Development of a new colorimetric chemosensor for selective determination of urinary and vegetable oxalate concentration through an indicator displacement assay (IDA) in aqueous media

Abstract

The paper proposes a method that exhibits operational simplicity for the indirect spectrophotometric determination of oxalate ion. We developed Reactive Blue 4 (RB4) as a sensor by complexation with copper ion as a simple, inexpensive yet selective colorimetric chemosensing ensemble for the recognition of oxalate over other available competitive analytes via indicator displacement assay (IDA) in both solution (aqueous medium) and solid state (paper-based experiment). The addition of oxalate to RB4-Cu2+ complex changed the colour from sky blue to dark blue due to the regeneration of RB4 by the chelation of oxalate as the competitive analyte with Cu2+. The absorbance band increase is linear with oxalate concentration from 1.76 to 49.4 μmol/L with a detection limit of 0.62 μmol/L. This measurement mode did not show any influence of interferences (available anions and ascorbic acid). This approach eliminated the need for the separation stages, enzymatic multiple-step reactions, sample preparation, organic solvent mixture, chemical modifications and equipment developed to a high degree of complexity. The oxalate determination gave results in different real samples such as urine, mushroom and spinach, which demonstrated the applicability of the existing method. Furthermore, this colorimetric system can serve as IMPLICATION molecular logic gate using Cu2+ and oxalate (C2O42-) as inputs and UV-Vis absorbance signal as the output with potential monitoring applications.