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Silica Nanoparticles-Enhanced Fluorescent Sensor Array for Heavy Metal Ions Detection in Colloid Solution Juanjuan Peng, Junyao Li, Wang Xu, Lu Wang, Dongdong Su, Chai Lean Teoh, and Young-Tae Chang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02883 • Publication Date (Web): 25 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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Analytical Chemistry

Juanjuan Peng,*,† Junyao Li,† Wang Xu,‡ Lu Wang,§ Dongdong Su,§ Chai Lean Teoh,§ and YoungTae Chang*,○,# †

State Key Laboratory of Natural Medicines, Key Laboratory of Drug Quality Control and Pharmacovigilance, Ministry of Education, School of Basic Medical Sciences and Clinical Pharmacy, China Pharmaceutical University, Nanjing, Jiangsu, 211198, China. Fax: +86-25-86185655, Email: [email protected] ‡

Department of Chemistry, Stanford University, Stanford, California, 94305, United States

§

Singapore Bioimaging Consortium, Agency for Science, Technology and Research (A* STAR), 138667, Singapore

○Center

for Self-assembly and Complexity, Institute for Basic Science (IBS), Pohang 37673, Republic of Korea

#

Department of Chemistry, Pohang University of Science and Technology, Pohang, Gyeongbuk, 37673, Korea. Fax: +8254-279-3399, Email: [email protected] ABSTRACT: Sensitivity and detection limit are two vital factors that affect fluorophores-based sensing and imaging system. However, it remains a challenge to improve the sensitivity and detection limit of fluorophores, largely due to their limited response and photophysical properties. In this study, we report for the first time, a novel approach to enhance the sensitivity and detection limit of probes using silica nanoparticles, also known as silica nanoparticles-enhanced fluorescence (SiEF). SiEF can drastically improve the fluorescence intensities and detection limit of fluorophores. A SiEF-improved fluorescent sensor array for rapid and sensitive identification of different heavy metal ions is achieved. And a 3D spatial dispersion graph is obtained based on the SiEF-improved fluorescent sensor array, which provides a lower concentration dependent pattern than fluorophores alone, allowing qualitative, quantitative and sensitive detection of heavy metal ions. Furthermore, with UV lamp irradiation of the sensor–metal ion mixtures, the output signals enable direct visual of heavy metal ions with low concentration. Thus, the SiEF approach provides a simple and practical strategy for fluorescent probes to improve their sensitivity and detection limit in analytes sensing. KEYWORDS: SiO2 nanoparticles, fluorescence enhancement, heavy metals ions, sensor array

INTRODUCTION

Silica nanoparticles with well-defined morphology, since the initial report by Stöber et al,12 has been exploited in the development of various hybrid silica nanomaterials with distinct properties for applications in different fields.13-16 Encapsulation of organic fluorophores in silica nanoparticles can increase their photostability and emission quantum yield.17,18 Organic fluorophores were pre-modified with organoalkoxysilanes, then the fluorophores can be doped into silica nanoparticles in the process of hydrolysis and condensation of silica precursors.19 Therefore, various organic fluorophores can be incorporated into the SiO2 nanoparticles to become probes for imaging application. However, since the fluorophores are doped inside of the nanoparticles, the recognition function of the fluorophores cannot be utilized efficiently.

The past decade has witnessed great development in the field of fluorescent chemosensors, due to their high selectivity, fast response time, and technical simplicity.1-4 In general, a reliable fluorescent response signals could be provided by the sensing systems under the conditions of test. Most often, the detection of signals of interest are hindered by the background noise. In this regard, Metal-Enhanced Fluorescence (MEF) has been developed,5-7 which studies the interactions between fluorophores and the surface of metallic colloids to amplify the fluorescence. Along with signal enhancement, there has been a growing interest in utilities of MEF for the development of methods, probes, and devices in the biosciences.8-11 While most of the MEF process was generated on the substrates, which makes it hard for imaging and sensing application in aqueous solution.

