Fluorescence of zirconium–naphthalene complexes: Effect of ortho-naphthalene substitution

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Spectrochimica Acta Part A 71 (2008) 907–914

Fluorescence of zirconium–naphthalene complexes: Effect of ortho-naphthalene substitution M.I. Rodr´ıguez-C´aceres a,∗ , R.A. Agbaria b , U.J. Luna b , S. White b , I.M. Warner b a

Department of Analytical Chemistry, University of Extremadura, 06071 Badajoz, Spain b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA

Received 27 May 2007; received in revised form 1 February 2008; accepted 5 February 2008

Abstract The effect of the position of substituents on the formation of metal–naphthalene complexes has been investigated. Two positional isomers, 1-hydroxy-2-naphthoic acid (1H2NA) and 3-hydroxy-2-naphthoic acid (3H2NA), have been chosen. A comparative study of the luminescence behaviour of the two isomers in the presence of Zr(IV) has been performed. Interesting results were obtained. While 1-hydroxy-2-naphthoic acid is quenched in the presence of Zr(IV), 3-hydroxy-2-naphthoic acid produced high-fluorescence enhancement. Several pH studies were performed between pH 2.5 and 5.0 and the stoichiometries of the complexes were also established at the different pH values tested, by use of the Benesi–Hildebrand method. In addition, the formation constants have been calculated. Finally, quenching and lifetime studies were performed in an attempt to establish the type of quenching (static or dynamic) that is produced when a complex is formed between 1-hydroxy-2-naphthoic acid and zirconium metal ion. © 2008 Elsevier B.V. All rights reserved. Keywords: 1-Hydroxy-2-naphthoic acid; 3-Hydroxy-2-naphthoic acid; Zirconium; Stability constants

1. Introduction Polynuclear aromatic hydrocarbons (PAHs) are important pollutants of the environment. It has previously been demonstrated that PAHs can be formed by thermal decomposition of any organic material containing carbon and hydrogen. Formation is based on two major mechanisms: (a) pyrolysis or incomplete combustion, and (b) carbonization processes. Due to the high toxicity of these compounds and, particularly their carcinogenic/mutagenic character, PAHs are listed by the Environmental Protection Agency (EPA) as priority pollutants. Two of these priority pollutants are phenanthrene and benz[a]anthracene. The biodegradation of these PAHs by a variety of bacteria may give several metabolites. Under such degradation, the main metabolite of phenanthrene [1] has been reported to be 1-hydroxy-2-naphthoic acid (1H2NA) and for the benz[a]anthracene [2], the two major metabolites are 3-hydroxy2-naphthoic acid (3H2NA) and 2-hydroxy-3-phenanthroic acid. It should be noted that 1H2NA and 3H2NA identified above are positional isomers of naphthalene. Identification of 1H2NA

∗

Corresponding author. Tel.: +34 924289375; fax: +34 924289375. E-mail address: [email protected] (M.I. Rodr´ıguez-C´aceres).

1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.02.011

and 3H2NA has been achieved by use of LC–electrospray ionization tandem mass spectrometry [3] and capillary electrophoresis [4]. In recent times, 3H2NA has been used as precursor for the synthesis of dyestuffs and drugs [5]. The purity of this compound for industrial use has been controlled by use of capillary zone electrophoresis (CZE) [6]. A survey of the literature shows that the study of metal complexes of ortho-hydroxycarboxylic aromatic compounds has been performed since 1960s. Thus, a variety of techniques such as potentiometry [7], conductimetry [8,9], spectrophotometry [7–12] and spectrofluorimetry [13–16] have been used to examine these complexes. The 1H2NA positional isomer has been used for the spectrophotometric determination of nitrite [9], while 3H2NA has been used for aluminum [8], copper [10], lanthanoids [11] and vanadium [12]. In addition, the 3H2NA positional isomer has been used for the spectrofluorimetric determination of beryllium [13,14] and scandium [15]. Other uses include the application of Calvin–Bjerrum pH titration technique, as used by Irving and Rossotti, for determining the stability constants of the complexes formed between 3H2NA with Cu(II), Ni(II), Co(II), V(V) and Mn(II) [16,17]. There are not many references in the literature about the fluorescence properties of both positional isomers. Thus, Kovi and

