Picric acid sensing by carbon nanodots: theoretical validation of selectivity

Abstract

Carbon nanodots (CNDs), known for their exceptional properties, have become a prominent member of the family of nanostructured carbon materials. The most appreciated application of carbon nanodots is sensing, evoked by the reflex response of fluorescence in the presence of specific analytes. Herein we unveil the mechanism of selective sensing of picric acid by carbon nanodots, among a group of analogous compounds. A solid explanation for the selectivity of the system among nitrocompounds is provided through DFT studies. The computed global reactive descriptors describe the relative reactivities of different nitroaromatics and the associated DAM simultaneously classifies them into electron donors and acceptors, from best to worst. Along with these data, a detailed FMO analysis clarifies the selective sensing of picric acid by CNDs, which is in complete agreement with the experimental observations. It is envisioned that this attempt would drive the design of CND-based systems with optimum energy levels, which can serve as promising sensors for analytes of significance.

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1. Introduction

Carbon nanodots (CNDs), the luminescent nanoparticles of carbon, have attracted much attention from the researchers owing to their desirable features. One of the most common applications of these particles is sensing, evoked by the reflex response of their inherent fluorescence in the presence of specific analytes. The ease of operation and sensitivity make carbon nanodot-based fluorimetric sensing a preferred method for tracing significant analytes. Herein, we unveil the decisive role of the band gap of carbon nanodots in tuning the selectivity towards specific analytes. p-Phenylenediamine-derived carbon nanodots are chosen as the model system and picric acid (PA), a priority pollutant falling in the category of nitroaromatics, is used as the analyte in the study. Nitroaromatic compounds (NACs) are widely used in different sectors and are produced in huge quantities due to their high global demand.¹ Although useful in several aspects, NACs have the potential to pollute the environment and pose serious dangers to both human health and the ecosystem. Numerous health concerns, such as skin irritation, respiratory difficulties, central nervous system damage, etc., result from the exposure to these chemicals,² which made these chemicals fall in the list of priority pollutant groups identified by the United States Environmental Protection Agency.³ Picric acid (PA) – the analyte of interest in the present study – has the most explosive potential among NACs. PA has been a component of landmines for many years. It is also used in the dye and leather industries, rocket fuel, match and firework preparation, and other sectors. Since picric acid is very acidic and readily soluble in water due to the presence of strong electron-withdrawing nitro groups, the soil- and ground water-mediated risk factor is quite high. Prolonged exposure to picric acid can lead to severe health complications, including eye and skin irritation, respiratory issues, liver malfunction, neurological disorders, infertility, and chronic diseases such as anemia, cancer, and cyanosis.⁴ The above concerns mandate the design of sensors that can easily detect picric acid in aqueous medium with selectivity and sensitivity. Nitroaromatic compound detection has been accomplished in recent years using a variety of analytical techniques, such as mass spectrometry,⁵ HPLC,⁶ SERS,⁷ X-ray imaging methods,⁸ etc. The aforementioned analytical techniques limit their integration into a portable system for real-time and on-site analysis, since they necessitate trained personnel, complex analytical facilities, laborious sample preparation processes, time-consuming analysis, etc.

Several fluorimetric sensors, including metal–organic frameworks,⁹ metal nanoclusters,¹⁰ quantum dots,¹¹ and metal halides,¹² have been extensively studied due to their commendable sensitivity and ease of manipulation. Nevertheless, their prospective applications are hampered by their toxicity, limited picric acid selectivity, and poor stability. In this context, carbon nanodots (CNDs), carbon particles of size under 10 nm, characterized by inherent luminescence, are an excellent choice for applications in the realm of sensing because of their exceptional qualities.

