Spectral studies on benzimidazole-based “bare-eye” probe for the detection of Ni2+: Application as a solid state sensor

Article abstract of DOI:10.1016/j.ica.2017.04.042

The colorimetric chemosensors for metal ion sensing containing azo group (–N[dbnd]N–) as a chromogenic signaling unit and 2-(2′-hydroxyphenyl)benzimidazole as binding unit are reported. Compound 2 and 3 were studied as metal ion chemosensors in the presen

Full text of DOI:10.1016/j.ica.2017.04.042

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Research paper
Spectral studies on benzimidazole-based “bare-eye” probe for the detection of
Ni2+: Application as a solid state sensor
Gargi Dhaka, Navneet Kaur, Jasvinder Singh
PII:
S0020-1693(17)30241-4
http://dx.doi.org/10.1016/j.ica.2017.04.042
ICA 17547
DOI:
Reference:
Inorganica Chimica Acta
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20 April 2017
21 April 2017
Please cite this article as: G. Dhaka, N. Kaur, J. Singh, Spectral studies on benzimidazole-based “bare-eye” probe
for the detection of Ni2+: Application as a solid state sensor, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/
10.1016/j.ica.2017.04.042
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Spectral studies on benzimidazole-based “bare-eye” probe for the detection of
Ni2+: Application as a solid state sensor
Gargi Dhaka, Navneet Kaur* and Jasvinder Singh
Department of Chemistry, Panjab University, Chandigarh 160014, India
Tel.: +91 172 2534430; fax: +91 172 2545074;
e-mail: neet_chem@yahoo.co.in; neet_chem@pu.ac.in (N. Kaur)
Abstract: The colorimetric chemosensors for metal ion sensing containing azo group (-N=N-) as
a chromogenic signaling unit and 2-(2’-hydroxyphenyl)benzimidazole as binding unit are
reported. Compound 2 and 3 were studied as metal ion chemosensors in the presence of Ni2+,
Cu2+, Zn2+/Co2+ in aqueous CH3CN solutions. Well-defined color change was observed upon
addition of metal ions to aqueous CH3CN solutions of chemosensors 2 and 3 enabling “bare-eye”
color change from colorless to yellow-gold upon Ni2+ or Cu2+ complexation. Furthermore, test
strips containing the receptor molecule were easily fabricated and offered a practical, efficient
and low-cost test kit for Ni2+ ions. The present sensing system has been successfully applied for
the detection of Ni2+ ions in real samples.
Keywords: Colorimetric chemosensors, Ni2+ sensing, solid state sensor, real water analysis
1. Introduction
Amongst other molecular sensors, colorimetric sensors exhibited superiority lying in a naked-eye
color change dispensing with spectrophotometer to distinguish optical perturbations [1-4]
without resort to any expensive equipment [5-6]. In colorimetric receptors, the coordination site
binds the guest in such a manner that signalling unit displays the changes in color. As a common
rule, the design of such receptors are founded on the receptor-chromophore general binomial,
which involves the binding of a specific analyte with receptor sites and a chromophore
responsible for translating the binding of receptor-analyte into an optical signal [7-9]. Due to
their interaction with analyte, hypsochromic or bathochromic shift of absorption spectra or visual
color change are affected by the relevant decrease or increase in electron densities on the
chromophore moiety [10]. Optically-loaded (both chromogenically and fluorogenically)
benzimidazole-based molecular organizations have been used for screening of diverse metal
ions. The presence of different binding sites viz hydroxyl, amine and imine and the property to
show appreciable change in its optical behaviour upon metal binding construct these
organisations suitable contender for cation recognition. Also, benzimidazoles are very important

