Colorimetric detection and ratiometric quantification of mercury(ii) using azophenol dye: 'Dip & read' based handheld prototype device development

Article abstract of DOI:10.1039/c8tc04720k

The extreme toxicity of mercury and its derivatives results from its high affinity for thiol groups in proteins and enzymes, leading to the dysfunction of cells and consequent health problems. Thus, developing a rapid, cheap and colorimetric sensor for detecting mercury ions at very low levels remains a challenge. Herein, we have developed a new chromogenic azophenol-based probe which allows the colorimetric detection of Hg(ii) metal ions and subsequently, showed suitability as a RGB chemodosimeter for the selective sensing of Hg(ii) in aqueous medium. The UV-vis absorption and colorimetric study showed that the azophenol-based sensor is highly selective towards Hg(ii) detection without the interference of other analytes. The color change from deep red to yellow in the complex solution after treating with mercury metal ions, which is visible by the naked-eye, makes this probe more convenient and simple to use in real sample analysis. Furthermore, for practical application of the sensor system in real time sample analysis, solid state silica based sensor chips have been developed which are impregnated with probe 1 displaying colorimetric changes for different concentrations of Hg²⁺, where the intensity of the red color decreased gradually towards a yellow color after the addition of Hg²⁺, apparently, makes them as potential candidate to conveniently monitor the concentration of Hg²⁺ in aqueous test solutions.

Full text of DOI:10.1039/c8tc04720k

Journal of
Materials Chemistry C
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PAPER
Colorimetric detection and ratiometric
quantification of mercury(II) using azophenol dye:
dip & read’ based handheld prototype device
Cite this: J. Mater. Chem. C, 2018,
, 12728
‘
6
development†
a
a
b
a
Gagandeep Singh, Pushap Raj, Harupjit Singh and Narinder Singh
*
The extreme toxicity of mercury and its derivatives results from its high affinity for thiol groups in
proteins and enzymes, leading to the dysfunction of cells and consequent health problems. Thus,
developing a rapid, cheap and colorimetric sensor for detecting mercury ions at very low levels remains
a challenge. Herein, we have developed a new chromogenic azophenol-based probe which allows the
colorimetric detection of Hg(II) metal ions and subsequently, showed suitability as a RGB chemo-
dosimeter for the selective sensing of Hg(II) in aqueous medium. The UV-vis absorption and colorimetric
study showed that the azophenol-based sensor is highly selective towards Hg(II) detection without the
interference of other analytes. The color change from deep red to yellow in the complex solution after
treating with mercury metal ions, which is visible by the naked-eye, makes this probe more convenient
and simple to use in real sample analysis. Furthermore, for practical application of the sensor system in
real time sample analysis, solid state silica based sensor chips have been developed which are
Received 18th September 2018,
Accepted 25th October 2018
2+
impregnated with probe 1 displaying colorimetric changes for different concentrations of Hg , where
2+
the intensity of the red color decreased gradually towards a yellow color after the addition of Hg
,
DOI: 10.1039/c8tc04720k
2+
apparently, makes them as potential candidate to conveniently monitor the concentration of Hg in
aqueous test solutions.
rsc.li/materials-c
+
4–6
+
MeHg in living systems.
It is well known that MeHg acts
Introduction
physiologically by binding to sulfhydryl groups in proteins or
In the last decade, the economies of developing countries have cysteine, forming water soluble complexes in tissues. Since the
been booming, and the excessive number of industrial plants recognition of the severe neurotoxic effects of mercury in the
constructed in these countries has resulted in serious air/water 1960s, the development of detection techniques for real-time
pollution, and discharged toxic heavy metal ions may ultimately and long-term monitoring of mercury contamination in environ-
1
accumulate in human bodies through the food chain. The mental and biological samples has become a high priority.
widespread contamination with pollutants such as highly poi- Accordingly various methods have been developed in the last
7,8
sonous mercury species could jeopardize our ecosystem, impos- couple of years, for instance, small organic molecules, biomole-
2
9–11
12,13
ing a great threat to human health. Various neurological effects cules,
and functionalized polymeric
and inorganic
1
4
of mercury exposure have been mainly attributed to the organic materials have been used to construct optical sensors. Although
form of mercury, predominantly methylmercury (MeHg ), which such findings help to overcome the severity of organic mercury
+
is known to accumulate in the food chain and cross the blood contamination, the threat of inorganic mercury, particularly
3
brain barrier after human ingestion. The literature reveals that mercury(II) ions, should not be underestimated. It is well estab-
+
there are very few colorimetric probes for MeHg , which is in lished that due to bacteria-assisted biotransformation processes,
15
strong contrast to the enormous interest in the detection of mercury(II) ions are the ‘‘precursor’’ form of methylmercury. The
relevant speciation for Hg toxicity via drinking water is inorganic
mercury in the form of labile Hg ions. The World Health
Organization and the U.S. Environmental Protection Agency
a
2+
Department of Chemistry, Indian Institute of Technology Ropar (IIT Ropar),
Rupnagar, Punjab 140001, India. E-mail: nsingh@iitrpr.ac.in;
Tel: +91-1881242176
(
EPA) have set the guideline for the tolerable limit of mercury in
b
Centre for Biomedical Engineering, Indian Institute of Technology Ropar (IIT Ropar),
Rupnagar, Punjab 140001, India
1
6
drinking water at 30 and 10 nM, respectively.
