SOLVOTHERMAL SYNTHESIS AND OPTICAL CHARACTERIZATION OF FLUORESCENT CARBON DOTS

ABSTRACT

Carbon dots (CDs) with tunable emission were synthesized solvothermally using different solvents. Blue (B-CDs), green (G-CDs), yellow (Y-CDs), and red (R-CDs) samples were obtained and rigorously purified to ensure uniformity and stability. The roles of solvent polarity, surface functional groups, and dopants in dictating structural and optical features were systematically examined. UV–vis spectra showed distinct absorption bands (385–567 nm) attributable to n–π* and π–π* transitions, highlighting contributions from surface groups and aromatic cores. Photoluminescence displayed excitation-dependent behavior, and TRPL revealed longer lifetimes at longer emission wavelengths, indicating suppressed recombination and enhanced carrier persistence. Computational modeling further accounted for band-gap variations underlying the color shifts. Collectively, the results establish a strong link between synthesis conditions, structure, and photophysical response, offering guidance for the rational design of CDs for optoelectronic, photocatalytic, and bioimaging applications.

АННОТАЦИЯ

Углеродные точки (CDs) с настраиваемым излучением были синтезированы сольвотермальным методом с использованием различных растворителей. Получены образцы с синим (B-CDs), зелёным (G-CDs), жёлтым (Y-CDs) и красным (R-CDs) излучением и тщательно очищены для обеспечения однородности и стабильности. Систематически изучены роли полярности растворителя, поверхностных функциональных групп и допантов в формировании структурных и оптических характеристик. Спектры UV–vis выявили отчётливые полосы поглощения в диапазоне 385–567 нм, относящиеся к переходам n–π* и π–π*, что подчёркивает вклад поверхностных групп и ароматических ядер. Фотолюминесценция продемонстрировала возбуждённо-зависимое поведение, а TRPL показала увеличение времени жизни при увеличении длины волны излучения, что указывает на подавление рекомбинации и повышенную устойчивость носителей. Вычислительное моделирование дополнительно объяснило вариации ширины запрещённой зоны, лежащие в основе цветовых сдвигов. В совокупности результаты устанавливают тесную связь между условиями синтеза, структурой и фотофизическим откликом и дают ориентиры для рационального конструирования углеродных точек для оптоэлектронных, фотокаталитических и биовизуализационных применений.

Keywords: Carbon dots (CDs), fluorescence, solvent effect, photoluminescence (PL).

Ключевые слова: Углеродные точки (CDs), флуоресценция, влияние растворителя, фотолюминесценция (PL).

 Introduction

Carbon dots (CDs) are a rapidly emerging family of carbon-based nanomaterials distinguished by their sub-10 nm dimensions and a two-component architecture: a quasi-spherical, sp²-rich carbon core built from stacked graphene-like domains, and a shell populated by diverse surface functional groups [1]. This structural duality underlies their notable physicochemical and optical traits—most prominently tuneable photoluminescence (PL), excitation-wavelength-dependent multicolor emission, low cytotoxicity, and excellent biocompatibility—which together motivate applications spanning biomedicine, chemical sensing, and optoelectronics [2].

A central objective in CD research is the reliable control of emission color. Steering the chemical structure during synthesis provides a practical route both to probe the origins of fluorescence and to program the optical response of the particles [3]. In broad terms, the PL output reflects a balance between core states and surface-related states. For CDs possessing extended π-conjugated domains and relatively few surface functionalities, emission is commonly attributed to intrinsic carbon-core states; in such cases, adjusting the size/extent of π-domains shifts the emission wavelength [4]. Conversely, when the surface bears abundant functional groups, a manifold of defect-related energy levels can form emissive traps; increased surface oxidation typically raises the trap density and drives red-shifted emission [5].

The present work addresses this gap by systematically outlining the operative PL mechanisms in CDs and surveying modulation approaches across blue, green, yellow, red, and multicolor emitters. Emphasis is placed on how core design, surface chemistry, and dopant incorporation dictate optical performance, thereby providing a framework for rational structure–property control and guiding the targeted deployment of CDs in optoelectronic, photocatalytic, and bioimaging applications.

