24-Analyst

Analyst

PAPER

School of Chemical & Biotechnology, SAST

Nadu, India. E-mail: [email protected]

4362264101

† Electronic supplementary informationacid and trisodium citrate AgNPs sensorof Cl-1–AgNPs. See DOI: 10.1039/c3an003

Cite this: Analyst, 2013, 138, 4370

Received 15th February 2013Accepted 18th April 2013

DOI: 10.1039/c3an00320e

www.rsc.org/analyst

4370 | Analyst, 2013, 138, 4370–437

Green synthesized silver nanoparticles for selectivecolorimetric sensing of Hg2+ in aqueous solutionat wide pH range†

Selvan Sukanya Ravi, Lawrence Rene Christena, Nagarajan SaiSubramanianand Savarimuthu Philip Anthony*

In the present study, the syntheses of crystalline silver and gold nanoparticles (NPs) has been demonstrated

in a green and environmentally friendly approach using citrus fruit extracts (lemon, Citrus limon (Cl-1) and

sweet orange, Citrus limetta (Cl-2)). In addition, potentially hazardous metal ion sensing properties of these

NPs in aqueous solution has been explored. Cl-1 and Cl-2 that predominantly contained citric and ascorbic

acid, exhibited different reducing abilities towards silver and gold ions into NPs. Cl-1 reduces silver ions into

AgNPs only in the presence of sunlight whereas AuNPs from gold ions were formed without exposure to

sunlight. In contrast, Cl-2 converts both silver and gold ions into the corresponding NPs in the absence of

sunlight. The colorimetric sensor studies of these green synthesized AgNPs (Cl-1–AgNPs) showed

selective sensing of the potentially hazardous Hg2+ ion in water at micromolar concentrations. More

importantly, green synthesized Cl-1–AgNPs sensor systems detected Hg2+ ions in water in a wide pH

range (3.2 to 8.5).

1 Introduction

The unique optical and electronic properties of nanostructuresof noble metals usually in the range of 1–100 nm has madethem potential candidates for various applications in modernscience and technology.1 Particularly silver nanoparticles(AgNPs) are of great interest both in fundamental research andindustrial applications. AgNPs by virtue of their versatility andunique characteristics such as low toxicity, biocompatibility,antimicrobial activity, optical and electrochemical propertieshave been extensively explored for various applications.2–4

Optical and electronic properties of AgNPs that are stronglydependent on the size,5 shape,6 capping layer7 and nature ofself-assembled nanostructures8 has spurred remarkableresearch efforts to develop several synthetic protocols to fabri-cate monodispersed AgNPs with controllable shape and crys-talline state.4 The wet chemical reduction of silver ions in thepresence of a stabilizer by reducing agents was the mostpreferred synthetic route until recently. However, potentialbiological and environmental problems caused by hazardouschemicals used in the wet chemical approach9–12 resulted inthe development of green nanotechnology that utilized

RA University, Thanjavur-613401, Tamil

a.edu; Fax: +91 4362264120; Tel: +91

(ESI) available: Digital images of citricstudies and tape water sensor studies20e

7

naturally occurring macromolecules such as plant extracts13

and microorganisms14 in synthesising and controlling the size,shape and assembly of the nanomaterials.15,16

Biosynthesis of NPs particularly using plant extracts whichact as reducing as well as capping agents have received greaterattention in recent years over microorganisms due to the ease ofhandling and cost effective nature compared to the elaborateprocess of maintaining cell cultures.17 Au and AgNPs were rstsynthesised using living plants by Jose-Yacaman et al.18 followedby several research groups using various plant extractsincluding hibiscus,19 black tea,20 Aloe vera,21 camphor22 leafextracts, neem leaf broth,23 and Indian gooseberry fruitextract.24 Coffee and green tea extracts have also been used tosynthesise stable NPs with various size and morphology.25 Thesodium salt of citric acid is one of the most extensively usedcapping and reducing agents in the synthesis of Ag and AuNPswith tunable size and morphology; although the role of citrateions in the growth of anisotropic nanostructures is still thesubject of investigation.26,27 Unsurprisingly, citrus extracts fromlemons and sweet and bitter orange plants have been exploredin the green synthesis of Au and AgNPs.28 Rapid reduction,assembly and sintering of ‘liquid-like’ spherical NPs lead to theformation of Au nanotriangles by lemon grass extracts.29

Of late there has been a great interest in the fabrication ofcolorimetric sensors for selective and sensitive detection ofheavy metal ions because of their simplicity, economical oper-ation and on-site environmental monitoring.30 Particularlynoble metal NPs have drawn signicant attention in recent

