Graphene fluorescence resonance energy transfer aptasensor for the thrombin detection

RNA extraction-free reduced graphene oxide-based RT-LAMP fluorescence assay for highly sensitive SARS-CoV-2 detection

Infectious diseases have always been a seriously endanger for human life and health. A rapid, accurate and ultra-sensitive virus nucleic acid detection is still a challenge to deal with infectious diseases. Here, a RNA extraction-free reduced graphene oxide-based reverse transcription-loop-mediated isothermal amplification (EF-G-RT-LAMP) fluorescence assay was developed to achieve high-throughput, rapid and ultra-sensitive SARS-CoV-2 RNA detection. The whole detection process only took ∼36 min. The EF-G-RT-LAMP assay achieves a detection limit of 0.6 copies μL-1 with a wide dynamic range of aM-pM. A large number (up to 384) of samples can be detected simultaneously. Simulated detection of the COVID-19 pseudovirus and clinical samples in nasopharyngeal swabs demonstrated a high-throughput, rapid and ultra-sensitive practical detection capability of the EF-G-RT-LAMP assay. The results proved that the assay would be used as a rapid, easy-to-implement approach for epidemiologic diagnosis and could be extended to other nucleic acid detections.

Modeling approach for Ti3C2 MXene-based fluorescent aptasensor for amoxicillin biosensing in water matrices

Amoxicillin, a member of the penicillin family, is primarily utilized for the treatment of various bacterial infections affecting ears, nose, throat, urinary tract, and skin. Given its widespread application in medicine, agriculture, environment, and food industry, the precise and sensitive detection of amoxicillin is important. This study introduces a novel approach to developing a sensitive and selective fluorescent aptasensor relying on fluorescence resonance energy transfer (FRET) for the specific detection of amoxicillin. The carboxyfluorescein-labeled aptamer serves as a energy donor, while MXene functions as an energy acceptor, and acting as a quencher. To achieve optimal detection efficiency, a dual optimization strategy utilizing RSM-CCD and ANN-GA was used to fine-tune experimental conditions. The fluorescence measurements revealed an expansive linear range extending from 100 to 2400 ng mL-1, accompanied by an exceptionally low detection limit of 1.53 ng mL-1. Additionally, it shows an excellent selectivity towards amoxicillin over other antibiotics commonly found in water matrices. The aptasensor demonstrates good stability and reproducibility; effectiveness of the aptasensor was validated by testing in real water samples. This remarkable sensitivity and broad dynamic range affirm the efficacy aptasensor in accurately detecting varying concentrations of amoxicillin in wastewater bodies.

Boronic Acid-Decorated Carbon Dot-Based Semiselective Multichannel Sensor Array for Cytokine Discrimination and Oral Cancer Diagnosis

Cytokines are essential components of the immune system and are recognized as significant biomarkers. However, detection of a single cytokine is not precise and reliable enough to satisfy the requirements for diagnosis. Herein, we developed a pattern recognition-based method for the multiplexed sensing of cytokines, which involves three-color-emitting boronic acid-decorated carbon dots (BCDs) and arginine-modified titanium carbide (Ti3C2 MXenes) as the sensor array. Initially, the fluorescence signals of the three BCDs were quenched by Ti3C2 MXenes. In the presence of cytokines, the fluorescence intensity of the BCDs was restored or further quenched by different cytokines. The fluorescence response occurs in two steps: first, boronic acid interacts with cis-diol functional groups of cytokines, and second, arginine headgroup selectively interacts with glycans. By exploiting the different competing binding of the BCDs and the cytokines toward Ti3C2 MXenes, seven cytokines and their mixtures can be effectively discriminated at a concentration of 20 ng mL-1. Furthermore, our sensor array demonstrated an excellent performance in classifying human oral cancer saliva samples from healthy individuals with clinically relevant specificity. The noninvasive method offers a rapid approach to cytokine analysis, benefiting early and timely clinical diagnosis and treatment.

Bibliometric analysis of aptamer-conjugated nanoparticles for diagnosis in the last two decades

Objective.Aptamer-conjugated nanoparticles for diagnosis have recently gained increasing attention. Here, we performed a bibliometric analysis to provide an overview of this field over the past two decades.Methods. The terms 'aptamer, nanoparticles and diagnosis' were used to search for relevant original articles published in English from 2003 to 2022 in the Web of Science database. VOSviewer and CiteSpace software were employed to analyze the development process, knowledge structure, research hotspots, and potential trends in the field of aptamer-conjugated nanoparticles for diagnosis.Results. A total of 1076 original articles were retrieved, with a rapid increase in the annual output and citation. The journal 'Biosensors and Bioelectronics' has contributed the most in this field, and the most influential researcher, institution and country were Weihong Tan, the Chinese Academy of Sciences, China, respectively. Gold nanoparticles and quantum dots were the most used, but in the past three years, research hotspots focused on carbon dots and graphene quantum dots. Diagnostic directions primarily focused on cancer. The most used strategy was label-free electrochemical detection, but in the past two years, colorimetric analysis and fluorescence imaging emerged as hot topics.Conclusion.The bibliometric analysis reveals a rapid increase in the research on aptamer-conjugated nanoparticles for diagnosis, major contributors at the levels of journals, authors, institutions, and countries, and research preferences in diagnostic objects, nanoparticle types, and detection methods, as well as the evolution of research hotspots and future trends.

