Ultra-sensitive and Selective Sensing of Selenium Using Nitrogen

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Ultrasensitive and Selective Sensing of Selenium Using Nitrogen-Rich Ligand Interfaced Carbon Quantum Dots Pooja Devi,*,†,‡ Anupma Thakur,‡,§ Shweta Chopra,§ Navneet Kaur,*,∥ Praveen Kumar,‡ Narinder Singh,⊥ Mahesh Kumar,¶ Sonnada Math Shivaprasad,□ and Manoj K. Nayak†,‡ †

Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research, New Delhi 110001, India CSIR-Central Scientific Instruments Organisation, Chandigarh 160030, India § Centre for Nanoscience and Nanotechnology, Panjab University, Chandigarh 160014, India ∥ Department of Chemistry, Panjab University, Chandigarh 160014, India ⊥ Department of Chemistry, Indian Institute of Technology, Ropar 140001, India ¶ National Physical Laboratory, CSIR, New Delhi 110012, India □ CPMU, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

ACS Appl. Mater. Interfaces 2017.9:13448-13456. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/19/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: This work reports a label-free, ultrasensitive, and selective optical chemosensory system for trace level detection of selenite (SeO32−), the most toxic form of selenium, in water. The probe, i.e., carbon quantum dots (CQDs), is designed from citric acid by means of pyrolysis and is interfaced with a newly synthesized nitrogen-rich ligand to create a selective sensor platform (functionalized CQDs, fCQDs) for selenite in a water matrix. Spectral (NMR, UV−vis, photoluminescence, Raman, and Fourier transform infrared analyses) and structural (high-resolution transmission electron microscopy) characteristics of the designed new probe were investigated. The developed sensor exhibits high sensitivity (limit of detection = 0.1 ppb), a wide detection range (0.1−1000 ppb range, relative standard deviation: 3.2%), and high selectivity even in the presence of commonly interfering ions reported to date, including Cl−, NO3−, NO2−, Br−, F−, As(V), As(III), Cu2+, Pb2+, Cd2+, Zn2+, Sr2+, Rb2+, Na+, Ca2+, Cs+, K+, Mg2+, Li+, NH4+, Co2+, etc. The observed selectivity is due to designed ligand characteristics in terms of strong Se−N chemistry. Ultrafast spectroscopic analysis of the fCQDs in the absence and presence of selenite was studied to understand the sensing mechanism. The sensor was successfully exemplified for real water samples and exhibits comparative performance to conventional ion channel chromatography as well as flame atomic absorption spectroscopy for selenite analysis. The promising results pave ways for realization of a field deployable device based upon a developed probe for selenite quantification in water. KEYWORDS: selenite, sensor, carbon quantum dots, ligand, fluorescence, UFS, water

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INTRODUCTION Of late, selenium contamination in water has gained significant attention of the scientific community to provide plausible remedial as well as quantifying solutions for this emerging water pollutant. Due to its adverse health and environmental impacts upon chronic exposure, which are chiefly associated with contaminated water, its monitoring at onset is of vital importance. The concentration of Se is found to increase gradually in water resources due to its presence in soils, rocks, and industrial wastes.1,2 Se contamination in water resources is reported in the northern hemisphere, some regions of Europe, and Asia. Selenium came into the scientific focus in Asian countries, in particular in India, in the mid-seventies after an animal disease locally called Degnala was found to be associated with high intake of Se in fodder grown on alkali soils of © 2017 American Chemical Society

Haryana state. The presence of up to 69.5 ppb Se in groundwater is observed at various locations in Punjab state. Selenium is reported to behave as an essential or toxic element owing to the narrow boundary between deficiency and toxicity associated with its uptake limit.3 As a micronutrient, it is known to play a critical role in normal growth and metabolism.4 Several selenoproteins such as thioredoxin reductases, glutathione peroxidases (GSHPx), and iodothyronine deiodinases are essentially required for defense against oxidative stress, regulation of thyroid hormone metabolism, and protection against carcinogenic diseases, respectively. Its Received: January 20, 2017 Accepted: March 31, 2017 Published: March 31, 2017 13448

