AuNCs-Catalyzed Hydrogen Selenide Oxidation: Mechanism and

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AuNCs-Catalyzed Hydrogen Selenide Oxidation: Mechanism and Application for Headspace Fluorescent Detection of Se(IV) Jing Xiong, Kailai Xu, Xiandeng Hou, and Peng Wu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00738 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Analytical Chemistry

AuNCs-Catalyzed Hydrogen Selenide Oxidation: Mechanism and Application for Headspace Fluorescent Detection of Se(IV) Jing Xiong,† Kailai Xu,*,† Xiandeng Hou, †, ‡ Peng Wu*,†,‡,‖

†College

of Chemistry, ‡Analytical & Testing Center, and ‖State Key Laboratory of

Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, 61004, China

*Corresponding Authors’ E-mails: [email protected], [email protected]

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Abstract The excellent fluorescence property of Au nanoclusters (AuNCs) has received great attention for various chemosensing and bio-related applications, but the sample matrix is still a cursing problem that causes undesirable fluorescence variation. Hydride generation (HG) is an effective strategy to separate the target analyte from the complex sample matrices, but the implementation of HG with AuNC-based fluorescent assays was not realized. On the other hand, due to the ultrasmall size of AuNCs and good catalytic performance of Au, AuNCs are also featuring intriguing catalytic applications. Herein, we proposed a new type of AuNC-based fluorescence assay for Se(IV) detection, in which hydride generation of Se(IV) was coupled with the fluorescence/catalytic dual functions of AuNCs. In a batch hydride generation mode, Se(IV) was first converted to volatile H2Se. Upon spreading in the headspace to contact with AuNCs supported paper, AuNC-catalyzed oxidation of H2Se by O2 to yield elemental selenium occurred, which further deposited on the surface of AuNCs to induce fluorescence quenching. The catalytic effect of AuNCs was studied in depth via both experimental and theoretical (density functional theory, DFT) investigations. Three main steps for H2Se oxidation were identified, with energy barriers in the presence of AuNCs significantly lower than those without. Benefiting from the reduced matrix interference by hydride generation and the unique catalysis/fluorescence of AuNCs, the proposed assay featured high selectivity, good sensitivity and simplicity, with successful applications for selenium detection in real samples.

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Introduction Gold nanoclusters (AuNCs), ultra-small gold nanoparticles (AuNPs) with core diameter less than 2 nm, have attracted considerable attention because of their distinctive electronic, catalytic and optical properties.1-4 Owing to their easiness to synthesis, excellent biocompatibility, facile surface functionalization, good photostability and strong photoluminescence, many AuNCs-based fluorescent assays have been developed for sensitive and selective sensing of various analytes, including metal ions, inorganic anions, small biomolecules and proteins.5-9 The interactions between the analytes and the Au core or ligands will result in the change of the valence state of Au core, the formation of complexes, the aggregation of the clusters, or the flow of electrons, which will eventually lead to perturbation of the fluorescence. Therefore, most of the sensing strategies using AuNCs are based on direct analyte-induced fluorescence change.10-16 Generally, these methods are vulnerable to environmental impact, leading to detection of real samples with complex matrices problematic. In order to eliminate the matrix effect, special functionalization of AuNCs and sample pretreatments are often needed, which leads to the tedious experimental process. Therefore, developing simple assays based on AuNCs with low sample matrix interference is of great significance. Chemical vapor generation (CVG) is an effective strategy to separate the target analyte from the complex sample matrices.17-19 For example, in a previous work, hydride generation (HG) was coupled with AuNP-based colorimetric assay to reduce the sample matrices, in which the target analyte (Se) was first converted to volatile hydrides before interaction with AuNPs.20 Since the signaling of AuNPs relies on the inter-particle distance change in the solution, the target analyte must undergo two step phase transfer, namely from liquid phase to gas phase (hydride generation) and next to 3



