Introduction

Fluorescence sensors for biorelated molecules have attracted great attention because of their high sensitivity, fast detection, and simplicity [1,2,3]. Developing sensors to detect anions is important because anions are involved in various biological events [4] For example, sulfate ions play a significant role in environmental and biochemical processes, such as biosynthesis, and are found in various compounds, such as sulfate binding proteins [5,6,7]. The necessary properties of anion sensors include high selectivity for a specific anion and strong binding ability [8]. Mimicking proteins is a promising design strategy to develop anion sensors that satisfy these demands. Similarly, introducing hydrogen bond donors, such as urea groups, in molecular cleft structures within the enzyme reaction pockets effectively achieves high affinity and selectivity when functional groups are sterically arranged [9, 10].

As a platform for realizing molecular recognition systems, we focused on polyhedral oligomeric silsesquioxane (POSS), which is a molecule with a cubic silica core and eight organic groups at each vertex [11,12,13,14,15,16,17,18,19,20,21,22]. By introducing luminophores at each vertex, thermally stable light-emitting diodes and solid-state luminescent materials were obtained [23,24,25,26,27]. In addition, chemiluminescence was accelerated in the presence of the accumulated dyes created by POSS [28]. It has been demonstrated that rigid three-dimensional POSS can effectively create a hydrophobic space around the POSS core in aqueous media, and various types of guest molecules can be captured through hydrophobic interactions [29,30,31,32,33,34,35,36,37,38,39,40]. Based on this capturing mechanism, fluorescence sensors can be generated by using POSS as a scaffold [36,37,38]. Moreover, we have demonstrated that molecular recognition through hydrogen bonds can be enhanced in POSS-core dendrimers and POSS-containing network polymers [39, 40]. In previous work, we synthesized a series of modified POSSs with polycyclic aromatic hydrocarbons, such as naphthalene, anthracene, and pyrene, and investigated their optical properties [41]. The results indicated that intramolecular interactions can be formed at the side-chains, and excimer formation and subsequent environment-sensitive emission were observed through the assembly and release of aromatic rings connected to POSS [42]. From these results, we presumed that by designing an appropriate structure for the ligand, the aggregation and dispersion of side chains could be controlled with specific target molecules, resulting in a unique sensor. Based on this idea, we designed a ligand structure in modified POSSs to achieve molecular recognition.

It is known that urea groups can bind to various anions through hydrogen bonding due to their hydrogen-bonded donor protons [9, 10, 43,44,45,46,47,48,49,50,51]. If eight urea groups are arranged in a three-dimensional POSS core, multiple intramolecular urea groups may chelate the anions, resulting in a high binding affinity. Furthermore, by modifying luminophores known to express excimer emission, the distance between the luminophores can be observed as a color change [52, 53]. Herein, we synthesized eight urea-modified POSS materials and found that POSS has a high binding affinity for sulfate ions. In addition, it was shown that POSS can be used as a fluorescent sensor for quantifying sulfate ions by modifying naphthalene as a luminophore via an excimer.

Results and discussion

We prepared UPOSS, which was not modified with luminophores, to avoid steric hindrance, as shown in Scheme 1. UPOSS was synthesized by reacting POSS with aminopropyl groups and phenyl carbamate in the presence of N,N-diisopropylethylamine. The structure of UPOSS was confirmed by 1H, 13C, and 29Si NMR spectroscopy and high-resolution mass spectrometry. We also prepared N-propyl urea (UP, Fig. 1), which has the same structure as the side chain of UPOSS, as a model compound to evaluate the influence of the POSS core on the optical properties.

Scheme 1
scheme 1

Synthesis of UPOSS and chemical structure of the model compound (UP)

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Fig. 1
figure 1

1H NMR spectra of UPOSS (2 mM, a) and UP (16 mM, b) with various anions of tetrabutylammonium salts (16 mM) in DMSO-d6. The asterisks (*) denote the signals from BPh4−

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Initially, 1H NMR measurements of UPOSS were performed with 8 equivalents of tetrabutylammonium salts of various anions in DMSO-d6 solutions to screen for anionic species capable of binding (Fig. 1a). According to the peak shifts and broadening of the protons on the urea groups at 5.5 and 6.0 ppm, most of the anions hardly induced significant changes. The peaks disappeared in the presence of hydroxide ions, and a slight low-field shift occurred in the presence of acetate ions. The most pronounced low-field shift was induced by sulfate ions. This is a typical response of urea groups when bonded to anions with hydrogen bonds [9, 10], indicating that UPOSS can bind to sulfate ions in DMSO solution. The same experiments were also performed with the model compound UP (Fig. 1b). Similar to UPOSS, significant low-field shifts of the peak derived from the urea groups were observed. This result indicates that UP can also bind sulfate ions.

To evaluate the binding ability of sulfate ions, titration was performed using 1H NMR spectroscopy (Fig. 2a). The horizontal axis shows the equivalents of sulfate ions to urea groups while the vertical axis shows change in the chemical shift of the urea NH proton. It was shown that UPOSS should have a greater binding ability than that of UP because the peak shift of UPOSS was saturated with fewer anion equivalents to urea groups. These data are explained by the cooperative effect caused by the binding of multiple urea groups within a single POSS molecule. To further evaluate the binding mode of UPOSS to sulfate ions, a job plot was created with the 1H NMR data (Fig. 2b). Accordingly, a clear relationship was found between the mole fraction (X, horizontal axis) and product generated by the change in NMR peak and the molar fraction (Δδ·X, vertical axis), in which the molar fraction (X) showing the maximum value of Δδ·X indicates the host and guest equivalents [54]. The molar fraction of POSS (XPOSS), which showed a maximum value of 0.34, suggested that UPOSS and sulfate ions form 1:2 complexes with four urea groups that chelate sulfate ions.

