In bio-neural systems, ionic fluxes dominate the regulation of neuroplasticity. Inspired by such biological strategies, indium-zinc-oxide (IZO) transistors gated by nanogranular SiO2 electrolyte films were proposed for synaptic behavior emulations. Due to the strong proton/electron coupling effect, the carrier density of IZO channel layer can be effectively tuned by a pH buffer solution on the SiO2 electrolyte dielectrics. The pH-dependent synaptic responses, such as excitatory postsynaptic current (EPSC) and paired pulse facilitation (PPF), were discussed in detail based on the surface charging effect. Most importantly, an enhanced pH sensitivity in terms of the relative gain of EPSC amplitude between pH = 4 and pH = 10 was demonstrated by multi-spike trains. Such IZO-based synaptic transistors are promising for low-power neuromorphic sensing systems.