Acoustic Vibration Modes of Gold–Silver Core–Shell Nanoparticles

Round 1

Reviewer 1 Report

In their manuscript entitled “Acoustic vibration modes of gold-silver core-shell nanoparticles”, Otomalo et al. report the study of plasmonic properties of silver-coated gold nanorods. This is a continuation of their previously published research [Otomalo T.O. et al. Adv. Opt. Mater. 2021, 9, 2001778, 642 doi:10.1002/adom.202001778].

The manuscript is well structured, and written in a very good language.

I recommend publication after addressing just several minor comments listed below.

1. Introduction, P. 2, L. 59-60: The authors state the following: “Due to their anisotropy, the mechanical behavior of rod-like core-shell NPs is richer than that of their spherical counterparts.” This statement should be supported by citing several previously published experimental papers, wherein a comparative study of such nanoparticles was performed.
2. Materials and Methods, P. 3, Section 2.2. I understand that the authors have just mentioned the use of transmission electron microscopy, and UV-visible spectroscopy for the characterization of the nanoparticles synthesized in their study. However, I recommend to expand this section by presenting a brief description of the techniques employed.
3. Results, P. 4, Section 3.1. I recommend the authors to give representative TEM images of the nanoparticles from the five samples mentioned in the Supplementary. This is important, since:

(1) the dimensions of the nanoparticles, and the core shell thickness influence their optical properties significantly, and

(2) the nanoparticles’ dimensions values were used in the study for the calculation of their mechanical properties.

4. The intended applications of the synthesized nanoparticles should be discussed (for instance, applications in highly sensitive biosensors). So, to date, gold nanorod-based LSPR biosensors are well known [see, for instance, the papers by Truong P.-L. et al.: Lab Chip, 2011,11, 2591-2597; Lab Chip, 2012,12, 1102-1109]. I also recommend the authors to mention possible future applications of the nanorods in the end of the abstract, and in the conclusions.

Sincerely,

The reviewer

Author Response

Please, see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments from the editors and reviewers:

The authors investigated the acoustic vibration modes of bimetallic AuNR@Ag core-shell NPs depending on the amount of silver deposited onto the gold nanorod core. The manuscript consists of many new results. The manuscript can be accepted for publication after minor revisions. 

Avoid breaking down words into syllables through the text.

Many words seem to be miswritten. Consider replacing it.

Check unnecessary double spaces through the text.

Avoid wordy sentences:  Too many non-content words may indicate wordiness. Consider rewriting to avoid some of these words: moreover, the, of, to, are, with, at, which, is:   For example:  (Moreover, the contributions of the vibrational modes to the experimental TA 21 spectra are shown to vary with the probe laser wavelength at which the signal is monitored)  

Many errors were found in the text. Consider a professional proofreading service for the article. 

Please decrease the size and shape of the arrow in figure 3.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

• Overview and general recommendation

In this manuscript, the FFT analysis of transient absorption spectra of bimetallic AuNR@Ag core-shell nanoparticles obtained by pump-probe spectroscopy was used to obtain their specific vibrational modes in the range of 15-150 GHz. Through the excitation of nanoparticles by light energy, after the redistribution of energy and the transfer of electron-electron and electron-phonon collisions, the rapid diffusion of thermal energy through electron transport in metals can lead to a sudden lattice expansion, resulting in a series of acoustic vibration patterns. Acoustic vibrations of metallic NPs have promising applications in nano-weighing of matrices deposited or grafted on NPs, and therefore, the study of acoustic vibrations of NPs is of interest to the reader.

Furthermore, previous studies have tended to focus on only two basic vibration modes, extensional and breathing. In this paper, the presence of more complex vibrational modes in bimetallic NPs, including face, edge, and corner displacements, is observed by FFT analysis of transient absorption spectra. Moreover, this manuscript also determines the effect of the probe laser wavelength on the vibration spectrum by analyzing the effect of different mechanisms involved in the acoustic-optical coupling. The results show that probing the vibrational landscape in the frequency domain of different plasmon modes is promising for quantitative assessment of the relative deposition of NP tips and sides. The research results of this paper are novel and promising in application and meet the research interests of the journal readers. However, there are several questions need to be addressed before considering publication.

