# Nitroxide-mediated polymerization of methacrylic acid onto silica nanoparticles for the preparation of molecularly imprinted materials

Soumaya Kouki, Najeh Jaoued, Yves Chevalier, Souhaira Hbaieb, Emmanuel Beyou

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Soumaya Kouki, Najeh Jaoued, Yves Chevalier, Souhaira Hbaieb, Emmanuel Beyou. Nitroxide-mediated polymerization of methacrylic acid onto silica nanoparticles for the preparation of molecularly imprinted materials. European Polymer Journal, 2025, 238, pp.114221. 10.1016/j.eurpolymj.2025.114221. hal-05217438

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# Nitroxide-mediated polymerization of methacrylic acid onto silica nanoparticles for the preparation of molecularly imprinted materials

Soumaya Kouki<sup>a,b,c</sup>, Najeh Jaoued<sup>d</sup>, Yves Chevalier<sup>b,*</sup> <sup>ID</sup>, Souhaira Hbaieb<sup>a,*</sup> <sup>DB</sup>, Emmanuel Beyou<sup>c,*</sup>

## ARTICLE INFO

### Keywords:

- Grafting through

- Grafting onto

- Nitroxide mediated radical polymerization

- Molecularly imprinted materials

- 3,5-Dichlorohydroxyacetophenone

## ABSTRACT

A new approach was investigated for the synthesis of molecularly imprinted polymers (MIPs) for the selective adsorption of 3,5-dichloro-2-hydroxyacetophenone (DCHA). This method involved poly(methacrylic acid-co-styrene) P(MAA-co-S) layers attached to the surface of silica particles utilizing polymers with either high or low molar masses grown on silica nanoparticles. Nitroxide mediated radical polymerization using a N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] based nitroxide as a control agent was combined with both “grafting onto” and “grafting through” surface modification techniques. The resulting P(MAA-co-S)-grafted silica nanoparticles showed excellent colloidal stability and dispersibility in 1,4-dioxane. DCHA-imprinted materials along with their non-imprinted counterparts were prepared by using ethylene glycol dimethacrylate as the cross-linker. The adsorption properties of these materials were evaluated and modeled by the Volmer and Langmuir–Volmer adsorption models. It was found that the materials synthesized with high molar mass polymers exhibited greater DCHA uptake compared to those with low molar mass; however, the low molar mass materials showed superior adsorption selectivity.

## 1. Introduction

The modification of inorganic surfaces with monomers plays a key role in the synthesis of nanocomposite materials and remains an area of considerable interest. Grafting thin layers of polymers is advantageous in the framework of “Molecularly Imprinted Polymer” (MIP) materials [1] because the selective molecular recognition takes place at the surface of the MIP and the selectivity of adsorption on molecular imprints requires a high rigidity of the material provided by the combined cross-linking and proximity of a solid surface [2–4]. Various strategies exist for grafting polymers onto surfaces [5,6]: the “grafting from” method for which polymer chains grow on a surface from an anchored initiator, the “grafting through” method which involves the polymerization of soluble monomers with surface anchored monomers to ensure covalent attachment, and the “grafting onto” method where pre-synthesized polymers are covalently grafted to the surface. Controlled radical

polymerization or “Radical Deactivation Reversible Polymerization” (RDRP) enable precise control over the growth of macromolecular chains in polymer layers, allowing for the covalent attachment of polymers to inorganic surfaces with accurately tuning of the grafting densities [5,6]. Therefore, the growing interest in having well-defined functionalized polymers has led to the exploration of different methods that control both the macromolecular architectures and the molar mass of grafted polymer layers. Three main RDRP techniques are commonly employed [7,8], such as reversible Radical Addition Fragmentation chain Transfer (RAFT), Atom Transfer Radical Polymerization (ATRP), and Nitroxide-Mediated Radical Polymerization (NMRP). The RAFT technique has been often used owing to its independence from metal catalysts [9] as well as the NMRP method as it allows the grafting of a variety of functional polymers onto different solid substrates and enables chain end-grafting on surfaces [10–13]. Most attention has been paid to the formation of “polymer brushes” [14] which consist of linear

<footnote><sup>a</sup> Laboratoire de Recherche: Caractérisations, Applications et Modélisation de Matériaux, Université de Tunis El Manar, Faculté des Sciences de Tunis, Campus universitaire El Manar, Tunisia</footnote>

<footnote><sup>b</sup> Laboratoire d'Automatique, de Génie des Procédés et de Génie Pharmaceutique, Université de Lyon 1, UMR 5007 CNRS, 69622 Villeurbanne Cedex, France</footnote>

<footnote><sup>c</sup> Laboratoire Ingénierie des Matériaux Polymères, Université de Lyon 1, UMR 5223 CNRS, 69622 Villeurbanne, France</footnote>

<footnote><sup>d</sup> Unité Spécialisée de développement des techniques analytiques, Institut National de Recherche et d'Analyse Physico-chimique, Biotechpole Sidi-Thabet, 2020 Ariana, Tunisia</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: yves.chevalier@univ-lyon1.fr (Y. Chevalier), souhaira.hbaieb@fst.utm.tn (S. Hbaieb), emmanuel.beyou@univ-lyon1.fr (E. Beyou).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1. Preparation of the MIP made of grafted cross-linked P(MAA-co-S) copolymer by the “grafting onto” three-steps process.</figcaption></figure>

macromolecules grafted at one chain end onto a surface. Until now, cross-linking reactions were not implemented in the radical polymerization processes used in “grafting onto” and “grafting through” methods. The successful formation of cross-linked poly(methacrylic acid) layers on silica surfaces via a “grafting from” NMRP process of methacrylic acid controlled by the N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide (DEPN) mediator has been recently reported [13]. The purpose of cross-linking was to freeze a “molecular imprint” of the 3,5-dichloro-2-hydroxyacetophenone (DCHA) template molecule into the polymer layer during the polymerization, resulting in a MIP material [1] that could be used for the selective recognition and capture of DCHA from the environment. DCHA was used as a model template molecule. This harmful compound [15] coming from the photodegradation of the octyl methoxycinnamate sunscreen in the chlorinated water of swimming pools [16] accumulates in groundwater up to a few µg·L⁻¹

concentration [17]. In that previous study [13], methacrylic acid (MAA), styrene (S) and ethylene glycol dimethacrylate (EGDMA) as the cross-linking agent were copolymerized by NMRP together with methacrylic ester monomers grafted onto the silica surface, resulting in the covalent attachment of a poly(methacrylic acid-co-styrene-co-ethylene glycol dimethacrylate) P(MAA-co-S-co-EGDMA) terpolymer via a grafting from mechanism. Styrene was included in the mixed monomers to better control of the polymerization kinetics of highly reactive MAA [18–21] and it is used at a low concentration (10 wt%) as advised in the above-mentioned references and also to avoid interfering with the binding of DCHA to the methacrylic acid units in the final MIP material. Indeed, the styryl radicals react faster with the DEPN than the methacryloyl radicals. Therefore, the presence of styrene reduces the overall concentration of radicals and limits the irreversible termination reactions by disproportionation. The role of styrene is to shift the

<figure><img src="image_4.png" /><figcaption>Fig. 2. The two steps approach for the preparation of the MIP materials using the “grafting through” process.</figcaption></figure>
activation-deactivation equilibrium towards deactivation and slow down the propagation rate of the radical polymerization. These processes have been modeled by Nicolas et al. [20]. The application to surface grafting processes have been introduced by Bartholome et al. [10], Lenarda et al. [22] and Blas et al. [23].

