European Polymer Journal 45 (2009) 272–277

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

A polymeric sensor for the chromogenic and luminescent detectionof anions

Narinder Singh, Navneet Kaur, John Dunn, Ruth Behan, Ray C. Mulrooney, John F. Callan *

School of Pharmacy and Life Sciences, The Robert Gordon University, Aberdeen, Scotland AB10 1FR, UK

a r t i c l e i n f o

Article history:Received 19 August 2008Received in revised form 16 October 2008Accepted 23 October 2008Available online 7 November 2008

Keywords:FluorescenceCo-operation bindingFluoridePolysiloxane

0014-3057/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.eurpolymj.2008.10.040

* Corresponding author.E-mail address: j.callan@rgu.ac.uk (J.F. Callan).

a b s t r a c t

A polysiloxane based thiourea coupled sensor has been developed for the determination ofanions from changes in the UV–Vis or fluorescence spectra. A comparative account of thephotophysical properties of the monomer and polymer units bearing the thiourea moietyrevealed better fluoride recognition in the polymeric framework. The fluoride recognitionby the polymeric sensor was attributed to co-operative binding involving a DMSO moleculeand a fluoride ion between thiourea groups on adjacent residues. The polymeric sensor canmeasure fluoride at two different concentration ranges by using either absorbance or emis-sion signalling.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Anions play key roles in a number of important physio-logical and environmental processes [1]. Consequently, thedevelopment of chromogenic and luminescent sensors toaccurately detect anions is of great current interest [2].The ionophore component present in these sensors variesgreatly and can consist of positively charged units suchas guanidinium groups [1a] or charge neutral ureas andthioureas [3] among others. Ditopic sensors have shownimprovements in selectivity/sensitivity over their mono-topic counterparts by providing an additional binding siteto assist with ion chelation [4]. We have also shown thatby incorporating ion receptors onto the surface of a pre-formed CdSe/ZnS core-shell quantum dot, the bindingaffinity of the receptor was modified [5]. As an extensionto this, the incorporation of receptors on a polymericframework may lead to a macromolecular assembly thatcan provide multiple binding sites. Here, we graft a knownfluorescent anion sensor onto a polysiloxane using thethiolene reaction. Specifically, 1-[2-(2-allyl-1,3-dioxo-2,3-

. All rights reserved.

dihydro-1H-benzo [de]isoquinolin-6-ylamino)-ethyl]-3-phenyl-thiourea (3) was grafted onto poly(mercaptopro-pylmethyl)siloxane (PMPMS) to furnish a side chain anionsensing polymer (4). The monomer 3 was almost structur-ally identical to that used by Gunnlaugsson et al. as a fluo-ride sensor, the only difference being the vinylic unitpresent in 3 [3e]. The monomer was designed accordingto the fluorophore-spacer-receptor format of photoinducedelectron transfer (PET) based sensors [6]. The napthala-mide fluorophore can absorb and emit photons in the vis-ible region of the electromagnetic spectrum and so isuseful when considering intracellular applications. Thespacer, an ethyl unit, functions to keep the fluorophoreand the charge neutral thiourea receptor close enough sothat electron transfer between them is possible, but alsofar enough away so that they do not directly influence eachother in the excited state. Therefore, upon binding a targetanalyte, the reduction potential of the receptor is raisedwhich increases the rate of PET from the receptor to thefluorophore and fluorescence is quenched. PMPMS waschosen as polymer as polysiloxanes typically have lowglass transition temperatures, low viscosity at room tem-perature and are generally biocompatible. The selectivityand sensitivity of the new polymeric sensor was

N. Singh et al. / European Polymer Journal 45 (2009) 272–277 273

determined against a range of anions and the results com-pared directly with its monomeric analogue.

2. Experimental

2.1. General

PMPMS was purchased from Fluorochem (6.6 kDa). Allother reagents used were of the highest grade obtainableand were purchased from Aldrich. Absorbance measure-ments were recorded on an Agilent UV–Vis spectrometerusing 10 mm quartz cuvettes. Fluorescence measurementswere recorded on a Perkin-Elmer LS55 luminescence spec-trometer using 10 mm quartz cuvettes. Excitation slit sizewas 10 nm and emission slit size was 10 nm. Scan speedwas set at 500. NMR spectra were recorded on a BrukerAVANCE 400 MHz spectrometer. Chemical shifts are re-ported in parts per million, downfield of TMS. AccurateMass data was provided by the EPSRC National Mass Spec-trometry Service, UK.

