Highly effective and specific way for the trace analysis of carbaryl insecticides based on Au42Rh58 alloy nanocrystals

Article abstract of DOI:10.1039/c7ta01197k

The exploitation of Au_xRh_1?x alloy nanocrystals for specifically and directly identifying carbamate pesticides is reported for the first time and could be easily further developed to meet the specific needs of emergency intoxication cases. In this study, Au_xRh_1?x alloy nanocrystals with a tunable composition were synthesized and then employed for the electrocatalytic oxidation of carbamate pesticides. Compared with other Au_xRh_1?x samples and monometallic analogues, the Au₄₂Rh₅₈ sample with an optimized composition showed advances in terms of its electrocatalytic activity. According to electrochemical assessments, we discovered that the Au₄₂Rh₅₈ sample had specific responses to carbaryl. Typical common existing substances including glucide, amino acids, metal ions, and OP pesticides showed no interference in carbaryl determination. Moreover, Au₄₂Rh₅₈ nanocrystals displayed a considerable response towards 1 nM carbaryl without the assistance of acetylcholinesterase (AChE), cholinesterase (ChE) or other biochemical recognition elements. The observed high-efficiency detection was driven by electrocatalyzing the hydrolysis product of carbaryl in a strong alkaline solution. Au₄₂Rh₅₈ alloy nanocrystals with high catalytic activity and good selectivity are promising sensing materials for the detection of trace levels of carbaryl. Furthermore, Au₄₂Rh₅₈ alloy nanocrystals with uniform shape and narrow size distribution could be used in micro/nanoelectrode fabrication technology such as in screen-printing or ink-jetting or in the further design and implementation of real-time identification systems.

Full text of DOI:10.1039/c7ta01197k

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Zhang, J. Fang, P. Xu, J. Xu, X. Li, C. Liu and W. Wen, J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA01197K.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Highly Effective and Specific Way for Trace Analysis of Carbaryl
Insecticides Based on Au42Rh58 Alloy Nanocrystals
Wei Chen,a,† Yingying Liu,b,† Yuan Zhang,a,* Jianhui Fang,b Pengcheng Xu,c Jiaqiang Xu,b Xinxin Li,c
Chung-Chiun Liud and Weijia Wena,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The exploitation of AuxRh1-x alloy nanocrystals for specific and direct identifying carbarmate pesticides is firstly reported,
which can be easily developed and meet the specific purpose for emergency intoxication cases. In this study, AuxRh1-x alloy
nanocrystals with tunable composition is synthesized, and employed for electrocatalytic oxidation of carbamate
pesticides. Compared with other AuxRh1-x samples and monometallic analogues, Au42Rh58 sample with optimized
composition shows its advances in terms of electrocatalytic activity. According to electrochemical assessments, we
discover that Au42Rh58 sample has specific responses to carbaryl. Typical common existing substances, including glucide,
amino acids, metal ions, and OP pesticides show no interference in carbaryl determination. Moreover, Au42Rh58
nanocrystals display considerable response towards 1 nM carbaryl without the assistance of acetylcholinesterase (AChE),
cholinesterase (ChE) or other biochemical recognition elements. The observed high-efficiency detection is driven by
electrocatalyzing the hydrolysis product of carbaryl in strong alkaline solution. Au42Rh58 alloy nanocrystals with high
catalytic activity and good selectivity are promising sensing material for the detection of trace level of carbaryl.
Furthermore, Au42Rh58 alloy nanocrystals with uniform shape and narrow size distribution could be used for micro/nano
electrode fabrication technology such as screen-printing or ink-jetting, further design and implement real time
identification system.
