Novel Amino-pillar[5]arene as a Fluorescent Probe for Highly Selective Detection of Au3+ Ions.

A novel fluorescent probe, amino-pillar[5]arene (APA), was prepared via a green, effective, and convenient synthetic method, which was characterized by nuclear magnetic resonance (NMR), infrared (IR), and high-resolution mass spectrometry. The fluorescence sensing behavior of the APA probe toward 22 metal ions in aqueous solutions were studied by fluorescence spectroscopy. The results showed that APA could be used as a selective fluorescent probe for the specificity detection of Au3+ ions. Moreover, the detection characteristics were investigated by fluorescence spectral titration, pH effect, fluorescence competitive experiments, Job's plot analysis, 1H NMR, and IR. The results indicated that detection of Au3+ ions by the APA probe could be achieved in the range of pH 1-13.5 and that other coexisting metal ions did not cause any marked interference. The titration analysis results indicated that the fluorescence intensity decreased as the concentration of Au3+ ions increased, with an excellent correlation (R 2 = 0.9942). The detection limit was as low as 7.59 × 10-8 mol·L-1, and the binding ratio of the APA probe with Au3+ ions was 2:1. Therefore, the APA probe has potential applications for detecting Au3+ ions in the environment and in living organisms.

Introduction

Au3+ ions, a notable heavy metal, have received great attention in chemistry and biology during recent decades and have been widely used in gold plating,[1] environmental studies,[2] anticancer agents,[3] nanomaterials, efficient catalysts, biological sensors, and drug/gene delivery systems based on their unique chemical properties and high biocompatibility.[4−7] In addition, these ions have also been developed for application in the jewelry industry.[8] Studies have shown that Au3+ ions can be used to prepare reduced graphene oxide/Au nanoparticles (NPs), drugs, and catalysts to detect dopamine, ascorbic acid, and uric acid; to treat tuberculosis and rheumatoid arthritis; and to activate carbon–carbon triple bonds.[9−11] Although Au3+ ions have versatile roles in materials science and biological systems, studies have demonstrated that Au3+ ion-based drugs exhibit potential toxicity at certain concentrations.[12] Alternatively, the Au3+ ions may be broken down and cause damage to the liver, kidney, and nervous system when bound to proteins and DNA.[13,14] Therefore, it was essential to explore a fast, operationally simple, and capable method to detect metabolic processes in living organisms, in view of the medically, physiologically, and environmentally important research value of gold derivatives.

Recently, fluorescent probes are the most effective methods in the field of ion detection because of their instant response,[15] operational simplicity,[16,17] and high sensitivity.[18−20] In recent years, the synthesis of macrocyclic compounds and their functionalization for applications in fluorescent probes have attracted the interest of many researchers. Pillararene was the only highly symmetrical tubular molecule found after the investigation of other macrocyclic molecules, such as crown ethers, cyclodextrin, calixarene, and cucurbituril[21] and has been widely applied in biology,[22] drug delivery,[23] separation process,[24] functional materials,[25,26] and the environment since 2008 owing to its unique rigid structural characteristics and good host–guest recognition performance.[5]arenes: Their Lewis Acid Catalyzed Synthesis and Host–Guest Property. J. Am. Chem. Soc.. 2008 ">27,28] To date, many researchers have successfully applied pillararenes to ion detection.[29−31] For example, Huang’s research group reported a fluorescent chemosensor anthracene-appended 2:3 copillar[5]arene to detect Fe3+.[5]arene: synthesis, computational studies, and application in highly selective fluorescence sensing for Fe(iii) ions. Chem. Commun.. 2015 ">32] Yuan and Feng et al. reported a nonsymmetric pillar[5]arene bearing triazole-linked 8-oxyquinolines, which was applied as a sequential fluorescence sensor for thorium(IV) followed by fluoride ions with high sensitivity and selectivity.[5]arene based on triazole-linked 8-oxyquinolines as a sequential sensor for thorium(iv) followed by fluoride ions. Dalton Trans.. 2015 ">33] Xia and Wang et al. developed a host–guest system between pillar[5]arene and a rhodamine B-containing amphiphile as a Cu(II) ion sensor in aqueous media.[5]arene and a Rhodamine B-Containing Amphiphile in Aqueous Media. Org. Lett.. 2017 ">34] In addition, the Stoddart et al. has reported gold recovery from gold-bearing raw materials by α-cyclodextrin.[35]

