# Diamonds for Life: Developments in Sensors for Biomolecules
## 1 Introduction  
A biosensor is, generally speaking, a device composed of a receptor layer (RL) and a transducer, as schematically depicted in Figure 1. Upon contact with a solution of the target analyte (e.g., DNA, proteins, lipids, saccharides), it binds to the RL, and the transducer typically translates the event into an output signal, such as an optical, thermal, piezoelectric, electrochemical, or electrochemimuminescent signal, to name a few.  
Diamond gathers a unique set of chemical and electronic properties that make it a quite a promising material for the composition of the transducer layer of a biosensor [1]. Chemical inertness makes diamond thin films safe to use in biological applications [2, 3] while the inherent hardness and mechanical resistance of diamond allow diamond-based sensors to be very durable and to operate under aggressive conditions such as high or low pH media [4].  
Figure 1: Schematic representation of a biosensor.  
Natural diamond is an insulator with a very wide bandgap (\(\approx\)5.5 eV) [5] and an exceedingly high electrical resistivity (10\({}^{16}\)\(\Omega\)-cm), hindering its application in electronics [6]. In turn, synthetic diamond deposited by chemical vapour deposition (CVD) in hydrogen-saturated conditions presents a much lower surface resistivity, with a value around 10\({}^{6}\)\(\Omega\)-cm [7]. The conductivity of diamond can be further tailored by including lattice-selected impurities, such as boron, nitrogen, and phosphorous, during the CVD process.  
The target-specific receptor layer depicted in Figure 1 can be achieved by the functionalisation of hydrogen-terminated CVD diamond films with amino or carboxylic pending moieties capable of linking a biomolecule. Photochemical methods (e.g., alkenes) or electrochemical modifications (via reduction of diazonium salts) are the most common methods to attach these pending groups to the diamond surface [8]. This is followed by a second reactional step, where a complementary biomolecule (to the target analyte) is attached to these linkers, thus conveying high sensitivity and specificity to the diamond layer.  
This review describes the main methods of fabricating diamond-based biosensors and is organised as follows: Section 2 describes the characteristics of diamond films deposited by CVD and the functionalisation of their surface using plasma treatments; Section 3 presents an overview of the photochemical, electrochemical, and green chemistry methods used in the surface modification of diamond films; Sections 4 and 5 describe the practical applications of diamond-based electrochemical sensors and of biosensors with a diamond receptor layer, respectively. We note that the sensing devices are listed according to the type of analyte, with electrodes finding applications in the analysis of small, redox-active biomolecules such as histamine, dopamine and other neurotransmitters, and biosensors with a larger surface area being used on the recognition of biomolecules such as DNA and proteins. At the end of the review, a comparison weighing the benefits and drawbacks of these various types of detectors is presented.