Coordinating ligand functionalized AgNPs for colorimetric sensing: Effect of subtle structural and conformational change of ligand on the selectivity

Article abstract of DOI:10.1039/c4ra13586e

Metal coordinating C₂-symmetric phenolic chelating ligands (1-3) were used to prepare surface functionalized AgNPs for selective colorimetric sensing of metal cations and anions. The effect of subtle structural and conformational change of 1-3

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ARTICLE TYPE
Coordinating ligand functionalized AgNPs for colorimetric sensing: Effect of
subtle structural and conformational change of ligand on the selectivity
V. Vinod Kumar and Savarimuthu Philip Anthony*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Metal coordinating C2ꢀsymmetric phenolic chelating ligands (1ꢀ3) were used to prepare surface
functionalized AgNPs for selective colorimetric sensing of metal cations and anions. The effect of subtle
structural and conformational change of 1ꢀ3 on the colorimetric sensing was also explored. UVꢀVisible,
IR spectroscopy and HRꢀTEM techniques were used to characterize AgNPs. Interestingly 1ꢀ, 2ꢀ and 3ꢀ
10 AgNPs showed different colorimetric sensing of metal cations and anions. Aliphatic flexible diamine
based 1ꢀAgNPs showed selective sensing of Co2+ ions (10ꢀ7 M), oxalate and dihydrogen phosphate anions
(10ꢀ6 M). Co2+ addition to 1ꢀAgNPs produced brownishꢀorange colour whereas oxalate and phosphate
resulted in brownishꢀpink colour due to aggregation. Alicyclic diamine based 2ꢀAgNPs exhibited
selective precipitate formation for Pb2+ ions without showing any anion sensing. The rigid diamine based
15 3ꢀAgNPs showed selective decolourisation of yellow colour for Hg2+ and nitrite anions due to
amalgamation and oxidation. Thus phenolic chelating ligands that have been used to generate versatile
coordination networks with metal ions offered simple way to develop AgNPs based potential colorimetric
sensor environmental screening.
key step in fabricating AgNPs colorimetric sensor.24 For example,
AgNPs with glutathione functionalization lead to selective
sensing of Ni2+,25 whereas pyridylꢀappended calix[4]arene and
50 dopamine functionalized AgNPs exhibited Fe3+ and Cu2+
colorimetric sensing, respectively.26,27 Triazole functionalized
AuNPs exhibited selective colorimetric sensing of Cr3+ ions
whereas aptamer functionalization lead to selective sensing of
K+.28,29 Mercaptothiadiazole fluorophore capped AuNPs showed
55 selective sensing of Hg2+ and selective Co2+ sensing was reported
with AuNPs modified with thioglycolic acid.30,31
Cobalt is an essential element for nutrition and its deficiency
may cause anaemia, retarded growth and loss of appetite.32 But
consumption of large doses of cobalt may cause lung effects,
60 which include respiratory irritation, asthma and pneumonia.33
Mercury is one of the most toxic elements in the environment and
poses serious health threats because of its high affinity for thiol
groups in proteins and enzymes, thereby leading to the
dysfunction of cells and consequently causing many health
65 problems in the brain, kidney, and central nervous system.34
Similarly lead, a second most toxic substance in the environment
due to its wide distribution and uses,35 can lead to neurological,
reproductive, cardiovascular and developmental disorders even at
low level exposure.36 Children with variants in iron metabolism
70 genes may be more susceptible to lead absorption and
accumulation. Whereas phosphate anion is the important
constituent of living systems and plays significant role in signal
Introduction
20 In recent years, there is a strong interest to design and develop
simple, reliable, rapid and inexpensive sensor techniques for
selective sensing of heavy metal ions and anions in water to asses
the toxicity as well as to understand the biological role.1–7 Hence
different methods have been developed for the detection cations
25 and anions, including atomic absorption spectroscopy, ICPꢀMS,
electrochemical approaches and fluorescent analysis.8ꢀ11
Although several fluorescent probes with high selectivity and
sensitivity were reported,4ꢀ7,11 colorimetric sensors have received
significant attention in recent years because of their advantages
30 such as naked eye detectable, cost effectiveness, simple and
portable configuration that makes colorimetric approach as an onꢀ
site method for real time monitoring of analytes.12ꢀ13 The recent
advances in the field of nanoscience and nanotechnology have
opened up new arenas for the applications of nanoscale materials
35 including the development of ultrasensitive sensing and imaging
methods in the analytical sciences.14ꢀ18 Particularly, noble silver
(Ag) and gold nanoparticles (AuNPs) draws greater attention for
fabricating colorimetric assays due to their strong and unique
localized surface plasmon resonance (SPR).19ꢀ21 The strong and
40 well defined colour of Ag and AuNPs due to excellent SPR
properties makes easy for visualization of colour change.22,23 The
interparticles distance dependent SPR properties Ag or AuNPs
provides practical platform for developing colorimetric sensors
for various chemicals including toxic heavy metal ions and bioꢀ
45 molecules in aqueous solution. The surface modification of
AgNPs with appropriate analyte interacting functionality is the
transduction
and
energy
storage
in
biological
systems.37 However, excess of phosphate leads to algae and
75 bacteria blooms, which is wellꢀknown as eutrophication.38
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Similarly, carboxylate functionality is part of a wide range of
dissolving in MilliꢀQ water.
