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Growth of Silver Colloidal Particles Obtained by Citrate Reduction To Increase the Raman Enhancement Factor L. Rivas,† S. Sanchez-Cortes,*,† J. V. Garcı´a-Ramos,† and G. Morcillo‡ Instituto de Estructura de la Materia, CSIC, Serrano 121, E-28006 Madrid, Spain, and Departamento de Quı´mica Orga´ nica y Biologı´a, Universidad Nacional de Educacio´ n a Distancia, Senda del Rey s/n, E-28040 Madrid, Spain Received July 24, 2000. In Final Form: December 8, 2000 A method is reported for obtaining silver colloids with a large particle size. It is based on the growth of seed colloidal particles obtained by chemical reduction of silver nitrate with citrate. These colloids were characterized by means of UV-visible spectroscopy and transmission electron microscopy. The nucleation mechanism is discussed on the basis of the morphology and size of the resulting particles. Moreover, the surface-enhanced Raman scattering activity of the resulting colloids was evaluated in the near-infrared region by using 1,5-dimethylcytosine as adsorbate.
Introduction Metal colloids have been widely employed in surfaceenhanced Raman spectroscopy (SERS), since the aggregation of metal particles leads to the formation of aggregates with a roughness and a fractal morphology necessary to render intense Raman spectra.1,2 Among the methods employed to obtain metal colloids, the chemical reduction of silver nitrate by citrate3 produces a more uniform distribution of particle sizes. In addition, it has been demonstrated that these colloidal suspensions are very stable, and highly sensitive and selective in comparison to other SERS substrates.4,5 The morphology of silver colloids has been extensively studied in recent years.6,7 However, despite the importance of these systems in SERS, few studies have been conducted thus far to improve their SERS effectiveness by varying their size and shape. One of the advantages of metal colloids is the possibility to control and modify the particle size and shape by choosing adequate experimental conditions.8 The particle size can be modified in Ag colloids by varying the temperature during the reduction process with citrate.6 The growth of initial Ag particles by metal deposition is an effective way of modifying the particle size,9 but the knowledge of the metal deposition mechanism as a function of the depositing atoms to metal surface ratio is crucial in determining the morphology and size of the resulting particles10 and, consequently, the SERS effectiveness of the resulting colloids. * Corresponding author. Phone: +34 91 5616800. Fax: +34 91 5645557. E-mail:
[email protected]. † Instituto de Estructura de la Materia, CSIC. ‡ Departamento de Quı´mica Orga ´ nica y Biologı´a, Universidad Nacional de Educacio´n a Distancia. (1) Albrecht, M. G.; Creighton, J. A. J. Am Chem. Soc. 1977, 99, 5215. (2) Sanchez-Gil, J. A.; Garcia-Ramos, J. V. J. Chem. Phys. 1998, 108, 317. (3) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3991. (4) Schmid, G. Chem. Rev. 1992, 92, 1709. (5) Sheng, R. S.; Zhu, L.; Morris, M. D. Anal. Chem. 1986, 58, 1116. (6) Sa´nchez-Corte´s, S.; Garcı´a-Ramos, J. V.; Morcillo, G.; Tinti, A. J. Colloid Interface Sci. 1995, 175, 358. (7) Munro, C. H.; Smith W. E.; Garner, M.; Clarkson, J.; White, P. C. Langmuir 1995, 11, 3712. (8) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (9) Shirtcliffe, N.; Nickel, U.; Schneider, S. J. Colloid Interface Sci. 1999, 211, 122. (10) Van Hyning, D. L.; Zukoski, C. F. Langmuir 1998, 14, 7034.
