Approach Based on Polyelectrolyte-Induced Nanoassemblies for

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Approach Based on Polyelectrolyte-Induced Nanoassemblies for Enhancing Sensitivity of Pyrenyl Probes Zhiyi Yao,*,† Yadong Qiao,‡ Haiqin Liang,§ Wenqi Ge,§ Li Zhang,‡ Zhong Cao,§ and Hai-Chen Wu*,† †

Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, China § School of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410114, China S Supporting Information *

ABSTRACT: We have developed a unique approach for enhancing the sensitivity of pyrenyl probes based on polyelectrolyte-induced nanoassemblies and explored its sensing application toward 2,4,6-trinitrophenol (TNP). The key issue of the method is the formation of the nanoassemblies which possess high-density charges, specific surface area, and inner hydrophobic regions. These properties would help increase the loading of analytes and promote probe−analyte interactions, thereby leading to the prominent enhancement of the sensitivity. In the course of TNP detection, pyrene nanoassemblies can bind TNP efficiently through cooperative noncovalent interactions including electrostatic, π−π stacking, and charge-transfer interactions, resulting in the distinct fluorescent responses of pyrene moieties. This system has excellent selectivity and sensitivity for TNP detection. The detection limit is as low as 5 nM. It may be used for monitoring the TNP concentrations in real-world samples.

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modulated because it relies on the responses of the formation and collapse of the whole assembly of pyrene complexes. As part of our interest in the development of highly sensitive sensing systems, we attempt to develop a unique approach for enhancing the sensitivity of pyrene-based probes by employing polyelectrolyte-induced nanoassemblies of pyrenes (PNPs). The principle that underlies the strategy is shown in Scheme 1. According to the traditional approach, a simple pyrenyl probe could be functionalized with the opposite charges of the analytes to enhance the interaction between the probe and analyte. Even so, the sensitivities are usually quite low because the contact probability between the probe and analyte is reduced in the diluted solutions (Scheme 1A). In the present strategy, we utilize polyelectrolytes to induce positively or negatively charged pyrenes to form assembled nanostructures through electrostatic interactions as well as π−π stacking and hydrophobic interactions between the polyelectrolytes and pyrene probes (Scheme 1B). The nanostructures possess highdensity charges, specific surface area, and inner hydrophobic regions. These properties would help increase the loading of analytes and promote probe−analyte interactions, thereby leading to the prominent enhancement of sensitivity. As a proof-of-concept application, we chose 2,4,6-trinitrophenol (TNP) as a model analyte to test the proposed strategy.

yrene derivatives are one type of particularly useful materials for sensing applications because of their easy syntheses, chemical stability, and excellent photophysical properties such as high quantum yield and long fluorescence lifetime.1,2 Especially, the formation of excimers of pyrene moieties under certain conditions provides an alternative signal output to monomer emission upon the sensing events,3 which may afford a very useful ratiometric method for the fluorescent detection of a wide variety of analytes. Besides, the fluorescence emission of pyrene excimers is normally in the range of 450− 550 nm, which could be easily observed by naked eyes and facilitate visual detection of analytes. To date, numerous pyrene-based probes have been developed for the detection of biomolecules,4−8 metal ions,3,9,10 explosives,11 environmental pollutants,12 pH,13 and temperature,14 etc. In these sensing systems, different receptors are designed and attached to pyrene moieties to enhance the selectivity of the probes, but little success has been achieved to improve the sensitivity which is another key criterion for the evaluation of a sensor. Therefore, an effective strategy that can enhance the sensitivity of pyrene-based sensing systems is still highly desirable. Moreover, this issue is particularly critical for trace analysis. Recently, controlled assembly and disassembly of pyrene compounds have been used as an effective approach in sensing applications.15−19 Assembly of functionalized pyrene probes is often accompanied by the pyrene monomer−excimer transition, which provides a distinct ratiometric signal output.20 However, the sensitivity of this sensing mode could hardly be © XXXX American Chemical Society

