An l-cysteine-mediated iodide-catalyzed reaction for the detection of I-

Article abstract of DOI:10.1039/c8nj04944k

In this study, a highly selective and eco-friendly fluorescent sensor consisting of upconversion (UCNPs) and gold nanoparticles (AuNPs) was developed for the detection of iodide (I^-). The negatively charged AuNPs were able to electrostatically adsorb on the positively charged UCNPs, leading to the fluorescence quenching of the UCNPs. In the presence of l-cysteine, AuNPs were released from the surface of the UCNPs and aggregated due to the Au-S interactions that occurred between the AuNPs and l-cysteine, which resulted in the fluorescence recovery of the UCNPs. Upon the addition of I^-, the fluorescence of the UCNPs was gradually quenched meaning that I^- catalyzes the oxidation of l-cysteine, preventing the aggregation of AuNPs and forming a fluorescence resonance energy transfer system. The I^--catalyzed oxidation reaction provides a method for the l-cysteine-triggered sensor to detect I^-, using l-cysteine to modulate the fluorescence signal in an eco-friendly manner. Based on the above special detection strategies, this sensor has excellent selectivity for I^- even in a complex matrix such as urine, which is much better than most assays for I^-. Under the optimal conditions, the sensor allows the quantitative analysis of I^- with a detection limit of 55 nM. The high selectivity, sensitivity and environmental friendliness of the sensor proves that it has the potential for the detection of I^- in actual samples. Most importantly, the excellent results of the sensor for the detection of I^- in urine prove that the probe can be used as an ideal tool in clinic diagnoses.

Full text of DOI:10.1039/c8nj04944k

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An L-cysteine-mediated iodide-catalyzed reaction
for the detection of Iˠ
Cite this: New J. Chem., 2019,
43, 1398
Kaili He, Hongyu Chen, Cuiyan Wu,* Meiling Liu
and Youyu Zhang
*
In this study, a highly selective and eco-friendly fluorescent sensor consisting of upconversion (UCNPs)
and gold nanoparticles (AuNPs) was developed for the detection of iodide (IÀ). The negatively charged
AuNPs were able to electrostatically adsorb on the positively charged UCNPs, leading to the
fluorescence quenching of the UCNPs. In the presence of L-cysteine, AuNPs were released from the
surface of the UCNPs and aggregated due to the Au–S interactions that occurred between the AuNPs
and L-cysteine, which resulted in the fluorescence recovery of the UCNPs. Upon the addition of IÀ, the
fluorescence of the UCNPs was gradually quenched meaning that IÀ catalyzes the oxidation of
L-cysteine, preventing the aggregation of AuNPs and forming a fluorescence resonance energy transfer
system. The IÀ-catalyzed oxidation reaction provides a method for the L-cysteine-triggered sensor to
detect IÀ, using L-cysteine to modulate the fluorescence signal in an eco-friendly manner. Based on the
above special detection strategies, this sensor has excellent selectivity for IÀ even in a complex matrix
such as urine, which is much better than most assays for IÀ. Under the optimal conditions, the sensor
allows the quantitative analysis of IÀ with a detection limit of 55 nM. The high selectivity, sensitivity and
environmental friendliness of the sensor proves that it has the potential for the detection of IÀ in actual
samples. Most importantly, the excellent results of the sensor for the detection of IÀ in urine prove that
the probe can be used as an ideal tool in clinic diagnoses.
Received 29th September 2018,
Accepted 25th November 2018
DOI: 10.1039/c8nj04944k
rsc.li/njc
detection limit of 1.39 Â 10À6 M.10 However, this method requires
1. Introduction
complicated sample preparation processes. Recently, on account of
Iodine deficiency causes a large range of health problems, their multifarious advantages such as reliability, high sensitivity
including mental retardation, increased perinatal mortality, and speed, and simple and straightforward operation, fluorescence
and hypothyroidism and cretinism, that mainly effect pregnant detection techniques have been regarded as promising and power-
women and young children.1 In contrast, excess ingestion of ful tools for the quantitative detection of IÀ,11 and fluorogenic
IÀ eventually leads to hyperthyroidism and the formation of a chemosensors have been proposed to determine IÀ concentration.
