Chinese Chemical Letters  2014, Vol.25 Issue (10):1403-1406   PDF    
Tuning plasmon absorption of unmodified silver nanoplates for sensitive and selective detection of copper ions by introduction of ascorbate
Xiao-Dong Xia , Tian-Lun Wang, Xiao-Yuan Yuan    
Key Laboratory of Theoretical Chemistry and Molecular Simulation, Minister of Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
Abstract: Silver nanoplates as novel optical sensors for Cu2+ detection have been demonstrated. Silver nanoplates are synthesized via previous H2O2-NaBH4 cyclic oxidation-reduction reactions. With introduction of ascorbate as mild reductants, Cu2+ ions are reduced into Cu+ and the Cu+ is further reduced to Cu, which is deposited on the surface of the silver nanoplates. The deposition of the Cu on the surface of the silver nanoplates allows a significant red-shift of their plasmon absorption. Therefore, trace Cu2+ can be detected. The shift of the plasmon absorption wavelength of silver nanoplates is proportional to the Cu2+ concentration over a range of 40-340 μmol L-1 with a limit of detection of 9.0 μmol L-1. Moreover, such silver nanoplate-based optical sensors provide good selectivity for Cu2+ detection, and most other metal ions do not disturb its detection. Moreover, the practicality of the proposed sensor was tested. This Cu2+ assay is advantageous in its simplicity, selectivity, and cost-effectiveness.
Key words: Silver nanoplate     Plasmon absorption     Wavelength shift     Optical sensor     Copper ions     Ascorbate    
1. Introduction

With unique optical/electronic properties for potential applications,silver nanoplates (also nanoprisms or nanodisks) have been intensively studied [1, 2, 3, 4]. The lateral dimensions being much larger than the thickness provides silver nanoplates an extreme degree of anisotropy,which favors highly tunable localized surface plasmon resonance absorption and maximum electromagneticfield enhancement [4, 5, 6, 7]. Such attractive features make them promising nanomaterials for chemo/bio sensing applications [1, 3, 7]. For instance,the shape-dependent plasmon absorption shifts of silver nanoplates have yielded various colorimetric sensors [3, 8, 9]. Also,silver nanoplates have been exploited for use in metal-enhanced fluorescence and surface-enhanced Raman scattering sensors in sensing applications [1, 7, 10, 11, 12, 13, 14].

Although copper is an essential element for life,excess copper levels exert toxicity to living organisms and lead to serious environmental contamination [15]. Therefore,Cu2+ recognition and detection is necessary,and optical sensors/probes for detection of copper ions have been continuously studied. For instance,various chromogenic/fluorophoric probes for copper ion detection have been reported [16, 17, 18, 19]. Despite their high sensitivity and selectivity,these sensors have certain drawbacks,including water-insolubility and time-consuming preparations. Fortunately,various emerging functional nanomaterials have been exploited as novel optical sensors/probes for copper ion detection with advantages of water-solubility and simple preparations. For example,a copper/silver nanocluster has served as a fluorescent probe for copper ion detection [20]. Also,a polyamine-function- alized carbon quantum dot probe for detection of Cu2+ has also been developed [21]. Moreover,nanomaterial-based colorimetric assays for Cu2+ detection by naked eye inspection have received intensive attention. For instance,a colorimetric assay for copper ions detection based on catalytic leaching of silver-coated gold nanoparticles has been established [22]. Also,functionalized-gold nanoparticles have been used as colorimetric probes for copper ions [23, 24, 25, 26, 27, 28]. However,these colorimetric probes involved timeconsuming modification of the gold nanoparticles. Therefore,modification-free nanomaterials for sensing copper ions are preferred.

