Sensing Organic Amines and Quantitative Monitoring Intracellular pH

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Sensing Organic Amines and Quantitative Monitoring Intracellular pH Change Using Fluorescent Self-Assembly System Xinhua Cao, Yiran Li, Aiping Gao, Yongsheng Yu, Xueping Chang, and Xiaohan Hei ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00238 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Polymer Materials

Sensing Organic Amines and Quantitative Monitoring Intracellular pH Change Using Fluorescent Self-Assembly System Xinhua Cao*a, Yiran Lia, Aiping Gaoa, Yongsheng Yua, Xueping Changa, Xiaohan Heib aCollege

of Chemistry and Chemical Engineering＆Henan Province Key laboratory of Utilization of Non-metallic

Mineral in the South of Henan, Xinyang Normal University, Xinyang 464000, China bCollege

of municipal and environmental engineering, Henan University of Urban Construction, Pingdingshan

467000, China *E-mail: [email protected] Abstract: A series of supramolecular self-assembly gelators (N-4, N-12 and N-18) were designed and synthesized. N-12 and N-18 could gelate some single or mixed solvents. Microbelts and nanofibres were obtained in the self-assembly process. The hydrophobic xerogel N-18 films surface were with water contact angles of 119-148.5°. Interestingly, these compounds exhibited solvatochromism properties. Interestingly, compound N-18 had ratiometric detection ability towards organic amines with the detection of limit for triethylamine (TEA) of 2.23 ×10-6 M. At the same time, compound N-18 in solution and organogel state could distinguish aromatic amines and fatty amines through its fluorescence changing. These compounds exhibited a specific pH-dependence fluorescence behivor in the pH value range of 3-11. Compound N-4 was successfully applied to bioimaging for quantitative monitoring pH change in cell. This work provides a new window for preparation self-assembly for sensitive detection organic amines and pH probes for accurate intracellular pH mapping and quantifying. Kewords: self-assembly; colorimetric detection; organic amine; quantitative monitoring; pH change

Stimuli responsive materials have received considerable attentions due to their potential applications as nano-medicine, drug

transport, bioengineering, catalysis, nanoelectronic, template, sensor and optoelectronic

devices.1-6 Supramolecular gels as soft materials usually have stimuli responsiveness ability due to their formation based on noncovalent interactions, such as H-bonding, π-π stacking, van der Waals interactions, hydrophobic interaction and metal coordination.7-9 External stimuli including temperature, pH, magnetic, light, mechanical stress, chemicals can give rise to different change, such as color, emission, physical state.10-16 Especially gel system as sensor, supramolecular self-assembly progress based on non-covalent interactions can fabricate various ordered nanostructures with well-defined shapes and dimensions which are in favor of the permeation of analyte

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ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

into interior and the release of drug.17,

18

Díaz and coworkers have reviewed application of gels for the

encapsulation and release of drug in a review paper.19 Smith and coworkers have published a review paper which has summarized applying gels as smart materials for pollutant removal.20 Organic amines are the important chemical materials and extensively used in dye chemistry and medical intermediate. However, organic amines may bring many environmental concerns and health problems.21 Therefore, it is of great significance for the study of sensing and recognition of amines. Xue and coworkers have reported a self-assembly system for sensing aromatic amines and acids vapor.22 We have designed some 1,8-naphthalimide based- gel systems, which can distinguish and sense organic amines.23-25 Fluorescence detection technology is a common detection method for organic pollutant. It is a pity that the absolute-intensity-dependent signal acquisition from single fluorescence probe is sometime accurate in traditional fluorescence sensing due to the existence of various analyte-independent confounding factors including instrumental parameters, background light scattering and microenvironmental variations. Compared with traditional fluorescence sensing, ratiometric fluorescence probe can effectively overcome these issue and lead to higher detection sensitivity through introducing another fluorescence emission band to achieve ratiometric signal readouts. Naphthalimide derivatives are a class of very important dye, and widely applied as fluorescence sensors and gelators.26 It is few reported that naphthalimide derivatives as a ratiometric fluorescence sensor. Yi and coworker reported a naphthalimide-based probe for cytoplasmic and hydrogen peroxide.27 Lu and coworkers reported a fluoride ion probe based on naphthalimide derivatives with the capability of differentiating cancer cells.28 There are many research papers about naphthalimide derivatives with the modifying the C-4 substitution, and the naphthalimide derivatives with the C-3 substitution was rarely concerned. Herein, we have incorporated different alkylamines into a 3-hydroxy-1,8-naphthalimide–based molecule (N-4, N-12 and N-18 in Scheme 1) which can form stable gel in some organic solvents. These gelators exhibited the typical structural character of naphthalimide based on the internal charge transfer (ICT) process.29 Interestingly, their emission behaviors were changed with the different solvents and different pH values. These compounds exhibte the solvatochromism properties and tunable emission behaviors through pH value change. This self-assembly system can be applied in colorimetric detection organic amines. More significant, compound N-4 with short alkyl chain can be applied in bioimaging for quantitative monitoring the intracellular pH change. The self-assembly process was investigated in details.


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Scheme 1 Molecule structures of compounds N-4, N-12 and N-18.