In this paper, we synthesized a novel fluorescent probe, amino-pillar[5]arene (APA), bearing water-soluble amino groups. The amino groups were successfully modified on the pillar[5]arene via aminolysis using ethylenediamine as both a reactant and a solvent. Subsequently, the application of APA in Au3+ ion detection with 22 metal ion species was studied. Moreover, pH effect analysis, fluorescence spectral titration, Job’s plot analysis, 1H NMR, and fluorescence competitive experiments were carried out. To the best of our knowledge, no reports concerning APA as a fluorescent probe for detecting Au3+ ions have been found in the literature. Therefore, the primary aim of this work was to synthesize a novel fluorescence probe, APA, which had potential for application in the detection of Au3+ ions in the environment and living organisms.

Results and Discussion

APA Fluorescence Performance Measurement

The APA probe showed yellow-white fluorescence (Figure a) under a UV lamp at 365 nm, as observed by the naked eye, and a sharp peak (Figure b) was detected at 323 nm in the fluorescence spectrum of aqueous APA. The experimental results and pictures show that APA can not only produce yellow-white light in solid form but also detect a strong fluorescence peak in aqueous solution. Although the emission peak of the APA aqueous solution was in the UV emission range and difference from the solid-state luminescence performance, due to the luminescent properties of substance were not only related to itself and form. Moreover, different solvents also affect the luminescence properties. Therefore, the luminescent properties of the substances in different forms were independent of each other.[36,37] Therefore, APA could be used as a fluorescence probe to study the recognition of the host–guest system of metal ions.

Figure 1

(a) Photograph of APA observed under a UV lamp at 365 nm and (b) APA fluorescence spectrum in aqueous solution.

Detection of Metal Ions

The fluorescence spectra (Figure ) showed an obvious decrease in fluorescence intensity from 703 to 20 au after adding 3 equiv of Au3+, but other metal ions did not show major changes. The results showed that the APA probe had high selectivity for Au3+ ions in aqueous solution and could thus be used as a fluorescence probe to detect Au3+ ions.

Figure 2

Fluorescence spectral responses of the APA probe (2 × 10–5 mol·L–1) in aqueous solution upon addition of 3 equiv of Ag+, Al3+, Ba2+, Bi+, Ca2+, Cd2+, Co3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Na+, Pb2+, Sb2+, Sn2+, Sr2+, Ni2+, and Au3+ (λex = 323 nm).

Fluorescence Spectral Titration

To further study the interaction between Au3+ ions and APA, fluorescence spectral titration experiments were carried out. A series of APA (2 × 10–5 mol·L–1) and Au3+ ion solutions were prepared. The concentrations of Au3+ ions ranged from 0 to 6 equiv (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, and 3.0). The fluorescence spectrum measurement method was the same as described in Section . The results are shown in Figure . The fluorescence intensity gradually decreased with increasing concentrations of Au3+ ions, and the APA fluorescence was almost completely quenched when 4 equiv of Au3+ ions was added. Furthermore, there was a good linear curve fitting between the fluorescence intensity and Au3+ ion concentration over a range of concentrations. As shown in Figure b, the linear equation was y = 659.93 – 209.40X, R2 = 0.99365.

Figure 3

(a) Fluorescence spectra of the APA probe in the presence of different concentrations of Au3+ ions and (b) linear fitting curve of the APA probe with increasing concentrations of Au3+ ions.

The lowest detection limit was calculated by the following formulaLOD (mol·L–1): detection limit. δ (mol·L–1): standard deviation of 10 blank measurements. S = 209.40: the slope of a linear fitting curve from fluorescence spectral titration experiments.

The lowest detection limit obtained from formula 1 was 7.59 × 10–8 mol·L–1.[38,39] The results showed that the APA probe had a high sensitivity to detect Au3+ ions and that APA could be used as a fluorescence probe to detect Au3+ ions quantitatively in aqueous solution.