Received: July 22, 2016 Accepted: October 5, 2016

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DOI: 10.1021/acs.analchem.6b02809 Anal. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Chemical Structures of PyBS, PyBTA, PDDA, and TNP and Schematic Illustration of the Detection of TNP Based on PyBTA and PyBS/PDDA Assemblies

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EXPERIMENTAL SECTION Sample Preparation. The stock solutions of TNP, poly(diallyldimethylammonium chloride) (PDDA), and common ions were prepared in pure water. The stock solutions of PyBS, PyBTA, and other organic interferences were initially dissolved in DMSO with a relatively high concentration, which were later diluted to the desired concentration with HEPES buffer. The detection of TNP was performed as follows: stock solutions of PyBS and PDDA were mixed directly to give PyBS/PDDA nanoassemblies with the desired concentration of each component in HEPES buffer (1 mM, pH 7.4) as the probe solution. Then, stock solution of TNP was added and the mixture was measured by fluorometric spectrometer immediately. Selective experiments were carried out under the identical conditions. The photographs of solution emission colors were taken under 365 nm UV light illumination. For real sample measurements, tap water was diluted 10 times with HEPES buffer and soil sample (1.0 g) was dissolved in 100 mL of HEPES buffer. After being filtered through a 0.22 μm membrane, fluorescent measurements for these samples spiked with varying concentrations of TNP were carried out following the similar procedures stated above. Measurements. Emission spectra and absorption spectra were collected by using a HORIBA Scientific Fluorolog-3 spectrofluorometer and a HITACHI U-3900 UV−vis spectrophotometer, respectively. NMR spectra were carried out on a Bruker Avance III 500 spectrometer. Transmission electron microscope (TEM) measurements were carried out a HITACHI H-7650B. Atomic force microscopy (AFM) analysis was conducted on a scanning probe microscopy (SPM, Multimode 8, Bruker) under ambient conditions.

TNP is one of the main explosives together with 2,4dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT), and often more powerful than these two analogues even at picomolar concentration levels.21,22 It has attracted increasing attention due to the growing concerns on homeland security, public safety,23 and its toxicity to organisms.24,25 Also, TNP is widely used in firework and matchbox factories, dye industries, and pharmaceutical fields. It is difficult to be degraded in both biosystems and the environment due to its electron-deficient nature, thus becoming one of the major environmental pollutants.26−28 Although various methods have been developed for the detection of TNP including high-performance liquid chromatography (HPLC),29,30 gas chromatography/mass spectrometry (GC/MS),31 surface-enhanced Raman spectrometry (SERS),32 ion mobility spectrometry,33 energy-dispersive X-ray diffraction,34 electrochemical assays,35,36 capillary electrophoresis,37 and fluorescent detection based on nanomaterials38,39 and synthetic receptors,40−43 some limitations still exist such as time-consuming sample processing, high cost, reliance on expensive equipment, use of organic solvent, poor water solubility, low sensitivity and selectivity, etc. Thus, it is still of great significance to detect TNP with high sensitivity, selectivity, and portability. In this work, we designed and synthesized two pyrenyl probes, namely, sodium 4-(pyren-1yl)butane-1-sulfonate (PyBS) and N,N,N-trimethyl-4-(pyren-1yl)butan-1-aminium bromide (PyBTA). The former is used for the detection of TNP based on the PNPs approach and the latter as the control based on the traditional approach. The PNPs approach affords a rapid detection of TNP in aqueous media with sensitivity ∼100 times higher than that of the traditional approach. Compared with the previous reports of TNP sensors, PNPs exhibit excellent selectivity toward TNP over other nitroaromatics including TNT, DNT, 4-nitrophenol (NP), and 2,4-dinitrophenol (DNP), whose interferences are very difficult to eliminate in most TNP sensing systems. Additionally, the PNPs approach provides a dual luminescence including monomer and excimer emission of pyrenes which permits a ratiometric detection of TNP and minimizes the interferences from the environmental background. The details for the sensing performance and the mechanism of the strategies will be discussed in the following sections.

