New trends in nanoclay-modified sensors

inorganics Review New Trends in Nanoclay-Modified Sensors Esperanza Pavón1,2,* , Rosa Martín-Rodríguez 3,4 , Ana C. Perdigón3,4 and María D. Alba 1   Citation: Pavón, E.; Martín-Rodríguez, R.; Perdigón, A.C.; Alba, M.D. New Trends in Nanoclay-Modified Sensors. Inorganics 2021,9, 43. https:// doi.org/10.3390/inorganics9060043 Academic Editor: Antonino Gulino Received: 11 May 2021 Accepted: 28 May 2021 Published: 2 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Instituto Ciencia de los Materiales de Sevilla, CSIC-Universidad de Sevilla, Avda. Américo Vespucio, 49. 41092 Seville, Spain; [email protected] 2Departamento Física de la Materia Condensada, Universidad de Sevilla, Av. Reina Mercedes s/n, 41012 Seville, Spain 3Departamento QUIPRE, Universidad de Cantabria, Avda. de Los Castros 46, 39005 Santander, Spain; [email protected] (R.M.-R.); anacarmen.per[email protected] (A.C.P.) 4Grupo de Nanomedicina, IDIVAL, Avda. Cardenal Herrera Oria s/n, 39011 Santander, Spain *Correspondence: [email protected]; Tel.: +34-954-48-9546 Abstract: Nanoclays are widespread materials characterized by a layered structure in the nano-scale range. They have multiple applications in diverse scientific and industrial areas, mainly due to their swelling capacity, cation exchange capacity, and plasticity. Due to the cation exchange capacity, nanoclays can serve as host matrices for the stabilization of several molecules and, thus, they can be used as sensors by incorporating electroactive ions, biomolecules as enzymes, or fluorescence probes. In this review, the most recent applications as bioanalyte sensors are addressed, focusing on two main detection systems: electrochemical and optical methods. Particularly, the application of electrochemical sensors with clay-modified electrodes (CLME) for pesticide detection is described. Moreover, recent advances of both electrochemical and optical sensors based on nanoclays for diverse bioanalytes’ detection such as glucose, H 2 O 2 , organic acids, proteins, or bacteria are also discussed. As it can be seen from this review, nanoclays can become a key factor in sensors’ development, creating an emerging technology for the detection of bioanalytes, with application in both environmental and biomedical fields. Keywords: nanoclays; sensor; electrochemical; optical; biosensors; bioanalytes 1. Introduction 1.1. Nanoclays: Definition and Classification Nanomaterials’ definition is still controversial, and scientists have not unanimously settled on a precise denotation, although there is consensus that they are partially characterized by their tiny size, in the range of nanometers. In 2011, in order to clarify this debate, the EU adopted the following definition for a nanomaterial: “a natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1 nm–100 nm” [ 1 ]. Among the first natural nanomaterials known, according to this definition, are nanoclays, which have been used since historical times. Nanoclays are a broad class of natural or synthetic clay minerals with at least one dimension in the nano-range order, as defined by the EU [ 1 ] and characterized by a layered structure. They can be divided into three main classes, according to their electrochemical charge (Table 1and Figure 1). • Neutral nanoclays do not possess any charge in their layers. Commonly, their layers are formed by two tetrahedral sheets that sandwich one octahedral sheet (2:1 nanoclay, pyrophyllite, and talc group) or by a tetrahedral sheet confronted with an octahedral sheet (1:1 nanoclays, kaolinite, and serpentine group). Tetrahedral sheets are formed by [SiO4]−4ions while octahedral sheets are formed by Mg+2 or Al+3 complex. Inorganics 2021,9, 43. https://doi.org/10.3390/inorganics9060043 https://www.mdpi.com/journal/inorganics Inorganics 2021,9, 43 2 of 23 • Cationic nanoclays have negatively charged layers. Two main groups can be found into this category, 2:1 and 2:1:1 nanoclays. The first one is formed by the same sheet configuration previously described for neutral 2:1 nanoclays. The most common minerals on the Earth’s surface can be found in this group: smectites, vermiculites, micas, and brittle micas. The second group is formed by stacking an octahedral sheet between a 2:1-layer configuration, forming the chlorite group. Negative layer charge is originated by the isomorphous substitutions that occur either in the tetrahedral (Si +4 by Al +3 ) or octahedral sheet (Al +3 /Mg +2 by Mg +2 /Li + , respectively, for example). Electrical neutrality is ensured by inorganic or organic ions in the interlayer space balancing the charge generated. When organic cations are introduced in the interlayer space, organo-nanoclays are created. • Anionic clays are positively charged hydroxide layers, also known as layered double hydroxides (LDHs). Their layer structure (Figure 1) is based in divalent metal ions (such as Mg +2 ), forming an octahedral sheet. Commonly, these divalent cations are substituted by trivalent cations (such as Al +3 ), creating a positive charge for each aluminum substituted by magnesium ion. To maintain the electrical neutrality, anions such as CO3−2, NO3−, and Cl−are located in the interlayer galleries. Table 1. Nanoclays’ classification. Type of Layer Charge Group Layer Type Layer Charge O10(OH,F)2Unit Neutral Kaolinite-serpentine 1:1 <0.01 Pyrophyllite-Talc 2:1 0 Cationic Smectite 2:1 0.2 to 0.6 Vermiculite 0.6 to 0.9 Mica ca. 1 Brittle mica up to 2 Chlorite 2:1:1 variable Anionic Layered double hydroxide (LDH) 0:1 variable Inorganics 2021, 9, x 2 of 25 are formed by [SiO 4 ] −4 ions while octahedral sheets are formed by Mg +2 or Al +3 complex. • Cationic nanoclays have negatively charged layers. Two main groups can be found into this category, 2:1 and 2:1:1 nanoclays. The first one is formed by the same sheet configuration previously described for neutral 2:1 nanoclays. The most common minerals on the Earth’s surface can be found in this group: smectites, vermiculites, micas, and brittle micas. The second group is formed by stacking an octahedral sheet between a 2:1-layer configuration, forming the chlorite