## 3 Synthetic Dyes  
Synthetic dyes have been largely used in the textile, paper printing, food, pharmaceutical, and other industries due to their several effective characteristics, such as cost-effectiveness in synthesis, firmness, and high stability to different factors such as light, temperature, detergent, and microbial attack [101; 102; 103]. Nowadays, these dyes represent a big problem for the environment. The color impedes the effect of the sunlight on water and negatively affects photosynthesis, which consequently reduces the food source of aquatic organisms and inhibits their growth [101; 103; 104; 105]. Additionally, many synthetic dyes have been considered toxic and carcinogenic [106]. Dyes are organic compounds with the property of absorbing light in the visible spectrum (400-700 nm). This property is possible due to the presence of a chromophore or \"color-bearing group\", described as a delocalized electron system with conjugated double bonds [107]. The chromophore is composed of groups of atoms, most commonly nitro (-NO\({}_{2}\)), azo (-N=N-), nitroos (-N=O), thiocarbonyl (-C=S), carbonyl (-C=O), and alkenes (-C=C-). Chemical classification of the dyes is based on these groups present in chromophores that are illustrated in Table 1[108]. For a chromophore to absorb electromagnetic waves, an excitation of the electrons in a molecule is required. Once this requirement is fulfilled, the molecule becomes chromogenic. In addition, most of the dyes comprise groups called auxochromes or \"color helpers\" that can be acidic (COOH, SO\({}_{3}\) and OH) or basic (NH\({}_{2}\), NHR and NHR\({}_{2}\)).These groups are mostly responsible for dye solubility and color shifting [109]. Azo dyes have been by far the most utilized class of dyes in the industry. The azo chromophore is composed of at least one azo group (-N=N-), but sometimes two (diazo) and three (triazo) azo groups can be present in the structure. This azo group is bonded to two groups, of which at least one is aromatic [107]. The main compound of anthraquinone dyes is anthracene, composed of three fused benzene rings with two carbonyl groups on the central ring. After the substitution of the aromatic rings, the whole structure is given the color [110]. In the anthraquinone structure, the carbonyl group acts as an electron acceptor, which is not the case in the structure of azo dyes. In order to break the structure of anthraquinone dye, an electron donor is required [111; 112] Moreover, anthraquinone dyes have a highly stabilized structure due to the resonance effect, which makes it harder to degrade them when compared with azo dyes [112; 113]. Indigoid dyes are one of the oldest known classes of organic dyes. These dyes, also members of vat dyes (based on their application that takes place in a bucket or a vat), are insoluble in water and require oxygen for color development. They have an aromatic or heteroaromatic complex with an indigoid carbon skeleton, shown in Table 1[108; 109]. Another important class of dyes widely used in industries producing textiles, paper, leather, plastic, food, and pharmaceuticals is the triphenylmethane class [114; 115; 116]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Class** & **Chromophore** & **Example** \\ \hline Nitrodyes & & C.I. Acid Yellow 24 \\ \hline Nitroso dyes & & N \\ \hline Azo dyes & & N \\ \hline Triphenyl methyl dyes & & C.I. Basic Violet 3 \\ \hline Phthalein dyes & & C.I. Acid Blue 74 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Synthetic dyes: classification and examples of dyes according to the chromophore present. These dyes are composed of a triphenylmethane backbone and are considered inexpensive colorants, but have shown cytotoxic effects on living organisms [117, 118]. Stamp inks used in libraries have a complex composition containing dyes or pigments, hydrocarbons, plant or mineral oil as solvent, and many other additives [119]. [PERSON] et al. [120] analyzed stamp pad inks by the TLC method and found that blue stamp ink contained Crystal Violet (CV) dye.