Analytical applications of aptamers

Interferon gamma (IFN-γ)-sensitive TB aptasensor based on novel chitosan-indium nano-kesterite (χtCITS)-labeled DNA aptamer hairpin technology

There has been increasing interest in the use of biosensors for diagnosis of infectious diseases such as tuberculosis (TB) due to their simplicity, affordability, and potential for point-of-care application. The incorporation of aptamer molecules and nanomaterials in biosensor fabrication explores the advantages of high-binding affinity and low immunogenicity of aptamers as well as the high surface-to-volume ratio of nanomaterials, for increased aptasensor performance. In this work, we employed a novel microwave-synthesized copper indium tin sulfide (CITS) substituted-kesterite nanomaterial, together with a natural biopolymer (chitosan), for signal amplification and increased loading of aptamer molecules. Study of the optical properties of CITS nanomaterials showed strong absorption in the UV region characteristic of kesterite semiconductor nanomaterials. X-ray diffraction analysis confirmed the presence of the kesterite phase with average crystallite size of 6.188 nm. Fabrication of interferon-gamma (IFN-γ) TB aptasensor with a chitosan-CITS nanocomposite (χtCITS) increased the aptasensor's electrochemical properties by 77.5 % and improved aptamer loading by 73.7 %. The aptasensor showed excellent sensitivity to IFN-γ concentrations with limit of detection of 6885 fg/mL (405 fM) and linear range of 850-17000 fg/mL (50 - 1000 fM). The aptasensor also exhibited excellent storage and electrochemical stability, with good selectivity towards IFN-γ and possible real sample application.

Fluorescent, phosphorescent, magnetic resonance contrast and radioactive tracer labelling of extracellular vesicles

This review focusses on the significance of fluorescent, phosphorescent labelling and tracking of extracellular vesicles (EVs) for unravelling their biology, pathophysiology, and potential diagnostic and therapeutic uses. Various labeling strategies, such as lipid membrane, surface protein, luminal, nucleic acid, radionuclide, quantum dot labels, and metal complex-based stains, are evaluated for visualizing and characterizing EVs. Direct labelling with fluorescent lipophilic dyes is simple but generally lacks specificity, while surface protein labelling offers selectivity but may affect EV-cell interactions. Luminal and nucleic acid labelling strategies have their own advantages and challenges. Each labelling approach has strengths and weaknesses, which require a suitable probe and technique based on research goals, but new tetranuclear polypyridylruthenium(II) complexes as phosphorescent probes have strong phosphorescence, selective staining, and stability. Future research should prioritize the design of novel fluorescent probes and labelling platforms that can significantly enhance the efficiency, accuracy, and specificity of EV labeling, while preserving their composition and functionality. It is crucial to reduce false positive signals and explore the potential of multimodal imaging techniques to gain comprehensive insights into EVs.

Therapeutic Applications of Aptamers

Affinity reagents, or target-binding molecules, are quite versatile and are major workhorses in molecular biology and medicine. Antibodies are the most famous and frequently used type and they have been used for a wide range of applications, including laboratory techniques, diagnostics, and therapeutics. However, antibodies are not the only available affinity reagents and they do have significant drawbacks, including laborious and costly production. Aptamers are one potential alternative that have a variety of unique advantages. They are single stranded DNA or RNA molecules that can be selected for binding to many targets including proteins, carbohydrates, and small molecules-for which antibodies typically have low affinity. There are also a variety of cost-effective methods for producing and modifying nucleic acids in vitro without cells, whereas antibodies typically require cells or even whole animals. While there are also significant drawbacks to using aptamers in therapeutic applications, including low in vivo stability, aptamers have had success in clinical trials for treating a variety of diseases and two aptamer-based drugs have gained FDA approval. Aptamer development is still ongoing, which could lead to additional applications of aptamer therapeutics, including antitoxins, and combinatorial approaches with nanoparticles and other nucleic acid therapeutics that could improve efficacy.

Biophysical characterization and design of a minimal version of the Hoechst RNA aptamer

RNA aptamers are oligonucleotides, selected through Systematic Evolution of Ligands by EXponential Enrichment (SELEX), that can bind to specific target molecules with high affinity. One such molecule is the RNA aptamer that binds to a blue-fluorescent Hoechst dye that was modified with bulky t-Bu groups to prevent non-specific binding to DNA. This aptamer has potential for biosensor applications; however, limited information is available regarding its conformation, molecular interactions with the ligand, and binding mechanism. The study presented here aims to biophysically characterize the Hoechst RNA aptamer when complexed with the t-Bu Hoechst dye and to further optimize the RNA sequence by designing and synthesizing new sequence variants. Each variant aptamer-t-Bu Hoechst complex was evaluated through a combination of fluorescence emission, native polyacrylamide gel electrophoresis, fluorescence titration, and isothermal titration calorimetry experiments. The results were used to design a minimal version of the aptamer consisting of only 21 nucleotides. The performed study also describes a more efficient method for synthesizing the t-Bu Hoechst dye derivative. Understanding the biophysical properties of the t-Bu Hoechst dye-RNA complex lays the foundation for nuclear magnetic resonance spectroscopy studies and its potential development as a building block for an aptamer-based biosensor that can be used in medical, environmental or laboratory settings.

Design and Fabrication of Biosensor for a Specific Microbe by Silicon-Based Interference Color System

In this paper, one of the great challenges faced by silicon-based biosensors is resolved using a biomaterial multilayer. Tiny biomolecules are deposited on silicon substrates, producing devices that have the ability to act as iridescent color sensors. The color is formed by a coating of uniform microstructures through the interference of light. The system exploits a flat, RNA-aptamer-coated silicon-based surface to which captured microbes are covalently attached. Silicon surfaces are encompassed with the layer-by-layer deposition of biomolecules, as characterized by atomic force microscopy and X-ray photoelectron spectroscopy. Furthermore, the results demonstrate an application of an RNA aptamer chip for sensing a specific bacterium. Interestingly, the detection limit for the microbe was observed to be 2 × 106 CFUmL-1 by visually observed color changes, which were confirmed further using UV-Vis reflectance spectrophotometry. In this report, a flexible method has been developed for the detection of the pathogen Sphingobium yanoikuyae, which is found in non-beverage alcohols. The optimized system is capable of detecting the specific target microbe. The simple concept of these iridescent color changes is mainly derived from the increase in thickness of the nano-ordered layers.

