Eosin Y (EY) is an acidic xanthene dye which is mainly used in food stuff and biological staining. Various analytical methods have been reported for the utility of this dye in the quantitative determination of several pharmaceutical compounds, heavy metals in addition to some surfactants and proteins. Most of the applied methods were based on the formation of association complexes between eosin Y and the target analytes in buffered aqueous solutions. The present article represents a comprehensive review for the use of eosin Y as a probe in analytical chemistry.
Eosin Y (2-(2,4,5,7-tetrabromo-6-oxido-3-oxo-3H-xanthen-9-yl)benzoate) is an acidic dye that belongs to the xanthene group, which has a yellowish-red color with green fluorescence (Figure 1). Other names of eosin Y are bromoeosine, eosin yellowish, bromofluoresceic acid, Acid Red 87, C.I. 45380 and D&C Red No. 22. It is used as a coloring agent in food, cosmetics, pharmaceuticals and textiles. Eosin Y is often used as a counterstain in biological tissues. The hematoxylin-eosin staining is most frequently used for staining histological material. The nuclei will appear blue to dark violet in color by hematoxylin, and the eosin Y solution will stain the cytoplasm and intercellular substances red-orange.
Almost all of the reported analytical method utilizing eosin reaction are based on the formation of an association complex between the dye and the analyte, with few exceptions (Srividya & Balasubramanian, 1997; Liang, Deng & Tan, 2015). The reaction was carried out mainly in slightly acidic medium. Therefore, the mechanism of complex formation will be discussed based on such media. According to the pH of the solution, eosin can exist in different forms (Figure 2) which can be represented by the following equation:
where R represents the anionic part of eosin Y. The reported (Batistela et al. 2011) values for pKa1, pKa2 and pKa3 of eosin Y were 2.10, 2.85 and 4.95, respectively. In a weak acidic medium (pH 3.0–4.5), eosin Y exists mainly in the monovalent anionic form (HR−). However, there are two possibilities for the formation of the monovalent anion of eosin, either by ionization of the hydroxyl or carboxylic groups. The ionization of the carboxylic group occurred first in fluorescein (eosin analogue without bromine substitutions), followed by hydroxyl group. But in the case of eosin Y, the presence of two strong electron withdrawing bromine atoms close to the hydroxyl group reduces the charge density at the oxygen atom of the hydroxyl moiety. As a result, the hydroxyl group tends to dissociate more easily than the carboxylic group. Therefore, eosin monovalent anion is formed by the ionization of the hydroxyl group. The complex formation occurs between eosin anion and the protonated analyte through the electrostatic attraction and hydrophobic forces as shown in Figure 3.
In most cases, the composition of the formed binary complex was in the ratio of 1:1, eosin:compounds. However, in some rare cases, a complex containing 2:1, drug:eosin, was formed due to the presence of two active basic centers in the drug such as azithromycin (Walash et al. 2007) and succinylcholine (Ayad et al. 2018). In the case of ternary complexes, the target analytes form chelates with metal ions followed by further complexation of the formed chelates with eosin anion. A presentation for the structures of both binary and ternary complexes is shown in Figure 3. Obviously, the complex formation is accompanied by a distinct change in the absorption spectrum, fluorescence quenching and change in the resonance Rayleigh scattering spectra of eosin Y. All these changes could be employed for the construction of various quantitative techniques for the determination of the involved analytes.
As an analytical reagent, eosin Y has been extensively used as ion-pairing agents for the spectrophotometric determination of some important pharmaceutical compounds with or without extraction. In addition, it was utilized in combination with metal ions for the determination of many drugs or heavy metals via ternary complex formation. The following sections summarize almost all the reported methods which applied eosin Y as analytical probe.