The development of sensitive and selective chromogenic
†
Electronic supplementary information (ESI) available. CCDC 1859342. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c8tc04720k sensors composed of chelating ligands that are used to detect
1
2728 | J. Mater. Chem. C, 2018, 6, 12728--12738
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Journal of Materials Chemistry C
heavy-transition-metal cations in biological environments has evaluated in terms of RGB based observation. Colorimetric
1
7–30
attracted extensive scientific interest.
Chelating agents are sensors are well established as an economical and easily minia-
well known for inhibiting the toxic effect of heavy metals, for turized alternative, where light is separated into a set of colors,
instance, particularly, ethylenediamine tetra acetic acid (EDTA), primarily red (R), green (G), and blue (B). Thus, we demonstrate
which normally inhibits the toxic effect of heavy metals, how- that the measurements can be carried out using conventional
ever, it has no inhibitory effect on the toxicity of mercury or may RGB sensors or on a smartphone application in terms of RGB
3
1
even increase it. Besides the development of these types of values of 8-bit resolution (256-bit color scale with 255, 255,
various effective nanosensors working in the solution phase, 255 corresponding to white, and 0, 0, 0 corresponding to black)
there is still a lack of a sensing method in the solid state circumventing all the limitations around the optical measure-
reaching a sensitivity below 1 pM which is in great need for ments with spectrometers and the data is frequently presented
practical applications. Therefore, a sensitive optical probe that in terms of total color differences (DC) using the equation
can detect ppb levels of inorganic mercury in drinking water will
be extremely useful for establishing the drinkability of water,
especially in remote areas where sophisticated instrumentation
for analysis of the metal content in water is not available.
Although there are numerous reports on optical sensors for
detecting Hg , major shortcomings are related to their limited
aqueous solubility and irreversibility. Very few sensors have a
combination of high aqueous solubility and reversibility along
with the requisite limit of detection.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
where: DR, DG, and DB are the change in red, green, and blue
colors from reference values, respectively.
2
2
2
DC ¼ ðDRÞ þðDGÞ þðDBÞ
2
+
Experimental section
General information
Thus, the development of irreversible reaction based chemo- All chemicals used were of analytical grade and were purchased
1
13
dosimeters for mercury has attained much attention.
from Sigma-Aldrich Co. H and C NMR spectra were recorded
2
+
Because of the strong thiophilic affinity of Hg , fluorescent on an Avance-II (Bruker) instrument, which operated at 400 MHz
1
13
changes associated with mercury-promoted desulfurization for H-NMR and 100 MHz for C-NMR (chemical shifts are
reactions which include hydrolysis, cyclization, and elimina- expressed in ppm). All chemical shifts were recorded in ppm
tion reactions have been used in the design of chemodosi- relative to tetramethylsilane as an internal reference. The
2
+ 32–35
meters for Hg .
However, significant challenges still exist particle size of nano-aggregates was determined with Dynamic
in this field. For instance, to drive these desulfurization reac- Light Scattering (DLS) using the external probe feature of a
tions to completion, it is often necessary to use either elevated Metrohm Microtrac Ultra Nanotrac particle size analyzer. The
2
+
temperature or excess quantities of Hg . Although, some other photo-physical properties and recognition properties of the
+
2+
metal ions such as Ag and Pb which are less thiophilic receptor and complex were determined with a Shimadzu
2
+
36,37
than Hg can also promote the desulfurization reactions.
UV-2400 spectrophotometer. The pH measurements were carried
2
+
Therefore, optimal ratiometric chemodosimeters for Hg must out on an ME/962P instrument. Mass spectra (HRMS) were
possess fast response times at ambient temperature and the analyzed on a Q-TOF high resolution mass spectrometer. The
ability to selectively and stoichiometrically detect Hg . Herein, TEM images were recorded on a Hitachi (H-7500) instrument
we report a novel azophenol dye-based Hg chemodosimeter working at 120 kV.
as a new advance in this field. Our design is based on the well-
known reaction mechanism for mercury induced desulfuriza-
2
+
2
+
Synthesis of probe 1
3
8
tion followed by cyclization leading to a heterocyclic moiety.
