Enhanced fluorescence of triphenylmethane dyes in aqueous surfactant solutions at supramicellar concentrations—effect of added electrolyte

Spectrochimica Acta Part A 58 (2002) 2547– 2555 www.elsevier.com/locate/saa

Enhanced fluorescence of triphenylmethane dyes in aqueous surfactant solutions at supramicellar concentrations —effect of added electrolyte Swati De *, Agnishwar Girigoswami, Suchismita Mandal Department of Chemistry, Uni6ersity of Kalyani, Kalyani, Nadia, West Bengal 741 235, India Received 5 November 2001; accepted 12 November 2001

Abstract The colour change of triphenylmethane (TPM) dyes induced by surfactants at concentrations much greater than their critical micellar concentrations is found to be accompanied by enhanced fluorescence. Thus, the otherwise weak fluorescence of TPM dyes can be detected using supramicellar surfactant concentrations. In this respect, the nonionic polyoxyethylene (POE) chain-containing surfactants are found to be more efficient compared with ionic surfactants. The POE surfactants, Triton X-100, Tween-20 and Tween-60 present a polymer-like surface to the dyes, which can thus easily bind to them. At supramicellar concentrations, the hydrophobic environment formed in these micelles is effective in preventing nonradiative relaxation processes of the dyes. As a result, there is enhanced fluorescence for even micromolar concentrations of the dyes. Among the Tween series, Tween-60 being more hydrophobic leads to greater fluorescence enhancement than Tween-20. From the fluorescence properties, binding constants for dye binding to the surfactants can be determined. Thus the relative efficiency of these surfactants as binding substrates can be assessed. Another interesting observation is that the electrolyte LiCl in presence of the surfactants leads to even larger fluorescence enhancement than the surfactants alone. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Fluorescence; Triphenyl methane dyes; POE surfactant; Supramicellar; Electrolyte

1. Introduction Triphenylmethane (TPM) dyes are an important class of commercial dyes and have been studied quite extensively in the past [1 – 4]. They are characterised by brilliant colours like red, blue, violet and green. They have versatile applications viz. in the textile industry [5], as sensitisers * Corresponding author E-mail address: [email protected] (S. De).

for photoconductivity [6], in medicine [7] (as these dyes exhibit antibacterial properties) and to stain tissue [8]. Due to their wide range of applicability, TPM dyes are often found in wastewater. Thus, a lot of research is directed towards determining the environmental hazards posed by this class of dyes, their detection and safe disposal [9,10]. Another useful application of the TPM series is as saturable absorber for mode locking in lasers [11]. In this particular aspect, malachite green (MG) has been found to be quite efficient [11]. More re-

1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 0 2 6 - 4

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S. De et al. / Spectrochimica Acta Part A 58 (2002) 2547–2555

Scheme 1.

cently, Baptista et al. have investigated the prospect of using these dyes for photodynamic therapy (PDT) which is an important method of cancer treatment [12]. Keeping in mind, the versatile uses of the TPM dyes and their special role in biological studies [7,8,12], this work was designed so as to have a method of studying micromolar concentrations of these dyes fluorimetrically. The absorbance and fluorescence spectra of TPM dyes are well studied in homogeneous and heterogeneous media [12– 15]. This work focuses on two commonly encountered TPM dyes, crystal violet (CV) and MG (Scheme 1). Both these dyes are weakly fluorescent in neat aqueous media due to rapid rotation of the phenyl rings which provides a non-radiative path for decay of the excited singlet state [16–18]. However in rigid media or on binding to a substrate, the fluorescence increases [16,17]. Baptista et al. have shown that there is a manifold increase in the fluorescence quantum yield (ƒf) and lifetime (~f) when CV binds to the protein bovine serum albumin (BSA) [12]. Jones et al. have found a similar increase in fluorescence when CV binds to the polyelectrolyte poly(methacrylic acid) [15a]. There have been previous studies involving TPM dyes in charged surfactant media. However, this is the first time the spectral (especially fluorescence) properties of these two dyes have been studied in presence of nonionic surfactants at supramicellar concentrations. The use of the latter eliminates complications arising due to electrostatic interactions between the charged dyes and the substrates to which they bind. Final concentrations used are very high, almost hundred times the critical micellar concentration (CMC), so as to ensure com-

plete binding of dye monomers to the surfactant. Another advantage of a high [Surfactant]/[Dye] ratio is elimination of the possibility of dye aggregation. Another interesting aspect of this work is the visual colour change of the dye solutions in presence of nonionic surfactants. This is novel in the sense that generally the colour change effect is prominent only when the charge on the surfactant aggregate is opposite in sign to that on the dye molecule or dissociated dye ion [18]. However, here even when the surfactant aggregate is uncharged, there is a colour change on binding of the positively charged dye cation. The same colour change effect has also been observed when ethyl violet (a related dye) binds to polymethacrylic acid (PMA), a polymer [13b]. Thus the nonionic POE surfactants maybe thought to present a polymer-like surface for dye binding.