Binary complex formation with eosin was incorporated in the spectrophotometric determination of several pharmaceutical compounds (Table 1). The solution was slightly acidic (pH 2.2–4.0) and the absorbance was measured at wavelengths in the range of 540–552 nm. The majority of the formed complexes were freely soluble in water and therefore, the extraction with an organic solvent or the addition of a surfactant was not required (Kovacs-Hadady & Fabian, 1998; El-Brashy, El-Sayed Metwally & El-Sepai, 2004; Walash et al. 2007; 2010; Li et al., 2011; Walash et al., 2011; Ammar, El-Brashy & Al-Sahly, 2014; Hussein, Fares & El-Kosasy, 2014; Ayad et al., 2018; Naggar et al., 2017; El-Masry et al., 2018). However, in a few cases (Gazy et al., 2002; Derayea et al., 2012; Derayea, 2014; El-Hay, El-Mammli & Shalaby, 2016), it was necessary to enhance the water solubility of the complex through the addition of non ionic surfactant such as methyl cellulose.
|Drug||Matrix||Linear range (μg/ml)||LOD (μg/ml)||Surfactant||References|
|Erythromycin, azithromycin, clarithromycin and roxithromycin||Pharmaceutical formulations and biological fluids||2.0–20|
|–||(Walash et al. 2007)|
|Succinylcholine||Ampoules||0.5–15||0.12||–||(Ayad et al. 2018)|
|Benzalkonium chloride||Eye drops||0.7–8.5||–||–||(Kovacs-Hadady and Fabian 1998)|
|Verapamil||Tablet and human serum samples||0.6–4.0||0.18||–||(Li et al. 2011)|
|Levofloxacin, norfloxacin and ciprofloxacin||Tablets and spiked human urine||2.0–8.0|
|–||(El-Brashy, El-Sayed Metwally, and El-Sepai 2004)|
|Tizanidine and orphenadrine||Pharmaceutical formulations||0.5–8.0|
|–||(Walash et al. 2011)|
|Azelastine||Dosage forms||0.5–15||0.13||–||(El-Masry et al. 2018)|
|Metoclopramide||Dosage forms||1.0–10||0.34||–||(Naggar et al. 2017)|
|CTAB*||Waste water||0.2–14||–||–||(Hussein, Fares, and El-Kosasy 2014)|
|Dothepine||Tablets and capsules||1.0–10||0.18||–||(Walash et al. 2010)|
|Doxazosin||Tablets||2.0–14||0.39||–||(Ammar, El-Brashy, and Al-Sahly 2014)|
|Cetirizine, fexofenadine, loratadine and acrivastine||Dosage forms||8.0–24|
|MC*||(Gazy et al. 2002)|
|Mebeverine||Dosage forms||1.0–12||0.53||MC*||(Derayea 2014)|
|Amlodipine and nicardipine||Pharmaceutical formulation||5.0–60|
|MC*||(Derayea et al. 2012)|
|Clemastine, desloratadine, losartan and moxepril||Pharmaceutical formulation||1.25–11.25|
|MC*||(El-Hay, El-Mammli, and Shalaby 2016)|
*CTAB is cetyltrimethyl ammonium bromide and MC is methylcellulose.
Methods based on ternary complex
Determination of pharmaceutical compounds
Ternary complex formation was applied for the spectrophotometric determination of many drug candidates (Table 2). In contrast to binary complexes, many of the formed ternary complexes had low water solubility. Therefore, several approaches were tried to solve this problem. The first approach was performed through extraction of the formed complex with an organic solvent. Perindopril (Abdellatef, Ayad & Taha, 1999; Rahman & Rahman, 2011), ramipril (Abdellatef, Ayad & Taha, 1999; Ayad et al., 2002), enalapril (Ayad et al. 2002) and carbinoxamine (Ramadan and Mandil 2006) were complexed with eosin and zinc (for perindopril) or copper (for all drugs) followed by extraction of the complex with chloroform and measurement of the absorbance of the organic layer at 510–538 nm. Another way to overcome the low solubility of the complex was achieved through the addition of nonionic surfactants, such as methyl cellulose (Fujita et al., 1987; el Walily, Belal & Bakry, 1996; Ayad et al., 2003; El-Enany, 2004; Krebs, Starczewska & Leszczyńska, 2006; Abdellatef, 2007; El‐Didamony, 2007; Omar, 2010; Al-Tamimi, 2013) or polyvinyl alcohol (Mori et al. 1986). The addition of surfactants increased the complex stability and prevented its precipitation. As a result, the extraction step was omitted and the measurements were carried out directly in the aqueous solution. Thus, the simplicity of the procedure was greatly enhanced, and the environmental safety of the methods was improved. However, in some cases, the addition of the surfactant adversely affected the reproducibility of the methods. Therefore, an alternative approach was followed through keeping the analyte concentration at maximum dilution before adding the dye solution and mixing well before the addition of the acidic buffer. By this procedure, the complex stability was greatly increased, with complete prevention of precipitate formation, and maximum precision was achieved (Kelani, Bebawy & Abdel-Fattah, 1999; Moustafa, 2000).
