PolymerChemistry

PAPER

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aNanotechnology Research Unit and Sup

Department of Chemistry and Center of

Faculty of Science, Mahasarakham Univer

E-mail: kchatthai@gmail.com; Fax: +66 043bSupramolecular Chemistry Research Unit

Science, Chulalongkorn University, Bangkok

Tel: +66 0221 87643

† Electronic supplementary informa10.1039/c3py00229b

Cite this: Polym. Chem., 2013, 4, 3039

Received 14th February 2013Accepted 4th March 2013

DOI: 10.1039/c3py00229b

www.rsc.org/polymers

This journal is ª The Royal Society of

Hybrid organic–inorganic nanomaterial sensors forselective detection of Au3+ using rhodamine-basedmodified polyacrylic acid (PAA)-coated FeNPs†

Noi Niamsa,a Chatthai Kaewtong,*a Weerapol Srinonmuang,a Banchob Wanno,a

Buncha Pulpokab and Thawatchai Tuntulanib

Rhodamine derivatives were grafted on poly(acrylic acid) or PAA to obtain hydrophobically modified PAA

(PAA-Rho1–4) bearing rhodamine moieties. Polymeric sensors were simply prepared by amidation

reactions between PAA and rhodamine of various mole ratios to obtain PAA-Rho1, PAA-Rho2, PAA-

Rho3 and PAA-Rho4 in 83%, 82%, 75% and 81% yields, respectively. Chemical compositions of PAA-

Rho1–4 were studied by IR and 1H NMR spectroscopy. Chemical structures and purity of polymeric

sensors were characterized by TGA, NMR, TEM and IR. It was found that polymeric sensors exhibited

high selectivity and sensitivity in colorimetric and fluorescence responses toward Au3+ over other metal

ions. The polymeric sensors were non-fluorescent in the spirolactam form and could be selectively

converted into the fluorescent ring-opened amide form in the presence of Au3+ ions that lead to

fluorescence enhancements and colorimetric changes. DFT calculation results suggested that the

polymeric sensor PAA-Rho2 formed stable complexes with Au3+ through a large number of cation–

dipole and ion–ion interactions. Moreover, the higher density carboxylic groups at the backbone of

PAA-Rho interacted strongly with monodisperse superparamagnetic Fe3O4 NPs (FeNPs) which provided

organic–inorganic hybrid fluorescence sensors (PAA-Rho2-FeNPs). The lower detection limit of the

hybrid sensor toward Au3+ (0.85 mM) was less than that obtained from PAA-Rho2 (11.40 mM), and the

response time was less than 40 seconds. Reusability was evaluated by repeating dipping and rinsing

cycles in aqueous Au3+ and EDTA solutions. This approach may provide an easily measurable and

inherently sensitive method for Au3+ ion detection in environmental and biological applications.

Introduction

Hybrid inorganic–organic nanomaterials, in which inorganicand organic components are combined to afford materials withunique properties, play a major role in the development ofadvanced functional nanomaterials.1 Due to their challengingcharacteristics, hybrid nanomaterials have recently beeninvestigated as promising platforms for devices displayingspecic single molecule properties. Receptors immobilized oninorganic materials such as SiO2, AgNPs, PtNPs, AuNPs, FeNPs,and TiO2 have important advantages as solid chemosensors inthe heterogeneous solid–liquid phase.2,3 For example, cationsensors based on gold, platinum or silver nanoparticles exhibit

ramolecular Chemistry Research Unit,

Excellence for Innovation in Chemistry,

sity, Mahasarakham, 44150, Thailand.

