Electrochemical aptamer-based sensors

Ultrasensitive Aptasensing Platform for the Detection of β-Amyloid-42 Peptide Based on MOF Containing Bimetallic Porphyrin Graphene Oxide and Gold Nanoparticles

The prompt detection of diseases hinges on the accessibility and the capability to identify relevant biomarkers. The integration of aptamers and the incorporation of nanomaterials into signal transducers have not only expedited but also enhanced the development of nanoaptasensors, enabling heightened sensitivity and selectivity. Here, the bimetallic nickel-cobalt-porphyrin metal-organic framework ((Ni + Cu)TPyP MOF) is regarded as an electron mediator, immobilization platform for an Alzheimer aptamer and to increase the electrochemical signal for the detection of the main biomarker of Alzheimer's disease (AD), amyloid β (Aβ-42). Furthermore, the ((Ni + Cu)TPyP MOF) was combined with reduced graphene oxide (rGO) and gold nanoparticles (AuNPs), on a gold electrode (GE) to provide an efficient interface for immobilizing aptamer strands. Concurrently, the incorporation of rGO and AuNPs imparts enhanced electrical conductivity and efficacious catalytic activity, establishing them as adept electrochemical indicators. Owing to the superior excellent electrical conductivity of rGO and AuNPs, coupled with the presence of ample mesoporous channels and numerous Ni and Cu metal sites within (Ni + Cu)TPyP MOF, this nanostructure with abundant functional groups is proficient in immobilizing a substantial quantity of aptamer. These interactions are achieved through robust π-π stacking and electrostatic interactions, alongside the high affinity between the thiol group of the aptamer and AuNPs concurrently. The as-prepared ternary (Au@(Ni + Cu)TPyP MOF/rGO) nanostructure electrode exhibited an enhancement in its electrochemically active surface area of about 7 times, compared with the bare electrode and the Aβ-42 redox process is highly accelerated, so the peak currents are significantly higher than those obtained with bare GE substrate. Under the optimized conditions, the designed aptasensor had the quantitative detection of Aβ-42 with a low detection limit of 48.6 fg mL-1 within the linear range of 0.05 pg mL-1 to 5 ng mL-1 by differential pulse voltammetry (DPV), accompanied by precise reproducibility, satisfactory stability (95.6% of the initial activity after 10 days), and minimal impact of interfering agents. Recorded results in human blood plasma demonstrated the high efficacy of porphyrin MOF system sensing even in the clinical matrix. The great performance of this aptasensor indicates that our new design of Au@(Ni + Cu)TPyP MOF/rGO nanostructure provides more opportunities for the detection of chemical signals in early diagnosis of Alzheimer's disease.

Optical nanoprobes for chiral discrimination

Chiral recognition can be achieved by exploiting chiral properties of nanoparticles within various colorimetric and luminescent sensing systems.

An aptamer-based fluorescence bio-sensor for chiral recognition of arginine enantiomers

In this study, a novel aptamer - based fluorescence bio-sensor (aptamer-AuNps) was developed for chiral recognition of arginine (Arg) enantiomers based on aptamer and gold nanoparticles (AuNps). Carboxyfluorescein (FAM) labeled aptamers (Apt) were absorbed on AuNps and their fluorescence intensity could be significantly quenched by AuNps based on fluorescence resonance energy transfer (FRET). Once d-Arg or l-Arg were added into the above solution, the aptamer specifically bind to Arg enantiomers and released from AuNps, so the fluorescence intensity of d-Arg system and l-Arg system were all enhanced. The affinity of Apt to l-Arg is tighter to d-Arg, so the enhanced fluorescence signals of l-Arg system was stronger than d-Arg system. What's more, the enhanced fluorescence were directly proportional to the concentration of d-Arg and l-Arg ranging from 0-300 nM and 0-400 nM with related coefficients of 0.9939 and 0.9952, respectively. Furthermore, the method was successfully applied to detection l-Arg in human urine samples with satisfactory results. Eventually, a simple "OR" logic gate with d-Arg &l-Arg as inputs and AuNps aggregation state as outputs was fabricated, which can help us understand the chiral recognition process deeply.

