Resonance energy transfer from a dye molecule to graphene

Observation of reversible and irreversible charge transfer processes in dye-monolayer graphene systems using Raman spectroscopy as a tool

Herein, we report the Raman spectroscopy of crystal violet (CV) and IR-780 Iodide molecules dispersed on the monolayer graphene film (MGF). In the CV-MGF system, the enhancement in the Raman scattering of CV molecules is observed irrespective of the location probed during the spectral measurements. This enhancement is due to the charge transfer from the MGF to CV molecules. However, in the case of the IR-780 Iodide - MGF system, the enhancement of Raman scattering of dye molecules or MGF is observed strongly depending upon the probed location. These observations indicate that the charge transfer is irreversible and reversible in the CV-MGF and IR-780 Iodide-MGF systems, respectively. Importantly, for the first time, this experimental study revealed that enhancing the Raman scattering of MGF is possible through the "chemical mechanism" with suitable dye molecules apart from the "electromagnetic mechanism" with plasmonic hot spots of the metal nanoparticles and photonic nanojets of single dielectric microparticles.

Expanding the range of graphene energy transfer with multilayer graphene

The interaction between single emitters and graphene in the context of energy transfer has attracted significant attention due to its potential applications in fields such as biophysics and super-resolution microscopy. In this study, we investigate the influence of the number of graphene layers on graphene energy transfer (GET) by placing single dye molecules at defined distances from monolayer, bilayer, and trilayer graphene substrates. We employ DNA origami nanostructures as chemical adapters to position the dye molecules precisely. Fluorescence lifetime measurements and analysis reveal an additive effect of graphene layers on the energy transfer rate extending the working range of GET up to distances of approximately 50-60 nm. Moreover, we show that switching a DNA pointer strand between two positions on a DNA origami nanostructure at a height of >28 nm above graphene is substantially better visualized with multilayer graphene substrates suggesting enhanced capabilities for applications such as biosensing and super-resolution microscopy for larger systems and distances. This study provides insights into the influence of graphene layers on energy transfer dynamics and offers new possibilities for exploiting graphene's unique properties in various nanotechnological applications.

Single-Molecule Fluorescence Probes Interactions between Photoactive Protein-Silver Nanowire Conjugate and Monolayer Graphene

In this work, we apply single-molecule fluorescence microscopy and spectroscopy to probe plasmon-enhanced fluorescence and Förster resonance energy transfer in a nanoscale assemblies. The structure where the interplay between these two processes was present consists of photoactive proteins conjugated with silver nanowires and deposited on a monolayer graphene. By comparing the results of continuous-wave and time-resolved fluorescence microscopy acquired for this structure with those obtained for the reference samples, where proteins were coupled with either a graphene monolayer or silver nanowires, we find clear indications of the interplay between plasmonic enhancement and the energy transfer to graphene. Namely, fluorescence intensities calculated for the structure, where proteins were coupled to graphene only, are less than for the structure playing the central role in this study, containing both silver nanowires and graphene. Conversely, decay times extracted for the latter are shorter compared to a protein-silver nanowire conjugate, pointing towards emergence of the energy transfer. Overall, the results show that monitoring the optical properties of single emitters in a precisely designed hybrid nanostructure provides an elegant way to probe even complex combination of interactions at the nanoscale.

Aptamer-Based fluorescent DNA biosensor in antibiotics detection

The inappropriate employment of antibiotics across diverse industries has engendered profound apprehensions concerning their cumulative presence within human bodies and food commodities. Consequently, many nations have instituted stringent measures limiting the admissible quantities of antibiotics in food items. Nonetheless, conventional techniques employed for antibiotic detection prove protracted and laborious, prompting a dire necessity for facile, expeditious, and uncomplicated detection methodologies. In this regard, aptamer-based fluorescent DNA biosensors (AFBs) have emerged as a sanguine panacea to surmount the limitations of traditional detection modalities. These ingenious biosensors harness the binding prowess of aptamers, singular strands of DNA/RNA, to selectively adhere to specific target antibiotics. Notably, the AFBs demonstrate unparalleled selectivity, affinity, and sensitivity in detecting antibiotics. This comprehensive review meticulously expounds upon the strides achieved in AFBs for antibiotic detection, particularly emphasizing the labeling modality and the innovative free-label approach. It also elucidates the design principles behind a diverse array of AFBs. Additionally, a succinct survey of signal amplification strategies deployed within these biosensors is provided. The central objective of this review is to apprise researchers from diverse disciplines of the contemporary trends in AFBs for antibiotic detection. By doing so, it aspires to instigate a concerted endeavor toward the development of heightened sensitivity and pioneering AFBs, thereby contributing to the perpetual advancement of antibiotic detection methodologies.

Optically active two-dimensional MoS2-based nanohybrids for various biosensing applications: A comprehensive review

Following the discovery of graphene, there has been a surge in exploring other two-dimensional (2D) nanocrystals, including MoS2. Over the past few decades, MoS2-based nanocrystals have shown great potential applications in biosensing, owing to their excellent physico-chemical properties. Unlike graphene, MoS2 shows layer-dependent finite band gaps (∼1.8 eV for a single layer and ∼1.2 for bulk) and relatively strong interaction with the electromagnetic spectrum. The tunability of the size, shape, and intrinsic properties, such as high optical absorption, electron mobility, mechanical strength and large surface area, of MoS2 nanocrystals, make them excellent alternative probe materials for preparing optical, photothermal, and electrical bio/immunosensors. In this review, we will provide insights into the rapid evolutions in bio/immunosensing applications based on MoS2 and its nanohybrids. We emphasized the various synthesis, characterization, and functionalization routes of 2D MoS2 nanosheets/nanoflakes. Finally, we discussed various fabrication techniques and the critical parameters, including the limit of detection (LOD), linear detection range, and sensitivity of the biosensors. In addition, the role of MoS2 in enhancing the performance of biosensors, the limitations associated with current biosensing technologies, future challenges, and clinical implications are addressed. The advantages/disadvantages of each biosensor technique are also summarized. Collectively, we believe that this review will encourage resolute researchers to follow up further with the state-of-the-art MoS2-based biosensing technology.

