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Ratiometric Phosphorescent Probe for Thallium in Serum, Water, and Soil Samples Based on Long-Lived, SpectrallyResolved Mn-doped ZnSe Quantum Dots and Carbon Dots Xiaomei Lu, Jinyi Zhang, Ya-Ni Xie, Xinfeng Zhang, Xiaoming Jiang, Xiandeng Hou, and Peng Wu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05365 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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Analytical Chemistry

Ratiometric Phosphorescent Probe for Thallium in Serum, Water, and Soil Samples Based on Long-Lived, Spectrally-Resolved Mn-doped ZnSe Quantum Dots and Carbon Dots

Xiaomei Lu,† Jinyi Zhang,‡ Ya-Ni Xie,† Xinfeng Zhang,§ Xiaoming Jiang,*,† Xiandeng Hou,†,‡ Peng Wu*,†,‡

†

Analytical & Testing Center, and ‡College of Chemistry, Sichuan University, 29

Wangjiang Road, Chengdu 610064, China

§

College of Materials and Chemistry & Chemical Engineering, Chengdu University of

Technology, Chengdu 610059, China

*Corresponding authors: [email protected] (PW); [email protected] (XJ)

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Thallium (Tl) is an extremely toxic heavy metal and exists in very low concentrations in the environment, but its sensing is largely underexplored as compared to its neighboring elements in the periodic table (especially mercury and lead). In this work, we developed ratiometric phosphorescent nanoprobe for thallium detection based on Mn-doped ZnSe quantum dots (QDs) and water-soluble carbon dots (C-dots). Upon excitation with 360 nm, Mn-doped ZnSe QDs and C-dots can emit long-lived and spectrally-resolved phosphorescence at 580 nm and 440 nm, respectively. In the presence of thallium, the phosphorescence emission from Mn-doped ZnSe QDs could be selectively quenched, while that from C-dots retained unchanged. Therefore, a ratiometric phosphorescent probe was thus developed, which can eliminate the potential influence from both background fluorescence and other analyte-independent external environment factors. Several other heavy metal ions caused interferences to thallium detection, but could be efficiently masked with EDTA. The proposed method offered a detection limit of 1 µg/L, which is among the most sensitive probes ever reported. Successful application of this method for thallium detection in biological serum, environmental water and soil samples were demonstrated.


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Introduction Because of its extreme toxicity to living organisms,1-3 thallium is included in the list of 129 so-called “priority pollutants” by the US Environmental Protection Agency (EPA). Thallium is a trace element and its abundance is very low. Similar to most of the other heavy metals, the major way of thallium entering the environment is through the byproduct of mining. The rapid development of superconductive industry also adds another way for environmental thallium threatening. As regulated by the US EPA, thallium in normal drinking water should be less than 2.5 to 10 nM. However, up to 100 nM level of thallium has been reported in groundwater, and even large concentration of 5-400 µM has been found in river areas proximity to metal mining drainage.4 At the present time, atomic spectrometric techniques are the most powerful for Tl determination, such as electrothermal atomic absorption spectrometry (ETAAS)5-7 and inductively coupled plasma mass spectrometry (ICP-MS).8-10 However, the volatile nature of thallium results in inefficient pyrolysis of sample matrix in ETAAS, while the high ionization potential of thallium makes atomization/ionization difficult in ICP-MS. Besides, the operation costs of ET-AAS and ICP-MS are relatively expensive. Therefore, optical sensors that can probe thallium in a highly sensitive and selective and yet simple manner should be promising.11-13 Semiconductor quantum dots (QDs) and carbon dots (C-dots) have garnered considerable interest for optical probes (especially metal ion probes14-18) during the past two decades due to their excellent luminescent properties.19-22 Compared with the



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fluorescence, phosphorescence detection can alleviate the autofluorescence and scattering light from common biological matrices and is thus promising for probe development.

