Composite Silica-Based Films as Platforms for Electrochemical Sensors

Sol-gel-derived silica thin films generated onto electrode surfaces in the form of organic-inorganic hybrid coatings or other composite layers have found tremendous interest for being used as platforms for the development of electrochemical sensors and biosensors. After a brief description of the strategies applied to prepare such materials, and their interest as electrode modifier, this review will summarize the major advances made so far with composite silica-based films in electroanalysis. It will primarily focus on electrochemical sensors involving both non-ordered composite films and vertically oriented mesoporous membranes, the biosensors exploiting the concept of sol-gel bioencapsulation on electrode, the spectroelectrochemical sensors, and some others.

Glucose Oxidase Loading in Ordered Porous Aluminosilicates: Exploring the Potential of Surface Modification for Electrochemical Glucose Sensing

Enzymatic electrochemical sensors have become the leading glucose detection technology due to their rapid response, affordability, portability, selectivity, and sensitivity. However, the performance of these sensors is highly dependent on the surface properties of the electrode material used to store glucose oxidase and its ability to retain enzymatic activity under variable environmental conditions. Mesoporous thin films have recently attracted considerable attention as promising candidates for enzyme storage and activity preservation due to their well-defined nanoarchitecture and tunable surface properties. Herein, we systematically compare pathways for the immobilization of glucose oxidase (GOx) and their effectiveness in electrochemical glucose sensing, following modification protocols that lead to the electrostatic attraction (amino functionalization), covalent bonding (aldehyde functionalization), and electrostatic repulsion (oxygen plasma treatment) of the ordered porous aluminosilicate-coated electrodes. By direct comparison using a quartz crystal microbalance, we demonstrate that glucose oxidase can be loaded in a nanoarchitecture with a pore size of ∼50 nm and pore interconnections of ∼35 nm using the native aluminosilicate surface, as well as after amino or aldehyde surface modification, while oxygen plasma exposure of the native surface inhibits glucose oxidase loading. Despite a variety of routes for enzyme loading, quantitative electrochemical glucose sensing between 0 and 20 mM was only possible when the porous surface was functionalized with amino groups, which we relate to the role of surface chemistry in accessing the underlying substrate. Our results highlight the impact of rational surface modification on electrochemical biosensing performance and demonstrate the potential of tailoring porous nanoarchitecture surfaces for biosensing applications.

Ordered Mesoporous Electrodes for Sensing Applications

Electrochemical sensors have become increasingly relevant in fields such as medicine, environmental monitoring, and industrial process control. Selectivity, specificity, sensitivity, signal reproducibility, and robustness are among the most important challenges for their development, especially when the target compound is present in low concentrations or in complex analytical matrices. In this context, electrode modification with Mesoporous Thin Films (MTFs) has aroused great interest in the past years. MTFs present high surface area, uniform pore distribution, and tunable pore size. Furthermore, they offer a wide variety of electrochemical signal modulation possibilities through molecular sieving, electrostatic or steric exclusion, and preconcentration effects which are due to mesopore confinement and surface functionalization. In order to fully exploit these advantages, it is central to develop reproducible routes for sensitive, selective, and robust MTF-modified electrodes. In addition, it is necessary to understand the complex mass and charge transport processes that take place through the film (particularly in the mesopores, pore surfaces, and interfaces) and on the electrode in order to design future intelligent and adaptive sensors. We present here an overview of MTFs applied to electrochemical sensing, in which we address their fabrication methods and the transport processes that are critical to the electrode response. We also summarize the current applications in biosensing and electroanalysis, as well as the challenges and opportunities brought by integrating MTF synthesis with electrode microfabrication, which is critical when moving from laboratory work to in situ sensing in the field of interest.

