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Selective Colorimetric Detection of Hydrogen Sulfide Based on Primary Amine-Active Ester Crosslinking of Gold Nanoparticles Zhiqin Yuan, Fengniu Lu, Meihua Peng, Chia-Wei Wang, Yu-Ting Tseng, Yi Du, Na Cai, Chia-Wen Lien, Huan-Tsung Chang, Yan He, and Edward S. Yeung Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b01302 • Publication Date (Web): 17 Jun 2015 Downloaded from http://pubs.acs.org on June 22, 2015

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Analytical Chemistry

Selective Colorimetric Detection of Hydrogen Sulfide Based on Primary AmineAmine-Active Ester Crosslinking of Gold Nanoparticles ‖

Zhiqin Yuan,†,‡ Fengniu Lu,§ Meihua Peng,₸ Chia-Wei Wang,‡ Yu-Ting Tseng,‡ Yi Du, Na Cai,† ChiaWen Lien,‡ Huan-Tsung Chang,*,‡ Yan He*,† and Edward S. Yeung† †

. College of Chemistry and Chemical Engineering, College of Biology, State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, P. R. China. ‡ . Department of Chemistry, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan. § . International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan. ₸ . Metabolic Syndrome Research Center, The Second Xiangya Hospital of Central South University, Changsha 410011, P. R. China. ǁ . Inspection and Testing Center for Agro-product Safety and Environment Quality, Institute of Applied Ecology Chinese Academy of Sciences (IAE CAS), 72 Wenhua Road, Shenyang 110016, P. R. China. Email: [email protected]. Phone: +86 731 88823074. Fax: +86 731 88821904. Email: [email protected]. Phone & Fax: +011 886 02 33661171. ABSTRACT: Hydrogen sulfide (H2S) is a highly toxic environmental pollutant and also an important gaseous transmitter, thus selective detection of H2S is very important; and visual detection of it with naked eye is preferred in practical applications. In this study, thiolated azido derivates and active esters functionalized gold nanoparticles (AE-AuNPs) based nanosensors have been successfully prepared for H2S perception. The sensing principle consists of two steps: first, H2S reduces azide group to primary amine; second, crosslinking reaction between primary amine and active ester induces the aggregation of AuNPs. The AE-AuNPs based nanosensors show high selectivity towards H2S over other anions and thiols due to the specific azide-H2S chemistry. Under optimal conditions, 0.2 µM H2S is detectable using UV-vis spectrophotometer and 4 µM H2S can be easily detected by the naked eye. In addition, the practical application of the designed nanosensors was evaluated with lake water samples.

H

ydrogen sulfide (H2S) with a characteristic smell of rotten egg is a flammable and water-soluble gas, which received attention as an environmental pollutant and toxic gas for many decades. It is also produced in humans; the production of H2S from cysteine is catalyzed primarily by two enzymes, cystathionine γ-lyase and cystathionine β-synthase.1 Recent studies have recognized H2S as the third gaseous signal transmitter besides carbon monoxide and nitric oxide in many biological processes.2 H2S is reportedly an ATP-sensitive potassium channel opener and affects the cardiovascular system, which can relax vascular smooth muscle and reduce blood pressure.3 Diseases such as Alzheimer’s disease and Down’s syndrome are also correlated to the abnormity of H2S level.4, 5 Therefore, it is very important to develop selective and sensitive methods for H2S detection. Towards this goal, electrochemistry, gas chromatography, inductively coupled plasma-atomic emission, UV-visible absorption spectrometry and fluorescence spectrometry based H2S detection methods have been developed.6-12 Since fluorescence-based detection methods exhibit high sensitivity and

low background, much effort has been dedicated to the synthesis of organic fluorophore based H2S chemosensors. These organic probes generally recognize H2S through three major chemical reactions, including H2S involved nucleophilic addition, copper-sulfide precipitation and H2S mediated azide reduction.13-16 For example, selective H2S detection was achieved using α, β-unsaturated acrylate methyl ester and aldehyde contained fluorophore through the combination of aldol condensation and Michael addition reactions.13 Both monobromobimane and dibromobimane compounds were developed for selective H2S detection based on the nucleophilic properties of H2S.17-19 And the sensitivity could be greatly enhanced when combing with reverse phase-high performance liquid chromatography technique. Due to paramagnetic properties of copper ions (Cu2+), the formation of fluorophoreCu2+ complexes would lead to the fluorescence quenching. Thereby, the precipitation reaction between H2S and Cu2+ can be employed as a turn-on strategy for H2S detection.14 Moreover, azide-caged rhodamine has also been synthesized for turnon H2S detection because H2S can specifically reduce azide group to primary amine and turn on the fluorescence of organ-

