Selective and sensitive colorimetric sensor of mercury (II) based on gold nanoparticles

Sensors and Actuators B 196(2014)106–111Contents lists available at ScienceDirectSensors and Actuators B:Chemicalj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s nbSelective and sensitive colorimetric sensor of mercury (II)based on gold nanoparticles and 4-mercaptophenylboronic acidYanli Zhou a ,Hui Dong b ,Lantao Liu a ,∗,Miaomiao Li a ,Kaixia Xiao a ,Maotian Xu a ,b ,∗aHenan Key Laboratory Cultivation Base of Nanobiological Analytical Chemistry,College of Chemistry and Chemical Engineering,Shangqiu Normal University,Shangqiu 476000,China bCollege of Chemistry and Molecular Engineering,Zhengzhou University,Zhengzhou 450001,Chinaa r t i c l ei n f oArticle history:Received 22October 2013Received in revised form 20December 2013Accepted 17January 2014Available online 8February 2014Keywords:Colorimetric sensor Gold nanoparticles Mercury ion4-Mercaptophenylboronic acid Anti-aggregationa b s t r a c tA simple and rapid colorimetric sensor for Hg 2+in aqueous solution was developed based on the observa-tion that the detection target,Hg 2+,could inhibit the aggregation of gold nanoparticles (AuNPs)induced by 4-mercaptophenylboronic acid (MPBA).Without a potent competitor,MPBA can bind to the AuNPs via Au-S bonds and induces the aggregation of AuNPs by self-dehydration condensation of boronic acid groups,resulting in a visible color change of the AuNPs solution from red to blue.In the presence of Hg 2+,the thiol group of MPBA preferentially binds to Hg 2+and thus MPBA loses the ability to induce aggregation of AuNPs,which is mechanism behind the color change of the solution from blue to red.Our results showed that the absorbance ratio (A 520/A 690)was linear with the Hg 2+concentration in the range of 0.01–5␮M (R =0.998).The detection limit of the colorimetric sensor was 8nM (S/N =3).The colori-metric changes of the sensor were extremely specific for Hg 2+and no obvious color change was observed when adding other competitive metal ions in parallel.Further promising results from Hg 2+detection in water samples indicated that our newly developed assay might open a new assay avenue for Hg 2+determination.©2014Elsevier B.V.All rights reserved.1.IntroductionMercury is one of the most toxic metals to human health and its ions (Hg 2+)are one of the most stable and widespread inorganic forms of mercury pollution.The World Health Organization (WHO)standard for the maximum allowable level of inorganic mercury is no more than 6ppb (30nM)in drinking water [1].Concerns over the safety of drinking water have prompted researchers to develop effective tools for monitoring Hg 2+in the aqueous environments.Conventional methods such as atomic absorption spectrometry [2],electrochemical analysis [3],inductively coupled plasma mass spectrometry [4],and selective cold vapor atomic fluorescence spectrometry [5],have been used to measure Hg 2+with high sen-sitivity and accuracy.However,these analytical methods demand bulky and expensive equipments,and complicated sample treat-ment,which limit their applications for rapid and on-field analysis.To reduce these drawbacks,chemical sensors based on biomateri-als [6,7],semiconductor nanocrystals [8,9],polymers [10,11],and organic small molecules [12,13]have also been developed for Hg 2+∗Corresponding author at:College of Chemistry and Chemical Engineering,Shangqiu Normal University,Shangqiu 476000,China.Tel.:+863702586802;fax:+863702586802.E-mail addresses:liult05@ (L.Liu),xumaotian@ (M.Xu).detection through the reading of the optical and electrochemical signals.So far,most of the sensors still suffer from sophisticated synthesis of the probe materials,high detection temperature,aque-ous solubility,and cross sensitivity toward other metal ions.Colorimetric sensors have drawn increasing attention due to their simplicity,high sensitivity and low cost [14].Gold nanopar-ticles (AuNPs)sensors are emerging as an important type of colorimetric assays because of visualization,ease of functionaliza-tion with organic ligands,photostability,and tunable properties [15].A well-dispersed AuNPs (10–50nm in diameter)solution exhibits a ruby red color,whereas an aggregated AuNPs solution appears purple or blue.The color change during the aggregation or dispersion of AuNPs is associated with the interparticle plas-mon coupling,which generates a significant