Grating Structures for Silver-Based Surface Plasmon Resonance Sensors with Adjustable Excitation Angle

Silver-based grating structures offer means for implementing low-cost, efficient grating couplers for use in surface plasmon resonance (SPR) sensors. One-dimensional grating structures with a fixed periodicity are confined to operate effectively within a single planar orientation. However, two-dimensional grating structures as well as grating structures with variable periodicity allow for the plasmon excitation angle to be seamlessly adjusted. This study demonstrates silver-based grating designs that allow for the plasmon excitation angle to be adjusted via rotation or beam position. The flexible angle adjustment opens up the possibility of developing SPR sensor designs with an expanded dynamic range and increased flexibility in sensing applications. The results demonstrate that efficient coupling into two diffraction orders is possible, which ultimately leads to an excitation angle range from 16° to 40° by rotating a single structure. The findings suggest a promising direction for the development of versatile and adaptable SPR sensing platforms with enhanced performance characteristics.

"Clickable" graphene nanoribbons for biosensor interfaces

We report on the synthesis of "clickable" graphene nanoribbons (GNRs) and their application as a versatile interface for electrochemical biosensors. GNRs are successfully deposited on gold-coated working electrodes and serve as a platform for the covalent anchoring of a bioreceptor (i.e., a DNA aptamer), enabling selective and sensitive detection of Interleukin 6 (IL6). Moreover, when applied as the intermediate linker on reduced graphene oxide (rGO)-based field-effect transistors (FETs), the GNRs provide improved robustness compared to conventional aromatic bi-functional linker molecules. GNRs enable an orthogonal and covalent attachment of a recognition unit with a considerably higher probe density than previously established methods. Interestingly, we demonstrate that GNRs introduce photoluminescence (PL) when applied to rGO-based FETs, paving the way toward the simultaneous optical and electronic probing of the attached biointerface.

CoNiFe alloy nanoparticles encapsulated into nitrogen-doped carbon nanotubes toward superior electrocatalytic overall water splitting in alkaline freshwater/seawater under large-current density

Developing bifunctional catalysts for overall water splitting with high activity and durability at high current density remains a challenge. In an attempt to overcome this bottleneck, in this work, unique CoNiFe-layered double hydroxide nanoflowers are in situ grown on nickel-iron (NiFe) foam through a corrosive approach and following a chemical vapor deposition process to generate nitrogen-doped carbon nanotubes at the presence of melamine (CoNiFe@NCNTs). The coupling effects between various metal species act a key role in accelerating the reaction kinetics. Moreover, the in situ formed NCNTs also favor promoting electrocatalytic activity and stability. For oxygen evolution reaction it requires low overpotentials of 330 and 341 mV in 1M KOH and 1M KOH + seawater to drive 500 mA cm-2. Moreover, water electrolysis can be operated with CoNiFe@NCNTs as both anode and cathode with small voltages of 1.95 and 1.93 V to achieve 500 mA cm-2 in 1M KOH and 1M KOH + seawater, respectively.

Laser-Induced Graphene-Functionalized Field-Effect Transistor-Based Biosensing: A Potent Candidate for COVID-19 Detection

Speedy and on-time detection of coronavirus disease 2019 (COVID-19) is of high importance to control the pandemic effectively and stop its disastrous consequences. A widely available, reliable, label-free, and rapid test that can recognize tiny amounts of specific biomarkers might be the solution. Nanobiosensors are one of the most attractive candidates for this purpose. Integration of graphene with biosensing devices shifts the performance of these systems to an incomparable level. Between the various arrangements using this wonder material, field-effect transistors (FETs) display a precise detection even in complex samples. The emergence of pioneering biosensors for detecting a wide range of diseases especially COVID-19 created the incentive to prepare a review of the recent graphene-FET biosensing platforms. However, the graphene fabrication and transfer to the surface of the device is an imperative factor for researchers to take into account. Therefore, we also reviewed the common methods of manufacturing graphene for biosensing applications and discuss their advantages and disadvantages. One of the most recent synthesizing techniques - laser-induced graphene (LIG) - is attracting attention owing to its extraordinary benefits which are thoroughly explained in this article. Finally, a conclusion highlighting the current challenges is presented.

