Amphiphilic Perylene Diimide-Based Aggregates Serve as Fluorescent Metal Ion Probes

In a recent study published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, authors Prajna Moharana and G. Santosh from Vellore Institute of Technology in India, investigated the self-assembly behavior of a newly synthesized amphiphilic perylene diimide derivative in various organic solvents (1). The research focused on the formation of fluorescent hemispherical aggregates and their potential as probes for metal ions.

The Erlenmeyer flask in the line with color range solvent using for analysis calibration curve of iron in waste water sample. The experiment in chemistry laboratory. | Image Credit: © Arpon - stock.adobe.com

The team synthesized an amphiphilic thioester appended with carboxylic acid functionalized perylene diimide and examined its self-assembly properties. In tetrahydrofuran (THF) solution, the compound formed J-type hemispherical aggregates, exhibiting distinct fluorescent properties.

To evaluate the interaction between the aggregates and metal ions, spectroscopic techniques were employed. The study revealed that the probes selectively bind to Fe³⁺ and Ba²⁺ ions. Interestingly, two equivalents of Fe³⁺ ions cooperatively bound to one equivalent of the perylene diimide derivative within the hemispherical aggregates, with a binding constant of 1.4 × 10⁷/M. The limit of detection (LOD) for Fe³⁺ ions was calculated to be 8.66 × 10^-6 M. This cooperative binding effect led to the supramolecular polymerization of the hemispherical aggregates.

Additionally, Ba²⁺ ions exhibited selectivity and sensitivity towards the fluorescent aggregates in THF, causing a complete quenching of the fluorescence intensity. The interaction between Ba²⁺ ions and the aggregates was characterized by a linear Stern-Volmer plot, with a Stern-Volmer constant value of 502.6/M. This observation suggests that the heavy atom effect of Ba²⁺ ions contributes to the fluorescence quenching phenomenon.

A linear Stern-Volmer plot is a graphical representation that relates the fluorescence quenching of a fluorophore to the concentration of a quenching agent. In this plot, the fluorescence intensity is plotted on the y-axis, while the concentration of the quenching agent is plotted on the x-axis. The plot follows a linear relationship, indicating that the quenching process is dynamic and follows the principles of the Stern-Volmer equation. The slope of the linear plot represents the Stern-Volmer constant, which provides information about the efficiency of the quenching process. A higher Stern-Volmer constant indicates a more effective quenching interaction between the fluorophore and the quencher. The linearity of the plot confirms that the quenching process follows a well-defined mechanism, enabling quantitative analysis of the quenching kinetics.

The researchers examined the morphological changes of the fluorescent hemispherical aggregates in the presence of Fe³⁺ and Ba²⁺ ions using electron microscopy. This detailed analysis provided insights into the structural transformations induced by the metal ions.

The study presents a promising approach for using amphiphilic perylene diimide-based aggregates as probes for metal ions. The selective binding of Fe³⁺ and Ba²⁺ ions to the hemispherical aggregates, along with their fluorescence properties, opens up possibilities for applications in metal ion detection and sensing. This research contributes to the field of molecular and biomolecular spectroscopy and lays the foundation for further investigations into the design and development of novel fluorescent probes with tailored metal ion selectivity.

Reference

(1) Moharana, P.; Santosh, G. Amphiphilic perylene diimide-based fluorescent hemispherical aggregates as probes for metal ions. Spectrochimica Acta Part A: Mol. Biomol. Spectrosc. 2023, 297, 122696. DOI: 10.1016/j.saa.2023.122696