The tuning of metal enhanced fluorescence for sensing applications

Article abstract of DOI:10.1039/c3dt52258j

Stable coinage metal nanoparticles (NPs) have been synthesized individually in an aqueous alkaline solution from the corresponding metal salts as precursors using the condensation product (CP) of salicylaldehyde and triethylenetetramine as a reagent. Silver and gold NPs are obtained with and without light illumination but UV irradiation is essential for Cu(0)NP formation. During nanoparticle formation the CP is oxidized to OCP which eventually becomes a fluorophore and also a stabilizer for the in situ produced NPs. It has been observed that silver and gold particle formation kinetics is accelerated by UV exposure. Thus the ease of evolution of coinage metal NP formation relates to their nobility. The as prepared OCP solutions containing coinage metals exhibit a fluorescence contrast behaviour (fluorescence enhancement by Cu and Ag; quenching by AuNP) due to the match and mismatch of wave vectors. The electric field evident from the FDTD simulation abreast of the scattering cross section of the NPs governed from Mie theory as a consequence of surface plasmon coupled emission (SPCE), near field electromagnetic intensity enhancement and lightening rod effect concentrating the electric field around the fluorophore are responsible for the Cu and AgNPs stimulated fluorescence. Again, lossy surface waves are anticipated for efficient quenching by the AuNPs. The most unprecedented observation is 'Turn On' fluorescence which is reported here as a result of the substitution of Au(0) or Cu(0) by Ag(0). Finally, the preferential fluorescence enhancement helps the selective detection of Ag(I) and Cu(II) well below the US Environmental Protection Agency (EPA) permissible level by tuning the experimental conditions. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3dt52258j

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The tuning of metal enhanced fluorescence for
sensing applications†
Cite this: Dalton Trans., 2014, 43,
1032
Mainak Ganguly,a Chanchal Mondal,a Joydeep Chowdhury,b Jaya Pal,a Anjali Palc
and Tarasankar Pal*a
Stable coinage metal nanoparticles (NPs) have been synthesized individually in an aqueous alkaline solu-
tion from the corresponding metal salts as precursors using the condensation product (CP) of salicylalde-
hyde and triethylenetetramine as a reagent. Silver and gold NPs are obtained with and without light
illumination but UV irradiation is essential for Cu(0)NP formation. During nanoparticle formation the CP is
oxidized to OCP which eventually becomes a fluorophore and also a stabilizer for the in situ produced
NPs. It has been observed that silver and gold particle formation kinetics is accelerated by UV exposure.
Thus the ease of evolution of coinage metal NP formation relates to their nobility. The as prepared OCP
solutions containing coinage metals exhibit a fluorescence contrast behaviour (fluorescence enhance-
ment by Cu and Ag; quenching by AuNP) due to the match and mismatch of wave vectors. The electric
field evident from the FDTD simulation abreast of the scattering cross section of the NPs governed from
Mie theory as a consequence of surface plasmon coupled emission (SPCE), near field electromagnetic
intensity enhancement and lightening rod effect concentrating the electric field around the fluorophore
are responsible for the Cu and AgNPs stimulated fluorescence. Again, lossy surface waves are anticipated
for efficient quenching by the AuNPs. The most unprecedented observation is ‘Turn On’ fluorescence
which is reported here as a result of the substitution of Au(0) or Cu(0) by Ag(0). Finally, the preferential flu-
orescence enhancement helps the selective detection of Ag(I) and Cu(II) well below the US Environmental
Protection Agency (EPA) permissible level by tuning the experimental conditions.
Received 19th August 2013,
Accepted 1st October 2013
DOI: 10.1039/c3dt52258j
www.rsc.org/dalton
SIFs can cause quenching, increased rates of excitation, and/or
increased quantum yields depending on the way of interaction
Introduction
Fluorescence measurement in bulk samples is a very common with the fluorophore.1 Not only SIFs, but other various silver
practice where the solutions are typically transparent to the surfaces like silver colloids,3 silver nanotriangles,4 silver nano-
emitted radiation and isotropic emission of the fluorophore in rods,5 and fractal-like silver surfaces6 are also found in the
the free space, which is observed in the far field. An alteration context of metal enhanced fluorescence (MEF).
of the refractive index is the main consequence of it. The free
One very promising advantage of colloidal suspension over
space spectral properties of the fluorophore are very minutely films is the homogeneous distribution of the metals and their
influenced with the change of refractive index.1 To explain the possible promising impregnation for medical imaging. But, a
huge change of fluorescence near the metallic surface, modifi- lot of interactions in solution make the study complicated.
cations of the free space conditions of the fluorophore have Moreover, to position the metals at the right distance from the
come into the field during the past decade.
fluorophore in solution phase is very difficult. An interesting
The report of Lakowicz et al. reveal that silver island films report by Aslan et al. to produce metal enhanced fluorescence
(SIFs) are interesting candidates for escalating the intensities in solution has been found to be a potential sensing platform.7
and photostabilities of fluorophores with low quantum yields.2 Again, the Tb3+ complex of salicylic-based open-chain crown
ether possesses enhanced fluorescence in silver chloride
collosol.8 A study by Gill et al. demonstrates that a dye-DNA
aDepartment of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
bDepartment of Physics, Sammilani Mahavidyalya, Kolkata-700075, India
cDepartment of Civil Engineering, Indian Institute of Technology, Kharagpur-721302,
composite becomes extremely fluorescent with silver nano
aggregates.9
Not only silver, but also gold and copper are known to
exhibit MEF. Of course, the number of publications2–6,10 is
much higher for silver as it has more favorable imaginary
component in the dielectric function than gold and copper. The
India. E-mail: tpal@chem.iitkgp.ernet.in; Fax: (+)91-03222-255303
†Electronic supplementary information (ESI) available: IR, mass, NMR, fluore-
scence, excitation, absorption and Raman spectra, FESEM image, mechanism of
NP formation. See DOI: 10.1039/c3dt52258j
1032 | Dalton Trans., 2014, 43, 1032–1047
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absorption cross section depends on the imaginary component in situ oxidized CP (OCP) in their individual capacity and
of the metal and it is higher for copper than gold or silver. So, experimental conditions. Surface plasmon coupled emission,
the close-proximity of the fluorophore would cause mostly near field electromagnetic intensity enhancement and the
quenching by the copper nanoparticles. Again, copper also has increasing local incident field around the fluorophore
a scattering component responsible for the MEF effect.11
(lightening rod effect) have been ascribed for such an
Localization of surface plasmon resonance (LSPR) at the enhancement.31–33 This MEF phenomenon has prompted us
surface of metal nanoparticles creating an enhancement of the to design a highly selective detection method for Cu(II) and
electromagnetic field as well as the coupling between the Ag(I) with the lower detection limit far below the prescribed
surface plasmon field of the metal and the molecular dipole of level as published by the US Environmental Protection Agency
the fluorophore are two mechanisms for the enhancement of (EPA) [∼460 nM for silver, ∼20 μM for copper]. On the contrary,
fluorescence. According to the radiating plasmon (RP) model, gold nanoparticles produced in the presence of a CP greatly
the enhancement as well as the quenching of the fluorophore quench the fluorescence of the OCP in solution due to lossy
near the surface of the metal are determined by the optical pro- surface waves.
