Monitoring Gold Nanoparticle Growth in Situ via the Acoustic

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Monitoring Gold Nanoparticle Growth in situ via the Acoustic Vibrations Probed by Four-Wave Mixing Jian Wu, Dao Xiang, and Reuven Gordon Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b05086 • Publication Date (Web): 27 Jan 2017 Downloaded from http://pubs.acs.org on January 29, 2017

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Analytical Chemistry

Monitoring Gold Nanoparticle Growth in situ via the Acoustic Vibrations Probed by Four-Wave Mixing Jian Wu, Dao Xiang, and Reuven Gordon* Department of Electrical and Computer Engineering, University of Victoria, Victoria, BC, V8P 5C2, Canada KEYWORDS: Nanomaterials, nonlinear optics, four-wave mixing, plasmonics

ABSTRACT: We monitor in situ gold nanoparticle growth in aqueous solution by probing the acoustic vibrations with four-wave mixing. We observe two acoustic vibrational modes of gold nanoparticles from the nonlinear optical response: an extensional mode with longitudinal expansion and transverse contraction; and a breathing mode with radial expansion and contraction. The mode frequencies, which show an inverse dependence on the nanoparticle diameter, allow monitoring the nanoparticle size and size distribution during synthesis. The information about the nanoparticle size and size distribution calculated based on the mode frequencies agrees well with the results obtained from the electron spectroscopy analysis, validating the four-wave mixing technique as an accurate and effective tool for in situ monitoring of colloidal growth.

Gold nanoparticles have found broad applications in biosensing, bioimaging, medicine, catalysis, photovoltaics, and nonlinear optics.1−9 Gold nanoparticles are commonly synthesized by the

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chemical reduction methods10−13 (e.g., the citrate reduction method and the sodium borohydride reduction method). Parameters such as reaction temperature, reaction time, and reactant concentration need to be carefully controlled and adjusted in order to achieve fine tuning of nanoparticles’ size and morphology.14−17 The UV-vis spectroscopy, the scanning electron microscopy (SEM), and the transmission electron microscopy (TEM) are the common methods for nanoparticle characterization.18−20 The UV-vis spectroscopy can achieve in situ monitoring the localized surface plasmon resonance (LSPR) band; however, the LSPR is strongly shape dependent with a much weaker size dependence.21 For example, the LSPR remains at 525 nm for gold spherical nanoparticles with diameter ranging from 5 to 25 nm.22 Therefore, UV-vis spectroscopy is not favored to accurately monitor the nanoparticle size change during synthesis. Although SEM and TEM can accurately characterize nanoparticles, the instrumental limitations and cost present barriers to in situ monitoring of nanoparticles’ size and morphology. The acoustic vibrations of nanoparticles can provide information about the size, shape, and elastic properties.23−25 Our recent works used four-wave mixing to probe the acoustic vibrations of dielectric and metallic nanoparticles.26−29 The mode frequencies of nanoparticles show a strong dependence on the size, shape, and material, allowing us to monitor nanoparticle growth (mainly the size and distribution change) in situ. In this work, we use the FWM technique as an accurate and effective in situ monitoring method by synthesizing gold spherical nanoparticles using both the sodium borohydride reduction and the citrate reduction methods. The acoustic vibrations of nanoparticles are probed in situ with FWM and the size and distribution information are calculated based on the vibrational frequencies. The gold nanoparticles are also analyzed by SEM. The results obtained from FWM and SEM agree well, validating the accuracy of the FWM method as an in situ monitoring tool for nanoparticle growth.


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EXPERIMENTAL SECTION Gold nanoparticle synthesis. Gold nanoparticles were synthesized by reduction of tetrachloroauric acid at room temperature using sodium borohydride30 and trisodium citrate31, respectively. For the sodium borohydride reduction, 20 mL of 0.5 mM tetrachloroauric acid (HAuCl4.3H2O, 99.9+%, Aldrich) aqueous solution and 20 mL of 7.5 mM sodium borohydride (NaBH4, 99.9+%, Aldrich) aqueous solution were prepared, respectively. The HAuCl4 solution was stirred vigorously by a Teflon-coated magnetic bar at 1000 rpm speed. The NaBH4 solution was quickly added to the HAuCl4 solution. An immediate color change of the solution could be observed. The solution was stirred for 3 min until the color of the solution turned wine red. The sample was labeled as Sample 1 for further analysis. In order to tune the size of the nanoparticles, we adjusted the concentration of the NaBH4 solution by preparing another two 20 mL NaBH4 solutions with concentrations of 10 mM and 20 mM (labeled as Sample 2 and Sample 3, respectively), followed by the same synthesis process described above. Gold nanoparticles were immediately characterized in aqueous solution by the UV-vis spectroscopy and FWM after the synthesis completed. For the trisodium citrate reduction, 20 mL of 0.5 mM HAuCl4 solution and 20 mL of 5 mM trisodium citrate (Na3Ct, 99+%, Biotech) solution were mixed as the growth solution and stirred by a Teflon-coated magnetic bar at 1000 rpm speed. The initial PH of the growth solution was adjusted to 7.2 by adding diluted NaOH solution. The color of the growth solution (pale yellow) gradually turned transparent, light pink after 2 h, and ruby-red after 24 h. FWM and UV-vis measurements were performed at certain intervals during the synthesis. In the meantime, samples from the growth solution were also drop-coated and quickly dried under nitrogen flow on a goldcoated slide (EMF Corp.) for SEM characterization.