High-performance paper-based biocathode fabricated by screen-printing an improved mesoporous carbon ink and by oriented immobilization of bilirubin oxidase

In this study, the performance of a paper-based, screen-printed biofuel cell with mesoporous MgO-templated carbon (MgOC) electrodes was improved in two steps. First, a small amount of carboxymethyl cellulose (CMC) was added to the MgOC ink. Next, the cathode was modified with bilirubin prior to immobilizing the bilirubin oxidase (BOD). The CMC increased the accessibility of the mesopores of the MgOC, and subsequently, the performance of both the bioanode and biocathode. CMC also likely increased the stability of the electrodes. The pre-modification with bilirubin improved the orientation of the BOD, which facilitated direct electron transfer. With these two steps, an open circuit potential of 0.65 V, a maximal current density of 1.94 mA cm⁻², and a maximal power density of 465 μW cm⁻² was achieved with lactate oxidase as bioanode enzyme and lactate as fuel. This is one of the highest reported performances for a biofuel cell.

Polyaniline nanowire arrays generated through oriented mesoporous silica films: effect of pore size and spectroelectrochemical response

Indium-tin oxide electrodes modified with vertically aligned silica nanochannel membranes have been produced by electrochemically assisted self-assembly of cationic surfactants (cetyl- or octadecyl-trimethylammonium bromide) and concomitant polycondensation of the silica precursors (tetraethoxysilane). They exhibited pore diameters in the 2-3 nm range depending on the surfactant used. After surfactant removal, the bottom of mesopores was derivatized with aminophenyl groups via electrografting (i.e., electrochemical reduction of in situ generated aminophenyl monodiazonium salt). These species covalently bonded to the ITO substrate were then exploited to grow polyaniline nanofilaments by electropolymerization of aniline through the nanochannels. Under potentiostatic conditions, the length of polyaniline wires is controllable by tuning the electropolymerization time. From cyclic voltammetry characterization performed either before or after dissolution of the silica template, it appeared that both the polyaniline/silica composite and the free polyaniline nanowire arrays were electroactive, yet with much larger peak currents in the latter case as a result of larger effective surface area offered to the electrolyte solution. At identical electropolymerization time, the amount of deposited polyaniline was larger when using the silica membrane with larger pore diameter. All polyaniline deposits exhibited electrochromic properties. However, the spectroelectrochemical data indicated more complete interconversion between the coloured oxidized form and colourless reduced polyaniline for the arrays of nanofilaments in comparison to bulky films. In addition, the template-free nanowire arrays (i.e., after silica dissolution) were characterized by faster electrochromic behaviour than the polyaniline/silica hybrid, confirming the potential interest of such polyaniline nano-brushes for practical applications.

Mesoporous Materials-Based Electrochemical Biosensors from Enzymatic to Nonenzymatic

Mesoporous materials have drawn more and more attention in the field of biosensors due to their high surface areas, large pore volumes, tunable pore sizes, as well as abundant frameworks. In this review, the progress on mesoporous materials-based biosensors from enzymatic to nonenzymatic are highlighted. First, recent advances on the application of mesoporous materials as supports to stabilize enzymes in enzymatic biosensing technology are summarized. Special emphasis is placed on the effect of pore size, pore structure, and surface functional groups of the support on the immobilization efficiency of enzymes and the biosensing performance. Then, the development of a nonenzymatic strategy that uses the intrinsic property of mesoporous materials (carbon, silica, metals, and composites) to mimic the behavior of enzymes for electrochemical sensing of some biomolecules is discussed. Finally, the challenges and perspective on the future development of biosensors based on mesoporous materials are proposed.

