Review on carbon-derived,solid-state,micro and nano sensors for electrochemical sensing applications

The aim of this review is to summarize the most relevant contributions in the development of electrochemical sensors based on carbon materials in the recent years. There have been increasing numbers of reports on the first application of carbon derived materials for the preparation of an electrochemical sensor. These include carbon nanotubes, diamond like carbon films and diamond film-based sensors demonstrating that the particular structure of these carbon material and their unique properties make them a very attractive material for the design of electrochemical biosensors and gas sensors.Carbon nanotubes (CNT) have become one of the most extensively studied nanostructures because of their unique properties. CNT can enhance the electrochemical reactivity of important biomolecules and can promote the electron-transfer reactions of proteins (including those where the redox center is embedded deep within the glycoprotein shell). In addition to enhanced electrochemical reactivity, CNT-modified electrodes have been shown useful to be coated with biomolecules (e.g., nucleic acids) and to alleviate surface fouling effects (such as those involved in the NADH oxidation process). The remarkable sensitivity of CNT conductivity with the surface adsorbates permits the use of CNT as highly sensitive nanoscale sensors. These properties make CNT extremely attractive for a wide range of electrochemical sensors ranging from amperometric enzyme electrodes to DNA hybridization biosensors. Recently, a CNT sensor based fast diagnosis method using non-treated blood assay has been developed for specific detection of hepatitis B virus (HBV) (human liver diseases, such as chronic hepatitis, cirrhosis, and hepatocellular carcinoma caused by hepatitis B virus). The linear detection limits for HBV plasma is in the range 0.5–3.0 µL^? 1 and for anti-HBVs 0.035–0.242 mg/mL in a 0.1 M NH₄H₂PO₄ electrolyte solution. These detection limits enables early detection of HBV infection in suspected serum samples. Therefore, non-treated blood serum can be directly applied for real-time sensitive detection in medical diagnosis as well as in direct in vivo monitoring.Synthetic diamond has been recognized as an extremely attractive material for both (bio-) chemical sensing and as an interface to biological systems. Synthetic diamond have outstanding electrochemical properties, superior chemical inertness and biocompatibility. Recent advances in the synthesis of highly conducting nanocrystalline-diamond thin films and nano wires have lead to an entirely new class of electrochemical biosensors and bio-inorganic interfaces. In addition, it also combines with development of new chemical approaches to covalently attach biomolecules on the diamond surface also contributed to the advancement of diamond-based biosensors. The feasibility of a capacitive field-effect EDIS (electrolyte-diamond-insulator-semiconductor) platform for multi-parameter sensing is demonstrated with an O-terminated nanocrystalline-diamond (NCD) film as transducer material for the detection of pH and penicillin concentration. This has also been extended for the label-free electrical monitoring of adsorption and binding of charged macromolecules. One more recent study demonstrated a novel bio-sensing platform, which is introduced by combination of a) geometrically controlled DNA bonding using vertically aligned diamond nano-wires and b) the superior electrochemical sensing properties of diamond as transducer material. Diamond nano-wires can be a new approach towards next generation electrochemical gene sensor platforms.This review highlights the advantages of these carbon materials to promote different electron transfer reactions specially those related to biomolecules. Different strategies have been applied for constructing carbon material-based electrochemical sensors, their analytical performance and future prospects are discussed.     

Chemical imaging sensor and its application to biological systems

The chemical imaging sensor is a semiconductor-based chemical sensor that can visualize the two-dimensional distribution of chemical species in the specimen. One of the prospective applications of the chemical imaging sensor is the visualization of biochemical activities in biological systems. The chemical imaging sensor was successfully applied for the detection of Escherichia coli colonies. For further applications to biological systems, enhanced adhesion of the sample to the sensing surface seems to be indispensable. As a candidate material for biocompatible sensors, porous Si was tested and was found to have a high pH sensitivity of 57.3 mV/pH.     

Optical,thermal and mechanical properties of CVD diamond

The exceptional combination of mechanical, thermal, chemical and optical properties of diamond make this material an ideal choice for numerous applications in which materials are required to operate in adverse environments exposed to abrasion or chemical attack, or in situations requiring the dissipation of extreme levels of power, such as windows for high power lasers. Progress in the synthesis and processing of chemical vapour deposition (CVD) diamond achieved at De Beers¹ Industrial Diamond Division (Debid) has opened many of these applications by bringing to the market a diamond material this is available in large areas, in industrial quantities and with the high quality and reproducible properties needed to meet the demanding requirements of the industry. This paper will review the properties of the different CVD diamond material grades used in optical, thermal and mechanical applications.     

