Complexation between Carrageenan and Methylene Blue for Sensor Design

摘要

Theoretical studies on the methylene blue (MB)-carrageenans complexation at solution and solid states have been carried out via ultraviolet spectrophotoscopy and reflectometry methods. The equilibrium constant (Ka) of the MBcarrageenans complexation follows the order of Iota > Lambda > Kappa carrageenans, which indicated Iota-carrageenan forms a stable complex. MB-carrageenan complexation reaction showed decrease in Ka value from 210.71 ppm~(-1) to114.57 ppm~(-1) when the reaction temperature increased from 298 K to 323 K. Le Chatelier’s principle and mass action law explained that the MB-carrageenan complexation was an exothermic reaction (?H=-18.54 kJmol~(-1)) that release heat. Thus MB-carrageenan complex was less stable at high temperature and tend to dissociate into free MB and carrageenan molecules. It was also supported by the van’t Hoff equation. The reaction is a spontaneous process (?G=-13.23 kJmol~(-1)) where the randomness of the molecules reduced (?S=-17.83 Jmol~(-1)K~(-1)) due to complexation. Besides, linear regression of the concentration and absorption of the MB-carrageenan reaction obeys the Beer Lambert law, which elucidated that the complexation process was not affected by any concentration dependent factors such as aggregation and self-quenching. Moreover, linear Benesi Hilderbrend plot revealed that the interaction between MB and carrageenan was a reversible and stoichiometric reaction with 1:1 ratio. However, the molar extinction coefficient (ε) and molar adsorption coefficient (μ_a) of the MB-carrageenan complex were lower compared to free MB, described that the complex was less adsorptive. The sensor constructed based on these theoretical investigations showed response behavior that was similar with solution test as both have attraction for carrageenans in the sequence of Iota-, Lambda-, Kappa- carrageenans. Likewise, carrageenan sensor was more selective towards Iota-carrageenan than to Lambda- and Kappa-carrageenans, and no response observed when tested with agar, alginate and glucose. Therefore the sensor is able to detect carrageenans specifically and offers rapid detection without the need of sample pretreatment when compared to conventional methods.