Inducible Sequential Oxidation Process in Water-Soluble Copper

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An Inducible Sequential Oxidation Process in Water-Soluble Copper Nanoclusters for Direct Colorimetric Assay of Hydrogen Peroxide in a Wide Dynamic and Sampling Range Yibing Du, Jun Fang, Hongli Wang, and Yang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01228 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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ACS Applied Materials & Interfaces

An Inducible Sequential Oxidation Process in Water-Soluble Copper Nanoclusters for Direct Colorimetric Assay of Hydrogen Peroxide in a Wide Dynamic and Sampling Range Yibing Du, Jun Fang, Hongli Wang, and Yang Yang*

†State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China.

ACS Paragon Plus Environment

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ABSTRACT

Direct and fast detection methods for H2O2 have great demand in materials science, biology, and medicine. Colorimetric assay of H2O2 has been regarded as one versatile approach, which can avoid tedious operation and complicated setup. In this report, we provided a cost-effective and time-saving H2O2 colorimetric assay strategy based on a mercaptosuccinic acid (MSA)stabilized Cu nanocluster (NC) probe without using any chromogenic reagent. Direct and fast colorimetric detection of H2O2 was realized based on the color change of MSA-capped Cu NCs in aqueous medium. It was found that the Cu NCs presented eligible resistance to natural oxidation either in concentrated solution or in the powder state. However, the dissolved oxygen in a highly diluted solution of the Cu NCs could trigger the aggregation of the Cu NCs and their further fusion into small Cu nanoparticles (NPs). When this diluted solution served as a probe solution for detecting H2O2, a sequential oxidation process occurred in the newly-formed Cu NPs including the cleavage of MSAs on the surface and conversion of Cu into Cu2O, leading to the probe with capacity for H2O2 assay in a wide dynamic and sampling range. The sensitive solution color change was attributed to the growth of the Cu NPs (fading of plasmonic absorption) upon the addition of low levels of H2O2 and the transition of the valence states of Cu (color reactions) upon the addition of high levels of H2O2. A concentration range of H2O2 from 1 µM to 1 M could be detected by a small dose of the probe. Moreover, the Cu NCs powder


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subsequent to storage for 10 months could maintain a similar sensitivity for H2O2 assay, which provides possibilities for a wide range of practical applications in water samples.

KEYWORDS: H2O2 assay, Cu nanocluster, mercaptosuccinic acid, plasmonic absorption, color reaction


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INTRODUCTION Hydrogen peroxide (H2O2) as one important reactive oxygen is an indispensable intermediate product in essential biological processes, which mediates diverse physiological functions such as cell proliferation, differentiation, and migration.1, 2 H2O2 is generated at a low level in normal biological processes, however, an excessive amount implies the pathogenesis of many diseases such as cardiovascular diseases, cancer, and neurodegenerative diseases.3,

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Besides, excess H2O2 used in food is harmful to our bodies. Therefore, developing uncomplicated, inexpensive, rapid, sensitive and quantitative H2O2-responsive signal generation systems applicable for both low and high levels of H2O2 would be of great importance in clinical medicine, chemical and pharmaceutical industries as well as in food security and environmental applications. Nowadays, many techniques for detection of H2O2 have been developed including fluorescence,5 electrochemiluminescence,6 electrochemical7 and colorimetric8–14 assays. Among these methods, colorimetric assay of H2O2 has been regarded as one of the most powerful and versatile approaches, which can avoid tedious operation and complicated setup. The change of H2O2 concentration could be measured directly by colorimetric observation via naked eyes or by spectroscopic detection.

Classically, the natural enzyme horseradish peroxidase (HRP), which is active for catalyzing H2O2 reduction, is exploited to promote fast oxidization of the peroxide substrate 3,3’,5,5’-tetramethylbenzidine (TMB) to its blue one-electron oxidation product for colorimetric


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monitoring of H2O2.8 Recent experiments have demonstrated that certain nanoparticles with large surface area and controlled catalytic potential possess peroxidase-like activity, which can serve as enzyme mimics in catalyzing H2O2 reduction for H2O2 colorimetric and spectroscopic detection.9–14 However, developing direct and fast approaches which are free from coloration species, long incubation processes and specific reaction conditions has great demand in sensitive and quantitative H2O2 assay applications in aqueous and biological media.

