Single-Component Chemical Nose with a Hemicyanine Probe for Pattern-Based Discrimination of Metal Ions?

Article abstract of DOI:10.1002/cjoc.202100013

We report here a chemical nose utilizing the nonspecificity of a hemicyanine dye (probe P) containing three acetates and one deprotonatable phenol groups. Unlike conventional pattern-based recognition that requires a combination of different probes, probe P alone is able to differentiate 9 different metals, generating distinctive absorption spectra and various patterns upon principle component analysis (PCA). River water samples and commercial mineral water samples were evaluated by the probe and were successfully distinguished.

Full text of DOI:10.1002/cjoc.202100013

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For published articles: https://onlinelibrary.wiley.com/journal/16147065
中
国 化 学
Cite this paper: Chin. J. Chem. 2021, 39, 1517—1522. DOI: 10.1002/cjoc.202100013
Single-Component Chemical Nose with a Hemicyanine Probe for
Pattern-Based Discrimination of Metal Ions†
Jingying Zhai,a Yaotian Wu,b and Xiaojiang Xie*,b
a Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China
b Department of Chemistry, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China
Keywords
Chemical nose | Cations | Sensors | UV/Vis spectroscopy| Principle component analysis
Main observation and conclusion
We report here a chemical nose utilizing the nonspecificity of a hemicyanine dye (probe P) containing three acetates and one depro-
tonatable phenol groups. Unlike conventional pattern-based recognition that requires a combination of different probes, probe P
alone is able to differentiate 9 different metals, generating distinctive absorption spectra and various patterns upon principle com-
ponent analysis (PCA). River water samples and commercial mineral water samples were evaluated by the probe and were success-
fully distinguished.
Comprehensive Graphic Content
*E-mail: xiexj@sustech.edu.cn
View HTML Article
Supporting Information
† Dedicated to Department of Chemistry, SUSTech, on the Occasion of Her
10th Anniversary.
Chin. J. Chem. 2021, 39, 1517—1522
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Zhai, Wu & Xie
Concise Report
crimination of ions is achieved through embedding the probe in a
complex environment. The probe itself is no doubt the key to
maximize the capacity for discrimination. Here, we present a col-
orimetric chemical nose sensor with only one optical sensing el-
ement probe P. The probe contains a hemicyanine chromophore
and multiple acetates and hydroxyl binding moieties, which gen-
erate information-rich absorption spectra during exposing to met-
al ions. The hydrophilic binding moieties also make this probe
water soluble without needing embed into membrane, hydrogel
or surfactants. By analysis of the absorbance data with principle
component analysis (PCA) method, the probe is able to distinguish
9 metal ions, including Ca2+, Co2+, Cu2+, Fe3+, Zn2+, Ag+, Ni2+, Cd2+,
and Pb2+. For practical applications, river samples were analyzed
and 15 different brands of driking water were satisfactorily diffr-
entiated with probe P.
Background and Originality Content
Metal ions are ubiquitous in our daily life. They are crucial to
human health and natural environment. Alkali metal ions such as
Na+ and K+ maintain the water and electrolytes balance in our
body.[1-2] Alkaline earth metal ions such as Ca2+ and Mg2+ partici-
pate in skeletal development, nerve conduction, enzyme catalysis
as well as gene transcription.[3-6] Fe2+, a central component of
hemoglobin, takes part in the gas transport in blood.[7] As for
transitional metal ions (also referred to as heavy metal ions), a
slight elevation in their concentration suffices to cause serious
problems to the health of human beings as well as various eco-
logical systems.[8-9] Considering the important roles of metal ions,
analytical tools are required so that ion concentrations could be
readily measured. A number of analytical instruments serve for
such a purpose, including ion chromatography, ion-selective elec-
trodes, atomic spectroscopy, mass spectroscopy, electrophoresis,
and so on.[10-15] However, these instruments are cumbersome and
require time-consuming sample preparation and pretreatment. To
meet the escalating challenges in modern analytical chemistry,
new methods suitable for real time and in-situ measurements are
in urgent demand.
