Gold Nanoparticle-Based Colorimetric Assay for Selenium Detection

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Gold Nanoparticle-based Colorimetric Assay for Selenium Detection via Hydride Generation Guoming Cao, Fujian Xu, Shan-Ling Wang, Kailai Xu, Xiandeng Hou, and Peng Wu Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 23, 2017

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Analytical Chemistry

Gold

Nanoparticle-based

Colorimetric

Assay

for

Selenium Detection via Hydride Generation Guoming Cao,† Fujian Xu,‡ Shanling Wang,‡ Kailai Xu,†,* Xiandeng Hou,†, ‡ Peng Wu†, ‡, *

†

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of

Chemistry, and ‡Analytical & Testing Center, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China

*Corresponding authors: [email protected] (KX), [email protected] (PW)

1

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ABSTRACT Gold nanoparticles (AuNPs)-based colorimetric assays are of particular interest since molecular events can be easily read out with the color changes of AuNPs by naked eye. However, the molecular recognitions occur almost exclusively in the liquid phase, i.e., the interaction between target analytes and AuNPs is always proceeded in the presence of sample matrix. Since the aggregation of the unmodified AuNPs is prone to be influenced by the ionic strength of the solution, sample matrix will cause undesirable interference. Here, we proposed a new type of AuNP-based colorimetric assay, in which target analyte selenium was first converted to its hydride chemical vapor (H2Se) and then delivered into the solution of AuNPs to induce color change. Therefore, sample matrix (for example, high salinity) were eliminated, leading to excellent selectivity and free of sample matrix. With the aid of hydride generation, the proposed method offered a detection limit of 0.05 µM with UV-vis detection and 1 µM with naked eye. Sucessful application of this method for selenium detection in biological and enviromental samples was demonstrated.

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INTRODUCTION Gold nanoparticles (AuNPs)-based colorimetric assays are of particular interest because of the unique chemical and physical properties of AuNPs, including the high extinction coefficients in the visible region, alterable surface plasmon resonance (SPR) absorption, simple preparation and easy functionalization.1-3 The color change is highly sensitive to the size, shape, capping agents, medium refractive index, as well as the aggregation state of AuNPs. Therefore, molecular events can be easily read out with the color changes of AuNPs by naked eye. Since the pioneering work of Mirkin and Alivisatos,4-5 AuNP-based visual assays have been extensively explored for detection of metal ions,6-11 small molecules,12-15 proteins,16-18 DNA,19-20 cancer cells,21-22 and etc. For the aggregation of AuNPs, there are mainly two driven forces: one is the target analyte-guided assembly for bringing close proximity of AuNPs (crosslinking); the other is salt-induced breaking of the electrostatic repulsion of original citrate-stabilized AuNPs (non-crosslinking). The latter mechanism, in combination with the differential stabilization effects of double strand and single strand DNA for AuNPs, is first explored by Li and Rothberg for the development of label-free colorimetric sensing platform.23-24 Such type of AuNP-based colorimetric assays (often called unmodified AuNPs) eliminates tedious functionalization of the AuNPs, and have received great attention in the past few years.25-29 Obviously, special care should be taken when unmodified AuNPs are used for target detection in sample matrix with high salinity. Besides, strongly acidic/alkaline media is also not favored 3



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for AuNPs,30 which restricts the practical application of unmodified AuNPs for detection of metal ions in complex sample matrices that needs sample digestion. Even for crosslinking AuNP-based assays, sample matrix with high salinity or strongly acidic/alkaline media will breakdown the surface functionalization of AuNPs and therefore their aggregation state, leading to undesirable interferences. When tracing the origin of the above interferences, one can see the interaction between target analytes with AuNPs occurs almost exclusively in the liquid phase, i.e., sample matrix with high salinity or strongly acidic/alkaline media also interacts with AuNPs at the same time. Even if the reaction of target analytes is highly selective, the aggregation state of AuNPs is vulnerable to the sample matrix. Of course, pre-separation of the sample matrix via solvent extraction or column-based preconcentration can alleviate the above interferences, but such steps add further tedious operations. Here, we proposed a new type of AuNP-based colorimetric assay, in which target analyte selenium was first converted to its hydride chemical vapor (H2Se) and then delivered into the solution of AuNPsto induce color change. Therefore, sample matrix (for example, high salinity) were eliminated, leading to excellent selectivity and sample matrix interference-free determination. It’s well-known that a variety of chemical vapor generation (CVG) modes, such as hydride generation, Grignard alkylation, halide generation, oxide generation, and carbonyl generation, exist in nature and have been explored for various applications.31-34 The interaction between gaseous H2Se and the solution of AuNPs perturbs the aggregation state of AuNPs, and 4


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therefore color change that can be readout with naked eye. Compared with solvent extraction or column-based preconcentration for sample matrix elimination, CVG used here is more convenient since it relies on simple mixing of acidic sample and KBH4 without repeated extraction operation or adsorption and elution.

