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Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 72– 80

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h om epa ge: www.elsev ier .com/ locate / jphotochem

new spectroscopic protocol for selective detection of water solubleulfides and cyanides: Use of Ag-nanoparticles synthesized byg(I)–reduction via photo-degradation of azo-food-colorants

iharendu Mahapatra, Shubhashis Datta, Mintu Halder ∗

epartment of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

r t i c l e i n f o

rticle history:eceived 2 July 2013eceived in revised form6 September 2013ccepted 30 October 2013vailable online 6 November 2013

a b s t r a c t

Slow and controlled visible photo-reduction of AgNO3 by synthetic azo food-dyes in aqueous mediaproduces spherical silver nanoparticles (Ag NPs). These show excellent selectivity toward inorganicanions (sulfide (S2−) and cyanide (CN−) with lowest detection limit (LOD) of ∼60 ppb. Completephoto-degradation of these dyes during nanoparticle formation can be very useful for environmentalremediation and removal of dye toxicity. Detection of water soluble S2− and CN− is achieved by spectro-scopic technique (fluorescence turn-on process), based on the etching of Ag NPs. Selectivity with respectto either of these two ions (S2− or CN−) is achieved by using suitable masking agents. This simple, rapid,

eywords:zo food dyehotochemical reductionye degradationnvironmental remediationighly stable Ag NP

and significantly low-cost sensing protocol can be employed at ease.© 2013 Elsevier B.V. All rights reserved.

nion sensing

. Introduction

Over the past two decades synthesis of metal nanoparticlesave drawn great attention toward chemists, physicists, biologistsnd engineers for their unique chemical and electronic proper-ies which is necessary for the development of new generationano-devices and sensors. Due to their wide range of applications,specially involving human contact, there is a growing tendency toevelop eco-friendly processes for the synthesis of nanomaterialsithout using toxic chemicals [1]. Hence the synthesis of nano-aterials coupled with photo-degradation of synthetic azo food

olorants can be a new method for the development of eco-friendlyrocesses.

Industrial waste-containing dyestuffs adversely affect wateresources, soil fertility, aquatic organisms and ecosystem integrity.

zo dyes are the most important group of synthetic colorantsxtensively used in textile, food, pharmaceutical and printingndustries. These industries discharge large amount of waste water

Abbreviations: Ag NP, silver nanoparticle; CTAB, cetyltrimethlyammoniumromide; CMC, critical micellar concentration; C314, coumarin 314; LOD, lowestetection limit; SERS, surface-enhanced Raman scattering; R6G, rhodamine 6G; AEF,pparent enhancement factors.∗ Corresponding author. Tel.: +91 3222 283314; fax: +91 3222 282252.

E-mail addresses: mintu@chem.iitkgp.ernet.in, mhchem@gmail.comM. Halder).

010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2013.10.015

in the dyeing process. Generally, azo dyes are electron-deficientxenobiotics containing electron-withdrawing azo (N N) and sul-fonate (−SO3−) functions. These are recalcitrant to biodegradationunder natural conditions and cannot be easily removed from wastewater by conventional waste-water-treatments. Hence degrada-tion of these colorants is a growing challenge for environmentalremediation. Several methods have been used to degrade differenttypes of azo dyes [2,3]. In most cases toxic reagents (such as TiO2)are used for dye degradation processes. Here in our case, diluteAg+ mediated complete photo-degradation of azo dyes in visiblelight, with a complementary Ag NP generation, is indeed a neweco-friendly way for greener environment. Moreover, Ag+ has beenknown to be effective against a broad range of microorganisms andis used to control bacterial growth in a variety of medical appli-cations, including dental work, catheters, and the healing of burnwounds [4]. Hence presence of Ag+ may also help to impair bacterialgrowth in various waste materials which are potential environmentpollutants.

