Sensitive sulfide ion detection by optofluidic catalytic laser using horseradish peroxidase (HRP) enzyme

We report an optofluidic catalytic laser for sensitive sulfide ion detection. In the catalytic reaction, horseradish peroxidase (HRP) enzyme is used for catalyzing the non-fluorescent substrate, 10-Acetyl-3,7-dihydroxyphenox-azine (ADHP), to produce highly fluorescent resorufin, which was used as gain medium for lasing. Using sulfide ions as inhibitors, the catalytic reaction slows down, resulting in a delay in the lasing onset time, which is used as the sensing signal. The sensing mechanism of the catalytic laser is theoretically analyzed and the performance is experimentally characterized. Sulfide anion is chosen as a model ion because of its broad adverse impacts on both environment and human health. Due to the optical feedback provided by the laser, the small difference in the sulfide ion concentration can be amplified. Consequently, a detection limit of 10 nM is achieved with a dynamic range as large as three orders of magnitude, representing significant improvement over the traditional fluorescence and colorimetric methods. This work will open a door to a new catalytic-laser-based chemical sensing platform for detecting a wide range of species that could inhibit the catalytic reaction.