*2.3. Characterization*

The morphology of the polymer nanoparticles was evaluated using transmission electron microscopy (TEM) (Tecnai G2 F20 TWIN Cryo-TEM, FEI Company), at 300 kV acceleration voltage, ata1Åresolution.

FTIR spectra were recorded on a Bruker VERTEX 70 spectrometer using 32 scans with a resolution of 4 cm−<sup>1</sup> in 4000–600 cm−<sup>1</sup> region. The samples were analyzed using the attenuated total reflection (ATR) technique.

The UV-Vis spectra were recorded using a V-550 Able Jasco spectrophotometer, using a bandwidth of 1 nm, and a scanning speed of 1000 nm min−1. The BCD content was quantified by determining the reducing sugars of the polymer using concentrated H2SO4 acidolysis and phenol colorimetric analysis [31]. The procedure employed for the determination of BCD content and the calibration curve are presented in the Supporting Info.

The thermogravimetric analyses (TGA) were performed using a Netzsch TG 209 F3 Tarsus equipment considering the next parameters: nitrogen atmosphere flow rate 20 mL min<sup>−</sup>1; samples mass: ∼3 mg; temperature range: room temperature −<sup>700</sup> ◦C; heating rate: 10 ◦C min−<sup>1</sup> in an alumina crucible.

NMR experiments were carried out on Bruker Avance III 500, 1H NMR spectra were recorded at 500 MHz using the solvent peaks as internal references and 13C NMR spectra were recorded at 125 MHz. All NMR experiments were conducted according to the literature, and they can be found in the supplementary material.

Matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry experiments were carried out on Shimadzu AXIMA Performance, operated in high-resolution reflectron mode using α-Cyano-4-hydroxycinnamic acid as matrix.

The fluorescence spectra have been registered using a FP-6500 Able Jasco spectrofluorometer.

#### **3. Results and Discussion**

The first stage of our study consisted in the synthesis of polymeric particles presenting the capacity for the chemical attachment of BCD derivatives (BCD-OH and BCD-NH2, Scheme 1). The spherical shape was selected due to its high specific surface and high surface-to-volume ratio. The presence of an oxirane functional group at the surface of the polymer particles also permits the grafting of both beta-cyclodextrin derivatives (BCD-OH and BCD-NH2). Therefore, the synthesis strategy involved the emulsion polymerization of ST-HEMA system [32], followed by the seeded polymerization of GMA on the surface of the polymer particles [33]. As a result, an oxirane functional group capable of reacting with both hydroxyl and amino functional groups will be present on the surface of the polymer particles after the seeded polymerization. Figure 1 presents TEM images of the polymer particles at different stages during the synthesis route: ST-HEMA (after the emulsion polymerization), ST-HEMA-GMA (after the seeded polymerization), and ST-HEMA-GMA-BCD-NH2 (after BCD-NH2 grafting to the polymeric particle).

**Figure 1.** TEM images of the samples: (**a**) ST-HEMA; (**b**) ST-HEMA-GMA; (**c**) ST-HEMA-GMA-BCD-NH2 and (**d**) ST-HEMA-GMA-BCD-OH.

From the analysis of Figure 1, it can be observed that the ST-HEMA polymer particles are around 200 nm with a monodisperse size distribution. The average particle size and the size distribution are increased after the GMA seeded polymerization. Thus, the average particle size increases to around 220–230 nm, and a core-shell structuring can be noticed (inset detail Figure 1b). In the case of BCD-NH2 and BCD-OH grafting on the surface of the polymer particles, aggregates with an average dimension of 20 nm [34] can be observed deposited on the polymer particles after the seeded polymerization (darker spheres around the ST-HEMA-GMA Figure 1c,d).

