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Fluorescence quenching of naphthols by Cu2+ in micelles
*Corresponding author. Tel.: +91 (0571) 2700920x3353 kabir7@rediffmail.com (Kabir-ud-Din)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Available online 3 November 2010
Peer review under responsibility of King Saud University.
Abstract
Effect of the micelles of anionic, cationic and non-ionic surfactants on the fluorescence quenching of 1- and 2-naphthols has been studied in the presence of copper ion. The excited state lifetime, dynamic and static quenching constants for these systems have been determined. Fluorescence quenching in water and SDS micelle is due to the collision of the fluorophore with the quencher with a small static component. The negatively charged naphtholate ions in the excited state are quenched with significantly higher rates than the neutral naphthol molecules, which are located further inside the mesophase. CTAB micelle is less effective than the SDS micelle for fluorescence quenching. The effect of CTAB on water-assisted excited-state deprotonation has been investigated in the presence of ZnSO4. For TX-100 micelle there is negligible quenching even at higher concentration of the quencher.
Keywords
Naphthol
Fluorescence
Micelle
Dynamic quenching
Static quenching
Excited state lifetime
1 Introduction
Fluorescence quenching has often been employed as a powerful tool to obtain information on the microstructure and dynamics of the complex aqueous media such as micelles, reversed micelles, and microemulsions. (Almgren and Swarup, 1982; Almgren, 1992; Gehlen and de Schryver, 1993; Panda et al., 1999). It has also been used as a convenient method to characterize the immediate environment of a fluorophore (Ganesh et al., 1982; Rodgers et al., 1978). If water contains a suitable quantity of the surfactant, micelles are formed and the fluorophore gets solubilized by inclusion inside them (Panda et al., 1999; Panda and Mishra, 2002). Naphthols have been known to be excellent probes for the microheterogeneous environments (Mishra et al., 2001); the excited-state deprotonation of 2-naphthol has been suitably used for the determination of the critical micelle concentration (Collins et al., 1999). Both 1-naphthol (1ROH) and 2-naphthol (2ROH) are extremely weak acids in the ground state (pKas of 1ROH and 2ROH are 9.39 and 9.49, respectively) (Albert and Serjent, 1984; Harris and Selinger, 1980; Ellis, 1966; Lee et al., 1986). Being photon-initiated acids, they show excited state proton transfer (ESPT) at neutral pH ( s of 1ROH and 2ROH are 0.4 and 2.8, respectively) (Mandal et al., 1998; Panda et al., 1995, 1998). Copper ion, a physiologically important heavy metal ion, has been known to be a very good quencher of naphthol fluorescence (Panda et al., 1995). In the present work, we have studied the fluorescence quenching of 1- and 2-naphthols in the anionic, cationic and non-ionic micelles. The following surfactants have been used: (i) sodium dodecyl sulphate (SDS), an anionic surfactant with cmc 8 mM, (ii) cetyltrimethylammonium bromide (CTAB), a cationic surfactant with cmc 0.92 mM, and (iii) Triton X-100, a neutral surfactant with the cmc 0.26 mM.
2 Experimental
1-Naphthol and 2-naphthol, obtained from Sisco-Chem, were purified by subliming after crystallization from alcohol. Copper sulphate (Sigma) was crystallized from water. Triple distilled water was used throughout the study. Freshly prepared solutions were used for all the experiments. Sodium dodecyl sulphate (Sigma) was crystallized from ethyl alcohol. Cetyltrimethylammonium bromide (Sisco-Chem) was crystallized from methanol. TX-100 (Sigma) was used as received. The surfactant concentrations in the solutions were 20 times of their critical micelle concentration.
Absorption and fluorescence spectra were recorded by Hitachi 220A UV–Vis spectrophotometer and Hitachi F-4500 spectrofluorimeter, respectively. Lifetime measurements were carried out using the Horiba-Jobin Yvon IBH single photon counting fluorimeter. The concentrations of the naphthols were 1 × 10−4 M (0.05% methanol) and those of the quenchers were varied in the range 0–0.1 M. The fluorescence intensities of naphthols were monitored at various concentrations of Cu2+ at 25 °C.
