Room temperature phosphorescence of five PAHs in a synergistic mesoporous silica nanoparticle-deoxycholate substrate

Accepted Manuscript Room temperature phosphorescence of five PAHs in a synergistic mesoporous silica nanoparticle-deoxycholate substrate

Jun Qin, Xiaomei Li, Feng Feng, Qiliang Pan, Yunfeng Bai, Jianguo Zhao PII: DOI: Reference:

S1386-1425(17)30150-6 doi: 10.1016/j.saa.2017.02.041 SAA 14963

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

9 October 2016 20 February 2017 20 February 2017

Please cite this article as: Jun Qin, Xiaomei Li, Feng Feng, Qiliang Pan, Yunfeng Bai, Jianguo Zhao , Room temperature phosphorescence of five PAHs in a synergistic mesoporous silica nanoparticle-deoxycholate substrate. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), doi: 10.1016/j.saa.2017.02.041

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ACCEPTED MANUSCRIPT Room temperature phosphorescence of five PAHs in a synergistic mesoporous silica nanoparticle-deoxycholate substrate

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School of chemistry and material science, Shanxi Normal University, Linfen 041004, P. R. China

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Jun Qina,b, Xiaomei Lib, Feng Fenga,b*, Qiliang Panb, Yunfeng Baib, Jianguo Zhaob

College of chemistry and environmental engineering, Shanxi Datong University, Datong 037009, P. R.

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China

*Corresponding author. Tel.: +86 352 7158662; Fax: +86 352 6100028

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E-mail address: [email protected] (F. Feng).

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ACCEPTED MANUSCRIPT Abstract: A synergistic mesoporous silica nanoparticle-sodium deoxycholate (mPS-NaDC) substrate was developed for room temperature phosphorimetry. The synergistic substrate exhibited rapid and strong RTP-inducing ability against temperature variation. NaDC might adsorb on the inner surface of mPS pore by

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possible hydrogen bonding and protected the triplet state of polycyclic aromatic

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hydrocarbons (PAHs) with different molecular sizes. Two mPSs named LPMS1 and LPMS2 with pore size of 3.05 and 3.83 nm were synthesized and optimized in

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inducing RTP, and the latter, LPMS2, was selected as an ideal substrate because of

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its stronger protection ability to the triplet and good stability. Dibromopropane and cyclohexane were also used as assistant phosphorescence-inducers. All results

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demonstrated the feasibility and application potential of synergistic mPS-NaDC

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substrate in phosphorimetry. The interaction detail of NaDC and inner surface of

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temperature

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KeyWords:

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selected mPS still needs to be explored in future.

phosphorescence;

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Mesoporous silica; Polycyclic aromatic hydrocarbon

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Sodium

deoxycholate;

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1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are known as toxic, mutagenic, carcinogenic, and persistent organic contaminants [1]. The major sources of PAHs

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in the environment include incomplete burning of fossil fuels, garbage, or other

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organic substances [2, 3]. With the rapid development and expansion of industries, increasing amounts of PAHs are released into the air. These pollutants either

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enrich in the human body through food chains or directly threaten human health

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when they are adsorbed by suspended particles in the air [4, 5]. To address the pollution problems, efforts on the analysis, detection, and control of PAHs have

pollutants[8],

and

the

European

commission

has

restricted

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controlled

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been conducted worldwide [6, 7]. The US EPA listed 16 PAHs as precedence-

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concentrations of eight carcinogenic PAHs in consumer products [9].

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PAHs consist of a π-π conjugated system in their molecular structure, and this system enables detections of PAHs on the basis of their room temperature

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phosphorescence (RTP) properties. Considering that the phosphorescence may be easily quenched by oxygen or solvent molecules, researchers have developed various protective techniques such as solid substrate RTP [10, 11], macrocyclic RTP [12, 13], and micelle-stabilized or halogen bonding enhanced RTP [14-16]. Target analytes were either absorbed into solid substrates (such as filter paper) or coated by β-cyclodextrin and surfactant micelle to acquire stable RTP emission 3

ACCEPTED MANUSCRIPT conditions. Other oxygen-removal methods were also combined with the above techniques to enhance the deoxygenation efficiency; these methods include purging of nitrogen and addition of sodium sulfite [17, 18].