intermediates for the growth of molecules of biological interest. Benzimidazole and its
derivatives have established applications in biological activities such as anticancer,
antimicrobial, anti-inflammatory, anticonvulsant and antivirus [11-15]. Colorimetric recognition
with visible color changes is very attractive due to the advantage of simplicity and low cost.
Azo-methines are organic dyes and enclose the characteristic chromophore groups –N=N- and –
C=N-. It is well known that azo colorants are the largest chemical class of industrial colorants for
many different textile products including silk, polyester, wool, leather and cotton which are
specifically intended for clothing and decoration [16]. Azo colors appear in different types which
comprise oxy-azo, amido-azo, diazo, tetrazo and other polyazo compounds. It has long been
known that the azo group has some capability to form a coordinate link with the different metal
ions [17]. Many azo dyes have been exploited as chromogenic reagents for colorimetric
determinations and as markers for complexometric titration [18]. It is well documented in
literature that insertion of azo-functionality in the chemosensors enhances chromogenic ability of
the chemosensors [19]. Azo dyes having aromatic heterocyclic moiety amplified the
chromophoric potency and thus, have been reviewed exhaustively [20-21]. Hetero-aryl azo dyes
are coined for their simplistic synthesis in addition to deep absorptions in the visible range and
thus can be employed reliably for developing colorimetric chemosensors [22].
The development of chemosensors with sensing aptitude towards metal ions (in particular
for transition metal ions) is of particular relevance [23]. Metal ions are desirable for a proper
metabolic activity; though, they are, when in excess, associated to several pathologies. Examples
of these are central nervous system disorder, cancer of lungs, nasal cavity, skin allergy, asthma,
acute pneumonitis [24-28], Wilson’s disease [29], Menkes disease [30], Prion and Alzheimer’s
diseases, Parkinson’s, epilepsy, ischemic stroke and infantile diarrhea, vasodilation and so on
[31-36].
2. Experimental
2.1 Methods and instrumentation
All solvents and reagents were purchased from Aldrich. All other chemicals were used as
received without further purification. Deionized water was used throughout the experiments.
Acetonitrile (CH3CN) was of HPLC grade. Chemical shifts are reported in ppm on the δ scale
and referenced to tetramethylsilane (TMS). The UV-vis. spectra were recorded on a Shimadzu
UV–240 UV/ Vis spectrophotometer. All the metal ions such as Na+, K+, Mg2+, Mn2+, Fe2+, Fe3+,

