White light emission from a mixture of silicon quantum dots and gold nanoclusters and its utilities in sensing of mercury(ii) ions and thiol containing amino acid

Article abstract of DOI:10.1039/c9ra02012h

White light emitting mixture (WLEM) was produced by controlled mixing of blue emitting silicon quantum dots (Si QDs) and orange red emitting gold nanoclusters (Au NCs). The chromaticity color co-ordinate of the WLEM studied using CIE (Commission Internationale del'Eclairage) diagram was found to be (0.33, 0.32), which was very close to that of perfect white light emitting source. The WLEM can also be achieved in the form of gel, solid and film with nearly the same CIE co-ordinates which enhances its utility as white light-emitting source in solid state devices. The reversible and thermo-responsive behaviour of the WLEM broadens its application in thermal sensing. Furthermore, the system was found to be showing fast, sensitive and selective detection of Hg²⁺ ions and thiol containing amino acid cysteine.

Full text of DOI:10.1039/c9ra02012h

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White light emission from a mixture of silicon
quantum dots and gold nanoclusters and its utilities
in sensing of mercury(II) ions and thiol containing
amino acid†
Cite this: RSC Adv., 2019, 9, 15997
*
Swati Tanwar,
Bhagwati Sharma, ‡ Vishaldeep Kaur and Tapasi Sen
White light emitting mixture (WLEM) was produced by controlled mixing of blue emitting silicon quantum
dots (Si QDs) and orange red emitting gold nanoclusters (Au NCs). The chromaticity color co-ordinate of
the WLEM studied using CIE (Commission Internationale del'Eclairage) diagram was found to be (0.33,
0.32), which was very close to that of perfect white light emitting source. The WLEM can also be
achieved in the form of gel, solid and film with nearly the same CIE co-ordinates which enhances its
utility as white light-emitting source in solid state devices. The reversible and thermo-responsive
behaviour of the WLEM broadens its application in thermal sensing. Furthermore, the system was found
to be showing fast, sensitive and selective detection of Hg2+ ions and thiol containing amino acid cysteine.
Received 15th March 2019
Accepted 13th May 2019
DOI: 10.1039/c9ra02012h
rsc.li/rsc-advances
The development of white light emitting mixtures in solution
phase is important because of their possible applications due to
1. Introduction
easy device fabrication on different types of substrates. Several
reports exist on various composites such as carbon dots, silica
hybrids or boronate microparticles which were exploited as
white light emitting materials owing to their easy solution
phase synthesis, good stability, bio-compatibility, cost effec-
tiveness and non-toxicity.15–19 Mandani et al. demonstrated the
solution based synthesis of white light emitting system by
controlled mixing of carbon dots with rhodamine B dye and
showed its application in Fe3+ sensing.20 Recently, uorescent
silicon quantum dots (Si QDs) have gained considerable interest
due to their unique optical, electronic and mechanical proper-
ties, and excellent biocompatibility.21–30 Fluorescent Si QDs have
been used for a myriad of applications, such as in bioimaging,
sensing, and optoelectronic devices.28,31,32 Although a handful of
reports exist in literature where several groups have tried inte-
grating Si QDs into light emitting devices,33–35 there are only
a few reports showing applicability of Si QDs for generation of
white light. In a very recent study, Bose et al. demonstrated the
production of white light by mixing green luminescent Si
nanoparticles (NPs) with blue emitting Si NPs and red lumi-
nescent Au nanoclusters.36 Tu et al. developed WLEDs by mixing
red emitting Si QDs with green and blue emitting rare earth
