catalysts Review Carbonaceous Nanomaterials Employed in the Development of Electrochemical Sensors Based on Screen-Printing Technique—A Review Alexandra Virginia Bounegru and Constantin Apetrei * Department of Chemistry, Physics and Environment, Faculty of Sciences and Environment, “Dunărea de Jos” University of Gala¸ ti, 47 Domnească Street, 800008 Gala¸ ti, Romania; alexandra.meresescu@ugal.ro * Correspondence: apetreic@ugal.ro; Tel.: +40-727-580-914 Received: 4 June 2020; Accepted: 15 June 2020; Published: 17 June 2020   Abstract: This paper aims to revise research on carbonaceous nanomaterials used in developing sensors. In general, nanomaterials are known to be useful in developing high-performance sensors due to their unique physical and chemical properties. Thus, descriptions were made for various structural features, properties, and manner of functionalization of carbon-based nanomaterials used in electrochemical sensors. Of the commonly used technologies in manufacturing electrochemical sensors, the screen-printing technique was described, highlighting the advantages of this type of device. In addition, an analysis was performed in point of the various applications of carbon-based nanomaterial sensors to detect analytes of interest in different sample types. Keywords: nanomaterials; graphene; carbon nanotubes; fullerene; carbon nanohorns; carbon nanofibers; quantum dots; electrochemical sensor 1. Introduction Electrochemical sensors have been extensively developed as a simple, fast, and accessible method of sensitive detection and quantification of a wide variety of analytes [1–3]. In particular, electrochemical sensors based on carbonaceous materials are often used due to their high sensitivity, low cost, good stability, and biocompatibility [4,5]. The most common carbon-based electrochemical sensors include electrodes of glassy carbon, graphite, carbon nanoparticles, carbon fibres, carbon microspheres, etc. [6–10]. Carbonaceous nanomaterials such as graphene, graphene oxide, fullerenes, carbon nanohorns, carbon nano-onion, diamond nanoparticles, carbon quantum dots, carbon nanofibers, carbon nanotubes, etc. have been extensively utilized in sensing applications [11]. However, nanomaterials of great importance in sensing were developed and used for developing novel sensors and biosensors with improved performance characteristics and ultrasensitivity [12]. The nanomaterials offer the advantages of high surface-volume ratio and large specific surface, characteristics that are of great importance in sensing [13]. In addition, carbonaceous nanomaterials have improved interfacial adsorption features, higher rate of electron transfer, better electrocatalytic properties, and biocompatibility compared to many materials used in classical electrochemical sensors [14,15]. Furthermore, the carbonaceous nanomaterials immobilized on electrode surface facilitate electron transfer during electrochemical reactions and these favourably interact with both electroactive ionic and covalent species [11,16]. 2. Nanomaterials used in Development of Electrochemical Sensors On 18 October 2011, the European Commission adopted the following definition of a nanomaterial: “A natural, incidental or manufactured material containing particles, in an unbound state or as an Catalysts 2020, 10, 680; doi:10.3390/catal10060680 www.mdpi.com/journal/catalysts