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Folic acid functionalized silver nanoparticles with sensitivity and selectivity colorimetric and fluorescent detection for Hg2+ and efficient catalysis

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 355702 (http://iopscience.iop.org/0957-4484/25/35/355702) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 25 (2014) 355702 (11pp)

doi:10.1088/0957-4484/25/35/355702

Folic acid functionalized silver nanoparticles with sensitivity and selectivity colorimetric and fluorescent detection for Hg2+ and efficient catalysis Dongyue Su1, Xin Yang1, Qingdong Xia1, Qi Zhang1, Fang Chai1,2, Chungang Wang2 and Fengyu Qu1 1

Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, Harbin Normal University, 150025, People’s Republic of China 2 Faculty of Chemistry, Northeast Normal University, Changchun, 130024, People’s Republic of China E-mail: [email protected], [email protected] and [email protected] (Fengyu Qu) Received 14 April 2014, revised 3 July 2014 Accepted for publication 11 July 2014 Published 12 August 2014 Abstract

In this research, folic acid functionalized silver nanoparticles (FA-AgNPs) were selected as a colorimetric and a ‘turn on’ fluorescent sensor for detecting Hg2+. After being added into Hg2+, AgNPs can emit stable fluorescence at 440 nm when the excitation wavelength is selected at 275 nm. The absorbance and fluorescence of the FA-AgNPs could reflect the concentration of the Hg2+ ions. Thus, we developed a simple, sensitive analytical method to detect Hg2+ based on the colorimetric and fluorescence enhancement of FA-AgNPs. The sensor exhibits two linear response ranges between absorbance and fluorescence intensity with Hg2+ concentration, respectively. Meanwhile, a detection limit of 1 nM is estimated based on the linear relationship between responses with a concentration of Hg2+. The high specificity of Hg2+ with FA-AgNPs interactions provided the excellent selectivity towards detecting Hg2+ over other metal ions (Pb2+, Mg2+, Zn2+, Ni2+, Cu2+, Co2+, Ca2+, Mn2+, Fe2+, Cd2+, Ba2+, Cr6+ and Cr3+). This will provide a simple, effective and multifunctional colorimetric and fluorescent sensor for on-site and real-time Hg2+ ion detection. The proposed method can be applied to the analysis of trace Hg2+ in lake water. Additionally, the FA-AgNPs can be used as efficient catalyst for the reduction of 4-nitrophenol and potassium hexacyanoferrate (III). S Online supplementary data available from stacks.iop.org/NANO/25/355702/mmedia Keywords: silver nanoparticle, folic acid, Hg2+, colorimetric, fluorescence, detection (Some figures may appear in colour only in the online journal) 1. Introduction

(WHO) standard for the maximum allowable level of inorganic mercury in drinking water is no more than 6 ppb (30 nM) [3, 4]. Therefore, environmental monitoring of aqueous Hg2+ has become increasingly important. Accordingly, great efforts have been devoted to the development of fluorescent and colorimetric sensors, which can selectively detect Hg2+ [1]. To meet this goal, a number of highly sensitive and selective Hg2+ sensors have been developed, based on gold

Mercury is one of the most toxic and dangerous heavy metal elements to human health and environmental safety because it is not biodegradable [1]. Mercury pollution comes from diverse sources including nature and human activities even at very low concentrations [2]. The water soluble divalent mercuric ion (Hg2+) is one of the most common and stable forms of mercury pollution. The World Health Organization 0957-4484/14/355702+11$33.00

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© 2014 IOP Publishing Ltd Printed in the UK

