# Microrobots from Toposelective Nanoparticle Attachment

M. R. Bailey,1∗ F. Grillo,1∗ N. D. Spencer2, L. Isa1∗

1Laboratory for Soft Materials and Interfaces, Department of Materials, ETH
Zürich,
Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland
2Laboratory for Surface Science and Technology, Department of Materials, ETH
Zürich,
Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland
∗To whom correspondence should be addressed:
E-mail<EMAIL_ADDRESS>
E-mail<EMAIL_ADDRESS>
E-mail<EMAIL_ADDRESS>
> Microrobots hold promise for applications ranging from targeted delivery to
> enhanced mixing at the microscale. However, current fabrication techniques
> suffer from limited throughput and material selection. Here, we demonstrate
> a versatile route enabling the synthesis of microrobots from off-the-shelf
> micro- and nano-particles. Our protocol hinges on the toposelective
> attachment of photocatalytic nanoparticles onto microparticles, exploiting a
> multi-functional polymer and a Pickering-wax emulsification step, to yield
> large quantities of photo-responsive active Janus particles. The polymer
> presents both silane and nitrocatechol groups, binding silica microspheres
> to a range of metal oxide nanoparticles. The Pickering-wax emulsions protect
> part of the microspheres’ surface, enabling asymmetric functionalization, as
> required for self-propulsion. The resulting photocatalytic microrobots
> display a characteristic orientation-dependent 3D active motion upon UV
> illumination, different to that conventionally described in the literature.
> By connecting the extensive library of heterogeneous nanoparticle
> photocatalysts with the nascent field of active matter, this versatile
> material platform lays the groundwork towards designer microrobots, which
> can swim by catalysing a broad range of chemical reactions with potential
> for future applications.

## Introduction

Machines increasingly perform tasks once believed the prerogative of
biological systems. Yet, advances in automation are mostly limited to large
machines such as cars, drones, and industrial robots. The advent of
microrobots, artificial objects that convert energy sources such as light or
chemicals into directed motion (?), promises to bring automation down to the
micro- and nano-scale. Microrobots can transport matter at the microscale, mix
and pump fluids without external agitation (?), thus offering tantalizing
opportunities for performing autonomous tasks at small scales in applications
ranging from targeted drug delivery (?), to environmental remediation (?), and
even energy conversion (?). However, significant technological hurdles must
first be overcome if microrobots are to realise their potential for real-world
applications.

Fabrication remains perhaps the most significant challenge to applied
microrobotics, as both top-down and bottom-up approaches suffer from
significant limitations with respect to scalability and modularity in
combining different materials (?). Designing novel multi-functional
micromachines requires increasingly expensive and specialized equipment, such
as for two-photon polymerization (?), with implications for the accessibility
of scientific research (?). In contrast to the complex microrobots produced by
such techniques, Janus microswimmers are arguably the simplest class of
synthetic active matter. These rely on surface patches with different
physicochemical properties for propulsion in self-generated gradients (?). In
particular, Janus chemical swimmers do not need external actuation, requiring
only a chemical fuel source to move. Nevertheless, these simple microrobots
often suffer from a very low fabrication throughput due to the methodology
used to produce the surface patches (?).

Janus microswimmers are typically produced by sputter-coating particle
monolayers, exploiting line-of-sight vapour-phase deposition and particle
self-shadowing (?). Metal coatings are thus selectively deposited as a
spherical cap, whose extension can be controlled by tilting the monolayer (?,
?). Despite its widespread popularity, this approach has clear downsides,
namely a yield on the order of micrograms and a highly inefficient use of
metal precursors. Recently, Archer et al. demonstrated the scalable
fabrication of Janus microrobots by functionalizing Pickering-wax emulsions
with a two-step nanoparticle seeding and film-growth protocol (?). This
represents a significant advance in the state-of-the-art of microswimmer
fabrication, but the technique is specific to platinum and does not provide
close control over the film morphology or composition. The constrained
material and synthetic options in the literature, although inconsequential for
fundamental studies, hamper the progress of applied microrobotics and, in
particular, inhibit the development of propulsion mechanisms based on useful
chemical reactions.

Here, we demonstrate a modular approach to achieve the asymmetric
functionalization of microparticles from the toposelective attachment of
different nanoparticle thin films, thereby obtaining large (100 mg) quantities
of photo-responsive Janus microrobots. Specifically, commercial nanoparticles
are asymmetrically attached to SiO2 microparticles, which are partially
embedded in Pickering-wax emulsion droplets, via a poly(acrylamide) modified
with silane and nitrocatechol groups (?). The approach is not only scalable
but also connects the vast literature on high-surface-area nanocatalysts (?,
?) to the fabrication of microswimmers, extending the range of targetable
reactions and enabling the facile introduction of new functionalities.

In particular, by utilizing silane and metal oxide-nitrocatechol chelation
chemistry, we demonstrate Janus particles functionalized by TiO2, SrTiO3, and
Fe2O3 nanoparticles. Furthermore, using a post-modifiable
poly(pentafluorophenylacetate) (pPFPAC) backbone presents the opportunity to
exploit other metal-coordination chemistries through the introduction of
various functional groups. Focusing on commercial TiO2 P-25 nanocatalysts, we
fabricate photoresponsive microswimmers, which not only self-propel under UV-
illumination, but also exhibit an interesting orientation-dependent motion
that could be exploited for navigation or directed transport (?).

## Results

### Polymer-assisted nanoparticle attachment on microspheres

The first step in the fabrication of our photocatalytic microrobots is to
identify a protocol for the robust attachment of nanoparticle thin films onto
the microparticle supports. Electrostatic attachment is frequently used as a
means for colloidal heteroaggregation, however, it cannot withstand the harsh
cleaning protocols required to remove the solidified wax in Pickering-wax
emulsions. Alternative strategies employing covalent bonds are therefore to be
sought. In particular, silane groups are frequently used as anchors for SiO2,
while a raft of coordination chemistries exist for transition metal complexes.
However, combining all these features into a single, facile, and robust
protocol presents significant challenges. Here, we overcome these obstacles by
means of a polymer bridge. This comprises an acrylamide polymer backbone
functionalized with both nitrocatechols and silanes to provide multiple
covalent linking sites that can stably bind the nanoparticles to the SiO2
surface.

