Highly portable fluorescent turn-on sensor for sulfide anions based on silicon nanowires

Abstract

By covalently modifying 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (ligand) and 4-amino-1,8-naphthalic anhydride (fluorophore) onto the surface of silicon nanowires (SiNWs) and subsequently complexing them with copper ions (Cu²⁺), a SiNWs-based fluorescent sensor for sulfide anions (S²⁻) was realized. Based on such a ligand/Cu²⁺ approach, the new type of sensor realizes rapid sensing of S²⁻, and exhibits high sensitivity and selectivity for S²⁻ in water. Moreover, the as-prepared SiNW arrays-based sensor was successfully used in real time and in-situ monitoring of S²⁻ in running water by directly inserting it into the water. The present SiNW arrays-based sensor can be developed into a portable commercial device applied in environmental analysis after further optimizing the technique and finely quantifying the response of the sensor.

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Introduction

As a traditional toxic pollutant in the environment, the sulfide anion (S²⁻) exists widely in industrial settings, such as conversion into sulfur and sulfuric acid, production of wood pulp, and dye and cosmetic manufacturing, where it is either used as a reactant or generated as a byproduct. S²⁻ is also produced in biosystems due to the microbial reduction of sulfate by anaerobic bacteria or formation from the sulfur-containing amino acids in meat proteins.¹ Both of above industrial processes and biological metabolism would release S²⁻ into the water.² It has been reported that exposure to low concentration of S²⁻ could cause dizziness, while high concentration contact of S²⁻ would lead to unconsciousness, irritation in mucous membranes, permanent brain damage or even asphyxiation.^3,4 Moreover, once protonated, the S²⁻ can be turned into HS⁻ or H₂S and more toxicity would be produced. Therefore, developing a method to rapidly and sensitively detect S²⁻ is of great significance for environmental and industrial requirement.

Several techniques, such as titration,⁵ inductively coupled plasma atomic emission spectroscopy,^6,7 electrochemical methods,⁸ ion chromatography,⁹ fluorimetry^10–12 et al. have been employed to detect S²⁻. Among these reported methods, fluorescence-based assay shows great advantages over others due to its high sensitivity, rapid detection and simplicity of operation. In the past few years, the strong reducing capacity and nucleophilicity of S²⁻ were often utilized to develop fluorescent probes for highly selective detection of S²⁻. However, these S²⁻ probes based on such approach always need a relatively long response time.^13–15 Recently, the affinity between anion and metal ion was used to detect anions and showed excellent kinetics. It is well known that S²⁻ can react with copper ion (Cu²⁺) to form a very stable CuS species, which owns a extremely low solubility product constant K_sp = 3.63 × 10⁻³⁶.¹⁶ Employing the Cu²⁺ binding sites on the organic fluorophores or fluorescent semiconductor quantum dots, as well as the strong affinity between S²⁻ and Cu²⁺, several fluorescent S²⁻ probes have been fabricated and a rapid detection for S²⁻ was realized.^17–24 Nevertheless, these fluorescent probes have not yet been integrated into a portable device, which would cause some inconvenience in environmental analysis. Developing a highly portable device and directly inserting it into the determinand would enable one to monitor the environmental pollution more conveniently. In order to realize a portable sensor device, anchoring the probe to the surface of carrier would be a rational strategy.^25,26

Recent studies show that one-dimensional (1D) nanostructures would be a good candidate for a carrier to fabricate various sensor devices with enhanced sensitivity and selectivity.^27–29 Among 1D nanostructures, silicon nanowires (SiNWs) are particularly favorable due to their stability, nontoxicity, and easy assembly as an array for the device.^30–32 Considering the virtues of the SiNWs and excellent kinetics of the Cu²⁺–S²⁻ affinity-based probes in S²⁻ detection, we covalently immobilize the 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (ligand) and 4-amino-1,8-naphthalic anhydride (fluorophore) onto the surface of SiNWs, and subsequently complex it with Cu²⁺, a SiNWs-based fluorescent turn-on sensor for S²⁻ was realized. This new type of sensor exhibits excellent sensitivity with a low detection limit of 0.88 μM, and good S²⁻ binding selectivity over other anions. Moreover, the as-prepared SiNW arrays-based sensor was successfully used in real time and in-situ monitoring of S²⁻ in running water by directly immersing it into the water. The present SiNW arrays-based sensor can be developed a portable commercial device applied in environmental analysis after further optimizing the technique and finely quantifying the response of the sensor. Furthermore, the methods used to construct the current S²⁻ sensor can be extended to realize other highly portable chem-sensor devices for environmental application.

