Colorimetric detection of cysteine using silver-hydroxyapatite nanoparticles

Cysteine is a sulphur-containing amino acid that acts as an effective antioxidant in human body. An excessive amount of cysteine leads to cystine formation that causes the stone formation in kidney, bladder, and ureters. A new method was developed for detection of cysteine in urine samples based on the aggregation of silver nanoparticles (AgNPs) on Hydroxyapatite (HAp) in the presence of cysteine. Transmission Electron Microscope (TEM) was used to observe the morphology of prepared HAp and AgNPs on HAp. The presence of cysteine of different concentrations resulted in different cluster sizes of AgNPs on HAp and hence different solid color. The color of the AgNPs containing HAp was observed by naked eyes and the color intensity was measured by the Image J software. To detect cysteine in urine sample, a sample preparation by extraction on cation exchange resin was introduced. The parameters affecting the analysis including extraction and elution time, concentration of Ca²⁺ eluent solution, addition of NaOH, eluted sample volume, contact time between Ag+ and HAp, concentration of Ag+, and volume of NaBH4 were optimized. The working range of this method was in a range from 0 to 105 µM. The standard addition method was applied to determine cysteine concentration in urine sample. The applicability of this method was then presented in terms of percent recovery and percent relative standard deviation (% RSD) which were in the range of 92.0 to 111.9 % and 1.4 to 6.9 %, respectively. The method exhibited a selectivity toward cysteine over amino acids commonly found in urine including homocysteine, methionine, and cystine.