# Deposition Routes of Molecularly Imprinted Silica
## for the Development of Highly Specific Electrochemical “in-field” Sensors

Taisiia Sikolenko, Elias Bou-Maroun, Thomas Karbowiak

## Abstract

Molecularly imprinted silica (MIS) electrode coatings have recently returned to the center of scientific research because of their advantages over organic imprinted polymers in electrochemical analysis. An easy-to-control and well-studied sol-gel process that uses “green solvents”, rigid structure and electrochemical inertness of the resulting material, applicability of electrochemical deposition techniques are the key elements that make MIS attractive for targeted and sensitive electrochemical detection. This review aims to draw attention to electrode modification using MIS. It highlights the main strategies for deposing MIS films on the electrode, indicates the adjustments that need to be made to known procedures to achieve an electrochemical sensing system with the best analytical performance, and discusses the advantages and weaknesses of each deposition route.

## Addresses

Univ. Bourgogne Franche-Comté, Institut Agro, PAM UMR 02 102, 1 Esplanade Erasme, 21000 Dijon, France

Corresponding authors: Elias Bou-Maroun (elias.bou-maroun@agrosupdijon.fr), Thomas Karbowiak (thomas.karbowiak@agrosupdijon.fr)

## Keywords

Molecularly imprinted silica; Sol-gel; Electrode modification; Coating; Electrochemical sensor
# 1. Introduction

Electrode surface modification is a common way to improve the performance of electrochemical analysis, particularly by increasing selectivity for the target analyte and lowering detection limits. Molecularly imprinted materials that are polymerized in the presence of the target molecule can provide selective binding of the analyte in a complex medium because of the chemical and steric complementarity created during the synthesis between the target and a functional monomer. Such sensing systems have already been proposed for the detection of pesticides [1–3], antibiotics [4], food contaminants [5], and biochemical sensing [6,7], including detection of the SARS-CoV-2 antigen [8], which can be adapted into portable systems for field analysis and screening [9].

However, the deposition method for imprinted film has a strong impact on the performance of the electrochemical sensor. It can cause coating inhomogeneity, unsuitable morphology, or insufficient adhesion of the synthesized layer; therefore, the deposition method must be considered carefully before developing the electrode [10,11]. Molecularly imprinted polymers (MIPs) are formed when organic monomers are used as precursors [12]. In addition to these organic precursors, inorganic functional monomers can also be used to form highly selective polymers [10]. Among these, MIS is the most promising because of its simple and well-known chemistry, robust framework, high surface area, and, most importantly, electrochemical inertness [10,13]. In contrast to MIPs, MIS display lower swelling, low toxicity, and are easy to handle.

Despite the numerous advantages of MIS electrode coatings, they have attracted much less interest than MIPs. However, recent advances in the electrochemical deposition of silica films have revived interest in the development of MIS systems. Therefore, the objective of this review is to highlight and critically evaluate the deposition procedures available for electrode modification by MIS to demonstrate their strengths and weaknesses in the electrochemical detection of various analytes.
## 2. Main steps in the preparation of MIS

The design of a MIS-modified electrode coating is based on the sol–gel process and consists of four main steps (Figure 1): (1) mixing the template molecule and silane monomer in the appropriate solvent to initiate the preassembly of the silane monomer around the template, (2) immobilizing the MIS on the electrode, (3) washing out the template, and (4) binding the analyte for detection by a suitable electrochemical method.

The silica-based sol for thin film deposition is prepared in an acidic ethoxy–ethanol [14] or water–ethanol [15] medium containing organosilane precursors and a target analyte that serves as a template. Subsequent steps include hydrolysis of the silica building blocks and their condensation around the template molecule, which is often induced by shifting to a basic pH. Tetraethoxysilane (TEOS) is the commonly used precursor for the synthesis of MIS. Silanes with organic functions complementary to the template (amine [15–20], phenyl [14,15,19,21–29], vinyl [30,31]) can be added to improve the affinity between the silica polymer and the template.
<figure><img src="image_1.png" /><figcaption>**Figure 1:** Schematic representation of the four main steps in the synthesis of MIS for electrode coating with emphasis on the most common immobilization routes: spin coating, drop casting, grafting, and electrochemical deposition
(in violet – monomer, in yellow – template, in blue – solvent)</figcaption></figure>
### 3. MIS immobilization routes

The formation of a thin MIS film on the working electrode can be implemented by physical coating with a sol. Spin-coating and drop-casting approaches are the most commonly used methods because their compatibility with the sol-gel process and because they allow thin films to be obtained. Chemical immobilization techniques include electrode modification by grafting or by direct electrochemical deposition of a thin film.