2. Experimental section CV (chloride salt, Sigma) and MG (oxalate salt, Sigma) were purified by repeated recrystallisation from methanol. The surfactant poly(oxyethylene) iso-octyl phenyl ether (TX-100) obtained from Aldrich, poly(oxyethylene) sorbitan monolaurate (TW-20) and poly(oxyethylene) sorbitan monostearate (TW-60) obtained from Sigma were used as received. Cetyltrimethyl ammonium bromide (CTAB, Aldrich) and sodium dodecylsulphate (SDS, Aldrich) were purified by recrystallisation. LiCl (anhydrous) obtained from SRL, India was used as received. Doubly distilled water was used to prepare all the solutions. Due to the adhering nature of the dyes, the following method was used for correction of final dye concentration. The solutions of varying surfactant concentrations were first prepared in distilled water. The dyes were added from a concentrated aqueous solution just prior to the fluorescence experiment and optical density at the wavelengths of excitation (uex) were determined. Final dye concentrations were maintained at 9.25 mM CV and 10 mM MG as at concentrations greater than 1× 10 − 5 M, the dyes may dimerise [18]. Moreover, the pH of the solutions is maintained at  7 so that the absorption

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and fluorescence spectra are pH independent [18]. Since the bulk medium is aqueous, the dyes will be present as their cations, i.e. CV+ and MG+.

2.1. Instrumentation UV-visible absorption spectra were measured with a Shimadzu UV-160 digital spectrophotometer (Kyoto, Japan) with a 1 cm quartz cuvette. The steady state fluorescence spectra were recorded on a Perkin Elmer Luminoscope, Model LS 50B. The wavelengths of exciting light were 550 and 590 nm for solutions of CV+ and 617 and 425 nm for solutions of MG+. Macro-viscosity measurements were done by using an Ostwald Viscometer.

3. Results and discussion

3.1. Absorption spectra The absorption spectrum of CV+ shows almost the same type of behaviour in presence of TX100, TW-20 and TW-60. On increasing the surfactant concentration, at points below the CMC (for all the surfactants), the usual absorption spectra of CV+ in water with a peak at 590 nm and a shoulder at 550 nm [4,13,15] is observed with very little variation of optical density. However, after the CMC is exceeded, with increasing surfactant concentration, there is a continuous red shift of the 590 nm peak to a final value of 600 nm at 20 mM surfactant concentration (Fig. 1a and Table 1). The red shift of the 590 nm peak after CMC is simultaneously accompanied by increasing prominence of the 550 nm shoulder. A similar change in I590/I550 ratio was observed for TPM dyes in polymers [15a]. This observation once again reiterates the fact that long-chain nonionic POE surfactants present a polymer-like surface to the dyes. For MG+, the main absorption peak is at 617 nm (x-band) with a smaller peak at 425 nm (y-band) [19]. On adding the POE surfactants TX-100, TW-20 and TW-60, there is a red shift of the 617 nm peak to 628 nm, while the 425 nm peak shows a slight red shift of 5 nm (Fig. 1b and Table 2). The red shift of the main absorption peak of CV+

Fig. 1. (a) Absorption spectra of 9.25 mM CV+ in: (i) aqueous solution ( – ); (ii) 0.02 mM TW-20 (·-·); and (iii) 20 mM TW-20 (---). (b) Absorption spectra of 10 mM MG+ in: (i) aqueous solution ( – ); (ii) 0.02 mM TW-20 (·-·); and (iii) 20 mM TW-20 (---).

