CdSe Quantum Dots to Quantum Rods: Transition Studies and Evaluation of Sensitivity as Transducers for Biosensing Glucose

Background: The estimation of glucose level in the blood serum, has been widely used as a clinical indicator of diabetes. Optical and electrochemical sensing of glucose widely uses Glucose Oxidase (GOD) enzyme, as the catalyst for glucose oxidation, which releases hydrogen peroxide (H2O2). Optical biosensors are superior to their electrochemical counter-parts as they are resistant to electromagnetic interference, easier to fabricate into a microdevice and require low power supply. The quantum-dot-based biosensors work on the phenomenon of fluorescence quenching following the release of H2O2. Methods: The CdSe nanoparticles are prepared in two series by room-temperature microemulsion method. In series A, only AOT surfactant is used to synthesize spherical CdSe nanoparticles. In series B, the mixed surfactant system of AOT and lecithin is used to synthesize anisotropic CdSe. The morphology and crystallography is studied as the CdSe shape changes from spherical to rod-like. As the CdSe nanoparticles are studied from spherical to rod-like morphology, the transducing sensitivity of these nanoparticles is evaluated with respect to glucose biosensing. The effects of size and shape are studied, based on the fluorescence quenching by H2O2 solutions. The sensitivity of proposed nanoparticles, is evaluated as a function of size, shape, surface area and number concentration of CdSe nanoparticles. Results: The spherical CdSe nanoparticles are found to increase in size as R(water-to-surfactant ratio) is increased from 4 to 12, in series A. Also, the aspect ratio of CdSe nanoparticle is found to increase from 4.2 to 12.8 as the ratio of AOT to lecithin is varied from 1:0.5 to 1:3. The decrease in sensitivity index is seen with increasing surface area for both series A and B. The sensitivity is decreasing again with increasing maximum dimension of the CdSe nanoparticle in the dispersion. While the trend is reverse in case of the number concentration for CdSe nanoparticles synthesized in series B. Conclusion: From the data presented, it can be safely concluded that the sensitivity indices for series A are better than those for series B, for the same values of a) the total surface area of CdSe nanoparticles, b) total number concentration, and c) maximum dimension of CdSe nanoparticles. Also, the single surfactant system (series A) is simple, cheaper and more reproducible to synthesize the CdSe nanosheres, as compared with the mixed surfactant system forming CdSe quantum rods (series B). With these points, it is reasonable to report that CdSe spherical QDs are better candidates for glucose biosensing, as compared to CdSe quantum rods.