Highly Sensitive and Selective Method for Detecting Ultratrace Levels

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Highly sensitive and selective method for detecting ultra-trace levels of aqueous uranyl ions by strongly photoluminescent responsive amine modified cadmium sulphide quantum dots Raj Kumar Dutta, and Ambika Kumar Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b01943 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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Analytical Chemistry

Visual detection of aqueous uranyl ion by photoluminescence quenching of CdS-MAA-TU QDs 52x30mm (300 x 300 DPI)

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Highly sensitive and selective method for detecting ultra-trace levels of aqueous uranyl ions by strongly photoluminescent responsive amine modified cadmium sulphide quantum dots R.K. Dutta* and Ambika Kumar Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India

* Corresponding Author: Email: [email protected] (R.K. Dutta) Fax: +91-1332-286202, Tel. +91-1332-285280

ABSTRACT Detection of ultra-trace levels of aqueous uranyl ions without using sophisticated analytical instrumentation and tedious sample preparation method is a challenge for environmental monitoring and mitigation. Here we present a novel, yet a simple analytical method for highly sensitive and specific detection of uranyl ions via photoluminescence quenching of CdS quantum dots. We have demonstrated a new approach for synthesizing highly water soluble and strong photoluminescence emitting CdS quantum dots (i.e., CdS-MAA and CdS-MAA-TU) of sizes less than 3 nm. The structural, morphological and optical properties of both the batches of CdS quantum dots were thoroughly characterized by XRD, high-resolution transmission electron microscopy (HRTEM), zeta potential, UV-visible absorption and photoluminescence spectroscopy. Compared to the batch of CdS quantum dots prepared by capping with only mercaptoacetic acid (CdS-MAA), the batch prepared by capping with mercaptoacetic acid and thiourea in tandem (CdS-MAA-TU) exhibited higher quantum yield= 16.64 ± 1.02 % and more importantly CdS-MAA-TU exhibited significantly higher order of photoluminescence quenching responses when treated with ultra-trace concentrations of uranyl ions. Under the optimized conditions, the sensitivity of detecting uranyl ion by CdS-MAA-TU was several folds better 1


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(0.316 L/ µg) than that of CdS-MAA (0.0053 (L/µg/), as determined from their respective SternVolmer plots. Qualitatively, CdS-MAA-TU probe can be used for visual detection of uranyl ions of concentration greater than 5 µg/L. But the instrumental method of analysis based on photoluminescence spectroscopy confirmed the feasibility for quantitative analysis of ultra-trace concentrations of uranyl ions as implied from a very low limit of detection (LoD = 0.07 µg/L) and limit of quantification (LoQ= and 0.231 µg/L). Systematic studies revealed very high selectivity for uranyl ion detection, though minor interference from Cu2+, Pb2+, Hg2+, CO32- and SO42- were found. The recovery analysis performed by spiking uranyl ions (0.5 µg/L to 10.0 µg/L) in groundwater and river water samples, confirmed the robustness of the as-developed CdS-MAA-TU QDs for detecting ultra-trace levels of uranyl ions in real water sample matrix. The very simple and effective strategy reported here should facilitate developing reliable sensors for detecting uranyl ion contamination in drinking water.

Keywords: Uranyl ions detection, CdS quantum dots, Photoluminescence quenching, SternVolmer equation, Spike analysis

INTRODUCTION Uranium is an important fuel material used in nuclear power industry and in nuclear research establishments. It is a radioactive material and is very toxic to ecosystem.1 It exists in various oxidation states (i.e., +2 to +6), but uranyl (UO22+) represents the most stable chemical species with maximum bioavailability in aerobic environment.2-4 Therefore its exposure and contamination in soil and natural water resources is a severe concern due to its adverse impact on human health and all living species.5-7 Apart from geochemical sources, uranium contamination in ground water and in aquifers are associated with activities related to uranium mining and 2



