Graphene oxide surface blocking agents can increase the DNA biosensor sensitivity

Sensitivity-Enhancing Strategies of Graphene Field-Effect Transistor Biosensors for Biomarker Detection

The ultrasensitive recognition of biomarkers plays a crucial role in the precise diagnosis of diseases. Graphene-based field-effect transistors (GFET) are considered the most promising devices among the next generation of biosensors. GFET biosensors possess distinct advantages, including label-free, ease of integration and operation, and the ability to directly detect biomarkers in liquid environments. This review summarized recent advances in GFET biosensors for biomarker detection, with a focus on interface functionalization. Various sensitivity-enhancing strategies have been overviewed for GFET biosensors, from the perspective of optimizing graphene synthesis and transfer methods, refinement of surface functionalization strategies for the channel layer and gate electrode, design of biorecognition elements and reduction of nonspecific adsorption. Further, this review extensively explores GFET biosensors functionalized with antibodies, aptamers, and enzymes. It delves into sensitivity-enhancing strategies employed in the detection of biomarkers for various diseases (such as cancer, cardiovascular diseases, neurodegenerative disorders, infectious viruses, etc.) along with their application in integrated microfluidic systems. Finally, the issues and challenges in strategies for the modulation of biosensing interfaces are faced by GFET biosensors in detecting biomarkers.

Ultra-stable threose nucleic acid-based biosensors for rapid and sensitive nucleic acid detection and in vivo imaging

The human genome's nucleotide sequence variation, such as single nucleotide mutations, can cause numerous genetic diseases. However, detecting nucleic acids accurately and rapidly in complex biological samples remains a major challenge. While natural deoxyribonucleic acid (DNA) has been used as biorecognition probes, it has limitations like poor specificity, reproducibility, nuclease-induced enzymatic degradation, and reduced bioactivity on solid surfaces. To address these issues, we introduce a stable and reliable biosensor called graphene oxide (GO)- threose nucleic acid (TNA). It comprises chemically modified TNA capture probes on GO for detecting and imaging target nucleic acids in vitro and in vivo, distinguishing single nucleobase mismatches, and monitoring dynamic changes in target microRNA (miRNA). By loading TNA capture probes onto the GO substrate, the GO-TNA sensing platform for nucleic acid detection demonstrates a significant 88-fold improvement in the detection limit compared to TNA probes alone. This platform offers a straightforward preparation method without the need for costly and labor-intensive isolation procedures or complex chemical reactions, enabling real-time analysis. The stable TNA-based GO sensing nanoplatform holds promise for disease diagnosis, enabling rapid and accurate detection and imaging of various disease-related nucleic acid molecules at the in vivo level. STATEMENT OF SIGNIFICANCE: The study's significance lies in the development of the GO-TNA biosensor, which addresses limitations in nucleic acid detection. By utilizing chemically modified nucleic acid analogues, the biosensor offers improved reliability and specificity, distinguishing single nucleobase mismatches and avoiding false signals. Additionally, its ability to detect and image target nucleic acids in vivo facilitates studying disease mechanisms. The simplified preparation process enhances practicality and accessibility, enabling real-time analysis. The biosensor's potential applications extend beyond healthcare, contributing to environmental analysis and food safety. Overall, this study's findings have substantial implications for disease diagnosis, biomedical research, and diverse applications, advancing nucleic acid detection and its impact on various fields.

Challenges for Field-Effect-Transistor-Based Graphene Biosensors

Owing to its outstanding physical properties, graphene has attracted attention as a promising biosensor material. Field-effect-transistor (FET)-based biosensors are particularly promising because of their high sensitivity that is achieved through the high carrier mobility of graphene. However, graphene-FET biosensors have not yet reached widespread practical applications owing to several problems. In this review, the authors focus on graphene-FET biosensors and discuss their advantages, the challenges to their development, and the solutions to the challenges. The problem of Debye screening, in which the surface charges of the detection target are shielded and undetectable, can be solved by using small-molecule receptors and their deformations and by using enzyme reaction products. To address the complexity of sample components and the detection mechanisms of graphene-FET biosensors, the authors outline measures against nonspecific adsorption and the remaining problems related to the detection mechanism itself. The authors also introduce a solution with which the molecular species that can reach the sensor surfaces are limited. Finally, the authors present multifaceted approaches to the sensor surfaces that provide much information to corroborate the results of electrical measurements. The measures and solutions introduced bring us closer to the practical realization of stable biosensors utilizing the superior characteristics of graphene.

