Chemosensing Test Paper Based on Aggregated Nanoparticles of a Barbituric Acid Derivative

The development of sensitive, cheap, and portable methods for detecting nitroaromatics explosives has a profound significance and value for public health and environmental protection. For this purpose, a new D-π-A barbituric acid derivative CB-CYH with aggregation-induced emission (AIE) behavior was synthesized, which can interact with picric acid through photoinduced electron transfer (PET). Scanning electron microscopy (SEM) and dynamic light scattering (DLS) indicate that the enhanced emission of the compounds is related to the formation of nano-aggregates. It is well known that an important source of mechanochromic fluorescence (MCF) characteristic materials is the compound with AIE characteristics. The chemosensing test paper prepared by aggregated nanoparticles based on AIE properties is often subjected to external friction or squeeze during transportation or storage, resulting in changes of their optical properties, and destruction of test paper followed. Therefore, the development of compounds with AIE properties and stable optical properties in the presence of external stimuli is particularly important for chemosensing test paper. Molecular dynamics simulation (MDS) shows that the presence of hydrophobic cycloalkyl group in CB-CYH, which caused the molecules to be closely interspersed with each other; hence, it is difficult to change the microstructure and stacking mode of molecules by external stimulation simultaneously; the optical properties are not changed by external stimuli. Therefore, the test paper based on the AIE effect of CB-CYH was developed as chemosensing test paper for the detection of nitroaromatics.


Introduction
The problem of human health is threatened by explosive residues from terrorist bombings, and military exercises are becoming more and more serious. The nitroaromatics are commonly used as explosive materials, and the methods of detection of nitroaromatic explosives are various [1], such as metal detectors [2], thermal neutron analysis [3], electrochemical assays [4], Raman spectroscopy [5], gas chromatography [6], X-ray imaging [7], and mass spectrometry [8]. However, the disadvantages are obvious, such as time-consuming, or expensive, or complicated processing. So many researchers turn their attention to photoluminescence (PL) sensor [9][10][11][12], which is a low-cost with high-sensitivity, easy-to-operate, and a new type of very promising method. The PL sensor is usually based on a photoinduced electron transfer (PET) mechanism that is from an electron-rich group of the sensor to an electron-deficient nitroaromatic. However, the practical application of traditional PL sensor is hindered by the phenomenon of aggregation quenching (ACQ) [13].
Since Tang's group [14][15][16] discovered compounds with aggregate-induced emission (AIE) feature, more and more people have paid attention to this; these compounds can not only be widely used in luminescent polymers, explosive detection [17,18], mechanochromic fluorescence [19], optoelectronic materials, biosensors, and gels but also successfully solve the ACQ problem [20][21][22][23][24][25][26][27][28]. Compounds with AIE properties are nonluminous when dissolved in a good solvent but are completely opposite in the aggregated state. The reason for strong emission in the aggregate state is that the formation of aggregated nanoparticles restricted intramolecular rotation (RIR), the nonradiative channels are blocked and radiation paths are opened [29]. PL sensors based on AIE have successfully solved the problems of traditional sensors with ACQ properties.
Chemosensing test papers are widely used due to their portability, low price, and easy storage [30][31][32]. It is well known that an important source of mechanochromic fluorescence (MCF) characteristic materials is the compounds with AIE characteristics. Chemosensing test paper based on AIE properties is often subjected to external friction or squeeze during transportation or storage, resulting in changes in their optical properties. Fortunately, a new D-π-A barbituric acid derivative CB-CYH with aggregation-induced emission (AIE) behavior was synthesized, of which the optical properties are not changed by external stimuli. Hence, the chemosensing test paper was prepared by aggregated nanoparticles of CB-CYH, which was developed for the detection of nitroaromatics.

2.2.
Characterizations. UV-vis spectra were registered by UV-2500 spectrometer. Emission spectra were recorded on an F-4600 fluorescence spectrophotometer. 1 H NMR (400 MHz) spectra were recorded on AVANCE II 400 spectrometer. FT-IR spectra were measured by a Nicolet 380 spectrometer using the KBr pellet method.