Targeted Liposomal Drug Delivery: Overview of the Current Applications and Challenges

In drug development, it is not uncommon that an active substance exhibits efficacy in vitro but lacks the ability to specifically reach its target in vivo. As a result, targeted drug delivery has become a primary focus in the pharmaceutical sciences. Since the approval of Doxil® in 1995, liposomes have emerged as a leading nanoparticle in targeted drug delivery. Their low immunogenicity, high versatility, and well-documented efficacy have led to their clinical use against a wide variety of diseases. That being said, every disease is accompanied by a unique set of physiological conditions, and each liposomal product must be formulated with this consideration. There are a multitude of different targeting techniques for liposomes that can be employed depending on the application. Passive techniques such as PEGylation or the enhanced permeation and retention effect can improve general pharmacokinetics, while active techniques such as conjugating targeting molecules to the liposome surface may bring even further specificity. This review aims to summarize the current strategies for targeted liposomes in the treatment of diseases.

DNA-modified Prussian blue nanozymes for enhanced electrochemical biosensing

Prussian blue nanoparticles exhibit the potential to be employed in bioanalytical applications due to their robust stability, peroxidase-like catalytic functionality, straightforward synthesis, and biocompatibility. An efficient approach is presented for the synthesis of nucleic acid-modified Prussian blue nanoparticles (DNA-PBNPs), utilizing nanoparticle porosity to adsorb nucleic acids (polyT). This strategic adsorption leads to the exposure of nucleic acid sequences on the particle surface while retaining catalytic activity. DNA-PBNPs further couple with functional nucleic acid sequences and aptamers through complementary base pairing to act as transducers in biosensors and amplify signal acquisition. Subsequently, we integrated a copper ion-dependent DNAzyme (Cu2+-DNAzyme) and a vascular endothelial growth factor aptamer (VEGF aptamer) onto screen-printed electrodes to serve as recognition elements for analytes. Significantly, our approach leverages DNA-PBNPs as a superior alternative to traditional enzyme-linked antibodies in electrochemical biosensors, thereby enhancing both the efficiency and adaptability of these devices. Our study conclusively demonstrates the application of DNA-PBNPs in two different biosensing paradigms: the sensitive detection of copper ions and vascular endothelial growth factor (VEGF). These results indicate the promising potential of DNA-modified Prussian blue nanoparticles in advancing bioanalytical sensing technologies.

A responsive disulfide bond switch aptamer prodrug exhibiting enhanced stability and anticancer efficacy

Aptamers have shown significant potential in treating diverse diseases. However, challenges such as stability and drug delivery limited their clinical application. In this paper, the development of AS1411 prodrug-type aptamers for tumor treatment was introduced. A Short oligonucleotide was introduced at the end of the AS1411 sequence with a disulfide bond as responsive switch. The results indicated that the aptamer prodrugs not only enhanced the stability of the aptamer against nuclease activity but also facilitated binding to serum albumin. Furthermore, in the reducing microenvironment of tumor cells, disulfide bonds triggered drug release, resulting in superior therapeutic effects in vitro and in vivo compared to original drugs. This paper proposes a novel approach for optimizing the structure of nucleic acid drugs, that promises to protect other oligonucleotides or secondary structures, thus opening up new possibilities for nucleic acid drug design.

The Development and Characterisation of ssDNA Aptamers via a Modified Cell-SELEX Methodology for the Detection of Human Red Blood Cells

Blood is one of the most commonly found biological fluids at crime scenes, with the detection and identification of blood holding a high degree of evidential value. It can provide not only information about the nature of the crime but can also lead to identification via DNA profiling. Presumptive tests for blood are usually sensitive but not specific, so small amounts of the substrate can be detected, but false-positive results are often encountered, which can be misleading. Novel methods for the detection of red blood cells based on aptamer-target interactions may be able to overcome these issues. Aptamers are single-stranded DNA or RNA sequences capable of undergoing selective antigen association due to three-dimensional structure formation. The use of aptamers as a target-specific moiety poses several advantages and has the potential to replace antibodies within immunoassays. Aptamers are cheaper to produce, display no batch-to-batch variation and can allow for a wide range of chemical modifications. They can help limit cross-reactivity, which is a hindrance to current forensic testing methods. Within this study, a modified Systematic Evolution of Ligands by Exponential Enrichment (SELEX) process was used to generate aptamers against whole red blood cells. Obtained aptamer pools were analysed via massively parallel sequencing to identify viable sequences that demonstrate a high affinity for the target. Using bioinformatics platforms, aptamer candidates were identified via their enrichment profiles. Binding characterisation was also conducted on two selected aptamer candidates via fluorescent microscopy and qPCR to visualise and quantify aptamer binding. The potential for these aptamers is broad as they can be utilised within a range of bioassays for not only forensic applications but also other analytical science and medical applications. Potential future work includes the incorporation of developed aptamers into a biosensing platform that can be used at crime scenes for the real-time detection of human blood.

Aptamers: A Cutting-Edge Approach for Gram-Negative Bacterial Pathogen Identification

Early and accurate diagnoses of pathogenic microorganisms is essential to correctly identify diseases, treating infections, and tracking disease outbreaks associated with microbial infections, to develop precautionary measures that allow a fast and effective response in epidemics and pandemics, thus improving public health. Aptamers are a class of synthetic nucleic acid molecules with the potential to be used for medical purposes, since they can be directed towards any target molecule. Currently, the use of aptamers has increased because they are a useful tool in the detection of specific targets. We present a brief review of the use of aptamers to detect and identify bacteria or even some toxins with clinical importance. This work describes the advances in the technology of aptamers, with the purpose of providing knowledge to develop new aptamers for diagnoses and treatment of different diseases caused by infectious microorganisms.

Wearable and Implantable Cortisol-Sensing Electronics for Stress Monitoring

Cortisol is a steroid hormone that is released from the body in response to stress. Although a moderate level of cortisol secretion can help the body maintain homeostasis, excessive secretion can cause various diseases, such as depression and anxiety. Conventional methods for cortisol measurement undergo procedures that limit continuous monitoring, typically collecting samples of bodily fluids, followed by separate analysis in a laboratory setting that takes several hours. Thus, recent studies demonstrate wearable, miniaturized sensors integrated with electronic modules that enable wireless real-time analysis. Here, the primary focus is on wearable and implantable electronic devices that continuously measure cortisol concentration. Diverse types of cortisol-sensing techniques, such as antibody-, DNA-aptamer-, and molecularly imprinted polymer-based sensors, as well as wearable and implantable devices that aim to continuously monitor cortisol in a minimally invasive fashion are discussed. In addition to the cortisol monitors that directly measure stress levels, other schemes that indirectly measure stress, such as electrophysiological signals and sweat are also summarized. Finally, the challenges and future directions in stress monitoring and management electronics are reviewed.