Initially, to prepare 4-nitrobenzenediazonium chloride, aromatic
This approach involves the reaction of the target analyte viz. amine (0.01 mol) was dissolved in a 1 : 1 ratio of conc. HCl and
2
+
ꢀ1
Hg cations with the molecular probe, called here the chemo- water. Sodium nitrite solution (0.02 mol mL ) was added to the
dosimeter, and is associated with significant chemical trans- reaction mixture dropwise with continuous stirring. The reaction
formation which involves the formation of covalent bonds and mixture was stirred for 45 min at a temperature of 0–5 1C. Then
results in the formation of a product differing from the starting the diazotization step was followed by coupling of the diazotized
chemodosimeter concomitantly with optically different proper- amines by drop wise addition to a solution of salicylaldehyde
ꢀ
1
ties. This type of chemical transformation, achieved by the (0.01 mol of salicylaldehyde dissolved in a 0.02 mol mL
specifically designed reaction, is relatively less affected by the solution of sodium hydroxide). The reaction was stirred at
environment and provides a distinct advantage in terms of high 0–5 1C for 2 hours under alkaline conditions. The conjugates
selectivity. Earlier, Tae et al. reported that a rhodamine-based of salicylaldehyde and aromatic amines were obtained as orange
thiosemicarbazide was easily transformed into 1,3,4-oxadiazole to red powders after the pH was neutralized to 6–7. Then, the
2
+
7
in the presence of a Hg ion, and Tian et al. also used this condensation of N,N-dimethylethylenediamine and obtained
desulfurization reaction to develop a naphthalimide-based 5-(2-(4-nitrophenyl)diazenyl)-2-hydroxybenzaldehyde results in
2
+
chemodosimeter for the Hg ion with a sensitive ratiometric the formation of schiff base compound 3, which was further
3
8
signal from green to blue emission. Therefore, in order to get subjected to reduction with NaBH
4
in methanol affording
better signal response, we use the azophenol dye chromophore compound 2. The progress of the reaction was monitored using
to give rise to a sensitive and sharp color change which can be thin layer chromatography. After completion of the reduction,
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the solvent was evaporated and the dark red crude compound 2 concentrations of various TBA salts (0–100 equiv.). pH titrations
was obtained and purified with column chromatography. Further, were also performed to explore the effect of pH on the recognition
the reaction of compound 2 (100 mg, 0.215 mmol) was carried out behavior by varying the acidity and basicity of the solution.
with phenyl isothiocyanate (120 mg, 0.645 mmol) in 15 mL dry
DCM under refluxing for 8 h, which leads to the formation of the Determination of mercury in water and plant samples
precipitated product probe 1, which was characterized by rigorous
Various water samples from a tap on campus and water from a
1
13
spectroscopic analysis ( H NMR, C NMR and HRMS).
-(((2-(Dimethylamino)ethyl)amino)methyl)-4-((4-nitrophenyl)-
diazenyl)phenol (2). R 0.33 (CHCl /hexane 8: 2); dark red powder,
yield = 75%.
nearby pond and river were used for environmental sample
testing. Sample solutions were collected and then filtered 3 times
2
2+
f
3
through a filter with a pore size of 0.2 mm. Hg at concentrations
over the range of 2–40 mM was spiked into the pond and river
water samples. The detection processes are identical to those
mentioned above for recognition studies. On the other hand, for
plant samples, the selected fresh plant samples were cleaned
with Milli-Q water, and the fresh weight (FW) of the samples was
recorded. Each individual plant sample was separated into root,
stem and leaf sub-samples. All the collected sub-samples were
air-dried, weighed to record the dry weight (DW) and then ground
to a fine powder and passed through a 0.18 mm-mesh sieve and
stored in polythene zip-bags at ambient temperature. Then for
analysis, the dried and pulverized samples were subjected to
digestion according to the method reported earlier (EPA method
1
H NMR (CDCl , 400 MHz): d 8.32 (m, 2H, Ar-H), 7.92 (m, 2H,
3
Ar-H), 7.83 (m, 1H, Ar-H), 7.67 (s, 1H, Ar-H), 6.92 (m, 1H, Ar-H),
5
.34 (br s, 2H, OH and NH), 4.09 (s, 2H), 2.73 (t, 2H, J = 6.0 Hz),
ꢁ 2).
, 100 MHz): d 163.74, 156.27, 148.05, 145.76,
2.46 (t, 2H, J = 6.0 Hz), 2.23 (s, 6H, CH
3
1
3
C NMR (CDCl
3
1
4
26.10, 124.80, 123.54, 123.11, 123.01, 117.33, 57.85, 52.11,
5.56, 29.78.
+
HRMS (ESI, m/z): calculated for C17
H
22
N
5
O
3
[M + H] ,
344.1723, found 344.1702.
1
-(2-(Dimethylamino)ethyl)-1-(2-hydroxy-5-((4-nitrophenyl)-
diazenyl)benzyl)-3-phenylthiourea (1). R 0.30 (CHCl /hexane
f
3
7
: 3); red powder, yield = 70%.
7473) with some modifications. Further, to avoid any external
1
H NMR (CDCl
3
, 400 MHz): d 12.6 (s, 1H, NH), 8.37–8.34 (m,
contamination, all the sample processing was performed in a
laminar flow fume cupboard. The final concentrations of mercury
in plant samples were analyzed using absorption spectrophoto-
metry. The results were analyzed with analyses of variance.
