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Highly photoluminescent silicon nanocrystals for rapid, label-free and recyclable detection of mercuric ions† Jia Zhang and Shu-Hong Yu* Hydrothermal treatment of 3-aminopropyltrimethoxysilane (APTMS) in the presence of sodium citrate generates a suspension of highly fluorescent silicon nanocrystals that fluoresces blue under UV irradiation. The photoluminescent quantum yield of the as-prepared silicon nanocrystals was calculated to be 21.6%, with quinine sulfate as the standard reference. Only mercuric ions (Hg2+) can readily prevent the fluorescence of the silicon nanocrystals, indicating a remarkably high selectivity towards Hg2+ over other metal ions. The optimized sensor system shows a sensitive detection range from 50 nM to 1 mM and a detection limit of 50 nM. The quenching mechanism was explained in terms of optical absorption
Received 5th November 2013 Accepted 7th January 2014
spectra and time-resolved fluorescence decay spectra. Due to the strong interaction of Hg2+ with the thiol group, the fluorescence can be fully recovered by biothiols such as cysteine and glutathione, therefore, a regenerative strategy has been proposed and successfully applied to detect Hg2+ by the
DOI: 10.1039/c3nr05896d
same sensor for at least five cycles. Endowed with relatively high sensitivity and selectivity, the present
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sensor holds the potential to be applied for mercuric assay in water.
1. Introduction The demand of uorescent probes to be used in biological staining, medical diagnosis, and chemical sensing is ongoing.1,2 Semiconductor quantum dots (QDs) have attracted enormous interest as a new generation of uorescent probes owing to their ultrahigh luminescence efficiency, excellent photostability, and size-tunable emission spectra.3 Among them, uorescent silicon nanocrystals (SiNCs) have been the subject of particular interest in recent years as prominent imaging agents since they exhibit favorable biocompatibility, low cytotoxicity, and even self-elimination.4 Although they have a number of advantages, a few technical difficulties impede the general applications of SiNCs. For example, most of current SiNCs require at least two independent routes to render them water-dispersible, rst to synthesize hydrophobic SiNCs by sophisticated physical or chemical methods,5–8 and second to cover them with hydrophilic molecules, including acrylic acid,9 allylamine,10 polymer,11 and micelle.12 This synthetic methodology achieves great success in producing uorescent SiNCs with a high prospect of bioimaging,13–15 however, they involve a set of relatively complicated procedures which are hard to reproduce. Very
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P.R. China. E-mail: [email protected]; Fax: +86-551-63603040 † Electronic supplementary information (ESI) available: Experimental details and additional gures. See DOI: 10.1039/c3nr05896d
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recently, He et al. has ingeniously developed a one-step microwave method for the quick synthesis of uorescent Si QDs.16 This method is simple and cost-effective. Until now, applications of the uorescent SiNCs are focused on bioimaging with few studies on chemical sensing.17 Mercury is one of the environmental contaminants that have been paid great attention in recent years. Emitted elementary mercury vapors from industrial and non-anthropogenic sources are easily transported in the atmosphere across oceans and continents and eventually accumulate on plants, in topsoil, and in waters as the ionized form (Hg2+).18 To date, the contamination of drinking water by Hg2+ ions remains a big problem. Long-term exposure to high Hg2+ levels is deleterious, causing permanent damage to the central nervous system and many other organs with serious sensory, motor, and cognitive disorders.19 In light of the infamous effects of mercury on the human body and the ecosystems, the World Health Organization (WHO) and United States Environmental Protection Agency (U.S. EPA) have regulated the maximum allowed levels of Hg2+ in drinking water to be 6 and 2 ppb, respectively. Although some traditional analytical techniques are viable for the sensitive determination of solvated Hg2+, they are generally reliant on massive instruments.20 As a result, the detection methods that meet with the criteria of facility, rapidness, and portability as well as accuracy are especially in demand. Fluorescence is one of the most powerful optical techniques for detecting trace concentrations of mercury. It has no inherent uorescence, suggesting that uorescence changes upon the specic interaction of Hg2+ with the probes, including small molecules,21
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semiconductor quantum dots,22 noble metal nanoclusters,23 polymer nanoparticles,24 uorescent carbon dots,25 carbon nanotube–DNA hybrid,26 and chemically modied nanoparticles.27 Most of these systems, however, suffer from limitations with respect to insufficient selectivity, utility of toxic materials, and incapability of reuse. Herein, we design a recyclable strategy for the rapid detection of Hg2+ in aqueous solutions, on the basis of label-free water-dispersible uorescent SiNCs. A homogeneous suspension of highly luminescent SiNCs was synthesized by a facile hydrothermal process, using 3-aminopropyltrimethoxysilane (APTMS) as the silicon source. We found that Hg2+ ion could attenuate the uorescence of the probe efficiently in a short time while many other cations can not, indicating a superb selectivity towards Hg2+. Moreover, the uorescence quenched by Hg2+ can be fully restored by biothiols such as cysteine and glutathione, which makes the SiNCs a fascinating recyclable sensor for Hg2+ that can be used for at least ve times.
2.