(590 nm) and MG+ (617 nm) in presence of the POE surfactants and a corresponding increase in I550 for CV+ matches with the corresponding peaks in alcohol [13a]. Now, once the CMC of the respective surfactants is reached, micellar aggregates are formed. The red shift of the u max abs indicates incorporation of the dyes in the nonionic micelles, the main driving force being hydrophobic interaction between the two moieties. To summarise, changes in u max abs and intensity of the 550 nm shoulder (I550) for CV+ reflect alteration of the microscopic environment available to the dye after incorporation within the nonionic micellar aggregates. The results obtained with the individual surfactants TX-100, TW-20 and TW-60 were used as a basis to study the spectral behaviour of

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Table 1 Spectral characteristics of CV+ in various media Solutions

u max abs (nm)

uem (nm)

Relative ƒf

1. Aqueous solution of 9.25 mM CV+ 2. (1)+20 mM TX-100 3. (1)+20 mM TW-20 4. (1)+20 mM TW-60 5. (1)+20 mM CTAB 6. (1)+20 mM SDS 7. (1)+20 mM PEG

590

625

1.0

600

638

35.0

600

635

15.0

600

635

24.0

596

635

6.0

594

632

1.2

594

625

1.0

Fig. 2. Absorption spectra of 9.25 mM CV+ in: (i) aqueous solution ( – ); (ii) 0.8 mM SDS (·-·); and (iii) 20 mM SDS (---).

the dyes CV+ and MG+ in mixed micellar aggregates formed by these surfactants. However, on varying the mole fraction of either of the two surfactants used in the mixture, the results obtained were almost the same as for the individual surfactants and hence inconclusive. For comparison of the effects exerted by the nonionic surfactants TX-100, TW-20 and TW-60 with those of ionic surfactants SDS and CTAB, the spectra were studied in the latter. For the surfactant CTAB (forming positively charged micelles), the results are almost comparable to those for the nonionic micelles, i.e. red shift of the 590 nm peak of CV+ to 596 nm and a corresponding increase of I550 after CMC, i.e. 0.8 mM. For MG+ in CTAB, the red shift is of 8 nm to 625 nm, while the 425 nm peak is unchanged. For

SDS (forming negatively charged micelles) the results below CMC are interesting. Unlike TX100, TW-20, TW-60 and CTAB, which below their respective CMCs have no remarkable effect on the absorption spectra of either CV+ or MG+, on increasing SDS concentration below its CMC, a new absorption peak at 530 nm in the absorption spectrum is formed along with the main 590 nm peak for CV+ (Fig. 2) while for MG+ a shoulder appears at 580 nm along with the main 617 nm peak. The new blue-shifted peak/shoulder may be ascribed to metachromasia induced by the dodecylsulphate anion on the cationic dyes. There are reports of such metachromasia induced by polyanions like polyvinyl sulphate (PVS) on CV+ [20]. Beyond the CMC of SDS, the absorption spectra of CV+ and MG+ show the usual pattern with slight red shift of the 590 nm peak of CV+ to 594 nm and corresponding increase of I550, while for MG+, there is a shift of u max abs from 617 to 625 nm and 425 to 428 nm. For both the dyes, it is observed that the nonionic micelles induce a larger red shift of the main absorption peak than

Table 2 Spectral characteristics of MG+ in various media Solutions 1. 2. 3. 4. 5. 6. 7.

Aqueous solution of 9.25 mM MG+ (1)+20 mM TX-100 (1)+20 mM TW-20 (1)+20 mM TW-60 (1)+20 mM CTAB (1)+20 mM SDS (1)+20 mM PEG

u max abs (nm)

uem (nm)

Relative ƒf

617 628 628 628 625 625 620

660 655 655 655 655 660 660

1.0 3.0 2.0 2.0 1.3 1.0 1.0

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the ionic micelles possibly due to their greater hydrophobicity.

3.2. Fluorescence spectra TPM dyes are weakly fluorescent in low-viscosity solvents due to their fast relaxation processes that arise due to rotational motion of the phenyl rings [4,12,15,16]. Thus in aqueous media, the fluorescence quantum yield is low for the dyes CV+ and MG+ [12]. In environments that hinder rotational relaxation processes, e.g. viscous media [13a], protein [12] and polymeric substrates [15a], the quantum yield increases due to prevention of radiationless deactivation to a low-lying twisted intramolecular charge-transfer (TICT) state. In this work, the effect of the nonionic longchain POE surfactants (TX-100, TW-20 and TW60) on the fluorescence properties of the two TPM dyes, CV+ and MG+, (differing in substitution at one phenyl ring) has been examined. On addition of surfactant to the aqueous dye solution, no perceptible change in intensity of the fluorescence spectra is observed before CMC is reached. However, as the surfactant concentration increases beyond CMC, there is a gradual increase of the fluorescence intensity (It) accompanied by a shift of the fluorescence emission maxima (uem) to longer wavelengths. In this study, final surfactant concentrations are maintained at hundred times the CMCs or even higher concentrations of the respective surfactants, so as to ensure maximum interaction between the dyes and the surfactants. For CV+, excited at 590 nm, there is a 35-fold increase in fluorescence quantum yield (ƒf) at a TX-100 concentration of 20 mM, i.e. hundred times its reported CMC (Fig. 3 and Table 1). This is accompanied by a red shift of uem from 625 to 638 nm (Table 1). The CV+ solutions were excited both at uex = 550 and 590 nm. However the latter gave a slightly larger increase in fluorescence intensity, 35-fold compared to the former (26-fold). To compare the fluorescence enhancement effects, exerted by different nonionic POE surfactants, concentrations of surfactants were maintained at the same value. It was found that the fluorescence enhancement for CV+ was 15-fold at 20 mM TW-20 and 24-fold for 20 mM TW-60. These

