Citrate-Capped Platinum Nanoparticle as a Smart Probe for

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Citrate-capped platinum nanoparticle as smart probe for ultrasensitive mercury sensing Gang-Wei Wu, Shao-Bin He, Huaping Peng, Hao-Hua Deng, Ai-Lin Liu, Xin-Hua Lin, Xing-hua Xia, and Wei Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac503544w • Publication Date (Web): 14 Oct 2014 Downloaded from http://pubs.acs.org on October 26, 2014

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Analytical Chemistry

Citrate-capped

platinum

nanoparticle

as

smart

probe

for

ultrasensitive mercury sensing

Gang-Wei Wu,a,c Shao-Bin He,a,b Hua-Ping Peng,a,b Hao-Hua Deng,a,b Ai-Lin Liu,a,b Xin-Hua Lin,a,b Xing-Hua Xia,d Wei Chen*a,b a

Department of Pharmaceutical Analysis, Fujian Medical University, Fuzhou 350004,

China b

Nano Medical Technology Research Institute, Fujian Medical University, Fuzhou

350004, China c

Department of Pharmacy, Fujian Provincial Hospital, Fuzhou 350001, China

d

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry

and Chemical Engineering, Nanjing University, Nanjing 210093, China. * Corresponding author. Tel./fax: +86 591 22862016. E-mail address: [email protected] (W. Chen). Gang-Wei Wu and Shao-Bin He contributed equally to this work.

1

ACS Paragon Plus Environment


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Abstract An easily prepared platinum nanoparticles (PtNPs) probe for the sensitive and selective detection of Hg2+ ions is developed here. The PtNPs with an average size of approximately 2.5 nm were prepared by a reduction method with sodium borohydride, trisodium citrate serving as reductant and stabilizer, respectively. The resulting PtNPs could catalyze the reduction of Hg2+ by surface-capping citrate. The effect of Hg2+ uptake implies amalgam formation, which leads to remarkable inhibition of the peroxidase-like activity of citrate-capped PtNPs. Based on this effect, a colorimetric mercury sensor was established through the use of citrate-capped PtNPs to catalyze the colorimetric system of 3, 3’, 5, 5’-tetramethylbenzidine (TMB) and H2O2. The high specificity of Hg-Pt interaction provides the excellent selectivity for Hg2+ over interfering metal ions. The sensitivity of this smart probe to Hg2+ is extremely excellent with a limit of detection (LOD) as low as 8.5 pM. In view of these advantages, as well as the cost-effective, minimized working steps and naked-eye observable, we expect that this colorimetric sensor will be a promising candidate for the field detection of toxic Hg2+ ions in environmental, biological and food samples.

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Peroxidase, known as ubiquitous oxidative heme-containing enzyme, can catalyze chemical reactions in various biological processes by donating electrons for binding to specific substrates.1 Due to its ability to yield chromogenic products at low concentration, peroxidase is probably the most common enzyme used in enzyme-liked immunosorbent assay (ELISA).2 It is also widely used in the determination of many compounds in combination with other enzymes in polyenzymatic systems. However, the versatile applications of native enzymes are inevitably restricted by limited natural sources, difficult and high-cost purification processes, and inherent instability.3 Therefore, more and more attentions have been paid to enzyme mimics in recent years. After the first report of ferromagnetic nanoparticles,4 a variety of inorganic nanomaterials with peroxidase-like activity came into sight, and thus enabled carboxyl-modified graphene oxide,5 gold nanoparticles,6-7 V2O5 nanowires,8 Pt/Au nanoparticles,9 CuO nanoparticles,10-11 Co3O4 nanoparticles,12 porous platinum nanoparticles on graphene oxide,13 BSA-Pt nanoparticles,14 carbon nanodots,15 and other materials to be potentially effective in biomedical and environmental detection. Mercury is one of the most hazardous metal pollutants, which is widely distributed in ambient air, water, and soil. Mercury exposure can greatly damage a series of organs and the human immune system.16 Accordingly, the United States Environmental Protection Agency (EPA) has set the maximum allowable levels of inorganic mercury in drinking water at 10 nM. It would be of great interest and challenge to develop reliable and rapid routine methodologies for the determination of Hg2+. Traditional methods for Hg2+ detection in aqueous systems, including inductively

coupled

plasma

mass

spectrometry

(ICP-MS)17

and

stripping

voltammetry,18 usually have the disadvantages of being labor-intensive and time-consuming, and the instrument requirements are complicated. On the part of

