“Fastening” Porphyrin in Highly Cross-Linked PolyphosphazeneHybrid Nanoparticles: Powerful Red Fluorescent Probe for DetectingMercury IonYing Hu,† Lingjie Meng,*,‡ and Qinghua Lu*,†,§

†School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, P. R. China‡MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter and Department of Applied Chemistry,School of Science, Xi’an Jiaotong University, Xi’an 710049, P. R. China§State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

*S Supporting Information

ABSTRACT: It is a significant issue to overcome the concentration-quenching effect ofthe small fluorescent probes and maintain the high fluorescent efficiency at highconcentration for sensitive and selective fluorescent mark or detection. We developed anew strategy to “isolate” and “fasten” porphyrin moieties in a highly cross-linkedpoly(tetraphenylporphyrin-co-cyclotriphosphazene) (TPP−PZS) by the polycondensa-tion of hexachlorocyclotriphosphazene (HCCP) and 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (TPP-(OH)4) in a suitable solvent. The resulting TPP−PZS particles were characterized with transmission electron microscopy (TEM),scanning electron microscopy (SEM), Fourier transform infrared (FTIR), 31P nuclearmagnetic resonance (NMR), and ultraviolet and visible (UV−vis) absorption spectra.Remarkably, TPP−PZS particles obtained in acetone emitted a bright red fluorescenceboth in powder state and in solution because the aggregation of porphyrin moieties in“H-type” (face-to-face) and “J-type” (edge-to-edge) was effectively blocked. Thefluorescent TPP−PZS particles also showed superior resistance to photobleaching, and had a high sensitivity and selectivity forthe detection of Hg2+ ions. The TPP−PZS particles were therefore used as an ideal material for preparing test strips to quicklydetect/monitor the Hg2+ ions in a facile way.

1. INTRODUCTION

Fluorescent nanoparticles have attracted great interest over thepast two decades in the fields of bioimaging and therapy,1−3

chemical sensors,4 heavy ion detection,5 display and lighting,6

and so on. Compared with the organic dyes, these nanoparticlesprovide competitive advantages in term of brighter emission,higher photobleaching resistance, and even capability formultiplexing.The frequently used fluorescent nanoparticles are semi-

conductor quantum dots (QDs),7 rare earth based nano-particles,8 noble metal nanoparticles,9 carbon dots,10 and dye-doped silica or polymer nanoparticles.11,12 The QDs and rareearth based nanoparticles have relatively high quantum yield,narrow emission, and high resistance to photobleaching.However, they have several fundamental problems, such aspoor chemical stability, susceptible emission to surfaceconditions, and potential cytotoxicity of their heavy metalsingredient.13 Noble metal nanoparticles and carbon dotspresent reduced cytotoxicity compared with QDs and rareearth, but their small size (<2 nm) makes them difficultachieving chemical stability, reproducibility, and comparabilityas well as quantification.14 Dye-doped silica or polymernanoparticles represent the most abundant structural fluo-rescent nanoparticles. They usually incorporate dye molecules

inside a silica or polymer particle by covalent attachment orphysical entrapment. Therefore, the photobleaching andphotodegradation of dyes can be greatly reduced.15 However,the organic dyes cannot be easily uniformly doped into thesilica or polymer matrix and tend to aggregate together or leakfrom the nanoparticles in practice.16 In addition, these dyemolecules with conjugate structure tend to aggregate at highconcentration by π−π stacking, leading to fluorescencequenching.17,18 It is still a challenge to design and synthesizechemically and optically stable, repeatable, and highlyfluorescent dye-doped nanoparticles.To overcome the aggregation-caused quenching of dyes, a

large number of aggregation-induced emission (AIE) materialshave been developed since Tang et al. found this phenomenonin 2001.19,20 In this work, we proposed a new strategy to“isolate” and “fasten” the fluorescent moieties such asporphyrins into a stable cross-linked polyphosphazene, whichcould meet all the above criteria. Porphyrins and theirderivatives are kinds of organic molecules with a macrocyclictetrapyrrole core and different substituent. They are ideal

