Intermolecular through‐space charge transfer enabled by bicomponent assembly for ultrasensitive detection of synthetic cannabinoid JWH‐018

Launching the intermolecular through‐space charge transfer (TSCT) from a bicomponent assembly for photophysical property manipulation is of great significance in fluorescence probe design. Here, we demonstrate the elaborate control of droplet evaporation dynamics for intermolecular TSCT can facilitate the ultrasensitive detection of JWH‐018, a representative synthetic cannabinoid. Driven by diverse intermolecular interactions, the probe, and JWH‐018 assemble in a closely stacked manner to emit strong fluorescence at 477 nm, ascribing to the intermolecular TSCT at the S2 state. The strategy realizes an ultra‐low limit of detection of 11 nmol/mL and great selectivity towards JWH‐018. The practicability is further verified by constructing a sensing chip for JWH‐018 aerosol detection, which facilitates the on‐site drug abuser screening with the naked eye. Moreover, the proposed assembly‐enabled TSCT is expected to find a variety of applications for optoelectronic materials design and photophysical mechanism‐dominated molecular recognition.


INTRODUCTION
During the past decade, the synthetic cannabinoids (SCs), as a family of human-made compounds with thousands of molecular structures designed to mimic the effects of tetrahydrocannabinol (THC) and cannabidiol, have become the new generation of psychoactive drugs and illegally marketed as "Spice" and "herbal blend" worldwide. [1] The SCs bind with the same CB1 [2] and CB2 [3] cannabinoid receptors as THC, but are up to 100 times stronger in potency, leading to serious side effects, such as cardiotoxicity, [4] psychosis, [5] epilepsy, [6] and even suicidal ideation. [7] Although the entire family of SCs is under strict regulation by the authorities, SC-related crimes still occur continually, accounting for a large proportion of drug crimes. In contrast, the detection of SCs was realized mainly in the laboratory by using time-consuming spectrometric and spectroscopic approaches, including high-performance liquid chromatography-mass spectrometry, [8] gas chromatography-mass spectrometry, [9] nuclear magnetic resonance, [10] surface-enhanced Raman spectroscopy, [11] electrochemical method, [12] and so forth. However, their onsite analysis application is severely limited due to the tedious sample preparations, bulky and expensive instrumentations, high technical thresholds, or susceptibility to the interference of coexisting substances. Therefore, it is of great significance for public security to develop a rapid and accurate in-field detection method of SCs with satisfactory sensitivity as an efficient forensic tool for drug crime. [13] Compared with those laboratory techniques, the visual detection methods, [14][15][16][17] namely colorimetry and fluorescence, owing to their direct result readout, portability, easy operation, and maintenance, [18] are preferred for the practical in-field detection but less developed due to the huge challenge of specific probe design towards SCs. The conventional strategy for organic fluorescent probe design is to combine a reactive site as a recognition unit with the organic fluorophore as a signal unit, such as 1,8-naphthimide. [19] Beneficial from the easy synthesis, spatial and temporal resolution, high sensitivity, high selectivity, and easy modification of structure to cover a wide range of absorption/emission characteristics, [20] the organic fluorescent probes have been widely applied in the fields of metal ions, [21,22] anions, [23,24] organic molecule, [25] and biosensing. [26] Upon encountering the target molecule, the reactive recognition process occurs with the chemical bond formation or cleavage, leading to the fluorescence signal variation based on the change of electron/energy transfer, for instance, intramolecular charge transfer (ICT), [27][28][29] twisted ICT, [30,31] photoinduced electron transfer (PET), [32] Föster resonance energy transfer, [33] and so forth. However, for a less reactive target like SCs without a reactive recognition site, the aforementioned active fluorescent probe design strategy with a well-established sensing mechanism doesn't work. Thus, whether an efficient fluorescent probe design strategy with comprehensive consideration of the overall feature of the target molecule by utilizing the noncovalent interaction as the binding force could be achieved, still remains unknown. Furthermore, since (1-pentyl-1Hindol-3-yl)-1-naphthalenylmethanone (JWH-018) is the most representative SCs if it could be rapidly and accurately detected for in-field application, the exploration of the fluorescent sensing methodology of the entire SCs family would undoubtedly be enlightened.
