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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.

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.


SERS spectra of crystal violet in samples A, B, and C and Raman spectrum of crystal violet.
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Fig4: SERS spectra of crystal violet in samples A, B, and C and Raman spectrum of crystal violet.

Mentions: Figure 4 shows the averaged SERS spectra of CV in samples A, B, and C, as well as the Raman spectrum of 1 × 10−4 mol L−1 CV. The SERS spectra of the three samples are of very high quality, even for 1 × 10−7 mol L−1 CV. In contrast with samples A and B, a lower concentration of CV for sample C was detected (6 × 10−8 mol L−1) (Figure 4 inset), but the normal Raman spectra of CV is not distinguished below concentrations of 10−4 mol L−1 in our experimental conditions. No significant differences exist in the spectral profiles, though the peak intensities increase in the following sequence: B < A < C. The SERS spectrum of CV contains peaks at 1,622 and 1,590 cm−1, corresponding to the C–C stretching vibration of the phenyl ring. The peak at 1,371 cm−1 is ascribed to the C–Ccenter stretching vibration, and those at 1,178 and 806 cm−1 are ascribed to C–H bending vibrations. The radical-ring skeletal vibration and C–N bending vibration occur at 914 and 423 cm−1, respectively [4,28]. Shifts in the SERS peaks are observed for all three samples, compared to the Raman spectrum of CV. This indicates chemical adsorption in all samples, as confirmed by the Ag–N stretching vibration at 214 cm−1 in the SERS spectra. CV likely interacts with Ag through the N lone-pair electrons [29], and SERS arises from electromagnetic and chemical enhancement. The former depends on particle size and morphology, and the latter is related to charge transfer between the adsorbate and the SERS-active substrate. Probe molecules must be adsorbed to the SERS-active substrate in either enhancement mechanism. All the current silver colloids are prepared from the same reactant concentration and thus exhibit the same adsorption force, as demonstrated by their equivalent zeta potentials. The similar particle sizes of samples A and C result in similar surface areas available to CV. The more intense SERS signals from CV in sample C may reflect its stronger SPR, because its absorption λmax is located near the excitation wavelength. The SERS signals of CV in sample B are weaker than those in samples A and C, though its absorption λmax approaches the excitation wavelength. Sample B has the largest particle size and lowest surface area, meaning that its surface available to CV and its number of silver particles probed by the laser are both reduced. This indicates that the amount of adsorbate also affects the SERS signal. The averaged Raman spectra, obtained by subtracting the minimum and maximum, show that sample C has the highest SERS activity. Using the CV band at 1,622 cm−1 as a reference, the average peak intensities of CV for sample A, sample B, and sample C are 4,428, 3,295, and 5,240 counts, respectively. The intensity of the peak at 1,622 cm−1 versus the measurement repetition for samples A, B, and C is shown in Figure 5; the relative standard deviation of sample C is 5% and those of sample A and B are 9% and 29%, respectively. Sample C exhibits high SERS activity and stable SERS signals, which benefits colloid applications in terms of quantitative analysis. Here, the CV band at 1,622 cm−1 is used as a reference to evaluate the SERS enhancement factor (EF), which is often estimated as:Figure 4


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)

SERS spectra of crystal violet in samples A, B, and C and Raman spectrum of crystal violet.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Fig4: SERS spectra of crystal violet in samples A, B, and C and Raman spectrum of crystal violet.
Mentions: Figure 4 shows the averaged SERS spectra of CV in samples A, B, and C, as well as the Raman spectrum of 1 × 10−4 mol L−1 CV. The SERS spectra of the three samples are of very high quality, even for 1 × 10−7 mol L−1 CV. In contrast with samples A and B, a lower concentration of CV for sample C was detected (6 × 10−8 mol L−1) (Figure 4 inset), but the normal Raman spectra of CV is not distinguished below concentrations of 10−4 mol L−1 in our experimental conditions. No significant differences exist in the spectral profiles, though the peak intensities increase in the following sequence: B < A < C. The SERS spectrum of CV contains peaks at 1,622 and 1,590 cm−1, corresponding to the C–C stretching vibration of the phenyl ring. The peak at 1,371 cm−1 is ascribed to the C–Ccenter stretching vibration, and those at 1,178 and 806 cm−1 are ascribed to C–H bending vibrations. The radical-ring skeletal vibration and C–N bending vibration occur at 914 and 423 cm−1, respectively [4,28]. Shifts in the SERS peaks are observed for all three samples, compared to the Raman spectrum of CV. This indicates chemical adsorption in all samples, as confirmed by the Ag–N stretching vibration at 214 cm−1 in the SERS spectra. CV likely interacts with Ag through the N lone-pair electrons [29], and SERS arises from electromagnetic and chemical enhancement. The former depends on particle size and morphology, and the latter is related to charge transfer between the adsorbate and the SERS-active substrate. Probe molecules must be adsorbed to the SERS-active substrate in either enhancement mechanism. All the current silver colloids are prepared from the same reactant concentration and thus exhibit the same adsorption force, as demonstrated by their equivalent zeta potentials. The similar particle sizes of samples A and C result in similar surface areas available to CV. The more intense SERS signals from CV in sample C may reflect its stronger SPR, because its absorption λmax is located near the excitation wavelength. The SERS signals of CV in sample B are weaker than those in samples A and C, though its absorption λmax approaches the excitation wavelength. Sample B has the largest particle size and lowest surface area, meaning that its surface available to CV and its number of silver particles probed by the laser are both reduced. This indicates that the amount of adsorbate also affects the SERS signal. The averaged Raman spectra, obtained by subtracting the minimum and maximum, show that sample C has the highest SERS activity. Using the CV band at 1,622 cm−1 as a reference, the average peak intensities of CV for sample A, sample B, and sample C are 4,428, 3,295, and 5,240 counts, respectively. The intensity of the peak at 1,622 cm−1 versus the measurement repetition for samples A, B, and C is shown in Figure 5; the relative standard deviation of sample C is 5% and those of sample A and B are 9% and 29%, respectively. Sample C exhibits high SERS activity and stable SERS signals, which benefits colloid applications in terms of quantitative analysis. Here, the CV band at 1,622 cm−1 is used as a reference to evaluate the SERS enhancement factor (EF), which is often estimated as:Figure 4

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.