elements (REEs) phosphors.37 The synthesis of red emitting Si
QDs used for development of WLED involved etching of Si wafer
with highly corrosive HF solution which is not environment
friendly. Ghosh et al. developed a hybrid WLED using red
emitting Si nanocrystals and poly[N,N0-bis(4-butylphenyl)-N,N0-
bis(phenyl)benzidine] lm which played the role of hole trans-
porter as well as blue green light emitter in the device.38
The emission of white light from various organic, inorganic and
hybrid materials is a highly desired phenomenon of immense
interest due to its prospective applications in displays or light
emitting devices.1 Besides the inorganic electroluminescence
devices and uorescent lamps, white organic light-emitting
diodes (OLEDs) have found enormous applications in the
development of solid state lighting as alternatives to incandes-
cent lamps. White light-emitting diode (LED) has the potential to
reduce consumption of light energy and pollution from fossil fuel
power plants by signicantly improving lighting efficiency,2 and
are of great interest due to their long lifetime, small size, and low
energy consumption. Presently, commercial white LEDs are
mainly achieved by combining rare-earth doped phosphor
materials. Commercial WLEDs, developed by mixing yellow
phosphor materials with blue LED chips,3,4 suffer from some
serious drawbacks in terms of their stability such as change in
chromaticity and color rendering with time.5 Therefore develop-
ment of novel, energy efficient and reliable white light emitting
sources remains a great challenge. Several strategies have been
adopted to generate and improve the quality of white light by
using different photofunctional sources such as polymers,6
metal–organic frameworks,7,8 semiconducting quantum dots,9,10
lanthanide co-doped systems,11,12 organic molecules13,14 etc.
Institute of Nano Science and Technology, Mohali, Phase-10, Sector-64,
Punjab-160062, India. E-mail: tapasi@inst.ac.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra02012h
‡ Present address: Materials Research Centre, Malaviya National Institute of
Technology, JLN Marg, Jhalana Gram, Malviya Nagar, Jaipur-302017.
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Luminescent noble metal clusters have received a lot of molecules (chemical or biological species) based on their
attention in various elds of nanotechnology due to their high specic chemical interaction with the individual components
uorescence, easy synthesis, water solubility, stability, and (i.e. either QDs or Au NCs) and can also be used in bio-imaging.
reduced toxicity.39 In particular, gold nanoclusters (Au NCs)
have been used as probes for sensing and assays in several
studies due to their behaviour like articial atoms with discrete
and size-tuneable electronic transitions from visible to near IR
2. Experimental section
2.1 Materials
region.40 Few reports exist on uorescent Au NCs used as light
emitting sources in optoelectronic devices. For example,
Barman et al. reported the composite of carbon dots and dye-
encapsulated BSA-protein-capped Au NCs for white light
generation.41 Goswami et al. demonstrated the formation of
white light emission by combining the blue and green uores-
cence of green uorescent protein (GFP)-expressing bacteria
and orange luminescence of Au nanoclusters.42 In most of the
previous studies, the generation of white light emission was
performed by combining different types of dyes or mixing dye
with uorescent NPs or mixing different types of NPs/NCs,
which consist of three components. Such systems can suffer
from self-absorption, color ageing, non-radiative energy trans-
fer and unwanted changes in the chromaticity coordinates.42,43
So far, to the best of our knowledge, there is no report on the
production of white light emission by mixing single emitting
NPs and single luminescent NCs.