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Schulman [18] studied the ionization sequences in the ground and lowest electronically excited singles states of 3H2NA. They concluded that 3H2NA has several prototropic forms. In addition, a study of the fluorescence properties of 1H2NA was also performed [19]. The main differences between the two positional isomers are: (1) while the neutral molecule derived from 1H2NA undergoes biprotonic phototautomerisation to the zwitterions, this process is not observed in 3H2NA, and (2) phototautomerism of the singly charged anion occurs in 3H2NA but not in 1H2NA. Mishra et al. [20] studied the photoinduced proton transfer in 3H2NA. Their experiments showed that 3H2NA on photoexcitation exhibits dual emission corresponding to normal and large Stokes-shifted fluorescence. The latter is attributable to emission following excited state intramolecular proton transfer (ESIPT). The absorption and emission characteristics are shown to be sensitive to the concentration of 3H2NA, the choice of the solvent, pH, temperature and excitation wavelength. Recently, it has been established that 1H2NA shows ESIPT reaction as well [21]. With regard to zirconium, this metal has often been used as an analytical reagent in fluorescence for the formation of complexes with several different fluorophores, due to the fact that the formation of a fluorescent metal complex can be expected from paramagnetic ions which have only one stable valency [22]. Thus, different complexes such as Zr–Alizarin Red S–EDTA [23], Zr–Oxalate–Alizarin Red S [24], Sulphate–Calcein–Zr [25], Zr–Morin [26] have been studied. Different surfactants such as hexadecyltrimethylammonium bromide [23], cetylpyridinium chloride [24], Triton X-100 [27] have been used to enhance the fluorescence of these complexes. Recently, on the basis of the studies reported in this work, a new spectrofluorimetric method has been developed by our research group for the determination of 3H2NA by formation of the ternary complex 3H2NA–Zr(IV)–␤-CD [28]. In this manuscript, the interaction of two positional isomers with zirconium metal ions has been studied. As a result of this interaction, the fluorescence intensity of both isomers presents different behaviour at all pH values tested. The Benesi–Hildebrand method was applied to determinate the stoichiometry of these complexes and the logarithms of the stability constants were calculated. In order to conclude if the quenching of the fluorescence intensity of 1H2NA was dynamic or static, quenching and lifetime experiments were carried out. The main purpose of this work is to establish the base for the further development of new fluorescent sensor molecules based on the different response to the zirconium metal ion. 2. Experimental 2.1. Instruments Steady-state fluorescence spectra were acquired on a Fluorolog-2 (Yvon-Horiba, NJ, USA) spectrofluorimeter equipped with a 450-W Xenon arc lamp. Samples were measured in a 1-cm quartz cell using 4 nm bandwidths for both excitation and emission slits. The instrument was controlled by use of software DataMax version 2.2. The excitation and emis-

