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Sensors and Actuators B 181 (2013) 288– 293

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o me pa ge: www.elsev ier .com/ locate /snb

new rapid colorimetric detection method of Mn2+ based on tripolyphosphateodified silver nanoparticles

ue-Xia Gaoa,b, Jun-Wei Xina, Zhe-Yu Shena, Wei Panb, Xing Lib, Ai-Guo Wua,∗

Key Laboratory of Magnetic Materials and Devices, and Division of Functional Materials and Nanodevices, Ningbo Institute of Materials Technology and Engineering,hinese Academy of Sciences, Ningbo 315201, ChinaThe School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China

r t i c l e i n f o

rticle history:eceived 11 November 2012eceived in revised form 24 January 2013ccepted 28 January 2013vailable online xxx

eywords:n2+

apid colorimetric detection method3O10

5−–AgNPs

a b s t r a c t

A new facile and rapid colorimetric method is proposed for detection of manganese ion (Mn2+)with high sensitivity and excellent selectivity using tripolyphosphate stabilized silver nanoparticles(P3O10

5−–AgNPs). The silver nanoparticles (AgNPs) were prepared by reducing silver nitrate with sodiumborohydride in the presence of sodium tripolyphosphate. Energy dispersive X-ray spectrum (EDS) andinfrared spectra (IR) indicate that tripolyphosphates anions (P3O10

5−) are capped on the surfaces of AgNPs.The P3O10

5−–AgNPs aggregate quickly in the presence of Mn2+, which leads to color change of the nanopar-ticle dispersion from yellow to reddish brown and decrease of the surface plasmon absorption intensity.The Mn2+ can form a six-coordinated structure with P3O10

5− coating on the AgNPs and leads to formationof large particles aggregation. The selectivity of P3O10

5−–AgNPs detection system for Mn2+ is excellent2+ 2+

queous solution

inear detectioncomparing with other ions and mixture of the above ions except Mn . Furthermore, this Mn detectionmethod based on surface plasmon resonance (SPR) absorption by ultraviolet and visible spectropho-tometer (UV–vis) offers a wide linear detection range from 0.05 �M to 20 �M. The lowest detectionconcentration by the naked eyes is 0.1 �M, which is also lower than the national table-water standard(1.8 �M). The results of Mn2+ detection in real water samples reinforce that our P3O10

5−–AgNPs detectionsystem is applicable for Mn2+ detection in the real environmental water samples.

. Introduction

Manganese ion (Mn2+) is one of the essential microelements inuman body, which plays an important role in the bone and otherissues formations, normal reproductive functions, carbohydratend lipid metabolism under the normal condition. Mn2+ deficiencyn human body is related to delayed blood coagulation and hyper-holesterolaemia. However, uptake of high concentration of Mn2+ isarmful. Signs of toxicity were manifested in the form of severe psy-hiatric abnormalities, including hyperirritability, violent acts andallucinations. As the diseases progressed, there was a permanentrippling neurological disorder of the extrapyramidal system withorphological lesions similar to those of Parkinson disease [1]. It isell known that Mn2+ can exist in water stably compared to other

alence states of manganese. Therefore, much more attention haseen attracted to develop methods for detecting Mn2+ at low con-

entrations in samples (such as foodstuffs, waters and beverages)ith high sensitivity and excellent selectivity.

∗ Corresponding author. Tel.: +86 574 86685039; fax: +86 574 86685163.E-mail address: aiguo@nimte.ac.cn (A.-G. Wu).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.01.079

© 2013 Elsevier B.V. All rights reserved.

In recent years, various methods have been reported for Mn2+

detection, including atomic absorption spectrometry (AAS) [2],inductively coupled plasma mass spectroscopy (ICP-MS) [3],and inductively coupled plasma-atomic emission spectrometry(ICP-AES) [4]. However, these methods need costly instrumentsor complicated procedures, making them inconvenient and time-consuming. Moreover, many dynamic methods have been reportedfor Mn2+ determination based on its catalytic effect on the oxida-tion of organic compounds. Although some of these methods haveextremely low detection limitation, most of them lack sufficientsensitivity for detecting Mn2+ and are cockamamie or uneconom-ical [5–8]. Recently, many colorimetric assay methods have beenreported for detection of heavy metal ions based on gold or silvernanoparticles due to their high extinction coefficient and strongsurface plasmon resonance properties [9–12]. Large numbers ofcolorimetric sensors have been established for detection of metalions (Co2+ [13,14], Hg2+ [15–17], Cr3+ [18,19], Cu2+ [20], Pb2+