The quantum-scale dimension of these nanomaterials is the ori-gin of their fascinating properties which is responsible for theirapplications in different research areas, such as sensing [5–8],catalysis [9], photonics [10], surface enhanced Raman scattering

(SERS) [11] etc. Nanomaterial-based sensing of several ions [5–7],biomolecules [8] are the mostly explored research areas till now.Among various anions, sulfides (S2−) and cyanides (CN−) are foundto be widely distributed in both environment and living systems

ry and

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N. Mahapatra et al. / Journal of Photochemist

12–14]. For example, sulfides (S2−) from industrial and microbialrocesses easily generates poisonous H2S gas which is responsibleor metal (e.g. iron, copper) corrosion, and thus is hazardous fornvironmental and industrial processing [12,15]. Cyanides (CN−)re generated by certain bacteria, fungi, and algae and also foundn a number of plants [16]. In plants, sugar molecules are usuallyound to cyanides in the form of cyanogenic glycosides [17]. Inuman body, cyanide inhibits the electron transport in mitochon-ria as it binds to the active site of cytochrome oxidase [13].s a matter of fact, exposure to higher concentrations of CN−

>300 ppm) will cause human death by depressing the central ner-ous system within a few minutes [14]. Thus an easy and selectiveetection protocol for sulfides (S2−) and cyanides (CN−) can be

mportant.Till now, several techniques have been proposed for the

etection of S2−, CN− in aqueous solutions, which include liq-id chromatography-mass spectrometry (LCMS); liquid chromato-raphy-atomic emission spectrometry (LC-AES); inductively cou-led plasma-atomic emission spectrometry (ICP-AES), liquidhromatography-inductively coupled plasma mass spectrometerICP-MS); ion chromatography and capillary electrophoresis cou-led with absorption or fluorescence detection [18–24]. Thesetrategies are rather time-consuming, and require tedious samplereparations, costly instruments, specific operating skills, and alsoas serious influence by the interference of coexisting ions.

Recently, different chemosensors are used to detect S2− or CN−

25,26], which also has some disadvantages such as their involvedrganic synthesis, environmental toxicity, water insolubility, poorhotostability and interference from other anions. For practicalpplications, an efficient sensor is expected to be not only highlyensitive and selective but also cheap, easy to operate, and it woulde even more important if the same sensor can selectively do mul-iple functions just by altering the associated conditions. Thus theevelopment of such multi-behaving sensors is even more chal-

enging. Most commonly, gold nanoparticles (Au NPs) are useds sensor due to their high extinction coefficients in the visibleegion [27,28]. Hence, many Au-NP-based colorimetric and fluo-escent sensing systems are developed to detect cations, anions,roteins, small molecules [29,30] etc. But recently, silver nanopar-icles (Ag NPs) have been used as an alternative sensing systemue to lower cost [31]. There are only few reports of Ag NP-baseduorescent sensors for the detection of (CN−) [32] and S2− [33]ut selective dual detection of either sulfide or cyanide in pres-nce of both of them using a multi-functioning CTAB stabilized AgP-based sensor has not been reported till date. Metal nanoparti-les synthesized by conventional chemical methods are generallytabilized by high concentration of CTAB. In general, CTAB stabi-ized spherical metal nanoparticle cannot be used in any type ofensing application due to the presence of long alkyl chain of surfac-ant around the nanoparticle surface, which inhibits the interactionetween metal surface and bimolecules or ions. Sensing applica-ions of those CTAB stabilized metal nanoparticle can be achievedy surface modification [34] or by anisotropic growth [35] or byggregation [36].

Herein for the first time, we have synthesized silver nanoma-erials via reduction of Ag+ complemented by photo-degradationf water soluble azo food colorants under eco-friendly condi-ions. A fluorescent sensor based on the synthesized Ag NP andoumarin 314 (C314) is employed for the selective detection of2− and CN− in aqueous solution in presence of other com-on anions, such as CH3COO−, S2O3

2−, SCN−, SO42−, SO3

2−,2O8

2−, CO32−, N3

−, NO3−, NO2

−, ClO4−, F−, Cl−, Br−, I−, PO4

3−,PO4

2−, H2PO4−. Use of significantly low concentration of CTAB

∼50 �M) as stabilizer, leads to much less surface coverage ofg NPs and facilitates the interaction between metal surface andnions.

Photobiology A: Chemistry 275 (2014) 72– 80 73

Use of Pb2+ as masking agent, our C314-Ag NP sensor is capableto selectively detect CN− in presence of both S2− and CN−. On theother hand, masking of CN− by Cu+ (CuI) enables our C314-Ag NPsensor to selectively detect S2−. On the practical side, our fluorimet-ric assay has been found to facilitate a simple, rapid and selectivedetection of either S2− or CN− in spiked pond water and tap watersamples.