group. Negative layer charge is originated by the isomorphous substitutions that occur either in the tetrahedral (Si +4 by Al +3 ) or octahedral sheet (Al +3 /Mg +2 by Mg +2 / Li + , respectively, for example). Electrical neutrality is ensured by inorganic or organic ions in the interlayer space balancing the charge generated. When organic cations are introduced in the interlayer space, organo-nanoclays are created. • Anionic clays are positively charged hydroxide layers, also known as layered double hydroxides (LDHs). Their layer structure (Figure 1) is based in divalent metal ions (such as Mg +2 ), forming an octahedral sheet. Commonly, these divalent cations are substituted by trivalent cations (such as Al +3 ), creating a positive charge for each aluminum substituted by magnesium ion. To maintain the electrical neutrality, anions such as CO 3−2 , NO 3− , and Cl - are located in the interlayer galleries. Table 1. Nanoclays’ classification. Type of Layer Charge Group Layer Type Layer Charge O 10 (OH,F) 2 Unit Neutral Kaolinite-serpentine 1:1 <0.01 Pyrophyllite-Talc 2:1 0 Cationic Smectite 2:1 0.2 to 0.6 Vermiculite 0.6 to 0.9 Mica ca. 1 Brittle mica up to 2 Chlorite 2:1:1 variable Anionic Layered double hydroxide (LDH) 0:1 variable Figure 1. (a) Tetrahedral sheet; (b) octahedral sheet. Layered nanoclay structures: (c) neutral nanoclays (type 0:1 and 2:1), (d) cationic nanoclays (type 2:1 and 2:1:1), and (e) anionic nanoclays (type 0:1). Figure 1. ( a ) Tetrahedral sheet; ( b ) octahedral sheet. Layered nanoclay structures: ( c ) neutral nanoclays (type 0:1 and 2:1), ( d ) cationic nanoclays (type 2:1 and 2:1:1), and ( e ) anionic nanoclays (type 0:1). In all cases, tetrahedral or octahedral sheets have thicknesses of 0.3 nm and 0.4 nm, respectively (Figure 1). Inorganics 2021,9, 43 3 of 23 Swelling capacity, cation exchange capacity, and plasticity are the main properties derived from the layer charge and interlayer cations that make nanoclays so useful for many applications. In fact, nanoclays, especially cationic ones, have been used for centuries in architecture, industry, and agriculture. Because they are environmentally friendly and readily available in large quantities at relatively low cost, they are nowadays applied in medical, food, and environmental fields [2]. Due to the cation exchange capacity, nanoclays can serve as host matrices for the stabilization of several molecules and, thus, they can be used as sensors by incorporating electroactive ions, biomolecules as enzymes, or fluorescence probes. The resulting nanoclaymodified electrodes (CLME) and optical sensors can be used for their electro-catalytic properties or analytical applications [3,4]. 1.2. Sensors A sensor is a device containing a recognition element that interacts with an analyte, leading to a chemical or physical change, which a transducer converts into a detectable signal [ 5 ]. They can be classified into several categories based on the transduction process (electrochemical, optical, piezoelectric, and thermal sensors) or according to the receptor type (enzymatic biosensors, microbial, biosensors, genosensors, or immunosensors) [ 6 ]. There is current interest in the development of different sensing systems for biomedical and environmental applications. Results in biology are usually complex, non-linear, and non-reproducible. Thus, the development of sensors with improved sensitivity and reproducibility represents an increasingly important area of research [ 7 , 8 ]. In the areas of agriculture and environment, sensors have been widely developed for the determination of pesticides, herbicides, and heavy metals in the soil and ground water, opening a new strategy for active prevention of forecast contamination. Different types of sensing techniques can be distinguished and are addressed in this review. Firstly, electrochemical sensors are defined as integrated systems that provide quantitative or semi-quantitative analytical response thorough interactions between biochemical receptors and electrochemical transduction elements [ 9 ]. Electrochemical signal transduction can occur through the measurement of current, potential, conductance, or field effect, giving rise to voltammetry, conductometry, or impedance spectroscopy techniques. A variety of electrode materials, namely, noble metals, glassy carbon (GC), semiconductors such as indium tin oxide (ITO), or conductive polymers are available to enable specific sensing applications [ 5 ]. Electrochemical sensors have been used for detection of many different analytes that can readily be subjected to or coupled with electrochemical reactions. Several challenges must be considered in the development of sensors based on electrochemical methods. (1) The electrochemical response is usually sensitive to electrode fabrication procedures and environmental contamination, which causes non-reproducibility. (2) The use of bare electrodes usually involves slow kinetics, which limits the sensitivity. (3) The overlapping of the target analyte and concomitant compounds’ signals may prevent accurate detection and selectivity [ 10 ]. Chemical modification of the bare electrodes’ surface holds great potential to overcome these problems [ 11 ]. Several studies have previously reported an increase in sensitivity and selectivity toward specific target molecules by electrode modification with nanomaterials enhancing surface conductivity, such as metal nanoparticles (NPs) [ 12 ], carbon-based materials [ 13 ], organic polymers [ 14 ], or ionic liquids [ 15 ]. Electrochemical sensors are most desirable among all sensors due to their high sensitivity, experimental simplicity, and low-cost production [16]. The electrode surface modification has strong influence on the sensing performance of the electrochemical sensor [ 17 ]. Nanoclays have been applied as surface modification materials to improve the sensing performance and, since the 1990s, CLME have been extensively studied, principally, their electron transfers’ phenomenon [ 18 – 21 ]. For example, the recent review by Goda et al. summarized the works on halloysite nanotubes’ (HNTs’) surface engineering