The influence of surface phenomena on the various physical properties of substances is most pronounced when their bodies are atomically thin. Consequently, one-dimensional (1D) materials such as carbon nanotubes and two-dimensional (2D) materials derived from the exfoliation of layered compounds such as graphite and transition metal dichalcogenides are regarded as promising platforms for detecting the adsorption of foreign molecules. This special issue highlights the current status and challenges of molecular sensors based on 1D nanotubes and 2D materials.

Chemical sensors, such as biosensors, gas sensors, and odor sensors, respond specifically or selectively to chemical substances, outputting signals according to their amounts, and possessing diverse characteristics. For this reason, there is a strong demand for their use in various fields, including the environment, medicine, and food. Here, we first describe the basic structure of a typical chemical sensor, and then provide an overview of its response characteristics using Langmuir adsorption isotherms. Next, we present molecular sensing using transistors that utilize recognition reactions on functionalized graphene surfaces, developed by our research group.

Self-assembled peptides provide a versatile strategy for functionalizing two-dimensional (2D) materials such as graphene and MoS₂. By rational sequence design, peptides spontaneously form ordered monolayers that enable the controlled immobilization of molecular receptors for biosensing. These receptors are positioned within nanometer-scale distances of conductive surfaces, thereby achieving high sensitivity and selectivity. Recent studies demonstrate odor detection with enantioselective recognition of limonene and protein sensing at femtomolar levels. Beyond sensing, hierarchical assemblies of peptides with cofactors such as hemin yield artificial enzyme interfaces capable of catalytic performance comparable to natural enzymes. This programmable approach integrates molecular recognition with electronic readout, providing a robust and scalable platform for next-generation biosensors, chemical sensors, and bioelectronic devices. Overall, peptide self-assembly offers a universal interfacial design principle bridging biomolecules and 2D nanomaterials, opening new opportunities for advanced sensing and catalytic applications.

The nitrogen dioxide (NO₂) gas response and recovery properties of ambipolar carbon nanotube (CNT) field-effect transistors were measured to investigate the effect of CNT amount on gas response. For the device with a small amount of CNTs, responses from the CNT bulk and CNT/electrode contacts were observed. For devices with a large amount of CNTs, in which a network-like structure of CNTs was observed near the electrodes, increased current in both electron and hole conduction regions was observed compared with that for the device with a small amount of CNTs. The increased current in the electron conduction region rapidly decreased during recovery. This response is consistent with that of CNT/CNT X-type contacts, which have high resistance before NO₂ adsorption. Equivalent circuits of CNT channels with CNT/CNT contacts were developed. Evaluation of time constants revealed that CNT/electrode contacts and CNT/CNT X-type contacts exhibited high NO₂ adsorption and desorption rates, respectively.

In order to establish a safer and secure society utilizing IoT devices, gas sensors have been increasingly demanded for healthcare, safety, and environmental monitoring applications. Conventional chemiresistive sensors, however, suffer from high energy consumption due to external heaters, low selectivity, and low stability. Here, we review a series of noble metal nanosheet sensors that satisfy the above requirements by utilizing the metal itself as both receptor and transducer. Platinum (Pt) nanosheets demonstrated reliable hydrogen (H₂) detection under high humidity, with sub-ppm sensitivity and immunity to water interference, unlike conventional. Density of states investigated by the density functional theory revealed that the resistance changes are due to the difference between oxygen- and hydrogen-induced electron scattering. Integrated Pt and PtRh nanosheets enabled simultaneous detection of hydrogen and ammonia by exploiting self-heating effects to achieve optimal operating temperatures. Although self-heating effects contribute to reduce the power consumption necessary to achieve temperature necessary to stimulate chemical reactions, power consumption was further reduced by scaling the device dimensions and pulsed operation of self heating without significant signal loss of sensor responses. Palladium-nanodot-functionalized suspended graphene nanosheets exhibited multifunctionality, switching their functionality between hydrogen and humidity sensing depending on applied bias. Gold (Au) nanosheets with TiN adhesion layers provided ppb-level detection of hydrogen sulfide (H₂S), with excellent selectivity, reproducibility, and long-term stability, achieving a limit of detection of 0.5 ppb. These results establish metal nanosheet sensors as a promising platform for breath analysis and IoT-based molecular sensing.

Measurement technologies that enable simple and rapid diagnosis have become increasingly important due to the experience of COVID-19 pandemic. The use of graphene in ion sensitive field effect transistor (ISFET)-based biosensors is expected to result in highly sensitive biosensing. However, the problem of Debye shielding limits the measurement range of FET sensors, making it difficult to detect biopolymers in solutions with physiological salt concentrations. In this paper, we introduce a graphene-based biosensor with molecular recognition capabilities by functionalizing the surface of suspended graphene. The graphene resonant mass sensor quantifies molecules by measuring the change in their resonant frequency associated with molecular adsorption. High sensitivity can be achieved in such mass sensors by thinning the vibrating membrane. The proposed technology is expected to be applied to measurement systems that can detect chemical substances and viruses in the environment, as well as disease-causing gas components in exhaled gases with ultra-high sensitivity.