Direct Mapping of Heterogeneous Surface Coverage in DNA-Functionalized Gold Surfaces with Correlated Electron and Fluorescence Microscopy

The characterization of biofunctionalized surfaces such as alkanethiol self-assembled monolayers (SAMs) on gold modified with DNA or other biomolecules is a challenging analytical problem, and access to a routine method is desirable. Despite substantial investigation from structural and mechanistic perspectives, robust and high-throughput metrology tools for SAMs remain elusive but essential for the continued development of these devices. We demonstrate that scanning electron microscopy (SEM) can provide image contrast of the molecular interface during SAM functionalization. The high-speed, large magnification range, and ease of use make this widely available technique a powerful platform for measuring the structure and composition of SAM surfaces. This increased throughput allows for a better understanding of the nonideal spatial heterogeneity characteristic of SAMs utilized in real-world conditions. SEM image contrast is characterized through the use of fluorescently labeled DNA, which enables correlative SEM and fluorescence microscopy. This allows identification of the DNA-modified regions at resolutions that approach the size of the biomolecule. The effect of electron beam irradiation dose is explored, which leads to straightforward lithographic patterning of DNA SAMs with nanometer resolution and with control over the surface coverage of specifically adsorbed DNA.

Beyond Simple Cartoons: Challenges in Characterizing Electrochemical Biosensor Interfaces

Design and development of surface-based biosensors is challenging given the multidisciplinary nature of this enterprise, which is certainly the case for electrochemical biosensors. Self-assembly approaches are used to modify the surface with capture probes along with electrochemical methods for detection. Complex surface structures are created to improve the probe-target interaction. These multicomponent surface structures are usually idealized in schematic representations. Many rely on the analytical performance of the sensor surface as an indication of the quality of the surface modification strategy. While directly linked to the eventual device, arguments for pursuing a more extensive characterization of the molecular environments at the surface are presented as a path to understanding how to make electrochemical sensors that are more robust, reliable with improved sensitivity. This is a complex task that is most often accomplished using methods that only report the average characteristics of the surface. Less often applied are methods that are sensitive to the probe (or adsorbate) present in nonideal configurations (e.g., aggregates, clusters, nonspecifically adsorbed). Though these structures may compose a small fraction of the overall modified surface, they have an uncertain impact on sensor performance and reliability. Addressing this issue requires application of imaging methods over a variety of length scales (e.g., optical microscopy and/or scanning probe microscopy) that provide valuable insight into the diversity of surface structures and molecular environments present at the sensing interface. Furthermore, using in situ analytical methods, while complex, can be more relevant to the sensing environment. Reliable measurements of the nature and extent of these features are required to assess the impact of these nonideal configurations on the sensing process. The development and use of methods that can characterize complex surface based biosensors is arguably required, highlighting the need for a multidisciplinary approach toward the preparation and analysis of the biosensor surface. In many ways, representing the surface without reliance on overly simplified cartoons will highlight these important considerations for improving sensor characteristics.

Electron Transfer Processes in Ferrocene-Modified Poly(ethylene glycol) Monolayers on Electrodes

Electrochemistry is a powerful tool to study self-assembled monolayers. Here, we modified cystamine-functionalized electrodes with different lengths of linear poly(ethylene glycol) (PEG) polymers end-functionalized with a redox-active ferrocene (Fc) group. The electron transport properties of the Fc probes were studied using cyclic voltammetry. The Fc moiety attached to the shortest PEG (M_n = 250 Da) behaved as a surface-confined species, and the homogeneous electron transfer rate constants were determined. The electron transfer of the ferrocene group on the longer PEGs (M_n = 3.4, 5, and 10 kDa) was shown to be driven by diffusion. For low surface densities, where the polymer exists in the mushroom conformation, the diffusion coefficients (D) and rate constants were increasing with polymer length. In the loose brush conformation, where the polymers are close enough to interact with each other, the thickness of the layers (e) was unknown and a parameter D^1/2/e was determined. This parameter showed no dependence on surface density and an increase with polymer length.