An electrochemical aptasensor for detection of prostate-specific antigen-based on carbon quantum dots-gold nanoparticles

In this work, an electrochemical aptasensor was described for the determination of prostate-specific antigen (PSA). Aptamer chains were decorated on the surface of a glassy carbon electrode (GCE) via carbon quantum dots/Au nanoparticles (Au/CQD). Structural analysis that was used to characterize the prepared materials shows that Au/CQD nanoparticles synthesized in a spherical shape with an average size of 70 nm. Furthermore, the combination of Au nanoparticles with CQD resulted in formation of crystalline the structure of the Au/CQD composite. To study the electrochemical performance of the prepared aptasensor, cyclic voltammetry, square wave voltammetry, and electrochemical impedance spectroscopy were used. The results show that the aptasensor has a good selectivity to PSA over other biomaterials with the time optimized about 30 min. K₄ [Fe(CN)₆ ] was used as an electrochemical probe with the limit of detection about 2 fg⋅mL^-1 . To avoid the hazardous nature of K₄ [Fe(CN)₆ ], a label-based aptasensor was prepared using methylene blue as an electrochemical signal producer. They provide the capability of electrochemical detection in buffer phosphate solution with high sensitivity.

Sensitive electrochemical detection of A549 exosomes based on DNA/ferrocene-modified single-walled carbon nanotube complex

Exosome is an emerging tumor marker, whose concentration level can reflect the occurrence and development of tumors. The development of rapid and sensitive exosome detection platform is of great significance for early warning of cancer occurrence. Here, a strategy for electrochemical detection of A549-cell-derived exosomes was established based on DNA/ferrocene-modified single-walled carbon nanotube complex (DNA/SWCNT-Fc). DNA/SWCNT-Fc complexes function as a signal amplification platform to promote electron transfer between electrochemical signal molecules and electrodes, thereby improving sensitivity. At the same time, the exosomes can be attached to DNA/SWCNT-Fc nanocomposites via the established PO₄^3--Ti⁴⁺-PO₄^3- method. Moreover, the application of EGFR antibody, which can specifically capture A549 exosomes, could improve the accuracy of this sensing system. Under optimal experimental conditions, the biosensor showed good linear relationship between the peak current and the logarithm of exosomes concentration from 4.66 × 10⁶ to 9.32 × 10⁹ exosomes/mL with a detection limit of 9.38 × 10⁴ exosomes/mL. Furthermore, this strategy provides high selectivity for exosomes of different cancer cells, which can be applied to the detection of exosomes in serum samples. Thus, owing to its advantages of high sensitivity and good selectivity, this method provides a diversified platform for exosomes identification and has great potential in early diagnosis and biomedical applications.

An off-on fluorescence aptasensor for trace thrombin detection based on FRET between CdS QDs and AuNPs

An off-on fluorescence aptasensor was developed for trace thrombin detection based on fluorescence resonance energy transfer (FRET) between CdS QDs and gold nanoparticles (AuNPs). Using DNA pairwise hybridization of the aptamer to the complementary DNA (cDNA), the CdS QDs (energy donor) were tightly coupled to the AuNPs (energy acceptor), resulting in the occurrence of FRET and there was a dramatic fluorescence quenching of CdS QDs (turn off). When the thrombin was added to the fluorescence aptasensor, the specific binding of the aptamer to the target formed a G-quadruplex that caused the AuNPs receptor to detach and the DNA duplex to be disassembled. The process would inhibit the FRET which contribute to the recovery of fluorescence (turn on) and an "off-on" fluorescence aptasensor for thrombin detection was constructed accordingly. Under optimal conditions, the fluorescence recovery showed good linearity with the concentration of thrombin in the range of 1.35-54.0 nmol L-1, and the detection limit was 0.38 nmol L-1 (S/N = 3, n = 9). Importantly, the fluorescence aptasensor presented excellent specificity for thrombin, and was successfully applied to the quantitative determination of thrombin in real serum with satisfactory recoveries of 98.60-102.2%.

Comparison of Optical and Electrical Sensor Characteristics for Efficient Analysis of Attachment and Detachment of Aptamer

Nucleic acid aptamer-based research has focused on achieving the highest performance for bioassays. However, there are limitations in evaluating the affinity for the target analytes in these nucleic acid aptamer-based bioassays. In this study, we mainly propose graphene oxide (GO)-based electrical and optical analyses to efficiently evaluate the affinity between an aptamer and its target. We found that an aptamer-coupled GO-based chip with an electrical resistance induced by a field-effect transistor, with aptamers as low as 100 pM, can detect the target, thrombin, at yields as low as 250 pM within five minutes. In the optical approach, the fluorescent dye-linked aptamer, as low as 100 nM, was efficiently used with GO, enabling the sensitive detection of thrombin at yields as low as 5 nM. The cantilever type of mechanical analysis also demonstrated the intuitive aptamer-thrombin reaction in the signal using dBm units. Finally, a comparison of electrical and optical sensors' characteristics was introduced in the attachment and detachment of aptamer to propose an efficient analysis that can be utilized for various aptamer-based research fields.