and application as electrochemical sensor [ 22 ]. HNTs have been Inorganics 2021,9, 43 4 of 23 applied for several bioanalytes’ identification, namely’ hydrogen peroxide, glucose, nitrite, ascorbic acid, amines, hydrazine, dopamine, or uric acid. Among electrochemical sensors, those based on enzymatic receptor have attracted wide attention in the biomedical and environmental fields. They can be defined as an analytical device that works on the basis of detecting an electric signal having an enzyme as a bioreceptor. They are usually named biosensors. Enzymes are biological macromolecules, proteins with catalytic activity, characterized by a high selectivity and specificity. However, the low stability of enzymes in organic solvents at widely elevated temperatures has frequently hampered the development of enzymatic biosensors [ 23 ]. To overcome this issue, the immobilization of enzymes onto organic or inorganic solid matrices has been suggested. Enzyme immobilization onto organic supports, such as polymer or organogels, has been demonstrated to satisfactorily maintain the high activity of the enzymes, but they present low thermal and chemical stability [ 24 ]. On the contrary, immobilization of enzymes on inorganic solids, such as zeolites, activated carbon, carbon nanotubes, silica mesoporous materials, and two-dimensional, layered clays, among others, results in improved activity, stability, and recyclability of the enzyme in a broader working pH and temperature range than the isolated enzyme [25,26]. The performance of an enzymatic biosensor depends also on maintaining its catalytic action. A reduction in the dimensions of the enzyme support material generally improves the biocatalytic efficiency [ 27 ]. In this sense, due to the tiny size of their particles, large specific surface area, good absorbance ability, and high cationic exchange capacity, nanoclays are one of the most relevant material for enzyme immobilization [ 28 ], promoting greater interaction with the enzyme and reducing diffusional limitations [ 27 ]. Most clay minerals used in clay-based biosensors are smectites (laponite, montmorillonite, nontronite) and have been applied in analytical applications [16]. Nanoclays have been widely exploited for biosensor design due to their mechanical and thermal stabilities, chemical inertness, and ion exchange properties, becoming versatile and non-expensive matrices. For example, montmorillonite, a naturally occurring cationic nanoclay, is applied in a variety of electrochemical studies as CLME. By functionalization of these clay minerals with specific organic groups like amines, the resulting organic–inorganic hybrid materials show better selectivity or sensitivity. In this review, we will expose the latest electrochemical sensors based on nanoclays, used in the determination of both harmful substances to the environment, such as pesticides, and diverse biological analytes. Secondly, optical sensors are composed of a light source, the sensing material that interacts with the analyte, and a light detector. Specifically, optical sensors based on fluorescence parameters, such as luminescence and excitation spectra, emission lifetime, or Stokes shift, can provide high flexibility and sensitivity, although the experimental apparatus might be expensive [ 29 ]. The sensing element containing the fluorescent probe must interact with the analyte and subsequently report on such interactions by the change of an optical property. Different kinds of fluorescence probes based on organic dye molecules [ 30 ], trivalent lanthanides [ 31 ], gold NPs, or semiconductor quantum dots [ 32 ] have been developed. Organic dyes usually show poor chemical stability, while quantum dots, composed of heavy metals, present toxicity and environmental hazards, which limit practical applications. On the contrary, lanthanide-based fluorescent probes present better performance, due to their low toxicity and better stability. In addition, lanthanide cations, particularly trivalent europium, Eu 3+ , show sharp-line emission bands, large Stokes shift, long lifetimes, and high resistance to photo-bleaching [ 33 ]. Additionally, magnetic resonance contrast agents have been the subject of extensive research in the past decade, with Gd 3+ ions being the most commonly used ions for T 1 -based magnetic resonance contrast enhancement [ 34 ]. In particular, colorimetric methods have recently gained attention due to their fast operation, low cost, and simplicity. They are based on the visual color change during interaction with the analytes and, thus, allow detection by immediate observation by the naked eye, without any expensive or sophisticated instrumentation. Inorganics 2021,9, 43 5 of 23 2. Nanoclay-Based Sensors for Environmental Applications Since 1960, several types of pesticides have been widely used in agriculture due to their high activity against fungi, insects, and weeds [ 35 ]. As a result, non-target organisms are inevitably exposed to these toxic compounds that can cause damages to human health and to ecosystems [ 36 , 37 ]. During the past decade, works devoted to the development of electrochemical devices suitable for the detection of pesticides have gained growing interest as innovative alternatives in terms of on-site detection, sensitivity, rapid response, miniature size, and low cost. Most reported papers have been devoted to the development of CLME for the detection of herbicides (such as mesotrione, glyphosate, phenyl urea herbicides, and paraquat) [ 3 , 20 , 38 – 41 ] and fungicides (such as propineb, chlorpyrifos, or carbendazim) [42–44]. Raw nanoclays [ 20 , 38 , 40 ] and modified nanoclays with metallic cations such as Cu 2+ [ 42 ] or organic cations have been tested as CLME. Among the possible organic molecules intercalated in the nanoclay interlayer, cetyltrimethylammonium (SaCTA) [ 20 ], didodecyldimethyl ammonium (SaDDA) [ 20 ], acetylcholinesterase (AChE) [ 43 ], or methyl viologen (MV) [ 38 ] are the most common ones. Moreover, nanoclays grafted with silane molecules have also been proposed as CLME [ 39 , 44 ]. In this case, several strategies have been