Simple colorimetric sensing platform for biothiols based on pyrrole-derived azo-Cu(II) complexes: Effect of ligand architecture.

In the pursuit of simple and cost-effective methods for biothiol detection, two pyrrole-bearing azo compounds - an acyclic and a macrocyclic derivative - were examined. A series of qualitative and quantitative studies was carried out to evaluate their potential application in colorimetric and spectrophotometric biothiol sensing, using L-cysteine as a model analyte. The most promising approach proved to be an indicator displacement assay (IDA) system with copper(II) ions, monitored by spectrophotometric and colorimetric methods. Both compounds exhibited a linear response toward increasing cysteine concentrations, with detection limits of 8.19 × 10-6 M and 0.54 × 10-6 M respectively. In parallel, several commonly used solid supports were evaluated to assess the feasibility of simple solid-phase sensing approaches. The copper(II) complex of the acyclic ligand enabled reliable visual cysteine detection in the millimolar concentration range (∼10-3 M) on all tested solid substrates. In contrast, the corresponding macrocyclic complex exhibited a less selective colorimetric response and showed limited suitability for solid-phase sensing under the same conditions. The presented system provides a simple and low-cost proof-of-concept for chromogenic biothiol sensing based on molecular receptors, demonstrated using L-cysteine as a representative analyte, without the need for advanced materials or sophisticated instrumentation.