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

The target molecule in the study was Acid Orange 10 (AO10), an anionic azo dye widely used in the textile industry and a representative of the broader class of synthetic colorants that resist conventional wastewater treatment. Azo dyes owe their vivid colors to the azo linkage—a nitrogen–nitrogen double bond—that also makes them chemically stable and, in many cases, toxic or recalcitrant in the environment. Textile effluents carrying such dyes reduce light penetration in receiving waters, disrupt aquatic photosynthesis, and pose health risks to humans and wildlife. Traditional treatment approaches, from adsorption onto activated carbon to biological degradation, often fall short because the dyes are designed from the ground up to resist fading, washing, and microbial attack.

The Algerian–Kuwaiti team’s answer was to build a photocatalyst that attacks these molecules at the molecular level using light. Titanium dioxide has long been the workhorse of photocatalysis: when photons with sufficient energy strike its surface, they excite electrons from the valence band to the conduction band, leaving behind holes. These electron–hole pairs migrate to the surface and trigger redox reactions with water and dissolved oxygen, generating reactive oxygen species—chiefly hydroxyl radicals and superoxide anions—that can oxidize organic pollutants into harmless smaller molecules, ultimately carbon dioxide, water, and inorganic ions. The problem is that pristine TiO2, with its wide bandgap of roughly 3.2 electron volts for the anatase phase, absorbs only ultraviolet light, which is a small fraction of natural sunlight and virtually absent from ordinary indoor lighting. Decorating the TiO2 surface with silver nanoparticles changes that equation dramatically.

Silver nanoparticles act as plasmonic antennas. Their conduction electrons oscillate collectively in response to visible light—a phenomenon known as surface plasmon resonance—which enhances light absorption and promotes the transfer of energetic electrons into the TiO2 conduction band. The silver also serves as an electron sink, trapping photoexcited electrons and thereby suppressing the recombination of electron–hole pairs, the wasteful process that otherwise limits photocatalytic efficiency. By extending the catalyst’s absorption into the visible spectrum and improving charge separation, the silver decoration makes the material responsive to the cool white light of household LEDs—an illumination source that is inexpensive, low-temperature, energy-efficient, and far more practical for real-world water treatment facilities than UV lamps.

What distinguishes this work is how the silver nanoparticles were made. Rather than employing chemical reducing agents such as sodium borohydride or citrate—reagents that are themselves hazardous and generate toxic byproducts—the researchers exploited the phytochemicals naturally present in rosemary extract. Plant-derived polyphenols, flavonoids, and terpenoids act as both reducing agents, converting silver ions into metallic silver nanoparticles, and capping agents, stabilizing the nascent nanoparticles and controlling their growth. The result is a one-pot, eco-friendly synthesis in which the plant chemistry performs the work of industrial reagents. The team confirmed the successful formation of the nanocomposite through a battery of characterization techniques: UV–Vis spectroscopy, which reveals the characteristic plasmon absorption of silver; X-ray diffraction (XRD), which identifies the crystalline phases; scanning electron microscopy (SEM), which images the morphology; and Fourier-transform infrared spectroscopy (FTIR), which detects the organic functional groups from the plant extract bound to the nanoparticle surfaces.

With the catalyst in hand, the researchers systematically varied the operating conditions of the photocatalytic degradation of AO10 under white LED irradiation. Four parameters emerged as critical: the pH of the solution, the catalyst loading, the concentration of hydrogen peroxide added as an oxidizing co-reagent, and the initial concentration of the dye itself. pH is particularly influential in TiO2 photocatalysis because it governs the surface charge of the catalyst particles and, consequently, the electrostatic attraction or repulsion between the catalyst and the dye molecules. At acidic pH values, the TiO2 surface is positively charged and attracts the anionic AO10 molecules; at high pH, the surface becomes negatively charged and repels the dye, while hydroxyl radicals are also scavenged more readily. Hydrogen peroxide, meanwhile, acts as a sacrificial electron acceptor that captures conduction-band electrons and generates additional hydroxyl radicals, accelerating the oxidative degradation—provided its concentration is optimized, since excess peroxide can quench radicals and even poison the catalyst surface. Catalyst loading presents a similar trade-off: more catalyst means more active sites, but excessive loading increases turbidity and scatters light, reducing the photon flux reaching each particle.