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antagonist, atropine, and oximes such as pralidoxime (2-
PAM).3 However, oximes have been shown to be
contraindicated for poisoning by some carbamates.4 For
Introduction
Carbaryl insecticide (trade name Sevin), one of the carbamate
pesticides, has been extensively used in agriculture due to its
generalizability and its capabilities to control over 100 species
of insect on crops, on lawns, ornamental plants, trees, in
forestry and even on animals. It is also used as a molluscicide
and acaricide.1 The primary toxic mechanism of action for
carbaryl poisoning is based on the inhibition of cholinesterase
(ChE), which is also the mode of toxic action of other
carbamate and organophosphate (OP) pesticides.2 As such, it is
logical to assume that the currently administration-approved
therapies for OP poisoning could afford therapeutic protection
for carbamates intoxication. The clinical treatment of
intoxication with a ChE inhibitor is the use of muscarinic
example, previous studies have reported some evidence that
2-PAM in patients with certain carbamates poisoning is
deleterious, and more so with carbaryl poisoning specifically.5
Unfortunately, based on symptomology alone, it is difficult to
discern whether the observed cholinergic crisis is because of
carbamates or OP intoxication. Therefore, it is critical to
identify the intoxication from carbamate or OP pesticides. So
that, by positively identifying patients poisoned by which kind
of pesticides and exclusively giving prehospital treatment, lot
of emergency patients could be saved.
So far, a variety of detection methods have been developed
and used for analysis of pesticides residues in food and water.6
The most widely used detection approach for the
determination of pesticide residues is chromatographic
technique, such as gas chromatography-mass spectrometry
(GC-MS) and high-performance liquid chromatography-tandem
mass spectrometry (HPLC-MS).7 Although sensitive and well-
established, these methods are not suitable for continuous in
situ analysis and often require complicated operation and
expensive instrument. In addition, the polarity and thermal
instability of most carbamates make their sensitive and
accurate determination by traditional GC methods difficult.8
Biosensors are especially useful for the inexpensive on-line
monitoring and control. Enzymatic biosensors based on
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inhibition of ChE or acetylcholinesterase (AChE) activity, has results showed that Au42Rh58 nanocrystals exhibited specific
been studied intensively and developed for carbamate or OP electrocatalytic performance towards carbaryl pesticides.
pesticides analysis.9 As we mentioned above, it is difficult to
distinguish solely on the basis of inhibition of ChE or AChE
Experimental Section
activity between carbamate and OP pesticides. Moreover,
complicated handling procedures, vulnerability to
Synthesis of AuxRh1-x nanocrystals
contamination and the requirement of special care with
enzymatic activity make biosensor systems more difficult to
work with and less robust.10 Also, the challenges include trace
pesticide residuals, multiple interferences, variable
environmental conditions (e.g., temperature, pH and
pollutants), and the request for low-cost, highly portable and
disposable in nature, still need to be considered. Direct
electrocatalysis of analytes with a non-enzymatic sensor avoids
the limitations associated with use of enzyme. Up to now,
many non-enzymatic biosensors have been explored,
especially Pt-based amperometric glucose sensors.11 However,
the present biosensors for detecting pesticides usually relay on
covalent immobilization of ChE or AChE, and non-enzymatic
pesticide sensors were reported rarely.12 Thus, it is really
needed to develop a simple and efficient method to analyze
and quantitate pesticides residues at real time.
Au42Rh58 nanocrystals were prepared through co-reduction of
metal precursors in the presence of oleylamine. Firstly, 10 mL
of oleylamine was added to a single-neck flask (25 mL) in air
and heated up to 110 °C, stirring and refluxing for 10 min. Then,
0.042 g of HAuCl4·4H2O (10 mM) and 0.021 g of RhCl3 (10 mM)
were added into the above solution. After the reactants
dissolved completely, the mixture was heated up to 220 °C,
maintained for 10 min at this temperature. The mixture was
then continuously heated up to 240 °C maintaining for another
10 min. After that, black precipitate was collected by
centrifugation at 12,000 rpm for 10 min. The slurry was
washed with n-hexane and ethanol for several times and
placed into a vacuum oven at 60 °C overnight. AuxRh1-x
nanocrystals with other atomic ratio could be synthesized by
adjusting the stoichiometric ratio of the HAuCl4·4H2O and
RhCl3.
