Inkjet-printed phosphorescent Iridium(III) complex based paper sensor for highly selective detection of Hg2+

Article abstract of DOI:10.1016/j.dyepig.2018.11.054

First of its kind inkjet-printed phosphorescent Iridium(III) complex [Ir(TPQ)₂(4-EO₂-pic)] paper based chemosensor has been developed for Hg²⁺ ions. The probe is highly selective towards mercury analyte over other metal io

Full text of DOI:10.1016/j.dyepig.2018.11.054

Accepted Manuscript
Inkjet-printed phosphorescent Iridium(III) complex based paper sensor for highly
2+
selective detection of Hg
Marimuthu Ponram, Umamahesh Balijapalli, Baskaran Sambath, Sathiyanarayanan
Kulathu Iyer, Kranthiraja Kakaraparthi, Giridhar Thota, Venkatachalapathy
Bakthavachalam, Ravichandran Cingaram, Jin Sung-Ho, Karthikeyan Natesan
Sundaramurthy
PII:
S0143-7208(18)31535-3
https://doi.org/10.1016/j.dyepig.2018.11.054
DYPI 7198
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 11 July 2018
Revised Date: 26 November 2018
Accepted Date: 26 November 2018
Please cite this article as: Ponram M, Balijapalli U, Sambath B, Kulathu Iyer S, Kakaraparthi K, Thota G,
Bakthavachalam V, Cingaram R, Sung-Ho J, Natesan Sundaramurthy K, Inkjet-printed phosphorescent
2+
Iridium(III) complex based paper sensor for highly selective detection of Hg , Dyes and Pigments
(2018), doi: https://doi.org/10.1016/j.dyepig.2018.11.054.
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Graphical Abstract
[Ir(TPQ)2(4-EO2-pic)] Printed Paper
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Inkjet-Printed Phosphorescent Iridium(III) Complex Based Paper Sensor for Highly
Selective Detection of Hg2+
Ponram Marimuthu,a
Sathiyanarayanan,d
Balijapalli Umamahesh,b
Kranthiraja,e,f
Sambath Baskaran,c Kulathu Iyer
Giridhar,g
Kakaraparthi
Thota
Bakthavachalam
Venkatachalapathy,a Cingaram Ravichandran,a Sung-Ho Jine and Natesan Sundaramurthy
Karthikeyan*a
aDepartment of Chemistry, SRM - Easwari Engineering College, Chennai 600 089, Tamil Nadu,
India.
bCentre of Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819 0395,
Japan
cDepartment of Chemistry, Tsinghua University, Beijing 100 084, People’s Republic of China
dDepartment of Chemistry, School of Advanced Sciences, Vellore Institute of Technology (VIT
University), Vellore 632 014, India
eDepartment of Chemistry Education, Graduate Department of Chemical Materials, Institute for
Plastic Information and Energy Materials, Pusan National University, Busan 46241, Republic of
Korea
fDepartment of Chemistry, National University Singapore, 3 Science Drive 3, 117543, Singapore
gSigma-Aldrich, Bangaluru 560 099, Karnataka, India
* E-mail: karthikeyan.ns12@gmail.com
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Abstract
First of its kind inkjet-printed phosphorescent Iridium(III) complex [Ir(TPQ)2(4-EO2-pic)] paper
based chemosensor has been developed for Hg2+ ions. The probe is highly selective towards
mercury analyte over other metal ions (Cu2+, Ca2+, Co2+, Cd2+, Pb2+, Zn2+, Ni2+, Mg2+, Mn2+,
Al3+, Fe3+, La3+, Ag+, K+, Na+ and Li+). [Ir(TPQ)2(4-EO2-pic)] complex solution gave a visible
color change from orange to yellow and significant phosphorescence quenching. Upon addition
of aqueous Hg2+, the [Ir(TPQ)2(4-EO2-pic)] inkjet-printed paper gave a significant color change
in both normal and UV light. The detection limit was found out as 1.78 × 10-8 M. Cyclic
voltammetry (CV) studies were conducted to explore the further utility of [Ir(TPQ)2(4-EO2-pic)]
as electrochemical sensor for Hg2+ ions in CH3CN / H2O (v/v = 1/1). The oxidation wave
intensity of [Ir(TPQ)2(4-EO2-pic)] at 0.98 V decreased and new reversible oxidation wave was
appeared at 0.91 V The binding ratio of the [Ir(TPQ)2(4-EO2-pic)]-Hg2+ complex was
determined to be 1:1 according to the Job’s Plot. The probable binding modes of these
[Ir(TPQ)2(4-EO2-pic)] with Hg2+ have also been suggested on the basis of the 1H-NMR titration,
absorption and luminescence studies.
