Your folate-grafted PHB nanoparticles contain a net positive surface charge, according to a positive zeta potential. This indicates that your nanoparticles' surfaces have more positively charged than negatively charged particles. A positive zeta potential can be used as a measure of the stability of a nanoparticle since it indicates that the particles are less likely to coalesce or aggregate as a result of attraction between similarly charged particles on their surfaces.

The stability of your nanoparticles is not always indicated by the zeta potential alone, though. Nanoparticle stability can also be influenced by other elements, including the size, shape, surface chemistry, and environmental conditions of your nanoparticles. Moreover, a number of elements over time might have an impact on the stability of nanoparticles,

More than the sign (positive or negative), the indicator of stability versus aggregation will be influenced by the absolute value of the load. In the literature, nanoparticles with a net charge above 30 mV (positive or negative) are said to exhibit good electrostatic stability due to repulsion from each other. However, non-ionic surfactants that can incorporate nanoparticles allow stabilisation of nanoparticulate systems by steric stabilisation as well.

When you say, Zeta potential has become positive, does that mean it was negative before? If so, this is an indicator of instability. Try adding a pH buffer and monitor the size of the nanoparticles to make sure they are not aggregating.