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Sensors and Actuators B 195 (2014) 239–245

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

co-friendly colorimetric detection of mercury(II) ions usingabel-free anisotropic nanogolds in ascorbic acid solution

i-Hua Jin, Chang-Soo Han ∗

chool of Mechanical Engineering, Korea University, Seoul 136713, South Korea

r t i c l e i n f o

rticle history:eceived 14 October 2013eceived in revised form 2 January 2014ccepted 6 January 2014vailable online 20 January 2014

a b s t r a c t

We report a simple, direct, and eco-friendly method of detecting mercury ions in water samples based onthe color change of hexadecyltrimethylammonium bromide (CTAB)-coated nanogold (NG) in the pres-ence of nontoxic ascorbic acid (AA). The surface plasmon resonance (SPR) absorption wavelength of theNG shifted from long (dark purple) to short (red) wavelengths results from the amalgamation process

eywords:anogoldsscorbic acidolorimetric

between Hg and Au. The detection scheme based on SPR property of NGs with a mixture of sphericaland rod-shaped gold nanoparticles offers excellent selectivity and good sensitivity. The lowest detectedconcentrations for mercury ions were 1 �M by the naked eye and 30 nM by UV–visible light spectroscopy.The total procedure takes less than 5 min without sample pre-treatment. It is successfully applied in mer-cury ion analysis in tap water and this provides a good potential for on-the-spot detection of mercuryions with simplicity, rapidity, cost-effectiveness, and harmlessness.

. Introduction

In modern society, heavy metal pollution is a serious problemhat must be solved because of its toxic effects on the environ-

ent and human health. Mercury ions (Hg2+) are one of the majornvironmental pollutants and are widely distributed in ambientir, water, soil, and even food [1,2]. Mercury exposure can destroy aariety of organs and the immune system, and have serious adverseffects on the brain, kidneys, and nervous and endocrine systemsn humans and animals [3–7]. Therefore, a method of detectingg2+ with high selectivity, efficiency, and sensitivity is impor-

ant in the environmental sciences, food analysis, and biomedicalssays. The usual adopted techniques for detecting mercury withigh sensitivity, with limits of detection at the parts-per-billion

evel, include atomic absorption spectrometry (AAS) [8,9], atomicmission spectrometry (AES) [10], inductively coupled plasmaass spectrometry (ICP-MS) [11–13], atomic fluorescence spec-

rometry (AFS) [14,15], and inductively coupled plasma-atomicmission spectrometry (ICP-AES) [16]. These techniques performell, but require complicated, time-consuming sample preparation

nd treatment, and high cost of the measurement equipment.In response to these shortcomings, a number of improved

ethods of detecting Hg2+ have been reported, including fluores-ein quenching [17–23], functionalized DNA sensors [24,25], DNAnzymes [26], electrochemical sensors [27,28], and colorimetric

∗ Corresponding author. Tel.: +82 232903354E-mail address: cshan@korea.ac.kr (C.-S. Han).

925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2014.01.020

© 2014 Elsevier B.V. All rights reserved.

assays [29–32]. Of these, colorimetric methods have attractedincreasing attention as they enable the direct determination of Hg2+

from color variation by the naked eye without special equipments.A colorimetric sensor enables on-the-spot detection of mercury.

Gold nanoparticles (Au NPs) have intrinsically strong surfaceplasmon resonance (SPR) absorption in the visible wavelengthsrange, and the colors of the Au NP solutions result from the extinc-tion cross-sections of the Au NPs and the wavelengths at whichthey absorb and scatter light, which depend on both their sizeand shape, the dielectric properties (refractive indices) of theirsurrounding medium, and their interactions with neighboring par-ticles [33–35]. For example, 10 nm Au NPs appear ruby red, whilewith large particles or when the distances between the Au NPsbecome less than the average particle diameter, the color of theAu NPs changes from red to purple. Consequently, Au NPs havebeen exploited to develop colorimetric sensors and devices forHg2+ detection with easy sample preparation, simple detection pro-cesses, and inexpensive sensing instruments [36–43]. For example,Chen et al. presented a colorimetric method with a blue-to-redcolor change for the determination of Hg2+ and Ag+ due to Au NPstabilization via a redox-formed metal coating, which is facilitatedby ascorbic acid (AA) [38]. An Hg/Ag–Au alloy coating formed onthe surface of Tween 20 Au NPs on reducing Hg2+ and Ag+ with theaid of AA, and the aggregation induced by self-assembling ligandN-acetyl-l-cysteine was prevented. Zhang et al. reported a one-

