Colorimetric Detection of Copper in Water Samples Using Dopamine

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Colorimetric Detection of Copper in Water Samples Using Dopamine Dithiocarbamate-Functionalized Au Nanoparticles Vaibhavkumar N Mehta,† M. Anil Kumar,‡ and Suresh Kumar Kailasa†,* †

Applied Chemistry Department, S. V. National Institute of Technology, Surat-395 007, India Department of Nanomaterial Chemistry, Dongguk University, Gyeongju 780-714, South Korea

‡

S Supporting Information *

ABSTRACT: This paper describes the sensing application of water-dispersible dopamine dithiocarbamate decorated gold nanoparticles (DDTC-Au NPs) as sensors for the colorimetric detection of Cu2+ ions in water samples. Dopamine dithiocarbamate (DDTC) molecules were successfully attached on the surfaces of Au NPs and were characterized by using UV-visible, FT-IR, proton NMR, TEM, and DLS (dynamic light scattering). The color of DDTC-Au NPs was changed from purple to blue by the addition of Cu2+ ions at pH 9.0 by using Tris-tricine buffer. These changes were measured by UV-visible spectrometry and DLS. The method was linear in the range of 1−10 mM with correlation coefficient (R2) 0.999. As a result, the present approach allows the detection of Cu2+ ions at 14.9 × 10−6 M. DDTC-Au NPs were effectively used as colorimetric sensors for the detection of Cu2+ ions in real samples (tap water).

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INTRODUCTION Metal pollution in air, soil, and water are global problems and it is a growing threat to humanity. There are hundreds of sources of heavy metal pollution, including the coal, natural gas, paper, and chlor-alkali industries.1 It can be noticed that certain metals remain toxic at trace amounts, which can enter into the body via a variety of routes and disturb enzyme activities and protein key functions in living systems.2 Copper is one of the vital transition metals and has significant role in various metabolic pathways.3 It has been confirmed that the excess amounts of copper can cause Alzheimer’s disease4 and inflammatory disorders.5 Due to its role and effects in metabolic pathways, a simple, selective and sensitive method is essentially required for the quantification and detection of Cu2+ in drinking water, industrial, environmental, and food samples. A wide range of methods such as atomic/molecular absorption spectroscopy,6 inductively coupled plasma emission/mass spectrometry,7 electrochemical methods,8 ion chromatography,9 and X-ray fluorescence10 have been used for the absolute identification of metal species in environmental samples. However, these methods are very expensive. In addition, these methods require tedious sample pretreatment and laborious clean up procedures prior to metal species analysis. Therefore, the development of a facile, inexpensive, selective and in situ method that allows realtime monitoring of metal species is a great challenge. Recent years, functionalized Au NPs have been integrated with UV-visible spectrophotometry for the selective and sensitive real-time monitoring of metal ions in environmental samples.11−15 In this connection, various mercapto molecules such as 6-mercaptonicotinic acid, L-cysteine, 2-mercaptoethanol, 3- mercaptopropionic acid, 11-mercaptoundecanoic acid, 4mercaptobenzoic acid capped Au NPs-assisted colorimetric © 2013 American Chemical Society

methods have been developed for the selective detection of Cd2+, Cu2+, Fe3+, and Hg2+ ions in various samples (drinking water, polluted water, lake water).11,16−20 Therefore, noble metal NPs-based colorimetric sensors have been received much consideration for the detection of analytes by the naked eye without any sample pretreatment.21 Recently, several organic molecules capped -Au and -Ag NPs based sensors have been gained much attention for the analysis of a wide variety of molecules in biological, chemical, and environmental fields due to their ease sample preparation, selectivity, high sensitivity, and biocompatibility.22 Since, organic molecules on the surfaces of Au NPs have key role to response specific guest molecules (analytes) through the covalent and noncovalent interactions, these can lead to form NPs-metal ion complexes between the electron rich groups (−OH, −NH2 and −COOH) on the surfaces of NPs and metal ions.23 Very recently, the applicability of dithiocarbamate derivatives on the surfaces of NPs has been described for the tracing of biomolecules.24 This is due to their easy preparation and very less interatomic distance between two sulfur atoms and these can facilitate a strong binding with NPs surfaces.25 So far, there were no reports on DDTC decorated Au NPs as colorimetric sensors for the monitoring of metal species. Moreover, dopamine has rich electron donating groups (−OH and −NH2) which permit host−guest interactions with target species.26,27 In this paper, we describe the potentiality of water-dispersible DDTC decorated Au NPs as sensors for the selective identification Received: Revised: Accepted: Published: 4414