Synthetic probes are a family of chemosensors able to detect
a range of analytes, including small molecules and ions.[16-20] Col-
orimetry refers to the color changes that occur when the probe
interacts with the analyte while fluorescence is the emission of
light from the probe after it is brought to the excited state by light
illumination. On the one hand, a number of synthetic probes have
been reported with excellent selectivity to a certain target ion. A
good selectivity allows the probe to detect the analyte in the
presence of interfering species. On the other hand, the discovery
of highly selective probes is usually a trial-and-error process, ac-
companied by countless unqualified products (typically regarded
as failures) with non-specificity. Recently, scientists showed that
such non-specific probes are after all not useless. On the contrary,
when properly assembled, they could function resembling the
olfactory systems of animals, leading to a unique analytical plat-
form coined “chemical nose”.[21-23]
A chemical nose usually combines sensor arrays with statisti-
cal data analysis methods to qualitatively and quantitatively de-
tect multiple analytes.[24-28] Optical probes are widely used to
form these sensor arrays and generate information-rich cross-
responsive signals. By taking advantages of the well-established
statistical methods such as principal component analysis (PCA)
and hierarchical clustering analysis (HCA), the optical signals could
be converted to more discriminate patterns.[29] These characteris-
tics make chemical nose capable of identification of various metal
ions even with small change in complex samples. Chang and
coworkers fabricate a sensors array composed of 5 fluorescent
probes that can distinguish 7 heavy metal ions.[30] Wolfbeis and
coworkers made use of 8 different unselective indicators to form a
sensor array to discriminate 5 metal ions.[31] Chemical noses offer
a good opportunity to discriminate different metal ions. However,
the more the sensor elements, the more complex for the data
acquisition and analysis. Some researchers make an effort to de-
sign sensor arrays with high separation ability but having minimal
number of sensor elements. Recently, Pavel Anzenbacher and
co-workers successfully reported a sensor assay with only one
sensing element, a probe 8-hydroxyquinoline attached with con-
jugated fluorophore, which could discriminate 10 metal ions.[32]
Ding and coworkers reported a chemical nose using a surfactant
assembled bis(2-picolyl)amine-modified pyrene derivative as the
only sensing element, which could well differentiate 13 metal ions
and different brands of drinking water.[33] Du and coworkers re-
ported a 1-(2-pyridinylazo)-2-naphthaleno (PAN)- and bromothy-
mol blue (BTB)-functionalized microgel sensor array for colorimet-
ric detection of 10 metal ions.[34] However, in these reports dis-
Results and Discussion
Scheme 1 shows the synthetic route of the water soluble col-
1
orimetric probe P and the structure was confirmed with H NMR
and 13C NMR (Figures S1 to S4). Probe P was designed with three
acetates and one hydroxyl binding arms, which combine with a
hemicyanine chromophore. The multiple binding arms are not
specific for metal ions (similar with EDTA) and offer more possibil-
ity to generate numerous changes in optical signals when it binds
with different meal ions. Also, the hydrophilic binding arms make
this probe water soluble. These properties make probe P a good
candidate as a chemical nose with simple sensing element. Bind-
ing with different cations will cause the deprotonation of the
phenol group with different intramolecular charge transfer effect
and thus generate optical spectroscopic changes.
The absorption spectra of probe P with different metal ions
are shown in Figure 1a, including Ca2+, Ag+, Cu2+, Co2+, Ni2+, Cd2+,
Zn2+, Pb2+ and Fe3+. The experiments were performed at fixed pH
(6.5) to avoid the pH interference. The absorption spectra of
probe P are all red shift with diverse change in curve shape and
absorbance (Figure 1a). At the same time, the color changes of
probe P can be visualized by the naked eye from yellow (proto-
nated form) to reddish (deprotonated form). Compared with dif-
ferent metal ions, the probe P shows distinct response with each
other which are not specific to just one metal ion but multiple
metal ions. This property makes probe P a promising chemical
nose to identify various metal ions at the same time. For better
visualization, the results were analyzed through principal compo-
nent analysis (PCA). PCA is a statistical method able to reduce the
data dimensions and find the most distinguishable components
for differentiation. Usually, the largest degree principle compo-
nent (PC1) of data variance and the second largest principle com-
ponent (PC2) were used to plot the pattern 2-dimensionally. As
shown in Figure 1b, the PC1 and PC2 were obtained based on
Figure 1a and all of the 9 metal ions are satisfactorily distin-
guished.
Scheme 1 Synthetic route for the probe P in this work
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Table 1 The complex formation constants Kf for probe P with different
metal ions
Metal ion
lg Kf
Pb2+
Ca2+
Cd2+
Co2+
Zn2+
Ag+
Cu2+
Ni2+
Fe3+
6.15
3.59
4.25
3.43
4.50
2.82
5.23
4.24
4.40
results of Kf showed that probe P has satisfactory binding affinity
and distinguishability to different metal ions. The affinity exhibit-
ed an order of Pb2+>Cu2+>Zn2+>Fe3+>Cd2+>Ni2+>Ca2+>Co2+>Ag+.
Other common cations such as Na+, K+, and Mg2+ were also tested
up to 10 mmol/L, but they did not result in any spectral change.
