# Green Approaches to Carbon Nanostructure-Based Biomaterials
## 2 Green Routes to Prepare Carbon Nanomaterials for Biomedical Applications
### Carbon Quantum Dots (CQDs)  
Carbon quantum dots (CQDs) are quasi-spherical nanoparticles characterized by pronounced luminescent properties, thus very appealing for sensing, and, recently, further morphologies have been attained to fine-tune their luminescent profiles [101], which depend also on the carbon source used [102]. Other applications include photodynamic (antibacterial) therapy [56; 103; 104], radiolabeled imaging [55], various forms of bioimaging [60; 71], and sensing [69; 60; 61; 61]. They have also been investigated for their antioxidant activity against radical oxygen species (ROS) mediated cell damage [54; 64], and as drug delivery agents [105].  
Graphene quantum dots (GQDs), which consist of nano-sized graphene monolayers that exhibit quantum confinement, can be produced in a variety of methods, of which the most popular are pyrolysis [69; 70] and hydrothermal [57; 58; 61; 63; 64] routes. Other methods that could be carried out at lower temperatures have attracted attention, especially using microwaves [67; 68; 106], but also ultrasound-assisted approaches [72]. Recently, an electrochemical green approach was developed that employed water and ethanol as solvents [55]. In this case, the dots were also radiolabeled with technetium-99m for imaging purposes, and in vivo experiments overall revealed a good profile in terms of biosafety, although some signs of mutagenic activity were noted [55]. Finally, GQDs were produced by UV-triggered radical polymerization of oxygen-containing aromatic compounds in water for bioimaging, producing just water and carbon dioxide as side-products [107].  
A plethora of natural sources have been recently reported for the green synthesis of CQDs. Citric acid and penicillin underwent pyrolysis to yield nitrogen- and sulfur co-doped CQDs that could detect mercury ions in living cells by means of fluorescence quenching [69]. Concerning plants, and plant derived-materials, various carbon sources have been used. They include fruit parts, such as seed extract [108], flesh [59], peel [61; 71], juice [60], and fruit waste [62]. Alternatively, hemicellulose [109] or cellulose, combined with caffe acid as a green reducing agent [67], or other plant parts, such as Gynostemma plant [54], red cabbage [64], palm-derived powder [110], green tea [63], mint [111], or turmeric [104] leaves, sunflower seeds [112], roasted chickpeas [68], wheat straw [66], soybean residues [113], and sandalwood powder [114]. Some of these carbon sources have been used with the prospect of recycling household kitchen waste, although in line of principle, the same concept could be applied to industrial waste. With this idea in mind, sugarcane bagasse pulp found a second use to be converted in antibacterial CQDs [65].  
In the majority of cases, CQDs are used in solution, as they were shown to enter cells by endocytosis and could be tailored to target specific subcellular organelles [108]. However, more research is emerging on their use in composites, for instance to reinforce films for tissue engineering applications [67], to yield luminescent hydrogels [115], or UV-responsive smart (bio)materials [58; 116], and antibacterial composites [117].