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Review

Silsesquioxanes as Promising Materials for the Development of Electrochemical (Bio)Sensors

by
Felipe Zahrebelnei
1,
Ariane Caroline Ribicki
1,
Aline Martins Duboc Natal
1,
Sérgio Toshio Fujiwara
1,†,
Karen Wohnrath
1,
Dhésmon Lima
2,*,‡ and
Christiana Andrade Pessôa
1,‡
1
Department of Chemistry, Universidade Estadual de Ponta Grossa, Avenida General Carlos Cavalcanti 4748, Ponta Grossa 84030-900, PR, Brazil
2
Department of Chemistry and Physics, Mount Saint Vincent University, 166 Bedford Highway, Halifax, NS B3M 2J6, Canada
*
Author to whom correspondence should be addressed.
†
Deceased author.
‡
These authors contributed equally to this paper.
Chemosensors 2024, 12(12), 259; https://doi.org/10.3390/chemosensors12120259
Submission received: 24 September 2024 / Revised: 21 November 2024 / Accepted: 27 November 2024 / Published: 12 December 2024
(This article belongs to the Special Issue Electrochemical Sensors and Biosensors for Food, Environmental and Biomedical Analysis)
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Abstract

:
Silsesquioxanes (SSQs) comprise an interesting and versatile class of three-dimensional organosilicate oligomers with diverse structural arrangements and interesting physicochemical properties. SSQs are of considerable technological interest, with applications that include the development of electrochemical detection devices. The presence of functional groups on their structures enables the anchoring of different electroactive and conductive species, such as complexes, metal nanoparticles and carbon nanomaterials, and biomolecules, including enzymes, nucleic acids, and antibodies, which boosts the sensitivity and selectivity of the obtained (bio)sensors. These materials can also be incorporated into conductive matrices using a range of methods, which enhances their versatility. This mini review provides an overview of the most recent applications of hybrid organic–inorganic SSQs in the preparation of modified electrodes for the development of electrochemical sensors and biosensors. Special focus is placed on the incorporation of nanomaterials in their polymeric structure and on the design and fabrication of electrochemical devices using different strategies.

Graphical Abstract

1. Introduction

The combination of inorganic and organic moieties to create new materials leads to the formation of organic–inorganic hybrid structures presenting unique properties. These hybrid materials are of simple obtention and can be easily functionalized, which results in versatile compounds suitable for a number of different applications [1]. Silsesquioxanes (SSQs) are a class of organic–inorganic hybrid materials that present the general composition R(SiO1.5)n, where R is an organic group or a hydrogen atom. These compounds are usually categorized based on their different molecular arrangements, including random structures and ladder-like networks, as well as open cage and cage-like arrangements, which form the interesting polyhedral oligomeric silsesquioxanes (POSSs) [2].
The good mechanical properties of SSQs arise from their inorganic core consisting of Si–O–Si networks, whereas the diverse organic moieties stably attached to the inorganic chains enable their interaction with other compounds and materials [1]. Therefore, these hybrid materials can display a range of interesting characteristics, such as high chemical and thermal stability, biocompatibility, and different chemical functionalities. As a result of their properties, they can find applications as nanofillers for polymer matrices [3], in the adsorption of metal ions [4], and in relevant biomedical topics, including tissue engineering [5], drug delivery [6], and sensors and biosensors [7,8].
Electrochemical (bio)sensors are miniaturized devices that can rapidly and inexpensively detect analytes with high sensitivity and selectivity. As a consequence, these sensing devices have found potential applications in clinical diagnostics, industrial quality control, and environmental monitoring, providing faster, more sensitive, and cost-efficient strategies to detect relevant analytes. The application of SSQs in the construction of electrochemical detection devices is growing considerably due to the interesting physicochemical features of such materials, such as biocompatibility and a rich surface chemistry, which enables the immobilization of conductive materials, biomolecules, and other electroactive species on the sensing platform. Furthermore, as a result of their porous matrix, SSQs can act as convenient agents for the stable immobilization of nanomaterials such as noble metal nanoparticles and graphene, leading to the creation of nanometric active sites for the detection of different chemical species. As a direct result, electrochemical sensors, immunosensors, and genosensors containing SSQs have been constantly fabricated and successfully applied for the detection of several analytes in real matrices [8,9,10,11,12,13,14].
Another advantage of using SSQs for the fabrication of detection devices is that these materials can be incorporated into the sensing platform using different methods, such as incorporation in carbon paste and carbon ceramic electrodes, as well as in the form of thin films obtained through techniques such as layer-by-layer (LbL) [15] and drop-coating [16]. The choice of the best method for surface modification will be dependent on the characteristics of the SSQs, including water solubility and the nature of the functional groups present in their structure [7].
Although some authors have recently reviewed aspects regarding the synthesis, structure, characterization, and applications of SSQs [17,18,19,20,21,22,23,24,25,26], no focus has been placed on the promising electrochemical sensing properties of these materials. Therefore, this mini review presents the most recent achievements on the applications of SSQs as functional materials for the design of new sensitive electrochemical sensors and biosensors. Firstly, the synthetic approaches for the obtention of these materials is briefly presented. Next, the most common methods for the modification of electrodes with SSQs and related nanocomposites, aiming for the fabrication of (bio)sensing devices, are discussed. In addition, common biosensing strategies and targeted analytes are presented and discussed regarding their advantages and limitations.