change of absorption band in the visible region of the electromagnetic spectrum [16].In recent years,several AuNPs colorimetric assays that made possi-ble for readout by the naked eyes have been reported for the Hg 2+detection.In these colorimetric assays,AuNPs were functionalized by oligonucleotides [17–20],oligopeptides [21,22],or functional molecules [23,24].Various functional molecules can either stabilize or aggregate AuNPs through covalent binding,electrostatic effect,or physical absorption.This can be simple,convenient,and versa-tile,and thus better than using oligonucleotides and oligopeptides.The functional molecules containing thiol groups such as,mer-captan carboxylic acid [25–27],quaternary ammonium compound0925-4005/$–see front matter ©2014Elsevier B.V.All rights reserved./10.1016/j.snb.2014.01.060Y.Zhou et al./Sensors and Actuators B196(2014)106–111107[28],cysteine[29],dithioerythritol[30],dithia-diaza[31],and tri-ethylene glycol ligands[32]can bind AuNPs via an Au S bond, which make them useful for assaying Hg2+in AuNPs-based col-orimetric recognition systems.Although the thiol-modified AuNPs assays have made great contributions toward Hg2+detection,some limitations including interference with other metal ions,insuffi-cient resolution in water samples,and use of masking agents,still exist.Thus,it should be desirable to develop a reliable AuNPs col-orimetric method for Hg2+detection to solve these problems.Boronic acid and its derivatives tightly bind to substances con-taining1,2-diol moieties,making them useful in the detection of carbohydrates and biologically important neurotransmitters[33]. Tian’s group reported that AuNPs functionalized with both boronic acid compound and carboxyl compound could interact with diols and amine functional groups,respectively,in dopamine to doubly recognize dopamine[34].In addition,formation of boroxine ring by the condensation of boronic acid monomers had some click-like attributes,which was very easy to take place[35,36].The self-dehydration condensation of boronic acids and the boronic acid-diol binding have been utilized for the colorimetric visu-alization of dihydronicotinamide adenine dinucleotide based on anti-aggregation of AuNPs[37].To the best of our knowledge,col-orimetric sensor by the use of boroxine chemistry has not yet been reported for the Hg2+detection.Here,by taking advantage of the self-dehydration condensation of boronic acid and high affinity of thiolates toward Hg2+,a very simple colorimetric sensor was developed for Hg2+detection based on the4-mercaptophenylboronic acid(MPBA)and -pared to the known sensors for Hg2+,the proposed method offers several important advantages including:(1)simplicity of the use of AuNPs and commercially available MPBA as the probe without any modification,design,and complex synthesis;(2)short assay time and lab-friendly condition(room temperature,aqueous solution);(3)high sensitivity due to the unique mechanism of the assay;(4) high selectivity for Hg2+over other metal ions without using any masking agents;(5)easy and direct visualization of end result with color change;and(6)low cost due to use of simple instruments.2.Experimental2.1.Chemicals and reagentsChloroauric acid trihydrate(HAuCl4·3H2O)and sodium citrate were obtained from Sinopharm Co.(Shanghai,China),and MPBA was purchased from Sigma–Aldrich.All of the metal salts and other reagents were purchased from commercial suppliers and used without afiltration system.Water was purified by a Millipore M-Q purification system(>18M cm).The pH of solutions was adjusted with0.01M citric acid/sodium citrated buffer solution and0.01M HCl(or0.01M NaOH)solution.2.2.Synthesis of gold colloidal solutionGold colloidal solution was synthesized by the well-known citrate reduction of HAuCl4,as reported by Frens[38].A150-mL aqueous solution of HAuCl4(0.015%,m/v)was brought to a vigor-ous boil with stirring in a round-bottomflaskfitted with a reflux condenser,and then5.25mL of sodium citrate(1%,m/v)was rapidly injected into the solution.The mixed solution was further refluxed for another15min into a ruby-red suspension.The suspension was gradually cooled to room temperature under stirring continu-ously.The gold colloidal solution was stored in a refrigerator at4◦C for future use.The size of AuNPs was measured by transmission electron microscopy(TEM)and the AuNPs surface was examined by X-ray photoelectron