7 54246; Tel: +66 0437 54246

, Department of Chemistry, Faculty of

10330, Thailand. Fax: +66 0221 87598;

tion (ESI) available. See DOI:

Chemistry 2013

red-shis upon forming nanoparticle aggregates because ofinterparticle plasmonic coupling in localized surface plasmonresonance.4 Azobenzene-coupled receptors functionalized onmesoporous silicas (AR-SiO2) or titania nanoparticles (AR-TiO2)via sol–gel or hydrolysis reactions were prepared as heteroge-neous “naked-eye” colorimetric and spectrophotometric Hg2+

sensors.5 Ma et al. showed that quinoline modied silicananoparticles having magnetic nanoparticles (NQTP–FeNPs@SiO2) can act as uorophores which have high selectivity andsensitivity to detect Zn2+ ions over other metal ions.6 Among allthese inorganic materials, magnetic nanoparticles (FeNPs) haveadvantages over other competitors for pharmaceutical andbiomedical applications.7 They could be simply separated orrecovered using an external magnetic eld and possessed lowtoxicity and biocompatibility. In addition, FeNPs can be used todesign biological uorescence probes.8 Rhodamine dyes arewidely used as uorescence probes owing to their highabsorption coefficient and broad uorescence in the visibleregion of the electromagnetic spectrum, high uorescencequantum yields and photostability aer complexes with metalions by activating a carbonyl group in a spirolactone or a spi-rolactam moiety.9 The mechanism is based on the switch off/on

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Scheme 2 Synthetic pathways to PAA-Rho1–PAA-Rho4.

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of the spirocyclic moiety mediated by guests. In general, spi-rolactam formation of rhodamine derivatives is non-uores-cent, whereas its ring-opened amide system by guests gives riseto a pink color and strong uorescence emission. Recently,rhodamine-based sensors for cations and other analytes havereceived ever-increasing interest in areas such as sensors forPb2+, Cu2+, Hg2+, Fe3+, Cr3+, NO, and OCl�.10–17 In our previousworks, we have successfully designed uorescence chemo-sensors for anions, cations and ditopic receptors utilizingrhodamine B as the uorophore.18

A variety of uorescent cation sensors have been developedbased on small molecules as cation receptors. Nevertheless,these materials have shown several problems such as lowmechanical and thermal stability, weak chemical union withthe metals, poor removal efficiency, high cost, etc. In contrastwith molecular sensors, lm sensors based on polymers exhibitprominent advantages, such as the ease of fabrication ofdevices, a wide choice of incorporating specic units intofunctional polymers, low cost, and so on.19 Recently, PAA hasfound a wide range of applications by surface modication withfunctional polymers and specic molecules. PAAs graed withp-t-butyl calix[4]arenediamine and p-t-butyl calix[4]arenediol areused to evaluate the sorption properties and show goodsorbents for heavy metal and alkali metal cations.20 Moreover,poly(acrylic acid) has a higher density of carboxylic groups at thebackbone which can interact strongly with FeNPs.21 In addition,the PAA polymers provide both electrostatic and steric repulsionagainst particle aggregation, and the stability of dispersions canbe controlled by adjusting the pH of the solution.21d,e

In addition, because of the commercial availability andindustrial importance of PAA, and the easy preparation ofrhodamines on a large scale, we select rhodamine derivatives asa good ionophore and distribute this compound in the PAAchain (protecting ligand) and gra onto the surface of Fe3O4

nanoparticles to make organic–inorganic hybrid uorescencesensors, which can be used as Au3+ chemosensors (Scheme 1).The detection mechanism is based on the switch off/on of thespirocyclic moiety. The rhodamine derivative is uorescenceinactive, whereas its ring-opened amide form activated by Au3+

yields a pink color and strong uorescence emission.

Scheme 1 Proposed selective detection mechanism of Au3+ by using rhoda-mine-based modified polyacrylic acid (PAA-Rho)-coated FeNPs.

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Amultiple-site model of these hybridmaterials has been used toincrease the selectivity and sensitivity of the measurements.

