Electrochemical biosensors based on nucleic acid aptamers

Cutting-edge biorecognition strategies to boost the detection performance of COVID-19 electrochemical biosensors: A review

Electrochemical biosensors are known for their high sensitivity, selectivity, and low cost. Recently, they have gained significant attention and became particularly important as promising tools for the detection of COVID-19 biomarkers, since they offer a rapid and accurate means of diagnosis. Biorecognition strategies are a crucial component of electrochemical biosensors and determine their specificity and sensitivity based on the interaction of biological molecules, such as antibodies, enzymes, and DNA, with target analytes (e.g., viral particles, proteins and genetic material) to create a measurable signal. Different biorecognition strategies have been developed to enhance the performance of electrochemical biosensors, including direct, competitive, and sandwich binding, alongside nucleic acid hybridization mechanisms and gene editing systems. In this review article, we present the different strategies used in electrochemical biosensors to target SARS-CoV-2 and other COVID-19 biomarkers, as well as explore the advantages and disadvantages of each strategy and highlight recent progress in this field. Additionally, we discuss the challenges associated with developing electrochemical biosensors for clinical COVID-19 diagnosis and their widespread commercialization.

Disposable electrochemical biosensors for the detection of bacteria in the light of antimicrobial resistance

Persistent and inappropriate use of antibiotics is causing rife antimicrobial resistance (AMR) worldwide. Common bacterial infections are thus becoming increasingly difficult to treat without the use of last resort antibiotics. This has necessitated a situation where it is imperative to confirm the infection to be bacterial, before treating it with antimicrobial speculatively. Conventional methods of bacteria detection are either culture based which take anywhere between 24 and 96 hor require sophisticated molecular analysis equipment with libraries and trained operators. These are difficult propositions for resource limited community healthcare setups of developing or less developed countries. Customized, inexpensive, point-of-care (PoC) biosensors are thus being researched and developed for rapid detection of bacterial pathogens. The development and optimization of disposable sensor substrates is the first and crucial step in development of such PoC systems. The substrates should facilitate easy charge transfer, a high surface to volume ratio, be tailorable by the various bio-conjugation chemistries, preserve the integrity of the biorecognition element, yet be inexpensive. Such sensor substrates thus need to be thoroughly investigated. Further, if such systems were made disposable, they would attain immunity to biofouling. This article discusses a few potential disposable electrochemical sensor substrates deployed for detection of bacteria for environmental and healthcare applications. The technologies have significant potential in helping reduce bacterial infections and checking AMR. This could help save lives of people succumbing to bacterial infections, as well as improve the overall quality of lives of people in low- and middle-income countries.

Development of a Multiplexed Electrochemical Aptasensor for the Detection of Cyanotoxins

In this study, we report a multiplexed platform for the simultaneous determination of five marine toxins. The proposed biosensor is based on a disposable electrical printed (DEP) microarray composed of eight individually addressable carbon electrodes. The electrodeposition of gold nanoparticles on the carbon surface offers high conductivity and enlarges the electroactive area. The immobilization of thiolated aptamers on the AuNP-decorated carbon electrodes provides a stable, well-orientated and organized binary self-assembled monolayer for sensitive and accurate detection. A simple electrochemical multiplexed aptasensor based on AuNPs was designed to synchronously detect multiple cyanotoxins, namely, microcystin-LR (MC-LR), Cylindrospermopsin (CYL), anatoxin-α, saxitoxin and okadaic acid (OA). The choice of the five toxins was based on their widespread presence and toxicity to aquatic ecosystems and humans. Taking advantage of the conformational change of the aptamers upon target binding, cyanotoxin detection was achieved by monitoring the resulting electron transfer increase by square-wave voltammetry. Under the optimal conditions, the linear range of the proposed aptasensor was estimated to be from 0.018 nM to 200 nM for all the toxins, except for MC-LR where detection was possible within the range of 0.073 to 150 nM. Excellent sensitivity was achieved with the limits of detection of 0.0033, 0.0045, 0.0034, 0.0053 and 0.0048 nM for MC-LR, CYL, anatoxin-α, saxitoxin and OA, respectively. Selectivity studies were performed to show the absence of cross-reactivity between the five analytes. Finally, the application of the multiplexed aptasensor to tap water samples revealed very good agreement with the calibration curves obtained in buffer. This simple and accurate multiplexed platform could open the window for the simultaneous detection of multiple pollutants in different matrices.

Nanoarchitectonics-based electrochemical aptasensors for highly efficient exosome detection

Exosomes, a type of extracellular vesicles, have attracted considerable attention due to their ability to provide valuable insights into the pathophysiological microenvironment of the cells from which they originate. This characteristic implicates their potential use as diagnostic disease biomarkers clinically, including cancer, infectious diseases, neurodegenerative disorders, and cardiovascular diseases. Aptasensors, which are electrochemical aptamers based biosensing devices, have emerged as a new class of powerful detection technology to conventional methods like ELISA and Western analysis, primarily because of their capability for high-performance bioanalysis. This review covers the current research landscape on the detection of exosomes utilizing nanoarchitectonics strategy for the development of electrochemical aptasensors. Strategies involving signal amplification and biofouling prevention are discussed, with an emphasis on nanoarchitectonics-based bio-interfaces, showcasing their potential to enhance sensitivity and selectivity through optimal conduction and mass transport properties. The ongoing challenges to broaden the clinical applications of these biosensors are also highlighted.