1. Introduction
Mercury is one of the most deleterious environmental pollutants. Despite the recent reduction of
its industrial use mercury contamination still occurs through a variety of natural and
anthropogenic sources.[1] Inorganic Hg2+ can be converted into methyl mercury by bio-organisms
and bioaccumulated in mammals by means of food chain.[2] Mercury causes severe neurotoxic,
genotoxic, kidney damage and serious cognitive disorders to the human even at low
concentration.[3-11] Henceforth, the detection of mercury is very important. In the past decades
many mercury chemosensors based on aggregation-induced emission (AIE), photoinduced
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electron transfer (PET), intramolecular charge transfer (ICT) and fluorescence resonance energy
transfer (FRET) signaling mechanisms have been developed.[12-13] However, there were very few
printer-based chemosensors reported early.[14- 19] Our ongoing research, aimed at the synthesis of
new heterocyclic molecules through rapid and simple synthetic approach[20-22] for instance, we
have recently reported mono and tetra sulfur substituted paper based fluorescent chemosensor.[23]
These results inspired us to further extend the first of its kind ink-jet printed phosphorescent
chemosensor (IJPCS) based on [Ir(TPQ)2(4-EO2-pic)] complex, which displayed high Hg2+
selectivity both in aqueous CH3CN-H2O (1:1 V/V) and in printed paper strips. Iridium(III)
complexes with thiophene substituted quinoline ligands have been synthesized by several
research groups, and also applied as phosphors in organic light-emitting devices (OLEDs). But
their potential application for heavy and transition metal ions detection has never been explored
in inkjet-printed based chemosensor. Moreover, the printed probes offer numerous advantages in
terms of sensitivity, selectivity, and response time, low cost, eco-friendly and provides a new
strategy for the detection of Hg2+. Hence the reported printing technique could be applied
successfully in OLEDs and luminescent biological labeling reagents in future.
2. Experimental Section
2.1 Materials and Reagents
The perchlorate salts Hg2+, Cu2+, Ca2+, Co2+, Cd2+, Pb2+, Zn2+, Ni2+, Mg2+, Mn2+, Al3+, Fe3+, Ag+,
Na+ and LaCl3, 1-(thiophen-2-yl)ethanone, 2-aminobenzophenone, diphenyl phosphate, 4-
chloropicolinic acid and all other reagents were purchased from Sigma Aldrich. IrCl3•xH2O was
purchased from alfa aesar. The solvents were of analytical grade purchased from Avra
Chemicals and used without further purification.
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2.2 Instrumentation
1
The H and 13C spectra were recorded on BRUKER 400 MHz FT-NMR spectrometers, using
tetramethylsilane as an internal reference. The absorption spectra were recorded with a Jasco V-
730 UV–Vis spectrophotometer at room temperature. Fluorescence spectra were recorded with
Hitachi FL 9000 series Spectrophotometer. HRMS values were obtained on Joel GC Mate II GC-
Mass Spectrometer.
Scheme 1 Synthesis of [Ir(TPQ)2(4-EO2-pic)]
2.3 Synthesis of 4-phenyl-2-(thiophen-2-yl) quinoline
A mixture of 1-(thiophen-2-yl)ethanone (3 g, 23.78 mmol), 2-aminobenzophenone (4.69 g, 23.78
mmol), diphenyl phosphate (5.95 g, 23.78 mmol) dissolved in m-cresol (30 mL) and degassed
with N2 while stirring at room temperature for 30 min and then refluxed for 12 hours at 140 °C.
