Ultrasensitive detection of sulfide ions through interactions between sulfide ions and Au(III) quenching the fluorescence of chitosan microspheres functionalized with rhodamine B and modified with Au(III)

Abstract

A rapid and facile fluorescence probe for detecting sulfide ions was developed. Sulfide is detected using the probe through interactions between sulfide ions and Au(III) quenching the fluorescence of chitosan microspheres functionalized with rhodamine B (RB-CSM) and modified with Au(III). The RB-CSM contain large numbers of positively charged amine groups and fluoresce strongly. Au(III) adsorb onto the RB-CSM surfaces through electrostatic interactions between AuCl₄⁻ and protonated amino groups in the RB-CSM, giving Au(III)/RB-CSM. The fluorescence of the Au(III)/RB-CSM is effectively quenched by sulfide ions through interactions between sulfide ions and Au(III), causing gold nanoparticles to form and allowing fluorescence resonance energy transfer between the gold nanoparticles and the RB-CSM to occur. Under optimum conditions, a good linear relationship (R² = 0.982) was found between the fluorescence quenching efficiency (F₀/F) and the sulfide ion concentration between 0.16 and 36 nmol L⁻¹. The detection limit for sulfide ions was 0.1 nmol L⁻¹. The probe can be completely regenerated using thiourea and was easily separated by centrifuging the mixture, meaning that the probe will cause much less environmental pollution than non-renewable fluorescence probes. The approach described here is a new and convenient way of developing reusable fluorescence probes.

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Introduction

Sulfide anions are found widely in aquatic systems because sulfide is released during industrial processes, sulfide is produced when anaerobic bacteria reduce sulfate, and sulfide is produced from sulfur-containing amino acids in meat proteins. Sulfide concentration is a very important aquatic pollution index, especially because sulfide is toxic to many aquatic organisms even at micromolar concentrations. High sulfide concentrations can irritate mucous membranes and lead to respiratory paralysis and loss of consciousness.¹ It has recently been found that sulfide is a critical mediator in the cardiovascular system and the nervous system and has various biological signaling functions. Accurate and reliable methods for determining sulfide concentration in environmental media are therefore of great interest. Various methods for detecting sulfide anions have been developed, and these methods involve such techniques as ion chromatography,² chemiluminescence,³ titration,⁴ spectroscopy,⁵ and electrochemical methods.⁶ Fluorescence methods are more powerful than the methods listed above for detecting ions because they are easy and relatively cheap to perform, are very sensitive and selective, and can be used to make real-time and online measurements.^7,8 Homogeneous solution systems, which are not recyclable, are the most commonly used fluorescence sensors.^9,10 Developing a reusable and relatively environmentally benign fluorescence system for detecting sulfide anions is a challenging task.

Chitosan is one of the most abundant naturally occurring amino-polysaccharides. Chitosan has a high positive charge density that allows it to interact strongly with negatively charged dyes and metal ions.^11,12 Chitosan is soluble in acidic media (at pH < 6.0), but it can be made less soluble and more stable by modifying it with glutaraldehyde to allow cross-linking to occur. Cross-linked chitosan has been used to selectively adsorb dyes and metal ions from industrial effluents, and it is easily removed from a liquid by centrifuging the liquid.¹³

Gold nanoparticles are promising nanomaterials. Gold nanoparticles have been used in many applications because they have remarkable properties. A solution containing gold nanoparticles will have a broad absorption band in the UV-vis region with a high extinction coefficient (10⁸ to 10¹⁰ L mol⁻¹ cm⁻¹), three to five orders of magnitude higher than the extinction coefficients of common organic molecules.¹⁴ Gold nanoparticles therefore efficiently quench most fluorophores through both the energy transfer and electron transfer processes.^15,16 Because they are “super-quenchers”, gold nanoparticles have been used in fluorescence assays and in fluorescence resonance energy transfer assays. Traces of sulfide ions have been found to react with Au(III) ions to form gold nanoparticles.^17,18 Here, we describe the first design and synthesis of chitosan microspheres functionalized with rhodamine B (RB-CSM) and modified with Au(III) for detecting sulfide ions through a fluorescence turn-off mechanism. The detection mechanism is shown in Scheme 1. The fluorescence of the Au(III)/RB-CSM will be effectively quenched by sulfide ions because interactions between the sulfide ions and Au(III) will cause gold nanoparticles to form and fluorescence resonance energy transfer between the gold nanoparticles and the RB-CSM to occur. Moreover, the probe can be completely regenerated using thiourea and was easily separated by centrifuging the mixture, meaning that the probe will cause much less environmental pollution than non-renewable fluorescence probes.