2. Synthesis of SSQs

SSQs are commonly synthesized utilizing the sol-gel process. This method is based on hydrolysis and condensation reactions between tetraalkoxylanes (Si(OR)4) and organosilanes (RxSi(OR’)4–x) under controlled conditions for the formation of the inorganic network [27,28,29], as represented in Figure 1 [17,30]. After this step, it is possible to modify the surface of the silica network through the attachment of functional groups by nucleophilic substitution reactions [31]. Tetraethylorthosilicate (TEOS-Si(OC2H5)4) is widely used as a silane precursor for the formation of the inorganic matrix, as it enables the obtention of a three-dimensional polysiloxane chain with moderate reactivity, which allows the control of the particle size, pores, and morphology of the resulting material [32,33].
The properties of the synthesis product are greatly influenced by the reaction conditions (the nature and molar proportion of precursors, pH, and temperature) and different chemical structures of SSQs have been reported, such as random, ladder, cage, and partial cage structures, as illustrated in Figure 2 [17,28,29,30,31,34,35,36,37]. The SSQs which present cage-like structures are designated POSSs and present high crystallinity [17,37]. For instance, Kaneko [17] described the preparation of ladder-type SSQs and POSSs with aminopropyltrimethoxysilane (APTMS). For the formation of ladder-like structures, HCl was used as a catalyst, whereas the formation of POSS structures required the use of other strong acids as catalysts, such as HNO3 and trifluoroacetic acids (TFA), in addition to a longer reaction time. The SSQ structures were confirmed by characterization techniques such nuclear magnetic resonance (29Si NMR) and X-ray diffractometry (DRX).
The nature of the catalyst is one of the factors that can influence the chemical structure of SSQs. The use of an acid catalyst allows the homogeneous hydrolysis of the alkoxy groups and provides materials with a texture closer to polymeric gels. On the other hand, the hydrolysis of the inorganic precursor carried out under basic conditions can accelerate the condensation reactions and originate heterogeneous networks with the formation of denser colloidal silica particles and colloidal gels [29,30]. From this perspective, Lorenzini and co-workers [38] synthesized SSQs through the sol-gel process in acidic and basic medium, and observed the formation of randomly branched structures in acidic conditions, whereas the basic hydrolysis promoted the creation of highly crosslinked structures or clusters.
Several studies have described the synthetic procedures, structures, and characterizations of SSQs [7,39,40,41,42,43,44,45,46]. For instance, the polymer 3-n-propylpyridinium silsesquioxane (SiPy+Cl−) was prepared by Alfaya and co-workers for the first time [47]. The synthesis of this polymer was performed in two steps. Firstly, tetraethylorthosilicate (TEOS) reacted with 3-chloropropyltrimetoxysilane (CPTMS) in ethanolic medium and under acid catalysis for 2 h. The solvent was then evaporated at 55 °C for 60 h to form the SSQs network. After the formation of the silica network, pyridine was added to the material in toluene medium for 24 h at reflux. This synthesis provided a material with a random structure and that was soluble in water, enabling its use in different applications, mainly in electrochemical sensors [8,11]. The synthesis procedure developed by Alfaya and co-workers has been used to obtain new SSQs functionalized with other organic groups, such as 4-(aminomethyl)pyridine [48], imidazole [49], and 2-amino-4-methylpyridine [50], which have been applied as functional agents in electrochemical sensing and heavy metal adsorption studies.
The structure and properties of SSQs obtained by the sol-gel method are highly dependent on the nature and molar proportion of their precursors. Cruz-Quesada and co-workers observed that the organic substituents in TEOS and alkyl or chloroalkyltriethoxysilane (RTEOS and ClRTEOS, R = methyl [M], ethyl [E] or propyl [P]) influence the properties of sol-gel materials obtained under acid catalysis, such as their inductive and steric effects, which directly affects their gelation times. In addition, it was observed that when a higher percentage of precursor is used, the local periodicity associated with four-membered rings (SiO)4 increases [51]. In another study, Cruz-Quesada and co-workers verified, in a series of new hybrid materials prepared by the co-condensation of tetraethoxysilane (TEOS) and 4-chlorophenyltriethoxysilane (ClPhTEOS) in different molar ratios, the production of different percentages of (SiO)6 rings associated with amorphous silica and (SiO)4 rings associated with ordered structures, such as POSSs. This study also determined that the molar percentage of the organic precursor (ClPhTEOS) led to an increase in the number of (SiO)4 rings in the final structures. Therefore, the increase in ClPhTEOS favours the formation of POSS [52].
The solvent identity and gelation temperature are additional experimental parameters that can significantly impact the structures and properties of SSQs. The research conducted by Fonseca and co-workers presents strong evidence for such an influence. Different silica adsorbent materials were synthesized without the use of a catalyst and using different alcohols as solvents, namely methanol, ethanol, isopropanol, and tert-butanol, and at gelation temperatures ranging from 60 to 90 °C. Larger pores on the SSQ structure were obtained using tert-butanol at 90 °C, suggesting a synergy between such factors to obtain SSQs with maximized pore sizes [53].
The presented overview of synthetic approaches for producing SSQs highlights how various experimental factors—such as precursor type, molar ratios, solvent choice, and gelation temperature—can influence the structure and properties of these materials. It highlights the rich chemistry and high versatility of SSQ synthesis, offering researchers significant control over reaction conditions and the ability to tailor the final properties for specific applications.