To sustain the chemical grafting of the BCD derivative through the oxirane group reaction, the materials were analyzed by FT-IR spectroscopy (Figure 2). Thus, the specific vibration of oxirane 907 cm−<sup>1</sup> decreased while the intensity of the signal at 3420 cm−<sup>1</sup> , specific for -OH vibration increased. Moreover, the appearance of C-O-C vibration signal at 1033 cm−<sup>1</sup> and 3266 cm−<sup>1</sup> signal specific for NH vibration confirm the chemical attachment of the BCD derivatives to the polymeric particles. The signal at 1026 cm−<sup>1</sup> specific for the vibration of primary alcohol functional groups can also be identified in the ST-HEMA-GMA-BCD-OH and ST-HEMA-GMA-BCD-NH2 samples. In addition, the C−N bond that is formed during BCD-NH2 grafting to the polymer particles can be highlighted in the BCD-NH2 and ST-HEMA-GMA-BCD-NH2 samples at 1143 cm<sup>−</sup>1.

**Figure 2.** FT-IR spectra for ST-HEMA, BCD-NH2, ST-HEMA-GMA-BCD-NH2, ST-HEMA-GMA-BCD-OH, ST-HEMA-GMA, ST-HEMA. (**a**)ST-HEMA; ST-HEMA-GMA; **(b**) BCD-NH2; ST-HEMA-GMA; Pol-BCD-NH2; Pol-BCD-OH.

After the BCD attachment to the surface of the polymer particles, the degree of grafting was determined by NMR, TGA, and UV-Vis spectroscopy (see Supporting info Figure S7 calibration curve and determination method). Thus, BCD-OH content was determined at 4.27%, while in the case of BCD-NH2 the content was significantly higher at 19.19%. The difference in BCD content in the final materials can be explained by the higher reactivity of the oxirane group towards the amino than hydroxyl groups.

The organic pollutants present in water, such as bisphenol A, can be removed relatively easily and at a low cost by adsorption processes that can exploit the presence of BCD cage structure immobilized on polymeric supports [35]. In this study, we have determined the quantity of bisphenol A adsorbed per gram of material using ST-HEMA-GMA-BCD materials (Qe (mg bisphenol A/g polymer)). Thus, using a calibration curve for fluorescence intensity depending on the bisphenol A concentration (Supporting info Figure S8), the amount adsorbed onto the BCD modified polymer spheres was determined using Equation (1):

$$\mathbf{Q\_e} = \frac{(\mathbf{c\_0} - \mathbf{c\_e}) \times \mathbf{V}}{\mathbf{m}} \tag{1}$$

where V is the solution volume (mL), c0 (mg L−1) and ce (mg L−1) are the initial and final solution concentrations of bisphenol A and m is the mass of BCD modified polymer particles (mg).

After 240 min of interaction at 25 ◦C, the adsorption capacity Qe values obtained were 9.51 and 12.57 mg/g for ST-HEMA-GMA-BCD-OH and ST-HEMA-GMA-BCD-NH2, respectively. The higher Qe value obtained for the polymers modified using the BCD-NH2 can be related to the higher BCD grafting degree. It is easily observed that the large difference in grafting efficiency is in contrast with the relatively small difference between the adsorption capacity at equilibrium. This can be explained by the formation of hydrogen bonds between the hydroxyl groups of BCD-OH and bisphenol A [20]. Nevertheless, the higher bisphenol A adsorption characteristics of ST-HEMA-GMA-BCD-NH2 are evident. Consequently, a kinetic analysis of the adsorption process was performed for ST-HEMA-GMA-BCD-NH2 (Figure 3).

Figure 3 illustrates the adsorption of bisphenol A onto the ST-HEMA-GMA-BCD-NH2 as a function of time in contact. It can be noted that the absorption rate is fast up to 60 min which is followed by a slight decrease up to 200 min, followed by an equilibrium tendency of the system in the 200–300 min. The first stage can be explained by a high bisphenol A concentration in solution which diffuses to the polymer particles surface. The second stage, from 60 to 200 min, represents intermediary behavior during which the adsorption rate decreases as the internal diffusion resistance increases, which is finally followed by the equilibrium characteristics after 200 min. Several mathematical models were analyzed to

