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Analytica Chimica Acta 820 (2014) 139–145

Contents lists available at ScienceDirect

Analytica Chimica Acta

journa l homepage: www.e lsev ier .com/ locate /aca

ncorporation of the fluoride induced Si O bond cleavage andunctionalized gold nanoparticle aggregation into one colorimetricrobe for highly specific and sensitive detection of fluoride

ie-Fang Sun, Rui Liu, Zhong-Mian Zhang, Jing-Fu Liu ∗

tate Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O.ox 2871, Beijing 100085, China

i g h l i g h t s

We report a novel colorimetric AuNPprobe for detecting F− in environ-mental waters.Most of the materials used in thismethod are cheap and available com-mercially.The sensing of the probe rely on bothhydrogen bond and specific chemicalreaction.The probe is able to accurately dis-criminate F− from a wide range ofions.This study provides a new thought todevelop superior AuNP probes.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 13 November 2013eceived in revised form 8 February 2014ccepted 18 February 2014vailable online 27 February 2014

eywords:

a b s t r a c t

A highly selective and sensitive probe was developed for the field test of F− in environmental waters.The probe was fabricated by anchoring 4-mercaptopyridine (MPD) on AuNPs via Au–S interaction toform MPD-AuNPs, and further assembling 3-aminopropyltrimethoxysilane (APTMS) on the surface ofMPD-AuNPs. The hydrolysis and cross-link of APTMS resulted in a thin monolayer of Si–O–Si protectinglayer to encapsulated MPD-AuNPs. In the assay, F− reacted with Si O bond and thus destroyed the outerprotecting layer of the probe, and further triggered the aggregation of internal MPD-AuNPs by forming

−

old nanoparticlesluorideolorimetric probepecific chemical reactionydrogen bonding

N H F hydrogen bond. The F induced aggregation of functionalized AuNPs gave rise to significantsolution color switch from red to blue, which facilitated visual assay of F− in the range of 1.0–7.0 �g mL−1

by naked eyes. The probe is able to discriminate F− from a wide range of environmentally dominant ions,thus it can be applied to detect F− in drinkable water with satisfactory results that is agreed well withthat of using ion chromatography.

© 2014 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +86 10 62849819; fax: +86 10 62849192.E-mail address: jfliu@rcees.ac.cn (R. Liu).

ttp://dx.doi.org/10.1016/j.aca.2014.02.026003-2670/© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Fluoride (F−) is the smallest anion with high charge density and

hard Lewis basic nature that result in unusual chemical propertiesrelevant to its many healthy and environmental issues. Fluoridedeficiency causes osteoporosis and poor dental health [1]. However,a number of epidemiologic studies speculated the unintentionally

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hronic overexposure to F− account for many medical issues includ-ng fluorosis and osteosarcoma [2–4]. To avoid the adverse healthroblems caused by the uptake of F− from water, governmentuthority, for example, the United States Environmental Protectiongency (USEPA) set a standard of 1 �g mL−1 to protect against oste-fluorosis and dental fluorosis, whereas 2 �g mL−1 is considereds health-risk and 4 �g mL−1 as the maximum contaminant levelMCL) [5]. Unfortunately, many people are still drinking water con-aining F− above the health-risk level [6], especially for those ruralnd remote areas, suggesting that it is of significance to developonvenient and economic assay for F−.

Determination of F− can be carried out using techniques like ionhromatography or capillary zone electrophoresis [7–9]. Althoughost of these methods show high sensitivity, they have seldom

een used in field analysis because of their non-portability, strictequirements for laboratory facilities and technical operators. Actu-lly, ion-selective electrode is the main method for on-field F−

etermination, but its low accuracy call for new improved testethods to monitor drinking water quality.Up to now, spectroscopic sensors, especially these colorimet-

ic ones, have gained great interest due to the quick feedback andacile quantification of signal. Colorimetric chemo-sensors, whichllow one to rationally design specific anion-responsive recep-ors with agreeable or even remarkable sensitivity, have beenidely exploited for F− detection [10–12]. In general, these sen-

