Cu Nanoclusters: Novel Electrochemiluminescence Emitters for

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Cu Nanoclusters: Novel Electrochemiluminescence Emitters for Bioanalysis Min Zhao, An-Yi Chen, Dan Huang, Ying Zhuo,* Ya-Qin Chai, and Ruo Yuan* Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China

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S Supporting Information *

ABSTRACT: Cu nanoclusters (Cu NCs), which emerged as a new class of nontoxic, economic, and excellent phosphors and catalysts, have attracted increasing interest for a wide variety of promising applications in biolabeling and biocatalysis. However, the electrochemiluminescence (ECL) behavior of Cu NCs has never been reported in previous works. Here, anodic and blue ECL emission of Cu NCs was observed for the first time with the efficient coreactant of hydrazine (HZ), and the possible luminescence mechanism of Cu NCs/HZ ECL system was studied in detail. Briefly, HZ was oxidized, and Cu NCs got the energy to generate excited state Cu NCs* for light radiation. Furthermore, a highly sensitive “signal-off” sensing platform for the determination of dopamine has been developed upon effectively quenching of dopamine toward the Cu NCs/HZ-based ECL system. As a result, this proposed method for dopamine detection possesses high selectivity, good stability, and excellent sensitivity with a detection limit down to 3.5 × 10−13 M. This indicates that Cu NCs show potential for applications in ECL bioanalysis as a new type of low-cost and superior luminophore candidates.

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biolabeling,16 and catalysis.17 It is reported that the preparation of stable Cu NCs suffered from the difficulties of oxidation susceptibility, size control, and irreversible aggregation.16,18,19 Thus, the introduction of protecting ligands, mainly referring to DNA,15,20 amino acids,21 sulfhydryl compounds,22 peptides,23 and protein,16,19,24 was an efficient way to improve the stability of Cu NCs. Ghosh et al.24 elaborated that highly fluorescent Cu NCs with the stabilizer of lysozyme exhibited high photostability and colloidal stability in an aqueous medium under ambient conditions. Goswami and co-workers19 stated that extremely stable bovine serum albumin (BSA)-protected Cu NCs aqueous solution with excellent fluorescence features was synthesized by a one-pot method and further applied for quantitative analysis of lead ion. Although Cu NCs-based fluorescence analysis has been widely reported, the ECL behavior of Cu NCs has never been uncovered in current research studies, to date. The difficulties to expand the ECL performance of Cu NCs may be attributed to the limited potential window available in aqueous solutions and the insufficient ECL emission without the satisfactory coreactants. Herein, stable Cu NCs were prepared by optimizing the amount of BSA protecting ligands, and then the anodic ECL emission of Cu NCs aqueous solution was observed for the first time. In order to amplify the ECL emission of Cu NCs, different coreactants [such as H2O2, TPA, and hydrazine (HZ)] were investigated, and the most highly intense ECL emission of Cu NCs was generated using the coreactant of HZ. This

ecently, nanocrystal-based electrochemiluminescence (ECL) has received increasing interest in immunoassays and DNA analysis1,2 and so forth due to the strongly sizedependent electronic, optical, and electrochemical properties of luminescent nanocrystals. Among them, the semiconductor nanocrystals of CdS,3 CdSe,4 and PdS5 and so forth were extensively chosen as ECL emitters due to their narrow emission band, good photostability, and high quantum yields. However, heavy metals (e.g., Cd2+, Pb2+) as the essential elements in available high-performance semiconductor nanocrystals have led to restrictions in the application of bioanalysis, which could further cause a critical environmental and health hazard. For a benign environment to be maintained, development of environmentally friendly and low-toxicity or nontoxic ECL emitters for bioassays has attracted explosive attention. Lately, noble metal nanoclusters with outstanding electrical conductivity and excellent biocompatibility,6,7 Au nanoclusters (Au NCs) and Ag nanoclusters (Ag NCs)-centric ECL substitutes, have become particular focuses in fundamental and practical application of chemical sensing8−10 and biosensor assays.11 Ding’s group has observed the near-infrared anodic ECL emission of Au38 nanoclusters and Au144 nanoclusters in acetonitrile solution with the tripropylamine (TPA) coreactant, respectively.12,13 Zhu’s group 9 and Lv’s group 10 have individually studied in detail the cathodic ECL of Au NCs and Ag NCs using K2S2O8 as the coreactant and then applied these materials to sensitively detect small biomolecules. Considering the high consumption of noble silver and gold, it remained a substantial challenge to seek a low-cost candidate with high-efficiency ECL emission. Cu nanoclusters (Cu NCs), which emerged as a kind of novel photoluminescent and catalytic nanomaterials, have gained growing popularity in various areas of sensing,14,15 © 2016 American Chemical Society