Synthesis, Solvatochromism and Fluorescence Quenching Studies of Naphthalene Diimide Dye by Nano graphene oxide

A naphthalene diimide dye with two side amine arm was prepared. Uv-Vis and fluorescence spectroscopic techniques are used for their photophysical and solvatochromic characteristics in different solvents. The Lippert-Mataga plot for naphthalene diimide demonstrated a negative linear dependence by increasing polarity. Results showed naphthalene diimide is more polar in the ground than in the excited state. A quenching study was conducted for interacting the naphthalene diimide as a fluorophore and graphene oxide as a quencher. Fluorescence quenching-based platforms in nanoscale have been used in sensing systems. Raman, FTIR, Uv-Vis and fluorescence spectroscopic techniques were used to study the quenching mechanism. The results indicated that graphene plays an effective quencher against the naphthalene diimide, with a quenching efficiency 91%. The Stern-Volmer analysis results show a mix of static and dynamic quenching mechanisms. The binding constant of the quencher-fluorophore and the number of binding sites have been reported. Thermodynamic parameters of their interaction were evaluated. The negative values of the Gibbs free energy confirm that the complexation process is spontaneous. Meanwhile, the positive entropy value confirms that the favorable pathway process.

Combining pMINFLUX, graphene energy transfer and DNA-PAINT for nanometer precise 3D super-resolution microscopy

3D super-resolution microscopy with nanometric resolution is a key to fully complement ultrastructural techniques with fluorescence imaging. Here, we achieve 3D super-resolution by combining the 2D localization of pMINFLUX with the axial information of graphene energy transfer (GET) and the single-molecule switching by DNA-PAINT. We demonstrate <2 nm localization precision in all 3 dimension with axial precision reaching below 0.3 nm. In 3D DNA-PAINT measurements, structural features, i.e., individual docking strands at distances of 3 nm, are directly resolved on DNA origami structures. pMINFLUX and GET represent a particular synergetic combination for super-resolution imaging near the surface such as for cell adhesion and membrane complexes as the information of each photon is used for both 2D and axial localization information. Furthermore, we introduce local PAINT (L-PAINT), in which DNA-PAINT imager strands are equipped with an additional binding sequence for local upconcentration improving signal-to-background ratio and imaging speed of local clusters. L-PAINT is demonstrated by imaging a triangular structure with 6 nm side lengths within seconds.

Optical Biosensor Based on Graphene and Its Derivatives for Detecting Biomolecules

Graphene and its derivatives show great potential for biosensing due to their extraordinary optical, electrical and physical properties. In particular, graphene and its derivatives have excellent optical properties such as broadband and tunable absorption, fluorescence bursts, and strong polarization-related effects. Optical biosensors based on graphene and its derivatives make nondestructive detection of biomolecules possible. The focus of this paper is to review the preparation of graphene and its derivatives, as well as recent advances in optical biosensors based on graphene and its derivatives. The working principle of face plasmon resonance (SPR), surface-enhanced Raman spectroscopy (SERS), fluorescence resonance energy transfer (FRET) and colorimetric sensors are summarized, and the advantages and disadvantages of graphene and its derivatives applicable to various types of sensors are analyzed, and the methods of surface functionalization of graphene and its derivatives are introduced; these optical biosensors can be used for the detection of a range of biomolecules such as single cells, cellular secretions, proteins, nucleic acids, and antigen-antibodies; these new high-performance optical sensors are capable of detecting changes in surface structure and biomolecular interactions with the advantages of ultra-fast detection, high sensitivity, label-free, specific recognition, and the ability to respond in real-time. Problems in the current stage of application are discussed, as well as future prospects for graphene and its biosensors. Achieving the applicability, reusability and low cost of novel optical biosensors for a variety of complex environments and achieving scale-up production, which still faces serious challenges.

One-Step Homogeneous Immunoassay for the Detection of Influenza Virus Using Switching Peptide and Graphene Quencher

One-step homogeneous immunoassay was developed for detecting influenza viruses A and B (Inf-A and Inf-B) using the switching peptide H2. As the fluorescence-labeled switching peptide dissociated from the binding pocket of detection antibodies, the fluorescence signal could be directly generated by the binding of Inf-A and Inf-B without washing (i.e., one-step immunoassay). For the one-step homogeneous immunoassay with detection antibodies in solution, graphene was labeled with the antibodies as a fluorescence quencher. To test the feasibility of the homogeneous one-step immunoassay, the stability of the antibody complex with the switching peptide was evaluated under different pH and salt conditions. The one-step homogeneous immunoassay with switching peptide was conducted using influenza virus antigens in phosphate-buffered saline and real samples with inactivated Inf-A and Inf-B spiked in serum. Finally, the one-step homogeneous immunoassay results were compared with those of commercially available lateral flow immunoassays.

Interfacial assembly of zinc(II) phthalocyanines on graphene oxide (GO): Stable "turn-off-on" nanoplatforms to detect G-quadruplexes (G4)

HYPOTHESIS

The aggregation of phthalocyanines (Pcs) enfeebles their suitability as G-quadruplex (G4) ligands over time. It is hypothesized that the interfacial assembly of Pcs on graphene oxide (GO) influences intermolecular interactions, thereby affecting their physicochemical properties and inducing stabilization of Pcs in solution. Hence, the stacking of Pcs on GO could be tuned to create nanosystems with the ability to detect G4 for longer periods through a slow release of Pcs.