The present article reports two alternative approaches for modifying silica surfaces in which the cross-linking step is performed separately, after the initial NMRP-based attachment of the poly(methacrylic acid-co-styrene) copolymer (P(MAA-co-S)). This attachment is carried out using either the “grafting onto” or “grafting through” method. These approaches are evaluated for their effectiveness in selectively adsorbing DCHA. The physicochemical properties and adsorption behavior of the resulting MIPs are compared both to each other and to those obtained from the one-step grafting through method involving in situ cross-linking and template incorporation [13]. Moreover, the influence of the molar mass of the P(MAA-co-S) copolymers on the final material properties is addressed.

The current “grafting onto” process involves three sequential steps as illustrated in Fig. 1: 1) Synthesis of P(MAA-co-S) copolymers; 2) Grafting these copolymers onto the surface of silica functionalized with methacryloyl groups (SiO₂-MPTS); 3) Cross-linking with EGDMA in the presence of DCHA followed by solvent extraction to remove DCHA.

The second approach is based on the “grafting through” process which consists in two main steps, as shown in Fig. 2: 1) Preparation of grafted P(MAA-co-S) copolymers on the SiO₂-MPTS silica; 2) Cross-linking with EGDMA in the presence of DCHA followed by solvent extraction to remove DCHA.

## 2. Experimental section

### 2.1. Materials

Fumed silica (Aerosil® 200) characterized by a primary particle diameter of 12 nm and a specific surface area of 200 m²·g⁻¹ was purchased from Evonik. γ-Methacryloxypropyltrimethoxysilane (γ-MPTS, 95 %), ethylene glycol dimethacrylate (EGDMA, 99.5 %), methacrylic acid (MAA, 99 %) and styrene (S, 99 %) were purchased from Sigma-Aldrich and used as received. N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide (DEPN, also called SG1) was supplied by the ATOFINA company. The styrene adduct of DEPN (Styryl-DEPN, Block-Builder) was prepared following the procedure described in reference [24]. Toluene, diethyl ether, 1,4-dioxane (99 %), methanol (99.7 %), acetic acid (99.5 %), deuterated dimethylsulfoxide (DMSO-d₆, 99 %) and 3,5-dichloro-2-hydroxyacetophenone (DCHA, 99 %) were purchased from Sigma-Aldrich and used as received.

### 2.2. Methods

Fourier Transform Infrared spectra were recorded using a Bruker ALPHA spectrometer in Attenuated Total Reflectance (ATR) mode with 24 scans at a resolution of 4 cm⁻¹. Solid-state ²⁹Si and ¹³C NMR, and liquid ¹H NMR spectra were collected at 125.76 MHz, 130.32 MHz and 500.13 MHz Larmor frequencies respectively, using a Bruker Avance III 500 ultra shield PLUS spectrometer. Polymer molar masses were measured by size exclusion chromatography (SEC) at 60 °C using a 410 Waters differential refractometer and a 996 Waters photodiode array as detectors. The setup included a 717 Waters autosampler, a 515 Waters HPLC pump, and two Waters Microstyragel columns (HR1 and HR4), for the analysis of polymers within a molar mass range from 5,000 to 600,000 g·mol⁻¹. Dimethyl sulfoxide (DMSO) was used as the mobile phase at a flow rate of 1 mL·min⁻¹. Number-averaged and mass-averaged molar masses (Mn and Mw) and polydispersity index (PdI = Mw/Mn) were determined using a calibration with poly(methacrylic acid) standards. Thermogravimetric measurements (TGA) were carried out using a Q500 thermal analyzer instrument (TA Instruments) by heating the sample from 25 to 800 °C in a helium gas flow (20 mL·h⁻¹)

<caption>**Table 1**</caption>

Composition of the polymerization recipe using 15 mL of 1,4-dioxane, and theoretical number-average molar mass of the copolymers (Mn).

<table><thead><tr><th rowspan="2"></th><th colspan="4">Masses and numbers of moles</th><th rowspan="2">Mn theo (g<br>mol<sup>-1</sup>)</th></tr><tr><th>MAA</th><th>Styrene</th><th>Styryl-<br>DEPN</th><th>DEPN</th></tr></thead><tbody><tr><td>P(MAA-co-<br>S)-1</td><td>12.01 g<br>139<br>mmol</td><td>1.276 g<br>12.3<br>mmol</td><td>0.928 g<br>2.32 mmol</td><td>0.015 g<br>3.4<br>µmol</td><td>5720</td></tr><tr><td>P(MAA-co-<br>S)-2</td><td>6.345 g<br>74 mmol</td><td>0.741 g<br>7.1 mmol</td><td>0.114 g<br>0.29 mmol</td><td>0.015 g<br>3.4<br>µmol</td><td>24,830</td></tr></tbody></table>

at 10 °C·min⁻¹. DSC measurements were recorded with a Q200 DSC instrument (TA Instruments) in the temperature range of 50–200 °C at a heating rate of 10 °C·min⁻¹. The glass transition temperature Tg was determined for each heating sequence. A Perkin-Elmer Lambda 2 UV–Vis spectrophotometer was used to obtain UV–vis absorbance spectra of DCHA and interfering molecules in solutions. Transmission electron microscopy (TEM) images were taken with a Philips CM120 electron microscope operating at 80 kV acceleration voltage at the “Centre Technologique des Microstructures” (CTµ) facility of the University of Lyon 1 (https://microscopies.univ-lyon1.fr/). A 0.2 % aqueous dispersion drop was deposited onto a copper grid covered with a Formvar film and air-dried before TEM observation.

### 2.3. Synthesis of silica grafted with methacrylic groups

First, silica was modified with 3-(trimethoxysilyl)propyl methacrylate (MPTS) in anhydrous toluene in the presence of N-ethyl-diisopropylamine as a catalyst [13]. The synthesis scheme of SiO₂-MPTS is illustrated in Fig. S1 (Supporting Information). In a typical procedure, 1 g of silica was dispersed in 50 mL of toluene under vigorous stirring. Subsequently, MPTS was gradually added to the suspension, followed by the addition of N-ethyldiisopropylamine. Then, the mixture was heated at reflux under nitrogen atmosphere for 24 h. After completion, unreacted MPTS was removed by centrifugation at 3,500 rpm for 15 min. The resulting MPTS-functionalized silica particles (SiO₂-MPTS) were collected by filtration, rinsed with 20 mL of THF and dried under vacuum at 40 °C overnight. The average grafting density, determined by TGA (Fig. S2, Supporting Information) and elemental analysis of carbon was 2.5 µmol m⁻². The dry materials were stored at room temperature for further use.