Materials and methods
2.1. Reagents and apparatus APTMS and Hg(NO3)2 were purchased from J&K Company. Glutathione and amino acids were purchased from Shanghai Sangon Biotechnology Co. Ltd. Sodium citrate and the other metal salts were obtained from Shanghai Chemical Company. All the materials are analytical reagent and used directly. Deionized water was used throughout the experiments. UV-Vis optical absorption spectra were recorded on a Shimadzu UV-2550 optical spectrophotometer. The uorescence spectra were recorded on a Hitachi F-7000 uorescence spectrophotometer. High-resolution transmission electron microscopy images were obtained using a JEOL-2010 transmission electron microscope equipped with energy dispersive X-ray (EDX) spectroscopy. Fourier transform infrared (FTIR) spectra were measured on a Bruker Vector-22 FTIR spectrometer from 4000 to 400 cm1 at room temperature. The C, H and N elemental contents were measured on a Vario EL III elemental analyzer system. The uorescence lifetimes were measured using a time-resolved/steady state uorescence spectrometer (Edinburgh Instrument Xe-900).
2.3. Detection of Hg2+ ion by the SiNCs An aqueous solution of SiNCs with a volume of 100 mL was mixed with different volumes of acetate buffer (pH 5.9, 20 mM) followed by the addition of diluted solutions of mercuric ions. The total volume of the solution was set at 1 mL, and the absorbance of SiNCs at 350 nm was equal to 0.013. Without time control, the uorescence spectra was recorded under excitation at 350 nm. The sloth widths of the excitation and emission were both 5 nm.
3.
Results and discussion
Following the preparation strategy previously reported,16 we employed APTMS for the hydrothermal synthesis of the SiNCs. The changes in the optical absorption spectra (Fig. 1) of the reactant precursor solutions identied the conversion. Aer hydrothermal treatment, a new absorption peak centered at 350 nm emerged, which can be explained by the generation of SiNCs. This agrees with the previous report.16 The result suggests that the pathways related to the APTMS reduction by citrate ions have no obvious difference between microwave heating and hydrothermal processing. Fig. 2a shows a representative transmission electron microscopic (TEM) image of the SiNCs. It revealed an image of densely packed, ill-shaped short nanobers, in large contrast to the monodispersed, spherical nanoparticles prepared by microwave irradiation. The selected-area electron diffraction (Fig. S1, ESI†) of the silicon nanobers showed a diffraction pattern of polycrystalline diamond cubic phase Si (JCPDS 271402), further conrming the synthesis of SiNCs. Fig. 2b shows a magnied image of the Si nanobers which possesses a kinked connection indicated by the arrow. A higher magnication of the selected part in Fig. 2b clearly reveals the interplanar spacing of 0.31 nm corresponding to {111} lattice fringe (Fig. 2c). The observed growth direction along h111i has been reported to be common for Si nanorods,28 while spherical Si particles tend to have h220i growth direction.8,29 This result suggests that the growth of the brous SiNCs experienced a different mechanism from that for the microwave grown Si nanoparticles. EDX (Fig. 2d) shows the existence of Si and O in the sample (C may not be conclusive since carbon exists in the copper grind matrix). The presence of C and N was further
2.2. Synthesis of uorescent SiNCs In a typical experiment, 1 g of sodium citrate was dissolved in 30 mL of water, followed by oxygen removal through purging with nitrogen gas of high purity for about 30 min. 1 mL of APTMS was added and homogeneously agitated, aerwards, the solution was transferred into a Teon-lined autoclave of 50 mL volume, which was sealed and maintained at a temperature of 160 C for 12 h and then naturally cooled to room temperature. The resulting transparent solution was dialyzed against stirred water in a dialysis bag with a 1 kDa cut-off molecular weight with change of water every 6 h before precipitate occurred in the bag. Finally, the solution in the bag was collected and stored at 4 C for use.
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Fig. 1 UV-Vis absorption spectra of the precursor reactant solutions and the purified solution of SiNCs.
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(a–c) TEM images of the SiNCs with different magnification and (d) EDX spectrum of the sample.
Fig. 2
identied by the FTIR spectrum and elemental analysis. The FTIR spectrum (Fig. 3d) clearly provides evidence for the amino group in the sample, reected by the N–H bending vibration and stretching vibration at 1570 and 3400 cm1, respectively. Moreover, the elemental analysis revealed contents of C and N to be 18.64% and 5.46%, respectively, giving an atomic ratio (C/ N) of 4. When irradiated by an UV light (365 nm), the solution of SiNCs gave off a strong blue uorescence (Fig. 3a). No uorescence was seen to emit from the reactant precursor solutions, further identifying the formation of SiNCs. By xing the emission wavelength at 440 nm, the excitation spectrum showed a well-resolved peak focused on 350 nm (Fig. 3b), reecting the optical absorption peak of the solution of SiNCs. The
Fig. 3 (a) Photographs of the reaction precursor solution and the asprepared solution of SiNCs under UV irradiation (365 nm). (b) The normalized FL excitation spectrum (dotted line) and emission spectra of the solution of SiNCs at different excitation wavelength. (c) The relationship of the integrated FL intensity with absorbance at 350 nm for the SiNCs and quinine sulfate. (d) FTIR spectrum of the SiNCs.