EXPERIMENTS

Four cationic structurally-related zinc(II) phthalocyanines (ZnPc) were non-covalently assembled on GO by ultrasonic exfoliation. A comprehensive characterization of ZnPcs@GO was carried out by spectroscopic techniques and electron microscopy to understand the organization of ZnPcs on GO. The fluorescence of ZnPcs@GO was studied in the presence of G4 (T₂G₅T)₄ and duplex ds26 through spectrofluorimetric titrations and monitored along time.

FINDINGS

GO induced a re-organization of the ZnPcs mostly to J-aggregates and quenched their original fluorescence up to 98 % ("turn-off"). In general, ZnPcs@GO recovered their fluorescence ("turn-on") after the titrations and showed affinity to G4 (K_D up to 1.92 μM). This is the first report that highlights the contribution of GO interfaces to assemble ZnPcs and allow their slow and controlled release to detect G4 over longer periods.

Role of Förster Resonance Energy Transfer in Graphene-Based Nanomaterials for Sensing

Förster resonance energy transfer (FRET)-based fluorescence sensing of various target analytes has been of growing interest in the environmental, bioimaging, and diagnosis fields. Graphene-based zero- (0D) to two-dimensional (2D) nanomaterials, such as graphene quantum dots (GQDs), graphene oxide (GO), reduced graphene oxide (rGO), and graphdiyne (GD), can potentially be employed as donors/acceptors in FRET-based sensing approaches because of their unique electronic and photoluminescent properties. In this review, we discuss the basics of FRET, as well as the role of graphene-based nanomaterials (GQDs, GO, rGO, and GD) for sensing various analytes, including cations, amino acids, explosives, pesticides, biomolecules, bacteria, and viruses. In addition, the graphene-based nanomaterial sensing strategy could be applied in environmental sample analyses, and the reason for the lower detection ranges (micro- to pico-molar concentration) could also be explained in detail. Challenges and future directions for designing nanomaterials with a new sensing approach and better sensing performance will also be highlighted.

Composite substrate of graphene/Ag nanoparticles coupled with a multilayer film for surface-enhanced Raman scattering biosensing

In this paper, we designed a surface-enhanced Raman scattering (SERS) substrate for graphene/Ag nanoparticles (Ag NPs) bonded multilayer film (MLF) using the hybrid nanostructures composed of graphene and plasmonic metal components with significant plasmonic electrical effects and unique optical characteristics. This paper achieved the advantages of efficient utilization of electromagnetic field and reduction of fluorescence background based on the electromagnetic enhancement activity of Ag NPs and unique physical/chemical properties of graphene with zero gap structures. Au/Al₂O₃ was stacked periodically to construct MLF. As indicated by the electric field intensity at the Au/Al₂O₃ interface of the respective layer, bulk plasmon polariton (BPP) in the MLF was excited and coupled with localized surface plasmon (LSP) in the Ag NPs, which enhanced the electromagnetic field on the top-layer of SERS substrate. To measure the performance of the SERS substrate, rhodamine 6G (R6G) and malachite green (MG) were used as the probe molecules, with the detection limits of 10^-11 M and 10^-8 M, respectively. The SERS substrate had high sensitivity and uniformity, which indicated that it has a broad application prospect in the field of molecular detection.

Plasmonic Metal Nanoparticles Hybridized with 2D Nanomaterials for SERS Detection: A Review

In SERS analysis, the specificity of molecular fingerprints is combined with potential single-molecule sensitivity so that is an attractive tool to detect molecules in trace amounts. Although several substrates have been widely used from early on, there are still some problems such as the difficulties to bind some molecules to the substrate. With the development of nanotechnology, an increasing interest has been focused on plasmonic metal nanoparticles hybridized with (2D) nanomaterials due to their unique properties. More frequently, the excellent properties of the hybrids compounds have been used to improve the drawbacks of the SERS platforms in order to create a system with outstanding properties. In this review, the physics and working principles of SERS will be provided along with the properties of differently shaped metal nanoparticles. After that, an overview on how the hybrid compounds can be engineered to obtain the SERS platform with unique properties will be given.

Quenching Efficiency of Quantum Dots Conjugated to Lipid Bilayers on Graphene Oxide Evaluated by Fluorescence Single Particle Tracking

A single particle observation of quantum dots (QDs) was performed on lipid bilayers formed on graphene oxide (GO). The long-range fluorescence quenching of GO has been applied to biosensing for various biomolecules. We demonstrated the single particle observation of a QD on supported lipid bilayers in this study, aiming to detect the quenching efficiency of lipid and protein molecules in a lipid bilayer by fluorescence single particle tacking (SPT). A single lipid bilayer or double lipid bilayers were formed on GO flakes deposited on a thermally oxidized silicon substrate by the vesicle fusion method. The QDs were conjugated on the lipid bilayers, and single particle images of the QDs were obtained under the quenching effect of GO. The quenching efficiency of a single QD was evaluated from the fluorescence intensities on the regions with and without GO. The quenching efficiency reflecting the layer numbers of the lipid bilayers was obtained.

Spectral Dependence of the Energy Transfer from Photosynthetic Complexes to Monolayer Graphene

Fluorescence excitation spectroscopy at cryogenic temperatures carried out on hybrid assemblies composed of photosynthetic complexes deposited on a monolayer graphene revealed that the efficiency of energy transfer to graphene strongly depended on the excitation wavelength. The efficiency of this energy transfer was greatly enhanced in the blue-green spectral region. We observed clear resonance-like behavior for both a simple light-harvesting antenna containing only two chlorophyll molecules (PCP) and a large photochemically active reaction center associated with the light-harvesting antenna (PSI-LHCI), which pointed towards the general character of this effect.

