# A Phthalocyanine Optical Probe Responding to Cationic Surfactants Containing Long Carbon Chains with High Selectivity in Total Water Phase and Its Applications

## TL;DR

A new water-soluble optical probe based on nickel phthalocyanine can detect long-chain cationic surfactants with high selectivity and accuracy in aqueous solutions.

## Contribution

A novel phthalocyanine-based optical probe for selective detection of long-chain cationic surfactants in water is developed.

## Key findings

- NiS4Pc shows a distinct and selective spectral response to cationic surfactants with 12 or more carbon atoms.
- The method allows for both single and total cationic surfactant quantification without using organic solvents.
- The method was successfully applied to real samples with a detection limit of 3 × 10−6 mol L−1.

## Abstract

The analysis of cationic surfactants with high selectivity is a source of great research interest. In this study, the absorption spectra of tetra-sulphonated metal phthalocyanine (coordinated by iron, zinc, cobalt, and nickel) in the presence of cationic surfactants in complete aqueous solutions were investigated. Interestingly, the absorption spectra of tetra-sulphonated nickel phthalocyanine (NiS4Pc) exhibits a remarkable response to the cationic surfactants compared with other water-soluble metal phthalocyanines. Further investigation has yielded important findings that cationic surfactants with carbon chains containing twelve or more carbons cause distinct spectral responses, and the response behaviors are highly similar, showing a typical structure–activity relationship. Studies on the mechanism of response indicate that the spectral behavior could be attributed to the dramatic binding effects of structure-matched cationic surfactants on the self-association equilibrium of nickel phthalocyanine. Based on the above findings, we applied NiS4Pc as a directly responsive optical probe for the quantitative analysis of long carbon chain cationic surfactants. Due to the high degree of similarity in optical responding, this method can be used to determine the single cationic surfactant and the total cationic surfactants. It is worth mentioning that NiS4Pc is a water-soluble optical probe that can be used in a complete aqueous phase. Therefore, this method is not only selective but also easy and fast to operate, without the need for organic solvents. Under optimized conditions, the average calibration curve equation of the method is y = 1.66 − 0.0173 x, r = 0.9987, with a limit of detection of 3 × 10−6 mol L−1. This method has been applied to the determination of real samples, for which we obtained satisfactory results. We not only describe the establishment of a new method for the direct quantitative analysis of cationic surfactants but also propose a new strategy for obtaining phthalocyanine-based optical probes in this study, which explored the novel application of phthalocyanine compounds in analytical sciences.