goiter.2 Therefore, it is crucial to determinate the level of IÀ in However, these fluorescence methods are based on the interaction
order to diagnose and control diseases and disorders that are between IÀ and toxic heavy metal ions, which may influence the
caused by the dietary deficiency of or abnormal metabolism of selectivity due to their similar chemical properties. For example, He
IÀ. A large number of countries monitor IÀ status through et al. prepared carbon dots as a novel probe for the selective
urinary IÀ levels, which considering that IÀ is mainly excreted determination of IÀ, which exhibited high sensitivity with a low
by the kidneys, urinary IÀ is a more sensitive indicator to use to detection limit of 0.234 mM.12 However, this sensor is based on the
detect recent changes in IÀ intake.1,3 To date, various methods strong affinity between IÀ and mercury ions, and mercury ions
have been reported for the detection of IÀ in urine,4–8 such as present a threat to human health and cause serious environmental
ion-pair reversed-phase HPLC and pulsed amperometric and health problems. In addition, some of the methods require
methods9 that have low quantification limits and high sample complicated synthetic procedures.13 These techniques are rather
throughput, but these are time-consuming procedures. Meanwhile, time-consuming and require tedious chemical modifications and
Ana et al. used an electrochemical method to detect IÀ with a low specific operating skills. Hence, it is of the utmost urgency to
develop a facile, low-cost, and environmentally friendly method for
the detection of IÀ.
Fluorescence resonance energy transfer (FRET) has sparked
great attention in the field of detection systems due to its
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research
(Ministry of Education), College of Chemistry and Chemical Engineering, Hunan
Normal University, Changsha 410081, P. R. China. E-mail: zhangyy@hunnu.edu.cn,
unique advantages, including accuracy, precision, and high
cuiyan.wu@hunnu.edu.cn; Fax: +86-731-88865515; Tel: +86-731-88865515
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj04944k sensitivity. The two major components in FRET are energy
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donors and acceptors. Among the many materials used in 2. Experimental
FRET-based analysis, upconversion nanoparticles (UCNPs), as
a class of materials, have been verified to be efficient and
2.1. Materials and apparatus
Lanthanide oxides (Y2O3, Yb2O3, Er2O3) were obtained from the
China National Pharmaceutical Group Corporation (Shanghai,
China). All of the rare earth lanthanide oxides were 99.9%
purity or higher. The lanthanide oxides were dissolved in hot
nitric acid and then diluted with deionized water to achieve
final concentrations of 0.4 M. Chloroauric acid (HAuCl4) and
hexadecyl trimethyl ammonium bromide (CTAB) were obtained
from Sigma (Shanghai, China), L-cysteine, Na2C2O4, Na2SO4,
NaF, NaCl, NaBr, Na2HPO4, NaNO3, NaIO3, NaClO3, citrate,
CH3COONa, NaH2PO4, NaClO4, NaNO2, Na2SiO3, and KI were
purchased from Beijing Chemical Corp. All other chemicals
(99%, Merck) used in this work were of analytical grade and
Millipore Milli-Q ultrapure water was used throughout the
experiments. A UV-2450 spectrophotometer (Shimadzu Co.,
Japan) was used to record the UV-vis absorption spectra. The
fluorescence spectra were measured by an F-4500 fluorescence
spectrophotometer (Hitachi Ltd, Japan) with an external
980 nm laser diode (Hi-Tech Optoelectronic Co., Ltd China)
as the excitation source. Transmission electron microscopy
(TEM) images were collected by a JEOL-1230 transmission
electron microscope (JEM-3010 Jeol, Japan). The z potential
promising energy donors. Rare-earth (RE) doped UCNPs, which
convert low-energy near-infrared (NIR) light into a higher-energy
output photon with a shorter wavelength via a two-photon
or multiphoton mechanism, have attracted wide attention.14
UCNPs have been extensively applied in in vivo fluorescence
imaging and the real-time monitoring of drug delivery,15–17 on
account of their low toxicity, narrow emission peaks and high