3. SSQs and Related Nanocomposites for Applications in Electrochemical Sensing

Due to their interesting physicochemical properties, SSQs have been used in recent years to obtain functional nanocomposites with great application versatility due to their organic–inorganic hybrid features [1]. These new nanocomposites have been mainly constituted of SSQs associated with metal nanoparticles, carbon nanomaterials, inorganic complexes, and organic polymers. In their recent review, Zhou and colleagues [54] presented and discussed diverse stable POSS–polymer architectures, in which the incorporation of POSSs into polymeric matrices can tune their properties as a result of changes in the polymer topology. Dong and colleagues [1] reported, in their review, the excellent ability of SSQs to originate nanocomposites with promising applications in catalysis, adsorption, sensing systems, energy storage, and biomedicine. The authors emphasized the hybrid properties of SSQs, which present reactive and non-reactive sites in their structure, which simultaneously ensure material stability and modification capacity.
The presence of SSQs in nanocomposites enhances their electrochemical properties by increasing the surface area of the nanocomposite compared to the precursor materials. Microscopy studies confirm that such an increase in surface area occurs [55]. In addition, the incorporation of charged organic functional groups improves the dispersion of such nanocomposites in liquid media, preventing agglomeration. The synergistic effect between SSQs and nanomaterials is further demonstrated by the improved electrochemical properties, including enhanced electrical conductivity and electrocatalytic performance, observed in the nanocomposites compared to the starting materials [13].
Several studies have described the synthesis of nanocomposites using SSQs as stabilizing agents to obtain metal nanoparticles (MNPs) in suspension, mainly those based on gold (AuNPs) [8,10,45,50], silver (AgNPs) [56,57], platinum (PtNPs) [58], magnetite (Fe3O4NPs) [58,59], copper (CuNPs) [60], palladium (PdNPs) [16], and Prussian Blue [61]. The organic groups in the SSQ structure are a source of functional moieties to promote nanoparticle functionalization, which makes the use of SSQs even more attractive for the design and synthesis of new metal-based nanomaterials. Another important advantage is based on the fact that SSQs can play important roles in nanoparticle stabilization, controlling their size and shape [45,50].
The synthesis procedure for the obtention of MNPs capped with SSQs is usually straightforward and can be completed in a single step, in which a metallic precursor (such as HAuCl4, AgNO3, or HPtCl6) is reduced to zero-valent MNPs in the presence of a soluble SSQ. Sodium borohydride (NaBH4) has been the most used reducing agent for this purpose [11,45,50]. After being generated, the MNPs are immediately capped by the SSQ chains, which enables the formation of highly stable nanocolloidal suspensions. A compilation of recent approaches for the synthesis of MNPs stabilized in SSQs for applications in electrochemical detection systems is presented in Table 1. Due to properties such as pronounced electrical conductivity, high specific surface area, biocompatibility, and electrocatalytic activity [62], AuNPs have been the most frequent types of MNPs synthesized using SSQs, followed by AgNPs, PtNPs, and Fe3O4NPs. Recently, PdNps have also been prepared using SSQs as a template. For instance, Goularte and co-workers [16] synthesized PdNPs stabilized in an SSQ hybrid material, 3-n-propyl(4-methylpyridinium) silsesquioxane (Si4Pic+NO3−), using Pd(OAc)2 as the metallic precursor. Transmission electron microscopy (TEM) analysis revealed that the PdNPs-Si4Pic+NO3− presented a mean size of 1.3 nm and high stability, with no apparent aggregation even after two years. MNPs capped with SSQs usually present stability for several months at ambient conditions. This is due to the efficiency of the SSQ structure in stabilizing nanocolloids, which is attained through a combination of steric and electrostatic effects [50].
The synthesis of SSQ-capped MNPs is usually preceded by the formation of complexes or molecular aggregates with the SSQ, which can take place either through the coordination of the functional groups of the SSQ structure (such as amino or thiol) [50,68] with the noble metal ions or electrostatic adsorption [69]. This process can favour the occurrence of interactions between the MNPs and the SSQ chains and enable the formation of smaller and stable nanoparticles, which is particularly important for the fabrication of electrochemical sensors and biosensors. The efficiency of SSQs in promoting the synthesis of stable and small nanoconjugates with MNPs is highly advantageous for biosensor production, since the resulting materials can provide a high load of immobilized biomolecules, preventing the loss of their bioactivity and enhancing their availability to recognize and bind to analyte molecules.
AuNPs, AgNPs, and PtNPs stabilized by SSQs have been synthesized using silica networks functionalized with several organic functional groups such as 3-n-propylpyridinium chloride [45,63,66,70], 3-n-propyl-4-methylpyridinium chloride [10,57,58], 3-n-propyl-2-amino-4-methylpyridium chloride [50], and 1,4-diazoniabicyclo-[2.2.2]octane [64]. Taking advantage of their high surface area, low toxicity, and magnetism, Fe3O4NPs have also been obtained for electrochemical applications using SSQ derivatives, including those containing the functional group 3-n-propyl-4-picolinium chloride [58,59].
In addition to their promising nanocomposites with MNPs, SSQs have also been used to obtain new composite materials through the incorporation of inorganic complexes or metallic ions. Considering that SSQs are usually electrochemically inert, which can sometimes restrict their electrochemical applications, the synthesis of composites containing ferrate complexes has been reported in the literature to obtain new electroactive materials that combine the good electrochemical properties of inorganic complexes with the structural features of SSQs. Recent reports include the synthesis of composites composed of octa(aminopropyl) silsesquioxane and cobalt nitroprusside [71].
Due to their outstanding properties such as pronounced electrical conductivity and electrocatalytic activity [72], carbon nanomaterials have been a common choice in the development of new nanocomposites with organic–inorganic silicon-based materials for applications in electrochemical sensing devices. For instance, Winiarski and co-workers [9] developed a nanocomposite based on oxidized multi-walled carbon nanotubes, Ni(OH)2, and 3-n-propyl-4-methylpyridinium silsesquioxane chloride, which was used to modify glassy carbon electrodes (GCEs) and construct voltametric sensors for the detection of folic acid in food samples. The presence of the nanocomposite on the electrodes greatly enhanced the folic acid oxidation current, as a result of the synergistic properties displayed by the nanocomposite constituents.
The synthesis of nanocomposites containing graphene oxide (GO) and SSQ derivatives for sensing applications has also been reported. Bai and co-workers [13] successfully obtained a nanocomposite consisting of a POSS and reduced graphene oxide through a one-step hydrothermal method. The new material significantly improved the detection properties of a GCE for nitrite determination in milk samples using chronoamperometry. The good analytical performance of the developed sensor was attributed to the electrocatalytic features of the nanocomposite.
Maraldi and do Carmo (2023) [61] developed a nanocomposite based on Prussian blue nanoparticles, GO, and octa(aminopropyl)silsesquioxane. In this material, GO was chemically modified with SSQ, which enabled the deposition of Prussian blue nanoparticles on the resulting material. The use of the modified GO and the electron transfer mediator complex made it possible to develop an electrochemical sensor for detecting the pesticide Diuron, which was based on the modification of a carbon paste electrode (CPE) with the nanocomposite. Due to the properties of the materials used, the sensor presented an analytical performance that was comparable to the most sensitive electrochemical sensors described in the literature for the detection of Diuron pesticide.
Composites based on SSQs, POSSs, and organic polymers have also displayed interesting electrochemical characteristics that can enable their application in electrochemical sensing. For example, taking advantage of donor–acceptor–donor (D-A-D) strategies for tailoring the intrinsic properties of polymeric semiconductors, Çakal et al. [73] used a phthalimide derivative bearing a POSS cage conjugated to ethylenedioxythiophene (EDOT) as a new monomer for electropolymerization to the corresponding electroactive polythiophene films on indium tin oxide coated glass (ITO) electrodes. The obtained polymeric films were shown to be electrochromic, displaying different colors when in their neutral and oxidized forms. In another study, a star-shaped silsesquioxane-polythiophene hybrid copolymer was synthesized and applied to modify single-walled carbon nanotubes (SWCNT). The formation of amphiphilic copolymer aggregates provided hydrophilic “buoys” to enhance the solubility of modified-SWCNT in aqueous solution [74]. The new nanocomposite displayed promising electrochemical features, which were studied by using cyclic voltammetry and electrochemical impedance spectroscopy.
In addition to applications in the field of electrochemical sensing, it is worth highlighting the use of SSQs in other sensor types, such as optical sensors [75]. Several examples of optical sensing systems have been described in the literature, mainly reporting the use of POSSs. Optical sensing applications of SSQs rely on properties such as the structure of the oligomeric silica core, solubility due to the identity of the side chains, and functionalization capacity for the insertion of luminophore groups. This last point is particularly important, as the presence of such groups allows the interaction of the material with the analyte to cause variations in the intensity of the emissions, enabling the detection process to occur [76]. Optical sensors based on SSQs have been applied to determine particle size [77] and identify fatty acid isomers [78], as well as fluoride [76,79] and cyanide [76] ions.