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nanoparticles, fluorophores, DNAzymesʼ polymer materials, streptavidin and proteins [5–14]. Among these sensors, noble metal nanoparticles (gold or silver nanoparticles) have attracted great interest as a colorimetric probe, which can directly detect analytes by monitoring the color change, using either UV–vis spectroscopy or even the naked eye. Due to the surface plasmon resonance (SPR) of gold or silver nanoparticles, the color change is highly sensitive to the size, shape, capping agent, medium refractive index, and state of noble metal nanoparticles [1–3, 15]. Many successful examples have been reported in the detection of Hg2+ by functionalized gold or silver nanoparticles [16–22]. Apparently, hardly any complicated instruments are involved in the detection procedures [23, 24]. Although many detection methods have been developed, ever-increasing health concerns still call for more sensitive ion-sensors with high selectivity to monitor the trace amounts of Hg2+. Recently it has been reported that AuNPs or Au nanoclusters can generate fluorescence in the visible, near-IR and red area [25]. It was found that AuNPs with the average diameter of 16 nm can emit stable fluorescence at 370 nm. The AuNPs could assemble with analytes to form larger aggregates through electrostatic interaction and coordinating interaction, which led to the significant enhancement of the fluorescence intensity [26, 27]. The advantage of these methods is that the concentration of Hg2+ can be determined by monitoring with both colorimetric and fluorescent method. In this paper, we present a simple colorimetric and fluorescent probe for the rapid detection of Hg2+ based on the folic acid modified AgNPs (FA-AgNPs). The quantification of Hg2+ can be determined according to the change in the surface plasmon resonance (SPR) absorption of the FAAgNPs solutions. Moreover, the fluorescence emission of FA-AgNPs can be turned on at about 440 nm after reacting with Hg2+. As expected, the experimental observations revealed that the fluorescence intensity of the FA-AgNPs was related to the concentration of the Hg2+ rather than to other heavy metal ions. Thus, a multifunctional colorimetric and ‘turn on’ fluorescence response to Hg2+ detection method was developed. This detection probe is simple (without complicated synthesis and modification techniques), cheap (no DNAzyme), rapid (colorimetric detection with 1 min), environmentally innoxious and highly active with two kinds of response (colorimetric and fluorescent detection) [25]. The Hg2+ can be determined by the naked eye or a UV–vis spectrometer, simultaneously, the Hg2+ can be used as a switch for turning on the fluorescence emission of FA-AgNPs. In addition, the typical catalysis of noble metal nanoparticles has the advantages of efficient activity and high selectivity under mild reaction conditions [16]. Since the reduction of 4-nitrophenol (4-NP) and potassium hexacyanoferrate (III) (K3[Fe(CN)6]) has received considerable interest for academic research as a model electron-transfer reaction, herein, the reduction of 4-NP and K3[Fe(CN)6] with NaBH4 was selected as a model reaction for monitoring the

catalytic activity of FA-AgNPs towards an organic and inorganic reaction, respectively [28].

2. Experimental section 2.1. Chemicals

All chemicals used were of analytical grade or of the highest purity available. Silver nitrate AgNO3 (99.99%), ascorbic acid, folic acid and 4-Nitrophenol (4-NP) (98%) were obtained from Aladdin and used as received. The used metal salts Cr(NO)3 · 9H2O, Pb(NO3)2, Ni(NO3)2 · 6H2O, Cd(NO3) · 4H2O, Mg(NO3)2 · 6H2O, FeCl2 · 4H2O, CaCl2 · 2H2O, Co(Ac)2 · 6H2O, Zn(Ac)2 · 2H2O, BaCl2 · 2H2O, CuSO4 · 5H2O, Mn(CHCOO)2, K2Cr2O7, Hg (NO3)2 · 2H2O and NaCl were purchased from Beijing Chemical Reagent Company (Beijing, China). Potassium hexacyanoferrate (III) (K3[Fe(CN)6]) (99.5%) was obtained from Tianjin Chemical Reagent Company (Tianjin, China). All glassware was thoroughly cleaned with freshly prepared 3:1 HCl/HNO3 (aqua regia) and rinsed thoroughly with Mill-Q (18.2 MΩ cm−1 resistance) water prior to use. Mill-Q water was used to prepare all the solutions in this study. 2.2. Characterization

The morphology and size of the FA-AgNPs were characterized by transmission electron microscopy (TEM) using a JEOLFETEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Absorption spectra were recorded on a UV–vis spectroscopy performed with a UV-2550 spectrophotometer (Shimadzu, Japan) at room temperature. Fluorescence spectra of FA-AgNPs were recorded by a PerkinElmer LS-55 fluorescence spectrometer. 2.3. Preparation of FA-AgNPs

With slight modifications from the reported paper, we synthesized folic acid functionalized silver nanoparticles (FA-AgNPs) to be used as the sensor for Hg2+ [29]. By using ascorbic acid as a reducing agent and folic acid as the capping agent, we prepared FA-AgNPs. In this experiment, 30 uL of 2 M AgNO3 was added into 50 mL of water and heated to boiling. Then 2 mL of 0.6% folic acid and 1 mL of 0.1 M ascorbic acid (pH was controlled to be between 11–12 by NaOH) were added simultaneously. After boiling for 10 min, the orange solution was formed and cooled to the room temperature. The FA-AgNPs solution was isolated by centrifugation at 10 000 rpm for 20 min and redispersed in water to remove the free folic acid and ascorbic acid molecules. The obtained FA-AgNPs was the probe for detecting Hg2+. 2.4. Detection for Hg2+

The Hg2+ stock solution was 3 mM, which was used for the Hg2+ sensitivity studies. Various concentrations (0.001 2