Here, we present p-phenylenediamine-derived carbon nanodots (PD-CNDs) that could detect picric acid through effective quenching of luminescence, among the selected aromatic compounds. CNDs have been utilized for the selective sensing of picric acid.^11b,13,14 Here a solid explanation for the selectivity of carbon nanodots towards PA is provided through DFT studies. The computed global reactive descriptors describe the reactivities of different nitroaromatics and the associated donor–acceptor map (DAM) simultaneously classifies them based on their donor/acceptor nature. Along with these data, a detailed FMO analysis explains the selective sensing of picric acid by CNDs. Although there are a few reports on CND-based PA sensors, analyzing the quenching pathway,^2b,15 most of the discussions are limited to experimental investigations. The current study upholds the decisive role of the band gap of CNDs in its response to external species. The band gap of CNDs relies on the size and anchored functional groups, among which the latter can be more easily manipulated through the proper choice of precursor molecules. The outcome of the study is thus utmost helpful for designing CND-based selective fluorimetric sensors.

2. Materials and methods

2.1. Materials

TCI Chemicals (India) Pvt. Ltd provided p-phenylenediamine (p-PD), which was used as received without further purification. The nitrocompounds selected for the analysis, o-nitrotoluene (o-NT), m-nitrotoluene (m-NT), p-nitrotoluene (p-NT), m-nitrobenzaldehyde (m-NBZA), p-nitrobenzaldehyde (p-NBZA), nitrobenzene (NB), p-nitrophenol (p-NP), p-nitrobenzoic acid (p-NBA), picric acid (PA) and 1-chloro-2,4-dinitrobenzene (CDNB), were purchased from HiMedia Laboratories Pvt. Ltd. All the commercially available reagent grade chemicals were used as received. Deionized (DI) water was used throughout the experiments.

2.2. Preparation of p-phenylenediamine-derived carbon nanodots

p-Phenylenediamine (5 mg in 60 ml of DI water) was subjected to hydrothermal treatment at 180 °C for 8 hours. Filtration, dialysis and centrifugation (3500 rpm for 20 min) were performed on the cooled aqueous solution. The filtrate was subjected to dialysis against deionised water (molecular weight cutoff: 1 kDa) for 24 h at a stirring speed of 80 rpm.

2.3. Characterization

A thorough characterization strategy was adopted to explore the structural, morphological, and optical properties of carbon nanodots. This involved the use of various analytical techniques, including high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), UV-visible absorption spectroscopy, and fluorescence spectroscopy. The morphology and particle size of the carbon nanodots (CNDs) were analyzed using a JEOL JEM-2100 high-resolution transmission electron microscope (HR-TEM) operated at an accelerating voltage of 200 kV. The X-ray diffraction (XRD) pattern was recorded using a Rigaku Miniflex-II diffractometer with Cu Kα radiation. The FT-IR spectrum was recorded using a JASCO FTIR-4100 spectrometer by the KBr pellet method. The surface elemental composition was examined via XPS using an Omicron spectrometer with 1253 eV Mg Kα radiation as the excitation source. Raman spectroscopy analysis was performed using a WITec alpha300 RRA system (WITec GmbH, Ulm, Germany) equipped with a 532 nm laser. Zeta potential analysis was performed using a Zetasizer Nano S (Malvern Instruments) at 25 °C. UV-visible absorption spectra were obtained using a JASCO V-550 spectrometer. Fluorescence measurements were carried out using a Cary Eclipse fluorescence spectrometer (Agilent Technologies). The fluorescence quantum yield was estimated using a combination of UV-visible and fluorescence spectroscopy of the reference (Rhodamine 6G (QY = 95%) in DI water) and CND sample solutions. Luminescent properties under UV illumination were monitored using an LZC-4X photoreactor and the photostability of the material was also evaluated using both an LZC-4X photoreactor and a Cary Eclipse fluorescence spectrometer (Agilent Technologies). Fluorescence lifetime studies were conducted using a HORIBA time-correlated single photon counting (TCSPC) system with a 370 nm Nano LED as the excitation source.

