Carbon dots are quasi-zero-dimensional carbon nanomaterials, typically smaller than ten nanometers, that have attracted intense interest for their photoluminescence, ease of surface functionalization, and low toxicity. In recent years, biomass has emerged as a sustainable and cost-effective feedstock for these nanoparticles, offering renewable supply chains and greener synthesis routes compared with synthetic molecular precursors. Yet crude biomass presents a fundamental problem. Composed of mixtures of polysaccharides, proteins, organic acids, and other constituents, it triggers competing reactions during carbonization, yielding structurally heterogeneous products with batch-to-batch variability. That heterogeneity translates into uneven distributions of anchoring sites on the resulting dots, which in turn causes uncontrolled aggregation of metal nanoparticles and poor signal reproducibility in the final SERS substrates.

The research team, led by Chengwei Hu and corresponding author Hong Bi of Anhui University, sidestepped this problem by starting from a structurally well-defined biomacromolecule rather than crude plant matter. Polygonatum cyrtonema Hua polysaccharide, abbreviated PcH, was extracted from dried rhizomes using hot-water extraction followed by ethanol precipitation, deproteinization, decolorization, dialysis, and lyophilization. Mass spectrometric analysis revealed a glucan-type backbone with a regular 162-dalton interval between dominant peaks, corresponding to hexose units, while chromatographic analysis of the acid hydrolysate confirmed glucose as the predominant monosaccharide. The polymer showed a moderately polydisperse population of oligomers with degrees of polymerization of roughly eight to sixteen, and thermogravimetric analysis demonstrated thermal stability up to about 300 degrees Celsius, a property that proved essential for the hydrothermal steps to follow.

The key innovation lies in what the researchers did next. Rather than carbonizing the polysaccharide directly, they first oxidized it selectively with sodium periodate under acidic conditions, converting vicinal diols into dialdehyde functionalities. Quantification using the Schiff reagent method showed the relative oxidation degree jumping from 1.26 percent in the pristine polysaccharide to 96.62 percent in the oxidized product, a near-complete conversion. The dialdehyde intermediate was then condensed with L-lysine, an amino acid whose primary amines react with the aldehyde groups to form imine linkages, the hallmark of Schiff-base chemistry. Nuclear magnetic resonance spectroscopy confirmed the formation of these -C=N- bonds with a characteristic resonance near 8.3 parts per million, while infrared spectroscopy showed the carbonyl band near 1720 wavenumbers weakening as a new band emerged around 1650 to 1680 wavenumbers, consistent with C=N stretching.

These imine motifs are far more than decorative. Nitrogen atoms in imine groups carry lone pairs of electrons embedded in an extended pi-conjugated system, making them excellent coordination sites for metal ions. Crucially, the team demonstrated that these functionalities survive the subsequent hydrothermal carbonization, in which the Schiff-base-functionalized precursor was heated at 160 degrees Celsius for six hours in a sealed autoclave. The resulting carbon dots, termed PcH-CDs, emerged as uniformly dispersed quasi-spherical particles averaging about 2.5 nanometers in diameter, with lattice fringes of roughly 0.20 nanometers corresponding to the (100) plane of graphitic carbon. X-ray diffraction showed the amorphous polysaccharide peak at 22 degrees shifting to 26 degrees after carbonization, indicative of a graphite-like sp2-conjugated framework, while Raman spectroscopy revealed the ratio of disorder to graphitic band intensities dropping from 1.59 to 0.75, signaling increased structural ordering.

With the imine sites preserved on the carbon dot surfaces, the researchers introduced silver through a silver-ammonia complex reduction. The surface-exposed -C=N- groups selectively coordinated silver ions, guiding uniform in-situ nucleation and suppressing the random aggregation that plagues conventional syntheses. Transmission electron microscopy showed the hybrid nanoparticles growing to an average of 13.23 nanometers, with silver nanocrystals of roughly 7 to 10 nanometers estimated from X-ray diffraction line broadening. X-ray photoelectron spectroscopy delivered perhaps the most telling evidence: the silver 3d binding energies in the hybrid were blue-shifted by 0.80 electron volts relative to silver nitrate, a shift attributable to strong coordination between silver and the imine nitrogen, which acts as a stronger electron donor than oxygen and withdraws electron density from the metal center. A control sample prepared from unmodified polysaccharide, lacking these nitrogen sites, showed only minor shifts characteristic of weaker silver-oxygen coordination and exhibited markedly weaker silver diffraction peaks.