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−50.0 μM) of Hg2+ were prepared using serial dilution of the stock solution to test the sensitivity limits of the FA-AgNPs. The colorimetric detection of aqueous Hg2+ was performed at room temperature. Briefly, 300 μL of FA-AgNPs solution was added to 300 μL of Hg2+ different concentrations of aqueous solution samples, which indicated that the ratio of volume between probe and analyte was 1:1. In the experiments of selectivity and practical assay, all samples were tested in a similar way. We investigated the selectivity of our new approach for Hg2+ over other metal ions (Pb2+, Cu2+, Mg2+, Zn2+, Ni2+, Co2+, Ca2+, Mn2+, Fe2+, Cr3+, Cr6+, Cd2+ and Ba2+) under the same conditions. Furthermore, the addition of enough salt would screen the repulsion between the negatively charged FA-AgNPs, and lead to aggregation of the AgNPs followed by a corresponding color change [30, 31]. To improve the sensitivity, we tested adding 75 μL of 0.1 M NaCl into 5 mL of FA-AgNPs as a probe to produce a color change quickly. After adding the NaCl solution, the color of FA-AgNPs changed from pink to orange, which can be observed in figure S1. Thus, the probe of detection was the FA-AgNPs with the addition of a certain amount of NaCl. In this work, UV–visible spectra and fluorescence emission were acquired to investigate the interaction between ions and FAAgNPs. After detection by colorimetric method, the samples were then left to react for 3 h prior to fluorescence measurements with excitation at 275 nm.

3. Results and discussion 3.1. Characteristic of FA-AgNPs

The prepared FA-AgNPs was a solution that displayed an orange color as observed by the naked eye. Figure 1(A) shows the optical absorption spectra of the prepared FAAgNPs: a typical UV–vis spectra with a plasmon band at approximately 414 nm, which can be attributed to the characteristic SPR peak of silver nanoparticles. Another SPR peak of FA-AgNPs at about 531 nm, which can be attributed to the coupled plasmon absorbance of nanoparticles in close contact, indicated that the state of nanoparticles of FA-AgNPs was linked to each other and took on chain structure [35–37]. The TEM images of FA-AgNPs observed in figure 1(C) proved the conclusion deduced from the SPR spectra. The emission spectrum of FA-AgNPs shows an emission maximum at 440 nm while being excited at 275 nm (figure 1(B)). The emission of FA-AgNPs was weak but stable, which we can see from related reports [26, 27]. From the TEM of FAAgNPs, the nanoparticles of FA-AgNPs can be observed with a diameter in the range of 6−9 nm. Though the particles of FA-AgNPs were dispersed, most of them arrayed to some chain’s structure, which was the result in the two characteristic SPR peaks of the FA-AgNPs. Energy-dispersive x-ray (EDX) microanalysis of the FA-AgNPs confirmed the presence of the peaks’ characteristic of pure silver (figure 1(D)). The presence of small amounts of oxygen can be ascribed to the carboxylate groups of FA, which is unavoidable. The Si peak is due to the grid used to perform the measurement.

2.5. Analysis of real samples

To confirm the practical application of the probes, we collected water samples from a freshwater lake on our campus and filtered them through a 0.2 μm membrane, then analyzed them using ICP-AES (table S1). We then prepared a series of samples by spiking them with a standard solution of Hg2+ in the range of 0.001−50 μM [32]. In the experiments of selectivity and practical assay, all samples were tested by the above method.

3.2. Mechanism of detection for Hg2+

The mechanism of the phenomenon that we propose is shown in figure 2(A). As illustrated in figure 2(A), the prepared spherical FA-AgNPs were stable throughout the distribution of the chain structure, because the folic acid protects the AgNPs from aggregation in the presence of a given high concentration of salt. Folic acid is an important vitamin and it has many donor atoms including two carboxylic groups [38]. It is expected that the amine groups bind with the AgNPs and that the acid groups stabilize the AgNPs from aggregation by electrostatic repulsion. The carboxylate group of FA-AgNPs have a much stronger affinity toward Hg2+ ions in water (log β4 = 17.6) [36]. Due to the FA-AgNPs containing free carboxylic groups, these free carboxylic groups strongly coordinate with Hg2+ to form a stable chain structure [27]. When in the presence of Hg2+, Hg2+ led to an aggregation of the AgNPs followed by a corresponding color change. The FA-AgNPs then assembled together and the aggregated FA-AgNPs formed a big block of particles. The absorbance of FA-AgNPs blue shifted and the corresponding color of FAAgNPs solution changed from orange to yellowish within 1 min. After incubation with Hg2+ for 3 h, significant aggregation caused the FA-AgNPs solution turned on blue fluorescence emission under excited at UV lamp. The AgNPs could conjugate with Hg2+ to form AgNPs-Hg aggregates through electrostatic interaction and coordinating interactions,

