Nano-sized semiconducting inorganic Nano-crystals have bright photoluminescence reference to their quantum confinement effect. Therefore, these semiconducting Nanocrystals have become popular for fluorescent making necessary for sensing, cellular probes, and bioimaging. Unfortunately, their high toxicity continues to handicap their in vivo and in vitro applications. In quest to overcome this shortcoming, researchers have identified nano-sized fluorescent carbon quantum dots as suitable alternatives for multicolor patterning, bioimaging, and sensors.
Photoluminescence of carbon quantum dots has been assigned controversially to surface energy traps and quantum confinement effects that are majorly based on the varying fabrication processes. However, their tunable light absorption-emission properties, high solubility in physiological media and excellent biocompatibility, make them attractive as opposed to fluorophores for bioimaging. This has resulted in their implementation as fluorescent labels for cellular imaging, in vivo optical imaging, and real-time molecular tracking in live cells.
Unfortunately, bare carbon quantum dots suffer from inadequate cell specificity. Therefore, modification by incorporation of biomolecules has been identified to mitigate this problem. In addition, while looking for quantum dots, which can be effectively accumulated in cells, incorporating particular proteins with targeting capabilities is appropriate.
Although a good number of sensors based on carbon quantum dots for metal-ions detection lack adequate selectivity, making them susceptible to interference phenomena with other materials and hence demanding the integration of functional species to carbon quantum dots for improved selectivity. In spite of the huge potential demonstrated for carbon quantum dots, more research is still needed.
Researchers led by Dr Nikos Tagmatarchis at the Theoretical and Physical Chemistry Institute, at the National Hellenic Research Foundation in Greece successfully synthesized fluorescent carbon quantum dots with amine-rich surface. In collaboration with Dr Stergios Pispas within the same organization, they took a step beyond by incorporating the di-block copolymer (CSS-IEO-3) with aqueous carbon quantum dots in an acidic environment, by making use of the electrostatic interactions between the di-block copolymer and the carbon quantum dots, which led to the formation of carbon quantum dot-CSS-IEO-3. Their research work is published in Journal of Materials Chemistry B.
The authors objective was to employ the carbon quantum dots-CSS-IEO-3 ensemble first as a sensitive and selective sensor for metal-ions of biological interest. Secondly, they applied it for bioimaging probing. For the latter application, the authors incorporated a protein, bovine serum albumin (BSA), to increase biocombatibility and allow efficient cell permeability of the carbon quantum dot-based three-component hybrid Nano-system (Figure 1). In future studies BSA can be replaced by other therapeutic proteins/peptides.
The authors electrostatically interacted spherical carbon quantum dots with a periphery rich in free amine units with the di-block copolymer. The material formed, carbon quantum dots/block copolymer nanoensemble, was observed to be stable in high saline and neutral environments. This was in fact critical when aiming for biological applications. The photoluminescence of carbon quantum dots within the formed material, with emission at 420 nm, remained unchanged by pH as well as salinity changes. This denoted the absence of considerable aggregation and highlighted the potential of the material for bioimaging.
The researchers adopted the photoluminescence of the carbon quantum dots/block copolymer in the development of a selective and sensitive approach for metal-ion sensing, in this case Fe3+, in the presence of other heavy metal cations (Figure 2). To this end, the negative charges of CSS-IEO-3 block copolymer were utilized for Fe3+ complexation and due to the proximity of the carbon quantum dots the decrease of emission was employed for efficient detection.
T. Skaltsas, M. Goulielmaki, A. Pintzas, S. Pispas and N. Tagmatarchis. Carbon quantum dots/block copolymer ensembles for metal-ion sensing and bioimaging. Journal of Materials Chemistry B, volume 5 (2017), pages 5397—5402.
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