another liquid phase (interaction with AuNPs). In this manner, interferences from the other chalcogens (Te) can occur, since both H2Se and H2Te are highly volatile and share similar affinity towards thiophilic AuNPs. Moreover, the equipping of a flow injection hydride generator with AuNPs hinders the field applications of this method. Headspace hydride generator, on the other hand, is a much simpler device that has been readily explored for on-site detection of various volatile hydrides in combination with the microextraction effects of several nanomaterials.21-23 Therefore in this work, headspace hydride generation was introduced to couple with AuNCs-based fluorescent sensing to decrease the potential interferences from complex sample matrices. To facilitate potential field applications, here fluorescent AuNCs is immobilized on a filter paper to achieve “test paper” format sensing. In order to accommodate the test paper with hydride generation, a headspace hydride generator is designed, in which the hydride generation is initialized via batch injection of the reductant KBH4 (Scheme 1). Hence, only one single step phase transfer was experienced for the target analyte Se(IV). Further investigations indicated that AuNCs here acted not only the indicator, but also the catalyst for H2Se oxidation (Scheme 1). However, no such catalyzed oxidation for H2Te was observed. The formation of elemental selenium on the surface of AuNCs eventually results in fluorescence quenching of AuNCs. Previously, catalytic decomposition of volatile hydrides (such as H2Se, AsH3, BiH3) was reported on nanostructured noble metal nanoparticles (e.g., PdNPs, AgNPs, etc.),21,24-25 but not on quantum-sized metallic nanoclusters with intriguing fluorescence for direct sensing of hydrides. Therefore, extra detection approaches such as total reflection X-ray fluorescence analysis are required. The dual role of AuNCs here therefore permit highly selective selenium detection, and the broad excitation of AuNCs endows simple visual readout of selenium via handheld UV lamp 4


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excitation.

Scheme 1. Scheme illustration of the AuNCs-based “ test paper” format fluorescent sensing of Se(IV) via headspace hydride generation.

Experimental Section Chemicals and materials. Reagents used in this work were of analytical grade or higher. HAuCl4‧3H2O and BSA used in this study were separately purchased from the National Chemical Reagent Company and Aladdin Reagent Database Inc. (Shanghai, China). HCl, KBH4 and NaOH were purchased from Kelong Reagent Factory (Chengdu, China). 1000 mg L-1 stock solutions of Se(IV), Na+, Mg2+, Ca2+, K+, Ba2+, Fe3+, Cu2+, Sb3+, Bi3+, Sn4+, Cr3+, Cd2+, Ag+, Au3+, Hg2+, Zn2+, As3+, Te4+, Mn2+, Co2+, Ni2+, and Pb2+ were purchased from the National Research Center for Standard Materials (NRCSM) of China. Selenomethionine, selenocysteine and selenocystine was purchased from the Bailingwei Technology Co., Ltd. (Beijing, China). High-purity 18.2 MΩ cm-1 deionized water (DIW) obtained from a Milli-Q water system (Chengdu Ultrapure Technology Co., Ltd., Chengdu, China) was used throughout this work. The accuracy of the proposed method was validated by analysis of Certified Reference

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Materials (CRMs) obtained from National Research Council Canada (NRCC), including dogfish liver (DOLT-5) and dogfish muscle (DORM-4). Synthesis of AuNCs. The synthesis of BSA-AuNCs was based on the well-known protocol developed by Xie and Ying.26 All glassware was washed with aqua regia. In a typical MW assisted synthesis experiment, 10.0 mL of 65 mg/mL BSA was added to 10.0 mL of 10 mM HAuCl4 3H2O, followed by 1.0 mL of 1.0 M NaOH. The mixture solution was heated with temporary pauses by using a microwave synthesis equipment (300W) for 2 min, including 40 sec MW irradiation, 1.0 min pause, 40 sec MW irradiation, 1.0 min pause, and 40 sec MW irradiation. MW irradiation was temporarily paused to prevent the reaction mixture from overheating in the process. Then the reaction mixture turned from light yellow to dark brown. It exhibited intensive red fluorescence emission under UV illumination, indicating the formation of AuNCs. Procedures for Se(IV) Detection. A chromatography paper (Whatman, UK) cut to a diameter of 1.6 cm by a puncher, circular paper sheets were immersed in AuNCs solution 30 min and dried naturally. Finally, the papers doped with AuNCs were then sealed in plastic bags and kept at room temperature prior to use. The AuNCsimmobilized paper and the gasket were adhered to the cap of a 25 mL headspace bottle (Taicang Fisher Instrument Co., Jiangsu, China) in turn. 10 mL samples or selenium standard solutions of Se(IV) containing 5% (v/v) HCl were added to the bottle which was then sealed with the lid. 1 mL 3% (m/v) KBH4 dissolved in 0.5% (m/v) KOH was injected into the bottle tardily for the generation of H2Se by HG reaction. The generated H2Se was instantaneously reacted with AuNCs on the paper. After a while, took off the circular paper and placed them in a UV test chamber with 365 nm UV illumination to complete the visual inspection. The paper was also put into solid sensing cells on a solid 6