Experimental

Reagents and materials

3-[2-(2-Aminoethylamino)ethylamino]propyl-trimethoxysilane (3-A) and 4-amino-1,8-naphthalic anhydride (4-A) were purchased from Alfa Aesar. Other reagents were purchased from Beijing Chemical Regent Co. All reagents and chemicals were AR grade and used without further purifications. Water used for measurement was purified by Millipore filtration system. Sodium sulfide was used as the source of sulfide.

Measurements

Transmission electron microscopy (TEM) image was recorded by a JEOL-2100 at an acceleration voltage of 200 kV. Scanning electron microscope (SEM) images were recorded by a Hitachi S-4800FEG. The X-ray photoelectron spectroscopy (XPS) was recorded by a PHIQUANTERA-II SXM at a voltage of 15 kV. Fluorescence spectra were recorded on a Hitachi F-4600 spectrophotometer. Fluorescence images were taken with Olympus BX51TRF microscope (λ_ex = 450–480 nm).

Preparation of SiNWs

SiNWs were prepared by a typical chemical vapor deposition method (CVD) using silicon monoxide powder as the single source.³³ An alumina boat containing SiO powder (Aldrich, 99%) was placed at the centre of a horizontal alumina tube mounted inside a high temperature tube furnace. The system was evacuated to 10⁻³ Pa. Mixture of Ar (95%) and H₂ (5%) as carrier gas was introduced at the flow rate of 49 sccm, then the system pressure was maintained at 800 Pa for ten minutes. And then the furnace was heated to 1350 °C, while the system pressure was maintained at 10⁴ Pa. The system was maintained for 6 h and then cooled to room temperature naturally. The slightly yellow cotton-like product was collected at both sides of the alumina boat. The structures and morphologies of the as-prepared SiNWs were characterized using TEM. The SiNWs have a crystalline Si core of 10–12 nm in diameter and a silicon oxide sheath of 1–2 nm in thickness, as shown in Fig. 1.

Fig. 1 The TEM image of SiNWs by CVD method.

Modification of SiNWs

The as-prepared SiNWs were immersed in a mixture containing 7.5 mL concentrated H₂SO₄ (98%) and 2.5 mL 30% H₂O₂ (v/v = 3 : 1) at 90 °C for 1.5 h and then cooled down to room temperature. After repeatedly washed with doubly distilled water, the clean SiNWs were immersed in a mixture of 5 mL H₂O, 1 mL 30% H₂O₂ and 1 mL NH₄OH (v/v/v = 5 : 1 : 1) at room temperature for 2 h to bring Si–OH bonds on the surface of the SiNWs. Then the OH-terminated SiNWs were repeatedly rinsed with doubly distilled water and dried under vacuum at 50 °C for the following chemical modification.

The modification procedure of the SiNWs was shown in Scheme 1. 20 mg of dried SiNWs were dispersed into 8 mL anhydrous toluene (distilled) under nitrogen in a two-neck round bottomed flask, then 0.78 mmol of 3-A was added by syringe. The suspension was heated to 90 °C and stirred for 36 h. After cooling down to room temperature, the product (defined as 3-A-SiNWs) was collected by a micro filter and repeatedly washed by ethanol.

Scheme 1 The procedure of SiNWs modification and the structure of 4-A-SiNWs.

The as-prepared 3-A-SiNWs were suspended in a round flask containing 6 mL absolutely anhydrous ethanol under nitrogen, then 0.1 mmol (21.2 mg) of 4-A was added. The mixture was stirred for 18 h at 80 °C. The product (defined as 4-A-SiNWs) were collected by filtration and repeatedly washed with ethanol. Unreacted organic materials were removed completely by monitoring the fluorescence of the washing liquid.