determine the adsorption efficiency and the mechanism that it follows: Lagergren's pseudofirst-order kinetic model (Equation (2)), Ho's pseudo-second-order model (Equation (3)), and Weber's intra diffusion model (Equation (4)):

$$\ln(\mathbf{Q\_e} - \mathbf{Q\_t}) = \ln \mathbf{Q\_e} - \mathbf{k\_1}\mathbf{t} \tag{2}$$

$$\frac{\text{t}}{\text{Q}\_{\text{t}}} = \frac{1}{\text{k}\_{2}\text{Q}\_{\text{e}}^{2}} + \frac{\text{t}}{\text{Q}\_{\text{e}}} \tag{3}$$

$$\mathbf{Q}\_{\mathbf{t}} = \mathbf{K}\_{\mathbf{P}} \sqrt{\mathbf{t}} \tag{4}$$

where Qe (mg g<sup>−</sup>1) and Qt (mg g<sup>−</sup>1) are the amounts of bisphenol A adsorbed per unit mass of ST-HEMA-GMA-BCD-NH2 at equilibrium and t (min), respectively. k1 is the pseudofirst-order adsorption rate constant (min<sup>−</sup>1), k2 is the pseudo-second-order adsorption rate constant (g (mg min)<sup>−</sup>1), and Kp is the intraparticle diffusion constant (mg g−<sup>1</sup> min<sup>−</sup>0.5).

**Figure 3.** Adsorption kinetic curve of ST-HEMA-GMA-BCD-NH2 at pH 5 and 25 ◦C.

The kinetic parameters of Lagergren's pseudo-first-order, Ho's pseudo-second-order, and Weber's intra particles diffusion models equations were calculated by slope-intercept of the linear fitting plots of ln(Qe-Qt) versus t, t/Qt versus t, and Qt versus t, respectively (Figure 4).

Comparing the experimental data with the results from the mathematical models for the adsorption process, the best fit was Lagergren's (both for Qe value and for R2) together with Weber's intraparticles diffusion model for the step that controls the mass transfer (Figure 3 and Table 1). Analyzing the values for Kp of Weber's model, this corresponds to a rapid adsorption process in the first stage of contacting [36].

**Table 1.** Kinetic parameters for the adsorption of bisphenol A by ST-HEMA-GMA-BCD-NH2.


The study of the adsorption isotherms offers information on the interaction between the adsorbate and the adsorbent and allows the determination of the adsorption capacity of the adsorbent, which is an important parameter for system evaluation. The most

intensively used isotherm adsorption model are Langmuir (Equation (5)) and Freundlich (Equation (6)).

$$\frac{1}{\mathcal{Q}\_{\text{e}}} = \frac{1}{\mathcal{Q}\_{\text{max}}} + \frac{1}{\mathcal{Q}\_{\text{m}}\mathcal{K}\_{\text{L}}} \times \frac{1}{\mathcal{c}\_{\text{e}}} \tag{5}$$

$$
\ln \text{Q}\_{\text{e}} = \ln \text{K}\_{\text{F}} + \frac{1}{\text{n}} \ln \text{c}\_{\text{e}} \tag{6}
$$

where Qe (mg g−1) indicates the amount of adsorbate at equilibrium; Qmax (mg g−1) indicates the maximum amount of adsorbate at equilibrium; ce (mg L<sup>−</sup>1) is the equilibrium concentration of the adsorbate in the solution; KL (L mg−1) and KF (L mg−1) are the Langmuir and Freundlich constants, respectively; and n is the heterogeneity factor.

**Figure 4.** The linear plots of Lagergren's pseudo-first-order (**A**), Ho's pseudo-second-order (**B**), and Weber's intraparticles diffusion models (**C**).

The isothermal adsorption at the equilibrium of bisphenol A by ST-HEMA-GMA-BCD-NH2 and ST-HEMA-GMA-BCD-OH are represented in (Supporting info Figure S9). From this, it can be noted that the adsorption capacity increases with the increase of adsorbate and reaches saturation as the bisphenol A concentration exceeds 45 mg/L and 40 mg/L in the case of ST-HEMA-GMA-BCD-NH2 and ST-HEMA-GMA-BCD-OH, respectively. The linearization of the two selected isotherms Equation (5), Equation (6), and the parameters calculated are presented in Figure 5 and Table 2.