Toward the Commercialization of Carbon Nanotube Field Effect Transistor Biosensors

The development of biosensors based on field-effect transistors (FETs) using atomically thick carbon nanotubes (CNTs) as a channel material has the potential to revolutionize the related field due to their small size, high sensitivity, label-free detection, and real-time monitoring capabilities. Despite extensive research efforts to improve the sensitivity, selectivity, and practicality of CNT FET-based biosensors, their commercialization has not yet been achieved due to the non-uniform and unstable device performance, difficulties in their fabrication, the immaturity of sensor packaging processes, and a lack of reliable modification methods. This review article focuses on the practical applications of CNT-based FET biosensors for the detection of ultra-low concentrations of biologically relevant molecules. We discuss the various factors that affect the sensors' performance in terms of materials, device architecture, and sensor packaging, highlighting the need for a robust commercial process that prioritizes product performance. Additionally, we review recent advances in the application of CNT FET biosensors for the ultra-sensitive detection of various biomarkers. Finally, we examine the key obstacles that currently hinder the large-scale deployment of these biosensors, aiming to identify the challenges that must be addressed for the future industrialization of CNT FET sensors.

A Brief Review of Graphene-Based Biosensors Developed for Rapid Detection of COVID-19 Biomarkers

The prevalence of mutated species of COVID-19 antigens has provided a strong impetus for identifying a cost-effective, rapid and facile strategy for identifying the viral loads in public places. The ever-changing genetic make-up of SARS-CoV-2 posts a significant challenfge for the research community to identify a robust mechanism to target, bind and confirm the presence of a viral load before it spreads. Synthetic DNA constructs are a novel strategy to design complementary DNA sequences specific for antigens of interest as in this review's case SARS-CoV-2 antigens. Small molecules, complementary DNA and protein-DNA complexes have been known to target analytes in minimal concentrations. This phenomenon can be exploited by nanomaterials which have unique electronic properties such as ballistic conduction. Graphene is one such candidate for designing a device with a very low LOD in the order of zeptomolar and attomolar concentrations. Surface modification will be the significant aspect of the device which needs to have a high degree of sensitivity at the same time as providing a rapid signaling mechanism.

Fabrication and Functionalisation of Nanocarbon-Based Field-Effect Transistor Biosensors

Nanocarbon-based field-effect transistor (NC-FET) biosensors are at the forefront of future diagnostic technology. By integrating biological molecules with electrically conducting carbon-based platforms, high sensitivity real-time multiplexed sensing is possible. Combined with their small footprint, portability, ease of use, and label-free sensing mechanisms, NC-FETs are prime candidates for the rapidly expanding areas of point-of-care testing, environmental monitoring and biosensing as a whole. In this review we provide an overview of the basic operational mechanisms behind NC-FETs, synthesis and fabrication of FET devices, and developments in functionalisation strategies for biosensing applications.

Fluorescent Biosensors for the Detection of Viruses Using Graphene and Two-Dimensional Carbon Nanomaterials

Two-dimensional carbon nanomaterials have been commonly employed in the field of biosensors to improve their sensitivity/limits of detection and shorten the analysis time. These nanomaterials act as efficient transducers because of their unique characteristics, such as high surface area and optical, electrical, and magnetic properties, which in turn have been exploited to create simple, quick, and low-cost biosensing platforms. In this review, graphene and two-dimensional carbon material-based fluorescent biosensors are covered between 2010 and 2021, for the detection of different human viruses. This review specifically focuses on the new developments in graphene and two-dimensional carbon nanomaterials for fluorescent biosensing based on the Förster resonance energy transfer (FRET) mechanism. The high-efficiency quenching capability of graphene via the FRET mechanism enhances the fluorescent-based biosensors. The review provides a comprehensive reference for the different types of carbon nanomaterials employed for the detection of viruses such as Rotavirus, Ebola virus, Influenza virus H3N2, HIV, Hepatitis C virus (HCV), and Hepatitis B virus (HBV). This review covers the various multiplexing detection technologies as a new direction in the development of biosensing platforms for virus detection. At the end of the review, the different challenges in the use of fluorescent biosensors, as well as some insights into how to overcome them, are highlighted.