ors were assigned to two categories. One class of sensors wereevised to incorporate receptors capable of forming hydrogenonding with the anionic guest F− [13–15], in which the hydrogen-onded receptors rely on either adjacent chromophore units oreprotonation followed by electron delocalization to display a col-rimetric response. Considering the bond energy, hydrogen bondHB) which belongs to weak intermolecular force scope is suscep-ible to be interfered by co-exist anions. The other class of sensorsas designed to incorporate special receptors of electron-deficient

ewis acidic centers such as boron or silicon [16–19]. In these probe,− as a ‘hard base’, could strongly interact with the center atom,hus converting the recognition event into a detectable opticalignal. However, these sensors commonly suffered from compli-

ated probe design, multistep synthesis strategy and consumptionf large amount organic agents. More unfavorable factor is that theajority of these sensors are unstable or dysfunction in aqueous

nvironment. Following these line of thought, quantum dots (QDs),

cheme 1. (A) Cartoon representation for the fabrication of MPD-AuNP@APTMS probuNP@APTMS.

Acta 820 (2014) 139–145

with fluorescence resonance energy transfer (FRET) initiated by thebreakage of hydrogen bond or Lewis acid–base reaction, have beenproposed as nano-probes for F− [20,21]. However, fluorescenceprobe still require instrument for readout.

In comparison to the maturely developed organic optical sen-sors, colorimetric assay based on gold nanoparticles (AuNPs) haverecently emerged as a fascinating research field [22–25]. Variousmetal ions were detected by monitoring the color changes occur-ring upon aggregation of ligand-functionalized AuNPs throughspecific molecular interactions [26–30]. However, studies on colori-metric anion recognition and sensing anions by AuNPs are limited[31–36]. Up to now, the thioglucose modified AuNPs had beendeveloped for colorimetric detection of F− based on interparticlecrosslink induced aggregation [37]. However, several issues in thismethod still have room to improve in term of the complication andover-consumption for thioglucose synthesis, besides its sensingrelied on hydrogen-bond alone is susceptible to interference.

We report herein our study on developing a selective and sen-sitive AuNP probe for spot testing F− in environmental waterswith naked eyes. It is well-known that pyridine group is aneffective hydrogen bonding donor, which is often used in chemo-sensors for anions detection [38–42]. In addition, F− inducedSi O bond cleavage reaction had also been well documentedin the chemosensors to selectively target F− due to the highlyspecial affinity between fluorine and silicon [43–45]. Based onthe previous research, 4-mercaptopyridine (MPD) was firstlymodified on AuNPs (15 nm) through ligand exchange, and then 3-aminopropyltrimethoxysilane (APTMS) was allowed to assembleon the MPD-AuNPs surface through the strong gold–amino bind-ing. The hydrolysis and condensation of surface siloxane moietiesresulted in a monolayer of Si–O–Si to encapsulated MPD-AuNPs.Thus, the achieved MPD-AuNP@APTMS probe selectively recog-nized F− through the following two sequent events (Scheme 1):F− reacted with Si O bond and destroyed the outer APTMS pro-tecting layer of the probe, which further triggered the aggregationof internal MPD-AuNPs via the formation N H F HB. The aggre-gation of interior MPD-AuNPs gave rise to significant color switchfrom red to blue and therefore facilitated visual assay of F−. The as-

fabricated AuNP probe improved its specificity efficiently throughsimultaneously integrating the F− induced HB interaction and thespecific Si O bond cleavage reaction into one probe in a simplemanner. In addition, most of the materials used here are cheap and

e. (B) Proposed mechanism for the F− induced colorimetric response of MPD-

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vailable commercially, making it particularly easy-to-use in theeveloped world and impoverished areas.

