Interaction between Diethyldithiocarbamate and Cu (II) on Gold in Non

sensors Article

Interaction between Diethyldithiocarbamate and Cu(II) on Gold in Non-Cyanide Wastewater Nguyễn Hoàng Ly 1 , Thanh Danh Nguyen 1,2 , Kyung-Duk Zoh 3, * and Sang-Woo Joo 1,2, * 1 2 3

*

Department of Chemistry, Soongsil University, Seoul 156-743, Korea; [email protected] (N.H.L.); [email protected] (T.D.N.) Department of Information Communication, Materials, Chemistry Convergence Technology, Soongsil University, Seoul 156-743, Korea Department of Environmental Health Sciences, School of Public Health, Seoul National University, Seoul 08826, Korea Correspondence: [email protected] (K.-D.Z.); [email protected] (S.-W.J.); Tel.: +82-2-880-2737 (K.-D.Z.); +82-2-820-0434 (S.-W.J.)

Received: 25 September 2017; Accepted: 10 November 2017; Published: 15 November 2017

Abstract: A surface-enhanced Raman scattering (SERS) detection method for environmental copper ions (Cu2+ ) was developed according to the vibrational spectral change of diethyldithiocarbamate (DDTC) on gold nanoparticles (AuNPs). The ultraviolet-visible (UV-Vis) absorption spectra indicated that DDTC formed a complex with Cu2+ , showing a prominent peak at ~450 nm. We found Raman spectral changes in DDTC from ~1490 cm−1 to ~1504 cm−1 on AuNPs at a high concentration of Cu2+ above 1 µM. The other ions of Zn2+ , Pb2+ , Ni2+ , NH4 + , Mn2+ , Mg2+ , K+ , Hg2+ , Fe2+ , Fe3+ , Cr3+ , Co2+ , Cd2+ , and Ca2+ did not produce such spectral changes, even after they reacted with DDTC. The electroplating industrial wastewater samples were tested under the interference of highly concentrated ions of Fe3+ , Ni2+ , and Zn2+ . The Raman spectroscopy-based quantification of Cu2+ ions was able to be achieved for the wastewater after treatment with alkaline chlorination, whereas the cyanide-containing water did not show any spectral changes, due to the complexation of the cyanide with the Cu2+ ions. A micromolar range detection limit of Cu2+ ions could be achieved by analyzing the Raman spectra of DDTC in the cyanide-removed water. Keywords: surface-enhanced Raman spectroscopy; plasmonic gold nanoparticles; diethyldithiocarbamate; industrial electroplating wastewater; cyanide removal

1. Introduction The resonant plasmonic enhancement of the local electric field on noble metal substrates could provide a highly sensitive platform for achieving single-molecule label-free detection [1]. SERS has been applied for detecting traces of organic compounds adsorbed on metal substrates [2]. A localized surface plasmon band of AuNPs can be utilized as a platform for studying chemical and biological reactions [3]. Plasmonic nanoparticles have been introduced to study heavy metal pollutants [4–6]. Detailed molecular interactions between organic compounds and metal atoms can be estimated with a combination of quantum-mechanical density functional theory (DFT) calculations [7,8]. Developments in sensing, identifying, and detecting heavy metal ions in aqueous solutions could contribute to the advanced treatment of wastewater [9]. The treatment of heavy metal ions in wastewater has been a significant challenge for environmental scientists [10]. However, because of the high toxicity of cyanide (CN) species, electroplating wastewater can be categorized into two kinds of wastewater: cyanide and non-cyanide [11]. Chlorination in highly alkaline conditions can be an efficient method for the reduction of cyanide ions in wastewater [12–15].