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ic fluorophores.15 Among these reactions, H2S mediated azide reduction strategy shows unparalleled extendability and versatility, making azido derivates popular in H2S detection. On the basis of this special chemical reaction, many organic molecules have been developed as fluorometric H2S probes.20-22 Azide-caged fluorophores (e.g., sulfonamide and coumarin) have been developed as turn-on fluorescent H2S sensors.23-25 To diminish the influence of probe concentration, azide group functionalized cyanine dye and cresyl violet have been synthesized as ratiometric fluorescent H2S sensors.26, 27 These organic probes provide admirable selectivity towards H2S over other anions and thiols, but their syntheses are difficult and/or reactants and reagents are expensive. Thus, it is preferable to utilize inorganic nanoprobes that can be readily produced at low cost for routine large scale H2S level measurements. Recently, inorganic nanomaterials (semiconductor quantum dots, gold nanocrystals and carbon nanomaterials, etc.) based probes have been widely used for analyte detection.28-31 In particular, gold nanoparticles (AuNPs) with localized surface plasmon resonance properties have received much attention in chemo/biosensing.32, 33 Aggregation of the AuNPs will result in color changes of the colloidal solution from red to blue, providing a gentle platform for absorption-based colorimetric detection with AuNPs as signal reporters, and the AuNPs aggregation could be induced by inter-AuNPs crosslinking. The color change can be easily observed by naked eyes, and do not require complicated instruments. Compared to other sensing strategies (e.g., fluorimetry), AuNPs based colorimetric assays show comparable sensitivity because of the high extinction coefficient of AuNPs. On the basis of these advantages, many AuNPs based nanosensors have been developed for colorimetric sensing of DNA, protein and metal ion, etc.34-37 It was reported that gold has high affinity to sulfide with a stability constant of 2 × 1036. Due to such strong Au-S interaction, several AuNPs based nanosensors have been utilized for colorimetric H2S detection.38-41 For example, glutathione capped AuNPs can serve as a colorimetric H2S sensor through a ligand exchange reaction induced aggregation.38 Glutathione capped AuNPs show high stability under high-ionic-strength solution, adsorption of H2S/HS¯ on to the AuNPs surface results in the ligand exchange, which leads to stability decrease and AuNPs aggregation. It was found that bare AuNPs possess intrinsic peroxidase-like activity, which is similar to the natural peroxidase and can catalyze the oxidation of 3,3’,5,5’tetramethylbenzidine substrate in the presence of hydrogen peroxide and subsequently generate blue products.40 After surface H2S adsorption, the activity is inhibited because of the shielding effect, and the decreased activity is proportional to the concentration of H2S. Although these AuNPs nanosensors show good sensitivity and even acceptable selectivity over other anions, some thiols still cause interference through strong sulfydryl-Au bonding. Hence, developing highly specific H2S sensors based on AuNPs is still appealing. In this study, we presented a colorimetric system for selective H2S detection using azido derivate and active ester cofunctionalized AuNPs (AE-AuNPs). The sensing principle is based on the fact that H2S triggered reduction of azide group and subsequently enhanced crosslinking efficiency among adjacent AE-AuNPs. This strategy exhibited high selectivity towards H2S over other anions and thiols due to the specificity of azide-H2S chemistry. To the best of our knowledge, this is the first work showing AuNP based H2S nanosensors through

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this specific chemical reaction, which can minimize interference from anions and thiols. The effects of parameters on the sensitivity of AE-AuNP for H2S were investigated. The practicality of this system was further validated by detecting H2S in lake water samples.