employed and the properties of the grafted nanoclays depended on the nature of the solvents and the silane molecule. For example, Mbokana et al. [ 39 ] successfully modified the internal and external surface of a smectite by triethoxyoctylsilane to prepare CLME for the electroanalysis of glyphosate. The material was first submitted to a soft acid pretreatment to provide abundant and available hydroxyl groups capable of reacting with the silane. They found that silane grafting was more effective in ethylene glycol (swelling) as solvent compared to toluene (non-swelling). Also, Yanke et al. [ 44 ] investigated the effect of these two solvents on the grafting of an organophilic silane (octyltriethoxisilane) onto a smectitic clay to evaluate their abilities for electrochemical detection of organophilic pesticide. There, the grafting occurred exclusively on the edges of the clay particles without intercalation. In both cases, the material functionalized in ethylene glycol was found to be the most efficient, due to the combined effect of the characteristics of the pristine clay and the grafted silane. Those sensors provided a very reproducible response and their abilities for electrochemical detection of organophilic pollutants reached the trace levels. Table 2summarizes the behavior of several CLME toward fungicide determination, mainly propineb, carbendazim, and chlorpyrifos [ 21 , 42 – 45 ]. A broad range of CLME configuration was observed, ranging from the application of raw nanoclays to advanced nanoclay composites with mainly carbon nanotubes and metal NPs. Moreover, the analysis of the results was even more complicated as several detection techniques were applied to characterize the electrochemical sensors. In general, a wide linear range was observed for all sensors; however, it was different for each specific configuration, as expected. As well, the limit of detection (LOD) was quite small for all of them, specifically for the detection of chlorpyrifos. A special case was the CLME with Cu +2 -montmorillonite for propineb detection. It presented the highest linear range observed for all the electrodes (5.0–30 µ M). However, its LOD was the highest observed, and ca. 100–1000 times higher than those previously reported for classical electrochemical sensors. Table 2. Performances of several chemically modified electrodes toward fungicide determination. Clay Modifier Analyte Detection Technique Analytical Range (µM) LOD (µM) Ref. Montmorillonite Cu2+ Propineb SWV 5.0–30 1 [42] Montmorillonite Octyltriethoxisilane Carbendazim CV 1–7 0.03 [44] Montmorillonite - DPV - 0.96 [45] Montmorillonite Heteropolyacid SWV 0.052–2.615 0.052 [21] CLDH AChE Chlorpyrifos DPV 1.43 ×10−4–0.43 1.43 ×10−4[43] SWV: square wave voltammetry; DPV: differential pulse voltammetry; CV: cyclic voltammetry; CLDH: Cu-Mg-Al calcined-layered double hydroxide; AChE: acetylcholinesterase. Inorganics 2021,9, 43 6 of 23 The same situation was observed for the determination of carbendazim by Namontmorillonite [ 45 ], showing a LOD considerably higher than the rest of the nanoclay sensors shown in Table 2. This drawback can be overcome with the montmorillonite organofunctionalization (Octyltriethoxisilane-Mt/CPE and Heteropolyacid Mt/GCE), which produces a decrease in the LOD in one order of magnitude [21,44]. The best performance was observed for the detection of chlorpyrifos. In this case, double-layered hydroxides loaded with acetylcholinesterase (AChE) were employed and this CLME exhibited a good response for the chlorpyrifos’ detection in the different configurations probed [43]. Herbicide detection has also been studied with CLME (Table 3) [ 20 , 21 , 38 – 40 , 45 – 47 ]. Diverse electrode configuration has been tested, with paraquat being the most common analyte detected. In general, the best performance was obtained when organonano-nanoclays were used. An example is the use of DDA-Saponite (SaDDA/GCE) vs. Na-Saponite (SaNa/GCE) for mesotrione detection [ 20 ] or organomontmorillonite film vs. sodium montmorillonite film/GCE for phenyl urea herbicide detection [ 21 , 45 ]. A particular case is the use of CLME for paraquat detection, where the use of raw smectites in the CLME resulted in good LOD and linear range [38,46,47]. Table 3. Performances of several chemically modified electrodes toward herbicide determination. Clay Modifier Analyte Detection Technique Analytical Range (µM) LOD (µM) Ref. Cameroonian smectite - Mesotrione SWV 5–19 - [20] Cameroonian smectite Cetyltrimethyl ammonium SWV 1.25–9.80 - [20] Cameroonian smectite DDA SWV 0.25–2.24 0.26 [20] montmorillonite Triethoxyoctylsilane Glyphosate SWV 10–100 0.98 [39] NiAl-LDH - phenyl urea herbicides CV 0.02–0.18 0.001 [40] montmorillonite film - CV 0.206–61.8 0.206 [45] montmorillonite Organo functionalized SWV 5×10−3–1.45 5 ×10−3[21] montmorillonite K+ Paraquat CV 0.03–0.4 0.14 ×10−3[38] Smectite film - SWV 1.6–2.8 3.8 ×10−3[46] clay composite film β-cyclodextrin DPV 0.099–1.99 2.8 ×10−3[47] SWV: square wave voltammetry; DPV: differential pulse voltammetry; CV: cyclic voltammetry; NiAl-LDH: layered double hydroxide; DDA: didodecyldimethyl ammonium. The LOD of glyphosate by electrochemical electrodes based in carbon is higher than those based in metallic particle but they provided a wider concentration of range of detection [39]. As stated in the introduction, biosensors are special electrochemical sensors, defined as an analytical device in which a biological response is converted into a signal that can be quantified and processed [ 48 ]. Enzymatic biosensors based on CLME can be used for the determination of pesticides through their inhibitory action on enzyme activity. In the last decade, the main studies have been centered in the determination of glyphosate and chlorpyriphos (see Table 4) [49–51]. Table 4. Behavior of several enzymatic biosensors toward herbicide determination. Clay Enzyme Analyte Detection Technique Analytical Range (mg/L) LOD (mg/L) Ref. Nanoclay Atemoya peroxidase Glyphosate SWV 100–4550 30 [49] CH-Mt AChE Chlorpyrifos CV 0.5–1000 0.448 [50] Kaol AChE EIS 0.5–200 0.28 [51] Mt AChE EIS 0.5–100 0.48 [51] Bent AChE EIS 2–200 0.65 [51] Sep AChE EIS 2–200 0.71 [51] AChE: acetylcholinesterase; Kaol: kaolinite; Mt: montmorillonite; Bent: bentonite; Sep: sepiolite; CH-Mt = chitosan-montmorillonite; SWV: square wave voltammetry; CV: cyclic voltammetry; EIS: electrochemical impedance spectroscopy. Inorganics 2021,9, 43 7 of 23 For example, a biosensor based on atemoya peroxidase immobilized into a modified nanoclay (montmorillonite containing 0.5–5 wt.