3 Results and discussion
3.1 Absorption and emission spectra
Both the naphthols behave differently on excitation. 1ROH, on excitation at 290 nm in water, dissociates completely exhibiting an emission peak at 464 nm for 1RO−∗ only. However, 2ROH, when excited at 330 nm, shows the emission peaks at 353 nm and 411 nm for neutral (2ROH∗) and anionic (2RO−∗) forms, respectively. The dissociation of both the naphthols is suppressed in the presence of SDS and TX-100, the suppression of dissociation being less in TX-100 micelle as compared to the SDS micelle. Naphthol molecule tries to go inside the micelle because of its nonpolar nature and thus less number of ROH is available at the interface for dissociation. In CTAB micelle, the emission spectra of naphthols are similar to those in water. Here, the dissociation is facilitated due to the specific Coulombic attractive interaction of RO−∗ with the positively charged nitrogen of CTAB at the interface showing a remarkable intensity enhancement for the anionic peak. The emission spectra of 1- and 2-naphthols in aqueous medium and in different micelles are given in Fig. 1 and the emission wavelengths are compiled in Table 1. For the anionic form of 1-naphthol, a blue shift of 12 nm in the emission spectrum was observed for SDS micelle, which is still more in the CTAB and TX-100 micelles. This indicates a more non-polar environment for 1RO−∗ in micelles than water. For 2-naphthol, a bathochromic shift of 3–4 nm in the emission spectra is observed. Error limits of KD and KS are ±3%.
Emission spectra of 1-naphthol (a) and 2-naphthol (b) in water and micelles (indicated).
Fluorophore
λem (nm)
τ0 (ns)
KD (mol−1 dm3)
KS (mol−1 dm3)
Water
SDS
CTAB
TX-100
Water
SDS
CTAB
TX-100
Water
SDS
CTAB
Water
SDS
CTAB
1ROH∗
–
361
355
360
0.50
1.84
0.50
3.08
–
152.22
–
–
–
–
1RO−∗
464
452
449
447
8.07
8.40
16.04
16.2
96.77
295.64
–
–
7.16
–
2ROH∗
353
354
357
357
4.72
8.28
3.77
8.64
38.59
386.03
–
–
–
–
2RO−∗
411
–
415
–
9.13
9.36
–
–
118.63
–
–
6.04
–
–
3.2 Fluorescence behaviour of naphthols in presence of Cu2+
3.2.1 1RO−∗, 2ROH∗ and 2RO−∗ in aqueous medium
1-Naphthol on excitation dissociates completely in aqueous medium and, therefore, it is not possible to determine the quenching parameters of 1ROH∗. For the other species, upon addition of Cu2+, an instantaneous decrease of the fluorescence intensity is observed and the intensity decreases as the concentration of Cu2+ is increased. Usually, in the case of collisional quenching (Panda et al., 1995), the Stern–Volmer (SV) plot, i.e., the plot of the ratio of fluorescence intensities in the absence and presence of quencher (Io/I) against the quencher concentration [Q], is a straight line with an intercept 1 and slope as KSV, the Stern–Volmer quenching constant. In the present study, the Stern–Volmer equation is obeyed by the species 1RO−∗ and 2ROH∗, but 2RO−∗ shows an upward deviation from linearity (Fig. 2). This indicates that quenching is purely dynamic in the first two cases, but for 2RO−∗, quenching is not purely collisional and some other factor is also responsible for quenching. In order to explain the non-linearity in the SV plot, the extended SV equation can be represented as (Behera and Mishra, 1993; Behera et al., 1995)
Stern–Volmer plots of 1RO−∗ (a), 2RO−∗ (b) and 2ROH∗ (c) in water.
The values of KD and KS have been calculated by Eq. (1) and are given in Table 1. The KD values are in the order 2RO−∗ > 1RO−∗ > 2ROH∗ (Table 1). The effect observed is a combined effect of the dynamic and static components. It is seen that the values of KS are very less as compared to KD, which explains the lack of spectral change on the addition of copper ion. The larger value of KD could be due to a more stable encounter complex (ROH∗·Cu2+).
3.2.2 1ROH∗ and 2ROH∗, 1RO−∗ and 2RO−∗ in micelles
The fluorophore and quencher can be solubilized in the micelle and remain in the same or different microenvironments. The absorption spectra of naphthols in the presence and absence of the quencher show no observable change in spectral shape and maxima. For both the fluorophores, no unusual fluorescence spectral changes and spectral shift have been observed in SDS micelle in the presence of Cu2+ other than diminishing the fluorescence intensity. This indicates that the electronic transition of the fluorophore is not perturbed by the micellar environment. The probable localization site of copper ion is the interface due to the Cu2+–Na+ exchange. 1RO−∗ is repelled by the negatively charged interface of SDS and thus resides in the bulk water. For 1RO−∗ there is a positive deviation in the SV plot (Fig. 3), because of the presence of a static component in addition with dynamic quenching process. For neutral species, the plots are linear (r > 0.99) indicating the presence of dynamic quenching. The KD values are in the order 2ROH∗ > 1RO−∗ > 1ROH∗ (Table 1). Higher KD value for 1RO−∗ than for 1ROH∗ is due to electrostatic effect. Mallick et al. (2006) have studied the quenching of fluorescence of 3-acetyl-4-oxo-6,7-dihydro-12H-indolo-[2,3-a]-quinolizine by Cu2+ ion as a function of the SDS concentration. They have also rationalized the enhancement of the fluorescence intensity as a function of SDS concentration by considering two competing processes: electrostatic interaction between the anionic SDS micellar surface and the positive metal ion (Cu2+), and lesser accessibility of the probe molecules towards the metal ions within the micellar environment.