Construction of a novel electrochemical biosensor based on a mesoporous silica/oriented graphene oxide planar electrode for detecting hydrogen peroxide

A constant magnetic field (CMF) was used to arrange the orientation of graphene oxide (GO) which was modified on a self-made screen-printed electrode. We evaluated the efficiency of this method for potential analytical application towards the sensing of hydrogen peroxide (H2O2). Mesoporous silica (MS)-encapsulated horseradish peroxidase (HRP) was immobilized on the electrode with vertically arranged GO to construct an H2O2 sensor (denoted as CMF/GO/HRP@MS). The linear range of the response of the CMF/GO/HRP@MS sensor to H2O2 was 0.1-235 μM, and the detection limit was as low as 0.01 μM. The results demonstrated that the vertical arrangement of GO resulting from the CMF on the electrode surface could increase the electron transfer rate. The excellent selectivity and anti-interference ability of this sensor to H2O2 in physiological samples may be attributed to the synergistic effect of mesoporous silica, GO and constant magnetic field.

Mesoporous Silica-Based Materials for Electronics-Oriented Applications

Electronics, and nanoelectronics in particular, represent one of the most promising branches of technology. The search for novel and more efficient materials seems to be natural here. Thus far, silicon-based devices have been monopolizing this domain. Indeed, it is justified since it allows for significant miniaturization of electronic elements by their densification in integrated circuits. Nevertheless, silicon has some restrictions. Since this material is applied in the bulk form, the miniaturization limit seems to be already reached. Moreover, smaller silicon-based elements (mainly processors) need much more energy and generate significantly more heat than their larger counterparts. In our opinion, the future belongs to nanostructured materials where a proper structure is obtained by means of bottom-up nanotechnology. A great example of a material utilizing nanostructuring is mesoporous silica, which, due to its outstanding properties, can find numerous applications in electronic devices. This focused review is devoted to the application of porous silica-based materials in electronics. We guide the reader through the development and most crucial findings of porous silica from its first synthesis in 1992 to the present. The article describes constant struggle of researchers to find better solutions to supercapacitors, lower the k value or redox-active hybrids while maintaining robust mechanical properties. Finally, the last section refers to ultra-modern applications of silica such as molecular artificial neural networks or super-dense magnetic memory storage.

Wireless Electromechanical Readout of Chemical Information

Collecting electrochemical information concerning the presence of molecules in a solution is usually achieved by measuring current, potential, resistance, or impedance via connection to a power supply. Here, we suggest wireless electromechanical actuation as a straightforward readout of chemical information. This can be achieved based on the concept of bipolar electrochemistry, which allows measuring the presence of different model species in a quantitative way. We validate the concept by using a free-standing polypyrrole film. Its positively polarized extremity participates in an oxidation of the analyte and delivers electrons to the opposite extremity for the reduction of the polymer. This reduction is accompanied by the insertion of counterions and thus leads to partial swelling of the film, inducing its bending. The resulting actuation is found to be a linear function of the analyte concentration, and also a Michaelis-Menten type correlation is obtained for biochemical analytes. This electromechanical transduction allows an easy optical readout and opens up very interesting perspectives not only in the field of sensing but also far beyond, such as for the elaboration of self-regulating biomimetic systems.

New insights into the structure-performance relationships of mesoporous materials in analytical science

Mesoporous materials are ideal carriers for guest molecules and they have been widely used in analytical science. The unique mesoporous structure provides special properties including large specific surface area, tunable pore size, and excellent pore connectivity. The structural properties of mesoporous materials have been largely made use of to improve the performance of analytical methods. For instance, the large specific surface area of mesoporous materials can provide abundant active sites and increase the probability of contact between analytes and active sites to produce stronger signals, thus leading to the improvement of detection sensitivity. The connections between analytical performances and the structural properties of mesoporous materials have not been discussed previously. Understanding the "structure-performance relationship" is highly important for the development of analytical methods with excellent performance based on mesoporous materials. In this review, we discuss the structural properties of mesoporous materials that can be optimized to improve the analytical performance. The discussion is divided into five sections according to the analytical performances: (i) selectivity-related structural properties, (ii) sensitivity-related structural properties, (iii) response time-related structural properties, (iv) stability-related structural properties, and (v) recovery time-related structural properties.

