“Turn-on” fluorescence sensor for organic amines fabricated via sustainable processing

Abstract

The detection of organic amines is a pivotal issue in chemistry. Here we present the synthesis of a novel fluorescence probe with “turn-on” greenish emissive (486 nm) response towards a series of organic amines for the first time with di-catechol moiety-based sensing materials. This sensor was fabricated from a 1,5-disubstituted naphthalene scaffold that was prepared via a sustainable process without the use of column chromatography or recrystallization. We reveal its dual-mode sensing capability covering visual and quantitative recognition for organic amines with a detection limit down to sub-ppm level. This system may find a broad spectrum of potential applications for monitoring organic amines in various fields, such as bioscience, biomedicine, pharmaceuticals, food science and environmental chemistry.

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Introduction

Organic amines are some of the most common compounds, whose basic structure could be ascribed to ammonia substituted with organic groups, and they have a highly significant role in biomedicine, pharmaceuticals, food science and environmental chemistry. The formation of volatile organic amines has proved to be associated with metabolic disorders, lung lesions, and kidney disease.^1,2 Small-molecule drugs containing amino groups such as Revlimid®, Imbruvica®, and Eliquis® are among the most consumed drugs in recent years. From another aspect, biogenic amine is always handled from toxicological viewpoints in food science, as ingesting excess amount of biogenic amine might induce foodborne diseases.^3,4 Monitoring of organic amines within the atmosphere, soil and water bodies deserves continuous research efforts in environmental science.^5,6 Thus, the development of an efficient detection method for organic amines in both gaseous and solution phases is of great significance.

Numerous methodologies, mainly covering chromatographic,^7–9 electrochemical,^10–13 colorimetric^14–17 and fluorescent approaches,^18–20 have been put forward to date for organic amine detection. Among these feasible protocols, the fluorescent sensor is remarkably appreciated for its high sensitivity and convenience in avoiding sophisticated pretreatments or expensive instrument operation. We could generally classify the fluorescent organic amine sensors into “turn-off” and “turn-on” type according to the reinforcement or attenuation of fluorescence signal upon contacting with the specific analyte. In most cases, signal transduction is realized through the analyte-induced quenching process.^21–25 Organic amines are regarded as electron-rich species which tend to interact with the photoexcited state of the specific luminogen and trigger a fluorescence quenching effect. However, these “turn-off” type fluorescence sensing procedures always suffer from low signal-to-noise ratio,²⁶ and it seems difficult for us to balance the requirements of high sensitivity and large detection range within these protocols. Thus, the fabrication of a “turn-on” type fluorescence organic amine sensor is fascinating and challenging. To date, rationally designed sensors based on MOF structures^27–30 or AIE molecules^31,32 have been sporadically reported. However, cumbersome preparation procedures and limited analyte scopes restrict their practical applications.

Here, we developed the novel fluorescent probe 4,4′-(naphthalene-1,5-diyl)bis(benzene-1,2-diol), abbreviated as DiCat, based on the 1,5-disubstituted naphthalene skeleton for the first time. As illustrated in Scheme 1, synthetic procedures and post-treatment are conducted under the guidance of green chemistry principles. DiCat, with its integrated catechol moiety, exhibits a broad-spectrum “turn-on” fluorescence sensing capability towards organic amines in both solution and gaseous phases. The detection limit using the spectroscopic method reaches sub-ppm level, and the resolution limit of the visual detection mode approaches ppm level. This protocol offers huge potential in organic amine monitoring in multidisciplinary fields.

Scheme 1 Brief synthetic procedure and working principle of this di-catechol integrated “turn-on” fluorescence sensor for organic amines. (Left side shows purification method yielding the target molecule via sustainable processing without use of column chromatography or recrystallization; right side shows the two-step synthetic procedure for this responsive luminogen.).

