Spectrophotometric determination of glutathione and cysteine based on aggregation of colloidal gold nanoparticles

We report herein the development of a highly sensitive colorimetric method for the determination of cysteine and glutathione, based on aggregation of the citrate capped gold nanoparticles (Au NPs). This was exploited from high affinity of low-molecular-weight aminothiols towards the Au NPs surface, which could induce displacement of the citrate shell by the thiolate shell of target molecules, resulting in aggregation of the NPs through intermolecular electrostatic interaction or hydrogen-bonding. As a result of aggregation, which can be affected by the ionic strength, pH and concentration of Au NPs, the plasmon band at around 521 nm decreases gradually, along with formation of a new red shifted band. The calibration curves, which are derived from the intensity ratios of absorbance at-640 nm and 650 nm for cysteine and glutathione, respectively, to the original wavelength of 521 nm, display a linear relation in the range of 1×10−6–100×10−6 M cysteine and 5×10−6–200×10−6 M glutathione. The obtained detection limits (3σ) were 2.1×10−6 M and 3.3×10−6 M for determination of cysteine and glutathione, respectively.