Advanced Functional Materials, Vol.26, No.48, 8991-8998, 2016
Controlled Modulation of Surface Coating and Surface Charging on Quantum Dots with Negatively Charged Gelatin for Substantial Enhancement and Reversible Switching in Photoluminescence
Here, the roles of coating and charging on the surface of CdSe quantum dots (QDs) are systematically investigated in tuning their emission via a simple change of pH in solution. A very thick layer of ZnS with a wide bandgap is chemically grown first to passivate the CdSe for retaining their high emission intensity, which is further encapsulated with a layer of degraded gelatin to achieve the phase transfer of QDs from hexane into aqueous solution. Interestingly, the degraded gelatin-capped CdSe/ZnS QDs (i.e., gelatinated QDs) greatly enhance their emission by adjusting pH from acidic to basic but greatly quench their emission by adjusting pH from basic to acidic. This highly reversible switching in photoluminescence is demonstrated for five cycles without obvious deterioration. With the increase of pH, there is a great decrease in radiative relaxation lifetime but a great increase in nonradiative relaxation lifetime as measured by time-resolved photoluminescence decays of the gelatinated QDs. As a result, the nonradiative lifetime becomes much longer than the radiative lifetime with the increase of pH so that the radiative emission occurs earlier before the nonradiative pathways take effect, resulting in greatly increased photoluminescence. Importantly, the successful demonstrations for the phase transfer of hydrophobic QDs into aqueous solution together with the reversible switching in photoluminescence arise from the reprocessing of acidic-degraded gelatin in weak alkaline solution, resulting in a great reduction in isoelectric point of the sequential acidic-alkaline degraded gelatin in free versus surface-bound forms from 7 to 4. With the exposure of more negatively charged carboxylic groups externally, the negative surface charging is greatly increased with the increase of pH while it is greatly decreased with the decrease of pH, leading to reversible switching in photoluminescence.