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improper disposal of nuclear waste.8-10 In addition, uranium can be released in the environment via phosphate fertilizer.11 The maximum permissible contamination level of UO22+ in drinking water is defined as 130 nM, which is equivalent to 35 µg/L by United States Environmental Protection Agency (USEPA).12 Because of crucial environmental concern, the assessment of ultra-trace level of uranium contamination in soil and water is of prime importance. Various instrumental techniques, e.g., inductively coupled plasma mass spectrometry,13 atomic absorption spectroscopy,14 total reflection X-ray fluorescence spectrometry,15 cold-vapor atomic fluorescence spectrometry,16 laser-induced kinetic phosphorimetry,17 surface enhanced Raman spectroscopy18 and cathodic stripping voltametry19,20 were developed for determining trace concentrations of uranium. These techniques involve expensive instruments and moreover the sample preparation is tedious and time consuming. It is therefore essential to develop simple and convenient method for detecting uranyl ions in aqueous environment. Lately, nanomaterial based uranyl ion sensing are reported, which were based on various detection principles, e.g., fluorescence,21,22 electroanalytical,23 colorimetry,24 magnetoelasticity25 and surface enhanced Raman scattering (SERS)26. Some of these studies, e.g. sensing by DNAzymes using gold nanoclusters22 and plasmonic nanowire interstice coupled with DNAzyme cleaved reaction using gold nanowires27 achieved very high selectivity and sensitivity at the cost of complicated method for fabrication of probe. One of the major limitations for developing gold based probes is its high cost of precursor materials. Because of this there is a continued interest to explore simpler, low cost and efficient method for fabricating of probe for detecting ultra-trace levels of uranium in natural water. In this regard, photoluminescent probes made of semiconductor type quantum dots (QDs) like CdS, CdSe, or their core shell type nanostructures were though developed for detecting heavy metals, e.g., Hg2+ Cu2+ or Pb2+ ions,28-34 but they are yet to be explored for detection of uranyl ions at trace concentrations. In the case of highly emissive CdS quantum dots, which are stabilized by 3


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thiol compounds like mercaptoacetic acid are excellent for detecting Hg2+ Cu2+ or Pb2+. The detection mechanism of these heavy metals, which have strong affinity towards sulfur, is based on photoluminescence quenching. But these semiconductor type nanoparticles or quantum dots are not reported for uranyl ion detection. It could be due to in-efficient interaction of uranyl ions with the available functional groups of mercatoacetic acid used for capping CdS nanoparticles. Since uranyl ions have affinity for amine groups, 35 we have introduced thiourea as a secondary capping agent for the synthesizing CdS quantum dots to facilitate interaction with uranyl ions and create analytical signal via photoluminescence quenching. The quantitative detection of uranyl ions was derived from the linear region of the Stern-Volmer equation. To the best of our knowledge, the photoluminescent probe developed by us is the first example of using a CdS based QDs for detection of ultra-trace (parts per billion and less) levels of uranyl ions with excellent selectivity.

EXPERIMENTAL SECTION Synthesis and characterization of the CdS based quantum dots for uranyl ion detection. In this study the aim was to synthesize stable CdS quantum dots in aqueous medium for which necessary modifications were done to eliminate the use toxic solvent - toluene as emulsifier for synthesizing CdS quantum dots as reported by us earlier.36 In view of this, two batches of CdS quantum dots were prepared using the chemical route, i.e., using only mercaptoacetic acid in one approach (referred to as CdS-MAA), and using mercaptoacetic acid and thiourea in tandem in the other approach (CdS-MAA-TU). These batches of quantum dots were characterized by array of techniques, e.g., X-ray diffraction, high resolution transmission electron microscopy, UV visible absorption and emission spectroscopy. The details of materials used, protocols developed for synthesizing the two batches of CdS quantum dots and their characterization are given as Supporting Information (Section S 2.1) 4



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Analytical Method: Optimization, Sensitivity, Selectivity and Real sample analysis. Several steps were optimized with respect to the photoluminescence property of the probe, e.g., pH of the pristine probes, pH of the probe treated with analyte (uranyl ion) and contact time between the probe and the analyte. The quantitative photoluminescence quenching of the as-developed CdSMAA-TU and CdS-MAA quantum dots were recorded against a selected range of uranyl ion concentrations which were used as input parameters in Stern-Volmer plot for obtaining analytical responses for determining limits of detection (LoD), i.e. sensitivity for uranyl ion detection by our method. The selectivity of detecting uranyl ions was studied by recording photoluminescence quenching of optimized probe, i.e., CdS-MAA-TU treated with 5 µg/L uranyl ions in presence of common cations e.g., Na+, K+, Mg2+, Ca2+, Fe2+, Mn2+, Zn2+, Cd2+, Co2+, Cr3+, Hg2+, Cu2+ and Pb2+) and anions e.g., F-, Cl-, NO3-, CO32-, SO42-, HCO3-, OAc-, and AsO43-. The robustness of the CdS-MAA-TU quantum dots as probe for uranyl ion detection in real sample was tested by means of spike analysis in groundwater and in river water samples. Details of the methodology developed for performing optimization, sensitivity, selectivity and spike analysis are given as Supporting Information (Section S 2.2).

