New Ni-Anthracene Complex for Selective and Sensitive Detection of 2 , 4 , 6-Trinitrophenol

Selective and sensitive detection of explosive materials through a simple approach is an attractive area of research having implications on public safety and homeland security. Considering this implication in mind, a new Ni-anthracene complex was designed and synthesized and has been demonstrated as an efficient fluorescence chemosensor for the selective and sensitive detection of 2,4,6-trinitrophenol. Firstly, a fluorescent anthracene ligand (A) was synthesized by treating anthracene9-carboxaldehyde with 1,3-diaminopropane in presence of a weak acid. To achieve superior selectivity and great quenching efficiency for 2,4,6-trinitrophenol (TNP), a Ni complex, namely, [Ni(μ 2 -L)(NO 3 )] (B), was synthesized via the reaction of A with Ni(NO 3 ) 2 ⋅6H 2 O. Complex B showed strong emission peak (λmax) at 412 nm and exhibited high selectivity towards TNP among other nitroaromatics and anions. 100 equivalents of TNP made 95% fluorescence quenching of B and its detection limit for TNP was calculated as 2.8μM.


Introduction
Nitroaromatic compounds are generally used in many industries for the preparation of dyes, pharmaceuticals, rubber products, explosives, chemical fibres and pesticides, and so forth [1].These nitroaromatic compounds were released into the environment as industrial wastage and are one of the main reasons for the pollution.Moreover, due to their poisonousness, carcinogenicity to living beings, and the risk to homeland safety, they cause serious intimidations to our lives [2].2,4,6-TNT; 2,4-DNT; 2,4-DNB; 2,6-DNT; and 2,4,6-TNP are the key constituents of environmental pollutants and explosive products [3].Out of these compounds, TNP in particular creates health hazards and chronic, skin, and eye diseases and is used as a powerful explosive similar to TNT [4].Furthermore, TNP is a more potent explosive than the more commonly used explosive TNT [5].
Various detection techniques such as electrochemical sensing, cyclic voltammetry, Raman spectroscopy, Liquid chromatography-mass spectrometry (LC-MS), photoluminescence spectroscopy (PL), and several other techniques have been used to detect different types of explosives including TNP.Nevertheless, most of them are pretty sophisticated, time taking, and/or difficult to operate [6,7].Among these reported procedures, PL based method offers shorter response time and improved sensitivity and is economically worthwhile [7,8].Hence, with the above stated problems from TNP and the benefits of PL, TNP sensing through optical detection is highly needed.
Till now, many fluorescent sensors have been developed for the detection of nitroaromatic explosives [7][8][9][10].Nitroaromatics are electron deficient in nature; due to this reason some electron-rich fluorescent probes can always produce nonfluorescent Meisenheimer or -stacking complexes with nitroaromatics [11,12].Selective detection of TNP is very difficult as it has strong electron affinity [13].Through donoracceptor electron transfer mechanism, TNP can quench the fluorescence emission of various molecules such as metal complexes, conjugated polymers, organic molecules, and metal-organic frameworks [14][15][16][17].Among various fluorophores used for designing of chemosensors, anthracene based compounds have been used for the detection of several analytes, due to their high chemical stability, richness of -electrons and strong fluorescence [18,19].Mainly anthracene based compounds are widely used for the sensing of anions and cations.Nevertheless, there are very limited compounds reported for the detection of explosives/nitroaromatics [20][21][22].In consequence, by considering the significance of the new photoluminescent material for TNP sensing, in this work, we synthesized a new anthracene based nickel complex with fluorescent activity for the detection of TNP.Solution phase spectral studies confirm the sensitivity of complex with TNP.

Synthesis of B.
To the solution of A (100 mg, 0.22 mM in THF), nickel nitrate [Ni(NO 3 ) 2 ⋅6H 2 O, 0.56 g, 1.93 mM] was added and stirred for two hours at room temperature.The yellowish green precipitate was separated out under reduced pressure.Yield: 83.96%.HR-MS [M + 1] + found 1082.34.