We base our polymer bridge on (poly)pentafluoroacetate (pPFPAC), a post-
modifiable polymer containing reactive ester linkages, which can be exchanged
with amine-containing functional groups by nucleophilic substitution. To
decorate our microparticles with various metal-oxide nanoparticles, we
functionalize this pPFPAC backbone with: i) N-boc-hexanediamine, ii)
nitrocatechols, and (iii) silane-based groups to obtain
poly(acrylamide)-g-(1,6-hexanediamine, 3-aminopropyldimethylethoxysilane,
nitrodopamine) (?). The N-boc protecting group on the diamine is removed with
TFA, exposing protonated amines on the polymer backbone. This electrostatic
component assists the polymer conformation when binding to negatively charged
inorganic surfaces. The silane groups covalently bind the polymer to the SiO2
microparticle support via siloxane groups, while nitrodopamine facilitates
chelation to a range of metals, including titanium and iron oxides (?, ?). In
this way, the multi-functional polymer acts as a bridge that provides
anchoring between the cores of silica microparticles and oxide nanoparticles
in a simple heteroaggreation process.

Figure 1: Overview of nanoparticle attachment and combination with Pickering-
wax emulsions to achieve Janus particles. a) Protocol to attach nanoparticles
to microparticle supports from the sequential mixing of the polymer and
nanoparticle suspensions. b) SEM image showing the absence of nanoparticles on
SiO2 without intermediate polymer functionalization step (scale bar 3 $\mu$m).
c,d) SEM images showing the successful attachment of TiO2 P-25 to polymer-
functionalized SiO2 microparticles (scale bars 4 $\mu$m, 0.4 $\mu$m
respectively). e) Loading curve of Ti/Si wt% as a function of polymer added
with constant TiO2 concentration. The amount of added polymer is limited by
its solubility in water. f) Loading curve of Ti/Si wt% as a function of TiO2
added with constant polymer concentration. g) Pickering-wax emulsion protocol
to toposelectively attach pre-synthesized nanoparticles. Wax is used as a
temporary mask to screen full coverage of the SiO2 microparticles. h) SEM
image of SiO2-wax colloidosomes produced after emulsification (scale bar 5
$\mu$m). i) SEM image showing TiO2 nanoparticle-functionalized SiO2
microparticles on SiO2-wax colloidosomes (scale bar 1 $\mu$m). j,k) SEM images
of Janus particles obtained after removal of the wax. All scale bars are 1
$\mu$m.

We optimize the attachment of uniform nanoparticle films focusing on TiO2 P-25
as our benchmark nanoparticle system, in concurrence with its status within
the field of photocatalysis. The process, schematized in Fig. 1a, starts with
the activation of the surface of the SiO2 microparticles by an initial
cleaning step, using a hot ammonia and hydrogen peroxide bath to provide
available hydroxyl groups to form siloxane bonds with the polymer. The polymer
is first dispersed at varying concentrations at $50^{\circ}$ C overnight.
Cleaned particles are then added dropwise to the polymer solutions under
magnetic stirring, and left to mix overnight at a final concentration of
0.1w/v%. After stirring, the SiO2 is then washed by centrifugation with
double-distilled water to remove excess polymer from solution. The SiO2
particles retain a yellow color from the polymer, due to the presence of
nitrodopamine. The functionalized polymer-SiO2 particles are then redispersed
in a PBS 7.0 buffer solution, adapting the protocol of Serrano et al. for flat
substrates (?). The particles obtain a pinkish hue, likely arising from
conformational changes of nitrodopamine in alkaline environments (?). TiO2
P-25 of varying concentrations is then added dropwise to a stirred 0.1 w%
solution of the polymer/SiO2 microparticles and then left mixing overnight.
Finally, the P-25/SiO2 microparticles are washed extensively with alternating
sonication and centrifugation steps to remove any excess TiO2 P-25 not bound
to the SiO2.

The produced P-25/SiO2 microparticles are imaged using SEM. We find that in
the absence of the polymer functionalization step, no TiO2 P-25 nanoparticles
are bound to the SiO2 microparticles after washing (Fig. 1b), even at the
highest TiO2 concentrations investigated. We also verify this with ICP-OES
elemental analysis, and find negligible quantities of Ti in the sample (Fig.
1e), highlighting the effectiveness of the cleaning protocol and the
requirement for the polymer to bind the TiO2 P-25 nanoparticles (Figs. 1b-e).
The TiO2 loading can be effectively controlled by varying the amount of
polymer and TiO2 P-25 added, and is retained after washing (Figs. 1e,f). We
observe a linear growth in the Ti/Si ratio with increasing polymer
concentration (Fig. 1e), which is limited by the solubility of the polymer in
water. The Ti/Si ratio would naturally saturate with increasing TiO2 P-25
(Fig. 1f), but, at the highest TiO2 concentrations we explore, aggregation of
the nanoparticles starts to occur.