The practical payoff came in SERS testing. When Rhodamine B was applied to the PcH-CDs-Ag substrate, the characteristic Raman peak at 1650 wavenumbers remained clearly visible down to a concentration of 1 x 10^-10 molar, with a linear calibration spanning ten^-10 to ten^-6 molar and a correlation coefficient of 0.99. The calculated enhancement factor reached 4.21 x 10^6, roughly three orders of magnitude higher than that of metal-free nitrogen-doped graphene quantum dots for the same analyte. Signal reproducibility, measured across twenty randomly selected points on a single substrate, yielded a relative standard deviation of 11.7 percent, an acceptable figure for practical analytical work. The substrate also proved durable: after six weeks of ambient storage, SERS intensities retained approximately 70 percent of their initial values, with the carbon dot matrix apparently protecting the silver nanoparticles from oxidation. Methylene blue served as a second probe molecule, detectable down to 1 x 10^-8 molar, confirming the platform’s versatility beyond a single analyte.

To understand why the nitrogen-coordinated system outperforms its oxygen-coordinated counterpart, the team turned to density functional theory. Optimized geometries showed an Ag-N bond length of 1.97 angstroms, significantly shorter than the 2.14-angstrom Ag-O bond, indicating a stronger and more localized interaction. Natural bond orbital analysis quantified the difference dramatically: the donor-acceptor interaction from the imine pi bond to the silver acceptor orbital exhibited a second-order stabilization energy of 71.6 kilocalories per mole with an orbital occupation number of 0.39639, whereas the corresponding oxygen-to-silver interaction registered below 3 kilocalories per mole with an occupation number of just 0.01314. The Ag-N system also displayed a substantially narrowed band gap of 1.63 electron volts compared with 3.46 electron volts for Ag-O, conditions that favor excited-state charge transfer across the molecule-metal interface and amplify the chemical enhancement contribution to the SERS signal.

Beyond the immediate analytical performance, the study carries a broader message about how natural polysaccharides can be used in materials design. The authors argue that glycans should be viewed not merely as sustainable carbon sources but as chemically tunable platforms whose repeating-unit backbones and abundant hydroxyl groups allow precise, site-specific molecular modification. By combining regioselective oxidation, Schiff-base condensation, hydrothermal carbonization, and coordination-directed metal deposition, the researchers demonstrated a molecular route to controllable carbon-metal hybrid nanoparticles in which interfacial chemistry, rather than chance, dictates performance. The aqueous processability of the hybrid, along with its uniform silver loading, suggests suitability for coating onto flexible substrates, opening possibilities for lightweight, portable SERS devices for environmental monitoring, food safety testing, and trace detection of organic pollutants. Extending this strategy to other polysaccharide systems, the team suggests, could open new avenues for sustainable materials in advanced sensing applications, turning an ancient medicinal plant into a blueprint for next-generation nanosensors.

Subject of Research: Schiff-base-functionalized carbon dots derived from Polygonatum cyrtonema Hua polysaccharide decorated with silver nanoparticles for surface-enhanced Raman scattering detection of trace organic pollutants

Article Title: Silver decorated Schiff base functionalized carbon dots derived from Polygonatum cyrtonema Hua polysaccharide for surface enhanced Raman scattering

Article References: Hu, C., Zhu, H., Cui, J., Xue, J., Liu, J., Liu, C., & Bi, H. (2026). Silver decorated Schiff base functionalized carbon dots derived from Polygonatum cyrtonema Hua polysaccharide for surface enhanced Raman scattering. Discover Green Chemistry, 1(1), Article 23. https://doi.org/10.1007/s44509-026-00024-0

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00024-0

Keywords: carbon dots, SERS, Polygonatum cyrtonema Hua, polysaccharide, Schiff base, silver nanoparticles, surface-enhanced Raman scattering, biomass, green chemistry, Rhodamine B, density functional theory, nanosensors

Cellulose nanofibers, or CNFs, are extraordinarily slender strands derived from the cellulose that gives plant cell walls their strength. Their nanoscale dimensions and high aspect ratio—the relationship between length and diameter—allow them to form networks with unusual mechanical and rheological properties. These characteristics make CNFs attractive for packaging, coatings, composite materials, filtration membranes and other applications where petroleum-based or energy-intensive materials might otherwise be used. Yet turning agricultural residues into high-quality CNFs generally requires removing lignin, pigments and other matrix components. Bleaching is therefore a crucial step, and the chemicals used at this stage can determine whether a process is genuinely sustainable or merely shifts environmental costs upstream.

Sugar beet pulp is particularly promising because it is a low-lignin residue. Much of the structural complexity that makes woody biomass difficult to process is already reduced, meaning that the pulp can be converted into cellulose-rich material with comparatively mild treatment. The researchers’ starting process uses enzymes to digest the pulp before chemical oxidation and mechanical fibrillation separate the cellulose into nanoscale fibres. In the established method, sodium chlorite performs the key bleaching step. Although effective, sodium chlorite can generate chlorinated compounds and requires careful handling, motivating the search for an alternative that removes chlorine from the process entirely.

