Post‑Treatment Strategies to Modulate Photoluminescence of Nitrogen‑Doped Carbon Dots: Solvent, Reduction, and Metal‑Induced Enhancements

Background

Carbon dots (CDs) are emerging as eco‑friendly, biocompatible nanoscale emitters, offering tunable photoluminescence and versatile synthesis routes from inexpensive precursors such as citric acid, fruit extracts, and food waste [1–4]. Compared with conventional quantum dots (e.g., InP, ZnSe, ZnInS, CuInS), CDs lack heavy metals, simplify fabrication, and broaden application prospects in bio‑imaging, LED displays, fluorescent sensing, and photodetection [5–10]. Despite these advantages, CD emission has historically been confined to the blue region. Reports of longer‑wavelength emission often rely on excitation‑dependent spectral shifts rather than true tunability, and the intensity of shifted bands is typically weak, limiting practical use [11,12]. The origin of CD PL remains contested, attributed to intrinsic core emission or surface‑defect states [13,14]. Addressing this knowledge gap requires controlled post‑treatments that modulate surface chemistry while preserving core integrity. Surface states profoundly influence a nanomaterial’s optical, electronic, and catalytic properties [15–17]. For example, surface passivation enhances perovskite optoelectronics, and acid–base characteristics improve CeO₂ catalysis [18,19]. Solvent‑dependent PL, driven by hydrogen bonding between surface functional groups and solvent molecules, has been observed in nitrogen‑rich CDs, producing blue‑to‑green shifts [20,21]. Metal‑ion addition can also induce aggregation‑induced emission enhancement (AIEE), as demonstrated for Fe³⁺‑triggered CD aggregation [22]. In this study, we synthesize yellow‑emitting NCDs via a single‑step hydrothermal method and apply three post‑treatments—solvent exchange, NaBH₄ reduction, and Ag⁺ addition—to systematically investigate the role of surface states in PL modulation.

Methods

Chemicals

O‑Phenylenediamine (OPD, 99.9 %), analytical‑grade solvents (water, ethylene glycol, ethanol, DMSO, acetone, toluene), metal chlorides/nitrates, and HEPES were sourced from commercial suppliers and used without further purification. Milli‑Q water was employed for all aqueous solutions.

Synthesis of NCDs

0.05 g OPD was dissolved in 10 mL Milli‑Q water under vigorous stirring until clear. The solution was transferred to a 25 mL Teflon‑lined autoclave and heated at 180 °C for 6 h. After cooling, the product was centrifuged at 9,000 rpm to remove solids, and the supernatant was freeze‑dried for 24 h, yielding a light‑yellow powder.

Post‑Treatment Protocols

For solvent‑dependent studies, 0.01 g of dried NCDs was dispersed in 10 mL of each solvent (water, EG, ethanol, DMSO, acetone, toluene). NaBH₄ solutions (10 mL, 0–0.04 g mL⁻¹) were added to 0.01 g NCDs to investigate reduction effects. Metal‑ion titrations involved adding 50 µM of Ag⁺, Cd²⁺, Cs⁺, Cu²⁺, Fe³⁺, In³⁺, Mg²⁺, Mn²⁺, Pb²⁺, or Zn²⁺ to 10 mL of 0.01 g NCDs in 10 mM HEPES buffer (pH 7.2). Ag⁺ concentrations were varied from 0 to 300 µM to study AIEE.

Characterization

NCD morphology and size were examined by HRTEM (JEOL JEM‑2s100F). Structural analysis used XRD (Bruker D8 Advance) and XPS (Thermo ESCALAB 250XI). FT‑IR spectra were recorded on a Nicolet 6700. UV‑vis absorption and steady‑state PL were measured with a Shanghai Lengguang 759S spectrophotometer and F97XP fluorescence spectrometer, respectively. Time‑resolved PL lifetimes were obtained using an Edinburgh Instruments FLS 920. Zeta potentials were determined with a Malvern Zetasizer.

Results and Discussion

Structural Characterization

HRTEM images (Fig. 1) reveal monodisperse NCDs with diameters ranging from 3 to 6 nm (average 3.96 nm). Lattice fringes at 0.21 nm and 0.32 nm correspond to the (100) and (002) planes of graphitic carbon. XRD shows a broad peak near 20° 2θ, indicating small crystalline cores with disordered surfaces.

