Biomimetic Sensitive Elements for 2,4,6-Trinitrotoluene Tested on Multi-Layered Sensors
, Mihai STOICA 5, Corina ORHA 2, Cornelia Elena BANDAS 2 and Andrei SARBU 1,*
Round 1
Reviewer 1 Report
The authors present a polysilane MIP templated against TNT for solution and vapour detection. They posit the formation of a ‘double Meisenheimer’ complex for binding the TNT.
The work is interesting, and thorough, but I have a few comments.
I would argue that Figure 1 doesn’t show a Meisenheimer complex, which is normally depicted as the covalent attachment of the nucleophile onto the aromatic ring, and instead shows a charge-based interaction.
It would also be good to see the UV-vis trace of the TNT (2.34 uM) in solution without the silane, to demonstrate that the peak at 508 nm is due to complex formation, not simply an impurity in the TNT.
The use of divide by signs confused me – e.g 0.002÷0.20 g on line 304. Do the authors mean to use a dash?
In figure 3 why are the dotted regions for the complex different in parts a and b?
Can the authors estimate a limit of detection/limit of quantification for their capacitive sensor?
There are a few typos, e.g. line 151 acetonitrile, line 169 titania, line 190 Scotch tape(?)
Author Response
Dear Reviewer,
Thank you for the constructive comments and suggestions for improving the manuscript. Please find the answers to your queries hereunder:
1. I would argue that Figure 1 doesn’t show a Meisenheimer complex, which is normally depicted as the covalent attachment of the nucleophile onto the aromatic ring, and instead shows a charge-based interaction.
We agree with the reviewer observation and, therefore, we have made the necessary modifications in Figure 1.
2. It would also be good to see the UV-vis trace of the TNT (2.34 uM) in solution without the silane, to demonstrate that the peak at 508 nm is due to complex formation, not simply an impurity in the TNT.
Thank you for this observation. The spectrum of TNT in ethanol is needed and was added in Fig.2a (and in the inset) to demonstrate that the peak at 508 nm is due to complex formation.
3. The use of divide by signs confused me – e.g 0.002÷0.20 g on line 304. Do the authors mean to use a dash?
The authors intended to highlight a range of concentrations. But a dash seems to be more proper. Therefore, we have made the corrections.
4. In figure 3 why are the dotted regions for the complex different in parts a and b?
Thank you for this observation. The regions should be the same for both graphs. We have made the necessary corrections.
5. Can the authors estimate a limit of detection/limit of quantification for their capacitive sensor?
Since the main goal of this study was to highlight only the potential application of the MIP films for sensors development we have only focused on rather basic tests. The installation from the Military Technical Academy is designed to test concentrated fluxes of explosives. The 0.2 g/L concentration was the actual limit of the installation. For thorough analysis we need a more delicate installation with sensitive control of vapors (for ppm concentrations of explosives), which unfortunately we cannot access at the moment. However, we are well aware that for a clear application of the mentioned sensors (using that particular multi-plate assembly) several studies should be performed to establish detection limits, sensitivity and cross-reactivity. We have applied for funds to continue our work and to bring this technology to a higher readiness level. Therefore, at this moment I cannot answer this question as we used just a rough methodology.
6. There are a few typos, e.g. line 151 acetonitrile, line 169 titania, line 190 Scotch tape(?)
Thank you for this observation. We have identified the mistakes and corrected them.
P.S. The corrected manuscript in TracK Changes is also available.
Reviewer 2 Report
This article is very interesting because of the development of sensitive elements based on silane layers and MIPs for the fabrication of TNT sensors.
However, there are certain details to be taken into account before publication.
Along the whole article authors talk about Meisenheimer complexes but no small indication is given as to what these concepts are based on, which might be useful for those readers who do not know.
In line 68 the authors talk about the interactions that are used for the application of MIPs for TNT recognition but do not indicate the disadvantages of these interactions or what this new approach would bring, which would be very useful to show the novelty of the study.
In section 2.2 we talk about the synthesis of the films and start by talking about the type of interaction that occurs between the functional monomer and the templates. This is a very useful explanation, but it is confusing because it is directly after the title. I would recommend the authors to add some sentence that would help to clarify what the paragraph means.
In table 1 the quantities in mg and mL are added but they do not contribute anything since the interest lies in the millimoles used and the relation between these, so it is rare and complicates the reading.
Line 169: "titan" should be "titanium"
In figure 2 it would be useful for the authors to add a job-plot or similar graph in order to check the molar relationship between the template and the functional monomer in a more intuitive way. In addition to adding the parameters of the fit more clearly and with the corresponding significant figures without directly including the table of the fit as reported by the program.
The article does not show the analytical characteristics of the developed sensor, which is understandable if only the manufacturing of the material is to be shown. However, in table 3 the response of the sensor to analyte, interferant and moisture is compared but cannot be correctly understood or is extracted from each study and would be used for its application as a sensor. I would therefore recommend the authors to put a little more emphasis on this part by explaining these results better.
That is why I consider that the authors should make small changes before accepting the article.
Author Response
Dear Reviewer,
Thank you for the constructive comments and suggestions for improving the manuscript. Please find the answers to your queries hereunder:
1. Along the whole article authors talk about Meisenheimer complexes but no small indication is given as to what these concepts are based on, which might be useful for those readers who do not know.
We agree with the reviewer observation and, hence, we have added some explanation of the concept in the Introduction section, as follows:
“A Meisenheimer complex is an anionic complex formed by the attack of a nucleophile upon one of the aromatic-ring carbons during the nucleophilic aromatic substitution reaction. In this respect, two types of Meisenheimer complexes, the σH-complex or σX-complex can be formed corresponding to the non-substituted or substituted aromatic ring.”