Results and discussion Compounds

N-4,

N-12

and

N-18

were

synthesized

through

the

reaction

between

3-hydroxyl-1,8-naphthalimide and the corresponding amine with high yield. The characterization data were given in the supporting information (Fig S1 and S2). The gelation abilities of N-4, N-12 and N-18 were investigated through the inverse flow method30. As shown in Table 1, N-4 did not form gel in the experimental solvents, and formed precipitate in methanol, acetonitrile, toluene and ethanol. The solutions N-4 were obtained in DMF, DMSO, 1,4-dioxane, acetone, ethyl acetate, THF and mixed solvents of DMF/H2O (4/1, v/v) and DMSO/H2O (4/1, v/v). N-4 could be insoluble in n-hexane and petroleum ether due to its short alkyl chain under 5 mg mL-1. With the alkyl chain of N-12 elongating, the gelation ability was obvious improved and organogels were obtained in methanol, acetonitrile, n-hexane, toluene and petroleum ether with the critical gel concentration (CGC) of 25.0 mg mL-1. N-12 solutions were formed in DMF, DMF/H2O (4/1, v/v), DMSO, DMSO/H2O (4/1, v/v) 1,4-dioxane, acetone, ethyl acetate, ethanol and THF. When the alkyl chain was elongated to 18, the gelation ability was further improved, and gels N-18 were formed in DMF/H2O (4/1, v/v), DMSO/H2O (4/1, v/v), acetonitrile, n-hexane, toluene and petroleum ether with the CGC of 12.5, 5.0, 25.0, 25.0, 25.0 and 25.0, respectively. Precipitate N-18 was obtained only in methanol. N-18 solutions were found in the other solvents. In view of N-18 with the relatively good gelation ability, the self-assembly process of N-18 in different solvents were investigated in detail. The pale gels N-18 could be stable for several months as shown in Fig. 1. Gels N-18 in n-hexane, toluene, petroleum ether and DMSO/H2O (4/1, v/v) were pale yellow, and gels N-18 in acetonitrile and DMF/H2O (4/1, v/v) were yellow. It was slightly different that gel N-18 in DMSO/H2O (4/1, v/v) was transparency and the others were opaque. Gels N-18 in DMSO/H2O (4/1, v/v) and DMF/H2O (4/1, v/v) emitted red light, and gels N-18 in other solvents emitted blue light. This showed that the emission properties could be affected by solvent.



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Table 1 Gelation ability of compounds N-4, N-12 and N-18 in commonly used solvents. Solvent

N-4

N-12

N-18

methanol

p

G (25.0)

P

DMF

S

S

S

DMF/H2O (4/1, v/v)

S

S

G (12.5)

DMSO

S

S

S

DMSO/H2O (4/1, v/v)

S

S

G (5.0)

acetonitrile

P

G (25.0)

G (25.0)

1,4-dioxane

S

S

S

n-hexane

I

G (25.0)

G (25.0)

acetone

S

S

S

toluene

P

G (25.0)

G (25.0)

ethyl acetate

S

S

S

ethanol

P

S

S

petroleum ether

I

G (12.5)

G (25.0)

THF

S

S

S

P = precipitate; I = insoluble; S = sol; G = gel; the values in the brackets denote the CGC with the unit of mg mL-1.

Fig. 1 Images of Gels N-18 formed in different solvents: a) and a´) n-hexane, b) and b´) acetonitrile c) and c´) toluene, d) and d´) petroleum ether, e) and e´) mixed solvents of DMF/H2O (4/1, v/v), f) and f´) mixed solvents of DMSO/H2O (4/1, v/v). a), b), c), d), e) and f) were under daylight. a´), b´), c´), d´) e´) and f´) were under 365 nm light.

The self-assembly was a complicated process due to the interaction between gelator and gelator, gelator and solvent, solvent and solvent. At the same time, self-assembly morphologies could be observed, even if only the same molecule in different solvents. The self-assembly structures of N-18 were studied by FESEM. The irregular microspheres structure with the diameter of about 1μm could be obtained through self-assembly of N-18 in n-hexane (Fig. 2a). The micorbelts with the length of dozens of micron and the width of 2-4 μm were found in organogel from acetonitrile (Fig.2b). The different microbelts were also found in organogels N-18 from toluene and petroleum (Fig. 2c and 2d). The nanofibers morphology was formed in organogel N-18 from DMF/H2O (4/1. v/v) and further entangled into three-dimensional networks (Fig. 2e). Compared with organogel N-18 from DMF/H2O (4/1. v/v), much large nanofibers structure with the width of 1μm and the length of dozen of micron


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was formed in gel N-18 from DMSO/H2O (4/1. v/v). EDS elemental analysis of xerogels N-18 from different solvents were carried out and shown in Table S1. As shown in Table S1, the experimental results were close to the theoretical value with C of 77.38 %, N of 3.01 % and O of 10.31 %. At the same time, the EDS experimental results of every xerogel samples from different solvents were consistent with each other.