2.1.1. Synthesis of 2-allyl-6-bromo-benzo[de]isoquinoline-1,3-dione (1)

The synthesis of 1 was performed following the litera-ture procedure [7a].

2.1.2. Synthesis of 2-allyl-6-(2-amino-ethylamino)-benzo[de]isoquinoline-1,3-dione (2)

Synthesis of 2 has been prepared before but was notcharacterised fully [7b]. We used a different method toprepare 2:1 (4.02 g, 12.72 mmol) was dissolved in an ex-cess of ethylene diamine (10 mL) and heated at 80 �C for18 h. The reaction mixture was poured slowly into waterand the resulting precipitate collected by filtration. Theproduct was dried in vacuo to yield the product as a yellowsolid (2.92 g, 77.80%). Mp 155-158 �C dH (400 MHz, CDCl3),8.53 (1H, d, J = 7.2 Hz, Naph-H) 8.41 (1H, d, J = 8.4 Hz,Naph-H) 8.12 (1H, d, J = 8.0 Hz, Naph-H) 7.56 (1H, t,J = 7.8 Hz, Naph-H) 6.65 (1H, d, J = 8.0 Hz, Naph-H) 6.13(1H, br s, NH) 5.94 (1H, m, CH), 5.23 (1H, d, Jac = 17.0,Jbc = 1.6 Hz, @CH) 5.10 (1H, d, Jab = 10.0, Jbc = 1.6 Hz, @CH)4.72 (2H, d, J = 5.6 Hz, @CACH2) 3.35 (2H, t, J = 5.6 Hz,ANCH2) 3.12 (2H, t, J = 5.6 Hz, ANCH2). dC (400 MHz,CDCl3) 164.5, 163.9, 149.8, 134.6, 132.7, 131.2, 129.9,126.4, 124.7, 122.9, 120.5, 117.0, 110.1, 104.4, 44.9, 42.1,40.1 HRMS (ES+): (M+H) calculated for C17H17O2N3,296.1394; found 296.1391.

2.1.3. Synthesis of 1-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1H-benzo [de]isoquinolin-6-ylamino)-ethyl]-3-phenyl-thiourea(3)

To a solution of 2 (2.00 g, 6.80 mmol) in DMF (�100 ml)was added phenylisothiocyanate (0.87 g, 6.42 mmol). Thesolution was stirred overnight at room temperature. Excesssolvent was removed under vacuum affording a yellow/or-ange solid which was stirred in cold chloroform, filteredand dried to yield the product as a yellow powder.(1.55 g, 53.10%) m.p. 218–220 �C, dH (400 MHz, DMSO-d6), 9.76 (1H, s, NHurea) 8.77 (1H, d, J = 8.0 Hz, Naph-H)8.52 (1H, d, J = 7.4 Hz, Naph-H7) 8.34 (1H, d, J = 8.4 Hz,

Naph-H) 7.99 (1H, br s, NHurea) 7.89 (1H, br s, NH) 7.78(1H, t, J = 8.0 Hz, Naph-H) 7.38 (4H, m, 4 � Ar-H) 7.19(1H, m, Ar-H) 7.05 (1H, d, J = 8.4 Hz, Naph-H) 5.99 (1H,m, CH@) 5.17 (1H, s, @CH) 5.13 (1H, d, J @ 5.8 Hz, @CH)4.70 (2H, d, J = 5.2 Hz, AC@CH2) 3.92 (2H, q, J = 6.0 Hz,NHCH2) 3.69 (2H, q, J = 5.8 Hz, NHCH2) dC (400 MHz,DMSO-d6) 180.5, 163.2, 162.4, 150.5, 138.7, 134.0, 133.3,130.5, 129.4, 128.8, 128.7, 124.4, 124.2, 123.5, 121.7,120.1, 116.0, 107.7, 103.9, 79.1, 42.2, 41.3, 35.7, 30.7 HRMScalcd for C24H22N4O2S 431.1356 (M+H)+ found 431.1356.

2.1.4. Synthesis of 4PMPMS (87.8 mg, 0.65 mmol), 3 (280 mg, 0.65 mmol of

repeat units) AIBN (0.05 mg, catalytic amount) and DMF(10 mL) were mixed in a tube flushed with nitrogen. Thetube was sealed and placed in an oven at 50 �C for 48 h.The contents were poured onto acidified methanol andthe product isolated by filtration. The product, a yellowsolid was re-dissolved in DMF and re-precipitated fromacidified methanol to yield the final product which wasfiltered and dried in vacuo (160 mg).