DOI: 10.1021/acsami.7b00991 ACS Appl. Mater. Interfaces 2017, 9, 13448−13456

Research Article

ACS Applied Materials & Interfaces

symmetrical emission, tunability, biocompatibility, high stability against photobleaching, and environment friendliness.28−30 Notably, CQDs have been successfully demonstrated for selective and sensitive detection of Zn, Cu, Ag, etc.; however, there is no report to the best of our knowledge for selenite detection, which is a major emerging water pollutant of the 21st century.31,32 In consideration of the above facts, we herein present the first label-free and economical carbon based fluorescent chemosensory system for highly selective, sensitive, and wide range detection of SeO32− in water. To impart selectivity, the designed probe is interfaced with nitrogen-rich ligand for selective detection of selenite by Se−N chemistry. Ultrafast spectroscopy (UFS) was used to understand the sensing mechanism. Further, the probe was well-characterized by NMR, UV−vis, Fourier transform infrared (FT-IR), X-ray photoelectron (XPS), and photoluminescence (PL) spectroscopic techniques and was also tested against real water samples. The analytical capability of probe against real samples is compared with ion channel chromatography as well as flame AAS (FAAS).

deficiency in humans leads to diseases such as cirrhosis, KashinBeck disease, colonic and pancreatic carcinoma, etc. However, at elevated concentrations, it is known to cause toxicity and oxidative stress.5 Among its various naturally existing forms [Se0, selenide (Se2−), selenite (Se(IV) or SeO32−), and selenate (Se (VI) or SeO42−)], selenite is highly mobile and 40 times more toxic over others.6,7 Its consumption above the recommended level (10 ppb) by the World Health Organization (WHO) can lead to chronic health effects, causing damage to critical cell components such as proteins, DNA, lipids, etc. by virtue of inhibitory effects through binding with thiols and producing oxygen free radicals such as superoxide anions.8 The pathological conditions such as selenosis, dermatitis, neurodegeneration, etc. are in close association with its toxicity.5,9,10 In view of above facts, its accurate determination in water at trace levels is of utmost importance, and there is a large unmet need of environment friendly cost-effective scientific solutions that can enable quick, sensitive, selective, and on-site determination of selenite in water resources to reduce threats to human health. To date, a plethora of analytical techniques, including catalytic kinetic spectrophotometric method,8 gas chromatography (GC),5 high-performance liquid chromatography (HPLC) coupled with ICP-MS,9 atomic absorption spectroscopy (AAS),10 atomic fluorescence spectroscopy (AFS),11 electrothermal atomic absorption spectroscopy (ETAAS),12 neutron activation analysis,13 and so forth are reported for qualitative and quantitative determination at trace levels. However, these techniques are limited to the laboratory sphere and rely upon expensive instrumentation; laborious procedures; trained labor; and accuracy in sampling method, pretreatment, and frequency. Electrochemical techniques are also reported for selenium quantification; however, they suffer from interference and matrix fouling issues. Alternatively, optical techniques have emerged as a plausible solution, wherein photoluminescence (PL) spectroscopy is not yet a successfully explored read out system for selenite monitoring in water. Moreover, it allows portability of a designed approach as well as a highly sensitive and user-friendly technique. The lower availability of literature for selenite detection using PL approach is limited due to sensitivity, selectivity, and stability issues of reported optical probes to date (Table S1, Supporting Information). Most of these probes (organic ligands, fluorophores, and colorimetric dyes) are −SH and −NH2-rich organic ligands that have limited scope for real time quantitative applications owing to photobleaching, stability, and related issues. Further, they are synthesized from expensive precursors using multistep reactions and complex work up procedures.11−24 Therefore, to overcome all of these challenges, synthesis of novel materials is necessitated to develop a stable, cost-effective, and sensitive probe for selenite determination in water. Although few attempts on nanomaterials intervention in quantitative determination of selenite by optical means have yet been made, they require conventional technique integration.25 More recently, Huang et al.26 explored CdTe quantum dots (QDs) immobilized on a paper for sensitive visual detection of selenite up to 0.1 ppb level. Despite high sensitivity observed by these semiconductor QDs, their inherent toxicity due to heavy metalbased composition environmentally confines their real world application.27 In comparison, recently developed carbon quantum dots (CQDs) can be a potential optical sensory system because they surpass all above limitations owing to their broad absorption,

