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Carbon nanotube-quantum dot heterostructures were formed using multiwalled carbon nanotubes and CdSe quantum dots, which were trapped as networks between electrodes via dielectrophoresis. Chemical exposure to 10 ppm CuSO4 displayed a size-dependent modulation of chemical sensitivity, with the heterostructures using 2.32 nm diameter quantum dots displaying an 11.4% resistance change, while those using 2.96 nm quantum dots displayed a 14.8% change and those using 4.57 nm quantum dots a 19.2% change. These results indicate a decrease in sensitivity when compared to amine-functionalized carbon nanotubes without quantum dot decoration, which show a 47.8% resistance change. This drop is partly attributed to charge transfer between the nanotubes and the CdSe quantum dots. The demonstrated modulation of nanotube properties has potential ramifications and applications for the rapidly emerging field of nanotube-based chemical sensors.