This journal is ª The Royal Society of Chemistry 2013

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years owing to their unique optical properties, excellentstability, good biocompatibility and water solubility.31 Unlikeorganic dyes, the metal NP colorimetric systems are photostableand do not undergo photobleaching.32 For example AgNPsappended with different chemical functionalities have beenused as analytes for Ni2+, Co2+, Hg2+, Pb2+ and Cu2+.33 Naturalbifunctional gallic acid based Ag and AuNPs have shownselective colorimetric detection of Pb2+.34 Azadirachta indicaextract produced AgNPs functionalized with Rh6G dye exhibitedselective uorescent ‘turn-on’ for Cu2+.35 Nevertheless, greensynthesised AgNPs from plant extracts that offer eco-friendlysystems which also possesses potential metal ion bindingchemical units such as phenol and heteroaromatic units havenever been explored in detail for metal ion colorimetric sensingexcept a recent report by Farhadia et al.36

Herein, we report the green synthesis of Ag and AuNPs usingcitrus fruit extracts (Cl-1 and Cl-2) and studied the effect ofsunlight exposure and electrochemical reduction potentialdifferences of silver and gold ions in the NPs formation. Thetime dependent studies revealed a much improved reactiontime (2 h) in the formation of NPs by this green approach. Theselective colorimetric sensing of hazardous Hg2+ pollutantusing Cl-1–AgNPs at a wide pH range (3.2–8.5) has beenexplored for the rst time to our knowledge. The selectivesensing of Hg2+ at wide pH range indicates that the green syn-thesised AgNPs might serve as potential sensor systems forheavy metal ions at different physiological conditions.

2 Materials and methods

Fresh lemon and sweet-lime were purchased from a fruit shopin Thanjavur, India. AgNO3 and HAuCl4 was obtained fromSigma-Aldrich and used as received. Milli-Q water was used forall the experiments. All the heavy metal salt solutions (1 mM)used for the experiments were prepared by mixing the requisiteamount of salt in Milli-Q water.

2.1 Preparation of Cl-1 and Cl-2 extract

Fresh lemon skin was peeled-off, washed with Milli-Q water anddried. Then it was squeezed to produce 2 gm of juice that wasdissolved in 10mL of water. The diluted lemon juice was lteredusing Whatman lter paper. A similar procedure was used toprepare sweet-lime extract.

2.2 Synthesis of Ag and AuNPs

3 mL of citrus fruit extract was added to 10�3 M AgNO3 aqueoussolution (1, 2 and 3 mL) with stirring at room temperature. Cl-1extract did not produce any colour change with AgNO3 solutioneven aer keeping for more than 24 h at room temperature. Thecolourless silver nitrate solution was converted into an intensebrownish-yellow colour upon exposure to sunlight. It is notedthat room light as well as heating did not have any signicantinuence on the AgNPs formation. Similarly, addition of Cl-1(3 mL) into HAuCl4 (10�3 M, 1 mL) solution at room tempera-ture under stirring produced Cl-1–AuNPs with a purple colourimmediately. Ag and AuNPs with Cl-2 were prepared by


following a similar procedure and the effect of sunlight was alsostudied. It is noted that each experiment was repeated threetimes to conrm the reproducibility.

2.3 Characterization

The quantitative measurements of the synthesized NPs wereanalysed in a Perkin-Elmer model UV-vis double beam spec-trophotometer from 250 to 800 nm, at the resolution of 1 nm.The bioreduction of silver or gold ions in aqueous solution as afunction of time was monitored by periodic sampling ofaliquots (0.2 mL) of the suspension, then diluting the sampleswith 2 mL deionized water and subsequently measuring theUV-vis spectra of the resulting diluents.

Ag and AuNPs morphology and sizes were analyzed usingeld emission scanning electron microscopy (FE-SEM) (JSM-6701F, JEOL Japan INC) with an accelerating voltage of 30 kVand lament current of 20 mA for 45 seconds. The samples arestuck onto a double-face conducting carbon tape mounted on abrass stub. The NP samples were coated with platinum which issputtered at a current of 20 mA for 45 seconds. The size,morphology and crystallinity of both Cl-1–NPs were also inves-tigated using HR-TEM (High Resolution Transmission ElectronMicroscopy). Samples for TEMmeasurements were prepared byplacing a drop of NP solution on the graphite grid and drying itin vacuum. Transmission electron micrographs were takenusing JEOL JEM-2100F operated at an accelerated voltage of200 kV and an ultra high-resolution pole piece. Powder X-raydiffraction measurements were carried out by using a Brukerdiffractometer (XRD-Bruker D8 Advance XRD) with Cu Karadiation.