State-of-the-art electrochemical biosensors based on covalent organic frameworks and their hybrid materials

As the development of global population and industry civilization, the accurate and sensitive detection of intended analytes is becoming an important and great challenge in the field of environmental, medical, and public safety. Recently, electrochemical biosensors have been constructed and used in sensing fields, such as antibiotics, pesticides, specific markers of cancer, and so on. Functional materials have been designed and prepared to enhance detection performance. Among all reported materials, covalent organic frameworks (COFs) are emerging as porous crystalline materials to construct electrochemical biosensors, because COFs have many unique advantages, including large surface area, high stability, atom-level designability, and diversity, to achieve a far better sensing performance. In this comprehensive review, we not only summarize state-of-the-art electrochemical biosensors based on COFs and their hybrid materials but also highlight and discuss some typical examples in detail. We finally provide the challenge and future perspective of COFs-based electrochemical biosensors.

Electrochemical biosensors for pathogenic microorganisms detection based on recognition elements

The worldwide spread of pathogenic microorganisms poses a significant risk to human health. Electrochemical biosensors have emerged as dependable analytical tools for the point-of-care detection of pathogens and can effectively compensate for the limitations of conventional techniques. Real-time analysis, high throughput, portability, and rapidity make them pioneering tools for on-site detection of pathogens. Herein, this work comprehensively reviews the recent advances in electrochemical biosensors for pathogen detection, focusing on those based on the classification of recognition elements, and summarizes their principles, current challenges, and prospects. This review was conducted by a systematic search of PubMed and Web of Science databases to obtain relevant literature and construct a basic framework. A total of 171 publications were included after online screening and data extraction to obtain information of the research advances in electrochemical biosensors for pathogen detection. According to the findings, the research of electrochemical biosensors in pathogen detection has been increasing yearly in the past 3 years, which has a broad development prospect, but most of the biosensors have performance or economic limitations and are still in the primary stage. Therefore, significant research and funding are required to fuel the rapid development of electrochemical biosensors. The overview comprehensively evaluates the recent advances in different types of electrochemical biosensors utilized in pathogen detection, with a view to providing insights into future research directions in biosensors.

DNA/RNA-based electrochemical nanobiosensors for early detection of cancers

Nucleic acids, like DNA and RNA, serve as versatile recognition elements in electrochemical biosensors, demonstrating notable efficacy in detecting various cancer biomarkers with high sensitivity and selectivity. These biosensors offer advantages such as cost-effectiveness, rapid response, ease of operation, and minimal sample preparation. This review provides a comprehensive overview of recent developments in nucleic acid-based electrochemical biosensors for cancer diagnosis, comparing them with antibody-based counterparts. Specific examples targeting key cancer biomarkers, including prostate-specific antigen, microRNA-21, and carcinoembryonic antigen, are highlighted. The discussion delves into challenges and limitations, encompassing stability, reproducibility, interference, and standardization issues. The review suggests future research directions, exploring new nucleic acid recognition elements, innovative transducer materials and designs, novel signal amplification strategies, and integration with microfluidic devices or portable instruments. Evaluating these biosensors in clinical settings using actual samples from cancer patients or healthy donors is emphasized. These sensors are sensitive and specific at detecting non-communicable and communicable disease biomarkers. DNA and RNA's self-assembly, programmability, catalytic activity, and dynamic behavior enable adaptable sensing platforms. They can increase biosensor biocompatibility, stability, signal transduction, and amplification with nanomaterials. In conclusion, nucleic acids-based electrochemical biosensors hold significant potential to enhance cancer detection and treatment through early and accurate diagnosis.

Electrochemical Aptasensing Platform for the Detection of Retinol Binding Protein-4

Here, we present the results of our the electrochemical aptasensing strategy for retinol binding protein-4 (RBP-4) detection based on a thiolated aptamer against RBP-4 and 6-mercaptohexanol (MCH) directly immobilized on a gold electrode surface. The most important parameters affecting the magnitude of the analytical signal generated were optimized: (i) the presence of magnesium ions in the immobilization and measurement buffer, (ii) the concentration of aptamer in the immobilization solution and (iii) its folding procedure. In this work, a systematic assessment of the electrochemical parameters related to the optimization of the sensing layer of the aptasensor was carried out (electron transfer coefficients (α), electron transfer rate constants (k0) and surface coverage of the thiolated aptamer probe (ΓApt)). Then, under the optimized conditions, the analytical response towards RBP-4 protein, in the presence of an Fe(CN)63-/4- redox couple in the supporting solution was assessed. The proposed electrochemical strategy allowed for RBP-4 detection in the concentration range between 100 and 1000 ng/mL with a limit of detection equal to 44 ng/mL based on electrochemical impedance spectroscopy (EIS). The specificity studies against other diabetes biomarkers, including vaspin and adiponectin, proved the selectivity of the proposed platform. These preliminary results will be used in the next step to miniaturize and test the sensor in real samples.

Beyond traditional biosensors: Recent advances in gold nanoparticles modified electrodes for biosensing applications

Gold nanoparticles (AuNPs) have emerged as powerful tools in the construction of highly sensitive electrochemical biosensors. Their unique properties, such as the ability to serve as an effective platform for biomolecule immobilization and to facilitate electron transfer between the electrode surface and the immobilized molecules, make them a promising choice for biosensor applications. Utilizing AuNPs modified electrodes can lead to improved sensitivity and lower limits of detection compared to unmodified electrodes. This review provides a comprehensive overview of the recent advancements and applications of AuNPs-based electrochemical biosensors in the biomedical field. The synthesis methods of AuNPs, their key properties, and various strategies employed for electrode modification are discussed. Furthermore, this review highlights the remarkable applications of these nanostructure-integrated electrodes, including immunosensors, enzyme biosensors, and DNA biosensors.