|Drug||Matrix||Linear range (μg/ml)||LOD (μg/ml)||Metal ion||References|
|Sparfloxacin||Dosage forms||1.6–16||–||Pd (II)||(El‐Didamony 2007)|
|Hydrochlorothiazide, indapamide and xipamide||Dosage forms||8.0–40|
|Lead (II)||(Omar 2010)|
|Ciprofloxacin and norfloxacin||Tablets||3.0–10|
|–||Pd (II)||(el Walily, Belal, and Bakry 1996)|
|Gemifloxacin||Dosage forms||1.0–10||0.16||Pd (II)||(Al-Tamimi 2013)|
|Chlorpromazine, thiamine, lincomycin, ofloxacin and theophylline||Pharmaceutical preparations||0–6.36||–||Pd(II)||(Fujita et al. 1987)|
|Timolol and enalapril||Pharmaceutical formulations||0.16–1.6|
|–||Pd (II)||(Ayad et al. 2003)|
|Gliclazide||Dosage forms||0.5–4.0||0.05||Pd (II)||(El-Enany 2004)|
|Bezafibrate||Dosage forms||0.06–0.3||–||Pd(II)||(Krebs, Starczewska, and Leszczyńska 2006)|
|Minocycline*||Capsules||0–40||–||Gallium||(Mori et al. 1986)|
|Astemizole#, terfenadine# and flunarizine#||Dosage forms||4.1–37.6|
|–||Lead (II)||(Kelani, Bebawy, and Abdel-Fattah 1999)|
|Lansoprazole# and pantoprazole#||Pharmaceutical formulations||3.7–16.6|
|–||Copper (II)||(Moustafa 2000)|
*Polyvinyl alcohol was used as surfactant.
#Methylcellulose was not used.
Determination of heavy metals
Some heavy metals were spectrophotometrically determined through ternary complex formation using counter ions and eosin Y with a limit of detection down to 0.005 μg ml−1. Copper (Yen et al. 1989) was reacted with neo-cuproin and eosin Y. The formed colored associate was extracted with chloroform and measured at 544 nm. This system was used to determine the contents of trace copper in some tablets and injections. Similarly, lead (Szczepaniak and Juskowiak 1987) was determined through its reaction with cryptand (2.2.2) and eosin Y followed by extraction with chlorobenzene. A non-extractive spectrophotometric method was reported for iron (Hashem, Seleim, and El-Zohry 2010) in pharmaceutical and water samples through its interaction with 5-(4-nitrophenylazo) salicylic acid in an acidic medium and measuring the absorbance at 545 nm. In the determination of mercury (I1) (Mudakavi 1984) with phenanthroline and eosin, the solubility of the formed ternary complex was enhanced by the addition of gelatine solution, and the absorbance of the aqueous solution was measured at 552 nm.
Indirect spectrophotometric method
Thiamine (Srividya and Balasubramanian 1997) in pharmaceutical preparations was determined based on the interaction of thiamine with a known excess of silver ions in a buffered medium of pH 9.0. The unreacted silver ions were estimated by the formation of an ion-pair complex with eosin Y and the absorbance was measured at 550 nm.