Co2+, Ni2+, Cu2+, Zn2+, Hg2+ and Cd2+ were added as their perchlorate for the different UV-vis.
spectroscopic experiments.
2.2 Synthetic procedures
2.2.1 Synthesis of 2-(1H-Benzoimidazol-2-yl)-phenol (1)
o-Phenylene diamine (250 mg, 5 mmol) and salicylaldehyde (395 mg, 5 mmol) were taken in a
50 mL round bottom flask and irradiated under 640 watt for 10 minutes with an interval of 2
minutes between the two halves of the total duration [37]. After completion of reaction (TLC
monitoring), the crude product was recrystallized to yield light colored product 1. Brown Solid,
Mp (oC) 149 (Mp litr 152 0C), Yield (%) 82; FT-IR (cm-1): 3318 (νO-H str.), 3232 (νN-H str.),
3055.06 (νC-H str., aromatic), 2867.09 (ν =C-H str.), 1629.35 (νC=N str.), 1319.75 (νC-O str.), 1129.90 (νC-N
str.); 1H NMR (DMSO, 400 MHz) δ (ppm): 6.96 (dd, J1= 11.8, J2 = 6.0 Hz, 2H, Ar-H), 7.24 (dd,
J1 = 5.16, J2 = 2.88 Hz, 2H, Ar-H), 7.32 (t, J = 5.85 Hz, Ar-H), 7.60 (s, 2H, Ar-H), 7.98 (d, J =
13
5.88 Hz, 1H, Ar-H), 13.11 (s, 2H, -OH phenolic ring, -NH- benzimidazole ring ); C NMR
(DMSO, 100 MHz) δ (ppm): 158.02, 151.62, 131.40, 125.94, 123.08, 118.83, 117.02, 112.49;
ESI-MS: m/z (relative abundance (%), assignment) = 211 [100, (M+1)+].
2.2.2 Synthesis of 2-(Benzimidazol-2-yl-4-phenylazo)phenol (2)
In a 100 mL round bottom flask, aniline (0.094 g, 1mmol) was suspended in 30.0 mL of distilled
water at the temperature of 0-5 oC. Then 1.0 mL of HCl solution was added to the mixture. After
15 min, aqueous solution of sodium nitrite (0.076 g, 1.1 mmol) was added, followed by THF
solution of 1 (0.227g, 1 mmol), then pH was adjusted to 8-9 with NaOH solution. After stirring
for 4h, the solution was neutralized with HCl solution. The produced precipitate was filtered and
washed several times with water and dried at vacuum. Column chromatography was performed
in hexane-ethyl acetate solvent system to obtain pure azo compound 2 in a yield of 60%. Solid,
Mp (oC) 183, Yield (%) 65; IR (cm-1) 3342 (νNH str.), 3064 (νAr-OH str.), 1632 (νC=N), 1586 (νC-N);
1H NMR (DMSO, 400 MHz) δ (ppm): 7.24 (d, 1H, J = 8.0 Hz, Ar-H), 7.33 (dd, 2H, J1 = 8.0 Hz,
J2 = 4.0 Hz, Ar-H), 7.56 (t, 1H, J1, J2 = 8.0 Hz, Ar-H), 7.63 (t, 2H, J1, J2 = 8.0 Hz, Ar-H), 7.72
(brs, 2H, Ar-H), 7.91 (d, 2H, J1, J2 = 8.0 Hz, Ar-H), 7.99 (d, 1H, J = 8.0 Hz, Ar-H), 8.78 (s, 1H,
Ar-H), 13.8 (brs, 2H, -OH phenolic ring, -NH- benzimidazole ring); 13C NMR (DMSO, 100
MHz) δ (ppm): 161.0, 152.0, 150.9, 144.9, 130.9, 129.5, 125.4, 122.4, 122.2, 118.3, 113.0; ESI-
MS: m/z (relative abundance (%), assignment) = 315.0781 [100, (M+1)+].
2.2.3 Synthesis of 2-(Benzimidazol-2-yl-4-(4-methoxyphenyl)azo)phenol (3)

Reaction of 1 with 4-mthoxy aniline in similar reaction conditions as described for synthesis of
2, yielded chemosensor 3. Solid, Mp (oC) 270, Yield (%) 60; IR (cm-1) 3323 (νNH str.), 3056 (ν Ar-
OH str.), 2982 (νAr-H str.), 2843 (νAr-C=C str.), 1627 (νC=N), 1495 (νC-N); 1H NMR (DMSO, 400 MHz) δ
(ppm): 3.81 (s, 3H, - OCH3), 7.16 (d, 2H, J = 8.0 Hz, Ar-H), 7.22 (d, 1H, J = 8.0 Hz, Ar-H),
7.33 (d, 2H, J = 8.0 Hz, Ar-H), 7.71 (brs, 2H, Ar-H), 7.88 – 7.95 (m, 3H, Ar-H), 8.69 (s, 1H, Ar-
H), 13.6 (brs, 2H, -OH phenolic ring, -NH- benzimidazole ring); 13C NMR (DMSO, 100 MHz) δ
(ppm): 161.6, 160.4, 151.1, 146.2, 145.0, 125.0, 124.1, 121.9, 118.2, 114.6, 112.9, 55.6; ESI-
MS: m/z (relative abundance (%), assignment) = 345.0873 [100, (M+1)+].
2.3 Experimental details for cation detection
UV-vis titrations were conducted on 10 µM solutions of 2 and 3 in 9:1 (v/v) aqueous CH3CN
(pH 7.0 HEPES buffer) solution. Stock solutions of the chemosensors 2 and 3 (10-2 M) and
different ions (10-1 M) were prepared in DMSO. This stock solution was further diluted with 9:1
(v/v) aqueous CH3CN and used further for different spectroscopic experiments. All the cations
were added as their perchlorate salts for the different absorption experiments. Aliquots of anions
under investigation were then injected into the sample solution through a rubber septum in the
cap. The solutions were allowed to get stabilized after each addition, and were then scanned.
3. Results and discussion
3.1 Synthesis and structural characterization
The general synthetic procedure for 2 and 3 is shown in Scheme 1. The azo coupling reaction of
1 with different aromatic amines, gave chemosensors 2 and 3 containing simple azo dye
functionalities. The desired products were explicitly characterized by NMR spectra (1H NMR,
13C NMR), FT-IR and Mass spectrum (Fig. S1-S6).
R
N N
H
N
(i) HCl
(ii) NaNO , 0 - 5 C
H
N
o
2
N
N
H N
2
R
HO
HO
1
2. R = H
3. R = -OCH3
Scheme 1. Synthetic pathway of chemosensors 2 and 3.
3.2 Colorimetric recognition of Ni2+ / Cu2+ by 2 and 3