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Although the above strategies have effectively reduced quenching and have

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been successfully applied in different analytical circumstances, some limitations

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still exist [19-21]. Deoxidants such as sodium sulfite may react with sample

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components, and quenching from solvent molecules remains inevitable. For macrocyclic compounds, their hydrophobic cavities often present a fixed size,

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which may limit their application range, given that various analytes exhibit

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different molecular sizes and structures. In solid substrate-RTP, phosphorescence emission may be affected by multi-step sample preparations, such as pointing and

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variation of temperature.

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drying operations. For micelle-stabilized RTP, micelle structures may change with

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Among numerous nanomaterials, mesoporous silica nanoparticle (mPS) has

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been widely applied as an analytical substrate because of its excellent adsorption property. mPS is used for drug-carrying [22], heavy ions detection [23], and the determination of biomacromolecules [24]. However, few applications of mPS in RTP have been reported, mainly because most phosphors often contain two or more aromatic rings and various groups in their molecules, indicating large molecular size and poor water solubility. Evidently, mPS with hydrophilic pore structure cannot easily adsorb these phosphors, and is unable to provide effective 4

ACCEPTED MANUSCRIPT protection as a RTP substrate. Even for the surface-modified mPS, how to effectively load target analytes remains a challenge [25-27]. Sodium deoxycholate (NaDC) micelle reportedly demonstrates strong RTP-

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inducing ability [28, 29]. Unlike many other surfactants, NaDC molecules exhibit

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a hydrophilic side and an opposite hydrophobic side, and these molecules may

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aggregate with their hydrophobic side face to face to form sandwich like micelles

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[30]. In the present work, we developed a novel "hard-soft" RTP substrate. Largepore mPSs were synthesized and dispersed into water solution to create a hard

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substrate. Afterward, NaDC was added as the soft component to the solution to

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form a mPS-NaDC synergistic RTP substrate. The RTP properties of five PAHs that have different molecule sizes, namely, naphthalene (Nap), phenanthrene (Phe),

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anthracene (Ant), pyrene (Pyr), and benzo(k)fluoranthene (Bkf), were investigated

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on the basis of their mPS-NaDC substrate. Furthermore, analytical performance

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experiments were conducted to determine the feasibility of the proposed method.

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2. Experimental section 2.1 Materials and instruments Cetyltrimethylammonium bromide (CTAB, 98%, TCI) was used as a structure-directing agent for MCM-41 like mPSs. Mesitylene (TMB, 99%, Aladdin) was used as a swelling agent. NaDC (Acros organics), Tetraethoxysilane (TEOS, Aladdin) and other reagents were of analytical grade. Nap, Phe, Ant, Pyr, 5

ACCEPTED MANUSCRIPT and Bkf were all reference standards (Aladdin). Deionized ultrapure water (MilliQ UV-Plus) was used throughout the experiment. mPS morphology was characterized by MAIA3 field emission scanning

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electron microscopy (SEM, Tescan), and the mesoporous structure was

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characterized by JEM-2100 transmission electron microscopy (TEM, JEOL).

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Small angle X-Ray diffraction determination was conducted on a D8 Advance

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XRD system (Bruker). Porosity analysis was conducted on an Autosorb IQ automated gas sorption analyzer (Quantachrome). All phosphorescence detections

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were conducted under the phosphorescence mode on a LS-55 spectrometer (Perkin

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Elmer), the delay time and the gate time were set at 1 ms and 5 ms, respectively. Excitation and emission slits were set at 5-20 nm. Ultraviolet (UV) absorption

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detections were conducted on a Lambda-35 spectrometer (Perkin Elmer).