Molecular assembly is a ubiquitous natural phenomenon, which has been widely applied in the field of molecular recognition, [34,35] biological imaging, [36][37][38] catalysis, [39] drug delivery, [40] and odor adsorption, [41] owning to the spontaneous association of molecules with the non-covalent interactions playing a crucial role. Driven by the overall intermolecular interactions, the dispersive elements get together initiatively under certain conditions to form more advanced, stable, and structurally well-defined aggregates with high internal order, resulting in diverse properties and functionalities. [42] The non-covalent interactions involved, including intermolecular hydrophobicity, hydrogen bonding, ion pairing, metal-ligand interaction, aromatic π stacking, [43,44] and so forth, are generally much weaker than covalent bonds apiece, but strong enough as a whole to build up assemblies. [45] Particularly, the self-assemblies could present entirely different photophysical properties compared with the dispersive elements, such as aggregationinduced emission (AIE), [46][47][48][49][50][51] aggregation-caused quenching (ACQ), [52] aggregation-enhanced emission (AEE). [53] The underlying mechanisms for these phenomena could be addressed as the electron or energy transfer process which only occurs in the assembled states, such as restriction of intermolecular motion, [54] metal-ligand charge transfer, [55] through-space conjugation, [56] through-space charge transfer (TSCT), [57] and so forth. It has been proven to be possible to utilize the self-assembly process and the corresponding photophysical properties to achieve accurate optical sensing of non-reactive targets by the fine-tuning of the specific noncovalent interactions through precise modulation of the probe molecular architecture. [58] For instance, the accurate and sensitive detection of the non-reactive perchlorate anion has been realized by the precise regulation of the self-assembly process via interaction control by modulating the alkyl chain length of the oligo(p-phenylenevinylene) derivative AIE probe, [59] and the ancillary ligand of the water-soluble Pt(II) terpyridyl complex AEE probe. [60,61] Furthermore, other than molecular architecture modulation, the molecular self-assembly behavior of the nonvolatile solute could be further regulated by the control of molecular motion or diffusion dynamics induced by the co-occurred droplet evaporation. [62,63] As an open thermodynamic system for mass, momentum, and energy transfer [64] with three phases involved, the evaporation of a sessile droplet on a solid substrate is a diffusion-controlled process affected by the solvent composition, surrounding atmosphere, the wetting properties of substrates, external forces, and so forth. [65] Effective regulation of the droplet evaporation dynamics and the simultaneous self-assembly has been achieved to obtain well-defined morphology and patterns, such as coffee rings, [66] yet study on utilizing the droplet evaporation to manipulate photophysical properties of the self-assembly and its sensing application is rare.
Here, we propose an intermolecular TSCT strategy, a brand-new fluorescence turn-on detection method, enabled by the bicomponent assembly to encounter the ultrasensitive detection demand for JWH-018. The intermolecular interactions, which are composed of H-bonding, π-stacking interactions, as well as the van der Waals force, as a whole drive the tailored probe and JWH-018 in the solution to assemble in a parallel manner at the phase boundary during the droplet evaporation process with dynamic control. The probe designed accordingly demonstrates bright greenish blue fluorescence towards JWH-018 with ultra-sensitivity and great specificity, compared with commonly abused traditional drugs, new psychoactive substances, and analogs.

RESULTS AND DISCUSSION
It is expected that driven by the intermolecular interactions, the disturber molecule, namely the target molecule, introduced to the ACQ fluorophore could form a closely packed dimer with the fluorophore within a distance of 2-4 Å, which is essential for the intermolecular molecular orbital overlap and TSCT enhanced fluorescence emitting ( Figure 1A). With excitation, the molecule transit from the ground state to the S 2 state, where the intermolecular TSCT between the two molecules occurs ( Figure 1B). Followed by the internal conversion (IC) from the S 2 state to the S 1 state, where the prominent transition dipole moment is formed, the molecule returns to the ground state to dissipate energy in a non-radiation way to emit fluorescence. In this case, the 4-hydrazine-N-butyl-1,8-naphthalimide (Figures S1-S3), namely the naphthalimide derivative probe, and JWH-018 were taken as an example to demonstrate the theory proposed above ( Figure 1C). The naphthalimide derivative probe is precisely designed to target the structural characteristics of JWH-018 to provide adequate driving forces for assembly formation. Specifically, 1) the naphthalimide core for sufficient π-π stacking with the indole ring of JWH-018, 2) the hydrazine group to form multiple hydrogen bonding with the carbonyl in JWH-018, and 3) the butyl chain for van der Waals interaction with the alkyl chain in JWH-018. The amorphous assembly of the naphthalimide derivative probes and JWH-018 molecules is achieved through a precisely controlled droplet evaporation process on a hydrophobic substrate, with the molecule pairs of the probe and JWH-018 as the repeating unit to form a short-range ordered multiple stacking structure ( Figure 1D).