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sample holder to measure fluorescence intensity. Computational methods. All of the calculations were carried out by employing DFT methods in the Dmol3 software of the Materials studio package.27-28 In this study, Au38 clusters were chosen as the model to explore the catalytic properties of AuNCs. The vibration frequency calculations have been completed to verify that all the reactants, intermediates and products have no imaginary frequency. The complete linear synchronous transit and quadratic synchronous transit (LST/QST) method was used to determine the transformation pathways and transition state structures. It was proved that each transition state had only one imaginary frequency and its vibration mode had the right direction connecting the reactant and product. The more details for the calculations were given in Supporting Information.

Results and Discussion Design and validation of the AuNCs-based fluorescent assay for the detection of Se(IV) via hydride generation. To facilitate the hydride generation, a ca. 25-mL headspace bottle was designed as the headspace hydride generator (Scheme 1). The acidified sample solution was placed inside the bottle, while the AuNCs supported paper was placed inside the bottle cap. After injection of the reductant (KBH4), volatile species containing H2Se was produced and released from the liquid phase to react with AuNCs in the headspace, resulting in the formation of elemental selenium and fluorescence quenching of AuNCs. The capture of H2Se by AuNCs and the formation of Se was confirmed by photographing. As can be seen from Figure 1A, without pre-deposition of AuNCs on the paper, no retention of H2Se or fluorescence quenching (original fluorescence of the filter paper) was observed. However, after loading of AuNCs, either BSA-AuNCs 7



(Figure S1)26 or GSH-AuNCs (Figure S2)29, distinct fluorescence quenching was received. Besides, reddish deposition was also observed on the paper of both BSAAuNCs and GSH-AuNCs after reaction with H2Se, which can be identified as element Se (see the characterization below) arisen from H2Se oxidation by O2. Unlike H2Se in solution that can be rapidly oxidized by dissolved oxygen in water to form elemental selenium,20 gaseous H2Se here is more stable and cannot be quickly oxidized by oxygen in air. Since the bare paper and AuNCs-loaded paper are both air-dry (no dissolved oxygen), AuNCs here may act as the catalyst for H2Se oxidation (without significant ligand effect). Moreover, such catalyst effect is not limited to AuNCs, but also plasmonic Au nanoparticles (AuNPs, Figure S3). As can be seen from Figure 1A, uncapped 5-nm AuNPs30 also featured similar Se deposition after reaction with H2Se. However, the deposition of Se on the surface of AuNPs is difficult than that of AuNCs, probably due to the size-dependent catalyst effect of Au.

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Figure 1. Conformation of the reaction of volatile H2Se with AuNCs-supported paper: (A) photographs of the bare paper and AuNCs and AuNPs-loaded paper before and after reaction with H2Se [generated from 2 mg/L Se(IV)]; and (B) fluorescence spectra of BSAAuNCs before and after reaction with H2Se [generated from 100 μg/L Se(IV)]. The inset in (B) shows the corresponding fluorescent images.