Received: January 21, 2014Revised: March 5, 2014Published: March 28, 2014

Article

pubs.acs.org/Langmuir

© 2014 American Chemical Society 4458 dx.doi.org/10.1021/la500270t | Langmuir 2014, 30, 4458−4464

organic molecules for all kinds of sensors due to the fact thatthey are remarkably robust under a variety of conditions andtheir optical properties can be well tuned via the choice ofmetal ion coordinated to the center of macrocyclictetrapyrrole.21,22 The polyphosphazenes are a class oforganic−inorganic hybrid polymers with several uniqueadvantages including tailored chemical and physical propertiesby adjusting the pendant groups on phosphorus atoms. Thoughtheir backbone formally consists of alternating single anddouble P−N bonds, the cyclotriphosphazene rings are notaromatic because there is a break in conjugation at eachphosphorus atom.23−25 Therefore, cyclotriphosphazene in thenanoparticles may serves as spacer to fasten and isolate theporphyrin moieties, effectively overcoming the concentration-quenching effect. As expected, the each fluorescent moiety inthe nanoparticles can exhibit single molecule performance (i.e.,high fluorescent efficiency) at any concentration. Thefluorescent nanoparticles also show an excellent chemicalstability and resistance to photobleaching due to their highlycross-linked structures. Interestingly, the fluorescent nano-particles exhibit high sensitivity and selectivity for detectingHg2+ ions. Therefore, fluorescent test strips were prepared bythe TPP−PZS particles, which opens a door for quickdetection/monitoring of the Hg2+ ions.

2. EXPERIMENTAL PART2.1. Chemicals. 5,10,15,20-Tetrakis(4-methoxyphenyl)porphyrin

(TPP-(OCH3)4, 95%), hexachlorocyclotriphosphazene (HCCP, 98%),BBr3 (99%), NaClO4, AgClO4, LiClO4·3H2O, Ca(ClO4)2, Mg(ClO4)2,Ni(ClO4)2, Co(ClO4)2·6H2O, Pb(ClO4)2, Zn(ClO4)2·6H2O, Cd-(ClO4)2·6H2O, Mn(ClO4)2·6H2O, Cu(ClO4)2·6H2O, and Fe(ClO4)3were purchased from J&K. Triethylamine (TEA) and all other organicsolvents were obtained from Shanghai Chemical Reagent Corporation.Methylene chloride (CH2Cl2) was distilled from CaH2 under dry N2purge. Water was purified using a Milli-Q system (Millipore, Bedford,MA).

2.2. Synthesis of (TPP-(OH)4). TPP-(OH)4 was prepared using amodified literature procedure.26 The TPP-(OCH3)4 (1 g, 1.3 mmol)was dissolved in dichloromethane (30 mL) in an ultrasonic bath (50W, 40 kHz) at room temperature. BBr3 in CH2Cl2 solution (1 M, 50mL) was then added dropwise at 0 °C. The resulting mixture wasstirred for 24 h at room temperature, quenched with methanol (10mL), and neutralized with TEA (30 mL) until the color of the solutionchanged from green to dark red. The products were collected andchromatographed (silica gel, acetone/light petroleum = 1:1) toprovide pure TPP-(OH)4 as a purple solid (644 mg, 73% yield).

2.3. Synthesis of Hollow TPP−PZS Nanoparticles. HCCP (10mg, 28.8 mmol) and TPP-(OH)4 (10 mg, 14.7 mmol) were placed in a100 mL round-bottom flask. Acetonitrile (30 mL) was then added andsonicated for 20 min (50 W, 40 kHz). After injecting of TEA (1.0 mL)into the mixture, the solution was then maintained at 20 °C under

Scheme 1. Preparation Procedure of TPP−PZSs

Langmuir Article

dx.doi.org/10.1021/la500270t | Langmuir 2014, 30, 4458−44644459

ultrasonic irradiation (50 W, 40 kHz) for 0.5 h. The resultant particleswere collected by centrifugation, washed with deionized water (3 × 30mL) and ethanol (3 × 30 mL) successively, and dried at 45 °C invacuum overnight.2.4. Synthesis of Solid TPP−PZS Nanoparticles. Solid TPP−

PZS nanoparticles were prepared in the same way as that describedabove for the hollow TPP−PZS nanoparticles by replacing the solventof acetonitrile with acetone, and the reaction time was prolonged to 18h.2.5. Preparation of Test Strips and Fluorescent Color Chart.