Extractive spectrofluorimetric methods
Extractive spectrofluorimetric methods were reported for some drugs based on either binary or ternary complex formation with eosin Y. Binary complex formation was applied for the determination of citalopram hydrobromide (Azmi et al. 2015) and doxepin hydrochloride (Rahman, Siddiqui, and Azmi 2009) in commercial dosage forms. The method was based on the formation of an ion-pair complex between the drug and eosin Y in the presence of acetate buffer (at pH 3.4 for citalopram and pH 4.52 for doxepin). The formed complexes were extracted into dichloromethane and the fluorescence intensity was measured at 554 nm (excitation at 259 nm) for citalopram and at 567 nm after excitation at 464 nm for doxepin. Labetalol (Rahman and Haque 2008) was determined in commercial tablets and human urine samples through ternary complex with zinc (II) and eosin. The complex was extracted with chloroform and measured at 452 nm after excitation at 317 nm. These methods were linear over concentrations of 2.0–26.0, 0.1–0.8 and 0.5–4.0 μg ml−1 with limits of detection of 0.12, 0.003 and 0.08 μg ml−1 for citalopram hydrobromide, doxepin hydrochloride and labetalol, respectively.
Highly sensitive extractive spectrofluorimetric methods were reported for some heavy metals based on the formation of ternary complex between the metal cation, cryptand 2.2.1 and the eosinate counter ion. The formed complexes were extracted with 1,2-dichloroethane and their fluorescence intensities were measured. Using the previous procedure, the following metal ions were determined: lead (Gomis et al. 1988), cadmium (Gomis et al. 1989), chromium (III) (García and Gomis 1997), mercury (Andrés, Fuente, and Blanco 1993) and thallium (Andrés García and Blanco Gomis 2002). The same principle was applied for the simultaneous determination of lead and cadmium (Gomis and Garcia 1990), lead and mercury (Andres, Fuente, and Blanco 1994) and strontium and calcium (Andrés, Fuente, and Blanco 1992) by sequential extraction of the ternary ion-association complexes formed between the cations, a cryptand as the ligand and eosin. Ligands, other than cryptand, were applied in the determination of some heavy metals (Haddad, Alexander & Smythe, 1976; Sanz-Medel et al., 1984; Afonso, Santana & Montelongo, 1986; Oue, Kimura & Shono, 1988; Talio et al., 2009) using the same technique. These methods are summarized in Table 3. The detection limits of metal ions were in the range of 0.5–3.0 ng ml−1.
|Cobalt (II)||pH 5.6||Pyridine-2-aldehyde-2-pyridylhydrazone||Chloroform/acetone mixture||(Haddad, Alexander, and Smythe 1976)|
|Zinc (II)||Acetate buffer (pH 5.5)||Pyrocatechol-1-aldehyde 2-pyridylhydrazone||50% ethanol/water mixture||(Afonso, Santana, and Montelongo 1986)|
|Lead (II)||18-crown-6||(Sanz-Medel et al. 1984)|
|Silver (I)||pH 8.6||Benzothiacrown ether||Dichloromethane||(Oue, Kimura, and Shono 1988)|
|Cadmium||Tris buffer pH 7.6||o-phenanthroline||PONPE 7.5*||(Talio et al. 2009)|
*The complex was extracted into polyoxyethylene (7.5) nonylphenyl ether (PONPE 7.5) using cloud point extraction.