It is important to note that our synthesized Ag NPs exhibitssurface-enhanced Raman scattering (SERS) activity even withoutany modification and this can also be utilized as an efficient ana-lytical tool for the detection of very low concentrations of chemicaland biological molecules adsorbed on the NP surfaces [37–39].Aggregated metal nanoparticles show greater SERS activity due tocoupling of surface plasmons between nanoparticles in close prox-imity [40–42]. Here ethanol induced aggregation of Ag NPs has beenfound to increase the SERS activity, as expected.

2. Materials and methods

2.1. Materials

Purest grade carmoisine, cochineal red A, orange G, coumarin314 (C314), rhodamine 6G (R6G), CTAB and AgNO3 are purchasedfrom Sigma Aldrich. NaOH (AR grade), methanol, ethanol (spectro-scopic grade), NaCN, Na2S, CH3COONa, Na2S2O3, NaSCN, Na2SO4,Na2SO3, Na2S2O8, Na2CO3, NaN3, NaNO3, NaNO2, NaClO4, NaF,NaCl, NaBr, NaI, Na3PO4, Na2HPO4, NaH2PO4, Pb(NO3)2, CuI arepurchased from Merck. The whole experiment is performed indeionized triply distilled water. All glasswares are cleaned thor-oughly by nitric acid and freshly prepared chromic acid, rinsedthoroughly with distilled water and acetone, and then dried inoven.

2.2. Instrumentation

Photochemical reaction has been performed in presence ofXenon light source (Newport, Model 66902, 300 W) in a 100 mlround bottomed borosilicate-glass flask with magnetic stirring.Light luminous flux per unit area is measured by using Lutron LX– 107HA digital light meter and each run is performed at a lightluminous flux per unit area of 50,000 (±100) Lux. The pH mea-surement is carried out with a Eutech-510 ion pH-meter, whichis pre-calibrated with standard pH buffer tablets. To deaerate thereaction mixture argon gas is purged slowly for about one hour andfollowed by air-sealing the reactor. Electronic absorption spectraare recorded with a UV-2450 (Shimadzu) absorption spectropho-tometer against solvent reference. The steady state fluorescencespectra are recorded on a Jobin Yvon – Spex Fluorolog-3 spec-trofluorimeter, using 1-cm path-length quartz cuvette. In order tomonitor the fluorescence of C314, samples are excited at 450 nmand the emission spectra are recorded from 470 to 600 nm. Trans-mission electron microscopy (TEM) images are acquired usingFEI-Tecnai G2 20 s-Twin with an operating voltage of 200 kV. X-raydiffraction (XRD) pattern of the sample is collected using Bruker D8Diffractometer unit with nickel-filtered Cu K� radiation (� = 1.54 A)in the 2� range of 36◦–90◦ at a scanning rate of 3.0◦ min−1. TheXRD data is analyzed using JCPDS software. For the identificationof ultimate products, the final irradiated reaction mixture isanalyzed by liquid chromatography–mass spectrometry (LC/MS)(Waters 2695, USA) coupled to a Micromass Quattro Micro triple-quadrupole mass spectrometer (Micromass, Manchester, UK) with

iments keeping 180◦ scattering geometry using a micro-Ramanspectrometer with 488 nm argon ion laser source. The spectrom-eter is equipped with an optical microscope (BX 41, Olympus,

7 ry and Photobiology A: Chemistry 275 (2014) 72– 80

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400 600 8000.0

0.2

0.4

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sorb

ance

Wavelength ( nm)

(b)

4 N. Mahapatra et al. / Journal of Photochemist

apan), single monochromator (Triax550, Jobin Yvon, Horiba,SA), an edge filter, and a Peltier cooled CCD (1024 × 256 pixel)etector. The laser power on the samples is 1.56 mW. The datacquisition time for each Raman spectrum is 20 s. A 50× objectives used to focus the laser beam onto a spot of approximately 2 �miameter.