Binding Force and Site-Determined Desorption and Fragmentation of Antibiotic Resistance Genes from Metallic Nanomaterials

Interfacial interactions between antibiotic resistance genes (ARGs) and metallic nanomaterials (NMs) lead to adsorption and fragmentation of ARGs, which can provide new avenues for selecting NMs to control ARGs. This study compared the adsorptive interactions of ARGs (tetM-carrying plasmids) with two metallic NMs (ca. 20 nm), i.e., titanium dioxide (nTiO₂) and zero-valent iron (nZVI). nZVI had a higher adsorption rate (0.06 min^-1) and capacity (4.29 mg/g) for ARGs than nTiO₂ (0.05 min^-1 and 2.15 mg/g, respectively). No desorption of ARGs from either NMs was observed in the adsorptive background solution, isopropanol or urea solutions, but nZVI- and nTiO₂-adsorbed ARGs were effectively desorbed in NaOH and NaH₂PO₄ solutions, respectively. Molecular dynamics simulation revealed that nTiO₂ mainly bound with ARGs through electrostatic attraction, while nZVI bound with PO₄^3- of the ARG phosphate backbones through Fe-O-P coordination. The ARGs desorbed from nTiO₂ remained intact, while the desorbed ARGs from nZVI were splintered into small fragments irrelevant to DNA base composition or sequence location. The ARG removal by nZVI remained effective in the presence of PO₄^3-, natural organic matter, or protein at environmentally relevant concentrations and in surface water samples. These findings indicate that nZVI can be a promising nanomaterial to treat ARG pollution.

Extended gate field-effect-transistor for sensing cortisol stress hormone

Cortisol is a hormone released in response to stress and is a major glucocorticoid produced by adrenal glands. Here, we report a wearable sensory electronic chip using label-free detection, based on a platinum/graphene aptamer extended gate field effect transistor (EG-FET) for the recognition of cortisol in biological buffers within the Debye screening length. The device shows promising experimental features for real-time monitoring of the circadian rhythm of cortisol in human sweat. We report a hysteresis-free EG-FET with a voltage sensitivity of the order of 14 mV/decade and current sensitivity up to 80% over the four decades of cortisol concentration. The detection limit is 0.2 nM over a wide range, between 1 nM and 10 µM, of cortisol concentrations in physiological fluid, with negligible drift over time and high selectivity. The dynamic range fully covers those in human sweat. We propose a comprehensive analysis and a unified, predictive analytical mapping of current sensitivity in all regimes of operation.

Electrochemical DNA detection of hepatitis E virus genotype 3 using PbS quantum dot labelling

The aim of this study was to develop a highly specific electrochemical DNA sensor using functionalized lead sulphide (PbS) quantum dots for hepatitis E virus genotype 3 (HEV3) DNA target detection. Functionalized-PbS quantum dots (QDs) were used as an electrochemical label for the detection of HEV3-DNA target by the technique of square wave anodic stripping voltammetry (SWASV). The functionalized-PbS quantum dots were characterized by UV-vis, FTIR, XRD, TEM and zeta potential techniques. As-prepared, functionalized-PbS quantum dots have an average size of 4.15 ± 1.35 nm. The detection platform exhibited LOD and LOQ values of 1.23 fM and 2.11 fM, respectively. HEV3-DNA target spiked serum is also reported.Graphical abstract.

Toehold-regulated competitive assembly to accelerate the kinetics of graphene oxide-based biosensors

With effective adsorption and quenching efficiency, graphene oxide (GO) can be utilized for sensing biomolecules such as nucleic acids and proteins. In these assays, the fluorophore-labeled nucleic acid (reporter) is usually adsorbed by GO first, followed by adding the target molecules to bind the reporter, thus restoring the fluorescence signal. However, the kinetics of fluorescence recovery is usually very slow because the target is probably adsorbed by GO and compromises the binding of the target and the reporter. Herein, we proposed a toehold-regulated strand displacement strategy to accelerate the kinetics of GO-based biosensing. In this strategy, the toehold of the duplex mediated a competitive assembly with the aim of eliminating the adsorption of the target by GO, facilitating the binding of the target and the reporter. While the duplex with the toehold of the target-blocker DNA or reporter-blocker DNA was formed, the rigid structure of the duplex weakened the adsorption of the target by GO and enhanced the recognition of the target by the reporter. This strategy achieved up to 2.6-fold enhancement in fluorescence signal restoration for nucleic acid detection, while there was 3.2-fold enhancement in fluorescence signal restoration for thrombin detection. It has also been demonstrated that this strategy can be used for the determination of DNA and thrombin in diluted serum with excellent specificity.

Oligonucleotide Detection and Optical Measurement with Graphene Oxide in the Presence of Bovine Serum Albumin Enabled by Use of Surfactants and Salts

As graphene oxide-based oligonucleotide biosensors improve, there is a growing need to explore their ability to retain high sensitivity for low target concentrations in the context of biological fluids. Therefore, we innovatively combined assay milieu factors that could influence the key performance parameters of DNA hybridization and graphene oxide (GO) colloid dispersion, verifying their suitability to enhance oligonucleotide–GO interactions and biosensor performance. As a model system, we tested single-strand (ss) DNA detection in a complex solution containing bovine serum albumin (BSA) and salts with surfactants. A fluorescein conjugated 30-mer oligonucleotide ssDNA probe was combined with its complementary cDNA target, together with solute dispersed GO and either non-ionic (Triton X-100 and Tween-20) or anionic sodium dodecyl sulfate (SDS) surfactants. In this context, we compared the effect of divalent Mg2+ or monovalent Na+ salts on GO binding for the quench-based detection of specific target–probe DNA hybridization. GO biosensor strategies for quench-based DNA detection include a “turn on” enhancement of fluorescence upon target–probe interaction versus a “turn off” decreased fluorescence for the GO-bound probe. We found that the sensitive and specific detection of low concentrations of oligonucleotide target was best achieved using a strategy that involved target–probe DNA hybridization in the solution with a subsequent modified “turn-off” GO capture and the quenching of the unhybridized probe. Using carefully formulated assay procedures that prevented GO aggregation, the preferential binding and quenching of the unhybridized probe were both achieved using 0.1% BSA, 0.065% SDS and 6 mM NaCl. This resulted in the sensitive measurement of the specific target–probe complexes remaining in the solution. The fluorescein-conjugated single stranded probe (FAM–ssDNA) exhibited linearity to cDNA hybridization with concentrations in the range of 1–8 nM, with a limit of detection equivalent to 0.1 pmoles of target in 100 µL of assay mix. We highlight a general approach that may be adopted for oligonucleotide target detection within complex solutions.

Investigating Adsorption/Desorption of DNA on ZIF-8 Surface by Fluorescently Labeled Oligonucleotides

As an important subclass of MOFs, ZIF-8, built from 2-methylimidazole and Zn(NO₃)₂·6H₂O, possesses excellent biocompatibility and high stability in aqueous solution. Recently, it has been found that ZIF-8 can efficiently adsorb DNA and quench the adsorbed fluorophores to a large extent. These properties make it possible to prepare DNA-based optical sensors using ZIF-8. Although practical analytical applications are being demonstrated, the basic understanding of the binding between ZIF-8 and DNA in solution has received relatively little attention. In this work, we report that the adsorption of 12-, 18-, 24-, and 36-mer single-stranded DNAs on ZIF-8 are affected by several factors. It is found from the outcomes that shorter DNAs are adsorbed more rapidly to the surface of ZIF-8. On the other hand, desorption of the probe DNA can be achieved using complementary strand DNA to restore the fluorescence value. Furthermore, the salt contributes to adsorption to some extent. These findings are important for further understanding of the interactions between DNA and ZIF-8 and for the optimization of DNA and MOF-based devices and sensors.

Well-Optimized Conjugated GO-DNA Nanosystem for Sensitive Ratiometric pH Detection in Live Cells

Intracellular pH is a vital parameter which can reflect the physiological process, and the detection of intracellular pH with a high signal-to-noise ratio (SNR) remains a challenge. Compared to pH biosensors based on a single-wavelength signal, it is much easier to obtain better sensitivity and higher SNR from the biosensors by two-wavelength ratiometric signals. In this study, we used DNA-grafted graphene oxide (GO) to ratiometrically detect intracellular pH ranging from basic to acidic. A high SNR with a 35-fold difference in the ratiometric output has been achieved through careful optimization: (1) A high DNA conjugation yield of 45% has been gained through utilizing the partial double-stranded assembly strategy. (2) Herring sperm DNA (HSD) plays an important role in improving the sensitivity of the nanosystem by purifying and passivating the surface of GO; therefore, the concentration of HSD has been optimized to pursue the most sensitive ratiometric response. Apart from the ultrahigh SNR, fabricated GO-AR-Cy5/IFO-Cy3 exhibited excellent stability and biocompatibility in biological environments. Further experiments demonstrated that the nanosystem worked well in live cells in response to pH changes. It is possible to distinguish small pH differences and realize quantitative detection based on ratiometric fluorescence imaging by laser scanning confocal microscope analysis, which makes the nanosystem a promising candidate for further biological study and clinical applications.

Graphene-based biosensors for the detection of prostate cancer protein biomarkers: a review

Prostate cancer (PC) is the sixth most common cancer type in the world, which causes approximately 10% of total cancer fatalities. The detection of protein biomarkers in body fluids is the key topic for the diagnosis and prognosis of PC. Highly sensitive screening of PC is the most effective approach for reducing mortality. Thus, there are a growing number of literature that recognizes the importance of new technologies for early diagnosis of PC. Graphene is playing an important role in the biosensor field with remarkable physical, optical, electrochemical and magnetic properties. Many recent studies demonstrated the potential of graphene materials for sensitive detection of protein biomarkers. In this review, the graphene-based biosensors toward PC analysis are mainly discussed in two groups: Firstly, novel biosensor interfaces were constructed through the modification of graphene materials onto sensor surfaces. Secondly, ingenious signal amplification strategies were developed using graphene materials as catalysts or carriers. Graphene-based biosensors have exhibited remarkable performance with high sensitivities, wide detection ranges, and long-term stabilities.

An ultrasensitive and simple assay for the Hepatitis C virus using a reduced graphene oxide-assisted hybridization chain reaction

Hepatitis C virus (HCV) is a major cause of chronic liver disease, which affects 2-3% of the world population. Until now, the early detection of HCV has been a great challenge, especially for those who live in developing countries. In this study, we developed a novel and ultrasensitive assay for the detection of HCV RNA based on the reduced graphene oxide nanosheets (rGONS) and hybridization chain reaction (HCR) amplification technique. This detection system contains a pair of single fluorophore-labeled hairpin probes that can freely exist in the solution in the absence of target RNA. The introduction of target RNA can robustly trigger a HCR with the two probes and produce long nanowires containing a double-stranded structure. The weak adsorption to rGONS makes the long nanowires emit a strong fluorescence. Using this enzyme-free amplification strategy, we developed a new method for the HCV RNA assay with a detection limit of 10 fM, which is far more sensitive than the common GO-based fluorescence method. Furthermore, the new method exhibits high selectivity for the discrimination of perfectly complementary and mismatched sequences. Finally, the new method was successfully used as a HCV RNA assay in biological samples with a strong anti-interference capability in complicated environments. In summary, these remarkable characteristics of the new method highlight its potential use in a clinical sample primary screening.

Aptasensor for multiplex detection of antibiotics based on FRET strategy combined with aptamer/graphene oxide complex

The development of a multiplexed sensing platform is necessary for highly selective, sensitive, and rapid screening of specific antibiotics. In this study, we designed a novel multiplex aptasensor for antibiotics by fluorescence resonance energy transfer (FRET) strategy using DNase I-assisted cyclic enzymatic signal amplification (CESA) method combined with aptamer/graphene oxide complex. The aptamers specific for sulfadimethoxine, kanamycin, and ampicillin were conjugated with Cyanine 3 (Cy3), 6-Carboxyfluorescein (FAM), and Cyanine 5 (Cy5), respectively, and graphene oxide (GO) was adopted to quench the fluorescence of the three different fluorophores with the efficiencies of 94.36%, 93.94%, and 96.97% for Cy3, FAM, and Cy5, respectively. CESA method was used for sensitive detection, resulting in a 2.1-fold increased signal compared to those of unamplified method. The aptasensor rapidly detected antibiotics in solution with limit of detection of 1.997, 2.664, and 2.337 ng/mL for sulfadimethoxine, kanamycin, and ampicillin, respectively. In addition, antibiotics dissolved in milk were efficiently detected with similar sensitivities. Multiplexed detection test proved that the fluorescently modified aptamers could work separately from each other. The results indicate that the aptasensor offers high specificity for each antibiotic and enables simultaneous and multicolor sensing for rapid screening of multiple antibiotics at the same time.

A versatile fluorometric aptasensing scheme based on the use of a hybrid material composed of polypyrrole nanoparticles and DNA-silver nanoclusters: application to the determination of adenosine, thrombin, or interferon-gamma

The authors describe a versatile aptasensing scheme based on the use of polypyrrole nanoparticles (PPyNPs) and DNA-silver nanoclusters (DNA-AgNCs) for multiple target detection. The DNA-AgNCs consist of two functional domains, viz. (a) a nucleation domain for attaching the metal core of the nanoclusters, and (b) a recognition domain which consists of a single-stranded aptamer. In the absence of analytes, the single-strand recognition domain will be absorbed onto the surface of the PPyNPs through π stacking and hydrophobic interactions. As a result, the red fluorescence of the DNA-AgNCs (with excitation/emission peaks at 535/625 nm) is quenched by the PPyNPs. On introducing the analytes, the DNA-AgNCs will bind them. This leads to the desorption of DNA-AgNCs and the recovery of the red fluorescence. Based on the above strategy, a versatile, sensitive and selective aptasensor was established for detection of adenosine, thrombin and interferon-gamma. The method was applied to the detection of the above targets in (spiked) serum samples and gave satisfactory results, with detection limit of 0.58 nM for IFN-γ, 0.39 nM for adenosine, and 2.2 nM for thrombin. The use of PPyNPs results in uniquely low non-specific absorption and in improved analytical results in case of real-sample analysis when compared to previously reported methods. Graphical abstract Schematic illustration of DNA-silver nanoclusters and polypyrrole nanoparticles in an aptasensor for detection of multiple targets.

A common anchor facilitated GO-DNA nano-system for multiplex microRNA analysis in live cells

The design of a nano-system for the detection of intracellular microRNAs is challenging as it must fulfill complex requirements, i.e., it must have a high sensitivity to determine the dynamic expression level, a good reliability for multiplex and simultaneous detection, and a satisfactory biostability to work in biological environments. Instead of employing a commonly used physisorption or a full-conjugation strategy, here, a GO-DNA nano-system was developed under graft/base-pairing construction. The common anchor sequence was chemically grafted to GO to base-pair with various microRNA probes; and the hybridization with miRNAs drives the dyes on the probes to leave away from GO, resulting in "turned-on" fluorescence. This strategy not only simplifies the synthesis but also efficiently balances the loading yields of different probes. Moreover, the conjugation yield of GO with a base-paired hybrid has been improved by more than two-fold compared to that of the conjugation with a single strand. We demonstrated that base-paired DNA probes could be efficiently delivered into cells along with GO and are properly stabilized by the conjugated anchor sequence. The resultant GO-DNA nano-system exhibited high stability in a complex biological environment and good resistance to nucleases, and was able to accurately discriminate various miRNAs without cross-reaction. With all of these positive features, the GO-DNA nano-system can simultaneously detect three miRNAs and monitor their dynamic expression levels.

