Availability of strategies for molecular biosensing over a finely adjustable dynamic range is essential for understanding and controlling vital biological processes. Herein we report design principles of highly responsive pH sensors based on a DNA i-motif where both response sensitivity and transition midpoint can be tuned with high precision over the physiologically relevant pH interval. The tuning is accomplished via rational manipulations of an i-motif structure as well as incorporation of allosteric control elements. This strategy delivers molecular sensing systems with a transition midpoint tunable with 0.1 pH units precision and with a total response range as narrow as 0.2 pH units which can be adjusted to a variety of outputs (e.g., fluorescent readout). The potential of the presented approach is not limited by pH sensing but may extend toward manipulation of other quadruplex based structures or the development of ultraresponsive elements for artificial molecular machines and signaling systems.