Results and discussion

The catechol moiety and relevant interactions based on this molecular substructure have raised extensive research interest within the academic world, starting with the most famous hormones dopamine, adrenaline and noradrenaline, to the functional materials designed based on the catechol motif.³³ Catechol and its derivatives have been revealed to undergo specific interaction with special structural domains of target proteins and play specific roles in biochemistry processes, such as anti-oxidative capability,³⁴ anti-microbial activity,³⁵ anti-inflammatory activity³⁶etc. Based on the coordinative property of the 1,2-diol motif, catechol-decorated materials could interact with various metal surfaces and act as effective adhesive for advanced applications.^37–39 In recent years, our group focused on the fabrication of polyphenol fluorophores and attempted to explore application scenarios in fluorescence imaging and sensing. We have published a series of research articles based on the luminogens with novel hydroquinone or catechol motif. Innovative sensing mechanism and remarkable detection capability have been achieved towards numerous analytes, including organic boron compound,⁴⁰ oxidative species⁴¹ and lithium metal.⁴² These species are of great interest in organic synthetic chemistry, analytical chemistry and material chemistry.

With the increasing realization of the versatility of polyphenol functionalized fluorophores, especially catechol moiety–decorated fluorophores, the development of novel molecular designs and feasible synthetic protocols has been an urgent demand. Here, we proposed the central symmetric bis-catechol fluorescent probe based on the di-substituted naphthalene scaffold. The synthetic procedure is briefly illustrated in Fig. 1, which involves a classical Suzuki-Miyaura reaction followed by BBr₃ deprotection, adopted to obtain the target molecule. The high molecular symmetry of DiCat and the precursor renders them with low solubility in common organic solvents.^43–45 Thus, we could improve the purification process by straightforward washing with small amounts of organic solvents instead of utilizing the exhausting column chromatography or recrystallization method. Table 1 shows a comparison of the purification process in our protocol with the conventional column chromatography method. Organic solvent consumption, isolation yield and product purity were comprehensively considered to evaluate the sustainability of these two processes. We find that under this rational molecular design, the bis-catechol fluorophore DiCat could be synthesized and feasibly purified by washing with lower solvent use. At the same time, the desired products could be obtained with satisfactory purity, and the total yield was determined as 74.4%. Detailed ¹H-NMR, ¹³C-NMR, and MALDI-TOF-MS results are found in ESI.† As suggested by the mainstream scientific community, procedures with lower solvent usage could be considered as preferred choices, consistent with the basic green chemistry principles.^46–48 From this aspect, our synthetic protocol exhibits huge potential in the development and construction of catechol-decorated fluorophores.

Table 1 Comparison of the purification process in our protocol with conventional column chromatography method

	Isolated yield (%)	Purity (%)	Usage amount of solvent (mL mmol^−1)
			EA	PE	DCM
a Purified with simple washing and drying under the green chemistry principle. Step 1: washing with EA/PE = 1 : 50 mobile phase first, then washing with DCM/PE = 1 : 4 to afford the crude product. Step 2: washing with EA several times to afford the target product. b Purified with conventional column chromatography process. Step 1: flushing with EA/DCM/PE = 1 : 1 : 2 to obtain the crude product (Rf = 0.75). Step 2: for the low-solubility DiCat, column chromatography could not afford the reaction product as expected. Note: solvent used in the extraction process is not included.
Step 1	90.0^a	98.2	0.1	9.0	1.0
	53.9^b	80.1	17.5	35.0	17.5
Step 2	82.7^a	99.1	20.0	0.0	0.0
	Trace^b

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Fig. 1 Detailed synthetic procedure for DiCat. The first step involves the typical Suzuki-Miyaura cross-coupling operated at 90 °C with a reaction yield of 90.0%. The second step is the deprotection of phenolic hydroxy group with BBr₃, with a reaction yield of 82.7%. Purification of each step is conducted with a simple washing and drying process instead of column chromatography or recrystallization, which obeys the green chemistry principle.

Fig. 2a shows the ground-state electron structure and conjugation length of DiCat, determined with UV-Vis spectra in THF solution. Maximum absorption wavelength is found at 311 nm, which shows a conspicuous bathochromic shift compared with naphthalene.⁴⁰ This indicates that effective conjugation might exist between two catechol moieties and the naphthalene core. This inference is verified with theoretical calculations based on the B3LYP functional and 6-31G* basis set.⁴⁹ As illustrated in Fig. 2c, we depict the optimal geometrical configuration from orthogonal perspectives. Torsion angle of 2.8° is identified between the linkage C–C bond and the naphthalene ring. The dihedral angle between the catechol moiety and naphthalene plane is determined to be 56.2°, which suggests that a certain degree of conjugation does exist, as mentioned above.