Fabrication of SiO2-stacked diamond membranes and their characteristics for microelectromechanical applications

Diamond is a promising microelectromechanical systems (MEMS) material due to its high Young's Modulus and very large thermal conductivity. In this work, ultrananocrystalline diamond was stacked between silicon dioxide to form thermally-stable and robust membranes. These SiO₂-stacked diamond layers were processed into MEMS-compatible membranes. For comparison, membranes composed of only SiO₂ were fabricated as well. The structural characteristics of these membranes are compared and analyzed for membranes of different diameters. Using finite element modeling, the experimental behaviors of SiO₂ and SiO₂-stacked diamond membranes are analyzed.     

Durable,Sensitive, and Wide‐Range Wearable Pressure Sensors Based on Wavy‐Structured Flexible Conductive Composite Film

Flexible pressure sensors have potential applications in human motion monitoring and electronic skins. To satisfy the practical applications, pressure sensors with a high sensitivity, a low detection limit, a broad response range, and an excellent stability are highly needed. Here, a piezoresistive pressure sensor based on wavy‐structured single‐walled carbon nanotube/graphite flake/thermoplastic polyurethane (SWCNT/GF/TPU) composite film is fabricated by a prestretching process. Due to the random wavy structure, high conductivity, and good flexibility, the prepared sensor displays a low detection limit of 2 Pa, a wide sensing range of 0–60 kPa, and a high sensitivity of 5.49 kPa^?1 for 0–50 Pa. Furthermore, the sensor shows a remarkable repeatability of over 1.1 × 10⁴, 9.0 × 10³, and 2.0 × 10³ pressure loading/unloading cycles at 50 Pa, 500 Pa, and 30 kPa, respectively, and a fast responsibility of 100–150 ms of loading response time and 400–600 ms of relaxation time. Therefore, the pressure sensor is successfully adopted to monitor both the large‐scale human activities (e.g., walk and jump) and the small‐scale signals (e.g., wrist pulse). Furthermore, a sensor array is assembled to map the weight and shape of an object, indicating its various potential applications including human–machine interactions, human health monitoring, and other wearable electronics.     

Diamond–carbon nanocomposites: applications for diamond film deposition and field electron emission

Diamond–carbon nanocomposites (DCN) containing diamond and graphitic particles, both a few nanometers in size, were studied as the material for field electron emission. Diamond–carbon mass ratio and grain size were varied to optimize field emission properties. The stable and uniform electron emission was observed at fields E=10 V μm⁻¹ with a negligible hysteresis of I–V curves. Treatment in microwave hydrogen plasma was found to reduce the threshold field for emission owing to preferential etching of carbon component and surface relief sharpening. Ultrathin chemical vapour deposition diamond films can be easily grown on DCN due to the very high nucleation density inherent to this composite.     

Diamond nano-wires,a new approach towards next generation electrochemical gene sensor platforms

A novel bio-sensing platform is introduced by combination of a) geometrically controlled DNA bonding using vertically aligned diamond nano-wires and b) the superior electrochemical sensing properties of diamond as transducer material. Ultra-hard vertically aligned diamond nano-wires are electrochemically modified to bond phenyl linker-molecules to their tips which provide mesospacing for DNA molecules on the transducer. The nano-wires are generated by reactive ion etching of metallically boron doped atomically smooth single crystalline CVD diamond. Surface properties are characterized by atomic force, scanning electron and scanning tunneling microcopy. Electro- and bio-chemical sensor properties are investigated using cyclic and differential pulse voltammetry as well as impedance spectroscopy with Fe(CN)₆^3-/4- as redox mediators which reveal sensitivities of 2 pM on 3 mm² sensor areas and superior DNA bonding stability over 30 hybridization/denaturation cycles. The fabrication of “all diamond” ultra-micro-electrode (UME) arrays and multi-gene sensors are discussed taking into account the unique properties of diamond.     

