Enzymes can be used as sensors. But they are difficult to handle. They degrade very easily. Nanozymes that mimic enzymes are more stable for field use. Recently, Rajiv Prakash and his students, Vinita and Narsingh, from the IIT Varanasi created a composite nanozyme with peroxidase-like enzymatic properties and used it to detect glucose.
The composite consisted of gold nanoparticles decorated on quantum dots – small semiconductor crystals, several nanometres in size – made of molybdenum sulphide. Gold forms a strong chemical bond with the sulphur atom on the outer side of the molybdenum sulphide molecule, making a stable composite stable.
The composite catalyses the oxidation of substrates in the presence of hydrogen peroxide, like a peroxidase enzyme. The research tested the peroxidase activity of the nanozyme to detect glucose in human serum, saliva and tears.
They added a solution containing the nano-composite, glucose oxidase enzyme and tetramethylbenzidine, a peroxidase substrate to agarose dissolved in water. Next they poured very small volumes of the solution into wells of a plastic plate. The solution solidified to form a hydrogel.
The researchers diluted minute amounts of human body fluid samples and added them to the hydrogel. Glucose oxidase in the hydrogel oxidised glucose present in the body fluid samples to gluconic acid and hydrogen peroxide. The nanoparticles transferred electrons quickly to the hydrogen peroxide molecules and free hydroxyl radicals were formed.
The free hydroxyl radicals oxidised the colourless tetramethylbenzidine substrate and the hydrogel turned bluish green after half an hour of incubation with the samples. The higher the absorbance value of the hydrogel colour, the greater the concentration of glucose in the sample. The composite could sense lower concentrations of glucose than could some commonly available glucose sensors.
The scientists followed up with experiments which showed that the nanozyme they synthesised was stable across a wide range of temperature and pH. The ready-to-use ELISA plate with the hydrogel could be stored for a long time in a refrigerator and used as a non-invasive, portable glucose detection kit.
Will the technology be accepted by the market and come into clinical use?
Only time will tell.
Sens. Actuator B-Chem., 263: 109-119
CSIR-Indian Institute of Chemical Biology