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EXPERIMENTAL SECTION

Materials and Methods. All reagents used in this work were of analytical grade and used as such without further purification. Citric acid anhydrous (CA, 98%, w/w) used for the synthesis of CQDs was purchased from Avra Synthesis, India. Hydrazine hydrate (99%, w/w, Loba Chemie), 2-pyridinecarboxaldehyde (Sigma-Aldrich Co.), and diethyl ether (Fisher Scientific Co.) were used for the synthesis of nitrogen-rich supramolecular ligand (SL) and functionalization of CQDs. The quantum yield (QY) of as-synthesized CQDs was calculated using quinine sulfate (QS) purchased from CDH, India. Standard stock solutions (1000 ppm) of Se(IV); Se (VI); various ions including NO2−, Br−, Cl−, NO3−, F−, As(V), As(III), Cu2+, Pb2+, Cd2+, Zn2+, Rb2+, Na+, Ca2+, Cs+, K+, Mg2+, Li+, NH4+, Co2+; and multielement mixtures such as M1 (SO42−, Cl−, Br−, F−), M2 (PO42−, NO3−,Cl−, SO42−), and M3 (Mg2+, Ca2+, Li2+, Na+, Cu2+) were procured from Merck and Inorganic Venture. Ultrapure deionized water obtained from Millipore (R = 18 M′Ω) was used for reagent preparation. Synthesis of CQDs and Se(IV) Selective SL. CQDs were synthesized from citric acid (CA) by a well-optimized and facile synthetic route using a laboratory hot air oven. The optimization studies related to synthesis time and temperature are provided in Figure S1 (Supporting Information). During synthesis, the precursor CA (2.0 g) underwent pyrolysis at high temperature (473 K) in hot air oven, which was witnessed as a pale yellow thick residual after synthesis. The as-synthesized CQDs were dissolved in methanol and stored at 4 °C until use. The obtained CQDs showed blue luminescence in a neutral pH environment under a UV illuminator. The QY of CQDs was investigated by comparing the integrated intensities (excited at 350 nm) and the absorbance value of CQDs at 350 nm with reference dye QS in 0.01 M H2SO4 (QY = 54%): QYS = QYR ×

ηR2 ηS2

×

IS A × R AS IR

where QY, I, A, and η are assigned to QY, integrated PL intensity, absorbance values, and refractive index, respectively. The subscripts R and S refer to reference and sample, respectively. To synthesize aminerich SL, 2-pyridinecarboxaldehyde (23 mmol, 2.28 mL, red color) and hydrazine hydrate (120 mmol, 3.73 mL) were mixed in methanol under stirring and refluxed at 65 °C for 24 h in oil bath. The excess solvent was removed by a rotary evaporator and washed with diethyl ether to obtain the final product, SL (Schiff base), a dark yellow colored product. 13449


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ACS Applied Materials & Interfaces Scheme 1. Interface Development between Esterified CQDs and As-Synthesized Ligand to Obtain fCQDs