4. Strategies for the Modification of Electrodes with SSQs and Related (Nano)Composites

One of the main advantages of using SSQs to develop electrochemical sensors and biosensors is the high versatility displayed by these materials regarding the methods for their immobilization on conductive matrices and substrates. The most used strategies are based on the incorporation of SSQs in carbon paste electrodes [4,10,61,80,81,82,83] and their drop-coating deposition on electrode surfaces such as graphite and glassy carbon [7,8,9,12,58,69,84,85,86]. Other promising strategies are based on the formation of organized thin films, including those obtained through layer-by-layer (LbL) [15,56,87,88,89,90,91,92], Langmuir-Blodgett (LB) [93,94], and self-assembled monolayer (SAM) [11] modification approaches. The choice of the best method for the modification of electrode surfaces will depend on the physicochemical characteristics of the SSQ, such as its water solubility and the nature of the functional groups present in its structure. For example, organofunctionalized SSQs obtained in a water-soluble form can be used to modify various conductive substrates such as ITO, glassy carbon, and graphite in the form of LbL or drop-coating films. While the modification of such electrodes with SSQs or SSQ-containing nanocomposites has been widely described, the use of SSQs for modifying gold and platinum electrodes for electrochemical sensing is not common. This gap may be due to the favourable electrochemical properties, ease of modification, and lower cost of these substrates, which likely make them the preferred choice over metallic surfaces.
Considering the formation of nanostructured self-assembled SSQ films on electrode surfaces, the LbL technique stands out mainly due to its simplicity and versatility. By using this technique, ultrathin films can be easily obtained through electrostatic interactions existing between positively and negatively charged molecules, which can be deposited on conductive surfaces as mono or multilayered films [95,96,97,98]. The film thickness can be easily controlled, and its influence on the electrochemical properties of the substrate is effectively investigated. The main advantage of using SSQs to obtain this type of film arises from the possibility of synthesizing highly charged compounds, which enables the obtention of stable arrangements with excellent sensing capabilities. For example, Ribicki and co-workers [48] obtained an SSQ functionalized with a 4-(aminomethyl)pyridine group that was applied in the construction of LbL films along with nickel phthalocyanine (NiTsPc) on ITO substrates. This novel SSQ allowed a higher adsorption of NiTsPc monomers on the substrate surface, originating stable nanostructured films. The presence of monomeric NiTsPc allows the metallic center to be exposed, which conferred an enhanced electrocatalytic activity for the platform towards the voltametric detection of nitrite ions. A similar behaviour was observed for nanostructured LbL films constructed with the same phthalocianine, NiTsPc, and a 3-n-propylpyridinium SSQ polymer. These films were utilized for the electrochemical detection of dopamine (DA) in the presence of interferents such as ascorbic acid (AA) and uric acid (UA) using square wave voltammetry, with a peak potential separation of 460 mV vs. Ag/AgCl [87]. The presence of SSQs in the films enabled the obtention of highly organized monolayers with a minimized formation of aggregates, leading to promising electroanalytical performances.
A similar electrochemical platform was employed in the research conducted by Winiarski and co-workers [15], which described an SSQ-based enzymatic biosensor for the detection of glucose in commercial beverages. The authors modified conductive glass substrates coated with fluorine tin oxide (FTO) with LbL films of SiPy+Cl− and NiTsPc. The highly stable LbL films acted as an effective support for the immobilization of glucose oxidase enzyme. Different methods for enzyme immobilization were investigated, including physical adsorption and covalent bonding (cross-linking), followed by the deposition of a layer of Nafion® polymer to prevent enzyme leakage from the modified surface (Figure 3). The authors demonstrated that physical adsorption led to an improved detection performance for the enzymatic biosensor when compared with the other immobilization methodologies. This biosensor showed high sensitivity towards glucose detection utilizing chronoamperometry and was successfully applied in the analysis of fermented kombucha samples.
The LB technique is not usually applied for the obtention of SSQ-based films since many of them present high solubility in aqueous media. However, ultrathin nanoporous SiO2 films were recently prepared using a photo-oxidation method from LB nanosheet films of a blend polymer based on a POSS and poly(N-dodecylacrylamide). A pH-responsive silane compound containing amine groups was used to modify the porous SiO2 surface. Selective ion permeation was demonstrated using cyclic voltammetry (CV) measurements under different pH conditions in the presence of negative and positive redox probes ([Fe(CN)6]3−/4− and [Ru(NH3)6]2+/3+). The results evidenced a different current variation behaviour for each probe, which was dependent on the medium pH. This suggests that the developed device presents an interesting ion selective permeation activity, which enables potential applications in sensing and molecular separation systems. This study is a clear example of how the LB technique coupled with the use of SSQs can also benefit the design and development of novel electrochemical detection systems [93].
Non-organized films can be obtained using soluble SSQs, usually through simpler methods, such as drop-coating. In this case, it is essential that the electrode surface presents strong interactions with the SSQ compound, in order to ensure enough stability for the resulting electrochemical sensor. By using this approach, Mossanha and co-workers [11] modified the surface of a GCE with AuNPs capped with 3-n-propylpyridinium silsesquioxane through drop-coating, followed by the formation of self-assembled monolayers (SAMs) of thiolactic acid. Horseradish peroxidase enzyme (HRP) was covalently immobilized on the surface of the functionalized electrode (Figure 4A), and the enzymatic biosensor was applied on the detection of catechol in tap water samples. Scanning electron microscopy analysis confirmed the immobilization of the SSQ-capped AuNPs on the GCE, and their functionalization with SAMs enabled the proper and stable immobilization of HRP molecules on the platform (Figure 4B). The high stability of the biosensor was a result of the high stability of the biosensor and demonstrated that the nanocomposite effectively anchored the enzyme molecules.
Although the described immobilization methods rely mostly on the use of water-soluble SSQs, insoluble materials can also be used to modify electrodes with the aim of developing improved electrochemical sensing platforms. In this scenario, the use of carbon paste as an efficient immobilization matrix has been the preferred choice [10,64,80]. In a recent report, octakis(3-chloropropyl)octasilsesquioxane was organofunctionalized with a PAMAM dendrimer, and the resulting material was used to form a new nanocomposite with copper chloride and [Fe(CN)6)]3−/4−. The obtained composite was incorporated in graphite paste electrodes for the efficient detection of ascorbic acid (vitamin C) using various electrochemical techniques such as cyclic voltammetry, differential pulse voltammetry, and chronoamperometry [99]. In another interesting study, graphite paste electrodes were used as a matrix to immobilize a composite consisting of 3-n-propyl(3-methylpiridinium) silsesquioxane and NiTsPc. The resulting electrodes showed to be greatly sensitive for the determination of the antibacterial drug sulfanilamide, both in clinical and pharmaceutical samples [80]. In both studies, the good electrocatalytic effect observed for the detection of the studied analytes can be attributed to the stable immobilization of the SSQ-based nanocomposites in the carbon paste matrix, confirming this method as an efficient strategy to develop electrochemical sensors.