2.6. Catalytic properties

The catalytic properties of FA-AgNPs were systematically examined by two experiments involving the reduction of 4NP and K3Fe(CN)6 [33, 34]. The catalytic reduction was carried out in a quartz cuvette and monitored using UV–vis spectroscopy at room temperature. For the catalytic reduction of 4-NP, aqueous solutions of 4-NP (0.01 M, 0.03 mL) and NaBH4 (0.5 M, 0.2 mL) were mixed with water (2.5 mL) in a quartz cuvette without stirring. After 5 μL of FA-AgNPs solution was added in, the reaction was monitored by measuring absorption spectra. The reduction of K3Fe(CN)6 was carried out according to a typical reaction, and 0.1 mL of 8 × 10−3 M K3Fe(CN)6 was added into 2 μL of FA-AgNPs solution, followed by the rapid addition of 0.2 mL of 0.040 M ice-cold fresh NaBH4. 3

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Figure 1. (A) UV–vis absorption spectra of FA-AgNPs, (B) fluorescence emission of FA-AgNPs, (C) TEM images of FA-AgNPs, (D) EDX spectra of FA-AgNPs.

which led to the significant enhancement of the fluorescence intensity of the FA-AgNPs [26, 39]. Thus, the quantification of Hg2+ can be obtained by colorimetric and intensity of fluorescence spectra of FA-AgNPs. The prepared FA-AgNPs displayed excellent salt resistance in NaCl (aq) thanks to the folic acid stabilizing agent. The UV–vis spectra of the solutions changed slightly, so the detection probe was a mixed solution created by adding 75 μL of 0.1 M NaCl to 5 mL of FA-AgNPs. Figure 2 shows the SPR of the FA-AgNPs probe and their corresponding UV–vis spectra with the addition of Hg2+. When 50 μM of Hg2+ solution was added, the color of the FA-AgNPs probe solution changed from orange to yellowish immediately, which reflected the aggregated states of the AgNPs induced by Hg2+. Meanwhile, as shown in figure 2(B), the UV–vis spectra of the sample exhibited a blue shift with decreasing intensity. The decrease of intensity and larger blue shift of SPR of FAAgNPs were due to their larger size distribution and aggregation, which was caused by the presence of Hg2+. To further confirm that these AgNPs aggregate clearly, we compared the TEM images of FA-AgNPs before and after incubation with Hg2+. Corresponding TEM images of FAAgNPs probe (figure 2(D)) and the sample after added Hg2+ (figure 2(E)) exhibited obvious contrast. Compared to FAAgNPs probe, the addition of Hg2+ made the dispersed Ag nanoparticles aggregate immediately. Thus, we confirm that the color change of the FA-AgNPs probe in the presence of

Hg2+ was caused by the aggregation of AgNPs. After incubation for 3 h, the further aggregation can be examined from figure 2(F), as shown by a large number of Ag nanoparticles accumulated in big blocks. The sample in which the FAAgNPs probe detected Hg2+ turn on a strong blue fluorescence emission from the original non-luminance. The quantification of Hg2+ can also be reflected by the intensity of the fluorescence spectra of FA-AgNPs (figure 2(C)). The corresponding fluorescence spectra of the FA-AgNPs probe (curve a) and the sample after Hg2+ is added (curve b) can be observed in figure 2(C). The FA-AgNPs show a slight emission maximum at 440 nm while being excited at 275 nm; no obvious emission can be seen from the photo image at the UV lamp. After being mixed with Hg2+, the fluorescence emission of FA-AgNPs turned on and increased intensely, which can be proved by the corresponding fluorescence spectra and photo image under the UV lamp. In comparison, the sample with Hg2+ turned on a significant blue brightness, however, the control was nonluminous under the same conditions. This observation clearly indicated that the FA-AgNPs could be used as a turn on fluorescent sensor for the detection of Hg2+. 3.3. Sensitivity 3.3.1. Colorimetric detection. To evaluate the detectable

minimum concentration of Hg2+ by colorimetric response and increase in fluorescence according to the above optimized 4

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Figure 2. (A) Strategy for Hg2+ detection using FA-AgNPs colorimetry, (B) UV–vis absorption spectra of FA-AgNPs and after the addition

of 50 μM of Hg2+, (C) fluorescence spectra of FA-AgNPs and after the addition of 50 μM of Hg2+. TEM images and diameter distribution of (D) FA-AgNPs, (E) FA-AgNPs in the presence of 50 μM of Hg2+ in a minute and (F) FA-AgNPs in the presence of 50 μM of Hg2+ for 3 h, respectively.

