Unique Effects of Cyclodextrins on the Formation and Colorimetric Transition of Polydiacetylene Vesicles

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.

Polydiacetylene (PDA) Liposome-Based Immunosensor for the Detection of Exosomes

Exosomes are extracellular vesicles (EVs) that have attracted attention because of their important biological roles in intercellular communication and transportation of various biomolecules, including proteins and genetic materials. However, due to difficulties in their selective capture and detection, further application of exosomes remains challenging. To detect EVs, we fabricated a liposomal biosensor based on polydiacetylene (PDA), a conjugate polymer that has been widely used in sensing applications derived from its unique optical properties. To confer selectivity and sensitivity to the sensory material, antibodies targeting CD63, a membrane protein exclusively found in exosomes, were attached to the PDA liposomes and phospholipid molecules were incorporated into the PDA vesicles. Signal analysis derived from PDA liposomes for exosome detection and quantification was performed by observing colorimetric changes triggered by the ligand-receptor interaction of PDA vesicles. Visual, UV-visible, and fluorescence spectroscopic methods were used to obtain signals from the PDA lipid immunosensor, which achieved a detection limit of 3 × 10⁸ vesicles/mL, the minimum concentration that can be used in practical applications. The strategies used in the system have the potential to expand into the field of dealing with exosomes.

Polydiacetylenyl β-cyclodextrin based smart vesicles for colorimetric assay of arginine and lysine

Selective visualization of arginine and lysine has been explored among 20 amino acids using the hybrid conjugate of β-cyclodextrin (β-CD) and polydiacetylene (PDA). The mono pentacosa-10,12-diynyl aminomethyl group was successfully coupled to either the primary or the secondary face of β-CD, where mono-6-amino-6-deoxy-β-CD or mono-3-amino-3-deoxy-β-CD reacted with the N-hydroxysuccinimide ester of 10,12-pentacosadiynoic acid. In this combinatorial system, the cylindrical β-cyclodextrin functions as a channel for the introduction of the cationic amino acids to the artificial membrane. The membrane perturbation and aggregation by the target amino acids could be exclusively visualized as a blue to red color change based on the responsive polydiacetylene domain. These interesting findings demonstrated that the developed β-CD conjugated PDA system may offer a new method of cell-penetrating mechanism, a promising vector system, as well as impact the production industry of arginine or lysine.

A 3D networked polydiacetylene sensor for enhanced sensitivity

Immobilization of polydiacetylene (PDA) vesicles on the surface of the modified carbon nanotube (CNT)-networked pillared structures afforded a 3D networked sensor system. A more than three order increase in the sensitivity was observed with the 3D networked sensor matrix in comparison with a conventional 2D PDA sensor system.

Host-guest-driven color change in water: influence of cyclodextrin on the structure of a copper complex of poly((4-hydroxy-3-(pyridin-3-yldiazenyl)phenethyl)methacrylamide-co-dimethylacrylamide)

In the present work we report the synthesis of poly((4-hydroxy-3-(pyridin-3-yldiazenyl)phenethyl)methacrylamide-co-dimethylacrylamide) and its reversible optical and complex-forming properties due to copper and cyclodextrin (CD) interactions. Color changing effects are characterized by UV-vis spectroscopy and the supramolecular behavior is investigated by dynamic light scattering experiments.

Naked eye screening of 11 phenolic compounds and colorimetric determination using polydiacetylene vesicles with α-cyclodextrin

The colorimetric response (CR) of poly(10,12-pentacosadiynoic acid) vesicles (PPCDA) induced by α-cyclodextrin (α-CD) in an aqueous solution has been studied. Various parameters affecting the CR, such as response time and concentration were investigated. The blue color of 0.01 mM PPCDA solution became pinkish red with the addition of α-CD at the concentration higher than 3 mM. The inhibition of the color transition from blue to red was investigated using 11 phenolic compounds. The color transition could be inhibited and observed by naked eye in the presence of 4 phenolic compounds, i.e. 4-nitrophenol (4-NP) and 4-bromophenol (4-BP) and 4-chlorophenol (4-CP) and 3-nitrophenol (3-NP). A colorimetric method for the determination of these compounds was validated and applied for surface water analysis. The linear range from the plot of CR against phenolic compounds concentration was in the order of 0.5-2.0 mM with R(2) more than 0.99. The recoveries were 90-95% with good precision (1-4%RSD, n=10).

Colorimetric biosensing using smart materials

In recent years, colorimetric biosensing has attracted much attention because of its low cost, simplicity, and practicality. Since color changes can be read out by the naked eye, colorimetric biosensing does not require expensive or sophisticated instrumentation and may be applied to field analysis and point-of-care diagnosis. For transformation of the detection events into color changes, a number of smart materials have been developed, including gold nanoparticles, magnetic nanoparticles, cerium oxide nanoparticles, carbon nanotubes, graphene oxide, and conjugated polymers. Here, we focus on recent developments in colorimetric biosensing using these smart materials. Along with introducing the mechanisms of color changes based on different smart materials, we concentrate on the design of biosensing assays and their potential applications in biomedical diagnosis and environmental monitoring.