### Toposelective nanoparticle attachment

Having established the success of the bulk surface modification, we combine it
with a Pickering wax emulsion approach to produce asymmetrically
functionalized SiO2 microparticles, en route to realizing photocatalytic
microrobots (?). This strategy consists in decorating the surface of molten
wax droplets in an aqueous medium with SiO2 microparticles. The particles are
irreversbily adsorbed at the water-wax interface and are immobilized when
solidifying the wax upon cooling. The surface of the particles immersed in the
wax is then protected from surface modifications that are carried out in the
aqueous medium. In particular, we prepare our Pickering emulsions by adapting
the methodology described by Perro et al. (?). Cleaned particles are dispersed
in didodecyldimethylammonium bromide (DDAB) solutions with concentrations
corresponding to an approximate surfactant monolayer coverage on all particles
(?). Wax is added to the suspension, which is heated to $75^{\circ}$ C, and
then subjected to a two-step vigorous stirring protocol (?) (see experimental
section for more details). The hot emulsion is then rapidly cooled in an ice-
bath to obtain solidified SiO2-wax colloidosomes (Figs. S1a-d). The
colloidosomes are then washed consecutively by gravitational sedimentation
with distilled water, a 0.1 M NaCl solution to remove the DDAB cationic
surfactant, then water once more to remove the salt. At this stage, we add the
multifunctional polymer to coat the exposed surface of the SiO2 particles. The
colloidosomes are dispersed in the polymer solution, gently agitated overnight
with an orbital mixer, then washed once more with distilled water to remove
excess polymer, redispersed in a pH 7.0 PBS solution, and gently mixed with
the nanoparticles overnight. Finally, the colloidosomes are collected and
dried, before mixing and filtering with chloroform to remove the wax (Fig.
1g).

For straightforward functionalization, the colloidosomes should be denser than
water (Fig. S3). Colloidosomes with a mean diameter of 33.4 $\mu$m are
produced using 2.12 $\mu$m particles (Supplementary Fig. 2). A particle
concentration of 5w/v% in water is emulsified with wax in a 1:10 wax:water
volumetric ratio. The process can be readily tuned to produce colloidosomes
with SiO2 particles of varying sizes (Figs. S1a-d). Microparticle size
provides a convenient handle on controlling physical properties of the final
microrobots such as the rotational diffusion coefficient, which could be
exploited for e.g. enhanced mixing (?) or navigation (?). However, unlike some
previous reports, we were not able to tune particle penetration into the wax
with surfactant concentration - instead we found that the surfactant
concentration only determined whether or not it was possible to obtain
Pickering emulsions (?).

Figure 2: HR-TEM and elemental mapping of asymmetric TiO2 P-25 nanoparticle
thin films on SiO2 microparticles. a-c) HAADF-STEM and EDS mapping of Janus
particles: the asymmetric functionalization with TiO2 is clearly visible
(scale bars 500 nm). d-f) HR-TEM of the nanoparticle films indicate direct
attachment of porous aggregate structures (scale bars 750 nm, 200 nm, 50 nm
respectively).

Our process gives an approximate 50-50 TiO2/SiO2 surface coating (Figs. 1i-k).
Ti loading is also confirmed by ICP-OES, which gives an approximate 50%
coverage for the Janus microrobots, as observed with SEM (Figs. 1e,f). The
value contrasts with the expected nanoparticle coverage based on the
penetration of the SiO2 microparticles into the wax (approximately 0.36R from
direct measurement of the three-phase contact angle) and on previous findings
using Pickering-wax emulsions (?, ?). We hypothesize that the closely packed
monolayers of SiO2 form an effective barrier to the transport of TiO2 P-25
nanoparticles onto the whole surface of the silica particles that is not
protected by the wax, thereby preventing a higher surface coverage. The
rotated particles in Fig. 1i, likely a result of subsequent filtration steps
after nanoparticle attachment, also suggest this shadowing effect. The Janus
morphology is retained after the harsh cleaning protocols necessary to remove
excess nanoparticles and wax, and the redispersal of the dried Janus
microrobots in water (Figs. 1j,k), indicating the durability of the polymer
linkage.

We confirm the Janus distribution of TiO2 by elemental mapping, namely HAADF-
EDS (Figs. 2a-c). Using TEM, we are also able to visualise the morphology of
the thin nanoparticle films. The thin films are networks of attached
nanoparticle clusters formed from multiple TiO2 primary particles rather than
individual nanoparticle structures. This is in agreement with the expected
morphology of commercial TiO2 P-25 (?). The formation of such porous
nanoparticle structures is favourable due to their enhanced surface area
compared to dense films (?), thereby increasing catalytic activity and thus
swimming speeds (?).

Utilizing nitrocatechol chelation chemistry extends the applicability of our
polymer-based nanoparticle attachment to a range of metal oxides, and could
therefore be exploited to obtain composite thin films of functional
nanoparticles on microparticle supports. To this end, and to demonstrate the
generality of our method, we also attach different phases of TiO2 (amorphous,
anatase, and rutile), Fe2O3, and SrTiO3 using the same protocols developed for
TiO2 P-25 (Fig. 3). Fe2O3 imparts both photocatalytic and magnetic properties,
and could be combined with TiO2 P-25 for enhanced speeds (?) and controlled
steering (?). SrTiO3 is a perovskite photocatalyst widely studied for its
water-splitting properties (?), and therefore is promising as the basis for a
fuel-free microswimmer. Toposelective nanoparticle attachment is thus a
promising modular route to obtaining large quantities of microrobots with a
range of functionalities that can be tuned by selection of the starting
materials.

Figure 3: Symmetric and asymmetric functionalization of SiO2 microparticles
with various commercial, pre-synthesized nanoparticles, highlighting the
versatility of the proposed approach (scale bars 500 nm).

### Microrobots from micro- and nano-particles swimming in 3D

To investigate the autonomous, photocatalytic motion of our TiO2 P-25
microrobots, we perform single-particle tracking studies with bright-field
microscopy, using in-house particle-tracking scripts (Fig. S8), under
different illumination conditions and H2O2 concentrations. TiO2 is known to
degrade H2O2 under UV light, and the Janus distribution of TiO2 P-25
nanoparticles on the SiO2 core’s surface thus leads to the formation of
asymmetric gradients around the microswimmers. These in turn develop flow
fields which result in self-diffusiophoresis of the particles (?) (Figs.
4a,b). We first confirmed the photo-responsive behavior of the microrobots by
alternating off-on UV illumination cycles and found that the UV illumination
is a necessary pre-condition for motion (Figs. 4c,d), with no evidence of
memory or photo-charging effects (?). We then evaluated the instantaneous
velocities of the microrobots under different illumination strengths and
wavelengths (Fig. S8).