However,

room-temperature

phosphorescence

(RTP)

from

semiconductor QDs are much less concerned over their fluorescence properties, except that Mn2+ triplet dopant emission (ca. 580 nm) from several Mn-doped QDs.23 Several Mn-doped QD-based phosphorescent metal ion probes were thus reported.24-25 However, quantification of the target analyte using only the emission intensity at a wavelength can sometimes be problematic, because interferences can arise from a variety of analyte-independent factors. To overcome such drawbacks, another phosphorescence peak can be taken into account for ratiometric detection.26-27 Fortunately, RTP from carbon dots was recently reported,28-31 with RTP emission spectrally-resolved from that of typical Mn-doped QDs. Therefore, in this work, we reported a ratiometric phosphorescent probe for thallium detection based the hybrid of Mn-doped ZnSe QDs and C-dots. Because of the characterized blue emission, C-dots have been frequently employed in ratiometric sensing.32-35 In the ratiometric sensor here, CDs was employed as a reference signal, while Mn-doped ZnSe QDs served as a response signal for thallium detection. As shown in Scheme 1, under the same excitation, both of the Mn-doped ZnSe QDs and C-dots are phosphorescently emissive, and thus can be explored in time-resolved detection for elimination of the short-lived background fluorescence after proper delay. Besides, the phosphorescence emission peaks of the Mn-doped ZnSe QDs and C-dots are well spectrally-resolved. In the presence of thallium (Tl+), only the


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phosphorescence emission of Mn-doped ZnSe QDs could be quenched, while that of C-dots could be reserved. Therefore, the presence of Tl+ could be ratiometrically probed in a time-resolved mode for correcting both short-lived background fluorescence and analyte-independent factors. Although there are many fluorescent ratiometric probes (either organic, inorganic, or hybrid of organic and inorganic materials36-40) and several organic phosphorescent ratiometric probes,41-42 this is the first ratiometric phosphorescent probe based on long-lived emission from nanocrystal materials.

Scheme 1. Schematic illustration of the working mechanism of the ratiometric phosphorescent probe for thallium detection.

Experimental Section Reagents and Materials. All reagents used were of at least analytical grade. Manganese chloride (MnCl2, 99.99%), stearic acid (HSt, 98%), selenium (Se, ≥ 99.999%), zinc stearate (ZnSt2, 99.99%), oleic acid (OA, AR), chloroform (CHCl3, 99.8%), citrate (AR, 99.5%) and ethanol absolute (AR, 99.5%) were brought from Aladin (Shanghai, China). 1-Octadecene (ODE, 90%), and oleylamine (OAm, 70%)



and 3-mercaptopropionic acid (MPA) were purchased from Sigma-Aldrich (Shanghai, China). Acetone (AR, 99.0%), thallium (I) nitrate (AR, 99.99%) and ethylenediamine were supplied by Kelong Chemical Reagent Company (Chengdu, China). Ultrapure water (18.2 MΩ·cm) produced with a purification water machine (PCWJ-10, Pure Technology Co. Ltd., Chengdu, China) was used throughout this work.

Instrumentation. The UV–vis spectra measurements were carried out on a UV-1700 UV/Vis spectrophotometer (Shimadzu, Japan). Fluorescence emission spectra were obtained with an F-7000 spectro-fluorometer (Hitachi, Japan). Time-resolved emission was collected with a FluoroMax-4P spectrofluorometer (Horiba Scientific). Fluorescence quantum yield and phosphorescence lifetime measurements were recorded using a Fluorolog-3 spectrofluorometer (Horiba Scientific) with a picosecond photon detection module (PPD-850) as the detector and a spectra LED laser (355 nm) as the excitation source. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) were acquired on a Tecnai G2 F20 S-TWIN transmission electron microscope (FEI Co., USA) with an accelerating voltage of 200 kV. X-Ray photoelectron spectroscopy (XPS) was carried out using an AXIS Ultra DLD (Kratos, UK) with Al Kα excitation (1486 eV). Binding energy calibration was based on C 1s at 284.6 eV.

Synthesis of Mn-doped ZnSe QDs. The synthetic protocol with Se powder as precursor was based on a previous publication (heterogeneous Se precursor).43 Briefly, Zn precursor (0.505 g Zn(St)2, 4 mL ODE, dissolved at 150 °C) and heterogeneous Se


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precursor (31.6 mg Se, 2 mL ODE) were prepared. Next, Mn(St)2 (8.0 mg), ODE (10 mL), and OAm (2 mL) were co-loaded into a 100 mL four-necked flask. The mixture was heated at 120 °C for 20 min together with bubbling with argon. Then, the temperature was raised to 280 °C, followed by addition of 1 mL Se-stock solution. After 30 seconds for formation of ultrasmall MnSe nanoclusters, 1 mL Zn-stock solution and 200 µL OAm were introduced for growing the ZnSe shell. After 2 min for cooling down to 260 ℃, 1 mL Zn-stock solution and 200 µL OAm were again added for further growth the outer ZnSe shell (10 min). Finally, the reaction was cooled to room temperature. For purification, the obtained nanocrystals were subjected to triple acetone/chloroform extraction. After purification, 20 mg of the as-prepared QDs were dissolved in 2 mL chloroform and ultrasonicated with 1 mL MPA (neutralized with NaOH) for 30 min for ligand exchange. The obtained MPA-capped QDs were washed 3 times with water/ethanol extraction for removal of excess MPA. The fluorescence quantum yield of the as-prepared Mn-doped ZnSe QDs was ~14.2%.