The aim of this work is to develop a simple method to modify the size of citrate-reduced Ag colloidal nanoparticles and the study of the nucleation mechanisms. The obtained colloids were characterized by means of UVvisible absorption spectroscopy and transmission electron microscopy (TEM). In addition, their SERS efficiency was tested by measuring the enhancement factors obtained when using 1,5-dimethylcytosine (DMC), which is known to yield a high SERS enhancement factor when adsorbed on Ag colloids.11 Experimental Part Silver nitrate (99.98%) and trisodium citrate (99%) were purchased from Merck. DMC was supplied by Sigma. All the reagents employed were of analytical grade. The aqueous solutions were prepared by using triply distilled water. The initial silver colloid (SC0) was prepared according to the Lee and Meisel method:3 200 mL of a 10-3 M AgNO3 aqueous solution was heated to boiling, then 4 mL of a 1% trisodium citrate solution was added, and, finally, the mixture was kept boiling for 1 h. The modified Ag colloids were prepared from this initial colloid by dispersing it in water up to the following Ag concentrations: 0.05 × 10-3 M (5%); 0.10 × 10-3 M (10%); 0.25 × 10-3 M (25%); and 0.50 × 10-3 M (50%). Afterward, the particles contained in these suspensions were grown by dissolving silver nitrate up to a 10-3 M concentration, adding 4 mL of a 1% trisodium citrate, and then keeping the suspension boiling for 1 h. This procedure leads to different final colloids depending on the proportion of added starting Ag particles: SC5 (5%), SC10 (10%), SC25 (25%), and SC50 (50%). Taking into account that the initial particles have an average diameter of about 45 nm, the [Ag+]/surface ratios deduced for each colloid were 1808 (SC5), 904 (SC10), 362 (SC25), and 181(SC50) ions per nm2. UV-visible spectra of the above Ag colloids were obtained by diluting them in water to 10% in a 1 cm optical path quartz cell. Samples for SERS measurements were prepared by adding a 10-2 M DMC aqueous solution (1 µL) to 1 mL of the Ag colloid. Samples for TEM were prepared following the method described in ref 6. FT-Raman spectra were recorded using a Bruker RSF 100/S model spectrophotometer. The 1064 nm line provided by a Nd: YAG laser was used as excitation source. The samples were placed in a 1 mm wide capillary, and the resolution was set to 4 cm-1. UV-vis absorption spectra were recorded on a Shimadzu UV2100 spectrometer. Micrographs were obtained with a Hitachi 7000 transmission electron microscope operating at 100 kV. (11) Sa´nchez-Corte´s, S.; Garcı´a-Ramos, J. V. J. Raman Spectrosc. 1990, 21, 679.
10.1021/la001038s CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001
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Langmuir, Vol. 17, No. 3, 2001 575 dispersion of 10% for each diameter range. The diameters of the spheroidal particles given below correspond to the average value calculated from the larger and smaller axis diameters measured for each particle in amplified TEM micrographs.
Results and Discussion
Figure 1. Absorption spectra of Ag colloids. The particle size distributions in Figure 3 were obtained after analyzing four to five different micrographs, showing a typical
UV-Visible Spectroscopy and Transmission Electron Microscopy. The SC0 colloid displays an asymmetric band centered at 434 nm (Figure 1). This band has been predominantly assigned to particles having a spheroidal shape. Figure 2a displays the TEM micrograph of this colloid showing a relatively homogeneous distribution of particles composed mainly of spherical or spheroidal particles. These particles have a diameter between 30 and 60 nm, with an average diameter of 45 nm (Figure 3), which is related to the absorption maximum at 434 nm. In addition, 200 and 1000 nm long rod-shaped particles are also observed. However, the number of rods is very low: less than 1% of the total particles. In the case of SC25 and SC50 (Figure 1), the splitting of the main absorbance band into two maximums is observed. In the SC50 colloid two maximums appear at 472 and 425 nm. However, in the SC25 colloid, the lower band appears as a shoulder, whereas the higher band does not show such an upshift (the maximum is seen at 470 nm). The presence of two maximums in these colloids is related to
Figure 2. TEM images of (A) SC0, (B) SC5, (C) SC10, (D) SC25, and (E) SC50 colloids.
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Figure 3. Particle size distribution of SC0, SC25, SC50, and SC5 colloids.
the existence of two main spheroidal particle sizes. Moreover, these colloids display a poor absorbance in the 550-850 nm region, thus revealing a low amount of aggregates or rods. The TEM micrograph of SC25 (Figure 2D) displays a highly homogeneous particle distribution, with diameters ranging between 45 and 60 nm, an average diameter of 50 nm, and a narrower dispersion of sizes (Figure 3). Even so, long rod-shaped particles (120-150 nm long) are also observed in this colloid. The larger particle diameters observed in the SC25 colloid indicate that a slight growth of the starting particles occurred, thus accounting for the 36 nm upshift of the SC0 absorption maximum from 434 to 470 nm. In the case of the SC50 colloid the TEM micrograph (Figure 2E) shows a broader particle distribution. The average diameter is slightly increased up to 55 nm (Figure 3), thus explaining the slight upshift of the absorption maximum to 472 nm and a general shift of the absorbance band toward higher wavelengths (Figure 1). In addition, a higher contribution of smaller particles (with a 30 nm mean diameter) is also seen. These smaller particles are responsible for the lower maximum observed at 424 nm in the absorption spectrum of the SC50 colloid (Figure 1). The existence of two main particle sizes in the above colloids can be attributed to the growth of particles having different sizes in the starting colloid (see size distribution of SC0 colloid in Figure 3). Therefore, the growth of
Letters
Figure 4. SERS spectra of DMC (10-5 M) on the different SC colloids (λex ) 1064 nm, 300 mW). Inset: Enhancement factor for DMC on each colloid as a function of the weighted particle diameter.
