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Spectrochimica Acta Part A 94 (2012) 119– 125

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j ourna l ho me page: www.elsev ier .com/ locate /saa

luorescence characteristics of 5-amino salicylic acid: An iodide recognitiontudy

riyanka Aroraa, Kanchan Suyala, Neeraj K. Joshia, Hem Chandra Joshib,∗, Sanjay Panta,∗∗

Photophysics Laboratory, Department of Physics, DSB Campus, Kumaun University, Nainital 263002, IndiaInstitute for Plasma Research, Laser Diagnostics Division, Bhat, Near Indira Bridge, Gandhinagar, Gujarat 382428, India

r t i c l e i n f o

rticle history:eceived 8 November 2011eceived in revised form 13 March 2012ccepted 21 March 2012

a b s t r a c t

In this paper we report the effect of iodide on the fluorescence of 5-amino salicylic acid (5-ASA). In theabsence of iodide, prominent blue green (BG) emission band at ∼465 nm (broad) is observed in aproticsolvents whereas violet (V) emission at ∼408 nm, blue green (BG) at ∼480 nm and green (G) at ∼500 nmare observed in case of protic solvents. On the addition of iodide ion (I−), the intensity of BG fluorescence

eywords:-Aminosalicylic acidSIPTodidenhancement

is enhanced in case of aprotic solvents. On the other hand the G band is enhanced in protic solvents anddecrease in the intensity of the V band is observed. The effect of hydrogen bonding as well as the interplayof neutral and ionic species is invoked to explain the observed results. The study projects the applicationof this system in iodide recognition in protic/aprotic environments.

© 2012 Elsevier B.V. All rights reserved.

ydrogen bonding

. Introduction

The development of the systems those are capable of recog-ising various biologically and/or chemically important negativelyharged species is emerging as a research area of great importance1–6]. One of the most attractive approaches in this field involveshe advancement in optical sensors. Changes in fluorescence andbsorbance are the output signals mainly used in the developmentf optical sensors [7].

The determination and recognition of halides in the phys-ological environment is important because of their role in itsunctioning [8–10]. The determination of halide using fluorescenceuenching is a popular technique because of high sensitivity ofuorescence and the simplicity of quenching reactions (wherenly a small volume of sample is required, the reactions aresually non-destructive and the reactions can be applied to almostny system that has an extrinsic or intrinsic fluorescent probe).luorescence quenching process has, often, been considered forhe sensing of halides from decades [11–16]. Particularly, in casef iodide, the explanation of this effect lies on the fact that the

fficiency of intersystem crossing to the excited triplet state isnhanced (promoted by spin–orbit coupling), which dependsn the mass of the quencher atom and hence the expression

∗ Corresponding author. Tel.: +91 79 2396 2056.∗∗ Corresponding author. Tel.: +91 59 4223 7450.

E-mail addresses: hem sup@yahoo.co.uk (H.C. Joshi),anjaypent@yahoomail.com (S. Pant).

386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.saa.2012.03.056

“heavy-atom effect” is sometimes used [17]. A major limitation ofthe earlier projected quenching based iodide probes is that theyonly show a fluorescence-quenching signal upon interaction withiodide [18–21].

It is only recently the enhancement in fluorescence for therecognition of halides has gained interest [22–25]. Moreover, thefluorescence quenching response usually results in low signal-to-noise ratio. Thus, from the analytical point of view, it is muchmore desirable to detect iodide by an enhanced fluorescencesignal.

Although several classical analytical methods were studied foriodine determination, most of them suffer from disadvantages, suchas being discontinuous, being time-consuming, or requiring a cum-bersome pre-treatment of the sample solution and a large sampleamount. With respect to the sensitivity, experimental simplicityand monitoring on-line, sensors have been proved to have advan-tages over the above methods [26]. Wolfbeis et al. reported theinitial concept of fluorescent sensor for iodine based on dynamicfluorescence quenching of immobilized fluorophores by the ana-lyte [27]. Since then there have been several reports on fluorescentiodine sensors and the quenching kinetics, which can be describedby the various models [28–31].

