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Facile synthesis of rhodamine-based highly sensitive and fast responsivecolorimetric and off-on fluorescent reversible chemosensors for Hg2+:preparation of a fluorescent thin film sensor†

Chatthai Kaewtong,*a Banchob Wanno,a Yuwaporn Uppa,b Nongnit Morakot,a Buncha Pulpokac andThawatchai Tuntulanic

Received 9th July 2011, Accepted 15th September 2011DOI: 10.1039/c1dt11307k

Fluorescence-active chemosensors (L1–L4), comprising a rhodamine scaffold and a pseudo azacrowncation-binding subunit, have been proposed and characterized as a fluorescent chemosensor for Hg2+.An on-off type fluorescent enhancement was observed by the formation of the ring-opened amide formof the rhodamine moiety, which was induced by the interactions between Hg2+ and the chemosensor.Upon the addition of Hg2+, an overall emission change of 350-fold was observed, and the selectivity wascalculated to be 300 times higher than Cu2+ for receptors L2–L4. A polymeric thin film can be obtainedby doping poly(methyl methacrylate) or PMMA with chemosensor L2. Such a thin film sensor can beused to detect Hg2+ with high sensitivity and can be recovered using diluted NaOH.

Introduction

Rhodamine is a dye used extensively as a fluorescent signal trans-ducer due to its excellent photophysical properties, such as longabsorption and emission wavelength, large absorption coefficient,and high fluorescence quantum yield. Indeed, a longer wavelengthemission (over 550 nm) of this dye was often employed as a sensingsignal to avoid the influence of background fluorescence (below500 nm).1 Many recent developments have shown that rhodamineis a promising structural scaffold for the design of selectivechemosensors. The cation-sensing mechanism of these probes isbased on a change from the spirolactam to an open-ring amide, re-sulting in a magenta-colored, highly fluorescent compound. Basedon this mechanism, some rhodamine-based chemosensors andchemodosimeters for Hg2+ have been developed.2–6 It was foundthat most rhodamine probes met serious interference from othertransition metal ions, especially from Zn2+ and Cu2+, which usuallyinduced a comparable fluorescence response to that of Hg2+. Forexample, a novel tren-based tripodal sensor was synthesized andfound to selectively bind the Hg2+ ion to give a visual colorchange, while other ions induced no color/spectral changes except

aSupramolecular Chemistry Research Unit, Department of Chemistryand Center of Excellence for Innovation in Chemistry, Faculty ofScience, Mahasarakham University, Mahasarakham, 44150, Thailand.E-mail: kchatthai@gmail.com; Fax: 66 0437 54246; Tel: 66 0437 54246bDepartment of Chemistry, Faculty of Engineering, Rajamangala Universityof Technology Isan Khon Kaen Campus, Khonkaen, 40000, Thailand;Fax: 66 0437 54246; Tel: 66 0433 36371cSupramolecular Chemistry Research Unit, Department of Chemistry,Faculty of Science, Chulalongkorn University, Bangkok, 10330, Thailand;Fax: 66 0221 87598; Tel: 66 0221 87643† Electronic supplementary information (ESI) available. See DOI:10.1039/c1dt11307k

Pb2+ and Cu2+ ions, which induced much smaller color/spectralchanges.2b Moreover, in 2008, Li and colleagues7 demonstratedthe use of Hg2+-selective fluorescence probes to construct acombinational logic circuit. However, the increasing absorbanceof Cu2+ (50-fold) and Zn2+ (30-fold) at 553 nm were also detected.Later, Das and co-workers reported a rhodamine-based reversiblechemosensor that could bind Hg2+ and Cu2+ in aqueous methanolsolution with detectable changes in colour. Previous studies haveshown that rhodamine derivatives have been used in synthesizingfluorescent sensors, but most of them have problems in selectivitybetween Hg2+ and Cu2+. Even though complicated moleculeswere synthesized, the results were exactly the same. Therefore,they were not suitable for integration into real applications.Recently, we demonstrated that the amide group connecting torhodamine could preferably bind metal cations.8 It is well knownthat the azacrown, one of the most extensively studied ligands,can coordinate strongly with Cu2+. However, a pseudo-azacrownfrom non-cyclic polyamines can probably regulate the bindingproperties towards Cu2+ and Hg2+ by varying chain lengths anddonor atoms. We envision that ethylene polyamine groups can actas pseudo azacrowns and can coordinate transition metal ions to adifferent extent. Herein, we describe a simple synthetic procedureto obtain sensors for Hg2+ from rhodamine derivatives containingethylene polyamine groups possessing different donor atoms andchain lengths. The sensors were expected to have high sensitivityand selectivity for possible applications as thin film sensors.