Fig. 2 SEM images of xerogels N-18 in different solvents. a) n-hexane; b) acetonitrile; c) toluene; d) petroleum ether; e) DMF/H2O (4/1. v/v) and f) DMSO/H2O (4/1. v/v). The scale bars for a), b), c), d), e) and f) are 5, 20, 10, 10, 5 and 5 μm.

UV-vis absorption spectrum can reveal self-assembly mode for gel. For example, the red or blue shift of π-π* transition peak in gel state indicate J-type or H-type aggregate with their corresponding side-by side and face-to-face stacking.31 UV-vis absorption spectra of N-18 in solution and gel state were investigated. The gel state sample was prepared through pulling gel onto quartz plate and formation sandwich mode. As shown in Fig. 3a, N-18 n-hexane solution exhibited an absorption band at 377 nm belonging to π-π* transitions, and the absorption band was broadened and shifted to 397 nm in its gel state, which indicated the J-type aggregate formation.32 For N-18 in acetonitrile, UV-vis absorption band was at 375 nm in its solution state, and did not have any shift for its gel state excluding the absorption band broadening (Fig. 3b). UV-vis absorption band of the gel N-18 in toluene was red-shifted from 374 nm to 378 nm (Fig. 3c). UV-vis absorption band at 371 nm of N-18 petroleum ether solution was red shifted to 386 nm of its gel state (Fig. 3d). For N-18 in DMF/H2O (4/1, v/v), UV-vis absorption band of N-18 solution had an obvious red-shift and was shifted to 381 nm compared with those of the above four solvents (Fig. 4e). The absorption band at 381 nm of solution was continuously red-shifted to 397 nm in the corresponding gel state, which showed that UV-vis absorption properties were disturbed by solvent. UV-vis absorption band of solution and gel state in DMSO/H2O (4/1, v/v) were all at 390 nm.


0.8 0.6 0.4 0.2 0.0 350

1.0

375

400

425

450

475

N-18 gel in toluene N-18 solution in toluene

0.8 0.6 0.4 0.2 0.0

1.0

380

400

420

440

460

Wavelength / nm

e

480

0.8 0.6 0.4 0.2 0.0 450

1.0

500

N-18 gel in DMF/H2O (4/1, v/v) N-18 solution in DMF/H2O (4/1, v/v)

400

1.0

500

Wavelength / nm

c

Normalized absorbance (a.u.)

N-18 gel in hexane N-18 solution in hexane


a

500



1.0

360


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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b N-18 gel in acetonitrile N-18 solution in acetonitrile

0.8 0.6 0.4 0.2 0.0 350

d

375

400

425

450

Wavelength / nm

475

500

N-18 gel in pertroleum ether N-18 solution in petroleum ether

0.8 0.6 0.4 0.2 0.0 350

1.0

375

400

425

450

475

500

Wavelength / nm

f

N-18 gel in DMSO/H2O (4/1, v/v) N-18 solution in DMSO/H2O (4/1, v/v)

0.8 0.6 0.4 0.2 0.0 350

400

Wavelength / nm

450

500

550

Wavelength / nm

Fig. 3 UV-vis absorption spectra of N-18 in solution and gel state in different solvents: a) n-hexane; b) acetonitrile; c) toluene; d) petroleum ether; e) DMF/H2O (4/1, v/v) and f) DMSO/H2O (4/1, v/v). The concentrations of solution and gel were 10-5 M and their corresponding CGC.

Naphthalimide as a class of most versatile fluorophore unit has been extensively applied in much fields.33 The emission spectra of N-18 in solution and gel state were studied one by one. N-18 of n-hexane solution with the concentration of 10-5 M could emit blue light with emission peak at 401 nm under the excitation of 350 nm in Fig. 4a. The fluorescence emission peak of N-18 in gel from n-hexane was red-shifted from 401 nm to 430 nm, indicating strong π-π stacking interaction existed in gel state (Fig. 4a).34 For N-18 in acetonitrile, the emission peaks of solution and gel state were at 410 and 422 nm, respectively (Fig. 4b). The similar change trend was found in toluene. The fluorescence emission peaks of N-18 solution and gel state in toluene were at 408 and 416 nm (Fig. 4c). Compared with N-18 petroleum ether solution with the emission peak of 399 nm, the emission of N-18 gel was red-shifted to 439 nm and accompanied with a new shoulder peak at 509 nm which showed that solvent affect the emission behavior of N-18 to some extent (Fig. 4d). This phenomenon was obviously appeared in DMF/H2O (4/1, v/v) and DMSO/H2O (4/1, v/v). As shown in Fig. 4e, N-18 in DMF/H2O (4/1, v/v) exhibited a high emission peak at 427 and a low and broad emission peak from 550nm to 650 nm which had a red-shift compared with N-18


Page 7 of 22

in solution n-hexane, acetonitrile, toluene and petroleum ether. When N-18 gel was formed in DMF/H2O (4/1, v/v), the emission peak at 427 nm was red-shifted to 440 nm and a new broad emission peak was appeared at 624 nm which explained why gel N-18 in DMF/H2O (4/1, v/v) could emit red light under 365 nm (Fig. 1e). It was possible hydrogen bonding between N-18 and solvent molecule. The fluorescence emission of N-18 in DMSO/H2O (4/1, v/v) was similar to that of N-18 in DMF/H2O (4/1, v/v). N-18 in DMSO/H2O (4/1, v/v) solution had a maximum emission peak at 428 nm and a weak and broad emission peak covered 550-650 nm (Fig. 4f). There were two

a

Fluorescence intensity (a.u.)