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Diethyldithiocarbamate (DDTC) has been known to bind with metal complexes [16–18]. Among the various metal ions, Cu2+ has been found to interact strongly with DDTC [19–22]. Plasmonic nanoparticle-mediated SERS has been employed to detect hazardous species, such as heavy metal ions, in environmental samples [23]. The SERS spectrum of the DDTC-Cu2+ complex has recently been reported in a combination of quantum mechanical calculations [24]. The development of new, convenient methods for the spectroscopic characterization of contaminants in water effluent is still of significant importance in environmental fields [25,26]. Despite various methods for detecting ionic species [27] in aqueous solutions, there has been no report of a Raman spectroscopy-based analytical approach to DDTC-correlated copper ion quantification in real industrial wastewater samples. In this work, we found that SERS could be applied for detecting Cu2+ ions with the micromolar sensitivity in non-cyanide wastewater samples. In the case of cyanide presence in wastewater, samples need to be treated with alkaline chlorination before Cu2+ detection. The potential application of this research is to develop a quick and easy spectroscopic tool to estimate the toxic amounts of harmful chemicals in wastewater. Our work may also be used to detect the presence of cyanide ions in wastewater samples before and after alkaline chlorination. 2. Materials and Methods 2.1. Materials and Preparation of AuNPs and DDTC-Metal Complex Sodium diethyldithiocarbamate trihydrate ((C2 H5 )2 NCSSNa·3H2 O) and the metal ionic substances were purchased from Sigma Aldrich (St. Louis, MO, USA). AuNPs were prepared by the previous method [28]. Our quantification methods for Cu2+ ions were based on our recent investigation [29]. We prepared AuNPs using a citrate reduction method. First, a triple-distilled water solution of hydrogen tetrachloroaurate trihydrate was mixed, stirred, and heated to the boiling point of water. Subsequently, sodium citrate was quickly added to this mixture and continuously stirred while the mixture was boiling. Finally, the level of the reaction solution was always kept at the beginning levels for 1 h by adding triple-distilled water slowly and continuously. The AuNPs were obtained at about 20 nm in diameter according to the measurements by Otsuka ELZ-2 and high-resolution transmission electron microscope (TEM) (JEOL JEM-3100). To prepare the UV-Vis experiment with the DDTC-Cu2+ complex, all of DDTC (0.89 mM in triple-distilled water, 500.0 µL) and Cu2+ (1.0 mM in triple-distilled water, 50.0 µL) were put into a 2.0 mL Eppendorf tube as a first step. This mixture solution (pH = 7.0) was stirred and kept stable for 30 min at room temperature. In the second step, 450.0 µL of triple-distilled water was added into this mixture to obtain 1000.0 µL solution of DDTC-Cu2+ complex with 50.0 µM final concentration of Cu2+ . After that, UV-Vis spectra of 1000.0 µL of DDTC-Cu2+ complex was recorded. Samples with other metal ions were prepared by substituting Cu2+ . For the SERS experiment of DDTC-Cu2+ complex on AuNPs, in the first step, all of the DDTC (8.9 mM in triple-distilled water, 50.0 µL) and Cu2+ (10.0 mM in triple-distilled water, 5.0 µL) was put into a 2.0 mL Eppendorf tube. This mixture solution (pH = 7.0) was stirred and kept stable for 30 min at room temperature. In the second step, 445.0 µL of AuNP solution was added into this mixture to obtain a 500.0 µL solution of the DDTC-Cu2+ complex on AuNPs with a 100.0 µM final concentration of Cu2+ . Then, SERS spectra of 500.0 µL of the DDTC-Cu2+ complex on AuNPs was recorded. Samples with other metal ions were prepared by substituting Cu2+ . 2.2. Instrumentations and DFT Calculations UV-Vis absorption spectral changes of the DDTC-metal complexes before and after applying to the AuNP colloidal solution were obtained with a 3220 PC spectrophotometer (Mecasys, Daejeon, Korea). Atomic percentages in wastewater was obtained using a NEXION 350 D ICP-MS spectrometer (Perkin-Elmer, Boston, MA, USA). DFT calculations [30] and potential energy distribution were

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performed using the previous literatures [31]. The Au6 cluster is one of the simplest models of gold models gold atoms. It includes calculations to predict the energetic stabilities and intramolecular atoms. Itofincludes calculations to predict the energetic stabilities and intramolecular interactions of the interactions of the adsorbates on Au surfaces. This model includes six gold atoms that can connect adsorbates on Au surfaces. This model includes six gold atoms that can connect with each other to with eachthe other to generate the triangular geometry needed to form gold clusters. generate triangular geometry needed to form gold clusters. All Raman data were obtained by using a Raman microscope system RM RM 1000 1000 spectrometer spectrometer All Raman data were obtained by using a Raman microscope system (Renishaw, Gloucestershire, Gloucestershire, UK) UK) with with aa 632.8 632.8 nm nm HeNe HeNe excitation excitation laser laser and and aa CCD CCD camera. camera. The (Renishaw, The SERS SERS 2+ detection spectra were recorded by using spectroscopic glass tubes after the preparation of the Cu 2+ spectra were recorded by using spectroscopic glass tubes after the preparation of the Cu detection samples. set up samples. The The integration integration time time for for SERS SERS measurement measurement was was set up at at 10 10 ss per per spectrum spectrum with with aa range range of of −1. Prior to performing SERS, the spectral positions were calibrated based on Si peak at 200–3200 cm − 1 200–3200 cm . Prior to performing SERS, the spectral positions were calibrated based on Si peak at −1 520 520 cm cm−.1 .

2.3. Preparation of Wastewater Samples and Removal of the Cyanide Species Using Alkaline Chlorination Chlorination The from thethe wastewater treatment center (Pusan, Korea). The The real realwater watersamples sampleswere wereobtained obtained from wastewater treatment center (Pusan, Korea). cyanide reactions could be derived from thethe previous literature The cyanide reactions could be derived from previous literature[32]. [32].Samples Samples“S1” “S1”and and “S2” “S2” were were obtained and after after the the process process of of removing removing the the cyanide cyanide species, species, respectively. respectively. obtained before before and 3. Results and Discussion 2+ on AuNPs 3.1. Adsorption of DDTC-Cu2+ on AuNPs 2+ ions ions with with a complex of DDTC and subsequent Scheme 1 is a diagram of our our detection detection of of Cu Cu2+ 2+ ions not As shown shown in Scheme 1a, our method would be able adsorption on AuNPs. As able to to detect detect Cu Cu2+ only using the UV-Vis method, but, more importantly, also based on the SERS tool, which could demonstrate a much higher selectivity and sensitivity capacity than the UV-Vis UV-Vismethod. method.

(a)

(b)

2+ ions with (a) a complex of DDTC and subsequent Scheme 1. Schematic of detection detection of of Cu Cu2+ Scheme 1. Schematic diagram diagram of ions with (a) a complex of DDTC and subsequent adsorption in electroplating electroplating wastewater wastewater after after alkaline alkaline chlorination chlorination under adsorption on on AuNPs AuNPs and and (b) (b) in under the the interference from the other ionic species. interference from the other ionic species.