perties of the metal structures calculated from electro-
dynamics.12 The extent of the enhancement also lies in the
geometry of the metallic nanostructures contributing different
Experimental section
surface plasmonic modes at “hotspots”.13 The quenching effect
Materials and instruments
in the proximity of the metal nanoparticles has been ascribed
All of the reagents were of AR grade. Throughout the experi-
ment, triple distilled water was used. Salicylaldehyde, triethy-
lenetetramine (TETA), silver nitrate, chloroauric acid, copper
sulphate and dioctyl sulfosuccinate sodium salt (AOT) were
obtained from Sigma-Aldrich. NaOH was purchased from
HiMedia Laboratories Pvt. Ltd. n-Heptane was purchased from
Merck. All glass wares were cleaned with freshly prepared aqua
regia, subsequently rinsed with copious amount of distilled
water and dried well before use.
to lossy surface waves (LSWs), dissipated losses, ohmic losses,
and such similar phenomena. All of them are members of non-
radiative dissipation of energy within metals for the proximity
based quenching. The energy of the quenching causing
trapped plasmons cannot be recovered as a fruitful signal.
A Schiff base [(2-(2′-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphe-
nyl)-3-azabut-3-enyl]-1,3-imidazolidine)], CP (the condensation
product of salicylaldehyde and triethylenetetramine) is mainly
used as a precursor of different metal–ligand coordinations.14
The CP is also known to form mixed valent complexes.15 The
structure of the CP and its metal complexes were confirmed
from single crystal XRD analysis. The luminescence of di-
nuclear lanthanide(III)–CP is also reported.16 Lijuan et al.17 have
described the fluorescence behaviour of La(III), Ce(IV), Eu(II)
complexes of CP.
The sample solution was irradiated with a TUV 15W/G 15
T8 ultra-violet light (Philips India) source. All of the UV-vis
absorption spectra were recorded with a SPECTRASCAN UV
2600 digital spectrophotometer (Chemito, India). The FT-IR
spectra were recorded with a FT-IR Nexus spectrophotometer
1
(Thermo Nicolet). The H-NMR spectrum was obtained with a
400 MHz Bruker NMR instrument. The X-ray photoelectron
spectroscopy (XPS) analysis was carried out with a VG Scienti-
fic ESCALAB MK II spectrometer (UK) equipped with a Mg Kα
excitation source (1253.6 eV) and a five-channeltron detection
system. The fluorescence measurement was carried out at
room temperature using a LS55 fluorescence spectrometer
(Perkin Elmer, USA). The particle morphology was examined
using a field emission scanning electron microscope (Supra
40, Carl ZEISS Pvt. Ltd). TEM analysis was performed with an
instrument H-9000 NAR, Hitachi, using an accelerating voltage
of 300 kV. The Raman spectra of the samples were obtained
with a Renishaw Raman microscope, equipped with the laser
sources (633, 785, and 514 nm) and a Peltiercooled (∼70 °C)
charge-coupled device (CCD) camera. A Leica DMLM micro-
scope was attached and was fitted with three objectives.
In spite of invasive utility and the broad prospects of sliver
in the electronic, photographic and imaging industry,18,19
much attention has been paid to detect Ag(I) ions owing to the
high toxicity of silver for aquatic organisms. Again, silver can
inactivate sulphydryl enzymes and accumulate in the body.20
Some efforts have been performed by different groups employ-
ing fluorescence, UV-visible absorption, atomic absorption,
and ICP atomic emission spectroscopy to detect silver
ions.21,22 Fluorescent sensors are mostly ready for in vivo and
in vitro cellular imaging claiming the superiority of the fluo-
rescence approach compared to other analytical methods.
Likewise copper, the third most abundant metal in the human
body, plays pivotal roles in many biological processes. Unfortu-
nately, free copper ions are lethal causing cellular toxicity with
serious diseases including Alzheimer’s disease,23 Indian child-
hood cirrhosis (ICC),24 prion disease,25 and Menkes and
Wilson diseases.26,27 So, a fluorescent sensor for copper detec-
tion becomes a hot research field.28–30
The synthesis of CP
A solution of 2.92 g (20 mmol) TETA in 20 mL of ethanol was
In our present study, a CP has been employed for the first added to a solution of 7.32 g (60 mmol) of salicylaldehyde in
time for the synthesis of coinage metal nanoparticles (MNPs) 10 mL of ethanol with vigorous stirring under ice cold con-
i.e., copper, silver and gold nanoparticles. As a corollary of ditions. A bright yellow precipitate was formed after a few
synthetic protocol, the as produced silver and copper nano- hours. The solid product was collected by filtration and dried
particles show a huge enhancement in the fluorescence of the at room temperature after washing repeatedly with diethyl
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where εs, ε∞, σ and τ represent the static permittivity, infinite
frequency permittivity, conductivity, and relaxation time
respectively, ω is the angular frequency, and ε0 is the permittiv-
ity of free space. The four parameters εs, ε∞, σ and τ are then
adjusted through a curve fitting technique to match the
complex permittivity correctly. The complex permittivity ε(ω),
as obtained from the generalized Drude model, faithfully rep-
resents the optical response of the silver and copper nano
materials over a wide frequency range, thereby ensuring the
accuracy of the FDTD simulation.35,36 The Yee cell used in the
calculation was set at 1 × 1 × 1 nm3 and the step size was fixed
at 1.93 atto second. The number of periods of the incident
sinusoidal wave was fixed at 10 to ensure the convergence of
the calculation. The amplitude of the incident sinusoidal wave
was set at 1 V m−1 for the excitation wavelength of 425 nm.