Recent applications of the combination of mesoporous silica nanoparticles with nucleic acids: development of bioresponsive devices, carriers and sensors

The discovery and control of the biological roles mediated by nucleic acids have turned them into a powerful tool for the development of advanced biotechnological materials. Such is the importance of these gene-keeping biomacromolecules that even nanomaterials have succumbed to the claimed benefits of DNA and RNA. Currently, there could be found in the literature a practically intractable number of examples reporting the use of combination of nanoparticles with nucleic acids, so boundaries are demanded. Following this premise, this review will only cover the most recent and powerful strategies developed to exploit the possibilities of nucleic acids as biotechnological materials when in combination with mesoporous silica nanoparticles. The extensive research done on nucleic acids has significantly incremented the technological possibilities for those biomacromolecules, which could be employed in many different applications, where substrate or sequence recognition or modulation of biological pathways due to its coding role in living cells are the most promising. In the present review, the chosen counterpart, mesoporous silica nanoparticles, also with unique properties, became a reference material for drug delivery and biomedical applications due to their high biocompatibility and porous structure suitable for hosting and delivering small molecules. Although most of the reviews dealt with significant advances in the use of nucleic acid and mesoporous silica nanoparticles in biotechnological applications, a rational classification of these new generation hybrid materials is still uncovered. In this review, there will be covered promising strategies for the development of living cell and biological sensors, DNA-based molecular gates with targeting, transfection or silencing properties, which could provide a significant advance in current nanomedicine.

Recent Trends on Electrochemical Sensors Based on Ordered Mesoporous Carbon

The past decade has seen an increasing number of extensive studies devoted to the exploitation of ordered mesoporous carbon (OMC) materials in electrochemistry, notably in the fields of energy and sensing. The present review summarizes the recent achievements made in field of electroanalysis using electrodes modified with such nanomaterials. On the basis of comprehensive tables, the interest in OMC for designing electrochemical sensors is illustrated through the various applications developed to date. They include voltammetric detection after preconcentration, electrocatalysis (intrinsically due to OMC or based on suitable catalysts deposited onto OMC), electrochemical biosensors, as well as electrochemiluminescence and potentiometric sensors.

Gold-loaded nanoporous iron oxide nanocubes: a novel dispersible capture agent for tumor-associated autoantibody analysis in serum

Autoantibodies are produced against tumor associated antigens (TAAs) long before the appearance of any symptoms and thus can serve as promising, non-invasive biomarkers for early diagnosis of cancer. Current conventional methods for autoantibody detection are highly invasive and mostly provide diagnosis in the later stages of cancer. Herein we report a new electrochemical method for early detection of p53 autoantibodies against colon cancer using a strategy that combines the strength of gold-loaded nanoporous iron oxide nanocube (Au@NPFe₂O₃NC)-based capture and purification while incorporating the inherent simplicity, inexpensive, and portable nature of the electrochemical and naked-eye colorimetric readouts. After the functionalisation of Au@NPFe₂O₃NC with p53 antigens, our method utilises a two-step strategy that involves (i) magnetic capture and isolation of autoantibodies using p53/Au@NPFe₂O₃NC as 'dispersible nanocapture agents' in serum samples and (ii) subsequent detection of autoantibodies through a peroxidase-catalyzed reaction on a commercially available disposable screen-printed electrode or naked-eye detection in an Eppendorf tube. This method has demonstrated a good sensitivity (LOD = 0.02 U mL^-1) and reproducibility (relative standard deviation, %RSD = <5%, for n = 3) for detecting p53 autoantibodies in serum and has also been successfully applied to analyse a small cohort of clinical samples obtained from colorectal cancer. We believe that the highly inexpensive, rapid, sensitive, and specific nature of our assay could potentially aid in the development of an early diagnostic tool for cancer and related diseases.