Figure 1. (a) Absorption and emission (excitation wavelength 370 nm) characteristics of as-synthesized CQDs; (b) emission behavior of CQDs, SL, and fCQDs at 420 nm excitation wavelength. Interface Development. To develop a CQDs-SL interface toward Se(IV), as-synthesized CQDs (5 mL, 4.5 mmol) were first esterified to methyl ester in the presence of an acid (HCl) as a catalyst under reflux conditions in methanol and thereafter functionalized with SL. The esterified CQDs were dialyzed overnight to completely remove leftover carboxylic acid containing CQDs moieties. For functionalization, esterified CQDs were refluxed with SL at 65 °C for 24 h to obtain fCQDs. SL was synthesized from its precursor, 2-pyridinecarboxaldehyde, which underwent condensation reaction in reflux condition in the presence of hydrazine hydrate to form a Schiff base. The mechanism of interface development between CQDs and SL is proposed in Scheme 1, which delineates the basic reaction involved in synthesis of fCQDs, in which SL was grafted onto esterified CQDs surface to impart selectivity toward selenite. The developed interface was well-characterized using FT-IR, Raman, and NMR spectroscopic techniques. To further probe the chemical composition of CQDs and fCQDs, XPS analysis was performed. The long aromatic chains of SL are introduced onto the CQD surface for selective binding toward selenite through Se−N chemistry. Quantification of Se with fCQD Probes. For selenite sensing, fCQDs were further purified by dialysis overnight and investigated in different pH windows to optimize their analytical performance. Thereafter, they were studied against different concentrations of Se(IV) in the 0.1−1000 ppb range. Interference Study and Real Water Samples Analysis. To measure selectivity of the fCQD probe, comparative and competitive ion studies were performed. For the comparative ion study, fCQD response toward various cation and anions such as Cl−, NO3−, NO2−, Br−, F−, As(V), As(III), Cu2+, Pb2+, Cd2+, Zn2+, Sr2+, Rb2+, Na+, Ca2+, Cs+, K+, Mg2+, Li+, NH4+, Co2+, etc., at 10 and 100 ppb concentrations

was monitored. On the other hand, a competitive ion study was done against several anions, cations, and multielement solutions (M1, M2, and M3) in the presence of Se(IV). The real sample analysis was done using water samples collected from different resources and were used as such without any chemical treatment. FAAS was used for a validation study of the developed probe. The real water samples as well as spiked samples were first converted into hydrides using a 3% solution of sodium borohydride in 1% NaOH in hydride generation system. An excitation wavelength of 204 nm was used for all measurements by FAAS. The sensor activities of the proposed sensor were also validated through comparing the results of metal ion concentration using ion channel chromatography. The standard solutions were prepared, and the concentrations were also authenticated. Characterization Techniques. The optical characteristic of assynthesized CQDs, SL, and fCQDs were probed by UV−vis (Hitachi Ltd., Japan), FT-IR (670 IR, Varian, United States), and PL (Shimadzu, RF-5301PC) spectrophotometers. HR-TEM (H-7500, Hitachi Ltd., Japan) and scanning (Hitachi S-4300) electron microscopes were used for the morphological and structural studies. Transient absorption spectroscopy was measured by using a visible Helios system (Ultrafast systems) with a 1 kHz femtosecond Ti:sapphire laser system (Libra) having a femtosecond pulse duration. Raman spectra for CQDs, SL, and fCQDs were collected with an inVia instrument (Reninshaw) at λex = 785 nm. The X-ray diffraction pattern was portrayed with an X-ray diffractometer (Xpert-Pro). The surface chemical composition was probed by XPS (Omicron) using Al Kα radiation equipped with high resolution 7-channeltron hemispherical analyzer. The data were acquired with pass energy of 100 eV for the survey scans and 25 eV for the core levels with a resolution of 0.1 eV at 13450


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Figure 2. (a) TEM and (b) SAED analysis of as synthesized CQDs.

Figure 3. (a) FT-IR and (b) Raman spectra of CQDs, SL, and fCQDs.