In this study, we report for the first time, a simple approach that uses silica nanoparticles to enhance the fluorescence signal of fluorophores (SiEF). To demonstrate this, we used the sensor array probes (Singapore tongue, 3

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SGTs), which is capable of identifying various heavy metal ions, previously developed by our group.20 Unfortunately,

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General. Tetraethylorthosilicate (TEOS), absolute ethanol, ammonium hydroxide (28.0 wt%) were supplied by SigmaAldrich, chemical reagents were used as received without further purification. Deionized water was used throughout. Physical Measurements. UV/vis spectra and fluorescence were performed using a fluorimeter and UV/Vis instrument, BioTek Synergy 4 Hybrid Microplate Reader. TEM measurements were carried out on a JEL-1400 transmission electron microscope operating at an acceleration voltage of 100 kV. The zeta potentials of nanoparticles in water were measured on a Zetasizer Nano ZS90 (Malvern) instrument. Synthesis of the SiO2. SiO2 nanoparticles were synthesized according to the Stöber method.12 In brief, water (5 mL), ammonium hydroxide (0.8 mL), and ethanol (20 mL) were mixed at R.T. in a flask. After continuous stirring for 15 min, TEOS (10 L) in ethanol (200 L) was added. Then, the reaction mixtures were stirred for another 2 h. The final SiO2 spheres were cleaned for three times using centrifugations and then dispersed in ethanol.

Figure 1. Schematic illustration showing the procedures for silica nanoparticles-enhanced fluorescence (SiEF). (a) Fluorescence spectrum of SGT2 (0.2 M). (b) Fluorescence spectrum of SGT2 (0.2 M) mixing with SiO2 nanoparticles (2 mg/mL). (c) Structures of SGT1, SGT2 and SGT3.

Fluorescence. The fluorescence emission spectra of SGT1−3 enhanced by SiO2, then mixed with seven different heavy metal ions were measured on a Synergy 4 plate reader. The heavy metal ion concentrations range from 0, 5, 10 20, 30, 40, 50, 60, 100, 125, 150 and 175 nM, while the SGT1, SGT2 and SGT3 concentration was 0.4, 0.2 and 1 μM, respectively, the concentration of SiO2 nanoparticles are 0.25-1 mg/mL. The fluorescence intensities of each sensor−heavy metal ion pair was measured at their respective emission maxima with excitation at 365 nm.

the detection limits of the SGTs were unable to meet the standards and guidelines as recommended by World Health Organization (WHO) for heavy metals in drinking water. In this paper, through physical mixing of SGTs and SiO2 nanoparticles in the solution, both fluorescence signal intensity and detection limits of SGTs for heavy metal ions are obviously improved. The mechanism for the SiO2enhanced fluorescence was also proposed (Figure 1a and b). Positively-charged fluorophores (Figure 1c) were adsorbed on the surface of the negatively-charged silica nanoparticles, as such, enriching the fluorophores, which leads to enhancement of the fluorescence intensities. Furthermore, the positively charged heavy metals ions can be further enriched through the SiO2 nanoparticles and become easily accessible by the SGTs. We accomplished the construction of silica nanoparticles improved fluorescent sensor array for simple, fast and sensitive sensing of heavy metal ions. The fluorescent signals also enable direct identification of heavy metal ions by exposure the sensor (SGTs+SiO2)–metal ion mixtures to 365 nm lamp, making this a promising candidate for point-of-use monitoring of heavy metal ions in environmental samples. Finally, the practical applications of the constructed system were verified by testing tap water and spiked different kinds or concentrations of heavy metals into DI water.

RESULTS AND DISCUSSION Construction of SiO2 Enhanced Fluorescent Probes. Small-molecule fluorescent dyes, SGT1-SGT3 were synthesized according to the published paper20, and the structures of the dyes are shown in Figure 1c. The SiO2 nanoparticles were prepared according to the literature12 with an average diameter of ∼60 nm (Figure 2a). The size and morphology of SiO2 nanoparticles did not change after mixing with SGT2 and Hg2+ (Figure S1). To examine the effect of SiO2 nanoparticles on the optical properties of SGT1-3, different concentrations of SiO2 were added into the aqueous solutions containing SGT1-3. From the spectral results of SGT2 mixed with SiO2 nanoparticles (Figure 2b), we can see that the fluorescence intensities of SGT2 were obviously enhanced and the enhancement was proportional to the concentrations of nanoparticles. Notably, the addition of Hg2+ further increases the fluorescence intensity of SGT2SiO2. As shown in Figure 2c, pure SGT2 in the presence of Hg2+ (50 nM) only gave slight enhancement of fluorescence

