Catalytic nanozyme Zn/Cl-doped carbon quantum dots as ratiometric fluorescent probe for sequential on-off-on detection of riboflavin, Cu(2+) and thiamine

A novel metal-doped Zn/Cl carbon quantum dots (Zn/Cl-CQDs) was developed successfully as ratiometric fluorescent probes for the sequential on-off-on detection of riboflavin, Cu(2+) ion and thiamine. The excellent catalytic performance of the Zn/Cl-CQDs nanozyme serves as an ideal platform for sensitive detection of thiamine. Due to the addition of riboflavin to the Zn/Cl-CQDs, the blue emission peak of Zn/Cl-CQDs at 440 nm remains unaffected and used as an internal reference approach, while the green emission peak of riboflavin at 520 nm appeared and increased remarkably. Following the presence of Cu(2+), a quenching blue fluorescence signal of Zn/Cl-CQDs was observed which resulted in consequent fluorescent ‘turn-off’ response toward Cu(2+) ion. Finally, upon the addition of thiamine to the above solution under alkaline condition, the blue emission of Zn/Cl-CQDs was gradually recovered. The prepared Zn/Cl-CQDs could act as a nanozyme catalyst for directly catalyzing the oxidation of non-fluorescent substrate of thiamine to produce highly fluorescent substrate of thiochrome. As a result, the blue fluorescence emission peak at 440 nm was recovered. Eventually, the sequential detection properties of ratiometric probes for riboflavin, Cu(2+) ion and thiamine were successfully applied in VB2 tablets, drinking water and VB1 tablet with good recoveries of 96.21%, 98.25% and 98.44%, respectively.