The reaction mixture was distilled to remove m-cresol, residue was diluted with DCM and
washed with 10% sodium hydroxide solution. The reaction mixture was washed with NaCl
solution and the resulting organic layer was dried over anhydrous MgSO4 and evaporated the
solvent. The residue was then purified by column chromatography on silica gel (hexane: ethyl
1
acetate (EA), 9:1 v/v) to furnish an off-white powder 3 (5.1 g, 74%). H NMR (400 MHz,
CDCl3, δ): 8.19 - 8.16 (d, 1H, Ar H), 8.06 - 8.05 (dd, 1H, Ar H), 7.90 - 7.86 (m, 2H, Ar H),7.74
- 7.69 (m, 2H, Ar H), 7.56 - 7.51 (m, 5H, Ar H), 7.48 - 7.43 (m, 2H, Ar H); 13C NMR (400 MHz,
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CDCl3, δ): 152.11, 149.29, 148.88, 145.64, 138.42, 129.88, 129.74, 128.84, 128.78, 128.70,
128.30, 126.36, 126.11, 125.71, 117.93, (Fig. S1 & S2).
2.4 Synthesis of (TPQ)2Ir2Cl2
4-phenyl-2-(thiophen-2-yl) quinolone (2.5 g, 8.7 mmol) and IrCl3•xH2O were added to mixture
of 2-ethoxy ethanol and water (40 ml, 3:1 v/v). The resulting mixture was refluxed at 140 °C for
24 hours under N2 atmosphere. The red solid was filtered, washed with water followed by 60 ml
of methanol, water mixture (1:2). Subsequently, the solid was dried under vacuum at 120 °C to
afford cyclometalated Ir(III) µ-chloro bridged dimer as a red solid (1.68 g, 60 %).
2.5 Synthesis of (TPQ)2Ir(4-EO2-pic)
The neutral iridium complex [Ir(TPQ)2(4-EO2-pic)] was obtained according to previous literature
method.[24-25] A solution of a cyclometalated Ir(III) µ-chloride bridged dimer (2 g, 1.53 mmol),
4-chloropicolinic acid (1.33 g, 7.65 mmol) and Na2CO3 (1.6 g, 15.3 mmol) in 2-ethoxyethanol
(30 mL) was stirred at 140 °C under N2 atmosphere for 12 hours. After cooling to room
temperature, the mixture was poured into water and extracted with ethyl acetate. The organic
layer was dried over anhydrous MgSO4 and concentrated in vacuo, giving a residue that was
subjected to silica gel column chromatography using ethyl acetate: hexane (4: 1) as the eluent.
The obtained product was further purified by recrystallization from methylene chloride–hexane
1
to (0.78 g, 58%) as a red solid. H NMR (400 MHz, CDCl3, δ): 8.79 - 8.77 (d, 1H, Ar H), 7.98 -
7.95 (m, 2H, Ar H), 7.76 - 7.68 (m, 4H, Ar H), 7.63 (s, 1H, Ar H), 7.59 - 7.57 (m, 10H, Ar H),
7.42 - 7.30 (m, 3H, Ar H), 7.20 - 7.12 (q, 3H, Ar H), 6.86 (t, 1H, Ar H), 6.67-6.66 (d, 1H, Ar H),
6.20-6.18 (d, 1H, Ar H), 4.18 - 4.05 (q, 2H; CH2), 3.78 - 3.67 (t, 2H; CH2), 3.57 - 3.46 (q, 2H;
CH2), 1.22 - 1.14 (t, 3H; CH3); 13C NMR (400 MHz, CDCl3): δ (ppm) 172.53, 166.72,
165.35,153.67, 153.59, 153.04, 151.39, 151.35, 149.93, 149.02, 147.03, 141.58, 139.31, 137.95,
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137.48, 137.44,134.84, 133.61, 132.09, 130.90, 130.03, 129.88, 129.21,129.08, 128.95, 127.97,
127.70,127.55, 126.48, 126.38, 125.76, 125.64, 124.85, 124.64, 124.14, 117.97,
116.99,111.91,15.04 (Fig. S3 & S4) Mass (m/z): 975.1774 [M+] (Fig. S5).
