The role of anionic micelles in enhancing fluorescence quenching of methylacridinium iodide by metal cations

Colloids and Surfaces,

23 (1987)

Elsevier Science Publishers

363-368

B.V., Amsterdam

363

-

Printed

in The Netherlands

The Role of Anionic Micelles in Enhancing Fluorescence Quenching of Methylacridinium Iodide by Metal Cations EL-ZEINY HABIB

M. EBEID*, MAHMOUD

Chem. Dept., Faculty of Science,

(Received

H. ABDEL-KADER**

and ABDEL-FATTAH

M.

Tanta Univ., Tanta (Egypt)

29 May 1986; accepted in final form 14 October 1986)

ABSTRACT Methylacridinium iodide (MAI) gives a green emission at 490 nm (A,, = 380 nm in water) that is characteristic of the acridinium cation. Methylacridinium cations (MA+ ) undergo solubilization in anionic micelles as revealed by spectroscopic and quenching studies. Both CU*+ and Fe’+ cations in aqueous medium show an insignificant ability to quench MA+ fluorescence but Fe3+ shows a quenching ability with a second-order quenching rate constant &=2.7X 10” dm” mall’ s-l revealing a static and/or electronic energy transfer quenching mechanism. In the presence of sodium dodecyl sulphate (SDS) micelles at concentrations above the critical micelle concentration, a substantial increase in quenching ability is observed for the three metal cations. The effect of increasing medium acidity is also reported.

INTRODUCTION

The application of micelles in the catalysis of chemical reactions is well known Micellar systems have also been used as media for studying some photophysical processes, e.g. the quenching of excited states in organic compounds [ 6-91 and the enhanced emission of many fluorophores [ lo]. The fluorescence quenching of acridinium ions by cobalt and bromide ions in the absence [ 111 and in the presence [ 12-141 of micellar media has been reported. Since the pK, of acridine is 5.58 [ 121. acridinium cations (AH+ ) have been generated by maintaining high medium acidity [ 11,121 which inevitably affects the micellar counter-ion distribution as well as both the ground and/or excited-state complex formation. Acridinium cations can also be generated in neutral media in the form of methylacridinium iodide (MAI). The effect of medium acidity on the quenching dynamics can thus be minimized. [l-5].

*To whom all correspondence should be addressed. **Present address: Arabian Gulf University, P.O. Box 26671, Bahrain.

0166-6622/87/$03.50

0 1987 Elsevier

Science Publishers

B.V.

364

In the present communication, we report the role of anionic micelles in enhancing the fluorescence quenching of methylacridinium ions (MA+ ) using cations. The mechanism of quenching is also considered. CUB+, Fe2+, and Fe3+ EXPERIMENTAL

Methylacridinium iodide (MAI) was prepared by stirring (for two days at room temperature) 3.2 g acridine with 12 ml of methyl iodide in 18 ml acetone as the solvent. The resultant precipitate was recrystallized from ethanol to give 3.4 g (53% ) of deep-red needles, m.p. 226-2265°C (decomposition). The crystals were then recrystallized twice from ethanol before use. Sodium dodecyl sulphate (SDS, Fluka, puriss) was used to prepare anionic micelles. The metal cations were obtained in the form of sulphates (analytical grade) and were used as supplied. Quenching and emission spectra were recorded using a Shimadzu RF 510 spectrofluorophotometer. The concentration of MA1 fluorophore was kept below 10m5 mol dmp3 to avoid precipitation of metal ions by the iodide ion. UV-visible absorption spectra were recorded using a Unicam SP 8000 spectrophotometer. Lifetimes of MA1 solutions were determined using Ortec single photon counting equipment as described elsewhere [ 121, RESULTS

Figure

AND DISCUSSION

1 shows the absorption,

excitation

(A,,= 480 nm) and emission 1O-2 mol dmp3 SDS media. The positions of the absorption peaks compare with excitation maxima. The relative intensities of the peaks at A> 300 nm are higher in excitation spectra, compared with absorption spectra as emission occurs from the low energy electronic state. The spectral peaks of MAI in lop2 mol dmp3 SDS solutions are slightly red-shifted ( -8 nm) compared with those for aqueous medium due to solubilization of MA+ at the anionic micellar interface. Further evidence of MA+ solubilization comes from quenching studies. Copper ( II) ions ( Cu2+ ) , for example, do not show a significant ability to quench MA+ in an aqueous medium. In SDS aqueous solutions, no appreciable quenching is observed at SDS concentrations below the critical micelle concentration (CMC) of - 8.3 x 10m3 mol dme3 [ 11. At concentrations higher than the CMC, efficient quenching is observed as shown in Fig. 2. This is due to the role of SDS micelles in bringing both Cu2+ and MA+ close together at the micelle interface with subsequent fluorescence quenching. This is also confirmed from studies of the (A,, = 380 nm) of MA1 in water and aqueous

