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  • Thi Huong Le1,2,
  • Van Quyen Nguyen1,2 and
  • Gaelle Trippe-Allard1,3
  • et al.

Reviewer 1: Anonymous Reviewer 2: Anonymous

Round 1

Reviewer 1 Report

In the present study, authors report the diode-like behaviour of a “semiconductive two-dimensional organic assembly” covalently grafted onto a carbon electrode toward redox probes in solution. This is a very interesting and relevant contribution to the surface modification. The authors have conducted and presented some thorough work and have produced plenty of good results. I do however have some concerns as noted:

Figure 2a : I do not understand why the CVs are recorded at such high scan rates, since all cyclic voltammetry studies in the presence of redox probes are achieved at 0.1 V/s. Authors must better define what they mean by “electrochemical stability”. Is there an evolution between the shape of the first CV and the following? In any cases, it is important to show a CV of the modified electrode at 0.1 V/s.

The structure of the organic layer onto the electrode surface is of crucial importance to the understanding of the electrochemical behaviour of the modified electrode. Authors must better discuss this point (this issue must not be discussed at the end of the conclusion part).

Figure 2b : since the faradaic current intensity increases linearly with the scan rate at the peak potential, but also at any values of the potential where the surface confined redox reaction takes place, it would have been more appropriate to show the linear increase of the faradaic charge with the scan rate, due to the difficulty in determining the current peak intensity from the Figure 2a.
Figure 3a: Why the modified electrode does not exhibits a typical diode-like behavior toward Fe(CN)63-? What is the influence of inner-and outer-sphere electron transfer mechanisms on the diode-like effect of the modified electrode?
Figures 3a-d: I am surprised to observe such a big difference in current peak intensities since all the concentrations are the same.  
Figure 5b: the peak at 1 eV is more probably due to C-N and/or C-S contributions.
Is there an effect of the thickness of the organic layer on the electrochemical behavior of the modified electrode toward redox probes in solution? Does the thickness of the organic layer is the same for all the redox probes investigated in the Figure 3?  

Comments for author File: Comments.pdf

Author Response

Response of reviewer 1

In the present study, authors report the diode-like behaviour of a “semiconductive two-dimensional organic assembly” covalently grafted onto a carbon electrode toward redox probes in solution. This is a very interesting and relevant contribution to the surface modification. The authors have conducted and presented some thorough work and have produced plenty of good results. I do however have some concerns as noted:

- Figure 2a : I do not understand why the CVs are recorded at such high scan rates, since all cyclic voltammetry studies in the presence of redox probes are achieved at 0.1 V/s. Authors must better define what they mean by “electrochemical stability”. Is there an evolution between the shape of the first CV and the following? In any cases, it is important to show a CV of the modified electrode at 0.1 V/s.

First we thank the referee for this interesting comment.

 The electrochemistry of the grafted film without redox probe in solution allows to study the electron transfer rate from the organic layer to the electrode. It is done at high scan rate and shows that it is possible to oxidize the grafted layer up to 400 V/s (ie: the electroactivity stability).

 This part has been clarified in the text where one can now read:

 “The faradic charge measured between 0.7 V and 1.1 V and is 9.5 10-6 C which gives a surface concentration of 7 10-10 molecules/cm2, a value close to that of a monolayer. Note also that this charge is independent of the scan rate and can thus be injected very fast in such layers as CV at scan rates as high as 400 V/s are still reversible.”

 The use of fast scan rate to study the film electroactivity without any redox probe in solution is here done to show that charge injection in the film is not limiting when redox probes in solution are added. In this later situation, the used scan rate is more usual and was set at 0.1 V/s. We added some precision in the manuscript (line 161).

 As we can see on figure 1 there is an evolution in the shape of the wave as the electroactivity start to be visible at around 50 V/s. We do not think that the CV of modified electrode at 0.1 V/s is important to show as the CV for 10 V/s shows any activity for electrografted layer.

 

- The structure of the organic layer onto the electrode surface is of crucial importance to the understanding of the electrochemical behaviour of the modified electrode. Authors must better discuss this point (this issue must not be discussed at the end of the conclusion part). 

 

We agree with the referee about the role of the structure of the organic layer in the electrochemical behavior. However due to the radical reaction during the grafting the structure of the organic film is not completely organized. [1,2]

We have added in the text, after the XPS results showing that the structure of the film is close to that of SNS, that the organization of the organic layer was not fully characterized

 

- Figure 2b : since the faradaic current intensity increases linearly with the scan rate at the peak potential, but also at any values of the potential where the surface confined redox reaction takes place, it would have been more appropriate to show the linear increase of the faradaic charge with the scan rate, due to the difficulty in determining the current peak intensity from the Figure 2a. 

 

In the figure 2b the current peak of the electroactivity wave of the organic layer was plotted versus the scan rate. However, the faradic current under this peak is constant (contrary to the reviewer comment).

 

We have added in the text (line 153):

“The faradic charge measured between 0.7 V and 1.1 V and is 9.5 10-6 C which gives a surface concentration of 7 10-10 molecules/cm2, a value close to that of a monolayer. Note also that this charge is independent of the scan rate and can thus be injected very fast in such layers as CV at scan rates as high as 400V/s are still reversible.”