sion wavelengths used in the steady-state measurements were 348/418 nm for 1H2NA and 356/504 nm for 3H2NA. Every single fluorescence datum was taken at the maximum emission wavelength of each fluorophore. No polarizer or magic angle conditions were used. All the measurements were performed at room temperature. pH measurements were made using an Orion 410A (Cambridge, MA, USA) digital pH-meter with a combination glass–calomel electrode. Lifetime measurements were acquired using a Spex Fluorolog-3 (model FL3-33TAU3; Jobin Yvon, NJ, USA) equipped with a 450-W xenon lamp and R928P photomultiplier tube (PMT) detector. The instrument was controlled by use of software DataMax version 2.2. The lifetime reference solution used was Ludox (colloidal silica) in water, with an assumed reference lifetime of 0.00 ns. Phase and modulation responses for both fluorophores were obtained using frequency-domain mode. The parameters fixed were: frequency range between 50 and 300 MHz in logarithmic scale, number of frequencies 10; average 5; and integration time 8 s. Frequency-domain phase and modulation decay profiles were analyzed using the GLOBALS UnlimitedTM software. Phase and modulation data sets were well fit by appropriate models as indicated by visual inspection of residual plots as well as by χ2 statistics. The χ2 -value will reflect the validity of the fit between the data and the calculated values (goodness-of-fit). A valid model should usually provide a χ2 -value between 1 and 2. If χ2 is greater than that, the rejection of the model should be considered. Rejection is judged from the probability that random noise could be the origin of the value of χ2 [29]. The accuracies of phase angle and modulation for the instrumentation need to be adjusted for a particular instrument, and were set, in this case, at 0.3◦ and 0.008, respectively, for consistency and ease day-to-day data interpretation. In all phase angle measurements, the measurement error was statistically pre-set at 0.2◦ . 2.2. Materials 1H2NA and 3H2NA were purchased from Aldrich (Milwaukee, WI, USA) and used as received. ZrCl4 was purchased from Sigma. Triple distilled de-ionized water (Purelab UV/UF, US filter) was used in all studies. 2.3. General procedure for fluorescence measurements Standard stock solutions of 1H2NA and 3N2NA were prepared in methanol. Diluted solutions were prepared by appropriate dilution of the standard stock solution with distilled water. The fluorescence spectra were recorded immediately after sample preparation using the excitation and emission wavelengths noted above for each fluorophore. For pH studies, 4 ␮g/mL of each fluorophore were placed in 5-mL volumetric flasks and the pH was adjusted with appropriate aliquots of sodium hydroxide or hydrochloric acid. Finally, water was added until 5-mL final volume to achieve the desired pH value. The interactions of 1H2NA and 3H2NA with zirconium were studied at several pH values by use of fluorescence measure-

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ments. For determining the stability constants of the complexes, the concentration of fluorophores were kept constant at 4 ␮g/mL, while the concentration of metal ion was varied in the range of 0.0–2.0 mM. The buffers used were Glycine/HCl (0.5 M) at pH 2.5 and NaAc/HAc (0.5.M) at pH 4.0 and 5.0. The addition order was keeping constant and was the following: fluorophore + buffer + metal ion. 2.4. Determination of stoichiometries and association constants of the complexes The stoichiometries and stability constants of the two complexes with zirconium at different pH values were established by use of the Benesi–Hildebrand method [30]. In this method, a graph of, 1/(I − I0 ) versus 1/[Me]0 was performed, where I is the observed fluorescence intensity at each concentration of metal ion tested, I0 is the fluorescence intensity of the fluorophore in absence of metal ion, and [Me] is the concentration of metal ion. If a linear plot was obtained (known as double-reciprocal plot), it could be concluded that the stoichiometry is 1:1 for the complex. The association constant was determined by dividing the intercept by the slope of the straight line obtained in the double-reciprocal plot. 3. Results and discussion 3.1. Influence of the pH Variations in the fluorescence of 1H2NA and 3H2NA in both, the presence and the absence of zirconium were studied. In the absence of zirconium, the fluorescence intensity of 1H2NA increases with the pH until it reaches a constant value at pH higher than 5.0 (see Fig. 1). When the influence of pH was carried

Fig. 2. Influence of pH for 3H2NA in both absence and presence of zirconium. [3H2NA] = 4 ␮g/mL; λex /λem = 356/504 nm.

out in the presence of zirconium, a decrease in the fluorescence intensity was observed and this decrease is greatest at pH values between 2.50 and 4.60. At pH higher than 5.0, a white precipitate appeared in the solution, probably due to the formation of ZrO4 . The fluorescence of 3H2NA in absence of zirconium (Fig. 2), increases between 2.35 and 4.50 and then remains constant at higher pH values. In the presence of zirconium, the fluorescence increases until a maximum is reached at pH 4.65. One more time, a white precipitate could be observed at higher pH values. Using these data, a pH range between 2.5 and 5.0 was chosen for the studies outlined below. 3.2. Influence of zirconium concentration

Fig. 1. Influence of pH for 1H2NA in both absence and presence of zirconium. [1H2NA] = 4 ␮g/mL; λex /λem = 348/418 nm.