Scheme 1 Schematic illustration of the mechanisms involved in detecting sulfide ions using the fluorescence quenching of chitosan microspheres functionalized with rhodamine B and modified with Au(III) (Au(III)/RB-CSM) through interactions between sulfide ions and Au(III).

Experimental

Materials and chemicals

Rhodamine B (RB) was obtained from the Tianjin Guangfu Fine Chemical Research Institute. Chitosan was purchased from Zhejiang Golden-shell Biochemical Co., Ltd. Acetic acid, span-80, magnesium stearate, glutaraldehyde, 11-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), chloroauric acid (AuCl₃·HCl·4H₂O), sodium sulfidenonahydrate (Na₂S·9H₂O), and N-hydroxysuccinimide (NHS) were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. Petroleum ether was supplied by the Beijing Yangtze River Chemical Plant. 1,4-Dioxane and liquid paraffin were acquired from Shanghai Lingfeng Chemical Reagent Co., Ltd. Stock solutions of Au(III) were prepared by dissolving appropriate amounts of AuCl₃·HCl·4H₂O in HCl_(aq). PBS buffer solutions were prepared using sodium dihydrogen phosphate, dibasic sodium phosphate or phosphoric acid under adjustment using a pH meter. All of the chemicals used were of analytical grade, and the water that was used was double distilled.

Instruments

Fluorescence spectra were obtained with a Hitachi F-4500 fluorescence spectrophotometer. The pH values of the solutions were measured using a pHS-3C pH meter (Hangzhou, China). Fourier transform infrared (FTIR) spectra were obtained using an FTIR spectrometer (FTIR-S-8400) instrument using KBr discs in the 400–4000 cm⁻¹ region. Scanning electron microscopy (SEM) images were acquired using an S-520 SEM instrument to characterize the surface morphology of the nanoparticles that were formed. UV-vis absorption measurements were made using a U-4100 spectrophotometer (Hitachi). All of the experiments were carried out at room temperature.

Preparation of chitosan microspheres (CSM) using an emulsion crosslinking method

The CSM were prepared using an emulsion crosslinking method.^19,20 A stock solution of 2% (v/v) glutaraldehyde was prepared by dissolving 2.0 mL of glutaraldehyde in 23 mL of 1,4-dioxane. Chitosan (0.25 g) was dissolved in 30 mL of 3 wt% acetic acid in water, and this was used as the aqueous phase. Liquid paraffin was mixed with 10 mL of span-80 (the emulsifier) and 2.5 g of magnesium stearate (the cosurfactant) in a magnetic stirrer at room temperature (until the solution was clear), and 200 mL of this solution was used as the organic phase. The chitosan and acetic acid solution was added to the organic phase and stirred for 2.0 h until the mixture was emulsified homogeneously. The glutaraldehyde solution in 1,4-dioxane was then added dropwise to the emulsion, and the mixture was kept at 40 °C for 6 h in an oil bath, continuously stirred using a magnetic stirrer. The product was then isolated by centrifuging the mixture at high speed. The product was washed with water several times to remove the unreacted chemicals.