.3. Synthesis of Ag nanostructures

Silver nanoparticles (Ag NPs) are prepared by the photochemicaleduction of silver nitrate solution with carmoisine in presence ofTAB at pH 9.0. A series of experiments are performed, varying theoncentrations of silver ions, reductant, stabilizer and also the pHbetween pH 3 and 11) of the starting reaction mixture. In a typicalrocedure, 50 ml of the reaction mixture at pH 9.0 (made by addingrops of aqueous 0.1 M NaOH) in a round bottom flask, dilute car-oisine (10 �M), CTAB and AgNO3 (mole ratio 1: 5: 5, respectively)

re taken. A broad-band visible light (obtained by using appropriatelter after the Xenon source) has been used to irradiate the mix-ure at room temperature. The light luminous flux per unit areas 50,000 Lux, and the typical irradiation time has been set up to80 minutes with constant stirring using a magnet bar. Character-

stic pale yellow colored silver sol appears in about ten minutesf irradiation. The color intensifies with the progress of reaction.his pale yellow silver sol is characterized by UV–vis spectroscopy,EM image and XRD analysis. The pale yellow silver sol after threeours irradiation is lyophilized, re-dissolved in methanol and thenubjected to liquid chromatography–mass spectrometry (LC/MS)nalysis for characterization of the products. Details of the reactionathways along with associated analysis, e.g., LC/MS and UV–visata are available in supplementary data S1, which also supportsomplete photo-degradation of the azo dye during Ag NP forma-ion. The plausible mechanism for the formation of Ag NP is asollows. Under the experimental conditions carmoisine forms aorresponding silver 1-naphthoxide which undergoes photolyticleavage at the O Ag bond resulting in naphthoxy radical. Hydrol-sis of the radical produces naphthyl hydrazine intermediate [43]hich reduces Ag+ to Ag. Nanoparticle formation does not takelace in the absence of carmoisine which indicates that the same

s an essential ingredient in the said process. Here we have usedight only above 400 nm i.e., visible light for the synthesis of silveranoparticles in order to eliminate the possibility of direct reduc-ion of Ag+ by UV photons. Importantly, formation of nanoparticleas also been observed with sunlight using the identical reactionomposition. Nanoparticles are obtained only when CTAB has beenresent below its critical miceller concentration (CMC). Probably atr above CMC, carmoisine gets solubilized within the micelle, andoes not have access to Ag+.

.4. Detection of sulfide (S2−) and cyanide (CN−) anions

5 �L of 0.3 mM methanolic C314 is added to 1.5 ml Ag NP solu-ion to prepare the fluorescent sensor. Then an appropriate volumef aqueous solution of the chosen anion is added to the C314-AgPs. Each fluorescence measurement is taken after 10 min of mix-

ng the anions. For the selective detection of S2− or CN−, we havequilibrated masking reagents with the anion mixture(s) for threeours prior to the addition of the C314-Ag NPs.

.5. Analysis of real samples

Pond water and tap water samples are collected from the IITharagpur campus, filtered and used. These water samples arepiked with solutions of S2− or CN−, then an appropriate volumef the spiked sample is added to C314-Ag NPs in order to achieve

Fig. 1. UV–vis spectra of initial (a) and 180 minutes irradiated (b) reaction mixture.Inset shows the photograph of the silver sol obtained after 180 min irradiation.

the desired concentration, and finally fluorescence measurementis taken.

2.6. Preparation of the samples for SERS measurement

Ethanol of different volumes (0–100 �L) are added to 1 ml ofthe synthesized pale yellow silver sol and incubated for about onehour to allow agglomeration. A 0.02 ml aqueous solution of rho-damine 6G (R6G), with different stock concentrations from 5 × 10−9

to 5 × 10−6 M, are added to 0.98 ml of ethanol treated silver sol andleft for one hour to equilibrate. 20 �L of each of the above final mix-tures is dropped onto the glass slide covered with clean aluminumfoil and allowed the solvent to evaporate. Raman measurementsare taken on these dried samples.

3. Results and discussion

3.1. Characterization of silver nanoparticles (Ag NPs)

The UV–vis spectra (Fig. 1) of prepared silver sol at pH 9.0 usingdilute carmoisine (10 �M), CTAB and AgNO3 at a mole ratio of 1:5:5after 180 min irradiation, shows absorption peak at 430 nm. TEMimage (Fig. 2a) of the Ag NPs shows mostly spherical and quasi-spherical particles with an average size of (6–14) nm, as evidentfrom the statistical histogram of the size of Ag NPs (Fig. 3a). Thehigh resolution TEM (HRTEM) image of the nanostructures (Fig. 2ainset) shows the lattice fringe spacing of 2.38 A, which correspondsto the interplanar spacing for (1 1 1) planes. The selected-area elec-tron diffraction (SAED) pattern (Fig. 4a) indicates the presence of(1 1 1), (2 0 0) and (2 2 0) planes of face-centered cubic (FCC) lat-tice. XRD pattern (Fig. 5a) further confirms the existence of suchplanes and are in good agreement with the standard values (Fm3m,a = 0.4086 nm, JCPDS file no. 04-0783).

