Template free synthesis of dendritic silver nanostructures and their application in surface-enhanced Raman scattering

Abstract

A facile method has been proposed to synthesize dendritic silver nanostructures without any template. UV-vis and TEM images revealed that the dendritic silver nanostructures with pronounced trunks and branches have been synthesized. Surface-enhanced Raman scattering (SERS) experiments showed that the synthesized dendritic silver gives an intense signal when p-aminobenzenethiol at very low concentration is used as a probe molecule.

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1. Introduction

Recently, nanomaterials with controlled morphology have attracted significant attention from researchers because of their excellent and fascinating optical, electronic, magnetic, catalytic and biosensing properties.^1–5 Therefore, a number of articles are available in the literature describing the synthesis of metal nanostructures with controlled size, shape, and crystallinity. Among the materials of nanoparticles, silver has utmost interest of the researchers because they have potential applications as antibacterial agent,⁶ super hydrophobic surface and a support material for surface enhanced Raman spectroscopy (SERS).^7–11 It is known that dendritic silver nanostructures with major trunks and branches give promising opportunity in the field of nanomaterials due to their large surface area, good conductivity and excellent connectivity compared with conventional silver nanoparticles. Due to the promising features, various routes have been proposed to synthesize dendritic silver nanostructures, i.e., replacement reaction,^12,13 pulse sonoelectrochemical,^14,15 thermal activation,¹⁶ microwave radiation,¹⁷ photochemical condition,¹⁸ solvothermal,¹⁹ reducing agent²⁰ and mixed solvent methods.²¹ Although many articles described the synthesis of dendritic silver, existing approaches could not fulfil the requirements of facile synthesis and green chemistry concept (i.e., aqueous medium, easy processing, surfactant or template free, low temperature, reproducibility). It is still challenging to develop a simple approach to overcome the existing drawbacks. From the past decades researchers have agreed that the development of the dendritic structures is due to the anisotropic crystal growth and diffusion-limited aggregation (DLA).^22–24

In the present contribution, we have demonstrated a simple process to synthesize a dendritic silver nanostructure without any surfactant or template. Furthermore, the synthesis is carried out in aqueous medium without any heavy metals or organic solvents, which could be classified as a green chemistry synthesis. The obtained dendritic silver is thoroughly characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis spectroscopy and wide angle X-ray diffraction (XRD). A promising application of dendritic silver as a support material for surface-enhanced Raman scattering (SERS) has also been described.

2. Experimental

Silver nitrate (AgNO₃) was purchased from Sigma Aldrich. Reducing agents, disodium citrate sesquihydrate (C₆H₆Na₂O₇·3/2H₂O) and sodium tetra hydroborate (NaBH₄) were purchased from Tokyo Chemical Industry. p-Aminobenzenethiol (PATP) was purchased from Sigma Aldrich. Ethanol was analytic grade and was purchased from Wako Co., Ltd. All reagents were used without further purification.

2.1 Preparation and characterization of dendritic silver nanoparticles

12 ml silver nitrate solution (1 mM) was heated at 80 °C for 20 min before adding reducing agent. A certain amount of reducing agent was added in the solution while stirring at 80 °C followed by further stirring for 30 min at the same temperature. The solution was cooled to room temperature for succeeding experiments.

The morphology of the dendritic silver nanostructure was observed through TEM (JEOL JEM-2100) with acceleration voltage of 200 kV and SEM (Hitachi SU-1500) with an accelerating voltage of 25 kV. TEM samples were prepared by casting a drop of water suspension on a copper grid. The samples were dried at room temperature. Absorption spectra were recorded on Hitachi U-3900 UV-vis spectrophotometer. XRD were measured on a Rigaku Rint-ultima diffractometer with Cu Kα radiation (40 KV, 20 mA) with 0.02° step and 2 s step time in 2θ range from 20° to 80°.

2.2 Surface enhanced Raman spectroscopy

500 μL of PATP ethanol solutions with various concentrations were mixed with 500 μL of dendritic Ag nanoparticles under vigorous stirring. After stirring for 30 min, 20 μL of each sample was deposited drop wise into an aluminum pan for SERS measurement. The typical exposure time for each SERS measurement was 10 s with two accumulations. A Raman spectrophotometer (Model RS-2100, Photon Design, Inc.) equipped with a charge-coupled device (CCD) (Princeton Instruments) was used for measuring SERS spectra of PATP. Radiation with the wavelength of 514.5 nm from an Ar⁺-ion laser (Spectra Physics) was employed for Raman excitation. The Raman band of the silicon wafer at 520.7 cm⁻¹ was used to calibrate the spectrometer.

3. Results and discussion

Fig. 1 shows the TEM images of the silver dendritic nanostructure (also called silver fern) as prepared. The size of the dendritic silver nanostructure is around 800 nm (ESI Fig. S1†), whereas the size of the component silver nanoparticles ranges from 5 to 10 nm. Fig. 1 clearly shows well defined, uniform and ordered dendritic silver nanostructure with pronounced trunk and branches on both sides of the trunk.

Fig. 1 TEM images of the dendritic silver nanostructure at low (a) and (b) high magnification.