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Instrumentation. Figure 1 shows the FWM setup. The gold nanoparticle solution was placed in a quartz cuvette. The sample was illuminated by two counter-propagating laser beams: a continuous-wave (CW) tunable external-cavity laser (ECL) (DL100, Topica Photonics) and a CW tunable distributed Bragg reflector laser (DBRL) (DBR852P, Thorlabs). In order to fully cover the thickness of the cuvette (1 mm) as the light-matter interaction region, two weak focusing lenses were used with focal lengths of 20 cm (Lens1) and 4 cm (Lens2) and two laser beams focused by the Lens1 (I1 and I3) were placed closely to minimize the angle between the two beams. The co-polarized illumination of the ECL and the DBRL was maintained by controlling the polarization of the DBRL laser through a polarization controller and a polarizer. An electrostrictive force induced by the interference between the two counter-propagating beams (I2 at the frequency of ω2 and I3 at the frequency of ω3) can stretch the nanoparticles along the beam polarization direction. When the beat frequency (ω3 − ω2) matches the acoustic resonance of the nanoparticles, the acoustic vibrations of the nanoparticles will be resonantly excited. The displacement of the nanoparticles generates a travelling periodic variation in the refractive index of the medium which is analogous to a moving Bragg grating. The FWM signal I4 at the frequency of ω4 (ω4 = ω2) is generated when the beam I1 at the frequency of ω1 (ω1 = ω3) diffracts from the Bragg grating. The FWM signal was collected by an avalanche photodetector (APD120A, Thorlabs) connected to a lock-in amplifier (SR510, Stanford Research Systems). The power of the ECL was set to the maximal output power of 67 mW and the wavelength was fixed at 853.4 nm. The power of the DBRL was maintained at 25 mW during the wavelength tuning and the wavelength (> 853.4 nm) was monitored by an optical spectrum analyzer (86142B, Agilent). The APD output voltage was recorded as a function of the beat frequency of the two lasers.


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The UV-vis spectrum was obtained by measuring the transmission spectrum of the nanoparticle solution placed in the quartz cuvette using a white light source (LS-1-LL, Ocean Optics Inc.) and a spectrometer (a spectrometer (QE65000, Ocean Optics Inc.). It should be noted that the path length is the thickness of the quartz cuvette (1 mm), not the common path length (10 mm) in a standard UV-vis spectrometer. The SEM images of the nanoparticles were obtained by a Hitachi S-4800 field emission SEM at 2kV. Nanoparticles were drop-coated and dried on a gold-coated slide for SEM imaging.

RESULTS AND DISCUSSION Figure 2 shows the FWM signal of gold nanoparticles synthesized by NaBH4 reduction as a function of the beat frequency between the ECL and the DBR lasers. The background signal from Rayleigh scattering of the DBR laser was subtracted. Two acoustic vibrations of gold nanoparticles were observed: a fundamental extensional mode with longitudinal extension and transverse contraction as shown in Figure 2A; a fundamental breathing mode with radial extension and contraction as shown in Figure 2B. We can estimate the nanoparticle diameter D and distribution by Lamb’s theory based on the fundamental extensional mode frequency υext and the fundamental breathing mode frequency υbr32: D=

ξν l πυext

(1)

D=

ην l πυbr

(2)

where νl (3240 m/s) is the longitudinal sound velocity in gold.33 ξ (0.985 for the fundamental extensional mode) and η (2.945 for the fundamental breathing mode) are proportionality factors under the free boundary condition.34 For example, the 52.7 GHz extensional mode corresponds