EXPERIMENTAL SECTION Materials. All reagents used in this work were of analytical grade or higher. HAuCl4·3H2O, trisodium citrate used in this study were purchased from Aladdin Reagent Database Inc. (Shanghai, China). The 100 mg/L of Se(IV) stock solution and 1000 mg L−1 stock solutions of Na+, Cu2+, Fe2+, Mg2+, Ca2+, K+, Ba2+, Sb3+, Bi3+, Sn4+, Cr3+, Cd2+, Ag+, Hg2+, Zn2+, As(III), Te(IV), Mn2+, Co2+, Ni2+, and Pb2+ were purchased from the National Research Center for Standard Materials (NRCSM) of China. All solutions were prepared by using ultrapure water (18.2 MΩ cm−1) from a water purification system (Chengdu Ultrapure Technology Co., Ltd., Chengdu, China).

Apparatus. Absorption spectra were recorded on an UV−vis spectrophotometer (UV-1750, Shimadzu, Japan). High-resolution transmission electron microscopy (HRTEM) images were obtained with JEM-100CX transmission electron microscope at an accelerating voltage of 80 kV (JEOL Co., Japan). X-ray photoelectron spectroscopy (XPS, PHI-5000 Versa Probe, ULVAC-PHI) was used to determine the valent state of the AuNPs before and after reaction. Raman measurement was accomplished with a confocal Raman system LabRAM HR800 (Horiba Jobin Yvon, 5



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France). The acquisition and analysis of Raman data were performed by using the LabSpec5 software. To facilitate the hydride generation and separation, a programmable intermittent reactor and gas–liquid separator were employed. A commercial two-channel hydride generation nondispersive atomic fluorescence spectrometer (AFS-9600, Beijing Haiguang instrumental Co., Beijing, China) was used for the detection of the intensity of the selenium atomic fluorescence.

Synthesis of AuNPs. AuNPs were prepared by citrate reduction of HAuCl4.35 In brief, 4 mL HAuCl4 (25 mM) was mixed with 96 mL deionized water in a 250 mL round-bottom flask equipped with a reflux condenser. When the solution was heated to a boil, a total of 10 mL of 38.8 mM sodium citrate was then added rapidly to the solution, and the mixture was heated under reflux for another 10 min, during which time its color changed from pale yellow to wine red. The size of the citrate-capped AuNPs verified by TEM analysis (JEM-100CX) was about 15 nm. The particle concentration of the AuNPs (ca. 15 nM) was determined according to Beer’s Law by using UV−vis spectroscopy.36

Procedures for Se(IV) Detection. 10 mL samples or selenium standard solutions of Se(IV) containing 15% (v/v) HCl were merged with 5% (m/v) KBH4 dissolved in 0.5% (m/v) KOH and the mixture was propel by a peristaltic pump. H2Se was produced as the result of the reduction of Se(IV) with KBH4. Meanwhile, a flow rate of 400 ml/min of Ar (carrier gas) flush the mixture to gas-liquid separation. The generated H2Se was then separated from the liquid and bumped into aliquots (600 µL) of 6


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Tris−HCl solutions (10 mM, pH 8.0) containing 1.7 nM AuNPs. Finally, the solutions were incubated at ambient temperature for 10 min and corresponding UV–visible spectra were measured.

Sample preparation. A microwave digestion method was used to digest DORM-4 and selenium rich egg. An aliquot of 0.2 g of DORM-4 and 2 g selenium rich egg were accurately weighed into pre-cleaned Teflon vessels and then 8 mL of HNO3 and 2 mL of H2O2 were added. The sample blanks were processed along with the samples. The sealed vessels were heated in a microwave oven (Master 40, Shanghai Sineo Microwave Chemistry Technology Co., China) operated under the following conditions: 15 min at 130 °C and 1000 W, 10 min at 150 °C and 1000 W, and 25 min at 180 °C and 1000 W. After cooling, the digested products were transferred into Teflon crucibles and heated to almost dry on an electric hot plate at 200 °C. The residues were transferred to pre-cleaned 10 mL volumetric flasks with 10% (v/v) HCl and added to the mark. Simulated water, tap water, and seawater samples added in hydrochloric acid were directly reacted with KBH4 without microwave digestion.

RESULTS AND DISCUSSIONS Design

and validation of

the

proposed

vapor generation-AuNP-based

colorimetric assay. Scheme 1 illustrates the working principle of the proposed colorimetric assay. We chose hydride generation, one of the most commonly used CVG methods, for converting target analytes to their corresponding volatile hydrides. Selenium is taken as the model analyte here since it can be easily converted to 7



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hydrogen selenide (H2Se).37 The importance of selenium has been recognized as essential nutrient at low concentrations, but it can be toxic at high levels. Besides, selenium is an important part of antioxidant enzymes that protect cells against adverse effects of free radicals produced during normal oxygen metabolism. After hydride generation, gaseous H2Se was isolated from the sample matrix via gas-liquid separation and then delivered to the solution of AuNPs. In the weak alkaline solution of AuNPs, H2Se was re-dissolved and quickly oxidized to elemental selenium by dissolved oxygen. Subsequent adsorption of Se0 onto the surface of AuNPs caused partial citrate detaching, leading to the aggregation of AuNPs along with a visible red-to-blue color change.

Scheme 1 Schematic illustration of the working principle of this colorimetric assay. Samples or standard solutions of Se(IV) (10mL, containing 15% HCl) were merged with KBH4 (5%, m/v) for hydride generation. The mixture was flushed into the gas-liquid separator by Ar carrier gas and then delivered into the AuNPs solution for inducing color change of AuNPs.

The feasibility of the proposed assay for Se(IV) detection was first confirmed via the H2Se-induced color change of AuNPs. As shown in Figure 1, unmodified citrate-capped AuNPs (15 nm) in aqueous solution is well dispersed owing to the

8


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electrostatic repulsion of the negatively charged citrate on the surfaces of AuNPs, appearing red with a SPR band at about 520 nm. In the presence of gaseous H2Se (hydride generation from 4 µM Se(IV)), the SPR band of AuNPs 520 nm decreased and displayed a red-shift. Meanwhile, the color of AuNPs solution changed from red to blue. However, direct mixing of Se(IV) with AuNPs did not cause any color change. TEM investigations indicated that aggregation of AuNPs occurred in the presence of H2Se. To our knowledge, this is the first report using AuNPs for colorimetric detection of Se(IV).

Figure 1 UV–vis spectra and TEM images of the AuNPs solution in the absence and presence of H2Se (generated from 4 µM Se(IV)).

The performance of the proposed colorimetric assay against sample matrix with high salinity. Next, the performance of the proposed assay against sample matrix with high salinity was investigated. It is well-known that unmodified citrate-capped AuNPs tend to aggregate in NaCl solution because of the breaking of the electrostatic repulsion. We found that 5 mM NaCl could induce SPR band change of AuNPs 9



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(Figure S1). The average amounts of salinity in biological samples (serum, urine, and sweat) is higher than 100 mM. Therefore, when unmodified AuNPs meet typical biological sample matrice, interferences will be unvoidable. However, after hydride generation for separation of sample matrix, the proposed assay can tolerate high level of ionic strength. As shown in Figure 2, even when the salt concentration was 5% (m/v), no impact of the colorimetric response of the AuNPs or interference to Se(IV) detection was observed. Such salt concentration is at least 200-fold higher than the minimal concentration that can cause SPR change of AuNPs, indicating that this colorimetric system was free from sample salinity.

Figure 2 The performance of the proposed assay against sample matrix with high salinity. Se(IV) concentration: 2 µM; salts concentration (m/v): 5% KNO3, CaCl2, NaCl, and MgSO4; 2% AlCl3 and FeCl3.

The aggregation mechanism of AuNPs in the presence of H2Se. The aggregation mechanism of AuNPs in the presence of H2Se was studied. In the TEM image of AuNPs after reaction with H2Se, a membrane-like coating on the surface of AuNPs was observed (Figure 3A). The corresponding EDX investigation showed that the new coating consisted majorly selenium (area 2 of Figure 3B). On the surface of AuNPs,

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selenium was also discovered (area 1 of Figure 3B). From the XPS spectra, the newly formed selenium species was identified as elemental selenium (Se0, Figure 3C). A strong peak appeared at 55.0 eV, which is corresponding to the electron binding energy of Se0 (3d). No further peaks for Se-2 (53 eV), Se+4 (59 eV) and Se+6 (61 eV) were observed.38 The formation of Se0 was further confirmed by the Raman spectroscopy (Figure 3D). An intensive peak was observed at 252 cm−1, which was attributed to monoclinic selenium. A weak peak at 233.7 cm−1, belonging to t-Se, was partially overlapped by the previous strong peak.39 Therefore, after delievering of gaseous H2Se into the solution of AuNPs, Se0 was formed on the surface of aggregated AuNPs .

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Figure 3 Identification of the Se0 from the AuNPs solution after the reaction with H2Se: (A) TEM image; (B) EDX spectra corresponding to area 1 and area 2 in the TEM image; (C) XPS patterns of AuNPs in the absence and presence of H2Se; and (D) Raman spectra of AuNPs in the absence and presence of H2Se (corresponding to 100 µM Se).

Obviously, the formed Se0 should originate from the oxidation of selenium hydride. When delivering H2Se into the pH 8.0 aqueous solutions that were free of AuNPs, oxidation of H2Se was observed: the color of the solution changed from colorless to pale yellow (25 µM Se(IV)) and then to orange-red (60 µM Se(IV), the same as that of the elemental selenium, Figure S2). However, if the solution was deoxygenated with N2, the oxidation would be largely alleviated (Figure S3). 12


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Therefore, the generation of elemental selenium should be ascribed to the following reaction: 2H2Se + dissolved O2 →2Se0 + 2H2O

(1)

Introduction of H2Se into the solution of AuNPs could induce a sensitive colorimetric response (Figure 1), but the response sensitivity would be reduced after deoxygenation of the AuNPs solution (Figure S4). Therefore, dissolved oxygen also played an important role in aggregation of AuNPs. It is well known that O2 can be tightly adsorbed onto the surface of AuNPs.40 Therefore, two kinds of O2, namely free oxygen and adsorbed oxygen, exist in the solution of AuNPs. Deoxygenation could only remove free oxygen away, but not adsorbed oxygen. Accordingly, deoxygenation of the AuNPs solution still yilded appreciable colorimetric response in the presence of H2Se (Figure S4), due to oxidation of H2Se by adsorbed oxygen. From the TEM image (Figure 3A), it can be seen that the formed Se0 was coated onto the surface of AuNPs and no Se0 was self-existent. On the basis of the above evidences, we proposed the mechanism of H2Se-induced aggregation of AuNPs as follows (Scheme 1). First, H2Se was oxidized to Se0 by the dissolved oxygen in the solution of AuNPs. Due to the thiophilic property of Au, the formed Se0 was strongly adsorbed by AuNPs, which caused partial dissociation of citrates from the surface of the AuNPs. As a result, the electrostatic repulsion between AuNPs was reduced, leading to the aggregation of the AuNPs. Similar aggregation was also observed for unmodified AgNPs (Figure S5). The adsorption of Se0 in fact competes with the dissociation of citrate on the surface of 13



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AuNPs. Therefore, the color change rate of AuNPs was slower than sole H2Se upon oxidation (Figure S6). Analytical performance of this assay for Se(IV) detection. To maximize the sensitivity of the proposed colorimetric assay, a series of experimental conditions were optimized, including the conditions for hydride generation and the amount of AuNPs (Figure S7-S11). Under the optimized conditions, the SPR of AuNPs red-shited continueously (Figure 4A) and the color of AuNPs changed from red to blue (Figure 4B). A linear correlation was obtained between the A600/A520 values and the concentration of Se(IV) in the range of 0.4-4 µM (inset in Figure 4B). The detection limit was 0.05 µM (S/N = 3) with UV-vis detection. With naked eye, the color change by 1 µM Se(IV) could be differentiated, indicating the high sensitivity of this colorimetric assay.

Figure 4 Analytical performance of the proposed colorimetric assay for Se(IV) detection: (A) UV–vis spectra of AuNPs in the presence of various amounts of Se(IV) after hydride 14


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generation; (B) plots of absorbance ratio (A600/A520) versus the concentration of Se(IV); and (C) selectivity of this assay for Se(IV) detection against other transition metal ions and anions. The concentrations in (C) are as follows: Se(IV), 2 µM; Cu2+, As(III), Sn4+, Sb3+, Bi3+, Cr3+, Mn2+, Zn2+, Pb2+,Co2+, Ni2+, CO32-, SO32-, NO2-, and IO3-, 100 µM; and Ag+, Cd2+, and Hg2+, 20 µM.