An Aptamer-Based Lateral Flow Biosensor for Low-Cost, Rapid and Instrument-Free Detection of Ochratoxin A in Food Samples

In this work, a simple and cost-efficient aptasensor strip is developed for the rapid detection of OTA in food samples. The biosensor is based on the lateral flow assay concept using an OTA-specific aptamer for biorecognition of the target analyte. The strip consists of a sample pad, a conjugate pad, a nitrocellulose membrane (NC) and an absorbent pad. The conjugate pad is loaded with the OTA-specific aptamer conjugated with gold nanoparticles (AuNPs). The test line of the NC membrane is loaded with a specific OTA-aptamer probe and the control line is loaded with a control probe. The assay is based on a competitive format, where the OTA present in the sample combines with the OTA aptamer-AuNP conjugate and prevents the interaction between the specific probe immobilized on the test line and the OTA aptamer-AuNP conjugates; therefore, the color intensity of the test line decreases as the concentration of OTA in the sample increases. Qualitative detection of OTA is performed visually, while quantification is performed by reflectance colorimetry using a commercial scanner and image analysis. All the parameters of the assay are investigated in detail and the analytical features are established. The visual limit of detection (LOD) of the strip is 0.05 ng mL-1, while the LOD for semi-quantitative detection using reflectance colorimetry is 0.02 ng mL-1. The lateral flow strip aptasensor is applied to the detection of OTA in wine, beer, apple juice and milk samples with recoveries in the range from 91 to 114%. The assay exhibits a satisfactory selectivity for OTA with respect to other mycotoxins and lasts 20 min. Therefore, the lateral flow strip aptasensor could be useful for the rapid, low-cost and fit-for-purpose on-site detection of OTA in food samples.

Aptasensors: employing molecular probes for precise medical diagnostics and drug monitoring

Accurate detection and monitoring of therapeutic drug levels are vital for effective patient care and treatment management. Aptamers, composed of single-stranded DNA or RNA molecules, are integral components of biosensors designed for both qualitative and quantitative detection of biological samples. Aptasensors play crucial roles in target identification, validation, detection of drug-target interactions and screening potential of drug candidates. This review focuses on the pivotal role of aptasensors in early disease detection, particularly in identifying biomarkers associated with various diseases such as cancer, infectious diseases and cardiovascular disorders. Aptasensors have demonstrated exceptional potential in enhancing disease diagnostics and monitoring therapeutic drug levels. Aptamer-based biosensors represent a transformative technology in the field of healthcare, enabling precise diagnostics, drug monitoring and disease detection.

A paper-based aptamer-sandwich assay for detection of HNP 1 as a biomarker for periprosthetic joint infections on an integrated microfluidic platform

BACKGROUND

Total joint arthroplasty (TJA) has significantly improved the quality of life for millions suffering from end-stage arthritis. However, periprosthetic joint infections (PJI) remain a serious complication, necessitating extensive interventions and prolonged antimicrobial treatments. The aging population is expected to lead to a rise in TJA cases, subsequently increasing the incidence of PJI, particularly in the elderly who face higher mortality rates. Current diagnostic methods for suspected PJI, such as radiographs and biochemical markers like CRP and ESR, exhibit limited sensitivity. Therefore, there is a critical need for a specific synovial fluid biomarker assay to enhance PJI diagnosis using specific SF-based assay.

RESULTS

This study introduces a novel microfluidic chip with a paper-based aptamer-sandwich assay for the quantitative detection of HNP 1, a crucial PJI biomarker, in synovial fluid. The assay leverages the advantages of aptamers over antibodies, demonstrating high selectivity and affinity for target molecules. The integration of a nitrocellulose (NC) membrane onto the microfluidic platform represents a significant advancement, reducing background signals and simplifying the assay procedure without intricate procedure and pre-treatment. The NC membrane-based microfluidic device offers rapid, cost-effective, and highly sensitive detection of HNP 1, with a limit of detection of 0.5 mg L-1. The microfluidic device demonstrates exceptional performance, detecting up to four clinical samples in approximately 42 min on a single chip with 100 % accuracy, as confirmed by analysis of 12 clinical samples and comparison with "gold-standard". Moreover, the assay exhibits a wide dynamic range of 0.5-100 mg L-1, underscoring its potential as a powerful tool for PJI diagnosis in clinical settings.

SIGNIFICANCE

This work introduces a paper-based microfluidic system tailored for rapid HNP 1 detection using synovial fluid near joint region (and not serum via blood) for better diagnosis. The innovative paper-based aptamer-sandwich assay yields results within 42-min. Significantly, it boasts a wide dynamic range, detecting levels from an impressive 0.5 mg L-1, crucial in the 2.6 mg L-1 threshold region. This heightened sensitivity and expansive detection capability establish our assay as a leader in PJI diagnostics, promising unmatched precision and efficiency in clinical applications.

Current Trends in Mycotoxin Detection with Various Types of Biosensors

One of the most important tasks in food safety is to properly manage the investigation of mycotoxin contamination in agricultural products and foods made from them, as well as to prevent its occurrence. Monitoring requires a wide range of analytical methods, from expensive analytical procedures with high-tech instrumentation to significantly cheaper biosensor developments or even single-use assays suitable for on-site monitoring. This review provides a summary of the development directions over approximately a decade and a half, grouped according to the biologically sensitive components used. We provide an overview of the use of antibodies, molecularly imprinted polymers, and aptamers, as well as the diversity of biosensors and their applications within the food industry. We also mention the possibility of determining multiple toxins side by side, which would significantly reduce the time required for the analyses.

CRISPR-based Biosensors for Human Health: A Novel Strategy to Detect Emerging Infectious Diseases

Infectious diseases (such as sepsis, influenza, and malaria), caused by various pathogenic bacteria and viruses, are widespread across the world. Early and rapid detection of disease-related pathogens is necessary to reduce their spread in the world and prevent their potential global pandemics. The clustered regularly interspaced short palindromic repeats (CRISPR) technology, as the next-generation molecular diagnosis technique, holds immense promise in the detection of infectious diseases because of its remarkable advantages, including supreme flexibility, sensitivity, and specificity. While numerous CRISPR-based biosensors have been developed for application in environmental monitoring, food safety, and point-of-care diagnosis, there remains a critical need to summarize and explore their potential in human health. This review aims to address this gap by focusing on the latest advancements in CRISPR-based biosensors for infectious disease detection. We provide an overview of the current status, pre-amplification methods, the unique feature of each CRISPR system, and the design of CRISPR-based biosensing strategies to detect disease-associated nucleic acids. Last but not least, the review analyzes the current challenges and provides future perspectives, which will contribute to developing more effective CRISPR-based biosensors for human health.

