Off-on fluorescent sensor from on-off sensor: Exploiting silver nanoparticles influence on the organic fluorophore fluorescence

Article abstract of DOI:10.1007/s10895-013-1333-4

Turn-off fluorescence of organic fluorophore, 2-{[4-(2H-Naphtho[1,2-d][1,2, 3]triazol-2-yl)-phenyl]carboxylic acid (NTPC), with metal ions (Fe³⁺, Cu²⁺, Pb²⁺) was converted into turn-on fluorescent sensor for biologically i

Full text of DOI:10.1007/s10895-013-1333-4

J Fluoresc
DOI 10.1007/s10895-013-1333-4
RAPID COMMUNICATION
Off-on Fluorescent Sensor from On-off Sensor: Exploiting
Silver Nanoparticles Influence on the Organic Fluorophore
Fluorescence
P. S. Hariharan & Arvind Sivasubramanian &
Savarimuthu Philip Anthony
Received: 25 September 2013 /Accepted: 20 November 2013
#
Springer Science+Business Media New York 2013
.
Keywords Heavy metal ions sensor Turn-on fluorescent
Abstract Turn-off fluorescence of organic fluorophore,
2-{[4-(2H-Naphtho[1,2-d][1,2,3]triazol-2-yl)-
phenyl]carboxylic acid (NTPC), with metal ions (Fe3+, Cu2+,
Pb2+) was converted into turn-on fluorescent sensor for bio-
logically important Zn2+, Cu2+ and Fe3+ metal ions in aqueous
solution at ppb level by exploiting strong fluorescence
quenching phenomena of metal nanoparticles when organic
fluorophores assembled in the vicinity of metallic surface.
Amino acid attached phenolic ligands (L) were used as reduc-
ing as well as functional capping agents in the synthesis of
silver nanoparticles (AgNPs). The hydrogen bonding func-
tionality of L facilitated the assembling of NTPC in the
vicinity of metallic surfaces that leads to complete quenching
of NTPC fluorescence. The strong and selective coordination
of L with metal ions (Zn2+, Cu2+ and Fe3+) separates the
NTPC from the AgNPs surface that turn-on the NTPC fluo-
rescence. HR-TEM and absorption studies confirm the metal
coordination with L and separation of NTPC from the AgNPs
surface. Mn2+ showed selective red shifting of NTPC fluores-
cence after 12 h with all sample. Effects of different amino
acid attached phenolic ligands were explored in the metal ion
sensitivity and selectivity. This approach demonstrates the
multifunctional utility of metal NPs in the development of
turn-on fluorescence sensor for paramagnetic heavy metal
ions in aqueous solution.
.
sensor fabrication Nanoparticles-organic fluorophore hybrid
.
sensor Supramolecular chemistry
Introduction
Exploration on chemosensors that can exhibit highly selective
and sensitive binding affinity towards transition metal ions has
attracted wide interest due to their crucial roles in various
biological and environmental processes [1–5]. Cu2+ and
Zn2+ ions are indispensable trace elements existing as catalytic
cofactors for a variety of metallo-enzymes and proteins and
plays important role in various physiological and pathological
processes [6, 7]. Under normal physiological conditions, brain
requires higher level of Cu2+ compared to other parts of the
body, but its excess accumulation causes neurodegenerative
ailments like Alzheimer’s, Wilson’s, and Menke’s diseases
[8]. Zinc-binding proteins such as metallothionein carry out
detoxification of lead by sequestering lead within enterocytes
[7]. Iron is a ubiquitous metal in cells, it being present in the
structure of many enzymes and proteins and therefore essen-
tial for cellular metabolism and enzyme catalysis [9].
Consequently, deficiency in Fe3+ leads to anemia, liver and
kidney damages, diabetes, and heart diseases [10, 11]. These
beneficial as well as detrimental roles of transition metal ions
have prompted researchers to develop efficient methods for
selective and sensitive assay of the metal ions both in vitro and
in vivo [12, 13]. Of the different kinds of sensing, fluores-
cence based approach has many advantages due to its high
sensitivity, straightforward application and real-time
:
:
P. S. Hariharan A. Sivasubramanian S. P. Anthony (*)
School of chemical & Biotechnology, SASTRA University,
Thanjavur 613 403, Tamil Nadu, India
e-mail: philip@biotech.sastra.edu