Electrochemical capacitive dengue aptasensor using NS1 in undiluted human serum

Non-structural 1 (NS1) is a protein biomarker that can be found in blood in the early stages of dengue and related infections (Zika and Chikungunya). This study aims to develop a biosensor to selectively quantify NS1 using DNA aptamer co-immobilized on gold electrodes with 6-(ferrocenyl)hexanethiol (FCH) using electrochemical capacitive spectroscopy. This technique uses a redox probe (FCH) immobilized on the self-assembled monolayer to convert impedance into capacitance information. The developed platform was blocked with bovine serum albumin before NS1 exposure and the ratio between aptamers and FCH was optimized. The aptasensor was tested using commercial NS1 serotype 4 in phosphate-buffered saline and commercial undiluted human serum. Using the optimum applied potential provides high sensitivity (3 and 4 nF per decade) and low limit of detection (30.9 and 41.8 fg/mL) with a large linear range (10 pg to 1 µg/mL and 10 pg to 100 ng/mL, respectively). Both results exhibit a residual standard deviation value < 1%. The results suggested that this aptasensor was capable of detecting NS1 in the clinical range and can be applied to any other specific aptamer with FCH, opening the path for label-free miniaturized point-of-care devices with high sensitivity and specificity.

Carbon-Based Electrochemical (Bio)sensors for the Detection of Carbendazim: A Review

Carbendazim, a fungicide widely used in agriculture, has been classified as a hazardous chemical by the World Health Organization due to its environmental persistence. It is prohibited in several countries; therefore, detecting it in food and environmental samples is highly necessary. A reliable, rapid, and low-cost method uses electrochemical sensors and biosensors, especially those modified with carbon-based materials with good analytical performance. In this review, we summarize the use of carbon-based electrochemical (bio)sensors for detecting carbendazim in environmental and food matrixes, with a particular interest in the role of carbon materials. Focus on publications between 2018 and 2023 that have been describing the use of carbon nanotubes, carbon nitride, graphene, and its derivatives, and carbon-based materials as modifiers, emphasizing the analytical performance obtained, such as linear range, detection limit, selectivity, and the matrix where the detection was applied.

(R)evolution of the Standard Addition Procedure for Immunoassays

A new method to transfer the standard addition procedure for concentration determination to immunoassays with non-linear calibration curves was developed. The new method was successfully applied to simulated data and benchmarked against a state-of-the-art algorithm, showing a significantly improved performance with improvement factors between 2 and 192. The logit function was used to transform the immunoassay signal response of test samples spiked with known analyte concentrations. The relationship between logit(signal) and log-transformed estimated total analyte concentration is linear if the estimated total analyte concentration is correct. Finally, the new method was validated experimentally using different assays in varying, relevant complex matrices, such as serum, saliva, and milk. Different concentrations of testosterone and amitriptyline between 0.05 and 3.0 µg L-1 were quantified using a binding inhibition assay in combination with reflectometric interference spectroscopy (RIfS) as the transduction principle. The sample concentration was calculated using a numerical method. Samples could be quantified with recoveries between 70 and 118%. The standard addition method accounts for individual matrix interference on the immunoassay by spiking the test sample itself. Although the experiments were carried out using RIfS, the method can be applied to any immunoassay that meets the analytical requirements.

A sensitive electrochemical aptasensor for zearalenone detection based on target-triggered branched hybridization chain reaction and exonuclease I-assisted recycling

Traditional methods for detecting antibiotic and mycotoxin residues rely on large-scale instruments, which are expensive and require complex sample pretreatment processes and professional operators. Although aptamer-based electrochemical sensors have the advantages of simplicity, speed, low cost, and high sensitivity, most aptamer-based sensors lack a signal amplification strategy due to their direct use of aptamers as probes, resulting in insufficient sensitivity. To solve the sensitivity problem in the electrochemical detection process, a novel electrochemical sensing strategy was established for ultrasensitive zearalenone (ZEN) detection on the basis of exonuclease I (Exo I) and branched hybridization chain reaction (bHCR) to amplify the signal. The amplification strategy showed excellent analytical performance towards ZEN with a low detection limit at 3.1×10-12 mol/L and a wide linear range from 10-11 to 10-6 mol/L. Importantly, the assay was utilized in the corn powder samples with satisfactory results, holding promising applications in food safety detection and environmental monitoring.

Wearable Insulin Biosensors for Diabetes Management: Advances and Challenges

We present a critical review of the current progress in wearable insulin biosensors. For over 40 years, glucose biosensors have been used for diabetes management. Measurement of blood glucose is an indirect method for calculating the insulin administration dosage, which is critical for insulin-dependent diabetic patients. Research and development efforts aiming towards continuous-insulin-monitoring biosensors in combination with existing glucose biosensors are expected to offer a more accurate estimation of insulin sensitivity, regulate insulin dosage and facilitate progress towards development of a reliable artificial pancreas, as an ultimate goal in diabetes management and personalised medicine. Conventional laboratory analytical techniques for insulin detection are expensive and time-consuming and lack a real-time monitoring capability. On the other hand, biosensors offer point-of-care testing, continuous monitoring, miniaturisation, high specificity and sensitivity, rapid response time, ease of use and low costs. Current research, future developments and challenges in insulin biosensor technology are reviewed and assessed. Different insulin biosensor categories such as aptamer-based, molecularly imprinted polymer (MIP)-based, label-free and other types are presented among the latest developments in the field. This multidisciplinary field requires engagement between scientists, engineers, clinicians and industry for addressing the challenges for a commercial, reliable, real-time-monitoring wearable insulin biosensor.