SL does not exhibit significant fluorescence characteristics of its own; however, fCQDs showed excitation dependent emission behavior, as shown in Figure S4 (Supporting Information) with maximum PL intensity at 420 nm excitation (blue-shifted as compared to CQDs), confirming functionalization, as observed in Figure 1b. Figure 2a is associated with morphological characteristic of as-synthesized CQDs, which clearly depicts their spherical morphology with an average diameter of ∼4−5 nm. The lattice structure (d = 0.38 nm) shown in the SAED pattern of CQDs in Figure 2b depicts their amorphous nature and is on higher side than graphitic carbon interlayer spacing. These observations are in line with X-ray diffraction pattern of CQDs and fCQDs, as shown in Figure S5 (Supporting Information), which further confirms their amorphous nature. The Gaussian deconvolution of the XPS C(1s) and O(1s) core level spectra into their components confirms the presence of CC, C−C, C−O, CO, and O−CO for the CQDS, where as an additional component N−CO was found for fCQDs, which confirms the successful functionalization of CQDs with N-rich ligands. The synthesized nitrogen-rich ligand, i.e. SL, was at first characterized with FT-IR, NMR, and mass spectroscopy (Figure S7, Supporting Information). A high intensity m + 1 peak at ∼121.06 was observed in mass spectra of SL, which was found equivalent to theoretical weight calculated from the chemical composition of SL. After confirmation, it was interface onto CQDs surface to provide nitrogen functionalities. The obtained fCQDs were characterized by FT-IR and Raman

90% of the peak height. After Shirley background corrections were done, Gaussian deconvolution for C (1s) and O (1s) was performed to quantify different possible chemical states present in the developed probes. NMR (13CNMR in DMSO-d6, ppm) spectra of as-synthesized as well as functionalized CQDs were recorded using a 400 MHz Avance-II spectrometer from Bruker. All pH measurements were made using a digital pH meter (LMPH 10, Labman Scientific Instruments Ltd.). Ion channel chromatography (Metrohm) was used for the sensor performance validation study. FAAS (PerkinElmer) was used for analysis of spiked real water samples using a hydride generation system (MHS 15) and argon as a carrier gas.

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RESULTS AND DISCUSSION Characterization of Synthesized CQDs, SL, and fCQDs. The optical spectrum recorded for CQDs shown in Figure 1a divulges characteristic absorption peaks at ∼370 nm associated with n → π* and π →π* transitions in C−C molecules with unsaturated centers at less energy.33 When excited at 390 nm, it exhibits a broad emission (400−570 nm) with maximum intensity centered at ∼460 nm, assigned to surface process and sp2 clusters in the core, and henceforth, radiative recombination of surface-confined electrons and holes are accounted for in the origin of bright fluorescence in CQDs.34 Further, their PL emission at different wavelengths showed their excitation independent emission behavior, as shown in Figure S2 (Supporting Information). The observed QY for CQDs with reference to QS was measured to be ∼32% (Figure S3, Supporting Information), which is highest among earlier reports on CQDs synthesized from citric acid. Alternatively, 13451


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Figure 4. 13C NMR spectra of (a) CQDs and (b) fCQDs.

Figure 5. (a) Sensing behavior of fCQDs toward selenite (inset: sensing mechanism) and (b) effect of pH on sensing performance of fCQDs.

peak at 1634 cm−1, corresponding to CN stretch for Schiff base. The shift in the −COOH group’s vibration to 1690 cm−1 in fCQDs confirms the functionalization of CQDs with synthesized SL. Furthermore, the presence of carbonyl amide vibrational peak associated with SL at 1579 cm−1 in fCQDs further ensure its binding onto the surface of CQDs. Likewise, D and G bands in Raman spectra, which are a molecular picture of carbon materials, were observed for CQDs at ∼1372 and ∼1638 cm−1, respectively. The relatively high intensity of the D band when compared to the G band of CQDs indicates the

analysis, as shown in Figure 3. The various characteristic vibrational modes were observed in FT-IR spectra for CQDs, SL, and fCQDs. It can be clearly seen that CQDs exhibit characteristic major peaks for the CC stretching mode of aromatic hydrocarbons, a constituent CQDs components, along with peaks for CO stretching mode of oxygenic groups and −COOH groups, confirming the presence of carboxyl groups on its surface, which makes it hydrophilic in nature and potential material for functionalization with desired ligand for various applications. SL itself exhibits a vibrational 13452


