Study of Green Synthesis of Ultrasmall Gold Nanoparticles Using Citrus Sinensis Peel
Round 1
Reviewer 1 Report
The work reports the study of the synthesis of gold nanoparticles from Au(III) using orange peel aqueous extract as source of reductant and nanoparticle stabilizer. This work is the continuation of a previous work by two of the authors (J. Phys. Chem. C 2017, 121, 8961, ref. 22) in which they use fruit juices/extract with the same purpose. In both works, by controlling the pH of the solution and the reaction steps, differente sizes of gold nanoparticles are obtained. In the present work, ultrasmall gold nanoparticles with narrow size distribution are obtained. In my opinion, the work is suitable for publication after some minor points are addressed.
- In section 3.1, the authors say that "the absorbance intensity of all the AuNPs groups decreases, accompanied by the increase in pH (pH= 6, 8 and 11)". However, as observed in Figure 1, at pH 11 the absorbance is higher than at pH 6 and 8, and the absorbance at pH 10 is higher than that at pH 8 (see also Table 1). Which is the reason for this behavior?
- With respect to the study of the composition of the mixture of AuNPs and peel extract by FT-IR (section 3.6), I think that the conclusions are a bit speculative. For instance, “a bigger shifting could be observed on curve c (3436 cm-1) than curve b (3434 cm-1) suggesting than more AuNPs were occurred and more carbonyl groups were participated…” Do the authors think that a 2 cm-1 shift in this broad band is significant? Moreover, the C=O band in curve c (1724 cm-1) do not change with respect to control curve a.
- In Section 3.7, could the authors provides a detailed experimental description of how the nanoparticles were incubated in 1 mM NaCl?
- References should be revised. In some cases, journal abbreviations are used and in other, the full name is used. Please also check that all references are correct. For instance, in ref. 32 the page numbers are not correct. Instead of 4 it should be 556-559.
- Finally, the English language should be revised.
Author Response
Point 1: In section 3.1, the authors say that "the absorbance intensity of all the AuNPs groups decreases, accompanied by the increase in pH (pH= 6, 8 and 11)". However, as observed in Figure 1, at pH 11 the absorbance is higher than at pH 6 and 8, and the absorbance at pH 10 is higher than that at pH 8 (see also Table 1). Which is the reason for this behavior?
Response 1: Firstly, the number of gold nanoparticles determines the absorbance of the solution, and the size of gold nanoparticles determines the number of gold nanoparticles. Secondly, the formation of gold nanoparticles and the aggregation of gold nanoparticles are hindered by the increase of pH value. Based on the above two points, when the pH value is low (pH=6), the amount of gold nanoparticles is larger, but the size is larger. When the pH value is higher (pH=11), the number of gold nanoparticles decreases, but the agglomeration degree of gold nanoparticles is smaller. Therefore, when ph=6 and ph=11, the absorbance of gold nanoparticles is higher. When the pH value is in the middle (8 and 10), neither a large number of large size gold nanoparticles nor enough small particles can be produced, so the absorbance is low.
Point 2: With respect to the study of the composition of the mixture of AuNPs and peel extract by FT-IR (section 3.6), I think that the conclusions are a bit speculative. For instance, “a bigger shifting could be observed on curve c (3436 cm-1) than curve b (3434 cm-1) suggesting than more AuNPs were occurred and more carbonyl groups were participated…” Do the authors think that a 2 cm-1 shift in this broad band is significant? Moreover, the C=O band in curve c (1724 cm-1) do not change with respect to control curve a.
Response 2: Thank you for your advice. In this paper, we focus on the synthesis of green gold nanoparticles from a biological waste, which is stable and has a smaller size than gold nanoparticles prepared from fruit extractes. Therefore, just like you said, this study only speculated about the possible reductants and protective agents in orange peel extracts by FTIR. We will focus on the contribution of different pulp and peel extracts to the synthesis of AuNPs by XRD and FTIR in our next experiment. At that time, if you are interested, I will be very honored to share the results with you.
Point 3: In Section 3.7, could the authors provides a detailed experimental description of how the nanoparticles were incubated in 1 mM NaCl?
Response 3: Please see section 2.6
2.6. Aging and 72 h-incubation test
The prepared AuNPs solution was placed in glass bottle and sealed , after 4 months at room temperature, the morphology and dispersion of the AuNPs solution were examined by TEM. At room temperature, 9 mL sodium chloride solution (1mM) and 1 mL AuNPs solution were added to form gold nanoparticles salt solution. The AuNPs salt solution was placed in room temperature for 72 hours, and then the AuNPs were characterized again for their stability by TEM.
Point 4: References should be revised. In some cases, journal abbreviations are used and in other, the full name is used. Please also check that all references are correct. For instance, in ref. 32 the page numbers are not correct. Instead of 4 it should be 556-559.
Response 4: Thank you for your advice and careful review. To ensure that the reference format is correct, the reference format is generated and imported directly from the corresponding template in the EndNote. The problem with full names and abbreviations of journals should be the setting of the template itself. I will reconfirm the template with the editor.
The pp. was revised from 4 to 556-559.