#### 3.1 Physical deposition

Spin coating is achieved by depositing a small amount of sol on the surface of the working electrode and spinning the electrode at high speed using a spin coater, which results in the formation of a thin layer under centrifugal forces. Following this deposition method, glassy carbon electrode (GCE) and indium tin oxide (ITO) can be modified with MIS for cyclic voltammetry (CV) [14,15,23] and square wave voltammetry (SWV) [23,24] detection of analytes. Unlike the spin-coating approach, the drop-casting approach does not require specific equipment for MIS immobilization. This technique involves the repeated deposition and drying of drops containing the silica precursor and template on the electrode, which results in the formation of thin MIS layers. In addition to sol casting [32], MIS layers have also been obtained from suspensions with core-shell nanoparticles [33], magnetic nanoparticles [16,34], or graphene oxide (GO) based composites [25,26]. Paper electrodes decorated with MIS-modified nanoparticles have also been reported [27]. In addition, sol-gel spraying was performed to obtain a MIS layer with a bisphenol A (BPA) template on the glass surface; however, no further electrochemical detection of the analyte was performed [35]. Although direct sol casting is technically straightforward, it takes about 4 hours to deposit eight MIS layers [32]. Several studies reported that the sensitivity of analyte detection depends on the thickness of the MIS, regardless of the deposition technique [23–25,27,32,34]. In particular, films that are too thin have few recognition binding sites, while thicker films have hindered mass transport, which decreases the electrochemical response.
## 3.2 Chemical deposition

Grafting MIS layers onto the working electrode aims at silanization, that is, the chemical modification of the surface with organosilanes. The most common method for grafting the silica layer is the prior introduction of a mercaptopropyl silane anchor on the electrode surface (ITO [36], Au [19,28]). Vinyl organosilane precursor can also be used to improve the adhesion of film on GCE [20]. The subsequent formation of MIS is performed on the predeposited layer by creating a Si-O-Si network. Interestingly, the deposition of (3-Mercaptopropyl)trimethoxysilane (MPS) on the ITO surface before MIP (ITO-MPS-MIP) sufficiently improves the detection of *trans*-resveratrol compared to the ITO-MIP configuration [36]. Although grafting offers flexible synthesis conditions that result in the exceptional stability of the immobilized layer, it is often technically complicated and time consuming [37].

