Microrobots
can expand our abilities to access remote, confined,
and enclosed spaces. Their potential applications inside our body
are obvious, e.g., to diagnose diseases,
deliver medicine, and monitor treatment efficacy. However, critical
requirements exist in relation to their operations in gastrointestinal
environments, including resistance to strong gastric acid, responsivity
to a narrow proton variation window, and locomotion in confined cavities
with hierarchical terrains. Here, we report a proton-activatable microrobot
to enable real-time, repeated, and site-selective pH sensing and monitoring
in physiological relevant environments. This is achieved by stratifying
a hydrogel disk to combine a range of functional nanomaterials, including
proton-responsive molecular switches, upconversion nanoparticles,
and near-infrared (NIR) emitters. By leveraging the 3D magnetic gradient
fields and the anisotropic composition, the microrobot can be steered
to locomote as a gyrating “Euler’s disk”, i.e., aslant relative to the surface and
along its low-friction outer circumference, exhibiting a high motility
of up to 60 body lengths/s. The enhanced magnetomotility can boost
the pH-sensing kinetics by 2-fold. The fluorescence of the molecular
switch can respond to pH variations with over 600-fold enhancement
when the pH decreases from 8 to 1, and the integration of upconversion
nanoparticles further allows both the efficient sensitization of NIR
light through deep tissue and energy transfer to activate the pH probes.
Moreover, the embedded down-shifting NIR emitters provide sufficient
contrast for imaging of a single microrobot inside a live mouse. This
work suggests great potential in developing multifunctional microrobots
to perform generic site-selective tasks in vivo.