Journal ofMaterials Chemistry B

PAPER

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School of Chemistry, Madurai Kamaraj Un

India. E-mail: pit12399@yahoo.com; Fax: +

† Electronic supplementary informationsynthesis of the thiol stabilized silver nanESI-mass spectra of 2-(2-aminophenylthicompound 1. UV-Vis, emission, powdeimages of thiol–silver nanoparticles, UV-Binding constants of 1 with all metal ions

Cite this: J. Mater. Chem. B, 2013, 1,1962

Received 15th December 2012Accepted 31st January 2013

DOI: 10.1039/c3tb00534h

www.rsc.org/MaterialsB

1962 | J. Mater. Chem. B, 2013, 1, 19

A highly selective ratiometric fluorescent chemosensorfor Cu(II) based on dansyl-functionalized thiol stabilizedsilver nanoparticles†

Vairaperumal Tharmaraj and Kasi Pitchumani*

A fluorescent chemosensor for Cu2+ ions based on dansyl-functionalized thiol stabilized silver nanoparticles

containing 2-aminothiophenyl units as the Cu2+ binding sites is developed. A decrease in fluorescence at

497 nm and an increase in fluorescence at 410 nm with an isoemissive point at 445 nm upon the

addition of Cu2+ ions is attributed to the formation of a Cu2+ complex in aqueous acetonitrile based on

an energy transfer mechanism. This sensor has excellent sensitivity and selectivity over other metal ions

and has a detection limit as low as 5.0 � 10�10 mol L�1.

1 Introduction

Transition-metal ions play crucial roles in the maintenance oflife in various organisms.1 Among them, copper ions play acritical role as catalytic cofactors for a variety of metal-loenzymes, including superoxide dismutase, cytochrome coxidase and tyrosinase. However, upon overloading, copperexhibits toxicity, in that it causes neurodegenerative diseases(e.g., Alzheimer's and Wilson's diseases), probably by itsinvolvement in the production of reactive oxygen species.2

Cellular copper uptake and release events are kinetically rapidand intracellular copper concentrations can increase up to20-fold within 1 h upon incubation in growth medium supple-mented with micromolar concentrations of copper,3 with facileexchange between cytosolic and mitochondrial stores.4

The facile redox chemistry and strong ligand binding prop-erties of copper ions are key factors that underlie its essentialbiological functions that include energy generation, dioxygentransport and activation and signal transduction. Under-standing the biological and environmental roles of copper(II)ions requires robust and versatile methods for quantication.5

The goal has been to devise “turn-on” uorescent sensors forcopper(II).6 The turn-on signals are, however, insufficient forquantication. An alternative approach involves sensors thatdisplay a change in the ratio of multiple emission bands,providing quantication as a signicant advantage,7 and only a

iversity, Madurai-625021, Tamil Nadu,

91 452 2459181; Tel: +91 0452-2456614

(ESI) available: General scheme of theoparticles. 1H NMR, 13C NMR, IR ando)acetyl bromide. 1H NMR spectra ofr XRD and EDAX spectra and AFMVis absorption selectivity studies of 1.. See DOI: 10.1039/c3tb00534h

62–1967

few ratiometric uorescent sensors for copper are currentlyavailable.8

Classical chemosensors typically combine two components:a recognition site that binds the target substrate and a readoutsystem that signals the binding.9 Generally, a typically synthe-sized probe of this type is constructed by covalently linkingthree components: a chelating unit, a spacer and a reportinggroup, though there are some examples of spacer-free probes.Upon the binding of metal ions, these sensor molecules displaycompletely different absorption/uorescence signals comparedto the free sensors in solution, enabling the quantitativedetermination of the amount of metal ions.10 The dansyl uo-rophore (5-dimethylamino-1-naphthalene sulfonate) is charac-terized by a charge transfer excited state exhibitingsolvatochromism and high emission quantum yields.11 Thesecharacteristics, together with the synthetic exibility of thesulfonic acid group, have led the dansyl uorophore to be acore-structure present in many uorescent sensors and labelsfor the detection of metal ions.12