Recent advances in nanomaterials for prostate cancer detection and diagnosis

Despite the significant progress in the discovery of biomarkers and the exploitation of technologies for prostate cancer (PCa) detection and diagnosis, the initial screening of these PCa-related biomarkers using current...

Hybrid Metal-Dielectric-Metal Sandwiches for SERS Applications

The development of efficient plasmonic nanostructures with controlled and reproducible surface-enhanced Raman spectroscopy (SERS) signals is an important task for the evolution of ultrasensitive sensor-related methods. One of the methods to improving the characteristics of nanostructures is the development of hybrid structures that include several types of materials. Here, we experimentally investigate ultrathin gold films (3–9 nm) near the percolation threshold on Si/Au/SiO₂ and Si/Au/SiO₂/graphene multilayer structures. The occurring field enhanced (FE) effects were characterized by a recording of SERS signal from Crystal Violet dye. In this geometry, the overall FE principally benefits from the combination of two mechanisms. The first one is associated with plasmon excitation in Au clusters located closest to each other. The second is due to the gap plasmons’ excitation in a thin dielectric layer between the mirror and corrugated gold layers. Experimentally obtained SERS signals from sandwiched structures fabricated with Au film of 100 nm as a reflector, dielectric SiO₂ spacer of 50 nm and ultrathin gold atop could reach SERS enhancements of up to around seven times relative to gold films near the percolation threshold deposited on a standard glass substrate. The close contiguity of the analyte to graphene and nanostructured Au efficiently quenches the fluorescent background of the model compound. The obtained result shows that the strategy of combining ultrathin nano-island gold films near the percolation threshold with gap plasmon resonances is promising for the design of highly efficient SERS substrates for potential applications in ultrasensitive Raman detection.

The Differences of Graphene Oxide Products Made from Three Kinds of Flake Graphites

Graphene oxide (GO), a widespread load platform in many research studies based on its microstructures, is largely made from flake graphite by a strong oxidation method. However, the differences of GO products made from different flake graphites have received little attention. Here, five GO products made from five different flake graphites by the Hummers method are investigated. The results reveal the differences in microstructures of the five GOs concerned with the ratio of C-C sp² structures to defects and the amount of oxygen-containing functional groups, which are further evidenced by their performances of quenching efficiencies by five DNA fluorescent probes. We demonstrated that the microstructural differences of GO products are transmitted from their parent flake graphites. Meanwhile, three kinds of parent flake graphites are proposed: (1) with large flakes and complete C-C sp² structures, (2) with large flakes but defective C-C sp² structures, and (3) with fine flakes but moderate C-C sp² structures, in which the performance of GO made from (1) is the best while the GO made from (3) shows comparable to or even better performance than that made from (2). Our work gives a reminder for precisely choosing graphite in the preparation of GOs and the potential value of tremendous natural fine-flake graphites.

Graphene oxide-based fluorescent biosensors and their biomedical applications in diagnosis and drug discovery

Graphene oxide (GO), an oxidized derivative of graphene, has received much attention for developing novel fluorescent bioanalytic platforms due to its remarkable optical properties and biocompatibility. The reliable performance and robustness of GO-based biosensors have enabled various applications in the biomedical field including diagnosis and drug discovery. Here, recent advances in the development of GO-based fluorescent biosensors are overviewed, particularly nucleic acid detection and enzyme activity assay. In addition, practical applications in biomarker detection and high-throughput screening are also examined. Lastly, basic design principles and remaining challenges of these types of biosensors are discussed for further progress.

Graphene and graphene oxide for bio-sensing: General properties and the effects of graphene ripples

The use of Graphene based materials, such as graphene oxide (GO), in biosensing applications is gaining significant interest, due to high signal output, with strong potential for high industrial growth rate. Graphene's excellent conduction and mechanical properties (such as toughness and elasticity) coupled with high reactivity to chemical molecules are some of its appealing properties. The presence of ripples on the surface (whether indigenous or induced) represents another property/variable that provide enormous potential if harnessed properly. In this article, we review the current knowledge regarding the use of graphene for biosensing. We discuss briefly the general topic of using graphene for biosensing applications with special emphasis on wearable graphene-based biosensors. The intrinsic ripples of graphene and their effect on graphene biosensing capabilities are thoroughly discussed. We dedicate a section also for the manipulation of intrinsic ripples. Then we review the use of Graphene oxide (GO) in biosensing and discuss the effect of ripples on its properties. We present a review of the current biosensor devices made out of GO for detection of different molecular targets. Finally, we present some thoughts for future perspectives and opportunities of this field. STATEMENT OF SIGNIFICANCE: Biosensors are tools that detect the presence and amount of a chemical substance, such as pregnancy tests and glucose monitoring devices. They are general portable, have short response times and are sensitive, making them highly effective. Gold and silver are used in biosensors and more recently, graphene.  Graphene is sheets of carbon atoms and is the only two-dimensional crystal in nature. It has unique features allowing its effective use in biosensing applications, including the presence of ripples (non-flat areas that give it its electronic properties). The last comprehensive review of this topic was published in 2016. This paper reviews the current knowledge of graphene based biosensors, with a focus on ripples and their effect on graphene biosensing capabilities.

Extraordinary Photostability and Davydov Splitting in BN-Sandwiched Single-Layer Tetracene Molecular Crystals

Two-dimensional molecular crystals have been beyond the reach of systematic investigation because of the lack or instability of their well-defined forms. Here, we demonstrate drastically enhanced photostability and Davydov splitting in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D crystals of graphene or hexagonal BN. Molecular orientation and long-range order mapped with polarized wide-field photoluminescence imaging and optical second-harmonic generation revealed high crystallinity of the 2D Tc and its distinctive orientational registry with the 2D inorganic crystals, which were also verified with first-principles calculations. The reduced dielectric screening in 2D space was manifested by enlarged Davydov splitting and attenuated vibronic sidebands in the excitonic absorption and emission of monolayer Tc crystals. Photostable 2D molecular crystals and their size effects will lead to novel photophysical principles and photonic applications.

