Fluorescence Sensing Approach for High Specific Detection of 2,4,6-Trinitrophenol Using Bright Cyan Blue Color-Emittive Poly(vinylpyrrolidone)-Supported Copper Nanoclusters as a Fluorophore.

In this paper, we illustrate an efficient, convenient, and simplistic fluorescence technique for the specific identification for nitro explosive 2,4,6-trinitrophenol (TNP) in 100% water medium by bright cyan blue color-emitting poly(vinylpyrrolidone)-supported copper nanoclusters (PVP-CuNCs) as a fluorescence probe. PVP-CuNCs exhibited linear fluorescence quenching response toward the increasing concentration of TNP analyte. Surprisingly, TNP only reduces the emission signal of PVP-CuNCs, whereas various nitro explosives cause very slight reducing emission intensity, validating the good specificity of the PVP-CuNC probe toward TNP. The highest Stern-Volmer quenching constant (K sv) value of 1.03 × 107 dm3 mol-1 and the extremely lowest limit of detection of 81.44 × 10-12 mol dm-3 were achieved solely for TNP in 100% water medium which is astonishing and exclusive for this nanoprobe. The sensing pathway for the high sensitivity of PVP-CuNCs assay to quantify the TNP is expected to combine with the inner filter effect process and static quenching. The static quenching mechanism between TNP and PVP-CuNCs is further verified by fluorescence decay measurements. Furthermore, the developed fluorescence sensing platform is applied for the quantification of a trace amount of TNP in real samples named dam water, sea water, and match stick.

Introduction

Nowadays, the quantification of explosive chemicals is essential for homeland security, military operations, humanitarian efforts, and environmental cleaning.[1] Nitroaromatic explosives such as 2,4,6-trinitrophenol (TNP, picric acid), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) are frequent components of industrialized explosive chemicals and originates in global landmines, that construct a main aim in dangerous nitro explosive determination.[1−3] Among them, TNP is a stronger explosive material which contains high explosion velocity and low security coefficient and is broadly utilized until World War I owing to the fact that it has potent explosive properties than TNT.[4−6] Furthermore, TNP has more significance in pharmaceuticals (antiseptic), militaries, and matchstick and dye industries.[7−9] Instead of such values, TNP induces several sensitive health effects: TNP is partly ascribed to the nitro and phenol functionalities. TNP is a reason for sturdy irritation to the skin/eye, and it is a main source of the prospective scratch to the organs concerned in the respiratory system.[10] Moreover, because of the electron deficient nature of TNP, the degradation of TNP is more complicated in the ecosystem which is possibly accountable for several chronic diseases, namely, cancer and cyanosis.[11,12] Hence, there is an imperative necessity to build up finer methodologies for examining the trace amount of TNP in aqueous solution to environmental pollution as well as to prevent terrorist fears.

Various analytical methodologies, including high-performance liquid chromatography,[13] surface-enhanced Raman spectroscopy,[14] nuclear quadrupole resonance,[15] electrochemical methods,[16] thermal neutron analysis,[17] ion mobility spectrometry,[18] and energy-dispersive electron diffraction techniques[19] have been documented for the determination of TNP. Even though, these methods are extremely selective and relatively high-priced, they need a long time for analysis and regular careful calibration is necessary. Moreover, the specific quantification of trace amounts of TNP has not been an easy assignment. In this consideration, fluorescent-based methodologies are mostly important and probable for the fast determination of TNP at room temperature because of good response, super sensitivity, and simple operation. Recently, luminescent nanomaterial-based fluorescent sensors are more interesting.