Over the past decade, electrochemical deposition of silica layers has become popular due to its compatibility with the sol-gel technique, rapid film generation, applicability for the deposition of hybrid organic-inorganic imprinted layers, control of the synthesis process, and chemical bonding of the layer to the electrode. The electrochemical generation of MIS is achieved by applying the specific potential that allows the release of the $OH^-$ catalyst at the working electrode, which causes the rapid condensation of silica and its cross-linking into an ordered network on the electrode surface. The formation of silica films can be held in a potentiostatic mode, galvanostatic mode, or by CV deposition. The simplest composition of electrogenerated MIS contains only silica precursors [18,31,38]. To increase the electroactive area of the electrode or to enhance the MIS conductivity and number of binding sites, gold nanoparticles (AuNPs) [17,22] or organic polymers such as pyrrole [21,39] can be introduced. Alternatively, modification of the electrode support with multiwall carbon nanotubes (MWCNTs) can be performed, leading to more sensitive detection of the analyte [17,29,30]. Du *et al.* demonstrated the one-step synthesis of a MIS-modified electrode containing chitosan and AuNPs by its deposition at -0.3 V (vs. SCE) [22]. Under the working potential, the generation of hydroxyl catalyst and the formation of AuNPs take place simultaneously, which results in the cross-linking of the silica-chitosan around the *p*-nitrophenol and freshly obtained AuNPs.
<table><caption>**Table 1** – Overview of the main parameters for electrode modification by MIS. The electroanalytical performance of the designed systems and their application to the analysis of real samples are classified according to the four deposition routes (spin coating, drop casting, grafting, and electrochemical deposition). Reproducibility was reported as the Relative Standard Deviation (RSD) of the measured values in the model systems from n different electrodes, when given in the corresponding articles.</caption><thead><tr><th>Deposition route</th><th>Analyte</th><th>Main MIS components</th><th>Working electrode</th><th>Method of detection</th><th>Linear range of the calibration curve</th><th>Limit of detection (LOD)</th><th>RSD</th><th>Selectivity towards</th><th>Model solution</th><th>Real sample analysis</th><th>Ref</th></tr></thead><tbody><tr><td rowspan="2">Spin coating</td><td rowspan="2">Dopamine</td><td>TMOS, PhTMOS, MTMOS</td><td>GCE</td><td>CV</td><td>1 × 10<sup>-4</sup> to 5 × 10<sup>-3</sup> mol L<sup>-1</sup></td><td>—</td><td>—</td><td>Ascorbic acid, dihydroxyphenyl acetic acid, 3-(3,4-dihydroxyphenyl)-L-alanine, catechol</td><td>PBS 0.1 mol L<sup>-1</sup> pH 7.4</td><td>—</td><td>[14]</td></tr><tr><td>Paraoxon</td><td>TMOS, PhTMOS, APTES</td><td>GCE, ITO</td><td>CV</td><td>—</td><td>—</td><td>—</td><td>—</td><td>PBS 0.1 mol L<sup>-1</sup> pH 7.0</td><td>—</td><td>[15]</td></tr><tr><td rowspan="2">D- and L-3,4-dihydroxyphenyl-alanine</td><td rowspan="2">TMOS, PhTMOS</td><td>ITO</td><td>CV, SWV</td><td>—</td><td>1 × 10<sup>-9</sup> mol L<sup>-1</sup></td><td>—</td><td>—</td><td>Dopamine, dihydroxyphenyl acetic acid, catechol</td><td>PBS 0.01 mol L<sup>-1</sup> pH 7.6</td><td>—</td><td>[23]</td></tr><tr><td>Tyramine, dopamine</td><td>TEOS, PhTEOS</td><td>GCE</td><td>SWV</td><td>1 × 10<sup>-5</sup> to 1 × 10<sup>-4</sup> mol L<sup>-1</sup> (dopamine)</td><td>—</td><td>—</td><td>Norepinephrine</td><td>PBS 0.01 mol L<sup>-1</sup> pH 7.2</td><td>—</td><td>[24]</td></tr><tr><td rowspan="2">Drop casting</td><td>L-histidine</td><td>TEOS, PhTMOS, MTMOS</td><td>Au</td><td>PQC, EIS</td><td>5 × 10<sup>-8</sup> to 1 × 10<sup>-4</sup> mol L<sup>-1</sup> (PQC)</td><td>2.5 × 10<sup>-8</sup> mol L<sup>-1</sup> (PQC)</td><td>—</td><td>—</td><td>PBS 0.1 mol L<sup>-1</sup> pH 7.4</td><td>—</td><td>[32]</td></tr><tr><td>Metronidazole</td><td>TEOS, APTES, MGCE<br/>Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub></td><td>DPSV</td><td>CV</td><td>5.0 × 10<sup>-8</sup> to 1.0 × 10<sup>-6</sup> mol L<sup>-1</sup></td><td>1.6 × 10<sup>-8</sup> mol L<sup>-1</sup></td><td>4.8 %</td><td>Dimetridazole, ronidazole</td><td>BRS 0.04 mol L<sup>-1</sup> pH 5.0</td><td>Milk, honey</td><td>[16]</td></tr><tr><td rowspan="2">tert-butylhydroquinone (TBHQ)</td><td rowspan="2">CIPTEOS, PEI, EGDMA</td><td>CV</td><td>CV</td><td>1 × 10<sup>-1</sup> to 5 × 10<sup>-1</sup> mg kg<sup>-1</sup></td><td>2.7 × 10<sup>-1</sup> mg kg<sup>-1</sup></td><td>4.1 % (n = 3)</td><td>Butyl hydroxy anisid, butylated hydroxytoluene, propyl gallate</td><td>PBS 0.2 mol L<sup>-1</sup> pH 6.05</td><td>Colza oil, Arachis oil, Beef tallow, Soybean oil, Blend oil</td><td>[33]</td></tr><tr><td>TEOS, PhTEOS</td><td>GCE/GO</td><td>DPV</td><td>1.0 × 10<sup>-7</sup> to 8 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>2.0 × 10<sup>-8</sup> mol L<sup>-1</sup></td><td>8.2 % (n = 5)</td><td>4-aminophenol, uric acid, ascorbic</td><td>PBS 0.1 mol L<sup>-1</sup></td><td>Drug tablets, Urine</td><td>[25]</td></tr></tbody></table>
acid pH 7.0