1.0

N-18 solution in hexane N-18 gel in hexane

0.8 0.6 0.4 0.2

1.0

400

450

500

Wavelength / nm

c

N-18 solution in toluene N-18 gel in toluene

0.8 0.6 0.4 0.2 0.0

1.0

400

e

450

500

Wavelength / nm

1.0

b

550

0.6 0.4 0.2 0.0

400

0.6 0.4 0.2 0.0 400 450 500 550 600 650 700 750 800

450

500

550

Wavelength / nm 1.0

d

N-18 solution in pertroleum ether N-18 gel in petroleum ether

0.8 0.6 0.4 0.2 0.0

N-18 solution in DMF/H2O (4/1, v/v) N-18 gel in DMF/H2O (4/1, v/v)

0.8

N-18 solution in acetonitrile N-18 gel in acetonitrile

0.8

550


0.0




emission peaks at 439 and 575 nm in the fluorescence emission spectrum of N-18 gel from DMSO/H2O (4/1, v/v).


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

400

450

500

Wavelength / nm

550

f N-18 solution in DMSO/H2O(4/1, v/v) N-18 gel in DMSO/H2O(4/1, v/v)

0.8 0.6 0.4 0.2 0.0

400

Wavelength / nm

450

500

550

600

650

Wavelength / nm

Fig. 4 Fluorescence spectra of N-18 in solution and gel state in different solvents. a) n-hexane; b) acetonitrile; c) toluene; d) petroleum ether; e) DMF/H2O (4/1, v/v) and f) DMSO/H2O (4/1, v/v). The concentration of solution of gel was 10-5 M and their corresponding CGC.

On the base of above results, we found that solvent had some effect on the photophysical properties of N-18. In order to fully understand the fact, UV-vis absorption and emission spectra of N-18 in different solvents were investigated. As shown in Fig. S3, UV-vis absorption spectra of N-18 in different solvents did not have large



change, and the absorption bands were located at 371-385 nm with molar absorption coefficient of 104 M-1 cm-1. And just looking at the face, N-18 in different solvents did not have any obvious distinction in Fig. S3a. But there was obvious change in their fluorescence emission. The maximum emission peak was in 399-434 nm. It was different that there was a new emission peak at 605 nm in fluorescence spectra of N-18 in DMF and DMSO solution expect for the peak at 434 nm (Fig. S3b). At the same time, N-18 in DMF and DMSO solution emitted purplish red and pale red light under 365 nm light, and the other N-18 solutions emitted blue light (Fig. S3c). In order to further understand the emission behavior of N-18 in DMF and DMSO, the emission spectra of N-18 in mixed solvents of acetonitrile/DMF and acetonitrile/DMSO with different proportion were carried out. With the increasing of DMF ratio, the fluorescence emission was gradually decreased and accompanied with a red-shifting. At the same time, an emission peak was appeared at 604 nm and increased with the DMF ratio increasing (Fig. 5a). The emission light of N-18 solution in mixed solvents of acetonitrile/DMF was changed from blue light into pale purplish red light under 365 nm when DMF ratio was up to 80% (Fig. 5b). The emission intensity ratio between 430 nm and 605 nm was gradually decreased with DMF ratio increasing. When the volume ratio of DMF and acetonitrile was 9/1, the emission intensity ratio was about 18. When N-18 was in mixed solvents of acetonitrile/DMSO, the similar fluorescence emission behavior was observed (Fig. 5c). It was different that the emission light transition from blue light into magenta light was at DMSO ratio of 40%. The emission intensity ratio of 430 nm and 604 nm was at 19. When the volume ratio of DMSO and acetonitrile was 9/1, the emission intensity ratio was about 4. It was shown that N-18 in DMSO and DMF solution had two molecule structures on the above experiment results and previous reference. In other word, one part of N-18 was deprotonated due to the weak alkalinity of DMSO and DMF. In order to verify this fact, the 1HNMR of N-18 in d6-DMSO was done (Fig. S4). The integration of the proton of hydroxyl at 4-position was only 0.48 which indirectly indicated partial the deprotonation of N-18 molecule in DMSO and DMF. This change behavior of N-18 would provide a good opportunity for ratiometric determination of organic amines. At the same time, the zeta potential of three compounds was investigated. The zeta potential of N-4, N-12 and N-18 were -9.79 mV, -6.03 mV and -5.39 mV, respectively which showed three compounds exhibited different weak acidity and had the ability of interaction with organic amines.