Several inorganic fluorescent nanostructures, named MoS2 quantum dots, graphitic carbon nitride (g-C3N4) nanosheets, and CdTe/ZnS quantum dots were utilized for the quantification of TNP.[20−22] These inorganic luminescent material-based TNP-sensing tactics are not suitable and also these materials are very toxic in nature. Hence, the ultrasensitive and selective determination of TNP in an eco-friendly medium remains a challenging task. To date, fluorescent gold (Au), silver (Ag), and copper (Cu) nanoclusters (NCs) have materialized as an attractive nanomaterials and these NCs are applied in the area of chemo-biosensors, bioimaging, and catalytic applications owing to their low toxic, outstanding photostability, better quantum yield, large Stokes shifts, biocompatibility, and water solubility.[23−25] Among them, fluorescent CuNCs have received superior interest over AgNCs and AuNCs as the precursor for the synthesis of CuNCs because they are comparatively abundant, cheap, and readily existing from commercial sources.[26,27] Furthermore, fluorescent CuNCs became good choice for biological cell labelling and imaging and these CuNCs were used as a fluorescent probe for the determination of different analytes.[27−32] Although, numerous ascertained applications were documented in modern times by using the CuNCs, however NCs applications in the quantification of TNP are very few. For example, very recently, cysteine-functionalized CuNCs were used for the detection of TNP with the limit of detection (LOD) being 0.19 μM.[33] Wu et al., reported that bovine serum albumin-stabilized CuNCs were applied as a fluorescence probe for the determination of TNP with the LOD value being 0.12 μM.[34]

In this work, Cu-based fluorescent NCs were applied for the quantification of trace amounts of TNP with high selectivity and sensitivity in aqueous medium. Poly(vinylpyrrolidine) (PVP)-supported CuNCs were prepared by the reduction of Cu2+ ions with ascorbic acid (AA) in water at ambient temperature. PVP-CuNCs exhibit the emission maxima at 430 nm and when excited at 380 nm show cyan blue color emission under an ultraviolet (UV)-lamp. Emission features of PVP-CuNCs were efficiently reduced and quenching efficiency is linearly enhanced with increasing concentration of TNP. With an outcome from emission spectral experiments of PVP-CuNCs with TNP, the concentrations of TNP are estimated. Other nitro aromatic compounds and some potentially interfering molecules which did not influence the emission intensity of PVP-CuNCs have suggested the excellent selectivity of this method. Furthermore, the developed sensing approach was applied for the various real samples such as dam water, sea water, and matchstick.

Results and Discussion

Characterization of PVP-CuNCs

CuNCs were prepared by treating the mixture of PVP and Cu2+ ions with AA in pH 6.0 at ambient temperature. In the reaction mixture, Cu2+ ions are connected to the nitrogen (N) atom by the relation of lone pair electrons of N atom with unfilled orbital of Cu2+ ions, ensuing in regions of their high concentration. AA is a gentle reducing reagent, capable to reduce Cu2+ ions preferably at such sites.[35] After completion of the reduction process, PVP-supported CuNCs were collected. The resultant PVP-CuNCs were characterized by emission, absorption, high resolution transmission electron microscope (HR-TEM), dynamic light scattering (DLS), and zeta potential measurements.

To investigate the emission characteristics of the collected PVP-CuNCs, emission and excitation spectra of PVP-CuNCs were monitored and analogous results are displayed in Figure . As depicted in Figure , PVP-CuNCs illustrated an emission maximum at 430 nm under excitation wavelength at 380 nm (solid line) and PVP-CuNCs show well-defined excitation spectrum at 380 nm (dotted line). As described in the inset of Figure a, the resultant PVP-CuNC colloidal dispersion is yellow in color under day light and shows strong cyan blue color emission under an UV lamp (λ = 365 nm). UV–vis absorption of the spectrum of PVP-CuNCs is presented in Figure S2. As illustrated in Figure S2, PVP-CuNCs did not observe any surface plasmon resonance peak at 560 nm which clearly indicates the creation of CuNCs instead of larger Cu nanoparticles. Similar kinds of absorption spectral results for CuNCs were reported earlier.[32,33,35]

Figure 1

Normalised excitation and emission spectra of PVP-CuNCs. Inset: (a) Color of the PVP-CuNCs under day light and (b) UV-light (λ = 365 nm).