Results and discussion

Polymeric sensors (PAA-Rho1–PAA-Rho4) were easily synthe-sized in good yields by a condensation reaction of rhodamine Band ethylenediamine under N2 at reux for 3 days. Then, ami-dation reactions between PAA and rhodamine–ethylenediamineof various mole ratios in the presence of DCC–DMAP as acoupling reagent under N2 at reux for 3 days yielded PAA-Rho1(82.5%), PAA-Rho2 (81.8%), PAA-Rho3 (75.3%) and PAA-Rho4(80.5%) (Scheme 2). Chemical structures and purity of poly-meric sensors were proven by TGA, NMR, TEM and IR. Theywere designed to chelate metal ions via its carbonyl O andamine N atoms.

Characterization data of the synthesized polymeric sensorsare shown in Fig. 1 and ESI.† The characteristic FTIR spectra ofpolymeric sensors showed aromatic rhodamine peaks (1622and 1510 cm�1) and a carboxylic PAA peak (3300–3500 cm�1).The SEM micrograph showed increase of the crystallinity andcrystalline sizes which was attributed to p–p interactions andhydrogen bonding of the rhodamine moieties. Thermal stabil-ities of PAA-Rho were similar to each other which had twothermal decomposition steps. TGA curves from 190–305 �C and

Fig. 1 Characterization data of polymeric sensors; (a) FTIR, (b) TGA, (c) SEM(PAA), and (d) SEM (PAA-Rho).

This journal is ª The Royal Society of Chemistry 2013

Fig. 3 Colour changes (A, B, and C) and fluorescence changes (D, E, and F) ofPAA-Rho2 (0.1 g L�1) in the presence of 10 mMAu3+: (A and D) PAA-Rho2 only, (Band E) PAA-Rho2 + other metal ions, and (C and F) PAA-Rho2 + Au3+.

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310–477 �C were attributed to the weight loss of PAA andrhodamine, respectively. Typical 1H NMR spectra of PAA-Rhoshowed the characteristic signals of –CH2 and –CH groups inthe region of 1.0–2.1 and 2.2–2.6 ppm and aromatic protons at7.8–5.5 ppm, respectively.

All synthesized sensors, PAA-Rho1–PAA-Rho4, gave colorlessDMSO solutions and are uorescence inactive in solution ateither low or high concentration, indicating that the spi-rolactam form predominantly exists. The uorescence intensitychanges of all polymeric sensors were investigated to determinethe cation binding abilities. Fig. 2a shows uorescence spectraof PAA-Rho2 in the presence and absence of 10 mM of variouscations. A high-intensity uorescence band at 590 nm wasobserved only upon addition of Au3+ into solutions of PAA-Rho2. The high responses were also observed in the case of PAA-Rho1 and PAA-Rho3, whereas the PAA-Rho4 showed a slightresponse in the presence of Au3+ (Fig. 2b). The color and uo-rescence changes of PAA-Rho2 upon the addition of variouscations are shown in Fig. 3 and S3.†

To further study the interaction between PAA-Rho2 and Au3+,1H NMR experiments were carried out in DMSO-d6 (Fig. S2†).For the free sensor, the chemical shi of the amide proton wasabout 7.94 ppm, whereas in the presence of Au3+ ion, it wasbroaden and shied downeld (9.64 ppm). In addition, thechemical shis of all protons also showed distinct downeldshis, especially in the case of aromatic protons. 1H NMRresults rmly supported that the carbonyl group of spirolactamwas involved in the Au3+ binding, thus inducing a ring-openingof the spirolactam in PAA-Rho2. In addition, infrared spectrawere also conducted to conrm the binding of the carbonylgroup of PAA-Rho2 with the Au3+. It was clearly observed thatthe carbonyl stretching band of PAA-Rho2 at 1629 cm�1 was

Fig. 2 (a) Fluorescence spectral changes of PAA-Rho2 after the addition of 10mM of various cations. (b) Fluorescence responses of PAA-Rho1–PAA-Rho4 with10 mM of various cations (0.1 g L�1 of sensors in 0.01 mol L�1 of TBAPF6 in DMSO).