Optical and Electrochemical Aptasensors Developed for the Detection of Alpha-Fetoprotein

Early diagnosis of hepatocellular carcinoma (HCC), a leading cause of cancer mortality, is decisive for successful treatment of this type of cancer and increasing the patients' survival rate. Alpha-fetoprotein (AFP) is a glycoprotein that has been currently employed as a potential serological biomarker for determination of HCC and several other cancers. Achieving highly sensitive and specific detection of this biomarker is an effective strategy to inhibit developing issues caused by the cancer. Though, traditional procedures cannot meet the requirements due to the technical drawbacks. Recently, growing number of aptamer-based biosensors (aptasensors) attracted important attention as superior diagnostic tools because of their unique properties such as high stability, target versatility and remarkable affinity and selectivity. Nanomaterials, which broadly employed in the structure of these aptasensors, can considerably enhance the detection limit and sensitivity of analytes determination. Therefore, this review selectively investigated the recent progresses in several different optical and electrochemical aptasensors and nano-aptasensors designed for AFP assay.

Spectral analysis on the acceptor concentration-dependent fluorescence resonance energy transfer process in CuInS2@ZnS-SQ complexes

Owing to the broad spectral response and flexible choices of donors and acceptors, fluorescence resonance energy transfer (FRET) system based on quantum dots (QDs) is a potential candidate for enhancing performance of solar cells and other optoelectronic devices. Thus it is necessary to develop such FRET systems with high efficiency and understand the involved photophysical dynamics. Here, with type I CuInS₂@ZnS core-shell quantum dots as the energy donor, series of CuInS₂@ZnS-SQ complexes are synthesized by adjusting the acceptor (squaric acid, SQ) concentration. The FRET dynamics of the samples is systematically investigated by virtue of steady-state emission, time-resolved fluorescence decay, and transient absorption measurements. The experimental results display a positive correlation between the energy transfer efficient (η). The best energy transfer efficient achieved from experimental data is 52%. This work provides better understanding of the photophysical dynamics in similar complexes and facilitates further development of new photoelectronic devices based on relevant FRET systems.

Recent Progress in Microneedles-Mediated Diagnosis, Therapy, and Theranostic Systems

Theranostic system combined diagnostic and therapeutic modalities is critical for the real-time monitoring of disease-related biomarkers and personalized therapy. Microneedles, as a multifunctional platform, are promising for transdermal diagnostics and drug delivery. They have shown attractive properties including painless skin penetration, easy self-administration, prominent therapeutic effects, and good biosafety. Herein, an overview of the microneedles-based diagnosis, therapies, and theranostic systems is given. Four microneedles-based detection methods are concluded based on the sensing mechanism: i) electrochemistry, ii) fluorometric, iii) colorimetric, and iv) Raman methods. Additionally, robust microneedles are suitable for implantable drug delivery. Microneedles-assisted transdermal drug delivery can be primarily classified as passive, active, and responsive drug release, based on the release mechanisms. Microneedles-assisted oral and implantable drug delivery mechanisms are also presented in this review. Furthermore, the key frontier developments in microneedles-mediated theranostic systems as the major selling points are emphasized in this review. These systems are classified into open-loop and closed-loop theranostic systems based on the indirectness and directness of feedback between the transdermal diagnosis and therapy, respectively. Finally, conclusions and future perspectives for next-generation microneedles-mediated theranostic systems are also discussed. Taken together, microneedle-based systems are promising as the new avenue for diagnosis, therapy, and disease-specific closed-loop theranostic applications.

Applications of graphene-based electrochemical and optical biosensors in early detection of cancer biomarkers

Graphene is a one-atom-thick carbon compound, which holds promises for detecting cancer biomarkers along with its derivatives. The atom-wide graphene layer is ideal for cancer biomarker detection due to its unique physicochemical properties like increased electrical and thermal conductivity, optical transparency, and enhanced chemical and mechanical strength. The scientific aim of any biosensor is to create a smaller and portable point of care device for easy and early cancer detection; graphene is able to live up to that. Apart from tumour detection, graphene-based biosensors can diagnose many diseases, their biomarkers, and pathogens. Many existing remarkable pieces of research have proven the candidacy of nanoparticles in most cancer biomarkers detection. This article discusses the effectiveness of graphene-based biosensors in different cancer biomarker detection. This article provides a detailed review of graphene and its derivatives that can be used to detect cancer biomarkers with high specificity, sensitivity, and selectivity. We have highlighted the synthesis procedures of graphene and its products and also discussed their significant properties. Furthermore, we provided a detailed overview of the recent studies on cancer biomarker detection using graphene-based biosensors. The different paths to create and modify graphene surfaces for sensory applications have also been highlighted in each section. Finally, we concluded the review by discussing the existing challenges of these biosensors and also highlighted the steps that can be taken to overcome them.

