Precise Control of Copper-Localized Surface Plasmon Resonance in the Near Infrared Region for Enhancement of Up-Conversion Luminescence

Round 1

Reviewer 1 Report

The English needs some general improvement but it is possible to follow the work. Maybe just some proof reading by someone whose first language is English would help.

I think the use of copper has been considered previously to replace Au and Ag. Practically there are some issues with contamination of deposition chambers (i.e. you need to have a vacuum chamber only for Cu). Anyway since the plasma wavelength of Cu is well known, so it can easily be seen that it can replace Au and Ag. Apart from the cost, the optical properties and advantages/disadvantages of Cu, over Au in particular, need more discussion in the introduction. I think most people would be put of by the rapid oxidization of copper and this would make it unusable in biosensor applications.

Author Response

I think the use of copper has been considered previously to replace Au and Ag. Practically there are some issues with contamination of deposition chambers (i.e. you need to have a vacuum chamber only for Cu). Anyway, since the plasma wavelength of Cu is well known, so it can easily be seen that it can replace Au and Ag. Apart from the cost, the optical properties and advantages/disadvantages of Cu, over Au in particular, need more discussion in the introduction. I think most people would be put off by the rapid oxidization of copper and this would make it unusable in biosensor applications.

We are thankful for this comment. In lines 77-83, we added the optical properties and advantages/disadvantages of Cu:
As the same time, compared with gold and silver nanoparticles by the method of sputtering, copper nanoparticles show the wide absorption and the peak of absorption appear at about 600 nm which is more suitable in the biological sensor applications than the peaks of absorption of gold and silver at 530 nm and 400 nm, respectively. Moreover, copper nanoparticles exposed on the air surface are
easily oxidized and this would make it unusable in biosensor applications. So, the TiO₂ film is considered as an optional material to protect nanoparticles from oxidation.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript represents a clear synthesis and characterization work on Cu/rutile films in the context of upconversion emission and is suitable for Metals/MDPI.  I have the following remarks that must be considered by the authors:

• The authors give reference to a rather limited range of authors/groups in the field of upconversion nanoparticles and in particular their applications in nanobiophotonics. It is in the responsible of the authors to correctly represent the various groups, such as the work of Xiaogang Liu.
• The authors claim that the upconversion process is a „nonlinear optical phenomenon“ (line 38). This however is not correct, since it is not related to the susceptibility of higher order. Instead the upconversion process is a multiple one-photon-process, that includes the intermediate population of higher energetic levels to sum up to (commonly) the doubled energy of photon emission. In addition there is a temporal delay between the two individual one-photon processes mediated by phonon interaction. The authors must correct this statement.
• L 43 please correct for „strong emission in the visible spectrum“
• In lines 45-50, the authors refer to works on efficiency enhancement, but they overlooked the latest approaches in increasing the dopant concentrations by the group of Marcus Haase.
• In the paragraph starting at line 51, the authors highlight the positive effect of surface plasmon resonances. I strongly recommend to add the limitations of this approach, as well, in order to give the reader a realistic picture of the system. In particular the question about size limitations need to be adressed in the context of applications in nanobiophotonics.
• My general question to the authors is related to the model they propose in their discussion and conclusion: Metal nanoparticles show plasmon resonance, without any doubt. If, however, the environment of the metal nanoparticle is altered from vacuum permittivity to a dielectric material, one may suggest that the induced electrons in the metal nanoparticle may be strongly affected by the insulating rutile. As far as i understand the authors argumentation they propose to attribute the wavelength shift of 235 nm to this phenomenon, and moreover, to attribute the redshift to it. Do the authors have some underlying model calculations that give a stronger evidence for this argumentation? Or is it just a guess of the authors? From all that is published on metal/insulator hybrids, the energy of plasmon emission shall be decreased, that is contrary to their findings.

Author Response

⚫The authors give reference to a rather limited range of authors/groups in the field of upconversion nanoparticles and in particular their applications in nanobiophotonics. It is in the responsible of the authors to correctly represent the various groups, such as the work of Xiaogang Liu.

We are thankful for this comment. In line 35, we added some applications and cited the corresponding article, the following is we added: biological imaging [4], photodynamic therapy [5], 3D displays [6,7] and solar energy
conversion [8].

[4] Jing, Z.; Zhuang, L.; Li, F. Upconversion Nanophosphors for Small-Animal Imaging.Chem. Soc. Rev. 2012, 41, 1323-1349, doi: 10.1039/c1cs15187h.
[5] Jiao, J.; Li, Y.; Shen, W.; Gai, S.; Tang, J.; Wang, Y.; Huang, L.; Liu, J.; Wang, W.;Belfiore, L.A. Fabrication and luminescence of KGdF₄ : Yb³⁺ /Er³⁺ nanoplates and their improving performance for polymer solar cells. Sci. Bull. 2018, S2095927318300161, doi: 10.1016/j.scib.2018.01.016.
[6] Deng, R.; Qin, F.; Chen, R.; Huang, W.; Hong, M.; Liu, X. Temporal full-colour tuning through non-steady-state upconversion. Nat. Nanotechnol. 2015, 10, 237-242, doi: 10.1038/nnano.2014.317.
[7] Zhang, Y.; Zhang, L.; Deng, R.; Tian, J.; Zong, Y.; Jin, D.; Liu, X. Multicolor Barcoding in a Single Upconversion Crystal. J. Am. Chem. Soc. 2014, 136, 4893-4896, doi: 10.1021/ja5013646.
[8] Haase, P.D.M.; Schfer, D.H. Upconverting Nanoparticles. Angew. Chem., Int. Ed. 2011, 50, 5808–5829, doi: 10.1002/anie.201005159.

