Phase transition, structure and color-transition behaviors of monocarboxylic diacetylene and polydiacetylene assemblies: The opposite effects of alkyl chain length

Supercooling suppression of microencapsulated capric acid-stearic acid eutectic via lignin-zein stabilized Pickering emulsion for highly efficient thermal energy storage

This study develops a fully biobased phase change material (PCM) by encapsulating capric acid-stearic acid eutectic (CSE) with a lignin-zein shell. It is driven by core-shell aggregation in Pickering emulsion during solvent exchange. Herein, the introduction of zein effectively promotes core-shell aggregation and stabilizes CSE Pickering emulsion. Consequently, CSE@lignin-zein capsules (zein/lignin: 10:90, w/w) achieve enhanced yield (up to 88.2 %) and encapsulation efficiency (up to 88.9 %) with a high melting enthalpy of 81.6 J g-1 at 28.2 °C. Furthermore, the lignin-zein shell significantly reduces the interaction between fatty acids (core) and lignin (shell), inducing homogeneous crystallization of fatty acids core. As a result, it suppresses supercooled phase transition at -2.5 °C and exhibits excellent thermal reliability over 200 cycles. The thermal conductivity of CSE@lignin-zein capsules is increased from 0.26 to 0.41 W m-1 K-1 with only 0.6 wt% graphene oxide nanosheet coating. Overall, this work offers a green solution to produce biobased PCM with highly efficient energy storage. The underlying mechanism of shell-induced supercooling suppression is studied systematically.

Fluorogenic Biosensing with Tunable Polydiacetylene Vesicles

Polydiacetylenes (PDAs) are conjugated polymers that are well known for their colorimetric transition from blue to red with the application of energetic stimulus. Sensing platforms based on polymerized diacetylene surfactant vesicles and other structures have been widely demonstrated for various colorimetric biosensing applications. Although less studied and utilized, the transition also results in a change from a non-fluorescent to a highly fluorescent state, making polydiacetylenes useful for both colorimetric and fluorogenic sensing applications. Here, we focus on the characterization and optimization of polydiacetylene vesicles to tune their sensitivity for fluorogenic sensing applications. Particularly, we look at how the structure of the diacetylene (DA) hydrocarbon tail and headgroup affect the self-assembled vesicle size and stability, polymerization kinetics, and the fluorogenic, blue to red phase transition. Longer DA acyl tails generally resulted in smaller and more stable vesicles. The polymerization kinetics and the blue to red transition were a function of both the DA acyl tail length and structure of the headgroup. Decreasing the acyl tail length generally led to vesicles that were more sensitive to energetic stimuli. Headgroup modifications had different effects depending on the structure of the headgroup. Ethanolamine headgroups resulted in vesicles with potentially increased stimuli responsivity. The lower energy stimulus to induce the chromatic transition was attributed to an increase in headgroup hydrogen bonding and polymer backbone strain. Boronic-acid headgroup functionalization led to vesicles that were generally unstable, only weakly polymerized, and unable to fully transform to the red phase due to strong polar, aromatic headgroup interactions. This work presents the design of PDA vesicles in the context of biosensing platforms and includes a discussion of the past, present, and future of PDA biosensing.

Cationic Amphiphilic Comb-Shaped Polymer Emulsifier for Fabricating Avermectin Nanoemulsion with Exceptional Leaf Behaviors and Multidimensional Controlled Release

The development of intelligent multifunctional nanopesticides featuring enhanced foliage affinity and hierarchical target release is increasingly pivotal in modern agriculture. In this study, a novel cationic amphiphilic comb-shaped polymer, termed PEI-TA, was prepared via a one-step Michael addition between low-molecular-weight biodegradable polyethylenimine (PEI) and tetradecyl acrylate (TA), followed by neutralization with acetic acid. Using the emulsifier PEI-TA, a positively charged avermectin (AVM) nanoemulsion was prepared via a phase inversion emulsification process. Under optimal formulation, the obtained AVM nanoemulsion (defined as AVM@PEI-TA) demonstrated exceptional properties, including small size (as low as 67.6 nm), high encapsulation efficiency (up to 87.96%), and high stability toward shearing, storage, dilution, and UV irradiation. The emulsifier endowed AVM@PEI-TA with a pronounced thixotropy, so that the droplets exhibited no splash and bounce when they were sprayed on the cabbage leaf. Owing to the electrostatic attraction between the emulsifier and the leaf, AVM@PEI-TA showed improved leaf adhesion, better deposition, and higher washing resistance in contrast to both its negatively charged counterpart and AVM emulsifiable concentrate (AVM-EC). Compared to the large-sized particles, the small-sized particles of the AVM nanoemulsion more effectively traveled long distances through the vascular system of veins after entering the leaf apoplast. Moreover, the nanoparticles lost stability when exposed to multidimensional stimuli, including pH, temperature, esterase, and ursolic acid individually or simultaneously, thereby promoting the release of AVM. The release mechanisms were discussed for understanding the important role of the emulsifier in nanopesticides.