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2.2 Synthesis of mesoporous silica

Large pore MCM-41 type mPSs were synthesized as previously described [31,

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32] with some modifications. Briefly, 1.0 g of CTAB, 480 mL of water, and 7.0 mL of 1.0 M NaOH were added into a Teflon bottle and then stirred to clear at 353 K in an oil bath. Mesitylene (TMB; 0, 2.0, and 6.0 mL) was added to the surfactant solution under vigorous stirring. TEOS (5.0 mL) was added dropwise, and a white precipitate formed. The mixture was stirred continually for 2 h and then cooled to room temperature. As-synthesized samples were collected by 6

ACCEPTED MANUSCRIPT filtration and named as MCM41, LPMS1, and LPMS2. After overnight drying at 323 K in a vacuum oven, the samples were calcined at 823 K for 6 h to completely remove CTAB.

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2.3 General procedure

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To prepare samples, mPS powder was dispersed into deionized water under ultrasonic homogenization. NaDC and Phe stock solutions were added to the

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dispersion in sequence; the mixture was then diluted to 10.0 mL with water,

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agitated well followed by 2 h of standing, and sampled for phosphorescence

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detection.

In UV detections, the mPS-Phe and mPS-NaDC-Phe dispersions were further

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centrifuged for 10 min at 5000 rpm after the 2 h of standing to separate mPS;

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leaving supernatants to be used for comparing the Phe content in above solutions

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with that in the single NaDC solution. Single NaDC samples were prepared by

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adding Phe to NaDC stock solution and diluted to 10.0 mL with water.

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2.4 Characterizations

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Fig. 1. Morphology and pore structure characterization of LPMS1 and LPMS2

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Part A and B are TEM images of LPMS1 and LPMS2, respectively.

Fig. 2. Small angle X-ray diffraction patterns of three mPSs

TEM characterizations of LPMS1 and LPMS2 are shown in Fig. 1. LPMS1 and LPMS2 particles were mainly spherical and short capsular in shape (see SEM image in Supplement Information); these particles exhibited an average diameter of 120 nm. Two characteristic features can be clearly identified: hexagonal 8

ACCEPTED MANUSCRIPT structures along the channel system and 2D pores arranged in form of wellorganized parallel stripes. Small angle X-ray diffraction spectra of all the samples are shown in Fig. 2. The strong diffraction peaks at 1.98 and 2.28 degree indicated

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the ordered porosity of LPMS1 and LPMS2.

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Fig. 3. N2 adsorption-desorption isotherms and pore distribution of three mPSs

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The N2 adsorption-desorption results of the three mPSs in Fig. 3 clearly conform to the IUPAC type IV isotherm. Although MCM41 and LPMS1 showed

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small hysteresis loops, LPMS2 with larger pores presented a hysteresis loop in the

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relative pressure range of 0.4-0.98. The Brunauer-Emmett-Teller (BET) surface areas of MCM41, LPMS1, and LPMS2 were 1139.780, 1063.087, and 711.789 m2/g, respectively; the corresponding pore volumes calculated by Barrett-JoynerHalenda (BJH) method were1.589, 1.347, and 0.949 cm3/g; the average pore diameters Dv(d) are 2.731, 3.049, and 3.835 nm, respectively. The pore sizes of

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ACCEPTED MANUSCRIPT LPMS1 and LPMS2 were larger than that of MCM41, and this finding demonstrates the pore-expanding ability of mesitylene.

3. Results and discussion

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3.1 Formation of NaDC primary micelle

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The RTP-inducing ability of the single-NaDC medium is offered by its

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primary micelle. However, in our previous study [29], we found that the formation of NaDC primary micelle is a slow process. The NaDC stock solution should be

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prepared few days before use to ensure sufficient formation of primary micelles.

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This process actually increases the detection period. To derive the formation trend of NaDC primary micelles, we selected Phe as the RTP probe, and a

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phosphorescence-standing time experiment was conducted.

