Non-adiabatic tapered optical fiber sensor for measuring the interaction between α-amino acids in aqueous carbohydrate solution

A single-mode non-adiabatic tapered optical fiber (NATOF) sensor was utilized for sensing the variation in refractive index (RI) with concentration of d-glucose in deionized water and measurement of the RI of amino acids (AAs) in carbohydrate solutions. This method showed a rewarding ability in understanding the basis of biomolecular interactions in biological systems. Due to high sensitivity, ease of application, low cost and real-time measurement, this method is more efficient in comparison with other techniques, such as calorimetric titration, NMR, UV absorption spectroscopy, x-ray crystallography, computer calculations, kinetic studies and chromatography data. The NATOF is fabricated by the heat pulling method, utilizing a CO₂ laser. The limit of detection of the NATOF was 55 ppm for a d-glucose concentration ranging from 0 to 80 mg ml⁻¹, and the limit of detection of the RI measurement corresponding to these concentrations in the range from 1.3330 to 1.3447 was 8.2 × 10⁻⁶ as a refractometer sensor. The response of the NATOF shows that different kinds of interactions of various groups of AAs, such as l-alanine, l-leucine and l-cysteine with d-glucose, sucrose and water molecules, depend on functional groups in AAs such as OH, SH, CH₂, NH⁺₃ and COO⁻. These results can be interpreted in terms of solute–solute and solute–solvent interactions and the structure making/breaking ability of solutes in the given solution. Such a study helps in the better understanding of the interactions occurring between AA molecules and entities present in biological matrices.