Cysteine

HPLC-grade formic acid was obtained from Sigma-Aldrich (USA). HPLC/MS-grade formic acid, ammonium formate, and acetonitrile were acquired from Fisher Scientific (USA). The internal standards Labeled Carnitine Standards (L-carnitine (N-trimethyl-D₉), L-acetylcarnitine (N-methyl-D₃), L-propionylcarnitine (Nimethyl-D₃), L-butyrylcarnitine (N-methyl-D₃), L-isovalerylcarnitine (N-trimethyl-D₉), L-octanoylcarnitine (N-methyl-D₃), L-myristoylcarnitine (N-trimethyl-D₉), L-palmitoylcarnitine (N-methyl-D₃)) were attained from Cambridge Isotope Laboratories. Seventeen amino acid standards (alanine, valine, leucine, isoleucine, phenylalanine, methionine, proline, glycine, serine, threonine, cysteine, tyrosine, histidine, lysine, arginine, aspartic acid, glutamic acid) were purchased from Sigma-Aldrich. Eight amino acid standards (tryptophan, asparaginate, glutamine, ornithine, taurine, citrulline, cysteine, γ-aminobutyric acid) were gained from J&K Scientific. Ultrahigh purity water was prepared by Millipore-Q Water Purification System (Millipore, Germany).

Dopamine hydrochloride, ZnSO₄·7H₂O, MnCl₂·4H₂O, Na₂S·9H₂O, p-aminophenol (PAP), and carbamide were obtained from Macklin Biochemical Co. Ltd. (Shanghai, China). Pyrocatechol, cysteine, methacrylic acid (MAA), ethylene glycol dimethyl acrylate (EGDMA) was purchased from J&K Scientific (Beijing, China). All the reagents are of analytical grade.
Fluorescence spectrometer (Techcomp FL-970, Shanghai, China). Ultraviolet spectrophotometer (Specord 210 plus, Jena, Germany). High-resolution transmission microscope (JEM-2100F, JEOL, Tokyo, Japan)