The work, published in the Journal of Materials Science, sits at the intersection of supramolecular chemistry and solid-state sensing. Calixarenes are bowl-shaped macrocyclic molecules whose cavities can be chemically tuned to capture specific guest molecules, much like a molecular lock accepting only certain keys. By incorporating sulfur atoms into the calixarene framework, the researchers created a family of thiacalixarene analogs whose electronic and host-guest properties differ meaningfully from their oxygen-rich cousins. When deposited as thin films, these compounds behave as organic semiconductors, and their electrical response changes measurably when chloroform molecules lodge themselves within the molecular cavities and between the polymer-like chains of the film.

The optical characterization of the films revealed a combination of properties that is highly desirable for sensing platforms. Using UV-Vis-NIR spectroscopy across the 300 to 2500 nanometer range, the team found that the films absorb strongly in the ultraviolet region while transmitting more than 90 percent of visible light. This transparency matters: it means the films could be integrated into optoelectronic devices or optical sensor architectures without interfering with light passing through them. The researchers then applied Tauc plot analysis, a standard technique for extracting optical band gaps from absorption spectra of amorphous semiconductors, and found band gap values ranging from 3.29 to 3.53 electronvolts across the compound series.

Those band gap numbers place the materials firmly in the wide-band-gap organic semiconductor regime, but the variation within the series tells a more interesting story. The lowest value, 3.29 electronvolts, belonged to the compound labeled 3b, which the authors attribute to improved conjugation within its molecular structure. Greater conjugation means the pi-electron systems of the molecule are more delocalized, which lowers the energy required to promote an electron across the gap. In practical sensing terms, a smaller band gap generally makes the film’s conductivity more sensitive to perturbations such as the adsorption of polarizable vapor molecules, and this expectation played out in the subsequent gas sensing experiments.

When the films were exposed to chloroform vapor, all of the tested compounds produced measurable responses, but compound 3b stood out decisively. It achieved a maximum response value of approximately 0.83, with a response time of just 3 seconds and a recovery time of 6 seconds. For context, many commercial gas sensors based on heated metal oxides require operating temperatures of several hundred degrees Celsius and exhibit response and recovery times measured in tens of seconds or minutes. A room-temperature organic sensor that completes a full detection cycle in under 10 seconds represents a meaningful advance in speed and energy efficiency, and it hints at applications in portable, battery-powered monitoring devices.

Selectivity is where many promising sensor materials stumble. A sensor that reacts equally to acetone, ethanol, benzene, and chloroform is of limited use in real environments, where gas mixtures are the norm rather than the exception. The sulfur-containing calixarene films, however, showed a strong preference for chloroform over other tested gases, with a selectivity value of roughly 83 toward chloroform. The researchers connect this selectivity to the host-guest chemistry inherent to the calixarene scaffold: the size, polarity, and polarizability of the chloroform molecule appear to match the cavity environment created by the sulfur-containing macrocycle particularly well, producing a stronger perturbation of the film’s electronic properties than other vapors can achieve.

Equally important for any practical sensor is a predictable, quantitative response to concentration. A detector that merely signals the presence of a gas without indicating how much of it is present offers only half the picture. The calixarene films delivered a linear response across chloroform concentrations from 50 to 200 parts per million, with a coefficient of determination, or R-squared value, of 0.998. That near-perfect linearity means the sensor output can be converted directly into a concentration reading with simple calibration, satisfying a key requirement for regulatory monitoring applications, where occupational exposure limits and water quality standards are defined in precise numerical terms.

Durability is the third pillar of sensor performance, and here too the results were encouraging. After 30 days of storage, the compound 3b sensor retained a response of 0.836, essentially indistinguishable from its initial performance. Organic semiconductor films are often vulnerable to oxidation, humidity, and structural degradation over time, so demonstrating month-scale stability suggests that the sulfur-containing calixarene chemistry is robust enough for real-world deployment rather than being a laboratory curiosity. The combination of speed, selectivity, linearity, and stability in a single material is rare, and it is what distinguishes this work from many earlier demonstrations of organic vapor sensors.

The broader context makes the achievement more compelling. Chloroform detection has previously been pursued with metal oxide composites such as zinc oxide and copper oxide structures, conductive polymer fibers, plasmonic nanostructures, graphene field-effect transistors, and even carbon dot-modified photonic crystals. Calixarenes themselves have a long history in sensing, having been incorporated into quartz crystal microbalances, surface acoustic wave devices, and Langmuir-Blodgett films for detecting a range of volatile organic compounds. What the new study adds is the demonstration that sulfur-containing calixarene analogs, used directly as semiconducting thin films, can deliver chloroform selectivity with exceptional kinetics, potentially simplifying device architectures by removing the need for separate transduction layers.