### 2.4. Synthesis of MIPs using the “grafting onto” process

The preparation of the MIPs involves the three-steps approach presented in Fig. 1.

#### 2.4.1. Synthesis of P(MAA-co-S) copolymers

Two copolymers of different molar masses were synthesized according to the recipes given in Table 1. A small amount of DEPN was added to enhance the control over molar masses of the copolymers. DEPN acts as a stable free radical, moderating chain growth without initiating polymerization, thereby not altering the final molar mass determined by the monomer-to-initiator (Styryl-DEPN) molar ratio. The mixtures were degassed through four freeze–thaw cycles in a pre-dried Schlenk flask under an argon atmosphere, and heated in an oil bath at 110 °C with stirring at 400 rpm for one hour. After polymerization, the copolymers were isolated by centrifugation at 10,000 rpm and dried under vacuum at 80 °C overnight, yielding the copolymers P(MAA-co-S)-1 and P(MAA-co-S)-2. The theoretical molar masses at full conversion were calculated based on the monomer (MAA and S) to the Styryl-DEPN initiator mole ratios.
<table><caption>**Table 2**
List of the studied materials together with their abbreviations.</caption><thead><tr><th>Materials</th><th>Abbreviations</th></tr></thead><tbody><tr><td>Silica powder</td><td>SiO<sub>2</sub></td></tr><tr><td>Modified silica by grafting γ-MPTS</td><td>SiO<sub>2</sub>-MPTS</td></tr><tr><td>N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide</td><td>Styryl-DEPN</td></tr><tr><td>Copolymer of methacrylic acid and styrene (90:10) with a number-average molar mass of 5700 g·mol<sup>-1</sup></td><td>P(MAA-co-S)-1</td></tr><tr><td>Copolymer of methacrylic acid and styrene (90:10) with a number-average molar mass of 24000 g·mol<sup>-1</sup></td><td>P(MAA-co-S)-2</td></tr><tr><td>SiO<sub>2</sub>-MPTS grafted with the copolymer P(MAA-co-S)-1 or P (MAA-co-S)-2</td><td>SiO<sub>2</sub>-GO-1 or SiO<sub>2</sub>-GO-2</td></tr><tr><td>MIP and NIP materials via the grafting onto process</td><td>MIP-GO-1 and NIP-GO-1<br>MIP-GO-2 and NIP-GO-2</td></tr><tr><td>Grafted silica by the grafting through process of copolymerization of methacrylic acid and styrene (90:10) together with SiO<sub>2</sub>-MPTS</td><td>SiO<sub>2</sub>-GT-1 and SiO<sub>2</sub>-GT-2</td></tr><tr><td>MIP and NIP materials via the grafting through process</td><td>MIP-GT-1 and NIP-GT-1<br>MIP-GT-2 and NIP-GT-2</td></tr></tbody></table>

### 2.4.2. Grafting the P(MAA-co-S) copolymers onto silica

To investigate the kinetics of the grafting reaction, 1 g of SiO₂-MPTS was dispersed in a solution of 0.5 g of P(MAA-co-S) in 15 mL of 1,4-dioxane. The mixture was degassed and heated at 110 °C for various durations ranging from 1 h to 8 h. The resulting grafted silicas were purified by successive centrifugation and redispersion steps in 1,4-dioxane and dried under vacuum at 80 °C. The resulting materials were designated SiO₂-GO-1 and SiO₂-GO-2 corresponding the grafts derived from P(MAA-co-S)-1 and P(MAA-co-S)-2 copolymers respectively.

### 2.4.3. Imprinting the grafted silica materials with DCHA

The preparation of imprinted materials involved cross-linking the silica-grafted P(MAA-co-S) using EGDMA in the presence of DCHA as a template molecule. Specifically, 1 g of silica-grafted P(MAA-co-S) was dispersed in 15 mL of 1,4-dioxane containing 2.18 g (0.011 mol) of EGDMA and 0.512 g (0.0025 mol) of DCHA, and the mixture was heated at 110 °C for 1 h. The reaction was stopped by cooling the sample in an ice-bath, and the DCHA target molecule was washed out with methanol by using a Soxhlet extractor to free the molecularly imprinted sites. The resulting materials were dried under vacuum at 80 °C overnight, yielding the MIP-GO-1 and MIP-GO-2 samples. The non-imprinted materials NIP-GO-1 and NIP-GO-2 were prepared using the same procedure, excluding the DCHA template.

## 2.5. Synthesis of MIPs using the “grafting through” process

The preparation of the MIPs requires a two steps procedure as presented in Fig. 2.

### 2.5.1. Grafting of P(MAA-co-S) on the silica surface

A “grafting through” strategy was employed to synthesize silica-grafted P(MAA-co-S) copolymers. In a pre-dried Schlenk flask, 0.5 g of SiO₂-MPTS was dispersed in a solution of methacrylic acid, styrene, Styryl-DEPN and DEPN in 15 mL of 1,4-dioxane. The amounts of these reagents were the same as for the grafting onto process (Table 1). The mixture was degassed by four cycles of freeze-pump-thaw and heated in an oil bath at 110 °C for one hour. The polymerization reaction was quenched by immersing the flask in an ice-water bath. The free P(MAA-co-S) copolymers were removed by four centrifugation cycles (10,000 rpm, 15 min). UV-vis analysis of the resulting supernatants confirmed the complete removal of free copolymers. These supernatants (total volume: 600 mL) were pooled and evaporated at 80 °C to recover the

free P(MAA-co-S) copolymers. Finally, the grafted samples SiO₂-GT-1 and SiO₂-GT-2 and the isolated free copolymers were dried under vacuum at 80 °C overnight.

### 2.5.2. Imprinting the grafted silica materials with DCHA

The cross-linking procedure whether in the presence or absence of DCHA was identical to that for the GO samples in section 2.4.3, yielding the molecularly imprinted samples MIP-GT-1 and MIP-GT-2, as well as the corresponding non imprinted NIP-GT-1 and NIP-GT-2 samples. The list of materials abbreviations is given in Table 2.

