Ultrasensitive and Selective Sensing of Selenium Using Nitrogen-Rich

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Ultra-sensitive 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 Kumar Nayak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00991 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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ACS Applied Materials & Interfaces

Ultra-sensitive and Selective Sensing of Selenium Using Nitrogen Rich Ligand Interfaced Carbon Quantum Dots Pooja Devi.1, 2,*, Anupma Thakur 2, 3, Shweta Chopra 3, Navneet Kaur 4,*, Praveen Kumar 2, Narinder Singh5, Mahesh Kumar 6, Sonnada Math Shivaprasad7 and Manoj K. Nayak 1, 2 1

Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research, New Delhi, India 2

3

CSIR-Central Scientific Instruments Organisation, Chandigarh-160030, India

Centre for Nanoscience and Nanotechnology, Panjab University, Chandigarh, India 4

5

Department of Chemistry, Panjab University, Chandigarh, India

Department of Chemistry, Indian Institute of Technology, Ropar, India 5

7

CSIR-National Physical Laboratory, New Delhi, India

CPMU, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India

*[email protected], [email protected] KEYWORDS: Selenite, Sensor, Carbon Quantum Dots, Ligand, Fluorescence, UFS, Water

ACS Paragon Plus Environment

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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 water matrix. Spectral (NMR, UV-Vis, PL, Raman and FT-IR) and structural (HR-TEM) characteristics of designed new probe are investigated. The developed sensor exhibit high sensitivity (limit of detection = 0.1 ppb), wide detection range (0.1 -1000 ppb range, RSD: 3.2 %) and high selectivity even in the presence of commonly interfering ions reported till 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 accounted to designed ligand characteristics in terms of strong Se-N chemistry. Ultrafast spectroscopic analysis (UFS) of the fCQDs in absence and presence of selenite is studied to understand sensing mechanism. The sensor is successfully exemplified for real water samples and exhibit comparative performance to conventional ion channel chromatography as well as flame atomic absorption spectroscopy (FAAS) for selenite analysis. The promising results pave ways for realization of field deployable device based upon developed probe for selenite quantification in water.


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Introduction Of late, selenium contamination in water has gained significant attention of peer community to provide plausible remedial as well as quantifying solution for this emerging water pollutant. Due to its adverse health and environmental impact on 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 northern hemisphere, some regions of Europe, and Asia. Selenium came into the scientiﬁc focus in Asian countries in particular 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 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 narrow boundary between deficiency and toxicity associated with its uptake limit 3. As a micronutrient, it is known to play 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 deficiency in humans leads to diseases like cirrhosis, kashin-beck disease, colonic & pancreatic carcinoma, etc. However, at elevated concentrations, it is known to cause toxicity and oxidative stress5. Amongst 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 recommended level (10 ppb) by WHO can lead to chronic health effects causing damage to critical cell components, such as proteins, DNA, lipids, etc., by virtue of


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inhibitory effects through binding with thiols, and producing oxygen free radicals such as superoxide anion8. 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 level 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 spectroscopy (AFS)

11

9

, atomic absorption spectroscopy (AAS)

10

, atomic fluorescence

, electro-thermal atomic absorption spectroscopy (ETAAS)

12

, neutron

activation analysis 13 and so forth are reported for its qualitative and quantitative determination at trace level. However, these techniques are limited to laboratory sphere and rely upon expensive instrumentation; laborious procedures, trained labor, and accuracy in sampling method, pretreatment & frequency. Electrochemical techniques are also reported for selenium quantification, however, they suffer from interference and matrix fouling issues. Alternatively, the optical techniques have emerged as a plausible solution, wherein photoluminescence (PL) spectroscopy is not yet successfully explored read out system for selenite monitoring in water. Moreover, it allows portability of designed approach as well as is a highly sensitive and userfriendly technique. The availability of less number of literature for selenite detection using PL approach is limited due to sensitivity, selectivity and stability issues of reported optical probes till date (Table S1, ESI). Most of these probes (organic ligands, fluorophores and colorimetric dyes) are –SH & -NH2 rich organic ligands and have limited scope for real time quantitative applications owing to photo-bleaching, stability and so forth issues. Further they are synthesized


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from expensive precursors using multistep reactions and complex work up procedures

11-24

.