relatively small and large initial particles, having diameters lower than 10 and 45 nm, respectively, leads to colloids integrated by particles with two main sizes, responsible for the band splitting observed in the absorbance spectra of these colloids. In particular, the high amount of smaller particles observed for the SC50 explains why the absorbance is more intense for this colloid at 424 nm. The absorption spectra of the SC5 and the SC10 colloids (Figure 1) display a maximum at about 420 nm in both cases. The downshift of the main absorbance band, in comparison to that of SC0 colloid, suggests the existence of colloidal particles with smaller diameter than those existing in the initial colloid (SC0). In addition, a notable increase in the absorbance is observed in the 500-900 nm region. These results are related to the micrographs of SC5 and SC10 (micrographs B and C of Figure 2), which show a broad distribution of particle sizes (Figure 3). Moreover, this broader distribution accounts for the wider absorbance bands observed in the UV-visible spectra, since the fwhh (full width at half-height) value of the absorbance bands can be used as a criterion to estimate the particle dispersion.12 Different groups of particles can be observed in the size distribution corresponding to the SC5 colloid (Figure 3): (1) small particles (30 nm) that may correspond to the absorption maximum at 420 nm (12) Mie, G. Ann. Phys. 1908, 25, 377.
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(Figure 1), (2) intermediate particles with a 45-50 nm average diameter, and (3) large particles, with a 125 nm average diameter, corresponding to the absorption at longer wavelengths (500-900 nm). This broader size distribution is again the result of several deposition mechanisms: (1) the smaller particles may correspond to those formed from new nucleation centers, (2) the intermediate ones may be created by the growth of the very small particles originally existing in the SC0 colloid, and, finally, (3) the larger particles are the result of the growth of the initial SC0 particles having a 45 nm average diameter. The existence of these three main size groups in the case of the SC5 and SC10 colloids is attributed to the much higher [Ag+]/(Ag surface) ratio in the deposition process (see Experimental Part) and the different mechanisms of deposition described above. The more homogeneous distribution observed in the SC25 colloid indicates that the prevalent mechanism during its formation is the deposition of Ag atoms on the starting initial Ag particles, which undergo a slight growth: from 40-45 nm to 50 nm. Thus, the formation of new nucleation particles is negligible in this case. SERS Activity of Size-Controlled Colloids. The SERS spectra of DMC (10-5 M) on the different Ag colloids prepared in this work are shown in Figure 4. The spectra show no significant changes in the spectral profile. However a progressive intensification can be observed in the order SC5 < SC10 < SC0 < SC50 < SC25. In the inset figure we show the enhancement factor (EF) measured for the different colloids, which are represented by their weighted particle diameter (Dw). This diameter takes into
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account the mass importance of the particles in the suspension and was deduced from the expression: Dw ) (∑diniVi)/(∑niVi), where ni and Vi are the number and volume of the particles with diameter di. On the other hand, the EF value was deduced from the corresponding SERS spectra and the FT-Raman spectra of a 0.1 M DMC aqueous solution by the following equation: EF ) (ISERSCAD)/(IADCSERS). Where ISERS and IAD are the intensities of the 792 and 780 cm-1 bands of DMC in the SERS and the FT-Raman spectra, respectively, while CSERS and CAD are the concentrations of DMC in the SERS sample (10-5 M) and the aqueous solution (10-1 M). EF reaches a maximum value for the SC25 colloid, being 10-fold higher than in the case of the SC5 colloid. The resulting EF/Dw ratios are different in each colloid due to the influence of two different factors: (i) the electromagnetic field intensity enhancement, which depends on the particle size and morphology, and (ii) the variation of the available surface on the particle as the particle size is modified. We have seen that the SERS intensity increases as the size of the particles becomes larger, in the case of the SC25 and SC50 colloids, obtaining a maximum in the case of the SC25 colloid. However, a further increase of the particle size, that is in the case of SC5 and SC10 colloids, leads to a lower SERS intensity signal. Acknowledgment. This work has been supported by DGESIC Project Number PB97-1221. S.S.-C. acknowledges the CSIC for a contract. LA001038S