spectroscopy(XPS)for possible molecular modification.2.3.Detection of Hg2+using the colorimetric sensorFor each measurement,a certain amount of mixed solution of MPBA(1mM)and Hg2+(1mM)was added into a1mL gold col-loidal solution in a2mL colorimetric tube at room temperature. Optimization of detection conditions for a pH value of2–11,a MPBA concentration of1–50␮M,addition order of MPBA and Hg2+,and reaction time were carried out for colorimetric measurements.Col-orimetric changes were detected by the naked eye and absorbance measurements.2.4.CharacterizationUV–vis absorption spectra of the assay solutions were recorded using an UV-2300spectrophotometer(Shimadzu,Japan)and pho-tographs for color changes of solutions were taken with a Canon Pc1262digital camera.TEM samples were prepared by dropping 20␮L gold colloidal solution onto a copper grid(3mm,300mesh) coated with carbonfilm,dried up by airing,and examined with Tecnai G220S-TWIN(FEI,America)transmission electron micro-scope.XPS samples were purified at high speed centrifugation (10,000rpm)for three times,and then20␮L of the samples were dropped onto a silicon chip and treated by overnight vacuum drying.XPS spectra were performed on an ESCALab220I-XL(VG Scientific,England)detector with a monochromatic Al␣K source (1486.6eV photon energy).3.Results and discussion3.1.Sensing mechanism of the assayThe strategy of our simple and rapid colorimetric assay for sensing Hg2+is shown in Scheme1,which is proposed based on anti-aggregation of AuNPs by the competition coordination of thiol group between the AuNPs and the aggregation agent(MPBA). The AuNPs in aqueous solution remain dispersed and the solu-tion appears ruby red because the AuNPs are capped by negatively charged citrate ions and the electrostatic repulsion in between keep them separate from aggregation.MPBA bearing boronic acid group is apt to self-dehydration condensation through forming a planar 6-membered boroxine ring with the elimination of three water molecules from three MPBA molecules[35,37].Meanwhile,the thiol group of MPBA can bind to AuNPs by Au S interaction.Accord-ingly,the AuNPs in the presence of MPBA are crosslinked and the solution color changes to blue because of the red-shifted absorp-tion band.When the mixture of MPBA and Hg2+are added,the extremely high affinity of thiolates toward Hg2+inhibits the bind-ing of thiol group with AuNPs.In this case,the MPBA does not attach to the surface of AuNPs surface and thus loses the ability of aggre-gating AuNPs.Therefore,molecular linker-based anti-aggregation offers a possible approach to Hg2+detection based on a simple and sensitive colorimetric assay.3.2.Feasibility of Hg2+detectionThe feasibility of using this assay for the colorimetric visualiza-tion of Hg2+is displayed in Fig.1.The AuNPs solution was originally ruby red and showed a maximal absorbance at520nm,which could be attributed to the surface plasmon resonance of the15nm AuNPs characterized by TEM(Fig.2a).Upon addition of MPBA into the AuNPs solution,a new strong absorbance peak appeared at690nm and the solution color clearly changed from ruby red to blue,indi-cating the aggregation of the AuNPs(Fig.2a).When MPBA and108Y.Zhou et al./Sensors and Actuators B 196(2014)106–111Scheme 1.Schematic presentation of the AuNPs colorimetric mechanism for Hg 2+detection.Fig.1.UV–vis absorbance spectra and photographic image (inset)of AuNPs solution (pH 4.0)(1),and MPBA (10␮M)-AuNPs solution in the absence (2)and presence (3)of 10␮M Hg 2+.Hg 2+were mixed in prior and added to the AuNPs solution,a single absorbance peak at about 520nm was similar to that of the original AuNPs solution,and the color of the solution remained red,sug-gesting that AuNPs were still in dispersion status (Fig.2a).Thus,the color was expected to change from blue to purple,and finallyto red with the increase of Hg 2+concentration due to change of AuNPs from an aggregation to a dispersion state.XPS was further employed to verify the molecular modification at the AuNPs surface,as shown in Fig.2b.The S 2p and B 1s spec-trum peaks of AuNPs containing MPBA were observed distinctly at the binding energy of 165and 190eV,respectively,indicating the successful binding of MPBA with AuNPs.By contrast,addition of the mixture of MPBA and Hg 2+,caused the disappearance of the above spectrum peaks,suggesting that MPBA could not bind to AuNPs.3.3.Optimization of parameters