<table><caption>Table 1 (Continued)</caption><thead><tr><th>Deposition route</th><th>Analyte</th><th>Main MIS components</th><th>Working electrode</th><th>Method of detection</th><th>Linear range of the calibration curve</th><th>Limit of detection (LOD)</th><th>RSD</th><th>Selectivity towards</th><th>Model solution</th><th>Real sample analysis</th><th>Ref</th></tr></thead><tbody><tr><td rowspan="3">Mesalazine (AZA)</td><td rowspan="3">TEOS, PhTEOS</td><td rowspan="3">GCE/GO</td><td rowspan="3">DPV</td><td>2 × 10<sup>-6</sup> to</td><td>9.7 × 10<sup>-7</sup></td><td>3.03 %</td><td rowspan="3">(n = 5)</td><td>Salicylic acid, 4-aminophenol, 2-aminobenzoic acid, 4-acetamidophenol</td><td>BRS 0.04 mol L<sup>-1</sup> pH 4.0</td><td>Drug tablets</td><td rowspan="3">[26]</td></tr><tr><td>2 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>mol L<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td>2 × 10<sup>-5</sup> to</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td>1.5 × 10<sup>-4</sup> mol L<sup>-1</sup></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Hemoglobin (Hb)</td><td rowspan="2">TEOS, Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub></td><td rowspan="2">MGCE</td><td rowspan="2">DVP</td><td>5 to 100 mg L<sup>-1</sup></td><td>1 mg L<sup>-1</sup></td><td>4.3 %</td><td rowspan="2">(n = 3)</td><td>Lysin, horse radish peroxidase, bovine serum albumin</td><td>PBS 0.1 mol L<sup>-1</sup> pH 5.0</td><td>Whole blood samples</td><td rowspan="2">[34]</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Serotonin</td><td rowspan="3">TEOS, PhTMOS, PVP, Fe<sub>3</sub>O<sub>4</sub>@AuNPs</td><td rowspan="3">GPE/ MWCNTs</td><td rowspan="3">LSV</td><td>1 × 10<sup>-8</sup> to</td><td>2 × 10<sup>-9</sup></td><td>8.35 %</td><td rowspan="3">(n = 5)</td><td>Ascorbic acid, uric acid, norepinephrine, dopamine, creatinine, glucose</td><td>PBS 0.1 mol L<sup>-1</sup> pH 8.0</td><td>Pharmaceutical al capsules, Urine</td><td rowspan="3">[27]</td></tr><tr><td>1 × 10<sup>-3</sup> mol L<sup>-1</sup></td><td>mol L<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="4">Grafting</td><td rowspan="4">trans-resveratrol (RES)</td><td rowspan="4">MPS, AA, EDMA, AIBN</td><td rowspan="4">ITO</td><td rowspan="4">DPV</td><td>2 × 10<sup>-6</sup> to</td><td>8 × 10<sup>-7</sup></td><td>–</td><td rowspan="4">Polydatin, bisphenol A</td><td>PBS 0.2 mol L<sup>-1</sup> pH 7.4</td><td>–</td><td rowspan="4">[36]</td></tr><tr><td>2 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>mol L<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Chlorogenic acid</td><td