The 3D-FDTD calculations were performed using XFDTD 6.3
software (RemCom XFDTD 6.3).
Scheme 1 A reaction scheme indicating the formation of the conden-
sation product (CP) and MNP.
ether. The crude CP (Scheme 1) was recrystallized from metha-
nol. The compound was characterized from melting point
(100 °C), elemental analysis (Table S1, ESI†), infrared (Fig. S1,
ESI†), mass (Fig. S2, ESI†), 1H-NMR (Fig. S3, ESI†) and
13C-NMR (Fig. S4, ESI†) analyses.
Results and discussion
The compound CP, a condensation product of salicylaldehyde
and triethylenetetramine (TETA), is known to form complex
with only one coinage metal copper, where the ligand to
copper ratio becomes 1 : 2.14 We have judiciously selected CP
and made proper use of it so that it inculcates dramatic fluo-
rescence behaviour with all of the three coinage metal nano-
particles (NPs) i.e. with Au(0), Ag(0) and Cu(0) NPs (Scheme 1).
The CP in our present study is a reagent for the synthesis of
AuNPs, AgNPs and CuNPs. The synthetic protocol keeps the
Cu(0), Ag(0) and Au(0) nanoparticles in such a position37,38
from the in situ oxidized CP (OCP) that the AgNPs and CuNPs
show an enhancement of fluorescence and the AuNPs exhibit
quenching (Scheme 2). In this context it should be mentioned
that CP is not water-soluble and we have carried out all our
experiments in a 0.1 M NaOH solution.
The synthesis of the coinage metal nanoparticles
In a 3.2 mL 0.1 M NaOH solution, 0.2 mL 2.5 × 10−3 M CP and
0.15 mL 10−2 M AgNO3 were mixed together with stirring. The
CP was dissolved in a 0.1 M NaOH solution as it is insoluble in
water. The solution was taken in a fluorescence cuvette and
irradiated for one hour to obtain silver nanoparticles (AgNPs).
Similarly, gold nanoparticles (AuNPs) were synthesized using
HAuCl4 instead of AgNO3 followed by UV exposure for
11 hours. For the preparation of the copper nanoparticles
(CuNPs), instead of AgNO3 or HAuCl4, 0.025 mL 10−2 M CuSO4
was introduced and the reaction mixture was exposed to UV
light for a period of 14 hours.
Computational details
The Mie theory calculations have been performed using Mie
Calc software.34 Mie theory calculations were performed for
the excitation wavelength of 425 nm which corresponds to the
emission wavelength of the fluorophore. In the above-men-
tioned calculations, the complex refractive index (np) for the
gold, silver and copper nanoparticles and the real refractive
index of the surrounding aqueous medium are taken as 1.45 +
i 1.948, 0.04 + i 2.462, 1.25 1 + i 2.305 and 1.331, respectively,
for the wavelength of incident radiation (λ) 425 nm.
The three dimensional finite difference time domain
(3D-FDTD) simulation technique has been utilized to simulate
the spatial distribution of the electric field around the silver
and copper nanoparticle surfaces. The dispersive properties of
the silver and copper nanomaterials have been incorporated in
the simulation using the generalized Drude model. According
to the generalized Drude model, the complex permittivity ε(ω)
is represented as:
Scheme 2 A schematic representation of the synthesis of coinage
metal nanoparticles with the condensation product (CP) of salicylalde-
hyde and triethylenetetramine (TETA) and then the intriguing fluor-
escence behaviour of the oxidized CP (OCP) by the in situ produced
metal nanoparticles.
εs ꢀ ε
σ
εðωÞ ¼ ε1 þ
1 þ
1 þ iωτ iωε0
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The oxidation of the CP and in turn the reduction of the place and a monodispersed gold hydrosol is generated with
metal salts take place simultaneously in solution. The phenolic smaller AuNPs (∼5 nm) bringing a tight size distribution
group (λem = 500 nm) of CP is oxidized to its quinone form profile after 8 days. And, a homogeneous red solution is
(λem = 430 nm) which might have the same mechanism as obtained. Importantly, only UV exposure makes the process of
suggested by Selvakannan et al.39 (Scheme S1, ESI†). The mass CuNPs nanoparticle formation feasible (Fig. 1). Without
spectrum (Fig. S1, ESI†) also supports the formation of a irradiation, the fluorescence of the alkaline CP is decreased in
quinone (not dimer of CP). The quinone formation is further presence of Cu(II) due to spin–orbit coupling.37
supported from the 70 nm blue shift and decreased fluo-
In the absence of any metal ions, the fluorescence of CP
rescence due to the loss of conjugation (Fig. 1). Previously we (λem = 500 nm) is destroyed due to photobleaching43 under UV
have reported that UV exposure is essential to reduce the metal light exposure due to polymerization.44 It is worth mentioning
ions for the formation of nanoparticles in the presence of that a peak at ∼430 nm appears due to a significant amount of
salen-like molecules.37,38 But in the present case, UV exposure OCP formation upon UV irradiation of an alkaline CP solution.
is not vital for the evolution of AgNPs and AuNPs in an alka- Finally, both the peaks due to CP and OCP are decreased with
line CP solution. However, for the formation of CuNPs, UV time under UV light irradiation. So, the instability of the
exposure is essential due to the comparatively low reduction exposed CP and OCP in the alkaline solution is verified.
potential (E0 = +0.34 V) of Cu(II)/Cu(0). Cu(II) needs drastic con- Fig. 2A represents the kinetics of decrement of the fluo-
ditions/a high activation energy which is supplemented by pro- rescence intensity of the CP under ∼365 nm UV light exposure.