EXPERIMENTAL SECTION Chemicals. Sodium citrate dehydrate, chloroauric acid (HAuCl4), 3,3’-dithiodipropionic acid di(Nhydroxysuccinimide ester) (DSP), 4-azidoaniline hydrochloride (4-AA), glutathione (GSH), 3-mercapto propionic acid (MPA), thioglycolic acid (TGA), dithiothreitol (DTT), tiopronin (TPN) and mercaptosuccinic acid (MSA) were purchased from Sigma Aldrich (Milwaukee, USA). Sodium benzoate (C6H5COONa), sodium phosphate (Na3PO4) and phosphoric acid (H3PO4), nitric acid (HNO3), hydrochloric acid (HCl) and other chemicals were obtained from Alfa Aesar (Heysham, U.K.). All chemicals were used without further purification. Ultrapure water was obtained from a Millipore system (Billerica, USA). Aqueous solution of F-, Br-, OAc-, EDTA2-, N3-, NO2-, NO3-, HCO3-, SO32- and SO42- were prepared from NaF, NaBr, NaOAc, EDTA-2Na, NaN3, NaNO2, NaNO3, NaHCO3, NaSO3 and Na2SO4, respectively. Phosphate buffer solutions (0.1 M) with pH values ranging from 5.0 to 11.0 were prepared according to a handbook (Handbook of biochemistry: selected data for molecular biology) and the pH values were measured with a benchtop Orion™ plus pH meter (Thermo-Fisher, USA). Synthesis of C-AuNPs. Approximately 13 nm diameter citrate stabilized AuNPs (C-AuNPs) were synthesized by a classical hydrothermal reduction method with minor modification. In a typical assay, an aqueous solution of HAuCl4 (1 mM, 250 mL) was brought to a refluxing solution quickly with rapid stirring, and then 25 mL of trisodium citrate (38.8 mM) solution was added rapidly. The resulting solution was boiled for an additional 15 min. During the reaction time, the color of the solution changed from yellow to blue-black and finally to deep red. The solution was kept in an ice bath for 15 min to terminate the reaction, and then filtered through a 0.22 µm syringe filter before stored in a refrigerator at 4 ℃ for further use. The concentration of the prepared C-AuNPs was about 15 nM according to Beer’s law using an extinction coefficient of 1.86×108 M−1·cm−1 at 520 nm for the 13 nm C-AuNPs.42 Preparation of thiolated azido derivatives and active esters co-functionalized AuNPs (AE-AuNPs). Thiolated azido derivates (TAD) were firstly synthesized by mixing 4-AA and DSP overnight under room temperature. AE-AuNPs were prepared based on the thiol self-assembly strategy. Briefly, 5 mL AuNPs solutions were added into 5 mL ultrapure water, and then 25 µL of DSP (0.1 mM) and 25 µL TAD (0.1 mM) was introduced into the solution. After stirring for 30 min, 225 µL MSA (0.1 mM) was introduced into the solution and stirring for another 4 hrs. The resulted AE-AuNPs solution (~7.2 nM) was directly used for H2S detection without further purification. This protocol was used to prepare other kinds of AEAuNPs for spacer tests by replacing MSA with TPN, MPA or TGA. Characterization. The UV absorption spectra of the AuNPs were obtained using a Cintra 10e double-beam UV-vis spectrophotometer (GBC, Australia). Transmission electron microscopy (TEM) images were collected with a H7100 transmission electron microscope (Hitachi, Japan). Mass spectra of

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the selectivity of the AE-AuNPs: F-, Cl-, Br-, I-, OAc-, EDTA2-, N3-, NO3-, SO32-, SO42-, GSH, TPN, TGA and MPA. Real sample analysis. Water samples obtained from a local lake in campus were firstly centrifuged (12,000 rpm, 30mins) and then filtered through a 0.2 µm membrane to remove large suspended particles. The H2S detection procedure is the same as sensitivity test, ten microliter solutions with different concentrations of Na2S were added into 1 mL of lake water diluted AE-AuNPs solution contains NaH2PO4–Na2HPO4–NaCl buffer at pH 9.0, in which the final concentrations of total phosphate and Cl- were 10 mM and 20 mM, respectively. The absorption spectra were collected using a UV-vis spectrophotometer. Before test, a concentration of 1 mM EDTA2- was added into the water samples.

