Electrochemical DNA Biosensors Based on Labeling with Nanoparticles

Scientific Insights into the Quantum Dots (QDs)-Based Electrochemical Sensors for State-of-the-Art Applications

Size-dependent optical and electronic properties are unique characteristics of quantum dots (QDs). A significant advantage is the quantum confinement effect that allows their precise tuning to achieve required characteristics and behavior for the targeted applications. Regarding the aforementioned factors, QDs-based sensors have exhibited dramatic potential for the diverse and advanced applications. For example, QDs-based devices have been potentially utilized for bioimaging, drug delivery, cancer therapy, and environmental remediation. In recent years, use of QDs-based electrochemical sensors have been further extended in other areas like gas sensing, metal ion detection, monitoring of organic pollutants, and detection of radioactive isotopes. Objective of this study is to rationalize the QDs-based electrochemical sensors for state-of-the-art applications. This review article comprehensively illustrates the importance of aforementioned devices along with sources from which QDs devices have been formulated and fabricated. Other distinct features of QDs devices are associated with their extremely high active surfaces, inherent ability of reproducibility, sensitivity, and selectivity for the targeted analyte detection. In this review, major categories of QD materials along with justification of their key roles in electrochemical devices have been demonstrated and discussed. All categories have been evaluated with special emphasis on the advantages and drawbacks/challenges associated with QD materials. However, in the interests of readers and researchers, recent improvements also have been included and discussed. On the evaluation, it has been concluded that despite significant challenges, QDs-based electrochemical sensors exhibit excellent performances for state-of-the-art and targeted applications.

Carbon nanomaterial-based aptasensors for rapid detection of foodborne pathogenic bacteria

Each year, millions of people suffer from foodborne illness due to the consumption of food contaminated with pathogenic bacteria, which severely challenges global health. Therefore, it is essential to recognize foodborne pathogens swiftly and correctly. However, conventional detection techniques for bacterial pathogens are labor-intensive, low selectivity, and time-consuming, highlighting a notable knowledge gap. A novel approach, aptamer-based biosensors (aptasensors) linked to carbon nanomaterials (CNs), has shown the potential to overcome these limitations and provide a more reliable method for detecting bacterial pathogens. Aptamers, short single-stranded DNA (ssDNA)/RNA molecules, serve as bio-recognition elements (BRE) due to their exceptionally high affinity and specificity in identifying foodborne pathogens such as Salmonella spp., Escherichia coli (E. coli), Listeria monocytogenes, Campylobacter jejuni, and other relevant pathogens commonly associated with foodborne illnesses. Carbon nanomaterials' high surface area-to-volume ratio contributes unique characteristics crucial for bacterial sensing, as it improves the binding capacity and signal amplification in the design of aptasensors. Furthermore, aptamers can bind to CNs and create aptasensors with improved signal specificity and sensitivity. Hence, this review intends to critically review the current literature on developing aptamer functionalized CN-based biosensors by transducer optical and electrochemical for detecting foodborne pathogens and explore the advantages and challenges associated with these biosensors. Aptasensors conjugated with CNs offers an efficient tool for identifying foodborne pathogenic bacteria that is both precise and sensitive to potentially replacing complex current techniques that are time-consuming.

DNA nanotechnology for cell-free DNA marker for tumor detection: a comprehensive overview

Advancements in DNA nanotechnology have led to new exciting ways to detect cell-free tumor biomarkers, revolutionizing cancer diagnostics. This article comprehensively reviews recent developments in this field, discussing the significance of liquid biopsies and DNA nanomachines in early cancer detection. The accuracy of cancer diagnosis at its early stages is expected to be significantly improved by identifying biomarkers. Liquid biopsies, offering minimally-invasive testing, hold the potential for capturing tumor-specific components like circulating tumor cells, cell-free DNA, and exosomes. DNA nanomachines are advanced molecular devices that exploit the programmability of DNA sequences for the ultrasensitive and specific detection of these markers. DNA nanomachines, nanostructures made of DNA that can be designable and switchable nanostructures, have a wide range of advantages for detecting tumor biomarkers, including non-invasiveness, affordability, high sensitivity, and specificity. Scientists also work on dealing with challenges like low marker concentrations and interference, which are addressed through microfluidic integration, nanomaterial amplification, and indirect signal detection. Despite advances, multiplex detection remains a challenge. In conclusion, DNA nanomachines bear immense promise for cancer diagnostics, advocating personalized treatment and improving patient outcomes. Continued research could redefine how we find and treat tumors, leading to better patient outcomes.