Herein, we demonstrate a solution phase synthesis of white
light emitting mixture (WLEM) by controlled mixing of blue
emitting Si QDs and orange red emitting Au NCs at a particular
ratio. The WLEM mixture can be incorporated in a gel, thin lm
as well as solid forms which will have potential utilities in
designing solid state devices. Our developed WLEM showed
reversible and thermo-responsive behaviour that can broaden
its application in the eld of thermal sensing. Furthermore, the
as obtained WLEM was employed for the reversible and selec-
tive detection of analyte molecules such as mercuric (Hg2+) ions
and cysteine amino acid (Cys). Presently, the ultrasensitive and
visual detection of heavy metal ions such as mercuric ions
(Hg2+) has been a focal point of research because of their
tremendous toxicity and potential threats to human health as
well as aquatic animals even at nanomolar concentrations due
to their high water solubility.44,45 On the other hand, the
detection of minute quantities of the amino acid, L-cysteine, in
biological uids is also very important to detect early onset of
diseases like edema, liver damage and slow growth rate in
children.46 It was found that our WLEM can function as a visual
sensor for the rapid, sensitive, selective and sequential sensing
of Hg2+ ions and cysteine molecule via a reversible color change
of the WLEM from white to blue to white again. The uores-
cence quenching of Au NCs component in the WLEM upon
addition of Hg2+ ions was due to metallophilic Hg2+–Au+ inter-
actions.47 Eventually the white emission of WLEM turned into to
blue. The white emission recovered again by subsequent addi-
tion of L-cysteine due to strong binding affinity between cysteine
(thiol containing amino acid) and thiophilic metal cation Hg2+
which resulted in weakening of Hg2+–Au+ interactions and
subsequent detachment of Hg2+ ions from the surface of Au
Gold chloride trihydrate (HAuCl4$3H2O), glutathione (GSH), L-
alanine, L-aspartic acid, L-histidine, L-phenylalanine, L-arginine,
L-glutamic acid, L-cysteine, polyvinylpyrrolidone (PVP), D-
glucose and 3-aminopropyltriethoxysilane (APTES) were
purchased from Sigma-Aldrich. Agarose was purchased from
Himedia. Carbon coated transmission electron microscopy
(TEM) grids used for imaging were purchased from Ted Pella.
Dialysis membrane with a molecular weight cut off of 0.5–1 kDa
was purchased from Spectrum labs. Dialysis membrane with
a molecular weight cut off of 7–10 kDa was purchased from
Himedia. Salts used for sensing namely calcium carbonate,
ferric nitrate nonahydrate, calcium chloride dihydrate, sodium
chloride, zinc nitrate hexahydrate, magnesium chloride hexa-
hydrate, sodium chlorate, cadmium nitrate tetrahydrate, nickel
chloride hexahydrate, copper nitrate trihydrate, lead chloride,
mercuric chloride, quinine sulphate dihydrate and sulphuric
acid were purchased from Merck, India. All the chemicals were
of analytical grade and were used without any further purica-
tion. Milli-Q water was used throughout the experiments.
2.2 Instrumentation
UV-visible absorption spectra were recorded at room tempera-
ture using a Shimadzu UV-2600 spectrophotometer. Photo-
luminescence (PL) spectra were recorded on an FS5 steady state
uorescence spectrometer from Edinburg instruments.
Dynamic light scattering (DLS) measurements were performed
on a Malvern zetasizer nano ZSP instrument. TEM measure-
ments were performed using a JEOL 2100 microscope with
a lanthanum hexaboride (LaB6) lament at an accelerating
voltage of 200 kV. Average nanoparticle size was measured using
Gatan microscopy suite soware. STEM-EDS mapping images
were obtained using a JEOL JEM 2100F system at an acceler-
ating voltage of 200 kV.
2.3 Synthesis of blue emitting silicon quantum dots (Si QDs)
The Si QDs were synthesized according to previously reported
protocol by our group.30 Briey, 450 mg of D-glucose was dis-
solved in 4 mL of water. To this solution 0.5 mL of APTES was
added slowly under stirring and the stirring was continued at
room temperature for 48 hours. Gradually, the color of the
solution changed from colorless to dark brown. The solution
was dialyzed against Milli-Q water to remove unreacted reac-
tants using a 1 kDa dialysis membrane for 6 hours and stored at
room temperature for further use.