Job’s Plot

The stoichiometry of APA and Au3+ ions was determined by the Job method.[40−43] The total molar concentrations of APA and Au3+ ions were maintained at 2 × 10–5 mol·L–1 in aqueous solution. The molar fraction of Au3+ ions ([Au3+]/([Au3+] + [A])) varied from 0 to 1.0. Then, the fluorescence intensity was recorded at 323 nm by a fluorescence spectrometer at room temperature. A Job plot (Figure a) was generated by taking the molar fraction of Au3+ ions as the X-coordinate and the fluorescence intensity multiplied by the molar fraction as the Y-coordinate. The binding stoichiometry of the APA probe with Au3+ ions was determined from the highest point of the Job plot. If the highest point of the Job plot corresponds to the X-coordinates of 0.33, 0.5, and 0.67, the binding stoichiometries of the APA probe with Au3+ ions were 2:1, 1:1, and 1:2, respectively. It can be seen from Figure a that the highest point corresponding to the X-coordinate was 0.33, from which it could be inferred that the binding ratio of APA to Au3+ ion was 2:1.

Figure 4

(a) Job’s plot for the APA probe and Au3+ ion system ([APA] + (Au3+) = 2 × 10–5 mol·L–1) at 323 nm in aqueous solution; (b) fluorescence spectra of the APA probe in the presence of Au3+ ions ([APA] + (Au3+) = 2×10–5 mol·L–1).

pH Effect Studies

In general, pH was also a significant factor affecting ion fluorescence, which determined the sensitivity of detecting ions. In this work, the pH effect was investigated by adding Au3+ ion (2 equiv) to the solution of APA probe (2 × 10–5 mol·L–1). The pH range of 1–13.5 was regulated by using 0.1 mol·L–1 HCl and 0.1 mol·L–1 NaOH. The APA fluorescence intensity increased as the pH increased (Figure ). In a strong acid environment, the fluorescence intensity of APA was relatively weak, which could be explained by protonation of the amino group. The APA fluorescence was relatively strong in neutral and alkaline environments, and the maximum intensity was 744.356 au. Minor changes were observed in the pH range of 1–13.5 when the Au3+ ion solution was added, which indicated that the APA fluorescent probe was not significantly affected by the detection of Au3+ ions in this pH range. APA showed very stable fluorescence characteristics in the pH range of 1–13.5. Therefore, this fluorescent probe could be used for detecting Au3+ ions in subsequent research in acidic, neutral, and alkaline environments.

Figure 5

Effect of pH on APA fluorescence intensity and Au3+ ion detection.

Competition with Other Metal Ions

For a good fluorescent probe, the most important factor is that the detected ions are not affected by the addition of competitive ions. In this research, the detectability of Au3+ ions by APA was evaluated by fluorescence competitive experiments, which were carried out by adding Au3+ ions (2 equiv) to the APA (2 × 10–5 mol·L–1) solution with Ag+, Al3+, Ba2+, Bi+, Ca2+, Cd2+, Co3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Na+, Pb2+, Sb2+, Sn2+, Sr2+, and Ni2+ (2 equiv) cations and Br–, I–, PO43–, HPO42–, Cl–, HSO3–, CO32–, HCO3–, SO42–, NO3–, NO2–, and F– (2 equiv) anions, the results are shown in Figure . It was found that other coexisting metal ions did not cause any marked interference. Therefore, APA had good selectivity for the detection of Au3+ ions.

Figure 6

(a) Fluorescence intensity of the fluorescent probe APA with Au3+ ions in the presence of different competitive cations (Ag+, Al3+, Ba2+, Bi+, Ca2+, Cd2+, Co3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Na+, Pb2+, Sb2+, Sn2+, Sr2+, and Ni2+). (b) Fluorescence intensity of the fluorescent probe APA with Au3+ ions in the presence of different competitive anions (Br–, I–, PO43–, HPO42–, Cl–, HSO3–, CO32–, HCO3–, SO42–, NO3–, NO2–, and F–).

1H NMR and infrared Spectra

To confirm the binding mode and mechanism of the fluorescent probe APA with Au3+ ions, the 1H NMR spectra of APA and different equivalents of Au3+ ions were measured in D2O. Figure showed that the obvious gradually disappeared APA peaks at 6.89 ppm (1), 4.54 ppm (3), and 3.88 ppm (5) when the concentration of Au3+ ions increased. Meanwhile, the obvious low-field shifts of APA peaks at 3.16 ppm (6) and 2.80 ppm (7) after addition of Au3+ ions. However, there were no significant changes at other peaks. Therefore, it inferred that the interaction between Au3+ ions with N signal peak (5) of APA, and at signals peaks 1, 2, and 3 disappearance of APA due to the electron cloud was shielded, which demonstrated again that APA and Au3+ ions formed a complex compound.