The size distribution and morphology of highly fluorescent cyan blue emissive PVP-CuNCs were characterized by HR-TEM analysis and resultant HR-TEM images at different magnifications are visualised in Figure . As noticed in Figure , HR-TEM images of PVP-CuNCs show roughly quasi spherical in morphology and monodispersed with average size ∼2.8 ± 0.3 nm. Moreover, HR-TEM images did not show larger Cu nanoparticles or aggregation because of the protective layer of PVP template. In addition, the morphological characteristics of PVP-CuNCs in aqueous medium DLS measurement were recorded and typical DLS observation is provided in Figure S3. The average hydrodynamic diameter size of PVP-CuNCs is around ∼5 nm (Figure S3). PVP-CuNCs were further distinguished by zeta potential analysis. The value of the zeta potential measurement data provides a suggestion of the stability of the nanostructures in solution medium.[36−38] Zeta potential result for PVP-CuNCs is shown in Figure S4. As depicted in Figure S4, zeta potential value of PVP-CuNCs is found as −30.6 mV, which suggested that the stability of the PVP-CuNC colloidal solution was based on steric stabilization by the PVP template. Similar kind of PVP-supported metal nanoparticles was reported earlier.[39] From the above results, PVP-supported CuNCs successfully prepared.

Figure 2

HR-TEM images of PVP-CuNCs at different magnifications.

Stability of PVP-CuNCs

The stability of highly fluorescent PVP-CuNCs was examined by evaluating the emission intensity of PVP-CuNCs at various time intervals up to 30 days and the collected emission spectral data are portrayed in Figure S5. It can be seen from Figure S5, we found that the emission intensity of freshly prepared and up to 30 days aged PVP-CuNCs remained the similar. These observations clearly designate that the resultant cyan blue emissive PVP-CuNCs are extremely stable in suspension medium in 30 days. Furthermore, photostability of PVP-CuNCs was inspected under constant irradiation of PVP-CuNCs with UV light for 120 min and corresponding results are given in Figure S6. As exposed in Figure S6, no noticeable alteration in the emission intensity was examined which clearly points out the good photostability of PVP-CuNCs. In addition, the stability of PVP-CuNCs at various ionic strengths was also evaluated by the emission intensity of PVP-CuNCs in the presence of various anions (50.00 × 10–3 mol dm–3). The relative emission intensity was calculated by the fraction of the emission intensities of PVP-CuNCs before and after the addition of various anions and is publicized in Figure S7. As described in Figure S7, most of the anions did not influence the emission intensity of PVP-CuNCs. Interestingly, PVP-CuNCs are tolerant at an ionic strength of up to 50.00 × 10–3 mol dm–3 concentrations of NaCl, which recommends that this nanoprobe is appropriate for the determination of TNP in the occurrence of higher ionic species in real water samples (Figure S8).

Sensitive Detection of TNP by PVP-CuNCs

For the good chemosensors, fast response to the target is extremely vital. To discover the probable application of the highly fluorescent PVP-CuNCs for the detection of TNP in aqueous solution, quantitative determination of TNP using the spectrofluorimetry approach by PVP-CuNCs was investigated. The fluorescence spectral changes of PVP-CuNCs with various amounts of TNP are given in Figure . In the absence of TNP, PVP-CuNCs show maximum emission at 430 nm while showing exciting wavelength at 380 nm (Figure ). As visualized in Figure , the emission intensity of PVP-CuNCs was steadily reduced with increasing concentration of TNP, simultaneously emission color of the solution also changes from cyan blue to colorless under UV light (inset of Figure ).

