Chinese Chemical Letters  2017, Vol. 28 Issue (3): 521-524   PDF    
A facile route to the hierarchical assembly of Au nanoparticles on carbon nanotubes through Cu2+ coordination for surfaceenhanced Raman scattering
Zhi-Qiang Duana, Xin-Bao Gaoa, Tian-Peng Lia, Kai Yaoa,b, Xu-Ming Xiec     
a Shijiazhuang Mechanical Engineering College, Shijiazhuang 050003, China;
b Department of Graduate Management, Equipment Academy, Beijing 101416, China;
c Key Laboratory of Advanced Materials (MOE), Department of Chemical Engineering, Tsinghua University, ; Beijing 100084, China
Abstract: We report the hierarchical assembly of Au nanoparticles on carboxylized carbon nanotubes (c-CNTs) through Cu2+ coordination. This route is facile and green, and can easily control the loading density of Au nanoparticles. The c-CNTs matrix ensures uniform distribution of Au nanoparticles, which is particularly important for the enrichment of hot spots while preventing their serious agglomeration. Moreover, the cCNT matrix also contributes to the electromagnetic enhancement due to its surface plasmon resonance, and the chemical enhancement due to the adsorption of the target molecules. The resulting Au@c-CNT nanohybrids exhibit a remarkable synergy in SERS compared to neat Au nanoparticles.
Key words: Au nanoparticles     Carbon nanotubes     Coordination     Hierarchical assembly     Surface-enhanced Raman scattering    
1. Introduction

Raman spectroscopy is one of the most widely used techniques for the noninvasive detection and molecular recognition, capable of providing the structural fingerprint information of various chemical and biological molecules. However, the cross section of conventional Raman scattering is only 10-6 and 10-14 of IR and fluorescence, respectively. This inherent deficiency, i.e., low sensitivity, of Raman spectroscopy unfortunately hampers its applications in the ultratrace detection and surface science. Nearly four decades ago, several groups disclosed a new effect, i.e., surface-enhanced Raman scattering (SERS). By this effect, the Raman signal of pyridine molecules adsorbed on the surface of noble metal electrodes was approximately 106 times higher than that in solutions [1-3]. SERS was able to increase the cross section of Raman scattering to the level of linear Rayleigh scattering by a combination of chemical enhancement involving the metal-molecule interactions and electromagnetic enhancement caused by the surface plasmon resonance of metal nanostructures, which laid a basis for broadening the applications of Raman spectroscopy in such fields as ultratrace detection and surface science [4-6].

Recently, it was found that when noble metal (e.g., Au) nanoparticles were localized on various carbonaceous matrices, a strong synergy to the intrinsic properties of these noble metal nanoparticles could be achieved, thereby enhancing the Raman signal in terms of both sensitivity and reproducibility [7-9]. However, all these reports exclusively employed in situ hydrothermal synthesis to obtain the Au@C nanohybrids, which was limited by its harsh experimental conditions including high temperature, complex equipment and tedious procedure. Generally, the positively charged metal ions were adsorbed to the defects (negatively charged oxygen moieties) on the surface of these carbonaceous matrices through electrostatic attraction, where they were reduced, nucleated and grew into nanoparticles, usually at a high temperature for a long time. In our previous work, we successfully developed a novel strategy to cross-link carboxylized carbon nanotubes (c-CNTs) and graphene oxide (GO) by means of metal ion coordination, resulting in a flexible and robust "allcarbon" film that might shine in the fields of transparent electrodes and high-performance nanocomposites [10, 11]. By this strategy, the oxygen moieties of c-CNTs and GO could be facilely interconnected by metal ions through coordination under ambient conditions, thus forming a uniform supramolecular network.

In this paper, we report the hierarchical assembly of Au nanoparticles on c-CNTs through Cu2+ coordination. This route is facile and green, and can easily control the loading density of Au nanoparticles. Au nanoparticles are selected due to their unique optical and surface properties, especially their prominent SERS effect. The CNT matrix ensures uniform distribution of Au nanoparticles, which is particularly important since we consider the extremely large surface area of these nanoparticles which often induces serious agglomeration. Moreover, the CNT matrix also contributes to the electromagnetic enhancement due to its surface plasmon resonance, and/or the chemical enhancement due to the adsorption of the target molecules. The resulting Au@c-CNT nanohybrids therefore exhibit a remarkable synergy in SERS compared to neat Au nanoparticles.