In general, our method could be promisingly applied to wastewater samples in both the presence In general, our method could be promisingly applied to wastewater samples in both the presence of CN−−and the absence of CN−.−Under wastewater conditions without CN−, the results indicate the of CN and the absence of CN . Under wastewater conditions without CN− , the results indicate ability for DDTC-Cu2+ complex detection via both the colorimetric indicator and the SERS tool (as the ability for DDTC-Cu2+ complex detection via both the colorimetric indicator and the SERS shown in Scheme 1b with the S2 sample). Moreover, in the case of the presence of CN− (the S1 sample), tool (as shown in Scheme 1b with the S2 sample). Moreover, in the case of the presence of CN− our approach may also be useful as a quick and simple method to identify the cyanide species in (the S1 sample), our approach may also be useful as a quick and simple method to identify the cyanide electroplating industrial water, since the copper cyanide complex cannot bind efficiently with DDTC, species in electroplating industrial water, since the copper cyanide complex cannot bind efficiently based on the results of UV-Vis and SERS in comparison with the S2 sample. with DDTC, based on the results of UV-Vis and SERS in comparison with the S2 sample. Figure 1a shows a photo of DDTC-metal complexes. The Cu2+ ion2+exhibited a yellow color, which Figure 1a shows a photo of DDTC-metal complexes. The Cu ion exhibited a yellow color, was supported by the UV-Vis absorption spectra of DDTC-metal complexes, as shown in Figure 1b. which was supported by the UV-Vis absorption spectra of DDTC-metal complexes, as shown in 2+ The inset in Figure 1b shows the UV-Vis absorption spectra, as well as a photo of DDTC-Cu2+ Figure 1b. The inset in Figure 1b shows the UV-Vis absorption spectra, as well as a photo of DDTC-Cu 2+ 2+ corresponding to various concentrations of the Cu ion. The absorption bands of DDTC-Cu exhibited prominent bands at ~450 nm, which is in different from other tested ions as previous reported [21]. The band at 520 nm is the spectrum of the pristine AuNPs just after synthesis, without

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corresponding to various concentrations of the Cu2+ ion. The absorption bands of DDTC-Cu2+ exhibited prominent bands at ~450 nm, which is in different from other tested ions as previous reported [21].17,The Sensors 2017, 2628band at 520 nm is the spectrum of the pristine AuNPs just after synthesis, without 4 of 11 any DDTC complexes. In Figure 1c, upon adsorption on AuNP surfaces, the plasmonic bands of any AuNPs DDTC complexes. In Figureconsiderably 1c, upon adsorption on to AuNP the plasmonic bands of of initial at 520 nm became redshifted ~700surfaces, nm, indicating the aggregation initial AuNPs at 520 nm became considerably redshifted to ~700 nm, indicating the aggregation AuNPs due to the strong binding of sulfur atoms in DDTC on Au. Although we found thatofthe AuNPs due to the color strongand binding ofspectral sulfur atoms in after DDTC on Au. Although we found that 1d thedid aggregation-induced UV-Vis changes adsorption upon AuNPs in Figure color andspecies, UV-Vispresumably spectral changes after upon AuNPs in Figure 1d in notaggregation-induced depend on the metal ionic due to theadsorption strong binding of the sulfur atoms did not depend on the metal ionic species, presumably due to the strong binding of the sulfur atoms 2+ DDTC on Au, the strong binding of the Cu ions with DDTC as shown in Figure 1a,b may change the in DDTC on Au, the strong binding of the Cu2+ ions with DDTC as shown in Figure 1a,b may change minute adsorption characteristics on AuNPs. Despite the extensive aggregation of AuNPs, regardless the minute adsorption characteristics on AuNPs. Despite the extensive aggregation of AuNPs, of the complex types of DDTC with different ions as indicated in Figure 1d, SERS may exhibit minute regardless of the complex types of DDTC with different ions as indicated in Figure 1d, SERS may spectral changes depending on the binding modes. As listed in Table 1, our DFT calculations predicted exhibit minute spectral changes depending on the binding modes. As listed in Table 1, our DFT that the ν(N=C) mode at 1490–1520 cm−1 for DDTC could be −1sensitively changed on Au atoms, calculations predicted that the ν(N=C) mode at 1490–1520 cm for DDTC could be sensitively depending on the binding modes and the presence of Cu(II) ions. To find any different interfacial changed on Au atoms, depending on the binding modes and the presence of Cu(II) ions. To find any interactions, we performed Ramanwe spectroscopy. different interfacial interactions, performed Raman spectroscopy.