Figure 3

Emission spectral changes of PVP-CuNCs with the incremental amounts of TNP analyte. [TNP]: (a) 0.00, (b) 6.00 × 10–9, (c) 12.00 × 10–9, (d) 18.00 × 10–9, (e) 24.00 × 10–9, (f) 30.00 × 10–9, (g) 36.00 × 10–9, (h) 42.00 × 10–9, (i) 48.00 × 10–9, (j) 54.00 × 10–9, (k) 60.00 × 10–9, (l) 66.00 × 10–9, (m) 72.00 × 10–9, (n) 78.00 × 10–9, (o) 84.00 × 10–9, (p) 90.00 × 10–9, (q) 96.00 × 10–9, (r) 102.00 × 10–9, (s) 108.00 × 10–9, (t) 114.00 × 10–9, (u) 120.00 × 10–9, (v) 126.00 × 10–9, (w) 132.00 × 10–9, (x) 138.00 × 10–9 and (y) 144.00 × 10–9 mol dm–3. (λexi = 380 nm, λemi = 430 nm). Inset shows PVP supported CuNCs without (i) and with (ii) the last addition of TNP. Condition: 100% water medium.

Interestingly, when 144.00 × 10–9 mol dm–3 of TNP solution was added into the PVP-CuNCs colloidal, the emission characteristics of the CuNCs were nearly 98% quenched. The achieved emission spectral response has clearly designated that PVP-CuNCs can rapidly quantify the trace amount of TNP. To find the linear relationship and lowest detection limit of the present approach, the emission intensities of the PVP-CuNCs were plotted against various amounts of TNP analyte and yielded calibration curve is shown in Figure . Calibration graph illustrates that the proposed sensing tactic is well linear fit with concentrations ranging from 6.00 × 10–9 to 144.00 × 10–9 mol dm–3. According to (3σ) the International Pure and Applied Chemistry (IUPAC) criterion, LOD is calculated as 81.44 × 10–12 mol dm–3.

Figure 4

Calibration curve for PVP-CuNCs with the increasing concentrations of TNP.

Emission Quenching Mechanism

The emission quenching follows various pathways such as fluorescence resonance energy transfer (FRET), exited state reactions, inner filter effect (IFE), ground state complex formation, and dynamic quenching.[21,33,34,40] Both in the IFE and FRET process, generally there must be an excellent spectral overlap between the emission or excitation spectrum fluorescence probe and UV–vis absorption spectrum of analyte. To explore the exact emission quenching mechanism for the PVP-CuNCs, ocular characteristics of TNP and PVP-CuNCs were tested. The spectral overlap between the emission/excitation spectra of PVP-CuNCs (donor) and UV-absorption spectrum of TNP (acceptor) is represented in Figure . As demonstrated in Figure , TNP exhibits a broad absorption spectrum of 300–480 nm with strong absorption maxima at 353 nm. Furthermore, upon the excitation of 380 nm, PVP-CuNCs give the strong and wide emission maximum at 430 nm. The extensive spectral overlap of emission/or excitation spectra of fluorescence probe and absorption spectrum of TNP proposed that the observed emission quenching can occur from IFE and FRET processes (Figure ).

Figure 5

Normalised spectral overlap between the emission/excitation spectra of PVP-CuNCs (donor) and UV-absorption spectrum of TNP (acceptor).

To differentiate the IFE and FRET processes, time-resolved fluorescence life time experiments of PVP-CuNCs in the absence and various concentrations of TNP molecules were recorded and collected life time decay profile is displayed in Figure . As demonstrated in Figure , the fluorescence decay data fitted by tri exponential function and yielded life time value for PVP-CuNCs is 3.36 ns. Upon the addition of different concentrations of TNP to PVP-CuNCs, very minor change of fluorescence decay lifetimes from 3.36 to 3.33 and 3.33 to 3.28 ns (Table S1) has occurred. Fluorescence lifetime observations eliminated the chance of the FRET process between TNP and PVP-CuNCs. Different from the FRET process, two molecules involved in the process IFE should not elementally interrelate with each other and the analyte does not alter the average decay lifetime of the fluorescence probe.[41,42] Hence, the IFE process is believed as one main process in the reducing emission intensity of PVP-CuNCs by TNP. Even though the IFE process is generally considered as a cause of error in fluorometric methods and the IFE process is utilized as budding luminescent sensors. There is no rejecting that the IFE process is significant, predictable, and a leading feature in spectrofluorimetric methods. In this tactic, the IFE process-based fluorescent chemo-biosensors did not necessitate ascertaining of any chemical connecting between the analyte and fluorescence probe, however, utilize the fluorescence moieties and acceptor which presents easy and significant flexibility.[43−45]