step colorimetric method of detecting Hg2+ based on color changes,in which CTAB-coated Au NPs were surrounded by a layer of HgSquantum dots to form in situ Au@HgS core–shell nanostructuresin the presence of thioacetamide. Although these methods show

2 d Actuators B 195 (2014) 239–245

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40 L.-H. Jin, C.-S. Han / Sensors an

ood sensitivity and seem simple, some problems remain [40].or example, thioacetamide works well under acidic conditions forercury detection in the presence of Au NPs, but the acidity makes

t more difficult to stabilize the Au NPs if their surfaces have noteen treated. In addition, some methods used more chemicals andgents during the sensing procedures, most of which are potentiallyarmful.

This study presents a simple, direct method of detecting mer-ury ions based on the color change of CTAB-coated nanogold (NG)n the presence of AA. The amalgamation of Hg and Au causes ahift in the absorption wavelength of the SPR of the NG from longdark purple) to short (red) wavelengths. Using NG, a simple, high-ensitivity, eco-friendly process for the colorimetric detection ofg was developed.

. Experimental

.1. Chemicals and instrumentations

Hexadecyltrimethylammonium bromide (CTAB) was purchasedrom Fluka. Hydrogen tetrachloroaurate(III), sodium borohydride99%), silver nitride, l-ascorbic acid (AA), methyl mercury chlo-ide, mercury chloride, sodium chloride, cadmium acetate, calciumhloride, silver nitrate, copper chloride, indium(III) acetate, ironhloride, lead chloride, cobalt chloride, and zinc chloride were pur-hased from Sigma–Aldrich. Deionized water (18 M�) was used inll of the experiments. Absorbance was measured using a single-eam ultraviolet–visible (UV–vis) spectrophotometer (Optizen120UV, Mecasys). For transmission electron microscopy (TEM),

FEI Tecnai microscope was operated at an accelerating voltagef 300 kV. To analyze composition using energy-dispersive X-raypectroscopy (EDS), the sample was tilted 15◦ toward the detector.or TEM sample preparation, we dropped 10 �L of the sample solu-ion onto a standard carbon-coated copper grid and then dried it atoom temperature.

.2. Synthesis of NG

The NG was fabricated following the process used to synthe-ize gold nanorods (Au NRs) [44]. The seed and growth solutionsere generated as described below. For the seed solution, 5 mL of

.2 M CTAB solution was stirred at 400 rpm and mixed with 5.0 mLf 0.0005 M HAuCl4. 0.60 mL of fresh ice-cold 0.010 M NaBH4 wasdded to the stirred solution, which resulted in the formation of

brownish-yellow solution. Vigorous stirring of the seed solutionas continued for 2 min, and then the solution was kept for 5 h

t 25 ◦C before use. To grow the NG, 125 mL of 0.20 M CTAB wasdded to 1.5 mL of 0.0040 M AgNO3 solution at 25 ◦C. To this solu-ion, 125 mL of 0.0005 M HAuCl4 was added and mixed gently. Then,0 �L of 0.0788 M AA was added. As a mild reducing agent, AAhanged the solution from dark yellow to colorless. Finally, 500 �Lf prepared seed solution was added to the growth solution. Then,he solution was kept undisturbed for 24 h at 27–30 ◦C to allowhe reaction to proceed to completion. The NG concentration wasstimated by assuming a 100% synthetic yield.