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solving these shorting-comings, various colorimetric and fluorescent mercury sensors based on noble metal (Ag and Au) nanoparticles have been developed.19-25 However, two key sensor features, selectivity and sensitivity, of many of such systems are largely determined by the choice of label molecules on the nanoparticle surface, making these systems relatively complicated and less practical for routine monitoring. In this paper, we present the findings that platinum nanoparticles could catalyze the reduction of Hg2+ by surface-capped citrate. The effect of Hg2+ uptake implies amalgam formation, which leads to remarkable inhibition of the peroxidase-like activity of citrate-capped PtNPs. Based on this effect, a colorimetric mercury sensor was established. Citrate-capped PtNPs were used to catalyze the oxidation of 3, 3’, 5, 5’-tetramethylbenzidine by H2O2. The high specificity of Hg-Pt interaction provides the excellent selectivity towards Hg2+ over other ions. Under the optimal conditions, we have obtained very low detection limit of Hg2+ (8.5 pM). This method enabled to develop a facile and fast way for Hg2+ in the tap water.

Experimental Section Apparatus. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were collected with a JEM-2100 microscope (JEOL, Japan). X-Ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250XI electron spectrometer (Thermo, USA) using monochromatic Al Kα radiation for analysis of the surface composition and chemical states of the product. The UV-visible spectra were carried out using a Shimadzu UV-2450 spectrophotometer (Shimadzu, Japan). Absorbance at 652 nm was monitored for quantitative analysis. Chemicals and Reagents. HgCl2 was purchased from TongRen Chemical Research Institute (Guizhou, China). 3, 3’, 5, 5’-tetramethylbenzidine was purchased

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from Aladdin Reagent Company (Shanghai, China). H2PtCl6·(H2O)6, trisodium citrate (Na3C6H5O7·2H2O), sodium borohydride (NaBH4), Na2HPO4, NaH2PO4·2H2O, and 30 wt% H2O2 were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Other reagents and chemicals were of at least analytical reagent grade and used without further purification. Solutions were prepared with water purified by a Milli-Q purification system (Millipore, USA). Synthesis of Citrate-Capped PtNPs. Glass wares were cleaned in aqua regia and all solutions were prepared using double distilled water. The synthesis of PtNPs was performed in a glass ware with the magnetic stir. Aqueous solutions of H2PtCl6 (1 mL, 16 mM) and trisodium citrate (1 mL, 40 mM) were mixed with 38 mL water and stirred for 30 min at room temperature. Subsequently, NaBH4 (200 µL, 50 mM) was added dropwise into the mixture. The colorless reactant mixture immediately turned into brownish yellow. Finally, the mixture was allowed to react and stirred at ambient temperature for 1 h. Mercury Sensing. For Hg2+ sensing, citrate-capped platinum nanoparticles (100 µL, 1.56 mg/L) were added into the solution which contains different amount of Hg2+ (400 µL). 2 minutes later, the solution was diluted with 2.3 mL ultrapure water. H2O2 (1 mL, 2 M) and TMB (200 µL, 16 mM) were then added and left to react at 45 oC for 10 min prior to spectroscopy measurement.