Urinary tract infections: a review of the current diagnostics landscape

Urinary tract infections are the most common bacterial infections worldwide. Infections can range from mild, recurrent (rUTI) to complicated (cUTIs), and are predominantly caused by uropathogenic Escherichia coli (UPEC). Antibiotic therapy is important to tackle infection; however, with the continued emergence of antibiotic resistance there is an urgent need to monitor the use of effective antibiotics through better stewardship measures. Currently, clinical diagnosis of UTIs relies on empiric methods supported by laboratory testing including cellular analysis (of both human and bacterial cells), dipstick analysis and phenotypic culture. Therefore, development of novel, sensitive and specific diagnostics is an important means to rationalise antibiotic therapy in patients. This review discusses the current diagnostic landscape and highlights promising novel diagnostic technologies in development that could aid in treatment and management of antibiotic-resistant UTIs.

A modified SELEX approach to identify DNA aptamers with binding specificity to the major histocompatibility complex presenting ovalbumin model antigen

Aptamers have sparked significant interest in cell recognition because of their superior binding specificity and biocompatibility. Cell recognition can be mediated by targeting the major histocompatibility complex (MHC) that presents short peptides derived from intracellular antigens. Although numerous antibodies have demonstrated a specific affinity for the peptide-MHC complex, the number of aptamers that exhibit comparable characteristics is limited. Aptamers are usually selected from large libraries via the Systemic Evolution of Ligands by Exponential Enrichment (SELEX), an iterative process of selection and PCR amplification to enrich a pool of aptamers with high affinity. However, the success rate of aptamer identification is low, possibly due to the presence of complementary sequences or sequences rich in guanine and cytosine that are less accessible for primers. Here, we modified SELEX by employing systemic consecutive selections with minimal PCR amplification. We also modified the analysis by selecting aptamers that were identified in multiple selection rounds rather than those that are highly enriched. Using this approach, we were able to identify two aptamers with binding specificity to cells expressing the ovalbumin alloantigen as a proof of concept. These two aptamers were also discovered among the top 150 abundant candidates, despite not being highly enriched, by performing conventional SELEX. Additionally, we found that highly enriched aptamers tend to contain fractions of the primer sequence and have minimal target affinity. Candidate aptamers are easily missed in the conventional SELEX process. Therefore, our modification for SELEX may facilitate the identification of aptamers for more application in diverse biomedical fields. Significance: we modify the conventional method to improve the efficiency in the identification of the aptamer, a single strand of nucleic acid with binding specificity to the target molecule, showing as a proof of concept that this approach is particularly useful to select aptamers that can selectively bind to cells presenting a particular peptide by the major histocompatibility complex (MHC) on the cell surface. Given that cancer cells may express mutant peptide-MHC complexes that are distinct from those expressed by normal cells, this study sheds light on the potential application of aptamers to cancer cell targeting.

Development of a label-free impedimetric aptasensor for the detection of Acinetobacter baumannii bacteria

Acinetobacter baumannii (A. baumannii) is responsible for various nosocomial infections, which is known as a clinically crucial opportunistic pathogen. Therefore, rapid detection of this pathogen is critical to prevent the spread of infection and appropriate treatment. Biological detection probes, such as aptamers and synthetic receptors can be used as diagnostic layers to detect bacteria. In this work, an electrochemical aptasensor was developed for the ultrasensitive detection of A. baumannii by electrochemical impedance spectroscopy (EIS). The aptamer was immobilized on the surface of a CSPE modified with the nanocomposite Fe3O4@SiO2@Glyoxal (Gly) for selective and label-free detection of A. baumannii. The charge transfers resistance (Rct) between redox couple [Fe(CN)63-/4-] and the surface of aptasensor in the Nyquist plot of EIS study was used as electroanalytical signal for detection and determination of A. baumannii. The obtained results showed that the constructed aptasensor could specifically detect A. baumannii in the concentration range from 1.0 × 103-1.0 × 108 Colony-forming unit (CFU)/mL and with a detection limit of 150 CFU/mL (S/N = 3). In addition to its sensitivity, the biosensor exhibits high selectivity over some other pathogens. Therefore, a simple, inexpensive, rapid, label-free, selective, and sensitive electrochemical aptasensor was developed to detect A. baumannii.

RNA-Cleaving DNAzyme-Based Amplification Strategies for Biosensing and Therapy

Since their first discovery in 1994, DNAzymes have been extensively applied in biosensing and therapy that act as recognition elements and signal generators with the outstanding properties of good stability, simple synthesis, and high sensitivity. One subset, RNA-cleaving DNAzymes, is widely employed for diverse applications, including as reporters capable of transmitting detectable signals. In this review, the recent advances of RNA-cleaving DNAzyme-based amplification strategies in scaled-up biosensing are focused, the application in diagnosis and disease treatment are also discussed. Two major types of RNA-cleaving DNAzyme-based amplification strategies are highlighted, namely direct response amplification strategies and combinational response amplification strategies. The direct response amplification strategies refer to those based on novel designed single-stranded DNAzyme, and the combinational response amplification strategies mainly include two-part assembled DNAzyme, cascade reactions, CHA/HCR/RCA, DNA walker, CRISPR-Cas12a and aptamer. Finally, the current status of DNAzymes, the challenges, and the prospects of DNAzyme-based biosensors are presented.

Harnessing Nucleic Acids Nanotechnology for Bone/Cartilage Regeneration

The effective regeneration of weight-bearing bone defects and critical-sized cartilage defects remains a significant clinical challenge. Traditional treatments such as autologous and allograft bone grafting have not been successful in achieving the desired outcomes, necessitating the need for innovative therapeutic approaches. Nucleic acids have attracted significant attention due to their ability to be designed to form discrete structures and programmed to perform specific functions at the nanoscale. The advantages of nucleic acid nanotechnology offer numerous opportunities for in-cell and in vivo applications, and hold great promise for advancing the field of biomaterials. In this review, the current abilities of nucleic acid nanotechnology to be applied in bone and cartilage regeneration are summarized and insights into the challenges and future directions for the development of this technology are provided.

