Dual nano-friction force microscopy/fluorescence microscopy imaging reveals the enhanced force sensitivity of polydiacetylene by pH and NaCl

Polydiacetylene (PDA) is a popular mechanochromic material often used in biosensing. The effect of its headgroup-headgroup interactions on the thermochromism such as pH or salt concentration dependency has been extensively studied before, yet, their effect on mechanochromism at nanoscale is left unstudied. In this work, the nano-friction force microscopy and the fluorescence microscopy were combined to study the effect of pH and ionic strength on the polydiacetylene (PDA) force-sensitivity at the nanoscale. We found that the increase in pH from 5.7 to 8.2 caused an enhancement in the force sensitivity by 8 folds. The elevation of NaCl concentration from 10 mM to 200 mM also made the PDA 5 times more force-sensitive. These results suggest that the PDA force sensitivity at the nanoscale can be conveniently enhanced by “pre-stimulation” with pH or ionic strength.