Fig. 4. RTP of Phe induced by NaDC with different stocking time cPhe: 2 μM cNaDC: 2(a), 4(b), 4(c) mM.

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ACCEPTED MANUSCRIPT In the first step, two NaDC stock solutions with different concentrations (2 and 4 mM) were prepared and sampled daily since the preparation, and Phe was added to these samples before RTP determination. By detecting the RTP of the samples, the variation of primary micelles was identified. As shown in Fig. 4, NaDC

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molecules aggregated slowly to form primary micelles in a stock solution with low

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concentration (curve a). When the concentration of the NaDC stock solution

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increased to 4 mM, the formation of primary micelles was accelerated (curve b). In the second step, Phe was directly added to a newly prepared 4 mM NaDC

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stock solution, and then the mixed solution was sampled and detected daily. In this

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case, because of the hydrophobic interaction between Phe and NaDC molecules, Phe was encapsulated in the sandwich-type NaDC aggregation as a core, and the

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formation of NaDC primary micelles was thus promoted. However, despite the

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promotion, RTP still need seven days to reach the maximum (curve c).

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3.2 RTP and UV spectra

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Next, single NaDC medium was compared with the mPS-NaDC binary dispersion. As shown in Fig. 5A, Phe exhibited weak RTP signal in the presence of 4 mM single NaDC. In contrast, Phe emitted strong RTP when it was added to LPMS2-NaDC. The maximum excitation and emission at 248 and 494 nm were the same as that in the single NaDC, but the intensities had increased over 50 times, respectively. Ant also showed strong RTP signals at 249 and 489 nm. Even 11

ACCEPTED MANUSCRIPT Nap, Pyr, and Bkf with weak intrinsic RTP emission presented notable RTP excitation/emission at 243/490 nm, 244/490 nm, and 247/497 nm, correspondingly

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(Fig. 5B).

Fig. 5. RTP spectra of five PAHs in the presence of NaDC and LPMS2-NaDC

In part A, curves a-a′and b-b′ are Ex-Em spectra of Phe in the presence of LPMS2-NaDC and single NaDC. In part B, curves a-d and a′-d′ are Ex and Em spectra of Pyr, Ant, Bkf, and Nap in LPMS2-NaDC, correspondingly. cNaDC in single-NaDC and mPS-NaDC samples are 4 mM and 2 mM, respectively. cLPMS2: 1 mg/mL. cPhe: 0.5 μM, cNap: 2 μM, cAnt: 0.1 μM, cPyr: 50 nM, cBkf: 20 nM. Standing time: 120 min. 12

ACCEPTED MANUSCRIPT In order to explore RTP-inducing effect of mPS-NaDC, UV absorption detections, separation by centrifugation, and luminescent imaging under UV light were conducted. As shown in Fig. 6, Phe in the presence of single NaDC and LPMS2 emitted blue-violet fluorescence (sample a and b). After centrifugations,

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the supernatants of these two samples strongly absorbed UV at 256 nm (curve a′,

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b′). The removal of LPMS2 caused small decrease in UV absorbance, which indicates that Phe was slightly adsorbed in LPMS2, but the adsorption did not

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offer effective protection for RTP.

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Phe-LPMS2-NaDC ternary sample shown a little weaker blue-violet

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fluorescence (sample c); meanwhile after a centrifugation, the supernatant exhibited a greatly decreased UV absorbance (curve c′). This is the evidence that a

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fair amount of Phe was adsorbed into LPMS2-NaDC. When the standing

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temperature rose to 313 K, Phe was more completely adsorbed (curve d′) in

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LPMS2-NaDC, the supernatant (sample e) from the heated ternary sample emitted apparent weaker fluorescence than that placed at room temperature as well.

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Sufficient adsorption of Phe into LPMS2-NaDC thus enabled bright RTP emission, green-yellow phosphorescence of Phe was captured immediately after the UV light turning off (sample d), as phosphorescence generally has fairly longer emission lifetime than florescence.