Aptamer-based nanointerferometer enables amplification-free ultrasensitive detection and differentiation of SARS-CoV-2 variants

The state-of-the-art SARS-CoV-2 detection methods include qRT-PCR and antibody-based lateral flow assay (LFA) point-of-care tests. Despite the high sensitivity and selectivity, qRT-PCR is slow, expensive and needs well-trained operators. On the other extreme, LFA suffers from low sensitivity albeit its fast detection speed, low detection cost and ease of use. Therefore, the continuing COVID-19 pandemic calls for a SARS-CoV-2 detection method that is rapid, convenient and cost-effective without compromise in sensitivity. Here we provide a proof-of-principle demonstration of an optimized aptamer-based nanointerferometer that enables rapid and amplification-free detection of SARS-CoV-2 spike protein-coated pseudovirus directly from human saliva with the limit of detection (LOD) of about 400 copies per mL. This LOD is on par with that of qRT-PCR, making it 1000 to 100,000-fold more sensitive than commercial LFA tests. Using various combinations of negative selections during the screens for the aptamer targeting the receptor binding domain of the spike protein of SARS-CoV-2, we isolated two aptamers that can distinguish the Omicron and Delta variants. Integrating these two aptamers with LFA strips or the nanointerferometer sensors allows both detection and differentiation of the Omicron and Delta variants which has the potential to realize rapid triage of patients infected different SARS-CoV-2 variants.

Small-molecule oligonucleotides as smart modality for antiviral therapy: a medicinal chemistry perspective

Small-molecule oligonucleotides could be exploited therapeutically to silence the expression of viral infection-causing genes, and a few of them are now in clinical trials for the management of viral infections. The most challenging aspect of these oligonucleotides' therapeutic success involves their delivery. Thus medicinal chemistry strategies are inevitable to avoid degradation by serum nucleases, avoid kidney clearance and improve cellular uptake. Recently small-molecule oligonucleotide design has opened up new avenues to improve the treatment of drug-resistant viral infections, along with the development of COVID-19 medicines. This review is directed toward the recent advances in rational design, mechanism of action, structure-activity relationships and future perspective of the small-molecule oligonucleotides targeting viral infections, including COVID-19.

Simultaneous screening of multiple diarrhetic shellfish poisons with group-specific split aptamers and silver nanocluster beacon

Poisoning events concerning diarrhetic shellfish poisons (DSPs) are increasing continually. It is extremely necessary to develop simple analysis methods for screening simultaneously different types of DSPs from food-related samples. Okadaic acid (OA) and its analogues, i.e., dinophysistoxin-1 (DTX-1) and dinophysistoxin-2 (DTX-2), are the prevalent DSPs. Herein, a facile and label-free fluorescent aptasensor targeting the three DSPs was constructed with a pair of group-specific split aptamers and silver nanocluster beacon. In presence of the targets, the DNA templates attached at the ends of the split aptamers would be dragged close to trigger enhanced fluorescence signals from silver nanoclusters. The aptasensor offered high sensitivity and good selectivity, with limit of detection of 2.282 nmolL^-1, 19.38 nmolL^-1, and 13.61 nmolL^-1 for OA, DTX-1, and DTX-2, respectively. Moreover, the applicability of aptasensor was well verified with shellfish and seawater samples. This study provides good reference for further exploration on analysis methods for food-related molecules.

Aptamers 101: aptamer discovery and in vitro applications in biosensors and separations

Aptamers are single-stranded nucleic acids that bind and recognize targets much like antibodies. Recently, aptamers have garnered increased interest due to their unique properties, including inexpensive production, simple chemical modification, and long-term stability. At the same time, aptamers possess similar binding affinity and specificity as their protein counterpart. In this review, we discuss the aptamer discovery process as well as aptamer applications to biosensors and separations. In the discovery section, we describe the major steps of the library selection process for aptamers, called systematic evolution of ligands by exponential enrichment (SELEX). We highlight common approaches and emerging strategies in SELEX, from starting library selection to aptamer-target binding characterization. In the applications section, we first evaluate recently developed aptamer biosensors for SARS-CoV-2 virus detection, including electrochemical aptamer-based sensors and lateral flow assays. Then we discuss aptamer-based separations for partitioning different molecules or cell types, especially for purifying T cell subsets for therapeutic applications. Overall, aptamers are promising biomolecular tools and the aptamer field is primed for expansion in biosensing and cell separation.

Reagentless Electrochemical Detection of Tumor Biomarker Based on Stable Confinement of Electrochemical Probe in Bipolar Silica Nanochannel Film

The development of simple and probe-integrated aptamer sensors for the electrochemical detection of tumor biomarkers is of great significance for the diagnosis of tumors and evaluation of prognosis. In this work, a probe-integrated aptamer sensor is demonstrated based on the stable confinement of an electrochemical probe in a bipolar nanochannel film, which can realize the reagentless electrochemical detection of the tumor biomarker carcinoembryonic antigen (CEA). To realize the stable immobilization of a large amount of the cationic electrochemical probe methylene blue (MB), a two-layer silica nanochannel array (SNF) with asymmetric charge was grown on the supporting electrode from bipolar SNF (bp-SNF). The inner SNF is negatively charged (n-SNF), and the outer-layer SNF is positively charged (p-SNF). The dual electrostatic interaction including the electrostatic adsorption from n-SNF and the electrostatic repulsion from p-SNF achieve the stable confinement of MB in bp-SNF. The recognitive interface is fabricated by the covalent immobilization of the CEA aptamer on the outer surface of bp-SNF, followed by the blocking of non-specific binding sites. Owing to the stable and abundant immobilized probes and highly specific aptamer interface, the developed aptamer sensor enables the sensitive detection of CEA in the range of 1 pg/mL to 1 μg/mL with a low limit of detection (LOD, 0.22 pg/mL, S/N = 3). Owing to the high selectivity and stability of the developed biosensor, reagentless electrochemical detection of CEA in serum was realized.

