Abstract
The multiresponsive behavior of functionalized water-soluble conjugated polymers (CPs) is presented with potential applications for sensors. In this study, we investigated the aqueous solubility behavior of water-soluble CPs with high photoluminescence and with a particular focus on their pH and temperature responsiveness. For this purpose, two poly(phenylene vinylene)s (PPVs)—namely 2,5-substituted PPVs bearing both carboxylic acid and methoxyoligoethylene glycol units—were investigated, with different amount of carboxylic acid units. Changes in the pH and temperature of polymer solutions led to a response in the fluorescence intensity in a pH range from 3 to 10 and for temperatures ranging from 10 to 85 °C. Additionally, it is demonstrated that the polymer with the largest number of carboxylic acid groups displays upper critical solution temperature (UCST)-like thermoresponsive behavior in the presence of a divalent ion like Ca2+. The sensing capability of these water-soluble PPVs could be utilized to design smart materials with multiresponsive behavior in biomedicine and soft materials.
Highlights
In our recent report [13], we described the synthesis of water-soluble poly(phenylene vinylene)s (PPVs) having both carboxylic acid and methoxyoligoethylene glycol pendant groups (Scheme 1)
We explored whether the acid-functionalized PPVs under study exhibit upper critical solution temperature (UCST) behavior in presence of calcium(II) analogs (Scheme 1)
Upon addition of half an equivalent of calcium(II) ions compared to the carboxylate groups, all polymer precipitated out of solution, ascribed to interpolymer crosslinking by calcium(II)–carboxylate coordination as schematically depicted in Scheme 2
Summary
Polymers that respond to external stimuli such as pH, temperature, ionic strength, light, and concentration are extensively studied for a broad range of applications, including biomedical applications like drug delivery [1,2], cell imaging [3,4,5], as well as biological [6] and optical sensing [7,8,9]. Polymer-based systems with responsive properties can straightforwardly be designed thanks to recent advances in polymer synthesis as well as better understanding of physicochemical properties at the molecular level [10,11,12] Such responsive functions arise from the chemical or topological structure of the polymer and lead to the design of stimuli-responsive smart materials with tailored functions. For biomedical applications and aqueous sensing, water-soluble CPs are required, which can be achieved by introducing ionic or polar functional groups to the side chains, such as sulfonate, phosphate, carboxylate, quaternary ammonium, and ethylene glycol units [8,13].
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