(a)

(c)

(b)

(d) Figure 1. (a) Photo of DDTC and DDTC-metal complexes for NH4+, K+, Ca2+, Mg2+, Cd2+, Pb2+, Hg2+, Figure 1. (a) Photo of DDTC and DDTC-metal complexes for NH4 + , K+ , Ca2+ , Mg2+ , Cd2+ , Pb2+ , Hg2+ , Zn2+, Co2+, Cr3+, Ni2+, Fe2+, Fe3+, Cu2+, and Mn2+ ions. (b) UV-Vis absorption spectra of DDTC-metal Zn2+ , Co2+ , Cr3+ , Ni2+ , Fe2+ , Fe3+ , Cu2+ , and Mn2+ ions. (b) UV-Vis absorption spectra of DDTC-metal complexes. The inset shows the absorption bands at ~450 nm and the photo of DDTC-Cu2+ complex complexes. The inset shows the absorption bands 2+at ~450 nm and the photo of DDTC-Cu2+ complex corresponding to various concentrations of the Cu ion. (c) UV-Vis absorption spectra of AuNPs and corresponding to various concentrations of the Cu2+ ion. (c) UV-Vis absorption spectra of AuNPs DDTC-metal complexes on AuNPs. The inset shows the TEM image of the AuNPs-DDTC-Cu2+ and DDTC-metal complexes on AuNPs. The inset shows the TEM image of the AuNPs-DDTC-Cu2+ complex. The scale bar is 100 nm. (d) High-resolution image of AuNPs with a scale bar of 5 nm. Photo complex. The scale bar is 100 nm. (d) High-resolution image of AuNPs with a scale bar of 5 nm. of AuNPs and DDTC-metal complexes on AuNPs. Photo of AuNPs and DDTC-metal complexes on AuNPs.

3.2. Raman Spectra of DDTC-Cu2+ on AuNPs Figure 2a exhibits normal Raman (NR) for the solid state of DDTC, and the SERS spectra of DTTC, DDTC-Zn2+, and DDTC-Cu2+ on AuNPs. Our spectra appear consistent with that in the previous report [24]. Notably, the vibrational band at ~1490 cm−1 was prominently blueshifted to

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3.2. Raman Spectra of DDTC-Cu2+ on AuNPs Figure 2a exhibits normal Raman (NR) for the solid state of DDTC, and the SERS spectra of DTTC, 2+ and DDTC-Cu2+ on AuNPs. Our spectra appear consistent with that in the previous DDTC-Zn Sensors 2017, 17,, 2628 5 of 11 report [24]. Notably, the vibrational band at ~1490 cm−1 was prominently blueshifted to ~1504 cm−1 , 2+ . Such spectral2+changes were not observed for the as marked arrowsin in red the case of DDTC-Cu ~1504 cm−1in , asred marked arrows in the case of DDTC-Cu . Such spectral changes were not SERS spectra of DDTC-metal on AuNPs, as illustrated in Figure 2b. The difference mayThe be observed for the SERS spectracomplexes of DDTC-metal complexes on AuNPs, as illustrated in Figure 2b. 2+ 2+ due to the exceptionally energyhigh of the Cu ion to DDTC. explain the spectral difference may be due tohigh the binding exceptionally binding energy of theTo Cu ion to DDTC. Tochanges, explain the quantum mechanical calculations were DFT introduced to better assign the Raman peaksassign of DDTC spectral changes, the DFT quantum mechanical calculations were introduced to better the with a complex of the metal ion in a free and adsorbed state on Au cluster atoms. An appropriate Raman peaks of DDTC with a complex of the metal ion in a free and 6 adsorbed state on Au6 cluster vibrational assignment vibrational is summarized in Table is 1. summarized in Table 1. atoms. An appropriate assignment

2+

1423

Cu

2+

Zn

AuNPs-DDTC

Raman Intensity (Arbitr. Unit)

1432 1454 1490 1504

1270

1147


n+

[M ]=50uM

NR of DDTC

500

AuNPs-DDTC

1000 -1 Wavenumber (cm )

(a)

1500

500


Cu(II) Zn(II) Pb(II) Ni(II) NH4(I) Na(I) Mn(II) Mg(II) K(I) Hg(II) Fe(II) Fe(III) Cr(III) Co(II) Cd(II) Ca(II)

1500

(b)