In this regard, gold (Au) nanoclusters (NCs) and nanoparticles (NPs) with a hydrophilic and biocompatible ligand shell have been used as a probe for the detection of H2O2 and glucose (H2O2 is generated from glucose by catalysis of glucose oxidase). A basic principle there is that the existence of H2O2 could destabilize Au NCs/NPs and induce their aggregation. As a result, the fluorescence of the Au NCs quenches15, 16 or the frequency of the plasmonic absorption bands of the Au NPs shifts in UV–vis spectrum with the alteration of their composition, shape and aggregation state, which leads to a change in color of solution.16–26 For example, Nam’s group harnessed glutathione (GSH)-modified Au NPs to detect H2O2 based on the detachment of the GSHs on the Au NPs surface via the formation of glutathione disulfides upon the addition of H2O2.21 The destabilized Au NPs could aggregate to generate the plasmonic couplings between Au NPs, which triggered the color change of the solution with gradual increase in H2O2 concentration. A H2O2 detection range was achieved from 1.29 µM to 1.29 M in the absence of catalyst. This process, however, required an incubation time of 2 h for each mixture at 25 °C.


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Recently, Zhao’s group engineered the surface of Au NPs by a specifically designed ligand. The naked-eye detection of H2O2 was realized based on the color change of Au NPs in aqueous medium upon aggregation via the removal of polyethylene glycol chains from the nanoparticle surface under H2O2 treatment.26 It indicated that a long incubation process up to 24 h was unavoidable for this strategy. This detection system showed a dynamic range in the µM scale with a distinguishable limit of 10 µM.

Relative to the expensive precursors for the synthesis of Au NCs/NPs, copper (Cu) precursors are relatively abundant, low-cost and readily available from commercial sources, which suggests that Cu NCs/NPs with similar properties are more favorable for diverse applications than the noble metal NCs/NPs.27 Recent advances in the analytical applications of Cu NCs/NPs have contained the exploitation of these new-emerging probes for fluorescence, chemiluminescence, electrochemical and colorimetric detection of various analyte, including metal ions, anions, proteins, nucleic acids, small molecules and pH.27–36 Therein, Wang’s group reported that poly(thymine)-templated fluorescent Cu NPs could work as an effective signal indicator for H2O2 sensing.34 The mechanism is derived from the length-dependent formation of Cu NPs on poly(thymine) and the hydroxyl radical (·OH)-triggered oxidative damage of DNA.34 However, in comparison to the extensively investigated Au NPs-based colorimetric sensors, label-free colorimetric assay of H2O2 by Cu NCs/NPs is relatively limited and still at an early


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stage.27 One critical reason lies in that the colloidal and optical stabilities of Cu NPs in physiological media are lower than those of Au NPs due to the higher reactivity of the metal Cu. Alternatively, it has been reported that Ti3+-doped TiO2 NPs synthesized by pulsed laser ablation method could be used for direct colorimetric detection of H2O2, considering the H2O2triggered color reaction through the oxidation of the blue Ti3+-doped TiO2 NPs into yellow H2TiO4.37 The coloration response of this system was free from incubation. The detection range for H2O2 was from 1 µM to 1 M with a detection limit at 0.5 µM. Similarly, Cu NPs with intrinsic reactivity are possible to possess multiple valence states when exposed to environments with reactive oxygen. In case Cu NPs are used as a probe, the varying color of oxidized Cu might widen the dynamic range in colorimetric detection of H2O2 in view of the coincident color reaction. In this work, we synthesized highly hydrophilic mercaptosuccinic acid (MSA)-stabilized Cu NCs in a one-pot reaction in aqueous solution. The as-prepared Cu NCs in both concentrated solution and the powder state presented eligible resistance to natural oxidation. However, the dissolved oxygen in a highly diluted solution of the Cu NCs triggered the aggregation/fusion of the Cu NCs to small Cu NPs. The surface property of the formed Cu NPs was found to be H2O2responsive based on a sequential oxidation process including the cleavage of MSAs on the surface and conversion of Cu into Cu2O. A sensitive color change of solution could occur, which was attributed to the aggregation-induced growth of the Cu NPs (fading of plasmonic absorption) and the transition of the valence states of Cu (color reactions) upon the addition of H2O2 with


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various concentrations. Using small doses of this cost-effective probe, we achieved the spectroscopic assay of H2O2 in a wide dynamic range from 1 µM to 1 M, exempting an incubation process. In addition, a distinctive red-to-yellow color change in the detection system facilitated the rapid definition of H2O2 with contrast levels by the naked eye.