Single vs sandwich aptamers: Towards the detection of human epidermal growth factor receptor 2 using composites of phthalocyanine and nanoparticles

The superiority of the sandwich over a single aptamer based aptasensor assay for the detection of the human epidermal growth factor receptor 2 (HER2) is demonstrated for the first time. Cobalt tris-3,5 dimethoxy-phenoxy pyridine (5) oxy (2)- carboxylic acid phthalocyanine (CoMPhPyCPc) and sulphur/nitrogen doped graphene quantum dots (SNGQDs) and cerium oxide nanoparticles (CeO₂NPs) nanocomposite (SNGQDs@CeO₂NPs) were used for electrode modification of glassy carbon electrode (GCE) both individually and combined to form the substrates: GCE/SNGQDs@CeO₂NPs, GCE/CoMPhPyCPc and GCE/SNGQDs@CeO₂NPs/CoMPhPyCPc. The designed substrates were used as immobilization platforms for the amino functionalized HB5 aptamer for the development of both single and sandwich aptasensor assays. A novel bioconjugate, made of the HB5 aptamer and nanocomposite (HB5-SNGQDs@CeO₂NPs) was fabricated, and characterized using ultra-violet/visible, Fourier transform infrared, and Raman spectroscopies as well as scanning electron microscopy. HB5-SNGQDs@CeO₂NPs was applied as a secondary aptamer in the design of novel sandwich assays towards the electrochemical detection of HER2. The performance of the designed aptasensors were evaluated using electrochemical impedance spectroscopy. The sandwich assay gave low limit of detection of 0.00088 pg/mL, high sensitivity of 773925 Ω pg^-1mL, showed stability, and good precision in real samples towards HER2 detection.

Aptamer-Based Electrochemical Microfluidic Biosensor for the Detection of Cryptosporidium parvum

Cryptosporidium parvum is a high-risk and opportunistic waterborne parasitic pathogen with highly infectious oocysts that can survive harsh environmental conditions for long periods. Current state-of-the-art methods are limited to lengthy imaging and antibody-based detection techniques that are slow, labor-intensive, and demand trained personnel. Therefore, the development of new sensing platforms for rapid and accurate identification at the point-of-care (POC) is essential to improve public health. Herein, we propose a novel electrochemical microfluidic aptasensor based on hierarchical 3D gold nano-/microislands (NMIs), functionalized with aptamers specific to C. parvum. We used aptamers as robust synthetic biorecognition elements with a remarkable ability to bind and discriminate among molecules to develop a highly selective biosensor. Also, the 3D gold NMIs feature a large active surface area that provides high sensitivity and a low limit of detection (LOD), especially when they are combined with aptamers,. The performance of the NMI aptasensor was assessed by testing the biosensor's ability to detect different concentrations of C. parvum oocysts spiked in different sample matrices, i.e., buffer, tap water, and stool, within 40 min detection time. The electrochemical measurements showed an acceptable LOD of 5 oocysts mL-1 in buffer medium, as well as 10 oocysts mL-1 in stool and tap water media, over a wide linear range of 10-100,000 oocysts mL-1. Moreover, the NMI aptasensor recognized C. parvum oocysts with high selectivity while exhibiting no significant cross-reactivity to other related coccidian parasites. The specific feasibility of the aptasensor was further demonstrated by the detection of the target C. parvum in patient stool samples. Our assay showed coherent results with microscopy and real-time quantitative polymerase chain reaction, achieving high sensitivity and specificity with a significant signal difference (p < 0.001). Therefore, the proposed microfluidic electrochemical biosensor platform could be a stepping stone for the development of rapid and accurate detection of parasites at the POC.

Detection of Biomarker Using Aptasensors to Determine the Type of Diabetes

Diabetes mellitus (DM) is a metabolic disorder characterized by elevated blood glucose levels. This disease is so serious that many experts refer to it as the "silent killer". The early detection of diabetes mellitus, whether type 1, type 2 or mitochondrial, is crucial because it can improve the success of treatment and the quality of life for patients. Aptamer-based biosensor diagnosis methods have been widely developed because they have high sensitivity and selectivity in detecting biomarkers of various diseases. Aptamers are short sequences of oligonucleotides or proteins that recognize specific ligands and bind to various target molecules, ranging from small ions to large proteins. They are promising diagnostic molecules due to their high sensitivity and selectivity, ease of modification, low toxicity, and high stability. This article aims to summarize the progress of detection methods, including detection principles, sensitivity, selectivity, and the performance of detection devices, to distinguish between types of diabetes mellitus using electrochemical aptasensors with biomarkers such as glucose, insulin, HbA1c, GHSA, and ATP.