Here,we describe a new silver nanoplate-based optical sensor for Cu2+ detection. Silver nanoplates are synthesized via previously established H2O2-NaBH4 cyclic oxidation-reduction reactions [29]. With introduction of ascorbate as a mild reductant,Cu2+ ions are reduced into Cu+,and the Cu+ is further reduced into Cu,which is deposited on the surface of the silver nanoplates. The deposition of the Cu on the surface of the silver nanoplates causes significant red-shifts of the surface plasmon resonance absorption. Therefore,trace Cu2+ can be detected. Such silver nanoplate-based optical sensors provide good selectivity for Cu2+ detection and most other metal ions do not disturb its detection. This sensing strategy for Cu2+ detection is advantageous for its simplicity,selectivity,and cost-effectiveness. cost-effectiveness. 2. Experimental 2.1. Materials

Silver nitrate,ascorbic acid,sodium borohydride,and sodium citrate were purchased from Sigma-Aldrich. H2O2 solutions (30 wt%) and analytical grade CuSO4·5H2O were obtained from Sinopharm Chemical Reagent Co.,Ltd 2.2. Instruments

TEM (HR-TEM) and energy dispersion spectroscopy element analyses were performed on a JEOL JEM-2100 transmission electron microscope and JSM-6380 scanning electron microscope,respectively. Absorption spectra were recorded by PerkinElmer- Lambda 35 UV-vis spectrophotometer. 2.3. Synthesis of silver nanoplates

Silver nanoplates were prepared via a previously reported method [29]. Typically,a 24.75 mL aqueous solution containing silver nitrate (0.05 mol L-1,50 mL),trisodium citrate (75 mmol L-1,0.5 μL) and H2O2 (30 wt%,60 μL) was vigorously stirred. Sodium borohydride (NaBH4,100 mmol L-1,250 μL) was rapidly injected into this mixture to initiate the cyclic oxidation- reduction reactions toward silver. When the color of the solution turned blue,silver nanoplates had been synthesized. The total 25 mL solution contains 125 mmol L-1 silver nanoplates (calculated with silver amount). 2.4. Detection of copper ions

The as-prepared silver nanoplates were purified by centrifugation and redispersed into citrate buffers (10 mmol L-1,pH 5.0). For copper ion detection,62.5 μmol L-1 silver nanoplate solution was incubated with 5.0 mmol L-1 ascorbate and various amount of Cu2+ ranging from 0.0-800 μmol L-1 for 30 min and then subjected to UV-vis spectroscopy measurements. 3. Results and discussion 3.1. Synthesis and characterization of silver nanoplates

By a facile previously reported cyclic oxidation-reduction reaction [29],blue silver nanoplate colloids were synthesized. TEM analysis in Fig. 1A shows their morphology and size features. Results indicate they are polymorphous. Fig. 1B shows their absorption spectrum with a characteristic peak at 670 nm.

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Fig. 1. (A) TEM image and (B) the absorption spectrum of silver nanoplates (125 μmol L-1).
3.2. Sensing mechanism

As shown in Fig. 2,the addition of copper ions causes the initial 670 nm plasmon absorption peak of silver nanoplates to be significantly red-shifted to 714 nm. On the other hand,the absorbance of free Cu2+ (340 mmol L-1) in this wavelength range is very small,even when ascorbate present. When ascorbate was present,Cu(II) was reduced into Cu(I),and the induced Cu(I) was further reduced to Cu(0) [30],and the reduced Cu was further deposited onto the surface of the silver nanoplates. The elemental analyses of energy dispersion spectroscopy of the dried silver nanoplates derived after the sensing reaction show that the surface composition of the silver nanoplates contains an average level of 27.4% Cu (ten silver nanoplates are measured). This result confirms that copper is indeed deposited on the surface of the silver nanoplates due to ascorbate reduction of copper ions. Although the trace deposited Cu obscures discernible variation of the silver nanoplate dimensions and shape,as confirmed by TEM analyses,the HR-TEM analyses indicate that the original clear surface crystal lattice structure of silver nanoplate turns indiscernible due to the deposition of a noncrystalline Cu layer,as shown in Fig. 2B and C. The in-plane dipole resonance of silver nanoplates has been proven to be very intense,and its wavelength is extremely dependent on the height,edge length,and tip sharpness of silver nanoplates [5, 7, 31]. In addition,the plasmon absorption is extremely sensitive to metal composition [32]. Therefore,the deposition of a copper layer makes the initial 670 nm plasmon absorption peak of silver nanoplates red-shift to 714 nm.