The stoichiometry between probe P and the metal ion was
obtained by the Job’s method. In this method, the total concen-
tration of probe and metal ion is fixed, but the mole fractions are
varied. The maximum position (intercept of the two linear fittings)
corresponds to the stoichiometry of the two species. Figure S6
demonstrates the Job’s plot for probe P and different metal ions
by plotting the absorbance with respect to the mole fraction of
the two components. Except for Fe3+ and Ag+, the maxima posi-
tion of Pb2+, Cu2+, Cd2+, Ni2+, Co2+, Zn2+, and Ca2+ are around at 0.5,
which indicates the 1 to 1 stoichiometry for probe P and metal
ions. The Job’s plot curves are abnormal for Fe3+ and Ag+ without
showing maximum points. This might be due to the existence of
ferric hydroxide at pH 6.5 and the detection limit of silver ions is
quite high and beyond the normal association constant range.
To further quantitatively evaluate the response of probe P to
metal ions, six metal ions were selected for the titration experi-
ments. This is because probe P is relatively not very sensitive to
Ag+ and Ni2+, and the concentration of Cd2+ in the real sample to
be analyzed is expected very low. The absorption spectra were
recorded with different ion concentrations during the performance
of titration experiments as shown in Figure 2. Notably, the initial
spectra without addition of metal ions are all the same, with
Figure 1 (a) UV-Vis spectra of probe P with 200 µmol/L of metal ions in 5
mmol/L pH 6.5 MES-NaOH buffer solutions. (b) The corresponding PCA
plot of Figure 1a. The concentration of P is kept at 3.33 µmol/L.
For metal ion detection, the affinity between the probe and
the target ions is very important. The complex formation con-
stants (Kf) for probe P and different metal ions were determined
and summarized in Table 1. The constants were obtained through
the calibration curves of probe P with different metal ions (Figure
S5). The absorbance signals were recorded in MES-NaOH buffer
solutions (pH 6.5, 5 mmol/L) and fixed P concentration (3.33×10–6
mol/L). The absorbance signals at 520 nm gradually increase with
continuous adding of metal ions. Theoretical curves were used to
fit the data to obtain the constants Kf (shown in Figure S5). The
Figure 2 Absorbance response of probe P to Ca2+, Cu2+, Fe3+, Pb2+, Zn2+ and Co2+ with indicated concentrations in 5 mmol/L pH 6.5 MES-NaOH buffer
solution. The concentration of P is kept at 3.33 µmol/L.
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Zhai, Wu & Xie
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strong absorbance around 420 nm. With increasing metal ions,
the spectra changed to different shapes by gradually decreasing at
420 nm and increasing at longer wavelength. The probe exhibited
submicromolar detection limits depend on the nature of the ions.
The response of probe P to various concentrations of metal
ions was also visualized with PCA in Figure 3a. The response
started all from the same point representing P in the blank and
dispersed to various directions. From the enlarged graph (Figure
3b), we could also visualize the separation of six metal ions at
micromolar concentrations.
Figure 4 PCA diagram analysis of the river samples through calibration
curves. Fiver river samples obtained from the same place at Dasha River,
Shenzhen during different days. All the metal ions and samples were
prepared with 5 mmol/L pH 6.5 MES-NaOH buffer solutions. Ca2+: 1, 5, 10,
20, 30, 50, 100 μmol/L; Co2+: 1, 5, 10, 20, 30, 50, 100 μmol/L; Cu2+:1, 5, 10,
20, 30 μmol/L; Fe3+: 1, 50, 100 μmol/L; Zn2+: 10, 20, 30, 50, 100 μmol/L;
Pb2+: 1, 1.5, 2, 3, 5 μmol/L.
Table 2 Ion chromatograph results of the river sample
Metal ions
Concentration/(mg·L–1)
Na+
Mg2+
Ca2+
1.556
0.228
3.247
of mineral water including 1 tap water sample obtained directly
from the lab. PCA analyses of the water samples are shown in
Figure 5. The experiments were easily performed by dissolving P
in different water samples without dilution or pretreatment. The
spectra were analyzed with the abovementioned PCA method and
the results showed that all the 15 kinds of drink water could be
well separated. Distinguishing the samples was successful mainly
due to the various concentrations of Ca2+ and pH, providing a two
co-ordination for each sample. Since Ca2+ and pH are very im-
portant parameters for commercial mineral water, we believe that
this simple chemical nose could potentially serve as an easy-to-
use quality control reagent.
Figure 3 (a) The calibration curves of probe P with various metal ions in
5 mmol/L pH=6.5 MES-NaOH buffer solutions by using PCA method to
analyze the absorption spectra. (b) The corresponding enlarged graph. The
concentrations of different metal ions are as follows: Ca2+: 1, 5, 10, 20, 30,
50, 100 μmol/L; Co2+: 1, 5, 10, 20, 30, 50, 100 μmol/L; Cu2+:1, 5, 10, 20, 30
μmol/L; Fe3+: 1, 50, 100 μmol/L; Zn2+: 10, 20, 30, 50, 100 μmol/L; Pb2+: 1,
1.5, 2, 3, 5 μmol/L.
