# Investigation of Gentamicin Release from
## 4 Conclusions  
In conclusion, this report sets out the detailed monitoring of in vitro gentamicin release from the G-PDA NPs in response to pH, using the dialysis method. The release test was performed for two batches with a variable gentamicin-loading amount, viz.; G-PDA NPs 1:1 and G-PDA NPs 0.6:1 at three pH conditions 3, 5 and 7.4. The PDA was found to be a stable nanocarrier in terms of drug release at an alkaline pH. The PDA, being zwitterionic in nature, provides negative active sites to chemically interact with gentamicin at a working alkaline pH. Therefore, at pH 7.4, the PDA exhibited good stability with a lower amount of gentamicin release over seven days. It is attributed to the deprotonation of the phenolic hydroxyl groups of PDA at an alkaline pH which strongly attaches to the positively charged amine groups of gentamicin. Unlike at pH 5 and 3, the amino groups of PDA are more prone towards protonation, making PDA positively charged, hence the repulsive forces cause the gentamicin to detach from its surface. Therefore, approx. 40% and 55% of the drug release was observed at pH 5 and 3, respectively. This study of gentamicin release at acidic pH was required to confirm the G-PDA NPs' behavior during local acidification caused by bacterial metabolism, hence assuring the efficacy of gentamicin. On the other hand, the G-PDA 0.6:1 with a lower loading percentage, showed a lower release under the same experimental conditions, which was attributed to the concentration gradient effect.  
Two kinetic models were implemented to the drug release data, namely Ritzer-Peppas and Higuchi's models, to investigate the drug release kinetics. This kinetic analysis further supported the pH-dependent drug release, hence confirming our observations, and PDA structural changes in response to pH. These findings suggested that the in situ loading method for gentamicin could provide long-term drug release over an extended period. Such an on-demand drug delivery system could be beneficial in treating various pathological tissues, meeting the drug efficacy and safety challenges.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12136319/s1](https://www.mdpi.com/article/10.3390/app12136319/s1)]([https://www.mdpi.com/article/10.3390/app12136319/s1](https://www.mdpi.com/article/10.3390/app12136319/s1)), Figure S1. Calibration curve for gentamicin; Figure S2. C1a isomer (\(m/z\) 450.4) of gentamicin at various concentrations; Figure S3. C1 isomer (\(m/z\) 464.5) of gentamicin at various concentrations; Figure S4. C1 isomer (\(m/z\) 478.5) of gentamicin at various concentrations; Table S1. Various drug release models and their mechanisms.  
Conceptualization, R.B. and A.Y.; methodology, R.B., M.B., A.K., A.A. and A.Y.; validation, R.B. and A.Y.; formal analysis, R.B., M.B., A.K., A.A. and A.Y.; investigation, R.B.; resources, R.B., M.B. and A.Y.; writing--original draft preparation, R.B.; writing--review and editing, M.B., A.K., A.A. and A.Y.; project administration, A.Y. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.