The trajectories of microswimmers are typically described with a 2D active
Brownian motion model (?), where a constant propulsion velocity is randomized
in 2D by rotational diffusion. More recently, there has also been a focus on
fabricating and studying microswimmers with unbounded 3D active motion (?).
However, our microrobots demonstrate more complex behavior, which does not
follow the 2D and 3D active Brownian motion equations (?). Specifically, their
motion is mostly confined to the 2D plane, with interdispersed short and rapid
periods of out-of-plane motion (Figs. 4e,h). The predominantly 2D motion may
be explained by hydrodynamic interactions with the glass substrate, which
favor in-plane motion (?). Competing effects from out-of-plane rotational
diffusion and angular velocity arising from a non-uniform nanoparticle coating
and wall-induced flows (?) could cause the observed random out-of-plane
ballistic segments. To characterise this 3D motion, we first confirm that in
the absence of chemical fuel the particles are able to rotate in 3D (Video S2)
and measure their rotational diffusion in solvents of varying viscosity in
line with theory (Table S1) (?). We then measure their 3D active trajectories
via a simple approach making use of the changing diffraction patterns of the
particles as they swim in and out of the focal plane (see Supplementary Text
for details).

Figure 4: Overview of particle motion under UV illumination in fuel-rich
media. a) Schematic of particle motion. Under the decomposition of hydrogen
peroxide, the microrobots swim with the functionalized cap forwards. b) Wide-
field image with superimposed trajectories (shown for 3 s) under UV
illumination (scale bar 20 $\mu$m). c) Distribution of instantaneous
velocities during off (black) and on (violet) cycles. d) Mean instantaneous
velocities of particles during alternating off-on cycles of UV illumination.
e) Example of a microrobot trajectory in 3D, with the magnitude of the
velocity vector color-coded to illustrate the occurrence of an orientation-
dependent velocity. f) Plot of particle velocity vs. out-of-plane orientation
angle. Particles swim faster out of plane, and the fastest motion is observed
when the particle swims towards the glass substrate from above
($\phi=-90^{\circ}$). g) Distribution of out-of-plane orientation angles
($\phi$) across all particle trajectories. h) Distribution of displacements
for all particles in x, y, and z. The distributions in x and y (in-plane) are
Gaussian and similar, while the distribution of z displacements shows
pronounced tails. Error bars correspond to 95% confidence intervals, which
were obtained with bootstrapping.

From the analysis of individual 3D trajectories, like the one reported in Fig.
4e, we measure median instantaneous velocities ranging from 2-13 $\mu$ms-1 on
a per particle basis, and note that the particles swim cap first (Fig. 4a,
Video S1). Under the conditions found to maximize swimming speeds (3v% H2O2,
340 nm, 9.4 mWmm-2), we find a median instantaneous velocity of approximately
6.5 $\mu$ms-1 on a per particle basis (Fig. S8). Lowering the fuel
concentration leads to lower swimming speeds (?), and we also note a
difference in swimming speed depending on the wavelength of illumination used.

By plotting the instantaneous velocities as a function of particle
orientation, we observe a significant asymmetry in the particle motion (Figs.
4f-h). In particular, the microrobots exhibit orientation-dependent
velocities, displaying faster motion as their orientation is increasingly
directed out of plane. The effect persists over multiple frames, indicating
that it is not a tracking artifact. Moreover, contrary to expectations where
shadowing dictates particle motion (?), the observed faster segments are not
uni-directional. Furthermore, the previously discussed on-off photo-
responsiveness of the particles excludes a memory or charging effect (?),
which might have explained the bi-directional fast swimming segments (Figs.
4c,d). Based on the fact that the transmission of 340 nm wavelength light
through 2 $\mu$m fused SiO2 is on the order of 90% (even after accounting for
Fresnel losses), we hypothesise that the SiO2 core does not block the UV
light. This implies that even when the catalytic cap is completely ”shadowed”,
upwards swimming is observed, and we attribute the fastest motion towards the
substrate (cap down) to the direct illumination of the catalytic cap and the
concurrent effect of gravity. The slower in-plane motion could be caused by
the orientation of the propulsion direction, and the near-wall hydrodynamic
interactions which increase drag forces (?). Although statistics for 2D active
Brownian trajectories with orientation-dependent velocities have been reported
(?), their extension to the the 3D case reported here presents potential for
future theoretical developments.

## Discussion

We have demonstrated a modular approach to fabricate large (100 mg) quantities
of photo-responsive microrobots from the asymmetric attachment of commercial
nanoparticles. Our method for toposelective nanoparticle attachment provides a
versatile material platform which can be extended to various functional
nanoparticles. We envision that it will offer an off-the-shelf modular
approach to obtain large quantities of Janus particles with mixed composite
films for a range of applications. The described protocol does not require
specialised equipment beyond that found in typical synthesis laboratories,
lending itself to wide-spread application.

Moreover, the motion of our photo-responsive microrobots highlights several
avenues for further research. Analysis of the observed 3D swimming behaviour
requires a new theoretical framework not currently found in the literature. We
also note that the TiO2 P-25 microrobots self-propel more slowly at higher
illumination wavelengths, which we attribute to TiO2’s large energy band gap.
This demonstrates the opportunities to exploit the wealth of catalysis
literature (?) to inform the design of microrobots with desirable attributes
(e.g. with faster swimming speeds or visible light activation) by appropriate
selection of nanocatalytic materials.