Bioaffinity Nanoprobes for Foodborne Pathogen Sensing

Bioaffinity nanoprobes are a type of biosensor that utilize the specific binding properties of biological molecules, such as antibodies, enzymes, and nucleic acids, for the detection of foodborne pathogens. These probes serve as nanosensors and can provide highly specific and sensitive detection of pathogens in food samples, making them an attractive option for food safety testing. The advantages of bioaffinity nanoprobes include their ability to detect low levels of pathogens, rapid analysis time, and cost-effectiveness. However, limitations include the need for specialized equipment and the potential for cross-reactivity with other biological molecules. Current research efforts focus on optimizing the performance of bioaffinity probes and expanding their application in the food industry. This article discusses relevant analytical methods, such as surface plasmon resonance (SPR) analysis, Fluorescence Resonance Energy Transfer (FRET) measurements, circular dichroism, and flow cytometry, that are used to evaluate the efficacy of bioaffinity nanoprobes. Additionally, it discusses advances in the development and application of biosensors in monitoring foodborne pathogens.

In silico selection of an aptamer for the design of aptamer-modified magnetic beads bearing ferrocene co-immobilized label for capacitive detection of acetamiprid

In silico evaluation of aptamer/target interactions can facilitate the development of efficient biosensor with high specificity and affinity. In this work, we present in silico, i.e. structural similarity, molecular docking and molecular dynamics selection of the aptamer with sufficient binding properties for acetamiprid (ACE), a nicotine-like pesticide, and its use to design aptamer-modified magnetic beads bearing ferrocene co-immobilized label for capacitive detection of ACE. Taking advantages of the aptamer higher stability and binding affinity, the specific properties of magnetic beads and the redox properties of ferrocene moiety, the developed aptasensor showed promising analytical performances for ACE detection, using electrochemical capacitance spectroscopy, with a linear response ranging from 1 fM to 100 pM and a limit of detection of 0.94 fM (S/N = 3). Furthermore, it was successfully applied to detect ACE in fortified tomatoes samples, proving a promising approach for routine detection of pesticide in real agricultural samples.

Aptasensor for Impedimetric Detection of Lysozyme

The impedimetric detection of a protein, lysozyme (LYS), was carried out herein by aptamer-immobilized single-use pencil graphite electrodes (PGEs). The aptamer was immobilized onto electrochemically activated surface of electrode without using any chemical agents, or any types of nanomaterials. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were applied to analyze the electrochemical behavior of unmodified PGE and aptamer immobilized PGE. The interaction of aptamer with its target protein, LYS, was then investigated by EIS. The limit of detection for LYS was found to be 1.44 μg/mL (equals to 100.65 nM). The developed aptasensor specific to LYS presented high selectivity against to bovine serum albumin and thrombin.

Review: 3-Aminopropyltriethoxysilane (APTES) Deposition Methods on Oxide Surfaces in Solution and Vapor Phases for Biosensing Applications

Surface functionalization and bioreceptor immobilization are critical processes in developing a highly sensitive and selective biosensor. The silanization process with 3-aminopropyltriethoxysilane (APTES) on oxide surfaces is frequently used for surface functionalization because of beneficial characteristics such as its bifunctional nature and low cost. Optimizing the deposition process of the APTES layer to obtain a monolayer is crucial to having a stable surface and effectively immobilizing the bioreceptors, which leads to the improved repeatability and sensitivity of the biosensor. This review provides an overview of APTES deposition methods, categorized into the solution-phase and vapor-phase, and a comprehensive summary and guide for creating stable APTES monolayers on oxide surfaces for biosensing applications. A brief explanation of APTES is introduced, and the APTES deposition methods with their pre/post-treatments and characterization results are discussed. Lastly, APTES deposition methods on nanoparticles used for biosensors are briefly described.

Potentiometric MIP-Modified Screen-Printed Cell for Phenoxy Herbicides Detection

In this study, a molecularly imprinted polymer (MIP)-based screen-printed cell is developed for detecting phenoxy herbicides using 2-methyl-4-chlorophenoxyacetic acid (MCPA) as the template. MCPA is a phenoxy herbicide widely used since 1945 to control broadleaf weeds via growth regulation, primarily in pasture and cereal crops. The potentiometric cell consists of a silver/silver chloride pseudo-reference electrode and a graphite working electrode coated with a MIP film. The polymeric layer is thermally formed after drop-coating of a pre-polymeric mixture composed of the reagents at the following molar ratio: 1 MCPA: 15 MAA (methacrylic acid): 7 EGDMA (ethylene glycol dimethacrylate). After template removal, the recognition cavities function as the ionophore of a classical ion selective electrode (ISE) membrane. The detected ion is the deprotonated MCPA specie, negatively charged, so the measurements were performed in phosphate buffer at pH 5.5. A linear decrease of the potential with MCPA concentration, ranging from 4 × 10^-8 to 1 × 10^-6 mol L^-1, was obtained. The detection limit and the limit of quantification were, respectively, 10 nmol L^-1 and 40 nmol L^-1. A Nernstian slope of about -59 mV/dec was achieved. The method has precision and LOD required for MCPA determination in contaminated environmental samples.