Figure 6

Fluorescence decay profile for PVP-CuNCs with various concentrations of TNP in aqueous medium.

In addition, the coexistence of different emission quenching pathways including dynamic (collisional) or static quenching process is too feasible. The emission quenching observations for PVP-CuNCs by TNP were treated with the traditional Stern–Volmer equation (eq )[40]where F0 and F denotes the IFE-corrected emission intensities of PVP-CuNCs before and after the addition of TNP; Ksv is the Stern–Volmer constant; [TNP] represents the concentration of picric acid; Kq denotes the quenching constant; and τ is the average lifetime of the fluorescence probe. According to the eq , the plot of F0/F versus [TNP] is displayed in Figure S9. It can be noticed from Figure S9 that the Stern–Volmer curve shows well linearity with the concentration ranging from 6.00 × 10–9 to 90.00 × 10–9 mol dm–3, and the correlation coefficient is 0.9916. From the slope of the linear Stern–Volmer graph, Ksv and Kq are estimated to be 1.07 × 107 dm3 mol–1 and 3.18 × 1015 dm3 mol–1 s–1, respectively. The obtained Kq value is much larger than the most scatter dynamic quenching constant (2 × 1010 dm3 mol–1 s–1), representing the ground state complex formation (static quenching process) between TNP and PVP-CuNCs.[40] Recently, Shanmugaraj and John, previously demonstrated the analogous behavior in the determination of TNP using cysteine-stabilized CuNCs.[33] In this case, highly sensitive detection of TNP by PVP-supported CuNCs is because of strong emission quenching and IFE process (Scheme ) between the nanoprobe and analyte.

Scheme 1

Schematic Illustration of Detection TNP by PVP-CuNCs

Effect of Other Interferences

Specificity is a crucial factor to appraise the characteristics of a fluorescent sensing methodology and selective identification of TNP is an exciting and interesting section of research. For validating the specific quantification of TNP by the present method, the emission characteristics of the PVP-CuNCs by TNP were examined in the presence of various nitro-substituted aromatic and potential interfering compounds. The relative emission intensity and fluorescence color changes of PVP-CuNCs with 50-fold higher concentrations of potential interfering compounds are displayed in Figure . As depicted in Figure , interestingly, TNP only showed remarkable response toward the emission spectrum of PVP-CuNCs compared to various other nitro substituted and potential interfering analytes. In addition, the fluorescence color of the PVP-CuNCs under a UV lamp (λ = 365 nm) also designated that insignificant influences for the elected potential interfering salts display the superb specificity of the present approach in the identification of TNP and this methodology can be easily visualised with naked eye (inset of Figure ). Moreover, it is important to observe that the emission or excitation spectrum of PVP-supported CuNCs largely overlaps the UV–vis absorption spectrum of TNP, whereas, other interfering substances have nearly no absorption peak over the emission or excitation spectrum of CuNCs involving the subsistence of IFE between PVP-supported CuNCs and TNP. Therefore, in this approach, the IFE process increases the emission quenching efficiency of PVP-CuNCs by TNP compared to other potential interfering substances. The examined superb specificity toward the TNP by PVP—CuNC-based selective recognition is elucidated through combined ground state complex formation and IFE process. Furthermore, PVP-CuNCs illustrated outstanding reaction toward the TNP only even in the existence of 50-fold higher concentration of major interfering nitro-substituted analytes which corroborates the utility of this nanoprobe for the analysis of real water samples.

