Highly selective and sensitive method for Cu2+

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 186 (2017) 76–81

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Highly selective and sensitive method for Cu2 + detection based on chiroptical activity of L-Cysteine mediated Au nanorod assemblies Shahryar Abbasi ⁎, Hamzeh Khani Department of Chemistry, Ilam University, Ilam, Iran

a r t i c l e

i n f o

Article history: Received 5 February 2017 Received in revised form 6 May 2017 Accepted 29 May 2017 Available online 30 May 2017 Keywords: Plasmonic circular dichroism Cu2+ detection Au NRs assembly L-Cysteine

a b s t r a c t Herein, we demonstrated a simple and efficient method to detect Cu2+ based on amplified optical activity in the chiral nanoassemblies of gold nanorods (Au NRs). L-Cysteine can induce side-by-side or end-to-end assembly of Au NRs with an evident plasmonic circular dichroism (PCD) response due to coupling between surface plasmon resonances (SPR) of Au NRs and the chiral signal of L-Cys. Because of the obvious stronger plasmonic circular dichrosim (CD) response of the side-by-side assembly compared with the end-to-end assemblies, SS assembled Au NRs was selected as a sensitive platform and used for Cu2+ detection. In the presence of Cu2+, Cu2+ can catalyze O2 oxidation of cysteine to cystine. With an increase in Cu2+ concentration, the L-Cysteine-mediated assembly of Au NRs decreased because of decrease in the free cysteine thiol groups, and the PCD signal decreased. Taking advantage of this method, Cu2+ could be detected in the concentration range of 20 pM– 5 nM. Under optimal conditions, the calculated detection limit was found to be 7 pM. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Copper is an essential trace element in the human body, acting as a cofactor and/or a structural component of numerous enzymes such as cytochrome oxidase, nitrate reductase and superoxide dismutase [1]. However, the accumulation of Cu2+ in humans and animal livers can cause DNA damage [2], low-density lipoprotein oxidation [3] which will lead to serious conditions, including neurodegenerative diseases (e.g., Wilson's diseases) and prion diseases [4]. Since the hazardous effects of high level of Cu2+ ion, the maximum allowable level of Cu2+ ion should not exceed 20 μM by EPA in drinking water [5]. Therefore, development of a portable but highly sensitive and selective method for the detection of Cu2+ ion is greatly important. As such, the identification and measurement of Cu2 + is emerging important. Currently, various techniques are available for the determination of copper ion, including inductively coupled plasma [6], electrochemical techniques [7] UV–Vis methods based on surface Plasmon resonance [8] and fluorescence assays [9,10]. However, these methods have a number of shortcomings; for instance, they are expensive or have higher detection limit than the standard of the EPA, making them unsuitable for on-site analysis. Therefore, development of an easy-handled and cost-effective approach for Cu2+ detection is important. Optical activity characterization based on circular dichroism (CD) spectrum is a powerful tool for chiral molecule conformational analysis. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Abbasi).

http://dx.doi.org/10.1016/j.saa.2017.05.064 1386-1425/© 2017 Elsevier B.V. All rights reserved.

Often, the CD bands of chiral molecules, such as biomolecules, are weak and located in the UV light range. Recently, it have been demonstrated that conjugation of noble metal nanoparticles (NPs), such as Au and Ag with chiral molecules could transfer the CD response to the visible wavelength region [11]. This new CD signal appeared at the frequency of surface plasmonic resonance (SPR) band of the noble metal NPs is named as plasmonic CD [12]. Such plasmonic responses is highly amplified with construction of three-dimensional (3D) assembly of plasmonic nanoparticles into chiral structures active in the visible region [11,13, 14], where the radiative plasmon − plasmon interaction between NPs is responsible for CD signal enhancement [15,16]. Regarding better sensitivity and reliability of the visible optical measurements compared to the UV ones, PCD has been demonstrated to be used as a new detection method for chiral recognition and enantioselective separation [17,18]. Wu et al. detected prostate-specific antigen based on a circular dichroism signal from plasmonic nanoparticle dimmers [19] or chiral Au NP pyramids were used for endonuclease analysis [20]. Compared with spherical NPs, the anisotropic noble metal NPs like Au nanorods (Au NRs) possess two directional electron oscillations in response to the polarization of the incident light, the transverse Plasmon band and the longitudinal Plasmon band, and moreover they are easily assembled to form spatial chirality, e.g., noncentrosymmetric structures [21]. Recent studies showed that these unique properties of Au NRs make them the excellent candidates to produce strong CD responses [22,23]. For examples, mercury ion was detected by using chiroptical activity of Au NRs assemblies [24]. Based on the Hg2 +-mediated T–T base pair of DNA, a concentration-dependent CD response for analysis of Hg2 +