thiolates were recorded in the linear positive-ion mode using a Microflex MALDI-TOF mass spectrometer (Bruker Daltonics, Germany) and a microOTOF-Q II electrospray ionization time-of-flight mass spectrometer (Bruker Daltonics, Germany). Infrared spectra (FTIR) were recorded by using a FT/IR410 Fourier transform infrared spectrophotometer (JASCO, Japan). Sensitivity and selectivity measurement. A high concentration stock solution of Na2S (0.1 M) was prepared and used to prepare standard solutions through a serial dilution. For H2S detection, ten microliter solutions with different concentrations (from 20 µM to 3 mM) of Na2S were added into 1 mL of NaH2PO4–Na2HPO4–NaCl buffer diluted AE-AuNPs (~6.5 nM, 900 µL AE-AuNPs solution mix with 100 µL buffer) solution at pH 9.0, in which the final concentrations of total phosphate and Cl- were 10 mM and 20 mM, respectively. The absorption spectra were collected using a UV-vis spectrophotometer. The following anions and thiols were used to evaluate

RESULTS AND DISCUSSION Principle of H2S detection. The AE-AuNPs were prepared by modifying citrate capped AuNPs (C-AuNPs) with thiolate

Scheme 1. Schematic illustration of AE-AuNPs-based nanoprobes for the detection of H2S.

a

b

427.02

465.08

O

S S

N

N H

O

Intensity

Intensity

c

N

O

H N

S

O

200

300

400

500

m/z 219.05

S

N H

O

N3

N3

O

H N O

O

200

d

300

400

307.05

200

N H

H N

O HS

N H

SH O

Intensity

HS

500

m/z

NH2

O

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

300

400

500

200

m/z

300

400

500

m/z

Figure 1. ESI-MS spectra of DSP (a), TAD (b), TA(c) and p-DC (d). The calculated mass peaks using isotope pattern software of these four compounds with Na+ are 427.024, 465.088, 219.056, and 305.053, respectively. Inset images are the corresponding chemical structures.

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ligands. The small red-shifted maximum absorption wavelength (~2 nm) suggests the successful surface modification with thiolate ligands (Figure S1).43 The existence of surface thiolate ligands was further supported by mass spectra spectra of C-AuNPs and AE-AuNPs (see discussion below). Under our experimental conditions (pH values 5.0 to 11.0), H2S and HS¯ are the main species (pK1 ≈ 6.9) and S2¯percentage (pK2 > 14) is very low. The mechanism for H2S detection consists of two parts as shown in Scheme 1. The azide group of TAD on the surfaces of AuNPs was reduced specifically by H2S to form primary amine, which has been widely used for designing H2S responsive organic probes.44, 45 The primary amine then reacts with N-hydroxysuccinimide ester to form stable acylamide compounds, resulting in the aggregation of AuNPs. To verify the proposed mechanism, chemical reactions of thiolates in solution were first characterized with ESI mass spectrometer. As can be seen in Figure 1a, a peak at m/z 427.02 is assigned for [DSP + Na]+. After adding 4-azidoaniline (4-AA), a peak at m/z 465.05 assigned for [TAD + Na]+ is dominant (Figure 1b), revealing the formation of TAD from DSP. A small peak at m/z 446.05 reveled that an incomplete reaction between DSP and 4-AA occurred. DSP has two active sites and thus it can react with 4-AA in fashions of 1:2 and 1:1 (incomplete reaction). Such an incomplete reaction can lead to competitive reactions between as-resulted primary amine and residual active ester with H2S, resulting in disordered signals and reducing sensitivity and linearity of the assay for H2S. To overcome the associated problems, excess dithiothreitol (DTT) was added to break down the disulfide-bond of TAD and DSP. DTT is an unusually strong reducing agent; it forms a stable six-membered ring with an internal disulfide bond once oxidized.46 Notice that DTT is a strong reducing agent, it may reduce azide group directly. Thus, DTT should be introduced into the solution after H2S reduction. With addition of H2S only, a peak at m/z 391.12 belongs to reduced TAD can be seen (Figure S2). While a new peak located at m/z 219.05 assigned for resulted thiolated aniline (TA) [TA + Na]+ is dominant (Figure 1c), revealing a successful reaction between the azide group and H2S to form a primary amine. The formation of p-diacylamide compounds (p-DC) upon the addition of active ester in the presence of excess of DTT that minimized the formation of huge polymer is supported by the detection of a peak at m/z 307.05 that is assigned for [p-DC + Na]+ (Figure 1d). To confirm the proposed reaction on the AuNP surface, mass spectra of thiolated probes functionalized AuNPs were investigated with MALDI-TOF mass spectrometer under different reaction stages. As shown in Figure S3, a peak at m/z 150.9 assigned for [MSA + H]+ appeared in AE-AuNPs. Adsorption of disulfide onto the metal surface results in the formation of Au-S bond and the break of RS-SR bond,47 thus only half of DSP or TAD will remain on the AuNP surface. The peak at m/z 226.8 assigned for [1/2 DSP + Na]+ and a peak at m/z 245.3 assigned for [1/2 TAD + Na]+, suggesting the existence of active ester and azide group on AuNP surface. After adding H2S, a peak at m/z 284.9 assigned for [p-DC + H]+ appeared, suggesting the successful azide group reduction and efficient crosslinking reaction. Fourier transform infrared spectra of Au NPs at different stages also supported the proposed mechanism. It was seen that a small peak around 2100 cm-1 belongs to aromatic azide group appeared in AE-AuNPs, suggesting the successful modification of TAD on AuNPs (Figure S4). After the addition of sodium sulfide, the peak around 2100 cm-1 disappeared, indicating the reduction of