.3. Sample preparation

Detection was performed in an aqueous NG solution at roomemperature. To examine the detection selectivity, In3+, Pb2+, Cu2+,a2+, Cd2+, Zn2+, Co2+, Na+, Fe3+, Ag+ and Hg2+ were added to 1.2 mL

f separated NG solution containing 0.214 nM NG and 5 mM AA.he final concentrations of Ag+ and Hg2+ were both 25 �M, andhat of the other metal ions was 500 �M. To examine the detectionensitivity, Hg2+ ions at concentrations of 0–50 �M were added to

Fig. 1. Temporal variation of SPR spectra of Au NPs in present of AA in aqueoussolution. The concentration of the Hg2+ is 25 �M.

1.2 mL of NG solution containing 5 mM AA. The absorption spectrawere collected after 10 min.

3. Results and discussion

Au NRs have two absorption bands, at short and long wave-lengths, which are related to the transverse and longitudinalmodes, respectively. Au NRs show better sensitivity of localizedSPR to the surrounding medium and surface changes than Au NPs[45]. Using this property of Au NRs, we planned to monitor theHg2+ by using NG in aid of AA, to increase sensitivity to the nakedeye and spectrophotometry. The NG we used contained sphericaland rod-shaped Au NPs, and their absorption spectra are shown inFig. 1 (black solid line), with two observed peaks at 528 nm (theSPR of spherical NPs and transverse mode of Au NRs) and 590 nm(longitudinal mode of Au NRs), respectively.

3.1. Effect of pH and temporal influence of AA on nanogolds

The effect of pH on the NG–AA system was investigated first.The pH value was adjusted from 2 to 5.5 by using HCl and NaOHsolutions. As shown in Fig. S1A, higher pH value (5 and 5.5) causedaggregation of NG in the absence of Hg ions, which was expressedan observable new peak at 700 nm wavelength. Then, excludingthe solutions with pH 5 and 5.5, 2 �M of Hg2+ was added to theremained NG–AA solutions (Fig. S1B). An aggregation of NG wasobserved for pH 2 at 700 nm wavelength. On the whole, pH higherthan 4.6 and lower than 2.5 was detrimental to the stability of theNG solution. The possible pH value in this NG–AA system is 2.5–4.6.It is to be noted that the pH of NG–AA solution without adding anyHCl and NaOH was about 3.8. Therefore, pH of 3.8 was selectedfinally in the following experiments to simplify the Hg2+ detectionprocess and prevent unnecessary influence.

AA, the main and only agent, played an important role as a mildreducing agent. It offered a reduction circumstance for the targetions. First, we considered the influence of AA on the NG. The absorp-tion spectra of 1.2 mL of NG in the absence (black solid line) andpresence (solid lines in other colors) of 5 mM AA are shown in Fig. 1.It was observed that the SPR peak shifted to shorter wavelengths.The mechanism is similar to that for NaBH4 [42]; i.e., the oxidizedform of Au was reduced to Au(0), which was deposited on the sur-face of the NG. The absorption peak shifted to shorter wavelengthsbecause of the reduced dielectric constant of the medium perceivedby the NGs. Then, the temporal variation in the SPR spectrum of NGwith AA was monitored for more than 3 h. There was no change in

the absorption of NG, indicating that the NG was stable enough inAA environment for use in further experiments.