Preparation of SiNW arrays

The high-quality SiNW arrays were fabricated by the chemical etching (CE) method.³⁴ A clean n-type silicon wafer with [100] orientation was immersed into the solution of 4.6 M hydrofluoric acid (HF) and 5 mM silver nitrate for 15 min to deposit Ag particles on its surface, and the particles would act as catalyst in the following etching process. Then the wafer attached with Ag particles was soaked into the etching solution consisting of 4.6 M HF and 0.2 M H₂O₂ at 50 °C for 20 min. After etching, the samples were immersed into a solution of condensed hydrochloric acid (HCl) and condensed nitric acid (HNO₃) (v/v = 3 : 1) for 1 h to remove the Ag particles. Finally, the SiNW arrays were obtained after copiously rinsing with water and ethanol. The SEM images were shown in Fig. 2. The diameters of the SiNWs are in the range of 150–300 nm, while the wire length is around 15 μm.

Fig. 2 SEM images of the side view (left) and the top view (right) of the SiNW arrays by CE method.

Modification of SiNW arrays

The modifying procedures of SiNW arrays were the same as that of SiNWs obtained by CVD method. The modified SiNW arrays were defined as 4-A-SiNW arrays.

Results and discussion

Characterization of the modification procedure

The X-ray photoelectron spectroscopy (XPS) was used to characterize the modifications of the SiNWs. As shown in Fig. 3a, it was found that little nitrogen can be detected from the bare SiNWs, while abundant nitrogen can be observed from 3-A-SiNWs. These results reveal that the molecules of 3-A have been covalently modified onto the surface of the SiNWs. After the 3-A-SiNWs further reacted with 4-A, the surface nitrogen content and carbon content of the 4-A-SiNWs changed compared with 3-A-SiNWs. From the insets of Fig. 3, it can be determined that the contents of N, C and Si were 16.25%, 55.45% and 9.82% on the surface of 3-A-SiNWs, while 11.23%, 65.38% and 5.46% on the surface of 4-A-SiNWs. Because the content of Si changed slightly, it can be employed as a standard to evaluate N content (16.25%/9.82% < 11.23%/5.46%) and C content (55.45%/9.82% < 65.38%/5.46%). Thus the contents of N and C on the surface of 4-A-SiNWs were more than that on the 3-A-SiNWs. Furthermore, the N(1s) spectrum of 3-A-SiNWs depicted in Fig. 3a shows a peak at around 398.8 eV corresponding to the C–N–H bonds from the 3-A.³⁵ While the N(1s) spectrum of 4-A-SiNWs depicted in Fig. 3b can be deconvoluted into three peaks: a same peak at 398.8 eV related to C–N–H bonds of the 3-A, a peak at 399.6 eV related to C–N–H bonds of the amino group from the fluorophore, and a peak at 400.6 eV related to the N–C bonds of naphthalic anhydride structure.³⁶ These results potently verify that the 4-A molecules has been covalently modified onto the surface of the 3-A-SiNWs successfully.

Fig. 3 XPS spectra of N(1s) region (a) Bare SiNWs and 3-A-SiNWs (b) 4-A-SiNWs. Inset: the relative atomic percentage of elements on the surface of 3-A-SiNWs and 4-A-SiNWs.

Thermogravimetric analysis (TGA) was employed to determine the content of the organic molecules on the surface of modified SiNWs. From the mass loss of the samples (Table S1†), the amount of the organic molecules on the surface of modified SiNWs was estimated to be 8.5 × 10⁻⁴ mol g⁻¹. The calculation method was following the literature.³⁷

Fluorescence response of 4-A-SiNWs to Cu²⁺

In order to investigate the binding between 4-A-SiNWs and Cu²⁺, the 4-A-SiNWs were dispersed into the HEPES-buffer to form a 50 μg mL⁻¹ suspended solution. After the Cu²⁺ with various concentrations were added into above 4-A-SiNWs suspended solution for 0.5 min, the fluorescent spectra were recorded as shown in Fig. 4a. The dependence of the fluorescence intensity of 4-A-SiNWs on Cu²⁺ concentration was inserted into Fig. 4a. It can be observed that the fluorescence intensity of 4-A-SiNWs gradually decreased with the increase of the Cu²⁺ concentration and finally reached at a plateau at around 14 μM. It reveals that the ligand-containing 4-A-SiNWs has bound with Cu²⁺ to form a 4-A-SiNWs/Cu²⁺ complex. The apparent binding constant K between the 4-A-SiNWs and Cu²⁺ was calculated to be 2.25 × 10⁵ by using a Langmuir-type analysis and fluorescence titration data.³⁸ The fluorescence response of 4-A-SiNWs to other metals ions such as Ca²⁺, Cd²⁺, Mg²⁺, Al³⁺, Co²⁺, Na⁺, Hg²⁺, Mn²⁺, K⁺, Ni²⁺, Pb²⁺, Zn²⁺ were also measured and the results were shown in Fig. S1.† From Fig. S1,† it can be found that Cu²⁺ exhibited the most excellent fluorescence quenching character.