5. Electrochemical Sensing Platforms Containing SSQ-Based Materials

The outstanding physicochemical properties of SSQs have allowed their use in promising electrochemistry-based sensing systems. Electrochemical sensors and biosensors are cost-effective, highly sensitive, portable, and specific at detecting several relevant analytes, such as drugs, biomolecules, and environmental contaminants [14,100,101,102,103]. The incorporation of SSQs and their (nano)composites in electrochemical sensing platforms have helped enhance the electroanalytical performance of such devices towards the detection of the target molecules, as a consequence of their improved conductivity and rich chemical functionality.
As displayed in Table 2, SSQ-based sensors have recently been designed, mainly targeting applications in the electroanalysis of pharmaceuticals [7,64,71,80,104], phenolic compounds [11,57,58,59,105], and food preservatives [10,48,106]. With regards to the electrochemical methods used, voltammetric techniques such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) have been the preferred choices. This is mainly due to the high sensitivity, versatility, and fast response of the mentioned methods, which is desirable for environmental, clinical, and industrial applications.
Regarding the detection of pharmaceuticals, antibiotics [64,71,80], anti-hypertensive [7], and analgesic drugs [64,104] have been the most common analytes determined with SSQ-containing electrochemical platforms. Recently, Tkachenko and co-workers [64] developed an interesting sensing device based on a carbon nanotube paste electrode modified with an ordered mesoporous silica material (named Santa Barbara Amorphous material—SBA-15 [107]) decorated with AgNPs supported on an ionic SSQ containing the group 1,4-diazoniabicyclo-[2.2.2]octane. The device was applied to the simultaneous detection of sulfamethoxazole and paracetamol in human urine samples using DPV. The good voltametric sensitivity of the sensor towards both drugs (LOD of 31.0 and 19.0 nmol L−1 for paracetamol and sulfamethoxazole, respectively) was attributed to the good electrical conductivity of carbon nanotubes, the high surface area of SBA-15 and the electrocatalytic properties of AgNPs supported on the ionic SSQ.
Taking advantage of the chemical versatility of SSQs, Kannan and co-workers [65] synthesized a cage-like POSS functionalized with thiol groups, which was employed as an inorganic capping agent for the synthesis of AuNPs. The obtained nanocomposite was deposited onto the surface of glassy carbon electrodes, and the prepared sensors were used to detect the antitumoral drug flutamide in drinking and tap water samples by using CV. The presence of the SSQ along with the nanoparticles favoured the electron transfer on the electrode interface, enhancing the reduction peak of flutamide. The attractive analytical performance of the developed sensor (LOD = 0.12 μmol L−1) and successful application in the analysis of water samples make this new approach attractive for flutamide detection in environmental matrices.
In the study conducted by Winiarski and co-workers [7], the anti-hypertensive drug nifedipine and its main metabolite, dehydronifedipine, were determined using a promising electrochemical platform consisting of a glassy carbon electrode modified with films of cobalt(II) tetrasulphophthalocyanine and 3-n-propyl(4-dimethylaminopyridinium) silsesquioxane. The synergistic effects of the phthalocyanine and the SSQ led to a pronounced increase in the sensitivity of the sensor, which achieved ultralow LOD values for both analytes (6.2 and 4.5 nmol L−1 for nifedipine and dehydronifedipine, respectively) employing DPV. Furthermore, the promising results for the analysis of synthetic urine and human serum samples evidenced the biomedical potential of the developed device to evaluate therapy-resistant hypertensive patients.
The most frequent phenol derivatives detected using electrochemical sensors containing SSQs have been nitrophenol [57,58] and bisphenol A [59,105]. In a promising approach, Gerent and Spinelli [58] were able to detect three nitrophenol isomers (2-, 3-, and 4-nitrophenol) in ppm levels in rainwater and human urine samples by using a voltametric sensor based on a glassy carbon electrode modified with Fe3O4 and platinum nanoparticles stabilized in 3-n-propyl-4-picoline silsesquioxane chloride. The SSQ polymer was essential to stabilize the nanoparticles, as well as to provide peak separation for the selective detection of the analytes using DPV. In another study, Ananthakrishnan and co-workers [105] used three different types of cage-like POSSs (OH-, SH-, and vinyl-functionalized) as modifiers of glassy carbon electrodes to detect bisphenol A in water samples. The authors found that the electrode modified with POSS-vinyl presented the most promising electroanalytical performance, as this SSQ was able to act as an electron mediator to enhance the bisphenol A oxidation. The electrochemical sensor presented high sensitivity in the range of 0.749 to 7.49 μmol L−1 and could detect bisphenol A in concentrations as low as 80 nmol L−1 by using DPV.
Sensing devices based on SSQs have also been devoted to food monitoring and quality control, mainly for the detection of ionic preservatives [10,48] and vitamins [9,99]. For example, a feasible electrochemical sensor for the detection of sulfite in acidic medium was reported by Winiarski and co-workers [10]. The authors applied a newly synthesized SSQ containing 3-n-propyl-4-methylpyridinium groups as the stabilizing agent for AuNPs (Figure 5A), which were used to modify graphite paste electrodes. The electrocatalytic properties of the SSQ-capped AuNPs conferred excellent sensitivity for the developed sensor, enabling the successful detection of sulfite in pH = 0 using square wave voltammetry in white wine and coconut water samples (LOD = 0.88 mg L−1) (Figure 5B). It is important to highlight that sulfite is in the form of SO2 molecules in pH = 0; therefore, this was the species actually detected by the electrochemical sensor.
SSQs are also outstanding materials for the design and construction of electrochemical biosensing platforms devoted to clinical diagnosis. The numerous possibilities for chemical functionalization enable these compounds to establish stable and strong interactions with different types of biomolecules, such as proteins and nucleic acids. In spite of these promising characteristics, the application of SSQs for biosensor production is yet to be deeply explored.
The SSQ-based biodevices developed so far were mainly applied in the biosensing of D-glucose [92], Chagas disease-related antibodies [50,63], cardiac troponin T [12], and Zika virus DNA [8]. Lima and co-workers [50] recently published a promising study reporting the use of 2-amino-4-methylpyridinium silsesquioxane (SiAMPy+) chloride as an effective capping agent for the synthesis of nanoconjugates with AuNPs. The nanoconjugates were shown to be nontoxic for white and red blood cells, and were properly deposited on a GCE to serve as anchors for the immobilization of Chagas disease antigens (Figure 6A). By monitoring variations in the oxidation current of the redox mediator [Fe(CN)6]3−/4− in the absence and presence of Chagas disease antibodies, the authors could effectively differentiate serum samples that were positive for the disease from those which were negative (Figure 6B). The nanoconjugates not only acted as signal amplifiers in the system, but also enabled the stable immobilization of Chagas disease antigens on the electrochemical platform, leading to an enhanced sensitivity for the diagnosis of the disease (Figure 6C).
In another study, Steinmetz and colleagues [8] employed an SSQ functionalized with 3-n-propylpirydinium chains to develop a promising label-free biosensing platform for Zika virus infection diagnosis. In this strategy, SSQ-capped AuNPs deposited on glassy carbon electrodes were used as anchors to immobilize thiolated Zika virus-related DNA probes (Figure 7A). The viral genome was detected in serum samples with high sensitivity using electrochemical impedance spectroscopy (Figure 7B), achieving an ultralow detection limit of 0.82 pmol L−1 for the detection of a specific Zika virus DNA sequence. This straightforward electrochemical biosensing strategy can potentially serve as a foundation for the development of commercial bioassays for the clinical diagnosis of Zika virus infection in patient samples.