[21], and so on), proteins [22], oligonucleotides [23,24], organicmolecules [25] and biological applications [26,27]. A small number

of researchers have been starting to explore Mn2+ detectionmethod based on gold or silver nanoparticles [28]. However, someof them used organic reagents as the functional selective reagents,which are instability and easily oxidized. In this work, we propose

Y.-X. Gao et al. / Sensors and Actuators B 181 (2013) 288– 293 289

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ahspapdsttsfmfsPFa

2

2

(Cwc

2

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Scheme 1. Schematic illustration on the aggregati

new facile and rapid colorimetric detection method of Mn2+ withigh sensitivity and excellent selectivity using tripolyphosphatetabilized silver nanoparticles (P3O10

5−–AgNPs). The sodiumolyphosphate is inorganic reagent which is stability, non-toxicnd biocompatible. The P3O10

5−–AgNPs aggregate quickly in theresence of Mn2+, which leads to color change of the nanoparticleispersion from yellow to reddish brown and decrease of theurface plasmon absorption intensity. As shown in Scheme 1,he Mn2+ is attracted to the surrounding of P3O10

5−–AgNPs viahe electrostatic interaction and then can form a six-coordinatedtructure [29] with P3O10

5− coating on the AgNPs, which leads toormation of large particles aggregation. Due to the aggregation

echanism, this P3O105−–AgNPs detection system is very sensitive

or Mn2+ detection. Furthermore, the selectivity of this detectionystem of Mn2+ is excellent comparing with other ions, such asb2+, Al3+, Ba2+, K+, Ca2+, Zn2+, Cr6+, Na+, Cr3+, Co2+, Cu2+, Cd2+,e3+, Ni2+, Mg2+, Hg2+, Cl−, NO3

−, SO42−, PO4

3−, and mixture of thebove ions except Mn2+.

. Experimental

.1. Materials

Silver nitrate (AgNO3 ≥ 99.8%) and sodium borohydrideNaBH4 ≥ 96.0%) were obtained from Sinopharm Chemical Reagento. Ltd. (Shanghai, China). Sodium tripolyphosphate (Na5P3O10)as purchased from Shanghai Aladdin Reagent Co. Ltd. All of the

hemicals were used as received without further purification.

.2. Methods

Transmission electron microscopy (TEM) images and the energyispersive X-ray spectra (EDS) data were recorded from a Tecnai20 instrument and were operated at 200 kV. Infrared spectra (IR)ata were obtained on Nicolet 6700 Fourier-Transform Infrared

pectrometer. Surface plasmon resonance (SPR) absorption dataf ultraviolet and visible spectrophotometer (UV–vis) were per-ormed using a Lambda 950 instrument. Inductively coupledlasma atomic emission spectroscopy (ICP-AES) was performed

chanism of the P3O105−–AgNPs induced by Mn2+.

with an Optima 2100 instrument from Perkin Elmer. MilliQ waterwas used for all experiments.

2.3. Preparation of AgNPs coated with P3O105−

AgNPs coated with P3O105− were first prepared by reducing

AgNO3 with NaBH4 in the presence of Na5P3O10. All the glasswarewas thoroughly washed with aqua regia (HCl/HNO3 (v/v) = 3:1) andrinsed with Milli-Q water. In this work, 1 mL of 20 mM AgNO3aqueous solution was rapidly added into 100 mL of 1.0–5.0 mMNa5P3O10 aqueous solution. 1.0 mL of 0.1 M NaBH4 solution (freshlyprepared) was then slowly injected into the mixture with vigorousstirring. The reaction was kept for 5 min, and then the color of theP3O10

5−–AgNPs dispersion turned into a vivid yellow. Finally, theprepared P3O10

5−–AgNPs dispersion was stored at room tempera-ture until use.

2.4. Detection of Mn2+

For detection of Mn2+ using P3O105−–AgNPs dispersion, 50 �L of

Mn2+ aqueous solutions with different concentrations were respec-tively added into a mixture of 900 �L P3O10

5−–AgNPs dispersionand 50 �L of 0.1 M NaOH solution (pH ≈ 10.8), then the total volumeof the mixture was 1 mL. Finally, the mixtures were maintained atroom temperature for several minutes, and then the color changedfrom yellow to reddish brown.