2.4 Synthesis of orange red emitting glutathione (GSH)
capped gold nanoclusters (Au NCs)
NCs.48,49 Our developed nontoxic WLEM will have potential The synthesis of GSH capped Au NCs was performed following
applications in selective sensing of a wide range of analyte a reported procedure.50 Briey, freshly prepared aqueous
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solutions of HAuCl4$3H2O (20 mM, 0.5 mL) and GSH (100 mM, emission with a maximum at 464 nm upon excitation at
ꢀ
0.15 mL) were mixed with 4.35 mL of ultrapure water at 25 C. a wavelength of 365 nm. Structural characterization of Si QDs
The reaction mixture was heated to 70 ꢀC under gentle stirring was further done using TEM study which conrms the forma-
(500 rpm) for 24 h and resulted in a pale yellow colored solution tion of Si QDs with an average diameter of ꢁ2.7 ꢂ 0.3 nm
which indicated the formation of Au NCs. The as obtained (Fig. S1c†), which matches well with the DLS data.
solution was dialyzed against water using a 7 kDa dialysis
membrane and was stored at 4 C for further use.
The UV-visible absorption spectrum of the as prepared Au
NCs shows a broad absorption in the region of 200 to 400 nm
without appearance of any prominent peak (Fig. S2a†). The
absence of localized surface plasmon resonance band (SPR) in
the absorption spectrum reveals the formation of very small
sized Au nanoparticles. The as synthesized Au NCs showed
orange red emission with a maximum centred at 620 nm upon
excitation with 365 nm wavelength (Fig. S2b†), indicating the
formation of Au nanoclusters. DLS study indicated the forma-
tion of particles with an average size of 2.4 nm (Fig. S2c†). The
size and morphology of the Au NCs were further ascertained
from the TEM studies, which conrmed the formation of
spherical Au NCs of average size ꢁ1.9 ꢂ 0.8 nm (Fig. S2d†).
ꢀ
2.5 Synthesis of white light emitting mixture (WLEM)
As synthesized solutions of Si QDs and Au NCs were diluted rst
so that the emission intensity of both the components become
almost identical. To generate white light emitting mixture, we
have mixed the diluted solutions of Si QDs and Au NCs in
different ratio. It was found that white light emission was
coming from a mixture of 200 mL of Au NCs to 2 mL solution of
Si QDs.
2.6 Metal ions sensing
For metal ions sensing, 2 mL of as obtained WLEM was titrated
with 4 mL (1 mM) solution of different metal salts. PL spectra of
WLEM in presence of different metal ions were recorded aer 5
minutes of incubation at room temperature.
3.2 Photoluminescence properties of WLEM
To generate white light emitting mixture we have mixed the
diluted solutions of Si QDs and Au NCs in different ratio. The PL
spectra coming from such mixture solutions have been recor-
ded and shown in Fig. 1a. We have also checked the Commis-
sion Internationale d'Eclairage (CIE) chromaticity co-ordinates
of pure Si QDs, Au NCs, and their mixtures and compared with
that of the perfect white light emitting source. It is well reported
3. Results and discussion
3.1 Optical and structural characterizations of Si QDs and
Au NCs
Dynamic light scattering measurement of as synthesized Si QDs that a perfect white light emitting source has CIE chromaticity
indicates the formation of tiny particles with an average coordinate of (0.33, 0.33). The CIE co-ordinate of as synthesized
hydrodynamic diameter of 3.6 nm (Fig. S1a†). The emission Si QDs was found to be (0.21, 0.26) in the blue region as rep-
spectrum of the Si QDs as presented in Fig. S1b† shows blue resented in Fig. 1b. On the other hand, the CIE co-ordinate for
Fig. 1 (a) PL spectra of Si QDs after addition of different volumes of Au NCs; (b) corresponding chromaticity plot for color co-ordinates; (c) digital
images of Si QDs, WLEM and Au NCs in day light; and (d) digital images of Si QDs, WLEM and Au NCs under UV light (lex ¼ 365 nm).
