Microengine-assisted electrochemical measurements at printable sensor strips

Article abstract of DOI:10.1039/c5cc02222c

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.

Full text of DOI:10.1039/c5cc02222c

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Microengine-assisted electrochemical
measurements at printable sensor strips†
Cite this: Chem. Commun., 2015,
1, 8668
5
ab
a
a
a
Stefano Cinti, Gabriela Valdes-Ramırez, Wei Gao, Jinxing Li,
´
´
b
a
Received 16th March 2015,
Accepted 16th April 2015
Giuseppe Palleschi* and Joseph Wang*
DOI: 10.1039/c5cc02222c
www.rsc.org/chemcomm
6
A new microengine-based built-in-platform exploiting a surprising dual confined into a stationary surface. Such fluid convection from
action with solution mixing and control of the reaction parameters, has the bubbles generating PANI/Pt tubular microengines has been
been applied for accelerating chemical reactions (organophosphorous demonstrated using the displacement of passive microparticle
nerve agents hydrolysis) and electrochemical detection of non- tracers. The fluid transport induced by water-driven Mg micro-
hazardous by-product (p-nitrophenol) using printable sensor strip. motors is shown here for a dramatic mixing effect for the
electroanalysis of microliter samples at planar strip electrodes.
The growing exploitation of advanced screen-printing fabrication Such unique coupling of microengines and sensor strips is
techniques allows for the widespread replacement of conventional demonstrated toward the amperometric measurements of OP
(
‘beaker-type’) electrochemical cells and bulky electrodes with easy- nerve agent degradation.
1
,2
to-use sensor strips. Such printable sensors permit simple,
OP chemical warfare agents and pesticides interrupt the
rapid, centralized and decentralized electrochemical analyses nervous stimuli communication by blocking the enzyme acetyl-
7,8
of microliter volumes. Yet, unlike traditional large cells that cholinesterase (AChE). OP electrochemical detection relies on
employ solution stirring or electrode rotation for enhancing the the enzymatic inhibition of cholinesterase (ChE) or hydrolysis of
3
response via convective mass transport, microcells based on OP nerve agents by organophosphorous hydrolase (OPH); however
printable strips are limited to quiescent solutions and hence these bio-approaches are characterized by long incubation times,
9
–11
solely to diffusion transport.
enzymes stabilization and enzymes engineering.
Recent advances in chemically-powered microscale motors
Here, we describe a new concept of microengine-assisted
1
2–14
electrochemical measurements using strip-based micro-volume have led to new capabilities and novel applications,
including
electroanalysis. We demonstrate the dramatic enhanced ampero- new biosensing strategies based on the direct isolation of target or
15–19
metric signal of organophosphorous (OP) nerve agent detection in changes in their movement in the presence of target analytes.
For
the presence of bubble-generating magnesium Janus microengines. example the motion of PANI/Pt micromotors has shown to enhance
20
The generated microbubbles induce localized convection and a colorimetric immunoassays. Here we exploit for the first time the
greatly enhanced mass transport and sensitivity. dual action of water-powered magnesium-based microengines, not
4
,5
As recently reported, the hydrogen-bubble generation from only to impart an enhanced fluid mass transport, but also to increase
magnesium-based microengines involves the oxidation of the dramatically the pH solution allowing a quick degradation and
Mg surface to reduce water:
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.
Mg(s) + 2H O(aq) - Mg(OH) (s) + H (g).
2
2
2
Bubble-propelled microengines have been shown recently to
generate a dramatically enhanced fluid transport even when
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:
0.1039/c5cc02222c
1
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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
4
layer, 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:
+
2+
ꢀ
Mg (aq) + H O(aq) - 1/2H (g) + Mg (aq) + OH (aq).
2
2
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 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
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
p-nitrophenol driven by microengines.
mixing’ and mass transport effects within the microliter sample
droplet, thus allowing a rapid and reproducible nerve-agent microengines. It is clearly visible that in the absence of the Mg
degradation with a subsequent detection without the need of microengines the solution remains colorless with the time
external stirrers or mixing devices. The presence of the nickel layer demonstrating absence of hydrolysis of paraoxon. Meanwhile,
in the Janus microparticles allows the magnetic confinement of in the presence of Mg Janus particles, paraoxon degrades
the microengines on the sensor strip, preventing their movement immediately and, after a minute of reaction the solution is
to the electrodes area avoiding interference during the electro- almost entirely yellow due to the produced p-nitrophenol and