Regulation of the Structure of Zirconium-Based Porphyrinic Metal-Organic Framework as Highly Electrochemiluminescence Sensing Platform for Thrombin

An electrochemiluminescence (ECL) sensor provides a sensitive and convenient method for early diagnosis of diseases; however, it is still a challenge to develop simple and sensitive sensing platforms based on efficient ECL signals and luminophore groups. Porphyrin-based metal-organic frameworks (MOFs) show great potential in ECL sensing; however, the mechanism and structure-activity relationship, as well as application, are rarely reported. Herein, hydrothermal reactions obtained porphyrin Zr-MOFs (PCN-222) with different specific surface areas, pore sizes, structures, and surface charge states by tuning the reaction time were developed, which served both as the ECL luminophore, coreaction promoter for S₂O₈^2-, and a connection in the ECL immunoassay. By progressively controlling the condition of the hydrothermal reaction, PCN-222 with large surface area-abundant micropores can be obtained, which has good conductivity and positively charged surfaces, obtaining excellent ECL performance. The ECL performance and the enhancement mechanism were investigated in detail. Using PCN-222-6h with the best ECL intensity as the immobilization matrix for the aptamer, a highly sensitive and selective assay for thrombin was developed. The decrease of the ECL signal was logarithmically linear with the concentration of thrombin in the range from 50 fg mL^-1 to 100 pg mL^-1 with a low detection limit of 2.48 fg/mL. This proposed strategy provides a brand new approach for tuning of the structures of MOFs as effective ECL signal probes, thus providing wider possibilities for effective ECL immunoassays in the detection of other biomarkers in diagnosis of diseases.

Rapid electrochemical detection of COVID-19 genomic sequence with dual-function graphene nanocolloids based biosensor

Discovered in December 2019, the Severe Acute Respiratory Syndrome Coronavirus 2 (aka SARS-CoV-2 or 2019-nCoV) has attracted worldwide attention and concerns due to its high transmissibility and the severe health consequences experienced upon its infection, particularly by elderly people. Over 329 million people have been infected till date and over 5.5 million people could not survive the respiratory illness known as COVID-19 syndrome. Rapid and low-cost detection methods are of utmost importance to monitor the diffusion of the virus and to aid in the global fight against the pandemic.

We propose here the use of graphene oxide nanocolloids (GONC) as an electroactive nanocarbon material that can act simultaneously as a transducing platform as well as the electroactive label for the detection of 2019-nCoV genomic sequences. The ability of GONC to provide an intrinsic electrochemical signal arising from the reduction of the electrochemically reducible oxygen functionalities present on its surface, allows GONC to be used as a simple and sensitive biosensing platform. Different intrinsic electroactivity of the material was obtained at each step of the genosensing process, starting from the immobilization of a short-stranded DNA probe and followed by the incubation with different concentrations of the target 2019-nCoV DNA strand. Monitoring such variations enabled the quantification of the target analyte over a wide dynamic range between 10⁻¹⁰ and 10⁻⁵ M.

All in all, this proof-of-concept system serves as a stepping stone for the development of a rapid, sensitive and selective analytical tool for the detection of 2019-nCoV as well as other similar viral vectors. The use of cost-effective electrochemical detection methods coupled with the vast availability and suitability of carbon-based nanomaterials make this sensing system a valid candidate for low-cost and point-of-care analysis.

Detection of small molecules by fluorescence intensity using single dye labeled aptamers and quencher transition metal ions

We describe herein an aptamer-based sensing approach that signal the presence of small-molecule targets when fluorescent DNA probes are challenged with the Ni²⁺ or Co²⁺ quencher metal ions. Functional oligonucleotides targeting L-tyrosinamide (L-Tym), adenosine (Ade) or cocaine (Coc) were end-labeled by the Texas-Red fluorophore. A fluorescence quenching occurred upon association of these transition metal ions with the free conjugates. The formation of the target-probe complex, by the way of variations in the overall binding of quencher metal ions along the DNA strands, led to a partial restoration (for the Ade and Coc systems) or a further attenuation (for the L-Tym system) of the fluorescence intensity. The absolute signal gain varied from 40 to 180% depending on the target-probe pair investigated. The approach was also used to detect the compound Ade in a spiked biological matrix in 1 min or less. The transition metal ion-based quenching strategy is characterized by its very simple implementation, low cost, and rapid signaling.

Fluorogenic "on-off" nanosensor based on dual-quenching effect for imaging intracellular metabolite of various microalgae

Sensing intracellular compounds such as ATP in living microalgal cells is of great importance in diverse fields. To achieve this, nanosensing platform composed of graphene oxide (GO) and ATP aptamer (APT) was applied to diverse microalgal cells (Chlamydomonas reinhardtii, Chlorella vulgaris, Anabaena flos-aquae, and Ochromonas danica). The nanosized GO was characterized with atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The nanosensing platform (GO-APT) was prepared by attaching fluorophore-labeled APT to GO. GO-APT was applicable to only cell wall-deficient species (O. danica and mutant strains of C. reinhardtii) and the existence of flagella did not affect the uptake of the GO-APT by the cells. These results indicate that the cell wall is the primary barrier of GO-APT internalization for sensing application. To reduce the background fluorescence signal elicited by nonspecific displacement of the fluorophore-labeled probe, APT was modified as molecular beacon (MB) type (APTMB). Owing to the double quenching effect (by GO and quencher-labeled complementary sequence), the background signal significantly reduced. Cytotoxicity of GO-APTMB on the microalgal species was also tested. The application of GO-APTMB had no effect on the growth of microalgae. Given that diverse aptamer sequences had been screened, the sensing platform is not limited for detecting ATP only, but also can be applied to other metabolite imaging by simply changing the aptamer sequences. Our research will contribute to broadening the application of GO and aptamer beacon complex for intracellular metabolite imaging and detecting.