Critical Design Factors for Electrochemical Aptasensors Based on Target-Induced Conformational Changes: The Case of Small-Molecule Targets

Nucleic-acid aptamers consisting in single-stranded DNA oligonucleotides emerged as very promising biorecognition elements for electrochemical biosensors applied in various fields such as medicine, environmental, and food safety. Despite their outstanding features, such as high-binding affinity for a broad range of targets, high stability, low cost and ease of modification, numerous challenges had to be overcome from the aptamer selection process on the design of functioning biosensing devices. Moreover, in the case of small molecules such as metabolites, toxins, drugs, etc., obtaining efficient binding aptamer sequences proved a challenging task given their small molecular surface and limited interactions between their functional groups and aptamer sequences. Thus, establishing consistent evaluation standards for aptamer affinity is crucial for the success of these aptamers in biosensing applications. In this context, this article will give an overview on the thermodynamic and structural aspects of the aptamer-target interaction, its specificity and selectivity, and will also highlight the current methods employed for determining the aptamer-binding affinity and the structural characterization of the aptamer-target complex. The critical aspects regarding the generation of aptamer-modified electrodes suitable for electrochemical sensing, such as appropriate bioreceptor immobilization strategy and experimental conditions which facilitate a convenient anchoring and stability of the aptamer, are also discussed. The review also summarizes some effective small molecule aptasensing platforms from the recent literature.

Supramolecular Enzymatic Labeling for Aptamer Switch-Based Electrochemical Biosensor

Here we report a novel labeling strategy for electrochemical aptasensors based on enzymatic marking via supramolecular host–guest interactions. This approach relies on the use of an adamantane-modified target-responsive hairpin DNA aptamer as an affinity bioreceptor, and a neoglycoconjugate of β-cyclodextin (CD) covalently attached to a redox enzyme as a labeling element. As a proof of concept, an amperometric aptasensor for a carcinoembryonic antigen was assembled on screen-printed carbon electrodes modified with electrodeposited fern-like gold nanoparticles/graphene oxide and, by using a horseradish peroxidase-CD neoglycoenzyme as a biocatalytic redox label. This aptasensor was able to detect the biomarker in the concentration range from 10 pg/mL to 1 ng/mL with a high selectivity and a low detection limit of 3.1 pg/mL in human serum samples.

In silico SELEX screening and statistical analysis of newly designed 5mer peptide-aptamers as Bcl-xl inhibitors using the Taguchi method

Drug development for cancer treatment is a complex process that requires special efforts. Targeting crucial proteins is the most essential purpose of drug design in cancers. Bcl-xl is an anti-apoptotic protein that binds to pro-apoptotic proteins and interrupts their signals. Pro-apoptotic Bcl-xl effectors are short BH3 sequences that form an alpha helix and bind to anti-apoptotic proteins to inhibit their activity. Computational systematic evolution of ligands by exponential enrichment (SELEX) is an exclusive approach for developing peptide aptamers as potential effectors. Here, the amino acids with a high tendency for constructing an alpha-helical structure were selected. Due to the enormous number of pentapeptides, Taguchi method was used to study a selected number of peptides. The binding affinity of the peptides to Bcl-xl was assessed using molecular docking, and after analysis of the obtained results, a final set of optimized peptides was arranged and constructed. For a better comparison, three chemical compounds with approved anti-Bcl-xl activity were selected for comparison with the top-ranked 5mer peptides. The optimized peptides showed considerable binding affinity to Bcl-xl. The molecular dynamics (MD) simulation indicated that the designed peptide (PO5) could create considerable interactions with the BH3 domain of Bcl-xl. The MM/GBSA calculations revealed that these interactions were even stronger than those created by chemical compounds. In silico SELEX is a novel approach to design and evaluate peptide-aptamers. The experimental design improves the SELEX process considerably. Finally, PO5 could be considered a potential inhibitor of Bcl-xl and a potential candidate for cancer treatment.

Generation and Selection of Specific Aptamers Targeting Brucella Species through an Enhanced Cell-SELEX Methodology

Brucellae are Gram-negative, aerobic, non-motile coccobacilli causing brucellosis in man and animals. The disease is one of the most significant yet neglected global zoonoses. Especially in developing countries, brucellosis is causing public health problems and economic losses to private animal owners and national revenues. Composed of oligonucleotides, aptamers are chemical analogues of antibodies that are promising components for developing aptamer-based rapid, sensitive, and specific tests to identify the Brucella group of bacteria. For this purpose, aptamers were generated and selected by an enhanced protocol of cell systematic evolution of ligands by exponential enrichment (cell-SELEX). This enhanced cell-SELEX procedure involved the combination of both conventional and toggle cell-SELEX to boost the specificity and binding affinity to whole Brucella cells. This procedure, combined with high-throughput sequencing of the resulting aptamer pools, comprehensive bioinformatics analysis, and wet lab validation assays, led to the selection of a highly sensitive and specific aptamer for those Brucella species known to circulate in Egypt. The isolated candidate aptamer showed dissociation constant (K_D) values of 43.5 ± 11, 61.5 ± 8, and 56 ± 10.8 nM for B. melitensis, B. abortus, and B. suis, respectively. This is the first development of a Brucella-specific aptamer using an enhanced combination of conventional and toggle cell-SELEX to the authors’ best knowledge.

Plasmonic Biosensors: Review

Biosensors have globally been considered as biomedical diagnostic tools required in abundant areas including the development of diseases, detection of viruses, diagnosing ecological pollution, food monitoring, and a wide range of other diagnostic and therapeutic biomedical research. Recently, the broadly emerging and promising technique of plasmonic resonance has proven to provide label-free and highly sensitive real-time analysis when used in biosensing applications. In this review, a thorough discussion regarding the most recent techniques used in the design, fabrication, and characterization of plasmonic biosensors is conducted in addition to a comparison between those techniques with regard to their advantages and possible drawbacks when applied in different fields.