3.2. Sensing of sulfide (S2−) and cyanide (CN−) using C314-Ag NPsensor

As shown in Scheme 1, addition of S2− or CN− dissolves theAg NPs through etching. Basically, the S2− ions react with metal-lic silver and generate Ag2S in air at room temperature as follows:[44–47]

2− −

4Ag + 2S + O2 + 2H2O → 2Ag2S + 4OH

In the presence of CN− ions, the Ag NPs dissolves to form solublesilver cyanide complex. Thus the metallic silver is finally oxidized to

N. Mahapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 72– 80 75

Fig. 2. TEM images of Ag NPs in absence (a) and in presence of 30 �M S2− (b) and 30 �M CN− (c). Inset shows the corresponding high resolution (HR) TEM images of silvernanostructures.

0 5 10 15 200

60

120

180

Nu

mber

Dia meter (nm)

(a)

0 5 10 15 200

100

200

300N

um

ber

Dia mete r (nm)

(b)

0 5 10 15 200

100

200

300

Nu

mber

Dia mete r (nm)

(c)

Fig. 3. Size distribution histogram of Ag NPs in absence (a) and in presence of 30 �M S2− (b) and 30 �M CN− (c). The red curve in each case indicates the lognormal fit of thesize distribution histogram.

Ps in absence (a) and in presence of 30 �M S2− (b) and 30 �M CN− (c).

A

4

mol∼ai(do(dea

o

40 50 60 70 80 900

1k

0

1k

0.0

3.0k

(c)

0.441

0.335

2 ( deg ree s)

Ag NP + 30 µM NaCN

(b)

0.455

0.329

Inte

nsi

ty (

a. u

.)

Ag NP + 30 µM Na2S

0.35 3

0.252

Ag NP

(a)

Fig. 4. Selected-area electron diffraction (SAED) pattern of Ag N

g(CN)2− by cyanide in the presence of oxygen (air) [48] as follows:

Ag + 8CN− + O2 + 2H2O → 4Ag(CN)2− + 4OH−

In both the cases (in presence of S2− and CN−) Ag NPs are ulti-ately destroyed. This is evident from the gradual disappearance

f the characteristic absorbance at 430 nm (Fig. 6) – the pale yel-ow Ag NP solution becomes completely colorless after addition of80 �M S2− or CN−. So monitoring the color disappearance can ben instrument-free visual method for the detection of S2− and CN−

ons (Fig. 7). TEM images of the Ag NPs in presence of S2− and CN−

Fig. 2b and c, respectively) shows that the size of nano-spheresecreases which is presumably due to etching. The particle sizesf Ag NPs in the absence and presence of S2− or CN− (30 �M) are6–14) nm and (2–7) nm, respectively, as obtained from the size-istribution histogram (Fig. 3). The decrease in particle size is also

vident from the broadening [49] of XRD peak (Fig. 5) after theddition of S2− or CN− to Ag NP.

Since fluorescence is a very sensitive tool, we utilize the turn-n fluorescence assay for the detection of S2− and CN− (as shown

Fig. 5. X-ray diffraction (XRD) pattern of Ag NPs in absence (a) and in presence of30 �M S2− (b) and 30 �M CN− (c). Full Width at Half Maximum (FWHM) is indicatedby numeric value shown against peak.

76 N. Mahapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 72– 80

So

io(qetaaatgNt

3

re

500 550 6000

1x105

2x105

3x105

4x105

(d)

(c)

(b)

I F (

a. u.)