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a

1 solution in acetonitrile 10% DMF 20% 1 solution in 30% 10% DMF 40% 20% 50% 60% 30% 70% 40% 80% 90% 50% 100% 60%

20

Fluorescence intensity


800

600

400

200

16 12 8

70% 80% 90% 100%

4 0

b

acetonitrile

560

600

640

680

Wavelength / nm

0

1000

400

450

c

40

800 600 400 200 0

500

30

20

600

650

450

700 1 solution in acetonitrile 10% DMSO 20% 30% 40% 50% 60% 70% 80% 90% 100%

1 solution in acetonitrile 10% DMSO 20% 30% 40% 50% 60% 70% 80% 90% 100%

d

10

0

400

550

Wavelength / nm



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

560

600

640

Wavelength / nm

550

600

650

680

700

Wavelength / nm Fig. 5 a) Fluorescence emission change of N-18 in mixed solvents of acetonitrile/DMF with different proportion; b) The images of N-18 in mixed solvents of acetonitrile/DMF with different proportion; c) Fluorescence emission change of N-18 in mixed solvents of acetonitrile/DMSO with different proportion; d) The images of N-18 in mixed solvents of acetonitrile/DMSO with different proportion. The concentration of N-18 solution was 10-5 M.

In order to further investigate the existence of deprotonation on these compounds, theoretical calculation of N-4 and deprotonated N-4 was done using the hybrid B3LYP density functional theory method (TD-DFT) at the level of 6-31G*(Table 2). 35 The calculated HOMOs and LUMOs energy of N-4 were -7.53 and -1.61 eV with the corresponding HOMO-LUMO difference of 5.92 eV. The calculated HOMOs and LUMOs energy of deprotonated N-4 were -2.06 and -2.53 eV with the corresponding HOMO-LUMO difference of 0.47 eV. The HOMO primarily resided on the negative ions of oxygen and naphthalene ring for deprotonated N-4. The HOMO primarily resided on the naphthalene ring and two carbonyl groups for N-4 molecule. The principal difference between the HOMO of N-4 and deprotonated N-4 was that negative ions of oxygen of deprotonated N-4 had the higher electron cloud density than that of hydroxyl group of N-4. The emission spectra of N-4 and deprotonated N-4 were speculated through theoretical calculation in Fig. S5. The maximum emission peak of N-4 was at 350 nm, and the maximum emission peak of deprotonated N-4 was at 280 nm along with a new emission peak at 610 nm which provided good agreement between experiment and theory. So the emission change of these compounds in DMF and DMSO was possible due to the toilless deprotonation of N-4 under the inducement of N atom of DMF and O atom of DMSO.



Page 10 of 22

Table 2 HOMOs and LUMOs of N-4 and deprotonated N-4 Molecule

HOMO

LUMO

-7.53 eV

-1.61 eV

-2.06 eV

-2.53 eV

HO O N O

N-4 O O N O

deprotonated N-4

Fluorescent probes toward organic amines had been reported in many previous works.22-25,

29

Few

ratiometric fluorescence probes for organic amines were reported. Some frequently-used organic amines including 4-chloroaniline, pyridine, aniline, TEA, diethylamine, ethylenediamine and n-butylamine were used to verify the response performance of this self-assembly system. UV-vis absorption spectra of N-18 in acetonitrile did not shift or generate new absorption peak except for absorbance change at its original absorption band after addition different amines in Fig. S6a. The colour of N-18 solution with addition of amines was still colourless in Fig. S7. The fluorescence behaviour of N-18 solution was distinctly changed after addition of amine. After addition of TEA, diethylamine, ethylenediamine and n-butylamine, the emission peak at 604 nm was appeared along with the fluorescence emission weakening of 409 nm in Fig. S6b. The emission light of N-18 solution was changed from blue to red under the irradiation of 365 nm light (Fig. S6d-g). For addition of 4-chloroaniline, pyridine and aniline, the fluorescence emission intensity was only decreased without any new emission peak appeared (Fig. S6b). N-18 solution with addition of above amines still emitted blue light with the decrement of intensity (Fig. S7a-c). It was found that aromatic amine and fatty amines could be distinguished via N-18 solution through its fluorescence change. To further investigate the ratiometric determination performance toward organic amine, triethylamine (TEA) was selected as a representative sample for titration experiment. The emission intensity of 409 nm was decreased with the addition of TEA in Fig. 6a. When the addition of TEA was up to 88 μL, the quenching efficiency was 77.9%. At the same time, a new emission peak at 604 nm was appeared and gradually increased with the titration going. Besides, the ratio of emission intensity at 604 and 409 nm plotted against the concentration of TEA gave linear response up to 7.1 μM of TEA concentration as shown in Fig. 6b. A linear response of N-18 as a function of TEA concentration was observed in the range of 0-710 μM. The linear equation was y = -0.000596+0.00242x (R2


Page 11 of 22

= 0.99063), where y was the emission intensity ratio at 604 and 409 nm (I604/I409) measured at a given TEA concentration, and x represented the concentration of TEA. The limit of detection of N-18 towards TEA was 2.23×10-6 M (Table S2).