Ultrasensitive detection of 5-hydroxymethylcytosine in genomic DNA using a graphene-based sensor modified with biotin and gold nanoparticles

Ten-eleven translocation (TET) proteins orchestrate deoxyribonucleic acid (DNA) methylation-demethylation dynamics by oxidizing 5-methylcytosine to 5-hydroxymethylcytosine (5hmC) and are frequently inactivated in various cancers. Due to the significance of 5hmC as an epigenetic biomarker for cancer diagnosis, pathogenesis, and treatment, its rapid and precise quantification is essential. Here, we report a highly sensitive electrochemical method for quantifying genomic 5hmC using graphene sheets that were electrochemically exfoliated and functionalized with biotin and gold nanoparticles (Bt-AuNPs) through a single-step electrical method. The attachment of Bt-AuNPs to graphene enhances the specificity of 5hmC-containing DNA and augments the oxidation of 5hmC to 5-formylcytosine in DNA. When coupled to a gold electrode, the Bt-AuNP-graphene-based sensor exhibits exceptional sensitivity and specificity for detecting 5hmC, with a detection limit of 63.2 fM. Furthermore, our sensor exhibits a remarkable capacity to measure 5hmC levels across a range of biological samples, including preclinical mouse tissues with varying 5hmC levels due to either TET gene disruption or oncogenic transformation, as well as human prostate cancer cell lines. Therefore, our sensing strategy has substantial potential for cancer diagnostics and prognosis.

A novel electrochemical biosensor based on palladium nanoparticles decorated on reduced graphene oxide-polyaminophenol matrix for the detection and discrimination of mitomycin C-DNA and acyclovir-DNA interaction

Both the design of molecules that will interact specifically with DNA and the determination of the mechanism of action of this drug on DNA are important as they allow the control of gene expression. In particular, rapid and precise analysis of this type of interaction is a vital element for pharmaceutical studies. In the present study, a novel reduced graphene oxide/ palladium nanoparticles/ poly(2-amino-4-chlorophenol) (rGO/Pd@PACP) nanocomposite was synthesized by chemical process to modify pencil graphite electrode (PGE) surface. Here, the performance of the newly developed nanomaterial-based biosensor for drug-DNA interaction analysis has been demonstrated. For this purpose, it was determined whether this system, which was developed by selecting a drug molecule (Mitomycin C; MC) known to interact with DNA and a drug molecule (Acyclovir; ACY) that does not interact with DNA, performs a reliable/accurate analysis. Here, ACY was used as a negative control. Compared to bare PGE, the rGO/Pd@PACP nanomaterial modified sensor exhibited 17 times higher sensitivity performance in terms of guanine oxidation signal measured by differential pulse voltammetry (DPV). Moreover, the developed nanobiosensor system provided a highly specific determination between the anticancer drug MC and ACY by discrimination the interactions of these drugs with double-stranded DNA (dsDNA). ACY was also preferred in studies for the optimization of the new nanobiosensor developed. ACY was detected in a concentration as low as 0.0513 μM (51.3 nM) (LOD), and limit of quantification (LOQ) was 0.1711 μM with a linear range from 0.1 to 0.5 μM.

A smartphone-based gold nanoparticle colorimetric sensing platform for kanamycin detection in food samples

The misuse of kanamycin in the breeding industry can pose a threat to human health through food exposure. Therefore, it is crucial to monitor kanamycin (Kana) levels in food. This study presents a novel colorimetric approach for detecting kanamycin based on the aggregation of gold nanoparticles (AuNPs) induced by kanamycin. To achieve this, a single-stranded DNA (ssDNA) aptamer was employed to bind the surface of AuNPs and maintain their dispersion under high salt concentrations. Upon adding Kana, the aptamer selectively binds to it and separates from the gold surface, resulting in the aggregation of AuNPs. This leads to a color change in the solution (from red to purple to blue) which can be observed under salt conditions. The proposed sensor demonstrated a linear range of 0.5-3 nM and a limit of detection (LOD) of 0.11 nM under optimal conditions. Its practicability was tested by monitoring kanamycin in six food samples, including milk, honey, vitamin C effervescent tablets, vegetable, and meat with satisfactory spiked recoveries. The sensor's miniaturization, convenience, simplicity, and low cost make it a desirable choice for fast and highly sensitive detection of Kana.

Label-Free Electrochemical Methods for Disease Detection

Label-free electrochemical biosensing leverages the advantages of label-free techniques, low cost, and fewer user steps, with the sensitivity and portability of electrochemical analysis. In this review, we identify four label-free electrochemical biosensing mechanisms: (a) blocking the electrode surface, (b) allowing greater access to the electrode surface, (c) changing the intercalation or electrostatic affinity of a redox probe to a biorecognition unit, and (d) modulating ion or electron transport properties due to conformational and surface charge changes. Each mechanism is described, recent advancements are summarized, and relative advantages and disadvantages of the techniques are discussed. Furthermore, two avenues for gaining further diagnostic information from label-free electrochemical biosensors, through multiplex analysis and incorporating machine learning, are examined.

Recent Developments in Nanotechnology-Based Biosensors for the Diagnosis of Coronavirus

The major challenge in today's world is that medical research is facing the existence of a vast number of viruses and their mutations, which from time to time cause outbreaks. Also, the continuous and spontaneous mutations occurring in the viruses and the emergence of resistant virus strains have become serious medical hazards. So, in view of the growing number of diseases, like the recent COVID-19 pandemic that has caused the deaths of millions of people, there is a need to improve rapid and sensitive diagnostic strategies to initiate timely treatment for such conditions. In the cases like COVID-19, where a real cure due to erratic and ambiguous signs is not available, early intervention can be life-saving. In the biomedical and pharmaceutical industries, nanotechnology has evolved exponentially and can overcome multiple obstacles in the treatment and diagnosis of diseases. Nanotechnology has developed exponentially in the biomedical and pharmaceutical fields and can overcome numerous challenges in the treatment and diagnosis of diseases. At the nano stage, the molecular properties of materials such as gold, silver, carbon, silica, and polymers get altered and can be used for the creation of reliable and accurate diagnostic techniques. This review provides insight into numerous diagnostic approaches focused on nanoparticles that could have been established for quick and early detection of such diseases.

Modern Electrochemical Biosensing Based on Nucleic Acids and Carbon Nanomaterials

To meet the requirements of novel therapies, effective treatments should be supported by diagnostic tools characterized by appropriate analytical and working parameters. These are, in particular, fast and reliable responses that are proportional to analyte concentration, with low detection limits, high selectivity, cost-efficient construction, and portability, allowing for the development of point-of-care devices. Biosensors using nucleic acids as receptors has turned out to be an effective approach for meeting the abovementioned requirements. Careful design of the receptor layers will allow them to obtain DNA biosensors that are dedicated to almost any analyte, including ions, low and high molecular weight compounds, nucleic acids, proteins, and even whole cells. The impulse for the application of carbon nanomaterials in electrochemical DNA biosensors is rooted in the possibility to further influence their analytical parameters and adjust them to the chosen analysis. Such nanomaterials enable the lowering of the detection limit, the extension of the biosensor linear response, or the increase in selectivity. This is possible thanks to their high conductivity, large surface-to-area ratio, ease of chemical modification, and introduction of other nanomaterials, such as nanoparticles, into the carbon structures. This review discusses the recent advances on the design and application of carbon nanomaterials in electrochemical DNA biosensors that are dedicated especially to modern medical diagnostics.

Conjugation monitoring of gold nanoparticles with alkanedithiols by capillary zone electrophoresis

Alkanedithiols were used for the conjugation of gold nanoparticles (AuNP) prepared by a solution plasma process. Capillary zone electrophoresis was utilized for the monitoring of the conjugated AuNP. When 1,6-hexanedithiol (HDT) was used as a linker, a resolved peak from the AuNP was detected in the electropherogram; the resolved peak was attributed to the conjugated AuNP. The resolved peak was developed with increasing concentrations of HDT, while the peak of the AuNP decreased complementary. The resolved peak also tended to develop along with the standing time at least up to 7 weeks. The electrophoretic mobility of the conjugated AuNP was almost identical over the HDT concentrations examined, suggesting that the conjugation of the AuNP did not proceed further, such as aggregate/agglomerate formation. The conjugation monitoring was also examined with some dithiols and monothiols. Resolved peak of the conjugated AuNP was also detected with 1,2-ethanedithiol and 2-aminoethanethiol.

Light-Amplified CISS-Based Hybrid QD-DNA Impedimetric Device for DNA Hybridization Detection

We design and build a novel light-amplified electrochemical impedimetric device based on the CISS effect to detect DNA hybridization using a hybrid quantum dot (QD)-DNA monolayer on a ferromagnetic (FM) Ni/Au thin film for the first time. Using spin as a detection tool, the current research considers the chiral-induced spin selectivity (CISS) phenomenon. After injecting a spin current into the QD-DNA system with opposite polarities (up and down), the impedimetric device revealed a large differential change in the charge-transfer resistance (ΔR_ct) of ∼100 ohms for both spins. Nearly, a threefold increase in the ΔR_ct value to ∼270 ohms is observed when light with a wavelength of 532 nm is illuminated on the sample, owing to the amplified CISS effect. The yield of spin polarization as extracted from the Nyquist plot increases by a factor of more than 2 when exposed to light, going from 6% in the dark to 13% in the light. The impact of light on the CISS effect was further corroborated by the observation of the spin-dependent asymmetric quenching of photoluminescence (PL) in the same hybrid system. These observations are absent in the case of a noncomplementary QD-DNA system due to the absence of a helical structure in DNA. Based on this, we develop a spin-based DNA hybridization sensor and achieve a limit of detection of 10 fM. These findings open a practical path for the development of spin-based next-generation impedimetric DNA sensors and point-of-care devices.

Screen-printed electrode-based biosensors modified with functional nucleic acid probes and their applications in this pandemic age: a review

Electrochemical methodology has probably been the most used sensing platform in the past few years as they provide superior advantages. In particular, screen-printed electrode (SPE)-based sensing applications stand out as they provide extraordinary miniaturized but robust and user-friendly detection system. In this context, we are focusing on the modification of SPE with functional nucleic acid probes and nanostructures to improve the electrochemical detection performance in versatile sensing applications, particularly in the fight against the COVID-19 pandemic. Aptamers are immobilized on the electrode surface to detect non-nucleic acid targets and complementary probes to recognize and capture nucleic acid targets. In a step further, SPE-based biosensors with the modification of self-assembled DNA nanostructures are emphasized as they offer great potential for the interface engineering of the electrode surface and promote the excellent performance of various interface reactions. By equipping with a portable potentiostat and a smartphone monitoring device, the realization of this SPE-based miniaturized diagnostic system for the further requirement of fast and POC detection is revealed. Finally, more novel and excellent works are previewed and future perspectives in this field are mentioned.

Forecasting the Post-Pandemic Effects of the SARS-CoV-2 Virus Using the Bullwhip Phenomenon Alongside Use of Nanosensors for Disease Containment and Cure

The COVID-19 pandemic has the tendency to affect various organizational paradigm alterations, which civilization hasyet to fully comprehend. Personal to professional, individual to corporate, and across most industries, the spectrum of transformations is vast. Economically, the globe has never been more intertwined, and it has never been subjected to such widespread disruption. While many people have felt and acknowledged the pandemic's short-term repercussions, the resultant paradigm alterations will certainly have long-term consequences with an unknown range and severity. This review paper aims at acknowledging various approaches for the prevention, detection, and diagnosis of the SARS-CoV-2 virus using nanomaterials as a base material. A nanostructure is a material classification based on dimensionality, in proportion to the characteristic diameter and surface area. Nanoparticles, quantum dots, nanowires (NW), carbon nanotubes (CNT), thin films, and nanocomposites are some examples of various dimensions, each acting as a single unit, in terms of transport capacities. Top-down and bottom-up techniques are used to fabricate nanomaterials. The large surface-to-volume ratio of nanomaterials allows one to create extremely sensitive charge or field sensors (electrical sensors, chemical sensors, explosives detection, optical sensors, and gas sensing applications). Nanowires have potential applications in information and communication technologies, low-energy lightning, and medical sensors. Carbon nanotubes have the best environmental stability, electrical characteristics, and surface-to-volume ratio of any nanomaterial, making them ideal for bio-sensing applications. Traditional commercially available techniques have focused on clinical manifestations, as well as molecular and serological detection equipment that can identify the SARS-CoV-2 virus. Scientists are expressing a lot of interest in developing a portable and easy-to-use COVID-19 detection tool. Several unique methodologies and approaches are being investigated as feasible advanced systems capable of meeting the demands. This review article attempts to emphasize the pandemic's aftereffects, utilising the notion of the bullwhip phenomenon's short-term and long-term effects, and it specifies the use of nanomaterials and nanosensors for detection, prevention, diagnosis, and therapy in connection to the SARS-CoV-2.

Highly stable gold nanoparticles in an aqueous solution without any stabilizer prepared by a solution plasma process evaluated through capillary zone electrophoresis

Gold nanoparticles (AuNP) were prepared by a solution plasma process in the presence of H₂O₂, and they were dispersed in an aqueous solution without any stabilizer generally used. The dispersion stability of the AuNP in an aqueous solution was evaluated by capillary zone electrophoresis (CZE). An anionic broad peak was detected with the AuNP by CZE based on its wide variations in size and net charge. The broad peak also suggests that the AuNP were well dispersed in an aqueous solution. The dispersion stability of AuNP was evaluated from the viewpoints of long-term dispersion, salt concentration, and organic co-solvent. The anionic broad peak attributed to the dispersed AuNP was successfully detected for at least 55 weeks from the preparation with less shot signals of the aggregates. The AuNP was also well dispersed in aqueous NaCl solutions with its concentrations up to 30 mmol L^-1, as well as with ethanol co-solvent up to 40%(v/v). The AuNP prepared by the solution plasma process was proved to be highly stable in an aqueous solution.

Nanomaterials and metal-organic frameworks for biosensing applications of mutations of the emerging viruses

The world today lives in a state of terrible fear due to the mutation of the emerging COVID-19. With the continuation of this pandemic, there is an urgent need for fast, accurate testing devices to detect the emerging SARS-CoV-2 pandemic in terms of biosensors and point-of-care testing. Besides, the urgent development in personal defense tools, anti-viral surfaces and wearables, and smartphones open the door for simplifying the self-diagnosis process everywhere. This review introduces a quick COVID-19 overview: definition, transmission, pathophysiology, the identification and diagnosis, mutation and transformation, and the global situation. It also focuses on an overview of the rapidly advanced technologies based on nanomaterials and MOFs for biosensing, diagnosing, and viral control of the SARS-CoV-2 pandemic. Finally, highlight the latest technologies, applications, existing achievements, and preventive diagnostic strategies to control this epidemic and combat the emerging coronavirus. This humble effort aims to provide a helpful survey that can be used to develop a creative solution and to lay down the future vision of diagnosis against COVID-19.

A fast and label-free detection of hydroxymethylated DNA using a nozzle-jet printed AuNPs@Ti3C2 MXene-based electrochemical sensor

5-hydroxymethylcytosine (5hmC) is a key epigenetic mark in the mammalian genome that has been proposed as a promising cancer biomarker with diagnostic and prognostic potentials. A new type of two-dimensional (2D) material called MXene includes transition metal carbides and nitrides and possesses unique physico-chemical properties suitable for diverse applications, including electrochemical sensors. Here, we report a new nozzle-jet printed electrochemical sensor using gold nanoparticles (AuNPs)@Ti₃C₂ MXene nanocomposite for the real-time and label-free detection of 5hmC in the genome. We utilized Ti₃C₂ MXene as a platform to immobilize AuNPs, which have been shown to exhibit different affinity interactions toward 5-methylcytosine (5 mC) and 5hmC, and thus produce distinct electrochemical responses. To fabricate the electrode, a highly conductive and adhesive silver ink was prepared to generate a silver line onto polyethylene terephthalate (PET) substrate using nozzle-jet printing, followed by deposition of AuNPs@Ti₃C₂ MXene ink at one end via dropcasting. Analyses of morphology and chemical composition showed that all steps of the sensor fabrication were successful. The fabricated sensor coupled with cyclic voltammetry showed excellent performance in distinguishing 5 mC- or 5hmC-enriched cellular genomic DNAs. As a proof-of-concept investigation, we confirmed that our sensor readily and consistently detected 5hmC diminution in multiple tumors, compared to the paired normal tissues. Thus, our simple and cost-effective sensing strategy using printable AuNPs@Ti₃C₂ MXene ink holds promise for a wide range of practical applications in epigenetic studies as well as clinical settings.

Recent Trends in Biosensors for Environmental Quality Monitoring

The monitoring of environmental pollution requires fast, reliable, cost-effective and small devices. This need explains the recent trends in the development of biosensing devices for pollutant detection. The present review aims to summarize the newest trends regarding the use of biosensors to detect environmental contaminants. Enzyme, whole cell, antibody, aptamer, and DNA-based biosensors and biomimetic sensors are discussed. We summarize their applicability to the detection of various pollutants and mention their constructive characteristics. Several detection principles are used in biosensor design: amperometry, conductometry, luminescence, etc. They differ in terms of rapidity, sensitivity, profitability, and design. Each one is characterized by specific selectivity and detection limits depending on the sensitive element. Mimetic biosensors are slowly gaining attention from researchers and users due to their advantages compared with classical ones. Further studies are necessary for the development of robust biosensing devices that can successfully be used for the detection of pollutants from complex matrices without prior sample preparation.

Singlet oxygen-based photoelectrochemical detection of DNA

The current work, designed for the photoelectrochemical detection of DNA, evaluates light-responsive DNA probes carrying molecular photosensitizers generating singlet oxygen (¹O₂). We take advantage of their chromophore's ability to produce ¹O₂ upon photoexcitation and subsequent photocurrent response. Type I, fluorescent and type II photosensitizers were studied using diode lasers at 406 nm blue, 532 nm green and 659 nm red lasers in the presensce and absence of a redox reporter, hydroquinone (HQ). Only type II photosensitizers (producing ¹O₂) resulted in a noticeable photocurrent in 1-4 nA range upon illumination, in particular, dissolved DNA probes labeled with chlorin e6 and erythrosine were found to give a well-detectable photocurrent response in the presence of HQ. Whereas, Type I photosensitizers and fluorescent chromophores generate negligible photocurrents (<0.15 nA). The analytical performance of the sensing system was evaluated using a magnetic beads-based DNA assay on disposable electrode platforms, with a focus to enhance the sensitivity and robustness of the technique in detecting complementary DNA targets. Amplified photocurrent responses in the range of 70-100 nA were obtained and detection limits of 17 pM and 10 pM were achieved using magnetic beads-captured chlorin e6 and erythrosine labeled DNA probes respectively. The presented novel photoelectrochemical detection can further be optimized and employed in applications for which enzymatic amplification such as polymerase chain reaction (PCR) is not applicable owing to their limitations and as an effective alternative to colorimetric detection when rapid detection of specific nucleic acid targets is required.

Bimetallic Nanomaterials-Based Electrochemical Biosensor Platforms for Clinical Applications

Diabetes is a foremost health issue that results in ~4 million deaths every year and ~170 million people suffering globally. Though there is no treatment for diabetes yet, the blood glucose level of diabetic patients should be checked closely to avoid further problems. Screening glucose in blood has become a vital requirement, and thus the fabrication of advanced and sensitive blood sugar detection methodologies for clinical analysis and individual care. Bimetallic nanoparticles (BMNPs) are nanosized structures that are of rising interest in many clinical applications. Although their fabrication shares characteristics with physicochemical methodologies for the synthesis of corresponding mono-metallic counterparts, they can display several interesting new properties and applications as a significance of the synergetic effect between their two components. These applications can be as diverse as clinical diagnostics, anti-bacterial/anti-cancer treatments or biological imaging analyses, and drug delivery. However, the exploitation of BMNPs in such fields has received a small amount of attention predominantly due to the vital lack of understanding and concerns mainly on the usage of other nanostructured materials, such as stability and bio-degradability over extended-time, ability to form clusters, chemical reactivity, and biocompatibility. In this review article, a close look at bimetallic nanomaterial based glucose biosensing approaches is discussed, concentrating on their clinical applications as detection of glucose in various real sample sources, showing substantial development of their features related to corresponding monometallic counterparts and other existing used nanomaterials for clinical applications.

