ConspectusOptical probes that can be used to measure certain
quantities with
subcellular resolution give us access to a new level of information
at which physics, chemistry, life sciences, and medicine become strongly
intertwined. The emergence of these new technologies is owed to great
advances in the physical sciences. However, evaluating and improving
these methods to new standards requires a joint effort with life sciences
and clinical practice. In this Account, we give an overview of the
probes that have been developed for measuring a few highly relevant
parameters at the subcellular scale: temperature, pH, oxygen, free
radicals, inorganic ions, genetic material, and biomarkers.Luminescent probes are available in many varieties, which can be
used for measuring temperature, pH, and oxygen. Since they are influenced
by virtually any metabolic process in the healthy or diseased cell,
these quantities are extremely useful to understand intracellular
processes. Probes for them can roughly be divided into molecular dyes
with a parameter dependent fluorescence or phosphorescence and nanoparticle
platforms. Nanoparticle probes can provide enhanced photostability,
measurement quality, and potential for multiple functionalities. Embedding
into coatings can improve biocompatibility or prevent nonspecific
interactions between the probe and the cellular environment. These
qualities need to be matched however with good uptake properties,
colloidal properties and eventually intracellular targeting to optimize
their practical applicability. Inorganic ions constitute a broad class
of compounds or elements, some of which play specific roles in signaling,
while others are toxic. Their detection is often difficult due to
the cross-talk with similar ions, as well as other parameters.The detection of free radicals, DNA, and biomarkers at extremely
low levels has significant potential for biomedical applications.
Their presence is linked more directly to physiological and clinical
manifestations. Since existing methods for free radical detection
are generally poor in sensitivity and spatiotemporal resolution, new
reliable methods that are generally applicable can contribute greatly
to advancing this topic in biology. Optical methods that detect DNA
or RNA and protein biomarkers exist for intracellular applications,
but are mostly relevant for the development of rapid point-of-care
sample testing.To elucidate the inner workings of cells, focused
multidisciplinary
research is required to define the validity and limitations of a nanoparticle
probe, in both physical and biological terms. Multifunctional platforms
and those that are easily made compatible with conventional research
equipment have an edge over other techniques in growing the body of
research evidencing their versatility.