A new microengine-based built-in-platform exploiting a surprising dual action with solution mixing and control of the reaction parameters, has been applied for accelerating chemical reactions (organophosphorous nerve agents hydrolysis) and electrochemical detection of non-hazardous by-product (p-nitrophenol) using printable sensor strip

Cinti, Stefano

Gao, Wei

Li, Jinxing

Palleschi, Giuseppe

Valdés-Ramírez, Gabriela

Wang, Joseph

Caltech Authors - Main

Electronic Supporting Info 
Microengine-assisted Electrochemical Measurements at Printable Sensor Strips 
Stefano Cinti,ab Gabriela Valdés-Ramírez,a Wei Gao,a Jinxing Li,a Giuseppe Palleschi,*b and 
Joseph Wang *a 
E-mail: josephwang@ucsd.edu 
E-mail: giuseppe.palleschi@uniroma2.it 
 
a Department of Nanoengineering, University of California San Diego, La Jolla, California 92093-
0448, United States 
b Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, Via della 
Ricerca Scientifica 1, 00133 Rome, Italy 
 
1. Experimental Section 
Reagents and Instrumentation  
Potassium Chloride (Catalog #P217) was obtained from Fisher Chemical (Fair Lawn, NJ), 
Paraoxon Methyl (Pestanal, Catalog #46192) was purchased by Sigma-Aldrich (St. Louis, MO), 
Magnesium (Mg) microparticles (Catalog #465666, powder, 325 mesh, 99.5% trace metals basis 
Sigma-Aldrich, St Louis, MO) All the chemicals were used without further purification. 
Electrochemical measurements were performed at room temperature using a portable CH 
Instruments electrochemical analyzer (model 1232A, Austin, TX). The applied potentials in all 
measurements were versus the screen-printed pseudo Ag/AgCl reference electrode. 
Screen-Printed Electrodes fabrication  
Alumina plates (10 cm x 10 cm) were purchased from CoorsTek, Inc. (CA, USA). A Speedline 
Technologies (Franklin, MA) screen printer (MPM-SPM model TF-100) was used to fabricate the 
sensor. An Ag/AgCl-based ink (124-36, medical grade), was purchased from Creative Materials 
Inc. (MA, USA) and it was employed to define the conductive underlayer as well as the reference 
electrode. A carbon-based ink (Acheson E440B) from Henkel Corp. (Madison Heights, MI) was 
overlaid on the conductor to define working and counter electrodes. The contacts pads were 
insulated. The working electrode has a diameter of 3 mm.  
Microengines fabrication  
The Janus microengines have been made as reported in a previous work,26 with a slight 
modification. Briefly, Mg microparticles were placed onto glass slide and coated with an 80 nm 
nickel layer and a 10 nm gold layer using Temescal BJD 1800 E beam Evaporator. The 
microengines were removed from the glass slide and dispersed into ethanol.  
Development of sensing platform 
 2 L of the Mg Janus microengine dispersion were drop casted onto the SPE's alumina substrate 
as displayed in Figure S1. The magnetic microparticles were disposed in 4 spots (0.20±0.02 mg) 
Electronic Supplementary Material (ESI) for ChemComm.
This journal is © The Royal Society of Chemistry 2015
sufficiently far from the working electrode area. In order to keep fixed the Mg Janus particles during 
the chemical reaction and electrochemical measurements, and external magnetic bar was 
positioned on the back of the microengine-modified electrode as shown in Figure S1. 
 
Figure SI1. Magnesium-based Janus particle microengines coupled to a screen printed electrode 
as  electrochemical sensing strip platform.  
Nerve Agents detection  
The electrode surface and Janus microengines were covered by a 50 L-drop of paraoxon 
prepared in 0.1 M KCl solution. Cyclic voltammetry and amperometry experiments were carried out 
after 4 minutes reaction, allowing microengine-driven degradation of the nerve agents. Cyclic 
voltammetry were performed from 0.3 to 1.2 V at 0.05 V s-1 scan rate. Amperometric 
measurements were carried out by applying a potential step to +0.9 V  for 60 s. Each 
measurement was performed by using a different electrode strips modified under the same 
conditions. 
2. Results 
OP degradation to p-nitrophenol 
Cyclic voltammetry experiments were performed to verify the success of the degradation carried 
out by using Mg-based microengines. 20 mM of a solution of p-nitrophenol at differente pH were 
analyzed and compared with the signal due to the OP degradation, as illustrated in Figure S2. 
 
Figure SI2. Cyclic Voltammetry of (a) 20 mM nitrophenol in 0.1 M KCl (pH=11) without 
microengines,  (b) 20 mM paraoxon in 0.1M KCl  with microengines , (c) 20 mM nitrophenol in 0.1 
M KCl (pH=7) without Janus microengines and (d) 20 mM paraoxon in 0.1M KCl  without 
microengines. Scan rate of 0.05 V/s. 
 