Computation.
The optimizations and electronic structure calculation for the CB-CYH were done at B3LYP/def2-SVP level with Gaussian09 software. Solvation effects were incorporated by molecular dynamics simulations (MDS) [33] solvation models with water as solvent. The dispersion correction was conducted by Grimme's D3 version with BJ damping function [34].
Eight optimized molecules of CB-CYH were randomly dissolved in 400 water molecules, respectively, to form initial systems with the same cubic simulation lattice (x = 28:89 Å, y = 28:89 Å, and z = 28:89 Å). Subsequently, all-atomic molecular dynamics simulations (MDS) were performed for the system in BIOVIA Materials Studio 2019 Forcite packages. The simple point charge (SPC) model [35,36], which can accurately describe the water solution environment [37], is adopted for all water molecules.
The MD simulation of all systems can be performed after charges and potentials are assigned to each atom. The longrange electrostatic interactions have been accounted for using the Ewald method. The total energy is written as a combination of valence terms including diagonal and offdiagonal cross-coupling terms and nonbond interaction terms, which include the Coulombic and Lennard-Jones functions for electrostatic and van der Waals interactions, where E VDW and E elec are given by the Eq. (2): The MDS was performed using the COMPASS force field [38,39], which is a force field for atomistic simulation of common organic molecules based on the state-of-the-art ab initio and empirical parametrization techniques. The simulations were equilibrated at constant temperature (298.15 K) and volume (NPT) for 30 ns.

Results and Discussion
3.1. Optical Properties. The absorption and emission spectra for CB-CYH are presented in Figure 1(a). The maximum absorption wavelength of CB-CYH was at 421 nm, which can be attributed to the intramolecular charge transfer (ICT) [42] from the carbazole group to the barbituric acid group, which can also be confirmed by density functional theory (DFT) calculations (Figure 1(b)). It should note that the absorption peak at 230-350 nm belongs to the π-π * electron transition of the corresponding benzene ring unit. In dilute THF solution, CB-CYH showed dim emission with emission wavelengths at 517 nm.
The AIE property of CB-CYH was studied in THF/H 2 O system. As shown in Figure 2(a), the emission intensity of CB-CYH increased by 4.4 folds with f w increased from 0% to 80%, because of the formation of nano-aggregates; however, as the f w increases further, the emission intensity decreases, which is due to the the low solubility of CB-CYH at high water content, resulting in rapid precipitation of the compounds, the total number of emission molecules decreases in the solution, and fluorescence reduction follows [43]. Fluorescent photograph of CB-CYH in THF/H 2 O mixtures with different f w , irradiated by 365 nm wavelength also demonstrate that CB-CYH is AIE active compound.
The fluorescence quantum yields of pure solution and aggregate state are determined relative to coumarin 307 in ethanol solution as a quantum yield standard (ΦF ðfluorescence quantum yieldÞ = 56%). The coumarin 307 is selected as a standard to study the ΦF due to its maximum absorption and emission wavelength (395 nm and 500 nm, respectively), which are close to the synthesized AIE compounds in this work; the fluorescence quantum yields of compound in the pure solution and aggregate state are 1.8% and 7.4%, respectively.
Scanning electron microscopy (SEM) and dynamic light scattering (DLS) were used to detect the morphology and size of nano-aggregates in f w = 80% (Figure 3) which show the presence of spherical and blocky aggregates. As shown in  3 Journal of Nanomaterials Figure 3, the solution was uniformly stable at a water content of 80%, and the average diameters (d) of the CB-CYH aggregates was 180 nm, respectively. These data indicate that the enhanced emission of the compound is related to the formation of nano-aggregates.
As shown in Figure S5, The fluorescence intensity of CB-CYH (f w = 80%) did not change within 30 minutes. These results mean that CB-CYH (f w = 80%) has excellent photo-stability. In addition, the average diameter (d) of CB-CYH aggregate has basically no change within half an hour ( Figure S6).
It is well known that an important source of mechanochromic fluorescence (MCF) property materials are compounds with AIE properties. However, it can be seen from Figure 4     Journal of Nanomaterials the XRD diffraction peak of CB-CYH almost unchanged (Figure 4(b)). It can also be seen from Figure 4(c) that the emission spectrum of CB-CYH has almost no change before and after grinding, regardless of the emission wavelength or intensity. The structure of the CB-CYH showed strong distortion, but not MCF property, which inspired our intense curiosity.