Results and discussion

As illustrated in Scheme 1, L1–L4 were easily synthesized ingood yields by condensation reactions of rhodamine B with

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Scheme 1 Synthetic pathways of L1–L4.

ethylenediamine, diethylenetriamine, triethylenetetraamine andtetraethylenepentaamine under N2 at reflux for 3 d to affordL1, L2, L3 and L4, respectively. Their molecular structures wereconfirmed by IR, MS and NMR spectroscopy and they weredesigned to chelate with metal ions via their carbonyl O and amineN atoms. In a similar manner to other rhodamine derivatives,ligands L1–L4 are colorless and fluorescence inactive in solutionin either low or high concentration (such as L1 in Fig. S1†),indicating that the spirolactam form predominantly exists. Thefluorescence intensity changes of L1–L4 were monitored uponadding metal ions to determine the cation binding abilities. Fig.1a shows fluorescence spectra of ligand L2 in the presence andabsence of 5 equiv. of cations. A high-intensity fluorescence bandat 580 nm was observed upon addition of Hg2+ into the solutionof ligand L2. High fluorescent responses were also observed inthe case of ligands L3 and L4, whereas the ligand L1 showed aslight response in the presence of Hg2+ (Fig. 1b). The color and

Fig. 1 (a) Fluorescence spectral changes of L2 after the addition of5 equiv. of various cations. (b) Fluorescence responses of L1, L2, L3 andL4 with 5 equiv. of various cations (10 mM of receptor in 0.01 mol L-1 ofTBAPF6 in MeCN).

fluorescence changes of L2 upon the addition of Hg2+ are shownin Fig. 2.

Fig. 2 Color changes (A, B) and fluorescence changes (C, D) of L2(10 mM) in the presence of 5 equiv. of Hg2+. (A, C) L2 only, (B, D) L2+ Hg2+.

To further study the interactions between L2 and the Hg2+ ion,1H NMR experiments were carried out in CD3CN, and the spectraare depicted in Fig. 3. For free L2 the chemical shift of the amineNH is around 4.20 ppm, whereas in the presence of 5 equiv. ofHg2+ ions it was broader and had shifted downfield (5.17 ppm). Inaddition, the chemical shifts of all the protons also showed distinctdownfield changes, especially the aromatic protons. The 1H NMRresults firmly supported that the pseudo-azacrown ether and thecarbonyl group of spirolactam were involved in the Hg2+ binding,thus inducing a ring-opening form of the spirolactam in L2.

Fig. 3 1H NMR spectra of L2 (10 mM) in CD3CN in (a) the presenceand (b) the absence of 5 equiv. of Hg2+.

As expected, the addition of Hg2+ to the solution of all ligandsin either CH3CN or DMSO caused a significant enhancementof absorbance intensity in the 500–650 nm range immediately,as a result of the Hg2+-induced ring opening of the spirolactamform. Moreover, UV-vis data were employed to determine thestoichiometry and stability constants for the complexes using theSIRKO program. According to the observed absorption spectrachanges (Fig. 4a), the association constants of all ligands wereobtained and summarized in Table 1. The Hg2+ preferred to bindwith L2 selectively (log b = 3.58, Table 1) over other ligandsby forming a 1 : 1 complex, which was supported by the moleratio plot (inset, Fig. 4a). The results were also consistent withthe selectivity from fluorescent titration. Upon the addition of5 equiv. of Hg2+, there were changes in the fluorescence spectraof all ligands. Continuous florescence enhancements at 580 nm

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Table 1 Stability constants (log b)a of 1 : 1 complexes of the ligands withHg2+ in 0.01 mol L-1 of TBAPF6 in MeCN by UV-vis methods (T = 25 ◦C)

Ligands log b

L1 3.00 (0.08)b, 8.91 (0.05)c

L2 3.58 (0.03)b

L3 3.39 (0.08)b

L4 3.24 (0.08)b

a Mean values derived from at least three independent determinations withthe standard deviation s n-1 in parentheses, b 1 : 1 complex (LM), c 2 : 1complex (L2M).