Microengine-assisted electrochemical measurements at printable sensor strips

8668 | Chem. Commun., 2015, 51, 8668--8671 This journal is©The Royal Society of Chemistry 2015
Cite this:Chem. Commun., 2015,
51, 8668
Microengine-assisted electrochemical
measurements at printable sensor strips†
Stefano Cinti,ab Gabriela Valdés-Ramı́rez,a Wei Gao,a Jinxing Li,a
Giuseppe Palleschi*b and Joseph Wang*a
A new microengine-based built-in-platform exploiting a surprising dual
action with solution mixing and control of the reaction parameters, has
been applied for accelerating chemical reactions (organophosphorous
nerve agents hydrolysis) and electrochemical detection of non-
hazardous by-product (p-nitrophenol) using printable sensor strip.
The growing exploitation of advanced screen-printing fabrication
techniques allows for the widespread replacement of conventional
(‘beaker-type’) electrochemical cells and bulky electrodes with easy-
to-use sensor strips.1,2 Such printable sensors permit simple,
rapid, centralized and decentralized electrochemical analyses
of microliter volumes. Yet, unlike traditional large cells that
employ solution stirring or electrode rotation for enhancing the
response via convective mass transport,3 microcells based on
printable strips are limited to quiescent solutions and hence
solely to diffusion transport.
Here, we describe a new concept of microengine-assisted
electrochemical measurements using strip-based micro-volume
electroanalysis. We demonstrate the dramatic enhanced ampero-
metric signal of organophosphorous (OP) nerve agent detection in
the presence of bubble-generating magnesium Janus microengines.
The generated microbubbles induce localized convection and a
greatly enhanced mass transport and sensitivity.
As recently reported,4,5 the hydrogen-bubble generation from
magnesium-based microengines involves the oxidation of the
Mg surface to reduce water:
Mg(s) + 2H2O(aq) - Mg(OH)2(s) + H2(g).
Bubble-propelled microengines have been shown recently to
generate a dramatically enhanced fluid transport even when
confined into a stationary surface.6 Such fluid convection from
the bubbles generating PANI/Pt tubular microengines has been
demonstrated using the displacement of passive microparticle
tracers. The fluid transport induced by water-driven Mg micro-
motors is shown here for a dramatic mixing effect for the
electroanalysis of microliter samples at planar strip electrodes.
Such unique coupling of microengines and sensor strips is
demonstrated toward the amperometric measurements of OP
nerve agent degradation.
OP chemical warfare agents and pesticides interrupt the
nervous stimuli communication by blocking the enzyme acetyl-
cholinesterase (AChE).7,8 OP electrochemical detection relies on
the enzymatic inhibition of cholinesterase (ChE) or hydrolysis of
OP nerve agents by organophosphorous hydrolase (OPH); however
these bio-approaches are characterized by long incubation times,
enzymes stabilization and enzymes engineering.9–11
Recent advances in chemically-powered microscale motors
have led to new capabilities and novel applications,12–14 including
new biosensing strategies based on the direct isolation of target or
changes in their movement in the presence of target analytes.15–19 For
example the motion of PANI/Pt micromotors has shown to enhance
colorimetric immunoassays.20 Here we exploit for the first time the
dual action of water-powered magnesium-based microengines, not
only to impart an enhanced fluid mass transport, but also to increase
dramatically the pH solution allowing a quick degradation and
subsequent detection of OP nerve agents without the need of any
external reagents or instruments. These effects are produced by
confining the microengines onto the strip surface keeping them
without propelling through the sample (Fig. S1, ESI†). The use of
bubble-generating Mg Janus microengines is visibly capable to
degrade in a short time paraoxon, producing p-nitrophenol which is
electrochemically detectable. The simple design and operation make
this approach a universal route for variety of electrochemical assays
of microliter samples at different sensor strips. The new concept
simplifies such measurements and can be readily extended to
different electrochemical sensing platforms and target analytes, while
eliminating the need for additional stimuli, instrument or external
stirrer to homogenize ultrasmall samples during their analyses.
a Department of Nanoengineering, University of California San Diego, La Jolla,
California 92093-0448, USA. E-mail: josephwang@ucsd.edu
b Dipartimento di Scienze e Tecnologie Chimiche, University of Rome Tor Vergata,
Via della Ricerca Scientifica 1, 00133 Rome, Italy.
E-mail: giuseppe.palleschi@uniroma2.it
† Electronic supplementary information (ESI) available: Additional information
pertaining to fabrication procedures, methods and characterization. See DOI:
10.1039/c5cc02222c
Received 16th March 2015,
Accepted 16th April 2015
DOI: 10.1039/c5cc02222c
www.rsc.org/chemcomm
ChemComm
COMMUNICATION
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As widely stated, regular magnesium microparticles in water