photostability, greater tissue penetration, and minimal photo-
damage to organisms.18 In comparison with traditional semi-
conductor quantum dots and organic fluorophores,19 UCNPs
are capable of increasing the sensitivity because there is no
autofluorescence interference from biomolecules.20 Therefore,
UCNPs have been extensively applied in biosensing.21–23 In this
work, an off–on–off fluorescence probe for IÀ sensing based on
the IÀ-catalyzed oxidation of cysteine to cystine, has been
designed and developed and a schematic of this process is
shown in Scheme 1. In this nanosystem, many negatively
charged AuNPs conjugate with positively charged UCNPs
through electrostatic interactions, resulting in the fluorescence
quenching of UCNPs (turn-off). The presence of Au–S bonds
between the L-cysteine and AuNPs led to the aggregation of the
AuNPs, and the simultaneous release of the AuNPs from the
surface of the UCNPs induced the UCNP fluorescence recovery
(turn-on). In contrast, since IÀ can catalyze the oxidation of
thiol compounds by dissolving oxygen to form disulfide
compounds,24 and the disulfide and the L-cystine did not cause
the aggregation of the AuNPs and formed an energy transfer
system between the UCNPs and AuNPs, the fluorescence recovered
by L-cysteine was effectively quenched (turn-off). More impor-
tantly, this work creates an eco-friendly system that uses
L-cysteine rather than mercury ions or other toxic heavy metal
ions to modulate the fluorescence signals to detect IÀ in urine
samples.
measurements were performed by
a Nano-ZS Zetasizer
ZEN3600 (Malvern Instruments Ltd, UK). FT-IR spectra of
samples were obtained using an FT-IR spectrometer (Nicolet
Instrument Co., USA).
2.2. Synthesis of the UCNPs
The NaYF4:Yb3+,Er3+ UCNPs were synthesized according to
previously reported methods.25,26 Firstly, 7.5 mL of 0.1 M
sodium trisodium citrate was added to a solution mixture
containing 8.8 mL of 0.1 M Y(NO3)3, 1.0 mL of 0.1 M
Yb(NO3)3 and 0.2 mL of 0.1 M Er(NO3)3 under stirring, then
25 mL of ethanol and 0.2 g of cetyl trimethylammonium
bromide (CTAB) were added to the above solution under
stirring for 15 min at room temperature. 12 mL of 0.1 M NaF
was added dropwise to the homogeneous solution under
continuous stirring over 1 h until the solution became clear.
Next, 2 mL of HNO3 was added into the precursor solution and
the solution mixture was transferred to a 100 mL Teflon-lined
autoclave and then sealed and heated at 180 1C for 4 h. Finally,
the reaction solution was cooled down to room temperature
and the nanocrystals were collected by centrifugation at
10 000 rpm. The product was cleaned by washing four times
with ethanol and Milli-Q ultrapure water in sequence and the
precipitate was dried in an oven at 98 1C.
2.3. Synthesis of the AuNPs
AuNPs were synthesized by the citrate reduction of HAuCl4
according to the method described in the previous literature.27,28
In general, an aqueous solution (including 0.5 mL of 2% HAuCl4)
of 100 mL of HAuCl4 was heated to 100 1C, then 1.8 mL of a 1%
sodium citrate solution was quickly mixed into the boiling HAuCl4
solution under vigorous magnetic stirring. The mixed solution
Scheme 1 Schematic illustration of the mechanism of IÀ detection using
the UCNP and AuNP probe.
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4
4
4
was boiled for 10 min, further followed by constant stirring transitions from the S3/2, and F9/2 excited states to the I15/2
without heating for another 15 min. The nearly colorless ground state, respectively.29
original solution turned pale yellow and finally wine-red, typical
The morphology and absorption spectrum of the AuNPs
phenomena observed during the preparation of AuNPs, and were investigated using TEM and UV-vis spectroscopy. As is
then a stable AuNP solution was obtained.
vividly shown in the Fig. 1B, the AuNPs show a spherical
structure with a size of approximately 25 nm in diameter.
In addition, the AuNPs have a characteristic surface plasmon
resonance (SPR) peak located at 524 nm when characterized by
UV-vis spectroscopy.
