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J. Phys. Chem. B 2002, 106, 6500-6506
Aggregation of Silver Particles Trapped at an Air-Water Interface for Preparing New SERS Active Substrates Jiawen Hu,† Bing Zhao,*,† Weiqing Xu,† Yuguo Fan,† Bofu Li,† and Yukihiro Ozaki*,‡ Key Laboratory for Supramolecular Structure and Spectroscopy of Ministry of Education, Jilin UniVersity, Changchun 130023, P. R. China, and Department of Chemistry, School of Science and Technology, Kwansei-Gakuin UniVersity, Sanda, Hyogo 669-1337, Japan ReceiVed: NoVember 27, 2001; In Final Form: March 21, 2002
In this paper, we propose a novel technique for the fabrication of active substrates of surface-enhanced Raman scattering (SERS). It was found that the silver particles themselves and the analyte-covered silver particles can be trapped at the air-water interface. In the presence of electrolyte, for example, KCl that is generally used to activate silver colloids for additional SERS enhancement, the trapped silver particles and clusters can attract each other spontaneously to form two-dimensional silver particle films. By this technique, the prerequisite aggregation of silver colloids for the SERS process is provided. At the same time, because the aggregation occurs only at the interface, problems of irreversible aggregation and instability of silver colloids can largely be overcome. The SERS enhancement ability of these silver particle films is larger by 1-2 orders than that of usual active substrates such as silver colloids and silver mirror. Their strong enhancement ability may arise from the unique two-dimensional structure itself.
Introduction Surface-enhanced Raman scattering (SERS) is a sensitive analytical tool for the investigations of molecules adsorbed on noble metal surface.1-10 It has been generally accepted that “surface roughness” is very important for SERS. The magnitude of the SERS effect depends not only on the nature of noble metal and excitation wavelength but also on the size, shape, and spacing of metal nanostructures for electromagnetic field enhancement.1-10 The investigations and preparations of nanostructures have been matters of extensive experimental challenges, and the evolution of an SERS-active surface plays a key role in the breadth of novel SERS applications as well as in the understanding of the mechanism of the SERS process. Recently, single-molecule probing by means of SERS has made remarkable progress, but it is restricted mainly to single Ag aggregates and metal colloids under strict experimental conditions.11-13 Developing stable, reproducibly prepared, and easy to make metal substrates that provide a remarkable enhancement factor is highly desirable for analytical applications of SERS in routine, on-line analyses. Various techniques have been used for preparing nanostructural substrates including electrochemically roughed electrodes,14 silver films deposited by vapor deposition or photoreduction,15 chemically deposited silver films,16 chemically etched silver foils,17 metal colloids18 and metal colloidal particles selfassembled into polymer-coated substrates.19 The requisite roughness can also be achieved by the deposition of metal on an inherently rough substrate, such as aluminum, titanium dioxide, and fused silica powders; filter paper; and latex beads.20 “Chemically” pure metal colloids can be made by laser ablation, which are nearly free from organic or ionic contaminants and * To whom all correspondence should be sent. Fax: +81-795-65-9077. E-mail:
[email protected]. † Jilin University. ‡ Kwansei-Gakuin University.