 

- Figure 3a: Why the modified electrode does not exhibits a typical diode-like behavior toward Fe(CN)63- ? What is the influence of inner-and outer-sphere electron transfer mechanisms on the diode-like effect of the modified electrode?

Fe(CN)63- is an inner-sphere redox probe and is very sensitive to surface adsorption effects. McCreery et al. gave us a good example of the difference between inner and outer sphere probes versus surface modification[3]. In the case of SNS, it seems that the layer can still block electron transfer to this probe even though it switched to its conductive state. Concerning this probe, other investigators have suggested different mechanism such as hydrophobic effects or the involvement of cations to explain the blocking behavior.[4,5] This is not the scope of this article but we have modified the main text to clarify this point (line 167).

 

- Figures 3a-d: I am surprised to observe such a big difference in current peak intensities since all the concentrations are the same.  

The area of the used electrodes are unfortunately not the same for some of these experiments when changing the used redox probe. However, for each redox probe used the signal of the probe on the bare electrode and on the modified electrode are obtained with an electrode with the same area so that the reported results are fully significant.

 

- Figure 5b: the peak at 286.1 eV is more probably due to C-N and/or C-S contributions.

We thanks the referee for this comments. Indeed the peak at 286.1 eV could be attributed to C-N and C-S contributions. We have changed this part in the manuscript.

 

- Is there an effect of the thickness of the organic layer on the electrochemical behavior of the modified electrode toward redox probes in solution? Does the thickness of the organic layer is the same for all the redox probes investigated in the Figure 3?   

 

There is probably an effect but it was not studied in the scope of this manuscript.

For the figure 3, the thickness of the organic layer is the same as the same electrode was used for all electrochemical experiments.

 

  1. Bousquet, A.; Ceccato, M.; Hinge, M.; Pedersen, S.U.; Daasbjerg, K. Redox grafting of diazotated anthraquinone as a means of forming thick conducting organic films. Langmuir 2012, 28, 1267–1275.
  2. Jiang, C.; Silva, S.M.; Fan, S.; Wu, Y.; Alam, M.T.; Liu, G.; Justin Gooding, J. Aryldiazonium salt derived mixed organic layers: From surface chemistry to their applications. J. Electroanal. Chem. 2016.
  3. Chen, P.; McCreery, R.L. Control of Electron Transfer Kinetics at Glassy Carbon Electrodes by Specific Surface Modification. Anal. Chem. 1996, 68, 3958–3965.
  4. Deakin, M.R.; Kovach, P.M.; Stutts, K.J.; Wightman, R.Mark. Heterogeneous mechanisms of the oxidation of catechols and ascorbic acid at carbon electrodes. Anal. Chem. 1986, 58, 1474–1480.
  5. Hu, I.Feng.; Kuwana, Theodore. Oxidative mechanism of ascorbic acid at glassy carbon electrodes. Anal. Chem. 1986, 58, 3235–3239.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript by Thi Huong Le et al. describes the electrografting of the diazonium based on 4-(2,5-di-thiophen-2-yl-pyrrol-1-yl)-phenylamine (SNS moieties) onto carbon/gold electrodes. I have a few questions or suggestions for reference.

 

Page 8 (Line 140 and Figure 2)– the potential scan rates applied should be 10 – 400 mV/s, instead of V/s? Page 8 – Figure 2b – At what potential were the anodic peak current values taken? The authors should refer to this figure too. It is also mentioned that poly(2,5-dithienylpyrrole) has an onset potential of 0.7 V (vs SCE). It is not clear to me where the authors derive this value, since there is no similarity to Figure 2a (CV of modified electrode). Page 12 and Figure 4 – From the description and the AFM results, I really cannot agree that the authors claimed the film thickness is around 2 nm. I’d rather say it is surface roughness than film thickness. Alternatively, the authors should elaborate how the hole or trench on the surface of the modified electrode was created. Page 15 (conclusions) – The authors write “As several studies shown[65–67], the conductance of the molecule can strongly depend on the angle value and strong decrease of conductance (quantum interference) can be observed due to this particular conformation.” I do not think this is proper, since there is no relevant evidence or data in the main text. Two electrodes were used for electrografting, namely carbon and gold electrode. Any difference? I presume that the electrografted film thickness is dependent on many electrochemical conditions. The authors should elaborate this point more.

 

Minor comment:

Page , line 61: many studies “have” been reported, not “has”

Author Response

Response of reviewer 2

 

Page 8 (Line 140 and Figure 2)– the potential scan rates applied should be 10 – 400 mV/s, instead of V/s?

There were no mistake in the X range of the figure 2b. The fast scan rate electrochemistry was done between 10 and 400 V/s as indicated in the initial manuscript

 

The use of fast scan rate to study the film electroactivity without any redox probe in solution  is here done to show that charge injection in the film is not limiting when redox probes in solution are added. In this later situation the used scan rates is more usual and was set at 0.1 V/s

 

Page 8 – Figure 2b – At what potential were the anodic peak current values taken? The authors should refer to this figure too.