% aminopropyltriethoxysilane and 15–35 wt.% octadecylamine) was developed [ 49 ]. The linear range of this biosensor for the detection of glyphosate was satisfactory. It was constructed using graphite powder, multiwalled carbon nanotubes, peroxidase immobilized on nanoclay, and mineral oil. The best performance was obtained using a 0.1 mol · L −1 phosphate buffer solution (pH 7.0), 1.9 ×10−4mol·L−1 hydrogen peroxide, a frequency of 30 Hz, a pulse amplitude of 50 mV, and a scan increment of 4 mV. This biosensor remained stable for a period of 8 weeks. For the detection of chlorpyriphos, several nanoclays functionalized with chitosan [ 50 ] or metallic NPs [ 52 ] were employed for the fabrication of enzymatic biosensors via immobilization of acetylcholinesterase (AChE) [50,51]. The hybridization of nanoclays and Au NPs provided remarkable composite materials with high electrochemical conductivity, high surface-to-volume ratio, high chemical reactivity, and chemisorption capacity of various types of molecules [ 53 ]. Phongphut et al. [ 51 ] found that Montmorillonite with Au NPs (Mt/Au NPs) was the best support for AChE loading, showing residual activities due to its high specific surface area and weak electrostatic interaction with the immobilized enzyme (Figure 2). However, when kaolinite was used as supported material for the Au NPs (Kaol/AuNPs), a lowest detection limit resulted, presumably due to the strong hydrophobic surface that helped to concentrate the nonpolar pesticide to the vicinity of the immobilized enzyme. Unfortunately, the highest electrostatic repulsion between the Kaol/AuNPs and AChE and the dehydrated kaolinite surface resulted in the lowest storage stability of the AChE pesticide sensors. Overall, Mt/AuNPs showed the highest potential as the AChE carrier and the modified electrode material for chlorpyrifos’ determination. Inorganics 2021, 9, x 8 of 25 Figure 2. Schematic representation of clay/AuNPs synthesis, AChE immobilization, and SPCE modification with (a) successful APTES modification and (b) unsuccessful APTES modification. Reprinted with permission from ref. [51]. Copyright 2020 Elsevier. 3. Nanoclay-Based Sensors for Biomedical Applications The development of diverse strategies for detection of bioanalytes shows great importance in biomedicine, clinical diagnosis, and treatment. Determination of small metabolic molecules, mainly glucose, hydrogen peroxide (H2O2), and dopamine, in biological fluids at low concentrations is fundamental for the early diagnosis of several diseases [54,55]. Two main sensors’ groups can be found related to this topic. The first one is classical electrochemical electrodes modified by the inclusion of nanoclays. In this group, special recognition to nanoclay-based biosensors will be given in this section. The second one corresponds to optical sensors based on the adsorption of a fluorophore molecule in nanoclays. In the following paragraphs, a description of the recent nanoclay-based sensors for the detection of several bioanalytes is presented, divided in terms of the bioanalyte detected and type of nanoclay sensor (electrochemical, biosensor, and optical). 3.1. Nanoclay-Based Electrochemical Sensors 3.1.1. Glucose Determination Unbalanced glucose levels in blood are used as an evidence of several chronic metabolic disorders, diabetes being the most common disease [56]. Elevated glucose levels in blood are also associated with other serious endocrine disorders, certain infections, and nervous, ocular, and cardiac diseases. Therefore, an inexpensive, sensitive, and fast method for monitoring glucose is needed at physiological concentrations of approximately 3–8 mM. Enzymatic glucose biosensors based on nanoclays have recently acquired attention due to their good performance in terms of detection limit, stability, sensibility, and selectivity for in vitro glucose determination from human biological fluids [57–59]. Several authors have applied raw and functionalized clays in biosensors’ preparation for glucose detection, as stated in Table 5 [60–72]. A schematic representation of an enzyme biosensor based on electrode with nanoclay for glucose determination is shown in Figure 3 [23]. In these biosensors, glucose is oxidized and gluconic acid is generated. The electrical signal is produced by a mediator that produces a redox reaction with the adenine dinucleotide (FAD), a component of glucose oxidase (GOx), the most common enzyme used in enzyme electrodes [23]. The reactions are depicted as follows: Figure 2. Schematic representation of clay/AuNPs synthesis, AChE immobilization, and SPCE modification with ( a ) successful APTES modification and ( b ) unsuccessful APTES modification. Reprinted with permission from ref. [51]. Copyright 2020 Elsevier. 3. Nanoclay-Based Sensors for Biomedical Applications The development of diverse strategies for detection of bioanalytes shows great importance in biomedicine, clinical diagnosis, and treatment. Determination of small metabolic molecules, mainly glucose, hydrogen peroxide (H 2 O 2 ), and dopamine, in biological fluids at low concentrations is fundamental for the early diagnosis of several diseases [ 54 , 55 ]. Two main sensors’ groups can be found related to this topic. The first one is classical electrochemical electrodes modified by the inclusion of nanoclays. In this group, special Inorganics 2021,9, 43 8 of 23 recognition to nanoclay-based biosensors will be given in this section. The second one corresponds to optical sensors based on the adsorption of a fluorophore molecule in nanoclays. In the following paragraphs, a description of the recent nanoclay-based sensors for the detection of several bioanalytes is presented, divided in terms of the bioanalyte detected and type of nanoclay sensor (electrochemical, biosensor, and optical). 