Fig. 2 Basic spectroscopic properties and structural optimization of DiCat. (a) UV-vis and fluorescence spectrum of DiCat, (b) fluorescence spectrum of DiCat in THF/H₂O binary solvent with varying water fractions, (c) structural optimization and illustration for DiCat.

Next, we focused on the excited-state property of DiCat and recorded the fluorescence spectra in THF solution. The maximum emission wavelength is found to be located at 393 nm, with a mild Stokes shift of 82 nm. DI water was introduced as poor solvent in increasing fraction; the photoluminescence (PL) intensity first undergoes a sluggish decrease and then a steep rise upon surpassing the water fraction of 95%. This behavior agrees with our fundamental understanding of the AIE effect. We could ascribe this abnormal aggregate-state emissive property to the restriction of intramolecular motion (RIM) in solid state according to our previous work⁴⁰ and generally accepted theory.^50–52 And we expect this property could contribute to the detection process in the binary solvent with the specific water content.

Based on the tetra-coordinated boron-intermediated three-component sensing system proposed by our group, the combination of a fluorescent catechol-type molecule and boronic acid with electron-rich ligand garners a fresh emissive species at the long-wavelength region.⁴⁰ We construct the di-catechol moiety integrated luminogen DiCat with intrinsic functionalization strategy⁴¹ in this work. Involving triethylamine (Et₃N) as an efficient ligand with DiCat/PBA (phenylboronic acid) solution stimulates the onset of greenish fluorescence under UV excitation. The response process could be finished within a few seconds by mixing of the DiCat/PBA ensemble and triethylamine. This phenomenon inspires us to exploit other novel sensing systems for organic amines with rational design.

After systematic optimization, we determined the optimal solvent as THF/DI H₂O (water fraction of 70%, vol%). The molar ratio of DiCat and PBA is fixed at 1 : 2. (Detailed optimization procedure can be found in ESI†). As shown in Fig. 3a, gradually increasing the concentration of Et₃N from 10 μM to 100 μM facilitates brighter emission and higher PL intensity at 486 nm. Focusing on the fluorescence change at 486 nm compared with pure DiCat in THF solution, a positive correlation between PL change and Et₃N concentration is disclosed. Deviating beyond the linear relationship induced us to attempt polynomial regression for the mathematic expression of the standard curve. A four-order polynomial fitting yields a satisfactory fitting curve with a correlation coefficient of 99.83%, as shown in Fig. 3b. The specific functional relation is depicted below:

ΔFL = 4646 − 415.8c¹ + 10.66c² + 0.1636c³ − 0.001530c⁴

where ΔFL stands for change of PL intensity at 486 nm, while c represents the concentration of Et₃N within the sensing system. To examine the practicability of this protocol, three unknown samples containing Et₃N are detected with our method. As summarized in Fig. 3b, modest precision is achieved with the polynomial regression equation. (Detailed spectroscopic information is provided in ESI.†).

Fig. 3 Detection capability and mechanistic discussions of DiCat. (a) Fluorescence spectrum of DiCat/PBA with increasing concentration of Et₃N under optimal detection condition (concentration of DiCat and PBA is fixed at 100 μM and 200 μM, respectively). (b) Polynomial regression of fluorescence change versus Et₃N concentration and examination for three unknown samples. (c) Job's plot with varying molar fractions of Et₃N. (d) Energy level diagram for the proposed intermediate through the sensing procedure (calculated with optimized molecular configuration; Et₃N is selected as the typical analyte). (e) Experimental and calculated fluorescence emission spectrum of the proposed intermediate (the experimental spectrum is collected in THF/DI H₂O solution; the concentration of DiCat, PBA and Et₃N is 100 μM, 200 μM and 100 μM, respectively).