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the Au NCs was found to be in the red region with value of (0.56, emission of Si QDs at 464 nm. In order to get insights into the
0.43) (Fig. 1b). A careful investigation of the CIE co-ordinates of nature of quenching, lifetime measurements of Si QDs and
both Si QDs and Au NCs suggested that the mixing of both WLEM were performed. The lifetime of Si QDs and WLEM were
components in a particular ratio could cover the entire visible collected at 464 nm with an excitation wavelength of 402 nm
region, thus leading to the generation of white light. The PL (Fig. S3†). It was found that lifetime of Si QDs remained almost
spectra of Si QDs upon addition of different volumes of Au NCs unaltered in Si QDs solution (9.5 ns) and WLEM (9.8 ns) which
have been shown in Fig. 1a. It is clearly visible from the emis- further conrmed the involvement of static quenching mecha-
sion spectra that the pattern of the spectrum of Si QDs was nism behind the PL quenching of Si QDs (Table S1†).
changing gradually aer addition of Au NCs to the Si QDs
solution. The appearance of a new band at 620 nm was observed
3.3 Structural characterizations of WLEM
upon addition of Au NCs in addition to the initial peak present
The as synthesized WLEM was characterized using TEM to get
at 464 nm. It was found that with increasing the concentration
information about the morphology of the mixture. Fig. 2a and
of Au NCs the intensity of the Si QDs peak centred at 464 nm
b show the low and high resolution TEM images of WLEM
was gradually decreasing along with an increase in the emission
which indicate the formation of agglomerated structures of
intensity of the Au NCs band at 620 nm. A clear iso-emissive
different shapes. The HRTEM images of the agglomerated
structures (Fig. 2c) show lattice fringes of d-spacing of 0.23 nm
point could be observed at 543 nm indicating the presence of
only two emissive species in the system. It was observed that
corresponding to (111) plane of Au and 0.16 nm corresponding
upon addition of Au NCs component, the CIE co-ordinates of
the mixture system started moving from blue region toward that
The structure formed is similar to that of a core–shell type
of perfect white source i.e. (0.33, 0.33) as shown in Fig. 1b and
Table 1. The analysis of CIE co-ordinates (Fig. 1b) revealed the
formation of WLEM with co-ordinate of (0.33, 0.32) when 200 mL
of Au NCs solution was added to 2 mL solution of Si QDs. This
a scanning transmission electron microscopy-energy dispersive
value is pretty close to that of perfect white light emitting source
(0.33, 0.33). The digital images under daylight and under UV
light for blue emitting Si QDs, orange emitting Au NCs and
to (311) plane of Si.
morphology where the aggregated Au NCs form the core and the
Si QDs form a continuous thin shell of thickness around 5 nm.
To conrm the nature of the core and the shell, we performed
X-ray spectroscopy (STEM-EDS) elemental mapping for the
WLEM (Fig. S4†). STEM-EDS elemental mapping images
conrmed that the Si QDs are homogeneously distributed over
white light emitting mixture are shown in Fig. 1c and d.
Au NCs in the agglomerated structures and the Au NCs aggre-
As we have seen from Fig. 1a, addition of increasing volume
gates form the core. The binding of the Si QDs with Au NCs can
of Au NCs to the Si QDs solution led to a quenching in the
be attributed to the presence of various functional moieties in
GSH that is present on the surface of Au NCs. The formation of
aggregates further supports the strong interaction between Si
Table 1 CIE co-ordinates of Si QDs with different volumes of Au NCs
QDs and Au NCs which can result in static quenching of PL
intensity of Si QDs.
Systems
X
Y
Si QDs
0.21
0.25
0.29
0.30
0.33
0.35
0.37
0.56
0.26
0.28
0.30
0.31
0.32
0.33
0.34
0.43
Si QDs + 50 mL Au NCs
Si QDs + 100 mL Au NCs
Si QDs + 150 mL Au NCs
Si QDs + 200 mL Au NCs
Si QDs + 250 mL Au NCs
Si QDs + 300 mL Au NCs
Au NCs
3.4 Stability studies of WLEM
For a uorescent material to be used for applications such as in
sensing, it is important for the material to be stable towards
light and temperature. Therefore, it was imperative for us to
study the stability of the WLEM under these conditions. It was
observed that the as synthesized WLEM showed photostability
Fig. 2 (a) and (b) TEM; and (c) HRTEM images of WLEM.
