Microengine-assisted electrochemical measurements at printable sensor strips

8668

Chem.Commun.,

2015,

, 8668--8671

This journal is

The Royal Society of Chemistry 2015

Cite this:

Chem.Commun.,

2015,

, 8668

Microengine-assisted electrochemical

measurements at printable sensor strips

†

Stefano Cinti,

Gabriela Valde

s-Ramı

rez,

Wei Gao,

Jinxing Li,

Giuseppe Palleschi*

and Joseph Wang*

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 widespre

ad replacement of conventional

(‘beaker-type’) electrochemical ce

lls 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,

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 indu

ce 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

Mgsurfacetoreducewater:

Mg(s) + 2H

O(aq)

-

Mg(OH)

(s) + H

(g).

Bubble-propelled microengines have been shown recently to

generate a dramatically enhanced fluid transport even when

confined into a stationary surface.

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 cholin

esterase (ChE) or hydrolysis of

OP nerve agents by organophosphorous hydrolase (OPH); however

these bio-approaches are characte

rized 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.

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 micro

engines is visibly capable to

degradeinashorttimeparaoxon,producing

-nitrophenol which is

electrochemically detectable. Th

esimpledesignandoperationmake

this approach a universal route for variety of electrochemical assays

of microliter samples at differe

nt sensor strips. The new concept

simplifies such measurements and can be readily extended to

different electrochemical sensing pl

atforms and target analytes, while

eliminating the need for additional stimuli, instrument or external

stirrer to homogenize ultrasmall samples during their analyses.

Department of Nanoengineering, University of California San Diego, La Jolla,

California 92093-0448, USA. E-mail: josephwang@ucsd.edu

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

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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,

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:

(aq) + H

O(aq)

-

1/2H

(g) + Mg

(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 actio

n 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 tran

sport 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 st

rip, 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

totheentiredrop(Fig.1).

Orozco

et al

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/

(

is the

distance from source), whereas, not anchored motors exert a

weaker convection field that decays quickly, 1/

. 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

-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

-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

the probe is only in

the presence of nerve agents (colorless) while at the end of the

process

the solution turns yellow, indicating degradation of

the starting OP compound to

-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,

-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. Pl

acing the microengines close to

the working electrode area could suppress the electrochemical

signal of interest. The microeng

ines were thus positioned as the

closest to the electrodes area where their presence does not affect

the electrochemical signal.

The electrochemical oxidation of

-nitrophenol at carbon

electrodes has been widely studied,

andweharnesstheMg-based

Janus microengines for chemical

hydrolysis of methyl paraoxon

and screen-printed electrodes fo

r the following electrochemical

detection of

-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

-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:

-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

-nitrophenol driven by microengines.

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Chem.Commun.,

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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 u

sing 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 th

at the current responses in the

presence of microengines are significantly higher than those

obtained using bare SPE. Fig. 5A illu

strates 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 (

B

15 fold) compared to electrodes without microengines.

Eight different microengines-modified SPEs were used for preci-

sion evaluation of the assay: as ill

ustrated 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

(

-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, u

p 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

-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 (a

) 0.1 M KCl solution without

microengines, (b

) 0.1 M KCl solution with microengines, (c

)20mM

paraoxon in 0.1 M KCl without microengines and (d

) 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 (

= 3); (B) Reproducibility of the response for 20 mM paraoxon

solution in KCl 0.1 M in presence of microengines (

= 8). Applied potential,

0.9 V (

vs.

Ag/AgCl).

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the hydrolysis (degradation) of OP compounds. The production

of an electroactive non-hazardous compound as

-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 H

solutions,

and used for the electrochemical measurements in different

environments.

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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