermediate layer to which the fluorophore attaches. Alternatively, there are other ways to approach the coating (or ‘functionalisation’) of the micro-structured fibres to try in future research. One of the challenges faced is in quantifying (or even seeing) the extent to which the flurophore has coated the fibre surface – the subject of current work by other IPAS researchers. Tanya informs me that this is the first time IPAS has produced a variant of a known flurophore that has been adapted to enable surface functionalisation. This approach means there is great potential for development that benefits from the economy of scale.
- Small scale of flushing equipment
Something that surprised me while collecting photographs of the equipment used in this research was the small scale of everything. While the paper does contain phrases such as “approximately 2 mL of the reaction mixture was used for the experiment” I did not imagine the nitrogen pumping apparatus, or the fluorescence cuvettes at a realistic scale. Stephen reminded me that one of the key features of micro-structured optical fibres is their tiny (less than a human hair) outer diameters which makes them ideal for sensing in-situ. Furthermore, they can be as short (or long) as required to transmit the fluorescence information from the sample to the detector. As you can hear in this short audio recording with Prof Monro, Stephen’s experimental work is the first to record this type of “dip-sensing” that works in only a few nano-litres of liquid.
Only 2x600mm lengths of the specially enhanced fibre were used for this work – and working lengths of 60mm after surface coating (or ‘functionalisation’) were used to perform the sensing measurements. I recorded some audio from a conversation with Stephen which gives a great behind the scenes look at the remarkable teamwork required to envisage, design, create and characterise this new platform for sensing. It goes something like this…
The micro-structured fibre lengths were flushed with a saline solution of polyelectrolyte (PAH) for 30 mintues. Each 600mm length of fibre had to be cleaned by pumping water through for 30 minutes then dried by flushing with nitrogen gas for a futher 30 minutes. Visual inspection was done using an optical microscope which is quite fiddly and laborious – especially since it is not possible to see fluids inside – only the meniscus that forms at the barrier between fluid and air can be seen.
Early work on the chemical attachment concept originated with Dr Markus Pietsch who is now at the University of Cologne hospital in Germany. Fellow IPAS researcher Alexandre Francois suggested the polyelectrolyte approach which first binds an intermediate layer to the glass fibre via electrostatic forces. The modified flurophore then attaches chemically to this intermediate layer via covalent bonds. Roger Moore (no, not that one!) found a way to ‘inflate’ the tiny holes inside the ‘wagon-wheel’ fibre using positive gas pressure during the drawing process. This is needed to avoid blockage of the holes during the flushing and coating steps required to functionalise the surface and to enable liquid sample to enter the holes during dip sensing. Peter Hoffmann (now deputy director of IPAS) provided access to a Typhoon imager used in the glass slide characterisation. The whole effort was made possible thanks to funding from the DSTO and ARC for Tanya Monro’s Federation Fellowship program.
This paper opens the door to a whole new platform for sensing that will allow IPAS to build tools for further research and to partner with industry in developing products with a diverse range of applications in fields from medicine, the environment and agriculture through to defence industries. The commercial opportunity for sensors based on this platform are enhanced by some of the new discoveries and potential for cost effective scale.
Post written by
Mike Seyfang for IPAS “talking papers” series