Carbon for Sensing Devices

Our own existence witnesses the peculiar role played by carbon among all atoms of the periodic table. The energy levels of the various bonding hybridizations of the carbon atom are so close that different configurations can exist in normal earth environments, bringing about life. The various hybridizations have an impact on the structure and properties of inorganic carbon based materials too. In this chapter we will review this last subject looking at the various carbon structures that arise from the different local bonding configurations.

In carbon materials technology the degree of development can be used to classify the various kinds of available carbon materials in three different stages. Conventional carbon materials include graphite blocks, the family of carbon blacks, activated carbons and diamond. Among the newly developed materials two types can be distinguished: nanotextured carbons and nanosized carbons. Nanotextured carbons comprise a wide range of carbon structures from carbon fibers, glass-like carbons or pyrolitic carbons to diamond-like carbon materials. Among nanosized carbons (or nanocarbons) fullerenes, carbon nanotubes and graphene can be quoted.

Polymer composites containing carbon-based fillers have recently received considerable attention due to their remarkable properties (i.e. mechanical, electrical, thermal and optical) together with low weight, easy processing.

Among the main carbon-based fillers, carbon nanotubes (CNTs), expanded graphite (EG), graphite nanoplateles (GNPs), graphene oxide (GO) and graphene (GR) attracted considerable attention in a great variety of applications such as chemical and biosensors, energy storage materials, field effect transistors, polymer composites, etc. In particular, CNTs and GRs are considered ideal materials for preparing “metal-free” conductive polymer composites or for reinforcing materials with potential applications in aerospace and automotive sectors, where lightweight and robust materials are needed.

Carbon nanomaterials represent a major area of transducers in (bio)chemical sensors. In the field, one common goal is to improve the analytical performances: the sensitivity, the selectivity and the stability. Such parameters are indeed mandatory to validate the sensor. Although carbon nanomaterials are very sensitive to their chemical surrounding, their lack of selectivity requires the use of a selective recognition element. Here, the functionalization process demonstrates to play a key role in producing such devices. Several approaches have been reported over the last decade within a broad range of detection techniques. In the present chapter book, we consider the analytical parameters of electrochemical sensors from the perspective of the chemical functionalization methodology. The chapter covers a brief introduction where the main challenges for a suitable functionalization process are described. Then, the two main functionalization strategies are treated: the covalent and the non-covalent approaches. A brief account on the characterization of functionalization is given before presenting the concluding remarks.

Carbon nanomaterials offer a number of possibilities for sensing platforms. The ability to chemically functionalize the surfaces of the nano-carbon, using hybrid or nano-composite structures, can further enhance the material properties. Complementary to the addition of any requisite chemical or biochemical functionality, such enhancements can take the form of improved electrical, optical or morphological properties which improve the transduction capabilities of the carbon nano-material, or facilitate detection of the transduced signal, for example by improving charge transfer to detection electronics. Here we review the methods of producing hybrid and nano-composite carbon structures for sensing systems, highlighting the advantages of the functionalization in each case and benchmark their performance against existing carbon-only devices. Finally, we detail some of the recent applications of hybrid and nano-composite carbon technologies in a wide variety of sensor technologies.

Electrochemical detection is one of the most powerful techniques in sensing applications, in particular for biomedical sensors. Its combination with carbon-based materials gives several advantages in terms of sensitivity, increase of electrode area and biocompatibility. In this chapter a first part will be focused on the electrochemical properties of carbon, then several electrode nanostructuration approaches will be presented. The last part will describe the use of these techniques for the detection of metabolites, and an application based on the use of Carbon NanoTubes in ElectroChemiLuminescence detection will conclude the chapter.

There is a growing interest in using technology to provide diagnostics, therapeutics, and measured drug delivery to individuals suffering from a myriad of issues. Many of these biotechnological devices need to interact with the human body indefinitely, and it has therefore become a priority that these devices interact with the body with an increased level of biocompatibility. Biocompatibility is a complex concept which has been redefined many times due to our ever increasing knowledge of human physiology. In essence, it is defined as the ability of a device to function as it was designed while not interacting physiologically in a deleterious manner. Numerous materials have been used to fabricate biotechnological devices but, unfortunately, many have not shown the level of biocompatibility necessary to perform their intended function within an appropriate period of time. In this chapter, we detail two materials that are ideal for long-term biotechnological devices. Silicon carbide has shown superior biocompatibility through international standard based testing, both in vitro and in vivo. This material possesses excellent physical robustness, chemical resistivity, and multiple options for smart devices through its electrical, chemical and optical properties. It is also an ideal surface for which to develop graphene, another important material with superior physical, chemical and electrical properties. We briefly detail some of the biotechnology already developed and used successfully in clinical trials. At the end of the discussion, we report on research being performed on two types of implantable devices: one that will allow continuous monitoring of blood glucose levels for diabetics, and an implantable neural interface system for use in brain machine interfaces (BMI) and therapeutics for diseases like Parkinson’s and Alzheimer’s diseases.

Nano composites (NC) are a class of materials widely investigated in the last decade. NC has broadened significantly to encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinct dissimilar components and mixed at the nanometer scale. The general class of organic/inorganic NC materials is a fast growing area of research. The properties of NCs depend not only on the properties of their individual component, but also on their morphology and interface characteristics. The large interface area between the matrix and the nano filler is a key issue for NCs.

Diamond is wide band gap semiconductor presenting many extreme properties. It is notably known as the most stable material with the highest chemical inertness, the highest mechanical hardness and the highest thermal conductivity. Since the mid 1970s it has been possible to grow synthetic diamond by several methods. High Pressure High Temperature techniques that mimic the diamond formation in the earth’s crust were first developed. Then Chemical Vapour Deposition (CVD) methods enable diamond growth at laboratory scale as well as the control the P-type and N-type doping of diamond. Besides, it is possible to tune the diamond electrical properties form very resistive to metallic thanks to the P-type doping with boron. Current achievements have enabled the development of diamond sensors that can operate in extreme conditions. After being used for its mechanical and thermal properties, diamond was considered for chemical sensing. In fact the chemical stability and the close-to-metallic conductivity of diamond make it a powerful tool for electrochemical detection in various environment. Furthermore, the diamond is an ideal substrate for surface functionalization thanks to the wide and very known carbon based chemistry. Such a feature combined to the outstanding electrochemical properties of the diamond electrodes have enable the production of very efficient biosensors and biochips. Diamond is also an interesting sensor for medical imaging. Its carbon nature, well tolerated by living tissues, are actually very useful for its use as a biosensor capable of working in contact with bio-environments as well as real neuronal interfaces. Both those topics will be discussed in details in the following pages. In a first part an overview on electrochemical based biosensors and their performance is described. Then in a second half of the chapter, novel applications where diamond is directly used as an electrode for neural tissue interfacing is presented in details.