RESULTS AND DISCUSSION Characterization of photo-luminescent CdS probes for uranyl ion detection. The batches of CdS nanoparticles prepared by (a) single capping agent - mercaptoacetic acid (CdS-MAA); and (b) two capping agents - mercaptoacetic acid and thiourea (CdS-MAA-TU), dispersed in de-ionized water appeared clear and colorless under normal light. When they were exposed to UV light (λ = 365 nm), the batch of CdS-MAA exhibited bluish green photoluminescence

(Figure

S1A)

while

CdS-MAA-TU

exhibited

intense

bluish

photoluminescence, (Figure S1B). The UV-visible spectroscopy of CdS-MAA-TU and CdS5


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MAA is given in Figure 1A and 1B, respectively. Their band gaps were calculated from the edge of the UV visible spectra corresponding to the intersection of the base line and the slope of the absorption plot corresponding to the minimum energy difference between the filled valence band and empty conduction band.37 The band gaps were 3.17 eV and 3.24 eV for CdS-MAA and CdSMAA-TU, respectively (compiled in Table-1), which were larger than that of bulk CdS (2.42 eV). The larger band gap in the as-synthesized CdS quantum dots is attributable to decrease in particle size.38 Furthermore, the particle sizes (D) of the both the batches of as-synthesized CdS were determined from UV-visible spectra using the empirical equation:39 D (nm) = (-6.6521 x 10-8)*λ3 + (1.9557 x 10-4)*λ2 – (9.2352 x 10-2)*λ + 13.29 where λ (nm) is the wavelength of the absorption maxima. From the above empirical equation, the particle sizes of CdS-MAA and CdS-MAA-TU QDs were calculated as 2.20 nm and 1.88 nm, respectively. The calculated particle sizes were much smaller than the Bohr excitonic radius of CdS.40 Due to this, the batches of CdS-MAA and CdS-MAA-TU are expected to exhibit quantum confinement effect and they can be referred to as quantum dots (QDs).41 The photoluminescence emission property of the batches of CdS-MAA and CdS-MAA-TU QDs were evident from their photoluminescence spectra. At an excitation of λ = 370 nm, the photoluminescence emission peak was recorded at 480 nm for CdS-MAA-TU (Figure 1C) and 505 nm for CdS-MAA (Fig. 1D). In addition, the photoluminescence peak of CdS-MAA-TU was less broad (full width at half maxima = 122 nm) and more intense than that of CdS-MAA (full width at half maxima = 130 nm). Broader photoluminescence peak for CdS-MAA is attributable to more surface defects.42 The reduction in the FWHM and enhancement in the photoluminescence emission intensity of CdS-MAA-TU is attributable to passivation of surface defects by introducing thiourea as a second capping agent. Similar photoluminescence enhancement was reported for amine bearing ligand (i.e., cysteine) as capping agent for synthesizing CdS QDs.43 The quantum yields were 6



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calculated as 16.64 ± 1.02 % (for CdS-MAA-TU) and 15.82±0.91% (CdS-MAA) with respect to Rhodamine 6G (Rh-6G) dye as a reference fluoroprobe (values compiled in Table-1).44,45 The corresponding photoluminescence spectra of Rh-6G, CdS-MAA and CdS-MAA-TU used for quantum yield calculations are given as Supporting Information (Figure S2A-S2C). The detailed method for quantum yield calculations is given as Supporting Information (Section S 3.1). Notably, the quantum yield derived for CdS-MAA-TU QDs was better than those reported for highly emissive batches of CdS QDs.46 Though both the batches of quantum dots were highly soluble and their quantum yields were comparable, but their XRD patterns were strikingly different (given as Supporting Information, Figure S3A and B). The XRD pattern of CdS-MAA did not match with any known structures of CdS by comparing with JCPDS file, indicating that the CdS nanoparticles prepared by capping with MAA did not attain the expected regular cubic structure. On the other hand, the XRD pattern of CdS quantum dots synthesized by capping with MAA and TU in tandem (CdS-MAATU) revealed two prominent peaks at 2θ = 27.44o and 46.75o, which corresponded to the reflection from (111) and (220) atomic planes of cubic phase of CdS. Our XRD pattern of CdSMAA-TU agreed well with the JCPDS file no. 80-0019.47 The parameters, e.g., 2θ values, the corresponding atomic planes and crystallite sizes derived from XRD measurements for both CdS-MAA and CdS-MAA-TU QDs are compared in Table-1. The transmission electron microscopy (TEM) studies of CdS-MAA-TU QD revealed formation of spherical particles of sizes less than 3 nm diameter (Figure 2A and 2B). The particle size measured by TEM was consistent with the crystallite size measured by XRD and also with the particle size calculated using the empirical equation (Table-1). The nanocrystalline property of CdS-MAA-TU QDs was evident from high resolution TEM (HRTEM) study, which revealed lattice fringes with inter-planar spacings of 0.320 ± 0.004 nm (mean ± standard deviation of four readings), shown in the inset of Figure 2B. The elemental composition of CdS-MAA-TU batch 7