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uorescence (FL) emission intensity is reliant on the excitation wavelength, increasing gradually and reaching a maximum at 350 nm. Meanwhile, the FL emission peak did not shi with the excitation wavelength, a phenomenon that was observed for the Si QDs prepared by the electrochemical etching of bulk silicon.17 The uorescence quantum yield (QY) of the SiNCs was measured by adapting a comparative method from the work of Sankaran et al.,30 using quinine sulfate as the standard reference (Fig. 3c). As presented, the integrated emission intensity for each sample is plotted versus the corresponding optical absorbance at the same excitation wavelength. Fletcher reported that no obvious deviations were observed for the quantum yield of quinine sulfate from a constant value with an excitation range from 240 to 400 nm.31 The excitation wavelength of the SiNCs solution has its maximum at 350 nm, hence we chose 350 nm as the excitation wavelength. The quantum yield of SiNCs was calculated to be 21.6%, derived from the ratio of the slopes of the tted linear lines, and the known quantum yield of quinine sulfate (54%). Such a high QY is comparable to those of uorescent Si nanoparticles in aqueous solutions or organic solvents.16,32 Prior to evaluating the metal ion selectivity, some fundamental issues about the uorescence of the SiNCs need to be addressed. Irrespective of what buffer was used, it revealed a similar tendency of pH-dependent FL intensity, suppressed in acidic environments and reaching the maximum at slightly alkaline conditions (Fig. S2a and b, ESI†). This behavior may be related to the presence of amino groups on the SiNCs surfaces. The time–scan FL intensity plot (Fig. S2c, ESI†) revealed that 90% of the initial uorescence was preserved aer 1 h of continuous excitation at 350 nm. This feature of FL instability may be attributed to the existence of crystallographic defects containing kinked brous SiNCs. Moreover, the uorescence of SiNCs was not reduced by the increase in the ionic strength of solution (Fig. S2d, ESI†), suggesting a possible use for biological imaging. The FL lifetime of the SiNCs was also measured (Fig. S3b, ESI†). The decay curve can be tted with a double exponential function, giving two distinct decay times of 1.37 (s1, 20%) and 10.8 (s2, 80%) ns, respectively. This implies the occurrence of competing fast and slow electron–hole recombination processes in the brous SiNCs.33 It has been reported that H2O2 can quench the uorescence of Si nanoparticles, based on which a sensor for glucose was designed.17 However, no attempt to investigate the interaction of SiNCs with metal ions has ever been conducted. For this purpose, incubation of SiNCs with Hg2+ ions (10 mM) and other metal ions (0.5 mM) was performed to measure the FL intensity (Fig. 4). Only Hg2+ suppressed the uorescence of SiNCs by 80% of original intensity, while others elicited minor effects, demonstrating excellent selectivity to mercury. Hg2+ is known as a common uorescence quencher due to the formation of nonradiative electron-transfer or energy-transfer.34 To elucidate the quenching mechanism, we recorded the optical absorption spectra and FL decay curve (Fig. S3, ESI†). An observed change in the absorption spectra of the SiNCs solution aer the addition of Hg2+ indicates the conjugation of silicon with Hg2+, which is a signature of static quenching. We believe that the interaction of SiNCs with Hg2+ originates from the presence of
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Fig. 4 The specificity of the SiNCs towards metal ions in a phosphate buffer (20 mM, pH 5.9).
amino groups on the Si surfaces via the affinity of Hg2+ to N.35 In the meantime, the tted FL decay trace experienced a dramatic shi from a double exponential function to single exponential function with the lifetime of 10.1 ns (s2). The change in the FL lifetime signies dynamic quenching. From these pieces of evidence, we propose that the FL quenching of SiNCs by Hg2+ involves both dynamic and static processes. Some parameters need to be assessed before the quantication of Hg2+ by the SiNC probe. It is shown that pH not only affects the initial FL intensity but also the quenched FL intensity (Fig. S4, ESI†). With respect to Hg2+ at 2 mM, higher quenching efficiencies were observed at more acidic conditions, suggesting that the weakly acidic media may be preferable for the detection of Hg2+. However, it was also noted that the quenching by Hg2+ of 0.5 mM at pH 4.9 was not better than that at pH 5.9, thus the optimal pH value was selected at 5.9. The concentration of SiNCs has a measurable inuence on the detection, as it is known that lowering the concentration of uorophore for a quencher will make the uorescent change (F0/F) larger and thus the limit of detection (LOD) lower.36 This is identied by the LODs as 0.1, 0.2, and 0.5 mM, respectively, in the order of decreasing SiNC concentration (Fig. S5, ESI†). Moreover, we found that the buffer composition could also affect the assay. Three kinds of buffer with the same pH value and concentration were chosen, and in the presence of same amount of SiNCs, the best detection performance (i.e. the lowest LOD and the highest sensitivity) was obtained for the acetate buffer (compared in Fig. S5f, S6b† and 5b). With regard to the time for assay, it was found that the FL quenching by Hg2+ was immediately stable within 1 min aer ion spiking (data not shown), thus making the assay a rapid one without strict time control. Fig. 5a shows the FL response upon the addition of Hg2+ ions. It presents a gradual decrease of FL intensity with increasing Hg2+, and along with the attenuation of intensity, the emission peak did not shi. The response toward Hg2+ over the concentration range from 0 to 1 mM can be tted to a highly linear line (inset of Fig. 5b) expressed as F/F0 ¼ 1 0.6 [Hg2+] (R2 ¼ 0.998), where F and F0 are the quenched and initial uorescence, respectively. The LOD was 50 nM, based on three times the response in the presence of Hg2+ (signal) versus the standard deviation of the blank (noise) by following the
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Fig. 5 (a) The detection of Hg2+ by the SiNC probe (A350nm ¼ 0.013), using 350 nm as the excitation wavelength. (b) The relationship between the FL intensity ratio and the mercury concentration. The inset shows the detection range from 0 to 1 mM with a fitted linear equation. The data are based on the average of two repetitive measurements.