The electrochemical detection of cardiac biomarkers using an SSQ-based biosensing platform was explored in the study reported by Zapp and co-workers [12]. The authors employed films of an ionic liquid crystal along with AuNPs capped with 3-n-propyl-4-picolinium silsesquioxane to construct an attractive platform for the immobilization of troponin T-specific antibodies. Their highly sensitive biosensor was able to detect this cardiac protein in concentrations as low as 0.076 ng mL−1 using square wave voltammetry. The successful application of this biodevice for the analysis of blood plasma samples evidenced its potential to support the fast diagnosis of acute myocardial infarction.
Table 2. Recent works reporting on the applications of SSQ-based materials in electrochemical sensing devices 1.
Table 2. Recent works reporting on the applications of SSQ-based materials in electrochemical sensing devices 1.
ElectrodeModifierAnalyteLOD (M)LOQ (M)Linear Range (M)Reference
GCEAuNPs-SiAMPy+Anti-T. cruzi antibodies–––[50]
GPEACCoNIsoniazid5.53 × 10−7–6.0 × 10−7–1.0 × 10−5[71]
GCEPOSS-S-AuFlutamide1.2 × 10−7–3.0 × 10−6–3.0 × 10−5[65]
CPEAPS, AEAPS
and DTOAS		Ag+APS: 1.0 × 10−6–APS: 1.6 × 10−6–1.0 × 10−2[18]
AEAPS: 1.0 × 10−8AEAPS: 1.0 × 10−6–1.0 × 10−2
DTOAS: 1.6 × 10−6DTOAS: 1.0 × 10−8–1.0 × 10−2
CPECuHSPDAscorbic acid1.66 × 10−65.54 × 10−65.0 × 10−6–4.0 × 10−5[99]
CNTPESBA-AgNPsSulfamethoxazol (SMZ)/paracetamol (PC)SMZ: 1.9 × 10−8–SMZ: 6.0 × 10−8–1.2 × 10−4[64]
PC: 3.1 × 10−8PC: 1.0 × 10−7–8.0 × 10−4
GCEPOSS-OH, POSS-SH and POSS-vinylBisphenol APOSS-OH 3.97 × 10−6–POSS-OH: 7.49 × 10−7–7.49 × 10−6[105]
POSS-SH 1.4 × 10−7POSS-SH: 7.49 × 10−7–8.99 × 10−6
POSS-vinyl 8.0 × 10−8POSS-vinil 7.49 × 10−7–7.49 × 10−6
GCEf-MWCNT-Ni(OH)2-Si4Pic+Cl−Folic acid9.5 × 10−83.0 × 10−75.0 × 10−7–2.6 × 10−5[9]
GPECuHCFSSQ-H and ZnHCFSSQ-HAscorbic acid2.99 × 10−4–4.0 × 10−4–4.0 × 10−3[4]
GCECoTsPc–Si4DMAP+Cl−Nifedipine (NIF)/
DehydroNIF (dHNIF)		NIF: 6.2 × 10−9
dHNIF: 4.5 × 10−9		–1.5 × 10−8–1.75 × 10−6[7]
GPEMTTiPNiHSulfite/sodium dipyrone (DIP)Sulfite: 3.0 × 10−5–Sulfite: 8.0 × 10−5–9.0 × 10−4[104]
DIP: 4.0 × 10−5Sodium dipyrone: 1.0 × 10−4–9.0 × 10−4
GCEAgNPs-Si4Pic+NO3−4-nitrophenol5.0 × 10−81.5 × 10−72.9 × 10−7–3.15 × 10−5[57]
GPEAgHSPAscorbic acid7.034 × 10−5–5.0 × 10−5–6.0 × 10−4[108]
GCEAuNPs-SiPyZika virus8.2 × 10−13–1.0 × 10−12–1.0 × 10−6[8]
GPEACCuNL-cysteine1.25 × 10−4–2.0 × 10−4–2.0 × 10−3[83]
CPESi3Pic+Cl−/NiTsPcSulfanilamide1.2 × 10−53.5 × 10−53.5 × 10−5–3.01 × 10−4[80]
GPEPOSS-SH/CuCl2/K3[Fe(CN)6]L-dopamine2.08 × 10−4–2.5 × 10−5–4.0 × 10−4[109]
GPEZTTiPNiHSulfite5.0 × 10−5–5.0 × 10−5–8.0 × 10−4[106]
Ceramic substratePOSS-TPPBrHumidity––11% to 95% (relative humidity)[110]
Ceramic substratePMDSHumidity––11% to 95% (relative humidity)[111]
ITO(Si4ampy+Cl−/NiTsPc)11Nitrite2.6 × 10−5–1.13 × 10−4–8.6 × 10−4[48]
CPEAu-Si4Pic+Cl−Sulfite0.88 mg L−12.68 mg L−12.54–48.6 mg L−1[10]
ITO(AuNPs-SiPy+/PB)2H2O2–––[45]
ITO(SiPy/NiTsPc)3
(S-SiPy/NiTsPc)3 (T-SiPy/NiTsPc)3		Dopamine(SiPy/NiTsPc)3:
9.672 × 10−6		(SiPy/NiTsPc)3:
2.971 × 10−5		9.9 × 10−6–9.09 × 10−5[90]
(S-SiPy/NiTsPc)3:
8.845 × 10−6		(S-SiPy/NiTsPc)3: 2.901 × 10−5
(T-SiPy/NiTsPc)3:
8.486 × 10−6		(T-SiPy/NiTsPc)3: 2.828 × 10−5
GCEFe3O4 NPs-Si4Pic+Cl−/AuNPs-Si4Pic+Cl−Bisphenol A7.0 × 10−9–2.0 × 10−8–1.4 × 10−6[59]
GCEAuNps/TLA/HRPCatechol8.52 × 10−7–6.0 × 10−6–4.6 × 10−5[11]
GCE(Fe3O4-Pt)NPs2-, 3-, and 4-nitrophenol (2-, 3-, 4-NP)2-NP: 3.37 × 10−82-NP:
1.022 × 10−7		1.0 × 10−7–1.5 × 10−6[58]
3-NP: 4.53 × 10−83-NP:
1.375 × 10−7
4-NP: 4.82 × 10−84-NP:
1.461 × 10−7
GPEGOSFeHDiuron4.96 × 10−9–1.00 × 10−8–9.00 × 10−7[61]
GCEPd-Si4Pic+NO3−Nimesulide3.90 × 10−8 1.30 × 10−7–6.00 × 10−5[16]
GCEV-POSS:MLG2-hydroxy benzophenone0.2 × 10−90.66 × 10−91.0 × 10−9–1.0 × 10−4[86]
GCEFODTryptophan0.01 × 10−6–0.06 × 10−6–1.0 × 10−4; 1 × 10−4–4.4 × 10−4[85]
CPESZnFe4-chlorophenol5.3 × 10−6–6.0 × 10−7–6.0 × 10−5[83]
CPESCuHNitrite2.65 × 10−5–2.0 × 10−4–2.0 × 10−3[82]
1 Acronyms: GPE (graphite paste electrode); CPE (carbon paste electrode); CNTPE (carbon nanotube paste electrode); ITO (indium tin oxide coated glass); AuNPs-SiAMPy+ (AuNPs stabilized in 3-n-propyl(2-amino-4-methyl)pyridinium silsesquioxane); ACCoN (nanocomposite prepared from octa(aminopropyl)silsesquioxane and cobalt nitroprusside); POSS-S-Au (polyhedral oligomeric silsesquioxane modified with octamercaptopropyl and AuNPs incorporated into the structure); APS (3-aminopropyltriethoxysilane); AEAPS (2-aminoethyl-3-aminopropylsyltriethoxysilane); DTOAS (silsesquioxane modified with dithiooxamide groups); CuHSPD (Octakis(3-chloropropyl)octasilsesquioxane organofunctionalized with the PAMAM Dendrimer G.0 and modified with [Fe(CN)6)]3−/4− and copper chloride); SBA-AgNP (Santa Barbara Amorphous material (SBA-15) decorated with silver nanoparticles stabilized in silsesquioxane containing the ionic 1,4-diazoniabicyclo-[2.2.2]octane group); POSS-OH (polyhedral oligomeric silsesquioxane functionalized with hydroxyl groups); POSS-SH (polyhedral oligomeric silsesquioxane functionalized with SH groups); POSS-vinyl (polyhedral oligomeric silsesquioxane functionalized with vinyl groups); f-MWCNT-Ni (OH)2-Si4Pic+Cl—(multi-walled carbon nanotubes functionalized with nickel hydroxide and 3-n-propyl(4-methylpyridinium) silsesquioxane chloride); CuHCFSSQ-H (octa-(3-chloropropryl)silsesquioxane modified with histidine and complexed with Cu2+); ZnHCFSSQ-H (octa-(3-chloropropryl)silsesquioxane modified with histidine and complexed with Zn2+); CoTsPc-Si4DMAP+Cl–(cobalt(II) tetrassulphophthalocyanine and 3-n-propyl(4-dimethylaminopyridinium); silsesquioxane chloride); MTTiPNiH (titanium (IV) silsesquioxane occluded in the cavities of a mesoporous silica MCM-41 and chemically modified with [Fe(CN)6)]3−/4−); AgNP-Si4Pic+NO3− (AgNPs stabilized in 3-n-propyl(4-methylpyridinium) silsesquioxane nitrate); AgHSP (octa-(3-chloropropyl) octasilsesquioxane (SS) was functionalized with a PAMAM dendrimer (G0), adsorbed Ag+, and reacted with [Fe(CN)6)]3−/4−); AuNPs-SiPy (AuNPs stabilized in 3-n-propylpyridinium silsesquioxane); ACCuN (copper pentacyanonitrosylferrate and octa(aminopropyl)silsesquioxane); Si3Pic+Cl−/NiTsPc (nickel(II) phthalocyanine-tetrasulfonic immobilized in 3-n-propyl(3-methylpyridinium) silsesquioxane chloride); POSS-SH/CuCl2/K3[Fe(CN)6] (polyhedral oligomeric silsesquioxane functionalized with 4-amino-5-phenyl-4H-[1,2,4]-triazole-3-thiol, CuCl2 and K3[Fe(CN)6]; ZTTiPNiH (titanium (IV) silsesquioxane and phosphoric acid occluded into the H-FAU zeolite, chemically modified with nickel and [Fe(CN)6)]3−/4−; POSS-TPPBr (polyhedral oligomeric silsesquioxane functionalized with tetraphenylphosphonium bromide); PMDS (mercaptopropyl polyhedral oligomeric silsesquioxane functionalized with 1, 4-divinylbenzene and sodium p-styrene sulfonate); (Si4ampy+ Cl−/NiTsPc)11 (LbL films of 3-n-propyl(4-aminomethyl)pyridinium silsesquioxane chloride and tetrasulfonated nickel phthalocyanine); Au-Si4Pic+Cl− (AuNPs stabilized in 3-n-propyl(4-methylpyridinium) silsesquioxane chloride); (AuNPs-SiPy+/PB)2 (LbL films of Prussian blue and AuNPs stabilized in propylpyridinium silsesquioxane); (SiPy/NiTsPc)3 (LbL films of nickel tetrasulfonated phthalocyanine and 3-chloride-n-propylpyridinium silsesquioxane); (S-SiPy/NiTsPc)3 (LbL films of nickel tetrasulfonated phthalocyanine and 3-chloride-n-propylpyridinium silsesquioxane with the surfactant sodium dodecyl sulfate); (T-SiPy/NiTsPc)3 (LbL films of nickel tetrasulfonated phthalocyanine and 3-chloride-n-propylpyridinium silsesquioxane with the surfactant Triton X); Fe3O4 NPs-Si4Pic+Cl−/Au NPs-Si4Pic+Cl− (ferroferric oxide nanoparticles and AuNPs stabilized in 3-n-propyl-4-picolinium silsesquioxane chloride); AuNps/TLA/HRP (AuNPs stabilized in 3-n-propylpyridinium silsesquioxane chloride, thiolactic acid and horseradish peroxidas); (Fe3O4 -Pt) NPs (magnetite and platinum nanoparticles stabilized in 3-n-propyl-4-picoline silsesquioxane); GOSFeH (nanocomposite consisting of GO and octa(aminopropyl)silsesquioxane with Prussian blue nanoparticles); Pd-Si4Pic+NO3− (palladium nanoparticles stabilized in 3-n-propyl(4-methylpyridinium) silsesquioxane nitrate); V-POSS:MLG (polyhedral oligomeric silsesquioxane-multi layer graphene nanocomposite); FOD (ferrocenyl-octasilsesquioxane dendrimer); SZnFe (octa-aminopropyl polyhedral oligomeric silsesquioxane-Zn2+-[Fe(CN)6]4− nanocomposite); SCuH (octa-aminopropyl polyhedral oligomeric silsesquioxane-Cu2+-[Fe(CN)6]4− nanocomposite).

