A Synergy Approach to Enhance Upconversion Luminescence Emission of Rare Earth Nanophosphors with Million-Fold Enhancement Factor

Round 1

Reviewer 1 Report

One of the major challenges to transform upconversion nanotechnology into real-world applications is to enhance the brightness and emission efficiency of UCNPs. In this work, Hsu and co-workers combined both nanoparticles and photonic engineering approaches to improve the brightness of UCNPs. They systematically studied the enhancement factors of main UCL peaks offered by three strategies adopted in this study, including surface passivation, Nd³⁺ doping, and excitation/extraction resonance. I agree with its publication after fixing the followed minor concerns.

 

1. In the experiment part on page 5, the authors reported “The monodispersed β-phase NaYF₄:Yb³⁺,Tm³⁺ UCNPs were synthesized through a thermal decomposition process”. The synthesis method in this study is the coprecipitation method (developed by Prof. Zhang’s group, Li et al, Nanotechnology 2008, 19 345606) instead of the thermal decomposition process (developed by Prof. Yan’s group).
2. In the experiment part on page 5, “10 mL of a methanol solution containing NH₄F (2.0 mmol) and NaOH (1.25 mmol) was slowly added into the flask”. Normally, for the NaYF₄ core synthesis through the coprecipitation method, the mol ratio for Re:Na:F is 1:2.5:4 (Wen et al. Nat Commun 2020, 11, 6047) instead of 1:1.25:2. Could the authors double-check it?
3. In figure 3a, the value 0,20 should be shown as 0.20 for the Y-axis. In figure 4a, the unit of the Y-axis is not correct. Also, the power density of 976 nm and 793 nm laser should be given for figures 3b,c,d, 4c, and 5a.
4. “It is clear that UCL intensities excited with 793 nm laser beam are about 6 to 9 times larger than those excited with 976 nm laser beam”. “Higher” could be better than “larger” in this sentence.
5. Compared with other figures, Figure 4 is not clear enough.
6. In figure S4, it seems the symbol for Non-RWG was not correctly displayed in this figure (red cycle), while the data with the symbol of the red triangle were displayed. Could the authors double-check the data?
7. On page 12, “Figure S4 shows the luminescence intensities of the 450 nm emission peak, obtained from the non-RWG area and the RWG area under excitation resonance + extraction resonance.” Actually, Figure S4 shows the lifetime data. I suggest the authors revise this sentence.

Author Response

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Reviewer 2 Report

This manuscript by Hsu and coworkers reports the synthesis, characterization and luminescence studies of three nanomaterials with the aim to achieve high UCL intensity.

The three nanoparticles are obtained through thermal decomposition process and characterize by a combination of XRD, TEM and UV-Vis-NIR spectroscopy. Remarkably, the excitation resonance + extraction resonance methodology presented by the authors, displays an enhancement of more than 10⁶ fold compared to Yb³⁺ doped nanoparticles. There are few examples in the literature reporting such enhancement, and this manuscript reports a fine example in which this is achieved. The work presented here is relevant for developing upconversion nanoparticles and materials for bio-imaging. The results reported here will be of interest to synthetic inorganic chemists interested in lanthanide upconversion proceses. But also to the broader bio-inorganic community or materials chemists with an interest in luminescence or bio-imaging.

 

I would recommend its publication addressing the following points prior to the publication of the manuscript:

• Figure 3. Please use dots instead of commas for decimal separators in the absorbance axis. As per the MDPI Style Guide.
• Figure 3 (a) caption. UV-Vis spectra of C an CS is missing information regarding the conditions used for recording the spectra, e.g. concentration, solvent, etc…
• Lines 384-387. Consider re-wording the following sentence: “Under this condition, i.e. λ = 793 nm and θ = 1.5º, which we refer to as resonant excitation condition, a strong local field can build up on the top surface of the TiO2 layer the evanescent field of the guided mode that concentrates a large fraction of the incident beam power in the GMR effect …”. It is not clear what the authors mean by “the evanescent field of the guided mode…” My understanding is that under those conditions there is a build up of a strong local field on the TiO2 surface that enhances the luminescence emission.

Author Response

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Reviewer 3 Report

In the paper authors proposed a way to correct the poor quantum yield efficiency of upconversion luminescence, UCL, of uponversion nano-
particles UCNPs. A new type of neodymium (Nd3+) sensitizer is proposed to solve the problem of low absorption cross-section of Ln3+ ions.
Nanoparticles were coated with an inactive shell of NaYF4 to avoid UCL quenching from surface defects and surface associated ligands. The 
excitation light intensity in the vicinity of UCNPs have been greatly enhanced using a waveguide grating structure thanks to the guided mode resonance. Through the synergy of these three approaches, authors demonstrated that the UCL intensity of UCNPs can be enhanced by at least milion folds compared with conventional Yb3+-doped UCNPs (by using plasmonic nanostructures or dielectric nanostructures to increase local photon density of states at the emission wavelength of UCNPs). Through the synergy approach presented in this work, authors demonstrated that the UCL intensity of Tm3+-doped UCNPs was remarkably enhanced more than 10^6 fold, which is particularly valuable for practical applications in bioimaging and biosensing.
Detailed comments
In the Introduction there are missing some papers that shold be called to present an actual state of UCL. 
E.g. 
Upconversion and anti-stokes processes with f and d ions in solids
F Auzel - Chemical reviews, 2004 - ACS Publications
History of upconversion discovery and its evolution
F Auzel - Journal of Luminescence, 2020 - Elsevier
Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF4:Yb3+,Er3+
RB Anderson, SJ Smith, PS May… - The journal of physical …, 2014 - ACS Publications
It is strange to me bearing in mind that among co-authors there are people from France.
All important parts of the paper are clearly described and do not require large corrections.

Author Response

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