Biomacromolecules exist and function
in a crowded and spatially
confined intracellular milieu. Single-cell analysis has been an essential
tool for deciphering the molecular mechanisms of cell biology and
cellular heterogeneity. However, a sound understanding of in vivo environmental effects on single-cell quantification
has not been well established. In this study, via cell mimicking with
giant unilamellar vesicles and single-cell analysis by an approach
called plasmonic immunosandwich assay (PISA) that we developed previously,
we investigated the effects of two in vivo environmental
factors, i.e., molecular crowding and spatial confinement, on quantitative
biochemistry in the cytoplasm of single cells. We find that molecular
crowding greatly affects the biomolecular interactions and immunorecognition-based
detection while the effect of spatial confinement in cell-sized space
is negligible. Without considering the effect of molecular crowding,
the results by PISA were found to be apparently under-quantitated,
being only 29.5–50.0% of those by the calibration curve considering
the effect of molecular crowding. We further demonstrated that the
use of a calibration curve established with standard solutions containing
20% (wt) polyethylene glycol 6000 can well offset the effect of intracellular
crowding and thereby provide a simple but accurate calibration for
the PISA measurement. Thus, this study not only sheds light on how
intracellular environmental factors influence biomolecular interactions
and immunorecognition-based single-cell quantification but also provides
a simple but effective strategy to make the single-cell analysis more
accurate.