For detection of Mn2+ in real water samples (lake water andtap water), different concentrations of Mn2+ was added into realwater samples, which was called standard addition method anddetected by our Mn2+ detection system. Typically, the lake watersamples and the tap water samples were used directly without fur-ther purification. Before detection, the lake water samples and thetap water samples were detected by ICP-AES, and then added dif-ferent concentrations of Mn2+. Finally these water samples were

also detected by ICP-AES. According to this method, 50 �L of Mn2+

aqueous solutions with different concentrations were respectivelyadded into a mixture of 900 �L P3O10

5−–AgNPs dispersion and50 �L of 0.1 M NaOH solution, the total volume of the mixture

2 Actuators B 181 (2013) 288– 293

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90 Y.-X. Gao et al. / Sensors and

as 1 mL. The Mn2+ aqueous solutions were prepared using MilliQater, Lake water and Tap water, respectively.

. Results and discussion

.1. Characterization of P3O105−–AgNPs without Mn2+

From Fig. 1(a), it can be observed that the size of P3O105−–AgNPs

s around 10 nm and the nanoparticles disperse well without aggre-ation. The EDS result (Fig. 1(b)) indicates that P3O10

5− is coatedn the surface of AgNPs due to the existence of P and O elements.

5−

he SPR absorption of the P3O10 –AgNPs was measured by aV–vis spectrophotometer and its maximum absorption is locatedt 390 nm. Fig. 2 shows IR spectra of Na5P3O10 and P3O10

5−–AgNPs.he strong peaks at 888 cm−1 and 1162 cm−1 are ascribed to P O

ig. 1. (a) TEM image of P3O105−–AgNPs and (b) EDS spectrum of the prepared

3O105−–AgNPs.

Fig. 2. IR spectra of Na5P3O10 and P3O105−–AgNPs.

Fig. 3. (a) The SPR absorption of P3O105−–AgNPs without or with 10 �M of Mn2+;

(b) TEM image of P3O105−–AgNPs with 10 �M of Mn2+; and (c) EDS spectrum of

P3O105−–AgNPs with 10 �M of Mn2+.

stretches in Na5P3O10. When Na5P3O10 are coated onto the sur-face of Ag NPs, the above two peaks are observed at 901 cm−1 and1125 cm−1.

3.2. Characterization of P3O105−–AgNPs with Mn2+

Fig. 3(a) shows the SPR absorption of P3O105−–AgNPs with-

out/with 10 �M of Mn2+. It can be found that the SPR absorptiondecreased sharply associated with introducing of 10 �M of Mn2+.Fig. 3(b) shows TEM image of P3O10

5−–AgNPs with 10 �M ofMn2+. Comparing with that of P3O10

5−–AgNPs without Mn2+

(Fig. 1), we can find that the P3O105−–AgNPs aggregated signifi-

cantly after introducing of Mn2+. Fig. 3(c) shows EDS spectrum ofP3O10

5−–AgNPs with 10 �M of Mn2+. This result indicates that Mn2+

cannot be washed out from P3O105−–AgNPs due to the existence of

Mn element in the EDS spectrum. Therefore, we can conclude thatthe P3O10

5−–AgNPs are stable due to the electrostatic repulsion,but aggregate in the presence of Mn2+.

3.3. Optimization of the P3O105−–AgNPs preparation and Mn2+

detection

Na5P3O10 concentration and pH value of the P3O105−–AgNPs

dispersion significantly influence the effect of Mn2+ detection(Figs. S1 and S2 in the Supporting Information). The sensitivity ofour detection system benefits from high Na5P3O10 concentration.However, a higher concentration [30] above 5 mM results in salt-ing out of the P3O10

5−–AgNPs colloid. Therefore, the subsequentexperiments were performed at 4 mM of Na5P3O10 concentration.Moreover, the P3O10

5−–AgNPs dispersion has a low sensitivity toMn2+ at a pH below 10.5 (Fig. S2). However, the interference to theMn2+ detection from other ions is obvious at a pH higher than 11.5.Therefore, the subsequent experiments were performed at pH 10.8.

