Dye removal remains one of the most challenging aspects of industrial waste management. The strong interactions between polyelectrolytes and dyes were pushed further by Cai et al. to develop a chitosan-based cationic polyelectrolyte microsphere (CCQM) for the ultra-efficient removal of Congo red (1500 mg g⁻¹) and methyl orange (MO, 179.4 mg g⁻¹) [62]. Based on the strong polyelectrolyte–dye interactions, hydrogels fabricated using poly([2-(acryloyloxy)ethyl] trimethylammonium chloride) and poly(ClAETA) with cellulose nanofibrillation (CNF) had an efficiency of 96% in the removal of methyl orange dye, which remains a major industrial contaminant [63]. Schwarze et al. developed polyelectrolytic emulsions based on quaternary ammonium surfactants and demonstrated a dye removal efficiency of 90% for methyl orange [64]. The dye–polyelectrolytic complex aggregates have an important role in determining the spectral behavior of the dye. It was observed that methyl orange demonstrated an absorption maximum at 368 nm in poly(l-ornithine) (PLO), compared to 462 nm in poly(vinyl benzyl triethylammonium chloride) (PVBTEA). This observation was attributed to the formation of larger aggregates in PLO compared to PVBTEA, which promoted electrolytic dye stacking via ion-pair formation [65].

Microgels are three-dimensional cross-linked structures of polymer colloidal particles with an adjustable size and strong response to environmental stimuli, such as pH, ionic strength, temperature, light, and ultrasound [66]. Self-assembled microgels comprised of poly(N-isopropylacrylamide-co-2-(dimethylamino) ethyl methacrylate) and sodium alginate (SA) have demonstrated a highly pH-sensitive response [66]. Methyl blue is an anionic hydrophilic dye molecule which has been reported to be adsorbed onto microgel core star ionic covalent organic polymers, polymers, fibrous materials, and cross-linked polymer particles with cavity and ammonium functionalization [62,67,68,69]. A quartz crystal microbalance (QCM) investigation of the interaction between anionic dyes and the SA/microgel multilayers in the aqueous phase revealed an enhanced electrostatic attraction between the dyes and the microgels deposited on the QCM sensor surface compared to that with SA in the multilayers, which caused the release of microgels from the self-assembled structure and a mass loss ratio of 27.6% [66]. This study showed a promising application of the QCM-based sensors in the detection of dye contaminants in wastewater. In another report, a linear polysaccharide chitosan (CTS) composed of β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit), was chemically modified to form a cationic polyelectrolyte, viz., N-[(2-hydroxy-3-trimethylammonium)propyl]chitosan chloride (HTCC) [70]. A comparative investigation of the interaction of three anionic dyes, viz., Reactive Black 5, Reactive Blue 19, and Reactive Red 195, with HTCC and organoclay-modified montmorillonite (OMMT) demonstrated a high efficiency of dye exclusion (>91%) compared to sole polyelectrolyte and organoclay adsorbents. The study further showed that structurally distinct anionic dyes localized at separate sites within the hybrid organoclay adsorbents, enabling the simultaneous adsorption of different dyes with improved efficiency [70]. Thus, the materials designed via the interactions between polyelectrolytes and cationic dyes have exhibited success in various fields.

Cationic dyes dissociate into positively charged ions and negative counterions in aqueous solutions and have been extensively explored to study their interaction with anionic polyelectrolytes. The strength of this interaction can be measured by the magnitude of metachromasy induced in its spectroscopic profile. Metachromasy, or the blue shift in the absorption spectrum, is one of the most common methods for spectroscopic detection of polyelectrolyte–dye interactions. Higher metachromic effects imply a stronger degree of interaction. Toluidine blue (7-amino-8-methylphenothiazin-3-ylidene)-dimethylammonium chloride) and methylene blue (3,7-bis(dimethylamino)-phenothiazin-5-ium chloride) both form a strong 2:1 dye–polyelectrolyte complex with polyacrylic acid polymer (PAA), exhibiting large hypsochromic shifts of 57 nm and 67 nm, respectively, in their UV-vis profiles. Consequently, due to the more hydrophobic nature of methylene blue, it formed a more stable complex with PAA compared to toluidine blue: the stability constants were 5332 dm⁻³/mol and 4358 dm⁻³/mol for methylene blue and toluidine blue complexes, respectively, at 298 K [71]. The interaction of toluidine blue with poly(potassium vinyl sulphate) (PPVS) resulted in observed metachromacy at 105 nm with a distinct color change of the blue uncomplexed form with a maximum absorption at 635 nm to a red-violet toluidine blue–PPVS complex with maximum absorption at 530 nm [72]. The metachromatic action of cationic dyes is particularly helpful in determining the charges of biopolymers and proteins [73]. The specific interaction of the polymerized cationic dye azure A and the biological polyelectrolyte DNA was utilized to detect and discriminate DNA damage [74]. The electrostatic forces and the difference in the negative charges on the repetitive polysaccharide units of sodium heparin and sodium alginate led to different degrees of metachromasy induced in the cationic dyes azure B and toluidine blue [75,76]. While azure B bonded more strongly with sodium alginate, a more favorable interaction was observed between toluidine blue and sodium heparin. The interactions between poly(2-acrylamide-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(diallyl dimethyl ammonium) chloride (PDDA) and the highly versatile cationic dyes methylene blue (MB) and methyl orange (MO) have been employed for the purification of colored wastewater by the polymer-enhanced ultrafiltration (PEUF) technique [6]. The maximum removal efficiency under optimal conditions (pH 6.0, initial MB and MO concentrations of 3.5 mg L⁻¹ and 80 mg L⁻¹, respectively) was reported to be 98% and 90% for MB and MO, respectively, together with an ultrafiltration membrane (molecular weight cut off value: 10 kDa). Polystyrene sulfonate (PSS) adsorbed on laterite soil (polymer modified laterite, PML) showed efficiency in the removal of methyl blue (83%) and crystal violet (92%) [77].

Dye removal remains one of the most challenging aspects of industrial waste management. The strong interactions between polyelectrolytes and dyes were pushed further by Cai et al. to develop a chitosan-based cationic polyelectrolyte microscope (CCQM) for the ultra-efficient removal of Congo red (1500 mg g⁻¹) and methyl orange (MO, 179.4 mg g⁻¹) [78].