Electrostatically assembled porphyrin aggregates: from supramolecular organization to metal ion sensing applications

Environmental contamination by metal ions, including toxic heavy metals, remains a persistent challenge because these species are non-degradable, can accumulate in ecosystems, and may pose risks even at low concentrations. 1 This PhD thesis investigates self-assembled porphyrin-based supramolecular systems whose analytical response is generated by coupling metal coordination with noncovalent organization in water. In particular, porphyrin units were studied under conditions where electrostatic interactions with oppositely charged nanomaterials or polyelectrolytes, modulated by pH control, drive the formation of organized aggregates and hybrid architectures. The overarching goal is to establish structure–organization–signal relationships that can be exploited to design optical and electrochemical platforms for the sensing of environmentally relevant metal ion. A first part of the study focuses on a supramolecular hybrid aggregate (GQDs@TPPS₄⁴⁻) formed by associating positively charged graphene quantum dots (GQDs) with the anionic porphyrin TPPS₄⁴⁻ (meso-tetrakis(4-sulfonatophenyl)porphyrin). Spectroscopic characterization (UV–Vis absorption and fluorescence) supports stable complex formation and a combined optical response arising from both components.2 This hybrid system is then evaluated as a fluorescent probe for metal ion, where signal changes reflect metal-induced perturbations of the porphyrin environment and, crucially, processes related to porphyrin metal insertion. An important outcome is that GQDs can accelerate metal insertion kinetics in the porphyrin macrocycle; this effect is particularly evident for larger ions such as Hg²⁺ and Cd²⁺, that can form “sitting-atop” type intermediates and promote Zn²⁺ incorporation into the porphyrin core, thereby enabling signal amplification pathways.3 While quantitative readout requires careful consideration of optical artifacts (e.g., inner filter contributions), the results demonstrate that a porphyrin–GQD assembly can be engineered into a responsive system for metal-ion recognition.3 The same supramolecular concept is translated into an electrochemical sensor by modifying screen-printed carbon electrodes (SPCEs) with GQDs@TPPS₄⁴⁻. Optimization of the assembly composition and measurement conditions yields markedly enhanced analytical performance compared with bare electrodes. In particular, selected formulations provide sensitive detection of Cd²⁺ and Cu²⁺, with 5-fold improvements for Cd²⁺ (for a 1:6 GQDs:TPPS ratio) and 4-fold for Cu²⁺ (for a 1:9 ratio), and an interference pattern consistent with the higher stability of Cu²⁺ porphyrin complexes. Notably, the approach is demonstrated on real seawater samples analyzed directly without pre-treatment, highlighting robustness under high ionic strength and complex-matrix conditions.4 A complementary research line examines how an oppositely charged polyelectrolyte template, carboxymethylcellulose (CMC), can organize a cationic porphyrin (t-H₂Pagg) into supramolecular species whose signatures can be monitored by UV–Vis, fluorescence, resonance light scattering, and circular dichroism. The comparative study of the free-base porphyrin and its metallated derivatives shows that the coordination geometry of the inserted metal ion is a primary determinant of both polymer–porphyrin interaction strength and aggregate type. Metal ions typically yielding square-planar porphyrins (e.g., Ni²⁺, Cu²⁺) favor more defined J-type organization, whereas Zn²⁺ (often associated with five-coordinate environments) leads to limited assemblies, and six-coordinate ions (e.g., Mn³⁺, Co³⁺) tend to suppress aggregation by hindering supramolecular contacts with neighboring porphyrins; charge effects and metal-specific chemistry further modulate the outcome.5,6 