A colorimetric indicator-displacement assay based on stable Cu2+ selective carbon dots for fluorescence turn-on detection of pyrophosphate anions in urine.

Determination of PPi levels in urine represents a measurable factor for diagnostic, treatment, and monitoring of urolithiasis. Owing to the quenching ability of Cu2+ on fluorescent carbon dots (CDs) and strong binding affinity between Cu2+ and PPi, we develop a new off-on assay for PPi detection using newly BPHA CDs (BPHA: N,N-bis(pyridin-2-ylmethyl)hexan-1-amine). The fluorescence intensity of BPHA CDs was significantly quenched by Cu2+ ("off") through forming BPHA CDs/Cu2+ complexes and the fluorescence intensity of BPHA CDs /Cu2+ system was completely resumed by PPi ("on") owing to the release of free Cu2+. The fluorescence turn-off/on approach showed a highly selective response to PPi over the large family of other anions. The detection limits were 0.094 μM for Cu2+ and 0.025 μM for PPi, respectively. A wide linear range for PPi was up to 4400 μM. The indicator displacement assay (IDAs) using pyrocatechol violet (PV) as a colorimetric indicator was carried out to detect PPi with the naked eyes. The "off-on" fluorescent sensor based on BPHA CDs shows many merits, including convenient operation, cost-saving, high sensitivity, selectivity, stability and wide detecting range, which is applied to PPi detection in human urine sample.