Challenges in Nano and Micro Scale Science and Technology
Volume:12 Issue: 2, Summer-Autumn 2024

With rapid advancements across industry, technology, and medicine, the demand for efficient gas and vapor detection has intensified. Among various sensing technologies, chemiresistor gas sensors have emerged as a leading choice due to their affordability, compactness, broad sensitivity range, and user-friendly operation. These sensors play a crucial role in applications spanning environmental monitoring, industrial safety, and healthcare. This review provides a comprehensive overview of chemiresistor gas sensors, beginning with their fundamental principles and operational mechanisms. Key performance factors, such as sensitivity, response time, selectivity, and stability, are examined, while the material innovations driving their progress are also considered. The evolution of these sensors is explored through recent research studies, shedding light on both breakthroughs and persistent challenges, including issues of selectivity and environmental interference. Finally, future perspectives are discussed, highlighting emerging technologies such as AI-assisted sensing, flexible electronics, and sustainable sensor materials that could shape the next generation of chemiresistor-based detection.

Magnetic nanoparticle hyperthermia is a preclinical treatment for bladder cancer that employs ferromagnetic nanoparticles exposed to an external alternating magnetic field to elevate the temperature within the tumor. In this study, magnetic hyperthermia using Fe3O4 nanoparticles was simulated for T1 non-muscle-invasive bladder cancer and T2 muscle-invasive bladder cancer, investigating the effect of presence of urine and its convection within the bladder in this treatment method. For this purpose, simulations for both stages of bladder cancer were conducted for three cases. First, the bladder without urine was modeled as a homogeneous solid muscle tissue. Next, to investigate the effect of the absence of perfusion, the bladder containing urine was modeled, but the effect of convection was neglected. Finally, the bladder containing urine was modeled, incorporating the effect of convection. The distribution of the volumetric heat generation rate due to induction heating, the volumetric power dissipation of the magnetic nanoparticles, and the temperature are presented for three simulation cases and two stages of bladder cancer. Pennes and Navier-Stokes equations were used to obtain the temperature distribution in solid tissues and fluid, respectively. The equations were solved using the finite element method in two dimensions. The results demonstrate that in magnetic nanoparticle hyperthermia, convection within urine increases heat generation, reduces temperature, and creates a non-uniform temperature distribution in the tumor for both stages of bladder cancer. Additionally, the more uniform temperature distribution in T1 compared to T2 suggests that this treatment is more effective for non-muscle-invasive bladder cancer than for muscle-invasive bladder cancer.

Piezoresistance, which is the change in resistance due to the applied stress, is a phenomenon that has been recognized in silicon. This study analyzes a micro-electro-mechanical-system (MEMS)-based force sensor that is both flexible and highly sensitive, utilizing a piezoresistive sensing mechanism. The design analysis focuses on enhancing the sensitivity of the microcantilever or beams by integrating various combinations of the stress concentration regions (SCRs). For simulation, four-point bending setup is used specifically for analyzing the piezoresistance effect in p-type silicon. The stress distribution in this setup is niform and aligned with the <110> crystal axis. The primary objective of this study is to investigate the impact of different shapes, distances, rotations, and the number of SCRs on the performance of piezoresistive beam. A finite element approach is employed to analyze different designs for obtaining relative resistance changes. The simulation results are compared with experimental data, demonstrating a good accuracy and it is also identified the appropriate element size for converging answers. As a result, a force sensor has been designed with high sensitivity and flexibility.

Magnetic hyperthermia is an emerging, minimally invasive modality for targeted cancer therapy, utilizing magnetic nanoparticles (MNPs) to induce localized heating within tumors. This study presents a comprehensive simulation-based analysis of magnetic hyperthermia applied to lung cancer. A two-dimensional axisymmetric finite element model was developed, incorporating key anatomical components tumor, air cavity, muscle, bone, skin to evaluate spatial thermal profiles under physiological respiratory dynamics. Magnetite (Fe₃O₄) nanoparticles (19 nm) were subjected to an alternating magnetic field (AMF) at 300 kHz and current intensity 300 A to induce localized heating through Néel and Brownian relaxation mechanisms. The model assessed thermal propagation during inhalation and exhalation, targeting tumor ablation temperatures (42–46 °C). Simulation results confirm the feasibility and thermal safety of MNP-assisted magnetic hyperthermia for lung cancer. These findings offer a clinically relevant framework for optimizing treatment planning and nanoparticle design. The novelty of this study lies in the development of a physiologically accurate finite element simulation that incorporates respiratory dynamics. This integrated approach, combining porous media modeling with thermal distribution analysis under dynamic breathing phases, offers new insights into optimizing safe and effective hyperthermia treatments for thoracic malignancies.

Thermoelectric (TE) devices offer a promising approach to harvesting wasted thermal energy from fluid flow without mechanical components. However, their performance is highly dependent on the of heat transfer between the TE legs and heat sinks, especially under varying fluid temperatures. This study investigates the enhancement of thermal performance at the leg/heat sink interface by integrating carbon nanotube (CNT) arrays as microfins on polymeric TE legs. A validated finite element model is used to simulate device behaviour and assess the impact of CNT geometry and fluid flow conditions. Simulation results reveal that CNT arrays significantly increase the temperature gradient across the TE legs, leading to improved energy conversion rates. Compared to devices without CNTs, the CNT-based thermopile exhibits 1.4 to 2 times higher open-circuit voltage and output power efficiency. When five TE units are connected to form a thermopile, the output reaches 7 mV and 0.3 μW approximately tenfold the performance of a single device. The amount of recovered waste heat from the hot flow source is about 0.0117 W. Optimal power output is achieved through impedance matching, which can be tuned by configuring the number of TE units in series or parallel. These findings highlight the potential of CNT-enhanced TE devices for efficient thermal energy harvesting in fluidic systems

Surfactants such as sodium dodecylbenzene sulfonate (SDBS) are extensively used in industrial and household applications, causing serious environmental concerns due to their persistence and toxicity in aquatic systems. Conventional treatment methods are often ineffective for complete SDBS removal. To address this issue, this study introduces an eco-friendly magnetic nitrogen-doped graphene (MNG) synthesized from orange peel as an efficient and sustainable adsorbent for SDBS elimination. The aim of this research is to investigate the adsorption behavior of MNG toward SDBS in batch experiments. Various characterization techniques, including vibrating sample magnetometry (VSM), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, elemental analysis, and Brunauer–Emmett–Teller (BET) surface area analysis, were used to confirm the successful synthesis of the nano adsorbent. The FTIR and Raman spectra verified the presence of nitrogen- and oxygen-containing functional groups and the graphitic structure of the material. SEM and AFM images revealed a wrinkled, layered morphology with a large surface area, while VSM results demonstrated strong magnetic properties enabling easy separation. The BET analysis indicated a high specific surface area suitable for efficient adsorption. Several parameters influencing adsorption performance, including adsorbent dosage, temperature, contact time, pH, and initial SDBS concentration, were systematically evaluated. The maximum adsorption capacity reached 556 mg/g at 45 °C and pH 3. The adsorption data fitted well with the Langmuir isotherm and pseudo-second-order kinetic models. Thermodynamic analysis confirmed a spontaneous and endothermic adsorption process.