J Fluoresc
monitoring with fast response time [1–5, 14–17]. Particularly,
developing a turn-on fluorescence sensor for heavy metal ions
received significant attention because of its ease of detection
[18, 19]. However, the paramagnetic nature of the transition
elements such as Cu2+ and Fe3+ often leads to selective turn-
off fluorescence and very rarely showed turn-on fluorescence.
The recent emergence of nanoscience and nanotechnology
offered interesting opportunities to fabricate materials with
enhanced/desired properties for various applications across
physics, chemistry, biology and other interdisciplinary area
of science and technology. For instance, the strong, unique
and distance dependent optical properties of silver (Ag) and
gold (Au) metal nanoparticles (NPs) that can be chemically
engineered via surface functionalization have been success-
fully employed to develop colorimetric sensor for biological
molecules as well as heavy metal ions [20–27]. Ag and
AuNPs surfaces also exert a strong influence on the fluores-
cence properties of fluorophores placed in their vicinity. Two
opposite phenomena was observed depending on the separa-
tion distance between NPs surface and fluorophores. Strong
quenching of fluorescence intensity with a dramatic reduction
on the excited states lifetimes were reported when the separa-
tion distance is smaller than 5 nm [28–31]. Whereas in the
separation distance range of 10–20 nm, enhanced fluores-
cence emission was observed because of local concentration
of the incident excitation field by the metallic nanoparticles
[32–34]. The metal enhanced fluorescence has been widely
exploited in the molecular fluorescence measurements and
biosensors to increase the sensitivity and adaptability
[35–37]. The strong fluorescence quenching properties of
metal NPs could be employed to develop turn-on fluorescence
sensor for heavy metal ions by choosing right capping ligands.
The capping ligands should have both NPs stabilization and
selective metal ions interacting functionality. The selective
binding of metal ions with surface functionality of NPs is
expected to separate the fluorophore from NPs surface vicinity
that would re-generate the fluorescence.
Organic ligands based on amino acid attached phenols exhib-
ited versatile coordination with different metal ions that re-
sulted in the formation of intriguing structures including heli-
cal supramolecular structures in the solid state [45–47].
Hence, amino acid attached phenolic ligands could be directly
used in the synthesis and stabilization of AgNPs that might
also provide metal ions interacting surface functionality.
Further such organic functionality could be used to assemble
organic fluorophore via supramolecular interactions near the
vicinity of metal NPs surface. In the present work, we report
the strong fluorescence quenching of NTPC in aqueous solu-
tion by assembling in the vicinity of AgNPs surface and
complete turn-on fluorescence upon addition of biologically
important metal ions such as Zn2+, Cu2+ and Fe3+ (Scheme 1).
The hydrogen bonding functionalities of phenolic amino acid
ligands (L) of AgNPs and NTPC plays important role to
facilitate the fluorophore assembling in the vicinity of NPs
surfaces. The strong and selective coordination of L with
metal ions (Zn2+, Cu2+ and Fe3+) separates the NTPC from
the AgNPs surface that leads to complete regeneration of
fluorescence. HR-TEM and absorption studies confirm the
metal ions interaction with L-AgNPs. Effect of amino acid
modification on the metal ions sensitivity and selectivity was
also explored. The present approach demonstrates the multi-
functional utility of metal NPs and an easy development of
turn-on fluorescence sensor for paramagnetic heavy metal
ions in aqueous solution.
Experimental Section
Chemicals Amino acid, ethanol, p-aminobenzoic acid,
Cu(OAc)2, pyridine, HCl, NaNO2, DMF, DMSO and NaOH
were obtained from Ranbaxy, India and used as received.
Salicylaldehyde, 2-aminonaphthalene, NaBH4, AgNO3,
poly(vinyl alcohol) (PVA, M.Wt) were obtained from
Sigma-Aldrich and were used as received. All the heavy metal
salt solutions used for the experiments were prepared by
mixing the requisite amount of salt in Mill-Q water. NTPC
was synthesized following the literature procedure [48].
Amino acid attached phenolic chelating ligands were synthe-
sized according to the reported procedure [45–47].
Chemical molecules with 1, 2, 3-triazole unit in the struc-
ture have been utilized as intermediates for the synthesis of
several products such as dyes [38] or optical brighteners [39].
Interestingly, naphthotriazoles exhibit intense fluorescence in
the near UVand visible regions with high fluorescence quan-
tum yield (>50 %, [40]) which is comparable to that of highly
fluorescent compounds such as 1-aminonaphthalene or 9,10-
diphenylanthracene [41]. Hence, naphthotriazoles are used as
whitening agents [42], fluorescent probes with peptides [41]
and sulfonated derivatives of 2H-naphthotriazoles are well-
known in the textile industry [43]. Naphthotriazoles with
different chemical functionalities including carboxylic and
sulfonic acid that makes it water soluble showed strong fluo-
rescence both in organic and aqueous solution.
General Procedures for the Synthesis
of 2-{[4-(2H-Naphtho[1,2-d][1,2,3]triazol-2-yl)-phenyl]
carboxylic acid (NTPC)
Diazotization
To a solution of p-aminobenzoic acid (6 mmol) in H2O
(13.0 mL), concentrated HCl (2.8 mL; 25 mmol) was slowly
added. The reaction mixture was cooled to 0–5 °C and 5 M
The easy ionization properties of phenolic groups have
been made use in the synthesis of AgNPs in the past [44].