rowspan="2">MPS, TEOS, PhTEOS, APTMS</td><td rowspan="2">Au</td><td rowspan="2">DPV</td><td>5.0 × 10<sup>-7</sup> to</td><td>1.48 × 10<sup>-7</sup></td><td>2.3 %</td><td rowspan="2">(n = 1, 50 DPV scans)</td><td>Caffeic acid, gallic acid, vanilic acid, catechol</td><td>PBS 0.1 mol L<sup>-1</sup> pH 7.0</td><td>Coffee, Black tea, Green tea, Mate tea</td><td rowspan="2">[28]</td></tr><tr><td>1.4 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>mol L<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Caffeine</td><td rowspan="2">VTMOS, TEOS, MTMS, APTMS</td><td rowspan="2">GCE/ MWCNTs</td><td rowspan="2">DPV</td><td>7.5 × 10<sup>-7</sup> to</td><td>2.2 × 10<sup>-7</sup></td><td>2.7 %</td><td rowspan="2">(n = 1, 50 DPV scans)</td><td>Xantine, theophylline, theobromine</td><td>Phosphoric acid 0.15 mol L<sup>-1</sup> pH 1.5</td><td>Coffee, Instant coffee, Energy drink</td><td rowspan="2">[20]</td></tr><tr><td>4.0 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>mol L<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Caffeic acid</td><td rowspan="2">MPS, TEOS, PhTEOS, APTMS</td><td rowspan="2">Au</td><td rowspan="2">DPV</td><td>5.0 × 10<sup>-7</sup> to</td><td>1.5 × 10<sup>-7</sup></td><td>6.95 %</td><td rowspan="2">(n = 10)</td><td>Cinnamic acid, ferulic acid, p-coumaric acid, vanilic acid, gallic acid, 1-hydroxy-2-naphthoic acid</td><td>Sulfuric acid 0.4 mol L<sup>-1</sup></td><td>Red wine, white wine</td><td rowspan="2">[19]</td></tr><tr><td>6.0 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>mol L<sup>-1</sup></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Electrochemical deposition</td><td rowspan="2">Thymidine</td><td rowspan="2">TEOS, MTMOS, VTMOS</td><td rowspan="2">Carbon/ MWCNTs</td><td rowspan="2">Amperometry</td><td>2 × 10<sup>-6</sup> to</td><td>1.6 × 10<sup>-9</sup></td><td>2.7 %</td><td rowspan="2">(n = 6)</td><td>Guanosine, cytidine, adenosine, 2'-deoxyguanosine, 2'-deoxyadenosine</td><td>PBS 0.2 mol L<sup>-1</sup> pH 7.0</td><td>Zidovudine tablets</td><td rowspan="2">[30]</td></tr><tr><td>2.2 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>mol L<sup>-1</sup></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>Table 1 (Continued)</caption><tr><td>L-hystidine</td><td>TEOS, APTES, ITO/ PhTMOS, MTMOS</td><td>DPV</td><td>2.0 × 10<sup>-6</sup> to 1.0 × 10<sup>-3</sup> mol L<sup>-1</sup></td><td>5.8 × 10<sup>-9</sup> mol L<sup>-1</sup></td><td>3.2 % (n = 3)</td><td>D-hystidine, D,L-methionine, L-norleucine, L-proline</td><td>PBS 0.2 mol L<sup>-1</sup> pH 7.0</td><td>Human blood serum</td><td>[29]</td></tr></table>