tend to be more stable.21 A controllable way of fabricating SERS substrates is provided by micro- or nanolithography, which is a powerful fabrication technique for the creation of arrays of nanoparticles with controlled size, shape, and interparticle spacing.22 It can be used to optimize the structural properties for the maximum SERS enhancement and to test the theory for electromagnetic enhancement by surface plasmon excitation at regularly arranged particles and metal grating. Some newly developed exotic substrates include nanocrystal tubules using self-assembled bolaamphiphile peptide tubules as templates,23 mesoporous-macroporous gold films assembled by colloidal crystals as templates,24 and a metal nanoshell, a new type of composite nanoparticle that has a tunable plasmon resonance by optimizing the size of the core and thickness of the noble metal shell.25 The arrays of nanorods fabricated by using anodic aluminum oxide templates have also been used as SERS active substrates.26 These methods provide the roughness on noble metal surfaces suitable for SERS. Metal colloidal systems have been employed extensively in SERS.5-8 Besides the easiness in their preparation, these systems exhibit remarkably strong enhancement. However, a major problem with the use of metal colloids is the tendency of colloidal aggregation after the addition of analyte, which makes the colloids unstable and leads often to the poor reproducibility of the SERS spectra. At the same time, the aggregation of colloidal particles is prerequisite for strong SERS enhancement.27 Although the essential criterion for obtaining SERS activity is the aggregation of metal colloids leading to either fractal aggregation or a string-like assembly, in many cases, controlling the aggregation is a difficult step. Stabilizers such as poly(vinyl alcohol), poly(vinylpyrrolidone), and sodium dodecyl sulfate have been used to minimize this coagulation problem, but the use of these stabilizers could produce interferences.28 There have been some trials to combine the advantages of colloidal suspensions and the stability of solid substrates. For example, porous sol-gel materials offer a unique matrix
10.1021/jp0143286 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/04/2002
Aggregation of Silver Particles for dispersing and stabilizing metal colloids.29 Colloid films immobilized by polymer-coated substrates19 and colloid aggregates gravitationally deposited from the colloids/adsorbate system30 more or less achieve the above motivation, although the aggregation process is not a controlled way. The essential aim of aggregation of silver colloids for strong SERS signals is to induce silver clusters in a solution. If we can select the silver clusters from the solution, then we can directly measure SERS signals from the silver clusters themselves and thus can overcome largely the instability of silver colloids. In this paper, we report such a method: silver particle films supported at an air-water interface. We found that silver particles or analyte-covered silver particles can migrate from the bulk solution to the air-water interface and be trapped. When more and more particles are trapped, they attract each other spontaneously to form nearly two-dimensional silver particle films (they are in fact silver clusters) that show fractal characters. The aggregation process can be viewed through an optical microscope. Because the aggregation occurs at the airwater interface, one can control the aggregation to some extent. There were a few interesting studies about the formation of silver films for SERS previously. Silver metal liquidlike films (MELLF)31-33 were reported to be produced within the interfacial region of two immiscible liquids, water and an organic medium, by chemical deposition techniques. This involves the reduction of a silver anisate salt layer deposited at the interfacial region to silver in the presence of a surfactant that would enhance the stability of the MELLF. This method was slightly modified to prepare mobile silver films from silver colloids and a solution of the transition-metal complex in an organic solvent.34 During this method, the metal complex has an important effect on the formation of the interfacial films and has been assumed to act as the surfactant to stabilize the formed silver films. In the present research, we have found silver particle films can be formed at the air-water interface from the silver colloids themselves without the need of the organic phase and any surfactant. Therefore, without the disturbance of a liquid phase, we can prepare the air-water supported silver particle films that are convenient for in situ applications and studying their optical properties. When an analyte is introduced into the silver colloids, the analyte-covered silver particles can also form the silver particle films. Therefore, combined with the elaborately devised U-shape capillary device that enables us to form the silver particle films quickly, this method has a great potential to be used extensively in the field of SERS. To test the potential of silver particle films as a SERS substrate, we measured the SERS of 4-mercaptopyridine. The application of the silver particle film method to SERS shows interesting properties: very high enhancement factor, low excitation power needed, high stability, and easiness in preparation. Experimental Section Preparation and Characterization of SERS Active Substrates. Silver colloids were prepared by the standard citratereduction procedure reported in ref 18b. Morphological measurement of the silver colloids was performed by a Hitachi 8100 type transmission electron microscopy (TEM), showing that the colloids were a mixture of heterogenerous particles. About 150 particles were selected to evaluate the average particle size, which was estimated to be about 73 nm. The typical concentration of silver colloids was 9 × 10-11 mol/L, corresponding to 5.4 × 1010 particles/mL. The average particle size was larger than that reported previously.35 This might be due to the differences in the conditions of preparation because the mor-
J. Phys. Chem. B, Vol. 106, No. 25, 2002 6501
Figure 1. Schematic of the U-type capillary device for forming silver particle films.