An aptamer-based depot system for sustained release of small molecule therapeutics

Delivery of hydrophilic small molecule therapeutics by traditional drug delivery systems is challenging. Herein, we have used the specific interaction between DNA aptamers and drugs to create simple and effective drug depot systems. The specific binding of a phosphorothioate-modified aptamer to drugs formed non-covalent aptamer/drug complexes, which created a sustained release system. We demonstrated the effectiveness of this system with small hydrophilic molecules, the site 1 sodium channel blockers tetrodotoxin and saxitoxin. The aptamer-based delivery system greatly prolonged the duration of local anesthesia and reduced systemic toxicity. The beneficial effects of the aptamers were restricted to the compounds they were specific to. These studies establish aptamers as a class of highly specific, modifiable drug delivery systems, and demonstrate potential usefulness in the management of postoperative pain.

Carbon Nanofiber-Ionic Liquid Nanocomposite Modified Aptasensors Developed for Electrochemical Investigation of Interaction of Aptamer/Aptamer-Antisense Pair with Activated Protein C

Selective and sensitive detection of human activated protein C (APC) was performed herein by using carbon nanofiber (CNF) and ionic liquid (IL) composite modified pencil graphite electrode (PGE) and electrochemical impedance spectroscopy (EIS) technique. A carbon nanomaterial-based electrochemical aptasensor was designed and implemented for the first time in this study for the solution-phase interaction of DNA-Apt with its cognate protein APC as well as APC inhibitor aptamer-antidote pair. The applicability of this assay developed for the determination of APC in fetal bovine serum (FBS) and its selectivity against different proteins (protein C, thrombin, bovine serum albumin) was also examined. CNF-IL modified aptasensor specific to APC provided the detection limit as 0.23 μg/mL (equal to 3.83 nM) in buffer medium and 0.11 μg/mL (equal to 1.83 nM) in FBS. The duration of the proposed assay from the point of electrode modification to the detection of APC was completed within only 55 min.

Recent Development in Biomedical Applications of Oligonucleotides with Triplex-Forming Ability

A DNA structure, known as triple-stranded DNA, is made up of three oligonucleotide chains that wind around one another to form a triple helix (TFO). Hoogsteen base pairing describes how triple-stranded DNA may be built at certain conditions by the attachment of the third strand to an RNA, PNA, or DNA, which might all be employed as oligonucleotide chains. In each of these situations, the oligonucleotides can be employed as an anchor, in conjunction with a specific bioactive chemical, or as a messenger that enables switching between transcription and replication through the triplex-forming zone. These data are also considered since various illnesses have been linked to the expansion of triplex-prone sequences. In light of metabolic acidosis and associated symptoms, some consideration is given to the impact of several low-molecular-weight compounds, including pH on triplex production in vivo. The review is focused on the development of biomedical oligonucleotides with triplexes.

Fluorescence-based aptasensors for small molecular food contaminants: From energy transfer to optical polarization

Small molecular food contaminants, such as mycotoxins, pesticide residues and antibiotics, are highly probable to be passively introduced in food at all stages of its processing, including planting, harvest, production, transportation and storage. Owing to the high risks caused by the unknowing intake and accumulation in human, there is an urgent need to develop rapid, sensitive and efficient methods to monitor them. Fluorescence-based aptasensors provide a promising platform for this area owing to its simple operation, high sensitivity, wide application range and economical practicability. In this paper, the common sorts of small molecular contaminants in foods, namely mycotoxins, pesticides, antibiotics, etc, are briefly introduced. Then, we make a comprehensive review, from fluorescence resonance energy transfer (in turn-on, turn-off, and ratiometric mode, as well as energy upconversion) to fluorescence polarization, of the fluorescence-based aptasensors for the determination of these food contaminants reported in the last five years. The principle of signal generation, the advances of each sort of fluorescent aptasensors, as well as their applications are introduced in detail. Additionally, we also discussed the challenges and perspectives of the fluorescent aptasensors for small molecular food contaminants. This work will offer systematic overview and inspiration for amateurs, researchers and developers of fluorescence-based aptasensors for the detection of small molecules.

Biofunctionalization of Multiplexed Silicon Photonic Biosensors

Silicon photonic (SiP) sensors offer a promising platform for robust and low-cost decentralized diagnostics due to their high scalability, low limit of detection, and ability to integrate multiple sensors for multiplexed analyte detection. Their CMOS-compatible fabrication enables chip-scale miniaturization, high scalability, and low-cost mass production. Sensitive, specific detection with silicon photonic sensors is afforded through biofunctionalization of the sensor surface; consequently, this functionalization chemistry is inextricably linked to sensor performance. In this review, we first highlight the biofunctionalization needs for SiP biosensors, including sensitivity, specificity, cost, shelf-stability, and replicability and establish a set of performance criteria. We then benchmark biofunctionalization strategies for SiP biosensors against these criteria, organizing the review around three key aspects: bioreceptor selection, immobilization strategies, and patterning techniques. First, we evaluate bioreceptors, including antibodies, aptamers, nucleic acid probes, molecularly imprinted polymers, peptides, glycans, and lectins. We then compare adsorption, bioaffinity, and covalent chemistries for immobilizing bioreceptors on SiP surfaces. Finally, we compare biopatterning techniques for spatially controlling and multiplexing the biofunctionalization of SiP sensors, including microcontact printing, pin- and pipette-based spotting, microfluidic patterning in channels, inkjet printing, and microfluidic probes.

Electrochemical biosensors based on saliva electrolytes for rapid detection and diagnosis

In recent years, electrochemical biosensors (ECBSs) have shown significant potential for real-time disease diagnosis and in situ physical condition monitoring. As a multi-constituent oral fluid comprising various disease signaling biomarkers, saliva has drawn much attention in the field of point-of-care (POC) testing. In particular, during the outbreak of the COVID-19 pandemic, ECBSs which hold the simplicity of a single-step assay compared with the multi-step assay of traditional testing methods are expected to relieve the human and economic burden caused by the massive and long-term sample testing process. Noteworthily, ECBSs for the detection of SARS-CoV-2 in saliva have already been developed and may replace current testing methods. Furthermore, the detection scope has expanded from routine indices such as sugar and uric acid to abnormal biomarkers for early-stage disease detection and drug level monitoring, which further facilitated the evolution of ECBSs in the last 5 years. This review is divided into several main sections. First, we discussed the latest advancements and representative research on ECBSs for saliva testing. Then, we focused on a novel kind of ECBS, organic electrochemical transistors (OECTs), which hold great advantages of high sensitivity and signal-to-noise ratio and on-site detection. Finally, application of ECBSs with integrated portable platforms in oral cavities, which lead to powerful auxiliary testing means for telemedicine, has also been discussed.

