Sulfur-Bearing Calixarene Films Sniff Out Chloroform in Seconds

Chloroform is one of those industrial chemicals that most people never think about, yet it quietly threads its way through drinking water, air, and workplace environments around the world. A byproduct of water chlorination and a common solvent in laboratories and manufacturing plants, this volatile organic compound is classified as a probable human carcinogen, which makes reliable, rapid detection a genuine public health priority. Now, a team of researchers from the University of Jeddah in Saudi Arabia and Sohag University in Egypt has reported a strikingly simple answer to this challenge: ultrathin films made from sulfur-containing calixarene molecules that can selectively sniff out chloroform vapor at room temperature, responding within seconds and recovering just as quickly.

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