Great progress has recently been made in employing highly
reactive catalytic nanomaterials to bio or chemical analysis.13
Owing to their specific physicochemical properties, this class of
nanomaterials displays the ability to function in response to
diverse analytes. For example, Zhang et al. have developed an
electrochemical sensor based on NiO nanowalls which
possessed good selectivity to non-enzymatic detection of
glucose with a detection limit of 200 nM.14 An iron nitride
nanoparticles-encapsulated nitrogen-doped graphene (FeN
NPs/NG) core-shell hierarchical nanostructure demonstrated
direct and mediator-less responses to nicotinamide adenine
dinucleotide (NADH) oxidation at a low potential.15 Featured
with good catalytic properties, high conductivity and fast
electron transfer capability, metallic nanocrystals have great
potential as catalysts for directly detecting target substances
on electrode surface.16 Moreover, being a special kind of
metallic nanomaterials, bimetallic nanocrystal is an emerging
catalyst, and the research on bimetallic catalysts is still
challenging and needs further attention. Bimetallic
nanocrystals with the introduction of heteroatoms and
heteroatomic bond will affect the geometric and electronic
structure of bimetallic catalysts, may meet the dual
requirements of catalytic activity and selectivity.17
Sample characterization
The phase and crystal structure of Au42Rh58 nanocrystals were
measured through X-ray diffractometer (XRD) (Rigaku-D/MAX-
2550, Japan) with a Cu Kα radiation (λ=1.5418 Å) as X-ray
source. Diffraction data were collected at the scan rate of
5 °/min with the step width of 0.02° over 2θ ranging from 30°
to 90°. Transmission electron microscopy (TEM)
characterization was performed with a JEM-200CX operated at
120 kV. Element analysis mapping was obtained by scanning
transmission electron microscopy (STEM) combined with
energy dispersive X-ray spectrometry (EDXS), which were
performed in a JEOL 2010F microscope. The overall weight
percentage of Au and Rh in prepared samples was measured
by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES) (Perkin-Elmer, P400, USA). The hydrolysis products of
carbaryl were identified by using a GC-MS apparatus (Agilent
7890A-5975C).
Preparation of AuxRh1-x nanocrystals modified electrode
The prepared AuxRh1-x nanocrystals (5 mg) were re-dispersed
in ethanol to form a uniform suspension, and then mixed with
carbon powder (20 mg, Vulcan XC 72R, Cabot) by ultrasound
treatment for 2 hours. The mixture was heated at 60 °C until
dry completely. The obtained AuxRh1-x/C composite (8 mg) was
added into the mixture of ethanol (250 μL) and Nafion solution
(50 μL, 5 wt%) by 30 min sonication to form a homogeneous
ink. Then, 8 μL of the obtained ink was loaded onto a glassy
carbon electrode (GCE) and dried at room temperature
forming a uniform thin film.
Group 11 elements [copper (Cu), silver (Ag), and gold (Au)]
in periodic table, primarily Au, is usually used for redox
reactions, in addition to the great biocompatibility as well as
not easily to be corroded and oxidized, which made them
excellent probes for the construction of electrochemical
sensors.18 And rhodium (Rh), a powerful noble metal catalyst,
possesses the ability to catalyze
a
wide range of
Typical Experimental Process for Pesticides Detection
heterogeneous reactions.19 In this work, we therefore choose
Au and then introduce Rh heteroatoms to form a bimetallic
catalyst. The explored AuxRh1-x bimetallic system is used for
non-enzymatic electrocatalysis of different pesticides. The
The detection of pesticides was based on direct
electrochemical method. The electrochemical measurements
were conducted in a lab-made electrochemical cell, using
AuxRh1-x nanocrystals modified GCE as the working electrode,
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saturated calomel electrode (SCE) as the reference electrode homogeneous distribution of Au and Rh in this area. Fig. 1b
and platinum mesh (1 cm2) as the counter electrode. All the indicated that the size distribution of these nanocrystals was
measurements were conducted in N2 saturated 0.2 M KOH at estimated ranging from 6-12 nm, and concentrated on ~8 nm.
room temperature and ambient pressure. A flow of N2 was Fig. 1c showed the XRD pattern of Au42Rh58 nanocrystals,
maintained during electrochemical measurements to ensure which was used to identify the phase and crystal structure of
the N2 saturation state. For the detection of pesticides, both Au42Rh58 bimetallic nanocrystals. XRD analysis revealed the
OP and carbamate pesticide molecules were selected as high angle shift of peaks position compared with Au standard
detecting species.
pattern (JCPDS No. 040784) and low angle shift compared with
Rh standard pattern (JCPDS No. 050685). The XRD peaks shift
indicated the contraction of lattice constant with substitution
of smaller Rh atoms in Au crystal lattice. According to the
result of XRD characterization, an alloy phase of Au42Rh58
nanocrystals is obtained.