To increase the selectivity and sensitivity, ratiometric measure-ments are utilized, which involve the observation of changes in theratio of the intensities of the absorption or the emission at two

wavelengths. Ratiometric fluorescent probes have the importantfeature in that they permit better signal ratio and thus increase thedynamic range and provide built-in correction for environmentaleffects. The perceived colour change would be useful not only for

1 ca Acta Part A 94 (2012) 119– 125

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fpfpaaiviiisp

2

2

ipflfdcstpsp

cpw

2

rtsTsereaps3wtwarrwt

20 P. Arora et al. / Spectrochimi

he ratiometric method of detection but also for rapid visual sens-ng. Hitherto, many investigations have been conducted to makeatiometric fluorescent probes for cations. In contrast, very fewatiometric fluorescent sensors for iodide have been found in theiterature. Thus, realization of ratiometric measurement for iodides still a subject of interest [32–36].

In earlier studies it was shown that 5-ASA exhibits proton trans-er reaction in excited state in aqueous medium [37,38]. In theresent work we studied the interaction of 5-ASA (a potential drugor tuberculosis and bowl infection) in protic polar and aproticolar medium and its interaction with iodide ion. Interestingly inprotic solvents its fluorescence is enhanced despite iodine being

strong fluorescence quencher because of heavy atom effect. Thenteraction with iodide is important from the following points ofiew (i) because of its medicinal importance and the presence ofodine in body fluids and (ii) if it is capable of sensing/recognitionn various protic/aprotic environments. The main goal of the studys to explore the interaction between 5-ASA and iodide and sub-equently to trace its applicability in the recognition of iodide inrotic/aprotic media.

. Experimental

.1. Materials

5-ASA (obtained from Aldrich) of 98% purity was tested forts fluorescence purity and used as such. The purity of the com-ound has been checked by fluorescence run test. We checked theuorescence spectra in acetonitrile and DMF by exciting from dif-

erent wavelengths but did not find any change in spectrum orevelopment of any new band. Moreover, the amplitude of decayomponents also did not change with excitation wavelength. Thishows that there is insignificant contribution from any other con-aminant. KI was 99.99% pure. The stock solution of KI (0.1 M) wasrepared from the stock solution of 5-aminosalicylic acid. From thisolution diluted solutions of KI were prepared. This ensured that therobe concentration did not change on the addition of KI.

All the solvents used were either of spectroscopic grades or werehecked for their fluorescence purity. Potassium Iodide (KI) withurity 99.99% was used for the present study. Dehydrated MeOHas used.

.2. Instrumentation

Steady state absorption spectra, at room temperature, wereecorded by dual beam JASCO V-550 spectrophotometer. The exci-ation and emission spectra were recorded by using JASCO FP-777pectrofluorimeter and data were analyzed by related software.ransparent quartz cuvette was used for the absorption and emis-ion measurements and frontal geometry was used in recordingmission to avoid inner filter effect. Fluorescence decay times wereecorded with the help of Edinburgh-199 time domain spectrom-ter and analyzed by TCC-900 software. The excitation source was

thyratron-gated (hydrogen filled) nanosecond flash lamp. Lamprofile was measured at the excitation wavelength using Ludoxcatterer. The pulse width was ∼1.5 ns with repetition rate of0 kHz. Time correlated single photon counting (TCSPC) techniqueas used to collect the decay curves and the resolution of the sys-

em was about 200 ps. The number of counts in the peak channelas at least 10,000. Time-resolved fluorescence decay curves were

nalyzed by deconvoluting the observed decay with the instrument

esponse function (IRF) to obtain the intensity decay function rep-esented as a sum of discrete exponentials; I(˛, t) = �i ˛iexp(−t/�i),here, I(t) is the fluorescence intensity at time t and ˛i is the ampli-

ude of the ith life time such that∑

i ˛i = 1. The average lifetime,

Scheme 1.

〈�〉 was calculated as follows; 〈�〉 = �˛i�i, gives the information onthe average fluorescence yield of the system.

3. Results and discussion

Briefly the steady state and time domain parameters of 5-ASAin some aprotic and protic solvents are given in Tables 1 and 2.

The absorption spectra of 5-ASA in aprotic solvents viz.dimethylformamide (DMF) and acetonitrile (ACN) show maximumat ∼360 nm (Fig. S1; supplementary material) while in the emissionspectrum, maximum is observed at ∼463 nm (BG) at �ex = 360 nm(Fig. S2). The corresponding Stokes shift is ∼6179 cm−1. The exci-tation spectrum of 5-ASA in aprotic solvent was found to beindependent of monitored emission wavelength and exhibits a sin-gle peak at ∼350 nm (which resembles to its absorption spectrum).In aprotic solvents (N type of neutrals (Scheme 1) should exist andmost possibly these neutrals should form intermolecular hydrogenbond (Scheme 1). The higher optical density (higher molar extinc-tion coefficient in case of DMF probably be due it stronger hydrogenbond accepting acceptor as compared to ACN (Table 1).