Aiming at deepening our understanding of the sensing mechanism dominating the abovementioned process, a series of control experiments was conducted. As shown in Fig. S14 (ESI†), the absence of DiCat diminishes the fluorescence feature of the sensing mixture, and no discriminative emission peak could be identified. Just a combination with PBA brings about mild influence on the emissive property of DiCat, while complexation of DiCat and Et₃N yields another emission peak at 466 nm. Et₃N is susceptible to induce the construction of a cross-linking network with DiCat through hydrogen bonding interaction, which suppresses intramolecular motion of DiCat. This blocking effect might engender the enhancement of PL intensity and bathochromic-shifted maximum emission wavelength. The extra addition of PBA boosts PL intensity and triggers further bathochromic shift of the emission wavelength. We could attribute this phenomenon to the synergistic effect, including formation of tetra-coordinated boron species and hydrogen-bonding-mediated cross-linking interaction, which renders restriction of intramolecular motion and extension of conjugation length. The stoichiometric ratio investigation depicted in Fig. 3c reveals that complexation of DiCat and Et₃N exists beyond the typical chemical stoichiometric compound, which agrees with our hypothesized plausible sensing mechanism as mentioned above. Theoretical calculation also supports our viewpoint, as shown in Fig. 3d and 3e. Using Et₃N as the typical analyte in our discussion, the energy level and electronic structure of the proposed tetra-coordinated boron species was acquired in Fig. 3d with TDDFT method. The calculated fluorescence spectrum depicted in Fig. 3e matches well with experimental results.

Success in Et₃N detection with this protocol raised our enthusiasm for exploiting the detection capability of DiCat towards other organic amines. Primary amine and secondary amine with simple chemical structures, such as n-butylamine and diisopropylamine, are scrutinized with DiCat/PBA under the optimal detection condition identified above. A similar positive correlation of PL intensity versus amine concentration is observed, which could likewise be constrained with polynomial regression. Detailed fitting parameters are summarized in Table 2, while fluorescence spectrum and fitting curves are provided in ESI.† It is quite inspiring to notice that our sensing method seems to be compatible with primary, secondary, and tertiary amines. We continued our research by examining more frequently used organic amines such as N,N-diisopropylethylamine (DIPEA) and 4-dimethylaminopyridine (DMAP) and acquired positive results from both of them. Detection methods for eight organic amines commonly discussed in food science, including 1,4-diaminobutane (putrescine), 1,5-pentanediamine (cadaverine), spermine, spermidine, tyramine, phenylethylamine, histamine, and tryptamine,⁵³ were further established. The versatility of this detection method based on DiCat/PBA is overviewed in Fig. 4, and we find the detection limit of this broad-spectrum protocol down to several μM (sub-ppm level). Outstanding selectivity towards amine substances, including primary amine, secondary amine, and tertiary amine, is developed, while other common nitrogen-containing species such as quaternary ammonium salt or amide induce minor change in PL intensity (Fig. S40, ESI†). Meanwhile, we checked other water-soluble organic compounds bearing electron-rich functional groups with our DiCat/PBA system (Fig. S41, ESI†). As shown in Fig. S42 (ESI†), selectivity towards organic amines is achieved with remarkable difference in relative intensity of the response compared with carbonyl, hydroxyl, carboxyl, and cyano containing compounds.

Table 2 Summary of fitting parameters with the organic amines of concern

Organic amine	P 0 (μM)	P 1 (μM^−1)	P 2 (μM^−2)	P 3 (μM^−3)	P 4 (μM^−4)	R ^2 (%)
Triethylamine	4646	−415.8	10.66	0.1636	−0.001530	99.83
Diisopropylamine	−1657	470.2	−15.70	0.3309	−0.001700	99.98
n-Butylamine	279.3	96.56	−10.12	0.3328	−0.001950	99.89
DIPEA	4154	−511.1	19.26	−0.09951	−0.00006874	99.91
DMAP	2232	−214.9	11.06	−0.09656	0.0002764	99.77
Putrescine	2222	−327.3	29.45	−0.2854	0.0008314	99.95
Cadaverine	5380	−975.5	57.42	−0.6772	0.002630	99.87
Spermine	5763	−1342	113.8	−1.527	0.006420	99.70
Spermidine	8091	−1312	86.07	−1.056	0.004220	99.96
Tyramine	9268	−1233	55.20	−0.5385	0.001720	99.90
Phenylethylamine	6510	−1023	53.42	−0.6015	0.002200	99.99
Histamine	4954	−648.2	24.61	−0.1843	0.0003816	99.71
Tryptamine	8371	−1049	45.93	−0.3973	0.001060	99.88

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Fig. 4 Response of DiCat towards diverse organic amines. (The graph shows relative fluorescence intensity of DiCat/PBA towards a series of organic amines under optimal detection conditions. The vertical axis represents ratio of photoluminescence intensity with 10 μM organic amines compared with blank control.)