Received: July 19, 2016 Accepted: November 4, 2016 Published: November 4, 2016 11527

DOI: 10.1021/acs.analchem.6b02770 Anal. Chem. 2016, 88, 11527−11532

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ECL measuring process, the detecting buffer was 0.1 M pH 8.0 phosphate buffer solution (PBS) composed of K2HPO4, NaH2PO4, and KCl. Apparatus. The electrochemiluminescent and electrochemical measurements were conducted with a model MPI-A ECL analyzer from Xi’an Remax Electronic Science & Technology Co. Ltd. (Xi’an, China) and a CHI 660C electrochemistry workstation from Shanghai CH Instruments (Shanghai, China), respectively. During the detection process, a conventional three-electrode system was used with a modified glass carbon electrode (GCE, Φ = 4 mm) as the working electrode, Ag/AgCl (saturated KCl solution) as the reference electrode, and a platinum wire as the counter electrode. Ultraviolet−visible (UV−vis) absorption spectra and photoluminescence spectra were carried out with the 2450 UV−vis spectrometer (Shimadzu, Japan) and the Hitachi F-2700 spectrofluorophotometer (Shimadzu, Japan), respectively. Morphology and structure characterization of Cu NCs was carried out by a high-resolution transmission electron microscopy (HRTEM, H600, Hitachi, Japan) at an acceleration voltage of 200 kV, a VG Scientific ESCALAB 250 spectrometer operating by Al Kα X-ray (1486.6 eV) as the light source for Xray photoelectron spectroscopy (XPS) analysis, and a Spectrum GX Fourier transform infrared (FTIR) spectroscopy system (PerkinElmer). Synthesis of Cu NCs. The synthetic steps of Cu NCs were outlined according to previously published literature studies19 with a minor modification as follows: 1 mL of 10 mM CuSO4 aqueous solution was put into 1 mL of 2 mg/mL BSA aqueous solution. After magnetic stirring for 5 min, 0.3 mL of NaOH (1.0 M) aqueous solution was added into the above mixture. In the process of the alkali addition, the mixture changed from blue to violet within 3 min and became light brown with stirring for 8 h at 65 °C. After centrifugation at 16 000 rpm for 15 min to retain supernatant, the as-prepared Cu NCs were filtered with a dialysis membrane (MWCO: 3500 Da) to adjust the Cu NCs solution to neutral. Finally, the Cu NCs were stored in the dark at 4 °C before use. HZ Assembled onto the Surface of GCE. First, the GCE was pretreated according to our previous work.11 Next, the assembly of HZ at the GCE (HZ/GCE) was performed in 3 mL of pH 8.0 PBS containing 50 mM HZ with cyclic voltammograms scanning for 40 cycles in the range −1.5 to 1.5 V (100 mV/s). The interfacial changes of GCE before and after assembling HZ were characterized by scanning electron microscopy (SEM) and CVs (Figure S1). Measurement Procedure. During the segment of ECL measurement, the HZ/GCE was investigated with a MPI-A ECL analyzer under the potential range 0−1.45 V in 2 mLof PBS (pH 8.0) containing 10 μg/mL Cu NCs and different concentrations of DA. With an increasing concentration of DA, the ECL response of the Cu NCs/HZ-based ECL system was gradually decreased, which could directly indicate the DA concentrations from the changes of ECL intensity.