2.4. Experimental procedure for the fluorescence detection
of IÀ
Various amounts of different concentrations of IÀ were
incubated with L-cysteine (12 mM) solution, then UCNPs
(0.01 mg mLÀ1), AuNPs (1.25 nM) and PBS (10 mM, pH = 8.0)
were added into each mixture (the final volume is 500 mL).
The mixtures were incubated for 45 min at 45 1C and then
fluorescence emission spectra were measured with an excitation
wavelength of 980 nm.
3.2. Fluorescence resonance energy transfer (FRET) between
the UCNPs and AuNPs
To verify the FRET between the UCNPs and AuNPs, the
z-potentials were determined. As presented in Fig. S3 (ESI†),
the z-potentials values of the dispersed UCNPs and AuNPs
in PBS buffer were measured as +38.3 mV and À31.3 mV,
respectively. These values manifest that the UCNP nanospheres
are positively charged, in contrast, the AuNPs are negatively
charged in solutions, which suggests that the UCNPs were
conjugated with AuNPs through electrostatic attractions. In
addition, the UV-vis absorption and fluorescence emission
spectra were also recorded. As shown in Fig. 2, curve b shows
the characteristic peaks of the AuNPs at 524 nm, and the main
emission peak of the UCNPs appeared in 550 nm, with the
absorption spectrum of the AuNPs largely overlapping
the emission spectrum of the UCNPs. Considering the above
two aspects, we speculate that the fluorescence quenched
mechanism of the AuNPs to the UCNPs was based on FRET.30
As shown in Fig. S4A (ESI†), the effect of the concentration
of the AuNPs on the fluorescence quenching efficiency was
studied in detail. Through analysis, we can conclude that the
fluorescence intensity of the UCNPs gradually decreased upon
an increase in the AuNP concentration. The Stern–Volmer
equation was applied to calculate the quenching constant:31
3. Results and discussion
3.1. Characteristics of the UCNPs and AuNPs
The morphology, crystallographic phase, distribution of the
surface groups, as well as the optical properties of the UCNPs
have been characterized. The TEM image shown in Fig. 1A
shows that the synthesized UCNPs possess a globular shape
distribution, with an average size of around 30 nm in diameter.
As presented in Fig. S1A (ESI†), the XRD pattern showed no
difference between the prepared UCNPs and the standard
values for JCPDS no. 77-2042, indicating that the UCNPs were
successfully synthesized in high crystallinity. The FT-IR spectrum
of the UCNPs (curve 2 in Fig. S1B, ESI†) was identical to that of
pure CTAB (curve 1 in Fig. S1B, ESI†). As shown in Fig. S1B
(ESI†) (curve 2), the UCNPs possessed broad absorption bands
at 2986 cmÀ1 and 2831 cmÀ1 that can be attributed to C–H
(–CH3 and –CH2–) stretching and had a weak absorption near
1597 cmÀ1 due to the absorptions in the region characteristic
of amine groups (–NH2). The FT-IR spectrum demonstrated
that CTAB partly covered the surface of the UCNPs. After amino
modification of the UCNPs, the UCNPs exhibited excellent
stability and dispersibility in water and the fluorescence inten-
sity remained unchanged within 60 min (Fig. S2A, ESI†).
As shown in Fig. S2B (ESI†), different pH values have little
influence on the fluorescence intensity of the UCNPs. The
fluorescence spectroscopy of the UCNPs collected in aqueous
solution revealed two characteristic peaks at 554 and 667 nm
under 980 nm laser excitation, which can be assigned to
I0/I = Ksv  [Q] + 1
I and I0 represent the fluorescence intensity in the absence and
presence of the AuNPs, respectively. The concentration of the
Fig. 2 Emission spectrum of the UCNPs (1) and absorption spectrum of
Fig. 1 TEM images of the UCNPs (A) and AuNPs (B).
the AuNPs (2).
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AuNPs is represented by Q. As described in Fig. S4B (ESI†), the
linear relationship between I0/I and the concentration of the
AuNPs can be written in a rearranged form of the well-known
Stern–Volmer equation:
I0/I = 1.928 Â [Q] + 1 (R2 = 0.9950)
The quenching constant (Ksv) was found to be 1.928 Â 109,
which was calculated using the Stern–Volmer equation. According
to the quenching constant, the AuNPs were found to remarkably
quench the fluorescence of the UCNPs.