The chemosensor 2/3 (10 µM) in 9:1 (v/v) aqueous CH3CN (pH 7.0 HEPES buffer) gave
colorless solution. Upon addition of competitive metal ions such as Na+, K+, Mg2+, Mn2+, Fe2+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ to solution of 2, a conspicuous color change from colorless
to yellow-gold in the case of Ni2+ and Cu2+ metal ions was observed. Moreover, addition of Co2+
ions to solution of 2 also caused color to change to orange (Fig. 1a).
(a) (b)
Fig. 1. Colorimetric analysis of (a) 2 and (b) 3 in 9:1 (v/v) aqueous CH3CN (pH 7.0 HEPES buffer) upon addition of
different cations.
On the other hand, upon addition of above mentioned metal ions to solution of 3,
conspicuous naked eye color change from colorless to yellow and pale yellow color was
observed with addition of only Ni2+, Cu2+ and Zn2+ ions (Fig. 1b).
3.3 Effect of pH on 2 and 3
In general, the chemosensing properties are highly dependent on the pH of the system, therefore,
the influence of pH on 2 and 3 have been evaluated in 9:1 (v/v) aqueous CH3CN. The absorbance
of 2 and 3 at 312 and 365 nm, respectively, remained by and large unaffected between pH 5.7-10
(Fig. 2). Significantly, on moving beyond the pH range 5.7-10, the absorbance gradually
decreased.
Fig. 2. Effect of pH on UV-vis spectra of (a) 2 and (b) 3.
3.4 UV-vis absorption studies of 2 and 3
To further explore the utility of chemosensors 2 and 3 as “bare-eye”, the absorption behavior of
2 and 3 were investigated with various metal ions. Among the competitive metal ions such as
Na+, K+, Mg2+, Al3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ tested in 9:1 (v/v) aqueous

CH3CN (pH 7.0 HEPES buffer), Ni2+, Cu2+ and Co2+ ions mainly perturbed the absorption
spectrum of chemosensor 2 (Fig. 2a). On the other hand, the absorption spectrum of
chemosensor 3 showed changes with addition of only Ni2+, Cu2+ and Zn2+ ions (Fig. 2b).
However, major changes in the absorption spectrum of both 2 and 3 were observed with Ni2+ and
Cu2+ ions. Along with aqueous CH3CN, the photophysical properties of both the dyes have also
been evaluated in other solvents like pure CH3CN, MeOH, as well as in pure aqueous media.
Both the receptors showed maximum selectivity and sensitivity towards Ni2+, Cu2+, Co2+/Zn2+
metal ions only in CH3CN-H2O [9:1 (v/v)] system. In all other solvents, non-selective behavior
of both the receptors as well as precipitation in case of pure H2O system was observed.
Fig. 3. (a) UV-vis. absorption spectra of (a) 2 and (b) 3 (10 µM) in 9:1 (v/v) aqueous CH3CN (pH 7.0 HEPES
buffer) upon addition of 100 equiv. of various cations as their perchlorate salts.
The qualitative metal ion sensing behaviour of 2/3 was determined by recording the
absorption spectra upon addition of various concentrations of metal ions (Ni2+, Cu2+, Co2+ and
Zn2+ ions). As shown in figure 4a, upon incremental addition of Ni2+ ions to 9:1 (v/v) aqueous
CH3CN (pH 7.0 HEPES buffer) solution of 2, decrease in absorption intensity at 312 and 365 nm
along-with a concomitant emergence of a new absorption maximum at 400 nm was observed
with an isosbestic point at 356 nm, indexing the generation of new ratiometric complex between
2 and Ni2+ ions (Fig. 4a). The binding curve reached to its plateau after the addition of 20 equiv.
of Ni2+ ions. The bathochromic shift of about 35 nm, upon addition of Ni2+ was in good
agreement with the observed color change. Similar type of ratiometric decrease of absorption
intensity at 321 and 365 nm and increase at 435 nm was observed upon addition of Ni2+ ions to
aqueous CH3CN solution of 3 (Fig. 4b). Next, Job’s plots were used to determine the

