Determination of methionine using solvent-based de-emulsification dispersive liquid–liquid microextraction coupled with surface plasmon resonance of silver nanoparticles prepared by a bio-stabilizer

Methionine, (2-amino-4-(methylthio)butanoic acid), is an essential amino acid with an important role in biological methylation reactions. Methionine is also important in the formation of blood proteins, globulins and albumins helping in the breakdown of lipids and acting as chelating agent for removal of heavy metals from body. This amino acid helps to reduce cholesterol level by increasing lecithin production in liver and is also essential for human growth. Methionine deficiencies have been attributed to toxaemia, muscle paralysis, hair loss, depression, schizophrenia, ParkinsonIs disease, liver deterioration and impaired growth. Since methionine cannot be synthesized in the body it must be obtained from food supplies and pharmaceutical supplements are commercially available [1, 2], Thus, the determination of this essential amino acid is very important in the clinical point of view. Due to the excellent LSPR properties of silver nanoparticles (AgNPs) and their color change visualization from yellow to brown by aggregate of AgNPs, they have been considered for colorimetric assays. Surface plasmon resonance (SPR) of dispersed silver nanoparticles in aqueous media depends on their size, shape, aggregation and structure which show very intense colors. Therefore AgNPs can be used as colorimetric probe for chemical or biological sensing [3]. In this study, a solvent-based de-emulsification dispersive liquid–liquid microextraction method coupled with AgNPs as a colorimetric probe was developed for determination of trace levels of methionine. The stable and dispersed AgNPs were synthesized by applying ascorbic acid as reducer and Stenotrophomonas maltophilia bacteria as bio-stabilizer and then preconcentrated in organic phase according to a facile dispersive liquid-liquid microextraction procedure based on 1-octanol as extraction solvent and acetone as disperser and demulsifier. The presence of methionine influenced on the intensity of plasmon resonance absorbance peak of AgNPs, which was employed for the spectrophotometric determination of this amino acid. The effect of different variables such as bio-stabilizer amount, ascorbic acid concentration and type and volume of extraction, disperser and demulsifier solvents was investigated and optimized. The calibration curve was linear in the range of 1.7 - 234.6 nM of methionine and limit of detection was 1.0 nM.