Synthesis of RB-functionalized CSM (RB-CSM)

RB was grafted onto the CSM through the amidation reaction of the RB with the CSM in the presence of EDC and NHS.²¹ This was achieved as described next. A 1.0 g aliquot of the CSM was dissolved in 150 mL of deionized water by applying ultrasonic radiation. A 0.05 g aliquot of RB was mixed with 0.25 g of EDC and 0.2 g of NHS, and this mixture was added to 50 mL of deionized water, then stirred in a magnetic stirrer for 30 min at room temperature. The mixture was then poured into the CSM solution, and this mixture was stirred continuously for 2 days. The product was then isolated by centrifuging the mixture at high speed. The product was washed with water several times to remove the unreacted chemicals.

Fluorescence titration experiments

Fluorescence measurements were made using a Hitachi F-4500 fluorescence spectrophotometer. A solution of HAuCl₄ (0.25 mL, 8 × 10⁻⁵ mol L⁻¹) was mixed with a solution of RB-CSM (0.5 mL, 0.4 g L⁻¹) in each of a series of test tubes at room temperature. The mixture in each tube was buffered to pH 5.3 by adding 0.5 mL of phosphate-buffered saline (PBS). The mixture was then allowed to equilibrate at room temperature for 20 min. A different amount of stock Na₂S solution was added to the mixture in each tube, then deionized water was added to bring the volume to 5.0 mL. The fluorescence intensity of the mixture in each tube (different tubes containing different amounts of Na₂S) was measured using an excitation wavelength of 520 nm and with the excitation and emission slits both set to 5.0 nm.

Results and discussion

Characterization of the synthesized chitosan microspheres

The morphologies of the as-prepared RB-CSM were characterized using scanning electron microscopy. A representative scanning electron microscopy image of the RB-CSM is shown in Fig. 1B. The chitosan microspheres had diameters of 1000–2500 nm. The RB-CSM had almost exactly the same sizes and morphologies as the chitosan microspheres, as is shown in Fig. S1 in the ESI.† The FTIR spectra of chitosan, crosslinked chitosan, RB, and the RB-CSM are shown in Fig. 1A. The chitosan molecule had a clear FTIR spectrum signature, with aliphatic hydrocarbon (C–H) stretching vibrations at 2929 cm⁻¹,²² O–H and N–H bond stretching at 3469 cm⁻¹,²³ and a β-anomer bond at 896 cm⁻¹.²⁴ Chitosan crosslinked with glutaraldehyde had a new peak at 1625 cm⁻¹, indicating that imine bonds (C=N) had formed during the crosslinking process.^25,26 The spectrum of the RB-CSM did not contain the carboxyl group absorption bands at around 1707 cm⁻¹ that were present in the spectrum of the pure RB. Instead, the RB-CSM spectrum contained the amide I band at 1647 cm⁻¹ that was predominantly produced by secondary amide stretching vibrations, and the amide II and amide III bands at 1540 cm⁻¹ and 1396 cm⁻¹, respectively, produced by N–H bending vibrations and C–N stretching vibrations, respectively. These absorption bands confirmed that the RB was successfully grafted to the surface of the crosslinked CSM. The stability of the RB-CSM was also studied as a function of time. It was found from the data in Fig. S2† that the fluorescence intensities of the RB-CSM were almost the same over 30 hours.

Fig. 1 (A) FTIR spectra of (a) rhodamine B, (b) chitosan, (c) chitosan microspheres (CSM), and (d) chitosan microspheres functionalized with rhodamine B (RB-CSM). (B) Scanning electron microscopy image of the RB-CSM.

Fluorescence quenching in the Au(III)/RB-CSM system caused by interactions between sulfide ions and Au(III)