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over a longer period of time (more than 3 months) at room good CIE co-ordinate (0.32, 0.28) (Fig. 4d–f). Further, we also
temperature, hence, making it a suitable candidate as phosphor studied white light emission from thin lm. A thin lm was
material in LED applications.
generated by mixing aqueous solutions of PVP, Au NCs and Si
Optical response of the WLEM toward temperature was QDs in appropriate ratio, which showed white light emission
studied by recording its temperature dependent PL spectra in upon illumination by a UV lamp (lex ¼ 365 nm) (Fig. S6†).
the range of 20 ꢀC to 80 ꢀC (Fig. S5a†). It was observed that the Thus, the WLEM, apart from liquid and solid forms, can also
PL intensity of the WLEM decreased signicantly with be changed to gel and lm states without losing its white light
increasing temperature from 20 ꢀC to 80 ꢀC. The emission emission property.
intensity of Si QDs component present in the WLEM decreased
by approximately half of its intensity at 80 ꢀC, whereas the
3.6 Application of WLEM for Hg2+ ions and reversible thiol
temperature induced PL quenching was more than 90% in case
sensing
of Au NCs component in the WLEM. The higher rate of
White light emitting mixtures are of great interest for sensing
applications as they can easily undergo a change in color upon
interaction with a particular analyte of interest because of the
presence of two or three components in WLEMs. Heavy metal
ions like Hg2+ ions are highly toxic to both human and aquatic
animals and is known as notorious environmental pollutant.
Therefore, we were interested in sensing Hg2+ ions using our
WLEM. It is well established that the interaction of Hg2+ ions
with Au NCs leads to quenching of emission of Au NCs due to
d10–d10 interaction.47 Therefore, it was reasonable to speculate
that the interaction of Hg2+ ions with WLEM would lead to
quenching of emission of Au NCs, in turn changing the color of
emission of the mixture. When different concentrations of Hg2+
ions were added to the WLEM, a gradual decrease in peak
centred at 620 nm due to Au NCs (Fig. 5a) was obtained with
increasing concentration of Hg2+ ions. On the other hand, the
emission due to Si QDs at 464 nm remained unaltered. Upon
temperature induced PL quenching for Au NCs compared to Si
QDs resulted in shiing of CIE co-ordinates of WLEM towards
bluish region (as shown in Fig. S5b†).
Interestingly, it was found that upon cooling the WLEM from
ꢀ
ꢀ
80 C to 20 C, its white light emission properties regained (as
shown in Fig. 3a). The reversible thermo responsive behavior
can be ascribed to synergistic effects between oxygen rich
functional groups present on the surface of QDs and hydrogen
bonding with water.30 This reversible thermo-response of
WLEM was repeated several times and was found to be stable
for several heating and cooling cycles (Fig. 3b and c). The
reversible thermo-response and water soluble luminescent
properties of as synthesized WLEM make it suitable for optical
thermometry and thermography applications.51,52
3.5 Optical properties of WLEM
For practical application of white light emitting materials, it is illumination of the mixture under UV lamp, it was observed that
essential that the white light be generated in solid or in gel the color of the mixture changed from white to blue aer
state. Therefore, to justify the candidature of WLEM as addition of 640 nM Hg2+ ions. The emission of Au NCs was
a potential phosphor material for utilities in WLEDs, white quenched due to its specic interaction with Hg2+ ions, which
light emission was studied in both gel form and solid state resulted in the emission of Si QDs only to observe. The present
condition. In order to check the emission in gel form, both the method is fast and sensitive toward sensing of Hg2+ ions and
components of WLEM were mixed together with melted the limit of detection was found to be 10 nM (Fig. 5a).