• Comments
  • Page 4, Line 177: Figure 1 reports the temporal profile of the TA signal probed at the edge of the LgSPR band in the stationary regime (699 nm). The author state that “…probed at the edge of the LgSPR band in the stationary regime (699 nm)”. There should be more elaboration about LgSPR band and TrSPR band below (Page 5, Line 206) or should add some references here. In addition, I see that there is also a temporal profile of AuNR@Ag at 526 nm in Fig. 1, which is not explained here. Still, in Fig. 1, The definition of the vertical coordinates in the graph may require some explanation.
  • Page 9, Line 326: This is due to the increase in the effective mass of the NP, which has an inverse relation with the mechanical oscillation mode frequency. The author state that the changes of mode frequencies is related to Ag coatings, I think the explanation here needs to be theoretical, like adding some references.
  • Page 10, Line 388: The acoustic vibration-induced volume change results in the modification of the free electron distribution. Consequently, the change in the bulk plasma frequency can be estimated from the modified free electron density (ED). Similarly, references can be added here to ensure the rigor of the article.
  • There are some formatting errors in the manuscript, such as Page 3, Line 145. The author should double check.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

In this manuscript, the authors use time-resolved pump-probe spectroscopy to explore the vibrational response of core-shell Au-Ag cuboid nanoparticles (NPs), which exhibit a complex plasmonic response dominated by a dipolar longitudinal mode and higher-order transverse modes in the near-UV, which may be exploited for a range of applications. Moreover, the authors use a combination of experiments and simulations of the elastic and optical properties of these nanoparticles to elucidate the main mechanisms involved in the acousto-plasmonic coupling. More in detail, the authors perform a fast Fourier transform analysis of the acousto-optical dynamics, revealing specific vibrational modes in the frequency range 15-150 GHz, further studied by numerical simulations based on the finite element method. Interestingly, the authors observe that while bare Au nanorods exhibit extensional and breathing modes, the bimetallic structures display more complex motions. The amplitude and frequency of these modes are shown to depend on the Ag-shell thickness, as the silver load modifies the NP aspect ratio and mass.

I find the manuscript correct and the results novel and interesting. Nevertheless, some points should be taken into account and addressed by the authors before publication.

1. The authors should explain better why their work is relevant for applications in the UV while they are focusing on effects arising from vibrational modes at GHz frequencies.
2. The Authors must give an explanation on why the contributions of the vibrational modes to the pump-probe spectra depend on the probe pulse wavelength. I would expect a stronger dependence on the pump wavelength, since the effect is purely thermal and by pumping at the resonances should have in principle a larger effect on the relaxation time of the system.
3. TEM/SEM images of the nanoparticles, as well as their absorption spectra, should be added, even if this characterization has been reported elsewhere. In this way it will be easy for the reader to see where the optical resonances are. This addition will be also useful to compare with the numerical simulations of the optical response, which is reported but never compared with the experimental data.
4. How can the Brownian motion of the solution affect the time-resolved measurements (I assume that in the measurements the delay stage stays at a particular time delay for some seconds and/or minutes)? Can the authors provide any detail/comment about this?
5. I might agree that the small resonance shift that the environment might induce in the spectral position of the plasmonic resonances can be neglected, but I am not sure that the mechanical coupling at the Au/Ag interface can be easily neglected. Can the authors provide a stronger support regarding this assumption? How would change the acoustic resonances if this effect is taken into account?
6. Instead of using “eq” to refer to the samples with different Ag thickness, I suggest to use an estimation of the real thickness. It is also visually better when plotting the curves.
7. Intensity in Figure 3 should be not normalized. If the NPs concentration is the same, the authors should put the absolute values to show if there is any change of the intensity when comparing the samples.
8. The cubes have been simulated with sharp edges, but I assume that in the real samples the edges are more round like in the nanorods (no images are provided so I just assume this). What happens if the shape is more rounded?
9. In Figure 6, what is the meaning of plotting “Delta_sigma”? To show better the effects of ED and DP? What happens if you consider only ED or only DP? Are these two phenomena coupled? Experimentally, how would the authors access this information? They can propose an experiment, they do not need to do it.
10. What is the purpose of figure 5? This figure is less important than figure 6 so I suggest flipping the order of the two and the related discussions.
11. What are the other mode in figure 4d? The authors highlight only 4 of them, but they do not explain what are the other modes. They also imply that previously modes 2 and 3 were not reported. From the measurements, it is also very difficult to assess the presence of those modes. Can they comment on this? Moreover, these modes have been assessed using numerical calculations. Maybe, since this has never been observed before, at least experimentally, neglecting the mechanical coupling at the Au/Ag interface might produce the appearance of these additional peaks in the simulations?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf