Water-Soluble Nonconjugated Polymer Nanoparticles with Strong

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Water-Soluble Nonconjugated Polymer Nanoparticles with Strong Fluorescence Emission for Selective and Sensitive Detection of Nitro-Explosive Picric Acid in Aqueous Media Shi Gang Liu, Dan Luo, Na Li, Wei Zhang, Jinglei Lei, Nian Bing Li, and Hong Qun Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07407 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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ACS Applied Materials & Interfaces

Water-Soluble Nonconjugated Polymer Nanoparticles with Strong Fluorescence Emission for Selective and Sensitive Detection of Nitro-Explosive Picric Acid in Aqueous Media Shi Gang Liu †, Dan Luo †, Na Li †, Wei Zhang ‡, Jing Lei Lei §, Nian Bing Li †*, and Hong Qun Luo †* Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of

†

Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, P.R. China Chongqing Institute of Green and Intelligent Technology, Chinese Academy of

‡

Sciences, Chongqing 400714, P.R. China School of Chemistry and Chemical Engineering, Chongqing University, Chongqing

§

400044, P.R. China

*

Corresponding Authors

*

Nian Bing Li

*

2, Tiansheng Road, BeiBei District, Chongqing, 400715, China. Tel: +86 23

68253237; fax: +86 23 68253237; E-mail address: [email protected] *

Hong Qun Luo

*

2, Tiansheng Road, BeiBei District, Chongqing, 400715, China. Tel: +86 23

68253237; fax: +86 23 68253237; E-mail address: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT A kind of water-soluble nonconjugated polymer nanoparticles (PNPs) with strong fluorescence emission was prepared by hyperbranched polyethyleneimine (PEI) and D-glucose

via Schiff base reaction and self-assembly in aqueous phase. The preparation

of the PEI- D-glucose (PEI-G) PNPs was facile (one-pot reaction) and environmentally friendly under mild conditions. Also, the PEI-G PNPs showed a high fluorescence quantum yield in aqueous solution, and the fluorescence properties (such as concentration-dependent and solvent-dependent fluorescence) and the origin of intrinsic fluorescence was investigated and discussed. The PEI-G PNPs were then used to develop a fluorescent probe for fast, selective, and sensitive detection of nitro-explosive picric acid (PA) in aqueous media, because the fluorescence can be easily quenched by PA whereas other nitro-explosives and structurally similar compounds only caused negligible quenching. A wide linear range (0.05 – 70 µM) and a low detection limit (26 nM) were obtained. The fluorescence quenching mechanism was carefully explored, and it was due to a combined effect of electron transfer, resonance energy transfer, as well as inner filter effect between PA and the PEI-G PNPs, which resulted in the good selectivity and sensitivity for PA. Finally, the developed sensor was successfully applied to detection of PA in environmental water samples. KEYWORDS: nonconjugated polymer nanoparticles, polyethyleneimine, intrinsic fluorescence, picric acid, fluorescent sensor

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INTRODUCTION Fluorescent polymers and polymer nanoparticles have attracted much interest in chemical sensors and bioanalysis applications in recent decades.1,2 Polymers with intrinsic fluorescence emission, namely constructed without the need of entrapment or covalent conjugation of fluorescent agents in the polymer, are almost conjugated polymers (CPs). Conjugated polymers possess conjugated main chain or π-aromatic building blocks (such as benzene ring and thiophene), which results in their strong fluorescence emission.3 However, conjugated polymers are often poorly water-soluble and thus, functional modifications to their side chains with charged moieties are inevitable for enhancement of their water solubility to fulfill analysis and biological applications. Another strategy to promote dispersity in water is to fabricate conjugated polymer nanoparticles (CPNs) by miniemulsion method, reprecipitation method, and so on. In recent years, numerous water-soluble conjugated polymers (WSCPs) and conjugated polymer nanoparticles have been developed and their applications in sensors, fluorescence imaging, and gene and drug delivery have been extensively explored.3-7 Nevertheless, several drawbacks still exist for their preparation and application, such as sophisticated multistep synthetic pathways, the use of environmentally unfriendly organic solvents, and the possibility of fluorescence self-quenching in aqueous solution. Therefore, the development of autofluorescent polymer materials with good water solubility and easy-preparation are very challenging and still in pursuit. Fortunately, in recent years researchers have been discovered that a few kinds of polymers like 3