The fluorescence quenching was further verified with fluorescence spectroscopy. As can be seen from Figure 1B, the fluorescence of BSA-AuNCs (peaked at 625 nm) excited with the 365 nm ultraviolet light was considerably quenched after reaction with H2Se (generated from hydride generation of 100 μg/L Se(IV)). Meanwhile, the original bright red fluorescence of BSA-AuNCs on the paper could be observed to be much weaker by naked eye. Therefore, Se(IV) can be quantitatively detected by fluorescence spectrophotometry or semi-quantitatively by naked eye. Compared with GSH-AuNCs, the red fluorescence of BSA-AuNCs is much more sensitive to the naked eye observations than that of yellow from GSH-AuNCs (Figure S4). Besides, the stability 9



of BSA-AuNCs is also higher than that of GSH-AuNCs. Fluorescence quenching mechanism: experimental and theoretical studies. To investigate the morphology change of AuNCs before and after reaction with H2Se, the sizes of AuNCs (in solution) were characterized with transmission electron microscopy (TEM). As shown in Figure 2A and 2B, the size of the BSAAuNCs changed from ~1.58 ± 0.43 nm (before reaction with H2Se) to ~2.26 ± 0.45 nm (after reaction with H2Se). Corresponding energy dispersive spectroscopy (EDS) analysis indicated that the enlarged particle size could be ascribed to the formation of a layer of Se. As shown in Figure 2C, only Au, C and O can be found in the EDS pattern of the AuNCs before reaction, while four elements including Se, Au, C and O can be seen after the reaction. X-ray photoelectron spectroscopy (XPS) analysis also confirmed such results. After reaction with H2Se, additional sign of Se was observed except Au, C, N, O and Na (Figure 2D). The Se peak was appeared at ~55.3 eV on the XPS spectrum (Figure 2E), which is consistent with the electron binding energy of Se0 (3d). No further peaks for Se2- (~53 eV), Se4+ (~59 eV) and Se6+ (~61 eV) was observed, indicating the absence of these valence states. Therefore, the newly generated red selenium was identified as elemental selenium (Se0).

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Figure 2. Identification of the formation of Se0 on the AuNCs after the reaction with H2Se: (A) TEM image of AuNCs before the reaction with H2Se; (B) TEM image of AuNCs after the reaction with H2Se; (C) EDX spectra before and after the reaction with H2Se; (D) XPS patterns of AuNCs before and after the reaction with H2Se; (E) Se 3d XPS patterns before and after the reaction with H2Se; and (F) fluorescence lifetime of AuNCs before and after the reaction with H2Se (corresponding to 100 μg/L Se).

The generated Se0 was deposited on the surface of AuNCs, probably because selenium can form stable Au-Se bond.31-32 Such AuNCs-seeded growth of Se0 can be evidenced from the fluorescence lifetime (τ) of AuNCs. As can be seen from Figure 2F, after interaction with H2Se (corresponding to 100 μg/L Se), the fluorescence lifetime of AuNCs decreased from ~1.43 μs to ~1.03 μs, demonstrating the distortion of the emission core of AuNCs by H2Se.33 As can be seen from Figure 1A, the oxidation of H2Se can occur on either fluorescent AuNCs (~1.5 nm) or plasmonic AuNPs (~5 nm). For even larger sized AuNPs (15 nm), the formation of elemental selenium was also observed (data not 11



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shown). Therefore, here AuNCs may act as the catalyst for H2Se oxidation. In fact, the catalytic performance of AuNCs has already been demonstrated in the literature.34-36 For better illustrating the oxidation of H2Se on AuNCs, theoretical calculations were carried out to investigate the above reaction with and without the Au cluster (see supporting information for details). Since the catalytic effect is not just limited to AuNCs, the classical model of Au38 with octahedron symmetric structure typically utilized in the literature for theoretical calculations was explored here.37-39 After calculation, it was found that the reaction of H2Se with O2 (2H2Se + O2 → 2H2O + Se2) took three steps to generate the end product of H2O and elemental selenium: H2Se + O2 → HSe‧ + HOO‧ (1) HSe‧ + HOO‧ + H2Se → HOOH + 2HSe‧ (2) HOOH + 2HSe‧ → Se2 + 2H2O (3) The key intermediates of such reaction is the HSe ‧