2.1. Synthesis of
60 diaminoethane),
diaminocyclohexane) and 3 (N, N’-bis(2-hydroxybenzyl)-1,2-
diaminobenzene).
1
(N, N’-bis(2-hydroxybenzyl)-1,2-
biologically active entities, and in many cases is responsible for
their biochemical properties.39ꢀ42 For instance, the specific
recognition and transport of dicarboxylates across biological
membranes is crucial in the metabolism of cells,39 and the
mechanism of action of the vancomycin antibiotics is highly
dependant on carboxylate binding.40
2
(N,
N’-bis(2-hydroxybenzyl)-1,2-
5
Reduced Schiff base phenolic chelating ligands, 1ꢀ3, was
synthesized by following the reported procedure.52 Typically 1
65 equivalent of diamine was dissolved 20 ml hot water and mixed
with ethanol solution of salicylaldehyde (1 equivalent, 10 ml).
The solution turned immediately bright yellow colour and the
solution was heated at 80 °C for 30 min. The reaction mixture
was cooled in iceꢀbath and NaBH4 (1.5 equivalent) was added
70 portionꢀwise. The bright yellow colour was slowly disappeared
and neutralization of the reaction mixture produced precipitates
of 1ꢀ3. The precipitate was filtered, washed with cold ethanol and
Subtle structural change of ligand is one of the simplest
strategies that have been extensively employed in coordination
10 chemistry to control or modify the mode of ligand coordination
and resultant coordination network structure.43ꢀ46 Phenol based
chelating C2ꢀsymmetric ligands have been widely used to
generate coordination polymers and complexes with diverse
topologies including helical structure.47,48 The easy ionization
15 properties of phenolic groups could be used to synthesis stable
surface functionalized AgNPs.49 This guest interacting surface
functionality coupled with interparticle dependent AgNPs optical
properties offered an opportunity to fabricate colorimetric sensor.
In our lab, we have been working on the Ag and AuNPs based
20 colorimetric assay for heavy toxic metal ions, anions and
biomolecules. Green synthesized bioꢀfunctionalized AgNPs and
amino acid attached phenolic ligands functionalized AgNPs for
selective colorimetric sensing of Hg2+, Pb2+, Cd2+ and Zn2+ was
reported recently.50ꢀ52 Kyong et al studied the AgNPs shape
25 dependent colorimetric sensing of metal ions.53 Subtle structural
change and conformationality of the ligand played important role
to tune and control the topology of metalꢀorganic framework in
coordination chemistry. However, the effect of such subtle
structural change of AgNPs surface functionality on the
30 colorimetric sensing has been rarely investigated.
dried in vacuum.
