Carbon Dots That Dim on Cue Turn Vitamin B12 Testing Into Paper and Swab Sensors

Vitamin B12 sits at the center of one of the most quietly important measurement problems in food science. The vitamin, essential for nerve function and red blood cell formation, is found mainly in animal-derived products, and deficiency can lead to serious neurological and hematological consequences. Yet confirming how much of it is actually present in a given food has long demanded exactly the kind of infrastructure that many producers, inspectors, and consumers do not have: chromatographic instruments, trained personnel, and a laboratory bench. A new study from Liaocheng University, published in Biomedical Analysis, describes a fluorescent sensing strategy that could shift part of that workflow out of the lab and onto something as simple as a strip of filter paper or a cotton swab.

The research centers on a class of nanomaterials known as carbon dots, nanoscale carbon-based particles that have attracted widespread attention for their strong fluorescence, low toxicity, ease of synthesis, and chemical tunability. In this work, the team prepared boron- and nitrogen-co-doped carbon dots, abbreviated B,N-CDs, using a one-step hydrothermal process starting from 3-aminophenylboronic acid and p-hydroxybenzaldehyde. The co-doping with boron and nitrogen is not decorative; heteroatom doping modifies the electronic structure and surface chemistry of carbon dots, and in this case it produced water-dispersible nanoparticles averaging about one nanometer in diameter that emit a bright blue fluorescence when excited by ultraviolet light.

The optical performance of the probe is central to its usefulness. Under 390 nm excitation, the B,N-CDs showed their strongest fluorescence emission at 475 nm, with a quantum yield of 18.15 percent, a figure that indicates a substantial fraction of absorbed photons are converted into emitted light. When vitamin B12 was introduced, the blue glow dimmed markedly. The probe displayed a linear fluorescence response across vitamin B12 concentrations from 0 to 100 micromolar, with a detection limit of 0.02 micromolar, a sensitivity level that places the method comfortably within the range needed for food analysis applications.

Selectivity is where many fluorescent probes falter, since real food samples contain a crowded mixture of molecules that can interfere with a signal. The Liaocheng team tested the probe against other B vitamins, amino acids, glucose, and a panel of common small molecules. Vitamin B12 produced pronounced quenching of the fluorescence, while the other tested substances caused little comparable response. That discrimination matters because a sensor that dims in response to anything would be useless in a food matrix, whereas one that responds specifically to the target analyte can deliver a meaningful reading even in chemically complex surroundings.

Understanding why the fluorescence fades was a substantial part of the study. The researchers assembled evidence from fluorescence-lifetime measurements, UV-visible absorption spectroscopy, Raman spectroscopy, FT-IR, X-ray photoelectron spectroscopy, particle-size analysis, and surface-charge measurements. The fluorescence lifetime of the carbon dots remained unchanged in the presence of vitamin B12, while Stern-Volmer plots recorded at different temperatures showed temperature-dependent behavior. Together, these observations support a static quenching mechanism: vitamin B12 associates with the carbon dots to form a non-luminescent complex in the ground state, effectively switching off the emission of the dots it binds. Zeta potential and dynamic light scattering measurements of the dots, the vitamin, and their combined system further corroborated this interaction, explaining why the blue fluorescence decreases predictably as vitamin B12 concentration rises.

What distinguishes this work from many fluorescence-probe studies is the deliberate translation of the liquid-phase assay into physical formats that a non-specialist could conceivably use. The researchers impregnated filter paper with the B,N-CDs to create test strips, and they designed a cotton-swab device that carries the fluorescent probe to a sensing region during use. Under 365 nm ultraviolet light, both formats became progressively darker as vitamin B12 concentration increased, turning the analytical signal into something visible to the eye rather than something requiring a spectrometer.

The paper strip format was quantified through image-based analysis of the green-to-blue intensity ratio extracted from photographs of the strips. This ratio showed a strong linear relationship with vitamin B12 concentration from 0 to 80 micromolar, with a coefficient of determination of 0.9927. In practical terms, that means a camera image of an illuminated strip can serve as the measurement instrument, an approach that aligns naturally with the ubiquity of smartphone cameras and opens a path toward quantitative readings without dedicated laboratory equipment.

The cotton-swab sensor represents a different kind of design ambition. Rather than merely displaying a signal, the swab was conceived to combine sample collection, transfer, mixing, and signal readout within a single disposable device. Once a suitable sample is available, the sensing step can be completed rapidly, collapsing several laboratory operations into one handheld object. Both the strips and the swabs retained useful fluorescence and vitamin B12 sensing performance for at least 20 days under the storage conditions evaluated, an important consideration for any sensor intended to be distributed, stored, and used outside a controlled laboratory environment.

To check whether the method could survive contact with real food, the researchers applied it to egg-yolk and goat-liver-powder samples, both genuine dietary sources of vitamin B12. They compared their quantitative measurements against high-performance liquid chromatography, the established reference technique. In spiked recovery experiments, recoveries ranged from 97.09 percent to 101.22 percent, and the measured vitamin B12 concentrations closely matched the HPLC results. For an alternative analytical method, agreement with a gold-standard technique at that level of accuracy is the key benchmark, and it supports the credibility of the carbon-dot approach in the food matrices studied.

The authors are candid about what remains before the technology can be considered field-ready. The current real-food workflow still involves laboratory sample pretreatment before measurement, so simplifying that step will be essential for truly portable testing. They also outline practical next steps, including replacing the laboratory ultraviolet lamp with a compact battery-powered UV-LED and adding smartphone-based image analysis for automated quantification. Even with those caveats, the study demonstrates that carbon-dot fluorescence can be adapted into visually readable portable sensors without sacrificing the sensitivity or selectivity that make the underlying chemistry useful, and it offers a template for how other analytes might be moved onto paper strips and swabs in the future.

Subject of Research: Fluorescence-based portable detection of vitamin B12 using boron- and nitrogen-co-doped carbon dots in food samples

Article Title: A blue glow for vitamin B12: carbon dots bring testing to paper strips and swabs

Article References: A blue glow for vitamin B12: carbon dots bring testing to paper strips and swabs. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: vitamin B12, carbon dots, fluorescence quenching, biosensors, food analysis, paper strips, cotton swab sensor, point-of-care testing, nanomaterials, static quenching, HPLC validation, Liaocheng University