The influence of Zr(IV) concentration on the fluorescence of 1H2NA was studied in the optimum pH range between 2.5 and 5.0. Surprisingly, fluorescence quenching was observed in all cases. At pH 2.5, the emission maximum of 1H2NA in the presence of zirconium shows a slight red shift of 6 nm as the concentration of zirconium is increased. The excitation spectra show even greater differences. There is a red shift of 10 nm, and the spectra present an iso-emissive point at 366 nm (Fig. 3A). The appearance of an iso-emissive point suggests that only two emissive species exist in the equilibrium mixture [31]. This could indicate that both, neutral and anionic form of the 1H2NA are complexed with Zr(IV) in the same extent [for 1H2NA the pK2 = 2.7 (19)]. At pH 4.0, no change was observed at the emission spectra with the maximum centred at 416 nm. However, in the excitation spectra, the fluorescence intensity decreases when the zirconium concentration increases and there is a red shift of 4 nm. It is worth noting that at concentrations higher than 1.50 mM of zirconium,

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Fig. 3. Excitation (1–6) and emission spectra (1 –6 ) of 1H2NA in the presence of zirconium at different pH values. (A) pH 2.5 (glycine/HCl 0.5 M); (B) pH 4.0 (NaAc/HAc 0.5 M); (C) pH 5.0 (NaAc/HAc 0.5 M); [1H2NA] = 4 ␮g/mL; λex /λem = 348/418 nm.

the spectra change significantly. While at low concentrations, the spectra show a maximum located at 346 nm, at high concentrations this maximum has a slight red shift (4 nm) and, besides, the spectrum presents a shoulder at around 366 nm (wavelength at which the iso-emissive point appeared at pH 2.5), as can be seen in Fig. 3B. This fact can be explained taken into account that, at this pH value, the equilibrium is more displaced toward the anionic form; and the shoulder could be associated at the anion of 1H2NA, whereas the maximum located at 346 nm could be associated at the neutral form. On the other hand, at pH 5.0, no changes in the emission spectra were observed; however, the excitation spectra show again some interesting changes. Around 1.00 mM of zirconium, the

maximum located at 344 nm decrease and the spectrum shows a shoulder around 358 nm, which becomes as a maximum at higher concentrations of zirconium (Fig. 3C). At this pH value the equilibrium is almost displaced to the anionic form, for that reason, it will be the prevailing; this can be observed in spectrum 6. Besides, visual inspection of spectra 5 and 6 show that the anionic form is not quenched by Zr(IV). A plot of the fluorescence intensity versus the concentration of zirconium at different pH values is shown in Fig. 4. A trend of decreased fluorescence quenching with the increase of pH values between pH 2.5 and 4.0 is observed. However, when the pH increases up to pH 5.0, an increase in the quenching effect is observed.

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Fig. 4. Fluorescence intensity of 1H2NA in the presence of variable concentration of zirconium at several pH values. [1H2NA] = 4 ␮g/mL; λex /λem = 348/418 nm.

For 3H2NA, the influence of zirconium concentration on the fluorescence intensity was studied at different pH values as well. Fig. 5 shows the excitation and emission spectra of 3H2NA in the absence and in the presence of zirconium. A significant increase of the fluorescence intensity is observed when a small concentration of zirconium (0.25 mM) is added. This experiment was carried out at pH 2.5. This increment is larger for the other pH values (pH 4.0 and 5.0). A red shift of 13 nm can be observed for the excitation spectra. However, a blue shift of 40 nm is

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Fig. 6. Fluorescence intensity of 3H2NA in the presence of variable concentration of zirconium at several pH values. [3H2NA] = 4 ␮g/mL; λex /λem = 356/504 nm.

observed for the emission spectra. The same trend is observed at the other pH values. A plot of the fluorescence intensity versus the concentration of zirconium at different pH values is shown in Fig. 6. The sharp curve at pH 5.0 clearly indicates that equilibrium between 3H2NA and the metal ion is not reached in the interval of Zr(IV) concentration studied. 3.3. Stoichiometry of the complexes with zirconium As previously noted, the stoichiometry of the two naphthalene positional isomer complexes with zirconium was studied using the Benesi–Hildebrand method. In all cases but one, a 1:1 stoichiometry was found. The binding constants for the 1H2NA:Zr(IV) and the 3H2NA:Zr(IV) at three different pH values were also determined. As the pH increases, the binding constant (K) decreases as shown in Table 1. The log K for the 1H2NA:Zr(IV) complex at pH 5.0 shows a high value, but this is likely due to the 1:2 stoichiometry. Thus, this constant is a global constant, log K1 K2 . As it has been explained above, the quenching of 1H2NA by Zr(IV) at pH 5.0 is not simple, for that reason, this global constant has to be carefully handle. Table 1 Stoichiometries and log K for the formed complexes pH

(1 ,

2 )

Fig. 5. Excitation (1, 2) and emission spectra of 3H2NA in the absence (1, 1 ) and the presence (2, 2 ) of zirconium. [3H2NA] = 4 ␮g/mL.