Fig. 4 (a) Absorption spectra of L2 (10 mM) in 0.01 mol L-1 of TBAPF6

in MeCN in the presence of different amounts of Hg2+. Inset: mol ratioplots by using absorbance at 556 nm as a function of Hg2+ concentration,indicating a 1 : 1 metal–ligand ratio. (b) Fluorescence spectra of L2 (10 mM)under the same conditions. Excitation was performed at 520 nm. Inset: molratio plots by using fluorescence at 580 nm, indicating a 1 : 1 metal–ligandratio.

were observed (Fig. 4b). The stability constant of ligands withHg2+ was calculated according to the 1 : 1 model and thus variesas follows: L2 (b = 83 000 M-1) > L3 (b = 55 000 M-1) > L4 (b =37 375 M-1) > L1 (b = 23 111 M-1). This can be explained by themost suitable coordination sphere of the receptor structure (L2)to accommodate a Hg2+ ion.

In addition, a competition experiment was also carried outby adding Hg2+ to the solution of L2 in the presence of othermetal ions. Fig. 5 showed the comparison between the fluorescenceresponse of L2 with each metal ion and the emission that occurredafter the addition of Hg2+. The results indicate that the sensing

Fig. 5 Fluorescence enhancement response of L2 (10 mM in 0.01 mol L-1

of TBAPF6 in MeCN) to 5 equiv. of various cations (black bars) and tothe mixture of 0.01 M different metal ions with 67 mM of Hg2+ (gray bars).Excitation and emission was at 520 and 580 nm, respectively.

of Hg2+ by L2 is hardly affected by these commonly co-existentions. The selectivity ratio of L1–L4 to Cu2+ and Hg2+ was alsoinvestigated. The fluorescence intensity ratios (IHg2+ /ICu2+ at 580nm) of L1–L4 showed the following the order: L2 (460) > L3(376) > L4 (335) > L1 (36). Furthermore, since the color andfluorescence of L2 + Hg2+ disappeared immediately when excessof a diluted solution of NaOH (1 N) was added, the sensingprocess was considered to be a reversible rather than ion-catalyzedreaction.

Recognition mechanism of ligand towards Hg2+: to explain therecognition abilities of L1–L4 towards Hg2+, the optimized geome-tries and the HOMO and LUMO energies of the complexes weredetermined by the density functional theory (DFT) calculationsat B3LYP/LanL2DZ level.9 The optimized structures of L1–L4complexes with Hg2+ are illustrated in Fig. 6. The results suggestedthat the receptors formed stable complexes with Hg2+ through alarge number of cation-dipole and ion-ion interactions. Further-more, the negative and high values of calculated complexationenergies found in all complexes indicated that Hg2+ could formstable complexes with synthesized receptors.10 Interestingly, for theL1 and Hg2+ complexation system, the calculated complexationenergy was lower for the 2 : 1 stoichiometry than that of 1 : 1model. This is in good agreement with experimental results thatthe association constant for the 2 : 1 model is higher than thatof 1 : 1 model. This implied that the 2 : 1 binding mode modelwas more favored than the 1 : 1 model. As shown in Fig. S2,† theHOMO distributions of L1–L4 are concentrated exclusively onthe thiourea and xanthene moieties, while those of the LUMO arelocated at the Hg2+. In the case of L1, an insignificant fluorescencespectral change upon the addition of Hg2+ ions was observed whencompared with L2–L4. It was assumed that the low sensitivityof L1 toward Hg2+ was induced by intermolecular fluorescenceself-quenching as shown in Fig. 7. The HOMO mainly locateson the xanthene ring of rhodamine B, and the LUMO showeda significant density on the other rhodamine of L1. These resultsalso showed that the intermolecular distance of the dimeric specieswas estimated to be about 11 A, which strongly supported theidea of the self-quenching processes. In addition, an increase inselectivity was observed on increasing the length of the polyamineunits due to the increased donor strength of the ethereal nitrogen

12580 | Dalton Trans., 2011, 40, 12578–12583 This journal is © The Royal Society of Chemistry 2011

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Fig. 6 The optimized structures of (1 : 1) L1–Hg2+, L1–Hg2+, L1–Hg2+,L1–Hg2+ and (2 : 1) L12–Hg2+ complexes, the complexation energies (DE)are in kcal mol-1).