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Fig.6. UV absorbance spectra and luminescence imaging

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Samples in luminescent image are NaDC-Phe at 293 K (a), centrifuged LPMS2-Phe at 293 K (b), LPMS2-NaDC-Phe at 293 K (c), LPMS2-NaDC-Phe at 313 K (d), and supernatant (e) of

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centrifuged sample d, correspondingly. Curves a′-d′ are UV absorption spectra of centrifuged

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sample a-d, correspondingly. cLPMS2: 1 mg/mL. cNaDC: 2 mM. cPhe: 20 μM. Standing time: 120 min.

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3.3 Analysis of RTP Enhancement

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In view of the UV detection results and remarkable RTP emission of five PAHs in the presence of LPMS2-NaDC, we can conclude that RTP protection is

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offered by the synergistic substrate formed by LPMS2 and NaDC. The RTPinducing mechanism of mPS-NaDC, which is different from that of single NaDC medium, has high correlation with hydrogen bonding effect between the two substances.

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Scheme 1. Schematic representation of the mPS-NaDC synergistic substrate

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Given that NaDC molecule possesses active groups of only two hydroxyls and

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one carboxyl on its hydrophilic side, mPS have Si-O structure and surface covered by Si-OH groups, NaDC molecules may easily connected onto mPS by hydrogen

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bonding. As depicted in Scheme 1, when NaDC was added to mPS dispersion,

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NaDC molecules were rapidly adsorbed into mPS pores. Through stable

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hydrogen-bonding interaction, NaDC molecules attached to the inner surface of mPS pores, while the hydrophobic side of NaDC faces the center of the pore. Thus,

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a “hard-soft” substrate with hydrophobic holes that can contain phosphors formed.

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The RTP protective effect was further illustrated by the RTP variation under different NaDC/mPS proportions. In the first series of tests, NaDC concentrations were controlled at 3 mM. As shown in Fig. 7, a weak RTP signal of Phe was detected in the single-NaDC medium (line a). Notably, the content of NaDC micelle is low when it is newly prepared. With the increase of LPMS2 content in the solutions, the phosphorescence increased significantly. 15

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Fig. 7. The RTP variation of Phe under different mPS:NaDC proportions

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cPhe: 1 μM. Standing time: 120 min. cNaDC in line a was fixed to 3 mM. cLPMS2 in line b and c were fixed to 1 and 3 mg/mL, respectively.

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In another series of tests, the contents of LPMS2 were fixed. Evidently, only

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LPMS2 was unable to ensure the phosphorescence of Phe. Hydrophilic mPS pores

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cannot sufficiently adsorb water-insoluble molecules. With the increase in NaDC content ratio, the RTP increased rapidly (line b); this finding indicates that NaDC

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improved the hydrophobicity of the inner pore by covering the pore wall. In this

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case, Phe molecules were gradually adsorbed, and were able to emit phosphorescence under stable protection. In previous works [29, 33], NaDC concentrations were controlled to lower than the CMC (4.9 mM) in detections to ensure the optimum RTP-inducing ability. In the current study, we found that the phosphorescence of Phe in the presence of mPS-NaDC was not limited by the CMC. At the LPMS2 content of 1 mg/mL, the 16

ACCEPTED MANUSCRIPT maximum RTP occurred at the NaDC concentration of 2 mM. When the LPMS2 content increased to 3 mg/mL, the RTP reached the maximum at NaDC concentration of 6 mM (line c). An optimal mPS/NaDC proportion of 0.5 mg/mM

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was determined.

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When overdose NaDC was added to LPMS2, the phosphorescence decreased

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significantly because the inner pore wall had been sufficiently covered with NaDC

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molecules, and excessive NaDC molecules may block the pore entrance and prevent the inclusion of phosphors. Conversely, when the addition of LPMS2 was

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excessive, the inner wall of the mPS pore was not completely covered by NaDC

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molecules, and the mPS pore with poor hydrophobicity cannot sufficiently adsorb phosphors. Besides, higher mPS concentration may also make the dispersion more

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opaque, RTP emission was thus blocked.