Synthesis of CDs. C-dots were prepared with the classical citrate-ethylenediamine route.44 Briefly, in a Teflon cup, citrate (1.0510 g) and ethylenediamine (548 µL) were dissolved with 10 mL deionized water. Then, the Teflon cup was sealed into a stainless steel autoclave heated at 200 °C for 8 h. Purification of the C-dots was made with a silica gel packed glass column (40 cm length, 2 cm i.d.) using water as mobile phase to remove excess citric acid or ethylendiamine complexes. After sample loading, collection of the purified C-dots was started from the first fluorescent drop (excited



with a 360 nm LED). The purified C-dots were diluted 10-fold before use. The fluorescence quantum yield of the as-prepared C-dots was ~49.7%.

Procedures for Tl+ Detection. 10 µL C-dots, 5 µL Mn-doped ZnSe QDs (10 mg/mL), 100 µL Tris-HCl (0.1 M), and proper amounts of Tl+ stock solution (10 ppm) were mixed and diluted to 1 mL with ultrapure water. The mixture was directly subjected to phosphorescence analysis. The excitation was set at 360 nm, and the excitation and emission slit were both set at 10 nm. For sample analysis, water samples were directly analyzed, serum samples were diluted 200-fold with ultrapure water before analysis, and soil sample digestions were diluted 10-fold before analysis.

Sample digestion for soil samples. Soil samples were first accurately weighted (~0.05 g) into PTFE plastic beakers. 5 mL concentrated HNO3 and 2 mL HF were added. Then, the beakers were placed on an electric heating furnace for digestion of soil samples. After vaporization of the liquid (1-2 drops left), 1 mL HClO4 was further added and the mixture was heated to near dry. Then, the residues were dissolved with water (no insolubles) and diluted to volume (50 mL).

Results and discussion Spectroscopic Characterization of Mn-doped ZnSe QDs and C-dots. Detailed physical characterizations of C-dots and Mn-doped ZnSe QDs were given in the Supporting Information (Figure S1-Figure S5). Spectroscopically, both C-dots and Mn-doped ZnSe QDs are phosphorescently emissive. As revealed from the 3-D time-resolved emission spectra (TRES), upon increasing the delay time, long-lived


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emission (~440 nm) could be identified from C-dots (Figure 1A), consisting with the previous reports.29,45 Besides, such long-lived emission was almost overlapped with their short-lived component. The mechanism of long-lived emission from C-dots is complicated. Probably, small energy gap exists between the excited singlet state and triplet state, which results in facile intersystem crossing (ISC) and reversed intersystem crossing (RISC) between these two excited states.45 Therefore, the collected overlapped emission spectra may be resultant from the mixture of phosphorescence and thermally activated delayed fluorescence. For Mn-doped ZnSe QDs, the characterized Mn2+ dopant emission (~580 nm) was collected (Figure 1B), while no clear sign of host emission from ZnSe was obtained, suggesting high-quality of the as-prepared QDs.46 Obviously, the phosphorescent emission of C-dots can be well-resolved from that of Mn-doped ZnSe QDs (Figure 1C).

Figure 1. Spectroscopic characterization of the hybrid of Mn-doped ZnSe QDs and C-dots: (A) TRES of C-dots; (B) TRES of Mn-doped ZnSe QDs; and (C) phosphorescent emission spectra (delay time of 50 µs) of individual C-dots and Mn-doped ZnSe QDs as well as their hybrid. All the spectra were collected with excitation wavelength of 360 nm.

Upon mixing C-dots with Mn-doped ZnSe QDs, the phosphorescent emission spectra (delay time of 50 µs) of the hybrid almost resemble those of the individual



spectra (Figure 1C), yielding two well-resolved emission peaks. Besides, the excitation of these two dots also shares considerable overlapped area. Therefore, they can be excited simultaneously for the following ratiometric investigations.