Looking ahead, the research opens several avenues. The tunability of the calixarene platform means that the cavity chemistry could, in principle, be re-engineered to target other hazardous VOCs, from industrial solvents to clinical breath biomarkers, an area of growing interest for noninvasive disease diagnostics. The high visible transparency and semiconducting behavior of the films also suggest dual use in optoelectronic circuits where sensing and signal processing could be integrated on the same substrate. For now, the immediate significance is clear: a molecularly engineered, room-temperature film that catches one of the world’s most common chlorinated pollutants in 3 seconds flat, holds its performance for a month, and tells you exactly how much of it is in the air. In the quiet race to make chemical sensing faster, cheaper, and more selective, a humble bowl-shaped molecule with a few sulfur atoms may have just taken a significant lead.

Subject of Research: Sulfur-containing calixarene thin films for selective chloroform vapor sensing

Article Title: Optoelectronic and gas sensing properties of sulfur-containing calixarene thin films: selective detection of chloroform vapor

Article References: Hajjar, D., Makki, A. A., Omran, O. A., Kamel, M. S., & Mohery, M. (2026). Optoelectronic and gas sensing properties of sulfur-containing calixarene thin films: selective detection of chloroform vapor. Journal of Materials Science, 61(43), 33626-33649. https://doi.org/10.1007/s10853-026-13797-3

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13797-3

Keywords: calixarene, thiacalixarene, chloroform, gas sensor, volatile organic compounds, organic semiconductor, thin films, optical band gap, Tauc plot, VOC detection, room-temperature sensing, selectivity

The significance of the work lies in how the memory is engineered. In conventional non-volatile memory, information persists even when power is removed because electrons are parked on a floating gate or inside a charge-trapping layer, typically a film of silicon nitride. The Korean team replaced that inorganic insulator with a self-assembled monolayer of poly-L-lysine nanoparticles, each acting as a discrete charging node. Because the nanoparticles sit at well-defined, separate locations on the dielectric surface, the density and distribution of charge traps can be controlled far more precisely than in continuous nitride films, where trap positions are essentially random. That control matters enormously for memory performance, since it governs how uniformly bits program and erase and how long data can be retained.

Synthesizing the nanoparticles was itself a feat of green chemistry. The researchers used a simplified thermal polymerization of lysine powder, stirred in a quartz reactor at 65 degrees Celsius, with potassium hydroxide as a catalyst to favor the alpha branch of the polymer. Dynamic light scattering and field-emission scanning electron microscopy showed that most of the resulting particles were around 10 nanometers in diameter, with a minority of larger aggregates reaching up to 200 nanometers. Crucially, because each nanoparticle functions independently as a charge-trapping center, the presence of a few oversized particles did not degrade device-to-device consistency—a practical advantage over many nanoparticle memory schemes that demand monodisperse particles.

Anchoring the particles to the dielectric required a clever piece of surface chemistry. The team first functionalized the insulating surface—either thermally grown silicon dioxide or a crosslinked polyvinyl alcohol film—with 3-glycidyloxypropyl trimethoxysilane, or GPTMS, a silane coupling agent that presents reactive epoxy groups. Lysine carries two amine groups, and the abundant amines on the surface of each poly-L-lysine nanoparticle react covalently with those epoxies, forming stable epoxy-amine bonds. A simple dip-coating process, followed by annealing at 80 degrees Celsius and a water sonication step to strip away any particles that had only physically adsorbed, left behind a robust monolayer of covalently tethered nanoparticles. Infrared spectroscopy confirmed the chemistry: a broadened hydroxyl stretching band between 3200 and 3600 reciprocal centimeters signaled epoxy ring opening, evidence that the conjugation had proceeded as designed.

To prove that the biological layer could genuinely store charge, the researchers built metal-insulator-indigo-silicon capacitors with and without the nanoparticle layer and measured their capacitance-voltage characteristics at 1 megahertz. The control device, lacking poly-L-lysine, showed a flatband voltage shift of less than 0.5 volts—essentially no memory window. With the nanoparticles in place, the window widened to 3.1 volts under a sweep from minus 5 to plus 5 volts, and grew to 5.8 volts when the sweep was extended to plus and minus 9 volts. The hysteresis loops were clockwise, the classic signature of charge trapping, and the asymmetry of the shifts with bias polarity pointed clearly to the amine-rich nanoparticles, not the dielectric interfaces, as the dominant storage sites.