RESULTS AND DISCUSSION

MSA is an inexpensive dicarboxylic acid containing a thiol functional group.38 The synthesis of Cu NCs in aqueous solution employed sodium borohydride as the reducing agent for the Cu(II) salt and MSA as the protecting reagent for the formed Cu NCs (molar ratio of Cu(II):MSA was 1:2). The as-prepared MSA-capped Cu NCs solution was concentrated (approximately 0.05 M based on stoichiometry) with a pH value of 10.0, whereas it was homogeneous and transparent, exhibiting dark brown color (inset in Figure S1a). Figure 1a shows the Fourier transform infrared (FT-IR) spectra of pristine MSA molecules and freezedried purified MSA-capped Cu NCs. In comparison with the two spectra, the peak at 2525 cm-1 that is consistent with the S–H stretching vibration mode of MSA disappeared in the absorption bands of the MSA-capped Cu NCs, indicating that MSA molecules were bound onto the Cu surface through Cu–S bonds. In addition, the C=O stretching band at 1710 cm-1 of carboxyl groups in MSA was substituted by two stretching bands at 1585 cm-1 (antisymmetric) and 1384 cm-1 (symmetric) in the spectrum of the MSA-capped Cu NCs, which is attributable to the


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vibration coupling in the polyelectronic conjugated system of carboxylate ions. Therefore, it could be identified that the thiol end-group of MSA coordinates the Cu surface to form a protective monolayer while the carboxylate ions of MSA render the Cu NCs water-soluble and biocompatible.38

Figure 1. (a) FT-IR spectra of pristine MSA and freeze-dried purified MSA-capped Cu NCs powder. (b) UV– vis absorption spectra of the as-prepared MSA-capped Cu NCs solution and the solution of the Cu NCs diluted in water. (c) UV–vis absorption spectra of the Cu NPs probe solution after the addition of H2O2 at a low (A) and a high (B) concentration; Inset shows the change of absorbance at 520 nm of the Cu NPs probe solution in the presence of 1 mM H2O2 with different pH values. Typical absorbance kinetics of the Cu NPs probe solution at 520 nm in the presence of 0.01 mM H2O2 (d) and at 375 nm in the presence of 500 mM H2O2 (e); Inset shows the corresponding photograph of each case.

The powder X-ray diffraction (XRD) pattern of the product subsequent to purification and freeze drying exhibited no diffraction peaks (Figure S1a), indicating the low crystallinity of the Cu clusters of sub-nanometer sizes.28, 31 The average hydrodynamic radius of the MSA-capped Cu NCs is extremely small, which was under the minimum detection limit of dynamic light


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scattering (DLS) characterization. By electron spray ionization-mass spectrometry (ESI-MS) measurements of the purified Cu NCs solution, it was confirmed that the Cu NCs have a wide mass distribution (Figure S1b), agreeing with the common feature of the Cu NCs directly grown in water recently reported in literature.33 As shown in Figure 1b, the absorption spectrum of the as-prepared Cu NCs solution showed no features in the visible region, but cluster-like optical transitions with absorbance bands below 470 nm.33 However, when the Cu NCs solution was drastically diluted in water, for example, 0.08 mL of the Cu NCs solution was diluted with 3.42 mL of water, the dark brown color gradually changed to a stable claret-red, which could maintain at least for 5 min after reaching a maximum at 25 °C. Accordingly, the UV–vis absorption spectrum of the diluted solution (Figure 1b) exhibited an obvious absorption peak centered at 520 nm, which is related to surface plasmon resonance (SPR) characteristic of Cu NPs. Subjected to H2O2, the above diluted Cu NCs solution accompanied various color changes involving decreased claret-red at low H2O2 concentrations and red-to-yellow at high H2O2 concentrations. Because the pH value in the aqueous solution can significantly influence the state of the MSA molecules capped on the Cu NPs, the effect of the solution pH in this system was first investigated. The maximum absorbance of the Cu NPs probe solution changed slightly in a pH range from 4.0 to 8.0 while it reduced noticeably under the basic condition (pH 8.0–10.0). The inset in Figure 1c presents ∆A520 (A0-A), the change of absorption response of the Cu NPs toward H2O2 in the pH range of 4.0–10.0, where A0 is the absorbance intensity at 520 nm of the Cu NPs probe solution and A is that in the presence of H2O2 with a concentration of 1 mM. It was observed that the optical response of the system showed perceptible pH-dependence and the


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maximum absorbance change in response to H2O2 was obtained at pH 7.0. As a result, we selected the pH value 7.0 as one optimal analytical condition in the subsequent experiments. Such neutral solution also validated potential applications of our assay in physiological environments. We further investigated the effect of performance temperature on the sensitivity of this assay system in neutral solution. It was verified that the absorption response toward H2O2 (e. g., 1 mM) altered slightly by varying the solution temperature from 20 °C to 40 °C (Figure S2). Figure 1c shows the UV–vis absorption spectra of the two typical response systems. The spectrum A exhibits an analogous SPR absorption peak with reduced intensity in the presence of 0.01 mM H2O2. In contrast, the spectrum B shows an absorption platform between 350–550 nm in the presence of 500 mM H2O2. The feature of the latter conforms to the band edge transition of semiconductors. Figure 1d and e exhibit the reaction kinetics results and the corresponding photographs for the two cases triggered by H2O2 with a low (0.01 mM) and a high (500 mM) concentration (the claret-red solution and the yellow solution, respectively), where A520 and A375 values were respectively plotted as a function of reaction time. The reaction progress for each case was monitored up to 300 s at 25 °C. The results revealed that both the reactions were nearly completed at ∼60 s, which did not progress noticeably afterwards. Hence, subsequent to the mixing of the detection target with the probe solution for a period of 60 s, spectroscopic detection of H2O2 can be initiated. Evidently, a long incubation process is not needed in the assay system due to the fast kinetics.