Polydiacetylene incorporated with peptide receptors for the detection of trinitrotoluene explosives

Because of their unique optical and stimuli-response properties, polydiacetylene-based platforms have been explored as an alternative to complex mechanical and electrical sensing systems. We linked chromic responsive polydiacetylene (PDA) onto a peptide-based molecular recognition element for trinitrotoluene (TNT) molecules in order to provide a system capable of responding to the presence of a TNT target. We first identified the trimer peptide receptor that could induce chromic changes on a PDA backbone. We then investigated the multivalent interactions between TNT and our peptide-based receptor by nuclear magnetic resonance (NMR) spectroscopy. We further characterized various parameters that affected the conjugated PDA system and hence the chromic response, including the size of end-group motifs, the surface density of receptors, and the length of alkane side chains. Taking these necessary design parameters into account, we demonstrated a modular system capable of transducing small-molecule TNT binding into a detectable signal. Our conjugated PDA-based sensor coupled with molecular recognition elements has already proven useful recently in the development of another sensitive and selective electronic sensor, though we expect that our results will also be valuable in the design of colorimetric sensors for small-molecule detection.

Size-controlled fabrication of supramolecular vesicles for the construction of conjugated polymer sensors with enhanced optical properties

Polymerizable supramolecular monomer vesicles are readily fabricated by employing a hydrodynamic focusing method on a microfluidic chip. The polymerized diacetylenene nanovesicles, generated using the microfluidic method, display an improved fluorescence property compared to those prepared by employing a conventional bulk method. The flexibility of the vesicle size control by manipulating the flow conditions is another significant feature of the new microfluidic approach.

Preparation, characterization, and sensing behavior of polydiacetylene liposomes embedded in alginate fibers

Polydiacetylene (PDA)-doped calcium alginate fibers were created by the solution blending of polymerized 10,12-pentacosadiynoic acid liposomes with sodium alginate in water prior to extrusion. The liposomes maintained their blue color during wet spinning and drying of the fibers but changed to red with exposure to specific external stimuli (heat, solvent, and chemical). In the latter case, the color change only occurred when the fibers were sufficiently permeable for the reacting species to reach the interior. A parameter termed the "Raman response" (RR) has been developed to quantify the amount of PDA liposomes in each of two critical conformations within the fibers. The RR attributes a quantitative measure of PDA response to individual stimuli. This method provides advantages over the commonly used "colorimetric response" in systems where sample limitations and chromophore activity make UV-vis spectroscopic measurements difficult or inaccurate. PDA liposomes are shown to effectively add a versatile sensing component to alginate fibers.

Fluorogenic polydiacetylene supramolecules: immobilization, micropatterning, and application to label-free chemosensors

This Account describes a new strategy for the preparation of label-free sensor systems based on the fluorogenic properties of the conjugated polymer, polydiacetylene (PDA). PDA has been extensively investigated as a sensor matrix, owing to a brilliant blue-to-red color transition that takes place in response to environmental perturbations. It has been known for some time that "blue-phase" PDAs are nonfluorescent while their "red-phase" counterparts fluoresce. For the most part, however, the significance of the different fluorogenic properties of PDAs has been ignored in the context of sensor applications. In the course of developing PDA-based sensors, we discovered that PDA vesicles can be readily immobilized on solid substrates. This is an attractive property of PDAs since it leads to the combined advantages of the vesicle sensors (which have three-dimensional interactions between sensor and target molecules) and film sensors (which are applicable to a two-dimensional array or chip format). Stable blue-phase immobilized PDAs can be prepared by employing one of three strategies involving the formation of covalent adducts, biotin-avidin complexes, or complexes formed through nonspecific physical adsorption. A procedure for generating well-patterned fluorescence images is necessary for the immobilized PDAs to function in chip-based sensor systems. Patterned fluorescence images are readily constructed by employing (1) the photolithographic technique, (2) the micromolding in capillaries (MIMIC) method, or (3) an array spotting system. Heat treatment of the patterned "blue-phase" PDA vesicles transforms the nonfluorescent images into their fluorescent red forms. The observation that finely resolved fluorescence patterns can be generated by heat treatment of microarrayed PDAs is highly significant in that it indicates that fluorescence signals might be produced by specific molecular recognition events. Indeed, red fluorescence emission is observed when immobilized PDAs are subjected to specific molecular recognition events, such as ligand--cyclodextrin or protein-protein interactions. The facile immobilization of PDA vesicles on solid substrates and the affinity-induced fluorescence emission combine to make this system applicable to the fabrication of label-free PDA sensors. Since in theory any molecular recognition event that promotes the blue-to-red color transition of PDAs should result in the generation of fluorescence, it should be possible to reformat a variety of previously described colorimetric PDA sensors into fluorescence-based sensor systems. The fluorescence properties of PDAs, when combined with modern methods for the fabrication of microarrays, should stimulate the development of a number of new label-free chemosensor systems.