International Union of Pure and Applied Chemistry (IUPAC) criteria.37 Albeit with a LOD slightly higher than the maximum level of Hg2+ in drinking water permitted by the WHO, the presented sensor may allow the on-spot detection of Hg2+ in some suspected highly contaminated sites in a prompt and specic way, in large contrast to those depending on complicated instruments. A recyclable sensor is one that can be adjusted to its original state by means of appropriate measures. A typical example of such a method is the use of chemical reagents. With respect to sensors for Hg2+, the addition of sodium EDTA, biothiols, or sulde ions have been tested for recovery, while the degree of reversibility depends on specied factors, including the sensor type and assay pH. For example, the EDTA solution induced a complete restoration of uorescence of the Hg2+-treated porphyrin-modied Au@SiO2 nanoparticle sensor,38 while cysteine and glutathione were found not to reverse the uorescence of Hg2+-treated CdTe quantum dots39 to their initial states. We rst investigated the effect of EDTA solution on the uorescence of the SiNCs–Hg2+ system. Even at ten times the concentration of Hg2+, EDTA did not completely recover the uorescence of the SiNCs (Fig. S7, ESI†). We then tested the effects of cysteine (Cys) and glutathione (GSH). A preliminary study clearly revealed the largest FL recovery in the presence of Cys and GSH, compared with other amino acids (Fig. S8, ESI†). In addition, both of the biothiols were found not to affect the
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uorescence of the SiNCs. When 10 mM of Cys and GSH was each added to the probe incubated with 5 mM of Hg2+, we observed 100% of the FL recovery for both of them (Fig. S9, ESI†). The result encouraged us to verify the reusability of the sensor. We noticed that there exists a discrepancy in the equilibrated Cys/Hg2+ or GSH/Hg2+ molar ratio,40,41 and thus before the recyclable test, the FL response of various concentrations of Cys or GSH to the SiNCs–Hg2+ system was recorded (Fig. S10, ESI†). Both FL intensities were observed to increase gradually with increasing the concentration of biothiol, and it revealed that uorescence of the SiNCs was nearly restored at 6 mM of Cys or GSH when Hg2+ was at 5 mM. This suggests that a complete recovery of uorescence can be obtained at a biothiol to mercury molar ratio slightly higher than one. Fig. 6 shows the recyclability of the sensor in the detection of 1 mM Hg2+, represented by the use of cysteine (1.2 mM). As presented, each FL quenching caused by Hg2+ is reversed by cysteine to the original FL, and this can be repeated for at least ve times. Aer the regeneration, the calibration of Hg2+ was also tested (Fig. S11. ESI†). Nearly no appreciable deviation (LOD ¼ 50 nM, Slope ¼ 0.59) from the results in Fig. 5b was revealed, indicating that the SiNCs are promising to be used for recyclable detection of Hg2+.
4. Conclusions In summary, highly photoluminescent silicon nanocrystals (SiNCs) prepared by a simple hydrothermal treatment of APTMS in the presence of sodium citrate can emit strong blue uorescence under UV irradiation. Hg2+ ions can immediately quench the uorescence of the SiNCs while other metal ions failed, prompting the development of sensor for Hg2+ with excellent selectivity by uorescence. More importantly, by means of the specic strong interaction between biothiols and Hg2+, the sensor has been made recyclable. In comparison to those sensors based on gold nanoclusters, carbon dots, or carbon nanotubes,42,43 the current one features both high selectivity and good reversibility; meanwhile, the favorable detection limit may satisfy the demand for mercury assay in some highly contaminated areas.
Fig. 6 The performance of the SiNC sensor for mercury (1 mM) detection for five cycles with the addition of cysteine (1.2 mM).
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Acknowledgements We acknowledge the funding support from the Ministry of Science and Technology of China (grant 2012BAD32B05-4), the National Basic Research Program of China (grants 2010CB934700, 2013CB933900, 2014CB931800), the National Natural Science Foundation of China (grants 91022032, 91227103, 21061160492), the Chinese Academy of Sciences (grant KJZD-EW-M01-1), and the China Postdoctoral Science Foundation (grant 2013M531515).