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RESULTS AND DISCUSSION Preparation and Characterization of Pyrenyl Probes and Nanoassemblies. Pyrenyl probes, PyBS and PyBTA, were prepared by reacting 1-pyrenebutanol with PBr3 and then treated with Na2SO3 and trimethylamine, respectively (Figures S1−S6). PyBS dissolves in HEPES (2-[4-(2-hydroxyethyl)-1piperazinyl]ethanesulfonic acid) buffer (1 mM, pH 7.4) with a dispersed form and exhibits strong monomer fluorescence, marked by emission maxima at 395 and 415 nm upon B

DOI: 10.1021/acs.analchem.6b02809 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry excitation with 365 nm light (Figure 1).1 Upon the addition of PDDA, the intensity of PyBS emission at 395 and 415 nm

decreased and a new broad emission band around 486 nm increased gradually, which indicated the formation of pyrene excimers resulting from the PyBS aggregation induced by PDDA.5 From Figure 1 we could see that when the concentration of PDDA was increased to 10 μM, the intensity of pyrene excimer emission (at 486 nm) reached maximum and the monomer emission (at 395 and 415 nm) reached minimum, respectively, and remained unchanged upon the further increase of PDDA concentration. Therefore, the concentration of 10 μM was chosen for the construction of the nanoassemblies of PyBS/PDDA and subsequent examination of its sensing ability toward TNP. To substantiate the formation of the nanoassemblies, the morphologies of PyBS/ PDDA and PyBS were investigated by TEM and AFM (Figure 1B and Figure S7). Under TEM observations, the as-prepared PyBS/PDDA nanoassemblies are monodispersed nanoparticles which are not observed in the PyBS sample under identical conditions. Sensing Performance of Pyrenyl Probes and Nanoassemblies. Figure 2A demonstrates the emission spectra of PyBS/PDDA upon titration with TNP in HEPES buffer. It was found that, upon the addition of increasing amounts of TNP, the emission bands associated with the pyrene excimer (486 nm) decreased gradually in intensity, and reduced by 87% when the concentration of TNP reached 1.0 μM. Interestingly, the monomer emission of pyrene in PyBS/PDDA was not affected by the addition of TNP, and there existed a good linear relationship between the ratio of fluorescence intensity at 486 nm (I486) and 395 nm (I395), I486/I395, and the concentration of TNP (R = 0.991 from 1.0 to 50 nM, Figure 2B). This means the sensor can be applied for the ratiometric detection and quantification of TNP within the above concentration range. We also investigated the responding time of PyBS/PDDA to

Figure 1. (A) Emission spectra of PyBS (10 μM) in the absence and presence of increasing concentrations of PDDA as indicated. λex = 365 nm. (B) Typical TEM images of PyBS/PDDA assemblies.

Figure 2. (A) Emission spectra of PyBS/PDDA in the presence of increasing concentrations of TNP as indicated. λex = 365 nm. (B) The relationship between the ratio of I486/I395 and the concentration of TNP. (C) Emission spectra of PyBTA in the absence and presence of increasing concentrations of TNP as indicated. λex = 345 nm. (D) The relationship between I/I0 and the concentration of TNP from 50 to 5000 nM, where I0 and I represent the fluorescence intensity of PyBTA at 402 nm in the absence and presence of TNP, respectively. [PyBTA] = 10 μM. C