. Experimental

.1. Materials

Chloroauric acid (HAuCl4), lipoic acid (LA), 4-mercaptopyrodineMPD), and 3-aminopropyltrimethoxysilane (APTMS) werebtained from Alfa-Aesar. Other chemicals were purchased fromeijing Chemical Reagent Company. All the chemicals used weref analytical grade or the highest purity available. Ultrapure water18.3 M�) produced with a Milli-Q Gradient system (Millipore,edford) was used throughout the experiment. All the solutions

or testing the interferences of anions were prepared by dissolvinghe corresponding sodium salts in pure water.

.2. Apparatus

TEM images were obtained through a JEOL model 1200EXnstrument operated at an accelerating voltage of 120 kV. Thelasma absorption of AuNPs was recorded on a double-beam Du-00 UV–vis spectrophotometer (Beckman). The Zeta potential ofhe AuNPs was measured using a dynamic light scattering systemMalvern Nano ZS instrument with a 633 nm laser diode). Raman

easurements were conducted with a Renishaw InVia Raman spec-rometer at an excitation laser of 785 nm. Nicolet 6700 Fourierransform infrared spectrometer (FT-IR, Thermo Scientific, USA)as also used to characterize AuNPs.

.3. Procedures

.3.1. Preparation of MPD-AuNPsUniform 15 nm AuNPs was firstly prepared through citrate-

ediated reduction of HAuCl4 in aqueous solutions [46]. Then,nto 200 mL of the obtained citrate-AuNPs (12 nM, pH 11.0) wasdded 1.0 mL of LA ethanol solution (0.1 M) and stirred overnight12 h) to prepare the LA capped AuNPs (LA-AuNPs) [47]. To purifyhe LA-AuNPs, the solution was centrifuged for 20 min at 15700 gnd decantated by supernatants, and the resulting AuNPs were re-uspended in water. The purified LA-AuNPs (15 nM, 200 mL) wasdjusted to pH 11.0 with NaOH and mixed with the ethanol solu-ion of MPD (1.0 mL, 0.1 M). After stirring for 24 h, the mixtureas centrifuged at 15700 g for 20 min followed by decantation of

upernatants to remove the excess MPD, and the precipitates wereedispersed in water for further use. The concentration of AuNPsan be calculated according to Beer’s law using an extinction coef-cient of 2.8 × 108 M−1 cm−1 at 520 nm for the 15 nm AuNPs [48].

.3.2. Fabrication of MPD-AuNP@APTMSTo encapsulate MPD-AuNPs with APTMS layer, freshly prepared

.0 mM APTMS ethanol solution was added into 100 mL of concen-rated MPD-AuNPs solution (20 nM). After reacting for 1 h, 1.0 mLH3·H2O (25%) was introduced into the mixture and kept con-

tant stirring for an additional 24 h for completing APTMS assemble.he thickness of APTMS layer was modulated by adding differentmount of APTMS to get a final concentration of 1, 5, 10, and 15 �M,espectively. Finally, the APTMS coated MPD-AuNPs (denoted asPD-AuNP@APTMS) was purified by centrifugation and washingith anhydrous ethanol. The precipitates were redispersed in water

or future use.

.3.3. Determination of F− in water samplesA tap water sample from our laboratory and 3 ground water

amples collected from Suozhou city in Shanxi province were usedo evaluate the applicability of the developed MPD-AuNP@APTMS

Acta 820 (2014) 139–145 141

probe. For masking some interfering cations, certain amount ofEDTA disodium salt was added to water samples with the finalconcentration of 20 mM. In a typical process, 100 �L of the concen-trated MPD-AuNP@APTMS (100 nM) was mixed with 3.0 mL watersample which was pre-adjusted to pH 6.0. After reacting for 4 h, thecolorimetric measurement was performed.

