Copper Dendrites: Synthesis, Mechanism Discussion, and Application in Determination of L-Tyrosine Xiaojun Zhang,*,†,‡ Guangfeng Wang,†,§ Xiaowang Liu,†,‡ Huaqiang Wu,†,‡ and Bin Fang†,§
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1430–1434
College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, P. R. China, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, P. R. China, and Anhui Key Laboratory of Chem-Biosensing, Anhui Normal UniVersity, Wuhu 241000, P. R. China ReceiVed NoVember 7, 2007; ReVised Manuscript ReceiVed January 12, 2008
ABSTRACT: Highly uniform copper dendrites were successfully prepared in large quantities using a facile hydrothermal approach, which was prepared from the reaction of cupreous nitrate and sodium hypophosphite with different ratios of diethanolamine (DEA)/ water. Well-defined assembly of uniform dendrites with an average size of 20 µm can be obtained after a simple size-selection process. The influences of the ratio of DEA/water, and different reaction times and temperatures have been discussed. In addition, a gold electrode modified with copper dendrites was constructed and characterized by electrochemical impendance spectrum (EIS) and cyclic voltammetry (CV). The resulting copper dendrite modified gold electrode was used to detect L-tyrosine in the solution. The results showed that the copper dendrites may be of great potential in the determination of L-tyrosine. Introduction Metal nanostructures have been widely exploited for use in photography, catalysis, biological labeling, photonics, optoelectronics, surface-enhanced Raman scattering (SERS), and formulation of magnetic ferrofluids.1 The intrinsic properties of a metal nanostructure are determined by its size, shape, morphology, composition, and crystallinity.2 To date, much effort has been made to design nanocrystals with welldefined sizes, shapes, and crystallinity. Quite a lot of nanostructures, such as discrete cobalt disks, rods, and cubes, platonic gold nanocrystals and other shapes, silver prisms, cubes, rods, and hexagonal plates, have been synthesized via efficient solution-phase reduction methods.3 Self-assembled hierarchical and repetitive superstructures are fascinating because of their promising complex functions. Dendritic patterns are essential phenomena that are observed in nonequilibrium conditions for metallurgic, inorganic, and organic crystal growth.4 Meanwhile, various approaches have been reported for fabricating Cu nanocrystals with various morphologies, such as wires, cubes, pyramids, and octahedra.5 Since single-crystalline Cu nanostructures with tailored architectures are expected to show novel properties,6 it would be desirable to fabricate single-crystalline copper dendrites with modulated morphologies. Herein, we used a facile onepot solution route to synthesize uniform single-crystalline copper dendrites. At the same time, the interest in nanomaterials has increased tremendously in recent years due to their potential use7 in electrode modification by enhancing the electrode conductivity, facilitating the electron transfer, and improving the analytical sensitivity and selectivity.8 As a result of their strong structure-, size-, and shapedependent physical and chemical properties,9 it is generally understood that various nanomaterials modified electrodes have distinct electrochemical characteristics. In the past few years, many investigations have been conducted on the nanomaterials modified * Corresponding author. E-mail:
[email protected]. † College of Chemistry and Materials Science. ‡ Anhui Key Laboratory of Functional Molecular Solids. § Anhui Key Laboratory of Chem-Biosensing.
Figure 1. XRD pattern of the sample.
electrode as chemical/biological sensors, such as a Ni hollow spheres modified electrode for detecting methanol and ethanol electrooxidation,10 a platinum nanoparticle and carbon nanotube comodified electrode to be excellent sensors for H2O2,11 gold nanoparticles modified electrode as arsenite analysis,12 etc. Among them, glassy carbon, gold, and platinum electrodes modified with electrochemically deposited crystallographically oriented copper nanoparticles have been examined as novel anodes for the electrocatalytic evolution as the sugar sensors, which are especially surprising. In this work, copper dendrites were successfully prepared in large quantities using a facile hydrothermal approach, which was prepared from the reaction of cupreous nitrate and sodium hypophosphite with different ratios of DEA/ water. Well-defined assembly of uniform dendrites with an average size of 20 µm can be obtained after a simple sizeselection process. The influences of the ratio of DEA/water, different reaction times, and temperature are discussed. We also study the character of the prepared copper dendrites. It was found that the copper dendrites modified gold electrode was prepared and used to detect L-tyrosine in the solution.The results show that the copper dendrites give a very high activity for detecting the L-tyrosine, which provide a new application of copper dendrites.