agarose in the same concentration as those were used for Comparison of previously developed small molecular sensors
preparing WLEM. Aer gelication, it was found that gel is for detection of Hg2+ ions shown in Table 2 indicates that our
also giving white light emission upon UV light illumination (as system is showing better sensitivity than the previously reported
shown in Fig. 4a). PL spectrum of gel was similar to that of ones. Further, the selectivity of the present system was also
liquid WLEM with CIE co-ordinate of (0.32, 0.30) (Fig. 4b and studied by recording the PL spectra of the WLEM in the pres-
c). The solid state emission coming from the WLEM in powder ence of different metal cations and anions with a concentration
form also showed high quality white light emission with very of 1 mM. It was observed that the addition of metal anions and
Fig. 3 (a) PL spectra of WLEM at 20 ꢀC and 80 ꢀC; (b) and (c) change in PL intensity of Si QDs and Au NCs in WLEM during consecutive heating–
cooling cycles.
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Fig. 4 (a) Digital image of WLEM incorporated in agarose gel under UV light (lex ¼ 365 nm); (b) PL spectrum of WLEM incorporated in agarose
gel; (c) corresponding chromaticity plot for color co-ordinate; (d) digital image of WLEM incorporated in powder form under UV light (lex ¼ 365
nm); (e) PL spectrum of WLEM incorporated in powder form; and (f) corresponding chromaticity plot for color co-ordinate.
cations such as CO32ꢃ, Fe3+, Ca2+, Na+, Zn2+, Mg2+, ClO3ꢃ, Cd2+, light are shown in Fig. 6. Thiols have a very strong binding
Ni2+, Cu2+, and Pb2+ showed no signicant change in the PL affinity toward Hg2+ ions. Thus, the rapid recovery of white light
intensity of WLEM (Fig. 5b), indicating high selectivity of the emission can be attributed to the binding of Cys with Hg2+ ions.
system for Hg2+ ions. Thus the present system could be Since, Hg2+ ions are now bound to the thiol group in Cys, the
employed for the fast, selective, sensitive and visual sensing of d10–d10 interaction of Hg2+ ions with the Au+ ion on the surface
Hg2+ ions. Interestingly, the white light emission can fully be of the Au NCs is obviously weakened, thus leading to regain the
recovered again by the subsequent addition of a thiol contain- emission of Au NCs, which in turn leads to recovery of the white
ing amino acid Cys with a limit of detection of 10 nM (Fig. 5c). emission of the mixture.48 This reversible “ON–OFF” of white
Comparative analysis of other known molecular sensors for Cys light emission was found to be very specic to Cys residue only.
detection (Table 3) indicates higher sensitivity of our system Scheme 1 illustrates the mechanism of reversible “ON–OFF” PL
than the previously reported ones. Digital images of WLEM, quenching of Au NCs in WLEM in presence of Hg2+ ions and
WLEM with Hg2+ and WLEM with Hg2+ ions and Cys under UV Cys. The control experiments were performed by taking
Fig. 5 (a) PL spectra of WLEM upon addition of different concentration of Hg2+ ions; and (b) change in PL intensity of Si QDs (black) and Au NCs
(pink) in the WLEM upon addition of different ions; and (c) PL spectra of WLEM having Hg2+ ions with different concentration of Cys.