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Analytical Chemistry TNP under the same conditions. Figure S8 shows that the introduction of TNP (final concentration 5.0 μM) into the PyBS/PDDA solution caused the fluorescence quenching at 486 nm in less than 10 s. These results indicated that the PyBS/ PDDA nanoassemblies could be used as a fluorescent sensor for the rapid detection of TNP. The limit of detection (LOD) was determined to be 5 nM, which is among the lowest reported for the fluorescent detection of TNP (Table S1). Notably, the emission color of the PyBS/PDDA solution changed in the presence of different concentrations of TNP which could be observed by naked eyes under 365 nm illumination with a UV lamp. As shown in Figure S9, upon increasing the concentrations of TNP, the luminescence of the solution turned weaker. When the concentration of TNP is greater than 0.5 μM, the changes in the solution fluorescence could be clearly discerned. Thus, 0.5 μM was set to be the LOD for the visual detection of TNP. To elucidate the sensitivity-enhancing effect of the nanoassemblies, we compared the sensing performance of PyBS/ PDDA with that of PyBTA, a pyrene derivative with a cationic ammonium moiety which can bind to TNP. It can be seen from Figure 2, parts C and D, that, upon the addition of increasing amounts of TNP, the emission of PyBTA was slightly quenched and there is a good linear relationship between the ratio of fluorescence intensity at 402 nm (I/I0) and the concentration of TNP from 50 to 5000 nM (R = 0.993). The LOD was measured to be 1.0 μM. Therefore, the nanoassembly approach enhanced the sensitivity of fluorescent sensing of TNP by approximately 2 orders of magnitude. This drastic enhancement should be attributed to the unique properties of the PyBS/ PDDA nanoassemblies. To evaluate the specificity of PyBS/PDDA toward TNP based on this approach, we examined the emission spectra of PyBS/PDDA in the presence of various organic interferences including TNT, DNT, methylbenzene (MB), nitrobenzene (NB), 3-nitropropionic acid (NPA), phenol (PHE), 4-nitrophenol (NP), 2,4-dinitrophenol (DNP), and common ions in water, such as K+, Na+, Ca2+, Mg2+, SO42−, CO32−, NO3−, Cl−, and H2PO4− at the same concentrations of 2.0 μM. From Figure S10 we could see that, with the addition of those analytes, the profiles of PyBS/PDDA emission spectra barely changed except for TNP, which caused remarkable fluorescence quenching. The value of (I0/I) was calculated to better illustrate the selectivity of PyBS/PDDA toward TNP (Figure 3A), where I0 and I are the emission intensities of PyBS/PDDA at 486 nm in the absence and presence of the analyte. It was found that all the values of (I0/I) corresponding to other analytes remained low (≤1.4), except that TNP gave a significantly high value (11.9), 8.5 times higher than DNP, which is a major interference in most TNP sensing systems. These results indicated that PyBS/PDDA nanoassemblies are highly selective for the detection of TNP. In addition, this selective assay for TNP could also be observed by monitoring the fluorescence color of the PyBS/PDDA solutions (Figure 3B). Mechanism of TNP Detection Using the PNPs Approach. It is critical to address the molecular mechanism behind the observations. According to the literature,5,44,45 there are two types of mechanisms for explaining the quenching of pyrene excimer: one is indirect quenching from the dissociation of the pyrene assembly and the other is direct quenching through the noncovalent interactions between the pyrene and quenchers. In the former case, the decrease of excimer emission is most often accompanied by the restoration of the monomer

Figure 3. Relative fluorescence intensity at 486 nm (A) and photographs (B) of PyBS/PDDA in the presence of various analytes. [PyBS] = [PDDA] = 10 μM; [all analytes] = 2 μM. Inset: chemical structures of TNP, TNT, DNP, and NPA.

fluorescence. However, in this work, the above process was not observed. Therefore, the mechanism of the approach in this work is very likely the direct quenching of the excimer fluorescence. To confirm this proposal, we examined the UV− vis spectra of PyBS/PDDA assembly in the absence and presence of TNP. As shown in Figure S11, a significant change was observed in the absorption spectra of PyBS after the addition of PDDA. Simultaneously, peaks related to the protons of the pyrene ring became broadened and hardly visible in the 1 H NMR spectra (Figure S13). These changes should be attributed to the favorable π−π interactions between the pyrene moieties and the masking effect provided by the polymer assembly, which further confirmed the formation of the PyBS/ PDDA assembly. Upon the addition of TNP, the profile of the UV spectra of PyBS/PDDA was barely affected, implying that the dissociation of the PyBS/PDDA nanoassemblies did not occur. Another proof was provided by the TEM images of PyBS/PDDA in the presence of TNP (Figure S12). It was found that the nanoparticles of PyBS/PDDA still existed with similar size upon the addition of TNP, indicating that the presence of TNP did not disassemble the PyBS/PDDA nanoassemblies. These results provide more direct evidence for the quenching of excimer fluorescence by TNP. To obtain more insight into the sensing mechanism, we compared the emission spectra of PyBS/PDDA in the presence of TNP and its structural analogues (TNT, DNP, and NPA in Figure 3A). Several considerations are discussed below. First, there was no obvious response in the fluorescence of PyBS/PDDA with the introduction of TNT, implying that the phenol group of TNP is essential and electrostatic interactions should be the primary driving force for the analyte recognition. For additional support of the conclusion, ζ-potential measurements were run to evaluate the surface charge status of the PyBS/PDDA nanoassemblies. It was shown that the initial ζ-potential value of PyBS/PDDA was 17.7 mV, and then it decreased to 8.02 mV after the addition of TNP. The results indicated that the added TNP was loaded in or around PyBS/PDDA assemblies driven by electrostatic interactions. Second, it is noted that DNP and NPA did not induce the fluorescence changes of the PyBS/ D