Figure 7

Relative emission intensity changes of PVP-CuNCs in the presence of 50-fold higher concentrations of common interfering nitroaromatics and TNP in 100% water medium. Inset: Corresponding photographs under UV light (wavelength = 365 nm).

Practical Applications

To further verify the specificity and practical benefit of this nanoprobe, it investigated the PVP-CuNCs to quantify the TNP in dam water, sea water, and matchstick head black powder mixed water samples. No alteration in the emission spectrum of PVP-CuNCs is monitored, PVP-CuNCs with real water samples which are considered because of the absence of TNP. Nevertheless, when real water samples were spiked with known concentrations of TNP level and treated with PVP-CuNCs, while, the emission intensity of PVP-CuNCs is reduced. It is clearly suggested that the observed reducing emission intensity of PVP-CuNCs is solely because of the addition of spiked concentrations of the TNP analyte. According to the emission quenching experiments, the recoveries (%) of TNP levels were calculated and are listed in Table . It is observed from Table , good recoveries (%) of TNP and the relative standard deviation of real water samples were less than 2.82%, informing the super precision and good reproducibility of the current method. Hence, it is worthy to note that the resultant PVP-CuNCs were appropriate to analytical application for the specific quantification of trace amount of TNP in real water (dam water, sea water and matchstick head black powder mixed water) samples. Furthermore, comparison of the current sensing technique with previously documented methodologies for TNP quantification were also made[6,20,21,33,34,41,42,45−49] and are listed in Table S2. Among all the sensing approaches, the current sensing technique is super-sensitive, efficient, convenient, low-toxic, rapid, simplistic, and precise and supplies consistent fluorescence spectral outcomes even at the trace level of TNP in aqueous medium.

Table 1

Analytical Recoveries (%) of TNP in Various Real Samplesa

water samples	spiked amount of TNP × 10–9 mol dm–3	calculated amount of TNP × 10–9 mol dm–3	recovery (%)	RSD
dam water	50.00	48.78	97.56	1.71
	100.00	98.49	98.49	2.16
sea water	50.00	49.12	98.24	2.36
	100.00	97.89	97.89	1.87
matchstick	50.00	51.45	102.9	2.82
	100.00	100.69	100.69	2.36

RSD: relative standard deviation.

Conclusions

To summarize all, a fast, simple, low-toxic, convenient, and extremely sensitive fluorescence platform is constructed for the specific identification of trace level of TNP in aqueous medium by high cyan blue color-emitting PVP-supported CuNCs. The resultant bright cyan blue color-emitting PVP-CuNCs showed the emission intensity is almost completely quenched (98%) with the final addition of TNP analyte. In addition, this methodology is applicable to the naked eye detection of TNP by PVP-CuNCs under an UV lamp. Lowest LOD of the PVP-CuNCs toward TNP in water medium was about 81.44 × 10–12 mol dm–3. Time resolved fluorescence lifetime decay data were revealed that the static quenching process involved between the TNP and PVP-CuNCs. The superb emission quenching performance of PVP-CuNCs by TNP is recognized to the ground state complex formation and IFE process. This simplistic methodology demonstrated that the super specificity and potential interference such as nitro-substituted analytes did not affect the detection of TNP in water medium. High selectivity and ultra sensitivity can assure PVP-CuNCs as a realistic nanoprobe for the quantification of the trace amount of TNP in real water samples such as dam water, sea water, and matchstick head black powder mixed water samples with good recoveries.

