In this article, we report the synthesis of mesoporous silica nanoparticles–silver nanoparticles (MSNs–AgNPs) for use in detection of breast cancer cells. The synthesized MSNs–AgNPs were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and particle size analysis. TEM images show that MSNs and AgNPs are spherical, with average particle sizes of 178 nm and 33 nm, respectively. Surface functionalization of MSNs–AgNPs with anti-HER2 antibodies results in MSNs–AgNPs–anti-HER2 conjugates. Exposure of these to MCF-7 breast cancer cells elicits a colorimetric response, with the color change being most pronounced for cell densities of 5000–50,000 cells/ml. The MSNs–AgNPs–anti-HER2 conjugates thus form the basis of a colorimetric assay for breast cancer cell detection.