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Quantum-sized ZnO nanoparticles with crystallite size about 6 nm were successfully synthesized by low temperature sol-gel process using tetraethylorthosillicate (TEOS) as capping agent to prohibit the crystallite aggregating and Ostwald ripening as well as accelerate the ZnO crystalline formation. The crystallite size was the smallest under the synthesized condition of pH = 8. The appropriate calcination temperature for as-synthesized quantum-sized ZnO was 400 °C for 1 h. The ZnO sensor exhibited the highest response of 264 to 40 ppm NO2 and the highest selectivity of 8.8 and 13.7 respectively to CO and CH4 at the same gas concentration and operating temperature of 290 °C. The mechanism of TEOS action and gas sensing were also discussed.