The broken line in Fig. 1 depicts the change in absorption of NG inAA solution after introducing Hg2+. The absorption peak due to the

L.-H. Jin, C.-S. Han / Sensors and Actuators B 195 (2014) 239–245 241

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ig. 2. (A) Schematic diagram showing the amalgamation of Hg with NGs. (B) and

nd 25 �M Hg2+.

ongitudinal mode of the Au NRs disappeared, resulting in a singleand for Au–AA–Hg2+. This suggests that the rods were converted

nto spheres. The chemical reactions involved are as follows [46]:

gCl2 + C6H8O6 → Hg0 + C6H6O6 + 2HCl, (1)

g0 + 3Au → Au3Hg. (2)

When Hg encounters NG, three possible mechanisms shoulde considered to explain the observed phenomena: amalgamationetween Au and Hg; surface covering of NG by the liquid metal;nd aggregation of the NG. Since the Hg could increase the effec-ive dielectric constant of the medium perceived by the NG [42], thebsorption spectrum of the NG would shift to a longer wavelength.ggregation would result in the precipitation of NG solution and aed shift in the spectrum. The blue shift of the SPR of the NG seenn Fig. 1 eliminates the possibility of aggregation. We believe thatmalgamation is the main mechanism of action because only amal-amation could reduce the effective aspect ratio of the Au NRs andead to a blue shift of the absorption wavelength. Fig. 2A shows achematic explanation of the sensing mechanism, and Fig. 2B and Chows TEM images of NG in AA solution in the absence and presencef Hg2+, respectively. Rod-shaped Au was seen in the AA solutionefore adding Hg2+ (Fig. 2B), while no rods remained after addinghe Hg2+ (Fig. 2C). In addition, the color of the solution changed fromurple (spherical and rod-shaped NG) to red (gold nanospheres), ashown in the figure insets.

Fig. 3 shows elemental maps of NG in the absence and presencef Hg2+. The dispersion of elemental Hg follows that of Au (Fig. 3B),hile Au alone was mapped as rod-shaped (Fig. 3A). This provides

ood evidence that the Hg forms a compound with Au. These resultstrongly support our postulate that the amalgamation process washe main mechanism explaining our observations. Given this mech-nism of action, we investigated the selectivity and sensitivity ofhe proposed method using standard solutions.

.2. Sensitivity and selectivity for mercury (II) ions

To assess the selectivity of AA for Hg2+, we examined differ-nt metal ions under identical conditions. Fig. 4 shows the UV–vis

M images of Au NRs in the presence of AA and Au NRs in the presence of 5 mM AA

spectrum (inset in B) and color (Fig. 4A) changes of NG on addingAg+, Hg2+, In3+, Ca2+, Pb2+, Zn2+, Co2+, Cu2+, Fe3+, Cd2+, and Na+

ions. Only Hg2+ caused the solution to change color, indicatingthat AA is selective for Hg2+. After adding Hg2+, the higher SPRpeak of NG shifted from 528 to 505 nm, while the second peak at590 nm disappeared. Therefore, we analyzed the selectivity usingthe ratio of the absorptions at 505 and 590 nm (i.e., A505 nm/A590 nm).As shown in Fig. 4B, the addition of 25 �M Hg2+ to a solution gaveA505 nm/A590 nm ≈ 3.6, whereas the other metal ions tested (500 �M)gave values similar to the blank, illustrating that 20-fold higherconcentrations of other metal ions did not interfere with the deter-mination of Hg2+.

Under the optimal conditions, we investigated the sensitivityof NG for Hg2+. Fig. 5 shows photographs of the color change andUV–vis absorption responses of 1.2 mL of CTAB-coated NG solu-tion containing 5 mM AA and different concentrations (0–50 �M)of Hg2+. Clearly, as the Hg2+ concentration increased, the higherSPR peak shifted to shorter wavelengths and the second peak grad-ually disappeared. Fig. 5A shows a relatively gradual visible colorchange, with which enabled 2.5 �M of Hg2+ in the NG solution to bedetected by the naked eye. For further analysis, the SPR peak wave-length was plotted against the Hg2+ concentration (Fig. 6A). TheSPR peak shifted to a shorter wavelength as the Hg2+ concentrationincreased. A linear relationship was obtained over Hg2+ concentra-tions of 0–10 �M, with a correlation coefficient of R2 = 0.985. Thewavelength shift was nearly 15 nm for 10 �M Hg2+. In addition,the A505 nm/A590 nm ratio was plotted against the Hg2+ concentra-tion (Fig. 6B). The inset in Fig. 6B shows a linear relationship overthe range 0–10 �M with R2 = 0.987. This probe could detect Hg2+ atconcentrations as low as 122 nM using UV–vis spectroscopy. Thelimit of detection (LOD) was calculated as LOD = 3 × SR/m, where mis the slope of the calibration curve and SR is the standard devia-tion.