phology of silver particles is sensitive to the ratio of silver ions to citrate concentration, the stirring rate, the adding rate of sodium citrate, and temperature. Chemically deposited silver films were prepared by the Tollen’s test.16 A carefully cleaned glass plate of 20 × 10 × 1 mm was put into a 10 mL beaker. A 5 mL aliquot of 0.02 mol/L silver ammonia solution and 5 mL of formaldehyde were mixed in the beaker. A few seconds after mixing, the solution turned to gray and black. In the meanwhile, the silver ions were reduced and deposited on the glass plate to form fine films that we call silver mirror. After withdrawing, the silver mirror was washed with distilled water. Newly developed silver particle films supported at an airwater interface were formed at a glass capillary device devised by our laboratory. The capillary tube with inner diameter of 0.9-1.1 mm was bent to form a U type tube, and then it was fixed on a microscope slide. The schematic of the device is shown in Figure 1. An optical microscope (Leica) attached to a Renishaw (1000) Raman spectrometer was used to observe the silver particle films formed at the air-water interface. It was difficult to obtain a good atomic force microscope (AFM) image because of the water subphase. Fortunately, we found that similar silver particle films with dimension of the scale of centimeters were also spontaneously formed at the air-water interface with freshly prepared silver colloids sealed in a conical flask for at least a few days without any disturbance. The latter silver particle films could be seen even with the naked eyes and were carefully picked up on a freshly cleaved piece of mica slide for AFM (Digital Instruments, Nanoscope IIIa) characterization. The properties of these two kinds of silver particle films prepared by the different procedures may resemble each other, because they are both formed at the air-water interface and probably through the same aggregation mechanism. Sample Preparation. 4-mercaptopyridine (4MPY) was used to test the enhancement ability of the SERS active substrates, silver colloids, silver mirror, and silver particle films. 4MPY purchased from Aldrich was recrystalized from methanol before use. Triply distilled water was used throughout the experiments.
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Figure 2. Typical optical images of the aggregation process of silver colloids at the air-water interface: (a-c) silver colloids in the presence of 1 × 10-3 mol/L KCl and (d-f) silver colloids in the presence of 1 × 10-3 mol/L KCl and 1 × 10-5 mol/L 4MPY.
A silver mirror was immersed into a 1 × 10-4 mol/L aqueous solution of 4MPY for about 15 min to yield a reproducible coverage of the monolayer range. After withdrawing, the film was gently washed with distilled water and then dried in the air. Samples with different 4MPY concentrations in silver colloids and those on silver particle films were prepared as follows. A 1 × 10-4 mol/L stock solution of 4MPY was consecutively diluted by a factor of 10. Then, 100 µL of the 4MPY solutions with a certain concentration was mixed with 0.9 mL of silver colloids. We added 10 µL of a 0.1 mol/L KCl solution to the mixture to activate the silver colloid.36 One part of the above mixture was incubated in a glass capillary tube for measuring SERS on the silver colloids, and the other part was injected into the U type capillary device for forming silver particle films at the air-water interface. Raman Measurements. SERS spectra were measured with a Renishaw 1000 model con-focal microscopy Raman spectrometer with a CCD detector and a holographic notch filter. Radiation of 514.5 nm from an air-cooled argon ion laser (Spectra-Physics model 163-C4260) was used for the SERS excitation with power of not more than a few miliwatts at the sample position. The microscope attachment was based on a Leica DMLM system, and a 50× objective was used to focus the laser beam onto a spot with approximately 1 µm in diameter. Images of the aggregation process were collected with a CCD camera and displayed on the computer. All of the spectra reported were the results of a single 10 s accumulation. Results and Discussion 1. Silver Particle Films Formed at the Air-Water Interface. A 50× objective was focused on the air-water interface from the right orifice of the U type capillary with a view field of 70 × 100 µm (Figure 1). Silver colloids were added into the capillary in such a way that the air-water interface was at the level with the orifice wall, which was achieved by the unique U type capillary device. The left side of the device is kept higher than its right side (Figure 1); an excess solution on the left side maintains the interface flat and serves as a reservoir that prevents rapid evaporation in the right orifice. By focusing the objective, the trapped particles and clusters could be readily distinguished from the air-water interface as bright spots. Figure 2a-c shows the aggregation process of silver colloids at the air-water interface with a 1 × 10-3 mol/L KCl solution. Figure 2d-f depicts the aggregation of the silver colloids with the 1 × 10-3 mol/L KCl solution mixed with 100 µL of a 4MPY solution of 1 × 10-4 mol/L (see Experimental Section). In the initial stage, we could even see a great number of small particles fluctuating under the interface, and only a few separated small clusters are