Construction of the Ratiometric Phosphorescent Probe for Tl+. After confirmation of the long-lived and spectrally-resolved emission from Mn-doped ZnSe QDs and C-dots, we next intended to construct the ratiometric phosphorescent probe. As shown in Figure 2A and Figure S6, upon addition of Tl+ (100 µg/L) to the solution of C-dots, the phosphorescence intensity of CDs kept almost unchanged. However, introduction of the same concentration of Tl+ caused obvious phosphorescence quenching to Mn-doped ZnSe QDs (Figure 2B and Figure S6). Similar to Figure 1C, the phosphorescence alternations in the ratiometric system almost resembled the response of individual dots (Figure 2C), namely unchanged at 440 nm and quenched at 580 nm, respectively. Therefore, a ratiometric phosphorescent probe could be potentially developed for Tl+ detection.


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Figure 2 Conformation of the ratiometric response for Tl+ (100 µg/L): (A) C-dots; (B) Mn-doped ZnSe QDs; and (C) the hybrid of Mn-doped ZnSe QDs and C-dots.

In conventional room-temperature phosphorescence analysis, thallium is typically employed as the heavy atom for inducing phosphorescence.47 The phosphorescence is mainly come from the analyte itself. The amount of thallium is typically at mM-M scale to ensure efficient collision between the heavy atoms and the triplet state of the analytes. While in this work, the phosphorescence comes from QDs. The probability of efficient collision between thallium and the triplet states of QDs is blocked by the outer shell of QDs, leading to different role of thallium here from the conventional room-temperature phosphorescence.

Possible Mechanism of the Tl+-induced phosphorescence quenching of Mn-Doped ZnSe QDs. There are generally two modes for fluorescence quenching,



namely dynamic and static quenching.48 Dynamic quenching refers to the perturbation of the excited state of the luminescent fluorophore by the quenching species, such as electron transfer and energy transfer.22 Besides steady-state emission intensity decreasing, dynamic quenching also accompanies with excited lifetime shortening. On the contrary, static quenching involves ground state interaction between the quenching species and the luminescent fluorophore. Accordingly, static quenching does not cause excited state lifetime change of the luminescent fluorophore. Both of the above two quenching modes can be described with the well-known Stern-Volmer (S-V) equation, namely F0/F = 1 + K[Q], where F0 and F are the fluorescence intensity of the fluorophore in the absence and in the presence of the quencher, respectively, and K is the quenching constant. In the presence of Tl+, a significant and regular decrease in the phosphorescence intensity of Mn-doped ZnSe QDs was observed (Figure 3A). Meanwhile, the phosphorescence lifetime of Mn-doped ZnSe QDs in the absence and presence of Tl+ was collected, showing a faster decay rate in the presence of Tl+ (Figure 3B). However, the change of the phosphorescence lifetime was much smaller than that of the phosphorescence intensity. Plotting the changed phosphorescence intensity (Ph0/Ph, where Ph 0 and Ph are the phosphorescence intensities of the Mn-doped ZnSe QDs in the absence and in the presence of a quencher, respectively) versus the concentration of Tl+ didn’t yield a linear S-V curve, but rather bending to the Y-axis (Figure 3C). Such upward curvature of the S-V plot is a clear sign of mixed quenching modes,49 i.e., both static and dynamic quenching are involved in


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Tl+-induced quenching towards Mn-doped ZnSe QDs.

Figure 3. Studying the quenching mechanism of Tl+ to Mn-doped ZnSe QDs: (A) phosphorescence spectra of Mn-doped ZnSe QDs in the presence of various amounts of Tl+; (B) phosphorescence lifetime of Mn-doped ZnSe QDs in the presence of Tl+; (C) Stern–Volmer plot in 0.1 M Tris-HCl buffer of pH 9.0; and (D) the quadratic Stern–Volmer plot.

To quantitatively analyze the phosphorescence quenching behavior of Mn-doped ZnSe QDs by Tl+, the regular S-V equation is changed to a quadratic Stern-Volmer model Ph0/Ph = (1 + KS[Tl+])(1 + KD[Tl+]),49 which incorporates both static (KS) and dynamic (KD) components. Deriving the above equation yields the following function: [(Ph0/Ph) - 1] × 1/[Tl+] = (KD + KS) + KD KS[Tl+] Then plotting [(Ph0/Ph) - 1] × 1/[Tl+] with the concentration of Tl+ yields an approximate linear curve (Figure 3D), in which the slope represents KD*KS, and the intercept is equal to KD + KS. The KD and KS are therefore determined as ~0.28 µM-1