The development of new sensors that can, at the same time,selectively recognize a target, signal the presence of the targetand quantify it in aqueous media has attracted widespreadattention in recent times because of the great demand ofspecic sensing systems that can operate in complex mediaencountered in biology or environmental monitoring.13 Usingthe set-up described above, a large number of molecular uo-rescent sensors have been devised for the detection of variousanalytes. While the covalent linkage ensures energy transferbetween the two components (receptor and uorophore), thesesensors must be properly designed for a given target andconsequently synthetic efforts are required.14 The specicrecognition of copper ions using nanoparticles/nanohybridssuch as gold,15 silica nanoparticles16 and ZnS quantum dots17

has been reported, however there are only a few reports with

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silver nanoparticles.18 Encouraged by the above observationsand also our interest in developing chemosensors for metalions, anions and neutral analytes,19 we were prompted todevelop a dansyl-functionalized silver nanoparticle basedratiometric uorescent chemosensor for Cu2+ ions using a 2-(2-aminophenylthio)acetyl moiety as the Cu2+ receptor.

2 Experimental section2.1 Instrumentation

UV-Vis absorption spectra were recorded using a JASCO-spectramanager (V-550) with a 1 cm path length quartz cuvette with avolume of 2 mL at room temperature. All uorescencemeasurements were recorded on a Fluoromax-4 Spectrouo-rometer (HORIBA JOBIN YVON) with an excitation slit set at2.0 nm band pass and emission at 4.0 nm band pass in a 1 cm�1 cm quartz cell. Scanning electron microscopy (SEM)measurements were carried out with a JEOL-JSM-6390. Atomicforce microscopy (AFM) images were recorded using an A-100SGS. High resolution transmission electron microscopy(HRTEM) images were obtained using a Jeol instrument at 300kV. The electron diffraction pattern was also recorded for theselected area. Electrospray ionisation mass spectrometry (ESI-MS) analysis was performed in the negative ion mode on aliquid chromatography ion trap mass spectrometer (LCQ Fleet,Thermo Fisher Instruments Limited, US).

2.2 Synthesis of thiol stabilized silver nanoparticles

The synthesis of thiol stabilized silver nanoparticles wasachieved by the reduction of silver nitrate (AgNO3) with 2-mer-captoethanol as a capping ligand. Silver nitrate solution(1.0 mmol) and 2-mercaptoethanol (1.0 mmol) were mixedwhilst vigorously stirring. Then, a freshly prepared aqueoussolution of sodium borohydride (NaBH4) was added drop bydrop. A white precipitate was formed slowly indicating theformation of silver nanoparticles. This solution was stirred for1 h at room temperature and the resulting particles were iso-lated by centrifugation (12 000 rpm, 10 min) aer washing threetimes with ethanol.

Fig. 1 (a), (b) HRTEM images of highly dispersed thiol stabilized Ag NPs. (c) The2D lattice fringes of the HRTEM image. (d) Selected area diffraction patternshowing the corresponding planes.

2.3 Synthesis of 2-(2-aminophenylthio)acetyl bromide(ligand)

The ligand for binding the Cu2+, 2-(2-aminophenylthio)acetylbromide was synthesized by stirring a solution of 2-amino-benzenethiol (0.11 mL, 1.0 mmol) in ethanol and bromoacetylbromide (0.09 mL, 1.0 mmol) in the presence of triethylaminefor 4 h at room temperature. The product was further puriedby column chromatography using petroleum ether/ethylacetate. A white solid was formed, mp: 183–185 �C, yield 80%,1H NMR (300 MHz, CDCl3): d 3.44 (s, 2H), 6.92–7.32 (m, 4H),9.41 (s, 2H) (ESI, Fig. S1†). 13C NMR (300 MHz, CDCl3): d 29.9,117.4, 199.8, 123.8, 127.2, 127.7, 136.3, 166.4 (ESI, Fig. S2†). IR(KBr, cm�1): 3317, 3203, 3110, 1783, 1657, 1477, 1392, 1193,1118 (ESI, Fig. S3†). An ESI-MS peak at m/z 283.61 (M + K+

adduct) was observed for the ligand (ESI, Fig. S4†).