The solid-state emission behavior of DiCat inspires us to study its sensing capability under aggregation state. Diverse sensing patterns as illustrated in Fig. 5a could be fabricated on silica gel plates or paper-based substrate without any difficulty. The solid-state sensing capability of DiCat towards gaseous organic amines is demonstrated with primitive homemade equipment. Et₃N atmosphere is created via vaporization of Et₃N solution in MeOH with varying concentrations within sealed vessels. DiCat is dissolved in THF with an equal molarity of PBA and cast onto silica gel plates. Customized plates are put into sealed vessels after totally drying in the air for further observation. As shown in Fig. 5b, it is easy for us to distinguish the color change of solid-state sensing materials within the environment containing 10 ppm of Et₃N vapor with the naked eye under hand-held UV lamp. “Turn-on” greenish fluorescence of DiCat/PBA complex occurs and tends to be brighter as the concentration of Et₃N vapor increased. Taking out the sensing materials into the atmosphere would recover the blue emission. We are dedicated to the further design and fabrication of a sensing device with in situ fluorescence spectrum collection capability, and we expect it could on-line determine the precise concentration of specific gaseous-phase organic amines. Recently, based on this homemade equipment, we could semi-quantitatively determine Et₃N vapor concentration through the atmosphere. The visual resolution limit is estimated to be 10 ppm, which is lower than the OSHA standard level for Et₃N vapor. It indicates that a sensing array based on DiCat offers an effective monitoring protocol for organic amines to avoid inhalation injury in the industry.

Fig. 5 Emission behavior of DiCat in the solid state. (a) Diverse sensing patterns fabricated with DiCat, (b) solid-state sensing capability of DiCat/PBA towards gaseous substrates.

Conclusions

Functional fluorophores decorated with catechol moiety have shown great promise in the development of future fluorescent chemical sensors. The feasible synthesis of these molecular skeletons is of vital importance as we consider the large-scale synthesis and application of catechol-based luminogens. Herein, we proposed the molecular design of the central symmetric bis-catechol fluorophore DiCat bearing a 1,5-disubstituted naphthalene scaffold. The unique molecular structure makes it possible to be fabricated with sustainable processing steps. Through avoiding the operation of conventional chromatography or recrystallization method, the entire purification process involves less solvent utilization and minor environmental disruption. We would further enrich viable fluorophore structures integrated with bis-catechol motif in the future, which could be synthesized with similar sustainable processing. Improvements on the agent use and catalyst selection of the whole synthetic procedure are also ongoing in our laboratory.

This fluorescence probe DiCat exhibits a rapid “turn-on” fluorescence response towards a broad spectrum of organic amines under warm conditions. The average detection limit is estimated as sub-ppm level with mathematic treatments. These inspiring results reveal the huge potential of the bis-catechol fluorophores, with sensing applications in bioscience, biomedicine and pharmaceutical fields, as it could respond to a wide range of amino-containing analytes in the liquid phase, which could be a key intermediate of metabolism or common amino-containing drugs. The gaseous phase–sensing capability also renders the possibility of atmospheric detection towards biogenic amines or volatile organic amines, which will be valuable for real-time monitoring in food science and environmental chemistry.

Author contributions

This project was designed by R. H., S. L., Z. S. and Y. W.; synthesis and characterization were conducted by R. H.; theoretical calculation was performed by S. L. and Z. S.; constructive suggestions on the writing of the manuscript were put forward by D. H., H. H., M. W., R. L. and M. T.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21788102). We are most grateful to Dr. Liucheng Mao, Mr. Wensheng Xie and Prof. Lei Tao of Tsinghua University for their valuable suggestions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qm01065h

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