and HOO ‧

radicals. The

involvement of oxygen on these reactions can be confirmed via deoxygenation (Figure S5). Here, Se2 was employed as the stable elemental selenium for calculations.40-41 As shown in Figure 3 and Table 1, the energy barrier for oxidation of H2Se in air is generally higher than that on Au38. Meanwhile, the Gibbs free energy (ΔG298) on Au38 is exclusively negative. Therefore, the catalytic role of Au38 in the oxidation of H2Se can be verified.

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Figure 3. Identification of the catalytic role of AuNCs through DFT calculations. The black and blue potential energy curves represent the H2Se oxidation in air and on the Au38, respectively. The optimized geometric structures of the species in the two reactions are depicted in the table at the bottom. The symbols outside/inside the brackets correspond to the reaction in air/on the Au38.

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Table 1. The activation energies ΔEa and the Gibbs free energy change ΔG298 (1 atm, 298.15 K) of the each reaction step.

Reaction step

ΔEa

ΔG298 Au38

Air

Au38

Air

H2Se + O2 → HSe‧ + HOO‧

27.7

17.0

30.2

-18.5

HSe‧ + HOO‧ + H2Se → 2HSe‧ + HOOH

15.8

2.0

-29.7

-25.1

2HSe‧ + HOOH → 2H2O + Se2

17.9

10.8

-67.5

-18.9

Specifically, for the reaction in air (without Au38), the first step has an energy barrier of 27.7 kcal/mol. The positive ΔG298 value (30.2 kcal/mol) suggests such step should be thermodynamically unfavorable at room temperature. Next, HOO‧ obtains a hydrogen atom from another H2Se to generate HOOH, with an energy barrier of 15.8 kcal/mol. Finally, HOOH reacts with HSe‧ to form Se2 and H2O as the end product with the energy barrier of 17.9 kcal/mol. While for the oxidation of H2Se on the Au38 cluster, the reaction proceeds via first adsorption of H2Se on the surface of gold, which activates the Se-H bond through increasing bond length by ~0.01Å. The co-adsorption energy of H2Se and O2 on Au38 is 39.6 kcal/mol, indicating their strong interaction with the gold clusters. Upon adsorption on gold and subtraction of H from H2Se by O2 (reaction step 1), the Se-H and O-O bond lengths increases by 0.95 Å and 0.23 Å, respectively, while the O-H distance decreases by 1.72 Å (Figure S6). Compared with the case in the air, the introduction of the gold cluster reduces the active energy barrier by 10.7 kcal/mol. Besides, the negative ΔG298 value (-18.5 kcal/mol) for the first step suggests that the reaction should be thermodynamically favorable. Therefore, the Au atom on the cluster 14


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stabilize the key intermediate HSe‧ and makes the reaction occurs more easily. For step 2, the calculated energy barrier is as low as 2.0 kcal/mol (15.8 kcal/mol for the case without Au38), indicating that such step occurs very quickly. Finally, step 3 on Au38 also features lower energy barrier (10.8 kcal/mol). Accordingly, the introduction of Au clusters decreases the energy barriers of all three steps, namely acting as a catalyst for H2Se oxidation. On the basis of the above experimental and theoretical investigations, the mechanism of H2Se-induced fluorescence quenching can be summarized as follows. In the presence of AuNCs catalyst, H2Se reacts with O2 to produce H2O and Se2 as final products through formation of HSe‧, HOO‧ and HOOH intermediates. The deposition of elemental selenium on the surface of AuNCs enlarges their particle size and distort the emission core of AuNCs, resulting in fluorescence quenching. Analytical performance for Se(IV) detection. The assay conditions were optimized to maximize the sensitivity, including the conditions for hydride generation, reaction time, volume and the concentration of AuNCs (Figure S7-S11). Under the optimized conditions, the fluorescence of AuNCs quenched gradually upon reaction with increasing amounts of gaseous H2Se (from batch hydride generation), accompanied by quenched image of the paper (Figure 4A). A linear calibration plot was obtained, with linear regression equation of I0/I = 0.023C + 0.397 (where C is the concentration of Se(IV)) and a correlation coefficient (R) of 0.995 (Figure 4B). The limit of detection (LOD), defined as the analyte concentration equal to three times of the standard deviation of the blank (n = 11), was 4 μg/L for Se(IV) (average trap efficiency for H2Se by BSA-AuNCs, ~13.5%). The reproducibility of the assay 15