3. Results and discussion

The synthesis of monomer 3 is shown in Scheme 1. Itwas prepared from 4-bromo-1,8-napthalic anhydride inthree steps following known procedures [7]. It was thengrafted onto PMPMS at one molar equivalence using thethiolene reaction to produce polymer sensor 4. Successfulgrafting was confirmed by 1H NMR spectroscopy withFig. 1 showing the stacked NMR spectra of 3, 4 and PMPMSin descending order. The olefinic protons present in 3 at5.95 and 5.15 ppm were absent in the spectrum of 4. Therewas also an upfield shift of the signal at 4.65 ppm in 3,reflecting the methylene protons adjacent to the olefinicgroup, to 3.95 ppm in 4. In addition, the thiol signal presentat 1.25 ppm in PMPMS was significantly reduced in 4, alsoindicating a substantial graft of the thiol groups with 3.New broad signals observed at 3.95, 3.50, 2.60, 1.75 and1.60 reflect the newly created and existing methylene pro-tons that flank either side of the thioether sulfur atom.Although the peaks were quite broad, we estimated thegrafting efficiency to be 87 ± 5% by relative integration.Based on this result the molecular weight of 4 was calcu-lated to be 15.0 ± 0.75 kDa IR analysis also showed thatthe peak at 2558 cm�1 in PMPMS, reflecting the S–Hstretch, was absent in the spectrum of 4.

The UV–Vis spectra of 3 and 4 recorded in DMSO areshown in Fig. 2. Both exhibited two main bands, the firstat kmax 270 nm being due to the phenyl chromophoreand the second at kmax 444 nm due to the internal chargetransfer (ICT) excited state of the napthalamide moiety.The ground state properties of 3 and 4 were also deter-mined in the presence of the tetrabutylammonium saltsof fluoride, chloride, bromide, acetate, hydrogensulfateand dihydrogenphosphate. No major changes were ob-served for 3 with any of the anions. In contrast, a similarcompound prepared by Gunlaugsson et al. showed in-creases in absorbance for the shorter wavelength bandattributed to the phenyl component of the thiourea recep-

O OO

Br

N OO

Br

N OO

NH

NH2

N OO

NH

NH

NHS

* Si O n

SH

N OO

NH

NH

NHS

Si O n

S(i) (iii)(ii)

2

3

1

(iv)

PMPMS

4

Scheme 1. Synthesis of 1–4. (i) Allyl amine, DMF, 50 �C, 18 h (ii) ethylene diamine, 80 �C, 18 h (iii) phenylisothiocyanate, DMF, 25 �C, 18 h. (iv) PMPMS,DMF, AIBN, 50 �C, 48 h.

a

b

c

H DMSO2O

Fig. 1. 1H NMR spectra of (a) 3 (b) 4 and (c) PMPMS. Solvent = d6-DMSOfor (a) and (b) and CDCl3 for (c).

0

0.2

0.6

0.8

1

1.2

1.4

260 310 360 410 460 510 560Wavelength (nm)

Abs

orba

nce

Abs

orba

nce

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

310 360 410 460 510 560Wavelength (nm)

[ F-] = 0 mM

[ F-] = 9.0 mM

[ F-] = 9.0 mM

[ F-] = 0 mM

0

0.2

0.4

0.8

1

1.2

260 310 360 41 460 510 560Wavelength (nm)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

260 310 360 410 460 510 560Wavelength (nm)

[ F-] = 0 mM

[ F-] = 9.0 mM

[ F-] = 9.0 mM

[ F-] = 0 mM

a

b

Fig. 2. UV–Vis titration of (a) 3 and (b) 4 with tetrabutylammoniumfluoride in DMSO. [3] = 3.4 � 10�5 M; [4] = 9.5 � 10�5 M.

274 N. Singh et al. / European Polymer Journal 45 (2009) 272–277

tor [3e]. As this chromophore is directly connected to thethiourea receptor it was expected that we would also seechanges in this band for 3 but surprisingly these wereabsent.