As is shown in Fig. S3 in the ESI,† a 0.04 g L⁻¹ RB-CSM solution fluoresced strongly at about 580 nm, and adding 4.0 × 10⁻⁶ mol L⁻¹ of HAuCl₄ decreased the fluorescence intensity only a little (by ∼13%) because of the Au(III)/RB-CSM complex forming. Abundant amino groups covered the RB-CSM surfaces, and AuCl₄⁻/RB-CSM conjugates will have formed through non-covalent adsorption because of the strong affinity between the protonated RB-CSM amino groups and AuCl₄⁻ ions. However, a trace of sulfide ions in the solution quenched the Au(III)/RB-CSM fluorescence, and increasing the sulfide concentration increased the amount of quenching that occurred, as is shown in Fig. 2A. The degree of quenching increased noticeably as the sulfide concentration increased, and reached 90% quenching at a sulfide ion concentration of 36 nmol L⁻¹. We investigated the effect of the presence of sulfide ions on the fluorescence of the RB-CSM and found that the fluorescence of the RB-CSM was not influenced by the presence of sulfide ions, as is shown in Fig. S4 in the ESI.† The decrease in the fluorescence of the system was therefore caused only by interactions between the sulfide ions and Au(III). Interestingly, the fluorescence intensity started to increase once the sulfide ion concentration exceeded 36 nmol L⁻¹, as is shown in Fig. 2B. About 98% of the fluorescence had been recovered at a sulfide ion concentration of 1.6 µmol L⁻¹. The mechanism involved in the fluorescence quenching process was investigated by acquiring UV-vis absorbance spectra of a RB-CSM solution in the absence and presence of HAuCl₄ and sulfide ions. A strong characteristic absorption peak was found for the solution containing only RB-CSM, as shown in curve “a” in Fig. 3A. The spectrum for Au(III)/RB-CSM (curve “b” in Fig. 3A) was almost exactly the same as the spectrum for RB-CSM. However, a new peak at around 520 nm was found when trace sulfide ions were added to the Au(III)/RB-CSM system, as can be seen from curve “c” in Fig. 3A. This was the characteristic absorption peak for gold nanoparticles, called the surface plasmon resonance peak.^27,28 This agreed with the results of a previous study in which Au(III) was found to be reduced to Au(0) when trace sulfide ions were present.^17,18 The high quenching efficiency achieved by the sulfide ions was caused by interactions between the sulfide ions and Au(III) causing gold nanoparticles to form on the RB-CSM surfaces and Förster resonance energy transfer and electron transfer between the dye and the gold nanoparticles occurring, as shown in Scheme 1. The surface plasmon resonance absorption peak of the gold nanoparticles disappeared when the sulfide ions exceeded a certain concentration, as is shown in curve “d” in Fig. 3A. This can be attributed to the gold nanoparticles reacting with the abundant sulfide ions and forming Au₂S nanoparticles.^17,18 This explains the recovery of the fluorescence intensity when the sulfide ion concentration was increased to 1.6 µmol L⁻¹, as shown in Fig. 2B.

Fig. 2 (A) Fluorescence emission spectra of 0.04 g L⁻¹ chitosan microspheres functionalized with rhodamine B (RB-CSM) in PBS (at pH 5.3) containing 4.0 × 10⁻⁶ mol L⁻¹ AuCl₄⁻ and different concentrations of sulfide ions (0 nmol L⁻¹ at the top to 36 nmol L⁻¹ at the bottom). The inset is the fluorescence intensity plotted against the sulfide ion concentration. (B) Fluorescence emission spectra of 0.04 g L⁻¹ RB-CSM in PBS (pH 5.3) containing 4.0 × 10⁻⁶ mol L⁻¹ AuCl₄⁻ and different concentrations of sulfide ions (36 nmol L⁻¹ at the bottom to 1.6 µmol L⁻¹ at the top). The inset is the fluorescence intensity plotted against the sulfide ion concentration.

Fig. 3 (A) UV-vis absorption of (a) 0.04 g L⁻¹ chitosan microspheres functionalized with rhodamine B (RB-CSM), (b) 0.04 g L⁻¹ RB-CSM with 4.0 × 10⁻⁵ mol L⁻¹ AuCl₄⁻, (c) 0.04 g L⁻¹ RB-CSM with 4.0 × 10⁻⁵ mol L⁻¹ AuCl₄⁻ and 10 nmol L⁻¹ sulfide ions, (d) 0.04 g L⁻¹ RB-CSM with 4.0 × 10⁻⁵ mol L⁻¹ AuCl₄⁻ and 1.0 µmol L⁻¹ sulfide ions, (e) 10 nmol L⁻¹ sulfide ions, and (f) 4.0 × 10⁻⁵ mol L⁻¹ AuCl₄⁻. (B) Fluorescence intensity ratio (F₀/F) of the chitosan microspheres functionalized with rhodamine B and modified with Au(III) (Au(III)/RB-CSM) in the presence (F) and absence (F₀) of sulfide ions as a function of the pH. The RB-CSM concentration was 0.04 g L⁻¹, the Au(III) concentration was 4.0 × 10⁻⁶ mol L⁻¹, and the sulfide ion concentration was 20 nmol L⁻¹.