The puried powders of Cl-1 and Cl-1–AgNPs were subjectedto FTIR spectroscopy measurements. These measurementswere carried out on a Perkin-Elmer Spectrum-One instrument inthe diffuse reectance mode at a resolution of 4 cm�1 in KBrpellets.

3 Result and discussion

Green synthesis of AgNPs using plant or fruit extract is usuallyobtained by mixing a silver nitrate solution with plant extractsthat generally produce yellow to intense brown colour whenvarying reaction time and temperature.13–25 Citrus limon plantextracts such as leaf extracts produced triangular shaped Aunanoplates upon mixing with a gold solution.29 Surprisingly,addition of a 1 mM solution of AgNO3 into citrus limon (Cl-1)juice extract (1 : 1, 1 : 2 and 1 : 3, VAgNO3

/Vextract) did not produceany signicant colour change and only resulted in the forma-tion of a very pale-pink coloured solution (Fig. 1). UV-Visibleabsorption spectra showed weak absorption peak (lmax) at470 nm and the pale-pink colour of the solution was stablefor more than ve days in ambient conditions (Fig. 2a).Even heating (30 min at 60 �C) could not improve/changethe pale-pink colour. It was observed that Cl-1 did not showany absorption in the visible range. However upon exposureto sunlight, the pale-pink Cl-1–AgNO3 solution turned imme-diately into a brownish yellow colour which was further

Analyst, 2013, 138, 4370–4377 | 4371

Fig. 1 Digital images of Cl-1–AgNPs (a and a1), Cl-1–AuNPs (b and b1), Cl-2–AgNPs (c and c1) and Cl-2–AuNPs (d and d1) before and after sunlight exposure.

Fig. 2 UV-Vis absorption spectra of Ag and AuNPs prepared from Cl-1 and Cl-2extracts.

Fig. 3 UV-Vis absorption spectra of Ag and AuNPs with different concentrationsof Cl-1 (a) and Cl-2 (b).

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intensied as the exposure time increased, which conrmed theformation of AgNPs (Fig. 1). The absorption studies revealed asurface plasmon resonance absorption peak (lmax) at 441 nm(Fig. 2a). It is a well known phenomenon that AgNPs exhibitvarious colours depending on the size and morphology. Thesecolours are observed due to excitation of surface plasmonresonance (SPR) in the metal nanoparticles.37 The 441 nm SPRband is indicative of spherical AgNPs formation in aqueoussolutions. Cl-1–AgNPs prepared from a higher volume of citruslimon juice with constant volume of AgNO3 showed a small blueshi in SPR absorption (Fig. 3a); a fact that could be attributedto the availability of more capping molecules at higherconcentrations. Time dependent absorption studies of AgNPs

4372 | Analyst, 2013, 138, 4370–4377

formation with Cl-1 clearly revealed colour intensication withtime, in the presence of sunlight and the reaction wascompleted within 2 h (Fig. 4a). This is a signicant improve-ment in the biosynthesis of AgNPs since, most of the reportedbiosynthetic approaches required much longer time forcompletion of AgNPs formation. Camellia extract producedstable AgNPs from a silver aqueous solution within 4 h, whereasAloe Vera extract in the presence of ammonia, which enhancedthe formation of the soluble silver complex, yielded stableAgNPs only aer 24 h. Microorganisms such as bacteria38 andfungi39 usually requires 24 h to 120 h to reduce silver ions intostable NPs.

Prathna et al.40 have recently reported the synthesis of AgNPsusing Cl-1. But AgNPs formation was observed only at higher


Fig. 4 Time dependent UV-Vis studies of AgNPs formation from (a) Cl-1 and (b)Cl-2.

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concentration of silver ions (10�2 M) and longer reaction times(4 h). But AgNPs formation was not observed at a lowerconcentration of silver ions (10�3 M). The absorption studiesshowed unusually broad SPR absorption spectra without asharp peak. This is quite unusual for AgNPs because SPR bandsof metal nanoparticles especially Ag and AuNPs are known toexhibit very strong and sharp absorption spectra. The broad andweak SPR absorption spectra might be due to incompletereduction of silver ions.