36

The reaction ratio of TEA to N-18 was investigated according to the previous

reference.37 As shown in Fig. S8, fluorescence of N-18 could be quenched by the addition of aqueous solution of TEA, which may because the destroying of ICT of N-18. The fluorescence titration curve exhibited a 17.5:1 stoichiometry for TEA and N-18.

a

b

8


800 600 400

1 solution in acetionitrile 8 L of TEA 16 24 32 40 48 56 64 72 80 88

6

4

2

0 525

550

575

600

625

650

675

Wavelength / nm

200

Y = -0.000596+0.00242x 2 R = 0.99063

0.020 1 solution in acetionitrile 8 L of TEA 0.015 16 24 32 40 0.010 48 56 64 0.005 72 80 88

I604 / I409

1000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.000

0

400

450

500

550

600

650

700

0

100 200 300 400 500 600 700 800

Wavelength / nm

-1

[TEA] (mol L )

Fig. 6 (a) Fluorescence emission change of N-18 acetonitrile solution (10 μmol/L) with the addition of increasing concentrations of TEA (0~710 μmol/L), Inset: The fluorescence emission change in the range of 530-675 nm of N-18 (10 μmol/L) upon the addition of 0~710 μmol/L of TEA; (b) The linearity of the emission intensity ratio I604/I409 at 604 and 409 nm as a function of TEA concentration in acetonitrile. The concentration of TEA in this titration was 1.0 M.

It is well known that it is a chance for analytes permeating into the inner of three-dimensional network from its surface due to the large superficial area and open-framework structure.20,

38, 39

Yang and coworkers have

reported a CO2 stimuli-responsive, injectable block copolymer hydrogels.40 Herein, organogel N-18 formed in acetonitrile was tried to detection organic amines. As shown in Fig. S6, organogel N-18 exhibited more sensitive detection ability than that of N-18 solution. At the same time, organogels N-18 exhibited the ability of distinguishing from aromatic amines and fatty amines through the emission behavior change of gel state. With 10 μL of organic amine or corresponding solution onto organogel surface, the color of organogels N-18 were changed from dull yellow to luminous yellow for 4-chloroaniline, pyridine, aniline, TEA and diethylamine (Fig. S9a-e). Otherwise, the color was changed from dull yellow to purplish red after addition of ethylenediamine and n-butylamine (Fig. S9f and S9g). Fluorescence emission of organogels N-18 with addition of pyridine and amiline were changed from blue to pink light and dark yellow light, respectively (Fig. S9b′ and S9c′). For addition of diethylamine, ethylenediamine and n-butylamine, the fluorescence emission light was changed to red light (Fig.



S9e′-g′). The blue light emitted from organogel N-18 was mainly quenched after addition of 4-chloroaniline (Fig. S9a′). Organogel N-18 with addition of TEA emitted orange under the excitation of 365 nm light (Fig. S9d′). The UV-vis absorption and emission spectra of organogel N-18 before and after TEA were done to investigate the color and emission changing process. As shown in Fig. S10a, the original absorption band was red-shifted from 377 nm to 382 nm and the absorbance covered 425-500 nm was enhanced to some extent with the addition of TEA. The fluorescence emission peak of organogel N-18 was red-shifted from 422 nm to 438 nm, and two broad emission peaks were appeared at 553 nm and 596 nm after addition of TEA which fully explained why its fluorescence emission change (Fig. S10b). pH homeostasis plays the important role of the function, apoptosis, proliferation and survival of living cells.41-46 Although many research works about monitoring pH values have been reported, the instant and colorimetric detection of intracellular pH change still remains a major challenge, especial through a simple organic fluorescence molecule. Yin and coworkers have reported a turn-on fluorescent pH probe based on naphthalimide fluorophore as a pinpoint diagnostic kit for heat stroke by monitoring lysosomal pH.47 An obvious difference between their molecules and ours was the hydroxyl place on naphthalimide molecule skelecton. The hydroxyl was at the C-4 and C-3 position of naphthalimide molecule skelecton for their molecules and ours, respectively. Inspired by their works, N-4 was tried to be applied as a colorimetric fluorescence probe for momitoring pH values change in cell. The spectroscopic properties of N-4 were examined under in vitro phosphate buffer at different pH values (Fig. 7a). Fluorescence emission behavior of N-4 in phosphate buffer solution was different from that of N-4 in organic solvent. N-4 in phosphate buffer solution (10-5 M) had two emission peaks at 398 and 572 nm under the excitation of 350 nm even if the pH value was 3.0 which did not affect its performance of colorimetric fluorescence probe. The fluorescence emission intensity ratio of 398 nm and 572 nm (I398/I572) was 1.2152. The fluorescence emission intensity of the two peaks was changed, and the ratio of 398 nm and 572 nm was also further changed with the pH values of phosphate buffer solution increasing. After the pH value came up to 11.0, the ratio of I398/I572 was changed from 1.2152 to 0.5670, which well proved the feasibility of N-4 as the colorimetric probe for monitoring the pH values change in cell. The ratio of red to blue fluorescence intensity illustrated a specific pH-dependence with a good linear correlation with R = 0.9738 over the pH range from 3.0 to 11.0 (Fig. 7b). The linear correlation provided a relatively precise way for quantitative determination of intracellular pH values. To further verify the application of N-4 in monitoring intercellular pH values, HepG2 cells were treated with nigericin sodium which provided the homogeneous endocellular pH with that of the surrounding medium. As