## 2.6. Adsorption studies

### 2.6.1. Adsorption isotherms

Batch adsorption experiments were performed at room temperature by dispersing 30 mg of adsorbent material in 5 mL of DCHA solution with initial concentrations ranging from 2.4 10⁻⁵ to 10⁻⁴ mol·L⁻¹. The mixtures were magnetically stirred overnight to ensure adsorption equilibrium. Following equilibrium, the suspensions were centrifuged and the supernatants were filtered through a 0.2 µm syringe filter. The residual concentration of DCHA in the filtrate was determined by UV-vis absorbance spectroscopy. The equilibrium adsorption capacity $Q_e$ (mol·g⁻¹) was then calculated using Eq. (1).

$$Q_e = \frac{C_0 - C_e}{m} V \quad (1)$$

where V (L) is the volume of the solution, m (g) is the weight of the absorbent, and $C_0$ and $C_e$ are the initial and equilibrium concentration of DCHA in solution (mol·L⁻¹), respectively. The balance of the adsorption process is given by the percentage of adsorbed DCHA (%A) as:

$$\%A = \frac{C_0 - C_e}{C_0} \quad (2)$$

### 2.6.2. Modeling the adsorption isotherms

The adsorption behavior and affinity of DCHA toward the synthesized adsorbents were studied using adsorption isotherms. The Langmuir–Volmer model [25] was selected for data analysis due to its ability to simultaneously describe both selective and non-selective adsorption processes that occur in molecularly imprinted polymers (MIPs). This hybrid model integrates two distinct adsorption mechanisms: i) localized selective adsorption onto molecularly imprinted sites, described by the Langmuir model (denoted as subscript S), and ii) the non-selective adsorption on the material surface as a mobile adsorbed layer, characterized by the Volmer model (denoted as subscript NS). The Langmuir model is defined by the following equation:

$$Q_s = Q_{\max,s} \frac{K_s C}{1 + K_s C} \quad (3)$$

where C is the equilibrium concentration of DCHA adsorbate, $Q_{\max,s}$ is the maximum adsorption capacity, and $K_s$ is the Langmuir equilibrium constant.

The Volmer model is described by the following equation:

$$C = \frac{1}{K_{\text{NS}}} \frac{\theta}{1 - \theta} e^{\frac{\theta}{1 - \theta}} \quad (4)$$

where θ is the surface coverage, $θ = \frac{Q_{\text{NS}}}{Q_{\text{max,NS}}}$, and $K_{\text{NS}}$ is the binding constant of the adsorption equilibrium.

Adsorption on MIPs is localized on molecular imprints (Langmuir model) and non-localized (mobile) off the molecular imprints (Volmer model). The overall adsorption capacity of MIPs is therefore modeled using the Langmuir–Volmer equation [25] which combines these two contributions: $Q_e = Q_s + Q_{\text{NS}}$.
<figure><img src="image_5.png" /><figcaption>Fig. 3. ¹H NMR of P(MAA-co-S)-1 (A) and P(MAA-co-S)-2 (B) copolymers in DMSO-d₆.</figcaption></figure>

### 3. Results and discussion

#### 3.1. MIPs synthesized on grafted silica via the "Grafting Onto" method

The "grafting onto" technique involves three sequential steps. First, the copolymer is synthesized via solution polymerization. In the second step, the preformed copolymer is grafted onto the silica surface. Finally, in the third step, the grafted polymer chains are cross-linked with EGDMA in the presence of DCHA template molecules which are then removed to generate molecular imprints.

A critical stage in this process is the grafting of polymer chains onto silica nanoparticles. Although this method is effective and relatively straightforward, it presents certain limitations. In particular, once the initial polymer chains are grafted, they create steric hindrance, which impedes the diffusion of additional chains toward the reactive double bonds (e.g., methacryloyl groups) on the silica surface. This restricted

accessibility results in a limited grafting density, which can constrain the performance and functionality of the final imprinted materials [26].

##### 3.1.1. Synthesis of the P(MAA-co-S) copolymers

The P(MAA-co-S) copolymers were prepared by radical polymerization of methacrylic acid with a small amount of styrene in 1,4-dioxane and using Styryl-DEPN as initiator. The inclusion of styrene helps to slow down the polymerization kinetics of methacrylic acid, allowing for better control over the copolymer chain length [27]. The copolymers were characterized by means of IR spectroscopy, ¹H NMR, TGA, CES and DSC measurements. The IR spectra (Fig. S3 in Supporting Information) showed the expected characteristic bands of the P(MAA-co-S) copolymer. The absorption bands at 3000 cm⁻¹ and 707 cm⁻¹ corresponded to =C-H stretching and bending vibrations from the aromatic ring of the styrene units, respectively. The strong band at 1713 cm⁻¹ was attributed to the C=O stretching vibration of the carboxylic acid groups in MAA

<figure><img src="image_6.png" /><figcaption>Fig. 4. DSC thermograms of copolymers P(MAA-co-S)-1 (a) and P(MAA-co-S)-2 (b).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. Thermogravimetric analyses of SiO₂-GO-2 grafted for 1 h (a), 2 h (b), 4 h (c) and 8 h (d), and of SiO₂-GO-1 grafted for 1 h (e), 2 h (f).</figcaption></figure>

units, while the band at 1260 cm⁻¹ corresponded to the C–O bond stretching. ¹H NMR spectra in DMSO-d₆ (Fig. 3) confirmed the chemical composition of the P(MAA-co-S)-1 and P(MAA-co-S)-2 copolymers. The spectra exhibited the expected lines at 1–2 ppm for the methylene and methine protons of both styrene and MAA units and lines in the range 6.5–7.5 ppm corresponded to the aromatic protons of styrene units [24]. The broad signal at 12–13 ppm was attributed to acidic protons of the carboxylic acid groups in MAA units. Finally, the line at 2.5 ppm is that of the DMSO-d₆ solvent and the one at 3.5 ppm is that of moisture present in DMSO-d₆. The compositions of the copolymers were calculated from the ratio of the proton integrals for the lines at between 6.5–7.5 ppm and at 1–2 ppm. The copolymer composition of both the P (MAA-co-S)-1 and P(MAA-co-S)-2 copolymers was calculated from the integration ratio of the aromatic region (6.5–7.5 ppm) to the aliphatic region (1–2 ppm). Both P(MAA-co-S)-1 and P(MAA-co-S)-2 were found to contain 90 mol% methacrylic acid and 10 mol% styrene, consistent with the initial monomer feed ratio used during polymerization.

SEC measurements provided the molar mass distribution and the molar mass distribution of the copolymers. The measured number-averaged molar masses of P(MAA-co-S)-1 and P(MAA-co-S)-2 were measured to be 5700 g·mol⁻¹ and 24000 g·mol⁻¹, respectively. These values were in close agreement with the corresponding theoretical molar masses of 5720 g·mol⁻¹ and 24830 g·mol⁻¹ which were calculated from the initial monomer feed ratio (MAA/S = 90/10 mol%) and the concentration of the Styryl-DEPN initiator, according to Equation (5).

$$M_n(\text{theo}) = \frac{(n_{\text{MAA}} M_{\text{mol.MAA}} + n_S M_{\text{mol.S}})}{n_{\text{initiator}}} \quad (5)$$

where n<sub>MAA</sub>, n<sub>S</sub> and n<sub>initiator</sub> are the number of moles of methacrylic acid, styrene and DEPN, M<sub>mol.MAA</sub> and M<sub>mol.S</sub> are the molar masses of the two monomers. The polydispersity index PdI = M<sub>w</sub>/M<sub>n</sub> was 2.2 for P(MAA-co-S)-1 and 1.4 for P(MAA-co-S)-2. The rather broad molar mass distribution of the copolymers is attributed to a poor control of the NMRP due to the high reactivity of methacrylic acid monomer [19].