However, in the excited state, these neutrals (N) can undergoESIPT (Tautomer of N) resulting in large Stokes shifted emission[39] (Scheme 1a) which emits at ∼463 nm. In all studied aproticsolvents fluorescence spectra are found to be independent of exci-tation wavelength. Again the higher intensity in case of DMF can beattributed to increase in quantum yield (Table 1) due to strongerhydrogen bonding.

In MeOH, absorption maxima are observed around 300 nm(O.D. ∼0.6 and 0.3, respectively) and 340 nm. Triple fluorescenceis observed at ∼408 nm, ∼480 nm and ∼500 nm referred here asV, BG and G bands, respectively (Fig. S3). The excitation spectrumcorresponding to V band shows a band with maximum around300 nm whereas for BG band two peaks at ∼310 nm (smaller) and340 nm (bigger) are observed (Fig. 1). However, corresponding to Gband, the excitation spectrum is red shifted with maximum around340 nm. The corresponding Stokes shifts for �abs = 300 nm are 8823,12,500, 13,333 cm−1 for V, BG and G bands, respectively. Similarly,the corresponding Stokes shifts for �abs = 340 nm are 4902, 8578.5,9412 cm−1 for V, BG and G bands. The excitation spectra clearly

reveal that V, BG and G emissions originate from different species.In earlier works [37,38], it has been demonstrated that G bandoriginates from anionic species hereby designated as A (Scheme 2).

P. Arora et al. / Spectrochimica Acta Part A 94 (2012) 119– 125 121

Table 1Steady state parameters of 5-ASA in various solvent.

Solvent aε a˛ a ̌ ϕ �max

ab(nm) �max

em (nm)

�ex = 300 nm �ex = 320 nm �ex = 340 nm �ex = 360 nm

MeOHb 25.3 0.83 0.77 303, 341 410 410, 470 (hump), 490 490 –EtOHb 24.5 0.86 0.75 306, 342 410 410, 470 (hump), 490 490 –Can 38.0 0.19 0.40 0.15 360 463 463 463 463DMF 36.7 0.00 0.69 0.25 356 470 470 470 470THF 7.58 0.00 0.55 0.22 364 460 460 460 460MeOH + H+c – – – 0.05 300 470 470 470 470MeOH + OH−d – – – 0.11 340 490 490 490 490

a ε, ̨ and ̌ are dielectric constant, hydrogen bond accepting and hydrogen bond donating parameters [44,45].b Have both neutral and ionic contribution and hence Q.Y. was not measured.c Neutral.d Anion.

Fl

ia(

aaMpwMas

TF

ig. 1. Excitation spectra of 5-ASA in MeOH (3 × 10−4 M) at different emission wave-engths.

In analogy with [37], the V band can be attributed to the speciesn which the amino group is intermolecularly hydrogen bondednd the carboxylic group is deprotonated (P type) as shown inScheme 2a).

In Ref. [37], it was established that emission at 408 nm andbsorption around 300 nm is due to zwitterions (protonated at themino group and deprotonated at carboxylic group). However, ineOH, it is unlikely that amino group can be protonated but the

ossibility of hydrogen bonding cannot be ruled out. Meanwhile it

as observed that carboxylic group partially gets deprotonated ineOH [40]. After ESIPT (which is very fast), the emission is observed

t 410 nm. Now we come to the BG emission at 470 nm. This emis-ion should originate in neutral molecules (N), which are hydrogen

able 2luorescence decay parameters of 5-ASA in different solvents.

Solvent �ex (nm) �em (nm) �1

MeOH 320

410 2.6440 3.1470 4.0500 4.7

ACN 360

420 6.6450 6.7460 6.6500 6.4

MeOH + OH− 320

440 6.6470 6.6500 6.6520 6.6540 6.6

a ˛i ’s are relative amplitude in percentage.

Scheme 2.

bonded with the oxygen of MeOH as the emission is observed,around the same position (of course with some shift) as in case ofaprotic solvents. This is possible in MeOH as it has both hydrogen

donating and accepting capabilities.