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by energy dispersive X-ray analysis (EDXA), revealed characteristic L-X-ray peaks of Cd and K-X-ray peak of S (Figure 2C). However, the TEM image of CdS-MAA batch lacked clarity in terms of identifying morphology of the particles (Figure 2D), and corroborated with the poorly crystalline XRD pattern. Interestingly, high resolution TEM image of CdS-MAA revealed very weak signatures of lattice fringes with inter-planar distance of 0.341 ± 0.006 nm (inset of Figure 2D, representing mean and standard deviation of four readings). The crystalline nature of the CdS-MAA-TU QDs was also studied from its selected area electron diffraction (SAED) image (given as Supporting Information Figure S4A). Two concentric rings were observed due to atomic layers corresponding to inter-planar spacings of 0.321 nm and 0.211 nm. The inter-planar spacing measured from SAED (i.e., 0.321 nm) matched with that of the lattice fringes (0.320 nm) and also agreed well with the XRD peak corresponding to (111) plane (the calculations for determining inter-planar distance from SAED image and XRD patterns are given in supporting information, Section S 3.2). Similarly, from the inter-planar spacing of δ = 0.211 nm matched with that of (220) plane recorded by XRD (δ = 0.216 nm). Unlike CdS-MAA-TU, the crystalline signature of CdS-MAA was not at all conclusive from its SAED image (given as Supporting Information Figure S4B). This corroborated with poor XRD pattern of CdS-MAA batch. The improved crystalline property observed in CdS-MAA-TU QDs is attributable to better surface chemistry between functional groups of capping agents or surfactants with that of the QDs.48 It is reported in literature that the crystalline nature of quantum dots improve when more than one surfactants or surfactants with larger chain lengths are used.49 It may be remarked that the improved crystalline property of CdS-MAA-TU QDs was due to formation of a longer chain or a network structure between MAA and TU, possibly via hydrogen bonding or by electrostatic interaction between the polar functional groups (Scheme 1, given as Supporting Information). Any chemical bonding due to mixing of thiourea and mercaptoacetic acid during synthesis of CdS QDs by our method could not be established as the 8



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FT-IR analysis of CdS-MAA-TU did not show emergence of any new peaks as compared to CdS-MAA batch (given as Supporting Information, Figure S5).

CdS QDs as uranyl ion sensor: The prospect of CdS QDs as a probe for detecting uranyl ions would depend on recording a measureable change in its photoluminescence response upon interacting with analyte, which follows Stern-Volmer equation:50 Io/I = 1 + Ksv [UO22+] where Io and I are the photoluminescence intensities of the QDs and the QDs treated with UO22+ ions, respectively; Ksv is the Stern-Volmer photoluminescence quenching constant (which corresponds to the sensitivity of the probe to the quencher) and [UO22+] is the concentration of uranyl ions. The suitability of CdS-MAA and CdS-MAA-TU QDs for quantitative uranyl ion sensing was evaluated from their respective Stern-Volmer plots treated with uranyl ions at pH 4. In the case of CdS-MAA-TU, complete photoluminescence quenching occurred when treated with 100 µg/L of uranyl ions concentration. On the other hand, the phenomenon of complete photoluminescence quenching of CdS-MAA was not observed even by treating with 1000 µg/L of uranyl ions concentration. The Stern-Volmer plots for CdS-MAA and CdS-MAA-TU are given as Figure 3A and 3B, respectively. The gradual decrease in the PL intensities of these batches of CdS probes treated with increasing concentration of uranyl ions is given in the inset of Figure 3A and 3B. The linear region of the Stern-Volmer plot is important for quantitative analysis as the slope of the linear trend corresponds to the analytical sensitivity for detecting uranyl ions. The sensitivity for uranyl ion detection by CdS-MAA was only 0.0053 (µg/L)-1, which is poor due to weaker photoluminescence quenching. On the other hand the linear region of the Stern-Volmer plot for CdS-MAA-TU probe corresponded to the uranyl ion concentration 9


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ranging between 0.1 µg/L and 10 µg/L (Figure 3C). The sensitivity of uranyl ions detection by CdS-MAA-TU was calculated as 0.316 (µg/L)-1, which was 60 times much higher than that of CdS-MAA. The limit of detection (LoD) by CdS-MAA-TU was calculated as 0.070 µg/L, which was 70 times better than that of CdS-MAA (i.e., 4.98 µg/L). Similarly, the limit of quantification (LoQ) for uranyl ions by CdS-MAA-TU and CdS-MAA probes were determined to be 0.231 µg/L and 16.43 µg/L, respectively. The details of LoD and LoQ calculations are given in Supporting Information in Section S 3.3.51 The sensitivity or the LoD of uranyl ion detection by CdS-MAA-TU QDs was better than or comparable with several recent reports on uranyl ion detection by more complex methods.21,22,27 It may be remarked that the drastic improvement in the sensitivity of uranyl ion detection by CdS-MAA-TU over CdS-MAA could be attributed to higher order of photoluminescence quenching due to greater affinity of uranyl ions for amine groups of thiourea.35 Further studies on uranyl ion sensing were conducted by optimizing CdS-MAA-TU QDs as probe.