Nanomaterial-Based Label-Free Electrochemical Aptasensors for the Detection of Thrombin

Thrombin plays a central role in hemostasis and its imbalances in coagulation can lead to various pathologies. It is of clinical significance to develop a fast and accurate method for the quantitative detection of thrombin. Electrochemical aptasensors have the capability of combining the specific selectivity from aptamers with the extraordinary sensitivity from electrochemical techniques and thus have attracted considerable attention for the trace-level detection of thrombin. Nanomaterials and nanostructures can further enhance the performance of thrombin aptasensors to achieve high sensitivity, selectivity, and antifouling functions. In highlighting these material merits and their impacts on sensor performance, this paper reviews the most recent advances in label-free electrochemical aptasensors for thrombin detection, with an emphasis on nanomaterials and nanostructures utilized in sensor design and fabrication. The performance, advantages, and limitations of those aptasensors are summarized and compared according to their material structures and compositions.

Bio-fabrication of thermozyme-based nano-biosensors: their components and present scenario

An amalgamation of microbiology, biocatalysis, recombinant molecular biology, and nanotechnology is crucial for groundbreaking innovation in developing nano-biomedicines and sensoristics. Enzyme-based nano-biosensor finds prospective applications in various sectors (environmental, pharmaceutical, food, biorefineries). These applications demand reliable catalytic efficiency and functionality of the enzyme under an extreme operational environment for a prolonged period. Over the last few years, bio-fabrication of nano-biosensors in conjunction with thermozymes from thermophilic microbes is being sought after as a viable design. Thermozymes are known for their robustness, are chemically resistant toward organic solvents, possess higher durability for constant use, catalytic ability, and stability at elevated temperatures. Additionally, several other attributes of thermozymes like substrate specificity, selectivity, and sensitivity make them desirable in developing a customized biosensor. In this review, crucial designing aspects of enzyme-based nano-biosensors like enzyme immobilization on an electrode surface, new materials derived from microbial sources (biopolymers based nanocomposites), improvisation measures for sensitivity, and selectivity have been addressed. It also covers microbial biosynthesis of nanomaterials used to develop sensoristic devices and its numerous applications such as wastewater treatment, biorefineries, and diagnostics. The knowledge will pave the way toward creating consistent eco-friendly, economically viable nanostructured-based technologies with broad applicability and exploitation for industrial use in the near future.

The First FRET-Based RNA Aptamer NanoKit for Sensitively and Specifically Detecting c-di-GMP

An effective method to identify c-di-GMP may significantly facilitate the exploration of its signaling pathways and bacterial pathogenesis. Herein, we have developed the first conjugated polymer-amplified RNA aptamer NanoKit with a unique core-shell-shell architecture, which combines the advantages of high selectivity of RNA aptamers and high sensitivity of strong fluorescence resonance energy transfer (FRET) effect, for precisely detecting c-di-GMP. We identified that NanoKit could selectively detect c-di-GMP with a low detection limit of 50 pM. Importantly, NanoKit could identify bacterial species and physiological states, such as planktonic, biofilm, and even antibiotic-resistance, on the basis of their different c-di-GMP expression patterns. Particularly, NanoKit could distinguish bacterial infection and inflammation and identify Pseudomonas aeruginosa associated pneumonia and sepsis, thereby guiding treatment choice and monitoring antibiotic effects. Therefore, NanoKit provides a promising strategy to rapidly identify c-di-GMP and its associated diseases and may benefit for pathophoresis management.

Hierarchical nickel hydroxide nanosheets grown on hollow nitrogen doped carbon nanoboxes as a high-performance surface substrate for alpha-fetoprotein cancer biomarkers electrochemical aptasensing

During recent decades, we have witnessed a great improvement in the performance of aptamer-based sensors, specifically when aptamers are combined with new nanomaterials; as a platform for biosensors. The design of hollow carbon-based materials has also received a lot of attention due to its excellent properties in various applications. Herein, we aim at designing hierarchical porous Ni(OH)₂ nanosheets on hollow N-doped carbon nanoboxes Ni(OH)₂@N-C n-box). In this sense, we obtained the hollow N-C n-box skeletons from the Fe₂O₃ nanocubes template. The development of label-free electrochemical aptasensor was carried out using the covalently immobilizing NH₂-functionalized aptamer on Ni(OH)₂@N-C n-box as an efficient substrate. The Ni(OH)₂@N-C n-box was characterized using scanning fourier transform infrared spectroscopy (FTIR), X-ray Diffraction (XRD), Brunauer, Emmett and Teller (BET), transmission electron microscopes (TEM) and electron microscopy (FESEM). The electrochemical evaluations clarified the fact that a linear relationship exists between the alpha-fetoprotein (AFP) contents and the charge transfer resistance (R_ct) (from 1 fg mL^-1 to 100 ng mL^-1) with a low detection limit of 0.3 fg mL^-1. Moreover, regarding the aptasensor, the superior detection recoveries were experienced in real biological samples, illustrating its great detection performance and practical feasibility. Considering the aptasensor application, these studies showed that Ni(OH)₂@N-C n-box possesses different enhanced electrochemical features, making it appropriate as an electrode material for aptasensor application.