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Highly photoluminescent silicon nanocrystals for rapid, label-free and recyclable detection of mercuric ions† Jia Zhang and Shu-Hong Yu* Hydrothermal treatment of 3-aminopropyltrimethoxysilane (APTMS) in the presence of sodium citrate generates a suspension of highly fluorescent silicon nanocrystals that fluoresces blue under UV irradiation. The photoluminescent quantum yield of the as-prepared silicon nanocrystals was calculated to be 21.6%, with quinine sulfate as the standard reference. Only mercuric ions (Hg2+) can readily prevent the fluorescence of the silicon nanocrystals, indicating a remarkably high selectivity towards Hg2+ over other metal ions. The optimized sensor system shows a sensitive detection range from 50 nM to 1 mM and a detection limit of 50 nM. The quenching mechanism was explained in terms of optical absorption
Received 5th November 2013 Accepted 7th January 2014
spectra and time-resolved fluorescence decay spectra. Due to the strong interaction of Hg2+ with the thiol group, the fluorescence can be fully recovered by biothiols such as cysteine and glutathione, therefore, a regenerative strategy has been proposed and successfully applied to detect Hg2+ by the
DOI: 10.1039/c3nr05896d
same sensor for at least five cycles. Endowed with relatively high sensitivity and selectivity, the present
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sensor holds the potential to be applied for mercuric assay in water.
1. Introduction The demand of uorescent probes to be used in biological staining, medical diagnosis, and chemical sensing is ongoing.1,2 Semiconductor quantum dots (QDs) have attracted enormous interest as a new generation of uorescent probes owing to their ultrahigh luminescence efficiency, excellent photostability, and size-tunable emission spectra.3 Among them, uorescent silicon nanocrystals (SiNCs) have been the subject of particular interest in recent years as prominent imaging agents since they exhibit favorable biocompatibility, low cytotoxicity, and even self-elimination.4 Although they have a number of advantages, a few technical difficulties impede the general applications of SiNCs. For example, most of current SiNCs require at least two independent routes to render them water-dispersible, rst to synthesize hydrophobic SiNCs by sophisticated physical or chemical methods,5–8 and second to cover them with hydrophilic molecules, including acrylic acid,9 allylamine,10 polymer,11 and micelle.12 This synthetic methodology achieves great success in producing uorescent SiNCs with a high prospect of bioimaging,13–15 however, they involve a set of relatively complicated procedures which are hard to reproduce. Very
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P.R. China. E-mail: [email protected]; Fax: +86-551-63603040 † Electronic supplementary information (ESI) available: Experimental details and additional gures. See DOI: 10.1039/c3nr05896d
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recently, He et al. has ingeniously developed a one-step microwave method for the quick synthesis of uorescent Si QDs.16 This method is simple and cost-effective. Until now, applications of the uorescent SiNCs are focused on bioimaging with few studies on chemical sensing.17 Mercury is one of the environmental contaminants that have been paid great attention in recent years. Emitted elementary mercury vapors from industrial and non-anthropogenic sources are easily transported in the atmosphere across oceans and continents and eventually accumulate on plants, in topsoil, and in waters as the ionized form (Hg2+).18 To date, the contamination of drinking water by Hg2+ ions remains a big problem. Long-term exposure to high Hg2+ levels is deleterious, causing permanent damage to the central nervous system and many other organs with serious sensory, motor, and cognitive disorders.19 In light of the infamous effects of mercury on the human body and the ecosystems, the World Health Organization (WHO) and United States Environmental Protection Agency (U.S. EPA) have regulated the maximum allowed levels of Hg2+ in drinking water to be 6 and 2 ppb, respectively. Although some traditional analytical techniques are viable for the sensitive determination of solvated Hg2+, they are generally reliant on massive instruments.20 As a result, the detection methods that meet with the criteria of facility, rapidness, and portability as well as accuracy are especially in demand. Fluorescence is one of the most powerful optical techniques for detecting trace concentrations of mercury. It has no inherent uorescence, suggesting that uorescence changes upon the specic interaction of Hg2+ with the probes, including small molecules,21
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semiconductor quantum dots,22 noble metal nanoclusters,23 polymer nanoparticles,24 uorescent carbon dots,25 carbon nanotube–DNA hybrid,26 and chemically modied nanoparticles.27 Most of these systems, however, suffer from limitations with respect to insufficient selectivity, utility of toxic materials, and incapability of reuse. Herein, we design a recyclable strategy for the rapid detection of Hg2+ in aqueous solutions, on the basis of label-free water-dispersible uorescent SiNCs. A homogeneous suspension of highly luminescent SiNCs was synthesized by a facile hydrothermal process, using 3-aminopropyltrimethoxysilane (APTMS) as the silicon source. We found that Hg2+ ion could attenuate the uorescence of the probe efficiently in a short time while many other cations can not, indicating a superb selectivity towards Hg2+. Moreover, the uorescence quenched by Hg2+ can be fully restored by biothiols such as cysteine and glutathione, which makes the SiNCs a fascinating recyclable sensor for Hg2+ that can be used for at least ve times.
2.