To navigate this multidimensional parameter space efficiently, the researchers turned to machine learning. Rather than relying on laborious one-factor-at-a-time experiments, they trained a polynomial regression model to capture the relationships among pH, catalyst loading, peroxide concentration, initial dye concentration, reaction time, and the resulting concentration ratio C/C0—the fraction of dye remaining relative to its starting value. Polynomial regression, a workhorse of statistical learning, fits a mathematical surface through the experimental data, allowing the model to interpolate and predict outcomes for conditions never tested in the laboratory. The team first screened several regression algorithms from the scikit-learn library before settling on the polynomial approach, and then coupled the trained model with Pareto optimization, a multi-objective technique that identifies conditions balancing competing goals—in this case, minimizing the residual dye concentration while minimizing reaction time. With the weighting parameter λ set to 1, the optimization predicted an ideal recipe: pH 1.50, a hydrogen peroxide concentration of 3.35 × 10⁻³ M, a catalyst loading of 80.24 mg L⁻¹, and an initial AO10 concentration of 26.40 mg L⁻¹. Under these conditions, the model forecast complete degradation of the dye in just 15.81 minutes—a remarkably short time for a process powered by an ordinary LED.

The machine learning results were complemented by quantum chemical calculations at the DFT level, which probed how the AO10 molecule actually attaches to the AgNPs/TiO2 surface. The team computed relative adsorption energies (Eads) for three possible binding orientations, in which the dye anchors to a silver site through its sulfonate group (SO3–Ag), through a nitrogen atom of the azo linkage (N–Ag), or through a hydroxyl group (OH–Ag). The calculations revealed that the SO3–Ag orientation is energetically the most favorable, meaning the dye preferentially binds through its sulfonate moiety—the acidic functional group characteristic of anionic dyes. This orientation positions the dye in a geometry that presumably facilitates electron transfer during photocatalysis. Perhaps more intriguingly, the DFT analysis showed that adsorption is strongly pH-dependent: at very high pH values, the adsorption process becomes endergonic—that is, thermodynamically unfavorable. This provides a molecular-level explanation for the experimentally observed collapse of photocatalytic activity under strongly alkaline conditions. The dye simply refuses to stick to the catalyst, and a molecule that never adsorbs can never be degraded. It is a satisfying example of theory and experiment converging on the same physical picture.

Kinetic analyses of the degradation experiments added further mechanistic depth. Photocatalytic degradation of dyes on TiO2 typically follows pseudo-first-order kinetics at low dye concentrations, where the rate is proportional to the dye concentration because active sites are not saturated. The kinetic data from this study helped the researchers characterize how the rate responds to each of the parameters they varied, feeding into the machine learning framework and reinforcing the understanding that adsorption of the pollutant onto the catalyst surface is the rate-determining first step in the overall process.

The significance of this work extends beyond a single dye. Acid Orange 10 serves as a model compound for an entire family of azo dyes, and the general strategy—an easily synthesized, plant-derived, silver-plasmonic TiO2 photocatalyst activated by low-energy LED light—could be adapted to other organic micropollutants, from pharmaceuticals to pesticides, that increasingly contaminate water supplies worldwide. The integration of machine learning for process optimization also points toward a broader trend in environmental catalysis: instead of exhaustive empirical screening, researchers can now train predictive models on relatively modest experimental datasets and use them to pinpoint optimal operating windows with minimal laboratory effort. And because the synthesis avoids toxic reagents and the illumination avoids energy-hungry UV lamps, the entire treatment chain aligns with the principles of green chemistry.

There remain, of course, the usual challenges between laboratory success and industrial deployment: scaling up nanomaterial synthesis, recovering and reusing catalyst particles from treated water, and validating performance in real, complex wastewater matrices rather than synthetic dye solutions. But the study offers a coherent proof of concept that a catalyst born from rosemary, powered by a reading lamp, and guided by an algorithm can dismantle one of the textile industry’s most stubborn pollutants in under sixteen minutes. For a field searching for sustainable answers to a global water contamination crisis, that is a compelling demonstration of what happens when green chemistry, surface science, quantum mechanics, and artificial intelligence are made to work together under the same light.

MXenes are two-dimensional materials made by selectively stripping elements from a class of layered ceramics known as MAX phases. Their unusual combination of metallic conductivity, mechanical strength, large surface area and chemically tunable surfaces has made them attractive for next-generation electronics, energy storage, catalysis, sensors and protective coatings. Unlike many other two-dimensional materials, MXenes can also be dispersed in liquids and processed into films, inks and composites. Those advantages have fueled intense interest, but manufacturing has remained a major obstacle because conventional synthesis relies on corrosive acids, long reaction times and difficult waste-handling procedures.

A typical MAX phase contains alternating layers of a transition metal and carbon or nitrogen, separated by a third element, commonly aluminum. In a titanium aluminum carbide precursor, for example, titanium and carbon form the structural layers while aluminum occupies the intermediate position. Producing a titanium carbide MXene requires removing the aluminum without seriously damaging the titanium-carbon framework. The resulting sheets are only a few atoms thick, and their exposed surfaces acquire chemical groups that influence how they interact with ions, molecules and other materials. Achieving that transformation requires atomic-level selectivity: an etchant must attack the targeted layer while preserving the conductive backbone.