longed (14 hours) UV irradiation for the formation of CuNPs. When a mixture of alkaline CP and Ag(I) is irradiated under
Nevertheless, we have used ∼365 nm UV light deliberately for UV, the fluorescence intensity (λem = 425 nm) of the exposed
the preparation of AgNPs and AuNPs also to accelerate the for- CP is increased rapidly for one hour indicating the formation
mation kinetics i.e., for time saving issues. Ag(I) needs just one of OCP capped AgNPs. Irradiation beyond one hour lowers the
hour under UV light to exhibit a maximum enhancement of fluorescence intensity (Fig. 2B). In the case of the Cu(II)
fluorescence. But, ∼7 hours of ageing is required to accom- system, UV irradiation results in the gradual appearance of a
plish the same fluorescence intensity for the dark reaction. ∼430 nm peak in the fluorescence spectrum with the decrease
The case of AuNPs formation is quite interesting. UV exposure of the 500 nm peak (due to CP) like that of the Ag(I) system.
for ∼11 hours produces a red colored gold hydrosol with a After ∼14 hours of UV irradiation, the band at ∼430 nm attains
plasmon band having a fixed λmax at ∼522 nm. But, without a maximum intensity indicating the quantitative transfor-
UV exposure, a mixture of alkaline CP and a HAuCl4 solution mation of Cu(II) into the OCP capped CuNPs. Over exposure
initially produces a brown precipitate containing AuNPs of (irradiation >20 hours) causes little decrease of (Fig. 2C) the
∼8 nm average diameter. Then digestive ripening40–42 takes CuNP enhanced fluorescence. Under the present situation,
Fig. 1 The formation of a coinage metal hydrosol for fluorescence and TEM studies. (A) The fluorescence intensity of the silver stimulated OCP (oxi-
dized CP) solution after one hour of UV exposure (black), a solution without UV exposure but after ageing the solution for 7 hours (red) and the solu-
tion of the CP (without silver) after one hour of UV exposure (blue). (B) The fluorescence intensity of the copper stimulated OCP (oxidized CP)
solution obtained after 14 hours of UV exposure (black), a solution without UV exposure but after ageing the solution for 20 hours (red), a solution of
the CP (without copper) after 14 hour of UV exposure (green) and the unexposed CP (without copper) solution (blue). (C) TEM images of precipitated
Au particles (left) after five hours of mixing of CP/HauCl4 and dispersed Au particles (right) after eight days ageing produced without UV irradiation.
Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M, [Au(III)] = 4.2 × 10−4 M.
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Fig. 2 The fluorescence spectra of the alkaline (A) CP, (B) CP–Ag(I), (C) CP–Cu(II) solution at different time intervals of UV exposure. (D) The UV-vis
spectra of the alkaline CP–Au(III) solution at different time intervals of UV exposure. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] =
0.70 × 10−4 M, [Au(III)] = 4.2 × 10−4 M.
over UV exposure kills the silver stimulated MEF (unlike wonderful compound to behave as a promising Ag(I) and Cu(II)
copper stimulated fluorescence) miserably, which is realized sensing platform just by fine-tuning the experimental con-
from the rapid decrease of the fluorescence intensity of the ditions. To examine the selective Ag(I) and Cu(II) sensing appli-
OCP capped AgNPs.
cations in their respective reaction conditions, we have
We can successfully monitor the formation kinetics of the introduced other different metal ions in lieu of Ag(I)/Cu(II). We
AuNPs by UV-vis spectroscopy by measuring the increase of have found that the fluorescence enhancement of the OCP by
the absorbance value at the λmax (∼522 nm) of the plasmon silver and copper is quite discriminating in their particular
peak due to AuNP. Thus, the efficient quenching of the fluo- reaction conditions in comparison to other metal ions (Fig. 3).
rescence of OCP by AuNPs is authenticated. After 11 hours of
The ratio of Ag(I) or Cu(II) to the CP concentration is very
UV irradiation, a stable band in the absorption spectrum important in the context of MEF. When the CP concentration
appears at ∼522 nm which is not at all perturbed by over time is higher, to obtain maximum silver enhanced fluorescence
UV irradiation (Fig. 2D). Capping in colloid chemistry is a the Ag(I) ion concentration should be ∼3 times that of the CP
reversible phenomenon. The ligand stabilizes metal nanoparti- concentration. But, with the decrease of the CP concentration,
cles and as a corollary, the ligand gets stabilized by the metal the Ag(I) : CP increases to have a maximum fluorescence
nanoparticles. So, the capping agent (OCP) in turn gets a enhancement. For trace level detection of Ag(I) in solution
different extent of stability by the coinage metal nanoparticles down to a very low concentration level, the CP concentration
under UV light which follows the order:
should also be as low as possible. At a low CP concentration,
silver enhanced fluorescence is found to be pronounced even
at a remarkably low Ag(I) precursor concentration. Employing
AuNPs > CuNPs > AgNPs
Ag(I) needs only one hour of UV irradiation to show a this idea in mind we have detected Ag(I) as low as 9 × 10−8
M
maximum fluorescence intensity, while Cu(II) demands when the CP concentration is kept at 1.4 × 10−6 M. The result
14 hours of irradiation in the presence of CP. Owing to the comes from seven replicate measurements. Fig. 4 represents
huge fluorescence enhancement of OCP in presence of the the fluorescence spectral profile of the Ag(I) concentration
AgNPs after one hour of UV exposure, we thought the fluoro- dependent silver enhanced fluorescence at different CP con-
phore CP could be a potential candidate for the Ag(I) sensor. centrations. The results have also been summarized in the
Again, the alkaline CP solution inherits an insignificant fluo- table denoting that the LOD (lower detection limit) is
rescence and the alkaline CP–Cu(II) solution exhibits a decent increased with the decrement of [CP]. On the contrary, the
fluorescence intensity after 14 hours of UV irradiation. So time linear detection range and (I/I0)max [I and I0 are fluorescence
dependent UV irradiation has paved the way to design a selec- intensity with and without metal] are enhanced with the
tive Cu(II) sensing strategy. So, we have found the CP to be a increase of [CP].
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Under appropriate conditions Cu(0) and Ag(0) exhibit 166 and
52 times enhancement of fluorescence.
