Rapid and facile quantification of surface amino groups on chitin nanowhiskers and nanofibers via spectrophotometry

Surface amino groups (SAGs) on chitin nanowhiskers and chitin nanofibers were rapidly and facilely quantified via spectrophotometry using three amino-labeling reagents (2,4,6-trinitrobenzensulfonic acid sodium salt (TNBS), o-phthalaldehyde (OPA) and ninhydrin) and two cationic dyes (reactive red 4 (RR4) and acid orange 7 (AO7)). After binding to the SAGs, the amounts of excess TNBS and OPA were determined via spectrophotometry and subtracted from the initial quantity. After ninhydrin was added to ChNWs/ChNFs, Ruhemann’s purple product was generated, which was quantified via spectrophotometry. When RR4 and AO7 were added in excess to the SAG of ChNWs/ChNFs, these dyes were adsorbed onto surface amino groups, and the excess amounts were similarly quantified. Each method yielded different amino group contents, which were compared with the titration values. Although the values obtained by labeling reagents were considerably underestimated, those obtained using TNBS and ninhydrin were proportional to those obtained by titration. The values obtained using AO7 adsorption at pH 2 or 3 corresponded well with the titration values for ChNWs but not ChNFs. Reliable results were attained using the two former labeling reagents with conversion equations or using AO7 adsorption. Surface amino groups (SAGs) on nanochitin materials were quantified using three amino-labeling reagents and two cationic dyes. After binding to SAGs, the excess labeling reagents or generated molecules were assessed by spectrophotometry. The dyes were adsorbed onto SAGs, and the excess was similarly quantified. The obtained values were compared with the titration values. Although the values by labeling reagents were underestimated, some of the values were proportional to those by titration. Reliable results were attained using the two labeling reagents with conversion equations or using Acid Orange 7 adsorption.