The established approach, known as liquid-phase chemical etching, generally uses hydrofluoric acid or fluoride-containing reagents. The acid penetrates the MAX material and dissolves the aluminum layer through a sequence of reactions that can take 12 to 24 hours or longer. Additional washing and treatment steps are then needed to remove residual chemicals and adjust the surface chemistry. Although this route has enabled much of the field’s progress, it produces hazardous waste and can expose researchers and manufacturers to highly toxic substances, including hydrogen fluoride. The lengthy process also increases energy, labor and equipment costs, limiting the ability to produce MXenes at industrial scale.

The Rice team sought a faster alternative by adapting flash Joule heating, a technique developed in James Tour’s laboratory. In flash Joule heating, an electrical pulse passes through a material, rapidly raising its temperature to extreme levels in a fraction of a second. The method has previously been used to transform carbon-rich feedstocks and process other difficult materials. In the new work, Shichen Xu, a postdoctoral researcher in the Tour lab, investigated whether the same ultrafast thermal approach could accelerate MXene synthesis. Rather than immersing the MAX phase in an acid solution, the researchers heated it rapidly and introduced reactive gases into the system.

The gas mixture contained chlorine and tetrafluoromethane, a fluorine-containing compound. Under the intense thermal conditions, these gases reacted with the MAX phase and selectively removed the aluminum layers. The chemistry effectively transferred the etching environment from a liquid solution to a controlled gas phase, while the rapid heating shortened the reaction to less than half a minute. By adjusting the composition of the gas mixture and the time for which the material was exposed, the researchers could alter the extent of etching. This control is particularly important because excessive reaction can attack the transition-metal layers or degrade the fragile two-dimensional structure.

The gas-phase process may offer more than a simple increase in speed. In a liquid system, the etchant must diffuse through a solvent and into the layered solid, while the reaction products and residual chemicals must later be separated through washing. Gas-phase chemistry can provide a more direct route to the exposed surfaces of the precursor, and the absence of a large liquid bath could simplify some aspects of processing and waste management. The researchers say that a properly engineered system could reduce the hazards associated with large volumes of hydrofluoric acid and toxic byproducts. However, chlorine and fluorinated gases are themselves hazardous, meaning that industrial adoption would require sealed reactors, precise gas monitoring and robust containment.

The study also revealed that the new method can display different efficiencies and selectivities depending on the material and etching conditions. That flexibility could allow researchers to tune the composition and surface terminations of MXenes rather than treating etching as a single, fixed operation. Surface terminations, such as fluorine-, oxygen- or hydroxyl-containing groups, influence electrical behavior, wettability, catalytic activity and interactions with ions. Controlling them is essential for designing MXenes for particular applications. A material optimized for electromagnetic shielding, for example, may require different surface chemistry from one intended for catalysis or electrochemical energy storage.

The implications reach beyond a faster laboratory protocol. MXenes are being explored for conductive coatings, flexible and miniature electronics, chemical sensors, batteries, supercapacitors, catalysts and aerospace materials that must withstand demanding environments. Their conductivity can support rapid charge transport, while their high surface area provides abundant sites for chemical reactions or interactions with surrounding molecules. If the Rice process can be scaled while maintaining uniformity and safety, it could lower production barriers for these technologies and make systematic experimentation with different MXene compositions more practical. The work also demonstrates how flash Joule heating can be paired with gas-phase reactions to manufacture advanced materials at speeds that conventional wet chemistry cannot easily match.

The researchers emphasize that the method is not simply a faster version of acid etching but a different manufacturing strategy based on rapid thermal activation and controllable gaseous chemistry. Further development will be needed to determine how broadly it applies across the large family of MAX phases, how consistently it produces defect-free sheets, and how its energy use compares with established methods at industrial scale. The team’s results nevertheless point toward a more rapid, tunable and potentially scalable route to MXenes. By replacing prolonged acid treatment with a reaction completed in seconds, the process could help move these atomically thin materials from specialized laboratories toward practical electronic, catalytic and aerospace applications.

Subject of Research: Rapid gas-phase synthesis of MXenes using flash Joule heating

Article Title: Flash Joule heating for rapid MXenes synthesis

News Publication Date: 10-Aug-2026

Web References: https://profiles.rice.edu/faculty/james-tour ; https://www.nature.com/articles/s44160-026-01132-2

References: Nature Synthesis, DOI: 10.1038/s44160-026-01132-2

Keywords

MXenes, flash Joule heating, gas-phase etching, MAX phases, two-dimensional materials, materials engineering, nanotechnology, electronic materials, aerospace coatings, chemical synthesis