A double signal amplification strategy was designed for highly sensitive and selective in situ monitoring of carbohydrate on living cells. The double signal amplification included the multiplex sandwich binding of functionalized quantum dots (QDs) to both glycan groups on the cell surface and glyconanoparticles and a cadmium cation sensitized fluorescence emission of Rhod-5N. Using the sialic acid-phenylboronic acid recognition system as a model, the 3-aminophenylboronic acid functionalized QDs (APBA-QDs) were synthesized by covalently binding APBA to mercaptopropionic acid capped CdS QDs, and the glyconanoparticles, polysialic acid stabilized gold nanoparticles (PSA-AuNPs), were prepared by a one-pot procedure. The APBA-QDs first recognized the sialic acid (SA) groups on BGC-823 human gastric carcinoma (BGC) cells and then the PSA on AuNPs, which were further used to bind more APBA-QDs on the cell surface for signal amplification. After the bound QDs were dissolved to release the Cd(2+), a Cd(2+)-sensitized fluorescence method was developed for the detection of BGC cells in a linear range from 5.0 × 10(2) to 1.0 × 10(7) cells mL(-1) with a limit of detection down to 210 cells mL(-1) (8 cells in 40 μL of solution) and the dynamic monitoring of SA expression variation on the cell surface. The monitoring result was identical with that from flow cytometric analysis. This approach showed high specificity and acceptable reproducibility. This strategy provided a promising platform for highly sensitive cytosensing and cytobiologic study.