EXPERIMENTAL SECTION 2


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solution (pH=3-5), being several times higher than that of SGTs-SiO2 dyes when dissolved in DI water. However, when SGTs-SiO2 was dissolved in acidic solution (pH=3-5), only weak and almost negligible response for heavy metal ions can be acquired. The response of SGTs-SiO2 towards Hg2+ at pH=3-5 is around 3 times lower than that in DI water (Figure S4). Therefore, all the tests were carried out in pH=6 DI water. Experiments were carried out to understand the mechanism of fluorescence enhancement by SGT1-3 when in the presence of SiO2 nanoparticles and Hg2+. Firstly, the SiO2 nanoparticles was non-fluorescent, demonstrating that the SiEF did not originate from the nanoparticles themselves (Figure S5). As the scattering of the nanoparticle to the excitation light depends greatly on their sizes, SiO2 nanoparticles with different sizes (60-500 nm) were prepared and employed to the sensing system. The results showed that all these nanoparticles can enhance the fluorescence of the SGTs (Figure S6). The increased extinction coefficient can also induce the enhancement of fluorescence. By testing the UV-vis spectrum of SGT2 with the addition of silica nanoparticles at different concentrations, we can see the excitation intensities of SGT2 was not enhanced by nanoparticles (Figure S7). Hence, the SiEF should not be ascribed to the change of extinction coefficient of the sensor.

Figure 2. (a) TEM image of the synthesized SiO2 nanoparticles. (b) Fluorescence spectra of SGT2 mixed with SiO2 nanoparticles with different concentration. (c) Fluorescence response of pure SGT2 or SGT2 enhanced by SiO2 to Hg2+. Insert: Fluorescence photography of different solution under UV lamp, 1: SGT2 (0.2 M), 2: SGT2 with 50 nM of Hg2+, 3: SGT2 with 0.25 mg/mL SiO2 , 4: SGT2 with SiO2 and 50 nM Hg2+. (d) Fluorescence intensity of SGT2 dispersed in aqueous solution with different pH.

signal, whereas SGT2-SiO2 emitted a remarkable increase in fluorescence under the same experimental condition, with a sensitivity enhancement of around 2.5 folds over direct detection. This means that SGT2-SiO2 can be used for detection of heavy metals with lower detection limit. When

Table 1. Hydrodynamic size and Z-potential change of SiO2 after mixing with SGT2 (1.25 M) and Hg2+ (6.25 nM).

the sensor–metal ion mixtures are irradiated with UV lamp, it shows that SGT2 alone reacted with Hg2+ bears very weak fluorescence change, while SGT2-SiO2 in the presence of Hg2+ shows clear fluorescence enhancement (Figure 2c insert). The same phenomena can also be observed for SGT1 and SGT3. Both fluorescent signals of SGT1 and SGT3 can be enhanced by SiO2 nanoparticles (Figure S2a and S3a) and the detection ability of SGT1-SiO2 and SGT3-SiO2 towards heavy metals ions are improved (Figure S2b and S3b).

Sample

Hydrodynam ic size (nm)

Z-Potential (mV)

SiO2

68.06

-21.8

SiO2+SGT2

78.82

-20.8

SiO2+SGT2+Hg2+

78.82

-19.8

As is well-known, SiO2 nanoparticles are negatively charged due to the abundant surface silanol groups. However, in the sensing system of SGTs to heavy metal ions, both the sensor SGTs and the analyte metal ions are positively charged, the repulsion forces between the two species can inhibit the binding. SiO2 nanoparticles can provide a negatively charged surface that reduces such repulsion, therefore enhance the fluorescence of SGTs with heavy metal ions. To testify the hypothesis, the hydrodynamic size and zeta-potential were employed to test the size and charge change of SiO2 after mixing with SGT2 and Hg2+ (Figure S8-13). The results were summarized in Table 1. After mixed with SGT2 (1.25 M), the hydrodynamic size of the SiO2 nanoparticles increased from 68.06 nm to 78.82 nm, whereas the zeta-potential changed