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changed to the lower wavenumber (1592 cm�1) aer bindingwith Au3+ as demonstrated in Fig. S4.†

UV-vis spectroscopy was also employed to determine bindingabilities of the complexes. Addition of Au3+ to PAA-Rho2 inDMSO caused a signicant enhancement of absorbance inten-sity (at 555 nm) as a result of the Au3+-induced ring opening ofthe spirolactam form (Fig. 4a). The results are also consistentwith the selectivity from uorescent titration. Upon the additionof 10 mMAu3+, there were changes in the uorescence spectra ofPAA-Rho2. Continuous uorescence enhancements at 590 nmwere observed (Fig. 4b). This can be explained by the suitabledistance and the so properties of the receptor structure (PAA-Rho2) to form a complex with Au3+ ions. In addition, thecompetition experiment was also carried out by adding Au3+ tothe solution of PAA-Rho2 in the presence of other metal ions as

Fig. 4 (a) Absorption spectra of PAA-Rho2 (0.1 g L�1 of the sensor in 0.01 molL�1 of TBAPF6 in DMSO) in the presence of different amounts of Au3+. (b)Fluorescence spectra of PAA-Rho2 (0.1 g L�1) under the same conditions.Ex ¼ 520 nm.

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shown in Fig. 5. The results indicate that the sensing of Au3+ byPAA-Rho2 is hardly affected by these common interfering ions.

The optimized geometries and the HOMO and LUMO ener-gies of the Au3+–PAA-Rho2 complex were calculated by thedensity functional theory (DFT) calculations at the B3LYP/LanL2DZ level using the Gaussian 03 program.22 The optimizedstructures of PAA-Rho2 complexes with Au3+ are illustrated inFig. 6 and S5.† The optimized location of Au3+ was foundbetween the oxygen atoms of two acyclic lactam groups inrhodamine moieties, conrming that the complexation indeedrequires the participation of amide and opening of the spi-rolactam ring which would lead to an increase of the C]O bonddistance, which was supported by the previous reports.18b Thesestructural changes were found to affect the HOMO/LUMOenergy levels of rhodamine chromophores (both HOMO andLUMO energy levels of the Au3+ complex became lower thanthose of the free ligand). Moreover, the energy gaps of thecomplex also decreased from 3.537 eV to 0.327 eV. The resultssuggested that the polymeric sensor formed stable complexeswith Au3+ through a large number of cation–dipole and ion–ioninteractions.

Monodisperse superparamagnetic Fe3O4 nanoparticles(FeNPs) were composed of a stable aqueous dispersion of ironoxide NPs with selective functionality through ligand exchange.The exchange process is illustrated in Scheme S2,† where theoriginal coatings of iron oxide NPs (oleic acid: OA; tri-octylphosphine oxide: TOPO) are replaced by PAA-Rho2. The

Fig. 5 Fluorescence enhancement response of PAA-Rho2 (0.1 g L�1) in 0.01 molL�1 of TBAPF6 in DMSO to 10 mM of different metal ions (the black bar portion)and to the mixture of 10 mMdifferent metal ions with 10 mMof Au3+ (the gray barportion).

Fig. 6 The optimized structures of PAA-Rho2 and PAA-Rho2 + Au3+ complexes,the gap energies (Eg ¼ ELUMO � EHOMO).