Emerging roles of the aptasensors as superior bioaffinity sensors for monitoring shellfish toxins in marine food chain

Shellfish toxins are derived from harmful algae and are easily accumulated in environment and marine food through the food chain, exposing high risks on human health. Preliminary rapid screening is one of the most effective monitoring ways to reduce the potential risks; however, the traditional methods encounter with many limitations, such as complicated procedures, low sensitivity and specificity, and ethical problems. Alternatively, bioaffinity sensors are proposed and draw particular attention. Among them, the aptasensors are springing up and emerging as superior alternatives in recent years, exhibiting high practicability to analyze shellfish toxins in real samples in the marine food chain. Herein, the latest research progresses of aptasensors towards shellfish toxins in the marine food chain in the past five years was reviewed for the first time, in terms of the aptamers applied in these aptasensors, construction principles, signal transduction techniques, response types, individual performance properties, practical applications, and advantages/disadvantages of these aptasensors. Synchronously, critical discussions were given and future perspectives were prospected. We hope this review can serve as a powerful reference to promote further development and application of aptasensors to monitor shellfish toxins, as well as other analytes with similar demands.

Sandwich-type electrochemical aptasensor based on hollow mesoporous carbon spheres loaded with porous dendritic Pd@Pt nanoparticles as signal amplifier for ultrasensitive detection of cardiac troponin I

Signal amplification is crucial to improve the sensitivity for the electrochemical detection of cardiac troponin I (cTnI), one of the ideal biomarkers for early acute myocardial infarction (AMI) diagnosis. Herein, we developed a novel signal amplification strategy to construct a sandwich-type electrochemical aptasensor for the detection of cTnI. Core-shell Pd@Pt dendritic bimetallic nanoparticles loaded on melamine modified hollow mesoporous carbon spheres (Pd@Pt DNs/NH₂-HMCS) was prepared as labels to conjugate with thiol-modification DNA aptamers probe for signal amplification. While introducing numerous amino groups, the melamine functionalized hollow mesoporous carbon spheres (NH₂-HMCS) retained the edge-plane-like defective sites for the adhesion and electrocatalytic reduction of H₂O₂. With the unique characteristics of NH₂-HMCS, it not only enhanced the dispersity and loading capacity of core-shell Pd@Pt dendritic bimetallic nanoparticles (Pd@Pt DNs), but also improved the stability of bonding by the affinity interaction between Pd@Pt DNs and amino groups of melamine. Meanwhile, the synergistic catalysis effect between Pd@Pt DNs and NH₂-HMCS significantly enhanced the electrocatalytic reduction of H₂O₂ and further amplified the signal. Under optimal conditions, this recommended aptasensor for cTnI detection displayed a wide dynamic range from 0.1 pg/mL to 100.0 ng/mL and a low detection limit of 15.4 fg/mL (S/N = 3). The sensor also successfully realized the analysis of cTnI-spiked human serum samples, meaning potential applications in AMI diagnosis.

Aptamer duo-based portable electrochemical biosensors for early diagnosis of periodontal disease

Recently, point-of-care tests (POCT) have gained much attention due to their convenient, fast, simple, and easy characteristics. For POCT, portability is an essential feature. In this study, we have successfully fabricated a portable mini-potentiostat. Using chronoamperometry, electrical signals of this portable mini-potentiostat were measured, and the analytical performance of electrochemical aptasensors was compared with a benchtop potentiostat. The electrochemical signals measured by mini-potentiostat can be displayed on the screen of a smartphone. To verify the analytical performance of this portable electrochemical aptasensor platform with a mini-potentiostat, two well-known model protein biomarkers, vaspin, a type 2 diabetes biomarker, and thrombin, a biomarker for pulmonary metastasis and cardiovascular disease, were confirmed to be detected by using corresponding aptamer duo. After solid verification of this portable electrochemical aptasensor platform, we have successfully implemented this portable mini-potentiostat system to develop a portable sandwich-type binding pair of aptamers-based electrochemical biosensor, which can diagnose periodontal disease by measuring ODAM biomarker. The linear range of this ODAM biosensor was 0 to 15 nM with a detection limit of 0.02 nM and 1 nM in buffer and saliva, respectively. The sensitivity of this biosensor has been greatly enhanced, compared to previously developed surface plasmon resonance (SPR) or lateral flow assay (LFA) based aptasensors. This study showed that this new portable aptamer duo-based biosensor is expected to diagnose the early stage of periodontal diseases from real samples, such as saliva or gingival crevicular fluid in a short time as a point-of-care (POC) testing.

Gold nanoparticle-based colorimetric aptasensor for rapid detection of multiple mycotoxins in rice

A novel, simple and rapid colorimetric aptasensor for multiple mycotoxins (ochratoxin A (OTA) and aflatoxin B₁ (AFB₁)) detection was developed using unmodified gold nanoparticles (AuNPs). In the work, the high affinities of OTA and AFB₁ aptamers were employed as the recognition elements for the colorimetric determination of OTA and AFB₁. In the absence of mycotoxins, the sulfhydryl-modified aptamers were directly adsorbed to the AuNP surface through Au-S bonds, further prohibiting the aggregation induced by a high concentration of salt, and the solutions remain red. In the presence of mycotoxins, the corresponding aptamer-target complexes were formed and the corresponding aptamers were detached from the surface of AuNPs, leading to the aggregation of AuNPs under the optimal salt solution and a color change. By spectroscopic quantitative analysis and visual analysis, the LODs of OTA and AFB₁ were down to 0.005 ng mL^-1 and 0.07 ng mL^-1, respectively. Furthermore, the colorimetric aptasensor showed a high specificity in the presence of other interfering mycotoxins and metal ions. Finally, the developed aptasensor was applicable to detect OTA and AFB₁ in rice samples with satisfactory performance. Our strategy has great potential for the rapid and sensitive detection of OTA and AFB₁ for on-site analysis.