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Figure 2. (a) Photographs of AE-AuNPs solution without (left) and with (right) addition of H2S. (b and c) Typical TEM images of AE-Au NPs correspond to the colored solution, scale bar = 100 nm.

azide group. Taken these data above together, the sensing mechanism is attributed to H2S induced azide group reduction of TAD and subsequently crosslinking reaction resulted AuNPs aggregation. The solution color of AE-AuNPs remained red and turned to purple without and with addition of sodium sulfide, respectively, indicating the formation of AuNPs aggregates (Figure 2a). In other words, the monodispersed AE-AuNPs and AuNPs aggregates have maximum absorption wavelengths around 520 and 720 nm, respectively. Further evidence for H2S induced aggregation of AE-AuNPs was supported by transmission electron microscopy (TEM) images (Figure 2b and 2c). The average diameter of dispersed AE-AuNPs was calculated to be 13.3 ± 1.6 nm (100 counts). Huge AE-AuNPs aggregates (Figure 2c) reveal efficient primary amine-active ester crosslinking induced aggregation of AE-AuNPs. Optimization of H2S detection. Since the electrostatic repulsion plays an important role on the dispersion of AuNPs, parameters (e.g., solution pH, azide/ester molar ratio and surface spacer, etc.) that affect the electrostatic repulsion may have effects on the aggregation behavior. For instance, the aggregation can be enhanced upon adding high concentration of salt because the electrostatic charges on AuNPs are screened and do not provide sufficient repulsion to keep the AuNPs colloidally stable.38, 48 And previous reports also suggest both azide group reduction and crosslinking reaction are pH sensitive.27, 49 Thus, some parameters may affect the aggregation behavior of AE-AuNPs upon adding H2S. In this study, a systematic study was performed to optimize these experimental conditions by monitoring the ratio of absorbance values at 720 and 520 nm (A720/A520). Greater extent of AuNPs aggregates has a higher absorbance ratio. The effect of solution pH on the degree of aggregation of AE-AuNPs was firstly investigated in the presence of H2S (Figure S5a). The pH value of the solution is a key factor that affects the assay through varying the efficiency of the azide group reduction and the primary amine-active ester crosslinking, which are both pH dependent. In basic media (pH > 7.9) HS¯, which is the major sulfide species (> 90%), enhanced the azide group reduction efficiency.27, 50 In addition, the primary amine-active ester crosslinking reaction is favoured under basic media. The degree of aggregation of the AE-AuNPs in the presence of H2S reached maximum value at pH 9.0. Therefore, all subsequent sensing experiments were conducted at pH 9.0. Next, we investigated the optimum molar ratio of azide/ester in the assay. As seen from Figure S5b, the molar ratio of azide/ester affected the degree of aggregation of AE-AuNPs upon adding H2S, showing that the degree of aggregation reached a plateau at azide/ester molar ratio of 1:1. At lower

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Figure 3. (a) Absorption spectra of AE-AuNPs solution with addition of various concentrations of H2S. (b) Plots of absorbance ratio (A720/A520) versus the concentration of H2S. (c) Corresponding photographs of AE-AuNPs solution upon adding various concentrations of H2S (From 1- 17, 0, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 20, and 30 µM).