A Simple and Ultrasensitive Colorimetric Biosensor for Anatoxin-a Based on Aptamer and Gold Nanoparticles

Here, we designed a simple, rapid, and ultrasensitive colorimetric aptasensor for detecting anatoxin-a (ATX-a). The sensor employs a DNA aptamer as the sensing element and gold nanoparticles (AuNPs) as probes. Adsorption of the aptamer onto the AuNP surface can protect AuNPs from aggregation in NaCl solution, thus maintaining their dispersion state. In the presence of ATX-a, the specific binding of the aptamer with ATX-a results in a conformational change in the aptamer, which facilitates AuNP aggregation and, consequently, a color change of AuNPs from red to blue in NaCl solution. This color variation is directly associated with ATX-a concentration and can be easily measured using a UV/Vis spectrophotometer. The absorbance variation is linearly proportional to ATX-a concentration across the concentration range of 10 pM to 200 nM, with a detection limit of 4.45 pM and high selectivity against other interferents. This strategy was successfully applied to the detection of ATX-a in lake water samples. Thus, the present aptasensor is a promising alternative method for the rapid detection of ATX-a in the environment.

Target-Activating and Toehold Displacement Ag NCs/GO Biosensor-Mediating Signal Shift and Enhancement for Simultaneous Multiple Detection

Herein, we demonstrate that a new multicolor silver nanoclusters/graphene oxide (Ag NCs/GO) hybrid material, upon target response, undergoes a configuration transformation, based on entropy-driven enzyme-free toehold-mediated strand displacement reaction, achieving emission shift and enhancement. To realize the aim above, two different synthesis routes (route I and II) of synthesizing fluorescent Ag NCs for constructing toehold displacement Ag NCs/GO biosensor is designed and performed. Influenza A virus subtype genes (H1N1 and H5N1) as a model can efficiently initiate the operation of entropy-driven displacement reaction, resulting in activatable fluorescence. Red-emitting and green-emitting Ag NCs tethering the complementary sequence of H1N1 (pDNA1) and H5N1 (pDNA2) are indirectly immobilized on GO surface through binding with capture DNA (cDNA1 and cDNA2), respectively, forming multicolor pDNA-Ag NCs/GO nanohybrid materials. However, they do not exhibit nearly fluorescence signals attributed to energy transfer from donor Ag NCs to acceptor GO. Upon adding targets H1N1 and H5N1 (tDNA1 and tDNA2), pDNA1-Ag NCs and pDNA2-Ag NCs detach from GO, based on toehold-mediated strand displacement reaction, which interferes the energy transfer and leads to significant fluorescence enhancement. More interestingly, the activatable process is accompanied by remarkable hypsochromic shift (19 nm) or bathochromic shift (21 nm) emission with quite high fluorescence recovery rates (823.35% and 693.62%). Therefore, based on these phenomena, a novel multiple approach has been developed with the assistance of toehold displacement and Ag NCs/GO nanohybrid materials. As for the remarkable emission recovery and multichannel signal, the proposed approach displays the promising application prospect in accurate diagnosis and treatment.

A Fluorescence Kinetic-Based Aptasensor Employing Stilbene Isomerization for Detection of Thrombin

It is important to detect thrombin due to its physiological and pathological roles, where rapid and simple analytical approaches are needed. In this study, an aptasensor based on fluorescence attenuation kinetics for the detection of thrombin is presented, which incorporates the features of stilbene and aptamer. We designed and synthesized an aptasensor by one-step coupling of stilbene compound and aptamer, which employed the adaptive binding of the aptamer with thrombin to cause a change in stilbene fluorescence attenuation kinetics. The sensor realized detection of thrombin by monitoring the variation in apparent fluorescence attenuation rate constant (k_app), which could be further used for probing of enzyme–aptamer binding. In comprehensive studies, the developed aptasensor presented satisfactory performance on repeatability, specificity, and regeneration capacity, which realized rapid sensing (10 s) with a limit of detection (LOD) of 0.205 μM. The strategy was successful across seven variants of thrombin aptasensors, with tunable k_app depending on the SITS (4-Acetamido-4′-isothiocyanato-2,2′-stilbenedisulfonic acid disodium salt hydrate) grafting site. Analyte detection mode was demonstrated in diluted serum, requiring no separation or washing steps. The new sensing mode for thrombin detection paves a way for high-throughput kinetic-based sensors for exploiting aptamers targeted at clinically relevant proteins.