Fig. 2
figure 2

a 1H NMR titration curves of the NH signals of UPOSS (2 mM) and UP (16 mM) with increasing amounts of sulfate ions in DMSO-d6/H2O (98/2, v/v). b 1H NMR Job plot for the binding of UPOSS to sulfate ions (total concentration = 10 mM) in DMSO-d6/H2O (98/2, v/v)

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We constructed UPOSS modified with naphthalene as a luminophore to verify whether this system can be used as a fluorescent sensor for quantifying sulfate ions, as shown in Scheme 2. Naphthalene can exhibit excimer emission when two naphthalenes are close to each other [41, 55,56,57]. Therefore, it was presumed that a photoluminescence spectrum alteration could be induced by the binding of sulfate ions and changes in the distance between the naphthalenes. To achieve this goal, NUA3POSS was synthesized by reacting Amino-POSS with naphthyl isocyanate under basic conditions. We also prepared NUP as a model compound, which has the same structure as the side chains of NUA3POSS. Furthermore, to evaluate the effect of the side chain length on the detection sensitivity, NUA5POSS and NUA7POSS were also synthesized, as shown in Scheme 3. These compounds were then characterized by NMR spectroscopy and high-resolution mass spectrometry.

Scheme 2
scheme 2

Synthesis of NUA3POSS and NUP

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Scheme 3
scheme 3

Synthesis of NUA5POSS and NUA7POSS

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The binding properties of NUA3POSS and NUP to sulfate ions were studied by 1H NMR measurements. Accordingly, the NH-derived peaks of the urea groups at 6.5 and 8.5 ppm were found to be significantly shifted in a low magnetic field, similar to UPOSS and UP. These data indicate that NUA3POSS and NUP can also capture sulfate ions even when they are modified with naphthalene. Next, the optical properties of NUA3POSS and NUP were measured in DMSO with or without sulfate ions (Fig. 3a). Only monomer emission was observed at approximately 380 nm, without a significant difference between NUA3POSS and NUP. The emission spectra of NUA3POSS changed when 100 equivalents of sulfate ions were added to the solution. For NUP, only monomer emission was detected after sulfate ions were added. On the other hand, for NUA3POSS, the emission intensity at approximately 450 nm increased after sulfate ions were added. From the fluorescence lifetime monitored at this peak wavelength, it was found that the lifetime was extended by approximately 25 ns. This finding suggested that the luminescence at approximately 450 nm was derived from the excimer binding to sulfate ions. Thus, the modification of POSS with luminophores and urea groups enables the detection of sulfate ions as the luminescence color changes. In this case, however, the luminescence remains in the blue area, and it is difficult to observe the color change with the human eye. Less significant changes were observed in the model compound NUP, implying that the naphthalene units might be far apart because they exhibited low binding ability in the dilute solution and could not bind to the sulfate ion.

Fig. 3
figure 3

a PL spectra of NUA3POSS (1.0 × 10−6 M) and NUP (8.0 × 10−6 M) with and without sulfate ions (1.0 × 10−4 M) in DMSO excited at the wavelengths of the absorption maxima. b PL spectra of NUA3POSS (1.0 × 10−6 M) with various anions (1.0 × 10−4 M) in DMSO excited at the absorption maxima

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To evaluate the selectivity of NUA3POSS for anion detection, optical measurements were performed with various anions. Accordingly, any spectral changes were hardly induced by acetate ions, while only a decrease in the emission intensity was observed when hydroxide ions were added (Fig. 3b). The results implied that hydroxide ions might form tight hydrogen bonds at the urea protons, whereas the distance between the naphthalene units should hardly be influenced. Therefore, the excimer state was not formed. In summary, NUA3POSS has high selectivity and high binding affinity for sulfate ions, followed by excimer emission.

Subsequent titration was performed with sulfate ions using photoluminescence spectroscopy to evaluate the detection sensitivity and quantification capability of NUA3POSS (Fig. 4a). Accordingly, marked changes in the spectrum were observed even in the concentration range of a few μM. Figure 4b shows the changes in the ratio of excimer emission to monomer emission on the vertical axis and the concentration of sulfate ions on the horizontal axis. The ratios linearly changed in the concentration range, indicating that NUA3POSS can quantify sulfate ions. NUA7POSS and NUA5POSS were also titrated. A comparison of the slopes of the changes for each compound revealed that the sensitivities were greater for NUA3POSS, NUA5POSS, and NUA7POSS in that order. A shorter linker length is likely favorable for assembling urea groups and subsequently enhancing the binding affinity for sulfate ions.

Fig. 4
figure 4

a PL spectra of NUA3POSS (1.0 × 10−6 M) with various concentrations of sulfate ions in DMSO excited at the absorption maxima. b Relationships between NUA3POSS, NUA5POSS, and NUA7POSS (1.0 × 10−6 M) with the ratio of the excimer emission intensity to the monomer emission intensity and the concentration of sulfate ions

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Conclusion

In this study, POSS derivatives modified with eight urea groups were successfully synthesized. UPOSS has a high binding affinity for sulfate ions. Furthermore, the data used for the job plot indicated that POSS forms a 1:2 complex with sulfate ions, suggesting that the high binding capacity is derived from cooperative effects of chelate-type bonding. Furthermore, naphthalene-modified POSS can be used as an anion sensor. Based on the preprogrammed molecular design of the hydrogen-bonding patterns, these results suggest that POSS is a versatile scaffold for strong anion receptors through hydrogen bonds.