1. Yield = 85 %. H NMR (CDCl3)
1
δ 7.12ꢀ7.23 (m, 4H), 6.76ꢀ
75 6.84 (m, 4H, hydroxyl benzene), 4.18 (s, 4H), 3.48 (s, 2H), 3.21
(s, 4H). 13C NMR (CDCl3)
δ
156.1, 134.2, 132.3, 128.6, 125.2,
119.6, 48.2, 43.3. C16H20N2O2 (272.34): calcd. C 70.56, H 7.40,
N 10.29; found C 70.80, H 7.31, N 10.20. m/z (LCꢀMS) 273.00
(M+H).
1
80 2. Yield = 83 %. H NMR (CDCl3)
δ 7.17ꢀ7.24 (m, 4H), 6.79ꢀ
6.87 (m, 4H), 4.11 (s, 4H), 3.42 (s, 2H), 3.18 (s, 2H), 1.78ꢀ1.89
(m, 4H), 1.45ꢀ1.61 (m, 4H). 13C NMR (CDCl3)
δ
155.8, 134.1,
132.5, 129.1, 124.3, 118.8, 59.4, 47.7, 32.4, 26.1. C20H26N2O2
(326.43): calcd. C 73.59, H 8.03, N 8.58; found C 73.64, H 8.35,
85 N 8.30. m/z (LCꢀMS) 327.20 (M+H).
1
3. Yield = 80 %. H NMR (CDCl3)
δ 6.85ꢀ6.94 (m, 4H), 6.56ꢀ
6.71 (m, 4H), 6.30 (d, 2H), 6.22 (d, 2H), 4.38 (s, 4H), 3.88 (s,
2H). 13C NMR (CDCl3)
δ
155.2, 133.3, 130.1, 129.4, 122.1,
Herein, we report the synthesis of AgNPs with metal ion
interacting surface functionality using three conformationally
different C2ꢀsymmetric phenolic chelating ligand and colorimetric
sensing of metal cations and anions in aqueous solution. UVꢀ
35 Visible, IR and HRꢀTEM studies confirmed the formation of
polydispersed AgNPs with phenolic ligands surface functionality.
Interestingly, all three samples showed different metal cations
and anions sensing (1ꢀAgNPs for Co2+, phosphate and oxalate, 2ꢀ
AgNPs for Pb2+ (no anions) and 3ꢀAgNPsꢀHg2+ and nitrite). Co2+
40 (10ꢀ7 M) addition to 1ꢀAgNPs produced brownish orange colour
whereas phosphate and oxalate with 1ꢀAgNPs showed brownishꢀ
pink colour. 2ꢀAgNPs with Pb2+ exhibited selective precipitate
formation. The rigid diamine based 3ꢀAgNPs showed selective
decolourisation of yellow colour for Hg2+ and nitrite ions. The
45 controlled experiments suggest that Co2+ formed strong strong
coordination with 1 and Pb2+ exhibited turbidity with 2 whereas 3
did not show any significant change with metal cations. Thus
potential colorimetric sensor for heavy metal ions and anions
with tunable selectivity has been developed using
50 conformationally different phenolic chelating ligand.
116.7, 112.3, 33.6. C20H20N2O2 (320.38): calcd. C 74.98, H 6.29,
90 N 8.74; found C 74.85, H 6.45, N 8.56. m/z (LCꢀMS) 321.00
(M+H).
2.2. Synthesis of 1-, 2- and 3-AgNPs
Silver nitrate (5 ml of 10ꢀ3 M) was added into aqueous solution of
1ꢀ3 (5 ml, 10ꢀ3 M) under stirring at 70 °C. 1ꢀ3 was dissolved in
95 water using 1:2 molar ratio of NaOH. The immediate appearance
of yellow colour suggested the reduction of silver ion into
AgNPs. The solution was allowed to stir at that temperature for
another 10 min. The reactions were repeated at least three times
to confirm the reproducibility of AgNPs formation. The
100 characterization of the synthesized AgNPs was carried out after
allowing the solution to stand at room temperature for one day.
2.3. Characterization
The UVꢀvisible measurement of the synthesized AgNPs were
analyzed in a Perkin Elmer model UV–Vis double beam
105 spectrophotometer from 250 to 800 nm, at the resolution of 1 nm.