2.4. Detection of picric acid using PD-CNDs

1 mM solutions of selected analytes (nitroaromatic compounds) were prepared in DI water. The detection of picric acid (2,4,6-trinitrophenol/TNP) was carried out at room temperature using the fluorescence quenching technique. A stock solution of PD-CNDs was prepared by adding 300 µL of PD-CNDs to 2 mL of DI water. 20 µL of picric acid solution (1 mM) was added to the stock solution, and the emission intensity of PD-CNDs at an excitation wavelength of 440 nm was observed before and after the addition of picric acid. A constant volume of picric acid (10 µL) was added gradually to the same solution to examine further quenching. For comparison, 10 µL of other nitrocompounds and aromatic compounds (1 mM) were added to the stock solution of PD-CNDs and the emission intensity at 440 nm was measured.

2.5. Theoretical investigations

Due to the ability to provide a precise idea at the molecular level, theoretical studies have become increasingly popular. Numerous programs, both free and paid, were made accessible so that a great deal of information about the system in question could be inferred quickly. This is helpful in preventing unintended steps in the experimental phase. In the current study, computational investigations have been used to correlate with the experimental findings in addition to investigating the preferential sensing of picric acid by carbon nanodots (CNDs).

As density functional theory (DFT) is very popular for the study of organic molecules, the present study also employed a DFT-based investigation at the B3LYP level of theory using the 6-31+G* basis set. All the computational calculations were performed using the Gaussian16 software package and GaussView 6.0 to illustrate the structures of nitroaromatic compounds. At first, the parent nitrobenzene is optimized and substitutions were made in the optimized structure of the parent to get the inputs for all the other nitroaromatic compounds. Using the same level of theory, the neutral, anionic, and cationic forms of the chosen nitroaromatic compounds were optimized, and the outcomes were assessed to compute the global reactive descriptors. As there are no imaginary frequencies, all the optimized structures were confirmed to be in the global minimum. The computed global reactive descriptors were used to plot the DAM, which provides an insight into the nature of reactivities and allows for comparison of the reactivities of various nitroaromatic compounds. Along with this, the mechanism of selective sensing was then deduced by analysing the molecular orbitals (MOs), particularly the highest occupied and lowest unoccupied MOs (HOMO and LUMO, respectively).

3. Results and discussion

3.1. Characterization of p-phenylenediamine-derived carbon nanodots

The formation of carbon nanodots from p-phenylenediamine was confirmed by different characterization techniques. TEM images (Fig. 1a and b) revealed the formation of spherical particles with an average size of 2.8 nm. The X-ray diffraction pattern of the system (Fig. 1c) displays characteristic peaks at 2θ values of 23.4, 42.4 and 60°. The first peak corresponds to the (002) plane of graphitic carbon,¹⁶ the second peak determines the longitudinal dimensions of the structural element¹⁷ and the third one represents the (103) plane of carbon in the hexagonal graphitic lattice,¹⁸ respectively. XPS and FT-IR spectroscopy enable the identification of the functional groups present in PD-CNDs. The broad absorption band at 3427 cm⁻¹ (Fig. 2a) indicates –O–H and/or –N–H stretching vibrations. The asymmetric and symmetric stretching vibrations of –C–H groups are observed at 2923 cm⁻¹ and 2848 cm⁻¹, respectively. The FT-IR bands at 1637 cm⁻¹ and 1021 cm⁻¹ are attributed to –C=O/–C=N and –C–O stretching, respectively. The bands at 1510 cm⁻¹ and 1425 cm⁻¹ are assigned to typical aromatic ring skeleton vibrations and the presence of –C–N moieties.¹⁹ Therefore, the core is suggested to contain amino (–NH,–NH2), carbonyl, carboxyl (–C=O/–COOH), and hydroxyl (–OH) groups as functionalities.

Fig. 1 (a) and (b) TEM images of PD-CNDs and (c) XRD pattern of PD-CNDs.