Materials and methods
2.1. Reagents and apparatus APTMS and Hg(NO3)2 were purchased from J&K Company. Glutathione and amino acids were purchased from Shanghai Sangon Biotechnology Co. Ltd. Sodium citrate and the other metal salts were obtained from Shanghai Chemical Company. All the materials are analytical reagent and used directly. Deionized water was used throughout the experiments. UV-Vis optical absorption spectra were recorded on a Shimadzu UV-2550 optical spectrophotometer. The uorescence spectra were recorded on a Hitachi F-7000 uorescence spectrophotometer. High-resolution transmission electron microscopy images were obtained using a JEOL-2010 transmission electron microscope equipped with energy dispersive X-ray (EDX) spectroscopy. Fourier transform infrared (FTIR) spectra were measured on a Bruker Vector-22 FTIR spectrometer from 4000 to 400 cm1 at room temperature. The C, H and N elemental contents were measured on a Vario EL III elemental analyzer system. The uorescence lifetimes were measured using a time-resolved/steady state uorescence spectrometer (Edinburgh Instrument Xe-900).
2.3. Detection of Hg2+ ion by the SiNCs An aqueous solution of SiNCs with a volume of 100 mL was mixed with different volumes of acetate buffer (pH 5.9, 20 mM) followed by the addition of diluted solutions of mercuric ions. The total volume of the solution was set at 1 mL, and the absorbance of SiNCs at 350 nm was equal to 0.013. Without time control, the uorescence spectra was recorded under excitation at 350 nm. The sloth widths of the excitation and emission were both 5 nm.
3.
Results and discussion
Following the preparation strategy previously reported,16 we employed APTMS for the hydrothermal synthesis of the SiNCs. The changes in the optical absorption spectra (Fig. 1) of the reactant precursor solutions identied the conversion. Aer hydrothermal treatment, a new absorption peak centered at 350 nm emerged, which can be explained by the generation of SiNCs. This agrees with the previous report.16 The result suggests that the pathways related to the APTMS reduction by citrate ions have no obvious difference between microwave heating and hydrothermal processing. Fig. 2a shows a representative transmission electron microscopic (TEM) image of the SiNCs. It revealed an image of densely packed, ill-shaped short nanobers, in large contrast to the monodispersed, spherical nanoparticles prepared by microwave irradiation. The selected-area electron diffraction (Fig. S1, ESI†) of the silicon nanobers showed a diffraction pattern of polycrystalline diamond cubic phase Si (JCPDS 271402), further conrming the synthesis of SiNCs. Fig. 2b shows a magnied image of the Si nanobers which possesses a kinked connection indicated by the arrow. A higher magnication of the selected part in Fig. 2b clearly reveals the interplanar spacing of 0.31 nm corresponding to {111} lattice fringe (Fig. 2c). The observed growth direction along h111i has been reported to be common for Si nanorods,28 while spherical Si particles tend to have h220i growth direction.8,29 This result suggests that the growth of the brous SiNCs experienced a different mechanism from that for the microwave grown Si nanoparticles. EDX (Fig. 2d) shows the existence of Si and O in the sample (C may not be conclusive since carbon exists in the copper grind matrix). The presence of C and N was further
2.2. Synthesis of uorescent SiNCs In a typical experiment, 1 g of sodium citrate was dissolved in 30 mL of water, followed by oxygen removal through purging with nitrogen gas of high purity for about 30 min. 1 mL of APTMS was added and homogeneously agitated, aerwards, the solution was transferred into a Teon-lined autoclave of 50 mL volume, which was sealed and maintained at a temperature of 160 C for 12 h and then naturally cooled to room temperature. The resulting transparent solution was dialyzed against stirred water in a dialysis bag with a 1 kDa cut-off molecular weight with change of water every 6 h before precipitate occurred in the bag. Finally, the solution in the bag was collected and stored at 4 C for use.
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Fig. 1 UV-Vis absorption spectra of the precursor reactant solutions and the purified solution of SiNCs.
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(a–c) TEM images of the SiNCs with different magnification and (d) EDX spectrum of the sample.
Fig. 2
identied by the FTIR spectrum and elemental analysis. The FTIR spectrum (Fig. 3d) clearly provides evidence for the amino group in the sample, reected by the N–H bending vibration and stretching vibration at 1570 and 3400 cm1, respectively. Moreover, the elemental analysis revealed contents of C and N to be 18.64% and 5.46%, respectively, giving an atomic ratio (C/ N) of 4. When irradiated by an UV light (365 nm), the solution of SiNCs gave off a strong blue uorescence (Fig. 3a). No uorescence was seen to emit from the reactant precursor solutions, further identifying the formation of SiNCs. By xing the emission wavelength at 440 nm, the excitation spectrum showed a well-resolved peak focused on 350 nm (Fig. 3b), reecting the optical absorption peak of the solution of SiNCs. The
Fig. 3 (a) Photographs of the reaction precursor solution and the asprepared solution of SiNCs under UV irradiation (365 nm). (b) The normalized FL excitation spectrum (dotted line) and emission spectra of the solution of SiNCs at different excitation wavelength. (c) The relationship of the integrated FL intensity with absorbance at 350 nm for the SiNCs and quinine sulfate. (d) FTIR spectrum of the SiNCs.