J Fluoresc
Scheme 1 Schematic diagram of
selective turn-on fluorescence
sensor fabrication for Zn2+, Cu2+
and Fe3+
NaNO2 (0.27 mL mmol−1 of substrate) was added drop wise;
the mixture was stirred for 30 min. The excess HNO2 (I2-
starch test) was destroyed using amidosulfuric acid.
128.50 (C-7, C-8), 129.40 (C-9), 129.91 (C-2′, C-6′),
130.94 (C-5), 132.23 (C-3a), 139.57 (C-4′), 142.67 (C-
1′), 142.83 (C-9b), 143.66 (C-3a), 160.22(C=O)
HRMS calcd for C17H11N3O2: 289.0851. Found: (M-
1)+288.0849.
Azo Coupling
To a solution of 2-aminonaphthalene (7 mmol) in H2O
(10.0 mL), concentrated HCl (1.0 mL; 9 mmol) was added
and the mixture heated for 10 min. After cooling, in an ice-
acetone bath, the diazonium salt solution was added drop
wise, pH was adjusted to 5 using a 5 M NaOH solution, and
the mixture stirred for 1 h. When the coupling was complete
(H-acid test), pH was adjusted to 7, the precipitated dye was
filtered off and used in the next step without drying.
General Procedure for the Synthesis of Ligand Functionalized
AgNPs
Silver nitrate (5 ml of 10−3 M) was added into VP and ILP
(5 ml, 10−3 M) solution under stirring at room temperature.
The immediate appearance of yellow colour indicated the
reduction of silver ion into AgNPs. The solution was allowed
to stir at room temperature for another 10 min. The reactions
were repeated at least three times to confirm the reproducibil-
ity of NPs formation. The characterization of the synthesized
AgNPs was carried out after allowing the solution to stand at
room temperature for more than 1 week. This was preserved
as a stock solution for metal nanoparticles fluorescence
quenching experiments.
Oxidation With Copper Acetate
To the above dye, pyridine (5.0 mL), H2O (2.0 mL) and
Cu(OAc)2 (2.5 g; 13.8 mmol) were added, the mixture was
refluxed for 15 min and poured into water. The precipitated
solid was filtered off and dried. Recrystallization from hot
ethanol afforded the final compound.
Characterization
Yield: 1.2 g, 49 %.
1H NMR (300 MHz, DMSO-d6): 7.71–7.80 (m, 2H (H-
7, H-8), 7.91 (d, J=9.3 Hz, 1H, H-4), 7.94 (d, J=9.0 Hz,
1H, H-5), 8.08 (dd, J=2.1, 6.6 Hz, 1H, H-9), 8.16 (d,
J=9.0 Hz, 2H, H-2′, H-6′), 8.54 (d, J=9.0 Hz, 2H, H-3′,
H-5′), 8.55 (dd, J=2.1, 6.6 Hz, 1H, H-6).
A Bruker DRX500 spectrometer recorded 1H and 13C NMR
spectra of the compound at 400 MHz and 100 MHz respec-
tively. 1H chemical shifts were reported in ppm downfield
from tetramethylsiane (TMS, δ scale with the solvent reso-
nances as internal standards). Absorption and fluorescence
spectra were recorded using Perking Elmer Lambda 1050
and Jasco fluorescence spectrometer-FP-8200 instruments.
13C NMR (75.4 MHz, DMSO-d6):, 116.20 (C-4), 120.04
(C-3′, C-5′), 122.93 (C-6), 123.75 (C-9a), 128.27 and