Next, a series of different concentrations of H2O2 were added in the Cu NPs probe solution with a pH value of 7.0 at 25 °C. After sufficient mixing for 60 s, colorimetric responses of the solutions were monitored (Figure 2a). The increase of H2O2 concentration from 0.001 mM to


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1000 mM resulted in the gradual fading of the initial claret red and its further evolvement into saffron yellow, which could be discriminated by the naked eye due to the perceptible chromaticity difference. The recorded UV–vis spectra in Figure 2b revealed a continuing decrease of the absorption peak at 520 nm with the H2O2 concentration increased from 0 to 1 mM. The minimum detectable concentration for H2O2 was found to be 0.001 mM. Figure 2c shows the dependence of ∆A520 (A0-A) as a function of H2O2 concentration (0.001–1 mM). The relationship between ∆A520 and H2O2 concentration could be well fitted by a natural logarithm curve with the correlation coefficient (R2) of 0.9924.

Figure 2. (a) Solution color images of the Cu NPs probe solution after adding different concentrations of H2O2. (b) UV–vis absorption spectra of the mixed solutions in the presence of H2O2 ranging from 0.001 mM to 1 mM. (c) Plot of ∆A at 520 nm versus the concentration of H2O2. (d) UV–vis absorption spectra of the mixed solutions in the presence of H2O2 ranging from 1 mM to 1000 mM. (e) Plot of ∆A at 375 nm versus the


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concentration of H2O2; Inset shows the logarithmic relationship of ∆A375 vs. H2O2 concentration in a range from 1 mM to 50 mM. Error bar: standard deviation calculated from the results of three parallel measurements.

With the H2O2 concentration further increased from 1 mM to 1000 mM, a progressive increase of a new absorption band saturated at ∼375 nm occurred (Figure 2d). Figure 2e shows the dependence of ∆A375 (A1-A0) as a function of H2O2 concentration (1–1000 mM), where A0 and A1 respectively relates to the absorbance intensity at 375 nm of the Cu NPs probe solution after adding H2O2 of 1 mM and higher concentrations. The change of ∆A375 also featured logarithmic characteristics with the H2O2 concentration ranging from 1 mM to 50 mM (R2 = 0.9921). The above results validated the capability of our Cu NPs probe for the colorimetric detection of H2O2 over a wide dynamic range from 0.001 mM to 1000 mM.

It was verified that the sensitivity of this system for H2O2 detection was not significantly improved by using purified Cu NCs solution to prepare the Cu NPs probe solution (Figure S3). This result indicated that dissociative ions in the probe solution had little effect on this assay strategy and the amount of remaining free MSAs was negligible. Hence, we could efficiently exploit the crude Cu NCs solution without purification. In addition, purified and freeze-dried Cu NCs solid powder was stored in a sealed container in dark at room temperature for 10 months. After the powder was re-dispersed in water, the reactions that produced the color changes were not affected (Figure S4).


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Figure 3. Selectivity analysis for the detection of H2O2 by monitoring the relative absorbance change ∆A at 520 nm. H2O2, Na+, K+, Ca2+, methanol, ethanol, acetone, Cr2O72–, MnO4–, and Fe3+ were at a concentration of 1 mM, respectively; GSH was at a concentration of 0.1 mM. Error bar: standard deviation calculated from the results of three parallel measurements.