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Figure 6. (a) Quenching pattern of fCQDs toward various concentrations of selenite and (b) relation between Io/I vs selenite concentration.

presence of localized sp3 defects within sp2 clusters. The Raman spectra of SL exhibit peaks for carbonyl amide, C−C stretch at ∼690 cm−1, and CN at ∼1670 cm−1 along with peak at ∼1368 cm−1 attributed to localized sp3 defects. On the other hand, fCQDs encompass all Raman peaks, including C−C, C N, CC, D band, and G band, which further confirms the functionalization of CQDs. Further, solid state 13C NMR analysis (DMSO-d6, 100 MHz, ppm) of CQDs and fCQDs further confirms functionalization (Figure 4). The clearly observed resonance peaks at 171.14 ppm are indicative of sp2 bonded carbon atoms and correspond to carbon nuclei of carboxylic acid groups at the surface of CQDs. The NMR spectrum further illustrates that the structure has common with a carbon QDs structure as carbon-aromatic peaks are practically present in zero dimensional carbon nanostructures that lead to aromatic chain polymerization of such structures, resulting in the formation of QD architecture. As compared to 13C NMR of CQDs, those of fCQDs show peaks of different carbon nuclei consisting of Schiff base at 154.8 ppm, amide carbon at 161.54, and carbon of aromatic pyridine ring between 118.40 and 149.5 ppm. However, peak of carboxylic group is absent in fCQD 13C NMR spectra, which confirms the functionalization of CQDs with SL ligand. The ζpotential analysis revealed a decrease in potential value from 19.3 mV to −0.83 V upon functionalization (Figure S8, Supporting Information) and could be assigned to presence of nitrogen-rich SL moieties on CQDs surface at a particular pH. Sensing Behavior of fCQDs toward Selenite. Figure 5a illustrates the sensing behavior of fCQD probe toward selenite in water. It can be seen that, upon addition of selenite, a decrease in fluorescence intensity of fCQDs is observed, which could be associated with nonradiative recombination of charge carriers on selective binding of electron deficient SeO32− to nitrogen-rich ligand present on fCQD surface (inset, Figure 5a), which even might have also led to elimination of vacancies at the CQD surface. The transfer of charge carriers during this binding process is therefore investigated by UFS analysis of fCQDs in the presence of selenite, and the results are shown in Figure S9 (Supporting Information). A broad excited-state adsorption (ESA) in the range of 450−750 nm was observed for fCQDs, whereas it is less broadened in the presence of SeO32−, as observed from ESA in the 675−750 nm range, which confirms binding of selenite ions with fCQDs35 and therefore electron transfer between donor nitrogen moieties present in ligand on fCQDs to SeO32−. The charge carrier lifetime study