Stern–Volmer plots of 1RO−∗ (a), 1ROH∗ (b) and 2ROH∗ (c) in SDS micelles.
In TX-100 micelle the highly hydrated head group surfaces (provided by the POE groups) increase the thickness of the interface. The state of water in the POE domain could be such that POE-water clusters are formed via hydrogen bonds. Because of the thickness of the layer (the so-called mantle), the probe is almost entirely confined to it and copper ion does not come near the naphthol molecules decreasing the encounter probability of the probe with the quencher; thus there is no quenching even at higher concentrations of the quencher. Molecules entrapped within the micelle are also protected from quenching.
Because of the positively charged interface of the CTAB micelle, copper ion resides in the bulk water away from the interface, 1ROH∗ resides in the micellar pocket and 1RO−∗ near the positively charged interface. Thus only few copper ions are accessible to both 1ROH∗ and 1RO−∗ for quenching; the SV plots show leveling effect after a particular concentration of copper ion (Fig. 4). Such a leveling effect is observed only when there are two populations of the fluorophores, out of which only one is accessible to the quencher. 1ROH∗ and 1RO−∗, present near the interface, are accessible to the quencher whereas 1ROH∗ in the micellar pocket and 1RO−∗ in the bulk water do not come in contact of the quencher which causes saturation in the SV plot after a particular concentration. Addition of salts screens the electrostatic head-group repulsion between amphiphiles within the micelle, allowing the surface area per micelle to be reduced. Because of this 2-naphthol molecule might be pulled towards the interface with the availability of more water for a better dissociation. For 2ROH in CTAB micelle, the dissociation goes on increasing till the concentration of the copper ion reaches 0.02 M, after which dissociation stops and quenching starts. To confirm whether there is any quenching process in addition to the increase in dissociation of 2ROH, ZnSO4, a non-quencher, was tried. As ZnSO4 does not quench the species, only the increase in dissociation of 2ROH∗ is observed till the concentration of the salt becomes 0.02 M (Fig. 5). Thus, when the salt concentration becomes equal to the concentration of CTAB (18.4 × 10−3 M) in the micellar solution, dissociation of 2ROH∗ stops. As both the dissociation and quenching processes are occurring simultaneously, it is difficult to suggest a particular quenching process for 2ROH-CuSO4 system.
Stern–Volmer plots of 1RO−∗ (a) and 1ROH∗ (b) in CTAB micelles.
Dissociation of 2-naphthol in CTAB micelles in the presence of ZnSO4.
4 Conclusions
Both the naphthols, in their excited singlet state, become stronger acid in the presence of CTAB than in water whereas in SDS micelle they become weaker acid. The present work has clarified the differential quenching behaviour of Cu2+ in different micelles. Emission from both the neutral and anionic forms of naphthols gets quenched by copper ion. Cu2+ is an efficient quencher of naphthol fluorescence for anionic micelles because its positive charge allows entry into the micelle, whereas in cationic micelle it is repelled and is less effective. Quenching in water and SDS micelle is due to the collision of the fluorophores with the quencher with a small static component. The negatively charged 1-RO−∗ and 2-RO−∗ are quenched with significantly higher rates than the neutral naphthol molecules, which were located further inside the mesophase. Excited state deprotonation of 2-naphthol becomes enhanced in CTAB micelle in the presence of CuSO4 or ZnSO4. In CTAB micelle, addition of salts screens the electrostatic head-group repulsion between amphiphiles within the micelle, allowing the surface area per micelle to be reduced. Here, 2-naphthol molecule might be pulled towards the interface with the availability of more water for a better dissociation.
Acknowledgements
We are thankful to Prof. A.K. Mishra, Department of Chemistry, IIT, Madras, for helpful discussion and for allowing to measure the fluorescence lifetime. Financial assistance by the Department of Science and Technology, Government of India, is gratefully acknowledged.
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