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of Cu2+ ion with the naked eye followed by UV-visible spectrometry.

can be observed that DDTC-Au NPs were responded to Cu2+ ions, resulting in the color change immediately from purple to blue along with the appearance of the new absorption band at 665 nm. Instrumentation. UV-visible spectra were measured with Maya Pro 2000 spectrophotometer (Ocean Optics, U.S.) at room temperature. 1H NMR spectra were recorded on a Varian 400 MHZ instrument. Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer (FT-IR spectrum BX, Germany). Transmission electron microscopy (TEM) images were taken on a Tecnai 20 (Philips, Holland) at an acceleration voltage of 100 kV. DLS measurements were performed by using Zetasizer Nano ZS90 (Malvern, UK).

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EXPERIMENTAL SECTION Chemicals and Materials. Hydrogen tetrachloroaurate hydrate (HAuCl4·xH2O), tris(hydroxymethyl) aminomethane (Tris), tricine, dopamine hydrochloride, metal salts (Zn(NO 3 ) 2 ·6H 2 O, Mn(NO 3) 2 ·4H 2 O, Co(NO 3 ) 2 ·6H 2 O, Pb(NO3)2, Cd(NO3)2·4H2O) were purchased from SigmaAldrich, U.S.. Cu(NO3)2·3H2O and carbon disulfide were purchased from Merck Ltd., India. Trisodium citrate dihydrate and triethylamine (TEA) were purchased from SD Fine Chemicals Ltd., India. Hg(NO3)2·H2O was purchased from Spectrochem Ltd., India. FeCl2·4H2O was purchased from Loba Chemie, India. All chemicals were of analytical grade and used without further purification. Milli-Q-purified water was used for the sample preparations. Preparation of DDTC-Au NPs. Au NPs were synthesized by the described procedure in literature.28 Briefly, 25 mL of 1.0 mM HAuCl4 solution was taken into 250 mL round-bottom flask and refluxed under constant stirring at 600 rpm. To this, 2.5 mL of 38.8 mM trisodium citrate was quickly injected and then the mixture was refluxed for 15 min, resulting in a color change from pale yellow to deep red which confirms the formation of Au NPs. The resulting solution was cooled to room temperature. Dopamine dithiocarbamate decorated Au NPs were prepared according to the previous method.24 However, we modified the method by reducing Au3+ ions with citrate ion instead of sodium borohydrate. Briefly, CS2 (80 μL, 0.549 mM) and triethylamine (10 μL) were added in dopamine (0.1896 g, 0.549 mM) and mixture was sonicated for 5 min at room temperature to acquire the ideal assembly of dopamine dithiocarbamate. These molecules were tailored on the surfaces of Au NPs as follows; 500 μL of DDTC was added to 1 mL of bare Au NPs and stirred for 30 min to ensure self-assembly of the dopamine dithiocarbamate onto the surface of Au NPs and color change was observed from deep red to purple (inset picture in Figure 1). DDTC-Au NPs As Sensors for Cu2+ Ion. To investigate the ability of DDTC-Au NPs for the metal species recognition, various metal solutions (200 μL of 10 mM) were added to 100 μL of DDTC-Au NPs by using Tris-tricine buffer at pH 9.0. It