3.1. Nanoclay-Based Electrochemical Sensors 3.1.1. Glucose Determination Unbalanced glucose levels in blood are used as an evidence of several chronic metabolic disorders, diabetes being the most common disease [ 56 ]. Elevated glucose levels in blood are also associated with other serious endocrine disorders, certain infections, and nervous, ocular, and cardiac diseases. Therefore, an inexpensive, sensitive, and fast method for monitoring glucose is needed at physiological concentrations of approximately 3–8 mM. Enzymatic glucose biosensors based on nanoclays have recently acquired attention due to their good performance in terms of detection limit, stability, sensibility, and selectivity for in vitro glucose determination from human biological fluids [ 57 – 59 ]. Several authors have applied raw and functionalized clays in biosensors’ preparation for glucose detection, as stated in Table 5[60–72]. Table 5. Electrochemical nanoclay sensors for glucose detection. Enzyme Clay Functional Molecule Electrode Analytical Ranges (mM) LOD (µM) Response Time (s) Ref. GOx Mt M/DM Carbon 0.05–1.0 38 7 [60] GOx Mt PAMAM dendrimer Carbon 0.01–16 4 <3 [61] PyOx Mt PVA/PAMAM Carbon 0.005–0.25 0.7 - [62] PyOx Mt Calixarene Carbon - 0.5 20 [63] G. oxydans Mt trimethylamine GCE 0.1–5.0 - - [64] GOx Mt Gly, Lys, Glu Carbon 0.1–1.0 - - [65] PyOx Dellite CTAB Carbon 0.01–0.5 0.081 - [66] GOx Pal PoPD Pt disk 0.05–3.7 0.0624 6 [67] PyOx Mt PGA Carbon 0.01–0.5 - - [68] PyOx Mt 4-ATP Carbon 0.01–0.5 - - [69] GOx Lap VBT/VBA Carbon - - - [70] GOx Lap PMMA/poly(BIPE)/ Graphite - - - [71] GOx Mt PAN/DDAC/MB -0.01–2.45 2.45–15 2 - [72] GOx: glucose oxidase; PyOx: pyranose oxidase; Mt: montmorillonite; Pal: palygorskite; Lap: laponite; M: methylamine; DM: dimethylamine; PAMAM: Poly(amidoamine); PVA: poly(vinyl) alcohol; Gly: glycine; Lys: lysine; Glu: glutamic acid; CTAB: cetyltrimethylammonium bromide; PoPD: Poly (o-phenyledediamine); PGA: polyglycolide; 4-ATP: 4-aminothiophenol; VBT: vinylbenzyl thymine; VBA: vinylbenzyl triethylammonium chloride; PMMA: polymethylmethacrylate; PAN: polyacrylonitrile; DDAX: dioctadecyl dimethyl ammonium chloride; MB: methylene blue. A schematic representation of an enzyme biosensor based on electrode with nanoclay for glucose determination is shown in Figure 3[ 23 ]. In these biosensors, glucose is oxidized and gluconic acid is generated. The electrical signal is produced by a mediator that produces a redox reaction with the adenine dinucleotide (FAD), a component of glucose oxidase (GOx), the most common enzyme used in enzyme electrodes [ 23 ]. The reactions are depicted as follows: GOx(FAD) + Glucose →GOx(FADH2) + Gluconic acid GOx(FADH2) + Medox →GOx(FAD) + Medred Seleci et al. [ 60 ] described a simple and accurate electrochemical method using methyl- and dimethylamine-exchanged montmorillonite as an immobilization matrix. GOx was chosen as a model enzyme in the presence of bovine serum albumin (BSA) and glutaralde- Inorganics 2021,9, 43 9 of 23 hyde, and they were immobilized on the clay-modified glassy carbon electrodes (GCE). In order to optimize the biosensor, the influence of the working pH, enzyme amount, and reproducibility of biosensor fabrication was characterized in detail. The optimized biosensor showed a very good linearity between 0.05 mM and 1.0 mM, a 7 s response time, and a LOD to glucose of 0.038 mM. Kinetic parameters and stabilities were also determined. Inorganics 2021, 9, x 10 of 25 Figure 3. A schematic drawing of enzyme biosensor based on electrode with nanoclay hybrids. Reprinted with permission from ref. [23]. Copyright 2015 Elsevier. Following the same approach, Han et al. [61] used dendritic macromolecules, in particular poly-amydoamine (PAMAM), to modify the electrode surface. PAMAM is able to prevent NPs’ aggregation and to increase the concentration of hydrophobic molecules at the surface. To improve the electron-transfer efficiency of the enzyme with the inorganic material, platinum NPs were encapsulated in the PAMAM polymer (Pt-DENs). Positively charged Pt-DENs were subsequently hybridized with negatively charged montmorillonite particles through electrostatic interaction. GOx was immobilized in this hybrid and used to modify a GCE to obtain an enzymatic glucose biosensor. The nanohybrid material and the sensor were characterized by cyclic voltammetry and amperometry. The biosensor had a response time below 3 s and a LOD as low as 4 µM. Demir et al. extensively worked on the development of clay-based biosensors for glucose determination. First, they used glycine- (Gly), lysine- (Lys), and glutamic acid- (Glu) modified clay mineral matrices in the biosensors [65]. GOx was selected as the model enzyme. After immobilizing the enzyme with amino acid-modified montmorillonite onto a GCE, working conditions like pH and modifier type were optimized. The linear range of optimum sensor design was 0.1–1.0 mM. Later, they proposed an innovative Gluconobacter oxydans cells’ immobilization strategy on trimethylamine-functionalized montmorillonite [64]. The usage of microbial cells provided several advantages. Enzymes inside cells do not need to be purified and can be regenerated in natural conditions since cofactors and coenzymes are already in the cell. Since Gluconobacter oxydans (G. oxydans) is an obligate aerobe, it has a fast and high oxidation efficiency. Hence, the G. oxydans genus is regarded as an ideal component for the fabrication of whole-cell biosensing systems. Measurements concerning analytical characteristics, operational stability, repeatability, and substrate specificity, depending on the carbon source, were investigated in a flow injection analysis (FIA) system. Linear ranges were found between 0.15 and 5.0 mM for the batch mode and 0.1 and 5.0 mM for the FIA system, respectively. By changing the inorganic material, Dellite (a commercial nanoclay derived from bentonite) instead of montmorillonite, and by using quaternary ammonium cations as organic, Maiga et al. [66] presented a novel support