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sensitivity of the hybrid organic–inorganic material by Au3+ wasalso investigated. From the TEM images of PAA-Rho2-FeNPsolution (Fig. 7), we observed the aggregation of PAA-Rho2-FeNPs in the presence of Au3+ ions. It has been noted that atleast two rhodamine units were bound to different magneticnanoparticles which could interact with Au3+. Moreover, thecolor and uorescence properties remarkably changed aeradding Au3+ to PAA-Rho2-FeNPs in water (Fig. 8). The separationprocess of PAA-Rho2-FeNPs + Au3+ was easily performed byusing amagnet. In addition, we evaluated the reversibility of theabove Au3+ detection procedure in aqueous solution treatedwith an aqueous EDTA solution under basic conditions. Asexpected, upon the addition of the EDTA solution, the uores-cence intensity of PAA-Rho2-FeNPs + Au3+ was quenched. Aerwashing, the hybrid sensor was re-exposed to Au3+, and theuorescence emission was restored completely. The uores-cence change was reproducible over several cycles of exposure–recovery (Fig. 9). Although some variations in the intensities ofthe emission minima and maxima are observed (�11%); thesechanges do not affect the overall hybrid sensor performancewhich might be due to wettability properties of the hybridsensor.23 In addition, the detection limit based on the formationof PAA-Rho2-FeNPs (0.1 g L�1) with Au3+ was also evaluated asillustrated in Fig. S6 and S7.† The quantitative measurements ofthe emission maximum of Au3+-bound PAA-Rho2-FeNPs indi-cated that the uorescence change correlated linearly with theconcentration of Au3+ over the 0.7–1.7 mM range. We deter-mined that the limit of detection of PAA-Rho2-FeNPs for Au3+

was 0.85 mM, lower than that observed using PAA-Rho2(11.40 mM) and the detection time was less than 40 seconds. As

Fig. 7 TEM images of PAA-Rho2-FeNP solution in the absence and presence ofAu3+ ions.

Fig. 8 Color change and fluorescence changes of PAA-Rho2-FeNPs (0.1 g L�1)in the (a) absence and (b) presence of Au3+ ions (c) collection of hybrid sensorsusing a magnet.

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Fig. 9 Changes in the fluorescence (measured at 590 nm) of the PAA-Rho2-FeNPs during the cyclic detection–reactivation processes.

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compared to PAA-Rho2, PAA-Rho2-bound FeNPs gave a lowerdetection limit and a higher sensitivity toward Au3+.

Conclusion

We have investigated a selective detection of Au3+ by usingrhodamine-based modied polyacrylic acid (PAA) sensors.Polymeric sensors were simply prepared by amidation reactionsbetween PAA and rhodamine of various mole ratios. Chemicalstructures and purity of polymeric sensors were proven by TGA,NMR, TEM and IR. It was found that polymeric sensor PAA-Rho2 exhibited the highest selectivity and sensitivity responsivecolorimetric and uorescence Au3+-specic sensor over othermetal ions. The polymeric sensors were non-uorescent in thespirolactam form and were selectively converted to the uo-rescent-active ring-opened amide form in the presence of Au3+

ions that lead to uorescence enhancements and colorimetricchanges. Moreover, the FeNP-based polymeric sensor PAA-Rho2-FeNPs were designed and synthesized. The Au3+ chela-tion-induced aggregation of FeNPs resulted in TEM, color anduorescence changes and the sensor properties could berestored using the EDTA solution. We believe that, thisapproach may provide an easily measurable and inherentlysensitive method for Au3+ detection in environmental and bio-logical applications.

ExperimentalChemical and methods

All reagents were of standard analytical grade, PAA with anaverage molecular weight of 2000 and rhodamine waspurchased from Aldrich. DCC, DMAP, ethylenediamine, MeOHwere purchased from Merck and used without further puri-cation. Commercial grade solvents, such as acetone, hexane,dichloromethane, methanol and ethyl acetate, were distilledbefore use. DMF was dried over CaH2 and freshly distilled undernitrogen prior to use. Oleic acid (95%) was purchased fromMerck. Sodium oleate was synthesized from oleic acid andmetallic sodium by reuxing at 50 �C until the metal dis-appeared completely, and was washed with pure ethanol for atleast six times. White powder of sodium oleate was obtainedupon drying in a vacuum oven at 45 �C.24

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Instrumentation

NMR spectra were recorded on a Varian 400 MHz spectrometerin deuterated chloroform and DMSO-d6. MALDI-TOF massspectra were recorded on a Biex Bruker Mass spectrometerusing 2-cyano-4-hydroxycinnamic acid (CCA) or 2,5-dihydroxy-benzoic acid (DHB) as the matrix. UV-vis absorption measure-ments were performed on a Perkin-Elmer Lambda 25 UV/VISspectrometer. Fluorescence spectra were recorded using a Per-kin-Elmer luminescence spectrometer LS50B. Infrared spectrawere obtained on a Nicolet Impact 410 using KBr pellet. Columnchromatography was carried out using silica gel (Kieselgel 60,0.063–0.200 mm, Merck).