The glass transition temperatures (T<sub>g</sub>) of the copolymers were then determined by DSC (Fig. 4) to be 62 °C and 150 °C for the P(MAA-co-S)-1 and P(MAA-co-S)-2 copolymers, respectively. The lower value obtained for P(MAA-co-S)-1 is consistent with its lower M<sub>n</sub> [28].

### 3.1.2. Grafting of copolymers onto functionalized silica

The DEPN free initiator enables effective control over the chain growth of both soluble and grafted polymers, as previously demonstrated for the copolymerization of methacrylic acid and styrene [29–31]. The optimum conditions for achieving a high grafting density of P(MAA-co-S) copolymer on the silica nanoparticles surface using the styryl-DEPN initiator were achieved at temperature ranging from 90 to 120 °C with 1,4-dioxane as the solvent.

This strategy relies on the reaction of nitroxide chain end groups of the P(MAA-co-S) chains and the vinyl groups located on the modified silica nanoparticles, causing the covalent attachment of the copolymer chains to the silica surface. The grafted silica nanoparticles were characterized by IR, TGA, DLS, and TEM.

The copolymer grafting was qualitatively confirmed through IR spectroscopy and solid-state NMR, providing evidence for the covalent attachment of the copolymer to the silica nanoparticles. The IR spectra of P(MAA-co-S) copolymer grafted silica nanoparticles are presented in Fig. S5 (Supporting Information). The spectra exhibited a prominent absorption band at 1713 cm⁻¹, corresponding to the characteristic carbonyl stretching vibrations (νC=O). This band was significantly more intense than that observed in the IR spectrum of silica grafted with MPTS. Additionally, the presence of characteristic hydroxyl stretching bands at 3031 cm⁻¹ and 3452 cm⁻¹ (νOH) further supported successful grafting. These spectral features collectively confirm the effective anchoring of the P(MAA-co-S) copolymer onto the silica surface.

The kinetics of the polymer grafting reaction were investigated over reaction times ranging from 1 h to 8 h with the aim of obtaining copolymers of both high and low molar masses and optimizing the grafting density. Quantitative determinations of the polymer content were performed by TGA (Fig. 5). The detailed interpretations of TGA results in terms of grafted polymer content along with DLS measurements of the
<table><caption>**Table 3**
Experimental conditions and TGA results of the grafting of P(MAA-co-S) copolymer chains onto modified silica yielding the SiO₂-GO materials.</caption><thead><tr><th>Material</th><th>Polymerization time (h)</th><th>Mass loss (%)</th><th>Grafting density (µmol·m⁻²)</th><th>Particle size (nm)</th></tr></thead><tbody><tr><td rowspan="6">SiO₂-GO-1</td><td>1</td><td>12.2</td><td>0.11</td><td>127</td></tr><tr><td>2</td><td>19.0</td><td>0.20</td><td>150</td></tr><tr><td>4</td><td>20.0</td><td>0.21</td><td>164</td></tr><tr><td>8</td><td>21.2</td><td>0.22</td><td>173</td></tr><tr><td>M<sub>n,tho</sub> =<br>5720</td><td></td><td></td><td></td><td></td></tr><tr><td>M<sub>n,exp</sub> =<br>5700</td><td></td><td></td><td></td><td></td></tr><tr><td>M<sub>w</sub>/M<sub>n</sub> =<br>2.2</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="6">SiO₂-GO-2</td><td>1</td><td>29.6</td><td>0.086</td><td>151</td></tr><tr><td>2</td><td>42.8</td><td>0.15</td><td>161</td></tr><tr><td>M<sub>n,tho</sub> =<br>24830</td><td></td><td></td><td></td><td></td></tr><tr><td>M<sub>n,exp</sub> =<br>24000</td><td></td><td></td><td></td><td></td></tr><tr><td>M<sub>w</sub>/M<sub>n</sub> =<br>1.4</td><td></td><td></td><td></td><td></td></tr></tbody></table>

hydrodynamic diameters of the SiO₂-GO particles are summarized in Table 3.

The grafting densities of the P(MAA-co-S)-1 copolymer increased during the first two hours of grafting reaction and did not significantly change after 2 h, indicating that grafting reached completion within 2 h of reaction. Within experimental errors, the final grafting density was 0.20 µmol·m⁻² for P(MAA-co-S)-1, and a lower value (0.15 µmol·m⁻²) was reached for the longer polymer chains of P(MAA-co-S)-2. This difference is likely due to steric hindrance arising from lateral interactions between the grafted polymer chains, which limited further attachment to the silica surface. However, this trend is reversed when the grafting densities are expressed in mass units: 1.2 mg·m⁻² for P(MAA-co-S)-1 against 3.7 mg·m⁻² for P(MAA-co-S)-2. This reflects the higher mass contribution of the longer polymer chains, despite their lower surface density in terms of chain number.

The diameters of the grafted silica particles measured by DLS (Table 3) ranged from 127 to 173 nm, which were consistent with the sizes of silica aggregates reported after grafting polystyrene onto similar silica with a specific surface area of 200 m²·g⁻¹ [10]. The particle sizes were not significantly affected by the molar mass of the grafted P(MAA-co-S) chains. This is likely because the thickness of the grafted polymer layer was relatively small compared to the overall diameter of the silica particles.

TEM images of SiO₂-GO-1 and SiO₂-GO-2 samples (Fig. 6) obtained by drying aqueous suspensions on the TEM grids showed that the grafted silica particles consisted of small, rounded elementary particles forming colloidal aggregates. Such agglomeration of particles may be attributed either to limited colloidal stability in suspension or to coagulation occurring during the drying process necessary for TEM sample preparation. These results suggest that while the particles retain their individual morphology, interparticle interactions can lead to secondary structure formation under certain conditions.

### 3.1.3. Synthesis of molecular imprinted polymer layers using the SiO₂-GO grafted silicas

The cross-linking reaction using EGDMA was carried out on surfaces bearing nitroxide-functionalized chain ends of the P(MAA-co-S) polymer chains grafted on silica for 2 h. Copolymerization of EGDMA was performed at 1,4-dioxane 110 °C for 1 h. Following cross-linking, the template molecule DCHA was removed via a washing step, aimed at liberating the molecular imprints and thus generating the molecularly imprinted polymers (MIPs). Methanol was selected as the washing solvent due to its polar, protic nature, which effectively disrupts the hydrogen bonding interactions between DCHA and the carboxylic acid groups of the polymer network within the imprints. Characterization of the MIP and NIP materials was performed using IR (Fig. S6, Supporting Information), solid state ¹³C NMR (Fig. S7, Supporting Information), TGA and TEM.