In the case of 4 substantial changes were observed uponthe addition of fluoride with the other ions having virtuallyno effect. In fact the intensity of the absorbance of bothbands was observed to increase substantially with increas-ing concentration of fluoride. As described earlier, this wasexpected for the shorter wavelength band as the chromo-phore is directly connected to the receptor. However, thereason for the enhancement of the ICT band was not ex-pected as the ethyl spacer should prevent any significantground state interactions between the thiourea and nap-thalimide units. Several groups have previously demon-strated using di- or polypodal anion sensors containingurea or 2-aminobenzamidazole receptors that DMSO may

assist in the co-ordination of anions between adjacentpods [8]. To determine if DMSO could assist 4 in the bind-ing of fluoride we repeated the titration in 100% acetoni-trile. As shown in Fig. 3a, no significant enhancementwas observed for either band, the only difference beinga slight red shift in the band at 450 nm, most likelycaused by the deprotonation of the thiourea protons athigh pH [3e]. Furthermore, when 4 was titrated with tet-

0

0.05

0.1

0.15

0.2

0.25

0.3

260 310 360 410 460 510 560Wavelength (nm)

Abs

orba

nce

NS

N

H

H

N OO

NH

SO

N OO

NHN

S

N

H

HF-N

SN

H

H

N OO

NH

SO

N OO

NHN

S

N

H

HF-

a

b

Fig. 3. (a) Titration of 4 with tetrabutylammonium fluoride in CH3CN.[4] = 9.5 � 10�5 M [F-] = 0 ? 9.0 mM. (b) Illustration of how DMSO mayco-operate in the binding of a fluoride ion between adjacent residues of 4.

0 2 4 6 8 101

1.1

1.2

1.3

1.4

1.5

[F -] mM

A /

Ao

(@ 4

44nm

)

a

N. Singh et al. / European Polymer Journal 45 (2009) 272–277 275

rabutylammonium hydroxide, the UV profile was muchdifferent to that with fluoride, with the appearance of anew red-shifted band at kmax 538 nm due to napthali-mide NH deprotonation (Fig. 4) [3e]. This suggests thatthere is no interaction between fluoride and the acidicnapthalimide NH proton and that DMSO must participatesomehow in the binding of fluoride by 4, most likely byfacilitating the association of receptors in adjacent resi-dues of the polymer, as depicted in Fig. 3b. This maylead to an aggregation of the napthalimide groups result-ing in an enhancement of the absorbance at 444 nm.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

260 310 360 410 460 510 560Wavelength (nm)

Abs

orba

nce

Fig. 4. Absorbance spectra of spectra of 4 upon increasing amounts oftert-butyl ammonium hydroxide. Solvent = DMSO; [4] = 9.5 � 10�5 M.

Unfortunately, we were not able to confirm this hypoth-esis by 1H NMR due to the broadness of the peaks. Nev-ertheless, sigmoidal binding profiles were obtained byplotting changes of absorbance at both 270 and 444 nmagainst fluoride concentration (Fig. 5). In fact both plotsexhibited very similar binding profiles. Using the equa-tion: log [Amax � A]/[A � Amin] = log [anion] � logb (whereAmax and Amin are the maximum and minimum absor-bance and A is the measured absorbance) the bindingconstants, logb, were calculated as 2.59 at 270 nm and2.61 at 450 nm (Table 1) [9].

When 3 and 4 were excited at 450 nm both compoundsexhibited a broad emission with kmax 540 nm, characteris-tic of the napthalimide fluorophore. No differences interms of peak position or shape was observed betweenthe two, however the quantum yield of 3 (/F = 0.41) wassignificantly greater than 4 (/F = 0.12). This could be dueto conformational reasons with the polymer adopting aconformation that alters the polarity of the local environ-ment of the fluorophore [10]. However, due to the ICT nat-ure of the napthalimide excited state, such a change inpolarity would be expected to manifest itself as a changein the emission kmax, which was not observed (see Fig. 7).Another possibility is that there may be a PET quenchingcontribution from neighboring thiourea receptors to a sin-gle fluorophore resulting in an increased residual PET for 4even in the unbound state.

The effect of anions on the quantum yield of both 3 and4 was investigated in DMSO. Addition of chloride, bromideand hydrogensulfate had no major effect on the intensity of

1

1.1

1.2

1.3

1.4

0 2 4 6 8 10[F -] mM

1

1.1

1.2

1.3

1.4

0 2 4 6 8 10[F - ] mM

A / A

o (@

270

nm

)

b

Fig. 5. Plot of relative absorbance against concentration of fluoride ion for4 measured at (a) 444 nm and (b) 270 nm. Solvent = DMSO;[4] = 9.5 � 10�5 M.