Mechanism study of fluorescence enhancement. To exclude the possibility of pH-induced fluorescence enhancement, SGT2 was dispersed in aqueous solution with different pH. The results show that SGT2 has the highest fluorescence in the presence of acidic solution (pH=5). The SiO2 nanoparticles were synthesized under basic conditions (ammonia, pH~8), while the fluorescence of SGT2 can only be raised under moderately acidic solution. Similarly, both SGT1 and SGT3 have higher fluorescence intensities in acidic solution rather than basic conditions (Figure S2c and S3c). Therefore, enhancement of the fluorescence cannot be attributed to the change of pH values by SiO2. Meanwhile, we also observed that fluorescence intensity of SGTs-SiO2 can be improved when they were dissolved in acidic 3



the surface charge is one important mechanism that the SiO2 nanoparticles can enhance the fluorescence of SGTs. Enhanced fluorescent probes for heavy metals ions detection. Next, the spectroscopic properties of SGT2 (0.2 µM) and SGT2-SiO2 in response to Hg2+ and Cd2+ were examined. In order to optimize the sensitivity and detection limit, different concentrations of SiO2 were added to a series of solution with SGTs or SGTs with Hg2+, the concentration of SGTs and Hg2+ were kept constant. Then the fluorescence of the solution with or without Hg2+ were compared. The value of ISGTs+SiO2+Hg2+/ISGTs+SiO2 were calculated to obtain the best concentration of SiO2 for the detection. As displayed in Figure 3a and 3c, only slight enhancement of SGT2 fluorescence in the presence of Hg2+ (0-50 nM) or Cd2+ (01.2 nM) can be observed. On the other hand, the response sensitivities towards Hg2+ or Cd2+ increased remarkably for SGT2-SiO2 under the same condition, (Figure 3b and 3d) with an improvement of around 2.5 to 3 folds. The enhancement of the fluorescence responses provides an opportunity to realize visual detection of Hg2+ and Cd2+. Aqueous solutions of the sensor (SGT2 or SGT2-SiO2) were placed in Black Greiner 96-well plates and were later mixed with heavy metals ions solutions. When the sensor–metal ion mixtures were irradiated with a 365 nm lamp, it can be observed that SGT2 alone shows very weak fluorescence change, while SGT2-SiO2 shows clear fluorescence color changes which enables direct identification of heavy metal ions (Figure 3a-d, top). The results demonstrate that the SiO2-enhancement approach can indeed strengthen the fluorescence response of the chemosensor towards heavy metal ions, facilitating rapid on-site detection.

Figure 3. (a) Photograph and fluorescence spectra of pure SGT2 (0.2 M) fluorescence response towards Hg2+ (5, 10, 20, 30, 40, 50 nM). (b) Photograph and fluorescence spectra of SGT2-SiO2 (0.25 mg/mL) fluorescence response towards Hg2+ (5, 10, 20, 30, 40, 50 nM). (c) Photograph and fluorescence spectra of pure SGT2 (0.2 M) fluorescence response towards Cd2+ (0.2, 0.4, 0.6, 0.8, 1.0, 1.2 nM). (d) Photograph and fluorescence spectra of SGT2SiO2 (0.2 M-0.25 mg/mL) fluorescence response towards Cd2+ (0.2, 0.4, 0.6, 0.8, 1.0, 1.2 nM). Excitation light is 365 nm UV lamps.

from -21.8 mV to -20.8 mV and finally to -19.8 mV after addition of Hg2+ (6.25 nM). This suggests that positively charged SGT2 and Hg2+ can be adsorbed on the negatively charged surface of SiO2 nanoparticles through electrostatic attraction.