dramatically with an increase in Hg2+ concentration, which is due to the fact that the Hg2+ accelerated the aggregation of the FA-AgNPs. When the concentration of Hg2+ was up to 10 μM, one of the two SPR peaks of the sample disappeared, while another decreased to an inconspicuous peak at about 531 nm. It can be clearly seen that the orange color of the solutions gradually became buff when the concentration of Hg2+ was increased from 0–50 μM (figure 3(C)). With the naked eye alone, we can detect Hg2+ at a concentration of 1.0 μM, which can be detected from figure 3(C). This absorbance is attributed to the coupled plasmon absorbance of nanoparticles in close contact. The SPR of the solution at

experimental conditions, after the different concentrations of Hg2+ reacted with FA-AgNPs, the UV–vis spectra and fluorescence emission of samples were recorded respectively. The SPR absorption of the FA-AgNPs solutions with various concentrations of Hg2+ was also recorded in a quantitative assay (figure 3(A)). In the absence of Hg2+, it showed an initial sharp and characteristic SPR band at 414 nm and 531 nm. With the increase of the concentration of Hg2+, the absorption peak at 414 nm decreases while that at 531 nm decreases and blue shifts, along with the color of the solutions gradually changing from orange to yellowish (figure 3(C)). It was clearly observed that the SPR absorption decreased 5

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The observed emission intensity enhancement can be rationalized in terms of photoinduced electron transfer and metal binding-induced conformational restriction upon complexation [26, 42]. According to the previous report, the mechanism of the ‘turn on’ fluorescence can be deduced that FA-AgNPs aggregation can reduce their vibration and rotation speed [27]. Weakness of the Brownian movement and decrease of collision probability between the AgNPs result in the increase of the external energy transfer rate and quantum yield. Another possible mechanism which has been proposed is a photoinduced electron transfer and metal ion binding that could restrict the excited-state rotation of a biaryl chromophore, suppressing intersystem crossing and leading to increased emission [42, 43]. Otherwise, the large-diameter AgNPs aggregation formed can lead to the self-absorption being significantly decreased. AgNPs have a strong optical absorption over a wide region, which leads to an enhancement of the fluorescence intensity of FA-AgNPs [26]. The related photo images in daylight and UV-light of different concentrations of Hg2+ reacted with FA-AgNPs can be observed in figure 4(B). In comparison, the fluorescence emission was enhanced gradually with the increase of concentration of Hg2+. Based on the enhancement of emission intensity, the concentration of Hg2+ was determined. The detectable limit of Hg2+ was 0.1 μM at UV-lamp by the naked eye. The inset of figure 4(A) showed a plot of fluorescent intensity versus the concentrations of Hg2+ in the range of 0.001–5 μM. A good linearity was obtained from 1 nM to 1 μM of Hg2+ (R2 = 0.9965) (the regression equation was I = 33.8778 C + 43.4598) and the detection limit was found to be 1 nM by fluorescent detection. The above results demonstrate the sensitivity of the FA-AgNPs probe for Hg2+ with colorimetric and fluorescence response.

Figure 3. (A) UV–vis absorption spectra of FA-AgNPs after the

addition of different concentrations of Hg2+ (0, 0.001, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 50 μM). (B) Plot of A531/414 versus the concentrations of Hg2+ in the range of 0.001–1 μM, respectively. The error bars represent standard deviations based on three independent measurements. (C) The corresponding photo images of detection (orientation of the arrowhead: 0.001, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 50 μM, blank).

414 and 531 nm are related to the quantities of dispersed and aggregated FA-AgNPs, respectively. Thus, we used the ratio of the values of absorbance 531/414 (A531/414) to express the molar ratio of aggregated and dispersed FA-AgNPs. So, the relationship of ratio (A531/414) relative to concentrations of Hg2+ was investigated (figure 3(B)). The value of A531/A414 is linear with the Hg2+ concentrations within a range from 1 to 1000 nM (R2 = 0.969) (equation I = 0.9299-0.082 78C). The lowest detectable concentration of Hg2+ is 1 nM, which was lower than the maximum level of 10 nM allowed by the US EPA and standard of 30 nM allowed by the WHO [40, 41].