In the FE-SEM images, it was observed that upon evaporation the pristine probe forms uniform spherical particles with an average diameter of 0.13 μm ( Figure 1E), while the JWH-018 forms spherical particles with bimodal size distribution in the range of 0.1-0.7 μm ( Figure 1F). In striking contrast, the mixture of the probe and JWH-018 forms irregularly shaped islands with crumpled surface morphology and an average diameter of around 6 μm ( Figure 1G), which is more than 10 folds larger than those of the probe and JWH-018, demonstrating the obvious morphology difference brought by the bicomponent assembly.
The assembly exhibits bright greenish blue-fluorescent emission at 477 nm ( Figure 1H), standing in sharp contrast to the probe and JWH-018 apiece, which show barely visible emission under the identical condition ( Figure S4). It is noteworthy that compared with that of the probe, the fluorescence emission of the assembly was not only dramatically enhanced over six times but also blue-shifted for 95 nm, corresponding to the obviously increased energy gap resulting from the S 1 →S 0 state transition. In addition, the assembly formation and its fluorescent emission remained the same with and without continuous UV light irradiation (Videos S1 and S2), demonstrating that there is no significant influence of the UV light on the assembly between JWH-018 and the probe. Besides, neither a new spot in thin layer chromatography analysis nor vibration band change in the IR spectra was observed, further proving that the fluorescent product is the assembly formed through intermolecular interactions instead of a new compound formed through chemical bonding ( Figures S5 and S6). Therefore, the intermolecular TSCT could be enabled by the bicomponent assembly of JWH-018 and the naphthalimide derivative probe with sufficient intermolecular interactions upon controlled evaporation to produce the blue-shifted fluorescence emission.
To get a clear picture of the formation mechanism and the fluorescence origin of the assembly, computational studies of the optimized dimer were conducted. In the IGMH visualized analysis based on the electron density gradient norm function, it is observed that a continuous green isosurface All excited-state calculations were conducted at the CAM-B3LYP/Def2-SVP level in methanol with wavefunction analysis with the assistance of Multiwfn [69] appears in the entire space between the naphthalene ring of the probe and the indole ring of JWH-018, extending to the alkyl chains region, indicating the relatively strong and broad intermolecular interactions (Figure 2A). The detailed characterization of these interactions was quantitatively conducted via Atoms-In-Molecules topological analysis as intermolecular attraction forces with the bond critical point (BCP) and a bond path between the attractive atom pairs marked as the orange spheres and lines, respectively ( Figure 2B). Fourteen independent BCPs can be found between the two molecules with the total energy per electron values (H(r)/ρ(r)) [67] ranging from 0.08 to 0.28, which quantitatively evaluate the strength of the interaction in the manner that smaller H(r)/ρ(r) represents stronger interaction ( Figure 2C). The first four BCPs (1-4) with H(r)/ρ(r) ranging from 0.15 to 0.23 represent the multiple hydrogen bonds with medium strength between the hydrazine group of the probe and the carbonyl oxygen of JWH-018. The strong face-to-face aromatic ππ stacking interaction between the naphthalene ring of the probe and the indole ring of JWH-018 is indicated by the following four BCPs (5-8) with H(r)/ρ(r) in the range of 0.08-0.11. The last six BCPs (9)(10)(11)(12)(13)(14) are located between the alkyl chains of the probe and JWH-018, with H(r)/ρ(r) in the range of 0.13-0.28, demonstrating the moderate van der Waals interaction between the two molecules. Therefore, all these three types of intermolecular interactions as a whole serve as the driving force for the effective assembly of the probe and the JWH-018. To further investigate the fluorescence mechanism of the assembly, the electron-hole distribution of the dimer at the excited states was calculated along with the corresponding charge transfer distance (D), [68] namely the distance between the centroids excited hole and electron distribution ( Figure 2D). It is observed that both holes (blue region) and electrons (green region) concentrate in the conjugate plane of the probe with a D value of 1.212 Å, demonstrating that the S 1 of the dimer is a typically located excitation (LE) state, in which the probe is predominant. Meanwhile, for the S 2 state, the electrons and holes parallelly distribute in the conjugate planes of the probe and JWH-018 respectively, resulting in the charge transfer from JWH-018 to the probe with a remarkable D value of 3.063 Å, which is approximate to the intermolecular distance of 3.04 Å, revealing the intermolecular TSCT nature of the S 2 state. Although a certain distribution of holes appears around JWH-018 in S 3 , the D value of 1.098 Å resulting from the overlapped electron-hole distribution indicates that S 3 is also a LE state. Subsequently, the energy in S 2 with TSCT transmit to S 1 via IC and enhance the emission of the S 1 →S 0 , which is the root cause of the fluorescence emission.