material for enzyme immobilization based on cetyltrimethylammonium bromide- (CTAB) modified Dellite (CTAB-Del). In this case, the use of inorganic matrix was proposed as an alternative to organic polymers for immobilization of biomolecules. Then, pyranose oxidase (PyOx) was immobilized onto the GCE, via glutaraldehyde cross-linking, and Chronoamperometric measurements were Figure 3. A schematic drawing of enzyme biosensor based on electrode with nanoclay hybrids. Reprinted with permission from ref. [23]. Copyright 2015 Elsevier. Following the same approach, Han et al. [ 61 ] used dendritic macromolecules, in particular poly-amydoamine (PAMAM), to modify the electrode surface. PAMAM is able to prevent NPs’ aggregation and to increase the concentration of hydrophobic molecules at the surface. To improve the electron-transfer efficiency of the enzyme with the inorganic material, platinum NPs were encapsulated in the PAMAM polymer (Pt-DENs). Positively charged Pt-DENs were subsequently hybridized with negatively charged montmorillonite particles through electrostatic interaction. GOx was immobilized in this hybrid and used to modify a GCE to obtain an enzymatic glucose biosensor. The nanohybrid material and the sensor were characterized by cyclic voltammetry and amperometry. The biosensor had a response time below 3 s and a LOD as low as 4 µM. Demir et al. extensively worked on the development of clay-based biosensors for glucose determination. First, they used glycine- (Gly), lysine- (Lys), and glutamic acid- (Glu) modified clay mineral matrices in the biosensors [ 65 ]. GOx was selected as the model enzyme. After immobilizing the enzyme with amino acid-modified montmorillonite onto a GCE, working conditions like pH and modifier type were optimized. The linear range of optimum sensor design was 0.1–1.0 mM. Later, they proposed an innovative Gluconobacter oxydans cells’ immobilization strategy on trimethylamine-functionalized montmorillonite [ 64 ]. The usage of microbial cells provided several advantages. Enzymes inside cells do not need to be purified and can be regenerated in natural conditions since cofactors and coenzymes are already in the cell. Since Gluconobacter oxydans (G. oxydans) is an obligate aerobe, it has a fast and high oxidation efficiency. Hence, the G. oxydans genus is regarded as an ideal component for the fabrication of whole-cell biosensing systems. Measurements concerning analytical characteristics, operational stability, repeatability, and substrate specificity, depending on the carbon source, were investigated in a flow injection analysis (FIA) system. Linear ranges were found between 0.15 and 5.0 mM for the batch mode and 0.1 and 5.0 mM for the FIA system, respectively. By changing the inorganic material, Dellite (a commercial nanoclay derived from bentonite) instead of montmorillonite, and by using quaternary ammonium cations as organic, Maiga et al. [ 66 ] presented a novel support material for enzyme immobilization Inorganics 2021,9, 43 16 of 23 Inorganics 2021, 9, x 17 of 25 magnetic resonance contrast agents, the calculated longitudinal relaxation rate was 12.36 mM−1s−1 for Gd(DPA)@Lapo and 8.41 mM−1s−1 for Eu(DPA)/Gd(DPA)@Lapo, which was higher than that from commercial agent Gd−DTPA, 4.63 mM−1s−1, at the same concentration. They demonstrated a linear dependence of the longitudinal relaxation time with the concentration of ascorbic acid in the 0.5 to 20 µM range, with a LOD of 0.2 µM. Secondly, the fluorescence response of Eu(DPA)/Gd(DPA)@Lapo was also investigated. The relationship between the Eu3+ luminescence intensity and the ascorbic acid concentration showed a good linear correlation from 0.5 to 20 µM, and the LOD was calculated to be 0.1 µM. Figure 4. Schematic representation of the Eu(DPA)/Gd(DPA)@Lapo nanohybrid system. Reprinted with permission from ref. [101]. Copyright 2017 Elsevier. Figure 4. Schematic representation of the Eu(DPA)/Gd(DPA)@Lapo nanohybrid system. Reprinted with permission from ref. [101]. Copyright 2017 Elsevier. Nanoclays, mainly montmorillonite, modified with different metal NPs or compounds have also been tested for H 2 O 2 detection by colorimetric methods, as stated in Table 8[104–108] . The response time for all these optical sensors was equal to or less than 30 s, demonstrating the fast response to H 2 O 2 detection of these nanoclay-based sensors. The detection limit was diverse and depended on the medium and metal used. In general, these values were higher than those observed with electrochemical sensors (see Table 6). LDH-based sensor exhibited by far the highest detection limit. In Figure 5 , the schematic representation of a colorimetric sensing of H 2 O 2 catalyzed by Co-doped CuS-montmorillonite is represented [107]. Inorganics 2021, 9, x 18 of 25 Nanoclays, mainly montmorillonite, modified with different metal NPs or compounds have also been tested for H2O2 detection by colorimetric methods, as stated in Table 8 [104–108]. The response time for all these optical sensors was equal to or less than 30 s, demonstrating the fast response to H2O2 detection of these nanoclay-based sensors. The detection limit was diverse and depended on the medium and metal used. In general, these values were higher than those observed with electrochemical sensors (see Table 6). LDH-based sensor exhibited by far the highest detection limit. In Figure 5, the schematic representation of a colorimetric sensing of H2O2 catalyzed by Co-doped CuS-montmoril- lonite is represented [107]. Figure 5. Schematic illustration of the colorimetric sensing of H2O2 catalyzed by Co-doped CuS- MMT. Reprinted with permission from [107]. Copyright 2020 Elsevier. Figure 5. Schematic illustration of the colorimetric sensing of H 2 O 2 catalyzed by Co-doped CuS-MMT. Reprinted with permission from [107]. Copyright 2020 Elsevier. Recently, a large body of research has also focused on the development of new sensors to replace conventional methods for bacteria detection. In particular, the possibility of combining synergistic detection, antibacterial activity, and recyclability has gained attention. Robby et al. designed a fluorescent