3. Results and discussions

3.1. Synthesis and characterization of MPD-AuNPs andMPD-AuNP@APTMS probe

Since direct modification of the citrate-AuNPs with MPDinduced irreversible aggregation, a two-step ligand exchangeapproach was adopted to prepare the MPD-AuNPs [47], i.e., thechloride and citrate physically adsorbed on AuNPs were at firstdisplaced by LA, which was further partly replaced by MPD. Theresidual of negatively charged LA on the prepared MPD-AuNPsprovided strong electrostatic repulsion to keep the MPD-AuNPs sta-bility. The estimated average loading of the MPD was 680 ± 70 unitsper particle (please see details in Supporting Information).

Virtually APTMS is well used to form hydrated silica mono-layer bonded to various materials and nanoparticles, and it showsgreat success in protecting surface characteristics [49–52]. In thismethod, APTMS was allowed to adsorb on the MPD-AuNP sur-face by its amine group while the silanol groups were believedto be pointing into solution. This binding process was driven bythe strong complexation capacity between gold and amine. Thesurface-bonded siloxane moieties hydrolyzed to silane triols withinminutes, and then condensation of silane triols occurred using thesilanol groups as anchor points, which promoted the formationof a thin and relatively homogeneous silica capping layer aroundMPD-AuNPs (MPD-AuNP@APTMS) [49].

The prepared MPD-AuNPs and MPD-AuNP@APTMS probesas well as their precursor citrate-AuNPs and LA-AuNPs werecharacterized by TEM, UV/Vis spectroscopy, surface enhancedRaman scattering (SERS) and FT-IR spectroscopy, respectively. TEMimages (Fig. 1A) confirmed that all of them are well-dispersedand have diameters of ∼15 nm. Because the thin APTMS con-densation layer on AuNPs could not be distinguished in TEMimages, MPD-AuNP@APTMS showed the same average diameterof 15 nm as its precursor. However, hydrodynamic diameters rep-resented increase from ∼18 nm for MPD-AuNPs to ∼22 nm forMPD-AuNP@APTMS (Table S1). The surface plasma peak displayedslightly red-shift from 520 nm (citrate-AuNPs) to 522 nm (MPD-AuNPs), and that of MPD-AuNP@APTMS further red-shifted to525 nm, which was due to the increase of surrounding refractiveindex after ligand-replacement and surface modification (Fig. 1B).The successful coating of MPD and further APTMS capping onthe AuNPs was demonstrated by SERS and FT-IR which are capa-ble of providing detailed structure information of the adsorbedorganic molecules on noble metal nanoparticles or clusters. Boththe citrate and LA molecules showed weak signals, which wasblamed for their inherent small Raman cross section, while both ofthe MPD-AuNPs and MPD-AuNP@APTMS gave similar strong SERSsignals referring to the MPD molecules (Fig. 1C). The SERS spectraof MPD ligand exhibited several prominent characteristic peaks,which were dominated by the bands such as the Au S stretch-ing mode (262 cm−1), the ring breathings vibration (1004 cm−1),the C S stretching mode (1090 cm−1), the C H in-plane-bendingstretch (1210 cm−1), and the ring-stretch mode (1574 cm−1). But

for the MPD-AuNP@APTMS, the characteristic SERS signal of APTMSwas not observed in the range of 400–3250 cm−1 (Fig. S1). Thisresult might be attributed to the fact that APTMS, which possessesinherent small Raman cross section, was not in close proximity to

142 J.-F. Sun et al. / Analytica Chimica Acta 820 (2014) 139–145

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AuNP@APTMS from 22 nm to 303 nm before and after addition ofF−. Preliminary experiment indicated that the MPD-AuNP@APTMSprobe showed nearly no response to SO4

2− and PO43−, suggest-

ing that the probe was promise for selective detecting F−. Detailed

ig. 1. TEM images (A), UV–vis absorbance (B), SERS (C) and FT-IR spectra (D) of diffenformation.

nner AuNPs but capped outside of MPD-AuNPs. Therefore, FT-IRpectra was used to verify the formation of silica monolayer. Ase expected, the MPD-AuNP@APTMS showed a distinct band at

042 cm−1 that was assigned to the Si O Si bond (Fig. 1D). It isoteworthy that the APTMS capping layer resulted in the differencef zeta potential from −8.2 mV (MPD-AuNPs) to –12.4 mV (MPD-uNP@APTMS) at pH 7.3, which was due to the ionization of thePTMS (their isoelectric point is near pH 3). We have characterizedifferent kinds of AuNPs achieved from five independent prepa-ation. It was demonstrated that the fabricating process for thePD-AuNP@APTMS showed good batch-to-batch reproducibility

Fig. S1 and Table S1).