produce hydrogen bubbles and magnesium hydroxide; yet
hydrogen production is not vigorous since the surface of the
Mg microparticles is readily passivated by the formation of
hydroxide layer. However, coating Mg microspheres with a gold
layer,4 and the presence of chloride ions allow the reaction to
proceed by exploiting the combination of galvanic corrosion
and pitting corrosion effects, resulting in a continuous hydro-
gen bubbles evolution as reported as follows:
Mg+(aq) + H2O(aq) - 1/2H2(g) + Mg
2+(aq) + OH(aq).
Here we exploit the generation of both microbubbles and
hydroxyl ions with the corresponding increased pH towards
degradation enhanced at non-enzymatic strip-based assays of
OP nerve agents. Such dual action of micromotors and improved
amperometric detection is accomplished with fixed Mg micro-
engines confining them onto the strip surface. The vigorous and
prolonged bubble production leads to a greatly enhanced ‘built-in
mixing’ and mass transport effects within the microliter sample
droplet, thus allowing a rapid and reproducible nerve-agent
degradation with a subsequent detection without the need of
external stirrers or mixing devices. The presence of the nickel layer
in the Janus microparticles allows the magnetic confinement of
the microengines on the sensor strip, preventing their movement
to the electrodes area avoiding interference during the electro-
chemical measurements. An effective fluid transport is achieved
in the presence of the generated bubbles, leading to a prolonged
coherent flow near their anchor points and dramatically enhan-
cing mass transport in their vicinity. The rising of a vertical
bubbles flow is capable to impart effective convective transport
to the entire drop (Fig. 1).
Orozco et al.,6 demonstrated how the fluid transport induced
by the rising bubbles is larger than that one induced by the freely
swimming catalytic micromotors or Janus-microparticles: fixed
motors produce a net force on the fluid depending by 1/r (r is the
distance from source), whereas, not anchored motors exert a
weaker convection field that decays quickly, 1/r2. We use micro-
engines capable of producing OH ions to increase the medium
pH and consequently promote the degradation of paraoxon into
a readily detectable p-nitrophenol. Fig. 2A shows optical images
of a 20 mM paraoxon solution (in 0.1 M KCl) on the sensor
strip, taken at different times in the absence and presence of
microengines. It is clearly visible that in the absence of the Mg
microengines the solution remains colorless with the time
demonstrating absence of hydrolysis of paraoxon. Meanwhile,
in the presence of Mg Janus particles, paraoxon degrades
immediately and, after a minute of reaction the solution is
almost entirely yellow due to the produced p-nitrophenol and
the mixing effect which is visible to naked eye; Fig. 2B shows a
schematic representation of the electrode before and after the
effect of microengine tasks. At time zero t0 the probe is only in
the presence of nerve agents (colorless) while at the end of the
process ti the solution turns yellow, indicating degradation of
the starting OP compound to p-nitrophenol.
The reactions involved in this Mg Janus/OP nerve agents
system are represented in Fig. 3.
Owing to the vigorous production of hydrogen bubbles, the
alkaline hydrolyzed by-product, p-nitrophenol, is transported
throughout the solution allowing the development of a homo-
geneous solution that is therefore measurable accurately without
external stirring. Due to the presence of the magnet located below
the strip, the Janus microengines remain confined far from the
area of the working electrode. Placing the microengines close to
the working electrode area could suppress the electrochemical
signal of interest. The microengines were thus positioned as the
closest to the electrodes area where their presence does not affect
the electrochemical signal.
The electrochemical oxidation of p-nitrophenol at carbon
electrodes has been widely studied,21 and we harness the Mg-based
Janus microengines for chemical hydrolysis of methyl paraoxon
and screen-printed electrodes for the following electrochemical
detection of p-nitrophenol. Fig. 4 illustrates the cyclic voltammo-
grams related to a 20 mM paraoxon solution. In the absence of
Janus particles, no peak is displayed near the 0.9 V (vs. Ag/AgCl)
whereas in presence of microengines a well-resolved peak is
observed at a 0.85 V (vs. Ag/AgCl). The oxidation peak position
and the intensity of the hydrolyzed paraoxon at 20 mM concen-
tration were compared with p-nitrophenol (20 mM) in KCl at
Fig. 1 Microengine-assisted mixing for electrochemical measurements.
Schematic illustration of microengine-enhanced bubbles flow. A: OP nerve
agents; P: p-nitrophenol; E: working electrode; D: distance between working
electrode and Mg-microengines (5 mm); M: magnet.
Fig. 2 (A) Time comparison between SPE without and with microengines
in the presence of 20 mM paraoxon in 0.1 M KCl after (a) 0, (b) 1, (c) 4
and (d) 10 minutes; (B) Schematic conversion of OP nerve agents to
p-nitrophenol driven by microengines.
Communication ChemComm