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.

Fluorometric Viral miRNA Nanosensor for Diagnosis of Productive (Lytic) Human Cytomegalovirus Infection in Living Cells

A human cytomegalovirus (HCMV) causes a persistent asymptomatic infection in healthy individuals and possesses unexpected dangers to newborn babies, immunocompromised people, and organ transplant recipients because of stealth transmission. Thus, an early and accurate diagnosis of HCMV infection is crucial for prevention of unexpected transmission and progression of the severe diseases. The standard method of HCMV diagnosis depends on serology, antigen test, and polymerase chain reaction-based nucleic acid detection, which have advantages for each target molecule. However, the serological test for an antibody is an indirect method assuming the past virus infection, and antigen and viral nucleic acid testing demand laborious, complex multistep procedures for direct virus detection. Herein, we present an alternative simple and facile fluorometric biosensor composed of a graphene oxide nanocolloid and fluorescent peptide nucleic acid (PNA) probe to detect the HCMV infection by simply monitoring the virally encoded microRNA as a new biomarker of lytic virus infection. We verify the sensing of HCMV-derived microRNA accumulated within 72 h after HCMV infection and examine the diagnosis of HCMV in living cells. We proceed with the time course and concentration-dependent investigation of hcmv-miRNA sensing in living cells as a direct method of HCMV detection at the molecular level on the basis of an intracellular hcmv-miRNA expression profile and graphene oxide nanocolloid-based simple diagnostic platform. The fluorometric biosensor enables the sequence-specific binding to the target HCMV miRNAs in HCMV-infected fibroblasts and shows the quantitative detection capability of HCMV infection to be as low as 4.15 × 10⁵ immunofluorescence focus unit (IFU)/mL of the virus titer at 48 h post-infection with picomolar sensitivity for HCMV miRNA.

Isothermal Detection of Canine Blood Parasite (Ehrlichia canis) Utilizing Recombinase Polymerase Amplification Coupled with Graphene Oxide Quenching-Based Pyrrolidinyl Peptide Nucleic Acid

Canine monocytic ehrlichiosis (CME), caused by transmitted Ehrlichia canis infection, is a major disease in dogs with worldwide distribution. Herein, a nucleic acid assay was established for the identification of E. canis infection employing a fluorescently labeled conformationally constrained pyrrolidinyl PNA probe (Flu-acpcPNA) designed to sequence-specifically target the 16S rRNA gene. The sensing principle is based on the excellent quenching ability of graphene oxide (GO) of the free PNA probe, that was diminished upon binding to the DNA target. The addition of DNase I improved the performance of the detection system by eliminating the nonspecific quenching capability of long-chain dsDNA and thus enhancing the fluorescence signaling. The assay was coupled with a recombinase polymerase amplification (RPA) technique, which could be performed under isothermal conditions (37 °C) without DNA denaturation and purification steps. The established method is simple to set up and execute, proving a rapid, specific, and sensitive detection of 16S rRNA gene of E. canis with a limit of detection at least 11.1 pM. This technique shows good potential for the visual detection of double-stranded DNA targets without the need for PCR or complicated instruments, which shows great promise for practical usage in resource limited areas.

Optical visualization and imaging of nanomaterials

Direct visualization and imaging of nanomaterials under ambient conditions is of great significance for their characterization and application. In most cases, the observation of individual nanomaterials usually requires high-resolution electron microscopes under high vacuum. In comparison, an optical microscope is much more convenient due to its facile operation and open space. However, the resolution of optical microscopes is much lower than that of electron microscope-based tools. Therefore, effective visualization and imaging strategies for nanomaterials are required to realize their direct observation, accurate location and controllable manipulation. In this review, we summarized the progress of optical visualization and imaging strategies for nanomaterials in recent years, including vapor-condensation-assisted optical visualization, nanoparticle-assisted optical visualization, substrate-assisted optical visualization and fluorescence visualization, and the applications of these techniques were also introduced. We believe that this review will inspire further improvement in optical visualization of nanomaterials and drive the application of nanomaterials in a broader domain.

Silver nanowires as plasmonic compensators of luminescence quenching in single up-converting nanocrystals deposited on graphene

Single nanocrystal spectroscopy is employed to demonstrate metal-enhanced optical response of Er³⁺/Yb³⁺ doped up-conversion nanocrystals deposited on graphene upon coupling with silver nanowires. Direct interaction between nanocrystals and graphene results in quenching of up-conversion emission and shortening of luminescence decay times, due to the energy transfer to graphene. The amount of the energy absorbed by graphene can be enhanced by coupling Er³⁺/Yb³⁺ doped up-conversion nanocrystals with silver nanowires. Microscopy studies with high spatial resolution together with time-resolved analysis of nanocrystal luminescence show increase of the emission rates with fourfold enhancement of the intensity for nanocrystals placed in the vicinity of silver nanowires. This strong enhancement emerges despite simultaneous interaction with graphene. The hybrid nanostructure provides thus a way to combine optical activity of up-conversion nanocrystals and enhancement provided by metallic nanowires with excellent electrical and mechanical properties of graphene.

Quantum Energy and Charge Transfer at Two-Dimensional Interfaces

Energy and charge transfer processes in interacting donor-acceptor systems are the bedrock of many fundamental studies and technological applications ranging from biosensing to energy storage and quantum optoelectronics. Central to the understanding and utilization of these transfer processes is having full control over the donor-acceptor distance. With their atomic thickness and ease of integrability, two-dimensional materials are naturally emerging as an ideal platform for the task. Here, we review how van der Waals semiconductors are shaping the field. We present a selection of some of the most significant demonstrations involving transfer processes in layered materials that deepen our understanding of transfer dynamics and are leading to intriguing practical realizations. Alongside current achievements, we discuss outstanding challenges and future opportunities.

