Carrageenan-based nanomaterials and nanocomposites: Synthesis, characterization, and applications

Carrageenans are hydrophilic and linear sulfated polysaccharides that are extracted from red seaweeds. It is commercially used for various food applications in meat, milk, dairy products, and ice cream due to its stabilizing and gelling properties. The carrageenan industry however is in danger if calls for its discontinuance as food additive succeed due to the reported health issues. There is a need to explore non-food applications to support the carrageenan industry. In this study, we explored other potential applications of carrageenan as a green and sustainable material for the synthesis of nanomaterials (silver nanoparticles (AgNP) and carbon nanodots (CND)), production of potassium sulfate, and as an electrolyte additive for dye-sensitized solar cells (DSSC). AgNPs were successfully synthesized using carrageenans as reducing agents of Ag+ ions. Carrageenans possess hydroxyl groups and negatively charged sulfate groups that reduce Ag+ ions and stabilize the AgNPs. The reaction conditions were optimized to afford carrageenan/AgNP nanocomposites in gel and film forms that have shown bioactivity against various microorganisms (Chapter 3). The treatment of carrageenan using the simple hydrothermal carbonization process yielded potassium sulfate and CND, which were separated by adding an antisolvent, ethanol, to precipitate the potassium sulfate. The preparation of potassium sulfate, which was characterized by Raman and XRD as K3H(SO4)2, offers an alternative, greener, and sustainable approach compared to the corrosive inorganic process of synthesizing this important fertilizer (Chapter 4). The obtained CND on the other hand, afforded a chemical structure containing hydroxyl, carboxylic acid, and sulfate groups, which can act as anchoring groups upon interaction with other substances. Indeed, the CND from carrageenan was used as a substitute sensitizer for nanostructured photovoltaic devices, albeit, affording low photoconversion efficiency of less than 0.1% (Chapter 6). Exploiting the gelling property of κ-carrageenan, it was utilized as a polymer matrix in a DSSC system replacing the volatile liquid electrolyte to prevent leakage (Chapter 5). The hydrophobic Z907 and MK-2 dyes replacing the typical ruthenium N719 dye were used to prevent water-initiated dye-desorption from the nano-sized TiO2. The photoconversion efficiency of DSSC with pure water as solvent was low due to poor electrolyte contact with the photoanode surface but the addition of relatively non-volatile polar solvent N-methylpyrrolidone improved the efficiency of the carrageenan/water/NMP-loaded DSSC to 2.42% using Z907 dye and 1.40% using MK-2 dye. These works demonstrate the utility of carrageenan for new applications adding value to the carrageenan.