for Hg 2+detectionThe pH value of buffer and MPBA concentration were found to play important roles in the aggregation of AuNPs [16].In order to optimize assay parameters,the UV–vis absorption spectra of AuNPs solution were recorded under different pH or different MPBA con-centration.The absorption ratio of A 520/A 690has been utilized as the flocculation parameter of AuNPs solution because the numerator and the denominator represent the degree of dispersion and aggre-gation,respectively [39].As shown in Fig.S1a,the AuNPs in the solution aggregated at pH 4.0or below,whereas they remained dis-persive at higher pH (>4)due to the electrostatic repulsion involved the deprotonation of COOH groups in citric acid [16].For the MPBA-AuNPs solution,AuNPs aggregation occurred especiallyatFig.2.TEM (a)and XPS spectra (b)of AuNPs solution (pH 4.0)(1),and MPBA (10␮M)-AuNPs solution in the absence (2)and presence (3)of 10␮M Hg 2+.Y.Zhou et al./Sensors and Actuators B196(2014)106–111109Fig.3.Photographic image(a),UV–vis absorbance spectra(b),and linear plot of the A520/A690vs.Hg2+concentration(c)of the MPBA(10␮M)-AuNPs solution(pH4.0) containing various concentration of Hg2+:(1)0,(2)0.01,(3)0.05,(4)0.1,(5)0.2,(6)0.8,(7)1.2,(8)2,(9)3,(10)4,(11)5,(12)6,(13)8,and(14)10␮M.low pH.The A520/A690value of AuNPs solution in the presence of MPBA and Hg2+regained the similar value to that of the AuNPs solu-tion at different pH,and had the maximum difference with that of the MPBA-AuNPs solution at pH4.0.To obtain the maximum response,a buffer solution of pH4.0was chosen.Fig.S1b showed the absorption ratio of MPBA-AuNPs solution obtained from using different MPBA concentrations in the absence or presence of Hg2+.In the presence of Hg2+,the MPBA-AuNPs solution had a high absorption ratio when MPBA concentration was below5␮M and limited aggregation occurred when the MPBA concentration was above20␮M.However,the baseline value of the assay increased when MPBA concentration decreased because more AuNPs would keep dispersive in the presence of less MPBA.A highest ratio of signal(curve2)to background(curve1)was obtained at the MPBA concentration of10␮M.Thus,10␮M MPBA was selected for the following experiments.The effect of order of MPBA and Hg2+addition was also explored on absorption spectra of AuNPs solution(pH4.0).As shown in Fig. S1c,upon either adding MPBA or Hg2+first into the AuNPs solution, the addition destabilized the AuNPs and the solution exhibited a color change from ruby red to blue.When the MPBA and Hg2+were mixed in prior and then added,the AuNPs stayed dispersive and the red color of the solution remained.The results suggested that for Hg2+detection,the order of adding reagent in the colorimetric assay is also important.As shown in Fig.S1d,the absorbance kinetics at690nm of the colorimetric probe was examined using varying concentra-tions of Hg2+.The absorbance at690nm decreased along with the increase of the concentrations because Hg2+could inhibit the aggre-gation of AuNPs.All the kinetic traces indicated that the absorbance increase was roughly linear in the20min window due to the inter-action of MPBA with AuNPs,which caused AuNPs to aggregate.The absorbance remained stable after20min.To obtain a sensitive and consistent result,data collection was done after20min reaction in each experiment for Hg2+detection.3.4.Sensitivity of the sensorUnder the optimized conditions,absorption spectra of MPBA-AuNPs in the presence of Hg2+with different concentrations were examined for the quantitative detection of Hg2+at room tempera-ture.More interestingly,the change of Hg2+concentration could be observed by the naked-eye,as presented in Fig.3a.Upon addition of increasing concentrations of Hg2+,the color of MPBA-AuNPs solu-tion changed gradually,initially from blue to purple,andfinally to ruby red.Analogously,the absorption peak increased at520nm and decreased at690nm(Fig.3b).The ratio of A520/A690was linear with a correlation coefficient of0.998within the range of0.01–5␮M of Hg2+concentration(Fig.3c).The detection limit for Hg2+was8nM at a signal-to-noise ratio of3.Although the sensitivity of the colori-metric assay of AuNPs was not comparable to that of conventional spectroscopic methods,it was higher than that of the other colori-metric assays based on the AuNPs modified by sulfur compounds [25–32](Table S1).Our results also met the requirement of the WHO standard for