Fluorescence quenching methods
The aqueous solution of eosin Y showed native fluorescence at about 545 nm after excitation at 301 nm. The addition of basic compounds (containing mainly amino groups) to the reagent solution dramatically reduced the fluorescence intensity of eosin Y. The fluorescence quenching was due to the formation of ion pair complexes between the basic compounds and eosin Y (Figure 4). Based on this fact, many analytical procedures were constructed for the determination of several pharmaceutical compound (de Vries, Ruben & Arends, 1991; Gazy et al., 2002; Belal et al., 2008; Liu et al., 2008; Arici & Kilinc, 2010; Walash et al., 2010; Ibrahim et al., 2011; Omar et al., 2011; Gan et al., 2012; Kaur & Malik, 2013; Ammar, El-Brashy & Al-Sahly, 2014; Derayea, 2014; Hussein, Fares & El-Kosasy, 2014; Wahba, El-Enany & Belal, 2015; Derayea et al. 2016; 2016; Hammad, Omar & Eltoukhi, 2016; Ayad et al., 2018; Azmi et al., 2017; Theia’a, Dhamra & Al-Ghabsha, 2017; Derayea, Omar & Abu-hassan, 2018) through the formation of binary complexes (Table 4). In many cases, metal ions (lead, copper, zinc and palladium) were chelated with the analyte. These chelates can induce the fluorescence quenching of eosin Y upon the formation of ternary complex (Figure 4). Table 5 summarizes the use of ternary complex formation for determination of some basic compound (Omar, 2010; Abdellatef, 2007; el Walily, Belal & Bakry, 1996; Al-Tamimi, 2013; Kelani, Bebawy & Abdel-Fattah, 1999; Attia & Omar, 2016; Wu et al., 2017; Shi et al., 2016) through fluorescence quenching of eosin Y. Stern-Volmer equation was applied in several studies (Liu et al., 2008; Gan et al., 2012; Kaur & Malik, 2013; Wahba, El-Enany & Belal, 2015; Azmi et al., 2017) to investigate the mechanism of the fluorescence quenching due to the complex formation. All the mentioned reports agreed that the quenching process followed static mechanism as the quenching constants were greater than the highest reported value for dynamic quenching. Further evidence for the static mode are the changes in the absorption spectra of the dye that accompanied the complex formation. In static quenching, the quencher reacts with a fluorescent agent in the ground state to form a non-fluorescent or a weakly fluorescent complex. The complex formation occurred through electrostatic attraction, hydrophobic forces and/or hydrogen bonding.
|Drugs||Matrix||Linear range (μg/ml)||LOD (μg/ml)||λem/λex (nm)||References|
|Cetyltrimethyl ammonium bromide||Industrial waste water||2.9–14.5||–||500/304||(Hussein, Fares, and El-Kosasy 2014)|
|Dothiepin||Dosage forms||0.3–8.0||0.11||543/304||(Walash et al. 2010)|
|Doxazosin||Tablets||1.0–10||0.079||570/340||(Ammar, El-Brashy, and Al-Sahly 2014)|
|Succinylcholine||Ampoules||0.05–1.0||0.012||550/480||(Ayad et al. 2018)|
|Cetirizine, fexofenadine, loratadine and acrivastine||Dosage forms||3.0–10|
|550/310||(Gazy et al. 2002)|
|Mebeverine*||Dosage forms||0.2–3.5||0.11||540/390||(Derayea 2014)|
|Chlorhexidine||Saliva||1.0–15||0.80||541/301||(de Vries, Ruben, and Arends 1991)|
|Olanzapine and fluphenazine||Dosage forms||0.05–1.0|
|547/323||(Belal et al. 2008)|
|Carbazochrome||Biological fluids||0.03–1.5||0.009||545/301||(Gan et al. 2012)|
|Ebastine||Dosage forms||0.1–1.0||0.021||553/457||(Ibrahim et al. 2011)|
|Losartan, irbesartan, telmisartan and valsartan||Pharmaceutical tablets||0.8–8.0|
|546/310||(Omar et al. 2011)|
|Betaxolol, carvedilol, labetalol, nebivolol and propranolol||Dosage forms||0.1–2.5|
|545/301||(Derayea et al. 2016)|
|Fluoxetine and paroxetine||Pharmaceutical formulations||0.2–2.4|
|545/301||(Derayea et al. 2016)|
|Amitriptyline and clomipramine||Tablets||0.08–2.0|
|550/310||(Kaur and Malik 2013)|
|Bleomycin A5 and bleomycin A2||Human serum and urine||0.12–2.5|
|527/542||(Liu et al. 2008)|
|Tetracycline and terbutaline||Dosage forms||0.50–18|
|545/350||(Theia’a, Dhamra, and Al-Ghabsha 2017)|
|Sunitinib||Dosage forms||0.80–5.0||0.041||547/350||(Arici and Kilinc 2010)|
|Almotriptan, rizatriptan, sumatriptan and zolmitriptan||Pharmaceutical preparations and biological fluid||0.07–1.0|
|543/301||(Hammad, Omar, and Eltoukhi 2016)|
|Clindamycin||Dosage forms||0.2–2.0||0.13||482/555||(Wahba, El-Enany, and Belal 2015)|
|Atomoxetine and fluvoxamine||Capsules and tablets||0.2–2.2|
|545/302||(Derayea, Omar, and Abu-hassan 2018)|
|Chlorpheniramine||Tablets||1.0–8.0||0.30||544/258||(Azmi et al. 2017)|
*Methyl cellulose was added to enhance the aqueous solubility of the complex.