J Fluoresc
Fluorescence metal ion selectivity measurement was per-
formed by adding excess concentration of metal salt (10:1)
into NTPC solution. FT-IR spectroscopy measurements were
carried out on a Perkin–Elmer Spectrum-One instrument in
the diffuse reflectance mode at a resolution of 4 cm−1 in KBr
pellets.
The size and morphology of AgNPs were investigated
using HR-TEM (High Resolution Transmission Electron
Microscopy). 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 JEM-2100 F operated at an accelerated
voltage of 200 kVand an ultra high-resolution pole piece.
Results and Discussion
Pure NTPC dissolves only in organic solvents and shows weak
to strong fluorescence with almost same λmax from different
Fig. 1 (a) NTPC fluorescence in different solvents. NTPC (b) fluores-
cence and (c) absorption with different metal ions in DMF
Fig. 2 Fluorescence of NTPC in (a) DMF and water and (b) with
different metal ions in water

J Fluoresc
solvents (Fig. 1a). In chloroform and dichloromethane, NTPC
showed weak fluorescence at λmax=384 nm (quantum yield,
ΦF =0.25 on comparison with quinine sulfate), but in DMF it
showed strong fluorescence (ΦF =0.71). Acetone, methanol
and ethanol solution showed similar fluorescence intensity. It
was found that the position and shape of the fluorescence
spectrum is independent of the excitation wavelength both at
342 or 360 nm. The small Stokes shift of the NTPC, demon-
strated that, there is no substantial geometry difference be-
tween vertical and relaxed (fluorescence) states, which could
be related to the planarity of the triazole chromophore. The
metal ion sensor studies of NTPC in DMF exhibited complete
fluorescence quenching for Fe3+ but only at very high concen-
tration (500 μl of 10−2 M, Fig. 1b). Absorption spectra of
NTPC in DMF showed two absorption peaks at 342 and
360 nm (Fig. 1c) and these band corresponds to π-π* transi-
tion. Addition of different metal ions did not show significant
change in the NTPC absorption except with Cu2+ and Fe3+.
Addition of Cu2+ with NTPC showed slightly red shifted λcut-
disappeared and showed small red shift in the λmax (384 to
391 nm). Metal ion sensor studies in water also showed
selective quenching of NTPC fluorescence with three metal
cations, Fe3+, Cu2+ and Pb2+ (Fig. 2b) but again only at higher
concentration (500 μl of 10-2 M). These results suggest that
Pb2+, Cu2+ and Fe3+ might be forming coordination complexes
with NTPC at higher concentration. The mechanism of NTPC
fluorescence quenching might be due to the electronic energy
transfer (EET) and/or photo-induced electron transfer (PET)
from ligand to metal centre that has paramagnetic unfilled
electronic configuration of d-orbital [49].
AgNPs functionalized with amino acid attached phenolic
chelating ligand was yellow and transparent, indicating the
good dispersity in water. Phenolic ligands with different ami-
no acids were used in the synthesis of AgNPs (Scheme 1). The
absorption spectrum of L-AgNPs showed a typical and intense
absorption peak between 409 nm to 418 nm which is due to
surface plasmon resonance vibration (Fig. 3a) [50]. The
without changing absorption peak position. However,
off
NTPC with Fe3+ exhibited completely different absorption
spectrum and confirms the selective interaction of NTPC with
Fe3+. Although pure NTPC is not soluble in water, conversion
of NTPC carboxylic acid into sodium salt makes it soluble and
importantly it retained its strong fluorescence in water also
(Fig. 2a). The small hump appeared at 368 nm in DMF has
Fig. 3 Absorption spectra of (a) L-AgNPs and (b) HR-TEM of (i) L3-
AgNPs and (ii) L5-AgNPs
Fig. 4 (a) IR spectra of L-AgNPs and (b) PXRD patterns of L3-AgNPs