.2. Response of the MPD-AuNPs to F−

Comparing with other anions, F− is the special one that pos-esses the maximal value of electronegativity and the minimaladius, so it reflects a distinct nucleophilic effect. In aqueous solu-ion, the MPD-AuNPs aggregated quickly in the presence of F−

hrough HB interaction, namely the terminal pyridine group ofPD formed N H F− HB, which resulted in the increase of the

atio of absorbance at 625 nm to that at 525 nm (A625/A525) forhe MPD-AuNPs. Since pyridine group must be protonated to formhe N H F− HB, the solution pH would greatly influence theesponse of MPD-AuNPs to F− [53,54]. At pH ≥ 9.0 (Fig. S2), theurface of MPD-AuNPs accommodated negative charges to providetrong Coulomb repulsion among nanoparticle. Under this condi-ion most of the pyridine groups were deprotonated, so the HBere inhibited. At pH < 9.0, the A625/A525 value gradually grownith the decrease of pH, indicating the protonated pyridine moiety

tarted to protonate and associated with F− through the forma-ion of HB, thus cross-linking among the MPD-AuNPs occurred.owever, at pH < 7.0, the MPD-AuNPs showed aggregation even

n the absence of F−, which was induced by the N H N HB medi-

ted self cross-linking among the individual MPD-AuNPs. Althoughhe MPD-AuNPs showed high sensitive detection of F− at pH 7.0,ts selectivity was poor. The blue bar in Fig. 2 demonstrated thatxcepting for F−, the MPD-AuNPs also responded to some other

odified AuNPs. The SERS experiment conditions have been described in Supporting

inorganic ions like SO42− and PO4

3− which are widespread innatural water with high contents. This result proved that the MPD-AuNPs probe containing only HB recognition motif was not enoughfor selective detection of F−.

3.3. Response of the MPD-AuNP@APTMS probe to F−

Upon the addition of F−, the MPD-AuNP@APTMS probe showedsignificant aggregation, thus changed the solution color from winered to blue. This F− induced aggregation was further confirmed byTEM images of the MPD-AuNP@APTMS and UV–vis spectra (Fig. 3),as well as by increase of the hydrodynamic diameter of the MPD-

Fig. 2. Selectivity of MPD-AuNPs and MPD-AuNP@APTMS (3.7 nM) in the presenceof various anions (0.35 mM).

J.-F. Sun et al. / Analytica Chimica

Fig. 3. (A) TEM images of the MPD-AuNP@APTMS (3.7 nM) before and after additionoda

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anions in aqueous solutions. SO4 caused less color change forMPD-AuNP@APTMS probe than that for MPD-AuNPs, demonstrat-ing that the proposed method was quite specific to F−. Interferencesfrom various coexisted cations were also evaluated (Fig. S6). It

f F− (7.0 �g mL−1). (B) Absorbance response of the MPD-AuNP@APTMS (3.7 nM) toifferent mount of F− (the concentration is 0, 1.0, 2.0, 3.0, 4.0, 5.0, 7.0 �g mL−1 fromto g). These results were all collected after reacting for 4 h under pH 6.0.

tudies on the performance of the MPD-AuNP@APTMS probeere conducted following with the optimized detection conditions

Fig. 2).