Interestingly, mixing Cl-1 with an aqueous solution ofHAuCl4 (10�3 M) converted the yellow solution into a purplecolour and conrmed the formation of AuNPs at roomtemperature immediately without exposure to sunlight.Sunlight exposure further enhanced the SPR absorption inten-sity with a little blue shi in the peak position (Fig. 1 and 2b).This reduction variation could be attributed to the electro-chemical differences between Ag+ and Au+ ions. The standardelectrochemical reduction potential for Ag is +0.80 V (Ag+ + e� ¼Ag) whereas Au is +1.50 V (Au+ + e� ¼ Au) and according to theelectrochemical series, the higher the reduction potential thefaster it undergoes reduction. Hence gold ions were easilyreduced to AuNPs whereas the lower reduction potential ofsilver ions required stronger reducing agents/conditions andhence, Cl-1 required sunlight to trigger the electron transfer toAg+ ions. However, citrus limon peel and leaf extracts instanta-neously reduced silver ions into AgNPs at room temperaturewithout the need of sunlight.28,29


To get the insight of metal ion reduction by Cl-1, we havecarried out controlled experiments using citric and ascorbicacid, the major chemical species of lemon juice. Though bothare weak reducing agents, citric and ascorbic acids are knownto reduce gold ions into AuNPs with and without stabilizingagents at room temperature.41 The controlled synthesis carriedout with commercial citric and ascorbic acid (individually or incombination) with AgNO3 did not produce the expectedAgNPs, rather they gave white colloids (Fig. S1†). However,addition of ascorbic acid to AgNO3 (10�3 M) solution in thepresence of poly(vinylalcohol) (PVA) and poly(styrenesulfonatesodium salt) (PSS) surfactant immediately produced a yellowcolour that indicates the formation of AgNPs (Fig. S2†).Althoughmixing of ascorbic acid to AgNO3–citrate solution didnot show an immediate colour change, sunlight exposureshowed a light yellow colour of AgNPs (Fig. S3†). Citrate saltalone has been used as a reducing, stabilizing and structuredirecting agent in synthesis of silver nanostructures,26,27

however, the reduction reaction takes place only at highertemperature. These experiments clearly indicate that ascorbicand citric acid alone in Cl-1 are not involved in the reduction ofsilver ions. Lemon juice is a rich source of minerals, citric andascorbic acid42 along with a number of phytonutrients such asavonones—hesperidin, eriocitrin and phenolic compoundsin the form of glycosides—quercetin and umbelliferone.43

Some avonoids have been investigated for their ability toreduce iron and copper ions.44 Therefore it can be concludedthat other bio-organics present in the lemon juice along withascorbic and citric acid synergistically play a role in theformation of the AgNPs.

Cl-2 which is another fruit with a high amount of citric andascorbic acid was explored to reduce both silver and gold ionsinto the corresponding Ag and AuNPs in a similar fashion(Fig. 1 and 2). Unlike with Cl-1, both silver and gold ions werereduced instantaneously into the corresponding Ag and AuNPsby Cl-2 at room temperature without exposure to sunlight(Fig. 1 and 2). Sunlight exposure on Cl-2–AgNPs reactionmixture increased SPR absorption intensity of AgNPs with asmall red shi in the absorption peak (Fig. 2). The 432 nmAgNPs SPR absorption was red shied to 441 nm upon expo-sure to sunlight. The change of Cl-2 volume ratio with respectto Ag+ ions did not show any corresponding signicant changein the SPR absorption (Fig. 3). Similarly exposure of Cl-2–AuNPs to sunlight did not exert a signicant effect, except for asmall blue shi of absorption maximum. Comparison ofAgNPs SPR absorption obtained from Cl-1 and Cl-2 did notshow any signicant change, however, AuNPs synthesizedfrom Cl-2 showed a 20 nm red shi in SPR absorptioncompared with Cl-1. Though it is difficult to quantify thereason for different reducing ability of Cl-1 and Cl-2, it couldbe due to the presence other chemicals that contribute tothe reduction process. Cl-2 contains a good amount ofsugar compared to Cl-1 and that might be the possiblechemical contributing to the reduction of silver ions withoutsunlight. Time dependent absorption studies of Cl-2–AgNPsshowed that the NPs formation was completed within 90 min(Fig. 4b).

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Fig. 6 FE-SEM images of Cl-2–AgNPs (a) and Cl-2–AuNPs (b).