The decay parameters of 5-ASA in different solvents are givenin Table 2. For aprotic as well as protic solvents the decay fits withbiexponential function. In case of ACN two decay components of

(ns) a˛1 �2 (ns) a˛2

4 92 7.91 85 75 8.10 250 51 8.28 492 30 8.52 707 78 11.58 223 84 10.47 163 82 10.05 183 70 8.73 300 100 – –7 100 – –8 100 – –8 100 – –3 100 – –

122 P. Arora et al. / Spectrochimica Acta Part A 94 (2012) 119– 125

Fo

6ndEw

3

ip

icI∼iitnc

ig. 2. Absorption spectra of 5-ASA in aprotic solvent (DMF) (3 × 10−4 M) in presencef [I−].

.3 ns and 9.5 ns are observed whereas in DMF these two compo-ents are 7.74 ns and 12.63 ns. Broad emission coupled with twoecay components indicates that there are two emitting species.SIPT coupled with CT can be evoked to explain the broad emissionith biexponential decay [41].

.1.1. Addition of KI (aprotic solvents)

Steady state and time resolved study of 5-ASA in aprotic solventn the presence of various concentration of iodide ion (I−) wereerformed at room temperature.

Interestingly, on keeping the solute concentration fixed andncreasing the I− ion concentration, hypsochromic as well as hyper-hromic effect in the absorption spectra was observed (Fig. 2).n DMF, the absorption maximum was shifted from ∼360 nm to340 nm while their corresponding optical density (O.D.) was

ncreased from ∼0.60 to ∼0.75. Similar behavior was also observedn ACN. The change in absorption with addition of I− ion indicates

hat there is complextaion between I− and 5-ASA. To know theature of complex we have plotted 1/�A versus inverse of iodideoncentration (Benesi-Hildebrand plot) and is shown in (Fig. 3). The

Fig. 3. Benesi-Hildebrand plot for 5-ASA in ACN.

Fig. 4. Fluorescence in ACN (a) in the presence of [I−] and (b) in the absence of [I−].

linear relationship indicates that the complex is 1:1 in nature. It canbe mentioned here that the excitation spectra also reflect similarchanges as observed in case of absorption.

Interestingly, in ACN, fourfold and in DMF twofold enhance-ments in the fluorescence intensity was observed with increasedconcentration of I− (Figs. S4 and S5). In addition to enhancement,a small red shift in fluorescence spectra is observed for the highestI− concentrations taken in the study. The visualization of enhance-ment in the fluorescence intensity can be viewed directly and isshown in Fig. 4. The enhancement as well as red shift in emissionis indicative of hydrogen bonding [42] of the carboxylic hydrogenwith I−.

For the sake of convenience we define intensity enhancementfactor (IEF) given by I/I0 where I0 and I are intensities in the presenceand absence of I− respectively. A plot between IEF and [I−] for ACNand DMF is depicted in Fig. 5. It is evident that it is linear for lower

concentrations (for DMF ∼0.005 M and for ACN up to 0.01 M) butslopes downward for higher concentrations.

Fig. 5. Plot of I/I0 (IEF) versus [I−] in aprotic solvent.

P. Arora et al. / Spectrochimica Acta Part A 94 (2012) 119– 125 123

Table 3Fluorescence decay parameters of 5-ASA in aprotic solvents with increasing concentration of I− ion at �ex = 360 nm, �em = 460 nm.

Solvent Concentration KI [M] �1 (ns) �2 (ns) a˛1a˛2

b�22 〈�〉 (ns)

ACN

0 6.30 9.51 80 20 1.01 6.946 × 10−4 6.42 9.54 61 39 1.05 7.641 × 10−3 6.44 9.63 52 48 1.02 7.974 × 10−3 6.43 10.00 34 66 1.07 8.798 × 10−3 6.44 10.28 24 76 1.07 9.362 × 10−2 6.28 10.31 12 88 1.16 9.836 × 10−2 6.32 10.82 11 89 1.30 10.331 × 10−1 6.33 10.91 10 90 1.00 10.45

DMF

0 7.74 12.63 28 72 1.08 11.266 × 10−4 7.68 12.65 23 77 1.11 11.511 × 10−3 7.71 12.58 20 80 1.05 11.614 × 10−3 7.73 12.72 8 92 1.08 12.328 × 10−3 – 12.83 – 100 1.09 12.832 × 10−2 – 13.11 – 100 1.07 13.116 × 10−2 – 13.29 – 100 1.05 13.291 × 10−3 – 13.31 – 100 1.02 13.31

mai(s(t

s

Fa

a ˛i ’s are relative amplitude in percentage.b Chi-square values for double exponential decay.

