## 3 Results and Discussion
### Experimental Conditions
#### 3.3.2 Drug Release  
The drug release conditions and maximum cumulative drug release for each of the publications are shown in Table S4. The temperature for in vitro drug release experiments was consistently kept at 37 \({}^{\circ}\)C, excluding three papers where the temperature was not recorded. The pH varied between the range of 2-10 between publications; however, drug release was most consistently recorded under (a) pH 7 or 7.4, which aims to represent cell surface or cytosolic neutral pH, or (b) a more acidic pH of 5.0-5.4, which reflects the lysosomal pH. Sixteen of the analysed publications show a controlled release of DOX from the nanocarrier over a period greater than twenty-four hours. One of the main target functions of a nanocarrier is to provide controlled release of the loaded drug. This functionality is important as a controlled system aims to reduce toxic side effects as compared to burst release by prolonging therapeutic efficacy, maintaining optimal concentrations, preventing degradation, and preventing fluctuations in concentrations [85]. A nanocarrier is also responsible for delivering the loaded drug to the target site. All of the analysed publications show a stimuli-responsive release of DOX from the nanocarriers, with fifteen studies being responsive to pH and two studies being stimulated by either NIR or ultrasound. With the presence of a stimulus, the amount of drug released is significantly increased. For example, the nanohybrid GO-PAC/HEC showed a release of 80% at pH 5.0 after 35 h; in contrast, 20% of the loaded drug was released after 35 h at pH 7.4 [64]. The increased release in acidic conditions is attributed to the partial dissociation of the hydrogen bonding between DOX and GO [72]. Under neutral conditions, DOX can form four hydrogen bonds with a GO sheet through the -COOH and -OH functional groups on GO and the -NH\({}_{2}\) and -OH group on DOX. Under acidic conditions, the DOX becomes protonated, changing the -NH\({}_{2}\) to an -NH\({}_{3}\)\({}^{+}\) group that cannot partake in hydrogen bonding. The hydrogen bonding strength is lower under acidic conditions, thereby increasing the hydrophilicity and solubility of DOX and facilitating its release [36,64,67]. pH-responsive nanocarriers are useful in cancer research as tumour microenvironments have an acidic pH [50]. This acidity can be utilised to trigger intratumoral release of drugs from nanocarriers by taking advantage of specific surface chemistry interactions [65]. The GO-TD-Fe\({}_{3}\)O\({}_{4}\)@PEG/DOX nanohybrid showed the most successful release of DOX, with 97.3% being released after 72 h at a pH of 5.0 [67]. Of the publications that use a pH response to monitor drug release, 73% show that more than half of the maximum cumulative release occurs in a controlled manner within the first twelve hours. This outcome may be attributed to the initial breaking of hydrogen bonding between the polar groups of DOX and the nanocarrier; the \(\pi\)-\(\pi\) stacking interactions are responsible for the observation of continued release throughout the time frame [75]. Typically, the publications that did not show this rapid initial DOX release were those that measured release over longer time frames. It should be highlighted that several publications demonstrate a high drug release at neutral conditions. The nanohybrids Zn-d3-rGO and FA-BSA/GO release over half of the loaded drug without any stimuli, thereby suggesting that less than half of the DOX loaded will reach the target site [50,68,70]. Alternatively, NIR- and ultrasound-stimulated release enhance the delivery of DOX from graphene nanocarriers through the absorption of energy. NIR release is used due to the high NIR absorbance of graphene and its derivatives; through light absorption and photothermal conversion, the loaded drug is released. The release can be enhanced by the incorporation of other NPs.