Probe Optimization. (a)

Equilibrium condition. The contact time between the CdS-MAA-TU QDs as probe and

uranyl ions was assessed from the kinetic study of PL intensity of the solution comprising probe (at pH 7) and the uranyl ion (at pH 4). The pH of the final mixture was measured to 6.7 ± 0.1 and remained constant for more than 4 h. The equilibration was obtained in 10 min, which corresponded to the saturation in the photoluminescence intensity (shown in Supporting Information, Figure S5 and time dependent photoluminescence spectra are given in the inset of Figure S6).

10



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(b) The pH effect. It should be mentioned here that uranium exists in various chemical forms depending on the pH of the medium.52 The photoluminescence intensities of only CdS-MAA-TU QDs as probe increased with the pH of the medium (given as supporting information, Figure S7A). The weak photoluminescence at pH 3 is attributable to unstable nature QDs due to protonation of surface binding thiolates.53 The unstable nature of the CdS QDs at pH 3 was reflected from the low measured zeta potential value, i.e., ζ = -1.94 mV (given as supporting information, Figure S8A). The maximum zeta potential was recorded for the probe at pH 7, which corresponded to highly stable CdS QDs, i.e., ζ = -33.8 mV (given as supporting information, Figure S8B), and corroborated with maximum photoluminescence intensity. At higher pH, the effect of protonation of thiol group of MAA is less likely to occur, and hence it is a favorable condition for capping of the CdS QDs via forming suitable S-S bond.54 Uranium primarily exists as UO22+ at pH 4. Therefore uranyl ion detection by CdS-MAA-TU QDs as probe should be conducted using probe at pH 7 and UO22+ at pH 4. At higher pH uranyl ions detection was not possible as photoluminescence quenching by uranyl ions at higher pH was not observed (given as supporting information, Figure S7B). At pH ≥ 6, uranium exists as UO2(OH)20 and UO2(OH)3,which perhaps does not interact with the amine groups of thiourea used in capping in CdS-MAA-TU QDs. The photoluminescence quenching by uranyl ion at pH 4 was nearly three folds higher that the probe treated with a blank solution (without uranyl ions) at pH 4, which implied the feasibility of detecting uranyl ions by CdS-MAA-TU QDs.

Visual detection of uranyl ions. It is important to have a simple qualitative technique with an option of visual detection for screening samples containing uranyl ions above a chosen threshold concentration. In this regard, the CdS-MAA-TU probe treated with different concentrations of uranyl ions exhibited visually detectable changes in its bluish photoluminescence intensities 11


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when excited by photons of λ = 365 nm in a UV chamber (Figure 4). In this regard, uranyl ion concentration above 5 µg/L could be qualitatively detected from recognizable changes in the photoluminescence quenching, while uranyl ion concentrations above 50 µg/L is certainly detected due to its complete photoluminescence quenching.

Mechanism of PL quenching. The phenomenon of photoluminescence quenching of quantum dots can occur due to several reasons, e.g., due to charge transfer processes at photoexcited state and/or aggregation of the quantum dots during interaction with analyte. The Stern-Volmer plot for CdS-MAA-TU treated with uranyl ions revealed non-linear photoluminescence quenching, where the Io/I value was saturated for the batch treated with 50 µg/L uranyl ions owing to complete quenching (Figure 3B). Such type of Stern-Volmer plot indicated a possible static or dynamic quenching mechanism due interaction of uranyl ions with the QDs. However, aggregation of quantum dots due to binding with uranyl ions by electrostatic interaction with amine group of CdS QDs could be most likely mechanism for photoluminescence quenching.55 The aggregation of CdS-MAATU QDs was implied from dynamic light scattering (DLS) measurement, which revealed hydrodynamic radii of the QDs treated with uranyl ions as 455 nm ± 60 nm (represented as mean ± standard deviation calculated from three independent measurements), and was significantly larger than the hydrodynamic radii of only probe (i.e., 311 ± 22 nm). It was further complimented by the decrease in the zeta potential of only probe (ζ= -33.8 mV) to ζ= -15.7 mV for probe + uranyl ion (given as supporting Information, Figure S8A-C). The drecrease in the zeta potential is associated with aggregation of quantum dots due to binding with uranyl ions. A schematic diagram showing a tentative binding of uranium with QDs is shown in Scheme 1 (given as Supporting Information). These results suggested static quenching mechanism due to interaction of uranyl ions with CdS-MAA-TU QDs. 12



