The Use of Plasmonic Spectroscopy for Detecting Ultra-Low Concentrations of Substances

Round 1

Reviewer 1 Report

This paper reports the results of a new method for detection on very low concentrations of H2O2 and NaCl using the optical method of surface electromagnetic waves (SEM). The results indicate that an effect can be measured  at such low concentrations which makes the technique at least feasible. However the sensitivity of the method is not clearly discussed  what is the expected uncertainty in the measurements? and how reproduceable is it?  If a concentration of 10-8 is measured is it to an order of magnitude or  eg 5 ± 1 x 10-8? I would like to see the results of two or three successive measurements of same 'stock solution' to see reproducibility. How does the sensitivity change as a function of temperature?  These are important questions to answer before it can be adopted as a method  indeed the authors should comment on perceived applications.

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Reviewer 2 Report

M.A. Kononov et al. have demonstrated detection of ultra-low H2O2 concentrations by detecting the excitation angle change in the surface plasmon resonance (referred to as surface electromagnetic wave) in the Kretschmann configuration. The authors have also demonstrated the detection of NaCl concentration change using the same setup. The author’s method of using the rate of angle shift over time as the differentiating signal between different samples is a very clear reading of the various H2O2 concentrations. Overall, this paper is a clear and solid demonstration of a detection system using surface plasmon resonance. I would thus recommend the acceptance of this manuscript after the authors addressing the following comments.

1. This paper is slightly under cited. The authors tend to mention an equation or a concept without giving corresponding references. To give a few examples, the equation in line 39 is given without reference or direct derivation; in line 105, the authors mentioned that AgI film will interact with H2O2 chemically, but no further description of this reaction or references were given.
2. In Figure 6, it’s demonstrated that the angle shift has abrupt changes with NaCl dilutes. However, we can find that the baseline of deionized water has a similar amount of change as compared to the NaCl+NaCl(C20) vs NaCl+H2O(C20) study. Is this fluctuation considered in the conclusion? Also, it seems that the change in angle shift is similar for the C20 vs C12 dilutions, which means that the current method cannot effectively differentiate between different NaCl concentrations. Can the author discuss the reason for this? Could it be due to the fact that the Na+ ion concentration in the double layer next to the Ag surface is much higher than the bulk concentration?

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Reviewer 3 Report

I have reviewed the manuscript titled “The use of plasma spectroscopy for detecting ultra-low concentrations of substances” by M.A. Kononov, E.A. Molkova, V.I. Pustovoy and V.V. Svetikov. The manuscript discusses an experimental procedure to determine the concentration of substances with low and ultra-low concentrations. This is shown by using the dependency on the excitation angle for the surface electron density waves for two set of solutions: H₂O₂ and NaCl in contact with Au/AgI, Au/Ag layers.

1. The title claims of using a plasma spectroscopy, but results show that authors are exciting surface electromagnetic or electron density waves referred as surface plasmons and it is apt to term as surface plasmon spectroscopy. Can the authors revise the title to reflect the actual technique used in this work? 
2. The abstract is a very confusing and suggested to rephrase for better reader understanding. Also, in abstract SLW is used without abbreviating what is means.
3. In Figure 1, it may be suggestible to show the schematic of the sample under consideration for better reader understanding.
4. The fonts used for figures 2 & 6 are not aligned with rest of the manuscript.
5. Texts used to indicate the concentration values in the images used for Fig 3 and Fig 4 are not clear.
6. Overall, the text in the whole manuscript in some sense is unclear and needs to be modified for better readability. It is obscure for the general audience in term of both language and scientific understanding.

Author Response

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