3.4. Selectivity and sensitivity for Mn2+

In order to test the selectivity of the proposed detection method,10 �M of various ions (Pb2+, Al3+, Ba2+, K+, Ca2+, Zn2+, Cr6+, Na+,

Y.-X. Gao et al. / Sensors and Actuators B 181 (2013) 288– 293 291

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CPda(ipfiiMstcCSaht[Tts

ovPttbn

Ff

ig. 4. (a) Photographic image and (b) UV–vis spectra of P3O105−–AgNPs dispersion

xcept Mn2+ and Mn+1 means the mixture of all above ions including Mn2+ (every io

r3+, Co2+, Cu2+, Cd2+, Fe3+, Ni2+, Mg2+, Hg2+, Cl−, NO3−, SO4

2− orO4

3−), Mn+ and Mn+1 were respectively added into P3O105−–AgNPs

ispersion. Mn+ means the mixture of all above ions except Mn2+

nd Mn+1 means the mixture of all above ions including Mn2+

every ion’s concentration is 10 �M). Fig. 4 shows photographicmage and UV–vis spectra of P3O10

5−–AgNPs dispersion in theresence of different metal ions (10 �M). From Fig. 4(a), we cannd that the color of the P3O10

5−–AgNPs dispersion with Mn2+

s quite different with that of P3O105−–AgNPs dispersions without

n2+. Moreover, Fig. 4(b) shows that the SPR absorption decreasedharply associated with introducing of Mn2+. Therefore, the selec-ivity of P3O10

5−–AgNPs detection system for Mn2+ is excellentomparing with other ions including Pb2+, Al3+, Ba2+, K+, Ca2+, Zn2+,r6+, Na+, Cr3+, Co2+, Cu2+, Cd2+, Fe3+, Ni2+, Mg2+, Hg2+, Cl−, NO3

−,O4

2−, PO43− and Mn+. The excellent selectivity may result from the

ggregation of P3O105−–AgNPs triggered by Mn2+. The extremely

igh selectivity for Mn2+ ascribes to the formation of the coordina-ion compounds by Mn2+ with P3O10

5− under the condition of alkali29,31], which results in the aggregation of silver nanoparticles.herefore, these results indicate that our P3O10

5−–AgNPs detec-ion system might be suitable for detecting Mn2+ in environmentalamples.

Under the optimized experimental conditions, the sensitivityf our detection system to Mn2+ was recorded by the naked eyesia the color change of the solution. Fig. 5 shows the image of the3O10

5−–AgNPs dispersions containing different Mn2+ concentra-ions. With decreasing of the Mn2+ concentration from 20 to 0.1 �M,

he color of the P3O10

5−–AgNPs dispersion changes from reddishrown to deep yellow, which could be obviously observed by theaked eyes. Therefore, the detection limit of Mn2+ by the naked

ig. 5. Image of P3O105−–AgNPs dispersions containing various Mn2+ concentrations

rom 20 to 0.1 �M.

presence of different metal ions (10 �M). Mn+ means the mixture of all above ionsncentration is 10 �M).

eyes is about 0.1 �M, which is much lower than the national table-water standard (0.1 mg/L ≈ 1.8 �M). Furthermore, UV–vis spectraof the P3O10

5−–AgNPs dispersions with various Mn2+ concentra-tions from 0.05 �M to 20 �M are recorded for quantitative assay.As shown in Fig. 6(a), the SPR absorptions gradually decrease withincreasing of Mn2+ concentration. Fig. 6(b) shows plot of A0/Aof different P3O10

5−–AgNPs dispersions versus Mn2+ concentra-tions ranging from 20 to 0.05 �M (A0 and A are the absorbance ofthe P3O10

5−–AgNPs dispersions at 390 nm without or with Mn2+,respectively). From Fig. 6(b) we can find a good linear relationship(R2 = 0.9957) between A0/A and Mn2+ concentrations from 0.05 �Mto 20 �M compared to other similar method [28]. Therefore, theseresults reinforce that the new facile and rapid colorimetric methodfor Mn2+ detection is achieved by exploring P3O10

5−–AgNPs dis-persion as Mn2+ detection system.

3.5. Detection of real samples

In order to confirm the practical application of our Mn2+ detec-tion system, different concentrations of Mn2+ were added intoreal water samples (lake water and tap water) which was calledstandard addition method and detected by our Mn2+ detectionsystem using Fig. 6(b) as the calibration curve according to mostrelative publications [20,28,32,33]. The Mn2+ concentration addedin the real water samples is from 15 to 0.5 �M. The observed resultsusing the calibration curve and the ICP detection results are given inTable 1. We can find that the observed Mn2+ concentrations are very

close to those added. Moreover, the color change of the nanoparti-cle dispersions for detecting of real water samples is in agreementwith that of the control group (Fig. S3 in Supporting Information).

Table 1Determination of Mn2+ in real water samples.