Figure 7

1H NMR spectra at (a) APA concentration of 2 × 10–3 mol·L–1; (b) APA concentration of 2 × 10–3 mol·L–1 with 2 equiv of Au3+ ions; (c) and APA concentration of 2 × 10–3 mol·L–1 with 5 equiv of Au3+ ions (D2O as solvent).

To further infer the binding mode and mechanism of the fluorescent probe APA and Au3+ ions, the infrared (IR) spectra of APA and complex compound were measured. The result is shown in Figure a; by comparing the IR spectra of APA with the complex, it can be clearly observed from Figure a that the peak intensity and peak shape at 3400 (N), 1650 (C=O), and 1150 (C–O–C) all have changed after Au3+ was complexed with APA. However, there was no significant change in the peak position corresponding to the benzene ring (1600, 1580, and 1500). Therefore, it can be inferred that Au3+ interacted with the N branch of the APA and without interacting with the benzene ring. On the basis of the findings of the above results, we inferred that the binding mode of APA with the Au3+ ions was shown in Figure b.

Figure 8

(a) IR spectra of APA and Au3+ ions and (b) binding mode of APA and Au3+ ions.

Conclusions

In summary, the fluorescent probe APA was successfully synthesized and characterized by nuclear magnetic resonance (NMR), IR, and high-resolution mass spectrometry (HRMS). Fluorescence spectroscopy showed that APA can be used as a probe to detect Au3+ ions. APA exhibited stable fluorescence characteristics in the pH range of 1–13.5, and other coexisting ions did not cause any marked interference. The binding mode was further confirmed by Job’s plot analysis, 1H NMR, and IR, showing that the binding ratio of APA to Au3+ ions was 2:1. Therefore, the fluorescent probe APA may open new prospects for the design and metabolism of gold-based drugs in living organisms.

Experimental Section

Materials

1,4-Diethoxybenzene, paraformaldehyde, 1,2-dichloroethane, boron trifluoride etherate, chloroform, boron tribromide, ethanol, potassium carbonate, anhydrous acetonitrile, methyl chloroacetate, dichloromethane, anhydrous sodium sulfate, methanol, ethylenediamine, n-hexane, and hydrochloric acid were obtained from Aladdin Reagent Co. Ltd. Ag+, Al3+, Ba2+, Bi+, Ca2+, Cd2+, Co3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Na+, Pb2+, Sb2+, Sn2+, Sr2+, Ni2+, Au3+, Br–, I–, PO43–, HPO42–, Cl–, HSO3–, CO32–, HCO3–, SO42–, NO3–, NO2–, F– (AgNO3, Al(NO3)3·9H2O, BaCl2·2HO2, Bi(NO3)3·5H2O, CaCl2, CdCl2, CoCl2·6H2O, Cs2CO3, CuSO4·5H2O, FeSO4·7HO2, FeCl3·6H2O, HgCl2, KCl, LiF, MgSO4·7H2O, Na2SO4, Pb(NO3)2, SbCl3, SnCl2·2H2O, SrCl2·6H2O, NiCl2·6H2O, HAuCl4, NaBr, NaI, Na3PO4, Na2HPO4, NaCl, NaHSO3, Na2CO3, NaHCO3, Na2SO4, NaNO3, NaNO2, and NaF) metal ions were purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water was used throughout all experiments.

Syntheses of the APA Probe

The synthetic route to fluorescent probe E is depicted in Figure . First, compound B was obtained in one step by a Lewis acid-catalyzed condensation reaction of commercial starting material A and paraformaldehyde. Subsequently, deprotection of compound B with boron tribromide in chloroform produced compound C, which was further applied in the substitution reaction with methyl chloroacetate to achieve compound D. Finally, the desired product E was obtained via amidation of compound D with ethylenediamine.

Figure 9

Synthetic route to fluorescent probe E from A–D.