Fig. 4 Fluorescence response of (a) the 4-A-SiNWs (50 μg mL⁻¹) in the presence of different concentrations of Cu²⁺ (0–14 μM) (b) the 4-A-SiNWs/Cu²⁺ complex system (the 4-A-SiNWs: 50 μg mL⁻¹, and Cu²⁺: 10 μM) in the presence of different concentrations of S²⁻ (0–30 μM) in HEPES buffer (pH = 7.0, 20 mM). The insets in (a) and (b) show the fluorescence change at 545 nm as a function of [Cu²⁺] and [S²⁻], respectively, λ_ex = 430 nm. (c) The linear relationship between the fluorescence intensity and the S²⁻ concentration in the range of 0–25 μM.

Fluorescence response of the 4-A-SiNWs/Cu²⁺ complex to S²⁻

To check the response of the 4-A-SiNWs/Cu²⁺ complex to S²⁻, the 4-A-SiNWs was preincubated with Cu²⁺, and then the fluorescence response of the 4-A-SiNWs/Cu²⁺ complex to S²⁻ was investigated after adding S²⁻ instantly. As shown in Fig. 4b, the fluorescence of the 4-A-SiNWs/Cu²⁺ complex system was enhanced gradually with the increase of the S²⁻ concentration, indicating the 4-A-SiNWs/Cu²⁺ complex could be a promising fluorescent turn-on sensor for S²⁻. The dependence of the emission intensity at 545 nm on the S²⁻ concentration was shown as inset of Fig. 4b. Furthermore, a good linear relationship in the range of 0–25 μM was obtained (Fig. 4c), and the detection limit of the 4-A-SiNWs/Cu²⁺ complex for the determination of S²⁻ was calculated to 0.88 μM.

Scheme 2 illustrates the proposed response mechanism of the S²⁻ sensor. The paramagnetic Cu²⁺ can bind with the ligand-containing 4-A-SiNWs to form a 4-A-SiNWs/Cu²⁺ complex, which would lead to the fluorescence quenching of the system. The S²⁻ added into the 4-A-SiNWs/Cu²⁺ complex system could coordinate with the Cu²⁺ to form the stable species CuS (K_sp = 3.63 × 10⁻³⁶), which resulted in the revival of the fluorescence.

Scheme 2 Proposed response mechanism of the S²⁻ sensor.

To investigate the selectivity, the fluorescence response of the 4-A-SiNWs/Cu²⁺ complex toward the common anions in environment was investigated. As shown in Fig. 5a, addition of 1 mM F⁻, Br⁻, Cl⁻, I⁻, NO₃⁻, HSO₃⁻, CO₃²⁻, CH₃COO⁻, PO₄³⁻, SO₃²⁻, NO₂⁻, SCN⁻, S₂O₃²⁻, SO₄²⁻ had little effect on the fluorescence emission from the sensing system. In contrast, addition of 30 μM S²⁻ to the 4-A-SiNWs/Cu²⁺ system resulted in the obvious fluorescence enhancement. Furthermore, the fluorescence response of the 4-A-SiNWs/Cu²⁺ complex towards S²⁻ in the presence of these anions were also investigated (Fig. 5a). It can be observed that these competing anions have ignorable influences on the detection of S²⁻. The results demonstrate that the 4-A-SiNWs/Cu²⁺ complex have sufficient selectivity for S²⁻ even coexisting with these competing anions. In addition, the pH effect on the sensor's performance was also studied. As shown in Fig. 5b, the fluorescence responses of the complex towards S²⁻ were relatively stable in the pH range from 6 to 10. This result indicates that the current sensor can be used for S²⁻ monitoring in a wide pH range.