Ashless filter papers were immersed in solid TPP−PZS alcoholsolution (2 mg mL−1) for 3 min and then dried in a vacuum at 45 °Cto obtain fluorescent test strips. The fluorescent color chart wasprepared by immersing test strips into the Hg2+ ion solution at variousconcentrations for 30 s and recorded the photographs and fluorescentimages under a 365 nm UV lamp.2.6. Characterization. Transmission electron microscopy (TEM)

was carried out on a CM120 (Philips). Field emission scanningelectron microscope (FE-SEM) images were obtained using a PhilipsSirion 200 instrument under an accelerating voltage of 20 kV. The sizeand distribution of all as-prepared nanomaterials were determinedfrom TEM and SEM micrographs using ImageJ (V1.41, NIH) forimage analysis. Photographs were taken with a digital camera (IXUS800IS, Canon, Japan). Fourier transform infrared (FTIR) spectra wererecorded on a Paragon 1000 (PerkinElmer) spectrometer. Sampleswere dried overnight at 45 °C under vacuum and thoroughly mixedand crushed with KBr to fabricate KBr pellets. Quantitative 31P NMRspectra were recorded on a Varian Mercury Plus-400 nuclear magneticresonance spectrometer (400 MHz) by an one-pulse sequence withthe high-power DD technique. The proton π/2 pulse duration was 5μs, and 2000 signal transients with a 100 s relaxation delay wereaccumulated. Magic-angle spinning was set at 6 kHz in order to ensurecomplete separation of side-band intensity from the central transition.Ammonium dihydrogen phosphate (ADHP) was utilized as a 31Pchemical shift reference (δ =0.8 ppm) and as an external reference.Ultraviolet and visible (UV−vis) absorption spectra were recorded ona Shimadzu UV-2550 spectrophotometer. The fluorescence spectrawere performed on a PerkinElmer LS 50B fluorescence spectrometer.Thermogravimetric analysis (TGA) curves (heating rate = 10 °Cmin−1 in nitrogen) were recorded on TGA-7 (PerkinElmer)instruments.

3. RESULTS AND DISCUSSION

The one-pot polymerization of HCCP and TPP-(OH)4 wascarried out in an organic solution to create a newpolyphosphazene-containing porphyrin, termed as TPP−PZS(Scheme 1). The hydroxyl groups on TPP-(OH)4 wereactivated by TEA under ultrasonic condition and tended toreplace the chlorine atoms on HCCP to generate a highlycross-linked structures. Excess of TEA was also used to absorbresulting HCl and accelerat the polymerization. However, onlyone chlorine atom might be substituted on each phosphorusatom due to the steric hindrance effect. With the proceeding ofsubstitution reaction, the obtained polymer precipitated fromthe reaction medium, leading to the termination of thepolymerization. Therefore, the organic solution is a key factorto affect the size and morphology of TPP-PZS because thedifferent organic solutions have various Hildebrand solubilityparameters.27 In addition, the solvent polarity also affected thereaction rate and the alignment of TPP in TPP−PZS. Here wechose two polar solvents (acetonitrile and acetone), which aregood solvents for both HCCP and TPP-(OH)4, to regulate themorphology and TPP alignment of TPP−PZS nanoparticles.TEM and FE-SEM were used to characterize the structure

and morphology of TPP−PZS particles (Figure 1). The TPP−PZS particles obtained in acetonitrile have a relatively smoothsurface, with an average size of 245.6 ± 17.1 nm. The centralportions of these particles were a bit lighter than their edges,indicating the formation of a hollow nanostructure (Figure 1a).The SEM image shows a direct evidence that most of thespherical particles have a hole (Figure 1c), while the TPP−PZSparticles prepared in acetone are irregular oval with a roughsurface, and their average size is 671.6 ± 20.1 nm. The centralportion of each particle was darker than its edge due to theirdifferent thickness of TPP−PZS along the solid particles(Figure 1b). The SEM images further proved that theseparticles are solid with rough surface (Figure 1d). As illustratedin Figure 1c,d, both TPP−PZS particles prepared in acetonitrile

Figure 1. TEM and FE-SEM images of (a, c) hollow TPP−PZS particles prepared in acetonitrile and (b, d) solid TPP−PZS particles prepared inacetone.