2+, Figure 2. and SERS SERS spectra spectra of of DDTC, DDTC, DDTC-Zn DDTC-Zn2+ Figure 2. (a) (a) Normal Normal Raman Raman (NR) (NR) for for the the solid solid state state of of DDTC DDTC and , 2+ on AuNPs. In the case of the DDTC-Cu 2+ complex, the vibrational band at ~1490 cm −1 and DDTC-Cu DDTC-Cu2+ and on AuNPs. In the case of the DDTC-Cu2+ complex, the vibrational band at ~1490 cm−1 −1, − was prominently cmcm as1 ,marked in red (b) The(b) SERS of DDTCwas prominently blueshifted blueshiftedtoto~1504 ~1504 as marked inarrows. red arrows. Thespectra SERS spectra of 2+ 2+ 2+ 2+ + 2+ 2+ + 2+ 2+ 3+, + 2+ , Zn 2+ , ,Pb 2+4, Ni 2+ ,, NH 2+ , Mg on AuNPs for Cu ,for ZnCu , Pb , Ni NH , Na Mn4 +, ,Mg , Hg , Fe2+ ,, K Fe+3+ Cr2+ metal complexes DDTC-metal complexes on AuNPs Na+,, K Mn , ,Hg , 2+ 2+ 2+ 2+ 3+ 3+ 2+ 2+ 2+ , Cd , and Ca . Co Fe , Fe , Cr , Co , Cd , and Ca . 2+ on Au. Table 1. 1. Spectral Spectral data data and and vibrational vibrational assignments assignments for forDDTC DDTCand andDDTC-Cu DDTC-Cu2+ Table on Au. a DFT b Assignments Based on PED SERS on b Assignments DFT DDTC-SERSSERS a DFT on on a DFT on Based on PED 2Cu(DDTC)SERS Au6 6 Calculations DDTC-Au Au Au Cu(DDTC)2Au -Au6 6 Au Au Calculations --- — β(C–N–C) + β(N–C–S) —--— --268 268 268 268 β(C–N–C) + β(N–C–S) 329 — --345 345 367 367 ν(Cu–S) + β(S–C–S) 350350 329 ν(Cu–S) + β(S–C–S) 415 435 435 407 407 434 434 β(C–C–N) + ν(S–C) + γ(N–S–S–C) 426426 415 β(C–C–N) + ν(S–C) + γ(N–S–S–C) 523 552 552 523 523 547 547 ν(S–C)ν(S–C) + γ(N–S–S–C) + β(C–N–C) 567567 523 + γ(N–S–S–C) + β(C–N–C) 756 — --756 756 — ν(N=C)(CH 2) 2) 775775 756 --ν(N=C)(CH 835 849 — 841 — δ(H–C–C–N) 835 849 --841 --δ(H–C–C–N) 910 935 902 942 885 ν(C–C) 910 935 902 942 885 ν(C–C) 1003 997 1005 997 998 ν(N=C)(CH2 ) + ν(S–C) + ν(C–C) 2) + ν(S–C) + ν(C–C) 1003 997 1005 997 998 ν(N=C)(CH 1074 1051 1080 1043 1075 ν(N=C)(CH2 ) + ν(C–C) 1074 1051 ν(N=C)(CH2) + ν(C–C) 1131 1144 1144 1080 1152 1043 1147 1075 δ(H–C–C–N) 1131 1144 δ(H–C–C–N) 1261 1292 1270 1144 1276 1152 1270 1147ν(N=C)(CS2 ) + δ(H–C–N–C) + β(H–C–C) δ(H–C–N–C) +2 )β(H–C–C) 1261 1292 ν(N=C)(CS2)++β(H–C–H)(CH 1367 1354 1350 1270 1354 1276 1366 1270 δ(H–C–N–C) 2) 1367 1354 δ(H–C–N–C) + β(H–C–H)(CH 1412 1416 1423 1350 1447 1354 1432 1366 ν(N=C)(CS 2 ) + β(H–C–H)(CH 2) 1449 1462 1454 1423 1470 1447 1454 1432 β(H–C–H)(CH ) + β(H–C–H)(CH 1412 1416 ν(N=C)(CS 2 ) + 2β(H–C–H)(CH 3 ) 2) 1474 1493 1490 1454 1517 1470 1504 1454 ν(N=C)(CS 2) + β(H–C–H)(CH 3) 1449 1462 β(H–C–H)(CH 2 ) + β(H–C–H)(CH 2) a The ) +out-of-plane β(H–C–H)(CH 2) 1474 1493 1504 β: in-plane ν(N=C)(CS scale factor of 0.97 was applied. b 1490 Abbreviations: δ: 1517 Torsion, ν: stretching, bending,2γ: bending.

NRNR of of DDTC DDTC

a

a The scale factor of 0.97 was applied. b Abbreviations: δ: Torsion, ν: stretching, β: in-plane bending, γ: out-of-plane bending. 3.3. DFT Calculations of DDTC-Cu2+ on AuNPs

The Calculations DFT calculated spectra of DDTC and DDTC-Cu2+ on the Au6 cluster under the polarizable 2+ on 3.3. DFT of DDTC-Cu AuNPs continuum model (PCM), appeared to match well with the SERS spectrum of the experimental Raman The DFT calculated spectra of DDTC and DDTC-Cu2+ on the Au6 cluster under the polarizable continuum model (PCM), appeared to match well with the SERS spectrum of the experimental Raman spectrum of DDTC. The bands at ~1423 and ~1490 cm−1 can be both ascribed to the vibrational modes of C=N stretching and CH2 bending modes. Of these, the stretching vibration of C=N is supposed to be more Raman-active dominant. These two bands, which are sensitive to the C=N

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80 60

10,000

40 20 10 1 0

1400 1500 1600 -1 Wavenumber (cm )

(b)

Raman Intensity at 1504 cm-1

[Cu2+]/ uM 100

0

(a)