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Selectivity of UO22+ ions detection The photoluminescence emission spectra of the batches of CdS-MAA-TU QDs as probe treated with 1000 µg/L of respective interfering cations (e.g., Na+, K+, Mg2+, Ca2+, Fe2+, Mn2+, Zn2+, Cd2+, Co2+, Cr3+, Cu2+ and Pb2+) and the one treated with 5 µg/L UO22+ are shown in Figure 5A. The corresponding Io/I values are given in Figure 5B, which revealed mild interferences from Cu2+ (Io/I = 1.76), Pb2+ (Io/I = 1.43) and Cr3+ (Io/I = 1.20), while interferences were negligible from other cations Io/I ≈ 1. Since the tolerance levels of Hg2+ in drinking water is only 2 µg/L, a ten folds higher Hg2+ ion concentration (20 µg/L) was used in the present study as an interfering cation, which can also be categorized as mild interfering agents (Io/I =1.45). The selectivity of determining ultra-trace concentration of uranyl ion (5µg/L) by CdS-MAA-TU QDs as a probe was evident from the significantly high Io/I value (Io/I =2.70). The scope for detecting 5µg/L UO22+ in the presence of the each of the above cations is further demonstrated via a plot of Io/I values for the batches of QDs treated with uranyl ions and interfering cations and compared with that of the batches of QDs treated with only interfering cations (Figure 5B). Similarly, the PL emission spectra of CdS-MAA-TU QDs treated with respective interfering anions (e.g., F-, Cl-, NO3-, CO32-, SO42-, HCO3-, OAc-, and AsO43-) at a concentration of 5 mM are shown in Figure 6C. Except for CO32- (Io/I = 1.12) SO42- (Io/I = 1.28) and NO3- (Io/I = 1.20), the Io/I values for other anions were nearly equal to 1, implying negligible interferences. The interference from SO42- could be suppressed in real samples where the concentration of SO42- is only 2.1 mM. It may be surmised that 5 µg/L of UO22+ ion could be detected with high selectivity in the presence of these interfering anions in real sample. The Io/I values of the batches of QDs treated with uranyl ions and interfering anions were significantly higher than the Io/I values of the QDs treated with only interfering anions (Figure 5D). It was inferred that the as-prepared CdS-MAATU-QDs at optimized conditions were highly selective towards for uranyl ion detection in 13


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aqueous medium, and our results were comparable or better than other types of uranyl ion probes.4,22,27,56

Real sample analysis: Spike and recovery study The feasibility study of using the CdS-MAA-TU QDs as probe for sensing UO22+ in real time water sample was performed by spiking a known concentration of UO22+ ions (ranging between 0.5µg/L and 10 µg/L) in groundwater (Roorkee, India) and river water (Ganga Canal, Roorkee India) samples. The groundwater and the river water samples were first filtered to avoid any particulate suspension prior to spiking with UO22+ ions at optimum contact time and pH conditions. These batches of spiked samples revealed linear Stern-Volmer plot (Figure 6), with similar slope as obtained for de-ionized water. The sensitivity of measuring uranyl ion concentration by the CdS-MAA-TU QDs was revealed from the LoDs measured in ground water (0.072 µg/L) and river water (0.076 µg/L) spiked samples, which were barely different from the LoD measured in the de-ionized water. The robustness of our analytical strategy was assessed by estimating back the spiked concentration of uranyl ions in real samples using the standard calibration plot obtained by spiking known concentrations of uranyl ions in de-ionized water. The results of spike recovery study are given in Table-2, which revealed significant positive error (32% to 38%) for detecting uranyl ions at very low concentrations (e.g., 0.5 µg/L). However, the error (positive) for estimation of uranyl ions of 5 µg/L and 10 µg/L were only 11% to 14%, respectively. The large error for estimating very low levels of uranyl ions (0.5 µg/L) was attributable to competitive quenching by interfering cations, especially by Cu2+ and Pb2+ ions in the ground water and river water samples. Further studies may be suggested for improving the recovery of very low concentrations of uranyl ions in groundwater and river water by suitable masking of Cu2+ and Pb2+ ions. The 14



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ability of detecting uranyl ions at the sub-nanomolar concentrations in aqueous medium is very encouraging to explore the methodology for detecting uranyl ions in biological systems including cells and tissues.

CONCLUSIONS Though the batches of CdS capped with MAA (CdS-MAA) and CdS capped with MAA and TU in tandem (CdS-MAA-TU) were highly soluble with high and similar order of quantum yields, the the batch of CdS-MAA-TU quantum was found to be more sensitive probe for uranyl ion detection at ultra trace level. The enhanced sensitivity for uranyl ion detection can be due to improved surface property of CdS quantum dots due to capping with two agents. Consequently, the phenomenon of photoluminescence quenching, which has been used as the analytical response, was due to interaction with the analyte, i.e., uranyl ions and not due to trapping of photoexcited electrons and holes at the surface defect sites. It has also been demonstrated that uranyl ions greater than 5 µg/L (5 ppb) can be detected by naked eyes from the differential photoluminescence quenching of the probe and probe treated with uranyl ions under UV light illumination. The ability for detecting uranyl ions in real water samples was demonstrated by spiking uranyl ions (especially for 5 µg/L and 10 µg/L) in ground water and river water samples. The methodology developed for detecting such ultra-trace levels of uranyl ions can be extended to detect uranyl ions in cells and tissues to assess the impact of accumulation of low concentration of uranyl ions on human health.