Langmuir Article

dx.doi.org/10.1021/la500270t | Langmuir 2014, 30, 4458−44644460

and acetone solution have a relatively narrow size distributionat ca. 265.4 and 684.3 nm, respectively, which are in goodagreement with the observations in TEM images.To explain the different shape and structure of the TPP−PZS

particles obtained in acetonitrile and acetone solution, werepeated the synthesis procedure without addition of HCCP.After adding TEA, the transparent red TPP-(OH)4 acetonitrilesolution becomes to cloudy pink liquid, while the TPP-(OH)4acetone solution remains a transparent red solution. Wespeculate that TPP-(OH)4 molecules might tend to assembletogether in acetonitrile under the assistance of TEA byhydrophobic and π-stacking effects (see Supporting Informa-tion, Figure S1).28 The resulting TPP−PZS may coat on thesurface of TPP-(OH)4 particles (see Supporting Information,Figure S2a,b). After thoroughly being washed by acetone andethanol, the encapsulated TPP-(OH)4 assemblies were easilydissolved (see Supporting Information, Figure S2c) and left thehollow TPP−PZS particles (Figure 1a,c). Compared toacetonitrile, acetone has better solubility to TPP-(OH)4 andTPP−PZS. Therefore, the TPP-(OH)4 can gradually react withHCCP with the assistance of TEA to render large solidparticles. These solid TPP−PZS particles can be well dispersedin acetone and ethanol, but no free TPP-(OH)4 can beextracted, indicating all the TPP-(OH)4 molecules have been“fastened” into the highly cross-linked polyphosphazenes.FTIR spectroscopy was used to confirm the successful

formation of TPP−PZS (Figure 2a). The absorption at 1246

cm−1 assigned to the Ph−OH in TPP-(OH)4 disappeared inthe TPP−PZS spectrum while a new intensive absorption at930 cm−1 belonging to P−O−Ph band appeared, which aredirect evidence of the polymerization of HCCP and TPP-(OH)4.

29 Other characteristic peaks of TPP−PZS can also beobserved, including 1217 cm−1 (PN) and 862 cm−1 (P−N)in the cyclotriphosphazene structure and 1601 and 1484 cm−1

(−Ph−) in the porphyrin units. In the solid state 31P NMRspectrum of solid TPP−PZS particles, two resonance signalsappeared at 12.6 and 20.4 ppm, indicating the presence of−NP(−OPh)(−Cl) and −NP(−Cl)2, respectively (Figure2b). The ratio of the integral values of the two resonancesindicates that about one-third of the phosphorus atoms exist inthe form of −NP(−OPh)(−Cl)2 in the cross-linked solidTPP−PZS particles. The incomplete substitution reactioncould be attributed to the steric hindrance effects of the bigTPP rings. And yet for all that, a highly cross-linked structurewas obtained as Scheme 1, and the solid TPP−PZS particlestherefore exhibit an excellent thermal stability (see SupportingInformation, Figure S3).The fluorescence emission spectra of solid TPP−PZS, hollow

TPP−PZS, and TPP-(OH)4 were measured to compare theirfluorescence behavior (Figure 3a). Solid TPP−PZS has similar

but a bit weaker emission peak at about 660 nm, whereas veryweak emission peak is observed for the hollow TPP−PZS. Thefluorescence quenching may come from the porphyrinaggregation in the hollow TPP−PZS particles, which can alsooccur during porphyrin conjugation to polymers.30 The strongfluorescence emission of solid TPP−PZS indicates that theporphyrin rings should be isolated in highly cross-linkedstructures. It has been reported that the cyclotriphosphazenerings are nonconjugated systems and photochemically inert.Therefore, the cyclotriphosphazene rings serve as spacers toisolate the porphyrin rings to prevent the excited electrons inporphyrin rings from transferring and thus to reduce the

Figure 2. (a) FTIR spectra of HCCP, TPP-(OH)4, and solid TPP-PZSparticles. (b) Quantitative solid state 31P NMR spectrum of solidTPP−PZS particles.

Figure 3. (a) Fluorescence spectra and (b) UV−vis spectra of TPP-(OH)4 solution (10 μM), solid TPP−PZS (Cporphyrin = 9.7 μM) andhollow TPP−PZS (Cporphyrin = 12.4 μM) suspension in ethanol.