Graphene oxide-controlled neutral versus cationic form of a red emitting dye: enhancement of fluorescence by graphene oxide

Fluorescence enhancement of fluorophores in neat solvent media in the presence of graphene oxide (GO) is less known. It is necessary to re-examine the role of GO from the fundamental scientific viewpoint. Herein, we have reported GO controlled conversion from the neutral to cationic form of a red emitting molecule. Besides this, the switching of the role of GO as an enhancer to a quencher of fluorescence depending on the concentration of GO in the presence of proton accepting solvent media was established. Intermolecular proton transfer from the GO surface to fluorophores is responsible for this phenomenon.

Three-Dimensional Quantitative Intracellular Visualization of Graphene Oxide Nanoparticles by Tomographic Flow Cytometry

Interaction of nanoparticles (NPs) with cells is of fundamental importance in biology and biomedical sciences. NPs can be taken up by cells, thus interacting with their intracellular elements, modifying the life cycle pathways, and possibly inducing death. Therefore, there is a great interest in understanding and visualizing the process of cellular uptake itself or even secondary effects, for example, toxicity. Nowadays, no method is reported yet in which 3D imaging of NPs distribution can be achieved for suspended cells in flow-cytometry. Here we show that, by means of label-free tomographic flow-cytometry, it is possible to obtain full 3D quantitative spatial distribution of nanographene oxide (nGO) inside each single flowing cell. This can allow the setting of a class of biomarkers that characterize the 3D spatial intracellular deployment of nGO or other NPs clusters, thus opening the route for quantitative descriptions to discover new insights in the realm of NP-cell interactions.

Graphene Oxide-Based Simple and Rapid Detection of Antibiotic Resistance Gene via Quantum Dot-Labeled Zinc Finger Proteins

With the increasing rise of antibiotic-resistant pathogens, a simple and rapid detection of antibiotic resistance gene (ARG) is crucial to mitigate the spreading of antibiotic resistance. DNA-binding zinc finger proteins (ZFPs) can be engineered to recognize specific double-stranded (ds) DNA sequences in ARG. Here, we designed a simple and rapid method to detect ARG in bacteria utilizing engineered ZFPs and 2D nanosheet graphene oxide (GO) as a sensing platform. Our approach relies on the on and off effect of fluorescence signal in the presence and absence of target ARG, respectively. By taking advantage of the unique quenching capability of GO due to its electronic property, quantum dot (QD)-labeled ZFPs are adsorbed onto the GO sheets, and their fluorescence signal is quenched by proximal GO sheets through fluorescence resonance energy transfer (FRET). In the presence of target DNA, ZFP binding to the target DNA induces dissociation from GO, thereby restoring the fluorescence signal. Our system detects target DNA through restoration of QD emission as the restored signal increases directly with target DNA concentrations. Engineered ZFPs were able to detect specific dsDNA of the tetracycline resistance gene tetM with high specificity after only 10 min incubation on our GO-based sensing system. Our sensing system employed one-step FRET-based ZFP and GO combined technology to enable rapid and quantitative detection of ARG, providing a limit of detection as low as 1 nM. This study demonstrated the application of GO in conjunction with engineered DNA-binding domains for the direct detection of dsDNA with great potential as a rapid and reliable screening and detecton method against the growing threat of antibiotic resistant bacteria.

Innovations in nanoscience for the sustainable development of food and agriculture with implications on health and environment

A rapid increase in world population is leading to the rise in global demand of food and agriculture (agri) products. Nanotechnology and its applications have emerged as one of the most pioneering and promising technology for transforming conventional food and agri industries, with the aim of sustainable farming, improving the food security, quality and safety which could revolutionize the food and agri industries. Current developments in nanotechnology have led to the new paths progressively and bringing the radical changes the way food is perceived throughout the farming, transportation, processing, packaging, storage, monitoring and consumption. This review brings the current updates on novel nanomaterials in food and agri industries. Emphasis is given on the importance of nanotechnological applications, offering complete food solutions from farm to fork; including nutraceutical and functional foods, improving bioavailability, efficiency, nutritional status, nano-additives, food texture, color, taste and packaging. Agricultural sector also witnessed several nano-based products, such as nano-fertilizer, nano-pesticide, nano growth promoters and many more for the development of sustainable farming and crop improvement. Despite of numerous advantages of nanotechnology, there are still toxicity challenges, safety concerns, which needs to be addressed and demands transformations in regulatory policies. Rapid development is projected to transform several foods and agri sectors, with rapid increase in market stake and investment. Government agencies, private research centers as well as academicians are also coming together to explore the benefits of nanotechnology to improve food scarcity in the coming years.

Graphene-on-Glass Preparation and Cleaning Methods Characterized by Single-Molecule DNA Origami Fluorescent Probes and Raman Spectroscopy

Graphene exhibits outstanding fluorescence quenching properties that can become useful for biophysics and biosensing applications, but it remains challenging to harness these advantages due to the complex transfer procedure of chemical vapor deposition-grown graphene to glass coverslips and the low yield of usable samples. Here, we screen 10 graphene-on-glass preparation methods and present an optimized protocol. To obtain the required quality for single-molecule and super-resolution imaging on graphene, we introduce a graphene screening method that avoids consuming the investigated sample. We apply DNA origami nanostructures to place fluorescent probes at a defined distance on top of graphene-on-glass coverslips. Subsequent fluorescence lifetime imaging directly reports on the graphene quality, as deviations from the expected fluorescence lifetime indicate imperfections. We compare the DNA origami probes with conventional techniques for graphene characterization, including light microscopy, atomic force microscopy, and Raman spectroscopy. For the latter, we observe a discrepancy between the graphene quality implied by Raman spectra in comparison to the quality probed by fluorescence lifetime quenching measured at the same position. We attribute this discrepancy to the difference in the effective area that is probed by Raman spectroscopy and fluorescence quenching. Moreover, we demonstrate the applicability of already screened and positively evaluated graphene for studying single-molecule conformational dynamics on a second DNA origami structure. Our results constitute the basis for graphene-based biophysics and super-resolution microscopy.