The fracture surface of the polymeric sensors and hybridmaterial before and aer addition of Au3+ were observed undera scanning electron microscope (SEM) (LEO 01455VP, Cam-bridge, England) operated at a voltage of 20 kV. Prior to exam-ination, the composite samples were immersed in liquidnitrogen for 30 min and then fractured. The specimens weresputter-coated with gold for enhanced surface conductivity.

Thermogravimetric analysis (TGA) was carried out using TAinstruments, SDT Q600 (Luken's drive, New Castle, DE). Theneat samples (8–10 mg) were loaded in an alumina crucible andthen non-isothermally heated from ambient temperature to1000 �C at heating rates of 20 �Cmin�1. The TGA was performedin a nitrogen atmosphere. The TGA data were simultaneouslyrecorded online in TA instrument's Q series explorer soware.

Determination of the acid number

The acid number for both PAA was determined according toASTM D 1045. The procedure was as follows: PAA (0.1 g) wasweighed into a 100 mL Erlenmeyer ask and dissolved in 25 mLof absolute ethanol. Then, a few drops of bromthymol blueindicator were added and the solution was titrated with asolution of 0.01 N NaOH. Also, the blank titration was carriedout on 25 mL of the solvent used to dissolve the sample. Theacid number expressed as the milligrams of NaOH per gram ofthe sample is as follows: acid number ¼ [(VNaOH(s) � VNaOH(b))N� 40]/C, which VNaOH(s) is the volume of NaOH used to titrate asample, VNaOH(b) is the volume of NaOH used to titrate a blank,N is the normality of the NaOH, and C is gram of the sample.The acid number for PAA was 21.6.

Synthesis

Synthesis of N-(rhodamine B) lactam-ethylenediamine(Rho). This was synthesized in amanner similar to the literatureprocedure with slight adaptation.25 Rhodamine B (0.20 g, 0.42mmol) was dissolved in 30 mL of ethanol and ethylenediamine(0.22 mL, excess) was added dropwise to the solution. Themixture was reuxed overnight (24 hours) until the solution lostits red color. The solvent was removed by evaporation. Water(20 mL) was added to the residue and the solution was extractedwith CH2Cl2 (20 mL � 2). The combined organic phase waswashed twice with water and dried over anhydrous Na2SO4. Thesolvent was removed by evaporation, and the product was driedin vacuo, affording a pale-yellow solid of Rho. Rho: (0.17 g, yield

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84%). 1H NMR (400MHz, CDCl3) d 7.86–7.81 (m, 1H, ArH), 7.45–7.32 (m, 2H, ArH), 7.08–7.03 (m, 1H, ArH), 6.42 (s, 1H, ArH), 6.39(s, 1H, ArH), 6.37 (s, 2H, ArH), 6.38–6.21 (m, 2H, ArH), 3.32 (q,J ¼ 6.8 Hz, 8H, NCH2CH3), 3.12 (t, J ¼ 6.8 Hz, 2H, NCH2CH2),2.23 (t, J ¼ 6.8 Hz, 2H, NCH2CH2NH2), 2.05 (s, 2H, CH2CH2NH2)and 1.16 (t, J ¼ 7.2 Hz, 12H, NCH2CH3). MS (MALDI-TOF); calcdfor [C30H36N4O2]

+: m/z 484.63. Found: m/z 485.91 [M + H]+.Synthesis of polymeric sensors (PAA-Rho1–PAA-Rho4).