J Fluoresc
synthesized AgNPs size and morphology were characterized
by using HR-TEM that confirmed the formation of poly-
dispersed spherical crystalline AgNPs (Fig. 3b). The amino
acid attached phenolic chelating ligand is expected to provide
metal ions interacting surface functionality to the AgNPs.
Prominent IR bands observed at 3,434, 2,931, 2,853, 1,696,
1,615, 1,430 and 1,385 cm−1 for all samples confirmed the
surface functionalization of AgNPs by amino acid attached
phenolic ligands (Fig. 4a). The phase structure of the prepared
AgNPs was characterized by powder X-ray diffraction
(XRD). Figure 4b shows the representative case for AgNPs
prepared by using L3 and peak at 38.2 and 44.1 agree well
with the (111) and (200) diffraction of face centered cubic
(fcc) silver (JCPDS file no. 04–0783). The other two peaks
assigned to the glass matrix.
[28–31]. The hydrogen bonding functionality of both NTPC
and amino acid based phenolic ligands are expected to facili-
tate closer assembling of fluorophore near NPs surface.
Assembling fluorophore near the vicinity of metallic nano-
particles surface is known to quench the fluorescence strongly
Fig. 5 Turn-on fluorescence studies of (a) NTPC-L3-AgNPs and (b)
NTPC-L5-AgNPs with different metal ions (λexc=340 nm)
Fig. 6 Concentration dependent studies of turn-on fluorescence with (a)
Zn2+, (b) Cu2+ and (c) Fe3+ in water (λexc=340 nm)

J Fluoresc
Addition of L-AgNPs (100 μl of 10−4 M) into aqueous solu-
tion of NTPC (200 μl of 10−6 M of DMF solution in 2 ml of
water) completely quenched the fluorescence (Fig. 5, S).
Amino acid attached phenolic chelating ligands, L, are strong-
ly coordinating ligands for metal ions and several coordination
polymers with intriguing solid state structures have been gen-
erated by self-assembling L with metal ions [45–47]. Hence
coordination complex formation of metal ions with L-AgNPs
is expected to separate NTPC fluorophore from the AgNPs
vicinity. The separation would lead to the regeneration of
NTPC fluorescence. Different metal ions (Na+, Mg2+, Cs2+,
Ca2+, Zn2+, Cd2+, Cr3+, Hg2+, Cu2+, Fe3+, Pb2+ and Ni2+) were
added into NTPC-L-AgNPs and monitored the fluorescence
change. Interestingly, all NTPC-L-AgNPs samples showed
selective turn-on fluorescence for biologically important
Zn2+, Cu2+ and Fe3+ metal ions in aqueous solution. Figure 5
shows the turn-on fluorescence of NTPC-L3-AgNPs and
NTPC-L5-AgNPs selectively with Zn2+, Cu2+ and Fe3+ in
aqueous solution. Addition of other metal ions either showed
partial regeneration of fluorescence or no fluorescence regen-
eration. The observed fluorescence turn-on for Zn2+, Cu2+ and
Fe3+ metal ions was due to the formation of metal coordination
complex with L-AgNPs that separates NTPC fluorophore
from the AgNPs surface vicinity. Concentration dependent
studies of NTPC-L5-AgNPs showed complete regeneration
of fluorescence intensity by addition of 650–700 μl of Zn2+,
Cu2+ and Fe3+ metal ions at ppb level (Fig. 6). Absorption
studies were performed to get the insight of the complex
formation with L-AgNPs (Fig. 7a). NTPC-L5-AgNPs showed
both absorption corresponding to NTPC and L5-AgNPs.
However, absorption peak of L5-AgNPs almost vanished with
Zn2+, Cu2+ and Fe3+ metal ions that confirm the formation of
coordination complex with L5. The metal coordination with L
is expected to produce smaller aggregates of AgNPs. HR-EM
analysis of Fe3+ metal ions added NTPC-L3-and L5-AgNPs
complexes clearly show the aggregate formation and supports
the metal coordination (Fig. 7b).
Interestingly, Mn2+ addition into NTPC-L-AgNPs did not
show any change of NTPC fluorescence or regeneration im-
mediately. However, after 12 h, all samples with Mn2+ exhib-
ited strongly red shifted fluorescence at 500 nm (Fig. 8a).
Other metal ions did not show any such fluorescence shift.
Fluorescence spectra of all Mn2+ added samples showed two
λ
max; one at 500 nm and other at 385 nm. Mn2+ with NTPC-
L3 and L5-AgNPs exhibited strong intensity at 500 nm along
Fig. 7 (a) Absorption spectra of NTPC-L5-AgNPs with Zn2+, Cu2+ and
Fe3+ in water and (b) HR-TEM images of (i) NTPC-L3-AgNPs and (ii)
NTPC-L5-AgNPs with Fe3+
Fig. 8 (a) Fluorescence spectra of Mn2+ with NTPC-L-AgNps and (b)
controlled experiment. Digital images of fluorescence changes are shown
in the inset