Results and Discussion Characterization of Citrate-Capped Platinum Nanoparticles. To prepare citrate-capped platinum nanoparticles, aqueous solutions of H2PtCl6 and trisodium citrate were mixed and stirred for 30 min at room temperature. After several drops of NaBH4 added, the color of the solution changed to brownish yellow which indicated

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the reduction of Pt ions and the formation of platinum nanoparticles. The size and morphology of the nanoparticles was analyzed by transmission electron microscopy (TEM). As is shown in Figure 1A, the platinum nanoparticles were largely monodispersed and roughly spherical. The average particle diameter is found to be approximately 2.5±0.4 nm (Figure S1). High-resolution TEM indicates the continuous lattice spacing of 0.223 nm corresponding to the (111) facet of the face-centered cubic (fcc) Pt crystal.26 X-ray photoelectron spectroscopy (XPS) measurements were performed for the surface elemental analysis. As is shown in Figure S2, the whole spectrum clearly revealed that platinum, carbon, oxygen, and sodium were present at the surface of PtNPs, indicating that the PtNPs are capped by citrate. The Pt 4f7/2 of citrate-capped PtNPs could be deconvoluted into two distinct components centered at binding energies of 72.82 and 74.71 eV, which could be assigned to Pt0 and Pt4+, respectively (Figure 1B).27

Figure 1. (A) TEM image of citrate-capped PtNPs. (B) XPS spectrum showing the binding energy of Pt 4f.

Inhibition Effect of Hg2+ on the Peroxidase-Like Activity of Citrate-Capped PtNPs. TMB, one of the most common chromogenic substrates of peroxidase, was

6


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employed to screen the peroxidase-like activity of citrate-capped PtNPs. As is shown in Figure 2, citrate-capped PtNPs could catalyze the oxidation of TMB by H2O2 to produce a blue color with maximum absorbance at 652 nm. However, having been added a trace amount of Hg2+, the catalytic activity of citrate-capped PtNPs gets dramatically inhibited.

Figure 2. UV-visible spectra of (a) TMB+H2O2, (b) TMB+H2O2+Hg2+ (5nM) +PtNPs, and (c) TMB+H2O2+PtNPs. Inset: the corresponding photographs.

To better understand the peroxidase-like activity of the citrate-capped PtNPs before and after being treated by Hg2+, kinetic data were obtained by changing the concentration of one substrate (TMB or H2O2) and maintaining the concentration of the other constant. These data were fitted to the Michaelis-Menten equation. Michaelis-Menten kinetics takes the form of an equation as v=vmax×[S]/(Km+[S]). The Michaelis constant (Km) is the substrate concentration at which the reaction rate is half of maximal velocity (vmax), which can reflect the affinity of enzyme to the substrates. For comparison, Km and vmax of citrate-capped PtNPs before and after

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being treated with different amount of Hg2+ were listed in Table 1. The apparent Km value of citrate-capped PtNPs increased while the maximal velocity decreased after the mercury treatment, suggesting that mercury-treated PtNPs have weaker affinity to the substrates and lower catalytic activity.

Table 1. Comparison of the kinetic parameter of citrate-capped PtNPs before and after being treated with Hg2+. 2+

-1

-1

Hg (nM)

TMB Km (mM)

TMB vmax (mM s )

H2O2 Km (mM)

H2O2 vmax (mM s )

0

0.1206

6.51×10-5

205.6

9.79×10-5

1

0.1337

3.85×10-5

221.9

2.69×10-5

10

0.2570

3.72×10-5

295.0

2.23×10-5

Peaks of Hg 4f and Hg 4d could be observed in the XPS spectrum of mercury-treated PtNPs (Figure S3). As is shown in Figure 3A, the Hg 4f electron spectrum could be well-resolved with two doublets with Hg 4f7/2 binding energies of 99.82 and 100.77 eV. The line at lower binding energy is attributed to Hg0, while the higher binding energy component is attributed to Hg2+. The presence of metallic Hg0 confirmed the reduction of Hg2+ at the PtNPs surface. The expanded spectrum of the peaks of the platinum element of mercury-treated PtNPs indicated that there were two new forming peaks at 71.06 and 74.41 eV different from the PtNPs in Figure 1C, which is probably due to the forming of the Hg-Pt alloys (Figure 3B). According to the experimental observations, Hg2+ is reduced to its metallic form, prior to the interaction with PtNPs. In principle, due to the noble nature of metallic Pt, one would not expect Hg2+ to oxide Pt0 under standard conditions. Previous studies confirmed 8