In addition, it is found that the positive (yellow) and negative (purple) regions of the transition dipole moment of S 1 are dissociated and coherent through the conjugate plane of the probe (Figure 2E), leading to an overall transition dipole moment of 2.691 pointing towards the alkyl chain ( Figure 2F), which yields the high oscillator strength for bright fluorescence emission. Furthermore, the same computational studies have been carried out for the molecular pairs between the probe and other JWHs with similar structures, including JWH-071, JWH-016, and JWH-250 ( Figures  S7-S9), demonstrating that the intermolecular TSCT exists universally in the S 2 state for these similar target structure with relatively high oscillator strength for bright fluorescence emission (Table S2). Therefore, it is proved theoretically that intermolecular TSCT, enabled by the close packing of JWH-018 and the probe, is the origin for the enhanced and blue-shifted fluorescence emission of the bicomponent assembly.
It is expected that the proper assembly of the naphthalimide probe and JWH-018 with fluorescent emission only occurs under thermodynamically controlled conditions. During droplet evaporation, owing to synergistic diffusion caused by capillary flow and Marangoni flow, [70,71] the solute enrichment at the three-phase boundary leads to a boosted molecular collision, allowing the intermolecular interactions induced assembly (Figure 3A left). It generally experiences three distinguishable stages with a continuous inward motion of the phase boundary, among which stage III is primary for the assembly due to its higher solute concentration ( Figure 3A right). Hence, a relatively uniform fluorescent assembly with maximized dimension and limited coffeering effect, could be achieved eventually by the cooperative dynamic control of the speed for the assembly, the boundary shrinking, and the solvent evaporation.
As a detailed demonstration, the evaporation-assembly process of a 20 μL mixture droplet consisting of JWH-018 and the probe at room temperature was investigated with the time-lapsed fluorescent images ( Figure 3B and Video S1). It is observed that during the first stage (0-124 s), the non-emissive transparent droplet remains the similar size of 5.1 mm as the initial state, while the droplet becomes offwhite fluorescent and turbid, which could be attributed to the precipitation of JWH-018 ( Figure S10) and drastically shrunk to half of the initial size during the second stage (124-352 s). In the third stage (352-1016 s), the assembly continuously accumulated at the shrinking phase boundary caused by the hydrophobic substrate, resulting in the relatively uniform greenish blue-fluorescent pattern with a diameter of ∼2 mm leftover on the substrate as the final state after complete evaporation. Meanwhile, although both the probe and JWH-018 experience a similar three-stage process with a fluorescence emissive intermediate state, neither results in an emissive pattern left over as the assembly.
Moreover, the three-stage diameter variation with contact angle changed accordingly is similar to the evaporation process of the ideal binary solvent system, [72] which may be due to the joint impact of the involatile mixed solutes and the hydrophobic nature of the substrate ( Figure 3C and Figures S11-S13). In addition, the fluorescent R/G/B values followed the exact same three-stage course as the droplet diameter with an opposite trend during the evaporation process ( Figure 3D). During stage III, in which the greenish bluefluorescent assembly started to form and accumulate, the B value undergoes the greatest growth, while the R-value stays at the lowest among the R/G/B curves. Thus, the ΔB/R value was chosen as the characteristic indicator for the formation of the assembly, which remained below 0 before approaching the final stage and raised sharply to a final value of 1.1 ( Figure 3E).
Furthermore, the influence of the evaporation temperature on the assembly formation was investigated to manipulate the evaporation dynamic condition ( Figure 3F). It is observed that by raising the temperature from 25 • C to 100 • C, the time course for the evaporation-driven self-assembly process reduced gradually from 1000 s to an instant. While the temperature rose to 120 • C, no assembly products formed at all as a result of the instant solvent vaporization with the absence of stage III. The 3D colormap surface analysis reflects that with the raising of temperature from 25 • C to 40 • C, the final fluorescent pattern remains the similar intensity, but the area of the pattern decreased dramatically over 60% ( Figure S14), which is undesirable for the acquisition of the fluorescent sensing signal. Thermodynamically, with elevated temperature, the molecular motion could be accelerated, which is beneficial to the reduction of the entire time course for all three stages but unfavorable for the formation of assembly. Therefore, it can be concluded that the three-stage evaporation of the droplet on the hydrophobic surface at room temperature with a balanced assembly rate and evaporation rate, is decisive to the proper bicomponent self-assembly of the probe and JWH-018 with satisfied intermolecular distance for fluorescent emission.