Acid-responsive polydiacetylene-Na+ assemblies with unique red-to-blue color transition

Polydiacetylenes (PDAs), conjugated and stimuli-responsive polymers, are of interest for colorimetric sensing technologies. Commercially available PDAs with carboxylic headgroup do not show any colorimetric response to acid. To achieve acid-responsive property, the headgroups of PDAs are often modified with some functional moieties, involving complicated synthetic processes. This contribution presents a facile approach to develop acid-responsive materials via co-assembly of PDA and excess sodium hydroxide (NaOH). After low-temperature incubation and photopolymerization, the mixtures of 10,12-tricosadiynoic acid (TCDA) and NaOH develop into red-phase poly (TCDA-Na+) assemblies. A unique red-to-blue color transition occurs when the poly (TCDA-Na+) assemblies are exposed to hydrogen chloride (HCl) acid both in aqueous solution and gas phase. Increasing the concentrations of NaOH and TCDA monomer during the self-assembly process affects the molecular organization and morphologies of the resultant poly (TCDA-Na+) assemblies, which in turn govern the sensitivity to acid. The results of this study offer a simple and inexpensive method for developing acid-responsive PDAs, extending their colorimetric sensing applications.

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

Polydiacetylenes are chromatic conjugated polymers formed upon the photopolymerization of self-assembled diacetylenes. They exhibit conformation-dependent colorimetric responses, usually irreversible, to external triggers. Here, we presented an approach to obtain a reversible colorimetric response to a pH change through structural modifications on the monomer and extended photopolymerization time. Both factors, enhanced hydrogen bond forming headgroups and longer UV exposure, impacted the rotational freedom of polydiacetylene conformation. Such a restricted conformation state reduced colorimetric response efficiency but enabled reversible colorimetric response to a pH change. These results highlight the possibility of obtaining a reversible colorimetric pH response of polydiacetylenes for customized sensing applications through monomer-level tailoring combined with tuning the photopolymerization time.

Dual Nanofriction 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 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.

Tracking Mechanical Stress and Cell Migration with Inexpensive Polymer Thin-Film Sensors

Polydiacetylene (PDA) Langmuir films are well known for their blue to red chromatic transitions in response to a variety of stimuli, including UV light, heat, bio-molecule bindings and mechanical stress. In this work, we detail the ability to tune PDA Langmuir films to exhibit discrete chromatic transitions in response to applied mechanical stress. Normal and shear-induced transitions were quantified using the Surface Forces Apparatus and established to be binary and tunable as a function of film formation conditions. Both monomer alkyl tail length and metal cations were used to manipulate the chromatic transition force threshold to enable discrete force sensing from ~50 to ~500 nN μm-2 for normal loading and ~2 to ~40 nN μm-2 for shear-induced transitions, which are appropriate for biological cells. The utility of PDA thin-film sensors was demonstrated with the slime mold Physarum polycephalum. The fluorescence readout of the films enabled: the area explored by Physarum to be visualized, the forces involved in locomotion to be quantified, and revealed novel puncta formation potentially associated with Physarum sampling its environment.

Quantitative Colorimetric Detection of Dissolved Ammonia Using Polydiacetylene Sensors Enabled by Machine Learning Classifiers

Easy-to-use and on-site detection of dissolved ammonia are essential for managing aquatic ecosystems and aquaculture products since low levels of ammonia can cause serious health risks and harm aquatic life. This work demonstrates quantitative naked eye detection of dissolved ammonia based on polydiacetylene (PDA) sensors with machine learning classifiers. PDA vesicles were assembled from diacetylene monomers through a facile green chemical synthesis which exhibited a blue-to-red color transition upon exposure to dissolved ammonia and was detectable by the naked eye. The quantitative color change was studied by UV-vis spectroscopy, and it was found that the absorption peak at 640 nm gradually decreased, and the absorption peak at 540 nm increased with increasing ammonia concentration. The fabricated PDA sensor exhibited a detection limit of ammonia below 10 ppm with a response time of 20 min. Also, the PDA sensor could be stably operated for up to 60 days by storing in a refrigerator. Furthermore, the quantitative on-site monitoring of dissolved ammonia was investigated using colorimetric images with machine learning classifiers. Using a support vector machine for the machine learning model, the classification of ammonia concentration was possible with a high accuracy of 100 and 95.1% using color RGB images captured by a scanner and a smartphone, respectively. These results indicate that using the developed PDA sensor, a simple naked eye detection for dissolved ammonia is possible with higher accuracy and on-site detection enabled by the smartphone and machine learning processes.