The powdered sample of 1ꢀAgNPs was subjected to FTꢀIR
spectroscopy measurement. These measurements were carried out
on a Perkin–Elmer SpectrumꢀOne instrument in the diffuse
reflectance mode at a resolution of 4 cmꢀ1 in KBr pellets.
Experimental Section
110
The size and morphology of AgNPs were investigated using
high resolution transmission electron microscopy (HRꢀTEM).
Samples for TEM measurements were prepared by placing a drop
of NPs solution on the graphite grid and drying it in vacuum.
Transmission electron micrographs were taken using JEOL JEMꢀ
Salicylaldehyde, cyclohexane diamine, NaBH4 and AgNO3 were
obtained from SigmaꢀAldrich. Ethylene diamine, 1,2ꢀ
diaminobenzene, ethanol and NaOH were obtained from Ranbaxy
55 fine chemicals. Ortho phenylenediamine was reꢀcrystallized from
hot water in presence activated charcoal. The metal cations
solution (10ꢀ3 M) used for the experiments were prepared by
115 2100F operated at an accelerated voltage of 200 kV and an ultra
highꢀresolution pole piece.
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change. Interestingly, addition of Co2+ to 1ꢀAgNPs resulted in the
formation of brownish orange colour selectively (Fig. 3).
Addition of other metal ions did not show any significant change
40 in the 1ꢀAgNPs colour. Absorption spectra of 1ꢀAgNPs with
different metal ions are shown in Fig. 3. 1ꢀAgNPs showed
absorption at 380 nm. The addition of Co2+ into 1ꢀAgNPs
produced new absorption at longer wavelength (527 nm)
selectively. 1ꢀAgNPs with other metal cations did not exhibit any
45 significant change in the absorption spectra. The appearance of
longer wavelength absorption was responsible for brownishꢀ
orange colour. The concentration dependent studies of 1ꢀAgNPs
clearly showed the appearance of longer wavelength absorption
with increasing Co2+ (10ꢀ7 M) concentration (Fig 4a). The
50 formation of new absorption peak at 527 nm was completed with
the addition of 300 µL of Co2+ (10ꢀ7 M). The interference studies
of 1ꢀAgNPs for Co2+ (10ꢀ6 M) in presence of various metal
cations (10ꢀ3 M) indicate the high selectivity. Co2+ addition to 1ꢀ
AgNPs in presence of other metal ions also showed longer
55 wavelength absorption (Fig. 4b) and suggest that other metal did
not have significant influence on the Co2+ colorimetric sensing.
Result and Discussion
3.1. Synthesis and characterization
The phenolic chelating ligands 1ꢀ3 was synthesized by Schiff
base condensation of diamine precursor with salicylaldehyde
followed by NaBH4 reduction in waterꢀethanol mixture. Addition
of silver nitrate (10ꢀ3 M, 2 ml) into aqueous solution of 1ꢀ3 (10ꢀ3
M, 10 ml) at 70 °C produced brownish yellow coloured AgNPs
solution. The intense colour of AgNPs is due to the strong surface
plasmon resonance (SPR) vibration.55 The phenolic compounds
5
10 are known to reduce the silver ions and stabilize the AgNPs via
electrostatic interactions of phenolic oxygen atoms (Fig. 1a).49,56
The formation of clear and transparent brownish yellow colour
confirmed the good dispersion of AgNPs. It is noted that aqueous
solution of ligands alone did not show any colour (Fig. S2). The
15 absorption studies of 1, 2 and 3ꢀAgNPs exhibited typical AgNPs
SPR absorption in the range of 380 to 435 nm (Fig. 1b). 1ꢀAgNPs
showed absorption λmax at 380 nm whereas 2ꢀAgNPs exhibited
absorption at 415 nm. 3ꢀAgNPs showed absorption λmax at 435
nm. The size, morphology and crystallinity of the samples were
20 analyzed using HRꢀTEM that showed the formation of spherical
crystalline polydispersed AgNPs with size range of 10 nm to 30
nm (Fig. 2). FTꢀIR studies of 1ꢀ3 showed peak at 3453 ꢀ 3458 (Nꢀ
Hꢀstretch), 2900 ꢀ 2944 (CꢀHꢀstretch), 1591 – 1611 (CꢀHꢀstretch)
and 1459 ꢀ 1479 (CꢀHꢀstretch) cmꢀ1 and similar peak position
25 with AgNPs confirms the surface coverage of phenolic chelating
ligand (Fig. S3). It is noted that the inclusion of AgNPs with 1ꢀ3
lead to small shift in the peak position.