Fig. 7 HOMO and LUMO distributions of L1 calculated by DFTcalculations.

of the azacrown. However, a slight decrease of sensitivity occurredfor both ligands L3 and L4. One reason for this might be their

high degree of conformational flexibility, which is supported bythe previous reports.11

Preparation of the polymeric thin films: to prepare the polymericthin film sensors, poly(methyl methacrylate) PMMA polymer (300mg) was dissolved in dichloromethane and poured onto a cleanglass surface and doped with the ligand L2 (1.5 mg, 0.5 wt%).The solvent was evaporated to dryness, and a homogeneous, non-fluorescent polymer sensor film was obtained. This thin film can beused for Hg2+ detection. A solution containing Hg2+ in acetonitrile(1 mM) was sprayed onto the film, and the solvent was evaporatedin air. A strong fluorescent image appeared on the regions exposedto Hg2+ ions (Fig. 8). For the erasing process, an acetonitrilesolution of NaOH was sprayed onto the thin film. The fluorescentHg2+ image disappeared, and the non-fluorescence of the Hg2+-exposed region of the film was restored. Infrared spectra were alsoconducted to confirm the binding of the carbonyl group of L2with the Hg2+ and the reversible process. It was clearly observedthat upon addition of the Hg2+, the carbonyl stretching band ofL2 at 1741 cm-1 was changed to the lower number (1725 cm-1)and turned back to 1741 cm-1 after treatment with diluted NaOHsolution and elution through a silica column (Fig. S3, ESI†).

Fig. 8 Fluorescence image of poly(methyl methacrylate) polymer sheetsdoped with ligand L2. The polymer film on the glass slide was irradiatedwith a hand-held UV lamp at 360 nm.

Conclusions

We synthesized simple, but showing good characteristics, Hg2+

sensors, containing various chain lengths of polyamine unitsconnecting to the rhodamine building block. UV-vis and fluores-cence spectrophotometry studies showed that L2 exhibited highselectivity and sensitivity towards Hg2+ compared to the otherligands. Common co-existing metal ions displayed insignificantinterference to the detection of Hg2+. Theoretical calculationsshowed that a suitable number of N-donor atoms and chainlength in L2 allowed the best accommodation for Hg2+. Aftersensing Hg2+, the sensor was found to be recovered by addingdiluted NaOH. Therefore, compound L2 can be used to fabricatea reversible polymeric thin film sensor for Hg2+.

Experimental

Synthesis of N-(rhodamine B)lactam-derivatives (L1–L4)

The derivatives were synthesized by adapting the synthesis proce-dure of similar compounds reported in the literature.12 RhodamineB (0.20 g, 0.42 mmol) was dissolved in 30 mL of ethanol andthen ethylenediamine, diethylenetriamine, triethylenetetraamineor tetraethylenepentaamine (0.22 mL, excess) were added dropwiseto the solution and refluxed overnight (24 h) until the solution lostits red color. The solvent was removed by evaporation. Water(20 mL) was added to the residue and the solution was extracted

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with CH2Cl2 (20 mL ¥ 2). The combined organic phase was washedtwice with water and dried over anhydrous Na2SO4. The solventwas removed by evaporation, and the product was dried in vacuo,affording a pale-yellow solid of L1–L4, respectively.

L1: (0.17 g, yield 84%). 1H NMR (400 MHz, CDCl3) d 7.86–7.81(m, 1H, ArH), 7.45–7.32 (m, 2H, ArH), 7.08–7.03 (m, 1H, ArH),6.42 (s, 1H, ArH), 6.39 (s, 1H, ArH) 6.37 (s, 2H, ArH), 6.38–6.21(m, 2H, ArH), 3.32 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.12 (t, J = 6.8Hz, 2H, NCH2CH2), 2.23 (t, J = 6.8 Hz, 2H, NCH2CH2NH2), 2.05(s, 2H, CH2CH2NH2) and 1.16 (t, J = 7.2 Hz, 12H, NCH2CH3).MS (MALDI-TOF) calcd for [C30H36N4O2]+: m/z 484.28. Found:m/z 485.91 [M + H]+.