and ~1.20 µM-1, respectively. The KS value is about ~4-5 times larger than the KD value, indicating that the static phosphorescence quenching between Mn-doped ZnSe QDs and Tl+ is dominant in the phosphorescence quenching. As is well-known that for Mn-doped ZnSe QDs, the emission spectrum generally presents two emission peaks, namely the host emission from ZnSe and dopant emission from Mn2+.46 For un-doped ZnSe QDs, thallium could also induce fluorescence quenching, but the quenching is much smaller than that of its doped counterpart (Figure S7). A series of other QDs, including CdSe、CdS、Cu-CdS and CdTe QDs, were also studied. As can be seen from Figure S8, no appreciable Tl+-induced fluorescence quenching were observed for these QDs except very weak quenching for ZnSe and CdSe QDs. However, for Mn-doped ZnS QDs, similar but less quenching as that of Mn-doped ZnSe QDs was found (Figure S8). Accordingly, the observed phosphorescence quenching is probably related to Mn2+ dopant and Se. It has been reported that the Mn2+ luminescence decay could be used for differentiation of the surface-bound Mn2+ impurities and the lattice-bound Mn2+ dopants,50 namely the short and long components represent the surface- and lattice-bound Mn2+, respectively. As can be seen from Figure 3B that in the presence of Tl+, the short component changed appreciably while the longer part remained almost unchanged. Plotting the short decay component (τ0/τ) with the concentration of Tl+ yields a KD’ value of ~0.11 µM-1 (Figure S9), somewhat close to the calculated KD (~0.28 µM-1). In other words, the part of dynamic quenching is probably related to the Tl+-induced perturbation of the surface-bound Mn2+. Besides, the Tl+-quenched


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phosphorescence of Mn-doped ZnSe QDs could be somewhat restored by polyacrylic acid (PAA), a cationic carboxyl-containing polymer that may coordinate with Tl+ (Figure S10). Possibly, the introduction of Tl+ resulted in partial detaching of the carboxyl ligands originally binding with surface-bound Mn2+,51 leading to dynamic quenching.

Meanwhile,

Tl+ may

bind

with

Se-containing

QDs to form

non-luminescent complexes, leading to static quenching.

Interference Study. It is well-known that QDs can response to a variety of metal ions.52 To further confirm that interference of co-existing ions on determination of thallium in this system, a variety of cations such as Li+, Na+, K+, NH4+, Ag+, Mg2+, Zn2+, Fe2+, Fe3+, Cu2+, Bi3+, Hg2+ and Pb2+ were introduced to the system to explore the interference effect. As can be seen from Figure 4A and Table S1 that Fe3+, Cu2+, Hg2+ and Pb2+ would cause interferences to Tl+ detection, i.e., these metal ions also caused phosphorescence quenching to the ratiometric system. The quenching mechanism of these metal ions is all dynamic, as revealed from the phosphorescence lifetime change of Mn-doped ZnSe QDs (Figure 4C- 4F). Fortunately, such interference could be largely inhibited by EDTA. As can be seen from Figure 4B, the presence of EDTA did not perturb the Tl+-induced phosphorescence quenching, but the quenching by Fe3+, Cu2+, Hg2+ and Pb2+ could be largely alleviated. When EDTA was added to the interferenced cases, i.e., Tl+ co-existing with Fe3+, Cu2+, Hg2+, Pb2+or Bi3+, the interferences from these metal ions could be large masked (Figure S11). Therefore, the proposed hybrid of C-dots and Mn-doped ZnSe QDs could be explored as a ratiometric phosphorescent probe for Tl+ detection.



Figure 4 Interference study about the ratiometric phosphorescent probe for Tl+ detection: (A) the response of the ratiometric phosphorescent probe for Tl+, Fe3+, Cu2+, Hg2+ and Pb2+ in the absence of EDTA; (B) the response of the ratiometric phosphorescent probe for Tl+, Fe3+, Cu2+, Hg2+ and Pb2+ in the presence of EDTA; and (C-F) phosphorescence lifetime of Mn-doped ZnSe QDs in the absence and presence of Fe3+, Cu2+, Hg2+ and Pb2+.