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3.4 Absorption dynamics

The RTP protection effect of mPS-NaDC substrate is based on the adsorption

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of NaDC and phosphors by mPS. Thus, the adsorption process in the presence of three mPSs with different pore sizes was investigated. Same amounts of NaDC and Phe were added to MCM41, LPMS1 and LPMS2 suspensions at room temperature (293 K). After quick shaking, RTP of the three samples were detected regularly with standing time. The variations of phosphorescence are depicted in Fig. 8. The RTP of Phe in the presence of MCM-41 increased slowly and required 17

ACCEPTED MANUSCRIPT over 24 h to reach the maximum intensity. LPMS1 exhibited a larger pore size (3.049 nm) and still needed almost 10 h to reach the maximum phosphorescence. For LPMS2, which presented the largest pore size (3.835 nm), the

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phosphorescence reached the maximum within only 260 min.

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Fig. 8. RTP variation of Phe with standing time in the presence of three mPSs cPhe: 2 μM, cNaDC: 2 mM, cmPS: 1 mg/mL

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Clearly, the increase speed of phosphorescence depends largely on the pore

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size of mPS. NaDC molecules can be more rapidly adsorbed into silica with larger

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pores, which enable faster subsequent adsorption of Phe. By comparing Fig.8 with the porosity of the three mPSs, we also concluded that the maximum RTP inducing-ability of the mPSs is closely related to their pore volume. mPS with larger pore volume offers more hydrophobic cavities for phosphors.

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ACCEPTED MANUSCRIPT To select mPS as the hard framework, we consider both adsorption speed and the maximum RTP inducing-ability. Faster adsorption speed certainly indicates shorter detection period, which is often a decisive criterion for analytical applications. Although LPMS2 possesses slightly lower pore volume than the two

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other mPSs, it adsorbed NaDC and Phe faster. LPMS2 also exhibited the same

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average particle size as that of MCM41. A nanoscale particle size indicates good dispersibility and stability in water solution. Thus, LPMS2 is an ideal hard

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substrate for the mPS-NaDC synergistic system.

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3.5 Influence of temperature

Fig. 9. RTP of Phe-mPSs-NaDC under different temperatures.

cPhe: 5 μM, cNaDC: 2 mM, cmPS: 1 mg/mL, sample Blank is 4 mM single-NaDC medium

In a single-component solution of surfactant, temperature generally exerts apparent effects on phosphorescence from two aspects. First, both the formation process of micelle and the micelle structure depend on the standing temperature of 19

ACCEPTED MANUSCRIPT the sample, similar to the hydrophobic cavity of the micelle. Second, the molecule motion also varies with temperature, and solvent and oxygen molecules may thus quench the phosphorescence under high temperatures. As shown in Fig. 9, RTP intensity of Phe in the presence of single-NaDC medium (sample Blank)

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decreased by 40% along with the standing temperature and detection temperature

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increasing from 283 K to 313 K. In comparison, RTP of Phe in mPS-NaDC mediums exhibited excellent thermostability, and variations of temperature

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showed no negative effect on the phosphorescence.

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This phenomenon is ascribed to the good structural rigidity of the “hard-soft”

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substrate. Although the micelle structure of single-NaDC varied significantly with temperature, the rigid frame structure of mPS was free from such variation. Once

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NaDC molecules were adsorbed into mPS pores, they attached to the interior

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mesopore wall. Consequently, the rigid pores restricted the motions of NaDC

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molecules, and the changes in temperature may not influence the shape of the hydrophobic cavity inside the mPS-NaDC complex. Thus, the phosphorescence

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remained stable.