2.6 Preparation of Stock Solution
Stock solutions (1×10-3) of perchlorate salts of Hg2+, Cu2+, Ca2+, Co2+, Cd2+, Pb2+, Zn2+, Ni2+,
Mg2+, Mn2+, Al3+, Fe3+, Ag+, Na+, and La3+ ions were prepared in HPLC grade acetonitrile and
water.
2.7 Electrochemical Measurements
Electrochemical measurements were performed with CHI 760E, electrochemical workstation
(CH Instruments, Austin, Texas). All measurements were carried out in a one-compartment cell
three electrode system , equipped with a glassy-carbon working electrode, a platinum wire
counter electrode, and a Ag/AgCl reference electrode. The supported electrolyte was 0.1 mol L-1
aqueous acetonitrile solution of tetrabutyl ammonium hexafluorophosphate (Bu4NPF6). The scan
rate was 100 mV‚s-1.
2.8 Limit of detection determination
The limit of detection was calculated based on fluorescence titration. Standard deviation
achieved by the fluorescence emission spectrum of probes was measured 5 times. To attain the
slope, the ratio of emission intensity was plotted against the concentration of analyte (Hg2+). The
limit of detection was calculated using the following equation.
The limit of detection = 3 × σ / s
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Where, σ = standard deviation of probe ; s = slope value of the plot [probe] plotted against
[Hg2+]
3. Results and Discussion
3.1 Absorption Studies
The metal affinity of [Ir(TPQ)2(4-EO2-pic)] towards various metal ions Hg2+, Cu2+, Ca2+, Co2+,
Cd2+, Pb2+, Zn2+, Ni2+, Mg2+, Mn2+, Al3+, Fe3+, La3+, Ag+ and Na+ was investigated by absorption
studies (Fig. 1) in mixed solvents of CH3CN / H2O (v/v = 1/1). Fig. 2, revealed the changes in
UV-Vis spectra of [Ir(TPQ)2(4-EO2-pic)] in CH3CN/H2O (v/v = 1/1) solution with various conc.
of Hg2+ ions. The complex displays very intense absorption band in the ultra violet region of 280
– 370 nm due to spin allowed inter ligand (π – π*) transitions. The weak absorption band in the
region of 406 – 576 nm are assigned to mixed singlet and triplet metal to ligand charge transfer
(MLCT).
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Fig.1 Changes in UV-Vis spectra of [Ir(TPQ)2(4-EO2-pic)] in CH3CN / H2O (v/v = 1/1) solution
with various metals ions
The absorption band at 483 nm has gradually decreased upon the addition of Hg2+ ions, whereas
a new band appeared at 442 nm with the blue shift of 41 nm with two isosbestic points at 408 nm
and 466 nm, and induced color change from orange to yellow in visible light indicates high
sensitivity and selectivity towards Hg2+ ions. The stoichiometry of [Ir(TPQ)2(4-EO2-pic)] and
Hg2+ was given by variation of A483 nm/A442 nm with various equivalence of Hg2+. The A483 nm/A442
nm curve gradually decreased up to 1 equiv. of Hg2+, further addition of Hg2+ gives only a minor
change, indicating that the (TPQ)2Ir(4-EO2-pic) forms 1:1 complex with Hg2+ ions.[26]
Fig.2 Changes in UV-Vis spectra of [Ir(TPQ)2(4-EO2-pic)] in CH3CN / H2O (v/v = 1/1) solution
with various conc. of Hg2+ ions. (Inset) Titration curve [Ir(TPQ)2(4-EO2-pic)] with Hg2+
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3.2 Fluorescence Studies
To evaluate the selectivity of [Ir(TPQ)2(4-EO2-pic)] with of 1 equiv. of different metal cations
(Hg2+, Cu2+, Ca2+, Co2+, Cd2+, Pb2+, Zn2+, Ni2+, Mg2+, Mn2+, Al3+, Fe3+, Ag+, Na+ and La3+) was
investigated with fluorescence spectroscopy and the observed spectral changes (Fig. 3).