The selectivity of the proposed assay was tested against a series of transition metal ions and several anions. As shown in Figure 4C, 100 µM of Cu2+, As(III), Sn4+, Sb4+, Bi3+, Cr3+, Mn2+, Zn2+, Pb2+, Co2+, Ni2+, CO32-, SO32-, NO2-, IO3- and 20 µM Ag+, Cd2+, Hg2+ didn’t show appreciable SPR change as 2 µM of Se(IV), indicating excellent selectivity of this assay. Possibly, some of the above metal ions and anions cannot generate volatile hydrides, therefore no response. For those metal ions capable of hydride generation, their interaction with AuNPs is much weaker than that between gold and Se0, also weaker than the electrostatic interaction between AuNPs and citrate. Accordingly, no perturbation of the aggregation of AuNPs occurred. Moreover, some of the volatile species are difficult to be dissolved in the solution, for example, Hg0. Although Hg0 can react with gold to form amalgam, the undissolved nature of Hg0 prevents it from alloying with gold, and therefore also no appreciable signal. The thiophilic property of Au should cover not only Se, but also S and Te since they belong to the same group. Therefore, the colorimetric responses of S and Te were compard with that of Se(IV). Direct delivering of gaseous H2S into the AuNPs solution did not cause any color change (Figure S12A), probably because H2S is difficult to be oxidized to elemental sulfur under the current experimental condition. When the sulfide (S2-, 100 µM) co-existed with 2 µM Se(IV), no significant 15



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interference was observed. For Te(IV), similar colorimetric response as Se(IV) was observed (Figure S12A), but the assay sensitivity was largely lower than that of Se(IV). It must be acknowledged that there is co-existing interference of Te(IV) to Se(IV) detection by this assay, as indicated in Figure S12B. Fourtunately, the abundance of tellurium is also significantly lower than that of selenium in typical samples. The potential applications of the proposed assay were verified for detection of selenium in biological and environmental samples, including fish protein certified reference material (Dorm-4), selenium-rich egg, simulated water reference material (GBW (E) 080395), tap water samples and high salinity seawater samples. As can be seen from Table 1, the analytical results for selenium are in good agreement with either certified values or those determined by hydride generation atomic fluorescence spectrometry (HG-AFS, a mature instrumental method for Se detection).

Table 1. Analytical results for the detection of Se(IV) in real sample matrices. Sample

Certified value

This method

Recovery (%)

Dorm-4

3.50 ± 0.34 µg/g

3.20 ± 0.22 µg/g

-

GBW (E) 080395

1.0 ± 0.02 mg/L

0.94 ± 0.17 mg/L

-

Se-rich egg

265.0 ± 9.0 µg/ga

273.4 ± 9.2 µg/g

-

Tap water

1.0 µMb

1.08 ± 0.02

108

Seawater-1

2.0 µMc

2.18 ± 0.13

109

Seawater-2

2.0 µMc

2.06 ± 0.08

103

16


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Seawater-2

2.0 µMc

2.06 ± 0.06

103

a: by HG-AFS b: no Se(IV) was detected, 1.0 µM Se(IV) was spiked. c: no Se(IV) was detected, 2.0 µM Se(IV) was spiked.

CONCLUSION In summary, a new type of colorimetric assay for Se(IV) detection was proposed based on the interaction of H2Se and AuNPs. With the aid of hydride generation, target analyte selenium was first converted to gaesous H2Se and then delivered into the solution of AuNPs to induce the aggreation of AuNPs. Due to the separation of sample matrix, such colorimetric assay could be explored for Se(IV) detection in samples with high salinity. However, potential interference from Te(IV) for Se(IV) detection exists, which limits the usefulness of this assay for samples containing relatively high levels of tellurium (such as ore samples). Considering the popularity of CVG methods and various volatile organic compounds, this new idea of gaseous sampling will be promising in developing of new colorimetric assays for biological and environmental samples.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21175093 and 21522505) and the Youth Science Foundation of Sichuan Province (Grant 2016JQ0019). 17



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Supporting Information Available: Additional information as noted in text, including The influence of salinity (NaCl concentration) to unmodified AuNPs, confirmation of the oxidation of H2Se by dissolved oxygen, optimization of the hydride generation parameters, and Investigation of the interference of S2- and Te(IV) to Se(IV) detection. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interest.

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For TOC only

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Gold Nanoparticle-Based Colorimetric Assay for Selenium Detection

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