Figure shows the molar absorption spectra of 6MP and Aza in PBS solution at pH 5.5, pH 7.4, and in borate buffer at pH 9.8. The lowest-energy absorption band for the neutral form of 6MP has a maximum at 323 nm, with a large molar absorption coefficient (ε 323 = 22.7 ± 0.3 mM - 1 cm -1 in PBS at pH 5.5). The absorption of 6MP does not change significantly in going from pH 5.5 to pH 7.4. However, there is a 5 nm blue shift in the absorption maximum (ε 318 = 19.7 ± 0.5 mM -1 cm -1 in PBS at pH 7.4). The deprotonated species of 6MP exhibits a further 5 nm blueshift in going from pH 7.4 to pH 9.8, which is accompanied by a decrease in molar absorption coefficient (ε 313 = 12.6 ± 0.3 mM -1 cm -1 in borate buffer at pH 9.8). Figure shows the molar absorption spectra of Aza in PBS at pH 5.5 and 7.4, and in borate buffer at pH 9.8. The nitroimidazole functionalization blueshifts the absorption maximum by 44 nm to 279 nm relative to the 6MP neutral species (ε 279 = 14.8 ± 0.4 mM -1 cm -1 in PBS at pH 5.5), while simultaneously broadening the spectrum into the near visible (Figure ). The absorption of the neutral and the deprotonated mixture at pH 7.4 is 2 nm redshifted but essentially of the same intensity as that of the neutral form of Aza (ε 281 = 14.4 ± 0.2 mM -1 cm -1 in PBS at pH 7.4). An additional 4 nm redshift and a slight decrease in molar absorption is observed for the deprotonated form of Aza (ε 285 = 11.7 ± 0.3 mM -1 cm -1 in borate buffer at pH 9.8).