Tetrahedral DNA nanostructure based biosensor for high-performance detection of circulating tumor DNA using all-carbon nanotube transistor

Adopting carbon nanotube (CNT) transistors as biosensors has been developed as a promising method for cancer biomarker detection, which has shown superior sensitivity and selectivity. However, the detection of circulating tumor DNA (ctDNA) by the CNT transistor based biosensors is still a challenge and no work has been reported. Here, direct label-free DNA detection of AKT2 gene related to triple-negative breast cancer by all-CNT thin-film transistor (TFT) biosensors incorporated with tetrahedral DNA nanostructures (TDNs) is proposed and achieved for the first time. The adoption of TDNs enables improved biosensor response for at least 35% and even as high as 98% as compared with single-stranded DNA (ssDNA) probes owing to the enhanced DNA hybridization efficiency. Influence of the TDNs' linker length on the biosensor performance is important and has been investigated. Concentration-dependent DNA detection is achieved by the all-CNT TFT biosensors with a broad linear detection range of six orders of magnitude and a theoretical limit of detection (LOD) of 2 fM. In addition, the all-CNT TFT biosensors exhibit favorable selectivity and repeatability. The platform of all-CNT TFT biosensors incorporated with TDNs has great potential for multiplexed detection of various cancer biomarkers, providing a simple yet high performance universal strategy for low-cost clinical applications.

Fabrication and microfluidic analysis of graphene-based molecular communication receiver for Internet of Nano Things (IoNT)

Bio-inspired molecular communications (MC), where molecules are used to transfer information, is the most promising technique to realise the Internet of Nano Things (IoNT), thanks to its inherent biocompatibility, energy-efficiency, and reliability in physiologically-relevant environments. Despite a substantial body of theoretical work concerning MC, the lack of practical micro/nanoscale MC devices and MC testbeds has led researchers to make overly simplifying assumptions about the implications of the channel conditions and the physical architectures of the practical transceivers in developing theoretical models and devising communication methods for MC. On the other hand, MC imposes unique challenges resulting from the highly complex, nonlinear, time-varying channel properties that cannot be always tackled by conventional information and communication tools and technologies (ICT). As a result, the reliability of the existing MC methods, which are mostly adopted from electromagnetic communications and not validated with practical testbeds, is highly questionable. As the first step to remove this discrepancy, in this study, we report on the fabrication of a nanoscale MC receiver based on graphene field-effect transistor biosensors. We perform its ICT characterisation in a custom-designed microfluidic MC system with the information encoded into the concentration of single-stranded DNA molecules. This experimental platform is the first practical implementation of a micro/nanoscale MC system with nanoscale MC receivers, and can serve as a testbed for developing realistic MC methods and IoNT applications.

An optic-fiber graphene field effect transistor biosensor for the detection of single-stranded DNA

Herein, a graphene field effect transistor (GFET) was constructed on an optic fiber end face to develop an integrated optical/electrical double read-out biosensor, which was used to detect target single-stranded DNA (tDNA). Two isolated Au electrodes were, respectively, prepared as the drain and source at the ends of an optic fiber and coated with a graphene film to construct a field effect transistor (FET). Probe aptamers modified with fluorophore 6'-carboxy-fluorescein (6'-FAM) were immobilized on the graphene for specific capture of tDNA. Graphene oxide (GO) was introduced to quench 6'-FAM and construct a fluorescence biosensor. Thus, a dual GFET and fluorescence biosensor was integrated on the end-face of an optic fiber. Following synchronous detection by fluorescence and FET methods, results showed satisfactory sensitivity for DNA detection. Compared with conventional biosensors using a single sensing technology, these dual sensing integrated biosensors significantly improved the reliability and accuracy of DNA detection. Furthermore, this proposed technique provides both a new biosensor for single-stranded DNA detection and a strategy for designing multi-sensing integrated biosensors.

Multiplexed femtomolar detection of Alzheimer's disease biomarkers in biofluids using a reduced graphene oxide field-effect transistor

Alzheimer's disease (AD) is a neurodegenerative disease that accounts for 70% of all dementia. Early stage diagnosis of AD is essential as there is no certain treatment after the lesion has progressed in the late stage. Nevertheless, there are still limitations of early diagnosis of AD using neuroimaging and psychological memory assessments. Here, we demonstrate ultrasensitive and multiplexed detection of pivotal AD biomarkers (Aβ_1-42 and t-Tau) in biofluids using a reduced graphene oxide field-effect transistor (gFET). The proposed approach provides a wide logarithmically linear range of detection from 10^-1-10⁵ pg mL^-1 and a femtomolar-level limit of detection in biofluids (human plasma and artificial cerebrospinal fluid) as well as phosphate-buffered saline (PBS). Furthermore, as these core biomarkers have different surface charges in physiological conditions based on the isoelectric point (pI), we achieved a distinctive output signal for each biomarker. The gFET biosensor platform presented in this paper has great potential and can be used for early diagnosis of AD in clinical practice as well as accurate analysis based on the surface charge of the analytes.