Therefore, in order to overcome all 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 nano-materials intervention in quantitative determination of selenite by optical means have also been made, yet, they require conventional techniques integration 25. More recently, Huang et al

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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 metals based 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 board absorption, symmetrical emission, tunability, biocompatibility, high stability against photo bleaching 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 21st century 31-32. In consideration of above facts, herein we 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 the selectivity, the designed probe is interfaced with nitrogen rich ligand for selective detection of selenite by Se-N chemistry. Ultra-fast spectroscopy (UFS) is used to understand the sensing mechanism. Further, the probe is well characterized by NMR, UV-Vis, FT-IR, XPS, PL spectroscopic techniques and 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).


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Experimental Materials & 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), 2pyridinecarboxaldehyde (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 sulphate (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 multi-element mixtures such as M1 (SO42-, Cl-, Br-, F-), M2 (PO42-, NO3-,Cl-, SO42-), M3 (Mg2+, Ca2+, Li2+, Na+, Cu2+) were procured from Merck and Inorganic Venture. Ultrapure deionized water obtained from Millipore (R = 18 MΏ) was used for reagents preparation. Synthesis of CQDs & Se (IV) selective SL CQDs were synthesized from citric acid (CA) by a well optimized and facile synthetic route using laboratory hot air oven. The optimization studies related to synthesis time and temperature are provided in Figure S1 (ESI). During synthesis, the precursor CA (2.0 g) underwent pyrolysis at high temperature (473 K) in hot air oven, which was witnessed as pale yellow thick residual after synthesis. The as synthesized CQDs were dissolved in methanol and stored at 4°C until used. The obtained CQDs showed blue luminescence in neutral pH environment under UV illuminator. The QY of CQDs was investigated by comparing the integrated intensities (excited


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at 350 nm) and the absorbance value of CQDs at 350 nm with reference dye, QS in 0.01 M H2SO4 (QY=54%):

Where, QY, I, A, and η are assigned to QY, integrated PL intensity, absorbance values, and refractive index, respectively. The subscript R and S refers to reference and sample, respectively. To synthesize amine rich SL, 2-pyridinecarboxaldehyde (23 mmoles, 2.28 mL, red color) and hydrazine hydrate (120 mmoles, 3.73 mL) were mixed in methanol under stirring condition and refluxed at 65°C for 24 hours in oil bath. The excess solvent was removed by rotary evaporator and washed with diethyl ether to obtain the final product, SL, (Schiff base), a dark yellow colored product. Interface Development To develop a CQDs-SL interface towards Se (IV), as synthesized CQDs (5 mL, 4.5 mmoles) were first esterified to methyl ester in the presence of an acid (HCl) as a catalyst under reflux condition in methanol and thereafter functionalized with SL. The esterified CQDs were dialyzed overnight to completely remove left over carboxylic acid containing CQDs moieties. For functionalization, esterified CQDs were refluxed with SL at 65 °C for 24 hours 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 towards selenite. The developed interface is well characterized using FT-IR, Raman and NMR spectroscopic techniques. Further to probe, the


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chemical composition of CQDs and fCQDs, XPS analysis was performed. The long aromatic chains of SL are introduced onto CQDs surface for selective binding towards selenite through Se-N chemistry. Quantification of Se with fCQDs 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 0.1 ppb -1000 ppb range. Interference study & Real water samples Analysis In order to measure selectivity of fCQDs probe, comparative and competitive ion study was performed. For comparative ion study, fCQDs response towards various cation & 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, competitive ion study was done against several anions, cations and multi-element solutions (M1, M2 and M3) in the presence of Se (IV). The real samples analysis was done using water samples collected from different resources and were used as such without any chemical treatment. FAAS was used for validation study of developed probe. The real water samples as well spiked samples were first converted into hydrides by using 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 proposed sensor are 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