FT‑IR analysis identifies abundant surface groups: O–H/N–H (3100–3600 cm⁻¹), C=O/C=C (1570–1750 cm⁻¹), C–N/C=N (1411/1239 cm⁻¹), and C–O–C (990–1170 cm⁻¹). XPS confirms the presence of C, N, and O (73.8 %, 22.6 %, 3.6 %). High‑resolution C 1s peaks correspond to sp² C–C/C=C, C–N, C–O, C=N, and C=O; N 1s peaks reveal pyridinic, amino, and pyrrolic nitrogen; O 1s peaks correspond to C–O and C=O bonds.

Solvent‑Dependent Photoluminescence

Emission spectra across six solvents (water, EG, ethanol, DMSO, acetone, toluene) show a red‑shift from 500 to 569 nm under 365 nm excitation, with intensity enhancements varying by solvent (Fig. 2a–b). UV‑vis absorption mirrors this trend, shifting to longer wavelengths with increasing solvent polarity (except water, due to limited solubility). PL lifetimes and Stokes shifts increase linearly with the E_T(30) polarity index, indicating stronger hydrogen bonding between solvent and surface functional groups enhances PL (Fig. 2d–e). The observed tunability is attributed to solvent‑mediated modulation of surface‑state contributions to the emissive transition.

Reduction with NaBH₄

NaBH₄ treatment (0–0.04 g mL⁻¹) significantly brightens PL and blue‑shifts the emission from 567 to 510 nm (Fig. 3a). Deconvolution reveals two Gaussian components (peak 1 ~ 565 nm, peak 2 ~ 496 nm); increasing NaBH₄ raises peak 2 intensity while peak 1 remains stable (Fig. 3c–d). XPS shows increased oxygen and decreased nitrogen content, consistent with C=O reduction to C–O bonds. UV‑vis absorption displays enhanced 320 nm band (n–π*) intensity, corroborating a shift toward intrinsic C–O–C emission. PL lifetime shortens from 3.48 to 2.2 ns, confirming reduced non‑radiative pathways.

Metal‑Enhanced Photoluminescence

Among tested metal ions, Ag⁺ uniquely amplifies PL intensity without shifting the emission peak (Fig. 4c). A linear relationship (R² = 0.992) exists between ΔF and Ag⁺ concentration (0–300 µM) (Fig. 4d). Time‑resolved PL shows a subtle lifetime change, suggesting complex formation. Zeta potential shifts from –34.0 mV (neutral) to –27.8 mV upon Ag⁺ addition, reducing electrostatic repulsion and promoting aggregation‑induced emission enhancement (AIEE). Aggregation observations (Fig. S9) confirm that Ag⁺‑induced clustering passivates surface defects, yielding brighter fluorescence.

Conclusion

We successfully synthesized yellow‑emitting NCDs via a simple hydrothermal route and demonstrated that PL can be precisely tuned by three post‑treatments: solvent exchange, NaBH₄ reduction, and Ag⁺ addition. Solvent‑mediated hydrogen bonding shifts and amplifies PL; chemical reduction transforms C=O to C–O, enhancing intrinsic (n–π*) emission; and Ag⁺ triggers AIEE, boosting fluorescence. These results validate the central role of surface functional groups in governing NCD photophysics and suggest practical strategies for designing advanced luminescent probes and lighting devices.

Abbreviations

AIEE:

Aggregation‑induced emission enhancement

CDs:

Carbon dots

DMSO:

Dimethyl sulfoxide

EG:

Ethylene glycol

FT‑IR:

Fourier transform‑infrared spectroscopy

HB:

Hydrogen bonding

HEPES:

4-(2‑Hydroxyethyl)piperazine‑1‑ethanesulfonic acid

HRTEM:

High‑resolution transmission electron microscopy

NCDs:

Nitrogen‑doped carbon dots

OPD:

O‑Phenylenediamine

PL:

Photoluminescence

QDs:

Quantum dots

rpm:

Revolution per minute

UV‑vis:

Ultraviolet‑visible

XPS:

X‑ray photoelectron spectroscopy

XRD:

X‑ray diffraction