2003004005006007008009001000

Inte

nsity

[F-] = 70 mM

[F-] = 0 mM

Table 1Photophysical properties of 3 and 4.

kmax (UV) nm kmax (EM) nm emax (mol�1 dm3 cm�1) UFLUa % F Red logb

3 444 525 4.18 0.41 – –4 440 525 3.75 0.12 – –3 + F- 446 523 – 0.15 62 1.923 + Cl- 444 525 – 0.36 12 –3 + Br- 444 525 – 0.36 12 –3 + AcO- 446 524 – 0.20 40 2.553 + HSO4

� 444 525 – 0.33 20 –3 + H2PO4

� 448 525 – 0.31 25 2.074 + F�b 441 524 – 0.06 53 2.59 (UV); 2.61 (UV); 1.90 (Flu); 4.43 (Flu)4 + Cl� 440 524 – 0.11 12 –4 + Br- 440 525 – 0.11 12 –4 + AcO 444 525 – 0.07 42 3.104 + HSO4

� 441 524 – 0.10 22 –4 + H2PO4

� 443 525 – 0.07 44 2.98

aQuantum Yields calculated with reference to fluorescein.bTwo binding constants were obtained from UV–Vis and fluorescence for fluoride.

276 N. Singh et al. / European Polymer Journal 45 (2009) 272–277

the emission of 3 or 4. Addition of acetate resulted in anapproximate 40% quenching of the original intensity forboth 3 and 4. The addition of fluoride and dihydrogenphos-phate also resulted in quenches of the original intensity,however the profile and magnitude was different for 3compared to 4. For 3, (Fig. 6a) there was clear evidenceof a single binding event with a large quench in intensityobserved at high concentrations of fluoride (logb = 1.92)[11]. For 4, there was evidence of two distinct binding

0100

480 530 580 630 680Wavelength

Fig. 7. Fluorescence spectra for 4 upon addition of increasing amount offluoride in DMSO. [4] = 5.0 � 10�6 M.

0

0.2

0.4

0.6

0.8

1

0 2 4 6

- log [F- ]

Rela

tive

Inte

nsity

0.9

- log [F- ]

Rel

ativ

e In

tens

ity

0

0.2

0.4

0.6

0.8

1

0 2 4 6- log [H2PO4

- ]

Rel

ativ

e In

tens

ity

0.73 5

- log [F- ]

a

b

Fig. 6. Plot of relative fluorescence intensity against concentration for (a)3 (filled circles) and 4 (open circles) titrated with fluoride (inset shows anexpansion of the lower concentration range) and (b) 3 (filled triangles)and 4 (open triangles) titrated with dihydrogenphosphate. Sol-vent = DMSO; [3] = 2.3 � 10�7 M; [4] = 5.0 � 10�6 M.

events. The first resulted in only a partial quench of fluo-rescence (�20%) and happened relatively low concentra-tions of fluoride (logb = 4.43). The second was almostidentical to that for 3 with a binding constant, logb of1.90, although the final relative intensity was higher for 4at 47% compared to 38% for 3 which was somewhat lowerthan that observed by Duke and Gunnlaugsson (Table 1)[3e]. The quenching of fluorescence at lower concentra-tions of fluoride may again be due to co-operative bindingbetween DMSO and fluoride in adjacent residues as shownin Fig. 3. At higher concentrations of fluoride the interac-tion between it and the polymer resembles that of themonomer 3 and the second binding event occurs with asimilar binding constant to that of the monomer.

A marked difference was also observed in the fluores-cence binding profiles of 3 and 4 upon addition of dihydro-genphosphate. Fig. 6b shows a plot of relative intensityagainst concentration of dihydrogenphosphate for 3 and4. A much more pronounced quench was observed for 4compared to 3 with a 25% reduction for 3 and 44% for 4.The binding constant was also greater for 4 (logb = 2.98)than for 3 (logb = 2.07). This is similar to that observedfor fluoride but without the appearance of two bindingevents. In all cases the quenching observed upon additionof anions is due to an increase in the reduction potential

N. Singh et al. / European Polymer Journal 45 (2009) 272–277 277

of the receptor upon binding resulting in a more effectivePET process.

In conclusion, we have shown that by incorporating athiourea receptor onto the side chain of polysiloxane thebinding profile against certain anions in DMSO was altered.Significantly, we were able to monitor the concentration offluoride at two different concentration levels by switchingbetween UV–Vis and fluorescence. We believe that DMSOassists the binding of fluoride between adjacent thiourearesidues. This offers the possibility of processing thesepolymers into thin films with potential use in diagnostics.To the best of our knowledge, this is the first reportedexample of a dual chromogenic/fluorescent sensor foranions.

Acknowledgements

The authors thank the EPSRC and RGU for financialassistance. We also thank the EPSRC national Mass Specservice for accurate mass data.

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[11] Binding constants were obtained from the equation – log (FMAX � F)/(F � FMIN) = log [Anion] + logb where FMAX is the maximumfluorescence intensity, FMIN is the minimum fluorescence intensityand F the observed fluorescence intensity. See Ref. [9].