Fig. 4 TEM images of the UCNPs in the presence of the AuNPs (A),
UCNPs–AuNPs after adding L-cysteine (B) and UCNPs–AuNPs upon
pretreating L-cysteine with IÀ (C). [UCNPs]: 0.1 mg mLÀ1; [AuNPs]: 1.25 nM;
[L-cysteine]: 12 mM; [IÀ]: 200 mM; PBS: 0.01 M.
3.3. The strategy of IÀ detection
The detection mechanism is primarily based on the following
reactions:24
IÀ catalyzed the oxidation of thiol compounds to form disulfide
compounds18 that prevented the aggregation of AuNPs and thus
resulted in a decrease in the fluorescence intensity of the UCNPs
(curve 4). The interaction between IÀ, L-cysteine, the UCNPs and
the AuNPs was further evidenced from TEM images. As shown in
Fig. 4A, in the presence of the UCNPs, the AuNPs remained in a
dispersed state and the solution mixture maintained its red color.
When L-cysteine was added into the UCNPs–AuNPs solution
mixture, as shown in Fig. 4B, the color of the solution became
blue and the AuNPs aggregated due to the Au–S bonds between
L-cysteine and the AuNPs. In contrast, when both of L-cysteine and
IÀ were present, the solution was red in color (Fig. 4C) and
the AuNPs remained dispersed in that IÀ acts as a catalyst in
the oxidation of L-cysteine to yield L-cystine, which prevents the
aggregation of the AuNPs. In addition, the IÀ catalyzed oxidation
of the thiol compounds was verified from FT-IR spectra, as shown
in Fig. S5 (ESI†). For L-cysteine, the peak of the stretching
vibration of –SH was located at 2556 cmÀ1. After the treatment
of the IÀ solution, the peak at 2556 cmÀ1 disappeared, indicating
that the oxidation of –SH was successfully catalyzed by IÀ.
Consequently, all of these results successfully explain the sensing
principle of the detection of IÀ.
Based on the above reactions, it is believed that IÀ acted as a
catalyst in the transformation of L-cysteine to L-cystine, which
caused the dispersion of the AuNPs. As can be seen from Fig. 3,
the fluorescence intensity of the as-prepared UCNP solution
was significantly enhanced centered at around 550 nm (curve
1), and the sensitivities were increased in that the UCNPs
resulted in an enhanced signal-to-background ratio. The intro-
duction of the AuNPs induced a decrease in the fluorescence
intensity of the UCNPs due to the occurrence of the FRET
process (curve 2). After adding L-cystine into the UCNP and
AuNP solution mixture, the L-cysteine caused the aggregation of
the AuNPs, which broke the FRET system and thus resulted in
the recovery of the fluorescence intensity of the UCNPs (curve 3).
On the contrary, after IÀ was incubated with L-cysteine,
3.4. Optimization of the detection conditions
To obtain high detection sensitivity and achieve better analy-
tical performances for the detection of IÀ, factors such as pH,
L-cysteine concentration, incubation time, and reaction tem-
perature were investigated. A large amount of L-cysteine
was found to assemble on the surface of the AuNPs, which
prevented the FRET process between the UCNPs and AuNPs
and thus allowed the recovery of the fluorescence intensity
of the UCNPs. As shown in Fig. 5A, the fluorescence recovery
was observed to have no obvious differences when the concen-
tration of L-cysteine was more than 12 mM. Moreover, a higher
concentration would be less efficient and unfavorable for
lowering the detection limit. Therefore, a concentration of
L-cysteine of 12 mM was employed for the following studies.