Rapid and Label-Free Detection of 5-Hydroxymethylcytosine in Genomic DNA Using an Au/ZnO Nanorods Hybrid Nanostructure-Based Electrochemical Sensor

Ten-eleven-translocation (TET) proteins modify DNA methylation by oxidizing 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). Loss of 5hmC, a widely accepted epigenetic hallmark of cancers, is proposed as a biomarker for early cancer diagnosis and prognosis. Thus, precise quantification of 5hmC holds great potential for diverse clinical applications. DNAs containing 5mC or 5hmC display different adsorption affinity toward the gold surface, thus producing different electrochemical responses. Here a novel, label-free electrochemical sensor based on gold nanoparticles (Au NPs)/zinc oxide nanorods (ZnO NRs) nanostructure for the facile and real-time detection of 5hmC-enriched DNAs is reported. The hybrid structure is fabricated by the vertical hydrothermal growth of ZnO NRs onto indium tin oxide glass substrate, followed by the decoration of ZnO NRs with Au NPs via sputtering. Successful fabrication is confirmed by analyzing the morphology and chemical composition of the sensor. By coupling the fabricated sensor with cyclic voltammetry, its functionality in distinguishing genomic DNAs containing different levels of 5hmC is validated. Notably, the sensor device successfully and consistently detects 5hmC loss in primary hepatocellular carcinoma, compared to the normal tissues. Thus, the novel sensing strategy to assess DNA hydroxymethylation will likely find broad applications in early cancer diagnosis and prognosis evaluation.

An electrochemical genosensor for differentiation of fully methylated from fully unmethylated states of BMP3 gene

The methylation state of a part of the BMP3 gene was detected by our genosensor. This epigenetic biomarker is involved in the biomarker panel of the sDNA test, which is an FDA approved test for colorectal cancer screening. In the present genosensor, polyethyleneimine-stabilized silver nanoparticles (PEI-AgNPs) were used as a non-specific nanolabel for signal generation/amplification and lowering the limit of detection. After immobilization of capture probes and mercaptoethanol molecules on the gold electrode, a thermally treated mixture of the BMP3 targets and reporter probes was introduced to the electrode. Because of the specificity of the reporter probes for fully methylated targets, complete sandwich-like complexes are formed only with them. Therefore, such full-length double-stranded hybrids compared to fully unmethylated targets have more negative charges and can more attract positively charged PEI-AgNPs. For discrimination between methylated and unmethylated targets, electroimpedance spectroscopy and cyclic voltammetry were used for electrode modification monitoring and signal measurement. The sharp and narrow anodic peaks of cyclic voltammograms, which resulted from silver oxidation, were utilized for calibration plot analysis. The genosensor showed a linear response for the target concentration range from 1fM to 100 nM, while the detection limit for methylated and unmethylated target discrimination was 1 fM.

Emerging materials for the electrochemical detection of COVID-19

The SARS-CoV-2 virus is still causing a dramatic loss of human lives worldwide, constituting an unprecedented challenge for the society, public health and economy, to overcome. The up-to-date diagnostic tests, PCR, antibody ELISA and Rapid Antigen, require special equipment, hours of analysis and special staff. For this reason, many research groups have focused recently on the design and development of electrochemical biosensors for the SARS-CoV-2 detection, indicating that they can play a significant role in controlling COVID disease. In this review we thoroughly discuss the transducer electrode nanomaterials investigated in order to improve the sensitivity, specificity and response time of the as-developed SARS-CoV-2 electrochemical biosensors. Particularly, we mainly focus on the results appeard on Au-based and carbon or graphene-based electrodes, which are the main material groups recently investigated worldwidely. Additionally, the adopted electrochemical detection techniques are also discussed, highlighting their pros and cos. The nanomaterial-based electrochemical biosensors could enable a fast, accurate and without special cost, virus detection. However, further research is required in terms of new nanomaterials and synthesis strategies in order the SARS-CoV-2 electrochemical biosensors to be commercialized.

Electrochemical Immunosensors with PQQ-Decorated Carbon Nanotubes as Signal Labels for Electrocatalytic Oxidation of Tris(2-carboxyethyl)phosphine

Nanocatalysts are a promising alternative to natural enzymes as the signal labels of electrochemical biosensors. However, the surface modification of nanocatalysts and sensor electrodes with recognition elements and blockers may form a barrier to direct electron transfer, thus limiting the application of nanocatalysts in electrochemical immunoassays. Electron mediators can accelerate the electron transfer between nanocatalysts and electrodes. Nevertheless, it is hard to simultaneously achieve fast electron exchange between nanocatalysts and redox mediators as well as substrates. This work presents a scheme for the design of electrochemical immunosensors with nanocatalysts as signal labels, in which pyrroloquinoline quinone (PQQ) is the redox-active center of the nanocatalyst. PQQ was decorated on the surface of carbon nanotubes to catalyze the electrochemical oxidation of tris(2-carboxyethyl)phosphine (TCEP) with ferrocenylmethanol (FcM) as the electron mediator. With prostate-specific antigen (PSA) as the model analyte, the detection limit of the sandwich-type immunosensor was found to be 5 pg/mL. The keys to success for this scheme are the slow chemical reaction between TCEP and ferricinum ions, and the high turnover frequency between ferricinum ions, PQQ. and TCEP. This work should be valuable for designing of novel nanolabels and nanocatalytic schemes for electrochemical biosensors.