S. Abbasi, H. Khani / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 186 (2017) 76–81

with LOD 0.03 ng/mL were obtained. In another work, chiral side-byside or end to end assemblies of Au NRs have been explored for the attomolar detection of DNA [25]. Previously, large chiral molecules were employed to link Au NRs or Au NRs first linked using non-chiral molecules, such as sodium citrate. In the latter case, Au NR assemblies didn't have any CD signal and by addition of small chiral molecules PCD response will be produced [26]. In this work, we use another approach to induce PCD. The Au NRs were linked directly using chiral molecules, such as L-Cysteine. By changing the concentration of cetyltrimethylammonium bromide (CTAB), two typical patterns of assembly, end-to-end (EE) or side-by-side (SS) are constructed between Au NRs in the presence of L-Cys. Before adding L-Cys, Au NRs exist in the form of stable monodisperse particles. Addition of L-Cys can induce the gold nanorods into end-to-end or side-byside assembly, giving rise to changes of UV–Vis absorption. Furthermore, L-Cys is a chiral molecule with distinct CD signals at UV region and the coupling between surface plasmon resonances (SPR) of Au NRs and the chiral signal of L-Cys will induce generation of plasmonic CD at the visible region. Then we use this PCD signal for detection of Cu2+. According to a previous report, Cu2+ can catalyze O2 oxidation of L-Cys to quickly form disulfide cystine [27,28]. With an increase in the concentration of Cu2+, the L-Cysteine-induced PCD decreased because of the decrease in the free cysteine thiol groups and therefore, the concentration of Cu2+ can be detected. 2. Experiment 2.1. Material and Methods Sodium borohydride (NaBH4), cetyltrimethylammonium bromide (CTAB), Copper (II) chloride dehydrate (CuCl2·2H2O), and L-Cysteine (Cys) were purchased from Sigma-Aldrich and used as received. Chloroauric acid (HAuCl4·3H2O) and silver nitrate (AgNO3) was received from Merck. Milli-Q water (18 MΩ cm) was used for all solution preparation and all the other substances were analytical-reagent grade. UV–Vis absorption spectra were obtained either simultaneously with the CD spectra on the CD spectrometer or from a Varian 300 Bio, Australia UV–Vis spectrophotometer with Czerny-Turner monochromator and PMT detector with a 1 cm quartz cuvettes, at a scan speed of 2000 nm/min with a bandwidth setting of 1 nm. CD measurements were conducted on a JASCO J-810 CD spectrometer in aqueous solution. A 1 mL portion of each sample was infused into a 0.5 cm quartz cell and measured at the scan speed of 500 nm/min with a bandwidth of 5 nm. The light source system was protected by nitrogen (flow rate: 5 L·min− 1). The baseline (CD signal of the pure Au NR solution) Subtracted from the spectrum of the sample. The pH adjustment was carried out using pH-meter (Metrohm, model-780, Switzerland). A thermostat bath (Mem-mert, Germany) maintained at the desired temperature was used gold nanorods synthesis. 2.2. Synthesis of Gold Nanorods Gold (Au) nanorods were prepared using the silver ion-assisted seed-mediated method [29]. Firstly, the seed solution was prepared in the following procedure: 0.25 mL of 10 mM HAuCl4 solution was mixed with 7.5 mL of 100 mM CTAB solution in 30 °C water bath with gentle mixing. To the mixture solution, 0.6 mL of 10 mM freshly prepared ice-cold NaBH4 solution was added, which resulted in formation of a brownish yellow seed solution. The seed solution was vigorously stirred for 2–3 min and stored at 30 °C. Secondly, growth solution was prepared by adding 0.5 mL of 10 mM HAuCl4 solution into 10 mL of 100 mM CTAB solution with gentle stirring for 1 min. To this solution, 60 μL of 10 mM AgNO3 solution and 60 μL of 100 mM ascorbic acid solution were added with rapid stirring. Ascorbic acid as a mild reducing agent changed the growth solution from dark yellow to colorless. Finally, 12 μL of as-prepared seed solution was added to the growth solution.

