Non-enzymatic colorimetric sensor for cardiac Troponin I (cTnI) based on self-assembly of gold nanorods on heparin

The paper reports a simple enzyme-free colorimetric sensor for the detection of cardiac marker, Troponin I (cTnI) based on the self-assembly of gold nanorods (AuNRs) on heparin. The sensing system consists of a purple colour solution of Hexadecyltrimethylammonium bromide (CTAB) stabilised AuNRs self-assembled in presence of heparin due to the electrostatic interaction resulting in the reduction of surface plasmon absorption (SPR) of AuNRs and hence colour quenching/change of solution from red to blue. However, in the presence of cTnI, the electrostatic balance was disturbed due to the strong complex forming tendency between heparin and cTnI which was attributed due to the stronger complex forming tendency between the sulphate and carboxylate group of glycosaminoglycans and heparin binding proteins present in cTnI. Hence, the AuNRs are left free in the solution and retains the native red colour of the solution. As the concentration of cTnI in the solution was increased, the colour gradually changes from blue to red there by enabling the sensing of cTnI. The above mechanisms were confirmed by transmission electron microscopy. The AuNR aggregation was found to be inversely proportional to the concentration of cTnI. The change in absorbance of AuNRs with different concentrations of cTnI was monitored by UV-Visible spectroscopy. Using this sensor system, cTnI in the range of 0.5–15 ng/mL could be measured with a detection limit of 0.4 ng/mL. The system also showed good selectivity in presence of different competing substances under the same experimental conditions. The method appears to be simple, cost effective, and would be highly be beneficial in rural health care centres where high tech diagnostic aids are inaccessible.