Narrowband nonlinear optical spectroscopy with spatially chirped broadband pulses

Nonlinear optical vibrational spectroscopies are powerful experimental tools for inspecting material properties that are difficult to acquire otherwise. As ultrafast lasers used in such experiments are typically of much broader bandwidth than vibrational modes, narrowband filtering is usually essential, and the utility of laser energy is often highly inefficient. Here we introduce an experimental scheme to break this trade-off. A broadband beam is spatially chirped as it reaches the sample, and generates sum-frequency signals upon overlapping with another broadband, unchirped beam. A narrowband spectrum can then be retrieved from the spatially dispersed image of signals, with both broadband pulses fully utilized. The scheme is also readily employed as a spatially resolved spectroscopy technique without scanning, and can be easily extended to other wave-mixing experiments.

Human virus detection with graphene-based materials

Our recent experience of the COVID-19 pandemic has highlighted the importance of easy-to-use, quick, cheap, sensitive and selective detection of virus pathogens for the efficient monitoring and treatment of virus diseases. Early detection of viruses provides essential information about possible efficient and targeted treatments, prolongs the therapeutic window and hence reduces morbidity. Graphene is a lightweight, chemically stable and conductive material that can be successfully utilized for the detection of various virus strains. The sensitivity and selectivity of graphene can be enhanced by its functionalization or combination with other materials. Introducing suitable functional groups and/or counterparts in the hybrid structure enables tuning of the optical and electrical properties, which is particularly attractive for rapid and easy-to-use virus detection. In this review, we cover all the different types of graphene-based sensors available for virus detection, including, e.g., photoluminescence and colorimetric sensors, and surface plasmon resonance biosensors. Various strategies of electrochemical detection of viruses based on, e.g., DNA hybridization or antigen-antibody interactions, are also discussed. We summarize the current state-of-the-art applications of graphene-based systems for sensing a variety of viruses, e.g., SARS-CoV-2, influenza, dengue fever, hepatitis C virus, HIV, rotavirus and Zika virus. General principles, mechanisms of action, advantages and drawbacks are presented to provide useful information for the further development and construction of advanced virus biosensors. We highlight that the unique and tunable physicochemical properties of graphene-based nanomaterials make them ideal candidates for engineering and miniaturization of biosensors.

Development of a Novel ssDNA Sequence for a Glycated Human Serum Albumin and Construction of a Simple Aptasensor System Based on Reduced Graphene Oxide (rGO)

Diabetes is one of the top 10 global causes of death. About one in 11 global adults have diabetes. As the disease progresses, the mortality rate increases, and complications can develop. Thus, early detection and effective management of diabetes are especially important. Herein, we present a novel glycated human serum albumin (GHSA) aptamer, i.e., GABAS-01, which has high affinity and specificity. The aptamer was selected by reduced graphene oxide-based systematic evolution of ligands by exponential enrichement (rGO-based SELEX) against GHSA. After five rounds of selection through gradually harsher conditions, GABAS-01 with high affinity and specificity for the target was obtained. GABAS-01 was labeled by FAM at the 5'-end and characterized by measuring the recovery of a fluorescence signal that is the result of fluorescence quenching effect of rGO. As a result, GABAS-01 had low-nanomolar Kd values of 1.748 ± 0.227 nM and showed a low limit of detection of 16.40 μg/mL against GHSA. This result shows the potential application of GABAS-01 as an effective on-site detection probe of GHSA. In addition, these properties of GABAS-01 are expected to contribute to detection of GHSA in diagnostic fields.

A versatile route to edge-specific modifications to pristine graphene by electrophilic aromatic substitution

Electrophilic aromatic substitution produces edge-specific modifications to CVD graphene and graphene nanoplatelets that are suitable for specific attachment of biomolecules.

A facile biosensor for Aβ40O based on fluorescence quenching of prussian blue nanoparticles

Amyloid β peptide oligomeFrs (AβOs) have been proved to be crucial biomarkers of Alzheimer's disease (AD). To explore an applicable method for the determination of AβOs is significant for the early AD diagnosis. Prussian blue nanoparticles (PBNPs), as one excellent nanomaterials, have the advantages of good stability, favorable biocompatibility, low cost, easy preparation and controllable shape. PBNPs was found to be of the fluorescence quenching ability to fluorophores, and the adsorption of DNA onto PBNPs surface occurred via the binding of phosphate skeleton in DNA to Fe²⁺/Fe³⁺ in PBNPs. On basis of this, carboxyl fluorescein (FAM) modified Aβ₄₀O-targeting aptamer (FAM-Apt_Aβ) was adsorbed onto PBNPs. And FAM-Apt_Aβ@PBNPs-based fluorescent aptasensor for the determination of Aβ₄₀O was developed. Upon incubating FAM-Apt_Aβ@PBNPs with Aβ₄₀O, the fluorescence intensity of the FAM-Apt_Aβ@PBNPs obviously increased comparing to the initial fluorescence intensity of the FAM-Apt_Aβ@PBNPs. The changes in the fluorescence intensity of the FAM-Apt_Aβ@PBNPs were linear with the Aβ₄₀O concentrations ranging from 1.00 nM to 100 nM. Moreover, AD patients and healthy persons can be distinguished using this method to determine Aβ₄₀O concentrations in human cerebrospinal fluid samples from AD patients and healthy persons. It demonstrates that this PBNPs-based aptasensor is not only simple and cost-effective, but also sensitive, selective and more applicable. This fluorescent sensing strategy is promising for the development of aptasensor in clinical fields.

