Methods and compositions for determining pH

Described herein are nucleic acid molecules and complexes useful as i-switch pH reporters that have increased sensitivities as a pH reporter and have alternate pH reporting capacity ranges. Aspects of the disclosure relate to a method for determining pH comprising providing a nucleic acid complex comprising: a first single-stranded nucleic acid molecule comprising the sequence CnXCnYCnZCn (SEQ ID NO. 6) wherein C is cytosine; X, Y and Z are each one or more of adenine, thymine, guanine, or combinations thereof; and n is greater than or equal to 2; and wherein at least 2 cytosine residues of the first single-stranded nucleic acid molecule are modified; and a second single-stranded nucleic acid molecule that is partially or fully complementary to the first single-stranded molecule, wherein a first label is conjugated to the first single-stranded nucleic acid molecule or the second single-stranded nucleic acid molecule; and wherein the first label is capable of producing a signal, wherein the intensity of the signal varies as a function of the conformation of the nucleic acid complex; and measuring the intensity of the signal and determining the pH from the measured signal.