To investigate the effect of I− concentration, transient experi-ents were carried out. The decay data are summarized in Table 3

nd the increase in decay time is depicted in Fig. 6. In ACN, it isnteresting to note that the longer component shows an increase�2 = 9.51 ns to 11.42 ns) with the addition of I− under the con-idered concentration range. However, the shorter component

�1 = 6.30 ns) does not change significantly. Of course, the ampli-ude corresponding to �1 goes on decreasing with addition of I−.

This increase in longer component can be attributed to thetronger hydrogen bond formation between the carboxylic

ig. 6. Decay profiles and corresponding residuals for 5-ASA in the presence of [I−]t 0 M (a), 10−2 M (b), 10−1 M (c) [�ex = 360 nm and �em = 460 nm].

hydrogen of 5-ASA and I−. It is to be noted that intermolecularhydrogen bonding stabilizes the tautomer (T) as reported in earlierstudies [42,43]. At the same time the decrease in the amplitude ofthe shorter component indicates that with addition of I− chargetransfer (CT) may be prohibited. Similar behavior is observed inDMF. Also, in DMF for higher concentration the decay becomesmono-exponential indicating that CT type of species are not presentafter this concentration which indicates that at higher iodide con-centration interaction of I− results in decreased CT character in5-ASA. Again the increase in longer decay component can beattributed to stabilization of the tautomer (T). The photo-cycle for5-ASA in aprotic medium in presence of KI is proposed in Scheme 3.

3.1.2. Addition of KI (protic solvents)

As mentioned earlier, in protic polar solvent (MeOH), twoabsorption bands are absorb at 300 nm and 340 nm. On addition of[I−], the absorption band at 300 nm diminishes and a band devel-ops at 340 nm (Fig. 7). At the highest concentration of studied I−

ion only a single absorption band at 340 nm is observed (Fig. 7).In the emission spectrum, at �ex = 320 nm, on increasing the con-

−

centration of I ion, the V band diminishes in intensity and the Gband shows enhancement (Fig. 7). It is interesting to see that the Gband gains in intensity with I− whereas V band starts diminishingand finally at higher I− concentrations it appears as a small hump.

Fig. 7. Absorption spectrum (left) and emission spectrum (right) of 5-ASA in MeOH(3 × 10−4 M) in presence of [I−].

124 P. Arora et al. / Spectrochimica Acta Part A 94 (2012) 119– 125

eme

ietaw

4fWatt

aid

TD

Sch

We have plotted the intensity ratio of G to V band (IG/IV) withodide concentration (Fig. 8). Using least-square fit procedure, ourxperimental data were fitted with linear equation and the correla-ion coefficient was found to be near unity. The change in intensitys well as colour on the addition of I− in MeOH can be easily viewedith eye and is shown in (Fig. S6).

Further, the decay behavior shows some interesting features. At10 nm (maximum of V band), the decay fits with a bi-exponentialunction with components around 2.64 ns and 7.91 ns (Table 4).

ith increase in I− concentration, �1 as well as �2 do not showny considerable change. However, the amplitude correspondingo �1 decreases significantly whereas the amplitude correspondingo �2; increases.

This clearly rules out any excited state reaction (as decay timesre almost constant with the addition of I−). However, the decreasen the amplitude together with fact the intensity of the V bandecreases with I−, suggests that P type of species (producing V

able 4ecay parameters of 5-ASA in protic solvent (MeOH) with increasing concentration of I−

�em (nm) [I−] M �1 (ns) �2 (ns)

410

0 2.64 7.91

5 × 10−4 2.87 7.88

1 × 10−3 2.87 7.97

3 × 10−3 2.89 8.04

6 × 10−3 2.88 8.02

1 × 10−2 2.87 8.01

3 × 10−2 2.90 8.12

6 × 10−2 2.85 8.33

1 × 10−1 2.79 8.37

500

0 4.72 8.52

1 × 10−3 4.91 8.21

1 × 10−2 4.82 8.08

6 × 10−2 4.71 8.09

1 × 10−1 4.71 8.18

a ˛i ’s are relative amplitude in percentage.b Chi-square values for double exponential decay.