1432

20,000

1454


1490 1504

spectrum of DDTC. The bands at ~1423 and ~1490 cm−1 can be both ascribed to the vibrational modes of C=N stretching and CH2 bending modes. Of these, the stretching vibration of C=N is supposed to be more Raman-active dominant. These two bands, which are sensitive to the C=N stretching modes, canSensors be expected to change considerably on AuNPs if the copper ion replaces the sulfur atom, 6leading 2017, 17, 2628 of 11 to change in the relative Raman signals based on the present theoretical calculation. Although not sulfurhere, atom, to change in of theDDTC relative Ramantosignals based thetopresent theoretical shown theleading bond length of C=N appeared decrease fromon1.39 1.34 Å after binding 2+ Although not shown here, thefrequencies. bond length of C=N of DDTC appeared to decrease from to calculation. Cu , resulting in increased vibrational 1.39 to 1.34 Å after binding to Cu2+, resulting in increased vibrational frequencies. 3.4. Quantification of the Cu2+ Ion on the Basis of Raman Spectra 3.4. Quantification of the Cu2+ Ion on the Basis of Raman Spectra According to a previous report [33], the C=N stretching vibrational frequency should increase According to a is previous [33],acids the C=N vibrational frequency that should when the Schiff base bound report to Lewis suchstretching as H+ and BF3 . Considering theincrease Cu2+ ion, when the Schiff base is bound to Lewis acids such as H+ and BF3. Considering that the Cu2+ ion, as an as an electron pair receptor, could play the role of a Lewis acid, its binding to the nitrogen atom electron pair receptor, could play the role of a Lewis acid, its binding to the nitrogen atom could could increase the C=N stretching vibrational frequencies. A recent SERS study also indicated that the increase the C=N stretching vibrational frequencies. A recent SERS study also indicated that the DDTC-based thiram and ziram may coordinate with a gold film as either a monodentate or a bidentate DDTC-based thiram and ziram may coordinate with a gold film as either a monodentate or a mode on Au [34]. bidentate mode on Au [34]. Figure 3a shows the concentration-dependent SERS spectra of DDTC on AuNPs. The peak Figure 3a shows the concentration-dependent SERS spectra of DDTC on AuNPs. The peak −1 steadily increased, depending on the concentration of the Cu2+ ion, intensities intensitiesatat~1504 ~1504 cm cm−1 steadily increased, depending on the concentration of the Cu2+ ion, as as magnified magnifiedininFigure Figure 3b. The calibration of the Raman peak intensities [Cu2+in] is 2+] is shown 3b. The calibration curve curve of the Raman peak intensities versus [Cuversus shown in3c. Figure 3c. Figure R2=0.98 4,000

3,000

2,000

1,000 0

10

20 30 40 [Cu2+] / uM

50

60

(c)

Figure 3. (a) Cu2+ concentration-dependent SERS spectra of DDTC on AuNPs in distilled water. (b) A Figure 3. (a) Cu2+ concentration-dependent SERS spectra of DDTC on AuNPs in distilled water. magnified view of the spectral region from 1380 to 1600 cm−1. (c) Three independent measurements (b) A magnified view of the spectral region from 1380 to 1600 cm−1 . (c) Three independent of Raman intensities of vibrational bands at ~1504 cm−1 were performed to provide the standard measurements of Raman intensities of vibrational bands at ~1504 cm−1 were performed to provide the deviations and a linear fit for the concentration range between 1 and 60 μM. standard deviations and a linear fit for the concentration range between 1 and 60 µM.

3.5. Raman Spectroscopic Quantification of Cu2+ Ions in Electroplating Wastewater Samples 3.5. Raman Spectroscopic Quantification of Cu2+ Ions in Electroplating Wastewater Samples Considering that alkaline chlorination can remove hazardous cyanide species from wastewater Considering alkalineour chlorination remove from wastewater samples [12–15],that we applied method to can detecting thehazardous presence ofcyanide cyanide species ions in real wastewater samples [12–15], our method to detecting presence of cyanide ions in wastewater samples before we andapplied after alkaline chlorination. Table 2the shows the atomic percentages of real various metal samples beforein and after alkaline chlorination. TableFigure 2 shows the atomic percentagesresults of various metal ionic species electroplating wastewater samples. 4 shows our spectroscopic from the electroplating wastewater samples: standard solution of [CN] = 100 our ppm, “S1” (cyanide-containing), ionic species in electroplating wastewater samples. Figure 4 shows spectroscopic results from the and “S2” (after alkaline chlorination). Undersolution our experimental UV-Vis absorption and electroplating wastewater samples: standard of [CN] = conditions, 100 ppm, “S1” (cyanide-containing), SERS DDTCchlorination). on AuNPs, depending onexperimental the concentration of Cu2+ UV-Vis for sample “S2” after and “S2”spectra (after of alkaline Under our conditions, absorption and 2+ alkaline chlorination, looked similar to those in the distilled water, whereas the cyanide-containing SERS spectra of DDTC on AuNPs, depending on the concentration of Cu for sample “S2” after samples of the standard and similar “S1” didtonot exhibit such behaviors. Thewhereas complexthe formation of the Cu2+ alkaline chlorination, looked those in the distilled water, cyanide-containing 2+ ion with the standard CN species may hamper theexhibit binding of Cu and DDTC. In Figureformation 4b, the absorption samples of the and “S1” did not such behaviors. The complex of the Cu2+ band was weakened for the cyanide-containing wastewater samples. This interpretation could also 2+ ion with the CN species may hamper the binding of Cu and DDTC. In Figure 4b, the absorption be supported by the relatively weak CN intensity at ~2114 cm−1 [29] of the wastewater after the band was weakened for the cyanide-containing wastewater samples. This interpretation could also be alkaline chlorination treatment of sample “S2”, as shown in Figure 4c. supported by the relatively weak CN intensity at ~2114 cm−1 [29] of the wastewater after the alkaline chlorination treatment of sample “S2”, as shown in Figure 4c.

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(a)

(b)

(c)

(d)

Figure 4. (a) Initial photo of the industrial wastewater samples: [CN] = 100 ppm, “S1” (cyanide-

Figure 4. (a) Initial photo of the industrial wastewater samples: [CN] = 100 ppm, containing), and “S2” (after alkaline chlorination). Photo of wastewater samples after dilution and “S1” (cyanide-containing), and “S2” (after alkaline chlorination). Photo of wastewater samples after treatment with DDTC. (b) UV-Vis absorption spectra of [CN], “S1”, and “S2” after treatment with dilution and treatment with DDTC. (b) UV-Vis absorption spectra of [CN], “S1”, and “S2” after DDTC. (c) SERS spectra of [CN], “S1”, and “S2” on AuNPs after treatment with DDTC. (d) A treatment with DDTC. (c) SERS spectra of [CN], “S1”, and “S2” on AuNPs after treatment with DDTC. magnified view of the C≡N triple bond stretching region at ~2114 cm−1. (d) A magnified view of the C≡N triple bond stretching region at ~2114 cm−1 . Table 2. Atomic percentages of various metal ionic species in electroplating wastewater samples.