10.1021/cg7011028 CCC: $40.75 2008 American Chemical Society Published on Web 02/28/2008
Synthesis of Copper Dendrites
Figure 2. FESEM view of copper dendrites (a) low-magnification, (b) typical copper dendrites, (c) TEM image of the sample, (d) SEAD of copper dendrites.
Crystal Growth & Design, Vol. 8, No. 4, 2008 1431 maintained at 140 °C for 12 h, and then cooled naturally to room temperature. The product was washed with distilled water and ethanol for several times to remove the impurities before characterizations. The samples, which were recorded at a scanning rate of 0.05°/s with the 2θ range from 10 to 80°, were characterized by X-ray diffraction (XRD) using an X-ray diffractometer with high-intensity Cu KR radiation (λ ) 0.154178 nm). Transmission electron microscopy (TEM) was performed using a Hitachi model H-800 transmission electron microscope with an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FE-SEM) was performed using JEOL JSM-6700 FESEM (operated at 10 kV). The modified electrode was prepared by mounting a drop of the solutions on a gold electrode and then allowing the drop to dry naturally. Electrochemical measurements were performed on a model CHI660B electrochemical analyzer (CH Instrumental, USA) controlled by a personal computer. Conventional three-electrode system was used with the modified gold electrode as the working electrode, a platinum wire as auxiliary electrode, and a saturated calomel electrode (SCE) as the reference one. Electrochemical impendance spectrum (EIS) experiments were performed in 0.1 mol L-1 KNO3 solution containing 5.0 mmol L-1 Fe(CN)63-/Fe(CN)64(1:1), using an alternating current–voltage of 5.0 mV. The impedance measurements were performed at an open circuit potential of 180 mV with a frequency range of 10-2 to 105 Hz. Cyclic voltammetry and differential pulse voltammetry measurements were done in an unstirred electrochemical cell at room temperature.
Results and Discussion
Figure 3. SEM images recorded during the representative growth process of dendritic copper nanostructures: (a) 1 h, inset of a is a SEAD image of copper particles, (b) 2 h, (c) 4 h, and (d) 8 h.
Experimental Procedures All of the chemical reagents used were of analytical grade. In a typical synthesis process, 0.24 g of copper nitrate, 0.2 g of sodium hypophosphite and diethanolamine (DEA) were mixed with distilled water. The mixture was stirred vigorously to homogeneity and then transferred into a 60 mL steel autoclave. The clave was sealed,
X-ray Diffraction. Powder X-ray diffraction patterns of the synthesized copper dendrites are shown in Figure 1. Three peaks at 2θ ) 43°, 50°, 74° could be indexed to the face-centered cubic copper with lattice constant R ) 0.3614 nm, which is very close to the reported data (JCPDS 85-1326, R ) 0.3615 nm). At the same time, no other peaks of impurities can be detected. This indicated that copper dendrites products were obtained under current synthetic conditions. Field-Emission Scanning Electron Microscopes and Transmission Electron Microscopy. The morphology and structure of the powders were observed by FESEM and TEM. These images (Figure 2a,b) clearly indicate that the obtained sample with a high aspect ratio consisted of a great many dendrites. A low-magnification FESEM image is shown in Figure 2a, revealing that the as-fabricated copper nanostructure has a dendritic morphology with lengths of up to tens of micrometers. The more highly magnified FESEM image (Figure 2b) gives more details about the morphology of the copper dendrites. It can be seen that these dendrites have a moderate size distribution, with the stem diameter in the range of 1–5 µm. The structure and morphology of the dendrites were further examined by TEM and selected-area electron diffraction (SAED). A typical TEM image of single a dendrite is shown in Figure 2c. The SEAD pattern (Figure 2d) related to the dendrite can
Figure 4. SEM images of dendritic copper nanostructures at 10 h (a) low-magnification view of copper dendrites. (b) Typical copper flower.
