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.

Tailoring molecular permeability of vertically-ordered mesoporous silica-nanochannel films on graphene for selectively enhanced determination of dihydroxybenzene isomers in environmental water samples

Herein we demonstrate a simple and rapid electrochemical method for selectively enhanced determination of catechol (CC) or hydroquinone (HQ) isomers in environmental water samples by tailoring the molecular permeability of vertically-ordered mesoporous silica-nanochannel films on electrochemically reduced graphene oxide (VMSF/ErGO). Such VMSF/ErGO composite film was fabricated on the gold electrode (AuE) surface using electrochemically assisted self-assembly approach. The as-prepared electrodes with surfactant micelles (SM) template inside silica nanochannels, designed as SM/VMSF/ErGO/AuE, possess hydrophobic hydrocarbon cores and show preferential response to CC via hydrophobic effect. After removing SM from silica nanochannels, the obtained VMSF/ErGO/AuE displays more sensitive response to HQ, which is due to the hydrogen bond effect between the silanol groups of silica walls and HQ. Given the potential-resolved and high electrocatalytic ability of ErGO, and molecular permeability and anti-fouling ability of VMSF, these two present sensors could detect CC and HQ in lake water with a low limit of detection (18 nM for CC and 16 nM for HQ), and a high sensitivity (0.33 μA/μM for CC and 0.37 μA/μM for HQ), without complicated sample pretreatment. Moreover, the proposed sensors provide a convenient, rapid and economic way for direct analysis of environmental water samples, exhibiting excellent long-term stability.

Antifouling Strategies for Electrochemical Biosensing: Mechanisms and Performance toward Point of Care Based Diagnostic Applications

Although there exist numerous established laboratory-based technologies for sample diagnostics and analyte detection, many medical and forensic science applications require point of care based platforms for rapid on-the-spot sample analysis. Electrochemical biosensors provide a promising avenue for such applications due to the portability and functional simplicity of the technology. However, the ability to develop such platforms with the high sensitivity and selectivity required for analysis of low analyte concentrations in complex biological samples remains a paramount issue in the field of biosensing. Nonspecific adsorption, or fouling, at the electrode interface via the innumerable biomolecules present in these sample types (i.e., serum, urine, blood/plasma, and saliva) can drastically obstruct electrochemical performance, increasing background "noise" and diminishing both the electrochemical signal magnitude and specificity of the biosensor. Consequently, this review aims to discuss strategies and concepts used throughout the literature to prevent electrode surface fouling in biosensors and to communicate the nature of the antifouling mechanisms by which they operate. Evaluation of each antifouling strategy is focused primarily on the fabrication method, experimental technique, sample composition, and electrochemical performance of each technology highlighting the overall feasibility of the platform for point of care based diagnostic/detection applications.

Electrodeposition of nickel nanostructures using silica nanochannels as confinement for low-fouling enzyme-free glucose detection

This work reports an enzyme-free glucose sensor based on nickel nanostructures electrodeposited on a fluorine-doped tin oxide (FTO) electrode modified with a silica nanochannel membrane (SNM).

Unraveling Mass and Electron Transfer Kinetics at Silica Nanochannel Membrane Modified Electrodes by Scanning Electrochemical Microscopy

An in-depth understanding of kinetic processes convoluting mass and charge transfer at nanoporous membrane modified electrodes is crucial for developing high-performance electrochemical sensors. In this work, we propose a theoretical model to unravel mass (k_m) and electron transfer rate (k_f) from the apparent electrochemical rate constant (k_app) at silica nanoporous membrane (SNM) modified indium tin oxide (ITO) electrodes (designated as SNM/ITO for simplicity). Using scanning electrochemical microscopy (SECM), the k_app of charged redox species was first determined at the SNM/ITO in the absence and presence of surfactant micelles inside SNM. On the basis of the theory, in the presence of micelles inside SNM, k_m equals zero for all charged probes (Ru(NH₃)₆²⁺, Ru(CN)₆^3-, and FcMeOH⁺), thus the SNM behaves as an insulating barrier and the overall electrode reactivity is dominated by the permeability of SNM. After excluding micelles from SNM, the k_m of Ru(CN)₆^3-/4- is strongly dependent on the KCl concentration in the solution, decreasing from 0.23/0.15 mm s^-1 to almost zero upon decreasing the KCl concentration from 1.0 to 0.01 M. In contrast, k_m increases from 1.33 to 2.4 mm s^-1 for Ru(NH₃)₆²⁺ and from 0.18 to 0.33 mm s^-1 for FcMeOH⁺, which are comparable to the electron transfer rate at the underlying ITO electrode surface (0.8 and 0.35 mm s^-1). In these cases, both mass and electron transfer processes are important in determining the overall redox activity of SNM/ITO electrodes. The methodology reported in this work can provide a quantitative way of unraveling these processes and their respective contributions.