Table 2. Atomic percentages of various metal ionic species in electroplating wastewater samples. Sample Cr Mn Fe Ni Cu Zn “S1” (cyanide wastewater) ND * ND 342.95 703.85 84.69 2447.67 Sample Cr Mn Fe Ni Cu Zn “S2” (after alkaline chlorination) ND 2.38 468.28 667.66 77.06 2175.26 “S1” (cyanide wastewater) ND * ND 342.95 703.85 84.69 2447.67 * ND: not detected. “S2” (after alkaline chlorination) ND 2.38 468.28 667.66 77.06 2175.26 Figure 5 shows the concentration-dependent SERS spectra of DDTC on AuNPs in electroplating * ND: not detected. the wastewater sample “S2”. The peak intensities at ~1504 cm−1 steadily increased, depending on the concentration of the the concentration-dependent Cu2+ ion, similar to the caseSERS of the distilled detection of the Figure 5 shows spectra of water. DDTC The on AuNPs in limit electroplating current SERS method was found to be around ten times lower than that of the colorimetric test under − 1 the wastewater sample “S2”. The peak intensities at ~1504 cm steadily increased, depending on our experimental conditions. The lowest concentration of our SERS detection of Cu2+ in wastewater the concentration of the Cu2+ ion, similar to the case of the distilled water. The detection limit of the samples was around 1 ppm, which is lower than the Environmental Protection Agency permission current SERS method was found to be around ten times lower than that of the colorimetric test under level (1.3 ppm = ~20 μM) for drinkable water [29]. Figure 6 illustrates the alkaline chlorination process our experimental conditions. The lowest concentration of our SERS detection of Cu2+ in wastewater of the influent sample “S1” to remove the cyanide species in electroplating the industrial wastewater samples was around 1 ppm, which is“S2”. lower than the Environmental Protection to produce non-cyanide wastewater The following equations can be applied toAgency treat thepermission cyanide levelwastewater (1.3 ppm =by ~20 µM) for drinkable water [29]. Figure 6 illustrates the alkaline chlorination process alkaline chlorination.

of the influent sample “S1” to remove the cyanide species in electroplating the industrial wastewater 8CN− + 2Cu2+ → 2Cu(CN)3− + (CN)2. (Stoichiometry cyanide reaction with Cu2+) to produce non-cyanide wastewater “S2”. The following equations can be applied to treat the cyanide wastewater alkaline chlorination. Clby 2 + 2NaOH → NaOCl + NaCl + H2O (alkaline chlorination) − 2+ − 8CN 2Cu(CN) + (CN)2 (Stoichiometry cyanide reaction with Cu2+ ) − + NaCl 3 (Destruction NaOCl++2Cu CN− →→ OCN of CN−) − +2NaOH Cl2 + NaCl H (alkaline chlorination) 2OCN− + 3OCl 2OH− →→ N2NaOCl + 3Cl− ++2CO 32− ++H 2O (Destruction of OCN− and OCl−) 2O

NaOCl + CN− → OCN− + NaCl (Destruction of CN− ) 2OCN− + 3OCl− + 2OH− → N2 + 3Cl− + 2CO3 2− + H2 O (Destruction of OCN− and OCl− )

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10 1

0

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5,000 1450 1500 -1 Wavenumber (cm )

(c)

2

R =0.98

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1550

Raman Intensity at 1504 cm

2+

1400

1500

(b) 1490 1504

1454

15,000

1432


(a)


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2,000

0

0

10

20 30 2+ [Cu ] / ppm

40

50

(d)

2+ Figure 5. ofof thethe DDTC-Cu in wastewater withwith bands at ~450 2+ complex Figure 5. (a) (a)UV-Vis UV-Visabsorption absorptionspectra spectra DDTC-Cucomplex in wastewater bands at 2+ ion. The inset shows a photo of the DDTC-Cu2+ nm, depending on the concentrations of the Cu ~450 nm, depending on the concentrations of the Cu2+ ion. The inset shows a photo of the DDTC-Cu2+ 2+] from 0 to 50 ppm in wastewater. (b) Cu2+ concentration-dependent SERS spectra complex with with [Cu [Cu2+ complex ] from 0 to 50 ppm in wastewater. (b) Cu2+ concentration-dependent SERS spectra of DDTC on AuNPs of DDTC on AuNPs in in wastewater wastewater samples samples after after alkaline alkaline chlorination chlorination treatment. treatment. (c) (c) A A magnified magnified view view −11. (d) Three independent measurements of Raman intensities at − of the region from 1400 to 1550 cm of the region from 1400 to 1550 cm . (d) Three independent measurements of Raman intensities at vibrational bands bandsof of~1504 ~1504cm cm−−11 were were performed performedtotoprovide providethe thestandard standard deviations and a linear fit vibrational deviations and a linear fit for for concentration the concentration between 50 The ppm. The samples for the calibration curve were the rangerange between 1 and150and ppm. samples for the calibration curve were obtained 2+) with the DDTC-Cu 2+ complex and the 2+ 2+ obtained by dilution of the initial wastewater (77.06 ppm Cu by dilution of the initial wastewater (77.06 ppm Cu ) with the DDTC-Cu complex and the AuNP made bythe changing AuNP solution. The samples other Cu2+ concentrations solution. The samples with the with otherthe Cu2+ concentrations could alsocould madealso by changing volumesthe of 2+ complex, accordingly. 2+ volumes of wastewater and DDTC-Cu wastewater and DDTC-Cu complex, accordingly.