the detection of the maximum allowable level of Hg2+in drinking water.3.5.Binding interaction of MPBA and Hg2+To estimate the binding constant of MPBA and Hg2+,the absorbance results in Fig.3was further analyzed based on a1:2 metal-to-ligand complex[28]:[Hg2+]=12K1[L]T1−˛˛2(1)˛=[L][L]T=A−A0A1−A0(2)where K is the binding constant of the MPBA and Hg2+,[Hg2+]is the free concentration of Hg2+,[L]is the free MPBA concentration,[L]T is the initial concentration of MPBA,˛is the ratio between[L]and[L]T, A1is the limiting ratio of A520/A690for˛=1(in the absence of Hg2+), and A0is the limiting ratio of A520/A690for˛=0(plateau).In Fig.S2,˛was given as a function of the logarithm of Hg2+concentration from the data of Fig.3c and Eq.(2).Log K=10.7was obtained,when the experimental data wasfitted to equation1by adjusting the K value[20,40].The high K value suggested that the binding of MPBA with Hg2+had high affinity in the AuNPs solution,and thus the sensor was expected to detect Hg2+with high selectivity.3.6.Selectivity of the sensorIn order to evaluate the selectivity of the colorimetric assay for Hg2+,our newly developed assay was used to detect other envi-ronmentally relevant metallic ions under identical conditions.As shown in Fig.4,it was clear that upon addition of the mixture of MPBA with each kind of metal ion,only Hg2+caused a signif-icant increase in the absorption ratio,whereas the other metal ions caused no noticeable increase of the absorption ratio after subtracting the baseline value(just addition of MPBA).Our assay was specific for Hg2+by direct visualization of the color change from blue to red,whereas the solution color unchanged for the other metal ions(inset of Fig.4).These results suggested that only110Y.Zhou et al./Sensors and Actuators B196(2014)106–111Fig.4.Absorption ratio at(A520/A690)of the MPBA(10␮M)-AuNPs solution(pH4.0) after addition of different metal ion(10␮M).Inset was photographic images of the colors.Blank was the result of MPBA-AuNPs solution.(1)was the AuNPs solution and simply used as color comparison.Hg2+could form very stable complexes with MPBA and therefore destroy the capacity of the latter to make AuNPs aggregate as com-pared with other metal ions.A similar observation that only Hg2+ could remove thiolates chemisorbed on Au surface was reported by another group[28].Thus,the obvious advantage of our probe (Table S1),with characteristics of high sensitivity and selectivity, and no need of any masking agents,can be very useful in detection for Hg2+in real samples.3.7.Analysis of Hg2+in water samplesWater samples were collected from the Chunming lake in our campus and the tap water in our lab,spiked with Hg2+at different concentration levels,and then analyzed with the proposed col-orimetric assay.The results are summarized in Table S2and the recovery rate of the spiked Hg2+was96.3–110.0%,indicating the applicability and reliability of the proposed method.4.ConclusionIn summary,a colorimetric sensor for the detection of Hg2+was developed using the MPBA and AuNPs in aqueous media,which was not only simple and cost-effective,but also rapid,sensitive and specific.The recognition mechanism of the sensor is based on the self-dehydration condensation of MPBA and the higher affinity of Hg2+toward thiolates than AuNPs.Although only detection of Hg2+is demonstrated here,the same principle of anti-aggregation can be used to develop assays for detection of other metal ions by substituting thiolates of phenylboronic acid with other functional group that will selectively bind the desired metal ions and metal nanoparticles.Furthermore,we believe that this approach has great potential for the routine detection of Hg2+in environmental sam-ples.AcknowledgmentsWe thank i(Shangqiu Normal University)for the helpful suggestions on the preparation of revised manuscript.We also thank the support from the National Natural Science Foun-dation of China(Grant Nos.21105062,21175091and21202095), Foundation for University Young Key Teacher of Henan Province (2013GGJS-162),Innovation Scientists and Technicians Troop Con-struction Projects of Henan Province,and Program for Innovative Research Team(in Science and Technology)in University of Henan Province.Appendix A.Supplementary dataSupplementary data associated with this article can be found,in the online version,at /10.1016/j.snb.2014.01.060. 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