Finally, the ballistic out-of-plane motion of our photocatalytic microrobots
suggests a photoreactor design where the competition between gravity and
activity is exploited. Realizing motion control in 3D, for example by dynamic
light patterning (?), could induce complex flows, which in turn can enhance
overall reactor efficiencies in traditionally difficult-to-mix settings. Such
control would not only be favorable for mixing in microfluidic channels (?),
which have similar dimensions to the observed Z displacements of the
microswimmers, but also in flat-panel reactors where mass transfer is a key
limiting factor (?). Increased reaction rates arising from microswimmer motion
have been previously demonstrated (?), but applicability has been limited by
materials and scalability. By targeting societally relevant reactions, such as
water splitting, we hypothesise that scalable microrobots could be exploited
in a novel photoreactor concept where the particles possess a dual catalyst-
stirrer functionality. Therefore, incorporating aspects of chemical reaction
engineering and soft matter physics could help overcome the four-phase mass-
transfer limitations inherent to current photocatalytic systems.

## Materials and Methods

### Nitrodopamine Synthesis

Nitrodopamine was synthesized following well-established protocols (?).
Briefly, dopamine(5g, 32.6 mmol) and sodium nitrite (6.3g, 91.3 mmol) were
dissolved in 150 mL water and cooled to $0^{\circ}$ C under stirring in an ice
bath. 25mL sulfuric acid (20v/v%) was added dropwise and left stirring at r.T.
overnight. The product mixture was cooled once more to $0^{\circ}$ C,
filtered, and washed with copious amounts of double-distilled water at
$0^{\circ}$ C and then ethanol at $0^{\circ}$ C. The resulting nitrodopamine
hydrogen sulfate was then dried under high vacuum overnight.

### poly(acrylamide)-g-(1,6-hexanediamine, 3-aminopropyl-dimethyloxysilane,
nitrodopamine) synthesis

Synthesis of the pentafluorophenyl acrylate monomer and its polymerization
were performed following previously published protocols (?). Briefly,
pentafluorophenol (87.21 g, 0.47 mol) was dissolved in 150 mL of
dichloromethane (DCM) at $0^{\circ}$ C and 2,6-dimethylpyridine (60.55 mL,
0.52 mol) was added slowly through a dropping funnel, which was afterwards
rinsed with another 150 mL of DCM. This second portion was added to the
reaction mixture. Acryloyl chloride (42.14 mL, 0.52 mol) was then added
dropwise to the reactor, still under cooling, and left to react overnight
under N2 atmosphere at room temperature. The resulting 2,6-dimethylpyridine
hydrochloride salt was removed by filtration and the residual solution was
washed three times with 100 mL of water, dried with magnesium sulfate and the
solvent evaporated under reduced pressure. The product monomer was purified by
distillation (in two portions) under reduced pressure (10 mbar) to give a
colorless liquid (97.09 g, 78%). The monomer pentafluorophenyl acrylate (14.31
g, 60.13 mmol), the initiator AIBN (23.83 mg, 0.15 mmol) and the chain-
transfer agent 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (158.45
mg, 0.43 mmol) were dissolved in 15 mL of toluene inside a Schlenk tube. The
solution was degassed via three freeze-pump-thaw cycles and left to react
under a nitrogen atmosphere at $80^{\circ}$C in an oil bath for 18h. After the
RAFT polymerization was completed, the mixture was left to cool to room
temperature and the resulting polymer (pPFPAC) isolated by precipitation in
methanol and dried under vacuum for 48h (12.90 g, 90%).

Likewise, the post-modification steps were carried out as outlined in the work
by Serrano et al., with the exception that the (poly)pentafluoroacetate
(pPFPAC) backbone was not first PEGylated, and instead is only post-modified
with the binding side groups (N-boc-hexanediamine,
3-aminopropyldimethylethoxysilane, nitrodopamine). Briefly, N-boc-
hexanediamine (227 mg, 1.05 mmol) was dissolved in 6.4 mL dimethylformamide
(DMF) with an excess of triethylamine (318 mg, 3.15 mmol). The mixture was
added drop-wise to pPFPAC (500 mg, 2.1 mmol) dissolved in 5.07 mL DMF and left
stirring overnight at $50^{\circ}$ C. A new solution containing 84.5 mg (0.525
mmol) of 3-aminopropyldimethylethoxysilane and triethylamine (160 mg, 1.58
mmol) was dissolved in 7.4 mL of DMF and added drop-wise to the previous
solution, still at $50^{\circ}$ C and under stirring overnight. An excess of
nitrodopamine was dissolved separately (154.4 mg, 0.525 mmol) in 7.4 mL of DMF
with 160 mg of triethylamine (1.58 mmol). The latter solution was added slowly
to the polymer solution and left stirring overnight at $50^{\circ}$ C. DMF was
evaporated under reduced pressure, the mixture re-dissolved in DCM (40 mL, 4
equivalents) and trifluoroacetic acid (TFA, 10 mL, 1 equivalent) and left to
react under stirring overnight. The resulting mixture was again evaporated
under reduced pressure and re-dissolved in twice-distilled water (80 mL). This
solution was purified by dialysis against water for two days using a membrane
with a MWCO of 3,500 Da and subsequently freeze-dried to obtain the yellow-
brown poly(acrylamide)-g-(1,6-hexanediamine, 3-aminopropyl-dimethyloxysilane,
nitrodopamine).

### Polymer-assisted nanoparticle attachment onto microspheres

Polymer solutions were prepared by dispersing the dry
poly(acrylamide)-g-(1,6-hexanediamine, 3-aminopropyl-dimethyloxysilane,
nitrodopamine) in $50^{\circ}$ C water overnight under magnetic stirring. The
maximum polymer concentration is limited by the solubility of the polymer
(approximately 60 mg/L). 1 w/v% SiO2 microparticles suspensions were first
added to a bubbling $70^{\circ}$ C H2O2/NH4OH solution (1:1:1 volumetric
ratio) under magnetic stirring for 10 minutes to activate the SiO2 surface
with reactive hydroxyl groups. The cleaned particles were then added dropwise
under magnetic stirring to the prepared polymer solutions and left stirring
overnight (final SiO2 concentration 0.1 w/v%). The polymer-SiO2 particles were
then washed by centrifugation to remove excess polymer and redispersed in
phosphate-buffered saline (PBS pH 7.0). Nanoparticle suspensions of varying
concentrations ((PBS pH 7.0 media) were then added dropwise to the polymer-
SiO2 suspensions under magnetic stirring and left mixing overnight (final SiO2
concentration 0.1 w/v%). Finally, the nanoparticle functionalized SiO2
microparticles were washed extensively with alternating sonication and
centrifugation steps to remove any excess nanoparticles not bound to the SiO2
microparticles.