Thermogravimetric analysis (TGA) of the MIP and NIP samples revealed similar thermal degradation profiles (Fig. S8, Supporting Information). As the cross-linked copolymer networks lack a defined molar mass, grafting densities were expressed in mass units (mg·m⁻²) and calculated based on the mass losses observed between 250 °C and 800 °C. The grafting densities were found to be 3.0 mg·m⁻² and 3.4 mg·m⁻² for MIP-GO-1 and NIP-GO-1, and 1.8 mg·m⁻² and 2.2 mg·m⁻² for MIP-GO-2 and NIP-GO-2, respectively. In both cases, the NIP samples exhibited slightly higher grafting densities than their MIP counterparts. This difference may be attributed to a radical scavenging effect of DCHA, which could interfere with the cross-linking reaction in the MIPs, thereby reducing the extent of polymer incorporation. Furthermore, the cross-linking efficiency was notably higher for the SiO₂-GO-1 samples compared to SiO₂-GO-2. This discrepancy is likely due to the greater accessibility of nitroxide end groups in the shorter P(MAA-co-S) chains (Mₙ = 5700 g·mol⁻¹), which are located closer to the silica surface after the grafting process. In contrast, the longer polymer chains in SiO₂-GO-2 (Mₙ = 24000 g·mol⁻¹) tend to bury the nitroxide end groups deeper within the polymer layer, hindering their reactivity with EGDMA. The cross-linking step led to a significant increase in the grafting density, rising from 1.1 mg·m⁻² (SiO₂-GO-1) to 3.8 mg·m⁻² (MIP-GO-1) and

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** TEM images of SiO₂-GO-1 (A) and SiO₂-GO-2 (B).</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. TEM images MIP-GO-2 (a, b), and NIP-GO-2 (c, d) at different magnifications.</figcaption></figure>

from 2.0 mg•m⁻² (SiO₂-GO-2) to 2.6 mg•m⁻² (MIP-GO-2). These results indicated that a substantial amount of poly(EGDMA) was successfully formed within the grafted P(MAA-co-S) layer, reinforcing the polymeric network at the silica surface.

The TEM images (Fig. 7) revealed that both MIP and NIP materials were composed of aggregated spherical particles with a homogeneous size distribution, ranging in diameter from 150 to 200 nm. The MIP and NIP exhibited similar morphologies, indicating that the grafting process did not significantly alter the particle morphology. It is worth noticing that the silica particles were not in a close contact because of the presence of the polymer layer grafted onto their surface. Its mean thickness was calculated from Eq. (6).

$$Thickness = \frac{Volume}{Area} = \frac{Massgraftingdensity(g \cdot m^{-2})}{Layerdensity(g \cdot m^{-3})} \qquad (6)$$

The estimated thickness of the polymer layer was 4 nm for MIP-GO-1 and 2.5 nm for MIP-GO-2. The mean distance between silica particles in the TEM images was ~ 5 nm, which correlates well with twice the thickness of the polymer layer of MIP-GO-2, supporting the presence of surface grafted polymer shells. Despite the stabilizing effect of the grafted polymer, the particles appeared to be assembled into aggregates, likely due to interparticle interactions during the drying process for TEM sample preparation or inherent colloidal instability in the dispersion medium.

## 3.2. Synthesis of MIPs using silica grafted via the "Grafting Through" process

The “grafting through” method was employed for the synthesis of molecularly imprinted polymers (MIPs) and non-imprinted polymers

(NIPs) using MPTS-functionalized silica (SiO₂-MPTS) as the solid support. In this approach, methacrylic acid (MAA) and styrene (S) monomers were copolymerized directly with the methacryloyl groups present on the surface of the SiO₂-MPTS, enabling the *in situ* growth of polymer chains from the silica surface. Following polymerization, a cross-linking step using ethylene glycol dimethacrylate (EGDMA) was carried out in the presence of the DCHA template molecule, under the same conditions as those used for the “grafting onto” approach. After the reaction, DCHA was removed through appropriate washing procedures to generate the molecular imprints within the polymer matrix.

### 3.2.1. Synthesis of polymers grafted on silica using the grafting through process

Two SiO₂-GT materials were synthesized by grafting P(MAA-co-S) copolymers of different molar masses onto silica nanoparticles. These materials were obtained by conducting the solution polymerization reaction for 1 h in the first step, followed by washing and drying. The resulting samples were characterized using IR spectroscopy, size exclusion chromatography (SEC), thermogravimetric analysis (TGA), dynamic light scattering (DLS), and transmission electron microscopy (TEM). IR spectroscopy (Fig. S9, Supporting Information) confirmed successful surface functionalization of silica following polymerization. The IR spectra of the SiO₂-GT materials resembled those of SiO₂-GO materials. A prominent band at 1707 cm⁻¹, attributed to the C=O stretching vibration, provided evidence of successful copolymerization and attachment of the polymer chains. The molar mass distributions of the free copolymer chains, isolated from the centrifugation supernatants, were analyzed by SEC. It was assumed that the molar masses of the free polymers in solution reflected those of the polymers grafted onto the silica surface [6]. After 1 h of polymerization, the number-average molar
<table><caption>**Table 4**
The grafting densities of the P(MAA-co-S) copolymer on the surface of silica.</caption><thead><tr><th rowspan="2">Material</th><th colspan="3">Free P(MAA-co-S) copolymers</th><th rowspan="2">Mass loss (%)</th><th colspan="2">Grafting density</th><th rowspan="2">Particle size (nm)</th></tr><tr><th>M<sub>n,theo</sub> (g·mol<sup>-1</sup>)</th><th>M<sub>n,exp</sub> (g·mol<sup>-1</sup>)</th><th>M<sub>w</sub>/M<sub>n</sub></th><th>μmol·m<sup>-2</sup></th><th>mg·m<sup>-2</sup></th></tr></thead><tbody><tr><td>SiO<sub>2</sub>-GT-1</td><td>5720</td><td>5900</td><td>1.3</td><td>8.5</td><td>0.077</td><td>0.42</td><td>166</td></tr><tr><td>SiO<sub>2</sub>-GT-2</td><td>24,830</td><td>23,150</td><td>2.4</td><td>37</td><td>0.127</td><td>1.9</td><td>175</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** TEM images of SiO₂-GT-1 (A) and SiO₂-GT-2 (B).</figcaption></figure>

masses ($M_n$) of the copolymers were 5900 g·mol⁻¹ and 23,150 g·mol⁻¹, respectively. These experimental $M_n$ values were in good agreement with the theoretical values calculated from the monomer-to-initiator ratios (Table 4), indicating efficient polymerization and minimal termination in solution. The success of the grafting suggested that chain

termination reactions may have been favored at the silica surface, contributing to the effective attachment of polymer chains.

