Diol responsive viscosity increase in a cetyltrimethylammonium bromide/sodium salicylate/3-fluorophenylboronic acid micelle system

We report a novel smart micellar system utilising a phenylboronic acid (PBA) derivative whose viscosity increases on adding diol compounds such as sugar or sugar alcohol. We prepared a typical worm-like micelle (WLM) system in 100 mM cetyltrimethylammonium bromide (CTAB)/70 mM sodium salicylate (NaSal), which showed high zero-shear viscosity (η0). Upon the addition of 20 mM 3-fluorophenylboronic acid (3FPBA) to the WLM system, η0 decreased by 1/300 that of the system without 3FPBA. Furthermore, upon the addition of 1.12 M fructose (Fru) and 1.12 M sorbitol (Sor) to the CTAB/NaSal/3FPBA system, η0 increased by 50-fold and 30-fold, respectively. 19F NMR spectral results of the systems using 4-fluorosalicylic acid (FSal) instead of NaSal demonstrated that the FSal/3FPBA-complex interacts with CTAB. Moreover, the addition of sugar or sugar alcohol to the micellar system leads to a decrease in the amount of FSal/3FPBA-complex interacting with CTA+ and an increase in the amount of 3FPBA/Fru or Sor-complex, which does not interact with CTA+. These changes in molecular interactions induce the elongation of the WLMs and increase the viscosity of the system. This system utilises the competitive cyclic ester bond between the NaSal/3FPBA and 3FPBA/sugar or sugar alcohol to induce viscosity changes.


Introduction
Worm-like micelles (WLMs) are molecular assemblies which are mainly formed by surfactants, and they show unique viscoelastic properties. 1,2 Stimuli-responsive WLMs are expected to be smart materials with applications in a wide range of areas. A variety of stimuli-responsive WLMs, with stimuli such as light, 3-6 redox reactions, 7,8 pH changes, [9][10][11] and CO 2 12 have been reported. 13 Phenylboronic acid (PBA) is a Lewis acid and a functional molecule, since it reversibly forms cyclic ester bonds with cis-diol compounds such as sugars (Fig. 1a). 14 This equilibrium shis to react with diol compounds, leading to a decrease in the molecular form of PBA and an increase in the anionic form. Utilising this change, PBA has been studied as a sugar sensor for analytical chemistry and supramolecular assemblies. [15][16][17] Sugar-responsive viscoelastic systems [18][19][20][21] are desired for applications in elds such as drug delivery, analytical chemistry, cell engineering or medical engineering. Recently, novel sugar-responsive WLMs composed of a PBA or PBA derivative and a cationic surfactant, cetyltrimethylammonium bromide (CTAB, Fig. 1b) were reported, whose viscosity is decreased on adding diols such as sugars. 22,23 However, to the best of our knowledge, there was no diolresponsive micellar system whose viscosity increases with the addition of diols. Herein, we report a novel diol-responsive viscosity-increasing micellar system.

Preparation of the mixed micellar systems
We prepared a mixed micellar system by mixing the two stock solutions under a few conditions. Stock solution A contained 200 mM CTAB or 200 mM CTAB with different concentrations of 3FPBA in distilled water. Stock solution B contained 140 mM NaSal and 200 mM phosphate, and its pH was adjusted to 7.4 using aqueous NaOH solution. Sugar or sugar alcohol was added as powder to System. The prepared mixed micellar systems were stored at 25 C for more than 24 h aer mixing at a temperature in the range of 15-25 C or with heating using a hot magnetic stirrer.

Observation of appearance
We prepared solutions (1.0 mL) in 1.5 mL microtubes or solutions (2.0 mL) in 6 mL glass vials. For microtubes, images were captured 3 s aer the microtubes were inverted.

Rheological measurements
A stress-controlled rotational rheometer (MCR-102, Anton Paar, Ostldern, Germany) was used for steady and dynamic rheological measurements at 25 C. We used a cone plate or concentric cylinder for both measurements. The strain (g) was xed at 10% for the dynamic viscoelasticity measurements.

Particle size measurements
The mean particle size was measured by dynamic light scattering (DLS) at 25 C with a Malvern Zetasizer Pro system (Malvern Panalytical, London, UK) equipped with a 4 mW and 633 nm He-Ne laser.
Fluorescence measurements for determination of apparent binding constant (K) Fluorescence spectra were recorded using a Shimadzu RF-5300PC instrument (Shimadzu Corporation, Kyoto, Japan). The K between 3FPBA and ARS, and K between 3FPBA and diol compounds was obtained according to a previously reported method. 14,24,25 NMR spectroscopy 1 H and 19 F NMR spectroscopy was conducted using a Varian 400 MR spectrometer (Agilent Technologies, CA, USA). For 19 F NMR spectroscopy, triuoroacetic acid was used as the external standard (chemical shis (d): À76.5 ppm), and the solvent used contained 10% D 2 O solution. For 1 H NMR spectroscopy, the pD values were calculated on the basis of the apparent pH values measured in D 2 O using the following equation: 26,27 pD ¼ apparent pH + 0.44 (1) The pD values were then adjusted using NaOD solution.