Fig. 2 (a) FT-IR spectrum of PD-CNDs, (b) XPS survey scan of PD-CNDs, and (c)–(e) fitting curves of C 1s, O 1s and N 1s.

In order to obtain more accurate information in this respect, XPS is conducted. Three sharp peaks at 288.75, 402.51, and 534.75 eV are observed in the XPS survey scan (Fig. 2b), which are assigned to C 1s, N 1s, and O 1s, respectively. The C 1s peak is further fitted to represent the C–C/C=C (284.35 eV), C–N/C–O (285.31 eV), and C=N/C=O (286.36 eV) moieties (Fig. 2c). Upon curve fitting, the O 1s peak (Fig. 2d) validates the presence of functional groups such as C=O, C–O/C–OH, and O–C=O, as envisaged by the peaks at 531.37, 532.47, and 533.14 eV, respectively.²⁰ The N 1s spectral peak (Fig. 2e) consists of three peaks at 398.39, 400.18, and 399.21 eV, representing pyridinic, pyrrolic, and amino nitrogen groups, respectively.²¹ Combining the information from XPS and FT-IR spectroscopy, it is predicted that the carbon framework of PD-CNDs contains pyridinic and pyrrolic nitrogens and surface functional groups including amino, carboxyl, and hydroxyl groups.

Raman spectroscopy was used to further elucidate the nature of the carbon core. Both the D-band at 1368 cm⁻¹ and the G-band at 1545 cm⁻¹, which correspond to the sp² and sp³ hybridized carbon atoms, are visible in the Raman spectrum shown in Fig. 3a. The G-band arises from the in-plane vibrations of sp²-bonded carbon atoms, whereas the D-band reflects structural defects in the graphite framework, caused by substituted carbon atoms, as evidenced by out-of-plane C–C vibrations.²² The zeta-potential measurement was carried out to determine the surface charge of PD-CNDs and was found to be 55 mV (Fig. 3b).

Fig. 3 (a) Raman spectrum of PD-CNDs, (b) zeta potential analysis of PD-CNDs, (c) UV-vis absorption spectrum, (d) photoluminescence spectrum and (e) fluorescence decay curve of PD-CNDs.

UV-visible absorption and photoluminescence spectroscopy were employed to conduct an in-depth analysis of the optical properties of PD-CNDs. Fig. 3c shows the UV-visible absorption spectrum of PD-CNDs. The two peaks at 240 nm and 278 nm are assigned to the π–π* transition that accounts for carbonyl groups and the aromatic –C=C– double bond and –C=N groups, respectively. One broad peak at 510 nm and a shoulder peak at 365 nm were also observed, which arise due to the n–π* transition of various surface functional groups.²³ The aqueous solution of PD-CNDs appeared brownish-red under visible light and emitted orange-red luminescence when exposed to UV light at 365 nm (Fig. 3d inset). The photoluminescence spectrum of PD-CNDs consists of two prominent peaks at 504 nm and 618 nm (Fig. 3d). It is clear from the figure that PD-CNDs are dual emissive in nature. Excitation using different energies reveals that the peak at 504 nm is excitation dependent and its position is redshifted upon increasing the excitation energy and vanishes, while the position of the other peak remains unaltered (Fig. S1, SI). This is typically due to the presence of different emissive traps and various surface functional groups on PD-CNDs.²⁴ The photostability of these carbon nanodots is monitored by continuous exposure to UV light and it is observed that the emission of these particles is noticeably steady in aqueous dispersion (Fig. S2, SI). The quantum yield measured using Rhodamine 6G as the reference was 61%, which is a comparatively good value. Quantum yield values above 50% are considered high-performance and are well-suited for demanding applications. Time-correlated single photon counting (TCSPC) analysis was carried out to measure the average fluorescence lifetime of PD-CNDs. The fluorescence decay profile of PD-CNDs is shown in Fig. 3e. The instrument response function (prompt) is obtained at 368 nm (excitation wavelength) using milk powder suspension (blue circles). The observed lifetime data of PD-CNDs were very well fitted to a mono-exponential function, and the observed average lifetime value of PD-CNDs was 2.40 ns. This indicates a homogeneous population of fluorophores in a uniform environment, with a single, distinct excited state.