 

The current value of the anodic peak was taken at 0.9 V where one can observed a peak in the reversible wave of the grafted layer. We have added the value in the manuscript.

 

On the figure 2b, It is also mentioned that poly(2,5-dithienylpyrrole) has an onset potential of 0.7 V (vs SCE). It is not clear to me where the authors derive this value, since there is no similarity to Figure 2a (CV of modified electrode).

 

We agree with the referee the value is not obvious for low scan rate. However, the onset potential can be defined as the inflexion point on the cyclic voltammogram (figure 2a). At 400 V/s we clearly observed the transition between low conductive electrode (below 0.7 V) and the increase of the conductivity of the grafted film.

To clarify this point we have changed the sentence in the manuscript.

One can now read:

This electrochemical signal has an onset at 0.7V/SCE and oxidative and reductive peaks close to 0.9V/SCE. It can be attributed to chemisorbed monomer or short oligo-monomer[55].

 

Page 12 and Figure 4 – From the description and the AFM results, I really cannot agree that the authors claimed the film thickness is around 2 nm. I’d rather say it is surface roughness than film thickness. Alternatively, the authors should elaborate how the hole or trench on the surface of the modified electrode was created.

 

We agree with the referee that the layer seems inhomogeneous. However the given value corresponds to the thickness and not to the roughness of the layer. During the scratch procedure, the AFM tip removes a part of the organic film due to the applied force but it is usual that in the middle of the scanned area, some molecules are still present. It is observed in the cross section at X = 1.5µm.

The thickness given in the manuscript allows the comparison with an electrodeposited polymer film (above 100 nm thickness) and shows that the value corresponds to only few layers of SNS moieties. We did not focus the study on a precise measurement of the thickness of the grafted layer.

On the other hand the measurement of thickness was also verified with a second procedure (not show in the manuscript) using a gold nanostructured subtract[1]. The same grafting was done on a gold nanostructured subtract with a 50 nm height gold line. The thickness of the layer was measured by comparison of the step height before and after grafting. The figure S1.A show an image of a gold modified step and a statistic analysis provided us the two histograms on the figure S1.B. A similar thickness was obtained on gold and carbon electrode.

Figure S1: a) AFM image of a gold modified nanostructured electrode b) histograms of bare (black) and modified (red) cross section along the step.

 

The presence of the hole or trench observed on AFM image could be explain by a change in the quality of AFM tip apex during the scratch procedure. In this process, the tip was scratched into the organic layer using a strong applied force. Then a second image of the same area was recorded with the same tip showing a less homogeneous surface. In comparison the homogeneity of the organic layer in the figure S1.A is better.

 

Page 15 (conclusions) – The authors write “As several studies shown[65–67], the conductance of the molecule can strongly depend on the angle value and strong decrease of conductance (quantum interference) can be observed due to this particular conformation.” I do not think this is proper, since there is no relevant evidence or data in the main text.

We agree with the referee and the sentence was removed to avoid some misunderstanding. Here we wanted to point out the fact that the orientation of the molecule could explain the specific electrochemical behavior observed. We plan to investigate such organic film in metal/molecule/metal device in order to study the electronic transport and the presence or not of quantum interference.

We deleted this sentence in the manuscript.

 

Two electrodes were used for electrografting, namely carbon and gold electrode. Any difference? I presume that the electrografted film thickness is dependent on many electrochemical conditions. The authors should elaborate this point more.

No obvious differences weres observed when we used the carbon or gold electrode for the electrografting. The electroreduction of diazonium salts was fully detailed in literature and several groups studied the influence of electrochemical conditions on the thickness. For example, the potential range, the diazonium concentration or the number of cycle allow to control the thickness of the film. In this study we did not focus on the control of the thickness through theses parameters. However, we used some usual conditions for diazonium grafting. To clarify this point we have added some details in the manuscript and written that films of various thicknesses can be obtained but that the effect of thickness on the properties of the film were not studied.

 

  1. Van Nguyen, Q.; Lafolet, F.; Martin, P.; Lacroix, J.-C.C. Ultrathin Molecular Layer Junctions Based on Cyclometalated Ruthenium Complexes. J. Phys. Chem. C 2018, 122, 29069–29074.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The CHI potentiostat has working potential ±10 V, so I think the unit of scan rates in Figure 2a and Figure 2b is not correct. 

I do not think it is proper to have citations in the conclusion, for most journals.

 

 

 

Author Response

Dear Editor,

Please find below the response letter for our manuscript entitled " Dithienylpyrrole electrografting on surface through the Electroreduction of Diazonium Salts ". We would like to thank the referees for their helpful comments which will certainly improve the quality of the manuscript.

 

Concerning the electrochemical setup and the scan rates used in the study, the CHI can deliver a potential range of 20V (potential window -10, +10V) but it can reach the scan rate of 5000 V/s with a 1mV potential increment (according to the specification http://chinstruments.com/chi600.shtml).

 

The references are removed in the conclusion and the sentence has been modified.

 

We sincerely hope the new amended version work will satisfy the journal’s standard.

Thank you for consideration of this work.

Sincerely,

Dr Pascal Martin, on behalf of all the authors.