(Invited) Smart Material for Smart Sensors: Addressing Future Challenges in Bio/Chemical Sensing with Diamond

Abstract

Introduction Synthetic diamond can be grown in the laboratory by Plasma-Enhanced Chemical Vapor Deposition (MP-CVD) both as single crystals or polycrystalline thin films [1]. This material exhibits outstanding physical and chemical properties, including typically a high optical transparency over a broad electromagnetic spectrum, high thermal conductivity (~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 and high resistance to fracture. Diamond may also be doped with boron during growth, offering electrical properties from semiconducting to quasi-metallic regimes. When heavily doped with boron, it becomes an exceptional electrode material featuring a high potential window > 3V in aqueous environments, and low capacitive background current. Moreover, diamond is extremely resilient to chemical attacks, and corrosion, and several studies have shown that it is a biocompatible material, which makes it very attractive for in-vivo sensing applications. Finally, the carbon terminated surface of diamond layers enables grafting of a wide range of chemical or biochemical functional groups through highly stable C-C covalent bonding. All those properties, which can be advantageously exploited to enhance the analytical performances and stability of chemical/biochemical sensors, have motivated our research over the last decade. Diamond-based sensor fabrication methods Polycrystalline diamond thin films are generally grown by MP-CVD in a hydrogen plasma, and in the presence of methane as the source of carbon. Several processes have been developed to micro-pattern diamond layers, either by top-up or top-down approaches, in order to design chemical transducers technologies including gravimetric MEMS devices [2], micro-electrodes or IDT electrodes [3], Field Effect Transistors, etc. (Fig.1). Some of these microstructures may also be transferred to flexible substrates like parylene or polyimide [4], thus making them attractive e.g. for wearable sensors. Further techniques have also been developed to enhance the active surface area of diamond transducer surfaces and thus the sensitivity of the resulting sensors. To this end, diamond can be grown for instance onto high aspect ratio templates such as vertically aligned CNTs or on some well selected high-porosity polymers. Most of these methods have been “standardized” and involve clean-room processing including dry etching, photolithography, and so forth. Results and applications Bare diamond electrodes, when heavily doped with boron (~2.1021 cm-3), have been developed successfully both as macro- and micro-electrodes for biomedical (e.g. detection of uric acid in human urine) or pharmaceutical applications. These applications benefit here both from the high analytical performances of diamond electrodes, and high stability and reliability. In particular 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. Such electrodes may also be functionalized by transition metal nanoparticles to enhance the catalytic behavior of the electrodes. From this concept, diamond multi-electrode arrays may be designed for chemical patterns identification. This approach was used for instance for coffee capsule discrimination [5]. Several assets of boron doped diamond electrodes were also be used in combination with electrochemiluminescence (ECL) techniques for foodstuff analysis, namely the selective detection of skatole in pig fat in the context of boar taint detection. Diamond microelectrode arrays have also been developed on flexible substrates for neural stimulation and recording, along with in-vivo neuromodulators measurements.Besides, a range of diamond based MEMS devices were developed (microcantilevers, SAW sensors), taking advantage both of the mechanical properties of diamond, along with steady carbon interface for convenient bio-functionalization. Our studies focused mainly on odor detection, 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 security applications or breathe analysis.