|Drug||Linear range (μg/ml)||LOD (μg/ml)||Metal ion||λem/λex (nm)||References|
|Hydrochlorothiazide, indapamide and xipamide||0.05–0.25|
|Lead (II)||545/462||(Omar 2010)|
|Ramipril||2.0–8.0||–||Copper (II)||543/300||(Abdellatef 2007)|
|Ciprofloxacin and norfloxacin||0.035–0.07|
|Palladium (II)||545/336||(el Walily, Belal, and Bakry 1996)|
|Gemifloxacin||0.6–4.0||0.11||Palladium (II)||544/337||(Al-Tamimi 2013)|
|Chlorpromazine, thiamine, lincomycin, ofloxacin and theophylline||0–2.4||–||Palladium (II)||545/462||(Fujita et al. 1987)|
|Astemizole, terfenadine and flunarizine||0.94–7.1||–||Lead (II)||462/545||(Kelani, Bebawy, and Abdel-Fattah 1999)|
|Thioridazine and flupentixol||0.5–3.0|
|Lead (II)||517/462||(Attia and Omar 2016)|
|Guanine*||0.5–17.5||0.22||Copper (II)||539/480||(Shi et al. 2016)|
|Quizalofop-p-ethyl||0.04–1.0||0.02||Palladium (II)||540/519||(Wu et al. 2017)|
*Methylcellulose was not used.
Catalytic spectrofluorimetric method
An indirect spectrofluorimetric method using eosin as a highly sensitive fluorescent probe for clenbuterol hydrochloride (Liu et al. 2014) detection has been designed. The method was based on its catalytic effect on sodium iodate oxidating eosin Y. The linear range was 0.02 × 10−18–24 × 10−18 g ml−1with detection limit of 6.8 zg ml−1 (10−21 g ml−1).
Switch “off-on” of eosin Y fluorescence
Perfluorooctane sulfonate (PFOS) (Liang, Deng, and Tan 2015) was determined in aqueous solution based on the “off-on” switch of eosin Y/polyethyleneimine (PEI) fluorescence system. In Britton-Robinson buffer solution (pH 7.0), eosin Y reacted with protonated PEI to form complex which resulted in a strong quenching of the native fluorescence of eosin Y. The fluorescence was recovered by the addition of PFOS due to the higher affinity of PEI to PFOS than eosin Y. The recovered fluorescence intensity was proportional to the concentration of PFOS in the range of 0–1.0 μg ml−1 with the limit of detection of 0.0075 μg ml−1.
Solid surface fluorescence (SSF)
Zinc (Talio et al. 2015) traces were determined in beverages and water samples by solid surface fluorescence. In Tris buffer (pH 7.5), zinc ion was complexed with o-phenanthroline and eosin and adsorbed in a piece of filter paper. The filter paper was used as a solid support in conventional quartz cuvette and the fluorescence intensity was measured at 440 nm (λex = 340 nm). The linear range of the method was 1.29–4.5 pg ml−1 (LOD, 0.36 pg ml−1). The nickel (Talio et al. 2010) level in human urine of smokers and non-smokers’ subjects was checked using SSF. Nylon membranes previously treated with eosin were applied to absorb the metal ions and the fluorescence emission was measured at 547 nm (λex at 515 nm). The linear metal concentration was in the range of 0.44–410 pg ml−1 (LOD, 0.13 pg ml−1). Nickel and cadmium (Talio et al. 2017) were simultaneously determination in tobacco after chemofiltration on nylon membrane. In phosphate buffer (pH 7), the metals were complexed with eosin and carbon nanotubes dispersed in sodium dodecyl sulfate solution and the solution was filtered through the nylon membrane. The solid support selectively retained Ni(II) and its fluorescence intensity was measured at 545 nm (λex = 515 nm). The Cd(II) content in the filtrate was determined by adjusting the pH to 5.0 with acetate buffer and the fluorescence was measured at 565 nm (λex = 540 nm). The calibration curves were linear in the range of 0.058–29.35 and 0.124–56.20 ng ml−1 with detection limits of 19 and 41 pg ml−1 for Ni(II) and Cd(II), respectively.