Fig. 2. TEM images of (a) 1ꢀ, (b) 2ꢀ and (c) 3ꢀAgNPs. The inset of the
figures shows the high resolution image of AgNPs.
Fig. 1. (a) Schematic diagram of AgNPs synthesis using 1ꢀ3 and
30 stabilization and (b) absorption spectra of AgNPs synthesized using 1ꢀ3.
The molecular strucutre of 1ꢀ3 and digital images of 1ꢀ, 2ꢀ and 3ꢀAgNPs
are shown in the inset.
60 Fig. 3. Absorption spectra and digital images of 1ꢀAgNPs with different
metal cations (10ꢀ3 M).
In contrast, 2ꢀAgNPs exhibited selective colorimetric changes
for Pb2+ ions. 2ꢀAgNPs with Pb2+ lead to the formation of
precipitates (Fig. 5). Other metal ions including Co2+ addition did
65 not show significant colour change or precipitate formation.
Absorption spectra showed that Pb2+ addition into 2ꢀAgNPs leads
to slight decrease of absorption intensity at 415 nm but enhanced
3.2. Colorimetric sensor studies of metal cations
The colorimetric sensing properties of 1ꢀ, 2ꢀ and 3ꢀAgNPs were
35 investigated by adding different metal ions into the AgNPs
solution and monitoring the absorption as well as visual colour
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absorption intensity at 338 nm (Fig. 5). Absorption spectra of 2ꢀ
AgNPs with other metal ions did not show any specific change 30 M) in presence of various metal cations (10ꢀ3 M) also confirmed
Hg2+ (10ꢀ8 M). The interference studies of 3ꢀAgNPs for Hg2+ (10ꢀ6
except small reduction of absorption intensity. The concentration
dependent studies of 2ꢀAgNPs showed gradual decrease of
absorption peak at 415 nm and enhancement of absorption at 338
nm with increasing Pb2+ (10ꢀ6 M) concentration (Fig. S4a). The
increase of absorption peak at 338 nm was completed with the
addition of 300 µL of Pb2+ (10ꢀ6 M). The interference studies of
other metal ions (10ꢀ3 M) on the selectivity of 2ꢀAgNPs for Pb2+
the high selectivity (Fig. S5b).
5
10 (10ꢀ6 M) suggest the high selectivity (Fig. S4b). Addition of Pb2+
to 2ꢀAgNPs in presence of other metal cations also leads to strong
absorption at 338 nm clearly.
Fig. 5. Absorption spectra and digital images of 2ꢀAgNPs with different
metal cations (10ꢀ3 M).
35 Fig. 6. Absorption spectra and digital images of 3ꢀAgNPs with different
metal cations (10ꢀ3 M).
Fig. 4. Change of 1ꢀAgNPs absorbance with Co2+ concentration (a) and
15 selectivity studies of 1ꢀAgNPs for Co2+ (10ꢀ6 M) in presence of different
metal ions (10ꢀ3 M).
3.3. Colorimetric sensor studies of anion
s
1ꢀ, 2ꢀ and 3ꢀAgNPs has also showed different colorimetric
sensing of anions depends on the phenolic chelating ligand. 1ꢀ
40 AgNPs showed selective colorimetric sensing of oxalate and
dihydrogen phosphate at ppm level. 2ꢀAgNPs did not show any
anions colorimetric sensing whereas nitrite ions were selectively
detected by 3ꢀAgNPs. 1ꢀAgNPs with oxalate and dihydrogen
phosphate showed brownishꢀpink color formation selectively
45 (Fig. 7). It is noted that oxalate addition produced immediate
color change whereas dihydrogen phosphate showed weak
orangeꢀpink color initially that was intensified within ten min.