Optimization of the conditions for detecting sulfide ions

The pH was one of the most important variables that affected the sensor system.²⁹ The pH at which the maximum fluorescence quenching efficiency F₀/F (where F₀ and F are the fluorescence intensities of the Au(III)/RB-CSM system in the absence and presence of sulfide ions, respectively) occurred was determined by performing fluorescence titration experiments using different initial pH values (between pH 2.5 and 7.5) with the sensor prepared in PBS. It can clearly be seen from Fig. 3B that the fluorescence quenching efficiency decreased gradually as the pH increased from pH 5.8 to 7.5. This was probably because the RB-CSM have many positively charged protonated amino groups at low pH values, meaning that AuCl₄⁻ ions adsorb onto the RB-CSM to form AuCl₄⁻/RB-CSM. An Au(III) hydroxide product will form at higher pH values, decreasing the amount of interactions between sulfide ions and Au(III) that will occur and decreasing the number of gold nanoparticles that will form. Further experiments were therefore performed at pH 5.3.

The kinetics of the reaction between the Au(III)/RB-CSM and sulfide ions was studied by monitoring the fluorescence intensity as a function of time. The changes that were found in the fluorescence intensity of the Au(III)/RB-CSM system when sulfide ions were added over time are shown in Fig. S5 in the ESI.† It can be seen that the minimum fluorescence intensity was reached more than 45 s after sulfide ions were added. The interactions between the sulfide ions and Au(III) caused gold nanoparticles to form on the RB-CSM surfaces and resulted in Förster resonance energy transfer and electron transfer from the dye to the gold nanoparticles, and this caused very efficient quenching to occur. Unless otherwise specified, the fluorescence experiments described below were performed in PBS at pH 5.3 using a contact time of 1.0 min.

An excellent fluorescence probe needs to be highly selective under most conditions. The selectivity of the Au(III)/RB-CSM probe was evaluated by determining the changes in fluorescence intensity caused by the presence of various potentially interfering substances. We measured the fluorescence intensity of the probe when Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, F⁻, Cl⁻, SO₄²⁻, NO₃⁻, or CH₃COO⁻ was present at 1.0 × 10⁻⁴ mol L⁻¹ or when vitamin PP, heparin, bovine serum albumin, or adenosine-5′-triphosphate disodium salt was present at 1.0 mg L⁻¹, using the same conditions. The effect of each potentially interfering substance on the fluorescence intensity of the probe is shown in Fig. 4A. Sulfide ions very efficiently quenched the fluorescence, and the presence of the other substances changed the efficiency at which fluorescence was quenched very little. We therefore concluded that the probe was outstandingly specific and selective for sulfide ions.

Fig. 4 (A) Fluorescence intensities of the chitosan microspheres functionalized with rhodamine B (RB-CSM) with 4.0 × 10⁻⁶ mol L⁻¹ AuCl₄⁻ (the green bars) and Au(III)/RB-CSM with 36 nmol L⁻¹ S²⁻ (the red bars) when various potentially interfering substances were present. The potentially interfering substances were (1) 36 nmol L⁻¹ S²⁻, (2) Na⁺, (3) K⁺, (4) Mg²⁺, (5) Ca²⁺, (6) Zn²⁺, (7) F⁻, (8) Cl⁻, (9) SO₄²⁻, (10) NO₃⁻, and (11) CH₃COO⁻, each at a concentration of 1.0 × 10⁻⁴ mol L⁻¹, and (12) vitamin PP, (13) heparin, (14) bovine serum albumin, and (15) adenosine-5′-triphosphate disodium salt, each at a concentration of 1.0 mg L⁻¹, in PBS (at pH 5.3). (B) Fluorescence quenching and recovery cycles of the Au(III)/RB-CSM. The RB-CSM concentration was 0.04 g L⁻¹, the Au(III) concentration was 4.0 × 10⁻⁶ mol L⁻¹, and the sulfide ion concentration was 20 nmol L⁻¹.