Aptamer Switches Regulated by Post-Transition/Transition Metal Ions

We introduced an aptamer switch design that relies on the ability of post-transition/transition metal ions to trigger, through their coordination to nucleobases, substantial DNA destabilization. In the absence of molecular target, the addition of one such metal ion to usual aptamer working solutions promotes the formation of an alternative, inert DNA state. Upon exposure to the cognate compound, the equilibrium is shifted towards the competent DNA form. The switching process was preferentially activated by metal ions of intermediate base over phosphate complexation preference (i.e. Pb²⁺ , Cd²⁺ ) and operated with diversely structured DNA molecules. This very simple aptamer switch scheme was applied to the detection of small organics using the fluorescence anisotropy readout mode. We envision that the approach could be adapted to a variety of signalling methods that report on changes in the surface charge density of DNA receptors.

Graphene-nucleic acid biointerface-engineered biosensors with tunable dynamic range

Programmed biosensors with tunable quantification range and sensitivity would greatly broaden their application in medical diagnosis, food safety and environmental analysis. Herein, we proposed a graphene-nucleic acid biointerface-engineered biosensor, allowing target molecules to be detected with adjustable dynamic ranges and sensitivities. The biosensors were programmed by simply tuning the poly A tail of aptamer probes. The tuning of the poly A tail would allow the interaction between aptamer probes and graphene oxide (GO) to be modulated, in turn programing the competitive binding processes of aptamer probes to target molecules and GO. The biosensors, termed affinity-tunable aptasensors (atAptasensors) could be easily tuned with different dynamic ranges by using aptamer probes with different tail lengths, and the dynamic range could be extended to be over 3 orders by a combined use of multiple aptamer probes. Remarkably, the specificity of aptamer probes could be increased by increasing the interaction between aptamer probes and GO. Reliability of atAptasensor for ATP detection was tested in serum and milk samples, and we also applied atAptasensor for culture-independent analysis of microorganism pollution.

A Cationic Surfactant-Decorated Liquid Crystal-Based Aptasensor for Label-Free Detection of Malathion Pesticides in Environmental Samples

We report a liquid crystal (LC)-based aptasensor for the detection of malathion using a cationic surfactant-decorated LC interface. In this method, LCs displayed dark optical images when in contact with aqueous cetyltrimethylammonium bromide (CTAB) solution due to the formation of a self-assembled CTAB monolayer at the aqueous/LC interface, which induced the homeotropic orientation of LCs. With the addition of malathion aptamer, the homeotropic orientation of LCs changed to a planar one due to the interactions between CTAB and the aptamer, resulting in a bright optical image. In the presence of malathion, the formation of aptamer-malathion complexes caused a conformational change of the aptamers, thereby weakening the interactions between CTAB and the aptamers. Therefore, CTAB is free to induce a homeotropic ordering of the LCs, which corresponds to a dark optical image. The developed sensor exhibited high specificity for malathion determination and a low detection limit of 0.465 nM was achieved. Moreover, the proposed biosensor was successfully applied to detect malathion in tap water, river water, and apple samples. The proposed LC-based aptasensor is a simple, rapid, and convenient platform for label-free monitoring of malathion in environmental samples.

Graphene, an Interesting Nanocarbon Allotrope for Biosensing Applications: Advances, Insights, and Prospects

Graphene, a relatively new two-dimensional (2D) nanomaterial, possesses unique structure (e.g. lighter, harder, and more flexible than steel) and tunable physicochemical (e.g. electronical, optical) properties with potentially wide eco-friendly and cost-effective usage in biosensing. Furthermore, graphene-related nanomaterials (e.g. graphene oxide, doped graphene, carbon nanotubes) have inculcated tremendous interest among scientists and industrials for the development of innovative biosensing platforms, such as arrays, sequencers and other nanooptical/biophotonic sensing systems (e.g. FET, FRET, CRET, GERS). Indeed, combinatorial functionalization approaches are constantly improving the overall properties of graphene, such as its sensitivity, stability, specificity, selectivity, and response for potential bioanalytical applications. These include real-time multiplex detection, tracking, qualitative, and quantitative characterization of molecules (i.e. analytes [H₂O₂, urea, nitrite, ATP or NADH]; ions [Hg²⁺, Pb²⁺, or Cu²⁺]; biomolecules (DNA, iRNA, peptides, proteins, vitamins or glucose; disease biomarkers such as genetic alterations in BRCA1, p53) and cells (cancer cells, stem cells, bacteria, or viruses). However, there is still a paucity of comparative reports that critically evaluate the relative toxicity of carbon nanoallotropes in humans. This manuscript comprehensively reviews the biosensing applications of graphene and its derivatives (i.e. GO and rGO). Prospects and challenges are also introduced.

