Smart phone assisted, rapid, simplistic, straightforward and sensitive biosensing of cysteine over other essential amino acids by β-cyclodextrin functionalized gold nanoparticles as a colorimetric probe†
In this paper, a β-cyclodextrin (β-CD) functionalized gold nanoparticle (β-CD AuNP) based simple, rapid, on-site and naked eye colorimetric sensor for the sensitive detection of cysteine (Cys) was reported. The β-CD AuNP nanoprobe was completely characterized with respect to morphology, optical properties and stability. The surface plasmon resonance (SPR) and colorimetric properties of the β-CD AuNPs sensitively react to Cys, whereby the SPR band decreases with a red shift, resulting in a distinctive colour changes from wine red to purple. This colour change is easily visualized by the naked eye or absorption spectrophotometry. The observed spectral and colour changes are due to Cys having a thiol group, which has a high binding affinity towards the β-CD AuNP surface via Au–thiol bond formation, which leads to aggregation (SPR band and colorimetric changes). Further, the lowest limit of detection of Cys is calculated to be 25.47 × 10−9 mol dm−3 and this method exhibits a linear detection range of Cys from 2.50 to 45.00 × 10−7 mol dm−3. The β-CD AuNP based colorimetric sensor demonstrates high selectivity towards Cys over other essential amino acids. Moreover, the present naked eye visual method is further applied for the quantification of Cys in clinical samples, like human urine and blood serum. In addition, the present method was further extended to a smart phone device with easy changes of the device enabling it to act as a quantitative Cys detector.