Dual Nanofriction Force Microscopy/Fluorescence Microscopy Imaging Reveals the Enhanced Force Sensitivity of Polydiacetylene by pH and NaCl

Abstract

Polydiacetylene (PDA) is a popular mechanochromic material often used in biosensing. The effect of its headgroup–headgroup interactions on thermochromism such as pH or salt concentration dependency has been extensively studied before; however, their effect on mechanochromism at the nanoscale is left unstudied. In this work, nanofriction force microscopy and 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 8-fold enhancement in force sensitivity. The elevation of NaCl concentration from 10 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.

Mechanical Properties of Polydiacetylene Multilayers Studied by AFM Force Spectroscopy

“…Micrometer-sized TRCDA/POPC hybrid films were deposited on coverslips by Langmuir–Blodgett methods, as reported previously. , POPC was added to increase the crystal size for facilitating the AFM experiments , and also because diacetylene/phospholipid hybrid films have been often used in biosensing as they present higher sensitivity. ,, In brief, TRCDA monomers and POPC were dissolved in chloroform at a 2.2:1 molar ratio at the total lipid concentration of 1.6 mg/mL, spread over the air–water interface in the Langmuir–Blodgett trough, compressed after the evaporation of solvent, and transferred vertically to oxygen-plasma-activated glass coverslips (Figure a–c). These samples were dried over 12 h under a vacuum in the monomer phase.…”

“…The transition from the blue to the red phase is marked by the reduction in the average conjugation length of the polymer, 10 despite the fact that the conjugated repeated unit (ene-eyn) itself remains unmodified for both chromatic phases. 11 This led researchers to believe that the twist of the alkyl side chains around the backbone is the origin of the color change, 2,12,13 which was supported by nuclear magnetic resonance (NMR) 14,15 and theoretical studies. 16 Mechanical properties of PDA have been traditionally studied by applying different types of macroscopic forces, as summarized in a recent review.…”

Dual Friction Force/Fluorescence Microscopy

“…Here, one of the pitfalls is that we need to perform zero-point correction to counteract the laser drift due to the fact that the hover mode is used (Figure S2). 40 Step 4: Registration of ΔFluorescence Image and the Lateral Force Map. The final step is the registration of the ΔFluorescence image obtained in step 2 and the lateral force map obtained in step 3.…”

“…So far, all of the experiments have been performed in air. Here, we characterized the nanoscale force sensitivity of PDA under aqueous solution 40 for the first time (Figure 6). The Langmuir−Blodgett film made by 10,12-tricosadiynoic acid (TRCDA) was characterized by the combined FFM and fluorescence microscopy setup in air and in buffer solution (Figure 6a−-e).…”

“…The scale bars are 10 μm. This figure has been reproduced from ref with permission from American Chemical Society, copyright 2023 …”