These trends support the use of polymer-templated assemblies as a practical screening platform in which metal insertion (or metal-mediated perturbation) produces characteristic supramolecular and spectroscopic responses. In addition, kinetic effects consistent with local cation enrichment at the polyelectrolyte interface are observed (e.g., rate enhancement for Cu²⁺ insertion under specific conditions), and catalytic-like behavior is found in which very low Hg²⁺ levels can facilitate conversion to Zn(II) porphyrin species when trace of Zn²⁺ ions are present.7 The thesis also investigates J-aggregates of the sterically distorted 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (Br₈TPPS₄⁴⁻) in strongly acidic aqueous media, emphasizing how protonation state and mixing protocol govern nucleation and growth. Compared with non-brominated analogues, aggregation shows a stronger concentration dependence and distinct morphology, supported by combined spectroscopic evidence and AFM imaging; computational analysis (DFT/TD-DFT) assists interpretation of vibrational and electronic features.8 Classical studies on TPPS aggregation dynamics and structure provide context for the observed excitonic coupling and aggregate organization.9 Together, these results broaden the accessible design space of porphyrin assemblies whose optical fingerprints can be tuned by chemical environment and metal-ion interactions. Finally, as part of a six month research period spent at the Department of Chemistry, University of Aveiro (Portugal) under the supervision of Prof. M. A. Faustino, new cationic porphyrins bearing triphenylphosphonium substituents (tetra- and octa-cationic species with varied alkyl chain lengths) have been synthesized and fully characterized (NMR, mass spectrometry, UV–Vis, fluorescence) to provide structurally defined porphyrin platforms optimized for strong electrostatic association with negatively charged partners and future development in sensing architectures.10 Overall, the thesis demonstrates that effective metal-ion sensing using porphyrin assemblies depends on the interplay among electrostatic co-assembly (with nanomaterials or polyelectrolytes), pH-controlled speciation and aggregation pathways, and metal coordination chemistry (geometry, insertion kinetics, and intermediate states). By linking supramolecular organization to measurable optical and electrochemical outputs, the work provides transferable guidelines for constructing responsive aqueous systems capable of detecting environmentally relevant metal cations in both model solutions and real samples. References: 1. Tchounwou, P. B.; Yedjou, C. G.; Patlolla, A. K.; Sutton, D. J. Heavy Metal Toxicity and the Environment. In Molecular, Clinical and Environmental Toxicology; Springer, 2012; pp 133–164 2. Sarà, M.; Giofrè, S. V.; Abate, S.; Trapani, M.; Verduci, R.; D’Angelo, G.; Castriciano, M. A.; Romeo, A.; Neri, G.; Monsù Scolaro, L. Molecules 2024, 29, 2015. 3. Sarà, M.; Romeo, A.; Lando, G.; Castriciano, M. A.; Zagami, R.; Neri, G.; Monsù Scolaro, L. Int. J. Mol. Sci. 2025, 26, 7295. 4. Sarà, M.; Abid, K.; Gucciardi, P. G.; Scolaro, L. M.; Neri, G. Synth. Met. 2025, 311, 117824. 5. Suzuki, M.; Nakata, K.; Kuroda, R.; Kobayashi, T.; Tokunaga, E. Chem. Phys. 2016, 469–470, 88–96. 6. T. Kobayashi (Ed.). J-Aggregates, Vol. 2; World Scientific: New York, 2012. 7. Zagami, R.; Castriciano, M. A.; Romeo, A.; Monsù Scolaro, L. Int. J. Mol. Sci. 2023, 24, 17371. 8. Abdelaziz, B.; Sarà, M.; Ayachi, S.; Zagami, R.; Patanè, S.; Romeo, A.; Castriciano, M. A.; Monsù Scolaro, L. Nanomaterials 2023, 13, 2832. 9. Akins, D. L.; Özçelik, S.; Zhu, H.-R.; Guo, C. J. Phys. Chem. 1996, 100, 14390–14396. 10. Inês Chaves, Filipe M. P. Morais, Cátia Vieira, Maria Bartolomeu, M. Amparo F. Faustino, M. Graça P. M. S. Neves, Adelaide Almeida, and Nuno M. M. Moura. ACS Applied Bio Materials, 2024 7 (8), 5541-5552