### Microrobot Fabrication

Wax-SiO2 Pickering Emulsions were prepared by adapting the methodology used by
Perro et al (?). Suspensions containing 5w/v% SiO2 particles, 10.8 mg/L
didodecyldimethylammonium bromide (DDAB) (?), and a 1:10 molten wax: water
volumetric ratio were heated to $75^{\circ}$ C, then stirred for 15 minutes at
3000 RPM before vigorous mixing at 13500 RPM for 160s using an IKA T-25
Digital Ultraturrax (?). After the emulsification step, the Pickering emulsion
was immediately placed in an ice bath to rapidly solidify the colloidosomes.
The emulsion was then washed in a 0.1M NaCl solution to remove surfactants,
before further washing in deionised water. The SiO2-Wax colloidosomes were
dispersed overnight by gentle agitation in an aqueous solution of a post-
modified (poly)pentafluoroacetate (pPFPAC) polymer. The pPFPAC-colloidosomes
were then washed thoroughly in deionized water before redispersion in a
phosphate-buffered saline (PBS) pH 7.0 suspension containing the functional
metal-oxide nanoparticles. After gentle mixing overnight, the nanoparticle
functionalized colloidosomes were collected by filtration and the wax is
finally removed with chloroform.

### Light-controlled motion experiments

Stock H2O2 (30 v/v%, manufacturer) was added to dilute particle suspensions of
the microrobots to obtain 300 uL of the desired H2O2 concentrations. 280 uL
thereof was then pipetted into a flow-through cell (cell 137-QS; Hellma
Analytics) with a light path length of 1 mm. Particles were imaged on an
inverted microscope under Köhler illumination using a 40x objective (CFI S
Plan Fluor ELWD 40XC) with adjustable collar (set to 1 mm), and videos were
taken at 10 fps on a Hamamatsu C14440-20UP digital camera. UV illumination of
the particles (340/380 nm) was achieved using a Lumencor SPECTRA X light
engine as the excitation source through the objective.

### Particle Tracking

Videos were first pre-processed with Fiji (ImageJ) for conversion to 8-bit and
cropped to obtain one particle per field of view. Particles centres were
tracked using a custom script combining the MATLAB implementation of the
Crocker and Grier IDL particle tracking method (?), and the radial symmetry
approach outlined by Parthasarathy (?). The first invariant moment (inertia)
of masks around the particle centres was determined using a MATLAB
implementation of Hu’s 7 invariant moments formulation (?). Inertia look-up-
tables (LUTs) were first obtained for stationary and diffusive particles. The
evolution of inertia with Z was then inverted, before fitting of a cubic
polynomial using an inbuilt MATLAB non-linear regression function (nlinfit).
From this cubic functional form, the Z values of active particles could be
determined with prediction intervals from inertia of their masks (see
Supplementary Text for more details).

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## Acknowledgments

The authors thank Dr. T Gmür and Dr. K Zhang for their assistance with polymer
synthesis and characterisation. They are also grateful to Dr. S.S Lee for his
assistance with the UV microscopy set up used for the particle tracking
experiments, to Dr. P Zeng for the images taken with SEM and TEM, to V Niggel
for the discussions on image analysis and lab safety, and to Dr. B Hattendorf
for the ICP-OES measurements. M.R Bailey would additionally like to thank Dr.
G.A Filonenko for his various insights pertaining to the synthesis. We
acknowledge the Scientific Center for Optical and Electron Microscopy (ScopeM)
of ETH Zurich for access to their instrumentation and expertise. Fabio Grillo
acknowledges financial support from the Swiss National Science Foundation,
grant number 190735.

### Author Contribution Statement

Author contributions are defined based on the CRediT (Contributor Roles
Taxonomy). Conceptualization: M.R.B., F.G., L.I. Formal Analysis: M.R.B., F.G.
Funding acquisition: F.G., L.I. Investigation: M.R.B. Methodology: M.R.B.,
F.G., L.I., N.S. Software: M.R.B., F.G. Supervision: F.G., L.I. Validation:
M.R.B., F.G. Visualization: M.R.B., F.G., L.I. Writing - original draft:
M.R.B., F.G., L.I. Writing - review and editing: M.R.B., F.G., L.I., N.S.

## Supplementary materials

  * Text S1: Preparation of SiO2-wax colloidosomes

  * Text S2: Polymer characterisation

  * Text S3: 3D tracking with invariant moments

  * Text S4: Polymer stability

  * Text S5: 3D rotation of microrobots

  * Text S6: Motion under different experimental conditions

  * Fig. S1: Colloidosomes from different particle sizes

  * Fig. S2: Size distribution of colloidosomes

  * Fig. S3: Colloidosome density

  * Fig. S4: 1HNMR spectrum

  * Fig. S5: ATR-IR spectra

  * Fig. S6: LUT of Inertia(Z)

  * Fig. S7: Microrobot stability under highly oxidizing conditions

  * Fig. S8: Box-plots of instantaneous velocity under different conditions

  * Table S1: Conditions to obtain colloidosomes

  * Table S2: 3D rotational diffusion of microrobots

## Supplementary Text

### Text S1: Preparation of SiO2-wax colloidosomes

SiO2-wax colloidosomes were prepared by adapting the protocol proposed by
Perro et al. as described in the main manuscript (?). In contrast to the
methodology presented in (?), here the particles are dispersed in the water
phase with surfactants, before vigorous stirring by means of an Ultra-Turrax,
to obtain the Pickering wax-in-water emulsion. We find that a 5 w/v% SiO2 in
water concentration provides the best results when preparing the emulsions (5
mL). For the 2.12 $\mu$m SiO2 particles that are the focus of this work, a
volumetric wax:water ratio of 1:10 was used. Surfactant concentrations were
selected such that an approximate monolayer coverage of the SiO2 particles is
achieved (?). The flexibility of the approach is demonstrated by producing
emulsions with various particle sizes (Table S1, Fig. S1), suggesting an
adaptable approach to obtain microrobots with different physical properties.