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The optical characteristic of as synthesized CQDs, SL and fCQDs were probed by UV–Vis (Hitachi Ltd, Japan), Fourier transform infra-red (FT-IR, 670 IR, Varian, USA) and photoluminescence (PL, Shimadzu, RF-5301PC) spectrophotometer. A high-resolution transmission (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 femto second pulse duration. Raman spectra for CQDs, SL & fCQDs were collected with inVia instrument (Reninshaw) at λex = 785 nm. X-ray diffraction pattern was portrayed with X-ray diffractometer (Xpert-Pro). The surface chemical composition was probed by X-ray Photoelectron Spectroscopy (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 90% of the peak height. After doing Shirley background corrections, 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 400 MHz AvanceII spectrometer from Bruker. All pH measurements were made using digital pH-meter (LMPH 10, Labman Scientific Instruments Ltd). Ion Channel Chromatography (Metrohm) was used for sensor performance validation study. FAAS (Perkin Elmer) was used for analysis of spiked real water sample using hydride generation system (MHS 15) and argon as a carrier gas.

Results and discussions Characterization of synthesized CQDs, SL & fCQDs


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The optical spectrum recorded for CQDs, shown in Figure 1(a), divulge 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 exhibit a broad emission (400570 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 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 (ESI). The observed QY for CQDs with reference to QS was measured to be ~32 % (Figure S3, ESI), which is highest amongst earlier reports on CQDs synthesized from citric acid. Alternatively, SL doesn’t exhibit significant fluorescence characteristics of its own, however, fCQDs showed excitation dependent emission behavior as shown in Figure S4, ESI, with maximum PL intensity at 420 nm excitation (blue shifted as compared to CQDs), confirming functionalization as observed in Figure 1(b). Figure 2(a) 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 SAED pattern of CQDs in Figure 2(b), 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, ESI, which further confirms their amorphous nature. The Gaussian deconvolution of the XPS C(1s) and O(1s) core level spectra into their components confirm 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 f-CQDs, which confirms the successful functionalization of CQDs with N-rich ligand. The synthesized nitrogen rich ligand i.e. SL, was at first characterized with FT-IR, NMR and Mass spectroscopy (Figure


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S7, ESI). 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 analysis as shown in Figure 3. The various characteristic vibrational modes were observed in FT-IR spectra for CQDs, SL & fCQDs. It can be clearly seen that CQDs exhibit characteristic major peaks for 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 exhibit a vibrational peak at 1634 cm-1 corresponding to C=N stretch for Schiff base. The shift in –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 CQDs surface. Likewise, D and G bands in Raman spectra, which are “molecular” picture of carbon materials, were observed for CQDs, at ~1372 cm-1 and ~1638 cm-1, respectively. The relatively high intensity of D band when compared to the G band of CQDs indicates the 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 ~1670 cm-1, respectively, along with peak at ~1368 cm-1 attributing to localized sp3 defects. On the other hand fCQDs encompasses all Raman peaks including C-C, C=N, C=C, D band and G band, which further confirms the functionalization of CQDs. Besides, solid state 13C NMR analysis (DMSO-d6, 100MHz, ppm) of CQDs and fCQDs further confirms functionalization (Figure 4). The clearly observed resonance peaks at 171.14 ppm are


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indicative of sp2 bonded carbon atoms and corresponds to carbon nuclei of carboxylic acid group 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 leads to aromatic chains polymerization of such structures resulting in the formation of QDs architecture. As compared to

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C NMR of CQDs, 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 118.40-149.5 ppm. However, peak of carboxylic group is absent in fCQDs