Langmuir Article

dx.doi.org/10.1021/la500270t | Langmuir 2014, 30, 4458−44644461

fluorescence quenching. The molecule structure of solid TPP−PZS is modeled and calculated by ChemBioDraw 3D 12.0 (seeSupporting Information, Figure S4). The porphyrin groups lineup almost perpendicularly above and below the nearly planarcyclotriphosphazene core, and this structural feature makes itpossible for the porphyrin rings to be retained in individualmolecules.31

Ultraviolet and visible (UV−vis) absorption spectra werefurther used to investigate the aggregation/arrangement state ofporphyrin rings in the TPP−PZS (Figure 3b). The absorptionspectrum of TPP-(OH)4 in a dilute solution has a sharp andintense absorption centered at 416 nm, belonging to thecharacteristic absorption of isolated porphyrin rings. Theabsorption spectrum of hollow TPP−PZS shows a notablebroadening compared to TPP-(OH)4, indicating manyporphyrin rings aggregate in the way of “H-type” (face-to-face) by π−π stacking. And a new feature around 447 nmemerged, which is assigned to the “J-type” (edge-to-edge)aggregates. Both the “J-type” and “H-type” aggregates affect theelectronic states of porphyrin rings and might induce theelectron and energy communication among the porphyrin ringsto severely quench the fluorescence. In contrast, the solidTPP−PZS nanoparticles exhibit a similar absorption spectrumto that of TPP-(OH)4 with a little shoulder peak at 447 nm. Itsuggests that most of the porphyrin rings in solid TPP−PZSparticles are isolated, and only a small quantity of porphyrin

rings aggregate in the way of “J-type”, which can well explainthe result of fluorescence emission spectra.The fluorescence performance in solid state and the

photobleaching properties of solid TPP−PZS particles andTPP-(OH)4 were compared. The solid TPP−PZS particlescould emit a bright red fluorescence even in solid state with theexcitation of 365 nm, whereas no fluorescence could be seen forthe TPP-(OH)4 powder at the same conditions due to thecomplete fluorescence quenching (see Supporting Information,Figure S5). Amazingly, the solid TPP−PZS particles showed asuperior resistance to photobleaching than TPP-(OH)4 (seeSupporting Information, Figure S6), indicating that theporphyrin moieties have improved photochemical stabilityafter being “fastened” in the cross-linked structures.The fluorescence spectra of solid TPP−PZS particles with

different metal ions including Na+, Ag+, Li+, Ca2+, Mg2+, Ni2+,Co2+, Pb2+, Zn2+, Cd2+, Mn2+, Cu2+, Fe3+, and Hg2+ wereinvestigated (Figure 4a). The solid TPP−PZS particle aqueoussuspension exhibited a strong emission at 657 nm, but theintensity of emission at 657 nm severely quenched after adding10 μM Hg2+. Interestingly, no significant emission changeswere observed in the parallel experiments with other ionsincluding Na+, Ag+, Li+, Ca2+, Mg2+, Ni2+, Co2+, Pb2+, Zn2+,Cd2+, Mn2+, Cu2+, and Fe3+. The solid TPP−PZS aqueoussuspension (10 mg L−1) have strong red fluorescence underirradiation of 365 nm light after being mixed with 10 μM allkinds of ions except Hg2+ (see the inset photograph in Figure

Figure 4. (a) UV−vis spectra (inset: photograph) and (b) fluorescence spectra (inset: fluorescence images) of solid TPP−PZS suspension (10 mgL−1) in the absence and the presence of different metal ions (10 μM). The wavelength of exciting light is 365 nm.

Langmuir Article

dx.doi.org/10.1021/la500270t | Langmuir 2014, 30, 4458−44644462

4a). The results imply that the solid TPP−PZS particles havepotential applications as a new organic−inorganic hybrid sensorfor selective detection of Hg2+ ion.The absorption spectra of the solid TPP−PZS suspension in

the presence of different metal ions showed a strong visibleabsorption band at 419 nm except Hg2+, corresponding to thecharacteristic absorption of porphyrin moiety (Figure 4b).When Hg2+ was added, the absorption showed a 37 nm red-shift and the color of the suspension turned from pink toyellow-green (see the inset photograph in Figure 4b). Itsuggests that the Hg2+ is strongly interacted with porphyrinmoiety, therefore affecting the electronic states of porphyrinmoiety. The coordination of Hg2+ with the four N atoms of theendocyclic porphyrin can be further proved by 1H NMRbecause the peak at −2.78 position (belonging to NH)disppeared with the addition of Hg2+ (see SupportingInformation, Figure S7). Based on these evidence, onereasonable explanation of the fluorescence quenching is that areverse photoinduced electron transfer (PET) occurs after theHg2+ connects the nitrogen atoms of the porphyrin rings.32