indicated that the novel Cu NCs-based ECL emission showed great potential for applications in the field of bioanalysis. Dopamine (DA), as a neurotransmitter within the brain, not only acts on the central nervous system to link to various diseases of schizophrenia, depression, and Parkinson’s disease, and so forth, but also has been involved in the immune system, cardiovascular system, and renal system, playing a critical role.25 Developing a highly sensitive method for DA detection was of great clinical significance. Herein, a sensitive “signal-off” sensing platform for the determination of DA was developed on the basis of a novel electrochemiluminescent system with Cu NCs as emitters and the efficient coreactant of HZ as signal label (Scheme 1B). First, the blue-emitting Cu NCs with BSA as Scheme 1. Schematic Showing the Preparation Process of Cu NCs and the Photo and Typical HRTEM Image of the AsPrepared Cu NCs (A), Sketch Diagram of the Sensing Platform for DA Detection Based on the Cu NCs/HZ ECL System (B), and Proposed ECL Enhancing Mechanism of the Cu NCs/HZ System and ECL Quenching Mechanism by DA toward the Cu NCs/HZ System (C)

protecting ligands were synthesized via a simple chemical reduction method (Scheme 1A). The occurrence of strong Cu NCs ECL was examined using the glassy carbon electrode modified HZ (HZ/GCE) under the potential scan range 0− 1.45 V. Subsequently, the possible luminescence mechanism of the Cu NCs/HZ ECL system was investigated in detail (Scheme 1C). Upon the effective quenching of DA to the Cu NCs/HZ-based ECL system, the responses of the proposed sensing decreased with the increasing concentration of DA. This DA biosensor exhibited high selectivity, good stability, and excellent sensitivity for detection down to a subpicomolar limit. This revealed that Cu NCs, a new type of nontoxic, economical, and excellent ECL candidates, showed promising applications in biosensing construction.

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RESULTS AND DISCUSSION Morphology and Structure Characterization of Cu NCs. The morphology of the as-synthesized Cu NCs was characterized by HRTEM (Figure 1A). It could be seen that Cu NCs were mainly distributed in the range 2−4.5 nm with an average diameter of 2.8 nm (the inset of Figure 1A). Furthermore, the surface bonds of Cu NCs were characterized using FTIR spectroscopy by the comparison of monomeric

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EXPERIMENTAL SECTION Reagents and Materials. Bovine serum albumin (BSA), dopamine (DA), and tripropylamine (TPA) were received from J&K Scientific Ltd. (Guangzhou, China). CuSO4·5H2O, NaOH, hydrazine (HZ), H2O2, and KCl were received from Chengdu Kelong Chemical Industry (Chengdu, China). In the 11528

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fluorescence spectrophotometer. As illustrated in Figure 2A, a weak absorption band of Cu NCs was observed around 325

Figure 2. UV−vis absorption of Cu NCs and PL spectra of Cu NCs with various excitation wavelengths (A). The inset in part A: photographs of the Cu NCs aqueous solution taken under visible light (lamp off) and 365 nm UV light (lamp on). The PL emission spectrum (a) of Cu NCs with the 325 nm excitation wavelengths and ECL emission spectrum (b) of Cu NCs with the help of optical filters (spaced 25 nm).