It is known that a strong anisotropic growth contributes to the evolution of silver nanostructure into a thermodynamic stable dendritic structure. However it is difficult to predict the reaction time for the formation of dendritic structure. Kim et al. described that silver dendritic structure depends on the reaction time. i.e., long reaction time makes the dendritic structure thick and fragile.¹⁰ It is also possible that high shear destabilizes the dendritic structure. To understand the stability, the synthesized dendritic silver was centrifuged at 12 000 rpm for 20 min. It has been shown in the Fig. S2† that the dendritic structure was broken and silver nanoparticles was organized in another fractal structure.^25,26

It is interesting to investigate the optical properties of dendritic silver nanostructure because of its surface plasmon resonance.²⁷ A strong absorption peak is observed around 303 nm (Fig. 2) similar to the reported dendritic silver nanostructure^9–11 but different from conventional silver nanoparticle.^28,29

Fig. 2 UV-vis absorption spectrum of silver dendritic nanostructure.

The structure of the dendritic silver was characterized by wide angle XRD (Fig. 3). All the diffraction peaks (111, 200, 220, 311, 222) can be indexed to the cubic structure of silver, which has unit cell parameter a = b = c = 4.0816 Å (4.0862 Å, from JCPDS 04-0783). Similar crystal structure of dendritic silver has been reported by Wen.⁹

Fig. 3 XRD pattern of the synthesized dendritic silver nanostructure.

There are different mechanisms so far proposed for the development of dendritic silver nanostructures.^{14,15,30–32} Most of the explanations are based on a diffusion limited aggregate model. It is described that the dendritic structure is formed from the asymmetric growth of silver nanoparticles when the growth rate is limited by the rate of diffusion of solute atoms to the interface. In presence of reducing agent, Ag¹⁺ ions convert to Ag nanoparticles. Therefore, at the initial stage, silver nanoparticles grow along the (111) direction to form a trunk (because the growth is more effective along the (111) direction). As the reaction continues, the additional silver nanoparticles were formed. The additional particles moved through a random walk until they reach to a low energy position or attach to a silver trunk. The growth of these attached silver nanoparticles also begins along the (111) direction to form further secondary and tertiary branches. As the time passes, the concentration gradient at its tip becomes more prominent, which increases the diffusion rate to the tip. As the reaction proceeded further with time, all the trunks, branches and leaves grew and finally interconnected to each other. This process occurs repeatedly until a dendrite structure is formed.^9,15,33

Another reducing agent NaBH₄ has been used to prepare dendritic Ag nanostructure. UV-vis and TEM image (Fig. S4†) shows that Ag nanoparticles are formed instead of dendritic structure. The main reason is not well understood, however one can argue that the formation of dendritic structure depends on the reduction process, namely, if the reduction process becomes fast then silver nanoparticles will form instead of dendritic structure.

To evaluate the potential application of such dendritic silver as SERS substrates, PATP was chosen as a probe molecule due to its well-established vibrational features. PATP can chemisorb on the surface of the dendritic silver nanostructure through the formation of Ag–S bonds. As shown in Fig. 4, the typical bands of PATP at 1578, 1438, 1394, 1146, and 1076 cm⁻¹ were observed and agreed well with previous work.³⁴

Fig. 4 SERS spectra of PATP molecules on dendritic Ag nanostructure with different concentrations (a) 1 × 10⁻⁴, (b) 1 × 10⁻⁵, (c) 1 × 10⁻⁶, (d) 1 × 10⁻⁷ and (e) 1 × 10⁻⁸ M.

There are some new bands appearing in the spectra after PATP concentration is down to 1 × 10⁻⁷ M. Actually, these bands stem from ethanol molecules (Fig. S5†). During the experimental process, ethanol is used as the solvent to dissolve and dilute the PATP molecule. As the concentration of PATP decreased, the SERS intensity of PATP decreased, and thus the bands from ethanol molecule were observed gradually. However, the Raman band of PATP at 1394 cm⁻¹ still can be clearly observed even if concentrations of PATP decreased to 1 × 10⁻⁸ M (Fig. 4). A rough calculation of the SERS enhancement factor (EF) was performed through the following equation: EF = (I_SERS/I_NR) × (C_NR/C_SERS), where I_SERS and I_NR are the band intensity of the selected band at 1146 cm⁻¹ obtained by SERS and corresponding band intensity of the bulk solution, respectively.³⁵ C_SERS and C_NR are the corresponding concentrations of PATP in the SERS and bulk solution. The EF for SERS detection of PATP in our experiment was estimated to be 7 × 10⁶, which is high enough for ultrasensitive detection. The dendritic silver exhibits high SERS performance, which is due to electromagnetic SERS enhancement.³⁶ More importantly, dendritic structure can provide more “hot spots” than a regular structure.³⁷ Thus, the as prepared dendritic silver could serve as powerful substrates for SERS-based determination in applications, such as in biological, environmental and food analyses.

4. Conclusion

A facile procedure has been proposed to synthesis dendritic silver nanostructure in an aqueous medium without any surfactant or template. TEM and SEM images manifest that the dendritic silver nanostructure contains a pronounced trunk and branches on both sides of the trunk. The mechanism of dendritic silver formation is not clear yet; however, concentration gradient of the silver ion between the bulk solutions and the surface of the silver nanoparticles could be a driving force to control the diffusion of solute. It was also found that the formation of the dendritic silver depends on the strength of the reducing agent; i.e., a strong reducing agent (NaBH₄) did not give the dendritic nanostructure. Surface-enhanced Raman scattering experiments show that the silver dendritic nanostructure gives an intense and enhanced signal to PATP (used as probe a molecule) although the concentration is lowered to 1 × 10⁻⁸ M. We believe that the present dendritic silver nanostructure can be useful not only in SERS but also in optics, biosensing, solar cell and catalysis.

Acknowledgements

One of the authors (KN) is grateful for a Grant-in-Aid for Scientific Research (24510139) from the Japan Society for the Promotion of Science (JSPS). We also thankful to Prof. Guoran Li, Nankai University, China for fruitful discussion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10113h

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