3.3.3. Effect of reaction time. In order to investigate the

effect of incubation time with different concentrations of Hg2+ on the fluorescence intensity, kinetic data were obtained by changing both the concentration of Hg2+ and reaction time. Thus, eight different incubation times with five concentrations were investigated. The results (figure 5) showed that the enhancement of intensity was related to both concentration and time. As expected, the incubation time indeed influenced the fluorescence emission responses of the FA-AgNPs in the presence of Hg2+. Additionally, that the intensity of fluorescence increased continuously with addition of Hg2+ can be proved adequately. With the addition of reaction time, the intensity of fluorescence emission of all samples raised gradually. In absence of Hg2+, the fluorescence emission of the FA-AgNPs gradually increased when extending the incubation time, but the intensity of fluorescence emission enhanced little within 7 h. By comparison, the increase in fluorescence intensity was more remarkable in the presence of all kinds of concentrations of Hg2+. With the same concentration of Hg2+, the fluorescence emissions of the FA-AgNPs increased dramatically after mixing for 3 h, indicating that the more severe aggregation appeared due to the action between FA-AgNPs and Hg2+. It was clearly

3.3.2. Fluorescent detection. The Hg2+ can be detected by

colorimetric method within several seconds by FA-AgNPs. The FA-AgNPs also can be used as a turn on fluorescent sensor for detecting Hg2+. In order to evaluate the sensitivity of the sensor further, the fluorescence emission of FA-AgNPs probe detecting samples were investigated. Figure 4(A) shows the fluorescence emission spectra of FA-AgNPs in the presence of different concentrations of Hg2+. It can be seen that as the concentration of Hg2+ increased after incubation for 3 h, the fluorescence intensity at 440 nm enhanced accordingly. On adding Hg2+ into FA-AgNPs, the fluorescence emission intensity at 440 nm was enhanced. 6

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Figure 4. (A) Fluorescence spectra of solutions of FA-AgNPs on addition of Hg2+ solution (0–50 μM) two days later. The inset shows a plot

of fluorescent intensity versus the concentrations of Hg2+ in the range of 0.001–5 μM. The error bars represent the standard deviations based on three replicate measurements. (B) The corresponding photo images of detection under UV lamp (orientation of the arrowhead: 0.001, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 50, 0 μM). 3.4. Selectivity

To verify that the sensor FA-AgNPs was highly selective toward mercury species, the colorimetric and fluorescent response were all measured. The selectivity was determined by testing other environmentally relevant metallic ions, including Fe2+, Ni2+, Cr6+, Cu2+, Co2+, Mg2+, Ca2+, Cd2+, Ba2+, Zn2+, Cr3+, Mn2+, and Pb2+, respectively, all at a concentration of 50 μM (figures 6 and 7). From the figure 6(B), only Hg2+ causes the aggregation of FA-AgNPs, resulting in a color change from orange to yellowish within several seconds. This selectivity can be seen with the naked eye. The colorimetric detection of Hg2+ over other ions can be measured by SPR of FA-AgNPs (figure 6(A)). The intensity of two SPR peaks of FA-AgNPs with chain states adding all of the ions did not obviously change except for Hg2+, which indicated the prominent selectivity of FA-AgNPs for Hg2+ by colorimetric method. The selectivity of detection could also be reflected by the fluorescence emission results shown in figure 7(A). In contrast to a significant fluorescence increase as observed for Hg2+, very little change in fluorescence intensity was observed upon exposure to other metal ions. The corresponding photo images were displayed in figure 7(B). In comparison, after adding Hg 2+, the FA-AgNPs turn on blue fluorescence emission by strongly aggregating. The result showed that only Hg2+ ions could significantly enhance the luminescence emission of FA-AgNPs. To quantify the spectral enhancements of the fluorescence emission of the samples, the ratio of intensity (I/I0) of FAAgNPs reacting to all metal ions is determined (figure 7(C)). In the presence of Hg2+, an obvious enhancement of the value can be observed. However, the other metal ions had no obvious effect on fluorescence emission, which demonstrated that FA-AgNPs are efficiently selective for Hg2+.

Figure 5. Fluorescence intensity of FA-AgNPs turned on by different

concentrations of Hg2+ (0−50 μM ) with different reaction time (0.5−7 h).

observed that the fluorescence emissions increase with addition of both concentration of Hg2+ and incubation time simultaneously. The photo images of fluorescent detection containing the sensitivity and selectivity with different reaction times were displayed in the figure S2. When in the presence of 50 μM of Hg2+, the FA-AgNPs turned on a strong fluorescent emission in 1 h, though the concentration of the Hg2+ sample had no obvious change. The intensity of fluorescent emissions of all samples with different concentrations of Hg2+ were increased with the addition of reaction time gradually. However, in the photo images of selectivity, the presence of other ions did not turn on the fluorescence emission of FA-AgNPs for a long time. 7

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Figure 6. (A) UV–vis absorption spectra of FA-AgNPs containing 50 μM other metal ions compared with 50 μM Hg . (B) The 2+

2+

2+

2+

2+

6+

2+

2+

corresponding photo images of detection (orientation of the arrowhead: Fe , Ni , Co , Cu , Cr , Mg , Ca , Cd2+, Ba2+, Zn2+, Cr3+, Pb2+, Mn2+, Hg2+, blank).