Materials and Methods

Carbon dots (CDs) with solvent-dependent fluorescence were prepared by a solvothermal route. Four solvent systems were used: 100% deionized water (B-CDs), 100% DMF (G-CDs), DMF/ethanol (1:1, v/v; Y-CDs), and DMF/acetic acid (1:1, v/v; R-CDs). Citric acid (5.764 g) and urea (2.402 g) were dissolved in 40 mL of the selected solvent under stirring (30 min) to obtain a clear solution, transferred to a 100 mL Teflon-lined autoclave, and heated at 200 ⁰C for 6 h; the reactor was then cooled to room temperature. The dispersions were purified by centrifugation filtration, followed by dialysis against ultrapure water using 1000 Da MWCO bags for 72 h with ultrasonic assistance. The products were vacuum-dried at 40 ⁰C for 48 h to yield four CDs with distinct emissions: blue (B-CDs), green (G-CDs), yellow (Y-CDs), and red (R-CDs).

Results and Discussion

After synthesis, the dispersions were examined under 365 nm excitation, showing broad PL due to size polydispersity. Size selection proceeded via centrifugation (10000 rpm, 20 min) and 0.22 μm membrane filtration. The filtrate was then purified by dialysis (1000 Da MWCO) against ultrapure water for 72 h with ultrasonic assistance, followed by two additional cycles in fresh bags 48 h total with periodic water exchange. The suspensions were again centrifuged and filtered to remove aggregates, yielding stable, well-dispersed CDs. Powders were obtained by vacuum drying at 40 ⁰C for 48 h. For longevity, CDs were stored under vacuum and refrigerated to mitigate sunlight oxidation-induced degradation. The solvent-dependence of fluorescence was systematically assessed: solvent polarity and acidity/basicity govern surface functionalization. Polar media (water, DMF, ethanol, acetic acid) promote –COOH, C=O, C–O–C groups, which favor red-shifted emission; nonpolar solvents reduce such groups [6]. Graphitic N content: DMF/ethanol mixtures increase graphitic N, a key contributor to red emission; DMF facilitates N functionalities, while acetic acid enriches –COOH, both strongly impacting optics. Size/defect effects: Solvent composition tunes nucleation/growth, altering particle size and crystallinity. Smaller CDs → stronger quantum confinement → blue emission; larger/defect-richer CDs → red-shifted PL. PL behavior: DMF and acetic acid typically enhance red emission (higher N and carboxyl content), whereas water often yields blue/green due to dominant C=O states [7]. Electronic transitions: Solvent-derived groups modulate donor–acceptor character and surface traps, commonly inducing red shifts in emission. Solvent polarity, acidity/basicity, and composition co-determine CD structure, functional groups, size, and optical response; hence, emission color and PL spectra can be precisely tuned by solvent choice [8]. Figure 1 shows the UV–Vis spectra (300–700 nm) and Tauc-derived band gaps (Eg) of color-tuneable CDs. B-CDs exhibit a UV peak at 385 nm, assigned to n→π* transitions of C=O, consistent with oxygen-rich surface functionalities. G-CDs display a visible peak at 505 nm arising from n→π* transitions associated with C–N/C=N/C–O–C groups; the broad band reflects prominent surface states. Y-CDs show a red-shifted maximum at 552 nm, indicative of higher electron density and/or N/O dopants at the surface, while R-CDs peak at 567 nm, the longest wavelength among the series.

Figure 1. (a) UV-vis absorption spectra of different fluorescent CDs, (b) Band gap energy of different fluorescent CDs

