Excitation-Wavelength- and Time-Dependent Fluorescent Ink Based on RGB Building Blocks for Advanced Anti-Counterfeiting
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis work in on the synthesis of three phosphors, namely red (Ca2YNbO6:0.4Eu3+), green (SrAlO4:0.01Eu2+,0.02Dy3+) and blue (CaAl2O4:0.012Eu2+, 0.06Nd3+, 13 0.036Gd3+). According to their PL excitation, each one reaches its maximum of emission at different wavelengths. Based in this principle, the mixture of the three phosphors (R: G: B=2:1:2, R: G: B=4:2:1, R: G: B=4:6:1) leads to different color emissions. The inherent persistence of emission of each phosphor also confers characteristic persistence of each mixture. Used as ink, the mixtures were used to probe anti-counterfeiting mechanism. The work requires revision before being considered for publication.
1) While R phosphor exhibits a narrow emission, G and B exhibit rather broad and overlapping emission. How far are these emissions from fundamental or primary colors? (see line 64). Please discuss on the limitations of overlapping of the G and B spectra. If possible, authors could mention strategies to narrow the emission band of G and B.
2) Please discuss the methodology followed to stablish the proportions 2:1:2, R: G: B=4:2:1, R: G: B=4:6:1 as optimum to show the excitation-wavelength and time dependent fluorescent. Maybe some other combinations could lead to other interesting emission colors.
3) If possible, also include in SI the absorption spectra of each phosphors and mixtures
4) Some wavelengths of excitations were not fixed in the experiments. For instance, in Fig. 4 the used wavelengths were 251 and 361 nm; in Figure 5 were 254 and 365. This is not a major issue as the wavelengths are almost the same, but please just check if it is not a writing error.
5) As for the persistence of emission, a better characterization must be presented. The discussion of the persistence effect is given just qualitatively. Data in figure 4d and 4e can be fitted and parameters of decay constant times can be provided to reader.
6) Line 14: “solid solutions”? maybe authors mean suspensions.
7) Figure 5 presents CIE coordinates. It would be interesting that authors present RGB coordinates obtained from photographs of each phosphor under excitation at 251 and 361 nm; the RGB coordinates from mixtures can also be obtained. Obtaining the RGB coordinates from digital photographs is straightforward. Such information could be part of SI.
8) I strongly recommend English revision. Only some examples:
Line 42-42 “…fluorescent materials have been developed which has a higher….”
Line 54: “All these above”
Line 64: “which means that the emission colors the whole spectrum with 64 them.”
Line 66 “three monochromatic phosphors above were synthesized”
Comments on the Quality of English LanguageI strongly recommend English revision. Only some examples:
Line 42-42 “…fluorescent materials have been developed which has a higher….”
Line 54: “All these above”
Line 64: “which means that the emission colors the whole spectrum with 64 them.”
Line 66 “three monochromatic phosphors above were synthesized”
Author Response
Reply to Reviewer’ Comments
- While R phosphor exhibits a narrow emission, G and B exhibit rather broad and overlapping emission. How far are these emissions from fundamental or primary colors? (see line 64). Please discuss on the limitations of overlapping of the G and B spectra. If possible, authors could mention strategies to narrow the emission band of G and B.
Answer: The red (Ca2YNbO6:0.4Eu3+), green (SrAlO4:0.01Eu2+,0.02Dy3+), and blue (CaAl2O4:0.012Eu2+, 0.06Nd3+, 0.036Gd3+) phosphors are all mature fluorescent powders that have been successfully commercialized, and their luminous performance has been fully tested. As two mature commercial fluorescent powders, the emission colors of G and B can be clearly distinguished by the naked eye. By varying the proportions of G and B in the mixed phosphors, the luminescence effect can be effectively adjusted; therefore, the overlap of the G and B spectra has no limitation on the final results. Additionally, the emission bands of G and B are unfortunately unable to narrow because they are both attributed to the f-d transition of Eu2+, unlike R which exhibits a narrow emission due to the f-f transition of Eu3+.
2.- Please discuss the methodology followed to stablish the proportions 2:1:2, R: G: B=4:2:1, R: G: B=4:6:1 as optimum to show the excitation-wavelength and time dependent fluorescent. Maybe some other combinations could lead to other interesting emission colors.