Control reagents including Na+, K+, Ca2+, ethanol, methanol and acetone were selected to investigate the selectivity of the Cu NPs probe solution for the detection of H2O2 at 25 °C. In relation to the response of the Cu NPs probe solution for low levels of H2O2 (e. g., 1 mM), all the other species presented much weaker effect on the absorbance change at 520 nm, as shown in Figure 3. In addition, the effect of thiol compounds such as glutathione (GSH) was investigated considering the presence of thiols generally induces the aggregation of coinage metal NCs/NPs.36 It was found that GSH at a low concentration (0.1 mM) had certain but limited impact on the absorbance change at 520 nm of the Cu NPs probe solution, whereas GSH at high concentrations (> 0.5 mM) significantly reduced the absorbance at 520 nm possibly due to the aggregation of the MSA-capped Cu NPs through electrostatic interactions induced by this positively charged thiol.39 It was also observed that the effect of oxidizing species such as Cr2O72–, MnO4–, and Fe3+


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on the absorbance change at 520 nm was not comparable with or much weaker than that of H2O2 with the same concentration. Especially, when the Cu NPs probe solution responded to high levels of H2O2 (e. g., 100 mM), all the control substances with the same or higher concentrations showed no evidence of converting the color of the probe solution from red to yellow. The above results revealed eligible selectivity and salt tolerance capability of this system toward H2O2 sensing in common water samples.

For further exploring the feasibility of this sensing system in potential biological applications, we conducted the detection of H2O2 with different concentrations at the physiological temperature of 37 °C in 10 mM phosphate buffer solution with a pH value of 7.4. A similar variation tendency was observed in both the solution color and the corresponding UV– vis absorbance. Figure 4a shows the typical colorimetric response of the Cu NPs solution to H2O2, where the claret-red color faded and gradually changed into yellow by increasing the H2O2 concentration. In the presence of low concentrations of H2O2 from 0.001 mM to 5 mM, the plot of ∆A520 versus H2O2 concentration could be well fitted by a natural logarithm curve (R2 = 0.9973), as depicted in Figure 4b, c. In the presence of high concentrations of H2O2, the change of ∆A375 featured linear characteristics with the H2O2 concentration ranging from 5 mM to 250 mM (R2 = 0.9986), as depicted in Figure 4d, e. We found that the gradually darken yellow color did not get saturated even at a high H2O2 concentration of 1000 mM. The above results indicated that our Cu NPs probe has a large response range toward H2O2 in phosphate buffer solution as


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well. Because H2O2 can be enzymatically generated after the oxidation of glucose by O2 in the presence of glucose oxidase (GOD), the current Cu NPs probing system was also competent for colorimetric detection of glucose in phosphate buffer solution. Figure S5 illustrates the resulting calibration curve of ∆A520 versus glucose concentration, which featured logarithmic characteristics over the range from 1 µM to 50 µM (R2 = 0.9439).

Figure 4. (a) Solution color images of the Cu NPs probe solution after adding different concentrations of H2O2 in phosphate buffer solution (pH=7.4, 10 mM) at 37 °C. (b) UV–vis absorption spectra of the mixed solutions in the presence of H2O2 ranging from 0.001 mM to 5 mM. (c) Plot of ∆A at 520 nm versus the concentration of H2O2. (d) UV–vis absorption spectra of the mixed solutions in the presence of H2O2 ranging from 5 mM to 1000 mM. (e) Plot of ∆A at 375 nm versus the concentration of H2O2, presenting the linear relationship of ∆A375 vs. H2O2 concentration in a range from 5 mM to 250 mM. Error bar: standard deviation calculated from the results of three parallel measurements.


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The practicality of the Cu NPs probe for colorimetric assay of H2O2 in the field of environmental monitoring was additionally evaluated by using real water samples such as tap water. The standard addition method was employed to eliminate any matrix effects.40 The obtained UV–vis absorption spectra in response to various concentrations of H2O2 along with the working curves of ∆A520/∆A375 vs. H2O2 concentration at 25 °C in water were exhibited in Figure S6. As listed in Table1, there were no detectable H2O2 in diluted tap water. The quantitative spike recoveries for the determination of H2O2 were achieved in the range from 96.7% to 104.1% for the tap water samples. The relative standard deviation (RSD) was below 4% for three replicate measurements, indicating the good repeatability and credibility of this analysis. The above results suggested that the Cu NCs-based colorimetric system is highly feasible for detection of H2O2 in real samples. Table 1 Detection of hydrogen peroxide in spiked water samples (n=3). Samplea 1 2 3 4 a

Detected b

ND ND ND ND

Added (mM)

Found (mM)

Recovery (%)

RSD (%)

0.028 0.14 0.56 28.0

0.0291 0.141 0.541 27.3

104.1 100.9 96.7 97.4

2.59 3.28 1.87 1.18

Water samples were collected from tap water and spiked at four different concentration levels.

b

Not detected.