2H, Ar-Hs), 7.97 (m, 3H, Ar-Hs), 7.76 (dist s, 1H, OH), 7.34–7.32
(
5
(
m, 5H, Ar-Hs), 7.18–7.14 (m, 1H, Ar-H), 7.08–7.06 (m, 1H, Ar-H),
.25 (s, 2H, CH ), 3.60–3.58 (m, 2H, CH ), 2.72 (m, 2H, CH ), 2.39
s, 6H, N-CH ꢁ 2).
2
2
2
3
1
3
C NMR (CDCl , 100 MHz): d 183.50, 161.65, 156.15, 148.25,
3
Handheld device design for RGB analysis
1
1
45.78, 140.50, 128.82, 128.77, 126.08, 125.20, 124.85, 124.11,
The 3D drawing of the sample holder was created using Free CAD
which is a free and open-source general-purpose parametric 3D
CAD modeler. The outer dimensions of the holder were fixed at
23.09, 121.53, 118.78, 58.93, 53.93, 47.44, 45.43.
+
HRMS (ESI, m/z): calculated for C24
H
26
N
6
O
3
S [M + H] ,
479.1865, found 479.1840.
1
33.8 mm ꢁ 68.6 mm ꢁ 100 mm (L ꢁ W ꢁ H) by keeping in mind
the dimensions of a cell phone (iPhone SE). The main holder is
divided into two compartments where one compartment contains
a square pocket (30 ꢁ 30 mm) at the bottom for holding samples
Synthesis of ONPs of probe 1
It is well established that the developed bio-analytical technique is
more powerful if it works efficiently in aqueous medium. Thus, the
fabrication of ONPs is generally governed by the field of applica-
tion. The solubility difference of the compound in aqueous/organic
solvent is the influencing factor for the fabrication of ONPs. In the
present investigation, organic nanoparticles were prepared by the
re-precipitation method. Probe 1 was dissolved in DMSO and slowly
injected into water with sonication. After the injection, sonication
was continued for another 30 min to ensure the preparation of
stable organic nanoparticles. Nanoparticle formation was examined
using an external probe for particle size analysis. The size distribu-
tion of the ONPs was continuously analyzed using DLS, whereas,
the size and shape of ONPs were studied through TEM analysis.
(Sensor Chip) and the other compartment houses the electric
circuit components. The holder was printed using the Dimension
uPrint SE Plus 3D printer and the printing filament was made of
ABS (acrylonitrile butadiene styrene). In order to fix all the lighting
conditions, the top of the holder was cut out in such a way that the
smartphone fit perfectly and did not allow external light to enter
the sample compartment. Two white light emitting diodes were
fitted in the sampling compartment for lighting the sample with
constant intensity. A two position on off switch was housed in the
circuit compartment which allowed turning on and off the LEDs.
A 9 V battery was utilised for powering the LED lights. Then digital
pictures were taken by the iPhone SE camera, and analysed for
RGB components by a freely downloaded application from the iOS
App Store i.e. Pixel Picker.
Recognition studies
UV-vis absorption spectral profiles were recorded at 25 ꢂ 1 1C. The
solutions were shaken sufficiently and sonicated before recording
the spectrum. The binding behavior of the organic nanoparticles
formed from probe 1 was first studied for various cations and
anions. Before the spectra were recorded, the volumetric flasks
Results & discussion
Synthesis and characterization of the organic receptor
were allowed to stand for half an hour. For the assessment of the The synthetic protocol employed for the preparation of target
effect of ionic strength, the spectrum was recorded at different receptor 1 is illustrated in Scheme 1. The azodye coupled
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Scheme 1
3
9
aldehyde was prepared using the literature method. The con- Table 1 Crystal data and structure refinement for probe 1
densation reaction between the obtained azo-aldehyde and
N,N-dimethylethylenediamine yielded the imine linked receptor
Empirical formula
Formula weight
24 26 6 3
C H N O S
478.57
273.15
Triclinic
P1%
9.1010(4)
9.2238(4)
14.9779(7)
80.357(2)
89.219(2)
82.426(2)
1228.71(10)
2
1.294
0.169
504.0
(
3; Fig. S
with NaBH
ESI†) in good yield, which was further reacted with phenyl
1
and S
2
, ESI†), which was further subjected to reduction Temperature/K
in methanol. This afforded compound 2 (Fig. S –S ,
3 6
Crystal system
Space group
a/Å
4
isothiocyanate leading to the formation of probe 1; characterized b/Å
1
13
c/Å
a/1
b/1
with spectroscopic techniques such as H NMR, C NMR and
HRMS (Fig. S –S ; ESI†). Probe 1 is designed in such a way that it
7
9
has oxygen, nitrogen and sulphur donor sites along with an g/1
optical-signaling chromophore unit (azophenol moiety) to selec-
3
Volume/Å
Z
ꢀ3
tively trigger color change upon complexation with the analyte.