2.5 4.0 5.0

1H2NA

3H2NA

Stoichiometry

log K

Stoichiometry

log K

1:1 1:1 1:2

3.30 2.70 7.48

1:1 1:1 1:1

3.35 3.00 2.94

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3.4. Characterization of the 1H2NA:Zr(IV) complex: quenching and lifetime studies At it has been mentioned in the introduction, when a complex is formed between a fluorophore and a paramagnetic ion with possess only one stable valency, an increase of the fluorescence intensity is expected and this happens with 3H2NA. However, 1H2NA is quenched by zirconium and this quenching resulted in a decrease of the fluorescence intensity as well as slight spectral shifts within the quencher concentration range used. In an attempt to understand this unexpected behaviour, quenching and lifetime studies were performed. In a quenching reaction, the fluorescence intensity, I, and quencher concentration [Q], are related by the Stern–Volmer equation, I0 = 1 + KSV [Q] I where I0 is the fluorescence intensity in the absence of quencher and KSV is the Stern–Volmer constant. Within the concentration range investigated, the fluorescence quenching of 1H2NA by zirconium does not show a linear dependence as predicted by the Stern–Volmer equation (Fig. 7). Over the three pH values examined, the Stern–Volmer plots derivate from linearity at the x-axis. This suggests that two fluorophore populations are present and one of them is not accessible to quencher [29]. In addition, inefficiency of the quenching can lead to a downward curving collisional quenching component when combined with static quenching can result in a downward curving Stern–Volmer plot, an apparently linear plot, or a plot showing only a slight upward deviation [32]. The non-linear (in the Stern–Volmer simple representation) quenching curves were analyzed by using the Stern–Volmer

equation modified to include static quenching, I0 V [Q] e = 1 + KSV [Q] I where V is the static quenching constant that represents an active volume element surrounding the excited solute molecule. Again, no lineal behaviour was observed. Thus, these results suggest that the mechanisms involved are complex in nature and difficult to elucidate. A quadratic behaviour may be described under the assumption of combined static and dynamic quenching processes [29]. However, it should be noted that the curvature is not absolute proof of the presence of both combined static and dynamic quenching, since heterogeneous emission systems can also result in a concave plot. Thus, KSV values could not be established for these data due to the non-linearity of Stern–Volmer plot and modified Stern–Volmer plot for this complex. In order to ascertain the factors that contribute to the downward deviation of the Stern–Volmer plots for the complex 1H2NA:Zr(IV), lifetime measurements were carried out. Initially, the lifetime values for the two fluorophores in the absence of Zr(IV) were calculated at pH 2.5 with frequency-domain measurements. The fluorescence lifetime of 1H2NA fit a single-exponential model. The lifetime calculated was 1.220 ± 0.023 ns (χ2 = 1.49). However, when the lifetime of the 3H2NA was measured under the same conditions as 1H2NA and fitted to a single-exponential model, χ2 = 22.80 was found. In order to decrease the χ2 -value, a double-exponential model was assayed. In this case, χ2 = 0.77 and the lifetime value was 1.563 ± 0.038. Average lifetimes (τ) for bi-exponential decays of fluorescence were calculated from the values of decay time (τ) and pre-exponential factor (f) using the following expression: τ¯ =

Fig. 7. Stern–Volmer plots for 1H2NA at several pH values. [1H2NA] = 4 ␮g/ mL.