Although the as-prepared Cu NCs were not observable under transmission electron microscopy (TEM) investigations due to their molecule-like tiny sizes and low contrast, the dilution-resulted Cu NPs in the probe solution consisted of many small particles (Figure S7). To determine the solution compositions which were responsible for the various colors, we further


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conducted TEM investigations on two distinctive samples upon the exposure to H2O2 with low and high concentrations. The TEM image in the left of Figure 5a confirmed that the claret-red solution (in the presence of 0.01 mM H2O2) contained a large number of well-dispersed crystallites without significant aggregation. The size histogram (right top) obtained from 200 particles indicated a wide size distribution from 2 nm to 10 nm with an average diameter of 4.5 nm. As exhibited in the HRTEM image of one single crystallite (right bottom), the lattice fringe spacing of 0.20 nm could be indexed to the {111} planes of cubic phase Cu. This result agreed well with the observed UV–vis absorption peak centered at 520 nm, which originated from the SPR of ultrafine Cu NPs. The TEM image in Figure 5b revealed that the crystallites in the yellow solution (in the presence of 50 mM H2O2) were also discrete without severe aggregation. The average diameter in the size histogram (right top) was ~ 6 nm, just slightly larger than that of the Cu NPs. The HRTEM image of one single crystallite (right bottom) indicated a lattice fringe spacing of 0.25 nm, which could be indexed to the {111} planes of cubic phase Cu2O. This result was also compatible with the UV–vis absorption spectrum of this yellow solution, which featured a typical band edge transition with an absorption edge at about 494 nm (2.51 eV). It should be noted that we failed to obtain the average hydrodynamic radius of the MSA-capped Cu NPs before and after reacting with either 0.01 mM or 50 mM H2O2 by DLS characterization due to its minimum detection limit. This result again confirmed that both the probe Cu NPs and the colored reaction products during the sensing process contained tiny sizes plus good solubility and dispersion in the aqueous solution.


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Figure 5. (a) Left: TEM image of the component in the claret-red solution (in the presence of 0.01 mM H2O2); Right top: particle size histogram; Right bottom: HRTEM image of a single crystallite. (b) Left: TEM image of the component in the yellow solution (in the presence of 50 mM H2O2); Right top: particle size histogram; Right bottom: HRTEM image of a single crystallite. (c) and (d) relate to the reduction solid products when the molar ratio of Cu/MSA was 1:0.5 in the synthesis: (c) XRD patterns of the as-precipitated sample (A) and the sample exposed to air for 2 days (B); (d) UV–vis diffuse reflectance spectra of the samples exposed to air for different time.

Because the size, stability and solubility of the probe Cu NPs are strongly dependent on the capping effect of the MSA monolayer anchored on the particle surface, we thus reduced the ratio of MSA molecules and cupric ions from 2:1 to 0.5:1 in the synthesis to intensify the following oxidation process of the as-prepared MSA-capped Cu NPs. The resultant reduction product in this MSA-insufficient environment was black precipitate instead of the dark brown solution. The


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XRD pattern (curve A in Figure 5c) of the newly-formed precipitate contained the diffraction peaks from both Cu and Cu2O of cubic phase. After the precipitate was exposed to air for 48 h, we found that the Cu species gradually vanished, accompanying with the increment of Cu2O (curve B in Figure 5c). This change was also verified by UV–vis diffuse reflectance spectra of the precipitate exposed to air for different time. As shown in Figure 5d, the strong SPR peak derived from Cu NPs gradually declined with the exposure time while the band edge absorption from Cu2O became more and more dominant. The above results confirmed that the formation of large Cu NPs as well as their oxidation into Cu2O NPs was facilitated when the protective MSA molecules became considerably deficient. Once the particle surface was capped by MSA molecules with an optimum density, the stability of the MSA-capped Cu NPs should be greatly improved whilst their sequential oxidation in aqueous solution could be selectively triggered by reactive oxygen species like H2O2.

Figure 6. Schematic diagram for the colorimetric assay of H2O2 based on the Cu NPs probe.


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Figure 6 illustrates the underlying mechanism for colorimetric assay of H2O2 based on the Cu NCs system. The MSA monolayer enwrapping the Cu NCs is supposed to have a more interlaced configuration in concentrated solutions and the solid state, which protects the Cu NCs from natural oxidation. When the Cu NCs are dispersed to a highly diluted solution, the oxygen molecules dissolved in water become numerically superior and capable of removing some weakly-coordinating MSAs from the Cu NCs surface, inducing the aggregation of the Cu NCs and their further conversion to very small Cu NPs. This process was confirmed by dilution of the Cu NCs in N2-saturated water, which could not result in the formation of Cu NPs (Figure S8). The characteristic SPR absorption of the Cu NPs induced by dissolved oxygen is relatively stable in the initial environment, which stands for the solution background color of claret-red and elevates the credibility of the following colorimetric test. Upon the addition of a strong oxidizing agent H2O2, partial MSAs on the Cu NPs surface can be readily detached via the cleavage of Cu– S bonds probably by hyperoxidation to certain -SOO--related species.2, 41, 42 The destabilized Cu NPs are inclined to grow via a ripening process or attachment, which causes the decrease of the plasmonic peak at 520 nm and corresponding solution color fading. Since the rate of color change depends on the concentration of H2O2, different concentrations of H2O2 can be discriminated by the spectroscopic detection. When the concentration of H2O2 is further increased, the excess H2O2 molecules can not only remove a portion of the residual bonded MSAs, but oxidize the Cu NPs with less capped surface, leading to the formation of Cu2O NPs with gradually enhanced red-to-yellow color change in the solution.43 The results of EDS