3.3. Detection of organic mercury

It is well known that the most toxic forms of mercury are itsorganic compounds since it can easily penetrate cell walls and iseasily absorbed in fatty tissues, nerve and brain cells. To validate

242 L.-H. Jin, C.-S. Han / Sensors and Actuators B 195 (2014) 239–245

A) in t

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Fig. 3. TEM and EDS analysis of NGs (

hat our proposed sensing strategy could apply to organomer-ury analysis, we introduced our NG–AA system to determine the

ethylmercury (CH3Hg+). UV–vis absorption responses of 1.2 mL

f CTAB-coated NG solution containing 5 mM AA and differentoncentrations (0–5 �M) of aqueous CH3Hg+ was collected, and

ig. 4. Selectivity of the NGs probe toward Hg2+ ions in AA aqueous solution. (A)hotographic images of the colors and (B) Value of absorption A505 nm/A590 nm ofhe CTAB-NGs solution with 5 mM AA upon the addition of 25 �M Hg2+ and Ag and00 �M other metal ions. Inset: UV–vis absorption spectra of NG solution with AAnd metal ions.

he absence and (B) presence of Hg2+.

for easy analysis, the A505 nm/A590 nm ratio was plotted against theCH3Hg+ concentration as shown in Fig. 7. Clearly, as could beseen in Fig. 7, a well linear relationship over the range 0–5 �Mwith R2 = 0.986 and the detection concentration as low as 205 nMwas obtained by using UV–vis spectroscopy. This result indicatedthat the NG–AA system was workable in organomercury samples.Although the LOD was not as low as we expected (lower than few

nM), it showed a possibility for simple organomercury determina-tion in the presence of AA. A lower LOD may be expected in a futuredevelopment.

Fig. 5. (A) Photographic images of the colors and (B) UV–vis absorption responsesof CTAB-NGs after the addition of various concentrations of Hg2+ ions (0, 0.25, 0.5,1, 2, 3, 4, 5, 10, 25, and 50 �M).

L.-H. Jin, C.-S. Han / Sensors and Act

Fig. 6. Plots showing (A) the SPR peak shifts and (B) the absorption A505 nm/A590 nm versus

solution with 5 mM AA. Inset: the enlarged portion of the plot in the Hg2+ concentration r

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ig. 7. Plots showing the absorption A505 nm/A590 nm versus the concentration ofethylmercury (1, 2, 3, 4, and 5 �M) in 1.2 mL NGs solution with 5 mM AA.

.4. Nanogold influences on the LOD

To investigate the influence of NG on the LOD of Hg, we alsoested NG II in our detection system. The absorption spectrum ofG II solution is shown in Fig. 8A (black solid line). The NG II sample

ppeared bluer (Fig. 8A, top). As mentioned above, the color of NGepends on its size, shape, and surface conditions. Fig. S2 showshat NG II consisted of various shape, including spheres, rods, andriangles. Combining the SPR properties of these gold nanoparticles,

ig. 8. Photographic images of the colors, (A) UV–vis absorption responses of CTAB-NGslots of the absorption A532 nm/A600 nm versus the concentration of Hg2+ in 1.2 mL NG II sol

uators B 195 (2014) 239–245 243

the concentration of Hg2+ (0, 0.25, 0.5, 1, 2, 3, 4, 5, 10, 25, and 50 �M) in 1.2 mL NGsange of 0.25–10 �M.