Results and discussion
It is known that boric acid forms cyclic ester bonds with salicylic acid 28 and CTAB forms typical WLMs upon the addition of NaSal. 1,2 Based on these reports, we hypothesised that adding a PBA derivative to the CTAB/NaSal WLM system induces a viscosity change. First, we prepared System A (pH 7.5) using 100 mM CTAB/70 mM sodium salicylate/100 mM phosphate, which is based on a typical WLM system, 1,2 and System A with 3FPBA. System B was then dened as System A with 20 mM 3FPBA. When microtubes containing the systems were inverted, System A maintained a gel-like appearance, while System B dropped rapidly (Fig. 2a). We also prepared System B with sugars, Fru and Glc, and sugar alcohol, Sor as diol compounds. The sample with 1.12 M Fru or 1.12 M Sor dropped slowly compared to System B (Fig. 2a). The sample with 1.12 M Glc dropped as rapidly as in System B. To study the effect of 3FPBA addition to the phase state of the CTAB/NaSal system, we prepared a 100 mM CTAB/110 mM NaSal/100 mM phosphate system (System C), a 100 mM CTAB/40 mM 3FPBA/100 mM phosphate system (System D), and a 100 mM CTAB/70 mM NaSal/40 mM 3FPBA/100 mM phosphate system (System E). Systems C and D were transparent and existed as a single phase, whereas System E resulted in phase separation (Fig. 2b).
To further investigate this unique phenomenon, we evaluated the rheological characteristics and the relationship between the shear rate (g) and viscosity (h). In System A, h was a constant at low ġand decreased aer a certain ġ (Fig. 3a). Such a characteristic rheological property is observed in typical WLMs. 29,30 Though System B and System B with 1.12 M Fru or 1.12 M Sor had different h at low g, they showed similar rheological behaviours to that of System A. We obtained the zeroshear viscosity (h 0 ) by the extrapolation of h, which is independent of ġat low g, onto the y-axis. The h 0 of System A decreased from 156 Pa s with increasing 3FPBA concentration, to 0.47 Pa s (1/300 compared to without 3FPBA) at 20 mM 3FPBA, and to 0.17 Pa s (1/900 compared to without 3FPBA) at 30 mM 3FPBA (Fig. 3b).
3FPBA was effective in decreasing the viscosity of System A. By contrast, the h 0 of System B with sugar or sugar alcohol increased from 0.47 Pa s with increasing diol concentration ( To obtain further rheological data, we measured dynamic viscoelasticity. The dynamic viscoelasticity measurements revealed the behaviours of both the storage modulus (G 0 ) and loss modulus (G 00 ) at different frequencies (u). These parameters are based on the Maxwell model in eqn (2) and (3), respectively: where s and G 0 are the relaxation time and plateau modulus, respectively. When the rheological behaviours follow the Maxwell model with a single s, G 0 , and G 00 can produce a semicircular curve in the Cole-Cole plot (G 0 versus G 00 ), as shown in eqn (4). 1,31 In both System A and System B with 1.12 M Fru, G 00 was higher than G 0 at low u, but lower at high u ( Fig. 4a and b). Neither System B nor System B with 1.12 M Glc had sufficient viscoelasticity for dynamic viscoelasticity measurements (data not shown). To study the formation of WLMs, we used a Cole-Cole plot, in which a semicircular curve is a rheological characteristic of typical WLMs. 1 System A and System B with 1.12 M Fru or 1.12 M Sor showed near-perfect semicircular curves (Fig. 4c). Based on these rheological characteristics, we presumed that these samples formed adequately long and entangled WLMs. We analysed the change in entanglement of WLMs in System B with the addition of sugar or sugar alcohol using s, an index of entanglement of WLMs. 29,32 We dened u c as the intersection point of G 0 and G 00 in the dynamic viscoelasticity measurements and obtained s as the inverse of u c . s increased with increasing concentrations of Fru or Sor in System B (Fig. 4d). The s of System A was 2.6 s, which is larger than that of System B with 1.12 M Fru (0.65 s). From these results, we deduce that adding 3FPBA to System A induces shortening of WLMs and adding Fru or Sor reverses this change.