We note here the importance of the overall colloidosome density. To ensure
effective dispersion of the colloidosomes during functionalization, their
density should be greater than that of water. However, paraffin wax has a
density of 900 kgm-3, and so colloidosome size can affect dispersibility. As a
rough estimation, we find that for 2 $\mu$m particles that fully cover the wax
droplet’s surface, a maximum colloidosome size of approximately 100 $\mu$m
ensures that the colloidosomes are denser than water (Fig. S3). The various
washing procedures, which depend on sedimentation, ensure that larger
colloidosomes are removed from the product before further functionalization
steps.

### Text S2: Polymer characterisation

The D2O 1HNMR spectrum of the post-modified
poly(acrylamide)-g-(1,6-hexanediamine, 3-aminopropyl-dimethyloxysilane,
nitrodopamine) (Fig. S4) is largely in agreement with literature (?). Chemical
shifts of 0.9 to -0.1 and 1.7 to 1.2 ppm, attributed to the presence of
3-aminopropyl-dimethyloxysilane and 1,6-hexanediamine respectively, are
observed. Likewise, the peak positions at 3.5 to 2.8 ppm correspond to the
polymer backbone. However, we are not able to identify the nitro-dopamine
group from the NMR spectrum, although we note that in the work by Serrano et
al. the reported signal is also very weak.

To further investigate the functional groups present on the post-modified
polymer backbone, ATR-IR measurements were also taken (Fig. S5). The
unmodified pPFPAC backbone displays two strong peaks at 990 cm-1 and 1520
cm-1, arising from the pentafluorophenyl groups (C-F stretch and C-C aromatic
stretch, respectively (?)). We note that after post-modification, the C-F
stretch disappears, while a new set of peaks at 1500-1550 cm-1 emerge. Looking
at the spectrum for the synthesized nitrodopamine, we identify the same
structure and attribute these multiple peaks to the combined contribution of
the aromatic C-C in-ring stretch and NO stretch of nitrodopamine. We also
identify; i) peaks which we attribute to 1,6-hexanediamine (3000-2850 cm-1,
C-H stretch), ii) multiple peaks in the fingerprint regime characteristic of
silanes, and iii) a strong merged peak around 1625 cm-1 which is likely a
combination of the signal from the C=O stretch and in-plane N-H bend of the
secondary amides formed when coupling the functional groups to the polymer.

### Text S3: 3D tracking with invariant moments

To characterise the 3D swimming behaviour of the microrobots, we developed a
simple image-invariant-based approach to determine the displacements in Z. We
exploit the changing diffraction pattern of particles imaged under bright-
field Köhler illumination as they move in and out of the focal plane. We
quantify this change in the image properties by taking the inertia (first
moment) of image masks centred at the particle. By studying trajectories in
the Z region where the inertia changes monotonically, we are therefore able to
track motion in 3D.

We first create a look-up-table (LUT) of inertia values by obtaining Z-stacks
of stationary particles with 0.2 $\mu$m increments (Fig. S6). A circular mask
around the particle center is taken for each Z-stack image, from which the
first invariant image moment (inertia) is calculated (?), (?). By performing
this operation for each Z slice, a series of inertia(Z) calibration curves are
obtained. These curves are then averaged, and the monotonic domain is inverted
to obtain Z(inertia), to which a cubic polynomial is fitted using an in-built
MATLAB function (nlinfit).

This cubic polynomial is then used to determine the Z-position of active
swimming trajectories. Briefly, we set the focal plane above the particle
centres so that as the particles swim upwards, they move into focus. This is
done to maximise the monotonically increasing regime of the particle
diffraction ring’s image inertia. We then image this plane under UV
illumination with H2O2 fuel as for normal 2D particle tracking experiments.
For each frame, the image moment of the particle is computed as previously
described for the Z-stacks. From the previously fitted cubic polynomial, a
prediction for the microswimmer out-of-plane position is thus obtained from
Z(inertia) with an average prediction error of approximately +/- 0.5 $\mu$m.
The described methodology carries a distinct advantage over more conventional
confocal approaches, in that a much higher FPS is achievable since only one Z
plane is imaged with wide-field microscopy (?). Such methods also require
computationally expensive point spread functions (PSFs) for accurate center
finding (?). They are therefore well suited for tracking fluorescent passive
particles in suspensions, but less well adapted for tracking photo-responsive
asymmetric active colloids in bright-field.

To capture the diffraction rings of the particles consistently, accurate
center finding is important for the image masks from which the first invariant
moment is calculated.To enhance particle localization and efficiency, the
MATLAB implementation of the Crocker and Grier algorithm (?) is used as a
first approximation to identify the particles. Masks are then created around
these particles, to which the Parthasarathy radial symmetry approach (?) is
applied. By combining the two approaches, we find that center finding is
improved with respect to the base cases, and works well when the particles are
both in and out of focus.