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values are lower than the corresponding fluorescence increase induced by 20 mM TX-100. Although, Tweens possess a larger number of oxyethylene groups than TX-100, the latter has a more chain like structure with an iso-octyl phenyl group at one end. The chain structure will facilitate better binding of the dyes to TX-100 micelles. Hence, a greater increase in ƒf is observed. Among the Tween series, hydrophobicity increases with increase in Tween number. Thus TW-60 forms micelles whose interior is more hydrophobic than the micelles formed by TW-20. This increased hydrophobicity is manifested in the larger fluorescence increase caused by TW-60 for the same concentration of it as TW-20, i.e. 20 mM. For the dye MG+, uex = 617 nm, the corresponding increase in ƒf is much smaller than for CV+, i.e. 3-fold for TX= 100 (20 mM), 2-fold for TW-20 (20 mM) and TW-60 (20 mM) (Table 2). In contrast to the behaviour of CV+, uem for MG+ shows a slight blue shift from 660 to 655 nm (Table 2). For aqueous MG+, excited at 425 nm, a small hump in emission spectra is observed with a peak at 485 nm. On increasing the surfactant concentrations to 20 mM, the intensity at 485 nm increases, however the hump disappears to give an almost flat shoulder. The fluorescence intensity results indicate that both the dyes inter-

Fig. 3. Fluorescence emission spectra of 9.25 mM CV+, at uex =590 nm in: (i) aqueous solution; (ii) 20 mM TX-100; and (iii) 20 mM TX-100 +3 M LiCl.

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Fig. 4. (a) Plot of (Ih − I0)/(It − I0) vs. 1/[M] for CV+ in TX-100 micelles. (b) Plot of (Ih −I0)/(It – I0) vs. 1/[M] for CV+ in TW-20 micelles. (c) Plot of (Ih −I0)/(It − I0) vs. 1/[M] for CV+ in TW-60 micelles.

act with the POE surfactants. The extent of interaction being much larger for CV+ than MG+. Since, the more significant changes in ƒf and uem of CV+ and MG+ occur when the surfactant concentrations exceed the respective CMCs, it can be assumed that the dyes bind to micellar aggregates formed by TX-100, TW-20 and TW-60. Thus one needs to estimate the magnitude of the binding constant. This can be done using the relation suggested by Almgren et al. [21a]. (Ih −I0)/(It − I0)=1 +1/KM[M]

(1)

where Ih =emission intensity at infinite micellar concentration; I0 =emission intensity in absence of micelles; It = emission intensity at intermediate micellar concentration; KM =binding constant; [M] = micellar concentration. From the slope of the plot of (Ih −I0)/(It −I0) versus inverse micellar concentration, the binding constant KM can be determined. The concentration of the micelles [M] can be determined using the relation below [21]. [M] =([surfactant]− CMC)/Nav

(2)

where [surfactant]=total surfactant concentration; Nav = aggregation number. Nav is ca. 134 for TX-100 [22], ca. 86 for TW-20 [23] and ca. 112 for TW-60. The KM values deter-

mined are 2.5×105 l mol − 1 for CV+ in TX-100, 1.25×105 l mol − 1 for CV+ in TW-20 and 1.7× 105 l mol − 1 for CV+ in TW-60 (Fig. 4 and Table 3). For MG+, from the fluorescence intensity in presence of the POE surfactants, KM can not be determined accurately as fluorescence enhancement is very little. From the relative ƒf values, shifts in uem and KM values, it can be inferred that CV+ binds to micelles of TX-100, TW-20 and TW-60. For MG+, the binding to the three types of micelles is very weak. The fact that the cationic dyes bind efficiently to nonionic micelles shows that in this case hydrophobic interaction prevails over electrostatic interaction. To elaborate, hydrophobic interactions play a role in solubilising the dye and during the process of solubilisation, the dye is transferred from a hydrophilic environment to a more hydrophobic one within the micellar aggreTable 3 Binding constants of CV+ in POE surfactants Solutions