<table><thead><tr><th>Deposition route</th><th>Analyte</th><th>Main MIS components</th><th>Working electrode</th><th>Method of detection</th><th>Linear range of the calibration curve</th><th>Limit of detection (LOD)</th><th>RSD</th><th>Selectivity towards</th><th>Model solution</th><th>Real sample analysis</th><th>Ref</th></tr></thead><tbody><tr><td></td><td>Methyl parathion</td><td>TEOS, VTEOS</td><td>GCE</td><td>SWV</td><td>1.0 × 10<sup>-8</sup> to 1.0 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>8.9 × 10<sup>-9</sup> mol L<sup>-1</sup></td><td>2.02 % (n = 6)</td><td>Parathion, p-nitrophenol, nitrobenzene, α-hydroxyl-4-nitrophenyl-dimethyl-phosphonate</td><td>PBS pH 5.9</td><td>Pears</td><td>[31]</td></tr><tr><td>Bisphenol A (BPA)</td><td>TEOS, APTES</td><td>Au/ MWCNTs/ AuNPs</td><td>Amperometry</td><td>1.13 × 10<sup>-7</sup> to 8.21 × 10<sup>-3</sup> mol L<sup>-1</sup></td><td>3.6 × 10<sup>-9</sup> mol L<sup>-1</sup></td><td>–</td><td>Phenol, 4-tert-butylphenol, 1,4-dihydroxybenzene</td><td>KCl 0.2 mol L<sup>-1</sup> pH 7.5</td><td>Honey, Tap water, Grape juice</td><td>[17]</td></tr><tr><td>Theophylline</td><td>TEOS, PHTEOS, Pyrrole</td><td>GCE</td><td>DPV</td><td>5.0 × 10<sup>-9</sup> to 1.0 × 10<sup>-3</sup> mol L<sup>-1</sup></td><td>6.6 × 10<sup>-8</sup> mol L<sup>-1</sup></td><td>4.3 % (n = 10)</td><td>Caffeine, theobromine, xanthine, guanine, adenine, paraxanthine</td><td>PBS pH 4.0</td><td>Green tea, Carbonated cola drink, Fermented milk drink, Preserved fruit</td><td>[39]</td></tr><tr><td>p-nitrophenol</td><td>PHTMOS, Chitosan, HAuCl<sub>4</sub>*</td><td>GCE</td><td>DPV</td><td>3.0 × 10<sup>-8</sup> to 3.5 × 10<sup>-4</sup> mol L<sup>-1</sup></td><td>5.0 × 10<sup>-9</sup> mol L<sup>-1</sup></td><td>1.74 % (n = 8)</td><td>o-nitrophenol, m-nitrophenol, 2,3-difluoro-o-nitrophenol</td><td>AcBS pH 5.2</td><td>–</td><td>[22]</td></tr><tr><td>Dopamine</td><td>TEOS</td><td>GCE</td><td>Amperometry</td><td>1 × 10<sup>-7</sup> to 8 × 10<sup>-7</sup> mol L<sup>-1</sup></td><td>5.51 × 10<sup>-7</sup> mol L<sup>-1</sup></td><td>–</td><td>Ascorbic acid</td><td>PBS 0.2 mol L<sup>-1</sup> pH 7</td><td>–</td><td>[38]</td></tr><tr><td>Naloxone</td><td>TEOS, PHTEOS, Pyrrole</td><td>ITO/ MWCNTs</td><td>DPV</td><td>0.0 to 1.2 × 10<sup>-7</sup> mol L<sup>-1</sup></td><td>2 × 10<sup>-8</sup> mol L<sup>-1</sup></td><td>9.3 % (n = 4)</td><td>Codeine, fentanyl, naltraxone, noroxymorphone</td><td>PBS 0.05 mol L<sup>-1</sup> pH 7.4</td><td>Synthetic urine</td><td>[21]</td></tr><tr><td>Clenbuterol</td><td>TEOS, APTES</td><td>Au</td><td>DPV</td><td>2 × 10<sup>-6</sup> to 1 × 10<sup>-4</sup> mol L<sup>-1</sup></td><td>3.1 × 10<sup>-8</sup> mol L<sup>-1</sup></td><td>4.1 % (n = 5)</td><td>–</td><td>PBS 0.1 mol L<sup>-1</sup> pH 7.0</td><td>Milk</td><td>[18]</td></tr><tr><td>Indole-3-acetic acid</td><td>TEOS</td><td>GCE</td><td>DPV</td><td>8.0 × 10<sup>-8</sup> to 1.0 × 10<sup>-5</sup> mol L<sup>-1</sup></td><td>5.0 × 10<sup>-8</sup> mol L<sup>-1</sup></td><td>3.6 % (n = 7)</td><td>Salicylic acid, tryptophan, abscisic acid, citric acid, dopamine, ascorbic acid</td><td>PBS 0.07 mol L<sup>-1</sup> pH 6.0</td><td>Soybean sprout, Sprouts of mung bean</td><td>[40]</td></tr></tbody></table>
- Not given