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that citrate can form complex anions with Hg2+ but has no reducing effect over Hg2+.28-29 However, the presence of citrate adsorbed on the surface of PtNPs could trigger a reduction mechanism catalyzed by the very reactive Pt surface atoms, in a similar way to what has been postulated for the reduction of Hg2+ catalyzed by gold nanoparticle.28

Figure 3. (A) Hg(4f) and (B) Pt(4f) XPS spectra of citrate-capped PtNPs after being treated with Hg2+. Due to the important role of citrate, the concentration of citrate used in the PtNPs fabrication on the inhibition ratio of 1 nM Hg2+ has been investigated. The results shown in Figure 4 confirmed that the inhibition effect of Hg2+ was limited to the bare PtNPs without citrate ions on their surface. Increasing amount of citrate used in the PtNPs fabrication enhanced the interaction between Hg2+ and PtNPs, resulting in increased inhibition ratio. In addition, excess amount of citrate used in the fabrication made no difference to the inhibition effect. These results confirmed that there exists a combined role of citrate as the capping, attracting and reducing agent in the adsorption/amalgamation process of Hg2+.

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Figure 4. The effect the concentration of citrate used in PtNPs fabrication on the inhibition ratio. 1 nM Hg2+ was used for the treatment of PtNPs. Inhibition ratio= [(A0-A)/A0] ×100, where A0 and A is the absorbance at 652 nm obtained from the system without and with Hg2+, respectively. Hg2+ sensing. The high specificity of Hg-Pt interaction provides excellent selectivity towards Hg2+ over other metal ions. Figure 5 shows that only Hg2+ ions can significantly inhibit the peroxidase-like activity of citrate-capped PtNPs. Other metal ions including major interfering ions such as Cu2+, Cd2+ and Pb2+ do not show distinct inhibition effect even with high concentration (100 times of the concentration of Hg2+ ions). We have further tested the system with 19 kinds of anions (SO42-, Cl-, F-, NO3-, Br-, BrO3-, HPO42-, ClO4-, Ac-, NO2-, CO32-, EDTA2-, SO32-, IO3-, S2O32-, S2O82-, I-, SCN-, and S2-). As shown in Figure S4, the interference from common anions can also be ignored.

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1.2

10 nM

1 µM

1.0

A/A0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4 0.2 0.0

+

+

+

2+

2+

2+

2+

2+

2+

2+

2+

2+

2+

2+

3+

3+

3+

NH4 Na K Ba Ca Cd Co Cu Mg Mn Ni Pb Fe Zn Al Cr Fe

2+

Hg

Figure 5. The effects of different cations on the peroxidase-like activity of citrate-capped PtNPs. Hg2+ is 10 nM, the other cations are 1 µM. A0 and A is the absorbance at 652 nm obtained from the system without and with Hg2+, respectively.

Under the optimized conditions, citrate-capped PtNPs were used to catalyze the TMB-H2O2 reaction after reacted with different concentration of Hg2+. As shown in Figure S5, the color of the solutions changed from dark blue to light blue when the concentration of Hg2+ increased. Even a small amount of Hg2+ (1.5 nM) was sufficient to generate an appreciable colorimetric change, which could be easily distinguished by naked eyes. Figure 6A shows the spectra and the gradual decrease at 652 nm of the TMB system when the concentration of Hg2+ increased from 0 to 4 nM. The absorbance at 652 nm against Hg2+ was linear in a range from 0.01 to 4 nM with a correlation coefficient of 0.998 (Figure 6B). The limit of detection for Hg2+ (signal-to-noise ratio=3) was calculated to be 8.5 pM, which is much lower than the maximum level (2.0 ppb=10 nM) of Hg2+ permitted in drinking water by the United 11



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States Environmental Protection Agency (EPA). The relative standard deviation (RSD) was 3.7% for 1 nM Hg2+ (n=6).