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poly(amidoamine) (PAMAM) could emit strong fluorescence under proper conditions,8-10 which own nonconjugated polymer structures and also lack typical chromophores. The fluorescence mechanism of these polymers is still under investigation. Increasing studies show that the fluorescence-emitting moieties may be aliphatic tertiary amines and some heteroatom-containing double bonds (C=O, C=N, N=O), which traditionally are not regarded as fluorescent chromophores.11 Because of their special structures and ample terminal groups, these fluorescent nonconjugated polymers are very promising as novel fluorescent materials for applications in chemical and biological fields. Picric acid (PA), namely 2,4,6-trinitrophenol (TNP), is a kind of nitroaromatic compound. Although, 2,4,6-trinitrotoluene (TNT) is the most widely used nitro-explosive, PA is another superior explosive material that possesses low safety coefficient and high detonation velocity. Meanwhile, PA has wide applications in various industries such as antiseptics, pesticides, and dye industry. When exposed, PA is possible to contaminate soil and groundwater because of its good solubility in water. At a low concentration, PA can cause severe effects on the skin, eyes, liver and kidneys, respiratory system, and metabolism, etc.12-14 Thus, it is very necessary to develop effective and reliable methods for the detection of trace amounts of PA in aqueous solution. A variety of techniques such as X-ray diffraction,15 surface-enhanced Raman spectroscopy,16 mass spectrometry,17 surface plasmon resonance,18 colorimetry,19 and electrochemistry20 have been used for the detection of PA. However, most of these methods are not accessible because of the requirements for expensive and bulky

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equipment, complicated manipulations, and time-consuming procedures. As an alternative approach, fluorescent sensors have such advantages as high sensitivity, low cost, and convenience. Hence, many fluorescent sensors has been designed for PA detection based on conjugated polymers,21 semiconductor quantum dots,22 carbon dots,23 metal-organic frameworks,24,25 small molecular fluorophores,26-28 metal nanoclusters,29 and so on.30-32 But most of the sensors for PA detection still suffer from some obstacles hampering their application, for example, the interference from other nitroaromatics, the complex synthesis of conjugated polymers, and the environmental and health risks of semiconductor quantum dots and metal-organic frameworks. So it is desirable and challenging to develop new fluorescent materials with a simple preparation, low or non-toxic nature, and high sensitivity and selectivity for PA detection. In this work, a kind of water-soluble nonconjugated polymer nanoparticles with strong fluorescence emission was constructed by hyperbranched polyethyleneimine (PEI) and D-glucose via Schiff base reaction and self-assembly. The preparation was under a mild condition (80 oC) without the need of organic solvents. The fundamental properties (such as concentration-dependent and solvent-dependent fluorescence) of the PEI-D-glucose polymer nanoparticles (PEI-G PNPs) have been investigated and also, their fluorescence origin was explored. It can be found that the preparation of the fluorescent PEI-G PNPs is not a modification of fluorescent conjugated polymers or incorporation of fluorescent dyes into polymer matrix. So in the study we not only have created a new class of fluorescent polymer nanoparticles but also provide a new

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strategy to synthesize fluorescent polymer materials. The fluorescence of the PEI-G PNPs can be easily quenched by PA whereas other nitro-explosives and structurally similar compounds only caused negligible quenching. Thus, we have used the PEI-G PNPs to develop a fluorescent probe for rapid, sensitive, and selective detection of PA in aqueous media. The possible mechanism of PA-induced fluorescence quenching was discussed and it was attributed to a combined effect of efficient electron transfer, resonance energy transfer, as well as inner filter effect between PA and the PEI-G PNPs.

EXPERIMENTAL SECTION Materials. Hyperbranched polyethyleneimine (PEI, Mw = 10 000, 99%) and D-glucose

(picric

were purchased from Aladdin Ltd., Shanghai, China. 2,4,6-Trinitrophenol

acid,

PA),

2,4,6-trinitrotoluene

(TNT),

m-dinitrobenzene

(m-DNB),

p-nitrotoluene (p-NT), nitrobenzene (NB), and 2,4-dinitrotoluene (2,4-DNT) were obtained from Tianjin Kermel Chemical Reagents Co., Ltd, Tianjin, China. Other reagents were of analytical reagent grade, and all the chemicals were used without further purification. Ultrapure water with a resistivity of 18.2 MΩ cm was used throughout the experiment. Britton-Robinson (BR) buffer solutions (0.04 M) were prepared according to standard protocols. Instruments. The fluorescence spectra including three-dimensional (3D) fluorescence spectra were collected with an F-2700 spectrofluorometer (Hitachi, Japan). The slits of both excitation and emission were fixed at 10 nm, and the scan wavelength speed was 1500 nm min−1. For 3D fluorescence measurement, the scan wavelength

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intervals were set as 10 nm. UV-vis absorption spectra were recorded using a UV-vis 2450 spectrophotometer (Shimadzu, Japan). Nuclear magnetic resonance (NMR) spectra were obtained on a Bruker AVANCE III 600 (600 MHz) (Bruker, Germany). Transmission electron microscopy (TEM) measurement was carried out with a JEM 1200EX transmission electron microscope (JEOL, Japan). The Fourier transform infrared (FT-IR) spectra were tested by the use of a Bruker IFS 113v spectrometer (Bruker, Germany). Zeta potential and hydrodynamic diameter measurements were performed on a Zetasizer Nano-ZS90 (Malvern Instruments Ltd., U.K.). Fluorescence lifetime decays were measured using an Edinburgh FL 920 fluorescence spectrometer (Edinburgh, U.K.). A PHS-3C pH meter (Shanghai Leici Instrument Company, Ltd., China) was utilized to detect pH values of solutions. Preparation of PEI-G Polymer Nanoparticles. The PEI-G PNPs were prepared by PEI and D-glucose via Schiff base reaction and self-assembly (Scheme 1). Typically, 50 µL of 0.1 g mL-1 PEI was first dissolved in 400 µL of Britton-Robinson (BR) buffer (0.04 M, pH 5.0) by stirring for about 1 min, and then 50 µL of 1 M D-glucose solution was added. Then, the mixture was stirred and heated at 80 oC for 4 h via hydrothermal treatment. Subsequently, the as-prepared PEI-G PNPs solution was dialyzed against ultrapure water through a dialysis membrane (molecular weight cutoff of 3000 Da) for 24 h. The product inside the dialysis bag was collected for further study. Procedures for Sensing Picric Acid. For a typical PA detection procedure, 5 µL of PEI-G PNPs was added to 395 µL of BR buffer (0.04 M, pH 7.0), and the solution was mixed. Then 100 µL of PA solution with various concentrations was added to the 7