Wavelength (nm)

(a)

Fig. 8. Fluorescence spectra of solutions of C314 in water (a), C314-Ag NPs (b), C314-2− −

2−

cheme 1. Cartoon representation of dissolution of Ag NPs through etching by S2−

r CN− .

n Scheme 2). Fig. 8a represents the fluorescence emission spectraf coumarin 314 (C314) in aqueous media with emission maxima�em

max) at 493 nm. As shown in Fig. 8b, addition of Ag NPs efficientlyuenches the fluorescence of C314. In the presence of S2− or CN−,tching of Ag NPs induces the release of C314 from the surface intohe solution and thereby restores fluorescence of the probe (Fig. 8cnd d respectively). The C314-Ag NP solution is also pale yellownd is very weakly fluorescencent due to quenching with a largemount of scattering under UV light. On addition of S2− or CN−,he pale yellow color changes to colorless immediately, and thereen fluorescence of C314 (with very little scattering due to AgPs) appears (inset of Fig. 8). The fluorescence of C314 is not found

o be quenched by either S2− or CN− (blank solution).

.3. Selectivity and sensitivity

To evaluate the selectivity and sensitivity of this fluorescence-ecovery technique, we have plotted the relative fluorescencenhancement [7], (IF /IF0 ) − 1, for the C314-Ag NP sensor in the

400 600 8000.0

0.2

0.4

(a)

Abso

rban

ce

Wavelength (nm)

[ S2-

] (µM)

0510205080

(b

Fig. 6. UV–vis absorption spectra of Ag NPs in absence and pr

Fig. 7. Photographs of Ag NPs in absence and presence

Ag NPs and S (30 �M) (c), C314-Ag NPs and CN (30 �M) (d). Inset: Photographsof the corresponding solutions in presence of UV light. The concentration of C314 iskept at 1 �M in each case.

presence of different anions (at 30 �M) (Fig. 9), where IF and IF0

represent the fluorescence intensities of C314-Ag NPs at 493 nm inthe presence and absence of anions, respectively. Fig. 9a–c revealsthat the addition of soluble S2− or CN− to C314-Ag NPs in presenceof different combinations of other anions (as stated earlier in theintroduction) has resulted in the recovery of the fluorescence.Hence the observed fluorescence turn-on is exclusively due to S2−

or CN−.For selective detection of CN− by C314-Ag NP sensor, we employ

soluble Pb2+ as a masking reagent due to high formation constant of

PbS (log Kf ∼ 28.0), whereas the log Kf values for Pb(CN)4 is ∼ 8.0[50,51]. As indicated in Fig. 9d, C314-Ag NPs containing the maskingagent (30 �M Pb(NO3)2 for sulfide) demonstrates high selectivityfor CN−. The fluorescence intensity of the C314-Ag NPs in presence

400 60 0 8000.0

0.2

0.4

0510205080

[ CN-

] (µM ))

Abso

rban

ce

Wavelength (nm)

esence of different concentration of S2− (a) and CN− (b).

of different concentration of S2− (a) and CN− (b).

N. Mahapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 72– 80 77

Scheme 2. Cartoon representation of C314-Ag NP-based

0.0

0.2

e

d

c

b

a

Anions

CO

3

2-C

lO4

-N

3

-H2P

O4

-H

PO

4

2-P

O4

3-

S2-S

2O

8

2-

S2O

3

2-S

O4

2-

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3

2-

NO

3

-

NO

2

-

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Br-

Cl-

F-

CH

3C

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-S

CN

-C

N-

0.0

0.20.0

0.2

(IF/I F

0)-1

0.0

0.20.0

0.2

Fig. 9. Sensitivity of C314-Ag NP sensor for different anions (30 �M) in the absenceof both CN− and S2− (a); presence of CN− but absence of S2− (b); presence of S2− butabsence of CN− (c); and (d and e) presence of masking reagent Pb(NO3)2 (30 �M) ⇒(d) and CuI (30 �M) ⇒ (e). Absence of box indicates the absence of correspondingata

ocb(pC

sd[tdflugatlNobC

nion; unshaded box indicates presence of corresponding anion with no responseo turn-on; shaded box indicates turn-on sensing in presence of the correspondingnion.

f the masking reagent gradually increases with the CN− ion con-entration (Fig. 10a). A good linear correlation (R = 0.994) existsetween the value of (IF /IF0 ) − 1 and the concentration of CN−

Fig. 10c). The limit of detection (LOD), calculated by using therocedure as discussed elsewhere [7], is found to be 2.0 �M forN−.