Moreover, higher standing temperature (than room temperature) enables faster adsorption speed of NaDC. By setting the standing temperature to 313 K after the addition of NaDC and Phe into LPMS2, the RTP intensity increased to the maximum value in 110 min. For the analytical method, reduction in standing time

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ACCEPTED MANUSCRIPT indicates faster detection speed and better application potential, which is benefited from the good tolerance of mPS-NaDC substrate to the temperature variation.

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3.6 Influence of heavy atom and cyclohexane

Fig. 10. RTP of Phe-LPMS2-NaDC in the presence of DBP cPhe: 0.2 μM, cNaDC: 2 mM, cLPMS2: 1 mg/mL

The heavy atom effect plays an important role in RTP detections [34, 35]. An ideal extrinsic heavy atom inducer should be as close as possible to phosphors. In this case, the intersystem crossing of π electrons from energy level S1 to level T1 21

ACCEPTED MANUSCRIPT can be apparently improved, and the maximum phosphorescence enhancement is achieved.

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assessed

and

compared

bromobutane,

dibromoethane,

dibromopropane (DBP), iodobutane, and iodoform for their RTP inducing efficiency. As shown in Fig. 10A, DBP demonstrated greater phosphorescence

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inducing ability than the four other chemicals. With the increase in addition of

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DBP, the RTP enhancement also increased. We can observe that the delayed fluorescence at 375 nm was apparently restricted (Fig. 10B). The RTP reached the

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maximum when the ratio of DBP:NaDC was 1:2, but continuous addition of DBP

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contributed no stronger phosphorescence because of the poor water-solubility of DBP. When an appropriate low amount of DBP was added after the addition of the

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phosphors, they may be well dissolved and absorbed into the mPS-NaDC substrate.

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However, when DBP was excessive, it is difficult for the extra DBP molecules to

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dissolve completely. Consequently, the mPS-NaDC substrate may be destroyed by

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mixing excessive DBP.

Fig. 11. RTP of Phe-LPMS2-NaDC in the presence of CYH 22

ACCEPTED MANUSCRIPT cPhe: 0.2 μM, cNaDC: 2 mM, cLPMS2: 1 mg/mL.

Cyclohexane (CYH) was reported to be an assistant RTP inducer [36, 37]. We added CYH to the LPMS2-NaDC substrate for further RTP enhancement. As

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shown in Fig. 11, the maximum phosphorescence was reached when the

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CYH:NaDC ratio was 7:20. Given that CYH is insoluble in water, its influence to

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the phosphorescence is similar to that of DBP, and it should not be overused as

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well.

In addition, the RTP-inducing mechanism of CYH differed from that of heavy

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atoms. CYH was adsorbed into mPS-NaDC together with Phe, causing two effects.

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On the one hand, CYH molecules filled the hydrophobic cavity of mPS-NaDC

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substrate to prevent the phosphorescence quenchers from entering the substrate. On the other hand, by space-regulation effect, the molecular motion of Phe was by the coexistent

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restricted

CYH.

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phosphorescence increased[38].

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Thus,

the

emission

efficiency of

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3.7 Analytical performance

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ACCEPTED MANUSCRIPT Fig. 12. Standard curves of five PAHs cNaDC: 2 mM, cLPMS2: 1 mg/mL. cCYH and cDBP were all 0.7 nM.

After optimizing the experimental conditions, different concentrations of the five PAHs were added into the LPMS2-NaDC substrate. Standard curves are

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depicted in Fig. 12, and the analytical results are listed in Table 1. Phe showed an

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excellent RTP emission; its RTP also presented a good liner correlation with a wide range of concentrations (Fig. 12A). Given the poor solubility of Ant in water,

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Ant showed a smaller linear concentration range (Fig. 12B), and the RTP of Ant

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decreased significantly when the concentration was over 250 nM. Nap exhibited a smaller π-π conjugated system in its molecular structure; thus, the RTP emission

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of Nap was weaker than that of the above two substances (Fig. 12C). Pyr and Bkf

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are insoluble in water, and their phosphorescence is very difficult to detect.