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The color of the solution gradually changed colorless after vigorously stirring for 3 min. Then the Au nanorod growth solution was left undisturbed and aged for 12 h at 30 °C. This pathway produced pure Au nanorod solutions with aspect ratios of 3.1. The Au nanorods were purified by centrifugation (10,000 rpm for 20 min) twice and re-dispersed into distilled water, and then sonicated for 30 min and stored for further use. 2.3. Preparation of Au NRs with Different Concentration of CTAB Au NRs synthesized above was further used as standard solution. Different volume of 100 μM pure CTAB solution as stock solution was added to the standard Au NR solution to get Au NRs with different concentration of CTAB. The prepared Au NRs were stored overnight before the following experiment. 2.4. Assembly of Au NRs Different concentration of L-Cys was added into 0.8 nM Au NRs suspension stabilized with different concentration of CTAB to initiate the SS and EE assembly of the Au NRs. After 3 min, the solution was subjected to CD or UV–Vis survey after stirring for 30 s. 2.5. Detection of Cu2+ The mixture of Cu2+ with different concentrations and 20 μM Cys (pre-incubated in a 27 °C waterbath for 15 min) was added to Au NR solutions. The total volume of the sample was 1 mL. Then, the CD spectra were recorded after 3 min. 3. Result and Discussion 3.1. Preparation and Characterization of Au NRs Au NRs were prepared using the silver ion-assisted seed-mediated method [29]. The characteristic transverse SPR (TSPR) and longitudinal SPR (LSPR) absorption peaks of as-prepared Au NRs are 515 and 725 nm, respectively (Fig. S1) and the nanorod concentration is estimated to be ≈0.8 nM. The corresponding extinction values at these two SPR peaks are 0.82 and 3.14, respectively, suggesting a high yield of Au nanorods. The average length, diameter, and aspect ratio, determined from ≈200 nanorods on transmission electron microscopy (TEM) images, are 40.5 ± 4.1 nm, 13.0 ± 1.1 nm, and 3.1 ± 0.4, respectively (Fig. S2). 3.2. Sensing Principle for Cu2+ ion Detection Assembly of the Au NR alters the localized surface plasmon resonance (LSPR) signals. Chiral Cys and Au NRs are intentionally assembled in two modes: end-to-end (EE) and side-by-side (SS) modes. Changing the concentration of cetyltrimethylammonium bromide (CTAB) surfactants in Au NR solution causes the different assembly modes in the presence of 10 μM L-Cys that is identified by UV–vis absorption and CD spectra [30]. Figs. 1A and B, present the extinction spectra of Au nanorod solutions and representative TEM image of the nanorods assembled at varying CTAB concentration, respectively. As-prepared Au NRs have two SPR band located at 515 and 725 corresponded to transverse SPR (TSPR) and longitudinal SPR (LSPR), respectively (Curve a in Fig. 1A). When the concentration of CTAB in solution was b10 μM, addition of 10 μM LCys would result in random aggregation of Au NRs. Inability of the low concentration of CTAB to capsulize and stabilize Au NRs cause the random aggregation. In the presence of 10 μM CTAB, by addition of 10 μM L-Cys intensity of the longitudinal SPR (LSPR) absorption reduced, while a new absorption shoulder at 800 nm appears (Curve b in Fig. 1A). This is the typical feature of the end-to-end Au NR assembly [31]. By increasing the concentration of CTAB to 20 μM (Curve d in Fig. 1A),

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Fig. 2. CD spectra of (a) original Au NRs and L-Cys assembled Au NRs under different CTAB concentrations. The concentration of CTAB is as following: (b) 10, (c) 15, (d) 20, and (e) 25 μM.

Fig. 1. (A) UV−Vis absorption of original Au NRs (a) and L-Cys assembled AuNRs under different CTAB concentrations. The concentration of CTAB is as following: (b) 10, (c) 15, (d) 20, and (e) 25 μM.(B) TEM images of (a) end-to-end and (b) side-by-side assembly of the Au NRs.

addition of L-Cys results in both intensity reduction and blue shift of LSPR peak while the transverse Plasmon peak slightly red shift, which is the characteristic of SS assembled Au NRs [31]. Further increasing of the CTAB concentration brings forth a red-shift of longitudinal plasmon peak and a dramatic increase in the intensity of the longitudinal plasmon band that shows the degree of SS assembly is gradually reduced (Curve e in Fig. 1A) [30]. The TEM image of the Au nanorods solution after assembly confirms the existence of the end-to-end and side-by-side Au NR assembly as shown in Fig. 1B. Fig. 2 present the CD spectra acquired from the formation of SS and EE Au NR assemblies recorded at varying CTAB concentration in the presence of 10 μM L-Cys. No noticeable CD signal is observed from the Pure Au NR solution (Fig. 2, curve a), that in agreement with the fact that Au nanorods are achiral. Meanwhile, L-Cys alone doesn't exhibit any CD signal above 300 nm (not shown). Thus, it can be concluded that any CD response at the visible light range should be attributed to the formation of complexes between Au nanorods and chiral Cys. By addition of 10 μM CTAB to the Au NRs solution in the presence of L-Cys, EE assembly of Au NRs occurs. Since Cys is a chiral molecule with distinct CD signals at UV region, the coupling between surface plasmon resonances (SPR) of Au NRs and the chiral signal of L-Cys result in generation of plasmonic CD at the visible light region (Curve b in Fig. 2). As expected, three CD peaks appear at the wavelength of 500–800 nm, corresponding to transverse and longitudinal SPR of Au NRs, respectively. These new CD peaks originated from chiral current inside Au nanorods that induced by the bounded Cys [22]. Which are thus called as plasmonic CD responses [32]. By increasing the CTAB concentration, Au NRs prefer to assemble in the SS fashion and therewith, the plasmonic