rcalc/g cm
1
ꢀ1
The H NMR spectrum of probe 1 showed signals at d 12.6 ppm m/mm
F(000)
Crystal size/mm
Radiation
2
and 7.76 ppm (exchangeable with D O) ascribed to NH (thioamide)
3
0.4 ꢁ 0.3 ꢁ 0.2
and OH (azophenol dye) protons, respectively. Resonances at
MoKa (l = 0.71073)
4.518 to 56.57
d 3.60 (m, 2H, CH ), d 2.72 (m, 2H, CH ), and d 2.39 (s, 6H, 2Y range for data collection/1
2
2
Index ranges
ꢀ12 r h r 12, ꢀ12 r k r 12,
CH ꢁ 2) were indicative of the presence of the N,N-dimethyl ethyl
3
ꢀ
19 r l r 19
13
chain, and the structure was also corroborated by C NMR spectral
Reflections collected
36 352
assignments. The peak corresponding to the mass of probe 1 with Independent reflections
5996 [Rint = 0.0486, Rsigma = 0.0417]
Data/restraints/parameters
5996/0/310
1.033
m/z 479.1840 (calc. m/z 479.1865) was observed in HRMS, con-
firming the molecular formula for the assigned structure, which
2
Goodness-of-fit on F
Final R indexes [I Z 2s(I)]
R
R
1
= 0.0887, wR
= 0.1251, wR
2
= 0.1992
= 0.2139
was also supported by proton–carbon connectivities via the HSQC Final R indexes [all data]
1
2
ꢀ
3
1
13
Largest diff. peak/hole/e Å
1.36/ꢀ0.73
spectrum. Overall H and C NMR spectral assignments, aided
1
13
by H– C hetero-COSY experiments, corroborated the assigned
structure. Finally, the structure of probe 1 was established by X-ray
crystallography (Fig. 1).
Probe 1 crystallizes in the triclinic crystal system with the
P1% space group, where the asymmetric unit consists of one
40
moiety of probe 1. The crystal structure diagram along with
atom numbering for probe 1 is given in Fig. 1a. Probe 1 is
stabilized by O–Hꢃ ꢃ ꢃS and NHꢃ ꢃ ꢃN intramolecular hydrogen
bonding and the selected bond lengths and bond angles are
1 3
given in Tables S –S (ESI†). The moieties of probe 1 are
arranged in the form of a one-dimensional chain running along
the b-axis. Different moieties in these chains are joined together
by the O–Hꢃ ꢃ ꢃC/CHꢃ ꢃ ꢃO type of hydrogen bonds as shown in
Fig. 1b. The crystal refinement parameters are given in Table 1.
Preparation of organic nanoparticles of probe 1
Due to the poor aqueous solubility of probe 1, it was decided to
compare the photo-physical properties of probe 1 in an organic
Fig. 1 (a) ORTEP view of probe 1; (b) packing arrangement of probe 1
viewed down the b-axis.
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Fig. 2 (A) Chemical structure of probe 1; (B) DLS histogram of probe 1 (ONP) showing a particle distribution under 65–75 nm; (C) TEM image showing
the formation of organic nanoparticles (ONPs) of receptor 1 in the DMSO/H O (1 : 99, v/v) solvent system.
2
solvent such as DMSO, DMF and acetonitrile, and compare by periodically recording UV-visible spectra of probe 1. It was
the functioning of probe 1 in a semi-aqueous medium with a observed that there is no change in absorbance shift after
varied fraction of water even up to 99% of water i.e. DMSO/H
2
O
irradiation with UV light, however, indicating only enhance-
(1 : 99, v/v), to make the developed analytical technique more ment in absorbance that exhibits an intense p–p* transition
powerful and efficient in aqueous medium. Consequently, organic band at 370 nm and a weak n–p* absorption band at 470 nm.
nanoparticles (N1) of probe 1 were prepared with DMSO, DMF Increasing formation of the band at 370 nm was observed with
and acetonitrile for its real time applicability in aqueous medium a decrease of the band at 470 nm over time (Fig. S13a, ESI†),
using the re-precipitation method. The size range of the obtained indicating that the formation of the trans isomer was proceed-
organic nanoparticles was confirmed by using the dynamic light ing. After 12 hours of relaxation in the dark, the trans isomer of
scattering method (DLS) and the hydrodynamic size distribution probe 1 was regenerated indicating little degradation during
of ONPs was observed under a 65–75 nm range for the DMSO/H
2
O
the photo switching process. Probe 1 can also undergo tauto-
system (1 : 99, v/v; Fig. 2B), which is lesser than the size expressed merisation as illustrated in Fig. 3, to form a tautomer that can
from the TEM image (about 90 nm; Fig. 2C), whereas, for both freely rotate, which may make the rate of cis–trans reversion
other DMF/H O and ACN/H O solvent systems, the particle size very fast.
2 2
is much higher as compared to the DMSO/H
and S11, ESI†).