To quantitatively investigate the relationship between thefluorescence intensity of TPP−PZS aqueous suspension andHg2+ ion concentrations, various molar concentrations of Hg2+

were added to the TPP−PZS aqueous suspension. Upongradual addition of Hg2+ ions in increasing concentration (0−10 μM), the emission intensity gradually declined (seeSupporting Information, FigureS8). When the concentrationof Hg2+ ions increased to 8 μM, TPP−PZS aqueous suspensionalmost completely lost their fluorescence. If the concentrationof TPP−PZS aqueous suspension decreases to 0.1 mg L−1, thedetection limit of Hg+ is about 10 ppb where the fluorecenceintensity drops by 10% (see Supporting Information, FigureS9).The selectivity of our method for Hg2+ detection over other

metal ions is studied (see Supporting Information, Figure S10and 11). Thirteen common metal ions (Na+, Ag+, Li+, Ca2+,Mg2+, Ni2+, Co2+, Pb2+, Zn2+, Cd2+, Mn2+, Cu2+, and Fe3+) werechosen to study their distraction to the detection of Hg2+ ions.All these 13 metal ions (10 μM) exhibited few backgroundfluorescence intensity changes, indicating that the solid TPP−PZS particles as well as TPP-(OH)4 have high selectivityagainst other interfering metal ions.The reusability is very important for the Hg2+ ions, which is

desired for industrial applications. To evaluate the reversibilityof solid TPP−PZS, ethylenediaminetetraacetic acid (EDTA)was used as a stripping agent for removal of Hg2+ ions in TPP−PZS particles. The fluorescence intensity of solid TPP−PZSsuspension can be restored to be four-fifths (see SupportingInformation, Figure S12), which is probably due to difficulty indetaching Hg2+ from the deep of the TPP−PZS particles.To conveniently detect Hg2+ ion without a heavy analysis

instrument, TPP−PZS test strips were prepared by immersingashless filter papers into the TPP−PZS alcohol solutions. Whendipped into the solutions of Hg2+ ions at differentconcentrations within 30 s, the color of test strips apparentlychanged which can be observed by visual means or uponexcitation at 365 nm under a UV lamp (Figure 5). Therefore,these strips offer a method to quickly detect/monitor the Hg2+

ions in a facile way.

4. CONCLUSIONSThe fluorescent porphyrin moieties were “isolated” and“ f a s t e n e d ” i n t h e h i g h l y c r o s s - l i n k e d p o l y -

(tetraphenylporphyrin-co-cyclotriphosphazene) (TPP−PZS)by the polycondensation of HCCP and TPP-(OH)4. Theparticle morphology and the aggregative state of porphyrinmoieties could be controlled by adjusting the reaction solvent.Hollow TPP−PZS particles prepared in acetonitrile have veryweak fluorescence due to the aggressive “H-type” (face-to-face)and “J-type” (edge-to-edge) aggregation of porphyrin moietiesin the particles. To the contrary, the solid TPP−PZS particlesobtained in acetone exhibited a bright fluorescent emission inboth powder and solution state because the porphyrin ringswere “fastened” in the cross-linked structures in isolation. Thesolid TPP−PZS particles also showed high resistance tophotobleaching. Intriguingly, the solid TPP−PZS showed ahigh sensitivity and selectivity for detection of Hg2+ ions.Therefore, the TPP−PZS strips were prepared and could beused for quick detecting/monitoring the Hg2+ ions in a facileway.

■ ASSOCIATED CONTENT*S Supporting InformationFigures S1−S12. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail menglingjie@mail.xjtu.edu.cn (L.M.).*E-mail qhlu@sjtu.edu.cn (Q.L.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by National Science Foundation forDistinguished Young Scholars (50925310), the NationalScience Foundation of China (20874059, 21174087), theMajor Project of Chinese National Programs for FundamentalResearch and Development (973 Project: 2009CB930400), theChina Postdoctoral Science Foundation (18420011), the

Figure 5. Fluorescence photographs (top) under UV (365 nm) lightand photographs (bottom) of TPP−PZS test strips after dipped inHg2+ ions solution with the concentrations of 0, 10, 25, 50, 75, and 100μM within 5 s.

Langmuir Article

dx.doi.org/10.1021/la500270t | Langmuir 2014, 30, 4458−44644463

Fundamental Funds for the Central Universit ies(DWHXC101000074), and the Shanghai Leading AcademicDiscipline Project (No.B202).