Typically, a mixture of Rho and PAA in 20 mL of dried DMF wasallowed to react in the presence of dicyclohexylcarbodiimide(DCC) and p-(dimethylamino) pyridine (DMAP). The resultingmixture was heated to 50 �C for 1 hour and heated at reuxovernight. Then, the mixture was cooled to room temperature,and the product was precipitated by adding an excess of water.The above dissolution–precipitation cycle was repeated forthree times. Aer drying in vacuo over overnight at 45 �C, PAA-Rho1–PAA-Rho4 were obtained as pale-yellow solids.

PAA-Rho1 (0.17 g). Rho 0.20 g, 0.40 mmol; PAA 0.04 g; DCC0.08 g, 0.40 mmol; DMAP 0.02 g, 0.21 mmol. 1H NMR (400 MHz,DMSO-d6): 7.81 (bs, NHCO), 7.75 (bs, ArH), 7.44 (bs, ArH), 6.91(bs, ArH), 7.33 (bs, ArH), 5.54 (d, J ¼ 8 Hz, ArH), 3.22–2.96 (m,NHCH2CH2), 1.82–1.76 (m, NCH2CH2), 1.74–1.43 (m, CHCH2),1.28–1.15 (m, NCH2CH3) and 1.09–0.97 (m, NCH2CH3); IRspectrum (KBr, (cm�1)): 3630–3400 (OH), 3336 (NH), 2962 (]C–H), 2935, 2852 (–C–H), 1682 (C]O), 1620, 1517 (C]C), 1227 (C–O); Mw (NMR) ¼ 2977 g mol�1.

PAA-Rho2 (0.32 g). Rho 0.39 g, 0.80 mmol; PAA 0.04 g; DCC0.17 g, 0.80 mmol; DMAP 0.05 g, 0.40 mmol. 1H NMR (400 MHz,DMSO-d6): 7.93 (bs, NHCO), 7.81–7.71 (m, ArH), 7.48 (bs, ArH),7.02–6.94 (bs, ArH), 6.41–6.20 (bs, ArH), 5.57 (d, J ¼ 8 Hz, ArH),3.23–2.15 (m, NHCH2CH2), 1.72–1.44 (m, CHCH2), 1.27–1.16 (m,NCH2CH3) and 1.11–0.87 (m, NCH2CH3); IR spectrum (KBr,(cm�1)): 3605–3408 (OH), 3330 (NH), 2973 (]C–H), 2930, 2850(–C–H), 1680 (C]O), 1627, 1516 (C]C), 1221 (C–O);Mw (NMR)¼3296 g mol�1.

PAA-Rho3 (0.25 g). Rho 0.29 g, 0.60 mmol; PAA 0.02 g; DCC0.12 g, 0.60 mmol; DMAP 0.04 g, 0.30 mmol. 1H NMR (400 MHz,DMSO-d6): 7.78 (bs, NHCO), 7.75–7.73 (m, ArH), 7.50–7.44 (m,ArH), 6.98–6.95 (m, ArH), 6.35–6.29 (m, ArH) 5.55 (d, J ¼ 8 Hz,ArH), 2.94 (d, J ¼ 8 Hz, NHCH2CH2), 2.17 (d, J ¼ 8 Hz,NCH2CH2), 1.73–1.44 (m, CHCH2), 1.28–1.67 (m, NCH2CH3) and1.15–0.98 (m, NCH2CH3); IR spectrum (KBr, (cm�1)): 3611–3420(OH), 3327 (NH), 2973 (]C–H), 2928, 2848 (–C–H), 1687 (C]O),1621, 1517 (C]C), 1223 (C–O); Mw (NMR) ¼ 4300 g mol�1.