f1 τ12 + f2 τ22 f 1 τ1 + f 2 τ2

Fig. 8 shows the single-exponential model for 1H2NA and the double-exponential model for 3H2NA. The formation of the complex between 1H2NA and Zr(IV) was also studied by use of frequency-domain lifetime measurements. Fig. 9 shows the frequency-domain decays for 1H2NA in both the absence and the presence of 0.25 mM of Zr(IV). In the presence of Zr(IV), 1H2NA required a double-exponential fit which could be attributable to the presence of two species which have different decay kinetics. In Table 2, the lifetime values for the complex are summarized using the double-exponential model. As can be seen, χ2 presents values comprises between 1.63 and 9.16. Assuming the validity of the fit, it could be observed that in presence of Zr(IV), the lifetime values decreases when Zr(IV) concentration increases. It is well established that static quenching does not decrease the lifetime. Thus, the decrease in lifetime must be attributed to dynamic quenching. When the Stern–Volmer equation is plotted using τ 0 /τ a straight line is observed at low concentrations of Zr(IV). The same can be observed if I0 /I is plotted. As seen in Fig. 10, when both τ 0 /τ and I0 /I are plotted against [Zr(IV)] straight lines are obtained. However, the required condition for dynamic quenching (τ 0 /τ = I0 /I)

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Fig. 8. Frequency response of 1H2NA and 3H2NA. The continuous line shows the best single-exponential fit for 1H2NA, while the discontinuous line shows the best double-exponential fit for 3H2NA. [1H2NA] = [3H2NA] = 4 ␮g/mL; pH 2.5.

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Fig. 10. Comparative Stern–Volmer plot using fluorescence intensity and lifetime data. [1H2NA] = 4 ␮g/mL; pH 2.5.

is not observed. In addition, the plots for these data are not conclusive for combined static and dynamic quenching. 4. Conclusions

Fig. 9. Frequency response of 1H2NA in both the absence and the presence of zirconium. [1H2NA] = 4 ␮g/mL; pH 2.5. Table 2 Lifetimes (τ i , ns) and fractional intensities (fi ) for 1H2NA:Zr(IV) complex at pH 2.5 [Zr] (mM)

0.00 0.25 0.50 0.75 1.00 1.50 2.00

Two-component τ1

f1

τ2

f2

XR2

1.208 1.765 1.689 1.436 1.353 1.728 1.588

4.13 52.47 46.99 49.73 47.55 34.08 41.14

1.221 0.4809 0.3526 0.2581 0.2549 0.3089 0.2757

95.87 47.53 53.01 50.27 52.45 65.92 58.86

1.63 3.61 5.51 5.93 7.10 5.99 9.16

The complex formation of zirconium with two positional isomers of naphthalene was studied by use of fluorescence measurements. Stoichiometries and association constants for the inclusion complexes were evaluated for these positional isomers with zirconium (d2 ) paramagnetic metal ion. As it has been mentioned above, zirconium only have one stable valency, thus, enhancement of the fluorescence intensity of the fluorophore is expected. In this work, it is worth noting that the Zr(IV) paramagnetic metal ion can quench or enhance the fluorescence intensity of two fluorophores, which only differ in the position of one hydroxyl group. On the one hand, 3H2NA displays a highfluorescence intensity in the presence of Zr(IV), which makes this fluorophore especially useful as a metal-ligand probe in several chemistry fields. On the other hand, 1H2NA, is unexpectedly quenched in the presence of Zr(IV). It can be concluded that in the quenching process two fluorescent species are involved, this has been observed in the steady-state and lifetime experiments carried out. However, no conclusive data about the mechanism were found, consequently, new experiments must to be performed in order to understand it. This leads to the conclusion that the mechanism of the quenching is extremely complex and further experiments need to be performed. At the moment, several experiments are planned with a variety of positional isomers PAHs derivatives in order to know their behaviour in the presence of Zr(IV). The studies reported here are interesting because the reasons of the fluorescence intensity