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The synthesized Ag and AuNPs size and morphology werecharacterized by using FE-SEM and HR-TEM. Fig. 5a and 5bclearly demonstrates that Cl-1 requires sunlight exposure toreduce silver ions into AgNPs. Whereas Cl-1 reduces gold ionsinto AuNPs without sunlight exposure and exposure ofsunlight light did not show any signicant effect on the NPsformation except for a small reduction in size (Fig. 5c and 5d).Ag and AuNPs of Cl-2 showed spherical morphology for bothNPs (Fig. 6). HR-TEM studies further conrmed the poly-dispersed spherical crystalline NPs formation both in Cl-1–AgNPs and Cl-1–AuNPs (Fig. 7). The size of the particlesobtained were in the range of 20–35 nm (AgNPs) and 10–25 nm (AuNPs). The crystalline nature of the AgNPs wasconrmed from the X-ray diffraction analysis (Fig. 8a). Thediffraction peaks at 38.6� and 45.2� correspond to the (1 1 1)and (2 0 0) facets of the face centered cubic crystal structure(JCPDS, le no. 04-0783). Prominent IR bands (Fig. 8b)observed at 3312, 2924, 2853, 1730, 1653, 1539 and 1438 cm�1

conrm the presence of citric acid, ascorbic acid and sugars inthe extract that act as the reducing as well as the stabilizingagents.

Hg2+ is one of the most hazardous metal pollutants, and iswidely distributed in water, soil, and even food.45 Exposure tomercury even at very low concentrations would result in diges-tive, kidney and especially neurological disorders.46 Varioussensor systems for the detection of Hg2+, based on uo-rophores,47 organic compounds48 and nanoparticles and nano-rods49 have been developed. However, green synthesized AgNPsthat serve as eco-friendly systems have rarely been explored fortheir metal ion sensing abilities. Cl-1–AgNPs were evaluated as acolorimetric sensor for various metal ions by adding a uniformconcentration (0.5 mL, 10�3 M) of metal salt solutions into afreshly prepared AgNPs solution (2 mL) that exhibited selectivedecolourising for Hg2+ addition (Fig. 9). Addition of metal ions(Cu2+, Co3+, Cd2+, Mg2+, Fe3+, Cr3+, Zn2+, Ca2+ and Ni2+) intoCl-1–AgNPs blue shied the SPR absorption peak with broad-ening except Pb2+ which caused a red shi in the absorption

Fig. 5 FE-SEM images of Cl-1–AgNPs (a and b) and Cl-1–AuNPs (c and d). (a andc) before (b and d) after sunlight exposure.

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peak (Fig. 10a). Interestingly, the addition of Hg2+ solution intoCl-1–AgNPs decolourised the solution from dark yellow anddemonstrated the selective sensing of Hg2+ ions. The minimumdetectable concentration of Hg2+ ions in aqueous solution(visible observation of colour change) was determined by add-ing different concentrations of mercury ions into Cl-1–AgNPsand monitoring the absorbance change (Fig. 10b and c). Initialaddition of Hg2+ ions (20 mL of 10�4 M) slightly blue shied thelmax with reduction of absorption intensity and during subse-quent additions, the absorption peak completely disappeared(180 mL). The mechanism of Hg2+ sensing by Cl-1–AgNPs couldbe explained based on the electrochemical differences of Ag+

and Hg2+ ions. The standard reduction potential for Ag is +0.80V (Ag+ + e� ¼ Ag) whereas for Hg2+ it is +0.92 V (2Hg2+ + 2e� ¼2Hg2+) and according to the electrochemical series, metals witha higher reduction potential act as better oxidising agents. Theoxidation of AgNPs due to electrochemical reduction potentialdifferences was further conrmed by adding higher reductionpotential metal ions such as K2Cr2O7 (+1.33 V) and KMnO4

(+1.51 V, 10�4 M, 0.5 ml) with Cl-1–AgNPs that exhibitedimmediate decolourisation of dark yellow colour (Fig. S4†). It isnoted that addition of a Mn2+ (reduction potential is �1.18 V)solution to Cl-1–AgNPs did not show any colour change(Fig. S4†). It was observed that Cl-2–AgNPs also selectivelydecolourise upon addition of Hg2+. Colorimetric sensingstudies of Cl-1–AuNPs with different metal ions did not showany signicant change including the addition of Hg2+ (Fig. S5†).Similarly, K2Cr2O7 and KMnO4 also did not oxidize Au since thereduction potential of gold is +1.50 V (Au3+ + 3e� ¼ Au) which is

Fig. 7 HR-TEM images of (a) Cl-1–AgNPs and (b) Cl-1–AuNPs. Inset shows highresolution images of Ag and AuNPs.