Hu et al. trapped and flux rapidly around the interface (Figure 2a,d). Because the interface is fluid, trapped silver particles and clusters have freedom of moving laterally along the surface. When a single particle comes into collision with existing clusters by the Brown motion, it attaches itself to them. Similar collision between one cluster and another cluster also allows many clusters to merge and to aggregate among themselves. When more and more particles are trapped, the number of trapped particles and clusters increase, and silver particle films grow up (Figure 2b,e). The formed clusters first fix themselves at the capillary wall especially at the defect site. Finally, through particle-cluster and cluster-cluster collisions, the growth of the fixed cluster leads to heterogeneous, highly ramified networks formed throughout the whole interface (Figure 2c,f), which can be characterized by a fractal approach.37 Once the networks are formed, no ordered domains and no rearrangement can be detected within the experimental time frame up to about 2 h, meaning that the aggregation at the air-water interface is irreversible one. Frequently, we can see blurs at the interface (see Figure 2). As a result of water evaporation, the interface becomes concave. Consequently, some places on the interface are not in the focal plane, and therefore, the blurs appear. The clusters supported at the interface are unstable. When irradiated by laser with high power (4.0 mW), strong coalescence leading to a small ball could be seen (Figure 2f). To avoid this, during the SERS experiment on the silver particle films, low laser power was used for the excitation. Silver particles in the bulk phase may be assumed to migrate toward the interface by the Brown motion,38 whereas the trapping of silver colloids is known to occur as the result of the surface tension of the air-water interface.39,40 Particle aggregation (considering doublet formation) at the surface depends primarily upon the particle pair interaction potential.41 For charged silver colloids, the interactions that stabilize the aggregates are thought to result from repulsive electrostatic forces and attractive van der Waals interactions.38,39,42,43 Another possible capillary attractive force, which is unique for particles at a fluid-liquid interface and may also lead to the formation of clusters, may be ruled out because the origin of this force is the particle weight. Small average silver particle size (73 nm) along with deep surface-energy potential well make gravitational force negligible.39,44,45 However, the capillary forces between a particle and capillary wall may be the reason for clusters to fix at the glass wall especially at the defect site.46,47 In the colloid science, the sum of these repulsive electrostatic and attractive van der Waals potentials gives the well-known Derjaguin-Landau-Vervey-Overbeek (DLVO) potential. An interaction potential with a secondary minimum in the DLVO case is due to the superposition of electrostatic repulsion and van der Waals attractive energy versus center-to-center separation of particles. Bonding energy in the vicinity of Brown motion (KT) can exist in the secondary minimum. In the presence of repulsive forces, one would expect the Brown motion to be biased away from the existing clusters. Hence, if the magnitude of bonding energy is of the same order as the Brown motion, rearrangement and ordered domains of silver particle films could be seen. On the other hand, if the Brown motion dominates, no clustering or ordered domains could be seen.43 A strong attractive van der Waals force results in the formation of heterogeneous clusters that is irreversible and ramifying. From the DLVO theory, the pair potential is a function of the particle diameter, the ion strength of the aqueous, the Hamaker constant, and the electric potential at the surface of the particle. The reduction of silver nitrate to prepare the silver colloids results
Aggregation of Silver Particles
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Figure 3. AFM image of silver particle films placed on a mica slide from silver colloids sealed on a conical flask.
in a stray electrolyte concentration (NaNO3) of 1 × 10-3 mol/ L, and therefore, considering the addition of 1 × 10-3 mol/L KCl, the total ion strength in silver colloids is 0.002 mol/L. In the presence of the high level stray electrolytes, the resulting shielding sharply diminishes the range of the electrostatic forces. Under these conditions, there is no secondary minimum, and van der Waals attractive forces prevail in all separations and overrun the energy of KT. It seems that the silver colloidal aggregation at the air-water interface results from this screening effect. Hurd and Schaefer39 put silica microspheres of 0.3 µm on a 1 mol/L NaCl aqueous solution and observed irreversible “stringy” structure. The bonding energy for their system estimated by Onoda43 was about 100 KT. Only under a very low electrolyte concentration (