of H2S (8 µM) as shown in Figure S5f in the Supporting Information. Therefore, incubation time of 15 min was selected. Sensitivity and specificity of the H2S sensing system. Upon increasing H2S concentration, the absorbance at 520 nm decreased, while that at 720 nm increased (Figure 3a). The absorbance ratio (A720/A520) increased gradually upon increasing H2S concentration from 0 to 30 µM (Figure 3b), and a typical S-shaped function curve was observed. The absorbance ratio A720/A520 displays a good linear relationship (R2 =0.983) versus H2S concentration ranging from 3 to 10 µM (Figure 3b inset) and was easily described by the linear equation: A720/A520 = -0.34 + K[Q], where the slope K acts as the aggregation constant and [Q] is the concentration of H2S. K was calculated to be 1.57 × 105 M¯1 by linear regression of the plot. The quantized detection limit was 0.2 µM with UV-vis spec-

1.2 1.0 0.8 0.6 0.4

TGA

TPN

GSH MPA

HS-

BenzoicSO32-

SO42-

NO2NO3-

FN3-

Br-

0.0

HCO3-

0.2

OAc-

and higher azide/ester molar ratios, low crosslinking efficiency and small degree of aggregation occurred, respectively. In addition to solution pH and azide/ester molar ratio, we expected the spacer of thiols plays some roles in determining the degree of aggregation of AE-AuNPs, mainly because it affects the crosslinking reaction on the surfaces of AuNPs. It was reported that thiol spacer on the DNA functionalized gold surface can control the DNA density and conformation, thereby affecting its hybridization efficiency.51 In addition, the primary amine may react with active ester spontaneously on the same AE-AuNP, leading to reduced aggregation of AE-AuNPs. To obtain high degree of aggregation of AE-AuNPs (sensitivity), four thiol spacers were tested. At high concentration (> 10 µM) of H2S, Tiopronin (TPN) and mercaptosuccinic acid (MSA) enhanced the degree of aggregation, while 3-mercapto propionic acid (MPA) and thioglycolic acid (TGA) inhibited the degree of aggregation (Figure S5c). We suspected that the branched chains of TPN and MSA near their sulfydryl group may inhibit overloading of thiolated azido derivate and active ester groups due to the steric effect, subsequently minimize the crosslinking reaction occurred on the surfaces of single AEAuNPs. Having a small steric effect, MSA (a shorter carbon chain) over TPN allowed greater degree of aggregation (Figure S5d). The optimal conditions with respect to sensitivity were solution pH at 9.0, azide/ester molar ratio of 1:1, and MSA as the surface spacer. Salt can reduce the electrostatic repulsion force between AuNPs and thus plays an important role in determining the sensitivity.52 Figure S5e shows that the AE-AuNPs were stable in low concentration of NaCl (≦30 mM), but became unstable at NaCl concentration higher than 40 mM. Since very low NaCl (e.g., 1 mM) has no significant assistance to promote the aggregation of AuNPs, relatively high concentration of NaCl is needed. To avoid the self-aggregation, 20 mM NaCl was selected. Under this condition, the absorbance ratio (A720/A520) of AE-AuNPs reached maximum within 15 min after addition

∆ ( A720 / A520)

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Analytical Chemistry

Figure 4. Increasement of absorbance ratio (∆(A720/A520)) of AEAuNPs upon adding various anions or thiolates. The concentrations are 10 µM for H2S and 1 mM for other anions and thiolates.

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Table 1. Comparison of this work with some established H2S detection methods. #

Probe

Strategy

Detection limit (µM)

Linear range (µM)

