Polysaccharide/Carbon Quantum Dots Composite Film on Model Colloidal Particles-An Electro-Optical Study

Abstract

Negatively charged carbon dots (Cdots) were successfully impregnated into chitosan/alginate film formed on model colloidal particles as a result of the attractive interactions with the chitosan molecules. The electrical properties of the produced films were studied by electrokinetic spectroscopy. In this study, the electric light scattering method was applied for first the time for the investigation of suspensions of carbon-based structures. The electro-optical behavior for the suspension of polymer-coated particles showed that the electric polarizability of the particle-covered layer from alginate was significantly higher compared to that of the layer from chitosan due to the higher charge density of alginate. The presence of a low concentration of Cdots in the film results in partial charge screening. It was confirmed that the polarizability of counterions with lower mobility along the adsorbed polyion chains was responsible for the registered electro-optical effect from the suspension of polymer-coated particles and that the participation of diffuse H⁺ counterions of Cdots in the creation of the electro-optical effect was negligible. The observed oscillation behavior in the evolution of the film thickness was interpreted through the participation of compensatory effects due to the additional adsorption/desorption of polyelectrolyte complexes from the film surface. The concentration of Cdots in the film was determined, and the loaded amount was ca. 6.6 µg/mL per layer.

Keywords: carbon dots; electrokinetic spectroscopy; layer-by-layer assembly; polysaccharides.

Figure 1

Deposition steps in the experimental procedure for preparation of alginate/chitosan/Cdots film on β-FeOOH particles.

Figure 2

Dependence of the surface charge density of bare β-FeOOH particles as a function of pH of the suspension (in the presence of 10⁻⁴ M NaCl).

Figure 3

Dependence of the rotary diffusion coefficient of bare β-FeOOH particles as a function of the electric field strength. Inset: dependence of the electro-optical effect as a function of the electric field strength (the frequency of the electric field is 3 kHz).

Figure 4

Representative TEM images of β-FeOOH particles. The length dimension bar is 100 nm.

Figure 5

Alteration in the electrophoretic mobility, U_ef, of β-FOOH particles (∗) covered by film after each adsorption step, n, from ALG (○) and CS (●) layer or Cdots (△).

Figure 6

Dependence of the electro-optical effect, α, as a function of the intensity of the applied electric field for particles with the last adsorbed layer from ALG2 (●), ALG8 (○), CS2 (■), CS8 (□), CDs2 (▲), and CDs8 (△). The frequency of the electric field is 1 kHz.

Figure 7

Dependence of the electrical polarizability, γ (●) and hydrodynamic thickness, L_H (○) as a function of the number of adsorption steps, n. (The dot line correspond to the line drawn by eye to illustrate the increased thickness of the film with the number of adsorbed layers.)

Figure 8

Dependence of the electro-optical effect, α, as a function of the frequency of the applied electric field from suspensions of particles with an adsorbed layer of ALG2 (●), ALG8 (○), CS2 (■), and CS8 (□). The electric field strength is 2.3 × 10⁻⁴ Vm⁻¹. The arrows indicate the relaxation frequency of the effect.

Figure 9

Dependence of the electro-optical effect, α, as a function of frequency of the applied electric field for (A) suspension of particles with last adsorbed layer CS1 (□), CS2 (■), CDs1 (△), and CDs2 (▲) and (B) suspension of particles with ALG2 (●), CS2 (■), and CDs2 (▲). The electric field strength is 2.3 × 10⁻⁴ V/m. The arrows indicate the relaxation frequency of the effect for particles.

Figure 10

Absorbance from dispersion of Cdots at different concentrations. The curve with the lowest absorbance corresponds to the supernatant of the suspension of particles coated with polysaccharide film in the presence of CDs2. Inset: calibration curve of absorbance from dispersion of Cdots at different concentrations (0.62 × 10⁻², 1.25 × 10⁻², 2.50 × 10⁻², 5 × 10⁻², 10⁻¹ mg/mL).