Recent progress of Ga2O3-based gas sensors

In recent years, gas sensors fabricated from gallium oxide (Ga₂O₃) materials have aroused intense research interest due to the superior material properties of large dielectric constant, good thermal and chemical stability, excellent electrical properties, and good gas sensing. Over the past decades, Ga₂O₃-based gas sensors experienced rapid development. The long-term stable Ga₂O₃-based gas sensors for detecting oxygen and carbon monoxide have been commercialized and renowned with extremely good gas sensing characteristics. Recent pioneering studies also exhibit that the Ga₂O₃-based gas sensors possess great potentials in applications of detecting nitrogen oxides, hydrogen, volatile organic compounds and ammonia gases. This article presents recent advances in gas sensing mechanism, device performance parameters, influence factors, and applications of Ga₂O₃-based gas sensors. The impacts of influence factors, doping, material structure and device structure on the performance of gas sensors are discussed in detail. Finally, a brief overview of challenges and opportunities for the Ga₂O₃-based gas sensors is presented.     

Diamond nucleation and growth under very low-pressure conditions

The synthesis of thin diamond films using various chemical vapor deposition methods has received significant attention in recent years due to the unique characteristic of diamond, which make it an attractive candidate for a wide range of applications. In order to grow diamond epitaxially, the proper control of diamond nucleation on mirror-polished Si is essential. Adding the negative bias voltage to the substrate is the most popular method. This paper has proposed a new method to greatly enhance the nuclear density. Under very low pressure (1 torr), the high-density nucleation of diamond is achieved on mirror-polished silicon in a hot-filament chemical vapor deposition (HFCVD). Scanning electron microscopy has demonstrated that the nuclear density can be as high as 10¹⁰–10¹¹ cm⁻². Raman spectra of the sample have shown a dominant diamond characteristic peak at 1332 cm⁻¹. The pressure effect has been discussed in detail and it has been shown that the very low pressure is a very effective means to nucleate and grow diamond films on mirror-polished silicon. Extraordinary pure hydrogen (purity=99.9999%) was used as the source. Compared with the highly pure hydrogen (purity=99.99%), we found that the density of nucleation was greatly increased. The residual oxygen in the hydrogen displayed a very obvious negative effect on the nucleation of diamond, although it can accelerate the growth of diamond. Based on these results, it was suggested that the enhanced nucleation at very low pressure should be attributed to an increased mean free path, which induced a high density of atomic hydrogen and hydrocarbon radicals near the silicon surface. Atomic hydrogen can effectively etch the oxide layer on the surface of silicon and so greatly enhance the nucleation density.     

Diamond detectors for hadron physics research

The application of diamond for the detection of charged particles in atomic, nuclear and high-energy physics experiments is described. We compare the properties of three undoped diamond types, all of them produced by Chemical Vapor Deposition (CVD), in particular homoepitaxial single-crystal CVD Diamond (scCVDD), polycrystalline CVD Diamond (pcCVDD) grown on silicon, and CVD Diamond on Iridium (DoI) grown on the multi-layer substrate Ir/YSZ/Si001. The characteristics of the transient current (TC) signals generated from ²⁴¹Am-α-particles in the samples are exploited to evaluate the potential of the diamond crystals for particle timing and spectroscopy applications. The TC technique (TCT) results are correlated to the dark conductivity and the structural defects of the bulk materials as well as to the morphology and roughness of the diamond surfaces. The deterioration of the sensors performance after heavy irradiations with 26 MeV protons, 20 MeV neutrons, and 10 MeV electrons is discussed by means of charge-collection efficiency results, TC technique, and optical absorption spectroscopy (OAS). The important role of the diamond signal processing is underlined, which influences both the quality of the CVDD characterization data as well as the in-beam performance of the diamond sensors.     

Optimal parameters to produce high quality diamond films on 3D Porous Titanium substrates

Diamond films grown on three-dimensional (3D) porous titanium substrate were obtained by hot filament chemical vapor deposition (HFCVD) technique. The growth parameters strongly influenced the film properties during this complex film formation process. The pressure inside the reactor as well as the methane concentration showed their influence on the film morphology, quality, and growth rate. The substrates were totally covered by a diamond coating including deeper planes leading to a 3D porous diamond/Ti composite material formation. The sp²/sp³ ratio as “purity index” (PI) and the “growth tendency index” (GTI), evaluated from Raman and X-ray spectra respectively, were obtained for these composite materials as a function of their growth parameters.     