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Table 2 Comparison of molecular sensors for mercury detectiona
Material
Chemical probe
LOD
Reference
Quantum dots
Organic molecules
CdS quantum dots
Nitrogen doped carbon dots
Squaraine dye
Azobenzene
Phenoxazinone
DNA oligonucleotides
DNA–Au nanoparticles conjugate
Gold nanoparticles
25.2 ng mLꢃ1
Anal. Methods, 2016, 8, 6512–6519
New J. Chem., 2018, 42, 6824–6830
0.65 mM
1.3 ꢄ 10ꢃ7
M
Sens. Actuators, B, 2012, 173, 874–881
Chem.–Eur. J., 2011, 17, 7276–7281
J. Am. Chem. Soc., 2003, 125, 3418–3419
Angew. Chem., Int. Ed., 2004, 43, 4300–4302
J. Am. Chem. Soc., 2008, 130, 3244–3245
J. Anal. Methods Chem., 2012, 2012
Angew. Chem., Int. Ed., 2007, 46, 4093–4096
Macromol. Rapid Commun., 2006, 27, 389–392
Adv. Mater., 2007, 19, 1471–1474
20 mM
100 nM
40 nM
10 mM
10 mM
100 nM
30 mM
Biomolecules
Nanoparticles
Polymers
Gold nanoparticles
Polythiophene with thymine moiety
Poly(3-(30-N,N,N-triethylamino-10-propyloxy)-
4-methyl-2,5-thiophene hydrochloride)
42 nM
a
LOD ¼ limit of detection.
Table 3 Comparison of molecular sensors for Cys detection
Material
Chemical probe
LOD
Reference
Organic molecules
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Fig. 6 Digital images of (a) WLEM; (b) WLEM with Hg2+; and (c) WLEM with Hg2+ ions and Cys under UV light (lex ¼ 365 nm).
different non-thiol amino acids such as alanine, aspartic acid, addition of Hg2+ ions, this further validates our hypothesis of
histidine, phenylalanine, arginine, and glutamic acid; however, dissolution of Au NCs structures due to binding of Hg2+ ions
no change in PL intensity was observed as shown in Fig. 7. To with GSH (Fig. S7a and b†). Furthermore, upon introduction of
get better insights into ion-induced structural changes, TEM Cys to this WLEM with Hg2+ ions, reformation of agglomerated
imaging of WLEMs aer addition of Hg2+ ions followed by Cys structures take place exactly like pure WLEM (TEM images
addition was done. TEM images represented in Fig. S7† revealed shown in Fig. S7c and d†), which results in recovery of white
the distortion of agglomerated structures of WLEM upon light emission from the mixture.
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Scheme 1 Schematic depiction of reversible “ON–OFF” PL quenching of Au NCs in WLEM.
Fig. 7 Fluorescence intensity change of WLEM with Hg2+ ions in presence of different amino acids (a) alanine; (b) aspartic acid; (c) histidine; (d)
phenylalanine; (e) arginine; and (f) glutamic acid.
16004 | RSC Adv., 2019, 9, 15997–16006
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RSC Advances
12 S. Dang, J.-H. Zhang and Z.-M. Sun, J. Mater. Chem., 2012, 22,
8868–8873.
4. Conclusions
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52, 4309–4312.
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In summary, we have designed a simple solution based
approach for generation of WLEM. As generated WLEM can be
converted to solid, gel or in the form of lm, making it a suit-
able candidate for solid state devices. Furthermore, this system
was found to be showing fast, sensitive and selective sensing of
Hg2+ ions and thiol containing amino acid Cys. The reversible
thermo-response property of WLEM could be utilized in optical
thermometry and thermography applications. Such biofriendly
white light emitting material holds promise for being used in
selective sensing of a wide range of chemical or biological
species based on their specic chemical interaction with the
individual components present in the WLEM and also in bio-
imaging.
Conflicts of interest
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2006, 1, 451–472.
There are no conicts to declare.
`
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Acknowledgements
T. S. is thankful to Department of Science and Technology (DST)
(SERB Project-ECR/2016/000436) for nancial support. B. S. and
S. T. acknowledge INST for Postdoctoral and Senior Research
Fellowships, respectively. V. K. is thankful to DST-SERB project
for providing Junior Research Fellowship. T. S. acknowledges
Advanced Instrumentation Research Facility (AIRF), Jawaharlal
Nehru University (JNU), New Delhi for providing access to the
TEM facility and Gajender Saini for technical help.
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