KM (M−1)

1. CV+ in aqueous TX-100 2. CV+ in aqueous TW-20 3. CV+ in aqueous TW-60

2.50×105 1.25×105 1.70×105

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gates. Hence, hydrocarbon chain length and consequently hydrophobic interactions affect the spectral changes induced by the surfactants. To determine the importance of hydrophobic interactions in binding of TPM dyes to POE surfactants, the results for the latter were compared with those obtained using ionic surfactants. To compare the effects exerted by the nonionic surfactants with the anionic surfactant SDS and the cationic surfactant CTAB, studies were carried out using these surfactants. It was found that at 20 mM SDS, there is no significant increase in ƒf of CV+ and MG+ compared to neat water. In SDS, the fluorescence increase is maximum (4fold) for CV+ below its CMC, i.e at a concentration of 0.5 mM, while for CTAB, a 6-fold increase for CV+ is observed although in the post-micellar region (Table 1). For MG+ in ionic surfactants, there is practically no change in ƒf (Table 2). The fact that the nonionic POE surfactants TX-100, TW-20 and TW-60 induce a much larger increase than the corresponding ionic surfactants SDS and CTAB at the same concentration, can be ascribed to the effect of the long chains present in the former, which facilitate binding of the dyes. The higher efficiency of the nonionic surfactants can thus be attributed to hydrophobic interactions. But, for the ionic surfactants CTAB and SDS, electrostatic interactions predominate. The fluorescence intensity increase is maximum for SDS below its CMC as then electrostatic interaction between dye cation and monomeric surfactant anion is strongest. Beyond CMC, the SDS micelles formed are surrounded by positive Na+ counterions in the Stern layer which may repel the dye cations. Hence, the fluorescence decreases at higher SDS concentrations. For CTAB, the monomeric surfactant cation repels the dye cations whereas in the region beyond the CMC, the dye cations can reside in the palisade layer of the CTAB micelles leading to a small increase in ƒf. Comparing the results obtained for nonionic and ionic surfactants, it can be concluded that for TPM dye binding to surfactants, hydrophobic forces exert stronger effects than electrostatic forces. In all the experiments, care was taken so that the viscosity values in the different surfactant

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media were almost comparable. Otherwise, comparison of the fluorescence enhancement effects will be difficult as for TPM dyes ƒf 8 p 2/3 (p being the viscosity of the medium) [24]. The concentrations of the surfactants used here do not lead to very high viscosity values. That the viscosity contribution to fluorescence enhancement by POE surfactants is negligible, is further proved by carrying out similar experiments using comparable concentrations of polyethylene glycol (PEG). It has been found that 20 mM PEG does not lead to any fluorescence enhancement of either of the dyes although the viscosity is high (Tables 1 and 2). Another proof in support of the prevalence of hydrophobic interactions in binding of TPM dyes to surfactants comes from the effects exerted by the salt LiCl on the fluorescence of these dyes. LiCl is well-known as a salting-out agent [25], i.e. it decreases the solubility of organic substances in water. In aqueous surfactant media, the role of LiCl is to help push the organic dyes into the micellar interior. Interestingly, in this work it has been found that in presence of 3 M LiCl, the fluorescence of TPM dyes in aqueous surfactant media increases almost 2-fold from that of the same in absence of salt (Fig. 3). Presence of salt does not, however, lead to any shift in uem. This is true for all the surfactants except SDS where the solution becomes turbid in presence of 3 M LiCl. However, at lower LiCl concentrations, there is a slight fluorescence increase even for SDS. Thus the salting-out agent LiCl causes increased solubilisation of the dye within the micelle leading to increase in ƒf. The difference in binding behaviour between MG+ and CV+ arises from the fact that MG+ (being a diamino TPM dye) is less hydrophobic than CV+. Thus it has a lower tendency to bind to hydrophobic substrates [12]. The fluorescence behaviour of CV+ and MG+ was also studied in mixed micellar aggregates formed by TX-100, TW-20 and TW-60. However, the results obtained were the same as those for the individual micelles. For mixed micellar aggregates, the ƒf and uem resemble the values in that surfactant which is present in greater proportion. Hence, these results lead to no new conclusion.