nm, which was attributed to the quasicontinuous electronic energy band structure and quantum confinement effects of Cu NCs.34 In addition, the strong absorption band around 560 nm appearing from the surface plasmon resonance of larger Cu nanoparticles was not observed,35 revealing the tiny size of the as-synthesized Cu NCs. Moreover, Figure 2A also depicted the excitation-dependent photoluminescence (PL) emission spectra of Cu NCs in the range 280−400 nm. It was found that, with the increasing excitation wavelength, the PL emission wavelength increased to the maximum at 408 nm and then decreased. Intuitively, the as-prepared Cu NCs solution was light brown under ambient daylight, but exhibited blue color with the 365 nm UV lamp radiation (the inset in Figure 2A). Figure 2B demonstrated the ECL emission spectrum (curve a) of Cu NCs with the maximum wavelength at 433 nm. In a comparison with the PL spectrum (curve b), the ECL spectrum exhibited a slight red-shift (408 to 433 nm) in the emission maxima, which could be attributed to the self-absorption12 of Cu NCs and instrument effects. Effects of Coreactants toward Cu NCs. Although the anodic ECL of Cu NCs has been observed, the ECL intensity was weak. Thus, it was necessary to seek an efficient coreactant for amplifying the ECL intensity of Cu NCs. Herein, three different anodic coreactants (TPA, H2O2, and HZ) were chosen to investigate the effects toward the ECL response of Cu NCs. Specifically, the bare GCE was first detected in 3 mL of PBS (pH 8.0) testing buffer containing 10 μg/mL Cu NCs. Then, the above-mentioned three coreatants were added into the testing buffer, respectively. As shown in Figure 3A,B, the ECL responses of Cu NCs with TPA and H2O2 as coreactants increased weakly compared to the ECL response of the pure Cu NCs. However, when the HZ was added into the testing buffer, the ECL response was significantly increased from 60 au to 2090 au (about 34-fold increase, Figure 3C), indicating that HZ could act as an efficient coreactant to amplify the ECL emission of Cu NCs. Thus, the HZ was selected as the desired coreactant in this work. Possible ECL Enhancement Mechanism of HZ toward Cu NCs. In order to save the reagent and reduce experimental error, HZ was assembled onto the surface of the GCE (HZ/ GCE), and the assembly process was illustrated in Supporting Information (The Assembly of HZ onto the Surface of GCE section). Additionally, the investigation of the ECL enhance-

Figure 1. HRTEM image of Cu NCs (A). Inset: size distribution of Cu NCs determined from HRTEM (A). FTIR spectra of the monomeric BSA (a, red line) and BSA-protected Cu NCs (b, blue line) (B). XPS spectra showing the full region of Cu NCs (C) and Cu 2p region (D).

BSA and BSA-protected Cu NCs. As the FTIR spectrum of monomeric BSA showed in Figure 1B (curve a), the broad peak at 3342 cm−1 was attributed to stretching of the amino group (NH2),26 and the peak at 1172 cm−1 was assigned to the CN bending vibration.27 The peaks at 2967 and 2875 cm−1 belonged to the characteristic asymmetric and symmetric vibrations of CH2, respectively.28 The strong peak at 1692 cm−1 was assigned to the CO and CN (amide I) stretching vibration, although the amide (II) peak at 1555 cm−1 originated from CN stretching and NH bending.29 The peaks at 1404 and 1126 cm−1 were attributed to CO stretching30 and CO deformation,31 respectively. The results showed that the BSA had the characteristic functional groups of carboxyl (COOH) and amino (NH2). When the BSA was employed to serve as a protecting ligand for the Cu NCs formation, the characteristic peaks of Cu NCs from the corresponding FTIR spectrum (curve b) exhibited a slight redshift in comparison to characteristic peaks of BSA. The reason could be that Cu(II) was first coordinated with the NH2 and SH of BSA and then was reduced to Cu(0) equipped with BSA as the protecting ligand. In addition, the component of Cu NCs and the oxidation state of copper in the Cu NCs were also investigated with XPS measurement. As displayed in Figure 1C, it could be seen that Cu NCs were made up of all the expected elements including C, O, N, S, Na, and Cu. Furthermore, as presented in Figure 1D, two characteristic peaks were observed at 933.08 and 952.98 eV, which were attributed to 2p3/2 and 2p1/2 features of Cu(0). 32 Also, a shakeup was noted at 943.58 eV, demonstrating the minimal presence of Cu(II) in the assynthesized Cu NCs.19 It was well-known that 2p3/2 binding energy of Cu(0) was only ∼0.1 eV different from that of Cu(I) species.33 Thus, the valence state of Cu NCs most likely fell between 0 and +1. UV−Vis Absorption and Photoluminescence Characterization of Cu NCs. In addition, the optical properties of Cu NCs were further explored by a UV−vis absorption and 11529