Highly sensitive and specific screening of EGFR mutation using a PNA microarray-based fluorometric assay based on rolling circle amplification and graphene oxide

Screening epidermal growth factor receptor (EGFR) mutations, especially deletions, is essential for diagnosis of non-small cell lung cancer (NSCLC) and also critical to inform treatment decisions for NSCLC patients. Here, we demonstrated a facile peptide nucleic acid (PNA) microarray-based fluorometric method for sensitive and specific detection of EGFR mutation, using rolling circle amplification (RCA), graphene oxide (GO), and a fluorescently-labeled detection probe (F-DP). First, the EGFR gene sequence was efficiently captured by the label-free PNA probe which was attached on the surface of a 96-well plate. And then, the EGFR mutation sequence was specifically amplified by RCA using the circular DNA, which was formed by the ligation of the padlock probe when hybridizing with the EGFR mutation, as a template. The single-stranded RCA product (RCAP) was then sensitively detected with the F-DP and GO system. This method has a detection limit of 0.3 pM for EGFR mutation and a high discrimination capability to target EGFR mutation against EGFR wildtype. The use of a PNA microarray and a fluorescence quenching platform make this system quite suitable for high-throughput analysis of EGFR mutations in resource-limited settings without the need of costly and cumbersome equipment. Furthermore, this detection system provides a novel way for the diagnosis of other diseases that are caused by gene deletion mutations.

Detachable nanoladders: A new method for signal identification and their application in the detection of ochratoxin A (OTA)

A highly sensitive fluorescence turn-off biosensor for the detection of ochratoxin A (OTA) was developed based on graphene oxide (GO) and an aptamer-induced detachable nanoladders. In this assay, two types of ssDNA (H1 and H2) were involved in the assembly of the DNA nanoladders, in which H1 was labeled with fluorophore, and H2 was the OTA binding aptamer. Self-assembly of the DNA nanoladders with the addition of GO weakened its adsorption and the fluorescence intensity remained strong. In the presence of OTA, the aptamer was specifically recognized and an aptamer-OTA complex was formed, leading to the detached of DNA nanoladders. With the addition of GO, the released H1 was adsorbed on the GO surface, thus efficiently quenching the fluorescence signal (turning off). The detection limit of OTA in this assay was 4.59 nM. To improve the sensitivity of this strategy, we creatively developed an alternative strategy to replace the sturdy nanoladders with frail nanoladders. More precisely, the sequences of H1 had mismatched bases, which, when hybridized with H2 could be used to create long non-perfect complementary nanoladders. For the mismatched bases-based frail nanoladders, it was easier for OTA to bind its aptamer sequence, thus enabling a more thorough and faster detachment of the nanoladders, along with a greater degree of fluorescence quenching. The detection limit for OTA was estimated to be 0.1 nM. The biosensors we developed were sensitive, specific, enzyme-free, cost-effective and can be applied in red wine samples spiked with known concentration of OTA.

Graphene Oxide: From Tunable Structures to Diverse Luminescence Behaviors

Since the first discovery of luminescent graphene oxide (GO), exponentially increasing investigations on the tunable structures and surfaces for modulating its optical properties have struggled to expand applications in imaging, sensing, biomedical diagnostics, and so on. Here, the latest works on reconstructing or modifying the structures and surfaces of GO to achieve diverse luminescence are systematically reviewed, including fluorescence, electroluminescence, and chemiluminescence. Moreover, the fundamental difficulties of the investigations and applications of luminescent GO nanomaterials are clarified to inspire more constructive thoughts for expanding their application boundaries.

Optical Detection of Intracellular Quantities Using Nanoscale Technologies

Optical probes that can be used to measure certain quantities with subcellular resolution give us access to a new level of information at which physics, chemistry, life sciences, and medicine become strongly intertwined. The emergence of these new technologies is owed to great advances in the physical sciences. However, evaluating and improving these methods to new standards requires a joint effort with life sciences and clinical practice. In this Account, we give an overview of the probes that have been developed for measuring a few highly relevant parameters at the subcellular scale: temperature, pH, oxygen, free radicals, inorganic ions, genetic material, and biomarkers. Luminescent probes are available in many varieties, which can be used for measuring temperature, pH, and oxygen. Since they are influenced by virtually any metabolic process in the healthy or diseased cell, these quantities are extremely useful to understand intracellular processes. Probes for them can roughly be divided into molecular dyes with a parameter dependent fluorescence or phosphorescence and nanoparticle platforms. Nanoparticle probes can provide enhanced photostability, measurement quality, and potential for multiple functionalities. Embedding into coatings can improve biocompatibility or prevent nonspecific interactions between the probe and the cellular environment. These qualities need to be matched however with good uptake properties, colloidal properties and eventually intracellular targeting to optimize their practical applicability. Inorganic ions constitute a broad class of compounds or elements, some of which play specific roles in signaling, while others are toxic. Their detection is often difficult due to the cross-talk with similar ions, as well as other parameters. The detection of free radicals, DNA, and biomarkers at extremely low levels has significant potential for biomedical applications. Their presence is linked more directly to physiological and clinical manifestations. Since existing methods for free radical detection are generally poor in sensitivity and spatiotemporal resolution, new reliable methods that are generally applicable can contribute greatly to advancing this topic in biology. Optical methods that detect DNA or RNA and protein biomarkers exist for intracellular applications, but are mostly relevant for the development of rapid point-of-care sample testing. To elucidate the inner workings of cells, focused multidisciplinary research is required to define the validity and limitations of a nanoparticle probe, in both physical and biological terms. Multifunctional platforms and those that are easily made compatible with conventional research equipment have an edge over other techniques in growing the body of research evidencing their versatility.