DLS has been used to conrm the changes in the micellar forms. 12,22,[33][34][35] We determined the particle size through DLS to conrm the changes in the micellar systems upon the addition of diol compounds. The size distributions of System B and System B with diol compounds are shown in Fig. 5a. Aer the addition of 0.28 M Fru, Sor, and Glc, the peaks of the DLS spectra broadened, indicating larger size distributions. The mean particle sizes, in terms of Z-average, increased with an increase in diol compounds (Fig. 5b). Fru and Sor substantially affected the Z-average of the micellar systems in relation to Glc. These results showed that the micelles elongated upon the addition of diol compounds, particularly Fru and Sor. These results are consistent with those from the rheology measurements (Fig. 3c).
To elucidate the mechanism, we studied the binding constant (K) between 3FPBA and NaSal, sugars and sugar alcohol using the uorescence method with alizarin red S 14,24,25 (Fig. 6). The compounds in decreasing order of K are Sor > Fru > NaSal > Glc (Table 1). The ordering of Sor > Fru > Glc corresponded with the case of PBA. 14 Considering the similarities in the chemical structures of 3FPBA and PBA, and pH conditions (pH 7.4), the obtained K values are reasonable. Although the ordering of diols by the effect on h 0 is not perfectly consistent  with the ordering by K, the increasing effect of h 0 almost reected K. These results are indicative that the decrease in h 0 in System A with the addition of 3FPBA and the increase in h 0 in System B with the addition of sugar or sugar alcohol is associated with the formation of a competitive cyclic ester bond between 3FPBA and diol compounds.
To study the intermolecular interactions in the micellar systems, we performed 1 H and 19 F NMR spectroscopy. Because 19 F NMR spectroscopy can reveal the hybridisation state of boron that binds to uorobenzene, it is can be conducted to study the interaction between 3FPBA and diol compounds. 23,36,37 Moreover, it is relatively easy to analyse signal changes in 19 F NMR spectra because 19 F NMR does not detect signals that are derived from the hydrocarbons of diol compounds, surfactants, and aromatic rings. For the NMR spectroscopy, we used FSal (a salicylic acid derivative with uorine) instead of NaSal to investigate the interaction between FSal/3FPBA-complex and CTAB. To conrm the interaction between the quaternary ammonium ion of the cetyltrimethylammonium cation (CTA + ) and the aromatic ring or carboxylate anion of FSal, we conducted 1 H NMR spectroscopy of CTAB and CTAB with FSal ( Fig. 7a-d).
The assignments of the chemical shi (d) for the 1 H signals of CTA + are shown as symbols "a"-"f" (Fig. 7a). In the absence of FSal, d f (3.0 ppm) and d e (3.2 ppm) correspond to d of the "f" and "e" symbols near the quaternary ammonium groups of CTA + , respectively (Fig. 7a). Both d f and d e shied upeld with increasing FSal concentration to 2.8 ppm (Fig. 7b-d), which indicates an increase in the electron density around the quaternary ammonium groups of CTA + . One of the reasons for these upeld shis is attributed to the interaction between the quaternary ammonium groups of CTA + and the benzene ring of aromatic compounds. [38][39][40] To verify this, we conducted 19 F NMR spectroscopy of FSal. The FSal signal appeared at À105.0 ppm, and it shied downeld with increasing CTAB concentration to À104.5 ppm (Fig. 7e-h), which indicates that the benzene ring of FSal interacts with the quaternary ammonium groups of CTA + , and the electron density near the aromatic ring decreases. Similar downeld shis have been reported for uorobenzene derivatives and cationic surfactant systems. 23,41,42 To study the interaction between FSal/3FPBA-complex and CTAB, we conducted 1 H NMR spectroscopy, focusing on the signals in the range of 6-8 ppm that are derived from the benzene ring. The signals derived from FSal appeared at 6.5 and 7.6 ppm, whereas those from 3FPBA appeared at 7.0, 7.2, and 7.3 ppm (Fig. 8a and b). In the 1 H NMR spectra of 10 mM FSal with 5 mM 3FPBA, new signals appeared at 6.8 and 7.1 ppm, which were attributed to FSal/3FPBA-complex (Fig. 8c). However, in the presence of 14 mM CTAB, the signals of the FSal, 3FPBA, and FSal/3FPBA-complex samples were too weak and complex to analyse, and new signals that appeared at approximately 6.3 ppm could not be assigned (Fig. 8d).