2. Results and discussion

The morphological information of as-synthesized Au nanoparticles is provided in Fig. 1. Fig. 1a shows a typical TEM image of Au nanoparticles, which reveals spherical nanostructures uniformly dispersed in deionized water. It can be seen that these Au nanoparticles are monodispersed, with a narrow size distribution within 11-15 nm (>80%). Fig. 1b presents the HRTEM image of an individual Au nanoparticle, which unambiguously discloses a lattice fringe distance of 0.236 nm agreeing well with the (111) plane of face-centered cubic (fcc) Au [15].

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Figure 1. (a) TEM and (b) HRTEM images of as-synthesized Au nanoparticles.

The concept for the hierarchical assembly of Au nanoparticles on c-CNTs through Cu2+ coordination is schematically depicted in Fig. 2. Basically, the aqueous solutions of Au nanoparticles and c-CNTs are mixed, to which Cu2+ ions are added. The empty orbitals of Cu2+ ions are able to coordinate with the lone-pair electrons possessed by the -COO-groups of both Au nanoparticles and c-CNTs, thereby interconnecting the two in a controllable way. By this route, Au nanoparticles can be facilely decorated on the side walls of c-CNTs, thus forming 0/1D Au@c-CNT nanohybrids.

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Figure 2. Scheme for the hierarchical assembly of Au nanoparticles on c-CNTss through Cu2+ coordination.

At first, the coordination ability between -COO- groups of Au nanoparticles and Cu2+ ions is studied, as shown in Fig. 3. It can be seen that Au nanoparticles are very sensitive to Cu2+ coordination. When Cu2+ ions are added, the color of the aqueous solution of Au nanoparticles turns quickly from brilliant red to light purple, indicating a strong interaction between Au nanoparticles and Cu2+ ions. After standing still for 48 h, all Au nanoparticles are interconnected, resulting in serious agglomeration and thus precipitation. This result clearly proves the coordination ability between the -COO- groups of Au nanoparticles and Cu2+ ions.

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Figure 3. Photographs of aqueous solution (6 ml) of Au nanoparticles before and after Cu2+ coordination.

Next, the aqueous solutions of Au nanoparticles and c-CNTs are mixed and subjected to Cu2+ coordination. It is found, interestingly, that Au nanoparticles, instead of agglomerating and precipitating, are decorated on the side walls of c-CNTs through Cu2+ coordination, forming Au@c-CNT nanohybrids. As seen from Fig. 4a and d, when the loading density of Au nanoparticles is relatively low, they are only sparsely distributed on c-CNTs. At a moderate ratio (Fig. 4b and e), the distribution of Au nanoparticles on c-CNTs follows a dense and uniform way. Note that there are few, if any, free Au nanoparticles scattering beyond c-CNTs, which demonstrates that the hierarchical assembly based on Cu2+ coordination is highly efficient, and the interaction between Au nanoparticles and c-CNTs is strong enough. When the ratio of Au nanoparticles is further raised (Fig. 4c and f), a substantial degree of aggregation, resulting from the interconnection of excess Au nanoparticles, is observed. Judged from this figure we can conclude that the appropriate volume ratio of the aqueous solutions of Au nanoparticles and c-CNTs is 1:2, from which Au@c-CNT nanohybrids consisting of densely and uniformly distributed Au nanoparticles are derived.

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Figure 4. TEM and SEM images of Au@c-CNT nanohybrids derived from the aqueous solutions of Au nanoparticles (10-4 mg mL-1) and c-CNTs (0.05 mg mL-1) at volume ratios of (a) and (d) 1:10, (b) and (e) 1:2, and (c) and (f) 1:1.

To elucidate the interaction between Au nanoparticles and c-CNTs through Cu2+ coordination, we provide the Raman spectrum of the Au@c-CNT nanohybrids in Fig. 5. For comparison, the Raman spectrum of the Au/c-CNT mixtures without Cu2+ coordination is also presented. As seen from this figure, both systems have peak D residing at~1350 cm-1 corresponding to the defects in the graphitic sheets, and peak G locating at~1580 cm-1 related to the vibration of the sp2-hybridized carbon atoms in the graphitic sheets [16]. The D/G intensity ratio represents the disorder degree, that is, the higher the D/G intensity ratio, the more the disorders. When Cu2+ ions are absent, the D/G intensity ratio of the Au/c-CNT mixtures is much lower than that of the Au@c-CNT nanohybrids. This phenomenon means that after Cu2+ coordination, the irregular vibration of the carbon atoms in the Au@c-CNT nanohybrids increases significantly. When Au nanoparticles are attracted to the side walls of c-CNTs through Cu2+ coordination, they inevitably influence the regular vibration of the neighboring carbon atoms, leading to an increase in the D/G intensity ratio.