■ REFERENCES(1) Qian, H. S.; Guo, H. C.; Ho, P. C.-L.; Mahendran, R.; Zhang, Y.Mesoporous-Silica-Coated Up-Conversion Fluorescent Nanoparticlesfor Photodynamic Therapy. Small 2009, 5, 2285−2290.(2) Qin, W.; Ding, D.; Liu, J.; Yuan, W. Z.; Hu, Y.; Liu, B.; Tang, B.Z. Biocompatible Nanoparticles with Aggregation-Induced EmissionCharacteristics as Far-Red/Near-Infrared Fluorescent Bioprobes for inVitro and in Vivo Imaging Applications. Adv. Funct. Mater. 2012, 22,771−779.(3) Wang, Z.; Xu, B.; Zhang, L.; Zhang, J.; Ma, T.; Zhang, J.; Fu, X.;Tian, W. Folic Acid-Functionalized Mesoporous Silica NanospheresHybridized with Aie Luminogens for Targeted Cancer Cell Imaging.Nanoscale 2013, 5, 2065−2072.(4) Schaeferling, M. The Art of Fluorescence Imaging with ChemicalSensors. Angew. Chem., Int. Ed. 2012, 51, 3532−3554.(5) Kim, H. N.; Ren, W. X.; Kim, J. S.; Yoon, J. Fluorescent andColorimetric Sensors for Detection of Lead, Cadmium, and MercuryIons. Chem. Soc. Rev. 2012, 41, 3210−3244.(6) Song, W.-S.; Yang, H. Efficient White-Light-Emitting DiodesFabricated from Highly Fluorescent Copper Indium Sulfide Core/Shell Quantum Dots. Chem. Mater. 2012, 24, 1961−1967.(7) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H.Quantum Dot Bioconjugates for Imaging, Labelling and Sensing. Nat.Mater. 2005, 4, 435−446.(8) Hemmer, E.; Venkatachalam, N.; Hyodo, H.; Hattori, A.; Ebina,Y.; Kishimoto, H.; Soga, K. Upconverting and NIR Emitting RareEarth Based Nanostructures for NIR-Bioimaging. Nanoscale 2013, 5,11339−11361.(9) Xie, J.; Zheng, Y.; Ying, J. Y. Protein-Directed Synthesis of HighlyFluorescent Gold Nanoclusters. J. Am. Chem. Soc. 2009, 131, 888−889.(10) Liu, J.-H.; Yang, S.-T.; Chen, X.-X.; Wang, H. FluorescentCarbon Dots and Nanodiamonds for Biological Imaging: Preparation,Application, Pharmacokinetics and Toxicity. Curr. Drug Metab. 2012,13, 1046−1056.(11) Yan, J.; Estevez, M. C.; Smith, J. E.; Wang, K.; He, X.; Wang, L.;Tan, W. Dye-Doped Nanoparticles for Bioanalysis. Nano Today 2007,2, 44−50.(12) Vijayasree, N.; Haritha, K.; Subhash, V.; Rao, K. R. S. S. Dye-Doped Nanoparticles: The Bioconjugated Nanoparticles for Bio-technology and Bioanalysis. Res. J. Biotechnol. 2009, 4, 61−66.(13) Luo, S.; Zhang, E.; Su, Y.; Cheng, T.; Shi, C. A Review of NIRDyes in Cancer Targeting and Imaging. Biomaterials 2011, 32, 7127−7138.(14) Resch-Genger, U.; Grabolle, M.; Cavaliere-Jaricot, S.; Nitschke,R.; Nann, T. Quantum Dots Versus Organic Dyes as FluorescentLabels. Nat. Methods 2008, 5, 763−775.(15) Ruedas-Rama, M. J.; Walters, J. D.; Orte, A.; Hall, E. A. H.Fluorescent Nanoparticles for Intracellular Sensing: A Review. Anal.Chim. Acta 2012, 751, 1−23.(16) Zhao, X. J.; Bagwe, R. P.; Tan, W. H. Development of Organic-Dye-Doped Silica Nanoparticles in a Reverse Microemulsion. Adv.Mater. 2004, 16, 173-+.(17) Jares-Erijman, E. A.; Jovin, T. M. Fret Imaging. Nat. Biotechnol.2003, 21, 1387−1395.(18) Wang, J.; Zhao, Y.; Dou, C.; Sun, H.; Xu, P.; Ye, K.; Zhang, J.;Jiang, S.; Li, F.; Wang, Y. Alkyl and Dendron SubstitutedQuinacridones: Synthesis, Structures, and Luminescent Properties. J.Phys. Chem. B 2007, 111, 5082−5089.(19) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-InducedEmission. Chem. Soc. Rev. 2011, 40, 5361−5388.(20) Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L.; Chen, H. Y.; Qiu,C. F.; Kwok, H. S.; Zhan, X. W.; Liu, Y. Q.; Zhu, D. B.; et al.Aggregation-Induced Emission of 1-Methyl-1,2,3,4,5-pentaphenylsi-lole. Chem. Commun. 2001, 1740−1741.