chemical measurements. An effective fluid transport is achieved the mixing effect which is visible to naked eye; Fig. 2B shows a
in the presence of the generated bubbles, leading to a prolonged schematic representation of the electrode before and after the
coherent flow near their anchor points and dramatically enhan- effect of microengine tasks. At time zero t
cing mass transport in their vicinity. The rising of a vertical the presence of nerve agents (colorless) while at the end of the
bubbles flow is capable to impart effective convective transport process t the solution turns yellow, indicating degradation of
0
the probe is only in
i
to the entire drop (Fig. 1).
the starting OP compound to p-nitrophenol.
6
Orozco et al., demonstrated how the fluid transport induced
The reactions involved in this Mg Janus/OP nerve agents
by the rising bubbles is larger than that one induced by the freely system are represented in Fig. 3.
swimming catalytic micromotors or Janus-microparticles: fixed
Owing to the vigorous production of hydrogen bubbles, the
motors produce a net force on the fluid depending by 1/r (r is the alkaline hydrolyzed by-product, p-nitrophenol, is transported
distance from source), whereas, not anchored motors exert a throughout the solution allowing the development of a homo-
2
weaker convection field that decays quickly, 1/r . We use micro- geneous solution that is therefore measurable accurately without
ꢀ
engines capable of producing OH ions to increase the medium external stirring. Due to the presence of the magnet located below
pH and consequently promote the degradation of paraoxon into the strip, the Janus microengines remain confined far from the
a readily detectable p-nitrophenol. Fig. 2A shows optical images area of the working electrode. Placing the microengines close to
of a 20 mM paraoxon solution (in 0.1 M KCl) on the sensor the working electrode area could suppress the electrochemical
strip, taken at different times in the absence and presence of 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
21
electrodes has been widely studied, 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.
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Fig. 3 Mechanism and reactions involved in the OP nerve agents degra-
dation to p-nitrophenol accelerated by 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).
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
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 observed in the presence of Janus microengines is significantly
0
.1 M KCl without microengines and (d) 20 mM paraoxon in 0.1 M KCl with larger (B15 fold) compared to electrodes without microengines.
0
microengines. Inset: amperograms of (a ) 0.1 M KCl solution without
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
0
0
microengines, (b ) 0.1 M KCl solution with microengines, (c ) 20 mM
0
paraoxon in 0.1 M KCl without microengines and (d ) 20 mM paraoxon
in 0.1 M KCl with microengines.
(
RSD o 5%) indicating high reproducibility for the microengine-
induced high performing mixing process towards paraoxon
same pH. The intensity as well as peak position were similar degradation and subsequent detection.
demonstrating not only that after 4 minutes of chemical reaction the
In conclusion, we have demonstrated the use of bubble-
paraoxon has been hydrolyzed with a high yield, (Fig. S2, ESI†). The generating microengines for generating a greatly enhanced fluid
bubbles produced by microengines are capable to produce strong transport during amperometric measurements at common printable
mixing effect generating a homogeneous solution, making attractive sensor strips. The presence of magnesium Janus microengines
the proposed coupled platform. By using similar criteria as for the along with OPH makes possible the rapid selective conversion
cyclic voltammetry measurements, amperometric experiments were of a non-detectable species (paraoxon) to an electroactive compound
carried out applying 0.9 V (vs. Ag/AgCl) constant potential during 1 ( p-nitrophenol) easily detectable with an electrochemical strip
min. Inset of Fig. 4 displays that the current responses in the platform. Such operation addresses the challenge of imparting
presence of microengines are significantly higher than those effective convective transport using microliter volumes strip-based
obtained using bare SPE. Fig. 5A illustrates the well-defined ampero- measurements. The microengines operation leads to increase the
metric response to different nerve agent concentrations up to pH of the solution and a built-in stirring increasing sensitivity
20 mM. The corresponding inset displays the corresponding towards paraoxon detection, up to 15-fold, compared to a non-
calibration plots and the enhanced sensitivity associated with engineered bare screen-printed electrode. The magnetically-
the presence of the Mg-Janus microengines.
anchored microengines can serve as an ‘artificial’ enzyme toward
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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
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