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SUPPORTING INFORMATION. It contains list of chemicals and solvents (materials) used synthesizing CdS-MAA and CdS-MAA-TU quantum dots; the details of characterization techniques; experimental methods used for optimization, sensitivity, selectivity and spiking analysis; details of calculations for quantum yield of CdS-MAA and CdS-MAA-TU QDs with precision, inter-planar distance from SAED and XRD, LoD and LoQ, and formula used for precision calculation; photograph showing greenish-blue and bluish photoluminescence emission of the QDs when illuminated under UV light; plots showing photoluminescence intensities measured at different absorbances required for quantum yield calculation; XRD patterns, SAED images and FT-IR spectra of CdS-MAA and CdS-MAA-TU QDs; time dependent photoluminescence spectra for QDs treated with UO22+ ions; plots showing pH effect of CdS-MAA-TU QDs and UO22+ on uranyl ion detection photoluminescence quenching; the zeta potential of probe and probe treated with uranyl ions; schematic representation showing a possible binding of uranyl ion with CdS-MAA-TU QDs.

ACKNOWLEDGEMENTS A.K. is grateful to University Grants Commission, Government of India for supporting with Senior Research Fellowship. R.K.D is thankful to the Board of Research in Nuclear Sciences (BRNS), Government of India for financial support (Grant No. 2011/37C/37/BRNS).

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FIGURE CAPTION Figure 1. UV-visible absorption spectra of (A) CdS-MAA-TU QDs and (B) CdS-MAA QDs; Photoluminescence emission spectra of (C) CdS-MAA-TU QDs and (D) CdS-MAA QDs Figure 2. (A) TEM image showing distribution of CdS-MAA-TU QDs; (B) HRTEM image showing sizes less than 3 nm of CdS-MAA-TU QDs, inset showing lattice fringes and inter planer spacing in CdS-MAA-TU QDs confirming crystalline nature of the as-synthesized QDs; (C) Energy dispersive X-ray spectrum of the imaged QDs showing elemental composition and (D) HRTEM of CdS-MAA QDs, inset showing signatures of lattice fringes Figure 3. (A) Stern-Volmer plot of CdS-MAA QDs (without thiourea modification) due to interaction with UO22+ ions concentration ranging between 25 µg/L and 1000 µg/L, at pH = 4 and contact time = 10 min, inset showing decrease in PL intensity of probe with increase in UO22+ ions concentration (B) Non-linear Stern-Volmer quenching profile of CdS-MAA-TU QDs with UO22+ ions concentration ranging between 0.10 and 50 µg/L at pH = 4 (contact time = 10 min), inset showing decrease in photoluminescence emission intensity of probe due to increase in the concentration of UO22+ ions; (C) A part of the above Stern-Volmer quenching profile representing linear dynamic range for UO22+ ions concentration ranging between 0.1 µg/L and 10 µg/L. Figure 4. Photographic image showing visually detectable change in the bluish photoluminescence intensities of the CdS-MAA-TU QDs treated uranyl ion, excited by light of λ = 365 nm in a UV chamber. Figure 5 (A) Emission spectrum of CdS-MAA-TU QDs treated with interfering cations (1000 µg/L, except Hg2+ ions with conc. = 5 µg/L) at pH = 4; (B) bar diagram representing Io/I values of CdS-MAA-TU QDs treated with respective interfering cations in the absence and presence of 5 µg/L of UO22+ ions (C) Photoluminescence emission spectrum of CdS-MAA-TU QDs with 21


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interfering anions (5 mM); (D) bar diagram representing Io/I values of CdS-MAA-TU QDs treated with respective interfering anions in the absence and presence of 5 µg/L of UO22+ ions. Figure 6. Stern-Volmer plots for batches of CdS-MAA-TU QDs spiked with uranyl ions in deionized water, groundwater and river water. The LoD, were calculated from the slope of the respective plots.

22



FIGURES

50k (C) (A) (B)

CdS-MAA-TU CdS-MAA

(D)

40k 30k

1 20k 10k 0 300

400

500

600

Emission intensity (a.u.)

2

Absorbance (a.u.)

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

Page 24 of 31

700

Wavelength (nm)

Figure 1.

23


Page 25 of 31

(A)

(B)

10 nm

20 nm

(D)

(C)

Counts

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


10 nm

Energy (keV)

Figure 2.