1432 Crystal Growth & Design, Vol. 8, No. 4, 2008
Figure 5. Representation of the proposed growth mechanism. Table 1. Summary of Various Morphologies of Copper Nanocrystals Fabricated at Different Experimental Conditions sample
H2O (mL)
diethanolamine (mL)
temperature (°C)
time (h)
morphology
1 2 3 4 5 6 7 8 9 10 11
20 20 20 20 20 20 20 30 35 10 5
20 20 20 20 20 20 20 10 5 30 35
100 120 140 160 180 140 140 140 140 140 140
12 12 12 12 12 4 8 12 12 12 12
particles rod dendritic dendritic (big) dendritic (bigger) rodlike dendritic like rodlike flower rodlike particles
be indexed to the [110] zone axis of face-centered cubic copper, suggesting that the dendrite is single crystalline growing along the [001] direction. The Possible Mechanism of the Formation of Copper Dendrites. To investigate the nucleation and growth process of copper dendrites, a series of experiments were carried out for different periods of time. The samples obtained at different stages of reaction time were examined by using TEM techniques. Figures 3 and 4 show the TEM images of the samples that were fabricated after the hydrothermal reaction was performed for 1, 2, 4, 8, and 10 h, respectively. These images clearly reveal the evolution of copper nanostructures from nanoparticles to dendrites over time at 140 °C. Figure 3a shows the stage product mainly consisted of nanoparticles with sizes of 300–500 nm. The SAED pattern
Zhang et al.
shown in the inset of Figure 3a further confirmed that these nanoparticles were all in the cubic phase. It confirms that cubic copper nanoparticles were formed at first. When the reaction time was increased to 2-4 h, the products consisted of three different forms of copper nanostructures, including rods, dendrites, and some sphere-like nanoparticles at the surface of the rod (Figure 3b,c), indicating that many nanopaticles had been developing into 3D nanostructures of copper and nanoparticles were gradually decreasing from panel b to panel c. Figure 3b,c shows a view of the surface of several dendrites, and these images clearly demonstrate that the copper dendrites were grown out from the surfaces of seeding nanocrystals, and at the same time, some junctions were also formed. Further extending the reaction time led to the formation of uniform dendrites. Figure 3d is a TEM image of a sample prepared after heating for 8 h, showing that the as-obtained products are dominated by the dendritic-like structures, and some copper nanoparticles were also observed. When the reaction time was increased to 10 h (as shown in Figure 4), the products consisted of the forms of copper dendrite structures, and some copper flowers at the surface of the copper dendrite were observed. However, if the reaction time increased to 12 h, copper flowers at the surface of the copper dendrite were not observed. According to the effect of different periods of time, we give a schematic representation of the proposed growth mechanism (Figure 5). It is well-known that the reaction conditions of not only heating time but also temperature and kinds and amount of reagents affect the morphology and size of the products in hydrothermal processes. In this paper, some primary comparisons are made (Table 1). As shown in Table 1, DEA played a crucial role in the growth of the copper dendrites. If the ratio of DEA/water increased to 30:10 and 35:5, the SEM images indicate that Cu rods and Cu particles are formed (Figure 6c,d). However, if the ratio of DEA/ water decreased to 10:30 and 5:35, the SEM images indicate that rodlike Cu and Cu flowers exist as products (Figure 6a,b).
Figure 6. TEM images of copper crystals obtained under different conditions: (a) VH2O/DEA ) 30/10, (b) VH2O/DEA ) 35/5, (c) VH2O/DEA ) 10/30, and (d) VH2O/DEA ) 5/35. Reaction over 12 h at 140 °C.
Synthesis of Copper Dendrites
Figure 7. The electrochemical impedance spectroscopy (EIS) of (a) bare gold electrode; (b) modified gold electrode in 0.1 mol L-1 KNO3 solution containing 5.0 mmol L-1 Fe(CN)63-/Fe(CN)64- (1:1).
