Depending on their characters, pH sensitive nanoparticles can be mainly divided into two types. One induces drug release at higher pH because of ionizable functional groups on the polymer backbone or side chain, for example, nanoparticles prepared using poly (methacrylic acid) [98], hydroxypropyl methylcellulose phthalate[99] and poly (acrylic acid) grafted poly(vinylidenefluoride)[100]. This kind of nanoparticles is typically used for oral drug delivery [97,101-109], where a physiological pH shifts along GIT facilitates the swelling or dissolution of the carriers. The other kinds of pH-sensitive nanoparticles have a reversed swelling or dissolution behavior of the former. It undergoes swelling in acidic environment and can be used to target tumors, lysosomes and endosomes, where pH is comparatively low [110].

Oral administration of poorly water-soluble drugs incorporated in pH-sensitive particles has been found to be very efficient through increasing drug bioavailability and achieving fast absorption [36,109, 111]. It has been considered that the good bioavailability of pH-controlled nanoparticulate drug systems depends on some properties, including the high specific surface area of the system, drug in an amorphous form or molecularly dispersed in the polymer matrix, and the release of drug close to the absorption site [36, 103, 111].
The dissolution of the particles should not be too fast, however, as this might cause drug precipitation [109] or crystallization [112], but should not be too slow which lead to the rapid elimination of the particles before absorption, either[109].

The advantages of pH-sensitive nanoparticles over other nanoparticles include: (a) the majority of carriers have been used as enteric-coating materials for a long time, and their safety has been approved. (b) The carriers show rapid drug release and then high drug concentration gradient as they undergo quick dissolving at definite pH and definite sites. The phenomenon is beneficial for the drug absorption. (c) They improve drug absorption comparing to the other conventional nanoparticles as they turn from the solid state to the hydrogel state at certain dissolution pH and so, the bioadhesion of the carrier to the mucosa becomes greater at specific fragment. (d) The drug stability can be enhanced more effectively using pH sensitive nanoparticles.