3.2. Detection of picric acid

The intense luminescent nature of the system was leveraged to trace the presence of significant analytes. A wide range of chemical entities were employed to investigate the potential changes in the fluorescence intensity of carbon nanodots upon the introduction of these analytes. Here, we monitored the variation in the luminescence intensity of PD-CNDs at an excitation wavelength of 440 nm upon adding certain nitroaromatic and organic compounds such as o-nitrotoluene (o-NT), m-nitrotoluene (m-NT), p-nitrotoluene (p-NT), m-nitrobenzaldehyde (m-NBZA), p-nitrobenzaldehyde (p-NBZA), nitrobenzene (NB), 1-chloro-2,4-dinitrobenzene (CDNB), p-nitrophenol (p-NP), picric acid (PA), p-nitrobenzoic acid (p-NBA), benzoic acid (BA), benzaldehyde (BZA), toluene (T) and aniline (A). The details of the selected analytes are provided in Table S1, SI. The concentrations of all the analytes were fixed at 1 mM. It is quite interesting to notice that the luminescence of carbon nanodots decreases gradually with increasing concentration of picric acid (Fig. 4a), and the corresponding changes in luminescence under UV exposure are depicted in the inset. The selectivity was examined by monitoring the change of the fluorescence intensity of PD-CNDs upon the addition of 20 µL of the analytes. The bar diagram (Fig. 4b) shows that maximum luminescence quenching occurs in the aqueous dispersion of PD-CNDs in the presence of picric acid. A plot of the relative fluorescence quenching efficiency as a function of the concentration of analytes shows good linearity at lower concentrations (Fig. 4c). The Stern–Volmer plot exhibits good linearity in the concentration range of 10–75 µM and the limit of detection (LOD) towards picric acid was estimated to be 17.68 µM.

Fig. 4 (a) Photoluminescence spectrum of PD-CNDs after the addition of PA; inset: quenching of the luminescence visualized under UV light exposure. (b) Selectivity of PD-CNDs towards PA. (c) The Stern–Volmer plot of relative fluorescence quenching as a function of PA concentration.

3.3. Possible sensing mechanisms

Typically, luminescence quenching occurs through various processes such as (a) electrostatic interactions or electron transfer, (b) resonance energy transfer, or (c) the inner filter effect (IFE).^11b,25 The strong acidity of picric acid in aqueous solution is attributed to the presence of three electron-withdrawing nitro groups on the benzene ring. According to Niu et al., the interaction between the electron-rich functional groups on the carbon nanodots and the picric acid moiety involves the formation of a picrate–carbon dot–NH₃⁺ ion pair through strong electrostatic interactions.²⁶ Such interactions are possible in this case. To investigate the possibility of complex formation, the UV-visible absorption spectrum of the aqueous solution of PD-CNDs was recorded in the presence and absence of picric acid solutions (Fig. 5a). An absorption peak appears around 355 nm upon the addition of PA, likely corresponding to the analyte itself, along with slight variations in other peaks. These results suggest that the analyte may form a non-fluorescent complex like the Meisenheimer complex with the electron-rich surface functional groups on PD-CNDs, leading to fluorescence quenching.²⁷ The possible electrostatic interaction and the proposed structure of the non-fluorescent complex are shown in Scheme 1.

Fig. 5 (a) UV-vis spectra of PD-CNDs with and without PA, (b) fluorescence lifetime decay curves, and (c) the combination of the excitation spectrum of PD-CNDs and the absorbance of PA.

Scheme 1 (a) Schematic representation of the interaction between PA and PD-CNDs. (b) Proposed structure of the non-fluorescent complex.

