(Invited) Diamond Based Chemical/Biochemical Sensors: State-of-the-Art and Perspectives
Authors: Scorsone Emmanuel 
Published: July 2021

Diamond is grown in some laboratories by either HPHT process or Plasma-Enhanced Chemical Vapor Deposition (MP-CVD) since a few decades. Single crystal diamond exhibits outstanding properties including a high optical transparency over a broad electromagnetic spectrum, high thermal conductivity approx. five times higher than copper, and acoustic wave velocity close to 19 000 m.s-1. It displays also remarkable mechanical properties with e.g. a Young’s modulus exceeding 1000 GPa along with high resistance to fracture, to name a few. Some of these properties remain also remarkable in its polycrystalline form when compare to most other materials. Furthermore, diamond can be doped with nitrogen or boron during growth, offering electrical properties from semiconducting to quasi-metallic regimes. When heavily doped with boron (~2.1021 cm-3), the so-called Boron Doped Diamond (BDD) electrodes become attractive electrodes featuring a high potential window > 3V in water and low double-layer capacitance. Moreover, diamond is extremely resilient to corrosion and more generally to chemical attacks. It is also biocompatible, which makes it very attractive for in-vivo sensing applications. Finally, the carbon nature of the diamond offers wide opportunities for surface grafting of chemical or biochemical functional groups through highly stable covalent carbon-carbon bonding. These properties can be exploited advantageously to enhance the analytical performances and stability of chemical/biochemical sensors and have motivated our research over the last 15 years. Our work focuses mainly on polycrystalline diamond thin films that can be grown typically on inches silicon substrates, thus offering access to some clean-room processes and potentially large-scale production.
Several processes were elaborated to micro-pattern diamond layers in order to design chemical transducers such as gravimetric MEMS devices, electrodes, field effect transistors, etc. Diamond microstructures may also be transferred to flexible parylene or polyimide substrates, thus making them attractive e.g. for wearable sensors and implantable medical electrodes. Further techniques have also been developed to enhance the active surface area of diamond transducer surfaces at the nanoscale and thus increase drastically the sensitivity of the resulting sensors by multiplying the number of active sites. Here diamond is typically grown onto well-chosen high aspect ratio templates that can withstand the growth conditions of diamond in high-density hydrogen plasma. Most of these methods have been “standardized”. They involve clean-room processing including dry etching, photolithography, and so on and forth.
As examples, heavily doped diamond electrodes were developed successfully both as macro- and micro-electrodes for biomedical, pharmaceutical or foodstuff analysis applications. These applications benefit both from the high analytical performances of diamond electrodes in particular due to their low background signals and high reactivity, and high stability and reliability. BDD electrodes may also be modified with transition metal nanoparticles to enhance their catalytic behavior. From this concept, diamond multi-electrode arrays were designed for chemical patterns identification, for instance for sensory analysis of coffee, or for the monitoring of environmental pollutants. BDD electrodes offer also significant advantages in electrochemiluminescence (ECL) techniques, which are being investigated for various applications ranging from foodstuff analysis to narcotics detection. A key benefit of BDD electrodes for all of the above applications is certainly that they can be electrochemically reactivated following fouling, sometimes directly in the analytical medium, to maintain high reactivity thus opening the way to reusable sensors and online monitoring. Besides, BDD microelectrode arrays have also been transferred to flexible substrates for neural stimulation and recording, along with in-vivo neuromodulators measurements. Finally, diamond based MEMS devices (microcantilevers, SAW sensors) take advantage both of the mechanical properties of diamond, along with steady carbon interface for convenient bio-functionalization. Our work here focused mainly on the detection of odorant molecules, using biomolecular receptors involved in olfaction in Nature as sensitive layers, including Odorant Binding Proteins (OBPs), Major Urinary Proteins (MUPs) and Olfactory Receptors (OR). Multisensor array instrumentations were developed around this concept, for applications ranging from breathe analysis to security applications.