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mixtures, followed by shaking well. At last, the fluorescence emission spectra were recorded under excitation at 345 nm. All measurements were carried out at room temperature. For analysis of environmental water samples, tap water was straight collected from our laboratory and river water was obtained from the Jialing River (Chongqing, China). All raw water samples were filtered through a 0.22 µm membrane to remove the suspended impurities, and ethylenediaminetetraacetate (EDTA) with final concentration of 50 µM was added to the samples. Then, the water samples were spiked with ultrapure water or different concentrations of standard PA solutions. Other procedures were the same as described above.

RESULTS AND DISCUTION Preparation and Characterization of PEI-G Polymer Nanoparticles. The synthesis of the PEI-G copolymer is based on Schiff base reaction, which refers to the reaction between compounds containing amino groups and aldehydes (or ketones) resulting in a product containing C=N bonds.33 Hyperbranched PEI, one of dendritic polymers, contains abundant amino groups. When PEI reacted with D-glucose, not only single Schiff base bonds but also conjugated double Schiff base bonds would be formed because in addition to aldehyde group, α-hydroxyl group of D-glucose was also able to react with amino group to produce a C=N bond because the α-hydroxyl group can be transformed to carbonyl group via oxidation or Amadori rearrangement.34,35 Scheme 1 illustrates the principle of reaction between PEI and D-glucose. The Schiff base reaction can occur under a mild condition, and a weak acidic condition will be conducive to it as

8


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in the reaction, a proton attacks carbonyl oxygen firstly and then nucleophilic attack takes place on the carbonyl carbon by the primary amine.36,37 Both PEI and D-glucose own favorable water solubility and thereby, the synthesis was under an aqueous solution with a pH 5.0 at 80 oC.

Different from the colorless PEI or D-glucose, the PEI-G PNPs are deep yellow, and the diluted PEI-G PNPs solution emits strong blue fluorescence under a 365 nm UV-lamp (inset of Figure 1A). As exhibited in Figure 1A, the UV-vis absorption spectra show that the PEI-G PNPs have a new absorption peak located at 358 nm, whereas PEI and D-glucose have no significant absorption at above 260 nm. The new absorption band can be ascribed to n→π* transitions of C=N bonds. Figure 1B shows FT-IR spectra of PEI, D-glucose, and PEI-G PNPs, respectively. Several feature absorption bands at the region from 1283 to 1593 cm−1 in PEI are associated with the stretching vibration of N-H bond, and their intensity is decreased in PEI-G PNPs which reveals that some amine groups have reacted with D-glucose. The absorption peaks at 2930 and 2835 cm−1 corresponding to the stretching vibration of CH2 bonds were found in the spectra of both PEI and PEI-G PNPs. The PEI-G PNPs have characteristic absorption centred at 3427 cm−1 that is the overlap of the absorptions of N-H bond and O-H bond, and the stretching vibration of C-O is situated at 1055 cm−1, demonstrating the presence of C-OH in PEI-G PNPs. Compared to the spectra of PEI and D-glucose, another remarkable new peak at 1627 cm−1 is observed in the PEI-G PNPs spectrum, which is assigned to the C=N bond.38 In addition, the 1H NMR spectra of PEI and PEI-G PNPs are shown in Figure S1 in the Supporting Information. A new peak at 8.00 ppm 9



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belonging to N=CH protons38 and an obvious increase in intensity of peaks centred at 2.65 ppm (contribution from CH, CH2 of D-glucose segment) on the 1H NMR spectrum of PEI-G PNPs are observed. The results demonstrate the formation of Schiff base bonds and PEI-G copolymer. The zeta potential of PEI-G PNPs at 25 oC in water was found to be 11.2 ± 0.7 mV, which was much lower than that of PEI (46.5 ± 1.3 mV), further indicating that amino groups of the PEI in the PEI-G PNPs had partly reacted with D-glucose. Figure 1C is a TEM image and reveals that the PEI-G PNPs are monodisperse with typical spherical shape and a size around 90 nm. The average size of the hydrodynamic size of the PEI-G PNPs in water was 342 nm measured by dynamic light scatting (DLS) (Figure 1D), which is larger than the diameter measured by TEM. The possible reason is that DLS gives a hydrodynamic diameter, while TEM image exhibits a size and shape of dry particles.39