J Fluoresc
with the 385 nm fluorescence. All other samples showed weak
fluorescence at 500 nm and strong fluorescence at 385 nm.
Controlled experiments were performed with different com-
bination of Mn2+ to get insight for the fluorescence change
(Fig. 8b). Mn2+-L4-AgNPs (without NTPC), Mn2+-NTPC-L4
(without AgNPs) and Mn2+-NTPC (without L4-AgNPs) did
not show any fluorescence change even after 24 h. These
studies indicate that presence of AgNPs, L as well as NTPC
fluorophore is important for the fluorescence red shift.
Absorption studies of Mn2+ with NTPC-L5-AgNPs revealed
no change immediately. However, absorption of L5-AgNPs
was completely disappeared after 12 h with appearance of new
peak at 500 nm. (Fig. 9a). HR-TEM studies confirmed the
presence of AgNPs in Mn2+ added NTPC-L5-AgNPs but with
aggregated form (Fig. 9b). This might be due to the formation
coordination complex very slowly.
Conclusion
We have demonstrated a simple approach in developing turn-
on fluorescent sensor based on AgNPs and organic
fluorophore hybrid materials for heavy metal ions including
paramagnetic metal ions by exploiting unique characteristics
of noble metal nanoparticles on the organic fluorophore fluo-
rescence. Assembling organic fluorophore, NTPC, near the
vicinity of AgNPs surface via hydrogen bonding interactions
of capping ligands with NTPC strongly quenches the organic
fluorescence. The strong and selective coordination of amino
acid attached phenolic chelating ligands with biologically
important metal ions, Zn2+, Cu2+ and Fe3+, separates
fluorophore from AgNPs surface and turn-on the NTPC fluo-
rescence. Concentration dependent studies showed selective
detection of Zn2+, Cu2+ and Fe3+ metal ions in aqueous
solution up to ppb level. Absorption and HR-TEM studies
further support the formation of coordination complexes with
metal ions. Change of amino acids did not show any influence
on the metal ion selectivity. Mn2+ selectively red shifted the
NTPC fluorescence λmax with all samples after 12 h. It is
expected that fine tuning of metal ion interacting capping
ligands of NPs would lead to highly selective and sensitive
turn-on fluorescence sensor for paramagnetic heavy metal
ions in aqueous solution.
Acknowledgments Financial supports from Department of Science
and Technology, New Delhi, India (DST Fast Track Scheme No. SR/
FT/CS-03/2011 (G), SR/FT/CS-10/2011 and SR/FST/ETI-284/2011(c))
are acknowledged with gratitude.
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