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to nanoparticles with a 19.3 nm average diameter according to eq 1 while the 157.9 GHz breathing mode gives the similar result of a 19.2 nm average diameter according to eq 2. The size distribution can be estimated from the full width at half maximum (FWHM) of the extensional mode FWHMext and the FWHM of the breathing mode FWHMbr based on eqs 1 and 2 and assuming a Gaussian size distribution: ∆D =

∆υ extξν l

∆D =

∆υbrην l

πυ

2 ext

πυ

2 br

=

FWHM ext

=

FWHM br

υ ext

υbr

D

(3)

D

(4)

where ∆D = 2.355σ and σ is the standard deviation. Table 1 summarizes the size and distribution results calculated based on the extensional and breathing modes, respectively. Figure 3 shows the characterization results of different samples synthesized by NaBH4 reduction from UV-vis spectroscopy and SEM for verifying the accuracy of the FWM method for in situ monitoring gold nanoparticles’ size and distribution. Figure 3A shows the extinction spectra of different samples. The extinction spectra show that the LSPR band does not shift with the nanoparticle size and remains at 524 nm. It indicates that the UV-vis spectroscopy is not suitable for monitoring nanoparticle’s size change during nanoparticle growth, especially for monitoring spherical nanoparticle growth. Figures 3B−3D show the nanoparticle size distribution obtained by measuring 100 nanoparticles in each sample from the SEM images and fitted by the Gaussian distribution function. The results obtained from the SEM analysis agree well with the results obtained from the FWM characterization. This verifies the accuracy of the FWM method for in situ monitoring of both nanoparticle size and distribution. The reduction of HAuCl4 by NaBH4 is a fast process for nanoparticle synthesis. Colloidal growth can complete within a few minutes and gold nanoparticles can remain stable during the


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following FWM characterization. To demonstrate the FWM method’s applicability to slower synthesis monitoring, we performed in situ monitoring of nanoparticle growth using Na3Ct reduction at room temperature. Figure 4 shows the FWM signal of the growth solution using Na3Ct reduction as a function of the beat frequency between the ECL and the DBR lasers at certain intervals. The background signal from Rayleigh scattering of the DBR laser was subtracted. Both extensional modes (Figure 4A) and breathing modes (Figure 4B) shift as the growth time increases, indicating the gradual size growth of gold nanoparticles. The synthesis completed after 24 h as both extensional and breathing modes became stabilized with time. Table 2 summarizes the size and distribution results calculated based on the extensional and breathing modes, respectively. Figure 5 shows the UV-vis and SEM characterization of gold nanoparticles from the growth solution at certain intervals. The extinction spectra (Figure 5A) do not show an appreciable shift during the entire synthesis. Figures 5B−5F show the nanoparticle size distribution obtained by measuring 100 nanoparticles from the SEM images and fitted by the Gaussian distribution function. Figure 6 shows that the nanoparticle size information obtained from the SEM analysis agrees well with the results obtained from the FWM measurements, verifying the accuracy of the FWM method for in situ monitoring of slow nanoparticle synthesis. Although the acoustic vibrations of nanoparticles could be probed by other methods such as the low-frequency Raman35 and the time-resolved spectroscopy36, the FWM technique shows advantages in terms of resolution with regard to Raman, and simplicity with regard to the timeresolved spectroscopy (no ultrafast lasers or delay stages required). Compared to the limited applicability of Raman and time-domain methods in analysis of colloidal synthesis, our work establishes the use of the FWM technique for in situ monitoring of both fast and slow colloidal



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growth by probing the acoustic vibrations of nanoparticles. The FWM technique also presents features such as high accuracy and low cost with regard to the common characterization methods (extinction and TEM/SEM). The FWM technique provides not only the advantages of the accuracy of TEM/SEM, but also can be carried out in-situ with an optical arrangement similar to extinction measurements, with orders of magnitude higher accuracy in monitoring particle size.

CONCLUSIONS Using a FWM setup to probe the acoustic vibrations of gold nanoparticles, the synthesis by the reduction of HAuCl4 using NaBH4 and Na3Ct was monitored in situ, respectively. Two acoustic vibrational modes of gold nanoparticles were observed: an extensional mode with longitudinal expansion and transverse contraction; a breathing mode with radial expansion and contraction. The size and distribution information was calculated based on the two modes. We also characterized the gold nanoparticles by SEM. The results obtained from the SEM analysis agree well with the results from the FWM measurements, validating the FWM method as an accurate technique for in situ colloidal growth monitoring.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The authors acknowledge financial support from the the Materials for Enhanced Energy Technologies NSERC CREATE program, the NSERC Discovery Grant program, and the Chinese Scholarship Council.