stoichiometries of the complexes formed between 2 or 3 and Ni2+. The results indicate a 1:2
(2/3:Ni2+) stoichiometry for the binding of 2 or 3 with Ni2+ (Fig. S7). The binding constant of 2
and 3 with Ni2+ were determined using Benesi-Hildebrand plot [38] and were found to be 4.7 x
109 M-2 (LOD = 56 µM) and 5.5 x 109 M-2 (LOD = 48 µM), respectively.
Fig. 4. UV-vis. spectra of (a) 2 and (b) 3 (10 µM) in 9:1 (v/v) aqueous CH3CN (pH 7.0 HEPES buffer) upon
addition of Ni2+; Inset: Binding isotherms at respective wavelengths.
Similar absorption changes were observed in the UV–vis. spectra of 2 upon the addition of
Cu2+ and Co2+ ions (Fig. S8). Addition of these ions resulted in the appearance of a new red
shifted band at 400 nm. The chemosensor 2 showed binding with Cu2+ and Co2+ ions in 1:2
manner (Job’s plot) with stability constant values of 4.4 x 108 M-2 (LOD = 22 µM) and 1.3 x 107
M-2 (LOD = 24 µM), respectively.
On the contrary, in case of 3 (10 µM) in 9:1 (v/v) aqueous CH3CN (pH 7.0 HEPES
buffer), incremental addition of Cu2+ ions decreased the absorption intensity of the bands at 321
and 365 nm. But, due to own absorption of Cu2+ ions in the region 260-300 nm, clear isosbestic
point was not observed (Fig. S9a). On the other hand, addition of Zn2+ ions to 3 in similar
conditions resulted in decrease in absorbance at both 321 nm and 365 nm bands and slight
increase in absorbance in region 410 - 440 nm (Fig. S9b). The binding constants of 3 with Cu2+
and Zn2+ ions were found to be 6.8 x 108 M-2 (LOD = 15 µM) and 1.2 x 107 M-2 (LOD = 39 µM),
respectively, considering 1:2 stoichiometry (Job’s plot) for the complexation.
3.4 Possible sensing mechanism
The appearance of a new red-shifted band with the addition of metal ions (Ni2+, Cu2+ and
Co2+/Zn2+) to solution of chemosensors 2 and 3 may be attributed to the possible hydrogen

bonding interaction and/or the deprotonation of the phenolic –OH [39-40]. Upon Mn+ (Mn+ =
Ni2+, Cu2+ and Co2+/Zn2+) coordination with 2/3, π delocalization might be enhanced, which
resulted in reduction of the energy of the π-π* transition, leading to new red-shifted absorption
band and “bare-eye” color change from colorless to yellow (Scheme-2) [41].
Scheme 2. Proposed binding mode of chemosensors 2/3 with metal ions in solution.
3.5 Application of 2 and 3 as solid state sensors
Inspired by the conspicuous colorimetric change of the 2 and 3 in solution, their practical
applicability as solid state sensors was tested for the detection of Ni2+ ions. For this, 2 and 3 were
adsorbed on the filter paper by immersing filter papers in the CH3CN-H2O solution of 2 and 3 (1
mM) and then drying them in the air. When Ni2+ solution was dropped in successive manner (5-
50 µM) on the 2/3-based test strips, the coated test papers were changed from colorless to
yellow-gold when 20-50 µM of Ni2+ was added (Fig. 5). Therefore, it is quite obvious that
chemosensors 2 and 3 demonstrated excellent colorimetric sensing performance even in the solid
state. This solid state method provides an easier, cost-effective and convenient method for
detection of Ni2+ in environmental samples.
Fig. 5. Photographs of test strips of (a) 2 and (b) 3 upon addition of increasing concentration from 5- 50 µM of Ni2+
ions.