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3.3. Colour change induced by the surfactants In addition to the spectral changes stated above, addition of TX-100, TW-20 and TW-60 to the dye solutions lead to visual colour changes at concentrations above the CMCs of the surfactants. In absence of TX-100 or at [TX-100]B 0.2 mM, aqueous CV+ solutions are violet in colour, whereas above 0.2 mM, i.e. CMC the solutions acquire a deep blue colour. The same change is observed at [TW-20]\0.05 mM and [TW-60]\ 0.02 mM, i.e their respective CMCs. The aqueous solution of MG+ which is Prussian blue in colour is transformed to a deep green solution in presence of the POE surfactants at concentrations greater than their CMCs. It has been found that aqueous surfactant solutions of CV+ retain their colour better on storage compared to their neat aqueous solutions. However, the green colour of aqueous surfactant solutions of MG+ fade on storage. This indicates that the binding of CV+ is more effective than that of MG+ and that binding on the whole has a stabilising effect on the dye. This binding is characterised by visual colour change, shifts in u max abs and uem and increase in fluorescence intensity. It has also been found that the rate of alkaline fading of CV+ and MG+ is slightly retarded in presence of the surfactants TX-100, TW-20 and TW-60. This is because the reductant, i.e. OH− being charged, experiences no electrostatic force driving it towards the micellar interior for interaction with the dye. In this context, it maybe recalled that previous literature shows that the rate of alkaline fading of cationic TPM dyes is enhanced in cationic surfactants [26].

4. Conclusions To summarise the results reported here, longchain POE surfactants forming nonionic micelles viz. TX-100, TW-20 and TW-60 can provide effective substrates for binding the TPM dyes CV+ and MG+. From the relative ƒf, shifts in u max abs , uem and KM values, it can be concluded that out of the surfactants studied, TX-100 provides the better substrate for due binding due to its resemblance to linear polymers. Among the Tween se-

ries, the more hydrophobic Tween-60 provides a better binding substrate than TW-20. Comparing the behaviour of the two dyes, it is found that binding of MG+ is weaker than CV+. This maybe due to the smaller molecular volume and larger dipole moment of the diaminosubstituted TPM dye MG+. The weaker binding efficiency of MG+ is also corroborated by its faster rate of self-decolourisation in surfactant media compared to CV+. These results indicate that hydrophobic effects are important in the binding of TPM dyes to nonionic POE surfactants. Further proof in this regard came from the comparison of the fluorescence effects observed with nonionic surfactants (TX-100, TW-20 and TW-60) and the ionic surfactants (SDS and CTAB). The latter lead to much lower increase in ff thus proving that electrostatic interactions indeed play a secondary role here, as otherwise the anionic micelles formed by SDS should have led to the maximum fluorescence increase for the TPM dye cations. Another strong evidence in support of the predominance of hydrophobic effects comes from the observation that the salting-out agent LiCl leads to further fluorescence increase over and above that obtained for 20 mM surfactant. LiCl facilitates burial of the dye cations deep within the hydrophobic micellar core leading to fluorescence increase as a consequence of prevention of nonradiative relaxation processes. The colour fading results for TPM dyes indicate that CV+ binding to the nonionic surfactants is strong and it stabilises the dye, thus its colour is preserved on standing. For MG+, binding is weak and hence fading fast. To conclude, in addition to the results summarised above, this method can be used for the fluorimetric detection of even micromolar concentrations of the otherwise weakly fluorescent TPM dyes CV+ and MG+ as presence of 20 mM POE surfactant and 3 M LiCl leads to an almost 70-fold increase in ƒf. Moreover, use of nonionic surfactants is advantageous as complications arising due to electrostatic interactions with ionic surfactants can lead to dye aggregation or even precipitation. In case of nonionic micelles formed by POE surfactants, the nonpolar portions of the

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dye will be localised in the hydrocarbon nucleus of the micelle while the polar portions will reside in the poly(oxyethylene) layer. Thus the observed spectral changes in nonionic micelles will be determined by lypophilicity of the surfactant.

Acknowledgements The authors wish to thank Professor K. Bhattacharyya of IACS, Professor S.C. Bera and Professor S. Bhattacharyya, Jadavpur University, Calcutta for use of their instrumental facilities. Professor S.P. Moulik, Jadavpur University and Professor M.K. Pal, University of Kalyani are thanked for helpful discussions.

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