Nitrogen-doped carbon dot threads as a "turn-off" fluorescent probe for permanganate ions and its hydrogel hybrid as a naked eye sensor for gold(III) ions
Authors: Daewon Sohn
Anil H Gore
Datta B Gunjal
Govind B Kolekar
Prashant V Anbhule
Vaibhav M Naik
Sheshanath V Bhosale
Journal: Analytical and bioanalytical chemistry (2020)
Discipline: CHEMISTRY
Keywords: Carbon dot hydrogel hybrid; Carbon dot threads; Fluorescent ink; Fluorescent probe; Ion sensing
Affiliations: Fluorescence Spectroscopy Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, 416004, India., Department of Chemistry, Uka Tarsadia University, Maliba Campus, Bardoli, Gujarat, 394350, India., Department of Chemistry, Hanyang University, Seoul, 04763, South Korea., Department of Chemistry, Goa University, Taleigao Plateau, Goa, 403206, India. ., Fluorescence Spectroscopy Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, 416004, India. ., Department of Chemistry, Hanyang University, Seoul, 04763, South Korea. .

Abstract:  Highly fluorescent nitrogen-doped carbon dot (NCD) threads were synthesized via simple pyrolysis of citric acid, p-hydroxybenzoic acid, and ammonia. The NCDs show excitation-independent behavior with maximum excitation and emission wavelengths of 350 nm and 435 nm, respectively. The developed probe was used as a turn-off fluorescent sensor for the selective and sensitive determination of permanganate ions in aqueous media. The probe's hydrogel hybrid displayed a beautiful purple color demonstrating its potential as a naked eye sensor for gold detection. The ratiometric sensor exhibited excellent selectivity towards permanganate ions over 27 other ions with a linear range of 510 nM to 2 μM, a detection limit of 170 nM, and a linear regression value (R2) of 0.9944. Similarly, the linear range and limit of detection for gold ions was 3.89-20 μM and 1.285 μM, respectively. The synthesized NCDs were also used as a fluorescent ink as well as a naked eye marker in association with a gold solution demonstrating its potential forensic and anti-counterfeiting applications. Graphical abstract. 

Retracted: No

PubPeer comments:
 The authors of the title paper report to have prepared so called N-doped carbon dot threads by heating an ammoniacal solution of p-Hydroxybenzoic acid (2 g) and citric acid (2 g) to 180oC and characterized the same with a variety of techniques excepting simple elemental analysis. The use of a combination of an aromatic hydroxy acid and an aliphatic hydroxy tricarboxylic acid in ammonia to get C=C containing carbon although appears very interesting is not explained in terms of the chemistry of the reagents employed. From the experimental details the choice of the reagents seems to be arbitrary. It is not clear why this is called N-doped carbon dot thread without even knowing the composition of C and N in the thread which could have obtained by an elemental analysis. Authors may like to clarify. , 
 #1 The N-doped carbon dot threads somehow reminded of the much recently discussed work on doping in C materials (for details see https: pubs.acs.org doi 10.1021 acsnano.9b00184 ) However the results claimed in the present paper are truly remarkable. The carbon nano thread displayed in Fig. 1 (see below) appears pale yellow with a greenish tinge, which is quite different from the colour expected for C. Based on the colour, one can question why not call this as a C doped N thread. Dissolution of citric acid in ammonia will result in formation of ammonium citrate (melting point 185oC). Likewise p-hydroxybenzoic acid is expected to form the corresponding ammonium salt. When the ammoniacal solution is heated to 180oC the excess ammonia, water etc. will evaporate first followed by the formation of the ammonium salts. The experimental section it is reported,  *The solution was transferred to a silica crucible and heated to 180 u00b0C on a hot plate until a molten mixture was obtained. Thereafter, a glass rod was dipped in the hot molten mixture and lifted above to obtain thread-like structures of NCDs. ..*. Since the temperature chosen is below the melting point of ammonium citrate it can be said that use of ammonium hydroxybenzoate reduces the melting point. Now if the molten solution is lifted to obtain the so called NCD s they can as well contain good amount of ammonium citrate processed at 180oC. It is not clear why this possibility has been considered by the present authors. The interpretation of the IR and UV-Vis data also requires clarification because authors report based on UV-Vis spectrum  *The peak at 257 nm is attributed to u03c0u2013u03c0* transitions of aromatic C=C bonds in sp2 domains and the peak at 338 nm is assigned to nu2013u03c0* transitions of C=N C=O, indicating the presence of nitrogen- and oxygen-containing functional groups* . However the IR spectrum of the N-doped reports a CO vibration at a much lower energy and the carbonyl vibration is not the most intense peak. In any case, I agree with #1 Probergrothius Angolensis that the elemental analytical data which is crucial is indeed missing. The Legend for Fig. 1 in the paper is  Photograph of NCD threads under a daylight and b UV irradiation (u03bbexu2009=u2009365 nm) . For the picture see below ![file] , 
 #2 The N-doped carbon dot thread is actually multi colored as some parts appear to be colorless indicating that the C- dot thread is actually the ammonium citrate (or a mixture of two ammonium salts). It is not clear if the authors have ruled out this possibility. However, the IR spectrum can be interpreted for ammonium citrate especially the low energy signal at ~1650 cm-1 for the -COO vibration. The intense signal at ~ 3325 cm-1 can be due to -NH vibration of the ammonium cation. The assignments of the UV-vis spectral signals is questionable because from the figure (Fig 3a) it is not clear if the weak features can be considered as a signal or it is due to electronic noise. A major drawback of the present claim is the absence of any data in the entire publication or in Supporting material due to ammonium citrate to rule out its presence. Authors may like to consider incorporating this data in the Supplementary material or in a corrigendum. , 
 In a 2014 Nanoscale paper titled,  Nitrogen-doped, carbon-rich, highly photoluminescent carbon dots from ammonium citrate Yang et al reported on N-doped carbon dots obtained from ammonium citrate. These C dots show blue fluorescence. For details of this published paper not cited by Vaibhav Naik et al in the present work see https: pubs.rsc.org en content articlelanding 2014 nr c3nr05380f#!divAbstract Instead of using ammonium citrate Vaibhav Naik et al are using citric acid and ammonia and also have added an equal amount by weight of p-hydroxybenzoic acid which probably makes it a thread. Authors are requested to explain. ,