Results and discussion, Experimental
Characterization of prepared AuxRh1-x nanocatalysts
Atomic composition effect in AuxRh1-x nanocrystals
Fig. 1 Characterization of Au42Rh58nanocrystals. (a) TEM image as well as STEM
image and corresponding element maps showing the distribution of Au (red) and
Rh (green), (b) size distribution, (c) XRD pattern.
For bimetallic system, particle size, morphology and
composition etc. are found to be the main factors that affect
catalytic properties in different reactions. Especially, there are
several advantages of optimizing composition; it reduces
poisoning effect, and creates a new reaction pathway leading
to a distinct selectivity.20 Moreover, it might also generate a
synergistic catalytic effect which alters the electronic band
structure to modify the adsorption strength of surface
species.21 Herein, AuxRh1-x nanocrystals with different atomic
ratio were employed to explore their application in pesticides
detection. Firstly, the chemical compositions of different
AuxRh1-x samples were precisely measured by ICP-AES. The
analysis results showed that the atomic ratio of Au to Rh was
61:39, 42:58, 26:74 at%, respectively. These results were
almost consistent with initial stoichiometric ratio of
HAuCl4·4H2O and RhCl3, indicating that the composition of
AuxRh1-x samples was easily tuned. The morphology
characterization and detailed atomic distribution analysis were
conducted by TEM and selected-area elemental mapping.
According to the characterization results, AuxRh1-x nanocrystals
even with different composition displayed same shape and
size distribution. Here, Au42Rh58 sample was selected as an
example to elucidate the morphology and atomic distribution.
Fig. 1a showed the TEM image of Au42Rh58 sample, which
confirmed that the obtained nanocrystals were high quality
and with good dispersion. The STEM image, together with
selected-area element analysis maps of Au and Rh in Fig. 1a,
implied that both Au (red) and Rh (green) atoms were
distributed through the selected whole area, revealing a
Fig. 2 The current change signal of different AuxRh1-x samples, single metallic Au
and Rh nanocrystals as well as stoichiometric mixture of Au and Rh (42%Au &
58%Rh) nanocrystals towards 500 nM carbaryl.
In this work, the sensing of carbaryl is based on the
electrocatalytic capability of AuxRh1-x nanocrystals. Here, the
electrocatalytic oxidation current of different AuxRh1-x samples
as well as single metallic Au and Rh nanocrystals towards 500
nM carbaryl was measured. From Fig. 2, it can be seen that all
the AuxRh1-x samples has much higher responses than that of
Au and Rh nanocrystals. Among different AuxRh1-x samples,
Au42Rh58 displayed superior electrocatalytic activity to other
samples. Especially, the current response of Au42Rh58 sample
towards carbaryl was about six times higher than that of Au or
Rh. In addition, the stoichiometric mixture of Au and Rh
(42%Au & 58%Rh) nanocrystals was also included for the
comparison with Au42Rh58 sample. Considering from the
electrocatalytic current, Au42Rh58 sample exhibited more than
three times response against the stoichiometric mixture. As for
bimetallic nanocrystals, atomic composition is an important
factor affecting the electro-catalytic performance. With the
introduction of second metal, the electronic structure (the d-
band center) of original metal would be modified, which in
turn change the adsorption energies of reactive intermediates
and surface coverage by blocking species on bimetallic
catalysts.16 After adjusting the atomic ratio to optimum, the
adsorption energies of surface reactive species will reach to
optimal state, and further attain the maximum catalytic
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activity. Accordingly, the electrocatalytic capability of Au42Rh58 esters is well known as a moderately electron withdrawing
sample with optimized atomic composition was superior to groups, it is postulated that the detection of hydrolysis
that of monometallic and stoichiometric mixture counterparts, products may get higher signal levels than that of their parent
revealing synergistic effect generated in Au42Rh58 nanocrystals. compounds.
Fig.
3 General structure for carbamate pesticides. For carbaryl, R1=CH3, R2=H,
R3=naphthyl.
Detection of carbaryl based on Au42Rh58 nanocrystals
Carbaryl belongs to a class of pesticides known as carbamates,
which features the carbamate ester functional group
containing amino (usually -NH-) and carboxylic acid esters. The
general structure of carbamate pesticides was shown in Fig. 3.
