Enhanced photoluminescence properties of a carbon dot system through surface interaction with polymeric nanoparticles

Abstract

Hypothesis

Carbon dot systems are highly surface sensitive fluorescent nanomaterials. In the presence of specific molecules or ions, the fluorescence properties can be strongly influenced. Often their fluorescent properties are activated or strongly enhanced through passivation agents such as polymer coatings. While several passivating polymers have been directly attached to the carbon dot systems, the interaction of carbon dot systems with the polymer surface of colloids has not been investigated as a way to activate or enhance the photoluminescent properties. Here, we show for the first time that the interaction of carbon dot systems with polymer colloids can strongly enhance the fluorescent properties of the carbon dot systems.

Experiments

To introduce carbon dot – polymer nanoparticle interactions, carbon dots are either generated directly in a microwave assisted synthesis in the presence of negatively charged polystyrene nanoparticles (in situ) or synthesized in the microwave separately and mixed afterwards with polymer nanoparticles (mixing). For the carbon dot system synthesis, chitosan, 1,2-ethylenediamine, and acetic acid are used as precursors. The produced carbon dot – polymer nanoparticle system are characterized by scanning electron microscopy, transmission electron microscopy, and flow cytometry measurements, and their interaction is assessed by fluorescence spectroscopy and fluorescence lifetime measurements.

Findings

We show that depending on the synthesis route (in situ or mixing), the carbon dot systems are either covalently attached (in situ) or electrostatically bound (mixing) to the surface of the nanoparticles. Regardless of the preparation methods of the investigated carbon dot – polymer nanoparticle system and the interaction (chemical or physical) with the surface, the fluorescence intensity is strongly enhanced and the fluorescence lifetime prolonged. These findings indicate a stabilization of the radiative trap states of carbon dot systems through interaction with the surface of the particles.

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.