Prospects of nanomaterials-enabled biosensors for COVID-19 detection

We are currently facing the COVID-19 pandemic which is the consequence of severe acute respiratory syndrome coronavirus (SARS-CoV-2). Since no specific vaccines or drugs have been developed till date for the treatment of SARS-CoV-2 infection, early diagnosis is essential to further combat this pandemic. In this context, the reliable, rapid, and low-cost technique for SARS-CoV-2 diagnosis is the foremost priority. At present reverse transcription polymerase chain reaction (RT-PCR) is the reference technique presently being used for the detection of SARS-CoV-2 infection. However, in a number of cases, false results have been noticed in COVID-19 diagnosis. To develop advanced techniques, researchers are continuously working and in the series of constant efforts, nanomaterials-enabled biosensing approaches can be a hope to offer novel techniques that may perhaps meet the current demand of fast and early diagnosis of COVID-19 cases. This paper provides an overview of the COVID-19 pandemic and nanomaterials-enabled biosensing approaches that have been recently reported for the diagnosis of SARS-CoV-2. Though limited studies on the development of nanomaterials enabled biosensing techniques for the diagnosis of SARS-CoV-2 have been reported, this review summarizes nanomaterials mediated improved biosensing strategies and the possible mechanisms that may be responsible for the diagnosis of the COVID-19 disease. It is reviewed that nanomaterials e.g. gold nanostructures, lanthanide-doped polysterene nanoparticles (NPs), graphene and iron oxide NPs can be potentially used to develop advanced techniques offered by colorimetric, amperometric, impedimetric, fluorescence, and optomagnetic based biosensing of SARS-CoV-2. Finally, critical issues that are likely to accelerate the development of nanomaterials-enabled biosensing for SARS-CoV-2 infection have been discussed in detail. This review may serve as a guide for the development of advanced techniques for nanomaterials enabled biosensing to fulfill the present demand of low-cost, rapid and early diagnosis of COVID-19 infection.

The functionalization of gold nanoparticles as a novel platform for the highly efficient electrochemical detection of silver ions

A novel platform was constructed by the functionalization of gold nanoparticles for the highly efficient electrochemical detection of silver ions.

DNA Nanomachines for Identifying Cancer Biomarkers in Body Fluids and Cells

Identifying molecular biomarkers promises to significantly improve the accuracy in cancer diagnosis at its early stage. DNA nanomachines, which are designable and switchable nanostructures made of DNA, show broad potential to detect tumor biomarkers with noninvasive, inexpensive, highly sensitive, and highly specific advantages. This Feature summarizes the recent DNA nanomachine-based platforms for the early detection of cancer biomarkers, both from body fluids and in cells.

Silver or gold? A comparison of nanoparticle modified electrochemical genosensors based on cobalt porphyrin-DNA

We applied a cobalt-porphyrin modified DNA as electrochemical marker, which was attached to nanoparticles, to detect specific DNA sequences. We compare the performance of gold and silver NPs in oligonucleotide sensors to determine if a change in metal will lead to either higher sensitivity or different selectivity, based on the redox behaviour of silver vs. gold. Surprisingly, we find that using either gold or silver NPs yields very similar overall performance. The electrochemical measurements of both types of sensors show the same redox behaviour which is dominated by the cobalt porphyrin, indicating that the electron pathway does not include the NP, but there is direct electron transfer between the porphyrin and the electrode. Both sensors show a linear response in the range of 5 × 10^-17-1 × 10^-16 M; the limit of detection (LOD) is 3.8 × 10^-18 M for the AuNP sensor, and 5.0 × 10^-18 M for the AgNP sensor, respectively, which corresponds to the detection of about 20-50 DNA molecules in the analyte. Overall, the silver system results in a better DNA economy and using cheaper starting materials for the NPs, thus shows better cost-effectivness and could be more suitable for the mass-production of highly sensitive DNA sensors.

Graphene field-effect transistors as bioanalytical sensors: design, operation and performance

Graphene field-effect transistors (GFETs) are emerging as bioanalytical sensors, in which their responsive electrical conductance is used to perform quantitative analyses of biologically-relevant molecules such as DNA, proteins, ions and small molecules. This review provides a detailed evaluation of reported approaches in the design, operation and performance assessment of GFET biosensors. We first dissect key design elements of these devices, along with most common approaches for their fabrication. We compare possible modes of operation of GFETs as sensors, including transfer curves, output curves and time series as well as their integration in real-time or a posteriori protocols. Finally, we review performance metrics reported for the detection and quantification of bioanalytes, and discuss limitations and best practices to optimize the use of GFETs as bioanalytical sensors.