It is known that carbamates are not persistent in the
environment. A water pH higher than 7 which creates alkaline
conditions can cause some carbamates to undergo
degradation or chemical breakdown, a process known as
hydrolysis. The hydrolysis half-life is mainly dependent on pH,
and varies from several seconds to several years.22
Scheme. 1 The detection of carbaryl based on Au42Rh58 nanocrystals.
Fig. 4 E1cB hydrolysis pathway for carbamate pesticides.
Fig. 5 GC-MS verification for the appearance of 1- naphthol after alkaline
hydrolysis. The inset shows the MS spectra of the produced 1- naphthol.
In general, the hydrolysis of carbamates in alkaline region
can be expressed schematically by Fig. 4, which followed
elimination unimolecular conjugate Base (E1cB) reaction
mechanism.23 Firstly, a base abstracts the proton from amino
group to generate a stabilized anion. The lone pair of electrons
on nitrogen atom then moves to the neighboring atom, thus
expelling the leaving group and forming double bond.
Therefrom it follows that the C-O bond is strongly split forming
R3O. In alkaline condition, electrocatalytic oxidation of
carbamates is actually occurred through their hydrolysis
product R3O.
In the case of carbaryl, the C-O cleavage leads to its
hydrolysis product 1-naphthol. As shown in Scheme 1, the
electrochemical detection of carbaryl was based on
electrocatalytic oxidation of its hydrolysis product 1-naphthol.
According to the previous reports, the hydrolysis half-life of
carbaryl at pH 9 is about 3.2 hours.25 And for every pH point
increase, the rate of hydrolysis will increase by approximately
10 times.22 Hence, the electrochemical detection of carbaryl
should be conducted under high pH value. Herein, KOH
concentration ranged from 0.1 to 0.5 M was chose for
evaluation. Fig. S1a showed the cyclic voltammograms (CV)
measurements of Au42Rh58 sample towards different KOH
solution. From the comparision, the current increased
correspondingly with the increase of KOH concentration from
0.1 M to 0.2 M, while the current response was reduced at a
higher KOH concentration (0.5 M). Carbaryl may be hydrolyzed
completely in 0.2 M KOH solution, and transferred to 1-
naphthol. The current signal decrease at 0.5 M KOH may be
ascribed to the corrosion of sensing materials. The
electrocatalytic oxidation measurement of 1-naphthol was also
Carbamates with amine group have the possibility to be
electro-oxidized because of the lone pair of electrons on
nitrogen atom, however, the presence of carboxylic acid ester
will provide opposite effect on the oxidation of amine group.
In the process of amine electrochemical oxidation, it usually
loses an electron firstly, thus forming radical cation.24 This step
is quite pivotal, which determines the whole electrochemical
oxidation dynamics. If there is electron withdrawing groups
coexisted in the amine compounds, the electrochemical
oxidation of amine would be suppressed. Carboxylic acid
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included for comparison. The current response curves of 500 nM carbaryl, it was observed that the current varied with
Au42Rh58 to 1-naphthol and carbaryl in 0.2 M KOH almost the increase of carbaryl concentration and reached a steady-
overlapped with each other (Fig. S1b). The phosphate buffered state current within 5s. From the magnified amperometric
saline solution (PBS buffer, pH=7.2) was chosen as a reference responses of low concentration of carbaryl (shown in inset of
to exam alkaline hydrolysis mechanism. From Fig. S1c, the Fig. 6a), we discovered that Au42Rh58 sample had considerable
electrochemical oxidation of carbaryl in PBS buffer (pH=7.2) response even towards 1 nM (0.2 μg/L) carbaryl. The detected
showed inconspicuous current signal. Compared with the lowest concentration is lower than the results reported in
considerable response got from alkaline solution (0.2 M KOH), literatures26,27 as well as the maximum permissible
the result obtained from PBS buffer (pH=7.2) further concentration of pesticide residues (0.1 mg/ L for individual
confirmed that it is effective to detect the alkaline hydrolysis and 0.5 mg/L for total according to the European Union
product of carbaryl. GC-MS characterization was used to verify directive on drinking water quality (98/83/EC)). The quick
the formation of 1-naphthol through carbaryl hydrolysis under response and considerable current responses signal indicated
0.2
M KOH. According to the chemical fingerprinting the effective electrocatalytic capability of Au42Rh58
information of GC-MS spectra in Fig. 5, the peak of 1-naphthol nanocrystals. The corresponding calibration curve of current
was clearly identified. These results indicated that 1-naphthol change versus carbaryl concentration was shown in Fig. 6b.