1

8-aminoquinoline-Cu2+ complex

fluorimetry

0.28

0.5-8

interference from thiol (e.g., GSH) --

2

sulfidefluor

fluorimetry

5

10-600

No

15

3

dansyl azide

fluorimetry

1

0-100

No

56

4

copper@gold nanoparticles

colorimetry

0.3

0-10

Y

57

5

Monobromobimane

fluorimetry

0.002

0.002-0.2

Y

18

¯6

¯6

ref. 55

0.6-10 ×10 0-50

--

19

fluorimetry

0.6×10 0.7

No

58

methylene blue

colorimetry

0.3

3.8-46.8

--

59

6

Dibromobimane

fluorimetry

7

luminol azide

8 9

7-nitro-1,2,3-benzoxadiazole

colorimetry

0.19

--

No

60

10

tricyanoethylene derivate

colorimetry

--

0-130

No

61

11

GSH-AuNPs

colorimetry

3

5-15

--

38

12

gold nanorods

colorimetry

24

20-100

Y

62

13

copper nanoclusters

fluorimetry

0.04

0.2-50

No

63

14

gold nanoclusters N,N-dimethyl-pphenylenediamine AE-AuNPs

fluorimetry

0.5

0.5-157

Y

64

colorimetry

1.7

1.7-8.8

--

65

colorimetry

0.2

3-10

No

this study

15 16 -- unavailable

trophotometer. Figure 3c displays that the AE-AuNPs solution color change upon adding various concentrations of H2S, and 4 µM of H2S was readily detected by the naked eye. To investigate whether the aggregation induced by H2S is specific, the absorption spectra of AE-AuNPs upon addition of various anions, including F¯, Br¯, I¯, OAc¯, EDTA2¯, N3¯, NO2¯, NO3¯, SO32¯ and SO42¯ were measured. As manifested in Figure 4, none of these anions (1 mM) could cause a conspicuous aggregation as H2S (10 µM) did. In addition, thiols (GSH, TPN, MPA and TGA; each at a concentration of 1 mM) were also investigated, showing negligible aggregation as induced by H2S (10 µM). Furthermore, neither anions nor thiols would interfere on the detection of H2S (Figure S6), revealing excellent selectivity of the assay toward H2S. Compared to other nanomaterials based H2S sensing assays,38, 53, 54 the present one provided better selectivity. Such a high selectivity of the AEAuNPs nanosensor can be attributed to the specific H2S-azide chemical reaction. Taken together, AE-AuNPs provide an effective colorimetric nanoprobe for the analysis of H2S. Table 1 compares different methods for analyzing H2S to the sensing results reported in the present study. For H2S analysis, the AEAuNPs are simple and low-cost, with comparable or better sensing performance as compared to other AuNPs, fluorophores or metal nanoclusters. Analysis of real samples. To investigate potential application of the designed system in real samples, we tested real and spiked lake water samples with various concentrations of H2S. The water samples did not induce the color change of AEAuNPs solution without extra H2S. The assay allowed detection of H2S in the spiked samples down to 0.5 µM (Figure S7). The sensitivity values obtained from Figure 3b and Figure S7b were 0.2 and 0.5 µM, revealing the AE-AuNPs based nanosensors are adaptable for H2S analysis in real water samples.

Since the maximum level of H2S in drinking water permitted by the World Health Organization is 15 µM (500 ppb), the high sensitivity suggests that AE-AuNPs based nanosensors may be applied to practical environmental analysis.

CONCLUSIONS In summary, we have reported a new colorimetric method to detect H2S based on primary amine-active ester crosslinking mediated aggregation of AuNPs. Highly specific H2S detection is achieved based on well-known azide-H2S chemical reaction, which endowing the nanosensors with satisfying interference rejection over 100 folds other anions and thiols. Under the optimal conditions, 4 µM of H2S could be readily detected by the naked eye. Our study also demonstrate an interesting combination of the specific H2S-azide reaction and crosslinking chemistry, thus new avenues for the design of AuNPs nanosensors for other analytes based on a similar strategy might be open up in the analytical and related fields.

ASSOCIATED CONTENT Supporting Information Additional figures as indicated in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Phone: +86 731 88823074. Fax: +86 731 88821904. * Email: [email protected]. Phone & Fax: +011 886 02 33661171.

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China with grant numbers of 20975036, 91027037, 21127009 and 21221003, Natural Science Foundation of Hunan Province 13JJ1015, and Hunan University 985 fund. This work was also supported by the Ministry of Science and Technology of Taiwan under contracts NSC 101-2113-M-002-002-MY3 and 103-2923-M-002-002-MY3. Z. Yuan is grateful to the Ministry of Science and Technology of Taiwan for a postdoctoral fellowship under contracts NSC 103-2811-M-002-169. Z. Yuan thanks Corina Leung from National Taiwan University Mass Spectrometry-based Proteomics Core Facility for assistance in ESI-MS measurements. The assistance of Ms. Ya-Yun Yang and Ms. Ching-Yen Lin from the Instrument Center of National Taiwan University for TEM measurement is appreciated.

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