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C NMR spectra, which confirms the functionalization of CQDs with SL

ligand. The Zeta potential analysis revealed decrease in potential value from 19.3 mV to -0.83 V upon functionalization (Figure S8, ESI) and could be assigned to presence of nitrogen rich SL moieties on CQDs surface at particular pH. Sensing behavior of fCQDs towards Selenite Figure 5(a) illustrates the sensing behavior of fCQDs probe towards selenite in water. It can be seen that on addition of selenite, a decrease in fluorescence intensity of fCQDs is observed, which could be associated with non-radiative recombination of charge carriers on selective binding of electron deficient SeO32- to nitrogen rich ligand present on fCQDs surface (inset, Figure 5 (a)), which even might have also led to elimination of vacancies at CQDs surface. The transfer of charge carriers during this binding process is therefore investigated by UFS analysis of fCQDs in presence of selenite and are shown in Figure S9 (ESI). 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 675 to 750 nm range, which confirms binding of selenite ions with fCQDs

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, henceforth, electron transfer between donor

nitrogen moieties present in ligand on fCQDs to SeO32-.The charge carrier lifetime study further justifies above observation since the fCQDs-Se complex exhibit shorter life time (~0.87 to 6.7


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ps) over fCQDs (~3.5 to 74ps) alone for ESA peak, which could be assigned to possible introduction of vibrational states by this complex leading to non-radiative recombination of photo excited electrons observed as quenching of 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 observed quenching phenomenon

38-40

. Figure 5(b), presents the

analytical performance of fCQDs towards selenite in acidic and basic environments and is observed highest at pH ≥7 depicted in terms of maximum intensity at physiological pH, which strengthens fCQDs sensing capabilities in physiological conditions. As the design of a selective fluorescent probe for selenite necessitates grafting of selenite selective ligand on CQDs surface. In previous report, 2,3-diaminanaphthalene is reported towards selective selenite detection utilizing Se-N chelating chemistry using chromogenic approach, however, color generation requires approximately two hours 14. In view of the strong interaction between Se-N complexes, we have synthesized a new ligand rich in nitrogen moieties, which induces selenite for selective quenching of CQDs probe. This is due to high bond dissociation energy between Se-N complexes (381 kJ/mol) formed between selenite species and nitrogen rich supramolecular ligand (SL) grafted on CQDs surface. The association constant of the fCQDs-Selenite complex in present was calculated using Benesi-Hildebrand method 41 and is found to be 7.6 × 106 M−1 as shown in Figure S10 (ESI). Analytical Characteristics of Optical Probe To determine detection window of the fCQDs the titration was carried out using different concentrations of selenite into fixed volume fCQDs sensor system. We observed more than 85fold decrease in the fluorescence intensity upon addition of 1000 ppb aqueous selenite solution as indicated in Figure 6(a). On addition of selenite concentration from 0.1 to 1000 ppb, a


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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 very low concentration (upto 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 w.r.t. calibration curve in 0.1 to 80 ppb range as shown in inset of Figure 6(a) and was found to be

, where y is selenite concentration in ppb and

. The

predicted values of concentration using the proposed equation was validated with various experimental values as shown in Table S2 (ESI). The theoretical detection (LOD, 3s) and quantification (LOQ, 10s) limit of fCQDs towards selenite was found to be ~0.1 ppb and 0.003 ppb, respectively, where, s is standard deviation, which is 0.033 in present case for six replicates (n=6). Further, relative standard deviation and precision of experiments (%) for n=6 was found to be better than 3.26 % and ~0.01 %, respectively. The observed analytical characteristic delineates high reproducibility, repeatability and sensitivity of presented probe towards selenite detection in water, which are comparative to other reported probes till 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 SV plot (Figure 6(b)) for linear equation Io/I= (7.47 × 106 )C + 1.2201 in 10-50 ppb linear concentration range was found to be 7.47×106 M-1. Further, the relative fluorescence quenching efficiency of selenite ( % Ksv) at 0.1 ppb, 1 ppb, 10 ppb, 100 ppb and 1000 ppb concentrations is calculated and found as 57.25 %, 86.90 %, 97.85 %, 98.52 % and 99.53 %, respectively (Figure S11, ESI), which supports the observed quenching of probe at different concentrations of selenite. The observed selective performance of probe could be attributed to strong association constant of Se-N complex (high bond dissociation energy of 381kJ/mol). Similar, hypothesis has been earlier utilized for selective selenite sensing by means


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of fluorescent organic polymers, chromogenic ligands, etc.