* HAuCl₄ precursor is added to sol to form AuNPs *in situ*.

TMOS – tetramethoxysilane, TEOS – tetraethoxysilane, PhTMOS – phenyltrimethoxysilane, MTMOS – methyltrimethoxysilane, APTES – (3-Aminopropyl)triethoxysilane, CIPTEOS – (3-Chloropropyl)triethoxysilane, PVP – poly(vinylpyrrolidone), PEI – polyethylenimine, EGDMA – ethylene glycol dimethylacrylate, MPS – (3-Mercaptopropyl)trimethoxysilane, AA – acrylamide, AIBN – 2,2'-azobisisobutyronitrile, VTMOS – vinyltrimethoxysilane; GCE – glassy carbon electrode, ITO – Indium Tin Oxide electrode, MGCE – magnetic glassy carbon electrode, GPE – graphite paste electrode, MWCNTs – multi-walled carbon nanotubes, Au – gold electrode, GO – graphene oxide, AuNPs – gold nanoparticles, Fe₃O₄@SiO₂ – silica coated magnetite nanoparticles, Fe₃O₄@AuNPs – magnetite decorated with gold nanoparticles; CV – cyclic voltammetry, SWV – square-wave voltammetry, EIS – electrochemical impedance spectroscopy, PQC – piezoelectric quartz crystal, DPV – differential pulse voltammetry, DPSV – differential pulse stripping voltammetry, LSV – linear sweep voltammetry; PBS – phosphate buffer solution, BRS – Britton-Robinson solution, AcBS – acetate buffer solution.
4. Application of MIS to electrochemical detection in real samples

Due to the smart design of MIS-modified electrodes, they have been successfully applied to the
electroanalytical detection of analytes in real samples (see **Table 1**). Regardless of the
deposition method, MIS electrochemical sensors generally exhibit selectivity towards the target.
The broadest application of MIS electrochemical sensors is the use for drug recognition in the
**pharmaceutical field**. In particular, the sensitive detection of paracetamol in drug tablets with a
limit of detection (LOD) of 2.0 × 10⁻⁸ mol L⁻¹ [25] became possible due to the introduction of
GO, which provided a higher affinity of the aromatic ring of paracetamol to the developed
conjugated graphene structure and was also assumed in the case of mesalazine recognition [26].
The addition of phenyltrimethoxysilane (PhTMOS) precursor to promote π-π interaction
allowed **enantiomeric recognition** between L- and D-dihydroxyphenylalanine in phosphate-
buffered saline (PBS) [23] and L- and D-histidine [29,32] at the nmol L⁻¹ level with a
calibration range up to four orders. Despite the complexity of **biological fluids**, it has been
possible to selectively capture a number of targets in urine, whole blood samples, and blood
serum. A drop-casted paper-based MIS electrode was used to detect serotonin in spiked urine
samples [27]. Recovery ranged from 100 to 111 %, and a relative standard deviation (RSD) of
1.3 %–6.3 % (n = 3) was reported for four urine samples. Recognition of relatively large
biological targets such as hemoglobin was specific towards horse radish peroxidase (imprinting
factor 2.86) despite using only the TEOS precursor to modify a magnetic GCE (MGCE) [34].
Moreover, hemoglobin concentration could be determined by differential pulse voltammetry
(DPV) in whole blood samples after the 3000-fold dilution in PBS with recoveries of 95.7 %–
105 %. Screening of **food samples** was also performed to quantify specific components and
contaminants. Santos *et al*. demonstrated the detection of caffeine in caffeine-containing
products using the standard addition method [20]. A dietary supplement, *tert*-butyl
hydroquinone (tBHQ), was detected by CV in different oils after its liquid–liquid extraction in
ethanol and further dilution of the supernatant in PBS [33]. Residues of antibiotics
(metronidazole [16]), pesticides (methyl parathion [31]), and BPA [17] were also quantified in
food products with LODs of 0.16, 8.9, and 3.6 nmol L⁻¹, respectively.
**5. Evaluation of MIS deposition routes for electroanalytical application**

The summarized information on the advantages and drawbacks of the deposition techniques discussed in this review is outlined in **Table 2**.

Regarding the simplicity of the deposition procedure, drop-casting and spin-coating approaches used to be the most attractive techniques. Although the effect of spin-coating deposition parameters is well-studied [41,42], the process is difficult to scale up and provides linearity that is not better than one order [14,24]. Drop-casted MIS sensors provide higher linearity of the current-concentration (up to five orders of magnitude, from $10^{-8}$ to $10^{-3}$ mol L$^{-1}$) and lower LOD (down to $2 \times 10^{-9}$ mol L$^{-1}$ [27]), which is mainly due to the switching to DPV detection and the use of conductive nanomaterials to make the sensor more sensitive compared to the spin-coating approach. Moreover, a single drop of sol can be sufficient to form a highly sensitive layer [16,25–27,34]. Nevertheless, the poor adhesion of MIS and the inhomogeneity of the resulting layers [43] often lead to an electrochemical response that is not very reproducible from one electrode to another, reaching an RSD of 8.2 % (n = 5) [25].