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elemental analysis of the freeze-dried Cu NCs powder and the products after the treatments by different amounts of H2O2 were shown in Figure S9. The gradually reduced atomic ratio of S:Cu in the products subjected to H2O2 with increased amounts clearly demonstrated the stepwise detachment of MSA molecules bonded on the particle surface.

Figure 7. Tauc plots of versus obtained from the absorption spectra for the mixed solutions by adding different concentrations of H2O2 into the Cu NPs probe solution. The table in the right lists the concentrations of the added H2O2 and the corresponding band gap values of Cu2O in the solutions.

The band gap for a semiconductor can be obtained using the following equation: / (where is the absorption coefficient, is the discrete photo energy, K is a constant, and is the band gap energy). Figure 7 shows Tauc plots of versus for the direct transition of the yellow suspensions, and the value of extrapolation to gives an absorption edge energy, which corresponds to the band gap . The band gap values listed in the table are apparently greater than the value of bulk Cu2O (2.2 eV)44 and moves towards lower energy by increasing the H2O2 concentration. These results were consistent with the report that Cu2O NPs with a decrease in size from 8.6 to 4.8 nm exhibited a band gap increase from


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2.25 eV to 2.6 eV as a consequence of the direct forbidden nature of the optical absorption gap of Cu2O along with the size distribution of the nanoparticles.45 This change produces more chrominance difference in the probe solution, which is favorable for direct colorimetric assay of higher levels of H2O2.

CONCLUSIONS In summary, we provided a cost-effective and time-saving H2O2 assay strategy based on MSA-capped Cu NCs. A concentration range of H2O2 from 1 µM to 1 M could be detected without using any chromogenic reagent or expensive instruments. We revealed that a sequential oxidation process in this system offered the Cu NPs probe with the capacity suitable for the colorimetric and spectroscopic detection of H2O2 in a wide dynamic and sampling range. Because the purified and freeze-dried Cu NCs solid powder presented eligible resistance to oxidation, a similar sensitivity for H2O2 assay could maintain when the probe solution was prepared from the powder which had been stored for 10 months. This versatile and simple H2O2 detection probe is expected to offer new opportunities in detecting and quantifying the amount of H2O2 applicable to the environments involving either low or high levels of H2O2. Our findings may also have general implications in the colorimetric assay of other reactive oxygen species.

EXPERIMENTAL SECTION


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Chemicals. Mercaptosuccinic acid (MSA, ≥98%) and glucose oxidase (GOD, ≥180 U/mg) were purchased from Aladdin. Sodium borohydride, hydrogen peroxide (30%), glutathione (GSH, ≥98%) and glucose were obtained from Sinopharm Chemical Reagent. Cupric nitrate was bought from Xinbao Reagent. The other chemical solvents were purchased from Shanghai Lengfeng Reagent. All the chemicals and solvents were used as received without further purification. Solutions were prepared with deionized water (18.2 M Ω).

Synthesis of Cu nanoclusters (NCs). The Cu NCs were prepared by one-step method. A freshly prepared aqueous NaBH4 solution (19.12 mmol NaBH4 was dissolved in 20 ml of ice-cold water with 5-fold molar excess than copper ion) was added through a syringe at a rate of 2.0 mL/min to 20ml of aqueous solution containing Cu(NO3)2 (2 mmol) and MSA (4 mmol) at 0 °C with stirring (ca. 350 rpm). The mixed solution immediately became dark brown. The reaction was allowed to proceed for 30min with stirring at 0 °C and then the mixed solution was sonicated for 2 min. The pH value of the final solution was ~10.

Purification of Cu NCs. To remove excess ligands and ions from the solution and eliminate ill effect on characterization of the Cu NCs, the crude Cu NCs solution was purified by using a RC membrane (cut-off 7000 g/mol) for 2 h (changing water every 30 minutes) in ice-cold water.