The fluorescence lifetime measurements of PD-CNDs with and without picric acid were performed using the time-correlated single-photon counting (TCSPC) technique. The corresponding decay curve is shown in Fig. 5b. It was noted that the average lifetime remained almost the same even in the absence and presence of the analyte, ruling out the possibility of Förster resonance energy transfer (FRET).²⁸ Meanwhile, the excitation spectrum of PD-CNDs was recorded and a large spectral overlap with the picric acid absorption band was found (Fig. 5c), which indicates that there is a possibility of the inner filter effect (IFE).²⁹ Therefore, the quenching of PD-CNDs caused by picric acid is presumably dominated by mechanisms such as electrostatic interactions/electron transfer and the IFE.

3.4. Theoretical studies

3.4.1. Frontier molecular orbital (FMO) analysis. Frontier molecular orbital analysis was carried out to identify the electron transport pathway in the system under study. This is very important when studying the reactions of charge transfer complexes. According to the FMO theory of chemical reactivity, the interaction between the interacting species, HOMO and LUMO is the reason for the occurrence of transition states in chemical processes. According to FMO theory, a system acts as an excellent electron donor when its HOMO is high in energy and an electron acceptor when its LUMO is low in energy.

One of the main mechanisms in the quenching of nitroaromatics is electron/charge transfer. In this case, nitroaromatics were able to extinguish the fluorescence intensity generated by CNDs. The HOMO and LUMO energies of the CNDs and quencher determine whether the process proceeds in an oxidative or a reductive manner. Transfer occurs via an oxidative pathway where the electron excited from the HOMO to the LUMO of CNDs is de-excited to the LUMO of the quencher, resulting in the quenching of fluorescence if the quencher's LUMO is less energetic than that of CNDs. The electron transfer, on the other hand, might occur via a reductive pathway in which electrons are stimulated from the HOMO to the LUMO of CNDs along with electron transfer from the HOMO of the quencher to the HOMO of CNDs if the quencher's LUMO is significantly more energetic than that of CNDs. In this instance, there should be a negligible energy difference between the HOMOs of the quencher and CNDs. The aforementioned electron transfer will be unlikely if there is a significant energy difference between the two.¹⁵

The optimized geometries of nitroaromatics are shown in Table S2, SI. The energies of the HOMO and LUMO (in eV) of all the nitroaromatics studied are tabulated in Table 1. The E_HOMO and E_LUMO of CNDs can be determined using cyclic voltammetry measurements.^2b,30 Details of the calculations and cyclic voltammograms are provided in the SI and Fig. S3 (SI). From the measurements, the E_HOMO and E_LUMO of PD-CNDs are −6.60 and −4.19 eV, respectively, and the band gap is estimated to be 2.41 eV, which is comparable to the band gap obtained from the Tauc plot (Fig. S4, SI). Comparing these values with those of nitroaromatics, it has been confirmed that the LUMO of picric acid is the only one lower than the LUMO of PD-CNDs. Thus, according to this principle, an oxidative pathway is possible here (Fig. 6), which is impossible for the other analytes under consideration. These findings are consistent with the experimental findings, which also demonstrate that PD-CNDs preferentially sense picric acid over the other nitroaromatics under investigation. Thus, the energy values of the HOMO and LUMO of the CNDs are significant here. The values of these orbitals and the band gap of the CNDs rely on the size and anchored functional groups, among which the latter can be more easily manipulated through the proper choice of precursor molecules. We envision that this attempt would initiate drives to design systems of optimum energy levels, which can function as promising sensors for analytes of significance. In the experimental investigation, p-nitrophenol is sensed by PD-CNDs next to picric acid. From the energy values of the MOs of p-nitrophenol, it is clear that the LUMO of p-nitrophenol is more energetic than that of PD-CNDs, which in turn prevents the oxidative pathway. There is a possibility of electron transfer from the HOMO of p-nitrophenol to the HOMO of the CNDs. Thus, there is a chance of reductive pathway of electron transfer, which may be the reason for its luminescence quenching efficiency.