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RESULTS AND DISCUSSION Characterization of DDTC Decorated Au NPs. We used UV-visible spectrometry to measure absorption maxima (λmax) of bare Au NPs and DDTC-Au NPs. Figure 1 shows the UVvisible spectra of bare Au NPs by the reaction of 1 mM of chloroauric acid with 38.8 mM of citrate ion at 85 °C. It can be observed that AuCl4− ions were reduced as Au NPs in the presence of citrate ions and lead to the appearance of new absorption band at 520 nm which is due to the surface plasmon resonance (SPR) peak of Au NPs (Figure 1). However, the characteristic SPR peak of Au NPs was red-shifted (longer wavelength, that is, 640 nm) by the tailoring of DDTC molecules onto the surfaces of Au NPs (Figure 1). Since, −SH group of DDTC was strongly bonded onto the surfaces of Au NPs via a simple “zero-length” covalent coupling (Supporting Information (SI) of Figure S1). Therefore, the characteristic SPR band at 640 nm clearly indicates that the DDTC molecules are successfully tailored onto the surfaces of Au NPs and these Au NPs are well-dispersed in water and those can be observed with naked eye. SI of Figure S2a shows the 4000−450 cm−1 region of the FTIR spectrum of pure dopamine. In this spectrum, pure dopamine contains the major functional groups such as two −OH groups and one −NH2 group; showed characteristic peaks at 935 and 1115 cm−1 for −CH2−NH2; peaks at 1261, 1320, 1499, and 3341 cm−1 corresponded to the −C−O−H symmetric bending, −C−O−H asymmetric bending, aromatic C−C symmetric stretching and −O−H asymmetric stretching vibrations, respectively. SI of Figure S2b represents the FT-IR spectrum of dopamine dithiocarbamate. This spectrum shows new peaks at 1013, 2583, 1212 cm−1 for the C−S, −S−H and CS−NH groups stretching vibrations, respectively. SI of Figure S2c shows the FT-IR spectrum of dopamine dithiocarbamate decorated Au NPs. It can be noticed that the mercapto group (−SH) peak is disappeared at 2583 cm−1, which confirmed bond formation between DDTC and Au NPs. Moreover, peaks at 3446, 1725 cm−1; 1594 and 1385 cm−1 confirmed that the formation of aromatic conjugated ketones (CCO) (SI Figure S2c). These results revealed that DDTC molecules were successfully attached on the surfaces of Au NPs. SI of Figure S3 represents 1H NMR spectra of pure dopamine, DDTC and DDTC capped Au NPs. 1H NMR spectrum of pure dopamine showed multiplet peaks around 6.0−7.0 ppm, which corresponded to aromatic protons in dopamine molecule. The peaks at 2.0−3.0 ppm and 8.8−9.0 ppm corresponded to −CH2 protons and −OH protons (doublet) in the dopamine molecule. The peak at 8.1 ppm corresponds to −NH2 protons in dopamine structure. It can be noticed that the peak intensities of −NH2 protons were

Figure 1. UV-visible spectra of (a) bare Au NPs and (b) DDTC-Au NPs. Inset picture shows (a) bare Au NPs (b) DDTC-Au NPs. 4415

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Figure 2. TEM images of (a) bare Au NPs and (b) DDTC-Au NPs.

Figure 3. UV-visible absorption spectra of Au NPs with the different concentration of DDTC (0.25−2.0 mM).

drastically decreased in DDTC molecule. It can be noticed that new peak at ∼1.2 ppm which corresponds to the proton of mercapto (-SH) group in DDTC structure (SI of Figure S3b). SI of Figure S3c shows the 1H NMR spectrum of DDTC capped Au NPs. By comparison with the 1H NMR spectra of pure dopamine, DDTC and DDTC capped Au NPs, dopamine dithiocarbamate decorated Au NPs showed upfield chemical shift of all the protons. Since, electronic environment of DDTC protons are affected by interactions of Au NPs with DDTC molecules, which resulted upfield shift in the spectrum.29 Based on the above results, it is confirmed that DDTC molecules are successfully attached on the surfaces of Au NPs. The sizes and morphology of bare Au NPs and DDTC-Au NPs were investigated by TEM. TEM images of bare Au NPs and DDTC-Au NPs are shown in Figure 2. These results revealed that the average diameter of Au NPs is ∼5 nm (Figure 2a). When DDTC was decorated onto the surface of Au NPs, the sizes of Au NPs were slightly increased by the attachment of DDTC molecules onto the surfaces of Au NPs and their sizes were found to be ∼25 nm (Figure 2b). These TEM results agree with UV-vis spectrometry and DLS results (Figure 1 and 5). Since, Au NPs are prepared by citrate reduction of HAuCl4. These NPs are spherical and well dispersed in solution phase (Figure 1). It is clearly observed that DDTC molecules are assembled as a faint “halo” on the surfaces of Au NPs and appeared as quasi superstructures (Figure 2b). In accordance with UV-vis spectrometry, DLS and TEM data, the DDTC-Au NPs are larger sizes and more aggregated than the bare Au NPs