Grafting of the electrode surface allowed to improve mechanical stability, resulting in better reproducibility of the electrode signal recorded on one electrode (RSD of 2.3 % [28] to 2.7 % [20] was calculated from 50 measurements) and a longer storage period (up to three months at room temperature [20]). Although grafting provides flexible synthesis conditions, resulting in outstanding stability of the immobilized layer, it is often technically complicated and time consuming [37]. Electrochemical deposition has overcome most drawbacks typical of other deposition techniques by precisely controlling the polymerization process through strictly defined electrochemical deposition parameters. This technique increased the linear range of analyte detection and lowered the LOD for most analytes tested. The RSD was thus reduced (to 1.74 % for the detection of *p*-nitrophenol, with n = 8, [22]), which allows more reproducible analysis. It is worth mentioning that a more consistent statistical treatment of the data reported in **Table 1** makes the calculated RSD values for electrochemical deposition more reliable than those for grafting. The complexity of electrochemically induced deposition normally requires
strict control of the precursor ratio to avoid steric hindrance of bulky functions during film
formation. Unfortunately, stability data for electrochemically deposited MIS are not often
discussed in the literature. Nonetheless, a GCE modified with MIS-CS-AuNPs was shown to
retain a stable DPV response after storage at room temperature for 60 days [22].
<table><caption>**Table 2** – Advantages and drawbacks of MIS deposition routes on the electrode surface for their application in electrochemical detection of analytes.</caption><thead><tr><th></th><th>Advantages</th><th>Drawbacks</th></tr></thead><tbody><tr><td>Spin coating</td><td>Simple and relatively fast<br>Applicable to any type of flat substrate</td><td>Poor film reproducibility<br>Low mechanical stability<br>Low linear calibration range<br>Limited to flat electrodes<br>Complicated scaling-up</td></tr><tr><td>Drop casting</td><td>Simple casting procedure<br>Applicable to any type of flat substrate<br>Improvement of the sensitivity by addition of nanomaterials (e.g. MWCNTs)</td><td>Poor film reproducibility<br>Poor mechanical stability<br>Limited to flat electrodes.</td></tr><tr><td>Grafting</td><td>Good mechanical stability<br>Flexible synthesis conditions<br>Applicable to non-flat surfaces</td><td>The use of organic solvents<br>Time consuming</td></tr><tr><td>Electrochemical deposition</td><td>Simple one-step deposition procedure<br>Good mechanical stability<br>Possible synthesis of a silica-polymer hybrid<br>Improvement of the sensitivity by addition of nanomaterials (e.g. MWCNTs)<br>Wide linear calibration range<br>Satisfactory film reproducibility</td><td>Conductive surface is required<br>Strict compliance with the ratio of silica precursors<br>Insufficient stability data</td></tr></tbody></table>
## 6. Conclusions

In this review, the main approaches for electrode modification by MIS have been critically discussed. Despite the simplicity of electrode coating by MIS following physical approaches, it is technically challenging to scale up to commercial levels while maintaining reproducible results. Therefore, electrochemical deposition has begun to replace physical modification routes and sophisticated grafting. This approach has overcome reproducibility and inhomogeneity issues while providing fast and robust attachment of the imprinted film with good scale-up prospects. Despite the obvious advantages of MIS, MIPs still dominate in the electrode modification field. Although the main challenge now lies in improving electrode stability, it is highly likely that once solved, MIS-based sensing systems will have a great future in portable analysis.

## Acknowledgements

This work was financially supported by the French "Investissements d'Avenir" program (Project ISITE-BFC 2021-112-SENSAAS).

## Author contributions

All authors equally contributed to this work.
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Immobilization of MIS on the electrode

<figure><img src="image_2.png" /></figure>

Spin
coating

<figure><img src="image_3.png" /></figure>

Drop
casting

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

Grafting

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Electro
chemical

<figure><img src="image_9.png" /></figure>