further justifies the above observation because the fCQD−Se complex exhibit shorter lifetime (∼0.87−6.7 ps) over that of fCQDs (∼3.5−74 ps) alone for the ESA peak, which could be assigned to possible introduction of vibrational states by this complex, leading to nonradiative recombination of photoexcited electrons observed as quenching of the probe. These observations are in line with similar reports.36,37 Besides, inner filter effect and electron transfer may be the possible facilitating processes behind the observed quenching phenomenon.38−40 Figure 5b presents the analytical performance of fCQDs toward selenite in acidic and basic environments and is observed highest at pH ≥ 7, depicted in terms of maximum intensity at physiological pH, which strengthens fCQD sensing capabilities in physiological conditions. The design of a selective fluorescent probe for selenite necessitates grafting of selenite selective ligand on CQDs surface. In a previous report, 2,3diaminanaphthalene is reported toward selective selenite detection utilizing Se−N chelating chemistry using a chromogenic approach; however, color generation requires approximately 2 h.14 In view of the strong interaction between Se−N complexes, we synthesized a new ligand rich in nitrogen moieties which induces selenite for selective quenching of the CQD probe. This is due to high bond dissociation energy between Se−N complexes (381 kJ/mol) formed between selenite species and nitrogen-rich SL grafted on the CQD surface. The association constant of the fCQD−selenite complex was calculated using the Benesi−Hildebrand method41 and is found to be 7.6 × 106 M−1, as shown in Figure S10 (Supporting Information). Analytical Characteristics of the Optical Probe. To determine the detection window of the fCQDs, the titration was carried out using different concentrations of selenite into a fixed volume fCQD sensor system. We observed more than 85fold decrease in the fluorescence intensity upon addition of 1000 ppb aqueous selenite solution, as indicated in Figure 6a. Upon addition of selenite concentration from 0.1 to 1000 ppb, a fashioned decrease in emission intensity was observed without any peak shift. Fluorescent fCQDs showed a good relationship with selenite concentrations and exhibit capability to detect even a very low concentration (up to 0.1 ppb), which is well below the WHO guidelines of 10 ppb. The observed analytical performance of fCQDs was validated using best fit power function model with respect to calibration curve in the 0.1−80 ppb range, as shown in the inset of Figure 6a, and was found to be y = 810 × x333, where y is selenite concentration in ppb and 13453


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Figure 7. (a) Response of fCQDs toward (a) cations and (b) anions.

Figure 8. Competitive ion study of fCQDs toward selenite in the presence of a library of (a) anions, cations, and (b) multielement solutions at 10 ppb.

x = 1 − F/Fo. The predicted values of concentration using the proposed equation were validated with various experimental values, as shown in Table S2 (Supporting Information). The theoretical detection (LOD, 3s) and quantification (LOQ, 10s) limit of fCQDs toward selenite were found to be ∼0.1 and 0.003 ppb, respectively, where s is standard deviation, which is 0.033 in present case for 6 replicates (n = 6). Further, relative standard deviation and precision of experiments (%) for n = 6 were found to be better than 3.26 and ∼0.01%, respectively. The observed analytical characteristic delineates high reproducibility, repeatability, and sensitivity of the presented probe toward selenite detection in water, which is comparative to other reported probes to-date. The Stern−Volmer (SV) quenching constant (Ksv) for quencher at a signal-to-noise ratio of 3 with fitting parameter (R2) of 0.9674 using an SV plot (Figure 6b) for linear equation Io/I = (7.47 × 106)C + 1.2201 in the 10−50 ppb linear concentration range was found to be 7.47 × 106 M−1. Further, the relative fluorescence quenching efficiencise of selenite (% Ksv) at 0.1, 1, 10, 100, and 1000 ppb were calculated and found to be 57.25, 86.90, 97.85, 98.52, and 99.53%, respectively (Figure S11, Supporting Information), which supports the observed quenching of the probe at different concentrations of selenite. The observed selective performance of the probe could be attributed to strong association constant of Se−N complex (high bond dissociation energy of 381 kJ/mol). A similar hypothesis was earlier utilized