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8670 | Chem. Commun., 2015, 51, 8668--8671 This journal is©The Royal Society of Chemistry 2015
same pH. The intensity as well as peak position were similar
demonstrating not only that after 4 minutes of chemical reaction the
paraoxon has been hydrolyzed with a high yield, (Fig. S2, ESI†). The
bubbles produced by microengines are capable to produce strong
mixing effect generating a homogeneous solution, making attractive
the proposed coupled platform. By using similar criteria as for the
cyclic voltammetry measurements, amperometric experiments were
carried out applying 0.9 V (vs. Ag/AgCl) constant potential during 1
min. Inset of Fig. 4 displays that the current responses in the
presence of microengines are significantly higher than those
obtained using bare SPE. Fig. 5A illustrates the well-defined ampero-
metric response to different nerve agent concentrations up to
20 mM. The corresponding inset displays the corresponding
calibration plots and the enhanced sensitivity associated with
the presence of the Mg-Janus microengines.
Linear correlation between current response vs. paraoxon
concentrations was found as is shown in the inset calibration
plot Fig. 5A (correlation coefficient of 0.995). The sensitivity
observed in the presence of Janus microengines is significantly
larger (B15 fold) compared to electrodes without microengines.
Eight different microengines-modified SPEs were used for preci-
sion evaluation of the assay: as illustrated in Fig. 5B, these series of
measurement at 20 mM paraoxon yielded a reproducible response
(RSD o 5%) indicating high reproducibility for the microengine-
induced high performing mixing process towards paraoxon
degradation and subsequent detection.
In conclusion, we have demonstrated the use of bubble-
generating microengines for generating a greatly enhanced fluid
transport during amperometric measurements at common printable
sensor strips. The presence of magnesium Janus microengines
along with OPH makes possible the rapid selective conversion
of a non-detectable species (paraoxon) to an electroactive compound
( p-nitrophenol) easily detectable with an electrochemical strip
platform. Such operation addresses the challenge of imparting
effective convective transport using microliter volumes strip-based
measurements. The microengines operation leads to increase the
pH of the solution and a built-in stirring increasing sensitivity
towards paraoxon detection, up to 15-fold, compared to a non-
engineered bare screen-printed electrode. The magnetically-
anchored microengines can serve as an ‘artificial’ enzyme toward
Fig. 3 Mechanism and reactions involved in the OP nerve agents degra-
dation to p-nitrophenol accelerated by microengines.
Fig. 4 Cyclic voltammograms of (a) 0.1 M KCl solution without micro-
engines, (b) 0.1 M KCl solution with microengines, (c) 20 mM paraoxon in
0.1 M KCl without microengines and (d) 20 mM paraoxon in 0.1 M KCl with
microengines. Inset: amperograms of (a0) 0.1 M KCl solution without
microengines, (b0) 0.1 M KCl solution with microengines, (c0) 20 mM
paraoxon in 0.1 M KCl without microengines and (d0) 20 mM paraoxon
in 0.1 M KCl with microengines.
Fig. 5 (A) Amperograms obtained in the presence of microengines
for different paraoxon concentrations (a) 0, (b) 4, (c) 8, (d) 12, (e) 16 and
(f) 20 mM in 0.1 M KCl. Inset: calibration plot comparison toward paraoxon
obtained without (square) and with (circle) microengines, current sampled
at 60 s (n = 3); (B) Reproducibility of the response for 20 mM paraoxon
solution in KCl 0.1 M in presence of microengines (n = 8). Applied potential,
0.9 V (vs. Ag/AgCl).
ChemComm Communication
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the hydrolysis (degradation) of OP compounds. The production
of an electroactive non-hazardous compound as p-nitrophenol
was easily detected by a disposable, cost effective and mass-
producible screen-printed carbon electrodes. The integration of
microengines within an electrochemical strip platform repre-
sents an attractive approach for built-in mixing that can be
extended to the enhancement of measurements using diverse
thick-film and thin-film planar sensor strips for a wide range
of practical applications. For example, microengines based
on different materials (e.g., Zn, Al, or Pt) can produce micro-
bubbles and mixing in acidic, alkaline or H2O2 solutions,
and used for the electrochemical measurements in different
environments.22 These bubble-generating microspheres can be
combined with different electrode materials for imparting the
necessary detection sensitivity and selectivity towards specific
analytes.
Defense Threat Reduction Agency-Joint Science and Technology
Office for Chemical and Biological Defense (Grant No. HDTRA1-13-
1-0002 and HDTRA1-14-1-0064) supported this project. S.C. was
economically supported by a PhD fellowship from the Italian
Ministry of University and Research (MIUR).
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Communication ChemComm
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Microengine-assisted electrochemical measurements at printable sensor strips

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