Determination of kinetic parameters. The kinetic measurements were carried out in time course mode by monitoring the absorbance change at single-wavelength for 300 seconds on a UV-3600 spectrophotometer. The typical absorbance kinetic was carried out at room temperature


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(25 °C) by diluting 0.08 mL of the as-prepared Cu NCs solution (0.05 M) in 3.32 mL of H2O, followed by adding 0.1 mL of H2O2 with different concentrations to form 3.5 mL of mixed solutions. Immediately H2O2 solutions were added, the absorbance kinetic measurements were started at 520 nm in the presence of 0.01 mM H2O2 and at 375 nm in the presence of 500 mM H2O2. In the measurements, the spectral slid width was 2 nm and the optical path of quartz colorimetric utensil was 10 mm. All the detection experiments were carried out without any buffers.

Colorimetric detection. The typical colorimetric detection was realized as follows: 0.08 mL of the prepared Cu NCs (0.05 M) was first diluted in 3.32 mL of H2O to form a probe solution. Then 0.1 mL of different concentrations of H2O2 was added into the probe solution at 25 °C. The concentration of H2O2 in 3.5 mL of the mixed solutions ranged from 0.001 mM to 1000 mM. The color change was monitored by the UV–vis absorption spectra in the spectral pattern between 300 nm and 800 nm after a period of 60 s since the mixing of the probe solution and the H2O2 solution. Furthermore, to obtain the standard error bar, each detection measurement was repeated at least three times. The corresponding photographs of the solution color were taken with a digital camera. The assay of H2O2 was also conducted by using the purified Cu NCs solution.

The time-dependent stability of the Cu NCs-based probe for the assay of H2O2 was evaluated by using the purified and lyophilized Cu NCs powder after storage in a sealed


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container in dark at room temperature for 10 months. First, 200 mg of the powder was dissolved in 5 mL of deionized water to form a transparent solution. Then 0.08 mL of the solution was diluted in 3.32 mL of H2O to form a probe solution. Afterward, 0.1 mL of different concentrations of H2O2 was added into the probe solution at 25 °C. The concentration of H2O2 in 3.5 mL of the mixed solutions ranged from 0.001 mM to 1000 mM. The spectroscopic measurements were conducted on the conditions identical to those described above.

The colorimetric detection of H2O2 was also performed in phosphate buffer solution (pH=7.4, 10 mM) at 37 °C. For the detection of glucose, 0.0036 g glucose was dissolved in 2 mL of phosphate buffer solution (0.5 mM, pH 7.0) and then mixed with 200 µL of GOD (40 mg/mL) solution. The mixed solution containing 10 mM glucose was incubated in a water bath at 37 °C for 1 h. Subsequently, 0.08 mL of the prepared Cu NCs solution (0.05 M) was diluted in 3.32 mL of the phosphate buffer solution, followed by the addition of 0.1 mL of the incubation solution with different concentrations. The spectroscopic measurements were conducted on the conditions identical to those described above.

Tap water sample was collected from our laboratory. The raw sample was centrifuged (12,000 rpm) and filtered through a 0.45-mm membrane to remove any suspended particles. The sample was spiked with various concentrations of H2O2 and then used for the colorimetric detection with the same procedure as those described above.


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Characterization. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) investigations were carried out using a JEOL JEM-2010 microscope operating at 200 kV. Electron spray ionization-mass spectrometry (ESI-MS) measurements were carried out on a Finnigan ESI-MS system. X-ray diffraction (XRD) patterns were acquired on a Rigaku D/Max2400 powder diffractometer (Cu Kα X-ray radiation, λ=0.154056 nm). The surface chemical structure was characterized by Fourier transform infrared (FTIR) spectroscopy (Thermo, USA). Ultraviolet–visible (UV–vis) absorption spectra and the UV–vis diffuse reflectance spectra were recorded on a Shimadzu UV-3600 spectrophotometer. The latter was equipped with an integrating sphere with BaSO4 as the reference and operated with a spectral slid width of 20 nm. Dynamic light scattering (DLS) measurements were performed on Malvern ZEN3690. Elemental analysis was conducted by energy dispersive spectrometer (EDS) on a Hitachi S-4800 microscope.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx. xxxx / xxxxx. XRD pattern and ESI–MS spectrum of the Cu NCs, stability and selectivity analysis for the detection of H2O2, detection of glucose, detection of H2O2 in spiked tap water samples, TEM image of the Cu NPs induced by dissolved oxygen, role of dissolved oxygen, EDS spectra of Cu NCs powder and the products treated by H2O2.


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AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by grants from the Jiangsu Natural Science Funds (BK20141455), the Key Programs of Educational Commission of Jiangsu Province (15KJA150005), an open research grant of the State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201315) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Financial support from “the Thousand Talents Plan” of China and “the Shuang Chuang Plan” of Jiangsu province for Y. Y. is also gratefully acknowledged.

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Table of Contents Graphic


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Inducible Sequential Oxidation Process in Water-Soluble Copper

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