.4. Optimization of detection conditions for F− by thePD-AuNP@APTMS

Fig. 1D show that the characteristic FT-IR band of the Si O Siond (1042 cm−1) in the MPD-AuNP@APTMS became attenuatefter adding F−, which suggested that the presence of F− couldestroy the protecting APTMS condensation layer thus made it pos-ible for the internal MPD ligand to interact with F−. Since theggregation of MPD-AuNP@APTMS probe could only occur afterhe dissolution of the outer APTMS layer by reacting with F−, its of great necessity to optimize the coating thickness to improvehe method selectivity and sensitivity. The coating thickness wasontrolled by adding different concentration of APTMS during theynthesis of MPD-AuNP@APTMS. As shown in Fig. S3, the increasef APTMS concentration reduced the probe response (A625/A525) tooth F− and SO4

2−, and the largest difference between F− and SO42−

ccurred when 5.0 �M APTMS was used. Although 10 �M APTMSould greatly eliminated the response to SO4

2−, it also gave rise toow sensitivity to F−. Consequently, 5.0 �M APTMS was adoptedn the subsequent experiments as a compromise of sensitivity andelectivity.

Given the sensing is dependent on F− induced the MPD-AuNPsggregation after its reaction with APTMS layer, the optimumetection pH for the MPD-AuNP@APTMS probe would differ fromhat for the MPD-AuNPs, thus it is important to study the influ-

nce of pH condition. Experiments showed that the respond ofPD-AuNP@APTMS for F− never happens at pH > 8.0, which was

ttributed to the low sensitivity of MPD-AuNPs to F−. Fig. S2 showshat the response of MPD-AuNP@APTMS probe for F− increased

Acta 820 (2014) 139–145 143

with the decrease of pH value in the range of 5.0–8.0. In thesame pH range, the MPD-AuNP@APTMS could keep the colloidstability, because the surface APTMS layer was protonated andprovided Coulomb repulsion. However, at pH ≤ 5.0, the aggrega-tion of MPD-AuNP@APTMS was observed even in the absence ofF−. This phenomenon was ascribed to two aspects: one was thestatic charge reduction for MPD-AuNP@APTMS with the decreaseof pH, the other was the N H N HB mediated cross-linking, whichwas blamed for the incomplete shielding of interior MPD ligandby APTMS layer. To avoid the high background value of A625/A525,pH 6.0 was adopted as the optimum pH for F− assay by the MPD-AuNP@APTMS.

The influence of incubation time was also studied under aboveoptimized conditions. At room temperature, the A625/A525 valuegrown gradually with the increase of time and ultimately reached toa plateau after 4 h (Fig. S4). Unlike the MPD-AuNPs that respondedto F− immediately through forming HB, the relatively long responsetime of the MPD-AuNP@APTMS was attributed to slow dissolutionof the APTMS condensation layer reacted by F−, in which multi-ple Si O bonds had to be broken. Here, the APTMS layer had atwo-side effect, one important role was as protecting layer againstinterference, but it also resulted in signal response delay.

The stability of both MPD-AuNPs and MPD-AuNP@APTMS dis-persing in ultrapure water have been investigated. As shown inFig. S5, the MPD-AuNPs could not keep long term stability becausethey tended to form N H N HB. However, the MPD-AuNP@APTMSshowed improved stability compared with the MPD-AuNPs, whichwas ascribed blocking of the formation of intermolecular HBbetween MPD ligands by the APTMS capping layer. At the sametime the surface APTMS layer (with a isoelectric point near pH 3)could be ionized, thus providing Coulomb repulsion among them.

3.5. Analytical performance

The performance of the proposed method was evaluated basedon the selectivity, sensitivity and reproducibility by using thestandard ion solutions. The yellow bar in Fig. 2 showed the respec-tive response of the MPD-AuNP@APTMS probe to each of common

2−

Fig. 4. Colorimetric detection of F− in aqueous solution. (A) Photograph of the cor-responding MPD-AuNP@APTMS solutions under the different F− concentrations.(B) Plot of A625/A525 of MPD-AuNP@APTMS as a function of F− concentration atpH 6.0 with the error bars representing the standard deviations of three parallelmeasurements.

