Micro- and nano-sensors lie at the heart of critical innovation in fields ranging from medical to environmental sciences. In recent years, there has been a significant improvement in sensor design along with the advances in micro- and nano-fabrication technology and the use of newly designed materials, leading to the development of high-performance gas sensors. Advanced micro- and nano-fabrication technology enables miniaturization of these sensors into micro-sized gas sensor arrays while maintaining the sensing performance. These capabilities facilitate the development of miniaturized integrated gas sensor arrays that enhance both sensor sensitivity and selectivity towards various analytes. In the past, several micro- and nano-gas sensors have been proposed and investigated where each type of sensor exhibits various advantages and limitations in sensing resolution, operating power, response, and recovery time. This paper presents an overview of the recent progress made in a wide range of gas-sensing technology. The sensing functionalizing materials, the advanced micro-machining fabrication methods, as well as their constraints on the sensor design, are discussed. The sensors’ working mechanisms and their structures and configurations are reviewed. Finally, the future development outlook and the potential applications made feasible by each category of the sensors are discussed.

Advanced Micro- and Nano-Gas Sensor Technology: A Review

High-quality optical fibers can be produced now at a low cost and large quantity, and this has further promoted the development of fiber optic (chemical) sensors. After over 30 years of innovation, fiber optic sensing technology has become mature because of acceptable costs, compact instrumentation, high accuracy and the capability of performing measurements at inaccessible sites, over large distances, in strong (electro)magnetic fields and in harsh environment. The technology is still proceeding quickly in terms of innovation, and respective applications have been found in highly diversified fields. This review covers work published in the time period between October 2015 and October 2019. It is written in continuation of previous reviews.

Fiber-Optic Chemical Sensors and Biosensors (2015-2019).

Zinc ferrite based gas sensors: A review

Semiconductor oxides with hierarchically hollow architecture can provide significant advantages as sensing materials for gas sensing by facilitating the diffusion of target gases and the surface reaction. A cost-efficient and facile strategy is developed to scalably fabricate triple-shelled hollow binary ZnO/ZnFe2O4 (ZZFO) metal oxides microspheres with excellent intimate heterojunctions through a morphology-inherited annealing treatment of single-resource Prussian Blue analogue of Zn3[Fe(CN)6]2·xH2O solid microspheres as self-sacrificial templates. The porous hybrid ZZFO triple-shelled hollow microspheres (TSHMSs) are demonstrated to be composed of crystalline ZnO and ZnFe2O4 nano-domains with well homogeneous dispersion at the nanoscale and large void spaces between adjacent shells. When evaluated as sensors towards acetone vapor, benefiting from the striking synergy and interplay between active bicomponents and the intrinsic structural advantages, the as-fabricated ZZFO TSHMSs sensor exhibited high performances, including excellent sensitivity, good selectivity and high reversibility, rapid response ability at ultralow working temperature (140 °C). Of great significance, these prominent findings may pave the way towards optimizational design and efficient fabrication of mixed metal oxides derived from other PBAs or even other MOFs as high-performance gas sensors in the future.

Triple-shelled ZnO/ZnFe2O4 heterojunctional hollow microspheres derived from Prussian Blue analogue as high-performance acetone sensors

Semiconductor oxide chemoresistive gas sensors are widely used for detecting deleterious gases due to low cost, simple preparation, rapid response and high sensitivity. The performance of gas sensor is greatly affected by the morphology of the semiconductor oxide. There are many semiconductor oxide morphologies, including zero-dimensional, one-dimensional, two-dimensional and three-dimensional ones. The semiconductor oxides with different morphologies significantly enhance the gas-sensing performance. Among the various morphologies, hollow nanostructures and core-shell nanostructures are always the focus of research in the field of gas sensors due to their distinctive structural characteristics and superior performance. Herein the morphologies of semiconductor oxides and their gas-sensing properties are reviewed. This review also proposes a potential strategy for the enhancement of gas-sensing performance in the future.

The Morphologies of the Semiconductor Oxides and Their Gas-Sensing Properties

Influence of surface functionalization on the gas sensing characteristics of ZnO nanorhombuses

In this work, ZnO nanoparticles were synthesized by simple aqueous chemical route method. The synthesized ZnO nanoparticles were characterized by X-ray diffraction andscanning electron microscope. The sensitivity of the nanoparticles was studied for different gases like acetone, ammonia andethanol in terms of variation in spectral light intensity. The XRD and SEM analysis confirms the formation of hexagonal wurtzite structure with the grain size of 11.2 nm. The small cladding region of the optical fiber was replaced with the synthesized nanoparticles. The light spectrum was recorded for different gas concentrations. The synthesized nanoparticles showed high sensitivity towards ammonia in low ppm level and acetone in high ppm level.

ZnO nanoparticles based fiber optic gas sensor

ZrO2-ZnO composite thin films were grown by reactive DC magnetron sputtering. X-ray diffraction studies reveal the composite nature of the films with separate ZnO and ZrO2 phase. Scanning electron microscopy studies confirm the nanocrystalline structure of the films. The films were studied for their impedometric relative humidity (RH) sensing characteristics. The complex impedance plot was fitted with a standard equivalent circuit consisting of an inter-granular resistance and a capacitance in parallel. The DC resistance was found to be decreasing with increase in RH.

R. Sivacoumar

Papers

Influence of surface functionalization on the gas sensing characteristics of ZnO nanorhombuses

ZnO nanoparticles based fiber optic gas sensor

ZrO2-ZnO composite thin films for humidity sensing

A Comparative Study on Structural, Optical and Humidity Sensing Characteristics of ZnO, SnO 2 and ZnO:SnO 2 Nanocomposite

Ammonia, acetone and ethanol gas sensing characteristics of CuO thin films grown by sputtering