Fluorescence turn-on sensing performances of the probe for JWH-018 via the evaporation-driven bicomponent assembly process with a droplet volume of 20 mL were further evaluated. The assembly formed with different concentrations of JWH-018 showed a gradual change from non-emissive to bright greenish-blue emission with the concentration increasing from 0 to 1460 nmol/mL ( Figure 4A and Figure S15). Particularly, with the JWH-018 concentration as low as 11 nmol/mL, equivalent to 78 ng, distinguishable greenish blue-fluorescent spots were observed on the edge of the pattern, with significantly increased G value of over 150 compared with the background (Figure 4B). With the progressive increase of the JWH-018 concentration, the G-G 0 value increased rapidly from 0 to 37 linearly first within the concertation range of 11-180 nmol/mL, and then slowly rose to 48, demonstrating the direct correlation between the fluorescent signal and the target concentration as well as the good sensitivity of this method ( Figure 4C).
From the fluorescent response images of the probe towards JWH-018 and common traditional drugs (heroin, cocaine, morphine, marijuana, opium, ecstasy, ketamine, and meth), new psychoactive substances (PCP, methadone, barbital, fentanyl, tryptamine, alprazolam, 5F-MDMB-PICA, and 5F-ADB), as well as the structural analogs (JWH-017, naphthalene, and indole) with chemical structures shown in  Figure S16, it can be observed that the probe has a good selectivity towards JWH-018 due to the unique greenish bluefluorescence ( Figure 4D and Figure S17). It should also be noted that the JWH-071 with a very similar structure as JWH-018 but a short alkyl chain, also exhibited a similar fluorescence. However, through further comparing the ΔB/R values of these fluorescent images, it is observed that only JWH-018 leads to a positive ΔB/R of 0.62, while all other drugs and analogs, could only cause the negative ΔB/R no matter if they could form the fluorescent emissive products with the probe or not. For instance, some traditional drugs, such as heroin, cocaine, and ecstasy, and some new psychoactive substances, such as PCP, fentanyl, tryptamine, and alprazolam, could also yield green-fluorescent products, but their ΔB/R values are all in the negative region, allowing the facile discrimination of JWH-018 from them. It is noteworthy that even JWH-071 could be distinguished from JWH-018, indicating that a slight structural difference could influence the molecular packing, which is a determinant for the electron state, and hence the photophysical property of the assembly, demonstrating that the intrinsic blue-fluorescent signal of the assembly induced by JWH-018 is highly reliable. Therefore, the proposed bicomponent assembly of the naphthalimide derivative probe and JWH-018 under certain circumstances with fluorescence emission is demonstrated to be an effective turn-on sensing method for JWH-018, which is much superior to existing methodologies in terms of outstanding specificity, low instrument dependency, portability, free of pretreatment, good simplicity, and fine sensitivity (Table S3), thus holding great promise for generalized practical in-field application.
To make this fluorescence sensing method applicable for rapid on-site detection of JWH-018, reducing the droplet size is considered the major path to accelerate the three-stage evaporation with retentive thermodynamic conditions for the assembly formation with the characteristic fluorescence emitting. According to the Maxwell-Fuchs equation, [73] compared with big droplets, owning the higher specific surface area and hence the promoted interphase solvent diffusion, the smaller droplet generally has a drastically accelerated evaporation process under identical thermodynamic conditions ( Figure 5A). Considering the presence of the drug residue in the breath blew aerosol, [74] a portable sensing chip for trace JWH-018 detection via breath sampling is fabricated by spotting deposition of the probe on the hydrophobic substrate to form the sensing area with the probe array in "SC" pattern ( Figure 5B). Once the JWH-018 abuser blows onto the device, the microdroplets carried by breath are collected on the sensing chip, which was mimicked by the spraying of JWH-018 solution experimentally to accelerate the evaporation-driven bicomponent assembly with the characteristic emission to light up the "SC" pattern as a positive output ( Figure S18).