The formation of the water-soluble nanoparticles is due to the following factors. In PEI-G copolymer, ample amine groups and hydroxyl groups are hydrophilic, whereas Schiff base bonds are hydrophobic. As a result, the hyperbranched structure of PEI-G copolymer trends to fold and collapse, shrinking and self-assembling into uniform polymer nanoparticles in aqueous media.40 And many hydrophilic groups on the surface of the PEI-G PNPs make the excellent water dispersity possible. The PEI-G PNPs can keep stable for at least six months when stored at 4 oC, suggesting satisfied long-term storage stability. Overall, the preparation of the PEI-G PNPs in this work is much facile and environmentally friendly in comparison with other methods for constructing polymer nanoparticles, such as emulsion polymerization and 10


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nano-reprecipitation that are complicated and often operated in organic solvents.7

Scheme 1. Synthetic strategy of PEI-G PNPs

Figure 1. (A) UV-vis absorption spectra of PEI (0.1 mg L-1), D-glucose (1 mM), and PEI-G PNPs

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(2% v/v), respectively. Inset is photographs of PEI-G PNPs under visible light and UV light of 365 nm. (B) FT-IR spectra of D-glucose (a), PEI (b), and PEI-G PNPs (c), respectively. (C) TEM image of PEI-G PNPs. (D) Hydrodynamic size of PEI-G PNPs measured by dynamic light scatting.

Fluorescence Properties of PEI-G Polymer Nanoparticles. Extraordinarily bright fluorescence is exhibited from the PEI-G PNPs under ultraviolet lamp (365 nm), in contrast with the sole PEI or D-glucose that has insignificant fluorescence. Figure 2A displays the fluorescence excitation and emission spectra of the PEI-G PNPs solution (2% v/v) and the maximum excitation and emission wavelengths are 345 and 465 nm, respectively. Using quinine sulphate as a reference, the quantum yield was estimated to be as high as 46% in water, compared to that of other fluorescent materials obtained from PEI, such as PEI-capped Ag nanoclusters (3.8%) and PEI-functioned carbon dots (42.5%).41,42 When the excitation wavelength was varied from 320 to 400 nm, there was no dramatic change of the maximum emission wavelength (Figure 2B) which is different from the fluorescence of carbon dots prepared using PEI as a precursor that is excitation-dependent.43,44 But the fluorescence of the PEI-G PNPs shows apparently concentration-dependent. When the PEI-G PNPs are at a relatively low concentration ranges (0 to 3% v/v), their fluorescence intensity is proportional to the concentration and there is no obvious change of the excitation and emission wavelengths (Figure S2). However, when the concentration further increases, it is intriguing that the maximum excitation and emission wavelengths would be gradually red-shifted. The fluorescence can be tuned from blue to green by adjusting the concentration of the PEI-G PNPs. Figure S3A, S3B, and S3C shows the three-dimensional fluorescence spectra of 2%,

12


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5%, 10% (v/v) PEI-G PNPs solutions, respectively. It can be seen that the peaks of excitation and emission are red-shifted from λex = 345 nm and λem = 465 nm (2% v/v) to λex = 440 nm and λem = 508 nm (10% v/v), respectively. Photographs of the PEI-G PNPs solutions with different concentrations under visible light and UV light (365 nm) are

exhibited

in

Figure

S3D

and

S3E,

respectively,

from

which

the

concentration-dependent fluorescence can also be found. The phenomenon might be because of the formation of higher order aggregated species at high concentration.45,46 The red shift of fluorescence was also observed when the PEI-G PNPs were dispersed

in

organic

solvents

including

dimethylsulfoxide

(DMSO)

and

dimethylformamide (DMF). Conventional Schiff base polymers are usually insoluble in almost all solvents.47,48 In this work, the good water solubility of the PEI-G PNPs stems from the ample hydrophilic amine groups and hydroxyl groups on their surface. Accordingly, the solubility of the PEI-G PNPs may dramatically descend with the decrease of solvent polarity. As anticipated, the PEI-G PNPs are hardly soluble in most organic solvents, except for relatively strong polar solvents (for example, DMSO, DMF, and ethanol) in which the PEI-G PNPs exhibit slightly soluble. As shown in Figure 2C, it can be observed that the fluorescence intensities of PEI-G PNPs in DMSO and DMF are increased and also the emission wavelengths exhibit some red-shifts in comparison with those in water because of the formation of aggregated species induced by the poor solubility. Figure 2D are photographs of the PEI-G PNPs dispersed in different solvents under visible and UV light (365 nm), illustrating the various solubility in water, DMSO, DMF, ethanol, and tetrahydrofuran (THF), respectively. Figure S4 reveals that the 13