Distance Dependence of Single-Molecule Energy Transfer to Graphene Measured with DNA Origami Nanopositioners

Despite the thorough investigation of graphene since 2004, altering its surface chemistry and reproducible functionalization remain challenging. This hinders fabrication of more complex hybrid materials with controlled architectures, and as a consequence the development of sensitive and reliable sensors and biological assays. In this contribution, we introduce DNA origami structures as nanopositioners for placing single dye molecules at controlled distances from graphene. The measurements of fluorescence intensity and lifetime of single emitters carried out for distances ranging from 3 to 58 nm confirmed the d^-4 dependence of the excitation energy transfer to graphene. Moreover, we determined the characteristic distance for 50% efficiency of the energy transfer from single dyes to graphene to be 17.7 nm. Using pyrene molecules as a glue to immobilize DNA origami nanostructures of various shape on graphene opens new possibilities to develop graphene-based biophysics and biosensing.

Simultaneous quantitative detection of multiple tumor markers in microfluidic nanoliter-volume droplets

We developed a simultaneous detection method for multiple tumor markers (TMs) in microfluidic droplets based on a multiple fluorescence resonance energy transfer (FRET) system. In this system, graphene oxide (GO) was used as the single quencher, while the multi-color quantum dots (QDs) labeled on different aptamers were employed as energy donors. When the aptamers were adsorbed onto GO due to the π-π stacking interaction, QDs were drawn to the surface of GO and quenched by it. Once the TMs were introduced, the corresponding fluorescence of QDs was recovered obviously owing to the preferential interaction of aptamers with the TMs. Here, the multi-FRET system was encapsulated into nanoliter-volume droplets by a simple T-junction microfluidic chip. The targets could be detected rapidly as the generated droplets flew through the integrated on-line detection zone. Three tumor markers, carcinoembryonic antigen (CEA), prostate-specific antigen (PSA) and vascular endothelial growth factor (VEGF₁₆₅) could be detected simultaneously in 33 nL-volume droplets, which is only 1/3000 of the volume of the sample consumed in the conventional fluorescence spectrophotometer. In addition, the signals corresponding to different TM targets in one nanoliter-volume droplet could be read out at the same time, and the signals could be output continuously owing to the uninterruptible generation of droplets. Even with a signal acquisition frequency of 55 droplets per minute, the multi-FRET biosensing system has linear ranges of 0.50-70 ng mL^-1 for CEA, 0.25-70 ng mL^-1 for PSA and 0.50-70 ng mL^-1 for VEGF₁₆₅. The detection limits of CEA, PSA and VEGF₁₆₅ were calculated to be 0.15 ng mL^-1, 0.035 ng mL^-1 and 0.11 ng mL^-1, respectively. The method was also validated by analyzing human serum sample dilutions. The proposed multi-FRET-based system has potential to become a powerful tool for rapid, low-cost and simultaneous detection of multiple tumor markers.

Topological Transformation-Based Nanobarcoding for Detection and Enumeration of MicroRNAs and Single Nucleotide Polymorphism

RNA biomarkers have been recently reported to be associated tightly with the diagnosis and prognosis of many diseases. Particularly, cancers considered to be a serious threat to primates are known to be vastly dominated by genetic networks where RNA plays a key role. RNAs are thus recognized as a major target group that can be used for numerous cancer treatments and it is still required to identify and enumerate them in an effective manner. Here, a new topological transformation-based nanobarcoding technique (TNT) is first reported using fluorescence-DNA barcodes engaged with graphene oxide (GO_x ) for effectively discriminating short RNAs such as miRNAs and their single nucleotide polyporphisms in tissue and plasma. Through topological transformation into 3D DNA-RNA polygonal structures, various kinds of microRNAs have been read at the same time and analyzed quantitatively. Also, it positively discerned epidermal growth factor receptor (EGFR) mutations known as single base variations of typical lung cancer specific RNAs. A single variant of 0.785% in target EGFR mutations is explicitly detected. It is speculated that the TNT may be a versatile method for polymerase chain reaction (PCR)-free practical diagnosis of several clinical genetic deviations such as significant biotic RNA and genic fragments and would be a promising alternative to conventional PCR terrains.

An “OFF–ON” quantum dot–graphene oxide bioprobe for sensitive detection of micrococcal nuclease ofStaphylococcus aureus

mAb-Strep-QDs-GO probe in an OFF state due to energy transfer from QDs to GO turns into an ON state when the energy transfer is inhibited by MNase, thus allowing the sensing of MNase (Micrococcal Nuclease, an extracellular endonuclease ofStaphylococcus Aureus).

Quenched Stochastic Optical Reconstruction Microscopy (qSTORM) with Graphene Oxide

Quenched Stochastic Optical Reconstruction Microscopy (qSTORM) was demonstrated with graphene oxide sheets, peptides and bacteria; a method of contrast enhancement with super-resolution fluorescence microscopy. Individual sheets of graphene oxide (GO) were imaged with a resolution of 16 nm using the quenching of fluorescence emission by GO via its large Resonant Energy Transfer (RET) efficiency. The method was then extended to image self-assembled peptide aggregates (resolution 19 nm) and live bacterial cells (resolution 55 nm, the capsular structure of E. coli from urinary tract infections) with extremely low backgrounds and high contrasts (between one and two orders of magnitude contrast factor improvements that depended on the thickness of the graphene oxide layer used). Graphene oxide films combined with STORM imaging thus provide an extremely convenient method to image samples with large backgrounds due to non-specifically bound fluorophores (either due to excess labelling or autofluorescent molecules), which is a common occurrence in studies of both biological cells and soft-condensed matter. The GO quenches the fluorescence across a thin layer at distances of less than 15 nm. Graphene oxide films coated with thin layers (≤15 nm) of polystyrene, polymethylmethacrylate and polylysine are shown to be effective in producing high contrast qSTORM images, providing a convenient modulation of sample/substrate interactions. The GO coatings can also provide an increased image resolution and a factor of 2.3 improvement was observed with the peptide fibres using a feature of interest metric,when there was a large non-specifically bound background.