On the other hand, selective detection of S2− by C314-Ag NPensor can be achieved by using CuI as masking reagent for CN−

ue to the high formation constant of Cu(CN)43− (log Kf ∼ 30.3)

52,53], however, Cu2S formation does not occur at room tempera-ure [54]. C314-Ag NPs containing the masking agent (30 �M Cu+)emonstrates high selectivity for S2− ion as shown in Fig. 9e. Theuorescence intensity of the C314-Ag NPs in presence of Cu+ grad-ally increases with the S2− ion concentration (Fig. 10b). Here, aood linear correlation (R = 0.996) also exists between (IF /IF0 ) − 1nd [S2−] (Fig. 10d). The LOD of S2− has been found to be 2.0 �M. Tohe best of our knowledge, the present fluorimetric assay is the first,abel-free, photochemically synthesized dilute CTAB stabilized Ag-P-based optical sensors for the sensitive and selective detection

f either S2− or CN− in presence of both ions and not interferedy other common anions. The advantages of this method are that314-Ag NP sensor is low cost, cheap and easy to handle.

fluorescence sensor for the detection of S2− or CN− .

3.4. Detection of sulfide (S2−) and cyanide (CN−) in real samples

To evaluate the practicality of our proposed sensing techniquefor the analysis of real samples, we have employed our C314-Ag NPsensor to determine the S2− and CN− levels in pond water and tapwater. A linear correlation (R = 0.96 − 0.98) of relative fluorescenceenhancement [7], (IF /IF0 ) − 1, with our sensor is observed for bothS2− and CN− ions spiked into different water samples (pond waterand tap water) over the range 0–100 �M (see supplementary dataS2). The minimum detectable concentration of either S2− (in pres-ence of Cu+) or CN− (in presence of Pb2+) in these water samples is∼60 ppb.

3.5. Surface-enhanced Raman scattering (SERS) studies

Our synthesized Ag NP shows SERS activity toward rhodamine6G (R6G) in the absence and presence of ethanol. In absence ofethanol, Ag NP shows SERS activity down to 10 nM concentrationof R6G (Fig. 11a). Raman peaks of R6G are in good agreement withthe literature reported results [55,56]. Addition of ethanol increasesthe SERS activity of Ag NPs. Fig. 11b shows the SERS spectra of10 nM R6G for different extents of aggregates induced by differ-ent volumes of ethanol. SERS enhancement ability increases withincreasing volume of ethanol and maximum enhancement occurswhen the volume of added ethanol reaches 50 �L (Fig. 11b). Fur-ther addition of ethanol causes a decrease in intensity of SERSsignal (Fig. 11b) which is presumably due precipitation of the AgNPs as a result of over-aggregation. Presence of ethanol removesthe protective CTAB layer from nanoparticles surface which causesaggregation and this can be controlled by varying the volume ofadded ethanol [36]. This aggregation increases the overall size ofthe nanoparticle with the increasing volume of ethanol. The great-est SERS enhancement occurs at the “junction” between two metalnanoparticles [57], which increases with increasing size of theaggregates [58]. Enhancement of electromagnetic field occurs atthe junction of the aggregated nanoparticles due to the coherentcoupling of surface plasmon oscillations of closely spaced nanopar-ticles [59]. This type of “junction” can act as an electromagnetic “hotspot” [60]. So addition of ethanol to the silver colloid will createsuch electromagnetic “hot spots” which causes an enhancement ofSERS activity by increasing electromagnetic field at the junction.It should be noted that 50 �L ethanol-induced aggregates givesthe greatest SERS signal and is used for concentration variationSERS of R6G. Fig. 11c represents the typical SERS spectra of vari-ous concentrations of R6G ranging from 10 to 0.1 nM after addition

of 50 �L ethanol in 1 ml silver sol (in this case background has beenremoved). The data shows that the addition of 50 �L ethanol in 1 mlsilver sol shows SERS activity with R6G down to 1 nM concentra-tion, i.e., nano molar sensitivity.

78 N. Mahapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 72– 80

Fig. 10. Fluorescence spectra of the C314-Ag NP solution used as sensor for the detection of CN− (0–100 �M) in the presence of 30 �M Pb(NO3)2 (a) and S2− (0–100 �M) inthe presence of 30 �M CuI (b). Plot of (IF /IF0 ) − 1 versus concentration of CN− (c) and S2− (d).