Recent Developments in Electrochemical Sensors for the Detection of Antibiotic-Resistant Bacteria

The development of efficient point-of-care (POC) diagnostic tools for detecting infectious diseases caused by destructive pathogens plays an important role in clinical and environmental monitoring. Nevertheless, evolving complex and inconsistent antibiotic-resistant species mire their drug efficacy. In this regard, substantial effort has been expended to develop electrochemical sensors, which have gained significant interest for advancing POC testing with rapid and accurate detection of resistant bacteria at a low cost compared to conventional phenotype methods. This review concentrates on the recent developments in electrochemical sensing techniques that have been applied to assess the diverse latent antibiotic resistances of pathogenic bacteria. It deliberates the prominence of biorecognition probes and tailor-made nanomaterials used in electrochemical antibiotic susceptibility testing (AST). In addition, the bimodal functional efficacy of nanomaterials that can serve as potential transducer electrodes and the antimicrobial agent was investigated to meet the current requirements in designing sensor module development. In the final section, we discuss the challenges with contemporary AST sensor techniques and extend the key ideas to meet the demands of the next POC electrochemical sensors and antibiotic design modules in the healthcare sector.

Ultrasensitive aptamer-functionalized Cu-MOF fluorescent nanozyme as an optical biosensor for detection of C-reactive protein

In the present work, an aptasensing method based on integration of RNA on Cu-MOF was developed for detection of C-Reactive Protein (CRP). Cu-MOF showed stimulated fluorescence and mimetic peroxidase enzymatic activity at the time and can be used as dual-signal transduction. CRP binding RNA was used as a highly selective recognition element and immobilized on the Cu-MOF. The immobilized RNA can block the peroxidase activity and fluorescence of the signal traducer probe. Adding CRP to the RNA/Cu-MOF will release RNA from the surface of Cu-MOF and recover both the stimulated fluorescence and peroxidase activity. A biosensor was built for detection of CRP using the two modes of transduction, either colorimetry or fluorometry. A dynamic linear range was obtained from 0.1 to 50 ng mL ^-1with a limit of detection (LOD) as small as 40 pg mL ^-1was calculated in fluorescence mode and 240 pg mL ^-1 as LOD in colorimetry mode. The LODs are lower than the LOD of nephelometric techniques used in clinical practice and is comparable to the normal clinical cutoff value in high-sensitivity CRP assays (1 μg/mL). The aptasensor was successfully applied for detection of CRP in Covid-19 patients with spike recoveries between 84 and 102% and RSD from 0.94% to 2.05%.

Label-Free Aptasensor for Detection of Fipronil Based on Black Phosphorus Nanosheets

A label-free fipronil aptasensor was built based on Polylysine-black phosphorus nanosheets composition (PLL-BPNSs) and Au nanoparticles (AuNPs). A PLL-BP modified glassy carbon electrode (GCE) was fabricated by combining BP NSs and PLL, which included a considerable quantity of -NH₂. Au nanoparticles (AuNPs) were placed onto the GCE, and PLL-BPNSs bonded to Au NPs firmly by assembling. The thiolated primers were then added and fixed using an S-Au bond, and competitive binding of the fipronil aptamer was utilized for fipronil quantitative assessment. The sensor’s performance was evaluated using differential pulse voltammetry (DPV) method. The linear equation is ΔI (μA) = 13.04 logC + 22.35, while linear correlation coefficient R² is 0.998, and detection limit is 74 pg/mL (0.17 nM) when the concentration of fipronil is 0.1 ng/mL–10 μg/mL. This aptasensor can apply to quantitative detection of fipronil.

Significance of Capping Agents of Colloidal Nanoparticles from the Perspective of Drug and Gene Delivery, Bioimaging, and Biosensing: An Insight

The over-growth and coagulation of nanoparticles is prevented using capping agents by the production of stearic effect that plays a pivotal role in stabilizing the interface. This strategy of coating the nanoparticles’ surface with capping agents is an emerging trend in assembling multipurpose nanoparticles that is beneficial for improving their physicochemical and biological behavior. The enhancement of reactivity and negligible toxicity is the outcome. In this review article, an attempt has been made to introduce the significance of different capping agents in the preparation of nanoparticles. Most importantly, we have highlighted the recent progress, existing roadblocks, and upcoming opportunities of using surface modified nanoparticles in nanomedicine from the drug and gene delivery, bioimaging, and biosensing perspectives.

Unveiling the underpinnings of various non-conventional ELISA variants: a review article

INTRODUCTION

Enzyme-linked immunosorbent assay (ELISA) is a key bio-analytical technique used for the detection of a large array of antigenic substances of scientific, clinical, food safety, and environmental importance. The assay primarily involves capturing and detecting target analytes using specific antigen-antibody interactions. The wide usage of ELISA shoulders on its high specificity and reproducibility. Notwithstanding, the conventional microwell plate-based format of ELISA has some major drawbacks, such as long assay time (4 - 18 h), large sample volumes requirement (100 - 200 μL), lack of multiplicity, and burdensome procedures that limit its utility in rapid and affordable diagnostics.

AREAS COVERED

Here, we reviewed microfluidic-ELISA, paper-ELISA, aptamer-ELISA, and those based on novel incubation such as heat-ELISA, pressure-ELISA, microwave-ELISA, and sound-ELISA. Further, the current trends and future prospects of these ELISA protocols in clinical diagnostics are discussed.

EXPERT OPINION

The reviewed non-conventional ELISA formats are relatively rapid, require low reagent volumes, are multiplexable, and could be performed in a low-cost setup. In our opinion, these non-conventional variants of ELISA are on a par with the conventional format for clinical diagnostics and fundamental biological research and hold added clinical translational potential for quick, inexpensive, and convenient measurements.

Nanostructured Materials for Water Purification: Adsorption of Heavy Metal Ions and Organic Dyes

Chemical water pollution poses a threat to human beings and ecological systems. The purification of water to remove toxic organic and inorganic pollutants is essential for a safe society and a clean environment. Adsorption-based water treatment is considered one of the most effective and economic technologies designed to remove toxic substances. In this article, we review the recent progress in the field of nanostructured materials used for water purification, particularly those used for the adsorption of heavy metal ions and organic dyes. This review includes a range of nanostructured materials such as metal-based nanoparticles, polymer-based nanomaterials, carbon nanomaterials, bio-mass materials, and other types of nanostructured materials. Finally, the current challenges in the fields of adsorption of toxic materials using nanostructured materials are briefly discussed.