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uorescence (FL) emission intensity is reliant on the excitation wavelength, increasing gradually and reaching a maximum at 350 nm. Meanwhile, the FL emission peak did not shi with the excitation wavelength, a phenomenon that was observed for the Si QDs prepared by the electrochemical etching of bulk silicon.17 The uorescence quantum yield (QY) of the SiNCs was measured by adapting a comparative method from the work of Sankaran et al.,30 using quinine sulfate as the standard reference (Fig. 3c). As presented, the integrated emission intensity for each sample is plotted versus the corresponding optical absorbance at the same excitation wavelength. Fletcher reported that no obvious deviations were observed for the quantum yield of quinine sulfate from a constant value with an excitation range from 240 to 400 nm.31 The excitation wavelength of the SiNCs solution has its maximum at 350 nm, hence we chose 350 nm as the excitation wavelength. The quantum yield of SiNCs was calculated to be 21.6%, derived from the ratio of the slopes of the tted linear lines, and the known quantum yield of quinine sulfate (54%). Such a high QY is comparable to those of uorescent Si nanoparticles in aqueous solutions or organic solvents.16,32 Prior to evaluating the metal ion selectivity, some fundamental issues about the uorescence of the SiNCs need to be addressed. Irrespective of what buffer was used, it revealed a similar tendency of pH-dependent FL intensity, suppressed in acidic environments and reaching the maximum at slightly alkaline conditions (Fig. S2a and b, ESI†). This behavior may be related to the presence of amino groups on the SiNCs surfaces. The time–scan FL intensity plot (Fig. S2c, ESI†) revealed that 90% of the initial uorescence was preserved aer 1 h of continuous excitation at 350 nm. This feature of FL instability may be attributed to the existence of crystallographic defects containing kinked brous SiNCs. Moreover, the uorescence of SiNCs was not reduced by the increase in the ionic strength of solution (Fig. S2d, ESI†), suggesting a possible use for biological imaging. The FL lifetime of the SiNCs was also measured (Fig. S3b, ESI†). The decay curve can be tted with a double exponential function, giving two distinct decay times of 1.37 (s1, 20%) and 10.8 (s2, 80%) ns, respectively. This implies the occurrence of competing fast and slow electron–hole recombination processes in the brous SiNCs.33 It has been reported that H2O2 can quench the uorescence of Si nanoparticles, based on which a sensor for glucose was designed.17 However, no attempt to investigate the interaction of SiNCs with metal ions has ever been conducted. For this purpose, incubation of SiNCs with Hg2+ ions (10 mM) and other metal ions (0.5 mM) was performed to measure the FL intensity (Fig. 4). Only Hg2+ suppressed the uorescence of SiNCs by 80% of original intensity, while others elicited minor effects, demonstrating excellent selectivity to mercury. Hg2+ is known as a common uorescence quencher due to the formation of nonradiative electron-transfer or energy-transfer.34 To elucidate the quenching mechanism, we recorded the optical absorption spectra and FL decay curve (Fig. S3, ESI†). An observed change in the absorption spectra of the SiNCs solution aer the addition of Hg2+ indicates the conjugation of silicon with Hg2+, which is a signature of static quenching. We believe that the interaction of SiNCs with Hg2+ originates from the presence of
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Fig. 4 The specificity of the SiNCs towards metal ions in a phosphate buffer (20 mM, pH 5.9).
amino groups on the Si surfaces via the affinity of Hg2+ to N.35 In the meantime, the tted FL decay trace experienced a dramatic shi from a double exponential function to single exponential function with the lifetime of 10.1 ns (s2). The change in the FL lifetime signies dynamic quenching. From these pieces of evidence, we propose that the FL quenching of SiNCs by Hg2+ involves both dynamic and static processes. Some parameters need to be assessed before the quantication of Hg2+ by the SiNC probe. It is shown that pH not only affects the initial FL intensity but also the quenched FL intensity (Fig. S4, ESI†). With respect to Hg2+ at 2 mM, higher quenching efficiencies were observed at more acidic conditions, suggesting that the weakly acidic media may be preferable for the detection of Hg2+. However, it was also noted that the quenching by Hg2+ of 0.5 mM at pH 4.9 was not better than that at pH 5.9, thus the optimal pH value was selected at 5.9. The concentration of SiNCs has a measurable inuence on the detection, as it is known that lowering the concentration of uorophore for a quencher will make the uorescent change (F0/F) larger and thus the limit of detection (LOD) lower.36 This is identied by the LODs as 0.1, 0.2, and 0.5 mM, respectively, in the order of decreasing SiNC concentration (Fig. S5, ESI†). Moreover, we found that the buffer composition could also affect the assay. Three kinds of buffer with the same pH value and concentration were chosen, and in the presence of same amount of SiNCs, the best detection performance (i.e. the lowest LOD and the highest sensitivity) was obtained for the acetate buffer (compared in Fig. S5f, S6b† and 5b). With regard to the time for assay, it was found that the FL quenching by Hg2+ was immediately stable within 1 min aer ion spiking (data not shown), thus making the assay a rapid one without strict time control. Fig. 5a shows the FL response upon the addition of Hg2+ ions. It presents a gradual decrease of FL intensity with increasing Hg2+, and along with the attenuation of intensity, the emission peak did not shi. The response toward Hg2+ over the concentration range from 0 to 1 mM can be tted to a highly linear line (inset of Fig. 5b) expressed as F/F0 ¼ 1 0.6 [Hg2+] (R2 ¼ 0.998), where F and F0 are the quenched and initial uorescence, respectively. The LOD was 50 nM, based on three times the response in the presence of Hg2+ (signal) versus the standard deviation of the blank (noise) by following the
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Fig. 5 (a) The detection of Hg2+ by the SiNC probe (A350nm ¼ 0.013), using 350 nm as the excitation wavelength. (b) The relationship between the FL intensity ratio and the mercury concentration. The inset shows the detection range from 0 to 1 mM with a fitted linear equation. The data are based on the average of two repetitive measurements.