Figure 6. (A) UV-visible spectra of TMB+H2O2+PtNPs in the presence of different concentrations of Hg2+. (B) The plot of the absorbance at 652 nm versus the concentration of Hg2+. The past few years have witnessed the development of many successful strategies for the detection of Hg2+. In comparison with other methods as shown in Table 2, the proposed method in our paper is relatively simple, facile and sensitive. The sensitivity of this probe to Hg2+ is extremely excellent with the limit of detection as low as 8.5 pM, which is at least 2-3 orders lower than other methods based on noble metal nanoprobes. Moreover, Hg2+ could be detected directly by citrate-capped PtNPs without further modification and the process is time-saving and visible to the naked eye.

Table 2. Comparison of the proposed approach with other reported methods for the detection of Hg2+ based on noble metal nanoprobes. Linear LOD Method

Probe

range

Time (nM)

(nM)

12


Real sample

Ref

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Tween 20 –AuNPs

200-800

100

5 min

Drinking and seawater

30

Colorimetric

Melamine-AuNPs

50-250

50

-

-

21

(X-AuNPs SPR)

Peptide-AuNPs

-

26

1 min

-

31

DNA-AuNPs

10-10000

10

10 min

Lake water

32

H2O2-TMB-AuNPs

1-600

0.3

20 min

Lake water

20

OliGreen-DNA-AuNPs

50-2500

25

1h

Pond water

33

TGA-AuNPs+R6G

0.5-35.5

0.06

15min

Colorimetric (AuNPs turn on)

Fluorescence River, tap and pond (turn on)

34 water

CTAB-GSH-AgNCs

0.1-125

0.1

10 s

-

BSA-AuNCs

400-43200

80

5 min

25

River, tap and mineral 24 water Fluorescence BSA-AuNCs

1-20

0.5

10 s

-

35

11-MUA-AuNDs

10-10000

5.0

10 min

Pond water

36

H2O2-AUR–AuNPs

5 -1000

4

2.5 h

Pond water

23

H2O2-AUR–Pt/AuNPs

10 -1000

2.5

2.5 h

Pond and tap water

9

H2O2-TMB-PtNPs

0.01-4

0.0085

10 min

Tap water

This work

(turn off)

Colorimetric (PtNPs turn off)

On the basis of the above results, the practicality of this method for the detection of Hg2+ in tap water was evaluated. There is no detectable Hg2+ existing in the tap water samples. Thus, using standard addition method, different concentrations of Hg2+ were respectively spiked in tap water samples and then analyzed with the method. As is shown in Table 3, the recoveries of the measurement were 93-103%, which can surely confirm that no significant difference existed between the measured value and added value. These results suggest that this method would be suitable for the detection of Hg2+ in tap water.

Table 3. The recovery of standard addition to measure the Hg2+ in tap water. Sample

Spiked (nM)

Found (nM)

Recovery (%)

RSD (%) (n=3)

Tap water

5

4.83

96.57

2.9

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10

9.96

99.63

2.3

25

25.47

101.88

0.6

Conclusions In summary, we have demonstrated that citrate-capped platinum nanoparticle can be used as smart probe for facile mercury sensing. The peroxidase-like catalytic ability of citrate-capped PtNPs can be inhibited by the in-situ reduction of Hg2+. The high specificity of Hg-Pt interaction provides the excellent selectivity for Hg2+ detection. The sensitivity of this probe to Hg2+ is significant with a LOD as low as 8.5 pM. In addition, the feasibility of the probe for the rapid analysis of Hg2+ in tap water has been demonstrated with satisfactory results. These features make this probe a potentially powerful tool for the quantitative determination of mercury ions in water or other environmental samples.

Acknowledgment The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (21175023), the Program for New Century Excellent Talents in University (NCET-12-0618), the Natural Science Foundation of Fujian Province (2012J06019), and the Medical Elite Cultivation Program of Fujian Province (2013-ZQN-ZD-25).

Supporting Information Available Size distribution histogram of citrate-capped PtNPs, XPS spectra of citrate-capped PtNPs before and after treated with Hg2+, the effect of different anions on the peroxidase-like activity of citrate-capped PtNPs, and photographs of TMB-H2O2

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system catalyzed by citrate-capped PtNPs after reacted with Hg2+. This information is available free of charge via the Internet at http://pubs.acs.org/.

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