An integrated microfluidic platform for selective and real-time detection of thrombin biomarkers using a graphene FET

Lab-on-a-chip technology offers an ideal platform for low-cost, reliable, and easy-to-use diagnostics of key biomarkers needed for early screening of diseases and other health concerns. In this work, a graphene field-effect transistor (GFET) functionalized with target-binding aptamers is used as a biosensor for the detection of thrombin protein biomarker. Furthermore, this GFET is integrated with a microfluidic device for enhanced sensing performances in terms of detection limit, sensitivity, and continuous monitoring. Under this platform, a picomolar limit of detection was achieved for measuring thrombin; in our experiment measured as low as 2.6 pM. FTIR, Raman and UV-Vis spectroscopy measurements were performed to confirm the device functionalization steps. Based on the concentration-dependent calibration curve, a dissociation constant of KD = 375.8 pM was obtained. Continuous real-time measurements were also conducted under a constant gate voltage (VGS) to observe the transient response of the sensor when analyte was introduced to the device. The target selectivity of the sensor platform was evaluated and confirmed by challenging the GFET biosensor with various concentrations of lysozyme protein. The results suggest that this device technology has the potential to be used as a general diagnostic platform for measuring clinically relevant biomarkers for point-of-care applications.

An Au Nanofilm-Graphene/D-Type Fiber Surface Plasmon Resonance Sensor for Highly Sensitive Specificity Bioanalysis

A highly sensitive Au-graphene structure D-type fiber surface plasmon resonance biosensor is presented in this study to specifically detect biomolecules. The method of growing graphene is employed directly on the copper, and then a gold film of optimum thickness is sputtered, and the copper foil is etched to obtain the structure. This method makes the contact closer between the gold layer and the graphene layer to improve surface plasmon resonance performance. The performance of this type of surface plasmon resonance (SPR) sensor has been previously verified both theoretically and experimentally. With the proposed Au-graphene structure D-type fiber biosensor, the SPR behaviors are obtained and discussed. In the detection of ethanol solution, a red shift of 40 nm is found between the refractive index of 1.3330 and 1.3657. By calculation, the sensitivity of the sensor we designed is 1223 nm/RIU. Besides, the proposed sensor can detect the nucleotide bonding between the double-stranded DNA helix structures. Thus, our sensors can distinguish between mismatched DNA sequences.

Magnetic Graphene Field-Effect Transistor Biosensor for Single-Strand DNA Detection

Herein, a magnetic graphene field-effect transistor biosensor was prepared through the transfer of a chemical vapor deposition graphene film onto a glass substrate to produce a sensing film and conductive channel. By fixing 1-pyrenebutanoic acid succinimidyl ester onto graphene film as an anchor, a probe aptamer was immobilized on the graphene film in order to capture magnetically labeled complementary single-stranded DNA. Our experiments showed that, within a periodic magnetic field, the biosensor impedance exhibited a periodic oscillation, the amplitude of which was correlated to the complementary DNA concentration. Based on this principle, the magnetic graphene field-effect transistor was utilized to detect single-stranded DNA with detection limition of 1 pM. The results were rationalized using a model wherein the magnetic force causes the DNA strand to bend, thereby resulting in magnetic nanobeads/DNA modulation of the double conductive layer of graphene transistors. Furthermore, since a periodic magnetic field could be introduced to produce a periodic impedance changes of MGFETs, sampling integration could be used to improve the signal-to-noise ratio efficiently by increasing the number of periods of the external magnetic field. Therefore, a novel biosensor for DNA detection with high sensitivity has been presented in this work. Based on the detection principle, this system may also be a potential tool for detecting other bio-molecules, cells, etc.

Experimental and theoretical investigation for surface plasmon resonance biosensor based on graphene/Au film/D-POF

A D-shape plastic optical fiber (D-POF) surface plasmon resonance (SPR) biosensor based on the graphene/Au film (G/Au) was proposed and experimentally demonstrated for detection of DNA hybridization process. To improve the detection performance of SPR sensors, the Physical Vapor Deposition (PVD) method was used to evaporate the Au film directly onto the graphene grown on copper foil, and the Au film acted as a role of traditional Polymethyl Methacrylate (PMMA). The process made graphene and Au film form seamless contact. Next, the G/Au was transferred onto the D-shape fiber together. We explored the G/Au SPR sensor by using the finite element method (FEM) and obtained the optimum materials thickness to form configuration. Compared to other plastic optical fiber experiments, the proposed sensor's sensitivity was improved effectively and calculated as 1227 nm/RIU in a range of glucose solution. Meanwhile, our proposed sensor successfully distinguishes hybridization and single nucleotide polymorphisms (SNP) by observing the resonance wavelength change. It also exhibits a satisfactory linear response (R² = 0.996) to the target DNA liquids with respective concentrations of 0.1nM to1µM, which shows this method's wide potential in medical diagnostics.

A graphene/Si Schottky diode for the highly sensitive detection of protein

Herein, a graphene/Si-based device was introduced for bovine serum albumin (BSA) sensing.