2
O system (Fig. S10
Further, for trans to cis photo isomerism, a solution of probe
1 in ACN was continuously irradiated with UV light at 365 nm
Photo-switching studies, influence of pH values and ionic for 20 min. UV-visible absorption spectra of probe 1 are shown
strength on the stability of prepared probe 1 in an aqueous in Fig. S13b (ESI†). The process was monitored by recording
solution. Azobenzenes are well-known for their characteristic UV-visible spectra of the sample periodically. It was observed
optical absorption properties and are, perhaps, the most widely that the absorbance of the band at 362 nm decreased with
4
1–48
studied photo switchable systems,
thus the obtained probe 1 hypsochromic shift and an increase in absorbance for the band
was initially investigated for its cis–trans behavior via UV-visible at 470 nm, and the latter absorbance has previously been
absorption spectroscopy. It is well established that azo-based attributed to the cis isomer. The UV-visible spectrum of probe
compounds undergo trans-to-cis conversion under UV irradia- 1 after 20 min of irradiation also showed an absorption band at
tion, while the reverse process occurs under visible light or via 320 nm. Thus, cis–trans isomerisation for probe 1 was observed
4
9–53
thermal relaxation.
Literature reports also revealed that to be much faster, and it is well known that in terms of appli-
p-hydroxy-substituted azo derivatives exhibit the typical spectral cation in photo switches, azo compounds that exhibit fast
profile of azobenzenes, i.e. a strong band with a maximum isomerisation are of most interest, suggesting probe 1 would
between 350 and 380 nm, assigned to the symmetry-allowed be a good candidate in this context.
p–p* transition of the trans isomer, and a weaker broad band
A suitable pH range for detection of required or desired
peaking at ca. 450 nm, associated with a symmetry-forbidden analytes is a key factor to consider in real application. Thus,
4
4
n–p* transition. For photo-switching studies in solutions,
probe 1 was dissolved in acetonitrile at suitable concentration.
The UV-vis absorption spectrum was recorded using a Shimadzu
UV-2400 spectrophotometer. To study the cis–trans isomerisation
of probe 1 in ACN (Fig. S , ESI†), a solution of this compound
1
2
was irradiated with UV light at 365 nm for 20 minutes, after
which the formation of the absorption spectrum for the cis
isomer was observed by UV-visible spectroscopy. The UV irradi-
ated solution was then kept in the dark to allow the cis–trans
thermal isomerisation to take place. This process was followed Fig. 3 Tautoisomerization in probe 1.
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to understand the effects of aquatic conditions on the prepared complex solution, however, no significant change was observed
2+
azo-dye based probe 1 for the detection of Hg , we have investi- in UV-visible absorption spectra (Fig. S14b, ESI†). Thus, probe 1
gated the stability of prepared organic nanoparticles under exhibited good stability under a wide pH range with no hindrance
different pH conditions (Fig. S14a, ESI†). The UV-visible absorp- from the salts present in the environment and this feature proves
tion spectra remain unchanged within a wide pH range covering its practical application for the determination of other analytes in
physiological pH. These results show that probe 1 works effi- aqueous medium.
ciently at physiological pH (pH 5–8) and can be applied suitably
for detection of Hg in real samples, such as in waste water and
industrial analysis. An experiment was also conducted to see the Further, upon the addition of Hg ions into probe 1 nano-
effect of high ionic strength. Therefore, tetrabutylammonium particles, apparent changes in the UV-visible absorption spectra can
salts were added (0–100 mM concentrations) to the probe 1 be observed from Fig. 4A, which shows the change in absorption
intensity as well as in color, from red to yellow, which can be easily
2
+
2
+
Ultrasensitive and selective detection of Hg ions
2
+
2+
detected with the naked eye (Fig. 4B) in the presence of Hg ions
0–10 mM) in phosphate buffered saline (PBS, 10 mM, 7.4) buffer
solution. This clearly indicates that probe 1 is a highly selective
(
2+
colorimetric sensor for Hg ions. It was also observed that the
absorption peak at lmax 477 nm disappeared and a new absorption
2+
peak at lmax 354 nm appeared with gradual addition of Hg metal
ions (0–10 mM; Fig. 4A), and the linearity was determined to be
0.9976. The detection limit was 8.1 nM based on S/N = 3.
Consequently, an isosbestic point at lmax 393 nm was also observed
which clearly indicates the complex formation due to complexation
2+
between probe 1 and Hg metal ions. The selective behavior of
probe 1 under study can be attributed to the complementarity of the
soft sulphur binding site as shown earlier. Additionally, to the solu-
tion of probe 1, various other metal salts have also been added along
2+
with Hg , inducing the same absorption changes in its response to
2+
Hg , indicating a negligible interference effect from other metal
ions on the recognition performance of probe 1 (Fig. S15, ESI†).
Next, in order to rationalize the interaction of probe 1 with
2
+
Hg metal ions (Fig. 5), and to study the binding ratio between
2
+
Hg and probe 1, ESI mass spectra were recorded for probe 1
2
+
with the addition of Hg ions (1 equiv.) to the probe 1 solution,
which showed an appearance of a signal at m/z 445.17 rapidly,
+
attributed to the molecular ion [4 + H] peak, while the peak at
Fig. 4 (A) Modulation of the UV-visible absorption band of probe 1 upon
+
m/z 479 (molecular ion [1 + H] ) decreased (Fig. S , ESI†),
2+
16
successive addition of Hg metal ions (0–10 mM) in the DMSO/H
2
O
directly revealing that the binding stoichiometry of probe 1
(
1: 99, v/v) solvent system; (B) color change from red to yellow after the
2
+
2+
with Hg ions was 1 : 1. At higher values, various ion peaks with
in absorption intensity with respect to the concentration of Hg metal ions. weak intensities are found which are attributed to the presence
addition of Hg metal ions; (C) linear regression graph showing the change
2+
Fig. 5 Tentative mechanism for desulfurization of probe 1.