Tracking Single DNA Nanodevices in Hierarchically Meso-Macroporous Antimony-Doped Tin Oxide Demonstrates Finite Confinement

Housing bio-nano guest devices based on DNA nanostructures within porous, conducting, inorganic host materials promise valuable applications in solar energy conversion, chemical catalysis, and analyte sensing. Herein, we report a single-template synthetic development of hierarchically porous, transparent conductive metal oxide coatings whose pores are freely accessible by large biomacromolecules. Their hierarchal pore structure is bimodal with a larger number of closely packed open macropores (∼200 nm) at the higher rank and with the remaining space being filled with a gel network of antimony-doped tin oxide (ATO) nanoparticles that is highly porous with a broad size range of textual pores mainly from 20-100 nm at the lower rank. The employed carbon black template not only creates the large open macropores but also retains the highly structured gel network as holey pore walls. Single molecule fluorescence microscopic studies with fluorophore-labeled DNA nanotweezers reveal a detailed view of multimodal diffusion dynamics of the biomacromolecules inside the hierarchically porous structure. Two diffusion constants were parsed from trajectory analyses that were attributed to free diffusion (diffusion constant D = 2.2 μm²/s) and to diffusion within an average confinement length of 210 nm (D = 0.12 μm²/s), consistent with the average macropore size of the coating. Despite its holey nature, the ATO gel network acts as an efficient barrier to the diffusion of the DNA nanostructures, which is strongly indicative of physical interactions between the molecules and the pore nanostructure.

How the Intricate Interactions between Carbon Nanotubes and Two Bilirubin Oxidases Control Direct and Mediated O2 Reduction

Due to the lack of a valid approach in the design of electrochemical interfaces modified with enzymes for efficient catalysis, many oxidoreductases are still not addressed by electrochemistry. We report in this work an in-depth study of the interactions between two different bilirubin oxidases, (from the fungus Myrothecium verrucaria and from the bacterium Bacillus pumilus), catalysts of oxygen reduction, and carbon nanotubes bearing various surface charges (pristine, carboxylic-, and pyrene-methylamine-functionalized). The surface charges and dipole moment of the enzymes as well as the surface state of the nanomaterials are characterized as a function of pH. An original electrochemical approach allows determination of the best interface for direct or mediated electron transfer processes as a function of enzyme, nanomaterial type, and adsorption conditions. We correlate these experimental results to theoric voltammetric curves. Such an integrative study suggests strategies for designing efficient bioelectrochemical interfaces toward the elaboration of biodevices such as enzymatic fuel cells for sustainable electricity production.

Electrochemical Study and Characterization of an Amperometric Biosensor Based on the Immobilization of Laccase in a Nanostructure of TiO₂ Synthesized by the Sol-Gel Method

Laccase amperometric biosensors were developed to detect the catechol compound. The laccase enzyme (LAC) immobilization was performed on nanostructures of (a) titania (TiO₂); (b) titania/Nafion (TiO₂/NAF) (both immobilized by the sol-gel method) and a third nanostructure, which consisted of a single biosensor composite of Nafion and laccase enzyme denoted as NAF/LAC. The Nafion was deposited on a graphite electrode and used to avoid "cracking" on the matrix. The TiO₂ particle size was an average of 66 nm. FTIR spectroscopy vibration modes of different composites were determined. The electrochemical behavior of the biosensor was studied using electrochemical spectroscopy (EIS) and cyclic voltammetry (CV). The biosensor based on TiO₂/NAF/LAC presented the best electro-chemical properties with regard to sensitivity, stability and detection limit after a period of 22 days.

Vertically ordered silica mesochannel films: electrochemistry and analytical applications

Vertically-aligned mesoporous silica films were used for electrochemical sensing and molecular separation in terms of molecular size, charge and lipophilicity.

Surface modification and porosimetry of vertically aligned hexagonal mesoporous silica films

Grafting vertically aligned mesoporous silica films with small organosilane precursors increases pore hydrophobicity, whereas larger reagents change only the film surfaces.