To observe the highest degree of selective fluorescence
enhancement in the presence of AgNPs and CuNPs, the choice
of excitation energy is crucial. The excitation energy for OCP
capped AgNPs and CuNPs is 290 and 300 nm respectively to
observe a maximum fluorescence intensity. To make the fluo-
rescence process more selective as well as sensitive, we have
chosen 290 nm as the excitation energy. Under these circum-
stances, we get maximum I/I0 [I0 = fluorescence intensity of
OCP; I = fluorescence intensity of OCP along with in situ pro-
duced metal NPs] for both the systems (Fig. 6). It is to be
noted that a sharp band with a λmax at ∼290 nm is observed in
the excitation spectra which is different from the absorption
spectra indicating a different interaction in the excited state
for both the copper and silver systems (Fig. S6 and S7, ESI†).
The time evolution fluorescence spectroscopy reveals a
single lifetime component (3.8 ns) of the AgNPs capped fluoro-
phore indicating that all the molecules are in the OCP form.
The CuNPs capped fluorophore has one slow (3.9 ns, 47%) and
one fast component (1.52 ns, 53%) indicating that the solution
contains both OCP and CP. Again, when the CP is irradiated,
both the OCP and CP are formed exhibiting the biexponential
(1.19 and 3.70 ns for fast and slow component) nature of the
life time plot (Fig. 7).
We have mentioned earlier that the fluorescence induced by
AgNPs is not stable enough in comparison to the gold and
copper systems under prolong UV exposure. Surprisingly, after
the formation of a gold or copper hydrosol from an individual
reaction mixture, the addition of Ag(I)NO3 causes ‘Turn On’
fluorescence. It is due to the formation of AgNPs and the
simultaneous expulsion of AuNPs or CuNPs from the solution.
As a result of the substitution by Ag(0), a precipitate due to
expelled Au(0) or Cu(0) from their respective hydrosol system is
observed rendering Ag(0) enhanced fluorescence. In this case
the birth of the fluorescence intensity is to some extent lower
than the pure silver hydrosol case. This is due to the removal
of some bound OCP along with AuNPs or CuNPs as a precipi-
tate. However, the maximum recovery of the Ag(0) enhanced
fluorescence is observed when Ag(I) is introduced to the red
Fig. 3 (A)
A bar diagram indicating the fluorescence intensity with
different metal ions used individually under the optimum condition where
silver causes maximum MEF and the inset indicates the normalized fluor-
escence spectra. (B) A bar diagram indicating the fluorescence intensity
with the variation of metal ions at the optimum condition where copper
causes maximum MEF and the inset indicates the normalized fluorescence
spectra. Conditions for (A): [CP] = 1.4 × 10−4 M, [Mn+] = 4.2 × 10−4 M,
exposure time = one hour, λex = 290 nm. Conditions for (B): [CP] = 1.4 ×
10−5 M, [Mn+] = 0.70 × 10−5 M, exposure time = 14 hours, λex = 290 nm.
Similar experiments are performed for Cu(II) in the appro- colored gold hydrosol. This is evidenced from the appearance
priate conditions for copper enhanced fluorescence. A similar of a fluorescent yellow solution which supports Ag(0) enhanced
type of observation is noticed regarding the LOD and linear
fluorescence conclusively. Besides, a black precipitate contain-
detection range like silver. But, for the case of copper, the 2 : 1 = ing Au(0) particles of average diameter ∼50 nm (Fig. S8, ESI†)
CP : Cu(II) produces the highest degree of enhancement what- is thrown out of the solution. Similar Ag(0) substitution experi-
ever the concentration of CP may be. It implies the 2 : 1
complex formation of CP with the Cu(II) before the formation ingly, we have found solutions with a variable fluorescence
ments have been done with other metal ions also. Interest-
of Cu(0). Seven replicate measurements with copper indicates
that we can go down to 9 × 10−8 M Cu(II) concentration when
the CP concentration is 1.4 × 10−6 M (Fig. 5). The maximum
copper stimulated fluorescence, (I/I0)max is obtained with
1.4 × 10−5 M CP and 7.15 × 10−6 M Cu(II) after ∼14 hours
irradiation.
intensity as depicted in Fig. 8. But for Co, Pd, Pt and Hg, no
fluorescence is recovered due to their association with Ag.
Similarly, CP has shown promise to design Cu(II) sensor
because after 14 hours of UV exposure, no metal produces
such fluorescence enhancement like Cu(II) as shown earlier. A
maximum enhancement of fluorescence is observed when
CP : Mn+ = 2 : 1 ratio. To observe the effect of the interfering
metal ions, we have repeated the experiment with other metal
Selective detection is achieved easily from short and long
times of UV exposure. Ag(I) and Cu(II) are detected selectively
through one hour and 14 hours of UV exposure respectively. ions along with Cu(II). The photoproduced CuNPs enhanced
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Fig. 4 Normalized fluorescence spectra (A1–D1) of OCP (oxidized CP after one hr UV irradiation) with the variation of [Ag(I)] at different CP concen-
trations using 290 nm excitation; (A1) [CP] = 2.8 × 10−4 M, (B1) [CP] = 1.4 × 10−4 M, (C1) [CP] = 1.4 × 10−5 M and (D1) [CP] = 1.4 × 10−6 M. The degree
of fluorescence enhancement (A2–D2) with the variation of [Ag(I)] at different CP concentrations and the inset represents the linear detection range;
(A2) [CP] = 2.8 × 10−4 M, (B2) [CP] = 1.4 × 10−4 M, (C2) [CP] = 1.4 × 10−5 M and (D2) [CP] = 1.4 × 10−6 M. The Table represents the linear detection
range, lower detection limit, (I/I0)max, Ag(I) conc. for (I/I0)max
.
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Fig. 5 Normalized fluorescence spectra (A1–D1) of OCP (oxidized CP after 14 h UV irradiation) with the variation of [Cu(II)] at different CP concen-
trations using 290 nm excitation; (A1) [CP] = 2.8 × 10−4 M, (B1) [CP] = 1.4 × 10−4 M, (C1) [CP] = 1.4 × 10−5 M and (D1) [CP] = 1.4 × 10−6 M. The degree of
fluorescent enhancement (A2–D2) with the variation of [Cu(II)] at different CP concentrations and the inset represents the linear detection range; (A2)
[CP] = 2.8 × 10−4 M, (B2) [CP] = 1.4 × 10−4 M, (C2) [CP] = 1.4 × 10−5 M and (D2) [CP] = 1.4 × 10−6 M. The Table represents the linear detection range,
lower detection limit, (I/I0)max, Cu(II) conc. for (I/I0)max
.
