alcohol vapor multisensory array was developed based on ZnO NR grown directly on a

multielectrode chip via a hydrothermal method [20]. The type of alcohol vapors studied

is ethanol, isopropanol, and butanol. The multisensory array chip was fabricated using

Si/SiO2 substrate, co-planar Pt/Ti sputtered cathode electrode, and connected using Au

wire. The working principle of the ZnO NR gas sensor is through a reaction of gas with

the ZnO surface, resulting in the change in resistance. The sign of reaction is dependent

on whether the chemisorbed species undergo a reduction or oxidation process. The

process may then cause electron exchange between the conductance band and the surface

local energy states in the gap. Theoretically, the morphology of the transducer may

greatly influence their gas sensing properties. The directly grown ZnO NR on a multi­

electrode chip with a diameter of 10–20 nm and length 90–150 nm show linear range

detection for alcohol vapor in the range of 0.2 to 5 ppm.

Pan and Zhao [21] have reported on the gas sensor LOC devices based on modification

with ZnO nanocombs for carbon monoxide (CO) detection. The LOC device was devel­

oped on a single Si chip integrated with a complementary-metal-oxide-semiconductor

(CMOS) microsensor. The ZnO nanocombs were grown on top of a silicon substrate. ZnO

nanocombs were synthesized via chemical vapor deposition (CVD) based on a vapor-

liquid-solid mechanism. ZnO nanocombs were employed as signal enhancers by pro­

viding a larger effective sensing area that exhibited high-sensitivity detection even at

room temperature. The detection mechanism was based on the reaction of CO target gas

with the generated oxygen ions, which released the combined electrons back to the

conduction band. This led to a significantly narrowed depletion region and a decrease in

resistance. Therefore, by measuring the overall resistance of the semiconductor metal

oxide in real-time, the CO gas concentration could be quantitatively measured. The de­

veloped ZnO nanocomb gas sensor exhibited high sensitivity of 7.22 and 8.93 for CO

concentrations of 250 ppm and 500 ppm at room temperature. This proved that the

proposed ZnO nanocomb greatly enhanced sensitivity even at room temperature and

promising nanoparticles used for CO gas sensors. The reason is that nanocomb structure

with a length of 58.7 μm and width of 4.35 μm have promoted a larger surface sensing

area by providing multiple conducting channels for gas detection.

Other types of MO nanomaterials commonly used for sensor signal enhancement is

iron oxide nanoparticle (IONPs) or also called magnetic materials. IONPs exist in various

phases such as MnFe2O4, Fe2O3, and Fe3O4. IONPs exhibit superparamagnetic properties,

biocompatible, high catalytic properties, and unique physicochemical properties. Another

important function of the magnetic properties of IONPs in LOC devices is the ability to

be integrated into transaction systems for efficient detection of the target analytes under

the influence of an external magnetic field. IONPs have been employed in numerous

applications such as biological separation, immunoassay, target delivery, and biosensor.

Recently, core-shell of AuNPs@Fe3O4 and ordered mesoporous carbon (CMK-8) in

chitosan were employed for signal enhancement in the electrochemical determination of

IgG antibodies anti-Toxocara canis (IgG anti-T. canis) for Toxocariosis disease [24]. The

microfluidic electrochemical immunosensor device was fabricated on PDMS/glass using

the photolithography process and sputtering of Ag/Au electrode acted as the transducer.

The IgG anti-T canis antibody was detected through a non-competitive immunoassay. The

combination of metal and metal oxide nanoparticles in forming core-shell of AuNPs@Fe3O4

features improved the electrochemical performance, good biocompatibility, and prepare a

larger surface area for binding with the recognition element, which is an antigen from

T. canis second-stage larvae. The sensor showed outstanding performance for IgG anti-T.

canis detection with LOD of 0.10 ng mL⁻¹ and linearity of 0.1–100 ng/mL.

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