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gradual changes of solubility and fluorescence in an aqueous alcoholic solution with increasing ethanol contents. The PEI-G PNPs with excellent solubility in water and poor solubility in organic solvents are different from water-soluble conjugated polymers which usually are amphiphilic through incorporating pendant charged functionalities on their hydrophobic backbones.3 Additionally, we also investigated the effects of preparation conditions (including concentration of reactants and reaction time) on the fluorescence. As shown in Figure S5A and S5B, increasing the concentration of D-glucose (or PEI), the fluorescence intensity would increase. But with higher concentration of reactants, some insoluble precipitates would be produced. As for the effect of reaction time, similarly, high fluorescence intensity would be obtained with the increase of heating time (Figure S5C), but precipitates also appeared when the reaction time exceeded 4 h. Therefore, the conditions of 0.1 M D-glucose and 0.01 g mL-1 PEI at 80 oC for 4 h were chosen for the preparation of PEI-F PNPs.

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Figure 2. (A) Fluorescence excitation and emission spectra of PEI-G PNPs (1% v/v). (B) Fluorescence emission spectra of PEI-G PNPs (1% v/v) under different excitations. (C) Fluorescence spectra of PEI-G PNPs (2% v/v) dispersed in H2O, DMF, and DMSO, respectively. Excitation: 345 nm. (D) Photographs of PEI-G PNPs (2% v/v) dispersed in H2O, DMSO, DMF, ethanol, and THF under visible light (a) and UV light of 365 nm (b), respectively.

Fluorescence Origin of PEI-G Polymer Nanoparticles. It is noticed that the PEI-G PNPs show strong intrinsic fluorescence without the conjugation to any external fluorochrome, and also the PEI-G PNPs lack typical chromophores such as conjugated main chain or π-aromatic building blocks functioning as emitting units. The high fluorescence emission is unexpected. In this regard, the PEI-G PNPs are very similar to a few kinds of polymers discovered in recent years which are fluorescent but have not conventional chromophores.9,10 And the fluorescence mechanism of these polymers is still under investigation. Published literature discovered that small molecular Schiff 15



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base compounds can exhibit strong fluorescence by the suppression of C=N isomerisation via coordinating with metal ions.49,50 In our recent work, we have demonstrated that the fluorescence from particles and gels formed by PEI and formaldehyde derives from the radiative decay from excited Schiff base bonds.51 Consequently, in this work, the fluorescence of the PEI-G PNPs also may originate from the n←π* transitions of Schiff base bonds. Even though the PEI-G PNPs lack of continuous conjugated chain, the formation of compact and rigid nanoparticles may confine the intramolecular rotations of Schiff base bonds which suppress the nonradiative pathway and results in the radiative decay, thus, making the PEI-G PNPs fluorescent. To verify the fluorescence mechanism, we utilized PEI with different molecular weights to react with D-glucose. When PEI with various molecular weights (Mw = 600, 1800 and 10 000) was used, the fluorescence intensity increased with increasing molecular weight, but the maximum emission wavelength was constant (Figure S6). The result demonstrates that a larger hyperbranched macromolecular structure, which is conducive to the formation of Schiff bases and nanoparticles, is crucial to the fluorescence. This is also confirmed by another control experiment in which

small-molecular

amine

compounds

(such

diethylenetriamine) were utilized to react with

as

ethylenediamine

D-glucose

and

and no significant

fluorescence was found in their products.

To further shed light on the fluorescence origin, another experiment was performed. Generally, Schiff base bonds would be easily reduced by sodium borohydride (NaBH4) at room temperature to secondary amines via reaction (1):52 16


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In our previous report,50 the fluorescence of the PEI-formaldehyde polymer particles containing only single Schiff base bonds would disappear after reduced by NaBH4 (Figure S7). However, a different result was found in the PEI-G PNPs. The PEI-G PNPs (1% v/v) was reduced by 0.1 M NaBH4 (reacting for 4 h at room temperature). As shown in Figure 3A, the UV-vis absorption peak at 358 nm of the PEI-G PNPs is replaced by a weaker absorption band after reduction, which is not similar to that of the PEI-formaldehyde polymer particles whose characteristic absorption of C=N disappears entirely (Figure S7A). Moreover, as shown in Figure 3B, the fluorescence intensity of the PEI-G PNPs increases remarkably and also, the emission spectrum is blue-shifted from 465 to 455 nm and the excitation spectrum is red-shifted from 345 to 355 nm. The results should be because of the existence of conjugated double Schiff base bonds in the PEI-G PNPs. When reduced by NaBH4, the conjugated double Schiff base bonds would turn to C=C bonds by reaction (2):

It is known that the C=C bonds are unable to be reduced by NaBH4. As a result, the fluorescence from the n←π* transition of C=N converts to the π←π* transitions of C=C, resulting in the increase of fluorescence intensity and the shifts of excitation and emission spectra.

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Figure 3. (A) UV-vis absorption spectra of PEI-G PNPs (1% v/v) before (i) and after (ii) reduced by NaBH4. (B) Fluorescence excitation and emission spectra of PEI-G PNPs (1% v/v) before (a, b) and after (c, d) reduced by NaBH4.