Page 12 of 22

Page 13 of 22

shown in Fig. 7c, the blue fluorescence of N-4 gradually decreased within the cells with the pH increasing from 3 to 9. Meanwhile, a corresponding enhancement of fluorescence in the red channel could be observed. The ratio (RB/R) images were established by fluorescence detection at 445 ± 25 nm (blue channel) and 615 ± 35 nm (red channel) using Carestream software. A significant pH-dependent decrease of the fluoresecnce ratio value in cells was observed. Simultaneously, the cytotoxicity of N-4 was evaluated through HepG2 cells via CCK8 assay. As shown in Fig. 8, the viabilities were estimated to be > 80% at 5 h or 10 h in the presence of 1-10 μM N-4, as determined by the MTT assay. Thus, N-4 had the potential to be used in biological applications.

a

PH=3 PH=4 PH=5 PH=6 PH=7 PH=8 PH=9 PH=10 PH=11

250 200 150 100

b

1.3 1.2

y = 1.37001pH-0.07553 2 R = 0.9738

1.1

I398/I572

300


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.9 0.8 0.7 0.6

50

0.5

400

450

500

550

600

650

2

3

Wavelength / nm

4

5

6

7

pH

8

9

10

11

12

Fig. 7 a) Fluorescence emission change of molecule N-4 (10 μM) in phosphate buffer at different pH values; b) Intracellular pH calibration curve of N-4 in HepG2 cells. R means the ratio of green to red fluorescence intensity of each cell (Igreen/Ired); C) Confocal microscopy images in HepG2 cells pre-loaded on N-4 (50 μg/mL, 20 min) and further incubated in buffers with various pH values (3.0, 5.0, 7.0 and 9.0) in the presence of 10 μM of nigericin for 30 min at 37 °C. Blue channel: 445 ± 25 nm, λex = 405 nm; Red channel: 615 ± 35 nm, λex = 405 nm scale bar = 10 μm.



5h

120

Cell Viability (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 h

100 80 60 40 20 0

con

1

2.5

5

7.5

Concentration (M)

10

Fig. 8 Cell viability values (%) estimated by CCK8 assay in HepG2 cells, which were cultured in the presence of 0-10 μM N-4 for 5 and 10 h.

Hydrogen bonding interaction is an important non covalent bonding interaction which is existed in many fields.48 For this self-assembly system, there is only hydroxyl group in N-18 molecule structure for production hydrogen bonding interaction. In order to understand the intermolecular hydrogen bonding of N-18 self-assembly system, FTIR experiments of xerogels N-18 from different solvents were carried out (Fig. S11). As shown in Fig. S11, the O-H stretching bands (νOH) in xerogels N-18 from DMF/H2O (4/1, v/v), DMSO/H2O (4/1, v/v), n-hexane, petroleum ether, acetonitrile and toluene were at 3371, 3371, 3376, 3376, 3376 and 3367 cm-1 at room temperature, indicating the formation of intermolecular hydrogen bonding in organogels N-18. Compared with the hydrogen bonding formed by N-H bond, the hydrogen bonding interaction intensity was not too strong which was possible due to O atom with more electronegativity than that of N atom. The C=O stretching bands of naphthalimides in xerogels from different solvents were all at 1696 cm-1. To gain a deeper insight into the assembly mechanism, powder X-ray diffraction (XRD) experiments of xerogels N-18 formed in different solvents were carried out in Fig. 9. As shown in Fig. 9a, there were a serial of diffraction peaks at 2θ = 3.99°, 8.03°, 12.06° and 16.12° with the corresponding d-space values of 2.23, 1.10, 0.73 and 0.55 nm in the XRD pattern of xerogel N-18 formed in n-hexane, which were in the ratio of 1: 1/2: 1/3: 1/4, indicating a lamellar structure with an interlayer distance of 2.23 nm.35 The diffraction peak at 2θ = 25.5o with the d-spacing at 0.35 nm was attributed to the interplanar space between naphthalimide rings of N-18 molecules based on the contribution of intermolecular π−π stacking interactions. The similar XRD patterns were found in the other xerogels N-18 formed in acetonitrile, toluene, petroleum ether, DMF/H2O (4/1, v/v) and DMSO/H2O (4/1, v/v) even if having some slight difference (Fig. 9b-f). From Fig. 9e and 9f, the XRD diffraction intensity of xerogels N-18 formed in DMF/H2O (4/1, v/v) and DMSO/H2O (4/1, v/v) were not strong like that of the xerogels N-18 in other four solvents, which indicated the degree of self-assembly order of N-18 molecule in DMF/H2O (4/1, v/v)


Page 15 of 22

and DMSO/H2O (4/1, v/v) was lower than that of N-18 in other four solvents. On the base of spectra data and XRD results, the probable self-assembly mode of N-18 molecule was speculated in Fig. S12. The molecule N-18 was firstly stacked into a fibres or belts structures, the fibres or belts were further intertwined and formed three-dimensional network.