Harnessing nanotechnology to expand the toolbox of chemical biology

Although nanotechnology often addresses biomedical needs, nanoscale tools can also facilitate broad biological discovery. Nanoscale delivery, imaging, biosensing, and bioreactor technologies may address unmet questions at the interface between chemistry and biology. Currently, many chemical biologists do not include nanomaterials in their toolbox, and few investigators develop nanomaterials in the context of chemical tools to answer biological questions. We reason that the two fields are ripe with opportunity for greater synergy. Nanotechnologies can expand the utility of chemical tools in the hands of chemical biologists, for example, through controlled delivery of reactive and/or toxic compounds or signal-binding events of small molecules in living systems. Conversely, chemical biologists can work with nanotechnologists to address challenging biological questions that are inaccessible to both communities. This Perspective aims to introduce the chemical biology community to nanotechnologies that may expand their methodologies while inspiring nanotechnologists to address questions relevant to chemical biology.

An ultrasensitive, homogeneous fluorescence quenching immunoassay integrating separation and detection of aflatoxin M1 based on magnetic graphene composites

A homogeneous fluorescence quenching immunoassay is described for simultaneous separation and detection of aflatoxin M₁ (AFM₁) in milk. The novel assay relies on monoclonal antibody (mAb) functionalized Fe₃O₄ decorated reduced-graphene oxide (rGO-Fe₃O₄-mAb) as both capture probe and energy acceptor, combined with tetramethylrhodamine cadaverine-labeled aflatoxin B₁ (AFB₁-TRCA) as the energy donor. In the assay, AFB₁-TRCA binds to rGO-Fe₃O₄-mAb in the absence of AFM₁, quenching the fluorescence of TRCA by resonance energy transfer. Significantly, the immunoassay integrates sample preparation and detection into a single step, by using magnetic graphene composites to avoid washing and centrifugation steps, and the assay can be completed within 10 min. Under optimized conditions, the visual and quantitative detection limits of the assay for AFM₁ were 50 and 3.8 ng L^-1, respectively, which were significantly lower than those obtained by fluorescence polarization immunoassay using the same immunoreagents. Owing to its operation and highly sensitivity, the proposed assay provides a powerful tool for the detection of AFM₁.

Graphene Oxide Derivatives and Their Nanohybrid Structures for Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Analysis of Small Molecules

Matrix-assisted laser desorption/ionization (MALDI) has been considered as one of the most powerful analytical tools for mass spectrometry (MS) analysis of large molecular weight compounds such as proteins, nucleic acids, and synthetic polymers thanks to its high sensitivity, high resolution, and compatibility with high-throughput analysis. Despite these advantages, MALDI cannot be applied to MS analysis of small molecular weight compounds (<500 Da) because of the matrix interference in low mass region. Therefore, numerous efforts have been devoted to solving this issue by using metal, semiconductor, and carbon nanomaterials for MALDI time-of-flight MS (MALDI-TOF-MS) analysis instead of organic matrices. Among those nanomaterials, graphene oxide (GO) is of particular interest considering its unique and highly tunable chemical structures composed of the segregated sp² carbon domains surrounded by sp³ carbon matrix. Chemical modification of GO can precisely tune its physicochemical properties, and it can be readily incorporated with other functional nanomaterials. In this review, the advances of GO derivatives and their nanohybrid structures as alternatives to organic matrices are summarized to demonstrate their potential and practical aspect for MALDI-TOF-MS analysis of small molecules.

Graphene/gold nanoparticle composites for ultrasensitive and versatile biomarker assay using single-particle inductively-coupled plasma/mass spectrometry

An ultrasensitive and versatile assay for biomarkers has been developed using graphene/gold nanoparticles (AuNPs) composites and single-particle inductively-coupled plasma/mass spectrometry (spICP-MS). Thrombin was chosen as a model biomarker for this study. AuNPs modified with thrombin aptamers were first non-selectively adsorbed onto the surface of graphene oxide (GO) to form GO/AuNPs composites. In the presence of thrombin, the AuNPs desorbed from the GO/AuNPs composites due to a conformation change of the thrombin aptamer after binding with thrombin. The desorbed AuNPs were proportional to the concentration of thrombin and could be quantified by spICP-MS. By counting the individual AuNPs in the spICP-MS measurement, the concentration of thrombin could be determined. This assay achieved an ultralow detection limit of 4.5 fM with a broad linear range from 10 fM to 100 pM. The method also showed excellent selectivity and reproducibility when a complex protein matrix was evaluated. Furthermore, the diversity and ready availability of ssDNA ligands make this method a versatile new technique for ultrasensitive detection of a wide variety of biomarkers in clinical diagnostics.

Ultrasensitive Fe3+ ion detection based on pH-insensitive fluorescent graphene nanosensors in strong acid and neutral media

Schematic illustration of the preparation of FRGO and the detection of Fe³⁺ ions in strong acid and neutral media.