Establishment of Fluorescence Method for Sensing Picric Acid. The PEI-G PNPs possess intrinsic fluorescence, good water solubility, and ample surface amine groups and hydroxyl groups, suggesting a variety of potential applications in chemical and biological fields. We found that the fluorescence of the PEI-G PNPs can be easily quenched by PA, which enables us to develop a sensor for the detection of PA. At the beginning, several tests were performed to optimize the sensing conditions including probe concentration, reaction time, pH, and ionic strength. The fluorescence-quenched efficiency

(∆F/F0,

which

is

a

common

representation

method

for

fluorescence-quenched efficiency.53) was used to optimize the probe (∆F = F0 – F, where F0 and F represent the fluorescence intensity of PEI-G PNPs in the absence and presence of PA, respectively). First, the concentration of probe was optimized. Generally, a low concentration of probe would result in an efficient fluorescence quenching in the presence of fluorescence quencher of a given concentration, and thus high sensitivity to analyte 18


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would be found. However, the linear range of the detection would be narrow with the decline of the concentration of fluorescent probe.41 Figure S8A clearly illustrates that the lower the PEI-G PNPs concentration, the higher the fluorescence-quenched efficiency is when the concentration of PA is fixed. The concentration of 1% (v/v) PEI-G PNPs was eventually chosen for the detection of PA in order to obtain the wider linear range and the lower limit of detection. Second, response time is another key factor in the practical application of a sensor. The response rate of the fluorescence signal of the PEI-G PNPs to reaction time upon addition of PA was monitored subsequently. Figure S8B reveals that the fluorescence quenching can be finished within 1 min. This result demonstrates that the quenching of the PEI-G PNPs fluorescence by PA is rapid and stable, implying a promising application in a fast and convenient sensing of PA without strict time control. Third, the effect of pH on fluorescence quenching was investigated. Figure S8C shows that the fluorescence intensities of PEI-G PNPs in the absence and presence of PA decrease with increasing pH value and then trend to keep constant under weak alkaline conditions. But the fluorescence-quenched efficiency of PA at various pH values has not dramatic change (Figure S8D). As a result, a neutral BR buffer with pH 7.0 was chosen to apply to the following experiments. Finally, we explored the effect of ionic strength on sensing PA. The fluorescence of PEI-G PNPs in different concentrations of NaCl solutions in the absence and presence of PA was tested, respectively. As presented in Figure S8E and S8F, in solutions with different NaCl concentrations, the fluorescence intensity can remain stable and the fluorescence-quenched efficiencies of PA are almost same, even

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in 1 M NaCl solution, suggesting that the PA sensor may have the ability to fit a complicated environment. Under optimum conditions, the linearity and the sensitivity was examined by recording emission spectra of the PEI-G PNPs after the addition of different concentrations of PA. Figure 4A exhibits that increasing the PA concentration gradually quenches the fluorescence, but no obvious change in the maximum emission wavelength was observed. And Figure 4B shows the relationship between the fluorescence intensity of PEI-G PNPs and the concentration of PA, which is not linear. It is known that the fluorescence quenching data generally follow the Stern-Volmer equation: F0/F – 1 = K[Q],54 where F0 and F denote fluorescence intensity of fluorophore in the absence and presence of quencher, respectively, K is the Stern-Volmer quenching constant, and [Q] is the quencher concentration. As shown in the insets of Figure 4B, the intensity ratio (F0 – F)/F (namely F0/F – 1) displays two good linear relationships versus PA concentration ranging from 0.05 to 1 µM (R2 = 0.9937) and 2 to 70 µM (R2 = 0.9922), where F0 and F are the fluorescence intensity of the PEI-G PNPs in the absence and presence of PA. The limit of detection (LOD) was estimated to be 26 nM based on a signal-to-noise of 3, indicating a high sensitivity. For the effective and specific detection of PA, minimal or no interference from other substances is an essential requirement. Therefore, the selectivity of PEI-G PNPs to PA was evaluated. Figure 5 shows the fluorescence responses of PEI-G PNPs to PA, 2,4,6-trinitrotoluene (TNT), m-dinitrobenzene (m-DNB), p-nitrotoluene (p-NT), nitrobenzene

(NB),

o-nitrophenol

(o-NP),

2,4-dinitrotoluene

(2,4-DNT), 20


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p-dihydroxybenzene (HQ), aniline (AN), benzoic acid (BA), o-dihydroxybenzene (CC), phenol (CA), 4-methylphenol (4-MP), and p-chlorophenol (p-CP). Among all these potential interferences and PA, it can be seen that only PA exhibits a substantial fluorescence quenching. Other nitroaromatics and phenols are unable to lead to remarkable interference, suggesting the highly selective sensing of PA by PEI-G PNPs. When applied to the analysis of PA in real water samples, the developed sensor may need the ability to resist interference of some common ions. As a consequence, the effect of 19 kinds of cations, including Na+, K+, Li+, Ag+, Ba2+, Mg2+, Zn2+, Ni2+, Co2+, Cd2+, Fe2+, Mn2+, Hg2+, Ca2+, Pb2+, Cu2+, Cr3+, Fe3+, and Al3+, on the fluorescence response of PEI-G PNPs were investigated. As exhibited in Figure S9A, most metal ions have no obvious effect on the fluorescence intensity, except that Cu2+, Ni2+, Fe3+, and Hg2+ may quench the fluorescence of the PEI-G PNPs, to some extent. Nevertheless,

