Enhanced antibacterial efficacy of carbon quantum dots-based metal complexes: a simple strategy against pathogenic microbes

This research requires a carbon source (mango peels) for CQDs synthesis and metal precursors (AgNO₃, CuCl₂, ZnCl₂, FeCl₃, NiCl₂, MnCl₂) for metal incorporation. Additional chemicals include ethanol for purification, NH₃ for pH adjustment, HCl, and NaOH for reaction facilitation. Antibacterial testing is conducted against Staphylococcus aureus, and Escherichia coli, using nutrient broth, LB broth, and MHA.

The synthesized samples with different functional groups were analyzed by a Fourier Transform Infrared (FTIR) spectrophotometer, 3000 Hyperion Microscope Vertex 80. FTIR analysis was carried out using KBr-pressed pellets over the 4000–500 cm⁻¹ spectral range, with spectra collected at a resolution of 1 cm⁻¹ to investigate the functional groups present in the samples. Scanning electron microscopy (SEM) was employed to investigate the surface morphology of the synthesized metal–CQDs using a ZEISS EVO 18 microscope. The elemental composition of the samples was determined by energy-dispersive X-ray spectroscopy (EDS) integrated with the same ZEISS EVO 18 system.

The synthesis of different carbon quantum dots (CQDs)-based metal complexes is explained owing to the interaction of CQDs with several metal ions (M⁺/M²⁺). Fig. 2 shows the synthesis mechanism of CQDs with metal ions. In the first instance, CQDs possess functional groups like hydroxyl (-OH) and carboxyl (-COOH), which present them with the capability of binding metal ions quite efficiently. After the introduction of different metal ions such as Ag⁺, Cu²⁺, Zn²⁺, Fe³⁺, Ni²⁺, and Mn²⁺, the above functional groups coordinated with respective metal species, forming strong coordination bonds. The subsequent linking of CQDs via metal ions led to enhanced antibacterial efficacy conferred by CQDs.

The antibacterial mechanism of CQDs-metal complexes must be a synergy of several aspects. The positive charge of the metal-loaded CQDs engenders electrostatic interactions with the bacterial membranes, hence facilitating the CQD binding on the bacterial surface. Once tightly bound, metal ions interfere with the integrity of the membrane, followed by an increase in permeability, thereby allowing leakage of intracellular constituents [15]. The generation of reactive oxygen species (ROS), leading to oxidative stress and destruction of nucleic acids, proteins, and lipids in the bacteria, reinforces the impact. The action of the metal ions will also interfere with another vital enzyme that is needed by the bacterial cell hence causing metabolic inhibition that will lead to cell death.

As a synergistic mechanism, this will greatly act together to render the CQDs with enhanced antibacterial actions through exploitation of their unique nanostructure and the cytotoxic potential of the metal ions, thus marking a very promising path for its application in the biomedical and antimicrobial fields [16].

To show how metal coordination alters the structure, FTIR spectroscopy was used to examine the functional groups in metal complexes and carbon quantum dots (CQDs). The presence of significant variations in the absorption bands in the FTIR spectra, as shown in Figure 3, suggests the presence of metal-ligand interactions.

A shift in significant vibrational bands in the FTIR spectra shows that CQDs effectively coordinate with Zn, Ni, Cu, Fe, and Mn. Any structural changes caused by metals are seen by the peaks at 1660 cm⁻¹ (C=O stretching) and 1598 cm⁻¹ (C=C stretching). The great change at 1420 cm⁻¹ (C-N stretching) suggests that there is a substantial interaction between the metal and the ligand through groups that include oxygen and nitrogen. These spectrum modifications indicate the enhanced stability and functional properties of CQD-metal complexes [17]. Antibacterial efficacy is increased due to the changes' promotion of ROS production and bacterial membrane breakdown. This means that FTIR studies can confirm that metal-functionalized CQDs are safe to employ in environmental and biological science.

SEM images of CQD composites display various changes in morphology arising from metal doping, as shown in Fig. 4. The pure CQD sample displays aggregate-like, almost spherical morphology with a comparatively smooth surface; this is expected for nanoscale dimensions and graphitic nature [18]. On addition of silver (Ag), the surface morphology displays sharp-edged, sheet-like features, indicating interaction of Ag ions with the CQD matrix.