Fig.3 Changes in emission spectra of [Ir(TPQ)2(4-EO2-pic)] in CH3CN / H2O (v/v = 1/1) solution
with different metal ions
To explore sensitivity the [Ir(TPQ)2(4-EO2-pic)] was titrated against Hg2+ ions (Fig. 4). Upon the
addition of Hg2+ ions to [Ir(TPQ)2(4-EO2-pic)] the emission band at 620 nm has decreased until
it was quenched completely and a distinct emission color change from red to colorless is shown
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in the inset of Fig. 4, thereby realizing the high selectivity, sensitivity and the naked-eye
detection of Hg2+. The detection limit was calculated as 1.78 × 10-8 M (Fig.S6). The quantum
yield of [Ir(TPQ)2(4-EO2-pic)] complex was 0.21.
Fig.4 Changes in emission spectra of [Ir(TPQ)2(4-EO2-pic)] in CH3CN / H2O (v/v = 1/1) solution
with various conc. of Hg2+
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Fig.5 Absorption response of [Ir(TPQ)2(4-EO2-pic)] in CH3CN / H2O (v/v = 1/1) in the presence
of various metals. Bars represent the absorbance intensity ratio (A483/A442). Solid bars represent
the addition of various metal cations to 20 µM solution of [Ir(TPQ)2(4-EO2-pic)]. White bar
represent the A483/A442 ratio after addition of Hg2+ to the above solution. 1, Blank; 2, Cu2+ ; 3,
Ca2+; 4, Co2+;5, Cd2+; 6,Pb2+; 7, Zn2+;8, Ni2+; 9, Mg2+; 10, Mn2+; 11, Al3+; 12, Fe3+; 13, La3+; 14,
Ag+ ; 15 Na+ .
3.3 Electrochemical studies
Cyclic voltammetry (CV) studies were conducted to explore the further utility of [Ir(TPQ)2(4-
EO2-pic)] as electrochemical sensor for Hg2+ ions in CH3CN / H2O (v/v = 1/1). A reversible
oxidation wave has been observed at 0.98 V for [Ir(TPQ)2(4-EO2-pic)] without Hg2+ ions. The
addition of 1 equiv. of Hg2+ to the solution of [Ir(TPQ)2(4-EO2-pic)] a significant changes
occurred in the cyclic voltammetry (Fig.6). Further addition of Hg2+ ions gives only a minor
change, indicating that the (TPQ)2Ir(4-EO2-pic) forms 1:1 complex with Hg2+ ions. The
oxidation wave intensity at 0.98 V decreased and new reversible oxidation wave was appeared at
0.91 V.
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Fig.6 Cyclic voltammograms of [Ir(TPQ)2(4-EO2-pic)] in CH3CN / H2O (v/v = 1/1) solution in
the absence and presence of 1 equiv. of Hg2+ ions.
Table 1
Some of the reported chemosensor for Hg2+ ions
Sensor probe
Medium
water (pH 1)
Water
LOD
Reference
[14]
N-alkylaminopyrazole
Rhodamine-fluorene
0.1 ppb
7.48 nM
[16]
Ferrocenyl-based thioethers Acetonitrile
Ferrocenyl-based carbazole CH3CN/H2O (7/3)
derivative
6.93×10−7 M
3.16×10−7M
[18]
[19]
Thiophene quinoline based CH3CN/H2O (1/1)
Iridium complex
1.78 × 10-8 M
present work
3.3 Binding mechanism
To gain further insight into the binding mode of the [Ir(TPQ)2(4-EO2-pic)] with Hg2+,
1
comparison experiments of H NMR spectroscopic titration of [Ir(TPQ)2(4-EO2-pic)] with
different concentration of mercury perchlorate were also recorded in CD3CN / D2O (Fig. S7).
According to Pearson’s hard acid soft base theory mercury is interacting with sulfur atom of the
1
thiophene ring of [Ir(TPQ)2(4-EO2-pic)] and an obvious change was observed in the H NMR
spectra. Upon addition of Hg2+ to the solution of [Ir(TPQ)2(4-EO2-pic)], a dramatic changes
occurred in aromatic region. These outstanding findings strongly proved that there exists obvious
interaction between [Ir(TPQ)2(4-EO2-pic)] and Hg(II) ions.