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Fig. 6 The silver (A1) and copper (B1) stimulated fluorescence spectra of OCP (normalized) with the variation of excitation wavelength. Both insets
represent the corresponding fluorescence spectra of OCP (normalized) without metal but with variations of the excitation wavelength; A2 and B2
represent the corresponding I/I0 with the variation of excitation wavelength. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, exposure time =
one hour for (A1) and (A2) i.e., for silver. [CP] = 1.4 × 10−5 M, [Cu(II)] = 0.70 × 10−5 M, exposure time = 14 hours for (B1) and (B2) i.e., for copper.
Fig. 7 The life time measurement of the exposed CP (UV exposure time
one hour), CP–Ag(I) (UV exposure time one hour) and CP–Cu(II) (UV
exposure time 14 hours). Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] =
4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M.
Fig. 8 A bar diagram indicating the fluorescence intensity (measured
after seven hours) of the reaction mixtures and normalized fluorescence
spectra (inset) after substitution of metal ions by silver. Conditions:
different metal ions in alkaline CP are separately exposed for one hour
fluorescence is not affected in the presence of other interfering
metal ions (Fig. S9, ESI†).
If we keep the OCP capped AgNPs [produced photochemi-
cally from alkaline CP by one hour exposure or Ag(I) in CP solu-
tion (unexposed) kept for 7 hours] in ambient (25 °C)
conditions, we find that the fluorescence intensity is increased
successively for 8 days. After that the solution remains stable
for >60 days without any alteration in the fluorescence inten-
sity. The successive increase of the fluorescence intensity is
due to the slow precipitation of extra silver(0) as a brown pre-
cipitate [authenticated from XPS45 and powder XRD46 analysis
under UV and then Ag(I) is introduced. Fluorescence measurements are
done after seven hours of waiting; [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 ×
10−4 M, [other Mn+] = 4.2 × 10−4 M, λex = 290 nm.
with the precipitate]. The time dependent fluorescence incre-
ment of the solution is presumably related to the removal of
Ag(0) from the quenching zone1 of the fluorophore OCP and
the further aggregation of the silver particles as reported by
Lukomska et al.47 But for the copper system, the fluorescence
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Fig. 9 (A) Powder XRD, (B) the XPS spectra for the element silver of the precipitate from the silver hydrosol after 8 days of ageing. (C) The normal-
ized fluorescence spectra and (D) TEM studies to investigate the effect of aging on the silver hydrosol. (E) The TEM image of the freshly prepared
photoproduced CuNPs and (F) the unaltered CuNP stimulated fluorescence with ageing. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M,
[Cu(II)] = 0.70 × 10−4 M, λex = 290 nm.
intensity does not change on ageing unlike the silver system coinage metal nanoparticles in the presence of CP. The
(Fig. 9).
binding energy of 366.21 eV and 372.16 eV represents Ag(0)
To investigate the donor capability of the capping agent, we 3d5/2 and Ag(0)3d3/2 respectively.38,48 The shift of the peak
have carried out the experiment in D2O. The 1H-NMR toward a lower binding energy for Ag(0) signifies tight capping
(Fig. S10, ESI†) spectrum reveals that in an alkaline D2O solu- by the OCP. Similarly, the 932.74 eV and 952.77 eV binding
37
tion, the peak at 9.8 ppm due to the –CHvN– proton45 energy symbolizes Cu(0)2p3/2 and Cu(0)2p1/2
.
The lattice
becomes acidic after the introduction of Ag(I) and is keep for fringes of the AgNPs and CuNPs are obtained as 0.234 and
7 hours. So, the peak vanishes after being exchanged with 0.211 nm respectively [for (111) plane] supporting their zero
D2O. This observation proves that the electron density of the oxidation state. The XPS spectra (binding energy of 81.58 eV
iminic bond is involved in capping the coinage metal nano- and 85.19 eV corresponds to Au(0)4f7/2 and Au(0)4f5/2 respect-
particles for long-term stability with interesting fluorescence ively) and lattice fringe (0.236 nm for (111) plane) indicate the
behaviours. Again, the metal–nitrogen interaction along with zero oxidation state of gold (Fig. 11).38
the quinone moiety has been substantiated from Raman spec-
The OCP molecule near the copper and silver nanoparti-
troscopy (Fig. S11, ESI†). Thus the long term stability of metal cles is not associated with the dramatic life time change in
NPs in CP the solution is due to the tight capping by the comparison to the OCP molecule in the absence of the
iminic bond.
metal. In order to understand the enhancement of fluo-
Whatever the concentration of Au(III) introduced to the alka- rescence, the surface plasmon-coupled emission (SPCE)
line CP solution, we have always found the quenching of fluo- model as proposed by Lakowicz31 has been considered.
rescence of OCP (photoproduced) by the highly stable (under According to the SPCE model, plasmons are created when
ambient conditions) AuNPs. With the increase of the precursor the excited-state fluorophore interacts with the nearby metal
HAuCl4 concentration, the plasmon band (λmax 522 nm) due to nanoparticle. The plasmons thus generated then radiate or
the formation of the AuNPs gradually increases without any are trapped into the substrate depending upon the scattering
shift of the band. I0/I exponentially (the Stern–Volmer plot) and absorption cross section of the nanoparticle. The
grows with respect to [Au(0)] indicating both static and plasmon radiation is generalized as the radiating plasmon
dynamic quenching by the AuNPs. It should be remembered (RP) model.
that with the increase of the precursor Au(III) concentration,
In order to estimate the scattering (QSca) and absorption
more OCP is formed as a corollary of evolution of more AuNPs. efficiencies (QAbs) of the copper, silver and gold nanoparticles,
Again, there is the factor of photobleaching of OCP and CP Mie theory49 has been successfully applied. When an isolated
under UV irradiation. So, at least it can be understood that the spherical particle is illuminated by electromagnetic radiation
I0/I vs. Au(III) concentration would be modified to some extent of a given wavelength, Mie theory49 helps one to estimate the
if the above points are taken into account (Fig. 10).
scattering (QSca) and extinction (QExt) efficiencies where extinc-
XPS measurements of the hydrosol under freeze drying tion is the removal of light from the incident beam by absorp-
conditions imply the zero oxidation state of the as prepared tion and scattering.