365

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:. : I. :/ :I :r :.i

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: z

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Fig. 1. Electronic spectra of 10m5 mol dm-3 MA1 in (....) water and (-.-.-.) lo-’ mol drn-” aqueous SDS solutions: (a) fluorescence (A,, = 380 nm) ; (b) excitation (A,,,,= 480 nm) ; and (c) absorption spectra.

quenching of MA+ fluorescence by CU’+, Fe2+ and Fe3’ in the absence and presence of lop2 mol dm-3 SDS. Figure 3 shows Stern-Volmer plots of the quenching of MA+ using Cu2+ and Fe2+ both in aqueous and in 10m2 mol dm-3 SDS solutions. No significant quenching is observed in the absence of SDS, whereas the quenching ability

366

Fig. 2. Changes in the quenching efficiency of MA1 (10m5 mol dme3) by CL?+ (lo-* mol dmm3) as a function of increasing SDS concentration. The arrow indicates the critical micelle concentration (CMC) .

r,

6 I

1 I 1 2 I M"1 x 10e'mol dme3

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Fig. 3. Stern-Volmer plots in the absence (0) and in the presence ( l) of lo-’ mol dn? SDS showingMA’quenchingby (a) Cu’+, (b) Fe’+ and (c) Fe3+ cations. Quenching in the presence of 0.05 M H,SO, is also shown ( 0 )

361

increases substantially in SDS micellar medium. Stern-Volmer plots are not linear and a plateau is reached, as shown in Fig. 3. Given the micellar aggregation number of SDS as n=75 at 20°C in water at a CMC of 8~ 1O-3 mol dmP3 [ 151, the micelle concentration in the micellar media employed is then N 2.67 x lop5 mol dmP3. The concentration of MA+ used was - lop5 mol dmP3 indicating that multiple occupancy of a micelle by the MA+ probe is insignificant. The non-linear Stern-Volmer plots indicate the adsorption of metal ions at the micellar interface as counter-ions and a saturation limit is associated with the appearance of a plateau in the plots. Such a plateau is attained for both CL?+ and Fe’+ at - lo-’ mol dmP3, but for Fe3+ it appears at a much lower concentration ( - 8 x 10M4 mol dmM3). Unlike Fe’+ and CL?+, Fe3+ shows an appreciable quenching ability in the absence of SDS micelles. The Stern-Volmer plot shown in Fig. 3c gives the slope k;z,=lOOO dm3 mol-’ according to the Stern-Volmer equation lo/I= 1 + kq*To [Q] . The lifetime of MA+ was determined as 70= 32 ? 1 ns, which is lower than the reported value of z. = 37.2 ns for the protonated AH+ species [ 111. The difference between the two values is presumably due to the quenching effect of the iodide ion in the case of MA1 as a result of the heavy atom effect. The rate constant k, for the bimolecular quenching is thus calculated as 2.69 x 10” dm3 mol-’ s-l, which is nearly four times higher than the diffusion rate constant in water at 2O”C, calculated to be 6.4~ 10’ dm3 mol-’ s-’ [ 161. This can be explained in terms of two possible mechanisms, namely static-type quenching with subsequent ground-state complex formation [ 171 and electronic energy transfer from donor (MA+ ) to acceptor (metal ion). A statictype quenching has been proposed earlier for a similar system to account for the fluorescence quenching of positively charged methylene blue (MB+ ) by ferric ions [ 181. The proposed ground-state complex, however, does not emit in the spectral range examined since the emission spectral pattern of MA+ is the same both in the absence and in the presence of Fe3+, despite changes in emission intensities. The absorption spectrum of MA1 in the presence of Fe3+ shows a slight consumption of Fe3+ upon adding MA1 as indicated by the decrease in its absorption band at 290 nm. Yet, no absorption bands characteristic of the proposed complex have been detected in the 200-500 nm range, presumably because of its low molar absorptivity. An electronic energy transfer mechanism is also feasible, since Fe3+ possesses an intense charge transfer absorption extending into the visible region [ 111 that obviously overlaps with the MA+ emission band at -490 nm. On the other hand, both Fe2+ and Cu2+ have transitions at 1000 and 800 nm [ 111, respectively, which are far from the MA+ emission band making these cations inefficient quenchers compared to Fe3+. It is worth mentioning that the quenching ability of Fe3+ in the presence of

368

lo-' mol dmP3 SDS micelles is lower in the presence of added electrolytes (e.g. 0.2 mol dm-” sodium sulphate) or in acidic medium (e.g. 0.05 mol dm-3 sulphuric acid) as shown in Fig. 3c. It is known [Z] that the addition of electro-

lytes causes an increase in the aggregation number of micelles thus lowering the micelle concentration and consequently the quenching efficiency. A second factor is that both Na+ and H+ can replace some Fe3+ and MA+ as counterions and thus suppress quenching. The increased medium acidity is also expected to affect the stability of any ground- and/or excited-state complexes with a subsequent decrease in quenching efficiency. ACKNOWLEDGMENT

We thank Dr N. James Bridge of the University of Kent at Canterbury preparing MA1 and assisting in the lifetime measurements.

for

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