A quantum dot-polymer nanocomposite for optical chemical and biological sensing is formed by stably incorporating functionalized quantum dots into a pH sensitive hydrogel polymer network. At least one monomer of the pH sensitive hydrogel has functional groups selectively chosen to correspond to functionalized groups on the quantum dots to enable conjugation between the hydrogel polymer network and the functionalized quantum dots. The resulting quantum dot-polymer nanocomposite is placed in a solution having a known pH and addition of a chemical composition or biological agent of interest generates a change in pH of that solution. The nanocomposite expands or contracts responsive to the pH change. The pH change is optically detected by measuring the intensity level of fluorescence from the quantum dots when the nanocomposite is subjected to an excitation light source.