Surfactants have attracted significant research interest over the last decade due to their potential to accelerate various reactions. The amphiphilic nature of surfactants makes them widely applicable in different fields such as nanoelectronics [1] and drug delivery [2], and as templates for mesoporous materials [3]. The global market for surfactants is anticipated to grow from USD 46.67 billion in 2022 to USD 67.92 billion in 2030, at a compound annual growth rate of 4.80% [4]. Anionic surfactants dominate the market, with a share of approximately 18.2% in 2022, due to their lower cost and availability. The presence of structural variations in surfactants, including a hydrophilic head group (mainly polar) and hydrophobic tail group (mainly non-polar long alkyl chain), gives them a special ability to be spontaneously adsorbed at interfaces. Further, they form well-defined aggregated structures at these interfaces, making them soluble in both polar and non-polar solvents [5].

To cope up with fresh challenges in modifying existing materials and synthesizing novel materials in colloid and interface science, there is an urgent need to modify their structures with application-oriented functionalities [6]. Multiple classes of surfactants have been synthesized by coordinating peptides [7], oligopeptides [8], carbohydrates [9], and metals with surfactants. Of these complexes, metal-surfactant complexes are of significant research interest as they blend organometallic chemistry and surface science. Metallosurfactants are a novel class of molecules incorporating a transition and/or inner-transition metal as an integral structural element. The integration of a transition metal in the head group of a surfactant offers several interesting properties, such as variable oxidation states, colors, paramagnetism, and pH sensitivity [10]. In detail, the flexible electron configuration of the metal allows for the complex to have variable oxidation states. Different colors are determined by ligand interactions with metal ions through electronic transitions. In response to external magnetic field, the complex exhibits paramagnetism due to unpaired electrons in the metal's d orbitals. Furthermore, pH sensitivity appears as a result of structural alterations that the metal-surfactant complex experiences in reaction to pH variations. It also supports localization of redox, spectral, magnetic, and catalytic properties at the air-water interface.

The self-aggregating behavior of surfactants is largely due to the interplay of hydrophobic interactions between lipophilic parts, repulsive steric or electrostatic interactions between hydrophilic head groups, and metal-ligand interactions [11]. As with conventional surfactants, metallosurfactants aggregate to form various self-assembled structures, such as micelles, vesicles, reverse vesicles, bilayers, and disks [12]. The molecular geometry and ionic charge of an amphiphile can be altered through the coordination of metal ions to a surfactant, leading to significant changes in aggregate morphology. The aggregation-controlled behavior of metallosurfactants makes them useful in biomedical applications [13], thin-film optoelectronic devices [14], nanomaterial fabrication [15], catalysis [16], light-driven hydrogen generation [17], and corrosion mitigation [18]. Coordination of metal ions with surfactant molecules improves their physicochemical properties. A surfactant‑copper(II) complex, Cu(L)Br3, exhibits superior surface-assimilation and enhanced biological potential in comparison with precursor surfactant molecules [19]. Changes in the position of metal ions in the surfactant also influence their applicability. Depending on the location of the metal ions, metallosurfactants can be classified into three classes (Fig. 1). In type 1, a core metal ion serves as a hydrophilic head group along with its primary coordination sphere, while the ligand with a lengthy hydrocarbon chain serves as a hydrophobic tail [20]. In type 2, the metal ion is part of the hydrophobic group and coordinates the surfactant [21], and in type 3, the metal ion is a counter ion that confirms the weak bonding of the metal ion to the surfactant [22]. This positioning of metal ions (as either a counter ion or an integral part of the head group) offers the opportunity to modify the features of the metallosurfactants.

A review of relevant literature revealed that the behavior of metallosurfactants changes depending on the nature of the metal ion, alkyl-chain length, head group, and solvent present (Aiad et al., 2012). Transition-metal complexes such as Cr(acac)3, Fe(acac)3, Zr(acac)3, and Hf(acac)4 cause the generation of amphiphilic surfactants in situ [23]. To explore their size, shape, and polydispersity in an aqueous solution, a variety of light-scattering techniques, including small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and dynamic and electrophoretic light scattering can be employed. These techniques examine how different parameters, such as metal ions and counter ions, increases in metallosurfactant concentration, and the addition of any biomolecule, affect the aggregation behavior of metallosurfactants.

Various aspects of the potential pharmaceutical applications of metallosurfactants in the form of metallodrugs have attracted research attention. To study the influence of metallodrugs on the human body, molecules must across the plasma membrane. Due to amphiphilic nature of surfactants, they can easily cross hydrophobic cell membranes. They can therefore be effectively used as solubilizing agents for the targeted delivery of drugs that are otherwise poorly soluble [24]. Because the primary target of these antibacterial, anticancer, and antiviral drugs are proteins and DNA, the interaction of metallosurfactants with these biomolecules has become an prominent research field [25].

Despite growing interest from the research community, only a few review articles that focus on metallosurfactant micelles for sustainable catalysis in water [22], metallo-vesicular structures for catalysis and biomedical applications [26], and biological activity of amphiphilic metal complexes [27] have been published. This prompted us to offer an updated review that explores the development of metallosurfactants, their aggregation behavior, structure-property relationships, interactions with biomolecules, and critical micelle concentrations (CMCs). The structural varieties of metallosurfactants (micelles, vesicles, lamellar phases, and complex self-assembled supramolecular aggregates) can be studied with the help of different scattering techniques. The discussion is then extended to the applicability of metallosurfactants in catalysis, nanoparticle synthesis, and biomedical fields such as diagnostics and therapeutics. The challenges involved in commercializing metallosurfactants are also explored. This article offers insights into metallosurfactants in terms of their design, structure-property relationships, and applicability in various fields.