Disclosed are nucleic acid-based molecular switches that respond to changes in pH. The switches may be used in DNA nanodevices. The switches may also act as sensors for measuring the pH of a sample, including cells, regions thereof, and whole organisms. The switch includes an A-motif that forms at acidic pH. Also disclosed are compositions and methods for measuring the pH of cells or regions thereof, such as vesicles, the nucleus, mitochondrial matrix, or the Golgi lumen.