Fig. 5 (a) Fluorescence response of 4-A-SiNWs/Cu²⁺ complex (the 4-A-SiNWs: 50 μg mL⁻¹, and Cu²⁺: 10 μM) in the presence of various anions alone (S²⁻: 30 μM, other anions: 1 mM) and interfering ions with S²⁻, respectively in HEPES buffer (pH = 7.0 20 mM). λ_ex = 430 nm, λ_em = 545 nm. 1–15 is successively for F⁻, Br⁻, Cl⁻, I⁻, NO₃⁻, HSO₃⁻, CO₃²⁻, CH₃COO⁻, PO₄³⁻, SO₃²⁻, NO₂⁻, SCN⁻, S₂O₃²⁻, SO₄²⁻ and S²⁻. (b) The pH effects on the fluorescence intensity at 545 nm of the 4-A-SiNWs/Cu²⁺ (4-A-SiNWs: 50 μg mL⁻¹, Cu²⁺: 10 μM) toward S²⁻ (30 μM).

To further evaluate the applicability of the SiNWs-based S²⁻ sensor in real sample, the fluorescent sensor was applied to determine the S²⁻ in running water. After the running water containing different concentrations of S²⁻ was added into the system, the fluorescence of the 4-A-SiNWs/Cu²⁺ complex was recorded instantly. From Table 1, it can be found that the recovery values are in the range of 95–103%, which indicates that the S²⁻ concentrations detected by the sensor are in good agreement with those added in the samples. The results also reveal that the composition of the running water does not have an obvious effect on the detection of S²⁻. According to Word Health Organization, the maximum recommended S²⁻ concentration in drinking water is less than 500 μg L⁻¹ (about 15 μM). With the detection limit of 0.88 μM and the good linear range of 0–25 μM, the sensor shows the potential in water quality monitoring.

Table 1 Determination of sulfide anion in running water

Samples	Amount of sulfide anion		Recovery (%)	RSD (%)
	Added (μM)	Found (μM)
Running water	0	—	—	—
	5.0	5.11	102.2	0.90
	10.0	9.69	96.9	3.85
	15.0	14.35	95.7	4.19
	20.0	19.52	97.6	2.73

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Fluorescence response of the highly portable SiNW arrays-based sensor device to S²⁻

In order to explore the application of our SiNW arrays-based sensor for real time and in-situ monitoring of S²⁻ in the determinand, the modified 4-A-SiNWs array was pre-incubated with Cu²⁺, then the unreacted Cu²⁺ was washed with deionized water and the SiNW arrays-based sensor device for S²⁻ was obtained. The sensor device was immersed into 1 mL running water, then different concentrations of S²⁻ were added into the system. The fluorescent images of the sensor device before and after being treated with 50 μM and 100 μM S²⁻ were recorded by a fluorescence microscopy and the results were shown in Fig. 6. As expected, the addition of different concentrations of S²⁻ resulted in the obvious enhancement with different levels determined from the fluorescence observation of the sensor device. This phenomenon exhibits the potential application of the portable sensor device in real time and in-situ monitoring of S²⁻ in environmental analysis after finely quantifying the response of the device. It is well known that S²⁻ is not stable in water and can be easily transformed into other substances like a more toxic gas H₂S, which is always produced in some specific working environment. Therefore, the response of the senor device to H₂S was also investigated. From Fig. S2,† it was found that the fluorescence of the sensor device increased obviously after being treated with H₂S. This phenomenon implied the potential use of sensor device in monitoring of H₂S in the environment.

Fig. 6 Fluorescence images of the SiNW arrays-based sensor device (Cu²⁺ preincubated 4-A-SiNW arrays) before (a) and after immersing in (b) 50 μM and (c) 100 μM S²⁻ for 0.5 minute.

Conclusions

In summary, we have designed and configured a rational fluorescent turn-on sensor for S²⁻ based on SiNWs and SiNW arrays. The present sensor displayed a good linear relationship between the fluorescence intensity and S²⁻ concentration from 0 to 25 μM, and exhibited high selectivity for S²⁻ against other common anions. In addition, the present S²⁻ sensor works well in a wide pH range. In view of all these merits, the SiNW arrays-based portable sensor device was successfully used in real time and in-situ monitoring of S²⁻ at micromolar concentration in 100% aqueous solution. This work paves a way to fabricate the portable sensor device for S²⁻ and exhibits potential application in environmental analysis.

Acknowledgements

This work was supported by Chinese Academy of Sciences (Grant KGZD-EW-T02), NSFC (Grants 61025003, 51272302, 21103211, 51272258, 61204128 and 91333119), National Basic Research Program of China (973 Program) (Grant 2012CB932400) and the Key Research Program of the Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10963e

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