sensor with antibacterial activity by a two-step process involving (1) incorporation of an alkaline phosphatase-sensitive polymer dot, containing β -cyclodextrin, in montmorillonite, and (2) immobilization of magnetic iron oxide (Fe 3 O 4 ) and NIR-responsive cesium tungsten oxide (CsWO 3 ) [ 102 ]. This nanocomposite exhibited intense luminescence, which was quenched in the presence of bacterial alkaline phosphatase due to hydrolysis of p-nitrophenyl phosphate in the hydrophobic site of β -cyclodextrin. Additionally, the nanocomposite allowed detection of both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus in the range of 10–10 7 CFU/mL, with a LOD of 5.09 and 4.62 CFU/mL, respectively, and showed high bacterial damage Inorganics 2021,9, 43 17 of 23 by inducing heat under NIR irradiation. This material also demonstrated recyclability via magnetic field exposure due to the presence of Fe3O4. Silicate nanoplatelets derived from natural montmorillonite clay were used by Lin et al. as a platform to detect both Escherichia coli and Staphylococcus aureus bacteria [ 103 ]. Specifically, the silicate nanoplatelets were modified with hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels, widely used for biomedical applications. In this study, atom transfer radical polymerization was used to grow the PHEMA chains. Then the nathphalimide-type fluorescent 6-(1,3-dioxo-1H-benzo[de]- isoquinolin-2(3H)-yl)hexanoic acid was bonded for sensing applications. In addition, several studies of cationic p-phenylene ethynylenes’ oligomers have shown both antimicrobial activity and strong photophysical changes upon aggregation with potential use for sensing bacteria. Hill et al. [ 110 ] analyzed the adsorption of these oligomers on laponite and their results suggested that they induce aggregation of both oligomers and laponite discs, leading to changes of their optical properties and interactions with bacteria. Specifically, the formation of molecular aggregates on laponite induced a slight red shift of the main absorption band from 325 nm to 335 nm and the appearance of a new absorption band at 360 nm, together with a red shift of the luminescence band from 370 nm to 430 nm. Additionally, solid oligomers-laponite films were shown to have fair resistance to dissolution in aqueous solution compared with laponite alone, and adhesion and minor killing of both Gram-negative and Gram-positive bacteria on the surface was observed. The results of this study are useful for development of sensors and biocides involving laponite as a scaffold. 4. Conclusions Nanoclays represent an important type of nanomaterials with unique layered structure and ion exchange capacity, and they hold great potential for new and existing applications, in particular for the development of new nanoclay-based sensors. Regarding electrochemical detection methods, CLME have improved stability, linear response range, and sensitivity compared to existing electrodes. Moreover, the inclusion of organic molecules has opened new opportunities in the development of advanced electrodes. Particularly, biosensors based on the incorporation of enzymes into the nanoclay structure is destined to be an emerging technology for the detection of small metabolic molecules, mainly glucose. Additionally, optical detection methods using nanoclay-based sensors have increased in the latest years. In this sense, nanoclays have proven to be an optimal matrix for the incorporation of fluorophores’ molecules with expecting applications to be found. However, a deeper analysis of the nanoclay role in the optical response would be desirable in order to design a specific optical sensor for a particular bioanalyte target. Thus, nanoclays have shown that they can become a key factor in emerging technologies for sensor development, with application in both environmental and biomedical fields. Author Contributions: All authors equally contributed to the conceptualization, bibliographic search, writing, reviewing, and editing of this manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: Pavón thanks University of Seville for the financial support of her current contract from VI PPIT-US program. Conflicts of Interest: The authors declare no conflict of interest. Inorganics 2021,9, 43 18 of 23 Abbreviations 4-ATP 4-aminothiophenol AChE acetylcholinesterase Bent Bentonite BSA bovine serum albumin Calix calixarene CH-Mt Chitosanmontmorillonite CLDH Cu-Mg-Al calcined layered double hydroxide CLME Nanoclay modified electrodes CMC carboxymethyl chitosan CTAB cetyltrimethylammonium bromide CV Cyclic voltammetry DDA didodecyldimethyl ammonium DDAX dioctadecyl dimethyl ammonium chloride DM dimethylamine DPA Pyridine-2, 6-dicarboxylic acid DPV differential pulse voltammetry EIS electrochemical impedance spectroscopy FcMe ferrocenemethanol FIA flow injection analysis GCE Glassy carbon electrode Glu glutamic acid Gly glycine GOx Glucose oxidase Hb hemoglobin HNTs Hallosyte nanotubes ITO indium tin oxide Kaol Kaolinite Lap Laponite LDH Layered double hydroxides LOD Limit of detection Lys lysine M Methylamine MB methylene blue. Mt Montmorillonite MTMA 2-(methacryloyloxy) ethyltrimethylammonium chloride MV methyl viologen NCs Nanoclays NPs Nanoparticles Pal Palygorskite PAMAM Poly(amidoamine) PAN polyacrylonitrile PAN polyacrylonitrile PGA polyglycolide PHEMA poly(2-hydroxyethyl methacrylate) PMMA polymethylmethacrylate PoPD Poly (o-phenyledediamine) PVA poly(vinyl) alcohol PyOx pyranose oxidase SaCTA cetyltrimethylammonium SaDDA didodecyldimethyl ammonium Sep Sepiolite SWV: Square wave voltammetry VBA vinylbenzyl triethylammonium chloride VBT vinylbenzyl thymine Inorganics 2021,9, 43 19 of 23 References 1. Official Journal of the European Union. Commission Recommendations; The European Commission: Brussels, Belgium, 2011; Volume 2011/696/E. 2. Bergaya, F.; Lagaly, G. Chapter 1—General Introduction: Clays, Clay Minerals, and Clay Science. In Handbook of Clay Science; Bergaya, F., Lagaly, G.B.T.-D., Eds.; Elsevier: Amsterdam, The Netherlands, 2013; Volume 5, pp. 1–19. ISBN 1572-4352. 3. Mousty, C. 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