Other anions did not show any significant color change with 1ꢀ
AgNPs. Absorption spectra of 1ꢀAgNPs with oxalate and
Rigid diamine based, 3ꢀAgNPs exhibited selective colorimetric
sensing of Hg2+ ions. Addition of Hg2+ into 3ꢀAgNPs resulted in
the formation of colourless solution from yellow selectively (Fig.
20 6). Other metal ions with 3ꢀAgNPs showed only dilution of the
yellow color slightly. 3ꢀAgNPs showed SPR absorption at 435
nm and addition of Hg2+ lead to complete disappearance of
absorption peak (Fig. 6). Absorption spectra of 3ꢀAgNPs with
other metal ions slightly reduced the intensity. The concentration
25 dependent studies of 3ꢀAgNPs showed decrease of SPR
absorption gradually with blue shift of peak with the
concentration of Hg2+ (10ꢀ8 M, Fig. S5a). Absorption peak of 3ꢀ
AgNPs was completely disappeared with addition of 300 ml of
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dihydrogen phosphate clearly showed a new absorption peak at
longer wavelength (515 nm, Fig. 7). The intensity of 380 nm peak
was substantially reduced. No characteristic change in the 1ꢀ
AgNPs absorption except small reduction in the intensity was
observed with other anions. The concentration dependent studies
of 1ꢀAgNPs clearly showed the appearance of longer wavelength
absorption with first addition of oxalate (10ꢀ6 M, Fig. 8a). Further
additions only lead to the reduction of absorption intensity at 380
nm that was completed with 240 µL of oxalate (10ꢀ6 M). The
5
10 concentration dependent studies for dihydrogen phosphate did not
perform due to delayed colorimetric response. 1ꢀAgNPs exhibited
good selectivity for oxalate (10ꢀ6 M) in presence of various anions
(10ꢀ3 M). Addition of oxalate into 1ꢀAgNPs in presence of other
anions also showed clear appearance of longer wavelength
15 absorption with reduction of intensity at 380 nm (Fig. 8b). 3ꢀ
AgNPs exhibited selective decolourisation of yellow colour with
nitrite anions. Highly selective sensing of nitrite ions by 3ꢀAgNPs
have been recently reported by our group.54
40 Fig. 8. Change of 1ꢀAgNPs absorbance with oxalate concentration (a) and
selectivity studies of 1ꢀAgNPs for oxalate (10ꢀ6 M) in presence of
different metal ions (10ꢀ3 M).
preferential coordination of ligand 1 with Co2+. However, the
solution remains clear (Fig. S6a). The selective coordination of
45 Co2+ with 1 in presence of AgNPs caused aggregation NPs. 2ꢀ
AgNPs with Pb2+ that showed the precipitate formation did not
show any colour change as well as absorption red shift. It only
exhibited small reduction in the AgNPs SPR absorption at 415
nm and enhancement absorption at 338 nm. The controlled
50 experiment of 2 with different metal ions showed a clear turbidity
formation with Pb2+ selectively (Fig. S6b). Addition of different
metal cations into aqueous solution of 3 did not show significant
change (Fig. S6c). The selective colour disappearance of 3ꢀ
AgNPs for Hg2+ is due to the amalgamation and oxidation of
55 AgNPs to silver ions due to the more positive electrochemical
reduction potential of Hg2+.57 The different selectivity of metal
ions arises from the difference in the molecular structure and
conformationality. However, the controlled studies 1 with anions
did not show any noticeable change (Fig. S7 and S9). This might
60 be due to the weak nature of supramolecular interactions
compared to coordination bond. The flexible amine based ligand
1 exhibited clear colorimetric changes for Co2+, oxalate and
dihydrogen phosphate ions via coordination and selective
supramolecular interaction with surface functionality of AgNPs
65 that caused AgNPs aggregation. The semiꢀflexible ligand based
2ꢀAgNPs showed selective precipitate formation with Pb2+
Fig. 7. Absorption spectra and digital images of 1ꢀAgNPs with different
20 anions (10ꢀ3 M).
3.4. Discussion and recovery studies
The selective colorimetric change of 1ꢀAgNPs with Co2+,
oxalate and dihydrogen phosphate ions could be due to the
aggregation of AgNPs driven by coordination and supramolecular
25 interaction (hydrogen bonding) of anion with phenolic chelating
surface functionality of AgNPs. FEꢀTEM studies of 1ꢀAgNPs
with Co2+ and anions also confirmed the aggregation of NPs (Fig.