Point 5: Finally, the English language should be revised.
Response 5: We will revise English grammar further before final submission.
Many thanks
Sincerely
Bo
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Study of Green Synthesis of Ultrasmall Gold 2 Nanoparticles Using Citrus sinensis peel
Yang and coworkers reports a sustainable methodology to produce gold NPs by using orange peel extract as a reducing agent. Synthesis of<5nm Au-NP is very intriguing and the TEM size characterizations are statistically significant. I recommend the publication of the manuscript with minor revisions are noted below,
1. In Figure 2 the EDS spectrum is provided. Authors should also add the image (TEM?) correspond to the spectrum.
2. Figure 2. Some peaks in the EDS spectra are not assigned (eg: ~1.8 keV (Si?), 3.2 keV). Please assign all of them.
3. Figure 3 caption: Maybe provide pH 3 in parenthesis after ‘control’
4. Figure 5D. The particle size showed in TEM image for ph11 is extremely small and the contrast is poor compare to the noise level. How ImageJ handle such systems. Manual counting is better?
5. Figure 5C. TEM image shows both larger and small particles and the standard deviation does not reflect this. Why?
4. The justification for size difference in TEM and DLS talk about the interference from the peel extract. This is not very clear. What is the DLS data of just the peel extract for comparison? Theoretically, DLS size should be bigger than TEM since it measures the hydrodynamic size rather than the bare particle as in TEM. Authors should re-evaluate the data.
5. Stability test: 1 mM NaCl cannot be referred as biological media but the ionic strength maybe comparable. In a biological media (for cell culture or in body) contains huge amount of proteins and amino acids, which can have effect on the particle size. Authors may re-phrase the paragraph.
General comments
1. What’s the active substance (reducing agent) in the peel extract causing the particle formation? Citric acid and vitamin E are potential candidates. Any available data on composition of the extract worth mentioning.
Author Response
Point 1: In Figure 2 the EDS spectrum is provided. Authors should also add the image (TEM?) correspond to the spectrum.
Response 1: Due to the limitation of experimental conditions, EDS is a part of SEM/EDS equipment. TEM photos and EDS can not be completed on the same equipment. And EDS is to qualitatively explain the gold content in samples, so no corresponding SEM photos are attached.
Point 2: Figure 2. Some peaks in the EDS spectra are not assigned (eg: ~1.8 keV (Si?), 3.2 keV). Please assign all of them.
Response 2: The main peaks (Mg and Si) were added into the spectra. Please see fig. 6.
Point 3: Figure 3 caption: Maybe provide pH 3 in parenthesis after ‘control’
Response 3: Revised. Please see the manuscript.
Point 4: Figure 5D. The particle size showed in TEM image for ph11 is extremely small and the contrast is poor compare to the noise level. How ImageJ handle such systems. Manual counting is better?
Response 4: Before using ImageJ statistics, each sample particle must be labeled manually. Because the original TEM image has high pixels (2MB single photo), it still can be clearly imaged when it is 20 times larger than the image in this manuscript. Although the workload is heavy, it is not difficult.
Point 5: Figure 5C. TEM image shows both larger and small particles and the standard deviation does not reflect this. Why?
Response 5: In the process of particle diameter analysis, a large number of TEM photographs were taken for samples prepared under each reaction condition, and all particles in at least 10 photographs were selected for statistics, which ensured the accuracy of the analysis. At the same time, we do labeled the lager and small particles in Fig. 5c, but the statistical results are not affected by the limited larger particles. The possible reason is that the weight of these particles is very small in ImageJ's algorithm.
Point 6: The justification for size difference in TEM and DLS talk about the interference from the peel extract. This is not very clear. What is the DLS data of just the peel extract for comparison? Theoretically, DLS size should be bigger than TEM since it measures the hydrodynamic size rather than the bare particle as in TEM. Authors should re-evaluate the data.
Response 6: revised. Please see the manuscript on section 3.5.
Point 7: Stability test: 1 mM NaCl cannot be referred as biological media but the ionic strength maybe comparable. In a biological media (for cell culture or in body) contains huge amount of proteins and amino acids, which can have effect on the particle size. Authors may re-phrase the paragraph.
Response 7: The “natural saline” was used to instead of “biological media”. Please see the manuscript in section 3.7.
General comments
1. What’s the active substance (reducing agent) in the peel extract causing the particle formation? Citric acid and vitamin E are potential candidates. Any available data on composition of the extract worth mentioning.
Response 1: We are really appreciated the patience and seriousness of the reviewer. It was found that the size of AuNPs prepared in the presence of citric acid or vitamin E was difficult to achieve the results obtained in this study. The main reason was the lack of protective agents or particle stabilizers used in chemical synthesis. As a mixture, plant extracts contain complex substances, besides citric acid and vitamin E, they also contain some unknown protective ingredients. Therefore, in the future research, we will continue to focus on how to effectively distinguish the substances which lead to weakening of AuNPs coagulation in the process of plant extracts synthesizing gold nanoparticles.
Many thanks
Sincerely
Bo
Author Response File: Author Response.pdf