6. Conclusions and Outlook

SSQ hybrid materials have attracted considerable attention in the last years due to their unique organic–inorganic structural features, which enable different applications in numerous fields, including electrochemical (bio)sensing technology. Considering such promising potential, this mini review summarized recent research advances on SSQ-based electrochemical sensors. The use of SSQs for electroanalytical purposes has been shown to be highly advantageous as a result of their biocompatibility and diverse chemical functionality. These attractive properties enable the establishment of interactions with biomolecules and electroactive species, enhancing the sensitivity and selectivity of the obtained devices. The use of nanocomposites based on SSQs and nanomaterials can further improve the analytical performance of the detection systems. In addition, the versatility of SSQs and related nanocomposites to be immobilized on conductive substrates and materials through different methods (such as incorporation into carbon paste electrodes and thin film formation by using LbL, LB, and drop-coating methods) is another important aspect that favours their use in the fabrication of electrochemical sensors.
Despite the advantages mentioned above, the low solubility of some SSQs may pose a challenge for their use in electrode modification. However, the functionalization of SSQs with charged or highly polar groups can be a strategy to overcome this solubility issue. Another limitation consists in their low electrical conductivity, which can be problematic for electrochemical sensing platforms, as the use of high SSQ concentrations might reduce conductivity and hinder electron transfer processes. This issue can be addressed by incorporating SSQs into nanocomposites with highly conductive materials, such as metal nanoparticles, carbon nanotubes, or graphene. In spite of their low conductivity, SSQs’ physicochemical properties enable the creation of stable nanocomposites with enhanced electrochemical features.
Considering the outstanding progress made in the field of sensor design and development with the incorporation of SSQs, we strongly believe that the use of such materials for the production of efficient sensing platforms will continue to grow in the next years. The production of new SSQ materials with enhanced physicochemical properties will help to tackle some challenges still faced by scientists working with SSQ-based electrochemical sensors, such as their low electrical conductivity and the stabilities of the developed platforms. The obtention of materials with specific organic moieties and structures can significantly tune their properties, which directly impacts the sensor performance and related applications. This will further accelerate the progress of the field, and increase the potential of these electrochemical devices to be inserted into the market and commercialized as competitive products.

Author Contributions

Conceptualization, S.T.F., K.W., D.L. and C.A.P.; Investigation, F.Z., A.C.R., A.M.D.N., K.W., D.L. and C.A.P.; Writing—original draft preparation, F.Z., A.C.R, A.M.D.N., K.W., D.L. and C.A.P.; writing—review and editing, D.L. and C.A.P.; Supervision, S.T.F, K.W., D.L. and C.A.P. 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