Synthesis of Compound B

Paraformaldehyde (2.8 g, 0.09 mol) was added to a solution of 1,4-diethoxy-benzene (5.0 g, 0.03 mol) in dry ClCH2CH2Cl (150 mL) under a nitrogen atmosphere at 25 °C. Next, BF3·OEt2 (4.2 mL) was added to the mixture, and the reaction was then stirred at 25 °C for 90 min. The mixture was then quenched with ethanol (20 mL). After stirring for 7 min, the filtrate was collected. The filtrate was extracted three times with deionized water, and the organic phase was combined and dried by anhydrous sodium sulfate. After removing the solvent, the obtained solid was dissolved in CH2Cl2 (30 mL) and precipitated in EtOAc. The solid was recrystallized from CH2Cl2 and n-hexane to obtain a white solid B (2.8 g, yield: 52%). 1H NMR (400 MHz, CDCl3): δ (ppm): 6.74 (s, 10H), 3.86 (q, J = 6.8 Hz, 20H), 3.77 (s, 10H), 1.28 (t, J = 6.8 Hz, 30H).

Synthesis of Compound C

Boron tribromide (4.5 mL, 0.044 mol) was slowly added to a solution of compound B (2.0 g, 0.022 mol) in CHCl3 (100 mL) under a nitrogen atmosphere at −12 °C. The reaction was then stirred at 25 °C for 24 h. The mixture was then quenched by ice water (50 mL) and stirred for 30 min. Finally, the solid was collected by filtration and washed by hydrochloric acid (36%). Then, the samples were washed three times with CHCl3 and dried to get a white solid C (1.42 g, yield: 90%). 1H NMR (400 MHz, DMSO): δ (ppm): 8.44 (s, 10H), 6.57 (s, 10H), 3.43 (s, 10H). 13C NMR (100 MHz, DMSO): δ (ppm): 146.5, 126.8, 117.7, 29.5.

Synthesis of Compound D

Potassium carbonate (10 g, 0.1 mol) and methyl chloroacetate (10 mL, 0.1 mol) were sequentially added to a solution of compound C (1.5 g, 0.0025 mol) in anhydrous CH3CN (60 mL) under a nitrogen atmosphere. The reaction was stirred at 85 °C for 36 h. After washing with dichloromethane, the filtrate was collected. Then, the solvent was removed, and the solid was recrystallized from CH2Cl2 and MeOH to obtain a white solid D (1.87 g, yield: 56%). 1H NMR (400 MHz, CDCl3): δ (ppm): 6.98 (s, 10H), 4.55 (s, 20H), 3.85 (s, 10H), 3.53 (s, 30H). 13C NMR (100 MHz, CDCl3): δ (ppm): 169.7, 148.8, 128.4, 114.4, 65.4, 51.9, 29.2.

Synthesis of Compound E

Compound D (1.33 g, 0.001 mol) was dissolved in ethylenediamine (5 mL) under a nitrogen atmosphere. The reaction was stirred at 110 °C for 12 h. Then, the solvent was removed, and the residue was dissolved in deionized water (15 mL) and finally extracted three times with CHCl3. The combined organic phase was washed with water, brined, dried by Na2SO4, filtered, and concentrated under reduced pressure to get a light-yellow solid E (1.4 g, yield: 87%). mp 126–127 °C. IR νmax (cm–1): 3.357, 2.931, 1.666, 1.541, 1.496, 1.438, 1.403, 1.206, 1.062, 934, 582. 1H NMR (400 MHz, D2O): δ (ppm): 6.88 (s, 10H), 4.54 (s, 20H), 3.88 (s, 10H), 3.53–3.16 (m, 20H), 2.97–2.40 (m, 20H). 13C NMR (100 MHz, DMSO): δ (ppm): 168.3, 149.4, 128.4, 115.1, 68.2, 42.0, 41.2, 29.3. HRMS (ESI) m/z calcd for C75H111N20O20 [M + H]+, 1611.8278; found, 1611.8277.

Fluorescence Spectral Measurement

Fluorescence Characteristics of APA

A stock solution of APA was prepared, which was further diluted to the appropriate concentration (2 × 10–5 mol·L–1). The emission spectrum was carried out using an excitation wavelength of 240 nm. The emission wavelength range was 280–450 nm. The excitation slit was adjusted to 10 nm, and the emission wavelength was adjusted to 10 nm.

Detection of Metal Ions

First, different metal ion solutions, including Ag+, Al3+, Ba2+, Bi+, Ca2+, Cd2+, Co3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Na+, Pb2+, Sb2+, Sn2+, Sr2+, Ni2+, and Au3+, were prepared. Then, different metal ions (5 equiv) were added to the APA solution (2 × 10–5 mol·L–1), and fluorescence spectra were measured and recorded under the conditions described in Section .