3.6 Application in real samples
The practical application of the designed chemosensors 2 and 3 have been evaluated by
determination of Ni2+ in drinking water, river water and tap water samples. The water samples
were first filtered to remove insoluble substances. To keep a stable system at given solvent ratio
and pH, HEPES buffer was added to maintain the pH 7.0. All the samples with or without
addition of Ni2+ at different concentrations as shown in table 1 were analyzed by chemosesnor 2
and the results obtained were found in good agreement with the concentration of Ni2+ ions in the
system. As shown in table 1, it can be confirmed that recovery studies of Ni2+ based on 2 are
satisfactory with RSD values ranging from 1.2 to 2.6%. This indicates the suitability and
practicality of the present chemosensor 2 for the detection of Ni2+ from real water samples
without any interferences from other environmentally relevant competitive metal ions. Simialr
practical applicability of chemosensor 3 was also observed for detection of Ni2+ ions in real
water samples (table S1).
Table 1. Results of Ni2+ sensing in drinking water, tap water and river water samples with 1.
Sample
Ni2+
added/mol L-1
0
4.50 x 10-6
8.00 x 10-6
11.00 x 10-6
15.00 x 10-6
19.50 x 10-6
0
4.50 x 10-6
8.00 x 10-6
11.00 x 10-6
15.00 x 10-6
19.50 x 10-6
0
4.50 x 10-6
8.00 x 10-6
11.00 x 10-6
15.00 x 10-6
19.50 x 10-6
Ni2+
Recovery
(%)
RSD (%)
found/mol L-1
Not detected
4.44 x 10-6
7.9 x 10-6
Drinking Water
˗˗
˗˗
98.7
98.8
103.6
97.3
105.6
˗˗
78.9
90.0
101.8
107.3
99.5
˗˗
1.4
1.2
2.0
1.2
1.2
˗˗
2.6
1.6
1.4
˗˗
11.4 x 10-6
14.6 x 10-6
20.6 x 10-6
Not detected
3.55 x 10-6
7.2 x 10-6
11.2 x 10-6
16.1 x 10-6
19.4 x 10-6
Not detected
4.9 x 10-6
Tap Water
2.3
River Water
˗˗
108.9
99.4
100.0
104.0
98.7
2.2
1.4
1.4
˗˗
7.95 x 10-6
11.00 x 10-6
15.6 x 10-6
19.25 x 10-6
2.3
4. Conclusions
Metal ion sensors 2 and 3 were designed and synthesized through microwave irradiation
followed by azo coupling reaction. These chemosensors resulted in distinct coloration and
appearance of a new red-shifted band with the addition of Ni2+, Cu2+, Zn2+ and Co2+ ions in 9:1
(v/v) aqueous CH3CN (pH 7.0 HEPES buffer). Chemosensors 2 and 3 have found applications in
real water analysis as well as for making solid sensor strips. These results proved that 2 and 3 are
promising sensors for “bare-eye” Ni2+ detection.
Acknowledgements

The authors are greatly thankful to SAIF, Panjab University Chandigarh for recording the NMR
and Mass spectra and are grateful to UGC (grant no. AB2/12/3115) and DST PURSE-II (Grant
no. 48/RPC) for the fellowship.
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1. Novel Benzimidazole based azo-dyes have been synthesized for Ni2+ and Cu2+ ions.
2. Distinct coloration and appearance of a new red-shifted band with Ni2+ and Cu2+ ions.
3. Synthesized azo-dyes have found applications in real water analysis and for making solid
sensor strips.