Answer: Thank you very much for your constructive comments. Actually, a series of works has been done on selecting the best proportions, but for the simplicity and clarity of the article, this part has been omitted. During the research process, we selected these three proportions that were most easily recognized by the naked eye after a series of repeated proportioning and testing experiments. Taking a few excluded proportions like R: G: B=1:1:1, R: G: B=2:1:1, R: G: B=1:2:1, R: G: B=1:1:2, R: G: B=1:2:2 as examples, their emission lights were all close to each other, lacking distinct features, which made them hard to distinguish.
3.- If possible, also include in SI the absorption spectra of each phosphors and mixtures
Answer: Thank you very much for your constructive comments. However, due to limitations in experimental conditions, we are temporarily unable to provide the absorption spectra of each phosphor and mixture as a part of the SI. If conditions permit in the future, we will supplement the research in this area.
4.- Some wavelengths of excitations were not fixed in the experiments. For instance, in Fig. 4 the used wavelengths were 251 and 361 nm; in Figure 5 were 254 and 365. This is not a major issue as the wavelengths are almost the same, but please just check if it is not a writing error.
Answer: Thank you for your reminder, and I have to say that it is not a writing error. The wavelengths used in Fig. 4 were the optimal excitation wavelengths and the optimal emission wavelengths of R, G, and B phosphors, which can better demonstrate their luminescent properties. The wavelengths used in Fig. 5 (254 nm and 365 nm) were the excitation wavelengths that we selected as the closest ones to the above experimental results under the limitation of equipment, to demonstrate the ability of mixed phosphors in practical anti-counterfeiting applications. Based on the data and results of a series of corresponding experiments (similar to what is shown in Fig. 5), we finally chose the optimal proportions from among other proportions.
5.- As for the persistence of emission, a better characterization must be presented. The discussion of the persistence effect is given just qualitatively. Data in figure 4d and 4e can be fitted and parameters of decay constant times can be provided to reader.
Answer: SrAlO4:0.01Eu2+,0.02Dy3+ and CaAl2O4:0.012Eu2+, 0.06Nd3+, 0.036Gd3+ are both long afterglow materials, and their fluorescence lifetimes cannot be simply fitted by data and calculated. The related mechanism has been mentioned in this paper. Please refer to the red lines on pages 8 and 9 for further details.
6.- Line 14: “solid solutions”? maybe authors mean suspensions.
Answer: "Solid solution" is a professional term that refers to the alloy phase in which solute atoms dissolve into the solvent lattice while still maintaining the solvent type. R, G, and B phosphors were all solid solutions, as verified by the results of XRD. “There are no other impurity diffraction peaks, and no obvious diffraction shifts were found, indicating that the doped ions were well integrated into the matrix lattice and formed a homogeneous solid solution without forming new phases.” Correlation analysis has been mentioned in this article.
7.- Figure 5 presents CIE coordinates. It would be interesting that authors present RGB coordinates obtained from photographs of each phosphor under excitation at 251 and 361 nm; the RGB coordinates from mixtures can also be obtained. Obtaining the RGB coordinates from digital photographs is straightforward. Such information could be part of SI.
Answer: Thank you very much for your constructive comments. Indeed, the paper would be clearer and more convincing if we included the CIE coordinates of R, G, and B phosphors excited at their optimal excitation wavelengths as a part of the SI. The related figure and figure caption have been added to this paper. Please refer to the red lines on pages 8 and 9 for details.
8.- I strongly recommend English revision. Only some examples:
Line 42-42 “…fluorescent materials have been developed which has a higher….”
Line 54: “All these above”
Line 64: “which means that the emission colors the whole spectrum with 64 them.”
Line 66 “three monochromatic phosphors above were synthesized”
Answer: Thank you very much for your constructive comments. All of the aforementioned problematic statements have been corrected, and the English revision has been completed by a native English speaker.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript submitted by Zhu and co-workers reports the mixed fluorescent ink for anti-counterfeiting. This is interesting piece of work even though the materials used had been already classical and well-developed. While the work needs further experimentations before being published, this paper might become suitable after suitably addressing following issues.
Major comments:
[Page 8] The characterization of Eu2+ is lacking. Please provide experimental evidence to support the presence of the reduced form. For example, XPS.
[Page 8] For CaAl2O4 including Eu2+, Nd3+, and Gd3+, the absence of emission originating from the latter two was explained by the cluster formation. However, once cluster forms, the structure of CaAl2O4 must be destroyed in part. This is significantly inconsistent with the observation by XRD. Therefore, the authors must provide a more rational reason or further experimental evidence.