Data retention proved equally encouraging. After biasing the capacitor’s top electrode at plus 9 volts for 20 seconds, the programmed state remained measurable for 10,000 seconds—nearly three hours—comparable to floating-gate memories built with metallic nanoparticles. That is a remarkable result for a layer made of nothing more exotic than polymerized amino acid, and it suggests that the epoxy-amine tethering holds charges tightly enough to resist spontaneous leakage through the thin 10-nanometer tunneling oxide.

The team then translated the capacitor results into a working transistor memory. Using a heavily doped silicon wafer as a common gate, they deposited indigo by thermal evaporation as the ambipolar semiconductor and patterned gold source and drain electrodes. The resulting transistor showed hole mobility of 1.81 times ten to the minus two square centimeters per volt and an on/off ratio above four orders of magnitude for hole transport. When the gate was swept, the transfer curves exhibited a wide hysteresis of 14.4 volts, and a programming pulse of minus 30 volts shifted the threshold voltage from minus 7.1 to minus 9.6 volts; an erasing pulse of plus 30 volts restored it to its initial value. In other words, the device could be written, read, and erased like a genuine non-volatile memory cell.

Perhaps the most forward-looking demonstration came on plastic. The researchers fabricated the same memory architecture on a transparent, flexible polyethylene terephthalate substrate, using a citric-acid-crosslinked polyvinyl alcohol dielectric spin-coated from water. Infrared analysis confirmed that ester bonds formed between the carboxyl groups of citric acid and the hydroxyl groups of PVA during a one-hour anneal at 110 degrees Celsius, stabilizing the dielectric against the humidity-driven degradation that plagues untreated PVA. The flexible transistor retained ambipolar behavior with a 7.2-volt hysteresis, and program and erase operations at plus and minus 30 volts shifted the threshold voltage as expected. A soft bending test—ten cycles at a 1.5-centimeter radius—produced no noticeable degradation, hinting at real mechanical robustness.

The devices also revealed an intriguing sensitivity that could be either a liability or a feature. Because both indigo and poly-L-lysine carry amine groups that readily protonate, the memory window depended strongly on ambient humidity: measurements taken at 20 to 60 percent relative humidity produced the large hysteresis values, while in very dry conditions below 10 percent the window shrank to under 1 volt. The authors attribute this to hydration and protonation of the epsilon amine groups on the nanoparticles, a mechanism reminiscent of melanin-based electronics, where ionic conduction intertwines with electronic transport. For memory applications, environmental control will be essential; but for biosensing, a device whose electrical response tracks moisture and pH could be exactly what is wanted.

Indigo itself deserves attention as more than a novelty. Unlike pentacene and other synthetic organic semiconductors, indigoids are biocompatible, and their ambipolar transport—handling both holes and electrons—makes them versatile channel materials even though their mobilities remain below one square centimeter per volt. The Myongji team argues that the combination of a biomolecule-derived charge-trapping layer, a natural-dye semiconductor, a water-processed dielectric, and a flexible substrate points toward fully biomaterial-based memory devices. Such devices could serve as the storage and signal-interface elements in bioelectronic systems, including brain-computer interfaces, where silicon’s rigidity and incompatibility with living tissue remain fundamental obstacles. The lysine nanoparticles would not replace flash memory in your laptop anytime soon, but as a demonstration that the essential function of a memory cell—trapping charge, holding it, and releasing it on command—can be delegated to molecules as ordinary as an amino acid, the work opens a genuinely provocative road toward electronics that speak biology’s language.

Subject of Research: Biocompatible organic non-volatile memory devices using poly-L-lysine nanoparticles as a charge trapping layer with an indigo semiconductor

Article Title: Organic memory device with poly-L-lysine nanoparticles embedded as charge trapping layer

Article References: Hur, H., Lee, P., Oh, S., Kim, T., & Lee, H. H. (2026). Organic memory device with poly-L-lysine nanoparticles embedded as charge trapping layer. Advances in Industrial and Engineering Chemistry, 2(1), Article 12. https://doi.org/10.1007/s44405-026-00052-1

Image Credits: AI Generated

DOI: 10.1007/s44405-026-00052-1

Keywords: poly-L-lysine, nanoparticles, charge trapping layer, non-volatile memory, indigo, organic semiconductor, thin-film transistor, floating gate, flexible electronics, bioelectronics, polyvinyl alcohol dielectric, PET substrate