Quantitative and Anisotropic Mechanochromism of Polydiacetylene at Nanoscale

Abstract

Quantitative and anisotropic mechanochromism of polydiacetylene over nanoscale distances remains unaddressed even after 50 years of extensive research. This is because its anisotropic structure on substrates necessitates the application of both vertical and lateral forces (shear forces) to characterize it, whereas atomic force microscopy, which is the usual technique used to investigate nanoscale forces, is only capable of quantifying vertical forces. In this study, we address this lacuna by utilizing quantitative friction force microscopy that measures lateral forces. Our data confirm that polydiacetylene reacts only to lateral forces, F // , and disprove the previously claimed hypothesis that the edges of the polymer crystals exhibit higher force sensitivity than the rest of the crystal. In addition, we report a correlation between mechanochromism and thermochromism, which can be attributed to the fact that both work and heat are different means of providing the same transition energy.

Recent progress in polydiacetylene mechanochromism

“…6b and c). 47 These data provide the first insight into quantitative, anisotropic PDA mechanochromism at the nanoscale, where the crystal-to-amorphous transition of the PDA structure seems to play an important role. Friction force microscopy combined with fluorescence microscopy can be also used to characterize other mechanosensitive polymers 78–80 and mechanophores 81 in the future.…”

“…6). 47 Friction force microscopy is an AFM-based technique that enables the quantification of forces lateral to the substrate by calibrating lateral laser deflection into forces. 75,76 The use of this experimental technique was partially enabled by our recent identification of an error source in the wedge calibration method over the nanonewton range.…”

“…Despite our general understanding of their mechanism and the large number of reports in biosensing, the lack of a quantitative understanding of the color-changing mechanism of PDAs at the nanoscale has hindered further development until recently, when AFM was introduced. 44,47,48 The recent technological advancement of AFM could shift the status of PDA as a quantitative mechanosensor at the nanoscale, which may lead to another leap in application development. Although there have been many informative reviews on PDAs, especially from the perspective of biosensing, 49–59 their mechanochromism, where external forces stimulate the blue-to-red transition, has not been reviewed since a brief report by Burns et al in 2004.…”

Mechanofluorochromic Material toward a Recoverable Microscale Force Sensor

“…Light, [ 7–11 ] heat, [ 10,13–15 ] solvent (and its vapor), [ 16–18 ] and mechanical forces, [ 2,6,19–22 ] modify the structure of materials at the (supra)molecular level to effect changes in both absorption and emission properties. Among these stimuli‐responsive processes, mechanofluorochromism, [ 20–50 ] the change in optical properties upon application of mechanical stimuli such as scratching or grinding, promises applications for force sensors. [ 20–36 ] Emission changes of coordination complex crystals, [ 24 ] polymeric microcrystals, [ 30 ] and organic fluorophores, [ 31–36 ] have been correlated to an applied force or pressure for new anticounterfeiting technologies [ 23–28 ] and biological stress probes.…”

“…Among these stimuli‐responsive processes, mechanofluorochromism, [ 20–50 ] the change in optical properties upon application of mechanical stimuli such as scratching or grinding, promises applications for force sensors. [ 20–36 ] Emission changes of coordination complex crystals, [ 24 ] polymeric microcrystals, [ 30 ] and organic fluorophores, [ 31–36 ] have been correlated to an applied force or pressure for new anticounterfeiting technologies [ 23–28 ] and biological stress probes. [ 29 ]…”