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Figure 3. ECL−potential curves of GCEs in Cu NCs solution without (curve a) and with (curve b) coreactants: TPA (A), H2O2 (B), and HZ (C). The concentrations of Cu NCs, TPA, H2O2, and HZ were 10 μg/mL, 10 mM, 10 mM, and 10 mM in PBS (pH 8.0), respectively. The working potential was set in the range 0−1.45 V (100 mV/s).

Figure 4. ECL−potential and the corresponding CV curves of GCE detecting in 3 mL of pH 8.0 PBS contaning 10 μg/mL Cu NCs (A) and HZ/ GCE detecting in 3 mL of pH 8.0 PBS contaning 10 μg/mL Cu NCs without (B) and with (C) 5 nM DA.

Immediately, H2O attacked the carbon atom of CN and induced the addition elimination reaction (eq 2) to obtain the Cu NCs-HZ coordination complex (eq 3). Next, HZ of the Cu NCs-HZ coordination complex was oxidized, and Cu NCs got the energy to generate excited state Cu NCs* (eq 4). Finally, the Cu NCs* excited state gave rise to strong ECL emission when Cu NCs* returned to Cu NCs (eq 5). The possible emission ECL mechanism was outlined as the following equations: ECL Quenching of Cu NCs by DA. An efficient ECL quenching of Cu NCs was observed using DA as the quencher, which could further construct a signal-off methodology for DA detection. Before designing the specific analysis model, the quenching mechanism of DA toward Cu NCs/HZ system was investigated. As displayed in Figure 4C, the ECL response was greatly reduced to 484 au from 1870 au in Figure 4B, when 5 nM DA was added in the working buffer containing 10 μg/mL Cu NCs. Additionally, the CV wave from Figure 4C showed a prominent irreversible oxidation peak at 0.67 V. In this process, DA was oxidized to produce the o-benzoquinone species (BQ) which quenched the excited state Cu NCs* by the energy transfer mechanism.36 The possible enhancing mechanism of HZ and the quenching mechanism of DA toward the ECL emission of Cu NCs were sketched in Scheme 1C. Analytical Performance of the ECL Biosensor toward DA. In order to synthesize stable Cu NCs with strong ECL emission, the amount of BSA should be sufficient as protecting ligands in the synthetic process. The optimization of the amount of BSA was exhibited in Figure S2 in the Supporting Information. It indicated that a strongest ECL signal was obtained when the concentration of BSA was 2.0 mg/mL BSA in Cu NCs synthesis. On the basis of the optimal Cu NCs/HZ ECL system, a simple and sensitive method for the determination of DA has been demonstrated by the HZ/ GCE monitoring in PBS buffer containing 10 μg/mL Cu NCs

ment mechanism of HZ toward Cu NCs was performed by comparing the ECL and CVs characteristics of GCE and HZ/ GCE in 3 mL of pH 8.0 PBS containing 10 μg/mL Cu NCs with potential scanning 0−1.45 V. As exhibited in Figure 4A, a weak anodic ECL signal about 60 au and a featureless voltammetric profile with low currents were observed on bare GCE in Cu NCs solution. After HZ assembled onto the surface of the bare GCE (HZ/GCE), the ECL intensity was raised to 1870 au from 60 au (Figure 4B). The possible enhancement mechanism was speculated as follows: Cu NCs provided an empty track to accept lone-pair electrons of the nitrogen atom of amino group in HZ assembled on GCE (eq 1). Then, the

formed coordination bond of Cu NCs-N made the electron density of the carbon atom reduce due to the inductive effect. 11530