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FIGURES AND TABLES

Figure 1. FWM setup. ECL: external cavity laser; DBRL: distributed Bragg reflector laser; OSA: optical spectrum analyzer; PC: polarization controller; FC: fiber coupler; BS: beam splitter; MR: mirror; IRS: iris; APD: avalanche photodetector; BLR: blocker; OC: optical chopper; PR: polarizer; FPC: fiber-port collimator.



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Figure 2. FWM signal of gold nanoparticles synthesized by NaBH4 reduction as a function of the beat frequency between the ECL and the DBR lasers. (A) Fundamental extensional modes of gold nanoparticles with mode frequencies shifting from 174.6 GHz to 52.7 GHz as the average size of gold nanoparticles increases; (B) Fundamental breathing modes of gold nanoparticles with mode frequencies shifting from 523.1 GHz to 157.9 GHz as the average size of gold nanoparticles increases. The error bar represents the standard deviation calculated by 148 data points at each beat frequency.


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Figure 3. Gold nanoparticle (synthesized by NaBH4 reduction) characterization by UV-vis spectroscopy and SEM. (A) Extinction spectra of different samples. All spectra show the same LSPR band at 524 nm. (B) Gaussian size distribution of Sample 1 with an average diameter of 6.0 nm and standard deviation of 0.3 nm. (C) Gaussian size distribution of Sample 2 with an average diameter of 10.3 nm and standard deviation of 0.7 nm. (D) Gaussian size distribution of Sample 3 with an average diameter of 19.7 nm and standard deviation of 2.0 nm. The size distribution is obtained by measuring 100 gold nanoparticles in each sample and fitted by the Gaussian distribution function. The insets are the SEM images of the corresponding samples obtained at 600k× magnification. The scale bar represents 50 nm length.



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Figure 4. FWM signal of gold nanoparticles synthesized by Na3Ct reduction as a function of the beat frequency between the ECL and the DBR lasers. (A) Fundamental extensional modes of gold nanoparticles with mode frequencies shifting from 236.1 GHz to 36.0 GHz as the average size of gold nanoparticles increases; (B) Fundamental breathing modes of gold nanoparticles with mode frequencies shifting from 675.2 GHz to 107.3 GHz as the average size of gold nanoparticles increases. The error bar represents the standard deviation calculated by 148 data points at each beat frequency.


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Figure 5. Gold nanoparticle (synthesized by Na3Ct reduction) characterization by UV-vis spectroscopy and SEM. (A) Extinction spectra of the growth solution at certain intervals. (B)–(F) Gaussian size distribution of gold nanoparticles. The insets are the SEM images obtained at magnifications of 500k× for (B), 400k× for (C), 250k× for (D), 200k× for (E) and (F), respectively.



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Figure 6. Nanoparticle size as a function of the growth time using Na3Ct reduction at room temperature. The error bar stands for the standard deviation. Nanoparticle size information obtained from the SEM analysis agrees well with the results obtained from the FWM measurements. Table 1. Size information of nanoparticles synthesized by NaBH4 reduction

peak

FWHM

size (nm) based on extensional modes

Sample 1

174.6

21.2

5.8 ± 0.3

523.1

51.6

5.8 ± 0.2

Sample 2

95.5

13.5

10.6 ± 0.6

286.4

39.0

10.6 ± 0.6

Sample 3

52.7

12.3

19.3 ± 1.9

157.9

35.0

19.2 ± 1.8

extensional (GHz)

peak

FWHM

size (nm) based on breathing modes

breathing (GHz)

Table 2. Size information of nanoparticles synthesized by Na3Ct reduction breathing (GHz)

FWHM

size (nm) based on extensional modes

peak

FWHM

size (nm) based on breathing modes

236.1

48.9

4.3 ± 0.4

675.2

162.9

4.5 ± 0.4

6

75.8

17.6

13.4 ± 1.3

223.9

56.0

13.6 ± 1.4

12

45.4

9.0

22.4 ± 1.9

135.1

31.1

22.5 ± 2.1

24

36.2

8.0

28.1 ± 2.6

108.7

27.5

28.0 ± 3.0

30

36.0

7.4

28.2 ± 2.5

107.3

27.4

28.3 ± 3.1

extensional (GHz) growth time (h)

peak

2


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Monitoring Gold Nanoparticle Growth in Situ via the Acoustic

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