The Mn doping shows wrinkling and crumpling, which imply that Mn ions modify the regular CQD structure by disordering and increasing surface roughness. The incorporation of Cu transforms CQDs into highly folded and dense structures, signifying a high packing density coupled with low levels of porosity. The Ni-doped sample shows fibrous interlinkages, which implies that coupling of Ni ions with the CQD will confer high mechanical strength upon the composite. Fe doping exhibits a rough irregular surface with high porosity, which contributes to enhanced adsorption and catalytic activity. Studies on these metals highlight the possibilities that metal doping has in tailoring CQD structure, porosity, surface roughness, and overall morphology, which in turn define potential applications for the material in catalysis, adsorption, and biocompatibility.

The results of EDS spectroscopy corroborate the doping of carbon quantum dots with various metals (Ag-Mn-Cu-Ni-Fe) in addition to a control sample of pure CQDs. Carbon (C) and oxygen (O) show strong peaks across all the samples, indicating the type of CQDs with the possibility of O-containing functional groups. Gold peaks are seen across all spectra possibly arising due to gold coating for EDS analysis. Fig. 5 depicts EDS images for CQDs and CQDs-metal complexes. CQDs-Ag shows peaks of Ag that can be considered for the enhancement of fluorescence and antibacterial properties.

Peaks of Mn in CQDs-Mn confirm the incorporation of manganese, whose presence can change magnetic and optical properties. The presence of Cu in CQDs-Cu gives credence to the possible application for catalytic and antibacterial properties, while Ni in CQDs-Ni may be considered for electrochemical sensing. In CQDs-Fe, a significant presence of Fe (~18.4% Wt) is clearly visible, which suggests it could have been strongly doped and is therefore of interest for magnetic and biomedical applications. We will take the lack of the appearance of any other metal peaks in the undoped CQDs sample as a control. These results confirm the incorporation of the metals and suggest that doping can, henceforth, modify the properties of CQDs in terms of prospective applications in catalysis, sensing, and biomedicine.

For each CQD-metal combination, Table 1 shows the results for the minimum bactericidal concentration (MBC) and minimum inhibitory concentration (MIC). According to the data in the table, the CQD-Ni, CQD-Ag, and CQD-Mn complexes outperform the CQD-Cu and CQD-Fe complexes in terms of bactericidal efficacy. One reason why different kinds of bacteria produce varied outcomes is that some, like gram-negative bacteria, have an outer membrane, while others do not. That one allows the bactericidal chemical to penetrate the cell by rupturing its membrane.

From antimicrobial testing, CQD-Ag exhibited the highest antimicrobial activity, requiring the lowest concentrations to inhibit both Staphylococcus aureus and Escherichia coli. Specifically, CQD-Ag exhibited a MIC/MBC of 78.5/220 µg/mL against S. aureus and 500/670 µg/mL against E. coli, which are significantly lower than those of CQD-Ni (135/280 and 620/1000 µg/mL) and CQD-Mn (149/650 and >1000/>1000 µg /mL), while CQD-Cu and CQD-Fe required concentrations of at least 800-1150 µg/mL. This notable efficacy at low doses of CQD-Ag is linked to the synergistic interaction between Ag and CQDs, where the CQD matrix allows for even dispersion and controlled release of Ag⁺ ions, promoting strong interactions with thiol-containing proteins and bacterial DNA. A key part of these CQDs' antibacterial effectiveness is their ability to generate ROS and physically harm the bacterial cell membrane. We noticed a much stronger reaction in S. aureus than in E. coli. This difference occurs because the LPS-rich outer membrane of Gram-negative bacteria creates a substantial permeability barrier. This barrier raises the necessary inhibitory concentrations (MIC/MBC). The primary reason for these CQDs' antibacterial activity is their ability to produce ROS and damage the bacterial cell membrane. We also observed a much higher reaction in S. aureus than in E. coli because the LPS-rich outer membrane of Gram-negative bacteria creates a significant permeability barrier that raises necessary inhibitory concentrations (MIC/MBC). Of all the metal-doped systems we examined, the CQD-Ag modification was without a doubt the best, yielding superior results at a quarter of the concentration required by the others.