Because 1 H NMR spectral results could not reveal the interaction between FSal/3FPBA-complex and CTAB, we performed 19 F NMR spectroscopy. The signals of 3FPBA at pH 5.0 and 11.0 appeared at À112.4 and À113.5 ppm, respectively ( Fig. 9a and b). Considering the pK a of 3FPBA is 8.4, 23 the signals at À112.4 and À113.5 ppm were ascribed to sp 2 -and sp 3hybridised boron, respectively. In the presence of 10 mM Fru with 10 mM 3FPBA at pH 7.4, the signal corresponding to sp 2hybridised boron was shied to À113.7 ppm, and a new signal due to sp 3 -hybridised boron of 3FPBA/Fru-complex appeared at À114.7 ppm (Fig. 9c). 23 Furthermore, the signal for 10 mM FSal appeared at À105.0 ppm (Fig. 9d). In the presence of 5 mM 3FPBA, four signals appeared at À102.0, À105.0, À113.7, and À114.4 ppm which correspond to FSal in FSal/3FPBA-complex, free FSal, free 3FPBA, and 3FPBA in FSal/3FPBA-complex, respectively (Fig. 9e). In the presence of 14 mM CTAB, the aforementioned four signals shied downeld to À101.8, À104.6, À113.4, and À113.9 ppm, respectively (Fig. 9f), indicating that FSal/3FPBA-complex interacts with CTA + . To investigate the interaction between 3FPBA and sugar or sugar alcohol in the presence of FSal and CTAB, we performed 19 F NMR spectroscopy. In the presence of Fru or Sor, the signals at  À101.8, À113.4, and À113.9 ppm disappeared, the signal of free FSal remained at À104.6 ppm, and new signals derived from sp 3 -hybridised boron of 3FPBA/Fru or Sor-complex appeared at À114.7 ppm (Fig. 9g and h). Therefore, the amounts of the FSal/ 3FPBA-complex and free 3FPBA decreased, whereas those of the 3FPBA/Fru and Sor-complex increased with the addition of Fru or Sor to the CTAB/FSal/3FPBA micellar system. In addition, because the signals of the 3FPBA/Fru and Sor-complex samples did not shi as in the absence of CTAB, it indicates that 3FPBA/ Fru and Sor-complex did not interact with CTA + (Fig. 9c, g and  h). In the presence of Glc, the signals at À101.8 and À113.4 ppm disappeared, the signal at À113.9 ppm slightly remained unchanged, the signal at À104.6 ppm did not shi, and the signal from sp 3 -hybridised boron of 3FPBA/Glc-complex appeared at À114.7 ppm (Fig. 9i). Similarly, these results showed that 3FPBA/Glc-complex did not interact with CTA + . We propose the following mechanism of the viscosity change in this system. In System A, CTA + interacts with the salicylate anion (Sal À ), 1,2 as shown in Fig. 7, which weakens the electrostatic repulsion with the head groups of CTA + . This tightens the packing of CTA + , leading to the formation of adequately long WLMs (Fig. 10a).
Previously, it has been reported that on adding diol compounds such as sugars into the CTAB/3FPBA micellar system, 3FPBA forms cyclic ester bonds with diol compounds, leading to an increase in the sp 3 hybridised boron of 3FPBA, which does not interact with CTA + . 23 In this study, 3FPBA forms a cyclic ester bond with Sal À (Fig. 8c and 9e). Although we expected that FSal/3FPBA-complex would not interact with CTA + , they interacted with each other (Fig. 9f). Based on these interactions, we assumed that the inuence of FSal/3FPBA-complex on CTA + is different from that of free Sal À or free 3FPBA. This presumption supports the occurrence of phase separation in System E, in which the content of NaSal/3FPBA-complex is higher than that in System B (Fig. 2). Owing to the strengthening of the interaction between NaSal/3FPBA-complex and CTA + , the packing state of CTA + changed (Fig. 10b), which shortened the WLMs and decreased the viscosity of System B. However, upon the addition of sugar or sugar alcohol to System B, 3FPBA bound to Sal À partially binds with sugar or sugar alcohol (Fig. 9g-i), leading to a decrease in the amount of NaSal/ 3FPBA-complex interacting with CTA + , and an increase in the amount of 3FPBA/Fru or Sor-complex which does not interact with CTA + (Fig. 10c). The changes in the molecular interactions induce the elongation of the WLMs and increase the viscosity of System B with sugar or sugar alcohol. Because Fru and Sor tends to bind with 3FPBA unlike Glc (Fig. 9g-i), the increase in viscosity of System B is larger for Fru and Sor than that for Glc (Fig. 3c). However, we cannot explain the difference in the increase in viscosity between Fru and Sor in System B.
In summary, the viscosity changes of the system are caused by the transformation of WLMs associated with the formation of competitive cyclic ester bonds between 3FPBA and diol compounds.

Conclusions
In conclusion, we prepared a novel micellar system utilising a PBA derivative whose viscosity increased in response to diol compounds such as sugar and sugar alcohol. This unique concept of utilising competitive cyclic ester bonds between PBA derivatives and diol compounds provides a new possibility for stimuli responsive WLMs as smart materials.

Conflicts of interest
There are no conicts to declare.