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2.4 Synthesis of compound 1

1 mL of 1% (w/v) thiol stabilized silver NPs (nanoparticles) wasdispersed in ethanol and ethanol solutions of 2-(2-amino-phenylthio)acetyl bromide (0.25 g, 2.0 mmol) and dansyl chlo-ride (0.27 g, 1.0 mmol) were added in the presence oftriethylamine for 4 h at room temperature. Characterization wasby 1H NMR (300 MHz, CDCl3): 1.43 (t, J ¼ 7.2 Hz, 2H), 3.42 (s,3H), 3.56 (q, J¼ 7.2 Hz, 4H), 5.55 (s, 1H), 6.98–7.31 (m, 8H), 9.49(br s, 2H) (ESI, Fig. S5†).

3 Results and discussion3.1 Synthesis and characterisation of thiol-stabilised silvernanoparticles

Aer the addition of NaBH4 to a solution of AgNO3 in thepresence of 2-mercaptoethanol, the solution became whiteindicating the formation of thiol stabilized silver nanoparticlesthrough a reduction as shown in Scheme S1 (ESI†).

The UV-Vis spectrum was recorded in the 300–600 nm range.The surface plasmon resonance peak appeared at 370 nm (ESI,Fig. S6a†) and was broad, indicating a uniform size distribution.The emission spectrum was also recorded for the thiol stabi-lized silver nanoparticles with an excitation wavelength of380 nm and uorescence emission occurred at 477 nm(Fig. S6b†).

The thiol stabilized silver nanoparticles were further char-acterized using the powder XRD technique to establish theirmetallic nature. Fig. S7 (ESI†) illustrates a typical X-ray diffrac-tion pattern for the thiol stabilized silver nanoparticles showingfour prominent peaks at 2q values of 38.2, 44.4, 64.5, and 77.5�,corresponding to the (111), (200), (220) and (311) planes of theface centered cubic (fcc) structure of metallic silver. The stan-dard XRD pattern for nano Ag is almost the same (JCPDS le no.04-0783).

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High resolution transmission electron microscopy (HRTEM)images of the highly dispersed thiol stabilised silver nano-particles are shown in Fig. 1(a) and (b). The particle size isaround 5.5 nm. The 2D lattice fringe spacing in the HRTEMimage is found to be 0.2 nm (Fig. 1(c)). Fig. 1(d) shows thecorresponding selected area electron diffraction (SAED) pattern,the square spot array is indexed to four clear diffraction ringsthat are assignable to diffractions from the (111), (200), (220),and (311) planes of the nano silver. The EDAX (energy dispersiveX-ray) spectrum (ESI, Fig. S8†) indicates that 19.70 atomic%silver is present in the thiol stabilised silver nanoparticles.

AFM was further used to dene the morphology of the silvernanoparticles and the data show that the surface roughness ofthe silver nanoparticles was about 1.0 mm. A cross-section of thesilver nanoparticles self-arranged in a similar direction as seenfrom the height prole of a three-dimensional (3D) AFM imagealso shows that the average out-of-plane thickness of the puresilver nanoparticles was about 0.1 mm (ESI, Fig. S9†).

3.2 Design of compound 1

When thiol stabilized silver nanoparticles were treated with anethanolic solution of 2-(2-aminophenylthio)acetyl bromide anddansyl chloride in the presence of triethylamine at roomtemperature (Scheme 1), compound 1 was obtained, which wascharacterised by 1H NMR spectroscopy (ESI, Fig. S4†). A cavitywas thus constructed for sensing Cu2+ by incorporating twothioether and two amine units present in the ligand 2-(2-ami-nophenylthio)acetyl bromide and dansyl chloride into a thiolstabilized silver nanoparticle.

3.3 Cu2+ ion sensing studies

Dansyl-functionalized silver nanoparticles (compound 1) in1 : 1 (v/v) aqueous acetonitrile (phosphate buffer, 10 mM) dis-played an absorption spectrum with a structured band havinglmax at 235 nm. Upon the addition of different metal ions, viz.Na+, K+, Li+, Ag+, Ba2+, Cd2+, Co2+, Fe2+, Hg2+, Mn2+, Ni2+, Pb2+

and Zn2+ to a solution of 1, there was no signicant change inthe UV-Vis spectrum except in the case of the Cu2+ ion. Upon theaddition of Cu2+ to a solution of 1, a new absorption band in theregion of 260 nm appeared (ESI, Fig. S10†).