Figure 8. Cyclic voltammograms of copper dendrites modified electrode in pH 7.0 PBS: (a) with 3.0 × 10-6 mol/L L-tyrosine at bare gold electrode; (b) with 1.0 × 10-5 mol/L L-tyrosine; (c) with 3.0 × 10-5 mol/L L-tyrosine. 13
According to the literature, diethanolamine molecules adsorbed on the samples determined morphology transition. Similarly, Cu2+ which is gradually released from a copper source forms Cu nanoparticles with Cu2+. These copper nanoparticles unceasingly adsorb diethanolamine molecules. Because of the coordinate action between copper nanoparticles and diethanolamine molecules, copper dendrites are formed. Electrochemical Character of the Copper Dendrite Modified Gold Electrode. EIS was applied to monitor the whole procedure in modified electrodes preparation, which could provide useful information between each step and often can be
Crystal Growth & Design, Vol. 8, No. 4, 2008 1433
used for probing the changes of surface-modified electrodes.14 In EIS, the semicircle diameter of EIS equals the electron transfer resistance (Rct). This resistance controls the electron transfer kinetics of the redox probe at the electrode interface. Figure 7 exhibited the EIS of fabricated procedure, shown as a Nyquist plot (Z”im versus Z′re). Figure 7a shows EIS of the bare gold electrode. There is an almost straight line that is characteristic of a diffusion limiting step of the electrochemical process. Assembly of copper dendrites on the electrode surface to form the modified electrode caused the appearance of the semicircle portion at higher frequencies, and a line at lower frequencies (Figure 7b). This may indicate that copper dendrites forming introduced a barrier to the interfacial electron transfer. Because of the pinhole lacuna of the film, the probe could arrive to the surface of the electrode. Thus, the electrochemistry reaction of [Fe(CN)6]4-/3- probe on the modified electrode was controlled by the diffusing and electrochemistry reaction together. The impedance changes of the modification process showed that copper dendrites had attached to the electrode surface. Figure 8 exhibited the electrochemical response of L-tyrosine at the copper dendrites modified electrode, and then cyclic voltammograms at 50 mV s-1 of the modified electrode in pH 7.0 phosphate buffer was shown (Figure 8, curve b). Under the same experimental conditions, there was almost no obvious redox peak of L-tyrosine at the bare gold electrode (Figure 8, curve a). However, it can be seen that the oxidation peak potential of L-tyrosine at copper dendrites modified electrode appeared compared with that at a bare gold electrode. Figure 8 curve c showed that with the addition of L-tyrosine, there was an enhancement in the anodic current. This suggested that the electrocatalytic activity of the modified electrode could be applied to the determination of L-tyrosine. Therefore, to improve the sensitivity for the quantitative determination of L-tyrosine, differential pulse voltammetry (DPV) has been adopted to record anodic peak current (Figure 9). The DPV peak currents increased linearly within the rage scanned (0 to +0.25 V) with various concentrations of Ltyrosine. The experiments of DPV showed that the oxidation current of L-tyrosine versus its concentration had a good linearity in the range from 2.0 × 10-7 to 5.0 × 10-4 mol/L with a correlation coefficient of 0.9937. The regression equation was Ipa ) -0.3747 - 0.2073C (Ipa: µA, C: µmol/L) and the detection limit (signal-to-noise ratio was 3) was 1.0 × 10-7 mol/L.
Figure 9. (A) Differential pulse voltammetry of L-tyrosine at the copper dendrites modified electrode in pH 7.0 PBS buffer (a-f) 1.0; 2.0; 3.0; 4.0; 5.0; 6.0 µmol L-1. (B) the relationship between the anodic peak current and the concentration of L-tyrosine.
1434 Crystal Growth & Design, Vol. 8, No. 4, 2008
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Table 2. Results for Determinations of L-Tyrosine in Pharmaceutical Samplesa sample
labeled values (g L-1)
found (mg L-1)
recovery (%)
1 2 3 4 5
0.500 0.500 0.500 0.500 0.500
0.510 0.501 0.496 0.480 0.491
102 100 99.2 96 98.2
a
Samples were obtained form Guangzhou Green Cross Pharmaceutical Corpor.
Determination of L-Tyrosine in Pharmaceutical Samples. To test the reliability of this method, the proposed method was applied to detect the concentration of L-tyrosine in amino acid injection. The determination was performed as follows: the amino acid injection was first diluted from 1.0 to 10.0 mL with double distilled water. One hundred microliters of the diluted sample was diluted again to 10 mL with pH 7.0 PBS and then transferred to an electrolysis cell. The solution was purged with purified nitrogen for 5 min to remove oxygen, and then a three electrode system was placed into the solution. The injection of L-tyrosine was analyzed by the standard addition method. The results are given in Table 1. The results were satisfactory, suggesting that the proposed method could be used for the determination of L-tyrosine in injection. Conclusion We have successfully prepared copper dendrites using a hydrothermal reduction process at a low temperature. The possible reaction process has been discussed. The results obtained from SEM and TEM indicate that the ratio of DEA/ water, different reaction times, and different reaction temperatures affect the formation of copper dendrites. In addition, a gold electrode modified with copper dendrites was used to detect L-tyrosine in the solution. The result shows that the copper dendrites show a very high activity for detecting L-tyrosine, and we think that copper dendrites may be of great potential use in L-tyrosine sensors. Acknowledgment. This work was supported by the Scientific Research Program of Anhui Province College Young Teacher
(2005JQ1048ZD), the Young Teacher Program of Anhui Normal University (2006xqn72), Anhui Provincial Natural Science Foundation (No. 070414179), and the National Natural Science Foundation of China (No. 20675001).
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CG7011028