**Figure 5.** (**A**,**C**) Langmuir plots illustrating the linear dependences of 1/Qe on 1/ce, and (**B**,**D**) Freundlich plot illustrating the linear dependences of lnQe on lnce for adsorption of bisphenol A by ST-HEMA-BCD-NH2 and ST-HEMA-BCD-OH, respectively.

**Table 2.** Isotherm parameters for the adsorption of bisphenol A by ST-HEMA-GMA-BCD-NH2 and ST-HEMA-GMA-BCD-OH.


The comparison of the Langmuir and Freundlich isotherm parameters as listed in Table 2 indicates better linearity in the case of the Langmuir plots, suggesting that the adsorption of bisphenol A by ST-HEMA-GMA-BCD-NH2 and ST-HEMA-GMA-BCD-OH can be regarded as a monolayer adsorption process. This means that the adsorption sites located on the surface of the polymer particles were homogeneously distributed and an equivalent adsorption force is displayed. Additionally, it could be evaluated from the Langmuir equation that the Qmax for bisphenol A was 148.37 mg g−<sup>1</sup> and 37.09 mg g−<sup>1</sup> for ST-HEMA-GMA-BCD-NH2 and ST-HEMA-GMA-BCD-OH, respectively. (Table 3) For a cross-linked polymer, three main adsorption mechanisms were proposed which involve (1) host–guest interactions in the polymers cavities, (2) interactions in the pores of the polymeric network, and (3) interactions on the surface (physical sorption) [37]. The difference between the adsorption characteristics of the two polymers (Qmax) can be correlated to the amount of BCD grafted to the polymer particles [38]. Thus, due to its

higher reactivity, BCD-NH2 afforded a larger substitution degree which is reflected by the enhanced Qmax value.


**Table 3.** Comparison of adsorption capacities of different polymer adsorbents for bisphenol A.

Comparing the values for Qmax for the adsorption of bisphenol A using different adsorbents (Table 3), it can be noted that the values obtained for our materials are reasonably high. The increased efficiency is due to the high ratio between the specific surface to the mass of the polymer particles and the high reactivity of the surface functional groups (oxirane). These characteristics permit the synthesis of BCD functionalized polymer particles with a good weight ratio (BCD to polymer support) and a proper distribution of BCD on the surface of the polymer particles. Therefore, the possibility of guest–host interaction is facilitated from the steric point of view (see Supplementary Info Figure S10). In addition to this interaction, there is also the possibility for adsorption processes in the pores of the cross-linked polymer particles, respectively physical interaction on the surface of the materials [20]. To confirm the interaction between the polymeric structures modified with BCD and bisphenol A (BPA) FTIR analysis was performed on the materials after the adsorption evaluation. From the spectra (see Supplementary Materials Figure S11), the appearance of a novel signal at 1150 cm−<sup>1</sup> corresponding to the C-O vibration from BPA [45] can be observed, confirming the presence of BPA on the surface of the polymer particles.

The thermal resistance is one of the most important properties of polymers and their derivatives, in our case the polymer particles modified with BCD-OH and BCD-NH2. Figure 6 presents the TGA and DTG curves for the synthesized polymers and starting materials.

Comparing the temperatures corresponding for a 10% weight loss (Table 4) it can be noted that BCD-OH has the highest thermal resistance, followed by ST-HEMA-GMA-BCD-OH and ST-HEMA-BCD-NH2, possibly due to the hydrogen bonds formed between the -OH groups. The ST-HEMA and ST-HEMA-GMA display lower thermal stability. The TGA analysis also establishes the temperature where the maximum weight loss takes place. In this case, the Tmax indicated that the most stable materials were ST-HEMA-GMA-BCD-OH and ST-HEMA-GMA-BCD-NH2, which display stability up to 400 ◦C.