9). Addition of metal cations and anions including Co2+, oxalate
and dihydrogen phosphate into aqueous solution of 1 did not
30 show any significant colour formation (Fig. S6a and S7a). This
clearly indicates that the colour changes are due to AgNPs
aggregation. Absorption studies of 1 with different metal cations
were performed to get the insight into the colour change
selectively for Co2+ (Fig. S8). 1 showed three absorption peaks at
35 235, 288 and 375 nm. Addition of metal ions increased the
absorption intensity with slightly blue or red shifting the peaks
position. Interestingly, Co2+ addition showed complete
disappearance of peak at 288 nm selectively. This indicates the
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without anion sensing. 3ꢀAgNPs that is based on rigid aromatic
due to aggregation. However, alicyclic diamine functionalized 2ꢀ
diamine showed selective decolourisation of yellow for Hg2+ 30 AgNPs showed selective precipitate formation for Pb2+ without
cation and nitrite anion colour due to amalgamation and oxidation
of AgNPs. The easy oxidation of 3ꢀAgNPs might be due to the
weak stability provided by 3.54 Zeta potential measurement
showed that 1, 2 ligand stabilized AgNPs stronger than 3 (Table
exhibiting any colour change. Whereas, weakly stabilized, rigid
aromatic diamine functionalized 3ꢀAgNPs showed selective
decolourisation of yellow colour for Hg2+ and nitrite anions due
to amalgamation and oxidation. Thus, subtle structural and
5
S1). We have also noticed that AgNPs settles down as precipitate 35 conformational change of phenolic chelating ligand could be used
on keeping 3ꢀAgNPs for more than a week whereas 1ꢀ and 2ꢀ
AgNPs were stable for months. Thus the flexible surface
for fabricating AgNPs based colorimetric assay for tunable metal
cations and anions sensing in water.
Table 1. Determination of Co2+ in water samples using the 1ꢀAgNPs
colorimetric assay.
10 functionality of AgNPs plays a important role in the colorimetric
sensing of metal cations and anions via coordination or
supramolecular interaction induced aggregation. The applications
of the 1ꢀAgNPs colorimetric assay for detection of Co2+ in
presence of different metal cations were also evaluated. As can be
15 seen in Table 1, the mean recoveries of such samples were
between 94% and 97% and suggest the potential application of 1ꢀ
AgNPs colorimetric assay for sensing Co2+ ions in water samples.
Mean
Mean
RSDb
(%)
Samples
Found Recoverya
(µM)
(%)
Co2+ (1.0)
Co2+ (1.0)+Hg2+(1.0)+
Pb2+(1.0)+Ca2+(1.0)+Zn2+(1.0)
Co2+ (1.0)+Pb2+(1.0)+
Mn2+(1.0)+Cd2+(1.0)+Ni2+ (1.0)
Co2+ (1.0)+Zn2+(1.0)+
Cu2+(1.0)+Mg2+(1.0)+Fe3+ (1.0)
Co2+ (1.0)+Ca2+(1.0)+
Pb2+(1.0)+Cd2+(1.0)+Zn2+ (1.0)
0.97
97
0.84
0.49
0.96
0.94
0.95
0.95
96
94
95
95
0.86
1.29
0.84
40
aMean recovery (%) =100 x (cmean found/cmean recovery).
bRelative standard deviation of three determination.
cValues in parentheses=concentration (µM) of the metal ion
added.
45 Acknowledgments
Financial supports from Department of Science and Technology,
New Delhi, India (DST Fast Track Scheme No. SR/FT/CSꢀ
03/2011(G) is acknowledged with gratitude.
Notes and references
50 School of Chemical & Biotechnology, SASTRA University, Thanjavur-
613401, Tamil Nadu, India. Fax: +914362264120; Tel: +914362264101;
E-mail: philip@biotech.sastra.edu
† Electronic Supplementary Information (ESI) available: IR,
concentration dependent, interference spectra for metal ions and anion
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