L2: (0.20 g, yield 91%). 1H NMR (400 MHz, CDCl3) d 7.90–7.88(m, 1H, ArH), 7.44–7.42 (m, 2H, ArH), 7.09–7.07 (m, 1H, ArH),6.43 (s, 1H, ArH), 6.41 (s, 1H, ArH) 6.37 (s, 2H, ArH), 6.28–6.25(m, 2H, ArH), 3.32 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.26 (t, J =6.8 Hz, 2H, NCH2CH2), 2.59 (t, J = 6.0 Hz, 2H, NCH2CH2NH),2.44 -2.38 (m, 4H, NCH2CH2NH), 1.70 (s, 3H, NCH2CH2NHand CH2CH2NH2) and 1.16 (t, J = 7.2 Hz, 12H, NCH2CH3). MS(MALDI-TOF) calcd for [C32H41N5O2]+: m/z 527.33. Found: m/z528.95 [M + H]+.

L3: (0.23 g, yield 95%). 1H NMR (400 MHz, CDCl3) d 7.89–7.88(m, 1H, ArH), 7.48–7.43 (m, 2H, ArH), 7.08 (s, 1H, ArH), 6.43–6.39 (m, 3H, ArH), 6.37 (s, 1H, ArH), 6.28–6.26 (m, 2H, ArH),3.32 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.28–2.3 (m, 12H, NCH2CH2,NCH2CH2NH and NCH2CH2NH), 2.04 (s, 4H, NCH2CH2NHand CH2CH2NH2) and 1.16 (t, J = 7.2 Hz, 12H, NCH2CH3). MS(MALDI-TOF) calcd for [C34H46N6O2]+: m/z 570.37 Found: m/z572.02 [M + H]+.

L4: (0.22 g, yield 87%). 1H NMR (400 MHz, CDCl3) d 7.82–7.81 (m, 1H, ArH), 7.38–7.36 (m, 2H, ArH), 7.15 (s, 1H, ArH),6.37–6.34 (m, 2H, ArH), 6.30 (s, 2H, ArH), 6.21–6.19 (m, 2H,ArH), 3.25 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.20–2.1 (m, 21H,NCH2CH2, NCH2CH2NH, NCH2CH2NH, NCH2CH2NH andCH2CH2NH2) and 1.09 (t, J = 7.2 Hz, 12H, NCH2CH3). MS(MALDI-TOF) calcd for [C36H51N7O2]+: m/z 613.41 Found: m/z615.86 [M + H]+.

Complexation studies of ligands by using UV-vis and fluorescencetitrations

The complexation abilities of ligands L1–L4 with cations wasinvestigated by spectrophotometric titration in MeCN at 25 ◦C.2 mL of the 10 mM L1, L2, L3 or L4 solution was placed ina spectrophotometric cell (1 cm path length). The solutions ofcations were added successively into the cell from a microburette.The mixture was stirred for 40 s after each addition and itsspectral variation was recorded. For UV-vis titration, the stabilityconstants were calculated from spectrometric data using theprogram SIRKO.13 For fluorescent titration, the stability constantfor a complex was obtained from a plot of the quantity Io/(Io-I)vs. 1/[M]. The ratio of intercept/slope gave the stability constant( Io = fluorescence intensity of free L and I = fluorescence intensityof the complex).14

Competition experiments

Hg2+ was added to the solution containing L2 and the other metalions of interest. All test solutions were stirred for 1 min and then

allowed to stand at room temperature for 30 min. For fluorescentmeasurements, excitation was provided at 520 nm, and emissionwas collected from 530 to 700 nm.

Preparation of the polymeric thin films15

Polymethylmethacrylate PMMA polymer (300 mg) was dissolvedin dichloromethane, poured onto a clean glass surface and dopedwith the ligand L2 (10 mM). The solvent was evaporated to dryness,and a homogeneous, non-fluorescent polymer sensor film wasobtained. This thin film was used for Hg2+ detection. For theerasing process, a solution of sodium hydroxide (NaOH) wassprayed onto the film. The non-fluorescent thin film was restored.

Acknowledgements

The authors gratefully acknowledge funding from The Asia Re-search Center, Chulalongkorn University and Thailand ResearchFund and Commission on Higher Education (RTA5380003) andThailand Research Fund Young New Staff (MRG5380167) andthe center of excellence for innovation in chemistry (PERCH-CIC).

Notes and References

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This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 12578–12583 | 12583

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