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Figure 5. Raman spectra of Au@c-CNT nanohybrids (with Cu2+ coordination) and Au/c-CNT mixtures (without Cu2+ coordination).

The superior SERS effect of the Au@c-CNT nanohybrids over neat Au nanoparticles is represented in Fig. 6. Here Rh6G is used as the probe molecule due to its well-established Raman features for easy recognition. When Rh6G is directly dropped on a naked SiO2/Si wafer, the Raman signal is rather ambiguous. Once the SiO2/Si wafer is coated with Au nanoparticles, their SERS effect renders a certain degree of signal enhancement, with some major peaks becoming visible. When the Au@c-CNT nanohybrids are coated on the SiO2/Si wafer, large quantities of Au nanoparticles are enriched on the c-CNT matrix as hot spots, and the SERS effect is therefore amplified significantly. In result, the intensity of the Raman signal increases by several orders of magnitude, and minor peaks can be clearly seen. The peak at 1127 cm-1 is ascribed to the C—H in-plane bending vibration, while the others correspond to the aromatic stretching vibration [15]. The superior SERS effect of the Au@c-CNT nanohybrids can be understood from the following twoaspects: first, the c-CNT matrix ensures uniform distributionof Au nanoparticles, which is particularly important for the enrichment of hot spots while preventing their serious agglomeration. Second, the c-CNT matrix also contributes to the electromagnetic enhancement due to its surface plasmon resonance, and/or the chemical enhancement due to the adsorption of the target molecules.

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Figure 6. SERS spectra of Rh6G (10-5mg mL-1) dropped on naked, Au-coated and nanohybrid-coated SiO2/Si wafers.

3. Conclusion

In conclusion, we have successfully achieved the hierarchical assembly of Au nanoparticles on c-CNTs through Cu2+ coordination. This route is facile and green, and can easily control the loading density of Au nanoparticles. The c-CNT matrix ensures uniform distribution of Au nanoparticles, which is particularly important for the enrichment of hot spots while preventing their serious agglomeration. Moreover, the c-CNT matrix also contributes to the electromagnetic enhancement due to its surface plasmon resonance, and/or the chemical enhancement due to the adsorption of the target molecules. The resulting Au@c-CNT nanohybrids exhibit a remarkable synergy in SERS compared to neat Au nanoparticles.

4. Experimental

Multi-walled CNTs (2g) were synthesized by catalytic pyrolysis of propylene [12], and refluxed in concentrated H2SO4/HNO3 (300mL/100mL) at 140 ℃ for 30min to obtain c-CNTs [13]. A stable, uniform aqueous suspension was prepared by sonicating cCNTs in deionized water without adding any surfactant. To synthesize Au nanoparticles, 0.01wt% HAuCl4 aqueous solution (50mL) was heated to boiling, and 1wt% Na3-citrate aqueous solution (0.5mL) was added. Within 2min, the mixed solution turned to light blue and then to brilliant red, indicating successful formation of Au nanoparticles. The boiling state was kept for 20min to complete the reduction process. Note that Na3-citrate served as a reducing agent and a surfactant to stabilize Na3-citrate in water, and endowed them with -COO-groups for Cu2+ coordination [14].

For Cu2+ coordination, the aqueous solutions of Au nanoparticles (10-4mg mL-1) and c-CNTs (0.05mg mL-1) were mixed at different volume ratios (1:10, 1:2 and 1:1), towhich CuSO4 aqueous solution (1mg mL-1) was added. The mixed solution was kept still under ambient conditions for 48h to obtain a complex system consisting of Au@c-CNT nanohybrids.

For SERS testing, naked SiO2/Si wafers (6 ×6mm2) were firstly soaked in Piranha solution (a mixture of H2SO4 and 30% H2O2 at a volume ratio of 7:3) at 60 ℃ for 30min. The aqueous solutions (100 mL) of Au nanoparticles and Au@c-CNT nanohybrids were dropped on the wafers and blow-dried by nitrogen, respectively. Next, 10-5mg mL-1 Rhodamine 6G (Rh6G) aqueous solutions (50 mL) was dropped on the wafers and blow-dried by nitrogen.

Acknowledgment

This research was financially supported by the National Natural Science Foundation of China (No. 21474058).

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