Inspired by the excellent results of SGT2-SiO2 responses towards Hg2+ and Cd2+, the fluorescent response of SGTs and SGTs-SiO2 toward different heavy metals (Hg2+, Cd2+, Zn2+, Cu2+, Pb2+, Cr3+) were evaluated in solution. In the presence of 5-175 nM heavy metals, all the probes including SGTs and SGTs-SiO2 show different degree of fluorescent responses for heavy metals. While pure SGT1-SGT3 without SiO2 show weak response or almost no response (Figure S2037a), the response becomes more sensitive after enhancement by SiO2 nanoparticles (Figure S20-37b). During this process, the SiO2 nanoparticles are working as a “magnifying glass”, whereby the response signals of SGTs towards analytes were enlarged significantly. Hence, original undetectable signals can be detected with the help of SiO2. It should be pointed out that SGT1-SGT3 showed almost no response in the presence of 5 nM Hg2+ (Figure S20-22a). After enhancement by SiO2 nanoparticles, SGTsSiO2 in the presence of Hg2+ (5 nM) show good practical responses, which meets the WHO standard for drinking water (Figure S20-22b). The practical response of SGT2-SiO2 for Cd2+ can reach 0.2 nM (Figure S38 and S39). The

The direct absorption of SGTs on the SiO2 nanoparticles is also demonstrated. Different concentrations of SGT1-3 were added to the SiO2 nanoparticles solution and centrifuged. Subsequently, spectroscopic absorption analysis of the supernatant showed that almost all the SGTs were absorbed by the nanoparticles (> 90%). The amount of SGT1-SGT3 on the surface of SiO2 (1 mg) were 3 nmol, 2.8 nmol, 3 nmol (Figure S14-S16), respectively, which reveals that positively charged SGTs can be captured by the negatively charged silica surface of the nanoparticles, and therefore facilitate their further binding with the analyte metal ions. In order to confirm that negatively charged surface is the major cause of SiEF, a typical negatively charged nanoparticle, polyacrylic acid (PAA) nanoparticles were made and the fluorescence of SGT2 after mixing with PAA nanoparticles was test. The results showed that the fluorescence of SGT2 were also enhanced by the PAA nanoparticles (Figure S17). Also, the detection limit of SGTs to the heavy metals can be improved by PAA nanoparticles (Figure S18 and S19). Based on this, it can be deduced that 4


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detection limits of SGT1-SGT3 towards four kinds of heavy metal ions (Hg2+, Cd2+, Zn2+ and Cu2+) after fluorescence enhanced by SiO2 nanoparticles are calculated (3s/k) and summarized in Table 2. Appealingly, comparing with the detection methods obtained by only SGT2 (Table S1), the SiEF effect can remarkably reduce the detection limits. From the table, we can see almost all the detection limits of the sensor-ion pairs, except SGT1-SiO2 for Hg2+, have reached the WHO guidelines for drinking water. The different detection limits of the sensor-ion pairs can be mainly attributed to the different dissociation constants.20 More significantly, the fluorescence and sensitivity enhancement of SGTs by SiO2 is very quick, and fluorescence changes can be achieved and tested immediately after adding the analytes to probes without incubation or other complex process. And the detection system kept the same at least for 60 min, no precipitation can be observed and the fluorescence spectra almost same (Figure S40).

discerned under a relatively high concentration (100 nM, Figure S42). In comparison, when SiO2 nanoparticles were added, these heavy metal ions can be differentiated in the 3D fluorescence plots within low concentrations (5 nM). Among the metal ions, Zn2+, Hg2+, Cu2+, and Cd2+ exhibited stronger activity in the interaction with the SGTs-SiO2 system, and can gain higher fluorescence intensity than other ions. Consequently, we pursue to realize identification of these metal ions using the sensor array, and further improve the detection limit with the assistance of SiEF effect (Figure 4a). Due to the diversified interaction of metal ions to the sensor array, each kind of metal ion can result in different pattern of fluorescence-growth for the 3 dyes-SiO2. Therefore, by connecting the data points for each single metal ion in the 3D matrix, a unique curve for a certain metal ion can be obtained. As displayed in Figure 4, the curves for Zn2+, Hg2+, Cu2+, and Cd2+ are stretching toward different directions with the increasing concentration of metal ions. Based on these curves in the 3D graph, discrimination of these heavy metal ions can be achieved by locating the position of the sample point on the corresponding curve. Moreover, the concentration of metal ions can be quantified by reading the X, Y and Z coordinates of the sample point. Thanks to the SiEF effect, the sensitivity of the sensor arrays is greatly improved, thus the quantification can be done in a low concentration range down to 5 to 175 nM, the lower limit of which is one twentieth of the detection without SiEF.20