14, 22, 42

. The sensing performance of

proposed sensor is 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, ESI) 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+, Co2+ at 10 ppb 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 1F/F0, (F/F0:ratio of fluorescence intensity in presence and absence of selenite) and is shown in Figure 7(a), which clearly manifest negligible interference due to these commonly reported interferents /ions in water at 10 ppb concentration, indicating that other ions hardly induce the structural and optical transformations of these fCQDs, which is very significant for selenite even at 10 ppb concentration. Moreover, at higher concentrations of these comparative ions (100 ppb) as shown in Figure 7(b), the trend still remains same. At higher concentration (1 ppm), the sensor response is still more prominent towards selenite (Figure S13, ESI). Whilst to test the practical applicability of the developed optical probe for selenite, competitive ion study results with library of anions, cations and multi-ions are shown in Figure 8. No significant change on fCQDs response towards selenite was observed even in the presence of various possible competitive ions


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in similar mixture. This concludes that the reported fCQDs are highly selective probes for selenite, and is 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 towards selenite, the practicality of same was testified against real water samples as shown in Figure S14 (ESI) collected from ground, lake and 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 concentration. fCQDs fluorescence intensity was decreased on addition of 10 ppb spiked water samples collected from above mentioned sources. The % recovery was found in 84 % to 116 % range as shown in Table S3 (ESI) and the results were in line with FAAS analysis of these samples. These observation implies 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) & USEPA (50 ppb) criterion of safe selenite level limit in potable water.

Conclusions In conclusion, we have reported the selective quantification of 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 on addition of ~10 µM selenite, and exhibit a theoretical detection


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limit of 0.1 µg/L, which is well below the WHO recommendation of 10 ppb. The probe is capable to detect 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 lab based flame atomic absorption spectrophotometer for selenite analysis in real water samples. This study paves promising feasibility for design of on-site selenite detection system in water matrix for a wide and practically found concentration range in contaminated water resources.

ASSOCIATED CONTENT Supporting Information (optical characteristics and QY measurement of CQDs, surface charge of CQDs and fCQDs, NMR, FT-IR & MS of synthesized ligand, XPS & XRD analysis of CQDs & fCQDs probes, and ultra-fast spectroscopic (UFS) analysis of fCQDs and fCQDs-Se (IV) complex)

AUTHOR INFORMATION Corresponding Author * [email protected], * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes The authors declare no competing financial interests.


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

ABBREVIATIONS WHO, world health organization, USEPA, U.S. Environment Protection Agency.

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Scheme 1: Interface development between esterified CQDs and as synthesized ligand to obtain functionalized CQDs (fCQDs)

Figure 1: (a) Absorption and emission (excitation wavelength 370 nm) characteristics of as synthesized CQDs ; (b) emission behavior of CQDs, SL & fCQDs at 420 excitation wavelength.


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Figure 2: (a) TEM and (b) SAED analysis of CQDs [Inset (a): SEM micrograph of as synthesized CQDs]

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


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Figure 4:

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C NMR Spectra of (a) CQDs and (b) fCQDs

Figure 5: (a) Sensing behaviour of fCQDs towards selenite [inset: sensing mechanism and (b) effect of pH on sensing performance of fCQDs


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

Figure 7: (a) Response of fCQDs towards (a) cations and (b) anions


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Figure 8: Competitive ion study of fCQDs towards selenite in the presence of library of (a) anions, cations and (b) multi-element solutions at 10 ppb concentration


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Table of Contents


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Ultrasensitive and Selective Sensing of Selenium Using Nitrogen-Rich

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