Folic acid encapsulated graphene quantum dots for ratiometric pH sensing and specific multicolor imaging in living cells

Folic acid (FA) encapsulated graphene quantum dots (FA-GQDs) were prepared via a one-pot microwaved alkali cutting method, using graphene oxide (GO) and FA as raw material. TEM and AFM images revealed that FA was successfully cross-linked on the surface of GQDs under alkali circumstance, giving rising to stable and homogeneous FA-GQDs with size of about 33.6 nm. XPS and FT-IR confirmed that there are abundant hydroxyl, carboxyl and amino groups on the encapsulated FA-GQDs. The as-prepared FA-GQDs possessed the fluorescence behaviors of multi-excitation and emission sites, and exhibited sensitive response to the pH variation within the pH range of 3–9, making it excellent fluorescence probe for pH sensing in cell lysate and environmental water samples. The broad emission with pH-sensitive characteristic and low toxicity of FA-GQDs also provided an alternative for the specific multicolor imaging and sensing of pH in living cells.