The team investigated peracetic acid, or PAA, an oxidising agent formed from acetic acid and hydrogen peroxide. PAA is already used in some disinfection and bleaching applications, and it decomposes primarily into compounds such as acetic acid, water and oxygen-related products rather than chlorinated residues. In the experiments, the acid was used in two ways: it was either added directly as an external reagent, described as exogenous PAA, or generated inside the reaction mixture from precursor chemicals. The researchers also compared PAA-driven peroxyl chemistry with a hydrogen-peroxide-based Fenton reaction, in which iron-mediated free radicals produce highly reactive oxidising species.

That distinction proved decisive. Oxidation is not a single chemical event but a family of reactions whose outcomes depend on the reactive molecules involved, their concentrations and the structures they encounter. Peroxyl-mediated bleaching brightened the sugar beet pulp, whereas free-radical bleaching darkened it. The difference suggests that the reactive intermediates attacked lignin and other coloured molecules through substantially different pathways. Peroxyl species appear to remove or modify chromophores—the molecular structures responsible for absorbing visible light—more effectively under the tested conditions. By contrast, the aggressive free-radical route may have fragmented matrix polysaccharides or transformed lignin into products that absorb more strongly, producing a darker material despite extensive oxidation.

In terms of appearance, the PAA-treated nanofibers came remarkably close to those produced with sodium chlorite. Both reached a relative brightness index of approximately 67 percent, according to the study. Brightness is not merely cosmetic in a biomaterial: it can influence the visual quality of paper-like films, coatings and packaging, as well as indicate how much coloured non-cellulosic material remains. However, the chemical composition of the products differed. PAA-oxidised CNFs retained more lignin than sodium-chlorite-oxidised CNFs, with lignin levels reported at about 9 percent and 4 percent, respectively. The Fenton-oxidised material contained about 10 percent lignin.

At first glance, retaining more lignin might seem to be a disadvantage, because lignin can contribute colour and interfere with cellulose processing. But the researchers found that the PAA- and Fenton-derived suspensions were more viscous than the sodium-chlorite product. Viscosity describes a fluid’s resistance to flow, and in a suspension of nanofibers it reflects how effectively the fibrils interact, entangle and create a three-dimensional network. The PAA-CNF suspension reached approximately 4,500 centipoise, compared with 2,500 centipoise for the Fenton material and 2,000 centipoise for the sodium-chlorite CNFs. Such differences could be valuable in applications requiring a thick, stable coating or a material that resists drainage and separation during processing.

The team also examined the electrical stability and shape of the fibrils. All of the CNF suspensions showed zeta potentials between approximately −17 and −23 millivolts. Zeta potential is the electrical potential at the slipping plane around particles or fibres suspended in a liquid; its magnitude provides an indication of how strongly those particles repel one another. More negative values generally help a colloidal suspension resist aggregation, although stability depends on ionic strength, pH and the chemistry of the fibre surface as well. The reported values indicate that the oxidised CNFs were at least incipiently stable, meaning they could remain dispersed under the tested conditions rather than rapidly clumping together.

Oxidation and fibrillation also increased the aspect ratio of the cellulose structures compared with unoxidised CNF material. A higher aspect ratio can improve network formation because long, thin fibrils span larger distances and create more points of contact. The researchers used microscopy and surface-related analyses to assess the products, including scanning electron microscopy and array-based microfibril surface assessment. These measurements are important because nanocellulose performance depends not only on chemical purity but also on fibril dimensions, surface charge, aggregation state and the extent to which individual fibres have been liberated from the original plant matrix.

One of the most significant findings was that PAA generated within the process produced CNFs of similar quality to those treated with PAA added from outside. In situ generation could simplify storage, transport and handling by avoiding the need to prepare and introduce a separate supply of peracetic acid. The results also demonstrate that three alternatives to sodium chlorite can work within the researchers’ production strategy: exogenous PAA, in situ PAA and hydrogen-peroxide Fenton oxidation. The products are not identical, and the choice of route would depend on whether brightness, viscosity, residual lignin or reaction safety is the dominant requirement. Rather than identifying one universal replacement, the work maps how chemistry can tune the properties of a biomass-derived nanomaterial.

The study’s broader significance lies in connecting waste valorisation with safer process chemistry. Sugar beet pulp is generated in large quantities by the sugar industry, and its conversion into CNFs could create a higher-value use for material that might otherwise be burned, composted or treated as low-value feedstock. An enzyme-mediated, totally chlorine-free process could reduce hazardous inputs while retaining the possibility of producing functional nanocellulose. The researchers caution implicitly that successful laboratory chemistry is only one stage in commercial development: future work will need to assess reagent recovery, water use, energy demand, lifecycle emissions, process scale-up and the performance of the materials in real products. Even so, the results show that removing chlorine does not require abandoning the desired properties of the fibres. By choosing the oxidation mechanism carefully, agricultural residue can be transformed into a versatile nanomaterial with a smaller toxicological footprint and a potentially stronger role in the circular bioeconomy.