In order to realize a complete reaction, the incubation time
was explored. Fig. 5B shows that the fluorescence intensity
decreased along upon an increase in the reaction time from
0 to 40 min, and remained constant after 40 min, illustrating
that the IÀ-catalyzed oxidation of L-cysteine to L-cystine could
be sufficiently completed within 40 min. Thus, the optimal
Fig. 3 Normalized fluorescence intensity of the UCNPs (1), UCNPs and
AuNPs (2), the of UCNP–AuNP–L-cysteine solution mixture (3), and the
UCNPs and AuNPs after adding L-cysteine and IÀ (4). [UCNPs]: 0.1 mg mLÀ1
[AuNPs]: 1.25 nM; [L-cysteine]: 12 mM; [IÀ]: 200 mM; PBS: 0.01 M.
;
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Fig. 6 (A) The overlaid fluorescence spectra of the UCNP–AuNP–L-
cysteine solution with various concentrations of IÀ. The concentrations
of IÀ (from top to bottom) are 0.1 mM, 1 mM, 5 mM, 10 mM, 50 mM, 100 mM,
and 200 mM, respectively. [UCNPs]: 0.1 mg mLÀ1; [AuNPs]: 1.25 nM;
[L-cysteine]: 12 mM; PBS: 0.01 M, and (B) the linear relationship between the
quenching efficiency (F À F0)/F and IÀ concentrations from 0.1 to 200 mM.
Selectivity and specificity are important factors in developing
an excellent sensor. In this work, the optical UCNP–AuNP–L-
cysteine–IÀ mixed nanosystem was extended to the detection of
other ions. IÀ with a final concentration of 200 mM was used to
evaluate the selectivity towards various metal ions (including
C2O42À, SO42À, FÀ, ClÀ, BrÀ, HPO42À, NO3À, IO3À, ClO3À, citrate,
CH3COOÀ, H2PO4À, ClO4À, NO2À, and SiO32À). Different metal
ions were added into the UCNP, AuNP, and L-cysteine solution
with a final concentration of 2 mM, then the fluorescence spectra
were recorded under excitation at 980 nm to study the selectivity.
Fig. 7 shows that the fluorescence intensity decreased remarkably
upon the addition of IÀ, whereas other ions had negligible or less
impact on the intensity. This indicates that the UCNP–AuNP–L-
cysteine–IÀ system has high selectivity towards IÀ.
Fig. 5 Effects of L-cysteine concentration (A), and incubation time (B),
incubation temperature (C) and pH (D) on the fluorescent sensor for IÀ
detection.
reaction time was determined as 40 min. The temperature
effects are summarized in Fig. 5C. The IÀ-catalyzed oxidation
of L-cysteine to L-cystine at different temperatures shows that
the increase in the temperature accelerated the reaction
process. The fluorescence of the nanoprobe hardly changed
when the temperature was greater than 45 1C, indicating that
the nanoprobe is stable under 45 1C. The pH value of the
solution had a great influence on the rate of the oxidation of IÀ,
which was investigated in our work. As shown in Fig. 5D, the
fluorescence quenching reached a maximum at pH 7, so it can be
concluded that at the pH value obtained in this experiment that
L-cysteine was rapidly oxidized to L-cystine by oxygen in solution.
3.6. Application in real samples
Human urine samples were collected from the Hospital
of Hunan Normal University (Changsha, China). The study
complied with all institutional and national guidelines, as per
the Institutional Ethics Committee. The protocol was approved
by the College of Chemistry and Chemical Engineering,
3.5. Performance of the sensor
Under the optimal conditions, solutions with different concentra-
tions of IÀ were detected using the abovementioned fluorescence
method. Fig. 6A manifest that a series of different concentrations
of IÀ contributed to different levels of fluorescence quenching
of the UCNPs in the PBS buffer solution. As discussed above,
(F À F0)/F was chosen as the output signal, in which F0 and F refer
to the fluorescence intensities of the UCNP–AuNP–L-cysteine–IÀ
solution mixtures in the presence of different concentrations of
and absence of IÀ, respectively. F1 refers to the fluorescence
intensity of the UCNPs–AuNPs at 550 nm in the absence of
L-cysteine and IÀ, and its variation is shown in Fig. 6B. There
was a significant inverse correlation between (F À F0)/F and the
concentrations of IÀ from 0.1 to 200 mM, as expressed by the linear
regression equation (F À F0)/F = 0.2235 + 0.1586 log[IÀ] (mM)
(R2 = 0.9887). The limit of detection (LOD) for IÀ calculated using
the 3s/s criteria was 55 nM, which represents a high enough
sensitivity for the detection of IÀ. In the summary and contrastive
analysis, a comparison between the developed sensors and other
reported methods for IÀ detection was obtained, as presented
in Table S1 (ESI†), and it was confirmed that the developed sensors
could achieve a more sensitive detection than other reported
methods.