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addition method-based recovery experiments were carried out using the human serum as real samples. First, the healthy human whole blood samples from Xinqiao Hospital of Chongqing were centrifuged at 3500 rpm for 25 min to get purified human serum, and then the human serum was diluted 40-fold with PBS buffer for DA dissolution. As displayed in Table S3, the recovery was in the range 96.0−102.2%, which confirmed that the proposed sensor could determine DA in human serum.

and DA with different concentrations. As displayed in Figure 5A, it was found that the ECL intensity decreased accordingly

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CONCLUSION In summary, the anodic ECL of the Cu NCs aqueous solution with blue emission was first demonstrated in this work. With the employment of HZ as a desired coreactant, the highintensity ECL emission of Cu NCs was generated. Herein, the emissive species Cu NCs* which further radiated light were most probably generated via getting the energy from the oxidation of HZ. On the basis of the new Cu NCs/HZ ECL system, a sensitive “signal-off” sensing platform for DA detection has been constructed with the detection limit down to 3.5 × 10−13 M. This indicated that Cu NCs with low cost provided alternative candidates for expensive and toxic ECL emitters and opened promising avenues to develop new ECL systems for sensing assays.

Figure 5. Standard ECL responses of the Cu NCs/HZ system upon addition of various DA concentrations in 2 mL of pH 8.0 PBS containing 10 μg/mL Cu NCs (A). DA concentrations follow: 0, 1.0 × 10−12 M, 5.0 × 10−12 M, 5.0 × 10−11 M, 1.0 × 10−10 M, 1.0 × 10−9 M, 5.0 × 10−9 M, 1.0 × 10−8 M (from a to h). The corresponding relationship between the ΔECL intensity and the logarithm of DA concentration (B). Selectivity of the Cu NCs/HZ system-based sensing assay for DA detection over other interference including glucose, L-Cys, L-Arg, AA, UA (C). The stability of the developed Cu NCs/HZ system-based sensing under 10 consecutive cyclic potential scans (D).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b02770. Characterization of the biosensor construction, the assembly of HZ onto the surface of GCE, and optimization of the BSA concentration in Cu NCs synthesis (PDF)

with increasing concentration of DA. More specifically, the ECL decrement (ΔI = I0 − I) was proportional to the logarithm of the DA concentration in the range from 1.0 × 10−12 to 1.0 × 10−8 M (Figure 5B) with a linear regression equation of ΔI = 286.2 lg(c/M) + 3765.9 (where ΔI stands for the change of ECL intensity and c stands for DA concentration). Also, an estimated detection limit of 3.5 × 10−13 M was calculated according to 3SB/m (where SB is the standard deviation of the blank, and m is the slope of the corresponding calibration curve).37 In a comparison with other ECL systems and other methods for DA detection listed in Tables S1 and S2, this proposed method based on the novel Cu NCs/HZ ECL system has achieved comparable or even better sensitivity. To further assess the selectivity of this proposed method for DA detection, several small biomolecules including glucose, Lcysteine (L-Cys), L-arginine (L-Arg), ascorbic acid (AA), and uric acid (UA) were selected as the interfering substances. As shown in Figure 5C, the presence of the interference substances with higher concentration (100-fold, 1.0 × 10−8 M) than DA (1.0 × 10−10 M) exhibited slight changes of ECL intensity compared with that of the target. This implied that this proposed method had superior selectivity to discriminate the target and nontarget biomolecules. Moreover, the stability investigation was also carried out by continuous CV scanning for 10 cycles using 5.0 × 10−9 M DA as a model, and Figure 5D displayed consecutive intensity-constant ECL peaks, suggesting good stability for this developed assay. Preliminary Analysis of DA in Samples. To discuss the feasibility of the developed DA sensor in real samples, standard

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86 23 68252277. Fax: +86 23 68253172. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the NNSF of China (51473136, 21575116, 21675129, 21675130) and the Fundamental Research Funds for the Central Universities (XDJK2015A002, XDJK2014A012), China.

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