To evaluate the ability of probe P to detect metal ions in real
samples, five river samples from Dasha River, Shenzhen were
gathered during different days and mixed with probe P in 5
mmol/L MES-NaOH pH=6.5 buffer solution. The absorbance sig-
nals were recorded and processed with the same principal com-
ponents (PCs) vectors as calibration curves shown in Figure 4. All
river water samples were located on the direction of Ca2+, impli-
cating a high level of Ca2+ in the river water samples. Notice that
the river water samples were collected at different time, which
explains why the points are slightly dispersed in Figure 4. We then
analyzed the samples by ion chromatography (IC) and atomic ab-
sorption spectroscopy (AAS). According to the IC data shown in
Table 2, the major cations in river sample are Na+, Mg2+ and Ca2+.
Other transition metal ions were found below the detection limit
of IC, while the AAS showed extremely low concentration of Pb2+
below the detection limit of AAS (ca. 1 mg/L). Therefore, the con-
tribution from the transition metal ions in the river water is con-
sidered negligible. Since probe P is not sensitive to Na+ and the
level of heavy metal ions is low in the river samples, PCA of the
sample should indeed locate on the calcium line.
Figure 5 The PCA diagram of probe P in response to 15 brands of
drinking water.
Conclusions
In this article, we reported a colorimetric probe P with the
ability to generate information-rich spectra upon binding with
various metal ions. The nonspecific binding enabled the probe as
a chemical nose to differentiate 9 metal ions including Ca2+, Ag+,
Cu2+, Co2+, Ni2+, Cd2+, Zn2+, Pb2+ and Fe3+. PCA was engaged to
provide clear visualization of the detection. Analyzing real water
samples with the probe led to the finding of high level of Ca2+
Moreover, probe P was utilized to differentiate various brands
of commercial drinking water. We purchased 14 different brands
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Pattern-Based Discrimination of Metal Ions
Chin. J. Chem.
which agreed well with IC and AAS. 14 different commercial min-
eral water and tap water were also successfully distinguished.
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Experimental
Reagents. Cd(NO3)2·4H2O, Ni(NO3)2·6H2O, CuCl2, ZnCl2,
FeCl3·6H2O, MgCl2·6H2O, CoCl2·6H2O, Pb(NO3)2, CaCl2·6H2O, AgNO3,
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3-ethyl-2-methylbenzothiazolium iodide, benzyl chloride, 3,4,5-tri-
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Instrumentation and measurements. Ultraviolet−visible
(UV−vis) absorption spectra were measured using an absorption
spectrometer (Evolution 220, Thermo Fisher Scientific). Ion chro-
matography was measured on the model Eco IC with Metrosep C
6 – 150/4.0 from Metrohm. Atomic spectra were measured by
atomic absorption spectrometer (AA400, Perkin Elmer). All solu-
tions except those specifically mentioned were prepared by dis-
solving appropriate salts into deionized water purified by Milli-Q
Integral 5.
Identification of stoichiometry between probe P and metal
ions by Job’s Plot approach. 100 μmol/L P and 100 μmol/L metal
ions were prepared for Job’s Plot experiment by using 5 mmol/L
MES-NaOH buffer solution (pH = 6.5). The Job’s Plot experiment
was performed by mixing P and metal ions. The mole fractions of
P were varied from 0 to 1 with fixed total concentrations of P and
metal ion. The corresponding UV-Vis absorption spectra were
recorded and the absorbance at 520 nm was used for analysis of
the metal-to-P stoichiometry.
Measurement of complex formation constants between
probe P and metal ions. The complex formation constants were
obtained through the calibration curves of metal ions with probe
P in absorbance mode. The experiments were performed under
fixed P concentration (3.33×10–6 mol/L) with different metal ion
concentration at pH 6.5 (5 mmol/L MES-NaOH buffer solution).
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different brand drinking water was mixed with 1 mL of 10–4 mol/L
P, respectively. Then the absorbance was recorded and analyzed
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Measurement of UV-Vis spectrum of P with river water sam-
ple. The river water sample obtained from Dasha River, Shenzhen
was filtered by the 0.22 μm syringe filter to remove the particu-
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signals were measured by UV-Vis spectroscopy.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.202100013.
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Acknowledgement
We thank the technical support from SUSTech Core Research
Facilities. This work is financially supported and dedicated to the
10th anniversary of SUSTech.
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Manuscript received: January 4, 2021
Manuscript revised: February 4, 2021
Manuscript accepted: February 5, 2021
Accepted manuscript online: February 7, 2021
Version of record online: XXXX, 2021
[29] Jolliffe, I. T. Principal Component Analysis, Springer, New York, 1986.
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Chin. J. Chem. 2021, 39, 1517—1522