Temperature responsive aluminum manganese doped carbon dot sensors for enhanced electrical conductivity with DFT calculations

Abstract

Agricultural wastes provide abundant cellulosic by-products, making them excellent candidates for sustainable material production. In this study, sugarcane bagasse was used to synthesize aluminum/manganese-doped carbon quantum dots (Al-Mn/CQDs) through a microwave-assisted process. Aluminum doping and subsequent thermal treatment progressively reduced the band gap of manganese-doped carbon quantum dots from 1.21 eV to 0.7 eV and 0.3 eV, respectively, demonstrating a tunable electronic structure with implications for applications requiring specific emission wavelengths. The resulting CQDs exhibit a spherical morphology (1.95-2.05 nm) and, upon aluminum incorporation, form uniform sheet-like structures decorated with these particles. Optical analysis shows a notable improvement in fluorescence quantum yield, reaching up to 42.65% at elevated synthesis temperatures, and a narrow full width at half maximum, demonstrating strong potential for bioimaging and sensing applications. Aluminum incorporation into Mn/CQDs lowers the LUMO energy level from - 0.12459 to - 0.14838 eV, indicating that aluminum creates or modifies acceptor states to favor electron acceptance. Moreover, the total energy decreases from - 1638.16 au in Mn/CQDs to - 1874.34 au in Al-Mn/CQDs, underscoring the enhanced stability and favorable formation of the aluminum-modified carbon dots. Density functional theory (DFT) calculations reveal a lower energy gap (0.0482 eV), higher softness (20.74 eV), and enhanced charge transfer, findings confirmed by stable and low-impedance conductivity across a wide frequency range. These properties make Al-Mn/CQDs ideal for antistatic protection, electromagnetic interference shielding, and RLC bridge calibration, while their temperature-sensitive behavior also shows promise for temperature sensing applications.

Keywords: Agricultural waste valorization; Aluminum/manganese-doped carbon quantum dots (Al-Mn/CQDs); Carboxymethyl cellulose (CMC); Electrical conductivity; Microwave-assisted synthesis; Sugarcane bagasse.

Fig. 1

(a) UV spectra, (b) XRD spectra, and fluorescence spectra of; (a) Mn/CQDs, (b) Al-Mn/CQDs 1, and (c) Al-Mn/CQDs 2.

Fig. 2

Fluorescence microscope of Mn/CQDs, Al-Mn/CQDs 1, and Al-Mn/CQDs 2.

Fig. 3

(a) FTIR spectra of Mn/CQDs, Al-Mn/CQDs 1 and Al-Mn/CQDs 2, and (b) DFT calculations of Mn/CQDs and Al-Mn/CQDs.

Fig. 4

Analysis of (a) Mn/CQDs, (b) Al-Mn/CQDs 1, and (c) Al-Mn/CQDs 2 with pore size distribution.

Fig. 5

(a) Permittivity versus frequency, (b) Loss versus frequency, (c) Conductivity versus frequency, (d) Real impedance versus frequency, (e) Cole–Cole plot of conductivity for Mn/C QDs, Al-Mn/CQDs 1, and Al-Mn/CQDs 2, and (f) Cole–Cole plot of conductivity for Al-Mn/CQDs 2 at different temperatures.