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expressed as the relative standard deviation (RSD) for a concentration of 50 μg/L of Se(IV), was about 2.8% (n = 7).

Figure 4. Analytical performance of the proposed assay for Se(IV) detection: (A) fluorescence spectra of BSA-AuNCs in the presence of increasing amounts of Se(IV); and (B) plots of I0/I versus the concentration of Se(IV). The inset of (A) shows the corresponding fluorescent images.

It should be noted that organoselenium compounds are also capable of reaction with borohydride to form volatile organoselenium species.42 Therefore, potential response of the volatile organoselenium species were investigated. Three organoselenium

compounds,

namely

selenomethionine,

selenocystine

and

selenocysteine, were tested. As shown in Figure 5A, no appreciable fluorescence quenching was observed when delivering the generated volatile organoselenium species to AuNCs-supported paper, even when the concentration of selenomethionine and selenocystine approached 10 mg/L. Considering that selenomethionine and selenocysteine are the major organoselenium species potentially existed in samples, the proposed assay thus exhibited minimal response towards organoselenium species.

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Figure 5. Selectivity and interference evaluation of the proposed assay for Se(IV) detection: (A) organoselenium compounds; (B) S and Te from the same group as Se; (C) ionic strength from K+, Na+, Ca2+, Mg2+, and Al3+ (5%, m/v); and (D) hydride-forming elements, transition metal ions, and noble metal ions. The concentrations in (D) are as follows: Se(IV), 100 μg/L; Fe3+, Pb2+, Sn4+, Zn2+, Cd2+, Sb3+, Cr3+, Mn2+, Co2+, Ni2+, and Au3+: 2 mg/L ; Ag+, Hg2+, Bi3+, As(III) and Cu2+: 1 mg/L.

Next special attention was paid to S and Te since they are both capable of hydride generation and belong to the same group as Se. The interferences from S could be easily eliminated via acidification of the sample. But for Te, no fluorescence quenching was observed at concentrations up to 20 ppm (Figure 5B), which is different from the previous work.20 The reduction potential of H2Te (-0.72 V) is much higher than that of H2Se (-0.40 V) in acidic solution. Therefore, H2Te generated from the batch hydride generation is easily oxidized by dissolved oxygen in the absence of strong purging (here 17



only the generated hydrogen, Figure S12), thus little H2Te could be released out to react with AuNCs. While for H2Se, most of the H2Se generated from the batch hydride generation could be purged out from the solution without appreciable oxidation (Figure S12). Accordingly, H2Se could receive catalyzed oxidation in the headspace. In this vapor-solid heterogeneous reaction system, the solid AuNCs are stable and their fluorescence change may occur only if they participate in the reaction. Therefore, the proposed one single step phase transfer protocol efficiently eliminate the Te interfaces that was observed in previous two step phase transfer involving AuNPs.20 Due to the introduction of hydride generation for separation of sample matrix, the proposed method show excellent tolerance towards high level of ionic strength. Up to 5% (m/V) salt concentration caused no significant impact on Se(IV) detection (Figure 5C), since Se(IV) was isolated from those coexisting ions prior to the fluorescence quenching process. A major drawback of hydride generation lies in the potential liquidphase interference from transition metal ions.43 Therefore, the selectivity and coexisting interferences of several hydride-forming elements, transition metal ions, and noble metal ions were evaluated. As shown in Figure 5D, all the 16 tested ions have no response in the proposed assay. When co-existing, some noble metal ions (Cu2+, Ag+ and Au3+) and Pb2+ resulted in decreased responses for Se detection, probably because of the liquid-phase interference during hydride generation. The applicability of the proposed assay was validated by the determination of selenium in certified reference materials (DOLT-5 and DORM-4), selenium-enriched rice and egg, tap water and seawater samples (Table 2 and Figure S13). For the certified reference materials and selenium-enriched rice and egg, the analytical results were in good accordance with the certified values or those determined by hydride generation 18