Selection and characterization of DNA aptamers for detection of glutamate dehydrogenase from Clostridium difficile

Rapid and accurate diagnosis of Clostridium difficile infections (CDI) is crucial for patient treatment, infection control and epidemiological monitoring. As an important antigen, glutamate dehydrogenase (GDH) has been proposed as a preliminary screening test target for CDI. However, current assays based on GDH activity or GDH immunoassays have suboptimal sensitivity and specificity. Herein, we describe the selection and characterization of single-stranded DNA aptamers that specifically target GDH. After 10 rounds of selection, high-throughput sequencing was used to identify enriched aptamer candidates. Of 10 candidates, three aptamers for GDH were identified. Gel shift assays showed that these aptamers exhibited low nanomolar affinities. One aptamer was optimized based on structural analysis and further engineered into a structure-switching fluorescence signaling aptamer, wherein desorption from reduced graphene oxide (RGO) upon binding of GDH led to an increase in fluorescence emission. This method allowed for quantitative detection of GDH with a detection limit of 1 nM, providing great potential for its further application in CDI diagnosis.

pH-Responsive Graphene Oxide-DNA Nanosystem for Live Cell Imaging and Detection

The interaction between graphene oxide (GO) and DNA is very sensitive to the environment. For example, under acidic conditions, the affinity of GO for DNA is enhanced, weakening the capability of GO to distinguish DNAs with different conformations. This effect has impeded the development of sensitive pH biosensors based on GO-DNA nanosystems. In this work, we systematically studied the affinity between GO and i-motif forming oligonucleotides (IFOs) at different pH values and developed a herring sperm DNA (HSD) treatment method. Using this method, HSD occupies the surface of GO, compromising the attractive force of GO that is significantly enhanced under acidic conditions. As a result, the ability of GO to distinguish between "open" and "closed" IFOs is successfully generalized to a wider pH range. Finally, a pH-sensitive GO-IFO nanosystem was fabricated that showed excellent sensing ability both in vitro and for intracellular pH detection. Because the interaction between GO and DNA is the basis for constructing GO-DNA biosensors, the strategy developed in this work shows great potential to be applied in a variety of other GO-DNA sensing systems.

Editorial: Translating the advances of biosensors from bench to bedside

Biosensors have been found with numerous applications in many areas including genetic analysis, detection of infectious diseases, environmental monitoring and forensic analysis. We have witnessed rapid advances in this field, especially with the emergence of nanotechnology in the past decade.

Comparison of Graphene Oxide and Reduced Graphene Oxide for DNA Adsorption and Sensing

Fluorescently labeled DNA adsorbed on graphene oxide (GO) is a well-established sensing platform for detecting a diverse range of analytes. GO is a loosely defined material and its oxygen content may vary depending on the condition of preparation. Sometimes, a further reduction step is intentionally performed to decrease the oxygen content, and the resulting material is called reduced GO (rGO). In this study, DNA adsorption and desorption from GO and rGO is systematically compared. Under the same salt concentration, DNA adsorbs slightly faster with a 2.6-fold higher capacity on rGO. At the same time, DNA adsorbed on rGO is more resistant to desorption induced by temperature, pH, urea, and organic solvents. Various lengths and sequences of DNA probes have been tested. When its complementary DNA is added as a model target analyte, the rGO sample has a higher signal-to-background and signal-to-noise ratio, whereas the GO sample has a slightly higher absolute signal increase and faster signaling kinetics. DNAs adsorbed on GO or rGO are still susceptible to nonspecific displacement by other DNA and proteins. Overall, although rGO adsorbs DNA more tightly, it allows efficient DNA sensing with an extremely low background fluorescence signal.