Dendronized polydiacetylenes via photo-polymerization of supramolecular assemblies showing thermally tunable chirality

Transformation of supramolecular chiral assemblies into covalent polymers integrates characteristics of supramolecular chemistry together with covalent entities, leading to fabrication of covalent chiral materials through versatile supramolecular chiral assemblies. Here, we report supramolecular assembly of an amphiphilic dendronized 10,12-pentacosadiynoic amide (PCDA) in aqueous solutions to form twisted ribbons, which were transferred into covalent dendronized polydiacetylenes (PDAs) via photopolymerization. These supramolecular dendronized PCDA and the corresponding covalent dendronized PDAs showed unprecedent thermoresponsive properties. The thermally-induced dehydration and aggregations tuned reversibly their chiralities, which can be visually inspected through colour changes.

Structures and strategies for enhanced sensitivity of polydiacetylene(PDA) based biosensor platforms

Polydiacetylene (PDA) is a versatile polymer that has been studied in numerous fields because of its unique optical properties derived from alternating multiple bonds in the polymer backbone. The conjugated structure in the polymer backbone enables PDA to possess the ability of blue-red colorimetric transition when π-π interactions in the PDA backbone chain are disturbed by the external environment. The chromatic property of PDA disturbed by external stimuli can also emit fluorescence in the red region. Owing to the unique characteristics of PDA, it has been widely studied in facile and label-free sensing applications based on colorimetric or fluorescence signals for several decades. Among the various PDA structures, membrane structures assembled by amphiphilic molecules are widely used as a versatile platform because facile modification of the synthetic membrane provides extensive applications, such as receptor-ligand interactions, resulting in potent biosensors. To use PDA as a sensory material, several methods have been studied to endow the specificity to PDA molecules and to amplify the signal from PDA supramolecules. This is because selective and sensitive detection of target materials is required at an appropriate level corresponding to each material for applicable sensor applications. This review focuses on factors that affect the sensitivity of PDA composites and several strategies to enhance the sensitivity of the PDA sensor to various structures. Owing to these strategies, the PDA sensor system has achieved a higher level of sensitivity and selectivity, enabling it to detect multiple target materials for a full field of application.

Digital analysis of polydiacetylene quality tags for contactless monitoring of milk

The incorporation of colorimetric sensors as quality indicators in food packaging is an exciting new area of research that could improve food management. The standard approach, however, demands a reliable interface between the sensor and the food and risks food contamination which is a significant consumer concern. To overcome this challenge, herein, we develop a polydiacetylene/phospholipid agarose-based sensor that encapsulates milk in the hydrogel matrix during synthesis. The chemical recognition of free fatty acids, a product of microbial spoilage of the encapsulated milk, induces a gradual blue to red color change in the sensor. We demonstrate that the new composite material exhibits the same spoilage kinetics as regular liquid milk (digital colorimetric response 28 ± 1% and 27 ± 3% respectively), indicating the agarose does not preserve the milk ingredients nor inhibit the detection mechanism of the polydiacetylene sensors. As a result, this sensor can be attached to the external surface of food packaging to provide an indirect indication of food quality without the need for contact with the milk product. The quality tags we present can be "switched" on and off using dehydration and rehydration, removing the need for in situ manufacturing and allowing storage before use. We show that the quality tags produce a similar digital colorimetric response of 21 ± 2% to indicate milk spoilage after rehydration. The color change of the quality tags could not be analyzed using absorption spectroscopy, the standard technique for polydiacetylenes, due to the opacity that milk imparts on the sample. To solve this problem, we develop digital colorimetric analysis software using the Python programming language to describe the extent of color change in polydiacetylene materials and develop a new metric termed the Digital Colorimetric Response that describes polydiacetylene response with excellent linearity (R² = 0.96). The software is programmed to employ statistical cleaning techniques that automatically remove image noise and outliers based on a pixel's grayscale Z-score. This new approach to sensor design increases practicality and could be extended to the contactless quality monitoring of other foods, medicines and other products whose safety or quality is jeopardized with direct sensor contact.

Naked-Eye Detection of Ethylene Using Thiol-Functionalized Polydiacetylene-Based Flexible Sensors

Ethylene is a hormone that plays a critical role in many phases of plant growth and fruit ripening. Currently, detection of ethylene heavily relies on sophisticated and time-consuming conventional assays such as chromatography, spectroscopy, and electrochemical methods. Herein, we develop a polydiacetylene-based sensor for the detection of ethylene via color change. The sensors are prepared through the reaction between polydiacetylene and Lawesson's reagent that results in decorating polydiacetylene with terminal thiol groups. Upon exposure to ethylene, the sensor changes color from blue to red which is visible to the naked eye. Our device shows a limit of detection for ethylene at 600 ppm in air and can be applied for monitoring ethylene released during the fruit-ripening process. Such easy-to-use ethylene sensors may find applications in plant biology, agriculture, and food industry.

Tuning chromatic response, sensitivity, and specificity of polydiacetylene-based sensors

In this review, we provide an overview of six major techniques to tune the sensitivity and specificity of polydiacetylene-based sensors.