Alkaline chlorination can be divided into two steps. (1) A highly alkaline condition (pH > 10) Alkaline chlorination can be divided into two steps. (1) A highly alkaline condition (pH > 10) will suppress the generation of gaseous HCN to maintain the free cyanide ions in wastewater will suppress the generation of gaseous HCN to maintain the free cyanide ions in wastewater (workers should be cautious about exposure to the gas in the atmosphere). A chlorine gas injection (workers should be cautious about exposure to the gas in the atmosphere). A chlorine gas injection at least seven times higher than that of cyanide will yield cyanate (OCN−), avoiding the formation of at least seven times higher than that of cyanide will yield cyanate (OCN− ), avoiding the formation the metal cyanide complexes and the other chlorine adducts, including cyanogen chloride (CNCl). of the metal cyanide complexes and the− other chlorine adducts, including cyanogen chloride (CNCl). (2) The cyanide adducts, such as OCN , can be destroyed by lowering the pH to 8.5 to decompose (2) The cyanide adducts, such as OCN− , can be destroyed by lowering the pH to 8.5 to decompose them to CO2 and N2. them to CO2 and N2 .

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Figure 6. An alkaline chlorination process of the influent sample “S1” to remove the cyanide species Figure 6. An alkaline chlorination process of the influent sample “S1” to remove the cyanide species in in electroplating industrial wastewater to produce the non-cyanide wastewater in sample “S2”. electroplating industrial wastewater to produce the non-cyanide wastewater in sample “S2”.

4. Conclusions 4. Conclusions Our study showed that a facile detection method for Cu2+ ions in the DDTC-metal complexes Our study showed that a facile detection method for Cu2+ ions in the DDTC-metal complexes could be achieved by monitoring specific marker bands in the SERS spectra. The SERS bands at ~1504 could be achieved by monitoring specific marker bands in the SERS spectra. The SERS bands at cm−1 increased after the introduction of DDTC-Cu2+ complexes on AuNPs. This could be interpreted ~1504 cm−1 increased after the introduction of DDTC-Cu2+ complexes on AuNPs. This could be as the conformation of the complex that would have different orientations on Au as supported by interpreted as the conformation of the complex that 2+would have different orientations on Au as 2+, Pb2+, Mg2+, Cd2+, Ca2+, Hg2+, NH4+, Cr3+, DFT calculations. The other ions of Ni2+, Fe2+, Co2+, Mn2+ , Zn2+ supported by DFT calculations. The other ions of Ni , Fe , Co2+ , Mn2+ , Zn2+ , Pb2+ , Mg2+ , Cd2+ , Fe3+ , and 2+ K+ did not exhibit such spectral behaviors. The UV-Vis absorption and a colorimetric method 2+ Ca , Hg , NH4 + , Cr3+ , Fe3+ , and K+ did not exhibit such spectral behaviors. The UV-Vis absorption were also introduced to check the [Cu2+]-induced spectroscopic changes. Our method can be and a colorimetric method were also introduced to check the [Cu2+ ]-induced spectroscopic changes. successfully applied to real electroplating wastewater samples. After removal of the CN species via Our method can be successfully applied to real electroplating wastewater samples. After removal alkaline chlorination, the DDTC spectral features could be correlated with the concentration of Cu2+. of the CN species via alkaline chlorination, the DDTC spectral features could be correlated with the concentration of Cu2+ . Acknowledgments: This work was supported by the Korea Environment Industry & Technology Institute (KEITI) through the Technologies for the Water Supply Sewerage Policy of Public Technology Program Based Acknowledgments: This work was supported by the&Korea Environment Industry & Technology Institute Environmental Policy Project funded by Korea Ministry of Environment (MOE) (No. 2016000700005). The (KEITI) through the Technologies for the Water Supply & Sewerage Policy of Public Technology Program Based Environmental Policy Project funded by Korea Ministry of Environment (MOE) (No. 2016000700005). The authors authors would like to thank Moon-Kyung Kim and Taekyung Kim for helping the experiments. would like to thank Moon-Kyung Kim and Taekyung Kim for helping the experiments. Author Contributions: S.-W.J. and N.H.L. conceptualized the study. T.D.N. conducted DFT calculations. K.-D.Z. Author Contributions: S.-W.J. and N.H.L. conceptualized the study. T.D.N. conducted DFT calculations. K.-D.Z. provided the insight of the ionic detection for environmental water samples. N.H.L. designed and conducted provided the insight of the ionic detection for environmental water samples. N.H.L. designed and conducted the the experiments. S.-W.J. analyzed the data. All authors reviewed the manuscript. experiments. S.-W.J. analyzed the data. All authors readread and and reviewed the manuscript. Conflictsof ofInterest: Interest:The Theauthors authorsdeclare declareno noconflict conflictof ofinterest. interest. Conflicts

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