**Figure 6.** (**a**) TGA curves for BCD-NH2, BCD-OH, ST-HEMA-GMA-BCD-NH2, ST-HEMA-GMA-BCD-OH, ST-HEMA-GMA, ST-HEMA and (**b**) DTG curves for BCD-NH2, BCD-OH, ST-HEMA-GMA-BCD-NH2, ST-HEMA-GMA-BCD-OH, ST-HEMA-GMA, ST-HEMA.

**Table 4.** Thermal properties of BCD-NH2, BCD-OH, ST-HEMA-GMA-BCD-NH2, ST-HEMA-GMA-BCD-OH, ST-HEMA-GMA, ST-HEMA.


T10% = decomposition onset temperature (measured at 10% weight loss); Tmax = the maximum decomposition temperature, corresponding to the maximum of DTG peak (the first derivative of the thermogravimetric curve).

#### **4. Conclusions**

A monoamine derivative of BCD was synthesized, characterized by NMR and mass spectroscopy. The BCD derivative was chemically grafted to ST-HEMA-GMA particles obtained by seeded emulsion polymerization. The GMA was used during the seeded polymerization due to its high reactivity towards -OH and NH2 functional groups. The chemical attachment of the BCD derivatives (BCD-OH and BCD-NH2) to the surface of the polymer particles was qualitatively confirmed by FT-IR spectroscopy and quantitatively by acidolysis and phenol colorimetric analysis. The degree of polymer functionalization using the BCD-OH and BCD-NH2 derivatives were 4.27 and 19.19 weight %.

The adsorption properties of the materials were evaluated using bisphenol A as target molecule. The results from the mathematical models for the adsorption kinetics process indicated that the best fit was Lagergren's model (both for Qe value and for R2), together with Weber's intraparticles diffusion model for the step that controls the mass transfer in the case of ST-HEMA-GMA-BCD-NH2. The isothermal adsorption evaluation indicated that both systems follow a Langmuir type behavior and afforded a Qmax value of 148.37 mg g−<sup>1</sup> and 37.09 mg g−<sup>1</sup> for ST-HEMA-GMA-BCD-NH2 and ST-HEMA-GMA-BCD-OH, respectively. Due to the higher reactivity of BCD-NH2, a larger substitution degree of the polymer particles was obtained, which is reflected by the enhanced Qmax value.

The thermogravimetric analysis of the materials indicated that the functionalization of the polymer particles afforded an increase of the thermal resistance. Thus, the BCD modified polymers present a degradation temperature of over 400 ◦C, which can be attributed to the hydrogen bonds and BCD thermal degradation pathway in the presence of the polymers.

**Supplementary Materials:** NMR and Maldi characterization Figures S1–S6; UV-Vis and fluorescence determinations procedures and calibration curves (Figure S7 and S8); adsorption isotherms (Figure S9), 3D rendering of bisphenol A insertion in BCD-NH2 (Figure S10) and FTIR spectra of the polymers modified with BCD after BPA adsorption study (Figure S11) are available online at https://www.mdpi.com/article/10.3390/polym13142338/s1.

**Author Contributions:** Conceptualization, E.R.; methodology, F.R.; software S.B.; validation, E.R., I.M. and A.D.; formal analysis, A.G.; investigation, R.S.; resources, A.M.; data curation, A.D.; writing original draft preparation, E.R.; writing—review and editing, E.R.; visualization, A.D.; supervision, E.R.; project administration, I.M.; funding acquisition, A.M., A.C.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Authors are thankful to the POSCCE-O 2.2.1, SMIS-CSNR 13984-901, No. 257/28.09.2010 Project, CERNESIM, for the NMR experiments. Raluca S, omoghi acknowledges financial support by Ministry of Research and Inovation, Nucleu Programme, Project PN.19.23.01.01 Smart-Bi. This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI—UEFISCDI, project number PN-III-P1-1.1-TE-2019-1387 (contract number TE141/2020), within PNCDI III.

**Conflicts of Interest:** The authors declare no conflict of interest.