Solid substrate-room temperature phosphorimetry (SS-RTP)
Trace of clenbuterol hydrochloride (Liu et al. 2009) was determined with high sensitivity (detection limit, 0.021 zg spot−1; corresponding concentration, 0.52 × 10−21 g ml−1) by SS-RTP based on its catalytic effect on the oxidation of eosin Y with sodium periodate. Oxidation of eosin was accompanied by a sharp quenching of the room temperature phosphorescence signal of eosin. Another SS-RTP has been reported for some bioactive matters (alpha-fetoprotein variant, alkaline phosphatase and glucose) (Liu et al. 2009) based on their adsorption affinity to eosin self-ordered ring. The ring was formed by the reaction of eosin with polyamide membrane in the presence of cetyl trimethyl ammonium bromide and ammonia water. Detection limits of this method were 0.040, 0.045 and 0.090 fg spot−1 for alpha-fetoprotein variant, alkaline phosphatase and glucose. As shown in Table 6, SS-RTP was the most sensitive technique which applied Eosin Y as a reagent.
|Organic compounds including pharmaceuticals|
|– Non-extractive spectrophotometry (binary complex)||0.10 × 10−6||(Walash et al. 2011)|
|– Non-extractive spectrophotometry (ternary complex)||50 × 10−9||(El-Enany 2004)|
|– Extractive spectrofluorimetry||3.0 × 10−9||(Rahman, Siddiqui, and Azmi 2009)|
|– Fluorescence quenching (binary complex)||9.0 × 10−9||(Gan et al. 2012)|
|– Fluorescence quenching (ternary complex)||20 × 10−9||(Wu et al. 2017)|
|– Switch “off-on” of eosin Y fluorescence||7.5 × 10−9||(Liang, Deng, and Tan 2015)|
|– Resonance Rayleigh scattering||0.95 × 10−9||(Li et al. 2011)|
|– Flow injection chemiluminescence||0.23 × 10−12||(Wang, Zhao, and Gao 2014)|
|– Catalytic spectrofluorimetry||6.8 × 10−21||(Liu et al. 2014)|
|– Solid substrate-room temperature phosphorimetry||0.52 × 10−21||(Liu et al. 2009)|
|– Spectrophotometry (ternary complex)||5.0 × 10−9||(Hashem, Seleim, and El-Zohry 2010)|
|– Extractive spectrofluorimetry (ternary complex)||0.5 × 10−9||(Andrés, Fuente, and Blanco 1992)|
|– Solid surface fluorescence (ternary complex)||0.13 × 10−12||(Talio et al. 2010)|
Streptomycin (Du et al. 2013) in rat plasma was determined by a chemiluminescence (CL)-based reaction. In the alkaline medium, N-bromosuccinimide-eosin system had a weak CL signal which was greatly enhanced by the addition of streptomycin. Ferulic acid (Shen et al. 2013) was also determined by flow injection chemiluminescence. The method was based on the significant enhancement of ferulic acid on the CL signal of the N-bromobutanimide-eosin-chromium (II) chloride system in alkaline solution. Lercanidipine (Wang, Zhao, and Gao 2014) was determined with high sensitivity using post-chemiluminescence (PCL) method combined with a flow injection technique. PCL reaction was performed by injecting lercanidipine solution into the CL reaction mixture of N-chlorosuccinimide with alkaline eosin Y (as the CL reagent) in the presence of cetyltrimethylammonium bromide as a surfactant. The linear concentration ranges were 0.008–1.0, 0.0004–0.1 and 0.0007–3.0 μg ml−1 with detection limits of 2.25, 0.28 and 0.23 ng ml−1 for streptomycin, ferulic acid and lercanidipine, respectively.