Table 2. Detection limits of SGTs or SGTs-SiO2 to different heavy metals (nM). Hg2+

Cd2+

Zn2+

Cu2+

SGT1-SiO2

12.6

3.01

5.20

10.1

SGT2-SiO2

2.01

0.109

2.08

11.2

SGT3-SiO2

1.81

0.0532

1.57

18.2

Real sample detection. In regards to applications, such sensor array-based 3D curves with enhanced fluorescence response can differentiate clean samples from polluted water with heavy metal ions at low concentration, as well as identify anonymous heavy metal ions pollutant in the sample. Water samples including tap water and DI water were analyzed with the established system. Even through the samples contain various other metal cations (Na+, K+, Ca2+ etc.) and anions (Cl-, HCO3-, etc.), no remarkable variation can be observed on the 3D graph. The DI water samples spiked with different type and concentrations of heavy metal ions were subjected to the general detection procedure. From Figure 4, it can be observed that each sample with the addition of specific cations (Hg2+, Zn2+, Cu2+ and Cd2+) is located at the very site of the corresponding curve, which not only reveals the type but also quantifies the concentration. These results demonstrated with the help of SiEF, our 3D analysis system based on the SGT chemosensor arrays can successfully identify water samples with heavy metal ions at low levels.

Enhanced fluorescent sensor array for heavy metal ions. Data processing methods were employed to transform the complex multi-dimensional data to a simple and easily visualized style. A statistical multivariate analysis model was established to identify the metal ions, whereby the fluorescence responses of the SGT1-3 and SGT1-3-SiO2 sensor array against the metal ions (Zn2+, Cu2+, Hg2+, Cr3+, Pb2+, and Cd2+) was plotted in 3D curve to provide differentiation information. Hence, by plotting 3D dispersion graph with the SGT1 or SGT1-SiO2 as X axis, SGT2 or SGT2-SiO2 as Y axis and SGT3 or SGT3-SiO2 as Z axis, all the fluorescence responses of the probes to heavy metals ions were included in the 3D graph, from which both the type and concentration of the heavy metals ions can be easily identified according to the location (Figure S41). Comparing the 3D fluorescence plots of SGT1-3 array with silica nanoparticles, it can be seen clearly that, without the assistance of SiO2, Hg2+, Zn2+ and Cd2+ can be only

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ACKNOWLEDGMENT This study was financially supported by NSFC ( 81701766) and A*STAR Joint Council Office (JCO/1231AFG028).

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Figure 4. (a) 3D plot of fluorescence spectra of SGTs-SiO2-heavy metals (0-175 nM), (b) fluorescence response (0-4 nM).

CONCLUSIONS In summary, we reported for the first time that silica nanoparticles can enhance the fluorescence of smallmolecule fluorescent dye SGT1-SGT3 and improve the detection limits of SGTs-SiO2 for heavy metal ions. With enhancement of around several folds over direct detection, the detection limits of SGT3-SiO2 for Hg2+ can reach 1.81 nM, whereas for Cd2+ is 0.0532 nM (3k/s). With the help of 3D dispersion graph of the responses, each heavy metal ion at the low concentration can be identified not qualitatively but also quantitatively. This SiEF enhanced sensitivity detection platform can serve as a convenient strategy in the development of new probes to improve their fluorescence intensity, sensitivity and detection limit towards analytes in environmental or biological samples at low levels.

ASSOCIATED CONTENT Supporting Information. TEM images of SiO2 after mixing with SGT2 and Hg2+, fluorescence spectra change of SGTs (SGT1, SGT2 and SGT3) mixed with SiO2 with different concentration, the adsorption rate of SiO2 to SGTs, the Z-potential and size change of SiO2 after mixing with SGT2, photography of SGTs or SGTs-SiO2 and fluorescence response towards different heavy metals including Hg2+, Cd2+, Cu2+, Zn2+, Pb2+ and Cr3+.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]

Notes

The authors declare no competing financial interest. 6


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