Fig. 7 Fluorescence spectra of the UCNPs–AuNPs nanosensor in the
presence of 200 mM IÀ or 2 mM other anions. respectively. [UCNPs]:
0.1 mg mLÀ1; [AuNPs]: 1.25 nM; [L-cysteine]: 12 mM; PBS: 0.01 M (a) C2O4
,
,
2À
(b) SO42À, (c) FÀ, (d) ClÀ, (e) BrÀ, (f) HPO4À, (g) NO3À, (h) IO3À, (i) ClO3
À
(j) citrate, (k) CH3COOÀ, (l) H2PO4À, (m) ClO4À, (n) NO2À, (o) SiO32À, and
(p) IÀ, respectively.
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Table 1 Detection of IÀ in urine samples
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Urine
Spiked
(mM)
Measured
(mM)
Recovery
(%)
RSD
(n = 3, %)
sample
5 Z. Zhang, Z. Chen, F. Cheng, Y. Zhang and L. Chen, Analyst,
2016, 141, 2955–2961.
6 L. Chen, W. Lu, X. Wang and L. Chen, Sens. Actuators, B,
2013, 182, 482–488.
1
2
3
0.1
10
100
0.11
9.82
95.83
110.0
98.20
95.83
4.86
1.67
1.90
7 H. Huang, Y. Weng, L. Zheng, B. Yao, W. Weng and X. Lin,
J. Colloid Interface Sci., 2017, 506, 373–378.
8 Q. Lin, P. P. Mao, Y. Q. Fan, L. Liu, J. Liu, Y. M. Zhang,
H. Yao and T. B. Wei, Soft Matter, 2017, 13, 7085–7089.
9 V. T. Nguyen, V. Piersoel and M. T. El, Talanta, 2012, 99,
532–537.
10 A. Machado, R. B. Mesquita, S. Oliveira and A. A. Bordalo,
Talanta, 2017, 167, 688.
11 L. Zhu, X. Peng, H. Li, Y. Zhang and S. Yao, Sens. Actuators,
B, 2017, 238, 196–203.
12 J. He, H. Zhang, J. Zou, Y. Liu, J. Zhuang, Y. Xiao and B. Lei,
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13 S. C. Wei, P. H. Hsu, Y. F. Lee, Y. W. Lin and C. C. Huang,
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Hunan Normal University. The person donating the human
urine gave informed written consent.
On further demonstrating the feasibility and repeatability
of the UCNPs–AuNPs probe for practical applications, urine
samples, spiked with different concentrations of IÀ, were used
as real samples. Insoluble substances in the urine samples were
removed by centrifugation (1000 rpm, 20 min) and filtration
through a 0.2 mm membrane, then the filtered urine was
diluted 50 times with a PBS buffer solution. The recovery
experiments were performed using urine samples as a matrix
to verify the method. The recoveries of IÀ are exhibited in
Table 1, ranging from 95.83% to 110.00% for the real samples,
with relative standard deviation (RSD, n = 3) values in the
range of 1.67% to 4.86%, indicating the prospect of using the
developed system.
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4. Conclusions
In summary, a simple and off–on–off fluorescence probe for the
detection of IÀ was developed. For this purpose, we used
L-cysteine to modulate the fluorescence signals of the UCNPs
for IÀ detection. By combining the highly selective IÀ-catalyzed
oxidation reaction with the nonautofluorescence benefits of the
UCNPs, a sensitive, eco-friendly and highly selective sensing
approach for IÀ detection was designed and applied to monitor
IÀ levels in human urine. It is believed that this strategy
will provide a new approach for developing highly sensitive
and selective sensors for use in environmental and biological
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21874042 and 21675051), and the Foun-
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