was produced via unimolecular elimination from carbaryl, and The linear fit of the current-concentration plots was Y=-
the electrochemical detection was based on electrocatalytic 3.35×10-7X-6.12×10-6, indicating a good linear relationship in
oxidation of 1-naphthol. Therefore, all the following detections the range of 1 nM to 500 nM carbaryl (R2=0.994, with
were accordingly carried out in 0.2 M KOH solution.
sensitivity of 1.71 A·M-1·cm-2).
For the analysis of pesticides residues, acetonitrile is an
effective solvent for the extraction and preconcentration of
pesticides. If the detection is carried out in electrochemical
environment, it must rule out that acetonitrile would not be
electrocatalytic oxidized or reduced. Fig. S1d showed the CV
curve of Au42Rh58 sample towards acetonitrile. From the
comparison with background curve, it can be seen that there is
no current change when acetonitrile was injected into KOH
supporting electrolyte. As a control, the CV curve of Au42Rh58
sample towards carbaryl with low concentration under the
same conditions was also checked, and obvious current change
For the sensing materials, it is essential to estimate its anti-
interference capability upon exposure to coexisting species in
the analyte except for high sensitive to the output parameter
measured. Since this work is aimed at developing a new
strategy for analysis pesticide residues, here we investigated
the effect of some common existing substances as potential
interferences, including glucide, amino acids, metal ions, and
OP pesticides. As shown in Fig. 6c, glucide, arginine and proline
only produced slight responses even at the concentration of
0.015 mM (30 times higher than that of carbaryl). Typical
signal was observed. Thus, it can be deduced that the usage of metal ions, OP pesticides, glycine, serine and aspartic acid
acetonitrile as solvent does not have interference for
pesticides detection.
showed no interference in carbaryl determination. These
results clearly demonstrated that such a high specificity for
carbamate pesticides detection was obtained over Au42Rh58
nanocrystals, without using of ChE, AChE, other mediators or
permselective membranes. Therefore, Au42Rh58 nanocrystals
could be used to develop high efficient electrochemical sensor
for pesticide residues monitoring.
Table S1 summarized the sensing performance for the
detection of carbaryl recently reported by other researchers.
Various kinds of sensing materials and techniques were used
for carbaryl biosensors fabrication. For example, Au
nanoparticle superlattices were used as an active substrate for
surface enhanced Raman scattering (SERS) that quantitatively
detected carbaryl with a limit of detection (LOD) of 1 ppm (∼5
μM).26 A modified QuEChERS (quick, easy, cheap, effective,
rugged, and safe) method and cloud point extraction
combined with visible spectrophotometric analysis was used
for carbaryl residue analysis in vegetables with a LOD of 0.1
mg/kg (∼
0.5 μM).27 In these works, the LOD is much higher
Fig. 6 (a) Current responses of Au42Rh58 sample upon step-wise addition of
carbaryl from 1 nM to 500 nM, (b) calibration curve of current change versus
carbaryl concentration, (c) anti-interference capability of Au42Rh58 sample (the
concentrtion of glucide, amino acids, and metal ions is 0.015 mM, the
concentration of carbamate and OP pesticides is 500 nM).
than that of the result obtained in this work. Furthermore,
most of carbaryl sensors were based on the inhibition of AChE
activity or other bio-recognizers.28-31 In that way, it is difficult
to distinguish with OP pesticides. Some drugs treatment, such
as 2-PAM is work for OP pesticides intoxication, but is
deleterious for patients with certain carbamates poisoning,
and more so with carbaryl poisoning specifically. It is critical to
identify the intoxication from carbamate or OP pesticides. In
Chronoamperometry was applied to evaluate the sensing
performance of Au42Rh58 sample towards carbaryl. Fig. 6a
presented the current responses of Au42Rh58 sample upon
step-wise addition of carbaryl. With the injection of 1 nM up to
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this work, Au42Rh58 alloy nanocrystals have shown the ability (shown in Table 1, Entry V). The electron-withdrawing group of
to perform high sensitivity, low detection limit as well as good carboxylic acid ester in methyl carbamate may dominate the
anti-interference property.