[Page 9] The authors prepared mixed inks. I'm wondering the quantum yields of the pristine indivisual ingredients and mixed form. Please provides experimental data. If quantum yields change by the mixing, please also give the reason.
[Page 9] The authors discussed the change in afterglow time and color by mixing. The reason is missing in the current manuscript. Please give more discussion.
[Page 13, Figure 8] I'm really wondering the repeatability of the afterglow. Please discuss the robustness of the reported ink preferably with data on repeating experiments. And, what about the effect on the operation temperature?
Minor comments:
[Page 3] Please show three digits for ionic radius to honestly compare the difference in size. See Y3+ and Ca2+.
[Page 8, 2nd paragraph, Line 226] with two peak --> with two peaks
Comments on the Quality of English LanguageShown in the above comments.
Author Response
Reply to Reviewer’ Comments
- [Page 8] The characterization of Eu2+ is lacking. Please provide experimental evidence to support the presence of the reduced form. For example, XPS.
Answer: A large number of literatures show that hydrogen atmosphere is a reducing atmosphere, which can reduce Eu3+ to Eu2+ [such as Inorg. Chem., 2005, 44, 489; J. Alloy. Compd., 2008, 458, 446-449; J. Am. Ceram. Soc., 2016, 99, 183; J. Rare. Earth., 2021, 39, 930-937]. “Then, the red phosphor was calcined at 1400oC for 6 h in air, while the green phosphor and blue phosphor were calcined at 1350oC and 1300oC for 2h and 5h respectively in hydrogen atmosphere.” the hydrogen atmosphere is used here.
2.- [Page 8] For CaAl2O4 including Eu2+, Nd3+, and Gd3+, the absence of emission originating from the latter two was explained by the cluster formation. However, once cluster forms, the structure of CaAl2O4 must be destroyed in part. This is significantly inconsistent with the observation by XRD. Therefore, the authors must provide a more rational reason or further experimental evidence.
Answer: Thank you very much for your constructive comments. Indeed, the unclear expression of "cluster formation" in the original text led to a misunderstanding by the reviewer. The author has checked and revised the related text. Please refer to the red lines on page 8 and 9 for the revised version.
3.- [Page 9] The authors prepared mixed inks. I'm wondering the quantum yields of the pristine indivisual ingredients and mixed form. Please provides experimental data. If quantum yields change by the mixing, please also give the reason.
Answer: SrAlO4:0.01Eu2+,0.02Dy3+ and CaAl2O4:0.012Eu2+, 0.06Nd3+, 0.036Gd3+ are both long afterglow materials, and their quantum yields cannot be calculated and evaluated. Thus, the quantum yields of the mixed form cannot be calculated and evaluated yet.
4.- [Page 9] The authors discussed the change in afterglow time and color by mixing. The reason is missing in the current manuscript. Please give more discussion.
Answer: Thank you very much for your constructive comments. The proportion and the different afterglow times of the three primary color phosphors are the main reasons for the change in color and afterglow time of the mixed phosphors. The original text lacked detailed and specific explanation. The related explanation has been supplemented in this paper. Please refer to the red lines on page 10 for the revised version.
5.- [Page 13, Figure 8] I'm really wondering the repeatability of the afterglow. Please discuss the robustness of the reported ink preferably with data on repeating experiments. And, what about the effect on the operation temperature?
Answer: All the experiments in the original text were conducted at room temperature (25°C), and the transparent screen-printing ink used in the experiments is commercially available polyvinyl chloride, which is highly temperature resistant. Thus, there is no need to worry about the problem of thermal decomposition.
6.- [Page 3] Please show three digits for ionic radius to honestly compare the difference in size. See Y3+ and Ca2+.
Answer: Thank you very much for your constructive comments. All the related text has been revised. Please refer to page 3 in red.
7.- [Page 8, 2nd paragraph, Line 226] with two peak --> with two peaks
Answer: Thank you very much for your constructive comments. The aforementioned problematic statements have been corrected, and the English revision has been completed by a native English speaker.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsIn my opinion the new version of the manuscript can be accepted for publication.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors addressed all issues raised by the reviewer. The current version of the manuscript is believed to meet the requests from the journal. Therefore, I would like to recommend the publication of this manuscript.
