# Carbon Nanoparticles and Their Biomedical Applications
## 10 Detection, Electrochemical Biosensors  
Biosensors are used to detect biomarkers associated with certain diseases. They allow the diagnosis of diseases and monitoring of their progress. Biomarkers are biologically important molecules, such as metabolites, proteins, nucleic acids, hormones, antigens, glucose, uric acid, cholesterol, acetone, hydrogen sulfide, glutathione, and vitamin B\({}_{12}\), which can be detected in blood, saliva, urine, sputum, or breath [98; 99]. The CNPs can be used to analyze the levels of hydrogen sulfide, toluene, acetone, benzene, ammonia, and nitrous oxide levels in the breath to diagnose distinct diseases, such as diabetes, lung or gastric cancer, halitosis, asthma, poor liver, and kidney function [100]. Specifically, CNP-detected substances associated with lung cancer include benzene, styrene (etheinylbenzene), hexanal, 2,2,4,6,6-pentamethyl heptanes, propyl benzene, 2-methyl heptanes, decane, 1,4-dimethyl benzene, 3-methyl nonane, 1-hexene, heptanal, undecane, 1-methyl-2-pentylcyclopropane, methyl cyclopentane, trichloro-fluorobenzene, 1-methylethenyl benzene, cyclohexane, 1-heptene, 3-methyl octane, 1,2,4-trimethyl benzene, 2-methyl-(isoprene)-1,3-butadiene, and 2,4-dimethyl heptanes [101]. For the detection of myocardial damage, graphene and its derivatives and carbon nanotubes can be used. They detect myoglobin, creatinine kinase, troponins, and B-type natriuretic peptides [90; 102]. Glucose monitoring is the standard practice for the diagnosis and control of diabetes. CNPs (nanotubes, graphene, graphene dots) with attached glucose oxidase detect glucose with high sensitivity and selectivity (carbon nanosensors were tested in the presence of interferents, such as acetaminophen and uric and ascorbic acid) [103]. [PERSON] et al. prepared a single-walled carbon nanotube biosensor with a detection limit of 50 \(\upmu\)M and an ultrafast response of less than 5 s. They designed wearable glucose sensors that can be combined with a smartphone to serve diabetic patients [103]. Glucose can also be measured using non-enzymatic detection with distinct graphene-based electrodes. These electrodes can be functionalized with a wide range of nanoparticles to improve their detection limits, for example, the detection limit of copper nanocubes with reduced GO is very low at 250 nM, while reduced GO (in combination with SnO\({}_{2}\)) has a detection limit of 13.35 \(\upmu\)M [104]. Monitoring cholesterol levels is clinically essential. For this purpose, both enzymatic and nonenzymatic methods can be used. The enzymatic method uses CNP electrodes enriched with cholesterol oxidase enzymes [105]. For nonenzymatic detection, graphene functionalized with \(\beta\)-cyclodextrintrim is often used. Electrochemical detection is mediated by methylene blue (MB) (redox indicator), which forms complexes with the nanomaterial. MB is replaced by cholesterol and released into the buffer solution [106]. Nanoparticles can also be used to detect compounds associated with neurodegenerative processes. [PERSON] measured the levels of the \(\alpha\)-synuclein protein (which is associated with [PERSON]'s disease) in human plasma by graphene conjugated with gold nanoparticles. The dynamic range and lower limits of quantification were determined to be from 4 to 128 ng/mL and 4 ng/mL, respectively [107]. Diagnoses of bacterial diseases can be prolonged and complicated. CNPs can greatly simplify and speed up the diagnostic process. Graphene loaded with specific antibodies can detect many types of pathogens, such as _E. coli_, _Salmonella typhimurium_, and Zika virus. GO might be used, for example, for the detection of dengue virus and rotavirus [108]. Recently, scientists have shown that CNPs can be used for SARS-CoV-2 detection from body fluid or exhaled breath. These methods are very effective, fast, and relatively inexpensive compared to commonly used methods [109; 110; 111]. CNP can also be designed for real-time monitoring, e.g., to monitor the release of neurochemicals from the nervous system.