Figure 4. (I) UV−visible absorption spectra of DDTC-Au NPs with various metal ions (Cu2+, Co2+, Cd2+, Fe2+, Hg2+, Mn2+, Pb2+, Zn2+) and (II) Photographic image of (a) bare Au NPs, (b) DDTC-Au NPs with various metal ions (c) Cu2+ (d) Co2+ (e) Zn2+ (f) Hg2+ (g) Cd2+ (h) Fe2+ (i) Mn2+ and (j) Pb2+, respectively.

(Figure 2 and 5). Furthermore, we also calculated the average size of bare Au NPs by using UV-visible spectrometry with the following eq 1.30 D = (9.8127 × 10−7)λ 3 − (1.7147 × 10−3)λ 2 + (1.0064)λ − 194.84

(1)

Where D (nm) is the size of a given Au NP sample, and λ is the wavelength (nm) of the SPR peak of Au NPs. By using the above formula, we estimated the average size of bare Au NPs is ∼2.81 nm for the SPR peak of Au NPs at 520 nm. Effect of the Concentration of Capping Ligand. It is well-known that the NPs solution can be precipitated by adding excess amount of mercapto molecules or organic derivatives on the surfaces of NPs system. Since, well-dispersed NPs system can be effectively acted as sensor to probe metal ions through the coordination bonds between metal ions and capped 4416

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Figure 5. DLS measurement of (a) Bare Au NPs, (b) DDTC-Au NPs, and (c) aggregation of DDTC-Au NPs after addition of Cu2+.

ligands.20 Therefore, we investigated the effect of DDTC concentration (0.25−2.0 mM) onto the surfaces of Au NPs (Figure 3). These results revealed that the SPR peak of bare Au NPs was decreased and shifted to longer wavelengths (from 520 to 640 nm) by increasing amounts of DDTC (0.25 to 1.0 mM) onto the surfaces of Au NPs. We also noticed that the color of bare Au NPs was completely changed from red to purple color by the addition of 1.0 mM of DDTC onto the surfaces of Au NPs (SI Figure S4). Moreover, DDTC-Au NPs were aggregated by the addition of DDTC concentration from 1.25 to 2.0 mM (SI Figure S4). Therefore, we selected 1.0 mM of DDTC concentration as the best optimal concentration onto the surfaces of Au NPs which facilitate to strong interactions with metal species. Effect of pH and Selectivity of DDTC-Au NPs. The pH of the solution is very important for the colorimetric detection of metal species by using NPs as sensors. Therefore, we studied that the effect of pH (from 4 to 12) for the selective sensing of Cu2+ ions with DDTC-Au NPs as sensors. It has been observed

that at lower pH (acidic) DDTC-Au NPs were not interacted with Cu2+ ion, since, DDTC ligands were detached and appeared as free −SH molecules from Au NPs. However, DDTC-Au NPs were effectively interacted with Cu2+ ions at pH 9.0 by using Tris-tricine buffer. Interestingly, when we added Cu2+ ions to DDTC-Au NPs, we noticed that the SPR peak of DDTC-Au NPs was completely shifted to longer wavelength (665 nm), which indicates the formation of coordination complexes between Cu2+ ions and −OH groups of DDTC-Au NPs (Figure 4(I)). Therefore, we selected pH 9.0 is the best condition for the high interactions of DDTC-Au NPs with Cu2+ ions. Next, we also studied the selectivity of DDTC-Au NPs as colorimetric sensing probes for Cu2+ ions in the presence of other relevant metallic species such as Cd2+, Co2+, Cu2+, Fe2+, Hg2+, Mn2+, Pb2+, and Zn2+ (Figure 4(II)) at 10 mM of each metal ion concentration. It can be observed that only Cu2+ ions showed strong tendency to interact with DDTC-Au NPs, whereas the remaining ions had insignificant effects under 4417