Nanostructured titanium nitride for highly sensitive localized surface plasmon resonance biosensing

Titanium nitride (TiN) as an alternative plasmonic ceramic material with superb properties including high hardness, outstanding corrosion resistance and excellent biocompatibility, has exhibited great potential for optical biochemical sensing applications. By sputtering about 35 nm–50 nm TiN on glass (f-TiN), the surface was found to provide sensing capability toward NaCl solution through the phenomenon of surface plasmon resonance. When the TiN film of about 27 nm–50 nm in thickness was sputtered onto a roughened glass surface (R–TiN), the sensing capability was improved. This was further improved when holes at nanoscale were created in the TiN film of about 19 nm–27 nm in thickness (NH–TiN). The roughened surface and nanohole patterns provided confinement of surface plasmons and significantly improved the sensitivity toward the local refractive index changes. In detail, the calculated refractive index resolution (RIR) of the optimal NH–TiN sensors for NaCl was found to be 9.5 × 10⁻⁸ refractive index unit (RIU), which had outperformed the f-TiN and R–TiN sensors. For biosensing, the optimized NH–TiN sensor was found to be capable to detect both small and large biomolecules, i.e. biotin (molecular weight of 244.3 g/mol) and human IgG (160,000 g/mol), in a label-free manner. Especially, the NH–TiN sensor significantly improved sensitivity in detecting small molecules due to the localized plasmonic confinement of electromagnetic field. Combining with the excellent mechanical and durability properties of TiN, the proposed NH–TiN can be a strong candidate for plasmonic biosensing applications.     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Atomistic aspects of ductile responses of cubic silicon carbide during nanometric cutting

Cubic silicon carbide (SiC) is an extremely hard and brittle material having unique blend of material properties which makes it suitable candidate for microelectromechanical systems and nanoelectromechanical systems applications. Although, SiC can be machined in ductile regime at nanoscale through single-point diamond turning process, the root cause of the ductile response of SiC has not been understood yet which impedes significant exploitation of this ceramic material. In this paper, molecular dynamics simulation has been carried out to investigate the atomistic aspects of ductile response of SiC during nanometric cutting process. Simulation results show that cubic SiC undergoes sp³-sp² order-disorder transition resulting in the formation of SiC-graphene-like substance with a growth rate dependent on the cutting conditions. The disorder transition of SiC causes the ductile response during its nanometric cutting operations. It was further found out that the continuous abrasive action between the diamond tool and SiC causes simultaneous sp³-sp² order-disorder transition of diamond tool which results in graphitization of diamond and consequent tool wear.     

Flexible and high-sensitivity piezoresistive sensor based on MXene composite with wrinkle structure

Due to the portability, good flexibility and excellent sensing performance, flexible piezoresistive sensors have received great attention in the field of transient electronic skin, intelligent robots and human-machine interaction. However, achieving both high sensitivity and wide sensing range by low-cost and large-scale method still remains a key challenge for developing high performance piezoresistive sensors. Here, a flexible and highly sensitive piezoresistive sensor was designed and realized by combining the 2D MXene material with wrinkle structure. The MXene composite based sensor with wrinkle structure was fabricated by spraying the active material onto the surface of a pre-stretched polyacrylate tape, which is facile, efficient and low-cost. The MXene composite based sensor demonstrates high sensitivity (148.26 kPa⁻¹), wide pressure range (up to 16 kPa), short response time (120 ms) and excellent durability (>13000 cycles). Moreover, benefiting from the extraordinary sensing performance and flexibility, the sensor can detect human physiological signals, monitor intelligent robot postures and map spatial pressure distributions, thus exhibiting great potential in physiological analysis systems, humanoid robotics and biomedical prostheses.     

A direct-write,resistless hard mask for rapid nanoscale patterning of diamond

We introduce a simple, resist-free dry etch mask for producing patterns in diamond, both bulk and thin deposited films. Direct gallium ion beam exposure of the native diamond surface to doses as low as 10¹⁶ cm^?2 forms a top surface hard mask resistant to both oxygen plasma chemical dry etching and, unexpectedly, argon plasma physical dry etching. Gallium implant hard masks of nominal 50 nm thickness demonstrate oxygen plasma etch resistance to over 450 nm depth, or 9:1 selectivity. The process offers significant advantages over direct ion milling of diamond including increased throughput due to separation of patterning and material removal steps, allowing both nanoscale patterning resolution as well as rapid masking of areas approaching millimeter scales. Retention of diamond properties in nanostructures formed by the technique is demonstrated by fabrication of specially shaped nanoindenter tips that can perform imprint pattern transfer at over 14 GPa pressure into gold and silicon surfaces. This resistless technique can be applied to curved and non-planar surfaces for a variety of potential applications requiring high resolution structuring of diamond coatings.     