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The binding stoichiometry ratio of [Ir(TPQ)2(4-EO2-pic)]-Hg2+ is recognized as 1:1 proved by
1
the method of absorption, fluorescence measurements and H-NMR, and the possible binding
mode of the sensing system is also proposed. Job’s plot was carried out to confirm the binding
ability of the [Ir(TPQ)2(4-EO2-pic)] complex (Fig. S8). According to the above investigated
results, we therefore proposed a possible binding mechanism of probe [Ir(TPQ)2(4-EO2-pic)]
with Hg2+, which was given in Scheme 2. We presume that the lone-pair electrons on the sulfur
atom of cyclometalated thiophene-quinoline ligand may interact with mercury analyte and
changes its local surroundings. This alteration probably leads to complicated energy transfer
from [Ir(TPQ)2(4-EO2-pic)] to the Hg2+ ions, which weakens its phosphorescent emission. To
gain further insight into the binding mode of the sulfur atom of thienyl group with Hg2+ ions, the
preliminary UV-Vis and fluorescence studies were also recorded for TPQ ligand (Fig.S9 –
Fig.S11).
Fig.7
Fig.8
Fig.7 & Fig. 8 are SEM images of [Ir(TPQ)2(4-EO2-pic)] without and with Hg2+ ions
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The surface morphology of pure complex and complex with metal were also observed using FE-
SEM analysis. Pure complex showed the crystalline morphology with microscale in Fig. 7. As
expected, the distinct morphology change was observed for the complex with Hg2+ in Fig. 8, due
to the interaction of mercury ions the crystalline morphology changed to amorphous. The
presence of mercury was also confirmed by Energy-Dispersive X-ray (EDX) spectrum (Fig. S12
&13).
O
O
O
O
O
O
N
N
Hg2+
O
Ir
O
Ir
N
N
2+
Hg
S
S
2
2
Scheme 2 Binding mechanism of [Ir(TPQ)2(4-EO2-pic)] with Hg2+
3.4 Theoretical studies
To understand the nature of the exited state, we have employed the DFT and time dependent
DFT (TD-DFT) calculations for Ir(III) complex [Ir(TPQ)2(4-EO2-pic)]. The Fig. 9 shows, the
ground state optimized geometry of the complex [Ir(TPQ)2(4-EO2-pic)]. The calculated
molecular orbital (HOMO-2, HOMO-1, HOMO, LUMO, LUMO+1, and LUMO+2)
distributions are displayed in Table S1 and the electronic excitation energies of low-lying singlet
and triplet excited states for complex [Ir(TPQ)2(4-EO2-pic)] are shown in Table S2. The
[Ir(TPQ)2(4-EO2-pic)] complex, the HOMO (highest occupied molecular orbital) mainly resides
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on the Iridium and thienyl part of the cyclometalated ligands and small contribution from the
quinolyl part of the cyclometalated ligands. Its LUMO (lowest unoccupied molecular orbital) is
distributed over the whole of N^C ligand and Iridium Centre.
Fig.9 Ground state optimized structure of the [Ir(TPQ)2(4-EO2-pic)] complex
The TD-DFT/B3LYP/LANL2DZ/6-31G* method using polarized continuum model (PCM) in
acetonitrile media has been used to understand the nature of the absorption spectra of the
[Ir(TPQ)2(4-EO2-pic)] complex, on the basis of the ground state (S0) optimized geometries, and
simulated absorption spectra shown in Fig. S14. As shown in the Table S2, the lowest S0‰S1
transition of [Ir(TPQ)2(4-EO2-pic)] complex is located at 495 nm. It can be seen that the
calculated absorbance band for 495 nm is in very good agreement with the experimentally
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observed value of 483 nm. The S1 state mainly composed on the HOMO‰LUMO transition.
