With colorimetric sensors, problems with selectivity and degradation give rise to stability issues. Jason Locklin and graduate student Kristen Fries at the University of Georgia, [Fries et. al., Chem. Commun., 2008 DOI: 10.1039/b818042c] have developed covalently bound polymer brushes that can bind metal ions when irradiated with UV light.

When asked the motivation behind this research Locklin comments, “In the course of a concurrent project related to methods we are developing to move droplets using light, we have utilized ionic complexation as a way to amplify wettability changes. The work of Professor Dermot Diamond at Dublin City University has inspired us to use these polymer brushes as sensors. The bindings mentioned above are completely reversible, and can be switched using visible light.”

Through a grafting-from approach, the number of functional groups present at a surface can be greatly enhanced through the three-dimensional arrangement of tethered polymer chains.

This allows for a brush-like morphology, with extended chain conformations and high density of molecules in a limited area. The increased functionality can also be used to amplify the stimuli responsive nature of the polymer coating at a surface.

Locklin comments, “The polymer brush geometry allows for an increase in functionality and high

density of molecules in a limited area. Tuning the microenvironment through copolymerization can lead to matrices with enhanced selectivity.

Covalently bound polymer chains can also lead to sensors with improved stability.

Using molecular switches that use light to trigger an active or a passive state allows for improved lifetime of the sensor. For example, it provides a simple way to recalibrate the sensor and account for any drift.

The changes in wettability can be used to control the motion of fluids in microfluidic channels. We are currently using surface energy differences to mix different fluids using light as the control element.”

Locklin went on to say, “Developing sensors that are reversible and also extremely sensitive is one potential challenge. The binding has to be sufficiently weak to achieve reversibility. We plan to pursue improved techniques to push the detection limit of the sensors, possibly by using fluorescence as detection strategy.”