Analytical Performance of the Ratiometric Phosphorescent Probe for Tl+ Detection. To maximize the sensitivity of the ratiometric probe for Tl+ detection, the assay buffer pH was investigated. As shown in Figure S12, pH 9.0 was optimal for ratiometric response. Under optimal conditions, the phosphorescence intensity (@580


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nm) decreased (Figure 5A) together decreased ratiometric response (I580 / I440, Figure 5B) with the with the Tl+ concentration. A linear correlation (Y = -0.0231X + 7.40) was obtained between the ratiometric response and the Tl+ concentration in the range of 5-100 µg/L (R2 = 0.9986, inset of Figure 5B). The limit of detection (LOD) was calculated (LOD = 3σ/slope, where σ is standard deviation of background variation) as 1 µg/L, representing one of the most sensitive probe for Tl+ detection (Table S2). Besides, the proposed method is also more sensitive than most of the commercial atomic spectrometric techniques (Table S3). Meanwhile, the calibration curve for Tl+ in the presence of EDTA was also measured (Figure S13) and similar assay sensitivity was obtained (Y = -0.0216X + 6.698, R2 = 0.9981). Therefore, for better elimination of potential interferences, EDTA was added into the buffer as media for the assay.



Figure 5 Analytical performance of the proposed assay for Tl+ detection: (A) The phosphorescence spectra of the hybrid system of Mn-doped ZnSe QDs and C-dots in presence of different amounts of Tl+; and (B) plots of the ratiometric response (I580 / I440) versus the the concentration of Tl+ added. The inset in (B) shows the linear assay range for Tl+ detection.

Determination of Tl+ in Human Serum and Environmental Water Samples. In order to evaluate the accuracy and reliability of the present assay under real conditions, the proposed sensing system was explored to detect Tl+ in human serum and environmental water samples. The human serum samples were collected from


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healthy volunteers and diluted by 200-fold with doubly distilled water before analysis. Water samples were analyzed as received. These samples featured severe fluorescence background, but such background was completely eliminated in phosphorescence mode (Figure S14, delay time of 50 µs). As can be seen from Table 1, no Tl+ was detected in the biological serum and environmental samples, and then the samples were analysed with a standard-addition method with adding certain amounts of Tl+ to the sample solutions. The quantitative spike-recoveries ranged from 94.5% to 107% and 98.3% to 106% for serum and water samples, respectively, demonstrating the potential application of the ratiometric probe for clinical diagnosis and environmental monitoring of toxic Tl+.

Table 1 Results of thallium determination in human serum and environmental water samples. Samples

Serum 1

Serum 2

Serum 3

Added (µg/L)

Found (µg/L)

Recovery (%)

0

N.D.

-

20

19.6 ± 0.9

98.2

50

47.3 ± 1.7

94.5

0

N.D.

-

20

21.4 ± 2.0

107

50

49.9 ± 1.6

99.8

0

N.D.

-

20

20.3 ± 0.9

102

50

47.8 ± 0.6

95.6

0

N.D.

-

20

21.1 ± 1.8

105

0

N.D.

-

Tap water Underground water



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20

22.7 ± 3.3

106

0

N.D.

20

19.0 ± 2.6

98.3

0

N.D.

-

20

20.2 ± 2.1

104

Peaceful pond

Funan River N.D.: not detected

The potential usefulness of the proposed ratiometric probe was further evaluated through determination of thallium in soil samples. Two certified reference soil materials, GBW07402 (GSS-2) and GBW07430 (GSS-16), were employed here. After sample digestion, the obtained clean solution of soil samples were first diluted 10-fold followed by addition of 200 µg/L EDTA, and then analyzed with the ratiometric probe. As can be seen from Table 2, the analytical results were in good agreement with the certified values.

Table 2 Results of thallium determination in soil standard samples. Sample No.

Determined value (µg/g)

Certified value (µg/g)

GBW07402 (GSS-2)

0.70 ± 0.12

0.62 ± 0.20

GBW07430 (GSS-16)

1.32 ± 0.10

1.12 ± 0.08

Conclusions In summary, a ratiometric phosphorescent probe for thallium was developed based on long-lived, spectrally-resolved Mn-doped ZnSe QDs and C-dots, which could effectively eliminate potential influence from both background fluorescence


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(phosphorescence) and other analyte-independent external environment factors (ratiometric). The obtained limit of detection (1 µg/L) is much lower than the previously reported optical probes. Since US EPA regulated less than 2.5 to 10 nM of thallium in drinking water, this method may be of potential usefulness in analyzing thallium in water samples.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21522505), the Sichuan Youth Science & Technology Foundation of Sichuan Province (Grant 2016JQ0019), and the Fundamental Research Funds for the Central Universities (2016SCU04A12). Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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