TGA analysis (Fig. S10, Supporting Information) was employed to determine the grafting densities of the SiO₂-MPTS and SiO₂-GT materials. The thermograms revealed two distinct mass loss regions: the first

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** IR spectra of SiO₂-MPTS, SiO₂-GT-1, MIP-GT-1, NIP-GT-1.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 10. TEM images of MIP-GT-1 (a, b) and NIP-GT-1 (c, d) at two magnifications.</figcaption></figure>

between 130 °C and 285 °C, corresponding to the loss of adsorbed moisture [15], and the second between 285 °C and 800 °C, attributed to the thermal decomposition of the organic component (i.e., grafted P(MAA-co-S) chains). The mass losses in the latter region were 8.5 % for SiO₂-GT-1 and 37 % for SiO₂-GT-2, confirming the successful grafting of copolymers with significantly different loading levels.

The nanoparticle sizes measured by DLS and TEM (Table 4) ranged from 166 nm to 175 nm, which were consistent with the particle diameters observed for P(MAA-co-S) copolymers prepared via the grafting onto technique.

TEM images of diluted suspensions of the P(MAA-co-S) grafted silica particles (Fig. 8) revealed that the samples of SiO₂-GT with molar masses between 5000 and 25000 g·mol⁻¹ formed aggregates. The silica particle sizes measured from TEM were consistent with the hydrodynamic diameters obtained from DLS measurements, confirming the reliability of the size characterization.

### 3.2.2. Synthesis of MIPs and NIPs using grafted silica via the “Grafting Through” method

The grafted silica samples, SiO₂-GT-1 and SiO₂-GT-2, were cross-linked with EGDMA in the presence of DCHA under inert atmosphere at 110 °C for 1 h. After cross-linking, the materials were crushed and subjected to Soxhlet extraction with methanol until the supernatant contained less than 1 % of the DCHA template molecule. Non-imprinted polymers (NIPs) were prepared following the same procedure but without the addition of DCHA. The resulting white powders were

characterized by IR spectroscopy, DLS, and TEM.

The IR spectra of both MIP-GT and NIP-GT samples displayed the same characteristic peaks as their parent SiO₂-GT material (Fig. 9). Notably, the peak at 1704 cm⁻¹ corresponded to the stretching vibration of the carbonyl (C=O) bond of the methacrylic acid (MAA) units. The broad peak at 3439 cm⁻¹ was attributed to the stretching of the hydroxyl (–OH) groups of MAA units, while the band at 707 cm⁻¹ was assigned to the aromatic –C=C– stretching of the styrene units.

The measured hydrodynamic diameters ranged between 194 and 212 nm for nanoparticles cross-linked with polymer chains of M_n = 23150 g·mol⁻¹, with specific values of 194 nm and 211 nm for MIP-GT-1 and NIP-GT-1, respectively. These sizes were consistent with those observed for materials synthesized using the grafting onto method, indicating that the cross-linking process and the synthetic route did not significantly alter the overall particle dimensions in dispersion.

TEM images of both MIP and NIP (Fig. 10) revealed spherical particles ranging from 120 to 150 nm. The sizes of the materials determined by DLS were slightly larger than those measured by TEM. This difference can be attributed to the fact that TEM images were taken from dried particles while DLS measurements were performed in aqueous suspensions of the PMAA-grafted silica particles. The hydrated polymer chains expanding into the solution contributed to the larger hydrodynamic diameter observed by DLS.
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Adsorption isotherms of DCHA at 25 °C on MIPs and NIPs. (A) MIP-GO-1 and NIP-GO-1, (B) MIP-GO-2 and NIP-GO-2, (C) MIP-GT-1 and NIP-GT-1, (D) MIP-GT-2 and NIP-GT-2.</figcaption></figure>

### 3.3. Adsorption of DCHA on MIPs

The adsorption of DCHA onto MIPs and NIPs was investigated by measuring adsorption isotherms at various concentrations (1, 20, 60, 100, 200, and 400 ppm) using the depletion method (Eq. (1)). Adsorption onto polymeric materials depends on multiple structural parameters such as molar mass, branching, and cross-linking density [29,32–34]. In this study, particular emphasis was placed on the influence of the polymer molar mass. A higher adsorption capacity in MIPs compared to NIPs is indicative of effective molecular imprinting. The extent of this imprinting effect at a given concentration is quantified by the Imprinting Factor (IF), defined as: $IF = Q_{MIP}/Q_{NIP}$.

However, the IF is not an absolute metric for comparing materials across different studies, as its value strongly depends on the specific concentration of the adsorbate—an important detail often omitted in literature. For a more reliable and universal comparison, it is preferable to analyze the full adsorption isotherms and apply appropriate modeling to extract equilibrium constants. In this context, the adsorption to the molecularly imprinted sites of the MIPs, being localized and specific, was modeled by the Langmuir model (Eq. (3)). In contrast, the adsorption onto NIPs and the non-imprinted regions of MIPs which involves surface mobility and non-selective interactions, was modeled using the Volmer model (Eq. (4)). Thus, the combined Langmuir–Volmer and Volmer isotherms [25] provided a suitable framework to describe and quantify the DCHA adsorption behavior MIPs and NIPs.

#### 3.3.1. Adsorption isotherms of DCHA to imprinted materials and their modeling

Adsorption isotherms were recorded over a concentration range of DCHA from $2.4 \times 10^{-5}$ to $1.0 \times 10^{-4} \text{ mol} \cdot \text{L}^{-1}$. Two batches of the MIP-

GO and MIP-GT were prepared for checking against the repeatability of the materials preparation and adsorption measurements. The full characterization and measurements of adsorption isotherms were performed with one batch only. A few adsorption measurements were done with the second batch; they were consistent within 20 % with the full isotherms measured with the first batch. The repeatability was then considered good enough because it fell in the expected range of such measurements. The data were modeled using the Langmuir–Volmer and Volmer adsorption isotherms to describe the adsorption behavior of DCHA on MIPs and NIPs, respectively. These models were selected to account for both the specific, localized adsorption onto the imprinted sites of the MIPs and the non-specific, mobile adsorption occurring on the surface of the NIPs and non-imprinted regions of the MIPs.

The adsorption capacities of MIPs and NIPs prepared with polymers of varying molar masses are illustrated in Fig. 11, highlighting the influence of polymer chain length on binding performance. The corresponding thermodynamic parameters, derived from the best fits of the Langmuir–Volmer and Volmer models to the experimental data, are summarized in Table 5.