Characterizations.
The 1 H and 13 C NMR spectra of the A have been recorded on a Bruker 400 MHz NMR instrument.Mass spectra of the B have been recorded on Kratos PC Axima HR-Mass spectrometer in linear mode.The fluorescence quenching measurements were performed using Cary Eclipse Fluorescence spectrophotometer.The excitation and emission slit widths (each 10 nm) and scan rate (500 nm min −1 ) were kept constant for all the measurements.The optical absorption spectral measurements were recorded using a Shimadzu UV-2450 spectrophotometer with a quartz cuvette (path length, 1 cm).Energy dispersive X-ray spectra (EDAX) were obtained using scanning electron microscope (SEM), FEI Nova Nano SEM-450.

Results and Discussion
Condensation of 1,3-diaminoprone 2 with anthracene-9carboxaldehyde 1 produced a fluorescent ligand A, which gave a nickel complex B with the addition of Ni(NO 3 ) 2 ⋅6H 2 O at RT (Scheme 1).The as-synthesized complex was characterized by mass spectrometry, where the parent ion (/) peak appeared at 1082 corresponding to the species [2A⋅Ni(NO 3 ) 2 ] (see supplementary Figure S3).The presence of all the elements in complex B is confirmed through elemental analysis (see supplementary Figure S4).As we are developing a chemosensor for TNP, the main motivation behind the synthesis of compound A is that it was expected to become a nonfluorescent moiety as photoinduced electron transfer (PET) process can take place due to presence of imino nitrogen atom [23].The binding of nickel ions to the imino nitrogen of compound A will prevent the PET process and it eventually results in the perseverance of fluorescence.Consequently, TNP can perhaps quench the fluorescence emission of B with electron transfer or energy transfer mechanism and could be sensed [24].
Complex B (1 M) showed reasonably good emission intensity of anthracene monomeric species at 412 nm in THF : HEPES (9.5 : 0.5) when excited at  ex = 365 nm [25].On addition of TNP (150 M) to the solution of B (1 M) in THF : HEPES (9.5 : 0.5), the emission band shows quenching of emission band (Figure 1).To observe the selectivity of B towards TNP, we carried out the fluorescence titration experiments with similar amount of 150 M other nitroderivatives, namely, nitrotoluene (NT), 2,4-dinitrotoluene (2,4 DNT), and 1,4-dinitrobenzene ( standard deviation calculated by measuring the fluorescence intensity of complex B for more than 15 times and  presents the slope of fitting curve between variation of the fluorescence intensity ( 0 − ) and the added amount of TNP at low concentration.According to the linear fitting curve (Figure S5 shown in supporting information), the detection limit of complex B for sensing of TNP was found to be 2.857 M [26].The complex B DL in sensing of TNP is as comparable with the existing materials (see Table S1 in supporting information).We also examined the meddling of other anions and nitroaromatics towards the sensing of TNP (Figure 3) and it was confirmed that the B was detecting TNP even in the presence of other potential analytes.The emission plot of B in the presence of TNP was studied using Stern-Volmer relationship.Stern-Volmer plot seemed like a hyperbolic curve (see supplementary Figure S6) which may be accredited to the combination of static and dynamic (collision) quenching [27].
We have also detected these variations with the assistance of UV-vis analysis.Nickel complex B (1 M) in THF : HEPES (9.5 : 0.5) showed an absorbance band at 260 nm (Figure 4).On addition of TNP (120 M) along with the formation of band at 340 nm corresponding to TNP (Figure 4), there was subsequent increase in the band at 260 nm presenting that there is some photochemical process happening between B and TNP.To have a deep insight into the mechanism, we made plot between the absorbance spectrum of TNP and emission spectrum of B (Figure 5).The overlap between the two plots suggests an energy transfer mechanism from photoexcited -electron-rich derivative B to ground state International Journal of Spectroscopy electron-deficient TNP.Thus, the changes occurring in the optical properties of B on addition of TNP are attributed to the energy transfer process.

Conclusions
In conclusion, we have designed and synthesized a new photoluminescent Ni-anthracene complex.The luminescence behaviour of the complex was assessed towards different nitroaromatics and other analytes.The synthesized Nianthracene complex shows selective and sensitive detection of TNP and could be a helpful tool for the people working in forensic sciences.

Figure 3 :Figure 4 :
Figure 3: Fluorescence quenching in B with the addition of TNP alone and TNP in the presence of other analytes.The molar ratio to B used in each case is 100 equivalents.

Figure 5 :
Figure 5: Spectral overlap of the absorption of TNP (red line) with the emission spectrum of B (blue line).