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Fig. 10 (A) The absorption spectra, (B1) fluorescence spectra and (B2) I/I0 with the variation of precursor Au(III) concentration. Conditions: [CP] =
1.4 × 10−4 M, exposure time = 11 hours.
According to Mie theory, the scattering (QSca), absorption electron clouds for which wave vector matching is not possible
(QAbs) and extinction (QExt) efficiencies are expressed as:
and the energy is dissipated as heat. This may result in the
quenching of the fluorescence of the molecule when adsorbed
on gold nanoparticles. However, the size and hence the dia-
meters of the copper and silver nanoparticles are much larger
than that of gold.
Though the fluorescence of the molecule in the vicinity of
both on the copper and silver nanoparticles is significantly
enhanced, their degree of enhancement is not the same. For
example the fluorescence enhancement is ∼52 fold for silver
while for copper it is only ∼5 fold if 1.4 × 10−4 M CP is
1
X
2
2
2
QSca
¼
¼
ð2n þ 1Þðjanj þ jbnj Þ
x2
n¼1
1
X
2
QExt
ð2n þ 1Þ½Reðan þ bnÞꢁ
x2
n¼1
QAbs ¼ QExt ꢀ QSca
where n is the summation index of the partial waves; n = 1, 2, 3 employed. In order to envisage the origin for the variations in
correspond to the dipole, quadrupole and octapole oscil- the fluorescent enhancement factors for the silver and copper
lations, an and bn are the Mie coefficients which can be evalu- nanoparticle, 3D-FDTD simulation studies have been per-
ated from the complex Bessel–Riccatti functions as reported formed. The snapshots indicating the spatial distributions of
elsewhere.50
the electric field around the silver and copper nanoparticles
Fig. 12A shows the variations of QSca, QAbs and QExt with the are shown in Fig. 12B. A significant ten fold enhancement of
wavelength (λ) for the copper, silver and gold nanoparticle the electric field around the silver nanoparticle has been esti-
having radii of ∼50, 100 and 2.5 nm, respectively. For the mated, while for copper, the amplitude of the electric field
copper and silver nanoparticles, the scattering efficiencies remains unperturbed. The enhancement of the electric field
(QSca) are quite appreciable particularly in the excitation wave- around the silver nanoparticle upon excitation with the fluoro-
length region which corresponds to the intense fluorophore phore emission (∼425 nm) wavelength in addition to the SPCE
emission (∼425 nm) as explained by Lakowicz.12 However, for may result in the increase in the fluorescence enhancement of
the gold nanoparticle, due to its reduced size (diameter molecule adsorbed on the silver nanoparticle.
∼5 nm), the scattering efficiency (QSca) is almost zero and the
entire absorption (QAbs) efficiency corresponds to the total copper is kept 4 times less than that of silver. In other words,
extinction cross section (QExt).
CP : Cu(II) = 2 : 1 exhibits a maximum fluorescence enhance-
To observe a maximum enhancement the concentration of
For the gold nanoparticle, the increase in absorption ment indicating the formation of a chelate followed by the
efficiency (Fig. 12A(a3)) may result in the oscillations of those evolution of the CuNPs due to photoirradiation. A computational
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Fig. 11 HRTEM images of the (A) gold, (B) silver and (C) copper hydrosol. The XPS spectra of the (A1) gold, (B1) silver and (C1) copper hydrosol. Con-
ditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M, [Au(III)] = 4.2 × 10−4 M.
Fig. 12 (A) The extinction, scattering and absorption cross section of (a1) 200 nm silver, (a2) 100 nm copper and (a3) 5 nm gold sphere. (B) Snap-
shots indicating the near field distribution around (b1) 200 nm silver sphere and (b2) 100 nm copper sphere obtained from a 3D-FDTD simulation.
study involving AuNPs and AgNPs agrees well with our experi- highly aggregated particle assembly and not accommodated in
mental fact. But the fluorescence enhancement, which we theory. As a corollary of SPCE, the enhancement of the electric
observed for the CuNPs in the prescribed condition, is not field around the metal may play a significant role. But, concen-
reflected in theory i.e., the FDTD simulated result. It is to be trating the field around OCP i.e., the lightening rod effect may
noted in this context of theoretical study that we have assumed also be of great importance.1 It is indeed true that the near-
a spherical particle geometry. Our experimental results speak field electromagnetic intensity enhancement is considered as
for a much improved fluorescence that may be due to the major mechanism for fluorescence enhancement by metal
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nanoparticles. The enhancement of the fluorescence intensity the remaining hard casing in a beaker containing 32 mL of
of the probe molecules in the proximity of the metal nano- 1.4 × 10−4 M aqueous alkaline CP solution (set 1, amount of
particles may not be exclusively considered to be due to the CP is much more than silver). In another set of experiments,
plasmon of the metal nanoparticles alone. It may be due to the egg casing with the membrane is placed in a beaker. In
the radiation from the plasmon–fluorophore association gen- that set, 6 mL of the CP solution (1.4 × 10−4 M) is taken
erated from the interaction between the excited fluorophores inside the cavity of the egg and 32 mL AgNO3 (2.8 × 10−4 M)
and the metal nanoparticles in their immediate proximity solution is taken in the beaker (set 2, amount of silver
(near-field). The radiating fluorophore is considered as an is much more than CP). In both of these cases the solu-
oscillating point dipole. According to the radiating plasmon tions (inside the cavity and in the beaker) maintain an
model51–53 the enhanced fluorescence emission observed in equal height for the equalization of hydrostatic pressure.
the MEF experiments is due to the radiation from the entire After 3 hours, it has been found that the colourless silver
excited state fluorophore–metal nanoparticle behaving as a nitrate solution (internal solution in the egg membrane, set 1
single radiating entity. However, in the present manuscript, we and external solution set 2, outside the egg membrane)
have simulated the near electric field distribution around the becomes blackish brown due to the formation of silver
metallic nanoparticles (Fig. 12B) arising from the localized hydroxide. After ageing for 2 days, it is observed that the
surface plasmon resonance (LSPR). The interaction of the near brown solution inside the egg of set 1 depicts a higher fluo-
electric field emitting from the fluorophore with that of the rescence and a pale yellow solution outside the membrane
plasmon of the nanoparticle could not be presented due to the shows a lower fluorescence intensity. For set 2, the solution
limitations of the software (XFDTD 6.5) used for the 3D-FDTD outside the membrane becomes dark brown with no fluore-
simulation.
