Quinine hemisulfate monohydrate

All the chemicals used were at least of analytical grade. Toluene was purchased from Honeywell (Honeywell, Charlotte, NC, USA). 4-nitrophenol, 2-nitrophenol, 3-nitrophenol, 2-methyl-4,6-dinitrophenol, 2,4-dichlorophenol, 2,3,5-trichrolophenol, humic acids, ammonia solution, ammonium chloride, sodium chloride and sodium sulfate were purchased from Sigma-Aldrich (Sigma-Aldrich, Steinheim, Germany). Quinine hemisulfate monohydrate (98%, suitable for fluorescence) was purchased from Alfa Aesar (Alfa Aesar, Karlsruhe, Germany). Stock metal solutions (Mn²⁺, Fe³⁺, Ag⁺, As⁵⁺, Se⁴⁺, Cr³⁺, Cu²⁺, Co²⁺, Ni²⁺, Li⁺, Cd²⁺, K⁺, Na⁺, Hg²⁺ and Ca²⁺) were obtained from Merck (Merck, Darmstadt, Germany). Amino acids (lysine, methionine, phenylalanine, leucine, histidine, arginine, tyrosine, threonine, proline, serine, glycine, tryptophan, asparagine, glutamine, cysteine, alanine and valine) were acquired from Fluka (Fluka, Seelze, Germany). Stock solutions of 4-NP were prepared in amber glass vials and stored at room temperature. All the experiments were conducted using double distilled water (DDW).

β-Galactosidase and the EDC required for the conjugation were purchased from MP Biomedicals (Irvine, CA, USA). Gel-like CDs were synthesized in the lab. For the synthesis of CDs, citric acid (99.5–100%) was purchased from VWR (Radnor, PA, USA), and EDA (99%) and quinine hemisulfate monohydrate from Alfa Aesar (Tewksbury, MA, USA). Hydrochloric acid, Sodium chloride (99%) and sodium hydroxide were obtained from MP Biomedicals (Irvine, CA, USA). Compressed argon gas with ultra-high purity (99.99%) was procured from Airgas (Radnor, PA, USA).