Resonance Rayleigh scattering
The formation of eosin Y association complex (binary or ternary types) was accompanied not only by change in the absorption spectrum, quenching of the eosin Y fluorescence, but also the large-scale enhancement of resonance Rayleigh scattering (RRS) and the appearance of new RRS spectra. The enhancement of RRS was attributed to the enlargement of the molecular volume as a result of increasing the molecular weight, increasing the molecular rigidity and planarity, and enhancement of the hydrophobicity as a result of complex formation (Li et al., 2017; Tian et al., 2014). The complex formation restricts the free rotation of the dye molecule, which increased its molecular rigidity and planarity. In addition, before the complex formation, the drug was positively charged, while eosin Y was negatively charged, so each of them can be easily hydrated and dissolved in water. However, when they reacted with each other, their charges are neutralized. Consequently, the produced ion-association complex is electrically neutral and hence the hydrophobicity is enhanced. The increase in the scattering intensity was directly proportional to the drug concentrations. RRS was used in the determination of bleomycin A5 and bleomycin A2 (Liu et al. 2007) in serum and urine samples with detection limits of 17 and 18 ng ml−1, respectively, verapamil (Li et al. 2011) in acetate buffer solution (pH 3.2) with linear range of 0.003–4.0 μg ml−1 (LOD, 0.95 ng ml−1), albendazole (Tian et al. 2014) in Britton-Robinson buffer solution (pH 3.3) with linear range of 0.2–3.2 μg ml−1 (LOD, 6.93 ng ml−1) and polyvinyl pyrrolidone (Yu et al. 2010) in acetate buffer (pH 3.2) with linear range of 0.01–0.16 μg ml−1 (LOD, 0.029 μg ml−1). Additionally, the new resonance Rayleigh scattering were appeared at 324, 620, 324, 356 and 276 nm for bleomycin A5, bleomycin A2, verapamil, albendazole and polyvinyl pyrrolidone, respectively. Linezolid (Thakkar, Gevriya, and Mashru 2014) and acetazolamide (Li et al. 2017) in acetate buffer (pH 4) reacted with eosin and palladium to form ternary ion association complexes in the presence of methyl cellulose. The RRS was measured at 538 and 298 nm for linezolid and acetazolamide, respectively. The linear concentration ranges were 0.01–0.5 and 0.014–2.5 μg ml−1. A ternary complex was formed between metallothioneins (Xiao et al. 2015), eosin Y and sodium dodecyl benzene sulfate in Britton-Robinson buffer (pH 3.9). The RRS of the complex was measured at 366 nm and the method had a linear concentration range of 0.04–14.0 μg ml−1.
Atomic absorption spectroscopy
Ramipril (Abdellatef, Ayad & Taha, 1999; Ayad et al., 2002), perindopril (Abdellatef, Ayad, and Taha 1999) and enalapril (Ayad et al. 2002) were determined indirectly through complex formation with copper (II) and eosin in aqueous buffered solutions. The formed ternary complexes were extracted into chloroform and the copper content of the organic extracts were determined by atomic absorption spectroscopy.
In this article, the use of eosin Y as an analytical probe was reviewed. The applications included mainly pharmaceutical analysis, in addition to several heavy metals, some surfactants and few proteins. These methods were based mainly on the formation of either binary or ternary complexes between the target analyte and eosin. Spectrofluorimetry was the most widely used technique and represents about 50% of all the reported spectroscopic methods (Figure 5). One half of the spectrofluorimetric methods were based on the quenching effect of the analyte on the native fluorescence of the dye. Fluorescence quenching methods are fairly sensitive (LOD down to 9 ng ml−1) and were applied to a wide range of pharmaceutical compounds and surfactants. Although RRS provided a more sensitive method for quantitative determination down to 0.95 ng ml−1, it was used in very few articles. In spite of the extreme sensitivity of the SS-RTP (Table 6) with a detection limit in the zepto-gram level (10−21 g), only two applications were reported using this technique. Other minor methods included atomic absorption spectroscopy (2%) and chemiluminescence (3%).
Conflict of interest: The authors of the paper declare no conflict of interest in publishing this paper.
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