Experimental Section

Chemicals

Copper sulphate pentahydrate (CuSO4·5H2O) and nitrobenzene were obtained from Merck. PVP (mean molecular weight, ∼40 000), AA, TNP, 3,5-dinito salicylic acid, 4-nitrophenol, 1,3-dinitrobenzene, 2-nitrotoluene, 4-nitrotoluene, and DNT were purchased from Sigma-Aldrich, USA. Aniline, phenol, benzene, 4-nitrobenzaldehyde, 2-nitrobenzaldehyde, 2-nitrobenzoicacid, 4-nitrobenzoicacid, and sodium hydroxide (NaOH) were collected from Loba Chemie Pvt. Ltd., India. All other chemicals were of at least analytical reagent grade and utilized as received. Doubly distilled water was used in the every experiment. All the analyses were performed minimum three times and at room temperature.

Apparatus

Emission spectral studies were monitored by a JASCO FP-6600 spectrofluorometer with excitation wavelength at 380 nm in the emission mode. Both the emission and excitation slit widths were kept at 5 nm. Absorption spectra were recorded from 800 to 200 nm by a JASCO V-630 spectrophotometer with 1.0 cm quartz cells. Nanostructure and morphology of NCs were verified by JEOL JEM 2100 HR-TEM operating at 200 kV. DLS information was evidenced to determine the average size (hydrodynamic diameter) of CuNCs. DLS analysis results were acquired by Malvern Zetasizer Nano. Zeta potential experiments were recorded by Zetasizer Nano, ZS with 633 nm He–Ne laser, equipped with a MPT-2 autotitrator (Malvern, UK). Fluorescence lifetime experiments were performed on Horiba Scientific equipped with an NL-C2 pulsed diode excitation source of 380 nm.

Preparation of Poly(vinylpyrrolidine)-Stabilized CuNCs (PVP-CuNCs)

CuNCs were prepared in accordance with an earlier reported method.[35] Briefly, 10 mL water was added to 0.5 g of PVP and this solution was sonicated for 10 min, then the pH of the solution was changed to 6.0 using 1.0 mol dm–3 concentration of NaOH. Next, 0.1 mL of CuSO4·5H2O (100.00 × 10–3 mol dm–3) and 1 mL of AA (100.00 × 10–3 mol dm–3) were dissolved with the PVP solution and this reaction mixture allowed reacting for 6 days at ambient temperature (25 °C). After 6 days, clear yellow color solution was collected, the collected colloidal solution exhibits cyan blue color emission under an UV lamp (λ = 365 nm) which clearly indicates that the formation of PVP-supported CuNCs. Finally, the obtained CuNCs were dialyzed against doubly distilled water and stored at 4 °C for further application purpose.

Detection Procedure for TNP

For the quantification of TNP, different known concentrations of TNP were added into the (0.750 mL) PVP-CuNC colloidal solution and make up to the mark of 5 mL standard measuring flask using doubly distilled water. This reaction mixture was shaken well and incubated for 5 min at ambient temperature (25 °C). Then, 2 mL of the reaction mixture was transferred to the emission cuvette and the emission spectrum was recorded in the range from 390 to 500 nm. All the experiments were carried out minimum three times and the given data were mean of three individual experiments. In the exploration of the selective determination of TNP in the present approach, some nitro aromatic and aromatic compounds were elected as interfering molecules. Specificity for TNP was verified by the mixing of various nitro aromatic and aromatic stock solutions rather than TNP by identical conditions. The relative emission intensity of PVP-CuNC colloidal solution with 12.00 × 10–9 mol dm–3 of TNP remained unaltered after 5 min which suggested that the reaction completed in 5 min. Hence, all the emission titration experiments were made 5 min after the successive addition of TNP (Figure S1).

Real Sample Analysis

Siruvani dam water was collected from Siruvani, Coimbatore, India and sea water was collected from the ocean near Pondicherry beach, India. This sea water was filtered thrice by qualitative filter papers. Matchstick head black powder is acquired and suspended in aqueous solution by sonication method and filtered. This filtrate of matchstick was applied for real sample analysis. These real water samples were mixed with known amounts of TNP and monitored by PVP-supported CuNCs. At last, emission spectral responses were monitored by standard addition procedure. The recovery (%) was determined as the ratio between amounts of added and estimated TNP.[36−38]