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Fig. 2. (A) Silver nanoplate (62.5 μmol L-1) absorption spectra in the absence (solid curve) and presence (dashed curve) of copper ions (340 μmol L-1). The HR-TEM images of the surface structure of single silver nanoplates in the absence (B) and presence (C) of copper ions by introduction of ascorbate as a reductant. The deposition of a noncrystalline Cu layer on the silver nanoplate completely obscures the crystal lattice of the original silver nanoplate.
3.3. Copper ion detection

We further exploit this novel finding for detection of copper ions. As shown in Fig. 3A,the absorption peak of silver nanoplates gradually red-shifts with the increase of the Cu2+ concentration along with an isosbestic point at 610 nm. The wavelength shift of silver nanoplates is proportional to the concentration of Cu2+ over a range of 40-340 μmol L-1 with a limit of detection of 9 μmol L-1,as shown in Fig. 3B.

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Fig. 3. (A) Copper ion-dependent red-shift of absorption peak of silver nanoplate. From a to f,the copper ion concentration is 0,40,80,160,260 and 340 μmol L-1,respectively. (B) Plot of wavelength shift versus concentration of copperions.

As the above mentions,the high sensitivity of plasmon absorption properties of silver nanoplates toward its composition and structure is the basis of the proposed sensor for Cu2+ detection. The selectivity of the silver nanoplate-based optical sensors for Cu2+ detection is further examined and the results are collected in Fig. 4. It exhibits satisfactory specificity. Notably,the presence of trace silver ions only increases the silver nanoplate absorbance and does not shift its absorption peak,because ascorbate can also reduce silver ions. The reduced Ag would be also deposited on the surface of the silver nanoplates,thus enhancing its absorbance.

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Fig. 4. The selectivity of the proposed sensor toward copper ions. The metal ion concentrations are all 150 mmol L-1

Without introduction of ascorbate,silver nanoplates cannot effectively respond to the copper ions. The introduction of ascorbate also plays important role in improving selectivity toward Cu2+ detection. For example,without ascorbate,the addition of oxidative Fe3+ allows gradual blue-shift of the silver nanoplate plasmon peak after a few hours,due to the possible oxidation etching reactions between Fe3+ and silver nanoplates. Enough ascorbate enables reduction of trace Fe3+ to Fe2+ and therefore protects silver nanoplates from Fe3+ etching. Additionally,when using sodium borohydride as an alternative reductant for the proposed sensing system,mercury ions also remarkably blueshift silver nanoplate plasmon peak and exert influence on the sensor selectivity toward copper ions. Sodium borohydride can also reduce mercury ions,and the reduced Hg would be deposited on the surface of the silver nanoplates,therefore shifting their plasmonic absorption spectrum. Therefore,the introduction of ascorbate as mild reductants in such sensing system is crucial. For consideration of the hydrolysis of Cu2+ under basic conditions,the detection of Cu2+ was performed under slight acidic conditions (pH 5.0). On the other hand,too much acidity would result in the aggregation of the silver nanoplates and disfavor the detection of copper ions.

To test the feasibility of the proposed silver nanoplate sensors for detection of copper ions,analysis of a pond water sample was also performed with the proposed method. A water sample collected from a pond on the campus of Hunan University of Science and Technology was filtered through a 0.2 μm membrane. This pond water was further spiked with various concentrations of standard Cu2+ solutions. The as-prepared samples were then analyzed separately using both ICPMS and the developed sensing technique. The obtained results are collected in Table 1. These results revealed that our new technology and the ICPMS-based approach do not differ significantly in their precision. Neither our sensor nor the ICPMS-based approach detected the presence of Cu2+ ions in this pond water sample.

Table 1
The comparison of the proposed method and ICPMS analyses of Cu2+ levels in real samples.
4. Conclusions

On the basis of the extreme sensitivity of the silver nanoplate absorption spectrum toward its composition and structure,a highly selective optical sensor for determination of Cu2+ has been described. By introduction of ascorbate,Cu2+ ions are reduced into Cu+,and Cu+ was further reduced into Cu,which is deposited on the surface of the silver nanoplates. The deposition of the Cu on the surface of the silver nanoplates allows its significant red-shifts of the surface plasmon resonance absorption peak. Thus,quantification of Cu2+ was established. This unmodified silver nanoplatebased probe for Cu2+ detection exhibits advantages of simplicity,selectivity,and cost-effectiveness.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (No. 21375036),and the Open Project Program of Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education (Hunan University of Science and Technology,No. E21201).

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