Fig. 8 PXRD pattern of Cl-1–AgNPs (a) and IR spectra of Cl-1 and Cl-1–AgNPs (b).

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higher or equal to the reduction potential of Hg2+, K2Cr2O7 andKMnO4 (Fig. S5†).

Sensor systems that detect metal ions in wide pH range arehighly desirable since they might serve as a good analyte tosense metal ions in water at different physiological conditions.The good stability of Cl-1–AgNPs at a wide pH range allowed usto explore its sensing capability of Hg2+ at higher pH. Cl-1–AgNPs solution reported in the present study shows a pH of3.2. From our above mentioned results it was clear that at a pH

Fig. 9 Cl-1–AgNPs selective sensing of Hg2+ in aqueous solution.


of 3.2 the green synthesized Cl-1–AgNPs selectively detectedHg2+ ions (Fig. 10a). To test its efficacy as a Hg2+ ion sensor athigher range, the pH, of Cl-1–AgNPs was increased to 8.5 by theaddition of dilute NaOH solution and its suitability as metalion sensor was investigated. Increasing the pH of Cl-1–AgNPssolution led to a blue shi in SPR absorption spectra with peakbroadening (lmax ¼ 410 nm). Addition of different metal ionsolutions (Cu2+, Co3+, Cd2+, Mg2+, Pb2+, Cr3+, Zn2+, Ca2+ andNi2+) did not show any signicant change in the absorptionexcept a small blue shi of lmax ¼ 400 nm (Fig. 10d). Fe3+

addition strongly blue shied the absorption to 375 nm.Similar to its effect at pH 3.2, addition of Hg2+ ions at pH of8.5, completely decolourised the yellow colour of Cl-1–AgNPswhich conrmed the selective sensing of Hg2+ ions at a pH of8.5. When tested with various metal ions at a pH higher than8.5, Cl-1–AgNPs did not exhibit good colorimetric sensorproperties.

In order to understand the role of citric acid in the metalion sensing properties, commercial citric acid was used as astabilizer in the synthesis of AgNPs using NaBH4 as a reducingagent that produced very light yellow coloured solution ofAgNPs. Absorption studies showed a weak peak at 405 nmwhich disappeared upon addition of Hg2+ ions. Nevertheless,the colour change was not distinguishable visibly due to a veryweak coloured solution (Fig. S6†). At higher pH (8.5), the citricacid of Cl-1–AgNPs will be a sodium salt of the citrate ions. In acontrol experiment, commercially available trisodium citratestabilized AgNPs were synthesized using NaBH4 and investi-gated for metal ion sensing properties. The colorimetricstudies exhibited no selective sensing towards any metal ions(Fig. S7†). These results indicate that selective Hg2+ sensing ofCl-1–AgNPs at lower as well as higher pH is due to the syner-gistic effect of different chemical species present in the juiceextract and not due to citric acid–citrate alone. We also foundthat Cl-1–AgNPs selectively detected Hg2+ in tap water(Fig. S8†). It is noted that the addition of tap water as well asdifferent metal ions dissolved in tap water into Cl-1–AgNPs didnot show any signicant change in colour except when Hg2+

was added which selectively caused a decolourisation of thebrownish-yellow colour.

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Fig. 10 UV-Vis spectra of Cl-1–AgNPs with different metal cations (a) pH ¼ 3.2,(d) pH ¼ 8.5, (b) change of Cl-1–AgNPs absorption vs. Hg2+ concentration and (c)change of absorbance vs. Hg2+ concentration.

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4 Conclusions

In the present study, we demonstrated a faster greener synthesisof AgNPs using citrus limon and citrus limetta fruit juice andshowed that the chemical species variations even in thiscommon family of plant extracts can have a different reducingability of silver ions. Both extracts produced crystalline Ag andAuNPs. Importantly the green synthesized Cl-1–AgNPs exhibitedselective Hg2+ sensing between 3.2 and 8.5 pH. Our resultsindicated that apart from its usual antibacterial properties,the green synthesized AgNPs can be effectively employed for

4376 | Analyst, 2013, 138, 4370–4377

selective sensing of environmentally hazardous metal ions at awide pH range. Development of a selective sensor system forother metal ions such as Pb2+, Fe3+, Zn2+ and Cd2+ based ongreen synthesized NPs are in progress.

Acknowledgements

Financial supports from Department of Science and Tech-nology, New Delhi, India (DST Fast Track Scheme no. SR/FT/CS-03/2011(G)) is acknowledged with gratitude.

Notes and references

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