NG II appeared dark blue, with three SPR peaks. To determine theLOD of Hg2+ using NG II, we obtained absorption spectra of NG IIwith Hg2+ concentrations from 0 to 5 �M (Fig. 8A). Via the nakedeye, 1 �M of Hg2+ could be detected in the NG II solution based ona gradual color change (photograph in Fig. 8A). After selecting theSPR absorption wavelength to analyze (data not shown here), theratio A532 nm/A600 nm was plotted against the Hg2+ concentration inFig. 8B. There was a good linear relationship with R2 = 0.9989 overthe range 0–5 �M, and the LOD using NG II was 30 nM. The LODwas improved by changing the SPR absorption of NG. This result isexciting, and better sensitivity and a lower LOD can be expected byoptimizing the SPR absorption for Hg2+ detection or even detectionsystems for other heavy metal ions.

3.5. Detection of Hg2+ in tap water

To test the practicality of the proposed method, the NG–AAsystem was evaluated to determine Hg2+ in tap water. The watersample was spiked with certain amounts of Hg2+. The final con-centration of Hg2+ in 1.2 mL of separated NG solution containing5 mM AA was varied from 0 to 5 �M. The linear correlations of theA505 nm/A590 nm with the concentration of Hg2+ for NG and NG IIwere shown in Fig. 9. The LODs of 133 nM with a correlation coef-

after the addition of various concentrations of Hg2+ ions (5 �M–100 nM), and (B)ution with 5 mM AA.

ficient of R2 = 0.9974 for NG (Fig. 9A) and 23 nM with R2 = 0.9937for NG II (Fig. 9B) were obtained. These results revealed that ourmethod was potentially applicable for the determination of Hg2+

ions in environmental water samples.

244 L.-H. Jin, C.-S. Han / Sensors and Actuators B 195 (2014) 239–245

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ig. 9. Plots showing the absorption (A) A505 nm/A590 nm for NG and (B) A532 nm/A600 n

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. Conclusions

Using AA as a reducing agent, we devised a simple, direct,co-friendly method of detecting mercury ions based on theolor change of CTAB-coated NG. The main mechanism, i.e.,n amalgamation process, was demonstrated using UV–vis andnergy-dispersive X-ray spectroscopy analyses. This method isasy, as it requires no sample pretreatment and only one step. Theotal procedure takes less than 5 min. Since AA is harmless, the

ethod does not harm the body or environment. The sensitivityan be improved by optimizing the SPR absorption of NG, resultingrom the mixture of spherical and rod-shaped gold nanoparticles.sing this technique, Hg2+ levels as low as 1 �M can be detectedy the naked eye and those of 30 nM using UV–vis spectroscopy.his method has good potential for the on-the-spot detection ofercury ions as it is simple, rapid, cost-effective, and safe.

cknowledgements

This work was supported by Top Engineering Co. and the Centeror Advanced Soft Electronics by MSIP in Korea.

ppendix A. Supplementary data

Supplementary material related to this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2014.01.020

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Biographies

Li-Hua Jin received her BS and MS degrees in 2003 and 2006, respectively from Yan-bian University, China. She received a PhD degree in 2012 from Korea AdvancedInstitute of Science and Technology (KAIST), Korea. Now she is working for KoreaUniversity as a research professor of school of mechanical engineering. Her researchis focused on heavy metal sensing process and application of nano materials includ-ing quantum dot, metal nanoparticles etc.

Chang-Soo Han received his BS and MS degrees in 1987 and 1989, respectivelyfrom Seoul National University, Korea. He joined Samsung Electronics for 6 years.

He received a PhD degree in 2000 from Korea Advanced Institute of Science andTechnology (KAIST), Korea. He worked in Korea Institute of Machinery & Materi-als (KIMM) for 11 years. Now he is working for Korea University as a professor ofschool of mechanical engineering. His research is focused on process and applicationof nano materials including quantum dot, carbon nanomaterials and nanowires.