Myocardial ischemia is featured by a significant increase in the cytoplasm proton concentration, and such a proton change may be applied as an index for earlier ischemic heart disease diagnostics. But such a pH change in a live heart cell is difficult to monitor as a normal fluorescent probe cannot specifically transport into the cytoplasm of an ischemic cell. This is because the heart cell contains condensed myofibrils which are tight barriers for a normal probe to penetrate. We design fluorescent probes, single-strand DNA wrapped single-wall carbon nanotubes (ssDNA-SWCNTs), where the ssDNA is labeled by the dye molecule hexachloro-6-carboxyfluorescein (HEX). This nanoprobe could transport well into a live heart cell and locate in the cytoplasm to sensitively detect the intracellular pH change of myocardial ischemia. Briefly, protons neutralize the negative charges of nanoprobes in the cytoplasm. This will weaken the stability of nanoprobes and further tune their aggregation. Such aggregations induce the HEX of some nanoprobes condensed together and further result in their fluorescence quenching. The nanoprobes are advantaged in penetrating condensed myofibrils of the heart cell, and their fluorescence intensity is sensitive to the proton concentration change in the live cell cytoplasm. This new method may provide great assistance in earlier cardiopathy diagnosis in the future.