Scheme 1 Schematic diagram of the synthesis of compound 1, involving thereaction of dansyl chloride and 2-(2-aminophenylthio)acetyl bromide with thiolstabilised Ag NPs.

1964 | J. Mater. Chem. B, 2013, 1, 1962–1967

3.4 Ratiometric uorescence response of 1

Compound 1 exhibited a strong emission band at 497 nm whenexcited at 260 nm. The uorescence response of 1 was alsostudied in the presence of various metal ions such as Na+, K+,Li+, Ag+, Ba2+, Cd2+, Co2+, Fe2+, Hg2+, Mn2+, Ni2+, Pb2+ and Zn2+

which gave little or poor response.However upon the addition of Cu2+ ions the intensity of the

emission band at 497 nm decreased with the appearance of anew band at 410 nm which was due to the formation of a 1 +Cu2+ complex (Fig. 2). The decrease in intensity of the 497 nmband and the appearance of the band at 410 nm show thatenergy transfer from the dansyl group to the copper complextook place.

3.5 Selectivity of the ratiometric sensing

The selectivity is a very important parameter for evaluating theperformance of any uorescent sensing system and the selec-tivity experiments for the ratiometric sensor were extended tovarious other metal ions, such as Na+, K+, Li+, Ag+, Ba2+, Cd2+,Co2+, Fe2+, Hg2+, Mn2+, Ni2+, Pb2+ and Zn2+. From the resultsshown in Fig. 3, it can be seen that Cu2+ ions induced a prom-inent uorescence ratio increase, whereas other metal ions ledto a very slight or even no uorescence ratiometric change. Thisresult indicates clearly that the present Ag NP-based sensor hasa good selectivity toward Cu2+ ions over other competitivecations and more importantly, the common quenchers for AgNPs, such as Hg2+ ions and other metal ions, do not interferewith the ratiometric detection of Cu2+ ions.

The effects of other coexisting cations on copper ion sensingwere also determined. The uorescence responses of thesensing system toward Cu2+ ions in the presence of alkali,alkaline earth, and other transition metal ions are shown inFig. 3 (inset). The presence of most of the selected metal ionsdoes not interfere with Cu2+ binding to the probe, indicating

Fig. 2 Emission spectrum of compound 1 (1 mg mL�1) in the presence of Cu2+

ions and various metal ions (5 � 10�6 mol L�1 Na+, K+, Li+, Ag+, Ba2+, Cd2+, Co2+,Fe2+, Hg2+, Mn2+, Ni2+, Pb2+ and Zn2+) at pH 7.4 (phosphate buffer, 10 mM) inCH3CN : H2O (1 : 1 v/v). (lex ¼ 260 nm, lem ¼ 410 and 497 nm, slit: 5 nm/5 nm).

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Fig. 3 Bar chart showing fluorescence response of compound 1 (1 mg mL�1) inthe presence of Cu2+ ions and various metal ions (5 � 10�6 mol L�1 Na+, K+, Li+,Ag+, Ba2+, Cd2+, Co2+, Fe2+, Hg2+, Mn2+, Ni2+, Pb2+ and Zn2+) at pH 7.4 (phosphatebuffer 10 mM) in CH3CN : H2O (1 : 1 v/v). (lex ¼ 260 nm, lem ¼ 410 and 497 nm,slit: 5 nm/5 nm). The inset shows the fluorescence intensity ratios (I410/I497) for aCH3CN : H2O dispersion of dansyl–silver nanoparticles upon the addition ofdifferent metal ions (5.0 � 10�6 mol L�1).

Scheme 2 Proposed binding mechanisms for 1 with a Cu2+ ion.

Fig. 4 Fluorescence response showing the sensitivity of the Cu2+ ion sensor atdifferent concentrations of Cu2+ ions (5.0 � 10�5 mol L�1 to 5.0 � 10�10 mol L�1)at pH 7.4 (phosphate buffer, 10 mM) in CH3CN : H2O (1 : 1 v/v). (lex ¼ 260 nm,lem ¼ 410 and 497 nm, slit: 5 nm/5 nm).