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variation with zirconium are not entirely apparent, and more studies about the subject are worthy to make principally for academic reasons. Acknowledgments Funding for this study was provided by a grant from the National Science Foundation of United States and Ministerio de Educaci´on y Ciencia (project CTQ2005-02389) of Spain. References [1] S.P. Story, S.H. Parker, S.S. Hayasaka, M.B. Riley, E.L. Line, J. Ind. Microbiol. Biotechnol. 26 (2001) 369. [2] M.A. Siddiqi, Z.X. Yuan, S.A. Honey, S. Kumar, H.C. Sikka, Polycycl. Aromat. Compd. 22 (2002) 621. [3] G. Ohlenbusch, C. Zweiner, R.U. Meckenstock, F.H. Frimmel, J. Chromatogr. A 967 (2002) 201. [4] S. Hamai, H. Sakurai, Anal. Chim. Acta 402 (1999) 53. [5] W. Herbst, K. Hunger, Industrial Organic Pigments, 2nd Ed., VCH, Weinheim, 1997. [6] A.L. Revilla, J. Havel, J. Borovcova, M. Vrchlabsky, J. Chromatogr. A 772 (1997) 397. [7] M.S. Rozk, N.T. Abdel-Ghani, Y.M. Issa, S.M. Atwa, Egypt. J. Chem. 36 (1995) 449 (volume date 1993). [8] S.L. Gupta, R.N. Soni, J. Indian Chem. Soc. 44 (1967) 195. [9] T.V. Ramakrishna, N. Balasubramanian, Z. Gesamte. Hyn. Ihre. Grenzgeb 30 (1984) 467. [10] E. Casassas, A. Izquierdo-Ridorsa, L. Puignou, Talanta 35 (1988) 199. [11] N.T. Abdel-Ghani, Y.M. Issa, A.A. Salem, Microchem. J. 39 (1989) 283.

[12] N.K. Rustamov, A.N. Gurbanov, Zavodskaya Laboratoriya Diagnostika Materialov 67 (2001) 12. [13] K. Kasiura, Chem. Anal. (Warsaw, Poland) 20 (1975) 389. [14] P.N. Kariuki, Kenya J. Sci. Technol., Ser. A 1 (1980) 27. [15] V. Kuban, J. Havel, B. Patockova, Collect Czech. Chem. Commun. 54 (1989) 1777. [16] D.B. Gladilovich, N.N. Grigor’ev, K.P. Stolyarov, Zh. Anal. Khim. 35 (1980) 1283. [17] S.S. Shandu, J.N. Kumaria, R.S. Sandhu, J. Indian Chem. Soc. 55 (1978) 670. [18] P.J. Kovi, S.G. Schulman, Anal. Chem. 45 (1973) 989. [19] S.G. Schulman, P.J. Kovi, Anal. Chim. Acta 67 (1973) 259. [20] H. Mishra, H.C. Joshi, H.B. Tripahi, S. Naheshwary, N. Sathyamurthy, M. Panda, J. Chandrasekhar, J. Photochem. Photobiol. A: Chem. 139 (2001) 23. [21] H. Mishra, S. Maheshwary, H.B. Tripathi, N. Sathyamurty, J. Phys. Chem. A 109 (2005) 2746. [22] H.M. Stevens, Anal. Chim. Acta 20 (1959) 389. [23] A. Manimekalai, Asian J. Chem. 10 (1998) 1019. [24] A.M. Garcia-Campa˜na, F. Alex-Barrero, L. Cuadros-Rodr´ıguez, M. Rom´an-Ceba, Luminiscence 15 (2000) 115. [25] J. Aybar-Mu˜noz, A.M. Garc´ıa-Campa˜na, F. Al´es-Barrero, Talanta 47 (1998) 387. [26] N. Chimpalee, D. Chimpalee, S. Suparuknari, B. Boonyanitchayakul, D. Thorburn-Burns, Anal. Chim. Acta 298 (1994) 401. [27] Y.X. Fan, Y.X. Zheng, Anal. Chim. Acta 281 (1993) 353. [28] F. Ca˜nada Ca˜nada, M.I. Rodr´ıguez C´aceres, J. Fluorescence 17 (2007) 23. [29] J.R. Lakowicz, Principle of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, USA, 1999. [30] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 98 (1954) 5247. [31] M. Sbai, S. Ait Lyazidi, D.A. Lerner, B. del Castillo, M.A. Martin, Anal. Chim. Acta 303 (1995) 47. [32] M.R. Eftink, C.A. Ghiron, Anal. Biochem. 114 (1981) 199.