3.

emission) decrease in number and at the same time more anions(A) are formed at the cost of the P type of molecules

At 500 nm the fluorescence decay again fits with biexponentialfunction with decay components of 4.72 ns and 8.52 ns. Interest-ingly again the decay components do not show any appreciablechange but exhibit change in amplitudes. This indicates that theconcentration of anion increases with the addition of I− as is alsoevident from the increase in the intensity of G band. This also rulesout the possibility presence of water in iodide, as in water theobserved decay time is shorter [37]. Thus it clearly shows that moreanions are produced at the cost of P type of conformers (Scheme 2).

It can be noted that recently fluorescence enhancement was pro-jected for the detection of iodide. However in these probes metals

like Hg and Cu are linked with organic molecules for the detectionof iodide as the probes work on redox mechanism [22].

Present system appears to be important in the sense that itdoes not contain any metallic element and hence is expected to

ion at �ex = 320 nm.

a˛1a˛2

b�22 〈�〉 (ns)

92 8 1.11 3.0691 9 1.01 3.3290 10 1.09 3.3889 11 0.98 3.4687 13 1.17 3.5586 14 1.16 3.5978 22 1.10 4.0568 32 1.09 4.6048 52 1.10 5.6930 70 1.03 7.3815 85 1.25 7.72

9 91 1.16 7.797 93 1.18 7.854 96 1.07 8.04

P. Arora et al. / Spectrochimica Act

bttspp[

rir

4

enimqpt

A

t

[[[[[

[[[

[[

[[

[

[[

[[

[[[[

[

[

[[

[[[[

[[

[

Fig. 8. Plot of IG/IV versus [I−] in protic (MeOH) solvent.

e free from any toxic effect. It is also important to mentionhat other halides, e.g. Cl− and Br− are rather poorly soluble inhese aprotic solvents, which makes this system attracting forensing dissolved iodide in aprotic media. In the same token theresent study demonstrates the role of iodide as fluorescenceromoter rather than quencher for which it is generally known17].

Finally we feel that IEF in case of aprotic medium and use ofatiometric analysis in protic medium can be projected to senseodide. Such molecular system appears to be interesting in iodideecognition/sensing in biological environments.

. Conclusion

The present study establishes the presence of various conform-rs and ionic species in 5ASA depending on the protic/aproticature of the solvent. The study demonstrates enhancement in the

ntensity in presence of iodide in aprotic solvents whereas in proticedia more anions are produced. The present system appears to be

uite interesting in the sensing/recognition of iodide ion in variousrotic/aprotic environments in a concentration range of 6 × 10−4 mo 10−1 M.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.saa.2012.03.056.

[[

[[

a Part A 94 (2012) 119– 125 125

References

[1] P.A. Gale, Coord. Chem. Rev. 213 (2001) 79–128.[2] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486–516.[3] B. Valeur, I. Leray, Coord. Chem. Rev. 205 (2000) 3–40.[4] A.P. Davis, R.S. Wareham, Angew. Chem. Int. Ed. 38 (1999) 2978–2996.[5] A.P. de Silva, D.B. Fox, A.J.M. Huxley, N.D. McClenaghan, J. Roiron, Coord. Chem.

Rev. 185–186 (1999) 297–306.[6] F.P. Schmidtchen, M. Berger, Chem. Rev. 97 (1997) 1609–1646.[7] B. Liu, H. Tian, J. Mater. Chem. 15 (2005) 2681–2686.[8] M.A.H. Khan, R.C. Rhew, M.E. Whelan, K. Zhou, S.J. Deverel, Atmos. Environ. 45

(2011) 977–985.[9] J.S. Kim, P.J. Shea, J.E. Yang, J.E. Kim, Environ. Pollut. 147 (2007) 634–641.10] Gieskes, J.M.C. Mahn, Appl. Geochem. 22 (2007) 515–533.11] C.D. Geddes, Sens. Actuators B: Chem. 72 (2001) 188–195.12] C.D. Geddes, J. Photochem. Photobiol., A 137 (2000) 145–153.13] C.D. Geddes, Meas. Sci. Technol. 12 (2001) 53–88.14] C.G. Niu, A.L. Guan, G.M. Zeng, Y.G. Liu, G.H. Huang, P.F. Gao, X.Q. Gui, Anal.