with

the

addition

of

a

strong

metal

ion

chelator,

ethylenediaminetetraacetate (EDTA), the fluorescence quenching by the four metal ions can be effectively avoided. Furthermore, we also measured the fluorescence of the PEI-G PNPs in the presence of common anions (F-, Cl-, Br-, I-, NO3-, NO2-, Ac-, SO32-, SO42-, S2-, and CO32-) and Figure S9B displays that the 11 common anions barely affect the fluorescence intensity of PEI-G PNPs. The results indicate that the developed sensor can serve as a favorable and reliable tool to probe PA in aqueous media. A comprehensive comparison with other PA sensors (e.g., carbon dots, conjugated polymer, and small molecule) reported by the literature in recent years is summarized in Table S1, from which we can find that our sensor has several prominent merits in both

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preparation and detection, such as facile synthesis in aqueous media with a relatively low temperature, lower detection limit, and wider linear range.

Figure 4. (A) Fluorescence emission spectra of PEI-G PNPs after adding various concentrations of PA. (B) Fluorescence intensity changes of the PEI-G PNPs in the presence of increasing concentration of PA (insets: linear relationships of (F0 – F)/F versus the concentration of PA over the range from 0 to 1 µM and 2 to 70 µM, where F0 and F denote the fluorescence intensity of the PEI-G PNPs before and after the addition of PA, respectively). Conditions: PEI-G PNPs, 1% (v/v); BR buffer (pH 7.0); excitation, 345 nm.

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Figure 5. Selectivity to PA using PEI-G PNPs as a sensor. ∆F = F0 – F, where F0 and F denote the fluorescence intensity of PEI-G PNPs before and after the addition of analytes, respectively. The concentration of all the analytes is 50 µM. Conditions: PEI-G PNPs, 1% (v/v); BR buffer, pH 7.0; excitation, 345 nm.

Mechanism of Picric Acid-Induced Fluorescence Quenching. As is known, nitro-explosive molecules are nitro-substituted electron-acceptor substrates, and they can bind to π-donor sites via donor-acceptor interactions.29 Fluorescence of most electron-rich fluorophores can be effectively quenched by the nitro-explosive molecules via electron-transfer, which has been extensively utilized to design fluorescence probes for explosives detection over the past years. Numerous electron-rich amine groups and hydroxyl groups are on the surface of the PEI-G PNPs. Thus, when electron-deficient PA is added to the PEI-G PNPs solution, it may be bound to the surface of PEI-G PNPs, which causes the fluorescence quenching because of electron-transfer. However, the good selectivity to PA and the weaker quenching caused by other nitro-explosives such as TNT indicate that the electron-transfer may not be the main factor in the quenching process. Figure 6A shows the fluorescence emission spectrum of PEI-G PNPs and UV-vis absorption spectrum of PA. It is observed that there is an evident overlap between the florescence emission spectrum of PEI-G PNPs and absorption spectrum of PA. In contrast, the absorption spectra of other latent interferences have insignificant overlap with the emission spectrum of PEI-G PNPs (Figure S10). Therefore, there is a strong possibility of resonance energy transfer from the fluorophores of PEI-G PNPs to the PA, which can effectively enhance the fluorescence-quenched efficiency and sensitivity.23 The UV-vis absorption spectrum of 23



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PEI-G PNPs with the addition of PA further demonstrates the energy transfer process. As shown in Figure 6B, the absorption of the mixture of PEI-G PNPs and PA is different from that of sole PEI-G PNPs or PA solution, and the shoulder peak of PA at around 410 nm disappeared, indicating that the PA has successfully bound to the surface of the PEI-G PNPs. A close proximity between PEI-G PNPs and PA makes the fluorescence resonance energy transfer possible.55 To further verify the nature of the fluorescence quenching, the fluorescence lifetime measurement was carried out. Generally, fluorescence lifetimes of fluorophores vary for dynamic quenching and are invariant for static quenching as a function of quencher concentration.56 Figure 6C and 6D show the time-resolved fluorescence spectra of PEI-G PNPs in the absence and presence of PA, respectively. The fluorescence lifetime of PEI-G PNPs keeps the same for both two cases (τ1 = 1.3 µs, τ2 = 8.5 µs), suggesting that a static quenching mechanism is accountable for the fluorescence quenching. Also, the fluorescence-quenched efficiency of PA at different temperatures further demonstrated the static quenching. It is known that higher temperatures result in faster diffusion and hence larger amounts of collisional quenching, while higher temperature will cause the dissociation of weakly bound complexes, and thus smaller amounts of static quenching.56 As shown in Figure 6E, the fluorescence-quenched efficiency decreases gradually with increasing temperature, proving the static quenching process. These facts support the energy transfer from PEI-G PNPs to PA. In addition, we found that the absorption band centered at 360 nm of PA has

24


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significantly overlapped with the excitation spectrum (345 nm) of the PEI-G PNPs (Figure 6F). Published literature reported that the absorption of light at the excitation or emission wavelength by compounds could reduce the fluorescence intensity of fluorophore which was called inner filter effect.57 As a result, the fluorescence quenching of the PEI-G PNPs by PA may also partly result from the inner filter effect related to the specific absorption of PA.