Fig. 5 Ratiometric calibration curve of I410/I497 as a function of Cu2+ concen-tration (triplicate). The concentration of Cu2+ was 5.0 � 10�5 mol L�1 to 5.0 �10�10 mol L�1 at pH 7.4 (phosphate buffer, 10 mM) in 1 : 1 CH3CN : H2O. (lex ¼260 nm, lem ¼ 410 and 497 nm, slit: 5 nm/5 nm).

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that these coexisting ions have negligible interference effects onCu2+ sensing by the dansyl–Ag nanoparticles 1.

3.6 Proposed binding mechanism for 1 with Cu2+ ions

A signicant advantage of the present sensing system is thatalthough the main function of the 2-(2-aminophenylthio)acetylbromide is to grasp the metal cation through direct N–metalinteractions, the additional thioether moiety contributes moreto the construction of the cavity in space resulting in it being theright size and in a suitable conformation. In most of thereported Cu2+ sensors, the uorescent moiety (usually dansyl) isplaced far away from the nanoparticle surface and the uo-rophore does not participate in complexation.20 Howeveranother advantage of the present system is that the uorophoreand binding unit are adjacent to each other. As a result energytransfer (ET) from the dansyl moiety to the copper complexoccurs and this causes the uorescent ratiometric response

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(Fig. 2) in aqueous acetonitrile (1 : 1, v/v) (phosphate buffer,10 mM). The whole process is shown in Scheme 2.

3.7 Fluorescence titration

A uorescence titration was performed in a solution of 1 inaqueous acetonitrile (1 : 1, v/v) (phosphate buffer, 10 mM). Asshown in Fig. 4, upon the addition of 5.0 � 10�10 to 5.0 � 10�5

mol L�1 of Cu2+ ions, a signicant decrease in the uorescenceintensity of the 497 nm band and a new uorescence emissionband centered at 410 nm with a clear isoemissive point at445 nm are observed, which is attributed to the copper complex(Cu2+–1).21

A plot of the emission intensity ratio versus the Cu2+ ionconcentration is shown in Fig. 5. From this gure, we can see

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that the ratio of the intensity at 410 nm to that at 497 nm(I410/I497) increased steadily as the concentration of Cu2+ ionswas increased, showing that the dansyl–Ag nanoparticle basedenergy transfer system is a sensitive ratiometric uorescentsensor for Cu2+ ions. It is likely that the energy transfer processis switched on by Cu2+ ions as excitation at 260 nm resulted inemission from the dansyl groups with a maximum intensity at497 nm. The energy transfer detection system has two separateemission bands with comparable intensities, which ensuresaccuracy in determining their intensities and ratios. In addi-tion, the low detection limit for this system (around 5.0 � 10�10

mol L�1) indicates that the dansyl–Ag nanoparticle system isstable in aqueous acetonitrile. Data (Table S1†) showing thebinding constants of 1with various metal ions recorded at roomtemperature in an acetonitrile–H2O (1 : 1, v/v) mixture showthat the Cu2+ ion has the strongest affinity for 1. The bindingconstant of 1 with Cu2+ was found to be 8345 M�1.

4 Conclusions

In summary, dansyl-functionalized thiol stabilized silvernanoparticles have been developed as a highly selective ratio-metric uorescent chemosensor for Cu2+ ions based on anenergy transfer mechanism. The synthesised thiol stabilisedsilver nanoparticles were characterised by UV-Vis and uores-cence spectroscopy, AFM, HRTEM and EDAX analysis. Adecrease in the uorescence at 497 nm and an increase in theuorescence at 410 nm with an isoemissive point at 445 nmupon the addition of Cu2+ ions is attributed to the formation ofa Cu2+ complex in aqueous acetonitrile. This system (1) showsexclusive selectivity for Cu2+ ions in the presence of a variety ofother metal ions such as Na+, K+, Li+, Ag+, Ba2+, Cd2+, Co2+, Fe2+,Hg2+, Mn2+, Ni2+, Pb2+ and Zn2+, evident from UV-Vis and uo-rescence studies. Also, this sensor system has a detection limitas low as 5.0 � 10�10 mol L�1.

Acknowledgements

Financial assistance from the University Grants Commission,New Delhi (for UPE programme to Madurai Kamaraj University)is gratefully acknowledged.

Notes and references

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This journal is ª The Royal Society of Chemistry 2013

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J. Mater. Chem. B, 2013, 1, 1962–1967 | 1967