By the continuous observation of the evaporation course of the droplets with different initial diameters, it is revealed that these tiny droplets all experienced the identical three-stage evaporation procedure with bicomponent assembly occurring but a much-shortened evaporation time compared with the big droplets ( Figure 5C and Figure S19A). For instance, for the droplet with a diameter of 0.33 mm, the time for evaporation to form the emissive assembly is 32 s, which is only 1/30 of that of the big droplet with a diameter of 5.1 mm. Besides, the evaporation times of the assembly formation, are in a good linear relationship with the initial droplet diameter ( Figure S19C), demonstrating the effectiveness of the droplet size reduction.
Moreover, the sensing performance of the chip was evaluated by JWH-018 aerosols with different concentrations. Remarkably, even with JWH-018 concentration as low as 29 nmol/mL, the characteristic fluorescent signal still appeared obviously after complete evaporation ( Figure 5D), demonstrating the good sensitivity of the sensing chip. The corresponding line scan of the characteristic B/R values carried out for the fluorescent images indicates that with the increase of the JWH-018 concentration, a higher B/R value with a larger signal area could be achieved ( Figure 5D). Besides, the interferents most commonly found in the saliva aerosol, namely ethanol, water, and acetone, could not lead to any fluorescence turn-on phenomenon ( Figure S20), indicating the effectiveness of this sensing chip for the direct visual on-site detection of JWH-018 breath sample with decent specificity.
Furthermore, we established a straightforward logical flow for the discrimination of JWH-018 based on the final-state fluorescent color signal analysis, which could be integrated with a smart screening device for intelligent on-site screening ( Figure 5E). The R/G/B values of the sensing region extracted from the fluorescent image are filtered with the criteria of "R > 10, G > 10, B > 10" to exclude the background noise first. Then, the B/R value over 1.8 is used as the standard for the detection of JWH-018, which is 12.5% higher than the average B/R value of 1.6 for the probe to ensure accuracy ( Figure S21). The logical flow for JWH-018 discrimination is inspected with a data set of 114 fluorescent images, including the blank probe, the sensing results of JWH-018 with various concentrations, and the sensing results for other drugs, proving that the criteria of B/R > 1.8 are effective despite of the concentration variation ( Figure 5F). Therefore, with minimized droplets, the evaporation time for proper bicomponent assembly, namely the sensing time for JWH-018, could be significantly shortened with retentive fluorescent signals in tens of seconds, providing the practical feasibility for sensing chip construction and its application for JWH-018 detection in respiratory gas.

CONCLUSION
In summary, this work presented a fundamental investigation of how to manipulate the fluorescence of the bicomponent assembly by launching the intermolecular TSCT at the exited state. Multiple intermolecular interactions as a whole serve as the driving force for the assembly of the naphthalimide probe and JWH-018 in a parallel manner, which enables the intermolecular TSCT at the S 2 state and hence the strong greenish-blue fluorescence emission at 477 nm. This assembly upon well-regulated droplet evaporation is demonstrated as an effective turn-on visual sensing strategy for JWH-018 with decent sensitivity (11 nmol/mL) and outstanding specificity over similar 19 drugs. This method is verified to be practicable for the on-site detection of JWH-018 in respiratory gas with a sensitivity of 29 nmol/mL by using the mimic aerosol spray, which could significantly facilitate drug abuser screening. Although the present exploration of the bicomponent assembly-enabled intermolecular TSCT is a proof-of-concept study, we expect this to provide a universal guiding inspiration for photophysical mechanismdominated molecular recognition benefited by intermolecular interactions for other non-reactive chemical species, the optoelectronic materials design, assembly-based functional material conceptualization, and so forth.

Experimental details
Materials, synthesis, and characterization of 4-hydrazo-Nbutyl-1,8-naphthimide, single crystal preparation and X-ray diffraction, hydrophobic treatment of the glass substrate, preparation of the naphthalimide probe solution and JWH-018 solutions with different concentrations for sensing test, sample preparation for SEM, video recording and data processing for single droplet evaporation process, selectivity evaluation procedure, the evaporation of droplets in different sizes, preparation of the sensing chip, the detection procedure of the sensor chip towards JWH-018 spray with different concentrations, the acquisition and data processing of the fluorescent images and videos, are all placed in the Supporting Information.

Calculation methods
The theoretical calculation in this work was conducted through the Gaussian software. [75] Subsequently, the wave function results were performed by the Multiwfn dev3.8. The VMD program [76] was used in the graphing of the molecular structure. More details of the calculations are placed in the Supporting Information.

A C K N O W L E D G M E N T S
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (52172168)

C O N F L I C T O F I N T E R E S T
The authors declare that they have no conflict of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.

E T H I C S S TAT E M E N T
Ethical approval is not applicable for this article as no animal or human experiments are involved.