CD response becomes stronger (Curve c in Fig. 2). When the concentration of CTAB increased to 20 μM, the plasmonic CD response of the Au NRs reached to the largest extent (Curve d in Fig. 2). Further increasing CTAB concentration, caused to decrease in the intensity of the CD signal (Curve e in Fig. 2). As CTAB concentration exceeds a certain threshold value, the amount of disorder increases and the CD intensity begins to decrease because of the reduction of hot spots [30]. Generally, when the SS assembly extent of Au NRs increases, the plasmonic CD response of the Au NRs obviously becomes stronger. Chirality of SS assemblies of Au NRs employing different CTAB concentration can be potentially used for biosensing of Cu2+. The possibility of improving the limit of detection (LOD) by taking advantage of the bisignate wave-shape of the CD signals can be one of the advantages of the method. According to previous report [27], L-Cysteine was oxidized to L-Cystine by O2 in the presence of Cu2+ ion and the catalytic reaction is as follows: Cu2þ

RSH + O2 → 2 RSSR + 2 H2O Generally, when Cu2 + adds to L-Cysteine solution, oxidation of L-Cysteine to form L-Cystine occurs that results in the decrease of the free cysteine thiol groups. Adding this mixture to the Au NRs solution, extent of the Au NRs assemblies and corresponding plasmonic CD peaks will be decreased. Fig. 3 shows the CD spectra of the SS and EE assembled Au NRs in the absence and presence of 10 nM Cu2+ ion. As can

Fig. 3. CD Spectra of the SS (a and c) and EE (b and d) assembled Au NRs in the absence (a and b) and presence (c and d) of 1 nM Cu2+ ion.


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be seen from Fig. 3, the CD signal of both SS and EE assembly of the Au NRs dramatically decrease by addition of Cu2+. This is an indication for the fact that, the presence of Cu2+ has a significant effect on the concentration of L-Cys. For as much as EE assembly of Au NRs has smaller CD signal intensity and the addition of Cu2 + caused to less significant change in the CD peak than SS fashion, therefore, in order to achieve a high sensitive sensor, plasmonic CD peak of the SS assembled Au NRs was chosen as a sensitive platform and used for further investigation. 3.3. Optimization of Experimental Conditions Thus CD signal of the side-by-side assembled Au NRs was used for ultrasensitive determination of Cu2+ ion. In this study, several factors that may affect CD response of the SS assembled Au NRs were optimized. The effect of the L-Cys concentration on the CD intensity of the SS assembled nanorods at the 620 nm was investigated. As displayed in Fig. S3, it can be obviously seen that by increasing the concentration of L-Cys, the PCD signal first increases and reaches a maximum at 20 μM. further increasing L-Cys concentration reduces the PCD signal. Based on this 20 μM L-Cys choose as the optimal concentration for the detection of Cu2+ ion. Furthermore, our experimental results demonstrate that CD response of the SS assembled Au NRs in the visible light region are sensitive to the pH value of the Au NRs solution. Fig. S4 shows the CD spectra of the Au NRs solutions at varying pH values in the presence of 20 μM CTAB and 20 μM L-Cys. A strong negative maximum is seen at all pH tested range from 3.5–7.5. The maximum CD signal appears at pH 6, and both increasing and decreasing pH of mixture result in the decrease in CD response which is in good agreement with previous reports [30]. Moreover incubation time fixing between L-Cys and Au NRs are of vital importance to the sensor performance. The effect of incubation time of the Au NRs with L-Cys on the CD signal was given in the Fig. S5. Fig. S5 displays the dependence of the CD response of the incubation time in the presence of 20 μM CTAB and 20 μM L-Cys with pH = 6.0. As can be seen from Fig.S5, the CD signal appears immediately after the samples are mixed and reduces with increasing time. Thus all measurements have been taken within 3 min. Assembly temperature is another effective parameter to the sensor performance (Fig. S6). Changes in the temperature cause negligible change in PCD. Therefore 30 °C was chosen as the optimal temperature. Next, the optimal Au NR concentration was investigated. As displayed in Fig. S7, when the concentration of Au NRs fixed at 0.8 nM, maximum PCD signal will be obtained. 3.4. Analytical Performance of Sensing System The ability of the proposed sensor for determination of Cu2 + ion under the optimized conditions was evaluated. The required time for L-Cys oxidation catalyzed by Cu2 + ions is determined by measuring the changes of CD signal at various incubation times. As displayed in Fig. S8, 20 μM L-Cysteine could be oxidized by O2 within 15 min at the greatest extent. Thus, first a mixture of 20 μM L-Cys and different concentrations of Cu2+ were incubated for 15 min and then added to Au NRs solution with 20 μM CTAB. The total volume of the sample was 1 mL. Then, the CD spectra were recorded. Under the optimal conditions, different concentrations of Cu2+ were used to assess the accuracy of the assay. The CD intensity at 620 nm versus the concentration Cu2+ was plotted in Fig. 4. It was clearly observed that CD intensity gradually decreased with increasing the concentration of Cu2 +. The CD amplitude versus Cu 2 + concentration could be fitted as the equation of CD = 2.3747C − 20.7240 and a linear relationship is found in the range of 20 pM–5 nM with the correlation coefficient of 0.9921. The calculated detection limit was found to be 7 pM (3σ/slope, n = 5), which is lower than the US Environmental Protection Agency-defined maximum contaminant level for copper in drinking water (20 μM). This sensitivity was approximately three orders of magnitude higher than that of previously reported