⚫ The authors claim that the upconversion process is a nonlinear optical phenomenon “(line 38). This however is not correct, since it is not related to the susceptibility of higher order. Instead the upconversion process is a multiple one-photon-process, that includes the intermediate population of higher energetic levels to sum up to (commonly) the doubled energy of photon
emission. In addition, there is a temporal delay between the two individual one-photon processes mediated by phonon interaction. The authors must correct this statement.

We apologize for this mistake and thank this Referee for bringing it to our attention. In lines 38-43, we correct this statement: The up-conversion process occurs by absorbing 980 nm laser irradiation and then successively transfers the excitation energy to nearby co-doped activators (Ho³⁺ , Er³⁺ , Tm ³⁺ ,etc.) [14]. The sensitizer Yb dopant ions can absorb 980 nm photons and then transfer them to an adjacent activator (Er), which emits luminescence with a short wavelength. The wavelength of up-conversion emissions can be tuned in the ultraviolet, visible and near-infrared regions.

[14] Ren, G.; Zeng, S.; Hao, J. Tunable Multicolor Upconversion Emissions and
Paramagnetic Property of Monodispersed Bifunctional Lanthanide-Doped NaGdF₄ Nanorods. J. Phys. Chem. C 2011, 115, 20141-20147,doi:10.1021/jp2064529.

⚫ L 43 please correct for “strong emission in the visible spectrum”

We are thankful for this comment. In lines 47-48, we correct the mistakes.

⚫ In lines 45-50, the authors refer to works on efficiency enhancement, but they overlooked the latest approaches in increasing the dopant concentrations by the group of Marcus Haase.

We apologize for this mistake and thank this Referee for bringing it to our attention. In lines 52- 53 we added the sample of increasing the dopant concentrations by the group of Marcus Haase: change the dopant concentration of ions [8].
[8] Haase, P.D.M.; Schfer, D.H. Upconverting Nanoparticles. Angew. Chem., Int. Ed. 2011, 50, 5808–5829, doi: 10.1002/anie.201005159.

⚫ In the paragraph starting at line 51, the authors highlight the positive effect of surface plasmon resonances. I strongly recommend to add the limitations of this approach, as well, in order to give the reader a realistic picture of the system. In particular the question about size limitations need to be adressed in the context of applications in nanobiophotonics.

We are thankful for this comment. In lines 55-59, we added the limitations of surface plasmon resonances effect: As the same time, surface plasmon resonance enhanced fluorescence has some limitations in the biomedical such as the nanoparticle size limitation which the materials are difficult to enter the cell
for the tumor treatment and some damages to organisms which using the surface plasma effect of nanomaterials on quartz glass.

⚫ My general question to the authors is related to the model they propose in their discussion and conclusion: Metal nanoparticles show plasmon resonance, without any doubt. If, however, the environment of the metal nanoparticle is altered from vacuum permittivity to a dielectric material, one may suggest that the induced electrons in the metal nanoparticle may be strongly affected by the insulating rutile. As far as i understand the authors argumentation they propose
to attribute the wavelength shift of 235 nm to this phenomenon, and moreover, to attribute the redshift to it. Do the authors have some underlying model calculations that give a stronger evidence for this argumentation? Or is it just a guess of the authors? From all that is published on metal/insulator hybrids, the energy of plasmon emission shall be decreased, that is contrary to their findings.

We apologize for this mistake and thank this Referee for bringing it to our attention. In our previous work, we studied the surface plasmon resonance effect between metal nanoparticles (Au) and oxides (ZrOx) in detail. the conjunctive effect of environmental dielectric properties and film interference on LSPR properties was systematically investigated. We found that the gold nanoparticles with different oxides have the same phenomenon which
the absorption have red shift and the reason for the red shift is ultra-high E-field intensity by FDTD method. However, the FDTD of copper nanoparticles with TiO₂ did not investigated. So, we guess the hybrid film (Cu/TiO 2 ) also have the ultra-high E-field. The following articleis our pervious work.

Mao, S.; Liu, J.; Pan, Y.; Lee, J.; Yao, Z.; Pandey, P.; Kunwar, S.; Zhu, Z.; Shen, W.;Belfiore, L.A., et al. Morphological and optical evolution of metallic oxide/Au nanoparticle hybrid thin film: High absorption and reflectance by plasmonic enhancement. Appl.Surf.Sci.2019,495,doi:10.1016/j.apsusc.2019.143575.

Author Response File: Author Response.pdf