### Text S4: Polymer stability

We investigate the stability of nanoparticle attachment via the polyacrylamide
by imaging the microrobots after a simulated particle tracking experiment. The
microswimmers are illuminated with high power UV (365 nm, 27 mWmm-2) in a H2O2
rich (5v/v%) environment for 30 minutes. The conditions used exceed the
experimental conditions used for the single particle tracking experiments, and
provide a highly oxidative environment which would degrade many organics. We
then collect the particles by centrifugation and image them with SEM. We find
no change to the morphology of the TiO2 nanoparticle thin films (Fig. S7),
indicating a stable attachment.

### Text S5: 3D rotation of microrobots

By illuminating the particles with a mixture of bright-field and 440 nm
wavelength light, we clearly observe 3D rotation of the passive microrobots
(see Video S2). This contrasts with descriptions of purely 2D microswimmers
possessing an orientation vector diffusing on the unit circle. The observation
also indicates that bottom-heaviness likely plays a less significant role for
our microrobots than in previous reports (?). We expect that our porous TiO2
nanoparticle films are less heavy than the 55 nm dense TiO2 films obtained by
e-beam evaporation, resulting in less constrained rotation (?).

We use the ”MOON” approach proposed by Anthony et al (?) to describe this
rotation. The centres of the particles, and the centroids of the nanoparticle
thin films are detected using in-house python scripts. The vector connecting
these structural points defines a diffusing point on a sphere using a
spherical coordinate system. The angle of variation between these points can
thus be obtained, from which the mean squared angular displacement (MSAD) can
be determined. We fit Perrin’s equation for small angular displacements at
short timescales $<\omega\textsuperscript{2}>=4D\textsubscript{R}\Delta t$ to
the MSAD to obtain DR. As in (?), it is necessary to include an intercept term
at $\Delta t=0$ to obtain a fit of the slope (DR), which approximates the
uncertainty in $\omega$. DR values thus obtained decay with viscosity as
expected from theory, indicating a physical origin for the values (Table S2).
We note occasional instabilities when finding the cap centroid which can lead
to outlier values, however we stress that the video clearly demonstrates 3D
rotation of the particles.

### Text S6: Motion under different experimental conditions

The instantaneous velocities of the P-25 microrobots were evaluated under
different experimental conditions (Fig. S8). The concentration of fuel (H2O2),
illumination intensity (mW mm-2), and the wavelength of illumination were
varied for dilute suspensions of the microswimmers. We find that decreasing
the 340 nm UV illumination intensity from 9.7 mWmm-2 to 4.9 mWmm-2 has little
effect on the speeds of the microswimmers, indicating that already at 4.9
mWmm-2 the illumination is almost saturating. A decrease in the instantaneous
velocities is observed upon reducing the H2O2 concentration by 4x, in line
with previous reports on the relationship between fuel concentration and
microswimmer speeds (?). We note that the relative decrease in the
instantaneous velocities is small compared to the reduction in H2O2,
indicating that low fuel concentrations are viable for motion experiments. The
most evident decrease in microrobot speeds is observed when increasing the
wavelength of UV illumination from 340 nm to 380 nm (decreasing the incident
photon energy). The band gap of TiO2 lies between 360 and 380 nm, which likely
explains the more significant reduction in instantaneous velocity observed.
For the analysis of photo-responsive behavior and orientation-dependent
velocities, we use the experimental conditions providing the highest median
velocity (340 nm illumination, 9.4 mWmm-2, 3v% H2O2 concentration).

## Supplementary Figures

Fig. S 1: Colloidosomes obtained with: a. 0.55 $\mu$m particles b. 1.2 $\mu$m
particles c. 2.12 $\mu$m particles d. 4.16 $\mu$m particles  Fig. S 2: a.
Colloidosomes obtained from 2.1 $\mu$m particles b. Size distribution of
pictured colloidosomes; from a sample size of 325, we observe a Gaussian
distribution centred at 33.42 $\mu$m with a standard deviation of 4.82 $\mu$m
Fig. S 3: a. Colloidosome (from 2 $\mu$m SiO2 particles) density as a function
of its size. b. Density of a 50 $\mu$m colloidosome with increasing
stabilizing SiO2 particle size. The red dashed line marks the density of
water, and the green dashed line marks the intersection of colloidosome
density and water Fig. S 4: D2O 1HNMR spectrum of post-modified
poly(acrylamide)-g-(1,6-hexanediamine, 3-aminopropyl-dimethyloxysilane,
nitrodopamine) Fig. S 5: ATR-IR spectra of a. Unmodified pPFPAC backbone b.
Nitrodopamine c. Post-modified poly(acrylamide)-g-(1,6-hexanediamine,
3-aminopropyl-dimethyloxysilane, nitrodopamine)  Fig. S 6: LUT produced of
Inertia(Z) from Z-stacks of immobilised particles. The monotonic region of
interest is studied. The LUT is inverted to obtain Z(Inertia), to which a
cubic polynomial is fit by nonlinear regression. Left: Images of particles
taken at different Z heights. Right: Inertia(Z) LUT averaged from 5 Z-stacks,
with the particles on the left indicated. Scale bars represent 2$\mu$m  Fig. S
7: a-c: Microrobots after exposure to highly oxidative conditions (long UV
exposure in an H2O2-rich solution). The morphology of the nanoparticle thin
films remains unchanged. Fig. S 8: Box-plots of instantaneous velocities
(evaluated over 0.2s) obtained under different conditions as follows: A) 340nm
9.4mW/mm2, H2O2 3v% B) 340nm 4.9mW/mm2, H2O2 3v% C) 340nm 9.4mW/mm2, H2O2
0.75v% D) 380nm 61.7mW/mm2, H2O2 3v%

## Supplementary Tables

Table S 1: Conditions used to obtain colloidosomes with particles of different
sizes Table S 2: Rotational diffusion for single particles in different
viscosity solutions found following the method proposed by Anthony et al (?).
The theoretical values expected from the Stokes-Einstein relationship
($D\textsubscript{R}=k\textsubscript{B}T/8\pi\eta R\textsuperscript{3})$ are
also shown as reference.