Label-free electrochemical biosensors based on 3,3',5,5'-tetramethylbenzidine responsive isoporous silica-micelle membrane

3,3',5,5'-Tetramethylbenzidine (TMB) has been frequently used as an indicator in G-quadruplex/hemin DNAzyme (G4zyme)-based chemical and biochemical analysis, and its oxidation products are usually monitored by electrochemical or optical methods to quantify G4zyme formation-related analytes. Herein we report a simple electrochemical approach based on isoporous silica-micelle membrane (iSMM) to measure TMB, instead of its oxidation products, in G4zyme-based detection of specific analytes. The iSMM was grown on the indium tin oxide (ITO) electrode, which was composed of highly ordered, vertically oriented silica nanochannels and cylindrical micelles of cetyltrimethylammonium. The iSMM-ITO electrode was selectively responsive to neutral TMB but not its oxidation products, thanks to the sieving and pre-concentration capacity of micellar structures in terms of molecular charge and lipophilicity. In other words, only TMB could be extracted and enriched into micelles and subsequently oxidized at the underlying ITO electrode surface (namely the micelle/ITO interface), generating an amplified anodic current. Since the depletion of TMB was catalyzed by G4zymes formed in the presence of specific analyte, the decrease of this anodic current enabled the quantitative detection of this analyte. The current variation relative to its initial value ((j₀-j)/j₀), termed as the current attenuation ratio, showed the obvious dependence on the analyte concentration. As proof-of-concept experiments, four substances, i.e., potassium cation (K⁺), adenosine triphosphate, thrombin and nucleic acid, were detected in aqueous media and the analysis of K⁺ in pre-treated human serum was also performed.

Detection of Metoprolol in Human Biofluids and Pharmaceuticals via Ion-Transfer Voltammetry at the Nanoscopic Liquid/Liquid Interface Array

Metoprolol (MTP) is one of the most widely used antihypertensive drugs yet banned to use in sport competition. Therefore, there has been an increasing demand for developing simple, rapid, and sensitive methods suited to the identification and quantification of MTP in human biofluids. In this work, ultrathin silica nanochannel membrane (SNM) with perforated channels was employed to support nanoscale liquid/liquid interface (nano-ITIES) array for investigation of the ion-transfer voltammetric behavior of MTP and for its detection in multiple human biofluids and pharmaceutical formulation. Several potential interfering substances, including small molecules, d-glucose, urea, ascorbic acid, glycine, magnesium chloride, sodium sulfate and large molecules, bovine serum albumin (BSA), were chosen as models of biological interferences to examine their influence on the ion-transfer current signal of MTP. The results confirmed that the steady-state current wave barely changed in the presence of small molecules. Although BSA displayed an apparent blockade on the transfer of MTP, the accurate determination of MTP in multiple human biofluids (i.e., urine, serum and whole blood) and pharmaceutical formulation were still feasible, thanks to the molecular sieving and antifouling abilities of SNM. A limit of detection (LOD) within the physiological level of MTP during therapy could be achieved for all cases, i.e., 0.5 and 1.1 μM for 100 times diluted urine and serum, respectively, and 2.2 μM for 1000 times diluted blood samples. These results demonstrated that the nano-ITIES array behaved as a simplified and integrated detection platform for ionizable drug analysis in complex media.

Electrochemical sensing of urinary progesterone with molecularly imprinted poly(aniline-co-metanilic acid)s

The sensing of urinary progesterone with molecularly imprinted poly(aniline-co-metanilic acid) coated electrodes.