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Despite the assistant dissolution effect of mPS-NaDC substrate and the phosphorescence enhancement by DBP and CYH, both Pyr and Bkf still showed

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weak RTP and small linear concentration ranges.

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Table 1 Analytical performance of five PAHs in the presence of LPMS2-NaDC Linear

LODa (nM)

concentration (μM)

Linear correlation coefficient

Nap

0.5-5

500

0.9979

Phe

0.01-2

5.0

0.9985

Ant

0.02-0.25

2.0

0.9960

Pyr

0.01-0.5

10

0.9945

Bkf

0.02-0.2

15

0.9966

a

Obtained according to the IUPAC definition[39] 25

ACCEPTED MANUSCRIPT The mPS-NaDC substrate was compared with previous reported methods in the analytical results. As shown in Table 2, the mPS-NaDC substrate offers ideal detection sensitivity and wide linear concentration ranges. Meanwhile, detection operations have higher safety because use of organic solvents was largely avoided,

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and the synthesis and application of large pore mPS have also become increasingly

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mature and cost effective.

Table 2 Comparisons of the mPS-NaDC substrate with previous reported methods Phosphor

LOD

Linear range

Ref

Filter paper

6-Thioguanine

4.6 ng

3.3-334.3 ng

[11]

Nylon powder

Thiabendazole

22.4 nM

64.1-546.6 nM

[18]

SDS micelle

Allopurinol

103 nM

183-514 nM

[40]

NaDC micelle

9-Bromophenanthrene

0.8 nM

0.1-10 μM

[29]

γ-cyclodextrin

Propranolol

ND a

ND

[13]

Cucurbit[5]uril

Naphthol

ND

ND

[41]

Molecularly imprinted polymer

Benzo[a]pyrene

0.16 nM

0.16-396 nM

[42]

Supramolecular gels

3-Bromoquinoline

ND

ND

[43]

Organic solvents

Phe, Ant

0.84, 11 nM

0.84-560, 11-1120 nM

[34]

mPS-NaDC

Phe, Ant

5, 2 nM

10-2000, 20-250 nM

This work

AN

M

ED

PT

Not discussed.

CE

a

US

Medium

AC

4. Conclusions

Compared with single-NaDC medium, the novel mPS-NaDC synergistic substrate proposed in this work showed faster speed and stronger ability to induce RTP, the induced RTP also showed good stability against temperature variation. After a series of detections and comparisons, we deduced that hydrogen-bonding interaction between NaDC and the inner pore surface of mPS is the possible RTP26

ACCEPTED MANUSCRIPT inducing mechanism, and further efforts to explore the direct evidence of the above mechanism are still necessary. Phosphorescence spectra of five PAHs were successfully detected; thus, the

T

mPS-NaDC substrate presents good adaptive capacity to target analytes with

IP

different molecular sizes. Based on these new features and the excellent adsorption

CR

property, the mPS-NaDC synergistic substrate may provide significant application

US

potential in the field of RTP, such as one-step extraction and detection of trace

Acknowledgements

ED

M

AN

components in real samples, and phosphorescent materials.

PT

We acknowledge the financial support from the National Natural Science

CE

Foundation of China (21375083) and the Youth Science Research Foundation

AC

Project of Shanxi Datong University, China (2013Q9 and 2012Q3). We appreciate Prof. Dr. Xiufang Qin (Shanxi Normal University) for her help on TEM characterizations.

27

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33

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ACCEPTED MANUSCRIPT

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Graphical abstract

34

ACCEPTED MANUSCRIPT Highlights The MSN-NaDC substrate shows strong RTP-inducing ability and good thermostability

T

The RTP-inducing ability is based on the hydrogen bonding between NaDC and MSN

CR

IP

The LPMS2 exhibits stable and rapid RTP-inducing ability

US

The MSN-NaDC substrate is adaptive to analytes with different molecular sizes

AC

CE

PT

ED

M

AN

The substrate enabled wide linear concentration range and high sensitivity

35