3.5. Practical experiments

In certain environmental samples, such as lake water, the concentrations of some metal ions or some unknown contaminants are significantly higher than that of Hg2+. Thus, potential practical assay is necessary, and it is a critical issue to the application of the most common sensors. To validate the practicality of our proposed colorimetric and fluorescent sensing strategy for the analysis of environmental samples, we used FA-AgNPs to determine the concentrations of Hg2+ spiked lake water samples. Hg2+ ions were not detected in the lake water samples, in good agreement with ICP-AES data. A series of samples were prepared by spiking them with a standard solution of Hg2+ in the range of 0.001−50 μM [32, 47, 48]. As depicted in figure 8(A), the corresponding UV–vis absorption spectra of FA-AgNPs with different concentrations (orientation of the arrowhead: 0, 0.001, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 50 μM) of Hg2+ were displayed, the intensity of absorbance of FA-AgNPs was decreased and blue shifting accompanied the increase of concentrations of Hg2+. The linear relation (range was 1 nM to 1 μM) with R2 of 0.9465 is described in the inset of figure 8(A). The minimum detectable concentration of Hg2+ in lake water by the naked eye was 3 μM, which is described in the related colorimetric detection images (figure S3A). The fluorescence emissions of FA-AgNPs detecting samples of lake water were also examined and the results can be seen in figure 8(B). With the increase in concentration of Hg2+, the fluorescence emission raised up gradually. Under the current lake water experimental condition, the lowest Hg2+ concentration that could be detected was 1 nM. The fluorescence intensity of FA-AgNPs toward Hg2+ increased linearly over the Hg2+ concentration range of from 1 nM to 3 μM. The linear relation (range was 1 nM to 2 μM) with R2 of 0.984 is described in inset of figure 8(B). The corresponding fluorescent detection images also proved the gradual increase of fluorescence emission with the addition of Hg2+ (figure S3B).

Figure 7. (A) Fluorescence spectra of solutions of FA-AgNPs on

addition of 50 μM other ions and 50 μM Hg2+ two days later. (B) The corresponding photo images of detection under UV lamp (orientation of the arrowhead: Mn2+, Pb2+, Cr3+, Zn2+, Ba2+, Cd2+, Ca2+, Mg2+, Hg2+, Cu2+, Cr6+, Ni2+, Fe2+, Co2+, blank). (C) Enhanced ratios (I/I0) of the fluorescence intensity of (A).

The aggregation-based scheme specific towards Hg2+ is probably due to the suitable coordination geometry conformation of the receptor, that simple carboxylic acids and amidogen of folic acid have a much-stronger affinity to the larger radius of the Hg2+, due to a strong interaction with the closed-shell d10 electronic configuration [44, 45]. The sensor showed superior selectivity both with the colorimetric and fluorescent detection for Hg2+. Colorimetric and fluorescence-based assay for mercury is multifunctional, simpler, more cost-effective and more selective [46]. 8

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Figure 8. Sensitivity of detection for lake water samples (A) UV–vis absorption spectra of FA-AgNPs containing different concentrations of Hg2+ (0–50 μM). Inset: plot of A531/414 versus the concentrations of Hg2+ in the range of 0.001−1 μM, (B) fluorescence spectra of FA-AgNPs on addition of Hg2+ solution (0–50 μM), inset shows a plot of fluorescent intensity versus the concentrations of Hg2+ in the range of 0.001–2 μM. The error bars represent the standard deviations based on three replicate measurements.

2+ 2+ 2+ 2+ Figure 9. (A) UV–vis absorption spectra and (B) fluorescence spectra of FA-AgNPs containing 50 μM other ions (Mg , Ni , Co , Pb , Ba2+, Zn2+, Ca2+, Cd2+, Cr3+, Mn2+, Cu2+, Fe2+, blank, Cr6+) compared with Hg2+ 50 μM in lake water.

practicality of using our probe for the determination of Hg2+ in environmental samples. All these results indicate that the FA-AgNPs provide both an effective colorimetric and fluorescent platform for Hg2+ detection with high sensitivity and selectivity. Apart from the visual detection of Hg2+ and its quantification by the UV–vis spectra, the fluorescence emission also permitted its quantification using the fluorescence spectra and color definition under a UV lamp.