PAA-Rho4 (0.34 g). Rho 0.39 g, 0.80 mmol; PAA 0.02 g; DCC0.17 g, 0.80 mmol; DMAP 0.05 g, 0.40 mmol. 1H NMR (400 MHz,DMSO-d6): 7.93 (bs, NHCO), 7.77–7.72 (m, ArH), 7.51–7.44 (m,ArH), 7.04–6.95 (m, ArH), 6.39–6.28 (bs, ArH), 5.55 (d, J ¼ 8 Hz,ArH), 2.94 (d, J ¼ 8 Hz, NHCH2CH2), 2.17 (m, d, J ¼ 8 Hz,NCH2CH2), 1.73–1.45 (m, CHCH2), 1.27–1.17 (m, NCH2CH3) and1.14–0.98 (m, NCH2CH3); IR spectrum (KBr, (cm�1)): 3607–3417(OH), 3327 (NH), 2973 (]C–H), 2928, 2853 (–C–H), 1691 (C]O),1621, 1512 (C]C), 1223 (C–O); Mw (NMR) ¼ 5891 g mol�1.

Complexation studies. Complexation studies of ligands werecarried out by using UV-vis and uorescence titrations. Thecomplexation abilities of ligands PAA-Rho1–PAA-Rho4 withcations were investigated by spectrophotometric titration in

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DMSO at 25 �C. 2 mL of the 0.1 g L�1 PAA-Rho1–PAA-Rho4solutions were placed in a spectrophotometric cell (1 cm pathlength). The solutions of cationswere added successively into thecell from amicroburette. The mixture was stirred for 40 secondsaer each addition and its spectral variation was recorded.

Competition experiments. Au3+ was added to the solutioncontaining PAA-Rho2 and other metal ions. All test solutionswere stirred for 1 min and then allowed to stand at roomtemperature for 30 min. For uorescence measurements, theexcitation was performed at 520 nm, and emission spectra wererecorded from 530 to 700 nm.

Synthesis of the iron oleate complex. The iron oleatecomplex was prepared according to a previously reportedmethod with modications.26 In brief, ferric chloride (3.50 g,0.01 mol) was mixed with sodium oleate (10.1 g, 0.03 mol) in asolvent mixture (hexane, 40 mL; ethanol, 20 mL; and 15 mL ofdeionized water) at 65 �C for 4 h. As the reaction proceeded, theinitial brown transparent solution became black. Aer phaseseparation, the organic phase containing the iron oleatecomplex was washed with deionized water and dried inside afume hood overnight at room temperature. The obtainedbrownish-black paste was used as the precursor for iron oxideNP synthesis.

Synthesis of iron oxide NPs. Iron oxide NPs were synthesizedby heating the iron oleate complex (2.5 g, 2.8 mmol) in 1-octa-decene (10 mL) in the presence of TOPO/OA (TOPO: 0.2 g, 0.5mmol; OA: 0.22 mL, 0.7 mmol). The mixture was heated at100 �C for 1 h to remove the residual solvents before heating upto 320 �C and then kept at this temperature for 2.5 h under N2.Aer the reaction, the mixture containing the nanoparticles wasrapidly cooled to room temperature. The as-synthesized NPswere precipitated out of solution by centrifugation and thendried under vacuum overnight. The well-dried powder was thenredissolved in MeOH under sonication to obtain the stocksolution of 5 mg mL�1 for the ligand exchange process.

Ligand exchange process of iron oxide NPs with PAA-Rho

The ligand exchange process was conducted by mixing the NPstock solution with the exchange ligands in MeOH at roomtemperature for 48 h. In the typical procedure, 5 mL of stocksolution (5 mg mL�1) was mixed with 5 mL of PAA-Rho (0.1 gL�1). Then, the mixture was kept at RT for 1 h before heating upto 60 �C and then kept at this temperature for 6 h. PAA-Rho-FeNPs were then magnetically collected and redispersed into5 mL of water (pH ¼ 7). These water-soluble NPs were stable atroom temperature for months without notable precipitation,even changes the pH from 5 to 10.

Acknowledgements

The authors gratefully acknowledge funding from Mahasarak-ham University and the Thailand Research Fund (RTA5380003)and Research Grant for Mid-Career University Faculty andCenter of Excellence for Innovation in Chemistry (PERCH-CIC),Office of the Higher Education Commission, Ministry ofEducation.

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Notes and references

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