90000

a

b

d=2.19nm

d=2.23nm

80000

Xerogel N-18 from hexane

60000 50000

50000

Intensity

Intensity

70000

d=1.10nm

40000 d=0.44nm

30000

40000 30000

d=0.44nm

d=1.10nm

20000

20000

d=0.74nm d=0.55nm

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d=0.73nm d=0.55nm

10000

0

0

c 50000

5

10

15

20

25

30

2 / degree

d=2.21nm

56000

xerogel N-18 from toluene

Intensity

d=1.09nm

25000 20000

d=0.74nm

15000 10000

e 5000

5

10

15

20

2 / degree

d=2.21nm

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30

xerogel N-18 from petroleum ether

40000

35000 30000

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d

d=0.40nm d=0.35nm

48000

45000 40000

Intensity

xerogel N-18 from acetonitrile

70000

60000

32000 d=1.09nm

24000 16000

d=0.55nm

d=0.39nm d=0.35nm

d=0.35nm d=0.44nm d=0.73nm d=0.55nm d=0.38nm

8000 0

5

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2 / degree

25

5

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f

d=2.24nm

1600

xerogel N-18 from DMF/H2O (4/1, v/v)

4000

10

15

d=0.44nm d=0.49nm d=0.42nm d=0.40nm

3000

d=1.11nm d=0.74nm d=0.62nm d=0.59nm

2000

d=0.38nm d=0.36nm d=0.35nm

1000

20

2 / degree

25

30

xerogel N-18 from DMSO/H2O (4/1,v/v) d=0.41nm

d=2.24nm

1400

Intensity

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


d=0.44nm

1200 d=0.97nm

1000

d=0.36nm

d=1.11nm

800

d=0.82nm d=0.59nm

600 400

5

10

15

20

2 / degree

25

30

200

5

10

15

20

2 / degree

25

30

Fig. 9 XRD patterns of xerogels N-18 from n-hexane (a), acetonitrile (b), toluene (c), petroleum ether (d), DMF/H2O (4/1, v/v) (e) and DMSO/H2O (4/1, v/v) (f).

Materials surface wettability was usually decided by two aspects including chemical composition and microstructure of material surface.49-54 In a way, surface wettability could reflect the self-assembly behavior in gel system.55 Surface wettability of the xerogel films N-18 from different solvents were investigated through water contact angle experiments. As shown in Fig. 10, all xerogel films exhibited hydrophobic properties with contact angles of 125°, 140°, 119°, 146°, 148.5° and 142.5°. The chemical composition was same, and their different surface wettability was due to their different microstructure. Xerogel films formed in n-hexane and toluene exhibited relative weak hydrophobicity. It was possible because the hydrophilic hydroxyl group was exposed to the materials surface and their small roughness. Xerogel films formed in DMF/H2O (4/1, v/v) and DMSO/H2O



(4/1, v/v) still exhibited strong hydrophobicity which was not affected by H2O.

Fig. 10 Water contact angle experiments results of the xerogels N-18 film formed in n-hexane (a), acetonitrile (b), toluene (c), petroleum ether (d), DMF/H2O (4/1, v/v) (e) and DMSO/H2O (4/1, v/v) (f).

Conclusion In conclusion, we developed a novel supramolecular self-assembly system (N-4, N-12 and N-18) based on naphthalimides by one step reaction between 3-hydroxy-1,8-naphthalic anhydride and alkyl amines with high yields. With the alkyl chain increasing, their gelation ability was ehanced from N-4 without gelation ability to organogels N-18 formed in six kinds of solvents. Hydrogen bonding and π-π stacking were the main driving forces of organogel formation. The J-type aggregate mode existed in the self-assembly system. The different self-assembly morphologies including microbelts structur and nanofibers were observed in the sol-gel transition process. The self-assembly materials surfaces obtained from different solvents all featured the hydrophobic properties with the contact angles of 119°-148.5°. Interestingly, these gelators exhibted solvatochromism behaivor, and the maximum fluorescence emission peak was in the range of 399-434 nm in the testing solvents. Especially, their emission light was changed from blue to red in DMF and DMSO. More significantly, these compounds could fast and sensitively respond toward organic amines. The detection limit for triethylamine (TEA) by solution N-18 in acetonitrile was 2.23 ×10-6 M. Fatty amine and aromatic amine could be distiguished by N-18 solution or organogel N-18. The ratio of red to blue fluorescence intensity illustrated a specific pH-dependence with a good linear correlation with R = 0.98883 over the pH range from 3.0 to 11.0. N-4 could be applied as the colorimetric fluorescence probe for quantitative monitoring the pH values change in cell. ASSOCIATED CONTENT

Supporting Information


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1H

NMR and

13C

NMR spectra of these compounds and Preparation of various solutions can be found in

supporting information.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

Notes

The authors declare no competing financial interest.

Acknowledgment The authors thanks for the financial support by the National Natural Science Foundation of China (U1704164), Henan Provincial Department of Science and Technology Research Project (172102210088), the Science & Technology Innovation Talents in Universities of Henan Province (No. 17HASTIT005) and the Nanhu Scholars Program for Young Scholars of XYNU.

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Injectable

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Hydrogels

Cross-Linked

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Discrete

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Graphical abstract


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Sensing Organic Amines and Quantitative Monitoring Intracellular pH

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