Molecular Dynamics Simulations (MDS).
The behavior of CB-CYH in the aggregated state was studied by MDS. As shown in Figure 5(a), all the molecules of the formed nanoaggregates together under high water content, and at the same time, the structure of CB-CYH shows a strong deformation. Among them, the electron repulsion and strong intermolecular interactions between aryl rings in carbazoles lead to extremely curved configurations ( Figure 5(b)), and from the distribution of dihedral angles between several moieties, the main dihedral angles of angle-1, angle-2, and angle-3 are 50°, 155°, and 155 o , respectively. Meanwhile, the cyclohexane on the N atom of the barbiturate acid exhibits extremely distorted chair conformation, which further increases the degree of distortion of the molecule CB-CYH ( Figure 5(c)). These data indicate that the molecule takes 3D conformation, which makes it easy to pack closely in the crystalline state. As shown in Figure 4(d), because of hydrophobic interactions, the substituents cyclohexane in CB-CYH tend to aggregate together. Hydrophobic interactions pull all molecules together and spread each other, which formed aggregated nanoparticles to lock the molecules. This also proves why CB-CYH does not have MCF property. 5 Journal of Nanomaterials the PA test was performed at solution state. As shown in Figure 6(a), the test paper emits bright blue-green fluorescence without adding PA solution, when adding 0.31 g/L PA solution, the test paper shows a slight quenching. It is found that the quenching efficiency increases with the increase of PA concentration, when the concentration reaches to 5 g/L, the area of adding PA solution is completely quenched. Next, perform a steam mode test. Place the chemical sensing test paper in a cylindrical glass container filled with solid PA (0.2 g) maintained at a constant temperature  Journal of Nanomaterials of 45°C for 5 minutes (under standard atmospheric pressure). The circular area of the test paper was exposed to PA vapor. It can be seen from Figure 6(b) that the fluorescence of the test paper is significantly quenched in the area exposed to the PA vapor. These results showed that CB-CYH-based chemosensing test paper can detect picric acid.
3.3.1. Sensing Mechanism. As shown in Figure 7, the absorption spectrum of PA and emission spectrum of CB-CYH were tested, which to explore the quenching mechanism. It noticed from Figure 7(a) that the absorption spectrum of PA and the emission spectrum of CB-CYH hardly overlap, and the results showed that the quenching mechanism of CB-CYH was photoinduced electron transfer (PET) [44][45][46]. In explosives containing nitro groups, which has a strong electron-withdrawing ability, resulting in that is extremely deficient in electrons. However, the fluorescent compound as the electron-donor is rich in electrons, and it is easy to interact with the electron-deficient substance by charge transfer interaction and bind under the condition of light irradiation. In the process of the photoinduced electron transfer (PET) mechanism, the fluorescent material provides electrons to the ground state of the electron-deficient nitroaromatic explosive, and part of it will return to the ground state in the form of a complex, thus losing fluorescence. Some electrons will return to the electron donor and still emit fluorescence. The driving force for the PET mechanism is the difference between the LUMO energy levels between the donor and acceptor, namely, the size of the energy gap difference.
As shown in Figure 7(b), the energy levels and electron cloud distributions of the HOMO and LUMO of TNT and CB-CYH were calculated by the B3LYP/6-325 31G (d) program. When the excited CB-CYH is exposed to TNT, the excited electrons are transferred from the LUMO of CB-CYH to the LUMO of PA. The main driving force of PET is the difference between the LUMO value of CB-CYH and PA, which is -1.924 eV.

Conclusions
In summary, the chemosensing test paper was prepared by aggregated nanoparticles of CB-CYH, which was developed for the detection of nitroaromatics. MDS showed that CB-CYH with a strong twist structure cannot cause changes in fluorescence, color, and emission wavelength by external stimuli due to the close interpenetration between molecules. Hence, the chemosensing test paper will not be damaged by squeeze and friction during transportation and storage.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.