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Preparation of silver colloids with improved uniformity and stable surface-enhanced Raman scattering.

Meng W, Hu F, Jiang X, Lu L - Nanoscale Res Lett (2015)

Bottom Line: Silver colloids of uniform shape and size are prepared by a two-step reduction.Small silver particles form initially by the rapid reduction of silver nitrate with sodium citrate at 100°C and then grow at 92°C.The reaction processes and resulting silver colloids are characterized by transmission electron microscopy, ultraviolet-visible absorption spectrophotometry, zeta-potential measurements, and Ag(+) concentration analysis.

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Affiliation: School of Sciences, China Pharmaceutical University, Nanjing, 211198 China ; Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing, 210094 China.

ABSTRACT
Silver colloids of uniform shape and size are prepared by a two-step reduction. Small silver particles form initially by the rapid reduction of silver nitrate with sodium citrate at 100°C and then grow at 92°C. The reaction processes and resulting silver colloids are characterized by transmission electron microscopy, ultraviolet-visible absorption spectrophotometry, zeta-potential measurements, and Ag(+) concentration analysis. The surface-enhanced Raman scattering (SERS) activity of the silver colloids is then investigated, using crystal violet (CV) as a SERS probe. The silver colloids exhibit uniform shape and size and stable SERS activity. The average size of the silver particles is 47 nm (14% relative standard deviation), while the average sizes of the silver colloids prepared at 100°C and 92°C are 41 (30%) and 71 nm (33%), respectively.

No MeSH data available.


UV–vis absorption spectra of samples A, B, and C.
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Fig3: UV–vis absorption spectra of samples A, B, and C.

Mentions: The LaMer model [22] and our previous study [23] suggest that particle size can be controlled by rapid nucleation followed by slow growth. A two-step reduction method involving a temperature change during reaction is thus carried out for sample C. After stage 1, the reaction vessel is transferred into a 92°C water bath, and the reaction mixture is quickly cooled to 92°C by adding cold water, allowing for slower growth. Figure 1c shows the change in λmax, the absorbance at λmax in the UV–vis absorption spectrum, and the Ag+ concentration of sample C. (Ag+ is reduced more slowly at 92°C.) Under these conditions, the formation of silver seeds is forbidden (Figure 2g), and seeds grow with a depletion of Ag+. Thus, Ag+ is almost completely reduced by the end of stage 2 (4 to 50 min); λmax occurs at 430 nm at the end of stage 2 and does not change substantially during stage 3. This suggests that growth is mainly due to molecular addition on silver-particle surfaces. Stable, quasi-spherical silver colloids are eventually obtained. The average particle size and relative standard deviation obtained from the TEM images are 47 nm and 14%, respectively (Figure 2h). The particle size of sample C is between those of samples A and B, and its relative standard deviation is lower than that of both the other samples, indicating more uniformly sized silver colloids. The peak half-width of the colloids prepared in the two-step reduction is larger than that of those prepared at 100°C (Figure 3). However, colloid quality cannot be unambiguously analyzed solely from UV–vis absorption spectra. The UV–vis absorption spectra of larger silver particles depend on particle size, because the higher-order terms contribute significantly. The plasmon bandwidth increases with increasing particle size, which is usually ascribed to extrinsic size effects because size dependence arises from the full expression of Mie’s theory [24,25]. Thus, the lower reaction temperature more effectively prevents continuous nucleation by decreasing the Ag+ reduction rate and supersaturation. However, sample D prepared by reaction at 100°C followed by 90°C is not uniform, since silver nanoparticles with a wide size distribution are observed in the final product (Figure 2i). Crystal growth is accompanied by dissolution. Smaller crystals and particle protrusions have higher interfacial energies and solubilities than larger crystals, which enhances the growth of bigger crystals and the uniformity of particles via Ostwald ripening. Further decreasing in temperature suppresses the oxidation of silver, making Ostwald ripening less significant. In addition, decreasing the temperature increases the required reaction time, and the reaction glassware becomes coated with silver nanoparticles. Of course, if the ratio of silver nitrate and citrate is changed, uniform silver colloids with different average sizes of nanoparticles may be prepared through two-step synthesis.Figure 3


Preparation of silver colloids with improved uniformity and stable surface-enhanced Raman scattering.

