Colorimetric Transition of Polydiacetylene/Cyclodextrin Supramolecular Assemblies and Implications as Colorimetric Sensors for Food Phenolic Antioxidants

Abstract

Molecular self‐assembly has significant potential in the field of sensing. Polydiacetylenes (PDAs) are conjugated polymers possessing peculiar optical properties obtained by photopolymerization of self‐assembled diacetylene monomers. Herein, the blue‐to‐red phase transition upon either thermal stimulus or interaction with cyclodextrins (CDs) of two PDAs, bearing either carboxylic (PCDA) or amino (PCDA‐NH2) polar heads, is investigated to develop a colorimetric sensor for food phenolic antioxidants. The change in the PDA polar head does not affect significantly thermo‐chromatic transition. Upon thermal stimulus, in both PDAs, color transition occurs straightforward between two distinct stable states and does not involve the disordering of the PDA crystal phase, as revealed by UV–vis spectroscopy and SAXS analysis. Contrarily, PDA/α‐CD interaction is influenced by intermolecular forces among PDA polar heads and is more efficient for PCDA. α‐CDs presumably cause changes in both PDA backbone conformation and local environment surrounding the individual PDA chains. The PCDA/α‐CD assemblies are investigated as colorimetric sensors for the detection of Tyrosol (Ty) and caffeic acid (CAF), by using the principle of competitive inclusion complex formation. The system results to be more sensitive to CAF than Ty and may permit the determination of CAF in concentration ranges suitable for different food products.

Effect of UV Irradiation Time and Headgroup Interactions on the Reversible Colorimetric pH Response of Polydiacetylene Assemblies

“…Transitions at low temperatures may result from side chain relaxation, while those at higher temperatures can be attributed to rigid core movement. , We believe that the low enthalpy values in modified monomers are due to the clearing enthalpies of - m BzA moieties . Therefore, the H-bond formation among headgroups plays a key role in the liquid-crystal phase formation of DA monomers where the strength of these H-bonds varies in a wide range, leading to broader transition endotherms in DSC curves. , The polymorphic structure of modified DA monomers is further confirmed by the first cooling curves. In PCDA- m BzA, the first cooling exhibits three exothermic transitions, indicating recrystallization near the temperatures of crystalline site melting (Figure C).…”

“…46 Therefore, the H-bond formation among headgroups plays a key role in the liquid-crystal phase formation of DA monomers where the strength of these Hbonds varies in a wide range, leading to broader transition endotherms in DSC curves. 38,49 The polymorphic structure of modified DA monomers is further confirmed by the first cooling curves. In PCDA-mBzA, the first cooling exhibits three exothermic transitions, indicating recrystallization near the temperatures of crystalline site melting (Figure 1C).…”

“…We see that the change of headgroup changes the thermal properties of DA monomers dramatically. Modified monomers exhibit multiple transition temperatures with broad peaks, indicative of the enantiotropic crystalline phases in both systems. ,, Table summarizes the transition temperatures and the corresponding enthalpy changes.…”

“…Modified monomers exhibit multiple transition temperatures with broad peaks, indicative of the enantiotropic crystalline phases in both systems. 31,38,46 The transition peaks in the second heating of PCDA-mBzA appeared at 51.4, 72.2, and 106.9 °C, with corresponding ΔH endo values of 2.6, 1.2, and 3.3 kJ/mol, respectively (Figure 1C). Similarly, TCDA-mBzA exhibited broad peaks at 38.8, 59, and 92.4 °C, along with the corresponding ΔH endo values of 0.6, 0.6, and 1.4 kJ/mol (Figure 1D).…”

“…The pH of an environment is a crucial factor for processes in biological and chemical reactions . Depending on the headgroup, deprotonation or protonation promotes the ionic repulsive interactions in PDAs, resulting in colorimetric pH responsiveness. , Their irreversible colorimetric response has been exploited to construct PDA-based sensors, especially for food sensing applications. , A few examples of reversible colorimetric responses to changes in temperature , and pH ,,– have been reported. Monocarboxylic headgroup bearing PDAs lack required strong headgroup interactions, i.e., H-bonds, to facilitate the molecular reorganization of polymer chains back to their original planar conformation upon removing external triggers .…”