144 J.-F. Sun et al. / Analytica Chimica Acta 820 (2014) 139–145

Table 1Determination of F− in water samples by the proposed method (MPD-AuNP@APTMS) and ion chromatography (IC).

Sample Spiked (�g mL−1) MPD-AuNP@APTMS IC (�g mL−1)

Detected (�g mL−1) Recovery (%)

Tap water 0 – – 0.090

Groundwater 1 0 Nda – 0.2983.0 3.07 102 –

Groundwater 2 0 1.02 – 0.9533.0 3.98 98.7 –

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as demonstrated that they had negligible effect for this assayxcept Al3+ and Fe3+ which could form stable complexes with F−

hus imposed negative deviation for this assay. However, due tohe contents of these ions are low (<1 �M) in drinkable water,he interferences from them were negligible. High concentrationf coexisted Ca2+ (>0.2 mM) and Mg2+ (>0.2 mM) also interferedith the detection of F−, possibly due to the bridging effects.owever, this interference can be eliminated by adding 20 mMDTA into the sample solutions. Fig. 4 showed the good linearelationship between A625/A525 and F− concentration (CF− ) in theange of 0.5–7.0 �g mL−1, with a linear regression equation of625/A525 = 0.086*CF− + 0.303, which was constructed with blankap water matrix spiked with different mount of F−. The calculatedetection limit (S/N = 3) was 0.45 �g mL−1. The detection limit withaked eyes was 1.0 �g mL−1 F−. The precision was determined byesting five standard solutions with 5.0 �g mL−1 F−. Similar color-hange was observed by naked eyes, and the resulted A625/A525alues gave a relative standard deviation (RSD) of 4.2% by the spec-rophotometer, demonstrating the satisfactory reproducibility ofhe proposed method.

.6. Real sample analysis

The applicability of the proposed method was evaluated byhe detection of F− in three groundwater samples. In order toliminate the interference of Ca2+and Mg2+, EDTA disodium saltas pre-added into the water samples to gave a final concen-

ration of 20 mM. Visual test with the proposed method showedhat the F− concentration was below 1 �g mL−1 in one ground-ater samples, and between 1 and 2 �g mL−1 in the other two

roundwater samples. These samples were further analyzed usingspectrophotometer based on the equation of linear regression

bove mentioned, as well as by ion chromatography (IC), respec-ively. It is shown that the F− contents determined by the proposed

ethod agreed with those values determined by IC (Table 1), andere all around the value evaluated by visual test with naked eyes.

he reliability of the proposed probe was further demonstratedy the good recoveries (98–102%) for all the tested samples at.0 �g mL−1 spiked level. Based on above results, it can be con-luded that the proposed probe is applicable for field screening andccurate determination of F− in real environmental water samplesy naked eyes, as well as spectrophotometer.

. Conclusions

A highly selective and sensitive colorimetric F− probe was fab-icated by incorporation of the specific F− induced Si O bond

leavage reaction, and the formation of intermolecular HB. Itas found that the color switch of the novel MPD-AuNP@APTMSrobe could not be triggered by common inorganic anions otherhan F−which could specifically destroy the APTMS condensation

[

– 1.8198.3 –

layer and induced the colorimetric response of internal MPD-AuNPs to F−. The proposed method offered a detection range of1.0–7.0 �g mL−1 with naked eyes, this capacity meet the drink-ing water quality control range (1–4 �g mL−1) defined by EPA andWHO. This work also provides a new route to develop AuNP probesthat functionalized through highly specific chemical reactions.

Acknowledgments

This work was supported by the National Basic Research Pro-gram of China (2011CB936001), the National Natural ScienceFoundation of China (21207142, 21025729) and the ResearchDevice Development Project of Chinese Academy of Sciences(YZ201147).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.aca.2013.12.001.

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