scence (due to very high silver concentration) and the solu-
We have disclosed in this piece of work a unique fluo- tion inside the membrane exhibits a lower fluorescent
rescence behaviour from a known compound in solution. It intensity. This indicates that the alkaline CP diffuses more
has been efficiently used to detect Ag(I) and Cu(II) ions using efficiently through the egg membrane than the silver ion
MEF phenomena. Except for Cu(II), no other coinage metal (Fig. 13).
requires any UV exposure unlike our previous report for the
More elaborately, the alkaline CP (solution in the beaker in
reduction to zero state.34,35 The quenching of the fluorescence set 1) gets hold of a small amount of silver that diffuses out
of OCP can be explained by the damping of dipole oscillations and exhibits a low fluorescence intensity. Again, a smaller
by the nearby field.1,54 The “lossy surface waves” originated by amount of the CP solution (set 2) exhibits a low fluorescence
the AuNPs quench fluorescence as induced electron oscil- intensity due to its interaction with trace silver due to the slow
lations cannot radiate to the far-field because of wave vector silver diffusion across the membrane. A highly intense fluo-
mismatch and optical constraints at the metal–sample inter- rescence is observed for the solution inside the egg membrane
face. Again, concentrating the local incident field55 on the (set 1) indicating the easy passage of the CP from the huge
fluorophore causes an increment of the rate of excitation with supply through the membrane into the egg. It is worth noting
enhanced fluorescence intensity which is called the lightening that the easy passage of the alkaline CP (supply is less) could
rod effect. The proximal metal can also enhance the radiating not bring an intense fluorescing solution in the beaker (set 2)
decay rate (at which the fluorophore emits photons). As a con- because of the quenching due to the presence of a large
sequence, the increase of fluorescence intensity and decrease amount of silver. This experiment broadens the applicability
of the life time are observed. SPCE is also pointed out by of the silver enhanced fluorescence study in vivo. Trace
several groups for enhancement.12 In our case, no virtual amounts of silver if present in the cell can be detected easily
change of lifetime of the fluorophore is observed for the case with the help of the CP exploiting its unique metal enhanced
of the CuNPs and AgNPs stimulated fluorescence eliminat- fluorescence capability.
ing the probability of modification of radiative decay rate.
The environment has a vital role to produce lossy trapped
Here, SPCE, the near field electromagnetic intensity enhance- plasmons, SPCE and the lightening rod effect. In other
ment and the lightening rod effect are responsible for the words, the fluorescence quenching and enhancement are
observed fluorescence enhancement by copper and silver in guided by not only the fluorophore and metal ions but also
solution.
by the surroundings. To understand this fact, we have per-
Piercing a small hole 0.5 mm in diameter in the sharp end formed the reactions in the water pool of the reverse
of a chicken egg, we have removed the yolk and egg white. micelle.56 We have found quenching (I/I0 = 0.23 in the water
The rest of the hard egg wall along with the internal semi- pool of the reverse micelle and I/I0 = 52 in aqueous alkali)
permeable membrane is kept intact. The interior of the and insignificant enhancement (I/I0 = 1.16 in the water pool
emptied egg membrane is repeatedly washed by running of the reverse micelle and I/I0 = 121 in aqueous alkali) of the
triple distilled water. Then two thirds of the outer hard fluorescence for brown silver sol and yellow copper sol
CaCO3 casing from the blunt end is dissolved by dilute HCl respectively when they are generated in the water/n-heptane/
to expose the internal semipermeable membrane to perform 0.1 M AOT (R = 40) reverse micelle instead of water as men-
diffusion experiments. We poured 6 mL 2.8 × 10−4 M AgNO3 tioned in the Experimental section. Again, the red colored
into the cavity of the egg and kept the egg membrane with gold sol with an efficient quenching (I/I0 = 0.25 in the water
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Fig. 13 (A) A schematic representation of the permeation experiment indicating the facile movement of the alkaline CP through the egg membrane.
(B) The absorption and (C) fluorescence spectra of the solutions in and outside the cavity of the egg after 2 days.
pool of the reverse micelle and I/I0 = 0.12 in aqueous alkali)
capability is produced in water/n-heptane/0.1 M AOT micro-
emulsions (Fig. 14).
Conclusions
A large fluorescence enhancement of the solution containing
OCP capped Cu or AgNPs (photoproduced) proves an intri-
guing phenomenon to study. Furthermore, OCP capped Cu
and AuNPs are selectively replaced by incoming AgNPs produ-
cing the respective precipitate and enhanced fluorescence.
Again, the efficient fluorescence quenching of OCP by in situ
generated gold nanoparticles is a notable contrast which
makes the study more interesting. Thus MEF (metal enhanced Fig. 14 The fluorescence spectra of gold (ai), silver (bi) and copper (ci)
systems; i = 1 stands for CP in aqueous alkaline medium and i = 2 stands
fluorescence) helps to configure selective Cu(II) and Ag(I)
sensing just by the judicious manipulation of the reaction con-
ditions. The fluorophore may be proved to be worthy of bio-
medical applications due to its selectivity regarding the fluo-
for CP in the water pool of water/heptane/AOT micro reactors. Con-
ditions: [CP] = 1.4 × 10−4 M, [Ag(III)] = 4.2 × 10−4 M, exposure time = 11
hours for (ai). [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, exposure time =
one hour for (bi). [CP] = 1.4 × 10−5 M, [Cu(II)] = 0.70 × 10−5 M, exposure
rescence behaviour involving coinage metals. Further study is time = 14 hours for (ci).
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Acknowledgements
The authors are thankful to the UGC, DST, NST, and CSIR,
New Delhi, India, and the IIT Kharagpur for financial assist-
ance. The authors are also thankful to Professor Anindya
Datta, IIT Bombay, India for life time measurements.
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