For R-CDs, the absence of a shoulder and the near-continuous absorption across the visible region point to combined n→π* and π→π* contributions from aromatic cores containing C–O/C–OH moieties [9]. This result indicates that progressive narrowing of Eg from B→G→Y→R, in line with the spectral red shifts. Collectively, the data support a model in which CDs comprise strongly delocalized, benzene-like sp² domains whose π-states govern the optics: enlarging the conjugated framework lowers Eg, while attachment of oxygen-containing groups enables n→π* transitions that further tune absorption and emission [10]. Figure 2 displays the emission spectra of B-, G-, Y- and R-CDs recorded under monochromatic excitation. As seen in panels (a)–(d), all samples exhibit excitation-dependent behavior: the emission maximum shifts with the excitation wavelength. Representative peak positions are 425 nm at 320 nm excitation for B-CDs, 520 nm at 420 nm for G-CDs, 554 nm at 460 nm for Y-CDs, and 616 nm at 500 nm for R-CDs. Accordingly, the dominant emission windows span 420–460 nm (blue), 480–520 nm (green), 530–570 nm (yellow), and 590–650 nm (red). These trends reflect differences in physicochemical structure, ordering, and surface states among the CDs and underpin their relevance for photocatalytic light harvesting by mapping photon absorption–emission pathways.

Figure 2. Emission spectra of (a) B-CDs, (b) G-CDs, (c) Y-CDs, (d) R-CDs when excited under different monochromatic light

The origin of the spectral shifts is attributed to a distribution of emissive states arising from (i) core π-conjugated domains and (ii) surface-related states/defects. Larger conjugated domains and deeper surface states favor red-shifted emission, consistent with the stronger long-wavelength output of R-CDs. Three factors rationalize both peak position and intensity variations: Particle size (quantum confinement): smaller CDs (e.g., B-CDs) possess larger effective band gaps and emit in the blue (~420–460 nm), whereas larger CDs (e.g., R-CDs) show narrower gaps and emit in the red (~600–650 nm) [11]. Functional groups such as –OH, –COOH, –NH₂, –C=O modify electronic levels, creating diverse emissive traps and shifting peak positions [12]. Heteroatom dopants (e.g., N, S, P) can passivate defects or introduce new states, altering n→π* and π→π* transitions and thereby tuning the emission wavelength. Recombination and dispersion: faster e⁻/h⁺ recombination suppresses PL intensity, while defect passivation (often more pronounced in R-CDs) stabilizes emission. Aggregation reduces absorption/emission efficiency; well-dispersed samples (e.g., B-CDs) typically show stronger PL. Differences in the type/abundance of surface groups (e.g., –COOH, –OH) open additional radiative pathways, enhancing PL intensity when their density/diversity is higher. The time-resolved photoluminescence (TRPL) decay profiles of each type of CD are presented, showing how their photoluminescence intensity decreases with time. The average fluorescence lifetimes (⟨τ⟩) were determined as follows: B-CDs ~4.8962 ns, G-CDs ~5.4547 ns, Y-CDs ~5.6835 ns, and R-CDs ~5.8573 ns. These results indicate that as the emission wavelength shifts toward longer regions, the carrier lifetimes increase, reflecting the greater structural and electronic complexity of CDs. The prolonged PL decay times are associated with intrinsic defects, dopants (e.g., N, O), and passivated surface groups within the CDs. The systematic increase in lifetimes from blue to red CDs suggests reduced recombination rates, thereby enhancing the persistence of charge carriers. Prior to their incorporation into TiO₂, the charge separation ability of CDs was evaluated. Computational models (Figure 4 (b) for B-CDs and (c,f) for R-CDs) were employed to illustrate the structural models of CDs, explaining their energy levels, band gaps, and electron density distributions. The red emission observed for R-CDs is attributed to the narrowing of the band gap [13]. These findings provide deeper insights into the optical properties of carbon dots and support their selective application in photocatalysis and bioimaging.

Conclusion

In this work, carbon dots exhibiting four distinct emission colors were synthesized solvothermally by varying the solvent environment. The solvent nature proved decisive in dictating chemical composition, surface functionalities, particle size, and hence optical behavior. UV–vis and PL measurements confirmed excitation-dependent emission and solvent-driven spectral shifts, while TRPL revealed longer carrier lifetimes for red-emissive CDs, consistent with suppressed recombination and greater structural complexity. Computational modeling corroborated these findings by linking the observed color evolution to bandgap narrowing and changes in electron-density distribution. Collectively, the results establish a clear structure–processing–property relationship and provide practical guidance for engineering carbon dots as multifunctional materials for energy conversion, sensing, and biomedical applications.

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