Folic acid capping Bi 3+ -doped Ag quantum dots for enzyme-like dual-mode recognition of toxic S 2- and visual sensing of NO 2 .

Background: NO ₂ ^- and S ^2- are two kinds of common toxic anions widely distributed in environmental water, soil and food products. Human beings have suffered a lot of diseases from intake of excessive NO ₂ ^- or S ^2- , i.e., infantile methemoglobin, cancer and even to death. Although tremendous efforts have been afforded to monitor NO ₂ ^- and S ^2- , most were high instrument-depended with complex processing procedures. To keep food safety and to protect human health, it will be a huge challenge to develop a convenient and efficient way to monitor S ^2- and NO ₂ ^- in practice.
Results: A kind of folic acid capping Bi ³⁺ -doped Ag quantum dots (FA@Bi ³⁺ -Ag QDs) was developed for the first time by one-pot homogeneous reduced self-assembly. Not only did FA@Bi ³⁺ -Ag QDs possess intrinsic fluorescent property, it expressed synergistic peroxidase-like activity to catalyze the redox of 3,3',5,5'-tetramethylbenzidine (TMB) and H ₂ O ₂ with K _m /v _max of 0.087 mM/6.61 × 10 ^-8  M s ^-1 and 6.42 mM/6.25 × 10 ^-7  M s ^-1 respectively. Interestingly, trace S ^2- could exclusively alter its fluorescent property and peroxidase-like activity, exhibiting significant hypochromic and "turn-on" fluorescent effects. While trace NO ₂ ^- could make FA@Bi ³⁺ -Ag QDs-TMB-H ₂ O ₂ system hyperchromic. Under the optimized conditions, FA@Bi ³⁺ -Ag QDs were applied for dual-mode recognition of S ^2- and visual sensing of NO ₂ ^- in real food samples with satisfactory recoveries, i.e., 100.7-107.9 %/95.8-104.7 % and 97.2-104.8 % respectively. The synergistic enzyme-mimic mechanism of FA@Bi ³⁺ -Ag QDs and its selective response mechanisms to S ^2- and NO ₂ ^- were also proposed.
Significance: This represents the first nanozyme-based FA@Bi ³⁺ -Ag QDs system for dual-mode recognition of S ^2- and visual sensing of NO ₂ ^- , well meeting the basic requirement in drinking water set by WHO. It will offer a promising way for multi-mode monitoring of different pollution using the same nanozyme-based sensor.
Competing Interests: Declaration of competing interest The authors declare no competing financial interest.
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