The thermodynamic parameters of the DCHA adsorption onto the MIP and NIP prepared by the “Grafting Onto” process revealed only minor differences between the binding constants for selective $K_S$ and non-selective $K_{NS}$ interactions. The main difference was a larger adsorbed amount on the MIP attributed to the additional contribution of selective adsorption onto the molecular imprints. The non-selective adsorption components for both the MIP and the NIP were essentially identical. Overall, the increased adsorption observed for MIPs was a result of additional adsorption sites provided by the imprinted cavities, although the binding constants remained unchanged, implying a lack of true selective recognition. In numerical terms, the amount of DCHA
<caption>**Table 5**</caption>

Volmer model parameters and fitting of the Langmuir–Volmer adsorption model to the experimental adsorption isotherms of DCHA on the MIP and NIP materials prepared by the “Grafting Onto” and the “Grafting Through” processes. The adsorption data of materials prepared by the “Grafting From” process [13] is added at the bottom of the table (MIP-GF and NIP-GF).

<table><thead><tr><th></th><th>Q<sub>max,S</sub><br>(µmol·g<sup>-1</sup>)</th><th>K<sub>S</sub></th><th>log<br>(K<sub>S</sub>)</th><th>Q<sub>max,NS</sub><br>(µmol·g<sup>-1</sup>)</th><th>K<sub>NS</sub></th><th>log<br>(K<sub>NS</sub>)</th></tr></thead><tbody><tr><td>MIP-GO-1</td><td>9.9</td><td>14,300</td><td>4.16</td><td>13.7</td><td>13,700</td><td>4.14</td></tr><tr><td>NIP-GO-1</td><td></td><td></td><td></td><td>14.7</td><td>11,150</td><td>4.04</td></tr><tr><td>MIP-GO-2</td><td>15.8</td><td>14,200</td><td>4.15</td><td>19.7</td><td>13,500</td><td>4.13</td></tr><tr><td>NIP-GO-2</td><td></td><td></td><td></td><td>16.7</td><td>12,400</td><td>4.09</td></tr><tr><td>MIP-GT-1</td><td>19.5</td><td>12,900</td><td>4.11</td><td>13.1</td><td>9900</td><td>4.00</td></tr><tr><td>NIP-GT-1</td><td></td><td></td><td></td><td>16.6</td><td>9200</td><td>3.96</td></tr><tr><td>MIP-GT-2</td><td>29.7</td><td>16,900</td><td>4.23</td><td>16.0</td><td>12,300</td><td>4.09</td></tr><tr><td>NIP-GT-2</td><td></td><td></td><td></td><td>20.0</td><td>11,700</td><td>4.07</td></tr><tr><td>MIP-GF<br>[13]</td><td>19.6</td><td>25,000</td><td>4.40</td><td>9.0</td><td>20,000</td><td>4.30</td></tr><tr><td>NIP-GF<br>[13]</td><td></td><td></td><td></td><td>16.3</td><td>11,000</td><td>4.04</td></tr></tbody></table>

bound to the MIPs was approximately twice that bound to the NIPs, but this increase did not reflect enhanced selectivity. Furthermore, the adsorption capacities of both MIP-GO-2 and NIP-GO-2 were higher than those of MIP-GO-1 and NIP-GO-1, respectively. This was a direct consequence of the higher molar mass of the grafted polymer chains (M<sub>n</sub> = 24,000 g·mol<sup>-1</sup> vs 5,700 g·mol<sup>-1</sup>), which resulted in a greater overall polymer loading and thus a higher total number of available adsorption sites. These findings indicate that the “Grafting Onto” strategy, while effective in increasing adsorption capacity through increased polymer mass, did not result in selective binding of DCHA.

The “Grafting Through” process enabled a slight but significant improvement in selective adsorption of DCHA onto molecularly imprinted polymers (MIPs), as indicated by the higher binding constants for specific adsorption (K<sub>S</sub>) compared to those for non-specific adsorption (K<sub>NS</sub>). For instance, the binding constant for MIP-GT-1 was K<sub>S</sub> = 12,900, exceeding the corresponding non-selective constants for MIP-GT-1 and NIP-GT-1 (K<sub>NS</sub> = 9,900 and 9,200, respectively). A similar trend was observed for MIP-GT-2, where K<sub>S</sub> = 16,900, while K<sub>NS</sub> values were 12,300 and 11,700 for MIP-GT-2 and NIP-GT-2, respectively. These findings confirm that the enhanced adsorption observed for MIPs over NIPs stems primarily from the additional specific binding sites created by molecular imprinting. As observed in the “Grafting Onto” process, increasing the molar mass of the polymer chains led to greater overall adsorption capacity. However, in the “Grafting Through” case, both specific and non-specific adsorption increased proportionally with molar mass, resulting in no further enhancement of selectivity at higher molar masses. Selectivity was quantitatively assessed using a “thermodynamic” Imprinting Factor, defined as the ratio IF = K<sub>S</sub>/K<sub>NS</sub> = 1.4, reflecting modest but real imprinting efficiency. Although alternative definitions of the imprinting factor may yield higher values, this thermodynamic approach offers a consistent basis for comparison. Notably,

the “Grafting From” process [13] was found to enable slightly greater selectivity than the “Grafting Through” approach, making it the preferred method for the preparation of MIPs when enhanced molecular recognition is desired.

## 4. Conclusion

The selective adsorption behavior of MIPs was found to strongly depend on the preparation method, with effectiveness following the order: Grafting From > Grafting Through > Grafting Onto. While selective binding is commonly attributed to the formation of a pre-polymerization complex between the template molecule (DCHA) and the functional monomer (MAA), the present findings suggest that such interactions, though important, are not solely sufficient to achieve selective adsorption. This is evidenced by the Grafting Onto process, where, despite the presumed presence of pre-polymerization interactions, no significant selectivity was observed. Across all preparation methods, the effect of the polymer’s number-average molar mass (M<sub>n</sub>) on DCHA adsorption followed a consistent trend: higher molar mass led to increased total adsorption capacity. However, this increase did not correlate with improved selectivity, as both selective and non-selective adsorption contributions rose concomitantly. Therefore, while polymer chain length influences the amount of adsorption, it does not inherently enhance molecular recognition.

A refined assessment of the adsorption process was made possible through the modeling of adsorption isotherms. While a cursory examination of the experimental isotherms simply indicates that adsorption onto MIPs exceeds that onto NIPs, this observation alone is insufficient to determine whether the difference arises from selective adsorption mechanisms or merely from a higher density of adsorption sites. To resolve this ambiguity, it is essential to take a model that reflects the actual adsorption mechanism, where the parameters used have clear physical and thermodynamic meaning.

## CRediT authorship contribution statement

Soumaya Kouki: Writing – original draft, Investigation. Najeh Jaoued: Investigation, Data curation. Yves Chevalier: Writing – review & editing, Validation, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. Souhaira Hbaieb: Writing – review & editing, Validation, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization. Emmanuel Beyou: Writing – review & editing, Validation, Supervision, Project administration, Methodology, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work was supported by the “PHC Utique” program for French-Tunisian cooperation (project number 19G1204).

## Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eurpolymj.2025.114221.

## Data availability

Data will be made available on request.
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