Durable and highly sensitive flexible sensors for wearable electronic devices with PDMS-MXene/TPU composite films

MXenes, as two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides, have very excellent electrical properties and surface activity and are increasingly used in supercapacitors, batteries, electromagnetic interference shielding, and composite materials. Still, the poor stability of MXene when exposed to aqueous oxygen and the poor ability to interact with the polymer matrix have become important factors limiting its’ practical applications. To enhance stability, highly conductive and stretchable Ti₃C₂MXene/TPU sensing elements were prepared by a simple spraying process using thermoplastic polyurethane (TPU) as a substrate, and the sensing elements were encapsulated by polydimethylsiloxane (PDMS) to obtain MXene-TPU/PDMS constructed flexible strain sensors with excellent performance. This strain sensor features low detection limits (less than 0.005%, 0.5 μm), a wide sensing range (0–90%), a short response time (120.1 ms), and excellent durability (>3000 cycles). This strain sensor can be applied to a range of applications such as health detection, motion signals, detection of robot movements, and wearable electronic devices.     

A ZnO thin-film driven microcantilever for nanoscale actuation and sensing

Zinc oxide (ZnO) thin film as a piezoelectric material for microelectromechanical system (MEMS) actuators and sensors was evaluated. ZnO thin films were deposited using radio frequency (RF) magnetron sputtering. Process parameters such as gas ratio, working pressure, and RF power were optimized for crystalline structure. The ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Good quality of ZnO thin films was further confirmed by a high transverse piezoelectric coefficient d ₃₁. A microcantilever was then designed, fabricated, and characterized. Design formulas of resonant frequency, actuation, and sensing sensitivities were derived. The resonant frequency was determined by an impedance analyzer. Tip deflection on nanometer level was demonstrated with the cantilever used as an actuator. The actuation sensitivity was found to be 12.2 nm/V. As a sensor, the cantilever was calibrated against a reference accelerometer. The sensing sensitivity was characterized to be 46 mV/g. The characterization results were compared with design specifications. The differences were caused mainly by thickness control in etching. This study showed that ZnO is a promising piezoelectric material for MEMS actuators and sensors in terms of excellent process compatibility and good piezoelectric performance.     

Effect of Al2O3-SiO2 substrate on gas-sensing properties of TiO2 based lambda sensor at high temperature

Alumina is widely used as substrate material for oxygen lambda sensors. It has been reported that response properties of sensors are highly dependent on surface state of substrates. In this work, Al₂O₃-x%SiO₂ (x = 2–30) substrates were specially prepared for TiO₂ based lambda sensors for high temperature operation. Structure and surface state of prepared substrates were characterized by XRD, SEM and XPS. TiO₂ sensing film was prepared by screen printing method. Results indicated that sensors fabricated on Al₂O₃-10%SiO₂ substrate exhibited the best sensing properties. Moreover, final steady-state voltages of all sensors were limited to less than 100 mV at 600–800 ℃.     

Surface acoustic wave properties of natural smooth ultra-nanocrystalline diamond characterized by laser-induced SAW pulse technique

Diamond is one of the best SAW substrate candidates due to its highest sound velocity and thermal conductivity. But conventional diamond films usually express facet structure with large roughness. Ultra-nanocrystallined diamond (UNCD) films grown in a 2.45 GHz IPLAS microwave plasma enhanced chemical vapor deposition (MPECVD) system on Si (100) substrates in CH₄-Ar plasma possess naturally smooth surface and are advantageous for device applications. Moreover, highly C-axis textured aluminum nitride (AlN) films can be grown by DC-sputtering directly on UNCD coated Si substrate. However, properties of UNCD films are much complex than microcrystalline diamond films, that is because this is a very complex material system with large but not fixed portion of grain boundaries and sp²/sp³ bonding. Properties of UNCD films could change dramatically with similar deposition condition and with similar morphologies. A simple and quick method to characterize the properties of these UNCD films is important and valuable. Laser-induced SAW pulse method, which is a fast and accurate SAW properties measuring system, for the investigation of mechanical and structure properties of thin films without any patterning or piezoelectric layer.