Fig. 4. Plots of CD intensity at 620 nm versus different concentration of Cu2+ in the range 20 pM–5 nM.

colorimetric Cu2 + detection wherein L-Cysteine-induced aggregation of Au NPs decreased in the presence of Cu2 + [33]. Obviously, the change in UV–vis absorption features is much smaller than that in plasmonic CD responses with respect to both the line shape and intensity of the peaks because in addition to electronic transitions, plasmonic CD responses are much more sensitive to the configuration transformation of nanostructures [22]. In addition, we compared the characteristics of this sensor with other nanoparticle based detection methods for copper ions. As seen from Table 1, the detection limit of our proposed method is comparable or better than other methods. 3.5. Selectivity of the Sensor for Cu2+ To evaluate the selectivity of the proposed method, we added various environmentally relevant metal ions instead of Cu2 +, including Co2 +, K+, Na+, Fe3 +, Ag+, Ni2 +, Hg2 +, Fe2 + and Al3 +. The results show that the existence of N30-fold excess of these metal ions had no evident effect on the PCD peak under optimized conditions. The results are summarized in Fig. 5. Except for Co2+ and Fe3+, the CD intensities upon the addition of various other metal ions were similar to that given by the blank. These results indicated that this method meets the requirements of Cu2+ assay. 4. Conclusion In conclusion, we have developed a facile and sensitive assay for determination of Cu2+ ion based on plasmonic circular dichroism. Compared to other CD probes, the CD signal of this case occurs in the visible light region by a significant enhancement in the intensity, which effectively eliminates interference from CD active analytes as well as improves the detection reliability (the visible light detector is more sensitive than the UV detector). The proposed method has several advantages, such as ease of operation and rapid results (below 20 min). It is expected that this sensor based on CD response of the assemblies between noble metal nanoparticles and chiral Cys will provide a facile and versatile platform for determination of other molecules because of many different reactions that Cu2+ involves them. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.05.064.

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Table 1 Analytical performances of various strategies based on the various nanoparticles/nanorods for copper detection in aqueous solution. Technique

LOD

Ref

Catalytic leaching of silver-coated gold nanoparticles Catalytic etching of silver nanoplates (AgNPls) by thiosulfate (S2O2− 3 ). Copper-mediated leaching of gold nanorods Assembly of magnetic nanoparticles induced by the Cu2+-dependent ligation DNAzyme Copper-catalyzed etching of gold nanorods Copper-modulated formation of core–shell gold nanorods Copper-catalyzed leaching of silver from silver-coated gold nanorods Oxidation of gold nanorods by copper-catalyzed H2O2 oxidation Solid-phase extraction and the RP-HPLC Disassembly of gold nanorods by copper catalyzed oxidation of Cys

1 nM 80 nM 0.5 nM 2.8 nM 2.7 nM 2 fM 3 nM 4.96 nM 32 pM 7 pM

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[22] Fig. 5. Effect of various metal ions on PCD intensity. [23]

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