Chim. Acta 547 (2005) 221–228.15] M.S. Mehata, H.B. Tripathi, J. Lumin. 99 (2002) 47–52.16] P.K. Khatua, S.K. Ghosh, S.C. Bhattacharya, J. Mol. Liquids 124 (2006) 45–50.17] J.R. Lakowicz, Principles of fluorescence spectroscopy, 2nd Ed., Kluwer, New

York, 1999.18] R. Giri, Spectrochim. Acta A 60 (2004) 757–763.19] B. Zelent, J. Kusba, I. Gryczynski, M.L. Johnson, J.R. Lakowicz, Biophys. Chem. 73

(1998) 53–75.20] C.D. Geddes, Dyes Pigment 45 (2000) 243–251.21] I. Texier, M.N. Berberan-Santos, A. Fedorov, M. Brettreich, H. Scho1nberger, A.

Hirsch, S. Leach, V. Bensasson, J. Phys Chem. A 105 (2001) 10278–10285.22] L.-R. Lin, W. Fang, Y. Yu, R.-B. Huang, L.-S. Zheng, Spectrochim. Acta A 67 (2007)

1403–1406.23] G. Xu, M.A. Tarr, Chem. Commun. (2004) 1050–1051.24] W. Lin, L. Yuan, X. Cao, B. Chen, Y. Fen, Sens. Actuators, B: Chem. 138 (2009)

637–641.25] S. Chakravorti, M.A. Tarr, Can. J. Chem. 85 (2007) 153–156.26] P. Grundler, Chemical Sensors: An Introduction for Scientist and Engineers,

Springer, New York, 2006.27] E. Urbano, H. Offenbacher, O.S. Wolfbeis, Anal. Chem. 56 (1984) 427–429.28] W.A. Wyatt, F.V. Bright, G.M. Hieftje, Anal. Chem. 59 (1987) 2272–2276.29] R.H. Yang, K.M. Wang, D. Xiao, X.H. Yang, Analyst 125 (2000) 1441–1445.30] C.X. Jiao, Q. Shen, S.Y. Huan, G.L. Shen, R.Q. Yu, Anal. Chim. Acta 528 (2005)

229–234.31] P.R. Bhagat, A.K. Pandey, R. Acharya, A.G.C. Nair, A.V.R. Reddy, Talanta 71 (2007)

1226–1232.32] C.G. Niu, A.L. Guan, G. Ming, Y.G. Liu, G.-H. Huang, P.F. Gao, X.Q. Gui, Anal. Chim.

Acta 547 (2005) 221–228.33] X.F. Yang, H. Qi, L. Wang, Z. Su, G. Wang, Talanta 80 (2009) 92–97.34] H. Chen, Y. Wu, Y. Cheng, H. Yang, F. Li, P. Yang, C. Huang, Inorg. Chem. Commun.

10 (2007) 1413–1415.35] V. Luxami, S. Kumar, Tetrahedron Lett. 48 (2007) 3083–3087.36] K. Tanaka, K. Inafuku, Y. Chujo, Bioorg. Med. Chem. 16 (2008) 10029–10033.37] H.C. Joshi, C. Gooijer, G. van der Zwan, J. Fluoresc. 13 (2003) 227–234.38] I.P. Pozdnyakov, A. Pigliuccib, N. Tkachenkoc, V.F. Plyusnina, E. Vauthey, H.

Lemmetyinen, J. Phys. Org. Chem. 22 (2009) 449–454.39] D.D. Pant, H.C. Joshi, P.B. Bisht, H.B. Tripathi, Chem. Phys. 185 (1994) 137–144.40] H.C. Joshi, H. Mishra, H.B. Tripathi, J. Photochem. Photobiol.: A 105 (1997)

15–20.41] D. Gormin, M. Kasa, Chem. Phys. Lett. 153 (1988) 574–576.

42] H.C. Joshi, H.B. Tripathi, T.C. Pant, D.D. Pant, Chem. Phys. Lett. 173 (1990) 83–86.43] M. Smoluch, H. Joshi, A. Gerssen, C. Gooijer, G. van der Zwan, J. Phys. Chem., A

109 (2005) 535–541.44] M.J. Kamlet, R.W. Taft, J. Am. Chem. Soc. 98 (1976) 377–383.45] R.W. Taft, M.J. Kamlet, J. Am. Chem. Soc. 98 (1976) 2886–2894.