Figure 6. (A) UV-vis absorption spectrum of PA (50 µM) (curve a) and fluorescence emission spectra of PEI-G PNPs (1% v/v) (curve b). (B) UV-vis absorption spectra of PA (50 µM), PEI-G PNPs (2% v/v), and PEI-G PNPs (2% v/v) added to 50 µM PA, respectively. (C) Time-resolved fluorescence spectrum of PEI-G PNPs (1% v/v). (D) Time-resolved fluorescence spectrum of PEI-G PNPs (1% v/v) with the addition of 50 µM PA. (E) Fluorescence-quenched efficiency of PA at

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different temperatures. (F) UV-vis absorption spectrum of PA (50 µM) (curve a) and fluorescence excitation spectrum of PEI-G PNPs (1% v/v) (curve b).

Hence, on the basis of above discussion, it can be concluded that the highly selectivity and sensitivity of PEI-G PNPs for PA detection in aqueous media is attributed to the efficient electron and energy transfer process, as well as the inner filter effect. Detection of Picric Acid in Environmental Water Samples. It is very challenging for a prepared fluorescent sensor to apply in real samples analysis because of potential unknown interferences. To assess the practicality, we applied the developed method to the detection of PA in tap water and Jialing River water samples. The samples were tested and the results are summarized in Table S2. No PA was found in both tap water and Jialing River water. For the PA-spiked samples, it can be seen that the values measured by this proposed method are consistent with the spiked values for the concentration of PA. The recoveries of PA in the samples range from 100.4 to 112.0%. In addition, the relative standard deviation (RSD) was obtained by repeating the experiment 6 times under the same conditions. The RSDs are in the range of 1.05% to 4.52%, suggesting a good repeatability of the proposed method. The data of the recovery value and the RSD are satisfactory, indicating that the sensor is reliable and applicable for PA detection in environmental water samples.

CONCLUSIONS In conclusion, we created a kind of nonconjugated PNPs with strong fluorescence

26


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emission by PEI and D-glucose via Schiff base reaction and self-assembly under a mild condition. The fundamental properties of the PEI-G PNPs have been studied and the possible mechanism of fluorescence was discussed. Ample hydrophilic groups (amine groups and hydroxyl groups) and hydrophobic Schiff base bonds make the hyperbranched structure of PEI-G copolymer fold and collapse, shrinking and self-assembling into uniform polymer nanoparticles in aqueous media. The formation of the nanoparticles makes fluorescence emission from the n←π* transitions of Schiff base bonds possible because compact and rigid nanoparticles confine the intramolecular rotations of Schiff base bonds. Because of the specific structure, the PEI-G PNPs exhibit excellent water solubility but poor dispersity in organic solvents. Furthermore, we have used the PEI-G PNPs to develop a fluorescent sensor of PA on the basis of the fluorescence quenching induced by PA due to the combined effect of the electron transfer, resonance energy transfer, and inner filter effect. This PEI-G PNPs provides a favorable and reliable platform for the rapid, sensitive, and selective detection of nitro-explosive PA in 100% aqueous media.

ASSOCIATED CONTENT Supporting Information 1

H NMR spectra, concentration-dependent fluorescence, 3D fluorescence spectra,

effects of solvent polarity, preparation conditions, and molecular weights (PEI) on fluorescence, optimization of sensing conditions, comparison of fluorescent sensors,

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UV-vis absorption spectra, and data of real water analysis. The Supporting Information is available free of charge on the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *

E-mail address: [email protected], [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (No. 21273174), the Municipal Science Foundation of Chongqing City (No. CSTC-2013jjB00002, CSTC-2015jcyjB50001) and Youth Innovation Promotion Association of CAS (No. 2015316).

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Their Use as Probes for Sensitive and Selective Detection of Mercury(II) Ions. Anal. Chem. 2012, 84, 5351-5357. (55) Yang, Y.; Huang, J.; Yang, X.; Quan, K.; Wang, H.; Ying, L.; Xie, N.; Ou, M.; Wang, K. FRET Nanoflares for Intracellular mRNA Detection: Avoiding False Positive Signals and Minimizing Effects of System Fluctuations. J. Am. Chem. Soc. 2015, 137, 8340-8343. (56) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006. (57) Weert, M.; Stella, L. Fluorescence Quenching and Ligand Binding: A Critical Discussion of a Popular Methodology. J. Mol. Struct. 2011, 998, 144-150.

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Water-Soluble Nonconjugated Polymer Nanoparticles with Strong

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