To test for specificity, we prepared a series of samples of other ions (50 μM) in lake water by the same method of Hg2+. The UV–vis absorption and fluorescent spectra of samples were recorded respectively. As described in figure 9(A), the intensity of SPR of FA-AgNPs reacted with all ions that were still strong except Hg2+. Though the blue shift of SPR of FAAgNPs with Cr6+ occurred, it is negligible compared to the result of detection for Hg2+, which indicated good selectivity. The results can also be proved by colorimetric images of all ions in figure S4A. The intensity of fluorescence emission of FA-AgNPs reacted with other ions can be clearly distinguished from Hg2+ in figure 9(B). The fluorescence emission of FA-AgNPs is turned on by Hg2+, but other ions could not ‘turn on’ or enhance the fluorescence emission of FAAgNPs. The related photo images (figure S4B) under UV light indicated that compared to other ions, only Hg2+ enhanced the fluorescence emission of FA-AgNPs. So, the selectivity of detecting Hg2+ by FA-AgNPs can be deduced by two methods in lake water. These results reveal the

3.6. Catalytic experiments

The catalytic properties of FA-AgNPs were systematically examined in a model reaction based on the reduction of 4nitrophenol (4-NP) with sodium borohydride to 4-aminophenol (4-AP). The catalytic performance of the FA-AgNPs was quantitatively evaluated in the liquid-phase reduction of 4-NP by NaBH4 (figure 10(A)). As usual, 4-NP exhibits an absorption peak at 317 nm in neutral or acidic solution. The 9

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transfer reaction, we have employed the system to evaluate the catalytic activity of FA-AgNPs towards an inorganic reaction [53]. The light yellow aqueous K3[Fe(CN)6] solution showed absorption at 420 nm. After the addition of NaBH4, the intensity of absorption gradually decreased due to the formation of K4[Fe(CN)6]. The whole reduction process took about 12 h, which can be observed in figure S5. After the addition of 2 μL of FA-AgNPs, the absorption at 420 nm dramatically decreased immediately (figure 10(B)). As shown in figure 10(B), the reaction process was within 1 min, indicating the high catalytic activity of FA-AgNPs.

4. Conclusions We have developed a colorimetric and ‘turn-on’ fluorescent sensor for determination of Hg2+ in aqueous media with excellent sensitivity and selectivity by FA-AgNPs. It combines the special advantages of both colorimetric and fluorescent sensors and exhibits well unanimous for detecting Hg2+. This sensor can offer additional advantages to efficiently detect Hg2+ over other ions in environmental lake water. The FA-AgNPs are able to perform the role of both colorimetric (fluorescent) sensor for the detection of Hg2+ and catalyst in the reduction of 4-NP and K3[Fe(CN)6]. With excellent sensitivity and selectivity, this sensor is potentially suitable for the monitoring of Hg2+ in more environmental applications.

Acknowledgments Figure 10. (A) UV–vis absorption spectra at different time intervals

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project No. 21205024, 21173038), the National Science Foundation for Post-doctoral Scientists of China (Grant No. 2012M520659, 2013T60307), the Natural Science Foundation of Heilongjiang Province (No. B201305), the project of Harbin Science and Technology bureau (No. 2014RFQXJ151), Program for Scientific and Technological Innovation Team Construction in Universities of Heilongjiang (No. 2011TD010).

indicate the disappearance of the peak of 4-nitrophenolate ion at 400 nm due to the reduction of 4-NP to 4-AP by NaBH4 in the presence of FA-AgNPs. Inset: plot of ln[C(t)/C(0)] against the reaction time, (B) UV–vis spectra of the reduction of K3[Fe(CN)6] by NaBH4 in the presence of FA-AgNPs recorded at within 1 min. Inset: the photograph of the solution of K3[Fe(CN)6] and NaBH4 (left) before and (right) after the addition of FA-AgNPs.

addition of NaBH4 deprotonates the OH group of 4-NP, and the absorption peak shifts to 400 nm which is the characteristic peak of 4-nitrophenolate ion [49, 50]. The reduction kinetics were monitored by UV–vis absorption spectroscopy of the reaction mixture after the addition of the catalyst. Figure 10(A) presents the absorption variation of 4-NP solution (0.1 mM) during catalysis reaction by FA-AgNPs. When the reduction of 4-NP is started, the absorption peak at 400 nm gradually decreases in intensity with a concomitant increase of about 300 nm peak of 4-AP [51, 52]. Figure 10(A) shows the plot of ln[C(t)/C(0)] against reaction time, where C (t) and C(0) are the concentrations of 4-NP at time t and 0, respectively. From the linear plot (inset of figure 10(B)), the reaction rate constant k was determined to be 0.55 min−1, which indicated good catalytic activity of FA-AgNPs. Since the reduction of K3[Fe(CN)6] has received considerable interest for academic research as a model electron-

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Folic acid functionalized silver nanoparticles with sensitivity and selectivity colorimetric and fluorescent detection for Hg2+ and efficient catalysis.

In this research, folic acid functionalized silver nanoparticles (FA-AgNPs) were selected as a colorimetric and a 'turn on' fluorescent sensor for det...
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