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2+
Fig. 6 LC-MS chromatograms: (A) probe 1 (10.0 mM) after addition of 0–2.0 equiv. of Hg (b–f); (B) overlay representation of LC-MS chromatograms.
5
4
of cluster ions only; i.e., there is no evidence for the formation mercury pollution. The use of mercury-containing pesticides,
2
+
of a Hg complex that can be isolated.
household bleach, caustic chemicals, and mercury-contaminated
To further confirm the sensing mechanism (Fig. 5), LC-MS irrigation water can also lead to the absorption and enrichment
5
5–57
analysis was also performed on probe 1 with gradual addition of of mercury in agricultural products.
Thus, in order to explore
2+
Hg to the probe 1 solution. Probe 1 and obtained dye 4 exhibited the real application of probe 1, we have utilized it to quantita-
a single peak with retention times at 2.37–2.39 min (Fig. 6a) and tively determine Hg in tap water, Sutlej River water and pond
.51 min (Fig. 6d), respectively. When 0.2 equiv. of Hg was water samples collected from near the Ropar region. All the water
2+
2+
2
added into the solution of probe 1, the intensity of the peak at samples under investigation were collected from three different
2
.38 min decreased with a concomitant occurrence of a new peak sites near to the Ropar region and filtered through a 0.25 mM
2+
at 2.51 min (Fig. 6b–e). The gradual addition of an excess of Hg
filter membrane prior to the experiment, and then were spiked
2+
(up to 2.0 equiv.) into the solution of probe 1 resulted in the with different concentrations of Hg (2 to 40 mM). It’s been
complete disappearance of the peak at 2.37–2.39 min and only observed that there is a linear relationship between the absorp-
2
+
one peak at 2.51 min remained (Fig. 6f), which is attributed to dye tion at 354 nm and Hg concentration in real water samples
4
as confirmed by its mass also.
(Fig. 7a–d) with a good recovery (Table S
5
, ESI†).
Furthermore, to support and understand theoretically the bind-
ing mechanism for probe 1 and Hg complexation leading to the Hg in plants was also evaluated. Literature studies revealed that
In addition, the potential application of probe 1 in detecting
2+
2+
5
8–60
formation of dye 4, keeping in mind that the strong thiophilic plants can absorb mercury from both air and soil.
Which
2+
affinity of Hg might lead to desulphurization and recyclization in part of the plant gets more contaminated with mercury is entirely
intermediates [A] and [B] as depicted in Fig. 5, the energies of the dependent upon the route of exposure of mercury. For instance,
highest occupied molecular orbital (HUMO) and lowest unoccupied when plants absorb mercury from the soil, the mercury content
molecular orbital (LUMO), and energy gaps were calculated for should be higher in the roots; whereas, the mercury contents
probe 1, and dye 4, by using DFT (density functional theory); the should be higher in shoots and leaves if air mercury is the main
calculations were performed using GGA-DFT package DMol-36. source of mercury contamination. Thus, for the detection of
The energies of the HOMO and LUMO characterize the electron mercury and its accumulation in different parts of a selected
donating and accepting abilities, respectively, whereas, the HOMO– plant (i.e. money plant), which is grown in the laboratory with
2
+
LUMO gap is related to the chemical stability/reactivity of a spiked soil samples of Hg , we have measured the mercury
molecule. The HOMOs and LUMOs of the reactant/product state concentrations via absorption spectroscopy in leaves, stems, and
4
analogs are shown in Table S (ESI†), which clearly reveals that the root extract of the money plant (Epipremnum aureum). The results
HOMO–LUMO gaps for both probe 1 and dye 4 are very comparable demonstrate the potential utility of probe 1 in real sample
to each other and thus, are stable in nature.
analysis which reveals that the mercury contents were much
higher in roots than in stems and leaves (Fig. 8).
Determination of mercury in real water/plant samples
RGB analysis for chromogenic sensing
Particularly in developing countries, aside from some natural
calamities, the burning and use of coal in industrial processes Keeping in mind the sharp color change of probe 1 from red to
2
+
such as mining and petrochemicals is the main cause of yellow (Fig. 4b) by the addition of Hg in the solution phase,
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2+
Fig. 7 (a) UV-visible absorption response of probe 1 (10.0 mM) in the presence of different concentrations of Hg ions in real water samples;
2+
linear relationships of absorption of probe 1 at 354 nm versus the spiked concentration of Hg (0.0–40.0 mM) in (b) river water, (c) pond and (d) tap
water samples.