(21) Delmarre, D.; Meallet, R.; Bied-Charreton, C.; Pansu, R. B.Heavy Metal Ions Detection in Solution, in Sol-Gel and with GraftedPorphyrin Monolayers. J. Photochem. Photobiol. Chem. 1999, 124, 23−28.(22) Li, C. Y.; Zhang, X. B.; Qiao, L.; Zhao, Y.; He, C. M.; Huan, S.Y.; Lu, L. M.; Jian, L. X.; Shen, G. L.; Yu, R. Q. Naphthalimide-Porphyrin Hybrid Based Ratiometric Bioimaging Probe for Hg2+:Well-Resolved Emission Spectra and Unique Specificity. Anal. Chem.2009, 81, 9993−10001.(23) Dewar, M. J. S.; Lucken, E. A. C.; Whitehead, M. A. TheStructure of the Phosphonitrilic Halides. J. Chem. Soc. 1960, 2423−2429.(24) Breza, M. The Electronic Structure of Planar PhosphazeneRings. Polyhedron 2000, 19, 389−397.(25) Potin, P.; Dejaeger, R. Polyphosphazenes - Synthesis, Structures,Properties, Applications. Eur. Polym. J. 1991, 27, 341−348.(26) Liu, S.-T.; Reddy, K. V.; Lai, R.-Y. Oxidative Cleavage ofAlkenes Catalyzed by a Water/Organic Soluble Manganese PorphyrinComplex. Tetrahedron 2007, 63, 1821−1825.(27) Goh, E. C. C.; Sto1ver, H. D. H. Cross-Linked Poly(MethacrylicAcid-Co-Poly(Ethylene Oxide) Methyl Ether Methacrylate) Micro-spheres and Microgels Prepared by Precipitation Polymerization: AMorphology Study. Macromolecules 2002, 35, 9983−9989.(28) Gong, X.; Milic, T.; Xu, C.; Batteas, J. D.; Drain, C. M.Preparation and Characterization of Porphyrin Nanoparticles. J. Am.Chem. Soc. 2002, 124, 14290−14291.(29) Schweiger, C.; Pietzonka, C.; Heverhagen, J.; Kissel, T. NovelMagnetic Iron Oxide Nanoparticles Coated with Poly(EthyleneImine)-g-Poly(Ethylene Glycol) for Potential Biomedical Application:Synthesis, Stability, Cytotoxicity and Mr Imaging. Int. J. Pharm. 2011,408, 130−137.(30) Lovell, J. F.; Jin, C. S.; Huynh, E.; Jin, H.; Kim, C.; Rubinstein, J.L.; Chan, W. C.; Cao, W.; Wang, L. V.; Zheng, G. PorphysomeNanovesicles Generated by Porphyrin Bilayers for Use as MultimodalBiophotonic Contrast Agents. Nat. Mater. 2011, 10, 324−332.(31) Liu, W.; Huang, X.; Wei, H.; Tang, X.; Zhu, L. IntrinsicallyFluorescent Nanoparticles with Excellent Stability Based on a HighlyCrosslinked Organic-Inorganic Hybrid Polyphosphazene Material.Chem. Commun. 2011, 47, 11447−9.(32) De Silva, A. P.; Gunaratne, H. N.; Gunnlaugsson, T.; Huxley, A.J.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Signaling RecognitionEvents with Fluorescent Sensors and Switches. Chem. Rev. 1997, 97,1515−1566.

Langmuir Article

dx.doi.org/10.1021/la500270t | Langmuir 2014, 30, 4458−44644464