Label-Free DNA Biosensor Using Modified Reduced Graphene Oxide Platform as a DNA Methylation Assay

This work reports the use of modified reduced graphene oxide (rGO) as a platform for a label-free DNA-based electrochemical biosensor as a possible diagnostic tool for a DNA methylation assay. The biosensor sensitivity was enhanced by variously modified rGO. The rGO decorated with three nanoparticles (NPs)—gold (AuNPs), silver (AgNPs), and copper (CuNPs)—was implemented to increase the electrode surface area. Subsequently, the thiolated DNA probe (single-stranded DNA, ssDNA−1) was hybridized with the target DNA sequence (ssDNA-2). After the hybridization, the double-stranded DNA (dsDNA) was methylated by M.SssI methyltransferase (MTase) and then digested via a HpaII endonuclease specific site sequence of CpG (5′-CCGG-3′) islands. For monitoring the MTase activity, differential pulse voltammetry (DPV) was used, whereas the best results were obtained by rGO-AuNPs. This assay is rapid, cost-effective, sensitive, selective, highly specific, and displays a low limit of detection (LOD) of 0.06 U·mL⁻¹. Lastly, this study was enriched with the real serum sample, where a 0.19 U·mL⁻¹ LOD was achieved. Moreover, the developed biosensor offers excellent potential in future applications in clinical diagnostics, as this approach can be used in the design of other biosensors.

Nanobioconjugates for Signal Amplification in Electrochemical Biosensing

Nanobioconjugates are hybrid materials that result from the coalescence of biomolecules and nanomaterials. They have emerged as a strategy to amplify the signal response in the biosensor field with the potential to enhance the sensitivity and detection limits of analytical assays. This critical review collects a myriad of strategies for the development of nanobioconjugates based on the conjugation of proteins, antibodies, carbohydrates, and DNA/RNA with noble metals, quantum dots, carbon- and magnetic-based nanomaterials, polymers, and complexes. It first discusses nanobioconjugates assembly and characterization to focus on the strategies to amplify a biorecognition event in biosensing, including molecular-, enzymatic-, and electroactive complex-based approaches. It provides some examples, current challenges, and future perspectives of nanobioconjugates for the amplification of signals in electrochemical biosensing.

Applications of Graphene Quantum Dots in Biomedical Sensors

Due to the proliferative cancer rates, cardiovascular diseases, neurodegenerative disorders, autoimmune diseases and a plethora of infections across the globe, it is essential to introduce strategies that can rapidly and specifically detect the ultralow concentrations of relevant biomarkers, pathogens, toxins and pharmaceuticals in biological matrices. Considering these pathophysiologies, various research works have become necessary to fabricate biosensors for their early diagnosis and treatment, using nanomaterials like quantum dots (QDs). These nanomaterials effectively ameliorate the sensor performance with respect to their reproducibility, selectivity as well as sensitivity. In particular, graphene quantum dots (GQDs), which are ideally graphene fragments of nanometer size, constitute discrete features such as acting as attractive fluorophores and excellent electro-catalysts owing to their photo-stability, water-solubility, biocompatibility, non-toxicity and lucrativeness that make them favorable candidates for a wide range of novel biomedical applications. Herein, we reviewed about 300 biomedical studies reported over the last five years which entail the state of art as well as some pioneering ideas with respect to the prominent role of GQDs, especially in the development of optical, electrochemical and photoelectrochemical biosensors. Additionally, we outline the ideal properties of GQDs, their eclectic methods of synthesis, and the general principle behind several biosensing techniques.

Graphene Quantum Dot-Based Electrochemical Immunosensors for Biomedical Applications

In the area of biomedicine, research for designing electrochemical sensors has evolved over the past decade, since it is crucial to selectively quantify biomarkers or pathogens in clinical samples for the efficacious diagnosis and/or treatment of various diseases. To fulfil the demand of rapid, specific, economic, and easy detection of such biomolecules in ultralow amounts, numerous nanomaterials have been explored to effectively enhance the sensitivity, selectivity, and reproducibility of immunosensors. Graphene quantum dots (GQDs) have garnered tremendous attention in immunosensor development, owing to their special attributes such as large surface area, excellent biocompatibility, quantum confinement, edge effects, and abundant sites for chemical modification. Besides these distinct features, GQDs acquire peroxidase (POD)-mimicking electro-catalytic activity, and hence, they can replace horseradish peroxidase (HRP)-based systems to conduct facile, quick, and inexpensive label-free immunoassays. The chief motive of this review article is to summarize and focus on the recent advances in GQD-based electrochemical immunosensors for the early and rapid detection of cancer, cardiovascular disorders, and pathogenic diseases. Moreover, the underlying principles of electrochemical immunosensing techniques are also highlighted. These GQD immunosensors are ubiquitous in biomedical diagnosis and conducive for miniaturization, encouraging low-cost disease diagnostics in developing nations using point-of-care testing (POCT) and similar allusive techniques.