  The viscosity of cell membranes is a crucial parameter that affects the
diffusion of small molecules both across and within the lipidic membrane and
that is related to several diseases. Therefore, the possibility to measure
quantitatively membrane viscosity on the nanoscale is of great interest. Here,
we report a complete investigation of the photophysics of an amphiphilic
membrane-targeted azobenzene (ZIAPIN2) and we validate its use as viscosity
probe for cell membranes. We exploit ZIAPIN2 the trans-cis photoisomerization
to develop a molecular viscometer and to assess the viscosity of Escherichia
coli bacteria membranes employing time-resolved fluorescence spectroscopy.
Lifetime measurements of ZIAPIN2 in E. coli bacteria suspensions correctly
indicate that membrane viscosity decreases as the samples were heated up. Our
results report a membrane viscosity value in live E. coli cells going from 10
to 5 cP, increasing the temperature from 22 {\deg}C up to 40 {\deg}C.