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high selectivity toward Cu2+ than the other metal species. Furthermore, the metal species (Pb2+, Co2+, Cd2+) are precipitated with DDTC-Au NPs solution. The effect of diverse ions on the absorbance of solution containing 5.0 mM of Cu2+ was studied. Tolerance limits of metal ions (mM) as follows: Cd2+ (5.0), Co2+ (2.5), Fe2+ (1.66) Hg2+(10), Mn2+ (10), Pb2+ (2.85), and Zn2+ (5.0), respectively. Quantification of Cu2+ Using DDTC-Au NPs as Colorimetric Sensors. Next, we used DDTC-Au NPs as sensors for the quantification of copper ion (1.0−10.0 mM) by using UV-visible spectrometry. Figure 6 clearly indicates that the peak intensity at 665 nm was gradually increased by increasing concentration of Cu2+ from 1.0 to 10 mM. These results indicate that the peak intensity at 665 nm is proportional to the concentration of Cu2+ up to 10.0 mM, after that the peak intensity is decreased. As a result, the solution color is changed from purple (DDTC-Au NPs) to blue (Cu2+ aggregated DDTC-Au NPs), which can perceptible with the naked eye. Moreover, DDTC-Au NPs system is able to detect Cu2+ ions up to minimum concentration of 1.0 mM. Moreover, the aggregation of DDTC-Au NPs is directly dependent on the concentration of Cu2+, which is due to an isosbestic point at 665 nm upon addition of Cu2+. SI of Figure S7 shows a linear correlation between absorbance of DDTC-Au NPs with Cu2+ and Cu2+ concentration. The correlation coefficient (R2) was found to be 0.999 for the quantification of Cu2+ within a range from 1.0 to 10.0 mM (y = 0.116x + 0.072). Meanwhile, the limit of detection (LOD) was calculated as 14.9 × 10−6 M by eq 2.

Figure 6. UV-visible spectra of DDTC-Au NPs solutions with various concentrations of Cu2+ with the range of (a) 1.0 mM, (b) 2.5 mM, (c) 5.0 mM, (d) 7.5 mM, (e) 10.0 mM, respectively.

Table 1. Comparison of DDTC-Au NPs as Colorimetric Sensors for the Detection of Cu2+ with the Reported Methodsa nanoparticles

capping agent

Ag NPs

4-mercaptobenzoic acid 3-azidopropylamine DNA dopamine DDTC

Au NPs Au NPs Ag NPs Au NPs

size (nm)

18.0 10.0 10.0

LOD (M)

reference

2.5 × 10−8

20

1.8 × 10−6 20 × 10−6 3.2−512 ppb 14.9 × 10−6

31 32 33 present study

LOD = K × S0/S

(2)

where K is a numerical factor chosen according to the confidence level desired, S0 is the standard deviation (SD) of the blank measurements (K = 3), and S is the slope of the calibration curve. The detection limit of DDTC-Au NPs system toward Cu2+ is quite higher than the other reported methods20,31−33 (Table 1). This can be attributed to the coordination problem of Cu2+ with the catechol group of dopamine, in which mono-, di-, or tricoordinated complexes may be generated and the species of formed complex is dependent on the metal−catechol ratios as well as the ion valence.34 Moreover, other reported system having the free −NH2 and/or −COOH group, which are more suitable to bind to the divalent metal ions as compare to the −OH group.35,36