International Union of Pure and Applied Chemistry (IUPAC) criteria.37 Albeit with a LOD slightly higher than the maximum level of Hg2+ in drinking water permitted by the WHO, the presented sensor may allow the on-spot detection of Hg2+ in some suspected highly contaminated sites in a prompt and specic way, in large contrast to those depending on complicated instruments. A recyclable sensor is one that can be adjusted to its original state by means of appropriate measures. A typical example of such a method is the use of chemical reagents. With respect to sensors for Hg2+, the addition of sodium EDTA, biothiols, or sulde ions have been tested for recovery, while the degree of reversibility depends on specied factors, including the sensor type and assay pH. For example, the EDTA solution induced a complete restoration of uorescence of the Hg2+-treated porphyrin-modied Au@SiO2 nanoparticle sensor,38 while cysteine and glutathione were found not to reverse the uorescence of Hg2+-treated CdTe quantum dots39 to their initial states. We rst investigated the effect of EDTA solution on the uorescence of the SiNCs–Hg2+ system. Even at ten times the concentration of Hg2+, EDTA did not completely recover the uorescence of the SiNCs (Fig. S7, ESI†). We then tested the effects of cysteine (Cys) and glutathione (GSH). A preliminary study clearly revealed the largest FL recovery in the presence of Cys and GSH, compared with other amino acids (Fig. S8, ESI†). In addition, both of the biothiols were found not to affect the
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uorescence of the SiNCs. When 10 mM of Cys and GSH was each added to the probe incubated with 5 mM of Hg2+, we observed 100% of the FL recovery for both of them (Fig. S9, ESI†). The result encouraged us to verify the reusability of the sensor. We noticed that there exists a discrepancy in the equilibrated Cys/Hg2+ or GSH/Hg2+ molar ratio,40,41 and thus before the recyclable test, the FL response of various concentrations of Cys or GSH to the SiNCs–Hg2+ system was recorded (Fig. S10, ESI†). Both FL intensities were observed to increase gradually with increasing the concentration of biothiol, and it revealed that uorescence of the SiNCs was nearly restored at 6 mM of Cys or GSH when Hg2+ was at 5 mM. This suggests that a complete recovery of uorescence can be obtained at a biothiol to mercury molar ratio slightly higher than one. Fig. 6 shows the recyclability of the sensor in the detection of 1 mM Hg2+, represented by the use of cysteine (1.2 mM). As presented, each FL quenching caused by Hg2+ is reversed by cysteine to the original FL, and this can be repeated for at least ve times. Aer the regeneration, the calibration of Hg2+ was also tested (Fig. S11. ESI†). Nearly no appreciable deviation (LOD ¼ 50 nM, Slope ¼ 0.59) from the results in Fig. 5b was revealed, indicating that the SiNCs are promising to be used for recyclable detection of Hg2+.
4. Conclusions In summary, highly photoluminescent silicon nanocrystals (SiNCs) prepared by a simple hydrothermal treatment of APTMS in the presence of sodium citrate can emit strong blue uorescence under UV irradiation. Hg2+ ions can immediately quench the uorescence of the SiNCs while other metal ions failed, prompting the development of sensor for Hg2+ with excellent selectivity by uorescence. More importantly, by means of the specic strong interaction between biothiols and Hg2+, the sensor has been made recyclable. In comparison to those sensors based on gold nanoclusters, carbon dots, or carbon nanotubes,42,43 the current one features both high selectivity and good reversibility; meanwhile, the favorable detection limit may satisfy the demand for mercury assay in some highly contaminated areas.
Fig. 6 The performance of the SiNC sensor for mercury (1 mM) detection for five cycles with the addition of cysteine (1.2 mM).
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Acknowledgements We acknowledge the funding support from the Ministry of Science and Technology of China (grant 2012BAD32B05-4), the National Basic Research Program of China (grants 2010CB934700, 2013CB933900, 2014CB931800), the National Natural Science Foundation of China (grants 91022032, 91227103, 21061160492), the Chinese Academy of Sciences (grant KJZD-EW-M01-1), and the China Postdoctoral Science Foundation (grant 2013M531515).
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