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water) were obtained, indicating the PyBS/PDDA nanoassemblies could be practically useful in the rapid detection of TNP with high sensitivity. The LOD for TNP in tap water and soil was determined to be 22.9 ppb and 10 nM, respectively.

PDDA nanoassemblies either. Given the fact that DNP has one less nitro group than TNP and NPA lacks aromatic ring, one may conclude that charge-transfer and π−π stacking interactions also play important roles in promoting the quenching of the excimer fluorescence of PyBS/PDDA, which were further confirmed by comparing NMR spectra of PyBS and PyBS/PDDA in the absence and presence of TNP (Figure S13). Application in Real Samples. To verify the feasibility of the application of this approach in practical TNP detection, we conducted emission spectra measurements of PyBS/PDDA nanoassemblies in the presence of interfering analytes. Figure S14 shows the fluorescence intensity ratios of I/I0 in the absence and presence of various interferences. It was found that the introduction of these interferences only slightly affected the sensing performance of the PyBS/PDDA nanoassemblies toward TNP. Prompted by the robustness of this sensing system, we attempted to explore its practical applicability for the rapid detection of TNP in the environmentally and healthrelevant samples including soil and tap water. After simple pretreatments, TNP spiked in these samples was detected by monitoring the emission spectra of PyBS/PDDA. Figure 4 and Figure S15 show the spectral responses of this sensing system in the presence of soil extraction and tap water, respectively. The results are similar to those obtained in HEPES buffer. Good linear relationships between I/I0 and the concentration of TNP in the range of 1−50 nM (R = 0.98 for soil, 0.99 for tap

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CONCLUSIONS In conclusion, we have developed a unique approach for enhancing the sensitivity of pyrene-based probes through the formation of polyelectrolyte-induced pyrene nanoassemblies. The properties of as-prepared nanoassemblies such as highdensity charges, specific surface area, and inner hydrophobic regions greatly increase the efficiency of its interactions with analytes and improve the sensitivity by approximately 2 orders of magnitude compared with simple probe−analyte bindings. The mechanism of this sensing strategy is attributed to the formation of a PyBS/PDDA/TNP complex through cooperative noncovalent interactions including electrostatic, π−π stacking, and charge-transfer interactions. The LOD is as low as 5 nM. We anticipate that the PNPs approach demonstrated here is not only useful for the rapid and sensitive detection of TNP but also can be applied to constructing new sensing systems based on different pyrene analogues which are useful for detecting other analytes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b02809. Detailed synthesis and characterization of probes, additional fluorescent spectral data, and TEM and AFM images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-88235745. Fax: +86-10-88235745. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Nos. 21204089, 21375130, 31571010, 21374106).

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Figure 4. (A) Emission spectra responses of PyBS/PDDA toward soil extract spiked with different amounts of TNP (0−5 μM). Inset: the emission color changes of PyBS/PDDA before and after the addition of TNP in spiked samples. λex = 365 nm. (B) The relationship between the ratio of I/I0 at 486 nm and the concentration of TNP. Inset: linear range of I/I0 vs TNP concentration at low-concentration range. [PyBS] = [PDDA] = 10 μM. E

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DOI: 10.1021/acs.analchem.6b02809 Anal. Chem. XXXX, XXX, XXX−XXX