The authors are grateful to the State University of Ponta Grossa (Ponta Grossa, PR, Brazil) and Mount Saint Vincent University (Halifax, NS, Canada). This study is dedicated to the loving memory of Sergio Toshio Fujiwara, who devoted his career to the study and electrochemical applications of silsesquioxanes.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. General procedure for the synthesis of SSQs through the sol-gel process. Adapted from references [17,30]. Reproduced with permission from Elsevier and the Brazilian Chemical Society.
Figure 1. General procedure for the synthesis of SSQs through the sol-gel process. Adapted from references [17,30]. Reproduced with permission from Elsevier and the Brazilian Chemical Society.
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Figure 2. Representative structures of SSQs. Adapted from reference [17]. Reproduced with permission from Elsevier.
Figure 2. Representative structures of SSQs. Adapted from reference [17]. Reproduced with permission from Elsevier.
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Figure 3. Schematic representation of the glucose biosensor reported by Winiarski and co-workers [15]. LbL films of the SiPy+Cl− silsesquioxane and NiTsPc were constructed on FTO substrates, followed by the immobilization of glucose oxidase molecules utilizing adsorption and cross-linking methods. Nafion® was utilized as a protective layer to prevent enzyme leakage. Reproduced with permission from MDPI.
Figure 3. Schematic representation of the glucose biosensor reported by Winiarski and co-workers [15]. LbL films of the SiPy+Cl− silsesquioxane and NiTsPc were constructed on FTO substrates, followed by the immobilization of glucose oxidase molecules utilizing adsorption and cross-linking methods. Nafion® was utilized as a protective layer to prevent enzyme leakage. Reproduced with permission from MDPI.
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Figure 4. (A) Schematic representation of the catechol biosensor reported by Mossanha and co-workers [11]. A GCE was functionalized with SSQ-capped AuNPs, followed by the formation of SAMs of thiolactic acid and covalent immobilization of HRP enzyme molecules. (B–D) Scanning electron microscopy images of the GCE surface after modification with the SSQ-capped AuNPs (B), thiolactic acid SAMs (C), and HRP enzyme (D). Reproduced with permission from Elsevier.
Figure 4. (A) Schematic representation of the catechol biosensor reported by Mossanha and co-workers [11]. A GCE was functionalized with SSQ-capped AuNPs, followed by the formation of SAMs of thiolactic acid and covalent immobilization of HRP enzyme molecules. (B–D) Scanning electron microscopy images of the GCE surface after modification with the SSQ-capped AuNPs (B), thiolactic acid SAMs (C), and HRP enzyme (D). Reproduced with permission from Elsevier.
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Figure 5. (A) Structure proposed by Winiarski and co-workers [10] for their nanocomposite based on 3-n-propyl-4-methylpyridinium chloride silsesquioxane and AuNPs (Au-Si4Pic+Cl−). (B) Schematic representation of the mechanism for the voltametric detection of sulfite using carbon paste electrodes modified with the Au-Si4Pic+Cl− nanocomposite. Reproduced with permission from Elsevier.
Figure 5. (A) Structure proposed by Winiarski and co-workers [10] for their nanocomposite based on 3-n-propyl-4-methylpyridinium chloride silsesquioxane and AuNPs (Au-Si4Pic+Cl−). (B) Schematic representation of the mechanism for the voltametric detection of sulfite using carbon paste electrodes modified with the Au-Si4Pic+Cl− nanocomposite. Reproduced with permission from Elsevier.
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Figure 6. (A) Schematic representation of the stepwise fabrication of the Chagas disease immunosensor as reported by Lima and co-workers [50]. A glassy carbon electrode (GCE) was initially oxidized in H2SO4 medium and functionalized with AuNPs capped with the SSQ SiAMPy+. Erythrocytes sensitized with Chagas disease protein antigens were then immobilized on the platform, followed by blocking non-specific binding sites with bovine-serum albumin (BSA). (B) Square wave voltammograms of the biosensor in the presence of the redox mediator [Fe(CN)6]3−/4− before and after incubation in a serum sample containing Chagas disease antibodies. (C) Percent current variations for the biosensor after incubation in serum samples that were positive or negative for Chagas disease. (D) Influence of the presence of the nanoconjugates on the detection properties of the biosensor (different star numbers above different columns represent statistical differences at p < 0.05). Reproduced with permission from Elsevier.
Figure 6. (A) Schematic representation of the stepwise fabrication of the Chagas disease immunosensor as reported by Lima and co-workers [50]. A glassy carbon electrode (GCE) was initially oxidized in H2SO4 medium and functionalized with AuNPs capped with the SSQ SiAMPy+. Erythrocytes sensitized with Chagas disease protein antigens were then immobilized on the platform, followed by blocking non-specific binding sites with bovine-serum albumin (BSA). (B) Square wave voltammograms of the biosensor in the presence of the redox mediator [Fe(CN)6]3−/4− before and after incubation in a serum sample containing Chagas disease antibodies. (C) Percent current variations for the biosensor after incubation in serum samples that were positive or negative for Chagas disease. (D) Influence of the presence of the nanoconjugates on the detection properties of the biosensor (different star numbers above different columns represent statistical differences at p < 0.05). Reproduced with permission from Elsevier.
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Figure 7. (A) Schematic representation of the fabrication of the biosensor for the detection of Zika virus DNA, as reported by Steinmetz and co-workers [8]. A glassy carbon electrode (GCE) was firstly oxidized in H2SO4 medium and modified with AuNPs capped with 3-n-propylpiridium silsesquioxane chloride (SiPy). A thiolated Zika virus probe DNA sequence was then anchored on the AuNPs and enabled the accurate detection of complementary Zika virus sequences through hybridization using electrochemical impedance spectroscopy. (B,C) Linear variation in the charge transfer resistance (ΔRct) with the increase in the concentration of Zika virus DNA (Zika virus DNA concentration increases from diagram a to h). Reproduced with permission from Elsevier.
Figure 7. (A) Schematic representation of the fabrication of the biosensor for the detection of Zika virus DNA, as reported by Steinmetz and co-workers [8]. A glassy carbon electrode (GCE) was firstly oxidized in H2SO4 medium and modified with AuNPs capped with 3-n-propylpiridium silsesquioxane chloride (SiPy). A thiolated Zika virus probe DNA sequence was then anchored on the AuNPs and enabled the accurate detection of complementary Zika virus sequences through hybridization using electrochemical impedance spectroscopy. (B,C) Linear variation in the charge transfer resistance (ΔRct) with the increase in the concentration of Zika virus DNA (Zika virus DNA concentration increases from diagram a to h). Reproduced with permission from Elsevier.
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Table 1. Recent reports on the synthesis of MNPs capped with SSQs for electrochemical sensing applications.
Table 1. Recent reports on the synthesis of MNPs capped with SSQs for electrochemical sensing applications.
MNPsMetallic
Precursor		SSQReducing
Agent		Diameter
(nm)		Reference
AgNPsAgNO33-n-propylpyridinium chlorideNaBH421.0[63]
AgNPsAgNO33-n-propyl(4-methylpyridinium) silsesquioxaneNaBH43.7[57]
AgNPsAgNO31,4-diazoniabicyclo-[2.2.2]octane silsesquioxaneNaBH4–[64]
AuNPsHAuCl4octamercaptopropyl-POSSNaBH4–[65]
AuNPsHAuCl43-n-propyl (4-methylpyridinium) silsesquioxane chlorideNaBH445.0[10]
AuNPsHAuCl43-n-propyl-4-picolinium silsesquioxane chlorideNaBH450.0[59]
AuNPsHAuCl43-n-propylpyridinium silsesquioxane chlorideNaBH412.3[45]
AuNPsHAuCl43-n-propyl(2-amino-4-methyl)pyridinium silsesquioxane chlorideNaBH45.8[50]
PtNPsH2PtCl63-n-propyl (4-methylpyridinium) silsesquioxane chlorideNaBH42.5[58]
PtNPsH2PtCl63-n-propylpyridinium silsesquioxane chlorideFormic acid3.0–4.0[66]
FeO3NPsFe3O4thiol-rich polyhedral oligomeric silsesquioxane (POSS-SH)Solvothermal method350–450[67]
Prussian blueK3[Fe(CN)6]octa(aminopropyl) silsesquioxane–200[61]
PdNPsPd(OAc)23-n-propyl(4-methylpyridinium) silsesquioxane nitrateNaBH41.3[16]
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Zahrebelnei, F.; Ribicki, A.C.; Martins Duboc Natal, A.; Fujiwara, S.T.; Wohnrath, K.; Lima, D.; Pessôa, C.A. Silsesquioxanes as Promising Materials for the Development of Electrochemical (Bio)Sensors. Chemosensors 2024, 12, 259. https://doi.org/10.3390/chemosensors12120259

AMA Style

Zahrebelnei F, Ribicki AC, Martins Duboc Natal A, Fujiwara ST, Wohnrath K, Lima D, Pessôa CA. Silsesquioxanes as Promising Materials for the Development of Electrochemical (Bio)Sensors. Chemosensors. 2024; 12(12):259. https://doi.org/10.3390/chemosensors12120259

Chicago/Turabian Style

Zahrebelnei, Felipe, Ariane Caroline Ribicki, Aline Martins Duboc Natal, Sérgio Toshio Fujiwara, Karen Wohnrath, Dhésmon Lima, and Christiana Andrade Pessôa. 2024. "Silsesquioxanes as Promising Materials for the Development of Electrochemical (Bio)Sensors" Chemosensors 12, no. 12: 259. https://doi.org/10.3390/chemosensors12120259

APA Style

Zahrebelnei, F., Ribicki, A. C., Martins Duboc Natal, A., Fujiwara, S. T., Wohnrath, K., Lima, D., & Pessôa, C. A. (2024). Silsesquioxanes as Promising Materials for the Development of Electrochemical (Bio)Sensors. Chemosensors, 12(12), 259. https://doi.org/10.3390/chemosensors12120259

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