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atomic fluorescence spectrometry (HG-AFS). While for water samples, no Se(IV) was detected, the accuracy was further confirmed with the standard spike-recovery test (recovery of 108% and 107%). The t-test proved no significant difference between the certified value and the determined value at the 95% confidence level. A comparison on the analytical performance of the proposed assay with other similar methodologies for Se determination is summarized in Table S1. Although the proposed assay is not the most sensitive, its simplicity and potentiality in on-site detection should be acknowledged. Table 2. Analytical results for the detection of selenium in real samples. recovery

sample

certified value

this method

t0.05 d

DOLT-5

8.3 ± 1.8 μg/g

7.8 ± 1.9 μg/g

2.11

—

DORM-4

3.50 ± 0.34 μg/g

3.31 ± 0.42 μg/g

1.94

—

Selenium-enriched rice

276.0 ± 7.5 μg/kga

265.6 ± 9.4 μg/kg

1.91

—

Selenium-enriched egg

221.3 ± 6.7 μg/kga

200.6 ± 8.6 μg/kg

4.16

—

Tap water

ND, 50 μg/L b

54.0 ± 2.7 μg/L

—

108

Seawater

ND, 100 μg/Lc

106.9 ± 6.5 μg/L

—

107

aBy

HG-AFS

bNo

Se(IV) was detected; 50 μg/L Se(IV) was spiked

cNo

Se(IV) was detected,100 μg/L Se(IV) was spiked

dCritical

(%)

value t0.05 = 4.30 (P = 0.95, N = 3)

Conclusion In summary, a simple “test paper” format was developed for headspace fluorescent detection of Se(IV). Here, fluorescent AuNCs was immobilized on a filter paper, which reacted with H2Se generated from headspace batch hydride generation to induce fluorescence quenching. We also found AuNCs here not only acted as the indicator for 19



H2Se, but also functionalized as a catalyst for H2Se oxidation. Under the catalysis of AuNCs, H2Se reacts with O2 to produce H2O and elemental Se as final products. The produced elemental Se deposited on the surface of AuNCs, thus resulted in fluorescence quenching of AuNCs. Benefiting from the ability of hydride generation to reduce the matrix and unique catalytic/fluorescence effect of AuNCs, the proposed assay featured the advantage of high selectivity, good sensitivity and simplicity for selenium detection. Overall, this work provides in-depth understanding of the synergistic fluorescence and catalytic properties of AuNCs, which may provide a new avenue for application of AuNCs in future chemical sensing.

Acknowledgement The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21876118) and the Sichuan Youth Science and Technology Foundation (grant 2016JQ0019). We would like to thank Dr. Yunfei Tian and Dr. Shanlin Wang of Analytical & Testing Center Sichuan University for their help in XPS and TEM data collection, respectively.

Supporting Information Available Additional information as noted in text, including instrumentation details, sample preparation, the computation details and geometric parameters, characterization of AuNCs, and the optimization of the sensing conditions. This material is available free of charge via the Internet at ACS Publications website.

Notes The authors declare no competing financial interest.

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Scheme 1 85x49mm (300 x 300 DPI)



Figure 1 85x103mm (300 x 300 DPI)


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Figure 2 160x113mm (300 x 300 DPI)



Figure 3 160x165mm (300 x 300 DPI)


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Figure 4 85x53mm (300 x 300 DPI)



Figure 5 160x124mm (300 x 300 DPI)


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AuNCs-Catalyzed Hydrogen Selenide Oxidation: Mechanism and

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