Regular ArticleEnhanced photoluminescence properties of a carbon dot system through surface interaction with polymeric nanoparticles
Graphical abstract
Introduction
Recently, a new class of fluorescent carbon nanomaterials, so-called carbon dot systems (CDSs), have attracted great attention due to their excellent photoluminescence (PL) properties [1], [2] and low-toxicity [3]. Additional advantages are a high water-dispersibility and photostability, a strong chemical inertness as well as easy surface functionalization [4].
One interesting feature of CDSs is the strong surface-dependent sensitivity of their PL properties. Small changes in the surrounding environment can lead to a strong change in the PL. As such, CDSs can be sensitive towards changes of the pH value of the surrounding medium [5], [6] which changes the protonation state of the surface functionalities of the CDS and affects the optical properties. Furthermore, the polarity of the surrounding medium affects their PL properties [7].
Additionally, molecules, such as ascorbic acid [8], or sugars [9], or specific ions, such as Fe3+ [10] or Cu2+ [11] can influence the PL properties. Therefore, CDSs are highly versatile fluorescent sensors for diverse applications such as a sensor for ferric ions in biological systems or for monitoring of glucose [12]. The surface sensitivity of the fluorescent properties of CDSs is also displayed in the activation of the fluorescence properties through passivation agents such as a polymer coating [13], [14], [15]. In this process of surface coating (e.g. with poly(ethylene imine) [14] or poly(ethylene glycol) [16]), CDSs which hardly possess fluorescent properties are being activated and display a much stronger fluorescence after coating. Such a thin layer of passivation agent around the CDS protects them from the environment and the effect of contaminants in the surrounding on the fluorescence properties is reduced [1]. Furthermore, the fluorescence lifetime is prolonged if the surface of the CDS is passivated [17]. It is supposed that CDSs become emissive due to the reduction of non-radiative recombination by stabilizing the trap states via passivation with surface passivation agent [16], [18]. Besides the activation of the PL properties through direct attachment of passivating polymers, enhanced PL properties of CDSs have been observed when embedding them in a polymer film [19], [20]. While it is an established procedure to activate the PL properties of CDSs through direct attachment of passivating polymers on the surface, the direct interaction of CDSs with the surface of polymer colloids has never been investigated with respect to an interaction that induces a stronger PL.
In this paper, the interaction of CDSs with polymer colloids is investigated. To introduce CDS – polymer nanoparticle (NP) interactions, carbon dots in the range of few nanometers are either generated in one step, in the presence of polystyrene (PS) nanoparticles in a microwave (MW) assisted synthesis (in situ) (Scheme 1, upper part) or in two steps by firstly synthesizing CDSs in the MW and afterwards mixing them with PS NPs (mixing) (Scheme 1, lower part). Interestingly, the interactions between the surfaces of both species (CDS and PS NP) strongly increased the fluorescence intensity and lifetime of the CDSs in the same order of magnitude for both approaches. The enhancement mechanism is however difficult to reveal and has to be further studied in the future in more detail. While the in situ reaction yielded a covalently attached CDS on the surface of the PS NPs, the mixing method resulted in physically adsorbed CDS which could be removed by changing the electrostatic interaction between the CDS and PS NPs via pH-change. Since both attachment mechanisms yield a similar increase in fluorescence properties, the enhancement mechanism can be assumed to be independent of the attachment type (covalently bound or electrostatically adsorbed). Furthermore, both systems were characterized by electron microscopy to investigate the surface morphology of the PS NPs before and after CDS coverage and flow cytometry to show the homogeneous attachment of CDS on the surface of the PS NPs. Additionally, cell uptake of CDS-labeled PS NPs was performed and visualized by fluorescence microscopy to demonstrate the efficient labeling of PS NPs with surface-activated CDS.
Section snippets
Chemicals
Acetic acid, acrylic acid, chitosan (low molecular weight), polyvinylpyrrolidone (PVP, MW 55000) and styrene were purchased from Sigma-Aldrich. 2,2′-Azobis(2-methylbutyronitril (V59) was procured from Wako, 1,2-ethylenediamine (EDA) from Fluka and hexadecane from Merck. Poly(ethylene glycol)-p-(1,1,3,3-tetramethylbutyl)-phenyl ether (Triton X-100) was purchased from Thermo Fischer and sodium dodecyl sulfate (SDS) was procured from Alfa Aesar. Sodium chloride (0.1 N) was purchased from Roth and
Results and discussion
To realize an interaction of CDSs with the surface of polymer NPs, two different procedures were developed (Scheme 1). In the one-step procedure, the CDS were synthesized directly in the presence of PS NPs (upper part Scheme 1). In the two-step procedure, the CDSs were synthesized separately and afterwards mixed with the PS NPs (lower part Scheme 1). In both cases, chitosan, acetic acid, and EDA were used as precursors for the CDS. Here, chitosan is the main carbon source. Acetic acid, which
Conclusion
In summary, we have shown that the photoluminescent properties of a carbon dot system can be strongly influenced and enhanced through interaction with polymer nanoparticles, a so far not investigated observation. Therefore, two different methods for the generation of a carbon dot-polymeric nanoparticle system were developed. Firstly, the in situ formation generated carbon dot systems on the surface of polystyrene beads. Secondly, the two species were synthesized separately and mixed after
Acknowledgment
The authors acknowledge the Max Planck Society for the financial support. We gratefully thank E. Muth for FTIR, zeta potential measurement, and particle charge detection support. We thank J. Pereira for the support of cells experiments. We thank G. Glasser for the help with SEM imaging and K. Kirchhoff and M. Hu for TEM imaging support.
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2020, Journal of Colloid and Interface ScienceCitation Excerpt :Within a reasonable concentration range, the aggregation could reduce the surface defects, induce non-radiative cross relaxation between CDs and bring about the increase of decay time. However, at a very high concentration, the energy or electron transfers between nanoparticles would shorten the fluorescence lifetime dramatically [52–55]. In contrast, because zirconium complexes hampers the aggregation and intermolecular interactions of ZrCDs, the emission lifetime of ZrCDs increased slightly first and then generally maintain stability (Table S4 and Fig. S8b).