# Diversity of Potential (Bio)Technological Applications of Amino Acid-Based Ionic Liquids
### Toxicity and Biodegradability  
The high stability of ILs is a desirable characteristic; however, degradation, disposal, or accidental release into the environment must be considered. Studies of their ecotoxicity generally indicate that the environmental impact of AA ILs depends on several factors, including their structure, concentration, and interactions with ecosystems. Overall, AAs with hydrophobic or bulky side chains tend to be more toxic. For their use in topical ionic liquid formulations, the cytotoxicity of AA esters on skin cells has been extensively evaluated. In general, AAOEt is less toxic than methyl esters (OMe), except when cytotoxicity increases with longer alcohol chain lengths. For various AAOEt-based ILs, the half-maximal inhibitory concentration (IC\({}_{50}\)) values range from 10 to 60 mM for keratinocytes and 1 to 20 mM for fibroblasts, with their salicylic acid salts further reducing toxicity [27]. Cations such as imidazolium, pyridinium, or quaternary ammonium are typically more toxic than naturally derived or simpler cations, such as choline and betaine. For example, our previous results showed that [Ch][AAs] had minimal effects on the growth of mouse fibroblasts (3T3 cells) even at 5.0 mmol/L [28]. Notably, significant reductions in cell growth (up to 50%) were observed only with [Ch][Arg], whereas [Ch][Glu] showed moderate antiproliferative activity (up to 25%) at the highest concentrations tested. In contrast, the IC\({}_{50}\) values for [Bmim][Gly], [Bmim][Ala], and [Bmim][Val] against 3T3 and CaCo-2 cells are in the millimolar range and comparable to those of second-generation ILs, such as [Bmim][BF\({}_{4}\)], [Bmim][PF\({}_{6}\)], and [Bmim][HSO\({}_{4}\)]. In these cases, the cation contributes significantly to the cytotoxicity [29]. Among ILs with the same BF\({}_{4}\) anion, those with AA cations such as [ValOMe], [GlyOMe], and [ValOMe] were found to be two to four times less toxic to 3T3 cells than those with [Emim] or [Bmim] cations [29]. Toxicity tests in aquatic organisms such as _Daphnia magna_, _Vibrio fischeri_, and _Pseudokircheriella subcapitata_ show that some AAILs can be moderately to highly toxic, depending on their chemical structure and concentration. Longer alkyl chains in the cation increase hydrophobicity, which often correlates with higher toxicity. An ecotoxicity analysis using the marine bacterial model _Vibrio fischeri_ revealed that the choline-based ILs have a different mechanism of toxicity compared to imidazolium-based ILs [30]. While choline-based ionic liquids are generally considered harmless, the ecotoxicity of some is indeed higher than that of common organic solvents such as chloroform or dichloromethane. In fact, certain choline-based ILs have toxicities similar to reference pesticides such as atrazine [30]. Additionally, when tested on three aquatic organisms, the half-maximal effective concentration (EC\({}_{50}\)) values for [Ch][AAs] were reported in the following ranges: brine shrimp (9000-15,000 mg/L), zebrafish (155-275 mg/L), and green algae (100-1000 mg/L), which can be considered non-toxic according to the toxicity ranking of [PERSON] and [PERSON] [31; 32]. In general, the cation is considered to be the major contributor to IL toxicity. In a toxicity comparison of three phenylalanine-based ILs containing [N4444], [P4444], or [Ch] cations against zebrafish, all were found to be non-toxic with EC50 values greater than 100 mg/L [33]. However, an increase in toxicity was observed when the cation was paired with the hydrophobic NTf2 anion. Furthermore, ILs composed of the 1-(2-hydroxyethyl-3-methylimidazolium) cation paired with glycinate, serine, alanine, or prolinate counteranions were found to be more environmentally friendly than [Emim][AAs] in antimicrobial tests against the green alga _Scenedesmus quadricaada_ and the bioluminescent marine bacterium _Vibrio fischeri_[34]. This reduced toxicity has been attributed to the presence of a short hydroxyl functional group in the cation [34].