Fig. 11. SERS spectra of varying concentrations of R6G in presence of Ag NPs without addition of ethanol (a); SERS spectra of R6G (1 × 10−8 M) in presence of Ag NPs mixedwith different volumes of added ethanol (b); SERS spectra of varying concentrations of R6G after addition of 50 �L ethanol in 1 ml silver sol (c) (in this case background hasbeen removed). In each case laser power on the sample is 1.56 mW; integration time: 20 s.

N. Mahapatra et al. / Journal of Photochemistry and

Table 1AEF for R6G molecule adsorbed on silver colloids formed from different added vol-ume of ethanol (monitored with 1652 cm−1 Raman band).

Concentration ofR6G (M)

Volume of added ethanolto 1 ml silver colloid (in �L)

AEF

1 × 10−7 0 5.93 × 103

1 × 10−8 0 3.22 × 104

1 × 10−8 25 4.18 × 104

1 × 10−8 50 6.49 × 104

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1 × 10−8 75 1.13 × 104

1 × 10−9 50 1.05 × 105

To have a precise idea regarding the interaction of the R6Gith Ag NPs, calculation of apparent enhancement factor (AEF)

s done by comparing the intensity of a selected Raman peak of6G (∼1652 cm−1) measured in the SERS experiments to the corre-ponding peak measured from normal Raman spectra (NRS) of anqueous 2 mM R6G solution [61,62] by keeping other experimentalonditions exactly same. AEF is given by [63]:

EF = ISERS[CNRS]INRS[CSERS]

here C and I represent the concentration and peak height of theelected Raman band measured from baseline, respectively. TheEF values at 1652 cm−1 are listed in Table 1. The AEF values indi-ate that in our case SERS enhancement is mainly due to an increasef the local electric field between Ag NPs which is also evident fromthanol variation study.

. Conclusions

We report here for the first time, the synthesis of spherical Aganostructure using water soluble azo food-dyes (less toxic andost effective) as source of reducing agent in presence of visibleight. Only very low concentration of CTAB (much less than CMC)as been used as stabilizer. The nanoparticles show prolonged sta-ility (stable for more than four months) at room temperature.his approach furnishes a complementary photo-degradation ofhe azo dye which can be an eco-friendly way for environmentalemediation. Moreover, use of Ag+ for photo-degradation of azoood-dyes has additional advantage toward environmental healingue to antifungal, antibacterial activities of silver ions.

For application aspect of our synthesized nanomaterials, weave developed a new C-314-Ag NP-based fluorescent sensor

or highly sensitive and selective detection of either sulfides oryanides in aqueous media. Restoration of fluorescence of C314hrough etching of Ag NPs is the key mechanism behind sensing.resence of Pb2+ as masking agent permits our C-314-Ag NP sensoro selectively detect CN− even at 2 ppm, whereas in the presencef Cu+, C-314-Ag NP sensor can detect only S2− down to 2 ppm.ence our sensor is not only highly sensitive and selective but also

ignificantly low cost, easy to operate and very importantly multi-ensing, that is the same sensor can selectively detect either S2− orN− just by masking one of them in their mixture.

The synthesized silver sol also shows SERS activity down to0 nM concentration of R6G. Addition of ethanol increases the SERSctivity of Ag NPs to 1 nM concentration of R6G. Electromagneticffect (EM) play dominant role in SERS enhancement which is evi-ent from AEF values.

Similar studies involving different NPs are under progress.

cknowledgments

We thank DST-India (Fund no. SR/FTP/CS-97/2006), CSIR-ndia (Fund no. 01/(2177)/07 EMR-II, dated 24/10/2007) andIT-Kharagpur (ISIRD-EEM grant) for financial support. NM thanks

[

Photobiology A: Chemistry 275 (2014) 72– 80 79

CSIR-India for his individual fellowship. SD thanks IIT-Kharagpurfor an institutional fellowship. We thank Prof. S. Das of Metallurgi-cal and Materials Engineering department IIT-Kharagpur for helpwith TEM measurements. We also thank Mrs. S. Bhattacharya ofPhysics & Meteorology department of IIT-Kharagpur for helpingwith SERS measurements. Thanks to Prof. N.D. Pradeep Singh forhelping in LC/MS analysis and Prof. D. Ray for his valuable sugges-tions.

Appendix A. Supplementary data

Supplementary material related to this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jphotochem.2013.10.015.

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