Table 1 Energies of molecular orbitals (in eV)

No.	Compounds	HOMO	LUMO
1	o-NT	−7.5562	−2.7563
2	m-NT	−7.5507	−2.8434
3	p-NT	−7.6541	−2.7917
4	m-NBZA	−7.8664	−3.2189
5	p-NBZA	−7.9371	−3.5672
6	NB	−7.8963	−2.9223
7	p-NP	−7.2950	−2.7454
8	p-NBA	−8.2010	−3.3985
9	PA	−8.5929	−4.3399
10	CDNB	−8.4242	−3.6679

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Fig. 6 E_HOMO and E_LUMO of PD-CNDs and PA (λ_ex = 440 nm and λ_em = 618 nm).

3.4.2. Global descriptive parameters and the donor–acceptor map (DAM). The use of global descriptive parameters enables a correlation between the chemical reactivity of a molecule and its sensitivity to structural disturbances and reactions to changes in the environment. The chemical potential, electronegativity, hardness, softness, and electrophilicity index are examples of global descriptive parameters. The linear responses of electron density to variations in the external voltage and electron count are represented by these numbers. Chemical hardness is essentially a measure of how the electron clouds of atoms, ions, or molecules are resistant to deformation or polarization under minor perturbations that occur during chemical reactions. The ability of a molecule to accept electrons is measured by its chemical softness, which is inversely proportional to its chemical hardness and more specifically connected to the groups or atoms that make up the molecule. The first derivative of energy with respect to the number of electrons accounts for the chemical potential in DFT, which measures the tendency of an electron to escape the equilibrium. It is also the opposite of electronegativity, which measures the tendency to attract electrons in a chemical bond. The degree of electrophilicity of a species is indicated by the electrophilic index. The following formulas have been used to calculate global reactive descriptors:³¹

Ionization potential (IP) = E_cation − E_neutral (1)

Electron affinity (EA) = E_neutral − E_anion (2)

Hardness (η) ≈ IP − EA/2 (3)

Electronegativity (χ) ≈ IP + EA/2 (4)

Softness (S) = 1/2η (5)

Chemical potential (μ) ≈ −χ (6)

Electrophilicity index (ω) ≈ μ2/2η (7)

The fractional charge transfer that takes place from one molecule to another can be demonstrated using DAMs (see Fig. 7). The necessary parameters electron-accepting power (ω⁺) and electron-donating power (ω⁻) can be calculated as:

ω⁻ = (3IE + EA)²/16 (IE − EA) (8)

ω⁺ = (IE + 3EA)²/16 (IE − EA) (9)

Fig. 7 Reference DAM.

where ω⁺ and ω⁻ indicate a compound's propensity to accept or donate charge. A high value of ω⁺ indicates a strong ability to accept electrons, whereas a compound with decreasing ω⁻ has a strong tendency to donate electrons.

ω⁺ and ω⁻ are more suited to describe the mechanisms of charge transfer, though the IE and EA are also connected to the ability of the compound to donate and accept electrons. This explains the tendency of occasional fractional charge transfer. The DAM displays both the electron-accepting and electron-donating capacities of a compound. The electron accepting index (R_a) and electron donating index (R_d) with respect to the reference are the respective parameters for the DAM, which are calculated as:³²

R_a = ω⁺L/ω⁺R (10)

R_d = ω⁻L/ω⁻R (11)