The ability to reuse a chemical sensor is important if the sensor is to be used extensively to detect specific analytes. We investigated the feasibility of regenerating the probe using acidified thiourea in which the gold nanoparticles were stripped from the sensor.³⁰ Used probes were mixed with 0.5 mol L⁻¹ thiourea and 0.1 mol L⁻¹ HCl and shaken for about 10 min, then the mixture was centrifuged to separate the Au(III)/RB-CSM, which were washed with water and used in subsequent fluorescence tests. This fluorescence “quenching and recovery” process was repeated for at least four cycles without the signal appearing to decrease, as is shown in Fig. 4B. The fluorescence was able to be recovered because the thiourea removed the gold nanoparticles from the Au(III)/RB-CSM, resulting in the fluorescence intensity of the sensor being restored.

Analytical parameters and analysis of samples

Under the optimum conditions, a plot of F₀/F (where F₀ and F are the fluorescence intensities of the sensor, at 580 nm, in the absence and presence of sulfide ions, respectively) against the sulfide ion concentration (C, in micromoles per liter) had a linear range of 0.16–36 nmol L⁻¹. The equation of the line was F₀/F = 0.9864 + 0.24322C, and the correlation coefficient (R²) was 0.982, as is shown in Fig. S6 in the ESI.† The limit of detection (C_lim) was determined using the equation C_lim = 3δ/k, where δ is the standard deviation of the concentrations found in the blanks (n = 10) and k is the slope of the calibration line. The detection limit was 0.1 nmol L⁻¹, which is lower than the detection limits for sulfide that have previously been achieved.^31–35

We investigated the practicality of using the system by using a standard addition method to determine the sulfide ion concentrations in two real samples, one of lake water and one of tap water. The water samples were passed through a 0.45 µm micropore membrane filter and centrifuged for 10 min at 10 000 rpm before being analyzed. As is shown in Table 1, the concentrations that were found in the lake water and tap water samples were 105.2% and 98.7%, respectively, of the expected concentrations. These results indicated that the method can be used to accurately and reliably determine sulfide ion concentrations in real samples.

Table 1 Sulfide ion concentrations found in lake water and tap water samples using the standard addition method

Sample	Added (nM)	Found (nM)	Recovered (%)	RSD (%)
Lake water	0.0	Not detected	—	—
	10	10.52	105.2	1.75
Tap water	0.0	Not detected	—	—
	20	19.74	98.7	2.13

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Conclusions

In summary, we developed a new type of fluorescent probe, a Au(III)/RB-CSM complex, for selectively and sensitively detecting sulfide ions in aqueous solutions. The fluorescence of the Au(III)/RB-CSM was effectively quenched by sulfide ions because interactions between sulfide ions and Au(III) caused gold nanoparticles to form and fluorescence resonance energy transfer between the gold nanoparticles and the RB-CSM to occur. Sulfide ion concentrations were found to be measured simply and with good sensitivity using the probe. The linear range of the probe was found to be 0.16 nmol L⁻¹ to 36 nmol L⁻¹, and the detection limit was found to be 0.1 nmol L⁻¹. The spent probe can be completely regenerated using a thiourea solution, then the regenerated probe can easily be retrieved by centrifuging the mixture. The probe can therefore be reused and will cause little environmental pollution each time it is used compared with the pollution caused by using non-renewable functionalized materials. We expect that the strategy used here may be used to develop other low-cost, relatively environmentally benign, sensitive sensors for other target substances.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21205002) and the Innovation Funds of Anhui Normal University (No. 2014cxjj09).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04407g

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