Electrochemical aptasensor for human osteopontin detection using a DNA aptamer selected by SELEX

A DNA aptamer with affinity and specificity for human osteopontin (OPN), a potential breast cancer biomarker, was selected using the SELEX process, considering its homology rate and the stability of its secondary structures. This aptamer exhibited a satisfactory affinity towards OPN, showing dissociation constants lower than 2.5 nM. It was further used to develop a simple, label-free electrochemical aptasensor against OPN. The aptasensor showed good sensitivity towards OPN in standard solutions, being the square wave voltammetry (SWV), compared to the cyclic voltammetry, the most sensitive technique with detection and quantification limits of 1.4 ± 0.4 nM and 4.2 ± 1.1 nM, respectively. It showed good reproducibility and acceptable selectivity, exhibiting low signal interferences from other proteins, as thrombin, with 2.6-10 times lower current signals-off than for OPN. The aptasensor also successfully detected OPN in spiked synthetic human plasma. Using SWV, detection and quantification limits (1.3 ± 0.1 and 3.9 ± 0.4 nM) within the OPN plasma levels reported for patients with breast cancer (0.4-4.5 nM) or with metastatic or recurrent breast cancer (0.9-8.4 nM) were found. Moreover, preliminary assays, using a sample of human plasma, showed that the aptasensor and the standard ELISA method quantified similar OPN levels (2.2 ± 0.7 and 1.7 ± 0.1 nM, respectively). Thus, our aptasensor coupled with SWV represents a promising alternative for the detection of relevant breast cancer biomarkers.

Conformational Changes of Enzymes and Aptamers Immobilized on Electrodes

Conformation constitutes a vital property of biomolecules, especially in the cases of enzymes and aptamers, and is essential in defining their molecular recognition ability. When biomolecules are immobilized on electrode surfaces, it is very important to have a control on all the possible conformational changes that may occur, either upon the recognition of their targets or by undesired alterations. Both enzymes and aptamers immobilized on electrodes are susceptible to conformational changes as a response to the nature of the charge of the surface and of the surrounding environment (pH, temperature, ionic strength, etc.). The main goal of this review is to analyze how the conformational changes of enzymes and aptamers immobilized on electrode surfaces have been treated in reports on biosensors and biofuel cells. This topic was selected due to insufficient information found on the actual conformational changes involved in the function of these bioelectrochemical devices despite its importance.

Voltammetric aptasensors for protein disease biomarkers detection: A review

An electrochemical aptasensor is a compact analytical device where the bioreceptor (aptamer) is coupled to a transducer surface to convert a biological interaction into a measurable signal (current) that can be easily processed, recorded and displayed. Since the discovery of the Systematic Evolution of Ligands by Enrichment (SELEX) methodology, the selection of aptamers and their application as bioreceptors has become a promising tool in the design of electrochemical aptasensors. Aptamers present several advantages that highlight their usefulness as bioreceptors such as chemical stability, cost effectiveness and ease of modification towards detection and immobilization at different transducer surfaces. In this review, a special emphasis is given to the potential use of electrochemical aptasensors for the detection of protein disease biomarkers using voltammetry techniques. Methods for the immobilization of aptamers onto electrode surfaces are discussed, as well as different electrochemical strategies that can be used for the design of aptasensors.

"Signal off" aptasensor based on enzyme inhibition induced by conformational switch

A novel sensing strategy for electrochemical aptamer-based sensors is presented. Nucleic acid aptamers are considered alternatives to antibodies. However, some of their intrinsic properties, such as that they can undergo conformational changes during the binding of the target, can be used to design novel sensing strategies. Unlike other electrochemical "signal off" aptamer-based sensors, we report a strategy based on enzymatic inhibition. Our approach shows the feasibility to detect small molecules based on the aptamer conformational change induced by the target that leads to the inhibition of the enzyme used as a label. Additionally, we prove the ability to regenerate the function of the aptasensor by simply applying a short potential pulse. As a proof-of-concept, the widely used aptamer for ochratoxin A (OTA) has been selected as a model. After self-assembling short oligonucleotides onto a gold electrode, complementary to the 3' end of the aptamer, hybridization of the aptamer takes place. To investigate the mechanism induced by the OTA-binding, surface plasmon resonance assays were performed, which confirmed the conformational switch of the aptamer rather than the aptamer displacement by dehybridization from the DNA-modified sensor surface. The electrochemical sensor can successfully detect OTA in wine at the limits stipulated by the European Commission. Given its sensitivity, rapid and easy detection, and regeneration, it can be envisaged as screening tool for OTA detection. Moreover, this sensing strategy has the potential to be applied to other aptamer-based biochemical assays for the detection of small molecules in the fields of food safety, environmental monitoring, and medical diagnostics.