Probing pH variation in living cells and assaying hemoglobin in blood with nitrogen enriched carbon dots

Nitrogen enriched carbon dots (CDs) are prepared by hydrothermal treatment with oxalic acid and ethylenediamine serving as raw materials. The obtained CDs display strong blue fluorescence emission at λ_ex/λ_em = 320/410 nm, along with a quantum yield of 14%. It is interesting to obtain a linear increase of fluorescence with the variation of pH value within pH 5.01-8.94. Meanwhile, a favorable stability of the fluorescence is achieved in a high saline matrix, i.e., up to 1.0 mol L^-1 NaCl, making the CDs particularly suitable for probing pH variation in living cells or real biological systems. In addition, hemoglobin (Hb) significantly quenched the fluorescence of CDs due to the inner filter effect and static quenching, providing a linear relationship between fluorescence and Hb concentration within 20-500 mg L^-1. The CDs exhibit favorable selectivity for both pH sensing and hemoglobin detection, which are further demonstrated by selective and sensitive intracellular pH monitoring in living HeLa cells and Hb assay in human blood.