When designing cationic nanocarriers using components like lecithin, squalene, Tween 80, Span 85, and DOTAP, it's important to consider the formulation and interaction of these components, as they can influence the surface charge of the nanoparticles. If your nanoparticles ended up with a negative charge instead of the expected positive charge, several factors could contribute to this outcome:

1. DOTAP Concentration:DOTAP is a cationic lipid that is commonly used to confer a positive charge to nanoparticles. The concentration of DOTAP in your formulation can significantly impact the overall charge of the nanoparticles. If the concentration of DOTAP is too low compared to the other components, it might not provide a strong positive charge to the nanoparticles.

2. Charge Interaction with Other Components:The interaction between the cationic and anionic components in your formulation can influence the overall surface charge. For example, if the negative charges from Tween 80 and Span 85 outweigh the positive charges from DOTAP, the nanoparticles could end up with a net negative charge.

3. Mixing and Temperature Conditions:The melting method at 80°C might lead to improper mixing of components, resulting in phase separation or uneven distribution of cationic components. Make sure the components are thoroughly mixed to ensure uniform distribution.

4. Lipid Ratio and Composition:The ratio of lecithin, squalene, and other lipids can also influence the surface charge. If the lipid composition is not properly balanced, it could affect the overall charge of the nanocarriers.

5. pH of the Formulation:The pH of the formulation can influence the ionization of functional groups in the components, affecting the overall charge of the nanoparticles. Make sure the pH of the formulation is appropriate for the intended cationic charge.

6. Characterization Methods:Ensure that the measurements of nanoparticle surface charge are accurate and reliable. Sometimes, issues with measurement techniques or instrument calibration can lead to incorrect readings.

7. Sample Preparation for Measurement:Improper sample preparation for charge measurement, such as using incorrect electrolyte solutions or not adequately dispersing the nanoparticles, can lead to inaccurate charge readings.

To troubleshoot the issue and achieve the desired positive charge, consider the following steps:

• Optimize the DOTAP concentration and balance it with other components.
• Adjust the ratio of cationic and anionic components in the formulation.
• Ensure proper mixing and distribution of components during the formulation process.
• Check the pH of the formulation and adjust if necessary.
• Validate the accuracy of the charge measurement techniques you're using.

By addressing these factors and optimizing your formulation, you should be able to achieve the desired positive charge on your cationic nanocarriers. Marzieh Attar