A Novel Sandwich ELASA Based on Aptamer for Detection of Largemouth Bass Virus (LMBV)

Largemouth bass virus (LMBV) is a major viral pathogen in largemouth bass culture, usually causing high mortality and heavy economic losses. Accurate and early detection of LMBV is crucial for diagnosis and control of the diseases caused by LMBV. Previously, we selected the specific aptamers, LA38 and LA13, targeting LMBV by systematic evolution of ligands by exponential enrichment (SELEX). In this study, we further generated truncated LA38 and LA13 (named as LA38s and LA13s) with high specificity and affinities and developed an aptamer-based sandwich enzyme-linked apta-sorbent assay (ELASA) for LMBV diagnosis. The sandwich ELASA showed high specificity and sensitivity for the LMBV detection, without cross reaction with other viruses. The detection limit of the ELASA was as low as 1.25 × 10² LMBV-infected cells, and the incubation time of the lysate and biotin labeled aptamer was as short as 10 min. The ELASA could still detect LMBV infection in spleen lysates at dilutions of 1/25, with good consistency of qRT-PCR. For the fish samples collected from the field, the sensitivity of ELASA was 13.3% less than PCR, but the ELASA was much more convenient and less time consuming. The procedure of ELASA mainly requires washing and incubation, with completion in approximately 4 h. The sandwich ELASA offers a useful tool to rapidly detect LMBV rapidly, contributing to control and prevention of LMBV infection.

A Review of Apta-POF-Sensors: The Successful Coupling between Aptamers and Plastic Optical Fibers for Biosensing Applications

Aptamers represent the next frontier as biorecognition elements in biosensors thanks to a smaller size and lower molecular weight with respect to antibodies, more structural flexibility with the possibility to be regenerated, reduced batch-to-batch variation, and a potentially lower cost. Their high specificity and small size are particularly interesting for their application in optical biosensors since the perturbation of the evanescent field are low. Apart from the conventional plasmonic optical sensors, platforms based on silica and plastic optical fibers represent an interesting class of devices for point-of-care testing (POCT) in different applications. The first example of the coupling between aptamers and silica optical fibers was reported by Pollet in 2009 for the detection of IgE molecules. Six years later, the first example was published using a plastic optical fiber (POF) for the detection of Vascular Endothelial Growth Factor (VEGF). The excellent flexibility, great numerical aperture, and the large diameter make POFs extremely promising to be coupled to aptamers for the development of a sensitive platform easily integrable in portable, small-size, and simple devices. Starting from silica fiber-based surface plasmon resonance devices, here, a focus on significant biological applications based on aptamers, combined with plasmonic-POF probes, is reported.

Biorecognition Engineering Technologies for Cancer Diagnosis: A Systematic Literature Review of Non-Conventional and Plausible Sensor Development Methods

Simple Summary

Approximately 70% of patients with cancer are diagnosed at late stages of the disease in developing countries. This is partly owed to the restricted access to cost-effective and accurate diagnostic tools in healthcare systems. Biosensor diagnostic tools based on conventional antibodies have been a valuable option for creating accessible detection systems for cancer. However, antibodies have certain limitations related to cost, stability, and applicability. The latter promoted the research and development of alternative approaches to generating molecules and molecule-based scaffolds with similar biorecognition properties to antibodies (non-conventional technologies). This review aimed to present and analyze the current trends of three of these emerging non-conventional technologies for biorecognition engineering in cancer diagnostics, named: molecularly imprinted polymers, recombinant antibodies, and antibody mimetic molecules. These non-conventional technologies are promising, relevant, and more accessible alternatives to conventional antibodies in developing cancer biosensors and worthy of being acknowledged by the scientific community, especially for their use in point-of-care cancer diagnostics in developing countries.

Abstract

Cancer is the second cause of mortality worldwide. Early diagnosis of this multifactorial disease is challenging, especially in populations with limited access to healthcare services. A vast repertoire of cancer biomarkers has been studied to facilitate early diagnosis; particularly, the use of antibodies against these biomarkers has been of interest to detect them through biorecognition. However, there are certain limitations to this approach. Emerging biorecognition engineering technologies are alternative methods to generate molecules and molecule-based scaffolds with similar properties to those presented by antibodies. Molecularly imprinted polymers, recombinant antibodies, and antibody mimetic molecules are three novel technologies commonly used in scientific studies. This review aimed to present the fundamentals of these technologies and address questions about how they are implemented for cancer detection in recent scientific studies. A systematic analysis of the scientific peer-reviewed literature regarding the use of these technologies on cancer detection was carried out starting from the year 2000 up to 2021 to answer these questions. In total, 131 scientific articles indexed in the Web of Science from the last three years were included in this analysis. The results showed that antibody mimetic molecules technology was the biorecognition technology with the highest number of reports. The most studied cancer types were: multiple, breast, leukemia, colorectal, and lung. Electrochemical and optical detection methods were the most frequently used. Finally, the most analyzed biomarkers and cancer entities in the studies were carcinoembryonic antigen, MCF-7 cells, and exosomes. These technologies are emerging tools with adequate performance for developing biosensors useful in cancer detection, which can be used to improve cancer diagnosis in developing countries.

Impedance Technique-Based Label-Free Electrochemical Aptasensor for Thrombin Using Single-Walled Carbon Nanotubes-Casted Screen-Printed Carbon Electrode

An impedance technique-based aptasensor for the detection of thrombin was developed using a single-walled carbon nanotube (SWCNT)-modified screen-printed carbon electrode (SPCE). In this work, a thrombin-binding aptamer (TBA) as probe was used for the determination of thrombin, and that was immobilized on SWCNT through π-π interaction. In the presence of thrombin, the TBA on SWCNT binds with target thrombin, and the amount of TBA on the SWCNT surface decreases. The detachment of TBA from SWCNT will be affected by the concentration of thrombin and the remaining TBA on the SWCNT surface can be monitored by electrochemical methods. The TBA-modified SWCNT/SPCE sensing layer was characterized by cyclic voltammetry (CV). For the measurement of thrombin, the change in charge-transfer resistance (R_ct) of the sensing interface was investigated using electrochemical impedance spectroscopy (EIS) with a target thrombin and [Fe(CN)₆]^3- as redox maker. Upon incubation with thrombin, a decrease of R_ct change was observed due to the decrease in the repulsive interaction between the redox marker and the electrode surface without any label. A plot of R_ct changes vs. the logarithm of thrombin concentration provides the linear detection ranges from 0.1 nM to 1 µM, with a ~0.02 nM detection limit.