2. In line 68 the authors talk about the interactions that are used for the application of MIPs for TNT recognition but do not indicate the disadvantages of these interactions or what this new approach would bring, which would be very useful to show the novelty of the study.
The authors agree again with this comment. We have indicated the drawbacks of former studies and highlighted the advantages of our method, right after line 72, as follows:
“For instance, the MIP films prepared by Florea et al. [27] were prepared using the wet phase-inversion method, but presented low adhesion to the substrate and detached after several washing procedures and the TNT-MIP coatings prepared by Gao et al. [43] and Xie et. al [44] were prepared using two-step procedures starting from surface pre-modification of silica and alumina, respectively, with functional silane followed by MIP grafting. In these MI approaches, as well, the recognition mechanism of TNT relied on weak hydrogen bonds or π-π interactions between mono-functional monomers and TNT."
"In light of aforementioned studies, the present study brings to the audience a very practical procedure to prepare thin TNT-MIP films by direct spray deposition of a sol-gel precursor solution on glass substrates. Another advantage of this newly presented method for films preparation refers to the low energy consumption in terms of low temperatures for polycondensation (i.e. room temperature) and for curing/ aging of films (55 °C).”
3. In section 2.2 we talk about the synthesis of the films and start by talking about the type of interaction that occurs between the functional monomer and the templates. This is a very useful explanation, but it is confusing because it is directly after the title. I would recommend the authors to add some sentence that would help to clarify what the paragraph means.
Thank you for this suggestion. We have added some clarifications here, as follows:
“The auto-assembly mechanism of TNT with 1141-D was based on charge transfer complexing interactions with the formation of particular σX-Meisenheimer complexes corresponding to the substituted aromatic ring of TNT (as depict in Figure 1).The formation mechanism of σX-Meisenheimer complex, where x is the methyl group of the substituted aromatic nucleus (position 1) of TNT, was proposed by several authors for monofunctional-amines and nitroaromatics [1, 2, 6, 7, 48] and adapted in this study for the formation of “double” σX-Meisenheimer complexes”
4. In table 1 the quantities in mg and mL are added but they do not contribute anything since the interest lies in the millimoles used and the relation between these, so it is rare and complicates the reading.
The main reason for adding both milimoles and mg or mL in the table was to have clear view of the recipe. But since it complicates the reading, only the quantities in milimoles were specified.
5. Line 169: "titan" should be "titanium"
Thank you for this observation. We have corrected the term accordingly.
6. In figure 2 it would be useful for the authors to add a job-plot or similar graph in order to check the molar relationship between the template and the functional monomer in a more intuitive way. In addition to adding the parameters of the fit more clearly and with the corresponding significant figures without directly including the table of the fit as reported by the program.
The suggestion is quite good, but for performing the Job-plot, the study for complex determination implies other methodologies than the one we used (the sum of the molar concentrations of the two binding partners is held constant, but their mole fractions are varied). Since we have studied the complex formation by titration of a fixed amount of monomer with TNT, and by doing so the sum of the molar concentrations of the two binding partners was not constant, we have determined the molar relationship between the template and the functional monomer using the “mole ratio method”, where the XTNT/1141D is the molar ratio of the two binding partners. The point of intersection of the two linear variations in the graph gave us the molar ratio of TNT and 1141D found in the complex.
An explanation of the method was added in section 3.1. Double Meisenheimer Complex formation:
“The molar relationship between the template and the functional monomer was determined using the “mole ratio method”, where the XTNT/1141D is the molar ratio between TNT and 1141-D. The point of intersection of the two linear variations in the graph represented the molar ratio of TNT and 1141-D in the complex. The resulted molar ratio of 1.74 suggested the formation of a double complex in the form of 1.53: 0.88 = TNT: 1141-D (M/M), similar to the one proposed in Figure 1.”
We have also modified Figure 2b, the figure caption and the conclusions with the new values.
7. The article does not show the analytical characteristics of the developed sensor, which is understandable if only the manufacturing of the material is to be shown. However, in table 3 the response of the sensor to analyte, interferant and moisture is compared but cannot be correctly understood or is extracted from each study and would be used for its application as a sensor. I would therefore recommend the authors to put a little more emphasis on this part by explaining these results better.
Indeed emphasis was made on the films preparation and for their potential as sensors elements we have only focused on rather basic tests. Yet, the explanation on the results may be improved. Therefore, we have also added the following paragraphs and another reference:
“The designed capacitive chemical sensors detect conductivity changes of the MIP sensitive film resulted from the interaction of TNT vapours”
“Considering the fact that all the measurements were performed using concentrated fluxes of TNT vapours the capacitance variations are appropriate (about 166 pF). For low concentrations of substance (up to ppm) other capacitance sensors were found to give slight changes of capacitance of around 0.15 pF [59].”
“Moreover, during exposure to TNT or DNB vapours, the sensors exhibited stable values for all the measured electrical parameters for over 10 minutes, which indicated a very good adhesion of the MIP-film to the support. A detachment of the MIP-film from the support would have led to a sharp decrease of capacitance [59].”
[59] Kavalenka, M.N.; Striemera, C.C.; DesOrmeaux, J.P.S.; McGrath, J.L.; Fauchet, P.M. Chemical capacitive sensing using ultrathin flexible nanoporous electrodes. Sens. Actuators B Chem. 2012, 162, 22-26.
P.S. The manuscript with Track Changes is also available.