24



8

(A)

2

R = 0.99 Slope = 0.00533

4

28k

Increasing UO22+

21k

2

7k 0 300 400

600

Wavelength (nm)

0

500

3

concentration

14k

0


2

35k

Io/I

Io/I

4

(B) 5


6

6

1000

1500

30k

2500

Concentration of UO2 (µg/L)

Increasing UO22+ concentration

20k 10k

1

800

2000

2+

40k

0 300 400

600

800

Wavelength (nm)

0

10

20

30

40

2+

50

60


5 (C) 2

R = 0.99 Slope = 0.316

4

Io/I

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

Page 26 of 31

3

2

1 0

2

4

6

8

2+

Concentration of UO2

10

12

(µg/L)

Figure 3.

25


Page 27 of 31

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


Blank

5 µg/L

10 µg/L

20 µg/L

50 100 µg/L µg/L

Figure 4.

26



5

60k


2+

2+

2+

(B) Interfering cations = 1 mg/L Interfering cations + UO22+ ion (5 µg/L) 2+ Hg = 20 µg/L

(A)

2+

Blank, Na+, Mg , Ca , Fe , Mn , 2+ 2+ 2+ 3+ Zn , Cd , Co , Cr

4

45k 2+

2+

3

Io/I

Pb , Hg 2+

Cu

30k

2

2+

UO2

15k

60k

45k

-

-

-

Blank, Cl , F , HCO3 , OAc , AsO4

-

Io/I

4

2-

SO4

UO22+

Hg2+

Cd2+ Co2+ Cr3+

(D)

Interfering anions = 5mM Interfering anions + 5 µg/L UO22+ UO22+ = 5 µg/L

5

3-

CO3 , NO3

30k

Zn2+

6

(C) -

Cu2+

Wavelength (nm)

Pb2+

800

Mn2+

700

Fe2+

600

Ca2+

500

Mg

0

400

2+

1

Na+ K+

0 300

3

2+

UO2

2

15k

2+

3-

UO2

AsO4

OAc

-

-

NO3

HCO3

-

0

2-

800

SO4

Wavelength (nm)

700

CO3

600

2-

500

-

400

F

0 300

-

1 Cl


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

Page 28 of 31

Figure 5.

27


Page 29 of 31

6 Deionized water, LoD = 0.070 µg/L Ground water, LoD = 0.072 µg/L River water, LoD = 0.076 µg/L

5 4

Io/I

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


3 2 1 0

2

4

6

8

2+

10

12


Figure 6.

28



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Table 1. Compilation of data of XRD, TEM, SAED, UV-visible absorption, Photoluminescence and quantum yield measurement Probe

CdS-MAA-TU

XRD

TEM

2θ(o), Atomic Plane

δ (nm)

27.8, (111 )

0.353

Crystallite size (d) in nm 1.44 ± 0.12

46.4, (220)

0.216

ND

SAED

Particle size (D in nm)

δ (nm)

< 3 nm

0.320 ± 0.004

δ (nm), (atomic plane) 0.321, (111)

Fluorescence Emission

UV-Vis absorption

λmax (nm)

Eg (eV)

340

3.24

D′ (nm) particle size 1.88

λex (nm)

λem (nm)

FWHM (nm)

370

480

122

% Quantum Yield (Φf)

16.64 ± 1.02

0.211, (220)

15.82 ± 0.91 41.9, (plane not ND ND < 3 nm 0.341 ± 0.006 Not 356 3.17 2.20 370 505 130 assigned) observed δ: Inter-atomic plane separation d: Crystallite size determined from Debye Scherrer equation, given as d = 0.89λ/β cos (θ); where λ is the wavelength of X-ray source, β is full width at half maximum (FWHM) in radians and θ is Bragg’s diffraction angle Band gap Eg: D: Particle size (diameter) measured from TEM D′: Particle size calculated using empirical formula based on UV-visible spectra of QDs λex: Excitation wavelength Emission peak wavelength λem: ND: Not determined

CdS-MAA

29


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Table 2. Recovery analysis of spiked uranyl ions in deionized water, groundwater and river water samples

Spiked concentration (µg/L)

De-ionized water Found (µg/L) Recovery (%)

Groundwater Found (µg/L) Recovery (%)

River water Found (µg/L) Recovery (%)

0 0.5 5.0 10.0

0.08 0.49 5.18 10.44

0.10 0.662 5.58 11.28

0.12 0.690 5.67 11.38

98 ± 1.8 103.6 ± 2.8 104 ± 2.9

132.4 ± 3.2 111.6 ± 2.4 112.8 ± 2.2

138 ± 3.4 113.4 ± 2.9 113.8 ± 2.5

The recovery concentration is given as mean ± standard deviation of three independent measurements (n = 3).

30


Highly Sensitive and Selective Method for Detecting Ultratrace Levels

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