identical conditions. This result was probably due to their rigid coordination geometry with the phenolic hydroxyl groups of DDTC. Since, Cu2+ ions have multivalent coordination with the phenolic hydroxyl groups of DDTC, resulting the solution color is changed from purple to blue and these changes can be read out with the naked eye (Figure 4(II)). SI of Figure S5 indicates the UV-visible spectra of bare Au NPs, DDTC-Au NPs and DDTC-Au NPs with copper ion. It can be noticed that SPR peak of DDTC-Au NPs was shifted to longer wavelength (665 nm), which confirms the formation of copper ion complex with DDTC-Au NPs. Dynamic light scattering (DLS) was used to measure sizes of bare Au NPs, DDTC-Au NPs, and DDTC-Au NPs with copper ion (Figure 5). These results revealed that the sizes of Au NPs were increased by addition of DDTC and copper ion. Since, the bare Au NPs have an average hydrodynamic diameter of ∼5 nm which maintains their size and stability (Figure 5a). After addition of DDTC as a capping agent of AuNPs, average hydrodynamic diameter increased to ∼45 nm (Figure 5b) that is close to average hydrodynamic diameter of Au NPs. In contrast, Au NPs were agglomerated and their sizes were increased to ∼214 nm by the addition of copper ion, which confirms the complex formation between DDTC-Au NPs and copper ion (Figure 5c). This result revealed that Cu2+ has high affinity to form complex with hydroxyl groups of DDTC-Au NPs via coordination bond. SI Figure S6 represents the sensing ability of DDTC-Au NPs with other metal species was expressed by the ratio of absorption at 665 nm to that at 520 nm (A665 nm/520 nm), indicating that DDTC-Au NPs showed

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APPLICATIONS In order to demonstrate that the practical applicability of DDTC-Au NPs as sensors to probe Cu2+, we assay the copper ions in tap water samples. To this, tap water samples were spiked with different concentrations of Cu2+ ions (1.0 to 10 mM). The sample pH was adjusted to 9.0 by adding Tris-tricine buffer and their absorbance spectra were measured for the construction of calibration graph. This graph exhibits linearity in the range of 1.0−10.0 mM and Cu2+ ion concentration is found to be 1.38 μg/mL. Furthermore, we also evaluated the potentiality of the proposed method for the quantification of Cu2+ in spiked tap water samples. For this, Cu2+ (1.27 μg/mL; 20 μM) was spiked in tap water and then analyzed by both methods (DDTC-Au NPs-based UV-visible spectrometry and inductively coupled plasma optical emission spectrometry (ICP-OES)). The results obtained by the proposed method were compared to those of ICP-OES by applying Student’s tand F-tests (SI of Table S1). Results obtained by the present 4418

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method were found to be very slightly higher (0.04 μg/mL) as compared to that of ICP-OES. This may be due to minor interference of water-soluble inorganic species. However, the results obtained from both methods were in excellent agreement, evaluated by the student t test at the 95% confidence level shows no significance difference (p < 0.02) between the results obtained by both methods, which confirmed that there is a good agreement between the results obtained by the proposed method and the reference method with respect to accuracy and precision. These results revealed that DDTC-Au NPs are effectively acted as sensors for the colorimetric detection of Cu2+ in water samples.

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CONCLUSIONS In conclusion, this method describes the sensing ability of DDTC-Au NPs for the simple, selective and sensitive detection of Cu2+ in water samples. The DDTC-Au NPs have showed high affinity toward Cu2+ with improved colorimetric sensitivity and accelerate the color change. By using this method, the lowest detectable concentration of Cu2+ is 14.9 × 10−6 M. Therefore, DDTC-Au NPs system would open a new avenue for the rapid, selective, and real-time in situ detection of Cu2+ and the color change can be readily seen by the naked eye. We hope that this system holds great prospective for practical applications to the simple and selective sensing of Cu2+ in various samples.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic representation of the method, FT-IR, 1HNMR, effect of DDTC concentration on Au NPs, UV-visible spectra of DDTC-Au NPs with Cu2+, sensing ability of DDTC-Au NPs with other metal ions and calibration graph are shown in Supporting Information (Figure S1−S7). Comparison of DDTC-Au NPs method with ICP-OES (Table S1) is also available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-261-2201730; fax: +91-261-2227334; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Director, SVNIT, Surat for the financial support to this work. We acknowledge construction of UV-visible spectrophotometer instrumentation supported by Department of Science and Technology, India. We thank Prof. Z. V. P. Murthy, Head, Department of Chemical Engineering, SVNIT, Surat, India for the DLS measurements. We acknowledge Center of Excellence, Vapi, India, for providing ICP-OES data.

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REFERENCES

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dx.doi.org/10.1021/ie302651f | Ind. Eng. Chem. Res. 2013, 52, 4414−4420