Fig. 9 Visible color changes in solid state chips: untreated silica chip
impregnated with probe 1, and after treatment with varied concentration
2+
of Hg
.
Fig. 8 Mercury distribution in organs of the Epipremnum aureum plant and clearly detected, probe 1 can also be used for practical
2+
grown in Hg spiked soil in lab conditions. Dashed line: maximum allowed applications in the solid state. Low naked eye detection via solid
ꢀ1
mercury level in vegetables (10 mg kg FW) (Food Safety Standard).
state silica based sensor chips coated with chemo-dosimeter probe
makes them a potential candidate to conveniently monitor the
concentration of Hg metal ions in aqueous solution.
1
2+
it was decided to investigate the developed probe 1 in the solid
Nowadays, smartphones are portable, widely available, user-
silica phase also, in order to develop a simple, portable, and friendly, and therefore well suited to act as an effective platform
6
1–63
affordable detection system that would significantly improve for on-site detection.
Many interesting biosensors have been
the ease of detection of heavy metal contamination in remote designed based on smartphones utilizing mobile apps and
locations. Thus, small size (4 ꢁ 4 cm) silica chips were dipped various connected devices, such as high-performance cameras
6
4,65
in a solution of probe 1 in CH CN (2 mL, 10 mM) and air dried and light sensors.
Thus, inspired by sharp color changing
3
2
+
which results in red colored silica chips (Fig. 9). Then these results from our developed solid sensor chips with Hg (Fig. 9),
prepared red colored probe 1 impregnated silica based sensor herein, we have developed a battery-powered mobile sensing
2
+
chips were treated with varied concentration solutions of Hg
1–20 nM) in double distilled water resulting in instant color attachment to a smart-phone along with an application for
change from red to yellow (Fig. 9). As the color change was rapid collecting, reporting and sharing RGB data, in order to provide
device that consists of a lightweight (B20 g) opto-mechanical
(
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2+
Fig. 10 (a) Digital images of the probe 1 solution taken by the iphone SE rear camera after addition of different concentrations of Hg metal ions; (b) RGB
2+
responses extracted from digital images of the probe 1 solution after addition of varied concentration of Hg metal ions.
a field-portable, economic, and wirelessly connected platform spectroscopy clearly shows 1 : 1 binding stoichiometry ratios of
2+
2+
to perceptibly quantify Hg metal ion concentration in water probe 1 with Hg , which was also corroborated by LC-MS analysis.
samples (Fig. S , ESI†). This lab-on a-phone device is based on The UV-visible absorption spectrum clearly revealed that the
17
fixed wavelength illumination using light-emitting diodes (LEDs) absorption peak at lmax 477 nm for probe 1 disappeared and a
and can quantify mercury-induced subtle transmission changes of a new absorption peak at lmax 354 nm appeared with gradual
2+
colorimetric assay utilizing solid state silica based chips impregnated addition of Hg metal ions. Consequently, an isosbestic point at
2
+
with probe 1. A series of experiments were conducted with Hg
lmax 393 nm was also observed which clearly indicates the com-
2+
solutions in the concentration range of 1–20 nM. Digital images for plexation between probe 1 and Hg metal ions. Naked-eye sensing
2+
the color changes after addition of Hg metal ions to the solution of helped in observing the color changes in the complex solution,
probe 1 are displayed in Fig. 10. As shown in Fig. 10, sample spots produced from the effect of mercury metal ions, thus making it
are shown from red, through orange to yellow, and the colors more convenient to use in real sample analysis. Furthermore, for
gradually became sharp in their intensity as the Hg concentration practical application of the sensor system in real time sample
increased. According to the color wheel theory, the color shown by analysis, we have developed solid state silica based sensor chips
the object is the complementary color of that which is absorbed by coated with probe 1 which display colorimetric changes for
2+
2+
the object, i.e. here the solution sample appears red, through orange different concentrations of Hg , where the intensity of the red
to yellow, because the red to yellow color is transmitted by the color decreased gradually towards a yellow color after the addition
2+
sample and the complementary color, the blue part of the spectrum, of Hg , which makes them a potential candidate to conveniently
2+
is absorbed, as revealed by the UV-visible spectrum also, which is monitor the concentration of Hg in aqueous test solutions.
attributed to blue light. The RGB responses of the digital images
were extracted and are illustrated in Fig. 10. From each part of
Fig. 10b, it can be seen that there is an increase in the value of the
Conflicts of interest
G-component and a decrease in the value of the B-component with
There are no conflicts to declare.
2+
the increase of Hg ion concentration. On the basis of the linear
2+
relationship between the RGB data and Hg concentration, the limit
of detection, which corresponds to the signal-to-noise ratio, was Acknowledgements
determined to be 13 nM which was very comparable with the
2+
Financial assistance in the form of sponsored research project
PDF/2017/000414 by SERB, Department of Science & Technology,
Government of India is gratefully acknowledged.
maximum tolerable level (10 nM) for Hg in drinking water
permitted by the United States Environmental Protection Agency,
whereas, lower than the WHO permitted standards (30 nM).
Conclusion
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