electrocatalytic reaction, and thus suppress hydrolysis and
following electrocatalytic reaction. While carbaryl with
naphthalene group was easily underwent electrocatalytic
oxidization than that of other four carbamates with phenyl
group (shown in Table 1, Entry VI). It has been suggested that
compounds with aromatic ring is favorable for electrocatalytic
oxidation and give rise to stable intermediates due to the
delocalization of the charge. Furthermore, naphthalene is
more reactive in many reactions compared with phenyl, which
makes carbaryl susceptible to be electrocatalyzed. These
results that have been obtained make it possible to detect
carbamate pesticides, especially carbaryl, with high specific
activity.
Discussion of sensing responses to other carbamates
The developed electrocatalytic system was effective not only
for carbaryl but also for other carbamate pesticides. As shown
the responses of Au42Rh58 to carbamate pesticides (Fig. 6c), it
was found that Au42Rh58 nanocrystals also exhibited mild
responses to isoprocarb, methiocarb, carbofuran and propoxur.
For most of carbamate pesticides, their structures exhibit the
characteristic properties of phenyl group, and having either
methylthio, dimethylamino or other substituents on the
aromatic ring system. As we mentioned before, carbamates
are easily hydrolyzed in strong alkaline solutions, and generate
R3O products. In order to gather further knowledge about the
electrocatalytic capability towards carbamates, we followed in
this work the electrocatalytic effect towards four substituted
phenyl N-methylcarbamates I - IV. Different substituent groups
on aromatic moieties may have an influence on the hydrolysis
dynamics, and further impact the following electrochemical
oxidation process.
Conclusions
In summary, AuxRh1-x alloy nanocrystals were successfully
prepared, and used for direct electrocatalysis of carbamate
pesticides. The electrochemical measurements indicated that
Au42Rh58 nanocrystals exhibited enhanced electrocatalytic
performance towards carbaryl, compared with monometallic
analogues (Au and Rh) and its stoichiometric mixture of Au and
Rh (42%Au & 58%Rh) nanocrystals. More specifically, Au42Rh58
nanocrystals showed good electrocatalytic responses towards
low concentration of carbaryl, even to the concentration of 1
nM. The ultra-sensitive and specific detection of carbaryl was
attributed to its hydrolysis in strong alkaline solution as well as
the following direct electrocatalysis of Au42Rh58 nanocrystals to
its hydrolysis product 1- naphthol. These results demonstrated
that the Au42Rh58 nanocrystals with high catalytic activity and
good specificity were a promising sensing material for the
detection of carbamate pesticides, especially carbaryl.
Furthermore, the exploited Au42Rh58 nanocrystals without the
assistance of ChE, AChE or other biochemical recognition
elements could simplify the design and fabrication of
micro/nanoelectrode biosensors by using screen-printing or
ink-jetting technology.
Table 1 Electrocatalytic oxidation of different class of carbamates over the Au42Rh48
nanocrystals.
Electro-
Electro-
Entry
Substrate catalytic Entry Substrate catalytic
signal
0.53
signal
I
IV
0.18
II
0.38
0.37
V
0
Acknowledgements
This research is supported by MOST of China
(2016YFA0200803), NSF of China (91323304, 61604163,
61527818), NSF of Shanghai (15ZR1447300) and Shanghai
Municipal Science and Technology Commission (14DZ2261200,
15DZ2260300). Y. Zhang developed the specific and direct
identifying carbarmate pesticides approach. Y. Zhang and W.
Wen undertook the experimental design and interpretation of
the findings. W. Chen and Y. Liu carried out the experiments. J.
Fang, P. Xu, J. Xu, X. Li and C. C. Liu provided advice on
experimental detection. Y. Zhang wrote the paper.
III
VI
2.5
Table 1 summarized the structures and corresponding
electrochemical oxidation current signal of different
carbamates. As listed in Table 1 (entries I to IV), the oxidation
of methiocarb, carbofuran and propoxur with strongly electron
donating group on phenyl ring was more efficient than that of
isoprocarb with weakly electron donating substituent on the
ring. It should be mentioned that methyl carbamate with
amine group did not produce any oxidation current signal
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