Samples Mn2+added(�M)

Mn2+ observed (�M)(mean ± E, n = 3)

ICP-AESobserved (�M)

Lake water 0.5 0.54 ± 0.067 0.541 0.86 ± 0.041 1.05

10 10.24 ± 0.135 10.0915 14.65 ± 0.876 15.10

Tap water 0.5 0.49 ± 0.038 0.531 0.85 ± 0.038 1.04

10 9.24 ± 0.218 10.115 13.52 ± 1.109 15.27

292 Y.-X. Gao et al. / Sensors and Actuators B 181 (2013) 288– 293

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ig. 6. (a) UV–vis absorption spectra of P3O105−–AgNPs dispersions containing diffe

n2+ concentration ranging from 20 to 0.05 �M. A0 and A are the absorbance (at 39

onsequently, our P3O105−–AgNPs detection system is applicable

or Mn2+ detection in the real environmental water samples.

. Conclusions

In this study, a new facile and rapid colorimetric method forn2+ detection has been developed by using tripolyphosphate

unctionalized silver nanoparticles (P3O105−–AgNPs). The experi-

ental results show that only Mn2+ can trigger the aggregationf P3O10

5−–AgNPs resulting in an obvious color change comparingith other ions. For Mn2+ detection, the optimal Na5P3O10 con-

entration and pH of the P3O105−–AgNPs dispersion are 4 mM and

0.8, respectively. The selectivity of P3O105−–AgNPs detection sys-

em for Mn2+ is excellent comparing with other ions, such as Pb2+,l3+, Ba2+, K+, Ca2+, Zn2+, Cr6+, Na+, Cr3+, Co2+, Cu2+, Cd2+, Fe3+, Ni2+,g2+, Hg2+, Cl−, NO3

−, SO42−, PO4

3−, and mixture of the above ionsxcept Mn2+. Furthermore, this Mn2+ detection method based onV–vis offers a wide linear detection range from 0.05 �M to 20 �M.he lowest detection concentration by the naked eyes is about.1 �M, which is also lower than the national table-water standard∼1.8 �M). The results of Mn2+ detection in real water samples rein-orce that our P3O10

5−–AgNPs detection system is applicable forn2+ detection in the real environmental water samples.

cknowledgements

This work was supported by the Program of National High-Techrogram (863 Program No. SS2012AA063202), the program of Zhe-iang Provincial Natural Science Foundation of China under Granto. R5110230; Hundred Talents and Academy-Locality Coopera-

ion Programs of Chinese Academy of Sciences, and Ningbo Sciencend Technology Bureau (Grants Nos. 2011A610140, 2011A610121,nd 2009B21005). And the Projects Sponsored by the Scientificesearch Foundation for the Returned Overseas Chinese Scholars,tates of Ministry of Human Resources & Social Security and Min-stry of Education.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2013.01.079.

[

n2+ concentrations; (b) plot of A0/A of different P3O105−–AgNPs dispersions versus

of the P3O105−–AgNPs dispersions without or with Mn2+, respectively.

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Biographies

Yue-Xia Gao received her bachelor degree in 2011, and now is a postgraduateof 2011 in Ningbo University. She is receiving united training in Ningbo Instituteof Materials Technology and Engineering, Chinese Academy of Sciences currently.Gao’s research interests include the preparation of the precious metal nanoparticlesand approach for the detection of the heavy metal ions.

Jun-Wei Xin has received his master degree of Ocean University of China in 2012.He was ever received training in Ningbo Institute of Materials Technology and Engi-neering, Chinese Academy of Sciences. His research interest was the detection of theheavy metal ions by colorimetric method.

Zhe-Yu Shen is an associate professor in Ningbo Institute of Materials Technologyand Engineering, Chinese Academy of Sciences since 2012 and engages in biomate-rials and nanomaterials study.

Wei Pan received his bachelor degree in 2011, and now is a postgraduate of 2011 inNingbo University. Pan’s current research interests lie in the areas of conducting andmagnetic molecular solids, crystal engineering, and main-group chemistry focusingon tetrathiafulvalene derivatives and dithiolene complexes.

Xing Li is an associate professor and postgraduate tutor in Ningbo University andengages in synthesis and properties of metal organic light-emitting materials study.

Ai-Guo Wu is a professor and postgraduate tutor in Ningbo Institute of Mate-rials Technology and Engineering, Chinese Academy of Sciences from 2009 andengages in the applications of various nanomaterials in biomedicine, bioanaylti-cal chemistry/environmental sciences and nanostructures fabricated based onbiomacromolecules study.