According to the molecular orbital distribution analysis, this absorption band is mainly assigned
to the dπ(Ir)‰π*N^C metal to ligand charge transfer (MLCT) transition and (π-π*) N^C intra
ligand charge-transfer (ILCT) transitions. The TD-DFT approach was used to calculate the
phosphorescent spectra of acetonitrile media on the basis of the lowest lying triplet exited state
geometries. In general, TD-DFT systematically overestimate the transition energies, to get
reliable results six DFT functionals (M062X, M052X, B3LYP, PBE1PBE, BP86 and CAM-
B3LYP) was studied (Table S3). The obtained results revealed that the calculated triplet excited
state energies of the [Ir(TPQ)2(4-EO2-pic)] complex with the M062X functional are closer to the
experimental results (Table S3). Therefore, we have employed the M062X function for the
further emission spectra studies. The calculated lowest-lying excited state emission energies,
transition nature are shown in Table S2. The lowest-lying triplet states (T1) of the complex
[Ir(TPQ)2(4-EO2-pic)] is originate from the HOMO-LUMO (80%), HOMO-1-LUMO (12%) and
HOMO-LUMO+4 (2%) transitions, respectively. The calculated emission wavelength of
[Ir(TPQ)2(4-EO2-pic)] complex is 657 nm, is in close agreement with experimental wavelength
of 620 nm. According to orbital distributions, both the dπ(Ir)‰π*N^C] metal to ligand charge
transfer (MLCT) transition and intra ligand charge-transfer (ILCT) (π-π*) N^C transitions are
responsible for the exited state of [Ir(TPQ)2(4-EO2-pic)] complex.
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Fig. 10 [Ir(TPQ)2(4-EO2-pic)] printed papers
Fig. 11 Emission response of printed paper strip with various metals ions under UV light
In order to explore the practical application of [Ir(TPQ)2(4-EO2-pic)], a portable printed paper-
based sensor has been developed. The synthesized [Ir(TPQ)2(4-EO2-pic)] was dissolved in
aqueous alcoholic medium and filled in the empty commercial ink-jet printer cartridge and the
probe solution was printed on the filter papers (Whatman®) using a commercial inkjet printer as
shown in Fig. 10. Selectivity is a very important parameter in discussing whether a probe is good
or not. The binding behavior of inkjet-printed [Ir(TPQ)2(4-EO2-pic)] strips towards different
cations (Hg2+, Cu2+, Ca2+, Co2+, Cd2+, Pb2+, Zn2+, Ni2+, Mg2+, Mn2+, Al3+, Fe3+, Ag+, Na+ and
La3+) was investigated (Fig. 11).
Upon the addition of Hg2+ solution on the printed paper, color change phenomenon can be
observed more directly both by naked eye and UV light, due to the binding of mercury ions to
the printed strips. There are no changes occurred in the absence of Hg2+. As shown in Fig. 12 ,
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the color intensity gradually increased by increasing the concentration of Hg2+ in the printed
paper strips.
Fig.12 Emission changes of [Ir(TPQ)2(4-EO2-pic)] printed strips with various conc. of Hg2+ ions
4. Conclusions
In summary, we have successfully demonstrated the inkjet-printed phosphorescent paper based
chemosensor for the detection of Hg(II) ions. This appealing phosphorescent printing technique
has never been explored in Iridium(III) complex based chemosensors for Hg2+ ions. In addition,
the [Ir(TPQ)2(4-EO2-pic)] showed obvious naked-eye colour and phosphorescent changes with
Hg2+ ions in the presence of other metal cations. The sulfur atom of cyclometalated ligands is
responsible for the significant variations in optical and electrochemical signals. The
lowest
detection limit was calculated as 1.78 × 10-8 M. Cyclic voltammetry (CV) studies explored the
further utility of [Ir(TPQ)2(4-EO2-pic)] as electrochemical sensor for Hg2+ ions. We are very
confident that the phosphorescent printing technology could contribute to the development of
heavy metal ion sensors and OLED devices.
Acknowledgements
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Dr. N. S. Karthikeyan is grateful for the financial support from the DST – SERB for Young
Scientist Scheme (Ref NO. SB/FT/CS-067/2013). We acknowledged Prof. K. Muralidharan,
School of chemistry, and University of Hyderabad for the support under UGC-NRC internship.
References
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Highlights
‹ First of its kind ink-jet printed phosphorescent chemosensor (IJPCS)
‹ [Ir(TPQ)2(4-EO2-pic)] complex showed high Hg2+ selectivity both in aqueous CH3CN-H2O
(1:1 V/V) and in printed paper strips.
‹ The detection limit was calculated as 1.78 × 10-8 M.