Meng W, Hu F, Jiang X, Lu L - Nanoscale Res Lett (2015)

UV–vis absorption spectra of samples A, B, and C.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4384897&req=5

Fig3: UV–vis absorption spectra of samples A, B, and C.
Mentions: The LaMer model [22] and our previous study [23] suggest that particle size can be controlled by rapid nucleation followed by slow growth. A two-step reduction method involving a temperature change during reaction is thus carried out for sample C. After stage 1, the reaction vessel is transferred into a 92°C water bath, and the reaction mixture is quickly cooled to 92°C by adding cold water, allowing for slower growth. Figure 1c shows the change in λmax, the absorbance at λmax in the UV–vis absorption spectrum, and the Ag+ concentration of sample C. (Ag+ is reduced more slowly at 92°C.) Under these conditions, the formation of silver seeds is forbidden (Figure 2g), and seeds grow with a depletion of Ag+. Thus, Ag+ is almost completely reduced by the end of stage 2 (4 to 50 min); λmax occurs at 430 nm at the end of stage 2 and does not change substantially during stage 3. This suggests that growth is mainly due to molecular addition on silver-particle surfaces. Stable, quasi-spherical silver colloids are eventually obtained. The average particle size and relative standard deviation obtained from the TEM images are 47 nm and 14%, respectively (Figure 2h). The particle size of sample C is between those of samples A and B, and its relative standard deviation is lower than that of both the other samples, indicating more uniformly sized silver colloids. The peak half-width of the colloids prepared in the two-step reduction is larger than that of those prepared at 100°C (Figure 3). However, colloid quality cannot be unambiguously analyzed solely from UV–vis absorption spectra. The UV–vis absorption spectra of larger silver particles depend on particle size, because the higher-order terms contribute significantly. The plasmon bandwidth increases with increasing particle size, which is usually ascribed to extrinsic size effects because size dependence arises from the full expression of Mie’s theory [24,25]. Thus, the lower reaction temperature more effectively prevents continuous nucleation by decreasing the Ag+ reduction rate and supersaturation. However, sample D prepared by reaction at 100°C followed by 90°C is not uniform, since silver nanoparticles with a wide size distribution are observed in the final product (Figure 2i). Crystal growth is accompanied by dissolution. Smaller crystals and particle protrusions have higher interfacial energies and solubilities than larger crystals, which enhances the growth of bigger crystals and the uniformity of particles via Ostwald ripening. Further decreasing in temperature suppresses the oxidation of silver, making Ostwald ripening less significant. In addition, decreasing the temperature increases the required reaction time, and the reaction glassware becomes coated with silver nanoparticles. Of course, if the ratio of silver nitrate and citrate is changed, uniform silver colloids with different average sizes of nanoparticles may be prepared through two-step synthesis.Figure 3

Bottom Line: Silver colloids of uniform shape and size are prepared by a two-step reduction.Small silver particles form initially by the rapid reduction of silver nitrate with sodium citrate at 100°C and then grow at 92°C.The reaction processes and resulting silver colloids are characterized by transmission electron microscopy, ultraviolet-visible absorption spectrophotometry, zeta-potential measurements, and Ag(+) concentration analysis.

View Article: PubMed Central - PubMed

Affiliation: School of Sciences, China Pharmaceutical University, Nanjing, 211198 China ; Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing, 210094 China.

ABSTRACT
Silver colloids of uniform shape and size are prepared by a two-step reduction. Small silver particles form initially by the rapid reduction of silver nitrate with sodium citrate at 100°C and then grow at 92°C. The reaction processes and resulting silver colloids are characterized by transmission electron microscopy, ultraviolet-visible absorption spectrophotometry, zeta-potential measurements, and Ag(+) concentration analysis. The surface-enhanced Raman scattering (SERS) activity of the silver colloids is then investigated, using crystal violet (CV) as a SERS probe. The silver colloids exhibit uniform shape and size and stable SERS activity. The average size of the silver particles is 47 nm (14% relative standard deviation), while the average sizes of the silver colloids prepared at 100°C and 92°C are 41 (30%) and 71 nm (33%), respectively.

No MeSH data available.