p-Phenylenediamine-derived carbon nanodots for probing solvent interactions

Abstract

Carbon nanodots, the luminescent nanoparticles of carbon with size restriction below 10 nm, have attracted inordinate attention in materials science due to their widespread applications in optoelectronic and biological fields. Low toxicity and facile synthesis pathways render them favourites in the above-mentioned areas in the context of green chemistry. This work presents fine applications of p-phenylenediamine-derived carbon nanodots (PD-CNDs) achieved via a facile one-pot hydrothermal method. Adequate characterization using X-ray diffraction and spectroscopic and microscopic studies confirmed spherical particles with an average particle size of 2.8 nm, functionalised with amino, carboxyl, and hydroxyl groups. The carbon framework was functionalised with pyridinic and pyrrolic nitrogens. Upon 365 nm UV light illumination, an aqueous dispersion of PD-CNDs showed red-orange fluorescence. Detailed spectral analysis using UV-visible absorption and fluorescence spectroscopy identified edge states and surface groups as luminescent centres, with a significant contribution arising from the latter. The investigation conducted using a collection of solvents, categorized into polar and nonpolar, indicated the potential of the system for applications based on its solvatochromic nature. The feature enabled the determination of different polarity parameters of the solvents, as well as dielectric constants of solvents and solvent mixtures, with considerable accuracy. The system was potent for predicting the composition of a given pair of solvents. The service of the system is also extended for moisture sensing in organic solvents within an error percentage < 1. High quantum yield values (0.61) combined with solvent composition-dependent optical features ensure broader applications of the system to probe solvent interactions.

Fig. 1. (a) and (b) TEM images of PD-CNDs at different magnification levels, inset of Fig. 1b shows the lattice fringes with d-spacing value of 0.19 nm. (c) Particle size distribution determined from the TEM, (d) powder XRD profile, (e) curve fitted Raman spectrum and (f) FT-IR spectrum of PD-CNDs.

Fig. 2. (a) XPS survey spectrum of PD-CNDs, (b)-(d) XPS fitting curves of C 1s, N 1s and O 1s of PD-CNDs, respectively.

Fig. 3. (a) Aqueous dispersion of the PD-CNDs under day light and 365 nm UV light, (b) UV-vis absorption spectrum, (c) fluorescence emission spectrum of PD-CNDs (λ_ex = 360 nm), (d) excitation dependent emission behavior of PD-CNDs, (e) photostability curve, and (f) fluorescence lifetime decay profile of PD-CNDs (λ_em = 618 nm) (red circles). Instrument response function (prompt) is obtained at 368 nm (excitation wavelength) using milk powder suspension (black circles).

Fig. 4. Photographs of dispersion of PD-CNDs in selected solvents. Upper panel under day light and lower panel under 365 nm UV illumination.

Fig. 5. (a) UV-vis absorption spectra of PD-CNDs dispersion in nonpolar solvents and (b) in polar solvents. The inset in (b) corresponds to absorption spectra in the visible region.

Fig. 6. (a) Jablonski diagram for fluorescence with solvent relaxation and (b) fluorophore-solvent excited state interaction.

Fig. 7. (a) Solvent polarity dependent fluorescence spectra of PD-CNDs (λ_ex = 420 nm) and (b) corresponding normalized fluorescence spectra to explicitly mark the shift in wavelengths with solvent polarity change.

Fig. 8. Fluorescence emission spectra of PD-CND dispersions in (a) xylene, (b) acetone, (c) ethanol and (d) methanol at different excitation levels.

Fig. 9. (a) Plot of fluorescence emission maximum values, λ_max (em), of PD-CNDs dispersion in selected solvents versus E_T(30) and (b) λ_max (em) values versus dielectric constant of solvents used for dispersing PD-CNDs.

Fig. 10. Photographs of PD-CND dispersions in different compositions of methanol–1,4-dioxane mixture (from 0 to 100%, with 10% increment) in the increasing order of methanol content. Images under day light (upper panel) and under 365 nm UV light illumination (lower panel).

Fig. 11. (a) Fluorescence emission spectra of PD-CNDs at selected compositions (0 to 100% with 10% increment), and (b) normalised emission spectra of PD-CNDs in selected composition of methanol–1,4-dioxane mixture (0, 20, 40, 60, 80 and 100% v/v) (λ_ex = 460 nm). (c) Plot showing the relationship of emission peak maximum versus methanol content in the binary mixture and (d) plot of emission peak maximum versus dielectric constant.

Fig. 12. Photographs of PD-CND dispersions in different compositions of 1,4-dioxane–water mixture (0, 1, 8, 9, 10, and thereafter with an increment of 10% up to 100% v/v) in the increasing order of water content. Images under day light (upper panel); under 365 UV illumination (lower panel).

Fig. 13. (a) Fluorescence emission spectra of PD-CNDs in 1,4-dioxane–water composition (0 to 100% with 10% increment) and (b) normalised emission spectra of PD-CNDs in selected 1,4-dioxane–water compositions (0, 20, 40, 60, 80 and 100%, v/v) (λ_ex = 420 nm). (c) Plot showing relationship of emission peak maximum versus water content in 1,4-dioxane.