for selective selenite sensing by means of fluorescent organic polymers, chromogenic ligands, etc.14,22,42 The sensing performance of the proposed sensor was further validated through ion channel chromatography. The standard solutions were prepared, and the concentrations were authenticated. Thereafter, the concentrations of these standard solutions were analyzed through our proposed sensor, and the results (Figure S12, Supporting Information) show the close agreement, which is within the permissible error range. Interference Study. To meet the real world application, the selectivity performance of fCQDs was evaluated against numerous ions such as Cl−, NO3−, NO2−, Br−, F−, As(V), As(III), Cu2+, Pb2+, Cd2+, Zn2+, Sr2+, Rb2+, Na+, Ca2+, Cs+, K+, Mg2+, Li+, NH4+, and Co2+ at 10 and 100 ppb, including other inorganic less toxic form of selenium, i.e. Se(VI), at different concentrations. The emission data was collected on the basis of relative change in fluorescence intensity as 1 − F/F0 (F/F0: ratio of fluorescence intensity in presence and absence of selenite) and is shown in Figure 7a, which clearly shows negligible interference due to these commonly reported interferents/ ions in water at 10 ppb, indicating that other ions hardly induce the structural and optical transformations of these fCQDs, which is very significant for selenite even at 10 ppb. Moreover, at higher concentrations of these comparative ions (100 ppb), as shown in Figure 7b, the trend still remains same. At higher 13454



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concentrations (1 ppm), the sensor response is still more prominent toward selenite (Figure S13, Supporting Information). To test the practical applicability of the developed optical probe for selenite, competitive ion study results with a library of anions, cations, and multi-ions are shown in Figure 8. No significant change of fCQD response toward selenite was observed even in the presence of various possible competitive ions in a similar mixture. This concludes that the reported fCQDs are highly selective probes for selenite and are unaffected by other reported interfering metal ions. Real Water Samples Analysis. In view of the observed excellent selectivity, high sensitivity, wide detection range, reproducibility, and fast response of fCQDs toward selenite, the practicality of same was testified against real water samples, as shown in Figure S14 (Supporting Information) collected from the ground, a lake, and a well. Before analysis, the water samples were filtered through a 0.22 μm membrane to remove suspended particles and spiked with standard solution to achieve 10 ppb selenite. fCQD fluorescence intensity was decreased upon addition of 10 ppb spiked water samples collected from above-mentioned sources. The percent recovery was found in the 84−116% range, as shown in Table S3 (Supporting Information), and the results were in line with FAAS analysis of these samples. These observations imply that, in spite of the numerous minerals in natural water samples, the fCQDs retained its relevance for selective and sensitive detection of selenite in water to 10 ppb level, satisfying WHO (10 ppb) and United States Environmental Protection Agency (50 ppb) criteria of a safe selenite level limit in potable water.

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CONCLUSIONS

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ASSOCIATED CONTENT

Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Pooja Devi: 0000-0001-9784-2121 Navneet Kaur: 0000-0002-0012-6151 Narinder Singh: 0000-0002-8794-8157 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The support and kind permission from Director, CSIO, to carry out this work in the laboratory is highly acknowledged. The financial support received from Department of Science & Technology for INSPIRE project (GAP0353) is also acknowledged.

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In conclusion, we reported the selective quantification of the most toxic form of selenium, i.e. selenite, in water with an inexpensive and label-free fluorescent chemosensory system based upon nitrogen ligand interfaced CQDs. The designed probe showed (>85 fold) selective quenching in PL intensity upon addition of ∼10 μM selenite and exhibited a theoretical detection limit of 0.1 μg/L, which is well below the WHO recommendation of 10 ppb. The probe is capable of detecting selenite in a wide detection range of 0.1−1000 ppb. The reported probe is economical, facile, and could discriminate against other heavy metal/metalloids ions present in water. The analytical capability of the probe is found comparative to labbased flame atomic absorption spectrophotometer for selenite analysis in real water samples. This study paves promising feasibility for design of an on-site selenite detection system in a water matrix for a wide and practically found concentration range in contaminated water resources.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00991. Optical characteristics and QY measurement of CQDs, surface charge of CQDs and fCQDs, NMR, FT-IR, and MS analyses of the synthesized ligand, XPS and XRD analyses of CQDs and fCQD probes, and UFS analysis of fCQDs and the fCQD−Se(IV) complex (PDF) 13455


Research Article

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Ultra-sensitive and Selective Sensing of Selenium Using Nitrogen

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