Table 2 provides the global parameters of the nitroaromatics under study. Fig. 8 displays the related DAM. A reference or parent molecule is essential for plotting the DAM. In the present work, different nitroaromatics were screened for their response towards CNDs. A comparison of their structure reveals that the aromatic ring with a nitro group is common to all the molecules. Thus, nitrobenzene is chosen as the reference or parent molecule to plot the DAM. There are four quadrants in DAM: I, II, III, and IV. The chosen nitroaromatic compounds fall into I, III, and IV, which represent a good electron acceptor, a good electron donor, and a good electron donor/acceptor (they may accept or donate electrons depending on the circumstance), respectively. Only compound 10 (CDNB) exhibits the ability to accept electrons, whereas compounds 1, 2, 3, and 7 (o-NT, m-NT, p-NT, and p-NP) exhibit the ability to donate electrons. All other compounds, with the exception of nitrobenzene, exhibit dual character. All are more reactive than the reference compound nitrobenzene according to the “S” value. Compound 10 (CDNB) is a stronger electron acceptor due to the presence of chlorine. However, molecules 1, 2, and 3 (o-NT, m-NT, and p-NT) are good electron donors because of the presence of the –CH₃ group. Similarly, p-nitrophenol is a suitable donor due to its –OH group. PA falls in quadrant IV, indicating that it can act as an electron donor as well as an electron acceptor depending on the situations.

Table 2 Global reactive descriptors of nitroaromatics (in eV)

No.	Compound	IP	EA	η	S	χ	μ	ω	Ra	Rd
1	o-NT	9.3151	1.1085	4.1033	0.1219	5.2118	−5.212	3.3099	0.839	0.8512
2	m-NT	9.3426	1.17	4.0863	0.1224	5.2563	−5.256	3.3807	0.8638	0.8561
3	p-NT	9.3249	1.1205	4.1022	0.1219	5.2227	−5.223	3.3246	0.8449	0.8534
4	m-NBZA	9.8955	1.6016	4.1469	0.1206	5.7485	−5.749	3.9843	1.1467	0.9976
5	p-NBZA	9.8859	2.0367	3.9246	0.1274	5.9613	−5.961	4.5274	1.285	0.9688
6	NB	9.8353	1.2081	4.3136	0.1159	5.5217	−5.522	3.5341	1	1
7	p-NP	9.0637	1.0541	4.0048	0.1249	5.0589	−5.059	3.1952	0.766	0.7851
8	p-NBA	9.8557	1.8846	3.9856	0.1255	5.8701	−5.87	4.3229	1.2268	0.9689
9	PA	10.121	2.9738	3.5735	0.1399	6.5473	−6.547	5.9979	1.6581	0.9759
10	CDNB	10.024	2.1605	3.9318	0.1272	6.0923	−6.092	4.72	1.3707	1.0039

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Fig. 8 DAM of selected nitroaromatics.

4. Conclusions

In summary, the study introduces fluorescent N-doped carbon nanodots (PD-CNDs) synthesized from p-phenylenediamine for the detection of picric acid. PD-CNDs exhibit orange-red luminescence with dual emission peaks at 504 nm and 618 nm upon excitation at 365 nm. Comprehensive characterization of PD-CNDs yields important information about their optical, morphological, and structural characteristics. The average particle size of PD-CNDs was observed to be 2.8 nm. The orange-red luminescence of PD-CNDs diminishes in the presence of picric acid, enabling its fluorimetric detection at micromolar concentrations. This investigation explores the sensing mechanism, indicating that fluorescence quenching is due to electrostatic interactions, electron transfer, and the inner filter effect (IFE). Additionally, theoretical investigations support these findings. The undeniable role of the band gap of CND systems in their selectivity towards specific analytes is further unveiled here.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available from the authors upon request.

Supplementary information (SI) is available. Optical properties of carbon dots, details of analytes and optimised geometries of analytes. See DOI: https://doi.org/10.1039/d5tc02999f.

Acknowledgements

N. V. acknowledges the University of Calicut for the financial support. The authors are grateful to the CSIF-University of Calicut for providing analytical facilities. The financial assistance received from the DST under the FIST program and the financial assistance obtained under the PAIR programme of ANRF are also acknowledged.

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